Instant Synthesis of 2,5‐Diformylfuran from Concentrated 5‐Hydroxymethylfurfural without Catalyst and Organic Solvent

Abstract 5‐Hydroxymethylfurfural (HMF) was transformed into 2,5‐diformylfuran (DFF) with nitric acid as the oxidant. The initial HMF content was as high as 12.5 wt %, and water was the sole solvent. This is an intensely exothermic reaction along with the emission of NO 2 gas. A high DFF yield of 83.2 % was achieved if the oxidation ability of nitric acid matched with the requirement of target oxidation reaction while mismatched those of side reactions by regulating the initial concentration of nitric acid (59.5 wt %) and reaction temperature (50–60 °C). The oxidation reaction of HMF, overoxidation of HMF, and skeleton‐cracking reaction of FDCA preferred concentrated HNO 3 , hot HNO 3 , and concentrated or hot HNO 3 , respectively. After liquid‐liquid extraction, filtration, and column chromatography, light‐yellow DFF powders with purity of 92.1 wt % and isolated yield of 40.2 % were obtained and confirmed by NMR, FTIR, UV‐Vis, and elemental analysis. HMF was the dominant impurity due to its affinity to DFF. In addition, chromogenic substances were obtained as a byproduct, and they were potential pH indicators. These chromogenic substances had highly conjugated molecular structures, and they were generated via successive β ‐protonation‐based ring‐opening reaction of HMF, aldol reaction, and dehydration reaction.