Influence of surface modification of Ti-SBA15 catalysts on the epoxidation mechanism for cyclohexene with aqueous hydrogen peroxide

The thermolytic molecular precursor method was used to introduce site-isolated Ti(IV) centers onto the surface of a mesoporous SBA15 support. The resulting surface Si-OH/Ti-OH sites of the Ti-SBA15 catalysts were modified with a series of (NN-dimethylamino)trialkylsilanes, Me2N-SiMe2(R) (where R = Me, Bu-n, or (n)Oc). Compared with the unmodified catalysts, the surface-modified catalysts are more active in the oxidation of cyclohexene with H2O2 and exhibit a significantly higher selectivity (up to 58%) for cyclohexene oxide formation (vs allylic oxidation products). In situ Fourier transform infrared (FTIR) and diffuse reflectance UV visible (DRUV-vis) spectroscopies were used to probe this phenomenon, and it was determined that active sites with capped titanol centers, (SiOsurface)(3)Ti(OSiR3), likely undergo Ti-OOH formation upon addition of H2O2 in a manner analogous to that for active sites of the type (SiOsurface)3TiOH. On the basis of the observation of similar Ti-OOH intermediates for both species, the electron-withdrawing effects on the Ti(IV) active site, resulting from the surface modification, are likely responsible for the observed increase in selectivity.