Tuning oxygen vacancy and missing linkers of defective MOF-808 for advanced adsorption of Yb(III) from aqueous solution

To improve the adsorption performance of rare earth elements by metal organic frameworks (MOFs) based adsorbents from aqueous solution, two kinds of defective MOF-808 with abundant oxygen vacancies are fabricated via the different strategies, and named as MOF-808-DLX and MOF-808-DSX, respectively. The as-prepared defective MOF-808 were characterized by X-ray diffraction (XRD), infrared spectrometer (IR), thermogravimetric analyzer (TGA), electron paramagnetic resonances (EPR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), confirming the crystal structures, the morphologies, the presences of oxygen vacancies and missing-linker defects. In addition, the adsorption kinetics, adsorption isotherms, and the effects of pH, different rare earth ions on adsorption performance, as well as the cyclic stability of two typical defective MOF-808 were carefully investigated. The results showed that MOF-808-DL1 and MOF-808-DS2 exhibited much higher adsorption capacities than the corresponding pristine MOF-808 at 30 °C and pH = 6. Furthermore, the adsorption mechanisms were clarified through energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Zeta potential, and density functional theory (DFT) calculations, verifying the complexation interaction between Yb(III) and oxygen vacancies originated from Zr6 clusters both for MOF-808-DLX and MOF-808-DSX. Meanwhile, the electrostatics attraction also contributed to the adsorption of Yb(III) for MOF-808-DSX. In summary, this work provides two feasible strategies for manufacturing MOF-808 with missing-linkers defects to promote their potential applications in adsorbing Yb(III) from aqueous solutions.