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

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 NO2 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 degrees C). The oxidation reaction of HMF, overoxidation of HMF, and skeleton-cracking reaction of FDCA preferred concentrated HNO3, hot HNO3, and concentrated or hot HNO3, 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 beta-protonation-based ring-opening reaction of HMF, aldol reaction, and dehydration reaction. HMF can be selectively transformed into DFF with concentrated nitric acid as the oxidant and solvent. DFF yield is up to 83.2 %. The overoxidation reaction of HMF and skeleton-cracking reaction of FDCA take place to a small degree in concentrated or hot HNO3. Chromogenic substances with highly conjugated structures are also generated, and they are potential pH indicators. image