Comparative study of Fischer-Tropsch synthesis with H2/CO and H2/CO2 syngas using Fe- and Co-based catalysts

Hydrogenation of CO, CO2 and their mixtures has been comparatively studied with a Co-MnO-Aerosil-Pt and a Fe-Al2O3-Cu-K catalyst at the University of Karlsruhe. With iron catalysts as promising for CO2 hydrogenation, their composition was varied: (1) several supports (SiO2, TiO2, Al2O3), (2) alkali promotion (Li, Na, K, Rb), (3) usage of Zeolite Y as catalyst component. The catalysts were characterised by adsorption methods, XRD, TPR and temperature programmed decarburisation after a H-2/CO2 treatment (Korea Research Institute of Chemical Technology). Iron and cobalt catalysts behaved differently in CO2 hydrogenation. With the alkalised iron catalyst the same hydrocarbon product composition was obtained from a H-2/CO2 and from a H-2/CO synthesis gas in spite of the CO partial pressure remaining low, specifically due to water gas shift equilibrium constraints. With the cobalt catalyst at increasing CO2 and respectively decreasing CO content of the syngas, the product composition shifted from a Fischer-Tropsch type (mainly higher hydrocarbons) to almost exclusively methane. These basically different catalyst behaviours are explained by different modes of formation of the kinetic regime of FT synthesis-selective inhibition of methane formation and the selective inhibition of product desorption as a prerequisite for chain growth-in the case of iron through irreversible carbiding and alkali surface coverage and in case of cobalt through strong reversible CO adsorption. Investigation of the various modified iron catalysts showed alumina to be the best support for CO2 hydrogenation and potassium to act as a powerful promotor. With the Fe-Y-zeolite-alkali catalysts, a decrease of methane selectivity was observed in the order Li < Na < K < Rb being applied as promoters. (C)1999 Elsevier Science B.V. All rights reserved.