Abstract
One molecule often cited as a key building block chemical in the development of a biorenewables-based chemicals industry is 5-hydroxymethylfurfural (HMF). Derived from fructose and glucose, HMF can be oxidized to 2,5-furandicarboxylic acid (FDCA), a potential replacement for the non-renewable polyethylene terephthalate monomer comprising PET plastics. The oxidation of HMF with dioxygen over Pt and Au catalysts in aqueous solution offers an environmentally benign process for the production of renewables-based plastics.
The oxidation of HMF to FDCA over Pt and Au catalysts at ambient temperatures required addition of base, such as NaOH, as well as the use of dioxygen. The rate of oxidation of HMF was an order of magnitude faster over Au than over Pt catalysts; however, at standard conditions (NaOH:HMF = 2:1, 295 K, 690 kPa O2) the major oxidation product over Au was the monoacid hydroxymethylfurancarboxylic acid (HFCA), whereas Pt produced a majority FDCA. Increasing the amount of NaOH (NaOH:HMF = 20:1) resulted in a majority product FDCA over Au.
Isotopically labeled H218O and 18O2 were used to determine the source of oxygen inserted in the acid products and showed that oxygen insertion proceeds through water rather than dioxygen. A mechanism was proposed in which molecular oxygen is reduced to peroxide and hydroxide, scavenging electrons from the metal surface, closing the catalytic cycle and regenerating hydroxide. Base-free HMF oxidation in water to FDCA was achieved through the use of higher temperatures (348 K) over Pt/C catalysts. Isotopic labeling studies confirm this oxidation reaction proceeds through the same mechanism as the one at high pH.
The reaction kinetics of HMF oxidation in aqueous solution with O2 to HFCA and FDCA were evaluated over a 3 wt% Pt/Activated Carbon catalyst in a semibatch reactor. In addition, the reaction kinetics of intermediate HFCA oxidation to FDCA over supported Pt were also investigated. The reactions were found to be zero order in substrate concentration, indicating high coverage of the Pt surface in substrate. The oxidation of HMF was found to be first order in initial NaOH concentration, though the oxidation of HFCA was found to be zero order in initial NaOH concentration above a 2:1 NaOH:HFCA ratio. Arrhenius plots of both HMF and HFCA oxidation revealed similar apparent activation energies, 29.0 kJ mol-1 and 29.6 kJ mol-1, respectively.
Solid bases as catalyst support may offer enhanced reaction rates for the oxidation of HMF to FDCA. Although attempts have been made at using hydrotalcites (HT) for this purpose, experiments demonstrate considerable leaching of these materials into solution was observed. Carbon nanofibers (CNF) treated with ammonia at high temperatures (1173 K) created supports with basic pyridine and pyrrole groups (CNF-N), on which Au and Pt particles were anchored. Likewise, CNF materials functionalized with acidic O-containing groups (CNF-ox) were used to support metal nanoparticles. Interesting synergy between Au and basic CNF-N materials was noted, wherein the diacid FDCA was produced in majority over this catalyst in just 2:1 NaOH:HMF at 295 K. Under the same conditions, Au particles supported on carbon black produced a majority of the monoacid HFCA, as did a physical mixture of basic CNF-N (no metal) and Au/Cblack.
