Functionalization of Spherical Carbons with Metal Complexes and Ionic Liquids for Application in Catalysis and Gas Purification

Transcription

1 Functionalization of Spherical Carbons with Metal Complexes and Ionic Liquids for Application in Catalysis and Gas Purification Funktionalisierung sphärischer Aktivkohlen mit Metallkomplexen und ionischen Flüssigkeiten zur Anwendung in Katalyse und Gasreinigung Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr.-Ing. vorgelegt von Heiko Klefer aus Jever

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3 Als Dissertation genehmigt von der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg. Tag der mündlichen Prüfung: 11. Dezember 2015 Vorsitzender des Promotionsorgans: Prof. Dr. Peter Greil Gutachter: Prof. Dr. Peter Wasserscheid Prof. Dr. Martin Hartmann

4 Vorwort Das Vorwort ist in der elektronischen Version der Dissertation nicht verfügbar.

5 Publications Parts of this work have been previously published in the following publications listed. Patent B. Böhringer, S. Fichtner, C. Schrage, J.-M. Giebelhausen, P. Wasserscheid, B.J.M. Etzold and H. Klefer. Katalysatorsystem und dessen Verwendung. DE Patent 102,014,103,351 A Publication H. Klefer, M. Munoz, A. Modrow, B. Böhringer, P. Wasserscheid and B.J.M. Etzold. Polymer-based spherical activated carbon as easy-to-handle catalyst support for hydrogenation reactions. Chem. Eng. Technol i

6 Contents 1. Introduction 1 2. Theoretical and technical background Activated carbon Activated carbon materials Polymer-based activated carbon particulates Activated carbon as catalyst support Activated carbon support materials in catalysis Mass transport in catalysis - aspects of diffusion High-purity polymer-based activated carbon particulates Post-treatment of catalyst - catalyst separation Carbon surface functionalization Surface oxidation Deposition of noble metal The role of metal precursors in catalyst preparation The role of C-π sites in catalyst preparation Reactions studied in this work Activated carbon in gas purification Activated carbon adsorbents in gas purification Functionalization of activated carbon with reactive surfaces Application of activated carbon adsorbents in gas purification Air filtration in clean rooms and for personal protection Industrial exhaust gas cleaning Functionalization of activated carbon with ionic liquids Supported ionic liquid phase (SILP) concept Properties of ionic liquids Reactive metal complexes dissolved in ionic liquids ii

7 Contents Application of SILP adsorbents in gas purification SILP adsorbents based on inorganic supports [9, 10] SILP adsorbents based on spherical carbon Gases studied in this work Objective of this work Experimental Catalyst preparation and characterization Oxidation of spherical carbon Deposition of noble metal Evaluation of catalytic activities Evaluation of filtration rates SILP material preparation and characterization Preparation of SILP materials Evaluation of SILP corrosivity Evaluation of gas purification performance Materials Overview of applied spherical carbon materials Synthesis of metal salts and ionic liquids Analytical methods Results and discussion Filtration and pressure drop of spherical carbon Spherical carbon in lab-scale filtration Spherical carbon in large-scale filtration Spherical carbon in flow chemistry applications Spherical carbon as novel catalyst support material Carbon surface functionalization Influences on the amount of functional surface groups Influences on the chemical composition of functional surface groups Influences on spherical carbon acidity Influences on carbon surface wettability Influences on carbon pore structure iii

8 Contents Important differences between nitric acid and sulfuric acid oxidation Active metal deposition Palladium: Influences on metal loading and dispersion Ruthenium: Influences on metal loading and dispersion Platinum: Influences on metal loading and dispersion Catalytic performance Palladium catalysts: Influences on catalytic activity Ruthenium catalysts: Influences on catalytic activity Platinum catalysts: Influences on catalytic activity The role of C-π sites Stability tests Catalyst leaching Catalyst recycling Proof of concept: SILP catalyst in slurry phase reaction Spherical carbon as support for SILP filter materials Preliminary investigations for the advancement of SILP technology in gas purification Influence of impregnation solvent on SILP product quality Influence of metal salt species on ammonia and hydrogen sulfide adsorption Corrosion investigations of halide-containing SILP materials Filter material improvements for irreversible adsorption of ammonia and other hazardous gases Development of halide-free SILP materials Alternative coating techniques of spherical carbon Organic copper salts in SILP materials Development of a filter material for formaldehyde removal Design of SILP materials for pressure and temperature swing adsorption processes Reversible adsorption of ammonia Reversible adsorption of hydrogen sulfide iv

9 Contents 6. Summary and Outlook Summary / Abstract Zusammenfassung / Abstrakt Outlook A. Lists of chemical substances and suppliers II B. Carbon surface functionalization VI B.1. Influences on the amount of functional surface groups VI B.2. Influences on the chemical composition of functional surface groups.... VII B.3. Influences on spherical carbon acidity VIII B.4. Influences on the pore structure of spherical carbon IX C. Catalytic experiments X C.1. Catalytic experiments concerning carbon pore structure X C.2. Catalytic experiments concerning carbon surface functionalization..... XII D. List of Algorithms XIV E. List of Figures XV F. List of Tables XX References XXV v

10 1. Introduction Activated carbon is a highly porous, carbonaceous substance with a characteristically large internal surface area. Society benefits from the adsorptive properties of activated carbon for more than 3500 years. In medicine, for instance, a variety of poisonous substances are adsorbed by activated carbon. Adsorption to activated carbon is technically applied in product purification by the food, chemical, pharmaceutical and hydrometallurgy industries, as well as in water treatment and air purification.[1 3] Furthermore, modified activated carbon materials find use in catalysis, sensors, batteries, fuel cells, capacitors, hydrogen storage, and magnetic materials.[4] Polymer-based spherical activated carbon (PBSAC, spherical carbon) is an advanced material with combined features derived from its polymer origin, spherical shape, and carbonaceous nature. The material is industrially produced on a large scale. Spherical polystyrene-divinylbenzene precursor material from a suspension polymerization process is classified into the desired particle size distribution by sieving. After the polymer is thermally stabilized by a sulfuric acid treatment, a carbonization step follows at elevated temperatures. A final steam and carbon dioxide activation process produces the spherical activated carbon material.[5, 6] The pore size distribution of spherical carbons is tunable in a broad range. Total pore volumes up to 1.3 cm 3 g -1 and BET surface areas as large as 1946 m 2 g -1 are realized. Due to its particulate character, spherical carbon is easy to handle and exhibits low pressure drops in flow applications. The constant chemical composition of the polymer precursor improves production reproducibility and results in high purity carbon with very low ash content. Thus, high mechanical stability is achieved. The large internal surface area and good flow characteristics render spherical carbon a very suitable filter material for organic molecules and metal ions.[5, 7] Due to its carbonaceous nature, spherical carbon features a simple disposal of spent material, resistance to high temperatures [8], chemical inertness [1], and hydrolysis stability [4]. Nevertheless, utilization of conventional activated carbon is economically more feasible in many 1

11 1. Introduction application scenarios. Spherical carbon excels in advanced processes profiting from most of its outstanding features. One current example is continuous gas purification for clean rooms and personal protection.[1, 5] In this work, two novel applications of spherical carbon are investigated, in the fields of catalysis and advanced gas purification. In the first part of this thesis, the development of catalysts based on metal-loaded spherical carbon is presented. The second part deals with continuous gas purification using spherical carbons coated with thin films of ionic liquids, so called supported ionic liquid phase (SILP) materials. In both cases, a fundamental understanding of the carbon surface and subsequent surface modifications are essential for successful application of the final products. The resulting materials are then screened for their performance in a broad variety of chemical reactions and gas adsorption/desorption processes, respectively. Spherical carbon as novel catalyst support material In heterogeneous catalysis, catalytically active species are immobilized in a second, solid phase and are contacted with the fluid reactant mixture at reaction conditions. The catalytic active species interacts with the reactants, reducing required activation energies and enhancing reaction rates. The advantage of heterogeneous catalysis is the ease of separating catalyst and product by filtration, which allows recycling of the catalyst. The selection of support media for catalytically active species depends on various process parameters. Activated carbon and polymer-based spherical activated carbon, in particular, are very interesting support materials for many reasons. The most relevant aspects are pointed out in the following. Here, the benefits of activated carbons compared to inorganic support materials as well as spherical carbon compared to conventional activated carbon powder are elaborated. Advantages of activated carbon compared to inorganic supports: The exceptionally large surface area of activated carbon materials enables the catalytically active species to be highly dispersed and the number of principally accessible catalytic sites to be maximized. This results in large intrinsic catalytic activities. Carbon materials can be functionalized, e.g. with oxygen surface groups, to facilitate deposition of catalytically active species. So, the activated carbon surface can be tuned to irreversibly retain the catalytically active species. With emerging green chemical technology, an increasing amount of sustainable platform chemicals and derived compounds are catalytically produced. These reactants are usually accompanied with traces of water. Water negatively affects the integrity of many today s catalysts, which are based on inorganic metal oxides. So, more hydrolysis- 2

12 1. Introduction stable materials need to replace conventional catalysts. Carbon supported catalysts are the prospective alternative. Advantages of spherical carbon compared to activated carbon powder: The particle size and pore structure of the spherical carbon support can be adjusted to fit the reaction system best. Smaller particles and a hierarchically structured pore system, for instance, enhance pore diffusion and mass transport of reactant molecules between catalytically active sites and bulk solution. In the industrial production of chemical commodities, as well as in analytical chemistry, reaction and post processing times are both economically important. Today s application of powdered activated carbon catalysts in chemical reactions makes product separation needlessly tedious. The particulate nature of spherical carbon allows for easy catalyst filtration and efficient product separation. Thus, post processing times are minimized. Catalyst recycling is possible, as well. In flow chemistry applications, the low pressure drop of the spherical catalyst beds is of great advantage. Fluids can pass the bed of spherical catalyst material with low resistance, thus enlarging the window of operation. Additionally, spherical shape and particulate character improve material handling during reactor (un-)loading and catalyst preparation due to good and dust-free material pourability. The reliably high purity of polymer-based carbon evidenced by the very low ash content is ideal for pharmaceutical and fine chemical processes. Thus, the reaction solution isn t contaminated with catalyst impurities. The good batch-to-batch reproducibility of spherical carbon further strengthens reliability of the final catalyst material to generate the desired outcome in terms of product selectivity and catalytic activity. Because one spherical carbon particulate is produced from one polymer particulate, mechanical stability is larger than that of traditionally molded carbon pellets based on activated carbon powder. The large mechanical stability and the resulting high integrity of the particulate material allow the application in stirred reactor setups with high-impact flow characteristics. In this research work, spherical carbon materials with different particle size and pore structure are functionalized with oxygen surface groups using various oxidation conditions. The materials are then loaded with noble metals applying the principle of electrostatic adsorption of metal ions and subsequent metal transformation. The spherical carbon surface modifications are characterized in detail. Catalytic activities are determined in hydrogenation test 3

13 1. Introduction reactions. The dehydrogenation of an organic liquid hydrogen carrier molecule using these PBSAC-based catalysts is investigated as technical relevant test reaction. Spherical carbon as support for SILP filter materials In continuous gas purification, as discussed in the second part of this thesis, specific molecules are removed from gas streams passing through filter materials. In this process, the specific molecules are physically or chemically bound to adsorption sites of the filter material. In certain applications, such as air filtration in clean rooms and for personal protection, the gas stream has to be completely purified from very low concentrations of hazardous molecules, usually by irreversible reaction with the filter material. In many industrial applications, on the other hand, a weaker interaction of specific molecules with adsorption sites is desired in order to economically regenerate the filter material. The engineering objectives to improve filter materials for continuous gas purification are increasing the number of adsorption sites for specific molecules and tuning adsorption processes for reversible or irreversible adsorption. The SILP technology is a novel approach to significantly improve filtration performance. SILP materials are porous solid materials coated with thin films of ionic liquids. The ionic liquids incorporate solubility and reactivity for specific molecules. The diffusion of molecules into the ionic liquid volume significantly increases the number of available sorption sites compared to adsorption to a plain solid surface. The strength of interaction with specific molecules is tuned by modifying the ionic liquid composition allowing for both irreversible and reversible filtration processes. With different reactive sites in the ionic liquid and at the solid support, SILP materials can retain a broad variety of molecules utilizing different reaction pathways.[9, 10] Several distinct material properties render spherical carbon a well suitable support material for ionic liquids in continuous gas purification compared to inorganic support materials and activated carbon powder. The majority of the following aspects have already been pointed out to apply to catalyst support materials, as well. Advantages of spherical carbon compared to inorganic supports: Due to the very large surface area of activated carbon, the gas/liquid interface area resulting from the ionic liquid coating is similarly large. Thus, mass transfer from the gas phase into the ionic liquid film is facilitated. The activated carbon material itself offers additional adsorption capacity for different hazardous gases. This adsorption capacity is currently utilized to realize SILP broadband filters. Other hazardous gases are removed by the thin film of ionic liquid partially 4

14 1. Introduction covering the carbon surface. Advantages of spherical carbon compared to activated carbon powder: Spherical carbon filter materials exhibit low pressure drops. The pressure drop isn t affected by the ionic liquid coating. Even smaller pressure drops result from open porous foam filter media, furnished with spherical carbon. Low pressure drops in filter materials are highly important for applications with a limited maximum pressure difference, e.g. personal protection and process extensions. For economic reasons, low pressure drops are generally desirable in industrial filtration processes. The handling benefits of spherical carbon are significant. Both, loading and unloading of the adsorber units, as well as SILP material preparation, are simplified. In the past years, the SILP group at the Institute of Chemical Reaction Engineering in Erlangen and other research groups worldwide have developed SILP filter materials based on inorganic support materials for a variety of applications.[11 15] The investigations focused on the reversible and irreversible adsorption of mercury compounds, organic sulfur compounds, sulfur dioxide, and carbon dioxide from a fluid phase. More recently, the research group in Erlangen investigated spherical carbon as a novel support material.[16, 17] Research started with the successful development of an ammonia filter for personal protection purposes with additional broadband capacities for other types of hazardous gases. In an ongoing project, this work continues with product optimizations and advances into other fields of application, i.e. industrial gas separation and clean room filtration. In this research work, spherical carbon is coated with ionic liquids that contain reactive properties for a variety of hazardous gases. These reactive properties are mainly introduced by dissolving metal salts within the ionic liquid. Already at room temperature, the metal ions selectively react with dissolved acidic gases like hydrogen sulfide or ammonia species forming metal complexes. For certain gases and ionic liquid impregnations, reducing pressure or increasing temperature regenerates the SILP materials. 5

15 2. Theoretical and technical background 2.1. Activated carbon Activated carbon materials Activated carbon materials are porous, carbonaceous substances with pore volumes of more than 0.2 ml g -1 and internal surface areas of at least 400 m 2 g -1. Pore sizes can range over five orders of magnitude, starting as low as 0.3 nm. Activated carbons exhibit adsorptive surface properties for a broad range of substances. They can be thermally and chemically well durable. Also, they can possess a good electrical conductivity and capacity. These properties render activated carbons an attractive material for many applications. In gas-phase and liquid-phase adsorption processes, activated carbons adsorb specific substances. For instance, they are technically applied in the purification of air and water, the decolorization of sugar solutions and the recovery of gold and silver. In catalysis, activated carbons are catalyst support or the catalyst itself. The electrical properties of activated carbons are utilized in capacitors and fuel cells. Here, activated carbon is the electrode material. Activated carbon products still under development are hydrogen storage materials, batteries and sensors [4].[1, 2, 8] Synthesis of activated carbons Commercially available activated carbon is typically produced from organic raw materials such as coal, wood, coconut shells and other fruit kernels. The raw materials are carbonized at temperatures between C in inert atmosphere, yielding a slightly porous material with a carbon content of more than 80 wt%. Afterwards, the carbon material is further activated by carbon gasification, thus creating the final pore structure. Physical activation with steam, carbon dioxide or air takes place at temperatures between C. The respective carbon gasification reactions are presented in scheme 2.1. Compared to the exothermic 6

16 2. Theoretical and technical background Scheme 2.1 Carbon gasification by steam, carbon dioxide and air [2] C + H 2 O CO + H 2 H = 121 k J mol 1 C + CO 2 2CO H = 163 k J mol 1 C + O 2 CO 2 H = 406 kj mol 1 air oxidation process, the endothermic gasification reactions with steam and carbon dioxide are easier to control by heat management. To enhance the steam gasification process, catalytically active species (e.g. alkali metal oxides, iron, copper) are added. Alternatively, raw materials are carbonized and activated chemically in a single process step. Therefore, the raw materials are mixed with dehydration agents like phosphoric acid, zinc chloride or sulfuric acid and treated in inert atmosphere at temperatures between C. Compared to physical activation, lower temperatures are applied in chemical activation processes. On the other hand, chemical activation involves corrosive chemicals that need to be washed out after the activation process.[2, 8, 18 20] Structure and composition of activated carbons Forms of activated carbon, that are commercially available, include pulverized and granular activated carbon as well as carbon extrudates. Extrudates consist of pulverized activated carbon mixed with a binder. The mixture is formed into pellets and the binder is carbonized in an additional carbonization step. However, mechanical stress is induced between the activated carbon and the binder during this process, which reduces mechanical stability of extrudates.[1, 2, 21] The overall structure of activated carbon is highly disorganized. Activated carbon is assembled of crystalline, graphitic microstructures and amorphous carbon. The degree of graphitization generally increases with increasing activation temperature. The edges of these graphitic microstructures consist of unsaturated carbon-carbon bonds. These so called C-π sites are important adsorption sites in activated carbon materials. Voids between the carbon layers resemble the slit-shaped pore system. A schematic representation of the activated carbon structure is shown in figure 2.1. A certain amount of heteroatoms like oxygen is incorporated into the activated carbon structure, as well. The fraction of noncombustible impurities is noted as ash content. These impurities, originating from the raw material, make precise control of the activation process more difficult and negatively affect batch-to-batch reproducibility. Also, a large ash content reduces mechanical stability of the activated carbon.[1, 2, 4, 22] Due to the carbon activation process, the bulk material is perforated by a hierarchically struc- 7

17 2. Theoretical and technical background Figure 2.1.: Schematic structure of activated carbons ([22], modified) tured pore system. The extended, slit-shaped pore network starts with macropores and mesopores leading into micropores. Depending on the pore width, pores are classified into three categories: macropores (d pore > 50 nm), mesopores (2 d pore 50 nm), and micropores (d pore < 2 nm) [23]. Especially the large quantity of small micropores contributes to the outstandingly large internal surface area of activated carbon materials. Internal surface areas up to 2500 m 2 g -1 are reached. The resulting pore structure depends on the raw material, as well as the carbonization and activation process conditions. Figure 2.2 shows typical pore size distributions of physically activated carbon derived from coconut shell and coal. For the coconut shell material, the pore size distribution is monodisperse around a pore diameter of 2 nm. In case of the coal-based material, the pore system is more hierarchically structured. Here, micropores, mesopores and macropores are present in significant quantities. The coconut shell activated carbon is ideal for the adsorption of small molecules. In applications dealing with larger molecules, e.g. catalysis, the coal-based material with a larger meso- and macropore content is more suitable.[1, 2] Characterization of activated carbons A qualitative impression of the bulk carbon material is observable in electron microscopy images. The degree of carbon graphitization is elucidated by X-ray diffraction. Thermogravimetric analysis allows the quantification of volatile compounds and the ash content. The pore structure is typically analyzed by nitrogen sorption measurements. Applying DFT calculations to sorption isotherms [24], pore size distributions between nm are gen- 8

18 dv / a.u. 2. Theoretical and technical background d pore / nm Coconut shell Coal Figure 2.2.: Pore size distributions of physically activated carbons derived from coconut shell and coal ([2], modified) erated, for instance. In carbon dioxide sorption experiments, the micropore structure is illuminated with higher resolution. Mercury porosimetry is a suitable analysis method for the characterization of mesopores and macropores. By these means, relevant parameters such as pore size distribution, surface area, total pore volume, and micropore volume are derived.[1] Polymer-based activated carbon particulates Polymer-based activated carbon particulates are a group of activated carbon materials with synthetic polymers used as raw material. The final activated carbon product derived from polymer materials features a particularly low ash content. The constantly high material purity results from the defined chemical composition of the polymer precursor. Particle size and pore geometry of the resulting activated carbon are specifically tailored by modifying the polymer structure. Batch-to-batch production of polymer-based carbon materials results in reproducible physicochemical properties.[4, 21] The complete production sequence of polymer-based activated carbon particulates, as shown in figure 2.3, generally consists of four main process steps: polymerization, chemical treatment, carbonization and physical activation. From the polymer precursor solution, the process chain begins with a polymerization reaction forming the polymer. The polymer particles are classified into size fractions. A chemical treatment follows to improve its thermal stabil- 9

19 2. Theoretical and technical background Polymer precursor solution Carbonization Physical activation No Polymerization Chemical activation? Yes Chemical activation Classification (Sieving) No Classification (Sieving) Chemical stabilization? Yes Chemical treatment Activated carbon particulates Figure 2.3.: Production sequence of polymer-based activated carbon particulates ity. The polymer is then carbonized by pyrolysis and further activated via partial oxidation and carbon gasification. Finally, the activated carbon particulates are classified into size fractions. The chemical stabilization treatment can be omitted, if the polymer material is stable enough. Instead of the carbonization and physical activation sequence, the polymer can be transformed into activated carbons with chemical dehydration agents. The final pore structure is established during the activation process. Nevertheless, in many cases, controlled polymerization already induces a pore structure. Porosity further increases with every process step.[2, 5, 6, 25] Polymer formation Several different polymer precursors have been successfully applied in the preparation of activated carbon particulates. Prominent examples are phenole-formaldehyde resins, resorcinolformaldehyde resins, acrylonitrile-divinylbenzene copolymers, and polystyrene-divinylbenzene copolymers (see figure 2.4). In the following, the formation of these polymer precursors and corresponding stabilization measures are discussed. Afterwards, an overview of carbonization and carbon activation procedures is given. Phenolic resins are synthesized in a polymerization process at very acidic conditions. The phenol and formaldehyde reactants are provided at a molar stoichiometry of e.g. 1 to After polymer chain growth and partial crosslinking, a macroporous resin is obtained. The resin can be further hardened with a curing agent like hexamethylene tetramine, introducing some nitrogen crosslinks. The critical step during polymerization is the controlled 10

20 2. Theoretical and technical background partial crosslinking of single polymer molecules. Controlled partial crosslinking guarantees successful particulate formation and the preservation of the resin s macroporous structure. If the resin is too soft, it will melt during thermal treatments and loose its porosity. On the other hand, if the resin is completely hardened, particulate formation by thermal sintering will be difficult. For particulate formation, the phenolic resin is at first pulverized and classified into size fractions of e.g μm. The desired form of the material is then produced with these small particles, which are thermally sintered together. The sintered polymer particulates determine the shape of the final carbon particulates. By this method, activated carbon particulates are produced in various forms and sizes. As no additional binder is necessary, mechanical stability of the material is high.[21, 26] Spherical resorcinol-formaldehyde polymers are obtained in an emulsion polymerization process. Therefore, a resorcinol-formaldehyde molar ratio generally between 1:1 and 1:4 is provided in an alkaline environment and mixed with an organic solvent like cyclohexane and a surface agent at temperatures between C for between 1-7 days. Sodium carbonate or potassium carbonate acts as polymerization catalyst. Emulsion viscosity and stirring speed determine the polymer particle size. Particle diameters are adjustable to values between μm. The polymer drying process significantly influences the carbon structure. One approach is to exchange water inside the polymer spheres with acetone and to then dry the material in supercritical carbon dioxide. This results in mesoporous activated carbon spheres.[20, 27] Another starting material for the production of polymer-based activated carbon is polystyrene-divinylbenzene. In a suspension polymerization process, the monomer mixture is stirred in an aqueous solution containing dispersing agents and polymerization initiator at around C. The monomer mixture is dispersed into small droplets and polymerizes into individual beads. So, spherical copolymers are formed consisting of polystyrene cross-linked with divinylbenzene. Crosslinking increases the thermal stability of the material. A sulfonation treatment with sulfuric acid further improves the thermal stability of the copolymer. An alternative to sulfonation is phosphorylation of the polymer material. Thus, the polymer precursor remains sulfur-free, but thermal stability lies between the untreated polymer and the sulfonated material. Also, air oxidation of the polymer material at 250 C increases thermal stability by introducing additional crosslinks.[25, 28 31] The addition of di- and trivalent metal ions to the sulfonated polymer resin via ion exchange results in microporous spherical carbon. The multivalent metal ions establish crosslinks between different functional groups, thus stabilizing the structure.[32] 11

21 2. Theoretical and technical background OH H 2 C (a) n (c) m n OH H 2 C N OH (b) n (d) m n Figure 2.4.: General molecular structures of polymer resins: phenol-formaldehyde resin (a), resorcinol-formaldehyde resin (b), polystyrene-divinylbenzene copolymer (c), acrylonitril-divinylbenzene copolymer (d) Acrylonitrile-divinylbenzene is an alternative starting material for spherical carbon. Spherical acrylonitrile-divinylbenzene copolymers are also formed in a suspension polymerization applying pore structure building solvents like toluene and nonane. The polymer structure is stabilized by partial oxidation in air at around C, with 250 C giving best carbon yields.[33 35] Polymer carbonization and carbon activation Carbonization and physical activation processes are similar for all applicable raw materials, in principle. In an oven, the materials are heated in an inert atmosphere to temperatures between C applying a certain temperature program. The exact temperature program of carbonization, including heating rates and durations of constant temperature, varies in dependence of the raw material and the desired carbon composition. Afterwards, the pore system is extended by partial oxidation with steam or carbon dioxide. These endothermic oxidation reactions require increased temperatures of around C. As an alternative, chemical activation of polymers is carried out to establish an extended pore structure. As chemical dehydration agent, potassium hydroxide or sodium hydroxide is applied. The materials are treated in inert atmosphere at temperatures between C. In a washing step, 12

22 2. Theoretical and technical background excess dehydration agents in the activated carbon are removed. A nitric acid treatment of polymers at room temperature also results in activated carbons.[20, 36 39] Phenolic resins are carbonized with a yield of around 40-50% by heating up to 900 C and activated with carbon dioxide at C. Depending on the degree of activation, BET surface areas up to 2130 m 2 g -1 and pore volumes of 2.13 ml g -1 are reached. The pore size is directly dependent on the size of the small, sintered particles. Pore geometry is influenced by the formaldehyde-phenol ratio, with a ratio below 0.5 leading to mesopores.[21, 40] Mesoporous activated carbon spheres are also produced by carbonization and steam activation at 800 C of polymer spheres containing ferrocene as pore-forming agent.[26] Heating spherical resorcinol-formaldehyde polymers up to temperatures between C, BET surfaces areas up to 779 m 2 g -1 and total pore volumes up to 0.68 ml g -1 are obtained after carbonization.[27] Highly-activated carbons with a dominating micropore content result from chemical activation with potassium hydroxide at temperatures up to 700 C. With increasing amount of applied potassium hydroxide, pore volume and BET surface area increase up to 1.35 cm 3 g -1 and 2760 m 2 g -1, respectively.[36, 37] After carbonization by heating up to 900 C, the defined molecular structure of the polystyrene-divinylbenzene copolymer remains preserved.[41] Thus, bimodal pore size distributions are obtained with large mesopore volumes of up to 0.44 cm 3 g -1.[31] With the help of steam and carbon dioxide activations at 800 C, the pore system is extended on demand and BET surface areas increase up to 2000 m 2 g -1. Chemical activation with potassium hydroxide at 770 C after carbonization of polymer spheres at C results in highly-activated spherical carbons. With increasing carbonization temperature, spherical carbon becomes more compact and its hardness increases. Increasing the amount of potassium hydroxide, total pore volumes up to 0.78 cm -3 g -1 and BET surface areas up to 2022 m 2 g -1 are reached. Carbon hardness slightly decreases with increasing pore volume. Still, mechanical stability of this polymer-based spherical activated carbon is significantly larger than that of traditional activated carbon materials. Particle size of spherical activated carbon is determined by the particle size of the starting material.[39] Technical considerations allowing efficient large-scale production of spherical carbon based on the polystyrene-divinylbenzene copolymer have been proposed, as well.[6] Polystyrene-divinylbenzene based spherical activated carbon with different particle size fractions is depicted in figure 2.5. Acrylonitrile-divinylbenzene-based material, carbonized at 850 C, exhibits BET surface areas between m 2 g -1, depending on the amount of pore structure building substances added during polymerization. Meso- and macroporous carbon materials result for less inten- 13

23 2. Theoretical and technical background Figure 2.5.: Light microscopy images of differently sized polystyrene-divinylbenzene based spherical activated carbons ([5], modified) sively cross-linked polymers.[35] Polymer-based activated carbon particulates used in this work As shown in this section, a variety of different polymers and copolymers are successfully transformed in subsequent process steps into activated carbon particulates. Polymer-based activated carbon materials excel due to their high material purity and large mechanical stability. Also, the pore system can be structured in detail, already starting at the polymerization step. Material properties are reproducible in batch-to-batch productions, as well.[42] In this work, the spherical activated carbon particulates are based on polystyrene-divinylbenzene polymers (see figure 2.5). A variety of particle sizes and pore structures is available. These spherical carbons are already produced at a larger scale and are thus readily available. The applications of activated carbon as catalyst support and in advanced gas purification are discussed specifically in the following sections of this chapter. 14

24 2. Theoretical and technical background 2.2. Activated carbon as catalyst support Activated carbon support materials in catalysis In heterogeneous catalysis, activated carbon is the catalyst support material of choice in the synthesis of many fine chemical and pharmaceutical products. Chemical reactions such as hydrogenations, dehydrogenations, oxidations, hydrogenolysis reactions and hydrodehalogenations are effectively and selectively promoted by carbon supported noble metal catalysts.[42 44] For instance, the hydrogenation of alkines and alkenes is catalyzed by palladium on carbon. Rhodium and ruthenium on carbon are generally the most active catalysts in the hydrogenation of carbocyclic rings. Heterocyclic compounds are easily hydrogenated by a variety of noble metal catalysts, with palladium and ruthenium being ideal in case of nitrogencontaining heterocycles. Platinum on carbon performs well in the selective hydrogenation of α,β-unsaturated aldehydes to the corresponding alcohols. Concerning dehydrogenation reactions, palladium and platinum on carbon are generally the most active catalysts. Palladium on carbon is applied in oxidations, hydrogenolysis and hydrodehalogenation reactions.[42 45] Technical applications of carbon supported noble metal catalysts are, for example, the hydrogenation of thymol and menthone to produce menthol [46, 47], the hydrogenation of cinnamaldehyde and crotonaldehyde [44], and the hydrogenation and dehydrogenation of liquid organic hydrogen carriers [48, 49]. Several unique features render activated carbons a very well suitable catalyst support material. Their large specific surface area allows for large dispersions of noble metals. Metal dispersion is the ratio of surface atoms to total metal content. Large dispersions are usually desired for good catalytic activities, i.e. many noble metal atoms are accessible to reactants. Also, the internal surface of activated carbons can be specifically functionalized or remain unmodified. Thus, metal-support and reactant-support interactions are tunable.[8] As activated carbon is durable to a certain degree in acidic and alkaline solutions [42] and withstands temperatures up to 1000 K in an oxygen-free atmosphere [8], chemical reactions can be catalyzed at harsh conditions. Compared to alumina and silica supports, the much higher hydrolytic stability of activated carbon is an advantage in water-containing reaction systems [4, 50 52]. Also, the absence of Lewis acid sites can prevent unwanted side reactions that would be catalyzed by an oxide catalyst support [1]. After their application in catalysis, the precious noble metals are easily recovered by burning the carbon [8, 42]. 15

25 2. Theoretical and technical background Micropore & metal clusters Macropore Support Mesopore Figure 2.6.: Schematic pore system of activated carbon catalysts Mass transport in catalysis - aspects of diffusion Activated carbon catalysts are commercially available in powder and granular form, or as extrudates. If the catalytically active species is not explicitly present at the outer particle surface, but dispersed throughout the inner carbon surface, mass transport effects inside the pore system need to be taken into account.[21] These mass transport effects significantly influence the overall reaction kinetics of the catalyst. Both pore structure and particle size of the activated carbon support affect the extent of mass transport limitation. In the following, after illustrating the general pathway of reactant molecules for heterogeneously catalyzed reactions, these two parameters are examined more closely. In the pathway of reactant molecules in heterogeneous catalysis, the molecules diffuse from the bulk solution through a boundary layer to the outer carbon surface. With further diffusion, the molecules are transported along the extended pore system to the catalytically active sites (compare figure 2.6). There, the reactants adsorb to the surface, react and desorb as products. The product molecules then diffuse through the pores and the boundary layer into the bulk solution. Diffusion through the boundary layer between bulk solution and the outer carbon surface is known as film diffusion, while pore diffusion describes the mobility inside the porous activated carbon material.[53, 54] 16

26 2. Theoretical and technical background Pore diffusion Depending on the pore structure of the catalyst support, reactant molecules are transported more or less easily. Larger pore diameters accelerate mass transport. If the pores leading to the catalytically active sites are dimensioned smaller than the reactant molecules, the catalytically active sites won t be accessible or the formed products won t reach the bulk solution. This effect is utilized by shape selective catalysts [2, 54, 55]. Mass transport inside the pore system by diffusion is quantified by the effective diffusion constant D e f f [54]. As shown in equation 2.1, it includes the porosity ϵ and tortuosity τ of the porous support, as well as the pore diffusivity D pore. The tortuosity is an empirical factor considering the random movement of reactant molecules in the pore system [53, 54]. The pore diffusivity can be expressed as a function of molecular diffusivity D mol and Knudsen diffusivity D Knu (see equation 2.2). Knudsen diffusion is taken into account, if the pore diameter is smaller than the mean free path of reactant molecules. An simplified estimation of the effective diffusion constant is given in equation 2.3, which solely depends on the molecular diffusion constant, neglecting Knudsen diffusion.[53] D e f f = ϵ τ D pore (2.1) 1 D pore = ( + 1 ) 1 (2.2) D mol D Knu D e f f = 0.1 D mol (2.3) The hierarchically structured pore system of activated carbon, with macropores and mesopores branching off into micropores, greatly benefits the overall performance of catalyzed reactions. The larger pores allow for sufficient mass transport, while the micropores provide a large surface area to finely disperse the catalytic species. 17

27 2. Theoretical and technical background Kinetics on pore diffusion limitation Concerning particle size, with increasing particle size, mass transport effects increasingly influence the overall reaction kinetics of the catalyst. The statistic pathway of reactant molecules between bulk solution and catalytically active site increases. The Thiele modulus ϕ is frequently applied to estimate the extent of pore diffusion limitation [56]. It describes the ratio of intrinsic surface reaction rate over effective pore diffusion and incorporates the particle size of the catalyst. Equation 2.4 presents the Thiele modulus for spherically shaped catalysts. ϕ = R k intr c n 1 b D e f f (2.4) Here, R is the particle radius, k intr the intrinsic catalytic activity of the active site without mass transport limitation, c b the bulk reactant concentration, n the reaction order, and D e f f the effective diffusion coefficient as defined in equation 2.1. The intrinsic reaction rate constant is determined experimentally. For large Thiele moduli, surface reaction is faster than the transport of reactants through the pore system, thus leading to a shortage of reactants at the active sites, especially near the center of the catalyst pellet. The intensity of mass transport limitation is also described by the effectiveness factor η [45, 54]. Looking at the catalytically active sites, the effectiveness factor represents the ratio of actually reacting molecules over the maximum possible reaction rate under isothermal conditions. It can be expressed to be solely a function of the Thiele modulus. Equation 2.5 shows the effectiveness factor for an irreversible first order reaction in a spherical catalyst pellet [54]. η = 3 ϕ ( 1 tanh ϕ 1 ϕ ) (2.5) Calculation of Thiele modulus and effectiveness factor both require knowledge of the intrinsic reaction rate constant. Therefore, the catalytic reaction has to be studied in absence of diffusion limitation. An alternative approach is the application of a-priori criteria. Here, only immediately measurable parameters are applied. The extent of pore diffusion limitation inside a spherical catalyst pellet is estimated a-priori by the Weisz-Prater criterion Φ (see equation 2.6) [54, 57]. 18

28 2. Theoretical and technical background Φ = r e f f R 2 c b D e f f = ϕ 2 η < 1 f or n = 1 < 6 f or n = 0 < 0.3 f or n = 2 (2.6) As equation 2.6 also shows, the Weisz-Prater criterion is a function of Thiele modulus and effectiveness factor. Critical values, at which the absence of pore diffusion limitation is expected, are given for individual reaction orders. Overall progress of catalyzed reactions The effective reaction rate r e f f can be expressed by a power law approach [54] (see equation 2.7), including the effective reaction rate constant k e f f, the reactant concentration c and the effective reaction order n e f f. r e f f = k e f f c n ef f (2.7) The effective reaction rate constant k e f f quantitatively describes the performance of a catalyst concerning catalytic activity, thus allowing quick comparison of different catalysts. The effective reaction rate constant incorporates the intrinsic performance of the catalytically active sites and mass transport limitations due to pore diffusion. The strong temperature dependence of the reaction rate constant is represented by the Arrhenius law (see equation 2.8). k e f f = k 0 e E A,app R T (2.8) In equation 2.8, k 0 is the pre-exponential factor, E A,app the apparent activation energy, R the ideal gas constant and T the temperature. Apparent activation energies below 30 kj mol -1 can result from mass transport limitation.[53, 54] Integrating the power law expression results in the course of conversion X over time t. The conversion function in dependence of the modified residence time τ mod and the effective reaction rate constant is shown in equation 2.9, assuming a first order reaction. The modified 19

29 2. Theoretical and technical background residence time accounts for the mass of applied noble metal m metal and the reaction volume V reaction (see equation 2.10) [58]. X (τ mod ) = 1 e k ef f τ mod (2.9) τ mod = t m metal V reaction (2.10) Another characteristic figure to compare catalytic activities is the turnover frequency TOF of reactant molecules at catalytically active sites [59, 60]. It s the rate of converted number of substrate molecules n substrate with regard to the number of catalytically active sites n catalyst (see equation 2.11). For supported noble metal catalysts, where only the surface metal atoms are accessible to the reactants, equation 2.12 can be applied. This equation incorporates molar metal mass M metal and metal dispersion D metal. TOF = n substrate n catalyst t (2.11) TOF = n substrate M metal τ mod V reaction D metal (2.12) High-purity polymer-based activated carbon particulates Polymer-based activated carbon pellets exhibit superior properties over traditional activated carbon materials for application as catalyst support. These advantages are presented in the following, together with examples of successful application in catalysis. Advantages as catalyst support Due to the complex chemical composition of natural raw materials, conventional activated carbons exhibit a disordered inner structure with large ash contents and volatile impurities. In catalysis, impurities on catalysts support can have negative effects regarding selectivity and activity of the reaction. The impurities can also contaminate the product.[8] Especially 20

30 2. Theoretical and technical background in pharmaceutical applications, product purity is essential [61]. Legislative guidelines limit the amount of different impurities within pharmaceutical products [62]. Application of purer activated carbon with known chemical composition is highly desired. Activated carbon materials derived from synthetic polymers are a potential solution, as the chemical composition of synthetic polymers is well-known. Thus, polymer-based activated carbon particulates are synthesized with reproducible structure and low ash content (see section for details). This reproducible chemical composition renders polymer-based activated carbon particulates a potentially well suitable group of catalyst support materials. Furthermore, the pore structure of these materials is widely adjustable to fit the catalyzed reaction best [42]. The pore system can already be established during polymerization and is further extended in carbonization and carbon activation treatments (see also section 2.1.2). The particulate character of the material has the advantage of an easier catalyst handling, but induces the risk of mass transport limitation issues. So, as pointed out in the previous section, polymer-based activated carbon catalysts need to be evaluated for pore diffusion limitation for a given reaction. To mitigate poor pore diffusion and allow for sufficient catalytic activity, the most suitable pellet size and pore structure need to be determined for a given reaction. Previous applications in catalysis Despite the advantages of polymer-based activated carbon particulates as catalyst support, in literature, only a few noble metal catalysts of this type were prepared and characterized. Mesoporous carbon was produced by carbonizing resorcinol-formaldehyde-based polymer gels. Compared to activated carbons derived from organic raw materials, the mesopore content was exceptionally large and the amount of micropores very low. This mesoporous carbon material was deemed highly suitable for application as catalyst support. Pore diffusion and accessibility of catalytically active sites was proposed to be improved in this carbon material.[63] Another mesoporous carbon material containing nickel nanoparticles was prepared by carbonization of a phenolic resin containing nickel nitrate. The size of the nickel nanoparticles varied between nm, depending on carbonization temperature and metal loading. Absolute metal loadings between 1-15 wt% were realized. But, only a limited amount of nickel was located at the carbon surface. This magnetic material was easily separable and was proposed to find application as nickel catalyst or catalyst support.[64] Spherical carbon with diameters between 1-40 μm were derived from petroleum residue as a platinum-ruthenium catalyst support. The carbon was treated with potassium hydroxide and 21

31 2. Theoretical and technical background impregnated with an aqueous solution of chloroplatinic acid and ruthenium chloride at 80 C. By addition of sodium dithionite, the metal salts were transformed to elemental platinum and ruthenium. The catalyst performed well in methanol oxidation reactions.[65] A pure, phenolic resin based, spherical carbon catalyst was prepared and successfully tested in the oxidation of cyclohexanone to C 4 -C 6 dicarboxylic acids. The spherical carbon was activated with carbon dioxide at 850 C and air at 450 C, which significantly increased pore volume and BET surface area to values up to 1477 m 2 g -1 and 1.33 ml g -1. Oxygen surface groups were formed in the process. The type of oxygen surface groups had a significant influence on the selectivity of cyclohexanone oxidation.[66] Post-treatment of catalyst - catalyst separation An efficient and economically feasible catalyzed chemical production is not solely determined by catalytic performance, but by the whole process including post-processing. If the catalyst is suspended in a liquid phase, post-processing includes separation of reactant solution and catalyst material by filtration. To evaluate filtration performance, the Darcy law can be applied. The Darcy law is a simplified expression of the Navier-Stokes equation, as shown in equation J = V A = p η R (2.13) It describes the fluid flux J through a porous medium. The fluid flux is the volume flow rate V of the fluid over the filter area A. The Darcy law incorporates the pressure drop p of the flow, the fluid viscosity η and the specific filter resistance R of the porous filter medium. The resistance can be seen as the sum of the resistances of filter membrane R m and filter cake R c. The filter cake resistance can be described according to the Carman-Kozeney extension with empirical constants and material properties (see equations ). R c = α L (1 ϵ) ρ c (2.14) 22

32 2. Theoretical and technical background α = k S 2 p (1 ϵ) ϵ 3 ρ c (2.15) S p = 6 d p (2.16) L = 4 m c π ρ c d 2 c (2.17) Where α is the specific cake resistance, k an empirical filter factor, S p the specific particle surface, ϵ the filter cake porosity, ρ c the packed bed density, d p the particle diameter, L the height of the filter cake, m c the mass of the filter cake, and d c the diameter of the filter cake.[67] Carbon surface functionalization The surface of carbon materials can be functionalized in many different ways. By surface engineering, carbon surfaces are modified with specific surface functionalities in order to inherit certain surface properties. Means of specific surface functionalization are heteroatom inclusion, oxidation, halogenation, sulfonation, grafting, nanoparticle attachment or polymer coating.[4] Heteroatoms are included during carbon synthesis. For instance, polyacrylonitrile polymer carbonization results in nitrogen functional surface groups. Various oxygen surface groups are created by carbon treatment with oxidation agents. Nitric acid oxidation is most common, predominantly resulting in carboxylic acid groups and is controlled by acid concentration, temperature and treatment time. Sulfonation with sulfuric acid at elevated temperature results in solid-acid catalysts. Fluorination with fluorine gas at C leads to hydrophobic carbon surfaces. Grafting of oxidized surfaces with organic reactants further modifies the surface. Impregnation enhances functionality of the material by incorporating e.g. catalytically active metals and metal oxides. Polymer coating by wet impregnation enhances mechanical strength, while preserving carbon porosity.[4] 23

33 2. Theoretical and technical background The two carbon functionalization methods applied in this work are surface oxidation and noble metal deposition by impregnation. These methods are eleborated in the following subsections Surface oxidation For preparation and application of activated carbon as catalyst support, formation of oxygen surface groups is most relevant. Depending on the production process of activated carbon (see sections and 2.1.2), some functional surface groups are already present. In the following, an overview of the types and features of oxygen surface groups is given. Afterwards, applicable functionalization procedures and characterization methods are described. Finally, previously conducted surface modifications of polymer-based spherical activated carbon are discussed. Types and features of oxygen surface groups Important oxygen surface groups are carboxylic acids, lactones, carboxylic anhydrides, phenols, quinones, and cyclic peroxides. Figure 2.7 schematically depicts these different types of oxygen functional groups on a generic activated carbon surface. Surface oxidation occurs at the edges of graphitic microstructures. The integration of these oxygen surface groups influences wettability, acidity and adsorption properties of activated carbons. Oxygen surface groups decrease the carbon s hydrophobicity, so that wettability of polar solvents is improved. In catalyst preparation, especially during the impregnation of activated carbon particulates, a polar metal precursor solution more easily penetrates a pore system with surface oxide functionalities. Also, oxygen surface groups act as adsorption sites. During catalyst impregnation, metal precursor compounds can adsorb to these surface sites (see section for details). Acidic or basic functional surface groups modify the alkaline nature of activated carbon. Carboxylic acids, lactones, carboxyl anhydrides and phenols are known to contribute to surface acidity. Carboxylic acids are the most acidic functional groups. Classification of other surface groups regarding their effect on carbon basicity is still ambiguous and under research [68, 69]. The acidity of a carbon suspension affects the strength of precursorsupport interactions during metal impregnation (also see section for details).[4, 8, 63, 70, 71] 24

34 2. Theoretical and technical background Carboxylic acid O OH Lactone O Carboxyl anhydride O O O O O O O Cyclic peroxide Quinone O OH O Phenol Ketone Figure 2.7.: Types of oxygen functional groups on activated carbon surfaces ([8], modified) 25

35 2. Theoretical and technical background Methods of surface oxidation Applicable functionalization procedures for carbon surface oxidation are conducted with either liquid-phase or gas-phase oxidation agents. Nitric acid, sulfuric acid and hydrogen peroxide are suitable liquid oxidizers. Oxygen, air, carbon dioxide, steam or ozone are applied in gas-phase oxidations. Nitric acid treatment creates mainly carboxylic acid groups and some carbonyls, as well as phenols.[68, 72 74] A mixture of nitric acid and sulfuric acid results in the introduction of nitrogen surface groups by amination, with sulfuric acid inducing the required nitronium ion formation [74]. Oxygen treatment generally produces carbonyl and phenol surface groups. The intensity of oxidation depends on the concentration of the oxidation agent, temperature, treatment time and the carbon amount.[7, 63, 73, 75, 76] Surface characterization A variety of analytical methods are available to characterize surface functionalization of carbon materials. Oxygen surface groups, in particular, are analyzed with titration methods and spectroscopic investigations. Additionally, thermogravimetric analysis and sorption measurements reveal information on oxygen surface modifications. The materials point of zero charge (PZC), resembling the sum of all surface charges, is determined by mass titration. With increasing amount of carbon material, the ph of an aqueous suspension approaches the ph PZC. Acidic surface groups decrease the PZC of carbon materials.[74] A classification into individual types of acidic and basic surface groups is possible by Böhm titration. Titration solutions of sodium hydroxide, sodium carbonate, sodium hydrogen carbonate and hydrochloric acid are applied. Sodium hydrogen carbonate neutralizes carboxylic acid and carboxylic anhydride groups. Sodium carbonate reacts with lactonic and carboxylic groups. Sodium hydroxide neutralizes four types of oxygen surface groups: carboxylic acid, carboxylic anhydride, lactonic and phenolic groups. Hydrochloric acid reacts with basic sites. Other types of surface oxides, i.e. quinones, carbonyls, and ethers, cannot be determined by Böhm titration.[68, 71, 77 80] A more advanced method to quantify different acidic and basic surface groups is potentiometric titration. Starting from a suspension s initial ph, two suspensions are titrated with an acid and a base, respectively. With the assumption that acid sites are characterized by an acid constant K a, titration curves are mathematically transformed into populations of surface sites pk a. As a result, a distribution function of pk a is obtained. Peaks below a pk a of 5 correspond to strongly acidic carboxylic acid groups. Peaks in the pk a range of 5-7 are 26

36 2. Theoretical and technical background Surface group Desorbing gas species Temperature range / C Carboxylic acid CO Carboxyl anhydride CO, CO Lactone CO Phenol CO Ethers CO Carbonyls, quinones CO Table 2.1.: Desorption temperatures and desorbing gas species of oxygen surface groups [86, 87] assigned to carboxylic acids, lactones and carboxylic anhydrides. Weak acids like phenolic groups appear in in the pk a range from 7-11.[71, 72, 81 85] A different carbon surface characterization approach is temperature programmed desorption with off-gas analysis by mass spectroscopy (TPD-MS). In the temperature range between C, surface groups desorb successively in distinct temperature regions. Also, surface groups decompose into characteristic gases like carbon dioxide and carbon monoxide, but also nitrogen or sulfur species. A comprehensive list of desorption regions of surface oxides with corresponding decomposition products is given in table 2.1. Strong acids desorb at lower temperatures, while weak acids, neutral groups and alkaline groups decompose at very high temperatures. Compared to the titration methods, TPD-MS distinguishes more surface groups such as neutral oxygen groups, but also nitrogen, phosphorous, and sulfur functional groups.[68, 71] Applicable, purely spectroscopic methods in carbon surface science are, for instance, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and dynamic light scattering (DLS).[68, 74, 88] With FTIR, characteristic peaks in each spectrum can be assigned to specific surface interactions. Thus, surface functionalities are qualitatively determined. But, only the outer carbon surface is spectroscopically analyzed. XPS measurements clarify the chemical composition of surfaces and sub-surfaces by monitoring the electrons being released from the material [89]. These spectroscopic methods are useful to validate results of TPD-MS measurements, for instance. DLS can be applied to determine the isoelectric point (IEP) of carbon suspensions by Zeta potential measurements at different ph values. In contrast to the ph PZC, the ph IEP of porous materials resembles solely the ex- 27

37 2. Theoretical and technical background ternal surface charges. The difference between both values indicates the charge distribution across carbon particles and, thus, the distribution of acidic and basic groups between the internal and external carbon surface.[71, 74] Nitrogen sorption experiments illuminate the structural influence of functionalization on pore geometry. Carbon material that was etched away or blocked micropores result in a noticeable change of total pore volume, micropore volume and BET surface area. Sorption measurements with water vapor characterize wettability of the carbon material. From water vapor isotherms, information such as a surface-specific oxygen number and the distance between oxygen atoms can be derived.[24, 45, 72, 75] Previous surface functionalization of PBSAC materials Polymer-based spherical activated carbon with 1 wt% of volatile components was previously oxidized in air at 450 C for min. With increasing processing time, the amount of volatile components increased up to 15 wt%. Correspondingly, the material s PZC decreased from ph 8-9 to around 6. Water vapor isotherms supported the decrease in hydrophobicity. After intensive oxidation, mechanical properties were affected. Functionalization of oxidized spherical carbon with nitrogen functional groups was also successful. The material was loaded with urea and post-treated at C in an inert atmosphere.[7] Deposition of noble metal Metal on carbon catalyst preparation generally consists of two succeeding steps. Impregnation of carbon support material with metal species. And, metal transformation into the catalytically active metal species by calcination or reduction of metal precursors. Typical metals applied in heterogeneous catalysis are palladium, platinum, ruthenium, rhodium and nickel [44]. Different impregnation and metal transformation techniques are described in the following. Afterwards, analysis methods for catalyst characterization are described. Impregnation of activated carbon with metal species The basic principles of most impregnation techniques involve the attraction of metal species to the inner surface of the support materials and the transition of dissolved metal species into small solid clusters. The physico-chemical reason, sequence and timing of these two steps vary for each impregnation technique.[90] In a standard impregnation, a metal precursor solution is introduced into the pore system of the catalyst support. The metal species adsorb to the inner surface of the support material 28

38 2. Theoretical and technical background due to different attractive forces. After the solvent is evaporated in a drying process, the metal species are fully deposited to the catalyst support. Attractive forces are for instance electrostatic interactions between the surface of the support material and ionic metal species in the impregnation solution. The surface of the support bears charges that are neutralized by adsorbing ions. Countercharged metal ions adsorb to the surface, thus replacing the previously adsorbed ions. In a washing step, non-adsorbing ions are removed from the catalyst. Besides electrostatic adsorption by ion-exchange, metal species can be chemically attached to surface functional groups.[91 93] In the deposition-precipitation process, transition of dissolved metal species into small solid clusters takes place in the pores that are still filled with impregnation solution. Here, nucleation is initiated by reducing the solubility of the metal precursor. Solubility can be decreased by changing the ph value of the solution. Alternatively, the metal precursor can be transformed to a less soluble species by changing its valency or by complexing/decomplexing the precursor adding/removing a compexing agent. The solid clusters are then attracted to the surface of the support. As an excess of impregnation solution is applied, the precipitation process needs to be controlled to prevent precursor nucleation in bulk solution. Depositionprecipitation works well for particulate support materials. In electrochemical depositionprecipitation, metal species dissolve from a metal anode and are deposited to the support. The electrochemical setup can also be used to induce ph or valency changes resulting in precipitation of metal species. These electrochemical processes are difficult to realize at a larger scale, though.[90] Instead of a metal precursor solution, metal nanoparticles are deposited in colloidal impregnations. The metal nanoparticles have either been previously prepared or are precipitated in-situ in the impregnation process.[94, 95] Depending on the amount of impregnation solution, catalyst preparation techniques are classified into incipient wetness and wet impregnation processes. In the incipient wetness impregnation, only the pore system is filled with impregnation solution. The volume of impregnation solution is limited to the total pore volume of the catalyst support. Wet impregnation utilizes an excess of impregnation solution. So, advantages of the incipient wetness technique are the complete deposition of precursor species inside the pore system and reduced solvent waste. In wet impregnations, chemical agents for precipitation or metal transformation can be added during the process.[90 93] Concerning the preparation of noble metal catalysts based on activated carbon support materials, the carbon surface can be either oxidized or non-functionalized, depending on the 29

39 2. Theoretical and technical background HO OH + PdCl 4 2- OH + OH C-π site Figure 2.8.: Adsorption of metal anions to protonated carbon surface groups by electrostatic interactions; C-π sites for alternative metal adsorption ([8], modified) applied impregnation mechanism and solvent. But, in order to fully utilize the outstanding diversity of carbon surface oxidations, as presented in section , the following part focuses on the electrostatic adsorption of metal ions to charged carbon surfaces. In a wet impregnation process applying the concept of electrostatic adsorption, an oxidized carbon support is suspended in an aqueous metal salt solution. The ph of the impregnation solution is adjusted by adding an acid or base, so that the ph difference to the carbon s PZC is sufficiently large. In consequence, metal ions adsorb to protonated or deprotonated surface sites (see figure 2.8). In case of a metal anion, the surface sites need to be protonated for successful ion adsorption by electrostatic forces. Here, the impregnation solution s ph needs to be smaller than the ph PZC. Metal cations adsorb to negatively charged surface sites.[96] It needs to be noted that metal salts can also electrostatically adsorb to basic C-π sites (see figure 2.8). These are non-oxidized, unsaturated carbon surface structures that provide free electrons to interact with the precursor. For more details on the influence of C-π sites see section Furthermore, metal ions can create new surface groups by oxidation, thus reducing themselves.[63, 76, 96, 97] Metal transformation of impregnated carbon materials Metal transformation of suspended catalysts after wet impregnation, reducing the metal precursor to the catalytically active species, is carried out by addition of alkaline formaldehyde, hydrazine, sodium borohydride, sodium formate, or hydrogen gas [98]. Alternatively, dried catalyst material is placed in hydrogen atmosphere at elevated temperatures until the metal is fully reduced. Here, the applied metal transformation temperature has significant influence on metal cluster size. For each noble metal, an optimum temperature exists, resulting in min- 30

40 2. Theoretical and technical background imum cluster sizes. Above this optimum, with increasing metal transformation temperature, metal clusters grow due to agglomeration and sintering.[76, 93, ] The metal specific Hüttig temperature, empirically derived from the melting temperature of the noble metal (see equation 2.18), points out the onset of thermal sintering by diffusion of metal atoms. For metal nanoparticles, metal diffusion and sintering can already occur at lower temperatures due to melting-point depression effects. Many other factors are also described to have an effect on the onset temperature of sintering such as metal-support and metal-chloride interactions.[44, 102, 103]. T Hüttiд [K] = 0.3 (T meltinд [K]) (2.18) C-π sites can spontaneously reduce some metal salts, as well (see section ). Catalyst characterization Heading to catalyst characterization, the three most important properties of supported metal catalysts are metal loading, metal dispersion and metal distribution. Besides, pore geometry and surface composition of the catalyst support material are relevant (see sections and ). These properties significantly influence catalytic activity.[63] The total amount of metal loaded to the carbon support is determined by e.g. ash analysis after thermal carbon decomposition. Also, a multi-stage metal extraction with aqua regia is possible. Alternatively, mass balances of metal concentrations in impregnation and washing solutions indirectly give information on the catalyst s metal loading. Metal dispersion refers to the number of catalytically active sites that are principally accessible to reactant molecules [104]. As shown in equation 2.19, it s defined as the ratio of surface metal atoms n sur f ace atoms to the total number of metal atoms n total in a catalyst. With increasing metal dispersion, the metal cluster size decreases. For spherical metal clusters, the metal cluster size d cluster is defined in equation 2.20 with v metal atom being the volume and s metal atom the surface area of a metal atom. D metal = n sur f ace atoms n total (2.19) d cluster = 6 v metal atom D metal s metal atom (2.20) 31

41 2. Theoretical and technical background O O O O C C C C (a) (b) (c) O C C O (d) (e) Figure 2.9.: Adsorption of carbon monoxide to metal surfaces: linear (a), bridged (b & c), triple bond (d), dissociative bonding (e) ([104], modified) Metal dispersion is determined in chemisorption experiments. Hydrogen or carbon monoxide gas adsorbs to surface metal atoms and the amount is correlated to the catalyst s metal loading. Hydrogen chemisorption on carbon supports can be difficult to interpret due to hydrogen spillover effects. In case of carbon monoxide chemisorption, it needs to be noted that five different adsorption behaviors are known, as figure 2.9 demonstrates. Stoichiometrically, a carbon monoxide molecule can adsorb to between one and three metal atoms or dissociatively adsorb to two metal atoms. Temperature programmed desorption of carbon monoxide loaded catalysts gives insight into these adsorption phenomena. At a palladium surface, for instance, linearly adsorbed carbon monoxide desorbs at lower temperatures than bridge bonded and triple bonded carbon monoxide, respectively. It was found that carbon monoxide chemisorption stoichiometry depends on particle size and decreases from 2 to 1 with decreasing particle size.[ ] An overview of metal cluster sizes is apparent from electron microscopy images (SEM or TEM). Alternatively, crystallite sizes are derived from X-ray diffraction (XRD) spectra using the Scherrer equation [104]. The third property of noble metal catalysts is the metal distribution over the cross-section of the material. Metal distribution in particulate catalysts is classified into uniform, egg shell, egg white, and egg yolk distributions (see figure 2.10). Egg yolk type catalysts perform well in kinetically controlled reactions, while egg white and egg shell distributions are preferable in diffusion limited reactions. Uniform type catalysts exhibit the largest metal dispersions, because the area for metal deposition is largest. The type of metal distribution is controlled during catalyst impregnation and drying. Tomographic methods are applied to 32

42 2. Theoretical and technical background (a) (b) (c) (d) Figure 2.10.: Metal distributions in catalyst particulates: uniform (a), egg-shell (b), egg-white (c), egg-yolk (d) ([108], modified) determine metal dispersion. The easiest way is to embed the catalyst particulate in a resin, polish the sample and scan the cross section using energy-dispersive X-ray spectroscopy (SEM-EDX).[107, 108] The role of metal precursors in catalyst preparation Palladium on activated carbon In case of palladium catalyst preparation to a microporous carbon support, the steric demand of the metal precursor influences the final position of the metal clusters in the pore system. Planar precursor molecules like tetrachloropalladic acid adsorb into slit-shaped micropores, while more bulky metal precursors like palladium acetate don t. This was indicated by differences in catalytic hydrogenations of cyclohexene, as cyclohexene doesn t adsorb well to palladium in micropores. Comparison with a non-microporous carbon support didn t exhibit these differences.[76] Concerning transformation of the palladate precursors to metallic palladium by reduction in hydrogen atmosphere, the reduction temperature influences the final metal dispersion. The highest palladium dispersions are observed at 80 C. Metal dispersion decreases with increasing reduction temperature.[76, 99] Halide-free metal precursors are also of particular interest, because some catalyzed reac- 33

43 2. Theoretical and technical background tions are affected by residual halides.[109] Chloride-free and highly dispersed palladium catalysts are prepared in an incipient wetness impregnation with palladium acetylacetonate in tetrahydrofuran.[92] The palladium complex of tetramethylammonium hydroxide is a suitable, halide-free precursor, as well, which is precipitated to activated carbon by increasing the impregnation solution s ph value.[110] Halide-free, colloidal palladium on carbon catalysts is prepared with electrostatically and sterically stabilized nanoparticles. Traditional palladium on carbon catalysts are outperformed by the colloidal palladium on carbon catalyst in terms of cinnamic acid hydrogenation activity and catalyst lifespan (see also section for details).[94] Platinum on activated carbon Platinum on activated carbon catalysts are successfully prepared in wet impregnation processes applying electrostatic adsorption of hexachloroplatinic acid or platinum tetramine chloride precursors. After metal transformation, platinum clusters are very small and narrowly distributed (1-2 nm) over a wide range of metal loadings ( wt%). During impregnation, it is important to account for ph buffering of the carbon. The platinum tetramine precursor is most suitable for impregnating carbon materials of low PZC applying neutral or alkaline impregnation solutions. The anionic platinate of hexachloroplatinic acid is ideally deposited to carbon materials of a higher PZC using an acidic impregnation solution.[96] In incipient wetness impregnations, these two platinum precursors, dissolved in a benzeneethanol mixture or in water, are also successfully deposited to carbon materials.[93] Here, surface functionalization doesn t strongly affect catalyst quality.[63] Metal transformations are performed in hydrogen atmosphere at C. In this temperature range, platinum dispersion decreases with increasing reduction temperature. Pretreatment of the impregnated materials in helium atmosphere at reduction temperature improves metal dispersion after metal transformation.[93] The role of C-π sites in catalyst preparation For graphitic carbon materials containing many surface C-π sites, two pathways of metal salt deposition were proposed by Simonov et al. correlating experimental results with electrochemical theory (see scheme 2.2). In the first case, tetrachloropalladic acid is spontaneously reduced to elemental palladium at the outer surface of the carbon material by the electrical double layer of the material. The electrical double layer, formed by adsorbing counterions to the charged carbon particle, 34

44 2. Theoretical and technical background Scheme 2.2 Reaction pathways of palladate deposition on graphitic carbon [97] H 2 PdCl 4 + C Pd/C + 2Cl/C + 2 HCl H 2 PdCl 4 + C PdCl 2 /C + 2 HCl increases the reductive strength of the graphitic carbon. Alternatively, palladium chloride adsorbs to the C-π sites. The different oxidation states of deposited palladium can be determined by XPS analysis. The formation of metallic palladium at the outer carbon surface is immediately visible as the black carbon turns gray. An egg-shell metal distribution results. Ion adsorption of palladate to C-π sites is more uniform across carbon pellets. A broad size range of metal clusters are formed in these processes with small clusters in the range of 1-3 nm and larger palladium clusters in the range of nm.[97] The in-situ reduction of metal salts at C-π sites generally results in larger cluster sizes than pure ion adsorption to oxygen surface groups.[63, 73, 99] Also, at longer impregnation times, hexachloroplatinic acid reacts with C-π sites in an adsorptive reduction reaction.[96] The degree of graphitization can be influenced by heat treatment. Heat treatment of activated carbon at C forms a graphitized surface consisting of C-π sites and lacking oxygen groups. With this support, a good platinum dispersion is obtained due to adsorption of metal precursor to C-π sites.[111] For oxidized graphitized carbon loaded with platinum, the presence of surface oxides negatively affects resistance to sintering and thus platinum dispersion.[112] Reactions studied in this work A variety of model reactions have been used in this work. All are slurry-phase hydrogenation or dehydrogenation reactions, i.e. the reaction systems are comprised of suspended catalyst material in reactant solutions with dissolved hydrogen gas. In these catalyst screening experiments, catalytic performance has been determined by monitoring the conversion of the substrate and deriving reaction rate constants or turnover frequencies. Besides catalytic activity, product selectivity was another relevant parameter studied in this work. In the following, the different model reactions are presented together with turnover frequencies of noble metal carbon catalyst found in literature. 35

45 2. Theoretical and technical background Scheme 2.3 Hydrogenation of cinnamic acid O O OH H 2 OH Cinnamic acid Hydrocinnamic acid Palladium catalysts screening In accordance to literature, the hydrogenation activity of palladium on carbon catalysts is evaluated in the hydrogenation of cinnamic acid to hydrocinnamic acid (see scheme 2.3).[94, 110, ] Only the unsaturated carbon-carbon bond is reduced at ambient reaction conditions. Reaction rates at ambient conditions regarding temperature and hydrogen pressure are already significant, so that palladium catalysts can be screened under mild conditions within reasonable reaction times. As found in literature, the cinnamic acid turnover frequency of a supported palladium nanoparticle catalyst amounted to 0.51 mol Cinnamic acid mol -1 Pd s -1 at 25 C and 1 bar hydrogen pressure. Here, stabilized palladium nanoparticles, with particle sizes of 3.2±0.98 nm, were deposited on activated carbon with a metal loading of 4.83 wt%.[114] For 5 wt% palladium on activated carbon materials, cinnamic acid turnover frequencies of up to 0.92 mol Cinnamic acid mol -1 Pd s -1 were determined at the same ambient process conditions.[113] Platinum catalysts screening Cinnamaldehyde hydrogenation is a model reaction to characterize platinum on carbon catalysts.[44, ] As scheme 2.4 shows, the unsaturated carbon-carbon bond and the aldehyde group is reduced at reaction conditions. Thus, product selectivity can also be studied for this reaction. Referring to literature reports, activated carbon loaded with 4 wt% platinum was tested in the hydrogenation of cinnamaldehyde at 100 C and 60 bar hydrogen pressure. It was demonstrated that the addition of iron chloride to the reactant solution up to an iron to platinum ratio of 0.2 significant improved catalytic activity and selectivity towards cinnamic alcohol. Initial turnover frequencies up to 0.02 mol Cinnamaldehyde mol -1 Pt s -1 and an initial cinnamic alcohol selectivity up to 70% were reached. After reaction, iron was found to be deposited on the platinum clusters and cluster sizes between nm were determined.[116] A monolithic structure with 1.23 wt% platinum on carbon exhibited a cinnamaldehyde turnover frequency of 0.07 mol Cinnamaldehyde mol -1 Pt s -1 at 30 C and 50 bar hydrogen pressure. The average 36

46 2. Theoretical and technical background Scheme 2.4 Hydrogenation of cinnamaldehyde O H H 2 Cinnamaldehyde H 2 O H OH Hydrocinnamaldehyde Cinnamic alcohol H 2 H 2 OH Hydrocinnamic alcohol platinum cluster size amounted to 4.1 nm.[117] The cinnamaldehyde turnover frequency of carbon nanofibers loaded with 5 wt% platinum reached values up to 0.83 mol Cinnamaldehyde mol Pt -1 s -1 at 30 C and 48 bar hydrogen pressure. Thermal treatment of the catalyst material at 700 C under nitrogen atmosphere significantly improved catalytic performance. Platinum cluster sizes of 1.8±0.7 nm were obtained.[118] Ruthenium catalysts screening Ruthenium on carbon is suitable for the hydrogenation of carbocyclic rings.[44, 119] In this work, multiple benzene derivatives have been used as reactants with such catalysts, such as toluene, m-cresol and thymol. Additionally, hydrogenation of the linear molecule 1-octene with ruthenium on carbon has been analyzed in this work. The molecule structures of the four substates are shown in figure In literature, toluene turnover frequencies up to 2.72 mol Toluene mol Ru -1 s -1 were reported for ruthenium on different carbon materials. Ruthenium metal loadings varied between wt% and toluene hydrogenation took place at 110 C and 40 bar hydrogen pressure. The most active ruthenium catalysts were prepared by thermal reduction of impregnated ruthenium chloride to ruthenium clusters at 900 C in nitrogen atmosphere. It was hypothe- 37

47 2. Theoretical and technical background 1-Octene Toluene OH OH m-cresol Thymol Figure 2.11.: Substrates for ruthenium catalyst screening - molecule structures of toluene, m-cresol, thymol, and 1-octene sized that thermal reduction significantly increased metal-support interactions, which positively affected toluene hydrogenation activity.[120] Ruthenium on carbon nanofibers exhibited turnover frequencies of up to 3.94 mol Toluene mol -1 Ru s -1 at 100 C and 30 bar hydrogen pressure. Here, metal loading varied between wt% and ruthenium clusters were between 2-4 nm in size.[121] Rhodium catalysts screening Rhodium metal complexes are, for instance, applied as catalyst species in the selective hydrogenation of cycloalkenes and its derivatives.[122, 123] In this work, the consecutive hydrogenation of 1,5-cyclooctadiene to cyclooctene and cyclooctane (see scheme 2.5) has been investigated as model reaction to test the performance of a rhodium metal complex supported on spherical carbon. According to literature, different cationic rhodium(i) complexes have previously hydrogenated 1,5-cyclooctadiene with high selectivity (62-92%) towards cyclooctene under ambient conditions (room temperature, 1 bar hydrogen pressure).[122] Platinum catalysts in a technical relevant test reaction A technical application of activated carbon based noble metal catalysts is the hydrogenation and dehydrogenation of liquid organic hydrogen carriers (LOHCs). LOHCs are of high economic interest as they provide a viable option to store excess electrical energy. To shortly 38

48 2. Theoretical and technical background Scheme 2.5 Hydrogenation of 1,5-cyclooctadiene H 2 H 2 1,5-Cyclooctadiene Cyclooctene Cyclooctane Scheme 2.6 (De-)hydrogenation of Marlotherm SH 9 H 2 H18-dibenzyltoluene Dibenzyltoluene describe the entire process, first of all, excess electrical energy is used to produce hydrogen by electrolysis. Then, in a hydrogenation step, unsaturated LOHC compounds are catalytically loaded with hydrogen. On demand, hydrogen is released and converted back to electrical energy within a fuel cell.[ ] A promising LOHC candidate is the commercially available heat-transfer oil known by the trade name Marlotherm SH. It is a mixture of isomeric dibenzyltoluenes and is toxicologically comparable to diesel fuel.[48] The (de-)hydrogenation scheme of Marlotherm SH is shown in scheme 2.6. In this work, the dehydrogenation activity of fully hydrogenated H18- dibenzyltoluene using platinum catalysts has been investigated. Previous catalyst screenings published in literature show that for a 1 wt% platinum on carbon catalyst, the dehydrogenation degree after 3.5 h reaction time at 170 C amounted to 71%. A 0.5 wt% platinum on aluminum oxide catalyst only reached 51% dehydrogenation degree at the same platinum to LOHC ratio of 0.15 mol%.[48] 39

49 2. Theoretical and technical background Table 2.2.: Selected gases and volatile organic compounds adsorbing to activated carbon surfaces Gas classification Gaseous species Molecular formula Organic compounds in general [ ] Organic Acidic Inorganic Cyclohexane C 6 H 12 [1] Benzene C 6 H 6 [1, 130] Chloropicrin CCl 3 NO 2 [1] Carbon dioxide CO 2 [131] Sulfur dioxide SO 2 [128, 129] Hydrogen chloride HCl [128, 129] Chlorine Cl 2 [130] Heavy metals [128, 129] 2.3. Activated carbon in gas purification Activated carbon adsorbents in gas purification In gas purification, activated carbons are applied as adsorbents for a variety of gases and volatile organic compounds. The gaseous species accumulate at the internal surface of activated carbons by adsorption to active surface sites or condensation in the pore structure. So, the large internal surface area of activated carbons and the extensive pore structure are highly beneficial to achieve large adsorption capacities. The inherent surface hydrophobicity and the resulting low wettability render activated carbon materials well suitable for filter applications at humid conditions. An incomplete list of species adsorbing to activated carbon is presented in table 2.2.[1, 7, 127] Adsorption kinetics Active adsorption sites are located across the internal surface area of activated carbons. These C-π adsorption sites, consisting of unsaturated carbon-carbon bonds, attract the gaseous species. Molecules are either adsorbed physically by van-der-waals forces (physisorption) or more strongly by chemical bonding (chemisorption). Upon adsorption, adsorption enthalpies between 5-65 kj mol -1 are released. Adsorption enthalpy increases with increasing interaction strength of adsorbate and adsorption site.[1] 40

50 2. Theoretical and technical background The equilibrium of a reversibly adsorbing gaseous species is influenced by temperature and pressure, as well as partial pressure and adsorption enthalpy of the adsorbate. The adsorption equilibrium immediately affects the adsorption capacity of a gaseous species. With increasing temperature, adsorption equilibrium is thermodynamically shifted to the gas-phase. Larger pressures increase surface coverage. The presence of other compounds, competing for adsorption sites, negatively affects the adsorption capacity of an adsorbate. A larger adsorption enthalpy results in a preferred adsorption.[1, 22, 132, 133] Additionally, if the adsorption temperature is lower than the critical temperature of the adsorbate, capillary condensation in the pore structure takes place. Thus, not only the surface area, but the whole pore volume is utilized to retain the adsorbate. This effect allows for significantly enhanced adsorption capacities, especially in microporous activated carbons.[1] Several models are available to describe isothermal adsorption processes of gaseous species to porous adsorbents. For activated carbon materials, the Dubinin model generally fits best. Here, micropores are filled with adsorbate at first, followed by multi-layer adsorption in meso- and macropores.[1, 132] Adsorption capacity is an important characteristic of adsorbents in gas purification applications. It describes the total amount of an adsorbate that can be retained to an adsorbent at defined process conditions. But also, effective adsorption kinetics are highly relevant. The gaseous species need to effectively diffuse through the pore system to the active site and adsorb to the surface. Pore diffusion limitation and slow intrinsic adsorption rates decrease the performance of a filter material. The hierarchical pore structure of activated carbon materials promotes efficient mass transport. The intrinsic adsorption rate is determined by the type of adsorbate and the carbon surface composition.[1] Reversible and irreversible adsorption modes Depending on the field of application, adsorption of gaseous species is either reversible or irreversible. Adsorbents are regenerated in closed systems, low-maintenance systems or in bulk filtrations. Regeneration of the adsorbents is realized in pressure swing adsorption (PSA) or temperature swing adsorption (TSA) setups. The adsorbate is desorbed by either decreasing the pressure of the adsorption column or increasing temperature. A PSA setup is less energy intensive than a TSA setup for adsorbates exhibiting adsorption enthalpies below 30 kj mol -1. In case of adsorbent regeneration, multiple adsorption and desorption cycles need to be monitored to determine long-term adsorption capacities. Irreversible adsorption of the gaseous species is applied in fine filtrations. The adsorbed species permanently remain 41

51 2. Theoretical and technical background in the filter material and are not released at changing process conditions, e.g. temperature increase or a strip gas stream.[1, 134] Adsorption performance measurements of filter materials The performance of adsorbents is characterized by measuring adsorption isotherms of specific compounds or in continuous breakthrough experiments. Information about the adsorption capacity, the type of adsorption isotherm and the adsorption enthalpy is derived from the discontinuous adsorption of specific compounds. At different relative pressures and isothermal conditions, the equilibrium surface coverage is determined. Using multiple adsorption isotherms and applying the Clausius-Clapeyron equation, the adsorption enthalpy is calculated from pressure and temperature points having equal surface coverages (see equation 2.21). d ln(p p 1 0 ( ) dt 1 ) θ=const = H ads R (2.21) Here, p is the pressure, T the temperature, θ the adsorbate surface coverage, H ads the adsorption enthalpy and R the ideal gas constant.[1, 134] In breakthrough experiments, a gas stream with the adsorbing species continuously flows through an adsorber unit. This adsorber unit contains a bed of adsorbents with defined height and diameter. Temperature and pressure in the adsorber unit are regulated according to the desired process conditions. Concentration differences of the adsorbing species between inlet and outlet of the adsorber unit are monitored over the process time. Optionally, the pressure difference is also determined for investigations concerning flow characteristics of different adsorbents. Results of a typical breakthrough experiment are shown in figure In the first section of the breakthrough curve, the outlet concentration of the adsorbing species is ideally zero. The gaseous compound is completely adsorbed to the filter material. At the breakthrough time, a certain threshold concentration is detected in the outlet stream. The breakthrough capacity has been reached and the filter material no longer adsorbs all tested gas compounds. In the following saturation phase, the outlet concentration increases until inlet and outlet concentration are equal. At this point of complete saturation, the saturation capacity of the filter material is reached. For most applications, breakthrough capacity is much more relevant than the saturation capacity, though. The slope of the saturation phase correlates to the effective adsorption rate. This is due to the conception that, 42

52 2. Theoretical and technical background c c -1 / (3) (2) (1) t / min Figure 2.12.: Typical breakthrough curve of a continuous breakthrough experiment: (1) exemplary breakthrough point, (2) curve inflection point, (3) saturation point within a certain adsorption zone, gaseous species passing the filter bed adsorb to consecutive active sites. The adsorption zone moves through the filter bed. When the adsorption zone reaches the end of the filter bed, concentration gradients are visible by measuring the outlet gas concentration and adsorption kinetics can be derived.[134, 135] Various mathematical models have been postulated to describe breakthrough curves of continuous, gas-phase adsorption experiments.[136] For instance, simplified mass balances of isothermal fixed-bed reactors were derived and integrated with respective boundary conditions. Steady decrease of active sites was also implemented. Some models ended up with more complex solutions that had to be applied iteratively.[137] One-dimensional, continuous, time-dependent concentration functions were also reported.[138] In this work, an empirical approach has been applied to fit individual data points of breakthrough experiments. The function of hyperbolic tangent type is shown in equation c c 1 0 = A tanh(b t + C) + A (2.22) In this time-dependent function of normalized test gas concentration c c 1 0, parameter A 43

53 2. Theoretical and technical background represents the normalized concentration regime. Ideally, A = 0.5, so that 0 c c Due to measurement uncertainties, often A 0.5 and the data series are normalized by the factor 0.5 A 1. Effective adsorption kinetics are essentially described by parameter B. Fast adsorption and filter material saturation is indicated by large values of B. Parameter C mathematically describes the function s horizontal alignment. This offset is directly influenced by breakthrough time and adsorption rate. Applying this fit function, breakthrough times can be estimated for any breakthrough concentration. Also, adsorption behavior of different filter materials can be compared. The slope m at the inflection point of the breakthrough curve effectively quantifies the adsorption rate (see equation 2.23).[16] m = A B (2.23) Functionalization of activated carbon with reactive surfaces The adsorption capability of pure activated carbon is limited to a certain range of compounds that interact with the unsaturated carbon-carbon active surface sites (see section 2.3.1). Adsorption of other species is realized by carbon surface functionalization. The surface of activated carbon is effectively modified by introducing surface functional groups or by impregnation with reactive components. Oxygen and nitrogen surface functional groups are the most prevailing carbon surface modifications that are directly integrated into the carbon structure (see also section ). Concerning impregnations, metals, metal salts, metal oxides and non-metal salts are predominantly applied.[1] The impregnations are located inside the pore system of activated carbon, dispersed at the carbon surface. In the following, these different surface modifications enhancing adsorption capabilities of activated carbon are described and summarized in table 2.3. The presented surface modifications allow for reactive adsorption of targeted gas compounds under ambient conditions. The combination of active sites with reactivity for different gaseous species is utilized in broadband filters. The activated carbon support also extends adsorption capacity for other gases that don t react with the impregnation, e.g. unpolar organic compounds like cyclohexane (see section 2.3.1). Nevertheless, the absolute gas adsorption capacity cannot be significantly increased by these types of surface modifications. This is, because the absolute number of accessible surface adsorption sites doesn t increase after surface group introduction or impregnation.[139] Under humid conditions, though, water dissolves salt impregnations increasing the number of accessible reaction sites by absorption into the liquid phase.[7] 44

54 2. Theoretical and technical background Activated carbons with oxygen functional groups Activated carbon functionalized with oxygen surface groups significantly increases ammonia adsorption capacity. An activated carbon material, functionalized by air oxidation at 300 C and then treated in a nitrogen atmosphere at 500 C exhibited an average ammonia capacity of 8.9 mg g -1. Compared to pure activated carbon, ammonia capacity increases by a factor of 6.4. Humid conditions promote the overall ammonia adsorption, likely because water allows the formation of ammonium ions. The ammonium ions then adsorb to deprotonated acidic surface groups.[81, 140] Oxidized carbon adsorbents also adsorb sulfur dioxide better than the pure carbon variant.[141] Activated carbons with nitrogen functional groups Nitrogen surface functionalization of polymer-based spherical activated carbon improves sulfur dioxide, hydrogen sulfide and formaldehyde adsorption capacities. With increasing nitrogen content, the adsorption capacities of sulfur dioxide and hydrogen sulfide increase. Formaldehyde adsorption performance strongly depends on the presence of water. The basecatalyzed polymerization to react away formaldehyde seems to require an aqueous environment at the carbon surface.[7] Activated carbons with alkali metal salt impregnations Activated carbon containing potassium hydroxide, potassium carbonate, potassium iodide, sodium hydroxide or sodium carbonate is applied to remove acidic gases like hydrogen sulfide from air. The reaction mechanism of hydrogen sulfide with potassium iodide is an oxidation to elemental sulfur. Also catalyzed by potassium iodide, phosphine is oxidized to phosphoric acid. Boron species are also removed using potassium hydroxide impregnated activated carbon.[1, 127, 139, 142, 143] Biological filters are comprised of antibacterial potassium iodide impregnated activated carbon.[142] Activated carbons with acid impregnations Phosphoric acid, sulfuric acid and citric acid impregnations adsorb and neutralize amine species.[127, 142] Activated carbons with transition metal oxide impregnations Porous carbon materials impregnated with molybdenum trioxide or vanadium pentoxide adsorb ammonia. Ammonia capacities up to 4 wt% were reached with 5-10 wt% metal oxide 45

55 2. Theoretical and technical background Scheme 2.7 Ammonia chemisorption by amine-complex formation with metal cations Cu NH 3 Cu(NH 3 ) 2+ 4 Zn NH 3 Zn(NH 3 ) 2+ 4 impregnations in continuous operation in air at 1 bar and 25 C.[81] Activated carbons with metal salt impregnations Metal salt impregnations with copper(ii) chloride or zinc(ii) chloride perform well in the removal of ammonia and other amine species. With an ammonia breakthrough capacity of 7 wt% at 1 bar and 25 C, metal salt impregnations performed even better than equallyloaded metal oxide materials. Correlating surface ph values to material acidities, the metal chloride impregnated carbon systems exhibited highest acidities. Thus, metal chloride impregnations offered the largest reactivities for ammonia. It was proposed that water enables Brønsted acid formation of inorganic metal salts, thus facilitating ammonia adsorption. Under dry conditions, Lewis sites are mostly responsible for ammonia adsorption. Spectroscopic measurements allowed in-depth investigation of ammonia adsorption mechanisms. It was demonstrated that metal chlorides form amine-complexes upon ammonia adsorption. In scheme 2.7, this complexation is presented for copper(ii) and zinc(ii) salts.[81, 142, 144] Zinc acetate impregnations also facilitate the adsorption of ammonia species. Concerning hydrogen sulfide removal, copper sulfate or lead acetate impregnated materials are suitable adsorbents. Arsine is adsorbed by a copper tetraamine salt impregnation via exchange of the complexing ligand.[127] Activated carbon impregnations with copper(ii) salts generally perform better than other metal salt impregnations due to the larger stability of copper complexes with ligands of all types. Due to the reciprocal ionic radii and second ionization potentials, zinc, nickel, or cobalt salts exhibit lower formation constants of metal-ligand systems.[145] In essence, copper(ii) salts form the most stable complexes with ligands such as ammonia species (e.g. ammonia, ethylendiamine, diethylentriamine, triethylentetramine), most amino acids, and cyanide. Of these systems, ammonia exhibits much larger formation constants than cyanide. Even though, most copper salts form complexes with water, ammonia displaces water as a ligand, because it s more polarizable than water. The good performance of copper(ii) salt impregnations in hydrogen sulfide adsorption can be explained similarly. It s well known that e.g. copper(ii) chloride forms more stable products with hydrogen sulfide than cadmium or zinc chloride. The metal sulfide solubility constant is the largest for zinc sulfide. It decreases by a factor of 250 for cadmium sulfide and by a factor 46

56 2. Theoretical and technical background Scheme 2.8 Hydrogen sulfide chemisorption by sulfide-complex formation with metal cations [147] Cu 2+ + H 2 S Cu(HS) + + H + Cu(HS) + + H 2 S Cu(HS) 2 + H + of 3x10 11 for copper sulfide. It s proposed that copper(ii) salts are reduced by sulfides to result in copper(i) hydrogen sulfide and later in copper(0) dihydrogen disulfide (see scheme 2.8).[ ] Activated carbons with metal impregnations Elemental copper on polymer-based spherical activated carbon shows very large capacities for hydrogen sulfide and sulfur dioxide removal. It s proposed that hydrogen sulfide more effectively yields copper(ii) sulfide with elemental copper impregnations than with copper salts.[7] Silver impregnated activated carbons are used in biological filters due to the antibacterial properties of silver ions.[142] Application of activated carbon adsorbents in gas purification Air filtration in clean rooms and for personal protection Activated carbon adsorbents are applied in clean rooms and for personal protection to filter airborne contaminants. In both applications, the air filtration takes place at ambient conditions, i.e. room temperature and atmospheric pressure. The concentration of contaminants in air is usually very low, within the ppm-level. To mitigate desorption of hazardous compounds, the adsorption process needs to be irreversible, at least under operation conditions. These process requirements necessitate specifically designed filter materials. In the following, air filtration in clean rooms and for personal protection is presented, including the role of activated carbon adsorbents in these two processes. Air filtration in clean rooms Clean rooms are intended to protect products (e.g. semiconductors) from environmental contamination or the environment from chemically or biologically hazardous substances. Air continuously circulates through the clean room and is primarily filtered for particulate matter. Particles down to 0.3 μm are separated by high-efficiency particulate air (HEPA) filters. In order to remove chemical substances, activated carbon filters are installed, additionally.[142] 47

57 2. Theoretical and technical background Table 2.3.: Overview of the adsorption capabilities of different carbon surface modifications Type of surface modification Active sites Examples of targeted gas compounds Carbon surface functionalization Oxygen surface groups Nitrogen surface groups NH 3, SO 2 [81, 141] SO 2, H 2 S, CH 2 O [7] Non-metal salt impregnations Metal oxide impregnations Metal salt impregnations Metal impregnations KOH H 2 S, H 3 BO 3 [127, 142] NaOH, Na 2 CO 3, H 2 S, HCl, HF [1, 127, K 2 CO 3 139, 142, 143] KI H 2 S, PH 3, AsH 3 [1] H 3 PO 4, H 2 SO 4, NH 3 [127, C 6 H 8 O 7 142] MoO 3, V 2 O 5 NH 3 [81] CuCl 2, ZnCl 2, NH 3 [81, 127, Zn(OAc) 2 142, 144] CuSO 4, Pb(OAc) 2 H 2 S [127] Cu(NH 3 ) 4 SO 4 AsH 3 [127] Cu H 2 S, SO 2 [7] 48

58 2. Theoretical and technical background In semiconductor processing, typically done in clean rooms, air contamination by boron compounds is an important issue. Airborne boron species affect doping levels of semiconductors. Traces of boron species are present in the atmosphere and standard HEPA clean room filters consisting of borosilicate glass also release boric acid. As already presented in section 2.3.2, boron species are removed using potassium hydroxide impregnated activated carbon. Furthermore, activated carbons containing specific salt impregnations are applied to remove acid gases, amine species and airborne minerals from air. Biological filters are comprised of antibacterial potassium iodide or silver impregnated activated carbons.[142] Air filtration for personal protection The other application of activated carbon adsorbents in air filtration is the protection of first responders from hazardous gases.[1, 5] A few broadband filters exist covering all toxic gases. But, most filters are made for a specific group of airborne substance, like particles, organic, inorganic, and acidic gases, as well as ammonia species.[16, 75, 149] Standardized test procedures exist to determine gas adsorption capacities. Also, minimum adsorption capacities are specified in product requirements. This allows for comparison of filter materials. Examples of standardized procedures are the DIN EN ABEK1 standard and the US NIOSH test parameters.[150, 151] Substances with high boiling points, i.e. organic gases, directly adsorb to carbon materials (see section 2.3.1). This behavior is also present for silica and alumina, but not at high relative humidity. This is why activated carbons are mainly used in air filters for personal protection. The filter materials need to perform well in environments with different relative humidity. With salt impregnations, similar to clean room filters, additional functionality for other gases is realized (see section for details). Acidic gases, ammonia species and inorganic gases are effectively removed by salt impregnated activated carbons. Mixing carbon adsorbents with different impregnations to allow broadband capabilities is possible without negatively affecting adsorption capacities. Irreversibility of adsorption until breakthrough is also important in filter materials for personal protection.[81, 127] Industrial exhaust gas cleaning European and national regulations limit emission of toxic gases to protect the environment. Threshold concentrations of many substances are specified in the European Union Industrial Emissions Directive 2010/75/EU and the German technical instructions for the maintenance of air purity ( TA Luft ).[152, 153] These values are oriented on environmental toxicity of 49

59 2. Theoretical and technical background a substance and the capability of available filtration technologies to remove this substance. Effective equipment and operation costs are also accounted for. With emerging filtration technologies or toxicological knowledge, threshold concentrations are reevaluated. These emission regulations apply to all industrial and agricultural processes. Common substances in off-gases, that are known to be hazardous to the environment, are hydrogen sulfide, sulfur dioxide, nitrogen oxides, ammonia species, heavy-metal compounds, organic compounds, and particles smaller 10 μm.[154] Solid particles are separated by electrical precipitation, for instance. By total oxidation with air, organic compounds are converted to inorganic oxides like carbon dioxide, sulfur dioxide, nitrogen oxides and metal oxides.[128, 129] Sulfur dioxide gas is industrially removed by absorption in calcium carbonate solutions at ph values between 6.5 and 7. Calcium sulfate is irreversible formed. Activated carbons can also be applied in sulfur dioxide removal. With activated carbons, sulfur dioxide is oxidized by air in the presence of water, resulting in the deposition of sulfuric acid in the pore system. At C, these carbon adsorbers can then be thermally regenerated yielding sulfur dioxide, steam and carbon dioxide. The carbon atoms of carbon dioxide originate from the activated carbon support itself.[1, 128, 129] Nitrogen oxides are catalytically reduced to nitrogen gas at temperatures between C. Therefore, e.g. ammonia is added as reducing agent in front of highly selective metal/metal oxide catalysts. Activated carbon also catalyzes the reduction of nitrogen oxide in the presence of ammonia and air.[1, 128, 129] Heavy metals and organic substances are removed by adsorption to activated carbon. Hydrogen chloride adsorbs to activated carbon, as well.[128, 129] Impregnated activated carbons are not yet reported to be applied in the fine filtration of off-gas streams, thus also removing the last remaining hazardous molecules. Possibly, fine filtration with impregnated adsorbents isn t currently economical due to limited adsorption capacities and limited regeneration abilities Functionalization of activated carbon with ionic liquids Supported ionic liquid phase (SILP) concept In the supported ionic liquid phase concept, a thin film of ionic liquid is dispersed on a porous support material. Macroscopically, a dry and pourable solid results, with the ionic liquid 50

60 2. Theoretical and technical background immobilized inside the pore system. SILP materials were initially developed for applications in catalysis. Homogeneous, catalytically active complexes were immobilized in the ionic liquid film. These SILP catalysts were successfully tested in hydroformylation and watergasshift reactions. The small film thicknesses of around 1-3 nm and the large exchange surface areas allow for fast diffusion and mass transfer. Next to pore volume and BET surface area, the ionic liquid loading α IL is an important parameter as defined in equation It s the ratio of ionic liquid volume V IL over the total pore volume V pore. α IL = V IL V pore (2.24) Compared to traditional two-phase liquid-liquid processes, only small amounts of ionic liquid are necessary, rendering the process efficient and economically feasible. Applying supported ionic liquid phase technology to gas purification is a fundamentally new approach to increase the capacity of solid adsorbents.[10] Compared to traditional solid impregnations (see section 2.3.2), not just surface adsorption sites are available, but absorption of gases into a liquid volume takes place. The gaseous substances dissolve in the supported, liquid salt film or undergo a chemical reaction in the liquid salt (see figure 2.13). It needs to be noted that gas purification using SILP materials is macroscopically an adsorption process, but microscopically an absorption process. In the following, important properties of ionic liquids are presented, as well as the effects of metal salt dissolution into ionic liquids.[9, 10, 16, ] 51

61 2. Theoretical and technical background Ionic liquid Pore Hazardous gas M + - M M + - Support Figure 2.13.: SILP concept ([16] - adapted by permission of The Royal Society of Chemistry) Properties of ionic liquids Ionic liquids are made of cations and anions. They have melting points below 100 C. Characteristically, ionic liquids exhibit very low vapor pressures and are thermally stable up to 250 C.[ ] By selectively combining cations and anions, ionic liquids can adopt a broad range of physicochemical properties. For application in gas purification, melting point, solubility, reactivity, and thermal stability are the most important parameters. If the ionic liquid is in fact liquid at operating temperature, mass transfer from the gas phase into the ionic liquid phase takes place. Solubility of certain gases can be selectively increased or decreased to facilitate gas separation. Ionic liquids can be tuned to physically solve significant amounts of specific gases. These physically loaded ionic liquids can be easily regenerated at decreased pressure or increased temperature.[162] Additionally, reactive functionalities can be integrated into the used ionic liquids. So, substances can be removed from the gas phase by forming chemical bonds. Ionic liquids can also solve regular salts in large quantities, while still remaining an ionic liquid. Thus, reactive functionalities of these regular salts are inherited.[17] Solubility parameters of ionic liquids are predictable by a priori calculations, for instance with COSMO-RS. Thus, the immense number of ion combinations is systematically screened to find ionic liquids with suitable properties.[163] The fact that ionic liquids have extremely low vapor pressures is a huge advantage in continuous gas-phase processes. Release of ionic liquid into the product gas feed is effectively mitigated. Very high product purities are achieved. For immobilized ionic liquids, long-term stable adsorbents result with large gas-liquid interfaces for efficient mass transfer.[10, 16] 52

62 2. Theoretical and technical background Reactive metal complexes dissolved in ionic liquids In conventional filter materials with metal salt dispersed on a solid support (see section 2.3.2), only metal salts at the solid-gas interface are accessible to the hazardous gases. This significantly limits adsorption capacity of these materials to roughly the surface area of the support. Dissolving metal salts in ionic liquids and preparing a SILP material results in increased gas adsorption capacities.[17] All metal species are accessible by absorption of the gaseous substances into the ionic liquid. For good accessibility and utilization, the metal salts need to be dissolved, or at least finely suspended, in the ionic liquid. Reactivity of the ionic liquid film is greatly enhanced by incorporating reactive metal salts. As presented in section 2.3.2, metal salts are known to readily react with hazardous gases such as hydrogen sulfide and ammonia, e.g. by complex formation. Complex formation already takes place at room temperature. Reactivity depends on complex formation constants of metal salts. The stability of the formed complex is characterized by product dissociation constants Application of SILP adsorbents in gas purification SILP adsorbents based on inorganic supports [9, 10] In 2012, the Malaysian company Petronas reported the application of a SILP material for the removal of mercury compounds from natural gas. An industrial adsorber unit utilizing 60 t of SILP material was presented. The exact SILP composition and reaction mechanisms weren t published, though. This is the first published, large-scale application of SILP technology in gas purification.[12] Concerning the flue gases sulfur dioxide and carbon dioxide, suitable ionic liquids with large absorption capacities were found. Several research groups immobilized these ionic liquids to study effects like mass transfer, stability and regeneration ability. Zhang et al. characterized a SILP material based on 1,1,3,3-tetramethylguanidinium lactate and silica in the adsorption of sulfur dioxide at C and 1 bar. With increasing ionic liquid loading, pore volume, specific surface area and porosity of the material decreased. Also, apparent density and average pore diameter increased. The average pore diameter increased as primarily the smaller pores were filled with ionic liquid. The mechanism of sulfur dioxide absorption in the ionic liquid was proposed to be a reaction with the amine functional group on the cation [164]. Due to the large gas-liquid interface, sorption equilibrium of SILP 53

63 2. Theoretical and technical background Scheme 2.9 Carbon dioxide chemisorption in amino-functional ionic liquids [15, 165] (Reprinted with permission from [15]. Copyright 2012 American Chemical Society) NH 2 + CO 2 NHCOOH NHCOOH + NH 2 NHCOO + NH + 3 materials was reached in less than 15 min, while the bulk ionic liquid required 45 min. After decreasing adsorption temperature from 30 to 20 C, sulfur dioxide capacity increased by 40%. Interestingly, SILP materials with lower ionic liquid loadings exhibited larger sulfur dioxide capacities of up to 0.95 g SO2 g IL -1. Sulfur dioxide could have condensed in the smaller pores prevailing at low ionic liquid loadings. Regeneration and recycling of SILP material was carried out at 90 C and 0.1 bar without loss in capacity.[13] Reversible adsorption of sulfur dioxide and carbon dioxide using different SILP materials was also reported by Shunmugavel et al..[14] For carbon dioxide adsorption, Ren et al. [15] immobilized liquid amine-functionalized amino acid salts on silica. The effects of both pure carbon dioxide and flue gas (14% CO 2 in N 2 ) were investigated. In experiments with a lysine-based SILP material at 1 bar and 25 C, the adsorption capacity decreased from 1.87 mmol CO2 g SILP -1 using pure carbon dioxide to 1.54 mmol CO2 g SILP -1 using flue gas. Contrary to previous assumptions, carbon dioxide sorption stoichiometry didn t appear to depend on the amine groups location, but rather on ion pair size. Carbon dioxide chemically reacted with the ionic liquid in a 1:2 mechanism. At first, carbamic acid was formed, which protonated a second amine functional group forming carbamate and an ammonium ion (see scheme 2.9). The influence of water was investigated, as well. Large water contents significantly decreased carbon dioxide capacity due to competition in hydrogen bonding. The SILP materials based on glycine and lysine anions were successfully regenerated at 0.08 bar and 90 C. Because the ionic liquids were thermally very stable, only minor losses in adsorption capacity (3-6%) were observed during recycling experiments. To remove sulfur species such as n-butyl mercaptan from nitrogen in a continuous process, SILP materials containing reactive metal salts were developed. Porous aluminum oxides coated with 1-alkyl-3-methylimidazilium chlorides and equimolarly dissolved zinc or tin chlorides were applied. The normalized breakthrough curves are presented in figure Until breakthrough, n-butyl mercaptan was always completely removed at 90 C ad- 54

64 2. Theoretical and technical background c c -1 / [C4C1IM]Cl-ZnCl2 [C4C1IM]Cl-SnCl2 [C8C1IM]Cl-SnCl [C12C1IM]Cl-SnCl time / min Figure 2.14.: Normalized breakthrough curves of n-butyl mercaptan using different SILP materials; T = 90 C, p = 1.05 bar, m SILP = 10.6 g, h filling = 9 cm, V N2 = 40 ml N min -1, V n-butyl mercaptan = 0.2 ml min -1 (adapted from Ref. [11] with permission from The Royal Society of Chemistry) sorption temperature and atmospheric pressure. High capacities were observed for acidic chlorostannate-based ionic liquids. The capacity increased with increasing alkyl-chain length of the cation, likely due to a lower packing density of the ionic liquid. The SILP material was effectively regenerated by a temperature increase to 130 C and a pressure decrease to mbar. A decrease in adsorption capacity was caused by dibutyldisulfide condensation during regeneration experiments.[11] SILP adsorbents based on spherical carbon Kohler et al. [16] prepared SILP materials based on spherical carbon by wet impregnation and tested these materials in the continuous removal of ammonia from nitrogen (1000 ppm NH 3 in N 2 ) at 1 bar and 30 C. Before SILP preparation, suitable ionic liquids were screened for ammonia solubility at 30 C. Solubility of ammonia was large in all tested samples, but significantly enhanced for metal salt containing systems, especially [C 8 C 1 IM]Cl-CuCl 2 1:1. This was due to the formation of metal-amine complexes, e.g. [Cu(NH 3 ) 4 ] 2+ (see also section 2.3.2). The chlorocuprate ionic liquid coated to spherical carbon exhibited noticeable ammonia 55

65 2. Theoretical and technical background c c -1 / [C8C1IM]Cl-CuCl2 [C4C1IM]Cl-CuCl2 [C2C1IM]Cl-CuCl2 CuCl2 fits according to model t / min Figure 2.15.: Normalized ammonia breakthrough curves of SILP materials based on spherical carbon with chlorocuprate ionic liquids (x CuCl2 = 0.5, α IL = 0.4) and a copper chloride impregnation as reference; T = 303 K, p = 1.21 bar, rh = 85%, h filling = 2 cm, V N2 = ml N min -1, V NH3 = 0.33 ml N min -1 ([16] - adapted by permission of The Royal Society of Chemistry) breakthrough times, already with 10 vol% ionic liquid loading. With increasing relative humidity from 0 to 80%, ammonia capacity increased by 74%. Varying the alkyl group of the imidazolium cation, ammonia capacity increased with decreasing alkyl chain-length and increasing copper content (see figure 2.15). At 85% relative humidity, SILP materials performed better than a pure copper chloride impregnation with comparable copper content (see figure 2.15). The ionic liquid seemed to improve accessibility of copper ions for complex formation and could have resulted in a more homogeneous impregnation. At dry and humid conditions, ammonia breakthrough times increased with increasing ionic liquid loading.[16] Irreversibility of a partially loaded SILP material (below breakthrough) was successfully demonstrated at 30 C. A completely saturated SILP material desorbed 56% of adsorbed ammonia at 85 C (see figure 2.16). Target applications of these one-time SILP filter materials are for personal protection and gas polishing, i.e. the removal of ammonia traces.[16] Broadband filtration capabilities of these irreversible SILP adsorbers were also analyzed with regard to DIN EN ABEK1. Spherical carbon coated with [C 2 C 1 IM]Cl-CuCl 2 1:1 (α IL =0.2) performed very well in the adsorption of ammonia, inorganic (chlorine) and acidic (hydrogen 56

66 2. Theoretical and technical background NH 3 flow / mg min -1 2 Absorption Desorption (T = 85 C) 30 C) t / min Figure 2.16.: Comparison of absorbed and desorbed amount of ammonia for the SILP absorber material coated with [C 2 C 1 IM]Cl-CuCl 2 (x CuCl2 = 0.57, α IL = 0.2); p = 1.21 bar, h filling = 2 cm, V N2 = ml N min -1, V NH3 = 0.33 ml N min -1 ([16] - adapted by permission of The Royal Society of Chemistry) sulfide) gases. Organic (cyclohexane) adsorption capacity was below ABEK1 requirements. This issue could be solved by decreasing ionic liquid loading, as organic gases directly adsorb to the unoccupied spherical carbon surface.[16] The resulting pressure drop of SILP materials based on spherical carbon was investigated, as well. 500 μm highly activated spherical carbon was coated with [C 2 C 1 IM]Br-CuBr 2 1:1 (α IL =0.2). Also, open porous foam filter media [5] were prepared, furnished with the same SILP material, and analyzed with regard to ammonia adsorption capacity and pressure drop. It needs to be noted that several ammonia adsorption parameters, including the geometries of the SILP fixed-bed and SILP foam adsorber, were different for structural reasons. Still, ammonia capacities of both filters were significant. Compared to the previous report of Kohler et al., ammonia capacity of the bromocuprat fixed-bed system was 23% larger than the chlorocuprat equivalent. The pressure drop of the SILP foam was substantially lower than the pressure drop of the SILP fixed-bed adsorber. Due to large adsorption capacity and low pressure drop, filter materials made of SILP foams were deemed generally well suitable for large-scale industrial application.[10] 57

67 Gases studied in this work 2. Theoretical and technical background In this work, filter materials for the continuous removal of ammonia, hydrogen sulfide, formaldehyde and cyclohexane are investigated. Concerning applications in gas purification, the specific properties of these gases and their occurrences in the environment are presented in the following. The resulting requirements for the filter materials are highlighted, as well. Ammonia (NH 3 ) is a colorless gas with a pungent odor. It s toxic to human health and the environment. Ammonia corrodes copper-based metals like brass and disintegrates some polymers like polypropylene. Ammonia gas is released in industries such as food, livestock breeding, rubber, leather, and waste decomposition plants.[140, ] In order to protect humans from immediate ammonia exposure, personal protection filter materials for ammonia retention are specifically designed. Ammonia gas species inherit a separate category in the classification of filter materials due to their alkaline properties, distinguishing ammonia from other gaseous compounds. Also, in broadband filters, ammonia adsorption capacity is a critical figure. Filter requirements for personal protection are irreversible adsorption of ammonia, fast adsorption kinetics and large breakthrough capacities at ambient adsorption conditions. Also, the filter materials need to perform well at varying relative humidity. As only few broadband filters with good ammonia capacity exist, yet, the development of SILP materials with large ammonia capacity and broadband capability is continued in this work.[7, 10, 16] Ammonia removal from biogas is an important industrial application that is elaborated in the following. Ammonia is a critical gas in the production of biogas for two reasons. Biogas production by bacteria in anaerobic fermentation plants is inhibited by too large ammonia concentrations in the plant. This is due to the toxic nature of ammonia to microorganisms. To limit the amount of free ammonia, parameters like temperature and ph value of the fermentation process are adjusted in dependence on the nitrogen content of the substrate.[169, 170] Also, biogas produced in an anaerobic fermentation plant contains of up to 0.05 vol% ammonia (see table 2.4). Ammonia is typically removed from these gases by chemical absorption processes.[171, 172] In both scenarios, adsorbents with large ammonia adsorption capacities could simplify the processes. Adsorption conditions vary between C and 1-10 bar.[173, 174] The effects of large relative humidity need to be considered. In large biogas production plants with significant ammonia quantities to be removed, regeneration of adsorbents is likely to be economically beneficial, as well. Consequentially, in this work, the reversible adsorption of ammonia in SILP materials for industrial applications is also investigated. Previous investigations on ammonia desorption 58

68 2. Theoretical and technical background Biogas compound Biogas fraction / vol% Methane Carbon dioxide Hydrogen sulfide 0-1 Ammonia Water (100% RH) Table 2.4.: Typical composition of biogas from an anaerobic digestion energy plant [171, 176] from copper-containing adsorbents and SILP materials indicated that adsorption is at least partially reversible. In filter materials saturated with ammonia, ammonia partially desorbed by temperature increase or flushing with inert gas.[16, 81] It s known that metal-amine complexes like Cu(NH 3 ) 4 SO 4 disintegrate at temperatures between C.[175] Thus, SILP materials are applied in temperature-swing and pressure-swing adsorption processes for further analysis. Hydrogen sulfide (H 2 S) is a toxic gas for human health and the environment. The gas smells like rotten eggs at concentrations between ppm, but isn t noticeable at larger concentrations due to failure of the olfactory sense. It s easily inflammable and forms explosive mixtures with air. The combustion product of hydrogen sulfide with air, sulfur dioxide, is similarly hazardous.[166, 167] Furthermore, hydrogen sulfide can act as catalyst poison in the production of chemicals.[177, 178] Large quantities of hydrogen sulfide are released by mankind in the processing of organic raw materials. Industrial examples are coal combustion in power plants, petroleum refineries, paper production, as well as natural gas and biogas processing plants.[179, 180] Concerning adsorption technologies to protect from hydrogen sulfide gas exposure, commercial acidic gas filters and broadband filters for personal protection are available. They include functionalities for hydrogen sulfide adsorption.[7, 16] In this work, hydrogen sulfide is applied as a test gas to determine the broadband capability of SILP materials for acidic gases. The gas needs to irreversibly adsorb to the SILP material at different relative humidity with sufficient breakthrough capacities. The process takes place at ambient conditions, i.e. room temperature and atmospheric pressure. As for ammonia adsorption, hydrogen sulfide removal is relevant in biogas purification. Biogas contains up to 1 vol% hydrogen sulfide (see table 2.4). Currently, hydrogen sulfide re- 59

69 2. Theoretical and technical background moval in biogas plants is technologically realized by biological desulfurization using appropriate microorganisms.[174] Quantitatively the largest gas processing operation worldwide, involving the removal of hydrogen sulfide, is the purification of raw natural gas.[181] So, besides irreversible adsorption, two potential applications of SILP materials adsorbing hydrogen sulfide, as discussed in this work, are natural gas and biogas purification. Large hydrogen sulfide capacities and the ability to regenerate the adsorber are demanded. Both, pressure and temperature swing adsorption modes are assessed. Based on the results, the competitiveness of SILP technology compared to conventional hydrogen sulfide scrubbers is evaluated. Cyclohexane (C 6 H 12 ) is a volatile organic liquid. Cyclohexane is toxic for human health and the environment. It s colorless and has a characteristic odor. Mixtures of air and cyclohexane vapor are explosive.[166] Cyclohexane vapor is the model compound to quantify the organic adsorption capacity of filter materials for personal protection purposes. In this work, the broadband capabilities of SILP materials are evaluated by the cyclohexane breakthrough capacity. As cyclohexane directly adsorbs to the activated carbon surface (see section 2.3.1), the cyclohexane capacity also indicates the number of remaining carbon adsorption sites after ionic liquid impregnation.[16] Formaldehyde (CH 2 O) is a toxic gas for human health. It s colorless, but has a pungent odor. The gas is easily inflammable and forms explosive mixtures with air.[166] Formaldehyde exposure is an issue in newly built houses. Formaldehyde gas and other aldehydes are mainly released from wood and wooden products. Composite materials containing urea-formaldehyde resins are the most dominant sources of formaldehyde air contamination. Water-based paints with Quaternium 15 are a source of formaldehyde exposure, as well. Indoors, ozone chemistry was found to also contribute to formaldehyde formation. Outdoors, formaldehyde is released from automotive exhaust gases. The formaldehyde content increases with increasing amount of oxygen species in the fuel, e.g. ethanol, methanol and methyl tertiary butyl ether.[ ] The development of SILP materials to remove toxic formaldehyde gas from air is part of this work. A first application scenario of irreversible formaldehyde adsorption is in household room filters for the Asian market. This market region has a climate with large relative humidity. A possible reaction pathway for formaldehyde conversion is, for instance, the reaction of aldehydes with alcohols forming acetale copolymers.[186] Such a specific functionality for formaldehyde removal could be introduced into ionic liquids. For good customer acceptance, ionic liquid impregnations with well-known non-toxicity are desired. A suitable salt with a 60

70 2. Theoretical and technical background hydroxide functional group could be choline chloride. Choline chloride is listed as animal feed additive and exhibits a well-known non-toxicity.[187, 188] 61

71 3. Objective of this work This thesis intends to demonstrate the applicability of polymer-based spherical activated carbon materials in catalysis and advanced gas purification. Furthermore, the inherent advantages of spherical carbon in comparison to activated carbon powder and inorganic support materials are discussed. Compared to powder materials, these advantages are, for instance, the easy material handling, the low pressure drop and the high filtration rate of spherical carbon. In contrast to inorganic supports such as alumina and silica, the broadly modifiable carbon surface for tailored catalyst preparation and the additional adsorption capability of spherical carbon for organic and inorganic gases in broadband gas filters are of high interest. In the following, the strategy for these scientific investigations is elaborated. Filtration and pressure drop of spherical carbons Faster filtration rates of spherical carbon over carbon powder can reduce the time needed for catalyst separation from the product solution. Due to the larger particle size and its spherical shape, spherical carbon materials are expected to exhibit a lower pressure drop in liquid phase filtration processes than carbon powder and, thus, faster fluid flow rates through spherical carbon beds. A lower pressure drop of spherical carbon beds will also be beneficial in continuous processes such as flow chemistry applications. The comparison of spherical carbon materials and carbon powder, as well as the influence of particle size on fluid flow rates and filtration times are demonstrated with the help of filtration experiments and numerical investigations. Spherical carbon as novel catalyst support material For successful application in catalysis, catalytically active species need to be well deposited to spherical carbon. Influencing parameters in catalyst preparation resulting in catalytically active materials can be the particle size, pore structure and surface properties of spherical carbon. In this work, noble metals such as palladium, ruthenium and platinum are chosen as catalytically active species. Important properties of the final catalysts that characterize the results of active metal deposition, i.e. metal dispersion and metal distribution, are analyzed 62

72 3. Objective of this work in detail. In screening experiments using a variety of test reactions, the performance of the prepared spherical catalysts is compared to the catalytic activity of respective carbon powder catalysts. Furthermore, correlations between catalytic activity and material properties, e.g. metal dispersion, metal distribution, particle size, pore structure and surface functionalization, are discussed. In order to apply spherical carbons as support materials for different noble metals, the spherical carbons are oxidized, at first. Then, the oxidized materials are loaded with noble metals by electrostatic ion adsorption and subsequent metal transformation. Parameters of catalyst preparation are chosen that keep a potential technical scale-up to larger quantities in mind. The spherical carbon materials, the oxidized spherical carbons and the prepared catalysts are characterized in detail and compared to the respective powder reference material. Correlations of experimental data are illuminated and conclusions are devised for each experimental series. A variety of spherical carbons are used as starting material, differing in particle size and pore structure. Palladium catalysts based on these differently sized spherical carbons are characterized regarding metal loading, metal dispersion and catalytic activity. To furthermore investigate the effects of different particle sizes, comminution experiments are conducted. The same characterization of metal loading, dispersion and catalytic activity is done for palladium loaded spherical carbons with different pore structures. Concerning the surface composition of spherical carbon, oxidation parameters of spherical carbon functionalization are varied. Oxidations are carried out with nitric acid and sulfuric acid, respectively. For each acid, two acid concentrations and two oxidation temperatures are chosen. The surface properties of these eight carbon samples are analyzed in detail. Important differences between nitric acid and sulfuric acid oxidations are specifically illuminated. The oxidized materials are loaded with palladium and characterized regarding metal loading, metal dispersion and catalytic activity. To study the influence of metal loading, palladium catalysts are prepared with desired metal loadings of 2, 5 and 10 wt%. The obtained metal loading and resulting catalytic activities are determined. Ruthenium catalysts with desired metal loadings of 0.5, 5 and 10 wt% are also characterized regarding metal loading, metal dispersion and catalytic activity. Catalytic activity of spherical ruthenium catalysts is tested in hydrogenation reactions of different molecules and compared to ruthenium on carbon powder. In a stepwise optimization of the platinum catalyst preparation procedures, the influence of various parameters is studied to yield a well performing catalyst for the technical dehydro- 63

73 3. Objective of this work genation of a liquid organic hydrogen carrier. Applying this derived preparation procedure, oxidized spherical carbon is loaded with 0.5, 5 and 10 wt% platinum. The catalysts are characterized regarding metal loading, metal dispersion and catalytic activity. Also, they are compared to platinum on carbon powder and platinum on alumina. In the end of the catalysis part, stability tests of palladium catalysts are conducted. Therefore, the extent of palladium and sulfur leaching is analyzed for two prepared spherical palladium catalysts and the palladium on carbon powder reference. Additionally, the recycling ability of a spherical carbon catalyst is demonstrated. Also, a rhodium metal complex dissolved in ionic liquid is supported on spherical carbon. In a proof of concept style, recycling ability and activation energy are determined applying this SILP catalyst in a test reaction. Spherical carbon as support for SILP filter materials In the second part concerning advanced gas purification, spherical carbon is gas adsorbent and support material for different liquid salts. Reactive metal salts are dissolved in ionic liquids and coated to the internal surface of spherical carbon by wet impregnation. Specific gases are either adsorbed to the carbon surface or absorbed into the ionic liquid film, where they react with dissolved metal salts. These SILP materials are prepared and tested for the applicability in two areas of gas purification with different functional requirements. In fine filtration processes, i.e. the removal of trace contaminants in the ppm-level, a fast and irreversible reaction of the contaminant with the filter material is necessary. The contaminants need to be completely removed from the gas stream. Applications of these filter materials are, for instance, in clean rooms and for personal protection. The second area, with large concentrations of contaminants, is the bulk filtration of gases, which is of particular interest in industrial gas purification processes. Here, the ability to regenerate the filter materials is most important. To minimize the energy input for regeneration, the contaminants need to weakly interact with the active sites of the filter material. As the filter capacity is of high interest in all applications, the SILP filter materials are primarily characterized regarding their breakthrough time in the continuous removal of hazardous test gases. Corrosion experiments and surface analysis of SILP materials are also conducted. In preliminary experiments, the influence of different impregnation solvents on SILP product quality is assessed. Also, SILP materials containing reactive cuprate and zincate salts are compared in the adsorption of ammonia and hydrogen sulfide. Corrosion of stainless steel and aluminum is determined for these halide-containing SILP materials, as well. 64

74 3. Objective of this work For the irreversible removal of hazardous gases, novel, halide-free SILP materials are prepared with ionic liquid loadings of 0.1, 0.2 and 0.3. Both, the ionic liquid and the reactive cuprate salt are halide-free. Ammonia breakthrough capacities are measured for a relative humidity of 25, 50 and 80%. Broadband capability of the medium-loaded SILP material is evaluated. Alternative coating techniques, i.e. incipient wetness impregnation and dropwise impregnation, are examined. Furthermore, organic metal salts are synthesized, dissolved in ionic liquid and coated to spherical carbon. In breakthrough experiments, ammonia capacity of these SILP materials is determined. Hygroscopic salts with reactive functionality for formaldehyde are also coated to spherical carbon. The resulting filter materials are applied in the continuous removal of formaldehyde gas at large relative humidity. Finally, the regeneration of SILP materials is investigated in pressure swing and temperature swing adsorption setups. Adsorption and desorption capacities of ammonia and hydrogen sulfide are measured. Metal-free impregnations of spherical carbon with reactive ionic liquids are also tested in the reversible adsorption of hydrogen sulfide. 65

75 4. Experimental 4.1. Catalyst preparation and characterization Oxidation of spherical carbon Standard oxidation parameters were 10% nitric acid with 2.5 h treatment time at RT. In detail, 3.2 ml of 65% nitric acid solution were mixed with 26.1 ml water in a round bottom flask. 22 g of spherical carbon were quickly added. The suspension was stirred at RT for 2.5 h. The carbon material was separated using suction filter and feeding bottle and intensively washed with 5 l water. In the end, the oxidized spherical carbon was dried at 120 C and 0.01 mbar for 20 h. Alternative functionalization conditions have also been applied varying oxidation agent (sulfuric acid), acid concentration (15-50%), oxidation temperature (90 C) or amount of spherical carbon (50 g). In all alternative functionalization processes, the volume of oxidizing solutions was set to three times the materials pore volume. In case of oxidations at 90 C, a reflux condenser was mounted to the round bottom flask. Oxidations with 15 and 36% nitric acid were performed at Blücher GmbH, Erkrath Deposition of noble metal Palladium, ruthenium and platinum salts were loaded to oxidized spherical carbon in a wet impregnation process. Specifically, the principle of electrostatic adsorption of metal anions to charged carbon surface groups was applied (see section ). 5 g of palladium catalyst with metal loadings between 2-10 wt% were prepared by dissolving palladium chloride in 5.5 ml 1 N hydrochloric acid and 29.5 ml water. Oxidized spherical carbon (5 g) was quickly added and the suspension was slowly stirred at RT for 24 h. Using suction filter and feeding bottle, the solid material was separated and rinsed with 200 ml water. The catalyst material was dried at 120 C and 0.01 mbar for 2 h. Metal transformation was 66

76 4. Experimental carried out in a horizontal flow reactor in hydrogen atmosphere. A temperature of 80 C was chosen for maximum palladium dispersion with reference to literature (see section ). A thin layer of catalyst material (ca. 2 g) was activated by 3.5 vol% hydrogen in nitrogen (reforming gas) at a flow velocity of m s -1 for 1 h. The procedure of ruthenium and platinum catalyst preparation was very similar. The impregnation solution for ruthenium catalysts only consisted of ruthenium chloride hydrate and water. Hexachloroplatinic acid hexahydrate was used as a precursor for platinum catalysts with metal loadings between wt%. Metal transformation temperature of ruthenium and platinum catalysts was set to 300 C, in accordance to literature (see section ). A rhodium SILP catalyst was also prepared. The SILP catalyst was based on spherical carbon coated with an ionic liquid containing a dissolved rhodium complex. 0.1 g precursor rhodium(i) dicarbonyl acetylacetonate, 0.45 g ligand triphenylphosphine-3,3,3-trisulfonic acid trisodium salt (TPPTS) and 4 ml ionic liquid [C 4 C 1 IM][PF 6 ] were solved in 40 ml water g of slightly activated 500 μm spherical carbon (V tot = 0.61 cm 3 g -1 ) were added. Water was slowly removed by rotary evaporation and the SILP material was further dried at 120 C and 0.1 mbar Evaluation of catalytic activities Catalytic activities of the prepared catalysts were determined on the basis of standardized hydrogenation experiments in a stirred-tank reactor. Reaction rates and turnover frequencies were calculated from conversion curves applying reaction engineering models. The dehydrogenation activity of the liquid organic hydrogen carrier Marlotherm SH, demonstrating the performance of prepared platinum catalysts in a technical application, was separately carried out in a glass reactor. The studied reactions are presented in section The stainless steel reactor with a volume of 600 ml made by Parr Instruments consisted of a gas entrainment impeller, a heating mantle, a cooling coil, temperature and pressure sensors, valves for hydrogen, nitrogen and off-gas, a safety relief valve, and a sampling pipe with a filter. Reactant, solvent, catalyst, and optionally, an internal standard were added into the reactor. The internal standard was used for calibration purposes of the gas chromatograph. In general, the total reaction volume amounted to 200 ml. The reactor was mounted, flushed with nitrogen and heated to reaction temperature. The stirrer velocity was set to rpm and a first sample was collected. The experiment was initiated by the addition of hydrogen gas with pressures between bar. Hydrogen gas pressure was kept 67

77 4. Experimental Table 4.1.: Standard hydrogenation parameters for testing catalyst performance Catalyst Pd on carbon Pt on carbon Ru on carbon Rh-SILP Educt molecule Cinnamic acid Toluene Cinnamaldehyde 1,5-Cyclooctadiene Solvent Ethanol Tetrahydrofurane/Water (3:1 v/v) Cyclohexane Cyclohexane Internal standard Ethane-1,2- diole - - n-heptane Educt concentration / mol m -3 Temperature / C Hydrogen pressure / bar Stirrer velocity / rpm GC column type FFAP FFAP Fused silica Fused silica constant throughout the experiment. In certain time intervals, samples were manually taken and analyzed in a gas chromatograph (GC) with FFAP or fused silica column. Using the fractions of feedstock and products, reactant concentrations were calculated and verified with the known concentration of the internal standard. Standard parameters for each hydrogenation test reaction are listed in table 4.1. The course of conversion over the modified residence time X (τ mod ) was fitted using an integrated power law expression, assuming a pseudo first order reaction (see section 2.2.2). Linear regressions of the logarithmized expression were performed in OriginLab Origin applying chi-square minimization, as well as further hypothesis tests and residuals plotting. Data points up to a conversion of around 80% were taken into account. The effective reaction rate constant k e f f was used to quantitatively describe the performance of a catalyst concerning catalytic activity. Alternatively, catalytic activities were compared via turn over 68

78 4. Experimental frequencies of substrate molecules at surface metal sites. Both parameters are explained in section The extent of mass transport limitation due to pore diffusion was investigated with classical reaction engineering methods, calculating Thiele moduli and effectiveness factors (see section 2.2.2). The required intrinsic reaction rate constant was determined in comminution experiments of particulate catalysts. The effective diffusion coefficient was estimated from the molecular diffusion coefficient of substrate molecules in solvent. For application of catalysts in the dehydrogenation of fully hydrogenated Marlotherm SH, experiments were conducted in a 100 ml glass reactor filled with ml of Marlotherm SH. The system was stirred with a magnetic stirrer and heated to 310 C. Then, the reaction was initiated by introducing the catalyst material. The molar ratio of noble metal to Marlotherm SH was always 0.1 mol%. The total reaction time amounted to 120 min. In certain time intervals, 0.1 ml samples were taken and quantitatively analyzed by 1H NMR to determine the dehydrogenation degree of Marlotherm SH Evaluation of filtration rates The filtration rates of carbon materials were determined experimentally and mathematically. The experimental setup consisted of a filtration apparatus from Sartorius made of transparent polycarbonate (Polycarbonate Filter Holder 16510). As a membrane, paper filters with 1 μm particle retention from Sartorius were used (Filter Discs Grade 393). For each filtration experiment, 1 g oxidized spherical carbon material or commercial palladium on carbon powder was used (Sigma-Aldrich, 5 wt% palladium on activated carbon). The carbon materials were suspended in 200 ml colored water (potassium permanganate) in the upper reservoir of the filtration apparatus. The closed system of the filtration apparatus allowed the application of 0.5 bar excess air pressure. The filtration process was recorded with a video camera and evaluated to result in a material-specific filtration rate. As a mathematical model, the Darcy law was applied to describe the filtration process and extrapolate filtration rates of different catalyst particle sizes for larger industrial reactors (see section for details). 69

79 4. Experimental 4.2. SILP material preparation and characterization Preparation of SILP materials SILP materials were usually prepared by wet impregnation and assisted ultrasonication. Ionic liquids were fully deposited inside spherical carbon, so that dry and pourable material resulted.[16] To calculate the required amount of ionic liquid melt, its density was determined by helium pycnometry at 20 C. Ethanol, acetonitrile, methylenchloride or water was used as impregnation solvent with a quantity of 1.9 times the spherical carbons total pore volume. In general, highly activated spherical carbon from Blücher GmbH, Erkrath with a particle diameter around 500 μm was applied (S BET = 2210 m 2 g -1, V pore = 1.33 ml g -1, ρ bulk = 0.38 g ml -1 ). In a round-bottom flask, ionic liquid melt was dissolved in solvent and spherical carbon was then suspended at room temperature. The system was treated in an ultrasonication bath for two hours at room temperature. In a rotary evaporator, solvent was removed at 50 C and pressures down to 20 mbar. Samples prepared with organic solvent were additionally dried at 120 C and 0.01 mbar. The standard production amount was 200 g SILP material. Two SILP materials based on copper bromide and zinc bromide were supplied by Blücher GmbH, Erkrath. Equation 4.1 describes the volume-specific molar metal content γ of a SILP material, with β being the ionic liquid loading as defined in equation 4.2. Also, ν being the molar fraction of each ionic liquid compound, z i,cu the molar number of copper atoms in the ionic liquid, M i the molar mass of each ionic liquid compound, V pore the pore volume of spherical carbon, α IL the ionic liquid loading and ρ the bulk densities of spherical carbon, SILP or the density of ionic liquid. In this equation of molar metal content, all parameters are usually known. γ = β ρ SILP i ν i z i,cu i ν i M i = β (1 + β 1 β ) i ν i z i,cu i ν i M i (4.1) β = α V pore ρ IL 1 + α V pore ρ IL (4.2) 70

80 4. Experimental Evaluation of SILP corrosivity Rudimentary corrosion tests derived from industrial standards were applied to characterize the general corrosion potential of SILP materials. Relative humidity was accounted for. Aluminum corrosion was analyzed by wrapping SILP material in aluminum foil and placing it in a climate chamber at 65-75% rh and 20 C for 7 days. Then, the degree of corrosion was optically evaluated. Aluminium corrosion tests were carried out by Blücher GmbH, Erkrath. For stainless steel corrosion tests, a glass bottle was partially filled with glass wool and 3 g of SILP material on top. A stainless steel (X5CrNi18-10) platelet of known mass was placed in each bottle. To account for the influence of relative humidity, 0.5 ml of water were injected into the bottom part of the bottle. The corrosion test was conducted at RT for 7 days. After that time, the stainless steel platelet was analyzed for corrosion. Additionally, 2 ml water was added to each bottle and the corrosion test was continued for another 7 days at RT. Finally, corrosion of stainless steel platelets was evaluated optically and gravimetrically Evaluation of gas purification performance Continuous gas-phase removal of ammonia was performed in the test rig shown in figure 4.1. SILP material was placed in the adsorber unit (18 mm diameter) with a bed height of 20 mm. The adsorber was heated to 30 C and operated at atmospheric pressure. Nitrogen gas and ammonia were fed by mass flow controllers (Bronkhorst), so that an ammonia concentration of 1000 ppm resulted. Nitrogen gas was optionally humidified in the saturator. Relative humidity was controlled by the saturator s water temperature and a humidity sensor. For calibration purposes, a bypass to the adsorber was available. With a constant velocity of 0.02 m s -1, the gas mixture passed through the SILP material. Behind the adsorber, the gas composition was analyzed by gas chromatography in short time intervals (Varian CP-3800, Volamine fused silica 60 m x 0.31 mm capillary column, thermal conductivity detector). The ammonia detection limit was around 100 ppm.[16] In a typical breakthrough experiment, the SILP material was preconditioned for 15+ hours in a nitrogen stream at adsorption conditions, but without ammonia. Then, using the adsorber bypass, ammonia was dosed into the system and its concentration was monitored for 30 min. The adsorption process was started by switching from bypass mode to the adsorber. When the initial ammonia concentration was reached, the experiment was finished. For formaldehyde adsorption experiments, this test rig was slightly modified. Nitrogen gas was effectively enriched with formaldehyde by passing through the saturator unit containing 71

81 4. Experimental TIC-3 PIC Mixer TI-2 rh I Saturator H 2O TIC-4 Adsorber TI-3 TIC-5 TIC-6 TIC-2 TI-1 TIC-1 Thermostat cycle TIC-7 GC Filter 1 Filter 2 F MFC-1 F MFC-2 N 2 NH 3 Figure 4.1.: Gas-phase test rig for continuous removal of ammonia ([16] - adapted by permission of The Royal Society of Chemistry) a 4% formalin solution. Breakthrough measurements of SILP materials with regard to DIN EN ABEK1 [150] and US NIOSH [151] were conducted by ProQares B.V., the Netherlands. These test rigs were similarly constructed to the previously described setup. Due to standardized specifications, adsorber geometry and process parameters varied. The removal of ammonia, hydrogen sulfide, cyclohexane, and formaldehyde was analyzed for selected SILP materials. 72

82 4. Experimental Table 4.2.: Particle size fractions and bulk densities of different spherical carbon materials Particle size / μm Carbon activation Particle size fraction / μm Bulk density / g l Increasing Increasing Increasing < Materials In the following, a characterization of the spherical carbon materials is given, as well as a description of the synthesis of metal salts and ionic liquids. All other materials and chemicals were commercially available. Appendix A includes several tables listing the different chemicals, purities and suppliers Overview of applied spherical carbon materials A broad selection of polymer-based spherical activated carbons was available for this study. All spherical carbon materials were produced and supplied by Blücher GmbH, Erkrath. Polystyrene-divinylbenzene copolymer was the respective polymer origin. Process parameters for sulfonation, carbonization and carbon activation weren t disclosed. The spherical carbons varied in particle size and carbon activation. Table 4.2 shows the particle size fractions after product classification, as well as bulk densities of the spherical carbon materials applied in this work. Additionally, table 4.3 lists the respective material properties derived from nitrogen sorption experiments. 73

83 4. Experimental Table 4.3.: Pore characteristics derived from nitrogen sorption data of different spherical carbon materials Particle size / μm Carbon activation V tot (0.990)* / cm 3 g -1 V micro / cm 3 g -1 Fraction of mesopores / % MP BET ( )* / m 2 g -1 Average pore diameter / nm Increasing Increasing (0.995)* n/a * p p Increasing

84 4. Experimental Depending on sieves used for classification, different particle size fractions were obtained. The large spherical carbon with μm in particle size and the smaller, highly activated material with μm in size were most narrowly distributed. Particles of the fine material were classified to be smaller than 100 μm. To allow for quick reference, the three major particle size fractions were indicated with a general particle size of 500, 200, and 50 μm. For each particle size, bulk density decreased with increasing carbon activation due to an increase in carbon discharge by gasification. Regarding nitrogen sorption measurements, the total pore volume increased corresponding to an increase in carbon activation. The total pore volume varied between cm 3 g -1. Micropore volume and BET surface area generally increased with increasing carbon activation, as well. One exception of this trend was observed for the highly activated 500 μm spherical carbon. Here, the carbon activation procedure was modified to result in a more mesoporous material. The significantly smaller ratio of micropore volume to total pore volume indicated that this modification was successful. Overall, values of micropore volumes between cm 3 g -1 and BET surface areas between m 2 g -1 were determined. The ratio of micropore volume to total pore volume decreased with increasing carbon activation, thus, leading to a larger fraction of mesopores. The average pore diameter also increased with increasing carbon activation. Average pore diameters between nm were realized, with the largest average pore diameter corresponding to the strongly mesoporous 500 μm spherical carbon. In essence, different degrees of carbon activation resulted in different pore geometries within the spherical carbon material. A larger quantity of mesopores and generally larger surface areas were formed after more intensive carbon activation Synthesis of metal salts and ionic liquids Copper(II) octylsulfate was synthesized via two different synthesis routes. In the first route (see scheme 4.1), an aqueous solution of 10 g sodium octylsulfate was percolated through a column containing Amberlite IR-120 ion exchange resin. The resulting acid was then neutralized with an excess of 2.4 g copper(ii) hydroxide/copper(ii) carbonate mixture. The aqueous copper(ii) octylsulfate solution was filtered and dried in a rotary evaporator. Alternatively, as shown in scheme 4.2, 500 g copper octylsulfate were synthesized by sulfatization of ml 1-octanol in 200 ml dichloromethane with 138 ml chlorosulfuric acid at room temperature and under slight vacuum. The solution was constantly stirred, chlorosulfuric acid was added dropwise and hydrogen chloride vapor was removed by a water jet pump. Then, the 75

85 4. Experimental Scheme 4.1 Synthesis of copper(ii) octylsulfate by cation exchange of the octylsulfate salt (1) and neutralization (2) Na + [C 8 H 17 OSO 3 ] + R SO 3 H H + [C 8 H 17 OSO 3 ] + R SO 3 Na (1) 4 H + [C 8 H 17 OSO 3 ] + CuCO 3 Cu(OH ) 2 2 Cu[C 8 H 17 OSO 3 ] 2 + H 2 CO H 2 O (2) Scheme 4.2 Synthesis of copper(ii) octylsulfate by sulfatization of 1-octanol (1) and neutralization (2) C 8 H 17 OH + HSO 3 Cl H + [C 8 H 17 OSO 3 ] + HCl (1) 4 H + [C 8 H 17 OSO 3 ] + CuCO 3 Cu(OH ) 2 2 Cu[C 8 H 17 OSO 3 ] 2 + H 2 CO H 2 O (2) aqueous phase was separated from the organic phase and 115 g copper(ii) hydroxide/copper(ii) carbonate mixture was added. Finally, the solution was filtered and dried in a rotary evaporator.[ ] Copper octanoate was synthesized by neutralizing commercially available octanoic acid (50 g) with a copper(ii) hydroxide/copper(ii) carbonate mixture (19.5 g) at room temperature (see scheme 4.3). Afterwards, copper octanoate was extracted using dichloromethane and dried in a rotary evaporator.[192] The hydrate ionic liquids tetramethylammonium acetate and tetramethylammonium malonate were synthesized by neutralizing an aqueous tetramethylammonium hydroxide solution with stoichiometric amounts of acetic acid and malonic acid, respectively. The substances were then dried in a rotary evaporator. Scheme 4.3 Synthesis of copper(ii) octanoate by neutralization of octanoic acid 4 H + [C 7 H 15 COO] + CuCO 3 Cu(OH ) 2 2 Cu[C 7 H 15 COO] 2 + H 2 CO H 2 O 76

86 4.4. Analytical methods 4. Experimental Nitrogen sorption experiments were conducted in a Quadrasorb SI from Quantachrome Instruments at 77 K. Nitrogen adsorption and desorption isotherms were recorded. Quenched solid density functional theory (QSDFT) modelling was applied to the obtained isotherms using a ratio of slit to cylindrical pores of 1:1 for activated carbon materials. The total pore volume was usually determined at a relative pressure of BET surface area was derived in the relative pressure region between Dynamic water vapor sorption experiments were carried out in Hydrosorb 1000 from Quantachrome Instruments. The PZC of the carbon materials was determined by ph measurement after hot filtration according to CEFIC [193]. 4 g of carbon material was suspended in 100 ml degassed water and boiled for 5 min. By filtration, the carbon material was separated at a temperature above 60 C. Then, the permeate solution was cooled down to RT and the ph was measured under constant stirring. Potentiometric titrations with sodium hydroxide were done with a Titrando 888 by Metrohm. For sample preparation, special carbon was comminuted in a Retsch mixer mill. 0.2 g carbon material was suspended in 100 ml 0.01 N sodium nitrate solution. With a 0.01 N hydrogen chloride solution, the suspension was adjusted to a ph value of 3. Then, a 0.1 N sodium hydroxide solution was automatically added and the ph continuously recorded. A pk a distribution was derived applying the numerical Saieus procedure [84]. Thermogravimetrical measurements were performed for more specific characterization. In a quick analysis of volatile components, a known amount of sample was placed in an oven, rapidly heated to 120 C and weighted. Afterwards, the sample was further heated to 800 C and its mass was recorded again. This approach allowed an easy partitioning of water content and amount of volatile components. All these previously described measurements were conducted by Blücher GmbH, Erkrath. Temperature programmed desorption with attached mass spectroscopy (TPD-MS) measurements were conducted in a Carlo Erba QTMD instrument at Zeta Partikeltechnik GmbH, Mainz. The helium flow amounted to 15 ml min -1 and the heating rate was 10 K min -1 from RT to 1000 C. Sample masses were gravimetrically determined before and after a TPD-MS measurement. The actual metal loading of the prepared catalysts was indirectly determined. The concentration of noble metal in the permeate solution after carbon impregnation was analyzed 77

87 4. Experimental by inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the Institute of Chemical Reaction Engineering, Erlangen. Pulse chemisorption experiments with carbon monoxide were carried out to analyze the metal surface of carbon supported noble metal catalysts. In particular, metal dispersion was determined. Around 0.2 g of catalyst sample was placed in a quartz glass tube of a Micromeritics AutoChem II The catalyst was preconditioned at its original metal transformation temperature (see section 4.1.2) in a continuous flow of 10 vol% hydrogen in argon for 60 min. After flushing with argon for 30 min, temperature was decreased to 40 C. At 40 C, defined amounts of carbon monoxide were added in pulse titration mode. The amount of non-adsorbing carbon monoxide was monitored. Pulse titration was continued until no carbon monoxide was adsorbed in succeeding pulse measurements. Metal dispersion and average cluster diameter were derived assuming a 1:1 adsorption stoichiometry (see section ). Measurements were conducted at the Institute of Chemical Reaction Engineering, Erlangen. Additionally, TEM images were taken of selected catalysts at the University of Erlangen- Nuremberg. Therefore, spherical catalyst material was comminuted, dispersed in acetone and applied to a TEM grid. An average metal cluster size was determined by image analysis. Metal distribution over the cross section of spherical carbon was determined by SEM-EDX. Spherical catalyst was embedded in proprietary epoxy resin (SpeciFix-20Kit) from Struers, Denmark. The samples were then polished and sputtered with gold. Sample preparation and analysis were carried out at the Institute of Glas and Ceramics (WW3), Erlangen. Metal leaching tests were conducted in order to determine the degree of catalyst reduction and to compare noble metal leaching of commercial catalysts. Concerning the degree of reduction, the assumption was that unreduced metal salts more readily dissolve in water than reduced metals. 0.5 g of catalyst was suspended in 10 g water and treated by ultrasonication at RT for 1-3 h. The catalyst was filtered off and metal concentration (and optionally sulfur concentration) in aqueous solution was analyzed by ICP-AES. Densities of ionic liquids and ionic liquid melts were determined by helium pycnometry measurements at 20 C using Pycnomatic ATC from Thermo Scientific. Around 5 g ionic liquid were weighted in a small crucible and placed in the pycnometry device. After flushing the sample with helium, an average ionic liquid density was determined after 20 measurements. 78

88 5. Results and discussion 5.1. Filtration and pressure drop of spherical carbon Spherical carbon in lab-scale filtration These sections take a look at the efficiency of filtration using spherical carbon materials. It is expected that spherical carbons are filtered off significantly faster than carbon powder. An influence of spherical carbon particle size on filtration time is also likely. The spherical carbons used in the filtration experiments had particle sizes of 500 and 200 μm. They were highly activated with total pore volumes of 1.18 and 1.67 cm 3 g -1, respectively. See section for additional material properties. The applied commercial 5 wt% palladium on carbon powder consisted of small particles between μm in size and larger particles up to 10 μm, as determined by analysis of light microscopy images. A timeline of the filtration experiments is shown in figure 5.1 and the results are summarized in table 5.1. Looking at the timeline, a clear trend of increasing filtration time with decreasing particle size is observed. The spherical carbon materials quickly settled forming a filter cake. Filtration of the largest spherical carbon material with 500 μm particle size was finished in less than two minutes. It took shortly more time for the 200 μm material and even longer in case of the powdered catalyst. While with the fine catalyst material, the colored water turned yellowish, the permeate color of the experiments with spherical carbon didn t change. The color change of the permeate stream in case of the powdered catalyst was due to a reduction reaction of the manganese(vii)-containing dye to e.g. manganese(iv) and manganese(ii) ions. Additionally, the powdered catalyst adhered to the walls of the filtration apparatus. The exact filtration times in table 5.1 show that the gap between the fastest filtration with 1.6 min and the slowest filtration with 7.5 min was large. The filtration factors quantify these differences. While filtration of the 200 μm spherical carbon material took 50% longer, the factor amounted to 4.7 and 3.1, respectively, for the powdered catalyst. Thus, it is apparent 79

89 5. Results and discussion Start 0.5 min 1 min 2 min 4 min 500 µm PBSAC 200 µm PBSAC Catalyst powder Figure 5.1.: Timeline of filtration experiments applying spherical carbon materials and a commercial powder catalyst (1 g material, 200 ml water, membrane with 1 μm particle retention, 0.5 bar excess pressure) Table 5.1.: Filtration experiments using spherical carbon materials and a commercial powder catalyst (1 g material, 200 ml water, membrane with 1 μm particle retention, 0.5 bar excess pressure) Material Filtration time Filtration factor to 500 μm spherical carbon Filtration factor to 200 μm spherical carbon 500 μm spherical carbon 200 μm spherical carbon 1 min 36 s 1-2 min 26 s Catalyst powder 7 min 30 s

90 5. Results and discussion Table 5.2.: Specification of constant parameters for large-scale filtration Parameter Symbol Value Pressure difference p 100,000 Pa Fluid viscosity (water) η Pa s Membrane resistance R m 1e11 m -1 Porosity ϵ 0.4 Filter factor k 4.8 Filter cake diameter d c 0.1 m Filter cake mass m c 0.1 kg Packed bed density ρ c 400 kg m -3 Filtrate volume V f iltrate 0.1 m 3 that systems with spherical adsorbents exhibit a significant filtration advantage compared to powdered activated carbon. Spherical carbon is quickly filtered off. The filtration advantage increases with increasing particle size. Reaction and product separation equipment was easily cleaned, because the spherical adsorbents weren t very adhesive. This advantageous behavior would also enable catalyst recovery and recycling. Primarily, these small-scale experiments apply to filtration processes in analytical chemistry. Generally, in analytical chemistry, the shortening of time consuming post-processing by application of spherical carbon based catalysts cannot be neglected Spherical carbon in large-scale filtration Large-scale filtration rates of spherical carbon materials were estimated a-priori with the Darcy equation and the Carman-Kozeney extension (see section 2.2.3). Here, it is assumed that the spherical carbons have formed a filter cake. Table 5.2 lists reasonable values chosen for calculations. In the calculations, the particle diameter of the spherical adsorbents d p was varied. The influences of particle size on the filtration rate J and filtration time t are shown in figure 5.2. The particle sizes of 500, 200 and 50 μm are highlighted in the diagrams, because these three particle sizes are used in this work. 81

91 J / l m -2 h Filtration rate function Highlighted performance of PBSAC materials d p / µm (a) 5. Results and discussion t / h Filtration time function Highlighted performance of PBSAC materials d p / µm (b) Figure 5.2.: Influence of spherical carbon particle size on filtration rate (a) and filtration time (b) It is obvious that the increase in filtration rate and the respective decrease in filtration time is most pronounced for small particle sizes up to μm. With further increasing particle size, a plateau is approached due to the dominating effect of the membrane resistance. In these calculations, the influence of particle size on filtration rate and filtration time is negligible for particle sizes larger than 100 μm. Compared to the filtration experiments in section 5.1.1, the filtration advantage of spherical carbon over carbon powder (with around 2 μm in particle size) is even more pronounced on a large, industrial scale. For a 10 μm powder, however, the large-scale filtration factor compared to 200 μm spherical carbon is about 4.1 and, thus, within the same dimension as the lab-scale filtration factor (see section 5.1.1). Catalysts based on spherical carbon materials can result in cost reduction of chemical productions on any scale due to a significantly enhanced catalyst/product separation. Reactor cleaning and refilling will also be much easier, as the spherical carbon materials are nonadhesive and non-dusting Spherical carbon in flow chemistry applications Another noteworthy aspect, already discussed for gas purification in section , is the low pressure resistance of spherical adsorbents. In continuously operated reactions, a reactant 82

92 Results and discussion Filtration rate function Highlighted performance of PBSAC materials J / l m -2 h d p / µm Figure 5.3.: Influence of spherical carbon particle size on fluid flux (R m = 0) fluid would more easily flow through a spherical catalyst bed, exhibiting a smaller pressure drop. In figure 5.3, the flow flux through a fixed bed of spherical carbon is estimated for different particle sizes using the filtration rate function and the parameters in table 5.2, but neglecting membrane resistance. Here, with increasing particle size, the fluid flux continuously increases. The factor f, which describes the increase of fluid flux with increasing particle size, simplifies to the expression f = d2 p,larдe. It solely is a function of the particle diameters of the materials in comparison. d 2 p,small For instance, an increase of particle size from 200 to 500 μm increases the fluid flux by a factor of 6.25 for the chosen parameters. So, in essence, an active catalyst based on spherical carbon can be very advantageous in flow chemistry applications of fine chemical and pharmaceutical productions due to a decreased flow resistance and pressure drop. 83

93 5. Results and discussion 5.2. Spherical carbon as novel catalyst support material Carbon surface functionalization A functionalized carbon surface is important for metal deposition by ion adsorption. In this section, the experimental window between mild carbon functionalization and possible structural disintegration is investigated. So, functionalization of spherical carbon with nitric acid and sulfuric acid was analyzed at different acid concentrations and temperatures. For the investigations in this section, 50 g of highly activated spherical carbon with an average particle diameter of 200 μm was oxidized in each experiment. Nitric acid concentrations of 15 and 36% and sulfuric acid concentrations of 25 and 50% were applied. Oxidation temperatures were set to RT and 90 C, respectively. To guarantee homogeneous oxidation, the volumes of oxidizing solutions were set to three times the materials pore volume. The suspensions with nitric acid were stirred by an external stirrer instead of a magnetic stirrer and later dried at 100 C. Here, less extreme drying conditions were chosen, because this would technically simplify a prospective scale-up of the oxidation process. Additionally, both nitric acid oxidations at RT were repeated a second time Influences on the amount of functional surface groups The total degree of surface functionalization was quantitatively determined by gravimetrical analysis before and after a thermal treatment. The resulting fractions of volatile components are compared to the manually performed gravimetrical measurements of TPD-MS experiments. Here, a well-defined temperature program was used, but water content wasn t distinguishable from other volatile components, as the samples were only weighted before and after TPD-MS measurements. The determined values were corrected by the separately determined water contents. Figure 5.4 shows the amounts of volatile components for differently oxidized spherical carbon. The table in appendix B.1 separately lists the water contents and the amounts of other volatile components. The water content of the nitric acid treated samples varied between wt% due to technically simplified drying conditions. This was acceptable and accounted for in the quantification of volatile surface functionalities. Nevertheless, sufficient sample flushing with water to remove residual nitric acid is important for precise surface analysis. The sulfuric acid functionalized materials were completely dry, with only one sample containing 0.4 wt% water. 84

94 m volatile components / wt% 5. Results and discussion 15% 12 HNO3 36% HNO3 25% H2SO4 50% H2SO4 Acid concentration Volatile components (TPD-MS) RT 90 C RT 90 C RT 90 C RT 90 C Oxidation temperature Figure 5.4.: Quantitative surface analyses of oxidized spherical carbon (amount of volatile components excluding water content) The fraction of volatile components was measured in order to directly determine the total amount of surface groups. Values varied between wt%. The amounts of volatile components fluctuated more strongly for the nitric acid treated samples than for the sulfuric acid treated materials. With increasing oxidation temperature, the amount of volatile components increased for nitric acid and sulfuric acid treatments. This trend is also supported by the TPD-MS gravimetrical results. The mass loss after TPD-MS was always larger than the values determined in the rapid test method for volatile components. This is due to a higher end temperature of 1000 C during TPD-MS (instead of 800 C), desorbing also the most stable surface groups. With increasing nitric acid concentration, there is a trend of decreasing surface functionalization. Though, influence of nitric acid concentration on the amount of volatile components seems to be small, except for samples functionalized with nitric acid at RT. Here, the total amount of surface groups significantly decreased with increasing acid concentration. This behavior is apparent in the reproduction samples, as well. So, oxidation with 36% nitric acid at RT in fact resulted in a significantly lower degree of surface functionalization. Possibly, this effect is caused by the specific composition of the spherical carbon itself. The nitric acid samples oxidized at 90 C exhibited at both acid concentrations a similarly high degree of surface functionalization. Compared to room temperature experiments, the carbon surface was more strongly oxidized. 85

95 5. Results and discussion In case of the sulfuric acid treated samples, the degree of oxidation increased with increasing temperature, too. The influence of acid concentration was small. In contrast to most nitric acid treated samples, sulfuric acid functionalization results in a lower absolute amount of weakly adsorbing surface groups. Due to the larger surface-specific acid molarity during oxidation treatment (see also the table in appendix B.1), more strongly adsorbing surface groups were introduced. This indicates a more intense reaction of carbon with sulfuric acid molecules compared to nitric acid Influences on the chemical composition of functional surface groups In the previous section, it was already indicated that different surface groups with different adsorption strength are present. The exact chemical composition can be derived from TPD- MS signals. Mass spectroscopy signals of released gases during temperature programmed desorption are plotted in figure 5.5 for nitric acid treatments and in figure 5.6 for sulfuric acid treatments. In all diagrams, signal intensity of carbon monoxide has been corrected to fit the baseline. All nitric acid treated samples exhibit a characteristic trend of the carbon monoxide signal. In the temperature regime between about C, two smaller overlapping peaks occur, followed by two larger overlapping peaks between C. For the material oxidized with 35% nitric acid at RT, these peaks are much less pronounced. In case of the carbon dioxide signal, the peak areas in the same temperature regimes are opposed to the carbon monoxide signal. Here, two large peaks overlap between C, followed by a decrease of signal intensity until 1000 C. Again, for the material oxidized with 36% nitric acid at RT, these peaks are much less pronounced. For nitrogen oxide, all nitric acid treated samples exhibit an increasing peak that abruptly decreases at around 500 C. The absolute signal intensity is much lower than the carbon oxide signal intensities. Nitrogen dioxide and sulfur dioxide show a similar trend as nitrogen oxide, but with even lower signal intensities. The hydrogen sulfide signal continually decreases from RT to 1000 C at very low absolute intensities. At around 1000 C, no further desorption of surface groups occurred. Sulfuric acid treated samples also featured characteristic trends. The carbon monoxide signal exhibits a small peak between C, followed by a constant value and a sharp peak around 900 C. The constant plateau between C is of very low intensity for the sample oxidized with 50% sulfuric acid at 90 C. The signal of carbon dioxide generally has a smaller peak between C and two larger overlapping peaks between C. With a much 86

96 5. Results and discussion RT 90 C intensity / a.u. intensity / a.u. 1.0x x x x x x x x x x CO2 NO2 H2S SO2 15% HNO 3 36% HNO CO2 NO2 H2S SO2 temperature / C (a) temperature / C (c) intensity / a.u. intensity / a.u. 1.0x x x x x x x x x x CO2 NO2 H2S SO CO2 NO2 H2S SO2 temperature / C (b) temperature / C (d) Figure 5.5.: TPD-MS spectra of nitric acid treated spherical carbon with 15% HNO RT (a), 36% HNO RT (b), 15% HNO 90 C (c), and 36% HNO 90 C (d) 87

97 5. Results and discussion RT 90 C intensity / a.u. intensity / a.u. 6.0x x x x x x CO2 NO2 H2S 25% H 2 SO 4 50% H 2 SO CO2 NO2 H2S SO2 temperature / C (a) temperature / C (c) intensity / a.u. intensity / a.u. 6.0x x x x x x CO2 NO2 H2S SO CO2 NO2 H2S SO2 temperature / C (b) temperature / C (d) Figure 5.6.: TPD-MS spectra of sulfuric acid treated spherical carbon with 25% H 2 SO RT (a), 50% H 2 SO RT (b), 25% H 2 SO 90 C (c), and 50% H 2 SO 90 C (d) 88

98 5. Results and discussion lower absolute intensity, nitrogen oxide reaches a maximum between C. Additionally, nitrogen dioxide exhibits two more broad peaks between C. Similarly to nitrogen oxide, both hydrogen sulfide and sulfur dioxide have a single peak between C at low absolute intensities. In case of the hydrogen sulfide signal, the absolute intensity increases with increasing oxidation temperature and acid concentration. Like the nitric acid treated material, no further desorption occurred at around 1000 C. The TPD-MS studies show that most surface groups desorb as either carbon monoxide or carbon dioxide. This is strongly indicated by the large absolute signal intensities of these two gases compared to the much lower signal intensities of the remaining gases. This behavior is observed for nitric acid and sulfuric acid treatments. The less pronounced signals of the sample oxidized with 36% nitric acid at RT indicate a lower degree of surface functionalization, which is consistent with the quantitative analysis presented earlier. Also, significant signal intensities are observed in the temperature range between C, plausibly explaining the deviations of the two thermo-gravimetrical analysis methods in section Together with the desorption temperature, the detected gases can be directly linked to specific surface groups (see section ). So, the functionalized surface of spherical carbon primarily consists of oxide surface groups. Based on absolute signal intensities, the nitric acid treated samples contain many strongly adsorbing groups like carbonyls and quinones, but also a significant amount of phenols, ethers and weakly adsorbing carboxylic acid groups. Sulfuric acid functionalized samples primarily consist of carbonyls, quinones, and less strongly adsorbing carboxylic anhydrides. Figure 5.7 shows ratios of differently strong chemisorbing oxide surface groups for each oxidized material, which are also listed in the table in appendix B.2. This ratio of surface groups was extracted from figures 5.5 and 5.6 by integration of the carbon monoxide and carbon dioxide signal intensities in the distinctive temperature regimes C and C. All oxidized materials exhibit a larger number of strongly adsorbing surface groups releasing carbon monoxide. Again, the material treated with 36% nitric acid at RT shows deviating properties with an enlarged ratio of strongly adsorbing surface oxides desorbing as carbon monoxide. Except for this material, the other nitric acid functionalized spherical carbons contain an excess of weakly adsorbing groups releasing carbon dioxide. The four sulfuric acid oxidized carbons have more strongly than weakly adsorbing surface groups, which is in accordance with the quantitative investigations of the previous section. Carboxylic acids seem to be more prevalent on nitric acid functionalized carbons than on sulfuric acid treated samples. Otherwise, surface composition in relation to adsorption strength appears to be 89

99 Ratio of strongly to weakly adsorbing groups / - 5. Results and discussion 10 15% HNO3 36% HNO3 25% H2SO4 50% H2SO4 Acid concentration 8 Groups releasing CO RT 90 C RT 90 C RT 90 C RT 90 C Oxidation temperature Figure 5.7.: Ratio of weakly adsorbing surface oxides desorbing between ( C) and strongly adsorbing oxides desorbing between ( C) similar for almost all samples. The dominant prevalence of carboxylic acid groups in nitric acid treated samples correlates well with literature reports (see section ). Next to pure oxygen chemisorption, nitric acid treatment incorporated some nitrogen functional groups into the carbon surface, observable by nitrogen oxide desorption. A small amount of sulfur is also found at the carbon surface of the nitric acid oxidized samples. The sulfur originates from the production process of the spherical carbon itself (see section 2.1.2). It s important to address the fact that after oxidation treatment, only sulfate is present, which is not a catalyst poison. Unexpectedly, nitrogen surface groups were detected in the samples with sulfuric acid treatment. These traces of nitrogen heteroatoms can originate from the production of spherical carbon, as well. An anticipated effect is that the sulfuric acid treatment apparently introduces additional sulfur functional surface groups. The amount of sulfur groups increases with intensifying reaction conditions, i.e. increasing acid concentration and temperature Influences on spherical carbon acidity Another analytic approach towards a diversified understanding of surface characteristics is possible by potentiometric titration. As described in section , this method provides detailed information of the surface s acid-base character. The results of potentiometric titration are shown in figure 5.8 in the form of distribution functions derived from the titration 90

100 5. Results and discussion curves. Pure spherical carbon exhibits three peaks, a larger one at a pk a of 10 and two smaller peaks between pk a 6-9. After oxidation, the total number and area of peaks increase. For nitric acid treated samples, three additional peaks between pk a 3-6 appear, except for the material treated with 36% nitric acid at RT. The sample oxidized with 15% nitric acid at 90 C has its first peak at a lower pk a of 3 compared to the other samples. Focusing on the larger peak at pk a 10, the peak area increases with increasing oxidation temperature. At constant temperature, this peak area decreases with increasing acid concentration. In case of the sulfuric acid functionalized samples, the differences to pure spherical carbon are small. The large peak at pk a 10 has a larger absolute area than the pure material, though the size then only slightly increases with increasing oxidation intensity. Three peaks exist in total with pk a values between 5-8, an exception of one additional peak being the oxidation with 50% sulfuric acid at 90 C. Interestingly, the pure spherical adsorbents already contain an observable amount of acidic surface groups. These are especially weak acids like phenols, but also some stronger carboxylic groups and lactones with pk a values between 6-7. This surface modification occurred during carbon activation with steam and carbon dioxide (see section 2.1.1). Nitric acid treatment introduces additional carboxylic groups and lactones with varying acidity. The exception is the material oxidized with 36% nitric acid at RT. Here, no additional surface groups were formed. This low degree of functionalization is consistent with TPD- MS results. Very acidic carboxylic acid groups are present in the sample treated with 15% nitric acid at 90 C. This behavior doesn t correlate with the TPD-MS studies. An explanation for especially acidic surface groups is the interaction of carboxylic acid groups with neighboring oxide or nitro surface groups [71, 83, 85]. In this case, this oxidation treatment leads to a particular surface distribution of functional groups. The amount of less acidic surface groups (pk a 10) still exceeds the amount of carboxylic acids. With increasing temperature these groups increase in number, correlating well with TPD-MS studies and the quantitative increase in functional groups. Noteworthy is the trend of decreasing amount of surface groups at pk a 10 with increasing acid concentration. This trend is also observed in the gravimetrical analysis. It is an indication of too excessive oxidation, resulting in carbon removal and a decreased carbon surface area. Consequentially, this smaller surface area has a lower capacity for functional groups. In contrast, sulfuric acid oxidation is more intense, as already discussed in the previous sections. Only elevated temperatures and acid concentrations lead to the formation of a few 91

101 5. Results and discussion f(pk a ) / mmol g -1 f(pk a ) / mmol g -1 f(pk a ) / mmol g % % HNO3 2 15% RT HNO3 36% 90 C 1 HNO pure PBSAC % 50% H2SO4 25% RT H2SO4 50% 90 C H2SO pk a / - Figure 5.8.: Potentiometric titration of functionalized and non-functionalized spherical carbon 92

102 ph PZC / - 5. Results and discussion 10 None 15% HNO3 Acid 36% concentration HNO3 25% H2SO4 Pure PBSAC 8 HNO3 50% H2SO4 H2SO4 oxidized PBSAC None RT 90 C RT 90 C RT 90 C RT 90 C Oxidation temperature Figure 5.9.: Point of zero charge ph values of functionalized and non-functionalized spherical carbon more weakly adsorbing, acidic carboxylic acid groups. Nevertheless, already at mild oxidation conditions, a significant amount of strongly adsorbing, weakly acidic oxide groups is created at pk a 10, correlating well with TPD-MS results. This amount doesn t increase further with increasing acid concentration or temperature. As a short summary, the potentiometric titrations generally validate gravimetrical measurements and TPD-MS results. Certainly, the acidic character of the oxidized spherical carbon has been clarified in more detail. In addition to potentiometric titrations, it s interesting to look at the overall acid-base character of differently oxidized spherical carbon. This is possible by analyzing the point of zero charge of each material. The point of zero charge measurements were conducted according to section 4.4. The corresponding ph values are shown in figure 5.9. Numerical data is listed in appendix B.3. Prior to oxidation, the pure spherical carbon had an initial point of zero charge at ph 9.1. The point of zero charge decreases after oxidation. The nitric acid treated samples exhibited zero charge at ph values between , with an exception for the sample oxidized with 36% nitric acid at RT. It had its point of zero charge significantly higher at ph 6.4. The samples oxidized with sulfuric acid all had a similar point of zero charge between ph The pure spherical carbon exhibits a basic character in water. Surface functionalization results in acidic materials. Sulfuric acid treated samples are more acidic than nitric acid treated 93

103 v ads (STP) / cm 3 g Results and discussion 25% 50% H2SO4 25% H2SO4 RT 50% H2SO4 pure H2SO4 PBSAC@ 90 C 15% 36% HNO3 15% HNO3 RT 36% HNO3 pure HNO3 PBSAC@ 90 C p p -1 0 / - Figure 5.10.: Water vapor isotherms of differently oxidized spherical carbon materials. Even though, nitric acid treated samples mostly have a larger number of surface groups and according to potentiometric titration some very acidic groups, they also have a large amount of less acidic groups. In sum of all surface charges, which represents the point of zero charge, the sulfuric acid treated materials are still more acidic. This correlates well with literature reports, where sulfuric acid treated carbons were applied as acid catalysts (see section 2.2.5). The sample oxidized with 36% nitric acid at RT had a much higher point of zero charge, because its overall degree of functionalization was lower Influences on carbon surface wettability Because carbon surface oxidation increases water affinity of the material, water vapor sorption experiments were performed to look at the hydrophilic character of differently oxidized spherical carbons. The results of water adsorption with increasing relative humidity and desorption with decreasing humidity are plotted in figure The bottom diagram shows isotherms of nitric acid pretreated samples and pure spherical carbon. The pure spherical carbon slowly began to adsorb water at around 70% rh. At 100% rh, the pore system was saturated with around 1.1 cm 3 g -1 water. Desorption proceeded slowly, but rapidly increased at around 60% rh. Oxidizing spherical carbon with nitric acid 94

104 5. Results and discussion significantly shifts sorption isotherms to lower relative humidity and larger total water adsorption. With increasing oxidation temperature and decreasing acid concentration, this shift is more pronounced. The largest total water adsorption amounted to about 1.3 cm 3 g -1 for the sample oxidized with 15% nitric acid at 90 C. Already at around 30% rh, this material began to adsorb water vapor. Looking at the upper diagram, a similar shifting of isotherms from the pure carbon to sulfuric acid treated samples is observed. With increasing temperature and increasing acid concentration, the isotherms are slightly shifted further to lower humidity and larger total water adsorption. Here, this effect is much less intense than for nitric acid treated materials. The maximum water adsorption amounted to around 1.25 cm 3 g -1 for the sample oxidized with 50% sulfuric acid at 90 C. Besides this basic description, mathematical analysis of these isotherms can be carried out. Thus, information such as surface-specific oxygen number and distance between oxygen atoms can be derived.[72] The described shifting of isotherms to lower relative humidity and larger water adsorption demonstrates an increase in water affinity of the material. Compared to pure spherical carbon, all oxidized samples are more hydrophilic. The hydrophilic nature of the materials further increases with increasing degree of functionalization. The order of isotherm shifting correlates exactly with the trends observed earlier in the previous sections by thermal desorption experiments and potentiometric titration. For the nitric acid treated samples, the surface functionalization increases with increasing oxidation temperature and decreasing acid concentration due to an increasing quantity of surface functional groups. In case of sulfuric acid treatment, more functional groups are introduced by increasing temperature and acid concentration. Nevertheless, these effects on the hydrophilic character are small compared to nitric acid treatments due to much smaller deviations in the quantity of surface functional groups for different oxidation treatments Influences on carbon pore structure Finally, the influence of surface oxidation on pore geometry and pore integrity was also analyzed in classical nitrogen sorption experiments. Out of all process steps in catalyst preparation, the oxidation procedure can most likely negatively affect stability of the support material due to total carbon oxidation. Theoretically, the growth of metal clusters during impregnation and metal transformation can also damage the pore structure. Figure 5.11 shows the volume-specific pore size distributions of pure spherical carbon and the oxidized samples. Additionally, figure 5.12 presents the change in BET surface area and total 95

105 dv(w) / cm 3 nm -1 g Results and discussion % 50% 7 H2SO % H2SO4 RT % H2SO4 pure H2SO4 PBSAC@ 90 C % 36% HNO3 15% HNO3 RT % HNO3 pure HNO3 PBSAC@ 90 C pore width / nm Figure 5.11.: Volume-specific QSDFT pore size distribution of differently oxidized spherical carbon pore volume after different surface oxidation treatments. Appendix B.4 includes a table of pore geometry properties derived from nitrogen sorption isotherms, including the deviation of BET surface area and pore volume from values of pure spherical carbon. In the pore size distribution of pure spherical carbon, all pores appear to be smaller than 3 nm. A large volume-specific amount of pores exhibits pore sizes between 1-3 nm, but there are also many pores smaller than 1 nm. Oxidation with nitric acid has a visible effect on the total pore volume. In the pore size distribution, the individual peaks decrease in size. The distribution of pores itself doesn t change, except for some pores apparently being formed between nm for samples treated with 36% nitric acid at RT and 15% nitric acid at 90 C. The pore size distributions of carbons treated with sulfuric acid at different temperatures and acid concentrations are very similar. Compared to pure spherical carbon, pore size distribution and pore volume aren t affected by sulfuric acid oxidation. Quantitatively looking at the pore geometry, nitric acid treatment has a significant impact on BET surface area and pore volume. With a 30% decrease at rather mild oxidation conditions, this effect was most pronounced for the sample treated with 15% nitric acid at RT. Not too surprisingly, as indicated by investigations of the previous sections, the sample oxidized with 96

106 5. Results and discussion Difference to pure PBSAC / % RT 90 C RT Oxidation 90 C temperature RT 90 C RT 90 C BET 15% HNO3 36% HNO3 25% H2SO4 50% H2SO4 V tot Acid concentration Figure 5.12.: Change in BET surface area and total pore volume of differently oxidized spherical carbon from values of pure spherical carbon 36% nitric acid at RT only shows very small deviations in pore geometry. The changes in surface area and pore volume are generally much smaller in case of sulfuric acid treatment. Here, the effect of decreasing deviations with increasing acid concentration is also observed. Oxidation of spherical carbon noticeably decreases internal surface area and pore volume. By nitric acid treatment, these quantities decrease by up to 31%. In case of sulfuric acid treatment, the deviation only amount to 12% and less. The decrease in specific surface area and pore volume is due to mass increase by heteroatom inclusion [194]. Consequently, using sulfuric acid, the carbon structure is oxidized with fewer effects on the pore structure Important differences between nitric acid and sulfuric acid oxidation In the previous sections, a variety of influences of different oxidation treatments on the surface properties of spherical carbon have been pointed out. The most important differences between the two applied acids are the effects on quantity and type of surface oxides, as well as on wettability, acidity, and pore geometry of spherical carbon. These differences are discussed in the following. The quantity of surface functional groups was similar for all sulfuric acid treated samples. For nitric acid treatments, on the other hand, the quantity of functional groups increased significantly more strongly with increasing oxidation temperature. Due to the strongly increas- 97

107 5. Results and discussion ing quantity of surface functional groups, the hydrophilic nature of the materials strongly increased for nitric acid treated samples with increasing temperature and decreasing acid concentration. This effect of increasing carbon wettability was much smaller for sulfuric acid treated samples with only small deviations in the degree of surface functionalization. The type of surface groups present in spherical carbon also vary for nitric acid and sulfuric acid treatments. While all oxidized materials consisted of many strongly adsorbing surface groups such as carbonyls and quinones, nitric acid treated samples also had a significant amount of phenols, ethers and weakly adsorbing carboxylic acid groups. Instead sulfuric acid functionalized samples contained additional, less strongly adsorbing carboxylic anhydrides. In total, sulfuric acid treated samples generally exhibit a larger fraction of strongly adsorbing oxides with thermal decomposition temperatures between C, due to a more intense reaction with the carbon surface. While the overall acidity of all oxidized spherical carbons was similar, most nitric acid treated samples contained some very acidic functional groups, likely due to the interaction of carboxylic acid groups with neighboring oxide or nitro surface groups. Concerning the pore geometry of spherical carbon, generally, pore volume and BET surface area strongly decrease after nitric acid treatment due to heteroatom inclusion, while the pore structure is much less affected by sulfuric acid treatments. Concluding the investigations on surface functionalization using nitric acid and sulfuric acid, the observed trends regarding the influences of oxidation temperature and acid concentrations were similar, but generally much less pronounced for sulfuric acid treatments. In the experimental window of oxidation temperature and acid concentration applied in this work, the results were most consistent for sulfuric acid treatments. 98

108 Active metal deposition 5. Results and discussion Palladium: Influences on metal loading and dispersion Influence of particle size In this section, the prepared 5 wt% palladium catalysts based on spherical carbon are characterized in order to study the influence of differently sized catalysts on metal loading and dispersion. Furthermore, material properties of the prepared catalysts are compared to a reference platinum on carbon powder catalyst. Therefore, highly activated spherical carbon with a particle diameter around 500 μm and a total pore volume of 1.18 cm 3 g -1 was functionalized with a 10% nitric acid solution at RT using standard oxidation parameters. Applying the ion adsorption method and subsequent metal transformation in hydrogen atmosphere, palladium metal was deposited. In addition, a 1 kg scale-up of the spherical catalyst was conducted by Blücher GmbH applying the very same conditions with proportionally increased masses. So, a sufficient amount of material was available for analysis. Moreover, experience of catalyst preparations on a larger scale was obtained. This material was also crushed and classified into multiple particle size fractions for further analysis in section In a parallel approach, the pure carbon material was intensively crushed in a ball mill. Then, it was oxidized with nitric acid and loaded with 5 wt% palladium using standard parameters. This crushed powder catalyst was prepared to better understand the influence of mass transfer during catalyst preparation. The metal loading was indirectly determined by analyzing the impregnation solution after solid separation using ICP-AES elemental analysis. The metal dispersion was determined by carbon monoxide pulse chemisorption analysis. TEM images of intensively crushed catalyst material were taken to gather information about the palladium cluster size. Also, the palladium distribution over the particle cross section was screened by SEM-EDX. Upon addition of carbon to the amber precursor solution, the solution discolored. According to ICP-AES results, the spherical carbon was loaded with 4.98 wt% palladium. As the quantity of non-adsorbed palladium salt was negligibly small, it can be stated that palladium chloride was completely deposited onto oxidized spherical carbon. For the 1 kg catalyst scale-up, almost no solid residues remained after the washing solution was evaporated, thus, demonstrating full metal salt deposition. Metal loading of the crushed catalyst wasn t explicitly determined. Table 5.3 lists metal dispersions of the scale-up catalyst and commercial 5 wt% palladium on carbon powder. 99

109 5. Results and discussion Table 5.3.: Metal dispersions of 5 wt% palladium catalysts based on spherical carbon and carbon powder Catalyst D Pd / % Derived metal cluster size / nm 5 wt% 500 μm PBSAC n/a n/a 5 wt% crushed PBSAC n/a n/a 5 wt% 500 μm PBSAC (1 kg scale-up) wt% PAC (commercial) With 52%, palladium dispersion on spherical carbon was very large. Compared to 30%, palladium was more finely dispersed on spherical carbon than on commercial carbon powder. The average metal cluster size derived from metal dispersion was in the range of 2.2 nm. A TEM image of crushed catalyst material, as shown in figure 5.13, gave a more accurate overview of metal cluster sizes. In figure 5.14, the results of a SEM-EDX scan over the cross section of a catalyst sphere is shown. In TEM images, palladium clusters were visible only with low contrast to the carbon support. Lattice planes of palladium weren t distinguishable. The clusters had an average particle diameter of 2.7±0.5 nm. With only 16 counts, sampling quantity was small, though. Compared to the carbon monoxide chemisorption result, the cluster size determined from TEM images was about 20% larger. This deviation is either due to a partially deviating carbon monoxide adsorption stoichiometry or due to uncertainties in TEM image analysis. Generally, palladium clusters were very small and homogeneously distributed across the sample, indicating successful catalyst preparation. In the SEM image, the cross section of the spherical catalysts showed a concentrical structure. The catalyst shell with a thickness of about 30 μm appeared much brighter than the core material. Also, the spherical carbons weren t perfectly spherical. The palladium concentration, determined by EDX at several positions across the radius, varied between 0.2 and 0.6%. With the exception of an elevated concentration in the outer shell, palladium was homogeneously distributed across the catalyst s diameter. The outer concentrical band was likely caused by epoxy resin partially penetrating the pore system. The elevated palladium content in this outer shell can result from a differing focus of the EDX beam. Because the resin penetrated shell had an increased hardness, a hollow can have been polished in the softer core. Never- 100

110 5. Results and discussion Pd-EDX content / % Figure 5.13.: TEM image of a crushed 5 wt% palladium on 500 μm spherical carbon catalyst with palladium clusters highlighted position 14 / Figure 5.14.: SEM image of a cross section of a 5 wt% palladium on 500 μm spherical carbon catalyst including the results of an EDX scan 101

111 5. Results and discussion theless, these findings further demonstrated that the internal surface area of spherical carbon likely was completely utilized in palladium deposition. A potential systematic error due to palladium smearing over the cross section during polishing cannot be ruled out, though. These results most importantly show that palladium was successfully deposited on spherical carbon with good dispersion. With the 1 kg catalyst preparation, the methods of carbon oxidation, subsequent electrostatic palladate adsorption and metal transformation were achieved on a significantly larger scale. Metal dispersion was even larger than palladium dispersion of the commercial powder catalyst. Influence of pore structure Spherical carbons with different pore structures were loaded with palladium and characterized to study the influence of pore structure on metal loading and dispersion. Almost all spherical carbon materials presented in section were tested as catalyst support. Thus, pore structure variations for three different particle sizes were investigated. The materials were oxidized with nitric acid and loaded with 5 wt% palladium according to standard procedures (see section and 4.1.2). The prepared catalysts were characterized with carbon monoxide pulse chemisorption for metal dispersion. The results are listed in table 5.4, together with indirectly determined metal loadings. In all cases, palladium chloride fully adsorbed to spherical carbon resulting in complete metal deposition. The spherical catalysts had metal dispersions in the range between 28-38%. A slight trend of increasing metal dispersion with increasing total pore volume was apparent, due to an increased BET surface area. Thus, more space was available for surface oxidation and metal deposition. The mesoporous 500 μm exhibited a slightly larger metal dispersion than the least activated 500 μm material, despite its lower BET surface area, though, possibly due to a lower degree of micropore filling with noble metal. Consequently, pore structure has a low influence on metal loading and metal dispersion. The effects of pore structure and metal dispersion on catalytic performance are addressed separately in section

112 5. Results and discussion Table 5.4.: Palladium metal loadings and dispersions of spherical catalysts with different particle size and pore volume Particle size / μm V tot / cm 3 g -1 Obtained Pd loading / wt% D Pd / % n/a 1.19* * spherical carbon with enhanced mesoporous pore structure 103

113 5. Results and discussion Table 5.5.: Palladium metal loadings and dispersions of differently oxidized spherical carbon catalysts Acid type Acid concentration / wt% Oxidation temperature / C Obtained Pd loading / wt% D Pd / % Nitric acid Sulfuric acid RT RT RT RT Influence of surface functionalization Next to the mild standard oxidation of spherical carbon, carbon functionalization was carried out with different acids, acid concentrations and oxidation temperatures. In section 5.2.1, these materials were analyzed in detail. After palladium deposition, the resulting materials were characterized further in order to investigate the influence of surface functionalization on metal loading and dispersion. The results are discussed in this section. Each 5 g sample of functionalized material was loaded with 5 wt% palladium according to standard procedures (see section 4.1.2). Metal dispersions were determined by pulse chemisorption experiments using carbon monoxide. The results are listed in table 5.5, together with indirectly determined metal loadings. Considering measuring uncertainties, palladium chloride was always fully deposited onto the differently functionalized carbon surfaces. Metal dispersions of the differently oxidized catalysts varied strongly between 23 and 63%. For nitric acid pretreated materials, metal dispersion increased with increasing acid concentration and oxidation temperature. Three samples oxidized with sulfuric acid showed similarly large palladium dispersions of 57-63%. The catalyst pretreated with 25% sulfuric acid at 90 C had a lower metal dispersion of 31%, though. The trend of increasing metal dispersion with intensifying nitric acid oxidation conditions didn t directly correlate with any material properties determined in section That is, 104

114 5. Results and discussion Table 5.6.: Obtained metal loadings of spherical carbon catalysts loaded with different amounts of palladium Expected palladium loading / wt% Obtained palladium loading / wt% neither the total amount of surface groups, the ratio of strongly to weakly adsorbing surface groups, the PZC values nor the pore structure alone did determine dispersion of palladium clusters. As a consequence, an interaction of antagonizing mechanisms is hypothesized. This can be the interaction of ion adsorption to charged surface oxides and reactive adsorption to unsaturated C-π surface sites. In case of poor surface oxidation, reactive C-π sites remain in large amounts on the carbon surface. Each C-π site can instantly reduce multiple palladate ions, forming large palladium clusters. Those palladate ions are then no longer available for ion adsorption. With increasing degree of surface functionalization, metal dispersion increases due to a lower number of C-π sites. Electrostatic adsorption of palladium salt to protonated surface oxides now is the primary mechanism of metal deposition. In order to clarify this hypothesis, further catalyst characterization, e.g. in the form of catalytic experiments, are necessary. See also section for concluding remarks on the role of C-π sites. Based on the same hypothesis, sulfuric acid apparently functionalizes spherical carbon well at mild conditions, thus leading to large metal dispersions through homogeneous ion adsorption. The anomaly of the low metal dispersion for the material oxidized with 25% sulfuric acid at 90 C is difficult to explain, though. Influence of metal loading In these experimental series, palladium metal loading of spherical carbon was varied between 2-10 wt%. Therefore, 200 μm highly activated spherical carbon was oxidized and loaded with the respective amount of palladium, applying standard procedures. Table 5.6 lists the applied metal loadings for each catalyst, as determined by indirect ICP-AES analysis. Metal dispersions have not been determined. Between 95-99% of palladium chloride did adsorb to spherical carbon during the impregna- 105

115 5. Results and discussion tion process. It can be stated that palladium can be almost completely deposited on spherical carbon up to a metal loading of 10 wt%. 106

116 5. Results and discussion Ruthenium: Influences on metal loading and dispersion Another noble metal that catalyzes hydrogenation reactions is ruthenium (see section 2.2.1). In a similar approach to the previous section, the deposition of ruthenium on spherical carbon was studied. Specifically, ruthenium loading was varied, metal dispersions were determined and compared to a reference ruthenium on carbon material. As catalyst support, highly activated 200 μm spherical carbon was chosen and oxidized with 50% sulfuric acid solution at 90 C for 2.5 h. Impregnation with ruthenium chloride hydrate and metal salt transformation were carried out according to standard procedures (see section 4.1.2). Three catalysts were prepared with desired metal loadings of 0.5, 5, and 10 wt%. The actual ruthenium loadings and ruthenium leaching were analyzed by ICP-AES measurements. Metal dispersion was determined by carbon monoxide pulse titration. In the beginning of the impregnation process, monitored ph values showed that aqueous ruthenium chloride solutions were acidic. For example, the precursor solution for the 5 wt% ruthenium catalyst had a ph value of Thus, a significant ph difference to the spherical carbon s PZC of 3.1 (see section 5.2.1) was given to allow for effective electrostatic adsorption. Nevertheless, after addition of carbon material and stirring for 24 h, the impregnation solutions didn t discolor, indicating incomplete adsorption of ruthenium ions. The quantitatively determined ruthenium depositions are plotted in figure 5.15 for different desired metal loadings. It should be noted that the impregnation time amounted to 65 h in case of the 5 wt% ruthenium catalyst. All three ruthenium catalysts with desired metal loadings between 0.5 and 10 wt% showed incomplete ruthenium deposition. Only between 55 and 63% of ruthenium were deposited. Metal deposition was already limited at a low desired metal loading of 0.5 wt%. Nevertheless, with increasing desired metal loadings up to 10 wt%, larger absolute amounts of ruthenium up to 5.54 wt% were still deposited. Consequentially, concerning the 0.5 and 5% ruthenium catalyst systems, the total number of carbon surface groups was sufficient for complete adsorption of ruthenium ions. Still, ruthenium deposition was incomplete. Also, a significant increase in impregnation time didn t improve metal deposition. These observations strongly indicated equilibrium of differently charged ruthenium species affecting ion adsorption. It is known in literature that aqueous solutions of ruthenium chloride consist of cationic and anionic ruthenate, as well as neutral ruthenium complexes [195, 196]. At a ph of 1, about 50% of ruthenium species were positively charged, more than one third was neutral and a small remaining fraction was negatively charged [195]. Apparently, negatively and positively charged ruthenium ions adsorbed to carbon surface groups, while neutral ruthenium 107

117 obtained Ru loading / wt% 5. Results and discussion 10 8 Ru Linear Ideal deposition Ru fitdepositon expected Ru loading / wt% Figure 5.15.: Comparison of desired and actual ruthenium loadings species weren t adsorbed. Adsorbed ruthenium ions still contributed to equilibrium. Otherwise, additional ruthenium ions would have been formed and ruthenium deposition would have eventually been completed. This equilibrium is likely responsible that, in contrast to the palladium system, ruthenium wasn t completely adsorbed by spherical carbon. However, ruthenium loadings of at least 5.54 wt% were accomplished by using the ion adsorption method. After metal salt transformation in hydrogen atmosphere, the prepared catalysts didn t leach ruthenium. It was assumed that the reduction parameters were sufficient to completely reduce the catalyst material. Table 5.7 lists corresponding metal dispersions and derived cluster sizes of the differently loaded ruthenium catalysts and a commercial 5 wt% ruthenium on carbon powder. Metal dispersions varied strongly between 19 and 59%. The catalyst with the lowest ruthenium loading exhibited the largest dispersion and the smallest derived cluster size. With 50% metal dispersion, the 5.54 wt% ruthenium catalyst also consisted of small ruthenium clusters. The 3.15 wt% ruthenium catalyst had a significantly smaller metal dispersion and an average cluster size of around 6.9 nm. The commercial powder catalyst with an actual metal loading of 5.26 wt% exhibited a metal dispersion of 34%. The large deviation in metal dispersion of the 3.15 wt% ruthenium catalyst can be due to the extended impregnation time. With increasing impregnation time, agglomerates of ruthenium ions can have allocated. The 0.31 wt% ruthenium catalyst exhibited a larger dispersion than the 5.54 wt% catalyst, because smaller 108

118 5. Results and discussion Table 5.7.: Metal dispersions and derived cluster sizes of ruthenium on spherical carbon, with different metal loadings and impregnation times, and commercial ruthenium on carbon powder Catalyst support material Ruthenium loading / wt% Impregnation time / h D Ru / % Derived Ru cluster size / nm PBSAC PAC amounts of ruthenium can be more homogeneously distributed across the carbon surface due to a lower metal surface concentration. In essence, ruthenium was deposited on spherical carbon with generally good metal dispersion. With the presented impregnation method, only about two-third of the precursor salt is deposited Platinum: Influences on metal loading and dispersion Investigations on metal deposition of palladium and ruthenium on spherical carbon were presented in detail in the previous sections. In case of spherical carbon loaded with platinum, the focus was to develop a highly active catalyst applying this previously acquired knowledge on catalyst preparation. Parameters, that affect catalytic activity most, were adjusted stepwise. Prepared materials were directly tested in catalytic experiments. Obtained results were interpreted to further improve catalytic activity. Influence of ion adsorption modification This experiment takes a closer look at the role of hydrogen chloride addition in the impregnation of platinum catalysts by ion adsorption. The additional chloride ions compete with metal anions for adsorption sites potentially affecting metal loading and dispersion. The first PBSAC-based platinum catalysts were produced in analogy to the established preparation method of palladium catalysts. A metal loading of 5 wt% was targeted. Highly activated spherical carbon was treated with nitric acid at standard oxidation parameters and impregnated with an aqueous solution of hexachloroplatinic acid and hydrochloric acid. Devi- 109

119 5. Results and discussion Table 5.8.: Platinum metal loadings of differently impregnated spherical catalysts Addition of hydrogen chloride to impregnation solution ph of impregnation solution / - Yes No Obtained Pt loading / wt% ating from standard procedures, platinum salt transformation in hydrogen atmosphere took place at 200 C. Additionally, an impregnation was conducted without addition of hydrochloric acid. In both cases, with and without hydrochloric acid, hexachloroplatinic acid readily dissolved into yellowish aqueous solutions. Similar to the ruthenium chloride system and in contrast to palladium chloride, hydrogen chloride addition wasn t necessary for dissociation of hexachloroplatinic acid into protons and chloroplatinate anions. After addition of spherical carbon both yellowish precursor solutions discolored. The obtained platinum loadings were indirectly determined by ICP-AES analysis (see table 5.8). Measurements of metal dispersions have not been conducted. Platinum was fully deposited onto spherical carbon after impregnation without hydrogen chloride addition. Likely due to chloride anion competitors at the adsorption sites, platinum loading was slightly lower after impregnation with hydrogen chloride. Still, metal deposition was almost complete. As expected, the ph value of the impregnation solution decreased with hydrogen chloride addition. This larger ph difference to the material s PZC can affect metal dispersion and distribution, because of increased strong metal support interactions. Influence of pore structure Catalysts with 0.5 wt% platinum loading were prepared on the basis of two spherical carbon materials with different pore structure. The idea was that less activated spherical carbon already provides a sufficiently large surface area for good dispersion of 0.5 wt% platinum. Spherical carbon materials with pore volumes of 1.67 and 0.95 cm 3 g -1 were oxidized and impregnated with platinum salt according to standard procedures. Salt transformation in hydrogen atmosphere took place at 200 C. Platinum was completely deposited onto both materials, as no platinum was detected by indirect ICP-AES measurements. Metal dispersions are listed in table

120 5. Results and discussion Table 5.9.: Platinum metal dispersions of two spherical carbon catalysts with different total pore volume V tot / cm 3 g -1 D Pt / % With values between 5 and 10%, platinum wasn t dispersed well, at all. The catalyst with a larger pore volume exhibited a larger metal dispersion. This could have originated from the larger surface area of the highly activated carbon material, but is difficult to explain without further knowledge about the differences in surface composition. Catalytic performance is expected to be low, in both cases. Most likely, the carbon surfaces weren t sufficiently functionalized to allow for homogeneous adsorption of platinate ions. This aspect is addressed in the following section. Influence of surface functionalization The cause of low metal dispersion of the catalysts prepared in the previous subsection likely was due to insufficient surface functionalization. If, for instance, C-π sites remain at the carbon surface, each C-π-site can possibly adsorb multiple metal ions resulting in large metal clusters (see section for concluding remarks on the role of C-π sites). In order to produce good 0.5 wt% platinum catalysts, the PBSAC surface had to be completely oxidized. Sulfuric acid is a potentially well suitable oxidation agent. It was demonstrated in section that sulfuric acid intensively oxidizes spherical carbon with large degrees of surface functionalization. Also, oxidation of spherical carbon with a 50% sulfuric acid at 90 C resulted in a very high palladium metal dispersion of 63% (see table 5.5). Thus, a different 0.5 wt% platinum catalyst was prepared with sulfuric acid functionalization. The highly activated PBSAC material was oxidized with a 50% aqueous solution of sulfuric acid at 90 C for 2.5 h. The parameters for impregnation and reduction remained unchanged. As yet another influential parameter had to be adjusted to obtain a well performing catalyst, metal loadings and dispersions of this material weren t determined, but complete platinum deposition was assumed. As indirect statement of metal dispersion, catalytic activity is discussed in section

121 5. Results and discussion Table 5.10.: Metal loadings and dispersions of 0.5, 5, and 10 wt% platinum on spherical carbon and 5 wt% platinum on pulverized activated carbon Catalyst support material Expected Pt metal loading / wt% Obtained Pt metal loading / wt% D Pt / % PBSAC PAC 5 n/a 28 Influence of metal transformation temperature To ensure complete reduction of the adsorbed platinum salt, reduction parameters were adequately adjusted. The biggest impact was expected by increasing temperature. So, some impregnated PBSAC material was reduced at 300 C, instead of the previously applied 200 C. Metal dispersion of this 0.5 wt% platinum catalyst increased to 50%. Influence of metal loading In the following, the platinum metal loading was varied between 0.5 and 10 wt% to study the influence of obtained metal loading and dispersion. Also, these material properties are compared to a reference platinum on carbon catalyst. Therefore, 200 μm highly activated spherical carbon was oxidized with 50% sulfuric acid solution at 90 C for 2.5 h. For the 0.5 wt% platinum catalyst, metal impregnation was conducted with hexachloroplatinic acid and hydrochloric acid according to standard procedures. The 5 and the 10 wt% spherical catalysts were prepared without addition of hydrogen chloride. Metal transformation was carried out in hydrogen atmosphere at 300 C. For details, see sections and Obtained metal loadings were indirectly determined by ICP-AES analysis. The spherical catalysts and commercial 5 wt% platinum on carbon powder were also analyzed by carbon monoxide chemisorption. Table 5.10 lists metal loadings and dispersions. In case of the 0.5 and 5 wt% metal loadings, chloroplatinate completely adsorbed to the oxidized spherical carbon. A platinum loading of 10 wt% wasn t achieved, though. Only 84% of the desired platinum metal was deposited. Platinum metal dispersions of the 0.5 and 5 wt% spherical catalysts were similarly large with 50-51% and significantly decreased to 15% for the 8.41 wt% platinum loading. The powder catalyst exhibited a metal dispersion of 28%. 112

122 5. Results and discussion The applied, sulfuric acid functionalized spherical carbon seems to have a maximum platinum adsorption of 8.41 wt%. For larger platinum loadings, the surface likely needs to be further optimized, increasing the number of available adsorption sites. A decrease of metal dispersion at increased metal loading can be explained by the larger chloroplatinate ion surface densities promoting sintering effects during the rapid temperature increase in the applied metal transformation [96]. In essence, metal dispersions of the 0.5 and 5 wt% spherical platinum catalysts were very high and significantly higher than the dispersion of the reference powder catalyst. 113

123 X / % 5. Results and discussion % 5% Pd 500 µm PBSAC 00 fits (1 kg according Pd PACto model mod / s kg Pd m-3 Figure 5.16.: Conversion of cinnamic acid using 5 wt% palladium catalysts based on spherical carbon and carbon powder, respectively; fits according to model applying equation 2.9 (T = 40 C, p H2 = 30 bar, c 0, cinnamic acid = 236 mol m -3, m catalyst = 0.5 g) Table 5.11.: Effective reaction rate constants and metal dispersions of 5 wt% palladium catalysts based on spherical carbon and carbon powder Catalyst k eff / m 3 kg -1 s -1 D Pd / % 5 wt% 500 μm PBSAC n/a 5 wt% 500 μm PBSAC (1 kg scale-up) wt% PAC (commercial) Catalytic performance Palladium catalysts: Influences on catalytic activity Influence of particle size Catalytic performance of the prepared spherical palladium catalysts was analyzed in cinnamic acid hydrogenations (see sections and 4.1.3) and compared to commercially available palladium on activated carbon powder. Figure 5.16 shows the conversion of cinnamic acid over time of two prepared 500 μm spherical catalysts and the reference catalyst. Table 5.11 lists the derived reaction rate constants and previously determined metal dispersions. Concerning the spherical catalysts, the rate of reaction was larger for the laboratory-scale 114

124 X / % 5. Results and discussion % 500 µm 00 fits according crushed PACto model PBSAC mod / s kg Pd m-3 Figure 5.17.: Conversion of cinnamic acid using a 5 wt% spherical carbon palladium catalyst and a 5 wt% palladium catalyst prepared on the basis of crushed spherical carbon, and commercial 5 wt% palladium on carbon powder; fits according to model applying equation 2.9 (T = 40 C, p H2 = 30 bar, c 0, cinnamic acid = 236 mol m -3, m catalyst = 0.5 g) catalyst than the 1 kg scale-up material. The powder catalyst resulted in a very fast conversion of cinnamic acid. The difference in effective reaction rate constant between the two spherical catalysts was a factor of 1.4. The powder catalyst was 8.7 times more active than the PBSAC-based catalyst. The differences in catalytic activity of the two prepared spherical catalysts can be explained by different masses applied in the preparation procedure. The lower catalytic activity of spherical carbon despite the larger metal dispersion compared to the powder catalyst is an indication of mass transport limitation inside the 500 μm spherical carbon. To characterize the influence of particle size during catalyst preparation, cinnamic acid conversion of the powder catalyst based on crushed 500 μm spherical carbon was compared to the result of the 500 μm spherical palladium catalyst, as well as the commercial powder catalyst (see figure 5.17). The palladium catalyst based on crushed spherical carbon fully converted cinnamic acid in a very short time and, thus, was significantly more active than the 500 μm spherical carbon catalyst. Regarding the effective reaction rate constants, catalytic activity increased by a factor of 3.2. The powder catalyst was still 2.7 times more active than the crushed PBSAC- 115

125 X / % 5. Results and discussion = µm 00 fits 63 according > x < > µm to model mod / s kg Pd m-3 Figure 5.18.: Palladium on spherical carbon (5 wt%, 1 kg scale-up, wet) with varying particle size fractions tested in cinnamic acid hydrogenations; fits according to model applying equation 2.9 (T = 40 C, p H2 = 30 bar, c 0, cinnamic acid = 236 mol m -3, m catalyst = 0.5 g) based catalyst. This demonstrates that spherical carbon itself is a suitable catalyst support material that can compete with commercial powder catalysts. Optimization of particle size and catalyst preparation procedure can possibly further improve the performance of PBSACbased catalysts. For traditional molded catalysts, it s well known that mass transport limitation can occur inside the porous network, thus affecting the overall catalytic activity (see section 2.2.1). In order to scientifically determine, if particle size is limiting catalytic activity, comminution experiments of the scale-up catalyst were conducted. The 500 μm scale-up catalyst was chosen, because a sufficient amount of material was available for comminution and the metal distribution is likely homogeneous due to the high metal dispersion. The material was crushed in a porcelain mortar and sieved into different size fractions. Catalyst material of each size fraction was tested in cinnamic acid hydrogenation experiments to compare catalytic activities. Figure 5.18 shows the resulting cinnamic acid conversions as a function of modified residence time. It s apparent that the effective reaction rate increased with decreasing particle size fraction. The increase in reaction rate was very pronounced between the size fractions x=500 μm, 200>x>100 μm, and 100>x>80 μm. The differences in cinnamic acid conversion were much 116

126 k eff / m 3 kg -1 Pd s < particle size fraction / µm (a) 5. Results and discussion Thiele modulus / - effectiveness factor / % < particle size fraction / µm (b) < particle size fraction / µm (c) Figure 5.19.: Effective reaction rate constants (a), Thiele moduli (b), and effectiveness factors (c) of different catalyst size fractions (k intr = s -1, D eff = 1.18e-10 m 2 s -1 ) smaller for the remaining particle size fractions, with particle sizes below 80 μm. Compared to the hydrogenation experiments in figure 5.16, the effective reaction rate of the 500 μm spherical scale-up catalyst was smaller, likely because here the wet catalyst wasn t dried before application. The increasing rate of cinnamic acid conversion with decreasing particle size demonstrates an influence of particle size on catalytic activity. It s an indication of internal mass transport limitation. Mathematical models, i.e. the determination of Thiele moduli and effectiveness factors, were applied using equations 2.4 and 2.5 to further analyze this effect and predict critical particle sizes of limiting mass transport. Therefore, effective reaction rate constants of the experimental data were derived by applying the power law (see equation 2.9). For very small particle sizes, absence of mass transport limitation was assumed, so that the intrinsic reaction rate can be estimated by the effective reaction rate. Figure 5.19 displays derived effective reaction rate constants of each catalyst size fraction, as well as calculated Thiele moduli and effectiveness factors. With decreasing particle size, the effective reaction rate constant increased and asymptotically approached a maximum value of about 0.06 m 3 kg -1 Pd s -1 for catalyst material smaller than 30 μm. In order to differentiate between intrinsic surface reaction and pore diffusion rate, the Thiele modulus was introduced. As intrinsic reaction rate constant, the largest ef- 117

127 5. Results and discussion fective reaction rate constant of this specific catalytic system from figure 5.19 (a) was applied. The effective diffusion constant was estimated from a molecular diffusion constant of cinnamic acid in toluene found in literature according to equation 2.3. The molecular diffusion constant from literature was 1.18e-9 m 2 s -1 [197, 198]. As seen in figure 5.19 (b), the Thiele modulus slowly increases from 0.12 to 0.58 with increasing particle size until a size fraction of μm. The non-comminuted catalyst with a particle size of around 500 μm has a significantly larger Thiele modulus of This means that the 500 μm spherical carbon catalyst exhibits a stronger disparity between intrinsic reaction rate and effective pore diffusion in a cinnamic acid hydrogenation. As, for spherical catalysts, a Thiele modulus larger than 3 indicates strong pore diffusion limitation, the PBSAC-based catalyst wasn t considered purely diffusion controlled [53]. The critical particle size resulting in negligible pore diffusion limitation was determined with the help of the effectiveness factor. The effectiveness factor was solely calculated from the Thiele modulus, assuming a pseudo first order reaction. Absence of pore diffusion limitation is defined for effectiveness factors between [45, 53]. Particles smaller 200 μm already complied with this criterion. Consequentially, catalytic systems can essentially be realized without pore diffusion limitation by using spherical carbon with particle sizes smaller 200 μm. However, as the wet palladium catalyst applied in this experimental study showed a reduced effective reaction rate constant compared to the 500 μm PBSAC catalyst in table 5.11, due to a decreased intrinsic reaction rate, the effect of pore diffusion limitation can be even stronger in other PBSAC catalysts than assumed here. Also, the estimated effective diffusion constant can be significantly smaller due to Knudsen diffusion and molecular sieve effects in the micropores of the PBSAC pore system [53]. Further investigations with different particle sizes and pore structures will follow in the next section. Influence of pore structure Spherical catalysts with different particle size and pore structure (see section ) were tested in cinnamic acid hydrogenation reactions to compare catalytic activities. Figure 5.20 contains experimental results of all performed cinnamic acid hydrogenation reactions in form of effective reaction rate constants, together with the previously determined palladium metal dispersions. Plots of cinnamic acid conversion and a table of effective reaction rate constants can be found in appendix C

128 k eff / m 3 kg -1 Pd s keff DPd Results and discussion Particle size / µm V tot / cm 3 g D Pd / % Figure 5.20.: Effective reaction rate constants of cinnamic acid hydrogenations and palladium metal dispersions of spherical catalysts with different particle size and total pore volume (T = 40 C, p H2 = 30 bar, c 0, cinnamic acid = 236 mol m -3, m catalyst = 0.5 g) In all cases of spherical carbon based catalysts with a particle diameter of 500, 200, and 50 μm, reaction rate increased with increasing total pore volume. Metal dispersion were very similar, but also followed the observed trend in reaction rate. Generally, with increasing degree of carbon activation, the effective reaction rate constant also increased due to two reasons. On the one hand, the intrinsic reaction rate increases with increasing metal dispersion. Additionally, pore diffusion is faster in more activated carbon materials with larger pore diameters. Comparing spherical catalysts with similar total pore volume, e.g cm 3 g -1, but different particle size, catalytic activity increased with decreasing particle size. This potential influence of mass transport in dependence on particle size was supported by previous comminution experiments (see the previous subsection). The fact that the highly activated 200 μm material exhibited enhanced catalytic activity over the second most activated 200 μm material, despite equal metal dispersions, also indicates an influence of pore diffusion. The same interpretation can be made for the mesoporous 500 μm spherical carbon that performed better than the similarly activated material with a total pore volume of 1.18 cm 3 g -1, despite similar pore volumes. The characterization of palladium catalysts based on different available spherical carbon materials shows that particle size and pore structure significantly influence the overall catalytic 119

129 5. Results and discussion activity. As previously demonstrated in comminution experiments, with decreasing particle size, the extent of internal mass transport limitation decreases. Increasing carbon activation results in larger surface areas leading to generally larger metal dispersions and catalytic activity. The increasing average pore diameter also positively affects pore diffusion and effective reaction rates. Of all tested materials, the catalysts based on highly activated 50 and 200 μm spherical carbon performed best. Thus, the 200 μm spherical carbon is the proposed catalyst support material of choice in the hydrogenation of cinnamic acid due to the ideal combination of good catalytic activity and ease in catalyst handling. Influence of surface functionalization Besides particle size and pore geometry, carbon surface functionalization is another parameter impacting catalytic performance, as the effects on metal dispersion were already significant. Prepared palladium catalysts based on differently functionalized spherical carbon (see section ) were tested in the hydrogenation reaction of cinnamic acid to compare catalytic activities. Note that in this experimental series, a hydrogen partial pressure of 5 bar was applied. For each catalyst, effective reaction rate constants and previously determined palladium metal dispersions are shown in figure Plots of cinnamic acid conversion and a table of effective reaction rate constants can be found in appendix C.2. Regarding the nitric acid pretreated catalysts, three of the four catalysts exhibited similar reaction rate constants between m 3 kg -1 Pd s -1. The catalyst material oxidized with 36% nitric acid at RT showed a significantly higher reaction rate of about m 3 kg -1 Pd s -1. So, in this nitric acid oxidation series, catalytic activity of the best catalyst was 82% larger than the slowest catalyst. With increasing metal dispersion, catalytic performance of the 15% nitric acid oxidized samples increased slightly. The two catalysts prepared using spherical carbon oxidized with 36% nitric acid exhibited an opposing trend of decreasing catalytic activity with increasing metal dispersion. The best catalyst with sulfuric acid pretreatment was slightly more active than the best catalyst with nitric acid pretreatment. Sulfuric acid pretreated catalysts showed a distinct trend of increasing reaction rate with harsher oxidation conditions. Catalytic activity doubled by doubling acid concentration from 25-50% at constant temperature. Also, with increasing oxidation temperature from RT to 90 C at constant acid concentrations, catalytic activity increased by around 80%. The fastest hydrogenation reaction occurred with a 50% sulfuric 120

130 k eff / m 3 kg -1 Pd s keff DPd 5. Results and discussion 15% HNO3 36% HNO3 25% H2SO4 50% H2SO4 Acid concentration RT 90 C RT 90 C RT 90 C RT 90 C Oxidation temperature D Pd / % Figure 5.21.: Effective reaction rate constants of cinnamic acid hydrogenation reactions and palladium metal dispersions of differently oxidized spherical carbon catalysts (T = 40 C, p H2 = 5 bar, c 0, cinnamic acid = 236 mol m -3, m catalyst = 0.5 g) acid pretreatment at 90 C. Reaction rate of the sample oxidized with 25% sulfuric acid at RT was lowest. The most active catalyst happened to also exhibit the largest metal dispersion. Otherwise, metal dispersion didn t appear to directly correlate with reaction rate constants. To determine the influence of carbon surface functionalization on catalytic activity, the results of hydrogenation and carbon monoxide chemisorption experiments were correlated to the numerous surface characterizations performed in section In the series of nitric acid pretreated samples, an optimum in catalytic activity seemed to occur. A plausible explanation is the interplay of three effects: surface functionalization, metal dispersion, and metal distribution (see also section for details). C-π sites can remain in an insufficiently oxidized carbon surface, competitively adsorbing palladium ions and promoting the formation of egg-shell catalysts. These egg-shell catalysts exhibit lower metal dispersions with lower intrinsic catalytic activity. But, this lower intrinsic activity can be compensated by a lower level of mass transfer restriction due to shorter diffusion distances. The role of C-π sites is explicitly discussed in section While the number of C-π sites cannot directly be determined, an anomaly in carbon surface functionalization, as discussed in section 5.2.1, can be another explanation for the optimum in catalytic activity of this experimental series. The most active catalyst is surprisingly based on an oxidized spherical carbon, which exhibits by far the lowest amount of func- 121

131 5. Results and discussion tional surface groups and a large point of zero charge. Possibly, the ph difference between the material s PZC and the ph of the impregnation solution impacted metal distribution and metal dispersion. A larger ph difference increases electrostatic interactions and promotes the formation of egg-shell catalysts. This hypothesis has already been discussed in the literature [96]. Another pronounced difference is the extent of strongly adsorbing surface oxides, which is significantly higher for the most active catalyst. So, the chemical surface composition can also impact the strength of electrostatic adsorption and thus metal dispersion and metal distribution. In order to clearly state the predominant effect resulting in large catalytic activity, further analysis is necessary. Comminution experiments, for instance, can lead to a more detailed insight in metal distribution, as the effective reaction rate of crushed samples is expected to increase more strongly for homogeneously distributed catalyst pellets than for egg-shell catalysts. Concerning sulfuric acid functionalization, the large metal dispersions indicate the absence of most C-π sites and the dominance of electrostatic palladate adsorption to protonated surface oxides. Thus, already at mild oxidation conditions, sulfuric acid has homogeneously oxidized unsaturated carbon bonds. In this experimental series, the pronounced increase in catalytic activity indicates a significant effect of surface functionalization. Nevertheless, most surface properties were similar, i.e. PZC, amount of volatile components, and pore geometry. The influence of surface functionalization is not expected to be that large. The increase of catalytic activity by % between each sample is more difficult to explain, because the different samples appear to have only an insignificantly different surface composition. But, the ratio of strongly to weakly adsorbing surface oxides correlates well with catalytic activity. An increasing total amount of strongly adsorbing surface oxides can have resulted in a more comprehensive egg shell metal distribution due to enhanced electrostatic adsorption. This plausible hypothesis needs to be verified by additional experiments. In essence, carbon surface functionalization has a large impact on a catalyst s metal dispersion, metal distribution and catalytic activity. Many observations regarding carbon functionalization were already published in the literature (see section 2.2.5). Even though, they generally apply to spherical carbon, too, this detailed characterization was necessary to determine the most suitable oxidation conditions. Spherical carbon can be oxidized with either nitric acid or sulfuric acid treatment. Because nitric acid strongly affects the carbon surface due to heteroatom inclusion, large acid concentrations, but mild temperatures are ideal to achieve a highly active catalyst with high cinnamic acid hydrogenation reaction rates. In contrast, sulfuric acid intensively reacts with the surface of spherical carbon without af- 122

132 X / % 5. Results and discussion % 2% 5% 00 fits according Pd 200 to µm µm model PBSAC mod / s kg Pd m-3 Figure 5.22.: Differently loaded 200 μm spherical catalysts with 2, 5, and 10 wt% palladium in the hydrogenation of cinnamic acid; fits according to model applying equation 2.9 (T = 40 C, p H2 = 30 bar, c 0, cinnamic acid = 236 mol m -3, m catalyst = 0.5 g) fecting the pore structure. Large acid concentrations and high temperatures promote advantageously functionalized surfaces without sacrificing carbon surface area. Metal dispersions and catalytic activities in the hydrogenation reaction of cinnamic acid were best for intensively oxidized materials. Because the effects of surface functionalization are more easily predictable, sulfuric acid treatment of spherical carbon is highly recommended for catalyst preparations. Influence of metal loading Concerning spherical carbon loaded with different amounts of palladium, hydrogenation experiments have been conducted to determine catalytic activities. Catalytic conversions of cinnamic acid are plotted in figure Derived effective reaction rate constants are listed in table In case of the prepared 2 wt% palladium on spherical carbon catalyst, conversion of cinnamic acid was fastest. Rate of reaction decreased with increasing metal loading. It needs to be noted that conversion is plotted in figure 5.22 as function of modified residence time, which incorporates palladium mass. Differences in effective reaction rate constants were most significant between the 5 and 10 wt% spherical catalysts, with an increase in catalytic activity of 43%. Between the 2 and 5 wt% catalysts, catalytic activity only increased by 19%. 123

133 5. Results and discussion Table 5.12.: Effective reaction rate constants of cinnamic acid hydrogenations of spherical catalysts with different palladium loading Palladium loading / wt% k eff / m 3 s -1 kg Pd This effect of increasing catalytic activity with decreasing metal loading is likely due to larger metal dispersions at low metal loadings. During impregnation via ion adsorption, palladate anions are homogeneously distributed across the internal carbon surface as they adsorb to the positively charged surface groups. With increasing metal loading, palladate anions are located more closely together, so that during metal transformation larger palladium clusters are formed. Generally, the application of palladium catalysts based on spherical carbon with low metal loadings is recommended. For low metal loadings, utilization of noble metal to catalyze cinnamic acid hydrogenation reactions is better. Also, the larger amount of applied spherical carbon support material doesn t affect product separation due to fast filtration rates Ruthenium catalysts: Influences on catalytic activity Influence of metal loading Spherical catalysts with different ruthenium loadings (see section ) were tested in the conversion of toluene to methyl cyclohexane (see section 2.2.6). Results of these hydrogenation experiments are plotted in figure Table 5.13 lists the corresponding turnover frequencies and previously determined metal dispersions of these catalysts. For catalyst screening purposes, turnover frequencies according to equation 2.12 were determined instead of effective reaction rates. Fitting of data points using equation 2.9 requires further investigations concerning the applied catalytic reaction systems. Turnover frequencies were determined around a modified residence time of 150 s kg Ru m -3. With the 3.15 wt% ruthenium catalyst, toluene was steadily hydrogenated until full conversion. In the same residence time frame, almost no product was formed with the 0.31 wt% ruthenium catalyst. Also, with the 5.54 wt% spherical catalyst, conversion of toluene was very slow and reached around 15% in the end. The turnover frequencies quantified these ob- 124

134 5. Results and discussion X / % % 3.15 Ru % PBSAC mod / s kg Ru m-3 Figure 5.23.: Ruthenium on spherical carbon with different metal loadings in the hydrogenation of toluene (T = 150 C, p H2 = 50 bar, c 0, toluene = 180 mol m -3, m catalyst = 0.1 g) Table 5.13.: Turnover frequencies of toluene hydrogenations and metal dispersions of ruthenium on spherical carbon with different metal loadings Ruthenium loading / wt% TOF / s -1 D Ru / %

135 5. Results and discussion servations. Toluene hydrogenation using the 3.15 wt% ruthenium catalyst was about a factor 37 faster than the second fastest hydrogenation. Looking at ruthenium metal dispersions, catalytic activity increased with decreasing metal dispersion. So, large ruthenium clusters appear to perform best in the hydrogenation of toluene. This thesis is supported by other research groups. They previously reported that hydrogenation activity of aromatic functional groups increases with increasing metal cluster size.[199, 200] The 3.15 wt% ruthenium spherical catalyst was reproduced according to the same preparation procedure and catalytic performance was successfully validated in a toluene hydrogenation experiment. Nevertheless, instead of varying impregnation duration, ruthenium cluster sizes can be most easily increased by applying increased metal transformation temperatures during catalyst preparation [100, 101]. In essence, the prepared ruthenium catalysts are catalytically active. As a specialty for ruthenium catalysts in the hydrogenation reaction of toluene, small metal dispersions and thus larger ruthenium clusters are necessary for good catalytic activities. Influence of reactant molecules The most active toluene hydrogenation catalyst, 3.15 wt% ruthenium on spherical carbon, was also tested in the hydrogenation of 1-octene, m-cresol and thymol (see also section 2.2.6). The results are presented in figure 5.24-a. The performance of a commercial 5 wt% ruthenium on carbon powder catalyst (5.26 wt% exact metal loading, 34% ruthenium dispersion) in these hydrogenation reactions is shown in figure 5.24-b, for comparison. Table 5.14 gives the respective turnover frequencies. This table also includes molecule dimensions determined using the Chem3D Pro modelling software by looking at the Cartesian coordinates. The corresponding molecule structures are drawn in figure With the prepared 3.15 wt% ruthenium on spherical carbon catalyst, 1-octene was very quickly converted. The rate of toluene conversion was significantly slower. m-cresol was only partially converted within the experiment s time frame. Thymol hydrogenation was negligibly small. In comparison, the powder catalyst rapidly converted 1-octene, as well. Toluene was also quickly hydrogenated. The reaction rates of m-cresol and thymol were similarly fast and much faster than with the PBSAC catalyst. In both cases, full conversion was reached during the experiment. The spherical catalyst exhibited a larger 1-octene turnover frequency than the powder catalyst. For toluene, m-cresol and thymol hydrogenations, the powder catalyst had larger turnover frequencies. With the exception of the 1-octene hydrogenation reaction, the powder catalyst performed significantly better than the spherical ruthenium catalyst in the conversion of toluene, m-cresol and thymol. 126

136 5. Results and discussion X / % Octene 80 Toluene m-cresol Thymol mod / s kg Ru m-3 0 (a) X / % Octene Toluene m-cresol 0 Thymol mod / s kg Ru m-3 (b) Figure 5.24.: Hydrogenation of 1-octene, toluene, m-cresol and thymol using 3.15 wt% ruthenium on spherical carbon (a) and commercial 5.26 wt% ruthenium on powdered carbon (b) (T = 150 C, p H2 = 50 bar, c 0 = 180 mol m -3, m catalyst = 0.1 g) Table 5.14.: Turnover frequencies in the hydrogenation of different educt molecules with 3.15 wt% ruthenium on spherical carbon and commercial 5.26 wt% ruthenium on powdered carbon catalysts Educt molecule Molecule dimensions / Å TOF Ru@PBSAC / TOF Ru@PAC / s -1 s -1 1-Octene 11.0 x 3.2 x Toluene 6.4 x 4.9 x m-cresol 6.4 x 5.8 x Thymol 8.4 x 5.2 x

137 5. Results and discussion Figure 5.25.: Molecule structures of 1-octene, toluene, m-cresol and thymol (from left to right) - view along the x-axis of the molecule Comparing molecule geometries, 1-octene is the longest molecule, but is small and flexible in the other two dimensions. Toluene and m-cresol exhibit a broader molecule width than 1-octene. Thymol is a molecule with larger width and height than 1-octene. Unexpectedly, thymol wasn t converted by the spherical catalyst, at all. Three highly influential factors are discussed in the following to explain the catalytic inactivity of the PBSAC catalyst for this specific reaction, i.e. metal dispersion, space confinement issues and chlorine/chloride inhibition. It was already pointed out in the previous section that ruthenium cluster size matters. The cluster size resulting from 34% metal dispersion of the commercial powder catalyst is apparently suitable to catalyze thymol hydrogenation reactions. It s unknown, if the larger cluster sizes lead to a significant decrease in catalytic activity. The hypothesis, that the larger cluster sizes of the spherical ruthenium catalyst result in catalytic inactivity, cannot be completely ruled out without an experiment applying a similarly dispersed ruthenium on spherical carbon catalyst. But, the spherical catalyst performed better in the hydrogenation of 1-octene than the commercial catalyst. This indicates that the spherical catalyst was intrinsically more active due to larger ruthenium clusters. Thus, space confinement issues of the spherical carbon pore geometry and chlorine/chloride inhibition originating from the applied ruthenium chloride precursor are more likely reasons for the catalytic inactivity against bulkier substrates. In the first case, a too restrictive 128

138 5. Results and discussion pore structure of spherical carbon hinders the thymol molecules from entering the pore system and accessing the catalytically active sites [55]. Transfer of reactants through the pore structure between bulk solution and active ruthenium clusters is prevented. Mass transport limitation of spherical carbon has already been discussed in section for cinnamic acid. However, cinnamic acid is a planar molecule and, thus, fits well into slit shaped, micro porous carbons. Thymol has a significantly larger molecule height than cinnamic acid and, thus, more difficulties penetrating this spherical carbon material (see also section ). Another possible explanation for inactivity of the thymol hydrogenation reaction is that chlorine/chloride inhibits this particular reaction. It was reported in literature that ruthenium catalyzed reactions are strongly affected by chlorine species dissociating from the applied chlorine-containing metal salt precursors [101, 201, 202]. The influence of chlorine/chloride inhibition can be validated and mitigated by using chlorine-free ruthenium precursors in catalyst preparation. In conclusion, spherical ruthenium catalysts are generally well suitable for hydrogenation reactions of planar and linear molecules such as carbocyclic rings and olefines. Further investigations and subsequent modification of the catalyst preparation procedure is necessary for the successful transformation of more complex and functionalized molecules Platinum catalysts: Influences on catalytic activity As a technically relevant reaction, PBSAC-based catalysts were tested for catalytic activity in the dehydrogenation of H18-dibenzyltoluene (see section for details). It has already been experimentally demonstrated that platinum catalysts performed best at hydrogen discharge. Two commercial powder catalysts based on activated carbon and aluminum oxide, respectively, were chosen as reference systems to benchmark catalytic activities of the PBSAC-based platinum catalysts prepared in this work. Influence of ion adsorption modification The prepared catalysts with and without addition of hydrogen chloride during impregnation were tested in dehydrogenation of H18-dibenzyltoluene in order to check, if the additional chloride ions affect metal dispersion. In figure 5.26, the results of two PBSAC-based 5 wt% platinum catalysts are shown. The performance of the two commercial powder catalysts is also plotted in figure Both commercial platinum catalysts supported on activated carbon powder and aluminum oxide powder showed similarly high catalytic activities. The dehydrogenation degrees were 129

139 dehydrogenation degree / % 5. Results and discussion % Al2O3 80 PAC 5% PBSAC (w/o HCl) time / min Figure 5.26.: Dehydrogenation of H18-dibenzyltoluene using 5 wt% platinum on spherical carbon, commercial 0.5 wt% platinum on aluminum oxide and commercial 5 wt% platinum on activated carbon (T = 310 C, p = 1 bar, n Metal / n LOHC = 0.1 mol%) 78 and 83%, respectively, after 120 min. While the reaction rates were alike within the first 30 min, the 0.5 wt% platinum catalyst on aluminum oxide flattened out less strongly than the 5 wt% platinum catalyst on activated carbon and, thus, finally reached a higher plateau. The conversion curves of the two spherical catalysts behaved similarly. In parallel, they continuously proceeded with decreasing rate until they reached dehydrogenation degrees of 43 and 44%, respectively. The difference was within measurement uncertainties. Apparently, the influence of the impregnations ph value on the catalytic activity was negligibly small. The additional chloride anions from the hydrochloric acid had no noticeable effect on the dispersion of the chloroplatinate anions during impregnation, even though, the anions compete with each other for adsorption sites. An excess of adsorption sites was available. Compared to the final dehydrogenation degree of the commercial platinum on carbon catalyst, only 56% of that reference value was reached after 120 min with the PBSAC-based system. The initial reaction rate was significantly lower. After 60 min reaction time, reaction rates of PBSAC-based and powder catalysts were similar. Admittedly, the first PBSAC-based platinum catalysts didn t reach the catalytic activities of the powdered commercial catalysts. Nevertheless, these catalysts with particle diameters around 200 μm already showed reasonable dehydrogenation performance together with the advantage in catalyst handling. 130

140 5. Results and discussion Due to the handling benefit of spherical carbon materials, lower platinum loadings are still technically interesting. A smaller platinum loading allows for a larger metal dispersion and, consequentially, a better utilization of the expensive noble metal [96]. So, the strategy was to develop highly active catalysts with a platinum loading of 0.5 wt%. 131

141 dehydrogenation degree / % 5. Results and discussion % Pt Al2O3 80 PBSAC 0.5% PBSAC (low Vtot) time / min Figure 5.27.: Dehydrogenation of H18-dibenzyltoluene using two 0.5 wt% platinum on spherical carbon catalysts with different carbon activation, a 5 wt% platinum on spherical carbon catalyst and commercial 0.5 wt% platinum on aluminum oxide (T = 310 C, p = 1 bar, n Metal / n LOHC = 0.1 mol%) Influence of pore structure The catalytic activity of two spherical catalysts loaded with 0.5 wt% platinum, but varying total pore volume is investigated in this section to see, if the highly activated carbon support is necessary. The results of the H18-dibenzyltoluene dehydrogenation experiments are presented in figure 5.27, together with the best performing experiments already discussed in the previous subsection. Within the first 35 min, both catalysts only lead to a dehydrogenation degree of less than 3%. Then, the catalyst based on the highly activated support showed increasing activity and a dehydrogenation degree of 26% was reached after 120 min. With a total conversion of merely 7%, the second catalyst performed even worse. The results of the catalytic experiments of these two PBSAC-based catalysts correlate well with the low platinum metal dispersions determined in section , as only few platinum atoms are available and accessible for reactants. Because reaction rates increased with time, a continuous activation process is observed. The catalytically active platinum species is obviously formed during the reaction. The catalysts are not completely reduced. The very low activity of the catalyst based on the less activated PBSAC material with the lower total pore volume can be explained by the small average pore diameter of 2.1 nm. 132

142 dehydrogenation degree / % 5. Results and discussion % Pt Al2O3 80 PBSAC 0.5% PBSAC (HNO3) (H2SO4) time / min Figure 5.28.: Dehydrogenation of H18-dibenzyltoluene using two 0.5 wt% platinum on spherical carbon catalysts with different surface functionalization, a 5 wt% platinum on spherical carbon catalyst and commercial 0.5 wt% platinum on aluminum oxide (T = 310 C, p = 1 bar, n Metal / n LOHC = 0.1 mol%) Thus, it is difficult for the planar H18-dibenzyltoluene molecules with a steric demand of at least 10.9 x 10.6 x 2.4 Å to reach the active sites within the catalyst. Mass transport limitation due to restricted pore diffusion plays a significant role in this catalytic system. In contrast, the highly activated PBSAC material exhibits an average pore diameter of 2.9 nm. Here, pore diffusion limitation didn t appear to be critical. Consequently, the pore structure of the highly activated spherical carbon material is sufficient to allow for pore diffusion of H18-dibenzyltoluene molecules. But, the low platinum dispersion of the 0.5 wt% platinum catalysts needs to be addressed to further improve catalytic activity. Influence of surface functionalization Differently functionalized spherical carbon materials, loaded each with 0.5 wt% platinum, were tested in H18-dibenzyltoluene dehydrogenation experiments to determine, whether the sulfuric acid treatment improves metal dispersion. The results are presented in figure 5.28, together with the best performing experiments already discussed in the previous subsections. During the first 5 min, catalytic activity of the catalyst oxidized with sulfuric acid was low. Then, until the 35 min data point, the reaction rate continuously increased and declined af- 133

143 5. Results and discussion terwards in accordance with reaction kinetics. Compared to the commercial catalyst, the catalytic activity was much lower in the region between 0 and 60 min reaction time. At around 90 min, the spherical carbon catalyst caught up with and, shortly later, even outperformed the commercial catalyst. The total dehydrogenation degree after 120 min amounted to 87%. In contrast to the first dehydrogenation catalysts based on nitric acid oxidized spherical carbon, a huge improvement in catalytic activity has been accomplished. By changing oxidation parameters, the number of remaining C-π sites was effectively reduced and strongly adsorbing surface oxides increase the strength of electrostatic platinate ion adsorption for a more suitable dispersion of platinum. The activation behavior in the beginning of the experiment is due to an incomplete metal transformation during catalyst preparation. In the course of the dehydrogenation experiment, the catalyst was obviously further transformed to metallic platinum by the produced hydrogen gas. Influence of metal transformation temperature To ensure complete transformation of the adsorbed platinum salt, metal transformation parameters were adjusted. The biggest impact is expected by increasing the metal transformation temperature. So, some impregnated spherical carbon material was treated at 300 C in hydrogen atmosphere and tested thereafter in the H18-dibenzyltoluene dehydrogenation reaction. Figure 5.29 presents the results of this catalytic experiment, together with the best performing experiments already discussed in the previous subsection. Conversion smoothly increased up to a total conversion of 94%. At any time, catalytic activity superseded the commercial catalyst s activity. This catalyst was reproduced according to the same preparation parameters and catalytic performance was successfully validated. The fact that this PBSAC-based catalyst outperforms the standard powder catalyst in the dehydrogenation of H18-dibenzyltoluene further demonstrates that pore diffusion limitation is not a general issue with PBSAC catalyst pellets. For this specific reaction, though, mass transport is enhanced by volume expansion through hydrogen formation at the active sites, subsequent flow of hydrogen gas to the outside and inherent uptake of liquid reactants by capillary forces of emptied pores [203, 204]. A high performance LOHC dehydrogenation catalyst has been successfully developed. Important steps in the preparation of the platinum catalyst were selecting a PBSAC starting material with suitable pore structure, appropriately modifying the carbon surface applying 134

144 dehydrogenation degree / % 5. Results and discussion % Al2O time / min 0.5% PBSAC (200 C) (300 C) Figure 5.29.: Dehydrogenation of H18-dibenzyltoluene using two 0.5 wt% platinum on spherical carbon catalysts with different metal transformation temperatures and commercial 0.5 wt% platinum on aluminum oxide (T = 310 C, p = 1 bar, n Metal / n LOHC = 0.1 mol%) sulfuric acid oxidation, impregnating the material with low platinum surface concentrations and transforming the catalyst at elevated temperatures. This PBSAC-based material marks a significant advancement in catalytic activity and ease of handling compared to the platinum on aluminum oxide standard catalyst. Influence of metal loading The well performing 0.5 wt% platinum on spherical carbon dehydrogenation catalyst, as developed in the previous subsections, was also tested in a hydrogenation reaction. Additionally, 5 and 8.4 wt% platinum on spherical carbon were characterized in the same model reaction. So, in essence, this resulted in an experimental series concerning the variation of platinum metal loading. The catalysts were tested in cinnamaldehyde hydrogenations. In this reaction system, selectivity effects can also be studied. See section for details on this reaction. In figure 5.30, catalytic conversion of cinnamaldehyde is plotted for differently loaded spherical carbon materials and a commercial 5 wt% platinum on pulverized activated carbon catalyst. Figure 5.31 shows the selectivity to hydrocinnamic alcohol. Table 5.15 lists reaction rate constants, final product selectivity and previously determined platinum metal dispersions. 135

145 X / % 5. Results and discussion %Pt 8.4%Pt 5% 00 fits PAC PBSAC to model mod / s kg Pt m-3 Figure 5.30.: Hydrogenation of cinnamaldehyde using 0.5, 5 and 8.4 wt% platinum on spherical carbon and a commercial 5% platinum on pulverized activated carbon; fits according to model applying equation 2.9 (T = 100 C, p H2 = 25 bar, c 0, cinnamaldehyde = 189 mol m -3 ) All prepared platinum catalysts were very active in the hydrogenation of cinnamaldehyde. Normalized to the platinum loading, the 0.5 wt% platinum spherical catalyst performed best. Cinnamaldehyde hydrogenation quickly approached full conversion. The 5 and 8.4 wt% platinum on spherical carbon materials showed lower catalytic activities. Full conversion was reached after longer modified residence times. The commercial platinum catalyst was less active than the equally loaded spherical carbon system. Interestingly, after 150 s kg Pt m -3, the reaction rate of the commercial catalyst strongly decreased before approaching full conversion. Between 50 and 80% conversion of cinnamaldehyde, selectivity towards hydrocinnamic alcohol remained constant with values between 34 and 48%. Approaching full conversion, selectivity increased. In case of spherical catalysts, final selectivity at full conversion varied between 49 and 65%. The 5 wt% platinum spherical catalyst exhibited a lower selectivity towards hydrocinnamic alcohol than the two other spherical materials. With 43%, the commercial catalyst had the lowest final selectivity. The concentration of cinnamic alcohol increased in the beginning of the reaction and decreased towards full conversion. A depletion of hydrocinnamic aldehyde wasn t observed. Interestingly, besides the expected hydrogenation products, propylbenzene was formed when using spherical catalysts. The amount of 136

146 5. Results and discussion S / % X / % 0.5%Pt 8.4%Pt PAC PBSAC Figure 5.31.: Selectivity of cinnamaldehyde hydrogenation towards hydrocinnamic alcohol using 0.5, 5 and 8.4 wt% platinum on spherical carbon and a commercial 5 wt% platinum on pulverized activated carbon (T = 100 C, p H2 = 25 bar, c 0, cinnamaldehyde = 189 mol m -3 ) Table 5.15.: Catalytic activity, final selectivity towards hydrocinnamic alcohol and metal dispersions of 0.5, 5 and 8.4 wt% platinum on spherical carbon and 5 wt% platinum on pulverized activated carbon Catalyst support material Platinum metal loading / wt% k eff / m 3 kg -1 Pt S final / % D Pt / % s -1 PBSAC PAC

147 5. Results and discussion propylbenzene increased steadily up to values between 13 and 17%. The commercial catalyst didn t catalyze propylbenzene formation. The 0.5 wt% spherical catalyst was 2.2 times more active than the 5 wt% spherical catalyst and 4.1 times more active than the 8.4 wt% spherical catalyst, with respect to applied platinum. Concluding this section on platinum-based PBSAC catalysts, it can be stated that spherical carbon loaded with 0.5 wt% platinum is an outstanding catalyst for both hydrogenation and dehydrogenation reactions. In the variation of platinum metal loading on spherical carbon, a clear trend is apparent. With increasing metal loading between wt% platinum, catalytic activity decreases. This partially correlates with metal dispersions. Utilization of metal is improved in highly dispersed systems. Also, the 5 wt% platinum spherical catalyst performed better than the commercial catalyst, as metal dispersion was increased. The large surface area of spherical carbon facilitates large metal dispersions. It s difficult to explain the enhanced catalytic activity of the 0.5 wt% over the 5 wt% spherical catalyst despite equal metal dispersions. A plausible effect is a different extent of pore diffusion limitation. Due to strong electrostatic interactions during catalyst preparation, the small amount of platinate ions necessary for a 0.5 wt% spherical catalyst can be predominantly adsorbed in the outer shell or the mesopores with large metal dispersion. Such a catalyst exhibits a decreasing influence of mass transport due to shorter distances between catalytically active sites and bulk solution, as well as easier diffusion in mesopores. For a similarly well dispersed 5 wt% platinum on spherical carbon catalyst, metal distribution is more homogeneous resulting in stronger pore diffusion limitations. The commercial catalyst deactivated stronger during cinnamaldehyde hydrogenation reaction than the PBSAC-based catalysts. According to literature, this deactivation behavior can be due to carbon monoxide poisoning of the catalytically active sites resulting from a decarbonylation side reaction of cinnamic alcohol [205]. In this experimental series, water was added to the reactant solution in order to vacate poisoned platinum sites by solvent effects. While the spherical catalysts didn t show signs of poisoning in this experimental series, possibly, the commercial catalyst exhibited deactivation due to different support interactions. In literature, a strong influence of carbon surface functionalization on cinnamaldehyde hydrogenation activity was reported, with the hypothesis that non-functionalized carbon sites assist in the adsorption of aromatic substrates [118, 206]. Concerning selectivity of the cinnamaldehyde hydrogenation reactions, interesting observations were made. Similar cases were observed in the literature and explained with effects discussed in the following. The intermediate formation and depletion of cinnamic alcohol 138

148 5. Results and discussion proved the conventional parallel reaction mechanism of cinnamaldehyde hydrogenation (see section 2.2.6). Interestingly, hydrocinnamic aldehyde didn t deplete, indicating stabilization of the aldehyde functional group after carbon-carbon bond saturation. Selectivity towards hydrocinnamic alcohol was enhanced in case of the 0.5 and 8.4 wt% platinum spherical catalysts. Impurities originating from the preparation procedure, such as traces of chlorine/chloride in varying amounts from the hexachloroplatinate precursor, can be responsible for the lower selectivity of the 5 wt% platinum spherical catalyst [116, 119]. Compared to spherical carbon, the lower aldehyde group hydrogenation activity of the carbon powder catalyst can be due to different steric effects within the pore system [117]. The reduction reaction to propylbenzene, catalyzed by spherical catalysts, can only be explained by differences in interactions between metal cluster, carbon support and reactant molecule. In a short summary, spherical carbon loaded with platinum metal performed better in the hydrogenation of cinnamaldehyde than a respective catalyst based on activated carbon powder. Catalytic activity and selectivity towards hydrocinnamic alcohol were generally enhanced. A low platinum loading of spherical catalysts is recommended for catalyzed reactions carried out in batch processes. Large metal loadings are desirable in flow chemistry processes with a limited catalyst bed volume [207] The role of C-π sites In some previous experimental series on active metal deposition and subsequent catalyst screening, trends were observed that cannot be explained with available information on material properties. As an explanation, it was already hypothesized in the individual sections that another influential factor can have affected metal deposition and, thus, catalytic performance. Besides metal ion adsorption to charged surface oxides by electrostatic interaction, remaining C-π sites at the carbon surface can competitively adsorb metal ions (see section ). As these C-π sites are known to instantly transform metal ions to elemental metal, the metal will be immobilized at a certain position within the carbon spheres. As C-π sites in the shell of the carbon spheres are reached at first, the presence of C-π sites will affect metal distribution resulting in more distinct egg-shell catalysts. Furthermore, if C-π sites adsorb multiple metal ions, metal dispersion will decrease as larger metal clusters are formed by instantaneous metal transformation. Experiments explicitly demonstrating the presence of C-π sites and their role in active metal deposition and subsequent catalytic performance are not possible. But, the number of performed experiments and previous reports in literature on graphitic carbon materials (see 139

149 5. Results and discussion sections and ) indicate that C-π sites are present and influence metal distribution, metal dispersion and, thus, catalytic activity of spherical catalysts. In the following, the role of C-π sites in the preparation of palladium and platinum catalysts based on pure and oxidized spherical carbon is discussed, also referring to and elaborating on experimental work of this thesis. Palladium deposition to pure spherical carbon During a 5 wt% palladium catalyst impregnation of non-functionalized PBSAC material (500 μm, 1.18 cm 3 g -1 ) with palladium chloride solution, a very interesting observation was made. The palladium salt was instantly transformed to elemental palladium when contacting the PBSAC surface. A lustrous palladium shell catalyst resulted. The following explanation was found, which directly correlates to literature reports on catalyst preparations based on graphitic carbon materials (see section ): As the non-functionalized PBSAC materials exhibit a large number of unsaturated C-π sites at the surface, their significant reduction potential has obviously transformed virtually all metal ions into metal immediately at first contact and, thus, immobilized the metal at the outer carbon shell. Palladium deposition to oxidized spherical carbon By oxidation treatment of the carbon material, the number of C-π sites is substantially reduced. After surface oxidation, the predominant mechanism of, for instance, palladium deposition to spherical carbon is ion adsorption. This is is apparently indicated by the absence of lustrous palladium metal at the outer surface of the final catalysts, which are prepared by applying oxidized spherical carbons. Furthermore, at least for nitric acid pretreated palladium catalysts, the results of catalytic experiments on the influence of surface functionalization (see section ) indicate that a certain amount of C-π sites remained, thus noticeably influencing metal deposition with regard to metal distribution and metal dispersion. In case of poor surface functionalization, unsaturated C-π sites remain in larger amounts at the carbon surface. Because C-π sites in the spherical carbon outer shell are reached at first, palladium distribution will primarily be located within this outer shell. Metal dispersion will be lower than in case of homogeneously distributed palladium. But, easier mass transport due to shorter diffusion distances in egg shell catalysts can balance catalytic performance. With increasing degree of surface functionalization, metal dispersion increases due to a lower number of C-π sites. Electrostatic adsorption of palladium salt to protonated surface oxides then is the primary mechanism of metal deposition. A more homogeneous metal distribution then negatively affects mass 140

150 5. Results and discussion transport and, thus, catalytic activity. While the catalyst pretreated with 15% nitric acid at RT exhibited a poor metal dispersion and a low catalytic activity (see section on the influence of surface functionalization), the amount of C-π sites likely was too large leading to large palladium clusters located in the spherical shell. With increasing oxidation conditions, metal dispersion increased. Oxidation with 36% nitric acid at RT apparently formed the best compromise between metal dispersion and metal distribution, because higher catalytic activity was achieved. Metal dispersions between 50-55% of more strongly oxidized samples indicate a more homogeneous metal distribution utilizing the whole carbon surface for metal deposition with increasing influence of pore diffusion limitation, thus decreasing the effective reaction rate. By controlling the number of C-π sites through specific carbon surface treatments, these additional adsorption sites can be used to tailor metal deposition by affecting metal distribution and metal dispersion. Such a variable is unique to graphitic carbon materials and poses another advantage of spherical carbon over inorganic catalyst supports. Platinum deposition to oxidized spherical carbon Concerning the catalytic performance of 0.5 wt% platinum on spherical carbon, the influence of surface functionalization was significantly larger than for the prepared 5 wt% palladium catalysts (compare sections and ). An interpretation is possible by discussing the presence and absence of C-π sites. For the 5 wt% palladium on spherical carbon materials, only a small fraction of palladium salt is instantly reduced by remaining C-π sites, forming larger palladium clusters. Most palladate ions adsorb to the charged functional surface groups, eventually leading to small palladium clusters. In the case of the 0.5 wt% platinum catalysts with the same oxidized support, the same absolute amount of platinum salt will be sacrificed at the reducing C-π sites. In a relative view, a much smaller quantity of salt remains for ion adsorption. Consequently, as demonstrated for a sulfuric acid treated material, a well oxidized carbon surface without any C-π sites allows for high dispersions of noble metals, especially at low metal loadings. 141

151 5. Results and discussion Table 5.16.: Palladium and sulfur leaching of two different spherical catalysts and a commercial powder catalyst with two different durations of ultrasonic treatment Catalyst Ultrasonication time / h Leached palladium fraction / ppm w Leached sulfur concentration in solution / ppm w 5 wt% 200 μm PBSAC 5 wt% 500 μm PBSAC (1 kg scale-up) 5 wt% PAC (commercial) Stability tests Catalyst leaching Palladium leaching tests have been established as a method to determine completeness of metal reduction after hydrogen treatment. Leaching tests were conducted as described in section 4.4. With 1 and 3 h, two different ultrasonication durations were applied. In addition to metal concentration, the sulfur content was analyzed in order to investigate potential effects of sulfur that is present in spherical carbon from PBSAC synthesis. Here, 5 wt% palladium on 200 μm highly activated spherical carbon (see section ), the 500 μm scale-up catalyst (also see section ) and a commercial palladium powder catalyst were tested for leaching. The PBSAC samples were oxidized with nitric acid. The results are listed in table Comparing palladium and sulfur leaching of each catalyst at different ultrasonic treatments, the differences were very small. The amount of leached substance remained almost constant at increasing ultrasonication time. All catalysts exhibited very low palladium leaching with ratios of less than 10 ppm w towards total palladium content. The catalyst based on 200 μm spherical carbon leached the least amount of palladium, closely followed by the commercial powder catalyst. The 500 μm spherical catalyst leached about six times more palladium than the 200 μm material. Concerning sulfur leaching, the amount was significantly lower in case of the 200 μm material compared to the other catalysts. The 500 μm spherical catalyst and 142

152 5. Results and discussion Table 5.17.: Effective reaction rate constants of four subsequent hydrogenation experiments with catalyst recycling # of recycling experiment k eff / m 3 kg Pd -1 s -1 Hydrogenation # Hydrogenation # Hydrogenation # Hydrogenation # the commercial powder catalyst leached notable amounts of sulfur. In conclusion, the effect of extended ultrasonication was negligibly small. All catalysts were well reduced. The scale-up catalyst exhibited more leaching possibly due to nitric acid and hydrochloric acid residues from catalyst preparation, as acids can dissolve palladium. To mitigate this, the material needs to be more intensively washed and dried. The fact that the 200 μm spherical catalyst leached less palladium than the commercial powder catalyst is an advantage in catalytic applications, in which palladium contamination is an issue, e.g. in pharmaceutical synthesis [61, 62]. Also, due to intensive washing and drying, the 200 μm catalyst leached almost no sulfur compared to the 500 μm material. TPD-MS measurements in section also showed that desorption of sulfur species from nitric acid treated surfaces was low. Interestingly, the commercial powder catalyst leached sulfur, as well. But, considering the results in section 5.2.3, catalytic activity of the studied test reactions obviously wasn t inhibited by these sulfur species Catalyst recycling The spent spherical scale-up catalyst (see section ) was recycled and reused for three further cinnamic acid hydrogenations to test its recycling capability. After each experiment, the catalyst was separated from the reaction solution by filtration. The catalyst material was washed with acetone and dried at 120 C and 0.01 mbar vacuum. The reaction parameters of cinnamic acid hydrogenation remained constant throughout the recycling experiment series. The results of cinnamic acid conversion are shown in figure 5.32 with effective reaction rate constants listed in table A significant decrease in rate of cinnamic acid reaction was observed between the hydrogena- 143

153 5. Results and discussion X / % #1 #2 00 Hydrogenation # fits according #4 250 to model 300 mod / s kg Pd m-3 Figure 5.32.: Conversion of cinnamic acid in four subsequent hydrogenation experiments with catalyst recycling; fits according to model applying equation 2.9 (T = 40 C, p H2 = 30 bar, c 0, cinnamic acid = 236 mol m -3, m catalyst = 0.5 g) 144

154 5. Results and discussion tion with fresh catalyst and the first recycling experiment. Concerning the effective reaction rate constant, the decrease amounted to 58%. The third and fourth hydrogenations exhibited similar, slightly decreasing, reaction rates than the second hydrogenation. Apparently, catalytic activity only decreased after the first experiment and remained almost constant in further reuses. The initial decrease is difficult to explain. It can be due to solvent effects or the observed palladium leaching of this scale-up catalyst as demonstrated in the previous section. Nevertheless, this series of experiments demonstrates that palladium catalysts based on spherical carbon can be recycled multiple times. The handling of spherical carbon in catalyst separation and drying is particularly easy, so that almost no precious material is lost in the recycling process. 145

155 X / % #1 Hydrogenation #2 0 fits according to #3 #4 model mod / s mol Rh m-3 (a) 5. Results and discussion S / % #1 Hydrogenation #2 00 fits according to #3 #4 model X / % Figure 5.33.: Conversion of 1,5-cyclooctadiene using a rhodium SILP catalyst that is recycled in three additional experiments with fits according to model applying equation 2.9 (a) and cyclooctene selectivity (b) (T = 60 C, p H2 = 30 bar, c 0 = 229 mol m -3 ) Proof of concept: SILP catalyst in slurry phase reaction The SILP concept, as introduced for gas adsorption and catalysis applications, was also applied to prepare a homogeneous rhodium catalyst immobilized on spherical carbon. The SILP catalyst preparation procedure is presented in section The SILP catalyst with an ionic liquid loading of 20 vol% with respect to the pore volume was tested in the hydrogenation of 1,5-cyclooctadiene. See section for details of the test reaction. After the initial hydrogenation at 60 C, the catalyst was separated from the reactant solution by filtration and dried at 50 C and 0.1 mbar. This SILP catalyst was reused in three recycling experiments at 60 C and three further temperature variation experiments. See figures 5.33 and 5.34 for respective results. The initial hydrogenation experiment showed that 1,5-cyclooctadiene was readily converted. Full conversion was quickly reached. After the first recycling, the initial reaction rate remained in a similar range, but full conversion wasn t approached anymore. The reaction rate leveled off at around 90% conversion. Though, full conversion was approached again in the next recycling experiment. The last hydrogenation run exhibited a larger reaction rate than the initial experiment. Here, full conversion was soon reached. So, the prepared rhodium SILP catalyst remained fully active in four consecutive hydrogenation runs. Up to 80% substrate conversion, selectivity of the intermediate product cyclooctene varied between (b) 146

156 ln c / ln(mol m -3 ) mod / s mol Rh m-3 (a) 5. Results and discussion 60 C T 70 C linear = 80 C 90 C fits ln k eff / ln(m 3 s -1 mol -1 ) Rh T -1 / K -1 (b) keff = f(t) linear fit Figure 5.34.: Conversion of 1,5-cyclooctadiene at temperatures between C reusing the rhodium SILP catalyst from previous recycling experiments (a) and Arrhenius plot of reaction rate constants (b) (p H2 = 30 bar, c 0 = 229 mol m -3 ) 47-67%. Then, cyclooctene molecules were also fully hydrogenated according to the reaction mechanism. This series of experiments demonstrates that spherical carbon immobilized well the ionic liquid catalyst solution. Leaching of catalyst complex into the reactant solution didn t occur. The large internal surface area of spherical carbon effectively immobilized the solution. Also, in this reaction system, the nonpolar reactant solution doesn t strongly interact with the immobilized polar ionic liquid and the dissolved polar catalyst complex due to different molecule polarities, thus further preventing leaching of the ionic liquid solution. More detailed future studies need to show that the homogeneous catalyst stays in a homogeneous form inside of the spherical carbon. The spherical SILP catalyst was easily separated from the reactant solution. The catalyst was stable in air and water atmosphere. Further analysis is necessary to explain the apparently reversible deactivation of catalyst in the second experiment. Also, the role of ionic liquid needs to be investigated, i.e. leaching and the influence towards catalytic activity and selectivity. In literature, a nickel on silica catalyst coated with the ionic liquid [C 4 C 1 IM][n-C 8 H 17 OSO 3 ] didn t exhibit leaching of ionic liquid during hydrogenation of 1,5-cyclooctadiene [208]. Also, ionic liquids are known to increase the yield of the intermediate cyclooctene [208]. Temperature variation experiments showed a trend of increasing catalytic activity with in- 147

157 5. Results and discussion creasing reaction temperature (see figure 5.34-a). Due to the demonstrated ability of SILP catalyst for recycling, the same catalyst was justifiably reused in this experimental study. The activation energy was derived from initial reaction rate constants applying Arrhenius law (see figure 5.34-b). Thus, the activation energy amounted to around 37 kj mol -1. This low value indicates pore diffusion influences due to the large particle size of spherical carbon and its low activation degree. The effect of particle size on pore diffusion limitation of 1,5-cyclooctadiene was investigated in literature in more detail [208]. Nevertheless, the prepared rhodium SILP catalyst based on 500 μm spherical carbon performed well in the hydrogenation of 1,5-cyclooctadiene. Material handling was simple due to the large particle size, air and water stability, and the absence of leaching. The product was easily separated from the catalyst by filtration. The SILP concept with PBSAC supports seems to be principally suitable to immobilize homogeneous catalysts by pure adsorption for application in slurry phase reactions. Previously, homogeneous catalyst complexes have been successfully immobilized to solid support materials by covalent bonding, as well [123]. 148

158 5. Results and discussion 5.3. Spherical carbon as support for SILP filter materials Preliminary investigations for the advancement of SILP technology in gas purification Influence of impregnation solvent on SILP product quality In some breakthrough measurements of SILP materials with ammonia and hydrogen sulfide conducted by Blücher GmbH, Erkrath, with reference to DIN EN ABEK1, undesired solvent artifacts appeared. Traces of residual ethanol were interfering with gas sensors and negatively affecting breakthrough times. Apparently, ethanol wasn t always sufficiently removed during SILP preparation, despite intensive drying under vacuum and elevated temperature. To mitigate solvent artifacts, acetonitrile was chosen as an alternative organic solvent. Acetonitrile is a toxicologically well-known solvent in chemical industry. As an advantage, acetonitrile exhibits a larger vapor pressure than ethanol (344 hpa instead of 293 hpa at 50 C), so that drying of SILP materials is less energy-intensive.[166] All tested ionic liquids (e.g. 1-ethyl-3-methylimidazolium chloride [C 2 C 1 IM]Cl, 1-ethyl-3-methylimidazolium bromide [C 2 C 1 IM]Br) and applied metal salts (e.g. CuCl 2, CuBr 2, ZnCl 2, and ZnBr 2 ) are well soluble in acetonitrile. After SILP preparation, the ionic liquid melts were completely deposited within the pore system of spherical carbon. The ammonia and hydrogen sulfide breakthrough times remained unaffected, indicating a similar distribution of ionic liquid. Solvent artifacts didn t appear during breakthrough measurements. Consequently, acetonitrile became the solvent of choice in SILP preparation for this application Influence of metal salt species on ammonia and hydrogen sulfide adsorption Well performing SILP materials based on spherical carbon have already been previously prepared for the irreversible adsorption of ammonia an hydrogen sulfide (see section ). Concerning further optimization of SILP materials for irreversible gas adsorption, many parameters can be tuned. But, attention was centered to parameters having the biggest potential impact. So, it was most relevant to understand the different underlying mechanisms and dependencies. One focus was set to the effectiveness of the ionic liquid film in gas cleaning. It was found that metal salts dissolved in ionic liquid were most responsible to react away hazardous gases at ambient temperature (see section ). Copper(II) salts generally 149

159 5. Results and discussion Table 5.18.: Ammonia and hydrogen sulfide adsorption capabilities of spherical carbon coated with [C 2 C 1 IM]Br-CuBr 2 1:1.3, [C 2 C 1 IM]Br-ZnBr 2 1:1.3, and [C 2 C 1 IM]Br CuBr 2 -ZnBr 2 1:0.65:0.65 in reference to previously tested [C 2 C 1 IM]Cl-CuCl 2 1:1, all with α IL =0.2 according to DIN EN ABEK1 (T=23 C, d adsorber =5 cm, h filling =2 cm, v air =0.1 m s -1, c=1000 ppm, rh=70%) Metal salt Metal content / 10-4 mol ml SILP -1 Ammonia breakthrough time / min CuBr ZnBr CuBr 2 -ZnBr CuCl [16] 83 [16] Hydrogen sulfide breakthrough time / min performed well in previous ammonia and hydrogen sulfide breakthrough experiments (see section ). But, are copper(ii) salts the most suitable metal salts for application in SILP materials? Several publications in the literature support this assumption (see section 2.3.2). In a short experimental series, the proposed superiority of copper(ii) salts was validated by comparing two SILP materials containing copper(ii) bromide and zinc(ii) bromide, respectively. A mixture of both salts was also tested for possible synergistic effects. SILP preparations were carried out according to standard procedures. Table 5.18 lists the corresponding molar metal contents and breakthrough times of adsorption experiments with ammonia and hydrogen sulfide. Also, the breakthrough times of a previously characterized SILP material with copper chloride impregnation is included. Ammonia capacities of the tested materials lay in a narrow range with breakthrough times between min, with the copper bromide material having a larger breakthrough time of 150 min. Adsorption capacities of hydrogen sulfide were large for both SILP materials containing only copper bromide or copper chloride with breakthrough after 260 and 83 min. The zinc bromide material exhibited a significantly lower breakthrough time of only 4 min. The mixture of copper bromide and zinc bromide had a negligibly larger breakthrough after 19 min. Apparently, ammonia complexation works similarly well with different transition metal salts. The metal salt mixture resulted in no significant synergetic effects. Ammonia capacity depends on the molar content of transition metal ions solved in ionic liquid. The copper bro- 150

160 5. Results and discussion mide material performed better than the copper chloride material due to a higher solubility of the bromide salt in the melt hydrate and consequently a higher copper utilization. On the contrary, hydrogen sulfide reactivity is more sensitive towards the transition metal cation and its anion. As proposed in literature, copper(ii) reacts more strongly with sulfides than zinc(ii) (see section 2.3.2). In fact, copper(ii) salts generally are more reactive than other transition metal salts due to their large complex formation constants. Interestingly, in a mixture of copper bromide and zinc bromide, zinc bromide appeared to inhibit sulfide complex formation. Also, copper bromide obviously is significantly more reactive than copper chloride. Consequentially, further development of SILP materials for ammonia and hydrogen sulfide adsorption was focused on copper(ii)-containing systems Corrosion investigations of halide-containing SILP materials In the following, the aspect of SILP corrosivity is discussed. It s well known that halide salts are corrosive to metals. Concerning the electrochemical series, a noble metal cation also oxidizes a less noble metal. In industrial applications, adsorber materials must not corrode instruments and process periphery usually made of stainless steel. Also, filter media for personal protection purposes are usually encapsulated in aluminum canisters. Because, in SILP materials, the halide ionic liquids and metal halides are immobilized within the spherical carbon, the critical compounds are not in immediate contact to the metal container. Thus, it was experimentally analyzed, if these halide and copper containing SILP materials were corrosive to aluminum and stainless steel. Corrosion tests of aluminum foil and stainless steel were conducted with different SILP materials at elevated relative humidity. The degree of corrosion is listed in table Corrosion of aluminum was most pronounced in the case of the copper bromide based SILP material. Here, the aluminum foil dissolved completely. The copper chloride system resulted in a partial dissolution of aluminum foil, while the copper bromide and zinc bromide mixture produced minor corrosion at the surface. The zinc bromide based material didn t noticeably affect the appearance of aluminum foil. In case of stainless steel, it was observed that, after 7 days, the spherical SILP particles with zinc bromide and a mixture of copper and zinc bromide sticked to the stainless steel platelets. No corrosion was noticed for the other SILP materials. After further addition of water and another 7 days, signs of corrosion accompanied with a significant mass loss were observed for the stainless steel platelets contacted with copper bromide and copper chloride SILP materials. In general, copper(ii) salts appeared to be more corrosive than zinc(ii) salts and bromide 151

161 5. Results and discussion Table 5.19.: Corrosion investigations of spherical carbon coated with [C 2 C 1 IM]Br-CuBr 2 1:1.3, [C 2 C 1 IM]Br-ZnBr 2 1:1.3, [C 2 C 1 IM]Br-CuBr 2 -ZnBr 2 1:0.65:0.65, [C 2 C 1 IM] Cl-CuCl 2 1:1, and [C 2 C 1 IM]Cl-ZnCl 2 1:1.3, all with α IL =0.2 in aluminum foil at 65-75% rh and 20 C for 7 days and with stainless steel (X5CrNi18-10) at 60-90% rh and 20 C for 14 days (+: no corrosion, -: slight corrosion, : corrosion, : strong corrosion) Metal salt Aluminum corrosion Stainless steel corrosion CuBr 2 ZnBr 2 + n/a CuBr 2 -ZnBr CuCl 2 more aggressive than chloride. The results of the corrosion experiments demonstrate that halide-containing SILP materials were corrosive, even though the critical substances were immobilized inside the spherical carbon. But, a minor amount of impregnation substance can still be located on the outside of spherical carbon. Thus, it is in direct contact to aluminum or stainless steel. This was strongly indicated by the sticking of spherical SILP material to the stainless steel platelets. Another possibility of corrosion is the release of hydrogen bromide or hydrogen chloride gas traces due to ionic liquid melt decomposition. This is another strong argument for halide-free materials in personal protection applications. For other applications, the excellent performance of this generation of SILP materials can still be leveraged by using plastic containers. Even in case of undetectable corrosion, the psychological effect of the presence of halides to process engineers would also be a relevant fact to be taken into account. Also, corrosion may occur at elevated temperature and humidity, which has not yet been tested. Consequently, alternative halide-free SILP materials are of high relevance and should be developed Filter material improvements for irreversible adsorption of ammonia and other hazardous gases Development of halide-free SILP materials For halide-free systems, the commercially-available ionic liquid 1-n-butyl-3-methyimidazolium octylsulfate ([C 4 C 1 IM][n-C 8 H 17 OSO 3 ]) was chosen, because it s well characterized and 152

162 5. Results and discussion its physical properties suit the application. It has a melting point between 34 and 35 C. Thermal decomposition starts at around 341 C. The ionic liquid is hydrolysis-stable at 80 C.[209] The octyl-group of the anion decreases the hygroscopic character compared to previously used halide ionic liquids. The ionic liquid is well soluble in many solvents, including ethanol, water, acetonitrile and methylenechloride. The low melting point and good thermal and chemical stabilities are advantageous in gas separation processes at room temperature and above. As metal salt, copper(ii) sulfate pentahydrate was initially used. It is soluble in [C 4 C 1 IM] [n-c 8 H 17 OSO 3 ] up to a molar ratio of 1:1.4. The best solvent for this copper salt is water. The limited solubility of copper sulfate in organic solvents necessitated the impregnation of spherical carbon with water. This had an immediate advantage. In contrast to organic solvents, water didn t need to be completely removed from the SILP material. This eliminates time and energy intensive drying processes in the final stage of SILP preparation. Nevertheless, water is known to penetrate hydrophobic activated carbon less deeply than organic solvents like alcohols and acetone [8]. This results in an inhomogeneous distribution of the ionic liquid within the spherical carbon. Surface areas for absorption and adsorption can be affected. For initial investigations, SILP materials with [C 4 C 1 IM][n-C 8 H 17 OSO 3 ]-CuSO 4 (5H 2 O) 1:1.4 based on spherical carbon were prepared according to standard procedures using water as solvent. SILP materials with different ionic liquid loadings between were obtained and characterized. The quality of impregnation was determined by nitrogen sorption experiments at Blücher GmbH, Erkrath (see table 5.20). Corrosion tests were carried out (see table 5.21). The ammonia separation efficiency was analyzed in breakthrough measurements at different relative humidity (see figure 5.35 and table 5.22). Additionally, the materials broadband capacities were characterized by cyclohexane and hydrogen sulfide breakthrough measurements (see table 5.23). After preparation of SILP materials with ionic liquid loadings of 0.2 and 0.3, white particles and powder were visible in the material. Also, the glass flask was coated with a white substance. It appeared that ionic liquid was partially precipitating outside the spherical carbons pore system during water removal in the rotary evaporator. Likely, the solubility of copper sulfate in water was limiting ionic liquid deposition inside spherical carbon as copper sulfate already precipitated in the bulk solution. In this case, a modification of the SILP preparation method should be developed to prevent ionic liquid precipitation to the outside of spherical carbon and, thus, improve product quality. Also, access to the pore system possibly is al- 153

163 5. Results and discussion Table 5.20.: Pore characteristics derived from nitrogen sorption experiments of PBSAC #1 coated with the halide-free ionic liquid melt [C 4 C 1 IM][n-C 8 H 17 OSO 3 ] CuSO 4 (5H 2 O) 1:1.4 and of PBSAC #2 coated with the halide ionic liquid melt [C 2 C 1 IM]Cl-CuCl 2 1:1.3, both with α IL =0.2, in reference to the pure 500 μm PB- SAC support materials PBSAC #1 and PBSAC #2 with varying pore structure Material V tot / cm 3 V micro / cm 3 cm -3 cm -3 MP BET area / m 2 cm -3 Loss in V tot / % PBSAC #1 (V tot = 1.33 cm 3 g -1 ) Halide-free PBSAC #1 PBSAC #2 (V tot = 1.18 cm 3 g -1 ) Halide PBSAC # ready blocked by solid copper sulfate preventing further inclusion of ionic liquid. The pore filling was investigated in nitrogen sorption experiments. Table 5.20 lists the remaining pore volume and other properties derived from nitrogen sorption experiments, also for a halidebased SILP reference material. The two SILP materials differ in the used activated carbon support material. To allow direct comparison, pore properties were correlated to the same filling volume applying bulk densities. Normalized to the bulk density, the two PBSAC support materials varied only slightly in micro pore volume and BET surface area. After impregnation, pore volume and surface area decreased. The loss in total pore volume was significantly lower for the copper sulfate based ionic liquid than for the copper chloride based melt. With a decrease of 24% in pore volume, the copper sulfate based melt occupied roughly its own volume. There is no indication of blocked access to empty pores, as postulated earlier. Despite utilization of water instead of ethanol during impregnation, the distribution of this ionic liquid in spherical carbon appeared to be more homogeneous than the chloride-based melt. A clear conclusion of this data is difficult, because several parameters were varied, i.e. PBSAC support material, ionic liquid and impregnation solvent. Corrosion experiments of SILP material with [C 4 C 1 IM][n-C 8 H 17 OSO 3 ]-CuSO 4 (5H 2 O) 1:1.4 were conducted as described in section The results are shown in table

164 5. Results and discussion Table 5.21.: Corrosion investigations of spherical carbon coated with [C 4 C 1 IM][n-C 8 H 17 - OSO 3 ]-CuSO 4 (5H 2 O) 1:1.4 with α IL =0.2 in aluminum foil at 65-75% rh and 20 C for 7 days and with stainless steel (X5CrNi18-10) at 60-90% rh and 20 C for 14 days (+: no corrosion, -: slight corrosion, : corrosion, : strong corrosion) Metal salt Aluminum corrosion Stainless steel corrosion CuSO At room temperature and elevated relative humidity, this halide-free SILP material exhibited no corrosion on aluminium foil and stainless steel platelets. Thus, it s potentially suitable for industrial applications. Results of ammonia breakthrough measurements are presented in figure Characteristic quantities of each breakthrough measurement are listed in table The adsorption rates were derived from the slopes of breakthrough curves at the inflection points. The molar breakthrough capacity describes the utilization of copper species at breakthrough. It s the ratio of adsorbed ammonia molecules over the number of cuprate ions in the SILP material. The assumption is that one cuprate ion forms complexes with four ammonia molecules. In case of the 10 vol% ionic liquid loading, breakthrough of ammonia appeared between 150 and 200 min. The breakthrough was delayed with increasing relative humidity. The maximum ammonia concentration was quickly reached for all three curves. A very similar behavior was observed with the SILP material loaded with 20 vol% ionic liquid. The breakthrough times were between min, increased with increasing humidity and were followed by quick saturation. The 30 vol% loaded material exhibited breakthrough times between min. Here, breakthrough time and slope of the breakthrough curve decreased with increasing relative humidity. Doubling the ionic liquid loading from 0.1 to 0.2 increased breakthrough capacity by %. Further increasing the loading by 50%, led to an increase of capacity by %. With increasing ionic liquid loading, absorption rates decreased. For each ionic liquid loading, ammonia absorption rates were generally similar. Only for the 30 vol% loaded spherical carbon, adsorption rates continuously decreased with increasing relative humidity. The molar breakthrough capacity of ammonia was between 61-78% in case of ionic liquid loadings between For a loading of 0.3, the molar breakthrough capacity reached maximum values between 86-99%. Generally, these SILP materials exhibited large ammonia breakthrough capacities and a low influence of relative humidity compared to previously tested halide-containing SILP materials [16]. The fast ammonia saturations after breakthrough indicate fast adsorption kinetics 155

165 5. Results and discussion c c -1 / rh 25% 50% fits= 80% rh 25% 50% fits= 80% t / min t / min (a) (b) rh 25% 50% fits= 80% c c -1 / - 0 c c -1 / - 0 t / min (c) Figure 5.35.: Ammonia breakthrough measurements of spherical carbon coated with [C 4 C 1 - IM][n-C 8 H 17 OSO 3 ]-CuSO 4 (5H 2 O) 1:1.4 with (a) α IL =0.1, (b) α IL =0.2 and (c) α IL =0.3 at different relative humidity; fits according to equation 2.22 (T=30 C, p=1.2 bar, d absorber =1.8 cm, h filling =2 cm, v N2 =0.02 m s -1, c NH3 =1000 ppm) 156

166 5. Results and discussion Table 5.22.: Ammonia absorption characteristics of spherical carbon coated with [C 4 C 1 IM]- [n-c 8 H 17 OSO 3 ]-CuSO 4 (5H 2 O) 1:1.4 at different ionic liquid loadings and relative humidity (T=30 C, p=1.2 bar, d absorber =1.8 cm, h filling =2 cm, v N2 =0.02 m s -1, c NH3 =1000 ppm) Ionic liquid loading / - Relative humidity / % Breakthrough time / min Absorption rate / 10-3 min -1 Molar breakthrough capacity / %

167 5. Results and discussion and good mass transfer properties of the SILP materials in a broad range of relative humidities. Also, utilization of available copper species until breakthrough was very large. Increasing ionic liquid loading from 0.1 to 0.2 led to a proportional increase of breakthrough time with similar molar breakthrough capacities. A further increase of ionic liquid loading to 0.3 resulted in an almost complete copper utilization until breakthrough. It can be hypothesized that, at low ionic liquid loadings, ionic liquid and copper sulfate partially segregate in the pore structure due to chromatographic effects. The ionic liquid [C 4 C 1 IM][n-C 8 H 17 OSO 3 ] adsorbes more deeply into micropores and/or the center of the carbon spheres, while copper sulfate remains in the outer shell, as already observed during SILP preparations. REM-EDX measurements over the cross section of SILP carbon spheres could be conducted to show cuprate and sulfate distributions clarifying this hypothesis. At an ionic liquid loading of 0.3, a sufficiently large amount of ionic liquid likely dissolved copper sulfate best. The breakthrough capacity and absorption rate of this SILP material slightly decreased with increasing relative humidity due to water absorption, volume expansion and thus a reduced boundary layer for ammonia absorption. Concerning broadband capability, the copper sulfate based SILP material with an ionic liquid loading of 0.2 was tested with multiple gases. The results are summarized in table For comparison purposes, the adsorption capacity of pure spherical carbon for each type of gas and the DIN EN ABEK1 requirements are listed, as well. The SILP material had capacities for cyclohexane, hydrogen sulfide and ammonia. Compared to pure spherical carbon, breakthrough times of cyclohexane decreased and capacities of hydrogen sulfide and ammonia significantly increased. Chlorine capacities weren t determined, but are expected to decrease by around 20%, in correlation to previous measurements [16]. The ABEK1 requirements were exceeded in case of hydrogen sulfide and ammonia adsorption and, even though not specifically tested for this SILP material, very likely for chlorine, as well. For cyclohexane, only 79% of the required capacity was reached. While ammonia and hydrogen sulfide react with the cuprate ions in the ionic liquid melt, cyclohexane physically adsorbs to the carbon surface. Because the ionic liquid melt occupies a certain amount of carbon surface, cyclohexane capacity decreases with increasing ionic liquid loading. As chlorine also adsorbs directly to the carbon surface, its capacity decreases with increasing ionic liquid loading, as well. Thus, slightly decreasing ionic liquid loading will result in a broadband filter meeting the ABEK1 requirements of the four gas classes. In summary, SILP materials based on copper sulfate adsorb large amounts of ammonia. They perform equally well at different relative humidities and are promising candidates for broad- 158

168 5. Results and discussion Table 5.23.: Broadband capabilities of spherical carbon coated with [C 4 C 1 IM][n-C 8 H 17 O- SO 3 ]-CuSO 4 (5H 2 O) 1:1.4 with α IL =0.2 in reference to pure spherical carbon according to DIN EN ABEK1 (T=23 C, d adsorber =5 cm, h filling =2 cm, v air =0.1 m s -1, c=1000 ppm, rh=70%) Class of gas Test gas ABEK1 requirement / min Pure spherical carbon breakthrough time / min Organic Cyclohexane Inorganic Chlorine [16] n/a Acidic Hydrogen sulfide 20 1 [16] 39 SILP breakthrough time / min Ammonia Ammonia 50 1 [16] 63* * Ammonia breakthrough time adapted from experiment with different measurement conditions band filters. Corrosion is no issue with these halide-free systems. SILP preparation still is a challenge, because copper sulfate partially precipitates outside the spherical carbon pore system. So, dust particles are released and the material cannot be processed to foam structures, for instance Alternative coating techniques of spherical carbon Alternative impregnation techniques of spherical carbon with [C 4 C 1 IM][n-C 8 H 17 OSO 3 ] CuSO 4 (5H 2 O) 1:1.4 have been tested to prevent precipitation of copper sulfate outside the spherical carbon. Two incipient wetness approaches were evaluated, with each utilizing a significantly lower amount of water compared to classical wet impregnation. See section on catalyst impregnation for differences between incipient wetness and wet impregnation processes. In the first approach, the standard impregnation procedure was slightly modified, so that water only amounted to 90% of the carbon s total pore volume. In the second approach, the impregnation solution (also: V = 0.9 V pore ) was slowly added dropwise to a continuously circulating mass of spherical carbon inside a rotary evaporator. The impregnation results are depicted in figure As already discussed earlier, white copper sulfate particles precipitated during solvent removal in the wet impregnation process. With the incipient wetness impregnation, spherical 159

169 5. Results and discussion (a) (b) (c) Figure 5.36.: Images of spherical carbon coated with [C 4 C 1 IM][n-C 8 H 17 OSO 3 ]-CuSO 4 (5H 2 O) 1:1.4 with α IL =0.2 applying different impregnation approaches: standard wet impregnation (a), incipient wetness impregnation (b) and dropwise impregnation (c) 160