Carbon isotopic fractionations during Fischer-Tropsch synthesis

To understand the carbon isotope fractionation during the mineral-catalyzed Fischer-Tropsch synthesis of hydrocarbons under hydrothermal conditions, experiments on formic acid were carried out at 300°C and 35 MPa using gold tubes in the presence of Fe as a catalyst. The experiments were composed of two groups: with and without water. Due to the limited volume of the gold tubes, only methane was available for isotopic analyses among all produced hydrocarbons. The results demonstrate that CO2 is the gas most enriched in 13C whereas methane is the gas most depleted in 13C. Moreover, methane becomes more and more depleted in 13C with an increase in reaction time. The carbon isotopic fractionation between CO2 and CH4 (α(CO2-CH4)) reached 1.052–1.059 at 144 h, which is similar to those of microbial reduction of CO2 to CH4 by methanogenic bacteria (1.048–1.079). This implies that carbon isotopic fractionation during Fischer-Tropsch synthesis is controlled by kinetic isotopic effects.