In-situ green assembly of spherical Mn-based metal-organic composites by ion exchange for efficient electrochemical oxidation of organic pollutant

Highlights: • Novel spherical metal-organic composites were in-situ assembled via ion exchange. • Functional groups on surface improved Mn distribution and metal-support interaction. • High activity and stability were achieved in electrochemical oxidation without H2O2. • Intramolecular electron transfer of solid accelerated MnII/MnIII autocatalytic cycle. • Unique surface chemistry and structure promote catalysis even in alkaline condition. -- Abstract: In this study, we develop a facile ion exchange strategy for in-situ assembly of novel spherical metal-organic composites on a large scale. The functional groups (-NH₂, -COOH and -SO₃H) on chelating and exchange resins had significant effects on improving uniform distribution of metallic sites and metal-support interaction. Without any addition of H₂O₂, Mn-based metal-organic composites realized the recovery of waste metallic ions and exhibited high activity for methylene blue (MB) electro-Fenton degradation (97.8% decoloration and 54.7% TOC removal) within 150 min under low current density (7.53 mA·cm⁻²) and 3.0 g·L⁻¹ catalyst dosage. Analyses of performance on different active sites (Fe^II, Mn^II, Co^II, Ce^III and Cu^II) and supports clearly indicated that synergetic effect of Mn^II and organic supports played crucial roles in electrochemical oxidation. Kinetic rate constant of 0.037 min⁻¹ and turn over frequency of 0.23 h⁻¹ were much better than those of inorganic supported catalysts, which were attributed to intramolecular electron transfer greatly accelerating Mn^II/Mn^III autocatalytic cycle. Meanwhile, possible degradation pathway of MB was proposed by analysis of oxidative intermediate products. Benefiting from excellent properties and millimeter-level size structure, metal-organic composites can be applied in wide pH range of 2.0–9.0 and easily separated in the industrial application.