(Cross-Linked Poly(Ionic Liquid)–Ionic Liquid–Zeolite) Mixed-Matrix Membranes for CO2/CH4 Gas Separations Based on Curable Ionic Liquid Prepolymers

A three-component (cross-linked poly(ionic liquid) (PIL)–ionic liquid (IL)–zeolite), mixed-matrix membrane (MMM) platform based on curable IL prepolymers of controlled length has been developed for separating CO2 from CH4. Solutions of these curable prepolymers demonstrate increased resistance to support penetration compared to comparable solutions of analogous cross-linkable IL monomers. By adjusting the curable IL prepolymer chain length, it is possible to manipulate polymer susceptibility to support penetration, polymer solution gelation time, and gas separation performance in MMMs based on these materials. When a 50 wt % solution of the curable IL prepolymer with a degree of polymerization (x) of 87 was cast on an ultrafiltration support membrane, only 3.7 wt % of the polymer penetrated into the support. As the degree of polymerization of the curable IL prepolymer increases, the CO2/CH4 gas separation performance of the resulting MMM performance also improves. For example, an MMM synthesized using 64 wt % curable IL prepolymer (x = 87), 16 wt % [EMIM][Tf2N] as the IL, and 20 wt % SAPO-34 zeolite exhibited a CO2/CH4 selectivity of (42 ± 5) and a CO2 permeability of (47 ± 1) barrers. This CO2/CH4 separation performance is comparable to the previous generation of MMMs based on curable small-molecule IL monomers with the same IL and zeolite. However, this new MMM system also exhibits faster curing gelation times and the ability to be solution-cast onto a porous support for formation of thin-film composite membranes without significant selective layer soak-in.