Fischer Tropsch synthesis over supported cobalt catalysts: effect of ethanol addition, precursors and gold doping
Date
2008-06-06T10:17:09Z
Authors
Jalama, Kalala
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Abstract
The effect of the addition of ethanol (2% and 6%) during the Fischer-Tröpsch (FT)
synthesis has been investigated using a 10%Co/TiO2 catalyst in a stirred basket
reactor (T = 220°C, P = 8 bar, H2/CO = 2). The transformation of ethanol vapour (2%
and 6% in nitrogen) over the Co/TiO2 catalyst was also studied in the absence of the
synthesis gas under FT reaction conditions. Ethanol was observed to be incorporated
in the growing chain and was found to (i) increase the selectivity to light products, (ii)
increase the olefin to paraffin ratio and (iii) significantly decrease the catalyst
activity. These effects were almost completely reversed when the ethanol in the feed
was removed. Thermodynamic predictions, TPR and XRD analysis have shown that
cobalt metal particles were oxidised to CoO by ethanol but that re-reduction to Co
metal was possible when ethanol was removed from the feed stream allowing the
catalyst to recover most of its initial performance, in particular when high flow rates
were used.
The effect of the cobalt carboxylate chain length (C2, C5 and C9) used in the
preparation of alumina supported cobalt catalysts has been studied by TPR, XRD and
hydrogen chemisorption techniques. The activity and selectivity of the prepared
catalysts have been evaluated for the Fischer-Tröpsch (FT) reaction in a stirred basket
reactor. It is shown that for catalysts with Co content of 10 wt.% the activity increases
as the carboxylate chain length increases while the selectivity towards methane and light hydrocarbons decreases with the carboxylate chain length. The catalyst prepared
using cobalt acetate was found to present the highest metal-support interaction and
the poorest performance for the Fischer-Tröpsch reaction. When the metal content
was increased to 15 wt.% Co and 20 wt.% Co respectively, the metal-support
interaction for the catalyst prepared from cobalt acetate significantly decreased
making it a better catalyst for the FT reaction compared to the catalysts prepared from
C5 and C9 cobalt carboxylates.
The effect of the addition of Au to a Co FT catalyst supported on titania, alumina and
silica respectively, has been investigated by varying the amount of Au (0.2 to 5 wt.%)
added to the catalyst. The catalysts were characterized by atomic absorption
spectroscopy, XRD, XPS and TPR analysis. The catalyst evaluation for the Fischer-
Tröpsch reaction activity and selectivity was achieved in a fixed bed micro-reactor
(H2:CO = 2; 20 bar; 220°C). Addition of Au to supported Co catalysts improved the
catalyst reduction and the cobalt dispersion on the catalyst surface. The catalyst
activity for the FT reaction and the methane and light product selectivity increased
with Au loading in the catalyst.
Description
Keywords
Fischer-Tropsch, cobalt catalysts, ethanol, precursors