Core–shell MOF@COF composites for ultra-efficient selective recovery of Pd(II)

Developing efficient and selective recovery of Pd(II) adsorbents is crucial for addressing environmental pollution issues. Among them, covalent organic framework (COF) and metal organic framework (MOF) composites due to the multi-site functional groups are expected to exhibit excellent Pd(II) adsorption. However, the design and preparation of COF and MOF composite adsorbents for highly efficient adsorption of heavy metal Pd(II) still face many challenges. Herein, we constructed a series of novel core–shell structure UiO-66-NH2@TAP-COF (denoted as M@C) adsorbents (M@C-X, X = 1–5, representing different MOF doses) by Schiff base reaction and applied in the adsorption of Pd(II). The findings indicated that the M@C-3 exhibited ultra-high adsorption performance, reaching a capacity of 2134.1 mg/g, and displayed excellent selectivity, making it the best among similar materials. Experimental data and density functional theory (DFT) calculations revealed that the efficient and selective adsorption of Pd(II) is attributed to coordination interaction and strong electrostatic between Pd(II) and the abundant N and O groups in M@C. This work demonstrates the significant potential of the constructed core–shell M@C composites for the efficient and selective recovery of precious metal Pd(II).