Highly active and stable Ru–(OH)-based catalysts supported on Ni–manganite for the base-free aerobic oxidation of 5-hydroxymethyl furfural to 2,5-furan dicarboxylic acid in a noble water–organic solvent system

Abstract

A new process for converting high concentrations of 5-hydroxymethyl furfural (HMF) into 2,5-furan dicarboxylic acid (FDCA) was studied. The method could produce FDCA in 92% yield from 5 wt% HMF in an acetone/water solvent mixture. HMF oxidation proceeded without a homogeneous base owing to the high solubility of FDCA in the solvent mixture. A series of Ru-doped Ni–manganite catalysts were prepared, characterized, and used for HMF oxidation. Ni addition stabilized Mn at higher oxidation states and formed the selective spinel phase NiMn₂O₄. CO-chemisorption analysis showed that Ru dispersed well over the Ni₁Mn₆ support as the spinel phase formed. Ru addition facilitated the reduction of Ni and Mn, and low-temperature reduction profiles were observed as the Mn content increased. XPS analysis also indicated cooperative electronic interactions between Ru and the support. XPS and EXAFS studies confirmed that the two main species that formed positively charged Ru were Ru–O and Ru–OH, with valences of +3 and +4, respectively. The Ru/Ni₁Mn₆ catalyst exhibited exceptional oxidation activity toward FDCA owing to the high number of oxygen atoms in its lattice and dispersion and interaction of Ru with the support. Stability tests of the catalyst in acetone/water and water-only solvent systems reflected its high stability and catalytic activity over several cycles of reuse in the acetone/water system. Overall, the present method is scalable for oxidizing HMF to FDCA under high concentrations without homogeneous base additives using organic/water solvent mixture.