Supercritical CO2 as sustainable solvent for selective photo-oxidation of monoterpenes with dioxo-Mo complex/TiO2-nanotubes

The selective oxidation of monoterpenes to the corresponding epoxides by the photo-stimulated Oxygen Atom Transfer (OAT) reaction was achieved in the supercritical CO2 medium (sc-CO2) under the action of molecular oxygen and the catalytic action of the dioxo-molybdenum complex anchored on TiO2 nanotubes, MoCl2O2(2,2′-bipyridine-4,4′-dicarboxylato)/TiO2-NT, [MoCl2O2(Bipy)/TiO2-NT]. It was observed that the supercritical medium enhanced the photo-stimulated oxygen transfer activity compared to the previously reported liquid-phase reaction conducted with acetonitrile as a solvent. The reaction rate in supercritical CO2 (sc-CO2) showed a pronounced dependence on pressure, reaching a maximum at about 14 MPa at 313.15 K. This fact can be attributed to the influence of pressure on the phase behavior of the reaction system, the diffusivity of the components, and the solvent power of sc-CO2. In order to propose future applications, turpentine oil was also evaluated in the photo-oxidation reaction with O2, showing that the mixture of monoterpenes can be oxidized to form the respective epoxides.