Hydrogenation of xylose to xylitol: Three-phase catalysis by promoted raney nickel, catalyst deactivation and in-situ sonochemical catalyst rejuvenation

Xylitol, the hydrogenation product of xylose, is a commonly used sweetening agent with anti-caries properties and no insulin requirement. Batchwise, three-phase hydrogenation of xylose to xylitol over Raney nickel and anomeric equilibria of xylose were studied. Raney Ni catalyst was active but deactivated relatively rapidly. Kinetic measurements, carried out with recycled catalysts revealed that the catalyst deactivates during the use, but an asymptotic activity level is finally attained. Catalyst characterization studies (nitrogen adsorption, XRD, ESCA-XPS and gravimetric reduction) suggested that the main reasons for the deactivation is the decay of accessible active sites through collapse of the pore structure and leaching of the promoter metal (Mo) as well as alumina. A novel kinetic model based on semi-competitive adsorption of dissociated hydrogen and organic species was developed. The proposed kinetic model was able to predict the xylose, xylitol as well as the by-product concentrations well. An integrated ultrasonic treatment of the catalyst was found to essentially retard catalyst deactivation. The sugar equilibria studies gave new information about the temperature dependence of the α-β-pyranose and pyranose-furanose equilibria of xylose. The amount of furanose increases clearly with temperature.