Carbon isotope effects in the open-system Fischer–Tropsch synthesis Yuri A. Taran a, * , George A. Kliger b , Vyacheslav S. Sevastianov c a Institute of Geophysics, UNAM, 04510 Mexico DF, Mexico b Institute of Petrochemical Synthesis, RAS, Moscow, Russia c Vernadsky Institute of Geochemistry, RAS, Moscow, Russia Received 28 November 2006; accepted in revised form 26 June 2007; available online 19 July 2007 Abstract Carbon isotopic composition was measured for products of the Fischer–Tropsch synthesis: catalytic reaction between CO and H 2 to produce CO, CO 2 , light hydrocarbons C1–C4 and ‘‘oil’’ fraction. Hydrogen isotopes were also measured in the oil fraction and the produced water. Experimental runs were conducted in the flow-through reactor at 260–310 °C and 30 bar using the synthesis gas composed of 5N 2 + 3H 2 + 2CO, on Fe-catalyst mixed with ZSM-5 synthetic zeolite. In the two of seven runs a Fe + Co-catalyst was used that gives a lower yield of unsaturated hydrocarbons in reaction products. The iso- topic effects depended on the conversion of the carbon monoxide. Under steady-state conditions (CO conversion more than 90%) a strong kinetic fractionation was observed between CO and CO 2 (10‰) and CO and hydrocarbons (+38‰). At low conversion a clear ‘‘inverse’’ isotopic trend of the depletion in 13 C of longer hydrocarbon chains was observed. On aver- age, D 12 = d 13 C(CH 4 )  d 13 C(C 2 H 6 ) correlates well with the CO conversion: the C 2 H 6 is 6‰ isotopically lighter than CH 4 at low conversion and 2‰ heavier at steady-state regime. Under steady-state conditions there almost no difference was observed in the isotopic composition of methane and ethane and higher hydrocarbons. The chemical composition of light hydrocarbons in the products of flow-through, dynamic FTS is different from that found in the static FTS-type experiments with Fe-catalyst, but isotopic effects are similar. Our results suggest that the isotopic distribution of carbon found in so-called ‘‘abiogenic’’ hydrocarbons from some natural gases (d 13 C1 > d 13 C2 > d 13 C3 >) is somewhat similar to that at low conver- sion of CO, but do not resemble the distribution characteristic for the high conversion products, at least, on Fe-catalyst. Other processes (a simple mixing of two or more endmembers) or other P–T conditions of the carbon reduction could be responsible for the ‘‘inverse’’ isotopic trend found in meteorites and some natural gases. Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The Fischer–Tropsch synthesis (FTS), which generally can be defined as the heterogeneous catalytic reduction of oxidized carbon compounds by molecular hydrogen, is widely accepted as a process potentially responsible for the presence of organic compounds in meteorites, submar- ine hydrothermal systems and igneous rocks (e.g. Lancet and Anders, 1970; Shock, 1990; Salvi and Williams-Jones, 1997; Yuen et al., 1984; Foustoukos and Seyfried, 2004; Horita, 2005). This ‘‘inorganic’’, ‘‘abiotic’’ synthesis has also been considered to be important in global geologic processes including production of methane and petroleum and finally, as a source of prebiotic compounds on the early Earth (Szatmari, 1989; Charlou et al., 2002; Sherwood Lol- lar et al., 2002; Horita, 2005, among others). However, despite this apparent agreement about the importance and existence of the abiotic synthesis on the early and modern Earth, there are still no rules how to dis- tinguish abiogenic and biogenic organic compounds, espe- cially, methane and light hydrocarbons which are common species in any natural gas manifestation, even in 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.06.057 * Corresponding author. E-mail address: taran@geofisica.unam.mx (Y.A. Taran). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 71 (2007) 4474–4487