Sulfonated polyimide/ionic liquid composite membranes for carbon dioxide separation

A series of composite membranes (SPI-NTf2 and SPI-PF6) consisting of a sulfonated polyimide (SPI) and an ionic liquid (IL), either [C(4)mim][NTf2] or [C(4)mim] PF6, where [C(4)mim] and [NTf2] stand for 1-butyl-3-methylimidazolium and bis(trifluoromethanesulfonyl) amide, respectively, were prepared, and their thermal, mechanical and gas (carbon dioxide (CO2) and N-2) permeation properties were systematically investigated. The ILs functioned as plasticizers of the SPI, making the composite membranes softer (lower Young's modulus and larger elongation at break). However, SPI-NTf2 (75), containing 75 wt % [C(4)mim][NTf2], still exhibited a Young's modulus higher than 10 MPa, which is much higher than that reported for common IL/polymer composites. The SPI membrane behaved as a gas barrier membrane; however, the addition of IL greatly enhanced gas permeability. Consequently, SPI-NTf2 (75) exhibited CO2 permeability (PCO2) of 412 Barrer at 30 degrees C and selectivity against N-2 (PCO2/PN2) of 27. The SPI-PF6 composite membranes showed similar behavior; however, gas permeability exhibited by these membranes was lower than that exhibited by the SPI-NTf2 composite membranes. In permeation tests under mixed-gas conditions, the PCO2 value decreased than that under single-gas conditions, whereas PN2 increased, and PCO2/PN2 decreased (from the range 29-27 to 12-16).