Poly(vinylbenzyl chloride)-based poly(ionic liquids) as membranes for CO2 capture from flue gas

Over the last decade, membrane-based CO2 capture using ionic liquids (ILs) has been demonstrated as a promising technology. However, elaborative synthesis of monomers and long-term instability of IL-based composite membranes have so far limited their industrial relevance. In this paper, novel membranes are introduced for CO2 separation using poly(ionic liquids) (PILs) based on polyvinylbenzyl chloride (PVBC). Three PIL-based membranes were prepared as thin-film composites (TFC) by solvent casting with subsequent sealing. They were tested for the CO2 removal from synthetic flue gas. An ammonium-derivatised PVBC-analogue was prepared as a first PIL-type by polymerisation of an IL monomer, whereas two other PILs, respectively with hydroxyethyl ammonium and pyrrolidinium cations, were obtained using a modification of commercial PVBC. Introduction of bis(trifluoromethylsulfonyl) imide (Tf2N) anions was accomplished by metathesis. A thorough characterisation of the material structure, composition, membrane morphology and gas separation properties demonstrates that the presence of hydroxyl groups in the polycation enhanced the interaction with CO2 molecules. The mixed-gas selectivity increases with the higher positive charge on the cation species (hydroxyethyl-dimethylammonium > trimethylammonium > pyrrolidinium). More importantly, experiments performed in humidified conditions particularly revealed a doubled CO2 permeance and a 20-30% increased selectivity in comparison to dry conditions. These developments are spurring the application of PIL-based TFC membranes for CO2 capture from flue gas streams.