Cobalt Catalyzed Fischer Tropsch Research Articles

Mechanistic Insights into the Effect of Sulfur on the Selectivity of Cobalt-Catalyzed Fischer–Tropsch Synthesis: A DFT Study

Sulfur is a common poison for cobalt-catalyzed Fischer–Tropsch Synthesis (FTS). Although its effects on catalytic activity are well documented, its effects on selectivity are controversial. Here, we investigated the effects of sulfur-covered cobalt surfaces on the selectivity of FTS using density functional theory (DFT) calculations. Our results indicated that sulfur on the surface of Co(111) resulted in a significant decrease in the adsorption energies of CO, HCO and acetylene, while the binding of H and CH species were not significantly affected. These findings indicate that sulfur increased the surface H/CO coverage ratio while inhibiting the adsorption of carbon chains. The elementary reactions of H-assisted CO dissociation, carbon and oxygen hydrogenation and CH coupling were also investigated on both clean and sulfur-covered Co(111). The results indicated that sulfur decreased the activation barriers for carbon and oxygen hydrogenation, while increasing the barriers for CO dissociation and CH coupling. Combining the results on elementary reactions with the modification of adsorption energies, we concluded that the intrinsic effect of sulfur on the selectivity of cobalt-catalyzed FTS is to increase the selectivity to methane and saturated short-chain hydrocarbons, while decreasing the selectivity to olefins and long-chain hydrocarbons.

Insight into the Intrinsic Active Site for Selective Production of Light Olefins in Cobalt-Catalyzed Fischer–Tropsch Synthesis

Selective production of light olefins remains a great challenge in cobalt-catalyzed Fischer–Tropsch synthesis (FTS). Understanding the nature of the active site and the structure–performance relati...

Accumulation of liquid hydrocarbons during cobalt-catalyzed Fischer–Tropsch synthesis - influence of activity and chain growth probability

It may take over one year in order to fill FT catalyst pores, depending on activity and chain growth probability.

Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer-Tropsch Reaction: Evidence of a Two-Site Reaction Model.

One of the well-known observations in the Fischer–Tropsch (FT) reaction is that the CH4 selectivity for cobalt catalysts is always higher than the value expected on the basis of the Anderson–Schulz–Flory (ASF) distribution. Depositing graphitic carbon on a cobalt catalyst strongly suppresses this non-ASF CH4, while the formation of higher hydrocarbons is much less affected. Carbon was laid down on the cobalt catalyst via the Boudouard reaction. We provide evidence that the amorphous carbon does not influence the FT reaction, as it can be easily hydrogenated under reaction conditions. Graphitic carbon is rapidly formed and cannot be removed. This unreactive form of carbon is located on terrace sites and mainly decreases the CO conversion by limiting CH4 formation. Despite nearly unchanged higher hydrocarbon yield, the presence of graphitic carbon enhances the chain-growth probability and strongly suppresses olefin hydrogenation. We demonstrate that graphitic carbon will slowly deposit on the cobalt catalysts during CO hydrogenation, thereby influencing CO conversion and the FT product distribution in a way similar to that for predeposited graphitic carbon. We also demonstrate that the buildup of graphitic carbon by 13CO increases the rate of C–C coupling during the 12C3H6 hydrogenation reaction, whose products follow an ASF-type product distribution of the FT reaction. We explain these results by a two-site model on the basis of insights into structure sensitivity of the underlying reaction steps in the FT mechanism: carbon formed on step-edge sites is involved in chain growth or can migrate to terrace sites, where it is rapidly hydrogenated to CH4. The primary olefinic FT products are predominantly hydrogenated on terrace sites. Covering the terraces by graphitic carbon increases the residence time of CHx intermediates, in line with decreased CH4 selectivity and increased chain-growth rate.

Insight into the structure and morphology of Run clusters on Co(111) and Co(311) surfaces

The adsorption configurations, growth modes and morphology of a Ru promoter under the approximate conditions of cobalt catalyzed Fischer–Tropsch synthesis (FTS) were investigated by density functional theory (DFT) calculations.

Accumulation of liquid hydrocarbons in catalyst pores during cobalt-catalyzed Fischer–Tropsch synthesis

Formation, vaporization and accumulation of liquid hydrocarbon products during FTS in a single catalyst pore.

Interplay of reaction and pore diffusion during cobalt-catalyzed Fischer–Tropsch synthesis with CO2-rich syngas

For cobalt-catalyzed Fischer–Tropsch synthesis (FTS), a model was developed to analyze numerically the reaction–diffusion performance in catalyst particles. The model is based on a mesoporous particle and is valid for CO2-free and -rich syngas. The kinetic parameters of all three relevant reactions, CO hydrogenation to C2+ hydrocarbons as well as CH4 and CO2 hydrogenation (mainly) to CH4, were derived at intrinsic conditions (dp=150μm). Thereafter, the effective kinetics considering pore diffusion limitations were derived by using a homemade catalyst in 5mm diameter. From the data measured, a kinetic approach for conversion of CO2 inside the catalyst particle was developed. The experimentally derived Langmuir–Hinshelwood type rate expressions and a variable chain growth parameter α, dependent on temperature and syngas ratio, were implemented into the model. Furthermore, the change of the local reaction rates and selectivities, as a consequence of changing syngas ratio due to diffusion limitation is taken into account. The simulated data of effective kinetics and selectivities are in agreement with the measured data. The simulation predicts that CO2 is only converted in the CO-free core region of large catalyst particles at high temperatures and strong pore diffusion limitations. For CO2 converts mainly to CH4 (selectivity 95%C), a slightly increased overall methane selectivity is expected indicating consumption of CO2. However, this effect was not measurable even with 5mm particles at high temperatures as methanation of CO2 occurs only to a minor extent even at pronounced diffusion limited conditions and is negligible at industrial conditions.

In Situ Scanning Tunneling Microscopy of the Dissociation of CO on Co(0001)

The dissociation of CO, a crucial step in the cobalt-catalyzed Fischer–Tropsch synthesis, was investigated by in situ scanning tunneling microscopy on Co(0001) at CO pressures up to 0.25 mbar and temperatures up to 493 K. A new type of surface reconstruction was observed that is formed by carbidic carbon, as shown by X-ray photoelectron spectroscopy. Using 13CO and titrating 13C with O2, the surface carbon is shown to originate from CO. The results are in contrast to predictions by theoretical work. However, the dissociation rates are 3 orders of magnitude lower than the previously measured methanation rates under equivalent conditions.

Cobalt-Catalyzed Fischer–Tropsch Synthesis: Chemical Nature of the Oxide Support as a Performance Descriptor

The effect of the chemical nature of the oxide support on the performance of cobalt Fischer–Tropsch catalysts is investigated. A series of supports is synthesized via monolayer coverage of porous γ-Al2O3 with various oxides representative of a wide range of Lewis acid–base character, as quantified by UV–vis spectroscopy coupled to alizarin adsorption. Incorporation of cobalt (20 wt %) results in model catalysts with identical porosities and similar Co particle sizes (>10 nm), allowing the study of support effects without overlap from diffusional or particle size factors. Under realistic reaction conditions, the initial TOF scales with the acidity of the oxide support, whereas the cobalt time yield and selectivity to industrially relevant C13+ hydrocarbons show a volcano dependence, with a maximum at an intermediate acid–base character. As inferred from in situ CO-FTIR, “selective” blockage of a few cobalt sites, though crucial for CO hydrogenation, by atoms from basic oxides and “unselective” site blockag...

Modeling of Multitubular Reactors for Iron- and Cobalt-Catalyzed Fischer-Tropsch Syntheses for Application in a Power-to-Liquid Process

Two-dimensional reactor modeling for iron- and cobalt-catalyzed Fischer–Tropsch syntheses (FTS) is shown within this work. The modeling includes the intrinsic and effective kinetics of the FT-reaction and side reactions such as methanation and water–gas shift (WGS), that is, the influence of pore diffusion was considered for both catalysts (Fe, Co). A parametric study was used to identify the most important parameters for designing a technical multitubular FT reactor; which include superficial velocity, tube diameter, and cooling temperature. Based on these considerations, optimized case simulations of a single reactor tube were performed to calculate the catalyst performance; the cobalt catalyst showed a productivity of 0.17 kg kgcat−1 h−1 compared to 0.12 kg kgcat−1 h−1 obtained with iron. Moreover, basic data for an industrial power-to-liquid (PTL) plant are outlined. An iron-based plant can produce 75 kt per year using a reactor with approximately 2600 single tubes with a diameters of 6 cm. In contrast, for the cobalt-catalyzed reaction, the same production volume requires a reactor with 8000 single tubes (of 3 cm diameter).

Cobalt Catalyzed Fischer–Tropsch Synthesis: Perspectives Opened by First Principles Calculations

Starting from an overview of general concepts in Fischer–Tropsch synthesis, this perspective article focuses on relevant theoretical works based on density functional theory (DFT) dealing with cobalt based Fischer–Tropsch (FT) catalysis. Our bibliographic analysis highlights key scientific questions in a challenging field for research and industry. Starting from the historical and empirical concepts, we discuss how recent DFT studies give insights into the catalyst structure, the FT reaction mechanisms and the deactivation pathways. There is still no clear consensus within the scientific community about the FT reaction mechanism. Some authors propose that the first elementary step in the FT synthesis consists on CO dissociation taking place at the steps of the cobalt active phase (carbide mechanism) while some others suggest that this molecule is hydrogenated prior to its dissociation no matter the structure of the active site (insertion mechanism). As for the crucial deactivation problem, according to the computed thermodynamic properties presented by some publications, the catalyst oxidation does not seem to play a major role in its deactivation process. By contrast, the interaction of C addatoms with the Co active phase can have a strong influence on the catalyst reactivity and selectivity, through a series of structural rearrangements: the formation of various more or less unsaturated hydrocarbon molecules (such as polyaromatic rings) and the formation of a surface carbide phase. However, the impact of these phenomena on the reactivity and selectivity of the catalyst remain to be investigated in the future.

Deuterium tracer studies on cobalt catalyzed Fischer–Tropsch synthesis: Addition of alcohols

Unlike the iron catalyzed Fischer–Tropsch (FT) synthesis, the additions of ethanol-d 6 and 2-propanol-d 8 during the Co/SiO 2 catalyzed FT reactions did not produce detectable amount of deuterated isomer in any alkane. However, when the FT reactions were conducted over a mixture of FT catalyst Co/SiO 2 and dehydration catalyst Al 2O 3, the d 1, d 2 and d 3 isomers of alkanes from C 7 to C 15 were obtained with a nearly constant D/molecule value of 1.4 when ethanol-d 6 was used as the tracer while d 1–d 6 isotopic isomers of alkanes were observed with a nearly constant D/molecule value of 2.3 when 2-propanol-d 8 was used. The similar results were obtained when 1-propanol was used as the tracer. These results lead us to conclude that the C 2 or C 3 species that initiates the chain growth in the FT reaction resembles ethene or propene. The pathways for the formations of ethylidene and propylidene were proposed.

Inverse kinetic isotope effects and deuterium enrichment as a function of carbon number during formation of C–C bonds in cobalt catalyzed Fischer–Tropsch synthesis

How the deuterium is enriched in hydrocarbons produced by Fischer–Tropsch (FT) synthesis is of broad scientific interest, particularly to those studying the mechanism of the FT reaction and the formation of hydrocarbons in the Earth's crust or in the nebula. In order to determine whether there is an inverse isotope effect and whether there exists the deuterium enrichment in hydrocarbons during the FT reaction, we performed the H 2/D 2 switching experiments in cobalt catalyzed FT reactions and found that the values of α H/ α D are in a range of 0.93–0.98 under five different reaction conditions, a clear indication of the inverse kinetic isotope effect. We also conducted the H 2/D 2 competition experiments using an equal molar ratio of D 2 and H 2 as the reagents and found that the H/D ratios in alkanes from C 6 to C 24 obtained are less than 1 and decrease with increasing molecular size. To account for these results, we proposed a modified alkylidene mechanism for the FT reaction. By combining the modified alkylidene mechanism with the Anderson–Schulz–Flory equation, the results obtained in this study can be quantitatively explained and the isotope fractionation patterns and deuterium enrichment observed in abiogenic hydrocarbons produced by the FT-type reactions can be predicted.

Deactivation of cobalt based Fischer–Tropsch catalysts: A review

To trace the origin of catalyst deactivation is in many cases difficult. It is usually a complex problem where several mechanisms contribute to the loss of activity/selectivity. Low temperature Fischer–Tropsch synthesis (FTS) is a three phase system having a wide range of products and intermediates. Additionally, high partial pressures of steam will arise during reaction. Thus, the chemical environment in the Fischer–Tropsch synthesis reactor encompasses a large number of interacting species which may negatively affect catalytic activity. Furthermore, it is an exothermic reaction and local overheating might occur. Utilization of the produced heat is crucial and the choice of the reactor should be done with respect to the catalyst stability properties. Catalyst deactivation in the Fischer–Tropsch reaction has been a topic of industrial as well as academic interest for many years. The main causes of catalyst deactivation in cobalt based FTS as they appear in the literature are poisoning, re-oxidation of cobalt active sites, formation of surface carbon species, carbidization, surface reconstruction, sintering of cobalt crystallites, metal–support solid state reactions and attrition. The present study focuses on cobalt catalyzed Fischer–Tropsch synthesis. The various deactivation routes are reviewed, categorized and presented with respect to the most recent literature.

Middle distillates from hydrocracking of FT waxes: Composition, characteristics and emission properties

A light cobalt catalyzed Fischer–Tropsch (FT) wax was subjected to hydrocracking in the range of temperature 319–351 °C and hydrogen pressure between 3.5 and 6.0 MPa. The catalyst used was platinum on amorphous silica–alumina. Hydrocracking reaction led to an increase of middle distillate yield up to 85% with a contemporary increase of iso-paraffins concentration which resulted in a remarkable improvement of cold flow properties of the products. The freezing point of C 10–C 14 fraction passed from −23 to −45 °C while the pour point of C 15–C 22 fraction decreased from 13 to −23 °C. The latter fraction displayed high cetane numbers ranging between 75 and 80. Changes in carbon distribution and molecular structure of products during hydrocracking have been rationalized in the light of the accepted hydrocracking mechanism where n-paraffins undergo to consecutive isomerization reactions leading to isomers with progressively higher branching degree and concomitant cracking reaction. Experimental evidences support the view that apparent reactivity of n-paraffins is chain length dependent, increasing with the molecular weight. Detailed characterization by NMR and GC showed that branching groups abundance in the middle distillate products was the following: methyl ≫ ethyl > propyl. Emission tests carried out with FT diesel and commercial ultra low sulfur diesel showed that FT diesel has excellent combustion properties and leads to a reduction of emissions.

Deuterium tracer study of pressure effect on product distribution in the cobalt-catalyzed Fischer–Tropsch synthesis

The pressure effect on the product distribution in the cobalt-catalyzed Fischer–Tropsch synthesis was investigated using a deuterium/hydrogen switching method with a fixed bed reactor. Reliable results were obtained at relatively high reactant conversion. On increasing the operating pressure from 0.2 to 2 MPa, the chain growth probability increased from 0.83 to 0.90, while the selectivity for methane and branched products decreased. Moreover, the obtained paraffin/olefin ratio increased with an increase in the carbon number, but was nearly constant at higher pressure. This study shows that higher chain growth probability and more olefins could be obtained under elevated operating pressure.

The effect of surface-active carbon on hydrocarbon selectivity in the cobalt-catalyzed Fischer–Tropsch synthesis

Carbon isotope transients ( 12CO→ 13CO) at reaction steady state were used to correlate the in situ surface inventories of active carbon species with product selectivity variations in the Fischer–Tropsch synthesis. The measurements were conducted on an unsupported cobalt catalyst at a variety of elevated pressure conditions (syngas pressures from 3 to 15 atm, H 2:CO ratios from 1 to 4, temperatures from 190 to 228 °C, and cofed water vapor pressures from 0 to 8 atm). The in situ adsorbed CO inventory is saturated and insensitive to the changes in reaction conditions, except for a small decrease in the presence of elevated water pressures. A large amount of active carbon intermediates is present as a monomeric species on the catalyst, and this amount varies widely with reaction conditions (adsorbed C ∗/ CO values from 0.3 to 0.8). Changes in active carbon inventory cause proportional changes in the polymerization probability at all carbon numbers. For the most part, carbon monoxide and water influence the polymerization probability indirectly by causing changes in the monomer C ∗ amounts. Hydrogen and temperature do not cause significant changes in monomeric carbon, but influence the rate coefficients directly. The influences of gas composition on the rate coefficients provide useful information regarding the site requirements for chain growth.

Support and rhenium effects on the intrinsic site activity and methane selectivity of cobalt Fischer–Tropsch catalysts

Catalyst composition effects on site activity, active carbon coverage, and methane selectivity in the cobalt-catalyzed Fischer–Tropsch synthesis were examined using 12CO→ 13CO isotope transients at reaction steady state. All catalysts (18 total) were tested at identical conditions: 221 °C, 6.5 atm of H 2/CO/Ne=2/1/1. The intrinsic site activity of unsupported Co and Co-supported on SiO 2, TiO 2, and Al 2O 3 (Co dispersion < 10% ) varies within a relatively narrow band ( ∼ 2x maximum rate difference). Re present at a Re/Co weight ratio of 0.1 does not affect Fischer–Tropsch activity and selectivity. Since its own activity is 20 times lower than that of Co, Re does not appear to block or modify active Co sites. While Re and the above-noted three common oxide supports do not affect the Fischer–Tropsch activity and selectivity of the relatively large cobalt crystallites, some modifications of SiO 2 and Al 2O 3 supports (with MgO, Y 2O 3, ZnO, and Ce 2O 3, by coating and coprecipitation) reduce site activity by up to an order of magnitude. Interestingly, the reduced site activity does not measurably affect methane selectivity. For all catalysts, the observed site-activity differences, whether small or large, do not correlate with bulk composition, and are possibly the result of unidentified impurities introduced during catalyst preparation. These impurities affect the reactivity of both the adsorbed CO and the active carbon present on the catalyst: higher site activity correlates with higher active carbon reactivity. Although this correlation is strong, some variability in the CO/C ∗ reactivity ratio does occur. This variability appears to play a key role in methane selectivity, namely, higher values correlate with lower methane selectivity. These observations can be rationalized by our previously published simple kinetic model.

The Effect of Water on the Cobalt-Catalyzed Fischer–Tropsch Synthesis

Carbon isotope transients at reaction steady state are used to examine the effect of water vapor on the amount and reactivity of the surface carbon intermediates involved in the Fischer–Tropsch synthesis on both a supported and an unsupported cobalt catalyst. Water increases the amount of active surface carbon, present predominantly as monomeric species. This increased surface concentration of monomeric carbon is caused by an acceleration of the CO dissociation rate without a matching reactivity increase in the downstream hydrocarbon synthesis steps. In turn, the proposed monomer dependencies in the Fischer–Tropsch synthesis mechanism explain the lower methane selectivity and higher molecular weight products observed at increased water concentrations.

Interpretation and kinetic modeling of product distributions of cobalt catalyzed Fischer–Tropsch synthesis

Carbon number distributions of Fischer–Tropsch products on iron and cobalt catalysts show deviations from the ideal Anderson–Schulz–Flory (ASF) distribution. For products obtained on cobalt catalysts these deviations are traced back by many authors to re-adsorption and incorporation of 1-alkenes followed by subsequent chain growth. In the present work, it could be shown by means of model calculations and based on experiments with co-feeding of ethene and 1-alkenes that such subsequent chain growth cannot be regarded as the main reason of observed deviations of the carbon number distribution from the ideal ASF distribution. The co-feeding experiments suggest that these deviations are the consequence of two different mechanisms of chain growth causing a superposition of two ASF distributions. Consequently, the carbon number distributions are represented by this superposition. In order to describe distributions as a function of reaction conditions the model parameter, the growth probabilities α 1 and α 2 as well as μ 1, the fraction of distribution ( α 1) are presented as function of the partial pressures of hydrogen and carbon monoxide. Finally, the typical model parameters of products formed on cobalt and iron are compared.