Entrapment vs. immobilization of polyoxometalates into silica and titania aerogels: Application in heterogeneous oxidation catalysis

Abstract

Silica and titania aerogels were successfully used as immobilization and entrapment media for phosphomolybdic and phosphotungstic acids. The hybrid materials were investigated in the heterogeneous oxidation catalysis of phenyl methyl sulfide. The characterization of the hybrid catalysts confirmed the immobilization and entrapment of the polyoxometalates (POM) and their dispersion onto the surface of the solid support. The entrapment of these clusters was found to affect less the surface properties of the inorganic network compared to the immobilization process, thus allowing smoother access to the active sites of the polyoxometalate molecules. The catalytic activity of the POM clusters was widely maintained after immobilization and incorporation, where the chemical nature of the surface functional groups was found to affect the behavior of the catalyst. The fastest oxidation of methyl phenyl sulfide was observed for the reactions catalyzed by POM-incorporated silica aerogels, coupled with a high selectivity towards the methyl phenyl sulfoxide.