Hydrothermal One-Electron Oxidation of Carboxylic Acids in the Presence of Iron Oxide Minerals

Hydrothermal experiments were undertaken to explore the reaction pathways of phenyl-containing carboxylic acids in the presence of iron oxide minerals. At 300 °C and 1 kbar (100 MPa), in addition to the previously reported decarboxylation and ketonic decarboxylation pathways, phenylacetic acid undergoes oxidation to form benzoic acid, which eventually forms 2-phenylacetophenone via ketonic decarboxylation with phenylacetic acid. The production of benzoic acid in the presence of magnetite (Fe3O4) or hematite (Fe2O3) parallels the production of benzoic acid in the presence of Cu(II) salt solutions observed in previous hydrothermal studies, which was attributed to a sequence of one-electron-transfer processes. We propose a similar one-electron oxidation reaction pathway in the presence of minerals. Complexity builds as the reaction options increase. Hydrothermal experiments with hydrocinnamic acid were performed to demonstrate the generality of the reaction pathways for carboxylic acids, although the rate of consumption of hydrocinnamic acid was slower than that of phenylacetic acid and yielded a complex variety of detected products. Hydrocarbons are produced at the longest observed time points of a reaction through either decomposition or C-C bond formation to larger compounds. These results indicate that minerals can enhance the complexity of organic product pathways for carboxylic acids and resulting products during hydrothermal transformations and may enable the production of hydrocarbons from organic acids and other precursors.