Study On Green Synthesis Of Benzil

Benzil plays an important role in synthesizing the active pharmaceutical ingredients(APIs)of biological and chemical drugs.At present,industrial production of benzil is to oxidize benzoin with concentrated nitric acid.The isolated yield of benzil reaches about 90%.However,concentrated nitric acid has high risks during production.Furthermore,large amounts of waste dilute nitric acid are generated.In addition,the producing process takes more than 2 h.This work is trying to solve the above problems by the following ideas:(1)The air oxidation method is adopted by replacing the concentrated nitric acid oxidation method to oxidize benzoin to benzil.The catalyst adopts copper acetate,which is low cost,easy availability and high catalytic activity.Meanwhile the air is purged in the solution.By recycling the catalyst and solvent,the waste water,gas and residue generated in the production process are reduced dramatically.(2)Ferric chloride is used to oxidize benzoin to benzil.By oxidizing ferrous chloride with air,the ferric chloride is recycled and thus the waste water,gas and residue are also reduced observably.(3)A microchannel continuous flow technique is used to synthesis benzil.The enhancement of mass and heat transfer by microchannel technology shortens the reaction time and improves the production efficiency.In this work,the method for synthesizing benzil by air catalytic oxidation of benzoin is studied at first.The method uses air as the oxidant,copper acetate as the catalyst,and acetic acid as the solvent.The effects of catalyst dosage,reaction temperature and reaction time on the yield of benzil are studied by single factor experiment,and the synthesis conditions are optimized.With using 15 mol% copper acetate,the synthesis takes 90 min at 100 °C,the chromatographic yield of benzil reaches 90.5%.After separating benzil from the solution,the catalyst copper acetate and solvent acetic acid is recycled five times.The average isolated yield of benzil is 82.6%.The reaction of synthesizing benzil from benzoin by air oxidation belongs to heterogeneous reaction.Oxygen in the air needs to be dissolved in the solution to participate reaction.Consequently,the long time(90 mins)is adopted in the actual production.Therefore,the homogeneous reaction,in which ferric chloride is as oxidant,is developed to reduce reaction time.The effects of oxidant/substrate ratio,reaction temperature and reaction time on the yield of benzil are studied by single factor experiment,and the synthesis conditions are optimized.By using oxidant/substrate ratio 3,the reaction is carried out for 50 min at 100 °C,the chromatographic yield of benzil reaches 96.5%.After separation and purification,the benzil isolated yield is 91.0%.After the reaction,ferrous chloride is oxidized by air to regenerate ferric chloride.In this work,the isolated average yield of benzil is 85.4% with using recycled oxidant for five times.To further shorten the reaction time,the synthesis of benzil is improved to a microchannel continuous flow method,in which the effects of different microchannel reactors,fluid flow states,reactant concentrations and reaction times on the yield of benzil are investigated,then the synthesis conditions are optimized.The chromatographic yield of benzil reaches 81.9% in the heart-shaped microchannel reactor with oxidant/substrate ratio of 3,reaction temperature of100 °C and reaction time of 25 min.The isolated yield of benzil reaches 80.3% after separation and purification.The three methods of synthesizing benzil in this work follow the 5R principle in green chemistry.Compared with the traditional nitric acid oxidation method,the methods show the advantages of safety,green,high efficiency,high yield and low cost.This work makes positive contributions to the technology innovation for the industrial production of benzil.