title: Influence of surface modification of Ti-SBA15 catalysts on the epoxidation mechanism for cyclohexene with aqueous hydrogen peroxide.
abstract: 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 (N,N-dimethylamino)trialkylsilanes, Me(2)N-SiMe(2)(R) (where R = Me, (n)Bu, or (n)Oc). Compared with the unmodified catalysts, the surface-modified catalysts are more active in the oxidation of cyclohexene with H(2)O(2) 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, (SiO(surface))(3)Ti(OSiR(3)), likely undergo Ti-OOH formation upon addition of H(2)O(2) in a manner analogous to that for active sites of the type (SiO(surface))(3)TiOH. 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.
authors.affiliation: Department of Chemistry, University of California-Berkeley, Berkeley, CA 94720-1460, USA.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Brutchey
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Ruddy
authors.forename: Lars K
authors.initials: LK
authors.lastname: Andersen
authors.forename: T Don
authors.initials: TD
authors.lastname: Tilley