Base-free conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid in ionic liquids

Base-free conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) was carried out by a simple and green process based on a novel ionic liquid (IL)-promoted non-noble metal catalytic reaction system. Among various of ILs, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) was found to be the best IL without addition of base as it has stronger hydrogen bonds formation ability and good thermal stability, as well as free to form definite aggregates. In addition, a series of non-noble metal catalysts containing Ce, Fe, Zr were synthesized and the Ce0.5Fe0.15Zr0.35O2 catalyst was found to have excellent activity due to the existence of active iron and zirconium species and the formation of CeO2-based solid solution, which could create more oxygen vacancies and improve the redox properties of catalyst. Under the optimal reaction condition, almost complete HMF conversion and 44.2% FDCA yield were obtained after 24 h at 160 °C over Ce0.5Fe0.15Zr0.35O2 in [Bmim]Cl. More importantly, the catalyst could be reused without significant decrease of its catalytic activity. This research not only develops a more environmentally friendly catalytic system that promotes the base-free oxidation of HMF into FDCA, but also provides an economic route to produce FDCA.