The use of H2O2 over titanium-grafted mesoporous silica catalysts: a step further towards sustainable epoxidation

The epoxidation of cyclohexene with aqueous hydrogen peroxide over mesostructured Ti(Cp)2Cl2-grafted silica catalysts is described for the first time. Three kinds of Ti-containing systems with different textural properties have been employed: Ti/SiO2, Ti/MCM-41 and Ti/MCM-48. A minimal local H2O2 concentration in the surroundings of Ti sites, obtained by a controlled dropwise addition of aqueous H2O2 (4.17 mmol H2O2 h-1 gcat -1), is crucial for an effective and highly selective epoxidation. Excellent selectivity (>98%) in cyclohexene epoxide is obtained at the end of the slow H2O2 addition in acetonitrile. Higher yields in epoxide are found over Ti/MCM-48 than over Ti/MCM-41 or Ti/SiO2 thanks to better isolation, dispersion and stability of Ti(IV) sites.