Thermally rearranged poly(benzoxazole-co-imide) composite membranes on α-Al₂O₃ support for helium extraction from natural gas

Membrane gas separation is deemed as the most energy-efficient approach to extract the rare He from natural gas. However, the techno-economic feasibility was far from the practical application because of the poor separation performance illustrated by Robeson's upper bound. Herein we reported a high-performance polymeric membrane for He extraction from natural gas. Besides the bulky Cardo segments, the pristine polymeric backbone was featured with reactive ortho-hydroxyl groups, which were thermally rearranged to benzoxazole rings benefitting micropore formation. The polymer was well dissolved in NMP then facilely coated on α-Al₂O₃ support for further thermal rearrangement. The thin and selective membrane showed He permeance of 40 GPU and He/CH₄ mixture selectivity of 74 surpassing the benchmark membranes for helium extraction. Mitigation of physical aging was demonstrated by the long-term (>1000 h) separation in He/N₂ and He/CH₄ binary mixtures. The excellent He separation performance and anti-aging property endowed the membrane appealing to practical He extraction from natural gas.