Identifying the Performance Descriptor in Direct Syngas Conversion to Long-Chain ?-Olefins over Ruthenium-Based Catalysts Promoted by Alkali Metals

Fischer-Tropsch synthesis (FTS) provides a non petroleum route for the production of value-added alpha-olefins from syngas. However, regulating the product distribution to promote the formation of long-chain alpha-olefins while maintaining low selectivity to C1 byproducts (CO2 and CH4) remains a great challenge. Herein, a series of alkali metal-promoted Ru/SiO2 catalysts were synthesized to investigate the effect of alkali metals and identify the performance descriptor for syngas to alpha-olefins. Xray absorption and photoelectron spectroscopy as well as CO diffuse reflectance infrared Fourier transformation spectra revealed that Ru's electronic structure can be tuned by the addition of alkali metal promoters. The Allen scale electronegativity deviation between the metallic Ru and the alkali metal promoter, which was identified as a performance descriptor, was correlated with the initial turnover frequency (TOF) and olefins distribution, as well as the chain growth probability. In addition, the fraction of olefin products was successfully adjusted from lower olefins (50.3%) to long-chain alpha-olefins (85.8%) by controlling the kinds of alkali metal promoters. Evidences from X-ray absorption spectroscopy and chemisorption characterizations revealed that the controllable Ru electron density could enhance CO adsorption and hinder the reactivity of chemisorbed H species, thus benefiting the alpha-olefin production. These results offer a comprehensive view of the electronic effect for regulating product selectivity during the Fischer-Tropsch reaction process.