Factors affecting the extent of branching in fischertropsch synthesis products with iron-based catalysts

Abstract

Branched products are mainly formed during secondary isomerization reactions, and not in the main synthesis reaction itself. The extent of branching is a function of the catalyst formulation. High acidity and a low hydrogenation strength of the catalyst (normally found in catalysts with a high basicity) favour branching. The latter can be explained by the fact that the rate of skeletal isomerization of alkenes is much higher than the rate of hydrogenation. If the former rate is higher than the latter, hydrogenation will take place rapidly before any isomerization can occur. Little variation with time on stream is observed in the extent of branching with non-acidic catalysts. Acidic catalysts yield initially a much more branched product, the extent of which decreases with time on stream, to eventually reach levels only marginally higher than those observed with non-acidic catalysts. The extent of branching is different for products with different carbon numbers. It does not follow a random probability pattern, as results from a hydrocarbon synthesis in which single carbon units are linked to the growing chains at random places, but rather a pattern which depends on the hydrogenation strength of the catalyst used. With low hydrogenation strength catalysts, branching occurs preferentially in the lighter products. Branching is favoured in the heavier products when higher hydrogenation strength catalysts are used. This is explained in terms of the higher surface mobility of lighter products.