Direct hydroxylation of benzene to phenol

The one-step process for direct hydroxylation of benzene to phenol is an attractive and challenging subject. So far, many research efforts made for direct hydroxylation of benzene to phenol are mainly focused on three preparation routes by adopting N2O, H-2 O-2 and O-2 as oxidants respectively. In this paper, the progress in these routes of direct hydroxylation of benzene to phenol is reviewed in detail. The characteristics and prospects of their industrial applications are also discussed. The route for direct oxidation of benzene to phenol with N2O as oxidant is technologically practicable, however, the process economics is strongly dependent on a cheap supply of N2O as a by-product in the production of adipic acid. On-purpose production of N2O as a feedstock for the process is economically unfeasible now. Thus the application of the technology is severely limited to just a few sites where N2O from adipic acid manufacture can be readily accommodated in the downstream direct oxidation process. The route for phenol preparation with H,02 as oxidant is an environmentally friendly process, however, it is technologically unfeasible now due to lower efficient utility for H2O2 and higher phenol production cost. And also expensive H2O2 shows negative effect on the economics of its industrial application. In comparison, the process for direct hydroxylation of benzene to phenol with O-2-H-2 system is an environmentally clean and sustainable route. It is of potential prospect in both academic research and technological development aspects. The route of direct oxidation of benzene to phenol with O-2-H-2 system by using inorganic membrane reactor is more attractive because of its higher conversion, selectivity and safer operation.