Trace-level halogen blocks CO₂ emission in Fischer-Tropsch synthesis for olefins production.

Sustainable production of fuels and olefins from syngas (carbon monoxide and hydrogen) through the Fischer-Tropsch synthesis process requires catalysts that deliver high selectivity, industrial productivity, and minimal carbon dioxide (CO2) emissions. Current industrial iron catalysts form substantial CO2 by-product that limits carbon efficiency. We report that introducing trace amounts [parts per million (ppm) level] of halogen-containing compounds into the feed gas can suppress CO2 formation using iron-based catalysts and boost olefin selectivity over paraffin and olefin productivity. Cofeeding 20 ppm bromomethane over an iron carbide catalyst decreased CO2 selectivity to <1% and increased olefin selectivity to ~85% among all carbon-containing products. Surface-bound halogens modulated the catalyst surface structure and selectively inhibited pathways responsible for CO2 generation and olefin hydrogenation. This strategy provides a simple, scalable, and broadly applicable route for carbon-efficient syngas conversion. Editor's summary: Two different strategies can produce olefins from synthesis gas (syngas, a mixture of carbon monoxide and hydrogen) with fewer CO2 by-products over iron-based catalysts (see the Perspective by Saeys). Cai et al. fed trace amounts of bromomethane with syngas over iron-based catalysts. Surface-bound bromine interacted with iron active sites to inhibit water dissociation, carbon monoxide and oxygen atom recombination, and olefin hydrogenation, and enabled near-zero CO2 production and high α-olefin selectivity. In another study, Gao et al. found that a sodium-promoted FeCx@Fe3O4 core-shell nanoparticle catalyst could couple water-gas shift and syngas-to-olefins reactions in situ. Starting from syngas with low hydrogen/carbon monoxide ratios, the authors achieved high olefin selectivity and hydrocarbon yield, along with a reduction in CO2 and water by-products that led to high hydrogen atom economy. —Phil Szuromi [ABSTRACT FROM AUTHOR]