Iron Fischer-Tropsch Catalysts Research Articles

Effect of reaction temperature and H2/CO ratio on deactivation behavior of precipitated iron Fischer-Tropsch synthesis catalyst

In this paper, a precipitated iron Fischer-Tropsch catalyst was synthesized, which is similar to commercial CNFT-1 catalyst. The effect of reaction temperature and H2/CO feeding ratio on the deactivation behavior were throughly investigated. The results revealed that deactivation rate was significantly affected by the reaction temperature, which showed 15 times of deactivation rate constant operated at 290 ℃ than that at 270 ℃. However, deactivation rate constant had less difference with different H2/CO ratio from 2 to 5. Further characterization results demonstrated that the sintering and surface oxidation of iron carbide crystallite might be the main reason responsible for the deactivation when increased the reaction temperature. An opposite effect of iron carbide oxidizing inhibition and crystallite agglomeration was observed when increasing of H2/CO feeding ratio, which would lead to the slight deactivation of the precipitated iron catalyst.

Deactivation behavior investigation on commercial precipitated iron Fischer–Tropsch catalyst for long time reaction

Deactivation behavior investigation on commercial precipitated iron Fischer–Tropsch catalyst for long time reaction

Active phases for high temperature Fischer-Tropsch synthesis in the silica supported iron catalysts promoted with antimony and tin

Fischer-Tropsch synthesis provides an important opportunity for utilization of biomass and plastic waste. Iron catalysts are the catalysts of choice for light olefin synthesis using Fischer-Tropsch reaction. In this paper, we investigate strong promoting effects of antimony and tin on the catalytic performance of silica supported iron Fischer-Tropsch catalysts using a combination of advanced and in-situ techniques. The catalyst doping with these elements added via impregnation results in a major increase in the reaction rate and much better catalyst stability. No enhancement of iron dispersion was observed after the promotion, while somewhat higher extent of iron carbidization was observed in the antimony promoted catalysts. Iron-bismuth bimetallic nanoparticles are detected by several techniques. In the working catalysts, the promoters are located in close proximity to the iron nanoparticles. The promotion leads to the 7–10 times increase in the intrinsic activity of iron surface sites due to their interaction with the promoters.

Conversion of synthesis gas to aromatics at medium temperature with a fischer tropsch and ZSM-5 dual catalyst bed

The production of base chemicals such as olefins and aromatics from synthesis gas is of great interest from both an academic and an industrial point of view. We prepared a bulk iron Fischer Tropsch catalyst promoted with potassium capable of converting synthesis gas to olefins and combined it with an H-ZSM-5 zeolite to convert the formed olefins to aromatics on the zeolite acid sites. All experiments were performed in stacked bed mode to avoid migration of potassium promoters from the iron catalyst to the zeolite, which can deactivate acid sites in the zeolite. Operating at a pressure of 20 bar and temperatures between 250 °C and 300 °C instead of high temperatures of 300−350 °C led to a methane selectivity of only 7–14 %C. Olefins in the range of C4–C8 were converted to aromatics in the initial section of the zeolite bed, while further into the zeolite bed C2-C3 olefins oligomerized to longer olefins and alkylated light aromatics forming heavier aromatic hydrocarbons. Investigation of the influence of CO conversion on the aromatization of olefins showed a maximum in aromatic selectivity (excluding CO2) of 18 %C at medium CO conversion (XCO = 26–47 %). This work provides detailed insight into the chemical events that occur in bifunctional catalyst beds and can contribute to the development of an industrial process to convert synthesis gas to aromatics in a single reactor.

Mobility and versatility of the liquid bismuth promoter in the working iron catalysts for light olefin synthesis from syngas.

Liquid metals are a new emerging and rapidly growing class of materials and can be considered as efficient promoters and active phases for heterogeneous catalysts for sustainable processes. Because of low cost, high selectivity and flexibility, iron-based catalysts are the catalysts of choice for light olefin synthesis via Fischer-Tropsch reaction. Promotion of iron catalysts supported by carbon nanotubes with bismuth, which is liquid under the reaction conditions, results in a several fold increase in the reaction rate and in a much higher light olefin selectivity. In order to elucidate the spectacular enhancement of the catalytic performance, we conducted extensive in-depth characterization of the bismuth-promoted iron catalysts under the reacting gas and reaction temperatures by a combination of cutting-edge in situ techniques: in situ scanning transmission electron microscopy, near-atmospheric pressure X-ray photoelectron spectroscopy and in situ X-ray adsorption near edge structure. In situ scanning transmission electron microscopy conducted under atmospheric pressure of carbon monoxide at the temperature of catalyst activation showed iron sintering proceeding via the particle migration and coalescence mechanism. Catalyst activation in carbon monoxide and in syngas leads to liquid bismuth metallic species, which readily migrate over the catalyst surface with the formation of larger spherical bismuth droplets and iron-bismuth core-shell structures. In the working catalysts, during Fischer-Tropsch synthesis, metallic bismuth located at the interface of iron species undergoes continuous oxidation and reduction cycles, which facilitate carbon monoxide dissociation and result in the substantial increase in the reaction rate.

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 (

Effect of Support Pretreatment Temperature on the Performance of an Iron Fischer–Tropsch Catalyst Supported on Silica-Stabilized Alumina

The effect of support material pretreatment temperature, prior to adding the active phase and promoters, on Fischer–Tropsch activity and selectivity was explored. Four iron catalysts were prepared on silica-stabilized alumina (AlSi) supports pretreated at 700 °C, 900 °C, 1100 °C or 1200 °C. Addition of 5% silica to alumina made the AlSi material hydrothermally stable, which enabled the unusually high support pretreatment temperatures (>900 °C) to be studied. High-temperature dehydroxylation of the AlSi before impregnation greatly reduces FeO·Al2O3 surface spinel formation by removing most of the support-surface hydroxyl groups leading to more effectively carbided catalyst. The activity increases more than four-fold for the support calcined at elevated temperatures (1100–1200 °C) compared with traditional support calcination temperatures of <900 °C. This unique pretreatment also facilitates the formation of ε′-Fe2.2C rather than χ-Fe2.5C on the AlSi support, which shows an excellent correlation with catalyst productivity.

Influence of copper and potassium on the structure and carbidisation of supported iron catalysts for Fischer–Tropsch synthesis

In silica supported iron Fischer–Tropsch catalysts, promotion with copper strongly enhances both hematite reduction and magnetite carbidisation, while potassium promotion hinders reduction of hematite to magnetite but enhances magnetite carbidisation in carbon monoxide.

Effect of Drying Temperature on Iron Fischer-Tropsch Catalysts Prepared by Solvent Deficient Precipitation

A novel solvent deficient precipitation (SDP) method to produce nanoparticles was studied for its potential in Fischer-Tropsch synthesis (FTS) catalysis. Using Fe(NO3)3·9H2O as the iron-containing precursor, this method produces ferrihydrite particles which are then dried, calcined, reduced, and carbidized to form the active catalytic phase for FTS. Six different drying profiles, including final drying temperatures ranging between 80 and 150°C, were used to investigate the effect of ammonium nitrate (AN), a major by-product of reaction between Fe(NO3)3·9H2O and NH4HCO3 in the SDP method. Since AN has two phase-transitions within this range of drying temperatures, three different AN phases can exist during the drying of the catalyst precursors. These AN phases, along with physical changes occurring during the phase transitions, may affect the pore structure and the agglomeration of ferrihydrite crystallites, suggesting possible reasons for the observed differences in catalytic performance. Catalysts dried at 130°C showed the highest FTS rate and the lowest methane selectivity. In general, better catalytic performance is related to the AN phase present during drying as follows: phase III > phase II > phase I. However, within each AN phase, lower drying temperatures led to better catalytic properties.

Size and Promoter Effects in Supported Iron Fischer–Tropsch Catalysts: Insights from Experiment and Theory

The fundamentals of structure sensitivity and promoter effects in the Fischer–Tropsch synthesis of lower olefins have been studied. Steady state isotopic transient kinetic analysis, switching 12CO to 13CO and H2 to D2, was used to provide coverages and residence times for reactive species on supported iron carbide particles of 2–7 nm with and without promoters (Na + S). CO coverages appeared to be too low to be measured, suggesting dissociative adsorption of CO. Fitting of CH4 response curves revealed the presence of parallel side-pools of reacting carbon. CHx coverages decreased with increasing particle size, and this is rationalized by smaller particles having a higher number of highly active low coordination sites. It was also established that the turnover frequency increased with CHx coverage. To calculate H coverages, new equations were derived to fit HD response curves, again leading to a parallel side-pool model. The H coverages appeared to be lower for bigger particles. The H coverage was suppress...

Development of a chemical selective iron Fischer Tropsch catalyst

In the chemical industry C2–C4 olefins are critical building blocks for high value commodity products. One of the key challenges in Fischer Tropsch Synthesis (FTS) technology development is to manipulate process conditions and/or catalytic systems in order to suppress methane selectivity while still producing mainly C2–C4 products with maximum olefin content. Sasol developed an iron catalyst promoted with sodium and sulphur that yielded a fairly high proportion of C2–C4 products with lower than expected methane selectivity. Due to the unexpected low methane selectivity in combination with high olefin selectivity necessitated the evaluation of other promoters similar to sulphur in combination with alkali promoters (i.e. sodium and potassium) in order to understand whether the combination of sulphur and sodium is unique or whether other alkali and non-metal promoters will yield similar results or even improve the selectivity further. By employing high throughput experimentation, a total of 306 catalysts were synthesised and tested in order to identify whether other possible non-metal FTS promoters could either match or improve the selectivities observed for sodium and sulphur and to also identify potential optimum promoter levels for maximum C2–C4 olefin selectivity and minimum methane formation. The results showed that from the series of catalysts evaluated sulphur and sodium yielded the highest olefin selectivity in combination with low methane selectivity and that a mole ratio of at least 2:1 Na:S is required for optimum C2–C4 selectivity. Apart from the statistical analysis of data and subsequent comparison of catalyst selectivity, the catalysts were also evaluated based on the expected Schulz Flory selectivity correlation. For unique catalyst selectivity a significant shift should be observed. In this paper the results of high throughput experimentation are presented and discussed using the direct relations of C2–C4 selectivity and methane selectivity in order to distinguish unique catalyst systems that are selective towards C2–C4 olefin production.

Deposition of Iron Oxide Nanoparticles onto an Oxidic Support Using a Novel Gas-Expanded Liquid Process to Produce Functional Fischer–Tropsch Synthesis Catalysts

Gas expanded liquids (GXL) are solvent systems which enable the controlled deposition of presynthesized iron oxide nanoparticles, based on their size, onto a surface through the use of compressible gas such as CO2. By controlling the applied CO2 pressure, and hence the solvent strength, one can systematically deposit/precipitate nanoparticles of desired sizes. In this study, a technique using a GXL has been developed to controllably deposit iron oxide nanoparticles onto oxidic materials such as alumina and silica. The nanomaterials generated using this technique were then tested for their effectiveness as catalysts in Fischer–Tropsch (FT) synthesis in a fixed bed reactor system under standard low temperature FT conditions. The catalytic performance of these nanoscale iron FT catalysts was compared to control catalysts that were prepared by traditional incipient wetness methods using the same iron loading. Characterization of these GXL-synthesized materials demonstrated that the iron oxide nanoparticles we...

Iron Fischer-Tropsch Catalysts Prepared by Solvent-Deficient Precipitation (SDP): Effects of Washing, Promoter Addition Step, and Drying Temperature

A novel, solvent-deficient precipitation (SDP) method for catalyst preparation in general and for preparation of iron FT catalysts in particular is reported. Eight catalysts using a 23 factorial design of experiments to identify the key preparation variables were prepared. The catalysts were characterized by electron microprobe, N2 adsorption, TEM, XRD, and ICP. Results show that the morphology of the catalysts, i.e., surface area, pore volume, pore size distribution, crystallite sizes, and promoter distribution are significantly influenced by (1) whether or not the precursor catalyst is washed, (2) the promoter addition step, and (3) the drying condition (temperature). Consequently, the activity, selectivity, and stability of the catalysts determined from fixed-bed testing are also affected by these three variables. Unwashed catalysts prepared by a one-step method and dried at 100 °C produced the most active catalysts for FT synthesis. The catalysts of this study prepared by SDP compared favorably in activity, productivity, and stability with Fe FT catalysts reported in the literature. It is believed that this facile SDP approach has promise for development of future FT catalysts, and also offers a potential alternate route for the preparation of other catalysts for various other applications.

Microwave radiation in the modification of iron Fischer–Tropsch catalysts

Potassium-promoted unsupported iron Fischer–Tropsch catalysts have been subjected to microwave radiation treatment as part of the catalyst preparation process. The treatment is applied after the catalysts have been dried and calcined. Solid-state modifications occur which result in changes in the manner and quantity of carbon monoxide adsorbed on the surface as detected by temperature programmed surface reaction (TPSR) with hydrogen. Methane is the essential product of the TPSR runs. The response to microwave treatment is an increase in the methane production at all potassium levels, but the magnitude of the increase is potassium-loading dependent and peaks at a potassium loading of 0.7wt%. Microwave-induced changes in TPSR characteristics appear to result from selective heating occurring almost immediately on exposure of the catalysts to radiation. Longer duration of microwave exposure does not appear to result in further changes.

Fischer–Tropsch Mechanism: 13C18O Tracer Studies on a Ceria–Silica Supported Cobalt Catalyst and a Doubly Promoted Iron Catalyst

Tracer studies were performed on cobalt and iron Fischer–Tropsch catalysts using a synthesis gas containing a 20:80 mixture of 13C18O and 12C16O. The objective of the work was to investigate the antecedents of the C–O bonds in alcohols and CO2 formed during Fischer–Tropsch (FT) synthesis. It was found that chain growth proceeded by a CO insertion mechanism over both cobalt and iron catalysts. Over the cobalt catalyst, the dominant pathway for methanol synthesis involved a partial hydrogenation of CO as well as CO2 by a reaction pathway separate from the Fischer–Tropsch pathway. Over the iron catalyst, the majority of the methanol was formed by partial hydrogenation of only CO through the FT reaction pathway. Iron is active for water gas shift conversion, which produced CO2. Oxygen exchange reactions of CO2 were likely over both catalysts and complicated the interpretation of the results.

Preparation of an Unsupported Iron Fischer–Tropsch Catalyst by a Simple, Novel, Solvent-Deficient Precipitation (SDP) Method

Recent breakthroughs in the preparation of transition metal oxide nanoparticles by solvent-deficient precipitation (SDP) provide exciting new possibilities for simpler and faster methods to prepare heterogeneous catalysts with small uniformly sized crystallites of the active phase. In this study, the authors apply the SDP method to prepare an unsupported iron Fischer–Tropsch (FT) catalyst with a nominal composition of 100 Fe/5 Cu/4 K/16 SiO2 (relative mass) and an average crystallite size of 10–11 nm. Three preparations of this catalyst were made using SDP, and each was tested for over 200 h in a fixed-bed reactor to determine activity, selectivity, and stability. Values of the apparent first-order rate constants for syngas consumption at 260 °C were 215–251 mmol gFe–1 h–1 MPa–1 for our catalyst compared to 155–265 mmol gFe–1 h–1 MPa–1 for literature catalysts not using a proprietary pretreatment (Bukur, D. B.; Lang, X. Ind. Eng. Chem. Res. 1999, 38, 3270−3275). Our catalyst was tested for more than 500 h...

The application of inelastic neutron scattering to investigate a hydrogen pre-treatment stage of an iron Fischer–Tropsch catalyst

The effect of a hydrogen pre-treatment step on an iron based Fischer–Tropsch catalyst applied to the CO hydrogenation reaction at 623 and 723K has been investigated by inelastic neutron scattering (INS). The catalyst cannot support methane production over a 6h period of reaction at 623K but it is active at 723K. The INS spectrum indicates the presence of a predominantly aliphatic hydrocarbonaceous overlayer for the post-reaction 623K sample, whereas the spectrum for the 723K sample is assigned to an overlayer comprising partially hydrogenated polycyclic aromatic compounds. The hydrocarbonaceous overlayer associated with the active phase of the catalyst is modified by the hydrogen pre-treatment step. The INS spectra are interpreted with reference to microreactor studies, powder X-ray diffraction, temperature-programmed oxidation and transmission electron microscopy. The hydrogen pre-treatment stage favours the formation of amorphous carbon and is deemed not to convey any advantage with respect to CO hydrogenation activity.

Highly active and stable supported iron Fischer–Tropsch catalysts: Effects of support properties and SiO2 stabilizer on catalyst performance

The effects of support properties including pore size, hydroxyl group concentration, and support stabilizer were investigated for six alumina-supported FeCuK Fischer–Tropsch catalysts containing 20% or 40% iron. Catalysts were supported on one of four aluminas stabilized with La2O3 or SiO2. A large pore support was found to accommodate 40% Fe without pore blockage. Catalyst activity was found to increase with increasing support pore size, lower OH group concentrations, and higher extents of reduction. SiO2 was found to be a more effective hydrothermal stabilizer than La2O3 as it suppresses the high-temperature transformation of γ-Al2O3 to α-Al2O3 up to 1200°C allowing SiO2-stabilized catalysts to be dehydroxylated at higher temperatures. SiO2 stabilization also inhibits deactivation by sintering and increases activity, possibly due to surface silicate groups. Indeed, the activity of Fe/Si–Al2O3 continues to increase after 700h on stream, while the Fe/La–Al2O3 catalysts lose activity over the same time period.

Fischer–Tropsch Synthesis: Branched Paraffin Distribution for Potassium Promoted Iron Catalysts

Iron Fischer–Tropsch catalysts with varying amounts of K (0, 2, 4, 6, 10 wt%) were utilized. Except possibly for the 10 wt% catalyst, the amount of the iso-alkanes produced was essentially constant for carbon numbers from 4 to 23. Iso-alkanes accounted for about 13–15 % of the hydrocarbon products that were produced. The 2- and 3-methyl isomers were present at about two times the amount of the other methyl isomers for each carbon number. Mechanisms to account for the branching were considered but a satisfactory one could not be found.

Supported Iron Fischer–Tropsch Catalyst: Superior Activity and Stability Using a Thermally Stable Silica-Doped Alumina Support

Fischer–Tropsch synthesis (FTS) is a technically proven and economically viable route for the conversion of coal, biomass, and natural gas to hydrocarbon fuels. Although unsupported Fe catalysts are proven for FTS, they lack the physical strength and durability that would make them more viable for large-scale commercial reactors, and their activity is still significantly less than that of Co FT catalysts. In this work, we report on a very active and stable supported Fe FT catalyst that is more active than any supported Fe FT catalyst previously reported and competitive with the best unsupported catalysts. In addition, its productivity, which takes into account selectivity to desired hydrocarbon products, is also very competitive. More importantly, the catalyst is extremely stable, as evidenced by the fact that after 700 h on stream, its activity and productivity are still increasing. These catalyst properties result from using a novel γ-alumina support material doped with silica and pretreated at 1100 °C. This unique support has high pore volume, large pore diameter, and unusually high thermal stability. The ability to pretreat this support at 1100 °C enables preparation of a material having a low number of acid sites and weak metal oxide–support interactions, all desirable properties for an FT catalyst. These results demonstrate that the surface chemistry of the support material plays a crucial role in the design of an active and stable supported Fe FT catalyst.