Iron-based Fischer-Tropsch Catalysts Research Articles

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.

Organic-solvent assisted synthesis of highly dispersed iron based Fischer-Tropsch catalysts with MCF support: The effect of organic-solvent

The development of efficient catalysts for Fischer-Tropsch synthesis has attracted much attention in recent years. Mesoporous cellular silica foams (MCFs) supported iron-based Fischer-Tropsch catalysts are prepared via the organic solvent-assisted impregnation methods. The organic solvents (ethanol, acetic acid, glucose, and citric acid) have a great impact on both the structure and performance of the catalysts. In the case of catalyst Fe/MCF-E, obtained by ethanol-assisted impregnation, forming a highly dispersed ɛ-iron phase supported on the supports during relevant industrial FTS process, exhibited excellent stability and activity for 100 h. The Fe/MCF-E catalyst showed C2-C4 olefin selectivity of 30.9% and FTY (iron-time-yield) of 4.68 mol/(gFe· s)·10−5 at 270 °C, and achieved a total olefin selectivity of 48.6% and FTY of 38.59 mol/(gFe· s)·10−5 at 300 °C. These results highlight a convenient strategy to prepare highly stable and active MCF-supported iron based catalysts for FTS.

Development of an Iron-Based Fischer—Tropsch Catalyst with High Attrition Resistance and Stability for Industrial Application

In order to develop an iron-based catalyst with high attrition resistance and stability for Fischer–Tropsch synthesis (FTS), a series of experiments were carried out to investigate the effects of SiO2 and its hydroxyl content and a boron promoter on the attrition resistance and catalytic behavior of spray-dried precipitated Fe/Cu/K/SiO2 catalysts. The catalysts were characterized by means of N2 physisorption, nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman spectrum, X-ray photoelectron spectroscopy (XPS), H2-thermogravimetric analysis (H2-TGA), temperature-programmed reduction and hydrogenation (TPR and TPH), and scanning and transmission electron microscopy (SEM and TEM). The FTS performance of the catalysts was tested in a slurry-phase continuously stirred tank reactor (CSTR), while the attrition resistance study included a physical test with the standard method and a chemical attrition test under simulated reaction conditions. The results indicated that the increase in SiO2 content enhances catalysts’ attrition resistance and FTS stability, but decreases activity due to the suppression of further reduction of the catalysts. Moreover, the attrition resistance of the catalysts with the same silica content was greatly improved with an increase in hydroxyl number within silica sources, as well as the FTS activity and stability to some degree. Furthermore, the boron element was found to show remarkable promotion of FTS stability, and the promotion mechanism was discussed with regard to probable interactions between Fe and B, K and B, and SiO2 and B, etc. An optimized catalyst based on the results of this study was finalized, scaled up, and successfully applied in a megaton industrial slurry bubble FTS unit, exhibiting excellent FTS performance.

Spatially-resolved reaction profiles in Fischer-Tropsch synthesis – influence of operating conditions and promotion for iron-based catalysts

The complex reaction mechanism of iron-based Fischer-Tropsch synthesis comprises of manifold primary and secondary reaction pathways. Within this work, an experimental setup for spatially-resolved measurements on promoted iron-based Fischer-Tropsch catalysts is presented, allowing for a general study of the influence of operating conditions and promotion as well as of the influence of secondary reactions on product selectivity. It was found that, depending on catalyst composition, considerable hydrogenation of 1-alcohols and olefins occurs, while no considerable influence of reinitiation of chain growth on product distribution could be found.

Stabilizing the active phase of iron-based Fischer-Tropsch catalysts for lower olefins: mechanism and strategy.

Fischer-Tropsch synthesis of lower olefins (FTO) is a classical yet modern topic of great significance in which the supported Fe-based nanoparticles are the most promising catalysts. The performance deterioration of catalysts is a big challenge due to the instability of the nanosized active phase of iron carbides. Herein, by in situ mass spectrometry, theoretical analysis, and atmospheric- and high-pressure experimental examinations, we revealed the Ostwald-ripening-like growth mechanism of the active phase of iron carbides in FTO, which involves the cyclic formation-decomposition of iron carbonyl intermediates to transport iron species from small particles to large ones. Accordingly, by suppressing the formation of iron carbonyl species with a high-N-content carbon support, the size and structure of the active phase were regulated and stabilized, and durable iron-based catalysts were conveniently obtained with the highest selectivity for lower olefins up to 54.1%. This study provides a practical strategy for exploring advanced FTO catalysts.

Relationship between Iron Carbide Phases (ε-Fe2C, Fe7C3, and χ-Fe5C2) and Catalytic Performances of Fe/SiO2 Fischer–Tropsch Catalysts

The influence of different iron carbides on the activity and selectivity of iron-based Fischer–Tropsch catalysts has been studied. Different iron carbide phases are obtained by the pretreatment of a binary Fe/SiO2 model catalyst (prepared by coprecipitation method) to different gas atmospheres (syngas, CO, or H2). The phase structures, compositions, and particle sizes of the catalysts are characterized systematically by XRD, XAFS, MES, and TEM. It is found that in the syngas-treated catalyst only χ-Fe5C2 carbide is formed. In the CO-treated catalyst, Fe7C3 and χ-Fe5C2 with a bimodal particle size distribution are formed, while the H2-treated catalyst exhibits the bimodal size distributed e-Fe2C and χ-Fe5C2 after a Fischer–Tropsch synthesis (FTS) reaction. The intrinsic FTS activity is calculated and assigned to each corresponding iron carbide based on the phase composition and the particle size. It is identified that Fe7C3 has the highest intrinsic activity (TOF = 4.59 × 10–2 s–1) among the three candidat...

Investigating the potential of carbon dioxide utilization in a gas-to-liquids process with iron-based Fischer-Tropsch catalyst

A Gas-to-liquids (GTL) process with two different synthesis gas (syngas) production configurations including steam methane reformer (SMR) and auto-thermal reformer (ATR) was optimized to maximize wax production rate and carbon efficiency. The kinetic model used was the one given by Van Der Laan for an iron-based Fischer-Tropsch (FT) reactor (Van Der Laan and Beenackers, 2000). In this paper, Wang's correlation (Wang et al., 2003) was used to calculate chain growth probability (α). It was assumed that there exists a 300 MW coal-fired power plant with a downstream post-combustion CO2 capturing unit nearby the GTL plant and all the captured CO2 is available to be utilized in the GTL process. The optimization results suggested that the ATR- and SMR-based GTL processes could produce 68.17 and 101.4 tons/h of wax, respectively. Furthermore, about 166.4 tons/h of CO2 was optimally imported from the power plant to the SMR, while there was no potential for CO2 utilization in the ATR-based configuration. The GTL process with either SMR or ATR reformers is net CO2 emitter and respectively releases ca. 194.1 and 131.3 tons/h of CO2 to the atmosphere.

The Effect of Microwave Radiation on Manganese-promoted Iron-based Fischer-Tropsch Catalysts

The Effect of Microwave Radiation on Manganese-promoted Iron-based Fischer-Tropsch Catalysts

Efficient diesel production over the iron-based Fischer–Tropsch catalyst supported on CNTs treated by urea/NaOH

Due to the complex control of nitrogen (N) species in functionalizing the carbon nanotubes (CNTs), the contribution of N species to catalysis is difficult to understand. In this study, we report a facile one-pot approach for the synthesis of N-doped CNTs with N present only in the pyrrolic form using a hydrothermal method and by tailoring the treatment conditions, the N content could reach 2.3at.%. Then the N-doped CNTs are used as a support to prepare an iron-based Fischer–Tropsch synthesis (FTS) catalyst. The FTS activity and C5+ selectivity of N-doped CNTs supported iron catalyst increases with an increase of N content up to 2.1% but it decreases with higher pyrrolic N content. The enhanced FTS catalytic performance is attributed to increased electron conduction, electron donor effect and improved surface basicity due to the pyrrolic N incorporated into the carbon framework. Furthermore, a urea/NaOH co-treatment of CNTs is conducted and the treated CNTs is used as a support for preparing an iron catalyst. The results indicate that the synergistic modification effect of NaOH and urea effectively functionalizes the CNTs and leads to the incorporation of the pyrrolic N, to a high graphitization degree, to easy reduction, to a structural integrity and to a strong metal-support interaction of the supported iron catalyst. This results in an increased FTS activity, superior stability and remarkable diesel selectivity (52%). This synergistic effect shows the potential applications of carbon materials in the field of energy storage, carbon dioxide capture and catalysis, with appropriate functionalization.

Comparing a cobalt-based catalyst with iron-based catalysts for the Fischer-Tropsch XTL-process operating at high conversion

The XTL-process is a capital-intensive process. Novel process configurations need to be explored to increase the economic viability of this process. This is typically performed using the known dependencies of the catalyst as a function of the targeted process conditions. However, new process conditions may require catalysts with different characteristics compared to those currently utilised. Here, a set of cobalt- and iron-based Fischer-Tropsch catalysts are evaluated for their suitability in the application of a single-pass XTL-process operating at high conversion of synthesis gas. It is shown that cobalt-based catalysts of a suitable activity may be preferred over iron-based catalysts. The higher selectivity for methane for cobalt-based catalysts is offset by a lower selectivity for C2-C4 resulting in a favourable C5+-selectivity.

Iron-based Fischer–Tropsch synthesis of lower olefins: The nature of χ-Fe5C2 catalyst and why and how to introduce promoters

As a sustainable and short-flow process, iron-catalyzed direct conversion of CO-rich syngas to lower olefins without intermediate steps, i.e., Fischer–Tropsch-to-Olefins (FTO), has received increasing attention. However, its fundamental understanding is usually limited by the complex crystal phase composition in addition to the interferences of the promoter effects and inevitable catalyst deactivation. Until recently, the combination of multiple in-situ/ex-situ characterizations and theoretical studies has evidenced Hägg iron carbide (χ-Fe5C2) as the dominant active phase of iron-based Fischer–Tropsch catalysts. This perspective attempts to review and discuss some recent progresses on the nature of χ-Fe5C2 catalyst and the crucial effects of promoters on the FTO performance from theoretical and experimental viewpoints, aiming to provide new insights into the rational design of iron-based FTO catalysts.

Selectively forming light olefins via macroporous iron-based Fischer–Tropsch catalysts

A highly dispersed macroporous iron-based catalyst (Fe/S80-E) was developed by the modification of a macroporous silica (80 nm) surface with ethylene glycol before the impregnation of iron precursors. Due to higher diffusion efficiency and highly dispersed iron particles, this kind of catalyst presents superb catalytic performance in Fischer–Tropsch synthesis with high selectivity of light olefins (46.2 c-mol%) as well as high activity (CO conv. 63.4 %) without any promoters. The obtained catalysts were characterized by N2 physisorption, mercury intrusion porosimetry, XRD, TEM, XPS, TPR and in situ DRIFT.

Size and Promoter Effects on Stability of Carbon-Nanofiber-Supported Iron-Based Fischer-Tropsch Catalysts.

The Fischer–Tropsch Synthesis converts synthesis gas from alternative carbon resources, including natural gas, coal, and biomass, to hydrocarbons used as fuels or chemicals. In particular, iron-based catalysts at elevated temperatures favor the selective production of C2–C4 olefins, which are important building blocks for the chemical industry. Bulk iron catalysts (with promoters) were conventionally used, but these deactivate due to either phase transformation or carbon deposition resulting in disintegration of the catalyst particles. For supported iron catalysts, iron particle growth may result in loss of catalytic activity over time. In this work, the effects of promoters and particle size on the stability of supported iron nanoparticles (initial sizes of 3–9 nm) were investigated at industrially relevant conditions (340 °C, 20 bar, H2/CO = 1). Upon addition of sodium and sulfur promoters to iron nanoparticles supported on carbon nanofibers, initial catalytic activities were high, but substantial deactivation was observed over a period of 100 h. In situ Mössbauer spectroscopy revealed that after 20 h time-on-stream, promoted catalysts attained 100% carbidization, whereas for unpromoted catalysts, this was around 25%. In situ carbon deposition studies were carried out using a tapered element oscillating microbalance (TEOM). No carbon laydown was detected for the unpromoted catalysts, whereas for promoted catalysts, carbon deposition occurred mainly over the first 4 h and thus did not play a pivotal role in deactivation over 100 h. Instead, the loss of catalytic activity coincided with the increase in Fe particle size to 20–50 nm, thereby supporting the proposal that the loss of active Fe surface area was the main cause of deactivation.

A DFT perspective of potassium promotion of χ-Fe5C2(1 0 0)

Potassium is a well-known promoter for, amongst others, iron-based Fischer–Tropsch catalysts. Its effect is typically viewed in terms of the interaction of coadsorbed CO and zero-valent potassium, increasing the strength of CO adsorption with simultaneous weakening of the CO bond. However, it is known that potassium also stabilises the adsorption of oxygen, which needs to be taken into consideration to evaluate the effect of potassium in a working Fischer–Tropsch catalyst. The presence of potassium results in a strong increase in the strength of adsorption of atomic oxygen on χ-Fe5C2(100). The presence of atomic oxygen does not destabilise adsorbed CO at the coverage considered, and the strength of CO adsorption is significantly increased by the presence of coadsorbed K with or without coadsorbed atomic oxygen. The stabilisation of atomic oxygen due to the presence of coadsorbed potassium makes the removal of this species in the form of water under typical Fischer–Tropsch conditions difficult. Alternative pathways for the removal of atomic oxygen from the carbide surface are discussed.

Selectivity control of Cu promoted iron-based Fischer-Tropsch catalyst by tuning the oxidation state of Cu to mimic K

The effect of activation gas on the Fischer-Tropsch synthesis (FTS) performance of copper and potassium promoted precipitated iron-based catalysts was investigated in an earlier study [Catal. Lett. 144 (2014) 1624-1635]. In that study, CO and syngas (H2/CO=0.7) activation methods were tested after varying the copper promoter loading (0, 2, and 5%, atomic ratios relative to iron). At similar CO conversion level, the selectivities for the CO-activated catalysts were identical for the various copper loaded catalysts, whereas for the syngas activated catalysts, light hydrocarbon (methane and C2–C4) selectivities decreased and higher hydrocarbon (C5+) selectivities increased with increasing copper loading. To understand the reason for that behavior, the catalyst samples were characterized by XANES/EXAFS spectroscopy following each activation procedure before and after exposure to FTS conditions. Cu K-edge normalized XANES results indicate that for the syngas activated catalyst copper is present primarily in the Cu1+ state, whereas for CO activated catalyst the copper is more reduced (greater fraction in the Cu0 state). The Cu1+ in the syngas activated catalyst behaved like K1+; therefore, the C5+ selectivity was enhanced and light hydrocarbon selectivity was suppressed.

Hydrocarbon selectivity models for iron-based Fischer–Tropsch catalyst

Two kinetic models of Fischer–Tropsch product selectivity have been developed based on reaction networks from the literature. The models were fitted to experimental data obtained using commercial iron-based catalyst in a stirred tank slurry reactor and under a wide range of process conditions. Results showed that both of the rival models were able to provide a satisfactory prediction of the experimental product distribution for n-paraffin, 1- and 2-olefin. The simpler of the two models, a reaction network with a single type of active sites and solubility enhanced 1-olefin readsorption term, was chosen as more adequate for practical use.

Promoting χ-Fe5C2(100)0.25 with copper – a DFT study

The role of copper in iron based Fischer–Tropsch catalysts was investigated using DFT with χ-Fe5C2(100)0.25 as a model surface. The presence of atomic copper on the iron-rich χ-Fe5C2(100)0.25-surface is more favorable than its presence in surface. Nevertheless, the segregation of copper from the surface yielding fcc-Cu remains an exergonic process. Carbon monoxide at a coverage of 2.2 CO per nm2 stabilizes atomic copper on this surface. The presence of copper results in the redshift in the stretching frequency of adsorbed CO. The mobility of copper atoms was investigated on χ-Fe5C2(100)0.25 in the presence of CO. The hopping frequency is reduced due to the presence of CO, although never enough to avoid formation of fcc-Cu on a shorter time scale than typically required for the formation of hydrocarbons in the Fischer–Tropsch synthesis.

Synthesis and characterization of iron-based catalyst on mesoporous titania for photo-thermal F-T synthesis

In this paper, a series of iron-based Fischer–Tropsch catalysts were prepared with mesoporous titanium dioxide as catalyst carrier. And the F-T catalytic activities of iron supported on the mesoporous TiO2 with different doping concentrations were also investigated in photo-thermal driven Fischer–Tropsch synthesis. Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Low temperature N2 adsorption-desorption, X ray powder diffraction (XRD), Temperature Programmed Reduction (TPR) were used to characterize the catalysts, and the catalytic performance were also investigated for Photo-thermal driven Fischer–Tropsch synthesis under the condition of atmospheric pressure at the temperature of 220 °C, n (H2)/n (CO) = 2:1 and GHSV = 1500 h−1. It is shown that, for different doping concentration of iron, the conversion of CO, the selectivity of methane and the selectivity of C2+ changes, resulting in the difference performance of Photo-thermal Fischer Tropsch synthesis reaction.

Sample environment issues relevant to the acquisition of inelastic neutron scattering measurements of heterogeneous catalyst samples

Refinements to established sample handling issues connected with the acquisition of inelastic neutron scattering (INS) spectra of heterogeneous catalyst samples are described. This involves modification of a reactor and improvements to gas handling apparatus. The latter includes provision of laboratory space and ventilation facilities that are suited to the increasingly wide range of catalytic samples being examined. INS measurements for CO hydrogenation over an iron-based Fischer-Tropsch catalyst recorded using the MARI spectrometer are presented to illustrate the applicability of the new arrangements.

An optimized simulation model for iron-based Fischer–Tropsch catalyst design: Transfer limitations as functions of operating and design conditions

Transfer limitations were successfully modeled and optimized for iron-based catalysts for Fischer–Tropsch synthesis. The simulation model predicts the effect of changing reaction temperatures, reaction pressures, catalyst pellet size, and the feed CO composition on pore diffusion, film heat transfer, internal heat transfer, and pressure drop. The comprehensive contour maps obtained from the model quantitatively display the effects of these various design variables to both optimize catalyst design and provide guidance for kinetic experiments. The optimization results favor higher reaction temperatures and pressures, smaller pellet sizes, and lower feed CO compositions to maintain high activity of kinetically-controlled reaction rates. The optimal temperature (255.8°C) was constrained by the rate of catalytic deactivation. The model was validated by experimental data acquired from a fixed-bed reactor and shows excellent agreement. The model predicts the observed rate to be 79% of the intrinsic rate at 250°C, 20bar, equimolar H2:CO, and 425μm pellet size, while experimental results showed this percentage was 74±7% for 250–600μm pellets. The model predicts no pore-diffusion limitations at pellet sizes smaller than 250μm, indicating that the reaction rate is kinetically-controlled. Furthermore, the resistance due to film temperature gradients is more limiting than that due to intraparticle temperature gradients. Finally, pressure drop was well below 10% of the inlet reactor pressure under laboratory-scale conditions. The model was used to predict the effect of using smaller catalyst pellets on pressure drop for a commercial-scale reactor, which showed that acceptable operation could be expected with a pressure drop of 20% of the inlet reactor pressure.