Influence of the textural properties of supports on the behaviour of titanium-supported amorphous silica epoxidation catalysts

A series of titanium-silica catalysts was prepared by grafting a titanium isopropoxide precursor on amorphous silica substrates with BET areas of 162-709 m2 g−1 and pore volumes of 0.5-1.4 mL g−1. The grafting process, consisting of hydrolysis of the Ti precursor by the hydroxyl groups of the silica surface, yielded catalysts containing Ti loadings of 0.65-2.37 wt%. The textural properties of the Ti/SiO2 catalysts were evaluated from the nitrogen adsorption-desorption isotherms, and their surface species were studied by ultraviolet-visible (UV-vis) spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS) techniques. These catalysts were tested in the liquid phase epoxidation of oct-1-ene with hydrogen peroxide as a model reaction. All catalysts showed very high (95-98%) hydrogen peroxide consumption at 353 K after 1 h reaction time, but selectivity to the epoxide was strongly dependent on the pore volume of the silica substrate. Under these reaction conditions, an epoxide selectivity as high as 90-93% was reached on the catalysts with the largest pore volumes (1.4 mL g−1). Spectroscopic techniques also revealed that TiIV ions coordinated tetrahedrally by oxide ions are essential to obtaining highly selective catalysts in the reaction.