Effect of thermally rearrangeable polyimide units on carbon molecular sieve membranes for enhanced C2H4/C2H6 separation

The structure of polymer precursors critically determines the performance of carbon molecular sieve (CMS) membranes, yet the role of hydroxyl-containing, thermally rearrangeable units in polyimides remains underexplored. In this work, hydroxylated 6FDA-HAB and non-hydroxylated 6FDA-DABP were employed to systematically investigate the effect of benzimidazole formation on CMS structure and ethylene/ethane separation. Thermal rearrangement (TR) affects both graphitization and microporous architecture: low TR temperatures yield amorphous, low-porosity membranes, moderate TR temperatures enhance conjugated aromatic formation and molecular sieving, while excessively high TR temperatures induce densification that reduces permeability and selectivity. The TR400 membrane pyrolyzed at 800 °C achieved a C 2 H 4 permeability of 204.6 Barrer and a C 2 H 4 /C 2 H 6 selectivity of 15.8, surpassing the latest upper bounds. These findings elucidate the pivotal role of thermally rearranged structures and provide a rational strategy for designing high-performance CMS membranes for C 2 H 4 /C 2 H 6 separation.