Hydrogenation of Lactose over Sponge Nickel CatalystsKinetics and Modeling

Abstract

Kinetics of lactose hydrogenation to lactitol, an alternative sweetener, over Mo-promoted sponge nickel slurry catalyst in aqueous solutions was studied. Hydrogenation experiments were carried out batchwise in a three-phase laboratory-scale reactor (300 mL, Parr Co.), operating at 20−70 bar and between 110 and 130 °C. The main hydrogenation product was lactitol, while small amounts of lactobionic acid, lactulose, lactulitol, sorbitol, and galactitol were detected as byproducts. The lactitol selectivity within the experimental range varied from 90 to 99%. The selectivity values improved as the hydrogen pressure increased and the reaction temperature decreased. The effect of catalyst loading and catalyst deactivation during consecutive hydrogenation batches was also studied. Fresh and recycled catalysts were characterized by nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), hydrogen temperature-programmed desorption (TPD), and particle-size analysis. Recycled sponge nickel catalyst was able to adsorb less hydrogen compared to fresh one, indicating active site poisoning. Of all byproducts, only lactobionic acid had an inhibiting effect on lactose conversion rate and deactivated the catalyst. By lactobionic acid, deactivated catalyst can be regenerated by controlled alkali wash. The kinetic data were modeled by Langmuir−Hinshelwood−Hougen−Watson (LHHW) kinetics, assuming surface reaction steps are rate determining. Noncompetitive adsorption of molecular hydrogen and lactose was assumed. The fitting of the experimental data to the kinetic model was carried out by a combined Simplex−Levenberg−Marquardt method. The model predicted the experimental concentrations of lactose and lactitol very well. A reasonably good description of the byproducts was obtained.