DOCTORAL THESIS. Structured Molecular Sieves. Valeri Naydenov. Department of Chemical and Metallurgical Engineering Division of Chemical Technology

Transcription

1 2003:39 DOCTORAL THESIS Structured Molecular Sieves Synthesis, Modification and Characterization Valeri Naydenov Department of Chemical and Metallurgical Engineering Division of Chemical Technology 2003:39 ISSN: ISRN: LTU - DT / SE

2 LULEÅ UNIVERSITY OF TECHNOLOGY Structured Molecular Sieves Synthesis, Modification and Characterization Valeri Naydenov Division of Chemical Technology Department of Chemical and Metallurgical Engineering Luleå University of Technology SE Luleå, Sweden 2003

3 SUMMARY The work presented concerns synthesis of structured molecular sieves. The preparation procedure is based on the use of ion exchange resins as macrotemplates. Macroporous cation or anion exchange resin types were used depending on the molecular sieve type to be synthesized. Anion exchange resin beads were employed as macrotemplates in the synthesis of zeolite type molecular sieves since negatively charged precursor species are present in the initial zeolite synthesis solutions. Cation exchange resins were used for the preparation of aluminophosphate molecular sieves, where precursor species in the synthesis solutions are positively charged. The method involves a number of steps, which can be summarized as: (i) introduction of precursor species into the resin by ion exchange; (ii) secondary treatment to transform the precursor into a desired phase; and (iii) removal of the ion exchanger by calcination. The first two steps can be performed simultaneously by hydrothermally treating the resin with synthesis solution, where inorganic material is initially ion exchanged into the resin and transformed into a zeolite upon hydrothermal treatment. The steps (i) and (ii) may also be separated by firstly treating the resins with precursor solutions and then converting the exchanged species into a molecular sieve by secondary treatment with additional components and/or structure directing agents. Upon calcination the ion exchange resin and zeolite structure directing agents are removed leaving porous molecular sieve structures with a shape determined by the shape of the resin macrotemplate. The calcined macrostructures can be employed in secondary synthesis for the preparation of composite macrostructures containing two types of molecular sieves. Modified molecular sieve macrostructures were also prepared by the method. These macrostructures correspond to molecular sieve macrostructures iii

4 SUMMARY containing various transition metal oxides. The modification is performed as a post-synthesis procedure. It is based on the fact that as synthesized resinmolecular sieve composites retain a certain degree of ion exchange capacity, which facilitates the introduction of charged metal ions. The procedure was demonstrated for both: (i) metal cations, which were exchanged into the cation exchange resin-molecular sieve composite; and (ii) negatively charged metal species, which were exchanged into anion exchange resin-molecular sieve composites. Upon calcination of a complex metal precursor-resin-molecular sieve composite modified molecular sieve macrostructures were obtained. The properties of non-modified and modified molecular sieve macrostructures were extensively characterized by various methods (X-ray diffraction, scanning electron microscopy, Raman and UV-vis DRS spectroscopy, nuclear magnetic resonance, nitrogen adsorption and chemisorption measurements) to study the crystallization mechanism, sample crystallinity and morphology, nature of the metal species, etc. The non-modified molecular sieve macrostructures are of interest for application as catalysts and catalyst supports, adsorbents and packing materials, whereas modified macrostructures are of interest mainly in the area of catalysis. Keywords: Molecular sieves; Silicalite-1; ZSM-5; Zeolite Beta; AlPO-5; MCM-41; Ion exchange resins; Macrostructures; Modification; Vanadium; Tungsten; Chromium; Palladium. iv

5 LIST OF PAPERS This thesis is based on the following papers, referred to in the text by Roman numbers: I Meso/macroporous AlPO-5 spherical macrostructures tailored by resin templating Valeri Naydenov, Lubomira Tosheva, Oleg N. Antzutkin and Johan Sterte Submitted to Chem. Mater. II III IV V ZSM-5 crystallization of preformed amorphous aluminosilicate beads Valeri Naydenov, Lubomira Tosheva and Johan Sterte To be presented at the 14 th International Zeolite Conference, Cape Town, South Africa, April 2004, accepted for publication in the proceedings. Self-bonded zeolite Beta/MCM-41 composite spheres Valeri Naydenov, Lubomira Tosheva and Johan Sterte Submitted to J. Mater. Chem. Spherical silica macrostructures containing vanadium and tungsten oxides assembled by the resin templating method Valeri Naydenov, Lubomira Tosheva and Johan Sterte Microporous Mesoporous Mater., 2002, 55 (3), 253. Vanadium modified AlPO-5 spheres through resin macrotemplating Valeri Naydenov, Lubomira Tosheva and Johan Sterte Microporous Mesoporous Mater., 2003, 66 (2-3), 321. v

6 LIST OF PAPERS VI VII Palladium-containing zeolite beta macrostructures prepared by resin macrotemplating Valeri Naydenov, Lubomira Tosheva and Johan Sterte Chem. Mater., 2002, 14 (12), Chromium containing zeolite beta macrostructures Valeri Naydenov, Lubomira Tosheva and Johan Sterte Published in the proceedings of 2nd FEZA Conference, 1-5 September 2002, Taormina, Italy, Stud. Surf. Sci. Catal., 142B, p vi

7 CONTENTS INTRODUCTION SETTING UP THE SCENE ION EXCHANGE RESINS MOLECULAR SIEVES STRUCTURED MATERIALS OVER MULTILEVEL LENGTH SCALES Top-down strategy Bottom-up strategy Other methods MOLECULAR SIEVE SUPPORTED CATALYSTS...16 EXPERIMENTAL SYNTHESIS PROCEDURE MODIFICATION PROCEDURE CHARACTERIZATION METHODS X-ray diffraction Scanning Electron Microscopy Nitrogen adsorption Flame atomic absorption spectrometry Raman and UV-vis absorption spectroscopy Nuclear magnetic resonance Chemisorption measurements...24 RESULTS AND DISCUSSION MOLECULAR SIEVE MACROSTRUCTURES Description of the method...25 vii

8 CONTENTS Aluminophosphate macrostructures Zeolite and composite molecular sieve macrostructures Macrostructures used in the modification procedure MODIFIED MOLECULAR SIEVE MACROSTRUCTURES Description of the procedure Modified molecular sieve macrostructures: general characterization Nature of the metal species in the modified macrostructures...41 CONCLUSIONS...45 FUTURE WORK...49 ACKNOWLEDGEMENTS...51 REFERENCES...53 PAPER I-VII viii

9 LIST OF FIGURES Figure 1. 1: Schematic representation of zeolite based acid catalyst structures over various length scale levels (micro- (<2 nm), meso- (2-50 nm) and macro- (>50 nm) regions are distinguished on the basis of common pore size classification)....2 Figure 1. 2: Styrene divinylbenzene copolymer...4 Figure 1. 3: Functionalized fragments of styrene-divinylbenzene based strong cation (a) and strong anion (b) exchange resins...4 Figure 1. 4: Schematic representation of gel (a) and macroporous (b) resin types...5 Figure 1. 5: MFI structure....7 Figure 1. 6: *BEA structure...7 Figure 1. 7: AFI structure...10 Figure 1. 8: MCM-41 structure...11 Figure 3. 1: Figure 3. 2: Schematic illustration of the procedures for the preparation of molecular sieve macrostructures using ion exchange resin beads as macrotemplates SEM images of cation exchange resin beads (a), resin-alpo-5 composite beads (b), calcined AlPO-5 spheres (c) and sphere surface (d)...27 ix

10 TABLE OF FIGURES Figure 3. 3: Figure 3. 4: Figure 3. 5: Figure 3. 6: Figure 3. 7: Figure 3. 8: Figure 3. 9: XRD pattern of calcined AlPO-5 macrostructures (a), adsorption-desorption isotherm (solid symbols adsorption, open symbols desorption) of the same sample (b) and corresponding BJH pore size distribution curve (insert in (b))...28 Single pulse 31 P (a) and 27 Al (b) solid-state NMR spectra of the calcined alumino-phosphate macrostructures obtained for various treatment times (indicated on the figure)...29 SEM images of anion exchange resin (a), calcined zeolite Beta/MCM-41 composites (b), calcined amorphous aluminosilicate spheres (c) and calcined ZSM-5 spheres (d)...31 XRD patterns (a), adsorption-desorption isotherms (solid symbols adsorption, open symbols desorption) (b) of the initial aluminosilicate beads (0 h) and spheres hydrothermally treated (28 h). Insert in (b) gives the corresponding BJH pore size distribution curves Raman spectra of samples prepared with NaOH for different treatment times: non-calcined (a) and calcined (b); 0 h corresponds to the initial beads aged in TPAOH...33 SEM images of the interior of the spheres prepared for 12 h (a) and 28 h (b)...33 XRD patterns (a), adsorption-desorption isotherms (solid sym-bols adsorption, open symbols desorption) (b) of the initial zeolite Beta beads and zeolite Beta composite spheres. Insert in (b) gives the corresponding BJH pore size distribution curves...34 Figure 3. 10: XRD patterns of the calcined S-1 of varying crystallinity (a), AlPO-5 and zeolite Beta (b) molecular sieve macrostructures used for modification Figure 3. 11: SEM images of the calcined molecular sieve macrostructures used for modification (a-d) and the corresponding sphere surfaces (a -d )...36 Figure 3. 12: Schematic representation of the procedure for the preparation of modified molecular sieve macrostructures Figure 3. 13: Nitrogen adsorption/desorption isotherms (solid symbols adsorption, open symbols desorption) for the V modified AlPO-5 macrostructures (displacement with 400 on the Y- axis for V3 sample) (a) and corresponding BJH pore size distribution plots (b)...40 x

11 TABLE OF FIGURES Figure 3. 14: XRD patterns of V, W, Cr and Pd modified macrostructures (*-peaks corresponding to V 2 O 5, WO 3, Cr 2 O 3 and PdO, respectively)...40 Figure 3. 15: Typical SEM image of a cross-sectioned Cr-Beta sphere (a) and typical EDS line scan analysis of chromium over it (b) Figure 3. 16: Raman spectra of V and W modified molecular sieve macrostructures: (a) Raman spectra representative for V in amorphous (spectrum 1), semi-crystalline (spectrum 2) and silicalite-1 (spectrum 3) spheres; (b) W in amorphous (spectrum 1) and on semi-crystalline and silicalite-1 (spectrum 2) spheres...41 Figure 3. 17: Raman spectra of calcined pure and vanadium-modified AlPO-5 samples (V loading increases from V1 to V3)...42 Figure 3. 18: UV-vis DRS spectra of V, W, Cr and Pd modified molecular sieve macrostructures: spectra corresponding to the samples of low V, W and Cr loading (a) of high V, W, Cr loading and Pd containing sample (0.5 wt.%) (b). The asterisks denote bands of the V 2 O 5, WO 3 and Cr 2 O 3 oxides, respectively...42 xi

12 LIST OF TABLES Table 2. 1: Resins used in this work Table 2. 2: Table 2. 3: Synthesis solutions, resin types, obtained macrostructures and reference papers...20 Composite types, metal salts used for the preparation of metal precursor solutions, modified macrostructures obtained by the procedure and reference papers...21 Table 3. 1: Table 3. 2: Table 3. 3: Properties of the calcined molecular sieve macrostructures used for modification and ion exchange capacities of the non-calcined resin-molecular sieve composites Metal precursor solutions and properties of some of the calcined modified molecular sieve macrostructures Pd loadings and some properties of the calcined Pd modified zeolite Beta macrostructures xiii

13 Chapter 1 INTRODUCTION 1. 1 SETTING UP THE SCENE This introductionary section describes the conception of structured materials in view of the work to be presented. The scope of the thesis is then defined. Recently there is an increasing interest in the field of design and preparation of so called structured porous materials. In general, the term structure refers to something arranged in a defined pattern of organization. For instance, it can be the arrangement of single particles in a substance, parts in a body, aggregates in an entity, etc. The structure then defines not only the arrangement of the constituents but also their relationships to each other. In addition learning from nature, the organization on a single level is often not sufficient for the functionality of a given system. It is rather the hierarchical organization that gives the specific functions e.g. proteins, which are not just a sequence of linked aminoacids but they form several higher order structures in order to operate in the living species. The hierarchy principle is also valid for synthetic inorganic materials, where it is used to design material with new specific function(s) or to tailor material properties in such a way that the materials formed conform better to the application requirements. Fig gives a schematic representation of how a zeolite based acid catalyst may be depicted on various length scale levels. Selecting suitable composition and synthesis conditions materials with crystalline framework containing channels and cavities may be obtained. The catalytic sites that are present in the crystals are spatially separated and accessible only for molecules that are able to fit into the pores. Therefore, designing the material on molecular size scale represents a level of organization. However, there are many catalytic sites in a single crystal which dimensions and morphology can also be a target for design thus giving a 1

14 INTRODUCTION Figure 1. 1: Schematic representation of zeolite based acid catalyst structures over various length scale levels (micro- (<2 nm), meso- (2-50 nm) and macro- (>50 nm) regions are distinguished on the basis of common pore size classification). second level of organization. Prior to application in e.g. fixed bed reactor zeolite crystals are assembled in shaped objects (spheres, extrudates, tablets) adding another level of organization. In this example, although preserved, the hierarchy principle does not yield a material with specific function as a result of the multilevel organization but rather contributes to the more efficient utilization of the catalyst. The term structure in its general definition can be applied to all organizational levels presented. The present work focuses on an alternative method for the preparation of shaped molecular sieves. Therefore we will refer to the obtained materials as structured molecular sieves or simply as macrostructures (macroscopic objects with sizes greater than 0.1 mm). Processing of the molecular sieves into macroscopic objects with various shapes is required since the synthetic molecular sieves are obtained as fine powders. Conventional methods used for the preparation of shaped objects involve mixing of the molecular sieve powder with a binding material (typically amorphous inorganic oxides) followed by forming processes such as spray drying, granulation, extrusion, etc. [1,2]. Binding material provides the object with mechanical stability but also dilutes the molecular sieve and/or partially blocks the pore openings thus slowing the mass transport to and from the internal pore architectures. Recently an alternative method for the preparation of binderless molecular sieve macrostructures was reported [3]. The method is conceptually based on the utilization of ion exchange resins as macrotemplates. The possibilities of the method were demonstrated for the preparation of silicalite-1 (S-1), zeolite Beta and ZSM-5 molecular sieve macrostructures using anion exchange resins as macrotemplates [3-5]. Ion exchange properties of the resins are used to introduce molecular sieve 2

15 1. 2 ION EXCHANGE RESINS nutrients into the resin pore structure, where they crystallize upon hydrothermal treatment. The ion exchange resin as being an organic material is removed in a subsequent step by calcination. An objective of the present work was to further extend the possibilities of the method for the preparation of other types of molecular sieve macrostructures or to further optimize synthesis of the existing ones. AlPO-5 macrostructures were obtained using cation exchange resins as macrotemplates thus opening a route for the preparation of aluminophosphate molecular sieves in self-bonded form. The original method was modified in search for lowering the coast of the procedure yielding ZSM-5 macrostructures. The possibility for the preparation of macrostructures containing more than one type molecular sieve was explored by the synthesis of Beta/MCM-41 composites. Another objective was to further develop the method for to produce modified macrostructures. It was found that the resin in as synthesized resinmolecular sieve composite retains a certain degree of residual ion exchange capacity, which can be used for the introduction of other component(s) possessing different functions e.g. transition metals. V, W, Cr and Pd containing macrostructures were prepared in the present work using either anion or cation exchange resin-molecular sieve composites [6]. Thus, after resin removal by calcination the obtained macrostructures are referred throughout the text as modified molecular sieve macrostructures. An objective was also to extensively characterize both non-modified and modified macrostructures thus evaluating the possibilities of the method. In the following introduction sections a short description of ion exchange resins and molecular sieves will be given. The literature survey on recent developments of methods for preparation of structured materials over multilevel length scales and conventional approaches used for the preparation of molecular sieve based catalysts intends to introduce the reader to the current trends in these subjects. This would help in the evaluation of the merits of the materials prepared in this work ION EXCHANGE RESINS Generally, ion exchange resins are referred to as cross-linked organic polymers carrying fixed functional groups [7,8]. The framework of the ion exchange resins is called matrix. It is a macromolecular three-dimensional (in most cases irregular) network of hydrocarbon chains. Functional groups are charged acidic, basic or chelating groups attached to the polymer matrix. The charge of each group is compensated by a mobile ion, which can be replaced by a stoichiometric equivalent of other ions having the same charge thus giving rise to ion exchange properties of the resin. Therefore, the carrier of exchangeable 3

16 INTRODUCTION cations is referred to as a cation exchange resin and a carrier of exchangeable anions is referred as an anion exchange resin. In the synthesis of the ion exchange resins first a three-dimensional crosslinked matrix is obtained. At present, the matrix of the most widely used resins is a result from an ethenylbenzene (styrene) and diethenylbenzene (divinylbenzene, DVB) copolymerization CH CH CH 2 CH 2 CH CH CH 2 CH 2 CH CH CH 2 CH 2 reaction initiated by a benzoyl peroxide as catalyst. A common technique used for the resin production is the suspension polymerization. By this technique the two immiscible monomers, styrene and DVB are mixed and dispersed as spherical droplets in an aqueous suspension, where they react forming discrete beads. If only styrene is polymerized Figure 1. 2: Styrene-divinylbenzene copolymer. linear polystyrene chains are obtained. The addition of DVB ensures the cross-linking of the polystyrene chains. Fig shows the cross-linked styrene-divinylbenzene copolymer. Varying the amount of DVB added to the reaction mixture, the degree of cross-linking can be adjusted in a reproducible manner. The degree of cross-linking impacts both the swelling ability and mechanical properties of the obtained resins. Introducing functional groups into the copolymer matrix results in the corresponding resin type. Thus treatment of the matrix with hot sulfuric acid introduces sulfonic functional group leading to a cation exchange resin (Fig. 1. 3a). An introduction of basic functional groups into the copolymer matrix requires two steps. First, chloromethyl ( CH 2 Cl) groups are introduced into the styrene nuclei by a Friedel-Crafts reaction. In the second step, the chloromethylated copolymer is reacted with aliphatic amines of various degree of substitution. Fig. 1. 3b shows a typical anion exchange resin with quaternary ammonium functional groups. The terms strong acid and strong base in their conventional electrolyte chemistry meaning i.e. CH CH 2 CH CH 2 + CH 2 N(CH 3 ) 3 Cl acid dissociates to a hydrogen cation (H + ) and base to a hydroxide anion (OH - ) may be applied to describe the obtained resins. Thus, resins depicted in Fig. 1. 3a and Fig. 1. 3b can be referred to as strong cation and strong anion exchange (a) SO 3 - H + (b) Figure 1. 3: Functionalized fragments of styrene-divinylbenzene based strong cation (a) and strong anion (b) exchange resins. 4

17 1. 2 ION EXCHANGE RESINS resins, respectively. The ion exchange reaction involves two basic steps shown with the following equations written for a strong cation exchanger (bar is the resin phase and R denotes copolymer matrix). hydration + RSO H H O RSO + H 3 anhydrous strong dissociation RSO ion exchange H + Aaq RSO + A + H aq hydrated The ion exchange capacity is probably one of most important characteristic that is specified for a given ion exchanger. Depending on the application area in which the resin will be used, different definitions for the exchange capacity are accepted. One of the widely used is the dry weight capacity (DWC). It is defined as the total equivalents of exchangeable ion (accepted standard ionic forms are H + and Cl - for cation and anion exchange resins, respectively) per dry kilogram of resin. The suspension polymerization technique described above yields so Figure 1. 4: Schematic representation of gel (a) and macroporous (b) resin types. called gel type resins (Fig. 1. 4a). The main characteristic of these resins is the absence of permanent pore structure. Their pores are very small, usually not greater than 30 Å, determined by the distance between the polymer chains and cross-links and are accessible only in the presence of an appropriate solvent. The suspension polymerization can be performed in such way that the resulting resins consist of two continuous phases, a continuous pore phase and a continuous gel phase. This type of resins is referred to as macroporous or macroreticular resins (Fig. 1. 4b). The difference in the preparation is that the monomers are firstly dissolved in an inert diluent, which is immiscible with the continuous phase. This mixture is then added to the continuous phase, where it is distributed in the form of droplets. The copolymerization and cross-linking occurs into the monomer-diluent droplets. After the separation of the resulting beads the diluent is extracted or evaporated thus leaving an open pore structure within the resin bead. The pores are prevented from collapsing by the high degree of cross-linking of the polymer phase. The polymeric phase is structurally composed of small spherical micro-gel particles that form agglomerates. Joining together these agglomerates results in common interfaces with the interconnected pore system of the final resin bead. Careful choice and control over the synthesis parameter may lead to macroporous ion exchange resins with a wide range values for the surface area and pore size distributions. 5

18 INTRODUCTION 1. 3 MOLECULAR SIEVES The term molecular sieve describes porous materials, which are able to selectively separate molecules on the basis of their size or shape [9,10]. A common classification of molecular sieve materials includes: (i) the family of the microporous crystalline solids, which comprises aluminosilicates (zeolites), allsilica molecular sieves, aluminophosphates and related materials; (ii) mesoporous solids from the M41S family; (iii) pillared clays, and (iv) non-crystalline solids such as charcoals and metal oxides [9,10]. In this work the term will be used to describe different members of the micro- (i) and mesoporous (ii) families. Zeolites are crystalline microporous hydrated aluminosilicate materials. Generally, their structure can be regarded as an inorganic polymer built from tetrahedral TO 4 units, where T is a Si 4+ or Al 3+ ion [10]. Each O atom positioned at the corners of a tetrahedral unit is in fact shared between two T atoms, thus forming an infinitely extended three-dimensional network. The formed zeolite framework structure contains channels and/or interconnected voids of discrete size (in the range Å). Typically, the chemical composition of a zeolite is represented by empirical formula, which written in an oxide form would be: M 2/n O Al 2 O 3 xsio 2 yh 2 O where M is the counterion, n is the counterion valence, x accounts for the SiO 2 /Al 2 O 3 ratio and y is the water content in the hydrated form the zeolite. The presence of a counterion is required, since each tetrahedral aluminum ion in the framework position generates one negative charge, which needs to be compensated. Typically, counterions are elements from the IA and IIA groups of the periodic table. The counterions are located into the zeolite channels and/or cavities in extra framework positions. They can reversibly be exchanged for other ions possessing the charge of same sign thus providing the material with ion exchange properties. The acidic form (Brønsted acidity) of the corresponding zeolite may be obtained, if the counterions are first exchanged for ammonium cations and the material is subsequently calcined. The water can reversibly be removed leaving an intact open structure. Zeolites can be classified based on different criteria. For instance based on framework Si/Al ratio zeolites are categorized as: (i) low Si/Al zeolites (ratio less then 2); (ii) intermediate Si/Al zeolites (Si/Al=2-5); (iii) high Si/Al zeolites (Si/Al from about 10 to 100) and (iv) all-silica molecular sieves. The Si/Al ratio is an important characteristic of each zeolite type. A gradual change of Si/Al ratio alters the properties of the synthesized zeolite. Thus increasing the Si/Al ratio the thermal stability, acid strength and hydrophobicity are increasing, whereas the ion exchange capacity decreases. In the extreme case, all-silica 6

19 1. 3 MOLECULAR SIEVES molecular sieves have a neutral framework, have no ion exchange capacity or catalytic activity and are hydrophobic. According to another common classification zeolites can be discriminated on the basis of their pore size (the number of T atoms forming the ring openings) as small (8-member ring), medium (10-member ring) and large (12-member ring) pore zeolites. The topology of the zeolite framework is defined by a unique three-letter code. In this work molecular sieves of the MFI and *BEA (the asterisks in front of the code denotes a disordered framework) structure types were synthesized [11]. The threeletter code is not related to the com- Figure 1. 5: MFI structure. position of the zeolite. Thus, ZSM-5 and S-1 are molecular sieves with a MFI topology depicted in Fig (each intersection of the lines corresponds to Si or Al atom with oxygen atom halfway between). Materials with this topology have a three-dimensional pore system consisting of sinusoidal 10-ring (5.1 x 5.5 Å) and perpendicularly intersecting straight 10-ring (5.3 x 5.6 Å) channels (latter shown in the figure). The difference between the ZSM-5 and S-1 is the aluminum content. The Si/Al ratio for ZSM-5 is usually in the range , which refers it to high silica zeolite category. The presence of Al provides the material with ion exchange properties and/or catalytic functions. The S-1 on the other hand has only negligible amounts of Al or no Al mainly depending on the grade of the reagents used in the synthesis. Thus the S-1 has a hydrophobic framework, which makes it more effective as adsorbent in e.g. organic separations. Therefore, it is more correct if the S-1 is referred to as molecular sieve rather than Figure 1. 6: *BEA structure. a zeolite. Zeolite β is a large pore (12-membered ring openings) high silica zeolite with *BEA topology, which has a three-dimensional channel system (5.5 x 5.5 and 6.4 x 7.6 Å) (Fig. 1. 6). Conventional zeolite synthesis involves hydrothermal treatment of a 7

20 INTRODUCTION reactive alkaline aluminosilicate mixture. The aluminosilicate mixture is commonly prepared by mixing the sources of silica, alumina and alkali hydroxides or/and organic bases. Thus prepared mixtures are highly heterogeneous and are usually referred to as synthesis gels. It is also possible to employ so-called clear synthesis solutions, which are characterized by a high degree of homogeneity (no macroscopic solid phases). Synthesis gels or clear synthesis solutions are then transferred into autoclaves and are kept for a certain interval of time (ranging from a few hours to a several days) at elevated temperatures (usually C). The pressure in the autoclave is autogeneous and depends on the mixture composition and the oven temperature. The temperature, alkalinity (ph), composition of the reaction mixture, nature of the reactants and pretreatment conditions can all affect the crystallization kinetics and the type of zeolite which forms. Often, a zeolite synthesis requires the addition of organic compounds such as quaternary ammonium salts, amines, alcohols, etc. to the synthesis mixture. The possible roles of the organic additives can be generalized as: (i) space filling-simply occupies void, around which solid crystallizes; (ii) structuredirecting agent (SDA), when the presence of a certain organic yields a solid with specific framework; (iii) template, when lattice of the resulting solid reflects the geometry of the organic additive [12]. After the synthesis, the organic additives remain in the channels of the material and can only be removed by calcination (temperatures above 400 C). Zeolites are widely used as detergents, drying agents, in gas purification processes, in separation processes such as n-paraffins from branched paraffins, p-xylene from its isomers etc., as catalysts and catalyst supports in petroleum refining processes and production of fine chemicals [13,14]. Often, zeolites are subject of various post-synthesis modifications with the purpose of conforming to the requirements of the application processes. Post-synthesis modifications are used to vary the Si/Al ratio (by steam treatment, acid leaching, ammonium fluorosilicate, EDTA and SiCl 4 treatment), to further control the size of the pore openings (e.g. by the addition of large organometallic species, which cannot diffuse into the pore system), to change the nature of the acid sites within the pores by adsorption of other materials, etc. Aluminophosphates represent another major class of microporous molecular sieves [5,10]. These solids contain phosphorous (present as P 5+ ion), which is four-coordinated and aluminum (as Al 3+ ion) with coordination number four, five or six. Usually, four of the aluminum coordination positions are filled by oxygen atoms that serve as bridges to the framework phosphorous and eventual fifth and sixth ligand positions are occupied of non-framework species such as hydroxo, aqua, fluoro, phosphate, etc. In the idealized case 8

21 1. 3 MOLECULAR SIEVES (neglecting the secondary coordination of alumina) the aluminophosphate structure can be represented as a neutral three-dimensional framework built of corner shared strictly alternating AlO 4 and PO 4 tetrahedra. Based on this generalized understanding these materials are often designated as AlPO-n, where the acronym AlPO reflects framework composition (Al/P=1) and n is a number that refers to a specific crystallographic structure. Similarly as zeolites, aluminophosphates are often classified on the basis of their pore size i.e. the number of T (Al or P) atoms forming the opening that provides access to the pore structure. Small, medium and large pore materials can be distinguished containing 8-ring, 10-ring and 12-ring apertures, respectively, whereas materials with openings greater than 12-ring are regarded as an extra-large (e.g. VPI-5 (18-ring), JDF-20 (20-ring)). Due to the rich coordination chemistry of aluminum in the AlPO-n molecular sieves in certain cases they are regarded as: (i) four coordinated AlPO-n (all framework Al is four coordinated); (ii) AlPO-n hydrates (Al atoms at specific crystallographic sites are six coordinated and have two aqua ligands); (iii) AlPO-n hydroxides (specific Al atoms are five or six coordinated and have as an additional ligands OH, OH and H 2 O or two H 2 O); (iv) AlPO-n phosphates (phosphate ions are trapped in a systematic way in specific cages as an extra-framework species). Synthesis of microporous crystalline AlPO-n involves hydrothermal treatment of a reactive gel containing alumina and phosphorous sources, an organic compound and water. A typical gel composition would be: R Al 2 O 3 P 2 O 5 xh 2 O where R is an organic compound, usually amines or quaternary ammonium salts and x varies in a wide range (from ten to above thousand). Orthophosphoric acid is typically used as a source of phosphorous, whereas pseudoboehmite and alumina alkoxide are utilized as Al sources. The hydrothermal treatment of the reactive gel is performed in an autoclave at elevated temperature ( C) for treatment times from a few hours to several days. The presence of alkali ions is detrimental for the synthesis of AlPO-n members. The ph of the starting gel is adjusted within the range Lower ph than 3 results in non-porous aluminophosphate solids, whereas a ph higher than 10 significantly decreases the yield of microporous AlPO-n. The organic compound has a dual role in the AlPO-n synthesis: (i) it is used to control the ph of the synthesis gel and (ii) it is essential for the synthesis of the microporous aluminophosphate. The latter is in contrast to the zeolite synthesis, where some of the zeolites can be synthesized in the absence of organic additive. However, a common feature with zeolites is that the additive is encapsulated within the material cavities during the synthesis and can experience different type of interaction with the host framework. It can act just as a space-filler often as non-dissociated ion pair such as tetraalkylammonium 9

22 INTRODUCTION hydroxide molecules. In this case the number of the additive molecules per structural unit depends on their nature, which may explain the large number of organics that can be employed in the synthesis of one type of AlPO-n (e.g. AlPO-5). In the other case the trapped molecule carries a positive charge, which is coupled with the negative charge of AlPO-n framework (e.g. AlPO-n hydroxides). For this kind of interaction a stoichiometric relationship between the additive and the structural element (such as cavities, repeating distances, etc.) can be observed. Three-letter codes are also ascribed to the aluminophosphate molecular sieves. In this work one of the Figure 1. 7: AFI structure. most studied members of the microporous AlPO-n family was synthesized, namely AlPO-5. It is a molecular sieve with an AFI type structure characterized by the parallel channels uniform in diameter (0.73 nm) and running along the c-direction of the crystals (Fig. 1. 7) [11]. The report on the preparation of solids with well-defined pores in the mesopore range (M41S family) gave rise to a new branch of the molecular sieve materials [15]. Although, the area is relatively new, these materials are extensively studied since it is anticipated that they will be materials of choice in the processing of very bulky molecules, where microporous molecular sieves would not be operative. Firstly, all-silica members denoted as MCM-41, MCM- 48 and MCM-50 of this family have been synthesized. These molecular sieves possess regular pores usually in the range 15 to 100 Å. Similarly as for microporous molecular sieves organic additives are used in their synthesis. As such surfactant molecules, typically C 12 -C 16 trimethylammonium salts are utilized, where direct correlation between the chain length and the pore size of the resulting solid has been found. An essential feature in the use of surfactants is that in an appropriate concentration and solvent (typically aqueous solutions) they arrange into supra-molecular aggregates with different interface curvature called micelles. It is possible under these conditions to force inorganic precursor species (e.g. silicates) to condense between thus formed micelles. As a result an inorganic-micelle entity is formed, which after calcination yields an inorganic open pore structure. Thus produced solids have a regular pore system but the pore walls are amorphous. Depending on the synthesis conditions the shape of micelle changes and thus the resultant pores of the synthesized material have 10

23 1. 4 STRUCTURED MATERIALS OVER MULTILEVEL LENGTH SCALES different shapes. For example for the synthesis of MCM-41, which has regular channels packed in a hexagonal fashion (Fig. 1. 8), the synthesis conditions in surfactant-silicate system are adjusted in such a way that micelles with cylindrical shape are favored. The MCM-48, which has two three-dimensional networks mutually intersecting, is synthesized at synthesis conditions, where micelles with spherical shape are generated. A stabilized lamellar MCM-50 phase is prepared at synthesis conditions, where laminar surfactant aggregates are favored. Mesoporous molecular sieves can be synthesized at room temperature as well as under hydrothermal conditions with synthesis times generally shorter than for microporous molecular sieves. The pore sizes can further be tailored by an addition of auxiliary organic compounds often referred to as swelling agents (e.g. trimethylbenzene). When present in the synthesis solution these agents prefer to occupy the center of the micelle, thus forcing them to increase in size. Different scientific groups have created various pathways for the synthesis of mesoporous molecular sieves mainly differing in the type of or- Figure 1. 8: MCM-41 structure. ganic additives used. However, in the majority of cases the obtained solids resemble the structural characteristics of the described above M41S members. One of the main drawbacks that still challenges scientists is the limited hydrothermal stability of the mesoporous molecular sieves, which is mainly due to the amorphous character of the pore walls STRUCTURED MATERIALS OVER MULTILEVEL LENGTH SCALES The preparation of materials with complex hierarchical structures is a challenging task for material chemists. Of particular interest are materials with a structured pore system over multilevel length scales. Such materials are expected to meet the optimal balance between the diffusion and confinement regimes, e.g. micro- and mesopores can provide size and shape selectivity towards guest molecules, whereas macropores should improve mass transport to the active sites. As it was stated in the opening section 1. 1 it is also often desirable to produce molecular sieves in the form of shaped objects. Strategies used for the preparation of hierarchically structured materials are often referred to as top-down or bottom-up strategies. 11

24 INTRODUCTION Top-down strategy Typically in this type of approaches an entity with structural organization on a certain level is utilized as starting material and involves processing steps that yield additional structural features at organizational levels of a lower rank. Conceptually using this strategy it should be possible to transform a pre-shaped object into a molecular sieve object while it retains its initial shape. However, this type of processing would be in contrast with the conventional molecular sieve syntheses, where hydrothermal treatment of reactive gels is involved. One solution can be for instance a method commonly denoted at present as dry gel conversion (DGC) [16]. Initially it has been demonstrated that a ZSM-5 powder can be prepared from an amorphous aluminosilicate gel under action of a vapor mixture containing water and volatile SDA s such as alkyl-amines. The method was denoted as vapor phase transport (VPT) [17]. The difference from the conventional molecular sieve syntheses is that the gel is not in a direct contact with the liquid phase, which has been achieved by an appropriate autoclave arrangement. The method is regarded as an alternative to hydrothermal methods in terms of reducing the coast of synthesis and generating less waste for disposal. The VPT method was extended for the synthesis of zeolites, which require non-volatile SDA s. In this synthesis the dry gel also contains organic additives such as quaternary ammonium hydroxides and the crystallization is realized only in presence of steam. This type of synthesis is referred to as steam assisted conversion (SAC) [16]. A number of aluminophosphates and their silica counterparts have been synthesized using both modifications of the DGC method, namely VPT and SAC [22]. Zeolite types such as MFI, MOR and FER have been synthesized using the VPT method [16]. In regard to the top-down strategy the method has first been utilized for the preparation of various zeolite membranes, ANA, FER, MOR, MFI and MOR-CHA composite membrane [16,18]. For these syntheses the amorphous aluminosilicate gel is deposited on the substrate, typically by dip coating, dried and exposed to the action of vapor mixture. A hierarchical porous zeolite material has been produced using a slightly modified VPT method [19]. In this synthesis natural diatomite was used as substrate. The difference with the standard VPT method is that the diatomite was seeded before the vapor treatment. As a result highly crystalline disc-shaped MFI structures have been obtained, which still retain macropores due to the substrate structure and micropores due to the formed MFI phase. Self-bonded MFI zeolite type pellets have been synthesized using the SAC variation of the DGC method [20]. In this syntheses the dry gels were processed into pellets before the steam treatment. The presence of certain amounts of Li + 12

25 1. 4 STRUCTURED MATERIALS OVER MULTILEVEL LENGTH SCALES in the pellets was indispensable for the retaining the macroshape. Using a similar approach, pellets containing MOR and MEL zeolite types have also been prepared [21]. A similar approach as DGC referred to as pseudomorphic synthesis has been utilized for the preparation of mesoporous MCM-41 spheres [23]. Preshaped commercial silica spheres have been treated with a NaOH/surfactant mixture for a certain period of time at room temperature and then autoclaved yielding pseudomorphic MCM-41 spheres. A few more examples can be mentioned here, which demonstrate the topdown strategy. Thus disc-shaped aluminosilicate gel has been used for the preparation of a disc-shaped MFI type zeolite through a solid-state reaction [24]. Porous glass beads have been transformed into beads containing MFI-type zeolite under the treatment with mixture containing Al source, NaOH and SDA [25]. A honeycomb body of mulite has been converted into a macroporous ZSM-5/mulite composite under hydrothermal treatment with mixture of NaOH and tetrapropylammonium bromide [26] Bottom-up strategy The bottom-up strategy is more often used for the preparation of materials with hierarchical structure in comparison with the top-down strategy. It is a stepwise synthesis that utilizes different structure controlling agents at each level with an increasing rank. In the syntheses of structured porous materials these agents are typically various sized removable templates. In this sub-section the emphasis is on the different type of templates that pattern the porosity in the macropore range (common molecular and supra-molecular templates used for the synthesis of micro- and mesoporous materials were described in section 1. 3). Colloidal crystal templating. Colloidal suspensions of polystyrene (PS or latex) spheres have the ability to assemble into e.g. close packed three-dimensional arrays upon slow sedimentation, centrifugation, etc. Thus formed arrays are known as colloidal crystals. Recently it has been reported that these colloidal crystals can be used as templates for the preparation of materials with ordered macropores [27]. For this synthesis the PS spheres are first assembled on a porous membrane by filtration, surface modified with surfactant and impregnated with the inorganic precursor (Si solution). Under these conditions the Si polymerizes in the interstitial space between the spheres. Removal of the PS spheres by calcination or dissolution yields Si flakes with periodic macroporous system. Controlling the size of the spheres used as templates lead to an ultimate control over the size of the pores in the inorganic solid. The voids of the colloidal crystal can also be infiltrated with larger building blocks such as nanosized particles. Gold flakes have been prepared using this approach [28]. Colloidal crystals have been grown in aqueous droplets suspended on 13

26 INTRODUCTION fluorinated oil yielding millimeter sized particles with various shapes such as spheres, ellipsoids, toroids, etc. controlled by alternation of the droplet composition [29]. In another similar approach the shape control has been achieved by performing the synthesis in the presence of electric field [30]. Compositional diversity of the macroporous materials produced by the colloidal crystal templating has been demonstrated by the preparation of Al 2 O 3, TiO 2, ZrO 2, Fe 2 O 3, Sb 4 O 6, WO 3 and mixed oxides such as yttria-stabilized zirconia [31,32]. Recently assemblies of two-dimensional binary colloidal crystals have been reported, which are expected to be useful as new generation of colloidal crystal templates for materials with hierarchical pore structure [33]. A layer-by-layer technique has been applied using colloidal templates to produce hollow S-1 spheres [34-38]. PS sphere surfaces were charge modified by cationic polymer before adsorption of S-1 nano-sized building blocks. The sequence of polymer-s-1 layer pairs can be repeated a number of times. Removal of the PS templates yields stable hollow S-1 spheres. S-1 monoliths have also been prepared using a combination of colloidal crystal assemblies and layer-by-layer technique [39-40]. Thus PS spheres with adsorbed S-1 seeds on the surface were allowed their close packing under slow sedimentation. Hydrothermal treatment of the closely packed layered spheres with a S-1 synthesis solution results in a monolithic structure, which after the PS and SDA removal posses regular macrocavities. Emulsion templating. This approach is based on the fact that droplets in a stable mono-disperse emulsion tend to self-assemble into nearly crystalline arrays. The general idea is to deposit an inorganic material at the exterior of the droplets in such an emulsion. Titania honeycomb structures with regular macropores have been synthesized using an oil-in-formamide emulsion to pattern the porosity [41]. The method allows a precise pore size control by controlling the droplet sizes. Millimeter-sized silica beads with hierarchical porosity have also been prepared by emulsion templating [42]. Vehicle templating. Surfactants or three-block copolymers with a carefully selected hydrophilic/hydrophobic ratio in solution at certain conditions tend to form large double-layered sheets. When the size of these sheets is large and the concentration is low they tend to close into a bubble-like shape, which is called vehicle. Using this approach silica-coated surfactant vehicles have been obtained by hydrolysis and condensation of silicon alkoxide at the vehicle interface surface [43]. A continuos macroporous niobium oxide has also been synthesized by vehicle templating [44]. Bio-templating. The wealth of materials with complex forms and hierarchical porosity produced by nature is well known. A simple approach to prepare synthetic materials with similar structural organization is to use suitable biomaterials as templating, agents thus obtaining their negative replicas. Various 14

27 1. 4 STRUCTURED MATERIALS OVER MULTILEVEL LENGTH SCALES macroporous inorganic structures have been prepared using bacterial superstructural templates [45,46]. In these syntheses bacterial fragments have been co-aligned into a macroscopic threads by slow drawing from the web-culture. Thus obtained super-structure resembles the arrangement of hexagonal packed cylinders. Silica sols or S-1 colloidal solutions have been utilized for the bacterial thread infiltration in order to obtain amorphous or crystalline microporous replicas of the thread assemblies [45,43]. Treatment of the bacterial threads with a MCM-41 synthesis solution yielded a macrostructured mesoporous material [45]. Lotus-shaped silica structures have been synthesized by sol-gel transcription of sugar based super-structures [47]. Zeolitic tissue has been prepared using the wood cell templating [48]. In these syntheses wood pieces were first seeded with nano-sized zeolite crystals followed by secondary growth from a zeolite synthesis solution, which after calcination yielded a selfbonded zeolitic structure. Sponge-like zeolite membranes and monoliths have also been synthesized using cellulose acetate filter membrane and starch gel as templating agents, respectively [49,50]. Multi-scale templating. Although, some of the above given examples describe materials structured on more than one level a few more syntheses should be mentioned in order to demonstrate the trend towards solids with hierarchical structures through templating. Porous silica, niobia and titania oxides have been patterned over multiple length scales using different templates responsible for the structural features on each level [51]. Three-block polymer (<10 nm), PS spheres (>10 nm) and polymer micro-mold (with imprinted features >0.1 mm) templates were used in these syntheses. Using a similar approach lower structural level was shifted to the micro-region by utilizing zeolite nano-crystals instead of metal alkoxides [52]. A silica monolith has been obtained in another synthesis with hierarchical pore structure on three different levels using PS spheres to produce macropores, block copolymer to pattern mesopores and individual polymer templating for micropores. Micro-/macroporous zeolite monoliths have been synthesized using SDA as a template for microporous zeolite phase, whereas PS or polyurethane foams have been utilized to create macropores [53-55] Other methods A few more approaches should be mentioned here, which contribute to the diversity of options available for the design and preparation of synthetic hierarchically structured materials. Instead of processing powdered molecular sieves into shaped objects in order to improve the mass transport a method for the preparation of large single crystals having mesopores has been developed [56]. In this approach microporous molecular sieves have been synthesized in the presence of carbon 15

28 INTRODUCTION nano-tubes (with diameter 1-20 nm). After the synthesis the carbon was removed by calcination leaving behind mesoporous molecular sieve crystals. Molecular sieves with MFI type topology (ZSM-5 and titanium silicalite-1) have been prepared and tested in catalytic reactions [56-58]. Another interesting approach is to utilize SiO 2 colloidal crystal to create its negative polymer replica [59]. This replica has been used as a mold in order to synthesize a second-generation of colloidal crystals of essentially any solid material. An additional option in this method is that due the plasticity of the polymer replica above the glass transition temperature it was possible to stretch it by applying uniaxial or biaxial forces and under quick cooling the replica retained its deformed state. Thus the shape of the second-generation colloidal particles can be varied from spheres through ellipsoids to discs. A monolith entirely built of zeolite phase has been synthesized by evaporation of the liquid phase of the S-1 sol followed by sintering [60]. The resulting monoliths have discrete micropores (S-1) and mesopores with relatively narrow size distribution (due to the crystal interstices). This method has been pushed further by adding polymer monomers into the zeolite sol and then the sol was cast into a mold, which can be designed into various complex shapes [61]. Thus the obtained monoliths have controlled micro- and mesoporosity and pre-defined shape. Various procedures have been reported for the preparation of molecular sieves having bimodal pore size distributions e.g. micro-/mesopores [62-68]. The general strategy is to prepare a composite material. Methods to develop composite materials of dual pore sizes include in-situ crystallization using a two-template synthesis gel system or only a CTA + template by adjusting the synthesis conditions, two-step crystallization employing dual templating or contacting mesoporous precursors with diluted zeolite gels followed by hydrothermal treatment. Using a composite strategy some improvement in the hydrothermal stability of the mesoporous molecular sieves has been achieved [69] MOLECULAR SIEVE SUPPORTED CATALYSTS This section mainly deals with molecular sieve supported transition metal catalytic systems [13,14]. Generally, these types of catalysts are composed of one or several catalytically active component(s) dispersed on a support. The support provides a large surface area, where the active component(s) is dispersed. However, the catalytic behavior of the supported catalysts has a much more complex character influenced by the nature of the metal species, e.g. oxidation state, coordination condition, dispersion and stability. On the other hand, the support is also of importance, for instance when shape selectivity is 16

29 1. 5 MOLECULAR SIEVE SUPPORTED CATALYSTS concerned, or when chemical and structural features of the support influence the metal dispersion and the ability to stabilize the species hosted. Molecular sieves are often used as catalyst supports. The advantages, which make them attractive as supports are: Controlled crystal sizes and morphologies allowing diffusion control of the catalytic reactions; High surface area and generally high thermal stability (an exception are mesoporous materials, although recently some advances in this respect have also been made [69]); Well-defined pores (specifically for zeolites three types of shape selectivity, towards the reactants, towards the transition (excited) states and towards the products have been recognized [10,13]); Existing pore architectures are accessible for further control by various post-synthesis modifications; Due to the so called cage effect reactions taking place in the presence of molecular sieves are alternative to those performed at elevated pressures. In addition some other options in the catalyst design that may be explored, when zeolites are used as supports: The extra framework ions can be ion exchanged by other cations thus altering e.g. acid/base characteristics; The active surface sites can be modified (selective poisoning ) thus giving priority to reactions taking place only within the inner architectures; The introduction of catalytically active component(s) into molecular sieves can be realized during or after the zeolite synthesis. The metal species may be present in framework positions, grafted to the support by various types of interactions, or as entrapped complexes and small metal oxide crystallites. Thus, V, W and Cr have been incorporated into molecular sieve framework positions by the hydrothermal treatment of synthesis mixtures containing the corresponding metal salts [70-78]. As a general trend using the direct incorporation approach (i) low metal loadings (usually up to 1 wt.%) have been achieved, (ii) lack of reproducibility in terms of effective incorporation of the heteroelement into the silica framework and (iii) lack of dispersion throughout the material. In comparison to zeolites a larger number of metals have been effectively incorporated into the aluminophosphate frameworks [79,80]. For the mesoporous molecular sieves, frameworks entirely built up of transition metal oxides have been reported [81]. Although a specific example, the incorporation of V ions into the framework of zeolite Beta by a post-synthesis incorporation into the lattice defects generated by Al extraction has also been reported [82]. A number of methods have been developed for the post-synthesis intro- 17

30 INTRODUCTION duction of the catalytically active phase(s) into the molecular sieves. The most common method to introduce metal species into molecular sieves is by liquid cation exchange of the charge compensating extra framework cations [83-87]. This technique is primarily used for zeolite based catalysts since they possess ion exchange properties. Due to the fact that the method is based on electrostatic interactions between the support and the metal species, materials with high metal dispersions containing stoichiometric metal amounts are obtained. Solid state ion exchange has also been employed [84-88]. In this method the metal precursor is physically mixed with the zeolite and under heating the metal migrates into the zeolite channels. The incipient wetness impregnation is another widely used technique for the metal modification of molecular sieves. Here, the metal precursor is dissolved in aqueous [89-93] or non-aqueous [94-96] media using volumes equal to the pore volume of the support and contacted with the carrier. For zeolites the impregnation method is also used in the cases when the amount of metal required is greater than their ion exchange capacity. Metal modifications have also been achieved by other methods, such as the adsorption of neutral volatile metal complexes such as carbonyls and halides by means of chemical vapor deposition (CVD) [97-99]. According to the ship-inbottle approach metal ions and metal ligands are introduced into the zeolite cavities and allowed to react [100,101]. As a result, metal complexes entrapped into the zeolite cages are obtained. Such materials have shown enantioselectivity for certain catalytic reactions. All of the techniques described above deal with metal modified molecular sieves. However, in these materials the metal is introduced into the molecular sieve structure either in the framework position or extra framework position but still in the pore system. Only a few reports have been found, where the modification aims to produce transition metal sites at the external surface. These sites have been formed after the molecular sieve synthesis. Thus, large transition metal-organic complexes, which are not able to access the inner pore structure, have been used for the preparation of external surface metal modified aluminophosphates or zeolites [102,103]. Zeolite surface has been also modified using various metal surfactant molecules [104,105]. A common feature for these approaches is that the deposition of the metal precursor on the surface is followed by the calcination step, which yields surface oxide species of the corresponding metal. 18

31 Chapter 2 EXPERIMENTAL The preparation of the various synthesis solutions, chemicals used, and some specifics of the synthesis procedures are discussed in the reference papers and will therefore not be repeated here. In this section an effort is made to describe main paths that lead to the preparation of various molecular sieve macrostructures and post-synthesis modified molecular sieve macrostructures through resin macrotemplating SYNTHESIS PROCEDURE In general, the synthesis procedure for the preparation of molecular sieve macrostructures consists of mixing resin macrotemplates with an appropriate synthesis solution followed by hydrothermal treatment in polyethylene reactors or autoclaves. Cation or anion type ion exchange resins were used in this work depending on the molecular sieve type to be synthesized. Details on resin properties specified by the supplier are listed in Table Synthesis solutions, resin type, obtained macrostructures and corresponding reference papers are Table 2. 1: Resins used in this work. Resin name MSA-1 Amberlite IRA-900 Amberlite IRA-200 Type Strongly basic, macroporous Strongly basic, macroporous Strongly acidic, macroreticular Active group Trimethylbenzyl Quaternary Sulfonic acid ammonium ammonium Ionic form Chloride Chloride Sodium Particle size mesh mesh mesh Moisture % % % Capacity (DWC) 4.1 meq g meq g meq g -1 19

32 EXPERIMENTAL Table 2. 2: Synthesis solutions, resin types, obtained macrostructures and reference papers. Solution type Resin type Macrostructure obtained (1) Locron L+TEACl Macroporous cation AlPO-5 a (2) H 3 PO 4 +TEAOH+H 2 O exchanger (1) Aluminosilicate gel Macroporous anion ZSM-5 a (2) TPAOH+NaOH+H 2 O exchanger (1) Zeolite Beta Macroporous anion Beta/MCM-41 a (2) MCM-41 exchanger Silicalite-1 Macroporous anion Silicalite-1 exchanger Zeolite Beta Macroporous anion Zeolite Beta exchanger a denotes macrostructures obtained in step procedures Paper I, V II III IV VI, VII summarized in Table Macrostructures obtained using multi-step syntheses involve utilization of different types of solutions in each step (I-III, V). For these macrostructures in the first step, component(s) is introduced into the resin yielding resin-solid composite: (i) aluminum was loaded into the resin under hydrothermal treatment with mixture (1) consisting of diluted aluminum chlorohydrate solution (Locron L) and tetraethylammonium chloride (TEACl) to form resin-al composite (I, V); (ii) aluminosilicate gel with the molar composition 5Na 2 O : Al 2 O 3 : 34SiO 2 : 1400H 2 O (1) was used to produce resinaluminosilicate composite (II) and (iii) resin-zeolite Beta composite was obtained under hydrothermal treatment of the resin with a zeolite Beta synthesis solution 0.31Na 2 O : 9TEAOH : 0.5Al 2 O 3 : 25SiO 2 : 295H 2 O (III) (this synthesis is described elsewhere [4]). In the second step, resin-solid composites were used as synthesized or calcined prior the next step. Using as synthesized resin-al composite and mixture of H 3 PO 4, tetraethylammonium hydroxide (TEAOH) and distilled water yielding the together molar composition (TEA) 2 O : Al 2 O 3 : 1.2P 2 O 5 : 100H 2 O, resin-alpo-5 composite was formed. Resinaluminosilicate composite was calcined, mixed with tetrapropylammonium hydroxide (TPAOH) and NaOH solutions and hydrothermally treated to form ZSM-5 macrostructures. Resin-zeolite Beta composite was calcined mixed with MCM-41 synthesis gel with molar composition 1.0SiO 2 : 0.17CTAB : 0.24TEAOH : 30H 2 O and yielded Beta/MCM-41 composite macrostructures under hydrothermal treatment. The other major path of the synthesis procedure leads directly to resinmolecular sieve macrostructure in one step. Thus S-1 and zeolite Beta (the later is the same synthesis used as first step toward zeolite Beta/MCM-41 composites) were prepared in a one step synthesis procedure by simply mixing 20

33 2. 2 MODIFICATION PROCEDURE the resin with the corresponding synthesis solutions followed by a hydrothermal treatment. For S-1 synthesis, a solution with the molar composition 9TPAOH : 25SiO 2 : 480H 2 O : 100EtOH was used. These syntheses were recently reported [3,4]. In the present studies, the as synthesized S-1 or zeolite Beta-resin composites were further used for the preparation of the modified molecular sieve macrostructures (see next section). In order to obtain large sample batches, the described syntheses of S-1 and zeolite Beta were up-scaled in this work. Finally, in order to remove the resin and/or SDA the resin-molecular sieve composite is calcined for 6 or 24 h at 600 C, after heating to this temperature at rate 1 C min -1, leaving behind self-bonded molecular sieve macrostructure. An exception was the Beta/MCM-41 composite, which was calcined at 550 C for 15 h. In the cases when modified molecular sieve macrostructures were to be prepared, only small portions of the resin-molecular sieve composites were calcined as reference samples MODIFICATION PROCEDURE Generally, the modification procedure consists of mixing the non-calcined resin-molecular sieve composite with a solution containing a metal precursor. During this step the metal is ion-exchanged into the resin. Table 2. 3 lists different composite types used, metal salts utilized in the preparation of the metal precursor solutions, obtained macrostructures and the corresponding reference papers. Resin-silica composites, in which the structure of silica varies from amorphous through semi-crystalline S-1 to highly crystalline S-1, were used for the preparation of various vanadium or tungsten modified macrostructures (IV). As synthesized resin-alpo-5 composite was utilized in the synthesis of vanadium modified AlPO-5 macrostructures (V), whereas in the preparation of palladium or chromium modified zeolite Beta macrostructures two types of resin-zeolite Beta composites were used (VI, VII). One type was as synthesized composite assuming that the resin is in OH - ionic form due to the high ph during the zeolite synthesis. Another type was a composite, in which Table 2. 3: Composite types, metal salts used for the preparation of metal precursor solutions, modified macrostructures obtained by the procedure and reference papers. Composite type Metal salt Macrostructure obtained Paper Resin-silica Resin-silica NH 4 VO 3 Na 2 WO 4 2H 2 O V-silica W-silica IV IV Resin-AlPO-5 VOSO 4 V-AlPO-5 V Resin-zeolite Beta Resin-zeolite Beta (NH 4 ) 2 PdCl 6 Na 2 Cr 2 O 7 2H 2 O Pd-Beta Cr-Beta VI VII 21

34 EXPERIMENTAL the resin was converted into the Cl - form after the zeolite synthesis. The ionic form of the resin was reversed by passing a 10 wt.% NaCl solution through an ion-exchange column loaded with the composites. The type of the resin, anion or cation exchanger, determines the type of the metal salt to be used. Thus salts yielding negatively charged metal ions in solution (VO 3-, WO 4 2-, PdCl 4 2- and Cr 2 O 7 2- ) were used as metal precursors in the preparation of V or W silicate and Pd or Cr zeolite Beta macrostructures, where anion exchange resin-molecular sieve composites were employed (IV, VI and VII). Salts yielding metal cations are suitable as precursors for the modification of cation exchange resinmolecular sieve composite exemplified by the preparation of V-AlPO-5 (V). After the secondary ion exchange, a complex resin-molecular sieve-metal precursor composite is obtained. The composite is separated, rinsed with distilled water, dried at room temperature and calcined for 6 or 24 h at 600 C, after heating to this temperature at a rate of 1 C min CHARACTERIZATION METHODS X-ray diffraction The crystallinity and phase purity of the calcined macrostructures were determined by powder X-ray diffraction (XRD) using a Siemens D5000 diffractometer (Cu K α radiation, λ= Å). The samples were ground into a powder prior to analysis. The relative degree of crystallinity of the modified S-1 or zeolite Beta macrostructures (IV and VII) was calculated from the areas of peaks in the range θ/ (S-1), θ/ (zeolite Beta) using the corresponding peak areas of the non-modified samples as references Scanning electron microscopy The morphology of the prepared macrostructures was studied with a Philips XL 30 scanning electron microscope (SEM) equipped with a LaB 6 emission source. Before SEM studies the samples were covered with a thin gold film by sputtering. A Link ISIS Ge energy dispersive X-ray detector attached to the SEM was used for the evaluation of elemental distribution over the macrostructures (IV, VI and VII). Prior to the energy dispersive spectroscopy (EDS) line scan analysis the macrostructures were embedded in an epoxy resin (Epofix, Struers) and polished to obtain flat cross sections of the spheres Nitrogen adsorption Nitrogen adsorption measurements were performed with a Micromeritics ASAP 2010 instrument. Samples were degassed at 300 C overnight prior to analysis. The specific surface area was calculated using the BET equation. The 22

35 2. 3 CHARACTERIZATION METHODS total pore volume was obtained by converting the amount adsorbed at a relative pressure of to the volume of liquid adsorbate. The micropore surface area and micropore volume was determined by the t-plot method. Pore size distributions were determined by the BJH method using the desorption branch of the isotherms Flame atomic absorption spectrometry Silicon, aluminum (II, III) and the metal contents of the metal modified macrostructures (IV-VII) were determined using flame atomic absorption spectrometry (AAS, Perkin-Elmer 3100). Prior to analysis, the calcined ZSM-5 (II) and vanadium (IV) or chromium (VII) modified macrostructures were fused with LiBO 2 at 1000ºC according to [106], whereas the tungsten modified spheres were dissolved in 2 wt% KOH (VII). The vanadium (V) and palladium (VI) contents of the prepared macrostructures were determined from the differences in concentrations of the corresponding metal precursor solutions before and after the ion exchange Raman and UV-vis absorption spectroscopy Raman spectra (II, IV, V and VII) were collected with a Perkin-Elmer PE 1700X NIR FT-Raman spectrometer equipped with a Nd YAG laser operating at 1064 nm. The spectra were measured at room temperature with a spectral resolution of 4 cm -1 using 500 or 1000 scans and a power of the incident light in the range 0.2 W to 1W depending on the samples. UV-vis diffusive reflectance spectroscopy (UV-vis DRS) spectra (IV-VII) were obtained with a Perkin-Elmer Lambda 2 UV-vis spectrometer equipped with a Labsphere RSA-PE 20 Reflectance Spectroscopy Accessory and operating in a single beam mode. A white, SRS-99 standard reference material was used for a background correction. The spectra were taken at room temperature. Prior to Raman and UV-vis DRS measurements, the samples were ground into a powder Nuclear magnetic resonance Solid-state 31 P magic-angle spinning (MAS) NMR spectra (I) were recorded on a Varian Chemagnetics Infinity CMX-360 (B 0 = 8.46 T) spectrometer using either single-pulse or cross-polarization (CP) from protons [107] experiments, both with proton decoupling. The 31 P operating frequency was MHz. The spinning frequency for all samples was 5000 Hz, stabilized to ±2 Hz with an inbuilt stabilization device. All 31 P isotropic shift data (in the deshielding, δ-scale) are given with respect to 85.5 % H 3 PO 4 (here, 0 ppm, externally referenced) [108], which was mounted in a 1.5 mm glass tube and placed in a 4 mm rotor to 23

36 EXPERIMENTAL avoid errors due to differences in the magnetic susceptibility. All 31 P solid state MAS NMR spectra were recorded at room temperature. 27 Al MAS NMR experiments (I) were performed at room temperature on a Varian Chemagnetics Infinity-600 spectrometer at 27 Al carrier frequency of MHz in 3.2 mm zirconium oxide rotors and at a spinning frequency of ± 10 Hz. Samples were externally referenced on a powdered YAG sample [109] Chemisorption measurements Chemisorption measurements of the palladium-modified samples (Paper VI) were performed with a Micromeritics ASAP 2010C instrument using carbon monoxide as a chemisorbent. The sample was first reduced with H 2 for 2 h at 400ºC followed by evacuation at the same temperature. A CO adsorption isotherm up to 400 mmhg at 35ºC was obtained. The sample was then evacuated at 35ºC for 30 min and the CO isotherm was repeated. The difference between the two straight lines extrapolated to zero pressure was taken as the amount of CO chemisorbed. Palladium dispersions were calculated on Pd : CO = 1 : 1 basis. Three consecutive measurements were performed on each sample. 24

37 Chapter 3 RESULTS AND DISCUSSION 3. 1 MOLECULAR SIEVE MACROSTRUCTURES Description of the method Fig shows a schematic representation of the synthesis procedure for the preparation of molecular sieve macrostructures using ion exchange resins as macrotemplates. Generally, molecular sieve macrostructures can be synthesized Figure 3. 1: Schematic illustration of the procedures for the preparation of molecular sieve macrostructures using ion exchange resin beads as macrotemplates. 25

38 RESULTS AND DISCUSSION following two major paths referred to as (i) step syntheses (A) and (ii) direct synthesis (B). In this section main focus will be on the various macrostructures produced via route (A), which in fact reflects the new developments of the resin macrotemplating method. Macrostructures synthesized via route (B) using anion exchange resin types were recently described [3]. However, a few examples of such macrostructures prepared by direct synthesis are briefly described in section since they were used as reference samples to the corresponding modified macrostructures. Following route (A) ion exchange resin beads can be mixed with various solutions and subjected for hydrothermal treatment. The solution may contain one (Al solution), several (aluminosilicate gel) or all (zeolite Beta synthesis solution) of the required components for the molecular sieve synthesis. Thereby, during this first step different inorganic species are introduced within the resin facilitated by the resin ion exchange properties and yielding particles referred to as resin-solid composites. Thus obtained composites can be either used in their as synthesized form (A1) or the resin can be removed by calcination before the next step (A2). In the later case solid particles are referred to as a negative resin replica. In fact these particles are negative replica of the resin pore structure but not the resin shape. In the next step, the resin-solid composite or the negative resin replica was mixed with additional components, which include inorganic precursors and/or SDA s. Hydrothermal treatment of those mixtures yields resin-molecular sieve composites or molecular sieve composites with pores blocked by the SDA s. In the final step of the procedure resin and SDA are removed by calcination. Aluminophosphate macrostructures (exemplified by AlPO-5, (I)) were synthesized through (A1), whereas ZSM-5 spheres (II) and zeolite Beta/MCM-41 composite macrostructures (III) were obtained through (A2). According to route (B) the ion exchange resin beads can be mixed with molecular sieve synthesis solution and hydrothermally treated. Molecular sieve nutrients are introduced within the resin facilitated by the resin ion exchange capacity, where they crystallize. However, the synthesis solution that crystallizes outside the resin has to be separated from resin-molecular sieve composites. In the final step, the resin-molecular sieve composite is calcined leaving self-bonded molecular sieve macrostructures Aluminophosphate macrostructures Macroporous cation exchange resin beads were used as macrotemplates for the synthesis of AlPO-5 macrostructures. The cation exchange resin type was chosen since in the aluminophosphate synthesis gels positively charged species have been identified as precursors that yield microporous AlPO-n s [110]. A step synthesis procedure was selected because attempts to prepare AlPO-5 26

39 3. 1 MOLECULAR SIEVE MACROSTRUCTURES Figure 3. 2: SEM images of cation exchange resin beads (a), resin-alpo-5 composite beads (b), calcined AlPO-5 spheres (c) and sphere surface (d). macrostructures via route (B) have failed. An additional motivation was to develop a procedure, which eventually would yield the desired phase crystallized only within the resin macrotemplate. Solutions containing polymeric Al cations of Keggin like type were used for the resin treatment in the first step. The solutions also contain a certain amount of tetraethylammonium chloride (TEACl). On the basis of obtained results the use of TEACl in this step was found to be essential for the AlPO-5 synthesis. Yet, the results showed that probable role of TEACl is not that of a structuredirecting agent but rather as an agent that stabilizes Al species within the resin and prevents their leakage during the synthesis. Taking into account the amount of Al introduced into the resin pore structure portions of the remaining components needed for AlPO-5 synthesis as phosphorous (supplied as H 3 PO 4 ), structure directing agent (tetraethylammonium hydroxide, TEAOH) and water were calculated using molar compositions typically reported in literature [111]. AlPO-5 can be synthesized using various organic additives as templates. TEAOH was selected as templating agent because it has been shown that it favors synthesis of nano-sized AlPO-5 [112]. A secondary hydrothermal treatment of a mixture consisting of resin-al composite, H 3 PO 4, TEAOH and distilled water produced a resin-alpo-5 composite and no bulk solid. In the last step the resin-alpo-5 composites were calcined to yield self-bonded AlPO-5 spheres. Fig shows SEM images of particles obtained at different steps of the (A1) route of the synthesis procedure. Upon AlPO-5 synthesis obtained resin- 27

40 RESULTS AND DISCUSSION Intensity/counts a Volume adsorbed/cm 3 g dv/dlog(d)/cm 3 g -1 0,6 0,5 0,4 0,3 0,2 0,1 0, Pore diameter D/Å b θ/ o 0 0,0 0,2 0,4 0,6 0,8 1,0 Relative pressure (p/p 0 ) Figure 3. 3: XRD pattern of calcined AlPO-5 macrostructures (a), adsorption-desorption isotherm (solid symbols adsorption, open symbols desorption) of the same sample (b) and corresponding BJH pore size distribution curve (insert in (b)). molecular sieve composites (Fig. 3. 2b) are similar in shape and size to the original resin beads (Fig. 3. 2a). The only difference is a slightly rougher surface due to the AlPO-5 agglomerates exposed on the surface. Upon calcination, intact spherical AlPO-5 macrostructures were obtained (Fig. 3. 2c). Judging from the SEM micrographs they preserve the shape of the resin macrotemplates but a slight shrinkage can not be excluded. Though some cracked spheres can be observed the macrostructures are mechanically stable. Close inspection of the sphere surfaces and interiors (not shown) revealed that they were built up of fine nano-sized particles (Fig. 3. 2d). This observation is expected since the crystallization takes place within the constrains of the resin polymer chains. In previous reports on the method, where bulk crystallization was also present, crystals larger than the resin pores could be observed attached on the macrostructure surfaces, which is explained by the absence of limitations imposed by the polymer [3-5]. Fig. 3. 3a shows XRD powder pattern of macrostructures obtained from the system with a molar composition 2TEAOH : Al 2 O 3 : 1.2P 2 O 5 : 100H 2 O after hydrothermal treatment for 10h at 150 C. Observed reflections are typical for materials with AFI type structure revealing that the obtained macrostructures consist of well-crystalline AlPO-5 phase. Variations in the system composition or the duration of the hydrothermal treatment yielded macrostructures containing other phases or mixtures of AlPO-5 and impurities e.g. tridymite. Fig. 3. 3b shows the nitrogen adsorption-desorption isotherm and corresponding BJH pore size distribution curve (the insert) of the AlPO-5 macrostructures described in Fig. 3. 3a. It can be seen that AlPO-5 spheres have a complex pore structure consisting of micro-, meso- and macropores. The 28

41 3. 1 MOLECULAR SIEVE MACROSTRUCTURES a b (24 h) (12 h) (10 h) (8 h) (6 h) (5 h) (4 h) (2 h) commercial AlPO 4 (24 h) (12 h) (10 h) (8 h) (6 h) (5 h) (4 h) (2 h) commercial AlPO 4 Al spheres δ/ppm Chemical shift/ppm Figure 3. 4: Single pulse 31 P (a) and 27 Al (b) solid-state NMR spectra of the calcined aluminophosphate macrostructures obtained for various treatment times (indicated on the figure). micropores are due to the presence of AlPO-5, whereas meso- and macropores are created upon resin removal. The AlPO-5 crystallization within the resin beads was extensively studied by 31 P and 27 Al solid state NMR spectroscopy (I). Single pulse 31 P and 27 Al NMR spectra of calcined spheres obtained from a system with molar composition 2TEAOH : Al 2 O 3 : 1.2P 2 O 5 : 100H 2 O at 150 C for various treatment times are shown on Fig The 31 P and 27 Al solid state NMR spectra of amorphous commercial AlPO 4 are also included in the figure for comparison. The 31 P spectra were taken on dehydrated samples (Fig. 3. 4a), whereas 27 Al spectra were recorded for samples exposed to the laboratory atmosphere (Fig. 3. 4b). It can be seen that already after 2 h of treatment the solid gives a strong phosphorous signal indicating that a substantial amount of P is already introduced into the resin. The peak is rather broad ca. 12 ppm and centered at ca. 27 ppm. There are two important features evident from this very first spectrum (2 h) namely: (i) the line width and unsymmetrical shape of the peak hints to the wide distribution of P sites with different chemical environments; and (ii) the spectrum is similar (position, line-width and shape) to the spectrum previously assigned to the anhydrous form of commercial AlPO 4 [113]. Based on this it is assumed that the solid may already contain alternating Al and P centers and even connectivities in between. In support to this assumption complex species containing Al or binuclear Al centers with H 3 PO 4 or H 2 PO 4 and H 2 O ligands have been reported to have P chemical shifts in this range [ ]. After the fifth hour of treatment, sample (5 h), a new sharper peak centered at -30 ppm 29

42 RESULTS AND DISCUSSION start to appear on the top of initially broad P peak. The position of the new peak is in the range typically reported for the anhydrous form of microporous AlPO- 5 [117]. With the prolongation of the treatment time the new peak becomes dominant and after 10 h according to the 31 P NMR no significant changes can be observed. Integral intensities of the 31 P resonances shown in Fi. 3. 4a were used also for quantitative estimation of the phosphorous content for these macrostructures. The results showed that most of the P present in the system is introduced into the resin after the first 2 h of the treatment. Fig. 3. 4b shows single pulse 27 Al NMR spectra of calcined resin-al composite (denoted as Al spheres) and the same set of samples described in Fig. 3. 4a. Three different Al sites are present in the initial resin-al composite particles characterized by broad peaks at 60, 40 ppm (corresponding to the Al in tetrahedral coordination) and 5 ppm (Al in octahedral coordination). After the first two hours of treatment these peaks completely disappear and new up-field shifted peaks appear at 45 (tetrahedral Al), 10 (penta-coordinated Al) and 13 ppm (octahedral Al). Again the peaks for the (2 h) sample are similar to those of the commercial AlPO 4. The up-field shift of the Al resonances and similarities with the spectrum of commercial AlPO 4 reinforce the assumption that new connectivities between alternating Al and P centers are created at the early stage of AlPO-5 crystallization. With a prolongation of the hydrothermal treatment the peaks became sharper and narrow. The position of the peak corresponding to the tetrahedral Al sites 37 ppm is in good agreement with the reported data for Al sites in the microporous frameworks [117,118]. It has also been shown that a certain number of Al sites in the aluminophosphates can additionally coordinate two water molecules i.e. become octahedrally coordinated although still in the framework positions [117,118]. Taking into an account that 27 Al NMR spectra were recorded for samples kept exposed to laboratory atmosphere this may explain the presence of 27 Al resonance at 12 ppm in our samples. Both 31 P and 27 Al NMR studies showed that macrostructures containing high quality AlPO-5 phase may be obtained after 10 h of hydrothermal treatment. Hydrothermal treatment times below and above 10 h yield AlPO-5 solid mixed with impurities Zeolite and composite molecular sieve macrostructures Macroporous anion exchange resins were used as macrotemplates in the syntheses of ZSM-5 spheres (II) and zeolite Beta/MCM-41 composite macrostructures (III) via the (A2) route (Fig. 3. 1). Although obtained through the same synthesis path, these two systems were conceptually quite different. In the case of ZSM-5 spheres, the negative amorphous aluminosilicate resin replica was transformed into a crystalline microporous phase. In the case of the composite macrostructures the negative resin replica (mesoporous zeolite Beta) was 30

43 3. 1 MOLECULAR SIEVE MACROSTRUCTURES Figure 3. 5: SEM images of anion exchange resin (a), calcined zeolite Beta/MCM-41 composites (b), calcined amorphous aluminosilicate spheres (c) and calcined ZSM-5 spheres (d). used as a macrotemplate for the synthesis of ordered mesophase. Fig shows the initial anion exchange resin beads and various product particles obtained during the syntheses of ZSM-5 and the zeolite Beta/MCM-41 composite macrostructures. The later macrostructures remain similar in shape and size through all steps of the synthesis procedure (Fig. 3. 5a and b). Therefore, the secondary hydrothermal treatment of the zeolite Beta spheres with MCM-41 synthesis gel does not appear to influence the macroscopic properties of the product spheres. In the case of ZSM-5 synthesis some differences can be mentioned. Although the shape is preserved, the negative aluminosilicate resin replica shrinks to a certain extent upon the macrotemplate removal (Fig. 3. 5c). Generally, the final ZSM-5 spheres preserve the size and the shape of the amorphous replica (Fig. 3. 5d). These macrostructures have rough surfaces and a certain number of them can be cracked or even broken. Fig shows XRD patterns and results from the nitrogen adsorption measurements corresponding to samples at the two extreme cases, initial aluminosilicate beads and beads treated at 150 C for 28 h. Based on these data, it can be concluded that the initial negative resin replica consists of amorphous solid and the macrostructures obtained after 28 h of treatment consist of a highly crystalline phase with MFI structure (Fig. 3. 6a). The initial aluminosilicate beads had a BET surface area of 730 m 2 g -1 and an isotherm indicating the presence of both micropores due to the amorphous character of the material and mesopores related to the resin macrotemplate (Fig. 3. 6b). The BET surface area for the ZSM-5 sample was about 300 m 2 g -1 and the isotherm contained 31

44 RESULTS AND DISCUSSION Intensity/a.u 28 h a Volume adsorbed/cm 3 g h 0 h 28 h ,0 0,2 0,4 0,6 0,8 1,0 2θ/ o Relative pressure (p/p 0 ) dv/dlog(d)/cm 3 g -1 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0, Pore diameter D/Å b Figure 3. 6: XRD patterns (a), adsorption-desorption isotherms (solid symbols adsorption, open symbols desorption) (b) of the initial aluminosilicate beads (0 h) and spheres hydrothermally treated (28 h). Insert in (b) gives the corresponding BJH pore size distribution curves. micropores due to the zeolite phase. However, the mesopore peak decreases and shifts towards the macropore range due to formation of large ZSM-5 crystals (see below) that are building up the obtained macrostructures. The changes in the pore size distributions for these two samples are more clearly seen in the corresponding BJH plots given in the insert. The sharp peak at about 40Å in the pore size distribution curve of the 28 h sample does not correspond to physical pores since it is not present in the corresponding BJH plot obtained from the adsorption branch of the isotherm. Before the secondary hydrothermal treatment the amorphous aluminosilicate spheres were aged with different solutions. Raman spectroscopy was used to explain the differences in the quality of the final samples depending on the aging procedure. Analysis showed that aging only with TPAOH solution gives the best results and the presence of Na + during the aging has little or no influence on the quality of the final spheres. On the other hand, spheres with higher degree of crystallinity were obtained in the presence of Na + during the hydrothermal treatment. The spheres aged only with TPAOH solution can also be transformed into a highly crystalline ZSM-5 material. However, the resulting particles were not intact. The aluminosilicate beads aged in TPAOH, mixed with NaOH solution and hydrothermally treated at 150 C for different times were analyzed by Raman spectroscopy in order to study the zeolitization process. Results are shown in Fig Firstly, changes in the TPA + cation into non-calcined samples were studied (Fig. 3. 7a). In the region cm -1 the changes in the band at about 1319 cm -1 were most noticeable. The band corresponds to the wagging modes of 32

45 3. 1 MOLECULAR SIEVE MACROSTRUCTURES a b Intensity/a.u. 28h 20h 14h 12h 7h 4h 0h Intensity/a.u. 28h 20h 14h 12h 7h 4h 0h Raman shift/cm -1 Raman shift/cm -1 Figure 3. 7: Raman spectra of samples prepared with NaOH for different treatment times: noncalcined (a) and calcined (b); 0 h corresponds to the initial beads aged in TPAOH. CH 2 groups [119,120]. The intensity of the band decreases with an increase in the duration of the hydrothermal treatment and a new band at about 1338 cm -1 appears. Noticeable changes were also observed in the C-H stretching region ( cm -1 ). Most affected bands with prolongation of the hydrothermal treatment are those assigned to the CH 2 groups proximal to the nitrogen atom. Similar changes have been shown to correspond to the change in conformation of the TPA + from S 4 (typical for free cation in solution) to the conformation of the cation located into the ZSM-5 channels [119,120]. The Raman spectra of the same calcined samples are shown in Fig. 3. 7b. The zeolitization process is clearly seen by the gradual decrease of the amorphous peak at about 495 cm -1 and appearance of the most pronounced MFI peak at about 382 cm -1 with the prolongation of the treatment time. The crystallization process for the samples prepared for different treatment times was followed by SEM. Two distinctive zones were distinguished for all macrostructures independently of the duration of the treatment, a dense shell Figure 3. 8: SEM images of the interior of the spheres prepared for 12 h (a) and 28 h (b). 33

46 RESULTS AND DISCUSSION Intensity/a.u. BSiM6 Beta θ/ o a Volume adsorbed/cm 3 g ,0 0,2 0,4 0,6 0,8 1,0 Relative pressure (p/p 0 ) and a lighter core. Substantial changes were found to take place in the core of the spheres. Based on these changes it can be concluded that the zeolitization process involves rearrangements of the fine particles building up the initial aluminosilicate beads into large well-defined ZSM-5 crystals (shown in Fig. 3. 8a) by complex dissolution-crystallization processes. The prolongation of the treatment time leads to dissolution of the crystals and decrease in the crystal size (Fig. 3. 8b). Fig shows XRD patterns and results from the nitrogen adsorption measurements of the initial zeolite Beta and the Beta/MCM-41 composite macrostructures (exemplified by the sample denoted BSiM6, which refers to zeolite Beta spheres treated with all-silica MCM-41 synthesis gel for 6 days at 100 C). It can be seen that the composite macrostructures consists of a highly crystalline zeolite Beta and in addition peaks at low angle of 2θ indicating the presence of a mesophase with a certain degree of ordering (Fig. 3. 9a). Based on the obtained results it was concluded that the mesophase forms utilizing nutrients from the MCM-41 synthesis gel, while the zeolite Beta serves more as a macrotemplate. The presence of the mesophase with narrow pores was further evident from the results of nitrogen adsorption measurements shown in Fig. 3. 9b. A distinctive step in the range of relative pressures can be noticed in the isotherm of the BSiM6 sample in comparison with the isotherm of the parent zeolite Beta. The meso-pore peak in the BJH pore size distribution plot is centered at ca. 30Å. In addition the amount of the larger pores due to the resin removal in the parent Beta decreased for the BSiM6 sample. This confirms the observations made by the SEM that the mesophase forms not only on the surface of the zeolite Beta spheres but also in their interior dv/dlog(d)/cm 3 g -1 2,0 b 1,6 1,2 0,8 0,4 0, Pore diameter D/Å BSiM6 Beta Figure 3. 9: XRD patterns (a), adsorption-desorption isotherms (solid symbols adsorption, open symbols desorption) (b) of the initial zeolite Beta beads and zeolite Beta composite spheres. Insert in (b) gives the corresponding BJH pore size distribution curves. 34

47 3. 1 MOLECULAR SIEVE MACROSTRUCTURES Macrostructures used in the modification procedure Different molecular sieve macrostructures were used for the preparation of the modified spheres. Most of the macrostructures were synthesized via route (B) (Fig. 3. 1). Exceptions were AlPO-5 macrostructures, which were prepared via (A1). Silicalite-1 of varying crystallinity was used for the synthesis of vanadium and tungsten containing macrostructures (IV). Highly crystalline AlPO-5 macrostructures were utilized in the synthesis of vanadium modified spheres (V). Zeolite Beta was employed for the preparation of chromium and palladium materials (VI, VII). Two series of experiments were performed in the latter case, in which the form of the resin in the resin-zeolite Beta composites was either OH - or Cl - (samples designated as B(OH) and B(Cl)). XRD patterns of the calcined pure molecular sieve macrostructures used are shown in Fig Amorphous, semi-crystalline silicalite-1 and highly crystalline silicalite-1 macro-structures prepared for different treatment times were the starting materials for preparing V and W spheres (Fig a). The synthesis of AlPO-5 was up-scaled to prepare starting macrostructures prior to modification, which required a slightly longer treatment time than at used in small batch synthesis (12 h instead of 10 h) (Fig b). The Cr and Pd containing spheres were synthe-sized from highly crystalline zeolite Beta samples, though a certain loss of crystallinity (reflection peaks of lower intensity) was detected for B(Cl) compa-red to B(OH) (Fig b). The BET surface areas and total pore volumes of the pure molecular sieve macrostructures are listed in Table These values were compared with the a b Intensity/a.u. Intensity/a.u. AlPO-5 B(Cl) B(OH) θ/ o 2θ/ o Figure 3. 10: XRD patterns of the calcined S-1 of varying crystallinity (a), AlPO-5 and zeolite Beta (b) molecular sieve macrostructures used for modification. 35

48 RESULTS AND DISCUSSION Table 3. 1: Properties of the calcined molecular sieve macrostructures used for modification and ion exchange capacities of the non-calcined resin-molecular sieve composites. Molecular sieve BET surface area, S BET (m 2 g -1 ) Total pore volume, V p (cm 3 g -1 ) Amorphous Semi-crystalline S S AlPO Zeolite Beta (OH) Zeolite Beta (Cl) Residual ion exchange capacity (meq g -1 ) ones obtained for the modified macrostructures to determine various metalsupport relationships. The residual ion exchange capacity of the resin-zeolite composites is also given in Table Higher values were obtained for the S-1 macrostructures in comparison to the zeolite Beta ones. Fig shows SEM images of the calcined pure molecular sieve spheres used for modification in this work amorphous silica (a), semi-crystalline S-1 (b), S-1 (c) and zeolite Beta (d). The corresponding SEM images taken from the sphere surfaces are shown in Fig (a -d ). The AlPO-5 spheres used for Figure 3. 11: SEM images of the calcined molecular sieve macrostructures used for modification (a-d) and the corresponding sphere surfaces (a -d ). modification were similar to those shown in Fig and are therefore not shown here. The spheres were solid and intact, except for the highly crystalline silicalite-1 particles, which were cracked and broken (Fig c). The interior of all the spheres was built up by fine particles with a size of about 100 nm, which is comparable to the pore size of macroporous resins (not shown). Larger crystals were observed on the sphere surfaces where there were no steric limitation effects for crystal growth due to the resin polymer chains (Fig b -d ). 36

49 3. 2 MODIFIED MOLECULAR SIEVE MACROSTRUCTURES 3. 2 MODIFIED MOLECULAR SIEVE MACROSTRUCTURES Description of the procedure Fig schematically illustrates different steps of the procedure for the preparation of modified molecular sieve macrostructures. The initial steps are omitted since they are described in Fig Figure 3. 12: Schematic representation of the procedure for the preparation of modified molecular sieve macrostructures. Thus, following the steps described by paths A1 and B (Fig. 3. 1), resinmolecular sieve composites were obtained. Instead of calcining these particles, they were further utilized in the present procedure. As synthesized resinmolecular sieve composites consist of two three-dimensional interconnected networks, an organic one originating from the resin polymer chains and an inorganic one from the molecular sieve phase formed. The resin-molecular sieve composites retain a certain degree of the resin ion exchange capacity (referred to as residual ion exchange capacity). This feature of the as synthesized composites was utilized for the introduction of the charged metal ions in the next step. Metal salt solutions containing positively (VO 2+, (V)) or negatively (VO 3-, WO 4 2-, Cr 2 O 7 2- and PdCl 4 2- ) (IV, VI and VII) charged metal species were used for ion exchange into the resin-molecular sieve composites depending on the resin type (cation or anion). As a result, complex Me-resinmolecular sieve composites are obtained. Finally, the resin is removed by calcination leaving behind self-bonded modified molecular sieve macrostructures Modified molecular sieve macrostructures: general characterization Visually, the modified macrostructures were colored with colors depending on the metals present (orangish, yellowish, yellowish to greenish or brownish for the V, W, Cr and Pd, respectively, spheres). SEM analysis showed that the modified macrostructures were similar in appearance to the pure molecular sieve spheres shown in Fig and Fig No changes due to the metal presence were observed inside the spheres or on the sphere surfaces. Exceptions were the Pd spheres, for which areas of varying 37

50 RESULTS AND DISCUSSION Table 3. 2: Metal precursor solutions and properties of some of the calcined modified molecular sieve macrostructures. Molecular sieve type Metal solution Solution/ composite ratio Metal content, (wt.%) BET surface area, S BET (m 2 g -1 ) Total pore volume, V p (cm 3 g -1 ) Amorphous A A Semi-crystalline S-1 A A S-1 A A Amorphous B B Semi-crystalline S-1 B B S-1 B B AlPO-5 C C C Zeolite Beta (OH) D D D Zeolite Beta (Cl) D D D A1=0.010M NH 4 VO 3 +buffer ph5; A2=0.10M NH 4 VO 3 +buffer ph5 B1=0.010M Na 2 WO 4 2H 2 O+buffer ph5; B2=0.10M Na 2 WO 4 2H 2 O+buffer ph5 C1=0.0005M VOSO 4 ; C2=0.001M VOSO 4 ; C3=0.0025M VOSO 4 D=0.010M Na 2 Cr 2 O 7 2H 2 O appearance were observed on the sphere surfaces (VI). The V modified AlPO-5 and Pd modified Beta spheres were of inferior stability compared to the pure Beta spheres and many of the particles were cracked and broken. Table 3. 2 lists compositions of the metal precursor solutions and properties of the calcined V, W and Cr containing spheres. Data about the Pd containing spheres are given in Table High metal loadings (up to 17 wt%) were achieved for the vanadium and tungsten modified silicate macrostructures. Lower values were obtained for V modified AlPO-5 macrostructures, which might be due to the low concentration of the solutions used for ion exchange. Similarly, lower values were obtained for the Cr samples (up to 4,3 wt%), 38

51 3. 2 MODIFIED MOLECULAR SIEVE MACROSTRUCTURES Table 3. 3: Pd loadings and some properties of the calcined Pd modified zeolite Beta macrostructures. Pd-zeolite Beta spheres Palladium loading, (wt. %) BET surface area, S BET (m 2 g -1 ) Total pore volume, V p (cm 3 g -1 ) B(Cl)Pd B(Cl)Pd B(Cl)Pd Palladium dispersion, (%) probably due to the lower residual ion exchange capacity of the resin-zeolite beta composites in comparison to the resin-silicate composites. Pd modified macrostructures containing controllable amounts of Pd (complete ion exchange) were prepared in paper VI (Table 3. 3). Generally, a higher concentration of the solution used for ion exchange or a higher metal solution to composite ratio resulted in higher metal loadings (Table 3. 2). The metal content was also dependent on the structure of the solid phase within the composites (V) and to a less extent on the resin counter ion of the composites (VII). The ph of the solution used for ion exchange was also of importance due to its influence on the nature of the metal species that are present in the solutions. Thus, at high ph values V, W and Cr are present as monomeric oxoanion species, whereas by decreasing the ph, anions with a higher negative charge are formed [121,122]. This explains the highest values for the V content when a buffer of ph 5 was added to the precursor solution and the slightly higher Cr contents for the spheres prepared from resin-b(cl) composites. Generally, the metal containing macrostructures showed lower BET surface areas with the decrease being proportional to the metal content (V, W in silicate macrostructures) or similar BET surface areas for V (in AlPO-5) Pd, Cr in comparison with the values for the non-modified molecular sieve macrostructures (Tables 3. 1, 3. 2 and 3. 3). The pore volumes of the vanadium and tungsten containing spheres were smaller, similar for V in the AlPO-5, whereas for chromium- or palladium-containing spheres higher pore volumes were measured in comparison to the pure molecular sieve macrostructures. The nitrogen adsorption/desorption isotherms recorded for the modified molecular sieve macrostructures were all of type IV typical of mesoporous materials with a substantial microporosity. Fig a shows nitrogen adsorption/desorption isotherms of the V modified macrostructures with lower (V1) and higher (V3) vanadium loadings. Generally, the micropores in the modified macrostructures are due to the presence of molecular sieve material. The mesopores originate from the resin removal. Changes in the pore size distributions, mainly in the mesopore range, depending on the metal content and the type of 39

52 RESULTS AND DISCUSSION a 1,8 1,6 b Volume adsorbed/cm 3 g V3 V1 dv/dlog(d)/cm 3 g -1 1,4 1,2 1,0 0,8 0,6 0,4 0,2 V3 V1 0 0,0 0,2 0,4 0,6 0,8 1,0 Relative pressure (p/p 0 ) 0, Pore diameter D, Å Figure 3. 13: Nitrogen adsorption/desorption isotherms (solid symbols adsorption, open symbols desorption) for the V modified AlPO-5 macrostructures (displacement with 400 on the Y-axis for V3 sample) (a) and corresponding BJH pore size distribution plots (b). the initial resin-molecular sieve composites were obtained for the modified macrostructures. The changes in the mesopore range can be clearly observed in Intensity/a.u. * * * * * * * * * Pd θ/ o * ** * Cr * * * Figure 3. 14: XRD patterns of V, W, Cr and Pd modified macrostructures (*-peaks corresponding to V 2 O 5, WO 3, Cr 2 O 3 and PdO, respectively). V W V the corresponding BJH pore size distribution plots (Fig b). Recalling the BJH pore size distribution of the non-modified AlPO-5 spheres (Fig. 3. 3b) it can be seen that upon V introduction the peak decreases in intensity and shifts to the right and an additional peak at about 250 Å appears. Figure shows XRD patterns of various calcined V, W Cr and Pd modified macrostructures. Depending on the metal loading, peaks corresponding to V 2 O 5, WO 3, Cr 2 O 3 and PdO, respectively were detected together with the molecular sieve reflection peaks. A certain decrease of the intensity of the zeolite peaks was also observed in the XRD patterns of the modified macro-structures. EDS line scan analysis was used to evaluate the palladium and chromium distribution over the macrostructures prepared (VI, VII). Fig shows a SEM 40

53 3. 2 MODIFIED MOLECULAR SIEVE MACROSTRUCTURES Figure 3. 15: Typical SEM image of a crosssectioned Cr-Beta sphere (a) and typical EDS line scan analysis of chromium over it (b). micrograph of a cross-sectioned Crbeta sphere (a) and the EDS line scan analysis of chromium over it (b). Chromium was evenly distributed across the sphere. Similar results were obtained for all the chromium and palladium spheres. Chemisorption measurements, however, showed poor palladium dispersions, which were decreasing with an in-crease in the Pd content (Table 3. 3) (VI) Nature of the metal species in the modified macrostructures The calcined metal containing macrostructures were characterized by Raman (IV, V) and UV-vis DRS (IV-VII) spectroscopy in order to determine the nature of the different metal species present in the final spheres. The Raman study of the vanadium and tungsten containing spheres showed that the nature of the metal species was dependent on the structure of a b Intensity/a.u. 3 2 Intensity/a.u Raman shift/cm -1 Raman shift/cm -1 Figure 3. 16: Raman spectra of V and W modified molecular sieve macrostructures: (a) Raman spectra representative for V in amorphous (spectrum 1), semi-crystalline (spectrum 2) and silicalite-1 (spectrum 3) spheres; (b) W in amorphous (spectrum 1) and on semi-crystalline and silicalite-1 (spectrum 2) spheres. 41