Highly active and selective Ti-incorporated porous silica catalysts derived from grafting of titanium(iv)acetylacetonate

Abstract

Metallosilicate catalysts with tetrahedrally coordinated transition metals covalently linked into silica frameworks are attractive because of their highly selective oxidation activities for a wide variety of hydrocarbons. Here, we report highly active and selective titanosilicate catalysts featuring an active titanium species with a uniform tetrahedral coordination state and homogeneous distribution derived from grafting Ti(IV)acetylacetonate (Ti(acac)₄) onto the surfaces of porous silicas, such as mesoporous silicas MCM-41 and SBA-15, and nanoporous silica HUS-6. Ti-grafted porous silicas showed excellent catalytic performances in the epoxidation of cyclohexene with tert-butyl hydroperoxide as an oxidant. The catalytic turnover number was approximately 8 times higher than that of Ti-containing mesoporous silica MCM-41 prepared by conventional one-pot hydrothermal synthesis. The Ti-grafted MCM-41 with a higher Ti content exhibited higher catalytic activity for epoxidation, with a turnover frequency of 346 h⁻¹ and reaction rate of 159 mmol g⁻¹ h⁻¹. We also found that the Ti-grafted porous silicas were effective for the epoxidation of a plant-derived olefin, limonene, using tert-butyl hydroperoxide as an oxidant.