Photocatalytic Oxygenation of Cyclohexane on Titanium Dioxide Suspensions:  Effect of the Solvent and of Oxygen

The photoassisted monooxygenation of C 6 H 12 to C 6 H 10 O and C 6 H 11 OH by molecular oxygen has been studied on TiO 2 powder catalyst dispersed in neat C 6 H 12 and in C 6 H 12 /CH 2 Cl 2 mixtures. The composition of the mixed solvent has a strong influence on the selectivity of the process: an increase in the content of CH 2 Cl 2 brings about both an enhancement in the rate of formation of mono-oxygenated products and a decrease in the production of CO 2 . 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 (C 6 H 11 OH and radicals) as the solvent composition is varied from pure C 6 H 12 to mixtures of it with increasing amounts of CH 2 Cl 2 . The results of experiments with different O 2 partial pressures are reported. The process is unaffected for O 2 partial pressures > 200 Torr. For 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(NO 2 ) 4 as the electron scavenger, the formation of C 6 H 10 O decreases markedly while that of C 6 H 11 OH 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.