Ti(IV) Catalytic Centers Grafted on Different Siliceous Materials: Spectroscopic and Catalytic Study

A detailed spectroscopic study was carried out on Ti(IV)-containing catalysts obtained by grafting titanocene dichloride on silica supports with different morphological features. The surface acidity and the local coordination and accessibility of Ti(IV) active centers in Ti(IV)-grafted materials were studied by FTIR and diffuse reflectance (DR) UV-vis-NIR spectroscopies supplemented by the use of probe molecules. DR UV-vis- NIR spectroscopy was also used to follow the formation of Ti(IV) catalytic sites and the removal of cyclopentadienyl moiety during thermal treatment. The use of CD3CN and CO as molecular probes has provided complementary information on the accessibility and coordination of the Ti(IV) centers. In particular, CO adsorption performed at 100 K has evidenced the presence of Ti-OH groups and was helpful in detecting different coordination environments and connectivities of Ti(IV) active centers. Ti(IV)-grafted materials were tested in the epoxidation reaction of limonene using both tert-butyl hydroperoxide (TBHP) and H2O2 as oxidants. The interaction of TBHP or H2O2 with Ti(IV) active sites was investigated by DR UV-vis spectroscopy. This study has clarified that the use of H2O2 leads to a rapid and irreversible deactivation of the grafted Ti(IV) active sites; in fact, when H2O2 was used as oxidant no production of limonene oxide was detected.