Enhancement of the Catalytic Activity of Titanium-Based Terminal Olefin Epoxidation Catalysts via Surface Modification with Functionalized Protic Molecules

Site-isolated Ti(IV) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. Prior to thermal treatment to generate Ti−OH sites, residual silanol groups were capped via reaction with Me 2 N-SiMe 3 to give TiMe cap SBA15 . After low temperature treatment in oxygen, the resulting Ti−OH sites of TiMe cap SBA15-O 2 were modified by reaction with a series of protic reagents: phenol, pentafluorophenol, acetic acid, and trifluoroacetic acid. The structure of the resulting TiSBA15 catalysts and the Ti(IV) epoxidation intermediates (formed upon treatment of Ti(IV) materials with TBHP or H 2 O 2 ) were probed using diffuse-reflectance UV−visible spectroscopy and infrared spectroscopy. A titanium-hydroperoxo species similar to that found in TS-1 is proposed for all catalysts. Samples modified with phenol and pentafluorophenol exhibited conversions of 1-octene that are 20 to 50% higher than those for TiMe cap SBA15-O 2, without a significant drop in selectivity for the epoxide product, 1,2-epoxyoctane, when TBHP was used as the oxidant. With aqueous H 2 O 2 as the oxidant, the phenol-treated materials exhibited 1-octene conversions that are 15 to 50% greater than those observed for TiMe cap SBA15-O 2, and an increased selectivity for 1,2-epoxyoctane of 10 to 30%. Additionally, the efficiency of H 2 O 2 usage, as monitored via 1 H NMR spectroscopy, increased by a factor of 2 to three for catalysts modified with phenol and pentafluorophenol, with respect to the efficiency observed over TiMe cap SBA15-O 2 . Catalysts modified with acetic acid and trifluoroacetic acid displayed decreased catalytic turnover numbers and epoxide selectivities when TBHP was used as the oxidant, but exhibited catalytic turnover numbers and epoxide selectivities similar to TiMe cap SBA15-O 2 when H 2 O 2 was used as the oxidant. After treatment of TiMe cap SBA15-O 2 with acetic acid, the H 2 O 2 efficiency decreased by a factor of 2 for the epoxidation of 1-octene with H 2 O 2 .