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Understanding the Crystallization of Nanosized Cobalt Aluminate Spinel from Ion-Exchanged Zeolites Using Combined in Situ QEXAFS/XRD Andrew M. Beale† and Gopinathan Sankar* DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K. ReceiVed April 15, 2005. ReVised Manuscript ReceiVed September 8, 2005
By using combined QEXAFS/XRD (supported by measurements obtained using DSC, XRD, and IR spectroscopy), we have studied, in situ, the evolution of nanosized cobalt aluminate spinel (CoAl2O4) particles from cobalt-ion-exchanged zeolite precursors. All of the cobalt-ion-exchanged zeolites were observed to first undergo dehydration, followed by amorphization before the CoAl2O4 phase was observed to form. For two of the zeolites studied, this phase was observed to form at temperatures as low as 450 °C. However, with the exception of zeolite Y, a minor amount of a secondary phase (Co2SiO4) was also observed to form from amorphized zeolites when the Si/Al ratio was >1.85 and the calcination temperature exceeded 850 °C. This unusual ability of zeolite Y to yield phase-pure CoAl2O4, even though it has a Si/Al ratio of ∼2.52 can be attributed to the tendency for the cobalt ion to predominantly locate within the double six rings (SI site) of the zeolite Y structure in close proximity to the Al3+ ions (as chargecompensating cations), thereby creating nucleation sites for the spinel material. In contrast, other zeolites that lose their framework structure allow random distribution of cobalt ions, away from Al3+, allowing the formation of other phases such as Co2SiO4.
Introduction Zeolites are known to exhibit a wide range of thermal stabilities. Some, for example, stilbite (STI), lose their microporosity at ca. 400 °C, whereas others, such as zeolites A (LTA) and X (FAU), retain their long-range ordering at temperatures in excess of 850 °C.1 It is well-known that, upon reaching the critical temperature for collapse, the zeolite structure undergoes an order-disorder transition leaving the material in an amorphous glasslike state.2,3 Further heating at or above the temperature of collapse often causes the formation of crystalline-dense framework structures; the abundance of Si(IV) and Al(III) in the zeolite framework often results in the production of a number of silicate and aluminate structures.4 However, the formation of crystalline structures containing the exchangeable countercation at temperatures lower than the melting point of the corresponding aluminosilicate glass has made this a viable synthetic route for the production of high-quality, relatively stressfree ceramics.5,6 To date, such approaches have yielded comparatively low-temperature preparation routes for a number of high-performance ceramics including cordierite * To whom correspondence should be addressed. E-mail:
[email protected]. † Current address: Department of Inorganic Chemistry and Catalysis, University of Utrecht, Sorbonnelaan 16, CA 3584 Utrecht, The Netherlands.
(1) Dyer, A. An Introduction to Zeolite Molecular SieVes; John Wiley & Sons: New York, 1988. (2) Greaves, G. N.; Meneau, F.; Sapelkin, A.; Colyer, L. M.; Gwynn, I. A.; Wade, S.; Sankar, G. Nat. Mater. 2003, 2, 622-629. (3) Colyer, L. M.; Greaves, G. N.; Carr, S. W.; Fox, K. K. J. Phys. Chem. B 1997, 101, 10105-10114. (4) Chandrasekhar, S.; Pramada, P. N. Ceram. Int. 2002, 28, 177-186. (5) Chowdhry, U.; Corbin, D. R.; Subramanian, M. A. U.S. Patent 4,814,303, 1989. (6) Subramanian, M. A.; Corbin, D. R.; Chowdhry, U. Bull. Mater. Sci. 1993, 16, 665-678.
and a number of first-row transition element containing normal and inverse spinels.3-12 Successful crystallization of stoichiometric condensed phases by this synthetic procedure is possible because of the atomic dispersion of cations in the zeolite framework. The framework consists of regular linked Al3+ and Si4+ tetrahedra that require charge compensation with ion-exchangeable cations.1 The distribution is maintained after collapse, thus enabling a uniform and homogeneous nucleation of phases from the amorphous materials. This synthetic method can therefore offer processing advantages in that it is possible to exert control over the properties of the final material as both temperature and time can be used as parameters to control the particle size of the crystallized phase. In principle, by careful optimization, it should be possible to prepare materials with surface areas in excess of 200 m2/g.12 During our investigations, we have attempted to understand further some of the criteria necessary to produce transitionmetal-containing spinels from zeolite precursors. We have focused on using in situ combined QEXAFS/XRD7,13-15 (a technique well-suited for investigating both long- and short(7) Sankar, G.; Wright, P. A.; Natarajan, S.; Thomas, J. M.; Greaves, G. N.; Dent, A. J.; Dobson, B. R.; Ramsdale, C. A.; Jones, R. H. J. Phys. Chem. 1993, 97, 9550-9554. (8) Colyer, L. M.; Greaves, G. N.; Dent, A. J.; Carr, S. W.; Fox, K. K.; Jones, R. H. Zeolites and Related Microporous Materials: State of the Art 1994; Elsevier: Amsterdam, 1994; Vol. 84, pp 387-394. (9) Colyer, L. M.; Greaves, G. N.; Dent, A. J.; Fox, K. K.; Carr, S. W.; Jones, R. H. Nucl. Instrum. Methods Phys. Res. B 1995, 97, 107110. (10) Weidenthaler, C.; Schmidt, W. Chem. Mater. 2000, 12, 3811-3820. (11) Schmidt, W.; Weidenthaler, C. Microporous Mesoporous Mater. 2001, 48, 89-94. (12) Schmidt, W.; Weidenthaler, C. Chem. Mater. 2001, 13, 607-612. (13) Thomas, J. M.; Greaves, G. N. Catal. Lett. 1993, 20, 337-343. (14) Clausen, B. S. Catal. Today 1998, 39, 293-300.
10.1021/cm050797z CCC: $33.50 © 2006 American Chemical Society Published on Web 12/14/2005
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range ordering in structures) to observe the stability of the zeolite and local structure around the charge-balancing transition metal (in this case, cobalt) before and after collapse of the open framework structure and during the subsequent recrystallization of transition-metal-containing phases. By correlating the changes between the two domains, it was anticipated that it would be possible to understand further their interrelationship, allowing some insight into why certain zeolites (such as zeolite Y) readily yield such spinel phases. Furthermore, in view of the wide range of thermal stabilities that zeolites exhibit, we investigated the possibility of using structures with lower thermal stability to yield pure spinel phases at lower synthesis temperatures, as the temperature of recrystallization and temperature of collapse are intrinsically related. Two zeolite members of the phillipsite group in particular were chosen: zeolite B with a gismondine framework structure type similar to that of zeolite P (we note that the structure of zeolite B has not been solved as it contains a large degree of disorder) and phillipsite (PHI).16,17 These zeolites were chosen because they are known to exhibit much lower thermal stabilities (with collapse temperatures of
