Ti-POSS covalently immobilized onto mesoporous silica. A model for active sites in heterogeneous catalytic epoxidation

A functionalized Ti-containing polyhedral oligomeric silsesquioxane was anchored on the surface of an ordered SBA-15 and a non-ordered amorphous mesoporous silica. The physico-chemical characterization of the textural, thermal and electronic properties of the final anchored materials led to the evaluation of the distribution of Ti-POSS moieties on the silica surface, of the oxidation state and of the coordination geometry of the titanium centres. It was discovered that Ti-POSS are accommodated mainly as dinuclear dimeric species on the external surface of ordered mesoporous silica or in the large mesopores of nonordered silica supports. Epoxidation tests on unsaturated terpenes (limonene, carveol and a-pinene) with TBHP as oxidant were performed to monitor the activity and accessibility of Ti(IV) sites in the anchored samples and compared with conventional titanocene-grafted Ti/SBA-15 and Ti/SiO2 systems. The different chemoselectivity recorded over anchored catalysts, with respect to the grafted ones, can be a potential diagnostic tool to shed light on the mechanism or the molecular intermediates occurring at the surface during the oxidation reaction.