Nanometric monodispersed titanium oxide particles on mesoporous silica: synthesis, characterization, and catalytic activity in oxidation reactions in the liquid phase

摘要

Subnanometric monodispersed titanium oxide particles have been dispersed on the surface of a large mesopore SBA-15 silica. TiSBA-15 materials were obtained by reaction between a hexanuclear titanium oxo cluster and a calcined SBA-15 silica in solution. Solids with various Ti loading have been prepared and characterized by chemical analysis, X-ray diffraction, nitrogen adsorption, and UV-visible spectroscopy. Titanium oxide particles containing 6 Ti octahedra show a UV-visible adsorption band at ca. 235 nm, close to those usually observed on TiMCM-41 or TiHMS materials obtained by direct synthesis. The catalytic performance of TiSBA compounds has been evaluated in a series of oxidation reaction with H_2O_2 or TBHP as oxygen donors and compared with that of TiHMS materials obtained by direct synthesis. Data indicate that TiO_2 nanoparticles are active and selective in epoxidation of cyclohexene and in oxidation of aniline and aromatic molecules. Activities and selectivities suggest that isolated tetrahedrally coordinated Ti species are not necessarily the only active species for these reactions. Moreover, results raise the problem of the validity of such oxidation reactions as a test for the determination of the coordination state of Ti species.