ARTICLE pubs.acs.org/IECR
Influences of Seeds on the Properties of Zeolite NaA Membranes on Alumina Hollow Fibers Jia Shao,† Qinqin Ge,† Lijun Shan,† Zhengbao Wang,*,† and Yushan Yan†,‡ † ‡
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P.R. China Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521, United States ABSTRACT: The influences of size (90 1500 nm) and amount (1 4 wt %) of as-synthesized seed crystals on the properties of zeolite NaA membranes on alumina hollow fibers are investigated. Seeds and membranes are characterized by dynamic light scattering, X-ray diffraction, and scanning electron microscope. It is found that in order to prepare dense NaA membranes with high separation performance, for seeds smaller than 100 and 200 nm, the optimal concentrations are 1 and 3 wt %, respectively; while for large-sized seeds (>1500 nm), membranes obtained using the seed suspension with 4 wt % seed concentration still have defects. Furthermore, the influence of crystal fragments (∼50 nm) obtained by ball-milling available crystals (e.g., 1500 nm) on the formation of zeolite membrane is also studied. Based on the morphology and performance of membranes grown from different seeds, seed-assisted membrane formation process is briefly discussed.
1. INTRODUCTION Among inorganic membranes, zeolite membranes, considering their molecular sieving properties, uniform pore size, high thermal stability, chemical inertness, and high mechanical strength, constitute a promising technology in separation processes, and also as catalytic membranes, sensors, electrodes, and optoelectronic devices.1 Hydrophilic zeolite membranes, for example, zeolite NaA with high Al content and small pore size (4.1 Å), have been found to be suitable for dehydrating water/ organic mixtures by pervaporation.2 Despite the successful commercialization of supported NaA membranes (Mitsui Engineering & Shipbuilding Co. Ltd.3), the fundamental understanding of membrane formation remains limited.4 Most of the zeolite NaA membranes have been prepared by the seeded growth method using either a milky-like gel or a clear solution on the surface of a porous support.5 9 Just a few have been synthesized by in situ hydrothermal synthesis.10,11 The aim of the seeded growth method is to separate the crystal nucleation and growth steps, and obtain a better control of the membrane microstructure.12 The seeding process on the supports has been recognized as a crucial factor to obtain high-quality zeolite membranes.13,14 Many seeding methods have been reported, such as dip-coating,6 rub-coating,15 spin-coating,16 vacuum method,17 and cross-flow filtration.18 The size and the concentration of seed crystals in the seed suspension are also very important. Pera-Titus et al.18 successfully synthesized NaA membranes on the inner side of R-Al2O3 supports (pore diameter: 1.9 μm) by means of a cross-flow filtration seeding technique, using seeds of 2 or 7 μm, and found that the optimal seeding weight gain was around 0.40 mg/cm2 of the membrane area. Pera-Titus et al.15 also reported that NaA membranes could be synthesized in a flow system by the action of gravity on the inner side of porous titania asymmetric tubular supports previously brush-seeded with zeolite NaA crystals (2 μm) by rotating the zeolite-loaded brush along the tube axis. Huang et al.17 prepared zeolite NaA r 2011 American Chemical Society
membranes on tubular R-Al2O3 supports (pore diameter: 1.0 2.0 μm) by a vacuum seeding method. The effects of the seed particle size (300, 1200, and 3000 nm), suspension concentration, coating pressure difference, and coating time on the formation of the membrane and its pervaporation properties were investigated. Results showed that high-quality zeolite NaA membranes could be reproducibly prepared by the seeded growth method with vacuum seeding under the following conditions: seed particle size of 1200 nm, suspension concentration of 0.7 wt %, coating pressure difference of 0.015 MPa, and coating time of 90 s. Compared with other reported seeding methods for zeolite NaA membrane synthesis, the dip-coating technique is economically advantageous and easy to be scaled-up, therefore, it constitutes the most widely used seeding techniques. Sato et al.19 employed R-Al2O3 tubular support (pore diameter: 1.3 μm) and NaA seed crystals (smaller than 1000 nm) to study the effect of the seed concentration (0.5 2.0 wt %) on the membrane separation performance. The seed crystals for the aqueous suspension were prepared by wet-milling of zeolite NaA powder from Mizusawa Industrial Chemicals Co. Ltd. It was concluded that the seed concentration of 0.5 wt % was the optimum condition, while both 0.1 and 2.0 wt % of seed suspension led to zeolite membranes with poor quality. We are interested in preparing zeolite membranes on hollow fiber supports because they have higher surface/volume ratio and thinner wall than tubular supports. Xu et al.20 explored the synthesis of zeolite NaA membranes on ceramic hollow fibers without seeding. To improve the quality of zeolite membranes, however, a multicycle synthesis (repeated synthesis) was adopted. Recently, we reported that a novel seeding method, dipcoating wiping, is key to obtaining zeolite membranes with Received: April 6, 2011 Accepted: July 11, 2011 Revised: July 1, 2011 Published: July 11, 2011 9718
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Table 1. Synthesis Conditions and Average Particle Size of Seeds molar composition seed
Na2O
crystallization
(TMA)2O
Al2O3
SiO2
H2O
aging timec (h)
temp. (K)
time (h)
average particle sized (nm)
ref
a
S1
0.1
3.59
0.5
1
170
2.5
333
24.0
90
22
S2a
0.1
3.59
0.5
1
170
2.5
353
6.0
120
22
S3a
0.1
3.59
0.5
1
170
2.5
353
9.0
160
22
S4a
0.1
3.59
0.5
1
170
2.5
353
13.0
190
22
S5a
0.044
1.6
0.2
1
80
3.5
373
6.5
260
23
S6b
2.9
0
0.48
1
98
24.0
353
3.0
800
-
S7
purchased from Mizusawa Industrial Chemicals Co. Ltd.
1500
-
a Silica source: colloidal silica; alumina source: aluminum isopropoxide. b Silica source: sodium metasilicate nonahydrate; alumina source: sodium aluminate. c Aging temperature: 298 K. d Measured by DLS.
high separation performance on the alumina hollow fiber support.21 To understand the function of wiping and improve the seeding method of dip-coating, the effects of the seed size and amount on the properties of zeolite membranes on hollow fibers should be clarified. However, available seeding methods and influences of seeds are mostly related to the tubular supports as summarized in the above. In this study, zeolite NaA membranes are synthesized on alumina hollow fibers by the seeded growth method using the dip-coating process, and the influences of seed size and the concentration of seed suspension on membrane morphology and separation performance of ethanol/water solution are evaluated. The optimum size and concentration of seed crystals for the synthesis of defect-free membranes is presented. The effects of crystal fragments obtained by ball-milling are also studied. By comparing membrane growth morphology, finally, seed-assisted membrane formation processes for small and large seed crystals are discussed respectively.
2. EXPERIMENTAL SECTION 2.1. Preparation of Seed Crystals. Zeolite NaA seeds S1 S4 and S5 were prepared by a method described in refs 22 and 23, respectively. For the synthesis of seeds S1 S5, aluminum isopropoxide, tetramethylammonium hydroxide (TMAOH), and silica sol were added into an aqueous solution of NaOH. Seed S6 was synthesized without the organic structure-directing agent (e.g., TMAOH) using sodium metasilicate nonahydrate as silica source and sodium aluminate as alumina source. Seed S7 was purchased from Mizusawa Industrial Chemicals Co. Ltd. in Japan. The detailed synthesis conditions are shown in Table 1. After the synthesis of seeds S1 S6, particles were recovered by repeatedly washing with deionized water and centrifugal separation, until the pH value of the seed suspensions became close to 8 9. The concentration of NaA crystals S1 S6 in aqueous suspensions used in the dip-coating process was in the range of 1 4 wt %. The aqueous suspension of Seed S0 was prepared by ballmilling of S7 powder followed by removing big crystals in water with centrifugation. Ball milling of zeolite NaA crystals was performed by means of a planetary ball mill (Nanjing Kexi Instrument Institute, XQM-04 L). The aluminum oxide grinding jars were arranged eccentrically on the sun wheel of the planetary ball mill. The direction of movement of the sun wheel was opposite to that of grinding jars (ratio: 1: 2), and changed every 5 min. The milling balls were made of aluminum oxide.
Seventy-five balls with 5 mm diameter, 15 balls with 10 mm diameter, and ∼10 g of zeolite sample were placed in a 100-mL grinding jar and ground for 1 h with the rotation speed of 600 rpm. Zeolitic phase identification and relative crystallinity of seeds was determined by X-ray diffraction (XRD, Rigaku, D/max-rA) using Cu KR radiation, and morphology was observed using a scanning electron microscopy (FE-SEM, Hitachi, S-4800) at 5 kV acceleration voltage. The particle size distributions were measured by dynamic light scattering (DLS, Malvern, ZEN 3600). 2.2. Seeding Process of Supports. Alumina hollow fibers (1.2 mm o.d. and 0.9 mm i.d.) with pore diameter of 100 200 nm and length of ∼5 cm were used as supports. One side of the supports was closed by Torr Seal (Varian Vacuum Technologies); another side was connected to a Teflon tube. The experimental procedure used for seeding was as follows. Aqueous suspensions of seed crystals were prepared by dispersing seed crystals into water and keeping in an ultrasonic bath for 30 min. The hollow fiber support was dipped into NaA seed suspension and kept in it for 10 s to adsorb seed crystals onto the support external surface, then was dried at 333 K for 1 h. This dipcoating step was repeated again to obtain a uniform distribution of seed particles on the support surface. Morphology of the supports before and after seeding was observed using a scanning electron microscope (FE-SEM, Carl Zeiss, Ultra 55) at 3 kV acceleration voltage. 2.3. Synthesis of Zeolite Membrane. The zeolite NaA membranes were prepared by seeded hydrothermal synthesis on the external surface of alumina hollow fiber supports. The milky-like hydrogel was prepared by mixing an aluminate solution and a silicate solution. The aluminate solution was prepared by dissolving 3.0 g of sodium aluminate (Wako Pure Chemical Industries Ltd., Japan, molar ratio of Al/NaOH = 0.81) in 11.0 g of deionized water. The silicate solution was prepared by mixing 7.8 g of sodium metasilicate nonahydrate (Wako) and 19.9 g of deionized water. To produce clear and homogeneous solutions, the silicate solution was vigorously stirred at 323 K for 15 min and the aluminate solution at ambient temperature for 15 min before mixing. Then, the mixture was obtained by slow addition of the aluminate to the silicate solution under continuous stirring. After aging for 30 min at ambient temperature, the precursor hydrogel was prepared. The molar ratio of this hydrogel was 3.4:1.0:2.0:155.0 Na2O/Al2O3/SiO2/H2O. Synthesis hydrogel (40 g) was carefully poured into a Teflonlined stainless steel autoclave (50-mL). The seeded support was 9719
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Figure 1. SEM images of seed particles with different size: (a) S1; (b) S5; (c) S6; (d) S7.
placed vertically in the autoclave to ensure the complete immersion of the support and partial exposure of the Teflon tube, and then the autoclave was sealed. The crystallization was carried out under autogenetic pressure in an electrically controlled convection oven at 373 K for 4 h. After each synthesis, the obtained membranes were rinsed with deionized water and dried at 333 K for more than 4 h. The as-synthesized membranes were designated as M-s-c, where s stands for size of the seed (nm) and c is the concentration of seed suspension (wt %). The structure of the as-synthesized membrane was determined by XRD (Rigaku, D/max-rA) operating at a tube voltage of 40 kV using Cu KR radiation. The step-size was 0.0167° and 2θ ranged from 5° to 50°. Before analysis, hollow fiber supports (1.2 mm o.d.) with the zeolite membrane layer were lined in parallel, and formed a square of 2 2 cm. Microstructure and morphology of growth layers were examined using scanning electronic microscopy (SEM, Hitachi TM-1000) with an accelerating voltage of 15 kV. The pervaporation experiments of 90 wt % ethanol aqueous solution were carried out in a laboratory scale setup at 348 K.21 Flux (kg/m2 3 h) was measured by weighing the cold trap before and after the pervaporation experiment. The water/ethanol separation factor (R) of the membrane was calculated as the proportion between the ratios of the weight fractions of water and alcohol at the permeation and feed sides.
3. RESULTS AND DISCUSSION 3.1. Seed Synthesis and Characterization. Seeds with various particle sizes were obtained by changing synthesis conditions. Table 1 lists the synthesis conditions and average particle sizes of seed crystals. Preparing nanosized zeolite crystals requires enough nucleation sites in the hydrogel. Many parameters can be adjusted, such as increasing the content of the organic structure-directing agent (SDA), extending the aging time, lowering the crystallization temperature, and so on. Here, the content of the SDA (TMAOH) was enhanced in the synthesis of zeolite nanocrystals S1 S4. A lower crystallization temperature (333 K) was used to obtain a smaller crystal size (90 nm) for the synthesis of seed S1. For seeds S2 S4, the
Figure 2. Particle size distribution of seed crystals: (a) S1; (b) S5; (c) S6; (d) S7.
increase in the crystal size can be ascribed to the prolongation of the crystallization time from 6 to 13 h. Less content of the organic SDA and higher crystallization temperature accomplished the synthesis of seed S5 (260 nm). Brar et al.24 showed the particle size distribution for zeolite NaA crystals synthesized with and without aging for 4 h at room temperature that mean particle diameters were centered at 2 and 3 μm, respectively, while the former had a narrower particle size distribution. It is known that structure rearrangement occurs during the aging of the synthesis solution leading to the formation of zeolite nuclei. The prolongation of the aging time results in the decrease in the size of aggregates formed during the structure rearrangement and the increase of the nuclei number. Therefore, considering the adverse effects of organic template on environment and cost, 24 h at room temperature without the SDA was chosen as the aging condition for seed S6 (800 nm). Seed S7, NaA zeolite powder purchased from Mizusawa Industrial Chemicals Co. Ltd., has the average particle size of 1500 nm. The SEM images recorded from representative seeds S1, S5, S6, and S7 are shown in Figure 1. Differences were observed in the size, shape, and morphology of the crystals. Seed S1 (Figure 1a) was uniform in size and all round in shape. Seed S5 (Figure 1b) had a relatively broad particle size distribution 9720
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Industrial & Engineering Chemistry Research (200 400 nm), and cubic crystals with round corners were the main fraction of the product. As shown in Figure 1c and d, there were many irregularly shaped entities attached to the surface of the cubic zeolite crystals (S6 and S7). That is, crystals tend to intergrow and conglobe, which is further described in Section 3.2. Because the distribution of seeds deposited on the support is an important factor in the quality control of zeolite membranes, the particle size of the seeds is investigated by dynamic light scattering (DLS). Figure 2 shows that the mean particle diameters were centered at 90, 260, 800, and 1500 nm for seeds S1, S5, S6, and S7, respectively. Seed S1 (Curve a) had narrow particle size variation (70 120 nm). As the content of the organic SDA fell to zero, the mean particle sizes of the resulting products increased from 260 nm (Curve b) to 800 nm (Curve c) with wide particle size distributions. The DLS results of zeolite NaA crystals were consistent with the SEM results in Figure 1. The XRD patterns of seeds S1, S5, S6, and S7 are shown in Figure 3. The position of the Bragg reflections is identical to LTA type crystals, indicating that only zeolite A crystals are grown in the reacting systems. Based on the peak intensity (Pattern a),
Figure 3. XRD patterns of the seeds: (a) S1; (b) S5; (c) S6; (d) S7.
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it appears that, for 90-nm-sized seeds, though the organic SDA is used, its crystallinity was relatively lower than others. 3.2. Effects of As-Synthesized Seeds on Membrane Formation. Taking into consideration the simplicity and practicality, a seeding method of dip-coating was used in this study. Many factors of the dip-coating method influence the final state of the seed crystals on the support surface, including the size of seed crystals, the concentration and solvent of the seed suspension, the properties of the support (the porosity and the thickness of the wall) and the operating factors, such as the immersion duration and the pulling rate. Here, we only studied the influences of the seed size and the seed suspension concentration by fixing other factors. SEM images of hollow fiber supports before and after seeding are shown in Figure 4. Figure 4d shows the top view of R-Al2O3 hollow fibers, having relatively smooth surface with alumina particles of 200 300 nm. Figure 4a and b show that, 90 nmsized seeds, smaller than alumina particles, were distributed throughout the lowlands, and alumina particles still partly exposed as highlands, preventing the loss of seeds in the dipping and pulling process. After the hydrothermal synthesis, the micrometer-level membranes were thick enough to cover the exposed alumina particles. But 1500-nm-sized seeds (Figure 4c), larger than alumina particles, formed a layer of their own upon the support, resulting in low adhesion strength and relatively nonuniform distribution. It is learned that the thicknesses of these membranes synthesized using different seed size and concentration are quite similar, indicating that the seed size and concentration have no significant effects on the membrane thickness under the synthesis conditions of this work (data not shown). Figure 5 shows SEM images of the top and cross-sectional view of the as-synthesized NaA membranes on alumina hollow fiber supports. From the top view, it is known that zeolite membranes with seeds of different particle sizes show distinct morphology. For membrane M-902.5 (seed 90 nm, 2.5 wt %) (Figure 5a), a clean multicrystal layer was composed of small crystals with cusps. For membrane
Figure 4. SEM top view images of hollow fiber supports after (a c) and before seeding (d). (a and b) 90 nm, 2.5 wt %; (c) 1500 nm, 4.0 wt %. 9721
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Figure 5. SEM images of the as-synthesized membranes with different seed sizes. (a and b) M-90-2.5; (c and d) M-260-4.0; (e and f) M-800-4.0; (g and h) M-1500-4.0. Left: top view; right: cross-sectional view.
M-260-4.0 (Figure 5c), crystals appeared to show double-edge structure, like the combination of sheet and cube. In a word, for membrane M-90-2.5 and M-260-4.0, an intergrown layer constituted by a randomly oriented distribution of crystals can be distinguished. It can be supposed that, under the induction of seeds, a layer derived from the dense accumulation of small crystals rapidly forms on the support surface. Cubic crystals, thought to be generated from the synthesis solution, inserted into the top of the dense layer. Because there was no space for them to insert in, they just fell on the dense layer. For membrane M-8004.0 (Figure 5e), crystals with volatile sizes and shapes constituted a relatively nonuniform membrane layer, while for membrane M-1500-4.0 (Figure 5g), the basic construction unit was the conglomeration of crystals. The probable reason for the formation of crystal agglomerates is that, in the early crystallization period, initial nuclei generated from hydrogel clung to the large
seed, and then both of them grew up along their respective crystal growth paths. Finally, the crystal agglomerates, full of corners, are obtained. Loose deposit of seed crystals results in insertion of cubic crystals and thus gaps. Although, in this study, the zeolite membranes were prepared with seeds of different particle size, their thicknesses were all around 3 μm (Figure 5b, 5d, 5f, and 5h). The cross-sectional view in Figure 5 shows that the demarcation line between the membrane layer and R-Al2O3 support is distinct, and the penetration of zeolite layer into support pores is not visible. That is, even if seed crystals are only 90-nm-sized, smaller than the average pore diameter of supports, due to the twists and turns of passageway, they did not penetrate into the support pores. Figure 6 presents the XRD patterns of membranes synthesized at 373 K for 4 h with seeds from 90 to 1500 nm. All membrane layers are composed of NaA crystals without any preferred 9722
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Figure 6. XRD patterns for as-synthesized zeolite NaA membrane layers. Peaks with index are from zeolite NaA and peaks with * are from R-alumina substrate: (a) M-90-2.5; (b) M-260-4.0; (c) M-800-4.0; and (d) M-1500-4.0.
orientation. The results are obtained by directly analyzing the outer surface of the membrane and each membrane sample used for analysis is limited in its amount of thin zeolite layer. In the range of 2θ used, alumina support has four significant reflections with stars (*): (012), (104), (110) and (113).25 The alumina lines are easily distinguished from the zeolite A pattern, indicating a very thin zeolite film, and the intensity remains unchanged from membrane M-260-4.0 to M-1500-4.0, which might be ascribed to the similar thickness of membranes synthesized with different seed size. The intensities of all peaks in membrane M-90-2.5 were lower than those in other membranes, which may be due to incomplete signal collection because of the small diameter of the hollow fiber. 3.3. Influence of As-Synthesized Seeds on Pervaporation Performance. The quality of the as-synthesized zeolite NaA membranes was evaluated by pervaporation. The results of the pervaporation tests in the dehydration of 90 wt % ethanol aqueous solution at 348 K are summarized in Figure 7. For 2 4 wt % seed concentrations, the separation factors decreased with the increase of the seed size from 90 to 1500 nm (Figure 7a). According to blank columns, separation factors of membranes synthesized with 90-, 120-, 160-, and 190-nm-sized seeds (2 3 wt %) were more than 10 000, while the separation factor of the membrane synthesized with 260-nm-sized seeds (4 wt %) was close to 2000, indicating a small amount of defects. For 800- and 1500-nm-sized seeds, separation factors even with the seed concentration of 4 wt % were still lower than 1000. According to shadow columns, when the seed concentration is as low as 1 wt %, only the separation factor of as-synthesized membrane from 90-nm-sized seeds was still higher than 10000 with high reproducibility, while others were lower than 5000. Especially, for 800and 1500-nm-sized seeds, separation factors are even below 10. In this case, there are so many defects in the membrane layers that fluxes were as high as 20 kg/m2 3 h (Figure 7b). In other words, to prepare dense zeolite NaA membranes, for seeds smaller than 100 nm, the seed suspension with 1 wt % concentration could provide enough seed crystals; for seeds between 100 and 200 nm, the corresponding value is 2 3 wt %. With the help of the theoretic calculation, we could infer the corresponding concentration for large size seeds to grow up into defect-free NaA membranes. Taking 1500 and 90 nm as examples, under the assumption that the optimum coverage areas of 1500- and 90-nm
Figure 7. Correlation of the separation factor (a) and the flux (b) with the particle size and concentration of seeds (blank columns: 2 4 wt %; shadow columns: 1 wt %).
seeds are equal, the concentration of 1500-nm-sized seeds is 1500/90 (≈16) times as much as that of 90-nm-sized seeds (1 wt %). That is, the optimum concentration value for the former should be as high as 16 wt %, which is in an unreasonable range for a homogeneous seed suspension. It should be noted that it is the coverage of seeds on the support surface, not the concentration of seed suspension that plays a key role in improving the membrane performance. The porosity of R-Al2O3 hollow fibers used in the study was 30% 40%. If the supports with higher porosity were used, which have great ability to adsorb much water and thus more seed crystals in suspension, the suitable concentration should be adjusted accordingly. Furthermore, the results obtained above were based on hollow fibers, and may have no application to the supports with bigger pores than 1 μm and rough surfaces, such as mullite tubes used by Mitsui Engineering and Shipbuilding Co. Ltd. It was reported that17 the small seed particles easily penetrated into the support pores, resulting in higher permeation resistance. However, as shown in Figure 7b, no significant correlation between the flux and the seed size was found. We have reported that the flux of zeolite membranes on hollow fibers increased with their porosity.21 Therefore, the fluctuation of the flux (5 7 kg/m2 3 h) in Figure 7b is attributable to the difference of the support porosity. 3.4. Effects of Ball-Milled Seeds. As concluded above, zeolite membranes with high separation performance can be obtained on alumina hollow fibers using crystals smaller than 90 nm as seeds. However, in order to synthesize the nanosized zeolite NaA particles, organic structure-directing agents, for instance, TMAOH, are usually needed, making the synthesis of colloidal zeolites a very expensive and environmentally unfriendly process. Moreover, the crystalline yield of nanosized zeolites is typically low.22 Therefore, crystal fragments, the size of which is close to small seeds, were obtained by ball-milling and centrifugal separation, and the effects of fragments on the properties of zeolite membranes is discussed below. 9723
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Figure 8. SEM images and DLS data of ball-milled seed fragments (a and b) and zeolite membrane on alumina hollow fibers seeded by dip-coating of ball-milled seeds (c and d); (c) top view; (d) cross-sectional view.
Figure 9. Pervaporation performance of the zeolite NaA membranes synthesized from ball-milled seeds (∼50 nm) in pervaporation at 348 K as a function of seed concentration. Numbers in brackets are the support porosity.
Figure 8 presents SEM images and DLS data of seed fragments and corresponding membranes. As shown in Figure 8a, seed fragments showed irregular shape, without inherent cubic structure of LTA crystals. The DLS data (Figure 8b) show that the mean particle diameter was centered at 50 nm. The DLS curve was asymmetric and trailing, because the crystals with the size of 90 150 nm could not be completely removed by centrifugal separation. Figure 8c and d, analogous to Figure 5a and b, shows SEM images of the top and cross-sectional view of membranes synthesized with 50-nm-sized crystal fragments (0.6 wt %).
Crystals with cusps were linked into layers, on which a small amount of cubic crystals generated from hydrogel fell. The thickness of the membranes is ∼2.5 μm. Figure 9 shows the influence of the concentration of ballmilled seed crystals (50 nm) on the pervaporation performance of as-synthesized membranes. Three samples on the supports with different porosity (Figure 9b, number in brackets) were prepared for the same seed concentration. Initially, a rise in the seed concentration leads to an increase of the separation factor (Figure 9a). When the concentration reaches a specific value, the separation factors maintain more than 10 000. That is, in order to ensure uniform and adequate coverage of seeds, the amount of seeds implanted in the support by dip-coating has to exceed a certain limit. The lowest seed concentration is ∼0.1 wt %. During the crystallization process, seed particles are gradually growing up and crowding into layers. The flux of the as-synthesized membrane had no significant correlation with the seed concentration (Figure 9b). It increased with the porosity of the support as reported in our previous paper.21 3.5. Time-Dependent Membrane Formation. Despite the successful commercialization of supported NaA membrane, the fundamental understanding of membrane formation remains limited, and the production of NaA membranes with high performance still seems to rely on a synthesize-and-test process to find suitable membrane systems under given synthesis conditions.4 The formation process of zeolite NaA membranes is discussed as follows. Figure 10 presents SEM images of zeolite NaA membranes synthesized on hollow fiber supports with different crystallization time using 50- and 1500-nm-sized seeds. When big crystals (1500 nm) were used as seeds, there were many uncovered places at reaction time of 1 2 h (Figure 10a1 c1), though their area reduced gradually with the reaction time. On the contrary, when small crystals (50 nm) were used as seeds, there existed only some small gaps on the surface of the membrane layer at the reaction time of 1 h (Figure 10a2), some pinholes at 1.5 h (Figure 10b2), and almost no pinholes at 2 h (Figure 10c2). 9724
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Figure 10. SEM top view images of zeolite NaA membranes with different crystallization time: (a) 1 h; (b) 1.5 h; (c) 2 h. (a1 c1) 1500-nm-sized seeds; (a2 c2) 50-nm-sized seeds.
According to the formation process of zeolite membranes and the crystallization mechanism of zeolite NaA, a growth model is proposed to clarify the effect of surface seeding on the formation of zeolite NaA membranes. In the aging stage (room temperature), the precursor species in the synthesis mixture, consisting of alkali ions, aluminate, silicate and aluminosilicate monomers, dimers and oligomers, rearrange into amorphous gel particles. Meanwhile, the zeolite synthesis solution is composed of an amorphous gel phase and a solution phase, even if there is no clear demarcation between them visually. In the crystallization (heating) stage, the following membrane formation process emerges from our observation. As can be seen from Figure 10a1, crystal nuclei from the synthesis hydrogel are aggregated around precoated seed crystals on the support surface, leading to framboid-like morphology crystals. On the exposed support surface covered with no seed crystals, just single crystal nucleus settle down, thus only isolated crystals form initially. Therefore, it is difficult to obtain continuous layers on these places. The inadequate coverage of seed crystals leads to the embedding of less-intergrown cubic crystals and defects (Figures 5g and 10c1). That is the reason why the high density of seeds is helpful for the formation of high-performance membranes. For small-sized seeds, seed crystals are relatively well-distributed on the surface of hollow fiber supports after dipcoating (Figure 4a). Initial nuclei generated from hydrogel surround the precoated seeds everywhere and grow up together
with them, leading to a relatively continuous layer at the early crystallization stage (Figure 10a2). That is why it is difficult to find cubic crystals of complete shape in the dense layer (Figures 5a and 10c2). The case of the dipcoating wiping method21 previously reported is similar to that of small-sized seeds, just much smaller seeds produced and uniformly dispersed on the support surface after wiping. Actually, results similar to Figure 10a2 c2 were also obtained when the dipcoating wiping method was used (data not shown). Possible reasons why small-sized seed crystals are suitable for preparing dense zeolite membranes are summarized as follows: (i) Sedimentation of seed crystals. For large seed particles, sedimentation in the gravity field affects the vertical uniformity of the suspension; 1500-nm-sized particles will settle down in 5 min, while 90-nm-sized suspensions can remain homogeneous for a couple of weeks. (ii) Adhesion of seed crystals. During seeding process, when the support is being pulled away from the suspension, large particles are likely to be scraped off from the support. In contrast, even the space between R-Al2O3 granules can be counted as a “pit” for small seeds. Therefore, small seed crystals can settle down into these pits. (iii) Coverage of seed crystals. In the case of similar amount and uniform dispersion of seed particles, surface coverage of large seeds on the support must be much less than that of small ones. The augmentation of nonseed region increases the workload of crystallization, bringing about the generation of intercrystalline pores. (iv) Growth 9725
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Industrial & Engineering Chemistry Research rate of seeds. For small seeds, large specific surface area means adequate contact area with nutrient solution. Therefore, compared with large ones, much more crystal nuclei surround small seed crystals uniformly dispersed on the support surface at the early crystallization stage, leading finally to a dense zeolite layer.
4. CONCLUSIONS Compared with large-sized seeds, zeolite NaA membranes prepared from as-synthesized small-sized seeds (less than 100 nm) on hollow fiber supports are apt to possess high separation factor and reproducibility. For seeds smaller than 100 and 200 nm, the optimal concentration is 1 and 3 wt %, respectively; while for large-sized seeds (>260 nm), membranes corresponding to 4 wt % seed concentration still have defects. It is easier for small seeds to uniformly cover the surface of the support with a low concentration while large seeds need unreasonably large concentration to get the same coverage. Initial nuclei generated from hydrogel surround the precoated small seeds everywhere and grow up together with them, leading to a relatively continuous layer at the early crystallization stage. For the large seeds, the inadequate coverage of seed crystals leads to the embedding of less intergrown cubic crystals and defects. Small-sized seeds can also be obtained by ball-milling instead of hydrothermal synthesis using organic SDA, and seed suspension of ball-milled crystals (>0.1 wt %) is suitable to obtain zeolite dehydration membranes with high separation factor (>10 000). ’ AUTHOR INFORMATION Corresponding Author
*Tel/Fax: +86-571-8795-2391. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (20876133, 21028002), Science and Technology Department of Zhejiang Province (2009R50020), and Qianjiang Rencai of Zhejiang Province (2008R10016) for financial support. Y.Y. thanks the Chinese Ministry of Education for the Visiting Changjiang Scholar Professorship. ’ REFERENCES (1) Coronas, J.; Santamaria, J. The Use of Zeolite Films in SmallScale and Micro-Scale Applications. Chem. Eng. Sci. 2004, 59, 4879–4885. (2) Caro, J.; Noack, M. Zeolite Membranes - Recent Developments and Progress. Microporous Mesoporous Mater. 2008, 115, 215–233. (3) 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, 25, 251–260. (4) Zah, J.; Krieg, H. M.; Breytenbach, J. C. Layer Development and Growth History of Polycrystalline Zeolite A Membranes Synthesised from a Clear Solution. Microporous Mesoporous Mater. 2006, 93, 141–150. (5) 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, 163–175. (6) Xu, X. C.; Yang, W. S.; Liu, J.; Lin, L. W. Synthesis of NaA Zeolite Membranes from Clear Solution. Microporous Mesoporous Mater. 2001, 43, 299–311. (7) Ge, Q. Q.; Wang, Z. B.; Yan, Y. S. High-Performance Zeolite NaA Membranes on Polymer-Zeolite Composite Hollow Fiber Supports. J. Am. Chem. Soc. 2009, 131, 17056–17057.
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(8) Ahn, H.; Lee, H.; Lee, S. B.; Lee, Y. Pervaporation of an Aqueous Ethanol Solution through Hydrophilic Zeolite Membranes. Desalination 2006, 193, 244–251. (9) Pina, M. P.; Arruebo, M.; Felipe, A.; Fleta, F.; Bernal, M. P.; Coronas, J.; Menendez, M.; Santamaria, J. A Semi-Continuous Method for the Synthesis of NaA Zeolite Membranes on Tubular Supports. J. Membr. Sci. 2004, 244, 141–150. (10) Han, Y.; Ma, H.; Qui, S. L.; Xiao, F. S. Preparation of Zeolite A Membranes by Microwave Heating. Microporous Mesoporous Mater. 1999, 30, 321–326. (11) Li, Y. S.; Chen, H. L.; Liu, J.; Yang, W. S. Microwave Synthesis of LTA Zeolite Membranes without Seeding. J. Membr. Sci. 2006, 277, 230–239. (12) Bernal, M. P.; Coronas, J.; Menendez, M.; Santamaria, J. On the Effect of Morphological Features on the Properties of MFI Zeolite Membranes. Microporous Mesoporous Mater. 2003, 60, 99–110. (13) Xu, X. C.; Bao, Y.; Song, C. S.; Yang, W. S.; Liu, J.; Lin, L. W. Synthesis, Characterization and Single Gas Permeation Properties of NaA Zeolite Membrane. J. Membr. Sci. 2005, 249, 51–64. (14) Huang, A. S.; Yang, W. S. Hydrothermal Synthesis of NaA Zeolite Membrane Together with Microwave Heating and Conventional Heating. Mater. Lett. 2007, 61, 5129–5132. (15) Pera-Titus, M.; Bausach, M.; Llorens, J.; Cunill, F. Preparation of Inner-Side Tubular Zeolite NaA Membranes in a Continuous Flow System. Sep. Purif. Technol. 2008, 59, 141–150. (16) Mintova, S.; Valtchev, V.; Petkov, N.; Jean, B.; Metzger, T. H.; Bein, T. Structural Properties of LTA Films Assembled from Aluminosilicate Clear Solutions and Dense Gels: A Gid X-Ray Study. Stud. Surf. Sci. Catal. 2004, 154, 717–724. (17) Huang, A. S.; Lin, Y. S.; Yang, W. S. Synthesis and Properties of A-Type Zeolite Membranes by Secondary Growth Method with Vacuum Seeding. J. Membr. Sci. 2004, 245, 41–51. (18) Pera-Titus, M.; Llorens, J.; Cunill, F.; Mallada, R.; Santamaria, J. Preparation of Zeolite NaA Membranes on the Inner Side of Tubular Supports by Means of a Controlled Seeding Technique. Catal. Today 2005, 104, 281–287. (19) Sato, K.; Nakane, T. A High Reproducible Fabrication Method for Industrial Production of High Flux NaA Zeolite Membrane. J. Membr. Sci. 2007, 301, 151–161. (20) Xu, X. C.; Yang, W. S.; Liu, J.; Lin, L. W.; Stroh, N.; Brunner, H. Synthesis of NaA Zeolite Membrane on a Ceramic Hollow Fiber. J. Membr. Sci. 2004, 229, 81–85. (21) Wang, Z. B.; Ge, Q. Q.; Shao, J.; Yan, Y. S. High Performance Zeolite LTA Pervaporation Membranes on Ceramic Hollow Fibers by Dipcoating-Wiping Seed Deposition. J. Am. Chem. Soc. 2009, 131, 6910–6911. (22) Larlus, O.; Mintova, S.; Bein, T. Environmental Syntheses of Nanosized Zeolites with High Yield and Monomodal Particle Size Distribution. Microporous Mesoporous Mater. 2006, 96, 405–412. (23) Hedlund, J.; Schoeman, B.; Sterte, J. Ultrathin Oriented Zeolite LTA Films. Chem. Commun. 1997, 1193–1194. (24) Brar, T.; France, P.; Smirniotis, P. G. Control of Crystal Size and Distribution of Zeolite A. Ind. Eng. Chem. Res. 2001, 40, 1133–1139. (25) Sand, L. B.; Sacco, A.; Thompson, R. W.; Dixon, A. G. Large Zeolite Crystals - Their Potential Growth in Space. Zeolites 1987, 7, 387–392.
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