Synergistic ether-flexible and bulky rigid diamine copolymers enable carbon molecular sieve membranes with enhanced C2H4/C2H6 separation

Carbon molecular sieve (CMS) membranes, with hierarchical pore structures and pronounced molecular sieving effects, are highly promising for C 2 H 4 /C 2 H 6 separation. Conventional CMS membranes typically achieve high C 2 H 4 permeability but only moderate selectivity. Here, we present a rational design strategy by copolymerizing 6FDA with a flexible ether-containing diamine (HFBAPP) and a bulky rigid diamine (FDA). During pyrolysis, ether linkages in HFBAPP generate micropores, while the flexible segments induce local chain packing that selectively hinders C 2 H 6 diffusion. Rigid FDA units reinforce the polymer backbone and increase free volume, creating a synergistic effect that simultaneously enhances C 2 H 4 permeability and C 2 H 4 /C 2 H 6 selectivity. Comparative studies with ether-free 6 F-DABP-FDA membranes confirm the critical role of ether linkages in achieving this balance. Optimizing the HFBAPP/FDA ratio and pyrolysis temperature, the 6 F–HF-FDA (1:1) membrane achieved a C 2 H 4 permeability of 382 Barrer and a C 2 H 4 /C 2 H 6 selectivity of 12.9 at 800 °C, surpassing previously reported CMS upper bounds. This work provides a generalizable design principle for high-performance CMS membranes and advances energy-efficient C 2 H 4 purification technologies.