Electro-mechanical factor affecting the Li+/Mg2+ selectivity performance of ion separation metal–organic frameworks

Abstract

Technology to selectively secure Li⁺ from various heavy metal complexes using a metal–organic framework (MOF)-based separator is important. This study theoretically investigated, for the first time, the method of maximizing selectivity between Li⁺ and Mg²⁺ ions with similar physical and chemical properties through UiO-66-(NH₂)₂. All-atom molecular dynamics simulations were used to analyze the structural and mechanical properties of hydrated ion clusters penetrating UiO-66-(NH₂)₂ under various electric fields. The results revealed that the robustness of the hydrates composed of ions is a key design factor for selective filtration properties. Li⁺ exhibits high permeability owing to repeated cleavage and recombination of water ligands in the process of penetrating the MOF. This hopping behavior is accelerated by the presence of amine groups and the application of an electric field. However, the hydrate formed by Mg²⁺ does not collapse its structure within the MOF because of the strong water ligand binding force. The differences between the physicochemical behavior of Li⁺ and Mg²⁺ ions discovered in this study can be directly used in MOF design and process management to maximize selective extraction of Li⁺.