Dual Anion–Cation Crosslinked Poly(ionic liquid) Composite Membranes for Enhanced CO₂ Separation

Herein, we report a strategy for developing dual anionic–cationic photocrosslinked polymerized(ionic liquid)-ionic liquid [poly(IL)-IL] composite gas separation membranes for enhanced CO2 selectivity. These materials represent the first dually photopolymerized anion–cation poly(IL) materials, in which the backbones of the poly(IL) formed feature both pendant anions and cations without any mobile counterions, unlike any poly(IL) material reported so far in the literature. This is enabled by the synthesis of a dually photopolymerizable anionic–cationic IL (DIL) monomer having a methacrylate functional group tethered with a highly delocalized sulfonimide anion (−SO2–N(−)–SO2–C7H7) and a N-vinylimidazolium counterion ([C4vim]+). To form these membranes, this DIL was photopolymerized in four compositions with a nonpolymerizable IL with a structurally analogous cation ([C4mim][Tf2N]) and 20 wt % polyethylene glycol diacrylate (PEGDA) crosslinker. The property characterizations of the corresponding four dual anionic–cationic poly(IL)-IL composite membranes were performed using Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction analysis. All of these dual anionic–cationic poly(IL)-IL composite membranes displayed outstanding permselectivities for CO2/CH4, CO2/N2, and CO2/H2 gas pairs together with reasonable CO2 permeabilities. As a result, all dual anionic–cationic poly(IL)-IL composite membranes outperformed the common poly(IL)-IL systems with respect to upper bound plots with a maximum CO2 permeability of 40 barrer and CO2/CH4 permselectivity of 85. This study provides a unique direction by which to further explore the potential of poly(IL)-IL composites for selective separation of CO2 from flue gas, natural gas, and syngas streams.