Novel cationic porous materials with open architectures for highly efficient palladium recovery: Integrated experimental studies and DFT simulations

Design of superior adsorbents for selective palladium recovery from secondary resources is of great significance but still remains a challenge. Herein, a series of covalent organic polymers (COPs) with abundant cationic open architectures were designed for selective palladium adsorption. Two classes of adsorbent materials COP-1 and COP-2 were created by polymerization of 1-methyl-3-phenyl-1H-imidazolium iodide with biphenyl and with pterphenyl, respectively. After treating with NaX (X = Cl, Br, I) for anion exchange, the resulting polymers COP-1 were selected for investigating the effect of counter anions on palladium sorption. Among these materials, COP-1Cl shows the highest uptake capacity (246.7 mg/g) for palladium at pH 3.0. In addition, COP-1-Cl and COP-2-Cl exhibited fast sorption kinetics (3 min) and excellent selectivity for palladium over 7 coexisting competing ions. The COP-1-Cl polymer also displayed good one-round enrichment capacity of 201 mg/g for palladium in the dynamic breakthrough experiment, demonstrating its potential in practical applications. The results from XPS experiments and SEM-mapping indicated that the sorption mechanism is involved in the anion exchange between Cl- in the materials and PdCl42- species in solution, which has been proved to be spontaneous according to DFT calculations. The sorption capacities of COP-1 with different counter anions showed the order of Cl- > I- > Br-. This is consistent with the adsorption energies for the anion exchange processes calculated with different anions of the materials. The present work has provided a series of promising materials for palladium recovery and insights into the effect of counter anions on the sorption performance by integrated experimental studies and DFT simulations.