Pathways for CO2 Formation and Conversion During Fischer–Tropsch Synthesis on Iron-Based Catalysts

Abstract

CO2 reaction and formation pathways during Fischer–Tropsch synthesis (FTS) on a co-precipitated Fe–Zn catalyst promoted with Cu and K were studied using a kinetic analysis of reversible reactions and with the addition of 13C-labeled and unlabeled CO2 to synthesis gas. Primary pathways for the removal of adsorbed oxygen formed in CO dissociation steps include reactions with adsorbed hydrogen to form H2O and with adsorbed CO to form CO2. The H2O selectivity for these pathways is much higher than that predicted from WGS reaction equilibrium; therefore readsorption of H2O followed by its subsequent reaction with CO-derived intermediates leads to the net formation of CO2 with increasing reactor residence time. The forward rate of CO2 formation increases with increasing residence time as H2O concentration increases, but the net CO2 formation rate decreases because of the gradual approach to WGS reaction equilibrium. CO2 addition to synthesis gas does not influence CO2 forward rates, but increases the rate of their reverse steps in the manner predicted by kinetic analyses of reversible reactions using non-equilibrium thermodynamic treatments. Thus the addition of CO2 could lead to the minimization of CO2 formation during FTS and to the preferential removal of oxygen as H2O. This, in turn, leads to lower average H2/CO ratios throughout the catalyst bed and to higher olefin content and C5+ selectivity among reaction products. The addition of 13CO2 to H2/12CO reactants did not lead to significant isotopic enrichment in hydrocarbon products, indicating that CO2 is much less reactive than CO in chain initiation and growth. We find no evidence of competitive reactions of CO2 to form hydrocarbons during reactions of H2/CO/CO2 mixtures, except via gas phase and adsorbed CO intermediates, which become kinetically indistinguishable from CO2 as the chemical interconversion of CO and CO2 becomes rapid at WGS reaction equilibrium.