Polybenzimidazole based polymeric ionic liquids possessing partial ionic character: Effects of anion exchange on their gas permeation properties

• Adequate anion exchanges were achieved with PILs possessing partial ionic character.
• Type of anion significantly governs the thermal stability even at lower DQ.
• PILs with BF4− anion exhibited high CO2 sorption and permeation.
• PILs with Tf2N− as an anion improved permeation of non-interacting gases also.

Polymeric ionic liquids (PILs) are emerging as promising membrane materials for CO2 separation. Present work is in continuation with our earlier efforts on evaluating gas permeation properties (especially, CO2) of film forming PILs based on polybenzimidazoles and efforts towards improving their permeability by structural tuning. This work specifically aims at evaluating effect of anion variation in chosen PILs possessing partial N-quaternization of imidazole moiety of PBI-BuI. Three anions (viz., Tf2N−, BF4− and Ac−) were chosen for the bromide exchange of two PILs, viz., [TBzPBI-BuI][Br]10 and [TBzPBI-BuI][Br]18, possessing degree of PBI N-quaternization (DQ) as 10 and 18%, respectively. Concurrent effects of variation in anion and DQ were analyzed in terms of physical and gas permeation properties of the resulting PILs. All of them possessed amorphous nature, adequate thermal stability (≥250 °C) and solvent solubility, which are primary requisites towards their applicability as a membrane material. PILs possessing BF4− anion exhibited improved CO2 permeability coefficient as well as its permselectivity over CH4 and N2, in comparison to their counterparts with other anions.

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