Kinetics of the catalytic hydrogenation of d-lactose on a carbon supported ruthenium catalyst

Catalytic hydrogenation of d-lactose to lactitol over various supported ruthenium catalysts was studied in a semi-batch slurry autoclave (300 ml, Parr Co.). 5% Ru/C (Johnson Matthey) had the best activity and gave the highest lactitol yield of the studied catalysts. Kinetic experiments of hydrogenation of aqueous d-lactose (1.31 mol/l) solutions on 5% Ru/C were performed at 40–60 bar hydrogen and 110–130 ◦C. The main hydrogenation product was lactitol, the selectivity varied between 96.5 and 98.5%, while small amounts of lactulose, lactulitol, sorbitol, galactitol and lactobionic acid were detected as by-products. The selectivity improved, as the hydrogen pressure increased and the reaction temperature decreased within the experimental range. The kinetic data were modelled by Langmuir–Hinshelwood–Hougen–Watson (LHHW) rate expressions, assuming surface reaction steps being rate determining. Non-competitive adsorption of molecular hydrogen and lactose on the catalyst surface was assumed. The kinetic model was fitted to the experimental data by a combined Simplex–Levenberg–Marquardt method. The model predicted the experimental concentrations of lactose and lactitol very well. A reasonable good description of the by-products was obtained.