Effects of manganese oxide and sulphate on the olefin selectivity of iron catalysts in the fischer tropsch reaction

Although it has been claimed by various authors that the addition of manganese oxide, MnO, to an iron catalyst gives a marked increase in the olefin selectivity of iron catalysts, we have been unable to confirm these claims in Fischer Tropsch experiments at 513 K for an iron manganese oxide catalyst made from P.A. chemicals. After the initial very low activity of the catalyst, the increase in activity is associated with an increase of the ethane to ethene ratio. This increase is explained in terms of the formation of special hydrogenation ensembles. The iron manganese oxide catalyst was also studied during Fischer Tropsch synthesis with Mo¨ssbauer spectroscopy and X-ray diffraction. These investigations also failed to find differences between a pure iron catalyst, on which we have reported previously, and an iron manganese oxide catalyst. The latter catalyst forms in the course of a Fischer Tropsch synthesis experiment at 513 K about the same quantities of the carbidesX-Fe5C2, ε'-Fe2.2C and FexC as a pure iron catalyst. At 623 K the iron manganese catalyst deactivates quickly and selectively produces methane. However, when small quantities of a sulphate salt are added to an iron manganese oxide catalyst it remains rather stable at 623 K and produces light olefins very selectively. The production of methane as well as the deposition of carbon are hindered by this sulphate treatment.