Studies on the reaction mechanism of the Fischer–Tropsch synthesis on iron and cobalt

Acetylene and Ethylene Adsorption during Floating Fe Catalyst Formation at the Onset of Carbon Nanotube Growth and the Effect of Sulfur Poisoning: a DFT Study

Here, we investigated the adsorption of acetylene and ethylene on iron clusters and nanoparticles, which is a crucial aspect in the nascent phase of carbon nanotube growth by floating catalyst chemical vapor deposition (FCCVD). The effect of sulfur on adsorption was also studied due to its indispensable role in the process and its commonly known impact on metal catalyst poisoning. We performed systematic density functional theory (DFT) computations, considering numerous adsorption configurations and iron particles of various sizes (Fen, n = 3-10, 13, 55). We found that acetylene binds significantly more strongly than ethylene and prefers different adsorption sites. The presence of sulfur decreased the adsorption strength only in the immediate proximity of the adsorbate, suggesting that the effect of sulfur is mainly of steric origin while electronic effects play only a minor role. Higher sulfur coverage of the catalyst surface significantly weakened the binding of acetylene or ethylene. To further investigate this interaction, Bader's atoms in molecules (AIM) analysis and charge density difference (CDD) were used, which showed electron transfer from iron clusters or nanoparticles to the adsorbate molecules. The charge transfer exhibited a decreasing trend as sulfur coverage increased. These results can also contribute to the understanding of other iron-based catalytic processes involving hydrocarbons and sulfur, such as the Fischer-Tropsch synthesis.

Effect of Different Iron Phases of Fe/SiO2 Catalyst in CO2 Hydrogenation under Mild Conditions

The effect of different active phases of Fe/SiO2 catalyst on the physio-chemical properties and the catalytic performance in CO2 hydrogenation under mild conditions (at 220 °C under an ambient pressure) was comprehensively studied in this work. The Fe/SiO2 catalyst was prepared by an incipient wetness impregnation method. Hematite (Fe2O3) in the calcined Fe/SiO2 catalyst was activated by hydrogen, carbon monoxide, and hydrogen followed by carbon monoxide, to form a metallic iron (Fe/SiO2-h), an iron carbide (Fe/SiO2-c), and a combination of a metallic iron and an iron carbide (Fe/SiO2-hc), respectively. All activated catalysts were characterized by XRD, Raman spectroscopy, N2 adsorption–desorption, H2-TPR, CO-TPR, H2-TPD, CO2-TPD, CO-TPD, NH3-TPD, and tested in a CO2 hydrogenation reaction. The different phases of the Fe/SiO2 catalyst are formed by different activation procedures and different reducing agents (H2 and CO). Among three different activated catalysts, the Fe/SiO2-c provides the highest CO2 hydrogenation performance in terms of maximum CO2 conversion, as well as the greatest selectivity toward long-chain hydrocarbon products, with the highest chain growth probability of 0.7. This is owing to a better CO2 and CO adsorption ability and a greater acidity on the carbide form of the Fe/SiO2-c surface, which are essential properties of catalysts for polymerization in FTs.

Detailed Kinetic Modeling of CO2-Based Fischer–Tropsch Synthesis

The direct hydrogenation of CO2 to long-chain hydrocarbons, so called CO2-based Fischer–Tropsch synthesis (FTS), is a viable future production route for various hydrocarbons used in the chemical industry or fuel applications. The detailed modeling of the reactant consumption and product distribution is very important for further process improvements but has gained only limited attention so far. We adapted proven modeling approaches from the traditional FTS and developed a detailed kinetic model for the CO2-FTS based on experiments with an Fe based catalyst in a lab-scale tubular reactor. The model is based on a direct CO2 dissociation mechanism for the reverse water gas shift and the alkyl mechanism with an H-assisted CO dissociation step for the FTS. The model is able to predict the reactant consumption, as well as the hydrocarbon distribution, reliably within the experimental range studied (10 bar, 280–320 °C, 900–120,000 mLN h−1 g−1 and H2/CO2 molar inlet ratios of 2–4) and demonstrates the applicability of traditional FTS models for the CO2-based synthesis. Peculiarities of the fractions of individual hydrocarbon classes (1-alkenes, n-alkanes, and iso-alkenes) are accounted for with chain-length-dependent kinetic parameters for branching and dissociative desorption. However, the reliable modeling of class fractions for high carbon number products (>C12) remains a challenge not only from a modeling perspective but also from product collection and analysis.

In Situ Characterization of Mixtures of Linear and Branched Hydrocarbons Confined within Porous Media Using 2D DQF-COSY NMR Spectroscopy

The analysis of 1D anti-diagonal spectra from the projections of 2D double-quantum filtered correlation spectroscopy NMR spectra is presented for the determination of the compositions of liquid mixtures of linear and branched alkanes confined within porous media. These projected spectra do not include the effects of line broadening and therefore retain high-resolution information even in the presence of inhomogeneous magnetic fields as are commonly found in porous media. A partial least-square regression analysis is used to characterize the mixture compositions. Two case studies are considered. First, mixtures of 2-methyl alkanes and n-alkanes are investigated. It is shown that estimation of the mol % of branched species present was achieved with a root-mean-square error of prediction (RMSEP) of 1.4 mol %. Second, the quantification of multicomponent mixtures consisting of linear alkanes and 2-, 3-, and 4-monomethyl alkanes was considered. Discrimination of 2-methyl and linear alkanes from other branched isomers in the mixture was achieved, although discrimination between 3- and 4- monomethyl alkanes was not possible. Compositions of the linear alkane, 2-methyl alkane, and the total composition of 3- and 4-methyl alkanes were estimated with a RMSEP <3 mol %. The approach was then used to estimate the composition of the mixtures in terms of submolecular groups of CH₃CH₂, (CH₃)₂CH, and CH₂CH(CH₃)CH₂ present in the mixtures; a RMSEP <1 mol % was achieved for all groups. The ability to characterize the mixture compositions in terms of molecular subgroups allows the application of the method to characterize mixtures containing multimethyl alkanes. The motivation for this work is to develop a method for determining the mixture composition inside the catalyst pores during Fischer–Tropsch synthesis. However, the method reported is generic and can be applied to any system in which there is a need to characterize mixture compositions of linear and branched alkanes.

Direct conversion of CO2 to a jet fuel over CoFe alloy catalysts

The direct conversion of carbon dioxide (CO₂) using green hydrogen is a sustainable approach to jet fuel production. However, achieving a high level of performance remains a formidable challenge due to the inertness of CO₂ and its low activity for subsequent C–C bond formation. In this study, we prepared a Na-modified CoFe alloy catalyst using layered double-hydroxide precursors that directly transforms CO₂ to a jet fuel composed of C₈–C₁₆ jet-fuel-range hydrocarbons with very high selectivity. At a temperature of 240°C and pressure of 3 MPa, the catalyst achieves an unprecedentedly high C₈–C₁₆ selectivity of 63.5% with 10.2% CO₂ conversion and a low combined selectivity of less than 22% toward undesired CO and CH₄. Spectroscopic and computational studies show that the promotion of the coupling reaction between the carbon species and inhibition of the undesired CO₂ methanation occur mainly due to the utilization of the CoFe alloy structure and addition of the Na promoter. This study provides a viable technique for the highly selective synthesis of eco-friendly and carbon-neutral jet fuel from CO₂.

•

An alloy is developed for the direct CO₂ hydrogenation to jet-fuel-range hydrocarbons

•

The selectivity of the hydrocarbons (63.5%) exceeds the theoretical maximum value

•

The CoFe alloy is the active phase in the coupling reaction between surface carbons

•

The CoFe alloy is a highly efficient catalyst in the presence of a sodium promoter

Thermodynamic Equilibrium Analysis of Product Distribution in the Fischer-Tropsch Process Under Different Operating Conditions

Thermodynamic equilibrium analysis is necessary to provide a fundamental understanding of the distribution of the products formed in the Fischer-Tropsch process. The thermodynamic equilibrium distribution of the products formed at constant temperature and pressure was studied based on the minimization of the total Gibbs free energy of the system. The effects of temperature, pressure, and feed ratio on the product distribution were investigated under typical operating conditions. The distribution of the total products obtained from the reactions of added ethylene or ethanol was also studied. The results indicated that the products formed in a state of thermodynamic equilibrium follow Anderson-Schulz-Flory's general polymerization distribution at carbon numbers greater than about three. Both olefins and paraffins are primary products and there are essentially no alcohol and water at high degrees of conversion when the conditions for thermodynamic equilibrium are satisfied. The olefins formed in the Fischer-Tropsch process consist essentially of propylene. The product distribution is very sensitive to feed composition, and to temperature and pressure to a lesser extent. The product spectrum can be described broadly by the probability of chain growth relative to chain termination. This parameter decreases with increasing temperature, the feed ratio of hydrogen to carbon monoxide, and after the addition of ethanol to the feed, but increases with increasing pressure and after the addition of ethylene to the feed. An increase in reaction temperature results in a shift in selectivity towards low carbon number hydrocarbons and more hydrogenated products.

Effect of Co-Feeding Inorganic and Organic Molecules in the Fe and Co Catalyzed Fischer–Tropsch Synthesis: A Review

This short review makes it clear that after 90 years, the Fischer–Tropsch synthesis (FTS) process is still not well understood. While it is agreed that it is primarily a polymerization process, giving rise to a distribution of mainly olefins and paraffins; the mechanism by which this occurs on catalysts is still a subject of much debate. Many of the FT features, such as deactivation, product distributions, kinetics and mechanism, and equilibrium aspects of the FT processes are still subjects of controversy, regardless of the progress that has been made so far. The effect of molecules co-feeding in FTS on these features is the main focus of this study. This review looks at some of these areas and tries to throw some light on aspects of FTS since the inception of the idea to date with emphasis and recommendation made based on nitrogen, water, ammonia, and olefins co-feeding case studies.

A Dinuclear Mechanism Implicated in Controlled Carbene Polymerization

Carbene polymerization provides polyolefins that cannot be readily prepared from olefin monomers; however, controlled and living carbene polymerization has been a long-standing challenge. Here we report a new class of initiators, (π-allyl)palladium carboxylate dimers, which polymerize ethyl diazoacetate, a carbene precursor in a controlled and quasi-living manner, with nearly quantitative yields, degrees of polymerization >100, molecular weight dispersities 1.2-1.4, and well-defined, diversifiable chain ends. This method also provides block copolycarbenes that undergo microphase segregation. Experimental and theoretical mechanistic analysis supports a new dinuclear mechanism for this process.

Fischer–Tropsch: Product Selectivity–The Fingerprint of Synthetic Fuels

The bulk of the products that were synthesized from Fischer–Tropsch synthesis (FTS) is a wide range (C1–C70+) of hydrocarbons, primarily straight-chained paraffins. Additional hydrocarbon products, which can also be a majority, are linear olefins, specifically: 1-olefin, trans-2-olefin, and cis-2-olefin. Minor hydrocarbon products can include isomerized hydrocarbons, predominantly methyl-branched paraffin, cyclic hydrocarbons mainly derived from high-temperature FTS and internal olefins. Combined, these products provide 80–95% of the total products (excluding CO2) generated from syngas. A vast number of different oxygenated species, such as aldehydes, ketones, acids, and alcohols, are also embedded in this product range. These materials can be used to probe the FTS mechanism or to produce alternative chemicals. The purpose of this article is to compare the product selectivity over several FTS catalysts. Discussions center on typical product selectivity of commonly used catalysts, as well as some uncommon formulations that display selectivity anomalies. Reaction tests were conducted while using an isothermal continuously stirred tank reactor. Carbon mole percentages of CO that are converted to specific materials for Co, Fe, and Ru catalysts vary, but they depend on support type (especially with cobalt and ruthenium) and promoters (especially with iron). All three active metals produced linear alcohols as the major oxygenated product. In addition, only iron produced significant selectivities to acids, aldehydes, and ketones. Iron catalysts consistently produced the most isomerized products of the catalysts that were tested. Not only does product selectivity provide a fingerprint of the catalyst formulation, but it also points to a viable proposed mechanistic route.

Confined small-sized cobalt catalysts stimulate carbon-chain growth reversely by modifying ASF law of Fischer-Tropsch synthesis

Fischer–Tropsch synthesis (FTS) is a promising technology to convert syngas derived from non-petroleum-based resources to valuable chemicals or fuels. Selectively producing target products will bring great economic benefits, but unfortunately it is theoretically limited by Anderson–Schulz–Flory (ASF) law. Herein, we synthesize size-uniformed cobalt nanocrystals embedded into mesoporous SiO₂ supports, which is likely the structure of water-melon seeds inside pulps. We successfully tune the selectivity of products from diesel-range hydrocarbons (66.2%) to gasoline-range hydrocarbons (62.4%) by controlling the crystallite sizes of confined cobalt from 7.2 to 11.4 nm, and modify the ASF law. Generally, larger Co crystallites increase carbon-chain growth, producing heavier hydrocarbons. But here, we interestingly observe a reverse phenomenon: the uniformly small-sized cobalt crystallites can strongly adsorb active C* species, and the confined structure will inhibit aggregation of cobalt crystallites and escape of reaction intermediates in FTS, inducing the higher selectivity towards heavier hydrocarbons.

Fischer–Tropsch synthesis (FTS) is theoretically limited by Anderson–Schulz–Flory (ASF) law. Here, the authors successfully tune the selectivity of products from diesel-range hydrocarbons to gasoline-range hydrocarbons in FTS by controlling the crystallite sizes of confined cobalt, and modify the ASF law.

Status and prospects in higher alcohols synthesis from syngas

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H₂) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis-structure-function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer-Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer-Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO₂ and recycle side-products in the process are described in the third section.

On the nature of active phases and sites in CO and CO2 hydrogenation catalysts

Advanced characterisation techniques are shedding new light on the identification of active CO_x hydrogenation phases and sites.

Insight into the preferred formation mechanism of long-chain hydrocarbons in Fischer–Tropsch synthesis on Hcp Co(10−11) surfaces from DFT and microkinetic modeling

DFT calculations, together with microkinetic modeling, have been employed to probe into the preferred mechanism of hydrocarbon C–C chain growth on Co(10−11) surfaces during Fischer–Tropsch synthesis.

First-principles study of structure sensitivity of chain growth and selectivity in Fischer–Tropsch synthesis using HCP cobalt catalysts

Co (0001) prefers the CO insertion mechanism with high methane selectivity, but Co (101̄1) prefers the carbide mechanism with high C₂-hydrocarbon selectivity.

Development of kinetic model for CO hydrogenation reaction over supported Fe–Co–Mn catalyst

After determining CO consumption rate, the production rate of methane, paraffin, olefin and chain growth probability factor (α) was derived and described.

Activation Energies for Chain Growth Propagation and Termination in Fischer–Tropsch Synthesis on Iron Catalyst as a Function of Catalyst Particle Size

The influence of the catalyst particle size in determining Fischer–Tropsch synthesis (FTS) performance for nano-structured iron catalysts was investigated. The catalysts were prepared by a microemulsion method and to achieve a series of catalysts with different iron particle size, the water-to-surfactant molar ratio (W/S) in the microemulsion system varied from 4 to 12. The results demonstrate that by decreasing the levels of active phase of the iron catalyst, the termination rates for chain growth are increased compared to the propagation rates. In addition, the activation energy for chain propagation is lower than for chain termination, and this difference (Et – Ep) for the hydrocarbon product distributions which is characterised by α1, is lower than the hydrocarbon product distribution which is characterised by α2 The results indicate the H2 concentration on the catalyst surface is decreased by increasing the catalyst particle size. Thus, the dependence of α (α1, and/or α2) on H2 partial pressures is increased by decreasing of catalyst particle size and the dependence of α2 on H2 partial pressures is weaker than for α1.

New size-dependent kinetic equations for hydrocarbon production rates from Fischer–Tropsch synthesis on an iron-based catalyst

The effect of catalyst nanoparticle size on hydrocarbon production rates in the Fischer–Tropsch synthesis (FTS) has been investigated on an iron-based catalyst. A series of iron oxide particles was prepared via precipitation by a microemulsion method. New size-dependent kinetic models for hydrocarbon production rates were developed using a thermodynamic analysis method. Size-dependent parameters of the models were evaluated using the experimental results with a non-linear optimisation routine by minimising the mean absolute relative residual. Because of the role of iron carbide in the FTS reaction by iron catalysts, the iron carbide particle sizes was considered as an important factor in this paper. Experimental results show that the hydrocarbon product distribution shows a slight shift to lower molecular weight hydrocarbons by decreasing catalyst particle sizes and increasing the reaction temperature. The value of the surface tension energy (σ) for paraffin and olefin production on the iron catalyst is calculated in the range 1.2–0.9 J m–2 and 0.62–0.49 J m–2 respectively. These values are lower than for metals and are related to the presence of iron carbide in the catalytic reaction. Also, σ for paraffin production is higher than that for olefin production. The size- and carbon number-independent activation energies for paraffin and olefin formation are 9.1 and 14.9 kJ mol−1, respectively.

Insight into the mechanism about the initiation, growth and termination of the C–C chain in syngas conversion on the Co(0001) surface: a theoretical study

A mechanism is proposed for the initiation, growth and termination of the C–C chain from syngas on the Co(0001) surface. R represents hydrogen or an alkyl group.

Upgrading oxygenated Fischer-Tropsch derivatives and one-step direct synthesis of ethyl acetate from ethanol - examples of the desirability of research on simple chemical compounds transformations

Oxygenates formed as by-products of Fischer-Tropsch syntheses can be transformed into other Fischer-Tropsch derived oxygenates instead of treating them as unwanted chemicals. One-step direct synthesis of ethyl acetate from ethanol is feasible with the use of some heterogeneous catalysts. Despite their apparent simplicity, both transformations are discussed as targeted fields of research. Furthermore, the two concepts are justified due to the environmental protection. Arguments regarding the Fischer-Tropsch process are focused on the opportunities of the utilization of undesirable by-products. The effective striving for their utilization can make the oxygenates the targeted products of this process. Arguments regarding the one-step direct synthesis of ethyl acetate underline the environmental protection and sustainability as a less waste-generating method but, above all, highlight the possibility of reducing the glycerol overproduction problem. The production of ethyl acetate from bioethanol and then transesterification of fats and oils with the use of ethyl acetate allows managing all the renewable raw materials. Thus, the process enables the biosynthesis of biodiesel without glycerine by-product and potentially would result in the increase in the demand for ethyl acetate. Graphical Abstract.

Straightforward synthesis of bimetallic Co/Pt nanoparticles in ionic liquid: atomic rearrangement driven by reduction-sulfidation processes and Fischer-Tropsch catalysis

Unsupported bimetallic Co/Pt nanoparticles (NPs) of 4.4 ± 1.9 nm can be easily obtained by a simple reaction of [bis(cylopentadienyl)cobalt(ii)] and [tris(dibenzylideneacetone) bisplatinum(0)] complexes in 1-n-butyl-3-methylimidazolium hexafluorophosphate IL at 150 °C under hydrogen (10 bar) for 24 h. These bimetallic NPs display core-shell like structures in which mainly Pt composes the external shell and its concentration decreases in the inner-shells (CoPt3@Pt-like structure). XPS and EXAFS analyses show the restructuration of the metal composition at the NP surface when they are subjected to hydrogen and posterior H2S sulfidation, thus inducing the migration of Co atoms to the external shells of the bimetallic NPs. Furthermore, the isolated bimetallic NPs are active catalysts for the Fischer-Tropsch synthesis, with selectivity for naphtha products.

Mechanism and microkinetics of the Fischer-Tropsch reaction

The increasing availability of quantum-chemical data on surface reaction intermediates invites one to revisit unresolved mechanistic issues in heterogeneous catalysis. One such issue of particular current interest is the molecular basis of the Fischer-Tropsch reaction. Here we review current molecular understanding of this reaction that converts synthesis gas into longer hydrocarbons where we especially elucidate recent progress due to the contributions of computational catalysis. This perspective highlights the theoretical approach to heterogeneous catalysis that aims for kinetic prediction from quantum-chemical first principle data. Discussion of the Fischer-Tropsch reaction from this point of view is interesting because of the several mechanistic options available for this reaction. There are many proposals on the nature of the monomeric single C atom containing intermediate that is inserted into the growing hydrocarbon chain as well as on the nature of the growing hydrocarbon chain itself. Two dominant conflicting mechanistic proposals of the Fischer-Tropsch reaction that will be especially compared are the carbide mechanism and the CO insertion mechanism, which involve cleavage of the C-O bond of CO before incorporation of a CHx species into the growing hydrocarbon chain (the carbide mechanism) or after incorporation into the growing hydrocarbon chain (the CO insertion mechanism). The choice of a particular mechanism has important kinetic consequences. Since it is based on molecular information it also affects the structure sensitivity of this particular reaction and hence influences the choice of catalyst composition. We will show how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options. The paper will be concluded with a short perspective section dealing with the needs for future research. Many of the current key questions on the physical chemistry as well as computational study of heterogeneous catalysis relate to particular topics for further research on the fundamental aspects of Fischer-Tropsch catalysis.

First-principles elucidation of the surface chemistry of the C(2)H(x) (x = 0-6) adsorbate series on Fe(100)

Ab initio total-energy calculations of the elementary reaction steps leading to acetylene, ethylene and ethane formation and their decomposition on Fe(100) are described. Alongside the endothermicity of all the formation reactions, the crucial role played by adsorbed ethyl as main precursor towards both ethylene and ethane formation, characterises Fe(100) surface reactivity towards C(2)H(x) (x = 0-6) hydrocarbon formation in the low coverage limit. A comprehensive scheme based on three viable mechanisms towards ethyl formation on Fe(100), including methyl/methylene coupling, methyl/methylidyne coupling followed by one hydrogenation and methyl/carbon coupling followed by two hydrogenations, is the main result of this article.