Covalent Organic Framework-Metal-Organic Cage hierarchically microporous membrane for efficient helium separation

Precision purification of helium (He) requires sub-nanometer control over molecular transport due to its extremely low polarizability and small kinetic diameter. The ability of covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) to discriminate He from methane (CH 4 ) is frequently restricted by the lack of effective pore synergy. To this end, in this study, negatively charged iron-based metal-organic cages (Fe-MOC) units were embedded into the pores of positively charged ionic covalent organic framework (iCOF) nanosheets via electrostatic coupling. This strategy constructs a hierarchically interconnected 2-dimensional (2D)-0-dimensional (0D) framework, where membrane formation generates effective diffusion nanochannels characterized by a heterogeneous electrostatic microenvironment. This integrated structure exerts a dual effect: the iCOF layers offer low-resistance planar diffusion channels for rapid He transport, while the incorporated Fe-MOC units create confined regions that effectively impede CH 4 permeation. The prepared iCOF@Fe-MOC membrane exhibited a He/CH 4 selectivity of 7.1 and a He permeance of 163 GPU, demonstrating stable separation performance across varying temperatures and pressures. The results demonstrate that the hierarchical microporous membrane, constructed through the synergy of COF nanosheets and MOCs, offers a promising architecture for efficient He enrichment. • The iCOF@Fe-MOC membrane forms by electrostatic 2D-0D assembly. • iCOF@Fe-MOC membrane hierarchical channels enable selective He transport. • Spatial confinement and interfacial electrostatics increase the selectivity of He/CH 4 . • The integrated structure delivers both high permeability and selectivity.