Performance optimization of imidazole containing copolyimide/functionalized ZIF-8 mixed matrix membrane for gas separations

The combination of functionalized metal organic frameworks with a polymer with structural similarity is a feasible route to enhance binary phase adhesion of hybrid membranes. Herein, we incorporate amine-modified ZIF-8 with a new type of much permeable imidazole-containing copolyimide (6FBD), which contains the bulky 2,4,6-trimethyl-1,3-diaminobenzene groups, to produce compatible MMMs with optimized gas separation performance. The addition of bulky groups in polymer matrix significantly increases the membrane permeability and the presence of imidazole groups in both polymers and ZIFs ensures the good compatibility. By contrast with ZIF-8 analogs, amine-modified ZIF-8 demonstrates much enhanced interfacial adhesion, ascribed to its hydrogen bonding created between amine groups and polymer matrix. This is manifested as an increase in glass transition temperature, an improvement in thermal stability and increased gas selectivity. Systematic studies of gas separation performance at various temperatures from −20 °C to 60 °C reveal that the hydrogen bonding-induced interfacial force enhancement is more favorable towards CO₂/CH₄ and H₂/CH₄ separations especially under sub-ambient temperatures. The CO₂/CH₄ ideal selectivity of 6FBD/30%ZIF-8-NH₂ membrane increases from 16.6 at 60 °C to 119 at −20 °C, which are increased by more than 6 times, while 6FBD/30%ZIF-8 membrane shows only around 2.5 times enhancement. This finding indicates tailoring hybrid membrane interfacial structure by creating hydrogen bonds offers more benefits when applied at sub-ambient operation conditions.