Cyano-Containing Protic Ionic Liquids for Highly Selective Absorption of SO2 from CO2: Experimental Study and Theoretical Analysis

The solubility of SO2 and CO2 in four cyano-containing protic ionic liquids (PILs) was experimentally measured at temperatures from 303.2 to 333.2 K and pressures up to 3.0 bar. Their physical properties, such as density, viscosity, and decomposition temperature, were also determined. It is found that [[DMPANH] [MOAc] and [DMAPNH] [EOAc] have the best selective absorption of SO2, from CO, at 303.2 K and 1.0 bar among the investigated PILs, and the ideal selectivities (119 and 107, respectively) of SO2/CO2 are significantly higher than those reported in literature for other ILs. The temperature-dependent Krichevsky-Kasarnovsky (K-K equation) and PR-NRTL equations are used to calculate the solubility data of SO2 and CO2, and the interactions between PILs and acid gases are analyzed thermodynamically. Quantum chemical calculations are also done to obtain the interaction configurations and energies. It is shown from the themodynamic analysis and the quantum chemical calculations that the interaction between SO2 and Pas is more energy favorable than that between CO2 and PILs, primarily due to the existence of the cyano group on the cation of PILs. The protic ionic liquids were reused for five absorption-desorption cycles without obvious loss in the absorption capacity, showing their potential as selective absorbents of SO2.