Reactivity of monocyclic aromatic compounds under hydrothermal conditions

Monocyclic aromatic compounds (MAC) represent a significant fraction of the total organic carbon in many geologic environments such as hydrothermal systems and petroleum reservoirs, yet the factors that control their abundance in these environments remain highly uncertain. In order to evaluate whether aqueous reactions involving MAC have a significant impact on their occurrence, laboratory experiments were conducted to observe the reactivity of several simple MAC under hydrothermal conditions that included the presence of mineral redox buffers. Aqueous solutions of individual MAC were heated at 300 to 330 degreesC and 350 bar in flexible gold reaction vessels with titanium fittings. Toluene in aqueous solution was found to gradually decompose during heating to form primarily benzene plus CO2, indicating the decomposition proceeded by an oxidative decarboxylation pathway. The rate of this reaction was enhanced by the presence of dissolved sulfur compounds and relatively oxidizing conditions, suggesting that intermediate oxidation state sulfur compounds (such as thiosulfate or polythionates) could play a role in promoting the reaction by facilitating the transfer of electrons among reactants. Benzoic acid decomposed rapidly to benzene plus CO2, suggesting that formation of benzoic acid is the rate-limiting step in the overall conversion of toluene to benzene. Both benzene and phenol were found to be essentially unreactive. An assessment of the reaction products was performed to evaluate whether reactions among MAC attained metastable thermodynamic equilibrium. The results of this assessment, however, were equivocal, with some observations suggesting approach to thermodynamic equilibrium while other data indicate that criteria to demonstrate equilibrium were not met. The laboratory results demonstrate that aqueous reactions can play a role in controlling the abundance of aromatic compounds in geologic environments. Copyright (C) 2001 Elsevier Science Ltd.