VOLUME 9, NUMBER 8
AUGUST 1997
© Copyright 1997 by the American Chemical Society
Communications A Highly Oriented Thin Film of Zeolite A Laura C. Boudreau and Michael Tsapatsis* Department of Chemical Engineering University of Massachusetts Amherst, Massachusetts 01003 Received March 18, 1997 Revised Manuscript Received May 22, 1997 The preparation of molecular sieve films has attracted considerable interest due to their potential uses as membranes, membrane-reactors, components of optical and electronic devices, and selective sensors.1-11 Progress in thin-film molecular sieve materials has been recently reviewed.12 Despite considerable advances in synthetic procedures,13-15 the challenge of preparing continuous, highly oriented, submicrometer thickness molecular sieve films remains to be met. A simple and general processing scheme particularly promising to meet this goal consists of using suspensions of small (nanometer size) zeolite crystals to prepare precursor particle layers, * Corresponding author. (1) Davis, M. E. Ind. Eng. Chem. Res. 1991, 30, 1675. (2) Yan, Y.; Tsapatsis, M.; Gavalas, G. R.; Davis, M. E. J. Chem Soc., Chem Commun. 1995, 227. (3) Rolison D. R. Chem. Rev. (Washington, D.C.) 1990, 90, 867 (4) Werner, L.; Caro, J.; Finger, G.; Kornatowski, J. Zeolites 1992, 12, 658. (5) Funke, H. H.; Kovalchick, M. G.; Falconer, J. L.; Noble, R. D. Ind. Eng. Chem. Res., 1996, 35, 1575. (6) Hennepe, H. J. C.; Bargeman D.; Mulder M. H. V.; Smoulders, C. A. J. Membr. Sci. 1987, 35, 39. (7) Ozin G. A.;, Kuperman, A.; Stein A. Angew. Chem., Int. Ed. Engl. 1989, 28, 359. (8) Yan, Y.; Bein, T. J. Am. Chem Soc. 1995, 117, 9990. (9) Yan, Y.; Bein, T. J. Phys. Chem. 1992, 96, 9387. (10) Yan, Y.; Bein, T. Chem. Mater. 1992, 4, 975. (11) Bein, T.; Brown, K. J. Am. Chem. Soc. 1989, 111, 7640. (12) Bein, T. Chem. Mater. 1996, 8, 1636. (13) Caro, J.; Finger, G., Kornatowski, J.; Mendau, J. R.; Werner, L.; Zibrowius, B. Adv. Mater. 1992, 4, 273. (14) Feng, S.; Bein, T. Nature 1994, 368, 834. (15) Yan, Y.; Chaudhuri, S. R.; Sarkar, A. Chem. Mater. 1996, 8, 473.
S0897-4756(97)00151-8 CCC: $14.00
Figure 1. Zeolite A particles used to prepare precursor layer: (a) SEM image; (b) X-ray powder diffraction pattern
followed by secondary growth of these particles to a continuous film.16-17 We have reported on the use of secondary growth of precursor layers to prepare intergrown zeolite films.16-19 (16) Lovallo, M. C.; Tsapatsis, M. Chem. Mater., 1996, 8, 1579. (17) Lovallo, M. C.; Tsapatsis, M. AIChE J. 1996, 42, 3020.
© 1997 American Chemical Society
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Figure 2. Top view SEM of precursor zeolite layer (a) and of film after secondary growth for 1 day (b), 2 days (c), and 3 days (d). Cross-section SEM view (e) of the film shown in (d). All films were calcined at 500 °C for 8 h.
In these reports the precursor layers consisted of randomly oriented particles. As a result of the random orientation in the precursor layer, the intergrown films are most often randomly oriented. In one case,17 preferentially oriented films of silicalite have been (18) Lovallo, M. C.; Boudreau, L.; Tsapatsis, M. Microporous and Macroporous Materials; Beck, J. S., Iton, L. E., Corbin, L. E., Lobo, R. F., Davis, M. E., Zones, S. I., Suib, S. L., Eds.; MRS: Pittsburgh, 1996. (19) Lovallo, M. C.; Tsapatsis M. Advanced Catalytic Materials; Ledoux, M. J., Lednor, P. W., Nagaki, D. A., Thompson, L. T., Eds.; MRS: Pittsburgh, in press.
prepared with the preferred orientation being induced by secondary growth. In these films most of the surface crystals were aligned with their straight and “sinusoidal” channels nearly parallel to the film surface. Although as a result of their microstructure these films displayed unique gas permeation properties, a distribution of orientations around the mean is evident by SEM17,18 examination and texture analysis.19 For the preparation of the highly oriented zeolite A films reported here, the precursor layer consisted of oriented
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Figure 3. X-ray diffraction pattern for the precursor zeolite A layer (a) and for the film after secondary growth for 1 day (b), 2 days (c), and 3 days (d). Figure 3e is the X-ray diffraction pattern for a randomly oriented film of zeolite A.
particles. This allowed for the preparation of thin (∼500 nm) zeolite films with a high degree of 2-dimensional orientational order. The micropores are oriented parallel and perpendicular to the substrate. Preparation of zeolite suspensions has been reported in the literature by several groups.20-23 The zeolite A suspension used here was prepared starting from a clear solution of composition 4.1 (TMA)2O:0.3 Na2O:1 Al2O3: 3.4 SiO2:239 H2O23 and heating at 90 °C for 24 h under rotation. The growth of zeolite A particles was followed by dynamic light scattering, and their morphology was examined by SEM. When the particle size reaches 200300 nm, they acquire a well-defined cubic shape. By repeated centrifugation and washings, suspensions of these particles can be prepared in water at pH 10 and a concentration of 20 g/L. Figure 1 shows a SEM image and a powder X-ray diffraction pattern of the particles. No particle agglomeration is observed, and the X-ray pattern is characteristic of zeolite A. We have achieved the formation of a closely packed monolayer by dipping the substrate (for instance a glass slide), in a suspension of the zeolite A particles, drying at room temperature, and calcining at 500 °C for 8 h. This process was repeated three times to obtain a sufficiently high coverage of zeolite A particles. As shown in Figure 2a, the cubic crystals are deposited face down making nearly a monolayer of zeolite A particles. Several variations of this procedure involving the use of addi(20) Verduijn, P. V.; Mechilium, J., De Gruijter, C. B.; Koetsier, W. T.; Van Oorschot, C. W. M. U.S. Patent 5,064,630, 1991. (21) Meng, X.; Zhang, Y., Meng, C.; Pang, W. In Proceedings of the 9th International Zeolite Conference; von Ballmoos, R., et al., Eds.; Butterworth-Heinemann: Boston, 1993. (22) Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J. E. Zeolites 1994, 15, 611. (23) Schoeman, B. J.; Sterte, J.; Otterstedt, J. E. Zeolites, 1994, 14, 110.
tives such as a bohemite suspension16 or pretreatment of the substrate with a surfactant24 can reduce the required number of coating cycles and affect the affinity and bonding of the particles with the substrate surface, but it was found that this was not necessary in this case. The zeolite A monolayer can be prepared with no modification of the glass surface and using only the zeolite A suspension in water. Figure 3a shows the X-ray diffraction (XRD) pattern collected in a BraggBrentano geometry from the glass slide coated with zeolite A particles. Only the (200) and (600) peaks appear, indicating the preferred orientation of the zeolite A layer in accordance with Figure 2a. The glass substrate causes the broad feature between 20 and 30° 2θ. Although oriented, the particulate layer is not continuous over the surface of the substrate. To prepare continuous films, we identified conditions under which, during secondary growth, the zeolite A particles can grow larger. To achieve this, a wide range of clear solution compositions were screened in the presence of the zeolite A particles used to prepare the precursor layer. The concentration of seeds present in the secondary growth solution affects the crystallization kinetics. As the amount of seeds is increased, fewer new crystallites are formed and the nutrients are mostly consumed for growth of the seed particles and less for the nucleation of new crystals. High concentrations of seed crystals most closely approach the environment in the vicinity of the precursor layer surface during secondary growth. Several of these secondary growth conditions were implemented for the preparation of continuous films starting from precursor layers. Figure 2b-d shows the evolution of a precursor layer during (24) Valtchev, V.; Schoeman, B. J.; Hedlund, J.; Mintova, S.; Sterte, J. Zeolites 1996, 17, 408.
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Figure 4. Pole figures for the (200) peak (a) and (220) peak (b) for the zeolite A film shown in Figure 2d.
secondary growth using a clear solution with composition 4.1 (TMA)2O:0.3 Na2O:1 Al2O3:4.4 SiO2:706.2 H2O under rotation at 90 °C. It can be seen that a continuous film gradually evolves which, after 3 days, covers the entire surface of the substrate. The cross section shown in Figure 2e shows that the film is ∼500 nm thick, nearly monograin in thickness, and well attached
to the substrate. The final film is oriented in the same manner as the precursor monolayer as evidenced by the corresponding XRD patterns shown in Figure 3a-d. For comparison, Figure 3e shows the XRD pattern of a zeolite A film with no preferred orientation that was grown using an in situ preparation.18 Figure 3a-d shows that the intensity of the (200) and (600) peaks
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Figure 5. Micrometer scale alignment in (a) precursor film and (b) regrown film.
increases and the amorphous feature attributed to the substrate decreases. No other peaks well above the noise level appear during secondary growth. Although the films reported here exhibit a high degree of orientation with the (h00) planes parallel (and perpendicular) to the substrate, this is not the case around the direction perpendicular to the substrate. Figure 4a,b shows the results of a texture analysis for the (200) and (220) peaks, respectively. The data are presented using isointensity contour lines, which have been normalized to the average intensity, and drawn on a polar plot where ψ is the radial axis and φ is the rotation axis. The nearly concentric isointensity lines on the (220) pole figure indicate random orientation around the perpendicular to the film surface. However, the concentrated placement of these concentric circles, around the 0° and 45° ψ axis, respectively, further reinforces the fact that the (h00) crystal planes are oriented parallel to the substrate surface and the (hh0) planes are at a 45° angle to the substrate. No amorphous phase is observed covering the substrate during secondary growth. Moreover, the regrowth process takes place slowly (over several days), eliminating mass-transport limitations. As a result, these conditions lead to uniform growth along the film thickness in contrast to our previously reported work which, by involving faster growth and/or formation of a precursor gel, results in asymmetric growth along the precursor film thickness.16-19 Regrowth experiments performed using millimeter thickness self-supported films of zeolite A particles result in a uniformly intergrown compact consisting of randomly oriented grains. The presence of the monolayer appears critical for the formation of a continuous film. Placing a glass substrate without the precursor layer in contact with the clear solution used for secondary growth results in very poor coverage of its surface with zeolite A crystals. However, the zeolite A crystals deposited on the substrate are similarly oriented. These results indicate
that the oriented film evolves mainly from grain growth of the particles in the precursor layer. Newly deposited crystals cannot be excluded from contribution in the film growth, but they do not disturb the orientation of the film. Oriented supported thin films can be similarly prepared on porous substrates such as porous alumina disks provided that the substrates have a surface roughness smaller than the dimensions of the zeolite crystal size (200-300 nm). Modification of this process is needed in order to prepare a zeolite film with three-dimensional preferential orientation. It appears that a complete alignment of the precursor particles will lead to such a film. We have observed that in some areas of the precursor film such an alignment is achieved over micrometer length scales as shown in Figure 5a. After secondary growth, intergrown areas exist over similar length scales exhibiting commensurate microstructure with threedimensional orientational order as shown in Figure 5b. We speculate that regions such as those shown in Figure 5b originate from the oriented domains shown in Figure 5a. If this is the case, the preparation of submicrometer thickness, supported zeolite films approaching singlecrystal quality may emerge from the simple processing scheme demonstrated here. Acknowledgment. Support for this work was provided by NSF [CTS-9624613 (CAREER) and CTS9512485 (ARI)]. We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society for the partial support of this work. M.T. is grateful to the David and Lucile Packard Foundation for a Fellowship in Science and Engineering. Finally, we acknowledge the W. M. Keck Polymer Morphology Laboratory for use of its Electron Microscopy facilities. CM970151+
