ABA-triblock copolymer ion gels for CO2 separation applications

The gas separation performance of two block copolymer ion gel systems were examined for CO2/N2 and CO2/CH4 gas pairs. The ion gels were formed by self-assembly of 10–15 wt% poly(styrene-b-ethylene oxide-b-styrene) (SOS) and poly(styrene-b-methyl methacrylate-b-styrene) (SMS) triblock copolymers in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIM][TFSA]), via association of the insoluble S blocks. Due to the high ionic liquid (IL) concentration, these gels exhibit rapid gas transport properties as well as high selectivity. The selectivity for both gas pairs was influenced by the type of polymer midblock: the SOS gel shows higher selectivity than both pure IL and SMS gels. Gas solubility tests on [EMI][TFSA]/PEO solutions indicate that poly(ethylene oxide) can suppress the solubility of N2 and CH4, and thereby increase the selectivity for CO2. This important result qualitatively agrees with the gel gas separation performance, and confirms the importance of the midblock chemistry. Additionally, since the selectivity mainly stems from the differential gas solubility rather than gas diffusivity, the real selectivities obtained from mixed gas experiments are almost the same as the ideal selectivities. Ion gels show favorable separation performance in comparison with the corresponding “upper bound” on the Robeson plots for CO2/N2 and CO2/CH4, highlighting the potential of these gels in future CO2-selective membrane separation.