The mechanisms underlying Li+/Mg2+ separation in ZIF-8 under an electric field from atomistic simulations

Abstract

In this study, we explore the mass transfer and separation mechanism of Li⁺ and Mg²⁺ confined within the flexible nanoporous zeolite imidazolate framework ZIF-8 under the influence of an electric field, employing molecular dynamics simulation. Our results highlight that the electric field accelerates the dehydration process of ions and underscore the critical importance of ZIF-8 framework flexibility in determining the separation selectivity of the ZIF-8 membrane. The electric field is shown to diminish ion hydration in the confined space of ZIF-8, notably disrupting the orientation of water molecules in the first hydration shells of ions, leading to an asymmetrical ionic hydration structure characterized by the uniform alignment of water dipoles. Furthermore, despite the geometrical constraints imposed by the ZIF-8 framework, the electric field significantly enhances ionic mobility. Notably, the less stable hydration shell of Li⁺ facilitates its rapid, dehydration-induced transit through ZIF-8 nanopores, unlike Mg²⁺, whose stable hydration shell impedes dehydration. Further investigation into the structural characteristics of the six-ring windows traversed by Li⁺ and Mg²⁺ ions reveals distinct mechanisms of passage: for Mg²⁺ ions, significant window expansion is necessary, while for Li⁺ ions, the mechanism involves both window expansion and partial dehydration. These findings reveal the profound impact of the electric field and framework flexibility on the separation of Li⁺ and Mg²⁺, offering critical insights for the potential application of flexible nanoporous materials in the selective extraction of Li⁺ from salt-lake brine.