Two studies point the way to cleaner syngas-to-hydrocarbons processes

The production of commercially important hydrocarbons from synthesis gas (syngas; mixtures of H₂ and CO) using the Fischer-Tropsch (F-T) process is a key strategy for making fuels and chemicals from a range of non-petroleum feedstocks. But F-T processes are relatively expensive and generate substantial amounts of carbon dioxide.

A pair of studies published in a recent issue of the journal Science could have major implications for improving processes in which syngas is converted into fuels and olefins. Both papers offer viable strategies for maximizing the incorporation of hydrogen and carbon atoms from syngas into hydrocarbon compounds, potentially opening pathways to more efficient and less polluting liquid-fuel manufacturing.

The first study took aim at the high amounts of byproduct CO₂ generated with current iron catalysts for F-T synthesis. Researchers led by Ding Ma at Peking University (Beijing, China) found that introducing a small amount of halogen-containing compounds into the feed gas could suppress CO₂ formation. Specifically, co-feeding 20 parts per million of methyl bromide over an iron carbide catalyst decreased the amount of CO₂ formed in the F-T process from around 30% to less than 1%.

The team describes how bromine atoms can bind to the iron active sites, thus blocking the water molecules’ access to the catalyst sites and preventing the reactions that lead to CO₂ formation. “This strategy provides a simple, scalable and broadly applicable route for carbon-efficient syngas conversion,” the researchers write.

In another study, researchers from Tsinghua University (Beijing) found that an iron-carbide-magnetite nanoparticle catalyst (specifically, a sodium-promoted FeC_x@Fe₃O₄ core-shell nanoparticle catalyst with iron carbides at the core and magnetite on the surface) could couple the water-gas shift (WGS) and syngas-to-olefins (STO) reactions in situ.

Coupling these two processes enables in-situ conversion of water generated during the STO process into hydrogen through the WGS reaction. This lessens the incorporation of hydrogen from water and improves hydrogen atom economy (HAE; the proportion of reactant hydrogen that ends up in the hydrocarbon product).

The new WGS-STO coupling route of the developed catalyst reduces steam consumption, wastewater generation and CO₂ emissions, with HAE reaching about 66 to 83%. This level exceeds the HAE of methanol-to-olefins (MTO) processes, which has a 50% upper limit, the researchers say.