Silica microspheres containing high density surface hydroxyl groups as efficient epoxidation catalysts

Abstract

Uniformly sized silica microspheres were synthesized by a hydrolysis–condensation method. The obtained material was etched with a mild aqueous potassium hydroxide solution for different periods of time to break their Si–O–Si bonds and increases the density of hydroxyl groups on their surfaces. The resulting materials were then used as transition metal-free catalysts for oxidation of olefins in the presence of hydrogen peroxide as a green oxidant. The materials were thoroughly characterized using various physicochemical techniques. These highly populated hydroxyl groups on the surface of silica microspheres were proven to be responsible for excellent conversion (up to 93%) and epoxide selectivity (up to 100%) for various olefins. Quantum mechanical calculations also corroborate the experimental findings. Furthermore, both experimental and theoretical studies show that tertiary silanols were present at the active sites of the catalyst surface and were responsible for olefin epoxidation.