Inverse kinetic isotope effects and deuterium enrichment as a function of carbon number during formation of C-C bonds in cobalt catalyzed Fischer-Tropsch synthesis

摘要

How the deuterium is enriched in hydrocarbons produced by Fischer-Tropsch (FT) synthesis is of broad scientific interest, particularly to those studying the mechanism of the FT reaction and the formation of hydrocarbons in the Earth's crust or in the nebula. In order to determine whether there is an inverse isotope effect and whether there exists the deuterium enrichment in hydrocarbons during the FT reaction, we performed the H2/D2 switching experiments in cobalt catalyzed FT reactions and found that the values of α_H/α_D are in a range of 0.93-0.98 under five different reaction conditions, a clear indication of the inverse kinetic isotope effect. We also conducted the H2/D2 competition experiments using an equal molar ratio of D2 and H2 as the reagents and found that the H/D ratios in alkanes from C6 to C_(24) obtained are less than 1 and decrease with increasing molecular size. To account for these results, we proposed a modified alkylidene mechanism for the FT reaction. By combining the modified alkylidene mechanism with the Anderson-Schulz-Flory equation, the results obtained in this study can be quantitatively explained and the isotope fractionation patterns and deuterium enrichment observed in abiogenic hydrocarbons produced by the FT-type reactions can be predicted.