Vanadium‐Substituted Tungstophosphoric Acids as Efficient Catalysts for Visible‐Light‐Driven Oxygenation of Cyclohexane by Dioxygen

This paper discloses that the oxygenation of cyclohexane by dioxygen (O 2 ) to cyclohexanol and cyclohexanone over three vanadium‐substituted tungstophosphoric acids (PW 11 V 1 , PW 10 V 2 , PW 9 V 3 ) can occur in MeCN under visible‐light irradiation, but provides very low cyclohexane conversion (0.8–2.4 %) with modest cyclohexanone selectivity (51–58.3 %). Importantly, an HCl aqueous solution was found to drastically promote this photooxygenation catalyzed by PW 10 V 2 and especially PW 9 V 3 acids, respectively providing approximately 20.4 and 23.4 % cyclohexane conversion and approximately 82.5 and 87.1 % cyclohexanone selectivity, with a concomitant formation of a small amount of chlorocyclohexane. However, such promoting effect was negligible in the PW 11 V 1 ‐ photocatalyzed oxygenation and the other acids and the chlorine‐containing salts did not show any promotion effect on the present photocatalysis reaction. Notably, the HCl‐promoted photocatalytic oxygenation was significantly influenced by the amount of water. The cyclohexanone selectivity continuously and significantly increased with water amount, but photooxygenation efficiency drastically decreased if a slight excess amount of water was added. Based on these findings and the UV/Vis spectral and cyclic voltammetric measurements, a free‐radical mechanism initiated by the Cl atoms generated in the present photocatalysis system was proposed.