CO2 Solubility in Bis(trifluoromethylsulfonyl)imide ([Tf2N]) Anion-Based Ionic Liquids: [BVIM][Tf2N], [P4441][Tf2N], and [N4222][Tf2N]

In this study, three types of [Tf2N]-based ionic liquids (ILs), 1-butyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide ([BVIM][Tf2N]), butyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N4222][Tf2N]), and tributylmethylphosphonium bis(trifluoromethylsulfonyl)imide ([P4441][Tf2N]), were selected to measure the solubility of CO2. A newly measured solubility database for these three ILs was generated in this study. It was found that the sequence of CO2 solubility was [BVIM][Tf2N] ≳ [P4441][Tf2N] > [N4222][Tf2N]. The CO2 solubility of the [BVIM] cation was approximately equal to that of the [P4441] cation at a low CO2 mole fraction region of less than 0.4; however, it was slightly higher than that of the [P4441] cation with increasing CO2 mole fraction. For the [N4222] cation, the CO2 solubility was similar to that of the [BVIM] and [P4441] cations when the CO2 mole fraction was less than 0.4; however, after this point, the CO2 solubility decreased drastically. To predict the ILs' CO2 solubility at various temperature and composition ranges, thermodynamic modeling was conducted using the Peng–Robinson equation of state and the van der Waals one-fluid mixing rule. The critical properties and acentric factor of the ILs were calculated using the modified Lydersen–Joback–Reid method. The overall average absolute deviation of pressure (AAD-P) values were 0.0174, 0.0148, and 0.0153 for the CO2 + [BVIM][Tf2N], CO2 + [N4222][Tf2N], and CO2 + [P4441][Tf2N] systems, respectively. The values were estimated to be less than 2% on average for the three ILs, which indicated that the modeling equation fit well with the experimental data. In addition, Henry's law constant was obtained using the extended Henry's law at different temperatures. It should be noted that the constant increased with increasing temperature.