Cobalt-based Nanoreactors in Combined Fischer-Tropsch Synthesis and Hydroprocessing: Effects on Methane and CO$_{2}$ Selectivity

Abstract

Fischer-Tropsch synthesis: Four types of bi-functional catalysts with cobalt nanoparticles supported on meso- or microporous silicates or aluminosilicates are investigated regarding the obtained CO$_{2}$ and CH$_{4l}$ selectivity under low-temperature Fischer-Tropsch reaction conditions. In situ x-ray absorption spectroscopy results under industrially relevant conditions reveal that strong cobalt-support interactions and oxidized cobalt species are the main factors determining the selectivity depending on the specific support material used. The production of liquid hydrocarbons from syngas (CO and H$_{2}$) via the combined Fischer-Tropsch (FT) synthesis and hydroprocessing (HP) is a promising strategy to provide valuable chemicals and fuels based on renewable feedstocks. High yields of liquid products are essential for industrial implementation since short-chain side products like methane and CO$_{2}$ reduce the overall carbon efficiency, which holds true especially for bi-functional Co/zeolite catalysts. In order to investigate the influence of the support material properties on the methane and CO$_{2}$ selectivities in the combined FT and HP reaction, we synthesized four well-defined catalyst materials with similar cobalt particle sizes. The active material is supported on either meso- or microporous silicates or aluminosilicates. The catalytic properties are investigated in FT experiments at industrially relevant conditions (20 bar, 200–260 °C) and correlated with in situ x-ray absorption spectroscopy results to determine the chemical environment responsible for the selectivity observed. The origin of the high methane selectivity detected for crystalline and amorphous aluminosilicate was mainly traced back to the strong cobalt-support interactions. The high CO$_{2}$ selectivity, observed only for crystalline zeolite materials, is driven by the presence of oxidized cobalt species, while the acidic support in combination with micropores and possible overcracking leads to the observed drop in the C$_{5+}$ selectivity