Influence of Reactive Additives on the Polyoxometalate‐Catalyzed Oxidation of Xylose to Formic Acid

Abstract Chemo‐catalytic biomass valorisation using polyoxometalate (POM) catalysts is a promising strategy for green and sustainable chemistry. Specifically, the production of short‐chain carboxylic acids like formic acid (FA) has received much attention. Recently, it has been shown that the selectivity of biomass oxidation processes can be drastically improved by the use of additives like methanol. In this study, the influence of various reactive additives on the kinetics of xylose oxidation to FA using the HPA‐5 (H 8 PV 5 Mo 7 O 40 ) POM‐catalyst has been investigated both in aqueous as well as in aqueous‐methanolic solutions. Herein, significant differences in the two reaction systems under investigation could be deduced. In aqueous solution, oxalic acid and acetic acid accelerate xylose conversion by initially reducing the HPA‐5 catalyst, generating a more active species. However, benzoic acid and especially propionic acid exert a detrimental effect on the xylose oxidation kinetics. This could be related to an inhibition induced by a competitive binding of the latter acids on the V site of the catalyst, as suggested by density functional theory (DFT) calculations. Moreover, this effect was less pronounced in aqueous‐methanolic solution, as the interaction of methanol with both the V and Mo sites of the POM catalyst shown via DFT calculations is much stronger compared to the binding of the reactive additives. This study shows interesting new correlations allowing for further pushing the limits of the OxFA technology toward higher FA selectivities and therefore increased productivity.