Abstract

Sugar monomers originating from well-controlled hydrolysis of hemicelluloses appearing in biomass are important platform molecules and they can be further valorized by catalytic hydrogenation, oxidation, and isomerization. Oxidation of sugar monomers with molecular oxygen in the presence of gold catalysts is a green pathway for obtaining sugar acids that are useful for alimentary, pharmaceutical, and construction industries. Catalytic oxidation of sugar mixtures on gold nanoparticles supported on aluminum oxide extrudates was studied in an aqueous environment to reveal the interaction of intrinsic kinetics, catalyst deactivation, and mass transfer effects. The oxidation kinetics of two sugar monomers, arabinose and glucose, and their mixtures in the presence of gold nanoparticles was determined. Several oxidation experiments were conducted in a laboratory-scale semibatch reactor at 70 degrees C, pH 8, and atmospheric pressure. SpinChem mixing technology was applied to immobilize the catalyst particles in the reactor and to create vigorous turbulence, suppressing external mass transfer limitations around the particles. The reaction kinetics were monitored by measuring the concentrations of the reactants and products by high-performance liquid chromatography (HPLC). The gold catalyst extrudates were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen physisorption, and particle size analysis. A decline in the oxidation activity was observed in successive semibatch experiments with recycled catalyst extrudates. It was confirmed by inductively coupled plasma analysis (ICP-OES) that the reason for the deactivation was leaching of gold. The kinetic results revealed strong internal mass transfer limitations inside the pores of the catalyst extrudates. A mathematical model was derived for the reaction kinetics, catalyst deactivation, and internal mass transfer, and the parameters of the model were successfully estimated by nonlinear regression analysis. With the mathematical model, the performance of catalyst particles of different sizes can be predicted, which is important for scale-up and in shifting from discontinuous to continuous oxidation technology in the future.