Efficient and selective effects of copper salts on hydrothermal oxidative decarboxylation have been discovered and studied.

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.

Efficient anaerobic oxidation of aldehydes using iron(iii) nitrate under hydrothermal conditions is reported.

Unlike ambient conditions, water at elevated temperature and pressure can increasingly favor dissolution of hydrophobic organic molecules, act as an acid/base catalyst, and trigger unique hydrothermal pathways. In a recent study, we found that amides could be readily synthesized from simple amines and carboxylic acids in hydrothermal solutions, which implied a potential peptide and biomolecule synthetic pathway in natural hydrothermal environments. Formation of amide or peptide bonds is critical in producing building blocks of peptides and proteins, which are essential to living organisms, such as extremophiles, in hydrothermal systems. However, amide formation and degradation pathways under hydrothermal conditions are still not well understood, particularly how these pathways would be influenced by surrounding minerals or dissolved metals is unknown. Here, we describe the effects of copper(II) salts, such as copper chloride and copper sulfate, on the hydrothermal formation and reactivity of amides at 250 °C and 40 bar (P sat ). The copper salts were chosen to be studied because they are commonly distributed in seawater and hydrothermal fluids. We found that the copper salts greatly inhibited the amide formation, with a decreased amide yield by up to 90% within 2 h. We also observed that aldehydes were quickly formed through the oxidation of amines by copper and became the dominant products. Compared to copper chloride/sulfate, copper acetate also facilitated the amine oxidation but did not suppress the amide formation, suggesting different roles of copper salts in amide hydrothermal synthesis. In addition, we found that hydrothermal reactivity of amides was strongly dependent on the solution pH, with lower reactivity at neutral pH than at very acidic or alkaline conditions. Our findings thus suggest that both copper and pH are important factors for amide hydrothermal synthesis and reactivity, which provide new insights into the prediction of amide or peptide bond synthesis in natural hydrothermal environments.