A kinetic modeling of the liquid-phase oxidation of lactose over Pt- and Au-supported catalysts

Abstract

Pt and Au catalysts, 2 wt% metal loading, supported on SiO2 and Al2O3 were used to study the effect of metal and support on the liquid-phase oxidation of lactose. Pt-based catalysts were prepared by incipient wetness impregnation while Au-based catalysts were obtained by the precipitation-deposition method. Catalytic tests were carried out in aqueous phase at 65 °C, using O2 as oxidizing agent and keeping pH constant at 9 by controlled addition of NaOH aqueous solution. In all of the cases, the only product of reaction detected and quantified was lactobionic acid. It was found that Pt supported on Al2O3 was more active than Pt supported on SiO2. This was explained on the basis that metal Pt dispersion on Al2O3 was three times higher than on SiO2. At the same time, Au/Al2O3 catalyst was more active than Pt/Al2O3 catalysts. The higher activity of Au/Al2O3 was attributed to Au nanoparticles interacting with the support, as determined by transmission electron microscopy. It was also verified that Au/Al2O3 activity was almost the same after two consecutive runs, indicating a good stability of the Au active phase. Kinetic studies were carried out by varying the initial concentration of lactose in the reaction mixture. A negative order respect to the reactant, determined applying a pseudo-homogeneous model, was estimated, which indicates that lactose molecules are strongly adsorbed on the surface of metal Au nanoparticles. A LHHW model, assuming that oxygen chemisorption was the controlling step, allowed to explain the negative order respect to lactose.