Molecular Simulation and Regular Solution Theory Modeling of Pure and Mixed Gas Absorption in the Ionic Liquid 1-n-Hexyl-3-methylimidazolium Bis(Trifluoromethylsulfonyl)amide ([hmim][Tf2N

Abstract

Monte Carlo simulations are carried out to compute the solubility of SO2, O2, and N2 in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]). Simulations are also used to compute the mixed gas isotherms for the mixtures CO2/O2, SO2/N2, and CO2/SO2. The pure gas isotherms agree well with available experimental data. For the mixtures CO2/O2 and SO2/N2, the simulations predict that the mixed gas solubilities are nearly ideal, with little enhancement or competition between the two solutes. This is contrary to published experimental studies, which found that in a gas mixture, CO2 enhances the solubility of the otherwise sparingly soluble O2 and that the poorly soluble N2 inhibits the solubility of the highly soluble SO2. For the SO2/CO2 mixture, it is found that the two gases absorb independently at low pressure but compete with one another at high pressure. An energetic analysis was performed for the different solutes in the ionic liquid. The van der Waals energy between the solutes and ionic liquid was greater than the electrostatic interactions, and the van der Waals interactions were roughly the same between the solutes and the two different ions. CO2 and SO2 interact more strongly with the anion than the cation due to stronger electrostatic interactions between the solute and the anion. N2 and O2 interact weakly with the ionic liquid and show little difference in interaction energy between the cation and anion. Regular solution theory (RST) was evaluated for its ability to predict pure and mixed gas isotherms. It was found that if RST was applied in a strictly predictive mode using experimentally derived parameters, solubilities of CO2 were underpredicted by a substantial margin.