Iron-based metal organic framework, MIL-88A, as a heterogeneous persulfate catalyst for decolorization of Rhodamine B in water

Abstract

While Metal Organic Frameworks (MOFs) have been extensively investigated as photocatalysts to eliminate toxic pollutants in water, studies using MOFs as chemical oxidative catalysts to degrade contaminants are still limited. MOFs used as catalysts for chemical oxidation reactions were prepared in N,N-dimethylformamide (DMF), a potential carcinogenic solvent. If such MOFs are not well activated and have not properly undergone solvent exchange, DMF can still be encapsulated inside MOFs, leading to secondary pollution. Considering the essence of wastewater treatment and pollutant reduction, DMF-free MOFs which still exhibit the catalytic activity to activate oxidants should be developed. Thus, we selected an iron-based MOF, MIL-88A, which can be prepared using Fe³⁺ with fumaric acid just in water. The as-synthesized MIL-88A is evaluated as the heterogeneous catalyst to activate persulfate for the decolorization of Rhodamine B (RB) dye. Iron oxide clusters (i.e., Fe₂O₃) form within MIL-88A through the coordination of Fe³⁺ and fumaric acid. Fe³⁺ of Fe₂O₃ is expected to induce the generation of persulfate radicals which in turn lead to the formation of sulfate radicals to decolorize RB dye. Factors influencing the activation of persulfate and RB decolorization were examined including persulfate dosage, MIL-88A loading, temperature, pH, UV and US irradiation as well as inhibitors. MIL-88A was assessed for the multiple-cycle activation of persulfate without additional regeneration of spent MIL-88A. These features make MIL-88A an effective and recyclable heterogeneous catalyst for the activation of persulfate.