Olefin pseudo-equilibrium in the Fischer–Tropsch reaction

Overall or global thermodynamic equilibrium predicts a predominately methane product for the Fischer–Tropsch (FT) reaction. Even though the FT product distribution may not be described by overall thermodynamic equilibrium, there may be aspects of it that can be described by equilibrium. We postulate that an equilibrium is set up in the FT reaction between species, i.e. between hydrocarbon or a surface precursor of chain length n and that of chain length n + 1, and thus although we do not have global equilibrium, we have an approach to equilibrium between the product species. As olefins are reactive under FT reaction conditions we initially postulate that an equilibrium distribution is set up between the α-olefins or a species that leads to α-olefins.The calculated results predict that equilibrium between the species of any homologous series would results in a Flory-type distribution, where the ratio of the moles of species n to that of the moles of species n + 1 would be constant. When the value of this ratio α is calculated for olefins, it is found that the experimentally measured values match those calculated and thus the measured and predicted distribution of the olefins agree. However, assuming that the alcohols or paraffin distribution approaches equilibrium, leads to calculated values of α that are not consistent with the experimental results. Therefore, there may be an equilibrium set up between species of either an olefin precursor or the olefins themselves which leads to the Flory-type distribution found in the FT reaction.

► Some aspects of Fischer–Tropsch (FT) product distribution are described by equilibrium.
► Equilibrium calculations for any homologous series results in an ASF type distribution.
► We have an approach to equilibrium between the olefin product species.
► Olefins are primary FT products since they determine the whole product distribution.