Visible light-triggered vanadium-substituted molybdophosphoric acids to catalyze liquid phase oxygenation of cyclohexane to KA oil by nitrous oxide

The development of a mild and highly efficient process for utilization of nitrous oxide (N2O) as a green oxidant has very important academic and applied values in the synthesis of oxygenated products. This paper first discloses that a series of vanadium-substituted molybdophosphoric acids (PMo12-nVn, n = 1-3), assisted by HCl aqueous solution, can efficiently catalyze the visible light-driven liquid phase oxygenation of cyclohexane by N2O in acetonitrile (MeCN), providing ca. 26.2% cyclohexane conversion and 90.2% selectivity for cyclohexanol and cyclohexanone (KA oil) under optimized conditions, along with a small quantity of chlorination. Among the catalysts examined, PMo10V2 and especially PMo9V3 showed a higher activity for this photocatalytic oxygenation than PMo11V. Furthermore, the amount of water added strongly influenced this HCl-promoted photocatalysis oxygenation. The selectivity of cyclohexanone was continuously and significantly improved from 21.8 to 82.7% with increasing water amount from 0 to 0.15 mL, but the conversion of cyclohexane obviously decreased when the amount of water added was higher than 0.12 mL. The promoting effect of HCl on the present photocatalysis reaction is probably due to the fact that the donor-acceptor (D-A) adduct between HCl and PMo12-nVn, can be excited by visible light to achieve its intra-molecular electron transfer from Cl- to PMo12-nVn, anions, which leads to the generation of Cl-center dot radicals and the reduction of catalysts. The subsequent reactions initiated by the Cl-center dot radicals result in the oxygenation of cyclohexane by N2O to KA oil and the regeneration of catalysts, as supported by UV-vis spectral and cyclic voltammetric measurements. (C) 2015 Elsevier B.V. All rights reserved.