Silicalite Membrane Preparation, Characterization, and Separation

High-Flux MFI Zeolite Membrane Supported on YSZ Hollow Fiber for ... Industrial & Engineering Chemistry Research 2012 51 (2), 978-993 ... A novel meth...
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Ind. Eng. Chem. Res. 2001, 40, 4069-4078

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Silicalite Membrane Preparation, Characterization, and Separation Performance Xiao Lin, Hidetoshi Kita, and Ken-ichi Okamoto* Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan

Silicalite membranes were prepared by a single hydrothermal synthesis on seeded porous tubular supports. The support tubes were simply seeded by a water slurry of silicalite particles or by repeated dip coating in a suspension of colloidal silicalite particles. It was found that a crystal growth behavior was strongly dependent on seeds. Silicalite crystals slowly grew onto the unseeded and nanoscale-silicalite-seeded supports under the present synthesis conditions, leading to the formation of an incompact crystal layer, especially on the unseeded support. On the other hand, silicalite crystals preferentially grew onto the support tubes seeded with silicalite powders (particle size up to 4 µm) under the same synthesis condition. The silicalite seed particles mainly provided nucleation sites and enhanced silicalite crystal growth in all directions onto the support, resulting in the formation of a thin and dense crystal layer. The higher crystallization temperature, faster crystal growth rate is the favorable condition for preparation of the silicalite membranes with high pervaporation performance. For example, the high ethanol/water separation factor of 89 with a high flux of 1.8 kg/m2‚h for a feed concentration of 5 wt % ethanol at 60 °C was obtained through the silicalite membranes prepared on silicalite-seeded R-Al2O3 tubes at 185 °C for 5.5 h of hydrothermal treatment. The silicalite membranes prepared by the same method also showed good separation for n-butane and isobutane (an ideal separation factor larger than 10 at 200 °C). Introduction In the past decade potential applications of zeolite membranes in gas separation, pervaporation, and membrane reactors have attracted significant attention in view of industrial, environmental, and scientific interests. Also, significant progress in synthesis and characterization of zeolite membranes has been seen.1,2 The membrane separation performance (permeability and selectivity) is related to the quality of zeolite layer formed on a porous support. In an ideal case, this layer with high crystallinity should be thin and defect-free. In general, to prepare zeolite membranes is to control the zeolite growth on porous supports. It is known that the formation of zeolite layers is involved in the complex chemical reactions, which are affected by many interacting factors such as the chemical composition, silica source, hydrothermal synthesis conditions, seeds, and so on. Under certain conditions, some factors play a more important role. There are two typical techniques which are often used to synthesize the zeolite membrane on a flat or tubular porous support under hydrothermal reaction conditions: (1) in situ crystallization, crystal growth on an untreated support from a solution or a gel of aluminosilicate;3-10 (2) seed-enhanced crystallization (a seeding method), formation of a crystal layer on a seeded support.11-23 Zeolite A and Y membranes have been successfully prepared on seeded porous tubes by our group,11-15 where seeds not only enhanced crystallization but also promoted the formation of a uniform amorphous gel layer on the seeded support, which resulted in a well-intergrown crystal layer.14 On the * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +81-836-85-9660. Fax: +81-836-85-9601.

other hand, preferentially oriented silicalite membranes were prepared on porous R-Al2O3 disks coated with an aqueous suspension of silicalite-seeded nanocrystals (∼100 nm). In this approach, nanoscale seeds grew directly to form the oriented silicalite membranes with columnar microstructures.16-18 A new synthesis technique has been recently developed to prepare silicalite membranes on cheap porous tubes by our group.23 In this case, the support with a pore size of 1 µm was simply rubbed with silicalite particles. Silicalite crystals randomly grew onto the seeded tubes, so that thin and compact membranes were prepared by this method. Zeolite A membranes have been successfully applied to remove water from aqueous solutions via a pervaporation process on a larger scale.24 In contrast, silicalite membranes exhibited preferential organic compound permeation from water such as ethanol/water mixtures because silicalite zeolite not only has strong hydrophobic property but also preferentially adsorbs organic compounds. A recent paper reported that improvements of the fuel ethanol production process from biomass resulting in even $0.02-0.05/gallon could significantly increase its demand.25 Pervaporation is proposed to be one of the most challenging techniques for continuous ethanol recovery. Thus, it is necessary to develop efficient membranes including inorganic and organic membranes with high fluxes and high separation factors. Silicalite membranes may be good candidates for such applications.8 In this paper, we report the preparation and characterization of silicalite membranes on porous tubular supports. The seeding, type of MFI seeds, crystallization temperature and time, and supports as well as the separation performance of the resulting membranes are systematically investigated. Compared with the sili-

10.1021/ie0101947 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/18/2001

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calite crystal growth on the porous R-Al2O3 disks coated with nanoscale silicalite seeds, a different mechanism of silicalite layer formation on our seeded porous tubular supports was proposed. Moreover, on the basis of this crystal layer formation, the good silicalite membranes were synthesized within the shortest time in the literature. Experimental Section Membrane Preparation. Silicalite membranes were prepared by a single hydrothermal treatment on three types of porous tubular supports with 5 cm in length: (1) mullite tube with a 1 µm diameter pore; (2) R-Al2O3 with a 2 µm diameter pore; (3) stainless steel with a 0.1 µm diameter pore. The dense layer was formed on the outside of the tubular supports. Most of the membrane preparation was carried out on mullite tubes, except for the comparison in which membranes were prepared on R-Al2O3 and stainless steel tubes. The surface of the mullite tube was very rough so that the outer surface was polished with SiC paper. All types of the tubes were cleaned three times by ultrasonic cleaner. Different MFI crystals were used as seeds, namely, silicalite, uncalcined silicalite, ZSM-5, and nanoscale silicalite. The silicalite (Si/Al ) ∞) and ZSM-5 (Si/Al ) 24) powders were provided by Toso, Ltd. The uncalcined silicalite was prepared by collecting the powder from the bottom of the Teflon vessel after the membrane synthesis with the silicalite-seeded tube at 175 °C for 12 h. The nanoscale silicalite seeds were prepared according to the method described by Moor et al.26 A synthesis mixture with the molar composition 1/0.244/ 0.17/11.4 SiO2/TPAOH/NaOH/H2O was prepared by mixing tetrapropylammonium hydroxide (Kanto Chem. Ltd., 22.5 wt % TPAOH in H2O) and sodium hydroxide at room temperature and then adding silica powder (Aldrich) to the solution. Thereafter, the homogeneous dispersion was heated at 80 °C by stirring until it became a clear solution. This clear solution was further heated at 95 °C for 48 h. The seeds were not separated from the mother liquid by repeated centrifugation and decantation. Twice the amount of deionized water was only added to the mother liquid. This colloidal silicalite suspension that might not be stable was used as to deposition of the nanoscale seed layer. Seeding was carried out by two methods: (1) The outer surface of the support tube was simply rubbed with a water slurry of MFI zeolite particles. (2) The support tube was dip-coated twice with the colloidal silicalite suspension. After seeding, the support tubes were first dried at room temperature and further dried at 100 °C for 1530 min. The former seeding method was first used for synthesis of zeolite A membranes by our group,11 which has been identified as an effective and reproducible seeding method.12-15,24 A clear solution for the synthesis of silicalite membranes was prepared by mixing and stirring tetraethyl orthosilicate (TEOS of Aldrich), TPAOH, and deionized water at room temperature for 1 h. The resultant molar composition is 0.17/1/120 SiO2/TPAOH/H2O. The seeded support was vertically immersed in the synthesis solution. The autoclave was placed in a convection oven preheated at the synthesis temperature from 145 to 185 °C for a given time. After crystallization, the autoclave was removed from the oven and cooled with water. The

Figure 1. Schematic illustration of a batch pervaporation system.

sample was taken out, washed carefully with hot distilled water, dried at 100 °C for several hours, and then calcined at 400 °C for 20-40 h with a heating rate of 0.5 °C/min. Characterization and Permeation Experiments. Scanning electron microscopy (SEM) observation of the membrane surface and cross section was carried out with SEM Hitachi S-2300. The compositions of the cross section were analyzed by electron probe microanalysis (EPMA; JEOL JXA-8800R) with a wavelength dispersive spectrometer (WDS). Pervaporation experiments were carried out with a batch system illustrated schematically in Figure 1. The silicalite membrane with one end blocked was sealed with silicone tubes. The silicone tubes were further covered with heat-shrunk (thermosetting) plastic tubes. The blank experiment carried out with a nonporous glass tube indicated no leaking. The amount of the feed solution of an ethanol and water mixture was 500 g. During pervaporation, a proper amount of ethanol was added to the feed solution at intervals of 15 or 30 min to keep the constant feed concentration due to high flux of the silicalite membranes. The inside of the membrane tube was evacuated through a vacuum line. The permeate vapor was collected by a cold trap cooled with liquid nitrogen. The downstream pressure was maintained below 0.1 Torr. The effective membrane area was about 10 cm2. The compositions of the feed and permeate were analyzed by a gas chromatograph (Shimalite F) equipped with 3 m column packed with Polarpack Q poly(ethylene glycol)-1000. The flux was calculated by weighing the condensed permeate. The separation factor was determined as

RA/B ) (YA/YB)/(XA/XB) where XA, XB, YA, and YB denote the mass fractions of components A (ethanol) and B (water) in the feed and permeate sides. Single-gas permeation experiments were carried out for H2, N2, n-butane, and isobutane at 200 °C using the permeation module described in our previous paper.14 The feed gas was introduced to the outside of the membrane at pressure differences of 1 atm, and the gas permeation was measured with a bubble flowmeter. Results and Discussion SEM Characterization. The mullite tube is made of Al2O3 (65 wt %) and SiO2 (35 wt %) and has an average pore size of 1 µm. After polishing, the surface is still very rough, as shown in Figure 2A. The big

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Figure 2. SEM surface views of mullite (A) and silicalite-seeded mullite (B) tubes.

cavities with size up to 100 µm were clearly observed. Figure 2B shows the SEM surface view of the silicaliteseeded mullite support. After rub-coated with a water slurry of silicalite powder, the support surface was covered with the silicalite particles and became much smoother without observation of the cavities, indicating that some of the silicalite seeds were also settled in the cavities. In close view, as shown in Figure 3, the silicalite particles were up to 4 µm in size which were much larger than those reported earlier,16-19 especially the nanoscale silicalite seeds. It is noted that there was no dense seed layer formed on the support surface, which resulted from the larger size seeds used and the very rough surface of the mullite tube. The larger voids among the seed crystals were clearly observed in Figure 3. However, the density of the silicalite seed layer was larger than that of the zeolite A seed layer seeded by the same method.14 Figure 4 shows SEM surface views of the membrane prepared on an unseeded mullite tube at 175 °C for 16 h of hydrothermal treatment. In the absence of seeds, silicalite crystals could not form a continuous layer on the surface of the tube. As Figure 4B shows, no gel was found on the surface. The distribution of the particle sizes was bimodal, and the larger size of the crystals was less than 1.5 µm. It seems that nucleation occurred in the bulk solution. During the process of crystallization, most of crystals were precipitated to the bottom with the formation of the bulk powder in the solution, and some of crystals were weakly attached onto the surface of the tube, resulting in the poor coverage. Figure 5 shows SEM surface views of the membrane prepared on a seeded (coated with a colloidal silicalite

Figure 3. SEM surface (A) and cross-sectional (B) views of a silicalite-seeded mullite tube.

suspension) mullite tube at 175 °C for 16 h of hydrothermal treatment. In contrast to the crystal growth on the unseeded tube, the surface of the tube coated with nanoscale seeds was entirely covered with silicalite crystals after the same hydrothermal treatment in Figure 5A. In close view, as shown in Figure 5B, crystals showed a well intergrowth behavior with less interzeolitic spaces. Figure 5C reveals that crystals also grew in the cavities of the mullite support, indicating that the dip coating with the colloidal silicalite suspension gave the uniform precursor layer on the tube. On the other hand, the crystals started to grow directly from the nanoscale seeds deposited on the support with the c axis normal to the support surface, leading to the morphology of the crystals similar to those formed on the R-Al2O316-18 and stainless steel27 flat disks coated with the nanoscale seeds. However, the crystal size was smaller than those of the previous reports probably because of the different compositions of the clear synthesis solution. SEM surface and corresponding cross-sectional views of the silicalite membranes prepared on silicalitecrystal-seeded mullite tubes at 175 °C for 4 and 8 h of hydrothermal treatment are shown in Figures 6 and 7, respectively, which are quite different from those formed on the unseeded and nanoscale-silicalite-seeded tubes. After 4 h of hydrothermal treatment, the morphology of the sample was similar to that of the silicalite-seeded tube and the crystal size was comparable to that of the silicalite seeds, indicating the seed crystals hardly grew at this stage. In addition, no gelation occurred under the present synthesis condition. However, it is noted that small crystals newly formed with a size of about 1

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Figure 4. SEM surface views of the membrane prepared on an unseeded mullite tube at 175 °C for 16 h.

µm were clearly observed in the voids among the silicalite seed crystals in Figure 6B. After 8 h of hydrothermal treatment, a well-intergrown silicalite crystal layer was formed on the silicalite-seeded support in Figure 7A. It is not surprising that silicalite crystals were also formed in the support shown in Figure 7B, even the central part of the support, because of the larger pore size of the support and the clear synthesis solution used. However, these crystals could not form the continuous layer because the limited nutrient was provided for their growth. On the other hand, it was very difficult to determine the interface between the mullite support and the silicalite crystal layer because of not only the very rough surface of the support but also the formation of the silicalite crystals inside the support. The thickness of the crystal layer after 8 h of hydrothermal treatment was estimated to be about 10 µm as shown in Figure 7C. It is well-known that a crystal growth rate is high at high crystallization temperature. Figure 8 shows a SEM surface view of the membrane prepared on the silicaliteseeded tube after 4 h of hydrothermal reaction at 185 °C. The surface of the silicalite-seeded tube was fully covered with silicalite crystals at 185 °C after only 4 h of hydrothermal treatment, meanwhile only small crystals were observed at 175 °C for the same reaction time, as shown in Figure 6. As expected, the later membrane did not have any separation property (this will be discussed later). The SEM characterization of the surface and cross section of the membranes indicates that silicalite crystals randomly grew onto the silicalite-seeded mullite tube, which is different in morphology from the colum-

Figure 5. SEM surface views of the membrane prepared on a nanoscale-silicalite-seeded mullite tube at 175 °C for 16 h.

nar microstructures of the membranes prepared by secondary growth. In the later case, nanoscale seeds grew directly to form the oriented silicalite membranes with columnar microstructures, exhibiting an increase in grain sizes with increasing crystal layer thicknesses. Although seeds promoted crystal growth onto the surface of the supports in both cases, they might play a different role in the process of crystal layer formation, resulting in different morphologies. In our case, silicalite crystals preferentially grew in the voids among the seed crystals in the early stage of crystallization, while the seeds hardly grew. For further crystallization, the surface of the seeds provided the nucleation sites. Thus, silicalite crystals grew in all directions around the surface of the silicalite seeds to form the random dense layer. In fact, whether seeds directly grow or provide nucleation sites strongly depends on seed properties, such as sizes and calcination, and synthesis conditions.28-32

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Figure 6. SEM surface (A and B) and corresponding crosssectional (C) views of the membranes prepared on a silicaliteseeded mullite tube at 175 °C for 4 h.

Figure 7. SEM surface (A) and corresponding cross-sectional (B and C) views of the membranes prepared on a silicalite-seeded mullite tube at 175 °C for 8 h.

EPMA Analysis. Pervaporation separation for an ethanol aqueous solution through silicalite membranes depends on the hydrophobic property of silicalite layers. The higher Si/Al ratio, stronger hydrophobicity would result in a higher separation factor. As in the previous papers reported,8,33 when Al leached from R-Al2O3 supports is incorporated into crystal layers, ZSM-5 membranes are obtained instead of silicalite membranes, resulting in lower separation for ethanol compared with those prepared on stainless steel supports under the same synthesis conditions. We thus analyzed the elemental compositions of the crystal layers prepared under the present synthesis conditions. The elemental composition profiles of the cross section of the silicalite membrane prepared on a silicaliteseeded R-Al2O3 tube at 185 °C for 5.5 h of hydrothermal treatment are shown in Figure 9. The Si content in a

depth of 1 µm from the top surface of the membrane was 100 atomic % without detection of Al. The Al content in the crystal layer was also at negligible levels. The Si/Al ratio of the top crystal layer is larger than 2000 (EPMA detection limit 0.05 atomic %). Therefore, the EPMA analysis confirms that the high siliceous MFI membrane was obtained on the R-Al2O3 tube with negligible Al leaching from the support. To further study the incorporation of alumina leached from the support into the zeolite layer, the membrane was also prepared on a mullite support seeded with ZSM-5 powder (Si/Al ) 24) at 175 °C for 16 h of hydrothermal treatment. Figure 10 shows the elemental composition profiles of the cross section of this membrane. As can be seen from this figure, the Al content in a depth of 1 µm from the top surface of the membrane was 0.01 atomic %, which is below the EPMA detection

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Figure 8. SEM surface view of the membrane prepared on a silicalite-seeded mullite tube at 185 °C for 4 h.

Figure 10. Elemental composition profiles of the cross section of the membrane prepared on a ZSM-5 seeded mullite tube at 175 °C for 16 h. Table 1. Pervaporation Performance of Silicalite Membranes for a 5 wt % Ethanol Feed Solution at 60 °C synthesis condition seeds (Si/Al) T (°C) no nanoscale silicalite nanoscale silicalite silicalite (∞) UC silicalitea ZSM-5 (24) ZSM-5 (24) a

Figure 9. Elemental composition profiles of the cross section of the membrane prepared on a silicalite-seeded R-Al2O3 tube at 185 °C for 5.5 h.

limit, and then increased toward the depth of the crystal layer. The EPMA characterization further reveals that the silicalite membrane could be prepared on even a ZSM-5-seeded support under the present synthesis conditions. Al leaching from the support is strongly dependent on the synthesis conditions, especially alkalinity. Under the strongly alkaline conditions, Al dissolution from the support, even the silicalite-seeded support,19 occurs; consequently, it is introduced into the zeolite layer. The MFI membranes with Si/Al ) 10-392 were obtained in aluminum-free solutions.8,34,35 According to these reports, the film growth at the first stage often follows formation of a gel layer.19,35 In contrast, secondary growth gives high silicalite membranes with the Si/Al ratio larger than 800.18 Under such synthesis conditions, no gel formation is observable. On the other hand, the Si/Al ratios are also reported to strongly relate to Si sources used for the preparation of a synthesis solution with an Al medium.6 In this case, three membranes were prepared on R-Al2O3 tubes under the same synthesis conditions, but with three different Si sources, namely, silica sol (Ludox AS 40), fumed silica (Aerosil200), and TEOS. For the membranes prepared using Ludox and Aerosil, the Si/Al ratio was about 300 compared with 600 in the gel, while for TEOS, it was

175 160 175 175 175 175 175

time (h)

flux (kg/m2‚h)

R

16 16 + 16 16 8 8 8 16

0.84 4.06 2.26 2.21 1.50 0.97

1 30 2 59 58 43 62

UC silicalite: uncalcined silicalite.

1200, which was twice as large as that in the gel. In the present study, not only TEOS used as a Si source with alkali and Al free media but also silicalite particles used as seeds resulted in the pure silicalite top layer with the Si/Al ratio larger than 2000. Pervaporation Performance. Under the certain gel composition for synthesis of zeolite membranes, a crystal growth behavior such as the growth rate, morphology, and orientation of crystals is strongly related to the crystallization temperature and time, seeds, supports, etc. Consequently, membrane separation properties are affected by these factors. In this section, the pervaporation performance of silicalite membranes corresponding to synthesis conditions was studied. Pervaporation results for silicalite membranes prepared under different synthesis conditions are listed in Table 1. The membrane prepared on the unseeded tube did not have a separation property, as confirmed by the SEM characterization shown in Figure 4A. Although the continuous crystal layer was formed on the nanoscalesilicalite-seeded tubes, the membranes still did not show good separation because of the much larger pore size and defects (cavities) of the support. The good silicalite membranes for separation of n-C4H10 and i-C4H10 were prepared by secondary growth on the supports with 10 times smaller pore size by Xomeritakis et al.16-18 On the other hand, the good silicalite membranes for separation of ethanol were obtained using silicalite seeds without considering calcination. In addition, the silicalite membrane prepared using ZSM-5 seeds for

Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4075 Table 2. Pervaporation Performance of Silicalite Membranes for a 5 wt % Ethanol Feed Solution at 60 °Ca synthesis condition T (°C) time (h) 185 185 185 185 175 175 175 175

4 5.5 6.5 8 4 8 12 16

flux (kg/m2‚h)

R

3.16 2.23 1.80 1.32 -2.26 1.41 1.65

37 62 68 65 1 59 72 41

a All membranes were prepared on silicalite-seeded mullite tubes.

Figure 11. Effect of the synthesis temperature on pervaporation performance for a 5 wt % ethanol feed solution at 60 °C.

twice the time of hydrothermal treatment (16 h) exhibited the same separation factor as those using silicalite seeds but lower flux. As mentioned above, only Si in a depth of 1 µm from the surface of this membrane was determined by EPMA, suggesting that the surface of the silicalite membrane might play an important role in the separation of ethanol. This table indicates that silicalite powder is the best seed for the preparation of silicalite membranes under the present synthesis condition, so that the following membranes were prepared using only silicalite powder as seeds. To optimize the crystallization temperature, the membranes were prepared on silicalite-seeded mullite tubes in temperature ranges of 145-185 °C for 8 h of hydrothermal treatment. Pervaporatoion results through these membranes as a function of the synthesis temperature are shown in Figure 11. It can be seen that separation factors for ethanol over water at 60 °C with 5 wt % ethanol solution increased with an increase in the synthesis temperature, combined with a decrease in their fluxes. A significant increase of the separation factor was observed at temperatures above 155 °C, combined with continuous loses of their fluxes. As mentioned above, the crystal growth rate is much faster at high temperature. The crystal layers of the membrane prepared at 145 °C for 8 h might be too thin, resulting in poor separation. Thus, a membrane was also prepared under the same conditions except for 24 h of hydrothermal treatment. However, the membrane still showed a low separation factor of 42 for ethanol over water. Considering both separation factors and fluxes, the better membranes were prepared at 175 or 185 °C. This result is inconsistent with Gora’s recent report.36 According to their results, the best silicalite membranes for gas separation were synthesized on TiO2-coated porous stainless steel disks at a temperature of 180 or 190 °C for 17 h. Vroon et al.37 found that the separation factor of n-C4H10/i-C4H10 increased with increasing synthesis temperature from 98 to 180 °C, although the good MFI membrane was also prepared at a low temperature of 120 °C. Tuan et al.6 also reported that ZSM-5 membranes prepared at higher temperature exhibited a better gas separation performance at high temperature compared with those prepared at lower temperature.

To optimize the crystallization time, silicalite membranes were prepared on silicalite-seeded mullite tubes at 175 and 185 °C for a given time. Pervaporation results through the resulting membranes are listed in Table 2. After 4 h of hydrothermal treatment, the membrane prepared at 185 °C showed a separation factor of 37, while the membrane prepared at 175 °C did not have any separation. This is evidenced by the SEM observations as shown in Figures 6A and 8. For further treatment, the separation factors first increased, combined with a decrease in the fluxes, and exhibited a maximum. The reason that the separation factor decreased with prolonged treatment might result from the formation of defects during calcination. Vroon et al.37 reported that the MFI membranes prepared by two subsequent hydrothermal treatments showed optimum quality, while the membranes prepared by three or more hydrothermal treatments became too thick (larger than 4 µm) and cracked during the removal of the template. To study the effect of supports on the pervaporation performance, silicalite membranes were also prepared on silicalite-seeded R-Al2O3 and stainless steel tubes at 185 °C for 5.5 h of hydrothermal treatment. The pervaporation results are listed in Table 3. As shown in Table 3, the best silicalite membranes for separation of an ethanol and water mixture were prepared on R-Al2O3 tubes compared with those on mullite and stainless steel tubes. The four membranes also possessed similar pervaporation performances, indicating that this membrane synthesis method was reproducible. These high separation factors of 82-89 through the silicalite membranes prepared on R-Al2O3 tubes are supported by the EPMA analysis as shown in Figure 9, in which only Si was found in a depth of 1 µm from the surface of the top crystal layer. On the other hand, the silicalite membranes with high separation factors were not prepared on the stainless steel supports. In the previous reports, the better silicalite membranes for separation of ethanol over water were prepared on the stainless steel supports compared with those on the R-Al2O3 supports,8 as will be shown in Table 6, because the dissolution of Al from the support occurred. In our case, the pure silicalite crystal layers should also be formed on the stainless steel support with an Al-free medium. Thus, the pervaporation performance was only related to the quality of the crystal layers. The silicalite seeds with a size of up to 4 µm could not be planted into 0.1 µm pore of the stainless steel tube, and then the crystals only grew on the surface, resulting in the formation of crystal layers with defects. Consequently, the membranes showed a pervaporation performance with low separation factors but high fluxes compared with those formed on other

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Table 3. Pervaporation Performance of Silicalite Membranes for Ethanol/Water Mixturesa

a

membrane

support

M1 M2 M3 M4 M5 M6

R tube R tube R tube R tube SS tube SS tube

synthesis condition T (°C) time (h) 185 185 185 185 175 175

feed concn (EtOH wt %)

PV temp (°C)

flux (kg/m2‚h)

R

5 5 5 5 5 5

60 60 60 60 60 60

1.81 1.80 2.01 2.09 3.67 4.02

89 89 83 82 35 30

5.5 5.5 5.5 5.5 8 8

R tube: R-Al2O3 tube. SS tube: stainless steel tube. Silicalite was used as seeds. Table 4. Gas Permeation Results for Silicalite Membranes at 200 °Ca gas permeance × 10-7 (mol/m2‚s‚Pa) H2 N2 n-C4H10 i-C4H10

membrane M7 M8 M9 M10

5.39 10.9 6.66 8.20

2.74 5.12 3.06 3.55

1.58 3.14 1.52 2.08

separation factor for n/i-C4H10

0.27 0.25 0.065 0.12

6 13 23 17

a All membranes were prepared on silicalite-seeded mullite tubes at 185 °C for 6 h of hydrothermal treatment.

Table 5. Pervaporation Performance of Polymer Membranes for Ethanol/Water Mixturesa

Figure 12. Effect of the pervaporation temperature on pervaporation performance for the membrane prepared on a silicaliteseeded R-Al2O3 tube at 185 °C for 5.5 h.

supports with more than 10 times larger pores under the same synthesis conditions. As expected, a better membrane could be prepared on a stainless steel support with larger pore size in which crystals could grow, or under different synthesis conditions such as using different chemical compositions and smaller size seeds. In other words, membrane preparation conditions should also be optimized regarding the state of supports. Figure 12 shows he temperature dependence of the separation factor and flux for M3 membranes prepared on the silicalite-seeded R-Al2O3 tube at 185 °C for 5.5 h of hydrothermal treatment. It can be seen that, with increasing temperature, the flux increased, while the separation factor did not change initially and then decreased slightly. At low temperatures, ethanol effectively blocks water permeation due to strong adsorption of ethanol. At high temperatures, water permeation increases a little bit faster than ethanol because of the low coverage of ethanol, leading to a decrease of the separation factor. Apparent activation energies of ethanol and water were 32 and 41 kJ/mol, respectively, which are similar to those of the previous report.38 Gas Permeation. For the gas permeation test, silicalite membranes with 10 cm length were prepared on silicalite-seeded mullite tubes at 185 °C for 6 h of hydrothermal treatment. Before calcination, two membranes were tested with N2 permeation. After evacuation at 200 °C for 24 h, both membranes showed impermeability to N2 (less than 10-12 mol/m2‚s‚Pa) at 200 °C due to the template-occupied zeolite pores, suggesting that the gastight membranes could be prepared by a single hydrothermal treatment.

membrane

feed concn (EtOH wt %)

T (°C)

flux (kg/m2‚h)

R

ref

PTMSP PVDMS PDMS-PS IPN PPDMS PDMS PDMS/silicalite PDMS PDMS/silicalite

5 7 10 8 5 5 7 5

30 37 60 30 22.5 22.5 22 22

0.39 0.007 0.016 0.025 0.024 0.051 0.026 0.15

6.2 5.6 5.5 10.8 5.4 9.3 7.3 34

39 40 41 42 43 43 44 44

a PTMSP: poly[1-(trimethylsilyl)-1-propyne]. PVDMS: poly(vinyldimethylsiloxane). PPDMS: plasma-polymerized dimethylsiloxane. PDMS: poly(dimethylsiloxane).

Single gas permeation of H2, N2, n-C4H10, and i-C4H10 was measured at 200 °C at a pressure difference of 1 atm. An ideal separation factor of n-C4H10 over i-C4H10 was used to evaluate a membrane quality. This is a typical method to detect defects of MFI membranes. The gas permeation was only carried out at 200 °C because of no adsorption effect.6,37 The definition of high-quality MFI membranes proposed by Vroon et al.37 was that if the ideal separation factor for n-butane/isobutane was larger than 10 at 200 °C, the membranes were considered as defect-free. Tuan et al.6 also reported that ideal separation factors for n-butane/isobutane were correlated to mixture selectivities at 200 °C but not at lower temperature because of preferential adsorption in mixtures. Table 4 lists gas permeation results for our silicalite membranes. It can be seen that our membranes showed high ideal separation factors for nbutane/isobutane which were larger than 10 at 200 °C. Therefore, on the basis of the above definition, our membranes would be defect free. These ideal separation factors are also in good agreement with our excellent pervaporation results as discussed above. Comparison of Pervaporation Separation. The pervaporation performances of polymer membranes and silicalite membranes in the literature are listed in Tables 5 and 6, respectively. Compared with silicalite membranes, polymer membranes show both lower separation factor and lower flux for ethanol over water, although separation factors for composite membranes made of polymer and silicalite crystal powder are

Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4077 Table 6. Pervaporation Performance of Silicalite Membranes for Ethanol/Water Mixturesa synthesis condition T (°C) time (h)

support

seeding

Ml tube Ml tube R tube SS disk R disk SS disk

yes yes yes no no no

175 185 185 170 170 170

12 5.5 5.5 48 48 144

SS disk Ml tube R tube γ tube

no no no no

170 170 170 170

48 148 48 24

a

feed concn (EtOH wt %)

PV temp (°C)

flux (kg/m2‚h)

R

ref

5 5 5 4 4 4 4 4.7 5 10 9.7

60 60 60 60 30 30 60 30 50 75 24

1.41 2.23 1.81 0.78 0.1 0.29 0.97 0.50 0.34 2.29 0.1

72 62 89 58 24 120 84 64 51 25 11.5

this work this work this work 8 8 45 45 48 46 47 33

Ml tube: mullite tube. R tube: R-Al2O3 tube. SS disk: stainless steel disk. R disk: R-Al2O3 disk. γ tube: γ-Al2O3 tube.

enhanced to some extent. The advantage of silicalite membranes for separation of ethanol from ethanol aqueous solutions is attributed to the strong hydrophobic property and well-defined pore size of silicalite crystals. Considering the kinetic diameter of water, ethanol, and silicalite pore size, separation is not based on molecular sieving but preferential adsorption, in which preferentially adsorbed ethanol blocks water permeation. This ethanol/water separation behavior is similar to that of hydrogen/n-butane and hydrogen/ isobutane, in which hydrogen permeation is hindered by strongly adsorbed n-butane or isobutane at low temperature.49,50 In 1994, Sano et al.8 first reported that silicalite membranes formed on a stainless steel disk with crystal layer thicknesses of 400-500 µm were successfully applied to separate ethanol from an aqueous ethanol solution. Several years later,45 they further improved the separation factor from 60 to 120 by using a little bit of diluted gel for prolonged hydrothermal treatment (144 h) and calcination at low temperature of 375 °C, leading to the formation of a more dense crystal layer with higher crystallinity. In our study, we tried different approaches such as seeding and different Si sources with an alkali-free medium, and high temperature to quickly synthesize silicalite membranes showed both high separation factor and flux. Recent simulation results51 suggested that ethanol might completely inhibit water access to the pores of silicalite, leading to an infinite separation factor for ethanol over water. The future research work is focused on further improvement of the separation property, which is underway. Conclusion High-performance silicalite membranes on seeded porous tubes could be prepared by a single hydrothermal treatment with a short synthesis time. Silicalite crystal growth behavior on a porous support was strongly controlled by MFI seeds. Silicalite powder was the best seed for synthesis of the silicalite membranes. In such cases, silicalite seeds mainly provided nucleation sites and enhanced preferential silicalite crystal growth onto the interparticle voids, leading to the formation of thin and dense crystal layers. For preparation of good silicalite membranes, not only should synthesis conditions be optimized but also the state of supports should be considered. Under suitable synthesis conditions such as a clear solution (Si source of TEOS) with an alkalifree medium and high synthesis temperature, high flux and high separation factor were obtained for the separation of ethanol from water.

Acknowledgment This work was supported 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) Bein, T. Synthesis and Applications of Molecular Sieve Layers and Membranes. Chem. Mater. 1996, 8, 1636. (2) Jasen, K.; Maschmeyer, T. Progress in Zeolitic Membranes. Top. Catal. 1999, 9, 113. (3) Suzuki, K.; Kiyozumi, Y.; Sekine, T.; Obata, K.; Shindo, Y.; Shin, S. Preparation and Characterization of a Zeolite Layer. Chem. Express 1990, 5, 793. (4) Geus, E. R.; Den Exter, M. J.; Van Bekkum, H. Synthesis and Characterization of Zeolite (MFI) Membranes on Porous Ceramic Supports. J. Chem. Soc., Faraday Trans. 1992, 88, 3101. (5) Jia, M. D.; Chen, B.; Noble, R. D.; Falconer, J. L. CeramicZeolite Composite Membranes and Their Application for Separation of Vapor/Gas Mixtures. J. Membr. Sci. 1994, 90, 1. (6) Tuan, V. A.; Falconer, J. L.; Noble, R. D. Alkali-Free ZSM-5 Membranes: Preparation Conditions and Separation Performance. Ind. Eng. Chem. Res. 1999, 38, 3635. (7) Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of Zeolite ZSM-5 Membranes by in-situ Crystallization on Porous R-Al2O3. Ind. Eng. Chem. Res. 1995, 34, 1652. (8) Sano, T.; Yanagishita, H.; Kiyozumi, Y.; Mizukami, F.; K. Haraya, K. Separation of Ethanol/Water Mixture by Silicalite Membrane on Pervaporation. J. Membr. Sci. 1994, 95, 221. (9) Bakker, W. J. W.; Kapteijn, F.; Poppe, J.; Moulijn, J. A. Permeation Characteristics of a Metal-Supported Silicalite-1 Zeolite Membrane. J. Membr. Sci. 1996, 117, 57. (10) Vroon, Z. A. E. P.; Keizer, K.; Gilde, M. J.; Verweij, H.; Burggraaf, A. J. Transport Properties of Alkanes through Ceramic Thin Zeolite MFI-Membranes. J. Membr. Sci. 1996, 113, 293. (11) Kita, H.; Horii, K.; Ohtoshi, Y.; Okamoto, K. Synthesis of a Zeolite NaA Membrane for Pervaporation of Water/Organic Liquid Mixtures. J. Mater. Sci. Lett. 1995, 14, 206. (12) Kita, H.; Inoue, T.; Asamura, H.; Tanaka, K.; Okamoto, K. NaY Zeolite Membrane for the Pervaporation Separation of Methanol-Methyl tert-Butyl Ether Mixtures. Chem. Commun. 1997, 45. (13) Kondo, M.; Komori, M.; Kita, H.; Okamoto, K. TubularType Module with Zeolite NaA Membrane. J. Membr. Sci. 1997, 133, 133. (14) Okamoto, K.; Kita, H.; Horii, K.; Tanaka, K.; Kondo, M. Zeolite NaA Membrane: Preparation, Single-Gas Permeation, and Pervaporation and Vapor Permeation of Water/Organic Liquid Mixtures. Ind. Eng. Chem. Res. 2001, 40, 163. (15) Kita, H.; Asamura, H.; Tanaka, K.; Okamoto, K. Preparation and Pervaporation Properties of X- and Y-Zeolite Membranes. In Membrane Formation and Modification; Pinnau, I., Freem, B. D., Eds.; ACS Symposium Series 744; American Chemical Society: Washington, DC, 2000; p 330. (16) Xomeritakis, G.; Gouzinis, A.; Nair, S.; Okubo, T.; He, M.; Overney, R. M.; Tsapatsis, M. Growth, Microstructure, and

4078

Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001

Permeation Properties of Supported Zeolite (MFI) Films and Membranes Prepared by Secondary Growth. Chem. Eng. Sci. 1999, 54, 3521. (17) Xomeritakis, G.; Tsapatsis, M. Permeation of Aromatic Isomer Vapors through Oriented MFI-Type Membranes Made by Secondary Growth. Chem. Mater. 1999, 11, 875. (18) Xomeritakis, G.; Nair, S.; Tsapatsis, M. Transport Properties of Alumina-Supported MFI Membranes Made by Secondary (Seeded) Growth. Microporous Mesoporous Mater. 2000, 38, 61. (19) Lai, R.; Gavalas, G. R. Surface Seeding in ZSM-5 Membrane Preparation. Ind. Eng. Chem. Res. 1998, 37, 4275. (20) Lin, X.; Kikuchi, E.; Matsukata, M. Preparation of Mordenite Membranes on R-Alumina Tubular Supports for Pervaporation of Water/Isopropyl Alcohol Mixtures. Chem. Commun. 2000, 957. (21) Lin, X.; Kikuchi, E.; Matsukata, M. Oriented Growth of Zeolite Mordenite on R-Alumina Tubular Support and Their Separation Properties. Proceeding of 4th International Conference on Catalysis in Membrane Reactors, Zaragoza, Spain, 2000; p 53. (22) Munoz, T.; Balkus, K. J. Preparation of Oriented Zeolite UTD-1 Membranes via Pulsed Laser Ablation. J. Am. Chem. Soc. 1999, 121, 139. (23) Lin, X.; Kita, H.; Okamoto, K. A Novel Method for the Synthesis of High Performance Silicalite Membranes. Chem. Commun. 2000, 1889. (24) Morigami, Y.; Kondo, M.; Abe, J.; Kita, H.; Okamoto, K. The First Large-Scale Pervaporation Plant Using Tubular-Type Module with Zeolite NaA Membrane. Sep. Purif. Technol. 2001, in press. (25) O’Brien, D. J.; Roth, L. H.; McAloon, A. J. Ethanol Production by Continuous Fermentation-Pervaporation: a Preliminary Economic Analysis. J. Membr. Sci. 2000, 166, 105. (26) Moor, P. E. A.; Beelen, T. P. M.; Santen, R. A. V. In situ Observation of Nucleation and Crystal Growth in Zeolite Synthesis. A Small-Angle X-ray Scattering Investigation on Si-TPA-MFI. J. Phys. Chem. B 1999, 103, 1639. (27) Holmes, S. M.; Markert, C.; Plaisted, R. J.; Forrest, J. O.; Agger, J. R.; Aderson, M. W.; Cundy, C. S.; Dwyer, J. A Novel Method for the Growth of Silicalite Membranes on Stainless Steel Supports. Chem. Mater. 1999, 11, 3329. (28) Loos, J. B. Growth Behavior of Silicalite and ZSM-5 Seed Crystals in Different Reaction Media. Zeolites 1997, 18, 278. (29) Warzywoda, J.; Edelman, R. D.; Thompson, R. W. Crystallization of High-Silica ZSM-5 in the Presence of Seeds. Zeolites 1991, 11 318. (30) Thompson, R. W.; Dyer, A. Nucleation of Zeolite Na-A Crystals in Hydrothermal Systems. Zeolites 1985, 5, 302. (31) Hou, L. Y.; Thompson, R. W. Stability and Growth of Silicalite Seeds: an Observation of Initial Breeding. Zeolites 1989, 9, 526. (32) Edelman, R. D.; Kudalkar, D. V.; Ong, T.; Warzywoda, J.; Thompson, R. D. Crystallization Phenomena in Seeded Zeolite Syntheses. Zeolites 1989, 9, 496. (33) Liu, Q.; Noble, R. D.; Falconer, J. L.; Funke, H. H. Organic/ Water Separation by Pervaporation with a Zeolite Membrane. J. Membr. Sci. 1996, 117, 163. (34) Sasaki, Y.; Shimizu, W.; Ando, Y.; Saka, H. Microstructure Analysis with TEM for Zeolite Layer Formed in Pore of Porous Alumina Substrate. Microporous Mesoporous Mater. 2000, 40, 63. (35) Lai, R.; Yan, Y.; Gavalas, G. R. Growth of ZSM-5 Films on Alumina and Other Surfaces. Microporous Mesoporous Mater. 2000, 37, 9. (36) Gora, L.; Nishiyama, N.; Jasen, J. C.; Kapteijn, F.; Teplyakov, V.; Maschmeyer, T. Highly Reproducible High-Flux Sili-

calite-1 Membranes: Optimization of Silicalite-1 Membrane Preparation. Sep. Purif. Technol. 2001, 22-23, 223. (37) 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. (38) Sano, T.; Hasegawa, M.; Kawakami, Y.; Kiyozumi, Y.; Yanagishita, H.; Kitamoto, D.; Mizukami, F. Potentials of Silicalite Membranes for the Separation of Alcohol/Water Mixtures. Stud. Surf. Sci. Catal. 1994, 84, 1175. (39) Hickey, P. J.; Juricic, F. P.; Slater, C. S. The Effect of Process Parameters on the Alcohols through Organophillic Membranes. Sep. Sci. Technol. 1992, 27, 843. (40) Lee, Y. M.; Bourgeois, D.; Belfort, G. Sorption, Diffusion, and Pervaporation of Organics in Polymer Membranes. J. Membr. Sci. 1989, 44, 161. (41) Liang, L.; Ruckenstein, E. Pervaporation of EthanolWater Mixtures through Poly(dimethylsiloxane)-Polystyrene Interpenetrating Polymer Network Supported Membranes. J. Membr. Sci. 1996, 114, 227. (42) Kashiwagi, T.; Okabe, K.; Okita, K. Separation of Ethanol from Ethanol/Water Mixtures by Plasma-Polymerized Membranes from Silicone Compounds. J. Membr. Sci. 1988, 36, 353. (43) Hennepe, H. J. C.; Bargeman, D.; Mulder, M. H. V.; Smolders, C. A. Zeolite-Filled Silicone Rubber Membranes. Part 1. Membrane Preparation and Pervaporation Results. J. Membr. Sci. 1987, 35, 39. (44) Jia, M. D.; Peinemann, K. V.; Behling, R. D. Preparation and Characterization of Thin Film Zeolite-PDMS Composite Membranes. J. Membr. Sci. 1992, 73, 119. (45) Matsuda, H.; Yanagishita, H.; Kitamoto, D.; Nakane, T.; Haraya, K.; Koura, N.; Sano, T. Preparation of Silicalite Pervaporation Membrane with High Ethanol Permeselectivity. Membrane 1998, 23, 259 (in Japanese). (46) Kondo, M.; Nakane, T.; Yanagishita, H.; Sano, T.; Kita, H.; Okamoto, K. Small-Scale Tubular Type Pervaporation Module Using Silicalite Membrane. Proceedings of 5th International Conference on Inorganic Membrane, Nagoya, Japan, 1998; p 528. (47) Kita, H. Pervaporation Using Zeolite Membranes. Proceedings of International Workshop on Zeolitic Membranes and Films, Gifu, Japan, 1998; p 43. (48) Nomura, M.; Yamaguchi, T.; Nakao, S. Ethanol/Water Transport through Silicalite Membranes. J. Membr. Sci. 1998, 144, 161. (49) Kapteijn, F.; Bakker, W. J. W.; van de Graaf, G.; Zheng, G.; Poppe, J.; Moulijn, J. A. Permeation and Separation Behavior of a Silicalite-1 Membrane. Catal. Today 1995, 25, 213. (50) Ciavarella, P.; Moueddeb, H.; Miachon, S.; Fiaty, K.; Dalmon, J. A. Experimental Study and Numerical Simulation of Hydrogen/Isobutane Permeation and Separation Using MFIZeolite Membrane Reactor. Catal. Today 2000, 56, 253. (51) Takaba, H.; Koyama, A.; Nakao, S. Dual Ensemble Monte Carlo Simulation of Pervaporation of an Ethanol/Water Binary Mixture in Silicalite Membrane Based on a Lennard-Jones Interaction Model. J. Phys. Chem. B 2000, 104, 6353.

Received for review February 26, 2001 Revised manuscript received June 19, 2001 Accepted June 22, 2001 IE0101947