(95d) Oxygenate/Hydrocarbon Distribution Compared Across Different Group I Promoters for Iron Catalysts

Oxygenate and Hydrocarbon Distribution Compared Across
Different Group I Promoters for Iron Catalysts.

Wilson D. Shafer, Robert Keogh, Burtron H. Davis,
Center for Applied Energy Research, University of Kentucky, Lexington, Ky

Abstract

A
main catalyst used in FT currently is iron; this is driven by cost and the
ability of Fe to play a role in water-gas-shift alongside the production of
liquid fuels. Iron catalysts can be dispersed amongst a support and promoted
with potassium or copper. The activity of iron catalysts promoted with
potassium display an increase in the FTS and WGS activity [1]. Specific testing
was done on different K promoter loading to find that greater than 5%molar K/Fe
loading did not increase the CO conversion [2]. 
Low atomic loadings of potassium have been shown to increase the FTS
conversion by playing a role in increasing the CO dissociation but slightly
decreasing the H₂ dissociation [3]. If this is the case for
potassium; the other alkali metals, containing the same chemical
characteristics should play a similar role.

A
factor in describing the effects seen is the basicity of the promoters
described by Roper et al [4], using a fluidized bed reactor, where the alkaline
basicity was inversely correlated to the amount methane produced. In addition, 2002
specific work was done with slurry phase operations investigating the role in
other row I alkali metals in comparison to potassium in iron catalysts [5]. This
specific work was done looking into the WGS and FT activity for a slurry
reactor system with similar alkaline promoter loadings and specific reactor
conditions.  The results based on the
previous work, display varying results for each promoter across the same Fe
catalyst. It was found that of the entire row I alkali metals, potassium
promotes both FTS and WGS the best, in fact Li, Cs, and Rb promoted catalysts
seem to show lower activity when compared to the unpromoted Fe catalyst.

This
specific analysis work was to investigate the role I metal promoters by looking
at the specific hydrocarbon/oxygenate distribution. The addition of each row I
metal exclusively adds to the mole per carbon amount of olefin and oxygenated
material over the unpromoted Fe catalyst. If, as previously mentioned, the
addition of K as a promoter assists in the dissociation of CO and in turn
suppression of H₂ dissociation, this then could be seen in the
overall product distribution for each row I promoter. The amounts of oxygenates
and olefins increase linearly with each promoter added, where Li and the
unpromoted produced the smallest amounts and Cs, Rb promoted Fe catalysts
displayed the most. This distribution work can then compared to the alkali
promoted Fe catalysts work done in 2002, which described basicity as a factor, others
include atomic size and the ability to suppress H₂ and assist in CO dissociation,
to better explain the metals activity and why K seems to be the best row I
promoter.

References

1.      D.B.
Bukur, D. Mukesh, S.A. Patel, Ind. Eng. Chem. Res. 29 (1990) 194.

2.     
M.
Luo, R. J. O'Brien, S. Bao, B. H. Davis; Appl. Catal. A: General 239 (2003)
111?120

3.      Gaube, J.; Klien, H.-F.; J. of Mol. Catal. A 283 (2008) 60

4.     
M.
Röper, in: W. Keim, (Ed.), Catalysis in C1 Chemistry, D. Reidel Publishing
Company, Dordrecht, 1983, pp. 41?88.

5.      W.
Ngantsoue-Hoc, Y. Zhang, R.J. O'Brien, M. Luo, B. H. Davis; Appl. Catal. A:
General 236 (2002) 77?89