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Continuity Control of b-Oriented MFI Zeolite Films by Microwave Synthesis Xianming Li,† Yushan Yan,†,‡ and Zhengbao Wang*,† Department of Chemical and Biological Engineering, Zhejiang UniVersity, Hangzhou 310027, P.R. China, and Department of Chemical and EnVironmental Engineering, UniVersity of California, RiVerside, California 92521
Influences of aging and crystallization conditions on the continuity of b-oriented MFI zeolite films on stainless steel substrates are systematically investigated. A long aging time (e.g., 20 h) at room temperature is needed to obtain a continuous b-oriented MFI zeolite film by the microwave heating at 165 °C. However, the aging time can be shortened from 20 h to 30 min by elevating the aging temperature to 80 °C. Compared with the conventional heating (2 h), continuous b-oriented MFI zeolite films are prepared on stainless steel substrates with a much shorter crystallization time by the microwave heating (35 min). Continuous b-oriented MFI zeolite films can be synthesized by controlling nuclei concentration and crystal size, which are mainly determined by the conditions of the aging and crystallization respectively. On the basis of the results from these experiments, a film continuity-synthesis condition map is obtained and a film formation mechanism is discussed. 1. Introduction Zeolite films and membranes have been widely studied for their unique properties, such as uniform molecular-sized pores, high thermal stability, and excellent solvent resistance. They might be applied to separation membranes,1–3 membrane reactors,4 chemical sensors,5 electrodes,6 low-k dielectric materials,7,8 heat pumps,9 and corrosion protection coatings.10 MFI zeolite films have been extensively studied because of their appropriate pore openings which are close to the size of many industrially important organic molecules. In the MFI zeolite crystal, the 0.53 nm × 0.56 nm straight channels (b-axis) are interconnected with the 0.51 nm × 0.55 nm sinusoidal channels (a-axis). Therefore, b-oriented MFI films are of interest from the viewpoint of both fundamental study and practical applications. Though a preferred orientation is often desirable, most of the zeolite films available up to the present are randomly oriented.2–4 Seeded growth and in situ hydrothermal synthesis are the two major methods to synthesize b-oriented MFI films. The former offers a way to control the growth of a zeolite film accurately due to the use of seeds.1,11,12 However, it is sometimes difficult to obtain a uniform seed layer. The advantages of the in situ method include its simplicity where the seeding process can be omitted and its ability to synthesize zeolite films on substrates with complex shapes. In our previous studies, b-oriented continuous MFI films on metal substrates have been synthesized using an in situ method by conventional heating.13,14 With conventional heating, the temperature gradient and low heating rate are often observed, leading to long operation time and high energy consumptions, and consequently, a high cost of zeolite film production. Since a relatively long time (more than an hour) is often needed for the synthesis mixture to reach the crystallization temperature, the real crystallization time with the conventional heating is much shorter than the total heating time. Moreover, the heating rate and temperature gradient change greatly when different autoclaves or ovens are used. With the conventional heating, it is difficult to investigate or control the heating process. Hence, it is difficult to accurately control * To whom correspondence should be addressed. Tel/Fax: +86-5718795-2391. E-mail:
[email protected]. † Zhejiang University. ‡ University of California.
the number and size of crystals in the conventional heating, while a certain number of zeolite crystal and a certain crystal size are required to manufacture a continuous zeolite film. For the past 10-15 years, extensive efforts have been made to synthesize zeolite films and membranes by the microwave heating method. A rapid heating throughout the material with little thermal gradient could be realized by the microwave heating because that energy is supplied by an electromagnetic field directly to the material in microwave processing.15–18 Therefore, microwave heating has several advantages over conventional heating. First, it offers a promising alternative to shorten the synthesis time of zeolite films and greatly improve the film synthesis efficiency. Second, the rapid heating could avoid or reduce some external influences during synthesis process, such as the dissolution of the substrates. And third, the heating process could be controlled accurately. The rapid heating and uniform temperature distribution in the microwave heating offer an opportunity to separate the nucleation and crystallization periods, which makes it possible to synthesize a continuous zeolite film by accurately controlling the microstructure of MFI film. Furthermore, these specific characteristics of the microwave heating facilitate the study of film formation mechanism. Although a large amount of work has been done on the preparation of zeolite films and membranes by microwave heating, most MFI films and membranes synthesized by the microwave heating method are randomly oriented.16,19,20 The (101) oriented silicalite-1 membranes have been synthesized by Motuzas et al. with microwave-assisted secondary growth method.21,22 Hwang et al. have prepared b-oriented MFI films on the substrate surface coated with metal oxide nanoparticles with or without a micropattern using microwave heating and investigated the influence of metal oxides with different dielectric constants.23 Very recently, we reported that continuous b-oriented MFI films on stainless steel substrates were synthesized by a two-stage microwave heating and that the crystal growth rate was accelerated greatly by microwave heating.24 It is widely known that the synthesis conditions affect the zeolite films significantly. However, few systematic and detailed studies exist on how the synthesis conditions influence the formation of b-oriented MFI zeolite films by the microwave heating method.
10.1021/ie1000136 2010 American Chemical Society Published on Web 05/07/2010
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In this paper, we systematically investigate the influences of the synthesis conditions on the zeolite film formation by accurately controlling of the heating progress and demonstrate that continuous b-oriented MFI films can be synthesized by controlling the nucleation and crystal growth using a microwave heating. Finally, a film continuity-synthesis condition map is developed and a film formation mechanism is proposed that is consistent with the experimental results. 2. Experimental Section Pure silica MFI (silicalite-1) films on stainless steel substrates were prepared as follows. Mirror stainless steel 202 plates (20 mm × 20 mm) were immersed in hydrogen peroxide solution for 45 min, then rinsed with deionized water, and dried at 60 °C before the synthesis of zeolite films. Synthesis solution of molar composition TEOS:0.32 TPAOH: 165 H2O was made by slowly adding tetraethylorthosilicate (TEOS, Acros Organics) to a solution of tetrapropylammonium hydroxide (TPAOH, Zhejiang Xianju Application Chemical Research Institute, China) and water under stirring. A clear synthesis solution was obtained after stirring at room temperature for 4 h. Conventional Heating. For this method, 20 g of synthesis solution was directly loaded without filtering into a 100 mL Teflon-lined stainless steel autoclave (cylinder shape, Φ4 cm ×10 cm). Then, a metal substrate was vertically placed with a Teflon holder at the bottom of the autoclave. The autoclave was then sealed and placed in a convection oven at 165 °C for 2 h (more than an hour is needed for the synthesis mixture to reach the set temperature). After the synthesis, the mixture was quenched. Finally, the sample was recovered, thoroughly washed with deionized water, and dried at 60 °C. Microwave Heating. Here, 20 g of synthesis solution was directly loaded without filtering into a 70 mL Teflon-lined microwave-proof autoclave (cylinder shape, Φ3 cm ×9.7 cm). Subsequently, a metal substrate was vertically placed with a Teflon holder at the bottom of the autoclave. The autoclave was then sealed and placed in a MDS-6 microwave oven (Sineo Microwave Chemical Technology Co. Ltd., Shanghai, China), in which the synthesis solution could be quickly heated to a desired temperature (only 1-3 min are needed for the synthesis mixture to reach the set temperature) and held at that temperature. The synthesis solution was further aged under stirring at room temperature for the time of t0 (0-20 h) or at elevated temperatures (T1, 50-110 °C) for the time of t1 (5-40 min) before the crystallization by the microwave heating. The crystallization temperature and time were defined as T2 (150-185 °C) and t2 (10-45 min), respectively. After the synthesis, the mixture was cooled naturally. Finally, the sample was recovered, thoroughly washed with deionized water, and dried at 60 °C. The morphology of silicalite films synthesized was examined by a field-emission scanning electron microscopy (FE-SEM) at 8 kV, using a Hitachi S-4800 microscope. X-ray diffraction (XRD) patterns were collected on a Panalytical X’ PertPro diffractometer using Cu KR radiation. 3. Results and Discussion 3.1. Aging at Room Temperature. A continuous film formed on the metal substrate at 165 °C for 2 h by the conventional heating at t0 ) 0 h (no further aging beyond the 4 h room temperature stirring) and the crystal size (along c-axis) was 1.5 µm. The X-ray diffraction (XRD) pattern shows that the film was predominantly b-oriented as previously reported.14
Figure 1. SEM micrographs of MFI zeolite films on stainless steel substrates synthesized by the microwave heating at 165 °C for 35 min after aging at room temperature for t0 ) (a) 6 and (b) 20 h.
For the microwave heating, only individual crystals, not a continuous film on the metal substrate, were observed at 165 °C for 35 min with t0 ) 0 h and the crystal size grew up to 1.9 µm.24 It is clear that the microwave heating can accelerate the growth of the crystals. However, the poor continuity of the film by the microwave heating was observed even when the crystals on the substrate already grew larger at a longer heating time than that by conventional heating. For example, even when the synthesis time of microwave heating was extended to 1 h, the crystals were 2.9 µm long, but the film obtained was still not continuous (data not shown). It seems that the crystal number by the microwave heating was smaller than that by the conventional heating.24 This observation can be explained by the possibility that by the microwave heating only the nuclei created at the room temperature aging were crystallized with no new nucleation. By contrast, with the conventional heating, more nuclei may have been produced during the slow ramping of temperature before the crystal growth takes over. For the microwave heating method, to increase the crystal number, the effect of the aging time (t0) at room temperature was further investigated here. As shown in Figure 1, by prolonging the aging time to 6 h, an increase in the crystal number was found and the crystals started to intergrow (Figure 1a). At the aging time of 20 h, the increase in the crystal number continued and a continuous film already formed (Figure 1b). The crystal size changed slightly with various aging times. In other words, the crystallization (crystal growth) only took place at high temperature by the microwave heating and the prolonged aging time at room temperature only increased the number of nuclei. 3.2. Aging at Elevated Temperature. To shorten the aging time (t0) at room temperature, the synthesis solution was aged at elevated temperatures (a microwave-assisted aging). After stirring at the room temperature for 4 h (t0 ) 0 h), the microwave-assisted aging of the synthesis solution at lower temperature (T1) was performed for a certain time (t1) first. And then, the mixture was crystallized at higher temperature (T2) for a certain time (t2) by microwave heating. In order to prepare continuous films by accurately controlling the crystal number and size, the influences of both the temperature and duration of the two stages on the MFI zeolite film synthesis were systematically studied. 3.2.1. Influence of the Microwave-Assisted Aging Temperature (T1). It is known that both the nucleation and crystallization rate are greatly affected by the temperature.14,17,18,25 Hence, the influence of the microwave-assisted aging temperature (T1) on the zeolite film synthesis was investigated first. The microwave-assisted aging was conducted at t1 ) 30 min, and T1 varied from 50 to 100 °C. The crystallization was conducted at T2 ) 165 °C and t2 )35 min. The SEM images of the as-synthesized films are shown in Figure 2, indicating that all of the films were predominantly b-oriented. XRD patterns also show that the films were predominantly b-oriented (Figure 3). A discontinuous film with some defects formed at T1 ) 50 °C (Figure 2a). And, a nearly
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Figure 2. SEM micrographs of MFI zeolite films on stainless steel substrates synthesized by the microwave heating at t1 ) 30 min, T2 ) 165 °C, t2 ) 35 min, and different microwave-assisted aging temperatures (T1) of (a) 50, (b) 60, (c) 70, (d) 80, (e) 90, and (f) 100 °C.
Figure 4. SEM micrographs of MFI zeolite films on stainless steel substrates synthesized by the microwave heating at T1 ) 80 °C, T2 ) 165 °C, t2 ) 35 min, and different microwave-assisted aging times (t1) of (a) 5, (b) 10, (c) 20, and (d) 40 min. Figure 3. XRD pattern of the MFI zeolite film synthesized by the microwave heating at t0 ) 0 h, T1 ) 80 °C, t1 ) 30 min, T2 ) 165 °C, t2 ) 35 min. (*) Peak from the stainless steel substrate.
continuous film with a few pinholes was observed at T1 ) 60 °C (Figure 2b). Continuous films were synthesized when T1 was elevated to 70, 80, and 90 °C (Figures. 2c-e) (The first layer was continuous and too smooth to be distinguished). Compared with the room temperature aging, the aging time at elevated temperatures was greatly shortened. Clearly, the microwaveassisted aging accelerated the nucleation. Further increase of T1 to 100 °C, films with several pinholes were observed again (Figure 2f). It was clear that T1 played an important role in the nucleation. A volcano profile relationship was found between
the nuclei concentration and T1. And the maximum nuclei concentration could be obtained when T1 was 80 °C. The crystal size changed slightly (1.6-1.8 µm) with microwave-assisted aging temperature. This implies that T1 had little effect on the crystal growth. 3.2.2. Influence of the Microwave-Assisted Aging Time (t1). The experiments were performed at T1 ) 80 °C, T2 ) 165 °C, and t2 ) 35 min. The results are shown in Figure 4. It is noted that only very short time (about 1 min) was needed for the synthesis mixture to reach the microwave-assisted aging temperature, and this time is included in t1. The crystals started to intergrow into each other at t1 ) 5 min (Figure 4a). Increasing t1 to 10 min, the film was nearly continuous (Figure 4b), and at t1 ) 20 min, a continuous film already formed (Figure 4c).
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Figure 5. SEM micrographs of MFI zeolite films on stainless steel substrates synthesized by the microwave heating at T1 ) 80 °C, t1 ) 30 min, t2 ) 35 min, and different crystallization temperatures of (a) 150, (b) 155, (c) 160, and (d) 170 °C. Figure 7. Film continuity-synthesis condition (temperature and time) map by the microwave synthesis method: (O) discontinuous films and (b) continuous films for different crystallization conditions at T1 ) 80 °C, t1 ) 30 min; (4) discontinuous films and (2) continuous films for different aging conditions at T2 ) 165 °C, t2 ) 35 min.
Figure 6. SEM micrographs of MFI zeolite films on stainless steel substrates synthesized by the microwave heating at T1 ) 80 °C, t1 ) 30 min, T2 ) 165 °C, and different crystallization times (t2) of (a) 10, (b) 15, (c) 20, and (d) 25 min.
Compared with the room temperature aging, the microwaveassisted aging accelerated the aging process greatly. More crystals were attracted to form the second layer from the bulk solution over the well-grown first layer when further increasing t1 to 30 and 40 min (Figure 2d and Figure 4d). It was found that the size of the crystals decreased slightly from 1.8 to 1.6 µm with t1 increasing from 5 to 30 min and decreased significantly to 1.4 µm when further increasing t1 to 40 min. This result is reasonable because the number of nuclei increased with t1, and the consumption of the nutrition was accelerated subsequently, leading to a slower growth of crystals. 3.2.3. Influence of the Crystallization Temperature (T2). To investigate the influence of the crystallization temperature on the zeolite film synthesis, the temperature varied in a range of 150-170 °C at T1 ) 80 °C, t1 ) 30 min, and t2 ) 35 min (Figure 5). At the low crystallization temperature of 150 °C, the crystal size was 0.85 µm and a film with several pinholes was synthesized (Figure 5a). As T2 increased to 155 °C, the crystal size increased to 1.1 µm and a continuous film formed (Figure 5b). Continuous films could also be synthesized at higher temperatures (160-170 °C). And the crystal size increased to 1.4-1.9 µm when further elevating T2 to 160-170 °C (Figures 5c and 5d). It was clear that the crystal size increased significantly with T2. 3.2.4. Influence of the Crystallization Time (t2). The influence of the crystallization time (t2) on the zeolite film synthesis was studied (10-35 min), and the results are summarized in Figure 6. It took about 2 min for the synthesis solution to reach 165 °C, and this time is included in t2. Only small individual crystals were observed at t2 ) 10 min (Figure
6a). As t2 increased to 15 min, the crystals grew up to 0.8 µm and started to intergrow into each other (Figure 6b). A continuous film consisting of crystals with ca. 1.1 µm in size formed at t2 ) 20 min (Figure 6c). The growth of the crystals continued with further increasing t2 to 25 and 35 min (Figures 6d and 2d). Note that a continuous film could be synthesized when crystals grew up to 1.1 µm whether by elevating the crystallization temperature or increasing the crystallization time. In other words, a continuous film could be synthesized as soon as the crystals grew up to a certain size at a certain nuclei concentration. On the basis of the above experiments, the following could be inferred. The nuclei concentration was mainly controlled by the temperature and duration of the microwave-assisted aging, which then determined the crystal number attracted onto the substrate surface. And the crystal size was mainly determined by the temperature and duration of crystallization. A certain number of zeolite crystals and a certain crystal size were required in order to manufacture a continuous MFI zeolite film, which could be obtained by controlling the conditions of the microwave-assisted aging and crystallization respectively. 3.2.5. Film Continuity-Synthesis Condition Map. As discussed above, microwave-assisted aging and crystallization conditions significantly affected the continuity of the MFI zeolite film synthesized by the microwave heating method. An extensive range of synthesis conditions were examined and the results of the film continuity as the function of the synthesis temperature and time are shown in Figure 7. The upper part of the map showed the relationship between the film continuity and crystallization conditions. Obviously, a certain crystal size is needed to form a continuous film with a given number of the zeolite crystal produced by aging at T1 ) 80 °C and t1 ) 30 min, and this critical crystal size can be achieved by prolonging the crystallization time or elevating the crystallization temperature, as shown in the upper right part of the map. The relationship between the film continuity and microwave-assisted aging conditions was shown in the lower part of the map with T2 ) 165 °C and t2 ) 35 min. As discussed in section 3.2.1, the maximum nucleus concentration was obtained at an appropriate aging temperature (80 °C). As shown in Figure 7, the shortest aging time (20 min) is needed at the aging temperature of 80 °C. It is well-known that the reaction velocity increases
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with the reaction temperature. Therefore, an adequate concentration could not form at a lower aging temperature than 80 °C when t1 ) 20 min. On the other hand, crystallization became more significant at much higher temperature than 80 °C. The nucleus concentration can be increased by extending the microwave-assisted aging time when the aging temperature (T1) was slightly lower or higher than 80 °C. However, this can not be achieved when the aging temperature is too high or too low. Therefore, if the microwave-assisted aging time was long enough and the temperature was appropriate, a continuous film with a certain crystal size could be synthesized, as shown in the lower right part of the map. We consider the film is continuous if there are no holes found in the SEM image of the surface. According to cyclic voltammograms of Ru(NH3)63+ and/or Co(Phen)32+ in aqueous solutions, Li et al.27 reported that the b-oriented MFI film synthesized by in situ crystallization possessed molecular sieving properties toward mixtures of ion species with different diameters and the film was absent of nonzeolitic defects. Therefore, if we want to know whether the film is continuous (without nonzeolitic pinholes) or not, we can measure cyclic voltammograms (CV) of Co(phen)32+ (diameter ∼ 13.0 Å). If there are no peaks observed in CV response, then we can consider this film to be continuous. From the above results, it is concluded that a continuous b-oriented MFI film can be obtained by the microwave heating at 165 °C for >20 min after the synthesis solution was aged by the microwave heating at 80 °C for 30 min. The aging of the synthesis solution by the conventional heating was also carried out. The synthesis solution aged in a water bath at 80 °C for 30 min was transferred into the autoclave with the substrate inside and crystallized by the microwave heating at 165 °C for 35 min. It is found that a continuous b-oriented MFI zeolite film could also be obtained by this process (data not shown). The crystallization by the convectional oven heating at 165 °C was also carried out after the synthesis solution was aged in a water bath at 80 °C for 30 min. The autoclave was preheated at 80 °C before the aged synthesis solution was transferred into it. The autoclave was kept in the convectional oven for 35 min, 1, and 1.5 h, respectively. There were no film and even no crystals found on the substrate after a 35 min heating. The film was not continuous in the case of 1 h and became continuous in the case of 1.5 h (data not shown). The crystal size is ∼1.0 µm after being heated at 165 °C for 1.5 h, smaller than that by the microwave heating. These results indicate that it still take time to reach the targeted temperature (165 °C) by the conventional heating even after the synthesis solution and the autoclave were preheated at 80 °C, because the heat transfer becomes slower when the temperature difference is smaller. The synthesis time (1.5 h) needed for obtaining a continuous film by the conventional heating is longer than that (35 min) by the microwave heating, indicating the fast heat transfer of the microwave heating. 3.3. Film Formation Mechanism for b-Oriented Films. Considering the homogeneous nucleation mechanism proposed previously,14,26 and based on our results above, a film formation mechanism by the microwave heating is proposed as follows. An invisibly small number of nuclei form in the synthesis solution during 4 h stirring at room temperature. The majority of the nuclei form during aging at elevated temperature. The nucleus concentration mainly depends on the aging conditions (T1, t1). As the synthesis solution is heated to the crystallization temperature by the microwave heating, the nuclei grow up to submicrometer crystals (
