Application of Protic Ionic Liquids to CO₂ Separation in a Sulfonated Polyimide-Derived Ion Gel Membrane

It is of great interest to determine whether a relatively easily prepared and inexpensive protic ionic liquid (PIL) could be used in place of an aprotic ionic liquid (AIL) in a composite membrane for CO2 separation. We prepared mechanically reliable sulfonated polyimide (SPI) composite membranes with a PIL, [N124][NTf2] (N,N,N-butylethylmethylammonium bis(trifluoromethylsulfonyl)amide) by a solution casting method, and their mechanical, thermal, and gas separation (CO2 and N2) properties were compared with those of membranes incorporating an isomeric AIL, [N1123][NTf2] ([N1123]+, N,N,N,N-ethyldimethylpropylammonium). We discussed the influence of the presence or absence of active protons in the ILs on CO2 separation characteristics and the applicability of the PIL to a CO2 separation membrane. The SPI/PIL membrane exhibited a slightly lower selectivity, but higher permeability, than the SPI/AIL membrane. It is supposed that these differences originate from the difference in membrane structure and CO2 interactions between PIL and AIL, which was investigated through a combination of ab initio calculations, CO2 solubility measurements, and dynamic mechanical analysis. The [N124][NTf2](PIL)/SPI membranes showed a CO2 permeability (PCO2 = 240 Barrer) with a relatively high selectivity (αCO2/N2 = 23) at 30 °C, which indicates that a PIL/SPI composite membrane can be employed for CO2 separation. We also explored CO2 separation under elevated temperatures and humidified conditions for confirming the applicability of the SPI/PIL membrane. Finally, the effect of PIL structures was studied by using [N122][NTf2] ([N122]+, N,N,N-diethylmethylammonium) in comparison with [N124][NTf2]. Composite [N122][NTf2]/SPI membranes exhibited a comparable selectivity and an improved permeability. We anticipate these results will unveil a new paradigm in functional ion gels for critical CO2 separation technology, toward utilization of effective, benign materials.