Thermal decarboxylation of acetic acid: Implications for origin of natural gas
Laboratory experiments on the thermal decarboxylation of solutions of acetic acid at 200°C and 300°C were carried out in hydrothermal equipment allowing for on-line sampling of both the gas and liquid phases for chemical and stable-carbon-isotope analyses. The solutions had ambient pH values between 2.5 and 7.1; pH values and the concentrations of the various acetate species at the conditions of the experiments were computed using a chemical model. Results show that the concentrations of acetic acid, and not total acetate in solution, control the reaction rates which follow a first order equation based on decreasing concentrations of acetic acid with time. The decarboxylation rates at 200°C (1.81 × 10 −8 per second) and 300°C (8.17 × 10 −8 per second) and the extrapolated rates at lower temperatures are relatively high. The activation energy of decarboxylation is only 8.1 kcal/mole. These high decarboxylation rates, together with the distribution of short-chained aliphatic acid anions in formation waters, support the hypothesis that acid anions are precursors for an important portion of natural gas. Results of the δ 13 C values of CO 2 , CH 4 , and total acetate show a reasonably constant fractionation factor of about 20 permil between CO 2 and CH 4 at 300°C. The δ 13 C values of CO 2 and CH 4 are initially low and become higher as decarboxylation increases.