Iron-based Fischer-Tropsch Synthesis Catalysts Research Articles

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 (

Syngas-derived olefins over iron-based catalysts: Effects of basic properties of MgO nanocrystals

A series of Fe/MgO catalysts with well-defined exposed crystal planes were synthesized by impregnation, deposition-precipitation and ultrasonic impregnation methods. The catalysts were characterized by X-ray powder diffraction, high-resolution transmission electron microscopy, CO2 temperature-programmed desorption, H2 temperature-programmed reduction, X-ray spectroscopy and N2 adsorption-desorption isotherms. The characterization results indicate that the basicity of MgO supports strongly affect the catalytic performance of iron-based catalysts for Fischer-Tropsch synthesis. It is found that the strong basicity sites of MgO supports remain during the ultrasonic impregnation process. The intrinsic basicity of Fe/MgO catalysts enhances dissociative CO adsorption and promotes the olefin selectivity. In addition, the catalyst of iron particles on the (111) crystal planes of MgO nanosheets presents higher TOF value and olefins selectivity than that of the catalyst using the (100) crystal planes of MgO nanocubes as a support. The effect of basic properties of MgO nanocrystals facilitates CO chemisorption, suppressing H2 adsorption and olefin desorption on the corresponding Fe/MgO catalysts.

Chemical and structural effects of strontium on iron-based Fischer-Tropsch synthesis catalysts

A series of Fe/SiO2 catalysts with different strontium/iron molar ratios were used to investigate the chemical and structural effects of strontium on the iron-based Fischer-Tropsch synthesis (FTS) catalysts. It was found that the chemical effect of strontium strengthened FeO bonds of Fe2O3 and hence suppressed the reduction of iron oxide. The chemical effect also suppressed the H2 adsorption but improved the CO adsorption, which is responsible for the low surface H/C ratios and methane selectivity. At the same time, the promotion of strontium improved the dispersion of iron oxides. The dispersion effect facilitated the reduction and carburization of catalysts. It was also helpful for the H2 adsorption and in turn led to high surface H/C ratios and methane selectivity. Under the combined action of the chemical and structural effects, the selectivity to methane of catalysts exhibited a parabola-like trend as a function of strontium loading and passed through a minimum at Sr/Fe molar ratio of 6/100. In contrast, the selectivity of heavy hydrocarbon demonstrated an opposite tendency.

Identification of carbon species on iron-based catalysts during Fischer-Tropsch synthesis

This paper focuses on the use of in situ and ex situ characterisation techniques to provide evidences of carbon species on a commercial iron-based Fischer-Tropsch synthesis catalyst as well as other indices of potential deactivation mechanisms. In situ XANES measurements demonstrate that re-oxidation or transformation of the active iron phase, i.e. the Hägg carbide phase, was not a significant deactivation mechanism at the studied conditions. Sintering of Hägg carbide nanoparticles is significant with increasing temperatures and time on stream. The sintering mechanism is proposed to be a hydrothermally-assisted process. In situ DRIFTS indicates the presence of different carbon species on the catalyst surface such as aliphatic hydrocarbons from wax products and oxygenate compounds such as alcohols, aldehydes/ketones and carboxylate species. Carboxylate species are resistant towards hydrogenation at 280 °C. The presence of different carbon species on the surface after wax product extraction is evident from TPH-MS measurements. GC-MS analysis shows that the strongly adsorbed carbon species remaining on the catalyst surface from wax products are mainly α-olefins and branched carboxylic species. The interaction of oxygenate compounds, especially carboxylate species with iron oxide, may form stable complexes limiting further iron catalyst carburization. STEM-EDX analysis shows that carbon is preferentially located on iron particles.

Nitrogen-rich mesoporous carbon supported iron catalyst with superior activity for Fischer-Tropsch synthesis

Superior iron-based Fischer-Tropsch synthesis (FTS) catalysts (Fe/NMCs) were developed by impregnating high amount of iron (40 wt%) over nitrogen-rich mesoporous carbons (NMCs) with high porosities. The large pore volumes and specific surface areas of NMCs realized the high loadings and proper dispersions of iron while the nitrogen-containing groups enhanced the basicities of the catalysts. Weak metal-support interactions were observed in Fe/NMCs, which improved the reduction, carburization of iron, and further the FTS activities. FTS tests indicated a medium amount of nitrogen (≤8.3 wt%) did not show obvious effect on the activity of the catalyst, but a much higher amount of nitrogen (≥16.5 wt%) could result in a significant decrease of activity. Nitrogen-containing groups could effectively suppress the methane selectivity and improve the lower olefins selectivity in the FTS reaction. The hydrocarbon productivity of the Fe/NMCs was up to 0.62 g HC/(h·g cat.) at 260 °C, 1 MPa, and H2/CO ratio of 1, much higher than that of any other iron catalyst reported under similar conditions. As a result of the superior performance, Fe/NMCs could be one of ideal candidates for iron-based FTS catalysts in the future.

Use of stability diagrams to predict catalyst speciation during Fischer Tropsch reduction stage: a mini-review

An alternative way of predicting phase evolution of iron-based Fischer Tropsch synthesis catalysts during activation.

Suppression by Pt of CO adsorption and dissociation and methane formation on Fe5C2(100) surfaces.

To understand the chemical origin of platinum promotion effects on iron based Fischer-Tropsch synthesis catalysts, the effects of Pt on CO adsorption and dissociation as well as surface carbon hydrogenation on the Fe5C2(100) facet with different surface C* contents have been studied using the spin-polarized density functional theory method. CO dissociation initiating from diverse sites was calculated through both direct and H-assisted pathways via the CHO intermediate. On the perfect (100) surface, CO can hardly dissociate, and the surface carbon can be facially hydrogenated to CH4. On the C*-defect and C*-free (100) surfaces, CO can strongly adsorb on the C* vacant sites and direct dissociation is favored to occur. The activity is higher with the decrease of the surface carbon content. When platinum atoms are added on the surfaces, the C*-vacancies have a higher activity for CO dissociation than the new sites generated by Pt adsorption. However, both the CO dissociation and the surface carbon consumption through CH4 formation are hindered. The evolution of surface carbon is predicted to be suppressed by the addition of Pt on the Fe5C2(100) surface.

Enhancing the light olefin selectivity of an iron-based Fischer-Tropsch synthesis catalyst by modification with CTAB.

The effects of the surfactant hexadecyltrimethylammonium bromide (CTAB) on the catalytic performance of a manganese-promoted iron (FeMn) catalyst for the Fischer–Tropsch to olefin (FTO) reaction were investigated. The use of the CTAB-assisted FeMn catalyst resulted in the production of light olefin (C2–4 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> ) selectivity of up to 55.45% with a ratio of olefin to paraffin among the C2–C4 hydrocarbons as high as 7.75 under industrially relevant conditions (320 °C, 1.0 MPa, H2/CO ratio of 1.5 (v/v), GHSV = 4200 h−1). The characterization results indicate that CTAB has a great influence on the structure, composition, chemical state, and catalytic performance of the iron-based catalyst. Most interestingly, a greater amount of Mn promoter was found to be dispersed on the surface of α-Fe2O3, rather than being dissolved into the α-Fe2O3 lattice when CTAB was employed, which contributed towards enhancing the promotional effects of the Mn promoter, leading to the formation of certain surface-specific activity sites.

The design principle of iron-based catalysts for fischer-tropsch synthesis: from theory to practice

Fischer-Tropsch process coverts syngas (CO and H2 which can be derived from nature gas, coal and biomass, etc.) into clean liquid fuels and high valuable chemicals. In China, the so-called Coal to Liquid (CTL) via Fischer-Tropsch synthesis (FTS) is not only to achieve the clean/efficient usage of coal, but also to alleviate China’s excessive dependence on oil imports with important strategic significance to national security. We have been performing systematical studies on FTS for more than 20 years across from the fundamental research to R&D to industrial practice. The FTS in two 160000 ton/year demo plants has passed the long-term operation and technical verification (more than 8 years), and achieved a remarkable successful application of the world’s largest single-set of Coal-to-Oil industrial plant in Shenhua Ningmei with a production capacity of 4 million ton/year. The archives in industrial applications benefit from our solid fundamental studies. In the article, the corresponding fundamental researches on FTS will be focused. The rational design of Fe-based catalysts for FTS will be presented via combining experimental and theoretical insights. In details, starting with theoretical studies, the quantum mechanics approach is extensively used to study the active phases of Fe-based catalyst for FTS (across from detailed benchmarks on method to predication on morphology/surface to design new generation Fe-based industrial catalysts), as well as FT reaction mechanism. The purpose of theoretical study is to provide theoretical insights for experimental designs, and to further guide the development of new generation FTS catalysts. Finally, the new generation Fe-based catalysts are designed rationally from theoretical prediction to experiment confirmation. In the meantime, the new challenge on designing Fe-based catalysts will be discussed.

Design of ultra-active iron-based Fischer-Tropsch synthesis catalysts over spherical mesoporous carbon with developed porosity

Iron-based Fischer-Tropsch synthesis (FTS) has received renewable interests due to the gradual depletion of crude oil resources and its flexible product adjustment from lower olefins to long-chain hydrocarbons. However, one of the main challenges of iron-based FTS catalysts is the relative lower activity or productivity (per gram catalyst) at low or middle temperature. In this work, ultra-active iron-based FTS catalysts (Fe/SMC) were developed by loading iron over spherical mesoporous carbon (SMC) with huge porosity. The large pore volume (2.22 cm3/g) and high specific surface area (767 m2/g) of SMC allowed the high iron loading (up to 50 wt%) with proper dispersion. Weak C-Fe interaction was observed in the Fe/SMC catalysts, which was favorable for the reduction of iron and its further carburization to form more active sites of FTS. Activity tests showed that the Fe/SMC catalysts exhibited ultra-high activities in FTS. With the increase of iron loadings (30–50 wt%), the FTS rates (FTY, mmol CO/(h·g cat.)) of Fe/SMC catalysts increased due to the larger iron surfaces exposed at higher iron loadings. Small amount of K promoter significantly enhanced the FTY and apparent turnover frequency (TOF, S−1) of the Fe/SMC catalysts. The hydrocarbon productivity of K-promoted catalyst (40FeK/SMC) was up to 0.91 g HC/(h·g cat.) (260 °C, H2/CO = 1, 2 MPa), much higher than that of any other supported or unsupported iron-based FTS catalysts reported at similar reaction conditions. The results obtained in this work will open a new avenue for the development of highly active iron-based FTS catalysts.

Comparative Study of Iron-Based Fischer–Tropsch Synthesis Catalysts Promoted with Strontium or Potassium

The effect of strontium as a chemical promoter on iron-based Fischer–Tropsch synthesis (FTS) catalysts was investigated and compared with that of potassium. Strontium was chosen for study because of its relationship to potassium through the Diagonal relationship in the Periodic Table. The catalysts were characterized by N2 physisorption, X-ray diffraction, laser Raman spectroscopy, Mossbauer effect spectroscopy, H2/CO temperature programmed reduction and temperature programmed hydrogenation. FTS reaction was tested in a fixed-bed reactor. It was found that strontium and potassium strengthened Fe-O bonds of iron oxides species, which is not favorable for the reduction of the catalysts in H2. Both of them enhanced the reduction and carbonization of the catalysts in CO and syngas atmosphere, suppressed the hydrogenation of surface carbon species, however, strontium is less effective than potassium. Besides, strontium improved the dispersion of iron oxide. Strontium did not significantly affect the activity of FTS and water gas shift (WGS), but facilitated the oxidation of iron carbides to Fe3O4 during FTS reaction process, while potassium significantly improved the FTS and WGS activity and inhibited the oxidation of iron carbide. Both of strontium and potassium decreased the selectivity of methane while facilitated the formation of heavy hydrocarbons and olefin, whatever, strontium exhibited a weaker effect compared to potassium.

Effect of alkalis on iron-based Fischer-Tropsch synthesis catalysts: Alkali-FeOx interaction, reduction, and catalytic performance

Alkalis (Li, Na, K, Rb and Cs) promoted Fe/SiO2 catalysts were used to investigate the influences of alkalis on the alkali-FeOx interaction, reduction behavior, and catalytic performance of iron-based Fischer-Tropsch synthesis (FTS) catalysts. Laser Raman spectroscopy (LRS), Fourier transform infrared spectroscopy (FTIR), Mössbauer effect spectroscopy (MES), H2 temperature programmed reduction (H2-TPR), in situ X-ray photoelectron spectroscopy (in situ XPS) and in situ X-ray diffraction (in situ XRD) were employed to characterize the catalysts. It was found that alkalis had strong interactions with iron species in catalysts. In as-prepared catalysts, the FeO bonds are strengthened by alkalis, which consequently influenced the reduction of catalysts. Li and Na strongly restrained the reduction of all iron oxides in catalysts. K, Rb and Cs inhibited the initial reduction step (reduction of Fe2O3 to Fe3O4), while improved the reduction of iron oxides to FeO and metallic iron. Besides, alkalis facilitated the carbonization of catalysts and the improvement increased with increasing the atomic number of alkalis. In FTS reactions, Li and Na decreased the FTS activity while K, Rb and Cs largely increased the FTS activity of iron catalysts. The selectivities to methane and alkane were decreased while those to olefin and heavier hydrocarbons were increased by alkalis.

Effect of zeolite acidity and mesoporosity on the activity of Fischer–Tropsch Fe/ZSM-5 bifunctional catalysts

The effect of acidic properties and mesoporosity created by desilication on ZSM-5 zeolites was investigated to evaluate changes in product selectivity during Fischer–Tropsch synthesis (FTS) over Fe-based bifunctional catalysts. The reaction was studied at 300°C, 10bar, and H2:CO=2:1, using a physical mixture of the iron-based FTS catalyst and zeolites with different Si/Al ratio. Hierarchical zeolites with micro- and mesoporosity were prepared by alkaline treatment. Higher acidity (lower Si/Al ratio) in the pristine zeolite was found to be responsible for cracking of heavy hydrocarbons and the formation of heavier aromatic products, while higher Si/Al ratio induced a higher selectivity towards gasoline-range products (isoparaffins and olefins). The presence of mesopores prompted a variable behavior depending on the final acidity of the resulting Fe-based hierarchical catalyst; zeolites that retained acidity led to a slight increase in C2-C4 paraffins due to overcracking of heavy molecules as a result of enhanced transport to the acidic sites through the mesopores. The products obtained with the Na+-containing alkali-treated zeolites with no acidity were similar to those obtained with the iron FTS catalyst alone, with most of the products in the paraffin range. Rather than improving the activity and selectivity to gasoline products, the better accessibility in the hierarchically-based Fe/ZSM-5 catalysts coupled to the preserved acidity of the zeolite induced the formation of aromatics and a higher amount of small paraffins (<C4+). This reinforces the idea of optimizing the catalyst towards a desired FTS hydrocarbon range through the control of its mesoporosity and acidity. Iron-based ZSM-5 catalysts with high Si/Al ratios and irrespective of the presence of mesoporosity shift the product distribution towards the formation of isoparaffins and lighter aromatics.

Nanocrystalline Iron-Ore-Based Catalysts for Fischer-Tropsch Synthesis.

Nanocrystalline iron ore particles were fabricated by a wet-milling process using an Ultra Apex Mill, after which they were used as raw materials of iron-based catalysts for low-temperature Fischer-Tropsch synthesis (FTS) below 280 degrees C, which usually requires catalysts with a high surface area, a large pore volume, and a small crystallite size. The wet-milling process using the Ultra Apex Mill effectively destroyed the initial crystallite structure of the natural iron ores of several tens to hundreds of nanometers in size, resulting in the generation of nanocrystalline iron ore particles with a high surface area and a large pore volume. The iron-ore-based catalysts prepared from the nanocrystalline iron ore particles effectively catalyzed the low-temperature FTS, displaying a high CO conversion (about 90%) and good C5+ hydrocarbon productivity (about 0.22 g/g(cat)(-h)). This demonstrates the feasibility of using the iron-ore-based catalysts as inexpensive and disposable catalysts for the low-temperature FTS.

Development of Hybrid Fischer-Tropsch Synthesis Catalysts for Direct Production of Synthetic Gasoline from Coal-Based Syngas: An Indonesian Perspective

The Fischer-Tropsch Synthesis (FTS) represents an environmentally friendly method for producing liquid fuelfrom coal-based syngas via the hydrogenation of carbon monoxide. In order for such a process to be feasible,better catalysts that are capable of enhancing the reaction performance are required. In response to these needs,new catalysts were investigated and introduced in this work. The incorporation of zeolite into the iron based FTScatalyst was expected to help refine the hydrocarbon products and shift the product distribution from the typicalFTS product range to the middle iso-paraffins, which is a gasoline range, and eventually increase the yield of theliquid fuel. This study aims to develop catalyst for producing liquid fuel, particularly gasoline, from carbonmonoxide and hydrogen. The pH of the catalysts was found to have significant effect on the catalytic activity dueto its ability to control the amount of promoter to be precipitated in the catalyst, which results in a lowerreduction temperature. Physically mixing the iron based FTS catalyst with zeolite was found to have little effecton the catalytic activity and the product distribution, apart from slightly increasing the selectivity of iso-paraffins,which is the indication of isomerization activity. Coating of zeolite onto the iron based FTS catalyst to form acore-shell structure was intended to enhance the ease of migration of the reactant and thus increasing thecatalytic activity and shifting the product distribution towards the gasoline range. While zeolite shell has beensuccessfully coated uniformly on the iron based core using hydrothermal synthesis technique, the formation ofthick zeolite shell might have blocked the active FTS sites on the iron based catalyst to some extent and isbelieved to have contributed to the low activity of the core-shell catalyst.

Study on reduction and carburization behaviors of iron phases for iron-based Fischer–Tropsch synthesis catalyst

Reduction and carburization behaviors of iron phases over a precipitated iron-based Fischer–Tropsch synthesis (FTS) catalyst were investigated by some techniques of Mössbauer effect spectroscopy (MES), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) as well as H2&CO temperature-programmed desorption (H2&CO-TPD). It was found that in H2 atmosphere phase transformation of iron phases involved α-Fe2O3→Fe3O4→FeO→α-Fe, both occurring in the bulk and on the surface layers. All of reduced iron species took place the carburization reaction, whereas carburizing ability was following the order α-Fe>FeO>Fe3O4. During FTS both iron carbides and Fe(II) oxide species reached a balance state without appearing the intermediate α-Fe. The conversion of reduced iron phases to iron carbides (especially for χ-Fe5C2) on the surface layers played a positive role in promoting the formation of hydrocarbons species.

Effects of alkali on iron-based catalysts for Fischer-Tropsch synthesis: CO chemisorptions study

The CO hydrogenation reactions over un-promoted and alkali (Li, Na, K and Rb) promoted iron-based catalysts were studied by pulsing CO method. The pulse experiments indicate that alkali metals suppress the CO hydrogenation or conversion and the methane formation. Through analyzing the data, it is found that alkali promoters decrease the surface concentration of hydrogen, improve the CO adsorption and increase the retention time of carbon species on the catalyst surface in the turn of Li, Na, K, Rb. These promotional effects well explain the hydrocarbon product variation over different alkali promoted catalysts in Fischer-Tropsch synthesis.

Promotion Effects of Nitrogen Doping into Carbon Nanotubes on Supported Iron Fischer–Tropsch Catalysts for Lower Olefins

Fischer–Tropsch synthesis (FTS) is a classical topic of great significance because of the approach of post-petroleum times. For decades, people have attempted to develop iron-based FTS catalysts with high selectivity for lower olefins. By means of the anchoring effect and the intrinsic basicity of nitrogen-doped carbon nanotubes (NCNTs), iron nanoparticles were conveniently immobilized on NCNTs without surface premodification. The so-constructed Fe/NCNTs catalyst presents superb catalytic performance in FTS with high selectivity for lower olefins of up to 46.7% as well as high activity and stability. The excellent performance is well-correlated with enhanced dissociative CO adsorption, inhibition of secondary hydrogenation of lower olefins, and promoted formation of the active phase of χ-Fe5C2. All of these merits result from participation of the nitrogen, as revealed by our experimental characterization. These results may lead to a new strategy for exploring advanced FTS catalysts with abundant N-doped c...

Study on Reduction and Carburization Behaviors of Iron-based Fischer-Tropsch Synthesis Catalyst

Fischer-Tropsch synthesis (FTS), as an important technology for producing liquid fuels and chemicals from syngas derived from coal, natural gas, and biomass materials, is receiving a renewed interest for both industrial and academic applications. Transformation and adsorption behaviors of iron species during reduction and carburization over a precipitated Fe-based FTS catalyst were investigated systemically. The results indicated that phase transformation of iron species during reduction is following as α-Fe2O3 → Fe3O4 → FeO → α-Fe, whereas the transformation trend of iron phases during carburization follows as α-Fe2O3 →Fe3O4 → iron carbides. Carburization ability of reduced iron species is following the order α-Fe > FeO > Fe3O4. The conversion of FeO and α-Fe to iron carbides provides the active sites for FTS.

Effects of alkaline-earth metals on the structure, adsorption and catalytic behavior of iron-based Fischer–Tropsch synthesis catalysts

Effects of alkaline-earth metals (magnesium, calcium, strontium and barium) on the physico-chemical properties and Fischer–Tropsch synthesis (FTS) performances of precipitated Fe/SiO2 catalysts were investigated by N2 physisorption, laser Raman spectroscopy, FT-IR spectroscopy, in situ X-ray diffraction, Mössbauer effect spectroscopy, temperature programmed reduction or desorption and fixed-bed reactions. The characterization results indicated that alkaline-earth metals strengthened FeO bonds of Fe2O3 in as-prepared catalysts, which consequently inhibited the reduction of catalysts under H2 atmosphere. Mg and Ca suppress the carbonization of catalysts while Sr and Ba enhance the carbonization. The H2 adsorption was suppressed while the CO adsorption was enhanced by alkaline-earth metals. In the FTS reaction, alkaline-earth metals had no apparent influence on the CO conversion of catalysts, whereas they suppressed the formation of methane and enhanced the selectivities to olefin and higher molecular weight products. Among these alkaline-earth metals, Sr has the most pronounced influence on inhibiting the light hydrocarbon production. Furthermore, the increased Sr content can further restrain the formation of light hydrocarbons.