A Kinetic and Mechanistic Study of the Selective Oxidation of 5-Hydroxymethylfurfural Over Supported Metal Catalysts

Interest in the production of commodity chemicals from renewable carbon sources instead of from fossil resources continues to grow. Oxidation of alcohols provides one such route for transformation of biorenewable feedstocks to value-added chemicals. One potential platform alcohol derived from fructose and glucose is 5-hydroxymethylfurfural (HMF). The HMF oxidation product 2,5-furandicarboxylic acid (FDCA) is a potential replacement for petroleum-derived terephthalic acid, the monomer used in production of PET plastic.  A high pH is required for HMF oxidation, and oxidation can proceed through multiple intermediate products.  Conversion and product selectivity is influenced by the choice of catalyst as well as the concentration of sodium hydroxide in the reactant mixture.  The mechanistic roles of sodium hydroxide, dioxygen and water in the oxidation of aqueous HMF are not fully understood and are key to the development of sustainable methods for biorenewable chemical production.

In the kinetic study, supported Pt, Pd and Au catalysts were evaluated in the aqueous-phase oxidation of HMF to FDCA at 295 K and high pH in a semibatch reactor.  The intermediate reaction product 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was formed in high yield over Au/C and Au/TiO₂ at 690 kPa O₂, 0.15 M HMF and 0.3 M NaOH, whereas the final reaction product FDCA was formed over Pt/C and Pd/C under identical conditions. However, the initial turnover frequency of HMF conversion was an order of magnitude greater on Au catalysts compared to either Pt or Pd.  Increasing the O₂ pressure and NaOH concentration facilitated the conversion of HFCA to FDCA over the supported Au.  The effect of base concentration, O₂ pressure, and metal on turnover frequency, conversion, and product selectivity will be discussed.

In the mechanistic study, Pt/C and Au/TiO₂ catalysts were utilized in labeling experiments conducted with ¹⁸O₂ and H₂¹⁸O. Results from these studies indicated that water was the source of oxygen atoms during the oxidation of HMF to HFCA and FDCA, presumably through direct participation of hydroxide in the catalytic cycle. Molecular oxygen was essential for the production of FDCA and played an indirect role during oxidation by removing electrons deposited into the supported metal particles. A detailed reaction path for HMF oxidation to FDCA will be proposed.