A Monte Carlo Simulation Study to Predict the Solubility of Carbon Dioxide, Hydrogen, and Their Mixture in the Ionic Liquids 1-Alkyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide ([cnmim+][tf2n–], N = 4, 6)

Ionic liquids are being investigated as potential solvents fot pre- and post-combustion CO2 capture. In precombustion applications, knowledge of the selectivity of CO2 over H-2 in high-pressure and high-temperature gas mixtures is crucial. In the present work, the solubility of catbon dioxide, hydrogen, and their mixture in the ionic liquids 1-alkyl-3-methylimidazoliurn bis(trifluoromethanesulfonyl)amide ([C(n)mim(+)][Tf2N-], n = 4, 6) is computed using the isothermal-isobaric Gibbs ensemble Monte Carlo (NPT-GEMC) method. The predicted solubility for CO2 and H-2 in 1-n-butyl-3methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(4)mim(+)][Tf2N-]) and 1-n-hexy1-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(6)mim(+)][Tf2N-]) are compared with experimental results. It is found that the predicted absorption isotherms for pure gases in the studied ionic liquids (ILs) agree reasonably well with the available experimental results. The Henry's Law constants, partial molar enthalpies of absorption, partial molar volumes, and the percentage change in the volumes of the IL phase upon absorption of CO2 and H-2 in [C(4)mim(+)][Tf2N-] and [C(4)mim(+)][Tf2N-] are estimated at different temperatures and are compared with the experimental data. The results indicate that the solubility of CO2 decreases with increasing temperature, whereas, for H-2, it slightly increases with increasing temperature, which is consistent with most previous experimental data. Mixed-gas (CO2/H-2) experiments are much harder to conduct than pure gas experiments, so it is usually assumed that absorption selectivity is ideal. To test this, mixed gas simulations were also carried out using the NPTGEMC method. The simulations indicate that the solubility selectivity of CO2 over H-2 in [C(4)mim(+)][Tf2N-] and [C(6)mim(+)][Tf2N-] varies, depending on the vapor phase composition. The selectivities are compared with the Henry's Law constant ratios and ideal selectivities that are obtained from pure gas isotherms for CO2 and H-2 in both ionic liquids. Both computed selectivities and ideal selectivities are close and are different from Henry's Law constant ratios at 333 K. The results indicate no enhancement in the solubility of H-2, because of the presence of CO2 in the IL phase. The selectivity decreases with increasing temperature, whith is consistent with the simulation study of Sin et al. [J. Phys. Chem. B 2010, 114, 6531]. This is due to the fact that CO2 solubility decreases with increasing temperature, whereas H-2 solubility increases.