- Email: [email protected]
Preparation of silicalite-1 microtube arrays supported on cordierite honeycomb by using palm fibers as templates
408
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
Preparation of silicalite-1 microtube arrays supported on cordierite honeycomb by using palm fibers as templates Weiwei Liua Lixiong Zhanga* Huanting Wangb and Nanping Xua a
College of Chemistry and Chemical Engineering, Key Laboratory of Materials-oriented Chemical Engineering, MOE, Nanjing University of Technology, Nanjing 210009. Tel: +8625-83587186; Fax: +86-25-83365813; E-mail: [email protected] b
Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia
ABSTRACT Preparation of silicalite-1 hollow fibers and silicalite-1 microtube arrays grown in the cells of the cordierite honeycomb using palm fibers as the templates was presented. XRD, SEM, nitrogen adsorption-desorption and ultrasonic vibration were used to characterize the silicalite-1/palm fiber and silicalite-1 microtube arrays /cordierite honeycomb composites. It was found that a layer of uniform silicalite-1 film was grown on the palm fiber after 18 h of hydrothermal synthesis. When a single palm fiber was immersed in the synthesis solution, single silicalite-1 hollow fiber with an inner diameter of ca. 230 ȝm was formed after syntheses for 2 cycles and remove-off of the template. By inserting a bundle of palm fibers into the cells of the honeycomb, silicalite-1 microtube arrays could be fabricated in the cells of the honeycomb through hydrothermal synthesis and after easily removing the palm fibers. The loadings of silicalite-1 on the honeycomb were as high as 91 and 117 g silicalite-1/ 100 g honeycomb after 2 and 3 cycles of hydrothermal syntheses, respectively. Ultrasonic vibration test results indicated that the composite was mechanically strong enough. 1. INTRODUCTION Zeolites are important microporous materials in both traditional and emerging fields, such as catalysis, adsorption, gas separation, information storage, optical materials, battery and sensors [1]. Zeolite crystals supported on inorganic supports, such as honeycomb ceramic, porous alumina, stainless steel, glass, gold and silicon wafers [2], could not only facilitate the full utilization of zeolites, but also stimulate more applications of zeolites as structured catalysts, membranes, low-k dielectrics films, etc. [3]. Among these supports, honeycomb ceramic is commonly used because zeolites supported on honeycomb ceramic can be used as effective catalysts for selective catalytic reduction of NOx [4]. Attempts had always been made to increase the zeolite loadings on the honeycomb so that more catalytic component could be supported. In situ synthesis of the honeycomb to form the zeolite films on the support was the main method presently used to increase the zeolite loadings. This in turn increased the thickness of the zeolite films, which is not benefit for the mass transfer during catalytic reaction. Recently, Ulla et al. [5] reported that mordenite loadings exceeding 50 g mordenite / 100 g cordierite honeycomb could be achieved using a seeded synthesis method. They found that the mordenite
409 crystals were highly accessible even at this high zeolite loading. However, other synthesis methods still have to be developed to increase the zeolite loadings on honeycomb as well as decrease the thickness of the zeolite films to facilitate the mass transfer. On the other hand, the development of microtechnology in chemical engineering requires fabrication of micro-units, such as micromembranes and microreactors. Zeolites, as important catalysts in industrial processes and promising candidates for inorganic membranes, had been directly grown within the confined space of micro-fabricated channels or fabricated as selfenclosed zeolite based microtunnel and microchannel structures [6]. These structures are very promising in applications in microreactors, microseparators, bio-chemical sensors or other microdevices. This paper reports our discovery that silicalite-1 crystals could be easily grown on a palm fiber by in-situ hydrothermal synthesis. After synthesis, the palm fiber could be easily withdrawn, leaving a silicalite-1 microtube. Based on this discovery, we designed the fabrication of silicalite-1 microtube arrays grown in the cells of the cordierite honeycomb. 2. EXPERIMENTAL SECTION 2.1. Preparation of synthesis solution Silicalite-1 synthesis solution was prepared following the procedure reported in literature [6] with modification. Briefly, after 1.776 g of tetra-propylammonium bromide (TPABr) was dissolved in 70 ml of deionized water, 1.332 g of NaOH was added with continuous stirring for 20 min. Then, 8.332 g of tetraethyl orthosilicate (TEOS) was dropwise added. After vigorous stirring overnight, the solution became clear with the final composition as 2Na2O: 6TEOS: 1TPABr: 600H2O. 2.2. Preparation of a single silicalite-1 microtube and silicalite-1 microtube arrays/cordierite honeycomb composites Palm fibers were obtained from local palm trees and washed with deionized water. Cordierite honeycomb with 72 cells cm-2 was sonicated in water for 10 min, rinsed and dried before use. The length of the palm fiber used to prepare single silicalite-1 microtube was usually 7 cm. It was just kept straight in the synthesis solution. For the preparation of silicalite-1 microtube arrays/cordierite honeycomb composites, palm fiber/cordierite honeycomb composites was first prepared by inserting a bundle of about 25 palm fibers into the one cell of the cordierite honeycomb. The length of the fiber was either longer than or equal to the height of the honeycomb. In-situ hydrothermal synthesis was carried out as follows. First, the silicalite-1 synthesis solution was transferred into the Teflon liner of an autoclave and samples were immersed in it. Then, the autoclave was put into a preheated 180 oC oven for 18 h. After synthesis, the samples were taken out, washed with deionized water, dried and immersed into a new synthesis solution for another cycle of synthesis. Finally, they were calcined in air at 550 °C for 3 h with a heating rate of 2.5 °C min-1. 2.3. Characterization Scanning electron microscope (SEM, Philips Quanta-200) was used to observe the products. X-ray diffractometer (XRD) was conducted on a Bruker D8 Advance with Cu KĮ radiation. Nitrogen adsorption-desorption isotherms were measured on BELSORP II (Ankersmid-Analytical Solutions). The strength of silicalite-1 / cordierite honeycomb composites was tested by in an ultrasonic water bath at 36 °C with a frequency of 50 Hz.
410 3. RESULTS AND DISCUSSION 3.1. Preparation of a single silicalite-1 hollow fiber Growth of silicalite-1 on the palm fiber was carried out at 180 oC for 18 h. We found that a layer of white powders was grown on the palm fiber when it was immersed in the silicalite1 synthesis solution and after synthesis for one cycle. XRD patterns of the palm fiber after the synthesis (not shown) were identical to those of pure silicalite-1 powders reported in literature [8], indicating that pure silicalite-1 crystals were grown on the palm fiber. Fig. 1a shows SEM photograph of the palm fiber, which was 220 ȝm in diameter. Figs. 1b and 1c show SEM pictures of the palm fiber after one and two cycles of synthesis. It could be seen a layer of silicalite-1 film was formed on the palm fiber, with the fiber shrunk to ca. 54 ȝm (b) and ca. 50 ȝm (c) in diameter after the syntheses for one and two cycles, respectively. Simultaneously, the thickness of silicalite-1 film increased from 30 ȝm to 40 ȝm, after one more synthesis. Therefore, the palm fiber could be easily withdrawn out, leaving a pure silicalite-1 microtube with ca. 300 ȝm of outer diameter and ca. 230 ȝm of inner diameter (Fig. 1d). When a few palm fibers were immersed in the synthesis solution, the resulting silicalite-1/palm fibers were turned to grown together (see Fig. 1e). We believed that if the palm fibers were tightly bound, the monolith silicalite-1 would be prepared. A close observation revealed that the microtube was composed of intergrown silicalite-1 crystals with a size of ca. 3 ȝm after two cycles of synthesis (see Fig. 1f).
Fig. 1. SEM images of the palm fiber (a), crosssection of silicalite-1/palm fiber composite after one (b) and two cycles (c) of syntheses, the silicalite-1 microtube (d), a few of silicalite-1 microtubes (e) and the surface of the silicalite-1 microtube with high magnification (f).
Fig. 2. SEM images of longitude (a) and crosssectional (b) view of the silicalite-1 honeycomb, cordierite honeycomb with silicalite-1 microtube arrays in its cells after one (c), two (d) and three (e) cycles syntheses, and silicalite-1 microtubes grown out of honeycomb (f).
411 3.2. Preparation of silicalite-1 honeycomb/cordierite honeycomb When a bundle of about 15 palm fibers was wrapped together by a Teflon tape and then immersed in the synthesis solution, silicalite-1 honeycomb with strong mechanic strength was prepared after syntheses for 3 cycles (Figs. 2a and 2b). It could keep integrity after free fall from a height of two meters. The inner diameter of the silicalite-1 honeycomb was ca. 180 ȝm and a wall thickness was ca. 30 ȝm. This result stimulated our idea that when a bundle of palm fibers was inserted into the cells of a cordierite honeycomb, supported silicalite-1 honeycomb could be produced after the hydrothermal synthesis. The fulfillment of this idea was shown in Figs. 2c~2e, from which we could apparently see that a number of silicalite-1 microtubes were arrayed in the cells of a cordierite honeycomb. The microtubes became thicker (from 16 ȝm, 26 ȝm to 30 ȝm) with the increase of the synthesis cycles, from one, two and to three (see Figs. 2c, 2d, 2e, respectively). Consequently, silicalite-1 loadings on the cordierite honeycomb also increased, as indicated in Table 1, where we also compared the silicalite-1 loadings on bare cordierite honeycomb after different cycles of hydrothermal treatments. The table shows that the weight gain was limited when bare cordierite honeycomb was used. The loadings of silicalite-1 were just about 40 g silicalite-1 /100 g cordierite honeycomb even after three cycles of hydrothermal syntheses. But the loadings of silicalite-1 on silicalite-1 microtube arrays/cordierite honeycomb were 52 g silicalite-1 /100 g cordierite honeycomb after one cycle of synthesis. Two and three cycles of syntheses could lead to a dramatic increase of the zeolite loadings to 91 g and 117 g silicalite-1 /100 g cordierite honeycomb, respectively. Fig. 3 shows XRD patterns of silicalite-1 grown on bare honeycomb and silicalite-1 microtube arrays on honeycomb. We could apparently see that the diffraction peaks of silicalite-1 on the silicalite-1 microtube arrays /cordierite honeycomb (Fig. 3b) were much stronger than those supported on bare cordierite (Fig. 3a). This result is the same as that listed in Table 1. When the length of the palm fibers was longer than that of the cordierite honeycomb, silicalite-1 microtubes could be grown out of the cordierite honeycomb, as shown in Fig. 2f. This can facilitate the sealing of the microtubes when they are used as micromembranes or microreactors. Table 1 Silicalite-1 loadings on cordierite honeycomb after different cycles of syntheses. Synthesis Silicalite-1 loadings (g silicalite-1/ g honeycomb) cycles Bare honeycomb Honeycomb inserted with palm fibers 1 2 3
34 38 40
52 91 117
Fig. 4 shows the nitrogen adsorption-desorption isotherms and the BJH pore size distribution of the silicalite-1 microtube arrays /cordierite honeycomb after 3 cycles of syntheses and calcined at 550 °C. The isotherms show a type IV isotherm, indicating that the composite contained abundant mesopores. The pore size distribution shows that the average pore diameter was ca. 2.2 nm and total pore volume was ca. 0.08 cm3 g-1. The specific surface area of the composite was 140.7 m2 g-1. By taking the specific surface area of the cordierite honeycomb as 1 m2 g-1 and that of the silicalite-1 powders as 350 m2 g-1 [9], we can calculate that the loading of silicalite-1 on the honeycomb is about 128 g silicalite-1/100 g cordierite honeycomb, when its surface area is 140.7 m2 g-1. Apparently, the calculated silicalite-1 loading is very close to the measured weight gain listed in Table 1.
412
Fig. 3. XRD patterns of the silicalite-1 supported on bare cordierite honeycomb (a) and silicalite-1 microtube arrays / cordierite honeycomb (b), after 3 cycles of syntheses and calcinations.
Fig. 4. N2 silicalite-1 honeycomb calcinations distribution
adsorption-desorption isotherms of microtube arrays / cordierite after three cycles of syntheses and at 550 oC and the BJH pore size (inserted).
Weight loss/%
100 90 80 70 60 50 40
0
100 200 300 400 500 600
Ultrasonic time/min Fig. 5. Weight loss of the silicalite-1 microtube arrays/cordierite honeycomb during the treatment of an ultrasonic vibration with a frequency of 50 Hz at 36 °C.
Fig. 6. SEM images of the silicalite-1 microtube arrays /cordierite honeycomb after ultrasonic vibrations with a frequency of 50 Hz for 180 min (a) and 420 min (b), respectively, at 36 °C.
In order to test the strength of the silicalite-1 microtube arrays supported on the cordierite, ultrasonic vibration with a frequency of 50 Hz was used and the weight loss was recorded with the time. Fig. 5 shows the weight loss of the silicalite-1 microtube arrays /cordierite honeycomb, which was prepared by hydrothermal syntheses for 3 cycles and after remove-off of palm fibers, with the increase of ultrasonic time. It can be seen that the composite had almost no weight loss after being treated 400 min, indicating that the strength of the composite was strong. After an ultrasonic vibration time of over 400 min, a sharp weight loss can be observed, referring that some of silicalite-1 crystals were lost from the honeycomb. Fig. 6 shows SEM pictures of the silicalite-1 microtube arrays/cordierite honeycomb after ultrasonic vibration for 180 min (Fig. 6a) and 420 min (Fig. 6b), respectively. We can see that some of silicalite-1 crystals lost at the opening of the microtubes, but the whole structure was still kept intact. 3.4. Discussion Silicalite-1 hollow fibers and silicalite-1 microtube arrays/cordierite honeycomb were prepared by using the palm fibers as the templates. The usage of the palm fibers has several advantages over the usage of carbon fibers [10], which were commonly used in the literature to prepare zeolite fibers or hollow structures. First, the palm fibers are much cheaper than carbon fibers. Second, it is very flexible and easy to be shaped. Third, no pretreatment is
413 needed. For carbon fibers, they had to be surface modified before use, so that the zeolite crystals can be grown on them. Fourth, not only silicalite-1 hollow fibers, but also other silicalite-1 hollow structures can be prepared by controlling the shape of the palm fiber because of its flexibility. Furthermore, the palm fiber could be removed off easily by mechanical withdraw other than calcination at high temperature. Recently, Ulla et al. [5] made mordenite coated on ceramic monolith at loadings exceeding 50 % by weight. By the method developed in this paper, liner silicalite-1 microtube arrays could be prepared and the silicalite-1 loadings on ceramic monolith increased to 117 g silicalite-1/100 g honeycomb ceramic after three cycles of hydrothermal syntheses. 4. CONCLUSION Palm fibers were successfully used as the templates for preparation of single silicalite-1 hollow fiber without any pretreatment. By inserting a bundle of palm fibers into the cells of the honeycomb, silicalite-1 microtube arrays with strong mechanical strength could be fabricated in the cells of the honeycomb. The loading of silicalite-1 on the honeycomb was 91 and 117 g silicalite-1/ 100 g honeycomb, respectively, after 2 or 3 cycles of syntheses. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NNSFC; No. 20141003 and No. 20201007) and Key Laboratory of Materials-Oriented Chemical Engineering of Jiangsu Province. H. W. thanks the Australian Research Council for the QEII Fellowship through the Discovery Project DP0559724. REFERENCES [1]
a) K. Okada, K. Kuboyama, T. Takei, et al., Microporous Mesoporous Mater., 37 (2000) 99; b) S. Mintova, V. Valtchev and B. J. Schoeman, J. Mater. Sci. Lett., 15 (1996) 840. [2] a) M. A. Ulla, R. Mallada, J. Coronas, et al. Appl. Catal. A: Genernal, 253 (2003) 257; b) Y. Yan, M. Tsapatsis, M. E. Davis, et al. Chem. Commun., (1995) 227; c) G. Clet, J. C. Jansen and H. Bekkum, Chem. Mater., 11 (1999) 1696; d) W. Y. Dong and Y. C. Long. Chem. Commun., (2000) 1067; e) S. Mintova, J. Hedlund, B. Schoeman, et al. Chem. Commun., (1997) 15; f) Z. B. Wang, H. T. Wang, A. Mitra, et al. Adv. Mater., 13 (2001) 746. [3] a) J. C. Jansen, J. H. Koegler, H. van Bekkum, et al. Microporous Mesoporous Mater., 21 (1998) 213; b) O. Larlus, S. Mintova, V. Valtchev, et al. Appl. Surf. Sci., 226 (2004) 155. [4] a) M. J. Heimrich, L. R. Smith and J. Kitowski. SAE 920847, (1992) 954; b) W. Held, A. Konig, T. Richter and L. Puppe. SAE 900496, (1990) 209. [5] M. A. Ulla, E. Miro, R. Mallada, et al, Chem. Commun., (2004) 528. [6] J. L. Chau and K. L. Yeung. Chem. Commun., (2002) 960. [7] Y. S. Yan, M. E. Davis and G. R. Gavalas. Ind. Eng. Chem. Res., 34 (1995) 1652. [8] J. L. Guth, H. Kessler and R. Wey. Stud. Surf. Sci. Catal., 28 (1986) 121. [9] J. M. Zamaro, M. A. Ulla and E. E. Mir. Chem. Eng. J., 16 (2005) 25. [10] J. W. Teng, G. L. Zhao, Z. K. Xie, et al. Chin. J. Catal., 25 (2004) 602. [11] a) V. Valtchev, B. J. Schoeman, J. Hedlund, et al. Zeolites, 17 (1996) 408; b) M. J. Garcia, A. D. Cazorla, S. A. Linares, et al. Microporous Mesoporous Mater., 42 (2001) 255; c) C. Ke, W. L. Yang, Z. Ni, et al. Chem. Commun., (2001) 783.
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
Preparation of silicalite-1 microtube arrays supported on cordierite honeycomb by using palm fibers as templates Weiwei Liua Lixiong Zhanga* Huanting Wangb and Nanping Xua a
College of Chemistry and Chemical Engineering, Key Laboratory of Materials-oriented Chemical Engineering, MOE, Nanjing University of Technology, Nanjing 210009. Tel: +8625-83587186; Fax: +86-25-83365813; E-mail: [email protected] b
Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia
ABSTRACT Preparation of silicalite-1 hollow fibers and silicalite-1 microtube arrays grown in the cells of the cordierite honeycomb using palm fibers as the templates was presented. XRD, SEM, nitrogen adsorption-desorption and ultrasonic vibration were used to characterize the silicalite-1/palm fiber and silicalite-1 microtube arrays /cordierite honeycomb composites. It was found that a layer of uniform silicalite-1 film was grown on the palm fiber after 18 h of hydrothermal synthesis. When a single palm fiber was immersed in the synthesis solution, single silicalite-1 hollow fiber with an inner diameter of ca. 230 ȝm was formed after syntheses for 2 cycles and remove-off of the template. By inserting a bundle of palm fibers into the cells of the honeycomb, silicalite-1 microtube arrays could be fabricated in the cells of the honeycomb through hydrothermal synthesis and after easily removing the palm fibers. The loadings of silicalite-1 on the honeycomb were as high as 91 and 117 g silicalite-1/ 100 g honeycomb after 2 and 3 cycles of hydrothermal syntheses, respectively. Ultrasonic vibration test results indicated that the composite was mechanically strong enough. 1. INTRODUCTION Zeolites are important microporous materials in both traditional and emerging fields, such as catalysis, adsorption, gas separation, information storage, optical materials, battery and sensors [1]. Zeolite crystals supported on inorganic supports, such as honeycomb ceramic, porous alumina, stainless steel, glass, gold and silicon wafers [2], could not only facilitate the full utilization of zeolites, but also stimulate more applications of zeolites as structured catalysts, membranes, low-k dielectrics films, etc. [3]. Among these supports, honeycomb ceramic is commonly used because zeolites supported on honeycomb ceramic can be used as effective catalysts for selective catalytic reduction of NOx [4]. Attempts had always been made to increase the zeolite loadings on the honeycomb so that more catalytic component could be supported. In situ synthesis of the honeycomb to form the zeolite films on the support was the main method presently used to increase the zeolite loadings. This in turn increased the thickness of the zeolite films, which is not benefit for the mass transfer during catalytic reaction. Recently, Ulla et al. [5] reported that mordenite loadings exceeding 50 g mordenite / 100 g cordierite honeycomb could be achieved using a seeded synthesis method. They found that the mordenite
409 crystals were highly accessible even at this high zeolite loading. However, other synthesis methods still have to be developed to increase the zeolite loadings on honeycomb as well as decrease the thickness of the zeolite films to facilitate the mass transfer. On the other hand, the development of microtechnology in chemical engineering requires fabrication of micro-units, such as micromembranes and microreactors. Zeolites, as important catalysts in industrial processes and promising candidates for inorganic membranes, had been directly grown within the confined space of micro-fabricated channels or fabricated as selfenclosed zeolite based microtunnel and microchannel structures [6]. These structures are very promising in applications in microreactors, microseparators, bio-chemical sensors or other microdevices. This paper reports our discovery that silicalite-1 crystals could be easily grown on a palm fiber by in-situ hydrothermal synthesis. After synthesis, the palm fiber could be easily withdrawn, leaving a silicalite-1 microtube. Based on this discovery, we designed the fabrication of silicalite-1 microtube arrays grown in the cells of the cordierite honeycomb. 2. EXPERIMENTAL SECTION 2.1. Preparation of synthesis solution Silicalite-1 synthesis solution was prepared following the procedure reported in literature [6] with modification. Briefly, after 1.776 g of tetra-propylammonium bromide (TPABr) was dissolved in 70 ml of deionized water, 1.332 g of NaOH was added with continuous stirring for 20 min. Then, 8.332 g of tetraethyl orthosilicate (TEOS) was dropwise added. After vigorous stirring overnight, the solution became clear with the final composition as 2Na2O: 6TEOS: 1TPABr: 600H2O. 2.2. Preparation of a single silicalite-1 microtube and silicalite-1 microtube arrays/cordierite honeycomb composites Palm fibers were obtained from local palm trees and washed with deionized water. Cordierite honeycomb with 72 cells cm-2 was sonicated in water for 10 min, rinsed and dried before use. The length of the palm fiber used to prepare single silicalite-1 microtube was usually 7 cm. It was just kept straight in the synthesis solution. For the preparation of silicalite-1 microtube arrays/cordierite honeycomb composites, palm fiber/cordierite honeycomb composites was first prepared by inserting a bundle of about 25 palm fibers into the one cell of the cordierite honeycomb. The length of the fiber was either longer than or equal to the height of the honeycomb. In-situ hydrothermal synthesis was carried out as follows. First, the silicalite-1 synthesis solution was transferred into the Teflon liner of an autoclave and samples were immersed in it. Then, the autoclave was put into a preheated 180 oC oven for 18 h. After synthesis, the samples were taken out, washed with deionized water, dried and immersed into a new synthesis solution for another cycle of synthesis. Finally, they were calcined in air at 550 °C for 3 h with a heating rate of 2.5 °C min-1. 2.3. Characterization Scanning electron microscope (SEM, Philips Quanta-200) was used to observe the products. X-ray diffractometer (XRD) was conducted on a Bruker D8 Advance with Cu KĮ radiation. Nitrogen adsorption-desorption isotherms were measured on BELSORP II (Ankersmid-Analytical Solutions). The strength of silicalite-1 / cordierite honeycomb composites was tested by in an ultrasonic water bath at 36 °C with a frequency of 50 Hz.
410 3. RESULTS AND DISCUSSION 3.1. Preparation of a single silicalite-1 hollow fiber Growth of silicalite-1 on the palm fiber was carried out at 180 oC for 18 h. We found that a layer of white powders was grown on the palm fiber when it was immersed in the silicalite1 synthesis solution and after synthesis for one cycle. XRD patterns of the palm fiber after the synthesis (not shown) were identical to those of pure silicalite-1 powders reported in literature [8], indicating that pure silicalite-1 crystals were grown on the palm fiber. Fig. 1a shows SEM photograph of the palm fiber, which was 220 ȝm in diameter. Figs. 1b and 1c show SEM pictures of the palm fiber after one and two cycles of synthesis. It could be seen a layer of silicalite-1 film was formed on the palm fiber, with the fiber shrunk to ca. 54 ȝm (b) and ca. 50 ȝm (c) in diameter after the syntheses for one and two cycles, respectively. Simultaneously, the thickness of silicalite-1 film increased from 30 ȝm to 40 ȝm, after one more synthesis. Therefore, the palm fiber could be easily withdrawn out, leaving a pure silicalite-1 microtube with ca. 300 ȝm of outer diameter and ca. 230 ȝm of inner diameter (Fig. 1d). When a few palm fibers were immersed in the synthesis solution, the resulting silicalite-1/palm fibers were turned to grown together (see Fig. 1e). We believed that if the palm fibers were tightly bound, the monolith silicalite-1 would be prepared. A close observation revealed that the microtube was composed of intergrown silicalite-1 crystals with a size of ca. 3 ȝm after two cycles of synthesis (see Fig. 1f).
Fig. 1. SEM images of the palm fiber (a), crosssection of silicalite-1/palm fiber composite after one (b) and two cycles (c) of syntheses, the silicalite-1 microtube (d), a few of silicalite-1 microtubes (e) and the surface of the silicalite-1 microtube with high magnification (f).
Fig. 2. SEM images of longitude (a) and crosssectional (b) view of the silicalite-1 honeycomb, cordierite honeycomb with silicalite-1 microtube arrays in its cells after one (c), two (d) and three (e) cycles syntheses, and silicalite-1 microtubes grown out of honeycomb (f).
411 3.2. Preparation of silicalite-1 honeycomb/cordierite honeycomb When a bundle of about 15 palm fibers was wrapped together by a Teflon tape and then immersed in the synthesis solution, silicalite-1 honeycomb with strong mechanic strength was prepared after syntheses for 3 cycles (Figs. 2a and 2b). It could keep integrity after free fall from a height of two meters. The inner diameter of the silicalite-1 honeycomb was ca. 180 ȝm and a wall thickness was ca. 30 ȝm. This result stimulated our idea that when a bundle of palm fibers was inserted into the cells of a cordierite honeycomb, supported silicalite-1 honeycomb could be produced after the hydrothermal synthesis. The fulfillment of this idea was shown in Figs. 2c~2e, from which we could apparently see that a number of silicalite-1 microtubes were arrayed in the cells of a cordierite honeycomb. The microtubes became thicker (from 16 ȝm, 26 ȝm to 30 ȝm) with the increase of the synthesis cycles, from one, two and to three (see Figs. 2c, 2d, 2e, respectively). Consequently, silicalite-1 loadings on the cordierite honeycomb also increased, as indicated in Table 1, where we also compared the silicalite-1 loadings on bare cordierite honeycomb after different cycles of hydrothermal treatments. The table shows that the weight gain was limited when bare cordierite honeycomb was used. The loadings of silicalite-1 were just about 40 g silicalite-1 /100 g cordierite honeycomb even after three cycles of hydrothermal syntheses. But the loadings of silicalite-1 on silicalite-1 microtube arrays/cordierite honeycomb were 52 g silicalite-1 /100 g cordierite honeycomb after one cycle of synthesis. Two and three cycles of syntheses could lead to a dramatic increase of the zeolite loadings to 91 g and 117 g silicalite-1 /100 g cordierite honeycomb, respectively. Fig. 3 shows XRD patterns of silicalite-1 grown on bare honeycomb and silicalite-1 microtube arrays on honeycomb. We could apparently see that the diffraction peaks of silicalite-1 on the silicalite-1 microtube arrays /cordierite honeycomb (Fig. 3b) were much stronger than those supported on bare cordierite (Fig. 3a). This result is the same as that listed in Table 1. When the length of the palm fibers was longer than that of the cordierite honeycomb, silicalite-1 microtubes could be grown out of the cordierite honeycomb, as shown in Fig. 2f. This can facilitate the sealing of the microtubes when they are used as micromembranes or microreactors. Table 1 Silicalite-1 loadings on cordierite honeycomb after different cycles of syntheses. Synthesis Silicalite-1 loadings (g silicalite-1/ g honeycomb) cycles Bare honeycomb Honeycomb inserted with palm fibers 1 2 3
34 38 40
52 91 117
Fig. 4 shows the nitrogen adsorption-desorption isotherms and the BJH pore size distribution of the silicalite-1 microtube arrays /cordierite honeycomb after 3 cycles of syntheses and calcined at 550 °C. The isotherms show a type IV isotherm, indicating that the composite contained abundant mesopores. The pore size distribution shows that the average pore diameter was ca. 2.2 nm and total pore volume was ca. 0.08 cm3 g-1. The specific surface area of the composite was 140.7 m2 g-1. By taking the specific surface area of the cordierite honeycomb as 1 m2 g-1 and that of the silicalite-1 powders as 350 m2 g-1 [9], we can calculate that the loading of silicalite-1 on the honeycomb is about 128 g silicalite-1/100 g cordierite honeycomb, when its surface area is 140.7 m2 g-1. Apparently, the calculated silicalite-1 loading is very close to the measured weight gain listed in Table 1.
412
Fig. 3. XRD patterns of the silicalite-1 supported on bare cordierite honeycomb (a) and silicalite-1 microtube arrays / cordierite honeycomb (b), after 3 cycles of syntheses and calcinations.
Fig. 4. N2 silicalite-1 honeycomb calcinations distribution
adsorption-desorption isotherms of microtube arrays / cordierite after three cycles of syntheses and at 550 oC and the BJH pore size (inserted).
Weight loss/%
100 90 80 70 60 50 40
0
100 200 300 400 500 600
Ultrasonic time/min Fig. 5. Weight loss of the silicalite-1 microtube arrays/cordierite honeycomb during the treatment of an ultrasonic vibration with a frequency of 50 Hz at 36 °C.
Fig. 6. SEM images of the silicalite-1 microtube arrays /cordierite honeycomb after ultrasonic vibrations with a frequency of 50 Hz for 180 min (a) and 420 min (b), respectively, at 36 °C.
In order to test the strength of the silicalite-1 microtube arrays supported on the cordierite, ultrasonic vibration with a frequency of 50 Hz was used and the weight loss was recorded with the time. Fig. 5 shows the weight loss of the silicalite-1 microtube arrays /cordierite honeycomb, which was prepared by hydrothermal syntheses for 3 cycles and after remove-off of palm fibers, with the increase of ultrasonic time. It can be seen that the composite had almost no weight loss after being treated 400 min, indicating that the strength of the composite was strong. After an ultrasonic vibration time of over 400 min, a sharp weight loss can be observed, referring that some of silicalite-1 crystals were lost from the honeycomb. Fig. 6 shows SEM pictures of the silicalite-1 microtube arrays/cordierite honeycomb after ultrasonic vibration for 180 min (Fig. 6a) and 420 min (Fig. 6b), respectively. We can see that some of silicalite-1 crystals lost at the opening of the microtubes, but the whole structure was still kept intact. 3.4. Discussion Silicalite-1 hollow fibers and silicalite-1 microtube arrays/cordierite honeycomb were prepared by using the palm fibers as the templates. The usage of the palm fibers has several advantages over the usage of carbon fibers [10], which were commonly used in the literature to prepare zeolite fibers or hollow structures. First, the palm fibers are much cheaper than carbon fibers. Second, it is very flexible and easy to be shaped. Third, no pretreatment is
413 needed. For carbon fibers, they had to be surface modified before use, so that the zeolite crystals can be grown on them. Fourth, not only silicalite-1 hollow fibers, but also other silicalite-1 hollow structures can be prepared by controlling the shape of the palm fiber because of its flexibility. Furthermore, the palm fiber could be removed off easily by mechanical withdraw other than calcination at high temperature. Recently, Ulla et al. [5] made mordenite coated on ceramic monolith at loadings exceeding 50 % by weight. By the method developed in this paper, liner silicalite-1 microtube arrays could be prepared and the silicalite-1 loadings on ceramic monolith increased to 117 g silicalite-1/100 g honeycomb ceramic after three cycles of hydrothermal syntheses. 4. CONCLUSION Palm fibers were successfully used as the templates for preparation of single silicalite-1 hollow fiber without any pretreatment. By inserting a bundle of palm fibers into the cells of the honeycomb, silicalite-1 microtube arrays with strong mechanical strength could be fabricated in the cells of the honeycomb. The loading of silicalite-1 on the honeycomb was 91 and 117 g silicalite-1/ 100 g honeycomb, respectively, after 2 or 3 cycles of syntheses. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NNSFC; No. 20141003 and No. 20201007) and Key Laboratory of Materials-Oriented Chemical Engineering of Jiangsu Province. H. W. thanks the Australian Research Council for the QEII Fellowship through the Discovery Project DP0559724. REFERENCES [1]
a) K. Okada, K. Kuboyama, T. Takei, et al., Microporous Mesoporous Mater., 37 (2000) 99; b) S. Mintova, V. Valtchev and B. J. Schoeman, J. Mater. Sci. Lett., 15 (1996) 840. [2] a) M. A. Ulla, R. Mallada, J. Coronas, et al. Appl. Catal. A: Genernal, 253 (2003) 257; b) Y. Yan, M. Tsapatsis, M. E. Davis, et al. Chem. Commun., (1995) 227; c) G. Clet, J. C. Jansen and H. Bekkum, Chem. Mater., 11 (1999) 1696; d) W. Y. Dong and Y. C. Long. Chem. Commun., (2000) 1067; e) S. Mintova, J. Hedlund, B. Schoeman, et al. Chem. Commun., (1997) 15; f) Z. B. Wang, H. T. Wang, A. Mitra, et al. Adv. Mater., 13 (2001) 746. [3] a) J. C. Jansen, J. H. Koegler, H. van Bekkum, et al. Microporous Mesoporous Mater., 21 (1998) 213; b) O. Larlus, S. Mintova, V. Valtchev, et al. Appl. Surf. Sci., 226 (2004) 155. [4] a) M. J. Heimrich, L. R. Smith and J. Kitowski. SAE 920847, (1992) 954; b) W. Held, A. Konig, T. Richter and L. Puppe. SAE 900496, (1990) 209. [5] M. A. Ulla, E. Miro, R. Mallada, et al, Chem. Commun., (2004) 528. [6] J. L. Chau and K. L. Yeung. Chem. Commun., (2002) 960. [7] Y. S. Yan, M. E. Davis and G. R. Gavalas. Ind. Eng. Chem. Res., 34 (1995) 1652. [8] J. L. Guth, H. Kessler and R. Wey. Stud. Surf. Sci. Catal., 28 (1986) 121. [9] J. M. Zamaro, M. A. Ulla and E. E. Mir. Chem. Eng. J., 16 (2005) 25. [10] J. W. Teng, G. L. Zhao, Z. K. Xie, et al. Chin. J. Catal., 25 (2004) 602. [11] a) V. Valtchev, B. J. Schoeman, J. Hedlund, et al. Zeolites, 17 (1996) 408; b) M. J. Garcia, A. D. Cazorla, S. A. Linares, et al. Microporous Mesoporous Mater., 42 (2001) 255; c) C. Ke, W. L. Yang, Z. Ni, et al. Chem. Commun., (2001) 783.