Titania−Silica Materials from the Molecular Precursor Ti[OSi(O<i>^t</i>Bu)₃]₄:  Selective Epoxidation Catalysts

Abstract

The tris( tert -butoxy)siloxy complex Ti[OSi(O t Bu) 3 ] 4 ( 1 ) was pyrolytically converted at low temperatures to the theoretical yield of homogeneous TiO 2 ·4SiO 2 materials. 1 also serves as a soluble model for isolated, tetrahedral titanium atoms in a silica matrix. The solid-state transformation of 1 to TiO 2 ·4SiO 2 affords a low-surface-area material ( 2a, 22 m 2 g - 1 for samples heated to 500 °C) consisting of roughly spherical particles with an average diameter of ca. 25 nm (by transmission electron microscopy). The solution-phase thermolysis of 1 affords a xerogel ( 2b ) consisting of smaller primary particles (≤ca. 5 nm) and possessing a much higher surface area (552 m 2 g - 1 for samples heated to 500 °C). Acid catalysis of the pyrolytic conversion in solution affords a xerogel ( 2c ) with a lower surface area (399 m 2 g - 1 for samples heated to 500 °C). Supercritical drying of the “wet-gel” in CO 2 affords an aerogel ( 2d ) with a morphology similar to that of the xerogel and a slightly higher surface area (677 m 2 g - 1 for samples heated to 500 °C). The TiO 2 ·4SiO 2 materials are amorphous as initially formed, and subsequent crystallizations of anatase ( 2a, 2b ), rutile, and cristobalite ( 2a ) occur at relatively high temperatures. 1 is a homogeneous catalyst for the selective epoxidation of cyclohexene to cyclohexene oxide (64% yield; 20 turnovers after 2 h) using cumene hydroperoxide (CHP) as the limiting reagent. 2a − d are heterogeneous catalysts that produce 6% ( 2a )−49% ( 2d ) yields of cyclohexene oxide after 2 h (with 0.05 g of catalyst). Supporting 1 on silica affords a highly active epoxidation catalyst that provides yields of 94% cyclohexene oxide after 2 h (with a standard amount of 0.05 g of catalyst). The molecular route to homogeneous titania−silica materials described here represents an alternative to the sol−gel method, affording materials with surface areas comparable to those of related aerogels and moderate-to-excellent activities for the epoxidation of cyclohexene.