Fischer-Tropsch Synthesis by Carbon Dioxide Hydrogenation on Fe-Based Catalysts

Impregnated and co-precipitated, promoted and unpromoted, bulk and supported iron catalysts were prepared, characterized, and subjected to hydrogenation of CO2 at various pressures (1-2 MPa) and temperatures (573-673 K). Potassium, as an important promoter, enhanced the CO2 uptake and selectivity towards olefins and long-chain hydrocarbons. Al2O3, when added as a structural promoter during co-precipitation, increased CO2 conversion as well as selectivity to C2+ hydrocarbons. Among V, Cr, Mn and Zn promoters, Zn offered the highest selectivity to C-2-C-4 alkenes. The different episodes involved in the transformation of the catalyst before it reached steady-state were identified, on the co-precipitated catalyst. Using a biomass derived syngas (CO/CO2/H-2), CO alone took part in hydrogenation. When enriched with H-2, CO2 was also converted to hydrocarbons. The deactivation of impregnated Fe-K/Al2O3 catalyst was found to be due to carbon deposition, whereas that for the precipitated catalyst was due to increase in crystallinity of iron species. The suitability of SiO2, TiO2, Al2O3, HY and ion exchanged NaY as supports was examined for obtaining high activity and selectivity towards light olefins and C2+ hydrocarbons and found Al2O3 to be the best support. A comparative study with Co catalysts revealed the advantages of Fe catalysts for hydrocarbon production by F-T synthesis.