Selective photo(catalytic)-oxidation of cyclohexane: Effect of wavelength and TiO2 structure on product yields

The liquid-phase photolytic oxidation of cyclohexane was studied and compared with photocatalytic oxidation over TiO2 with varying wavelengths of light exposure, slurry densities, and sources and pretreatments of catalyst material. Photolytic oxidation at λ<275 nmλ<275 nm (i.e., in the absence of catalyst) yielded a high selectivity to cyclohexanol (>85%). By adding a TiO2 catalyst to cyclohexane exposed at λ<275 nmλ<275 nm, the selectivity shifted to the ketone, with the amount of catalyst added determining the obtained cyclohexanone:cyclohexanol ratio. When a combination of a TiO2 catalyst and a Pyrex reactor was used (the latter preventing photolytic formation of cyclohexanol), an almost complete selectivity to cyclohexanone was obtained (>95%). The activity toward ketone formation was affected by catalyst structure, with surface hydroxyl group density being the most important parameter. Based on the observed correlation between the hydroxyl group density and activity, as well as the observed negative effect of cyclohexanol addition on cyclohexanone production rate, a preliminary reaction mechanism is proposed involving the light-induced formation of surface cyclohexyl radicals, followed by formation of a peroxide intermediate and decomposition and desorption to cyclohexanone. Accumulation of cyclohexanol on the TiO2 surface is proposed to deteriorate the photocatalytic activity and to contribute to CO2 formation.