Peroxide oxidation of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde over titanium(IV)-functionalized mesostructured silica

Ti(IV)-functionalized derivatives of mesostructured HMS and MCM-41 silica with 3D wormhole and 1D hexagonal framework structures, respectively, were examined as catalysts for the oxidation of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde using tert-butylhydroperoxide (TBHP) and aqueous hydrogen peroxide as oxidizing agents in acetonitrile solution. In an effort to achieve optimal site-isolation of reactive Ti(IV) centers, functionalization was carried out through the grafting of titanocene dichloride to the framework walls of the pre-assembled mesostructures, followed by calcination. Clear evidence for the undesirable clustering of Ti(IV) centers with increasing Ti(IV) loading was obtained by UV–Vis spectroscopy. Thus, the Ti(IV)-loading in the silica framework was limited to values below 8 mol%. Maximal 23.6% and 21.8% conversions of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde with >99% selectivity were observed for 4.8% Ti-HMS and 4.8% Ti-MCM-41 catalysts using TBHP as the oxidant. Substantially lower conversions were obtained with H2O2 as the oxidant due to clustering of Ti(IV) centers under reaction conditions and the catalyzed disproportionation of H2O2 by those clusters. A 2.0% Ti-HMS derivative prepared by direct assembly from tetraethylorthosilicate and tetraisopropylorthotitanate, denoted Ti-HMS(DS), was less active than Ti(IV)-functionalized mesostructures prepared by post-assembly grafting reaction with titanocene dichloride. Direct assembly places too much Ti(IV) at inaccessible positions within the framework walls, whereas grafting ensures optimal access to site-isolated Ti(IV) sites at the pore surfaces.