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 Me2N-SiMe3 to give TiMecapSBA15. After low temperature treatment in oxygen, the resulting Ti−OH sites of TiMecapSBA15-O2 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 H2O2) 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 TiMecapSBA15-O2, without a significant drop in selectivity for the epoxide product, 1,2-epoxyoctane, when TBHP was used as the oxidant. With aqueous H2O2 as the oxidant, the phenol-treated materials exhibited 1-octene conversions that are 15 to 50% greater than those observed for TiMecapSBA15-O2, and an increased selectivity for 1,2-epoxyoctane of 10 to 30%. Additionally, the efficiency of H2O2 usage, as monitored via 1H NMR spectroscopy, increased by a factor of 2 to three for catalysts modified with phenol and pentafluorophenol, with respect to the efficiency observed over TiMecapSBA15-O2. 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 TiMecapSBA15-O2 when H2O2 was used as the oxidant. After treatment of TiMecapSBA15-O2 with acetic acid, the H2O2 efficiency decreased by a factor of 2 for the epoxidation of 1-octene with H2O2.