Titania on Silica:\nA Case Study for Cyclohexene Epoxidation\ntoward Flow Applications

We report the synthesis of a titanium-based catalyst\nsupported\non silica pellets and its use as a heterogeneous catalyst for the\nepoxidation of cyclohexene. Commercially available SiO<sub>2</sub> trilobe-shaped pellets (NorPro) were modified in a two-step procedure\nincluding Ti species grafting followed by surface passivation using\nhexamethyldisilazane (HMDS). The amount of active metal could be controlled\nvia variation of the quantity of the titanium precursor. The materials\nwere characterized via EDX, UV–vis, XPS, and <sup>29</sup>Si\nsolid-state NMR. All investigations confirmed the presence of dispersed\ntitanium(IV) species grafted as well on the outer as on the inner\nsurface of the silica pellets, mainly in a tetrahedral environment.\nThe successful passivation via trimethylsilylation was monitored via\nsolid-state <sup>29</sup>Si NMR spectroscopy. The use of the materials\nas heterogeneous epoxidation catalyst showed that the amount of immobilized\nTi(IV) species has a strong effect on the catalytic properties of\nthe materials. In particular, a material displaying a Si/Ti molar\nratio of 33 offered a good compromise between optimized Ti content,\ncatalytic activity/selectivity, and reusability. A kinetic study of\nthe epoxidation reaction was carried out in an environmentally friendly\nsolvent, cyclopentyl methyl ether. The apparent kinetics were successfully\nfitted by an Eley–Rideal model describing product inhibition\nwith negligible adsorption of cyclohexene.