Kinetics and Mechanistic Insight into Efficient Fixation of CO2 to Epoxides over N‑Heterocyclic Compound/ZnBr2 Catalysts

In this contribution, a series of N-heterocyclic compounds cooperated with ZnBr2 catalyst for chemical fixation of CO2 to cyclic carbonates was developed without utilization of additional organic solvents. It was found that the catalytic activity of N-heterocyclic compounds could be obviously enhanced in the presence of ZnBr2, and the N-methylimidazole (Mim)/ZnBr2 catalytic system was the most efficient among the catalysts employed. Under the optimum reaction conditions, 99% yield of propylene carbonate was achieved with TOF of 474 h–1, and the catalysts were also versatile for CO2 cycloaddition with less active epoxides such as styrene oxide and cyclohexene oxide. Furthermore, a possible synergistic catalytic mechanism was proposed. Moreover, the rate constants were determined as a function of reaction temperature in the range of 130–160 °C, the activation energy was determined to be 41.1 kJ·mol–1, and the kinetic equation on the synthesis of propylene carbonate catalyzed by Mim/ZnBr2 was also obtained.