Regulating effects of anthraquinone substituents and additives in photo-catalytic oxygenation of p-xylene by molecular oxygen under visible light irradiation

This paper reports that the optical properties of anthraquinone (AQ) and its photo-catalytic performance in the visible light-driven p-xylene (PX) oxidation can be fine-tuned and improved through varying its skeleton substituents at the molecular level, and the electron-withdrawing substituent exhibits a better improvement effect than the electron-donating one. The best catalyst 2-carboxyanthraquinone (AQ-COOH) can achieve 70.9% PX conversion and 88.2% p-toluic acid selectivity under visible light illumination and 1 atm O2 pressure. The addition of benzenesulfonic acid in the above photocatalytic system can further increase PX conversion to 86.7%, along with a significant improvement in the selectivity of p-carboxybenzaldehyde and terephthalic acid, which likely originates from the unique recombination behavior of the photo-generated charge carriers of the catalyst and the additive driven by the strong interactions between their aromatic skeletons and acidic substituents. Furthermore, AQ-COOH can achieve almost quantitative yield of acetylacetone in ethylbenzene photooxidation and the regulating effect of toluene's para-position substituents on its reactivity is also reported in the AQ-COOH photocatalysis system. Based on quenching tests and EPR spectral characterizations, the oxygen vacancies (h+), superoxide free radicals (O2•-) and singlet oxygen (1O2) as the photo-generated active species are involved in the proposed photo-catalysis mechanism.