Amphiphilic Polyaromatic Hydrocarbon Assisted Synthesis of Amorphous Coordination Polymer Monolayer on Water Surface for He/CH₄ Separation

2D coordination polymer membranes, characterized by ultrathin thickness and nanoscale porosity, exhibit exceptional selectivity and permeability for molecular separations. While amorphous coordination polymer (aCP) monolayers are particularly attractive for scalable fabrication, as their separation performance is less dependent on crystallinity, aCP monolayers have never been synthesized and their gas separation capabilities have been completely unexplored. Herein, we report an amphiphilic polyaromatic hydrocarbon (PAH) assisted interfacial assembly strategy to fabricate nanometer thick aCP monolayer. An amphiphilic hexa(terpyridine)hexaphenylbenzene (HTPHPB) monolayer is first formed at the air/water interface, which adsorbed pyrazine and coordinated with Co²⁺ to generate an active layer, subsequently inducing the interfacial co-assembly of water-soluble pyrazine and tetracyanonickelate into continuous aCP films with a thickness of 2.2 nm. Remarkably, the six layer stacking aCP membranes exhibit unprecedented He/CH₄ separation performance with a He permeance of 136.6 GPU and a He/CH₄ separation factor of 110, representing a nearly threefold improvement in separation factor over the best-performing MOF materials reported to date. Furthermore, the membrane maintains its separation efficiency without significant performance loss under 50 times bending, demonstrating excellent structure stability. This work not only establishes a facile strategy for synthesizing aCP monolayers but also opens new avenues for designing high-performance gas separation membranes.