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

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 ([C2mim]+) counter cations were successfully synthesized and photopolymerized with two distinct amounts of free IL containing the same structural cation ([C2mim][Tf2N]) and 20 wt% PEGDA crosslinker, 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 TGA, DSC, and XRD analysis. 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(1eq.)) 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 the polymer membrane design with strong candidate materials for practical applications.