Significantly enhanced gas separation properties of membranes by debromination and thermal rearrangement simultaneously

One important challenge in advanced gas separation membranes is breaking the trade-off effect between gas permeability and selectivity. Here, we developed a method by combining the debromination and thermal rearrangement (TR) techniques together to significantly enhance the gas separation properties of the resulting membrane for the first time. First, we designed a co-polyimide (DBOH) containing both hydroxyl and bromine (50/50) groups in the ortho position of the imide group. After thermal treatment at 450 °C, the TR and debromination happened simultaneously as proved by TG-MS, FT-IR and XPS results, the resulting DBPO showed a larger d-spacing (7.03–9.71 Å), a 18-fold enhanced BET surface area, and 2-fold improved in ultra-microporosity (<7 Å) than its DBOH precursor. Hence, DBPO demonstrated not only an over 100 times improved CO2 permeability (7933 vs 76 Barrer) but also keep a high CO2/CH4 selectivity of 31, and the overall performance surpassed the latest 2019 trade-off curve. Besides, wonderful anti-plasticization and mixed-gas separation properties were also observed for DBPO, which showed a CO2 permeability of 3324 Barrer and CO2/CH4 selectivity of 14.7 even under the upstream CO2/CH4 mixed-gas pressure of 435 psi (1 psi = 6894.8 Pa), outperforming the latest 2018 CO2/CH4 mixed-gas upper limit. It showed 100 times improved CO2 permeability while maintain selectivity of DBPO is attributed to the significantly enhanced ultra-microporosity that increases the diffusion and solubility coefficient, whereas maintain their diffusion and solubility selectivity. Conclusively, the debromination coupled with TR provides a novel direction for designing advanced TR membranes.