Fischer–Tropsch Synthesis: Effects of Hydrohalic Acids in Syngas on a Precipitated Iron Catalyst

Abstract

The current investigation was undertaken to identify limits of hydrohalic acid (HX, X = F, Cl, Br) impurities in syngas and shed light on the mechanism of HX poisoning of a 100 Fe/5.1 Si/2 Cu/3 K FTS catalyst under industrially relevant conditions using a 1-L slurry-phase reactor. Co-feeding <2.0 ppm of HCl or HBr in syngas for 72–170 h did not significantly deactivate the Fe catalyst. On the other hand, cofeeding 3.0–5.0 ppm of HCl or HBr in syngas for a similar time period resulted in slow deactivation. A higher tolerance, that is, 5–10 ppm, was observed for HF, indicating a weaker poisoning effect. In all cases, rapid deactivation was accompanied by decreases in C5+ and CO2 selectivities at higher levels of HX (i.e., 20 ppm and above). Mössbauer spectroscopy and XRD were used to study the Fe phases in the catalyst before and after deactivation induced by each of the HX poisons. Results of Mössbauer spectroscopy suggest that an adsorptive mechanism is responsible for the deactivation of Fe catalysts by HX poisoning. Co-feeding HX neither significantly changed the distribution of iron carbides (χ-Fe5C2 and ε′-Fe2.2C) and magnetite nor formed Fe–X compounds. In addition, it was found that Hägg carbide (χ-Fe5C2) converted to ε′-Fe2.2C during the startup period or under low temperature FTS conditions. In this study, the poisoning strengths of F–, Cl–, and Br– on the Fe catalyst were also quantified by calculating the loss in surface Fe atoms per halide ion (Fe/X–), with the following trend being observed at 260 °C: Fe/Br– (1.0) > Fe/Cl– (0.9) > Fe/F– (0.4). This order remained the same at 270 °C, but the Fe/X– ratio decreased slightly, as expected for an adsorptive mechanism.