Aerobic oxidation of glucose: 2. Catalysis by colloidal gold

The selective oxidation of d-glucose to d-gluconic acid was performed in aqueous phase at atmospheric pressure, controlled pH value and different glucose and oxygen concentrations, in the temperature range from 303.2 to 333.2 K, using a colloidal metal gold catalyst (average gold diameter 3.5 nm). Initial rate was measured as a function of glucose and oxygen concentration: in the experimental conditions it was found that gluconic acid is produced together with hydrogen peroxide, which later decomposes in a fast way, due to the presence of alkali. The measurements were interpreted by considering different models based on different reaction pathways. Among the considered models, the experimental data fit with an Eley-Rideal mechanism where a glucose molecule, adsorbed on the catalyst, interacts with an oxygen molecule coming from the liquid phase. The model includes a kinetic parameter kcat and the equilibrium constant KG for the adsorption of glucose on the gold surface. The activation energy for kcat was found to be 47.0 ± 1.7 kJ mol-1. It has been observed that KG decreases when temperature is increased, but the experimental uncertainty did not allow to obtain a precise value of the adsorption enthalpy. The values of the rate parameters here calculated for the colloidal gold catalyst have been compared with those previously obtained using the homogeneous enzymatic catalyst Hyderase under similar experimental condition. Considering geometric constraints, the specific activity of gold catalysis resulted quite similar to the enzymatic one.