Highly Efficient Two-Step Synthesis of 2,5-Furandicarboxylic Acid from Fructose without 5-Hydroxymethylfurfural (HMF) Separation: In Situ Oxidation of HMF in Alkaline Aqueous H2O/DMSO Mixed Solvent under Mild Conditions

2,5-Furandicarboxylic acid (FDCA) is a promising biobased aromatic monomer that can be used to synthesize novel biobased polymeric materials, but efficient synthesis of FDCA is still a great challenge to date. In this study, a two-step method is proposed to synthesize FDCA directly from fructose under mild conditions without 5-hydroxymethylfurfural (HMF) separation, using commercially available catalysts. In the first step, HMF was formed in 97.1% HPLC yield by dehydrating fructose in DMSO under the catalysis of Amberlyst-15 at 120 °C for 1 h. In the second step, the in situ formed HMF was oxidized in 3/1 (w/w) alkaline (K2CO3) H2O/DMSO medium under the catalysis of Pt/C at 100 °C for 10 h to form FDCA in 91% HPLC yield. The overall yield of FDCA from fructose reached as high as 88.4%. The conversions of HMF to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and HMFCA to 5-formyl-2-furancarboxylic acid (FFCA) are fast, but FFCA to FDCA is a slow and rate-limiting step in HMF oxidation. H2O/DMSO ratio, alkali type, alkali/HMF ratio, and reaction temperature are important factors affecting the reaction rate and selectivity. The synergy between alkaline aqueous solution and DMSO is the key to high FDCA yield.