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 sy stems and petroleum reservoirs, yet the factors that control their abundanc e in these environments remain highly uncertain. In order to evaluate wheth er aqueous reactions involving MAC have a significant impact on their occur rence, laboratory experiments were conducted to observe the reactivity of s everal simple MAC under hydrothermal conditions that included the presence of mineral redox buffers. Aqueous solutions of individual MAC were heated a t 300 to 330 degreesC and 350 bar in flexible gold reaction vessels with ti tanium fittings. Toluene in aqueous solution was found to gradually decompo se during heating to form primarily benzene plus CO2, indicating the decomp osition proceeded by an oxidative decarboxylation pathway. The rate of this reaction was enhanced by the presence of dissolved sulfur compounds and re latively 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 r eactants. Benzoic acid decomposed rapidly to benzene plus CO2, suggesting t hat formation of benzoic acid is the rate-limiting step in the overall conv ersion of toluene to benzene. Both benzene and phenol were found to be esse ntially unreactive. An assessment of the reaction products was performed to evaluate whether reactions among MAC attained metastable thermodynamic equ ilibrium. The results of this assessment, however, were equivocal, with som e observations suggesting approach to thermodynamic equilibrium while other data indicate that criteria to demonstrate equilibrium were not met. The l aboratory results demonstrate that aqueous reactions can play a role in con trolling the abundance of aromatic compounds in geologic environments. Copy right (C) 2001 Elsevier Science Ltd.