Time-Resolved in Situ X-ray Powder Diffraction Study of the Formation of Mesoporous Silicates Stephen O’Brien, † Robin J. Francis, † Andrew Fogg, † Dermot O’Hare,* ,† Nanae Okazaki, ‡ and Kazuyuki Kuroda ‡,§ Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, U.K., Department of Applied Chemistry, Waseda University, Ohkubo 3, Shinjuku-ku, Tokyo 169, Japan, and Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda 2, Shinjuku-ku, Tokyo 169, Japan Received January 25, 1999. Revised Manuscript Received April 12, 1999 In situ, time-resolved energy-dispersive X-ray diffraction has been used to investigate the formation of the mesoporous silicates FSM-16 and MCM-41. The data suggest that the silica- surfactant mesophases formed are highly dependent on the reactant medium, the effect of the silica source being one of the main determining factors. Kanemite, a layered polysilicate, proves to be an excellent silicate source, giving rise to relatively ordered mesophases and subsequent highly ordered mesoporous silicate products. The time-resolved in situ X-ray diffraction data of the kanemite-alkytrimethylammonium system indicated that the silica- surfactant mesophase precursor to FSM-16 forms from a medium containing a number of intercalated silicate phases, while in contrast, the hexagonal mesophase precursor to MCM- 41 forms from a medium containing no other ordered silicate-surfactant phases detectable by in situ X-ray diffraction. Introduction The determination of mechanistic information relat- ing to solid-state reactions should ultimately lead to a more rational approach to the synthesis of microporous and mesoporous materials. Understanding how mol- ecules and ions can preferentially organize themselves under certain conditions to form extended crystalline solid structures is a well-established goal in modern inorganic chemistry. However, it is often difficult to acquire sufficient data to substantiate a particular mechanistic theory that can adequately describe het- erogeneous reactions (i.e., involving solid and liquid reagents): Such systems involve a complicated variety of species, nucleation, and crystallization processes. In situ methods can often provide invaluable insights into the nature and the rate of transformation from reac- tants to products and can therefore shed light on the stages of nucleation and the molecular species involved. 1 The mechanisms involved in the formation of meso- porous silicates are an important issue in contemporary solid-state chemistry. An additional feature to the problem of examining these reaction mechanisms is that the concept of templating is applied to aggregates, as opposed to single molecules, and involves a variety of inorganic-organic species. Some of the currently re- ported synthetic routes to silica-surfactant mesophases which are precursors to mesoporous silicates are sum- marized in Table 1. It appears that there seems to be effectively two independent routes to the synthesis these materials. The synthetic approach originally reported by Beck, Vartuli, and co-workers 2 led to the development and investigation of the M41S family of mesoporous molec- ular sieves. 3-6 The three phases obtained from the M41S synthesis methods are hexagonal (MCM-41), cubic (MCM-48), and lamellar (MCM-50), whose microscopy and diffraction data are reminiscent of surfactant/water binary systems. This has led to many extensive in situ NMR studies of the systems, 7-9 and prompted Beck et al. to initially propose a liquid crystal template (LCT) mechanism. 3 Beck et al. describes the process as the formation of liquid crystals by the surfactants, followed by condensation of the silicate framework around the † University of Oxford. ‡ Department of Applied Chemistry, Waseda University. § Kagami Memorial Laboratory for Materials Science and Technol- ogy, Waseda University. (1) Cheetham, A. K.; Mellot, C. F. Chem. Mater. 1997, 9, 2269- 2279. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (3) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; Mccullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (4) Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W. J.; Schramm, S. E. Chem. 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