The use of hydrotalcite derived catalyst precursors in the Fischer-Tropsch synthesis

Master Thesis


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University of Cape Town

The current fused iron catalyst used in the high temperature Fischer-Tropsch process is not very responsive to chemical promotion. Thus the optimisation of the selectivity of this catalyst is restricted. In the development of a catalyst the aim is to obtain the best combination of activity, selectivity and stability with the optimum number of effective active sites of the catalyst exposed to the reaction medium [Gibson, 1999]. Catalysts that have a large catalytically active surface area exposed to the reaction can be obtained by having very small catalyst particles well dispersed on a support material [Augustine, 1996]. Supporting the iron metal used in the HTFT process onto a basic support should eliminate the problem associated with supported iron catalysts namely, the effect of "alkali dilution" caused by the migration of the alkali onto the support material [Dry,1981 ]. Hydrotalcites have been shown to provide a high surface area basic support [Schaper, 1989]. Supporting the iron metal used in Fischer-Tropsch synthesis onto the basic hydrotalcite eliminates the problems associated with supported alkali promoted iron catalyst and results in a catalyst system that can be controlled and optimised to produce catalysts with specific compositions and characteristics. Alternatively, the iron can be incorporated into the hydrotalcite structure to form an Mg-Fe hydrotalcite. This allows for interaction between the elements which may result in possible synergistic effects.The use of a Mg-Fe hydrotalcite in the Fischer-Tropsch synthesis was investigated in this study. Mg-Fe hydrotalcites were prepared with a molar ratio of Mg to Fe of 2:1. The catalyst was thereafter heat treated and activated prior to synthesis. The hydrotalcite derived catalyst showed an overall CO conversion of ca. 75% and conversion to FT products of ca. 40%. The performance was comparable to that of a standard fused catalyst which is typically used in the High Temperature Fischer-Tropsch process.

Includes bibliographical references (leaves 92-96).