| Summary: | Poly(ethylene oxide) (PEO)-based copolymers are at the forefront of advanced membrane materials for selective CO<sub>2</sub> separation. In this work, free-standing composite membranes were prepared by blending imidazolium-based ionic liquids (ILs) having different structural characteristics with a PEO-based copolymer previously developed by our group, targeting CO<sub>2</sub> permeability improvement and effective CO<sub>2</sub>/gas separation. The effect of IL loading (30 and 40 wt%), alkyl chain length of the imidazolium cation (ethyl- and hexyl- chain) and the nature of the anion (TFSI<sup>-</sup>, C(CN)<sub>3</sub><sup>-</sup>) on physicochemical and gas transport properties were studied. Among all composite membranes, PEO-based copolymer with 40 wt% IL3-[HMIM][TFSI] containing the longer alkyl chain of the cation and TFSI<sup>-</sup> as the anion exhibited the highest CO<sub>2</sub> permeability of 46.1 Barrer and ideal CO<sub>2</sub>/H<sub>2</sub> and CO<sub>2</sub>/CH<sub>4</sub> selectivities of 5.6 and 39.0, respectively, at 30 °C. In addition, almost all composite membranes surpassed the upper bound limit for CO<sub>2</sub>/H<sub>2</sub> separation. The above membrane showed the highest water vapor permeability value of 50,000 Barrer under both wet and dry conditions and a corresponding H<sub>2</sub>O/CO<sub>2</sub> ideal selectivity value of 1080; values that are comparable with those reported for other highly water-selective PEO-based polymers. These results suggest the potential application of this membrane in hydrogen purification and dehydration of CO<sub>2</sub> gas streams.
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