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

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 HNO₃, 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.