Physical Absorption of CO _<b>2</b> , CH _<b>4</b> , and N _<b>2</b> in Nine Imidazolium-Based Ionic Liquids

Ionic liquids (ILs) are promising alternative candidates for CO2 capture due to their negligible vapor pressure and the tunability of their cation–anion pairs, enabling precise control over solvent properties to meet specific performance demands. While ILs have been widely studied for CO2 absorption, limited data on CH4 and N2 solubilities make it challenging to identify selective ILs for CO2/CH4 and CO2/N2 separations. We evaluated nine imidazolium-based IL candidates ([1O2O1mim][Tf2N], [bmim][Tf2N], [bmim][DCA], [bmim][TfO], [bmim][SCN], [bmim][NO3], [bmim][TCM], [amim][DCA], and [amim][TCM]), all of which engage in purely physical gas sorption. In addition to the CO2 solubility, we measured CH4 and N2 solubility in these ILs using a gravimetric apparatus at pressures up to 140 bar, all at 308.2 K. From this data, we determined pure gas CO2/CH4 and CO2/N2 solubility selectivities. The choice of cation and anion has a much larger effect on CH4 and N2 solubility than on CO2 solubility. This indicates that selectivity enhancements can be more effectively achieved by suppressing N2 and CH4 solubility rather than increasing CO2 solubility. Notably, cyano-anion-based ILs that have small molar volumes exhibit high CO2/CH4 and CO2/N2 solubility selectivities, up to 27 ± 3 and 134 ± 32, respectively. Additionally, these ILs have viscosities as low as 12 mPa·s at 308.2 K, compared to ILs containing fluorinated anions with viscosities as high as 60 mPa·s at the same temperature. The pure gas CO2/CH4 solubility selectivities of several of the ILs investigated are comparable to or higher than those of conventional solvents like SelexolTM, Purisol®, Rectisol®, Fluor SolventTM, and sulfolane.