Enhancing the performance of a supported titanium epoxidation catalyst by modifying the active center

Significant effects on catalytic activity in the epoxidation of cyclohexene with alkyl hydroperoxides are observed when the surface of mesoporous silica (MCM-41) is modified with either Ge(IV) or Sn(IV) prior to the grafting of tetracoordinated Ti(IV). Titanium centers attached via oxygen to two silicons and one germanium show an increase in turnover frequency of up to 140% compared to centers attached via three Si-O bonds to a purely siliceous MCM-41. The tin-modified catalyst, in contrast, shows poor activity. The environments of the Ti, Ge, and Sn centers were probed directly by in situ X-ray absorption spectroscopy and also by ex situ transmission electron microscopy. The germanium-modified silica contains evenly distributed, anchored tetrahedral Ge(IV) species, whereas the tin-containing material forms extraframework particles of SnO(2).