(425b) Direct Production of Higher ?-Olefins from Carbon Dioxide over Iron-Based Catalysts—Catalyst Life Cycle and Reaction Kinetics
AIChE Annual Meeting
2019 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Thermochemical CO2 Reduction II: C2+ Chemicals
Tuesday, November 12, 2019 - 3:48pm to 4:06pm
Here we report the development of iron-based modified Fischer-Tropsch processes tailored for direct production of higher Î±-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 Î±-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 Î±-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 FeOx/FeCx/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 coveragesâwhich 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 Î±-olefins from CO2 and H2. Transient kinetic studies are ongoing to illuminate surface reaction pathways and dynamic formation of active sites.
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