Phosphine-based ionic liquids for CO2 chemical fixation: Improving stability and activity by asymmetric flexible steric hindrance

Among various strategies for CO2 chemical fixation, the CO2 cycloaddition with epoxides is one of the most promising and efficient approaches, in which ionic liquid (ILs) is a typical relatively environmentally friendly catalyst and medium. However, ILs decomposition and catalytic stability have not been in-depth investigated, and the structure-activity relationship of cations and anions has remained obscure, which would affect the understanding of catalytic behavior and catalyst loss, and limit the theoretical research and industrial application in CO2 utilization. Therefore, based on a variety of phosphonium ILs we have designed and synthesized for CO2 cycloaddition, the structure-activity relationship, steric hindrance and electronic effect of various anions and cations have been systematically studied in main and side reactions. Especially, a regulation strategy of cationic flexible steric hindrance and anionic electron effect have been developed to improve both ILs’ activity and stability. Remarkably, the synergistic action of electron-donating effect, hydrogen bond and decreased acidity from flexible steric alkyl groups result in the spatial shielding of cation center and enhancement of the Br's leaving. Consequently, the quaternary phosphonium ILs with asymmetric flexible steric hindrance exhibit the optimal performance, showing propylene oxide (PO) conversion of 99.15% and propylene carbonate (PC) yield of 98.86%, which can efficiently decrease the formation of phosphine oxide, avoiding the decomposition of ILs. This discovery provides a new idea for developing stable and efficient CO2 fixation system in sustainable industry.