Hierarchical Design of COFs via NHC-Grafting for Selective Gold Recovery from E-Waste: Experimental and MD Analyses

In this study, a highly crystalline imine-linked covalent organic framework (COF) was synthesized via Schiff base condensation, followed by postsynthetic modification (PSM) to introduce N-heterocyclic carbene (NHC) ligands. The two-step PSM involved reduction of imine bonds to secondary amines and the subsequent covalent attachment of NHCs, while preserving the COF's crystallinity and porosity. Structural analyses confirmed successful functionalization and high stability of the modified COF. The resulting NHC-functionalized COF demonstrated strong affinity and high selectivity toward Au ions, making it a promising adsorbent for efficient gold recovery from complex e-waste matrices. XPS analysis confirmed the selective adsorption of Au(III) ions and their reduction to Au(I) via electron donation from NHC sites, enabling efficient and selective uptake from solution through coordination interactions within the COF matrix. Adsorption analysis demonstrated that Au(III) uptake by COF-NHC follows a PSO-Freundlich model, characterized by rapid adsorption kinetics and heterogeneous surface binding. Molecular dynamics simulations revealed that carbon functional groups within the postsynthetically modified COF play a crucial role in stabilizing and potentially reducing gold ions, underscoring the framework's effectiveness for selective gold recovery from e-waste.