Pretreatment effect studies with a precipitated iron Fischer–Tropsch catalyst in a slurry reactor

Abstract

Effects of pretreatment procedures, using H 2, CO, and syngas (H 2/CO = 2/3) as reductants, on the performance (activity, selectivity and stability with time) of a precipitated iron catalyst (100 Fe/3 Cu/4 K/16 SiO 2 on a mass basis) during Fischer–Tropsch (F–T) synthesis were studied in a stirred tank slurry reactor. The catalyst reduced with hydrogen to magnetite only reached its steady state activity faster than the catalyst partially reduced to metallic iron (2 h versus 4 h). Activity of the catalyst partially reduced to metallic iron under mild reduction conditions (250°C for 4 h) was about 40% higher than that of the catalyst reduced at 280°C for 8 h. The steady state activities of the catalyst reduced to magnetite only (240°C for 2 h) and of the unreduced catalyst were the same as that of the catalyst partially reduced to metallic iron at 280°C for 8 h. Initial activity of the CO activated catalyst (280°C for 8 h) was relatively low, and increased with time, reaching a steady state level at about 50 h on stream. However, its steady state activity was the highest among all the pretreatment procedures used. Methane and gaseous hydrocarbon selectivities on the hydrogen reduced and the unreduced catalyst increased with time before reaching a steady state, whereas the opposite trend was observed on the CO and the syngas (280°C for 8 h) activated catalysts. The catalyst which was initially in an oxide form (unreduced catalyst and catalyst reduced to magnetite) had lower gaseous hydrocarbon selectivities than the catalyst partially reduced to α-Fe. The unreduced catalyst had the lowest gaseous hydrocarbon selectivity, whereas the syngas activated catalyst had the highest. Total olefin and 1-olefin contents decreased in the following order: CO activated ≈ unreduced > syngas > H 2 reduced.