Tuning Acid–Base Properties of Carboxyl–Functionalized Ionic Liquids for Synergistic Catalysis of CO2 Cycloaddition

The cycloaddition of carbon dioxide (CO2) with epoxides represents an attractive route for the high–value utilization of CO2; however, achieving efficient catalysis under mild conditions remains challenging. In this work, a series of carboxyl–functionalized imidazolium ionic liquids (ILs) with different acid–base properties and structural rigidity were designed and synthesized by tuning the side–chain structures. The acid–base characteristics and local electronic structures of the ILs were systematically investigated using potentiometric titration method, NH3 temperature–programmed desorption (NH3–TPD), and CO2 temperature–programmed desorption (CO2–TPD), in combination with density functional theory (DFT) calculations. The results reveal that, although the overall Brønsted acidity of the ILs gradually decreases, the local acidity of the imidazolium C2–H site as well as the basicity and electron density of the Cl– anion are significantly enhanced, leading to the formation of a favorable “mild acid–strong base” cooperative microenvironment. The C2–H site in the cation acts as a Lewis acid site, cooperating with the nucleophile Cl⁻ to promote the ring–opening of epichlorohydrin, while the electrostatic interaction of Cl⁻ and the π–π interactions involving the benzene rings and imidazolium rings effectively adsorb and activate CO2. Under the cooperation of these multi–sites, CO2/epichlorohydrin can efficiently react under mild conditions (0.1 MPa, 80 °C) to produce chloropropylene carbonate with a yield of up to 99%. The catalyst maintains structural stability after 5 cycles, and has good applicability to various epoxide substrates. This study clarifies the intrinsic relationship between overall acidity and local acid–base properties in ILs and provides new insights into the rational design of efficient catalysts for CO2 cycloaddition.