Theoretical and Experimental Investigations of Acetal Formation During the Oxidation of Xylose to Formic Acid Catalysed by H ₅ PV ₂ Mo ₁₀ O ₄₀ in Methanolic‐Aqueous Solution

The BioValCat (Enhanced Biomass Valorization by Engineering of Polyoxometalate Catalysts) project aims at developing homogeneously catalysed, selective biomass transformation technologies ultimately leading to an industrial viable biomass valorisation process. Herein, investigations of the conversion pathways for selected model substrates is one of the major tasks. Besides the catalyst-solvent interactions, the substrate-solvent interactions also play an important role. In this study, we aimed at answering the fundamental research question: what are the key mechanisms in aqueous-alcoholic solutions preventing decarboxylation and where are the limits with respect to process conditions and solvent composition? Therefore, a special focus was placed on the influence of the functional groups in different reaction intermediates from HPA-2 (H₅PV₂Mo₁₀O₄₀) catalysed xylose oxidation on the way to formic acid. We found that the protection of reactive functional groups of the model carbohydrates depends on the formation and stability of several reaction intermediates, which are formed via acetalization or esterification. This is strongly dependent on solvent composition and reaction conditions. The present findings were supported by DFT-calculations explaining the different effects of the solvent composition on both substrate and catalyst, emphasising the catalyst-solvent interactions are of superior importance for catalysis.