(57c) Prediction of Pure Component and Mixed-Gas Adsorption and Transport Properties of CO2 and CH4 in [C4mim][NTf2] Confined in Carbon Slit Pores from Molecular Simulations

Most gas separation processes for which ionic liquids have been evaluated have focused on traditional contacting devices in which the solubility of the gases in the bulk phase of the ionic liquid is of most relevance. Recently, hybrid devices based on “Supported Ionic Liquid Phase” or SILP concepts have gained popularity due to improved mass transfer rates and the ability to reduce the amount of ionic liquid needed. In SILP technologies, ionic liquids are imbibed into a porous support material such as a membrane, and gas separation is carried out via a solution-diffusion mechanism. Understanding how confinement of ionic liquids at the nanometer length scale affects sorption and diffusion of gases is therefore of paramount importance.

Here we present results of Monte Carlo molecular simulations of single component absorption isotherms of CO₂ and CH₄ in the ionic liquid 1-n­-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C₄mim][NTf₂] confined in carbon slit pores, and compare this against absorption in the bulk ionic liquid phase. The effect of different slit pore widths on the single component absorption will be discussed by considering the confinement of the ionic liquids in pores with 2 nm and 5 nm widths. Results of simulations of binary mixtures of CO₂ and CH₄ in the confined ionic liquid will be presented and used to show the effectiveness of separating CO₂ and CH₄. Additionally, results of molecular dynamics simulations are presented in which the diffusion coefficients of the gases in the pores filled with the ionic liquid are computed as a function of gas loading. When combined with the solubility calculations, this allows for the prediction of gas permeabilities. The structure of the ionic liquid under confinement and interactions responsible for absorption tendencies of gases in these systems will be discussed.