Titania on Silica: A Case Study for Cyclohexene Epoxidation toward Flow Applications

We report the synthesis of a titanium-based catalyst supported on silica pellets and its use as a heterogeneous catalyst for the epoxidation of cyclohexene. Commercially available SiO2 trilobe-shaped pellets (NorPro) were modified in a two-step procedure including Ti species grafting followed by surface passivation using hexamethyldisilazane (HMDS). The amount of active metal could be controlled via variation of the quantity of the titanium precursor. The materials were characterized via EDX, UV–vis, XPS, and 29Si solid-state NMR. All investigations confirmed the presence of dispersed titanium(IV) species grafted as well on the outer as on the inner surface of the silica pellets, mainly in a tetrahedral environment. The successful passivation via trimethylsilylation was monitored via solid-state 29Si NMR spectroscopy. The use of the materials as heterogeneous epoxidation catalyst showed that the amount of immobilized Ti(IV) species has a strong effect on the catalytic properties of the materials. In particular, a material displaying a Si/Ti molar ratio of 33 offered a good compromise between optimized Ti content, catalytic activity/selectivity, and reusability. A kinetic study of the epoxidation reaction was carried out in an environmentally friendly solvent, cyclopentyl methyl ether. The apparent kinetics were successfully fitted by an Eley–Rideal model describing product inhibition with negligible adsorption of cyclohexene.