Insight into the Specific Adsorption of Cu(II) by a Zinc-Based Metal–Organic Framework Mediated via a Proton-Exchange Mechanism

In the context of scarce metal resources, the one-step separation and recovery of high-value copper metal ions from secondary resources is of significant importance and presents substantial challenges. This study identified a Zn-based triazole MOF (Zn(tr)(OAc)) with accessible and noncoordinated terminal hydroxyl groups within its framework. The Zn(tr)(OAc) surpasses most currently reported Cu-specific MOF adsorbents regarding adsorption capacity and Cu²⁺ selectivity. Furthermore, in the one-step separation and recovery experiment of Cu²⁺, the Cu²⁺ concentration was increased from 79.64% in the simulated secondary copper resource solution to 98.62% in the adsorbed phase. Both experimental and theoretical studies indicated that the high ion selectivity for Cu²⁺ is primarily due to the specific recognition ability of the terminal hydroxyl (-OH) group, enabling only Cu²⁺ to undergo proton exchange with the hydrogen. The strong adsorption capacity of the material was attributed not only to proton exchange between the hydroxyl groups in the framework and Cu²⁺ but also to interactions between the nitrogen (N) and oxygen (O) atoms in the heterocyclic rings and Cu²⁺. In summary, Zn(tr)(OAc) demonstrates great potential in the separation and recovery of Cu²⁺ from secondary copper resources and provides additional possibilities for enhancing Cu²⁺ selectivity in MOF-based adsorbents.