Effects of solvent polarity on the hydrogenation of xylose

Abstract

The three-phase hydrogenation of xylose to xylitol and anomeric equilibria of xylose were studied batchwise in aqueous and alcoholic solvents. Rate equations based on a novel model for semi-competitive adsorption of dissociated hydrogen and organic species were developed. The proposed kinetic model was well able to predict the xylose, xylitol and also the by-product concentrations. The model parameters were related to the polarity of the solvent. The sugar equilibria studies gave new information about the temperature dependence of the α–β-pyranose and pyranose–furanose equilibria of xylose, in both water and aqueous ethanol solutions. It was found that the equilibria in both D2O and C2D5OD follow an S-shaped curve, the equilibria being shifted towards the α-form at higher temperatures. The enhancing effect of alcoholic (eg ethanol, 2-propanol) solvents on the hydrogenation of sugars 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. © 2001 Society of Chemical Industry