Hydrocarbon production via Fischer–Tropsch synthesis from H2-poor syngas over different Fe-Co/γ-Al2O3 bimetallic catalysts

Fischer–Tropsch synthesis (FTS) at 20 bar and 483 K, with H2-poor syngas (H2/CO ratio = 1.0) in order to simulate gasified biomass, was performed over Al2O3-supported catalysts with various ratios of Fe:Co (12 wt% bimetal) prepared by co-impregnation. Co was found to be incorporated into the Fe2O3 phase after calcination, at least for the iron-rich samples, while no evidence of Fe incorporated into Co3O4 was found. Upon reduction, most probably FeCo alloys were formed in the iron-rich bimetallic samples. The degree of reduction of the catalysts showed a non-linear behavior with respect to the Fe:Co ratio, but it is obvious that Co increases the reducibility of Fe. Alloying Co with small/moderate amounts of Fe improved the FT activity compared to the 100% Co catalyst at low conversion levels. Alloying Fe with small/moderate amounts of Co lowered the FT activity, but increased the relative water–gas-shift (WGS) activity compared to the 100% Fe catalyst. However, the overall WGS activity was very low for all catalysts, even with external water addition to the feed, resulting in low FT productivities (per gram catalyst) due to the low partial pressure of H2. A higher Fe:Co ratio in the bimetallic catalyst generally resulted in higher relative WGS activity, but did not lower the H2/CO usage ratio to the desired value of 1.0. For the Fe-containing catalysts, the space–time yield of hydrocarbons (HCs) decreased with increasing partial pressure of water or reduced space velocity, indicating an inhibition of water on the FT activity, most often resulting in low FT productivity under the conditions with highest relative WGS activity (usage ratios closest to the inlet H2/CO ratio). Moreover, the co-impregnation technique resulted in a surface enrichment of Fe, at least for the Co-rich samples, covering the Co sites. For the bimetallic catalysts, both FT and WGS activities rapidly declined at high partial pressure of water due to deactivation by oxidation and sintering. However, the results indicate that WGS and FT proceeded over sites of different nature in the bimetallic catalysts.The bimetallic catalysts showed essentially no synergy effects with respect to HC selectivities and olefin/paraffin ratios, which partly can be explained by the use of a sub-stoichiometric H2/CO ratio as feed. The higher the Fe content, the lower were the C5+ selectivity and C3 olefin/paraffin ratio. Water addition increased the C5+ selectivity and C3 olefin/paraffin ratio and reduced the CH4 selectivity.