Rational process design for the efficient oxidation of crude HMF-solution using AuPd/C catalysts

The oxidation of 5-(Hydroxymethyl)furfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is of high interest for the production of renewable monomers. To achieve a cost-efficient, sustainable process for the large-scale production of FDCA, it is important to use the as-synthesized crude HMF-solution without extensive purification steps. In this work, we present the direct oxidation of crude HMF-solution produced from fructose syrup with an AuPd/C catalyst. The catalyst shows good tolerance against various remaining impurities from the HMF-preparation. A 95% FDCA yield with a productivity of 75.5 molFDCA molAuPd-1 h-1 could be achieved under increased temperature (140 °C) and pressure (40 bar) with only 2 eq. Na2CO3. We further improved the process' efficiency by using a two-step oxidation with a split addition of the base in the same reactor. First, a mild oxidation (1 eq. Na2CO3, 100 °C, 1.5 h) of HMF afforded the intermediates HFCA and FFCA, which were then further oxidized under harsher conditions (3.3 eq. NaOH, 140 °C, 4 h) to FDCA. Due to the higher efficiency, this process enabled increasing the HMF concentration up to 0.3 M for crude solution and 0.6 M for purified solution while minimizing the degradation of HMF, leading to a more cost-efficient oxidation of crude HMF-solution, and facilitating the industrial implementation of renewable FDCA.