Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Abstract

Aldehydes represent an intermediate redox state of organic carbon and can be precursors to carboxylic acids via disproportionation. A model aldehyde, benzaldehyde, was subjected to hydrothermal experiments (250–350 °C, saturation pressure) to assess the kinetics and mechanisms of the reactions leading to carboxylic acids. The concentration dependence demonstrates the kinetics are second-order in benzaldehyde, consistent with a disproportionation reaction, which is reminiscent of the base-promoted Cannizzaro reaction known at lower temperatures. Arrhenius parameters for these rate constants trend well with data from most, but not all, previous studies for the reaction under supercritical conditions. The rate constants yielded an entropy of activation (ΔS‡) of −161 J mol–1 K–1, consistent with a bimolecular transition state at the rate-limiting step. Experimental yields of benzoic acid and benzyl alcohol were not equal, unlike what is expected for the disproportionation reaction. A kinetic model that includ...