Fischer–Tropsch principles of co-hydrogenation on iron catalysts

Abstract

The temporal changes of product composition together with changes of the catalyst in composition and structure have been investigated for Fischer–Tropsch synthesis with an alkalized precipitated iron catalyst at 250°C, 1 MPa, using a special synthesis gas with a molar H₂/CO₂-ratio of three. It was observed that the steady state of synthesis developed in processes of self-organization during several episodes with individual kinetic regimes. The“true FT catalyst” apparently was “constructed” at reaction conditions under complete consumption of α-iron and formation of iron carbide (Fe₅C₂). The magnetite phase disappeared partially and a new “unknown” (probably FeO _x ) phase was formed. It has been concluded from the data of chain growth and branching probability that during self-organization only the number of sites increased but their nature remained unchanged. Strong spatial constraints appear to apply at the sites.  On iron catalysts, the FT sites are very stable, invariant against changes in reaction conditions, in contrast to FT synthesis on cobalt. There the sites show a dynamic behavior.