Functionalized metal-organic-framework CMPO@MIL-101(Cr) as a stable and selective rare earth adsorbent

In this contribution we report the innovative use of metal-organic frameworks (MOFs), more specifically MIL-101(Cr), as a novel stable supporting material for rare earth adsorption from aqueous environment. The mesoporous MIL-101 was covalently functionalized with chelating carbamoylmethylphosphine oxide-type ligands (CMPO) via a three-step procedure, yielding 0.5 ± 0.15 mmol of active groups per gram. These new materials were fully characterized by X-ray diffraction spectrometry (XRD), infrared spectrometry (DRIFTS), N2-sorption measurements, X-ray fluorescence spectrometry (XRF), and elemental analysis (CHNS), which confirm the successful anchoring of the CMPO ligands and the preservation of the porous structure. The stability of the material after contact with the acidic solutions used in adsorption tests was determined as well (XRD, XRF) and it was found that the functionalized MOF perfectly survives the applied conditions with full preservation of the structure and minimal ligand leaching (≤5 %). The adsorption performance of the functionalized MOF was investigated through a europium equilibrium study and selectivity tests with elements (Eu, Y, and Zn) which are commonly found in end-of-life products, such as fluorescent lamps, color TVs, and PC monitors. The europium adsorption isotherm shows good correlation (R 2 > 95 %) to the Langmuir model, which suggests a calculated maximum adsorption capacity of 12.5 mg Eu/g. Moreover, selectivity results show a considerable affinity for europium over yttrium (SFEu/Y 3.2) and a high affinity over zinc (SFEu/Zn 8.5), which is promising for urban mining applications.