Experimental and theoretical studies on structure-reactivity relationships of titanium-modified silicas in the hydrogen peroxide-promoted oxidation of cyclohexene

Theoretical calculations demonstrate that the presence of ligands with different basicity modifies the Lewis acidity (expressed both as the LUMO energy and the enthalpy of coordination of a NH3 molecule) of the active centers in silica-supported titanium catalysts. Several catalysts containing different organic ligands bonded to the titanium centers have been prepared, characterized, and tested in the benchmark reaction of cyclohexene with hydrogen peroxide. Both the activity and the selectivities of the catalysts depend on the relative basicity of the ligands and their variation is proportional to the calculated Lewis acidity, expressed as the LUMO energy. With more basic ligands, hydrolysis of the epoxide to diol occurs to a lesser extent at the expense of the catalytic activity and the epoxidation/allylic oxidation selectivity, showing the coincidence of the active sites for epoxidation and hydrolysis reactions