Niobium-silica catalysts for the selective epoxidation of cyclic alkenes: the generation of the active site by grafting niobocene dichloride

Niobium-containing silica materials obtained by deposition via liquid-phase grafting or dry impregnation of niobocene(IV) dichloride are active and selective catalysts in the epoxidation of alkenes in the presence of aqueous hydrogen peroxide. The generation of the catalytically-active Nb species was followed step-by-step, and investigated using a combined DR-UV-Vis, NIR, Raman, XRD, XANES and EXAFS analyses. At the end of the grafting procedure, the nature of the surface active species can be described as an oxo-Nb(V) site, tripodally grafted onto the silica surface in close proximity to other Nb(V) centres. The liquid-phase methodology provides a better dispersion of the metal sites onto the siliceous support than the dry-impregnation approach. The niobium-silica catalysts were then tested in the epoxidation of cyclohexene and 1-methylcyclohexene, as model substrates.