Although Fischer-Tropsch (F-T) synthesis can convert coal-, biomass-, and natural gas-derived synthesis gas into hydrocarbon derivatives, selectivity toward lower olefins tends to be low. As a result, non-petroleum production routes for olefins usually involve at least two conversion steps, involving either cracking of FT-derived hydrocarbons or the methanol to olefins (MTO) process.

For several decades, research groups have attempted to develop iron-based catalysts to direct product selectivity of the FT synthesis toward light olefins. Relative to other FT catalysts such as cobalt, iron disfavors competing formation of methane, and furthermore catalyzes the water-gas shift reaction, enabling the use of a CO-rich syngas feed without an H₂/CO ratio adjustment.

...Despite...promising results, however, the bulk iron catalysts are mechanically unstable when the reaction is performed at high temperature, which is necessary to steer product selectivity to lighter hydrocarbons...The poor mechanical stability of the bulk iron oxide catalysts may lead to plugging of the catalyst bed in fixed-bed operation or to fouling of separation equipment in a fluidized-bed process.

Supported iron catalysts display enhanced dispersion of the active phase and may withstand the mechanical degradation that threatens bulk iron catalysts. Research on supported iron catalysts has met with limited success, however.

...To overcome the low activity and mechanical stability problems, we explored the use of support materials weakly interactive toward iron.

—Galvis et al.

The team formulated and tested their supported and bulk Fe catalysts in the FT reaction initially at 1 bar and 350°C at low CO conversion (0.5 to 1%) to restrict secondary hydrogenation of olefins. Among their findings:

• High initial activity was observed for Fe/β-SiC and Fe/CNF.

• The activity of Fe/CNF decreased continuously during the 15 hours of reaction; the activity of the Fe/β-SiC catalyst increased during the first 5 hours of reaction, then decreased slowly afterward.

• Fe/α-Al₂O₃ exhibited a lower catalytic activity than Fe/CNF and Fe/β-SiC; however, it showed remarkable stability, as the activity remained constant over 15 hours.

• Fe/γ-Al₂O₃ and Fe/SiO₂ displayed a low catalytic activity, comparable to the bulk Fe catalysts.

• Fe/CNF and Fe/α-Al₂O₃ exhibited high selectivity toward lower olefins (~60% C) while directing comparatively little carbon to methane (<25% C)—an important attribut of an FTO catalyst.

• Fe/β-SiC and Fe/SiO₂ also showed high selectivity to C₂ through C₄ olefins, but the CH₄ product fraction was higher than 30% C.

• Fe/γ-Al₂O₃ and the bulk catalysts displayed a high selectivity to methane (>40% C).

Based on the results obtained at 1 bar, they prepared and tested additional α-Al₂O₃–supported catalysts with different iron loadings (6 and 25 wt % Fe) to study the effect of iron content on catalytic performance at 20 bar.

The promoted catalysts prepared using supports with low interaction with iron showed high catalytic activities combined with high selectivities to the desired products. Fe/CNF and 25 wt % Fe/α-Al₂O₃ exhibited high selectivities toward light olefins (>50% C) while yielding a methane product fraction lower than 15% C.

The FTO process represents a strong alternative route for the sustainable production of lower olefins from biomass-derived synthesis gas. The industrial potential of this process is greatly enhanced by the reported development of active, selective, and mechanically stable catalysts that consist of promoted iron nanoparticles dispersed on weakly interactive supports. Further suppression of methane production, maximization of the C₂-C₄ olefins fraction, and reduction of carbon lay-down by addition of promoters and by optimization of physical properties (e.g., Fe particle size, distribution of Fe nanoparticles on the support) will allow us to further understand and develop the performance of these catalysts.

—Galvis et al.

Resources

• Hirsa M. Torres Galvis, Johannes H. Bitter, Chaitanya B. Khare, Matthijs Ruitenbeek, A. Iulian Dugulan, and Krijn P. de Jong (2012) Supported Iron Nanoparticles as Catalysts for Sustainable Production of Lower Olefins. Science 335 (6070), 835-838.doi: 10.1126/science.1215614