Highly selective adsorption of rare earth elements by honeycomb-shaped covalent organic frameworks synthesized in deep eutectic solvents.

One of the key factors to obtain a highly pure individual rare earth element (REE) is to prepare adsorbents with high selectivity and adsorption capacity. Covalent organic frameworks (COFs), which encompass a variety of properties, including regular/tunable pore size, high specific surface area and easy functionalization, could be effective as adsorbents for separating rare earth elements (REEs). In this paper, TpPa COFs were successfully synthesized using an eco-friendly deep eutectic solvent (DES) as the reaction medium instead of toxic organic solvents at room temperature. TpPa COFs have a good separation effect on the nine REEs investigated in this work. Among them, the separation factors (β) of Eu/Yb, Eu/Tm and Eu/La are 15.34, 14.70 and 10.78, respectively, indicating that the TpPa COFs have good separation performance. Further discoveries showed that the adsorption and separation mechanism of the TpPa COFs for REEs in this experiment may be due to the coordination of REE ions with O to form a stable structure. This study blazed a trial for a green and facile synthesis strategy of TpPa COFs and expanded its implementation as a solid adsorbent in the separation of REEs.