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Synthesis of NaA zeolite membrane by microwave heating
Separation and Purification Technology 25 (2001) 241– 249 www.elsevier.com/locate/seppur
Synthesis of NaA zeolite membrane by microwave heating Xiaochun Xu, Weishen Yang *, Jie Liu, Liwu Lin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
Abstract The synthesis of NaA zeolite membrane on a porous a-Al2O3 support by microwave heating (MH) was investigated. The formation of a NaA zeolite membrane was drastically promoted by MH. The synthesis time was reduced from 3 h for conventional heating (CH) to 15 min for MH. Surface seeding cannot only promote the formation of NaA zeolite on the support, but also inhibit the transformation of NaA zeolite into other types of zeolites. The thickness of the NaA zeolite membrane synthesized by MH was about 4 mm, thinner than that of NaA zeolite membrane synthesized by CH. The permeance of NaA zeolite membrane synthesized by MH was four times higher than that of the NaA zeolite membrane synthesized by CH, while their permselectivities were comparable. Multi-stage synthesis resulted in the transformation of NaA zeolite into other types of zeolites, and the perfection of the as-synthesized membrane decreased. The formation mechanism of NaA zeolite membrane on the porous a-Al2O3 support by MH was proposed. The promotion effect on the formation of NaA zeolite membrane by MH can be divided into two parts: the ‘‘thermal effect’’ and the ‘‘microwave effect’’. The formation of a homogeneous and thin NaA zeolite membrane resulted from both the ‘‘thermal effect’’ and the ‘‘microwave effect’’, while the fast formation of NaA zeolite membrane was mainly caused by the ‘‘microwave effect’’. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Gas permeation; Hydrothermal synthesis; Microwave heating; NaA zeolite; Zeolite membrane
1. Introduction The zeolite membrane constitutes a special class of inorganic membranes that has developed very rapidly in recent years [1 – 7]. The zeolite membrane has distinctive advantages, such as uniform molecular-sized pores and a high thermal stability. Moreover, the pore size and affinity of the zeolites can be finely controlled by various methods, for example, ion exchange and vapor-phase * Corresponding author. Tel.: +86-411-4671991; ext: 744; fax: + 86-411-4694447. E-mail address: [email protected] (W. Yang).
deposition. Until now, the zeolite membrane has shown a very good separation performance with several industrial important mixtures, e.g. butane isomers and xylene isomers [7–15]. However, the permeance is too low for practical applications. Thus, one of the most challenging works in the field of zeolite membrane is to prepare a zeolite membrane with a high permeance, while keeping the separation factor high. The microwave is a kind of electromagnetic radiation with a high frequency between 0.3 and 300 GHz. The microwave technique was widely used in all kinds of research fields, e.g. biology and medicine. After the 1980s, the microwave
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technique began to be used in the field of chemistry, and a new interdisciplinary of microwave chemistry appeared [16]. At the beginning of the 1990s, the microwave technique began to be applied in the synthesis of zeolite. In 1990, Chu et al. [17] firstly reported the synthesis of A-type and ZSM-5 zeolite by MH. Later, more studies were reported [18–27]. Compared with the conventional hydrothermal synthesis, microwave synthesis of zeolites has the advantages of a very short synthesis time, narrow particle size distribution, broad synthesis composition and high purity. These advantages prompt us to explore its use as a method for the synthesis of zeolite membranes. However, until now, there have been few reports on the synthesis of zeolite membranes by MH [28– 32], and no gas permeation data have been reported. Recently, we reported the synthesis of a high-permeance NaA zeolite membrane by MH [33]. In this paper, a more detailed investigation was reported, and the effect of microwave was distinguished for the first time.
2. Experimental
2.1. Synthesis of NaA zeolite membrane A porous a-Al2O3 disk (30 mm in diameter, 3 mm in thickness, 0.10.3 mm pore radius, about 50% porosity, and homemade) was used as the support. The surface of the support was polished with sandpaper on both sides, after which the support was cleaned with deionized water in a ultrasonic cleaner for c. 3– 5 min to remove the loose particles created during polishing. The cleaned support was calcined in air at 400°C for 3 h to burn off the organics on the support surface before hydrothermal synthesis or coating the seeds. One side of the support was coated with NaA zeolite crystals as nucleation seeds before synthesis [34]. The synthesis solution was prepared by mixing an aluminate solution and 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 was stirred vigorously for 15 min. The molar ratio of this mixture solution was 5SiO2:Al2O3:50Na2O: 1000H2O. The support was placed vertically with a Teflon holder in a polyethylene bottle to avoid any precipitation of zeolite crystals onto the support during the membrane synthesis. The synthesis mixture was added to the bottle carefully without hitting the support, and 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 909 5°C in 60 s, and then held at the final temperature for the desired time. In some cases, multi-stage synthesis was carried out or the unseeded a-Al2O3 disk was used as the support. 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 NaA zeolite and the a-Al2O3 support, a low heating and cooling rate, c. 1 K/min, was adopted to avoid crack formation during the thermal treatment
2.2. Membrane characterization The structure of the as-synthesized membrane was determined by X-ray diffraction (XRD) patterns. XRD was carried out on a Ragaku D max/b powder diffractometer using Cu Ka (l= 1.54 A, ) radiation operating at 40 kV and 50 mA. The morphology and thickness of the as-synthesized membrane were examined using a scanning electron microscope (SEM). The SEM photographs were obtained on a JEM-1200E scanning electron microscope. Gas permeation was carried out to evaluate the perfection of the as-synthesized membrane. The as-synthesized membrane was sealed in a permeation module with the zeo-
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lite membrane on the high-pressure side. The permeances of the membranes were measured by a soap-film flowmeter under a pressure difference of 0.10 MPa at 25°C. The permselectivity of A/B is defined as the permeance ratio of gas A and gas B.
3. Results
3.1. Synthesis of NaA zeolite membrane by microwa6e heating The synthesis of NaA zeolite membrane on the seeded a-Al2O3 support by MH was investigated, and the as-synthesized membranes were characterized by XRD and SEM. Fig. 1 shows the XRD patterns of the as-synthesized membrane. After 1 min of synthesis, the synthesis solution became cloudy, and NaA zeolite was detected in the synthesis mixture by XRD. However, the intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane did not increase significantly compared with that of the seeded support.
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After 10 min of synthesis, the intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane increased. After 15 min of synthesis, the synthesis solution became clear again, and the intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane was the highest among the zeolite membrane investigated. In addition, only the diffraction patterns of NaA zeolite and a-Al2O3 appeared in the XRD diagram, which indicated that only NaA zeolite membrane formed on the support. Further increasing the synthesis time resulted in the transformation of NaA zeolite into other types of zeolites. A hydroxy-sodalite zeolite membrane formed after 45 min of synthesis. Compared with the formation of NaA zeolite membrane by CH [34], the formation of NaA zeolite membrane was drastically promoted by MH. The synthesis of NaA zeolite membrane needs 2–3 h by CH, while only needs 15 min by MH. The synthesis time is eight to 12 times shorter by MH than by CH. The surface and cross-section of the NaA zeolite membranes synthesized by MH of 15 min and CH of 3 h were further compared by SEM, and the SEM images are shown in Fig. 2. The surface of the a-Al2O3 support after MH synthesis was completely covered with randomly oriented NaA zeolite crystals with a uniform size of about 2 mm, and the crystals were highly intergrown. The surface of the a-Al2O3 support after CH synthesis was also covered with NaA zeolite crystals. However, the surface of the zeolite membrane was rough, and the zeolite crystals were not uniform in size. The zeolite membrane synthesized by MH was 4 mm thick, thinner than the 5–8 mm of the membrane synthesized by CH.
3.2. Effect of surface seeding
Fig. 1. X-ray diffraction patterns of the as-synthesized membranes by MH. (1) support; (2) seeded support; (3) 1 min; (4) 10 min; (5) 15 min; (6) 45 min. (*) a-Al2O3 support; ( ) NaA zeolite; (") hydroxy-sodalite zeolite.
The effect of surface seeding on the formation of NaA zeolite membrane by MH was also investigated. The XRD patterns and the SEM images of the as-synthesized membranes are shown in Figs. 3 and 2, respectively. The intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane with surface seeding was stronger than that of the as-synthesized membrane without surface seeding, which indicated
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Fig. 2. SEM images of the as-synthesized membrane by MH with surface seeding (1); by MH without surface seeding (2) and by CH with surface seeding (3).
that surface seeding promoted the formation of NaA zeolite membrane on the porous a-Al2O3 support. In addition, hydroxy-sodalite zeolite formed on the unseeded support, while no other types of zeolites formed on the seeded support, which implied that surface seeding also inhibited the transformation of NaA zeolite into other types of zeolites. The SEM images confirmed the XRD results. After synthesis, the sup-
port surface was completely covered with NaA zeolite crystals for the seeded support and was sparse for the unseeded support. As the cross-section images shown, there was a continuous membrane formed on the seeded support, and no continuous membrane formed on the unseeded support. The same phenomenon was also observed in the synthesis of NaA zeolite membrane by CH [34].
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Fig. 3. XRD patterns of the as-synthesized membranes with (1) and without (2) the aid of surface seeding. (*) a-Al2O3 support; ( ) NaA zeolite; (") hydroxy-sodalite zeolite.
3.3. Effect of multi-stage synthesis In the synthesis of NaA zeolite membrane by CH, the perfection of the membrane was improved by employing the multi-stage synthesis method [34]. In order to improve the perfection of NaA zeolite membrane, multi-stage synthesis was also employed in the MH synthesis. Figs. 4 and 5 show the XRD patterns and SEM images of the as-synthesized membrane after multi-stage synthesis, respectively. The XRD patterns showed that the diffraction patterns of NaA zeolite disappeared after a two-stage synthesis. Only two weak peaks appeared at 15 and 25°. These peaks can be ascribed to the diffraction patterns of hydroxy-so-
Fig. 4. XRD patterns of the as-synthesized membrane with multi-stage synthesis by MH. (1) one-stage synthesis; (2) twostage synthesis; (3) three-stage synthesis. (*) a-Al2O3 support; ( ) NaA zeolite; (") hydroxy-sodalite zeolite.
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dalite zeolite. From the SEM images, it can be seen that the NaA zeolite membrane dissolved, and new types of zeolites began to form on the support. After a three-stage synthesis, the intensity of the diffraction patterns of hydroxy-sodalite zeolite increased. The SEM image showed that a new type of zeolite membrane formed, and the thickness of the membrane was about 10 mm. Comparing the MH synthesis with the CH synthesis, the transformation of NaA zeolite on the support surface was totally different. In the synthesis by CH, the NaA zeolite transformed gradually. With the dissolution of the NaA zeolite, other types of zeolites, e.g. NaX zeolite and hydroxy-sodalite zeolite, formed. The NaA zeolite even existed in the membrane with a synthesis time of 4 h after a three-stage synthesis [34]. On the contrary, the NaA zeolite was suddenly transformed in the MH synthesis. The NaA zeolite totally dissolved after a two-stage synthesis. After a three-stage synthesis, hydroxy-sodalite zeolite formed on the support.
3.4. Perfection e6aluation of the as-synthesized membrane The perfection of the as-synthesized membrane was evaluated by gas permeation, and the permselectivity of H2/n-C4H10 was selected as the yardstick of the perfection of the NaA zeolite membrane [34]. Table 1 shows the permeance of H2, n-C4H10 as well as the permselectivity of H2/n-C4H10 of the as-synthesized membranes. After a one-stage synthesis, the H2 permeance of the as-synthesized membrane decreased by one magnitude compared with that of the seeded support, which indicated that a dense NaA zeolite membrane formed on the support. The permselectivity of H2/n-C4H10 was 11.9, higher than that of the Knudsen diffusion selectivity of 5.39, which meant that the gases mainly permeated through the NaA zeolite channels. However, the permeation of n-C4H10 indicated that the NaA zeolite membrane had certain defects with the diameter larger than the pore size of the NaA zeolite channels. The perfection of the membrane after a two-stage synthesis and a three-stage synthesis decreased.
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Fig. 5. SEM images of the as-synthesized membrane with multi-stage synthesis by MH. (1), (2) two-stage synthesis; (3), (4) three-stage synthesis.
4. Discussion
4.1. Effect of microwa6e in the synthesis of zeolite membrane The effect of microwave in the synthesis of zeolites can be divided into two parts — the ‘‘thermal effect’’ and the ‘‘microwave effect’’. The ‘‘thermal effect’’ refers to the fast and homogeneous heating of microwave. The ‘‘microwave effect’’ means the changes of the characteristic of the substance in the microwave field. In the synthesis of zeolites, the ‘‘microwave effect’’ mainly refers to the change of the characteristic of water in the microwave field. Jansen et al. [19] considered that the hydrogen bridges of the water molecules are destroyed in the microwave field and the ‘‘active’’ water forms. The ‘‘active’’ water has a high activation energy, and the synthesis gel can be easily dissolved by ‘‘active’’ water. Therefore, the synthesis of zeolite was promoted. The formation of a zeolite membrane on a porous support is a heterogeneous nucleation pro-
cess. First, a gel layer formed on the porous support surface, followed by nucleation and crystal growth to form a membrane [35,36]. Accordingly, the synthesis process of zeolite membrane by MH and CH is proposed in Fig. 6. In the microwave environment, because of the ‘‘microwave effect’’, the water became ‘‘active’’ water, and the dissolution of the gel layer in the support/ Table 1 Perfection evaluation of the as-synthesized membrane by gas permeation Membrane
Permeance (10−8mol m−2 s−1 Pa−1)
Permselectivity
H2
n-C4H10
h(H2/n-C4H10)
1120 17.9
1.51 11.9
11.8
6.11
39.2
3.39
Seeded support 1690 One-stage 213 synthesis Two-stage 72.1 synthesis Three-stage 133 synthesis
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Fig. 6. Comparative synthesis model of zeolite membrane by MH and CH.
solution interface was accelerated. At the same time, because of the ‘‘thermal effect’’, the synthesis mixture was heated up quickly and homogeneously. Under this situation, a great number of zeolite nuclei formed on the support surface simultaneously and homogeneously. In addition, because of the simultaneous nucleation and the homogeneous heating, uniform and small zeolite crystals can be synthesized. As a result, a thin zeolite membrane can be formed in a very short synthesis time. When CH was used, because of the low dissolution rate of the gel layer and the low heating rate, the nuclei did not form on the support surface simultaneously. Therefore, the assynthesized zeolite crystals were not uniform in size. In order to form a continuous membrane, a long synthesis time will be needed and the zeolite membrane will be inhomogeneous and thick. The different transformation process of the NaA zeolite membrane by CH and by MH can also be explained by the different formation mechanism of zeolite membrane. In the case of MH, because the zeolites nucleated and grew simultaneously, NaA zeolite transformed at the same time. Therefore, a pure zeolite membrane can be formed. In the case of CH, because of the low heating rate and inhomogeneous heating, NaA zeolite did not form at the same time. The NaA zeolite crystals formed at the early stage began to transform into other types of zeolites, while the other NaA zeolite crystals was stable. In order to distinguish between the ‘‘thermal effect’’ and the ‘‘microwave effect’’ in the synthesis of the NaA zeolite membrane by MH, the following experiment was designed. A seeded sup-
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port was placed in the synthesis mixture and heated up quickly to 90°C by MH, and then the synthesis system was transferred to a conventional oven, which was preheated to 90°C. After 15 min of synthesis, the as-synthesized membrane was characterized by XRD. If the intensity of the diffraction patterns of NaA zeolite does not change significantly compared with that of the seeded support, it can be concluded that the promotion effect of the microwave is mainly caused by the ‘‘microwave effect’’. If the intensity of the diffraction patterns of NaA zeolite is similar to that of the NaA zeolite membrane synthesized by MH, it can be concluded that the ‘‘thermal effect’’ is the main effect. It was shown, from the XRD patterns (Fig. 7) that there is no significant difference in the intensity of the diffraction patterns of NaA zeolite between the as-synthesized membrane and the seeded support. The intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane is smaller than that of the NaA zeolite membrane synthesized by MH of 15 min. Thus, the formation of NaA zeolite membrane by MH was mainly promoted by the ‘‘microwave effect’’. However, because of the fast and homogeneous heating and the easy dissolution of the gel layer, a great number of zeolite nucleated and grew on the support surface simultaneously and homogeneously. Therefore, uniform and small NaA zeolite crystals can be synthesized. As a result, thin NaA zeolite membrane can be obtained, and so the formation of a thin and homogeneous NaA zeolite membrane by MH was caused by both the ‘‘thermal effect’’ and the ‘‘microwave effect’’.
Fig. 7. XRD patterns of the as-synthesized membranes by MH (1) and MH +CH (2). (*) a-Al2O3 support; ( ) NaA zeolite.
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5. Conclusion
Fig. 8. Gas-permeation properties of the seeded support ("), the NaA zeolite membranes synthesized by MH ( ) and by CH ().
Microwave-assisted hydrothermal synthesis method is an effective method for the synthesis of a high-permeance and high-selectivity zeolite membrane. The synthesis time of NaA zeolite membrane is eight to 12 times shorter by MH than by CH. The permeance of the NaA zeolite membrane synthesized by MH is four times higher than that of the NaA zeolite membrane synthesized by CH, while their permselectivities are comparable. The promotion effect of microwave can be divided into two parts: the ‘‘thermal effect’’ and the ‘‘microwave effect’’. The fast formation of NaA zeolite membrane was mainly caused by the ‘‘microwave effect’’, while the formation of a homogeneous and thin NaA zeolite membrane resulted from both the ‘‘thermal effect’’ and the ‘‘microwave effect’’.
4.2. Gas-permeation properties Acknowledgements Fig. 8 shows the gas-permeation properties of the seeded support, the NaA zeolite membrane synthesized by MH of 15 min and CH of 3 h. The permeance of H2, O2, N2 and n-C4H10 of the NaA zeolite membrane synthesized by MH decreased as the gas molecular kinetic diameter increased. The permselectivities of O2/N2 and H2/n-C4H10 were 1.02 and 11.8, respectively, higher than those of the Knudsen diffusion selectivity of 0.96 and 5.39, which showed the molecular sieving effect of the NaA zeolite membrane and indicated that the gases mainly permeated through the NaA zeolite channels [34]. The permselectivities of O2/N2 and H2/n-C4H10 were 1.10 and 9.83, respectively, for the NaA zeolite membrane synthesized by CH [34], similar to those for the NaA zeolite membrane synthesized by MH. However, the NaA zeolite membrane synthesized by MH had a higher permeance, e.g. the permeance of H2 was four times higher than that of the NaA zeolite membrane synthesized by CH. The higher permeance of the NaA zeolite membrane may be attributed to the formation of a thinner NaA zeolite membrane by MH than by CH.
The authors gratefully acknowledge the funding from the National Science Foundation of China (59789201) and the National Advanced Materials Committee of China (715-006-0120).
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Synthesis of NaA zeolite membrane by microwave heating Xiaochun Xu, Weishen Yang *, Jie Liu, Liwu Lin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
Abstract The synthesis of NaA zeolite membrane on a porous a-Al2O3 support by microwave heating (MH) was investigated. The formation of a NaA zeolite membrane was drastically promoted by MH. The synthesis time was reduced from 3 h for conventional heating (CH) to 15 min for MH. Surface seeding cannot only promote the formation of NaA zeolite on the support, but also inhibit the transformation of NaA zeolite into other types of zeolites. The thickness of the NaA zeolite membrane synthesized by MH was about 4 mm, thinner than that of NaA zeolite membrane synthesized by CH. The permeance of NaA zeolite membrane synthesized by MH was four times higher than that of the NaA zeolite membrane synthesized by CH, while their permselectivities were comparable. Multi-stage synthesis resulted in the transformation of NaA zeolite into other types of zeolites, and the perfection of the as-synthesized membrane decreased. The formation mechanism of NaA zeolite membrane on the porous a-Al2O3 support by MH was proposed. The promotion effect on the formation of NaA zeolite membrane by MH can be divided into two parts: the ‘‘thermal effect’’ and the ‘‘microwave effect’’. The formation of a homogeneous and thin NaA zeolite membrane resulted from both the ‘‘thermal effect’’ and the ‘‘microwave effect’’, while the fast formation of NaA zeolite membrane was mainly caused by the ‘‘microwave effect’’. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Gas permeation; Hydrothermal synthesis; Microwave heating; NaA zeolite; Zeolite membrane
1. Introduction The zeolite membrane constitutes a special class of inorganic membranes that has developed very rapidly in recent years [1 – 7]. The zeolite membrane has distinctive advantages, such as uniform molecular-sized pores and a high thermal stability. Moreover, the pore size and affinity of the zeolites can be finely controlled by various methods, for example, ion exchange and vapor-phase * Corresponding author. Tel.: +86-411-4671991; ext: 744; fax: + 86-411-4694447. E-mail address: [email protected] (W. Yang).
deposition. Until now, the zeolite membrane has shown a very good separation performance with several industrial important mixtures, e.g. butane isomers and xylene isomers [7–15]. However, the permeance is too low for practical applications. Thus, one of the most challenging works in the field of zeolite membrane is to prepare a zeolite membrane with a high permeance, while keeping the separation factor high. The microwave is a kind of electromagnetic radiation with a high frequency between 0.3 and 300 GHz. The microwave technique was widely used in all kinds of research fields, e.g. biology and medicine. After the 1980s, the microwave
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technique began to be used in the field of chemistry, and a new interdisciplinary of microwave chemistry appeared [16]. At the beginning of the 1990s, the microwave technique began to be applied in the synthesis of zeolite. In 1990, Chu et al. [17] firstly reported the synthesis of A-type and ZSM-5 zeolite by MH. Later, more studies were reported [18–27]. Compared with the conventional hydrothermal synthesis, microwave synthesis of zeolites has the advantages of a very short synthesis time, narrow particle size distribution, broad synthesis composition and high purity. These advantages prompt us to explore its use as a method for the synthesis of zeolite membranes. However, until now, there have been few reports on the synthesis of zeolite membranes by MH [28– 32], and no gas permeation data have been reported. Recently, we reported the synthesis of a high-permeance NaA zeolite membrane by MH [33]. In this paper, a more detailed investigation was reported, and the effect of microwave was distinguished for the first time.
2. Experimental
2.1. Synthesis of NaA zeolite membrane A porous a-Al2O3 disk (30 mm in diameter, 3 mm in thickness, 0.10.3 mm pore radius, about 50% porosity, and homemade) was used as the support. The surface of the support was polished with sandpaper on both sides, after which the support was cleaned with deionized water in a ultrasonic cleaner for c. 3– 5 min to remove the loose particles created during polishing. The cleaned support was calcined in air at 400°C for 3 h to burn off the organics on the support surface before hydrothermal synthesis or coating the seeds. One side of the support was coated with NaA zeolite crystals as nucleation seeds before synthesis [34]. The synthesis solution was prepared by mixing an aluminate solution and 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 was stirred vigorously for 15 min. The molar ratio of this mixture solution was 5SiO2:Al2O3:50Na2O: 1000H2O. The support was placed vertically with a Teflon holder in a polyethylene bottle to avoid any precipitation of zeolite crystals onto the support during the membrane synthesis. The synthesis mixture was added to the bottle carefully without hitting the support, and 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 909 5°C in 60 s, and then held at the final temperature for the desired time. In some cases, multi-stage synthesis was carried out or the unseeded a-Al2O3 disk was used as the support. 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 NaA zeolite and the a-Al2O3 support, a low heating and cooling rate, c. 1 K/min, was adopted to avoid crack formation during the thermal treatment
2.2. Membrane characterization The structure of the as-synthesized membrane was determined by X-ray diffraction (XRD) patterns. XRD was carried out on a Ragaku D max/b powder diffractometer using Cu Ka (l= 1.54 A, ) radiation operating at 40 kV and 50 mA. The morphology and thickness of the as-synthesized membrane were examined using a scanning electron microscope (SEM). The SEM photographs were obtained on a JEM-1200E scanning electron microscope. Gas permeation was carried out to evaluate the perfection of the as-synthesized membrane. The as-synthesized membrane was sealed in a permeation module with the zeo-
X. Xu et al. / Separation/Purification Technology 25 (2001) 241–249
lite membrane on the high-pressure side. The permeances of the membranes were measured by a soap-film flowmeter under a pressure difference of 0.10 MPa at 25°C. The permselectivity of A/B is defined as the permeance ratio of gas A and gas B.
3. Results
3.1. Synthesis of NaA zeolite membrane by microwa6e heating The synthesis of NaA zeolite membrane on the seeded a-Al2O3 support by MH was investigated, and the as-synthesized membranes were characterized by XRD and SEM. Fig. 1 shows the XRD patterns of the as-synthesized membrane. After 1 min of synthesis, the synthesis solution became cloudy, and NaA zeolite was detected in the synthesis mixture by XRD. However, the intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane did not increase significantly compared with that of the seeded support.
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After 10 min of synthesis, the intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane increased. After 15 min of synthesis, the synthesis solution became clear again, and the intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane was the highest among the zeolite membrane investigated. In addition, only the diffraction patterns of NaA zeolite and a-Al2O3 appeared in the XRD diagram, which indicated that only NaA zeolite membrane formed on the support. Further increasing the synthesis time resulted in the transformation of NaA zeolite into other types of zeolites. A hydroxy-sodalite zeolite membrane formed after 45 min of synthesis. Compared with the formation of NaA zeolite membrane by CH [34], the formation of NaA zeolite membrane was drastically promoted by MH. The synthesis of NaA zeolite membrane needs 2–3 h by CH, while only needs 15 min by MH. The synthesis time is eight to 12 times shorter by MH than by CH. The surface and cross-section of the NaA zeolite membranes synthesized by MH of 15 min and CH of 3 h were further compared by SEM, and the SEM images are shown in Fig. 2. The surface of the a-Al2O3 support after MH synthesis was completely covered with randomly oriented NaA zeolite crystals with a uniform size of about 2 mm, and the crystals were highly intergrown. The surface of the a-Al2O3 support after CH synthesis was also covered with NaA zeolite crystals. However, the surface of the zeolite membrane was rough, and the zeolite crystals were not uniform in size. The zeolite membrane synthesized by MH was 4 mm thick, thinner than the 5–8 mm of the membrane synthesized by CH.
3.2. Effect of surface seeding
Fig. 1. X-ray diffraction patterns of the as-synthesized membranes by MH. (1) support; (2) seeded support; (3) 1 min; (4) 10 min; (5) 15 min; (6) 45 min. (*) a-Al2O3 support; ( ) NaA zeolite; (") hydroxy-sodalite zeolite.
The effect of surface seeding on the formation of NaA zeolite membrane by MH was also investigated. The XRD patterns and the SEM images of the as-synthesized membranes are shown in Figs. 3 and 2, respectively. The intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane with surface seeding was stronger than that of the as-synthesized membrane without surface seeding, which indicated
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Fig. 2. SEM images of the as-synthesized membrane by MH with surface seeding (1); by MH without surface seeding (2) and by CH with surface seeding (3).
that surface seeding promoted the formation of NaA zeolite membrane on the porous a-Al2O3 support. In addition, hydroxy-sodalite zeolite formed on the unseeded support, while no other types of zeolites formed on the seeded support, which implied that surface seeding also inhibited the transformation of NaA zeolite into other types of zeolites. The SEM images confirmed the XRD results. After synthesis, the sup-
port surface was completely covered with NaA zeolite crystals for the seeded support and was sparse for the unseeded support. As the cross-section images shown, there was a continuous membrane formed on the seeded support, and no continuous membrane formed on the unseeded support. The same phenomenon was also observed in the synthesis of NaA zeolite membrane by CH [34].
X. Xu et al. / Separation/Purification Technology 25 (2001) 241–249
Fig. 3. XRD patterns of the as-synthesized membranes with (1) and without (2) the aid of surface seeding. (*) a-Al2O3 support; ( ) NaA zeolite; (") hydroxy-sodalite zeolite.
3.3. Effect of multi-stage synthesis In the synthesis of NaA zeolite membrane by CH, the perfection of the membrane was improved by employing the multi-stage synthesis method [34]. In order to improve the perfection of NaA zeolite membrane, multi-stage synthesis was also employed in the MH synthesis. Figs. 4 and 5 show the XRD patterns and SEM images of the as-synthesized membrane after multi-stage synthesis, respectively. The XRD patterns showed that the diffraction patterns of NaA zeolite disappeared after a two-stage synthesis. Only two weak peaks appeared at 15 and 25°. These peaks can be ascribed to the diffraction patterns of hydroxy-so-
Fig. 4. XRD patterns of the as-synthesized membrane with multi-stage synthesis by MH. (1) one-stage synthesis; (2) twostage synthesis; (3) three-stage synthesis. (*) a-Al2O3 support; ( ) NaA zeolite; (") hydroxy-sodalite zeolite.
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dalite zeolite. From the SEM images, it can be seen that the NaA zeolite membrane dissolved, and new types of zeolites began to form on the support. After a three-stage synthesis, the intensity of the diffraction patterns of hydroxy-sodalite zeolite increased. The SEM image showed that a new type of zeolite membrane formed, and the thickness of the membrane was about 10 mm. Comparing the MH synthesis with the CH synthesis, the transformation of NaA zeolite on the support surface was totally different. In the synthesis by CH, the NaA zeolite transformed gradually. With the dissolution of the NaA zeolite, other types of zeolites, e.g. NaX zeolite and hydroxy-sodalite zeolite, formed. The NaA zeolite even existed in the membrane with a synthesis time of 4 h after a three-stage synthesis [34]. On the contrary, the NaA zeolite was suddenly transformed in the MH synthesis. The NaA zeolite totally dissolved after a two-stage synthesis. After a three-stage synthesis, hydroxy-sodalite zeolite formed on the support.
3.4. Perfection e6aluation of the as-synthesized membrane The perfection of the as-synthesized membrane was evaluated by gas permeation, and the permselectivity of H2/n-C4H10 was selected as the yardstick of the perfection of the NaA zeolite membrane [34]. Table 1 shows the permeance of H2, n-C4H10 as well as the permselectivity of H2/n-C4H10 of the as-synthesized membranes. After a one-stage synthesis, the H2 permeance of the as-synthesized membrane decreased by one magnitude compared with that of the seeded support, which indicated that a dense NaA zeolite membrane formed on the support. The permselectivity of H2/n-C4H10 was 11.9, higher than that of the Knudsen diffusion selectivity of 5.39, which meant that the gases mainly permeated through the NaA zeolite channels. However, the permeation of n-C4H10 indicated that the NaA zeolite membrane had certain defects with the diameter larger than the pore size of the NaA zeolite channels. The perfection of the membrane after a two-stage synthesis and a three-stage synthesis decreased.
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Fig. 5. SEM images of the as-synthesized membrane with multi-stage synthesis by MH. (1), (2) two-stage synthesis; (3), (4) three-stage synthesis.
4. Discussion
4.1. Effect of microwa6e in the synthesis of zeolite membrane The effect of microwave in the synthesis of zeolites can be divided into two parts — the ‘‘thermal effect’’ and the ‘‘microwave effect’’. The ‘‘thermal effect’’ refers to the fast and homogeneous heating of microwave. The ‘‘microwave effect’’ means the changes of the characteristic of the substance in the microwave field. In the synthesis of zeolites, the ‘‘microwave effect’’ mainly refers to the change of the characteristic of water in the microwave field. Jansen et al. [19] considered that the hydrogen bridges of the water molecules are destroyed in the microwave field and the ‘‘active’’ water forms. The ‘‘active’’ water has a high activation energy, and the synthesis gel can be easily dissolved by ‘‘active’’ water. Therefore, the synthesis of zeolite was promoted. The formation of a zeolite membrane on a porous support is a heterogeneous nucleation pro-
cess. First, a gel layer formed on the porous support surface, followed by nucleation and crystal growth to form a membrane [35,36]. Accordingly, the synthesis process of zeolite membrane by MH and CH is proposed in Fig. 6. In the microwave environment, because of the ‘‘microwave effect’’, the water became ‘‘active’’ water, and the dissolution of the gel layer in the support/ Table 1 Perfection evaluation of the as-synthesized membrane by gas permeation Membrane
Permeance (10−8mol m−2 s−1 Pa−1)
Permselectivity
H2
n-C4H10
h(H2/n-C4H10)
1120 17.9
1.51 11.9
11.8
6.11
39.2
3.39
Seeded support 1690 One-stage 213 synthesis Two-stage 72.1 synthesis Three-stage 133 synthesis
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Fig. 6. Comparative synthesis model of zeolite membrane by MH and CH.
solution interface was accelerated. At the same time, because of the ‘‘thermal effect’’, the synthesis mixture was heated up quickly and homogeneously. Under this situation, a great number of zeolite nuclei formed on the support surface simultaneously and homogeneously. In addition, because of the simultaneous nucleation and the homogeneous heating, uniform and small zeolite crystals can be synthesized. As a result, a thin zeolite membrane can be formed in a very short synthesis time. When CH was used, because of the low dissolution rate of the gel layer and the low heating rate, the nuclei did not form on the support surface simultaneously. Therefore, the assynthesized zeolite crystals were not uniform in size. In order to form a continuous membrane, a long synthesis time will be needed and the zeolite membrane will be inhomogeneous and thick. The different transformation process of the NaA zeolite membrane by CH and by MH can also be explained by the different formation mechanism of zeolite membrane. In the case of MH, because the zeolites nucleated and grew simultaneously, NaA zeolite transformed at the same time. Therefore, a pure zeolite membrane can be formed. In the case of CH, because of the low heating rate and inhomogeneous heating, NaA zeolite did not form at the same time. The NaA zeolite crystals formed at the early stage began to transform into other types of zeolites, while the other NaA zeolite crystals was stable. In order to distinguish between the ‘‘thermal effect’’ and the ‘‘microwave effect’’ in the synthesis of the NaA zeolite membrane by MH, the following experiment was designed. A seeded sup-
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port was placed in the synthesis mixture and heated up quickly to 90°C by MH, and then the synthesis system was transferred to a conventional oven, which was preheated to 90°C. After 15 min of synthesis, the as-synthesized membrane was characterized by XRD. If the intensity of the diffraction patterns of NaA zeolite does not change significantly compared with that of the seeded support, it can be concluded that the promotion effect of the microwave is mainly caused by the ‘‘microwave effect’’. If the intensity of the diffraction patterns of NaA zeolite is similar to that of the NaA zeolite membrane synthesized by MH, it can be concluded that the ‘‘thermal effect’’ is the main effect. It was shown, from the XRD patterns (Fig. 7) that there is no significant difference in the intensity of the diffraction patterns of NaA zeolite between the as-synthesized membrane and the seeded support. The intensity of the diffraction patterns of NaA zeolite of the as-synthesized membrane is smaller than that of the NaA zeolite membrane synthesized by MH of 15 min. Thus, the formation of NaA zeolite membrane by MH was mainly promoted by the ‘‘microwave effect’’. However, because of the fast and homogeneous heating and the easy dissolution of the gel layer, a great number of zeolite nucleated and grew on the support surface simultaneously and homogeneously. Therefore, uniform and small NaA zeolite crystals can be synthesized. As a result, thin NaA zeolite membrane can be obtained, and so the formation of a thin and homogeneous NaA zeolite membrane by MH was caused by both the ‘‘thermal effect’’ and the ‘‘microwave effect’’.
Fig. 7. XRD patterns of the as-synthesized membranes by MH (1) and MH +CH (2). (*) a-Al2O3 support; ( ) NaA zeolite.
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5. Conclusion
Fig. 8. Gas-permeation properties of the seeded support ("), the NaA zeolite membranes synthesized by MH ( ) and by CH ().
Microwave-assisted hydrothermal synthesis method is an effective method for the synthesis of a high-permeance and high-selectivity zeolite membrane. The synthesis time of NaA zeolite membrane is eight to 12 times shorter by MH than by CH. The permeance of the NaA zeolite membrane synthesized by MH is four times higher than that of the NaA zeolite membrane synthesized by CH, while their permselectivities are comparable. The promotion effect of microwave can be divided into two parts: the ‘‘thermal effect’’ and the ‘‘microwave effect’’. The fast formation of NaA zeolite membrane was mainly caused by the ‘‘microwave effect’’, while the formation of a homogeneous and thin NaA zeolite membrane resulted from both the ‘‘thermal effect’’ and the ‘‘microwave effect’’.
4.2. Gas-permeation properties Acknowledgements Fig. 8 shows the gas-permeation properties of the seeded support, the NaA zeolite membrane synthesized by MH of 15 min and CH of 3 h. The permeance of H2, O2, N2 and n-C4H10 of the NaA zeolite membrane synthesized by MH decreased as the gas molecular kinetic diameter increased. The permselectivities of O2/N2 and H2/n-C4H10 were 1.02 and 11.8, respectively, higher than those of the Knudsen diffusion selectivity of 0.96 and 5.39, which showed the molecular sieving effect of the NaA zeolite membrane and indicated that the gases mainly permeated through the NaA zeolite channels [34]. The permselectivities of O2/N2 and H2/n-C4H10 were 1.10 and 9.83, respectively, for the NaA zeolite membrane synthesized by CH [34], similar to those for the NaA zeolite membrane synthesized by MH. However, the NaA zeolite membrane synthesized by MH had a higher permeance, e.g. the permeance of H2 was four times higher than that of the NaA zeolite membrane synthesized by CH. The higher permeance of the NaA zeolite membrane may be attributed to the formation of a thinner NaA zeolite membrane by MH than by CH.
The authors gratefully acknowledge the funding from the National Science Foundation of China (59789201) and the National Advanced Materials Committee of China (715-006-0120).
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