Ind. Eng. Chem. Res. 2005, 44, 4035-4041
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SEPARATIONS Modification of Zeolite Membranes for H2 Separation by Catalytic Cracking of Methyldiethoxysilane Mei Hong, John L. Falconer,* and Richard D. Noble Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424
Boron-substituted ZSM-5 and SAPO-34 membranes were silylated by the catalytic cracking of methyldiethoxysilane (MDES) to increase their selectivity for H2 separation from light gases. The MDES reacted in the B-ZSM-5 pores and reduced their effective pore diameter, so that silylation significantly increased their H2 selectivity. The H2/CO2 separation selectivity at 473 K increased from 1.4 to 37, whereas the H2/CH4 separation selectivity increased from 1.6 to 33. However, silylation decreased the H2 permeances more than 1 order of magnitude in the B-ZSM-5 membranes. The H2 permeance and H2/CO2 and H2/CH4 separation selectivities increased with temperature. At 673 K, the H2 permeance was 1.0 × 10-7 mol‚m-2‚s-1‚Pa-1 and the H2/CO2 separation selectivity was 47. Methyldiethoxysilane does not fit into SAPO-34 pores, but silylation apparently decreased the pore size of the nonzeolite pores in the SAPO-34 membranes. After silylation, the H2 permeances and H2/CO2 and H2/N2 separation selectivities were almost unchanged in the SAPO-34 membranes because H2, CO2, and N2 permeate mainly through SAPO34 pores. In contrast, H2/CH4 separation selectivity increased from 35 to 59, and CO2/CH4 separation selectivity increased from 73 to 110, apparently because CH4 permeates mainly through non-SAPO-34 pores. 1. Introduction Zeolite membranes have the potential to efficiently separate gas mixtures because they have pores in the appropriate molecular size range and they have a narrow size distribution. The high thermal, chemical, and mechanical stabilities of zeolite membranes make them ideal for use in high-temperature applications such as H2 separation from coal gasification products. Most zeolite membrane studies have focused on MFI membranes.1-5 The MFI zeolite has straight channels (0.54 × 0.56 nm) perpendicular to sinusoidal channels (0.51 × 0.54 nm).6 Because MFI channel sizes are similar to the sizes of hydrocarbons, MFI membranes effectively separate hydrocarbon isomers, such as nbutane from isobutane and n-hexane from 2,2-dimethylbutane (DMB). Of the many studies on MFI membranes, only a few have reported high ideal selectivities for H2 over other light gases. Lovallo et al. prepared an oriented silicalite membrane that had a H2/N2 ideal selectivity of 60 at 423 K, and its H2 permeance was 1.2 × 10-7 mol‚m-2‚s-1‚Pa-1.7 Lai et al. observed a H2/ CH4 ideal selectivity of 1000 at 298 K and a H2/CO2 ideal selectivity of 15 at 423 K for a ZSM-5 membrane; the H2 permeances were 6.7 × 10-9 mol‚m-2‚s-1‚Pa-1 at 298 K and 1.2 × 10-7 mol‚m-2‚s-1‚Pa-1 at 423 K, respectively.8 More recently, Dong et al. prepared oriented boron-substituted Al-ZSM-5 membranes from porous glass; their membrane had a H2/N2 ideal selectivity of * To whom correspondence should be addressed. Tel.: (303) 492-8005. Fax: (303) 492-4341. E-mail: john.falconer@ colorado.edu.
16.8 at 293 K, but the H2 permeance was only 2.3 × 10-8 mol‚m-2‚s-1‚Pa-1.9 Most MFI membranes have not been effective for light gas separations, however, apparently because MFI pores are much larger than the sizes of the light gas molecules. The high selectivities for these few membranes may be caused by occlusion of the amorphous species inside the channel structure 8 and/or the orientation of the zeolite crystals at the intergrown surface layer.7 Masuda et al. used catalytic cracking of organosilanes to modify the effective pore opening of an Al-ZSM-5 zeolite.10 Methyldiethoxysilane (MDES) and methyldimethoxysilane, both of which are small enough to fit into the pores of an Al-ZSM-5 zeolite, were adsorbed and cracked on zeolite crystals and membranes. They determined from the change in the NH3 temperatureprogrammed desorption (TPD) spectra of the Al-ZSM-5 powder before and after silylation, and the fact that the number of chemisorbed MDES molecules was equal to the number of acid sites in the zeolite, that MDES reacted on acid sites. After calcination, SiO2 units formed at the acid sites within the zeolite. This treatment reduced the effective pore size and markedly reduced the benzene adsorption capacity of Al-ZSM-5 powders, but the CO2 adsorption capacity did not change. The H2 permeance through the treated membrane decreased to 2 × 10-8 mol‚m-2‚s-1‚Pa-1, which is 10% of its original value, whereas the N2 permeance decreased to 0.2% of its original value. Thus, the H2/N2 separation factor at 383 K increased from 1.5 to 4.5 to 90-140 when separating 0.2-0.8 mol % H2 from N2. The separation factor for a H2/O2 mixture also increased to 110-120.
10.1021/ie048739v CCC: $30.25 © 2005 American Chemical Society Published on Web 04/23/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005
Instead of the modification of ZSM-5 membranes, the synthesis of small-pore zeolite membranes was also used to obtain sieving zeolite membranes for small light gas molecules. For A-type zeolite membranes in one study, H2 permeances ranged from 10-7 to 1 order of magnitude at low temperatures, the treated membranes had H2 permeances of 1.0 × 10-7 mol‚m-2‚ s-1‚Pa-1 above 673 K when separating a H2/CO2 mixture. The current study demonstrates that silylation has the potential to modify membranes to make them selective toward H2. Thinner MFI membranes with a thickness of 0.5 µm have been reported to have H2 permeances as high as 2.2 × 10-5 mol‚m-2‚s-1‚Pa-1 at room temperature.25 Silylating such membranes might yield H2-selective membranes with higher permeances than obtained in the current study, and that might have potential for commercial application. 4.2. SAPO-34 Membranes. The H2/CH4 separation selectivity was high for the original SAPO-34 membranes because the SAPO-34 pores are almost the same size as the CH4 kinetic diameter, which is 0.38 nm. Both CO2 (0.33 nm) and N2 (0.36 nm) are smaller than the SAPO-34 pores, so the H2/CO2 and H2/N2 selectivities were lower than the H2/CH4 selectivity. The H2/CO2 selectivity was 1 order of magnitude. Both the H2 permeance and the H2/CO2 and H2/CH4 separation selectivities significantly increased with temperature up to 773 K. The highest H2/CO2 and H2/CH4 separation selectivities were 48 and 41, respectively. The highest H2 permeance during separations was 1 × 10-7 mol‚m-2‚s-1‚Pa-1. Methyldiethoxysilane did not fit into SAPO-34 membranes, but it partially blocked nonzeolite pores and increased the H2/CH4 and CO2/CH4 separation selectivities, while the H2 permeance was unchanged. The highest H2/CH4 separation selectivity for a SAPO membrane was 59 at 298 K. Acknowledgment We gratefully acknowledge support by the U.S. Department of Energy (Grant No. DE-FG26-02NT41536). We thank Dr. Vu A. Tuan for providing B-ZSM-5 membranes, Dr Shiguang Li for preparing SAPO-34 membranes and conducting CO2/CH4 separation experiments, and Dr. Manuel Arruebo for providing the SEM pictures and valuable discussions. Literature Cited (1) Mizukami, F. Application of zeolite membranes, films and coatings. Stud. Surf. Sci. Catal. 1999, 125, 1. (2) Tavolaro, A.; Drioli, E. Zeolite membranes. Adv. Mater. 1999, 11, 975. (3) Coronas, J.; Santamaria, J. Separations using zeolite membranes. Sep. Purif. Methods 1999, 28, 127. (4) Caro, J.; Noack, M.; Kolsch, P.; Schafer, R. Zeolite membranessstate of their development and perspective. Microporous Mesoporous Mater. 2000, 38, 3. (5) Chiang, A. S. T.; Chao, K. J. Membranes and films of zeolite and zeolite-like materials. J. Phys. Chem. Solids. 2001, 62, 1899. (6) Gump, C. J.; Lin, X.; Falconer, J. L.; Noble, R. D. Experimental configuration and adsorption effects on the permeation of C-4 isomers through ZSM-5 zeolite membranes. J. Membr. Sci. 2000, 173, 35. (7) Lovallo, M. C.; Tsapatsis, M. Preferentially oriented submicron silicalite membranes. AICHE J. 1996, 42, 3020. (8) Lai, R.; Gavalas, G. R. ZSM-5 membrane synthesis with organic-free mixtures. Microporous Mesoporous Mater. 2000, 38, 239. (9) Dong, W.; Long, Y. Preparation and characterization of preferentially oriented continuous MFI-type zeolite membranes from Porous glass. Microporous Mesoporous Mater. 2004, 76, 9. (10) Masuda, T.; Fukumoto, N.; Kitamura, M.; Mukai, S. R.; Hashimoto, K.; Tanaka, T.; Funabiki, T. Modification of pore size of MFI-type zeolite by catalytic cracking of silane and application to preparation of H-2-separating zeolite membrane. Microporous Mesoporous Mater. 2001, 48, 239.
(11) Aoki, K.; Kusakabe, K.; Morooka, S. Gas permeation properties of A-type zeolite membrane formed on porous substrate by hydrothermal synthesis. J. Membr. Sci. 1998, 141, 197. (12) Guan, G.; Kusakabe, K.; Morooka, S. Gas permeation properties of ion-exchanged LTA-type zeolite membranes. Sep. Sci. Technol. 2001, 36, 2233. (13) Guan, G. Q.; Tanaka, T.; Kusakabe, K.; Sotowa, K. I.; Morooka, S. Characterization of AIPO(4)-type molecular sieving membranes formed on a porous alpha-alumina tube. J. Membr. Sci. 2003, 214, 191. (14) Poshusta, J. C.; Tuan, V. A.; Pape, E. A.; Noble, R. D.; Falconer, J. L. Separation of light gas mixtures using SAPO-34 membranes. AICHE J. 2000, 46, 779. (15) Poshusta, J. C.; Tuan, V. A.; Falconer, J. L.; Noble, R. D. Synthesis and permeation properties of SAPO-34 tubular membranes. Ind. Eng. Chem. Res. 1998, 37, 3924. (16) Yan, Y. S.; Davis, M. E.; Gavalas, G. R. Preparation of highly selective zeolite ZSM-5 membranes by a postsynthetic coking treatment. J. Membr. Sci. 1997, 123, 95. (17) Nomura, M.; Yamaguchi, T.; Nakao, S. Silicalite membranes modified by counterdiffusion CVD technique. Ind. Eng. Chem. Res. 1997, 36, 4217. (18) Tuan, V. A.; Falconer, J. L.; Noble, R. D. Isomorphous substitution of Al, Fe, B, and Ge into MFI-zeolite membranes. Microporous Mesoporous Mater. 2000, 41, 269. (19) Yuan, S. P.; Wang, J. G.; Li, Y. W.; Jiao, H. J. Bronsted acidity of isomorphously substituted ZSM-5 by B, Al, Ga, and Fe. Density functional investigations. J. Phys. Chem. A 2002, 106, 8167. (20) Tuan, V. A.; Noble, R. D.; Falconer, J. L. Boron-substituted ZSM-5 membranes: Preparation and separation performance. AICHE J. 2000, 46, 1201. (21) Funke, H. H.; Kovalchick, M. G.; Falconer, J. L.; Noble, R. D. Separation of hydrocarbon isomer vapors with silicalite zeolite membranes. Ind. Eng. Chem. Res. 1996, 35, 1575. (22) Xomeritakis, G.; Lai, Z. P.; Tsapatsis, M. Separation of xylene isomer vapors with oriented MFI membranes made by seeded growth. Ind. Eng. Chem. Res. 2001, 40, 544. (23) Lai, Z. P.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Microstructural optimization of a zeolite membrane for organic vapor separation. Science 2003, 300, 456. (24) Au, L. T. Y.; Yeung, K. L. An investigation of the relationship between microstructure and permeation properties of ZSM-5 membranes. J. Membr. Sci. 2001, 194, 33. (25) Hedlund, J.; Sterte, J.; Anthonis, M.; Bons, A. J.; Carstensen, B.; Corcoran, N.; Cox, D.; Deckman, H.; De Gijnst, W.; de Moor, P. P.; Lai, F.; McHenry, J.; Mortier, W.; Reinoso, J. High-flux MFI membranes. Microporous Mesoporous Mater. 2002, 52, 179. (26) Li, S. G.; Falconer, J. L.; Noble, R. D. SAPO-34 membranes for CO2/CH4 separation. J. Membr. Sci. 2004, 241, 121. (27) Campelo, J. M.; Lafont, F.; Marinas, J. M.; Ojeda, M. Analysis of occluded templates in silicoaluminophosphate molecular sieves by high-resolution mass spectrometry. Rapid Commun. Mass Spectrom. 1999, 13, 521. (28) Mees, F. D. P.; Van Der Voort, P.; Cool, P.; Martens, L. R. M.; Janssen, M. J. G.; Verberckmoes, A. A.; Kennedy, G. J.; Hall, R. B.; Wang, K.; Vansant, E. F. Controlled reduction of the acid site density of SAPO-34 molecular sieve by means of silanation and disilanation. J. Phys. Chem. B 2003, 107, 3161.
Received for review December 28, 2004 Revised manuscript received March 22, 2005 Accepted April 1, 2005 IE048739V
