Effect of manganese on a potassium-promoted iron-based Fischer-Tropsch synthesis catalyst

The effects of manganese promoter on the reduction–carburization behavior, surface basicity, bulk phase structure and their correlation with Fischer-Tropsch synthesis (FTS) performances have been emphatically studied over a series of spray-dried Fe–Mn–K catalysts with a wide range of Mn incorporation amount. The catalysts were characterized by means of H₂ and CO temperature-programmed reduction (TPR), CO₂ temperature-programmed desorption (TPD), Mössbauer spectroscopy etc.. The results indicated that small amount of Mn promoter can promote the reduction of the catalyst in H₂. However, FeO phase formed during reduction is stabilized by MnO phase with the further increase of Mn content, making FeO phase difficult to be reduced in H₂. The addition of Mn promoter can stabilize the Fe²⁺ and Fe³⁺ ions, and suppresses the reduction and carburization of the catalyst in syngas and CO. Mn promoter can also enhance the amount of the basic sites and weaken the strength of the basic sites, which possibly come from the reason that the Mn–K interaction is strengthened with the addition of Mn promoter. The change of surface basicity can modify the selectivity of hydrocarbons and olefins, and the change of bulk structure phase derived from the addition of Mn promoter will affect the catalyst activity and run stability. The synergetic effects of the two main factors result in an optimized amount of Mn promoter for the highest catalyst activity and heavy hydrocarbon selectivity in slurry FTS reaction of Fe–Mn–K catalysts.