Copolyimide membranes fabricated by nonsolvent-induced phase separation for helium extraction from natural gas

Helium extraction from natural gas is energy extensive since >95% feed stream needs to be liquefied in multi-cycles. Membrane gas separation is an energy-efficient alternative because of room temperature operation and no phase change. However, the practical application is significantly retarded by the limited permeation flux, which is mainly attributed to the thick selective layer. Herein, asymmetric 6FDA-APAF_0.5-Cardo_0.5 copolyimide membranes with effective thickness of ∼ 200 nm were prepared by the one-step phase inversion method. The He permeance reached up to 120 GPU, one order magnitude higher than that of the free-standing one. The selectivity was 44 for He/N₂ and 67 for He/CH₄ binary mixtures. The performance surpassed 2019 upper bound and the benchmark membranes for He separation from N₂ or CH₄. Meanwhile, the performance is resistant to the impurities of C₂H₆ and CO₂ in the natural gas. Together with long-term stability (>200 h), the thin 6FDA-APAF_0.5-Cardo_0.5 membrane would pave the way to He extraction from natural gas for practical application.