Tailored CO<sub>2</sub>-Philic Anionic Poly(ionic liquid) Composite Membranes: Synthesis, Characterization, and Gas Transport Properties
Irshad Kammakakam, Jason E. Bara, Enrique M. Jackson, Josu Lertxundi, David Mecerreyes, Liliana C. Tomé
Abstract
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 ([C2 mim]+) countercations were successfully synthesized and photopolymerized with two distinct amounts of free IL containing the same structural cation ([C2 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/N2 selectivities together with moderate CO2/H2 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 ∼20 barrer and αCO2/CH4 ∼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.