5-Hydroxymethylfurfural oxidation to Maleic acid by O2 over graphene oxide supported vanadium: Solvent effects and reaction mechanism

Oxidation of 5-hydroxymethylfurfural (HMF) with molecular oxygen to maleic acid (MAc) was performed on graphene oxide (V-GO) supported vanadium composite. The solvent effects (γ-valerolatctone (GVL), acetic acid and H2O) on the reaction were investigated by combing the characterizations of X-ray Photoelectron Spectroscopy (XPS), 13C and 1H Nuclear Magnetic Resonance (13C NMR and 1H NMR), Inductive Coupled Plasma Emission Spectrometer (ICP), and Elemental Analysis (EA), and kinetic studies. GVL was the most efficient solvent with the highest yield of MAc, while acetic acid and H2O benefited the C–C cleavage, giving more formic acid (FAc) than GVL. These variation in the solvent effects originated from the different ability of solvents to convert HMF molecules and 5-hydroxyl-2(5H)-furanone (HF) intermediate. Besides, the protons (H+) in graphene oxide were identified to promote HMF oxidation to MAc, whereas the added NaHCO3 caused the catalyst deactivation. By combing the fabrication of model catalyst and XPS analysis, this negative effect of NaHCO3 resulted from both the inhibition in the reduction of V5+ → V4+ and/or V4+ → V3+ and the loss in the H+ sites on the GO support caused by neutralization reaction. Furthermore, the reaction intermediates were identified by NMR. Oxidation of HMF to MAc was a two-step process, i.e., HMF oxidization to HF intermediate and its following oxidation to MAc. Both oxidation steps required the presence of vanadium and O2. A feasible path for HMF oxidation to MAc was proposed based on the NMR observations.