Ionic liquid enhancement of interface compatibility in mixed-linker ZIF-based mixed matrix membranes for advanced CO2/CH4 separation

Mixed matrix membranes (MMMs) are highly promising for natural gas sweetening due to their high microporosity and outstanding gas separation performance. However, MMMs are frequently challenged by the polymer/filler interfacial incompatibility, leading to the deterioration of gas separation performance. In this study, the mixed-linker ZIF-7-8 was employed as a filler by finely tuning the molecular sieving for CO2/CH4 separation. An amino-functionalized ionic liquid ([C3NH2Mim][Cl]) was introduced for constructing the interface connection between the ZIF-7-8 filler and polymer matrix through coordination and hydrogen bonding, which was verified by extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR). The design of interface interaction through an IL is crucial to improve the interface compatibility and the CO2/CH4 separation performance. The newly developed 20% ZIF-7-8-IL (1%)/PI and 25% ZIF-7-8-IL (1%)/PI MMMs displayed a dramatic improvement of CO2 permeability by 244.2% and 291.6%, from 308 to 1060 and 1206 barrer, respectively, for pure-gas separation, compared to the pristine polyimide matrix. Moreover, the 25% ZIF-7-8-IL (1%)/PI MMM also demonstrated outstanding low-temperature mixed-gas CO2/CH4 separation performance in the temperature range of −45–35 °C, far beyond the 2018 mixed-gas CO2/CH4 upper bound.