Synthesis of Benzil by Air Oxidation of Benzoin and M(Salen) Catalyst

Bis salicylaldehyde ethylenediamine Schiff base (Salen) and its complexes with three metal ions (Co2+, Ni2+, Zn2+) were prepared, and characterized by infrared spectroscopy(IR). Using air as oxygen source, the optimum reaction conditions for the catalytic oxidation of 0.05 mol benzoin by Co (Salen) were obtained by orthogonal test as follows: base KOH 2 g, catalyst 1.5 g, N, N-dimethylformamide(DMF) as solvent, reaction temperature 40 °C, reaction time 1 h. Under these conditions, the catalytic performances of different metal complexes were investigated. The catalytic activity of Co(Salen) was the best one, the yield of benzil was up to 93.6%, the number of Ni(Salen) and Zn(Salen) was 86.3% and 82.1%, respectively. The reused catalytic performance of M(Salen) complex was also studied. The catalytic activity of Co(Salen), Ni(Salen) and Zn(Salen) was stable after 4 times recycle, the yield of benzil was 71.4%, 63.3% and 57.4%, respectively, and it was easy for catalyst recycling. The oxidation product was certainly benzil with high purity according to the characterization results of melting point(MP), IR, high performance liquid chromatography(HPLC) and 1H nuclear magnetic resonance(1H NMR). Compared with the common synthetic method of benzil, this one has the advantages of friendly environment, low cost and easy operation. It is a simple and green way to synthesize benzoyl efficiently.