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Hydrothermal synthesis of uniform and dense NaA zeolite membrane in the electric field
Microporous and Mesoporous Materials 102 (2007) 58–69 www.elsevier.com/locate/micromeso
Hydrothermal synthesis of uniform and dense NaA zeolite membrane in the electric field Aisheng Huang b
a,b,*
, Weishen Yang
b,*
a Department of Chemistry, Tongji University, Shanghai 200092, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Received 30 August 2006; received in revised form 4 November 2006; accepted 5 December 2006 Available online 1 February 2007
Abstract Uniform and dense NaA zeolite membranes were prepared on the tubular porous a-Al2O3 supports in the electric field, where the charged zeolite particles attracted and transported to the support surface for membrane preparation homogeneously. The membranes properties were characterized by XRD, SEM and pervaporation for dehydration of 95 wt% isopropanol/water mixture at 343 K, respectively. The applied potential had great effect on membrane morphology, membrane thickness and separation performance. Under the action of the applied electric field, the negatively charged zeolite particles could migrate to the support surface homogenously and rapidly, facilitating to form uniform and dense membranes in a short time. Most zeolite particles transported to the support surface for membrane formation in the electric field, which increased the percentage of the product contained in the support surface and thus fully utilized the zeolite particles. High-quality NaA zeolite membrane, i.e., with a separation factor (water/isopropanol) of 3281 and a flux of 1.24 kg/(m2 h), could be prepared with the electrical potential of 1.0 V. Ó 2006 Elsevier Inc. All rights reserved. Keywords: NaA zeolite membrane; Hydrothermal synthesis; Electrophoresis; Pervaporation
1. Introduction Because of their potential applications as separators, reactors and sensors, zeolite membranes have been attracted much interest in the last two decades [1,2]. For the mentioned applications, the zeolite membrane should have high selectivity and high flux at the same time. Zeolite membranes have been usually prepared by in situ hydrothermal synthesis method, i.e., the porous support is immersed into the synthesis solution, and then the membrane is formed by direct crystallization [3–6]. Because the physico-chemical properties of support have a strong effect on the membrane formation [7–9], quality of the assynthesized membrane considerably depends on the char* Corresponding authors. Tel.: +86 21 65981180; fax: +86 21 65982287 (A. Huang), Tel.: +86 411 84379073 (W. Yang). E-mail addresses: [email protected] (A. Huang), [email protected] (W. Yang).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.12.005
acter of the support surface. Since preparation of highquality zeolite membranes by a direct in situ hydrothermal synthesis is encountered with certain difficulties [10], various methods, such as secondary growth method [11–17] and microwave synthesis [18] have been developed to improve the quality of membrane. Zeolite membranes were conventionally synthesized by static hydrothermal process. Two problems can be encountered during the synthesis of zeolite membranes by conventional static hydrothermal process: (a) a rapid depletion of nutrients concentration in the boundary layer, caused by the growth process [19]; and (b) the negative influence of gravitation, e.g., deposition of the zeolite crystals formed in solution onto the growing membrane [20,21]. The first problem can be solved by a continuous supplying of nutrients system. On the other hand, the negative influence of gravitation can be evaded by applying dynamic synthesis conditions, e.g., rotation of reaction vessel, or circulation of the precursor solution inside the
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
reaction vessel. This also can help in the preventing of formation of a stationary layer on the crystal-solution interface, and thus in the supplying the crystal surface with nutrients. Tiscaren˜o-Lechuga et al. [20] synthesized NaA zeolite membranes under a centrifugal force field. They reported that the centrifugal forces produced by high rotational speed drove the crystal nuclei towards the support surface, promoting formation of more continuous and denser layer. Yamazaki et al. [21] compared the formation of NaA zeolite membranes by both static synthesis and a method with circulation of the precursor solution. For the static method, they reported formation of by-products such as gmelinite, chabazite and faujasite along with zeolite A, while when the circulated solution system was used, only zeolite A was detected. Richter et al. [22] synthesized MFI type membranes on the inner surfaces of alumina tubes and capillaries in a continuous system. In this study, a fresh synthesis gel was supplied to the autoclave continuously from a reservoir. C ¸ ulfaz et al. [23] recently synthesized MFI type membranes in a continuous system with circulation of the synthesis solution, while Pina et al. [24] synthesized NaA zeolite membranes using a semi-continuous system in which fresh gel was periodically supplied to the synthesis vessel. It is expect that additional supply of fresh gel provide better control of the synthesis and crystallization conditions. As indicated in the above report, it is evident that migration of zeolite particles (nuclei, crystals) plays an important role in the formation of zeolite membrane. Electrophoretic deposition (EPD) is an effective technique for preparation of thin films [25,26]. In the EPD process, charged particles migrate to the support surface homogenously and rapidly under the action of the applied electric field, resulting in forming uniform, dense and thickness-controllable membranes in short time. The charged zeolite particles, successively produced during the hydrothermal synthesis, and migrate onto the support surface under the action of an applied electric field. There are two types application of electrophoretic technique for preparation of zeolite membrane. One was used for coating the support surface with zeolite seed crystals to prepare seeding layer before hydrothermal treatment. Mintova et al. [27], Berenguer-Murcia et al. [28] and Seike et al. [29] used electrophoretic technique to coat the support surface with zeolite seed crystals for preparing ZSM-5, Silicate-1 and Y-type zeolite membrane, respectively. It is indicated that electrophoretic technique is available for depositing a uniform, continuous and orderly seeding layer on the support surface, consequently improving the zeolite membrane quality. Another application was used to drive zeolite particles migrate to the support surface for membrane preparation during hydrothermal synthesis. Oonkhanond et al. [30] prepared ZSM-5 membranes on the tubular a-Al2O3 supports by electrophoretic technique, investigating the influence of the precursor concentration, applied potential and synthesis time on membrane properties. They reported that the electrophoretic technique was
59
effective for preparing continuous membranes on the support surface in shorter time. Mohammadi et al. [31] suc-
DC power
Rubber plug Autoclave
Thermocouple
α-Al2O3 support Synthesis solution Oven Stainless steel rod
Fig. 1. Synthesis apparatus for membrane preparation on tubular alumina support in the electric field.
6 3 6
7
2 4 1
5
Fig. 2. Experimental apparatus for pervaporation. (1) Water bath, (2) liquid tank, (3) liquid pump, (4) membrane model, (5) cold traps, (6) ball value, (7) vacuum pump.
Table 1 Some details of the synthesis conditions and properties of the NaA zeolite membranes prepared in this work Membranes
Potentials (V)
Concentrationa (%)
T (K)
Time (h)
Membranes thickness (lm)
M01 M02 M03 M04 M05 M06 M07 M08 M09 M10 M11 M12 M13 M14 M15 M16 M17
0 0.5 1.0 1.5 0 1.0 1.0 1.0 1 0 0 0 0 1.0 1.0 1.0 1.0
100 100 100 100 50 50 33.3 25.0 100 100 100 100 100 100 100 100 100
363 363 363 363 363 363 363 363 363 363 363 363 363 363 363 363 363
6 6 6 6 6 6 6 6 6 4 8 12 20 2 4 6 8
12 5 7 10 5 4 2.5 / / 8 10 12 10 2.5 4 8 10
a
Relative concentration to the original synthesis solution.
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A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
cessfully prepared zeolite A membrane from kaolin by the electrophoretic technique, farther showing application prospect of the electrophoretic technique in membrane preparation owing to better controlling homogeneous nucleation and crystallization conditions.
(a)
We recently reported preparation of A-type zeolite membranes on nonporous metal supports by using electrophoretic technique [32]. To our knowledge, no studies have been reported for preparation of NaA zeolite membranes on the tubular a-Al2O3 supports in the electric field until
(b)
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(c)
Cross-section
(d)
Cross-section
Top view
(e)
(f)
Cross-section
Top view
(g)
(h)
Top view
Cross-section
Fig. 3. SEM images of membrane: (a,b) M01: 0 V, 100% concentration, (c,d) M02: 0.5 V, 100% concentration, (e,f) M03: 1.0 V, 100% concentration and (g,h) M04: 1.5 V, 100% concentration.
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now. In this paper, tubular NaA zeolite membranes are prepared on the porous a-Al2O3 supports by using electrophoretic technique. The effects of the electrical potential on the membrane formation and pervaporation properties are investigated. The effects of electrical potential on the percentage of the product contained in the support surface and the rate of membrane formation are also investigated. Based on the observed results, the formation mechanism of zeolite membrane in the electric field is discussed.
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the autoclave and the autoclave was sealed. After crystallizing 1–24 h at 363 K with different potentials, the solution was decanted off and the membrane was washed with deionized water, and then dried. In all cases, the zeolite product in the solution was filtered, washed and then dried to weight. The amount of zeolite product contained in the support surface was measured by the weight gain of the support after crystallization. 2.2. Characterization of zeolite membrane
2.1. Synthesis of zeolite membrane Porous a-Al2O3 tubes (home-made: 11 mm in outer diameter, 7 mm in inner diameter, 120 mm in length, 0.5– 1.0 lm pore radius, about 40% porosity) were used as supports. The surface of the support was polished with 700 grit-sand papers, and then was cleaned with deionized water in a Branson SB2200 ultrasonic cleaner three times to remove loose particles created during polishing. Before hydrothermal synthesis, the cleaned support was calcined in air at 673 K for 3 h. A DC power supply (WYJ-T303, Buxin Sensor Factory, China) was used to generate electric field for NaA zeolite membrane synthesis. The solution for synthesizing NaA zeolite membrane was prepared according to the procedure reported previously [17]. The aluminate solution was prepared by dissolving sodium hydroxide (AR, Shenyang Reagent Factory, China) in deionized water, then adding aluminum foil (AR 99.8%, Beijing Hongxin Chemical Factory, China) to the solution at room temperature. The silicate solution was prepared by dilution of silica sol (27 wt% SiO2, q 1.16 g/l, Qingdao Haiyang Chemical Factory, China) with deionized water with vigorous stirring at 333 K. After 10 min of stirring, preheated (333 K) aluminate solution was added into the stirred silicate solution. The stirring was continued for 15 min to produce a clear and homogeneous aluminosilicate solution having molar composition of 50Na2O:Al2O3:5SiO2:1000H2O. For preparation diluted synthesis solution, the above experiments were carried out only correspondingly increasing the water quantity. The diluted synthesis solution with composition of 50Na2O: Al2O3:5SiO2:2000H2O, 50Na2O:Al2O3:5SiO2:3000H2O and 50Na2O:Al2O3:5SiO2:4000H2O was used for membrane preparation, respectively. The synthesis unit is illustrated schematically in Fig. 1. As shown in Fig. 1, the bottom end of the a-Al2O3 support was fixed with a Teflon holder. The middle of the support tube was filled with a stainless steel rod (about 6 mm in diameter), which was used as an anode. The support was placed vertically in stainless steel autoclave (30 mm in inner diameter), and the stainless steel autoclave was used as a cathode. The DC power supply was used to generate potential between the two electrodes. The electrical potential varied from 0 to 2.0 V. The synthesis solution was poured into
The structure of the as-synthesized membranes was confirmed by X-ray diffraction (XRD) patterns. XRD was carried out on a Ragaku D max/rB power diffractometer using CuKa radiation operating at 40 K and 50 mA. The morphology and thickness of the as-synthesized membranes were examined by scanning electron microscopy (SEM). The SEM photographs were obtained on a JEM-1200E scanning electron microscopy. All the samples used for XRD and SEM test were selected middle position of the support tube. The properties of the as-synthesized zeolite membranes were also evaluated by pervaporation for dehydration of 95 wt% isopropanol/water mixture at 343 K. The apparatus used for the pervaporation experiments is illustrated schematically in Fig. 2. The 95 wt% isopropanol/water mixtures, preheated to 343 K, were fed to the out side of the zeolite membrane in the membrane model that was heated in a thermostatic water bath. The inside of the membrane was evacuated with a vacuum pump. The cold traps with liquid N2 cooling were used to collect the permeate. The compositions of the feed and the permeate were analyzed by gas chromatogram (HP5890). The most important variables for pervaporation are the selectivity and the flux. The total flux (J), the component flux (Ji) and the separation factor (a) are defined as respectively:
(b)
Intensity
2. Experimental
(a)
10
20
30
40
50
2θ/degrees Fig. 4. XRD pattern of the NaA zeolite membrane: (a) M01 and (b) M03 (j) A-type zeolite, (m) X-type zeolite, () P- type zeolite, (d) a-Al2O3.
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j¼
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W ; DtA
J i ¼ J vi;p ;
ai;j ¼
vi;p vj;f vi;f vi;n
3. Results and discussion
where W is total weight of the permeate (kg), Dt is collecting time (h), A is separation area of the membrane, xi,p is the weight fraction of species i in the permeate and xi,f is the weight fraction of species i in the feed.
(a)
3.1. Membrane preparation in the electric field During the formation of zeolite membrane, the force acting on the zeolite particle has important influence to
(b)
Cross-section
Top view
(c)
(d)
Top view
Cross-section
(f)
(e)
Cross-section
Top view
(g)
(h)
Top view
Cross-section
Fig. 5. SEM images of membrane: (a,b) M06: 0 V, 50% concentration, (c,d) M07: 1.0 V, 50% concentration, (e,f) M08: 1.0 V, 33.3% concentration and (g,h) M09: 1.0 V, 25% concentration.
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the membrane formation [20,32,33]. Table 1 summarizes some details of the synthesis conditions and properties of the NaA zeolite membranes prepared in this work. Fig. 3 shows the SEM images of the as-synthesized NaA zeolite membrane after 6 h hydrothermal synthesis at 363 K with different electrical potential. As shown in Fig. 3 and Table 1, the applied electrical potential had great effect on membrane morphology and thickness. At 0 V of the electrical potential (where M01), the size of the crystals was nonhomogeneous, with crystals size of about 3–10 lm. Judged by the cross-section of the as-synthesized zeolite membrane, the zeolite membrane was very rough and loose with thickness of about 12 lm. Moreover, spherical crystals as well as octahedral crystals were formed among A-type zeolite crystals, which indicated that A-type zeolite crystals had transformed to other crystals. As reported previously [10,21], the A-type zeolite crystals easily transform to other crystals by hydrothermal synthesis. Fig. 4 shows the XRD patterns of the as-synthesized zeolite membrane M01 and M03, respectively. As shown in Fig. 4a, the XRD patterns of the zeolite membrane M01 farther approve of A-type zeolite crystals transformed to other crystals. In the static hydrothermal synthesis, the zeolite particles successively produced in the solution are deposited by gravitation, and also diffused by hydrodynamics. The gravitation force has a negative influence on the membrane formation because it makes the zeolite particles precipitate [34]. Most zeolite particles have precipitated on the bottom of the autoclave before forming dense membrane. There were abundant of zeolite particles for membrane formation on the bottom parts, and conversely there were deficient zeolite particles for membrane formation on the upper part. Consequently, on one hand, the bottom parts formed too much thick NaA zeolite membrane, and on the other hand, dense membrane could not form on the upper part before other crystals produced. These may be the reason why it is difficult to prepare high-quality NaA zeolite membrane by the conventional hydrothermal synthesis method. As it can be seen in Fig. 3, with increasing of the electrical potentials, the zeolite crystals became uniform and the
(a)
packaging density of the zeolite membrane increased. With increasing the applied electrical potential to 0.5 V (where M02), the a-Al2O3 support surface was fully covered with homogeneous A-type zeolite crystals, and the membrane thickness was about 5 lm. Further increasing the electrical potential to 1.0 V (where M03), the support surface was completely covered with uniform and compact cubic A-type zeolite crystals, and about 7 lm dense zeolite membrane was formed. It was found that it was effective to restrain NaA zeolite from transforming to other crystals prepared in the electric field. As shown in Fig. 4b, the XRD patterns of membrane M03 indicated that high purity A-type zeolite layer was formed on support surface. One possible explanation is that zeolite particles can rapidly immigrate to the support surface under the action of electric field, thus (a) decreasing the time of zeolite particles staying in the active synthesis solution, and (b) producing more nucleus centers on the support surface, which are benefit to restrain from forming impure crystalline phase. When the electrical potential increased to 1.5 V, the properties of the as-synthesized zeolite membrane M04 did not change much (uniform, smooth and dense consistently), although the thickness of the membrane increased to about 10 lm, as shown in Fig. 3g and h. Fig. 5 shows the SEM images of the as-synthesized NaA zeolite membrane prepared by using diluted synthesis solution at 363 K. When 50% concentration solution was used, the zeolite membrane M05, prepared by conventional hydrothermal synthesis, was uncontinuous with thickness of about 5 lm, and there were obvious gap on the support surface. As the electrical potential increased to 1.0 V under the same synthesis concentrations, the coverage of the zeolite membrane M06 increased, and continuous membrane with thickness of about 4 lm could be obtained. When more diluted synthesis solution was used, thinner membrane could be formed (i.e., M07), and even no membrane (i.e., M08) was formed when 25% concentration solution was used. There were not enough nutrients for zeolite formation by using diluted synthesis solution, and thus decreasing the rate of nucleation and crystal growth. Therefore, it is not easy to form continuous zeolite
(b)
Top view
63
Cross-section
Fig. 6. SEM images of membrane M05 prepared by electrophoretic technique with 1.0 V electrical potential at 100% concentration.
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membrane during conventional hydrothermal synthesis, while more nutrients could be supplied to the growth interface at a sufficiently high rate under the action of the applied electric field, facilitating to form continuous zeolite membrane.
(a)
In order to further understand the effect of electric field on membrane formation, the electrode polarity was reversed, i.e., the stainless steel rod was used as the cathode and the stainless steel autoclave was used as the anode. Fig. 6 shows the SEM images of the as-synthesized zeolite
(b)
Cross-section
Top view
(c)
(d)
Cross-section
Top view
(e)
(f)
Cross-section
Top view
(g)
(h)
Top view
Cross-section
Fig. 7. SEM images of membrane prepared with 0 V and 100% concentration at different crystallization time: (a,b) M10: 4 h, (c,d) M11: 8 h, (e,f) M12: 12 h and (g,h) M13: 20 h.
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
membrane M09 after 6 h crystallization at 363 K with potential of 1.0 V. As shown in Fig. 6, it can be seen, there was no zeolite membrane formed on support surface, and only a few separated crystals formed. Results further indicated that the applied potential had great effect on the membrane formation.
(a)
65
3.2. Effect of applied potential on rate of the membrane formation Fig. 7 shows the SEM images of the as-synthesized NaA zeolite membrane prepared by conventional hydrothermal synthesis at different crystallization time. As shown in
(b)
Cross-section
Top view
(c)
(d)
Cross-section
Top view
(e)
(f)
Cross-section
Top view
(g)
(h)
Top view
Cross-section
Fig. 8. SEM images of membrane prepared with 1 V and 100% concentration at different crystallization time: (a,b) M14: 2 h, (c,d) M15: 4 h, (e,f) M16: 6 h and (g,h) M17: 8 h.
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A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
surface, and the membrane M13 dissolved and transformed to other crystals. Fig. 8 shows the SEM images of the as-synthesized NaA zeolite membrane prepared by hydrothermal synthesis in the electric field at different crystallization time. After 2 h crystallization, the support surface was covered by zeolite particles, with about 2.5 lm in thickness, but there were many voids obviously. As crystallization time increased to 4 h, the particles grew and formed membrane, and the thickness of the zeolite membrane M15 increased to about 4 lm. As crystallization time increased to 6 h, a dense zeolite membrane M16 could be formed, with about 8 lm in thickness. As crystallization time increased to 8 h, the zeolite membrane produce crack as shown in Fig. 8.
80
Percentage of zeolite crystal (100%)
70 zeolite crystals in the support surface 60 50 40 30 20 10 0.0
0.5
1.0
1.5
2.0
Potentials (V) Fig. 9. The percentage of zeolite crystals in the support surface as a function of the electrical potentials assuming a 100% crystallization of the zeolite synthesis bath.
3.3. Effect of applied potential on percent of the product contained in the support surface
Fig. 7, after 4 h crystallization, no continuous film formed, and only scattered crystals existed on the support surface. As crystallization time increased to 8 h, the coverage increased but still no continuous membrane formed. Moreover, as revealed before, spherical crystals as well as octahedral crystals were formed after 8 h crystallization. As crystallization time increased (even increased to 20 h), there were still no continuous membranes formed on the support
After hydrothermal synthesis, the zeolite product in the solution was filtered, washed and dried to weight. The amount of zeolite crystals in the support surface was measured by the weight gain of the support after crystallization. The total zeolite amounts were defined to the sum of the zeolite crystals product in the solution and in the support surface. Fig. 9 shows that percentage of zeolite crystals contained in the support surfaces increases linearly
a
Autoclave
Support Stainless steel rod
-
-
--
-
-
FC
--
-- --
-
-
--Migrate slowly and randomly - - -- - - -
--
-
-
FG FR
--
--
--
Charged particles
Long formation time
Zeolite membrane
b
-
-
-
-
-
FC+FE
FG
FR
Migrate rapidly and orientedly
-
-
(–)
-
-
-
-
Short formation time
(+) FC: capillary force, FE: electric field force, FG: gravitation force, FR: resultant force
Fig. 10. Comparisons of zeolite membrane prepared on alumina support surface by hydrothermal synthesis: (a) without electric field and (b) with electric field.
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
with increasing electric potential. Only 20.4% of zeolite is contained in the support in the absence of electric potential (conventional synthesis), while more than 50% of zeolite is contained in the support when the electric potential of 2.0 V was applied. This result can easily be explained by the fact that influence of gravitational settling of the formed zeolite crystals decrease with increasing electric potential, as it is schematically presented in Fig. 10. The zeolite crystals formed in the absence of electric field migrate to the support surface randomly, under the common influence of gravitational sedimentation and diffusion (Fig. 10a). Under such conditions, a large number of zeolite crystals formed in the solution are attracted and transported toward the bottom of autoclave before a dense membrane can be formed on the support surface. Consequently, a membrane of inappropriate characteristics can be obtained: the upper part of the membrane is tenuous and insufficiently thick, while the bottom part of the membrane is too thick which can cause the formation of cracks. In the cases when electric field is applied during hydrothermal synthesis, the negative influence of gravitation can be reduced to same degree and the charged zeolite crystals formed in the solution rapidly migrate toward the support surface under the action of the electric field, which results in a rapid formation of uniform and dense zeolite membrane (Fig. 10b). 3.4. Water–isopropanol separation performance NaA zeolite membrane, with low ratio of silicon to aluminum (Si/Al = 1), has high affinity for water in nature, which has been reported to be extremely effective for dehydration of alcohol/water mixtures by pervaporation [6,17]. Table 2 shows the Water/isopropanol separation performance of the as-synthesized NaA zeolite membrane prepared with different applied potential. As shown in Table
67
2, when the applied potential is 0 V, there was no dense membrane formed on the support surface and A-type zeolite crystals transformed to other crystals, which resulted in poor separation performance of the as-synthesized membrane. The separation factor (water/isopropanol) is only 107. With increasing the applied electrical potential to 0.5 V, the separation performance improved. The separation factor increases to 916 and the flux is 1.48 kg/(m2 h). When the applied potential is 1.0 V, the support surface was completely covered by A-type zeolite, and the separation performance improved continuously. The separation factor increases to 3281 and the flux is 1.24 kg/(m2 h). When the applied potential increases to 1.5 V, the density of the zeolite membrane increased, resulting in the separation selectivity increased (the separation factor increased to 9481). Many experiments indicated high-quality NaA zeolite membranes could be easily prepared in the electric field. At 343 K, the separation factor (water/isopropanol) of the NaA zeolite membranes were 1000–5000 for 95 wt% isopropanol/water mixture, and the fluxes are 1.0–1.5 kg/ m2 h, respectively. The reproducibility of the membrane preparation was satisfactory. Table 2 also compares the separation performances of the zeolite membranes prepared in this study with literature data. While pervaporation results from different laboratories were often difficult to compare because the different experimental conditions was used, it can be said that the as-synthesized zeolite membranes prepared by hydrothermal synthesis in the electric field were among those with good separation performance. 4. Conclusions Uniform and dense NaA zeolite membrane could be easily prepared on the a-Al2O3 support by hydrothermal
Table 2 Comparisons of the pervaporation properties of the as-synthesized zeolite membrane prepared in this study with literature data Membranes
Mixtures (A/B)
T (K)
Concentration (A wt%)
aA/B
Flux (kg/m2 h)
References
Silicalite Silicalite NaA NaA NaA NaX NaA NaA NaA NaA Mordenite NaY NaY M01 M02 M03 M04
EtOH/H2O EtOH/H2O H2O/EtOH H2O/EtOH H2O/i-PrOH H2O/EtOH H2O/EtOH H2O/EtOH H2O/DMF H2O/i-PrOH H2O/i-PrOH MeOH/MTBE EtOH/benzene H2O/i-PrOH H2O/i-PrOH H2O/i-PrOH H2O/i-PrOH
303 303 348 393 343 348 348 348 333 343 348 323 333 343 343 343 343
5 4.65 10 10 5 10 10 10 5 10 10 10 10 5 5 5 5
63 64 42000 47000 10000 400 23000 10000 200–500 4500 3360 7000 930 107 916 3281 9481
0.6 0.6 2.08 8.37 1.67 0.5 2.5 2.15 2.65 0.5 0.1 1.02 0.22 1.69 1.48 1.24 1.02
[5] [6] [7] [7] [17] [22] [22] [35] [36] [37] [38] [39] [39] This This This This
study study study study
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A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
synthesis in the electric field. The applied potential had great effect on membrane morphology, membrane thickness, separation performance and the percentage of the zeolite product contained in the support surface. Under the action of an applied electric field, the charged zeolite particles attracted and transported to the support surface for membrane preparation homogeneously and rapidly, facilitating to form uniform and dense membranes in shorter time. Most zeolite particles transported to the support surface for membrane formation in the electric field, which increased the percentage of the product contained in the support surface. High-quality NaA zeolite membrane, i.e., with a separation factor (water/isopropanol) of 3281 and a flux of 1.24 kg/(m2 h), could be prepared with the electrical potential of 1.0 V. The reproducibility of the membrane preparation in the electric field was satisfactory. Acknowledgments This work was supported by the National Advanced Materials Committee of China (2003AA328010), the National Science Foundation of China (20607015) and Program for Young Excellent Talents in Tongji University (2006KJ057). References [1] J.C. Jansen, J.H. Koegler, H. van Bekkum, H.P.A. Calis, C.M. van den Bleek, F. Kapteijn, J.A. Moulijn, E.R. Geus, N. van der Puil, Zeolite coating and their potential use in catalysis, Micropor. Mesopor. Mater. 21 (1998) 213. [2] J. Caro, M. Noack, P. Ko¨lsch, R. Scha¨fer, Zeolite membranes-state of their development and perspective, Micropor. Mesopor. Mater. 38 (2000) 3. [3] Y. Yan, M.E. Davis, G.R. Gavalas, Preparation of zeolite ZSM-5 membranes by in-situ crystallization on a-Al2O3 porous, Ind. Eng. Chem. Res. 34 (1995) 1652. [4] J. Sterte, S. Mintova, G. Zhang, B.J. Schoeman, Thin molecular sieve films on noble metal substrates, Zeolites 18 (1997) 387. [5] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami, K. Haraya, Separation of ethanol/water mixture by silicate membrane on pervaporation, J. Membr. Sci. 95 (1994) 221. [6] M. Kondo, M. Komori, H. Kita, K. Okamoto, Tubular-type pervaporation module with zeolite NaA membrane, J. Membr. Sci. 133 (1997) 133. [7] V. Valtchev, S. Mintova, The effect of the metal substrate composition on the crystallization of zeolite coatings, Zeolites 15 (1995) 171. [8] J.L.H. Chau, C. Tellez, K.L. Yeung, K. Ho, The role of surface chemistry in zeolite membrane formation, J. Membr. Sci. 164 (2000) 257. [9] R. Lai, Y.S. Yan, G.R. Gavalas, Growth of ZSM-5 films on alumina and other surfaces, Micropor. Mesopor. Mater. 37 (2000) 9. [10] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Synthesis of NaA zeolite membranes from clear solution, Micropor. Mesopor. Mater. 43 (2001) 299. [11] M.C. Lovallo, M. Tsapatsis, Preferentially oriented submicron silicalite membranes, AICHE J. 42 (1996) 3020. [12] R. Lai, G.R. Gavalas, Surface seeding in ZSM-5 membrane preparation, Ind. Eng. Chem. Res. 37 (1998) 4275.
[13] L.C. Boudreau, J.A. Kuck, M. Tsapatsis, Deposition of oriented zeolite A film: in situ and secondary growth, J. Membr. Sci. 152 (1999) 41. [14] K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Formation of a Y-type zeolite membrane on a porous a-alumina tube for gas separation, Ind. Eng. Chem. Res. 36 (1997) 649. [15] V. Engstro¨m, B. Mihailova, J. Hedlund, A. Holmgren, J. Sterte, The effect of seed size on the growth of silicalite-1 films on gold surfaces, Micropor. Mesopor. Mater. 38 (2000) 51. [16] D. Coutinho, K.J. Balkus Jr., Preparation and characterization of zeolite X membranes via pulsed-laser deposition, Micropor. Mesopor. Mater. 52 (2002) 79. [17] A.S. Huang, Y.S. Lin, W.S. Yang, Synthesis and properties of A-type zeolite membranes by secondary growth method with vacuum seeding, J. Membr. Sci. 245 (2004) 41. [18] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Synthesis of a high-permeance NaA zeolite membrane by microwave heating, Adv. Mater. 12 (2000) 195. [19] M. Noack, P. Ko¨lsch, R. Schafer, P. Toussaint, J. Caro, Molecular sieve membranes for industrial application: problems, progress, solutions, Chem. Eng. Technol. 25 (2002) 221. [20] F. Tiscaren˜o-Lechuga, C. Te´llez, M. Mene´ndez, J. Santamarı´a, A novel device for preparing zeolite-A membranes under a centrifugal force field, J. Membr. Sci. 212 (2003) 135. [21] S. Yamazaki, K. Tsutsumi, Synthesis of A-type zeolite membrane using a plate heater and its formation mechanism, Micropor. Mesopor. Mater. 37 (2000) 67. [22] H. Richter, I. Voigt, G. Fischer, P. Puhlfur´ß, Preparation of zeolite membranes on the inner surface of ceramic tubes and capillaries, Sep. Purif. Technol. 32 (2003) 133. [23] P.Z. C ¸ ulfaz, A. C ¸ ulfaz, H. Kalıpc¸ılar, Preparation of MFI type zeolite membranes in a flow system with circulation of the synthesis solution, Micropor. Mesopor. Mater. 92 (2006) 134. [24] M.P. Pina, M. Arruebo, M. Felipe, F. Fleta, M.P. Bernal, J. Coronas, M. Mene´ndez, J. Santamarı´a, A semi-continuous method for the synthesis of NaA zeolite membranes on tubular supports, J. Membr. Sci. 244 (2004) 141. [25] Y. Kaneko, S. Mori, J. Yamanaka, Synthesis of electrochromic praseodymium-doped vanadium oxide films by molten salt electrolysis, Solid State Ionics 151 (2002) 35. [26] E.M. Wong, P.C. Searson, ZnO quantum particle thin films fabricated by electrophoretic deposition, Appl. Phys. Lett. 74 (1999) 2939. [27] S. Mintova, J. Hedlund, V. Valchev, B.J. Schoeman, J. Sterte, ZSM-5 films prepared from template free precursors, J. Mater. Chem. 10 (1998) 2217. ´ . Berenguer-Murcia, E. Morallo´n, D. Cazorla-Amoro´s, A ´ . Linares[28] A Solano, Preparation of thin silicate-1 layers on carbon materials by electrochemical methods, Micropor. Mesopor. Mater. 66 (2003) 331. [29] T. Seike, M. Matsuda, M. Miyake, Fabrication of Y-type zeolite films by electrophoretic deposition, Solid State Ionics 151 (2002) 123. [30] B. Oonkahanond, M.E. Mullins, The preparation and analysis of zeolite ZSM-5 membranes on porous alumina supports, J. Membr. Sci. 194 (2001) 3. [31] T. Mohammadi, A. Pak, Making zeolite A membrane from kaolin by electrophoresis, Micropor. Mesopor. Mater. 56 (2002) 81. [32] A.S. Huang, J. Liu, Y.S. Li, Y.S. Lin, W.S. Yang, Preparation of Atype zeolite membrane on nonporous metal supports by using electrophoretic technique, Chin. Sci. Bull. 49 (2004) 1226. [33] E.N. Coker, J.C. Jansen, J.A. Martens, P.A. Jacobs, F. DiRenzo, F. Fajula, A. Sacco Jr., The synthesis of zeolites under micro-gravity conditions: a review, Micropor. Mesopor. Mater. 23 (1998) 119. [34] M. Lassinatti, J. Hedlund, J. Sterte, Faujiasite-type films synthesized by seeding, Micropor. Mesopor. Mater. 38 (2000) 25. [35] K.I. Okamoto, H. Kita, K. Horii, K. Tanaka, Zeolite NaA membrane: preparation, single-gas permeation, and pervaporation
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69 and vapor permeation of water/organic liquid mixtures, Ind. Eng. Chem. Res. 40 (2001) 163. [36] D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results, J. Membr. Sci. 179 (2000) 185. [37] J.J. Jafar, M. Budd, Separation of alcohol/water mixtures by pervaporation through zeolite A membranes, Micropor. Mater. 12 (1997) 305.
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[38] X. Lin, E. Kikuchi, M. Matsukata, Preparation of mordenite membraneson a-alumina tubular supports for pervaporation of water–isopropyl alcohol mixtures, Chem. Commun. (2000) 957. [39] H. Kita, K. Fuchida, T. Horita, H. Asamura, K. Okamoto, Preparation of Faujasite membranes and their permeation properties, Sep. Purif. Technol. 25 (2001) 261.
Hydrothermal synthesis of uniform and dense NaA zeolite membrane in the electric field Aisheng Huang b
a,b,*
, Weishen Yang
b,*
a Department of Chemistry, Tongji University, Shanghai 200092, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Received 30 August 2006; received in revised form 4 November 2006; accepted 5 December 2006 Available online 1 February 2007
Abstract Uniform and dense NaA zeolite membranes were prepared on the tubular porous a-Al2O3 supports in the electric field, where the charged zeolite particles attracted and transported to the support surface for membrane preparation homogeneously. The membranes properties were characterized by XRD, SEM and pervaporation for dehydration of 95 wt% isopropanol/water mixture at 343 K, respectively. The applied potential had great effect on membrane morphology, membrane thickness and separation performance. Under the action of the applied electric field, the negatively charged zeolite particles could migrate to the support surface homogenously and rapidly, facilitating to form uniform and dense membranes in a short time. Most zeolite particles transported to the support surface for membrane formation in the electric field, which increased the percentage of the product contained in the support surface and thus fully utilized the zeolite particles. High-quality NaA zeolite membrane, i.e., with a separation factor (water/isopropanol) of 3281 and a flux of 1.24 kg/(m2 h), could be prepared with the electrical potential of 1.0 V. Ó 2006 Elsevier Inc. All rights reserved. Keywords: NaA zeolite membrane; Hydrothermal synthesis; Electrophoresis; Pervaporation
1. Introduction Because of their potential applications as separators, reactors and sensors, zeolite membranes have been attracted much interest in the last two decades [1,2]. For the mentioned applications, the zeolite membrane should have high selectivity and high flux at the same time. Zeolite membranes have been usually prepared by in situ hydrothermal synthesis method, i.e., the porous support is immersed into the synthesis solution, and then the membrane is formed by direct crystallization [3–6]. Because the physico-chemical properties of support have a strong effect on the membrane formation [7–9], quality of the assynthesized membrane considerably depends on the char* Corresponding authors. Tel.: +86 21 65981180; fax: +86 21 65982287 (A. Huang), Tel.: +86 411 84379073 (W. Yang). E-mail addresses: [email protected] (A. Huang), [email protected] (W. Yang).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.12.005
acter of the support surface. Since preparation of highquality zeolite membranes by a direct in situ hydrothermal synthesis is encountered with certain difficulties [10], various methods, such as secondary growth method [11–17] and microwave synthesis [18] have been developed to improve the quality of membrane. Zeolite membranes were conventionally synthesized by static hydrothermal process. Two problems can be encountered during the synthesis of zeolite membranes by conventional static hydrothermal process: (a) a rapid depletion of nutrients concentration in the boundary layer, caused by the growth process [19]; and (b) the negative influence of gravitation, e.g., deposition of the zeolite crystals formed in solution onto the growing membrane [20,21]. The first problem can be solved by a continuous supplying of nutrients system. On the other hand, the negative influence of gravitation can be evaded by applying dynamic synthesis conditions, e.g., rotation of reaction vessel, or circulation of the precursor solution inside the
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
reaction vessel. This also can help in the preventing of formation of a stationary layer on the crystal-solution interface, and thus in the supplying the crystal surface with nutrients. Tiscaren˜o-Lechuga et al. [20] synthesized NaA zeolite membranes under a centrifugal force field. They reported that the centrifugal forces produced by high rotational speed drove the crystal nuclei towards the support surface, promoting formation of more continuous and denser layer. Yamazaki et al. [21] compared the formation of NaA zeolite membranes by both static synthesis and a method with circulation of the precursor solution. For the static method, they reported formation of by-products such as gmelinite, chabazite and faujasite along with zeolite A, while when the circulated solution system was used, only zeolite A was detected. Richter et al. [22] synthesized MFI type membranes on the inner surfaces of alumina tubes and capillaries in a continuous system. In this study, a fresh synthesis gel was supplied to the autoclave continuously from a reservoir. C ¸ ulfaz et al. [23] recently synthesized MFI type membranes in a continuous system with circulation of the synthesis solution, while Pina et al. [24] synthesized NaA zeolite membranes using a semi-continuous system in which fresh gel was periodically supplied to the synthesis vessel. It is expect that additional supply of fresh gel provide better control of the synthesis and crystallization conditions. As indicated in the above report, it is evident that migration of zeolite particles (nuclei, crystals) plays an important role in the formation of zeolite membrane. Electrophoretic deposition (EPD) is an effective technique for preparation of thin films [25,26]. In the EPD process, charged particles migrate to the support surface homogenously and rapidly under the action of the applied electric field, resulting in forming uniform, dense and thickness-controllable membranes in short time. The charged zeolite particles, successively produced during the hydrothermal synthesis, and migrate onto the support surface under the action of an applied electric field. There are two types application of electrophoretic technique for preparation of zeolite membrane. One was used for coating the support surface with zeolite seed crystals to prepare seeding layer before hydrothermal treatment. Mintova et al. [27], Berenguer-Murcia et al. [28] and Seike et al. [29] used electrophoretic technique to coat the support surface with zeolite seed crystals for preparing ZSM-5, Silicate-1 and Y-type zeolite membrane, respectively. It is indicated that electrophoretic technique is available for depositing a uniform, continuous and orderly seeding layer on the support surface, consequently improving the zeolite membrane quality. Another application was used to drive zeolite particles migrate to the support surface for membrane preparation during hydrothermal synthesis. Oonkhanond et al. [30] prepared ZSM-5 membranes on the tubular a-Al2O3 supports by electrophoretic technique, investigating the influence of the precursor concentration, applied potential and synthesis time on membrane properties. They reported that the electrophoretic technique was
59
effective for preparing continuous membranes on the support surface in shorter time. Mohammadi et al. [31] suc-
DC power
Rubber plug Autoclave
Thermocouple
α-Al2O3 support Synthesis solution Oven Stainless steel rod
Fig. 1. Synthesis apparatus for membrane preparation on tubular alumina support in the electric field.
6 3 6
7
2 4 1
5
Fig. 2. Experimental apparatus for pervaporation. (1) Water bath, (2) liquid tank, (3) liquid pump, (4) membrane model, (5) cold traps, (6) ball value, (7) vacuum pump.
Table 1 Some details of the synthesis conditions and properties of the NaA zeolite membranes prepared in this work Membranes
Potentials (V)
Concentrationa (%)
T (K)
Time (h)
Membranes thickness (lm)
M01 M02 M03 M04 M05 M06 M07 M08 M09 M10 M11 M12 M13 M14 M15 M16 M17
0 0.5 1.0 1.5 0 1.0 1.0 1.0 1 0 0 0 0 1.0 1.0 1.0 1.0
100 100 100 100 50 50 33.3 25.0 100 100 100 100 100 100 100 100 100
363 363 363 363 363 363 363 363 363 363 363 363 363 363 363 363 363
6 6 6 6 6 6 6 6 6 4 8 12 20 2 4 6 8
12 5 7 10 5 4 2.5 / / 8 10 12 10 2.5 4 8 10
a
Relative concentration to the original synthesis solution.
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cessfully prepared zeolite A membrane from kaolin by the electrophoretic technique, farther showing application prospect of the electrophoretic technique in membrane preparation owing to better controlling homogeneous nucleation and crystallization conditions.
(a)
We recently reported preparation of A-type zeolite membranes on nonporous metal supports by using electrophoretic technique [32]. To our knowledge, no studies have been reported for preparation of NaA zeolite membranes on the tubular a-Al2O3 supports in the electric field until
(b)
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(c)
Cross-section
(d)
Cross-section
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(e)
(f)
Cross-section
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(g)
(h)
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Fig. 3. SEM images of membrane: (a,b) M01: 0 V, 100% concentration, (c,d) M02: 0.5 V, 100% concentration, (e,f) M03: 1.0 V, 100% concentration and (g,h) M04: 1.5 V, 100% concentration.
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now. In this paper, tubular NaA zeolite membranes are prepared on the porous a-Al2O3 supports by using electrophoretic technique. The effects of the electrical potential on the membrane formation and pervaporation properties are investigated. The effects of electrical potential on the percentage of the product contained in the support surface and the rate of membrane formation are also investigated. Based on the observed results, the formation mechanism of zeolite membrane in the electric field is discussed.
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the autoclave and the autoclave was sealed. After crystallizing 1–24 h at 363 K with different potentials, the solution was decanted off and the membrane was washed with deionized water, and then dried. In all cases, the zeolite product in the solution was filtered, washed and then dried to weight. The amount of zeolite product contained in the support surface was measured by the weight gain of the support after crystallization. 2.2. Characterization of zeolite membrane
2.1. Synthesis of zeolite membrane Porous a-Al2O3 tubes (home-made: 11 mm in outer diameter, 7 mm in inner diameter, 120 mm in length, 0.5– 1.0 lm pore radius, about 40% porosity) were used as supports. The surface of the support was polished with 700 grit-sand papers, and then was cleaned with deionized water in a Branson SB2200 ultrasonic cleaner three times to remove loose particles created during polishing. Before hydrothermal synthesis, the cleaned support was calcined in air at 673 K for 3 h. A DC power supply (WYJ-T303, Buxin Sensor Factory, China) was used to generate electric field for NaA zeolite membrane synthesis. The solution for synthesizing NaA zeolite membrane was prepared according to the procedure reported previously [17]. The aluminate solution was prepared by dissolving sodium hydroxide (AR, Shenyang Reagent Factory, China) in deionized water, then adding aluminum foil (AR 99.8%, Beijing Hongxin Chemical Factory, China) to the solution at room temperature. The silicate solution was prepared by dilution of silica sol (27 wt% SiO2, q 1.16 g/l, Qingdao Haiyang Chemical Factory, China) with deionized water with vigorous stirring at 333 K. After 10 min of stirring, preheated (333 K) aluminate solution was added into the stirred silicate solution. The stirring was continued for 15 min to produce a clear and homogeneous aluminosilicate solution having molar composition of 50Na2O:Al2O3:5SiO2:1000H2O. For preparation diluted synthesis solution, the above experiments were carried out only correspondingly increasing the water quantity. The diluted synthesis solution with composition of 50Na2O: Al2O3:5SiO2:2000H2O, 50Na2O:Al2O3:5SiO2:3000H2O and 50Na2O:Al2O3:5SiO2:4000H2O was used for membrane preparation, respectively. The synthesis unit is illustrated schematically in Fig. 1. As shown in Fig. 1, the bottom end of the a-Al2O3 support was fixed with a Teflon holder. The middle of the support tube was filled with a stainless steel rod (about 6 mm in diameter), which was used as an anode. The support was placed vertically in stainless steel autoclave (30 mm in inner diameter), and the stainless steel autoclave was used as a cathode. The DC power supply was used to generate potential between the two electrodes. The electrical potential varied from 0 to 2.0 V. The synthesis solution was poured into
The structure of the as-synthesized membranes was confirmed by X-ray diffraction (XRD) patterns. XRD was carried out on a Ragaku D max/rB power diffractometer using CuKa radiation operating at 40 K and 50 mA. The morphology and thickness of the as-synthesized membranes were examined by scanning electron microscopy (SEM). The SEM photographs were obtained on a JEM-1200E scanning electron microscopy. All the samples used for XRD and SEM test were selected middle position of the support tube. The properties of the as-synthesized zeolite membranes were also evaluated by pervaporation for dehydration of 95 wt% isopropanol/water mixture at 343 K. The apparatus used for the pervaporation experiments is illustrated schematically in Fig. 2. The 95 wt% isopropanol/water mixtures, preheated to 343 K, were fed to the out side of the zeolite membrane in the membrane model that was heated in a thermostatic water bath. The inside of the membrane was evacuated with a vacuum pump. The cold traps with liquid N2 cooling were used to collect the permeate. The compositions of the feed and the permeate were analyzed by gas chromatogram (HP5890). The most important variables for pervaporation are the selectivity and the flux. The total flux (J), the component flux (Ji) and the separation factor (a) are defined as respectively:
(b)
Intensity
2. Experimental
(a)
10
20
30
40
50
2θ/degrees Fig. 4. XRD pattern of the NaA zeolite membrane: (a) M01 and (b) M03 (j) A-type zeolite, (m) X-type zeolite, () P- type zeolite, (d) a-Al2O3.
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j¼
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W ; DtA
J i ¼ J vi;p ;
ai;j ¼
vi;p vj;f vi;f vi;n
3. Results and discussion
where W is total weight of the permeate (kg), Dt is collecting time (h), A is separation area of the membrane, xi,p is the weight fraction of species i in the permeate and xi,f is the weight fraction of species i in the feed.
(a)
3.1. Membrane preparation in the electric field During the formation of zeolite membrane, the force acting on the zeolite particle has important influence to
(b)
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Top view
(c)
(d)
Top view
Cross-section
(f)
(e)
Cross-section
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(g)
(h)
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Fig. 5. SEM images of membrane: (a,b) M06: 0 V, 50% concentration, (c,d) M07: 1.0 V, 50% concentration, (e,f) M08: 1.0 V, 33.3% concentration and (g,h) M09: 1.0 V, 25% concentration.
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
the membrane formation [20,32,33]. Table 1 summarizes some details of the synthesis conditions and properties of the NaA zeolite membranes prepared in this work. Fig. 3 shows the SEM images of the as-synthesized NaA zeolite membrane after 6 h hydrothermal synthesis at 363 K with different electrical potential. As shown in Fig. 3 and Table 1, the applied electrical potential had great effect on membrane morphology and thickness. At 0 V of the electrical potential (where M01), the size of the crystals was nonhomogeneous, with crystals size of about 3–10 lm. Judged by the cross-section of the as-synthesized zeolite membrane, the zeolite membrane was very rough and loose with thickness of about 12 lm. Moreover, spherical crystals as well as octahedral crystals were formed among A-type zeolite crystals, which indicated that A-type zeolite crystals had transformed to other crystals. As reported previously [10,21], the A-type zeolite crystals easily transform to other crystals by hydrothermal synthesis. Fig. 4 shows the XRD patterns of the as-synthesized zeolite membrane M01 and M03, respectively. As shown in Fig. 4a, the XRD patterns of the zeolite membrane M01 farther approve of A-type zeolite crystals transformed to other crystals. In the static hydrothermal synthesis, the zeolite particles successively produced in the solution are deposited by gravitation, and also diffused by hydrodynamics. The gravitation force has a negative influence on the membrane formation because it makes the zeolite particles precipitate [34]. Most zeolite particles have precipitated on the bottom of the autoclave before forming dense membrane. There were abundant of zeolite particles for membrane formation on the bottom parts, and conversely there were deficient zeolite particles for membrane formation on the upper part. Consequently, on one hand, the bottom parts formed too much thick NaA zeolite membrane, and on the other hand, dense membrane could not form on the upper part before other crystals produced. These may be the reason why it is difficult to prepare high-quality NaA zeolite membrane by the conventional hydrothermal synthesis method. As it can be seen in Fig. 3, with increasing of the electrical potentials, the zeolite crystals became uniform and the
(a)
packaging density of the zeolite membrane increased. With increasing the applied electrical potential to 0.5 V (where M02), the a-Al2O3 support surface was fully covered with homogeneous A-type zeolite crystals, and the membrane thickness was about 5 lm. Further increasing the electrical potential to 1.0 V (where M03), the support surface was completely covered with uniform and compact cubic A-type zeolite crystals, and about 7 lm dense zeolite membrane was formed. It was found that it was effective to restrain NaA zeolite from transforming to other crystals prepared in the electric field. As shown in Fig. 4b, the XRD patterns of membrane M03 indicated that high purity A-type zeolite layer was formed on support surface. One possible explanation is that zeolite particles can rapidly immigrate to the support surface under the action of electric field, thus (a) decreasing the time of zeolite particles staying in the active synthesis solution, and (b) producing more nucleus centers on the support surface, which are benefit to restrain from forming impure crystalline phase. When the electrical potential increased to 1.5 V, the properties of the as-synthesized zeolite membrane M04 did not change much (uniform, smooth and dense consistently), although the thickness of the membrane increased to about 10 lm, as shown in Fig. 3g and h. Fig. 5 shows the SEM images of the as-synthesized NaA zeolite membrane prepared by using diluted synthesis solution at 363 K. When 50% concentration solution was used, the zeolite membrane M05, prepared by conventional hydrothermal synthesis, was uncontinuous with thickness of about 5 lm, and there were obvious gap on the support surface. As the electrical potential increased to 1.0 V under the same synthesis concentrations, the coverage of the zeolite membrane M06 increased, and continuous membrane with thickness of about 4 lm could be obtained. When more diluted synthesis solution was used, thinner membrane could be formed (i.e., M07), and even no membrane (i.e., M08) was formed when 25% concentration solution was used. There were not enough nutrients for zeolite formation by using diluted synthesis solution, and thus decreasing the rate of nucleation and crystal growth. Therefore, it is not easy to form continuous zeolite
(b)
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63
Cross-section
Fig. 6. SEM images of membrane M05 prepared by electrophoretic technique with 1.0 V electrical potential at 100% concentration.
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membrane during conventional hydrothermal synthesis, while more nutrients could be supplied to the growth interface at a sufficiently high rate under the action of the applied electric field, facilitating to form continuous zeolite membrane.
(a)
In order to further understand the effect of electric field on membrane formation, the electrode polarity was reversed, i.e., the stainless steel rod was used as the cathode and the stainless steel autoclave was used as the anode. Fig. 6 shows the SEM images of the as-synthesized zeolite
(b)
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(c)
(d)
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(e)
(f)
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(g)
(h)
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Fig. 7. SEM images of membrane prepared with 0 V and 100% concentration at different crystallization time: (a,b) M10: 4 h, (c,d) M11: 8 h, (e,f) M12: 12 h and (g,h) M13: 20 h.
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
membrane M09 after 6 h crystallization at 363 K with potential of 1.0 V. As shown in Fig. 6, it can be seen, there was no zeolite membrane formed on support surface, and only a few separated crystals formed. Results further indicated that the applied potential had great effect on the membrane formation.
(a)
65
3.2. Effect of applied potential on rate of the membrane formation Fig. 7 shows the SEM images of the as-synthesized NaA zeolite membrane prepared by conventional hydrothermal synthesis at different crystallization time. As shown in
(b)
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(c)
(d)
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(e)
(f)
Cross-section
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(g)
(h)
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Fig. 8. SEM images of membrane prepared with 1 V and 100% concentration at different crystallization time: (a,b) M14: 2 h, (c,d) M15: 4 h, (e,f) M16: 6 h and (g,h) M17: 8 h.
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A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
surface, and the membrane M13 dissolved and transformed to other crystals. Fig. 8 shows the SEM images of the as-synthesized NaA zeolite membrane prepared by hydrothermal synthesis in the electric field at different crystallization time. After 2 h crystallization, the support surface was covered by zeolite particles, with about 2.5 lm in thickness, but there were many voids obviously. As crystallization time increased to 4 h, the particles grew and formed membrane, and the thickness of the zeolite membrane M15 increased to about 4 lm. As crystallization time increased to 6 h, a dense zeolite membrane M16 could be formed, with about 8 lm in thickness. As crystallization time increased to 8 h, the zeolite membrane produce crack as shown in Fig. 8.
80
Percentage of zeolite crystal (100%)
70 zeolite crystals in the support surface 60 50 40 30 20 10 0.0
0.5
1.0
1.5
2.0
Potentials (V) Fig. 9. The percentage of zeolite crystals in the support surface as a function of the electrical potentials assuming a 100% crystallization of the zeolite synthesis bath.
3.3. Effect of applied potential on percent of the product contained in the support surface
Fig. 7, after 4 h crystallization, no continuous film formed, and only scattered crystals existed on the support surface. As crystallization time increased to 8 h, the coverage increased but still no continuous membrane formed. Moreover, as revealed before, spherical crystals as well as octahedral crystals were formed after 8 h crystallization. As crystallization time increased (even increased to 20 h), there were still no continuous membranes formed on the support
After hydrothermal synthesis, the zeolite product in the solution was filtered, washed and dried to weight. The amount of zeolite crystals in the support surface was measured by the weight gain of the support after crystallization. The total zeolite amounts were defined to the sum of the zeolite crystals product in the solution and in the support surface. Fig. 9 shows that percentage of zeolite crystals contained in the support surfaces increases linearly
a
Autoclave
Support Stainless steel rod
-
-
--
-
-
FC
--
-- --
-
-
--Migrate slowly and randomly - - -- - - -
--
-
-
FG FR
--
--
--
Charged particles
Long formation time
Zeolite membrane
b
-
-
-
-
-
FC+FE
FG
FR
Migrate rapidly and orientedly
-
-
(–)
-
-
-
-
Short formation time
(+) FC: capillary force, FE: electric field force, FG: gravitation force, FR: resultant force
Fig. 10. Comparisons of zeolite membrane prepared on alumina support surface by hydrothermal synthesis: (a) without electric field and (b) with electric field.
A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
with increasing electric potential. Only 20.4% of zeolite is contained in the support in the absence of electric potential (conventional synthesis), while more than 50% of zeolite is contained in the support when the electric potential of 2.0 V was applied. This result can easily be explained by the fact that influence of gravitational settling of the formed zeolite crystals decrease with increasing electric potential, as it is schematically presented in Fig. 10. The zeolite crystals formed in the absence of electric field migrate to the support surface randomly, under the common influence of gravitational sedimentation and diffusion (Fig. 10a). Under such conditions, a large number of zeolite crystals formed in the solution are attracted and transported toward the bottom of autoclave before a dense membrane can be formed on the support surface. Consequently, a membrane of inappropriate characteristics can be obtained: the upper part of the membrane is tenuous and insufficiently thick, while the bottom part of the membrane is too thick which can cause the formation of cracks. In the cases when electric field is applied during hydrothermal synthesis, the negative influence of gravitation can be reduced to same degree and the charged zeolite crystals formed in the solution rapidly migrate toward the support surface under the action of the electric field, which results in a rapid formation of uniform and dense zeolite membrane (Fig. 10b). 3.4. Water–isopropanol separation performance NaA zeolite membrane, with low ratio of silicon to aluminum (Si/Al = 1), has high affinity for water in nature, which has been reported to be extremely effective for dehydration of alcohol/water mixtures by pervaporation [6,17]. Table 2 shows the Water/isopropanol separation performance of the as-synthesized NaA zeolite membrane prepared with different applied potential. As shown in Table
67
2, when the applied potential is 0 V, there was no dense membrane formed on the support surface and A-type zeolite crystals transformed to other crystals, which resulted in poor separation performance of the as-synthesized membrane. The separation factor (water/isopropanol) is only 107. With increasing the applied electrical potential to 0.5 V, the separation performance improved. The separation factor increases to 916 and the flux is 1.48 kg/(m2 h). When the applied potential is 1.0 V, the support surface was completely covered by A-type zeolite, and the separation performance improved continuously. The separation factor increases to 3281 and the flux is 1.24 kg/(m2 h). When the applied potential increases to 1.5 V, the density of the zeolite membrane increased, resulting in the separation selectivity increased (the separation factor increased to 9481). Many experiments indicated high-quality NaA zeolite membranes could be easily prepared in the electric field. At 343 K, the separation factor (water/isopropanol) of the NaA zeolite membranes were 1000–5000 for 95 wt% isopropanol/water mixture, and the fluxes are 1.0–1.5 kg/ m2 h, respectively. The reproducibility of the membrane preparation was satisfactory. Table 2 also compares the separation performances of the zeolite membranes prepared in this study with literature data. While pervaporation results from different laboratories were often difficult to compare because the different experimental conditions was used, it can be said that the as-synthesized zeolite membranes prepared by hydrothermal synthesis in the electric field were among those with good separation performance. 4. Conclusions Uniform and dense NaA zeolite membrane could be easily prepared on the a-Al2O3 support by hydrothermal
Table 2 Comparisons of the pervaporation properties of the as-synthesized zeolite membrane prepared in this study with literature data Membranes
Mixtures (A/B)
T (K)
Concentration (A wt%)
aA/B
Flux (kg/m2 h)
References
Silicalite Silicalite NaA NaA NaA NaX NaA NaA NaA NaA Mordenite NaY NaY M01 M02 M03 M04
EtOH/H2O EtOH/H2O H2O/EtOH H2O/EtOH H2O/i-PrOH H2O/EtOH H2O/EtOH H2O/EtOH H2O/DMF H2O/i-PrOH H2O/i-PrOH MeOH/MTBE EtOH/benzene H2O/i-PrOH H2O/i-PrOH H2O/i-PrOH H2O/i-PrOH
303 303 348 393 343 348 348 348 333 343 348 323 333 343 343 343 343
5 4.65 10 10 5 10 10 10 5 10 10 10 10 5 5 5 5
63 64 42000 47000 10000 400 23000 10000 200–500 4500 3360 7000 930 107 916 3281 9481
0.6 0.6 2.08 8.37 1.67 0.5 2.5 2.15 2.65 0.5 0.1 1.02 0.22 1.69 1.48 1.24 1.02
[5] [6] [7] [7] [17] [22] [22] [35] [36] [37] [38] [39] [39] This This This This
study study study study
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A. Huang, W. Yang / Microporous and Mesoporous Materials 102 (2007) 58–69
synthesis in the electric field. The applied potential had great effect on membrane morphology, membrane thickness, separation performance and the percentage of the zeolite product contained in the support surface. Under the action of an applied electric field, the charged zeolite particles attracted and transported to the support surface for membrane preparation homogeneously and rapidly, facilitating to form uniform and dense membranes in shorter time. Most zeolite particles transported to the support surface for membrane formation in the electric field, which increased the percentage of the product contained in the support surface. High-quality NaA zeolite membrane, i.e., with a separation factor (water/isopropanol) of 3281 and a flux of 1.24 kg/(m2 h), could be prepared with the electrical potential of 1.0 V. The reproducibility of the membrane preparation in the electric field was satisfactory. Acknowledgments This work was supported by the National Advanced Materials Committee of China (2003AA328010), the National Science Foundation of China (20607015) and Program for Young Excellent Talents in Tongji University (2006KJ057). References [1] J.C. Jansen, J.H. Koegler, H. van Bekkum, H.P.A. Calis, C.M. van den Bleek, F. Kapteijn, J.A. Moulijn, E.R. Geus, N. van der Puil, Zeolite coating and their potential use in catalysis, Micropor. Mesopor. Mater. 21 (1998) 213. [2] J. Caro, M. Noack, P. Ko¨lsch, R. Scha¨fer, Zeolite membranes-state of their development and perspective, Micropor. Mesopor. Mater. 38 (2000) 3. [3] Y. Yan, M.E. Davis, G.R. Gavalas, Preparation of zeolite ZSM-5 membranes by in-situ crystallization on a-Al2O3 porous, Ind. Eng. Chem. Res. 34 (1995) 1652. [4] J. Sterte, S. Mintova, G. Zhang, B.J. Schoeman, Thin molecular sieve films on noble metal substrates, Zeolites 18 (1997) 387. [5] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami, K. Haraya, Separation of ethanol/water mixture by silicate membrane on pervaporation, J. Membr. Sci. 95 (1994) 221. [6] M. Kondo, M. Komori, H. Kita, K. Okamoto, Tubular-type pervaporation module with zeolite NaA membrane, J. Membr. Sci. 133 (1997) 133. [7] V. Valtchev, S. Mintova, The effect of the metal substrate composition on the crystallization of zeolite coatings, Zeolites 15 (1995) 171. [8] J.L.H. Chau, C. Tellez, K.L. Yeung, K. Ho, The role of surface chemistry in zeolite membrane formation, J. Membr. Sci. 164 (2000) 257. [9] R. Lai, Y.S. Yan, G.R. Gavalas, Growth of ZSM-5 films on alumina and other surfaces, Micropor. Mesopor. Mater. 37 (2000) 9. [10] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Synthesis of NaA zeolite membranes from clear solution, Micropor. Mesopor. Mater. 43 (2001) 299. [11] M.C. Lovallo, M. Tsapatsis, Preferentially oriented submicron silicalite membranes, AICHE J. 42 (1996) 3020. [12] R. Lai, G.R. Gavalas, Surface seeding in ZSM-5 membrane preparation, Ind. Eng. Chem. Res. 37 (1998) 4275.
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