Clay materials modified with amino acids for purification processes of biogas and natural gas

Transcription

1 Clay materials modified with amino acids for purification processes of biogas and natural gas Joanna Juźków Thesis to obtain the Master of Science Degree in Chemical Engineering Supervisor: Prof. Dr. Moisés Luzia Gonçalves Pinto Examination Committee Chairperson: Prof. Carlos Manuel Faria de Barros Henriques Supervisor: Prof. Moisés Luzia Gonçalves Pinto Memeber of the Committee: Prof. João Manuel Pires da Silva July 2016

2 ACKNOWLEDGEMENTS In the first place I would like to thank my supervisor, prof. Moisés Luzia Gonçalves Pinto for the opportunity to conduct my work in Lisbon, valuable guidance during the research, sharing the knowledge, patience and motivation that allowed me to successfully complete my thesis. I would like to offer my special thanks to prof. João Pires da Silva for helpful suggestions, encouragement and enjoyable cooperation. I am also very grateful to Ana Cristina Fernandes for her assistance in the research and helping hand. I am very thankful to all the workers and students of Grupo de Adsorção e Materiais Adsorventes of Center of Chemistry and Biochemistry of the Faculty of Sciences of the University of Lisbon for wonderful collaboration and friendly atmosphere. I also want to thank my dearest parents and sister for their constant support, faith in me and love.

3 ABSTRACT Biogas and natural gas purification is an issue of a great industrial importance. Due to decrease of calorific value of the fuel, corrosion and dry ince formation as a result of carbon dioxide presence in methane, the concentration of CO 2 needs to be reduced to ppm level. Adsorption on a natural sorbent has drawn the attention of researchers recently, as a low-cost, environmentaly friendly and effective way of gas treatment. In the present research samples of montmorillonite were intercalated with amino acids: glycine, arginine and L-histidine at ph 7 and ph 5 in order to enhance adsorption properties of the clay materials. The obtained adsorbents were analyzed with FTIR, XRD, thermogravimetry with DSC and nitrogen adsorption-desorption. Conducted tests confirmed retention of amino acid molecules in the clay structure and increase of porosity of materials in the result of intercalation. Investigations of methane and carbon dioxide adsorption on the prepared samples were conducted and adsorption isotherms were plotted. Clays intercalated with arginine and L-histidine adsorbed more CO 2 than in case of glycine. Materials prepared at ph 5 showed better results than samples obtained at ph 7. The adsorbent with the highest adsorption capacity was ARG-5 with 0.80 mmol/g of CO 2 adsorbed. The most selective material for CH 4 /CO 2 separation was L-HIST-5, which up to 0.7 molar fraction of CH 4 adsorbed only CO 2 from the mixture at relatively low pressure 100 kpa. The obtained results showed a promising possibility for further application of intercalated clay materials in industrial gas treatment processes. Keywords: adsorption, montmorillonite, amino acids, methane purification, carbon dioxide separation

4 OBJECTIVES The main aim of present research was to characterize the adsorption properties of amino acid intercalated monmorillonite materials for the methane and carbon dioxide separation. The samples were investigated with analytical methods: FTIR, XRD, thermogravimetry and nitrogen adosrptiondesprtion to prove the amino acids renention in the caly structure and their influence on the surface area and porosity of clay material. Adsorption measurements of CO 2 and CH 4 were conducted and adsorption isotherms were plotted in order to evaluate the adsorption capacity of the intercalated clays in the function of pressure. Selectivity of the samples was calculated with MathCAD software and phase diagrams of adsorption of carbon dioxide and methane from binary mixture were prepared to determine the separation abilities of prepared materials.

5 TABLE OF CONTENTS I. INTRODUCTION...1 II. THEORETICAL PART Natural gas and biogas Fossil fuels and CO 2 issue Biomass Biogas energy Natural gas Biogas production anaerobic digestion Biogas and natural gas upgrading Gas purification methods Physical adsorption Chemical adsorption Adsorption on a solid surface Membrane separation Cryogenic separation Pressure swing adsorption Clay materials as adsorbents Low-cost adsorbents Clays Clay structure Smectites Properties of clays Ion exchange Swelling Acidity Intercalation Montmorillonite Amino acid intercalation Adsorption of carbon dioxide on amino acid intercalated montmorillonite III. EXPERIMENTAL PART Materials preparation Characterization methods Fourier-transform infrared spectroscopy XRD Thermogravimetry Nitrogen adsorption-desorption High pressure adsorption line Methodology of methane and carbon dioxide adsorption measurements... 29

6 5. Data evaluation IV. RESULTS AND DISCUSSION Fourier-transform infrared spectroscopy XRD Thermogravimetry Nitrogen adsorption-desorption Adsorption isotherms Carbon dioxide and methane adsorption at ph 7 at 25 o C Carbon dioxide and methane adsorption at ph 5 at 25 o C Carbon dioxide adsorption at ph 5 at 25 o C and 45 o C Selectivity Phase diagrams V. CONCLUSIONS VI. REFERENCES VII. ANNEX VIII. ANNEX

7 LIST OF FIGURES Figure 1. Comparison of different CO 2 emissions connected with different options of electricity production...4 Figure 2. Number of plants and total installed capacity in Europe in years Figure 3. Energy produced from biogas power plants in Poland in years Figure 4. Anaerobic digestion steps...7 Figure 5. Dependence of Wobbe index of CO 2 content and relative density of the biogas Figure 6. Origin of low-cost adsorbents...13 Figure 7. Schema of 1:1 and 2:1 clay minerals structures...15 Figure 8. Classification of clays Figure 9. The structure of monmorillonite Figure 10. Structural formulas of glycine, arginine and L-histidine...20 Figure 11. Forms of glycine ions in aqueous solution depending on ph...21 Figure 12. Intercalation of amino acids on montmorillonite depending on ph value...22 Figure 13. Mechanism of CO 2 adsorption on amino acid intercalated montmorillonite Figure 14. Scheme of amino-acid intercalated MMT materials preparation Figure 15. Simplified schema of the adsorption line Figure 16. FTIR spectra for pure glycine, arginine and l-histidine Figure 17. The FTIR spectra for MMT, L-histidine intercalated MMT samples L-HIST-7 and L-HIST-5 and pure L-histidine Figure 18. The FTIR spectra for MMT, arginine intercalated MMT samples GLY-7 and GLY-5 and pure arginine Figure 19. The FTIR spectra for MMT, glycine intercalated MMT samples GLY-7 and GLY-5 and pure glycine Figure 20. XRD plots for amino acid modified MMT materials obtained at ph Figure 21. XRD plots for amino acid modified MMT materials obtained at ph Figure 22. TG DSC analysis of mass loss and heat flow against temperature for adsorbent materials prepared at ph Figure 23. TG DSC analysis of mass loss and heat flow against temperature for adsorbent materials prepared at ph Figure 24. Isotherms of nitrogen adosprtion-desorption for pure MMT and amino acid intercalated MMT samples at ph Figure 25. Isotherms of nitrogen adosprtion-desorption for pure MMT and amino acid intercalated MMT samples at ph Figure 26. Adsorption isotherms of carbon dioxide and methane adsorption at 25 o C on amino acid intercalated MMT samples prepared at ph Figure 27. Adsorption isotherms of carbon dioxide and methane adsorption at 25 o C on amino acid intercalated MMT samples prepared at ph

8 Figure 28. Adsorption isotherms of carbon dioxide adsorption at 25 o C and 45 o C on amino acid intercalated MMT samples prepared at ph Figure 29. Selectivity of aminoacid intercalated MMT adsorbents against pressure Figure 30. Phase diagrams at 100 kpa describing separation of CH 4 between gaseous and adsorbed phase as a function of mole fraction of CH 4 in adsorbed phase Figure 31. Phase diagrams for L-HIST-7, ARG-7, L-HIST-5, ARG-5 and GLY

9 LIST OF TABLES Table 1. Top natural gas exporters in Table 2. Typical composition of natural gas and biogas...9 Table 3. Typical specifications of composition on feed to LNG plant and on pipeline gas Table 4. θ values for basal peak obtained in XRD plots for MMT and amino acid modified MMT materials, corresponding interlayer spacing and expansion of MMT in the effect of amino acid intercalation Table 5. Porosity and surface properties characteristics of pure MMT and amino acids modified MMT materials... 41

10 LIST OF ABBREVIATIONS ARG-5 montmorillonite intercalated with arginine prepared at ph 5 ARG-7 montmorillonite intercalated with arginine prepared at ph 7 ARG pure pure arginine CEC concentration of exchangeable cations CHP combined heat and power C 1 C 2 C 3 d FTIR G GHG constant constant constants the interplanar distance [Ǻ] Fourier-transfer infrared spectroscopy Gibbs free energy [J] greenhouse gas GLY-5 montmorillonite intercalated with glycine prepared at ph 5 GLY-7 montomrillonite intercalated with glycine prepared at ph 7 GLY pure pure glycine K equilibrium constant L-HIST-5 montomrillonite intercalated with L-histidine prepared at ph 5 L-HIST-7 montomrillonite intercalated with L-histidine prepared at ph 7 L-HIST pure pure L-histidine LNG liquiefied natural gas MMT raw montmorillonite n the positive integer n ads amount of moles of gas adsorbed on the unit of mass of adsorbent [mmol/g] n 0 CO2 standard-state loading for pure CO 2 n 0 CO2 standard-state loading for pure CO 2 p pressure [kpa] PSA pressure-swing adsorption R gas constant S CO2/CH4 selectivity of the clay material for CO 2 /CH 4 mixture T temperature [ o C] TG DSC thermogravimetry with differential scanning calorimetry XRD X-ray diffraction x a CO2 x a CH4 x g CO2 x g CH4 CO 2 molar fraction in the adsorbed phase CH 4 molar fraction in the adsorbed phase CO 2 molar fraction in the gas phase CH 4 molar fraction in the gas phase λ the wavelength of incident wave (λ= Ǻ) θ the scattering angle [ o ]

11 I. INTRODUCTION Nowadays, due to proceeding exploitation of limited fossil fuel resources, alternative sources of energy need to be developed. Moreover, because of high concentration of CO 2 in the atmosphere steps have to be undertaken in direction of cutting the emissions released to the environment. Implementation of biomass as renewable energy source offers a considerable solution for both issues mentioned. Use of this variant of green energy allows to avoid exploitation fossil fuels and rise of the level of CO 2, because of emission of so called neutral CO 2. Among fuels derived from biomass, biogas gained a lot of attention as a green alternative for natural gas. Biogas is a product of anaerobic digestion, composed in major part of methane and carbon dioxide. In order to be applied as substituent for natural gas, biogas needs to undergo treatment in order to remove contaminants and upgrading to eliminate CO 2, which has negative influence on the calorific value of biogas. Hence, an efficient way of carbon dioxide and methane separation needs to be found, which could be possible to implement in a large-scale industrial process. Among available adsorbents clay minerals drew attention, because of their extraordinary properties allowing for their implementation in gas separation. Layered structure, porosity and cation exchange properties are main characteristics, that make these materials highly interesting for investigation in the field of adsorption. A lot of research over these materials has been already done, however, still new applications for these materials are being discovered. The proper modification of clays tunes properties allowing for adsorption of specific compounds with high selectivity and efficiency. In the present work investigations on separation of carbon dioxide and methane were conducted using montmorillonite a clay material. Amino acid intercalation of montmorillonite was applied in order to enhance its adsorptive properties. The obtained samples were characterized with different analytic methods, what allowed for description of their structural, surface and porous properties. The adsorption isotherms for retention of carbon dioxide and methane on modified clays were plotted and analyzed. Finally, selectivity of the tested materials was evaluated and estimation of adsorption from binary mixture was conducted using phase diagrams. 1

12 II. THEORETICAL PART 1. Natural gas and biogas 1.1. Fossil fuels and CO 2 issue Since ancient times mankind utilized bioenergy and biofuels. Burning of wood provided heat and light needed for cooking, heating of shelters, illumination and pottery. Until 19 th century wood remained the main fuel for cooking and heating purposes and vegetable oil was used as a fuel for lightning [1]. Nowadays fossil fuels are by far the most dominant energy sources and provide about 80% of total world energy consumption. There is a strong necessity to cut the utilization of fossil fuel energy resources due to their significant environmental impact connected with the emission of greenhouse gases and pollutants [2]. As effect of fossil fuels consumption, the atmospheric CO 2 concentration has been elevated from 280 ppm in pre-industrial period to nearly 400 ppm in present times, what contributed to a significant climate change [1]. Due to this fact steps aiming in decreasing the GHG emission have to be undertaken. The recent solution for this problem is the implementation of renewable energy sources as an alternative of fossil fuels. This idea is supported by the European Union, that on 9 th December 2008 passed the Renewable Energy Directive, which set a target of achieving the production of 20% of Europe s energy from renewable sources by the year 2020 [3, 4] Biomass One of most promising renewable energy sources is biomass. Biomass can be defined as the contemporaneous (non-fossil) biological material generated from the conversion of solar energy into vegetable matter and it is considered as one of the most suitable ways of energy storage, being a real alternative to fossil fuels, as it is abundant, clean and carbon neutral [5]. In 2009 biomass contributed 13,1% of global energy demand. The global primary energy supplied from biomass reached approximately 55 EJ in Heating accounted for the majority of use of biomass (46 EJ), including heat produced from modern use of biomass (biofuels) and also traditional uses in the form of wood and peat. By the end of 2012, global bio-power approached 83 GW. In Europe, biomass currently accounts for around 2/3 of renewable energy and will play a key role in reaching the target approved by the renewable sources by 2020 [2]. Biomass resources can be defined as byproducts with no or low profit from agricultural crops or industrial processes and as crops grown specially for the purpose of energy production. Other biomass resources are part of agricultural or industrial waste streams representing negative profit. In Europe and North America, agricultural byproducts used for energy production include wheat straw, corn stalks and soybean residues, while large industrial waste streams, in e.g. the United States, originate mainly from the paper-making industry. The major crops presently grown for energy include 2

13 sugar or starch crops such as sugar cane and corn. In addition, crops containing mainly lignocelluloses e.g. willow and poplar are getting more attention with respect to their application for energy production [6] Biogas energy Biogas is produced by anaerobic digestion of biomass; organic feedstock, the most common being: animal waste and crop residues, dedicated energy crops, domestic food waste and municipal solid waste (MSW) [7]. Industrial biogas is generated at sewage treatment plants, landfills, sites with industrial processing industry and at digestion plants for agricultural organic waste [8]. Biogas is an alternative for energy production, that has advantages of being eco-friendly source of energy, in that the calorific value of biogas is equal to that of half litre of diesel oil. (6 kwh/m 3 ). This biofuel is fully capable of replacing other rural energy sources like wood, hard coal, kerosene, plant residues or propane [9]. Use of biogas is highly beneficial for the environment, as it contributes to reduction of emission of greenhouse gas and air pollution in effect of reduction of the use of fossil fuels. Moreover, the process of anaerobic digestion applied in biogas production reduces odours, pathogens and other components, that could be harmful to plants (e.g. organic acids) [7]. Biogas is mainly used for Combined Heat and Power (CHP) and in electricity generation and feed-in to the national grid. Like with any other renewable energy resources, the application of biogas technology will contribute to reduction in greenhouse gas (GHG) emissions and air pollution, due to the expected reduction of the use of fossil fuels. Fig. 1. compares GHG emissions associated with different electricity production options from a life-cycle perspective for biogas, fossil fuels, and other renewable energy sources. The calculated negative emissions for biogas-chp are result of the substitution of oil fuel with biogas [7]. 3

14 Fig 1. Comparison of different CO 2 emissions connected with different options of electricity production [10]. To the countries with biggest biogas production are Germany, UK, France, Italy and Netherlands. Biogas is utilized in combined heat and power (CHP) units to produce electricity and heat. In 2013, primary production of biogas in Europe (including landfill and sewage gas) was estimated 13,4 million tons of oil equivalents (Mtoe) [2, 11-13]. It is significant, that a clear increase in energy obtained from renewable sources in Europe is being noted, for instance, in 2003 the proportion of renewable energy obtained was 11.1% of primary energy in general, and in 2012 this value increased to 22.3%. In Fig. 2. the number of plants and total installed capacity of biogas powered power plants in Europe during years are presented. Constant growth of the number of power plants can be observed, followed by the rise in amount of energy produced. 4

15 Fig 2. Number of plants and total installed capacity in Europe in years [14]. The rise in renewable energy utilization can be also noticed in Poland. In 2012 the share of the obtained renewable energy was 11.7% of the primary energy in general, while in 2003 it was 5.2%. Electricity production in power plants powered with biogas should also be considered, because the progress in this field can be clearly seen. In the years an almost twelve-fold increase in the production of energy from biogas was recorded; it grew from 56 GWh in 2003 to 670 GWh in 2013 [15]. This tendency is presented in the Fig. 3. below, with specification of origin of biogas used for energy production. It can be clearly seen, that renewable energy use has been intensively developed in Poland during recent decade and is likely to increase even more in following years. 5

16 Fig. 3. Energy produced from biogas power plants in Poland in years [15]. In Poland, due to a large area of agricultural lands (14.6 million hectares) and well developed cattle and pig raising, the opportunities for development of the renewable energy market are seen in agricultural biogas plants. Poland has a great biogas potential, which is comparable to that of Germany, which is a leader country in generation energy from biogas in Europe [16] Natural gas Natural gas production is estimated to be over 3300 billion cubic meters per year worldwide [17]. In 2010 natural gas supplied 23,81% of world s energy demand and the rise in consumption of natural gas was noted by 7,4% comparing to The increase of the demand led to re-evaluation of gas reserves, that earlier were considered as unviable economically due to contamination. Moreover, many significant reserves of natural gas are located far from the gas markets in Western Europe, Japan and South Korea. For this reason immerse volumes of gas have to be transported for long distances from exporting countries by pipeline or in tankers as LNG liquefied natural gas. The production of LNG is essential to international trade and its importance is going to increase in following decades [18]. In Table 1. top natural gas exporters were gathered with amounts of gas exported by them in As newer resources of natural gas are being discovered, the natural gas market is predicted to expand up to 65% in 2035 [19]. 6

17 Table 1. Top natural gas exporters in 2010 [18]. Country Pipeline LNG Total Russian Federation Norway Qatar Canada Algeria Although natural gas cannot be referred to as the green energy, its use has many advantages over the use of other conventional fuels. Burning NG produces less CO 2 and more water vapour per energy unit than burning gasoline or diesel. NG presents an alternative to oil-derived fuels. In the automotive sector, NG provides a secure, clean and efficient combustion, almost free of sulphur and lead oxides, benzene and solid particles. As it is less dense than air, it spreads in the case of leak, minimizing risk of explosion. The emissions of carbon dioxide and carbon monoxide in NG-powered vehicles can be reduced 23% and 85% in comparison to conventionally-powered vehicles. The main drawback of NG is low volumetric heat of combustion than in case of liquid fuels [20] Biogas production - anaerobic digestion Anaerobic digestion is a natural biological process of biogas production, in which organic matter (biomass) is broken down by bacteria in conditions of small or no access of oxygen. The final result of a series of decomposition reactions are methane, carbon dioxide and water as main products. The process of anaerobic digestion can be divided into three main steps, as shown in the Fig 4. Complex organics carbohydrates, fats, oils, proteins Hydrolysis Liquefaction Simpler organics amino acids, fatty acids, sugars Acetogenesis Organic acids formic, acetic, propionic Methanogenesis CH 4 CO 2 H 2 O Fig. 4. Anaerobic digestion steps. 7

18 In the first stage, called hydrolysis or liquefaction, the insoluble complex organic matter like e.g. cellulose is converted by fermentative bacteria into soluble molecules like sugars, amino acids and fatty acids. The complex polymeric matter is hydrolyzed to monomers, e.g. cellulose to sugars or alcohols and proteins to peptides or amino acids, by hydrolytic enzymes, (lipases, proteases, cellulases, amylases, etc.) secreted by microbes (1-4). lipids fatty acids (1) polysaccharides monosaccharides (2) proteins aminoacids (3) nucleic acids purines and pyrimidines (4) In the second stage products of the first phase are converted by acetogenic bacteria to simple organic acids, carbon dioxide and hydrogen. The acetogenesis reaction is presented below (5): C 6 H 12 O 6 2C 2 H 5 OH + 2CO 2 (5) In the final third stage methane is produced by bacteria called metanogens in two possible ways by cleavage of acetic acid molecules to carbon dioxide and methane or by reduction of carbon dioxide with hydrogen. The first reaction is dominant, as the reduction of CO 2 is dependent of hydrogen concentration in digesters. The methanogenesis reaction can have following forms (6-8) [21-23]: CH 3 COOH CH 4 +CO 2 (6) 2C 2 H 5 OH + CO 2 CH 4 + 2CH 3 COOH (7) CO 2 + 4H 2 CH 4 + 2H 2 O (8) The obtained biofuel can be use as alternative energy source. When CO 2 and other impurities are removed during the upgrading process, the methane concentration increases and thus the resulting biomethane can be utilized as an alternative to natural gas [24]. After treatment the biogas can be used as a vehicle fuel or injected in existing natural gas grids [25] Biogas and natural gas upgrading The main aims of the biogas treatment are: - The cleaning process, in which the trace components harmful to the natural gas grid, appliances and end-users are removed - The upgrading process, in which CO 2 is removed in order to adjust the calorific value and relative density and to meet the specifications of Wobbe index, which is dependent on both calorific value and relative density [26]. 8

19 Typical composition of natural gas and biogas are presented in the Table 2. below. Table 2. Typical composition of natural gas and biogas [27]. Natural gas component %mol methane ethane 0-20 propane butane CO N H 2 S 0-5 O rare gases (He, Ar, Xe, Ne) trace Biogas component %mol methane CO H 2 S H 2 O 0-10 volatile compounds trace NH 3 N 2 O 2 CO For methane distribution through pipelines or liquefaction in order to transport it for long distances the impurities contained in methane (in form of biogas or natural gas) need to be removed, because they can cause corrosion in the pipelines, compressors, gas storage tanks and engines [12]. Methane for domestic use is required to have 97% or higher purity. Presence of CO 2, as it is a inert gas in terms of combustion, lowers the calorific value of the fuel. In order to eliminate the formation of dry ice and corrosion in the liquefaction step, impurities contained in the natural gas or biogas, such as CO 2 or H 2 O have to be reduced to ppm level [29]. Upgrading biogas to the quality of natural gas is a multiple step process. After removal of water (vapour), H 2 S, siloxanes, carbon hydrates and NH 3, the removal of CO 2 is necessary to obtain quality that meets the Wobbe index. As the CO 2 in the upgraded gas is removed, the relative density decreases and the calorific value increases, increasing the Wobbe index [26]. In the Fig. 5. dependence of the Wobbe index on CO 2 content and relative density is shown. 9

20 Fig. 5. Dependence of Wobbe index of CO 2 content and relative density of the biogas [30]. After transformation, the final product is referred to as biomethane and typically contains 95-97% CH 4 and 1-3% CO 2 [26]. The calorific value of raw biogas is to kj/m 3. After CO 2 is removed, the methane gas increases calorific value up to kj/m 3 [31]. Typical requirements, which have to be met in order to apply the gas on feed to NG plants and for pipeline transport were gathered in the Table 3. Table 3. Typical specifications of composition on feed to LNG plant and on pipeline gas [32]. Impurity Feed to LNG plant Pipeline gas H 2 O <0.1 ppmv 150 ppmv H 2 S <4 ppmv mgsm -3 CO 2 <50 ppmv 3-4 vol.% total sulphur <20 ppmv mgsm -3 N 2 <1 vol.% 3 vol.% Hg <0.01 mg/nm 3 - C 4 <2 vol.% - C 5+ < 0.1 vol.% - aromatics <2 ppmv - 10

21 1.7. Gas purification methods Removal of carbon dioxide from methane in order to meet specifications required for pipeline transport (typically 2-3vol% CO 2 ) or LNG production (typically less than 50 ppm CO 2 ) can be achieved using different technologies available [33]. Some of them are discussed below Physical adsorption One of the easiest and cheapest method of the gas treatment is physical adsorption. It involves use pressurized water as an adsorbent. Instead of water, organic solvents such as methanol and dimethylethers of polyethyleneglycol (DMPEG) can be used to absorb CO 2 [34]. The raw biogas is pressed and introduced to the column from the top, while pressurized water is sprayed from the top. The CO 2 (as well as H 2 S) is dissolved in the water, which is collected in the bottom of the column. Methane, which has very low solubility in water, is not retained. This method is effective also at low flow rates, at which power plants are normally operating. It is also simple method, that does not require much infrastructure and is cost-effective [35, 36] Chemical adsorption Chemical adsorption involves formation of a reversible chemical bond between solute and solvent. Regeneration of the solvent requires breaking of the bonds, which needs more energy input. Solvents used contain usually aqueous solutions of amines or aqueous solution of alkaline salts [36]. The absorption with organic amine solution is highly efficient, but the regeneration of amine solution usually involves high energy consumption and potential high corrosion, which is not environmentally friendly [31] Adsorption on a solid surface Adsorption process involves transfer of solute in the gas stream to the surface of solid material, on which they concentrate by the effect of physical or Van der Waals forces. Commercial adsorbents are usually granular solids with a large surface area per unit volume. Depending on the choice of adsorbent, simultaneous or selective removal of CO 2, H 2 S, moisture and other impurities can be obtained. Among commonly used adsorbents are silica, alumina, activated carbon and silicates. This type of adsorption process is usually conducted at high temperature and pressure. The advantages are significant moisture removal capacities, design simplicity and easiness of operation [36]. 11

22 Membrane separation The principle of this method is the transport of some components of the raw through a thin membrane (< 1mm), while others are retained. The transport of the molecules in driven by the difference in partial pressure over the membrane and is dependent on the permeability of the membrane material. For the high methane purity, the permeability of membrane needs to be high. For example, a solid membrane constructed from acetate-cellulose polymer shows permeability for CO 2 up to 20 times higher than for CH 4. The disadvantage of the process is that it requires application of high pressure of bar [36, 37]. For the membrane separation, the membrane itself is expensive, often suffers thermal shock and chemical corrosion, and is easily contaminated [31] Cryogenic separation The cryogenic method involves the separation of gas mixtures by fractional condensations and distillations at low temperatures. During cryogenic the raw biogas is compressed to ca. 80 bar. Then the gas is dried in order to avoid freezing in the cooling process. Next, the biogas is cooled down to - 45 o C and condensed CO 2 is removed in the separator. The CO 2 is further processed to recover the dissolved methane, which is recycled to the gas inlet. With this process methane of 97% purity can be obtained. The advantage of the process is the recovery of the pure methane in the form of liquid, which can be transported conveniently. Disadvantages of the method are high capital cost and complicated equipment requirements [33] Pressure swing adsorption (PSA) Among the available adsorption technologies, pressure swing adsorption (PSA) has gained interest as a method for separation and capture of CO 2 due to the low energy requirements and low capital costs in comparison to common separation methods. PSA is based on selective adsorption of the undesired gas on a porous adsorbent at high pressure and recovery of the gas at low pressure. After the process the porous adsorbent can be reused in a subsequent adsorption cycle [38]. 12

23 2. Clay materials as adsorbents 2.1. Low-cost adsorbents Nowadays there is a strong concern risen around the proceeding degradation of the natural environment caused by human activity. It is essential to take the steps that would minimize the impact on the nature. Hence, new criteria appeared in terms of design of new materials and processes. The attention of the scientific research is focused to find not only cheap and effective materials that can be applied in the big-scale industrial processes, but also environmental friendly. The recent trend in the science is the development of materials that are biocompatible, non-toxic and neutral towards ecosystem. Conventional adsorbents utilized in the industry usually involve high cost of production, complicated treatment and regeneration, also generating pollutants and residues, which pose danger to the natural environment. In the adsorption processes a new type of materials is gaining a growing popularity in the last few decades. These are so-called low-cost adsorbents, a group of natural materials, which are abundant in the nature, easily accessible and exploitable and relatively cheap in comparison to conventional adsorbents. This group also encloses materials that are agricultural or industrial residues, which after simple treatment can be successfully used as adsorbents. The origin of these materials, diversity of properties and low costs enable their wide application in various processes [39]. The division of low-cost adsorbents regarding their origin is presented in the Fig 6. below: LOW-COST ADSORBENTS Natural materials Agro-industrial residues Mineral and soil materials orange peel fly ash clays coconut shell sawdust peat wheat straw red mud iron oxides sawdust tea waste natural zeolites Fig. 6. Origin of low-cost adsorbents [40]. As presented above, mineral materials are one of the groups of low-cost adsorbents. They are inorganic materials building the earth s crust, abundant in the nature. 13

24 Natural clay minerals are known to mankind since the beginning of the civilization. They are naturally occurring nanomaterials, which are the youngest members of the family of minerals in the earth s crust. Regarding their abundance in most continents of the world, high sorption properties and potential for ion exchange, clay materials found wide application in adsorption processes [41, 42] Clays Clays are natural, earthy, fine-grained materials that develop plasticity when mixed with limited amount of water. They are cheap, easily available and green materials possessing a wide variety of structures [43, 44]. Clays are the main components of the mineral fraction of soils and are one of most abundant natural low-cost materials (~$50/ton) [45]. Clays are very versatile materials and hundreds of millions of tons currently find application in ceramics and building materials, paper coating and fillings, drilling muds, foundry moulds, pharmaceuticals etc. Clays have been extensively used as bricks and insulation materials due to their abundance, high mechanical strength and good thermal stability. They have attracted attention due to their unique properties such as selective adsorption and the capability for the surface modification. They found application as adsorbents, catalysts or catalysts supports, ion exchangers etc, depending on their specific properties [45-47]. Clays have also been incorporated as fillers for fabricating nano-composite polymers either to enhance the mechanical/physical properties [43]. Two broad classes of clays may be identified: - Cationic clays (or clay minerals), widespread in nature. They have negatively charged alumina-silicate layers, with small cations in the interlayer space to balance the charge - Anionic clays (or layered double hydroxides LDHs), more rare in nature, but relatively simple and inexpensive to synthesize in the laboratory and industrial scales. The anionic clays have positively-charged brucite-type metal hydroxide layers with balancing ions and water molecules located interstitially [46] Clay structure Clay minerals are built of layered silicates. They are crystalline materials of very fine particle size varying from 150 to less than 1 micron. The basic building block for clay materials are tetrahedral and octahedral layers [41]. Tetrahedral layers are composed of continuous sheets of silica tetrahedral linked via three corners to form a hexagonal mesh and the fourth corner of each tetrahedron (normal to the plane of the sheet) is shared with octahedral in adjacent layers. Octahedral layers in the clay minerals, are composed of flat layers of edge-sharing octahedra, each formally containing cations in its centre (usually Mg 2+ or Al 3+ ) 14

25 and OH - and O 2- at its apices. The Al 3+ is generally found in six fold or octahedron coordination, while the Si 4+ cation takes place in four fold or tetrahedral coordination with oxygen. Octahedral layers may be trioctahedral or dioctaherdral, which depends on the degree of occupancy of the octahedral sites [41, 43]. The different arrangement of tetrahedral and octahedral layers leads to creation of two basic structures, in which clay minerals can occur; namely 1:1 and 2:1 structure. - 1:1 structure, also known as OT structure, has alternating tetrahedral and octahedral sheets, occurs in e.g. kaolinite group - 2:1 structure or TOT structure is composed of a sandwich of one octahedral sheet between two tetrahedral sheets and can be found in e. g. smectite clay materials, like montmorillonite. These layered crystals, which are approximately 1 nm thick with lateral dimensions from 30 nm to several microns, are piled parallel to each other and are bonded by local Van der Waals and electrostatic forces [41, 43]. The schema of discussed clay structures are presented in Fig. 7. below: 1:1 structure (OT) 2:1 structure (TOT) Fig. 7. Schema of 1:1 and 2:1 clay minerals structures [42]. Clays are a wide family of minerals, which can be divided into groups, considering their structure. The general systematization of clay minerals is presented in the Fig 8. below: 15

26 CLAYS 1:1 MINERALS 2:1 MINERALS pyrophyllite dioctahedral the Kaolin Group kaolinite dickite nacrite halloysite trioctahedral the Serpentine Group lizardite antigorite chrysotile amesite talc micas smectites vermiculite illite montmorillonite beidelite hectorite saponite hinsingerite carlosturanite chlorite greenalite Fig. 8. Classification of clays [42] Smectites Smectites belong to the family of 2:1 phylosilicates and are constructed from layers formed in the result of the condensation of one central octahedral sheet and two tetrahedral sheets. [48]. Smectites are a group of clay minerals that have a dioctahedral or trioctahedral structure, with isomorphous substitution that leads to a negative layer charge of less than 1.2 formula unit. Interlayer spacings vary between ~10 and 15 Ǻ and are generally dependent of the nature of the exchangeable cation and the relative humidity [41]. Smectites can be divided into four sub-classes, depending upon: - The type of octahedral layer (dioctahedral or trioctahedral) - The predominant location of the layer charge sites (octahedral or tetrahedral) 16

27 Montmorillonite and beidellite groups are dioctahedral smectites with composition presented in formulas below: Montmorillonite group: (M x + ) ex [(Si8) tet (M(III) 4-x )M(II) x ) oct O 20 (OH)] x-4 Beidellite group: (Mx+) ex [(Si 8-x Al x ) tet (M(III) 4 oct O 20 (OH) 4 ] x- Where M + is an exchangeable cation present in the interlayer (e.g. Na + ) and M(III) and M(II) are nonexchangable octahedrally coordinated trivalent and divalent cations (e.g. Al 3+ and Mg 2+ ), respectively and the layer charge is 0.5<x<1.2. Minerals belonging to hectorite and saponite groups are trioctahedral smectites, characterized by formulas: Hectorite group: (M x + ) ex [(Si 8 ) tet (M(II) 6-x M(I) x ) oct O 20 (OH) 4 ] x- Saponite group: (M x + ) ex [(Si 8-x Al x ) tet (M(II) 6 ) oct O 20 (OH) 4 ] x- M(II) and M(I) are non-exchangeable octaherally coordinated divalent and univalent cations (e.g. Mg 2+ and Li + ) respectively and the layer charge is 0.5<x<1.2 [41] Properties of clays Clay minerals present a number of extraordinary properties, because of which these materials find wide application as adsorbents, catalysts and supports. Most significant of them were shortly described below Ion exchange The clay materials posses ion exchange properties, which allow for isomorphous substitution of metal cations in the lattice by lower-valent ions, e.g. the aluminium ions can be replaced by silicon ions, what in effect leaves the residual negative charge in the lattice. The created negative charge can be balanced by other cations. The cations can be exchanged when they are brought into contact with other ions in aqueous solution [41]. The concentration of exchangeable cations of clay material is called CEC and is usually expressed in miliequivalents per 100 g of the dried clay. Because smectites are clays with the highest concentration of interlayer cations, they show highest cation exchange capacities (typically mequiv./100 g). Structural defects at layer edges additionally contribute CEC and also to a small amount of anion exchange capacity. 17

28 The exchangeable ions influence microporosity and surface area affecting also adsorption capacity of the clay material. Hence, the possibility to introduce desired cations to the interlayer position allows for tuning the clay s properties. It was reported, that saturation of clay material with exchangeable ions Na +, K +, Ca 2+, H +, Al 3+ enhanced its adsorption capacity towards C 2 H 2 and CO 2. The gas adsorption was influenced by the ionic radius of exchangeable cations, the interlayer separation and surface area of montmorillonite [49] Swelling Clay minerals can absorb water between layers, moving them apart, what causes the swelling of clay. In order to obtain efficient swelling the energy released by cations or layer salvation needs to be sufficient to overcome the attractive forces (like hydrogen bonding) between the layers in the clay structure. In 1:1 (OT) type clay minerals (kaolinite), water forms strong hydrogen bonds with hydroxyl groups on hydrophilic octahedral layers, allowing swelling to occur. In 2:1 (TOT) clay minerals the ability to swell depends on the solvation of interlayer cations and the layer charge. Clays with low layer charge (e.g. talc and phyrophillite) have very low concentration of interlayer cations and their swelling is more difficult. On the other hand, clay minerals that have very high charges (e.g. mica) present strong electrostatic forces, which hold together anionic layers and the interlayer cations together, also preventing from swelling. The expansion proceeds most easily in case of materials with univalent cations in the interlayer. With the increase of valence number of the cation, swelling decreases accordingly. The extent of swelling can be observed by measuring interlayer separations using powder X-ray diffraction method [41] Acidity The cations located in the interlayer space contribute to the acidity of the clay minerals. The protons H + and polarizing cations (e.g. Al 3+ ) give rise to strong Brönsted acidity. The higher the electronegativity of M +, the stronger are the acidic sites generated. Brönstedt acidity also derives from the terminal hydroxyl groups and from the bridging oxygen atoms in the clay structure. Moreover, presence of layer surface and edge defects in clay minerals results in weaker Brönsted or Lewis acidity, specially at low concentrations [25] Intercalation Considering cation exchange properties and layered structure of clay materials, it is possible to introduce molecules of other compounds, which can be used to change and tune their properties. Different guest molecules can be introduced between layers of the clay, depending on the desired 18

29 result. Because of swelling properties of clays, exchanged molecules can be bigger than original ones, what is compensated by expanse of interlayer spacing in the clay structure [42, 50, 51] Montmorillonite Montmorillonite is a smectite clay and many of its industrial uses are related to the manufacture of wine, beer, oils, moulding sand, ore pellets, petroleum products, pesticides, catalysts, adsorbents, cosmetics, ceramics, paintings, etc. Montmorillonite is a 2:1 (or T O T) layer phyllosilicate clay formed by an octahedral sheet containing Al 3+ or Mg 2+ ions between two tetrahedral silica sheets The isomorphic substitution in the octahedral and tetrahedral sheets results in a deficit of surface charges that is balanced by exchangeable cations, e.g. Na +, K +, Ca 2+, Mg 2+ situated in the interlayer position [52]. The structure of montmorillonite is presented in Fig. 9. Ca 2+ Mg 2+ Na + Mg 2+ K + Ca 2+ Na + Mg 2+ K + Na + Fig. 9. The structure of monmorillonite. Montmorillonite presents a relatively high cationic exchange capacity and is easily expandable, which allows the intercalation of a wide range of organic species [53]. Researchers have studied the properties of clays and found that clays contain weak base character, due to presence of OH groups in its structure and ion-exchange properties which can attract CO 2 gas easily [54]. The interactions performed by the formation of weak Van der Waals forces and the immobilization of CO 2 molecules. Thus, clays can potentially be applied as CO 2 adsorbents [47]. 19

30 2.7. Amino acid intercalation Clay minerals can be substantially modified by replacing the natural inorganic interlayer cations with selected organic cations. An example of organic compounds used for intercalation are amines [55] and amino acids [51]. In the present research, in order to enhance the adsorption properties of montmorillonite towards CO 2 an intercalation of clay materials with amino acids was conducted. Amino acids are cheap, under appropriate ph they are stable as cations and anions, so they can be exchanged with interlayer ions of cationic clays. These compounds can be used for carbon dioxide adsorption purposes in order to enhance the adsorption properties presented by clay materials. It was proved that amino acids can be successfully intercalated to the montmorillonite, causing the extension of intermolecular spacing % increase of BET surface area was indicated [50]. Three types of amino acids were used for clay intercalation in the presented research: glicyne, arginine and L-histidine. Their structural formulas are presented in the Fig. 10. Fig. 10. Structural formulas of glycine, arginine and L-histidine [56]. Amino acids can be considered as green compounds, as being one of main building blocks of living organisms are eco-friendly and do not cause contamination when released to the environment. Among the amino acids three of them were chosen for the research: glycine, arginine and L-hisitidine. The choice was made in order to compare the influence of functional groups of amino acid on adsorption properties. Glycine, as the simplest amino acid, contains only one NH 2 group in the structure. L- histidine and arginine, having additional basic amino groups side chains are expected to show enhanced adsorption properties [56]. 20

31 Amino acids in water solutions dissociate and turn into ions. Depending on the ph applied, they can be present in the form of cations, anions or zwitterions. They are adsorbed to MMT by exchange with exchangeable ions present in interlayer space of clay. This amino acid can be located in the interlayer space as both glycinium and zwitter ion [50]. There is an electrostatic interaction between the carboxylic acid species and the edge surface sites. When smectite edge sites are saturated, glycine is adsorbed by intercalation [57]. Forms, in which amino acids will be present in aqueous solution at given ph is dependent on their isoelectric point. For the chosen amino acids the values of isoelectric points are as follows [58]: pi = 6.0 for glycine pi = 7.6 for l-histidine pi = 11.2 for arginine Predominant forms in which the amino acid is present in the aqueous solution depending on ph are shown for the example of glycine on Fig. 11. below: Fig. 11. Forms of glycine ions in aqueous solution depending on ph [59]. In case, when applied ph is lower than pi, the amino acids are protonated and NH + 3 are dominant in chains, the strong electrostatic interaction between NH + 3 of amino acid molecules and negatively charged surface of montmorillonite is the main driving force for intercalation, in case when ph > pi there are more COO - groups in amino acid chains and the intercalation proceeds by coordination between COO - groups and silicate layer of montmorillonite, the amino acids are inserted in the interlayer and bind with the clay by carboxylate groups [60]. In general, positively charged basic amino acids are more strongly adsorbed than neutral or acidic amino acids, due to ion-exchange reactions onto negatively charged clay surfaces [57]. Two possible ways of amino acid intercalation are presented in Fig. 12 below: 21

32 ph < pi ph > pi Fig. 12. Intercalation of amino acids on montmorillonite depending on ph value [60] Adsorption of carbon dioxide on amino acid intercalated montmorillonite Amino acid intercalated clay materials are capable of CO 2 retention due to the presence of amine groups in the amino acid structure. However, the exact mechanisms leading to the retention of carbon dioxide inside the clay structure have not been investigated yet. According to one of hypothesis the adsorption of CO 2 can proceed through a reaction of a carbon dioxide molecule with amine group creating carbamic acid group, which can transform to carbamate due to dissociation. This model is based on better known system of CO 2 adsorption on amine intercalated clays, already discussed in another work [61]. The proposed mechanism of retention of CO 2 by amino acid intercalated clay was presented in the Fig

33 Fig. 13. Mechanism of CO 2 adsorption on amino acid intercalated montmorillonite. The created carbamic acid group is unstable and can release CO 2 upon heating, what allows for easy regeneration of adsorbent, not requiring application of other chemical solutions, what is an important advantage of this process [62]. The manner of bonding between clay material and amino acid molecules is also not known yet. It can proceed through creation of a chemical bond or electrostatic forces. The bonding presented in Fig. 13 is a proposed model for the system. 23

34 III. EXPERIMENTAL PART 1. Materials preparation Natural montmorillonite (MMT) clay from Wyoming, USA has been used as a raw material for adsorbents preparation. For the clay s surface functionalization three different amino acids: glycine, arginine and l-histidine (Sigma Aldrich, purity >99%) were used. Two samples, at ph 5 and ph 7 were prepared for each kind of amino acid and also a sample of nonintercalated MMT. In order to prepare amino acid intercalated clay adsorbents the following procedure was applied. 2 g of clay was added to 100 ml of distilled water and mixed for 3 h using magnetic stirrer. 50 ml of 0.03 M solution of amino acid was prepared by dissolving adequate amount of amino acid in distilled water. All solutions were obtained at room temperature. Then, the amino acid solution was added to the clay and ph of the mixture was adjusted to the required value using 0.05 M HCl solution. The acid solution was prepared by dilution of 37 % HCl (Carlo Erbo) with distilled water. The clay mixture was mixed with amino acid solution overnight in the room temperature. The next day it was removed from stirrer and centrifuged with centrifuge (NF400 N400R Nüve). Next the supernatant was discarded and the sample was washed with distilled water at room temperature and dried in the oven overnight. The obtain material was milled with a mortar and stored in a plastic container. For non-intercalated MMT sample preparation the same procedure as above was used, without steps of amino acid solution addition and ph adjustment. Scheme of adsorbents preparation steps are presented on the Fig. 14 below. For the description simplification following abbreviations for prepared adsorbents were used: MMT for pure montmorillonite material GLY-7 for glycine intercalated montmorillonite prepared at ph 7 GLY-5 for glycine intercalated montmorillonite prepared at ph 5 ARG-7 for arginine intercalated montmorillonite prepared at ph 7 ARG-5 for arginine intercalated montmorillonite prepared at ph 5 L-HIST-7 for L-histidine intercalated montmorillonite prepared at ph 7 L-HIST-5 for L-histidine intercalated montmorillonite prepared at ph 5 24

35 2 g clay 100 ml distilled water 50 ml 0.03 M amino acid 0.05 M HCl for ph adjustment centrifugation drying mincing stirring AMINO ACID INTERCALATED CLAY MATERIAL Fig. 14. Scheme of amino-acid intercalated MMT materials preparation. 2. Characterization methods Several analytical methods were applied in order to characterize obtained adsorbent materials. Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetry with differential scanning calorimetry and nitrogen adsorption-desorption were used Fourier-transform infrared spectroscopy Infrared spectroscopy is an analytical method to detect the vibrations of the atoms of a molecule. The infrared spectrum is recorded by passing infrared radiation through a sample and determining which part of incident radiation was absorbed at a particular energy. The energy at which peaks on the obtained spectra appear indicate the frequency of vibration of a part of a given molecule. Fouriertransform infrared spectroscopy uses the interference of radiation between two beams. The interaction of the beams creates an interferogram as a signal resulting from a function of change of pathlength between them. During the FTIR spectrophotometer experiment the radiation emitted from the source is passed through an interferometer to the sample, which absorbs certain part of radiation. The unabsorbed radiation reaches detector and is directed to an amplifier, which allows for elimination of high-frequency disturbances. The data are converted to digital form by an analog-to-digital converter and transferred to the computer for Fourier-transformation, which transforms the data of intensity of radiation falling on the detector into wavelength value. By subtraction of background spectra from 25