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Preparation of zeolite MFI membranes on alumina hollow fibers with high flux for pervaporation
Journal of Membrane Science 378 (2011) 319–329
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Preparation of zeolite MFI membranes on alumina hollow fibers with high flux for pervaporation Lijun Shan a , Jia Shao a , Zhengbao Wang a,∗ , Yushan Yan a,b a b
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521, USA
a r t i c l e
i n f o
Article history: Received 11 February 2011 Received in revised form 20 April 2011 Accepted 9 May 2011 Available online 13 May 2011 Keywords: Zeolite membrane Pervaporation MFI Hollow fiber Support leaching
a b s t r a c t Polycrystalline zeolite MFI (silicalite-1) membranes are successfully prepared by the secondary growth method on the ␣-Al2 O3 hollow fibers with a high surface/volume ratio. Influences of the seed suspension concentration, the crystallization time and the content of the structure-directing agent (SDA, e.g., TPAOH) in the synthesis solution are investigated. As-synthesized zeolite seeds and membranes are characterized by X-ray diffraction (XRD), dynamic light scattering (DLS) and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX). The membrane formation and integrity are found to be sensitively depending on the SDA contents in the synthesis solution and the seed amount of the seeded layer. High TPAOH content may cause aluminum leaching from the alumina support seriously and low TPAOH content may lead to non-continuous zeolite layer, leading to poor separation performance. A continuous and dense zeolite seed layer is necessary to obtain the silicalite-1 membrane with good quality. However, too thick seed layer may cause peeling and cracking of the membrane and thus decrease the separation performance. Appropriate TPAOH and seed amount may prevent the ␣-Al2 O3 support leaching effectively. The silicalite-1 membranes prepared in this paper show the highest flux and relatively high separation factor of ethanol/water mixtures compared with the literature data. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past two decades, the potential use in a wide range of industrial applications, such as separation membranes, membrane reactors and catalysis [1,2], leads to the considerable research on the development of zeolite membranes. Membrane-based pervaporation separation of azeotropic mixtures and near boiling point mixtures, has great advantages in that the energy consumption is lower than the traditional distillation and extraction [3]. Compared with organic membranes, zeolite membranes show high mechanical strength, thermal and chemical stability. Moreover, MFI-type zeolite membrane, especially silicalite-1 membrane, has been studied most extensively because of its welldefined pore structure (ca. 0.5 nm) and hydrophobicity, which is suitable for separation of many industrial hydrocarbon molecules and organic/water mixtures by pervaporation (PV). At present, the MFI-type membranes were mainly synthesized by in situ crystallization [4,5] and secondary-growth methods [6–8]. The secondary-growth method for preparing zeolite membranes separates the crystal nucleation and growth steps, and requires lower temperature and shorter synthesis time compared
∗ Corresponding author. Tel.: +86 571 8795 2391; fax: +86 571 8795 2391. E-mail address: [email protected] (Z. Wang). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.011
with the in situ crystallization method, thus helps to obtain a better control of the membrane microstructure [9]. Many methods have been used to coat the zeolite seeds onto the porous support, such as rub-coating [10], spin-coating [11], dip-coating [12], vacuumcoating [13], electrophoretic deposition method [14], etc. It is known that the preferred adsorption of organic molecules onto the surface of hydrophobic silicalite-1 crystals provides the primary separation selectivity for organic/water mixtures by pervaporation. However, incorporation of aluminum atom into the silicalite-1 layer would reduce its hydrophobicity and this would lead to worse organic selective separation properties. Sano et al. [15] reported that the silicalite-1 membranes synthesized on the porous sintered stainless steel disk support showed better separation selectivity of ethanol than on the porous alumina support due to aluminum leaching from the alumina support and its incorporation into the zeolite framework. However, for silicalite1 membranes synthesized on the stainless steel support, cracks formed easily during the calcination because of the thermal expansion coefficient mismatch between the steel support and the silicalite-1 layer and the adhesion of the zeolite membrane to the support was weak [16]. Kanezashi et al. [17] synthesized the MFI-type membranes on zirconia coated alumina supports by a template-free secondary growth method. They used the yttrium stabilized zirconia as the intermediate layer to reduce the incorporation of aluminum from the support into the membrane layer.
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But the zirconia coating step was complicated and made the process more expensive. Recently, Chen et al. [18] reported that the separation performance of MFI membranes synthesized on silica tubes was better than that of membranes synthesized on ␣-alumina tubes. Although a solution-filling method was adopted, the permeation flux of silicalite-1 membranes on silica tubes was low for ethanol/water separation. On the other hand, one of the main constraints of zeolite silicalite-1 membranes in ethanol/water pervaporation for industrial application is the low permeation flux, although the separation factor is reasonably high. The properties of the porous support, the crystal orientation and hydrothermal synthesis conditions are the major factors that influence the flux. At present, most membranes reported in literatures were synthesized on flat plates [6,7,15,19] or large diameter tubular supports [20–22]. The thick wall of the support leads to high resistance of the permeate transport through the support. Wang et al. [23] successfully synthesized zeolite LTA membranes on ␣-Al2 O3 hollow fibers with higher flux than on the mullite tubular supports. They stated that the lower flux of the zeolite membranes on the mullite tubular supports was due to the thicker wall of the support than ␣-Al2 O3 hollow fibers. Lai et al. [24] synthesized the b-oriented silicalite-1 membrane on porous alumina supports with a mesoporous silica intermediate layer using trimer-TPA as organic template. The as-prepared membrane showed superior performance both in the flux and the separation factor for the separation of xylene isomers mixtures. Zhao et al. [25] reported high-permeance silicalite-1 membranes with a modified secondary growth method on the macroporous ␣-Al2 O3 tubular support for the gas separation. In this paper, we synthesize silicalite-1 membranes by the secondary (seeded) growth method on ␣-Al2 O3 hollow fiber supports because the hollow fibers allow separation modules that can have higher separation surface area-to-volume ratio (>1000 m2 /m3 ) than conventional tubular supports (<500 m2 /m3 , if the outer diameter was larger than 10 mm). We demonstrate for the first time that silicalite-1 membranes on alumina hollow fiber supports have high flux for the separation of ethanol/water with a relative high separation factor by controlling the SDA amount and seed concentration. 2. Experimental 2.1. Preparation of nanocrystal seeds The silicalite-1 nanocrystal seeds used in this paper were prepared by a traditional hydrothermal synthesis following a previously reported method [26]. A clear solution was obtained by aging the synthesis solution at ambient temperature for 24 h under stirring. The molar composition of the synthesis solution was TPAOH:TEOS:H2 O = 1:2.8:40, using tetrapropylammonium hydroxide (TPAOH, Zhejiang Xianju Application Chemical Research Institute, China) as the structure-directing agent (SDA), and tetraethyl orthosilicate (TEOS, Acros Organics) as silicon source. The solution was transferred into a Teflon-lined autoclave. Then the autoclave was kept rotating at 30 rpm for 3 days in an oven at 80 ◦ C. The resulting seed crystals were purified by repeated washing with deionized water and centrifugation (12,000 rpm) until the pH value of the seed suspension was close to 8–9. The final seed suspension for the dip-coating process was obtained by dispersing the seeds in deionized water (1–10 wt.%). 2.2. Preparation of silicalite-1 membranes The silicalite-1 membranes were prepared by the seeded secondary growth onto the external surface of ␣-Al2 O3 hollow fiber supports with pore diameters of 100–200 nm. Alumina hollow
Fig. 1. SEM images of ␣-Al2 O3 hollow fiber. (a) Top view and (b) cross-sectional view.
fibers were prepared according to Refs. [27,28] by spinning a polymer slurry containing suspended aluminum oxide powder to a hollow fiber precursor, which was then sintered at elevated temperatures (e.g., 1500 ◦ C). Polyethersulfone (PES), N-methyl-2pyrrolidone (NMP), and polyvinyl pyrrolidone (PVP) were used in the polymer slurry as a polymer binder, a solvent, and an additive, respectively. Fig. 1 shows SEM images of top view and crosssectional view of the hollow fibers. The porosity of the supports was obtained by measuring the amount of absorbed water within a certain amount of time (e.g., 10 s). The hollow fibers were 1.1 mm o.d., 0.8 mm i.d. and 20–40% porosity. The supports were seeded by the dip-coating method with the above prepared silicalite-1 seed suspension prior to the membrane synthesis. The hollow fiber support was dipped into the seed suspension and kept in it for 10 s to adsorb seed crystals onto the support external surface and dried at 60 ◦ C for 1 h. The solution used for the secondary growth was prepared by mixing deionized water, TPAOH, and TEOS with the molar composition TPAOH:TEOS:H2 O = 0.12–0.32:1:165. The solution was aged at ambient temperature for 4 h under stirring. Then the seeded hollow fiber supports were placed vertically in the secondary synthesis solution in a Teflon-lined autoclave. The crystallization was carried out under autogenetic pressure in an electronically controlled convection oven at 175 ◦ C for 2–12 h. After each synthesis, the obtained membranes were rinsed with deionized water and dried at 60 ◦ C for more than 4 h. The calcination for removing the SDA (TPA+ ) occluded in the zeolite pores during synthesis was performed in
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Fig. 2. XRD pattern and particle size distribution (inset) of silicalite-1 nanocrystal seeds.
a Muffle Furnace at 500 ◦ C for 12 h with heating and cooling rates of 1.0 ◦ C/min. 2.3. Characterization The crystalline microstructure and particle size distribution of the as-prepared crystal seeds were determined by X-ray diffraction patterns (XRD, Rigaku, D/max-rA) using CuK␣ radiation and dynamic light scattering (DLS, Malvern, ZEN 3600), respectively. Microstructure and morphology of seeds and membranes were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800) and scanning electron microscopy (SEM, Hitachi TM-1000). The Al distribution in the zeolite layer was measured by energy dispersive X-ray (EDX, Horiba EX350). The surface hydrophobicity of the zeolite membranes was determined by contact angle measurement (Dataphysics, OCA 20) using sessile drop methods with a 0.1 L droplet of deionized water. Three measurements were carried out for each sample. The pervaporation experiments of ethanol/water mixtures were carried out in a laboratory scale setup which is the same as the one reported in Ref. [23]. Zeolite membranes on the hollow fiber supports (5–8 cm long) were pasted into a non-porous stainless steel tube by Torr Seal (Varian Vacuum Technologies), and then connected to the vacuum system. 5 wt.% or 3 wt.% ethanol aqueous solution in a flask of 550 mL was heated at a certain temperature and kept stirred during PV. The permeate was collected in a trap for 40–60 min. The total flux (Q) and separation factor (˛) were determined as Q = W/(A × t) and ˛ = (Yethanol/Ywater)/(Xethanol/Xwater), where W is the total weight of the permeate (kg), A is the separation area of zeolite membrane, t is the collecting time (h), and X and Y are the mass fraction of components in the feed and permeate, respectively. The weight of the permeate was measured by weighing the cold trap before and after the collection of the permeate. The mass fraction of ethanol in the feed and permeate was analyzed by gas chromatograph. 3. Results and discussion 3.1. Influences of seed concentration According to the DLS data (Fig. 2, inset), the particle size of the as-prepared seed was about 70 nm. The XRD pattern (Fig. 2) showed that the seeds were silicalite-1 zeolites with no evidence of other crystalline phase. The peak width at half maximum at the Bragg diffraction angle 2 of 23.1◦ was 0.32◦ , and the average seed size calculated using the Debye–Scherrer formula (d = 0.9/Bcos , where B is the peak width at half maximum in radians, is the Bragg
Fig. 3. SEM images of seed layers on the ␣-Al2 O3 hollow fiber supports with different seed concentration, (a) 1 wt.%, (b) 5 wt.% and (c) 10 wt.%.
diffraction angle and d is the average grain size.) is 27.4 nm, which is lower than the size from DLS, which is understandable in that DLS usually measures the hydrodynamic diameter of the particles and tend to be dominated by large particles. It is also possible that the seed crystals aggregated under the DLS measuring conditions. The average seed size from XRD also indicates that the seeds were nano-sized. Secondary growth was considered to be one of the most effective methods to obtain the MFI zeolite separation membranes and oriented thin films. For the secondary growth method, the support coated with seed crystals was subjected to the hydrothermal synthesis to obtain a continuous zeolite layer. The property of seed layer has a great influence on the membrane performance
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Table 1 Effects of seed concentration and TPAOH content on the PV performance of silicalite-1 membranes. Mem. no.
TPAOH content (x)
B1 B2 B3 B4 B5 B6 B7 B8
0.17 0.17 0.17 0.17 0.32 0.27 0.22 0.12
Seed concentration (wt.%) 1 10 5 5 5 5 5 5
Porosity (%)
Flux (kg/(m2 h))
Separation factor, ˛
21 ± 5 32 ± 5 33 ± 5 42 ± 5 34 ± 5 36 ± 5 36 ± 5 22 ± 5
0.7 ± 0.2 – 1.3 ± 0.2 2.9 ± 0.2 1.5 ± 0.2 1.1 ± 0.2 1.3 ± 0.2 0.6 ± 0.2
11 ± 2 – 44 ± 2 66 ± 2 25 ± 2 45 ± 2 66 ± 2 9±2
Note: The synthesis solution composition TPAOH:TEOS:H2 O = x:1:165; PV conditions, feed 3 wt. % ethanol, temperature 60 ◦ C.
[7]. Zhang et al. [29] prepared the silicalite-1 membranes by the hydrothermal synthesis method with different seed sizes. They indicated that the intergrowth of the silicalite-1 layer was poor with large seed size, whereas using zeolite seeds of 100 nm, the as-prepared membranes were continuous and dense. For further understanding the effect of seeding on the membrane quality, we studied the impact of seed concentration on the membrane performance for the separation selectivity of ethanol/water mixtures. The deposition of the seed layer on the outer side of the hollow fiber in this paper was made by the dip-coating method, which was a simple process. SEM images of porous ␣-Al2 O3 hollow fibers seeded with different seed concentrations are shown in Fig. 3. The configuration of seed crystals was spherical. It is clear from the surface of the seeded supports that the seed layer was denser and more uniform with increasing the seed concentration. Some bared supports could be observed clearly with the seed concentration of 1 wt.%, and many nanocrystal seeds inserted into the support pores or located at the lowland of the support surface. When the concentration increased to 5 wt.%, the support was almost covered with a dense seed layer, and only few bared spaces (circled places) could be seen. The complete coverage was achieved using the 10 wt.% seed concentration. The SEM images of the silicalite-1 membranes prepared on the supports seeded with the seed concentrations of 0 wt.%, 1 wt.%, 5 wt.%, and 10 wt.% from the same composition of the synthesis solution (TPAOH:TEOS:H2 O = 0.17:1:165) are shown in Fig. 4. Without seeds, the support was covered by randomly oriented and slice-shaped silicalite-1 crystals with size of ∼20 m after the hydrothermal synthesis at 175 ◦ C for 12 h, and some exposed support can be seen clearly from Fig. 4a with a non-uniform and non-continuous layer (Fig. 4b). For the secondary growth method, continuous zeolite layer composed of c-oriented crystals formed after synthesis for 12 h (Fig. 4d, f, and h). However, many holes can be seen clearly for 1 wt.% seed concentration from Fig. 4c. With increasing the seed concentration, the membranes on the top layer become more compact (Fig. 4e and g). The thickness of the membranes obtained by the seeded method was about 12 m (Fig. 4d, f, and h), which was much thicker than the membranes prepared by the in situ method (Fig. 4b). Table 1 shows the concentration of the seed suspension has great impact on the separation performance of as-prepared membranes (B1–B4). When the concentration was 1 wt.%, the separation factor was very low (B1). When the concentration was 10 wt.%, the as-prepared membrane was leak and could not be used for pervaporation (B2). When the concentration was 5 wt.%, the separation factor was high (B3 and B4). Membrane B4 with higher porosity of the support had much higher flux and separation factor than membrane B3. According to the literatures [30,31], the seed layer should be dense enough to cover the support surface for the preparation of MFI membranes. This is different from synthesis of zeolite LTA membranes in which a trace distribution of seed on the support surface was sufficient [23]. The support surface was incompletely
covered by seed with low concentration from Fig. 3a, and some exposed places could be seen clearly. After crystallization for 12 h, weak intergrowth could be found in the silicalite-1 layer due to the poor continuity of the seed layer (Fig. 4c), and so the performance of the as-prepared membrane would not be high. However, when the seed concentration was too high, the seed layer deposited on the support surface was too thick, leading to that the seeds close to the support could not intergrow each other after the hydrothermal synthesis (Fig. 5b). Gaps and non-zeolitic pores among the seed crystals between the dense zeolite layer and the support may cause the crack formed during the calcination of SDA (Fig. 5a), which may decrease the membrane quality. More seriously, the less compact silicalite-1 layer would detach from the support. Lovallo et al. [30] also reported the support should be covered by the seed layer to obtain high performance silicalite-1 membranes, but too thick seed layer could cause the membrane to peel or crack and thus decrease the quality of the membrane. Consequently, increasing the thickness of seed layer may have a negative impact on the membrane performance. During the hydrothermal treatment, the aluminum would leach from the ␣-Al2 O3 support [15,19,32–35], which would reduce the hydrophobicity of the silicalite-1 membranes and cause the separation selectivity of organic/water mixtures reduce. This concern was also investigated in this paper. However, we found the seeding layer could prevent the ␣-Al2 O3 support leaching effectively. Fig. 6a shows the support leaching was serious for the low coverage with 1 wt.% seed concentration, and reduced the hydrophobicity of membrane surface, so the separation factor decreased. The aluminum leaching from ␣-Al2 O3 support can be efficiently prevented if the surface was covered by a dense seed layer appropriately. With the seed concentration increasing, the support leaching reduced dramatically. When the seed concentration was 10 wt.%, the aluminum leaching could not even be seen from Fig. 6c, leading to much more hydrophobic surface of zeolite layer. This is one of the reasons why silicalite-1 membranes from 1 wt.% seed concentration showed lower separation factor. 3.2. Influences of crystallization time The evolution of the zeolite membrane microstructure during the hydrothermal synthesis was examined ex situ using SEM. Fig. 7 presents SEM images of zeolite silicalite-1 membranes synthesized on ␣-Al2 O3 hollow fiber supports with different crystallization time. Starting from a dense seed layer (5 wt.% seed concentration) consisting of 70 nm silicalite-1 seeds, a 1.5 m thick zeolite layer was deposited after 2 h of hydrothermal treatment at 175 ◦ C in a synthesis solution with the molar composition TPAOH:TEOS:H2 O = 0.17:1:165. It was clear from Fig. 7a that the hollow fiber support after the hydrothermal treatment of 2 h was almost fully covered by a zeolite layer, although many holes were still seen. The ultra-thin zeolite layer followed the contour of the support surface (Fig. 7b). The zeolite grain size in the membrane layer increased with increasing the crystallization time
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Fig. 4. SEM images of the as-synthesized membranes with different seed suspension concentration for the synthesis solution molar composition TPAOH:TEOS:H2 O = 0.17:1:165 at 175 ◦ C for 12 h. (a and b) 0 wt.%, (c and d) 1 wt.%, (e and f) 5 wt.% and (g and h) 10 wt.%. Left: top view. Right: cross-sectional view.
(Fig. 7a–c–e), and the crystal edges became more clear, the holes in the membrane surface significantly smaller. As shown in Figs. 7b, d and f, with the increase of the synthesis time from 2 h to 8 h, the membrane thickness increased from 1.5 m to 7.8 m. It can be clearly seen from Fig. 7 that the longer the crystallization time is, the larger the crystals and the better continuity of the membrane layer. So in order to obtain a dense membrane layer with high separation performance, the crystallization time of 12 h was chosen.
3.3. Influences of TPAOH content and support porosity Fig. 8 shows the XRD patterns of the silicalite-1 membranes on the ␣-Al2 O3 hollow fibers with different TPAOH content. All of the membranes after secondary growth at 175 ◦ C exhibited (h 0 h)- and oblique c-orientation. The as-prepared silicalite-1 membranes with the TPAOH content x = 0.32–0.17 showed the same or nearly the same peak heights (Fig. 8a–d), indicating that all these membranes have similar thickness. The lower peak height of the membrane
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Fig. 5. Magnified SEM images (a: top view, b: cross-sectional view) of the as-synthesized membranes with 10 wt.% seed concentration. Other conditions: same as Fig. 4.
Fig. 6. EDX images of the as-synthesized membranes with different seed suspension concentration (a) 1 wt.%, (b) 5 wt.% and (c) 10 wt.%. Other conditions: same as Fig. 4.
Fig. 7. SEM images of the as-synthesized membranes from the molar composition TPAOH:TEOS:H2 O = 0.17:1:165 at 175 ◦ C with different crystallization time. (a and b) 2 h, (c and d) 4 h and (e and f) 8 h. Left: top view. Right: cross-sectional view.
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Fig. 8. XRD patterns for the as-synthesized silicalite-1 membranes with the molar composition TPAOH:TEOS:H2 O = x:1:165 at 175 ◦ C for 12 h. (a) x = 0.32, (b) x = 0.27, (c) x = 0.22, (d) x = 0.17 and (e) x = 0.12. Peaks with index (*) are from alumina substrate.
with the TPAOH content x = 0.12 (Fig. 8e), illustrates that this membrane was thinner, which is consistent with the following SEM results. TPAOH have a dual role in this synthesis study. It served as structure directing agent in assembling the zeolite framework and also provided the alkalinity of the synthesis solution. The effect of the TPAOH amount on the membrane quality was evaluated on the basis of SEM micrographs. As shown in Fig. 9, the support surface was fully covered by silicalite-1 layers in all batches. In contrast to the synthesis of silicalite-1 particles [36], the crystal size observed from the top view increased slightly with increasing TPAOH content during the membrane synthesis. With the reduction in the amount of TPAOH (Fig. 9c, e and g), the membrane surface becomes more smooth. The silicalite-1 layer prepared with the extremely low TPAOH content (x = 0.12) consisted of crystals with poor intergrowth (Fig. 9i); grains in the polycrystalline membrane showed the shape of platelet, and many inter-crystalline pores could be seen clearly. This is due to preferential growth of the seed crystals along their c-axis and the severe suppression of nucleation at very low TPAOH leading to the growth of crystals with a high aspect ratio [37]. From the SEM images of the fractured cross-sectional views of the membranes (Fig. 9), the silicalite-1 layer grew only on the seeded surface and not inside the support cavities, and the as-synthesized membranes exhibit columnar grain microstructure, which is consistent with the literature [7]. The thickness from the cross-sectional view was about 12 m for membranes B3–B7 (Fig. 9b, d, f, h), much thicker than membrane B8 (Fig. 9j), which was approximately 9 m. Table 1 shows the effect of the TPAOH content on the pervaporation properties of silicalite-1 membranes from the composition TPAOH:TEOS:H2 O = x:1:165 on ␣-Al2 O3 hollow fibers at 175 ◦ C for 12 h (B3–B8). When the TPAOH concentration is too high (B5) and too low (B8), the separation factors were low. When the TPAOH concentration (or alkalinity) was high aluminum leached from the hollow fiber support. The leaching decreases with decreasing the TPAOH concentration, which can be seen from the EDX results (Fig. 10). The hydrophobicity of the membrane surfaces was evaluated by contact angle measurements using deionized water
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(Fig. 11). For the membranes prepared from the TPAOH/TEOS ratio (x) = 0.12, the average contact angle was found to be the highest as a value of 67◦ . The contact angle decreased to 51◦ when the TPAOH/TEOS ratio (x) was increased to 0.32. This result also indicates that the support leaching increased with the increase of TPAOH concentration, reducing the surface hydrophobicity of the silicalite-1 membrane. For the TPAOH content x = 0.12, the separation factor ˛ of membrane B8 is extremely low, e.g., ˛ = 9, which is due to the existence of the pinholes as shown in SEM image (Fig. 9i). Membranes from the moderate TPAOH concentration showed relatively high separation factor. For example, the separation factors of membranes B4 and B7 were higher than 60 at x = 0.17 and 0.22. It has been reported that the flux of zeolite LTA membranes increased almost linearly with the porosity of alumina hollow fibers [23]. The results in Table 1 also suggest that the porosity of the supports has a great impact on the flux of silicalite-1 membranes. A linear relationship between the flux of silicalite-1 membranes and the support porosity was observed in Fig. 12. The high porosity of the support reduced mass transfer resistance according to the adsorption–diffusion model leading to the flux increased [38]. However, the extremely high porosity could reduce the mechanical strength, so the appropriate porosity of the support should be available. It should be noted that some data points were far from the line, probably indicating the existence of some unknown factors. For example, the support porosity of membrane B3 and B4 was 33% and 42% in Table 1, respectively, however, the flux of membrane B4 (2.9 kg/(m2 h)) is more than twice that of membrane B3 (1.3 kg/(m2 h)). Work is ongoing in identifying the effects of some other factors on the flux of zeolite membranes, e.g., the structure of the hollow fiber and the surface porosity and so on. 3.4. Comparison with literature data Sano et al. already studied the effects of the PV temperature and the feed composition in detail [15]. And their results showed that the separation factor slightly decreased with the PV temperature and greatly with the feed ethanol concentration (>3 wt.%) and that the flux increased with the PV temperature and the feed ethanol concentration. The pervaporation performance of silicalite1 membranes (B4 and B7) for the 5 wt.% ethanol feed solution at 75 ◦ C was compared with the 3 wt.% ethanol feed solution at 60 ◦ C in Table 2. It can be seen that the separation factor decreased, but the flux increased dramatically. The separation performance depends on the sorption of the permeating molecules in the feed side and their diffusion through the membrane according to the adsorption–diffusion model. The diffusion rate increases with elevated feed temperature which leads to the flux increase. For higher ethanol concentration in the feed solution, more ethanol molecules could adsorb to the surface of the hydrophobic silicalite-1 zeolite, and then transport through the membrane, also leading to the increase in the flux. The result of the as-prepared membranes in this work is similar to that reported in literatures [15,33]. The comparison of the separation results of the membranes synthesized on the hollow fibers with that reported in literatures is presented in Table 2. Sano et al. [15] first reported that the silicalite-1 membranes synthesized on the stainless steel support preferentially permeated ethanol with separation factor of 58, but the flux was too low for the membrane because the membrane was too thick, i.e., 400–500 m by Si line analysis with EDX. Chen et al. [33] reported the synthesis of silicalite-1 membranes by solutionfilling method on the silica tubular support, and they showed that these membranes had better separation performance than the membranes synthesized on ␣-Al2 O3 tubes. They indicated that the flux of membrane synthesized by the solution-filling method could be improved by 90%, but it was still lower than 1.0 kg/(m2 h). Lin
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Fig. 9. SEM images of the as-synthesized membranes with 5 wt.% seed suspension concentration for the molar composition TPAOH:TEOS:H2 O = x:1:165 at 175 ◦ C for 12 h. (a and b) x = 0.32, (c and d) x = 0.27, (e and f) x = 0.22, (g and h) x = 0.17 and (i and j) x = 0.12. Left: top view. Right: cross-sectional view.
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Fig. 10. EDX images of the as-synthesized membranes with the molar composition TPAOH:TEOS:H2 O = x:1:165 at 175 ◦ C for 12 h. (a) x = 0.32, (b) x = 0.27, (c) x = 0.22, (d) x = 0.17 and (e) x = 0.12.
et al. [39] reported the flux of 1.81 kg/(m2 h) and 2.23 kg/(m2 h) for the silicalite-1 membranes synthesized on the ␣-Al2 O3 tubular and mullite tubular supports, respectively, and both supports had large pore diameter. These were the best separation results of silicalite-1 membranes for the ethanol/water pervaporation to date. Sebastian et al. [40] reported the flux of 1.5 kg/(m2 h) and the separation factor of 54 using the membrane synthesized on the ceramic ␣-Al2 O3 capillary by microwave-assisted heating method. The membrane thickness was about 2.5 m. They indicated that the high flux could be obtained by decreasing the membrane thickness. To the best of our knowledge, the silicalite-1 membranes synthesized in this study on the outside of ␣-Al2 O3 hollow fiber supports showed the highest flux for ethanol/water mixtures separation (5.4 kg/(m2 h)) by a simple secondary growth. The transport mechanism of ethanol/water mixtures by pervaporation through zeolite membrane can be described by the adsorption–diffusion model [38]. Based on this model, each component of the permeating molecules is adsorbed to the inlet of the zeolite micropores and diffuses through the zeolite pores due to the concentration gradient. Thus, the thickness of the membrane and support may affect the transport rate of the permeates to some extent. The thickness of the hollow fiber support used in this paper was about one order of magnitude lower than other supports reported in the literature [33,40], leading to decreased resistance for permeate transport through the support and thus to higher fluxes.
Fig. 11. Contact angle results of the as-synthesized membranes on hollow fibers as a function of TPAOH concentration (x).
Table 2 Comparison of PV separation results of as-prepared membranes with those in literatures. Support
Membrane thickness (m)
Pervaporation conditions ◦
Hollow fiber Hollow fiber Hollow fiber Hollow fiber Stainless steel ␣-Al2 O3 tube Silica tube ␣-Al2 O3 tubular Mullite tubular ␣-Al2 O3 capillary
12 12 12 12 400–500 50 30 Not mentioned Not mentioned 2.5
Note: Feed, ethanol aqueous solution.
Temp ( C)
Feed (wt.%)
75 60 75 60 60 80 60 60 60 65
5 3 5 3 5 3 3 5 5 5
Flux (kg/(m2 h))
Separation factor, ˛
Ref.
2.7 ± 0.2 1.3 ± 0.2 5.4 ± 0.2 2.9 ± 0.2 0.76 0.41 0.87 1.81 2.23 1.50
57 ± 2 66 ± 2 54 ± 2 66 ± 2 58 33 69 89 62 54
B7 B7 B4 B4 [15] [33] [33] [39] [39] [40]
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Fig. 12. Relationship between the flux of MFI membrane and the porosity of alumina hollow fiber support.
4. Conclusions Polycrystalline silicalite-1 membranes on the ␣-Al2 O3 hollow fibers were successfully synthesized by the secondary growth method. It was found that the seed concentration and TPAOH content showed great influences on the membrane formation and its separation performance. When TPAOH/TEOS molar ratio was 0.12, the zeolite crystal on the membrane surface showed platelet shape, and the separation performance was poor. However, the support leaching was serious for the high TPAOH content (TPAOH/TEOS>0.32), and caused the membrane to become less hydrophobic. A continuous and dense zeolite seed layer is necessary to obtain a silicalite-1 membrane with good quality. However, 10 wt.% seed concentration led to a thick seed layer and caused the membrane to peel or crack. The membrane from the synthesis solution with TPAOH/TEOS = 0.17 and the seed suspension of 5 wt.% concentration showed the flux of 2.9, 5.4 kg/(m2 h) and the separation factor of 66, 54 for 3 wt.% ethanol feed at 60 ◦ C and 5 wt.% ethanol feed at 75 ◦ C, respectively. These fluxes are the highest fluxes to date with moderate separation factors for the pervaporation separation of ethanol/water mixtures. Acknowledgements We thank the National Natural Science Foundation of China (20876133 and 21028002), Zhejiang Nature Science Foundation (ZJNSF) (R4090099) and Science and Technology Department of Zhejiang Province (2009R50020) for financial supports. Y.Y. thanks the Chinese Ministry of Education for the Visiting Changjiang Scholar Professorship. References [1] L. Gora, J.C. Jansen, Hydroisomerization of C-6 with a zeolite membrane reactor, J. Catal. 230 (2005) 269–281. [2] J. Caro, M. Noack, Zeolite membranes – recent developments and progress, Micropor. Mesopor. Mater. 115 (2008) 215–233. [3] T.C. Bowen, R.D. Noble, J.L. Falconer, Fundamentals and applications of pervaporation through zeolite membranes, J. Membr. Sci. 245 (2004) 1–33. [4] Z.B. Wang, Y.S. Yan, Oriented zeolite MFI monolayer films on metal substrates by in situ crystallization, Micropor. Mesopor. Mater. 48 (2001) 229–238. [5] X.M. Li, Y.S. Yan, Z.B. Wang, Continuity control of b-oriented MFI zeolite films by microwave synthesis, Ind. Eng. Chem. Res. 49 (2010) 5933–5938.
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Schafer, Zeolite membranes – state of their development and perspective, Micropor. Mesopor. Mater. 38 (2000) 3–24. [17] M. Kanezashi, J. O’Brien, Y.S. Lin, Template-free synthesis of MFI-type zeolite membranes: Permeation characteristics and thermal stability improvement of membrane structure, J. Membr. Sci. 286 (2006) 213–222. [18] H.L. Chen, C.S. Song, W.S. Yang, Effects of aging on the synthesis and performance of silicalite membranes on silica tubes without seeding, Micropor. Mesopor. Mater. 102 (2007) 249–257. [19] T. Sano, S. Ejiri, K. Yamada, Y. Kawakami, H. Yanagishita, Separation of acetic acid-water mixtures by pervaporation through silicalite membrane, J. Membr. Sci. 123 (1997) 225–233. [20] H.L. Chen, Y.S. Li, J. Liu, W.S. Yang, Preparation and pervaporation performance of high-quality silicalite-1 membranes, Sci. China Ser. B: Chem. 50 (2007) 70–74. [21] C.L. Kong, J.M. Lu, J.H. Yang, J.Q. 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Lu, Preparation of high-permeance MFI membrane with the modified secondary growth method on the macroporous alpha-alumina tubular support, J. Membr. Sci. 320 (2008) 303–309. [26] L.M. Huang, Z.B. Wang, J.Y. Sun, L. Miao, Q.Z. Li, Y.S. Yan, D.Y. Zhao, Fabrication of ordered porous structures by self-assembly of zeolite nanocrystals, J. Am. Chem. Soc. 122 (2000) 3530–3531. [27] X.Y. Tan, S.M. Liu, K. Li, Preparation and characterization of inorganic hollow fiber membranes, J. Membr. Sci. 188 (2001) 87–95. [28] S.M. Liu, K. Li, R. Hughes, Preparation of porous aluminium oxide (Al2 O3 ) hollow fibre membranes by a combined phase-inversion and sintering method, Ceram. Int. 29 (2003) 875–881. [29] X.F. Zhang, H.O. Liu, K.L. Yeung, Influence of seed size on the formation and microstructure of zeolite silicalite-1 membranes by seeded growth, Mater. Chem. Phys. 96 (2006) 42–50. [30] M.C. Lovallo, A. Gouzinis, M. 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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation of zeolite MFI membranes on alumina hollow fibers with high flux for pervaporation Lijun Shan a , Jia Shao a , Zhengbao Wang a,∗ , Yushan Yan a,b a b
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521, USA
a r t i c l e
i n f o
Article history: Received 11 February 2011 Received in revised form 20 April 2011 Accepted 9 May 2011 Available online 13 May 2011 Keywords: Zeolite membrane Pervaporation MFI Hollow fiber Support leaching
a b s t r a c t Polycrystalline zeolite MFI (silicalite-1) membranes are successfully prepared by the secondary growth method on the ␣-Al2 O3 hollow fibers with a high surface/volume ratio. Influences of the seed suspension concentration, the crystallization time and the content of the structure-directing agent (SDA, e.g., TPAOH) in the synthesis solution are investigated. As-synthesized zeolite seeds and membranes are characterized by X-ray diffraction (XRD), dynamic light scattering (DLS) and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX). The membrane formation and integrity are found to be sensitively depending on the SDA contents in the synthesis solution and the seed amount of the seeded layer. High TPAOH content may cause aluminum leaching from the alumina support seriously and low TPAOH content may lead to non-continuous zeolite layer, leading to poor separation performance. A continuous and dense zeolite seed layer is necessary to obtain the silicalite-1 membrane with good quality. However, too thick seed layer may cause peeling and cracking of the membrane and thus decrease the separation performance. Appropriate TPAOH and seed amount may prevent the ␣-Al2 O3 support leaching effectively. The silicalite-1 membranes prepared in this paper show the highest flux and relatively high separation factor of ethanol/water mixtures compared with the literature data. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past two decades, the potential use in a wide range of industrial applications, such as separation membranes, membrane reactors and catalysis [1,2], leads to the considerable research on the development of zeolite membranes. Membrane-based pervaporation separation of azeotropic mixtures and near boiling point mixtures, has great advantages in that the energy consumption is lower than the traditional distillation and extraction [3]. Compared with organic membranes, zeolite membranes show high mechanical strength, thermal and chemical stability. Moreover, MFI-type zeolite membrane, especially silicalite-1 membrane, has been studied most extensively because of its welldefined pore structure (ca. 0.5 nm) and hydrophobicity, which is suitable for separation of many industrial hydrocarbon molecules and organic/water mixtures by pervaporation (PV). At present, the MFI-type membranes were mainly synthesized by in situ crystallization [4,5] and secondary-growth methods [6–8]. The secondary-growth method for preparing zeolite membranes separates the crystal nucleation and growth steps, and requires lower temperature and shorter synthesis time compared
∗ Corresponding author. Tel.: +86 571 8795 2391; fax: +86 571 8795 2391. E-mail address: [email protected] (Z. Wang). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.011
with the in situ crystallization method, thus helps to obtain a better control of the membrane microstructure [9]. Many methods have been used to coat the zeolite seeds onto the porous support, such as rub-coating [10], spin-coating [11], dip-coating [12], vacuumcoating [13], electrophoretic deposition method [14], etc. It is known that the preferred adsorption of organic molecules onto the surface of hydrophobic silicalite-1 crystals provides the primary separation selectivity for organic/water mixtures by pervaporation. However, incorporation of aluminum atom into the silicalite-1 layer would reduce its hydrophobicity and this would lead to worse organic selective separation properties. Sano et al. [15] reported that the silicalite-1 membranes synthesized on the porous sintered stainless steel disk support showed better separation selectivity of ethanol than on the porous alumina support due to aluminum leaching from the alumina support and its incorporation into the zeolite framework. However, for silicalite1 membranes synthesized on the stainless steel support, cracks formed easily during the calcination because of the thermal expansion coefficient mismatch between the steel support and the silicalite-1 layer and the adhesion of the zeolite membrane to the support was weak [16]. Kanezashi et al. [17] synthesized the MFI-type membranes on zirconia coated alumina supports by a template-free secondary growth method. They used the yttrium stabilized zirconia as the intermediate layer to reduce the incorporation of aluminum from the support into the membrane layer.
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But the zirconia coating step was complicated and made the process more expensive. Recently, Chen et al. [18] reported that the separation performance of MFI membranes synthesized on silica tubes was better than that of membranes synthesized on ␣-alumina tubes. Although a solution-filling method was adopted, the permeation flux of silicalite-1 membranes on silica tubes was low for ethanol/water separation. On the other hand, one of the main constraints of zeolite silicalite-1 membranes in ethanol/water pervaporation for industrial application is the low permeation flux, although the separation factor is reasonably high. The properties of the porous support, the crystal orientation and hydrothermal synthesis conditions are the major factors that influence the flux. At present, most membranes reported in literatures were synthesized on flat plates [6,7,15,19] or large diameter tubular supports [20–22]. The thick wall of the support leads to high resistance of the permeate transport through the support. Wang et al. [23] successfully synthesized zeolite LTA membranes on ␣-Al2 O3 hollow fibers with higher flux than on the mullite tubular supports. They stated that the lower flux of the zeolite membranes on the mullite tubular supports was due to the thicker wall of the support than ␣-Al2 O3 hollow fibers. Lai et al. [24] synthesized the b-oriented silicalite-1 membrane on porous alumina supports with a mesoporous silica intermediate layer using trimer-TPA as organic template. The as-prepared membrane showed superior performance both in the flux and the separation factor for the separation of xylene isomers mixtures. Zhao et al. [25] reported high-permeance silicalite-1 membranes with a modified secondary growth method on the macroporous ␣-Al2 O3 tubular support for the gas separation. In this paper, we synthesize silicalite-1 membranes by the secondary (seeded) growth method on ␣-Al2 O3 hollow fiber supports because the hollow fibers allow separation modules that can have higher separation surface area-to-volume ratio (>1000 m2 /m3 ) than conventional tubular supports (<500 m2 /m3 , if the outer diameter was larger than 10 mm). We demonstrate for the first time that silicalite-1 membranes on alumina hollow fiber supports have high flux for the separation of ethanol/water with a relative high separation factor by controlling the SDA amount and seed concentration. 2. Experimental 2.1. Preparation of nanocrystal seeds The silicalite-1 nanocrystal seeds used in this paper were prepared by a traditional hydrothermal synthesis following a previously reported method [26]. A clear solution was obtained by aging the synthesis solution at ambient temperature for 24 h under stirring. The molar composition of the synthesis solution was TPAOH:TEOS:H2 O = 1:2.8:40, using tetrapropylammonium hydroxide (TPAOH, Zhejiang Xianju Application Chemical Research Institute, China) as the structure-directing agent (SDA), and tetraethyl orthosilicate (TEOS, Acros Organics) as silicon source. The solution was transferred into a Teflon-lined autoclave. Then the autoclave was kept rotating at 30 rpm for 3 days in an oven at 80 ◦ C. The resulting seed crystals were purified by repeated washing with deionized water and centrifugation (12,000 rpm) until the pH value of the seed suspension was close to 8–9. The final seed suspension for the dip-coating process was obtained by dispersing the seeds in deionized water (1–10 wt.%). 2.2. Preparation of silicalite-1 membranes The silicalite-1 membranes were prepared by the seeded secondary growth onto the external surface of ␣-Al2 O3 hollow fiber supports with pore diameters of 100–200 nm. Alumina hollow
Fig. 1. SEM images of ␣-Al2 O3 hollow fiber. (a) Top view and (b) cross-sectional view.
fibers were prepared according to Refs. [27,28] by spinning a polymer slurry containing suspended aluminum oxide powder to a hollow fiber precursor, which was then sintered at elevated temperatures (e.g., 1500 ◦ C). Polyethersulfone (PES), N-methyl-2pyrrolidone (NMP), and polyvinyl pyrrolidone (PVP) were used in the polymer slurry as a polymer binder, a solvent, and an additive, respectively. Fig. 1 shows SEM images of top view and crosssectional view of the hollow fibers. The porosity of the supports was obtained by measuring the amount of absorbed water within a certain amount of time (e.g., 10 s). The hollow fibers were 1.1 mm o.d., 0.8 mm i.d. and 20–40% porosity. The supports were seeded by the dip-coating method with the above prepared silicalite-1 seed suspension prior to the membrane synthesis. The hollow fiber support was dipped into the seed suspension and kept in it for 10 s to adsorb seed crystals onto the support external surface and dried at 60 ◦ C for 1 h. The solution used for the secondary growth was prepared by mixing deionized water, TPAOH, and TEOS with the molar composition TPAOH:TEOS:H2 O = 0.12–0.32:1:165. The solution was aged at ambient temperature for 4 h under stirring. Then the seeded hollow fiber supports were placed vertically in the secondary synthesis solution in a Teflon-lined autoclave. The crystallization was carried out under autogenetic pressure in an electronically controlled convection oven at 175 ◦ C for 2–12 h. After each synthesis, the obtained membranes were rinsed with deionized water and dried at 60 ◦ C for more than 4 h. The calcination for removing the SDA (TPA+ ) occluded in the zeolite pores during synthesis was performed in
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Fig. 2. XRD pattern and particle size distribution (inset) of silicalite-1 nanocrystal seeds.
a Muffle Furnace at 500 ◦ C for 12 h with heating and cooling rates of 1.0 ◦ C/min. 2.3. Characterization The crystalline microstructure and particle size distribution of the as-prepared crystal seeds were determined by X-ray diffraction patterns (XRD, Rigaku, D/max-rA) using CuK␣ radiation and dynamic light scattering (DLS, Malvern, ZEN 3600), respectively. Microstructure and morphology of seeds and membranes were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800) and scanning electron microscopy (SEM, Hitachi TM-1000). The Al distribution in the zeolite layer was measured by energy dispersive X-ray (EDX, Horiba EX350). The surface hydrophobicity of the zeolite membranes was determined by contact angle measurement (Dataphysics, OCA 20) using sessile drop methods with a 0.1 L droplet of deionized water. Three measurements were carried out for each sample. The pervaporation experiments of ethanol/water mixtures were carried out in a laboratory scale setup which is the same as the one reported in Ref. [23]. Zeolite membranes on the hollow fiber supports (5–8 cm long) were pasted into a non-porous stainless steel tube by Torr Seal (Varian Vacuum Technologies), and then connected to the vacuum system. 5 wt.% or 3 wt.% ethanol aqueous solution in a flask of 550 mL was heated at a certain temperature and kept stirred during PV. The permeate was collected in a trap for 40–60 min. The total flux (Q) and separation factor (˛) were determined as Q = W/(A × t) and ˛ = (Yethanol/Ywater)/(Xethanol/Xwater), where W is the total weight of the permeate (kg), A is the separation area of zeolite membrane, t is the collecting time (h), and X and Y are the mass fraction of components in the feed and permeate, respectively. The weight of the permeate was measured by weighing the cold trap before and after the collection of the permeate. The mass fraction of ethanol in the feed and permeate was analyzed by gas chromatograph. 3. Results and discussion 3.1. Influences of seed concentration According to the DLS data (Fig. 2, inset), the particle size of the as-prepared seed was about 70 nm. The XRD pattern (Fig. 2) showed that the seeds were silicalite-1 zeolites with no evidence of other crystalline phase. The peak width at half maximum at the Bragg diffraction angle 2 of 23.1◦ was 0.32◦ , and the average seed size calculated using the Debye–Scherrer formula (d = 0.9/Bcos , where B is the peak width at half maximum in radians, is the Bragg
Fig. 3. SEM images of seed layers on the ␣-Al2 O3 hollow fiber supports with different seed concentration, (a) 1 wt.%, (b) 5 wt.% and (c) 10 wt.%.
diffraction angle and d is the average grain size.) is 27.4 nm, which is lower than the size from DLS, which is understandable in that DLS usually measures the hydrodynamic diameter of the particles and tend to be dominated by large particles. It is also possible that the seed crystals aggregated under the DLS measuring conditions. The average seed size from XRD also indicates that the seeds were nano-sized. Secondary growth was considered to be one of the most effective methods to obtain the MFI zeolite separation membranes and oriented thin films. For the secondary growth method, the support coated with seed crystals was subjected to the hydrothermal synthesis to obtain a continuous zeolite layer. The property of seed layer has a great influence on the membrane performance
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Table 1 Effects of seed concentration and TPAOH content on the PV performance of silicalite-1 membranes. Mem. no.
TPAOH content (x)
B1 B2 B3 B4 B5 B6 B7 B8
0.17 0.17 0.17 0.17 0.32 0.27 0.22 0.12
Seed concentration (wt.%) 1 10 5 5 5 5 5 5
Porosity (%)
Flux (kg/(m2 h))
Separation factor, ˛
21 ± 5 32 ± 5 33 ± 5 42 ± 5 34 ± 5 36 ± 5 36 ± 5 22 ± 5
0.7 ± 0.2 – 1.3 ± 0.2 2.9 ± 0.2 1.5 ± 0.2 1.1 ± 0.2 1.3 ± 0.2 0.6 ± 0.2
11 ± 2 – 44 ± 2 66 ± 2 25 ± 2 45 ± 2 66 ± 2 9±2
Note: The synthesis solution composition TPAOH:TEOS:H2 O = x:1:165; PV conditions, feed 3 wt. % ethanol, temperature 60 ◦ C.
[7]. Zhang et al. [29] prepared the silicalite-1 membranes by the hydrothermal synthesis method with different seed sizes. They indicated that the intergrowth of the silicalite-1 layer was poor with large seed size, whereas using zeolite seeds of 100 nm, the as-prepared membranes were continuous and dense. For further understanding the effect of seeding on the membrane quality, we studied the impact of seed concentration on the membrane performance for the separation selectivity of ethanol/water mixtures. The deposition of the seed layer on the outer side of the hollow fiber in this paper was made by the dip-coating method, which was a simple process. SEM images of porous ␣-Al2 O3 hollow fibers seeded with different seed concentrations are shown in Fig. 3. The configuration of seed crystals was spherical. It is clear from the surface of the seeded supports that the seed layer was denser and more uniform with increasing the seed concentration. Some bared supports could be observed clearly with the seed concentration of 1 wt.%, and many nanocrystal seeds inserted into the support pores or located at the lowland of the support surface. When the concentration increased to 5 wt.%, the support was almost covered with a dense seed layer, and only few bared spaces (circled places) could be seen. The complete coverage was achieved using the 10 wt.% seed concentration. The SEM images of the silicalite-1 membranes prepared on the supports seeded with the seed concentrations of 0 wt.%, 1 wt.%, 5 wt.%, and 10 wt.% from the same composition of the synthesis solution (TPAOH:TEOS:H2 O = 0.17:1:165) are shown in Fig. 4. Without seeds, the support was covered by randomly oriented and slice-shaped silicalite-1 crystals with size of ∼20 m after the hydrothermal synthesis at 175 ◦ C for 12 h, and some exposed support can be seen clearly from Fig. 4a with a non-uniform and non-continuous layer (Fig. 4b). For the secondary growth method, continuous zeolite layer composed of c-oriented crystals formed after synthesis for 12 h (Fig. 4d, f, and h). However, many holes can be seen clearly for 1 wt.% seed concentration from Fig. 4c. With increasing the seed concentration, the membranes on the top layer become more compact (Fig. 4e and g). The thickness of the membranes obtained by the seeded method was about 12 m (Fig. 4d, f, and h), which was much thicker than the membranes prepared by the in situ method (Fig. 4b). Table 1 shows the concentration of the seed suspension has great impact on the separation performance of as-prepared membranes (B1–B4). When the concentration was 1 wt.%, the separation factor was very low (B1). When the concentration was 10 wt.%, the as-prepared membrane was leak and could not be used for pervaporation (B2). When the concentration was 5 wt.%, the separation factor was high (B3 and B4). Membrane B4 with higher porosity of the support had much higher flux and separation factor than membrane B3. According to the literatures [30,31], the seed layer should be dense enough to cover the support surface for the preparation of MFI membranes. This is different from synthesis of zeolite LTA membranes in which a trace distribution of seed on the support surface was sufficient [23]. The support surface was incompletely
covered by seed with low concentration from Fig. 3a, and some exposed places could be seen clearly. After crystallization for 12 h, weak intergrowth could be found in the silicalite-1 layer due to the poor continuity of the seed layer (Fig. 4c), and so the performance of the as-prepared membrane would not be high. However, when the seed concentration was too high, the seed layer deposited on the support surface was too thick, leading to that the seeds close to the support could not intergrow each other after the hydrothermal synthesis (Fig. 5b). Gaps and non-zeolitic pores among the seed crystals between the dense zeolite layer and the support may cause the crack formed during the calcination of SDA (Fig. 5a), which may decrease the membrane quality. More seriously, the less compact silicalite-1 layer would detach from the support. Lovallo et al. [30] also reported the support should be covered by the seed layer to obtain high performance silicalite-1 membranes, but too thick seed layer could cause the membrane to peel or crack and thus decrease the quality of the membrane. Consequently, increasing the thickness of seed layer may have a negative impact on the membrane performance. During the hydrothermal treatment, the aluminum would leach from the ␣-Al2 O3 support [15,19,32–35], which would reduce the hydrophobicity of the silicalite-1 membranes and cause the separation selectivity of organic/water mixtures reduce. This concern was also investigated in this paper. However, we found the seeding layer could prevent the ␣-Al2 O3 support leaching effectively. Fig. 6a shows the support leaching was serious for the low coverage with 1 wt.% seed concentration, and reduced the hydrophobicity of membrane surface, so the separation factor decreased. The aluminum leaching from ␣-Al2 O3 support can be efficiently prevented if the surface was covered by a dense seed layer appropriately. With the seed concentration increasing, the support leaching reduced dramatically. When the seed concentration was 10 wt.%, the aluminum leaching could not even be seen from Fig. 6c, leading to much more hydrophobic surface of zeolite layer. This is one of the reasons why silicalite-1 membranes from 1 wt.% seed concentration showed lower separation factor. 3.2. Influences of crystallization time The evolution of the zeolite membrane microstructure during the hydrothermal synthesis was examined ex situ using SEM. Fig. 7 presents SEM images of zeolite silicalite-1 membranes synthesized on ␣-Al2 O3 hollow fiber supports with different crystallization time. Starting from a dense seed layer (5 wt.% seed concentration) consisting of 70 nm silicalite-1 seeds, a 1.5 m thick zeolite layer was deposited after 2 h of hydrothermal treatment at 175 ◦ C in a synthesis solution with the molar composition TPAOH:TEOS:H2 O = 0.17:1:165. It was clear from Fig. 7a that the hollow fiber support after the hydrothermal treatment of 2 h was almost fully covered by a zeolite layer, although many holes were still seen. The ultra-thin zeolite layer followed the contour of the support surface (Fig. 7b). The zeolite grain size in the membrane layer increased with increasing the crystallization time
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Fig. 4. SEM images of the as-synthesized membranes with different seed suspension concentration for the synthesis solution molar composition TPAOH:TEOS:H2 O = 0.17:1:165 at 175 ◦ C for 12 h. (a and b) 0 wt.%, (c and d) 1 wt.%, (e and f) 5 wt.% and (g and h) 10 wt.%. Left: top view. Right: cross-sectional view.
(Fig. 7a–c–e), and the crystal edges became more clear, the holes in the membrane surface significantly smaller. As shown in Figs. 7b, d and f, with the increase of the synthesis time from 2 h to 8 h, the membrane thickness increased from 1.5 m to 7.8 m. It can be clearly seen from Fig. 7 that the longer the crystallization time is, the larger the crystals and the better continuity of the membrane layer. So in order to obtain a dense membrane layer with high separation performance, the crystallization time of 12 h was chosen.
3.3. Influences of TPAOH content and support porosity Fig. 8 shows the XRD patterns of the silicalite-1 membranes on the ␣-Al2 O3 hollow fibers with different TPAOH content. All of the membranes after secondary growth at 175 ◦ C exhibited (h 0 h)- and oblique c-orientation. The as-prepared silicalite-1 membranes with the TPAOH content x = 0.32–0.17 showed the same or nearly the same peak heights (Fig. 8a–d), indicating that all these membranes have similar thickness. The lower peak height of the membrane
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Fig. 5. Magnified SEM images (a: top view, b: cross-sectional view) of the as-synthesized membranes with 10 wt.% seed concentration. Other conditions: same as Fig. 4.
Fig. 6. EDX images of the as-synthesized membranes with different seed suspension concentration (a) 1 wt.%, (b) 5 wt.% and (c) 10 wt.%. Other conditions: same as Fig. 4.
Fig. 7. SEM images of the as-synthesized membranes from the molar composition TPAOH:TEOS:H2 O = 0.17:1:165 at 175 ◦ C with different crystallization time. (a and b) 2 h, (c and d) 4 h and (e and f) 8 h. Left: top view. Right: cross-sectional view.
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Fig. 8. XRD patterns for the as-synthesized silicalite-1 membranes with the molar composition TPAOH:TEOS:H2 O = x:1:165 at 175 ◦ C for 12 h. (a) x = 0.32, (b) x = 0.27, (c) x = 0.22, (d) x = 0.17 and (e) x = 0.12. Peaks with index (*) are from alumina substrate.
with the TPAOH content x = 0.12 (Fig. 8e), illustrates that this membrane was thinner, which is consistent with the following SEM results. TPAOH have a dual role in this synthesis study. It served as structure directing agent in assembling the zeolite framework and also provided the alkalinity of the synthesis solution. The effect of the TPAOH amount on the membrane quality was evaluated on the basis of SEM micrographs. As shown in Fig. 9, the support surface was fully covered by silicalite-1 layers in all batches. In contrast to the synthesis of silicalite-1 particles [36], the crystal size observed from the top view increased slightly with increasing TPAOH content during the membrane synthesis. With the reduction in the amount of TPAOH (Fig. 9c, e and g), the membrane surface becomes more smooth. The silicalite-1 layer prepared with the extremely low TPAOH content (x = 0.12) consisted of crystals with poor intergrowth (Fig. 9i); grains in the polycrystalline membrane showed the shape of platelet, and many inter-crystalline pores could be seen clearly. This is due to preferential growth of the seed crystals along their c-axis and the severe suppression of nucleation at very low TPAOH leading to the growth of crystals with a high aspect ratio [37]. From the SEM images of the fractured cross-sectional views of the membranes (Fig. 9), the silicalite-1 layer grew only on the seeded surface and not inside the support cavities, and the as-synthesized membranes exhibit columnar grain microstructure, which is consistent with the literature [7]. The thickness from the cross-sectional view was about 12 m for membranes B3–B7 (Fig. 9b, d, f, h), much thicker than membrane B8 (Fig. 9j), which was approximately 9 m. Table 1 shows the effect of the TPAOH content on the pervaporation properties of silicalite-1 membranes from the composition TPAOH:TEOS:H2 O = x:1:165 on ␣-Al2 O3 hollow fibers at 175 ◦ C for 12 h (B3–B8). When the TPAOH concentration is too high (B5) and too low (B8), the separation factors were low. When the TPAOH concentration (or alkalinity) was high aluminum leached from the hollow fiber support. The leaching decreases with decreasing the TPAOH concentration, which can be seen from the EDX results (Fig. 10). The hydrophobicity of the membrane surfaces was evaluated by contact angle measurements using deionized water
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(Fig. 11). For the membranes prepared from the TPAOH/TEOS ratio (x) = 0.12, the average contact angle was found to be the highest as a value of 67◦ . The contact angle decreased to 51◦ when the TPAOH/TEOS ratio (x) was increased to 0.32. This result also indicates that the support leaching increased with the increase of TPAOH concentration, reducing the surface hydrophobicity of the silicalite-1 membrane. For the TPAOH content x = 0.12, the separation factor ˛ of membrane B8 is extremely low, e.g., ˛ = 9, which is due to the existence of the pinholes as shown in SEM image (Fig. 9i). Membranes from the moderate TPAOH concentration showed relatively high separation factor. For example, the separation factors of membranes B4 and B7 were higher than 60 at x = 0.17 and 0.22. It has been reported that the flux of zeolite LTA membranes increased almost linearly with the porosity of alumina hollow fibers [23]. The results in Table 1 also suggest that the porosity of the supports has a great impact on the flux of silicalite-1 membranes. A linear relationship between the flux of silicalite-1 membranes and the support porosity was observed in Fig. 12. The high porosity of the support reduced mass transfer resistance according to the adsorption–diffusion model leading to the flux increased [38]. However, the extremely high porosity could reduce the mechanical strength, so the appropriate porosity of the support should be available. It should be noted that some data points were far from the line, probably indicating the existence of some unknown factors. For example, the support porosity of membrane B3 and B4 was 33% and 42% in Table 1, respectively, however, the flux of membrane B4 (2.9 kg/(m2 h)) is more than twice that of membrane B3 (1.3 kg/(m2 h)). Work is ongoing in identifying the effects of some other factors on the flux of zeolite membranes, e.g., the structure of the hollow fiber and the surface porosity and so on. 3.4. Comparison with literature data Sano et al. already studied the effects of the PV temperature and the feed composition in detail [15]. And their results showed that the separation factor slightly decreased with the PV temperature and greatly with the feed ethanol concentration (>3 wt.%) and that the flux increased with the PV temperature and the feed ethanol concentration. The pervaporation performance of silicalite1 membranes (B4 and B7) for the 5 wt.% ethanol feed solution at 75 ◦ C was compared with the 3 wt.% ethanol feed solution at 60 ◦ C in Table 2. It can be seen that the separation factor decreased, but the flux increased dramatically. The separation performance depends on the sorption of the permeating molecules in the feed side and their diffusion through the membrane according to the adsorption–diffusion model. The diffusion rate increases with elevated feed temperature which leads to the flux increase. For higher ethanol concentration in the feed solution, more ethanol molecules could adsorb to the surface of the hydrophobic silicalite-1 zeolite, and then transport through the membrane, also leading to the increase in the flux. The result of the as-prepared membranes in this work is similar to that reported in literatures [15,33]. The comparison of the separation results of the membranes synthesized on the hollow fibers with that reported in literatures is presented in Table 2. Sano et al. [15] first reported that the silicalite-1 membranes synthesized on the stainless steel support preferentially permeated ethanol with separation factor of 58, but the flux was too low for the membrane because the membrane was too thick, i.e., 400–500 m by Si line analysis with EDX. Chen et al. [33] reported the synthesis of silicalite-1 membranes by solutionfilling method on the silica tubular support, and they showed that these membranes had better separation performance than the membranes synthesized on ␣-Al2 O3 tubes. They indicated that the flux of membrane synthesized by the solution-filling method could be improved by 90%, but it was still lower than 1.0 kg/(m2 h). Lin
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Fig. 9. SEM images of the as-synthesized membranes with 5 wt.% seed suspension concentration for the molar composition TPAOH:TEOS:H2 O = x:1:165 at 175 ◦ C for 12 h. (a and b) x = 0.32, (c and d) x = 0.27, (e and f) x = 0.22, (g and h) x = 0.17 and (i and j) x = 0.12. Left: top view. Right: cross-sectional view.
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Fig. 10. EDX images of the as-synthesized membranes with the molar composition TPAOH:TEOS:H2 O = x:1:165 at 175 ◦ C for 12 h. (a) x = 0.32, (b) x = 0.27, (c) x = 0.22, (d) x = 0.17 and (e) x = 0.12.
et al. [39] reported the flux of 1.81 kg/(m2 h) and 2.23 kg/(m2 h) for the silicalite-1 membranes synthesized on the ␣-Al2 O3 tubular and mullite tubular supports, respectively, and both supports had large pore diameter. These were the best separation results of silicalite-1 membranes for the ethanol/water pervaporation to date. Sebastian et al. [40] reported the flux of 1.5 kg/(m2 h) and the separation factor of 54 using the membrane synthesized on the ceramic ␣-Al2 O3 capillary by microwave-assisted heating method. The membrane thickness was about 2.5 m. They indicated that the high flux could be obtained by decreasing the membrane thickness. To the best of our knowledge, the silicalite-1 membranes synthesized in this study on the outside of ␣-Al2 O3 hollow fiber supports showed the highest flux for ethanol/water mixtures separation (5.4 kg/(m2 h)) by a simple secondary growth. The transport mechanism of ethanol/water mixtures by pervaporation through zeolite membrane can be described by the adsorption–diffusion model [38]. Based on this model, each component of the permeating molecules is adsorbed to the inlet of the zeolite micropores and diffuses through the zeolite pores due to the concentration gradient. Thus, the thickness of the membrane and support may affect the transport rate of the permeates to some extent. The thickness of the hollow fiber support used in this paper was about one order of magnitude lower than other supports reported in the literature [33,40], leading to decreased resistance for permeate transport through the support and thus to higher fluxes.
Fig. 11. Contact angle results of the as-synthesized membranes on hollow fibers as a function of TPAOH concentration (x).
Table 2 Comparison of PV separation results of as-prepared membranes with those in literatures. Support
Membrane thickness (m)
Pervaporation conditions ◦
Hollow fiber Hollow fiber Hollow fiber Hollow fiber Stainless steel ␣-Al2 O3 tube Silica tube ␣-Al2 O3 tubular Mullite tubular ␣-Al2 O3 capillary
12 12 12 12 400–500 50 30 Not mentioned Not mentioned 2.5
Note: Feed, ethanol aqueous solution.
Temp ( C)
Feed (wt.%)
75 60 75 60 60 80 60 60 60 65
5 3 5 3 5 3 3 5 5 5
Flux (kg/(m2 h))
Separation factor, ˛
Ref.
2.7 ± 0.2 1.3 ± 0.2 5.4 ± 0.2 2.9 ± 0.2 0.76 0.41 0.87 1.81 2.23 1.50
57 ± 2 66 ± 2 54 ± 2 66 ± 2 58 33 69 89 62 54
B7 B7 B4 B4 [15] [33] [33] [39] [39] [40]
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Fig. 12. Relationship between the flux of MFI membrane and the porosity of alumina hollow fiber support.
4. Conclusions Polycrystalline silicalite-1 membranes on the ␣-Al2 O3 hollow fibers were successfully synthesized by the secondary growth method. It was found that the seed concentration and TPAOH content showed great influences on the membrane formation and its separation performance. When TPAOH/TEOS molar ratio was 0.12, the zeolite crystal on the membrane surface showed platelet shape, and the separation performance was poor. However, the support leaching was serious for the high TPAOH content (TPAOH/TEOS>0.32), and caused the membrane to become less hydrophobic. A continuous and dense zeolite seed layer is necessary to obtain a silicalite-1 membrane with good quality. However, 10 wt.% seed concentration led to a thick seed layer and caused the membrane to peel or crack. The membrane from the synthesis solution with TPAOH/TEOS = 0.17 and the seed suspension of 5 wt.% concentration showed the flux of 2.9, 5.4 kg/(m2 h) and the separation factor of 66, 54 for 3 wt.% ethanol feed at 60 ◦ C and 5 wt.% ethanol feed at 75 ◦ C, respectively. These fluxes are the highest fluxes to date with moderate separation factors for the pervaporation separation of ethanol/water mixtures. Acknowledgements We thank the National Natural Science Foundation of China (20876133 and 21028002), Zhejiang Nature Science Foundation (ZJNSF) (R4090099) and Science and Technology Department of Zhejiang Province (2009R50020) for financial supports. Y.Y. thanks the Chinese Ministry of Education for the Visiting Changjiang Scholar Professorship. References [1] L. Gora, J.C. Jansen, Hydroisomerization of C-6 with a zeolite membrane reactor, J. Catal. 230 (2005) 269–281. [2] J. Caro, M. Noack, Zeolite membranes – recent developments and progress, Micropor. Mesopor. Mater. 115 (2008) 215–233. [3] T.C. Bowen, R.D. Noble, J.L. Falconer, Fundamentals and applications of pervaporation through zeolite membranes, J. Membr. Sci. 245 (2004) 1–33. [4] Z.B. Wang, Y.S. Yan, Oriented zeolite MFI monolayer films on metal substrates by in situ crystallization, Micropor. Mesopor. Mater. 48 (2001) 229–238. [5] X.M. Li, Y.S. Yan, Z.B. Wang, Continuity control of b-oriented MFI zeolite films by microwave synthesis, Ind. Eng. Chem. Res. 49 (2010) 5933–5938.
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