Fischer–Tropsch synthesis: A review of the effect of CO conversion on methane selectivity

• Cobalt and ruthenium behave similarly.
• Up to about 80% CO conversion, CH4 selectivity decreases.
• CH4 selectivity increases when conversion is higher than 80% due to enhanced WGS.
• The variation of methane selectivity is small for iron catalyst.
• Water plays an important role in determining methane selectivity.

Methane is the least desired product for the Fischer–Tropsch synthesis so that it is of paramount importance to reduce methane selectivity in the FT process. Despite numerous efforts devoted to the reduction of methane selectivity, the effect of CO conversion on methane selectivity is still not well defined. For cobalt and ruthenium based catalysts, methane selectivity generally decreases monotonically with increasing CO conversion within the range 20–80%, while on iron catalysts, the methane selectivity is more influenced by its water–gas shift activity and potassium promotion. Methane selectivity remains more or less constant at conversion lower than 70% for potassium promoted iron catalysts. Pressure and temperature have a greater influence on methane selectivity for Co and Ru based catalysts. Pressure and temperature change the preference of the secondary reactions of primary olefins and tune methane selectivity at different CO conversions. An increased extent of olefin readsorption may compete with methyl intermediates for surface sites and hence reduce methane selectivity. Water seems to play an important role in determining the dependence of CH4 selectivity on CO conversion for the Co and Ru based catalysts by either inhibiting the hydrogenation reaction or by increasing the amount of surface carbon for chain growth. Choosing appropriate promoters and process conditions may reduce methane production.

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