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

Abstract

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 H 2O 2 as the oxidant due to clustering of Ti(IV) centers under reaction conditions and the catalyzed disproportionation of H 2O 2 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.