Supported ionic liquid membranes (SILMs) with exceptional selectivity and permeability for dilute CO₂ separations

Supported ionic liquid membranes (SILMs), containing phosphonium ionic liquids with aprotic N-heterocyclic anions (AHA ILs) in an inert inorganic support, were tested under both dry and humidified (40 % RH) mixed-gas conditions down to 420 ppm CO₂ in N₂ at 35 °C. In the dry case, the best performing IL, triethyl(octyl)phosphonium 4-bromopyrazolide ([P₂₂₂₈][4-BrPyra]) exhibited mixed-gas CO₂ permeabilities and CO₂/N₂ permeability selectivities as high as 26,800 barrer and 7,000, respectively. In the presence of humidity, the CO₂ permeability and CO₂/N₂ selectivity increased to 49,100 barrer and 13,200, respectively, and these are the highest reported combination in the literature. Humidity amplifies CO₂ permeabilities and CO₂/N₂ permeability selectivities through increases in CO₂ capacity due to bicarbonate formation and through faster mobility of the mobile carrier from decreased viscosity. N₂ permeability stayed roughly invariant in the presence of humidity, likely from competing effects of viscosity reduction and lower N₂ solubility.