Confined ionic association and its effect on Li+/Mg2+ permselective transport through the HKUST-1 nanopore

Metal-organic frameworks (MOFs) are considered as ideal materials for ion separation. HKUST-1, as a subclass of MOFs, has been employed to construct novel membranes for Li⁺/Mg²⁺ separation due to its appropriate pore size. However, HKUST-1 hardly achieves the expected Li⁺/Mg²⁺ selectivity. To this end, molecular dynamics simulation is employed to investigate the ionic transmembrane permeation from the viewpoint of atomic level. The effects of window and cavity in HKUST-1 are separately explored. The crystal window behaves a combined effect of size-sieving and electrostatic affinity on the Li⁺/Mg²⁺ permselective transport. The presence of Mg²⁺ leads to an enhanced ionic association, which greatly blocks the confined Li⁺ transport in the cavity. Accordingly, it is necessary to block the entry of Mg²⁺ into the HKUST-1 cavities for relieving the confined ionic associations. This work not only helps to understand the mechanism of Li⁺/Mg²⁺ permselective transport, but also promotes the design of high-performance membranes with the help of HKUST-1 for Li⁺/Mg²⁺ separation.