Abstract
Oxidation is a key reaction in organic synthesis and will likely play a significant role in the development of value-added chemicals from biomass. The application of heterogeneous catalysis and molecular oxygen to oxidation reactions offers a green alternative to traditional, toxic chemical oxidants. An example of a biorenewable substrate that can be derived from fructose is 1,6-hexanediol. Adipic acid can be produced by the selective oxidation of 1,6-hexanediol and is a monomer for the production of nylon-6,6, which is a widely used polymer in the textiles, plastics, and automotive industries. The current production of adipic acid from fossil resources contributes considerably to greenhouse gas emissions. Thus, the production of adipic acid from biorenewable feedstocks with an environmentally friendly chemistry is an attractive target. In addition, so-called green oxidation could be applied to many other multi-functional alcohols whose acid products are valuable chemicals.
In this work, the selective catalytic oxidation of a variety of terminal alcohols was performed over Pt/C with 10 bar dioxygen at 343 K in aqueous solvent at low pH. The influences of Pt particle size, carbon support, alcohol structure, and start-up conditions were explored. Although the turnover frequency was not affected by particle size or the carbon support, the structure of the alcohols affected their initial rate of conversion. Both the rate of oxidation of -diols and selectivity of the diols to the diacids increased with increasing carbon chain length. Although the rate of 1,6-hexanediol oxidation was independent of dioxygen pressure, the order of reaction with respect to diol concentration depended on the start-up conditions. A kinetic model involving two types of metal sites was proposed to account for the experimental observations. The mechanism of alcohol oxidation over supported metal catalysts was discussed in light of these new results and prior published works.
The oxidation of alcohols with O2 at 343 K over Pt was explored to determine the origin of catalyst deactivation. The catalyst deactivation was observed during the oxidation of a variety of terminal alcohols over platinum supported on C, BN, SiO2, TiO2, and Al2O3. A decrease in TOF for 1,6-hexanediol oxidation after the exposure of Pt/C to dioxygen was easily reversed by reduction with the alcohol substrate. However, Pt that had been deactivated during alcohol oxidation could not be regenerated in a similar manner, suggesting that over-oxidation of Pt was not the cause of deactivation. The sintering of Pt nanoparticles, dissolution of Pt, and strong adsorption of chelating species also did not contribute significantly to the observed deactivation. In-situ ATR-IR spectroscopy of the Pt surface determined that CO was produced before alcohol oxidation, but was easily oxidized by O2 to CO2. A strongly adsorbed species was observed during a temperature programmed desorption reaction experiment on a recycled Pt/BN catalyst recovered after ethanol oxidation.
Bimetallic Pt and Au nanoparticles were prepared by sol-immobilization synthesis and subsequently supported on carbon, titania, and silica prior to evaluation in the aqueous oxidation of 1,6-hexanediol with 10 bar O2 at 343 k and a low pH. The removal of the polymer stabilizer (polyvinyl alcohol) increased the TOF of 1,6-hexanediol oxidation. Although the initial TOF of 1,6-hexanediol oxidation over the monometallic Pt/C was 0.20 s-1, the initial TOF over the bimetallic AuPt/C catalyst was substantially greater at 0.68 s-1. Interestingly, a Au/C catalyst was inert at these reaction conditions. The bimetallic AuPt/C catalyst was characterized by hydrogen chemisorption, XRD, and TEM with EDS. The EDS of single nanoparticles indicated that they contained both Au and Pt. The AuPt/TiO2 and AuPt/SiO2 catalysts had significantly lower TOF’s than AuPt/C.
