Aqueous-phase selective aerobic oxidation of 5-hydroxymethylfurfural on Ru/C in the presence of base

The aerobic oxidation of 5-hydroxymethylfurfural (HMF) was performed on an activated carbon-supported ruthenium (Ru/C) catalyst in water. The presence of Mg-Al hydrotalcite (HT, Mg/Al molar ratio = 3/1) as a base afforded higher selective oxidation of HMF to 5-formyl-2-furancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA) than with the bases MgO, Ca(OH)2 and NaOH owing to its appropriate strength of basicity. X-ray photoelectron spectroscopy characterization confirmed that metallic Ru0 species were the active sites for HMF oxidation. Isotopic tracer experiments conducted with 18O2 and 16O2 indicated that H2O rather than O2 provided the oxygen atom for the oxidation of HMF to FFCA and FDCA via hydration of the formyl group. These results and kinetic studies of the oxidation of HMF and 2,5-diformylfuran (DFF) led to the proposition that the aerobic oxidation of HMF to FFCA follows a Langmuir-Hinshelwood mechanism. The oxidation involved dissociative adsorption of HMF and O2 to form adsorbed alcoholate and atomic oxygen species followed by kinetically relevant abstraction of β-H from the alcoholate species via the atomic oxygen species to adsorbed DFF species on the Ru surface, which then underwent hydration and oxidation to FFCA under basic conditions.

Graphical AbstractAerobic oxidation of 5-hydroxymethylfurfural (HMF) to 5-formyl-2-furancarboxylic acid on Ru/C with base in water follows a Langmuir-Hinshelwood mechanism, involving kinetically relevant β-H abstraction from alcoholate species, which are formed from dissociative adsorption of HMF and O2 on the Ru surface by atomic oxygen.Figure optionsDownload as PowerPoint slide