Theoretical Insights into the Effect of Cations, Anions, and Water on Fixation of CO2 Catalyzed by Different Ionic Liquids

Chemical fixation of CO2 is an efficient means for decreasing amount of CO2 in the atmosphere. One of promising technologies is the cycloaddition of CO2 with epoxides to synthesize cyclic carbonates. In this reaction, ionic liquid (IL) catalysts show versatile and unique advantages. However, the reaction mechanism using ILs is not clear. In this work, a detailed theoretical investigation was performed by DFT calculations. The energetic profile shows that the reaction consists of three steps, with the ring-opening step being the rate-determining step. Based on the results, effects of cations, anions and water were calculated. Cations show strong hydrogen bonding interactions with epoxides, which decreases the energy barrier of the ring-opening step, indicating that hydrogen bonds play a positive role in promoting the reaction. The effect of anions was evaluated by nucleophilicity index (NNu ); anions with a larger NNu (stronger nucleophilicity) value show lower energy barriers. The influence of water was investigated by implicit and explicit models. Compared with the solvent-free case, water as an implicit solvent decreases the energy barriers through polarization with epoxides. In the explicit solvent model, the water molecules form new hydrogen bonds with epoxides and cations, which can efficiently reduce the energy barriers. The result indicates that there is a new synergic catalytic mechanism, in which the water acts not only as solvent but also as a catalyst in the reaction. Supporting experiments further confirm the calculation results.