Ring-opened heterocycles: Promising ionic liquids for gas separation and capture

We report on a new class of highly fluid ionic liquids integrating a cation that resembles an opened imidazolium structure with two distinct anions, bis(trifluoromethylsulfonyl)imide, [Tf2N], and a nitrile-containing anion, [C(CN)3]. These new ionic liquids show exceptional CO2 permeability values in liquid membrane gas separations with results that equal or exceed the Robeson upper bound. Moreover, these ionic liquids offer ideal CO2/N2 selectivities competitive with the best results reported to date, exhibiting values that range from 28 to 45. The nitrile containing ionic liquid displayed the highest ideal CO2/N2 selectivity with a value of 45 which primarily results from a reduction in the nitrogen permeability. In addition to permeability results, CO2 solubilities were also measured for the this new class of ionic liquids with values similar to the popular 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The CO2 solubility results were compared to predicted values obtained using both a modified regular solution theory and the quantum chemical Conductor-like Screening Model for Real Solvents (COSMO-RS) method. Agreement between predicted and measured solubility values is also discussed.

► Novel ionic liquids with structure of ring-opened heterocycle are described.
► The ionic liquids show high CO2 permeability and CO2/N2 selectivity.
► Performance attains and exceeds Robeson limit.
► Regular solution theory and COSMO-RS are used to describe solubility.