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

Abstract

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.