Fischer-Tropsch Activity Research Articles

Role of Carbonaceous Supports and Potassium Promoter on Higher Alcohols Synthesis over Copper–Iron Catalysts

The identification of an effective copper–iron catalyst for the direct conversion of synthesis gas into higher alcohols is hindered by the low solubility limit of Cu in Fe and the limited understanding of structural and electronic descriptors in such multicomponent systems. Here, commercial carbonaceous carriers are shown to produce an efficient material only if they enable control of the size and location of metal species through confinement in adequately sized channels, with conical carbon nanofibers being more adequate than carbon nanotubes. Application of a sol–gel route was preferred to other deposition methods to avoid excessive Cu aggregation, associated with enhanced CO2 formation. A bulk Cu/Fe ratio of 2 permitted one to balance the different tendencies of Cu and Fe toward agglomeration, i.e., to form numerous Cu particles of moderate size and hinder the dispersion of the Fe phase and in turn the Fischer–Tropsch activity. Promotion by tiny amounts of potassium was instrumental to further increase...

Effect of Gallium Additions on Reduction, Carburization and Fischer–Tropsch Activity of Iron Catalysts

Undoped and gallium-doped iron oxide catalysts (100Fe, 100Fe:2Ga, and 100Fe:5Ga) were prepared by following a continuous co-precipitation technique using ammonium hydroxide as precipitant. The catalysts were characterized by BET surface area, X-ray diffraction, H2-temperature programmed reduction, 57Fe Mossbauer spectroscopy, and temperature programmed decarburization techniques. The addition of gallium affects both reduction as well as carburization of iron oxide. The CO conversion decreases with an increase of gallium content relative to iron. The gallium-doped iron catalyst (100Fe:2Ga) exhibits initially a lower CO conversion after H2 activation than an undoped iron catalyst; however, the activity of the doped catalyst kept increasing with time. A strong interaction between iron oxide and gallium could explain the suppressed formation of χ-Fe5C2 and metallic iron during the carburization and reduction of iron oxide, respectively. The relative percentage of iron in χ-Fe5C2 was found to correlate with the initial rates of FT and WGS activity indicating that iron carbide is the main active component for both FT and WGS reactions.

Bifunctional Co/Al-SBA-15 Catalyst with Tunable Acidity for Selective Production of Aviation Fuel

Production of alternative, clean aviation fuel from biomass synthesis gas via Fischer–Tropsch (FT) synthesis has attracted much attention. In this paper, Al-SBA-15 supported cobalt catalysts with different Al/Si ratios were successfully prepared for the production of aviation fuel by FT synthesis. The physical and acidic properties of the Co/Al-SBA-15 catalysts were characterized by XRF, XRD, N2 adsorption–desorption, TEM, H2-TPR, 27Al NMR, NH3-TPD, and Py-FTIR. The formation of long-chain hydrocarbons was inhibited by the ordered mesoporous structure. Fischer–Tropsch activity and selectivity were demonstrated to be affected by the acidity of the Al-SBA-15 supports. The selectivity turned to lighter iso-paraffin products. Co-based catalyst with Al/Si ratio of 0.01 exhibited the highest selectivity (52.4%) for aviation fuel (C8–C18 products) and much higher ratio of iso-paraffin/normal-paraffin (iso/n) (20.2%). This study might provide useful information for the selective production of aviation fuel via FT...

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.

Deactivation of Co-Based Fischer–Tropsch Catalyst by Aerosol Deposition of Potassium Salts

A 20%Co/0.5%Re/γAl2O3 Fischer–Tropsch catalyst was poisoned by four potassium salts (KNO3, K2SO4, KCl, and K2CO3) using the aerosol deposition technique, depositing up to 3500 ppm K as solid particles. Standard characterization techniques (H2 chemisorption, BET, TPR) showed no difference between treated samples and their unpoisoned counterpart. The Fischer–Tropsch activity was investigated at industrially relevant conditions (210 °C, H2:CO = 2:1, 20 bar). The catalytic activity was significantly reduced for samples exposed to potassium, and the loss of activity was more severe with higher potassium loadings, regardless of the potassium salt used. A possible dual deactivation effect by potassium and the counterion (chloride and sulfate) is observed with the samples poisoned by KCl and K2SO4. The selectivity toward heavier hydrocarbons (C5+) was slightly increased with increasing potassium loading, while the CH4 selectivity was reduced for all the treated samples. The results support the idea that potassium...

Direct Synthesis of the Reduced Co–C/SiO2 As an Efficient Catalyst for Fischer–Tropsch Synthesis

The coimpregnation of the glucose and cobalt nitrate aqueous solution into SiO2 followed by the carbonization in an inert gas was demonstrated for the preparation of the directly reduced Co-xC/SiO2 (x: glucose to SiO2 mass ratio) Fischer–Tropsch (FT) catalysts. The as-prepared Co-xC/SiO2 (x = 0.1, 0.5, 1.0, 1.5) without any pretreatment was evaluated for FT synthesis under typical conditions. Significantly, the completely reduced Co-xC/SiO2 were obtained when the x was at or above 1.0, and the maximum normalized FT activity (cobalt-time-yield, CTY) of 3.5 × 10–4 molCO·g–1Co·s–1 over Co-1C/SiO2 was doubled the activity of the high-temperature reduced Co/SiO2. Moreover, light hydrocarbons (C1 ∼ C4) were favorably formed over Co-xC/SiO2, and the product distribution can be regulated via simply changing the ratio of the glucose to SiO2. The simple procedure and the cheap glucose make the directly reduced Co-xC/SiO2 catalysts very promising for the development of a more efficient FT process.

A sinter resistant Co Fischer-Tropsch catalyst promoted with Ru and supported on titania encapsulated by mesoporous silica

One of the pathways responsible for the deactivation of Fischer-Tropsch catalysts is the loss of active metal surface area due to nanoparticle agglomeration. To combat this effect efforts have been made to increase the interaction between the metal nanoparticles and the support using materials like silica. In this study, the supported metal particles were covered with a highly porous layer of silica to stabilize the Co nanoparticles on a titania support both during reduction and under reaction conditions. Co3O4 nanoparticles (size range: 8–12 nm) supported on titania were stabilized by coating them with a thin layer of mesoporous silica (∼4 nm) to make Fischer-Tropsch catalysts that are less prone to sintering (Co/TiO2@mSiO2). To mitigate the strong metal support interactions brought about by the titania and silica a Ru promoter was loaded together with the cobalt nanoparticles onto the titania (CoRu/TiO2@mSiO2). Temperature programmed XRD studies on the evolution of the Co metal nanoparticles showed that there was no significant particle size growth under reduction conditions in the temperature range from 30 to 600 °C. Chemisorption studies following reduction under hydrogen at 350 °C and 450 °C gave results consistent with the in situ XRD data when compared to the Co/TiO2. Fischer-Tropsch synthesis on the Co/TiO2@mSiO2 and CoRu/TiO2@mSiO2 catalysts encapsulated inside the mesoporous silica shell exhibited good catalytic performance without any display of significant mass transport limitations that might arise due to a silica shell coating of the active sites. For these two catalysts the Fischer-Tropsch activity increased with reduction temperature without any significant negative changes in their selectivity due to sintering, while the activity on Co/TiO2 decreased due to Co nanoparticle sintering.

Deactivation of alumina supported cobalt FT catalysts during testing in a Continuous-stirred tank reactor (CSTR)

A set of alumina supported cobalt catalysts was prepared, varying the synthesis method and formulation to produce a range of cobalt particle sizes. A combination of degree of reduction (DOR) measurements and H2 chemisorption was used to calculate the cobalt particle sizes on the fresh catalyst samples (5–19nm). The catalysts were pre-reduced and encapsulated in wax prior to testing for Fischer-Tropsch (FT) activity in a continuous-stirred tank reactor (CSTR) system for between 500 and 6000h on-line. The catalysts all demonstrated rapid deactivation over the first 200h of FT testing, with the catalysts with the highest initial activity deactivating most rapidly. Characterisation of the discharged catalysts samples using XRD, TEM and H2 chemisorption indicated that the cobalt particles had undergone a sintering process consistent with an Ostwald ripening mechanism. Catalyst stability in CSTR testing can be enhanced by the presence of large cobalt particles (>20nm).

The Influence of Alkali Treatment for Synthesizing Hierarchical Zeolite on Behavior of Cobalt Fischer–Tropsch Synthesis Catalysts

Hierarchical zeolites were synthesized by alkali treatment and their applications in Fischer–Tropsch (FT) synthesis were studied. It was found that alkali treatment not only created hierarchical structure but also could tune cobalt-support interaction. The dissolution of Si by alkali treatment became easier with the increase of Si/Al ratio, and thus the amount of mesopority increased. An optimal Si/Al molar ratio was identified over the zeolite with Si/Al ratio of 80, which was found to be superior to other catalysts in terms of better diesel selectivity and lower CH4 selectivity due to its relatively narrow bimodal pore size distribution and moderate cobalt-support interaction. Meanwhile alkali treatment could enhance cobalt-support interaction via the formation of ɑ-SiO2, Co/MZ-120 catalyst showed the lowest FT activity and higher CH4 selectivity due to the increase of such new phase.

Effect of support of Co-Na-Mo catalysts on the direct conversion of CO2 to hydrocarbons

This study of the effect of support of Co-Na-Mo based catalysts on the direct hydrogenation of CO2 into hydrocarbons (HC) provides guidelines for the design of catalysts for CO2 conversion. We demonstrate that the surface area of the support and the metal-support interaction have a key role determining the cobalt crystallite size and consequently the activity of the system. Cobalt particles with sizes <2nm supported on MgO present low reverse water gas shift conversion with negligible Fischer-Tropsch activity. Increasing the cobalt particle size to ∼15nm supported on SiO2 and ZSM-5 supports not only substantially increases the CO2 conversion but it also provides high HC selectivities. Further increase of the cobalt particle size to 25–30nm has a detrimental effect on the global CO2 conversion with HC:CO ratios below 1, however, lower methane selectivity and enhanced formation of unsaturated HC products are achieved. Additionally, the metal-support interaction potentially also has a strong effect on the growth chain probability of the formed hydrocarbons, increasing as the metal-support interaction increases. These evidences demonstrate that CO2 conversion and hydrocarbon distribution can be tuned towards desired products by controlled catalyst design.

On the superior activity and selectivity of PtCo/Nb2O5 Fischer Tropsch catalysts

In this study Co/Nb2O5 catalysts and the effect of Pt-promotion thereon are investigated in comparison with γ-Al2O3- and α-Al2O3-supported catalysts for the Fischer Tropsch (FT) synthesis. Upon Pt-promotion of Co/Nb2O5 the cobalt-weight normalized FT activity was found to increase by a factor of 2.4, while the high C5+ selectivity of 85wt% was maintained. Based on environmental TEM results no indications were found that Pt affected the cobalt particle size in Co/Nb2O5 catalysts. A kinetic study indicates an increased number of active sites upon Pt-promotion whereas Steady-State Isotopic Transient Kinetic Analysis experiments show that a combination of an increased number of active sites and an increased turnover frequency is at the origin of the enhanced activity in Co/Nb2O5 catalysts upon Pt-promotion. Pt was tentatively proposed to bring about more efficient promotion of Co by NbOx being present as smaller clusters.

Investigations into enhanced wax production with combustion synthesized Fischer–Tropsch catalysts

Combustion synthesized (CS) cobalt catalysts deposited over two supports, alumina and silica doped alumina (SDA), were characterized and tested for its Fischer–Tropsch (FT) activity. The properties of CS catalysts were compared to catalysts synthesized by conventional impregnation method (IWI). The CS catalysts resulted in 40–70% increase in the yield of C6+ hydrocarbons compared to IWI catalysts. The FT activity for CS catalysts showed formation of long chain hydrocarbon waxes (C24+) compared to the formation of middle distillates (C10–C20) for IWI synthesized catalysts, indicating higher hydrocarbon chain growth probability for CS catalysts. This is ascribed to the smaller crystallite sizes, increased degree of cobalt reduction and consequentially, a higher number of active metal sites exposed over the catalyst surface. Additionally, 12–13% increase in the overall C6+ hydrocarbon yield is realized for SDA-CS catalysts, compared to Al2O3-CS catalysts. The improved performance of CS-SDA catalysts is attributed to 48% increase in cobalt dispersion compared to Al2O3 supported CS catalysts, which is again caused by the decrease in the cobalt-support interaction for SDA supports. The metal support interactions were analyzed using XPS and H2 TPR–TPD experiments. Combustion method produced catalysts with smaller crystallite size (17–18nm), higher degree of reduction (∼92%) and higher metal dispersion (16.1%) compared to the IWI method. Despite its enhanced properties, the CS catalysts require prominently higher reduction temperatures (∼1100–1200K). The hydrocarbon product analysis for Al2O3 supported catalyst showed higher paraffin wax concentrations compared to SDA supported catalysts, due to the lower surface basicity of Al2O3. This work reveals the impact of the CS catalysts and the nature of support on FT activity and hydrocarbon product spectrum.

In Search of Initial Predictors of Fischer–Tropsch Catalytic Activity

All Fischer–Tropsch mechanisms known to date begin with the adsorption of carbon monoxide followed by its dissociation on a given catalysts surface. Understanding how those catalysts modify reactivity descriptors such as CO adsorption and dissociation energies is a key for nanoengineering materials for this type of applications. Cluster models of Co, Fe, Ni, Pd, Pt, and Ru have been explored using the density functional theory, in search of initial descriptors of Fischer–Tropsch activity for pure and binary combinations of those elements. Carbon monoxide adsorption energies were calculated for adsorption on all possible catalytic sites, and the most preferred CO adsorption sites were found in each case. An initial predictor that can be used to anticipate potentially effective catalysts was identified as a percentage difference, using CO adsorption energy results in combination with CO dissociation energies from the previously found sites. A greater catalysts performance is expected when that percentage difference is maximized. On pure clusters, the predictor indicates that ruthenium is expected to be the best catalyst followed by cobalt, in very good agreement with the current knowledge in this field. Thus, this paper presents a mechanism to quickly explore the natural potential of a catalyst material to break the CO bond. The predictor presented here can guide the synthesis of new catalysts, involving modifications of known and currently used catalysts for the Fischer–Tropsch process, and investigations on catalysis in general.

Back Cover: Selective Formation of Hägg Iron Carbide with g‐C3N4 as a Sacrificial Support for Highly Active Fischer–Tropsch Synthesis (ChemCatChem 21/2015)

Back Cover: Selective Formation of Hägg Iron Carbide with g‐C₃N₄ as a Sacrificial Support for Highly Active Fischer–Tropsch Synthesis (ChemCatChem 21/2015)

Characterisation and Catalytic Fischer–Tropsch Activity of Co–Ru and Co–Re Catalysts Supported on γ-Al2O3, TiO2 and SiC

A number of supported cobalt catalysts for Fischer–Tropsch synthesis were considered. Catalysts were prepared by impregnation of the supports with a cobalt precursor resulting in cobalt concentrations of 15 or 20 wt%. The active metal was supported onto aluminum oxide, titanium dioxide and silicon carbide supports while rhenium or ruthenium metals were used as promoters, whose concentrations were 0, 0.2 or 1.0 wt%. The catalysts were characterized by a number of methods including BET, chemisorption, and XRD. Some of the catalysts were tested for catalytic activity and selectivity in the Fischer–Tropsch reaction using a fixed bed reactor. Based on the results, the addition of promoter metals increases the dispersion of the active metal cobalt and correspondingly a decrease in the cobalt metal particles, this effect was most evident for the alumina supported catalysts. Further, increased concentrations of the ruthenium promoter (1.0 wt%) slightly increased the selectivity to CH4 and decreased selectivity to C5+.

Hydrocarbon synthesis using Iron and Ruthenium/SiO2 with FISCHER-TROPSCH catalysis

Fe2(CO)9, Fe3(CO)12 and Ru3(CO)12 clusters were used as precursors for silica supported metals. The impregnated silica solids were obtained in organic solvents under inert atmosphere and the adsorbed complexes and reduced metals characterized by FT-IR, SEM EDX and HRTEM. The catalysts showed good Fischer-Tropsch (FT) activity; the main products were alkanes, alkenes and medium and higher alcohols as analyzed by GCMS. The Ru catalysts showed higher alcohols selectivity. HRTEM showed Ru nanoparticle size.

Cobalt-Based Fischer–Tropsch Activity and Selectivity as a Function of Crystallite Size and Water Partial Pressure

Using well-defined supported cobalt nanocrystallites in a novel in situ sample presentation device for laboratory X-ray diffractometers, we investigated the extensively studied structure sensitivity of Fischer–Tropsch (FT) catalysts under simulated high conversion conditions, that is, at high water to synthesis gas ratios. This study has to be regarded as a further small step toward a full understanding of the various processes governing FT activity and selectivity. We were able to show, for two different crystallite sizes, that water has an overall enhancing effect on carbon monoxide conversion and surface specific turnover frequency on metallic surfaces and improves the overall product selectivity with a decrease of methane selectivity. For small crystallites oxidation was observed at elevated water partial pressures, which caused a decrease of activity. The selectivity to the undesired product methane is suppressed in favor of chain growth. This influence on the selectivity might originate from water-i...

Correlating Fischer-Tropsch activity to Ru nanoparticle surface structure as probed by high-energy X-ray diffraction.

Synchrotron X-ray diffraction coupled to atomic pair distribution function analysis and Reverse Monte Carlo simulations is used to determine the atomic-scale structure of Ru nanoparticle catalysts for the Fischer-Tropsch reaction. The rate of CO hydrogenation strongly correlates with the abundance of surface atoms with coordination numbers of 10 and 11. DFT calculations confirm that CO dissociation proceeds with a low barrier on these Ru surface atom ensembles.

Fischer-Tropsch performance correlated to catalyst structure: Trends in activity and stability for a silica-supported cobalt catalyst

The influence of the cobalt particle size and particle size distribution in determining Fischer-Tropsch (FT) performance for silica supported catalysts was investigated. Using a line broadening powder X-ray diffraction analysis, both the particle size and particle size distribution of cobalt were estimated and compared to the FT activity and stability at industrially relevant conditions (210°C, H2/CO=2, 20bar). It was shown that the apparent turnover frequency increased linearly with Co0 particle size up to 9.8nm, the largest size tested. For the first time, it has been shown that the breadth of the particle size distribution correlates directly to catalyst stability. Using these correlations, a highly active and stable catalyst was developed, showing a deactivation rate of −0.08% CO converted/day in a microchannel reactor with an activity of 0.25molCOgCo−1h−1.

Study of temperature-programmed calcination of cobalt-based catalysts under NO-containing atmosphere

Temperature-programmed calcination (TPC) experiments were performed under NO containing atmosphere for alumina supported cobalt catalyst. During these experiments the NO and NO2 concentration of the effluent was measured continuously. It was observed that NO concentrations below 1vol% significantly influence the TPC profile. With increasing NO content in the gas phase from 0 to 1vol% an increasing amount of NO2 evolves at temperatures below 100°C, which is correlated with consumption of NO. These results indicate decomposition of cobalt nitrate forming cobalt hydroxynitrates at low temperatures in the presence of NO. In agreement with literature, increased cobalt dispersion was obtained for materials calcined in NO containing atmosphere. Thus, the present method seems to be interesting for preparation of highly active Fischer–Tropsch catalysts.