Microkinetic Modelling of the Heterogeneously Catalyzed Hydrogenation of Glycolic Acid over Ru/C

Abstract

The heterogeneously catalyzed hydrogenation of biomass- or CO₂-derived glycolic acid (GA) is a renewable pathway for obtaining the bulk chemical ethylene glycol (EG). In this study, the reaction network of the aqueous-phase hydrogenation of GA is investigated over a Ru/C catalyst. Beside this target reaction, both undesired parallel reactions (such as the hydrogenation of GA to acetic acid (AcA)) and unwanted consecutive reactions (such as over-hydrogenation) can occur. These reactions are experimentally assessed by employing different (possible) reaction intermediates and products (EG, AcA, glycolaldehyde, acetaldehyde, and ethanol) as reactants. The data for all individual reactions as well as the overall GA hydrogenation at different reaction temperatures (120–180 °C) is combined to propose a catalytic reaction pathway network and to develop a corresponding microkinetic model. The kinetic model describes the catalytic data in the range up to 150 °C, where EG is the main reaction product, with high accuracy. It is shown that selectivity for EG is highest at 120 °C (up to ca. 70%) and decreases strongly with increasing temperatures both due to the parallel reaction of GA into acetic acid (AcA), which has a significantly higher activation energy than the overall hydrogenation of GA into EG, and due to degradation of EG into ethanol and other volatile products. Temperatures exceeding 150 °C predominantly yield volatile products, suggesting low yields of EG or AcA at higher temperatures despite enhanced GA conversion.