Separation of Gases with an A-Type Zeolite Membrane - Industrial

Jun 2, 2000 - A highly hydrophilic A-type zeolite membrane was prepared by a hydrothermal synthesis on the outer surface of a porous α-alumina tube. ...
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Ind. Eng. Chem. Res. 2000, 39, 2245-2251

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MATERIALS AND INTERFACES Separation of Gases with an A-Type Zeolite Membrane Kanna Aoki, Katsuki Kusakabe, and Shigeharu Morooka* Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-8581, Japan

A highly hydrophilic A-type zeolite membrane was prepared by a hydrothermal synthesis on the outer surface of a porous R-alumina tube. Increasing and decreasing the permeation temperature between 308 and 473 K over 40 cycles in a period of 4 months did not damage the membrane. Permeation tests with a variety of permeates showed that the membrane possessed two types of pores: zeolitic pores of 0.4-0.43 nm diameter and nonzeolitic pores. Molecules larger than C2H6 were not able to permeate through the zeolitic pores. The separation factors of the membrane were dependent on molecular size, affinity to the pore walls, and hydrophobicity of the permeating molecules. The combination of permeants also affected the permeation rates. H2O molecules could be concentrated in the zeolitic and nonzeolitic pores, thus reducing the permeation of the hydrophobic permeants. The H2O/H2 separation factor was larger than 160 at permeation temperatures of 303-473 K. Introduction Zeolites are microporous aluminosilicate crystals which contain pores equivalent to the diameters of some molecules. The pore size, as well as the affinity for the molecules, can be adjusted via ion exchange with cations.1 Because of these attractive properties, zeolite membranes have been prepared and utilized in the separation of gases. However, most of the studies on zeolite membranes have been focused on MFI zeolites,2-11 which have pores of 0.53-0.56 nm. This pore size is suitable for the separation of relatively large organic molecules but is excessive for the separation of smallsized molecules such as H2, H2O, CO2, and CH4. A-type zeolite is a candidate as a material of the membrane, which can separate these small molecules. In general, the difficulty in preparing zeolite membranes increases with increasing aluminum content in the zeolite.3,12 Thus, no highly permselective A-type membranes have been reported. Yamazaki and Tsutsumi13 prepared A-type zeolite membranes on porous supports. Masuda et al.14 prepared an A-type zeolite film on the outer surface of an alumina filter by hydrothermal synthesis but did not evaluate its separation properties. Kita15 synthesized an A-type zeolite membrane on a seeded porous alumina support. This membrane was hydrophilic and, therefore, highly permselective to water against organic solvents. The water selectivities against ethanol, methanol, acetone, dioxane, and dimethylformamide in the operation of pervaporation were 16 000, 2500, 6800, 9300, and 8700, respectively. However, the permeation tests showed that noncondensable gases permeated through the membrane by a Knudsen diffusion mechanism and, there* To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-92-651-5606. Phone: +81-92-642-3551.

fore, that these molecules passed mainly through the grain boundaries of the membrane. Wang et al.16 also conducted gas permeation tests using an A-type zeolite membrane, which was deposited on a porous substrate by a hydrothermal process. The order of permeabilities of gases was ethylene > carbon dioxide > methane > nitrogen > oxygen. This also suggests that permeation rates were dominated by a surface diffusion mechanism through the grain boundaries and not by molecular sieving diffusion through zeolite crystals. Aoki et al.12 synthesized an A-type zeolite membrane which was exchanged with Na ions. At permeation temperatures of 308-393 K, the permselectivity of the membrane for H2-N2 systems was 4.5-4.8, which was higher than the value based on the Knudsen diffusion mechanism, i.e., 3.7. The order of permeances for single-component gases was H2 > O2 > CH4 > CO2 ) N2 > C3H8. The permeation of C3H8 (molecular size ) 0.43 nm) indicated that the membrane contained some defects. Recently, several research groups succeeded in fabricating oriented A-type zeolite films.17-19 They claimed that a thin film of oriented crystals would possess fewer defects than conventional randomly grown polycrystalline membranes. Hedlund et al.17 formed an oriented A-type zeolite film on a seeded single-crystal alumina (001) substrate. The X-ray diffraction (XRD) pattern showed that the (h00) face of the A-type zeolite layer was parallel to the substrate. However, the orientation of the membrane could not be confirmed by scanning electron microscopic (SEM) observation. Boudreau and Tsapatsis18 and Boudreau et al.19 formed an oriented A-type zeolite film by a method similar to that described by Hedlund et al.17 The (h00) face was oriented to the substrate, because strong (200) and (600) peaks were observed by XRD. The orientation of the crystals was also confirmed by SEM techniques. However, none of the above studies reported the gas separation properties of these membranes.

10.1021/ie990902c CCC: $19.00 © 2000 American Chemical Society Published on Web 06/02/2000

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Aoki et al.20 synthesized A-type zeolite membranes by repeating a short-term hydrothermal synthesis. Zeolite crystals grew on a porous R-alumina support with the (h00) faces parallel to the substrate surface. The membranes showed a H2 permeance and a H2/N2 separation factor higher than those of A-type zeolite membranes, which were prepared by a conventional method and composed of randomly grown crystallites. However, the H2/N2 separation factor was too low for industrial applications. From the studies of Kita,15 A-type zeolite membranes can be highly useful for separating water from water-organic mixtures, although the technique for fabricating defect-free A-type zeolite membranes has not yet been accomplished. The result reported by Kita15 suggests that the affinity between pore walls and permeates is more important than the pore size, for the utilization of A-type zeolite membranes in the separation of liquid mixtures. In this study, we prepared an A-type zeolite membrane by a hydrothermal synthesis and investigated its selectivities among a variety of gases.

Figure 1. SEM images of the A-type zeolite membrane: (a) top surface; (b) cross section.

Experimental Section Membrane Formation. An A-type zeolite membrane was synthesized by hydrothermal crystallization on the outer surface of a porous R-alumina tube (a product of NOK, Japan, average pore size ) 150-170 nm, outer diameter ) 2.8 mm, inner diameter ) 1.9 mm, length ) 0.2 m). Prior to the synthesis of the membrane, the outer surface of the support tube, other than the permeating central portion (14 mm in length), was glazed with a SiO2-CaO-BaO sealant (Nippon Electric Glass, No. GA-13) and calcined at 1173 K for 30 min. To avoid the dissolution of the glass sealant into the solution during the hydrothermal synthesis, the glazed portions were further coated with a hydrofluorocarbon polymer. The outer surface of the support tube was then rubbed with NaA zeolite particles (Wako Pure Chemical, Japan) to implant crystals as nucleation sites. A gel was prepared using a water glass (Na2O ) 1719 wt %, SiO2 ) 35-38 wt %, Wako Pure Chemical, Japan) as the Si source, sodium aluminate (Al/NaOH ) 0.81) as the Al source, and NaOH as the Na source. The final molar composition of the gel was Na:Al:Si:H2O ) 2:1:1:240. This solution was stirred for 1 h at 333 K. A 25 mL portion of the gel and the seeded substrate were placed in a poly(tetrafluoroethylene) (PTFE) reactor, which was then placed horizontally in an electric oven. The hydrothermal reaction was carried out at 373 K for 20 h. After the hydrothermal synthesis, the membrane was washed with deionized water and dried at room temperature. The morphology of the membrane was observed by SEM (Hitachi S-900). Permeation Tests. Both ends of the support tube were connected to a gastight permeation cell using O-rings. The feed gas was then introduced into the outside of the membrane. Argon was used as a sweep gas to reduce the concentration of the permeating gases in the permeate side. The total pressure on each side of the membrane was atmospheric. The permeation cell was placed in a temperature-controlled electric furnace. The flow rate of the retentate and that on the permeate side were measured with bubble flow meters. The concentrations of gases were determined with a chromatograph equipped with a thermal conductivity detector (TCD). Single-component permeation tests were conducted at 308-473 K using He, H2, CO2, O2, N2, CH4,

Figure 2. Single-component permeances as a function of molecular size.

C2H6, n-C4H10, i-C4H10, and SF6. Equimolar mixtures of n-C4H10 and either He, H2, O2, or C2H6 were also used. To determine the separation of H2O from other gases, water was vaporized by bubbling with either H2, He, CO2, N2, O2, CH4, C2H6, n-C4H10, i-C4H10, or SF6 at room temperature. The concentration of H2O was approximately 2 mol %. Permeances were calculated based on the partial pressure difference between the feed and permeate sides, averaged logarithmically between the inlet and outlet of the membrane. Results and Discussion Morphology. Parts a and b of Figure 1 show the top surface and cross section of the A-type zeolite membrane, which was synthesized in this study. The membrane was composed of randomly grown crystallites of the A-type zeolite. The membrane thickness was 1.7 µm after a 20-h hydrothermal reaction. Single-Component Permeation. Figure 2 shows the single-component permeation rates at permeation temperatures of 308, 373, and 473 K, as a function of molecular sizes. The permeances were nearly independent of permeation temperature and did not greatly decrease with increasing molecular size. The gases with sizes between those of He and C2H6 showed similar permeances. However, n-C4H10, i-C4H10, and SF6 showed

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Figure 3. Permeances to He and n-C4H10 for single-component and binary mixture systems.

permeances which were much lower than those of smaller gases. The molecular size of C2H6 is 0.4 nm, and that of n-C4H10 is 0.43 nm. Thus, the membrane is capable of separating gases at the boundary of 0.4-0.43 nm, which is equivalent to the pore size of the A-type zeolite crystal. However, the finite permeation rates of n-C4H10, i-C4H10, and SF6 indicate that this membrane contains nonzeolitic pores (defects), which are larger than the zeolitic pores, as reported by Wegner et al.21 The difference in permeations to n-C4H10 and i-C4H10 is not fully understood at the present moment. Binary Gas Permeation. He and n-C4H10 Systems. Figure 3 shows the single- and mixed-component permeances to He and n-C4H10, determined at 308, 373, and 473 K. Except for the He permeance at 473 K, the permeances to He and n-C4H10 for both the single- and mixed-component systems are approximately the same. This suggests that He, a small species, mostly permeates through the zeolitic pores, whereas n-C4H10, a large species, mainly permeates through the nonzeolitic pores. Thus, there is no competition between He and n-C4H10 in the zeolitic pores, and the permeance of one is not affected by the existence of the other. Because of permeation through the nonzeolitic pores, however, the He permeance is decreased by the coexisting n-C4H10 with a lower diffusivity and the n-C4H10 permeance is increased by the coexisting He with a higher diffusivity. H2 and n-C4H10 Systems. Figure 4 shows the H2 and n-C4H10 permeances for single- and mixed-component systems. The permeances for the H2 and n-C4H10 systems are similar to those for the He and n-C4H10 systems, described above. The molecular size of H2 is larger than that of He, but the sizes of both are smaller than those of the other gases under consideration here. Thus, the same permeation mechanism prevails for the combination of H2 and n-C4H10. However, the permeance to H2 in the case of the mixed feed is approximately 10% higher than that in the case of the single-component feed, and this cannot be explained by the available permeation models. O2 and n-C4H10 Systems. Figure 5 shows the permeances for the single- and mixed-component O2 and n-C4H10 systems at 308, 373, and 473 K. The permeation of O2 is different from that of H2 and He. The permeances to O2 in the mixed feed are half the values of the single feed. Meanwhile, the permeances to n-C4H10 are approximately the same for both the single- and

Figure 4. Permeances to H2 and n-C4H10 for single-component and binary mixture systems.

Figure 5. Permeances to O2 and n-C4H10 for single-component and binary mixture systems.

mixed-component systems. In the mixed feed, n-C4H10 molecules diffuse through the nonzeolitic pores at a slower rate than the O2 molecules. Thus, n-C4H10 molecules may reduce the diffusion rate of the O2 molecules through the nonzeolitic pores. In contrast, the permeation of n-C4H10 is somewhat enhanced by the coexisting O2 molecules, which permeate at a rate faster than the n-C4H10 molecules. Because the molecular size of O2 is larger than that of He and H2, the O2 molecules may pose some difficulties in terms of the permeation through the zeolitic pores. Thus, the fraction of the O2 molecules, which permeate through the nonzeolitic pores, is higher than that in the case of He and H2. C2H6 and n-C4H10 Systems. As shown in Figure 2, C2H6 is the largest of the group of molecules, which are smaller than the zeolitic pores. Figure 6 shows the permeation behaviors for the C2H6 and n-C4H10 systems. The permeation rates of C2H6 in the mixture are lower than those in the single feed. This is similar to the permeation for the O2-n-C4H10 systems especially at lower permeation temperatures, where C2H6 are adsorbed on the pores and, as a result, the permeation through the zeolitic pores dominates. Because the molecular size of C2H6 is close to the zeolitic pores, however, the C2H6 molecules are able to partially permeate through the nonzeolitic pores, especially at

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Figure 6. Permeances to C2H6 and n-C4H10 for single-component and binary mixture systems.

Figure 8. Permeances to H2 and H2O for single-component and moistened systems.

Figure 7. Permeances to He and H2O for single-component and moistened systems.

Figure 9. Permeances to CO2 and H2O for single-component and moistened systems.

higher permeation temperatures, where no adsorption of C2H6 occurs. The permeation of C2H6 is then similar to that of n-C4H10. Thus, the n-C4H10 molecules, which fundamentally migrate through the nonzeolitic pores at a slower rate, reduce the permeation of the C2H6 molecules, which would have diffused at a more rapid rate. The coexisting C2H6 molecules, as is observed for the other combinations, increase the permeance to n-C4H10. He and H2O Systems. Figure 7 shows the permeances to He and H2O for the single-component and moistened systems, as a function of permeation temperature. Although He (0.26 nm) and H2O (0.27 nm) are approximately of the same molecular size, the permeation rates of H2O are more than 2 orders of magnitude higher than those of He in the moistened feed. Moreover, the permeation rates of He in the moistened feed are decreased to 16-20% of the corresponding values in the single feed. Because the A-type zeolite possesses a strong affinity to H2O, the pores of the A-type zeolite membrane may be filled with H2O molecules, and, as a result, it would be difficult for He molecules to diffuse through the membrane. The He/H2O separation factor is estimated to be 2.1, if these molecules permeate via the Knudsen diffusion mechanism. In this study, however, the H2O molecules permeate at rates which are

much faster than those of the He molecules, and the H2O/He separation factors in the moistened feed are 555, 544, and 460 at the permeation temperature of 308, 373, and 473 K, respectively. Because all of these temperatures are lower than the critical temperature of H2O ()647 K), H2O can be condensed in the pores. The permeance to He can be reduced as a result of the blockage by H2O molecules. It is noteworthy that the high selectivity to H2O was observed at 473 K, which was 100 K higher than the boiling temperature of H2O at ambient pressure. H2 and H2O Systems. Figure 8 shows the permeation behaviors of H2 in the single- and moistened-feed systems. The combination of H2 and H2O shows the permeation behaviors, similar to those observed for the combination of He and H2O. The permeance to H2 in the moistened feed is 48-53% lower than that in the single-component feed. The H2O/H2 separation factors are as high as 235, 217, and 160 at 308, 373, and 473 K, respectively. CO2 and H2O Systems. Figure 9 shows the permeation behaviors of CO2 and H2O for the single- and moistenedfeed systems. The permeation of CO2 is different from that of He and H2 in the moistened systems. The permeance to CO2 in the moistened feed is higher than that in the single-component feed. Because CO2 has an

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Figure 10. Permeances to O2 and H2O for single-component and moistened systems.

Figure 11. Permeances to N2 and H2O for single-component and moistened systems.

affinity for H2O, it is possible that CO2 molecules become entrained in the pores along with the H2O flux, which is much higher than the CO2 flux. The H2O/CO2 separation factors are 8.5, 13, and 22 at 308, 373, and 473 K, respectively. O2 and H2O Systems. Figure 10 shows the permeances to O2 and H2O obtained for the single- and moistenedfeed systems. The O2 permeance for the moistened feed is larger than that of the single feed, but the differences are quite small. This can be explained by the weak affinity of O2 for H2O. The H2O/O2 separation factor is 32, 48, and 41 at 308, 373, and 473 K, respectively. These values are much higher than those expected from the Knudsen diffusion mechanism, 1.6. N2 and H2O Systems. Figure 11 shows the permeances to N2 and H2O for the single- and moistenedfeed systems. The N2 permeances for the moistened feed are larger than those for the single-component feed. Because the affinity of N2 for H2O is weaker than that of CO2 and O2, this increase cannot be explained by affinity alone and is discussed below. The H2O/N2 separation factors are 26, 26, and 28 at 308, 373, and 473 K, respectively. CH4 and H2O Systems. Figure 12 shows the permeances to CH4 and H2O for the single- and moistenedfeed systems. The coexistence of H2O decreases the

Figure 12. Permeances to CH4 and H2O for single-component and moistened systems.

Figure 13. Permeances to C2H6 and H2O for single-component and moistened systems.

permeances to CH4, which is a hydrophobic molecule. This trend is the same as those observed for the HeH2O and H2-H2O systems. The H2O/CH4 separation factors are 50, 122, and 247 at 308, 373, and 473 K, respectively. C2H6 and H2O Systems. As shown in Figure 13, the permeances to C2H6 for the single- and moistened-feed systems are fundamentally similar to those for the combinations of He-H2O, H2-H2O, and CH4-H2O systems. n-C4H10, i-C4H10, SF6, and H2O Systems. The separation factors for the moistened feeds of n-C4H10, i-C4H10, and SF6 were determined at 308, 373, and 473 K. In the single-component systems, the permeances to these gases are lower than those to the small-molecule species, as shown in Figure 2. This suggests that these largemolecule species permeate through the nonzeolitic pores. In the moistened systems, however, the permeation of n-C4H10, i-C4H10, and SF6 was not observed within the limit of the detection in the present study (less than 10-11 mol‚m-2‚s-1‚Pa-1). Meanwhile, the H2O permeance was in the same range for the CH4-H2O and C2H6-H2O systems. In the moistened systems, H2O molecules are adsorbed or condensed in the nonzeolitic pores. Molecules, such as n-C4H10, i-C4H10, and SF6, are large and hydrophobic and are easily blocked by the H2O molecules in the pores. The mole fraction of H2O in the

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Figure 14. Permeances to H2O in moistened systems.

feed was 2%, which was sufficient to prevent the permeation of the coexisting gas, at a level of 98%, in the feed. Permeation Mechanism for H2O in the MixedComponent Systems. Figure 14 shows the temperature dependency of the permeances to H2O in the moistened-feed systems. The permeances can be divided into two regions. When the dominant species are small molecules (He, H2, CO2, O2, N2), the permeances to H2O can be summarized in the upper group in Figure 14. Meanwhile, the permeances to H2O in the presence of CH4, C2H6, n-C4H10, i-C4H10, and SF6 can be summarized in the lower group in Figure 14. This classification of coexisting gases is fundamentally the same as the case of the single-component permeances, which are shown in Figure 2, although CH4 and C2H6 are classified to the group of small molecules in Figure 2. The H2O molecules are small, easily adsorb on the A-type zeolite, and can diffuse through both zeolitic and nonzeolitic pores of the membrane as well. The intensive flow of H2O molecules entrains the small molecules, except for He and H2, which are highly hydrophobic. Thus, the permeances to CO2, O2, and N2 are increased in the presence of H2O. The large, heavy, and hydrophobic molecules (shown as the solid symbols in Figure 14) permeate through only the nonzeolitic pores. The permeances to these species are decreased by the coexisting H2O molecules, because a part of the nonzeolitic pores are blocked by H2O molecules. The H2O permeances through the nonzeolitic pores are also decreased by the coexisting species, the mole fraction of which in the gas phase is 0.98. Stability of the Membrane. Figure 15 shows the He permeances, which were determined before and after the series of permeation tests. The He permeances decreased from the initial values by approximately 80% after 40 cycles of temperature changes between 308 and 473 K over a 4-month period. This suggests that the network of the zeolite structure shrunk during these measurements. However, it should be noted that the A-type zeolite membrane was thermally stable. Conclusions A polycrystalline A-type zeolite membrane was formed on the outer surface of a porous R-alumina tube by a hydrothermal process. The permeances of the membrane for single-component feed systems decreased

Figure 15. Permeances to He determined before and after the series of permeation tests.

clearly between C2H6 and n-C4H10, suggesting that the majority of the pores were zeolitic. However, the coexisting nonzeolitic pores, through which large molecules were able to diffuse, complicated the permeation properties of the membrane. For binary mixed-feed systems, the separation factors were dependent on the size of the permeants, the adsorptivity of molecules to the pore walls, and the hydrophobicity of permeants. The A-type zeolite membrane showed a strong affinity for H2O molecules. For the moistened-feed systems, H2O molecules were condensed in the nonzeolitic pores and increased the permeances to small and hydrophilic species but reduced the permeances to large and hydrophobic species. The H2O/H2 separation factor was greater than 100 at permeation temperatures of 303473 K. The membrane was not damaged by repeated cycles of temperature changes between 308 and 473 K over 40 times for 4 months. Acknowledgment This work was supported by the Japan Society for the Promotion of Science, “Research for the Future” Program, Production- and Utilization-Technology of Hydrogen Aiming at the Hydrogen Energy Society (JSPSRFTF97P10110; Project leader, Prof. M. Shioji, Kyoto University), and the Ministry of Education, Science, Sports, and Culture, Japan, and the New Energy and Technology Development Organization, Japan. K.A. also gratefully acknowledges Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. Literature Cited (1) Aoki, K.; Tuan, V. A.; Falconer, J. L.; Noble, R. D. Gas Permeation Properties of Ion-Exchanged ZSM-5 Zeolite Membranes. Microporous Mesoporous Mater., submitted for publication. (2) Liu, Q.; Noble, R. D.; Falconer, J. L.; Funke, H. H. Organics/ Water Separation by Pervaporation with a Zeolite Membrane. J. Membr. Sci. 1996, 117, 163. (3) Sano, T.; Ejiri, S.; Yamada, K.; Kawakami, Y.; Yanagishita, H. Separation of Acetic Acid-Water Mixtures by Pervaporation through Silicalite Membrane. J. Membr. Sci. 1997, 123, 225. (4) Smetana, J. F.; Falconer, J. L.; Noble, R. D. Separation of Methyl Ethyl Ketone from Water by Pervaporation Using a Silicalite Membrane. J. Membr. Sci. 1996, 114, 127.

Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2251 (5) Tuan, V. A.; Falconer, J. L.; Noble, R. D. Alkali-free ZSM-5 Membranes: Preparation Conditions and Separation Performances. Ind. Eng. Chem. Res. 1999, 37, 3924. (6) Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of Highly Selective Zeolite ZSM-5 Membranes by Post-synthetic Coking Treatment. J. Membr. Sci. 1997, 123, 95. (7) Oh, H. S.; Kim, M. H.; Rhee, H. K. Synthesis of ZSM-5 Zeolite Membrane on the Inner Surface of A Ceramic Tube. Stud. Surf. Sci. Catal. 1997, 105, 2217. (8) Bai, C.; Jia, M.-D.; Falconer, J. L.; Noble, R. D. Preparation and Separation Properties of Silicalite Composite Membranes. J. Membr. Sci. 1995, 105, 79. (9) Coronas, J.; Noble, R. D.; Falconer, J. L. Separation of C4 and C6 Isomers in ZSM-5 Tubular Membranes. Ind. Eng. Chem. Res. 1998, 37, 166. (10) Coronas, J.; Falconer, J. L.; Noble, R. D. Characterization and Permeation Properties of ZSM-5 Tubular Membranes. AIChE J. 1997, 43, 1797. (11) Bakker, W. J. W.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. Temperature Dependence of One-Component Permeation through a Silicalite-1 Membrane. AIChE J. 1997, 43, 2203. (12) 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. (13) Yamazaki, S.; Tsutsumi, K. Synthesis of an A-type Zeolite Membrane on Silicon Oxide Film-Silicon, Quartz Plate and Quartz Fiber Filter. Microporous Mater. 1995, 4, 205.

(14) Masuda T.; Hara H.; Kouno M.; Kinoshita H.; Hashimoto K. Preparation of an A-Type Zeolite Film on the Surface of an Alumina Ceramic Filter. Microporous Mater. 1995, 3, 565. (15) Kita, H. Pervaporation through Zeolite Membranes. Membrane (Tokyo) 1995, 20, 169. (16) Wang, J.; Wang, Y.; Fan, S.; Shi, X. Preparation and Gas Permeabilities of Zeolite A Membrane. Proceedings of the 3rd International Conference on Inorganic Membranes, Worcester, MA, 1994. (17) Hedlund, J.; Schoeman, B.; Sterte, J. Ultrathin Oriented Zeolite LTA Films. J. Chem. Soc., Chem. Commun. 1997, 1193. (18) Boudreau, L. C.; Tsapatsis, M. A Highly Oriented Thin Film of Zeolite A. Chem. Mater. 1997, 9, 1705. (19) Boudreau, L. C.; Kuck, J. A.; Tsapatsis, M. Deposition of Oriented Zeolite A Films: in situ and Secondary Growth. J. Membr. Sci. 1999, 152, 41. (20) Aoki, K.; Kusakabe, K.; Morooka, S. Preparation of Oriented A-type Zeolite Membranes. AIChE J. 2000, 46, 211. (21) Wegner, K.; Dong, J.; Lin, Y. S. Polycrystalline MFI Zeolite Membrane; Xylene Pervaporation and its Implication on Membrane Microstructure. J. Membr. Sci. 1999, 158, 17.

Received for review December 14, 1999 Revised manuscript received March 22, 2000 Accepted April 21, 2000 IE990902C