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 1 H 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 1 H NMR and the titration method were characterized in Fourier transform infrared, 13 C 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.