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

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.