Reaction mechanism and intrinsic kinetics of sugar hydrogenation to sugar alcohols on solid foam Ru/C catalysts – From arabinose and galactose to arabitol and galactitol

Abstract

Solid foam catalysts have two great benefits: thin catalyst layers guaranteeing a high effectiveness factor and a low pressure drop in continuous operation. The ability of ruthenium-based solid foam catalysts in the hydrogenation of monomeric sugars was illustrated with extensive experiments of L-arabinose and D-galactose hydrogenation at 90-120˚C and 20 bar hydrogen pressure. Kinetic experiments were carried out with individual sugars and binary sugar mixtures at different D-galactose-to-L-arabinose molar ratios to reveal the molecular interactions in the presence of the solid foam catalyst. High conversion of sugars and high selectivity to sugar alcohols were achieved in the isothermal and isobaric laboratory-scale reactor which operated under intrinsic kinetic control. The sugar hydrogenation process was considered from a viewpoint of elementary steps on the catalyst surface. By assuming plausible surface reaction mechanisms, it was possible to derive rate equations for the formation of sugar alcohols, both in case of individual sugars and binary sugar mixtures. The kinetic model based on the non-competitive adsorption of sugars and hydrogen on the ruthenium surface gave a very good description of the hydrogenation kinetics and product distribution on the solid foam catalysts. The work opens a perspective to the selective and very effective hydrogenation of several sugars to valuable sugar alcohols in the presence of open foam Ru/C catalysts, both in batch and continuous operation modes.