Fischer-Tropsch synthesis from bio-syngas models over cobalt and cobalt-iron alumina supported catalysts
Fischer-Tropsch (FT) synthesis is a process in which synthesis gas (syngas), a mixture of H2 and CO, is converted into long chain hydrocarbons mainly over Co- or Fe-based catalysts. The FT synthesis requires an H2/CO molar ratio (usage ratio) of approximately 2.1 above the catalyst surface. If the inlet syngas H2/CO ratio is less than 2.1, an in situ water-gas shift (WGS) activity is desired in order to increase the syngas ratio.
The purpose of this work is to study the FT and the WGS activities of 12% Co/γ-Al2O3, 12% Co-0.5% Re/γ-Al2O3 and different Fe:Co ratios of 12 wt% bimetal Co-Fe/γ-Al2O3 catalysts and their selectivity to hydrocarbon products with and without an addition of water. The FT synthesis was investigated in a fixed-bed reactor at 20 bars and 483 K - 523 K using syngases with H2/CO molar ratios of 2.1, 1.5 and 1.0 simulating syngas derived from biomass. The catalysts were characterized by temperature program reduction, H2-chemisorptions, CO-chemisorptions, O2-titration, N2-adsorption and X-ray diffraction.
The results indicated a significant drop in hydrocarbon production rate when changing the syngas inlet ratio from 2.1 to 1.5. The reason for this drop is that the H2/CO usage ratio was still too high (≈ 2.0). The catalysts used in this study had low WGS activities, even when the water added was increased from 20% to 33% for all types of catalyst and syngas.
Water addition resulted in an increase in selectivity to C5+ and propene selectivity and a decrease in selectivity to CH4. All catalysts were deactivated by water addition but the catalyst activity is partly recovered in H2/CO ratio inlets of 1.0 and 1.5. The Re-promoted Co catalyst was considerably more active and selective to longer hydrocarbons than the un-promoted one.
A higher Fe:Co ratio in the Fe-Co/Al2O3 catalyst resulted in higher WGS activity, but did not lower the usage H2/CO ratio. The higher the Fe-content, the lower were the CO conversion, the C5+ selectivity and the C3-(olefin/paraffin) ratio. The CH4 selectivity and the CO2 selectivity increased with increasing Fe content. Water addition increased the C5+ selectivity and C3-(olefin/paraffin) ratio but reduced CH4 selectivity.
The addition of Re promoted the reduction of highly dispersed cobalt phases. A decrease in CO adsorptions and a higher Fe2O3 reduction temperature were observed with an increase in Fe loading. The experiments pointed out that the WGS and FT reactions were competing for the same active sites (on the Fe-Co bimetallic catalysts), giving a decrease in productivity for the catalyst with highest WGS activity. The increase in reaction temperature up to 523 K did not vary the hydrocarbon product distribution of the Fe-Co/Al2O3 bimetallic catalysts.