2D redox-active COF with the anthraquinone structure for photocatalytic reduction of uranium

Photocatalysis can be an effective technology for the conversion of U(VI) to U(IV) because it is efficient, facile and free of secondary pollution. In this work, anthraquinone-functionalized covalent organic framework (DQTP) was fabricated by an irreversible tautomerization of 2,6-diaminoanthraquinone (DQ) and 2, 4, 6-triformylphloroglucinol (TP) in the β-ketoenamine structure. The resultant DQTP could be effectively used as a metal-free heterogeneous catalyst for the photoreduction of U(VI) (k = 0.00287 min-1). The pivotal role of anthraquinone moieties as the active sites for U(VI) reduction was evidenced, i.e., to promote charge separation and thus enhance photocatalytic activity, as well as the production of H2O2 followed by the formation of (UO2)O2·2H2O with U(VI) through in-situ reactions. Additionally, free radical capture experiments and spectrometry analysis were conducted to determine the synergistic reduction mechanism of photogenerated electrons, superoxide radicals and hydrogen peroxide. It was reassuring that this work could provide the research basis for the subsequent development of more practical metal-free photocatalysts for the purification of U(VI)-containing wastewater.