Effects of solvent polarity on the hydrogenation of xylose

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.