MIL-100 (Fe) with mix-valence coordinatively unsaturated metal site as Fenton-like catalyst for efficiently removing tetracycline hydrochloride: Boosting Fe(III)/Fe(II) cycle by photoreduction

Iron-based MOFs have been considered as the candidate for Fenton-like reagents owing to their open pore size and uniformly dispersed Fe active centers. However, the strong coordination of metal ions with organic ligands greatly reduces the exposed active sites of most iron-based MOFs, thus reducing Lewis acid sites and limiting the activation efficiency of H2O2. At the same time, low Fe3+/Fe2+ cycle efficiency also restricts H2O2 activation efficiency. Hence, in this work, a series of MIL-100 (Fe) containing mixed valence coordinatively unsaturated metal site (CUS) with different Fe2+/Fe3+ ratios was synthesized by heat treatment and used for tetracycline hydrochloride (TC-HCL) degradation with the assistance of visible light. Interestingly, accelerated Fe3+/Fe2+ cycle was observed with the aid of visible light. Among the prepared samples, CUS-MIL-250 prepared under vacuum at 250℃ has superior degradation performance, primarily owing to its large specific surface area and pore volume, improved visible light absorption, and the introduction of more Fe2+ CUS. Meanwhile, compared with Fenton-like system and photocatalytic system, the CUS-MIL-250/H2O2/Vis system, with effective separation of photogenerated electrons and holes, demonstrates a synergistic effect to generate more reactive oxidation species, thereby improving the degradation efficiency. In addition, the catalyst shows good stability and recyclability.