Photocatalytic metal-free oxidation of alcohols with molecular oxygen in supercritical CO₂ medium

Near-UV-light-induced oxidative conversion of cyclic and linear alcohols into corresponding carbonyl compounds was achieved in the supercritical CO2 medium under the action of molecular oxygen in the presence of 2-fluoroanthraquinone (1 mol %), a simple and available metal-free photocatalyst. A thorough examination of the impact of various process parameters on the reaction outcome allowed to identify a narrow density area at similar to 0.3 g/cm(3) in the vicinity of the medium critical point and optimal reaction conditions (similar to 45 degrees C, similar to 8.4 MPa) where up to 99 % conversions and 65-93 % yields of the oxidation products could be attained. Furthermore, it has been shown that compressed atmospheric air, a far cheaper and safer oxidizer than pure oxygen, can be applied to the reaction. Based on the experimental data, a plausible mechanism of the photocatalytic process has been proposed comprising photocatalyst excitation with near-UV light followed by the hydrogen atom transfer (HAT) as the key stages.