Comparative study of Fischer-Tropsch synthesis with H-2/CO and H-2/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 w as 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 program med decarburisation after a H-2/CO2 treatment (Korea Research Institute of Chemical Technology). Iron and cobalt catalysts behaved differently in CO2 hydrogenation. With th e alkalised iron catalyst the same hydrocarbon product composition was obta ined from a H-2/CO2 and from a H-2/CO synthesis gas in spite of the CO part ial pressure remaining low, specifically due to water gas shift equilibrium constraints. With the cobalt catalyst at increasing CO2 and respectively d ecreasing CO content of the syngas, the product composition shifted from a Fischer-Tropsch type (mainly higher hydrocarbons) to almost exclusively met hane. These basically different catalyst behaviours are explained by differ ent modes of formation of the kinetic regime of FT synthesis-selective inhi bition of methane formation and the selective inhibition of product desorpt ion as a prerequisite for chain growth-in the case of iron through irrevers ible carbiding and alkali surface coverage and in case of cobalt through st rong reversible CO adsorption. Investigation of the various modified iron catalysts showed alumina to be t he best support for CO2 hydrogenation and potassium to act as a powerful pr omotor. With the Fe-Y-zeolite-alkali catalysts, a decrease of methane selectivity w as observed in the order Li < Na < K < Rb being applied as promoters. (C)19 99 Elsevier Science B.V. All rights reserved.