CO2/Hydrocarbon Selectivity of Trihexyl(tetradecyl)phosphonium-Based Ionic Liquids

In a previous study, a conductor-like screening model for the real solvents (COSMO-RS) method and machine learning were used to predict CO2 Henry's law constants for several ionic liquids (ILs), where trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P-66614][TFSA]), trihexyl(tetradecyl)phosphonium perfluorooctanesulfonate ([P-66614]-[PFOS]), and trihexyl(tetradecyl)phosphonium hexafluorophosphate ([P-66614][PF6]) were selected as ILs exhibiting superior CO2 absorption. Based on the prediction results, the selected phosphonium-based IL CO2 solubilities were measured at 313.15 and 333.15 K. Here, a magnetic suspension balance was used to measure the solubilities of hydrocarbons such as methane (CH4), ethane (C2H6), and ethylene (C2H4) in [P-66614][TFSA], [P-66614][PFOS], and [P-66614][PF6] at 313.15 and 333.15 K. The CO2, CH4, C2H6, and C2H4 Henry's law constants were calculated from the experimental solubility data for mole fractions of <0.1. The CO2/hydrocarbon selectivities were calculated using the ratio of Henry's law constants of CO2 and hydrocarbons. The IL CO2/CH4 selectivities increase in the order of the anion PFOS- < TFSA(-) < PF6-, whereas CO2/C2H6 selectivities increase in the order of the anion PFOS- < TFSA(-) <= PF6-. The CO2/C2H4 selectivities of the three IL systems were identical at 313.15 K, whereas at 333.15 K, the selectivities increased in the order PFOS- < TFSA(-) <= PF6-. When only the IL anions were changed, the CO2/hydrocarbon selectivity increased with a decreasing anion molecular weight.