Zeolite NaA Membrane - American Chemical Society

Ind. Eng. Chem. Res. 2001, 40, 163-175

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Zeolite NaA Membrane: Preparation, Single-Gas Permeation, and Pervaporation and Vapor Permeation of Water/Organic Liquid Mixtures Ken-ichi Okamoto,* Hidetoshi Kita, Kohji Horii, and Kazuhiro Tanaka Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan

Masakazu Kondo Mitsui Engineering and Shipbuilding Co. Ltd., Tamano, Okayama 706-8651, Japan

Zeolite NaA membranes were prepared reproducibly by a one-time-only hydrothermal synthesis with a short reaction time of 3 h at 373 K using a gel with the composition Al2O3:SiO2:Na2O: H2O ) 1:2:2:120 (in moles) and porous R-alumina support tubes seeded with zeolite NaA crystals. A dense intergrown zeolite crystal layer of about 30 µm in thickness was formed on the outer surface. The zeolite NaA membranes were highly permeable to water vapor but impermeable to every gas unless dried completely. The completely dried membranes displayed gas permeation behavior attributed to Knudsen diffusion, indicating the presence of interstitial spaces between the zeolite crystal particles, or nonzeolitic pores. The membranes displayed excellent waterpermselective performance in pervaporation (PV) and vapor permeation (VP) toward water/ organic liquid mixtures. With an increase in temperature, both the permeation flux Q and the separation factor R increased, and the membrane performance was much better for VP than for PV. For VP at 378 K and 10 wt % of feedwater, Q values were 4.5, 3.5, and 7.8 kg/(m2 h) and R values were >30000, 5700, and >9000 for the water/ethanol, /methanol and /dioxane systems, respectively. A mechanism of PV and VP based on the capillary condensation of water in the zeolitic and nonzeolitic pores and the blocking of other molecules from entering the pores was proposed. Introduction Pervaporation (PV) has attracted increasing attention as an effective and energy-efficient technique for the separation of azeotropic or close-boiling liquid mixtures.1,2 Polymeric membranes have been widely investigated for PV of these liquid mixtures.1-3 However, practical application of polymeric membranes has been limited to dehydration of alcohols4,5 because of the insufficiency of their thermal, mechanical and chemical stability. Inorganic membranes are generally superior to polymeric membranes in terms of thermal, mechanical, and chemical stability. Recently, many papers have reported on the PV properties of inorganic membranes including silica,6,7 zeolite A,8-15 faujasite-type zeolites (X and Y),16,17 MFI-type zeolites (silicalite and ZSM5),18-21 and mordenite.22,23 Among these inorganic membranes, only zeolite NaA membrane modules, developed by Mitsui Engineering and Shipbuilding Co. Ltd. in cooperation with Yamaguchi University, have been used for the dehydration of organic liquids on a large commercial scale.11,24-26 We first reported on the development of zeolite A membranes with high PV performance in 1993.8 Since then, we have reported on the PV properties of zeolite A membranes in a letter9 and in the proceedings of conferences concerning membrane separation.10,12 In this paper, we comprehensively report on the preparation and characterization of zeolite NaA * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 81-836-85-9601. Tel.: 81-836-85-9660.

membranes and their PV and vapor permeation (VP) properties toward water/organic liquid mixtures. Experimental Section Membrane Preparation. Zeolite A membranes were synthesized hydrothermally on porous support tubes. The materials used for the synthesis were sodium methasilicate nonahydrate, aluminum hydroxide, sodium aluminate (Al/NaOH ) 0.58), sodium hydroxide, and distilled water. They were purchased from Wako Pure Chemical Industries Ltd. Zeolite NaA powder (200 mesh, Wako) was used as the seed crystal. Porous R-alumina tubes with dimensions of 1.0 cm o.d., 0.8 cm i.d. and 20 cm length, supplied by Mitsui Grinding Wheel Co., were used as the substrates of the zeolite membranes. They had an average pore size of 1 µm and a porosity of about 50%. An aluminate solution was prepared by dissolving aluminum hydroxide or sodium aluminate in a hot aqueous sodium hydroxide solution, and then it was added to an aqueous silicate solution at a temperature of 303-333 K. The resulting mixture was stirred vigorously for 15-30 min to produce a homogeneous gel. The molar composition of the gel used for the synthesis was Al2O3:SiO2:Na2O:H2O ) 1:2:2-2.4:80-600. Prior to the hydrothermal synthesis, the outer surface of the support tubes was seeded with zeolite NaA crystals by coating them with a slurry of zeolite 4A powder and water and then drying them at 343 K. The gel (360 g) was placed in a glass tube reactor (3 cm i.d. and 40 cm length)

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Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001

Table 1. Synthesis Conditions of Zeolite NaA Membranes and PV Performance for a Water/Ethanol Mixture (xW ) 10 wt %) at 348 K membrane synthesis conditions experiment

gel compositiona Al2O3:SiO2:Na2O:H2O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1:2:2:120 1:2:2.4:144 1:2:2.4:144 1:2:2.4:144 1:2:2.4:240 1:2:2.4:480 1:2:2:120 1:2:2:120 1:2:2:120 1:2:2:120 1:2:2:120 1:2:2:120 1:2:2:120 1:2:2:120 1:2:2:80 1:2:2:400 1:2:2:120 1:2:2.4:480

temp [K]

reaction time [h]

373 373 373 353 353 353 373 373 373 373 373 373 373 373 373 373 353 353

4 3 3 3, 5, 8 12 12 4 4.5 0.5 1 2 3 6 3 3 3 3 3

PV performance

seedingb

number of synthesis times

Q [kg/(m2 h)]

+c +d + + + + + e + + + +

4 4 3 4 4 4 2 1 1 1 1 1 1 1 1 1 1 1

1.1 1.1

0.90 1.2 2.5 2.2 1.9 1.9

RW/E 2000 400 1 1 1 1 2000 170 1 1 1400 >10000 2700 1 5200 1 1 1

a As the Al source, sodium aluminate was used in experiments 2-7, and aluminum hydroxide was used in the other experiments. b ) No seed treatment, + ) seeded with zeolite NaA crystals as described in the Experimental Section, except for experiments 7 and 8. c Seeded by rubbing dry powder of zeolite 4A on the outer surface of support tube. d For seeding, the support tube was immersed in a suspension of zeolite 4A/water, and the inside of the tube was evacuated to deposit seed crystals on the tube by filtration. e Seed crystal powder (zeolite 4A) was dispersed in the synthesis gel, and the support tube was not seeded.

The permeate vapor was collected by a cold trap cooled by liquid nitrogen. The downstream pressure was maintained below 13.3 Pa unless otherwise noted. The effective membrane area was 46 cm2 (15 cm in effective length) or 30 cm2 (10 cm in effective length). Composition analysis of the feed and permeate was performed on a gas chromatograph equipped with 3-m column packed with Polarpack Q poly(ethylene glycol)-1000 supported on Shimalite F. The PV performance of the membrane was evaluated by permeation flux Q in kg/ (m2 h) and separation factor R. The separation factor of water over organic liquid is defined as

RW/O ) (yW/yO)/(xW/xO)

Figure 1. One-end-open type membrane modules used for (a) PV and (b) VP.

equipped with a water condenser. The seeded tube was put in the gel, and the hydrothermal synthesis reaction was carried out at 353 or 373 K for 0.5-12 h under atmospheric pressure. After the synthesis, the tube was washed with water and dried in vacuo at 343 K. In some cases, especially for unseeded support tubes, this synthesis procedure was repeated several times. Characterization and Permeation Experiments. Zeolite membranes were characterized by X-ray diffraction (XRD) using a Shimadzu XD-3 instrument and by scanning electron microscopy (SEM) using Hitachi S-2300 instrument. PV experiments were carried out using the permeation cell illustrated in Figure 1a. Each end of the membrane tube was sealed in the cell with double Viton O rings. The permeation cell was set in a thermostated air bath. Feed liquid preheated to a constant temperature was fed to the outer side of the membrane tube at a flow rate of about 30 cm3/min. The inside of the membrane tube was evacuated through a vacuum line.

where x and y are the weight fractions of the water components in the binary feed mixture and in the permeate, respectively. Subscripts W and O refer to water and organic liquid components, respectively. VP experiments were carried out at 373-473 K using the permeation cell illustrated in Figure 1b. The feed liquid was vaporized by a super heater and then fed to the outer side of the membrane tube at a flow velocity of 140-170 cm/s. The feed vapor pressure was controlled in the range of 0.1-0.3 MPa by a needle bulb at the outlet of the feed vapor. VP measurements were done for a smaller effective membrane area of 15 or 30 cm2 because of the very high permeation flux. Other experimental procedures for VP were similar to those for PV. The VP performance of the membrane was evaluated by permeance R in cm3 (STP)/(cm2 s cmHg), as well as by Q and RW/O. Single-gas permeation experiments were carried out for H2, O2, N2, CO2, CH4, and SF6 at 308, 378, and 473 K using the permeation cell for PV. Prior to the gas permeation measurements, the membrane in the cell was completely dried by evacuating both sides of the membrane for more than 3 days at 473 K. The measurement gas was fed to the feed and permeate sides of

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 165

Figure 2. XRD patterns of (a) R-alumina support tube and (b-e) zeolite NaA membranes after 1, 2, 3, and 6 h, respectively, of the synthesis and (f) zeolite NaA powder.

the membrane at pressures of 0.2-0.6 and 0.1 MPa, respectively, and then the flow rate of the permeate was measured with a liquid film flowmeter. Results and Discussion Membrane Preparation and Characterization. The membrane synthesis conditions investigated and the PV performance of the resulting zeolite NaA membranes are summarized in Table 1. Based on refs 27 and 28 and our preliminary experiments in preparing zeolite NaA crystals, membrane preparation was investigated mainly for the gel composition Al2O3:SiO2:Na2O ) 1:2:2-2.4. The preparation procedure and concentration of gel, reaction temperature and time, seed treatment, and number of the hydrothermal synthesis times were considered as the operating factors. In the case of unseeded support tubes, the permselective zeolite membranes were prepared only in experiments 1 and 2, where the hydrothermal synthesis was repeated four

times at 373 K in the high-concentration gel of Al2O3: H2O ) 1:120-140. Several membrane samples were prepared under these synthesis conditions (1 and 2), and the PV performance varied largely from sample to sample. The PV performance data for the membrane samples with the highest R values are listed for experiments 1 and 2 in Table 1. Thus, it was difficult to prepare zeolite NaA membranes with high PV performance reproducibly without the seed treatment. The seed treatment was very effective for the preparation of highly permselective zeolite NaA membranes without repetition of the hydrothermal synthesis. The membrane with the best PV performance was prepared by the synthesis conditions in experiment 12, where the hydrothermal synthesis was carried out once for the seeded tube in a gel with the composition Al2O3:SiO2: Na2O:H2O ) 1:2:2:120 for 3 h at 373 K. No permselective membrane was prepared in the more dilute gel (Al2O3:H2O ) 1:400) or at the lower temperature (353

166


Figure 3. SEM photographs of the (a) surface and (b) cross section of the seeded support tube and (c-f) surface of zeolite NaA membranes after 1, 2, 3, and 6 h, respectively, of the synthesis.

K). Under such conditions, the repeated synthesis might be necessary to prepare a permselective membrane. In the series of experiments 9-13, the effect of reaction time on the PV performance was investigated under the optimum conditions for gel composition and reaction temperature. The membrane formed after 1 h of the synthesis did not show selectivity. The membrane after 2 h showed a slightly larger permeation flux but a much lower separation factor compared with the membrane after 3 h. The membrane after 6 h also showed a much lower separation factor. These membranes were also characterized by XRD. Figure 2 shows the XRD patterns of the zeolite NaA membranes, zeolite NaA crystal powder, and the R-alumina support tube. The membrane formed after 1 h showed very weak peaks of zeolite A besides strong peaks of the R-alumina substrate. For the membrane after 2 h, the zeolite A peaks appeared more clearly but were still weak compared with the substrate peaks. The membrane after 3 h clearly displayed every peak observed for zeolite A crystal powder; that is, the XRD pattern consisted of the zeolite A crystal peaks and the substrate peaks. However, the zeolite A peaks became much weaker for the membrane after 6 h.

The morphologies of these membranes were observed with SEM. Figure 3 shows SEM photographs of the outer surface of the seeded R-alumina support tube and the zeolite NaA membranes after 1-6 h of the synthesis. The surface of the support tube was rather rough and larger pores (depressions) of up to several micrometers in size were present in the surface. The seed crystals, whose size was less than about 2 µm, were not deposited continuously in a layer but rather preferentially in places such as larger pores. After 1 h of the synthesis, the amorphous gel layer was deposited on the surface. After 2 h, cubic crystals characteristic of zeolite A, 2-4 µm in size, were formed on the surface with rather loose packing. There were observed many cavities of micrometer size between zeolite crystal particles. After 3 h, the surface was completely covered with zeolite NaA crystals, and the packing of the crystals was very dense. No space (cavity) between the crystal particles was found by SEM. It was observed that, after 6 h, roundshaped crystals characteristic of zeolite P were formed on the zeolite A crystals. After 6 h, the reflection intensities of the zeolite peaks in the XRD pattern reduced, but no peaks of zeolite P were observed, probably because the main peaks due to zeolite P appear


Figure 4. SEM photographs of the zeolite NaA membrane after 3 h of the synthesis: (a) outer surface, (b) cross section of the outer side, (c) cross section of the middle, (d) cross section of the whole, (e) inner surface, and (f) cross section of the inner surface.

at positions similar to those of the peaks due to zeolite A. However, the XRD pattern of the powder formed at the bottom of the vessel after the prolonged hydrothermal synthesis showed the presence of zeolite P as well as zeolite A. The significant decrease in PV selectivity after 6 h seemed to be due to the formation of zeolite P crystals. Zeolite P crystals formed on the zeolite A crystal layer, being less hydrophilic, tend to spoil the preferential adsorption of water in the membrane. Thus, the optimum reaction time was around 3 h. Figure 4 shows SEM photographs of the cross-sections and the inner surface of the membrane (experiment 12). The dense intergrown zeolite crystal layer, of which the thickness was around 30 µm, was formed on the outer surface of the support tube, whereas zeolite crystals were not formed in a layer but rather in places on the inner surface that had not been seeded. The inside of the support tube remained unchanged. The depth distribution of the Si/Al atomic composition in the cross section measured by electron probe microanalysis (EPMA) showed the presence of an intermediate layer

between the zeolite crystal layer and the unchanged substrate layer.11 The thickness of the intermediate layer was about 10 µm. Based on the aforementioned results, the formation of zeolite NaA membrane might proceed as illustrated in Figure 5. First, the amorphous gel begins to deposit on the support tube coated with seed crystals. After 1 h of the hydrothermal synthesis under the optimum conditions, the amorphous gel layer is deposited on the support tube thickly to some extent. The seed crystals promote homogeneous crystallization of the amorphous gel layer, probably because the surface of the seed crystals provides nucleation sites. After 2 h, the crystallization of the amorphous gel layer has proceeded almost completely. From the SEM top view, zeolite A crystals with rather similar sizes of 2-4 µm were packed loosely at the top side. However, the membrane had a fairly high separation factor of 1400, indicating the presence of the dense zeolite crystal layer on the inner surface. In the second stage, the growth of new crystals in spaces between the crystals proceeds to form the well-inter-

168


Table 2. Compositions of Reaction Mixtures and Reaction Conditions for Synthesis of Zeolite A Membranes

code I-1 I-2 I-3 I-4 I-5 I-6 I-7 II-1 II-1′ II-2 II-3 II-3′ II-4 III-1

gel or solution composition Al2O3:SiO2:Na2O:H2O 1:2:2:120 1:2:2:120 1:2:2:120 1:0.85:4.1:200 1:2.5:4.25:222 1:2:2:240 1:2:2:480 1:5:55:1005 1:5:55:1005 1:5:50:1000 1:5:50:1000 1:5:50:1000 1:9:80:5000, secondary growth 1:4.4:0.036:706 + (TMA)2O:4.1 secondary growth

support (average pore size)

reaction conditionsa

seedb

separation

ref

R-alumina tube (1 µm) 65% R-alumina tube (1 µm) R-alumina tube (0.17 µm) alumina tube, porous quartz fiber filter 65% R-alumina tube (1 µm) R-alumina tube (0.17 µm) porous zirconia sheet ZrO2-TiO2/carbon tube(0.14 µm) stainless steel plate R-alumina disk (0.1-0.3 µm) R-alumina disk (0.1-0.3 µm) R-alumina disk (1 µm)

373 K, 3 h, 1 time 373 K, 3.5 h, 1 time 368 K, 3-5 h, 1 time 353-358 K, 10-12 h, 1-4 times 353, 368 K, 4-6 h, 1-4 times 373 K, 0.25 h, M-W, 1 time 373 K, 5-3 h, 1-5 times 323K, 48 h or 353 K, 3 h, 1 time 323 K, 48 h, 2 times 330-358 K, 3-48 h, 363 K, 2-4 h, 1 time 363 K, 0.25 h, M-W, 1 time 353 K, 1-25 h, 1-several times

+ + + + + + + + + + +, 0.7 µm

gas, PV PV gas PV gas PV PV gas gas PV

9, 10 11 34 30 31 12 35 14 14 36 37 37 15

glass slide

363 K, 3 day

+

-

32, 33

colloidalc

a Reaction temperature, reaction time, and synthesis times. b + ) Seeded, - ) unseeded from a solution of Al2O3:SiO2:Na2O:H2O ) 1:4.4:0:035:239 and (TMA)2O:4.1.

Figure 5. Schematic illustration of formation process of zeolite NaA membrane: (a) seeded support tube and (b-d) membranes after 1, 2, and 3 h, respectively, of the synthesis.

grown crystal layer, as can be seen in the SEM top view of the membrane after 3 h. The outer surface of the R-alumina support tubes is rough rather than smooth and has some pores large enough for seed crystals to enter. Therefore, it is reasonable to consider that zeolite A crystals could also grow in such pores near the surface of the support tube. Such zeolite crystals might correspond to the intermediate layer observed by EPMA. Many papers have reported on the preparation of zeolite A membranes, as listed in Table 2.8-15,29-37 They are classified into three types according to the compositions of reaction mixtures in the hydrothermal synthesis, namely, milk-like gel (a series of code I),9-12,30,31,34,35 clear solution (a series of code II)14,15,29,36,37 and clear solution with template (code III).32,33 The results of our method (codes I-1 and I-2) is classified as the milk-like gel type. The typical optimum composition of the gel is Al2O3:SiO2:Na2O:H2O ) 1:2:2:120, and the optimum reaction temperature and time are 373 K and 3-4 h, respectively. This method has an advantage that zeolite NaA membranes with high PV performance can be prepared reproducibly by a one-time-only synthesis with a short reaction time using properly seeded support

c

Dip coated in colloidal suspension prepared

tubes. Aoki et al. (code I-3) prepared zeolite A membranes using a gel with the same composition and porous R-alumina tubes with smaller dimensions (2.8 mm in o.d. and 14 mm in effective length) and a smaller pore size (170 nm).34 Their membranes had a morphology that was slightly different from that of our membranes, that is, smaller zeolite crystals (less than 2 µm) and a much thinner zeolite layer (0.4-3.8 µm for 1-6 h reaction). This is probably because of differences in the seed crystals, support tube, and reaction temperature. They also prepared zeolite A membranes with a different morphology using a dilute synthesis solution with the composition Al2O3:SiO2:Na2O:H2O ) 1:2:2:480 (code I-7).35 In this case, the reaction was repeated several times at 373 K to achieve a higher selectivity of H2/N2. On the other hand, the clear-solution type method was developed, with the expectation of potential advantages in terms of reproducibility and control in zeolite membrane preparation.14,29 The typical composition of the clear solution is Al2O3:SiO2:Na2O:H2O ) 1:5:50:1000 (codes II-I-II-3′). The characteristics are a higher ratio of SiO2/Al2O3, lower contents of Al2O3 and SiO2, and a higher alkali content compared to those of the milk-like gel. The reaction temperature is in the range of 363323 K with a relatively long reaction time of up to 48 h. The lower reaction temperature and longer reaction time, typically 323 K and 48 h (codes II-1 and II-2), are characteristic of the clear-solution type, compared to the milk-like gel type. Jafar and Budd have reported on the preparation of zeolite A membranes by means of the clear-solution type method without pretreatment of the substrate (codes II-1 and II-1′).14 Their disk-shaped membranes prepared by a one-time synthesis at 323 K for 48 h displayed high performance for PV of water/ipropanol mixtures. For the tubular membranes, a twotimes synthesis was necessary to achieve permselectivity. The clear-solution type might be superior to the milk-like gel type in preparing zeolite A membranes without seed treatment of the substrate. However, the nucleation is sensitive to the synthesis conditions. This might be a serious obstacle in producing high-performance membranes reproducibly on a large scale. Kumakiri et al. have reported on the preparation of zeolite A membrane by growing seed crystals on a support disk in a clear solution with the composition Al2O3:SiO2:

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 169 Table 3. Gas Permeances for the Zeolite NaA Membranes membranea

temp [K]

R and R ratiob H2

H2/N2 H2/CO2 H2/SF6 H2/n-C4H10

m-1 m-2 m-3

378 378 378

3.1 1.27 0.30

This study 3.8 4.3 3.7 5.0 4.0 5.8

(1) (2) (3)

308 308 308

3.3 0.24 2.4

9.4 4.6 6

(4) (5)

298 298

8.9 9.8 10.7

ref NA NA NA

Aoki et al. 34 34, 35 35 Xu et al. 15.9 64

3.2 3.2

9.8 11.8

37 37

a (1) and (2), milk-like gel; (3), dilute solution and 5-times synthesis; (4), clear solution and conventional heating; and (5), clear solution and microwave heating. b R is in 10-4 cm3 (STP)/ (cm2 s cmHg).

Na2O:H2O ) 1:9:80:5000 at 353 K (code II-4).15 After 5 h of the synthesis, the seed crystal grew from 0.7 µm to 1.2 µm, but the membrane showed low PV selectivity. A several-times synthesis was necessary to prepare the membranes with a high separation factor. Tsapatsis et al. have reported on the preparation of oriented zeolite A films on glass plates (code III-1).32,33 Using monodispersed zeolite A colloidal suspensions and slow-speed dip coating, the precursor layers of closepacked hexagonal colloidal crystals were prepared on glass slides. Then, the precursor layers were subjected to secondary growth in a clear solution containing a template to prepare continuous intergrown zeolite A films. Later in the process, incorporation of crystal particles from solution also occurred. However, neither the preparation of such zeolite A membrane on a porous substrate nor its permeation properties have been reported. Permeation Properties. Many samples of zeolite NaA membranes were prepared under the same conditions as experiment 12 in Table 1 and were subjected to investigations of single-gas permeation properties and of PV and VP properties toward water/organic liquid mixtures. Single-Gas Permeation. It is important for elucidation of the membrane morphology and permeation mechanism of the zeolite NaA membranes to investigate their single-gas permeation properties. Once a zeolite NaA membrane had been exposed to an atmosphere, it became impermeable to every gas investigated. Prior to the gas permeation experiments, the membrane, set in the permeation cell, was kept in vacuo for more than 3 days at 473 K. The completely dried membrane gave reproducible gas permeance values. The gas permeances of zeolite NaA membranes varied rather significantly from sample to sample. Table 3 lists the permeance values of several gases for three typically different membrane samples prepared under the same synthesis conditions, which had relatively high (m-1), medium (m-2), and low (m-3) gas permeances. It is noted that these three samples had high PV performances (permeation flux higher than 1.6 kg/(m2 h) and separation factors greater than 10000) for a water/ethanol mixture (xW ) 10 wt %) at 348 K, as will be mentioned below. The gas permeance was independent of feed pressure in the range of 0.2-0.6 MPa. Figure 6 shows plots of the permeance R versus the reciprocal of the square root of the molecular weight M of the gas for the m-2 sample at different temperatures. A linear

Figure 6. Plots of R versus M-1/2 of several gases for the zeolite NaA membrane (m-2 sample) at different temperatures.

relationship between R and M-1/2 was observed at each temperature, suggesting a Knudsen diffusion mechanism. A linear relationship was also observed for the m-1 and m-3 samples. The gas permeances of SF6, which has a kinetic diameter (0.55 nm) that is larger than the zeolitic pores (0.41 nm) of zeolite NaA, were close to the values expected for Knudsen diffusion. That is, the R ratios of H2/SF6 were only slightly larger than the Knudsen diffusion ratio (8.54), as listed in Table 3. These results suggest that interstitial spaces between the zeolite crystal particles, or nonzeolitic pores, were still present in the dense intergrown zeolite crystal layer responsible for membrane separation and that gas molecules permeated through the nonzeolitic pores by the Knudsen diffusion mechanism. The gas permeance increased with an increase in temperature, as can be seen in Figure 6. The apparent activation energy was in the range of 2-3 kJ/mol. As shown in Figure 7, the modified permeance R(MT)1/2 increased with an increase in temperature. This is different from the prediction of Knudsen diffusion that it should be independent of temperature. This might be due to the expansion of the crystals (and, therefore, of the zeolitic and nonzeolitic pores) with temperature and the effect of the adsorption of the remaining water (smaller at higher temperature), as well as the contribution of activated diffusion. The gas permeance data reported in the literature34,35,37 are also listed in Table 3. Some zeolite NaA membranes have been reported to display higher R ratios for H2/N234,35 and H2/n-C4H1037 than the Knudsen diffusion ratios (3.7 and 5.4, respectively). However, the differences are not so large that those zeolite NaA membranes are evaluated to be dense and defectless and that a permeation mechanism other than Knudsen diffusion is originally predominant for gas permeation. We should take into account the fact that the presence of water vapor, even at extremely low content, significantly reduces the permeance of other gases and affects the R ratio of a gas pair.

170


Figure 7. Plots of modified permeance R(MT)1/2 versus T of several gases for the zeolite NaA membrane (m-2 sample).

It is well-known that zeolite A is a strong water adsorbent because of its strong hydrophilicity due to the presence of cations such as Na+; zeolite A has a Si/Al ratio of unity, and therefore one cation per Al atom. The zeolite NaA membranes displayed extremely high permeances for water vapor, for example, R ) 1 × 10 -2 cm3 (STP)/(cm2 s cmHg) for the m-2 sample at 378 K, which was 200 times larger than the R value expected from Knudsen diffusion. Based on the aforementioned results, the following points are considered about gas permeation through the zeolite NaA membranes. In the completely dried state, Knudsen diffusion through the nonzeolitic pores is predominant, and the contribution of zeolitic pores is very small for most zeolite NaA membranes. When present in the measurement cell, water vapor is strongly adsorbed in the zeolitic and nonzeolitic pores and condensed there, and as a result, it significantly inhibits permeation of other gases by blocking them from entering the pores. When the water vapor content is very low, water vapor is not adsorbed significantly enough to block the nonzeolitic pores, but the adsorbed water decreases the effective size of the pores. In such a case, the R ratio of a gas pair might be observed to be larger than the Knudsen diffusion ratio. When water vapor is continuously fed to the system, it condenses in the zeolitic and nonzeolitic pores and is vaporized at the downstream side of the membrane. Thus, the zeolite NaA membranes are extremely permeable to water vapor through the capillary condensation mechanism. PV Performance. The gas permeance largely varied from sample to sample as mentioned above, whereas variations in the PV performance among the samples were rather small. For 9 membrane samples prepared under the same synthesis conditions (No 12 in Table 1), the permeation flux was 1.86 ( 0.05 kg/(m2 h), ranging from 1.62 to 2.15 kg/(m2 h), and the separation factor was 15000 ( 2000, ranging from 8000 to 30000 for a water/ethanol mixture (xW ) 10 wt %) at 348 K. For a water/methanol mixture (xW ) 10 wt %) at 323 K, the permeation flux was 0.57 ( 0.02 kg/(m2 h), ranging from 0.50 to 0.70 kg/(m2 h), and the separation factor was 1960 ( 100, ranging from 1600 to 2400. The typical results are presented below. PV performance of the zeolite NaA membranes toward various water/organic liquid mixtures is summarized in Table 4. The zeolite NaA membranes were

Figure 8. Feed composition dependence of Q and R in PV of water/ethanol for the zeolite NaA membrane at 348 K.

Figure 9. Feed composition dependence of Q and R in PV of water/methanol for the zeolite NaA membrane at 323 K. Table 4. PV Performance of the Zeolite NaA Membranes toward Water/Organic Liquid Systems separation system ethanol methanol n-propanol i-propanol acetone dioxane DMF

temp [K]

xW [wt %]

Q [kg/(m2 h)]

R [-]

348 348 348 323 323 348 348 323 323 333 333

10 5 0.5 10 5 10 10 10 5 10 10

2.15 1.10 0.012 0.57 0.23 1.91 1.76 0.91 0.83 1.87 0.95

10000 16000 5100 2100 2500 18000 10000 5600 6800 >9000 >9000

highly water-permeable toward these organic liquids and displayed extremely high permeation fluxes and separation factors. At xW value of 10 wt %, the water content in permeate, yW, was higher than 99.9 wt % for every organic liquid except for methanol. This suggests that a small scatter in the composition analysis of the permeate leads to a large scatter in R. This should be taken into account in discussing some difference in such high values of R. Figures 8 and 9 show the feed composition dependence of the permeation flux and separation factor in PV of water/ethanol and water/methanol systems, respectively. In the entire feed range, the total permeation


Figure 10. Downstream pressure dependence of qW and R at xW of 1 and 10 wt % in PV of water/ethanol for the zeolite NaA membrane at 348 K.

Figure 11. Feed composition dependence of qW, RW, and R in VP of water/ethanol for the zeolite NaA membrane at 378 K.

Table 5. VP Performance of the Zeolite NaA Membranes toward Water/Organic Liquid Systems at 378 K separation system ethanol methanol dioxane

xW [wt %]

Q [kg/(m2 h)]

10 3 11 3.4 10

4.5 1.47 3.5 0.84 7.8

RW × 103 [cm3 (STP)/ (cm2 s cmHg)] 9.2 9.2 9.3 9.8

R [-] >30000 >110000 5700 3600 >9000

flux Q was substantially equal to the water flux qW, and the ethanol and methanol fluxes qE and qM were below 4 × 10-4 and 3 × 10-3 kg/(m2 h), respectively. With an increase in xW, Q increased initially linearly and then sublinearly and finally saturated. With an increase in xW, R increased initially, reached a maximum around xW of 5-10 wt %, and then decreased to a constant value. However, the variation in R with xW is not very important, because yW was held very close to 100 wt % for the whole range of xW, for example, yW ) 99.97 and 96.1 wt % for xW ) 50.0 and 0.5 wt %, respectively, for the water/ethanol system. With increasing temperature, qW increased significantly, whereas qE and qM hardly changed, and as a result, R increased (see an example of the water/ethanol system shown later in Figure 14). The activation energies of qW at xW ) 10 wt % were 35 and 43 kJ/mol for the water/ethanol and water/methanol systems, respectively. Thus, the PV performance was much better at a higher temperature. In this study, most of the PV experiments were carried out at a downstream pressure below 13.3 Pa. However, practical PV plants are usually operated at a much higher downstream pressure, typically above 2 kPa. Therefore, the influences of the downstream pressure on the PV performance for the water/ethanol system were measured. As shown in Figure 10, both qW and R at xW ) 10 wt % were independent of the downstream pressure in the range up to 2 kPa. On the other hand, at xW ) 1 wt %, qW decreased by a factor of

Figure 12. Feed composition dependence of qW, RW, and R in VP of water/methanol for the zeolite NaA membrane at 378 K.

10 at a downstream pressure of 2.6 kPa compared with the value at 13 Pa and R also decreased fairly. This is attributed to a decrease in the driving force of qW, as a result of an increase in the downstream water-vapor partial pressure. VP Performance. The VP performance of the zeolite NaA membranes toward water/organic liquid mixtures is summarized in Table 5. It is noted that both Q and R in the systems investigated were higher for VP at 378 K than for PV at 323-348 K. Figures 11 and 12 show the feed composition dependence of qW and R in the VP of the water/ethanol and water/methanol systems, respectively, at 378 K. With

172


Figure 13. Upstream pressure dependence of qW and RW in VP of (a) pure water, (b) water/ethanol (xW ) 3 wt %), (c) water/ethanol (xW ) 10 wt %), and (d) water/methanol (xW ) 10 wt %) for the zeolite NaA membrane at 378 K.

an increase in xW, qW continued to increase, and the permeance of the water component RW was hardly dependent on xW, indicating that the water flux increased almost linearly with the driving force of water permeation. In the water/ethanol system, yW was 99.97 wt % for the whole range of xW. It is noted that the zeolite NaA membrane substantially permeated only the water component but not ethanol one in whole feed range, although with an increase in xW, R decreased from 1.1 × 105 at xW ) 2.9 wt % to 450 at xW ) 88 wt %. In the water/methanol system, when xW approached zero, the permeate contained a little more methanol and R decreased slightly, although it still remained at a very high level. Figure 13 shows the upstream pressure dependence of qW and RW in the pressure range of 1-3 atm. With increasing feed pressure, the water flux increased, but the increasing rate was sublinear. Thus, the permeance of the water component decreased with an increase in upstream pressure. The dependency was similar among the water/ethanol, water/methanol, and pure water systems. It is deduced from Figures 11-13 and Table 5 that the RW values at 378 K and 0.1 MPa were in the range of 8-10 × 10 -3 cm3 (STP)/(cm2 s cmHg), independent of the kind of organic component and feed composition. That is, the RW values for water/organic liquids were equal to that for pure water within the experimental errors and within the difference of the membrane sample. This indicates that the condensed state of water molecules in the zeolitic and/or nonzeolitic pores is almost the same irrespective of the presence of an organic component in feed. Figure 14 shows the temperature dependence of qW, qE, and R for VP of a water/ethanol mixture through two membrane samples (m-4 and m-5) that had a similar PV performance. For comparison, the temperature dependence of qW, qE, and R for PV was also shown. The qW and qE values for VP at 378 K are on the extrapolated line of the correlation of log qW and log qE vs 1/T for PV. A similar relation was also observed

Figure 14. Temperature dependence of qW, qE, and R in PV (closed symbols) and VP (open symbols) of a water/ethanol mixture of xW ) 10 wt % through two zeolite NaA membrane samples, m-4 (circles) and m-5 (triangles).

for the water/methanol system. This suggests that there is no significant difference in the permeation mechanism between PV and VP unless the measurement temperature is very different. However, in the region of much higher temperature, the situation was different. For the m-4 sample, with increasing temperature, qW decreased slightly, whereas qE increased largely in a temperature range of 378-423 K and then slightly. As a result, R decreased significantly down to 560 at 473 K. This was not due to any breakdown of the membrane, because it again displayed both high qW and R values at lower temperatures. On the other hand, for the m-5 sample, with an increase in temperature from 378 to 423 K, both qW and qE increased slightly and R was maintained at a high level of 90000. This clearly shows a difference in the membrane quality between these two samples, as will be discussed below, although they displayed similar high performances of PV and VP at lower temperatures. Permeation and Separation Mechanism of PV and VP. As discussed above, water vapor is strongly adsorbed in the zeolitic and nonzeolitic pores of the zeolite NaA membranes and condensed there, and as a result, it permeates with an extremely high permeance by the capillary condensation mechanism and significantly inhibits the permeation of other gases by blocking them from entering the pores. It is reasonable to consider that a similar mechanism is operative in the PV and VP of water/organic liquid mixtures through the zeolite NaA membranes. As shown in Figure 15, the adsorption amounts of vapors on zeolite NaA crystal powder were in the order water > methanol > ethanol . i-propanol. At 298 K, the adsorption of water vapor was only 2.4 and 4 times larger than those of methanol and ethanol, respectively, whereas the separation factors of PV of water/methanol and water/ethanol were as high as 2000 and 10000,

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 173 Table 6. PV and VP Performances of Inorganic and Polymeric Membranes Reported in Literature toward Water/ Organic Liquid Systems membranea modified porous glass zeolite NaA, tubular zeolite NaA, tubular zeolite NaA, sheet zeolite NaA, disk silica, tubular silica, tubular silica, tubular silica, tubular silica, tubular silica, tubular silica/zirconia, tubular silica/acrylamide GFT (PVA composite) PI hollow fiber PI hollow fiber PAA/polyion complex Chitosan PI/PSF composite a

separation system water/ethanol water/ethanol water/ethanol water/ethanol water/ethanol water/ethanol water/ethanol water/acetic acid water/acetic acid water/n-butanol water/ethanol water/ethanol water/ethanol water/ethanol water/ethanol water/methanol water/ethanol water/ethanol water/ethanol

temp [K]

xW [wt %]

Q [kg/(m2 h)]

RW/E [-]

ref

372 368 343 343 303 351 343 343 378 358 343 348 323 363 463 408 333 333 313

5 5 5 5 90 4 2 3.2 11.4 5 4 10 10 5 20 21 5 10 10

0.10 2.35 0.23 0.65 0.7 0.2 0.15 0.9 3.6 3-4 1.1 2.5 0.3 0.24 1 × 10-3 b 1 × 10-3 b 1.63 0.1 1.70

1630 >5000 2000 6000 >10000 110 200 400 1000 300-400 250 2500 3200 9500 150 20 3500 6000 240

13 11 14 14 15 41 42 43 44 45 45 46 47 4 5,48,49 5,48 50 51 52

PI ) polyimide, PAA ) poly(acrylic acid), PSF ) polysulfone. b RW is in cm3 (STP)/(cm2 s cmHg).

Figure 15. Adsorption isotherms of water, methanol (MeOH), ethanol (EtOH), and i-propanol (i-PrOH) for the zeolite NaA powder at 298 K. The open and closed symbols refer to the adsorption and desorption runs, respectively.

respectively, at an xW of 10 wt %. This suggests that the adsorption of alcohol is almost completely inhibited by the presence of water, that is, the adsorbed water completely blocks alcohol molecules from entering the zeolitic and nonzeolitic pores. It is well-known for inorganic membranes that the separation selectivities for mixtures are much higher than the ideal selectivities because of competitive preferential adsorption.17,18,38-40 However, high selectivities of more than a few thousand could be achieved for only a few membranes.7,17 Both qW and R in PV were larger at a higher temperature. Both qW and R values in VP at 378 K were on the extrapolated line of their temperature dependence in PV. Thus, water permeation was a significantly thermally activated process at temperatures below 378 K. An increase in the driving force of water permeation in the feed side, such as xW in PV and the feed pressure

in VP, caused only a smaller increase in qW in the range of the larger driving force. These results suggest that the rate-determining step of water permeation is not in the adsorption of water into the zeolitic and nonzeolitic pores or in the capillary condensation but rather in the diffusion and/or desorption of water in and/or from the pores. With an increase in temperature from 378 to 423 K in VP, for the m-4 membrane sample, qE increased significantly, but qW decreased slightly, and as a result, R decreased significantly. On the other hand, the m-5 sample did not display such significant changes. Both qW and qE increased slightly, and R was maintained at a high level. In the case of the m-4 sample, at temperatures above 378 K, the capillary condensation of the water component in the nonzeolitic pores became incomplete, probably because of the presence of relatively larger-size nonzeolitic pores with larger content, and therefore, the blocking of ethanol molecules from entering such pores became incomplete. In the case of the m-5 sample, the capillary condensation and therefore the blocking were maintained sufficiently, even at high temperatures, probably because of the relatively smaller size of the nonzeolitic pores with smaller content. Thus, the membrane quality was much better for the m-5 sample than for the m-4 one. It is probable that, under the operating conditions where adsorption of water component in nonzeolitic pore is apt to be insufficient, the permeation of the organic component through the nonzeolitic pores becomes appreciable, especially for the membrane samples not well fabricated. In such cases, the following might be considered: very high temperature, extremely low feedwater content, and the water/methanol system (relatively small difference in adsorption affinity between components). At present, we have no data on the size distribution and content of the nonzeolitic pores in the dense zeolite crystal layer responsible for the separation. Judging from the high selectivity of about 5000 for the VP of water/methanol at 378 K, it is reasonable to consider that the membranes did not substantially contain larger penetrating nonzeolitic pores. Further study is necessary. Comparison of PV and VP Performance between Zeolite NaA Membranes and Other Representative Mem-

174


branes. The PV and VP performance data for the representative membranes reported so far are listed in Table 6. The PV performance of zeolite A membranes prepared by other groups is not as high as that of our membranes. This clearly shows a difference in membrane quality. Kondo et al. chose porous substrates composed of mullite, R-alumina, and/or cristobalite instead of R-alumina tubes to reduce the membrane cost significantly with only a slight reduction in the membrane performance.11 They succeeded in preparing tubular-type zeolite NaA membrane modules on a commercial level.24-26 Amorphous porous silica membranes were first developed by Asaeda et al. in 1986,41 but their performance for the separation of water/alcohol vapors was rather low and changed gradually on exposure to wet atmosphere. However, recently, stable and high-performance silica and silica/zirconia membranes have been developed.44-46 Polymer membranes for the dehydration of organic liquids have been widely investigated.4,5,48-52 Compared with these polymer membranes, the zeolite NaA membranes have the following advantages: (1) They have both high water flux and high selectivity in the entire feed range for most organic liquids such as methanol, acetone, dioxane, and DMF. (2) They have higher performance at higher temperature and can be used at high temperatures at least up to 473 K. (3) They are not swollen at all in water and organic liquids. Polymer membranes can be used only in a certain range of feed composition to prevent them from breaking down. This means that start-up and shut-down of the process are much easier for the zeolite NaA membranes. Conclusions (1) Zeolite NaA membranes were prepared reproducibly by a one-time-only hydrothermal synthesis with a short reaction time of 3 h at 373 K using a gel with the composition Al2O3:SiO2:Na2O:H2O ) 1:2:2:120 (in moles) and porous R-alumina support tubes seeded with zeolite NaA crystals. They were composed of densely intergrown zeolite crystal layer of about 30 µm in thickness on the outer surface of the tubes. (2) The membranes were highly permeable to water vapor but impermeable to every gas unless dried completely. In the completely dried state, their gas permeation was attributed to Knudsen diffusion through the nonzeolitic pores. (3) The membranes displayed the excellent waterpermselective performance (high flux and high selectivity) in PV and VP toward water/organic liquid mixtures in the entire feed range. Both Q and R were higher at a higher temperature, and as a result, the membrane performance was much better for VP than for PV. For the well-fabricated membrane samples, the high selectivity was maintained even at a high temperature of 423 K. In such membranes, the nonzeolitic pores were suggested to be smaller in number and in size. (4) The high PV and VP performance was due to the capillary condensation of water in the zeolitic and nonzeolitic pores and the blocking of other molecules from entering the pores. Acknowledgment This work was supported by a Grand-in-Aid for Development Scientific Research (08555193) from the Ministry of Education, Science, and Culture of Japan,

and by the Proposal-Based New Industry Creative Type Technology R&D Promotion Program (98Ec-04-002) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Literature Cited (1) Lipnizki, F.; Field, R. W.; Ten, P.-K. Pervaporation-Based Hybrid Process: A Review of Process Design, Applications and Economics. J. Membr. Sci. 1999, 153, 183 and references therein. (2) Feng, X.; Huang, R. Y. M. Liquid Separation by Membrane Pervaporation: A Review. Ind. Eng. Chem. Res. 1997, 36, 1048 and references therein. (3) Wang, H.; Ugomori, T.; Wang, Y.; Tanaka, K.; Kita, H.; Okamoto, K.; Suma, Y. Sorption and Pervaporation Properties of Cross-linked Membranes of Poly(ethylene oxide imide) Segmented Copolymer to Aromatic/Nonaromatic Hydrocarbon Mixtures, J. Polym. Sci., Polym. Phys. Ed. 2000, 38, 1800, and references therein. (4) Bruschke, H. E. A.; Tusel, G. F.; Rautenbach, R. Pervaporation Membranes; Application in the chemical process industry. In ACS Symposium. Series 281, Reverse Osmosis and Ultrafiltration; Sourirajan, S., Matsuura, T., Eds.; American Chemical Society: Washington D.C., 1985; p 467. (5) Nakagawa, K.; Kusuki, Y.; Ninomiya, Separation of WaterOrganic Mixtures by Vapor Permeation through Aromatic Polyimide Hollow Fibers; Proceedings of the 7th International Conference on Pervaporation Processes in the Chemical Industry; Bakish, K. R., Ed.; Bakish Materials Corp.: Ft. Lauderdale, FL, 1989; p 250. (6) Asaeda, M.; Okazaki, K.; Nakatani, A. Preparation of Thin Porous Silica Membrane for Separation of Nonaqueous Organic Solvent by Pervaporation. Ceram. Trans. 1992, 31, 411 (7) Nair, B. N.; Okubo, T.; Nakao, S. Structure and Separation Properties of Silica Membranes. Membrane 2000, 25, 73 and references therein. (8) Horii, K.; Tanaka, K.; Kita, H.; Okamoto, K. Proceedings of 26th Autumn Meeting of Society of Chemical Engineering, Japan; 1993, p 99. (9) Kita, H.; Horii, K.; Ohtoshi, Y.; Tanaka, K.; Okamoto, K. Synthesis of a Zeolite NaA Membrane for Pervaporation of Water/ Organic Liquid Mixtures. J. Mater. Sci. Lett. 1995, 14, 206. (10) Kita, H.; Horii, K.; Tanaka, K.; Okamoto, K.; Miyake, N.; Kondo, M. Pervaporation of Water/Organic Liquid Mixtures Using a Zeolite NaA Membrane. Proceedings of the 7th International Conference on Pervaporation Processes in the Chemical Industry; Bakish, R., Ed.; Bakish Materials Corp.: Englewood, NJ, 1995; p 364. (11) Kondo, M.; Komori, M.; Kita, H.; Okamoto, K. TubularType Module with Zeolite NaA Membrane. J. Membr. Sci. 1997, 133, 133. (12) Kita, H.; Harada, T.; Asamura, H.; Tanaka, K.; Okamoto, K. Conventional vs Microwave Hydrothermal Synthesis of Zeolite Membranes and Their Pervaporation Properties. Proceedings of the 5th International Conference on Inorganic Membranes, ICIM '98, Nagoya, Japan, 1998; p 318. (13) Ishikawa, A.; Chiang, T. H.; Toda, F. Separation of WaterAlcohol Mixtures by Pervaporation through a Zeolite Membrane on Porous Glass. J. Chem. Soc., Chem. Commun. 1989, 764. (14) Jafar, J.; Budd, P. M. Separation of Alcohol/Water Mixtures by Pervaporation through Zeolite A Membrane. Microporous Mater. 1997, 12, 305. (15) Kumakiri, I.; Yamaguchi, T.; Nakao, S. Preparation of Zeolite A and Faujasite Membranes from a Clear Solution. Ind. Eng. Chem. Res. 1999, 38, 4682. (16) Kita, H.; Inoue, T.; Asamura, H.; Tanaka, K.; Okamoto, K. NaY Zeolite Membrane for the Pervaporation Separation of Methanol-Methyl tert-Butyl Ether Mixtures. J. Chem. Soc., Chem, Commun. 1997, 45. (17) Kita, H.; Asamura, H.; Tanaka, K.; Okamoto, K. Preparation and Pervaporation Properties of X- and Y-Type Zeolite Membranes. In ACS Symposium Series 744, Membrane Formation and Modification; Pinnau, I., Freemann, B. D., Eds.; American Chemical Society: Washington D.C., 2000; p 330. (18) Sano, T.; Yanagishita, H.; Kiyozumi, Y.; Mizukami, F.; Haraya, K. Separation of Ethanol/Water Mixture by Silicalite Membrane on Pervaporation. J. Membr. Sci. 1994, 95, 221.

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(39) 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. (40) Vroon, Z. A. E. P.; Keizer, K.; Burggraaf, A. J.; Verweij, H. Preparation and Characterization of Thin Zeolite MFI Membranes on Porous Supports. J. Membr. Sci. 1998, 144, 65. (41) Asaeda, M.; Du, L. D. Separation of Alcohol/Water Gaseous Mixtures by Thin Ceramic Membrane, J. Chem. Eng. Jpn. 1986, 19, 72. (42) van Gemert, R. W.; Cuperus, F. P. Newly Developed Ceramic Membranes for Dehydration and Separation of Organic Mixtures by Pervaporation. J. Membr. Sci. 1995, 105, 287. (43) Kitao, S.; Asaeda, M. Separation of Organic Acid/Water Mixtures by Thin Porous Silica Membrane. J. Chem. Eng. Jpn. 1990, 23, 367. (44) Asaeda, M.; Kawasaki, N. Pervaporation of Aqueous Organic Acid Solutions by Porous Silica Membrane Prepared by Sol-Gel Methods. Proceedings of the 5th International Conference on Inorganic Membranes, ICIM '98, Nagoya, Japan, 1998; p 540. (45) Bakker, W. J. W.; Bos, I. A. A. C. M.; Rutten, W. L. P.; Keurentjes, J. T. F. Application of Ceramic Pervaporation Membranes in Polycondensation Reactions. Proceedings of the 5th International Conference on Inorganic Membranes, ICIM '98, Nagoya, Japan, 1998; p 448. (46) Asaeda, M.; Yang, J.; Sakou, Y. Porous Silica-Zirconia (50%) Membranes for Pervaporation of IPA/Water Mixtures. International Congress on Membranes and Membrane Processes, ICOM ‘99, Toronto, Canada, 1999; p 89. (47) Sakohara, S.; Maekawa, Y.; Takeishi, Y.; Asaeda, M. Effects of Gel Composition on Separation Properties of Ethanol/ Water Mixtures by Acrylamide Gel Membranes. J. Chem. Eng. Jpn. 1992, 25, 598. (48) Nakagawa, K.; Asakura, Y.; Nakanishi, S.; Hoshino, H.; Kokufuda, H.; Kusuki Y. Water-alcohol vapor separation through asymmetric polyimide hollowfiber membranes. Koubunnshironnbunnshu 1989, 46, 405. (49) Tanihara, N.; Tanaka, K.; Kita, H.; Okamoto, K.; Nakamura, A.; Kusuki, Y.; Nakagawa, K. Vapor-permeation separation of water-ethanol mixtures by asymmetric polyimide hollowfiber membrane modules. J. Chem. Eng. Jpn. 1992, 25, 388. (50) Karakane, H.; Tsuyumoto, M.; Maeda, Y.; Honda, Z. Separation of Water- Ethanol by Pervaporation through Polyion Complex Composite Membrane. J. Appl. Polym. Sci. 1991, 42, 3229. (51) Mochizuki, A.; Sato, Y.; Ogawara, H.; Yamashita, S. J. Appl. Polym. Sci. 1989, 37, 3375. (52) Kim, J.-H.; Lee, K.-H.; Kim, S. Y. Pervaporation separation of water from ethanol through polyimide composite membranes. J. Membr. Sci. 2000, 169, 81.

Received for review June 21, 2000 Revised manuscript received September 26, 2000 Accepted October 6, 2000 IE0006007

Zeolite NaA Membrane - American Chemical Society

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