Clay materials modified with amino acids for purification processes

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1 Clay materials modified with amino acids for purification processes of biogas and natural gas Instituto Superior Técnico, Lisbon, Portugal Poznan University of Technology, Poznań, Poland 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.8 mmol/g of CO 2 adsorbed. The most selective material for CH 4 /CO 2 separation was L-HIST-5, which up to.7 molar fraction of CH 4 adsorbed only CO 2 from the mixture at relatively low pressure 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. Introduction Nowadays fossil fuels are by far the most dominant energy sources and provide about 8% 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 []. As effect of fossil fuels consumption, the atmospheric CO 2 concentration has been elevated from 28 ppm in pre-industrial period to nearly 4 ppm in present times, what contributed to a significant climate change [2]. One of most promising renewable energy sources is biomass, as 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 [3]. In Europe, biomass currently accounts for around 2/3 of renewable energy and will play a key role in reaching the

2 target approved by the renewable sources by 22 []. Biogas is produced by anaerobic digestion of biomass, in the industry is generated at sewage treatment plants, landfills, sites with industrial processing industry and at digestion plants for agricultural organic waste [4]. 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 ) [5]. The final result of a series of decomposition reactions of organic matter are methane, carbon dioxide and water as main products [6]. 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 [7]. Natural gas itself, although 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 [8]. 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 [9]. Presence of CO 2 lowers calorific value of the fuel and can cause dry ice formation. Methane for domestic use is required to have 97% or higher purity []. Among different technologies developed for methane treatment, adsorption on the low-cost adsorbents in gaining more attention, as method relatively cheap, simple and environmental-friendly. Natural clays are an interesting group of adsorbents, regarding their layered structure and extraordinary properties, like ion-exchange, swelling and possibility of intercalation of particles between the layers [-3]. Due to them it is possible to introduce molecules of other compounds into the clay s structure, tuning their properties. In the present work a natural clay - montmorillonite (MMT) was intercalated with three different amino acids: glycine, arginine and L-histidine in order to enhance its adsorptive properties. Amino acid intercalated clay materials are capable of CO 2 retention due to the presence of amine groups in the amino acid structure. 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 [3, 4]. 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. 2. 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 non-intercalated MMT. In order to prepare amino acid intercalated clay adsorbents the following procedure was applied. 2 g of clay was added to ml of distilled water and mixed for 3 h using magnetic stirrer. 5 ml of.3 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.5 M HCl solution. The acid solution was prepared by dilution of HCl solution (Carlo Erba, 37%) with distilled water. The clay mixture was mixed with amino acid solution overnight in 2

3 the room temperature. The next day it was removed from stirrer and centrifuged with centrifuge (NF4 N4R 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. Six kinds of adsorbents were prepared: three at ph 7, intercalated with glycine (GLY-7), arginine (ARG- 7) and L-histidine (L-HIST-7). Also three materials at ph 5 were obtained, GLY-5, ARG-5 and L- HIST-5, respectively. Schema of adsorbents preparation is shown in the Fig.. Fig.. Schema of adsorbents preparation. 3. Characterization methods FTIR spectra of prepared adsorbents were collected along with spectra of pure amino acids used for montmorillonite intercalation glycine, arginine and L-histidine. The analysis was conducted using KBr pellet method. First KBr pellet was obtained by milling KBr with a mortar and pressurizing it. The KBr pellet was used to obtain the background spectra. The pellets of investigated samples were prepared by mixing adsorbent material with KBr in proportion 2:3, milling with a mortar and pressurizing to produce the final pellet. The sample spectra was collected by subtraction of background KBr spectrum from the spectrum of pellet containing KBr and sample material. The investigation of FTIR was conducted with use of a Nicolet 67 Fourier transform IR spectrophotometer (256 scans, resolution 4 cm - ) in the wavenumber range from 4 to 4 cm -. X-ray diffraction spectra of pressed powder samples were prepared for pure MMT and each type of amino acid-intercalated MMT. X-ray powder diffraction patterns in range from 3 o to o were obtained with a Phillips PW 73 diffractometer with automatic data acquisition (APD Phillips v3.6b software using a Cu anode (λ=.546 Ǻ)). The values of interatomic spacing were calculated on the basis of θ values from the Bragg law:, where: d the interplanar distance [Ǻ], θ the scattering angle [ o ], n the positive integer (n=), λ the wavelength of incident wave (λ=.546 Ǻ). The clay expansion in result of intercalation was calculated by subtracting the value of the interlayer spacing of MMT from the value of interlayer spacing of intercalated materials. Experiments of thermogravimetry with differentialscanning calorimetry were conducted using an apparatus Setaram TG-DSC. All the samples were analyzed regarding mass loss and heat flow during heating. The analysis was conducted in the temperature range from 25 o C to 7 o C and speed 5 o C/min. In the nitrogen adsorption-desorption analysis the capillary condensation of nitrogen inside pores allows for evaluation of porous volume and pore size distribution in the sample. Clay adsorbents were analyzed by nitrogen (Air Liquide, %) adsorption-desorption with NOVA 22e Quantachrome at -96 o C. The samples were previously degassed for 2.5 h at 5 o C under vacuum conditions. 3

4 4. CO 2 and CH 4 adsorption measurements Pure gas adsorption isotherm experiments were performed on a high-pressure adsorption line. The central element of the adsorption line was a stabilization cell of calibrated volume enclosed between three valves and connected with pressure tranducer, which allowed for precise measurement of pressure of the gas inside. The left valve was connected to the pressurized gas bottles (switched between methane, carbon dioxide and helium), that supplied the gas used for adsorption properties investigation. The right valve was connected to the diffusion pump and allowed for creating vacuum conditions inside the line and degassing of the sample. The bottom valve was connected to the cell with powder of investigated adsorbent material inside. The cell and the stabilization cell were submerged in the water bath, in order to conduct the experiments in the conditions of controlled temperature. For degassing of the sample vacuum pump and diffusion pump were applied in the line. The liquid nitrogen was used for removal of contaminations from the line by their condensation in the trap. The general scheme of high pressure adsorption line installation is presented in the Fig 2. Fig. 2. Scheme of the adsorption line The sample powder ca..5 g was placed inside the cell, which was connected to the adsorption line and degassed under vacuum conditions with diffusion pump for 2.5 h at 5 o C using oven with thermocouple for temperature control. Next, the cell was cooled down to the room temperature and the cell factor measurements were conducted. In order to calculate the cell factor extrapure helium (Praxair, purity %) was introduced into the cell at pressures increasing accordingly to the order: 2, 4, 6 and kpa. After opening the cell valve, the resulting pressure decrease was used to calculate the cell factor. For adsorbents prepared at ph 7 the cell factor was calculated at 25 o C, for adsorbents at ph 5 at 25 and 45 o C. The cell with sample and calibrated cell were submerged in waterbath for temperature control. After finishing the calibration helium was eliminated from the cell under vacuum. Next, investigation of adsorption isotherms were conducted using pressurized gases: methane (Air Liquide, purity %) and carbon dioxide (Criolab, purity %). In order to obtain the isotherms portions of gas were purged into the cell starting from 2 kpa and doubling the pressure each time up to the value of kpa. The value of pressure drop was noted after reaching equilibrium pressure and the amount of the gas adsorbed was calculated. After finishing the experiment the powder was cleaned removing the gas by heating for ca. h at 5 o C in vacuum conditions. On the basis of obtained results adsorption isotherms of carbon dioxide and methane adsorption on amino acid intercalated MMT were plotted. The isotherms show results obtained at 25 o C for clay based adsorbents at ph 7 and at 25 and 45 o C for adsorbents produced at ph 5. The obtained results were fitted into virial equation. The analytical expressions were further applied for calculations of selectivity of materials and adsorption of carbon dioxide and methane from the binary mixture. 4

5 intensity [a. u.] intensity [a. u.] Clay materials modified with amino acids for purification processes of biogas and natural gas Table. 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. MMT ph 7 ph 5 GLY-7 ARG-7 L-HIST-7 GLY-5 ARG-5 L-HIST-5 θ [ o ] d [Ǻ] expansion [Ǻ] ph7 ph MMT GLY ARG L-HIST 8 6 MMT GLY ARG L-HIST θ [ o ] θ [ o ] Fig. 3. XRD plots for amino acid modified materials obtained at ph 7 and ph Results and discussion Regarding the data obtained with FTIR method for amino acid intercalated samples, the FTIR analysis could not provide reliable proof for the presence of amino acid molecules in the clay structure for all the investigated materials. According to a big dilution of amino acids in clay material in was very difficult to obtain noticeable peaks for respective peaks in the spectra. Also the peaks coming from the raw material montmorillonite influenced to the great extent the plot of spectra, covering the presence of amino acids. Materials obtained at ph 7 gave no characteristic peaks visible in spectra and only peaks characteristic for MMT were observed. Samples prepared at ph 5 showed some peaks that could correspond to the amino acids presence, although it was hard to relate them to specific vibrations. In XRD analysis a characteristic, strong and sharp basal reflection for the MMT plot can be observed samples shift of the basal peak to smaller θ values was noticed (Fig. 3.). It indicates increase of interlayer spacing in the structure of investigated materials, what can be explained by expansion of clay structure caused by intercalation of amino acid molecules. The results of calculated interlayer spacing along with values of MMT structure expansion are presented in the Table. The basal spacing for the pure MMT was calculated as 2.27 Ǻ, which is characteristic value of basal spacing for clay materials, usually enclosed between 2 to 4 Ǻ. It can be noticed that the greatest expansion of spacing is present in case of ARG-7, equal to.2 Ǻ, which is followed by L- HIST-7 with the value of.7 Ǻ. The least expanded material of all the investigated samples was GLY-7 with expansion corresponding to.4 Ǻ. Adsorbent samples obtained at ph 5 presented medium expansion with values located between for θ equal to 3.6 o. For all the amino acid modified marginal results for materials at ph 7. Among 5

6 mass [%] heat flow [mv] mass [%] heat flow [mv] Clay materials modified with amino acids for purification processes of biogas and natural gas MMT GLY ARG L-HIST 2 ph7 MMT GLY ARG L-HIST ph temperature [ o C] temperature [ o C] Fig. 4. TG DSC plots for adsorbents prepared at ph 7 and ph 5. them the most expanded material was L-HIST-5 amounting to.65 Ǻ. The least expanded material was GLY-5 with expansion of.59 Ǻ. In the TG DSC analysis (Fig. 4) the raw MMT the first loss of mass of ca. 7% was observed between 3 and 5 o C, which contributed to the release of water physically adsorbed on the clay. It corresponded to the endothermic peak that could be observed on the heat flow in the same range of temperatures. During further heating the mass of MMT remained constant until 6 o C. The sharper decrease of mass beyond 6 o C resulted from destruction of clay structure under high temperature. For intercalated clays prepared at ph 7 analogous mass loss due to water evaporation can be noticed, but with smaller mass decrease indicated; approximately 4% for L- HIST-7 and 6% for GLY-7 and ARG-7. Next, another small decrease of mass (around %) was observed, which also reflected in slight endothermic peak on the heat flow plot, that contributed to the loss of water adsorbed in the pores. Further slow decrease of mass was observed for GLY-7 and L-HIST-7 (ca. 5% and 6% respectively) in the region of 3 to 6 o C In case of ARG-7 no significant mass decrease was observed in this region. However, corresponding strong endothermic decrease in the heat flow can be noticed. It indicated decomposition of adsorbed amino acids from the interlayer space of clays. In case of materials obtained at ph 5 analogical mass loss along with characteristic endothermic peaks can be observed in the range of 5 to 5 o C contributing to the water loss. The mass loss contributing to amino acids removal amounted to 3% for ARG-5, 6% for GLY-5 and 7% for L- HIST-5., what indicates more efficient intercalation Table 2. Porosity and surface properties characteristics of pure MMT and amino acids modified MMT materials. MMT ph 7 ph 5 GLY-7 ARG-7 L-HIST-7 GLY-5 ARG-5 L-HIST-5 BET surface area [m 2 /g] micropore volume [cm 3 /g] micropore area [cm 2 /g] 2 external surface area [cm 2 /g] total pore volume [cm 3 /g]

7 The results of nitrogen adsoption-desorption analysis were gathered in the Table 2. GLY-5 and ARG-5 show the best porosity and surface properties among all the adsorbents prepared, with surface area amounting to 35 m 2 /g and total pore volume.74 cm 3 /g for ARG-5 and.72 cm 3 /g for GLY-5. Both adsorbents intercalated with L- histidine; L-HIST-7 and L-HIST-5 exhibited relatively low development of surface area, 4 cm 2 /g and 7 cm 2 /g respectively, which was lower than for MMT with 8 cm 2 /g. ARG-7 and GLY-7 presented medium values of surface area with 22 cm 2 /g and 34 cm 2 /g respectively and pore volume.44 cm 3 /g and.54 cm 3 /g respectively. As presented in the Fig. 5, at ph 7 the highest adsorption capacity towards carbon dioxide was shown by ARG-7 with the value of.46 mmol/g at high pressures and it was the only material among intercalated clays, that showed better adsorption abilities than raw MMT with CO 2 retention amounting to.25 mmol/g at high pressures. Both GLY-7 and L-HIST-7 exhibited lower amount of CO 2 adsorbed with results of.8 and.23 mmol/g respectively. In case of methane adsorption, amounts of gas adsorbed were lower than for carbon dioxide for all the investigated adsorbents. Amount of CH 4 retained by GLY-7 was very low (.3 mmol/g) and below the sensitivity of the method. For this reason results of this adsorption isotherm are not presented on the graph. ARG-7 L-HIST-7 GLY-7 MMT CO 2 L-HIST-7 ARG-7 CH 4,8,8 nads [mmol/g],6,4 nads [mmol/g],6,4,2, Fig. 5. Adsorption isotherms of carbon dioxide and methane adsorption at 25 o C on amino acid intercalated MMT samples prepared at ph 7. ARG-5 L-HIST-5 GLY-5 MMT GLY-5 ARG-5 L-HIST-5 CO 2 CH 4,8,8 nads [mmol/g],6,4 nabs [mmol/g],6,4,2, Fig. 6. Adsorption isotherms of carbon dioxide and methane adsorption at 25 o C on amino acid intercalated MMT samples prepared at ph 5. 7

8 Selectivity nads [mmol/g] Clay materials modified with amino acids for purification processes of biogas and natural gas L-HIST-5 ARG-5 GLY-5 MMT 25 o C L-HIST-5 ARG-5 GLY-5 45 o C,8,8 nads [mmol/g],6,4,6,4,2, Fig. 7. Adsorption isotherms of carbon dioxide adsorption at 25 o C and 45 o C on amino acid intercalated MMT samples prepared at ph 5. Methane retained on ARG-7 and L-HIST-7 reached value of. mmol/g. Due to low amounts of gases adsorbed at 25 o C, for the materials prepared at ph 7 adsorption at higher temperature (45 o C) was not conducted. For ph 5 (Fig. 6.) the amounts of carbon dioxide adsorbed increased for all the intercalated clays in comparison to the values obtained at ph 7, with the best result for ARG-5, equal to.8 mmol/g. It was the highest amount of carbon dioxide adsorbed among all experiments conducted in presented research. The second value belonged to L-HIST-5 with.53 mmol/g of CO 2 adsorbed. The material of lowest adsorptive properties was GLY-5, which retained.32 mmol/g. All adsorbents prepared at ph 5 exhibited better adsorption of carbon dioxide than raw MMT material. ARG-5 and L-HIST-5 are capable of adsorbing over.4 mmol/g of CO 2 at pressures lower than 2 kpa. It indicates the possibility to conduct the adsorption process at lower pressure, what is favourable from technical and economical point of view for further implementation to industrial-scale processes. The amounts of methane adsorbed were significantly lower than CO 2. As it can be noticed in Fig. 7, at 45 o C ARG-5 showed the highest adsorption properties with.72 mmol/g of all the samples investigated at ph 5. Although L-HIST-5 presented slightly lower results with final amount of carbon dioxide adsorbed equal to.64 mmol/g, it obtained better results than ARG-5 at lower pressures, reaching.5 mmol/g at 24 kpa. GLY-5 was the material with the lowest adsorptive properties, reached.32 mmol/g. It should be pointed out, that even though it was expected, that at higher temperature the adsorbents would exhibit lower properties of adsorption, GLY- 5 and L-HIST-5 reached higher amounts of carbon dioxide adsorbed at 45 o C than in case of the experiment conducted at 25 o C L-HIST-5 ARG-5 GLY-5 L-HIST-7 ARG p / kpa Fig. 8. Selectivity of aminoacid intercalated MMT adsorbents against pressure. 8

9 y CH4,8,6,4,2 L-HIST-5 ARG-5 GLY-5 L-HIST-7 ARG-7,5 adsorbed amount / mmol g -,5,4,3,2, ARG-7 CO2 CH4 Total,5 adsorbed amount / mmol g -,5,4,3,2, L-HIST-7 CO2 CH4 Total,5 adsorbed amount / mmol g -,5,4,3,2, GLY-5 x CH4 CO2 CH4 Total,5 adsorbed amount / mmol g -,5,4,3,2, ARG-5 ych 4 CO2 CH4 Total,5 adsorbed amount / mmol g -,5,4,3,2, L-HIST-5 ych 4 CO2 CH4 Total,5 ych 4 ych 4 ych 4 Fig. 9. Phase diagrams at kpa describing separation of CH 4 between gaseous and adsorbed phase as a function of mole fraction of CH 4 in adsorbed phase (left up) and phase diagrams for L-HIST-7, ARG-7, L-HIST-5, ARG-5 and GLY-5. Among materials used for experiences L-HIST-5 presented the best selectivity properties. (Fig. 8.) The second adsorbent was ARG-7 followed by ARG-5. The least selective materials were L-HIST- 7 and GLY-5. It is remarkable, that for while samples with low selectivity, the selectivity is not much dependent on pressure (GLY-5, L-HIST-7, ARG-5). For materials highly selective, selectivity grows significantly with increase of pressure. The graphs presented in the Fig 9. show that L- HIST-5 has best adsorptive properties among all the investigated samples. Up to.7 molar fraction of CH 4 contained in the mixture, no adsorption of this gas is exhibited and only CO 2 is being retained with amount of.3 mmol/g of CO 2 adsorbed. The retention of CO 2 remains predominant up to.96 molar fraction of CH 4 in the binary mixture. ARG-7 shows exclusivity of CO 2 adsorption until.25 molar fraction of CH 4 with retention of CO 2 equal to.34 mmol/g at this point. Interesting properties were also shown by ARG-5 with the highest retention of both pure gases, with values amounting to.44 mmol/g for CO 2 and. mmol/g for CH 4. Very poor selectivity and adsorption properties were shown by L-HIST-7 and GLY Conclusions Intercalation process was successful for all the materials prepared. Due to insertion of amino acid molecules into the interlayer space expansion of montmorillonite structure was observed on XRD spectra, as well as for all the samples, except for L- HIST-5, increase of surface area and total pore volume was noted. The CO 2 adsorption was strongly enhanced by intercalation with amino acids.higher adsorption capacity was obtained for adsorbents prepared at ph 5. It can be explained by greater retention of amino acids by clay material at this ph due to protonation of amine groups of amino acids, what causes the increase of their availability for carbon dioxide adsorption. The material with highest adsorption capacity towards CO 2 was ARG-5 with.8 mmol/g. Both arginine 9

10 and L-histidine intercalated clays showed satisfactory results and their adsorption capacity towards carbon dioxide was higher than in case of raw montmorillonite. Adsorption on glycine intercalated samples did not give promising results. For the tests conducted at temperature 45 o C at ph 5, the amount of carbon dioxide adsorbed was higher for ARG-5 and GLY-5 than at 25 o C. Retention of methane by intercalated clays gave lower results than for CO 2 for all materials. In terms of selectivity L-HIST-5 was the best adsorbent, adsorbing exclusively CO 2 up to.7 molar fraction of CH 4 in the binary mixture and predominant CO 2 retention up to.96 molar fraction of CH 4. Amino acid modified montmorillonite adsorbents showed promising results for CO 2 and CH 4 separation. Regarding their low-cost, environmental friendly character and selectivity, they can be successfully applied in the industrial separation processes. 7. References [] Hijazi O., Munro S., Zerhusen B., Effenberger M., Review of life cycle assessment for biogas production in Europe, Renewable and Sustainable Energy Reviews, 54 (26) [2] Guo M., Song W., Buhain J., Bioenergy and biofuels: History, status, and perspective, Renewable and Sustainable Energy Reviews, 42 (25) [3] Lourinho G., Brito P., Assessment of biomass energy potential in a region of Portugal (Alto Alentejo), Energy, 8 (25) [4] Ryckebosch E., Drouillon M., Vervaeren H., Techniques for transformation of biogas to biomethane, Biomass and bioenergy, 35 (2) [5] Poeschl M., Ward S., Owende P., Prospects for expanded utilization of biogas in Germany, Renewable and Sustainable Energy Reviews, 4 (2) [6] Molino A., Nanna F., Ding Y., Bikson B, Braccio G, Biomethane production by anaerobic digestion of organic waste, Fuel, 3 (23) 3 9. [7] Starr K., Gabarrell X., Villalba G, Talens L, Lombardi L Life cycle assessment of biogas upgrading Technologies Waste Management, 32 (22) [8] Esteves I. A. A. C., Lopes M. S. S., Nunes P. M. C., Mota J. P. B., Adsorption of natural gas and biogas components on activated carbon, Separation and Purification Technology, 62 (28) [9] Budzianowski W. M., A review of potential innovations for production, conditioning and utilization of biogas with multiple-criteria assessment, Renewable and Sustainable Energy Reviews, 54 (26). [] AlMamun M. R., Karim M. R., Rahman M. M., Asiri A. M., Torii S., Methane enrichment of biogas by carbon dioxide fixation with calcium hydroxide and activated carbon, Journal of the Taiwan Institute of Chemical Engineers, 58 (26) [] Varma R. S., Clay and Clay-supported reagents in organic synthesis, Tetrahedron, 58 (22) [2] Volzone C., Ortiga J., Influenece of the exchangeable cations of montmorillonite on gas adsorptions, Process Safety and Environmental Protection, 82 (24) [3] Kollar T., Palinko I., Konya Z., Kiricsi I., Intercalating amino acid guests into montmorillonite host, Journal of Molecular Structure, (23) [4] Fernandes A. C., Pinto M. L., Antunes F., Pires J., L-histidine-based organoclays for the storage and release of therapeutic nitric oxide, Journal of Materials Chemistry B, 3 (25)