Polymeric ionic liquids with mixtures of counter-anions: a new straightforward strategy for designing pyrrolidinium-based CO2 separation membranes

Abstract

Polymeric ionic liquids (PILs) are interesting membrane materials for CO₂ separation. In order to increase the flexibility in tailoring the permeability and selectivity of PIL-based membranes for flue gas separation and natural gas purification, this work explores the use of PILs with mixtures of counter-anions employing a straightforward strategy. A new family of PIL random copolymers having pyrrolidinium cation pendant units combined with different counter-anion mixtures was synthesized and characterized. A simple and quantitative anion exchange procedure was successfully applied to the commercially available poly(diallyldimethylammonium) chloride as confirmed by NMR, FTIR and titration experiments. Composite membranes of the copolymers with 20 wt% of free ionic liquid ([pyr₁₄][NTf₂]) were prepared and their CO₂, CH₄ and N₂ permeation properties were measured at 20 °C using a time-lag apparatus. In addition, their tensile mechanical properties were also assessed. The results show that the permeability of all gases in the composite membranes is related to their gas diffusivities which are strongly dependent on the second counter-anion. The prepared membranes exhibit permselectivities ranging from 10.8 to 29.3 for CO₂/CH₄ and from 21.4 to 32.0 for CO₂/N₂. Furthermore, their CO₂ separation performance as a function of permeability can be tuned by using PILs with different counter-anion mixtures, which opens new possibilities for designing the CO₂ separation of these materials.