Direct hydroxylation of benzene to phenol with molecular oxygen over vanadium oxide nanospheres and study of its mechanism

Direct hydroxylation of benzene to phenol using molecular oxygen is a green route with high atom economy but still a great challenge when compared with the existing method of production. The activation of oxygen is necessary and reductive agents were used to activate dioxygen in a so-called reductive activation process. Here, nano vanadium oxides that consist mainly of low valence vanadium to activate dioxygen were prepared under different conditions via a hydrothermal method. Under the optimized conditions, an excellent phenol selectivity of 96.3% with benzene conversion of 4.2% was achieved over the VOC2O4-N-5 without reductive agents. Characterizations revealed that VOC2O4-N-5 was composed of a mesoporous nanosphere structure with medium strong acid sites and low valence vanadium species. A mechanism was proposed as follows: dioxygen was activated by low valence vanadium in VOC2O4-N-5 to produce the active oxygen species which oxidized acetic acid to peracetic acid. Then the active oxygen species was subsequently transferred from peracetic acid to benzene and inserted into the C-H bond to give phenol.