Highly Selective and Active Niobia-Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

The performance of Co/Nb₂O₅ was compared to that of Co/γ-Al₂O₃ for the Fischer–Tropsch synthesis at 20 bar and over the temperature range of 220–260 °C. The C₅₊ selectivity of Nb₂O₅-supported cobalt catalysts was found to be very high, i.e. up to 90 wt% C₅₊ at 220 °C. The activity per unit weight cobalt was found to be similar for Nb₂O₅ and γ-Al₂O₃-supported catalysts at identical reaction temperature. However, due to the low porosity of crystalline Nb₂O₅, the cobalt loading was limited to 5 wt% and consequently the activity per unit weight of catalyst was lower than of Co/γ-Al₂O₃ catalysts with higher cobalt loadings. This low activity was largely compensated by increasing the reaction temperature, although the C₅₊ selectivity decreased upon increasing reaction temperature. Due to the high intrinsic C₅₊ selectivity, Nb₂O₅-supported catalysts could be operated up to ~250 °C at a target C₅₊ selectivity of 80 wt%, whereas γ-Al₂O₃-supported catalysts called for an operation temperature of ~210 °C. At this target C₅₊ selectivity, the activity per unit weight of catalyst was found to be identical for 5 wt% Co/Nb₂O₅ and 25 wt% Co/Al₂O₃, while the activity per unit weight of cobalt was a factor of four higher for the niobia-supported catalyst.     

Fischer–Tropsch Synthesis on SiC-Supported Cobalt Catalysts

Seven SiC supports provided by SICAT with different surface areas and pore volumes were impregnated with 12,5 wt% Co. H₂-chemisorption, N₂-adsorption, temperature programmed reduction and Fischer–Tropsch synthesis in a fixed-bed reactor at 483 K, 20 bar and H₂/CO = 2.1 were performed in order to characterize and test the samples. The performances were compared with well characterized Co/Al₂O₃ and Co-Re/Al₂O₃ reference catalysts. The selectivity towards heavier hydrocarbons (C₅₊) was found to be moderately higher for the SiC supported catalysts while the site-time yields was 20 to 66 % lower than the Co-Re/Al₂O₃ catalyst. Elemental analysis showed the presence of several impurities in the SiC material. Alkali and alkaline earth elements, such as Na, K and Ca, are all known to lower the catalytic activity and also to influence the selectivity. It is proposed that these impurities in addition to sulfur and phosphorus known to be present in SiC, are responsible for the significantly lower catalytic activity of the SiC supported catalysts.     

Influence of a Zeolite-Based Cascade Layer on Fischer–Tropsch Fuels Production over Silicon Carbide Supported Cobalt Catalyst

In this work, selective production of middle distillate from synthesis gas was conducted over a cascade catalytic system in a single unit. Co/β-SiC was chosen as an efficient Fischer–Tropsch synthesis (FTS) catalyst (first layer) while proton-type H-ZSM-5 and H-βeta zeolites (second layer) were tested for the subsequent hydroprocessing to produce middle distillate from waxes. Moreover, in order to compare, a prior FTS reference experiment was performed. Catalytic materials were characterized by means of Atomic Absorption, Thermogravimetric analysis, X-Ray diffraction, N₂ adsorption–desorption, Temperature-Programmed Reduction and Temperature-Programmed Desorption. From catalytic results, a distinguishable enhancement of commercial fuels products was observed under the proposed cascade operation compared to the stand-alone configuration or physical mixture. Regardless the zeolite type, FTS over Co/β-SiC with subsequent upgrading was demonstrated to result in the complete elimination of waxes, solving the main weakness of a conventional fixed-bed reactor. Moreover, apart from a selective production of gasoline, the proposed concept provided a significant enhancement of both kerosene and diesel yields, particularly when zeolite H-βeta is incorporated to the cascade system due to its mild acidity and larger pore size.     

Fischer–Tropsch Synthesis: Characterization Rb Promoted Iron Catalyst

Rubidium promoted iron Fischer–Tropsch synthesis (FTS) catalysts were prepared with two Rb/Fe atomic ratios (1.44/100 and 5/100) using rubidium nitrate and rubidium carbonate as rubidium precursors. Results of catalytic activity and deactivation studies in a CSTR revealed that rubidium promoted catalysts result in a steady conversion with a lower deactivation rate than that of the corresponding unpromoted catalyst although the initial activity of the promoted catalyst was almost half that of the unpromoted catalyst. Rubidium promotion results in lower methane production, and higher CO₂, alkene and 1-alkene fraction in FTS products. Mössbauer spectroscopic measurements of CO activated and working catalyst samples indicated that the composition of the iron carbide phase formed after carbidization was χ-Fe₅ C₂ for both promoted and unpromoted catalysts. However, in the case of the rubidium promoted catalyst, ?′-Fe_2.2C became the predominant carbidic phase as FTS continued and the overall catalyst composition remained carbidic in nature. In contrast, the carbide content of the unpromoted catalyst was found to decline very quickly as a function of synthesis time. Results of XANES and EXAFS measurements suggested that rubidium was present in the oxidized state and that the compound most prevalent in the active catalyst samples closely resembled that of rubidium carbonate.     

Cobalt Fischer–Tropsch Catalyst Deactivation Modeled Using Generalized Power Law Expressions

Ten sets of deactivation data from five previously reported studies of cobalt Fischer–Tropsch synthesis (FTS) catalysts were found to be modeled well using concentration-independent first and second order generalized power law expressions (GPLEs) which predict that activity approaches a non-zero asymptote. Concentration dependencies of reactants and products were generally not addressed in the model regressions, although selected simulations which incorporated CO, H₂, and/or H₂O concentrations in deactivation rate equations showed very little or no dependence on concentrations of these species. For reaction temperatures in the range of 220–230 °C, pressures of 15–30 bar, and H₂/CO ratios of 1.6–2.6, first order and second order deactivation rate constants average 0.12 ± 0.06 and 0.11 ± 0.05 day^?1, respectively. Limiting (asymptotic) activities are largely in the range of 30–40 % of initial activity based on the generally superior extrapolations of second order GPLE. This consistency is impressive considering significant differences among catalyst properties and operating conditions in the five studies that apparently involve different mechanisms of deactivation, including sintering, carbon formation, and/or cobalt aluminate formation. Second order models predict significant longevity for cobalt FTS catalysts; for example, based on the 2nd order models, normalized activities for commercial catalysts in two different pilot slurry reactor facilities are projected to be 56 and 45 % of initial activity after 200 days on stream. For two of the previous studies providing data over periods of 40–55 days, it was possible to identify two different causes of deactivation, one rapid (reaching completion in 10–20 days) and one slow (apparently continuing beyond 40–50 days). A method was developed for calculating first and second order model parameters for the two regions of operation. Rapid activity loss (path 1) is observed for either sintering or Co surface aluminate formation, while poisoning/fouling by deposited carbon or coke (path 2) occurs relatively slowly over the entire process run of 40–55 days and is the dominant mechanism after 10–20 days for both sets of data. The results show that simple GPLE models are surprisingly generally useful for predicting activity versus time behavior of supported cobalt FTS catalysts under typical process conditions.     

ZSM-5 Supported Cobalt Catalyst for the Direct Production of Gasoline Range Hydrocarbons by Fischer–Tropsch Synthesis

Abstract  

Fischer–Tropsch synthesis (FTS) reaction for the direct production of gasoline range hydrocarbons (C₅–C₉) from syngas was investigated on cobalt-based FTS catalyst supported on the ZSM-5 possessing a four different Si/Al ratio. The FTS catalysts were prepared by impregnation method using cobalt nitrate precursor in a slurry of ZSM-5, and they were characterized by surface area, XRD, H₂-TPR and NH₃-TPD. Cobalt supported catalyst on ZSM-5 having a low Si/Al ratio of 15 was found to be superior to the other catalysts in terms of better C₅–C₉ selectivity due to the formation of small cobalt particle and the presence of larger number of weak acidic sites. It also exhibited the highest catalytic activity because of the higher reducibility and the small cobalt particle size.     

Effect of Pt Impregnation on a Precipitated Iron-based Fischer–Tropsch Synthesis Catalyst

Effect of Pt impregnation on the textural properties, surface element distributions and catalytic behavior of a precipitated iron-based catalyst for Fischer–Tropsch synthesis (FTS) was investigated by N₂ physical adsorption, temperature-programmed reduction (TPR), Mössbauer effect spectrometer (MES), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). Low levels of Pt addition lead to an increase in BET surface area. The result of XPS indicates that Pt enriches on the catalyst surface after being calcined. HRTEM shows that Pt crystallites with diameter about 2 nm are well dispersed on the surface of the catalyst (100Fe/1Pt/4 K/16SiO₂). The results of TPR and MES clearly indicate that Pt facilitates the reduction and carburization of Fe₂O₃ to some extend. The reaction tests in a slurry reactor give the result that the Pt impregnation remarkably increases the FTS activity, and suppresses the selectivities of the light hydrocarbons and the olefins.     

Characterization of Modified Fischer–Tropsch Catalysts Promoted with Alkaline Metals for Higher Alcohol Synthesis

Abstract  

Two series of Cu/Co/Cr modified Fischer–Tropsch catalyst promoted with Zn or Mn and an alkaline metal (Me: Li, Na, K, Rb, Cs) were prepared by co-precipitation method and tested for high alcohol synthesis (HAS) at one hour on-stream and at two temperatures, 300 and 350 °C. The results indicate that the best selectivity toward high alcohols depends on temperature and catalysts composition and is obtained as follows: a) at 300 °C over catalysts without Zn and containing K, Na and Rb; b) at 350 °C over catalysts without Zn and containing K; c) at 350 °C over catalysts containing Zn as well as Li and Cs.     

Fischer–Tropsch Synthesis over Ordered Mesoporous Carbon Supported Cobalt Catalysts: The Role of Amount of Carbon Precursor in Catalytic Performance

Abstract  

Ordered mesoporous carbon supported cobalt-based catalysts (Co/MC) were synthesized via incipient wetness impregnation with different amounts of furfuryl alcohol (FA) as carbon precursor. The characterizations of obtained Co/MC were subjected to N₂ adsorption, XRD, XPS, TEM, H₂-TPR, H₂-TPD and H₂-TPSR. The results indicate that the reducibility and dispersion of Co active species vary significantly due to the difference of FA amount. By simply tuning the FA content from 25 to 100 wt%, the reduction temperature of deriving metallic Co shifts gradually to lower. The catalytic performance of Co/MC was evaluated for Fischer–Tropsch (FT) synthesis. The observed FT activity exhibits a volcano-type curve with the amount of FA due to the effect of both reducibility and dispersion of active species. As the precursor concentration overweighs 50 wt%, the ability of CO to dissociate over the active surface and the selectivity to the C₅₊ products level off after experiencing an initial increase. Substantially, the catalysts with higher concentration of FA render the larger crystallites having an average size of more than 6 nm, which facilitates the CO hydrogenation by way of carbon chain propagation. It seems that the sample with FA content of 50 wt% is optimum in terms of FT activity and C₅₊ selectivity.     

Electron Microscopy Study of γ-Al2O3 Supported Cobalt Fischer–Tropsch Synthesis Catalysts

Three supported catalysts containing 20 wt% cobalt and 0.5 wt% rhenium were subjected to electron microscopy studies in their calcined state. The catalysts were prepared by incipient wetness impregnation of γ-Al₂O₃ supports of different pore characteristics with aqueous solutions of cobalt nitrate hexahydrate and perrhenic acid. The influence of the support on the Co₃O₄ crystallite size and distribution was studied by X-ray diffraction and electron microscopy. There was a positive correlation between the pore diameter of the support and the post calcination Co₃O₄ crystallite size. On all three γ-Al₂O₃ supports, Co₃O₄ was present as aggregates of many crystallites (20–270 nm in size). Cobalt oxide did not crystallise as independent crystallites, but as an interconnected network, with a roughly common crystallographic orientation, within the matrix pore structure. The internal variations in crystallite size between the catalysts were maintained after reduction. Fischer–Tropsch synthesis was carried out in a fixed-bed reactor at industrial conditions (T = 483 K, P = 20 bar, H₂/CO = 2.1). Although the cobalt-time yields varied significantly (4.6–6.7 × 10^?3 mol CO/mol Co s), the site-time yields were constant (63–68 × 10^?3 s^?1) for the three samples. The C₅₊ selectivity could not be correlated to the cobalt oxide aggregate size and is more likely related to the cobalt particle size and chemical properties of the γ-Al₂O₃ support.     

Carbon Coated Supports for Cobalt Based Fischer–Tropsch Catalysts

Cobalt based Fischer–Tropsch synthesis catalysts were prepared on carbon coated alumina supports. Carbon coating resulted in a decrease in the average cobalt crystallite size (down to 6 nm) and increased active cobalt metal surface area. Very importantly, the use of carbon on the alumina surface also altered the cobalt nitrate mechanism of binding and thermal decomposition, resulting in a significant change in the macroscopic cobalt distribution with improved inter-particle distances. The enhanced cobalt metal surface areas together with the significantly improved cobalt distribution/inter-particle distances resulted in cobalt Fischer–Tropsch synthesis catalysts with an activity that was increased by 40–75 % without having a negative influence on the methane selectivity.     

Promotion Effects of Platinum and Ruthenium on Carbon Nanotube Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

Abstract  

Carbon nanotube supported nano-size monometallic and noble metal (Pt and Ru) promoted cobalt catalysts were prepared by incipient wetness impregnation (IWI) using solution of cobalt nitrate and characterized by nitrogen adsorption isotherm, X-ray diffraction (XRD), temperature programmed reduction, in situ magnetic method and TEM. Analysis of the magnetization and H₂-TPR data suggested promotion with platinum and ruthenium significantly decreased the cobalt species reduction temperature. TEM and XRD results showed that the presence of noble metal promoters had no significant effect on the size of cobalt for carbon naotube as catalytic support. Promotion of cobalt carbon nanotube-supported catalysts with small amounts of Pt and Ru resulted in slight increase in Fischer–Tropsch cobalt time yield. The Pt and Ru promoted cobalt catalyst exhibited carbon monoxide conversion of 37.1 and 31.4, respectively. C₅+ hydrocarbon selectivity was attained at 80.0%. The Pt promoted cobalt supported on carbon nanotube yielded better catalytic stability than that of the monometallic cobalt catalyst.     

Silica-Supported Cobalt Catalysts for Fischer–Tropsch Synthesis: Effects of Calcination Temperature and Support Surface Area on Cobalt Silicate Formation

Cobalt silicate formation reduces the activity of the catalyst in Fischer–Tropsch synthesis (FTS). In this article, the effects of calcination temperature and support surface area on the formation of cobalt silicate are explored. FTS catalysts were prepared by incipient wetness impregnation of cobalt nitrate precursor into various silica supports. Deionized water was used as preparation medium. The properties of catalysts were characterized at different stages using FTIR, XRD and BET techniques. FTIR-ATR analysis of the synthesized catalyst samples before and after 48 h reaction identified cobalt species formed during the impregnation/calcination stage and after the reduction/reaction stage. It was found that in the reduction/reaction stage, metal-support interaction (MSI) added to the formation of irreducible cobalt silicate phase. Co/silica catalysts with lower surface area (300 m²/g) exhibited higher C₅₊ selectivity which can be attributed to less MSI and higher reducibility and dispersion. The prepared catalysts with different drying and calcination temperatures were also compared. Catalysts dried and calcined at lower temperatures exhibited higher activity and lower cobalt silicate formation. The catalyst sample calcined at 573 K showed the highest CO conversion and the lowest CH₄ selectivity.