CO₂ separation of novel bis(trifluoromethylsulfonyl)imide [TFSI]⁻-based plastic crystal composite membranes are reported. A range of organic ionic plastic crystals based on [TFSI]⁻ anion paired with different cations are co-cast with polymers PVDF and PVDF-HFP, and the effect on the thermal properties, gas separation performance, and ionic mobility discussed.

Abstract

Membrane-based gas separation technologies are one solution towards mitigating global emissions of CO₂. New membrane materials with improved separation performance are still highly sought after. Composite membranes based on organic ionic plastic crystals (OIPCs) have shown preferential interaction for CO₂ over N₂, leading in some cases to competitive CO₂/N₂ selectivities. However, these ionic materials have been scarcely studied in the field of gas separation. Here, OIPCs based on the bis(trifluoromethylsulfonyl)imide ([TFSI]⁻) anion were investigated for use as gas separation membranes for the first time. The effect of the polymer type was also investigated, through the comparison of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) OIPC membranes. A strong temperature dependence of the gas separation performance was found, particularly in the N-methyl-N-ethylpyrrolidinium-based composites where the material undergoes a solid-solid phase transition within the testing temperature range. The polymer type was noted to induce a strong effect on the structure of the composites, as well as affecting the gas and ionic transport. Thus, this research provides insights on the influence of the [TFSI]⁻ anion on the structure and separation properties of OIPC-based composites, and new information towards the development of novel OIPC-based membranes with enhanced gas separation performance.