The three-phase hydrogenation of xylose to xylitol and anomeric equilibria
of xylose were studied batchwise in aqueous and alcoholic solvents. Rate eq
uations based on a novel model for semicompetitive adsorption of dissociate
d hydrogen and organic species were developed. The proposed kinetic model w
as well able to predict the xylose, xylitol and also the by-product concent
rations. The model parameters were related to the polarity of the solvent.
The sugar equilibria studies gave new information about the temperature dep
endence of the alpha-beta -pyranose and pyranose-furanose equilibria of xyl
ose, in both water and aqueous ethanol solutions. It was found that the equ
ilibria in both D2O and C2D5OD follow an S-shaped curve, the equilibria bei
ng shifted towards the cr-form at higher temperatures. The enhancing effect
of alcoholic leg ethanol, 2-propanol) solvents on the hydrogenation of sug
ars is notable. We propose that the rate accelerating effect of alcohols is
mainly due to the improved hydrogen solubility. Thus, the mass transfer of
hydrogen from the gas phase to the liquid bulk, and further to the surface
of the catalyst is clearly improved by the use of an alcohol solvent. (C)
2001 Society of Chemical Industry.