Some Current Challenges in the Conversion of Biomass-Derived Molecules over Supported Metal Catalysts

Robert J. Davis, University of Virginia

The conversion of biomass-derived feedstocks to fuels and chemicals is one approach to partially relieve our growing dependence on fossil fuels.  However, there are many scientific and engineering challenges to overcome.  In this work, the selective catalytic oxidation of the model polyol glycerol to glyceric acid has been investigated over supported Au catalysts in alkaline water solutions.  The influences of solution pH, reactor configuration, and gold particle size were studied in detail. Although the smallest of Au nanoparticles revealed the highest turnover frequency, large Au particles and even bulk Au powder catalyzed the oxidation reaction at high pH.  Results from isotopic labeling experiments with 18O showed direct participation of hydroxide from the solution instead of gaseous oxygen in the oxidation reaction, which accounts for the very strong influence of solution pH on the reaction rate and product selectivity as well as the production of peroxide during the reaction.  These results are extended to the oxidation of hydroxymethylfurfural in basic solution.

The selective reduction of glycerol to propanediols was also studied over supported metals in aqueous solution.  By directly comparing the reaction of glycerol to propylene glycol and ethylene glycol over Pt and Ru catalysts under identical conditions, we concluded that a C-C cleavage path exists on Ru that is not favored on Pt.  Moreover, the higher rate of glycerol conversion and shift in product distribution with increasing solution pH indicated solution-phase reactions can be strongly coupled to metal-catalyzed reactions.  Although supported Pt catalysts produced mainly 1,2-propanediol under the conditions of study, promotion of Pt by Re resulted in the formation of some 1,3-propanediol that was not observed with monometallic Pt catalysts.  A new model of a bifunctional active site will be presented. The influences of catalyst composition and solution pH on the reaction rate and product distribution during oxidation and reduction reactions will be discussed.