Tunable Ionic Sieving Membrane via Reactive Layer‐By‐Layer Assembly of Porous Organic Cages

Abstract Selectively permeable membranes with subnanosized pores show potential for applications in saline wastewater treatment and resource recovery. Here, a novel ionic sieving membrane (ISM) is fabricated via reactive layer‐by‐layer assembly of reduced porous organic cages, providing confined pores for the entry of monovalent ions and the interception of divalent ions. The ion separation performance of ISMs can be tuned by varying the concentration of reduced porous organic cages during the assembly. The highly permeable ISM featuring a surface potential of −52 mV and widely distributed aperture (µ_p = 0.69 nm, σ_p = 1.37) preferentially permits the passage of Cl⁻ over SO₄²⁻, resulting in remarkable permeance of 9.4 L m⁻² h⁻¹ bar⁻¹ and Cl⁻/SO₄²⁻ selectivity of 50.3. However, the highly selective ISM with a surface potential of −30 mV and narrowly distributed aperture (µ_p = 0.66 nm, σ_p = 1.27) exhibits superior Cl⁻/SO₄²⁻ selectivity of 277.3 and Li⁺/Mg²⁺ selectivity of 64.7, indicating that the regular subnanosized pore structure plays a dominant role in achieving precise separation of mono‐/divalent ions. Moreover, owing to the acquired excellent Li⁺/Mg²⁺ selectivity, for the first time, lithium extraction from simulated salt lake brine is achieved by a two‐stage nanofiltration process with reduced Mg²⁺/Li⁺ mass ratio from 40 to 0.3.