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

The tris(tert-butoxy)siloxy complex Ti[OSi((OBu)-Bu-t)(3)](4) (1) was pyrol ytically converted at low temperatures to the theoretical yield of homogene ous TiO2.4SiO(2) materials. 1 also serves as a soluble model for isolated, tetrahedral titanium atoms in a silica matrix. The solid-state transformati on 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 pa rticles with an average diameter of ca. 25 nm (by transmission electronmicr oscopy). The solution-phase thermolysis of 1 affords a xerogel (2b) consist ing of smaller primary particles (ca. 5 nm) and possessing a much higher su rface area (552 m2 g(-1) far samples heated to 500 degrees C). Acid catalys is of the pyrolytic conversion in solution affords a xerogel (2c) with a lo wer surface area (399 m(2) g(-1) for samples heated to 500 degrees C). Supe rcritical drying of the "wet-gel" in CO2 affords an aerogel (2d) with a mor phology 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) materia ls are amorphous as initially formed, and subsequent crystallizations of an atase (2a, 2b), rutile, and cristobalite (2a) occur at relatively high temp eratures. 1 is a homogeneous catalyst for the selective epoxidation of cycl ohexene to cyclohexene oxide (64% yield; 20 turnovers after h) using-cumene hydroperoxide (CHP) as the limiting reagent. 29-d are heterogeneous cataly sts that produce 6% (2a)-49% (2d) yields of cyclohexene oxide after 2 h (wi th 0.05 g of catalyst). Supporting 1 on silica affords a highly active epox idation catalyst that provides yields of 94% cyclohexene oxide after 2 h (w ith a standard amount of 0.05 g of catalyst). The molecular route to homoge neous titania-silica materials described here represents an alternative to the sol-gel method, affording materials with surface areas comparable to th ose of related aerogels and moderate-to-excellent activities for the epoxid ation of cyclohexene.