Mechanistic Study of Glycerol Oxidation over Supported Gold Catalysts

Current progress on the conversion of cellulose has established glycerol and ethanol as potential biorenewable feedstocks. Large volumes of glycerol are also obtained as a co-product of biodiesel production. Oxidation of alcohols with molecular oxygen over supported gold catalysts in liquid water provides a sustainable, environmentally-benign route for traditional processes utilizing harmful organic solvents or expensive inorganic oxidants. Water is a solvent of choice for the oxygen-rich substrates encountered during the chemical transformations of biomass and plays an important role during gold catalyzed alcohol oxidation. Experiments with isotopically-labeled compounds showed that hydroxide groups are critical to the activity of Au in liquid water. Hydroxide served as the source of O-atoms in the acid products and the predominant reaction path involved both solution-mediated and metal-catalyzed steps. The oxidation mechanism over gold was shown to be similar to that over Pt group metals wherein hydroxide reacted with alcohols on a Pt catalyst despite the fact that molecular oxygen can dissociate on this metal. Molecular oxygen is believed to play an indirect role by removing electrons deposited into the supported metal nanoparticles during oxidation.;The oxidation of glycerol by molecular oxygen in the aqueous phase over Au/TiO2 was also investigated in both a batch reactor and a continuous up-flow fixed bed reactor. The effects of catalyst particle size, gas flow rate, liquid flow rate, reaction temperature, dioxygen pressure, and solution pH were examined in the fixed bed system. The high catalyst to fluid volume of the fixed bed system allowed for secondary oxidation products such as tartronic acid and oxalic acid to form in substantial amounts, which contrasts the product distribution observed in a batch system under similar conditions. To further understand the contrasting product distribution in the two reactor systems, the influence of reaction conditions such as O2 pressure, base loading and catalyst loading on product distribution of glycerol oxidation was evaluated. Subsequent oxidation of the primary product, glyceric acid, was also studied separately to get insights into diacid production.