# New Strategy Based on Click Reaction for Preparation of 3-Acyl-4-hydroxycoumarin-Modified Silica as a Perspective Material for the Separation of Rare Earth Elements

## TL;DR

A new silica-based material was developed for efficiently separating rare earth elements using a click reaction strategy.

## Contribution

The first method for covalently immobilizing 3-acyl-4-hydroxycoumarin on silica via a click reaction is proposed.

## Key findings

- The material showed high extraction efficiency (~98%) for several REEs like Gd(III), Dy(III), and Er(III).
- Fast adsorption kinetics (30 min) and high selectivity were observed under pH 5–5.5 conditions.
- The material retained over 95% extraction efficiency for Dy(III) and Er(III) in the presence of common metal ions.

## Abstract

The separation of rare earth elements (REEs) with similar chemical properties remains a relevant challenge today, most often addressed using liquid–liquid and solid-phase extraction with various chelating agents. Excellent complexing agents for REEs are 1,3-diketones and their analogs. We have for the first time proposed a method for preparing a material consisting of a covalently immobilized 3-acyl-4-hydroxycoumarin ligand on silica. For its synthesis, we employed a strategy based on the “click” reaction of 3-azidopropyl silica with a propargyl-containing coumarin–chalcone conjugate—this approach is the most tolerant and does not affect the coordinationally active fragment of the ligand. The material was characterized by thermal analysis, IR spectroscopy, and 13C NMR. The potential of the synthesized material for REE preconcentration was demonstrated at pH 5–5.5: high extraction efficiency for Gd(III), Dy(III), Er(III), Eu(III), Sm(III), and Yb(III) was observed, with fast adsorption kinetics (30 min) and extraction degrees of ~98%. Under unified conditions of static and dynamic extraction for Gd(III), Dy(III), Er(III), Eu(III), Sm(III), and Yb(III), affinity series toward the surface were obtained as a function of the distribution coefficient. It was shown that 10-fold molar excesses of Fe(III), Al(III), Cu(II), Ni(II), and Co(II) allow retention of more than 95% extraction for Dy(III) and Er(III). After adsorption of Dy(III) and Er(III), shifts in the carbonyl group absorption bands are visible in the IR spectra of the material, indicating a chelating mechanism of sorption. Additional studies are required for implementation in analytical and preparative REE separation schemes; however, preliminary data show that the material is a highly active adsorbent.