Stable, Long-Term, High-Temperature, and High-Pressure Operation of Poly(ionic liquid)-Ionic Liquid Membranes Made with Ionic-Liquid-Based Multifunctional Cross-Linkers for CO2/CH4 Separation

The long-term stability and mixed-gas CO2/CH4 separation performance of free-standing poly(ionic liquid)-ionic liquid (PIL-IL) composite membranes made with multifunctional imidazolium bistriflimide IL-based cross-linkers were evaluated at elevated temperatures (up to 50 ?) and feed pressures (up to 40 atm). PIL-IL membranes made with the IL-based cross-linkers exhibited higher CO2 permeability and CO2/CH4 selectivity compared to the membranes made with a conventional, nonionic cross-linker, divinylbenzene (DVB). The gas separation perform-ance of PIL-IL membranes can be further enhanced and tuned by varying the wt % of the cross-linker, the PIL:IL ratio, and both simultaneously. For example, a cross-linked PIL-IL composite membrane prepared using a PIL-IL ratio of 70:30 (i.e., 1-vinyl-3-methylimidazolium bistriflimide as the IL monomer and 1-ethyl-3-methylimidazolium bistriflimide as the free IL) with 8 wt % 1,3,5-tris(1'-methylene-3'-vinylimidazolium bistriflimide)benzene as the IL cross-linker exhibited a ~ 50% increase in CO2 permeability and a comparable CO2/CH4 selectivity compared to an analogous PIL-IL-zeolite MMM cross-linked with DVB. In addition, a similar PIL-IL membrane exhibited 90% retention of its initial mixed-gas CO2 permeability after testing at 50 ? and 40 atm feed pressure for 42 weeks (7100 h). In comparison, an analogous PIL-IL composite membrane cross-linked with DVB shattered after testing for 5 h under the same testing conditions.