Hydrothermal Transformations of Alcohols with Copper(II) and Iron(III) Salts

Organic reactions in hydrothermal systems are important and unique for many geochemically relevant processes such as petroleum maturation and carbon cycling in the deep ocean. Reaction pathways that link different types of organic functional groups have been proposed and verified by laboratory-simulated hydrothermal experiments. In these organic functional group interconversions, alcohols serve as a redox intermediate that connects hydrocarbons and carboxylic acids in deep sedimentary basins. While dehydration of alcohols to form hydrocarbons such as alkenes is thermodynamically favorable under hydrothermal conditions (e.g., above 200 °C), oxidation of alcohols to aldehydes and carboxylic acids is often less likely to occur due to the lack of an oxidizing power. In this study, we examined the effects of six different copper­(II) and iron­(III) salts on hydrothermal reactions of model alcohol compounds, including primary, secondary, and tertiary alcohols. In the absence of dissolved metals, we find that dehydration is the dominant pathway for alcohols in the hydrothermal fluids. However, in the presence of copper­(II) or iron­(III) salts, the oxidation of alcohols is greatly promoted and becomes a competitive pathway to form aldehydes and carboxylic acids as the major products. Geochemical calculations on the aqueous properties of reactions further support that alcohol oxidations could be thermodynamically favorable with the metal ions under hydrothermal conditions. Our results suggest an important role of dissolved metal ions in hydrothermal transformations of alcohols, which may provide new understanding of how inorganic materials control organic transformations in natural hydrothermal environments.