Mechanistic insights in the selective catalytic oxidation of glycolaldehyde: an industrially feasible route from biomass to bio-glycolic acid

Abstract

Glycolic acid (GAc) is used in a variety of applications, e.g., in the textile and leather industry, in dermatology and cosmetics and as a monomer to produce biodegradable polyglycolic acid (PGA) for e.g. biodegradable packaging. Currently, GAc is mainly produced from fossil feedstocks. To improve sustainability and circularity, a bio-based route is highly desirable. Pyrolysis of glucose and other monomeric sugars is an effective way to produce glycolaldehyde (GAl) from agricultural side streams. Hence, the selective catalytic oxidation of GAl to GAc is of scientific as well as industrial interest. Due to high reactivity of GAl, it is challenging to reach high selectivities (required for many applications) at industrially relevant conversions. Here we report the catalytic oxidation of GAl to glycolic acid using a Pt/C catalyst. Our experimental results show high selectivity at high conversion, while maintaining a high carbon balance under DoE optimised reaction conditions. The impact of oxygen pressure, catalyst:substrate ratio, and substrate concentration, as well as pH effects and product inhibition are addressed. Catalyst stability tests and characterisations (HAADF-STEM, BET, XRD, XPS, Elemental analysis) confirm that Pt/C is a promising catalyst for the selective oxidation of GAl to GAc. The experimental results are rationalized by employing DFT-metadynamics simulations at the finite reaction temperature and heterogeneous phase, where the entire reaction mechanism and network of main and side products are investigated and the key-elementary steps identified.