Covalent/metal–organic framework membranes with tailored pore functionality for accurate ion separation

Ion-selective membranes present a promising solution for efficient ion extraction, a critical need addressing both water resource cycling and critical metal supplies. Conventional polymer membranes often exhibit an intrinsic permeability-selectivity trade-off, stemming from their disordered and random pore architecture. This structural limitation further complicates the understanding of how diverse ion species are transported within their non-uniform sub-nanochannels. Covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) stand out as a promising solution for ion separation due to their ordered pore architecture, designable topological configuration, and tunable pore microenvironment. Herein, recent advancements in ion-selective membranes fabricated using these advanced COF and MOF materials are critically reviewed. We begin by unravelling the currently dominant ion-differentiation mechanisms within their sub-nanochannels, including size exclusion, electrostatic interactions, and chemically specific binding. Particular emphasis is placed on the pore engineering design principles of these crystalline framework materials, highlighting the role of specific ion-binding moieties, pore dimensions and charge, and asymmetric structures in selective ion transport. Advanced fabrication methods and ion-related applications of crystalline framework-based membranes are subsequently described. We conclude by presenting fundamental prospects and challenges for the on-demand design of next-generation ion-selective framework membranes.