(472f) Molecular Simulation of Pure and Mixture Gases Absorption In Ionic Liquids

Gases solubility for the mixtures of CO2/O2, SO2/N2, and CO2/SO2, and pure gases of SO2, O2, and N2 absorption in ionic liquid of 1-n-hexyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) were computed using isothermal-isobaric Gibbs continuous fractional component (CFC) Monte Carlo (MC) and osmotic CFC simulations. The predictive capability of regular solution theory (RST) for these systems was also investigated.

Computed isotherms for the pure gases agreed well with available experimental data. Energy analyses showed that the van der Waals (VDW) energy for pure CO2, SO2, N2, and O2 interacting with the ionic liquid dominates over the electrostatic energy. CO2 and SO2 interact more strongly with the anion than with the cation. Typically, CO2 interacts with anion through VDW and electrostatics forces about 10%-15% more strongly than it does with cation. For SO2, the difference in interaction energy between the solute-anion and solute-cation is 15%-30%. For N2 and O2, the cation and anion interact with solutes comparably.

In the mixed gas simulations, it was observed that the selectivity for CO2 absorption in [hmim][Tf2N] over O2 varies between 8 and 20, close to the ideal selectivity of 11.2 computed from the pure gas Henry's law constants. Absorption of CO2 decreases the solubility of O2, rather than enhancing the solubility of O2. This is in contrast to the experimental finding by Hert [1] et al.. The mixture simulation of SO2/O2 absorption in [hmim][Tf2N] shows N2 does not compete significantly for the absorption of SO2. This is in contrast with the experiment for the same mixture absorption in two ionic liquids of [BMIM][BTA] and [BMIM][BF4] [2] , but in agreement with experimental findings for three other ionic liquids [2]. For the CO2/SO2 mixture, both gases compete with each other leading to the decrease of the solubility for both gases. The selectivity of SO2 absorption over CO2 varies between 10 and 12, close to the pure Henry's law ratio of 14±2 bar between CO2 and SO2.

Overall, calculations show that the RST does not perform well. For pure gases of CO2,

O2, and N2 absorption in [hmim][Tf2N], the mole fractions predicted from RST are about 20% -60% different than those from experiments and simulations. For the mixture of CO2/O2 absorption in [hmim][Tf2N], the RST predicts the mole fraction to be about 30% to 40% different from the simulation.

References:

D. G. Hert, J. L. Anderson, S. N. V. K. Aki, J. F. Brennecke, Chem. Commun., 2603-2605, 2005

J. Huang , A. Riisager, P. Wasserscheid, R. Fehrmann, Chem. Commun., 4027-4029, 2006