Solid-state protic and aprotic salt–polymer composite membranes for CO₂/N₂ separation

Selectively removing CO₂ from gas streams is central to carbon capture. Membranes are an attractive option, but their performance is generally limited by low CO₂ solubility and the permeability-selectivity trade-off. This study investigates a new class of composite membranes in which solid-state organic salts are immobilized in a poly(ethylene oxide) (PEO) matrix blended with ~9 wt% polydimethylsiloxane (PDMS), aiming to combine two modes of CO₂ uptake: physical absorption (dissolution) and chemical absorption (reversible reaction). Three protic guanidinium salts, guanidinium methanesulfonate ([Gdm][Ms]), benzenesulfonate ([Gdm][Bs]) and nitrate ([Gdm][NO₃]), are investigated for the first time and expected to absorb CO₂ by physical dissolution, while the aprotic salt, tetraethylammonium benzimidazolide ([N₂₂₂₂][BnIm]), incorporates a CO₂-reactive aprotic heterocyclic anion (AHA) within the membrane. Across the protic series, CO₂ solubility follows the basicity of the anion, while CO₂ permeability and selectivity reflect the combined contributions of sorption affinity and ionic diffusivity; [Gdm][Ms] gave the best protic performance (CO₂ permeability of 83 ± 8 Barrer, CO₂/N₂ ideal selectivity of 60 ± 11). The [N₂₂₂₂][BnIm] membrane outperformed the protic systems, reaching a CO₂ permeability of 705 ± 18 Barrer and a CO₂/N₂ ideal selectivity of 542 ± 20, surpassing the Robeson upper bound. ATR-FTIR spectra of the neat salt showed bands consistent with reversible CO₂ binding through carbamate formation. Together with the pressure-dependent permeability, this suggests that physical dissolution and a facilitated-transport pathway operate together in this membrane. Thus, this study introduces solid CO₂ reactive salts combined with polymers as a promising route to high-performance membranes for CO₂ capture.