Silicalite Membranes Modified by Counterdiffusion CVD Technique

Counterdiffusion CVD technique was carried out to fill the intercrystalline region. Tetraethyl ..... The silicalite membrane was not broken by the CVD...
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Ind. Eng. Chem. Res. 1997, 36, 4217-4223

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MATERIALS AND INTERFACES Silicalite Membranes Modified by Counterdiffusion CVD Technique Mikihiro Nomura,* Takeo Yamaguchi, and Shin-ichi Nakao Department of Chemical System Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

A new technique to modify the zeolite molecular sieve membrane has been developed. Zeolite membranes have polycrystalline structure, and penetrant molecules can transport through zeolite crystals and among crystal faces. When the intercrystalline region are plugged, the zeolite crystal pores are the only pathway for membrane transport. That is a promising way to obtain a good molecular sieving effect. Counterdiffusion CVD technique was carried out to fill the intercrystalline region. Tetraethyl orthosilicate (TEOS) was employed as a silica source, and O3 was used as an oxidizing agent; these reactants form amorphous silica by CVD reaction. TEOS hardly penetrated into zeolitic pores because of its molecular size, and zeolitic pores may not have been changed by the reaction. Thus, only the intercrystalline region was filled due to the silica formation. The CVD-modified silicalite membrane produced a n-butane selectivity of 87.8 over isobutane at 288 K. Introduction Recently, much attention has been paid to the preparation of inorganic membranes. Inorganic membranes have chemical, thermal, and mechanical stability, and they have the potential for high-temperature gas separation. Zeolites are hydrated aluminosilicates composed of crystalline structure with pores of the same size as single molecules, and unique properties can be obtained by their molecular sieving effect. Zeolites are commercially used as sorbents, catalysts, and detergent builders. Several research groups were trying to make zeolite membranes for the best use of the molecular sieving effect (for example, Sano et al., 1991, 1994; Kusakabe et al., 1996; Funke et al., 1996a,b; Yan et al., 1995; Matsukata et al., 1993; Baertsch et al., 1996; Vroon et al., 1996; Bakker et al., 1996; Kita et al., 1995). Silicalite (ZSM-5) is one of the hydrophobic zeolites having 0.53-0.56 nm pores. Sano et al. (1991, 1994) prepared silicalite and ZSM-5 type zeolite membranes. Hydrothermal syntheses were performed with clear hydrogels (H2O/SiO2 ratios of more than 70) without stirring, and the zeolite crystals accumulated on the support and grew in an interlocking fashion during the synthesis. Kusakabe et al. (1996) made a ZSM-5 type zeolite membrane on a porous R-alumina tube and the permeability of n-C4H10 to i-C4H10 was 10-50. Funke et al. (1996a) separated n-heptane from n-heptane/ isooctane mixtures through a silicalite membrane by vapor permeation, and the highest separation factor was 57. Yan et al. (1995) put the support upside of the autoclave to reduce accumulation of the crystals, and then only the ZSM-5 crystals on the support grew tightly. A different preparation scheme was employed by Matsukata et al. (1993). This scheme involved two steps. The first step was preloading a porous alumina support with the synthesis gel and the second was providing water by vapor phase. Kita et al. (1995) made a NaA zeolite membrane by hydrothermal synthesis. The NaA zeolite membrane was a water-selective mem* Author to whom correspondence is addressed. Fax: +813-5684-8402. E-mail: [email protected]. S0888-5885(97)00338-2 CCC: $14.00

brane, and the separation factor of the water/ethanol system was over 10 000 by pervaporation at 348 K. However, all the zeolite membranes reported have polycrystalline structure. The polycrystalline layer is composed of zeolite crystals and an intercrystalline region having a larger size than zeolitic pores. If the space between the crystals were filled with some inorganic materials, we can make a true molecular sieving membrane. Sano et al. (1995) modified the silicalite membranes using silane coupling reagents and improved the ethanol separation performance of the water/ethanol system. This improvement was explained by enhancement of the hydrophobicity of the membrane. Yan et al. (1997) filled the intercrystalline region of the zeolite ZSM-5 membranes by coking of 1,3,5-triisopropylbenzene. The single gas ratio of n-butane over isobutane at 458 K was 322 after coking treatment. This membrane maintained the performance after calcining at 773 K for 30 h. The coke barrier, however, could be damaged at high temperature by oxidation. In this study, modification was carried out using amorphous silica made by CVD technique because silica is stable against oxidizing agent. CVD technique is a strong tool to modify inorganic materials and was employed to make microporous membranes or thin SiO2 films by several research groups (Okubo and Inoue, 1989; Gavalas et al., 1989; Tsapatsis et al., 1991; Nakao et al., 1994, 1995). There are two methods to make CVD molecular sieving membranes, one-sided CVD and counterdiffusion CVD (two-sided CVD). One-sided CVD supplies all the reactants from the same side of the support membrane. The CVD reaction occurs at the surface of support. The substrate pores are plugged by the amorphous silica, and finally the amorphous silica is deposited on the substrate. The property of amorphous silica determines selectivity of the gas separation. On the other hand, the counterdiffusion CVD method can control pore size by changing the reactant size. The counterdiffusion CVD method needs two reactants, and reaction takes place when both reactants contact each other. Each reactant must supply from the opposide side of a porous © 1997 American Chemical Society

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Figure 1. Chemical structures of TMOS and TEOS.

substrate, and they can diffuse through the substrate pores from opposite directions. Those reactants can contact in the pores and react. The reaction makes amorphous silica in the pores and narrows the pores. Finally, both reactants cannot diffuse through such narrow pores, and the reaction will stop due to poor reactants supply. Uniform pore size can be obtained using this technique. Okubo and Inoue (1989) carried out one-sided CVD of thermal decomposition of TEOS on the porous glass tube. The separation factor of He/O2 reached 6, which is bigger than Knudsen’s diffusion. Yan et al. (1994) also employed one-sided CVD of thermal decomposition of TEOS. Porous γ-alumina substrate was filled by the amorphous silica efficiently by vacuuming the other side of the membrane. The separation factor of H2 to N2 was over 1000 at 873 K. Tsapatsis et al. (1991) compared the one-sided and two-sided CVD of the SiCl4/water system. They noted that the one-sided CVD membrane has significantly higher selectivity of He/N2 than the two-sided CVD membrane. Nakao et al. (1994, 1995) successfully made a silica molecular sieve membrane using the TEOS-O3 counterdiffusion CVD technique. The highest separation factor of He/N2 obtained was 950 at 313 K. Both gases had activated energy through the membrane. Thus, the pore size was considered about 0.3 nm from the kinetic diameter of N2. To employ the CVD technique for the modification of the intercrystalline region of the zeolite membrane, there are some differences between one-sided CVD and counterdiffusion CVD. The one-sided CVD technique is usually employed for making a thin film on the substrate. The amorphous silica made by the reaction might deposit on the surface of the zeolite crystals which will fill the zeolite pores. On the other hand, counterdiffusion CVD can control the silica formation region by the reactants’ size. We can modify only the intercrystalline region by using bigger reactants than the zeolite pores. If the reactants can pass through the intercrystalline region, the region can be filled by this technique for any shape of the intercrystalline region. To fill the pinholes of the sol-gel membranes, Kitao and Asaeda (1991) used the SiH4/O2 counterdiffusion CVD method and the permeability ratio of He/N2 reached more than 300. Niwa et al. (1986) used CVD of silicon methoxide (TMOS) to modify only the outside of the powder silicalite crystals. This phenomenon indicates that TMOS cannot diffuse through the silicalite pores. The kinetic diameter of TEOS is about 0.9 nm. That is also bigger than silicalite pores (0.53-0.56 nm). TEOS and TMOS have similar chemical structures (Figure 1), and they might have similar effects on the silica source of the CVD reaction. We can modify only the intercrystalline region using TEOS or TMOS as silica sources. The pore size of TEOS-O3 counterdiffusion CVD was considered about 0.3 nm (Nakao et al., 1995). There is a 0.3 nm pore in the intercrystalline region after the modification of TEOS-O3 counterdiffusion CVD. This

Figure 2. Apparatus for the modification of counterdiffusion CVD.

pore size is small enough compared with the silicalite pores. Thus, CVD using the TEOS-O3 system was employed to fill only the intercrystalline region of the silicalite membrane. In this paper, we will report the preparation and characterization of silicalite membranes modified by TEOS-O3 counterdiffusion CVD. Experimental Section Synthesis of the Silicalite Membrane. The synthesis method of the silicalite membrane was followed by Sano et al. (1994). A hydrogel was made with Colloidal Silica (Cataloid SI-30: Shokubai Kasei Co.; 30.4 wt % SiO2, 0.38 wt % Na2O, 69.22 wt % water), tetrapropylammonium bromide (TPABr), sodium hydroxide, and pure water (0.1 TPABr-0.05 Na2O-1 SiO280 H2O). A porous stainless steel disk of 5 cm diameter with an average pore diameter of 10 µm was used for the support. The support was placed at the bottom of a 300 mL stainless stell autoclave. The hydrogel was stirred for more than 1 h at room temperature, and the hydrogel was poured into the autoclave. The autoclave was placed in an air-heated oven at 443 K for 12-48 h. The hydrothermal synthesis was carried out under autogenous pressure without stirring. After the synthesis, the silicalite polycrystalline membrane on the support was taken out and washed by pure water. In order to decompose the organic amine occluded in the silicalite framework, the silicalite membrane was calcinated in air at 773 K for 20 h. Counterdiffusion CVD Modification. The schematic diagram of the apparatus is shown in Figure 2. TEOS or TMOS was put in the bubbler, and the bubbler temperature was controlled between 333 and 373 K. The silica source was carried by O2 or N2 gas. The flow rate of the carrier gas was 3.33 × 10-6 m3 s-1. The carrier gas of the silica source was kept at 403 K to prevent condensation in the apparatus. The concentration of TEOS was 0.73 or 2.08 mol m-3 at the inlet of the reactor. O3 was generated by the usual ozone generator in which oxygen was passed through the silent discharger. The concentration of O3 was 0.2 mol m-3. The flow rate of oxygen was kept at 1.67 × 10-5 m3 s-1. The silica source was supplied from one side of the silicalite membrane, and O3 gas was introduced from the other side of the membrane. Each gas counterdiffused through the silicalite membrane, and reaction should have occurred in the membrane. CVD was carried out at 473 K for 1-27 h. Permeation Measurement. Permeation measurements were made with a single gas of He, N2, n-butane, isobutane, and SF6 between room temperature and 473

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Figure 3. Schematic diagram of the single-gas permeation experiment.

K. The permeation apparatus is shown in Figure 3. Permeance was measured by the flow rate of the permeation side or pressure change of the vacuumed permeate side compartment. The flow rate method was employed for higher permeance measurements, and the pressure change method was carried out for lower permeance measurements. The permeances of both methods were checked by n-butane permeances at room temperature, and both systems showed the same results in the region of 10-8-10-9 mol m-2 s-1 Pa-1. The feed pressure of He, N2, and SF6 was kept at 0.20 MPa, and those of n-butane and isobutane were 0.06 and 0.15 MPa, respectively. Before the transport experiments, the permeance of N2 was measured to monitor the desorption of any previously adsorbed hydrocarbon gas to the membrane. The membrane was calcined in an oxygen atmosphere at 473 K until the N2 permeance recovered. Permeability of 1,3,5-triisopropylbenzene was measured by evaporation at 303 K to examine pinholes after the CVD modification. Results and Discussion Microstructure of the Silicalite Membrane. Figure 4 shows the SEM microphotographs of the surface, support side, and cross section of the silicalite membrane. The silicalite layer was peeled off from the stainless steel substrate for this observation. The photograph showed that grain sizes at the surface are larger than those at the bottom. The surface polycrystalline structure is obviously denser, and the support side is looser from the cross-sectional view.

Figure 4. SEM image of the silicalite membrane.

First, the crystallization took place in the hydrogel, and the crystals fell on the stainless steel substrate due to gravity (Sano et al., 1992). Next, the crystallization in the hydrogel stopped by the decrease in concentration of each component in the hydrogel, and the crystals contacting the hydrogel were intergrown with each other. Thus, the surface crystals were larger and denser than the support side crystals. Diffusion Direction of the Reactants. The direction of counterdiffusion for supplied gases was changed. The TEOS concentration was 2.08 mol m-3 at the inlet of the reactor. TEOS was supplied from the support side, and O3 was supplied from the surface for 5.5 h. At the start of the CVD treatment, the reactor temperature increased by 10.5 K in 2 min by the reaction. The polycrystalline structure was broken after the CVD treatment. This might be due to rapid CVD reaction. Figure 5 shows SEM microphotographs of the surface and the cross section of the CVD-modified silicalite membranes. Much of amorphous silica was deposited on the membrane although the reaction time was only 5.5 h. This phenomenon might be due to leakage of the ozone through the broken membrane. TEOS was supplied from the surface side for 25 h using a silicalite membrane having almost the same gas permeation properties. There was no remarkable temperature change in the reactor. The silicalite membrane was not broken by the CVD reaction. Figure 6 shows an SEM photograph of the CVD-modified membrane surface. There is no indication of the existence of amorphous silica on the membrane surface. The reaction can be controlled for this operation. Figure 7 shows a model comparing with CVD reactants supplied direction. In this model, the silicalite membrane is divided into a tight surface side and a loose support layer. As shown in Figure 7a, TEOS was supplied from the loose substrate side and ozone was introduced from the surface side. TEOS can hardly diffuse through the tight surface side by the diffusion resistance, while oxidizing agents pass through it, and the concentration of both TEOS and O3 might be very high in the loose support side. The high TEOS concentration in the membrane causes fast reaction. As a result, the membrane was

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Figure 6. SEM image of the surface of a 25 h CVD-modified membrane. The silica source was supplied from the surface. The initial SF6 permeance was 4.23 × 10-9 mol m-2 s-1 Pa-1.

Figure 5. SEM image of a 5.5 h CVD-modified membrane. The silica source was supplied from the support side.

cracked due to rapid temperature change or silica formation by the reaction. Figure 7b schematically represents the membrane cross section when TEOS was supplied from the surface side. For this case, resistance in the tight surface membrane against TEOS diffusion is high, and TEOS concentration in the membrane is low compared with Figure 7a. The mild reaction would occur in the dense surface part of the silicalite membrane, and no crack was detected. TEOS was introduced from the tight surface side in the following sections. Effect of the Concentration of TEOS. The effect of the supplied TEOS concentration was examined. The TEOS concentration was 2.08 mol m-3 at the inlet of the reactor for CVD modification. Table 1 shows the pure gas permeance of n-butane and isobutane. The permeation ratio of n-butane over isobutane at room temperature was 4.04 after 9 h of CVD modification. The permeances of both n-butane and isobutane increased, and the permeance ratio decreased after an additional 8 h of CVD. Thus, the membrane was suggested to be cracked due to the extra CVD reaction.

Figure 7. Schematic diagram of the CVD reaction (effect of the supply side of the silica source). Cross section of the silicalite membrane. Table 1. Pure Gas Permeation Measurements for a CVD-Modified Silicalite Membrane at Room Temperature (TEOS 2.08 mol m-3) permeance [10-9 mol m-2 s-1 Pa-1] CVD 1 h CVD 9 h CVD 17 h CVD 25 h

n-butane

isobutane

permeance ratio

17.5 3.66 5.86 1.82

9.58 0.91 2.83 0.78

1.82 4.04 2.07 2.33

To maintain mild CVD reaction condition, the TEOS bubbler temperature was reduced to 333 K, and then the TEOS concentration became 0.73 mol m-3 at the inlet of the reactor. Table 2 shows the single-component permeances at room temperature through the membrane. The permeance of isobutane seriously decreased by 8 h of CVD modification while the n-butane permeance slightly changed. As a result, the permeances ratio of n-butane over isobutane became 87.8. Figure 8 shows the permeances of each gas at room tempera-

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4221 Table 2. Pure Gas Permeation Measurement for a CVD-Modified Silicalite Membrane at Room Temperature (TEOS 0.73 mol m-3)

membrane silicalite CVD 4 h CVD 8 h CVD 16 h

ratio of permeance [10-9 mol m-2 s-1 Pa-1] n-butane/ He N2 n-butane isobutane SF6 isobutane 106 281 97.7 240 50.2 73.5

50.9 40.9 19.7a

1.64 0.466a 0.367a

4.49 2.80 1.12

9.1b 31.1 87.8 53.7

a Permeance was measured by pressure change. b Average of 11 silicalite membranes.

Figure 10. Permeances of the CVD modification of the loose silicalite membrane at room temperature.

Figure 8. Permeances at room temperature and 473 K by the CVD modification.

Figure 9. Each gas permeance at 473 K as a function of the CVD treatment time.

ture and 473 K for 4 and 16 h of CVD treatment. The permeances of isobutane and SF6 decreased both at room temperature and 473 K, although the permeances of He and N2 were kept constant. This phenomenon indicates that only the intercrystalline region was filled with amorphous silica by the counterdiffusion CVD. Figure 9 shows gas permeances at 473 K plotted against the CVD treatment time. The permeances of all gases decreased by the first 8 h of CVD modification. The permeance was, however, kept constant by an additional 8 h of CVD modification. The CVD reaction stopped after the first 8 h of CVD treatment. The intercrystalline regions might be well modified, and the reactants cannot diffuse through the membrane.

The permeability of 1,3,5-triisopropylbenzene was not detected for 6 h of pervaporation after 16 h of CVD modification (flux was less than 0.001 kg m-2 h-1). The silicalite membrane which was not modified by CVD has the permeability of 0.018 kg m-2 h-1 at 303 K. All the CVD-modified silicalite membranes were stable by calcining in an oxygen atmosphere at 473 K. The intercrystalline region must be filled with amorphous silica by the CVD modification using the mild reaction condition. Influence of the Intercrystalline Region Size of the Initial Silicalite Membrane. The kinetic diameter of SF6 is 0.55 nm, and the SF6 diffusion in the silicalite crystals must be very slow. SF6 permeance at room temperature was used to check pinholes in the membrane. For all the silicalite membranes used in these experiments, initial permeances of SF6 were between 3.98 and 7.19 × 10-9 mol m-2 s-1 Pa-1. The looser silicalite membrane was tested for the limitation of this modification. Figure 10 shows the singlecomponent permeances at room temperature by each CVD modification for the looser silicalite membrane. The initial SF6 permeance was 2.54 × 10-8 mol m-2 s-1 Pa-1. TEOS and TMOS were employed for the silica source in this modification. First 5.5 h of treatment was carried out using TEOS for a silica source. The permeances of small molecules like He and N2 decreased, while the permeances of isobutane and SF6 slightly decreased. TMOS is smaller than TEOS (Figure 1), and TMOS could diffuse well in the intercrystalline region. TEOS/TMOS mixtures (0.9 TEOS-0.1 TMOS) were used as a silica source for an additional 4.5 h of treatment, and pure TMOS was employed after that. The permeation results, however, were almost the same as those of the TEOS modification as shown in Figure 10. Figure 11 shows a SEM photograph of the membrane surface after 27 h of CVD modification. The amorphous silica was observed slightly on the surface of the membrane. The amorphous silica accumulated on the membrane and covered the silicalite pores without filling the crystal faces. As shown in Figure 6, no amorphous silica was observed on the membrane surface when a dense silicalite membrane was used. The silicalite membrane has a SF6 permeance of 4.23 × 10-9 mol m-2 s-1 Pa-1. For the denser silicalite membrane case, only intercrys-

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Conclusion The counterdiffusion CVD technique was employed to fill the intercrystalline region of a zeolite membrane. Nonzeolitic pathways among crystal faces were successfully filled with amorphous silica formed by CVD, and the membrane selectivity was improved. A CVDmodified membrane produced n-butane selectivity of 87.8 over isobutane at 288 K and 14.2 at 473 K. The SF6 permeance of the initial silicalite membrane should be below 10-8 mol m-2 s-1 Pa-1 for this operation. Acknowledgment This research was supported by a NEDO International Joint Research Grant. The authors also acknowledge helpful advises and discussions with Mr. Takashi Sugawara and Dr. Tatsuya Okubo. Figure 11. SEM image of the surface of a 27 h CVD-modified membrane. The initial SF6 permeance was 2.54 × 10-8 mol m-2 s-1 Pa-1.

Figure 12. Schematic diagram of the CVD reaction (effect of the properties of the initial silicalite membrane).

talline regions were filled by the amorphous silica made by the reaction. The permeance of SF6 should be below 10-8 mol m-2 s-1 Pa-1 for this operation. Figure 8 shows that permeances of bigger molecules decreased by the CVD modification, and Figure 10 shows different results. This is because of differences of the silicalite base membranes. The reaction model of dense and looser silicalite membranes is shown in Figure 12. As shown in Figure 12a, the intercrystalline region could be filled by the amorphous silica made by the reaction when the region is not broad. The leakage of bigger molecules like isobutane or SF6 is reduced by the modification, and the true molecular sieving effect is obtained. Figure 12b shows the looser silicalite membrane case. The pinholes were too big for the amorphous silica to fill them completely. As a result, the oxidizing agent leaks to the TEOS vapor side through the pinholes, and the amorphous silica will lay on the membrane surface as shown in Figure 11. The amorphous silica deposited on the silicalite pores to reduce the permeance of all molecules.

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Received for review May 12, 1997 Revised manuscript received July 2, 1997 Accepted July 2, 1997X IE970338A

X Abstract published in Advance ACS Abstracts, August 15, 1997.