Tailored CO2-Philic Anionic Poly(ionic liquid) Composite Membranes: Synthesis, Characterization, and Gas Transport Properties

Polymeric membranes either containing, or built from, ionic liquids (ILs) are of great interest for enhanced CO2/light gas separation due to the stronger affinity of ILs toward quadrupolar CO2 molecules and hence high CO2 solubility selectivity. Herein, we report the development of a series of four novel anionic poly(IL)-IL composite membranes via a photopolymerization method for effective CO2 separation. Interestingly, these are the first examples of anionic poly(IL)-IL composite systems in which the poly(IL) component has delocalized sulfonimide anions pendant from the polymer backbone with imidazolium cations as free counterions. Two types of photopolymerizable methacryloxy-based IL monomers (MILS) with highly delocalized anions (-SO2-N(-)-SO2-CF3 and -SO2-N(-)-SO2 C7H7) and mobile imidazolium ([C-2 mim](+)) countercations were successfully synthesized and photopolymerized with two distinct amounts of free IL containing the same structural cation ([C-2 miM][Tf2N]) and 20 wt % PEGDA cross-linker to serve as a composite matrix. The structure-property relationships of the four newly developed anionic poly(IL)-IL composite membranes were extensively characterized by thermogravimetric analysis, differential scanning calorimetry, and X-ray diffraction. All of the newly developed anionic poly(IL)-IL composite membranes exhibited superior CO2/CH4 and CO2/N-2 selectivities together with moderate CO2/H-2 selectivity and reasonable CO2 permeabilities. The membrane with an optimal composition and polymer architecture (MIL-C7H7/PEGDA((20%))/IL(1 equiv)) reaches the 2008 Robeson upper bound limit of CO2/CH4 due to the simultaneous improvement in permeability and selectivity (CO2 permeability similar to 20 barrer and alpha CO2/CH4 similar to 119). This study provides a promising strategy to explore the benefits of anionic poly(IL)-IL composites to separate CO2 from flue gas, natural gas, and syngas streams and open up new possibilities in polymer membrane design with strong candidate materials for practical applications.