Titanium-silica catalyst derived from defined metallic titanium cluster precursor: Synthesis and catalytic properties in selective oxidations

A class of titanium-grafted mesoporous silica catalysts has been designed and prepared starting from molecularly defined metal clusters. The organosol mixture of zerovalent Ti13 clusters was impregnated onto the surface of ordered mesoporous silica molecular sieves (MCM-41 and MMM-2) and, after hightemperature calcination, an evenly dispersed non-single-site Ti(IV)nOx-like silica-supported catalyst was obtained. The catalytic solids, fully characterized by microscopic, spectroscopic and porosimetric techniques, showed standard performance in the liquid-phase epoxidation of a cyclic alkene, as limonene, but remarkably high selectivity values in the oxidative carboxylation of styrene, with tert-butylhydroperoxide and carbon dioxide in the presence of tetrabutylammonium bromide as a cocatalyst. Unprecedented high yields, up to 67%, in styrene carbonate were achieved after 24 h, under solvent-free conditions. The catalysts displayed also a noteworthy stability of the performance to repeated recovery and reuse cycles.