Geometrically stable nanopores in Zr-MOF membranes for effective Li+/Mg2+ separation

Membranes featuring structural stability and precise ion selectivity are indispensable for efficient lithium recovery from magnesium-rich brines. Despite the promising potential of metal-organic framework (MOF) membranes in ion separation, the flexibility of aromatic linkers in conventional MOF structures often induces pore size fluctuations, which compromises size-based discrimination efficiency. Herein, defect-free Zr-Frustrated Trigonal-symmetry Rotor (Zr-FTR) membranes were fabricated on polyethylene terephthalate (PET) substrates via a counter-diffusion interfacial growth method. The Zr-FTR framework is constructed with bicyclopentane linkers, whose rigid geometric configuration and substantial torsional energy barriers ensure the retention of a fixed pore aperture (∼4.3 Å). This rigid confinement effect significantly suppresses transient pore expansion, enabling effective ion size sieving while retaining exceptional structural stability. The optimized Zr-FTR-24 membrane exhibits a high Li + permeability of 0.09 mol m −2  h −1 and a Li + /Mg 2+ selectivity of 235, with sustained performance stability over a 7-day continuous operation in mixed-ion solutions. This work establishes a novel design paradigm for suppressing linker dynamics in MOF membranes through rigid linker engineering, ultimately achieving structurally stable and size-exclusive ion transport.