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

Abstract

Herein, we report a strategy for\ndeveloping dual anionic–cationic\nphotocrosslinked polymerized­(ionic liquid)-ionic liquid [poly­(IL)-IL]\ncomposite gas separation membranes for enhanced CO₂ selectivity.\nThese materials represent the first dually photopolymerized anion–cation\npoly­(IL) materials, in which the backbones of the poly­(IL) formed\nfeature both pendant anions and cations without any mobile counterions,\nunlike any poly­(IL) material reported so far in the literature. This\nis enabled by the synthesis of a dually photopolymerizable anionic–cationic\nIL (DIL) monomer having a methacrylate functional group tethered with\na highly delocalized sulfonimide anion (−SO₂–N⁽⁻⁾–SO₂–C₇H₇) and a <i>N</i>-vinylimidazolium counterion ([C₄vim]⁺). To form these membranes, this DIL was photopolymerized\nin four compositions with a nonpolymerizable IL with a structurally\nanalogous cation ([C₄mim]­[Tf₂N]) and 20 wt %\npolyethylene glycol diacrylate (PEGDA) crosslinker. The property characterizations\nof the corresponding four dual anionic–cationic poly­(IL)-IL\ncomposite membranes were performed using Fourier transform infrared\nspectroscopy, differential scanning calorimetry, thermogravimetric\nanalysis, and X-ray diffraction analysis. All of these dual anionic–cationic\npoly­(IL)-IL composite membranes displayed outstanding permselectivities\nfor CO₂/CH₄, CO₂/N₂, and\nCO₂/H₂ gas pairs together with reasonable CO₂ permeabilities. As a result, all dual anionic–cationic\npoly­(IL)-IL composite membranes outperformed the common poly­(IL)-IL\nsystems with respect to upper bound plots with a maximum CO₂ permeability of 40 barrer and CO₂/CH₄ permselectivity\nof 85. This study provides a unique direction by which to further\nexplore the potential of poly­(IL)-IL composites for selective separation\nof CO₂ from flue gas, natural gas, and syngas streams.