Oxalic Acid Hydrogenation to Glycolic Acid: Toward Stable and Selective Ruthenium Catalysts.

If we want to address climate change and loss of biodiversity, we need to move toward a circular economy that uses closed-loop recyclable plastics, reduces CO emissions, and replaces fossil feedstocks for chemicals and materials. Fossil-based polymers can be replaced by CO- or biobased polymers starting from monomers such as glycolic acid. Glycolic acid can be obtained from oxalic acid by the direct hydrogenation of only one of the two carboxylic acid groups with a very high selectivity up to 100%. In this work, we studied a set of improved ruthenium-tin-based two- and three-metallic catalysts. We achieved a 95% glycolic acid yield after 4 h in batch reactors (100 bar H; 75 °C), and 100% yield at 70-100 °C and 60 bar H in flow reactors, with improved selectivity toward glycolic acid consistently above 90%, acetic acid formation below 5%, and improved catalyst stability in the harsh acid environment (pH < 1). We established the ideal loading and ratio of ruthenium and tin, explored the influence of supports, and showed that avoiding the presence of chloride increases the catalyst stability. We study the electronic properties of chloride-free ruthenium-tin catalysts during the reaction and identify insufficient Ru/Sn metal reduction as the main cause of catalyst deactivation. The addition of platinum as a third metal significantly improved the catalyst stability while maintaining the high activity and selectivity reducing activity loss to only 9% over multiple uses. This work enables the efficient direct reduction of oxalic acid to glycolic acid and, consequently, the utilization of CO and biomass-derived oxalic acid as monomers for polyesters.