Exploring the effect of fluorinated anions on the CO₂/N₂ separation of supported ionic liquid membranes

Abstract

The CO₂ and N₂ permeation properties of ionic liquids (ILs) based on the 1-ethyl-3-methylimidazolium cation ([C₂mim]⁺) and different fluorinated anions, namely 2,2,2-trifluoromethylsulfonyl-N-cyanoamide ([TFSAM]^-), bis(fluorosulfonyl) imide ([FSI]^-), nonafluorobutanesulfonate ([C₄F₉SO₃]^-), tris(pentafluoroethyl)trifluorophosphate ([FAP]^-), and bis(pentafluoroethylsulfonyl)imide ([BETI]^-) anions, were measured using supported ionic liquid membranes (SILMs). The results show that pure ILs containing [TFSAM]^- and [FSI]^- anions present the highest CO₂ permeabilities, 753 and 843 Barrer, as well as the greatest CO₂/N₂ permselectivities of 43.9 and 46.1, respectively, with CO₂/N₂ separation performances on top of or above the Robeson 2008 upper bound. The re-design of the [TFSAM]^- anion by structural unfolding was investigated through the use of IL mixtures. The gas transport and CO₂/N₂ separation properties through a pure [C₂mim][TFSAM] SILM are compared to those of two different binary IL mixtures containing fluorinated and cyano-functionalized groups in the anions. Although the use of IL mixtures is a promising strategy to tailor gas permeation through SILMs, the pure [C₂mim][TFSAM] SILM displays higher CO₂ permeability, diffusivity and solubility than the selected IL mixtures. Nevertheless, both the prepared mixtures present CO₂ separation performances that are on top of or above the Robeson plot.