Selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and propylene glycol on supported Ru catalysts

The selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and propylene glycol was carried out on different catalysts in the presence of Ca(OH)(2). The catalysts included Ru supported on activated carbon (C) and, for comparison, on metal oxides, Al(2)O(3), TiO(2), ZrO(2) and Mg(2)AlO(x) as well as C-supported other noble metals, Rh, Pd and Pt, with similar particle sizes (1.6-2.0 nm). The kinetic effects of H(2) pressures (0-10 MPa), temperatures (433-513 K) and solid bases including Ca(OH)(2), Mg(OH)(2) and CaCO(3) were examined on Ru/C. Ru/C exhibited superior activities and glycol selectivities than Ru on TiO(2), ZrO(2), Al(2)O(3) and Mg(2)AlOx, and Pt was found to be the most active metal. Such effects of the metals and supports are attributed apparently to their different dehydrogenation/hydrogenation activities and surface acid-basicities, which consequently influenced the xylitol reaction pathways. The large dependencies of the activities and selectivities on the H(2) pressures, reaction temperatures, and pH values showed their effects on the relative rates for the hydrogenation and base-catalyzed reactions involved in xylitol hydrogenolysis, reflecting the bifunctional nature of the xylitol reaction pathways. These results led to the proposition that xylitol hydrogenolysis to ethylene glycol and propylene glycol apparently involves kinetically relevant dehydrogenation of xylitol to xylose on the metal surfaces, and subsequent base-catalyzed retro-aldol condensation of xylose to form glycolaldehyde and glyceraldehyde, followed by direct glycolaldehyde hydrogenation to ethylene glycol and by sequential glyceraldehyde dehydration and hydrogenation to propylene glycol. Clearly, the relative rates between the hydrogenation of the aldehyde intermediates and their competitive reactions with the bases dictate the selectivities to the two glycols. This study provides directions towards efficient synthesis of the two glycols from not only xylitol, but also other lignocellulose-derived polyols, which can be achieved, for example, by optimizing the reaction parameters, as already shown by the observed effects of the catalysts, pH values, and H(2) pressures.