Tuning the active sites of supported cobalt Fischer-Tropsch catalysts to enhance efficiency for hard wax production

Abstract

The Fischer-Tropsch (FT) process yields high-quality hydrocarbon products, including hard waxes used in adhesives, polymer processing, cosmetics, and pharmaceutical applications. Sasol commercially produces these hard waxes using precipitated iron-based catalysts, which are cost-effective compared to cobalt catalysts. With proper chemical promotion, these iron catalysts can produce a high-alpha (C_25–40 = 0.95) product slate, suitable for hard wax production. However, iron catalysts are characterised by short reactor lifetimes, waste generation during production, sensitivity to high water partial pressures, and high CO₂ production. These issues can be mitigated by using cobalt slurry catalysts. However, cobalt’s limited responsiveness to chemical promotion poses a significant obstacle, making it challenging to achieve hard wax selectivity under the same conditions. This study aims to enhance hard wax selectivity by tuning the active sites of a supported cobalt catalyst for stable operation at high per pass conversion. During activation and FT synthesis, nanoparticulate cobalt mainly exists in two phases: hexagonal close-packed (HCP) and face-centred cubic (FCC). Theoretical simulations indicated that the HCP phase has superior activity due to a greater variety of site arrangements. A reduction-carbiding-reduction (RCR) technique was developed to prepare HCP-rich cobalt catalysts. The performance of HCP-rich and FCC-HCP mixed catalysts (the latter produced by standard H₂ activation) were assessed via in-situ magnetometry and lab-scale FT testing. The catalyst preparation and activation methods were scaled up to a pilot level and tested in a 2-inch slurry bubble column to evaluate catalyst hard wax yield, and to generate samples for application testing.