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Molecular Simulation Study of the Solubility, Diffusivity and Permselectivity of Pure and Binary Mixtures of CO2 and CH4 in the Ionic Liquid 1‑n‑Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

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journal contribution
posted on 09.09.2015, 00:00 authored by Samir Budhathoki, Jindal K. Shah, Edward J. Maginn
The knowledge of the mixture solubility and diffusivity of gases in ionic liquids (ILs) is critical for the design of supported ionic liquid membranes (SILMs). Because mixed gas solubilities and diffusivities in ILs are much more difficult to measure than pure gas properties, pure gas solubility and diffusivity data are typically used along with an ideal solubility/diffusivity assumption to estimate permselectivities. It is not clear, however, if the ideal solubility and diffusivity assumptions are valid. In this work, molecular dynamics (MD) and Gibbs ensemble Monte Carlo (GEMC) simulations were used to compute the diffusion selectivity and solubility selectivity of CO2 and CH4 in the IL 1-n-butyl-3-methylimidazolium bis­(trifluoromethyl­sulfonyl)­imide ([C4mim]+[Tf2N]), along with the pure gas solubilities and diffusivities, in order to test the ideal permselectivity assumption. Pure gas solubilities of CO2 and CH4 in [C4mim]+[Tf2N] at 333 K and pressures ranging from 1 to 100 bar were found to be in excellent agreement with literature values. Simulated CO2:CH4 solubility selectivities in [C4mim]+[Tf2N] at CO2:CH4 gas phase mole ratios of 4:96, 8:92 and 16:84 were found to be less than the ideal solubility selectivities computed from pure gas solubilities. Self-diffusion coefficients of pure CO2 and CH4 dissolved in the IL were independent of the concentration of the dissolved gases, within the studied pressure range. In CO2/CH4–[C4mim]+[Tf2N] mixtures, self-diffusion coefficients of CO2 and CH4 were similar for all CO2:CH4 mole ratios. Computed permselectivites were only slightly smaller than the ideal permselectivites computed from pure gas properties. Because the self-diffusion coefficients for CO2 and CH4 are similar, only solubility selectivity was found to influence the overall permselectivity of CO2 over CH4 in the IL.