Abstract
Various silylated Ti-TUD-1 samples were synthesized by treating Ti-TUD-1 samples of differing Ti loadings with hexamethyldisilazane (HMDS). The silylation of silanol (Si-OH) and titanol groups (Ti-OH) on the mesoporous silica was confirmed by FTIR spectroscopy, hydrophobicity (TGA), and NH3-TPD measurements. The relatively unreactive olefins, such as 1-octene and p-tert-butylphenylallyl ether, were epoxidized using both silylated and unsilylated Ti-TUD-1 samples with different Ti loadings in the range of 1-10 wt% Ti. The catalytic epoxidation reactions were performed with commercial solutions of organic hydroperoxides as oxidants without the addition of any other organic solvent. The selectivities in peroxide utilization were found to depend on the type of olefin, oxidant, and catalyst. Using unsilylated TiTUD-1, low selectivity to epoxide was achieved for cumyl hydroperoxide (CHP), which is attributed to its facile acid-catalyzed decomposition to phenol and acetone under the reaction conditions. However, both t-butyl hydroperoxide (TBHP) and cumyl hydroperoxide yielded high selectivity to epoxide (based on converted hydroperoxide) with silylated Ti-TUD-1 samples. The selectivity to epoxide was lower for the electron-poor olefin p-tert-butylphenylallyl ether than for simple 1-octene. The experiments with CHP as the oxidant were used as a diagnostic tool to demonstrate that the enhanced epoxidation activity and selectivity of the silylated samples with TBHP can be credited to two effects of silylation. The elimination of protic Ti-OH sites of the Ti-TUD-1 catalyst resulted in a significant reduction of the competing acid-catalyzed decomposition of the peroxide, whereas the enhanced hydrophobicity of the catalyst surface enhanced the accommodation of the apolar olefinic substrate. (C) 2008 Elsevier Inc. All rights reserved.