Microwave-Assisted Direct Hydroxylation Of Benzene

Recently, with the green chemistry becoming more and more concerned, using catalyst to replace the traditional reagent to improve the atomic efficiency in organic synthsis is developing rapidly. The direct hydroxylation of the aromatic ring which is related to the activation of C-H bond, needs only one step reaction from aromatic hydrocarbon to the final product, which is one of the most applicable significance in synthetic and catalytic chemistry.As a new technology of synthetic chemistry, microwave irradiation has many advantages, such as short reaction time, convenient operation, high selectivity, lower energy consumption, mild condition and so on. Many organic reactions that do not occur by conventional heating can proceed under microwave irradiation. This paper will focus on the direct catalytic hydroxylation of benzene to phenol by hydrogen peroxide under signal-mode focused microwave irradiation system.This paper reports that the Fe/activated carbon, Fe/SiO₂, Fe/TiO₂, Fe/β-zeolite and Fe/γ-Al₂O₃ catalysts were prepared under microwave irradiation and conventional impregnated methods. Catalysts were characterized by XRD, BET and SEM. The catalytic activity for direct hydroxylation of benzene to phenol over different catalysts under microwave irradiation was investigated. Study on effecting of different preparation and reaction conditions on the catalytic activity under microwave irradiation, the reaction conditions were optimized. Results show that: Compared to conventional heating, microwave irradiation improves the catalytic activity for direct catalytic hydroxylation of benzene to phenol at room temperature, especially, the Fe/activated carbon catalysts. (5 wt %) FeCl₃/activated carbon catalyst which was prepared by conventional impregnated method exhibits the high activity for the hydroxylation of benzene with 100 % selectivity of phenol and 28.2 % phenol yield under microwave irradiation at (35°C) the optimum reactions conditions: 1 mL benzene, 3 mL 30 % aqueous solution of H₂O₂, 0.4 g catalyst, 15 mL acetonitrile and 20 min reaction time. The optimum preparation conditions of microwave irradiation method: 5 % contents, 45 W irradiation energy, 20 min irradiation time, and the optimum reactions conditions: 1 mL benzene, 3 mL 30% aqueous solution of H₂O₂, 0.4 g catalyst, 15 mL acetonitrile and 15 min reaction time. Compared to conventional impregnated method, (5 wt %) FeCl₃/activated carbon catalyst which was prepared by microwave irradiation method exhibits the higher activity of 100 % selectivity of phenol and 29.7 % phenol yield. The research focus on the improvement of catalytic performance of Fe/activated carbon, Fe/SiO₂, Fe/TiO₂, Fe/β-zeolite and Fe/γ-Al₂O₃ catalysts by adding second metal oxides (Cu and Co) at different compositions to the catalysts. The catalysts were prepared under microwave irradiation and conventional impregnated methods. Results show that: the presence of the second metal can improve the phenol yield of the typical Fe/activated carbon, Fe/SiO₂, Fe/TiO₂, Fe/β-zeolite and Fe/γ-Al₂O₃ catalysts, although the BET surface area and pore volume were reduced.In this work, a pyridine-modified heteropoly acid catalyst Py3PMo11V was prepared and used for the direct hydroxylation of benzene by hydrogen peroxide in acetonitrile. The catalyst shows 100 % of phenol selectivity and 28.2 % of phenol yield under the optimum reaction conditions: 1.0 mL benzene, 1.8 mL 30% aqueous solution of H₂O₂, 0.15 g catalyst, 70°C and 20 min.