While molar volume-based models for gas solubility in ionic liquids (ILs) have been proposed, free volume within the IL can be shown to be the underlying property driving gas solubility and selecitivity. Previously published observations as to the distinct differences in solubility trends for gases such as CH4 and N2 relative to CO2 in systematically varied ILs can be attributed to positive and negative effects arising from increasing free volume with increasing alkyl chain length. Through the use of COSMOtherm as a powerful and rapid tool to calculate free volumes in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium ([Cnmim]­[X]) ILs, a previously unreported, yet speculated, critical underlying relationship between gas solubility in ILs is herein described. These results build upon previous assertions that Regular Solution Theory is applicable to imidazolium-based ILs, which appeared to indicate that a global maximum had already been observed for CO2 solubility in imidazolium-based ILs. However, the findings of this computational study suggest that the perceived maximum in CO2 solubility might be exceeded through rational design of ILs. We observe that although Henry’s constants in ILs are dependent on the inverse of molar volume and free volume, the volume-normalized solubility of CH4 and N2 are proportional to free volume, while CO2 is inversely proportional to the square root of free volume. Our free volume model is compared to experimental data for CO2/CH4 and CO2/N2 selectivity, and a nearly identical plot of selectivity relative to IL molar volume can be generated from the computational method alone. The overall implication is that large, highly delocalized anions paired with imidazolium cations that have minimally sized alkyl chains may hold the key to achieving greater CO2 solubility and selectivity in ILs.

1-n-Alkylimidazoles are a class of tunable solvents with low volatility and low viscosities. Although imidazoles have been known for some time in the pharmaceutical industry, and as convenient precursors for synthesizing imidazolium-based ionic liquids (ILs), only recently have they been given consideration in some of the same solvent-based separations applications that ILs have been studied for, such as post-combustion CO 2 capture and natural gas treating. "Sweetening", the removal of CO 2 , H 2 S, and other "acid" gases from natural gas (CH 4 ), is an existing industrial application where low volatility, low viscosity physical solvents are already applied successfully and economically at large scale. Physical solvents are also used for syngas cleanup and in the emerging application of pre-combustion CO 2 capture. Given the similarities in physical properties between 1-n-alkylimidazoles, and physical solvents currently used in industrial gas treating, the 1-n-alkylimidazole class of solvents warrants further investigation. Solubilities of CO 2 and CH 4 in a series of 1-n-alkylimidazoles were measured under conditions relevant to the use of physical solvents for natural gas treating: ∼5 atm partial pressure of CO 2 and temperatures of 30À75 °C. Solubilities of CO 2 and CH 4 were found to be strongly dependent on temperature, with the solubility of each gas in all solvents diminishing with increasing temperature, although CO 2 exhibited a stronger temperature dependence than CH 4 . Ideal CO 2 /CH 4 solubility selectivities were also more favorable at lower temperatures in 1-n-alkylimidazole solvents with shorter chain lengths. CO 2 solubility decreased with increasing chain length, while CH 4 solubility exhibited a maximum in 1-hexylimidazole. The solubility trends observed with temperature and chain length can be explained through calculation of solution enthalpies and solvent fractional free volume as approximated from van der Waals volumes as calculated via atomic contributions. Of the solvents examined, 1-methylimidazole displays the most favorable CO 2 solubility and CO 2 /CH 4 selectivity, and has the lowest viscosity. A comparison of 1-methylimidazole to commercially used solvents reveals similar physical properties and the potential for use in industrial gas processing. Imidazolium-based ILs are also compared, although they appear less favorable for use within established process schemes given their higher viscosities and reduced capacity for CO 2 .

Previously, we investigated 1-n-alkylimidazoles as low viscosity, low vapor pressure physical solvents for CO2/CH4 separation and noted a decrease in performance as the length of the n-alkyl chain was extended. Here, we examine imidazoles featuring oligo(ethylene glycol) substituents (“PEG n -imidazoles”) as an opportunity to improve upon the separation performance of this class of molecules. In the current work, we have characterized the density and the viscosity of PEG n -imidazoles over the temperature range 20–80 °C. PEG n -imidazoles are slightly more viscous than 1-n-alkylimidazoles but still fall below 20 cP. Ideal gas solubilities of CO2 and CH4 were measured in PEG n -imidazoles at gas partial pressures of ∼5 bar and temperatures of 25–70 °C. Solubilities of CO2 and CH4 were both found to decrease with increasing temperature, with a stronger dependence for CO2. However, better CO2/CH4 selectivity was achieved in PEG n -imidazoles at lower operating temperatures than was observed for 1-n-alkylimidazoles. Physical properties and gas separation performances were correlated with fractional free volume calculated via COSMOtherm, as well as solubility parameters. The results show trends of decreased FFV when polar ether groups comprise the substituent, and that CO2 solubility and solubility selectivity for CO2/CH4 are improved compared to their nonpolar, hydrocarbon-based analogues.