Acidity Scale in a Quaternary Phosphonium Ionic Liquid and Its Implication on Carbon Dioxide Absorption

In this work, an acidity scale that comprises 23 organic Brønsted acids of different structural and electronic features was established in a representative quaternary phosphonium ionic liquid (QPIL), i.e., [P_4,4,4,16][OTf] (with water content of 100 ppm), by the UV-vis spectroscopic method. The established acidity scale ranges from 5.4 to 22.2, covers about 17 pK units. We found that [P_4,4,4,16][OTf] has fairly good stability toward some very strong bases. Acidity comparisons and regression analyses among various solvents show that the counteranion of [P_4,4,4,16][OTf] is freer than those of imidazolium-based counterparts, which governs the solvation behavior of the QPIL. With the established acidity scale, we also studied CO₂ absorptions in QPILs. To investigate the CO₂ absorption in QPILs, a series of anion-functionalized QPILs, i.e., [P_4,4,4,16][X] were synthesized, and their absorption capacities and kinetics were measured. The results show that the maximum absorption capacity of [P_4,4,4,16][X] is much greater than that of other types of ILs, and a more basic QPIL leads to a faster absorption rate and a higher absorption capacity. The kinetic profiles and correlation analysis suggest that the capacity and mechanism of CO₂ absorption in [P_4,4,4,16][X] are closely associated with the basicities of anions of these QPILs.