Influence of Surface Modification of Ti−SBA15 Catalysts on the Epoxidation Mechanism for Cyclohexene with Aqueous Hydrogen Peroxide

• Published in: Langmuir Vol. 21; no. 21; pp. 9576 - 9583
• Main Authors: Brutchey, Richard L, Ruddy, Daniel A, Andersen, Lars K, Tilley, T. Don
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 11.10.2005
• Subjects: Catalysis; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Porous materials; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Modification; Hydrogen peroxide; Hydrocarbon; Support; In situ; Active site; Cycloalkene; Oxides; Modified catalyst; Selectivity; Porous material; Precursor; Mechanism; Mesoporosity; Infrared spectrometry; Heterogeneous catalysis; Epoxidation; Oxidation; Ethylenic compound; Electrons; Reflectance
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la051182j

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, Me2N−SiMe2(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 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)3Ti(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.