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

Among various strategies for CO 2 chemical fixation, the CO 2 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 CO 2 utilization. Therefore, based on a variety of phosphonium ILs we have designed and synthesized for CO 2 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 CO 2 fixation system in sustainable industry. • Phosphine oxide generated from ILs' decomposition is first concerned and systematically investigated in CO 2 conversion. • Cationic steric hindrance and anionic electron effect are regulated to enhance both activity and stability. • Quaternary phosphonium ILs with asymmetric flexible groups exhibit excellent performance, avoiding phosphine oxide's formation. • Flexible alkyls' electron-donating effect and hydrogen-bond interaction lead to the spatial shielding and protection to cation center. • This discovery provides a novel avenue for developing CO 2 mitigation and fixation system in sustainable industry. [ABSTRACT FROM AUTHOR]