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. (Chemical Equation Presented).