Enhanced gas separation by free volume tuning in a crown ether-containing polyimide membrane

Tuning coupled free volume and pore size distribution is of great significance for membrane-based gas separation. In the present work, a polyimide membrane containing bulky -CF₃ group and crown ether ring structure was developed using 4, 4′ - (hexafluoroisopropylidene) diphthalic anhydride as dianhydride monomer, 2, 2′-bis [4-(4-aminophenoxy)phenyl] -hexafluoropropanane and di(aminobenze)-21-crown-7-ether as diamine monomers. Diamine monomer with large bulky -CF₃ grant the polyimide membrane more free volume, which was helpful for gas permeability enhancement. Besides, crown ether could act as a pores size distribution tuning monomer, which endowed membranes with improved gas selectivity. Results showed that the introduction of crown ether into polyimide backbone exhibited a significant impact on free volume and pores size distribution. Typically, with the increasing crown ether content, the free volume decreased, whereas the relative available free volume ratio (FAV_i/FAV_j) achieved a remarkable increment. It's also worth noting that the pores size distribution was changed by copolymerization with crown ether. The intensity of macro-pore gradually decreased accompanied with an increase in micro-pore structure. As a result, the gas selectivity significantly increased. Compared the membrane without crown ether, the maximum selectivity enhancement was achieved at 30 mol% di(aminobenze)-21-crown-7-ether content. It exhibited remarkable selectivity increments about 204.8%, 198.8%, 123.2%, and 115.4% for He/CH₄, H₂/CH₄, CO₂/CH₄, and CO₂/N₂, respectively. These results took a huge step forward which nearly hit up to the 2008 Robeson's upper bound, especially for CO₂/CH₄.