- Email: [email protected]
Growth of oriented zeolite crystal membranes
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
2233
G r o w t h o f oriented zeolite crystal m e m b r a n e s Mojie, Cheng; Liwu, Lin*; Weishen, Yang; Yashu, Yang; Yide, Xu and Xinsheng, Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics Chinese Academy of Sciences, Dalian 116023, P.R.China SUMMARY
Silicalite-1 crystals were grown with vertical orientation on porous alumina supports and with lateral orientation on dense fiat glass slabs. The crystallization of the oriented silicalite-1 crystals on porous alumina supports were investigated, and it was found that the growth mechanism lies in both nucleation and crystal growth of crystallization. A model of pentasil layer structures preferred orientation was proposed for the explanation of crystals on glass slabs. 1. INTRODUCTION Zeolites have attracted increasing attentions for the developing of molecular sieving membranes and host frameworks to orient guest molecules[I-6]. The in situ hydrothermal synthesis is a novel process for the fabrication of zeolite membranes. The aligned crystal orientation will improve membrane separation efficiency and macro chemiphysical properties of guest molecules. By the control of synthesis conditions, zeolite membranes with the oriented crystal orientation can be achieved. J. G. Tsikoyianni et al.[7] first found the preferred orientation of zeolite crystals on Teflon slabs from the X-ray diffraction patterns of silicalite-1 films. J. C. Jansen et a1.[8,9] reported that silicalite-1 crystals can either be vertically grown on metal supports or laterally grown on silicon wafers, but only random oriented silicalite-1 membranes were obtained on porous supports[10]. S. Feng and T. Bein[11,12] introduced the idea of supra molecule structure preorganization and templating at the organic and inorganic interface to the art of zeolite synthesis and achieved membrane with vertical growth of AIPO-5 crystals on zirconium organic phosphate modified gold wafers. However, there is no report on the preparation of the oriented crystal zeolite membrane on porous supports for separation and on transparent support for the design of host-guest materials. In this paper, we report the synthesis of the vertical growth of silicalite-1 crystals on porous alpha alumina supports and the lateral growth of silicalite-1 crystals on dense fiat glass slabs and discuss their growth mechanism. 2. EXPERIMENTAL Alpha alumina discs, with 0.1-0.3 ~tm pore radius, were used as porous supports. The mixtures for the synthesis of silicalite-1 were prepared by mixing silica sol, sodium hydroxide solution, tetrapropylammonium bromide solution and water, and aged for a given time at room
2234 temperature. The molar con,position of the synthesis mixture was 3Na20 100SiO 2 10000H20 4TPABr. The alumina discs or glass slabs were hold by Teflon supports and vertically immersed in the mixture in stainless steel autoclves. The crystallization was carried out at 180~ in a convection oven. After the synthesis, as-synthesized membranes were recovered, ultrasonic cleaned in distilled water and dried. The As-synthesized silicalite-1 membranes were calcined at 500~ with a careful control of heating procedure. X-ray powder diffraction(XRD) patterns were taken on Ragaku D max/rb. Scanning electron microscopy (SEM) photograph were taken on JEM-1200. 3. RESULTS AND DISCUSSION 3.1. Identification of the vertical growth of silicalite-1 crystals A typical characteristic of the oriented growth of silicalite-1 membranes is their different peaks' intensities compared with powdery sample. Fig.1 shows the XRD patterns of assynthesized silicalite-1 powdery sample, the as-synthesized silicalite-1 membrane and the calcined silicalite-1 membrane. It is found that the vertical growth of silicalite-1 membrane has the same crystal structure as that of powdery sample. The intensities of 002, 102, 103, 104 diffraction peaks of silicalite-1 membrane increased greatly, but the intensities of 200, 501 and 051 peaks of silicalite-1 membrane decreased greatly, which implies that the c-directions of silicalite-1 crystals are vertical to the support surface. The vertical growth of silicalite-1 crystals is also confirmed by SEM photograph as shown in Fig 2. Both the acute edges of prism silicalite1 crystals from top view and the vertical figures of prism crystals from cross-section view are powerful evidences for the vertical growth of silicalite-1 crystals. 12K 002 103
104
103 0 ~
104
101
A 002 101 A
~2
2 J 501 051 15~I
i
1
It 303
101 200
11
.--~-_ I 20
40
0 10 01100 0
A-_
50
2O
Fig.1. X-ray diffraction patterns of (1) silicalite-1 powder sample, (2) as-synthesized and (3) calcined vertical oriented silicalite-1 crystal membrane on porous ct-A1203 support.
2235
Fig.2. Scanning electron microscopy photograph of vertical growth silicalite-1 crystal membrane on porous ct-A1203 support. (a) top view; (b) cross-section view.
12K ~
.
9
.
.
.
6
.
20
40
50
2(1 Fig.3. X-ray diffraction patterns of vertical growth silicalite-1 membranes crystallized at 180~ with different crystallization time(I). 1, 2hours; 2, 4hours; 3, 8hours; 4, 12hours; 5, 24 hours and 6, 48 hours.
2236
3.2. Vertical growth of silicalite-1 membranes on porous alumina supports The formation of silicalite-1 membranes prefer a low alkalinity and high water content synthesis mixture[13], so the oriented growth silicalite-1 membranes can only be obtained at these particular conditions. It has been found that silicalite-1 crystals can be vertically grown at an alkalinity range as wide as that for the synthesis of random oriented silicalite-1 crystal membranes, and the best range is with the Na20/SiO2 ratio from 0.02 to 0.04[ 14]. The vertical growth process of silicalite-1 crystals was investigated to elucidate the crystallization mechanism. The effects of crystallization time on the vertical growth is shown in Fig.3 and Fig.4. It was found that silicalite-1 membrane can either vertically growth at early stage of crystallization as shown in Fig.3, or turn to the vertical growth from a radom oriented silicalite-1 membrane at the late stage of crystallization as shown in Fig. 4. The SEM photograph in Fig.5 also shows that silicalite-1 crystals can be vertically grown on porous alumia support at the early stage of crystallization. 12K
5 4
ix
........
3
k,-~;
A , 5
I 20
It
~
i
I 40
A
2
~
1 50
20 Fig.4. X-ray diffraction patterns of vertical growth silicalite-1 membranes crystallized at 180~ with different crystallization time(II). 1, 2hours; 2, 4hours; 3, 8hours; 4, 12hours and 5, 48 hours.
3.3 Lateral growth of silicalite-1 crystals on dense fiat glass slabs The lateral growth of silicalite-1 crystals on porous supports may result in a high efficient kinds of membrane for separation. Unfortunately, this kind of silicalite-1 membrane haven't been obtained at present. However, it was found that silicalite-1 crystals can be laterally grown on dense fiat glass slabs at about the same synthesis conditions as that of the vertial growth on porous alumina supports, which may give us some indications on the design of lateral growth of silicalite-1 on porous supports. The composition of the synthesis mixture was 4Na20 100SiO 2 10000H20 4TPABr, and the crystallization was carried out at 180~ for 24 hours. Fig.6 shows the XRD patterns of as-synthesized and calcined lateral growth silicalite-1 crystals. It can seen that only 200, 020, 400, 040, 600, 060, 800, 080, 10 0 0 and 0 10 0 diffraction peaks are present
2237 with high intensity, which depicts that the silicalite-1 crystal were highly oriented with their cdirections parallel to the surface of the glass slab. The high intensities of these diffraction peaks implies that the large crystals and the intergrowth of silicalite-1 crystals have constructed a large scale of crystal periodicity. The SEM photograph, as shown in Fig.7, depicts that the silicalite-1 membrane on the dense glass slabs consisted of large shelf-like silicalite-1 crystals intergrown with each other. Silicalite-1 crystals of about 70 ~tm in length, 10 ~tm in height and width are laterally grown on the surface of glass slab, which results in a 10 ~tm thickness of zeolite membrane.
Fig.5. Scanning electron microscopy photograph of vertical growth of silicalite- 1 crystals at the early stage of crystallization. 100K
200 020
040 4001
L
5
l
1
0 10 0
I
_
600'~60
800' 080
,
,,
20
100
I
40
01
I,
2
J
50
20 Fig.6. X-ray diffraction patterns of lateral growth of silicalite-1 membrane on a dense fiat glass slab. 1, as-synthesized and 2, calcined.
2238
Fig.7. Scanning electron microscopy photograph of lateral growth of silicalite-1 crystal membrane on a dense fiat glass slab. (a) top view; (b) cross-section view. 3.4. Growth mechanism of the oriented silicalite-1 crystals The oriented growth of silicalite-1 crystals lies in both nucleation and crystal growth of crystallization. The preferred orientation of silicalite-1 nucleus may play an important role in the early stage of crystallization, such as the lateral growth on glass slabs and the vertical growth on porous alumina supports at the early stage of crystallization. Orientation of nucleus A gel layer model has been postulated for the lateral growth of silicalite-1 crystals on silicon wafer[8,9]. It is assumed that at first a gel layer is formed on the surface of the silicon wafer. Nucleation takes place at the interface of the supported gel and liquid. Due to the flatness of the surface, the crystals are oriented with their ac faces parallel to the surface of support. The layer growth model gives a good explanation for the lateral growth of silicalite-1 crystals with their b-directions vertical to the silicon surface. It can be seen from this model that the nucleus must be large enough to achieve ac face to be the largest face. If the nucleus is very small, the orientation may be not control by ac plane. As we know, silicalite-1 and ZSM-5 are pentasil zeolites, and double-five-ring building blocks have been found in the synthesis mixture. Pentasil layer precursors may form from these double five ring building blocks[15]. In the synthesis of silicalite-1 membrane, pentasil layer precursors will interact with the support surfaces, prefer to adsorb on the support surface with their faces, and combine with the support through the condensation of their terminal hydroxyl groups with those of the support surface. Two kinds of pentasil layer structures result in two kind of nucleus orientation as shown in Fig.8, one with [100] direction parallel to the support surface and another with [010] parallel to the support surface, from which silicalite-1 grow into crystals with their [100] or [010] directions perpendicular to the support surface. The orientation of nucleus leads to the lateral crystal growth on flatness supports and vertical growth on porous supports at the early stage of crystallization. The vertical growth of silicalite-1 crystals is due to the preferred orientation of nucleus on the pore wall. The lateral growth of silicalite-1 crystals with their [100] and [010] direction perpendicular to the support surface can
2239 found a direct explanation from the two kinds of pentasil layer orientations. However, the oriented growth on glass can also be explained by the gel layer growth model. At first, silicalite-1 crystals laterally grow with their b-direction vertical to the support surface. Because the intergrowth of silicalite-1 crystals mainly occurs on (010) faces with 90 ~ around c-direction of the main crystals[16], the 90 ~ intergrowth on the substrate crystals creates component crystals with their a-directions perpendicular to the glass surface. It can be found from Fig.7 that the silicalite-1 membrane is actually composed of two layers of intergrowth crystals. A thin layer of about l lam thickness is bounded to the glass surface, which may crystallize from primary nucleation process. The crystals of the top layer is intergrown from the main crystals of the bottom layer. Therefore, the nucleus preferred orientation and the intergrowth of silicalite-1 crystals may also account for the lateral growth of silicalite-1 on glass slabs.
TP
(a) (b) Fig.8. Scheme of two kinds of pentasil layer preferred orientation on flat surface. (a) (100) layer structure; (b) (010) layer structure.
Orientation in crystal growth It is known that the growth rate of silicalite-1 crystals is different in different crystal directions, which results silicalite-1 crystals with c > a > b. In the late stage of crystallization, due to the decrease of base content and TPA ions leads to the decrease of building block nutrients, the competition of the crystal growth in different crystal directions becomes greatly. Only the crystals intergrown with their c-direction perpendicular to support surface grow fast. Therefore silicalite-1 crystals develop to the vertical growth in crystal growth. 4. CONCLUSIONS Continuos large crystal silicalite-1 membranes have been synthesized with vertical crystal orientation on porous alpha alumina supports and with lateral crystal orientation on dense fiat slabs from a synthesis mixture without the presence of TPAOH. The orientation of silicalite-1 crystals on glass slabs and that on porous alpha alumina supports at the early stage of crystallization is ascribed to the nucleus orientation on the local support surface. The
2240 development of a oriented silicalite-1 membrane from a random oriented silicalite-1 membrane shows that crystal growth also play an important role in the oriented growth. ACKNOWLEDGMENT The financial support of the National Nature Science Foundation of China is gratefully acknowledged. REFERENCES:
1. H. Suzuki, Zeolite Membrane, US Patent NO. 4 699 892. 2. T. Sano, Y. Kiyozumi, et al., Zeolites, 12(1992) 131. 3. M. D. Jia, K. V. Peinemannn and R. D. Behling, J. Membrane Sci., 82(1993)15. 4. Y. S. Yan, M. Tsapatsis, G. R. Gavalas and M. E. Davis, J. Chem. Soc., Chem. Commun., (1995)227. 5. J. Caro, F. Marlow and M. Wubbenhorst, Advanced Materials, 6(1994)413. 6. G. A. Ozin, A. Kupermann and A. Stein, Advanced Materials, 4(1992)11. 7. J.G. Tsikoyiannis, W.O. Haag, Zeolites, 4(4), 273, 1992. 8. J.C. Jansen, W. Nugroho and H. van Bekkum, in Eds. R von Ballmos, et al., Proceeding of the 9th Internal Conference Zeoilites, 247, 1992. 9. J. C. Jansen, D. Kashchiev and A. Erdem-Senatalar, Stud. Surf. Sci. and Catal., 85(1994)215. 10. E. R. Genus, H. van Bekkum, et al., J. Chem. Soc., Chem. Commun., 15(1992)1056. 11. S. Feng and T. Bein, Scinence, 265(23),1839, 1994. 12. S. Feng and T. Bein, Nature, 368(28), 834, 1994. 13. M. J. Cheng, W. S. Yang, et al., Chinese J. Catal.(CUIHUA XUEBAO), 16(2)(1995)89. 14. M. J. Cheng, L. W. Lin, et al., submitted to KEXUE TONGBAO. 15. R. A. van Santen, J. Keijsper, et al., in New Developments in Zeolite Science and Technolgy, Y. Murakami, A. Ijima and J. W. Ward(Eds.), Elsevier, Amsterdam, 1986,169. 16. D. G. Hay, H. Jaeger and K. G. Wilshier, Zeolites, 14(1990)571.
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
2233
G r o w t h o f oriented zeolite crystal m e m b r a n e s Mojie, Cheng; Liwu, Lin*; Weishen, Yang; Yashu, Yang; Yide, Xu and Xinsheng, Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics Chinese Academy of Sciences, Dalian 116023, P.R.China SUMMARY
Silicalite-1 crystals were grown with vertical orientation on porous alumina supports and with lateral orientation on dense fiat glass slabs. The crystallization of the oriented silicalite-1 crystals on porous alumina supports were investigated, and it was found that the growth mechanism lies in both nucleation and crystal growth of crystallization. A model of pentasil layer structures preferred orientation was proposed for the explanation of crystals on glass slabs. 1. INTRODUCTION Zeolites have attracted increasing attentions for the developing of molecular sieving membranes and host frameworks to orient guest molecules[I-6]. The in situ hydrothermal synthesis is a novel process for the fabrication of zeolite membranes. The aligned crystal orientation will improve membrane separation efficiency and macro chemiphysical properties of guest molecules. By the control of synthesis conditions, zeolite membranes with the oriented crystal orientation can be achieved. J. G. Tsikoyianni et al.[7] first found the preferred orientation of zeolite crystals on Teflon slabs from the X-ray diffraction patterns of silicalite-1 films. J. C. Jansen et a1.[8,9] reported that silicalite-1 crystals can either be vertically grown on metal supports or laterally grown on silicon wafers, but only random oriented silicalite-1 membranes were obtained on porous supports[10]. S. Feng and T. Bein[11,12] introduced the idea of supra molecule structure preorganization and templating at the organic and inorganic interface to the art of zeolite synthesis and achieved membrane with vertical growth of AIPO-5 crystals on zirconium organic phosphate modified gold wafers. However, there is no report on the preparation of the oriented crystal zeolite membrane on porous supports for separation and on transparent support for the design of host-guest materials. In this paper, we report the synthesis of the vertical growth of silicalite-1 crystals on porous alpha alumina supports and the lateral growth of silicalite-1 crystals on dense fiat glass slabs and discuss their growth mechanism. 2. EXPERIMENTAL Alpha alumina discs, with 0.1-0.3 ~tm pore radius, were used as porous supports. The mixtures for the synthesis of silicalite-1 were prepared by mixing silica sol, sodium hydroxide solution, tetrapropylammonium bromide solution and water, and aged for a given time at room
2234 temperature. The molar con,position of the synthesis mixture was 3Na20 100SiO 2 10000H20 4TPABr. The alumina discs or glass slabs were hold by Teflon supports and vertically immersed in the mixture in stainless steel autoclves. The crystallization was carried out at 180~ in a convection oven. After the synthesis, as-synthesized membranes were recovered, ultrasonic cleaned in distilled water and dried. The As-synthesized silicalite-1 membranes were calcined at 500~ with a careful control of heating procedure. X-ray powder diffraction(XRD) patterns were taken on Ragaku D max/rb. Scanning electron microscopy (SEM) photograph were taken on JEM-1200. 3. RESULTS AND DISCUSSION 3.1. Identification of the vertical growth of silicalite-1 crystals A typical characteristic of the oriented growth of silicalite-1 membranes is their different peaks' intensities compared with powdery sample. Fig.1 shows the XRD patterns of assynthesized silicalite-1 powdery sample, the as-synthesized silicalite-1 membrane and the calcined silicalite-1 membrane. It is found that the vertical growth of silicalite-1 membrane has the same crystal structure as that of powdery sample. The intensities of 002, 102, 103, 104 diffraction peaks of silicalite-1 membrane increased greatly, but the intensities of 200, 501 and 051 peaks of silicalite-1 membrane decreased greatly, which implies that the c-directions of silicalite-1 crystals are vertical to the support surface. The vertical growth of silicalite-1 crystals is also confirmed by SEM photograph as shown in Fig 2. Both the acute edges of prism silicalite1 crystals from top view and the vertical figures of prism crystals from cross-section view are powerful evidences for the vertical growth of silicalite-1 crystals. 12K 002 103
104
103 0 ~
104
101
A 002 101 A
~2
2 J 501 051 15~I
i
1
It 303
101 200
11
.--~-_ I 20
40
0 10 01100 0
A-_
50
2O
Fig.1. X-ray diffraction patterns of (1) silicalite-1 powder sample, (2) as-synthesized and (3) calcined vertical oriented silicalite-1 crystal membrane on porous ct-A1203 support.
2235
Fig.2. Scanning electron microscopy photograph of vertical growth silicalite-1 crystal membrane on porous ct-A1203 support. (a) top view; (b) cross-section view.
12K ~
.
9
.
.
.
6
.
20
40
50
2(1 Fig.3. X-ray diffraction patterns of vertical growth silicalite-1 membranes crystallized at 180~ with different crystallization time(I). 1, 2hours; 2, 4hours; 3, 8hours; 4, 12hours; 5, 24 hours and 6, 48 hours.
2236
3.2. Vertical growth of silicalite-1 membranes on porous alumina supports The formation of silicalite-1 membranes prefer a low alkalinity and high water content synthesis mixture[13], so the oriented growth silicalite-1 membranes can only be obtained at these particular conditions. It has been found that silicalite-1 crystals can be vertically grown at an alkalinity range as wide as that for the synthesis of random oriented silicalite-1 crystal membranes, and the best range is with the Na20/SiO2 ratio from 0.02 to 0.04[ 14]. The vertical growth process of silicalite-1 crystals was investigated to elucidate the crystallization mechanism. The effects of crystallization time on the vertical growth is shown in Fig.3 and Fig.4. It was found that silicalite-1 membrane can either vertically growth at early stage of crystallization as shown in Fig.3, or turn to the vertical growth from a radom oriented silicalite-1 membrane at the late stage of crystallization as shown in Fig. 4. The SEM photograph in Fig.5 also shows that silicalite-1 crystals can be vertically grown on porous alumia support at the early stage of crystallization. 12K
5 4
ix
........
3
k,-~;
A , 5
I 20
It
~
i
I 40
A
2
~
1 50
20 Fig.4. X-ray diffraction patterns of vertical growth silicalite-1 membranes crystallized at 180~ with different crystallization time(II). 1, 2hours; 2, 4hours; 3, 8hours; 4, 12hours and 5, 48 hours.
3.3 Lateral growth of silicalite-1 crystals on dense fiat glass slabs The lateral growth of silicalite-1 crystals on porous supports may result in a high efficient kinds of membrane for separation. Unfortunately, this kind of silicalite-1 membrane haven't been obtained at present. However, it was found that silicalite-1 crystals can be laterally grown on dense fiat glass slabs at about the same synthesis conditions as that of the vertial growth on porous alumina supports, which may give us some indications on the design of lateral growth of silicalite-1 on porous supports. The composition of the synthesis mixture was 4Na20 100SiO 2 10000H20 4TPABr, and the crystallization was carried out at 180~ for 24 hours. Fig.6 shows the XRD patterns of as-synthesized and calcined lateral growth silicalite-1 crystals. It can seen that only 200, 020, 400, 040, 600, 060, 800, 080, 10 0 0 and 0 10 0 diffraction peaks are present
2237 with high intensity, which depicts that the silicalite-1 crystal were highly oriented with their cdirections parallel to the surface of the glass slab. The high intensities of these diffraction peaks implies that the large crystals and the intergrowth of silicalite-1 crystals have constructed a large scale of crystal periodicity. The SEM photograph, as shown in Fig.7, depicts that the silicalite-1 membrane on the dense glass slabs consisted of large shelf-like silicalite-1 crystals intergrown with each other. Silicalite-1 crystals of about 70 ~tm in length, 10 ~tm in height and width are laterally grown on the surface of glass slab, which results in a 10 ~tm thickness of zeolite membrane.
Fig.5. Scanning electron microscopy photograph of vertical growth of silicalite- 1 crystals at the early stage of crystallization. 100K
200 020
040 4001
L
5
l
1
0 10 0
I
_
600'~60
800' 080
,
,,
20
100
I
40
01
I,
2
J
50
20 Fig.6. X-ray diffraction patterns of lateral growth of silicalite-1 membrane on a dense fiat glass slab. 1, as-synthesized and 2, calcined.
2238
Fig.7. Scanning electron microscopy photograph of lateral growth of silicalite-1 crystal membrane on a dense fiat glass slab. (a) top view; (b) cross-section view. 3.4. Growth mechanism of the oriented silicalite-1 crystals The oriented growth of silicalite-1 crystals lies in both nucleation and crystal growth of crystallization. The preferred orientation of silicalite-1 nucleus may play an important role in the early stage of crystallization, such as the lateral growth on glass slabs and the vertical growth on porous alumina supports at the early stage of crystallization. Orientation of nucleus A gel layer model has been postulated for the lateral growth of silicalite-1 crystals on silicon wafer[8,9]. It is assumed that at first a gel layer is formed on the surface of the silicon wafer. Nucleation takes place at the interface of the supported gel and liquid. Due to the flatness of the surface, the crystals are oriented with their ac faces parallel to the surface of support. The layer growth model gives a good explanation for the lateral growth of silicalite-1 crystals with their b-directions vertical to the silicon surface. It can be seen from this model that the nucleus must be large enough to achieve ac face to be the largest face. If the nucleus is very small, the orientation may be not control by ac plane. As we know, silicalite-1 and ZSM-5 are pentasil zeolites, and double-five-ring building blocks have been found in the synthesis mixture. Pentasil layer precursors may form from these double five ring building blocks[15]. In the synthesis of silicalite-1 membrane, pentasil layer precursors will interact with the support surfaces, prefer to adsorb on the support surface with their faces, and combine with the support through the condensation of their terminal hydroxyl groups with those of the support surface. Two kinds of pentasil layer structures result in two kind of nucleus orientation as shown in Fig.8, one with [100] direction parallel to the support surface and another with [010] parallel to the support surface, from which silicalite-1 grow into crystals with their [100] or [010] directions perpendicular to the support surface. The orientation of nucleus leads to the lateral crystal growth on flatness supports and vertical growth on porous supports at the early stage of crystallization. The vertical growth of silicalite-1 crystals is due to the preferred orientation of nucleus on the pore wall. The lateral growth of silicalite-1 crystals with their [100] and [010] direction perpendicular to the support surface can
2239 found a direct explanation from the two kinds of pentasil layer orientations. However, the oriented growth on glass can also be explained by the gel layer growth model. At first, silicalite-1 crystals laterally grow with their b-direction vertical to the support surface. Because the intergrowth of silicalite-1 crystals mainly occurs on (010) faces with 90 ~ around c-direction of the main crystals[16], the 90 ~ intergrowth on the substrate crystals creates component crystals with their a-directions perpendicular to the glass surface. It can be found from Fig.7 that the silicalite-1 membrane is actually composed of two layers of intergrowth crystals. A thin layer of about l lam thickness is bounded to the glass surface, which may crystallize from primary nucleation process. The crystals of the top layer is intergrown from the main crystals of the bottom layer. Therefore, the nucleus preferred orientation and the intergrowth of silicalite-1 crystals may also account for the lateral growth of silicalite-1 on glass slabs.
TP
(a) (b) Fig.8. Scheme of two kinds of pentasil layer preferred orientation on flat surface. (a) (100) layer structure; (b) (010) layer structure.
Orientation in crystal growth It is known that the growth rate of silicalite-1 crystals is different in different crystal directions, which results silicalite-1 crystals with c > a > b. In the late stage of crystallization, due to the decrease of base content and TPA ions leads to the decrease of building block nutrients, the competition of the crystal growth in different crystal directions becomes greatly. Only the crystals intergrown with their c-direction perpendicular to support surface grow fast. Therefore silicalite-1 crystals develop to the vertical growth in crystal growth. 4. CONCLUSIONS Continuos large crystal silicalite-1 membranes have been synthesized with vertical crystal orientation on porous alpha alumina supports and with lateral crystal orientation on dense fiat slabs from a synthesis mixture without the presence of TPAOH. The orientation of silicalite-1 crystals on glass slabs and that on porous alpha alumina supports at the early stage of crystallization is ascribed to the nucleus orientation on the local support surface. The
2240 development of a oriented silicalite-1 membrane from a random oriented silicalite-1 membrane shows that crystal growth also play an important role in the oriented growth. ACKNOWLEDGMENT The financial support of the National Nature Science Foundation of China is gratefully acknowledged. REFERENCES:
1. H. Suzuki, Zeolite Membrane, US Patent NO. 4 699 892. 2. T. Sano, Y. Kiyozumi, et al., Zeolites, 12(1992) 131. 3. M. D. Jia, K. V. Peinemannn and R. D. Behling, J. Membrane Sci., 82(1993)15. 4. Y. S. Yan, M. Tsapatsis, G. R. Gavalas and M. E. Davis, J. Chem. Soc., Chem. Commun., (1995)227. 5. J. Caro, F. Marlow and M. Wubbenhorst, Advanced Materials, 6(1994)413. 6. G. A. Ozin, A. Kupermann and A. Stein, Advanced Materials, 4(1992)11. 7. J.G. Tsikoyiannis, W.O. Haag, Zeolites, 4(4), 273, 1992. 8. J.C. Jansen, W. Nugroho and H. van Bekkum, in Eds. R von Ballmos, et al., Proceeding of the 9th Internal Conference Zeoilites, 247, 1992. 9. J. C. Jansen, D. Kashchiev and A. Erdem-Senatalar, Stud. Surf. Sci. and Catal., 85(1994)215. 10. E. R. Genus, H. van Bekkum, et al., J. Chem. Soc., Chem. Commun., 15(1992)1056. 11. S. Feng and T. Bein, Scinence, 265(23),1839, 1994. 12. S. Feng and T. Bein, Nature, 368(28), 834, 1994. 13. M. J. Cheng, W. S. Yang, et al., Chinese J. Catal.(CUIHUA XUEBAO), 16(2)(1995)89. 14. M. J. Cheng, L. W. Lin, et al., submitted to KEXUE TONGBAO. 15. R. A. van Santen, J. Keijsper, et al., in New Developments in Zeolite Science and Technolgy, Y. Murakami, A. Ijima and J. W. Ward(Eds.), Elsevier, Amsterdam, 1986,169. 16. D. G. Hay, H. Jaeger and K. G. Wilshier, Zeolites, 14(1990)571.