Kinetics and Mechanisms of Hydrothermal Ketonic Decarboxylation

The formation of ketone products from carboxylic acids in the presence of minerals has not been considered in the interpretations of aqueous geochemistry, even though the formation of ketones is a well-known industrial process that occurs on mineral surfaces. This study demonstrates the formation of ketone products through ketonic decarboxylation from phenylacetic and hydrocinnamic acid in the presence of the mineral surfaces of magnetite (Fe3O4), hematite (Fe2O3), corundum (Al2O3), and spinel (MgAl2O4) at hydrothermal conditions (300 °C, 1000 bar). These minerals were chosen to deconvolve the mechanism of ketonic decarboxylation and explore the difference in abundance and rate of product formation on different kinds of oxide minerals. The presence of minerals increased the number and variety of reaction paths available to phenylacetic acid, compared to reactions without minerals. Magnetite and spinel favored the ketonic decarboxylation reaction more strongly than hematite and corundum, resulting in greater product yields. In the case of spinel, the presence of mineral both increases the formation of dibenzylketone and the decomposition of the same ketone into toluene.