CO2/light gas separation performance of cross-linked poly(vinylimidazolium) gel membranes as a function of ionic liquid loading and cross-linker content

A series of cross-linked poly(vinylimidazolium)-RTIL gel membranes was synthesized and evaluated for room-temperature, ideal CO2/N2, CO2/CH4, and CO2/H2 separation performance. The membranes were formed by photo-polymerization of oligo(ethylene glycol)-functionalized cross-linking (i.e., di-functional) and non-cross-linking (i.e., mono-functional) vinylimidazolium RTIL monomers with non-polymerizable, “free RTIL.” The effect of free RTIL ([emim][Tf2N]) loading on CO2 separation performance was evaluated by varying RTIL loading at three levels (45, 65, and 75 wt.%). The effect of cross-linker content on CO2 separation performance was also evaluated by varying the copolymer composition of cross-linked membranes from 5 to 100 mol% di-functional monomer. The substituent on the mono-functional RTIL monomer was also varied to investigate the effect of substituent structure and chemistry on CO2 separation performance. CO2 permeability was dramatically increased with higher loading of free RTIL. Increased RTIL loading had no effect on CO2/N2 or CO2/CH4 permeability selectivity, but significantly improved CO2/H2 permeability selectivity. Reducing the cross-linking monomer concentration generally improved CO2 permeability. However, anomalous permeability and selectivity behavior was observed below critical concentrations of cross-linker. The effect of the substituent on the mono-functional monomer on CO2 separation performance was minimal compared to the effects of RTIL loading and copolymer composition.