(216b) Hollow, Mesoporous Titano-Silicate Micro-Spheres As Heterogeneous Catalysts
A major challenge in the synthesis of titano-silicates is chemical non-uniformity in the framework and irregularity of particle size. A high concentration of isolated Ti sites is desired for applications as heterogeneous catalysts for the selective oxidation of hydrocarbons. However, at high Ti doping concentrations, Ti-O-Ti domains in the framework are often observed, instead of Ti-O-Si. Also, the formation of hollow microspheres instead of solid particles is of interest to eliminate poorly accessible material inside the particles, which could cause low catalytic effectiveness.
Li and Coppens have synthesized stable, hollow, mesoporous titanium-silica micro-spheres, in which the mesopores are templated using a surfactant, and the hollow core is formed by carrying out the synthesis in a rapidly stirred oil/water/oil emulsion. However, the particles had a broad size distribution, and shell thickness was only controlled within a narrow range. Also, while Ti sites were quite well isolated at low concentrations2, some of the Ti was still present as Ti-O-Ti microdomains at higher Ti concentrations.
Further experimental work is required to improve the structure, and associated catalytic performance, by altering the synthesis conditions. We use a Doehlert matrix approach, which is a second-order statistical technique, widely utilized as a chemometric tool in analytical sciences, to discover the most informative experimental conditions. Using such a rigorous statistical approach drastically reduces the required number of experiments. The method allows experimenters to achieve uniformity in space filling and flexibility in defining the reaction boundaries. Nitrogen adsorption/desorption, SEM, TEM, FTIR, Raman and UV-Vis absorption are used to characterize the structure of the materials.
Subsequently, catalytic characterization of the microspheres is carried out by extending the approach of Baca et al., which involves the epoxidation of cyclohexene with tert-butylhydroperoxide. This reaction is employed to validate the improved materials properties obtained via the statistically guided synthesis optimization. The relationship between the controlled hierarchical structure and chemical composition, on the one hand, and catalytic efficiency, on the other hand, is discussed.
 W. Li and M-O. Coppens, “Synthesis and characterization of stable hollow Ti-silica microspheres with a mesoporous shell”, Chem. Mater. 2005, 17, 2241-2246
 M. Baca, W. J. Li, P. Du, G. Mul, J.A. Moulijn, and M.-O. Coppens, “Catalytic characterization of mesoporous Ti–Silica hollow spheres”, Catalysis Letters 2006, 109, 207-210