Klaus Herzog

Kinetic studies for elucidation of the promoter effect of alkali in Fischer-Tropsch iron catalysts

The kinetics of the formation of hydrocarbons in Fischer-Tropsch synthesis and of consecutive reactions, i.e., hydrogenation and double bond isomerization of primarily formed 1-alkenes, were studied in a slurry-phase reactor using alkalized and nonalkalized precipitated iron catalysts. Both consecutive reactions have an order of about −2 with respect to carbon monoxide and an order of about 1 with respect to 1-alkenes. The order with respect to hydrogen is 1 for the hydrocarbon formation and also 1 for the consecutive hydrogenation, but 0 for the consecutive isomerization. The reaction orders are not changed by addition of K2CO3 to the catalyst. The kinetics of consecutive reactions can be interpreted by competitive adsorption of 1-alkene and carbon monoxide. Alkali addition causes an increase in the strength of carbon monoxide adsorption and consequently a decrease in 1-alkene adsorption so that the rates of the consecutive reactions are reduced. This hypothesis could be proved by studies of 1-hexene hydrogenation in the presence and absence of carbon monoxide for alkalized and nonalkalized iron catalysts. The kinetics were studied in the ranges 1.3 < PCO(105Pa) < 7, 1.6 < PH2(105Pa) < 9, and 490 K < T < 530 K.