Partial oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid using O2 and a photocatalyst of a composite of ZnO/PPy under visible-light: Electrochemical characterization and kinetic analysis

This work describes a new application of a composite of ZnO and polypyrrole (ZnO/PPy) in the partial photocatalytic oxidation of 5-hydroxymetilfurfural (5-HMF) to 2,5-furandicarboxylic acid (FDCA) using air as an oxygen source under visible light illumination. The composite of ZnO/PPy was synthesized by polymerization method and characterized by Raman spectroscopy, cyclic voltammetry (CV) and Tafel curves (electrochemical characterization). The electrochemical characterization of ZnO/PPy shows this composite is active under visible light, due to the presence of a higher value of exchange current density (J(corr)) for ZnO/PPy compared with ZnO and PPy; this shows its improved reactivity, demonstrating faster electron transfer and suggesting the PPy acts as electron sensitizer and donator in the composite photocatalysts. A reaction mechanism with a combination of parallel-reversible steps was evaluated, showing that the 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was the principal intermediate due to the oxidation of aldehyde groups of 5-HMF.