Niobium metallocenes deposited onto mesoporous silica via dry impregnation as catalysts for selective epoxidation of alkenes

A series of niobium catalysts for the selective epoxidation was synthesised by post-synthesis modification of a commercial silica, starting from niobocene dichloride through solventless organometallic precursor dry impregnation (OM-DI) or conventional liquid-phase grafting technique. OM-DI showed to be cheaper, more versatile, less time-consuming and avoided the use of environmentally unfriendly chlorinated solvents. Nb–SiO2 catalysts displayed an excellent performance in the epoxidation of limonene, using aqueous hydrogen peroxide as oxidant. Niobium–silica catalysts were obtained via OM-DI for the first time in this occasion. They showed conversions up to 78% and chemoselectivity to epoxide of 98%. An unexpected regioselectivity to exocyclic epoxide was also observed.