CO2 separation applying ionic liquid mixtures: the effect of mixing different anions on gas permeation through supported ionic liquid membranes

Abstract

In order to increase flexibility in tailoring the permeability and selectivity of supported ionic liquid membranes (SILMs) for flue gas separation and natural gas purification, this work explores the use of ionic liquid mixtures. For that purpose, gas permeation properties of CO₂, CH₄ and N₂ in several binary ionic liquid mixtures based on a common cation ([C₂mim]⁺) and different anions such as bis(trifluoromethylsulfonyl)imide ([NTf₂]⁻), acetate ([Ac]⁻), lactate ([Lac]⁻), dicyanamide ([DCA]⁻) and thiocyanate ([SCN]⁻) were measured at 293 K using a time-lag apparatus. In addition to gas permeation results, the thermophysical properties of those mixtures, namely viscosity and density, were also determined so that trends between the two types of properties could be evaluated. The results show that mixing [Ac]⁻ or [Lac]⁻ with [NTf₂]⁻ promotes the decrease of gas permeability and diffusivity of the SILMs based on those binary mixtures, essentially due to their high viscosities. The pure ionic liquids containing anions with nitrile groups, [DCA]⁻ or [SCN]⁻, and also their mixtures with [C₂mim][NTf₂] exhibit permselectivities ranging from 19.1 to 23.0 for CO₂/CH₄, and from 36.6 to 67.8 for CO₂/N₂, as a consequence of a reduction in the CH₄ and N₂ permeabilities, respectively. Furthermore, it is shown that mixing anions with different chemical features allows variations in ionic liquid viscosity and molar volume that impact the gas permeation properties of SILMs, offering a clear pathway for the optimization of their CO₂ separation performances.