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Formation mechanism of microwave synthesized LTA zeolite membranes
Journal of Membrane Science 281 (2006) 646–657
Formation mechanism of microwave synthesized LTA zeolite membranes Yanshuo Li a,b , Jie Liu a , Weishen Yang a,∗ a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Graduate School of the Chinese Academy of Sciences, China Received 10 January 2006; received in revised form 27 March 2006; accepted 21 April 2006 Available online 5 May 2006
Abstract In order to optimize the synthesis conditions for a specific zeolite membrane with high efficiency, and generalize this method to other types of zeolite membranes, a fundamental understanding of the membrane formation mechanism is of great importance. In present paper, gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR), and gas permeation were carried out to characterize the whole formation process of LTA zeolite membrane which was synthesized by “in situ aging-microwave heating” method (AM method), and the formation mechanism of LTA zeolite membrane was proposed. With this mechanism, a gel layer is first formed on the support after in situ aging, which contains plenty of pre-nuclei. During the following microwave assistant crystallization, these pre-nuclei rapidly and simultaneously develop into crystal nuclei, and then crystal growth by propagation through the amorphous primary particles (with size of ca. 50 nm) goes on, and finally, the amorphous particles transform into LTA crystal particles with the same size. This propagation growth process is followed by or parallel with the agglomeration and densification of the primary particles. In this way, compact LTA zeolite membrane consisting of sphere grains with undefined crystal facets is obtained. © 2006 Elsevier B.V. All rights reserved. Keywords: LTA; Zeolite membrane; Microwave synthesis; Formation mechanism; Gas permeation
1. Introduction Since the mid of 1990s, owing to the potential molecular sieving action, controlled host–sorbate interactions and high thermal and chemical stability, extensive efforts have been devoted to the preparations, characterizations, and applications of zeolite membranes. The progresses achieved in this rapidly growing field are reflected by a number of excellent reviews [1–8] and book chapters [9]. Various synthetic strategies and methods have been invented and developed. Besides the general demands of zeolite powders synthesis (such as pure phase and high crystallinity), morphology control [10], orientation control [11–13] and membrane thickness control [13,14] are the more challengeable targets when zeolite membranes are proposed to be fabricated. Different types of supported polycrystalline zeolite membranes, such as LTA [15,16], MFI [10–14,17], FAU [18], SAPO-34 [19], and isomorphous substituted MFI zeolite membranes [20], have
∗
Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447. E-mail address: [email protected] (W. Yang). URL: http://yanggroup.dicp.ac.cn/.
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.04.051
been successfully prepared under laboratory-condition. Nevertheless, the optimization of zeolite membrane synthesis is still a trial-and-error procedure at present, which is low effective and difficult to be generalized. Therefore, a fundamental understanding of the formation mechanisms of zeolite membranes is of great significance. Extensive efforts have been made to elucidate the formation mechanisms of zeolite membranes. A homogeneous nucleation model was reported on the synthesis of MOR [21], LTA [16], and MFI zeolite membranes [17,22]. According to this model, nucleation occurs homogeneously in the bulk solution and the formed zeolite nuclei or crystals move and deposit on the substrate surface. A heterogeneous nucleation model was reported on the synthesis of LTA [23] and MFI [24] zeolite membranes. With this model, at the early stage of preparation, a gel layer is formed on the substrate surface and then heterogeneous nucleation and crystal growth occur at the interface between the gel and solution. Boudreau et al. proposed a model for the secondary growth of the seed layer on synthesis of LTA zeolite membrane [25]. With this model, the seeded crystals epitaxially grow at the early stage. With longer synthesis time, zeolite crystals could form homogeneously in the synthesis solution and
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deposit on the membrane. Recently, Li et al. investigated the formation mechanism of b-oriented MFI zeolite film directly by TEM [22]. Abundant TEM information including crystallographic orientation relationships among crystals in the film (both out-of-plane and in-plane), grain boundaries, and each crystal grain was obtained. This TEM investigation provides direct evidence to support the homogeneous nucleation mechanism. However, this mechanism was indirectly deduced from the information of the final obtained zeolite film, and only SEM and HRTEM characterizations have been performed. In order to achieve a better understanding of the formation mechanism of zeolite membrane, the events that take place during the whole synthesis process are needed to be monitored by comprehensive characterization techniques. Recently, we reported a novel microwave synthesis method, namely in situ aging-microwave synthesis method (AM method) [26]. High quality LTA zeolite membranes could be synthesized through this method with high reproducibility. It was found that the “in situ aging” step was necessary for a successful microwave synthesis, and the so obtained zeolite membrane consisted of sphere grains without well-developed crystal faces. In present study, other than the commonly used characterization techniques, such as SEM, XRD and permeation/separation tests, gravimetric method was used to monitor the weight evolution of the membranes as synthesis proceeded, and XPS was used to analyze the Si/Al ratios of the membranes with different synthesis times. In addition, attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR) was used to characterize the framework vibrations of the zeolite (or its precursor) that formed on the support during the synthesis process. Based on these comprehensive characterizations, the function of in situ aging was made clear, and the extraordinary morphology of AM synthesized LTA zeolite membrane was interpreted. A formation mechanism of LTA zeolite membrane prepared by AM method
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was proposed, which can provide generalized guidance for the preparation of other types of zeolite membranes. 2. Experimental 2.1. Membrane preparation LTA zeolite membranes were prepared according to our reported method (“in situ aging-microwave synthesis” method) [26]. The whole synthesis procedure is illustrated in Fig. 1. The synthesis mixture was prepared by mixing an aluminate solution and a silicate solution. In a typical synthesis, an aluminate solution was prepared by dissolving 40.0 g sodium hydroxide (purity > 96%) in 159.0 g deionized water, then adding 1.0 g aluminium foil (purity > 99.5%) into the caustic solution. A silicate solution was prepared by mixing 34.1 g sodium hydroxide, 20.6 g silica sol (d = 1.16 g/ml, containing 26.0% SiO2 ), and 159.0 g deionized water. After being stirred for 1 h at room temperature, the silicate solution was poured into the aluminate solution under vigorous stirring. The obtained mixture was stirred for 30 min at room temperature to produce a clear homogeneous solution with the following chemical composition: 5SiO2 :Al2 O3 :50Na2 O:1000H2 O. A porous ␣-Al2 O3 tube (self-made, 12 mm outside diameter, 9 mm inside diameter, 7 cm length, ca. 0.3 m pore radius, and ca. 40% porosity) was sealed with two Teflon caps at both the ends and placed vertically in a Teflon autoclave. For disk-shaped membrane synthesis, a porous ␣-Al2 O3 disk (self-made, 28 mm in diameter, 1.5 mm in thickness, ca. 0.3–0.5 m pore radius, and ca. 50% porosity) was placed vertically with a Teflon holder in the autoclave. After the synthesis solution was added, the autoclave was sealed. Before microwave heating, the autoclave was put in an air oven and aged at 50 ◦ C for 7 h (“in situ aging”). Then, the crystallization was carried out in a microwave oven
Fig. 1. Illustration of the process of AM synthesis.
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with a working frequency of 2450 MHz (Haier, HR-8801M). The synthesis solution was quickly heated to 90 ◦ C and then held at that temperature for a certain time (“microwave synthesis”). After synthesis, the membrane was immersed in deionized water for 1 day. Meantime, the alkalified water was exchanged for several times. The treated membrane was then dried in an air oven at 110 ◦ C over night. In our previous report, high quality membranes were obtained after two-stage synthesis. Therefore, in this paper, we will investigate and discuss the first and second synthesis stage, respectively. 2.2. Characterization The amount of reaction product that deposited on the support was determined by gravimetric method. The obtained membranes with different synthesis time were dried at 110 ◦ C for 24 h to get constant weights, and then were weighed in an electronic balance (Sartorius BS 323 S). Flat membranes were used to obtain the X-ray diffraction (XRD) patterns (Rigaku DX2000, Cu K␣ radiation, operating at
40 kV and 250 mA). The obtained XRD patterns were processed with MDI Jade (Materials Data, Inc., Version 5.0). In the present study, the intensity of the (2 0 0) peak of LTA zeolite (I(2 0 0) ) was adopted to monitor the evolution of zeolite membrane. X-ray photoelectron spectroscopy (XPS, VG ESCA-LAB MK-II, VG Scientific Ltd., Al K␣ radiation, hν = 2381.7963282 × 10−19 J (1486.6 eV)) was performed to determine the Si/Al ratio of the as-synthesized membranes. Attenuated total reflectance/Fourier transform infrared spectroscopy was used to characterize the framework vibrations of the zeolite (or its precursor) that formed on the support during the synthesis process. The ATR–FTIR spectra were recorded with a Nicolet-continum micro-FTIR spectroscopy. All samples were scanned for 64 times at the spectral resolution of 4 cm−1 . Due to the limitation of our FTIR–ATR instrument (frequency region: 650–4000 cm−1 ) and background spectra from the alumina support (400–900 cm−1 ), only the frequency region between 850 and 1150 cm−1 was intensively studied. The texture and thickness of the membranes were examined by scanning electron microscopy (SEM, JEM-1200EX scanning electron microscope, JEOL, operating at 40 kV).
Fig. 2. SEM images of: (a) top view of porous ␣-Al2 O3 support, (b) top view of the support after aging, (c) cross-sectional view of (b), and (d) XRD patterns of the support before/after aging; the insets show the corresponding enlarged images.
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2.3. Gas permeation measurements Before gas permeation measurements, the disk-shaped membranes were dried at 110 ◦ C under vacuum for at least 2 days to remove the water absorbed in the zeolite channels. The dried zeolite membrane was sealed in a permeation cell for gas permeation measurements. The effective membrane area was ca. 3 cm2 . The single gas permeance was measured by a soap-film flowmeter at room temperature (20 ◦ C) 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. 3. Results and discussions 3.1. The first-stage synthesis SEM top views of the membranes before and after aging are shown in Fig. 2a and b. It can be seen that after 7 h aging, the surface of the alumina support was covered throughout by a gel layer with amorphous-looking aggregates. The corresponding SEM cross-sectional view (Fig. 2c) also shows that the porous support was covered by a layer after aging. This layer was loose in appearance and about 4 m in thickness. It also can be seen that the pores of the support near the surface were blocked or partly blocked by the intruded gel. From the XRD pattern (Fig. 2d) of the membrane after aging, it can be seen that only the peaks of alumina support (␣-Al2 O3 ) were detected, which indicates that no detectable crystal growth occurred during aging. Meanwhile, it can be deduced that a kind of X-ray amorphous layer did cover the surface of the support because the XRD intensity of alumina support (␣-Al2 O3 ) decreased after aging (Fig. 2d). The amount of the deposited gel was ca. 2.0 wt.% (see Fig. 3, “microwave heating time” of 0 min means “after aging”), which was rich in Si, with a Si/Al ratio of 3.9 (see Table 1). Fig. 3 shows the weight gain and the intensity of the (2 0 0) peak of LTA zeolite (I(2 0 0) ) on the support versus microwave heating time. Here, the weight gain is corresponding to the total cumulative amount of reaction product, including un-
Fig. 3. Weight gain and diffraction intensity of LTA (2 0 0) of the as-synthesized membrane vs. microwave synthesis time during the first-stage synthesis.
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Table 1 The Si/Al ratios of the as-synthesized membrane with different microwave synthesis times Microwave synthesis time (min)
First-stage synthesis Second-stage synthesis a
0a
5
15
25
3.90 1.49
1.77 1.29
1.31 1.32
1.41 1.43
After aging.
crystallized amorphous phase and crystallized phase. While, I(2 0 0) can be regard as the amount of crystalline LTA zeolite formed on the support. After aging, a 2.0 wt.% weight gain was obtained. At the beginning of microwave synthesis (after 5 min of microwave synthesis), a sharp weight loss of ca. 50% occurred (down from the weight gain of 2.0–1.1 wt.%). After that, the cumulative amount of reaction product maintained nearly constant (ca. 1.1 wt.%) until the reaction time prolonged to 35 min (the weight gain increased slightly ∼1.3 wt.%). The I(2 0 0) curve shows a quite different trend, which is much like the S-shape crystal growth curve in zeolite synthesis. The existence of induction period (0–5 min) indicates that no appreciable nucleation took place during the in situ aging. The breakthrough of LTA zeolite growth happened at 15–25 min. Afterwards, a limited growth continued, consistent to the slight increase of weight gain. It should be noted that, during the whole first-stage synthesis, LTA zeolite was the only crystalline phase which was confirmed by XRD characterization. The Si/Al ratios of the reaction product deposited on the support with different synthesis times were determined by XPS, as shown in Table 1. After aging, an aluminosilicate gel was deposited on the support, with a Si/Al ratio of 3.90. In the case of preparation of LTA zeolite membranes, the formation of a gel layer on the support which was rich in silica has also been observed by other authors [16]. After 5 min of microwave synthesis, the Si content in the reaction production decreased, resulting in a Si/Al ratio of 1.77. As microwave synthesis proceeding, the Si/Al ratio decreased to 1.31 after 15 min of microwave synthesis. Afterwards, the Si/Al remained almost constant, and LTA zeolite membrane with a Si/Al ratio of 1.41 was obtained after 25 min of microwave synthesis. Generally, LTA zeolite should have a Si/Al ratio of 1.0. In the present study, however, the so obtained LTA zeolite membrane showed a higher Si/Al ratio of 1.41. Other authors also reported that microwave synthesized zeolite possessed a higher Si/Al ratio as compared with that obtained under conventional heating [27]. The reason why microwave heating synthesis results in products with higher Si/Al ratio is not clear now. IR spectroscopy is often used to characterize zeolite structure and examine changes in the solid phase prior to and during crystallization of zeolite. In the present study, ATR/FTIR was applied to directly characterize the structural evolution of aluminosilicate species on the support as synthesis progressed. In the frequency region between 850 and 1150 cm−1 , no band was detected for the alumina support as shown in Fig. 4. After aging, a wide band centered around 1000 cm−1 was observed, which is attributed to the asymmetric stretching vibrations of TO4
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Fig. 4. ATR/FTIR spectra of the as-synthesized membrane with different microwave synthesis times during the first-stage synthesis.
(T = Si or Al) tetrahedra. This verified the formation of the gel layer during in situ aging. After 5 min of microwave synthesis, this band developed into a bimodal peak, with a wide band around 1010 cm−1 and a small should near 970 cm−1 . As crystallization progressed, the band around 1010 cm−1 kept almost unchanged, while the adsorption near 970 cm−1 enhanced and shifted to lower frequency (960 cm−1 after 25 min of synthesis).
In order to clarify the assignments of these peaks, commercial LTA zeolite powder and the LTA zeolite powder that simultaneously precipitated in the bulk phase were scanned for ATR/FTIR spectra. The obtained IR spectra (second-order derivative spectra are also shown in order to distinguish the overlapping peaks) and the corresponding SEM images are shown in Fig. 5. The spectrum of commercial LTA zeolite powders (Fig. 5a) comprises a bimodal peak, around 1014 and 970 cm−1 , respectively. The spectrum of AM synthesized LTA zeolite powder (Fig. 5b) also comprises a bimodal peak. Whereas, the peak positions are different from that of commercial one but similar to that of the LTA zeolite membrane after the first-stage AM synthesis, i.e. two peaks at 1014 and 960 cm−1 , respectively. The different infrared vibrations between commercial LTA zeolite powder and AM synthesized one might result from their different morphologies. It can be seen from the SEM images that the commercial LTA zeolite is typical cubic single crystal (Fig. 5a), while the LTA zeolite powder synthesized by AM method appears twin growth and intergrowth of polycrystals (Fig. 5b). Further discussion of the bimodal peak will be given later. It should be noted that the appearance of the characteristic bimodal peak of LTA zeolite after 5 min of synthesis indicates that LTA phase or at least some LTA structure units had already established at that time, which coincides with the XRD results. The morphological evolution of the deposited gel layer during microwave assistant crystallization was monitored by SEM, as shown in Fig. 6. After 5 min of crystallization, some uniform particles with sizes below 100 nm emerged in the gel layer, as shown in Fig. 6a-1. Combined with XRD and IR results, it is speculated that these particles already had crystalline structures in some extent. Subsequently, these particles developed into larger ones (Fig. 6b-1), and after 25 min of microwave synthesis, round-shaped particles with size around 400 nm were obtained (Fig. 6c-1). From the high magnification image (Fig. 6c-1, inset),
Fig. 5. (a) SEM top view and ATR/FTIR spectrum of commercial LTA zeolite powder, and (b) SEM top view and ATR/FTIR spectrum of AM synthesized LTA zeolite powder.
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it can be seen that the round-shaped particles were made up of smaller particles with sizes around 50 nm, as pointed out in the picture with arrows. When microwave assistant crystallization prolonged, well-shaped cubic LTA zeolite crystals with sizes around 1 m were obtained, as shown in Fig. 6d-1. The terrace feature of these crystals (as shown in Fig. 6d-1, inset with arrows) implies that a layer growth is the controlling mechanism during the crystallization from 25 to 35 min. From the cross-section views (Fig. 6a-2–c-2), it can be seen that the layer thickness did not increase observably with synthesis time, i.e. ca. 2 m all the time, except the one after 35 min of synthesis which was a little thicker (ca. 3 m, Fig. 6d-2). This kind of layer thickness evolution coincides with the weight gain curve (Fig. 3). From the cross-section views, it also can be seen that the pores of the support were occupied with reaction products, which resulted in an unclear interface between the external product layer and the support. The obtained crystallized LTA zeolite membranes after the first-stage AM synthesis (after 25 or 35 min of microwave synthesis), however, were not compact enough. Neither the aggregated zeolite particles (400 nm in size, composing of 50 nm particles, Fig. 6c-1) nor the cubic zeolite crystals (1 m in size, Fig. 6d-1) were well intergrown together. This was further confirmed by gas permeation experiments. Both the membrane after 25 min and that after 35 min of synthesis showed poor separation performances. Therefore, repeated synthesis (the second-stage AM synthesis) is necessary. Based on our previous studies, the optimized synthesis procedure is 25 min of microwave synthesis in the first-stage AM synthesis. Thus, in the next section, the second-stage AM synthesis following a 25 min first-stage synthesis will be discussed. 3.2. The second-stage synthesis Fig. 7 shows the weight gain (relative to the value after the first-stage synthesis) and the intensity of the (2 0 0) peak of LTA zeolite (I(2 0 0) ) on the support versus microwave heating time. In the second-stage synthesis, the total cumulated amount of reac-
Fig. 6. SEM top views of the as-synthesized membrane after (a-1) 5 min, (b-1) 15 min, (c-1) 25 min, and (d-1) 35 min of microwave synthesis during the firststage synthesis; SEM cross-sectional views of the as-synthesized membrane after (a-2) 5 min, (b-2) 15 min, (c-2) 25 min, and (d-2) 35 min of microwave synthesis during the first-stage synthesis; the insets show the corresponding enlarged images. Fig. 7. Weight gain and diffraction intensity of LTA (2 0 0) of the as-synthesized membrane vs. microwave synthesis time during the second-stage synthesis.
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Fig. 8. ATR/FTIR spectra of the as-synthesized membrane with different microwave synthesis times during the second-stage synthesis.
tion product on the support (weight gain) maintained constant and finally processed a slight raise. As compared with the firststage synthesis, the average weight gain in the second-stage synthesis was apparently larger, i.e. ca. 3.9 wt.% versus 1.1 wt.%. After an initial sharp increase (0–5 min), the I(2 0 0) curve shows an approximate flat line (5–25 min) except at the point of 35 min of microwave synthesis, where I(2 0 0) declines obviously which coincides with the dissolution of LTA and growth of SOD zeolite. In consideration of the obvious weight gain (3.79 wt.%) and increase of LTA zeolite (increase of I(2 0 0) ) just after aging, it is speculated that the nano-sized LTA zeolite that formed after the first-stage synthesis has grown up in certain degree and substantial secondary nucleation has accomplished during the second in situ aging. The sharp increase of I(2 0 0) at the beginning of crystallization and rapid reach of a plateau indicate that the transformation of the gel layer into crystal phase complete very soon, and the major part of the second-stage synthesis is not a crystallization process but a morphological evolution process of the already formed LTA crystals. The Si/Al ratio of the reaction product on the support kept almost unchanged throughout the second-stage synthesis. This indicates that the chemical evolution of the gel layer has already finished during second in situ aging. The ultimate obtained LTA zeolite membrane had a Si/Al ratio of 1.43, as shown in Table 1. Fig. 8 illustrates the ATR/FTIR spectra of the membranes synthesized with different microwave synthesis times during the second-stage synthesis. For comparison, the spectrum of the membrane after the first-stage synthesis (25 min of microwave heating) is also shown in the same figure. After second aging, the band around 1010 cm−1 enhanced (for the second-stage syn-
thesis series, this band is near 1013 cm−1 ; considering that the resolution of our experiment is 4 cm−1 , this 3 cm−1 peak shift is negligible), while the band at 960 cm−1 disappeared, replaced by a rather broad band at 940–960 cm−1 . As crystallization progressed, a band near 940 cm−1 emerged, which maintained during the whole crystallization process. Eventually, the spectrum of the membrane after two-stage synthesis shows an apparent bimodal peak. In addition, it can be seen that the relative intensity of these two peaks (960 cm−1 /1013 cm−1 ) increases gradually with synthesis time. Kyotani et al. [28] used ATR/FTIR to characterize LTA zeolite membrane and also found the appearance of bimodal peak (1012 and 930 cm−1 ). They attributed the 930 cm−1 peak to the amorphous substances and/or LTA crystals embedded in the alumina porous support. A proportional relationship between the relative ratio of the two peaks and the dehydration performance (separation factor) in pervaporation was clarified by them. In the present study, it was found that with the enhancement of the intergrowth of LTA zeolite, i.e. from single crystals (commercial LTA powders) to polycrystals (LTA powder precipitated during AM synthesis), and from noncontinuous membrane (after the first-stage synthesis) to compact membrane (after the second-stage synthesis, as shown Fig. 9d1), the higher frequency of the bimodal peak (1014, 1010, and 1013 cm−1 ) maintained almost unchanged, while the lower frequency 970 cm−1 peak moved to 960 cm−1 after the first-stage synthesis (or polycrystalline LTA powder) and further moved to 940 cm−1 after the second-stage synthesis. Therefore, we prefer that the lower frequency of the bimodal peak (960 or 940 cm−1 ) is derived from the crystal boundary structures. The different arrangement of Si–O–Al bonds at the crystal boundary to that of the bulk phase might cause the different infra vibrations. Despite that the accurate assignments of 960 and 940 cm−1 vibration are not clear now, a higher ratio of the two peaks (lower frequency/higher frequency) associates with a more compact LTA zeolite membrane (higher degree of intergrowth), as reported by Kyotani et al. [28] and reconfirmed by present study. SEM images in Fig. 9 show the morphological evolution of the reaction product on the support during the second-stage microwave synthesis. After second aging, a gel-looking layer covered the zeolite layer that formed during the first-stage synthesis again, as shown in Fig. 9a-1. Some portions of the gel can be found to compose of small particles with sizes around 50 nm, as pointed out in Fig. 9a-1 with arrows (inset, high magnification image). After sequential 5 min of microwave synthesis, the 50 nm particles boomed, which began to packed together and form some aggregates with sizes of ca. 0.5–1 m. Whereas, most of these aggregates were not packed together tightly and individual 50 nm particles can be distinguished in the high magnification SEM image, as pointed out with arrows in Fig. 9b-1. After 15 min of microwave synthesis, the 50 nm particles began to agglomerate together, and some of the former 0.5–1 m aggregates became larger (1–3 m). From the high magnification image (Fig. 9c-1, inset), it can be seen that the 50 nm particles conjoin with each other, and some fuse together to form larger particles. Nevertheless, the boundaries between these particles are still visible. When microwave synthesis prolonged to 25 min, the 50 nm particles well fused together, and
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Fig. 9. SEM top views of the as-synthesized membrane after (a-1) 0 min, (b-1) 5 min, (c-1) 15 min, (d-1) 25 min, and (e-1) 35 min of microwave synthesis during the second-stage synthesis; SEM cross-sectional views of the as-synthesized membrane after (a-2) 0 min, (b-2) 5 min, (c-2) 15 min, (d-2) 25 min, and (e-2) 35 min of microwave synthesis during the second-stage synthesis; the insets show the corresponding enlarged images, except (e-1), where the inset shows the corresponding XRD pattern.
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the former aggregates (1–3 m) inter-grew with each other. As a result, the grain boundaries are hardly to be distinguished, as shown in Fig. 9d-1 (inset). After further microwave synthesis (35 min), some acicular crystals emerged on the surface of LTA zeolite layer (Fig. 9e-1). These were proved to be SOD (hydroxyl-sodalite) crystals by XRD characterization, as shown in Fig. 9e-2 (inset). From the cross-sectional view (Fig. 9a-2), it can be seen that after second in situ aging the thickness of the membrane increased from 2 m (after the first-stage synthesis) to 5 m. During the consequential microwave assistant crystallization, the layer thickness did not increase distinctly (Fig. 9b-2–d-2), which coincides with the weight gain curve (Fig. 7). Here, we speculated that these 50 nm particles (named as “primary particles” hereafter) were transformed from the preformed amorphous gel particles with similar size. The crystalline zeolite particles retained the size and morphology of the initial amorphous particles. Aggregation of the primary particles leads to the formation of larger particles (secondary particles, e.g. the 400 nm particles in the first-stage synthesis; 0.5–1 and 1–3 m particles in the second-stage synthesis). These secondary particles can undergo a densification process to fuse together. Mintova et al. [29] studied the formation of LTA zeolite at room temperature from a system containing (TMA)2 O as template by means of HRTEM. They found that the aggregation of the primary colloidal silica particles (5–10 nm) leaded to the formation of 40–80 nm aggregates. Smaihi et al. [30] investigated the crystallization stages of LTA zeolite at 40 ◦ C from an organic-free system (the same as the synthesis solution used in our study) by small-angle X-ray scattering (SAXS) and SEM. They found that the primary particles were in the range of 40–100 nm. Nevertheless, our naming the 50 nm particles as primary particles is somewhat non-strict. Further characterization, e.g. HRTEM, is needed to clarify the primary particles and their corresponding sizes. SEM characterization indicates that the major part of the second-stage synthesis is an aggregation and densification process of primary particles. This was further confirmed by gas separation test and the results are shown in Fig. 10. The membrane after second aging showed a H2 permeance of 20.6 × 10−7 mol m−2 s−1 Pa−1 , and H2 /N2 permselectivity was only 2.77, which is lower than that of Knudsen diffusion ratio (3.74). After 5 min of microwave synthesis, N2 permeance decreased from 7.44 × 10−7 to 4.51 × 10−7 mol m−2 s−1 Pa−1 while H2 permeance decreased largely at the same time to 11.9 × 10−7 mol m−2 s−1 Pa−1 , as a result, the H2 /N2 permselectivity decreased to 2.64. After 15 min of microwave synthesis, the permeances of H2 and N2 kept on decreasing and the permselectivity increased to 3.06 which was still lower than that of Knudsen diffusion ratio. When microwave synthesis prolonged to 25 min, the H2 /N2 permselectivity increased to 4.71 with a corresponding H2 permeance of 2.40 × 10−7 mol m−2 s−1 Pa−1 . This indicates that the zeolite membrane after 25 min of second microwave synthesis was compact and free of considerable defects and the gases mainly permeated through the LTA zeolite channels (with pore size of 0.41 nm × 0.41 nm). When microwave synthesis continued, due to the formation of SOD
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Fig. 10. H2 /N2 permeation properties of the as-synthesized membrane with different microwave synthesis times during the second-stage synthesis.
impurity, the integrity of the zeolite membrane was destroyed in certain degree, and the H2 /N2 permselectivity decreased to 3.97. The further decrease of H2 (N2 ) permeance is probably due to the further increase of membrane thickness as indicated from the weight gain curve (Fig. 7) and seen from the SEM cross-sectional view (Fig. 9e-2). It is believed that the essential mechanism of zeolite formation is similar, whether in powder form or in film and membrane form. As to LTA zeolite, owing to its relatively
simple composition and fast crystallization kinetics, many fundamental researches have been focused on its formation process, and the commonly accepted formation mechanism of LTA zeolite has been described [29,30]. In the present study, based on the comprehensive characterization of the whole formation process of LTA zeolite membrane, and referred to the reported formation mechanism of LTA zeolite, the formation mechanism of LTA zeolite membrane by AM method is proposed, as illustrated in Fig. 11. For the first-stage synthesis, after the 7 h in situ aging, an amorphous gel layer was formed on the support surface. This is caused by the capillarity function of the support pores and the dissolution of support. Other authors have also reported that an amorphous gel layer was indispensably formed on the support surface before the initiation of nucleation and crystal growth [16,31]. During the course of aging, Na+ -assisted rearrangement of the Si–O–Al bonds in the amorphous phase leads to the formation of areas of local order, which can be regarded as pre-nuclei or germ-nuclei. This is necessary to overcome the nucleation-related bottleneck in microwave synthesis of zeolite [32,33]. This X-ray amorphous gel is a pseudo-steady-state intermediate with increased ordered, and can be named as secondary amorphous phase [34]. During the following microwave assistant crystallization, under the rapid and uniform dielectric heating, the deposited amorphous gel underwent some chemical evolutions, i.e. partial dissolution of the silica from the amorphous gel layer and re-equilibration between the gel phase and the solution phase. Isometric amorphous primary particles with
Fig. 11. Schematic illustration of the proposed formation mechanism of LTA zeolite membrane synthesized by AM method.
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sizes around 50 nm emerged. At the same time, those pre-nuclei rapidly and simultaneously developed into crystal nuclei (areas of sufficient order for a periodic structure). After this induction period (pre-nucleation and nucleation periods), crystal growth by propagation through the gel network went on. Growth by propagation through the gel phase has already been observed in the formation of zeolite LTA [29,30], LTL [35], MFL [36], and FAU [37]. After 25 min of microwave heating, the 50 nm amorphous particles that contained nuclei were transformed into the final LTA crystal particles remaining with the same sizes. This propagation growth process was followed by or parallel with the agglomeration and densification of the nano-particles, i.e. secondary particles became larger by aggregation of more primary particles and became more compact, which leaded to the formation of LTA zeolite crystals with rough surfaces. Aggregation of nano-sized zeolite particles during their crystallization has also been reported by many authors [38–41]. Microwave adsorption through Maxwell–Wagner polarization mechanism, which results from the accumulation of charge at the material interface, might be applied to explain the strong aggregation trend between zeolite particles under microwave irradiation. With prolonged microwave heating, the formed zeolite crystals grew up by acquisition of nutrients from bulk solution or nearby unreacted amorphous gel (which did not contain nuclei) or small LTA crystals (Ostwald ripening). Layer growth is the controlling mechanism in this stage which leads to well-shaped zeolite crystals [42]. The zeolite membrane formation process in the second-stage synthesis is substantially similar with that in the first-stage synthesis. Whereas, unlike the naked support used in the first-stage synthesis, the second-stage synthesis begins with a “seeded support”. Nano-sized LTA crystals that formed after the first-stage synthesis covered the support completely, which can be regarded as a seed layer. Therefore, the first-stage synthesis can also be seen as an in situ seeding step. During second in situ aging, those seed crystals grew larger in certain degree, while the dominating change was formation of an amorphous gel layer on the seed layer and nucleation in the gel phase. The phenomenon that a gel layer was first formed on the seeded support during synthesis of LTA zeolite membrane has also been reported by Kita et al. [43]. As compared with precipitation of zeolite on the support, formation of an amorphous layer is more kinetic favorable. Besides, the activation energy of nucleation of LTA zeolite is smaller than that of growth of LTA zeolite. Therefore, at low temperature, i.e. in situ aging temperature of 50 ◦ C, nucleation is prevailing. In addition, the existence of seed crystals facilitated secondary nucleation remarkably. As a result, a gel layer containing plenty of nuclei covered on the support after second aging. The nucleus density in the second aging generated gel layer was much higher than that in the first-stage synthesis. During the following microwave assistant crystallization, transformation from the gel layer into crystalline LTA zeolite layer (propagation through the gel network) progressed rapidly, and almost completed after only 5 min of microwave synthesis. Afterwards, agglomeration and densification went on, which were the dominating courses during the second-stage synthesis, and made the second-stage synthesis more like a morphological evolution process. As synthesis progressed (35 min in the present study),
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base concentration in the bulk phase increased. This led to the dissolution of formed LTA zeolite and the heterogeneous nucleation and solution-mediated growth of sodalite product (SOD) on the LTA layer. As discussed above, no fundamental difference exists between the first-stage and the second-stage synthesis. The phenomenological differences are originated from the higher nucleus density at the beginning of microwave heating in the second-stage synthesis. This high nucleus density accelerates the digestion of the gel layer and enhances the agglomeration and densification. In this way, compact LTA zeolite membrane which consisted of round-shaped crystals with un-defined facets is obtained. By comprehensive characterizations of the process of LTA zeolite formation, Smaihi et al. concluded that the crystallization process of LTA zeolite should be subdivided into two stages [30]. The first stage of crystal growth process proceeded by propagation through the aluminosilicate gel phase, and nano-crystals with a rough surface were obtained at the end of the process. During the second stage, a layer growth, where a solution-mediated transport of nutrients dominates, is the controlling mechanism, and well-shaped zeolite crystals were obtained at last. In a recent review of McLeary and Jansen [44], the authors pointed out that the ever-occurring triangularly shaped gap between growing crystals on the support would lead to the systematic imperfections of synthesized membrane, and the formation of sphere grains with undefined crystal facets was necessary to achieve a pinhole-free continuous zeolite layer. In the present study, taking advantage of MH, the membrane formation was limited to the first stage of LTA crystal growth, and compact LTA membrane that consisted of sphere grains without well-developed crystal faces was thus obtained. These advantages include: (I) fast synthesis, i.e. the dissolution of synthesis gel is promoted and the crystallization time is shortened by the “active water” [45]. Therefore, the zeolite membrane synthesis can be achieved in short time (25 min in the present study), and Ostwald ripening is suppressed in certain degree. (II) Selective heating, i.e. microwaves selectively couple with the gel layer because that the gel phase has a higher dissipation factor (tan δ, which defines the ability of a medium to convert electromagnetic energy into heat) than the bulk solution phase. This makes AM synthesis a kind of gel conversion synthesis. (III) Volumetric heating, i.e. the gel layer is heated quickly and entirely and homogeneously, which results in simultaneous nucleation and synchronous propagation type crystallization. (IV) Surface activation, i.e. microwave adsorption through Maxwell–Wagner polarization mechanism, which results in a remarkable aggregation of LTA nano-particles, and thus strengthens the agglomeration and densification type crystallization. A detailed comparison between MH and CH and further discussion of the effect of microwave in synthesis will be given elsewhere. 4. Conclusion Based on the above results, some conclusions can be drawn: (I) In situ aging process was proved to be a simple but effective way to increase the nucleation density on the support
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surface. The rapid and synchronous nucleation is realized under fast volumetric microwave heating. (II) The essential mechanism of LTA zeolite membrane formation is similar to its powder-formed analogue. In order to obtain high quality zeolite membrane, the membrane formation should be limited to the first stage of crystal growth, i.e. propagation growth through the gel phase. This is realized by microwave heating assistant crystallization, owing to the special characteristics of microwave heating, i.e. “active water”, selective heating, volumetric heating, and surface activation. (III) The formation process by AM method is something like a gel layer conversion process. This indicates that we can designedly adjust the thickness of obtained zeolite membrane by controlling the thickness of the gel layer or by varying the times of synthesis stage. Acknowledgments This work was supported by the National Science Foundation of China (Grant No. 20321303) and the Ministry of Science and Technology of China (Grant No. 2003CB615802). References [1] J. Caro, M. Noack, P. K¨olsch, R. Sch¨afer, Zeolite membranes—state of their development and perspective, Microporous Mesoporous Mater. 38 (2000) 3. [2] E. Tavolaro, E. Drioli, Zeolite membranes, Adv. Mater. 11 (1999) 975. [3] A.S.T. Chiang, K. Chao, Membranes and films of zeolite and zeolite-like materials, J. Phys. Chem. Solids 62 (2001) 1899. [4] J. Jansen, J.H. Koegler, H. Bekkum, H.P.A. Calis, C.M. Bleek, F. Kapteijn, J.A. Moulijn, E.R. Geus, N. Puil, Zeolitic coatings and their potential use in catalysis, Microporous Mesoporous Mater. 21 (1998) 213. [5] M. Matsukata, E. Kikuchi, Zeolitic membranes: synthesis, properties, and prospects, Bull. Chem. Soc. Jpn. 70 (1997) 2341. [6] T. Bein, Synthesis and applications of molecular sieve layers and membranes, Chem. Mater. 8 (1998) 1636. [7] M. Noack, P. K¨olsch, R. Sch¨afer, P. Toussaint, J. Caro, Molecular sieve membranes for industrial application: problems, process, solutions, Chem. Eng. Technol. 25 (2002) 221. [8] Y.S. Lin, I. Kumakiri, B.N. Nair, H. Alsyouri, Microporous inorganic membranes, Sep. Purif. Method 31 (2002) 229. [9] S. Nair, M. Tsapatsis, in: S.M. Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Zeolite Science & Technology, Marcel Dekker, New York, 2003, p. p. 867. [10] W.C. Wong, L.T.Y. Au, P.P.S. Lau, C.T. Ariso, K.L. Yeung, Effects of synthesis parameters on the zeolite membrane morphology, J. Membr. Sci. 193 (2001) 141. [11] Z.P. Lai, G. Bonilla, I. Diaz, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W. Thompson, M. Tsapatsis, D.G. Vlachos, Microstructural optimization of a zeolite membrane for organic vapor separation, Science 300 (2003) 456. [12] L.C. Boudreau, M. Tsapatsis, A highly oriented thin film of zeolite A, Chem. Mater. 9 (1997) 1705. [13] A. Gouzinis, M. Tsapatsis, On the preferred orientation and microstructural manipulation of molecular sieve films prepared by secondary growth, Chem. Mater. 10 (1998) 2497. [14] J. Hedlund, J. Sterte, M. Anthonis, A.J. Bons, B. Carstensen, N. Corcoran, D. Cox, H. Beckamn, W.D. Gijnst, P.P. de Moor, F. Lai, J. McHenry, W. Ortier, J. Reinoso, J. Peters, High-flux MFI membranes, Microporous Mesoporous Mater. 52 (2002) 179.
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Formation mechanism of microwave synthesized LTA zeolite membranes Yanshuo Li a,b , Jie Liu a , Weishen Yang a,∗ a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Graduate School of the Chinese Academy of Sciences, China Received 10 January 2006; received in revised form 27 March 2006; accepted 21 April 2006 Available online 5 May 2006
Abstract In order to optimize the synthesis conditions for a specific zeolite membrane with high efficiency, and generalize this method to other types of zeolite membranes, a fundamental understanding of the membrane formation mechanism is of great importance. In present paper, gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR), and gas permeation were carried out to characterize the whole formation process of LTA zeolite membrane which was synthesized by “in situ aging-microwave heating” method (AM method), and the formation mechanism of LTA zeolite membrane was proposed. With this mechanism, a gel layer is first formed on the support after in situ aging, which contains plenty of pre-nuclei. During the following microwave assistant crystallization, these pre-nuclei rapidly and simultaneously develop into crystal nuclei, and then crystal growth by propagation through the amorphous primary particles (with size of ca. 50 nm) goes on, and finally, the amorphous particles transform into LTA crystal particles with the same size. This propagation growth process is followed by or parallel with the agglomeration and densification of the primary particles. In this way, compact LTA zeolite membrane consisting of sphere grains with undefined crystal facets is obtained. © 2006 Elsevier B.V. All rights reserved. Keywords: LTA; Zeolite membrane; Microwave synthesis; Formation mechanism; Gas permeation
1. Introduction Since the mid of 1990s, owing to the potential molecular sieving action, controlled host–sorbate interactions and high thermal and chemical stability, extensive efforts have been devoted to the preparations, characterizations, and applications of zeolite membranes. The progresses achieved in this rapidly growing field are reflected by a number of excellent reviews [1–8] and book chapters [9]. Various synthetic strategies and methods have been invented and developed. Besides the general demands of zeolite powders synthesis (such as pure phase and high crystallinity), morphology control [10], orientation control [11–13] and membrane thickness control [13,14] are the more challengeable targets when zeolite membranes are proposed to be fabricated. Different types of supported polycrystalline zeolite membranes, such as LTA [15,16], MFI [10–14,17], FAU [18], SAPO-34 [19], and isomorphous substituted MFI zeolite membranes [20], have
∗
Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447. E-mail address: [email protected] (W. Yang). URL: http://yanggroup.dicp.ac.cn/.
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.04.051
been successfully prepared under laboratory-condition. Nevertheless, the optimization of zeolite membrane synthesis is still a trial-and-error procedure at present, which is low effective and difficult to be generalized. Therefore, a fundamental understanding of the formation mechanisms of zeolite membranes is of great significance. Extensive efforts have been made to elucidate the formation mechanisms of zeolite membranes. A homogeneous nucleation model was reported on the synthesis of MOR [21], LTA [16], and MFI zeolite membranes [17,22]. According to this model, nucleation occurs homogeneously in the bulk solution and the formed zeolite nuclei or crystals move and deposit on the substrate surface. A heterogeneous nucleation model was reported on the synthesis of LTA [23] and MFI [24] zeolite membranes. With this model, at the early stage of preparation, a gel layer is formed on the substrate surface and then heterogeneous nucleation and crystal growth occur at the interface between the gel and solution. Boudreau et al. proposed a model for the secondary growth of the seed layer on synthesis of LTA zeolite membrane [25]. With this model, the seeded crystals epitaxially grow at the early stage. With longer synthesis time, zeolite crystals could form homogeneously in the synthesis solution and
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deposit on the membrane. Recently, Li et al. investigated the formation mechanism of b-oriented MFI zeolite film directly by TEM [22]. Abundant TEM information including crystallographic orientation relationships among crystals in the film (both out-of-plane and in-plane), grain boundaries, and each crystal grain was obtained. This TEM investigation provides direct evidence to support the homogeneous nucleation mechanism. However, this mechanism was indirectly deduced from the information of the final obtained zeolite film, and only SEM and HRTEM characterizations have been performed. In order to achieve a better understanding of the formation mechanism of zeolite membrane, the events that take place during the whole synthesis process are needed to be monitored by comprehensive characterization techniques. Recently, we reported a novel microwave synthesis method, namely in situ aging-microwave synthesis method (AM method) [26]. High quality LTA zeolite membranes could be synthesized through this method with high reproducibility. It was found that the “in situ aging” step was necessary for a successful microwave synthesis, and the so obtained zeolite membrane consisted of sphere grains without well-developed crystal faces. In present study, other than the commonly used characterization techniques, such as SEM, XRD and permeation/separation tests, gravimetric method was used to monitor the weight evolution of the membranes as synthesis proceeded, and XPS was used to analyze the Si/Al ratios of the membranes with different synthesis times. In addition, attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR) was used to characterize the framework vibrations of the zeolite (or its precursor) that formed on the support during the synthesis process. Based on these comprehensive characterizations, the function of in situ aging was made clear, and the extraordinary morphology of AM synthesized LTA zeolite membrane was interpreted. A formation mechanism of LTA zeolite membrane prepared by AM method
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was proposed, which can provide generalized guidance for the preparation of other types of zeolite membranes. 2. Experimental 2.1. Membrane preparation LTA zeolite membranes were prepared according to our reported method (“in situ aging-microwave synthesis” method) [26]. The whole synthesis procedure is illustrated in Fig. 1. The synthesis mixture was prepared by mixing an aluminate solution and a silicate solution. In a typical synthesis, an aluminate solution was prepared by dissolving 40.0 g sodium hydroxide (purity > 96%) in 159.0 g deionized water, then adding 1.0 g aluminium foil (purity > 99.5%) into the caustic solution. A silicate solution was prepared by mixing 34.1 g sodium hydroxide, 20.6 g silica sol (d = 1.16 g/ml, containing 26.0% SiO2 ), and 159.0 g deionized water. After being stirred for 1 h at room temperature, the silicate solution was poured into the aluminate solution under vigorous stirring. The obtained mixture was stirred for 30 min at room temperature to produce a clear homogeneous solution with the following chemical composition: 5SiO2 :Al2 O3 :50Na2 O:1000H2 O. A porous ␣-Al2 O3 tube (self-made, 12 mm outside diameter, 9 mm inside diameter, 7 cm length, ca. 0.3 m pore radius, and ca. 40% porosity) was sealed with two Teflon caps at both the ends and placed vertically in a Teflon autoclave. For disk-shaped membrane synthesis, a porous ␣-Al2 O3 disk (self-made, 28 mm in diameter, 1.5 mm in thickness, ca. 0.3–0.5 m pore radius, and ca. 50% porosity) was placed vertically with a Teflon holder in the autoclave. After the synthesis solution was added, the autoclave was sealed. Before microwave heating, the autoclave was put in an air oven and aged at 50 ◦ C for 7 h (“in situ aging”). Then, the crystallization was carried out in a microwave oven
Fig. 1. Illustration of the process of AM synthesis.
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with a working frequency of 2450 MHz (Haier, HR-8801M). The synthesis solution was quickly heated to 90 ◦ C and then held at that temperature for a certain time (“microwave synthesis”). After synthesis, the membrane was immersed in deionized water for 1 day. Meantime, the alkalified water was exchanged for several times. The treated membrane was then dried in an air oven at 110 ◦ C over night. In our previous report, high quality membranes were obtained after two-stage synthesis. Therefore, in this paper, we will investigate and discuss the first and second synthesis stage, respectively. 2.2. Characterization The amount of reaction product that deposited on the support was determined by gravimetric method. The obtained membranes with different synthesis time were dried at 110 ◦ C for 24 h to get constant weights, and then were weighed in an electronic balance (Sartorius BS 323 S). Flat membranes were used to obtain the X-ray diffraction (XRD) patterns (Rigaku DX2000, Cu K␣ radiation, operating at
40 kV and 250 mA). The obtained XRD patterns were processed with MDI Jade (Materials Data, Inc., Version 5.0). In the present study, the intensity of the (2 0 0) peak of LTA zeolite (I(2 0 0) ) was adopted to monitor the evolution of zeolite membrane. X-ray photoelectron spectroscopy (XPS, VG ESCA-LAB MK-II, VG Scientific Ltd., Al K␣ radiation, hν = 2381.7963282 × 10−19 J (1486.6 eV)) was performed to determine the Si/Al ratio of the as-synthesized membranes. Attenuated total reflectance/Fourier transform infrared spectroscopy was used to characterize the framework vibrations of the zeolite (or its precursor) that formed on the support during the synthesis process. The ATR–FTIR spectra were recorded with a Nicolet-continum micro-FTIR spectroscopy. All samples were scanned for 64 times at the spectral resolution of 4 cm−1 . Due to the limitation of our FTIR–ATR instrument (frequency region: 650–4000 cm−1 ) and background spectra from the alumina support (400–900 cm−1 ), only the frequency region between 850 and 1150 cm−1 was intensively studied. The texture and thickness of the membranes were examined by scanning electron microscopy (SEM, JEM-1200EX scanning electron microscope, JEOL, operating at 40 kV).
Fig. 2. SEM images of: (a) top view of porous ␣-Al2 O3 support, (b) top view of the support after aging, (c) cross-sectional view of (b), and (d) XRD patterns of the support before/after aging; the insets show the corresponding enlarged images.
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2.3. Gas permeation measurements Before gas permeation measurements, the disk-shaped membranes were dried at 110 ◦ C under vacuum for at least 2 days to remove the water absorbed in the zeolite channels. The dried zeolite membrane was sealed in a permeation cell for gas permeation measurements. The effective membrane area was ca. 3 cm2 . The single gas permeance was measured by a soap-film flowmeter at room temperature (20 ◦ C) 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. 3. Results and discussions 3.1. The first-stage synthesis SEM top views of the membranes before and after aging are shown in Fig. 2a and b. It can be seen that after 7 h aging, the surface of the alumina support was covered throughout by a gel layer with amorphous-looking aggregates. The corresponding SEM cross-sectional view (Fig. 2c) also shows that the porous support was covered by a layer after aging. This layer was loose in appearance and about 4 m in thickness. It also can be seen that the pores of the support near the surface were blocked or partly blocked by the intruded gel. From the XRD pattern (Fig. 2d) of the membrane after aging, it can be seen that only the peaks of alumina support (␣-Al2 O3 ) were detected, which indicates that no detectable crystal growth occurred during aging. Meanwhile, it can be deduced that a kind of X-ray amorphous layer did cover the surface of the support because the XRD intensity of alumina support (␣-Al2 O3 ) decreased after aging (Fig. 2d). The amount of the deposited gel was ca. 2.0 wt.% (see Fig. 3, “microwave heating time” of 0 min means “after aging”), which was rich in Si, with a Si/Al ratio of 3.9 (see Table 1). Fig. 3 shows the weight gain and the intensity of the (2 0 0) peak of LTA zeolite (I(2 0 0) ) on the support versus microwave heating time. Here, the weight gain is corresponding to the total cumulative amount of reaction product, including un-
Fig. 3. Weight gain and diffraction intensity of LTA (2 0 0) of the as-synthesized membrane vs. microwave synthesis time during the first-stage synthesis.
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Table 1 The Si/Al ratios of the as-synthesized membrane with different microwave synthesis times Microwave synthesis time (min)
First-stage synthesis Second-stage synthesis a
0a
5
15
25
3.90 1.49
1.77 1.29
1.31 1.32
1.41 1.43
After aging.
crystallized amorphous phase and crystallized phase. While, I(2 0 0) can be regard as the amount of crystalline LTA zeolite formed on the support. After aging, a 2.0 wt.% weight gain was obtained. At the beginning of microwave synthesis (after 5 min of microwave synthesis), a sharp weight loss of ca. 50% occurred (down from the weight gain of 2.0–1.1 wt.%). After that, the cumulative amount of reaction product maintained nearly constant (ca. 1.1 wt.%) until the reaction time prolonged to 35 min (the weight gain increased slightly ∼1.3 wt.%). The I(2 0 0) curve shows a quite different trend, which is much like the S-shape crystal growth curve in zeolite synthesis. The existence of induction period (0–5 min) indicates that no appreciable nucleation took place during the in situ aging. The breakthrough of LTA zeolite growth happened at 15–25 min. Afterwards, a limited growth continued, consistent to the slight increase of weight gain. It should be noted that, during the whole first-stage synthesis, LTA zeolite was the only crystalline phase which was confirmed by XRD characterization. The Si/Al ratios of the reaction product deposited on the support with different synthesis times were determined by XPS, as shown in Table 1. After aging, an aluminosilicate gel was deposited on the support, with a Si/Al ratio of 3.90. In the case of preparation of LTA zeolite membranes, the formation of a gel layer on the support which was rich in silica has also been observed by other authors [16]. After 5 min of microwave synthesis, the Si content in the reaction production decreased, resulting in a Si/Al ratio of 1.77. As microwave synthesis proceeding, the Si/Al ratio decreased to 1.31 after 15 min of microwave synthesis. Afterwards, the Si/Al remained almost constant, and LTA zeolite membrane with a Si/Al ratio of 1.41 was obtained after 25 min of microwave synthesis. Generally, LTA zeolite should have a Si/Al ratio of 1.0. In the present study, however, the so obtained LTA zeolite membrane showed a higher Si/Al ratio of 1.41. Other authors also reported that microwave synthesized zeolite possessed a higher Si/Al ratio as compared with that obtained under conventional heating [27]. The reason why microwave heating synthesis results in products with higher Si/Al ratio is not clear now. IR spectroscopy is often used to characterize zeolite structure and examine changes in the solid phase prior to and during crystallization of zeolite. In the present study, ATR/FTIR was applied to directly characterize the structural evolution of aluminosilicate species on the support as synthesis progressed. In the frequency region between 850 and 1150 cm−1 , no band was detected for the alumina support as shown in Fig. 4. After aging, a wide band centered around 1000 cm−1 was observed, which is attributed to the asymmetric stretching vibrations of TO4
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Fig. 4. ATR/FTIR spectra of the as-synthesized membrane with different microwave synthesis times during the first-stage synthesis.
(T = Si or Al) tetrahedra. This verified the formation of the gel layer during in situ aging. After 5 min of microwave synthesis, this band developed into a bimodal peak, with a wide band around 1010 cm−1 and a small should near 970 cm−1 . As crystallization progressed, the band around 1010 cm−1 kept almost unchanged, while the adsorption near 970 cm−1 enhanced and shifted to lower frequency (960 cm−1 after 25 min of synthesis).
In order to clarify the assignments of these peaks, commercial LTA zeolite powder and the LTA zeolite powder that simultaneously precipitated in the bulk phase were scanned for ATR/FTIR spectra. The obtained IR spectra (second-order derivative spectra are also shown in order to distinguish the overlapping peaks) and the corresponding SEM images are shown in Fig. 5. The spectrum of commercial LTA zeolite powders (Fig. 5a) comprises a bimodal peak, around 1014 and 970 cm−1 , respectively. The spectrum of AM synthesized LTA zeolite powder (Fig. 5b) also comprises a bimodal peak. Whereas, the peak positions are different from that of commercial one but similar to that of the LTA zeolite membrane after the first-stage AM synthesis, i.e. two peaks at 1014 and 960 cm−1 , respectively. The different infrared vibrations between commercial LTA zeolite powder and AM synthesized one might result from their different morphologies. It can be seen from the SEM images that the commercial LTA zeolite is typical cubic single crystal (Fig. 5a), while the LTA zeolite powder synthesized by AM method appears twin growth and intergrowth of polycrystals (Fig. 5b). Further discussion of the bimodal peak will be given later. It should be noted that the appearance of the characteristic bimodal peak of LTA zeolite after 5 min of synthesis indicates that LTA phase or at least some LTA structure units had already established at that time, which coincides with the XRD results. The morphological evolution of the deposited gel layer during microwave assistant crystallization was monitored by SEM, as shown in Fig. 6. After 5 min of crystallization, some uniform particles with sizes below 100 nm emerged in the gel layer, as shown in Fig. 6a-1. Combined with XRD and IR results, it is speculated that these particles already had crystalline structures in some extent. Subsequently, these particles developed into larger ones (Fig. 6b-1), and after 25 min of microwave synthesis, round-shaped particles with size around 400 nm were obtained (Fig. 6c-1). From the high magnification image (Fig. 6c-1, inset),
Fig. 5. (a) SEM top view and ATR/FTIR spectrum of commercial LTA zeolite powder, and (b) SEM top view and ATR/FTIR spectrum of AM synthesized LTA zeolite powder.
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it can be seen that the round-shaped particles were made up of smaller particles with sizes around 50 nm, as pointed out in the picture with arrows. When microwave assistant crystallization prolonged, well-shaped cubic LTA zeolite crystals with sizes around 1 m were obtained, as shown in Fig. 6d-1. The terrace feature of these crystals (as shown in Fig. 6d-1, inset with arrows) implies that a layer growth is the controlling mechanism during the crystallization from 25 to 35 min. From the cross-section views (Fig. 6a-2–c-2), it can be seen that the layer thickness did not increase observably with synthesis time, i.e. ca. 2 m all the time, except the one after 35 min of synthesis which was a little thicker (ca. 3 m, Fig. 6d-2). This kind of layer thickness evolution coincides with the weight gain curve (Fig. 3). From the cross-section views, it also can be seen that the pores of the support were occupied with reaction products, which resulted in an unclear interface between the external product layer and the support. The obtained crystallized LTA zeolite membranes after the first-stage AM synthesis (after 25 or 35 min of microwave synthesis), however, were not compact enough. Neither the aggregated zeolite particles (400 nm in size, composing of 50 nm particles, Fig. 6c-1) nor the cubic zeolite crystals (1 m in size, Fig. 6d-1) were well intergrown together. This was further confirmed by gas permeation experiments. Both the membrane after 25 min and that after 35 min of synthesis showed poor separation performances. Therefore, repeated synthesis (the second-stage AM synthesis) is necessary. Based on our previous studies, the optimized synthesis procedure is 25 min of microwave synthesis in the first-stage AM synthesis. Thus, in the next section, the second-stage AM synthesis following a 25 min first-stage synthesis will be discussed. 3.2. The second-stage synthesis Fig. 7 shows the weight gain (relative to the value after the first-stage synthesis) and the intensity of the (2 0 0) peak of LTA zeolite (I(2 0 0) ) on the support versus microwave heating time. In the second-stage synthesis, the total cumulated amount of reac-
Fig. 6. SEM top views of the as-synthesized membrane after (a-1) 5 min, (b-1) 15 min, (c-1) 25 min, and (d-1) 35 min of microwave synthesis during the firststage synthesis; SEM cross-sectional views of the as-synthesized membrane after (a-2) 5 min, (b-2) 15 min, (c-2) 25 min, and (d-2) 35 min of microwave synthesis during the first-stage synthesis; the insets show the corresponding enlarged images. Fig. 7. Weight gain and diffraction intensity of LTA (2 0 0) of the as-synthesized membrane vs. microwave synthesis time during the second-stage synthesis.
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Fig. 8. ATR/FTIR spectra of the as-synthesized membrane with different microwave synthesis times during the second-stage synthesis.
tion product on the support (weight gain) maintained constant and finally processed a slight raise. As compared with the firststage synthesis, the average weight gain in the second-stage synthesis was apparently larger, i.e. ca. 3.9 wt.% versus 1.1 wt.%. After an initial sharp increase (0–5 min), the I(2 0 0) curve shows an approximate flat line (5–25 min) except at the point of 35 min of microwave synthesis, where I(2 0 0) declines obviously which coincides with the dissolution of LTA and growth of SOD zeolite. In consideration of the obvious weight gain (3.79 wt.%) and increase of LTA zeolite (increase of I(2 0 0) ) just after aging, it is speculated that the nano-sized LTA zeolite that formed after the first-stage synthesis has grown up in certain degree and substantial secondary nucleation has accomplished during the second in situ aging. The sharp increase of I(2 0 0) at the beginning of crystallization and rapid reach of a plateau indicate that the transformation of the gel layer into crystal phase complete very soon, and the major part of the second-stage synthesis is not a crystallization process but a morphological evolution process of the already formed LTA crystals. The Si/Al ratio of the reaction product on the support kept almost unchanged throughout the second-stage synthesis. This indicates that the chemical evolution of the gel layer has already finished during second in situ aging. The ultimate obtained LTA zeolite membrane had a Si/Al ratio of 1.43, as shown in Table 1. Fig. 8 illustrates the ATR/FTIR spectra of the membranes synthesized with different microwave synthesis times during the second-stage synthesis. For comparison, the spectrum of the membrane after the first-stage synthesis (25 min of microwave heating) is also shown in the same figure. After second aging, the band around 1010 cm−1 enhanced (for the second-stage syn-
thesis series, this band is near 1013 cm−1 ; considering that the resolution of our experiment is 4 cm−1 , this 3 cm−1 peak shift is negligible), while the band at 960 cm−1 disappeared, replaced by a rather broad band at 940–960 cm−1 . As crystallization progressed, a band near 940 cm−1 emerged, which maintained during the whole crystallization process. Eventually, the spectrum of the membrane after two-stage synthesis shows an apparent bimodal peak. In addition, it can be seen that the relative intensity of these two peaks (960 cm−1 /1013 cm−1 ) increases gradually with synthesis time. Kyotani et al. [28] used ATR/FTIR to characterize LTA zeolite membrane and also found the appearance of bimodal peak (1012 and 930 cm−1 ). They attributed the 930 cm−1 peak to the amorphous substances and/or LTA crystals embedded in the alumina porous support. A proportional relationship between the relative ratio of the two peaks and the dehydration performance (separation factor) in pervaporation was clarified by them. In the present study, it was found that with the enhancement of the intergrowth of LTA zeolite, i.e. from single crystals (commercial LTA powders) to polycrystals (LTA powder precipitated during AM synthesis), and from noncontinuous membrane (after the first-stage synthesis) to compact membrane (after the second-stage synthesis, as shown Fig. 9d1), the higher frequency of the bimodal peak (1014, 1010, and 1013 cm−1 ) maintained almost unchanged, while the lower frequency 970 cm−1 peak moved to 960 cm−1 after the first-stage synthesis (or polycrystalline LTA powder) and further moved to 940 cm−1 after the second-stage synthesis. Therefore, we prefer that the lower frequency of the bimodal peak (960 or 940 cm−1 ) is derived from the crystal boundary structures. The different arrangement of Si–O–Al bonds at the crystal boundary to that of the bulk phase might cause the different infra vibrations. Despite that the accurate assignments of 960 and 940 cm−1 vibration are not clear now, a higher ratio of the two peaks (lower frequency/higher frequency) associates with a more compact LTA zeolite membrane (higher degree of intergrowth), as reported by Kyotani et al. [28] and reconfirmed by present study. SEM images in Fig. 9 show the morphological evolution of the reaction product on the support during the second-stage microwave synthesis. After second aging, a gel-looking layer covered the zeolite layer that formed during the first-stage synthesis again, as shown in Fig. 9a-1. Some portions of the gel can be found to compose of small particles with sizes around 50 nm, as pointed out in Fig. 9a-1 with arrows (inset, high magnification image). After sequential 5 min of microwave synthesis, the 50 nm particles boomed, which began to packed together and form some aggregates with sizes of ca. 0.5–1 m. Whereas, most of these aggregates were not packed together tightly and individual 50 nm particles can be distinguished in the high magnification SEM image, as pointed out with arrows in Fig. 9b-1. After 15 min of microwave synthesis, the 50 nm particles began to agglomerate together, and some of the former 0.5–1 m aggregates became larger (1–3 m). From the high magnification image (Fig. 9c-1, inset), it can be seen that the 50 nm particles conjoin with each other, and some fuse together to form larger particles. Nevertheless, the boundaries between these particles are still visible. When microwave synthesis prolonged to 25 min, the 50 nm particles well fused together, and
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Fig. 9. SEM top views of the as-synthesized membrane after (a-1) 0 min, (b-1) 5 min, (c-1) 15 min, (d-1) 25 min, and (e-1) 35 min of microwave synthesis during the second-stage synthesis; SEM cross-sectional views of the as-synthesized membrane after (a-2) 0 min, (b-2) 5 min, (c-2) 15 min, (d-2) 25 min, and (e-2) 35 min of microwave synthesis during the second-stage synthesis; the insets show the corresponding enlarged images, except (e-1), where the inset shows the corresponding XRD pattern.
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the former aggregates (1–3 m) inter-grew with each other. As a result, the grain boundaries are hardly to be distinguished, as shown in Fig. 9d-1 (inset). After further microwave synthesis (35 min), some acicular crystals emerged on the surface of LTA zeolite layer (Fig. 9e-1). These were proved to be SOD (hydroxyl-sodalite) crystals by XRD characterization, as shown in Fig. 9e-2 (inset). From the cross-sectional view (Fig. 9a-2), it can be seen that after second in situ aging the thickness of the membrane increased from 2 m (after the first-stage synthesis) to 5 m. During the consequential microwave assistant crystallization, the layer thickness did not increase distinctly (Fig. 9b-2–d-2), which coincides with the weight gain curve (Fig. 7). Here, we speculated that these 50 nm particles (named as “primary particles” hereafter) were transformed from the preformed amorphous gel particles with similar size. The crystalline zeolite particles retained the size and morphology of the initial amorphous particles. Aggregation of the primary particles leads to the formation of larger particles (secondary particles, e.g. the 400 nm particles in the first-stage synthesis; 0.5–1 and 1–3 m particles in the second-stage synthesis). These secondary particles can undergo a densification process to fuse together. Mintova et al. [29] studied the formation of LTA zeolite at room temperature from a system containing (TMA)2 O as template by means of HRTEM. They found that the aggregation of the primary colloidal silica particles (5–10 nm) leaded to the formation of 40–80 nm aggregates. Smaihi et al. [30] investigated the crystallization stages of LTA zeolite at 40 ◦ C from an organic-free system (the same as the synthesis solution used in our study) by small-angle X-ray scattering (SAXS) and SEM. They found that the primary particles were in the range of 40–100 nm. Nevertheless, our naming the 50 nm particles as primary particles is somewhat non-strict. Further characterization, e.g. HRTEM, is needed to clarify the primary particles and their corresponding sizes. SEM characterization indicates that the major part of the second-stage synthesis is an aggregation and densification process of primary particles. This was further confirmed by gas separation test and the results are shown in Fig. 10. The membrane after second aging showed a H2 permeance of 20.6 × 10−7 mol m−2 s−1 Pa−1 , and H2 /N2 permselectivity was only 2.77, which is lower than that of Knudsen diffusion ratio (3.74). After 5 min of microwave synthesis, N2 permeance decreased from 7.44 × 10−7 to 4.51 × 10−7 mol m−2 s−1 Pa−1 while H2 permeance decreased largely at the same time to 11.9 × 10−7 mol m−2 s−1 Pa−1 , as a result, the H2 /N2 permselectivity decreased to 2.64. After 15 min of microwave synthesis, the permeances of H2 and N2 kept on decreasing and the permselectivity increased to 3.06 which was still lower than that of Knudsen diffusion ratio. When microwave synthesis prolonged to 25 min, the H2 /N2 permselectivity increased to 4.71 with a corresponding H2 permeance of 2.40 × 10−7 mol m−2 s−1 Pa−1 . This indicates that the zeolite membrane after 25 min of second microwave synthesis was compact and free of considerable defects and the gases mainly permeated through the LTA zeolite channels (with pore size of 0.41 nm × 0.41 nm). When microwave synthesis continued, due to the formation of SOD
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Fig. 10. H2 /N2 permeation properties of the as-synthesized membrane with different microwave synthesis times during the second-stage synthesis.
impurity, the integrity of the zeolite membrane was destroyed in certain degree, and the H2 /N2 permselectivity decreased to 3.97. The further decrease of H2 (N2 ) permeance is probably due to the further increase of membrane thickness as indicated from the weight gain curve (Fig. 7) and seen from the SEM cross-sectional view (Fig. 9e-2). It is believed that the essential mechanism of zeolite formation is similar, whether in powder form or in film and membrane form. As to LTA zeolite, owing to its relatively
simple composition and fast crystallization kinetics, many fundamental researches have been focused on its formation process, and the commonly accepted formation mechanism of LTA zeolite has been described [29,30]. In the present study, based on the comprehensive characterization of the whole formation process of LTA zeolite membrane, and referred to the reported formation mechanism of LTA zeolite, the formation mechanism of LTA zeolite membrane by AM method is proposed, as illustrated in Fig. 11. For the first-stage synthesis, after the 7 h in situ aging, an amorphous gel layer was formed on the support surface. This is caused by the capillarity function of the support pores and the dissolution of support. Other authors have also reported that an amorphous gel layer was indispensably formed on the support surface before the initiation of nucleation and crystal growth [16,31]. During the course of aging, Na+ -assisted rearrangement of the Si–O–Al bonds in the amorphous phase leads to the formation of areas of local order, which can be regarded as pre-nuclei or germ-nuclei. This is necessary to overcome the nucleation-related bottleneck in microwave synthesis of zeolite [32,33]. This X-ray amorphous gel is a pseudo-steady-state intermediate with increased ordered, and can be named as secondary amorphous phase [34]. During the following microwave assistant crystallization, under the rapid and uniform dielectric heating, the deposited amorphous gel underwent some chemical evolutions, i.e. partial dissolution of the silica from the amorphous gel layer and re-equilibration between the gel phase and the solution phase. Isometric amorphous primary particles with
Fig. 11. Schematic illustration of the proposed formation mechanism of LTA zeolite membrane synthesized by AM method.
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sizes around 50 nm emerged. At the same time, those pre-nuclei rapidly and simultaneously developed into crystal nuclei (areas of sufficient order for a periodic structure). After this induction period (pre-nucleation and nucleation periods), crystal growth by propagation through the gel network went on. Growth by propagation through the gel phase has already been observed in the formation of zeolite LTA [29,30], LTL [35], MFL [36], and FAU [37]. After 25 min of microwave heating, the 50 nm amorphous particles that contained nuclei were transformed into the final LTA crystal particles remaining with the same sizes. This propagation growth process was followed by or parallel with the agglomeration and densification of the nano-particles, i.e. secondary particles became larger by aggregation of more primary particles and became more compact, which leaded to the formation of LTA zeolite crystals with rough surfaces. Aggregation of nano-sized zeolite particles during their crystallization has also been reported by many authors [38–41]. Microwave adsorption through Maxwell–Wagner polarization mechanism, which results from the accumulation of charge at the material interface, might be applied to explain the strong aggregation trend between zeolite particles under microwave irradiation. With prolonged microwave heating, the formed zeolite crystals grew up by acquisition of nutrients from bulk solution or nearby unreacted amorphous gel (which did not contain nuclei) or small LTA crystals (Ostwald ripening). Layer growth is the controlling mechanism in this stage which leads to well-shaped zeolite crystals [42]. The zeolite membrane formation process in the second-stage synthesis is substantially similar with that in the first-stage synthesis. Whereas, unlike the naked support used in the first-stage synthesis, the second-stage synthesis begins with a “seeded support”. Nano-sized LTA crystals that formed after the first-stage synthesis covered the support completely, which can be regarded as a seed layer. Therefore, the first-stage synthesis can also be seen as an in situ seeding step. During second in situ aging, those seed crystals grew larger in certain degree, while the dominating change was formation of an amorphous gel layer on the seed layer and nucleation in the gel phase. The phenomenon that a gel layer was first formed on the seeded support during synthesis of LTA zeolite membrane has also been reported by Kita et al. [43]. As compared with precipitation of zeolite on the support, formation of an amorphous layer is more kinetic favorable. Besides, the activation energy of nucleation of LTA zeolite is smaller than that of growth of LTA zeolite. Therefore, at low temperature, i.e. in situ aging temperature of 50 ◦ C, nucleation is prevailing. In addition, the existence of seed crystals facilitated secondary nucleation remarkably. As a result, a gel layer containing plenty of nuclei covered on the support after second aging. The nucleus density in the second aging generated gel layer was much higher than that in the first-stage synthesis. During the following microwave assistant crystallization, transformation from the gel layer into crystalline LTA zeolite layer (propagation through the gel network) progressed rapidly, and almost completed after only 5 min of microwave synthesis. Afterwards, agglomeration and densification went on, which were the dominating courses during the second-stage synthesis, and made the second-stage synthesis more like a morphological evolution process. As synthesis progressed (35 min in the present study),
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base concentration in the bulk phase increased. This led to the dissolution of formed LTA zeolite and the heterogeneous nucleation and solution-mediated growth of sodalite product (SOD) on the LTA layer. As discussed above, no fundamental difference exists between the first-stage and the second-stage synthesis. The phenomenological differences are originated from the higher nucleus density at the beginning of microwave heating in the second-stage synthesis. This high nucleus density accelerates the digestion of the gel layer and enhances the agglomeration and densification. In this way, compact LTA zeolite membrane which consisted of round-shaped crystals with un-defined facets is obtained. By comprehensive characterizations of the process of LTA zeolite formation, Smaihi et al. concluded that the crystallization process of LTA zeolite should be subdivided into two stages [30]. The first stage of crystal growth process proceeded by propagation through the aluminosilicate gel phase, and nano-crystals with a rough surface were obtained at the end of the process. During the second stage, a layer growth, where a solution-mediated transport of nutrients dominates, is the controlling mechanism, and well-shaped zeolite crystals were obtained at last. In a recent review of McLeary and Jansen [44], the authors pointed out that the ever-occurring triangularly shaped gap between growing crystals on the support would lead to the systematic imperfections of synthesized membrane, and the formation of sphere grains with undefined crystal facets was necessary to achieve a pinhole-free continuous zeolite layer. In the present study, taking advantage of MH, the membrane formation was limited to the first stage of LTA crystal growth, and compact LTA membrane that consisted of sphere grains without well-developed crystal faces was thus obtained. These advantages include: (I) fast synthesis, i.e. the dissolution of synthesis gel is promoted and the crystallization time is shortened by the “active water” [45]. Therefore, the zeolite membrane synthesis can be achieved in short time (25 min in the present study), and Ostwald ripening is suppressed in certain degree. (II) Selective heating, i.e. microwaves selectively couple with the gel layer because that the gel phase has a higher dissipation factor (tan δ, which defines the ability of a medium to convert electromagnetic energy into heat) than the bulk solution phase. This makes AM synthesis a kind of gel conversion synthesis. (III) Volumetric heating, i.e. the gel layer is heated quickly and entirely and homogeneously, which results in simultaneous nucleation and synchronous propagation type crystallization. (IV) Surface activation, i.e. microwave adsorption through Maxwell–Wagner polarization mechanism, which results in a remarkable aggregation of LTA nano-particles, and thus strengthens the agglomeration and densification type crystallization. A detailed comparison between MH and CH and further discussion of the effect of microwave in synthesis will be given elsewhere. 4. Conclusion Based on the above results, some conclusions can be drawn: (I) In situ aging process was proved to be a simple but effective way to increase the nucleation density on the support
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surface. The rapid and synchronous nucleation is realized under fast volumetric microwave heating. (II) The essential mechanism of LTA zeolite membrane formation is similar to its powder-formed analogue. In order to obtain high quality zeolite membrane, the membrane formation should be limited to the first stage of crystal growth, i.e. propagation growth through the gel phase. This is realized by microwave heating assistant crystallization, owing to the special characteristics of microwave heating, i.e. “active water”, selective heating, volumetric heating, and surface activation. (III) The formation process by AM method is something like a gel layer conversion process. This indicates that we can designedly adjust the thickness of obtained zeolite membrane by controlling the thickness of the gel layer or by varying the times of synthesis stage. Acknowledgments This work was supported by the National Science Foundation of China (Grant No. 20321303) and the Ministry of Science and Technology of China (Grant No. 2003CB615802). References [1] J. Caro, M. Noack, P. K¨olsch, R. Sch¨afer, Zeolite membranes—state of their development and perspective, Microporous Mesoporous Mater. 38 (2000) 3. [2] E. Tavolaro, E. Drioli, Zeolite membranes, Adv. Mater. 11 (1999) 975. [3] A.S.T. Chiang, K. Chao, Membranes and films of zeolite and zeolite-like materials, J. Phys. Chem. Solids 62 (2001) 1899. [4] J. Jansen, J.H. Koegler, H. Bekkum, H.P.A. Calis, C.M. Bleek, F. Kapteijn, J.A. Moulijn, E.R. Geus, N. Puil, Zeolitic coatings and their potential use in catalysis, Microporous Mesoporous Mater. 21 (1998) 213. [5] M. Matsukata, E. Kikuchi, Zeolitic membranes: synthesis, properties, and prospects, Bull. Chem. Soc. Jpn. 70 (1997) 2341. [6] T. Bein, Synthesis and applications of molecular sieve layers and membranes, Chem. Mater. 8 (1998) 1636. [7] M. Noack, P. K¨olsch, R. Sch¨afer, P. Toussaint, J. Caro, Molecular sieve membranes for industrial application: problems, process, solutions, Chem. Eng. Technol. 25 (2002) 221. [8] Y.S. Lin, I. Kumakiri, B.N. Nair, H. Alsyouri, Microporous inorganic membranes, Sep. Purif. Method 31 (2002) 229. [9] S. Nair, M. Tsapatsis, in: S.M. Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Zeolite Science & Technology, Marcel Dekker, New York, 2003, p. p. 867. [10] W.C. Wong, L.T.Y. Au, P.P.S. Lau, C.T. Ariso, K.L. Yeung, Effects of synthesis parameters on the zeolite membrane morphology, J. Membr. Sci. 193 (2001) 141. [11] Z.P. Lai, G. Bonilla, I. Diaz, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W. Thompson, M. Tsapatsis, D.G. Vlachos, Microstructural optimization of a zeolite membrane for organic vapor separation, Science 300 (2003) 456. [12] L.C. Boudreau, M. Tsapatsis, A highly oriented thin film of zeolite A, Chem. Mater. 9 (1997) 1705. [13] A. Gouzinis, M. Tsapatsis, On the preferred orientation and microstructural manipulation of molecular sieve films prepared by secondary growth, Chem. Mater. 10 (1998) 2497. [14] J. Hedlund, J. Sterte, M. Anthonis, A.J. Bons, B. Carstensen, N. Corcoran, D. Cox, H. Beckamn, W.D. Gijnst, P.P. de Moor, F. Lai, J. McHenry, W. Ortier, J. Reinoso, J. Peters, High-flux MFI membranes, Microporous Mesoporous Mater. 52 (2002) 179.
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