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.

Graphical abstractNiobium–silica mesoporous catalysts can be easily prepared via solventless dry impregnation from niobocene dichloride. They showed good activity, excellent chemoselectivity and unexpected regioselectivity in the liquid-phase epoxidation of limonene with aqueous hydrogen peroxide as oxidant.Figure optionsDownload full-size imageDownload high-quality image (52 K)Download as PowerPoint slideHighlights► Preparation of niobium–silica mesoporous catalysts via solventless dry impregnation. ► Good activity and excellent selectivity in liquid-phase epoxidation with aqueous H2O2. ► Good yield in limonene epoxide production with unexpected regioselectivity.