(111g) Kinetics Study of the Hydrodeoxygenation of Xylitol over a ReOx-Pd/CeO2 Catalyst

Xylitol is a sugar alcohol derived from the hemicellulose portion of lignocellulosic biomass, and is mass produced and commonly used as a sugar substitute. Xylitol can be chemically upgraded into value added platform chemicals such as 1,2-dideoxypentitol and 1,2,5-pentanetriol via a simultaneous hydrodeoxygenation (S-HDO). 1,2-dideoxypentitol and 1,2,5-pentanetriol are 300-5000 times more valuable than the initial cost of xylitol. The S-HDO removes two vicinal hydroxyl groups by undergoing a deoxydehydration (DODH) to form a double bond which is then hydrogenated to a single bond. The ReO_x-Pd/CeO₂ catalyst was determined to be the best catalyst tested in literature for the S-HDO of sugar alcohols.

In this study, we elucidate the reaction kinetics for the S-HDO of xylitol to 1,2-dideoxypentitol and 1,2,5-pentanetriol over a ReO_x-Pd/CeO₂ (2.0 weight% Re, 0.30 weight% Pd) catalyst. The reaction was determined to be a zero-order reaction with respect to xylitol. The activation energy was elucidated through an Arrhenius relationship and non-Arrhenius kinetics. The Arrhenius relationship was investigated at 150–170 °C and a constant H₂ pressure of 10 bar resulting in an activation energy of 48.7 ± 10.5 kJ/mol. The investigation of non-Arrhenius kinetics was conducted at 120–170 °C and a sub-Arrhenius relation was elucidated with activation energy being dependent on temperature, and ranging from 10.2–51.8 kJ/mol. The sub-Arrhenius relation is indicative of quantum tunneling occurring during this reaction. Internal and external mass transfer were investigated through evaluating the Weisz–Prater criterion and the effect of varying stirring rate on the reaction rate, respectively. There were no internal or external mass transfer limitations present in the reaction. The structure of the ReO_x was found to change as a function of temperature in Raman spectroscopy. The di-oxo (O=Re=O) species of ReO_x was found to increase in concentration as temperature is increased and the mono-oxo (Re=O) species decreases