MINERAL-ASSISTED ORGANIC TRANSFORMATIONS OF CARBOXYLIC ACIDS AT HYDROTHERMAL CONDITIONS

The opening of novel hydrothermal transformation pathways for organic compounds at mineral surfaces, which differ dramatically from reactions in water alone [1] highlights the potential influence of mineral surfaces on the emergence of complexity among the organic inventory of the early Earth and other water- and mineral-rich planets. In the present study, hydrothermal (300°C, 100 MPa) experiments were performed to analyze the effects of magnetite on transformations of carboxylic acids. The acids used in this study, phenylacetic acid (C₆H₅CH₂COOH) and hydrocinnamic acid (C₆H₅(CH₂)₂COOH), differ by the addition of a phenyl ring from acetic and valeric acids [2,3] used in previous mineral-assisted studies. Ring structures facilitate the investigation of mechanistic pathways for product formation and are analogs for lipids or other aromatic organic compounds.

In the absence of minerals, decarboxylation is the major reaction pathway for carboxylic acids under the experimental conditions. Phenylacetic acid (PAA) reaches 80% conversion (50 h) [4] while hydrocinnamic acid (HCA) reaches only 20% conversion (1000 h). For PAA, the only product is toluene. The primary product from HCA is ethyl benzene with additional minor products from intramolecular reactions.

In the presence of magnetite (Fe₃O₄) hydrothermal reactions yield organic products larger than the reactants. For PAA the magnetite-activated product pathways lead to diphenyl alkanes, alkenes and ketones. Magnetite enhanced the conversion of HCA from 20% in water alone to 80% (1000 h). In addition to forming the same variety of compounds observed in the PAA experiments, HCA experiments also produced polymerization products and an array of products resulting from secondary reaction pathways.

Formation of new products may be the result of hydrogen atoms attracted to the magnetite. Affiliation of the carboxyl group with the protonated surface could create an environment where molecules readily interact. Interactions like this are not possible in water alone, providing a compelling need for mineral surfaces to encourage complex organic reactions.

[1] Shipp JA et al. (2014) PNAS [2] Bell JLS et al. (1994) Geochim Cosmochim Acta [3] McCollom TM and Seewald JS (2003) Geochim Cosmochim Acta [4] Glein CR (2012) PhD Dissertation, Arizona State University