Promoted Iron-Based Catalysts for the Fischer–Tropsch Synthesis: Design, Synthesis, Site Densities, and Catalytic Properties

Iron-based catalysts were prepared by using promoters (K, Ru, Cu) and synthesis and activation protocols that inhibit sintering of oxide precursors and favor the nucleation of small Fe carbide crystallites. The effects of promoters on reduction/carburization behavior, on Fischer–Tropsch synthesis (FTS) rates, and on the number of CO binding sites formed during reaction were examined by combining steady state and transient rate measurements, titration of active sites, and X-ray absorption spectroscopy. K, Ru, and Cu promoters increased reduction/carburization rates of Fe–Zn oxide precursors, steady-state FTS rates, and the number of CO binding sites present after activation and FTS. These promoters increased the number of active sites formed during activation by favoring the nucleation of smaller Fe3O4 and FeCx domains as Fe2O3 precursors were transformed into active catalysts during initial contact of oxide precursors with synthesis gas. These smaller crystallites, in turn, provide higher surface areas, a larger number of CO binding sites, shorter distances for lattice oxygen diffusion during carburization, and higher steady state FTS rates. The use of surface-active alcohols during drying and thermal treatment of oxide precursors also led to higher active site densities and FTS rates; these methods minimized sintering of oxide precursors during these thermal treatments. Turnover rates on Fe-based catalysts were about three times lower than on Co-based catalysts, at conditions typical used for the latter (473 K, 2.0 MPa). Hydrocarbon synthesis rates (per catalyst mass or volume) on Fe–Zn–Cu–K catalysts prepared by the methods described here were similar to those on representative Co-based catalysts at these conditions. Fe–Zn–Cu–K catalysts gave much lower CH4 selectivities than Co-based catalysts. Fe-based catalysts also showed much weaker effects of temperature and of synthesis gas composition on CH4 and C5+ selectivities. CO2 selectivities were lower than on previous Fe-based catalysts, predominately because of the lower reaction temperatures made possible by the high active site densities attained on the promoted Fe–Zn catalysts reported in this study.