Selective Photooxidation of Cyclohexane to KA-Oil by Mo-Doped Mesoporous TiO2 and the Role of Electron-Shuttling Quinones

Selective oxidation of cyclohexane (CHA) to KA-oil (cyclohexanol/cyclohexanone) is an important reaction in the chemical industry, which is constrained by high energy consumption and low selectivity. In this study, Mo-doped mesoporous TiO2 (Mo-MT) has been successfully synthesized via calcinating MIL-125(Ti) doped with molybdenum acetylacetonate. A series of characterizations and density functional theory (DFT) calculations reveal that the doped Mo not only decreases the band gap from 3.02 eV (MT) to 2.71 eV (Mo-MT-2) and enhances efficiencies in charge separation and photogenerated carrier transfer but also directly reduces O2 to Mo-η2-O2 peroxo on Mo-MT-2, achieving a 3.2 times higher yield of KA-oil (62.7 μmol) than that by P25 and a high selectivity toward KA-oil of 99.9% under visible-light irradiation. Meanwhile, electron shuttling of benzoquinone facilitates electron transfer from CHA to O2, thus significantly enhancing the KA-oil yield from 62.7 to 431.0 μmol. Diffuse reflectance infrared Fourier transform spectroscopy and electron paramagnetic resonance analyses confirm the electron-shuttling roles of benzoquinone in H-abstraction and H-transfer during selective photooxidation of CHA by Mo-MT-2. DFT calculations further reveal that the benzoquinone-hydroquinone/semiquinone-benzoquinone cycle shuttles electrons from CHA to Mo-η2-O2 peroxo on the surface of Mo-MT-2, thus facilitating H-abstraction from CHA to form C6H11·, coupled with the formation of H2O2. The produced C6H11· then eventually reacts with O2 to form KA-oil. This study enriches our understanding of the importance of peroxo species and electron shuttles in selective photooxidation of C(sp3)–H.