Finely tailoring microstructure of hyperbranched polyimide membrane for facile natural gas upgrading

Polyimide membranes offer the potential of industrial natural gas sweetening to effectively diminish carbon dioxide from raw streams, but it remains a challenge to improve the separation selectivity and further overcome the trade-off limitation through synthesis design. Here, we present an A 2 +B 2 +B 3 strategy to fabricate a series of polyimides containing hyperbranched network structures by copolymerizing the triamine monomer (TAPA) in conjunction with linear 6FDA-6FAP. The chemical structures and physicochemical properties of polyimides were confirmed by SEM, FTIR, XPS, TGA and XRD. The molar ratio changes of 6FAP/TAPA finely tailored the fractional free volume from 16.9 % to 13.1 % and brought about high packing efficiency, leading to a surge in separation selectivity. The 6FDA-6FAP/TAPA (7:3) membrane exhibited a maximum CO 2 /CH 4 selectivity of 134.5 with a permeability of 26.9 Barrer, improving the CO 2 /CH 4 separation performance of 6FDA-6FAP from far below the 1991 limitation to beyond the 2008 Robeson's upper bound line. Diffusivity and solubility acting as the main factors that govern the gas separation were also carefully investigated. Additionally, 6FDA-6FAP/TAPA(7:3) demonstrated high CO 2 /CH 4 binary gas separation performance, good high-temperature (105 °C) reversibility, great plasticization resistance (20 bar) and moderate long-term stability (360 days) under practical working conditions. These results indicate that hyperbranching of polyimide can be successfully controlled to enhance the separation properties, which facilitate the development of high-performance gas separation membranes with topological microstructures.