Elucidating activating and deactivating effects of carboxylic acids on polyoxometalate-catalysed three-phase liquid-liquid-gas reactions

Three-phase liquid-liquid gas reactions are promising strategies to combine highly active and selective homogeneous-catalysed oxidation reactions with in-situ product isolation. Several approaches like the three-phasic oxidation of biomass to formic acid (OxFA process) or the three-phasic extraction-coupled oxidative desulfurization (ECODS process) of fuels have been developed. Hereby, an aqueous polyox-ometalate (POM) catalyst solution in combination with molecular oxygen as oxidant is used. However, contrary effects on the oxidation performance of the POM-catalyst were observed for different carboxylic acids as reaction products or intermediates. By using model calculations employing Density-Functional Theory (DFT) in combination with magnetic and optical spectroscopy, we could explain the experimental observations that oxalic acid activates the POM catalyst and therefore promotes its oxidation perfor-mance while acetic and especially formic acid form stable complexes with the active centres and there-fore inhibit the oxidation activity. These findings greatly improve the understanding of POM-acid complexes and their effect on the catalytic performance. (c) 2022 Elsevier Ltd. All rights reserved.