In situ thermally rearranged poly(benzoxazole-co-imide) membranes on α-alumina substrates for He/CH₄ and He/N₂ separation

Membrane gas separation is an energy-efficient approach to extract helium from natural gas. However, the limited separation performance shown as Robeson’s upper bound has hindered the techno-economic feasibility. This study introduces an advanced copolyimide membrane engineered for He extraction from natural gas. The membranes were facilely achieved by dip-coating the α-alumina substrates in the copolyimide solution followed by in situ thermal rearrangement. In addition to the rigid 5-amino-2-(4-aminobenzene)benzimidazole segments, the active ortho-hydroxyl groups were converted to benzoxazole rings, contributing to extra micropores. The membrane showed an improved mixture selectivity of 120 and He permeance of 23.5 GPU, far surpassing the performance of benchmark membranes for helium separation over CH₄. The membrane also demonstrated long-term stability as evidenced by the continuous operation over 250 h. Additionally, the membrane exhibited resistance to impurities such as CO₂ and C₂H₆, enduring the asymmetric membranes promising for practical helium extraction from natural gas.