The surface of a Ti/SiO2 catalyst was silylated using hexamethyldisilazane (HMDS) and tetramethyldisilazane (TMDS) as silylating reagents in vapor phase. The silylation of silanol (Si–OH) on the catalysts was confirmed by diffuse reflectance UV-Vis, DRIFT spectroscopy and solid-state 29Si MAS NMR techniques. Silylation with TMDS improves the catalytic performance of Ti/SiO2 catalyst more significantly than with HMDS and this is mainly due to the steric bulk effects of trimethylsilyl groups from HMDS as compared to dimethylsilyl groups from TMDS. The silylation degree with TMDS was found to be higher than with HMDS and no changes in the silylation degree with TMDS was observed for silylation times longer than 2 h. Despite the silylation route employed, the silylated surface is hydrophobic which enhances not only in hydrogen peroxide efficiency but also and in selectivity to epoxide in the epoxidation of alkenes with H2O2. These effects are more evident when the hydrogen peroxide concentration is higher and particularly for the catalyst modified with TMDS.
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机译：使用六甲基二硅氮烷（HMDS）和四甲基二硅氮烷（TMDS）作为气相甲硅烷基化试剂将Ti / SiO2催化剂的表面进行甲硅烷基化。通过漫反射UV-Vis，DRIFT光谱和固态29Si MAS NMR技术证实了硅烷醇（Si-OH）在催化剂上的甲硅烷基化。与TMDS进行甲硅烷基化反应相比，HMDS可以显着改善Ti / SiO2催化剂的催化性能，这主要是由于HMDS中的三甲基甲硅烷基与TMDS中的二甲基甲硅烷基相比具有空间体积效应。发现TMDS的甲硅烷基化程度高于HMDS，并且对于大于2小时的甲硅烷基化时间，未观察到TMDS的甲硅烷基化程度变化。尽管采用了甲硅烷基化途径，但是甲硅烷基化的表面是疏水的，这不仅提高了过氧化氢的效率，而且还提高了烯烃与H 2 O 2环氧化时对环氧化物的选择性。当过氧化氢浓度较高时，特别是对于用TMDS改性的催化剂，这些效果更明显。
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