(425b) Direct Production of Higher ?-Olefins from Carbon Dioxide over Iron-Based Catalysts—Catalyst Life Cycle and Reaction Kinetics

Development of substitutional production technology for value-added chemicals from CO₂ is appealing for mitigating industrial carbon emissions while offsetting the costs of carbon capture and sequestration. α-olefins—generally referring to 1-olefins with carbon numbers greater than four—are high-end and value-added chemical raw materials traditionally produced through oligomerization of light olefins. α-olefins are the primary product in iron-based high-temperature Fischer-Tropsch process, providing a potentially alternative route for higher α-olefins production from CO and CO₂ hydrogenation [1-3].

Here we report the development of iron-based modified Fischer-Tropsch processes tailored for direct production of higher α-olefins from CO₂. Previous reports demonstrate that iron-based catalysts derived from a precursor containing zinc ferrite exhibit excellent performance in both CO and CO₂ hydrogenation reactions [3-5]. In our most recent work, a series of Fe-Zn-Na catalysts with different Fe:Zn atomic ratios were prepared. Catalyst performance of α-olefin synthesis was tested at 573-623 K, 1.5-2.5 MPa using a feed gas composition of H₂:CO₂ = 3:1. It is shown that the overall C₄₊ selectivity could be tuned to 65% with α-olefin fractions as high as 90% at CO₂ conversion levels around 40%, meanwhile the selectivity of CH₄ and CO remained less than 20%. The performance was maintained for two weeks using the optimized catalyst.

Evolution of catalyst structure during its full life cycle, including activation, plateau period, deactivation and regeneration, was elaborated combining operando Raman spectroscopy, in-situ XRD, in-situ XPS, HRTEM-EELS and extensive temperature-programmed experiments (TPH/TPO/TPD). Operando Raman under industrially relevant conditions for the first time reveals the direct correlation between excessive formation of Fe₃O₄ and loss in catalyst activity. Zinc proves to play a pivotal role in modulating the contact between FeO_x/FeC_x/Na and enhancing the resistance to oxidation under severe hydrothermal conditions of high-pressure CO₂ hydrogenation. Kinetics study was carried out for Fe-Zn-Na and Fe-Na model catalysts during the plateau period. It is found that low amounts of zinc effectively improve the coupling of reverse water-gas shift and subsequent Fischer-Tropsch reactions. Alkaline promoters facilitates alkenes formation and desorption accompanied by high carbon coverages—which could be neutralized by introduction of zinc. These new findings shed light on the synergetic effects of iron, zinc and sodium in the synthesis of higher α-olefins from CO₂ and H₂. Transient kinetic studies are ongoing to illuminate surface reaction pathways and dynamic formation of active sites.

References

[1] Wei, J., Ge, Q., Yao, R., et al. Directly converting CO₂ into a gasoline fuel. Nat Commun. 2017, 8, 15174.

[2] Guo, L., Sun, J., Ji, X., et al. Directly converting carbon dioxide to linear α-olefins on bio-promoted catalysts. Commun. Chem. 2018, 1(1), 11.

[3] Zhai, P., Xu, C., Gao, R., et al. Highly tunable selectivity for syngas-derived alkenes over zinc and sodium-modulated Fe₅C₂ catalyst. Angew. Chem. Int. Ed. 2016, 55(34): 9902.

[4] Choi, Y., Ra, E., Kim, E., et al. Sodium-containing spinel zinc ferrite as a catalyst precursor for the selective synthesis of liquid hydrocarbon fuels. Chemsuschem 2017, 10(23), 4764.

[5] Cui, X., Gao, P., Li, S., et al. Selective production of aromatics directly from carbon dioxide hydrogenation. ACS Catal., 2019, 9, 3866.