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 CO 2 in N 2 at 35 °C. In the dry case, the best performing IL, triethyl(octyl)phosphonium 4-bromopyrazolide ([P 2228 ][4-BrPyra]) exhibited mixed-gas CO 2 permeabilities and CO 2 /N 2 permeability selectivities as high as 26,800 barrer and 7,000, respectively. In the presence of humidity, the CO 2 permeability and CO 2 /N 2 selectivity increased to 49,100 barrer and 13,200, respectively, and these are the highest reported combination in the literature. Humidity amplifies CO 2 permeabilities and CO 2 /N 2 permeability selectivities through increases in CO 2 capacity due to bicarbonate formation and through faster mobility of the mobile carrier from decreased viscosity . N 2 permeability stayed roughly invariant in the presence of humidity, likely from competing effects of viscosity reduction and lower N 2 solubility. • SILMs exhibit dual-mode facilitated transport membrane behavior. • Humidity enhanced CO 2 permeability and CO 2 /N 2 selectivity for all AHAs. • SILMs achieve CO 2 /N 2 selectivity of 13,200 at 420 ppm CO 2 . • SILMs achieve CO 2 permeability of 49,100 barrer at 420 ppm CO 2 .