Carbon isotope effects in the open-system Fischer Tropsch synthesis

Details Carbon isotope effects in the open-system Fischer Tropsch synthesis Carbon isotope effects in the open-system Fischer Tropsch synthesis

Sep 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.4474t&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 71, Issue 18, p. 4474-4487.

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Carbon isotopic composition was measured for products of the Fischer Tropsch synthesis: catalytic reaction between CO and H2 to produce CO, CO2, 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 5N2 + 3H2 + 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 isotopic 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 CO2 (˜-10‰) and CO and hydrocarbons (˜+38‰). At low conversion a clear “inverse” isotopic trend of the depletion in 13C of longer hydrocarbon chains was observed. On average, Δ12 = δ13C(CH4) - δ13C(C2H6) correlates well with the CO conversion: the C2H6 is ˜6‰ isotopically lighter than CH4 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 (δ13C1 > δ13C2 > δ13C3 >⋯) is somewhat similar to that at low conversion 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.

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