Langmuir 1997, 13, 4193-4196
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Synthesis of Oriented Zeolite Membranes at the Interface between Two Fluid Phases V. Tricoli,*,† J. Sefcik,‡ and A. V. McCormick‡ Dipartimento di Ingegneria Chimica e Scienza dei Materiali, University of Pisa, 56126-Pisa, Italy, and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received December 10, 1996. In Final Form: May 22, 1997X A method was developed for the synthesis of free-standing silicalite-1 membranes. Intergrown polycrystalline films were synthesized at the interface between a colloid-free aqueous phase and an organic phase or air. These films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that the crystallites of the films formed at the air-water interface were oriented with their (010) axis orthogonal to the interface.
Introduction Zeolite molecular sieves have found widespread application as catalysts, adsorbents, and ion-exchangers throughout the chemical industry for over two decades. More recently it has been proposed that the unique pore system of these materials and their ability to discriminate between molecules on the basis of size or specific interactions may be exploited in a number of nonconventional ways. Zeolites have been proposed as filters operating continuously at a molecular scale1-3 and as high-performance chemical sensors.4,6 Inclusion chemistry has opened additional perspectives for these materials. The zeolite framework has been utilized as a host for polymer molecular wires,7,8 semiconductor and metal clusters,9,10 and organic species.11 Comprehensive reviews are available of zeolite host-guest systems and the impact that these may have on a number of areas: molecular electronics, nonlinear optics materials, photo-electrochemistry.12,13 Many of these potential applications require that these materials be made as defect-free thin films. Generally, previous attempts achieved disordered two-dimensional structures consisting of zeolite crystallites intergrown on various solid supports.1-3 Ordered zeolite films with pore channels oriented orthogonal to the plane may be more attractive. The growth of oriented silicalite coatings onto silicon wafer platelets, with the b-axis of the crystals perpendicular to the surface, was reported.14,15 Silicalite films with similar crystal orientation were obtained on fused silica16 and †
University of Pisa. University of Minnesota. X Abstract published in Advance ACS Abstracts, July 15, 1997. ‡
(1) Tsikoyiannis, J. G.; Haag, W. O. Zeolites 1991, 12, 126. (2) Jia, M. D.; Peinemann, K. V.; Behling, R. D. J. Membr. Sci. 1993, 82, 15. (3) Geus, E. R.; den Exter, M. J.; van Bekkum, H. J. Chem. Soc., Faraday Trans. 1992, 88, 3101. (4) Shaw, R. B. ACS Symp. Ser. No. 1989, 390, 318. (5) Bein, T.; Brown, K.; Frye, G. C.; Brinker, C. J. J. Am. Chem. Soc. 1989, 111, 7640. (6) Alberti, K.; Haas, J.; Plog, C.; Fetting, F. Catal. Today 1991, 8, 509. (7) Enzel, P.; Bein, T. J. Phys. Chem. 1989, 93, 6270. (8) Enzel, P.; Bein, T. Chem. Mater. 1992, 4, 819. (9) Wang, Y.; Herron, N. J. Phys. Chem. 1988, 92, 4988. (10) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 530. (11) Cox, S. D.; Gier, T. E.; Stucky, G. D.; Bierlein, J. J. Am. Chem. Soc. 1988, 110, 2986. (12) Ozin, G. A.; Kuperman, A.; Stein, A. Angew. Chem., Intl. Ed. Engl. 1989, 28, 359. (13) Ozin, G. A. Adv. Mater. 1992, 4, 612.
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mercury17 surfaces from a precursor hydrogel solution. The synthesis of composite polycrystalline silicalite membranes with the a and b crystal axes both parallel to the surface was also reported.18 Preorganized interfaces containing chemical functionalities, such as compressed langmuir monolayers, may induce oriented crystallization of a variety of species from aqueous solutions.19-22 That was ascribed to a molecular recognition process between the interface and the inorganic ions involving electrostatic interactions, latticematching, steric complementarity,23 and hydrogen bonding.24 This approach was utilized to grow oriented molecular sieve crystals onto organophosphonate selfassembled monolayers.25,26 Very recently, films of mesostructured porous silica have been prepared using airwater27 and organic-water interfaces.28 Here we report on oriented silicalite-1 thin films formed beneath the interface between an organic or (humid) air phase and a colloid-free aqueous phase. Methods The aqueous phase was a typical bath for silicalite-1 synthesis. Sodium hydroxide (Baker) and tetrapropylammonium (TPA) bromide (templating agent) (Aldrich) were dissolved in deionized water (18 MΩ‚cm), and the solution was filtered. Silica particles (Aldrich, average size 3 mm or 200 µm, purity > 99%) were utilized as a source of zeolite precursors. All reagents were used as supplied without further purification. A crucial point in this (14) Koegler, J. H.; Zandbergen, H. W.; Harteveld, J. L. N.; Nieuwenhuizen, M. S.; Jansen, J. C.; van Bekkum, H. Stud. Surf. Sci. Catal. 1994, 84, 307. (15) Jansen, J. C.; van Rosmalen, G. M. J. Cryst. Growth 1993, 128, 1150. (16) Yan, Y.; Chaudhury, S. R.; Sarkar, A. Chem. Mater. 1996, 8, 473. (17) Kiyozumi, Y.; Mizukami, F.; Maeda, K.; Toba, M.; Niwa, S. Adv. Mater. 1996, 8, 517. (18) Lovallo, M. C.; Tsapatsis, M. AIChE J. 1996, 42, 3020. (19) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261, 1286. (20) Gavish, M.; Popowitz-Biro, R.; Lahav, M.; Leiserowitz, L. Science 1990, 250, 973. (21) Popowitz-Biro, R.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1991, 113, 8943. (22) Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (23) Mann, S. Nature 1993, 365, 499. (24) Frostman, L. N.; Bader, M. M.; Ward, M. D. Langmuir 1994, 10 (2), 576. (25) Feng, S.; Bein, T. Nature 1994, 368, 834. (26) Feng, S.; Bein, T. Science 1994, 265, 1839. (27) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589. (28) Schacht, S.; Huo, Q.; Voight-Martin, I. G.; Schuth, F. Science 1996, 273, 768.
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4194 Langmuir, Vol. 13, No. 16, 1997 work was to utilize a truly clear solution for the synthesis. All the syntheses were carried out in the absence of pre-existing silica colloidal gel. When dry silica particles are soaked in water, they disintegrate, generating smaller particles (which settle at the bottom of the beacker) and a silica colloidal suspension. Thus, the silica particles were first placed in a stream of water. After a few hours the colloidal suspension had been completely washed off by the stream. The particles that had settled were recovered and rinsed several times with deionized water first and then with part of the solution used for the synthesis. The particles were transferred to a clean Teflon-lined autoclave (inner diameter 65 mm), and the TPA solution was added. Dust and other small particles which we could see floating on the water surface were removed. The solution always appeared perfectly clear to eye inspection. Samples drawn from the aqueous phase before and after the hydrothermal treatment were also analyzed by quasielastic light scattering (Coulter mod. N4SD). Our instrument showed that no colloidal silica with particle size in excess of 3 nm was present in the synthesis solution. The molar ratio between water and sodium hydroxide was always 1000, whereas the molar ratio between TPA bromide and water, TPA/H2O, was varied between 5 × 10-4 and 4 × 10-3. The amount of silica particles at the bottom of the autoclave was always greater than the saturation limit. Finally an oil phase was settled on top of the aqueous solution. We tried various interfaces with the oil phase being one of the following organics: 1-dodecanol, benzene, or hexadecane with a small amount of dissolved stearic acid (0.1 wt %). In the last case our intent was to form a functionalized interface. It has been shown that negatively charged chemical functionalities on organic surfaces may substantially reduce the free energy barrier for the nucleation of various oxides.29 Amphiphile stearic acid forms a monolayer at the interface with carboxylate head groups pointing toward the aqueous phase. Owing to electrostatic attraction, the concentration of TPA cations might be higher at the interface. We hoped this promoted zeolite nucleation. In other experiments we simply let the aqueous phase be in contact with the humid air phase. Several syntheses were carried out at temperatures ranging from 150 to 190 °C. The reaction time was 3, 5, or 8 days. SEM photographs of the films were taken on a JEOL (mod. JSM-T300) microscope. XRD data were collected on a Philips (mod. PW 2273/20) diffractometer, using Cu KR radiation.
Results and Discussion Free-standing polycrystalline films routinely formed on the underside of the interface between water and 1-dodecanol or benzene over the whole range of temperature for 1 × 10-3 < TPA/H2O < 2 × 10-3. When we used benzene, we obtained continuous films extending over the entire surface except for a region close to the wall. However, the films formed at the water-1-dodecanol interface were inhomogeneous and exhibited flaws even to eye inspection. In both cases the films were opaque, which suggests random intergrowth of the crystallites. This was confirmed by SEM (Figure 1a). The XRD pattern for the as-synthesized films is consistent with that of pure powder silicalite-1 (Figure 2a). However, some of the peaks for the zeolite film have substantially higher intensity than those for powder silicalite-1. These peaks correspond to the (200) or (020) and the (400) or (040) reflections. This indicates that the crystallites are oriented to some degree with their (100) or (010) axis orthogonal to the plane of the film. No zeolite crystals grew beneath the stearic acid monolayer. This may be due to electrostatic repulsion between carboxylate head groups and silicate anions. One may consider investigating zeolite nucleation onto interfaces functionalized with positively charged surfactants. Also, zeolite membranes formed at the air-water inter(29) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48.
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Figure 1. SEM photographs of a silicalite-1 film formed at the 1-dodecanol-water interface (a, top) and at the air-water interface (b, bottom).
face, only for certain conditions of temperature, composition, and reaction time. This indicates that the surfaces of the benzene and of the long chain alcohol phase may promote the crystallization of the zeolite precursors with respect to the air or stearic acid in hexadecane phase. Crystallization of silicalite-1 at the air-water interface was very sensitive to the synthesis conditions. For TPA/ H2O ) 1 × 10-3 and T ) 150 °C, the whole water surface appeared to be covered by a single layer of zeolite crystals. However, the crystals were disconnected from one another and did not form a continuous sheet. When we operated at higher temperatures (170-190 °C) and lower TPA concentrations (5 × 10-4 < TPA/H2O < 1 × 10-3), we often found many small fragments of film lying at the bottom of the autoclave. In such cases, because no crystals floating at the interface were observed, we argue that these membranes formed at the interface and then precipitated. Unlike the films grown at the oil-water interface, these fragments were translucent. SEM revealed strong orientation of the crystallites in the plane of the interface (Figure 1b). Also, the two sides of the films displayed the same morphology. SEM images of the film cross-section (not shown) indicated that the film thickness is the equivalent of one to three layers of crystals. The XRD pattern for the as-synthesized films at the air-water interface (Figure 2b) displays only four peaks. The peaks correspond to the (2i,0,0) or (0,2i,0) reflections of silicalite-1 (i ) 1, 2, 3, 4). This confirms that a high degree of crystal orientation was achieved for films
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Figure 2. X-ray diffraction patterns of a silicalite-1 film grown at the 1-dodecanol-water interface (a) and at the air-water interface (b).
synthesized at the air-water interface. Because the a and b unit-cell dimensions are almost equal for silicalite, it cannot be ascertained from the XRD pattern whether the crystallites are oriented with their (100) or (010) axis orthogonal to the film. Also, twinning of the (100) and (010) axes has been observed in fluoride silicalite and some other MFI-type structures.30,31 However, in pure silica MFI molecular sieve (silicalite-1) the (010) axis runs perpendicular to the largest crystal face. Figure 1b shows that the coffin-shaped crystallites lay flat with their largest face in the plane of the film. This suggests that they are oriented with their (010) axis normal to the film. Straight pores run parallel to (010) in these structures, interconnected by sinusoidal pores parallel to (100).32 Accordingly, the two sides of the zeolite membrane would be connected by an array of straight molecular size channels orthogonal to the plane. The films synthesized at the air-water interface usually extended over several square millimeters. In a few cases we obtained films as large as several square centimeters. Unfortunately, the synthesis of such large sheets was not readily reproducible and in most cases we obtained many small films. While in the presence of the oil phase the films remained suspended at the interface, for the air-water system the (30) Weidenthaler, C.; Fischer, R. X.; Shannon, R. D.; Medenbach, O. J. Phys. Chem. 1994, 98, 12687. (31) Price, G. D.; Pluth, J. J.; Smith, J. V.; Bennett, J. M.; Patton, R. L. J. Am. Chem. Soc. 1982, 104, 5971. (32) Flanigen, E. M.; Bennet, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature 1978, 271, 512.
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film pieces, once formed, precipitated. This might explain why it was difficult, though possible, to form a single large sheet. SEM did not reveal defects in the films grown at the benzene-water interface (however, it should be pointed out that the absence of intercrystalline porosity cannot be ascertained merely from SEM examination). For the films synthesized at the interface between air and water, SEM revealed that there are zones where the crystallites are highly intergrown, as well as zones exhibiting void space, due to less complete intergrowth. However, it is noteworthy that our membranes were produced during a single hydrothermal stage. The presence of intercrystalline porosity is quite common in zeolite membranes. Usually, defective zeolite membranes are exposed to several hydrothermal treatments, until the pinholes are filled either with crystals which grow larger at each hydrothermal stage18 or with new crystals, which form at each stage.2 The effect of calcination on the structure of the films was also investigated. Calcination often produces cracks in the structure of supported zeolitic membranes, owing to thermal stresses arising during the heating process. Such stresses arise because of different thermal expansion of the zeolitic layer and of the support. Accordingly, the effect of calcination as to the formation of cracks in freestanding zeolite films is expected to be less dramatic, if not absent. We calcined some of the films in flowing nitrogen at 460 °C for 8 h. The heating and cooling rates were 3 °C/min. We examined by SEM both as-synthesized and calcined films formed at the air-water interface coming from the same synthesis batch. No significant difference of structure between the two films could be observed. We have already pointed out that no colloidal silica was present in solution before and after the hydrothermal treatment. Moreover, since silicate anions came only from partial dissolution of the silica particles placed at the bottom of the autoclave, it is likely that no colloidal silica was present in solution during the synthesis as well. It was proposed that the TPA cation has the role of a structure directing agent in the synthesis of a pure silica ZSM-5.33,34 According to these studies, soluble silicate species interact with TPA cations forming preorganized inorganic-organic composite aggregates prior to nucleation. The present work supports that such primary composite structures may form in a truly clear solution in the absence of a pre-existing (colloidal) gel. Whether these composite structures aggregate directly into zeolite nuclei or form much larger gellike structures in which zeolite nucleation subsequently occurs cannot be ascertained from our results. According to a recent study,35 the latter hypothesis appears more likely, although it should be pointed out that a colloidal silica suspension was present at an early stage in that synthesis solution. However, it appears from our results that the interface is capable of orienting preformed zeolite nuclei or, perhaps, of producing a preferential orientation in the zeolite precursor during the nucleation process. Conclusion We demonstrated a route to make oriented silicalite-1 thin films from clear solutions. This method utilizes the interface between an aqueous phase and an organic or air phase as a template for aligning the zeolite-forming (33) Chang, C. D.; Bell, A. T. Catal. Lett. 1991, 8, 305. (34) Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4647. (35) Dokter, W. H.; van Garderen, H. F.; Beelen, Th. P. M.; van Santen, R. A.; Bras, W. Angew. Chem., Intl. Ed. Engl. 1994, 34, 73.
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species. Orientation is most complete at the air-water interface. The silicalite-1 crystallites are oriented with their (010) axis normal to the film. Thus, a regular array of straight pores runs perpendicularly to the zeolite film. If membranes can be built routinely large enough, self supporting, oriented, and with a variety of zeolite structures, this can give a significant boost to the development of molecular electronic and optic devices and chemical sensors.
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Acknowledgment. The authors are grateful to Professor E. L. Cussler of the University of Minnesota for his helpful advice. We thank Professor F. Marchetti of the University of Pisa for his help with XRD and Mr. P. Narducci for his help with SEM. V.T. is indebted to Professor P. F. Marconi of the University of Pisa for financial support. LA962095H
