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Synthesis of B-Substituted β-Zeolite Membranes Vu A. Tuan, Laura L. Weber, John L. Falconer,* and Richard D. Noble Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
Alumium-free, B-β-zeolite membranes with high quality and reproducibility were prepared for the first time. In situ crystallization from an alkali medium and a tetraethylammonium hydroxide template was used. The membrane selectively separated organic mixtures in the liquid and gas phases, including both structural isomers and stereoisomers. Small- and medium-pore (LTA and MFI) zeolite membranes have been studied much more extensively than large-pore zeolite membranes, such as MOR1 and FAU types,2 which have been used to separate larger molecules. β Zeolite, which also has large pores, has exhibited a similar linear/branched alkane separation performance as MOR zeolite.3 β Zeolite is a high silica zeolite, possessing a three-dimensional, 12-ring, interconnected channel system with pore diameters of 0.53 × 0.57 nm and 0.71 × 0.73 nm.4 Because β zeolite is of industrial interest as a catalyst for fluid catalytic cracking,5 hydrotreating,6 dewaxing,7 alkylation,8 synthesis of methyl tert-butyl ether (MTBE) and ethyl tertbutyl ether,9 and total hydroisomerization of C5-C7 fractions,10 significant effort has been devoted to synthesize and modify it. Boron-β zeolite has lower acidity than Al-β and is expected to be more selective for total hydroisomerization because the side products formed from catalytic cracking on strongly acidic sites are suppressed. Boron-β zeolite can also be used as a starting structure for incorporating other cations into the framework. For example, boron can be removed from the framework by acetic acid treatment, and Si can be incorporated into the boron sites.11,12 This procedure can form pure silicaβ, which cannot be prepared directly in an alkali medium. Likewise, large cations such as Ti, V, and Mo can be isomorphously substituted by gas-phase modification of B-β,13 and these catalysts showed a high performance for selective oxidation of organics. However, Deruiter et al. claimed that pure B-β zeolite (Alfree) cannot be prepared in an alkali medium using tetraethylammonium hydroxide (TEAOH) as a template.12 We previously14 reported the synthesis of an Al-βzeolite membrane (Na-form) that exhibited a higher separation selectivity for 1,3-propanediol/glycerol separations than X-, Y-, or MOR-type membranes. In the current study, Al-free, B-β-zeolite (Si/B ) 15) membranes were prepared by in situ crystallization using TEAOH as a template. Because NaOH was not used in the synthesis gels, the hydrogen form of B-β-zeolite membranes (acid form) formed after calcination to remove the template. A porous stainless steel tube (0.5-µm pores, 0.65 cm i.d., 2.5 cm length; Mott Co.) was used as a support to avoid incorporation of Al from an Al2O3 support, which is often used for zeolite membranes. Nonporous stainless * To whom correspondence should be addressed. Tel.: 303 492-8005. Fax: 303 492-4341. E-mail: john.falconer@ colorado.edu.
steel tubes were welded onto the ends of the porous tube to provide a sealing surface for the O rings used in the membrane modules. Before synthesis, the supports were cleaned by brushing their inner surfaces and then treating them in an ultrasonic bath that contained deionized water. The tubes were then boiled in deionized water for 1 h and dried at 373 K under vacuum for 30 min. The zeolite synthesis gel had the molar composition 0.67:1.0:0.066:10.7 TEAOH/TEOS/B(OH)3/H2O, where TEOS is tetraethyl orthosilicate. The reaction mixture was vigorously stirred at room temperature for 8 h to remove ethanol that formed by TEOS hydrolysis. Note that ethanol must be completely removed to avoid formation of the MFI structure. Before synthesis, one end of the support was wrapped with Teflon tape and plugged with a Teflon cap. The tube was filled with approximately 2 mL of the synthesis gel and then placed vertically in a Teflon-lined autoclave. Crystallization was carried out at 403 K for 6 days. After growth of the first layer, the membranes were impermeable to N2 at room temperature, indicating that both intercrystalline and zeolite pores were filled with template molecules. The membranes were then calcined in N2 flow in the presence of NH3 gas (N2/ NH3 molar ratio of 5; total flow rate of 3 cm3/s) at 653 K for 10 h to remove the template. Ammonia was used because Deruiter et al.12 reported that calcination of B-containing β zeolite in air caused a loss of crystallinity. Similarly, we observed that calcination in air at 625-675 K caused a 50% loss of crystallinity of powders that formed at the same time as the membrane. The decrease in X-ray diffraction (XRD) peak intensities with calcination was used to determine the loss in crystallinity. After calcination the membrane contained intercrystalline pores, as indicated by high N2 permeances and similar permeances of H2, N2, CH4, and i-C4 at 300 K. Three synthesis layers (each 6 days) were required to obtain membranes with lower N2 permeances (10-7 mol/m2 s Pa) and n-C4H10/i-C4H10 selectivities greater than 1. The powders collected from the bottom of the membrane tubes were analyzed by XRD with a Scintag PAD-V diffractometer with Cu KR radiation. All peak positions and intensities for the powder collected for a Si/B ratio of 15 (Figure 1) match those reported by Robson15 for β zeolites, and no additional peaks were observed, indicating that only β zeolite was obtained. The low, flat background and the high intensities of the peaks indicated high crystallinity. As indicated in Table 1, pure B-β zeolites were only obtained for Si/B ratios between 10 and 15. In the absence of boron, either amorphous material or the MTW zeolite structure
10.1021/ie030035i CCC: $25.00 © 2003 American Chemical Society Published on Web 05/17/2003
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Figure 1. XRD pattern of B-β-zeolite powder collected from the bottom of the membrane tube. Table 1. Influence of the B Concentration in the Synthesis Gel on Zeolite Membrane Synthesis molar gel composition (0.67:1.0:10.7 TEAOH/TEOS/H2O plus)
Si/B ratio
structure
0.1 NaOH 0.033 B(OH)3 0.05 B(OH)3 0.066 B(OH)3 0.1 B(OH)3
∞ ∞ 30 20 15 10
amorphous MTW MFI BEA + MFI (trace) BEA BEA
Figure 3. Pervaporation fluxes of pure hydrocarbons through a B-β-zeolite membrane at 303 K as a function of the kinetic diameter of the hydrocarbon.
Figure 4. Methanol permeate concentration versus feed concentration for methanol/MTBE separation by pervaporation through a B-β-zeolite membrane at 303 K.
Figure 2. Single gas permeances through a B-β-zeolite membrane at 303 and 473 K as a function of the kinetic diameter.
formed, whereas for Si/B ratios between 20 and 30, the MFI zeolite structure was obtained. Because previous studies have shown that MFI powders and membranes formed at the same time had the same XRD patterns, we assumed that the B-β membrane had a structure similar to that of the powder. Thus, the β membranes were not destroyed to obtain an XRD pattern. Single gas permeances (using a transmembrane pressure drop of 138 kPa) at 303 and 473 K decreased as the kinetic diameter increased (Figure 2), but the ideal selectivities (H2/N2, H2/n-C4, and H2/i-C4) were less than the Knudsen selectivities. The n-C4/i-C4 ideal selectivities at 303 and 473 K were 1.8 and 1.4, however. These values indicate that transport was through small pores because the Knudsen selectivity is one for isomers. These n-C4/i-C4 ideal selectivities are much lower than selectivities obtained using MFI membranes (∼10-50). The selectivity is not expected to be as high for β zeolite, however, because both n-C4 (0.43 nm) and i-C4 (0.50 nm) are much smaller than the β-zeolite pores (0.71 × 0.73 nm), whereas i-C4 is close to the MFI pore size (0.520.57 nm). For MFI membranes, the H2/N2 ideal selectivity is also less than Knudsen values because both H2 and N2 are significantly smaller than the MFI pore
diameter. Thus, H2 and N2 permeations are not effective for evaluating the quality of medium- and large-pore zeolite membranes. Some membranes may be selective for molecules that adsorb more strongly, such as CO2, and thus selectivities can be greater than Knudsen for some light gases in some zeolites. Indeed, the CO2/N2 selectivity is greater than 1, whereas the CO2/N2 Knudsen selectivity is 0.8. Thus, permeation of larger molecules by pervaporation may be a better indication of membrane quality. To check the reproducibility of membrane synthesis, two additional membranes (Si/B ) 15) were prepared using the same gel composition and synthesis conditions. The single gas permeances were within (2% of those in Figure 2. Figure 3 shows that the pervaporation fluxes of hydrocarbon liquids through the B-β-zeolite membrane decreased as their kinetic diameter increased. These measurements were made at 303 K. Note that triisopropylbenzene (TIPB), which is significantly larger than the β-zeolite pores, is expected to permeate only through nonzeolite pores. The low value of the TIPB flux (0.5 g/m2‚h) indicates a few nonzeolite pores that are larger than the zeolite pores. Pervaporation of a methanol/MTBE mixture at 303 K was used to evaluate membrane separation properties. Methanol preferentially permeated, and the separation selectivity of the B-β-zeolite membrane (Figure 4) was higher than that for a silicalite-1 membrane and higher than that for vapor-liquid equilibrium. Because silicalite-1 (MFI structure) has a smaller zeolite pore
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the DMC permeances were 2 orders of magnitude higher for the B-β membrane. Acknowledgment We acknowledge support by the University of Colorado and MemPro Corp. Literature Cited
Figure 5. Hexane isomer permeances and separation selectivities versus temperature for a B-β-zeolite membrane. Table 2. DMC Permeances and Separation Selectivities zeolite membrane temp (K) 373 453
DMC permeance × 109 (mol/m2‚s‚Pa) trans cis Al-ZSM-5 Zeolite 2.4 1.3 3.1 3.1
trans/cis separation selectivity 1.8 1.0
B-β Zeolite 373 423
210 260
100 130
2.1 2.0
(0.53-0.56 nm) than β zeolite, MTBE (0.62 nm) only slowly permeates through MFI pores. The lower selectivity for silicalite-1 may be due to 1.0-nm nonzeolite pores reported by the authors.16 The B-β membrane also separated a 50/50 mixture of n-hexane/2,2-dimethylbutane (DMB) in the vapor phase. As shown in Figure 5, n-hexane permeated faster than DMB; the selectivity was greater than 5 over the temperature range. High selectivities are not expected for the β-zeolite membrane because both n-hexane (0.43 nm) and DMB (0.62 nm) are smaller than the zeolite pores. Jeong et al.17 reported that a Y-type membrane did not separate n-hexane/DMB mixtures. Because both β- and Y-type zeolites have similar pore sizes (0.7-0.74 nm), the separation selectivity obtained for the B-βzeolite membrane may be due to its shape-selective structure. Denayer et al.3 reported differences in adsorption capacities of linear/branched alkanes on β-, Y-, and MOR-type zeolites. As a more severe test of membrane quality, the B-β membrane was used to separate stereoisomers. The B-β membrane and a ZSM-5 membrane separated a 36/64 mixture of trans/cis-1,4-dimethylcyclohexane (DMC) in the vapor phase. To our knowledge, only one previous study reported using a zeolite membrane to separate stereoisomers.18 Song et al.19 measured diffusion coefficients of DMC in silicalite powders and observed that the trans isomer had a diffusivity more than 55 times higher than the cis isomer. Table 2 compares permeances and separation selectivities for Al-ZSM-5 (Si/ Al ) 100) and B-β membranes. For both membranes, the trans isomer permeated faster. The large-pore B-β membrane had a similar separation selectivity at 373 K as the medium-pore Al-ZSM-5 membrane. Moreover, the β-zeolite membrane was also selective at 423 K, whereas the ZSM-5 membrane was not. Furthermore,
(1) Nishyaama, N.; Ueyama, K.; Matsukata, M. A defect-free mordenite membrane synthesized by vapor-phase transport method. J. Chem. Soc., Chem. Commun. 1995, 1967. (2) Nair, S.; Nikolakis, V.; Xomeritakis, G.; Bonilla, G.; Tsapatsis, M. Separation of close-boiling hydrocarbon mixtures by MFI and FAU membranes made by secondary growth. Microporous Mesoporous Mater. 2001, 48, 219. (3) Denayer, J. F.; Souverijns, W.; Jacobs, P. A.; Martens, J. A.; Baron, G. V. High-temperature low-pressure adsorption of branched C5-C8 alkanes on zeolite beta, ZSM-5, ZSM-22, zeolite Y, and mordenite. J. Phys. Chem. B 1998, 102, 4588. (4) Baerlocher, Ch.; Meier, W. M.; Olson, D. H. Atlas of zeolite framework types; Elsevier: New York, 2001; p 77. (5) Bonetto, L.; Camblor, M. A.; Corma, A.; Perezpariente, J. Optimization of zeolite-β in cracking catalysts influence of crystallite size. Appl. Catal. A 1992, 82, 37. (6) Kiricsi, I.; Flego, C.; Pazzucoui, G.; Parker, W. O.; Millini, R.; Perego, C.; Bellussi, G. Progress toward understanding zeolitebeta aciditysan IR and Al27 NMR spectroscopic study. J. Phys. Chem. 1994, 98, 4627. (7) Taylor, R. J.; Petty, R. H. Selective hydroisomerization of long chain normal paraffins. Appl. Catal. 1994, 119, 121. (8) Corma, A.; Martinez, A.; Arroyo, P. A.; Monteiro, J. I. F.; Sousa-Aguiar, E. F. Isobutane/2-butene alkylation on zeolite beta: Influence of post-synthesis treatments. Appl. Catal. A 1996, 142, 139. (9) Collignon, F.; Poncelet, G. Comparative Vapor Phase Synthesis of ETBE from Ethanol and Isobutene over Different Acid Zeolites. J. Catal. 2001, 202, 68. (10) Van Santen, R. A., Van leeuwen, P. W. N. M., Moulijn, J. A., Avenrill, B. A., Eds. Catalysis: an Integrated Approach. Stud. Surf. Sci. Catal. 1999, 123. (11) Jones, C. W.; Hwang, S. J.; Okubo, T.; Davis, M. E. Synthesis of hydrophobic molecular sieves by hydrothermal treatment with acetic acid. Chem. Mater. 2001, 13, 1041. (12) Deruiter, R.; Pamin, K.; Kentgens, A. P. M.; Jansen, J. C.; van Bekkum, H. Synthesis of molecular-sieve [B]-beta and modification of the boron site. Zeolites 1993, 13, 611. (13) Niederer, J. P. M; Holderich, W. F. Oxidation capabilities of BEA isomorphously substituted with molybdenum, vanadium and titanium: an explorative study. Appl. Catal. A 2002, 229, 51. (14) Tuan, V. A.; Li, S.; Falconer, J. L.; Noble, R. D. In Situ Crystallization of Beta Zeolite Membranes and their Permeation and Separation Properties. Chem. Mater. 2002, 14, 489. (15) Robson, H. Verified Synthesis of Zeolitic Materials; Elsevier: New York, 2001; p 117. (16) Sano, T.; Hasegawa, M.; Kawakami, Y.; Yanagishita, H. Separation of methanol/methyl-tert-butyl ether mixture by pervaporation using silicalite membrane. J. Membr. Sci. 1995, 107, 193. (17) ) Jeong, B. H.; Hasegawa, Y.; Sotowa, K.; Kusakabe, K.; Morooka, S. Vapor permeation properties of an NaY-type zeolite membrane for normal and branched hexanes. Ind. Eng. Chem. Res. 2002, 41, 1768. (18) van de Graaf, J. M.; Zweip, M.; Kapteijn, F.; Moulijn, J. A. Application of a zeolite membrane reactor in the metathesis of propene. Chem. Eng. Sci. 1999, 54, 1441. (19) Song, L.; Sun, Z. L.; Rees, L. V. C. Experimental and molecular simulation studies of adsorption and diffusion of cyclic hydrocarbons in silicalite-1. Microporous Mesoporous Mater. 2002, in press.
Received for review January 14, 2003 Revised manuscript received April 17, 2003 Accepted April 21, 2003 IE030035I
