Solvent- and catalyst-free synthesis of an azine-linked covalent organic framework and the induced tautomerization in the adsorption of U(vi) and Hg(ii)

Abstract

A green and facile solvent- and catalyst-free method, without inert gas protection and/or mechanical grinding to synthesize an azine-linked covalent organic framework (ACOF) with high efficiency, was proposed for the first time. Characterization techniques and experimentations show that the as-synthesized ACOF has high crystallinity and superior physical and chemical stability and exhibits good adsorption performance for both radioactive heavy metal U(VI) and ordinary heavy metal Hg(II), with adsorption capacities of 169 mg g⁻¹ for U(VI) and 175 mg g⁻¹ for Hg(II), which makes the ACOF a potential adsorbent material for heavy metal removal and uranium recycling. Tautomerization between the enol-form and keto-form was verified in the structure of the ACOF which can be induced by the adsorption of metal ions and also affect the adsorption properties simultaneously. It was found that tautomerization from the enol-form to keto-form was promoted in the adsorption, which can be ascribed to the better coordination ability of CO in the keto-form towards the target metal ion, while the selectivity of the ACOF towards U(VI) increases with the decreasing solution pH and reaches 96.2% when the pH is 1.5, which is due to the tautomerization from the keto-form to enol-form under acidic conditions and the consequent size-matching effect. The study on tautomerization in solid materials is rarely reported and is of great importance for deep understanding of the relationship between material structures and properties. The synthesis strategy proposed in this paper provides references for the convenient, efficient and green synthesis of COFs.