Performance of nitrile-containing anions in task-specific ionic liquids for improved CO₂/N₂ separation

This work explores the performance of a series of ionic liquids that incorporate a nitrile-containing anion paired to 1-alkyl-3-methylimidazolium cations in tailoring the selectivity and permeance of supported ionic liquid membranes for CO₂/N₂ separations. The permeance and selectivity of three ionic liquids, each with an increasing number of nitrile groups in the anion (i.e., two, three, and four), were measured using a non-steady-state permeation method. By predictably varying the molar volume and viscosity of the ionic liquids, we show that the solubility, selectivity, and permeance can be optimized for CO₂/N₂ separation through controlled introduction of the nitrile functionality into the anion. Of the three nitrile-based ionic liquids studied, 1-ethyl-3-methylimidazolium tetracyanoborate, [emim][B(CN)₄], showed the highest permeance with a value of 2.55×10^-9mol/(m²Pas), a magnitude 30% higher than that of the popular ionic liquid [emim][Tf₂N]. This same nitrile-bearing ionic liquid also exhibited a high CO₂/N₂ selectivity of approximately 53. Additionally, the carbon dioxide solubility for each ionic liquid was measured at room temperature with [emim][B(CN)₄] again exhibiting the highest CO₂ solubility. Results from our study of the nitrile-based ionic liquids can be rationalized in terms of regular solution theory wherein the selectivity and permeance of a given SILM system are largely determined by the molar volume and viscosity of the corresponding ionic liquid phase.