Influences of melting method on fused iron catalysts for Fischer–Tropsch synthesis

Abstract

The impact of melting method on the performance of fused iron catalysts for the Fischer–Tropsch Synthesis (FTS) has been investigated. Characterization technologies including ICP, XRD, H₂-TPR, CO-TPD, and Mössbauer effect spectroscopy (MES) were used to study the bulk phase composition, and reduction and adsorption behaviors. The syngas conversion and CH₄ selectivity of AAW was very similar to a commercial fused Fe catalyst reported by Sasol, slightly better than OT, but much better than IF. The much inferior IF catalyst was attributed to contamination with Al (Si) from the container, as the melting pool leached the wall of the crucible severely. The introduction of Al (Si) resulted in the formation of an irreducible FeAl₂O₄ (Fe₂SiO₄) phase that resisted carburation, thus IF exhibited the lowest content of active carbide. Both AAW and OT were excluded from this effect, since no crucible was adopted during fusion. The OT catalyst appeared mainly in the wustite phase (FeO), showing slightly lower reducibility and higher abundance in supermagnetic Fe³⁺ or Fe²⁺ than AAW. The slightly negative performance of OT compared with AAW might be attributed to a lower extent of reduction originated from a difference in crystal lattice type, which led to a difference in both lattice defects and the solubility of exterior ions. The AAW catalyst, composed of mainly magnetite phase (Fe₃O₄) besides a trace amount of FeO, was reduced in steps and formed two active sites. The spent catalyst possessed the largest carbide content, which correlated well with its high activity and olefin selectivity. In addition, the AAW might benefit from the mixing precursor (Fe₃O₄ plus a small amount of FeO) that brought out assorted active sites.