Highly Selective Benzimidazole-Based Polyimide/Ionic Polyimide Membranes for Pure- and Mixed-Gas CO2/CH4 Separation

•Hydrogen bonding and electrostatic interaction tighten microporous structure and restrict polymer chain mobility.•BET surface areas and d-spacing values decrease with benzimidazole molar content and the degree of ionization.•Hydrogen bonding and electrostatic interaction cause gain of CO2/CH4 selectivity at 35 °C by up to 85%.•Hydrogen bonding and electrostatic interaction induce charge transfer complexes formation and boost mixd-gas selectivity. This paper reports two new series of benzimidazole functionalized polyimides and ionic polyimides for highly selective membranes with great potential for natural gas sweetening. It has been demonstrated that both the –NH groups in the benzimidazole moieties and the corresponding ionic groups after N-quaternization tighten the microporous structure and restrict polymer chain mobility through hydrogen bonding and electrostatic interaction. The BET surface areas and d-spacing values decrease with benzimidazole molar content or the degree of ionization. Consequently, a linear correlation between CO2 permeability coefficients with benzimidazole molar content or degree of ionization was observed due to the decrease of CO2 diffusivity, and the monotonic increase of CO2/CH4 selectivities is ascribed to the increase of both diffusivity selectivity and solubility selectivity. The benzimidazole-based copolyimide and the ionic copolyimide membranes exhibited high CO2/CH4 selectivity under high-pressure mixed-gas conditions. In particular, the copolyimide PI-0.75 membrane displayed a mixed-gas CO2 permeability of 27 Barrer and CO2/CH4 selectivity of 47 at 20 bar total pressure. The performance was much higher than those of the state-of-the-art commercial cellulose triacetate membranes for natural gas upgrading. The facile polymer synthesis and microporosity tunability, as well as the excellent mixed-gas separation performance, render the copolyimide membranes in this study promising towards economic membrane-mediated natural gas upgrading.