Iron-based Fischer-Tropsch Synthesis Catalysts Research Articles

Morphological effect of MnO2 promoter on iron-based Fischer-Tropsch synthesis catalysts

In this work, flower-like spherical, sea urchin-like and tubular MnO2 are prepared via hydrothermal synthesis and used as promoters of iron-based Fischer-Tropsch synthesis (FTS) catalysts to investigate their morphological effects. It is found that the different morphology of MnO2 exhibits observably distinct promotion functions on iron-based FTS catalysts. Tubular MnO2 contains moderate amount of surface oxygen vacancies accompany with weak Fe-Mn electronical interaction, as a result, tubular MnO2 promotes the formation of long-chain hydrocarbon and olefins, and restrains the generation of CH4. Flower-like spherical MnO2 presents negative roles in the CO conversion and formation of long-chain hydrocarbon, due to its insufficient oxygen vacancies and moderate Fe-Mn electronical interaction. With the most abundance of oxygen vacancies and strongest Fe-Mn interaction, sea urchin-like MnO2 modified catalysts exhibits the highest initial CO conversion, but deactivates rapidly. Besides, the sea urchin-like MnO2 enhances the formation of long-chain paraffins. This work presents new insights of the promotion roles of Mn additive, which is beneficial for the rational design of efficient iron-based FTS catalysts.

Effect of alkaline-earth metals (Mg, Ca, Sr, and Ba) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis of light olefins

For the high-temperature Fischer-Tropsch synthesis (HTFT) of light olefins (C2=-C4=), alkaline-earth metals (Mg, Ca, Sr, and Ba) modified FeMn catalysts were prepared using precipitation and impregnation methods. It was demonstrated that the electron-giving interaction of Ca, Sr, and Ba decreased the adsorption of H species while improving the dissociative adsorption of CO. This interaction resulted in the production of χ-Fe5C2, which increased CO conversion and C2=-C4= selectivity. Compared with Ca and Ba, the FeMnSr catalyst had the strongest CO dissociative adsorption capacity and the highest χ-Fe5C2 content with the best FTS performance. When Sr and Na were introduced simultaneously, the synergistic effect of Sr and Na inhibited the formation of θ-Fe3C and the aggregation of catalyst particles, and also increased the χ-Fe5C2 content. This synergistic effect resulted in the highest surface basicity of FeMnSrNa catalyst, the highest electron cloud density on the surface of Fe atoms, the weakest H species adsorption, the strongest CO dissociative adsorption, and the highest total amount of active iron carbide, which improved the FTS performance. The light olefins yield of FeMnSrNa catalyst (427.0 g/(h·kgCat)) was significantly higher than that of FeMnSr catalyst (341.4 g/(h·kgCat)) and FeMnNa catalyst (80.6 g/(h·kgCat)).

Surface modification of Fe5C2 by binding silica-based ligand: A theoretical explanation of enhanced C2 oxygenate selectivity

Elucidating the role(s) of support-like ligands remains a challenge in catalytic reaction like the Fischer-Tropsch Synthesis (FTS) catalyzed by the iron catalyst supported on silica. We herein theoretically investigated surface modification of Fe5C2 by silica-based ligand and its influence on C2 oxygenate selectivity in FTS, by carrying out DFT calculations on dissociation of CO and formations of CH4 and C2 on ha-SiO2/Fe5C2(510). To mimic the structure of surface modification, the ha-SiO2/Fe5C2(510) model was built up by binding the silica cluster (the ha-SiO2 ligand) to Fe5C2(510). DFT calculations elucidated that the C + CH coupling with the lowest activation barrier among all the possible routes of C2 formation on Fe5C2(510) is suppressed after modification with the ha-SiO2 ligand because the binding ha-SiO2 ligand limits the geometry relaxation caused by the C+CH coupling. However, CO molecule is anchored by the ha-SiO2 ligand via hydrogen bond, suppressing the CO cleavage because the d-valence band center of Fe5C2(510) lowers in energy by surface modification with the ha-SiO2 ligand, but facilitating the C + CO coupling with the lowest activation barrier among all the possible routes of C2 formation. Also, CH4 formation in the ha-SiO2/Fe5C2(510) case is not so easy as that on Fe5C2(510). Therefore, C2 oxygenate is formed more easily in the ha-SiO2/Fe5C2(510) case than in the Fe5C2(510) case. The result agrees with the experimental observation that C2 oxygenate selectivity became high for iron-based FTS catalyst after surface modification of by silica-based ligand.

Role of SiO2 in different iron-based catalysts for Fischer-Tropsch synthesis to light olefins

Co-precipitated FeSi-cp catalysts, impregnated FeSi-im catalyst and core–shell Fe@Si catalysts synthetized through different method were characterized by N2 adsorption, ICP, XRD, TEM, H2-TPR, CO-TPD, Mössbauer spectroscopy and TG. The configuration between Fe and Si played an important role in the catalyst performance. Both FeSi-cp and FeSi-im showed high dispersion, but easily to be agglomerated in FTS. In contrast, Fe@Si has high crystallinity and strong CO chemisorption capacity, which is conducive to the formation of Fe5C2. The SiO2 shell could isolate the active components, reduce carbon deposition and restrain the production of C5+. The results of FTS performance revealed that core–shell Fe@Si catalysts show higher C2∼4= selectivity and better FTS stability in 120 h.

Unravelling acidity–selectivity relationship in the bifunctional process of Fischer-Tropsch synthesis and catalytic cracking

Fischer-Tropsch synthesis (FTS) was carried out over bifunctional model catalysts prepared by physically mixing precipitated iron-based FTS catalysts (P-Fe) and H-ZSM-5. We adjusted the acidity of model catalysts by varying the ratio of P-Fe/H-ZSM-5 and the ratio of Si/Al in the H-ZSM-5. The physical mixing of H-ZSM-5 was adequate to supply the Brønsted acid sites (BAS), which are nonexistent in the P-Fe, to the bifunctional model catalysts without deteriorating the catalytic activity of P-Fe. As a result, we found a linear correlation between the cracking rate and the BAS concentration in the bifunctional process of FTS and cracking for the first time. Furthermore, the model catalysts showed high CO conversion (73–78 %), comparable to that of P-Fe (73 %), and improved C5–C20 selectivity with increased BAS concentration. The highest C5–C20 selectivity obtained in this study (72 wt%) was twice higher than that obtained in the P-Fe (36 wt%).

The promotion role of different crystal phase MnO2 in iron-based Fischer-Tropsch catalysts

Manganese oxide additive is an effective promoter for modulating the catalytic performance of iron-based Fischer-Tropsch synthesis (FTS) catalysts and studied extensively, however, the promotion effects of MnO2 crystal structures on iron-based FTS catalysts have never been reported. In this work, four types of MnO2 with crystal phases corresponding to α-, β-, γ- and δ-MnO2 were prepared, and their promotion functions on iron-based FTS catalysts were studied. These four types of MnO2 exhibit dramatically distinct promotion effects on the structure evolution and catalytic performances. The ferrite MnFe2O4 only forms in the α-MnO2 modified catalysts, while the β-, γ- and δ-MnO2 also present different existing forms in the catalysts. The presence of abundant oxygen vacancies on the surface of α- and δ-MnO2 notably facilitates the cleavage of Fe-O bonds and the dissociation of CO, leading to the enhancing formation of iron carbides in the activated and spent catalysts. Meanwhile, the strong Mn-Fe electronical interactions formed in α- and δ-MnO2 modified catalysts lead to high density of surface carbon species with a long residence time, dramatically improving the selectivity of linear high carbon α-olefins and heavy hydrocarbons. In contrast, the β- and γ-MnO2 modifying dramatically increases the catalytic activity, but suppresses the formation of olefins and long chain hydrocarbons, due to their relatively few amounts of surface oxygen vacancies and moderate Mn-Fe electronical interactions. This work provides new insights and in-depth understanding of the promotion roles of Mn additive, benefiting for the rational design of high performance iron-based FTS catalysts.

Effect of alkalis (Li, Na, and K) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis

Alkalis (Li, Na, and K) decorated FeMnZr catalysts prepared by coprecipitation and impregnation methods were used to investigate the effects of three alkali metal promoters and promoter loading (for Na and K) on the iron-based catalyst for high-temperature Fischer-Tropsch synthesis (HTFT). The results showed that alkalis donated electrons to iron species, which promoted CO dissociative adsorption while suppressing hydrogen adsorption, then boosted the selectivity to light olefins and the improvement was enhanced with the increase of the atomic number of alkali metals and the content of alkali metals. Moreover, compared to alkalis-free catalysts, alkalis-promoted catalysts facilitated the formation of χ-Fe5C2 and ɛ’-Fe2.2C. In alkalis decorated catalysts, the χ-Fe5C2 content decreased and the ɛ’-Fe2.2C content increased with increasing basicity of the catalyst surface. In addition, compared with χ-Fe5C2, ɛ’-Fe2.2C has a stronger CC coupling ability and promotes chain growth more. Compared to Li and Na promoters, K was easier to facilitate the formation of ɛ’-Fe2.2C due to its strongest basicity and CO dissociative adsorption ability. The lowest CH4 and highest C5+ selectivity were accompanied by the highest content of ɛ’-Fe2.2C on K-promoted catalysts, which suggested that ɛ’-Fe2.2C had outstanding CC coupling ability.

Effects of Potassium Loading over Iron–Silica Interaction, Phase Evolution and Catalytic Behavior of Precipitated Iron-Based Catalysts for Fischer-Tropsch Synthesis

Potassium (K) promoter and its loading contents were shown to have remarkable effects on the Fe–O–Si interaction of precipitated Fe/Cu/K/SiO2 catalysts for low-temperature Fischer-Tropsch synthesis (FTS). With the increase in K content from 2.3% (100 g Fe based) up to 7% in the calcined precursors, Fe–O–Si interaction was weakened, as reflected by ATR/FTIR, H2-TPR and XPS investigations. XRD results confirmed that the diffraction peak intensity from (510) facet of χ-Fe5C2 phase strengthened with increasing K loading, which indicates the crystallite size of χ-Fe5C2 increased with the increase in K contents either during the syngas reduction/carburization procedure or after FTS reaction. H2-TPH results indicated that more reactive surface carbon (alpha-carbon) was obtained over the higher K samples pre-carburized by syngas. Raman spectra illustrated that a greater proportion of graphitic carbon was accumulated over the surface of spent samples with higher K loading. At the same time, ATR-FTIR, XRD and Mössbauer spectra (MES) characterization results showed that a relatively higher level of bulk phase Fayalite (Fe2SiO4) species was observed discernibly in the lowest K loading sample (2.3 K%) in this work. The catalytic evaluation results showed that the CO conversion, CO2 selectivity and O/P (C2–C4) ratio increased progressively with the increasing K loading, whereas a monotonic decline in both CO conversion and O/P (C2–C4) ratio was observed on the highest K loading sample during c.a. 280 h of TOS.

Role of Sio2 in Different Iron-Based Catalysts for Fischer-Tropsch Synthesis to Light Olefins

Role of Sio2 in Different Iron-Based Catalysts for Fischer-Tropsch Synthesis to Light Olefins

Boron adsorption and its effect on stability and CO activation of χ-Fe5C2 catalyst: An ab initio DFT study

Developing advanced Fischer-Tropsch synthesis (FTS) technology to acquire fuel and valuable chemicals from non-petroleum sources more efficiently is one of the important tasks in heterogeneous catalysis and chemical engineering research. With the purpose of improving iron-based FTS catalyst, we explored the modification effect of boron on the χ-Fe5C2 catalyst by density functional theory (DFT) calculations. Boron atom adsorption on χ-Fe5C2 surfaces is considered to modulate the stability as well as the activity of CO activation on FTS process. Anti-coking can be observed when boron atom adsorbs on χ-Fe5C2 catalyst due to the competitive adsorption and repulsive interactions between carbon and boron atoms. More open and active planes, such as (311), (021), and (312̅) facets, can expose in the morphology of χ-Fe5C2 when boron atom adsorbs on χ-Fe5C2. It is found that co-adsorbed boron atoms have little effect on the barrier of CO direct dissociation in most cases, while there is also particular situation that CO is adsorbed and activated on boron with carbon termination. Our work shed light on the promoting effects of boron adsorption on CO activation, which can provide useful guide for the design of novel iron-based FTS catalysts.

Quick microwave assembling nitrogen-regulated graphene supported iron nanoparticles for Fischer-Tropsch synthesis

Facile preparation of highly efficient iron-based catalysts is vital to improve the performance of Fischer-Tropsch synthesis (FTS), a sustainable process for converting non-petroleum carbon resources to valuable hydrocarbons. In this work, a convenient preparation for nitrogen-regulated graphene supported iron nanoparticles, with the assistant of microwave treatment, was developed. Thereinto, the nitrogen-containing groups in graphene support were introduced by NH3·H2O treatment. By comparison with ordinary calcination, microwave treatment could significantly shorten the heating time to few seconds, and promote the reduction of iron species. The existence of a small number of nitrogen-containing groups could effectively anchor iron species, and improve the dispersion of iron species. Meanwhile, the duration length of NH3·H2O treatment time could regulate the content of N configuration (pyridinic-N and pyrrolic-N). The size of supported iron species from 120 nm to 10 nm could be well controlled via increasing microwave time from 0 to 14 s. For N-regulated graphene supported iron catalysts, evenly dispersed small particles of iron species, improved surface reduction behavior and reasonable N configuration all promoted FTS. Under relevant industrial conditions (320 °C, 2.0 MPa, 5 g h mol−1), the optimized 15%Fe/AG(12 h)-W(10) catalyst achieved CO conversion as high as 97.2% and C5+ selectivity of 40.0% while maintaining a mild CO2 selectivity (28.2%). The superior preparation and catalytic performance disclosed that the catalyst could be one of ideal candidates for industrial iron-based FTS catalysts in future.

Promotive effect of boron oxide on the iron-based catalysts for Fischer-Tropsch synthesis

Experimental improvement on the suppression of coke deposition remains a substantial challenge in iron-catalyzed Fischer-Tropsch Synthesis. Here, we performed a thorough study to provide in-depth understanding of the coking resistant as well as intrinsic promotive effect of boron oxide on the iron-based catalysts, by excluding the possible interference from another components. It turns out that the dispersion of catalysts is improved by boron oxide doping, which simultaneously inhibit the reduction of iron species in the catalysts. Strikingly, both the coking resistant behavior and hydrogenation activity of single active site of catalysts is greatly enhanced with addition of boron oxide. Such dual pronounced influence has been tentatively reasoned as the electronic interaction between Fe and B that render the electron deficiency of iron species, which thereby result in weaker adsorption capability of CO while inversely promote the chemisorption of H2 for B2O3-doped catalysts. As a consequence, the ratio of H/C on the surface of B2O3-doped catalysts is greatly enhanced with respect to undoped one, thus facilitate the formation of hydrocarbons via hydrogenation of surface carbon species rather than the competitive coke deposition.

Brief Review of Precipitated Iron-Based Catalysts for Low-Temperature Fischer–Tropsch Synthesis

Fischer–Tropsch synthesis (FTS) is a promising way to produce clean liquid fuels and high value-added chemicals from low-value carbon-containing resources such as coal, natural/shale gas, waste wood/plastics, and CO2 via syngas (CO + H2). Precipitated iron-based catalysts are one group of commercially viable FTS catalysts that are important in academic circles as well as in industry. Here, we briefly review the promoters for precipitated iron-based FTS catalysts, which critically influence the catalyst performance. Precipitated iron-based catalysts typically contain three types of promoters: reduction promoters, alkali promoters, and structural promoters. The major effects of reduction promoters on catalytic performance include shortening of the induction period and increasing the CO conversion. Alkali promoters significantly influence the catalyst activation and product selectivity. They facilitate carburization of iron and increase selectivity for the target hydrocarbons C5+ and 1-olefin. The heavier the alkali metal, the stronger the effect. The structural promoters are effective for increasing the resistance of precipitated iron-based catalysts to thermal sintering and physical attrition. However, the impact of structural promoters on chemical properties and resultant catalytic performance is still considered a matter of debate. In the case of alkali promoters and structural promoters, the optimal amount of loading is very important for obtaining high catalytic performance, and this deteriorates severely if loading exceeds the optimal range.

Theoretical exploration of intrinsic facet-dependent CH4 and C2 formation on Fe5C2 particle

Elucidation of intrinsic working principle of single-phase iron carbide and its facet-dependent catalytic behavior remains a substantial challenge in iron-catalyzed Fischer-Tropsch synthesis. Here, we provided in-depth understanding of the iron carbide phase-dependent and facet-dependent properties on theoretically established Fe5C2 particle model through an approach combining Wulff construction, density functional theory, and microkinetics. We studied two key probe reactions, C2 formation via C1+C1 coupling and CH4 formation, by monitoring surface-dependent thermodynamics and microkinetics. Integration of results thereby allows us to assess the macroscopic catalytic properties at single particle level and evaluate contribution from individual exposed surface. The surface (111) and (101-) are kinetically more viable for C2 formation, whereas CH4 formation is kinetically more facile on surface (010), (110) and (111-). This study enriches the knowledge of the intrinsic working mechanism for single-phase iron carbide and provides fundamental insights into rational design of improved iron-based Fischer-Tropsch synthesis catalysts.

Effects of Al, Si, Ti, Zr Promoters on Catalytic Performance of Iron-Based Fischer–Tropsch Synthesis Catalysts

Precipitated iron catalysts containing structural promoters were prepared and their Fischer–Tropsch synthesis (FTS) performance was tested under the same reaction condition. BET, XRD, H2-TPR, and MES were conducted to characterize the phases, structure, morphology, reducibility and carbonization characteristics of the catalysts. It was observed that the reduced catalysts under syngas(H2/CO = 5) promoted with Al/Zr generate more Fe5C2 phase than Si/Ti, which showed that Al/Zr promoted catalysts are more easily reduced than the Si/Ti promoted ones. Addition of Zr and Al promoters improved the reduction and carbonization process of iron-based FTS catalysts. FTS performance data showed that the addition of Al and Zr can increase the CO hydrogenation activity compared to Si and Ti. Moreover, these two promoted catalysts also exhibited excellent stability.

Quantitative Comparison of Zeolite Addition to Iron-Based Catalysts for Fischer-Tropsch Synthesis in a Separate vs Mixed Reactor Bed Setup

Quantitative Comparison of Zeolite Addition to Iron-Based Catalysts for Fischer-Tropsch Synthesis in a Separate vs Mixed Reactor Bed Setup

Hydrogen Partitioning as a Function of Time-on-Stream for an Unpromoted Iron-Based Fischer–Tropsch Synthesis Catalyst Applied to CO Hydrogenation

Inelastic neutron scattering (INS) is employed to examine the evolution of a promoter-free iron-based Fischer–Tropsch synthesis catalyst (∼10 g catalyst charge) that is exposed to ambient pressure ...

A gas-solid fluidized bed reactor for activating the iron-based Fischer-Tropsch synthesis catalyst

A gas-solid fluidized bed process was developed to activate the iron-based F-T synthesis catalyst. The experiment system was built and the influence of activation parameters (H2/CO of activating gas, activation temperature and duration) on synthesis performance of the catalyst was investigated and the crystal phase composition of activated catalyst was characterized by Mössbauer spectrum. The synthesis performance of activated catalyst was evaluated in the stirred tank under the condition of H2/CO = 2.0, pressure = 2.3 MPa, space velocity (sv) = 6000 Nml·g−1·h−1 with temperature = 250 °C and 260 °C. The results indicated that the activity increases and selectivity decreases with increasing H2/CO of activating gas, and the activity and selectivity increase with increasing activation temperature. Activation duration had a complex effect. Under the activation condition of 260 °C and H2/CO = 20, the catalyst was activated completely in 6 h and little effect was found for increasing activation time further. Under the activation condition of 280 °C and H2/CO = 12.5, the activity and CH4 selectivity increase with increasing the activation time. The optimized activation condition are 260 °C, H2/CO = 12.5 and duration = 6.0 h. The synthesis performance of the catalyst activated by this innovative process is much better than that activated by traditional gas-liquid-solid slurry bed process. The yield of C3+ and C5+ increases 4.5% and 8.6% respectively under the evaluating condition of 260 °C, H2/CO = 2.0, pressure = 2.3 MPa and sv = 6000 Nml·g−1·h−1. The main crystal phase components of the activated catalyst are Fe5C2, Fe2.2C, Fe3+(spm), Fe2+(spm) and α-Fe. The activated catalyst with 20%–50% active phase (Fe5C2, Fe2.2C and α-Fe) will give better performance with high CO conversion, low selectivity of CO2 and CH4.

Perspectives on the effect of sulfur on the hydrocarbonaceous overlayer on iron Fischer-Tropsch catalysts

Abstract Fischer-Tropsch synthesis (FTS) is commonly viewed as an alternative approach to the production of diesel fuels via sources independent of crude oil. The adaptability of the FTS process allows for the selective production of shorter chain C2 to C6 hydrocarbons and has the potential to be a legitimate source of useable chemical feedstocks with high value to the chemical manufacturing industry. Interestingly, although recognised as a poison in most catalytic systems, small amounts of sulfur in iron-based FTS catalysts has been demonstrated to promote catalyst reducibility and activity towards shorter chain hydrocarbons. However, it is not known what impact sulfur has on the formation of hydrocarbonaceous surface species that have been proposed to play a pivotal role in the mediation of reactants during iron FTS. Here we apply ambient pressure CO hydrogenation at 623 K on a selection of sulfur promoted iron FTS catalysts to investigate the effect of sulfur content on hydrocarbonaceous species formation. For the first time, we report the application of inelastic neutron scattering to quantify the presence of hydrocarbonaceous species under the presence of sulfur promotion. In combination with temperature programmed oxidation, X-ray diffraction, and Raman spectroscopy, we observe how low sulfur loadings (

Examining the temporal behavior of the hydrocarbonaceous overlayer on an iron based Fischer–Tropsch catalyst

In order to examine fundamental processes connected with the use of an unpromoted iron based Fischer–Tropsch synthesis (FTS) catalyst, model studies examining the temporal formation of hydrocarbonaceous species that form over the catalyst are undertaken using a combination of temperature-programmed oxidation, powder X-ray diffraction, Raman scattering, transmission electron microscopy and inelastic neutron scattering (INS). Catalyst samples were exposed to ambient pressure CO hydrogenation at 623 K for defined periods of time-on-stream (3, 6, 12 and 24 h) prior to analysis. INS reveals a progressive retention of hydrogenous species that is associated with the evolution of a hydrocarbonaceous overlayer, as evidenced by the presence of sp2 and sp3 hybridized C–H vibrational modes. Correlations between the formation of aliphatic and olefinic/aromatic moieties with post-reaction characterization leads to the proposal of a number of chemical transformations that, collectively, define the conditioning phase of the catalyst under the specified set of reaction conditions. A comparison between the inelastic neutron scattering spectra of the 24 h sample with that of an iron catalyst extracted from a commercial grade Fischer–Tropsch reactor validates the relevance of the experimental approach adopted.