Kinetic and Mechanistic Studies on the Kölbel-Engelhardt Reaction over an Iron Oxide Catalyst

Abstract

Abstract The yield and selectivity to hydrocarbons in the Kölbel-Engelhardt (KE) reaction over a Fe2O3 catalyst at 548–593 K under atmospheric pressure were as low as 0.6–2.2% and 2.0–3.8%, respectively, but the observed four characteristic features of the reaction, i.e., the distribution of produced hydrocarbons following the Schulz-Flory plot, the decrease in propagation probability with increasing reaction temperature, nearly the same value of propagation probability irrespective of a great change in contact time of the reaction, and the remarkable decrease in the yield ratio of olefins to paraffins with increasing contact time of the reaction agreed with the common features of the synthesis of hydrocarbons by the Fischer-Tropsch reaction. The catalyst was composed of Fe3O4 with a low concentration of χ-type carbide (Fe5C2) during the KE reaction, and the results of the effect of contact time on the reaction revealed that formation of hydrocarbons took place following the water-gas shift (WGS) reaction. Furthermore, both the rates of formation of hydrocarbons in the reactions of carbided iron catalyst (Fe5C2) with H2O and with H2 at 573 K were smaller by a factor of >5 than the rate of formation of hydrocarbons in the KE reaction at a steady state at 573 K. Thus, it is concluded that a much greater portion of hydrocarbons produced in the KE reaction was via a sequential reaction pathway involving the WGS reaction followed by the Fischer-Tropsch reaction, not via the decomposition of carbided iron catalyst with H2O and its reduction with H2. Based on the observed increase in the catalytic activity of Fe2O3 upon addition of a small amount of such metal oxides as Cu, Pt, Co, Ni, and Rh oxides, the mechanical mixture of a WGS reaction catalyst and a Fischer-Tropsch reaction catalyst is proposed to be an effective catalyst for the production of hydrocarbons and one illustration of this view is presented.