Tuning Local Charge‐Density in COFs to Suppress Protonation and Unlock Binding Sites for Efficient Palladium Recovery

Abstract

ABSTRACT Imine‐linked COFs are increasingly being used to recover palladium from aqueous waste streams, but the binding sites undergo protonation in acidic media, thereby losing palladium adsorption capacity. Herein, we report a new strategy that effectively suppresses imine protonation in COFs by tuning the local charge density, thereby allowing highly efficient recovery of Pd(II) from acidic solutions. By judicious placement of electron‐donating hydroxyl substituents on aromatic units next to imine groups, the electron cloud density around imine N atoms increases, suppressing protonation and preserving a high density of accessible Pd(II) binding sites. Further incorporation of extended π‐conjugated naphthalene units increases local charge density at the imine centers, strengthening the Pd(II) affinity and boosting adsorption capacity. As a result, the optimized adsorbent (COF‐3) exhibits rapid adsorption kinetics, exceptional selectivity, and an unprecedented Pd(II) uptake of 942.02 ± 24.61 mg/g in 0.1 M HNO 3 , surpassing all reported crystalline adsorbents thus far. Subsequently, COF‐3 demonstrates robust performance in dynamic recovery of Pd(II) from both acidic laboratory waste streams and simulated high‐level radioactive liquid waste, while maintaining excellent adsorption efficiency across multiple adsorption‐desorption cycles. Our rational strategy opens a new avenue for designing next‐generation sorbents for precious metal recovery and other applications.