Physical Absorption of CO 2 , CH 4 , and N 2 in Nine Imidazolium-Based Ionic Liquids

Ionic liquids (ILs) are promising alternative candidates for CO 2 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 CO 2 absorption, limited data on CH 4 and N 2 solubilities make it challenging to identify selective ILs for CO 2 /CH 4 and CO 2 /N 2 separations. We evaluated nine imidazolium-based IL candidates ([1O2O1mim][Tf 2 N], [bmim][Tf 2 N], [bmim][DCA], [bmim][TfO], [bmim][SCN], [bmim][NO 3 ], [bmim][TCM], [amim][DCA], and [amim][TCM]), all of which engage in purely physical gas sorption. In addition to the CO 2 solubility, we measured CH 4 and N 2 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 CO 2 /CH 4 and CO 2 /N 2 solubility selectivities. The choice of cation and anion has a much larger effect on CH 4 and N 2 solubility than on CO 2 solubility. This indicates that selectivity enhancements can be more effectively achieved by suppressing N 2 and CH 4 solubility rather than increasing CO 2 solubility. Notably, cyano-anion-based ILs that have small molar volumes exhibit high CO 2 /CH 4 and CO 2 /N 2 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 CO 2 /CH 4 solubility selectivities of several of the ILs investigated are comparable to or higher than those of conventional solvents like Selexol TM, Purisol®, Rectisol®, Fluor Solvent TM, and sulfolane.