Investigation of gas permeation properties of film forming polymeric ionic liquids (PILs) based on polybenzimidazoles

• Film forming PILs based on PBI were investigated for gas sorption and permeation.
• These PILs possessed attractive combination of CO2 permeability and PCO2/PCH4PCO2/PCH4 selectivity.
• PBI to PILs conversion largely improved CO2 sorption and permeation than for other gases.
• Some of the PILs showed remarkably high PHe/PH2PHe/PH2 (~1.7–2).
• [TMPBI-BuI][Ac] exhibited highest CO2 sorption as well as selectivity over N2 and CH4.

Polymeric ionic liquids (PILs) are emerging as new generation membrane materials for CO2 separation. Gas permeation properties of a recently developed PIL family based on three structurally different polybenzimidazoles (PBIs) possessing excellent film forming characteristics are reported. Effects of cationic backbone and anions on gas permeation properties of PILs using pure gases (He, H2, N2, CH4 and CO2) were examined at 20 atm upstream pressure and correlated with their physical properties. In comparison to the parent PBIs, PILs exhibited generally higher CO2 permeability, without largely affecting CO2 based selectivity. Some of these PILs exhibiting high PCO2/PCH4PCO2/PCH4 selectivity and PH2PH2/PCO2PCO2 approaching ~1 indicated dominance of CO2 sorption over diffusion. The CO2 sorption specificity of these PILs was distinctly observed based on their improved SCO2/SN2SCO2/SN2, SCO2/SCH4SCO2/SCH4 and SCO2/SH2SCO2/SH2 selectivity than their parent PBIs. A large variation in permeation properties of PILs based on a common polycation but different anions indicated that anions play a crucial role on governing gas permeation properties of these PILs.