Insights into the influence of support and potassium or sulfur promoter on iron-based Fischer–Tropsch synthesis: understanding the control of catalytic activity, selectivity to lower olefins, and catalyst deactivation†
The fundamental understanding of the control of catalytic activity, selectivity and deactivation in iron-based FTS is of prime scientific and industrial importance. In this work, the inter-effects of various supports and promoters (K, Na and S) in FTS have been studied intensively to elucidate and draw the key points of a better reaction performance. Logical results from abundant experimental work indicate that the catalytic activity is mainly affected by the reducibility of iron oxides which is related to the particle size, interaction with the support, and promoter characteristics, and by the particle size-dependent carburization. In order to reach a desired selectivity to lower olefins (C2=–C4=) beyond the limitation of the Schulz–Flory distribution, the use of K or Na alone is insufficient due to its favorable chain growth to form higher hydrocarbons. It is well confirmed that S can shift the selectivity toward short-chain C2–C4 hydrocarbons without an increased selectivity to methane. Combining this distinctive function of S with the favoring of β-hydride abstraction termination of K and a suitable support to have a weak interaction with iron particles, a substantially higher selectivity to lower olefins at about 45–55% with 10–15% methane and ∼6% C2–C4 paraffins could be obtained at a milder reaction temperature and elevated reaction pressure (300 °C, 1.0 MPa). In this study, the catalyst deactivation is mainly ascribed to K-induced carbon deposition while the reversible transformation of χ-Fe5C2 into Fe3O4 is another reason to some extent.