(537f) Effect of Ionic Liquid Molar Volume on CO2/N2 and CO2/CH4 Pure and Mixed Gas Solubility Selectivity

Ionic liquids (ILs) are alternative candidates for CO₂ capture due to their negligible vapor pressure, and ability to tune the cations and anions based on the application needs. This class of solvents have been evaluated extensively to capture CO₂ from post-combustion and natural gas. However, solubility data of the less soluble N₂ and CH₄ are very scarce, which makes it difficult to select the right IL that would yield the best CO₂/N₂ and CO₂/CH₄ solubility selectivity for these industrial applications. Using COSMO-RS and available literature data, we screened multiple ILs and selected eight candidates ([bmim][Tf₂N], [bmim][DCA], [bmim][TfO], [bmim][SCN], [bmim][NO₃], [bmim][TCM], [amim][DCA], [10201mim][Tf₂N]) based on estimated solubility selectivity. We measured N₂ and CH₄ solubility in these ILs, using a gravimetric apparatus up to 140 bar to reduce uncertainty, and obtain more accurate solubilities, and pure gas CO₂/N₂ and CO₂/CH₄ selectivities. ILs with small molar volumes yield higher solubility selectivity because N₂ and CH₄ solubilities are lower, presumably due to less free volume in the IL. We have also investigated mixed gas solubility selectivity for the ILs with the highest selectivities based on the pure gas measurements since real mixed gas selectivities are frequently lower than the estimates from pure gas solubilities. For this purpose, we developed a mixed gas sorption system to investigate the phase behavior of ILs with CO₂/CH₄ and CO₂/N₂ mixtures. In particular, we seek to gain a fundamental understanding on the effect of highly soluble CO₂ dissolution on the solubility of the less soluble gas (CH₄ or N₂). We have investigated CO₂/CH₄ and CO₂/N₂ mixtures at various gas compositions in ILs at partial pressures up to 30 bar at 35 ËšC. The mixed gas solubilities in the IL are obtained by analyzing the number of moles of desorbed gas from the IL in a 5 cm³ sample chamber that expands into a 500 cm³ ballast volume, with the compositions determined with gas chromatography. The real mixed gas selectivity is compared to the ideal pure gas selectivity of these gases previously measured gravimetrically and the subsequent Henry’s law constants derived from these isotherms.