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Microwave-assisted hydrothermal synthesis of hydroxy-sodalite zeolite membrane
Microporous and Mesoporous Materials 75 (2004) 173–181 www.elsevier.com/locate/micromeso
Microwave-assisted hydrothermal synthesis of hydroxy-sodalite zeolite membrane Xiaochun Xu a
a,b,*
, Yun Bao a, Chunshan Song b, Weishen Yang
a,*
, Jie Liu a, Liwu Lin
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Clean Fuels and Catalysis Program, The Energy Institute, 209 Academic Project Buildings, Pennsylvania State University, University Park, PA 16802, USA Received 29 January 2004; received in revised form 30 June 2004; accepted 16 July 2004 Available online 25 September 2004
Abstract A high quality pure hydroxy-sodalite zeolite membrane was successfully synthesized on an a-Al2O3 support by a novel microwave-assisted hydrothermal synthesis (MAHS) method. Influence of synthesis conditions, such as synthesis time, synthesis procedure, etc., on the formation of hydroxy-sodalite zeolite membrane by MAHS method was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and gas permeation measurements. The synthesis of hydroxy-sodalite zeolite membrane by MAHS method only needed 45 min and synthesis was more than 8 times faster than by the conventional hydrothermal synthesis (CHS) method. A pure hydroxy-sodalite zeolite membrane was easily synthesized by MAHS method, while a zeolite membrane, which consisted of NaX zeolite, NaA zeolite and hydroxy-sodalite zeolite, was usually synthesized by CHS method. The effect of preparation procedures had a dramatic impact on the formation of hydroxy-sodalite zeolite membrane and a single-stage synthesis procedure produced a pure hydroxy-sodalite zeolite membrane. The pure hydroxy-sodalite zeolite membrane synthesized by MAHS method was found to be well inter-grown and the thickness of the membrane was 6–7 lm. Gas permeation results showed that the hydrogen/n-butane permselectivity of the hydroxy-sodalite zeolite membrane was larger than 1000. 2004 Elsevier Inc. All rights reserved. Keywords: Gas separation; Hydrothermal synthesis; Hydroxy-sodalite; Microwave heating; Zeolite membrane
1. Introduction During the past ten years, increasing attempts have been made to develop zeolites into membrane forms for separation and catalytic applications based on utilizing their molecular sieving properties, high thermal resistance, chemical inertness, high mechanical
*
Corresponding authors. Present address (X. Xu): 209 Academic Project Buildings, The Energy Institute, Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1 814 865 6617; fax: +1 814 863 7432 (X. Xu), tel.: +86 411 4379073; fax: +86 411 4379073 (W. Yang). E-mail addresses: [email protected] (X. Xu), [email protected] (W. Yang). 1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.07.019
strength and uniform pore sizes [1–5]. Several preparation methods have been developed, such as in situ hydrothermal synthesis [6–8], vapor phase transport method [9], secondary growth method [10–12] or embedding microcrystals of zeolite into a matrix [13]. Ideally, for a better separation performance, zeolite membrane should preferably be made of pure zeolite crystals with uniform and small particle size. Because the intergrowth between different types of zeolite crystals is not as good as that between the same types of zeolite crystals, the existence of impure zeolite crystals results in many inter-crystalline pores, which can have a detrimental effect on the separation performance of the zeolite membrane [14]. It is more difficult to form high quality zeolite membrane using zeolite crystals
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with broad particle size distribution than with narrow particle size distribution [10,15]. In addition, zeolite membranes made by large particles are thicker than that made by small particles and therefore has a lower permeance. However, conventional synthesis methods, such as, in situ hydrothermal synthesis and vapor phase transport method, need long crystallization times of a few hours to a few days. The long crystallization time usually results in the formation of impure zeolites [10]. Because of the low heating rate and the inhomogeneous heating, zeolite nuclei do not form on the support surface simultaneously and zeolite crystals are not uniform in size [15,16]. Therefore, the improvement in the separation performance of zeolite membranes can be achieved by developing a new synthesis approach to synthesize zeolite membranes, which are derived from zeolite crystals with uniform and small particle size and with high purity. Recently, a new synthesis method that combines hydrothermal crystallization with a microwave heating technique has been developed for the synthesis of zeolites, molecular sieves as well as zeolite membranes [15–36]. In the late 1980Õs, Chu et al. [17,18] first reported the synthesis of A and ZSM-5 zeolite by microwave heating. Since then more studies have been reported using microwave heating [17,18,26–35]. Different types of zeolites and molecular sieves such as, A [17,18,27], MFI [17,18,28,32], Beta [31], X/Y [27,28], AlPOs [29,30], SAPOs [34] and MCM-41 [35], have been synthesized by this novel microwave-assisted hydrothermal synthesis (MAHS) method. Compared with the conventional hydrothermal synthesis (CHS), MAHS of zeolites has the advantages of a short synthesis time, broad synthesis composition, small zeolite particle size, narrow particle size distribution and high purity [15,27]. These advantages prompt researchers to explore its use as a method for the synthesis of zeolite membranes. Up to date, A-type [15,16,19,21], Sodalite [20], X/Y-type [21,22], MFI [21], SAPO-5 [23] and AlPO4-5 [24,25] zeolite membranes have been synthesized. However, few separation data has been reported. Kita et al. [21,37] reported the liquid pervaporation separation by zeolite membrane synthesized by MAHS method. Previously, we reported the synthesis of a high gas permeance NaA zeolite membrane on a porous a-Al2O3 support prepared by MAHS method [15,16]. The synthesis time for NaA zeolite membrane was drastically reduced from 3 h for CHS to 15 min for MAHS. Zeolite crystals in the NaA zeolite membrane synthesized by MAHS were smaller and more uniform in particle size than those synthesized by CHS. The thickness of the NaA zeolite membrane synthesized by MAHS was about 4 lm, which was thinner than 8 lm for the NaA zeolite membrane synthesized by CHS. Therefore, the gas permeance of the NaA zeolite membrane synthesized by MAHS was four times higher than that of the NaA zeolite membrane synthe-
sized by CHS, while their permselectivities were comparable. MAHS method has proved to be a promising method for the synthesis of high quality zeolite membrane. Hydroxy-sodalite has the same framework structure as sodalite and consists of the cubic array of b-cages. Hydroxy-sodalite has a six-membered ring aperture ˚ . The pore size of hydroxywith a pore size of 2.8 A sodalite is smaller than that of the zeolites with an eight-membered ring aperture, e.g., NaA zeolite. Only small molecules, such as helium, hydrogen and water, etc., can enter the pore of hydroxy-sodalite. Therefore, hydroxy-sodalite zeolite membrane is expected to possess a better performance on the separation of small molecules from gas or liquid mixtures than the zeolite membranes with a bigger pore size. Recently, Julbe et al. [20] synthesized a sodalite/a-Al2O3 composite membrane by MAHS method with a relatively low He/N2 permselectivity of 6.2 at 115 C. In the present work, MAHS method was further explored in the synthesis of hydroxy-sodalite zeolite membrane. Through optimizing the synthesis conditions, a pure hydroxy-sodalite zeolite membrane with a high H2/nC4H10 permselectivity was synthesized.
2. Experimental 2.1. Support A porous a-Al2O3 disk (home-made, 30 mm in diameter, 3 mm in thickness, 0.1–0.3 lm pore radius, with 50% porosity) was used as the support. The surface of the support was polished with 700 grit sand paper on both sides, whereupon the support was cleaned with deionized water in a Branson SB2200 ultrasonic cleaner for 3–5 min to remove the loose particles created during polishing. Before hydrothermal synthesis, the cleaned support was calcined in air at 673 K for 3 h with a heating and cooling rate of 4.0 K/min to burn off any organics on the support surface. 2.2. Synthesis solution The synthesis solution was prepared by mixing an aluminate solution and a silicate solution. The aluminate solution was prepared by dissolving sodium hydroxide (40 g) in deionized water (159 g), then adding aluminum foil (1.0 g) to the caustic solution at room temperature. The silicate solution was prepared by mixing sodium hydroxide (34.1 g), silica sol (SiO2 27 wt%) (20.6 g) and deionized water (159 g). The aluminate solution, preheated to 50 C, was added to the silicate solution with stirring. In order to produce a clear homogeneous solution, the resulting mixture
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was stirred vigorously for 15 min. The molar ratio of this mixture solution was 5SiO2:Al2O3:50Na2O: 1000H2O. 2.3. Microwave synthesis of zeolite membrane
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3. Results and discussion 3.1. Microwave-assisted hydrothermal synthesis of hydroxy-sodalite zeolite membrane
2.4. Characterization
The formation of hydroxy-sodalite zeolite membrane on porous a-Al2O3 support by microwave-assisted hydrothermal synthesis (MAHS) method was investigated and the as-synthesized membranes were characterized by XRD and SEM. Fig. 1 shows the XRD patterns of the powder and as-synthesized membranes by MAHS method with different synthesis times. After 15 min of synthesis, the diffraction patterns of the as-synthesized membrane were represented by the sum of the diffraction patterns of a-Al2O3 support, NaA zeolite and hydroxy-sodalite zeolite. This meant that the as-synthesized membrane was not a pure hydroxy-sodalite zeolite membrane. After 45 min of synthesis, the diffraction patterns of NaA zeolite disappeared and the intensity of the diffraction peaks of hydroxy-sodalite intensified, which indicated that the NaA zeolite transformed into hydroxy-sodalite zeolite and a pure hydroxy-sodalite zeolite membrane formed on the porous a-Al2O3 support. Fig. 2 shows the surface and the cross-section SEM images of the as-synthesized membranes. After 15 min of synthesis, the surface of the porous a-Al2O3 support was covered with zeolite crystals with different shapes. According to their morphology, they were mainly NaA zeolite and hydroxy-sodalite zeolite. The cross-section image showed that the as-synthesized membrane was not continuous. After 45 min of synthesis, the surface of the aAl2O3 support was completely covered with randomly
The structure of the as-synthesized membranes was determined by X-ray diffraction (XRD) patterns. X-ray diffraction was carried out on a Ragaku D/ max II powder diffractometer using Cu Ka ˚ ) radiation operating at 40 KV and 50 mA. (k = 1.54 A The morphology and the thickness of the as-synthesized membrane was examined by scanning electron microscope (SEM). The SEM photographs were obtained on a JEM-1200E scanning electron microscope. The surface of the samples was sputtered with a gold film. Gas permeation measurements were carried out to evaluate the quality of the membranes. Before gas permeation measurements, the as-synthesized membranes were dried at 200 C overnight to remove the water absorbed in the zeolite channels. The dried zeolite membrane was sealed in a permeation cell with the zeolite membrane facing on the high-pressure side for gas permeation measurements. The single gas permeance was measured by a soap-film flowmeter at room temperature under the pressure difference of 0.10 MPa. The permselectivity of A/B was defined as the permeance ratio of gas A and gas B.
Fig. 1. XRD patterns of the powder and the as-synthesized membranes by microwave-assisted hydrothermal synthesis. (1) Support; (2) powder after 45 min synthesis; (3) membrane after 15 min synthesis; (4) membrane after 45 min synthesis. ( ) a-Al2O3 support; (d) NaA zeolite; () Hydroxy-sodalite zeolite.
The a-Al2O3 disk was placed vertically with a Teflon holder in a polyethylene bottle to avoid any precipitation during membrane synthesis. 100 ml synthesis solution was poured into the bottle carefully without hitting the support and the support was fully immersed in the synthesis solution. Then the bottle was covered with a cap. The crystallization was carried out in a modified domestic microwave oven operating at 2450 MHz. The synthesis mixture was heated up quickly from room temperature to 90 ± 5 C in 60 s with full power, and then held at the final temperature for the desired times with reduced power. In some cases, multi-stage synthesis [10] was carried out. For comparison, the synthesis was also carried out at 90 C by conventional heating. After synthesis, the as-synthesized membranes were washed several times with deionized water until the pH value of the washings was neutral, and then dried at 150 C for 3 h. Because of the different thermal expansion coefficient of the hydroxy-sodalite zeolite and the aAl2O3 support, a low heating and cooling rate, ca. 1 C/ min, was adopted to avoid crack formation during the thermal treatment.
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Fig. 2. SEM images of the as-synthesized membranes by microwave-assisted hydrothermal synthesis. (1) and (2) Support; (3) and (4) membrane after 15 min synthesis; (5) and (6) membrane after 45 min synthesis.
oriented pure hydroxy-sodalite zeolite crystals with uniform size of about 4 lm. There were no other types of zeolites present on the support surface. The zeolite crystals were found to be highly inter-grown and there were no observable inter-crystalline gaps in the zeolite membrane. These observations are in accordance with the XRD characterization. As the cross-section image shows, the hydroxy-sodalite zeolite membrane was bound firmly to the support with a thickness of approximately 6–7 lm. For comparison, the formation of hydroxy-sodalite zeolite membrane on the porous a-Al2O3 support by conventional hydrothermal synthesis (CHS) method was also investigated. Fig. 3 shows the XRD patterns of the as-synthesized membranes by CHS method with different synthesis times. After 15 min of synthesis, zeolitic materials did not form in the bulk clear synthesis solution or on the porous a-Al2O3 support surface. Diffraction patterns other than the a-Al2O3 support emerged after 1 h of synthesis. These diffraction patterns were attributed to NaA zeolite. After 2 h of synthesis, the intensity of the diffraction peaks of NaA zeolite in-
creased, which implied that more NaA zeolite crystals formed on the support. When the synthesis time was extended to 3 h, the intensity of the diffraction peaks of NaA zeolite became weak. With further extending the synthesis time to 4 and 6 h, diffraction patterns of zeolite X and hydroxy-sodalite zeolite appeared. These phenomena indicated that the NaA zeolite on the support surface transformed into other types of zeolites in the alkaline synthesis solution. However, NaA zeolite did not fully transform into hydroxy-sodalite zeolite even after 6 h of synthesis and pure hydroxy-sodalite zeolite membrane did not form. SEM images of the as-synthesized membranes by CHS are shown in Fig. 4. After 1 h of synthesis, discrete NaA zeolite crystals were observed on the support surface. When the synthesis time increased to 2 h, the coverage of the NaA zeolite crystals on the support surface increased and small amounts of hydroxy-sodalite zeolite formed. However, there was no continuous zeolite membrane formed on the support, as revealed from the cross-section SEM image (not shown). When the synthesis time was extended to 3 h, the NaA zeolite on the support surface dissolved and
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Fig. 3. XRD patterns of the as-synthesized membranes by conventional hydrothermal synthesis with different synthesis times. (1) Support; (2) 1 h; (3) 2 h; (4) 3 h; (5) 4 h; (6) 6 h. ( ) a-Al2O3 support; (d) NaA zeolite; () Hydroxy-sodalite zeolite; (j) NaX zeolite.
other types of zeolites formed, which is consistent with the XRD results. When the synthesis time was further extended to 6 h, NaA, NaX and hydroxy-sodalite co-existed in the as-synthesized membrane. The morphology of the hydroxy-sodalite crystals was a ‘‘ball of yarn’’. The morphology of the hydroxy-sodalite crystals syn-
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thesized by CHS and by MAHS was different. Whether the difference is caused by microwave heating is unknown. Comparing the formation of hydroxy-sodalite zeolite membrane by MAHS with that by CHS, MAHS significantly promoted the formation of hydroxy-sodalite zeolite membrane with a shorter synthesis time. The synthesis of hydroxy-sodalite zeolite membrane by MAHS only needed 45 min, while NaA, NaX and hydroxy-sodalite co-existed in the as-synthesized membrane even after 6 h of synthesis by CHS. Secondly, small hydroxy-sodalite zeolite crystals with uniform size formed by MAHS, while larger zeolite crystals formed by CHS. In addition, hydroxy-sodalite zeolite crystals in the zeolite membrane synthesized by MAHS were found to be well inter-grown, while there were big gaps between the zeolite crystals in the zeolite membrane synthesized by CHS. Lastly, pure hydroxy-sodalite zeolite membrane was synthesized by MAHS, while a zeolite membrane consisting of NaA, NaX and hydroxy-sodalite was synthesized by CHS. The reason for the promoting effect of microwave heating on the formation of hydroxy-sodalite zeolite membrane will be discussed later. 3.2. Synthesis procedure A multi-stage synthesis procedure was carried out to investigate the effect of synthesis procedure on the formation of hydroxy-sodalite zeolite membrane by MAHS. The synthesis time for each stage was 15 min
Fig. 4. SEM images of the as-synthesized membranes by conventional hydrothermal synthesis with different synthesis times. (1) 1 h; (2) 2 h; (3) 3 h; (4) 6 h.
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Fig. 5. XRD patterns of the as-synthesized membranes by microwaveassisted hydrothermal synthesis with different stages of synthesis (synthesis time for each stage is 15 min). (1) One-stage; (2) two-stage; (3) three-stage. ( ) a-Al2O3 support; (d) NaA zeolite; () Hydroxysodalite zeolite; ( ) unknown phase.
was not continuous when prepared by one-stage synthesis. After two-stage synthesis, XRD patterns of NaA zeolite disappeared and the intensity of the XRD patterns of hydroxy-sodalite intensified, which indicated that NaA zeolite transformed into hydroxy-sodalite zeolite. However, hydroxy-sodalite zeolite crystals were very small and were not observed in the SEM images even at high magnification. One again, the as-synthesized membrane was still not continuous when two-stage synthesis was performed. After three-stage synthesis, the as-synthesized zeolite membrane was a mixture of hydroxy-sodalite and other unidentified phases. One such unidentified phase may possibly be NaZ-21 [38]. SEM image showed that crystals cover the support surface, however, the as-synthesized membrane was still not continuous. Therefore, multi-stage synthesis did not promote the formation of hydroxy-sodalite zeolite membrane under the current experiment conditions. 3.3. Gas permeation properties
and the synthesis procedure was repeated up to 3 times. Therefore, the total synthesis time for a three-stage synthesis was 45 min, which was the same as the synthesis time for the formation of pure hydroxy-sodalite in one-stage synthesis. In each stage, a fresh synthesis solution was used. Figs. 5 and 6 show the XRD patterns and SEM images of the as-synthesized membranes by MAHS with different synthesis stages, respectively. After one-stage synthesis, NaA zeolite and hydroxysodalite zeolite formed on the support. SEM images (Fig. 2(3) and (4)) confirm the XRD results. The crosssection image showed that the as-synthesized membrane
The best method to evaluate the quality of the zeolite membranes is gas permeation [6,10]. Since the opening pore size of hydroxy-sodalite zeolite is around 0.28 nm [20], only gases such as helium and hydrogen can diffuse through the pores, and gases with larger size, such as nbutane, are excluded from the pores. Therefore, the permselectivity of H2/n-C4H10 was selected as a yardstick for the quality of the hydroxy-sodalite zeolite membranes since n-C4H10 should not permeate through a defect-free hydroxy-sodalite zeolite membrane. Thus a higher the H2/n-C4H10 permselectivity implies fewer
Fig. 6. SEM images of the as-synthesized membranes by microwave-assisted hydrothermal synthesis with different stages of synthesis (synthesis time for each stage is 15 min). (1) and (2) Two-stage; (3) and (4) three-stage.
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Table 1 Gas permeation properties of the hydroxy-sodalite zeolite membrane and other types of zeolite membranes reported in the literatures Membranes
Alumina support Hydroxy-sodalite zeolite membrane NaA zeolite membrane [39] SAPO-34 zeolite membrane [40] SSZ-13 zeolite membrane [41] MFI zeolite membrane [6] X zeolite membrane [42]
Pore size (nm)
200–600 0.28 0.40 0.43–0.50 [43] 0.38 0.55 0.70
defects in the hydroxy-sodalite zeolite membrane. The permeance of H2 and n-C4H10 through the pure hydroxy-sodalite zeolite membrane (synthesis time of 45 min with one-stage synthesis) and the a-Al2O3 support under a pressure difference of 0.10 MPa were measured at room temperature and the results are shown in Table 1. The H2 permeance of the hydroxy-sodalite zeolite membrane decreased by two orders of magnitudes when compared to the a-Al2O3 support, which indicated that a compact zeolite membrane formed on the porous a-Al2O3 support. The permeance of n-C4H10 was below the limit of the practical flow rate measurement of 1.0 · 10 10 mol m 2 s 1 Pa 1. The permselectivity of H2/n-C4H10 was estimated to be larger than 1000 for the hydroxy-sodalite zeolite membrane. The gas permeation property of the hydroxy-sodalite zeolite membrane is compared with those of the other types of zeolite membranes in Table 1. The H2/nC4H10 permselectivity of the hydroxy-sodalite zeolite membrane is higher than those of the other types of zeolite membranes reported in the literature. The high H2/n-C4H10 permselectivity of hydroxy-sodalite zeolite membrane can be explained by the small pore size of hydroxy-sodalite zeolite. Hydroxy-sodalite zeolite has a six-membered ring of Si–O–Si bond and its pore size is 0.28 nm. The pore size of hydroxy-sodalite is smaller than those of the zeolites with an eight-membered ring of Si–O–Si bond, e.g., NaA zeolite (0.40 nm) and SAPO-34 (0.43–0.50 nm [43]), or with a ten-membered ring of Si–O–Si bond, e.g., MFI zeolite (0.55 nm) or with a twelve-membered ring of Si–O–Si bond, e.g., X zeolite (0.7 nm). Therefore, it is easy to understand that hydroxy-sodalite zeolite membrane possesses a higher H2/ n-C4H10 permselectivity than those of the other types of zeolite membranes. 3.4. Discussion Compared with the CHS method, MAHS method drastically reduces the crystallization time of zeolites. The synthesis of hydroxy-sodalite zeolite membrane by MAHS only needed 45 min, while only small levels of hydroxy-sodalite formed in the as-synthesized mem-
Permeance (10 7 mol m
Permselectivity 2
s
1
Pa 1)
H2
n-C4H10
a(H2/n-C4H10)
134 1.14 2.86 0.32 1.35 1.6 35
112 <0.001 0.027 <0.0007 0.09 0.048 8.9
1.12 >1000 106 >450 15 33 3.93
brane after 3 h of synthesis by CHS. Even after 6 h of synthesis by CHS, pure hydroxy-sodalite zeolite membrane did not form, and NaA, NaX and hydroxysodalite co-existed in the as-synthesized membrane. Therefore, the crystallization rate of hydroxy-sodalite zeolite on the porous a-Al2O3 support by MAHS improved by at least 8 times when compared with preparation by CHS. Pang et al. [44] also reported that the synthesis time for hydroxy-sodalite zeolite was 3–96 h by CHS method using the same synthesis mixture. The reason for the fast formation of zeolite membrane in the microwave environment is still unclear. Jansen et al. [26] suggested that the fast formation of zeolite by microwave heating could be attributed to the fast, homogeneous heating and the formation of ‘‘active’’ water molecules. Because of the fast and homogeneous heating by microwave, the nucleation of zeolites by MAHS is faster and more simultaneous than that by CHS. Compared with the hydrogen-bonded water molecules, the ‘‘active’’ water molecules has a higher potential to dissolve the gel, because the electron lone pairs and OH groups of the ‘‘active’’ water molecules are available to attack the gel bonding. Therefore, the fast formation of zeolite membrane on the porous support surface in the microwave field can be explained by the fast, homogeneous heating and the easy dissolution of the gel at the support/solution interface. Compared with the hydroxy-sodalite zeolite membrane synthesized by CHS method, the hydroxy-sodalite zeolite membrane synthesized by MAHS method was made of smaller and more uniform size hydroxy-sodalite zeolite crystals with higher purity. The formation of a zeolite membrane on a porous support is a heterogeneous nucleation process [10,45,46]. First, a gel layer forms on the porous support surface, followed by nucleation and crystal growth to form a membrane. In the microwave environment, because of the fast, homogeneous heating and the formation of ‘‘active’’ water molecules [27], the gel layer at the support/solution interface dissolves quickly that results in a faster and more simultaneous nucleation of zeolite on the support surface than if heating conventionally. Moreover, because of the simultaneous nucleation and homogeneous heating,
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small zeolite crystals with uniform size are synthesized. In the case of CHS, the nuclei are not formed on the support surface simultaneously because of the low dissolution rate of the gel and the low heating rate. Thus the zeolite crystals are not uniform in size. In order to form a dense membrane, a long synthesis time is needed and zeolite crystals are large [10,15]. The formation of zeolite with high purity can also be explained by the above hypothesis and by the different transformation process of the zeolite by CHS and by MAHS. In the case of MAHS, because the zeolite crystals nucleate and grow simultaneously, zeolite crystals formed at the early stage of synthesis (NaA zeolite) transform at approximately the same time into other types of zeolite crystals (hydroxy-sodalite zeolite). Therefore, a pure zeolite membrane forms. In the case of CHS, because of the low heating rate and inhomogeneous heating, zeolite crystals do not form at the same time. When the zeolite crystals formed at the early stage of synthesis (NaA zeolite) begin to transform into other types of zeolites (hydroxysodalite zeolite), the same types of zeolite crystals formed later are still stable. The formation of zeolite membrane with high purity is very important for the gas separation performance [14]. 4. Conclusions 1. High quality pure hydroxy-sodalite zeolite membrane was successfully synthesized on an a-Al2O3 support by a novel microwave-assisted hydrothermal synthesis method (MAHS). MAHS not only promoted the formation of hydroxy-sodalite zeolite membrane, but also inhibited the formation of impure zeolite in the hydroxy-sodalite zeolite membrane. Synthesis procedures drastically affected the formation of hydroxy-sodalite zeolite membrane. When a multi-stage synthesis procedure was employed, the hydroxy-sodalite zeolite membrane was impure. 2. The pure hydroxy-sodalite zeolite membrane synthesized by MAHS was found to be well inter-grown and the thickness of the membrane was 6–7 lm. Gas permeation results showed that the hydrogen/nbutane permselectivity of the hydroxy-sodalite zeolite membrane was larger than 1000. Therefore, hydroxysodalite zeolite membrane is a promising candidate for the separation of hydrogen from gas mixtures and important for the emerging hydrogen energy fuel system.
Acknowledgment This work was supported by the Prestigious Outstanding Scholars Overseas Award by the Chinese Aca-
demy of Sciences (POSOA-CAS) and the Ministry of Science and Technology of China (Grant No. 2003CB615802).
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Microwave-assisted hydrothermal synthesis of hydroxy-sodalite zeolite membrane Xiaochun Xu a
a,b,*
, Yun Bao a, Chunshan Song b, Weishen Yang
a,*
, Jie Liu a, Liwu Lin
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Clean Fuels and Catalysis Program, The Energy Institute, 209 Academic Project Buildings, Pennsylvania State University, University Park, PA 16802, USA Received 29 January 2004; received in revised form 30 June 2004; accepted 16 July 2004 Available online 25 September 2004
Abstract A high quality pure hydroxy-sodalite zeolite membrane was successfully synthesized on an a-Al2O3 support by a novel microwave-assisted hydrothermal synthesis (MAHS) method. Influence of synthesis conditions, such as synthesis time, synthesis procedure, etc., on the formation of hydroxy-sodalite zeolite membrane by MAHS method was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and gas permeation measurements. The synthesis of hydroxy-sodalite zeolite membrane by MAHS method only needed 45 min and synthesis was more than 8 times faster than by the conventional hydrothermal synthesis (CHS) method. A pure hydroxy-sodalite zeolite membrane was easily synthesized by MAHS method, while a zeolite membrane, which consisted of NaX zeolite, NaA zeolite and hydroxy-sodalite zeolite, was usually synthesized by CHS method. The effect of preparation procedures had a dramatic impact on the formation of hydroxy-sodalite zeolite membrane and a single-stage synthesis procedure produced a pure hydroxy-sodalite zeolite membrane. The pure hydroxy-sodalite zeolite membrane synthesized by MAHS method was found to be well inter-grown and the thickness of the membrane was 6–7 lm. Gas permeation results showed that the hydrogen/n-butane permselectivity of the hydroxy-sodalite zeolite membrane was larger than 1000. 2004 Elsevier Inc. All rights reserved. Keywords: Gas separation; Hydrothermal synthesis; Hydroxy-sodalite; Microwave heating; Zeolite membrane
1. Introduction During the past ten years, increasing attempts have been made to develop zeolites into membrane forms for separation and catalytic applications based on utilizing their molecular sieving properties, high thermal resistance, chemical inertness, high mechanical
*
Corresponding authors. Present address (X. Xu): 209 Academic Project Buildings, The Energy Institute, Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1 814 865 6617; fax: +1 814 863 7432 (X. Xu), tel.: +86 411 4379073; fax: +86 411 4379073 (W. Yang). E-mail addresses: [email protected] (X. Xu), [email protected] (W. Yang). 1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.07.019
strength and uniform pore sizes [1–5]. Several preparation methods have been developed, such as in situ hydrothermal synthesis [6–8], vapor phase transport method [9], secondary growth method [10–12] or embedding microcrystals of zeolite into a matrix [13]. Ideally, for a better separation performance, zeolite membrane should preferably be made of pure zeolite crystals with uniform and small particle size. Because the intergrowth between different types of zeolite crystals is not as good as that between the same types of zeolite crystals, the existence of impure zeolite crystals results in many inter-crystalline pores, which can have a detrimental effect on the separation performance of the zeolite membrane [14]. It is more difficult to form high quality zeolite membrane using zeolite crystals
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with broad particle size distribution than with narrow particle size distribution [10,15]. In addition, zeolite membranes made by large particles are thicker than that made by small particles and therefore has a lower permeance. However, conventional synthesis methods, such as, in situ hydrothermal synthesis and vapor phase transport method, need long crystallization times of a few hours to a few days. The long crystallization time usually results in the formation of impure zeolites [10]. Because of the low heating rate and the inhomogeneous heating, zeolite nuclei do not form on the support surface simultaneously and zeolite crystals are not uniform in size [15,16]. Therefore, the improvement in the separation performance of zeolite membranes can be achieved by developing a new synthesis approach to synthesize zeolite membranes, which are derived from zeolite crystals with uniform and small particle size and with high purity. Recently, a new synthesis method that combines hydrothermal crystallization with a microwave heating technique has been developed for the synthesis of zeolites, molecular sieves as well as zeolite membranes [15–36]. In the late 1980Õs, Chu et al. [17,18] first reported the synthesis of A and ZSM-5 zeolite by microwave heating. Since then more studies have been reported using microwave heating [17,18,26–35]. Different types of zeolites and molecular sieves such as, A [17,18,27], MFI [17,18,28,32], Beta [31], X/Y [27,28], AlPOs [29,30], SAPOs [34] and MCM-41 [35], have been synthesized by this novel microwave-assisted hydrothermal synthesis (MAHS) method. Compared with the conventional hydrothermal synthesis (CHS), MAHS of zeolites has the advantages of a short synthesis time, broad synthesis composition, small zeolite particle size, narrow particle size distribution and high purity [15,27]. These advantages prompt researchers to explore its use as a method for the synthesis of zeolite membranes. Up to date, A-type [15,16,19,21], Sodalite [20], X/Y-type [21,22], MFI [21], SAPO-5 [23] and AlPO4-5 [24,25] zeolite membranes have been synthesized. However, few separation data has been reported. Kita et al. [21,37] reported the liquid pervaporation separation by zeolite membrane synthesized by MAHS method. Previously, we reported the synthesis of a high gas permeance NaA zeolite membrane on a porous a-Al2O3 support prepared by MAHS method [15,16]. The synthesis time for NaA zeolite membrane was drastically reduced from 3 h for CHS to 15 min for MAHS. Zeolite crystals in the NaA zeolite membrane synthesized by MAHS were smaller and more uniform in particle size than those synthesized by CHS. The thickness of the NaA zeolite membrane synthesized by MAHS was about 4 lm, which was thinner than 8 lm for the NaA zeolite membrane synthesized by CHS. Therefore, the gas permeance of the NaA zeolite membrane synthesized by MAHS was four times higher than that of the NaA zeolite membrane synthe-
sized by CHS, while their permselectivities were comparable. MAHS method has proved to be a promising method for the synthesis of high quality zeolite membrane. Hydroxy-sodalite has the same framework structure as sodalite and consists of the cubic array of b-cages. Hydroxy-sodalite has a six-membered ring aperture ˚ . The pore size of hydroxywith a pore size of 2.8 A sodalite is smaller than that of the zeolites with an eight-membered ring aperture, e.g., NaA zeolite. Only small molecules, such as helium, hydrogen and water, etc., can enter the pore of hydroxy-sodalite. Therefore, hydroxy-sodalite zeolite membrane is expected to possess a better performance on the separation of small molecules from gas or liquid mixtures than the zeolite membranes with a bigger pore size. Recently, Julbe et al. [20] synthesized a sodalite/a-Al2O3 composite membrane by MAHS method with a relatively low He/N2 permselectivity of 6.2 at 115 C. In the present work, MAHS method was further explored in the synthesis of hydroxy-sodalite zeolite membrane. Through optimizing the synthesis conditions, a pure hydroxy-sodalite zeolite membrane with a high H2/nC4H10 permselectivity was synthesized.
2. Experimental 2.1. Support A porous a-Al2O3 disk (home-made, 30 mm in diameter, 3 mm in thickness, 0.1–0.3 lm pore radius, with 50% porosity) was used as the support. The surface of the support was polished with 700 grit sand paper on both sides, whereupon the support was cleaned with deionized water in a Branson SB2200 ultrasonic cleaner for 3–5 min to remove the loose particles created during polishing. Before hydrothermal synthesis, the cleaned support was calcined in air at 673 K for 3 h with a heating and cooling rate of 4.0 K/min to burn off any organics on the support surface. 2.2. Synthesis solution The synthesis solution was prepared by mixing an aluminate solution and a silicate solution. The aluminate solution was prepared by dissolving sodium hydroxide (40 g) in deionized water (159 g), then adding aluminum foil (1.0 g) to the caustic solution at room temperature. The silicate solution was prepared by mixing sodium hydroxide (34.1 g), silica sol (SiO2 27 wt%) (20.6 g) and deionized water (159 g). The aluminate solution, preheated to 50 C, was added to the silicate solution with stirring. In order to produce a clear homogeneous solution, the resulting mixture
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was stirred vigorously for 15 min. The molar ratio of this mixture solution was 5SiO2:Al2O3:50Na2O: 1000H2O. 2.3. Microwave synthesis of zeolite membrane
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3. Results and discussion 3.1. Microwave-assisted hydrothermal synthesis of hydroxy-sodalite zeolite membrane
2.4. Characterization
The formation of hydroxy-sodalite zeolite membrane on porous a-Al2O3 support by microwave-assisted hydrothermal synthesis (MAHS) method was investigated and the as-synthesized membranes were characterized by XRD and SEM. Fig. 1 shows the XRD patterns of the powder and as-synthesized membranes by MAHS method with different synthesis times. After 15 min of synthesis, the diffraction patterns of the as-synthesized membrane were represented by the sum of the diffraction patterns of a-Al2O3 support, NaA zeolite and hydroxy-sodalite zeolite. This meant that the as-synthesized membrane was not a pure hydroxy-sodalite zeolite membrane. After 45 min of synthesis, the diffraction patterns of NaA zeolite disappeared and the intensity of the diffraction peaks of hydroxy-sodalite intensified, which indicated that the NaA zeolite transformed into hydroxy-sodalite zeolite and a pure hydroxy-sodalite zeolite membrane formed on the porous a-Al2O3 support. Fig. 2 shows the surface and the cross-section SEM images of the as-synthesized membranes. After 15 min of synthesis, the surface of the porous a-Al2O3 support was covered with zeolite crystals with different shapes. According to their morphology, they were mainly NaA zeolite and hydroxy-sodalite zeolite. The cross-section image showed that the as-synthesized membrane was not continuous. After 45 min of synthesis, the surface of the aAl2O3 support was completely covered with randomly
The structure of the as-synthesized membranes was determined by X-ray diffraction (XRD) patterns. X-ray diffraction was carried out on a Ragaku D/ max II powder diffractometer using Cu Ka ˚ ) radiation operating at 40 KV and 50 mA. (k = 1.54 A The morphology and the thickness of the as-synthesized membrane was examined by scanning electron microscope (SEM). The SEM photographs were obtained on a JEM-1200E scanning electron microscope. The surface of the samples was sputtered with a gold film. Gas permeation measurements were carried out to evaluate the quality of the membranes. Before gas permeation measurements, the as-synthesized membranes were dried at 200 C overnight to remove the water absorbed in the zeolite channels. The dried zeolite membrane was sealed in a permeation cell with the zeolite membrane facing on the high-pressure side for gas permeation measurements. The single gas permeance was measured by a soap-film flowmeter at room temperature under the pressure difference of 0.10 MPa. The permselectivity of A/B was defined as the permeance ratio of gas A and gas B.
Fig. 1. XRD patterns of the powder and the as-synthesized membranes by microwave-assisted hydrothermal synthesis. (1) Support; (2) powder after 45 min synthesis; (3) membrane after 15 min synthesis; (4) membrane after 45 min synthesis. ( ) a-Al2O3 support; (d) NaA zeolite; () Hydroxy-sodalite zeolite.
The a-Al2O3 disk was placed vertically with a Teflon holder in a polyethylene bottle to avoid any precipitation during membrane synthesis. 100 ml synthesis solution was poured into the bottle carefully without hitting the support and the support was fully immersed in the synthesis solution. Then the bottle was covered with a cap. The crystallization was carried out in a modified domestic microwave oven operating at 2450 MHz. The synthesis mixture was heated up quickly from room temperature to 90 ± 5 C in 60 s with full power, and then held at the final temperature for the desired times with reduced power. In some cases, multi-stage synthesis [10] was carried out. For comparison, the synthesis was also carried out at 90 C by conventional heating. After synthesis, the as-synthesized membranes were washed several times with deionized water until the pH value of the washings was neutral, and then dried at 150 C for 3 h. Because of the different thermal expansion coefficient of the hydroxy-sodalite zeolite and the aAl2O3 support, a low heating and cooling rate, ca. 1 C/ min, was adopted to avoid crack formation during the thermal treatment.
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Fig. 2. SEM images of the as-synthesized membranes by microwave-assisted hydrothermal synthesis. (1) and (2) Support; (3) and (4) membrane after 15 min synthesis; (5) and (6) membrane after 45 min synthesis.
oriented pure hydroxy-sodalite zeolite crystals with uniform size of about 4 lm. There were no other types of zeolites present on the support surface. The zeolite crystals were found to be highly inter-grown and there were no observable inter-crystalline gaps in the zeolite membrane. These observations are in accordance with the XRD characterization. As the cross-section image shows, the hydroxy-sodalite zeolite membrane was bound firmly to the support with a thickness of approximately 6–7 lm. For comparison, the formation of hydroxy-sodalite zeolite membrane on the porous a-Al2O3 support by conventional hydrothermal synthesis (CHS) method was also investigated. Fig. 3 shows the XRD patterns of the as-synthesized membranes by CHS method with different synthesis times. After 15 min of synthesis, zeolitic materials did not form in the bulk clear synthesis solution or on the porous a-Al2O3 support surface. Diffraction patterns other than the a-Al2O3 support emerged after 1 h of synthesis. These diffraction patterns were attributed to NaA zeolite. After 2 h of synthesis, the intensity of the diffraction peaks of NaA zeolite in-
creased, which implied that more NaA zeolite crystals formed on the support. When the synthesis time was extended to 3 h, the intensity of the diffraction peaks of NaA zeolite became weak. With further extending the synthesis time to 4 and 6 h, diffraction patterns of zeolite X and hydroxy-sodalite zeolite appeared. These phenomena indicated that the NaA zeolite on the support surface transformed into other types of zeolites in the alkaline synthesis solution. However, NaA zeolite did not fully transform into hydroxy-sodalite zeolite even after 6 h of synthesis and pure hydroxy-sodalite zeolite membrane did not form. SEM images of the as-synthesized membranes by CHS are shown in Fig. 4. After 1 h of synthesis, discrete NaA zeolite crystals were observed on the support surface. When the synthesis time increased to 2 h, the coverage of the NaA zeolite crystals on the support surface increased and small amounts of hydroxy-sodalite zeolite formed. However, there was no continuous zeolite membrane formed on the support, as revealed from the cross-section SEM image (not shown). When the synthesis time was extended to 3 h, the NaA zeolite on the support surface dissolved and
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Fig. 3. XRD patterns of the as-synthesized membranes by conventional hydrothermal synthesis with different synthesis times. (1) Support; (2) 1 h; (3) 2 h; (4) 3 h; (5) 4 h; (6) 6 h. ( ) a-Al2O3 support; (d) NaA zeolite; () Hydroxy-sodalite zeolite; (j) NaX zeolite.
other types of zeolites formed, which is consistent with the XRD results. When the synthesis time was further extended to 6 h, NaA, NaX and hydroxy-sodalite co-existed in the as-synthesized membrane. The morphology of the hydroxy-sodalite crystals was a ‘‘ball of yarn’’. The morphology of the hydroxy-sodalite crystals syn-
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thesized by CHS and by MAHS was different. Whether the difference is caused by microwave heating is unknown. Comparing the formation of hydroxy-sodalite zeolite membrane by MAHS with that by CHS, MAHS significantly promoted the formation of hydroxy-sodalite zeolite membrane with a shorter synthesis time. The synthesis of hydroxy-sodalite zeolite membrane by MAHS only needed 45 min, while NaA, NaX and hydroxy-sodalite co-existed in the as-synthesized membrane even after 6 h of synthesis by CHS. Secondly, small hydroxy-sodalite zeolite crystals with uniform size formed by MAHS, while larger zeolite crystals formed by CHS. In addition, hydroxy-sodalite zeolite crystals in the zeolite membrane synthesized by MAHS were found to be well inter-grown, while there were big gaps between the zeolite crystals in the zeolite membrane synthesized by CHS. Lastly, pure hydroxy-sodalite zeolite membrane was synthesized by MAHS, while a zeolite membrane consisting of NaA, NaX and hydroxy-sodalite was synthesized by CHS. The reason for the promoting effect of microwave heating on the formation of hydroxy-sodalite zeolite membrane will be discussed later. 3.2. Synthesis procedure A multi-stage synthesis procedure was carried out to investigate the effect of synthesis procedure on the formation of hydroxy-sodalite zeolite membrane by MAHS. The synthesis time for each stage was 15 min
Fig. 4. SEM images of the as-synthesized membranes by conventional hydrothermal synthesis with different synthesis times. (1) 1 h; (2) 2 h; (3) 3 h; (4) 6 h.
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Fig. 5. XRD patterns of the as-synthesized membranes by microwaveassisted hydrothermal synthesis with different stages of synthesis (synthesis time for each stage is 15 min). (1) One-stage; (2) two-stage; (3) three-stage. ( ) a-Al2O3 support; (d) NaA zeolite; () Hydroxysodalite zeolite; ( ) unknown phase.
was not continuous when prepared by one-stage synthesis. After two-stage synthesis, XRD patterns of NaA zeolite disappeared and the intensity of the XRD patterns of hydroxy-sodalite intensified, which indicated that NaA zeolite transformed into hydroxy-sodalite zeolite. However, hydroxy-sodalite zeolite crystals were very small and were not observed in the SEM images even at high magnification. One again, the as-synthesized membrane was still not continuous when two-stage synthesis was performed. After three-stage synthesis, the as-synthesized zeolite membrane was a mixture of hydroxy-sodalite and other unidentified phases. One such unidentified phase may possibly be NaZ-21 [38]. SEM image showed that crystals cover the support surface, however, the as-synthesized membrane was still not continuous. Therefore, multi-stage synthesis did not promote the formation of hydroxy-sodalite zeolite membrane under the current experiment conditions. 3.3. Gas permeation properties
and the synthesis procedure was repeated up to 3 times. Therefore, the total synthesis time for a three-stage synthesis was 45 min, which was the same as the synthesis time for the formation of pure hydroxy-sodalite in one-stage synthesis. In each stage, a fresh synthesis solution was used. Figs. 5 and 6 show the XRD patterns and SEM images of the as-synthesized membranes by MAHS with different synthesis stages, respectively. After one-stage synthesis, NaA zeolite and hydroxysodalite zeolite formed on the support. SEM images (Fig. 2(3) and (4)) confirm the XRD results. The crosssection image showed that the as-synthesized membrane
The best method to evaluate the quality of the zeolite membranes is gas permeation [6,10]. Since the opening pore size of hydroxy-sodalite zeolite is around 0.28 nm [20], only gases such as helium and hydrogen can diffuse through the pores, and gases with larger size, such as nbutane, are excluded from the pores. Therefore, the permselectivity of H2/n-C4H10 was selected as a yardstick for the quality of the hydroxy-sodalite zeolite membranes since n-C4H10 should not permeate through a defect-free hydroxy-sodalite zeolite membrane. Thus a higher the H2/n-C4H10 permselectivity implies fewer
Fig. 6. SEM images of the as-synthesized membranes by microwave-assisted hydrothermal synthesis with different stages of synthesis (synthesis time for each stage is 15 min). (1) and (2) Two-stage; (3) and (4) three-stage.
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Table 1 Gas permeation properties of the hydroxy-sodalite zeolite membrane and other types of zeolite membranes reported in the literatures Membranes
Alumina support Hydroxy-sodalite zeolite membrane NaA zeolite membrane [39] SAPO-34 zeolite membrane [40] SSZ-13 zeolite membrane [41] MFI zeolite membrane [6] X zeolite membrane [42]
Pore size (nm)
200–600 0.28 0.40 0.43–0.50 [43] 0.38 0.55 0.70
defects in the hydroxy-sodalite zeolite membrane. The permeance of H2 and n-C4H10 through the pure hydroxy-sodalite zeolite membrane (synthesis time of 45 min with one-stage synthesis) and the a-Al2O3 support under a pressure difference of 0.10 MPa were measured at room temperature and the results are shown in Table 1. The H2 permeance of the hydroxy-sodalite zeolite membrane decreased by two orders of magnitudes when compared to the a-Al2O3 support, which indicated that a compact zeolite membrane formed on the porous a-Al2O3 support. The permeance of n-C4H10 was below the limit of the practical flow rate measurement of 1.0 · 10 10 mol m 2 s 1 Pa 1. The permselectivity of H2/n-C4H10 was estimated to be larger than 1000 for the hydroxy-sodalite zeolite membrane. The gas permeation property of the hydroxy-sodalite zeolite membrane is compared with those of the other types of zeolite membranes in Table 1. The H2/nC4H10 permselectivity of the hydroxy-sodalite zeolite membrane is higher than those of the other types of zeolite membranes reported in the literature. The high H2/n-C4H10 permselectivity of hydroxy-sodalite zeolite membrane can be explained by the small pore size of hydroxy-sodalite zeolite. Hydroxy-sodalite zeolite has a six-membered ring of Si–O–Si bond and its pore size is 0.28 nm. The pore size of hydroxy-sodalite is smaller than those of the zeolites with an eight-membered ring of Si–O–Si bond, e.g., NaA zeolite (0.40 nm) and SAPO-34 (0.43–0.50 nm [43]), or with a ten-membered ring of Si–O–Si bond, e.g., MFI zeolite (0.55 nm) or with a twelve-membered ring of Si–O–Si bond, e.g., X zeolite (0.7 nm). Therefore, it is easy to understand that hydroxy-sodalite zeolite membrane possesses a higher H2/ n-C4H10 permselectivity than those of the other types of zeolite membranes. 3.4. Discussion Compared with the CHS method, MAHS method drastically reduces the crystallization time of zeolites. The synthesis of hydroxy-sodalite zeolite membrane by MAHS only needed 45 min, while only small levels of hydroxy-sodalite formed in the as-synthesized mem-
Permeance (10 7 mol m
Permselectivity 2
s
1
Pa 1)
H2
n-C4H10
a(H2/n-C4H10)
134 1.14 2.86 0.32 1.35 1.6 35
112 <0.001 0.027 <0.0007 0.09 0.048 8.9
1.12 >1000 106 >450 15 33 3.93
brane after 3 h of synthesis by CHS. Even after 6 h of synthesis by CHS, pure hydroxy-sodalite zeolite membrane did not form, and NaA, NaX and hydroxysodalite co-existed in the as-synthesized membrane. Therefore, the crystallization rate of hydroxy-sodalite zeolite on the porous a-Al2O3 support by MAHS improved by at least 8 times when compared with preparation by CHS. Pang et al. [44] also reported that the synthesis time for hydroxy-sodalite zeolite was 3–96 h by CHS method using the same synthesis mixture. The reason for the fast formation of zeolite membrane in the microwave environment is still unclear. Jansen et al. [26] suggested that the fast formation of zeolite by microwave heating could be attributed to the fast, homogeneous heating and the formation of ‘‘active’’ water molecules. Because of the fast and homogeneous heating by microwave, the nucleation of zeolites by MAHS is faster and more simultaneous than that by CHS. Compared with the hydrogen-bonded water molecules, the ‘‘active’’ water molecules has a higher potential to dissolve the gel, because the electron lone pairs and OH groups of the ‘‘active’’ water molecules are available to attack the gel bonding. Therefore, the fast formation of zeolite membrane on the porous support surface in the microwave field can be explained by the fast, homogeneous heating and the easy dissolution of the gel at the support/solution interface. Compared with the hydroxy-sodalite zeolite membrane synthesized by CHS method, the hydroxy-sodalite zeolite membrane synthesized by MAHS method was made of smaller and more uniform size hydroxy-sodalite zeolite crystals with higher purity. The formation of a zeolite membrane on a porous support is a heterogeneous nucleation process [10,45,46]. First, a gel layer forms on the porous support surface, followed by nucleation and crystal growth to form a membrane. In the microwave environment, because of the fast, homogeneous heating and the formation of ‘‘active’’ water molecules [27], the gel layer at the support/solution interface dissolves quickly that results in a faster and more simultaneous nucleation of zeolite on the support surface than if heating conventionally. Moreover, because of the simultaneous nucleation and homogeneous heating,
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small zeolite crystals with uniform size are synthesized. In the case of CHS, the nuclei are not formed on the support surface simultaneously because of the low dissolution rate of the gel and the low heating rate. Thus the zeolite crystals are not uniform in size. In order to form a dense membrane, a long synthesis time is needed and zeolite crystals are large [10,15]. The formation of zeolite with high purity can also be explained by the above hypothesis and by the different transformation process of the zeolite by CHS and by MAHS. In the case of MAHS, because the zeolite crystals nucleate and grow simultaneously, zeolite crystals formed at the early stage of synthesis (NaA zeolite) transform at approximately the same time into other types of zeolite crystals (hydroxy-sodalite zeolite). Therefore, a pure zeolite membrane forms. In the case of CHS, because of the low heating rate and inhomogeneous heating, zeolite crystals do not form at the same time. When the zeolite crystals formed at the early stage of synthesis (NaA zeolite) begin to transform into other types of zeolites (hydroxysodalite zeolite), the same types of zeolite crystals formed later are still stable. The formation of zeolite membrane with high purity is very important for the gas separation performance [14]. 4. Conclusions 1. High quality pure hydroxy-sodalite zeolite membrane was successfully synthesized on an a-Al2O3 support by a novel microwave-assisted hydrothermal synthesis method (MAHS). MAHS not only promoted the formation of hydroxy-sodalite zeolite membrane, but also inhibited the formation of impure zeolite in the hydroxy-sodalite zeolite membrane. Synthesis procedures drastically affected the formation of hydroxy-sodalite zeolite membrane. When a multi-stage synthesis procedure was employed, the hydroxy-sodalite zeolite membrane was impure. 2. The pure hydroxy-sodalite zeolite membrane synthesized by MAHS was found to be well inter-grown and the thickness of the membrane was 6–7 lm. Gas permeation results showed that the hydrogen/nbutane permselectivity of the hydroxy-sodalite zeolite membrane was larger than 1000. Therefore, hydroxysodalite zeolite membrane is a promising candidate for the separation of hydrogen from gas mixtures and important for the emerging hydrogen energy fuel system.
Acknowledgment This work was supported by the Prestigious Outstanding Scholars Overseas Award by the Chinese Aca-
demy of Sciences (POSOA-CAS) and the Ministry of Science and Technology of China (Grant No. 2003CB615802).
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