Nature of Active Sites in Sol–Gel TiO2–SiO2 Epoxidation Catalysts

A series of titania–silica aerogels with 0–100 wt% TiO2 content were synthesized and characterized by N2 physisorption, DRIFT, UV-Vis, XPS, and 29Si CP/MAS NMR analysis. It is shown that kinetic analysis of the epoxidation of 2-cyclohexene-1-ol (1) with TBHP is an informative test reaction providing insight in the nature of active sites. The surface area, pore volume, hydrophobicity, and relative abundance of Ti–O–Si linkages in the aerogels decreased with increasing Ti/Si ratio. Parallel to these changes, the initial rate of epoxide formation per Ti site (TOF) and the epoxide selectivity decreased but the productivity of the catalysts went through a maximum at 10 wt% TiO2. We propose that due to kinetic effects in the sol–gel synthesis the whole range of active Ti sites may be present in the mixed oxides, spanning from tetrahedral Ti isolated by four SiO groups to octahedral Ti surrounded by six TiO groups in titania nanodomains. Ether formation from 1 was catalyzed by Brønsted sites present only on high titania-containing aerogels. Oligomerization was a major side reaction on all catalysts including Ti-free silica. Si-free titania was the most active in allylic oxidation of 1 to cyclohexenone. Silylation, or amine (Me2BuN) addition to the reaction mixture, eliminated ether formation and suppressed oligomerization.