pubs.acs.org/Langmuir © 2009 American Chemical Society
New Insights on CO2-Methane Separation Using LTA Zeolites with Different Si/Al Ratios and a First Comparison with MOFs Miguel Palomino, Avelino Corma,* Fernando Rey, and Susana Valencia Instituto de Tecnologı´a Quı´mica (CSIC-UPV), Campus de la Universidad Polit ecnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain Received July 21, 2009. Revised Manuscript Received August 26, 2009 LTA zeolites can be synthesized with tailored adsorption properties by controlling the Al content in the framework. In this work, we have demonstrated that it is possible to adjust the polarity of the zeolitic adsorbent to optimize its thermodynamic adsorption properties for the energetically relevant CO2/CH4 separation process. The thermodynamic study has been made from the corresponding adsorption isotherms of the pure gases carried out at different pressures and temperatures, as well as breakthrough separation experiments of CO2/CH4 mixtures and the results were compared to those reported on MOFs. The separation values obtained allow us to conclude that LTA zeolites offer unique possibilities for CH4 upgrading from natural gas.
Introduction The development of microporous materials with controlled pore apertures and large adsorption capacities is of paramount importance to perform selective adsorption and separation of gases. A clear example is found for the separation of branched from linear hydrocarbons with small pore zeolites due to the pore restriction imposed to the diffusion of the branched paraffins through pores having diameters of 0.4 nm.1,2 Recently, the advent of pure silica zeolites, with complete absence of acidity, has allowed performing these separations even in the presence of olefins, something which was not possible before with Al-containing zeolites due to the occurrence of olefin oligomerization that causes pore blocking of the zeolites.3-6 On this regard, propene could be successfully separated from propane using pure silica ITQ-3,6 ITQ-12,7 Deca-Dodecasil,8 and ITQ-32,9 isobutene may be separated from linear C4 olefins using ITQ-29,3,10 which is the pure silica analogue of the Linde Zeolite A (LTA). Finally, pure silica ITQ-32 and RUB-41 were found to be able to separate linear 1-butene and/or trans-2-butene from butane and cis-2-butene in the C4 raffinate stream.9,11 Additionally, medium pore zeolites with micropore apertures close to 0.55 nm, such as ZSM-5, can discriminate between monobranched and dibranched paraffins, the latest with higher octane number.12 In all the above-mentioned *Corresponding author. E-mail:
[email protected]. (1) Barrer, R. M. In Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978; Chapters 1, 2, and 6. (2) Breck, D. W. In Zeolite Molecular Sieves: Structure, Chemistry and Use; John Wiley: New York, 1974; Chapters 1 and 2. (3) Corma, A.; Rey, F.; Rius, J.; Sabater, M. J.; Valencia, S. Nature 2004, 431, 287. (4) Olson, D. H. U.S. Patent 2002/0144597, 2002. (5) Reyes, S. C.; Olson, D. H.; Liu, H.; Strohmaier, K. G.; Santiesteban, J. G. U.S. Patent 2005/0096494, 2005. (6) Olson, D. H.; Camblor, M. A.; Villaescusa, L. A.; Kuehl, G. H. Microporous Mesoporous Mater. 2004, 67, 27. (7) Olson, D. H.; Yang, X.; Camblor, M. A. J. Phys. Chem. B 2004, 108, 11044. (8) Zhu, W.; Kapteijn, F.; Moulijn, J. A. Chem. Commun. 1999, 2453. (9) Palomino, M.; Cantin, A.; Corma, A.; Leiva, S.; Rey, F.; Valencia, S. Chem. Commun. 2007, 1233. (10) Corma, A.; Rey, F.; Valencia, S. WO Patent 2006/035090, 2006. (11) Tijsebaert, B.; Varszegi, C.; Gies, H.; Xiao, F. S.; Bao, X.; Tatsumi, T.; M€uller, U.; De Vos, D. Chem. Commun. 2008, 2480. (12) Denayer, J. F.; Souverijns, W.; Jacobs, P. A.; Martens, J. A.; Baron, G. V. J. Phys. Chem. B 1998, 102, 4588.
1910 DOI: 10.1021/la9026656
separations, the key feature of zeolites is the pore aperture that must be adequate to the kinetic diameters of the molecules that are attempted to separate. Moreover, besides the pore aperture, the most important variable for achieving good separations is the polarity of the adsorbent.13 This is particularly true for gas separation processes in which small molecules are involved, such as N2 and O2 separation from air14 or the upgrading natural gas by removal of CO2 and/or N2.15-17 For the specific cases named above, aluminum and cation containing zeolites have been considered because the high electrostatic fields present inside the channels and cavities of the zeolites results in a preferential adsorption of the most polar molecule. One of the today most important application of zeolites as adsorbent is the upgrading of natural gas. Indeed, methane is seen as a strategic source of energy in a midterm scale and from an economical point of view, the key point for the profitability of natural gas landfills is its transportation as liquefied gas or through pipelines.15 The natural gas should not be directly liquefied or introduced into the pipelines when there is CO2 in relatively high concentration, because it is highly corrosive in the presence of water and could affect the integrity of the pipelines. Also, condensable CO2 at liquid-methane temperature could provoke pipeline and valve blocking of the equipments. Then, the actual regulations for natural gas transportation strongly limit the CO2 concentration in the stream. The actual technology for CO2 removal from natural gas resources is based on the amine scrubbing that must be performed on site. Frequently, this is not economically viable for relatively small natural gas resources or for wells located at remote places. North-African, Siberian, and Sea-Fields are good examples of these cases. Then, other simpler technologies must be applied for revalorization of these natural gas resources. (13) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity; Academic Press: London, 1982; p 82. (14) Yang, R. T. In Gas Separation by Adsorption Processes. Series on Chemical Engineering; Imperial College Press: London, 1997; Vol. 1, pp 7 and 25. (15) Tagliabue, M.; Farrusseng, D.; Valencia, S.; Aguado, S.; Ravon, U.; Rizzo, C.; Corma, A. Chem. Eng. J. 2009, accepted. (16) Harlick, P. J. E.; Tezel, F. H. Microporous Mesoporous Mater. 2004, 76, 71. (17) Kuznicki, S. M.; Bell, V. A.; Nair, S.; Hillhouse, H. W.; Jacubinas, R .M.; Braunbarth, C. M.; Toby, B. H.; Tsapatsis, M. Nature 2001, 412, 720.
Published on Web 09/16/2009
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The use of zeolites as adsorbents for natural gas upgrading must be accompanied by the appropriate separation technology. On this regard, Pressure Swing Adsorption (PSA) process seems to be the most adequate one, since it is based on the selective adsorption of one component of the gas mixture at relatively high pressures and its release upon decreasing pressure. The main benefit of this technology for natural gas treating is (1) there is no chemical disposal in the whole process and (2) only very little energy is needed because no compression of the natural gas is required because of its very high pressure at the wellhead.15 In consequence, porous materials have been studied for methane separation. Recently, it has been proposed that porous metal-organic frameworks (MOFs) possessing giant adsorption capacities are excellent candidates for CH4/CO2 separation.18-22 Particularly, it was claimed that imidazolate based frameworks (ZIFs) exhibit extraordinary capacity for storing CO223 (1 L of ZIF-69 can hold approximately 83 L of CO2 at 273 K under ambient pressure) and good selectivity for CO2 capture from CO2/CH4 mixtures.18,20 These numbers, which may look impressive, need to be regarded in perspective with those obtained with other existing microporous inorganic materials. On this regard, zeolites have been studied for upgrading of natural gas and mostly large pore zeolites of low Si/Al ratios have been employed. Particularly, zeolite 13X,24,25 zeolite Y26,27 (which are Fuajasitetype of zeolites with Si/Al ratio close to 1 and 2.5, respectively), and Mordenite28 have shown a good ability for methane purification from mixtures of gases containing CH4/CO2/N2. Also, medium and small pore zeolites have been employed for this purpose, particularly ZSM-5,29 pure-silica DD3R,30 Zeolite T31 (for which because of its characteristic stacking faulting only 8MR channels are effective for adsorption purposes), cationexchanged Linde A zeolites32 and some silico-aluminophosphates, such as SAPO-34,33 AlPO-18, and STA-7,34,35 with excellent properties for gas separation. This is particularly true when these microporous solids are synthesized as membranes, being observed that the gas separation ability greatly increases. This has been clearly observed in zeolite T31 where the selectivity (expressed as QCO2/QCH4) increases from 3 in powder form to (18) Hayashi, H.; C^ote, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Nat. Mater. 2007, 6, 501. (19) Llewellyn, P. L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.; Hamon, L.; De Weireld, G.; Chang, J. S.; Hong, D. Y.; Hwang, Y. K.; Jhung, S. H.; Ferey, G. Langmuir 2008, 24, 7245. (20) Wang, B.; C^ote, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Nature 2008, 453, 207. (21) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 3875. (22) Cavenati, S.; Grande, C. A.; Rodrigues, A. E.; Kiener, C.; M€uller, U. Ind. Eng. Chem. Res. 2008, 47, 6333. (23) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939. (24) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. J. Chem. Eng. Data 2004, 49, 1095. (25) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Chem. Eng. Sci. 2006, 61, 3893. (26) Kusakabe, K.; Kuroda, T.; Murata, A.; Morooka, S. Ind. Eng. Chem. Res. 1997, 36, 649. (27) Ghoufi, A.; Gaberova, L.; Rouquerol, J.; Vincent, D.; Llewellyn, P. L.; Maurin, G. Microporous Mesoporous Mater. 2009, 119, 117. (28) Vansant, E. F.; Voets, R. J. Chem. Soc. Faraday Trans. I 1981, 77, 1371. (29) van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. Chem. Eng. Sci. 1999, 54, 259. (30) Himeno, S.; Tomita, T.; Suzuki, K.; Yoshida, S. Microporous Mesoporous Mater. 2007, 98, 62. (31) Cui, Y.; Kita, H.; Okamoto, K. J . Mater. Chem. 2004, 14, 924. (32) Sircar, S.; Golden, T. C. Sep. Sci. Technol. 2000, 35, 667. (33) Venna, S. R.; Carreon, M. A. J. Phys. Chem. B 2008, 112, 16261. (34) Deroche, I.; Gaberova, L.; Maurin, G.; Castro, M.; Wright, P. A.; Llewellyn, P. L. J. Phys. Chem. C 2008, 112, 5048. (35) Deroche, I.; Gaberova, L.; Maurin, G.; Llewellyn, P.; Castro, M.; Wright, P. Adsorption 2008, 14, 207.
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266 in membranes; and in SAPO-34, where the observed selectivity was 9 for powdered SAPO-34,33 while in membrane form ranged from 170 to 506 depending on the synthesis conditions and operation procedure.36-39 It is generally accepted that the separation ability increases with increasing electrostatic field in the zeolite cavities, which is mostly affected by the framework charge, which can be affected either by the nature of the charge compensating cations,40-43 or by the Al content of the zeolite framework.43 An interesting material for CH4/CO2 separation would be LTA zeolite. However, this material is normally synthesized with a very low Si/Al ratio and then, the desorption of CO2 becomes the limiting factor. We will show here that by preparing zeolite LTA with an optimum Si/Al ratio, it is possible to greatly improve the desorption properties, while maintaining a good adsorption capacity. Thus, in this paper, we have synthesized five Zeolites A (LTA) with different aluminum contents ranging from Si/Al = 1 (Zeolite 4A) to the pure silica LTA material (ITQ-29).3,44-46 Their adsorption capacities of CO2 and CH4 have been studied at different temperatures (close to ambient temperature) and the corresponding thermodynamic analyses of the resulting isotherms have been established.
Materials and Methods Adsorbents. Syntheses of LTA zeolites with different Si/Al ratios were performed following reported methods in the existing literature.3,45,46 Pure-silica LTA, also named as ITQ-29, was synthesized in fluoride medium at 408 K using 4-methyl-2,3,6,7tetrahydro-1H,5H-pyrido[3.2.1-ij] quinolinium (ROH) and tetramethylammonium (TMAOH) hydroxides as organic structure directing agents (SDA) from a gel of molar composition 1:0.25: 0.25:0.5:3 SiO2:ROH:TMAOH:HF:H2O. LTA zeolites with Si/Al ratios of 5, 3.5, and 2 were prepared at 373 K using diethyl-dimethylammonium (DEDMAOH), tetraethylammonium (TEAOH), TMAOH, and TMACl as organic SDAs from gels of the following compositions: Sample LTA-5: [1:0.05:0.30:0.20:0.05:0.05:17SiO2:Al2O3:TEAOH:DEDMAOH: TMACl:NaCl:H2O] Sample LTA-3.5: [1:0.06:0.85:012:0.05:30 SiO2:Al2O3:DEDMAOH:TMACl:NaCl: H2O] Sample LTA-2: [1:0.17:0.78:0.20:33 SiO2:Al2O3:TMAOH:Na2O:H2O] 1:1:0.17:0.78:0.20:33 LTA-2:SiO2:Al2O3:TMAOH:Na2O:H2O Finally, LTA zeolite with a Si/Al ratio of 1 in its sodium form was commercially available from Aldrich as Molecular Sieves 4A. The zeolites prepared in the presence of organics were submitted to calcination processes at temperatures comprised between 773 and 973 K in air and Na+ sodium exchange before starting the adsorption measurements with the aim of removing the occluded organic. The integrity of the samples used in this study was checked by means of X-ray diffraction (see Figure S1 in the Supporting Information) and micropore volume and surface area (36) Poshusta, J. C.; Tuan, V. A.; Pape, E. A.; Noble, R. D.; Falconer, J. L. AIChE J. 2000, 46, 779. (37) Li, S.; Falconer, J. L.; Noble, R. D. Adv. Mater. 2006, 18, 2601. (38) Carreon, M. A.; Li, S.; Falconer, J. L.; Noble, R. D. Adv. Mater. 2008, 20, 729. (39) Carreon, M. A.; Li, S.; Falconer, J. L.; Noble, R. D. J. Am. Chem. Soc. 2008, 130, 5412. (40) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5896. (41) Delgado, J. A.; Uguina, M. A.; Gomez, J. M.; Ortega, L. Separ. Purif. Technol. 2006, 48, 223. (42) Vansant, E.; Voets, R. J. Chem. Soc., Faraday Trans. I 1981, 77, 1371. (43) Llewellyn, P. L.; Maurin, G. Stud. Surf. Sci. Catal. 2007, 168, 555. (44) Kerr, G. T. U.S. Patent 3 314 752, 1967. (45) Wadlinger, R. L., Rosinski, E. J.; Plank, C. J. U.S. Patent 3 375 205, 1968. (46) Moscoso, J. G.; Lewis, G. J.; Gisselquist, J. L.; Miller, M. A.; Rohde, L. M. WO Patent 03/068679, 2003.
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Palomino et al. Table 1. Physicochemical Properties of LTA Zeolites
sample
Si/Al
SBET (m2 g-1)
VMICROPORE (cm3 g-1)
¥ 695 0.32 LTA-SiO2 LTA-5 5.0 654 0.27 LTA-3.5 3.5 642 0.30 LTA-2 1.9 583 0.28 a a LTA-1 1.0 a N2 is not able to access the porosity of this zeolite because of the presence of a large amount of Na cations.
determination from the N2 adsorption isotherms at 77 K measured in a Micromeritics ASAP 2010 instrument. Both techniques show that the structural integrity of the zeolites was preserved upon organic removal by calcination. Chemical composition of the calcined LTA samples was determined by ICP on a Varian 715-ES ICP-Optical Emission Spectrometer. The characteristics of the LTA samples used in this work are summarized in Table 1. Adsorption Measurements. CO2 and CH4 adsorption isotherms in the range of pressures comprised between vacuum and 500 kPa were measured in a volumetric apparatus (up to 100 kPa) and in a gravimetric instrument (up to 500 kPa). The adsorption isotherms in the low pressure range were measured in a Micromeritics ASAP 2010 instrument using approximately 200 mg of adsorbent placed in a sample holder that was immersed into a liquid circulation thermostatic bath for precise temperature control. Before each measurement, the sample was treated overnight at 673 K under vacuum. CO2 and CH4 adsorption isotherms were then acquired at 273, 283, and 303 K. The adsorption isotherms in the pressure range up to 500 kPa were measured in an IGA-3 gravimetric analyzer (Hiden Isochema) using approximately 50 mg of sample, that were placed in the balance and outgassed at 673 K under vacuum during 4 h before each adsorption experiment. The temperature of the sample was subsequently reduced under vacuum until the target temperature (303 K) was reached. The CO2 and CH4 adsorption measurements were performed by introducing gas to build up the desired pressures into the gravimetric system. The equilibrium conditions were fixed at 98% of the calculated uptake using the Avrami-Erofe’ev model47 or a maximum equilibration time of 120 min for each point of the isotherm. CO2/CH4 Separation Experiments. The gas separation properties of LTA zeolites were examined by breakthrough experiments using CO2/CH4/He gas mixtures (48:48:4 v/v/v). He flow was used as internal standard and it was assumed that it can freely diffuse through the zeolite bed. The zeolite samples (2.5 mL) were packed into a stainless-steel column and activated at 723 K under Ar flow before running the experiment. The separations were carried out at 303 K and dosing the gas mixture (140 kPa) at a flow rate of 20 mL min-1. The relative amounts of the gases passing through the column were monitored on a Balzers Instruments mass spectrometer detecting ion peaks at m/z 44, 28, and 12 for CO2, and m/z 15 for CH4, and using He (m/z = 4) as internal standard.
Results and Discussion CO2 Adsorption. High-resolution CO2 adsorption isotherms were conducted at 273, 283, and 303 K in the pressure range from 110-2 to 100 kPa on the different LTA zeolite samples. The semilogarithmic plots of the isotherms measured at 303 K are presented in Figure 1a to magnify the Henry’s law regime, where there is a linear correlation between pressure and adsorption. The same isotherms are plotted in linear scale in Figure S2 of the Supporting Information. Finally, high-pressure CO2 adsorption isotherms in the range from 0.1 to 500 kPa are shown in Figure 1b. It can be seen there that the adsorption capacity of the LTA samples increases as the Al content increases, reaching a maximum value at Si/Al ratio (47) Erofe’ev, B. V.; Dokl, C. R. Acad. Sci., USSR 1946, 52, 511.
1912 DOI: 10.1021/la9026656
equals to 2. Above this aluminum content, the adsorption capacity decreases because the volume occupied by the sodium cations placed at extraframework positions is not negligible and the available pore volume for CO2 adsorption diminishes. More importantly than the adsorption capacity is the shape of the isotherms, which are better seen in the semilogarithmic plots (Figure 1a). It is clear from this figure that the CO2 adsorption capacities at low pressure regime increases as the Al content does (i.e., the isotherm plots are closer to the y-axis). This behavior unambiguously indicates that the interaction between adsorbed CO2 molecules and the LTA zeolite walls increases with the Al content. This qualitative assertion can be quantified by applying typical isotherm models, such as Langmuir,48 Toth,24,49 or multiple and dual site Langmuir (DSL)50-52 models, which can conveniently describe the experimental adsorption data. The typical fittings of the different models for the CO2 adsorption isotherms obtained for the LTA zeolite having a Si/Al ratio of 2 are shown in Figure S3 in the Supporting Information. There it can be seen that Langmuir model does not describe properly the experimental isotherm, whereas Toth model provides a slight improvement of the fitting, being the DSL model the one that better describes the experimental isotherms. This conclusion could be expected, because Toth and DSL models are able to describe heterogeneities in the adsorption processes, such as those produced by the presence of different channels or preferential adsorption sites.24,52 However, the fitted parameters using these models must follow reasonable tendencies as the adsorption temperature changes. The best fitting parameters for Langmuir, Toth and DSL models with the experimental CO2 isotherms on LTA zeolite of Si/Al = 2 obtained at different temperatures are given in Table 2. There, it can be seen that the fitted parameters obtained using the DSL model do not follow any reasonable tendency when changing temperature. This is an indication that none of the models employed is able to properly describe the adsorption isotherms of CO2 on LTA zeolites obtained in this work, by providing thermodynamic parameters with real physical-chemical meaning. Then, we have applied the Virial isotherm for fitting experimental data points, because this approach is just a polynomial fitting without any starting assumption about the adsorption process, but which provides a precise extrapolation of the adsorption constant at zero coverage (Henry constant, KH).53,54 It has been found that a fourth grade polynomial is able to properly describe the CO2 isotherms on the LTA samples studied in this work at the different temperatures. Isosteric heats of adsorption provide information about the energy released during the adsorption process and depend on the temperature and surface coverage. The isosteric heats can be determined from a set of isotherms according to the ClausiusClapeyron equation55 Dðln PÞ 2 Dðln PÞ qst ¼ RT 3 R DT Q ¼cte Dð1=TÞ Q ¼cte The derivate must be calculated at constant CO2 uptake. Here, we have calculated the isosteric heats of the CO2 adsorption on the (48) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361. (49) Toth, J. Acta Chim. Acad. Sci. Hung. 1971, 69, 311. (50) Nitta, T.; Shigetomi, T.; Kuro-Oka, M.; Katayama, M. J. Chem. Eng. Jpn. 1984, 17, 39. (51) Drago, R. S.; Webster, C. E.; McGilvray, J. M. J. Am. Chem. Soc. 1998, 120, 538. (52) Zhu, W.; Kapteijn, F.; Moulijn, J. A.; den Exter, M. C.; Jansen, J. C. Langmuir 2000, 16, 3322. (53) Sun, M. S.; Shah, D. B.; Xu, H. H.; Talu, O. J. Phys. Chem. B 1998, 102, 1466. (54) Talu, O.; Li, J.; Kumar, R.; Mathias, P. M.; Moyer, J. D.; Schork, J. Gas Sep. Purif. 1996, 10, 149. (55) Hill, T. L. J. Chem. Phys. 1949, 17, 520.
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Figure 1. CO2 adsorption isotherms measured at 303 K in the (a) low- and (b) high-pressure ranges of LTA zeolites having Si/Al ratios of 1 (9), 2 (b), 3.5 (2), 5 (1), and ¥ ((). Table 2. Best Fitting Parameters of the Experimental CO2 Adsorption Isotherms on LTA Zeolite of Si/Al = 2 Using Different Adsorption Models Langmuir
Toth
T (K)
Q¥ (mol/g)
K (Pa-1)
Q¥ (mol/g)
K (Pa-1)
273 283 303
0.00499 0.00498 0.00496
0.0024 0.0013 0.0004
0.00538 0.00529 0.00510
0.0044 0.0019 0.0005
DSL m
Q¥,1 (mol/g)
0.65 0.72 0.89
0.00156 0.00160 0.07377
K1 (Pa-1) 2.64 10-5 1.42 10-5 6.78 10-8
Q¥,2 (mol/g)
K2 (Pa-1)
0.00439 0.00449 0.00471
0.0032 0.0016 0.0005
Figure 3. Isosteric heat of adsorption of CO2 at zero coverage (qst0) obtained for LTA zeolites with different Al content. Figure 2. Isosteric heat of adsorption (qst) vs CO2 coverage (Q) obtained for LTA zeolites of different Si/Al ratios (shown in the figure).
different LTA samples from isotherms taken at 273, 283, and 303 K. Figure S4 (Supporting Information) shows a typical plot for the calculation of the isosteric heat obtained for the LTA zeolite of Si/Al = 2. The isosteric heats of adsorption (qst) at different CO2 loadings (Q) for the LTA zeolites studied in this work are plotted in Figure 2. The adsorption energies of CO2 at zero coverage (qst0) increase from 21 to 49 kJ mol-1 from the pure silica LTA zeolite to the Zeolite A having an Si/Al ratio of 1 following a nearly linear correlation with the Al content, as is shown in Figure 3. The isosteric heats of adsorption versus coverage (Figure 2) indicate that this tendency is also observed at coverages up to Langmuir 2010, 26(3), 1910–1917
2.5 mmol g-1. However, above this loading, different adsorption behaviors start to appear. Indeed, the flatness of the isosteric heat curve observed for pure silica LTA zeolite indicates that gas-gas interactions are dominant along the whole adsorption process up to the maximum loading reached in this work, being this value close to 20 kJ mol-1. However, the LTA zeolites with the highest Al contents (Si/Al = 1 and 2, respectively) show a different behavior and the heat of adsorption decreases upon reaching a value close to 2.5 mmol g-1 of adsorbed CO2. This indicates that the adsorption regime at low CO2 coverage is dominated by solid-gas interactions, while this interaction becomes less important as the CO2 loading increases. The LTA zeolite having a Si/Al ratio equals to 3.5 is an intermediate case, and the isosteric heat of adsorption is smoothly decreasing along the whole CO2 loading, but showing a significantly higher isosteric heat of adsorption than the pure silica zeolite. DOI: 10.1021/la9026656
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Table 3. Uptake, Working Capacity, and Regenerability of LTA Zeolites Calculated from CO2 Adsorption Isotherms sample
CO2 uptakea (mmol/g)
CO2 working capacityb (mmol/g)
regenerabilityc (%)
3.11 75 LTA-SiO2 4.15 LTA-5 5.58 2.51 45 LTA-3.5 5.76 1.44 25 LTA-2 6.12 1.04 17 LTA-1 5.04 0.81 16 a CO2 uptake measured at 500 kPa and 303 K. b (Uptake at 500 kPa Uptake at 100 kPa). c (Working capacity/CO2 uptake) 100.
Figure 5. Isosteric heat of adsorption (qst) vs CH4 coverage (Q) obtained for LTA zeolites of different Si/Al ratios (shown in the figure).
Figure 4. CH4 adsorption isotherms measured at 303 K of LTA zeolites having Si/Al ratios of 1 (9), 2 (b), 3.5 (2), 5 (1), and ¥ (().
The isosteric heat of adsorption is an important point to be considered since the regenerability of the LTA adsorbent will depend on the CO2-adsorbent interaction. Then, low adsorption energy provides better regeneration value, but lower adsorption capacity for a given pressure than high adsorption heat for a pressure swing adsorption (PSA) process. On the contrary, high adsorption capacities can be achieved when high energy is associated with the adsorption process, but poor regeneration of the adsorbent will be obtained. Consequently, there is a window for the heat of adsorption value, which optimizes the adsorption capacity and regeneration, simultaneously. In the case of LTA zeolite, as it has been shown above, the adsorption energy value can be nicely tailored by controlling the Al content of the adsorbent and therefore, the optimization of adsorption capacity and regenerability can be afforded. To do that, we conducted relatively high pressure (up to 500 kPa) experiments of CO2 adsorption on the different LTA samples (Figure 1b). A convenient procedure for PSA process based on CO2 adsorption could be a process in which the adsorption step takes place at moderate pressures above atmospheric pressure (then, expensive compression units are not required), and desorption must be performed preferentially at atmospheric pressure (to avoid vacuum and/or heating systems). Consequently, we need to define two adsorption parameters: (1) the CO2 working capacity, which is the difference of the uptake at 500 kPa minus the uptake at atmospheric pressure (100 kPa); and (2) the regenerability, which can be defined as the ratio between the working capacity and the uptake at 500 kPa expressed as percentage. These two values can be directly calculated from the isotherms plotted in Figure 1b. The CO2 working capacities and regenerabilities of the different Al-content LTA zeolites are given in Table 3. 1914 DOI: 10.1021/la9026656
Figure 6. Isosteric heat of adsorption of CH4 at zero coverage (qst0) obtained for LTA zeolites with different Al content.
Figure 7. CO2/CH4 selectivity obtained with LTA zeolites having Si/Al ratios of 1 (9), 2 (b), 3.5 (2), 5 (1), and ¥ (() at 303 K and different pressure. The inset shows the low-pressure range.
It is clear that the working capacity increases as the Al content of the LTA sample diminishes. This is a consequence of the Langmuir 2010, 26(3), 1910–1917
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Table 4. Henry Constants Obtained from the CO2 and CH4 Adsorption Isotherms at 273, 283, and 303 K Using LTA Zeolites with Different Si/Al ratio As Adsorbents and the Corresponding CO2/CH4 Selectivity Coefficients r0 Derived from Them Henry constant, KH (mol/g /Pa) 108 CO2
R0 (CO2/CH4)
CH4
sample
273 K
283 K
303 K
273 K
283 K
303 K
273 K
283 K
303 K
LTA-SiO2 LTA-5 LTA-3.5 LTA-2 LTA-1
2.57 98.40 256.08 1365 7785.2
1.84 60.64 146.68 710.6 4540.0
1.04 23.37 56.29 209.4 963.1
0.573 0.921 1.049 1.242 2.460
0.469 0.722 0.811 1.162 1.713
0.298 0.444 0.505 0.643 0.758
4.5 106.8 244.1 1099.0 3164.7
3.9 84.0 180.9 611.5 2650.3
3.5 52.6 111.5 325.7 1270.6
improvement of the adsorbent regenerability (even considering the reduction of the observed CO2 uptakes discussed above) when the Al content decreases. Then, the optimum CO2 adsorption performance for PSA processes is found for LTA zeolites having Si/Al ratio higher than 5. Additionally, the methane adsorption parameters on LTA adsorbents must also be considered for CO2/CH4 separation purposes, because low adsorption energy of CO2 could give raise to a poor selectivity for preferential adsorption. We are then showing below the CH4 isotherms and discussing the calculated thermodynamic parameters on the same LTA zeolites. CH4 Adsorption. Figure 4 shows the high-pressure CH4 isotherms measured at 303 K up to 500 kPa, whereas Figure S5 in the Supporting Information provides the high-resolution CH4 isotherms up to 100 kPa. (Note that for comparison purposes in these and all figures given below, the same scale in y axes than in CO2 plots has been maintained). The isosteric heat of adsorption versus the CH4 coverage calculated using the Virial fitting of the isotherms is shown in Figure 5. Finally, the dependence of the extrapolated isosteric heat of adsorption at zero coverage with the Al content is displayed in Figure 6. It is clear that LTA zeolites show a lower affinity for CH4 than for CO2 adsorption, as was expected. This is evident from the much lower CH4 uptake since nearly linear adsorption isotherms are obtained on all the LTA samples, indicating that the CH4 adsorption processes on LTA samples is close to the Henry’s law regime. The calculated isosteric heats of adsorption are close to 17 kJ mol-1 except for LTA-1, which gives the highest CH4 adsorption energy at zero coverage (27 kJ mol-1), but it decays to the same value than other adsorbents as the coverage increases. This behavior for the CH4 adsorption on zeolite LTA having a Si/Al ratio of 1 could be attributed to the presence of the large amount of sodium cations within the cavities of the LTA zeolite of Si/Al = 1. CO2/CH4 Separation. A quantitative analysis of the influence of the Al-containing zeolites on the selective CO2 adsorption over CH4 uptake can be obtained from the study of the equilibrium selectivity factor (R),56,57 which is expressed as R = (QCO2)/ (QCH4), where (QCO2) and (QCH4) are the equilibrium uptakes of CO2 and CH4 at a given pressure taken from the corresponding single component isotherms. The equilibrium selectivities of the different LTA zeolites studied in this work are shown in Figure 7. In this figure, the inset shows the derived selectivities at low pressures using volumetric high-resolution isotherms, while main figure shows the variation of the R factor up to relatively high pressures (500 kPa). It can be seen in the figure that the selectivity of CO2 adsorption versus CH4 at relatively high pressures (higher than 300 kPa) is (56) Xu, X.; Zhao, X.; Sun, L.; Liu, X. J. Nat. Gas Chem. 2008, 17, 391. (57) Ribeiro, R. P.; Sauer, T. P.; Lopes, F. V.; Moreira, R. F.; Grande, C. A.; Rodrigues, A. E. J. Chem. Eng. Data 2008, 53, 2311.
Langmuir 2010, 26(3), 1910–1917
Figure 8. Schematic representation of a system proposed for achieving CO2/CH4 separation using different LTA zeolites as adsorbents.
practically the same for all the LTA samples, except for the LTA-1, which shows a lower selectivity because of its lower CO2 adsorption capacity as a consequence of the presence of a high concentration of Na cations located at the cavities of this zeolite. A selectivity higher than 3 is acceptable for separation purposes,41 and in the case of the LTA zeolites studied in this work excluding LTA-1, the selectivity is always higher than 3.5, which meets the selectivity criterion for PSA processes. The influence of the Al content on the CO2/CH4 selectivity coefficient becomes more important as the solid-gas interactions are more pronounced, that is at lower coverages. Then, the variation of the R factor calculated at low pressures (inset of Figure 7) provides useful information about the capabilities of LTA zeolites for CH4/CO2 separation processes. Indeed, Figure 7 indicates that R parameter abruptly increases at low pressures for high-Al content LTA zeolites (zeolites LTA-1 and LTA-2). At adsorption pressures lower than 150 kPa, the R values calculated for the isotherms increase except for the pure silica LTA zeolite, which gives a nearly constant R value along the whole isotherm. The increasing rate of R coefficient with the Al content becomes more important, when lower is the adsorption pressure. This can be clearly seen in the inset of Figure 7, where the selectivity coefficient was calculated from the Virial fittings of the CO2 and CH4 isotherms at 303 K. Finally, the equilibrium selectivity factors at zero coverage (R0) were estimated from the Henry constants of the CO2 and CH4 isotherms at different temperatures and the corresponding values are shown in Table 4. These results clearly confirm that LTA zeolites can effectively separate CO2 and CH4, and the effectiveness of this separation is highly dependent on the Al-content of the LTA zeolite. Then, better separations can be afforded by using low Si/Al ratio LTA zeolites. However, as it has been discussed above, the regenerability follows exactly the opposite tendency. (58) www.iza-structure.org (59) Miller, S. R.; Wright, P. A.; Devic, T.; Serre, C.; Ferey, G.; Llewellyn, P. G.; Denoyel, R.; Gaberova, L.; Filinchuk, Y. Langmuir 2009, 25, 3618.
DOI: 10.1021/la9026656
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Figure 9. Breakthrough curves corresponding to separation experiments of CO2/CH4 gas mixtures (1:1) run at 303 K and 140 kPa, using different LTA zeolites as adsorbents. Solid black lines are CH4, dash black lines are CO2, and gray lines are He, which was used as internal standard.
Therefore, from the practical point of view, it is feasible to achieve moderate R factor on an adsorbent with excellent regeneration properties, such as that obtained for the pure-silica LTA zeolite, and then a relatively impure CH4 stream will be obtained. This stream could then be passed through a second LTA bed having a higher Al content than the first one. In this second adsorption vessel, the CO2 concentration will be further reduced, and therefore the outlet flow will be enriched in CH4, and the adsorbent could be regenerated using this stream with lower CO2 concentration, as is shown schematically in Figure 8. This procedure can be repeated as many times as needed using different LTA zeolites with growing Al contents to achieve the CH4 purity required for its final use. It has to be noticed that the amount of adsorbent (and therefore, the vessel volume) required for each adsorption step will decrease as the purity of the CH4 stream increases. Then from this thermodynamic discussion, it seems feasible that LTA zeolites could be useful adsorbents for performing CH4 upgrading from Natural Gas. To confirm this hypothesis, we have carried out more realistic breakthrough experiments of CO2/CH4 mixtures of ratio 1:1 at 303 K at pressure close to ambient using He as internal standard using very similar conditions to those described previously in reference 23. The corresponding curves obtained for LTA-1, LTA-3.5, LTA-5, and LTA-SiO2 are shown in Figure 9. On all LTA samples, CO2 is preferentially retained, whereas CH4 passes through the adsorbent bed, in close agreement with the thermodynamic results, and the separation performance can be nicely controlled by modifying the Al content of the LTA zeolite. Pure silica LTA then gives the less selective separation; on the contrary, CO2 is strongly adsorbed on the LTA-1 zeolite, providing a nearly pure CH4 stream. Also, the regenerability of the adsorbents has been measured by fluxing air for 30 min at 303 K through the adsorbent in between two consecutive CO2/CH4 breakthrough experiments. The percentages of regeneration of the LTA samples under these conditions are given in Figure 10. There, it is evident that LTA zeolites 1916 DOI: 10.1021/la9026656
Figure 10. Regenerability of the different LTA zeolites used as adsorbents in CO2/CH4 breakthrough separation experiments after fluxing air for 30 min at 303 K through the adsorbent in between two consecutive experiments.
of Si/Al = ¥ and Si/Al = 5 provide full regeneration of the adsorbent. From these Si/Al values, the LTA regenerability decreases as the Al content increases, down to a 70% of regeneration obtained for LTA of Si/Al = 1. However, even more important than the regenerability values are the time required to reach these values, since this is an important parameter in the design of PSA units. It is then highly desirable to desorb as fast as possible the CO2 out of the LTA zeolite. The CO2 desorption curves of LTA zeolites having Si/Al of 1 and 5 are shown in Figure 11. There, it is observed that LTA with the highest Al content needs more than 30 min to reach 70% regeneration, whereas the LTA zeolite of Si/Al ratio of 5 was fully regenerated in only 5 min. It is then clear that the more realistic breakthrough experiments and the thermodynamic parameters obtained above are in full agreement and clearly indicate that the Langmuir 2010, 26(3), 1910–1917
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Figure 11. CO2 desorption curves of LTA zeolites having Si/ Al ratios of 5 (solid line) and 1 (dash line) obtained after CO2/ CH4 separation experiments by fluxing air at 303 K through the adsorbent. Table 5. Comparison of the CO2/CH4 Selectivity Coefficients (r) at Zero Coverage and Adsorption Capacities of CO2 and CH4 at 100 KPa and 273 K Obtained for LTA Zeolites of Different Si/Al Ratios with Those Reported in the Literature for MOFs R(CO2/ CH4)
Q(CO2) (mmol/g)
Q(CH4) (mmol/g)
Q(CO2) (mmol/ cm3)
Q(CH4) (mmol/ cm3)
2.8 0.5 4.0 c 0.7 c LTA-SiO258 4.5 LTA-558 106.8 4.7 0.7 7.1 c 1.1 c 58 c LTA-3.5 244.1 5.3 0.8 8.1 1.2 c LTA-258 1099.0 5.5 1.0 8.7 c 1.6 c LTA-158 3164.7 4.9 1.2 8.2 c 2.0 c ZIF-2018 10.7 3.1 0.8 3.9 d 1.0 d ZIF-6821 5.3 2.9 0.8 3.0 d 0.8 d ZIF-6921 5.2 3.2 0.9 4.1 d 1.2 d ZIF-7021 5.2 2.4 0.7 2.1 d 0.6 d ZIF-7821 10.6 3.2 1.0 3.8 d 1.2 d ZIF-7921 5.4 2.4 0.8 2.6 d 0.9 d ZIF-8121 5.8 2.9 0.8 3.8 d 1.0 d ZIF-8221 9.6 3.8 0.8 3.6 d 0.8 d MIL-10019 585 a MIL-10119 31a Cu-BTC22 6 b 3.5 b 0.7 b 2.0 e 0.4 e Sc-BDC59 415 b 0.91 b 0.18 b 1.1 d 0.2 d a Selectivity coefficient calculated from the Henry constants at 303 K. b Values estimated from the corresponding isotherms measured at 303 K. c LTA density was calculated from the crystallographic data given in the corresponding reference, but each molecular weight was adjusted to the Si/Al ratio of each LTA sample studied in this work. d Density was calculated from the crystallographic data given in the corresponding reference. e Density value was reported by the authors in the corresponding reference.
polarity of the adsorbent is one of the key parameters in the design of PSA units for CO2/CH4 separation. On this regard, zeolites offer exceptional benefits with respect to other adsorbents recently described in the open literature,23 such as MOFs even though the latest possess higher pore volumes and surface areas than zeolites. Indeed, the employ of zeolites provides a very straightforward way of controlling the polarity by modifying the Al content in the framework that, as it has been shown above, is of paramount importance in the energetics of the CO2 adsorption and in the regenerability of the zeolite. The achievement of a similar precise control of the polarity using MOFs is far from being proved at this moment, being the highest CO2/CH4 selectivity range at zero coverage in between 12 and 4, depending on the functionality of the organic linker of the MOF.21 This is clearly demonstrated in Table 5 that provides the best separation selectivities using MOFs reported in the open literature with those obtained in this work using LTA zeolites. There, it is clear Langmuir 2010, 26(3), 1910–1917
that LTA zeolites are able to provide much better and tunable separation capabilities than MOFs. Also, it must be said that MOFs offer much higher CO2 capacities than LTA zeolites, but this is only true at very high pressures. However, low pressure PSA processes for Natural Gas upgrading will have more chances of commercial application because of the low energy demand. Then, it is important to compare the adsorption capacities at relatively low pressures. Table 5 shows the adsorption capacities at 100 KPa of pressure of CO2 and CH4 for the essayed LTA zeolites in this work as well as a number of MOFs for which CO2/CH4 separation capabilities have been claimed. There, it is seen that Zeolite LTA provides the highest adsorption capacities even expressed as mmol/g. These differences are even more pronounced when the adsorption comparison is carried out on basis of the adsorbent volume, since MOFs possess lower densities than LTA zeolites. Finally, zeolites show a much better thermal and mechanical stability than MOFs.
Conclusions From the results obtained in this work, we can conclude that zeolites offer unique possibilities for CH4 upgrading from natural gas. This conclusion is supported on the following findings obtained using LTA zeolites with different Si/Al ratios as adsorbents: (1) The adsorption energy of CO2 increases as the Al content in LTA zeolite increases, and consequently the adsorbent regenerability will decrease as the Al content becomes higher. (2) On the contrary, the Al content has little effect on the isosteric heat of CH4 adsorption on LTA samples. (3) The CO2/CH4 selectivity (R factor) strongly depends on the pressure and the Al content. Then, R factor exponentially decreases with the adsorption pressure, but this exponential decay strongly decreases as the Al-content diminishes. Indeed, pure silica LTA gives a nearly constant R factor close to 3.5 along the whole isotherms, whereas LTA with a Si/Al ratio of 1 gives a R factor of 1271 at zero coverage and this parameter decreases to reach 1.2 at high coverage. (4) Nearly full regeneration in very short time can be achieved by using relatively high-silica LTA zeolites as selective CO2 adsorbent in CO2/CH4 separation processes. Further studies with different large adsorption capacity eightmember ring zeolites with slightly smaller pore apertures than LTA zeolites must be pursued to obtain a real molecular sieve separation of CO2 from CH4 owing to the smaller kinetic diameter of former molecule than the latter. Acknowledgment. This work was mostly supported by the Spanish CICYT (MAT2009-14528-C02-01, MAT2006-14274C02-01, and CTQ2007-66614/PPQ) and the European Project TOPCOMBI. Also, the authors gratefully acknowledge financial support from Universidad Politecnica de Valencia and Generalitat Valenciana (PAID-06-08:3279 and GV/2009/066). M.P. thanks CSIC for a JAE doctoral fellowship. The authors are indebted to Aroa Alos for her invaluable work on breakthrough experiments. Supporting Information Available: X-ray diffraction patterns of LTA zeolite samples used as adsorbents, supplementary figures of CO2 and CH4 adsorption isotherms, fitting of the experimental data using different models, and calculation of the isosteric heat of adsorption (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. DOI: 10.1021/la9026656
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