The catalytic mechanism of KI and the co-catalytic mechanism of hydroxyl substances for cycloaddition of CO2 with propylene oxide

Many experiments have demonstrated that KI/hydroxyl (-OH) substance catalytic systems are very effective for the cycloaddition of CO2 with epoxides to produce cyclic carbonates, which is among the most successful and important routes to convert CO2 into value-added chemicals. But the catalytic mechanism is not clear. In this work, we conducted the first theoretical research to clarify the catalytic mechanism of KI and the co-catalytic mechanism of hydroxyl substances. Density functional theory (DFT) method was used, propylene oxide (PO) was selected as the epoxide, and glycerol and propylene glycol (PG) as the hydroxyl substances. The transition structures, rate-determining steps and lowest energy barrier reaction pathways were identified for both gas phase and solvent environments. It was found that the hydroxyl groups, the potassium cation and the iodine anion form a ternary synergistic catalytic system, I--(-OH)-K+. The polar solvent environment provided by the hydroxyl substance was also favorable to the cycloaddition reaction. The results provide a clear profile for the cycloaddition of CO2 and epoxides promoted by KI/hydroxyl substance catalytic systems, and can explain satisfactorily the previous experimental observations.