(nBu4N)4W10O32-catalyzed selective oxygenation of cyclohexane by molecular oxygen under visible light irradiation

The development of mild and efficient process for the selective oxygenation of organic compounds by molecular oxygen (O-2) can be one of the key technologies for synthesizing oxygenates. Here, the photo-oxygenation of cyclohexane to cyclohexanol and cyclohexanone over tetrabutylammonium decatungstate (W10O324-) was carried out under visible light irradiation and pure O-2 atmosphere. The W10O324- was found to be active to this photo-oxygenation in a pure acetonitrile (MeCN), which can achieve ca. 8.1% cyclohexane conversion with ca. 64.3% selectivity for cyclohexanone under sustained visible light irradiation of 12 h. Notably, the above-described photo-catalysis oxygenation was improved to some extent in the presence of some acidic additives such as 10 M HCl, H2SO4, or H3PO4 aqueous solution and benzenesulfonic acid, providing ca. 12.4-14.3% cyclohexane conversion with ca. 59.7-69.4% cyclohexanone selectivity. Moreover, this acid-promoted effect was further magnified in the case of adding a suitable amount of water, which can lead to enhancing conversion (16.8-20.0%) and improving cyclohexanone selectivity (68.2-78.3%). This acidic aqueous solution-promoted effect was also observed upon the W10O324--photocatalyzed oxygenation of other substrates like toluene, ethylbenzene and butanone. This is likely due to the acidic aqueous solutions playing important roles in improving photo-redox cycling of W10O324- and preserving its stability, as supported by the UV-vis spectra and cyclic voltammetry measurements. (C) 2014 Elsevier B.V. All rights reserved.