Copolyimide membranes for helium separation: Balancing He/ CH 4 and He/ N 2 selectivity

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Abstract Membrane separation technology exhibits significant advantages for helium extraction from low‐helium natural gas. Nevertheless, its performance remains constrained by the inherent trade‐off between permeability and selectivity. Here, we systematically tailor the nanoscale architecture of copolyimide membranes by adjusting the ratio of 9,9‐bis(4‐amino‐3‐chlorophenyl)fluorene (CFDA) to 9,9‐bis(4‐amino‐3‐fluorophenyl)fluorene (FFDA), enabling selective separation of He/CH 4 and He/N 2 . At a CFDA/FFDA ratio of 1:3, the membrane shows a moderate fractional free volume (FFV = 24.06%), which restricts the diffusion of CH 4 more effectively than that of He, leading to a He/CH 4 selectivity of 78.0. At a ratio of 1:5, the denser structure (FFV = 23.72%) with a lower FFV enhances size‐exclusion for N 2 , achieving a He/N 2 selectivity of 37.7. Both membranes exhibit excellent 210‐day stability and maintain high selectivity in a ternary mixture (He/CH 4 /N 2 = 0.5/98/1.5, vol%). This work elucidates the composition‐nanostructure‐performance relationship, offering a rational design strategy for targeted gas separation membranes.

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