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

Abstract

Fischer-Tropsch synthesis (FTS) provides a nonpetroleum route for the production of value-added α-olefins from syngas. However, regulating the product distribution to promote the formation of long-chain α-olefins while maintaining low selectivity to C1 byproducts (CO 2 and CH 4 ) remains a great challenge. Herein, a series of alkali metal-promoted Ru/SiO 2 catalysts were synthesized to investigate the effect of alkali metals and identify the performance descriptor for syngas to α-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 α-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 α-olefin production. These results offer a comprehensive view of the electronic effect for regulating product selectivity during the Fischer-Tropsch reaction process.

Control of metal–support interaction for tunable CO hydrogenation performance over Ru/TiO2 nanocatalysts

“…The catalytic performance of the Ru/TiO 2 catalyst with two crystal phases of TiO 2 support was evaluated at 1 MPa, 3000 mL g −1 h −1 , H 2 /CO = 2 and 260 °C. Before reaction, the catalysts were reduced with pure H 2 at 300 °C for 4 h. Since prior work suggested that Na plays a crucial role in shifting the product selectivity of a metallic Ru-based catalyst from paraffins to olefins during CO hydrogenation, 7,21 most catalysts described here contained an Na promoter with a fixed weight ratio Na/Ru of 0.1. As shown in Fig.…”

“…7 Subsequent work also suggests that the types of promoter and support could greatly affect the performance of Fischer-Tropsch synthesis to olefins (FTO) for supported Ru-based catalysts. 8,9 Tiejun Lin Tiejun Lin earned his BS degree from Zhejiang University of Technology and received his PhD degree in 2015 from East China University of Science and Technology. He then joined in Shanghai Advanced Research Institute (SARI), Chinese Academy of Science (CAS), where he was promoted to full professor in 2023.…”

“…7 Subsequent work also suggests that the types of promoter and support could greatly affect the performance of Fischer–Tropsch synthesis to olefins (FTO) for supported Ru-based catalysts. 8,9…”

K-promoted cobalt-based catalysts for heterogeneous hydroformylation

“…To improve the catalytic performance of Co-based heterogeneous catalysts for hydroformylation, various noble metals are always added. 17,18 In addition, the promotion effect of alkali metals as electron promoters or structural promoters has received great attention, which has shown extensive applications in many reactions such as ammonia synthesis, 19 Fischer–Tropsch synthesis, 20,21 aromatic hydrocarbon conversion, 22 and carbon dioxide hydrogenation. 23,24 As electronic modification agents, alkali metals can induce the formation of specific active centers (such as M δ + cations 25 and carbide phases 26 ) and regulate the adsorption/activation of reactants and intermediates, thereby modulating catalytic activity and selectivity.…”

Reversing the Selectivity of Alkanes and Alkenes in Iron-Based Fischer–Tropsch Synthesis: The Precise Control and Fundamental Role of Sodium Promotor

“…Moreover, alkali metals are frequently employed as promoters with a strategic inclusion of minute quantities for iron-based catalysts in FTS processes, − while they exert a significant improvement on the performances of FTS by leading to a boosted yield of olefins. − Among the various alkali metals, soduim (Na) was most widely used due to its remarkable effects on the reactivity of iron-based catalysts toward FTS, and two possible mechanisms are suggested in terms of the structure–reactivity relationship between Fe-based catalysts and FTS. For one thing, the presence of Na was considered to improve the generation of the active phase of iron carbide which serves as the active site for olefin production in FTS according to the linear relationship between the amount of iron carbide and olefin yield. − For the other, the Na dopant is also regarded as an electronic promoter, which could modulate the catalytic behavior of Fe active sites. , In this case, the electron transfer from Na to Fe-based active sites is considered, which thus leads to a preferred reaction route toward olefin desorption from catalyst surfaces.…”