Composite organic ionic plastic crystal membranes: The effect of ether-functionalized cations on light-gas separation performance

In light of increasing concerns about climate change, there is a growing need for innovative solutions to address CO₂ emissions. Addressing this urgency, this work presents a unique approach to CO₂ separation utilizing composite membranes of organic ionic plastic crystals (OIPCs) with ether-functionalized cations and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). Here we report the gas separation performance of OIPC-based membranes of 3,3-dimethyloxazolidinium [C₁moxa]⁺, 4-ethyl-4-methylmorpholinium [C₂mmor]⁺ and 4-isopropyl-4-methylmorpholinium [C_i3mmor]⁺ cations paired with the bis(fluorosulfonyl)imide [FSI]^- anion. These composites demonstrated very good gas separation properties, especially [C₁moxa][FSI] which produced a permeability of 63 barrer for CO₂ and an overall selectivity (CO₂/N₂) of 205, which is the highest amongst all the OIPCs reported so far. Additionally, a large change in separation performance was observed for the [C_i3mmor][FSI] membrane upon heating above the solid-solid phase transition (II - I) at 48 °C; the CO₂ permeability increased from 7 to 163 barrer and an approximately 3-fold increase in selectivity was observed. These findings advance the design of composite membranes based on OIPCs towards increased selectivity and sustained effectiveness in the separation of light gases.