Ionic liquids with asymmetric thioether-TFSI anions: Thermal properties and gas separation

A straightforward and efficient thiol-ene Michael addition “click” chemistry route was developed to introduce asymmetry into the TFSI anion, enabling the synthesis of four ionic liquids (ILs) by coupling 1-ethyl-3-methylimidazolium (EMIM) cation with thioether-functionalized anions. This method offers high yields, excellent purities, mild reaction conditions, and versatile functionalization. The resulting ILs exhibit suppressed crystallization and vitrification behavior, with low glass transition temperatures (−75 to −62°C), ensuring liquid-state stability over a wide temperature range. Compared to common ILs, such as EMIM TFSI, EMIM 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide (TFSAM) and EMIM 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide (TSAC), the synthesized ILs display higher viscosities, which reduce CO₂ diffusion and lower overall permeabilities (133–221 Barrer). Nevertheless, they demonstrate enhanced CO 2 /N 2 selectivity (up to 41), with EMIM C₂H₅-S-TFSI combining the highest permeability (221 Barrer) with excellent selectivity. These findings highlight the potential of thioether-modified TFSI ILs for gas separation applications where selectivity is of primary importance.