Enhanced ethylene/ethane separation using carbon molecular sieve membranes derived from polybenzoxazole-based polyimides

Carbon molecular sieve (CMS) membranes have garnered substantial attention for their ability to separate similarly sized gas mixtures. Despite their high gas permeability, CMS membranes exhibit moderate selectivity for the C2H4/C2H6 gas pair. To address these issues, this study explores two types of polyimide precursors made from 2-(4-aminophenyl)benzo[d]oxazol-5-amine (APBO) and p-diaminobiphenyl (DABP) with 6FDA monomer to prepare CMS membranes. The integration of the flat, rigid polybenzoxazole (PBO) structure with the bulky, twisted 6FDA structure facilitates the formation of ultramicropores and micropores in the CMS membrane. The 6FDA-APBO-based CMS membrane, pyrolyzed at 700 degrees C, exhibits superior C2H4 permeability and C2H4/C2H6 selectivity compared to the 6FDA-DABP-derived CMS membrane. Moreover, 6FDA-APBO-based CMS achieves an exceptional C2H4 permeability of 160.8 Barrer with a C2H4/C2H6 selectivity as high as 13.7, significantly surpassing the C2H4/C2H6 upper bounds for both the polymeric and CMS membranes. The design principle provides an effective approach to enhance the C2H4/C2H6 separation performance of membranes, paving the way for the development of energy-efficient C2H4/C2H6 separation processes.