Sulfonated Polyimide/Ionic Liquid Composite Membranes for CO2 Separation: Transport Properties in Relation to Their Nanostructures

The transport properties of ionic liquid (IL)/sulfonated polyimide (SPI) composite membranes for CO2 separation were explored in relation to their nanostructures. 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C4mimNTf2), 1-butyl-3-methylimidazolium hexafluorophosphate (C4mimPF6), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide (P14NTf2) were selected as the ILs. These composite membranes enable favorable CO2 separation and superior mechanical properties. SPI is plasticized by the IL, which induces a decrease in the elastic modulus; however, due to the formation of bicontinuous nanostructures of IL-rich and SPI-rich phases, a modulus of >10 MPa is retained even with the incorporation of 75 wt % IL. C4mimNTf2 and C4mimPF6 exhibit stronger plasticization effects on SPI than P14NTf2. The diffusion coefficients of CO2 and N2, which were measured by the time-lag method, abruptly increase with increasing IL content in the C4mimNTf2/SPI composite membranes, which coincided with a change from an isolated to a continuous IL-rich phase structure. On the other hand, the selective solubility of CO2 in the C4mimNTf2/SPI composite membranes exhibits no relation with the IL content. Consequently, the permeation coefficient (PCO2) of the membranes increases with increasing IL content without decreasing the separation factor (αCO2/N2), showing PCO2 = 431 barrer and αCO2/N2 = 30 at 30 °C.