(761f) Effect of Preparation Variables On the Performance of Supported Iron Fischer-Tropsch Catalysts

The
typical iron FT catalyst used by Sasol is unsupported iron promoted with copper,
potassium and silica (Fe/Cu/K/SiO₂). Bukur et al. [1] reported a
weight-time yield of 450 mmol (CO+H₂)/g_Fe/h/MPa and a C₂₊
hydrocarbon productivity of 0.86 g_HC/g_Fe/h for
100Fe/3Cu/4K/16SiO₂ at 260 °C and 2.2 MPa.  Despite their high
activity and favorable selectivity, unsupported iron catalysts are generally
too mechanically weak to be used in slurry bubble column reactors (SBCRs): the
most thermally efficient and economical reactors [2].

This
study chooses four important variables in the preparation of alumina-supported
Fe catalysts and holds all other variables constant to systematically
investigate the effect of each variable.  Catalysts were prepared to examine
the effects of (1) iron loading level, (2) potassium loading level, (3) impregnation
method (aqueous Incipient wetness or non-aqueous slurry), and (4) timing of
potassium addition.

Table 1 below catalogs the
values of each of the 4 preparation variables for the six catalysts of this
study.

Table 1. 
Preparation variable values for each of the six catalysts studied in this paper

Adding K to the support which increased it's basicity
largely increased the activity of the catalyst. The co-impregnation preparation
(K5) significantly enhanced the activity since iron particles may have better
contact with potassium. Doubling the potassium level (K4 vs. K3) slightly decreased
the activity and methane selectivity indicating a threshold for potassium level
of around 4K/100Fe. Although K5 and K6 were the most active catalysts,
stability analysis conducted at 250C revealed that neither of these two
catalysts was very stable, i.e., K6 particularly deactivates to nearly half its
former rate in only 175 hours on stream.  The non-aqueous catalysts were the
most stable.

These alumina-supported catalysts had
activities ranging from 211-267 mmol(CO+H₂)/g_cat/h/MPa),
and are nearly three-fold more active than the silica-supported catalyst
reported by Bukur (101 mmol(CO+H₂)/g_cat/h/MPa), and 5-fold
more active than Bukur's alumina-supported (50 mmol(CO+H₂)/g_cat/h/MPa)
catalyst.

References:

1- Bukur, D.B. and X. Lang, Highly Active and
Stable Iron Fischer-Tropsch Catalyst for Synthesis Gas Conversion to
Liquid Fuels. Ind. Eng. Chem. Res., 1999. 38(9): p. 3270?3275.

2- Xu, J., C.H. Bartholomew, J. Sudweeks and
D.L. Eggett, Design, synthesis, and catalytic properties of silica-supported,
Pt-promoted iron Fischer?Tropsch catalysts. Topics in Catalysis, 2003. 26(1-4):
p. 55.