Fe-based Fischer-Tropsch Synthesis Research Articles

High-Temperature Fe-Based Fischer–Tropsch Synthesis: Experimentally Validated Kinetic Models Implemented at Pellet and Reactor Scales

High-Temperature Fe-Based Fischer–Tropsch Synthesis: Experimentally Validated Kinetic Models Implemented at Pellet and Reactor Scales

Machine learning insights into catalyst composition and structural effects on CH4 selectivity in iron-based fischer tropsch synthesis

Fe-based Fischer-Tropsch Synthesis (FTS) enables the selective conversion of syngas into long-chain hydrocarbons, which can be further refined to produce highly demanded liquid fuels and high-value chemical products. However, developing novel heterogeneous catalysts for FTS with desirable performance characteristics is a challenging task, as their performance depends on various factors such as precursor, support material, promoters, pretreatment conditions and the catalyst structures. Thus, it remains difficult to understand the structure-performance relationship of FTS and to optimize the catalyst formulations and operating conditions rationally. By integrating traditional chemistry with machine learning, we herein establish intrinsic correlations among reduction, reaction conditions, phase information and the methane selectivity of Fe-based FTS, using high quality experimental data. The content of the iron phases in the post-reaction phase, particularly χ-Fe5C2, significantly influences the methane selectivity of the catalyst. Four types of additives K, Cu, SiO2, and Ca could effectively suppress the methane selectivity, most likely by promoting or stabilizing the iron carbide phases, indicated by their strong correlation. The machine learned structure-performance relationships offers new insights into the design of Fe-based FTS catalysts, and could guide the further optimization of the preprocessing conditions and various parameter factors to minimize the methane selectivity of FTS.

Eco-friendly solid-state synthesis of Na-promoted Mn-Fe/ZrO2 catalyst for Fischer-Tropsch synthesis

The advancement of large-scale and environmentally-friendly solid-state synthesis methods is of paramount importance for the production of precipitated catalysts, notably Fe-based catalysts utilized in Fischer-Tropsch synthesis (FTS). In this study, we present an innovative solvent-independent, wastewater-free solid-state approach for the synthesis of highly efficient Fe/ZrO2 catalysts, elucidating the mechanisms of promotion within this specific methodology. The prepared catalysts underwent rigorous evaluation under industrially-relevant FTS conditions (230–270 °C, 20 bar, and WHSV = 5400 mL/(gcat⋅h)). A comprehensive suite of characterization techniques was employed to establish the intricate interplay between catalytic activity and structural attributes. We meticulously examined the impact of Na as an alkali promoter and the role of transition metals (Cu and Mn) in enhancing FTS catalytic performance. The catalytic outcomes obtained clearly demonstrate the pivotal role of the synergistic interaction between Mn and Na within the Na-Mn-Fe/ZrO2 catalyst system in driving the FTS reaction. This synergism significantly enhances FTS performance, yielding an impressive CO conversion rate (89.91 %) while maintaining low selectivity to CH4 (7.75 %). The incorporation of Mn and Na into the catalyst matrix enhances the dispersion of Fe2O3, thereby improving the reducibility of Mn/Fe oxides and elevating the electron density on the catalyst surface. These factors facilitate the formation of active Hägg iron carbide (χ-Fe5C2), stimulating chain propagation and ultimately leading to the generation of valuable C5+ hydrocarbon products, which underscores its potential as an optimal candidate for future industrial applications in Fe-based Fischer-Tropsch synthesis.

Interface-Induced Phase Evolution and Spatial Distribution of Fe-Based Catalysts for Fischer–Tropsch Synthesis

Fe-based catalysts undergo complex reduction, carburization, and phase interconversion as well as oxidation during Fischer–Tropsch synthesis (FTS). Unraveling the evolution of carbides and establishing the dominant factors are essential for rational design and modulation of Fe-based catalysts. In this work, Fe–oxide interfaces were constructed to investigate the effect on phase transformation and FTS performance. By correlating performance to the composition of iron species, the crucial role of Fe2.2C in high CO conversion and C5+ productivity has been confirmed. More importantly, a heterogeneous spatial distribution of iron carbides and oxides induced by Fe–oxide interfaces was observed. Fe3O4 preferentially forms at interfaces with the coexistence of carbon-rich Fe2.2C at subadjacent interfaces, while Fe5C2 locates at the region away from oxides or in the bare Fe catalyst. Further ex/in situ characterizations, including XPS, TPO, in situ XRD, and in situ IR, were conducted for revealing the mechanism. This study provides insights for iron carbides formation and stability by modulating interfaces, which guides the further development of Fe-based FTS catalysts.

A DFT study towards dynamic structures of iron and iron carbide and their effects on the activity of the Fischer-Tropsch process.

The Fe-based Fischer-Tropsch synthesis (FTS) catalyst shows a rich phase chemistry under pre-treatment and FTS conditions. The exact structural composition of the active site, whether iron or iron carbide (FeCx), is still controversial. Aiming to obtain an insight into the active sites and their role in affecting FTS activity, the swarm intelligence algorithm is implemented to search for the most stable Fe(100), Fe(110), Fe(210) surfaces with different carbon ratios. Then, ab initio atomistic thermodynamics and Wulffman construction were employed to evaluate the stability of these surfaces at different chemical potentials of carbon. Their FTS reactivity and selectivity were later assessed by semi-quantitative micro-kinetic equations. The results show that stability, reactivity, and selectivity of the iron are all affected by the carbonization process when the carbon ratio increases. Formation of the carbide, a rather natural process under experimental conditions, would moderately increase the turnover frequency (TOF), but both iron and iron carbide are active to the reaction.

Revisiting the syngas conversion to olefins over Fe-Mn bimetallic catalysts: Insights from the proximity effects

Fe-Mn bimetallic catalysts have shown great promises in direct conversion of syngas to olefins via Fischer-Tropsch Synthesis (FTS). However, due to the structural complexity of Fe-based catalysts and the interference of secondary promoters, the promotional effects of Mn on phase transition of Fe-based FTS catalysts and catalytic performance are still under debate. Herein, Fe-Mn bimetallic catalysts were prepared from two different methods, namely solution combustion method (SCM) and co-precipitation (CP) method. Fe4Mn1-SCM exhibited an excellent C5+ selectivity (45 %) and stability over 100 h. In contrast, the product distribution given by Fe4Mn1-CP was dominated by C1-C4 hydrocarbons (85 %). The superior chain growth ability of the Fe4Mn1-SCM was attributed to the abundant interfacial Fe-MnOx structure, which retarded the oxygen removal and carbon diffusion process, resulting in less χ-Fe5C2 phase but more ε-Fe2.2C phase that was highly active for C–C coupling. Moreover, Mn-induced promotional effects were analyzed via kinetic measurements, physiochemical characterizations, Operando Raman-mass spectroscopy and theoretical calculations. It was found that the Fe-Mn proximity played a key role in controlling the interaction of structural and electronic impacts between Mn and Fe species. These findings may provide new insights for fabrication of Fe-based FTS catalysts with tunable product distribution.

Activating nitrogen-doped carbon nanosheets by KOH treatment to promote the Fischer-Tropsch synthesis performance

Both appropriate support and promoter decoration are vital to improve the performance of Fe-based catalysts in Fischer-Tropsch Synthesis (FTS). In this work, KOH chemical activation was applied to modify N-doped carbon nanosheets supported Fe catalysts, which achieves support modification and promoter introduction simultaneously. The influence of KOH activation on pore structure, N configuration and residual K effect were discussed on Fe-based FTS catalyst. Upon KOH activation, the micro-porosity was enriched which improved the Fe dispersion. Meanwhile, the N configuration was regulated through KOH in-situ reacting with carbon matrix. Further investigation on residual K revealed a combined effect of K and N. The physical structure, electronic properties and phase compositions after activation were characterized by N2-physisorption, XRD, Raman, TEM, XPS and Mössbauer spectroscopy. Combing with the catalytic performance, we demonstrated that the pyrrolic N sites and suitable K residuals contribute to electron donation to Fe species, collectively promoting Fe carburization accompanied by higher C2–C4= productivity. This finding is expected to offer an activation method for preparation of N-doped carbon supported metal catalysts.

Illustrating the overall reaction network of the synthesis-gas-to-hydrocarbons process over iron-zeolite bifunctional catalysis

Since its discovery in the early 20 th century, Fischer-Tropsch synthesis (FTS) has opened a path, as an alternative to crude oil, to produce fuels and chemicals. When classical FTS catalysts are combined with acidic zeolites, the scope of this gas-phase polymerization can be narrowed, thus maximizing the production of value-added commodities and eliminating energy-consuming separation steps. However, from a mechanistic standpoint, even now, little is known about the role of the different reaction intermediates. Here, we present a comprehensive, in-depth, mechanistic investigation using solid-state NMR spectroscopy and well-designed control experiments on combining a classical Fe-based FTS catalyst and zeolites with different topologies to establish the impact of “co-catalytic” key organic carbon-based reaction intermediates, including carbonylated/oxygenated species (ester/ketone/alcohol/ether/epoxide/ketene). Consequently, this work provides experimental evidence supporting the “co-existence” of oxygenate (cf. surface-enol and CO-insertion) mechanisms (together with the traditional carbide-based FTS mechanism). The significance of “supramolecular reactive centers” within zeolite and host-guest chemistry has also been illuminated. • Zeolite topologies altered preferential hydrocarbon product selectivities • Mobility-based solid-state NMR identified crucial carbon-based reaction intermediates • Co-involvement of oxygenated/carbonylated species is evaluated during syngas conversion • Significance of inorganic-organic supramolecular reaction centers has been established To alleviate pressure from increased environmental regulations over carbon emissions, the direct hydrogenation of CO/CO 2 to diverse hydrocarbons over bifunctional catalysts is a priority research topic worldwide. Identifying new catalytic routes to valorize C1-feedstock is a key to combating climate change, whereby Fischer-Tropsch synthesis (FTS) over bifunctional metal-zeolite catalysts offers elegant solutions to diversify global fuel/chemical supply. However, their overall reaction network to deliver preferential final product selectivity remains elusive and controversial. Through this work, we achieve this goal by identifying key carbon-based reaction intermediates and establishing a full reaction network of the synthesis-gas-to-hydrocarbons process over Fe 2 O 3 @KO 2 /zeolite catalyst. This established structure-reactivity relationship can upgrade the current FTS process and improve the fundamental understanding of bifunctional catalysis. Identifying “descriptor”-style carbon-based reaction intermediates is significant in upgrading the current state of metal-zeolite bifunctional “cascade” catalysis. In this work, four different product selectivities (short olefins, light paraffins, long-chain hydrocarbons, and aromatics) were observed during the syngas-to-hydrocarbon process (Fischer-Tropsch type) over potassium superoxide-doped iron oxide/zeolite-based bifunctional catalysts. Here, we demonstrate that the zeolite topology and “co-catalytic” hybrid supramolecular reactive centers (i.e., organic hydrocarbons pool species trapped by the inorganic zeolite) regulate the reaction mechanism and the final product selectivity.

Upcycling of CO2 into sustainable hydrocarbon fuels via the integration of Fe-based Fischer-Tropsch synthesis and olefin oligomerization: A comparative case study

Power-to-X (PtX) technologies, especially for the Power-to-Liquids (PtL) and Power-to-Gas (PtG) have attracted extensive attention recently, as promising pathways for carbon upcycling via converting CO2 into high-value products including liquid fuels and substitute natural gas (SNG). Herein, aiming at further improving the PtX process efficiency, we proposed two novel PtL/PtG hybrid processes, namely an indirect process (with RWGS unit) and a direct process (without RWGS unit) by integrating Fe-based Fischer-Tropsch synthesis (FTS) and olefin oligomerization technologies, which co-produce syncrude and SNG. Both process simulation and techno-economic analysis were implemented to evaluate the overall process performances, through various indicators involving technical indicators (e.g., syncrude production, energy efficiency, and net CO2 reduction), and economic indicators such as total capital investment, net CO2 reduction costs together with total product costs. Both proposed PtL/PtG processes are efficient in converting CO2 into valuable hydrocarbon fuels, and the syncrude production and total product revenues of indirect process are 2.35–14.58% and 7.55–8.51% higher than those of the direct process, respectively. Whereas, the direct process has lower net CO2 reduction cost of 206.09 $/tonne CO2. Moreover, the present PtL/PtG processes have higher syncrude production and total product revenues than those of our previous studies including a direct PtL/PtG process coupled with Fe-based FTS and two indirect PTL/PTG processes combined with RWGS and Fe/Co-based FTS reaction, with rates of 30.95 and 12.73% at most.

In Situ Active Site for Fe-Catalyzed Fischer-Tropsch Synthesis: Recent Progress and Future Challenges.

Fischer-Tropsch synthesis (FTS) that converts syngas into long-chain hydrocarbons is a key technology in the chemical industry. As one of the best catalysts for FTS, the Fe-based composite develops rich solid phases (metal, oxides, and carbides) in the catalytic reaction, which triggered the quest for the true active site in catalysis in the past century. Recent years have seen great advances in probing the active-site structure using modern experimental and theoretical tools. This Perspective serves to highlight these latest achievements, focusing on the geometrical structure and thermodynamic stability of Fe carbide bulk phases, the exposed surfaces, and their relationship to FTS activity. The current reaction mechanisms on CO activation and carbon chain growth are also discussed, in the context of theoretical models and experimental evidence. We also present the outlook regarding the current challenges in Fe-based FTS.

Efficient production of renewable hydrocarbon fuels using waste CO2 and green H2 by integrating Fe-based Fischer-Tropsch synthesis and olefin oligomerization

Recently, Power-to-Liquids (PtL) and Power-to-Gas (PtG) technologies have been regarded as promising pathways for renewable energy storage and CO2 mitigation. Herein, we newly proposed two PtL/PtG hybrid processes (Cases A and B) by integrating the Fe-based Fischer-Tropsch (F-T) synthesis and olefin oligomerization, to further enhance the production of value-added liquid hydrocarbons. The process modelling and case study were implemented to evaluate their process performances by using Aspen Plus. In addition, the effects of different feeding conditions (i.e., CO/CO2 feeding) and process configurations (i.e., F-T synthesis only or two-stage reactor) were also comparatively analyzed. It is found that both Cases A and B are efficient technologies for converting CO2 into value-added hydrocarbons, and Case A is found to be more beneficial in the aspects of the carbon and thermal efficiencies, and net CO2 reduction. While, Case B is competitive in producing high-value liquid hydrocarbons. Moreover, the options of CO2 feeding and two-stage reactor are more preferable than the options of CO feeding and F-T synthesis only, and both Cases A and B are more competitive in the aspects of syncrude production, thermal efficiency, and CO2 reduction, as compared to the Base cases 1–3.

FeMn@HZSM-5 capsule catalyst for light olefins direct synthesis via Fischer-Tropsch synthesis: Studies on depressing the CO2 formation

For Fe-based Fischer-Tropsch synthesis catalyst, how to decrease CO2 formation is a big challenge. In this work, a capsule catalyst (FeMn@HZSM-5) with FeMn as core and HZSM-5 as shell was prepared and used for Fischer-Tropsch to olefins (FTO) reaction, compared with bare FeMn catalyst and several hybrid catalysts by physically mixing FeMn and HZSM-5 in different ways. Among these catalysts, the FeMn@HZSM-5 capsule catalyst showed the best catalytic performance with the highest light olefins selectivity and the lowest CO2 selectivity. Compared with FeMn catalyst, the CO2 selectivity of FeMn@HZSM-5 catalyst decreased more than 10%. However, the CO2 selectivity of other physically mixing catalysts was similar to that of bare FeMn catalyst, indicating that randomly adding HZSM-5 had no effect on depressing the CO2 formation. Benefiting from the HZSM-5 shell, the FeMn@HZSM-5 capsule catalyst could effectively affect the diffusion of H2O and thus suppress the water-gas shift reaction in FTO reaction.

Green liquid fuel and synthetic natural gas production via CO2 hydrogenation combined with reverse water-gas-shift and Co-based Fischer-Tropsch synthesis

In recent years, significant attention has been paid to the CO2 mitigation via the power-to-liquids (PTL) and power-to-gas (PTG) processes because they can efficiently transform CO2 into high-value products such as liquid hydrocarbons and synthetic natural gas (SNG), and provide a promising solution for the storage of the intermittent renewable energy. Herein, we suggested a PTL process and a PTL/PTG process combined with the reverse-water-gas-shift (RWGS) and Co-based Fischer-Tropsch synthesis (FTS), which respectively produce liquid hydrocarbons only and the combination of liquid hydrocarbons and high-calorie SNG, based on the previous study regarding the PTL and PTL/PTG processes composed of Fe-based FTS. We conducted the process modelling and comparative techno-economic analysis to determine the key performance of the proposed PTL and PTL/PTG processes including the CO2 mitigation rate, energy efficiency, total product cost and CO2 mitigation cost. The proposed PTL and PTL/PTG processes can be regarded as feasible technical solutions to convert wasted CO2 into high-value liquid hydrocarbons and SNG. Additionally, the indirect PTL and PTL/PTG processes were helpful for improving the production of liquid hydrocarbons. Meanwhile, the direct PTL/PTG processes and indirect PTL processes favor the energy efficiency and CO2 mitigation.

Efficient Production of Renewable Hydrocarbon Fuels Using Waste CO2 and Green H2 by Integrating Fe-Based Fischer-Tropsch Synthesis and Olefin Oligomerization

Efficient Production of Renewable Hydrocarbon Fuels Using Waste CO2 and Green H2 by Integrating Fe-Based Fischer-Tropsch Synthesis and Olefin Oligomerization

Synthesis of Iron-Carbide Nanoparticles: Identification of the Active Phase and Mechanism of Fe-Based Fischer–Tropsch Synthesis

Despite the extensive study of the Fe-based Fischer–Tropsch synthesis (FTS) over the past 90 years, its active phases and reaction mechanisms are still unclear due to the coexistence of metals, oxi...

Identification of Iron Carbides in Fe(−Na−S)/α‐Al2O3 Fischer‐Tropsch Synthesis Catalysts with X‐ray Powder Diffractometry and Mössbauer Absorption Spectroscopy

AbstractIn Fe‐based Fischer‐Tropsch Synthesis (FTS), the Fe carbides form under the carburizing H2 : CO reaction atmosphere providing the active phases for hydrocarbon synthesis. H2 reduced Fe(−Na−S)/α‐Al2O3 catalyst materials, with and without Na−S promotion, were carburized under CO at 240–440 °C to form Fe carbides. X‐ray Powder Diffractometry (XRPD) with Rietveld Quantitative Phase Analysis (R‐QPA) and Mössbauer Absorption Spectroscopy (MAS) were used to identity and quantify the formed Fe carbide phases. The Fe carbides formed in order of increasing temperature are ϵ‐Fe3C, η‐Fe2C, χ‐Fe5C2 and θ‐Fe3C. θ‐Fe7C3 and a distorted χ‐Fe5C2 phase are formed at 25 bar CO (340 °C) from a Fe oxide precursor. Fe carbide formation was unaffected by Na−S addition, but it did increase Fe oxidation (≤290 °C) and preferred formation of χ‐Fe5C2 over θ‐Fe3C phase (≥390 °C). The results unify the often ambiguous Fe carbide identification and nomenclature and specify the role of Na−S in the carburization process.

Combined In Situ X-ray Powder Diffractometry/Raman Spectroscopy of Iron Carbide and Carbon Species Evolution in Fe(−Na–S)/α-Al2O3 Catalysts during Fischer–Tropsch Synthesis

A Na–S promoted Fe-based Fischer–Tropsch synthesis (FTS) catalyst converts a H2/CO gas mixture into hydrocarbons with enriched C2–C4 olefin content. Above 300 °C, the carbon-depositing Boudouard reaction competes with the FTS reaction for CO as reactant. By making use of a combined in situ X-ray powder diffractometry (XRPD)/Raman spectroscopy setup, the simultaneous evolution of the FexOy/α-Fe/FexC phases and various formed carbon species has been monitored at 340 °C and 10 bar. CO carburized, Na–S promoted and unpromoted Fe­(−Na–S)/α-Al2O3 catalysts were investigated. The various Fe phases present were quantified with Rietveld quantitative phase analysis (R-QPA) from the in situ collected XRPD patterns. The observed D- and G-bands in the in situ Raman spectra were analyzed for their relative intensities, band widths, and positions and compared to reference carbon materials. It was found that amorphous carbon with C sp3 and C sp2 in chain-like ordering evolved toward carbon nanofiber-like structures during FTS. Na–S promotion and initial CO carburization at temperatures ≥340 °C led to an increased amount of cyclic sixfold C sp2 species. Preliminary carbon deposits present in the catalysts decreased the initial fast increase of the Raman band intensities, while Na–S promotion increased Raman band intensity growth after the initial fast increase period. The carbon species evolution was unaffected by the presence of specific Fe carbides or by carbide-to-carbide transitions. Na–S promotion aided in the reduction of Fe3O4 by (H2:)­CO to carbon-depositing Fe carbides. The results obtained add to our further understanding on the role of Fe and carbon species during a high-temperature FTS reaction.

Experimental Investigation on the Two-Sided Effect of Acidic HZSM-5 on the Catalytic Performance of Composite Fe-Based Fischer–Tropsch Catalysts and HZSM-5 Zeolite in the Production of Aromatics from CO2/H2

A negative effect of an HZSM-5 zeolite was observed for CO2 hydrogenation toward the production of aromatics when combined with Fe-based Fischer–Tropsch synthesis (FTS) catalysts in an improper manner. The conversions of CO2 and CO were seriously hindered due to suppression in the formation of Fe carbides. With various especially designed experiments, it is confirmed that the Brønsted acid sites in the zeolite serving as an electron acceptor should be the major factor influencing the electron transfer from the Fe phase to CO molecules, resulting in the suppression of CO dissociation. Additionally, we suggested a practical way to eliminate the negative effect of the acidic HZSM-5 zeolite by coating an inert layer on either the Fe-based catalyst or the zeolite. With these observations, it is beneficial to design a multifunctional catalyst system that can be scaled up by paying more attention toward the two-sided effect of zeolites.

Reduction and carburization of iron oxides for Fischer–Tropsch synthesis

Abstract The activation of iron oxide Fischer–Tropsch Synthesis (FTS) catalysts was investigated during pretreatment: reduction in hydrogen followed by carburization in either CO or syngas mixture, or simultaneously reduction and carburization in syngas. A combination of different complementary in situ techniques was used to gain insight into the behavior of Fe-based FTS catalysts during activation. In situ XRD was used to identify the crystalline structures present during both reduction in hydrogen and carburization. An increase in reduction rate was established when increasing the temperature. A complete reduction was demonstrated in the ETEM and a grain size dependency was proven, i.e. bigger grains need higher temperature in order to reduce. XPS and XAS both indicate the formation of a small amount of carbonaceous species at the surface of the bulk metallic iron during carburization.

Exploring the Reaction Paths in the Consecutive Fe-Based FT Catalyst–Zeolite Process for Syngas Conversion

Direct conversion of syngas into aromatics with tandem catalysts containing Fe-based Fischer–Tropsch synthesis (FTS) catalysts and zeolites has been attracting significant attention because of the ...