The effect of palladium dispersion and promoters on lactose oxidation kinetics

Lactose oxidation was investigated at 70 °C and at pH 8 using oxygen as an oxidant over a comprehensive set of commercially available mono- and multi-metallic as well as promoted Pd catalysts with active carbon, alumina and calcium carbonate as catalyst supports. An optimum cluster size of 6–10 nm resulted in the highest initial turnover frequencies. High conversion levels above 90% were achieved on Pd/C catalyst, as well as over Pd/Al₂O₃ and (Pd–Pb)/CaCO₃, whereas (Pd–V)/C catalyst gave only 30% conversion after 200 min. The latter catalyst was relatively inactive due to its high support acidity and profound deactivation during oxidation. Besides the main oxidation product, lactobionic acid, also, lactulose was generated as a result of lactose isomerisation under alkaline conditions. The electrochemical potentials of the catalysts were measured during lactose oxidation. The main result of these measurements was that, when the electrochemical potential of the catalyst increased very quickly, its oxidation activity was low due to metal over-oxidation. The selectivities to the desired product, lactobionic acid, were relatively high, above 80% for most of the catalysts, except for (Pd–V)/C. Furthermore, the selectivity to the lactobionic acid decreased with increasing metal dispersion, thus, indicating that the optimum metal particle sizes for producing high amounts of lactobionic acid is above 3 nm.