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

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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