(442d) 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.