Optimisation of extraction performance for uranium with covalent organic frameworks of β-ketoenamines with different functional groups

[Display omitted] •COFs are crystalline materials with high adsorption capacity and adjustable structure.•COFs material is the most promising material for uranium adsorption.•COFs can effectively synergize with adsorption/catalytic reduction for uranium removal.•β-keto-enamine COFs have different electron transfer capabilities. Investigating the modulation of building blocks and composition of COFs to elucidate the influence of the electronic structure on photocatalytic activity is a challenging yet crucial endeavor for achieving a comprehensive understanding of this phenomenon. Herein, three β-keto enamine COFs featuring various hydroxyl functional groups were synthesized for the purpose of photocatalytic uranium reduction. Research results have revealed that the reversibility of bonding in supramolecular chemistry plays a pivotal role in determining the photocatalytic activity of COFs. Compared with Dh-Tat, Tp-Tat, Hb-Tat shown the better photocatalytic reduction performance and the separation of photogenerated charges and holes due to the flexible and reversible tautomeric forms of keto-enol tautomerism and conjugation extent of frameworks. Compared to Dh-Tat, Tp-Tat, Hb-Tat showed superior photocatalytic performance in reduction. This superiority was linked to the existence of flexible and reversible tautomeric keto-enol forms and extended conjugation within the frameworks, enhancing the segregation of photogenerated charges and holes. The localized polarization effect within the Hb-Tat molecule, in conjunction with its inherent electric field and heightened electronegativity, has enhanced its catalytic reduction properties towards uranyl ions. Furthermore, the order of photocatalytic removal rate was presented as follows: Hb-Tat (5.88×10-4 g•mg−1•min−1) > Dh-Tat (3.779×10-4 g•mg−1•min−1) > Tp-Tat (1.57×10- g•mg−1•min−1). The photocatalytic reduction of uranium by COFs was performed as follows: water-soluble U(VI) was reduced to insoluble U(IV) by e+ and ∙O2- generated by photoactive COFs, so then the insoluble U(IV) was transform into stable crystalline phases ((UO2)O2·2H2O) and deposited in the surface and the open channels. Through the design of molecular precursors, this study contributes to a better understanding of the structure-performance relationship to achieve efficient photocatalysis, thus advancing the design and development of COF photocatalysts.