Epoxidation of Soybean Oil Catalyzed by Deep Eutectic Solvents Based on the Choline Chloride–Carboxylic Acid Bifunctional Catalytic System

Deep eutectic solvents based on choline chloride–carboxylic acids, as a low-cost effective bifunctional catalyst, were successfully used to catalyze epoxidation of soybean oil with peroxyformic acid as the oxygen supplier under solvent-free conditions. The influence of reaction temperature and time on the epoxidation process evaluated by 1H nuclear magnetic resonance (NMR) quantitative analysis was extensively investigated. The optimal mild conditions were 50 °C and 8 h for choline chloride–oxalic acid dihydrate with 88.80% conversion of double bonds and a high selectivity of 93.86%. Meanwhile, choline chloride–malonic acid at 50 °C for 10 h exhibited better catalytic performance, 90.87% selectivity versus only 83.52% in the choline chloride–citric acid monohydrate system for 8 h. The epoxy products analyzed simultaneously by 1H NMR and the titration method were characterized in Fourier transform infrared, 13C NMR, and thermogravimetric analysis. The results suggested choline chloride–oxalic acid dihydrate with a favorable recyclability was more efficient for the promotion of epoxidation of soybean oil.