Precise Recognition of Palladium Through Interlaminar Chelation in a Covalent Organic Framework

Palladium isotopes as fission products in used nuclear fuel (UNF) represent precious alternative resources of this noble metal besides its natural reserves and therefore has a high extraction value. However, although solvent extraction of Pd from UNF has been considered for several decades, its practical use is still challenging mostly originating from the harsh fuel reprocessing conditions that lead to the extractant decomposition and the vast co-existing metal ions that depress the extraction efficiency. Herein, we present an interlayer synergistic binding strategy, a metal-recognition manner markedly deviating from chelation within a single ligand molecule in solvent extraction, for selective palladium chelation by a radiation-resistant and acid-stable salicylaldimine-based covalent organic framework (COF-TzDa), where the binding sites are pre-organized and spatially separated for the preference of Pd coordination between adjacent COF layers. COF-TzDa, as expected, shows ultrahigh extraction selectivity towards Pd ions both in static and dynamic conditions, primarily due to that the preorganized Pd-specific binding sites are less prone to match other undesired metal ions. Fast adsorption kinetics, high adsorption capacity (265.4 mg g-1), and more importantly one-round enrichment and purification of Pd from the simulated high level nuclear waste solution are unprecedentedly achieved. The powder X-ray diffraction analysis, X-ray photoelectron spectroscopy results, and in-depth density functional theory corroborate the interlayer synergistic binding to be the result of enol-to-keto tautomerization where oxygen donors from two adjacent layers work together in a cooperative fashion. This work demonstrates the feasibility of structure-engineering of framework materials in designing upgraded adsorbent for targeted guests.