Photocatalytic oxygenation of cyclohexane on titanium dioxide suspensions: Effect of the solvent and of oxygen

The photoassisted monooxygenation of C6H12 to C6H10O and C6H11OH by molecular oxygen has been studied on TiO2 powder catalyst dispersed in neat C6H12 and in C6H12/CH2Cl2 mixtures. The composition of the mixed solvent has a strong influence on the selectivity of the process: an increase in the content of CH2Cl2 brings about both an enhancement in the rate of formation of mono-oxygenated products and a decrease in the production of CO2. At the same time, the alcohol to ketone ratio increases in the mixed solvent. An explanation of this behavior is proposed which is based on the observed decrease in the adsorption strength of intermediates (C6H11OH and radicals) as the solvent composition is varied from pure C6H12 to mixtures of it with increasing amounts of CH2Cl2. The results of experiments with different O-2 partial pressures are reported. The process is unaffected for O-2 partial pressures > 200 Torr. Far lower values the formation of dicyclohexyl becomes significant and reaches a maximum at a pO(2) of 60 Torr. In O-2-free media containing C(NO2)(4) as the electron scavenger, the formation of C6H10O decreases markedly while that of C6H11OH is essentially the same as that in oxygenated media. In the mechanism proposed, the reaction of cyclohexyl radicals with O-2 and/or activated oxygen species is the main route leading to the ketone.