High performance membranes based on ionic liquid polymers for CO₂ separation from the flue gas

Three vinyl functionalized imidazolium based room-temperature ionic liquids (RTILs): 1-vinyl-3-ethylimidazolium dicyanamide ([veim][dca]), 1-vinyl-3-butylimidazolium dicyanamide ([vbim][dca]) and 1-vinyl-3-heptylimidazolium dicyanamide ([vhim][dca]) were synthesized and UV-polymerized to form free standing membranes. The pure gas permeabilities of CO₂ and N₂ of these newly developed membranes acquired at 1 atm 35 °C increased with an increase in the number of N-alkyl group in the monomers but their ideal CO₂–N₂ selectivities decreased. The three vinyl functionalized monomers were blended with three free RTILs: 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]), 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)₄]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF₄]) and subsequently subjected to UV-polymerization to form the poly(RTIL)–RTIL composite membranes. The incorporation of free RTILs not only significantly increases the CO₂ permeability but also greatly improves the CO₂–N₂ selectivity. The best separation performance is achieved for the poly([vbim][dca])–[emim][B(CN)₄] (1 : 2) and poly([vbim][dca])–[emim][dca] (1 : 2) composite membranes, which have CO₂ permeabilities of 340 and 273 barrers and CO₂–N₂ selectivities of 42 and 53, respectively, at 1 atm 35 °C. The mixed gas permeabilities of these two composite membranes are 297 and 253.5 barrers with corresponding CO₂–N₂ selectivities of 38.8 and 50.6, respectively, using a 50 : 50 CO₂–N₂ mixed gas at 2 atm 35 °C. These two composite membranes have separation performance very close to the 2008 “Robeson Upper Bound”, suggesting their potential for industrial applications, especially for the post-combustion flue gas treatment.