Titania-silica materials from the molecular precursor Ti[OSi(OtBu)3]4:: Selective epoxidation catalysts

The tris(tert-butoxy)siloxy complex Ti[OSi((OBu)-Bu-t)(3)](4) (1) was pyrolytically converted at low temperatures to the theoretical yield of homogeneous TiO2.4SiO(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 TiO2.4SiO(2) affords a low-surface-area material (2a, 22 m(2) g(-1) for samples heated to 500 degrees C) consisting of roughly spherical particles with an average diameter of ca. 25 nm (by transmission electronmicroscopy). 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 m2 g(-1) far samples heated to 500 degrees 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 degrees C). Supercritical drying of the "wet-gel" in CO2 affords an aerogel (2d) with a morphology similar to that of the xerogel and a slightly higher surface area (677 m2 g(-1) for samples heated to 500 degrees C). The TiO2.4SiO(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 h) using-cumene hydroperoxide (CHP) as the limiting reagent. 29-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.