Photocatalytic activity of MCM-organized TiO2 materials in the oxygenation of cyclohexane with molecular oxygen

The photo-oxygenation of cyclohexane by molecular oxygen has been investigated on two mesoporous TiO2 materials, which have been prepared using colloidal nanoparticles as building blocks. One of the structured systems (mpTiO2-50) is a mixture of 50% TiO2 and 50% SiO2; the second one (mpTiO2-100) is constituted by 100% of TiO2. Both mpTiO2-100 and mpTiO2-50 can induce cyclohexane photooxidation in repeated cycles, but with the former the yield in cyclohexanone is higher and only traces of cyclohexanol are observed. The results of experiments with different incident light intensities are reported: contrary to mpTiO2-50, the selectivity of mpTiO2-100 towards cyclohexanone is not significantly affected by the photonic flux. Based on the substrate conversion rates, incident photonic flux effects, photoluminescence and EPR spectra of the mesoporous materials, we infer that the photoreactivity of mpTiO2-100 and mpTiO2-50 is mainly controlled by textural effects. In particular, we propose that the inter-particle electron mobility that characterizes the mpTiO2-100 material, which is constituted exclusively of TiO2 nanoparticles, entails a better utilization of electron traps for converting the photogenerated cyclohexyl-peroxide radicals to cyclohexanone.