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

Abstract

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 Ru 0 species were the active sites for HMF oxidation. Isotopic tracer experiments conducted with 18 O 2 and 16 O 2 indicated that H 2 O rather than O 2 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 O 2 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. Aerobic 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 O 2 on the Ru surface by atomic oxygen.