Preparation of High‐Performance Carbon Molecular Sieve Membranes Derived From Network Polyimide Containing Bromine Groups

ABSTRACT Network polyimide was a promising precursor for advanced carbon molecular sieves (CMS) membrane fabrication, since its firm cross‐linked structure could effectively prevent the collapse of micropores during the high‐temperature pyrolysis process. In this study, the network polyimide (PI) was prepared by polycondensing reaction among the triamine monomer TAPA, the brominated diamine monomer MPD‐Br, and the anhydride monomer 6FDA, and the resulting PI was then used as a precursor for CMS membranes. Notably, the introduction of the TAPA monomer can build a robust network structure in the PI precursor, effectively prevent the shrinkage of micropores during the carbonization process, and further optimize the proportion of ultra‐micropores within the membrane. Meanwhile, the brominated diamine monomer MPD‐Br has the ability to adjust the cross‐linking density of PI. Besides, it can assist with the carbonization by means of the thermally induced debromination process to regulate the gas separation performance of the CMS membrane. Experimental results demonstrated that after the debromination and carbonization process, the d ‐spacing, BET specific surface area, and the proportion of ultra‐microporous structures of the resultant CMS membrane were obviously increased. For instance, the d ‐spacing of the 6F‐TA:MPD‐Br (5:1)‐550 membrane was enlarged to 11.61 Å, and its specific surface area by BET was increased to 655.10 m 2 /g, with a significant increase in the proportion of ultra‐microporous structures to about 76%. The change in structure resulted in a significant improvement in gas separation performance. Specifically, the 6F‐TA:MPD‐Br (5:1)‐550 membrane achieved a CO 2 permeability as high as 2399.78 Barrer and a CO 2 /CH 4 selectivity of 43.07, approaching the 2019 Robeson upper bound. We anticipate that this current study could offer a new insight into the rational design of CMS membranes having excellent gas separation performance, particularly those derived from the network PI precursors containing bromine groups.