Oxygenates in iron Fischer-Tropsch synthesis : is copper a selectivity promoter?

Doctoral Thesis


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University of Cape Town

The Fischer-Tropsch synthesis is regarded as a stepwise polymerisation reaction between adsorbed hydrogen, carbon monoxide and monomers formed from them. Commercially, a supported precipitated iron catalyst promoted with small amounts of potassium and copper is one of the catalysts used in this reaction. Precipitated iron catalysts are chemically promoted with potassium in order to enhance the product selectivity, while copper is added as a reduction promoter which increases the reduction rate and decreases the reduction temperature of the iron catalyst. The effect of copper on the product selectivity however, remains unclear. This study falls into three distinct categories: firstly the preparation of co-precipitated iron-copper catalysts of varying copper loading and the characterisation of the calcined, reduced and spent catalysts; secondly Fischer-Tropsch synthesis in both fixed-bed and Berty reactors to investigate the effects on product selectivity in regard to copper, and finally the co-feeding of C8 oxygenates, over a pure iron, and an iron catalyst promoted with 50 wt% copper, to investigate the mechanism and pathways of interaction. Co-precipitated iron-copper catalysts were prepared from their nitrates andsubsequently promoted with potassium. Characterisation of these catalysts showedthat the addition of copper formed small (x-ray amorphous) iron crystallites that decreased in size with copper loading. It was also found that the added potassium had a higher affinity for the iron than the copper. Upon reduction the iron crystallites agglomerated to a constant size while the size of the copper crystallites increased with copper loading. Examination of the spent catalysts showed constant ratios of Hägg carbide to magnetite but a decrease in the size of the iron carbide crystallites with increased copper promotion.

Includes synopsis.

Includes bibliographical references (p. 188-197).