Comparative study of iron-based Fischer–Tropsch synthesis catalyst promoted with potassium or sodium

FeCu/SiO₂ catalysts, in which K or Na promoter is incorporated respectively, are prepared by a combination method of continuous co-precipitation and spray drying technology. The catalysts were characterized by temperature-programmed desorption and Mössbauer spectroscopy. The Fischer–Tropsch synthesis (FTS) performance of the catalysts was studied in a continuously stirred tank slurry reactor. The basicity of the K-promoted catalyst is enhanced, as demonstrated by CO₂-TPD results. MES results show that sodium can weaken the dispersion of α-Fe₂O₃ phase; either potassium or sodium can promote carburization of the catalyst, while the effect of sodium is weaker. FTS results indicate that the addition of K or Na can improve the catalyst activity, and shift the product distribution to heavy hydrocarbons to the different extent.     

A simulation study on gas-to-liquid (natural gas to Fischer–Tropsch synthetic fuel) process optimization

A simulation study on gas-to-liquid (natural gas to Fischer–Tropsch synthetic fuel) process was carried out in order to find optimum reaction conditions for maximum production of synthetic fuel. Optimum operating condition for GTL (gas-to-liquid) process was determined by changing reaction variable such as temperature. During the simulation, overall synthetic process was assumed to proceed under steady-state conditions. It was also assumed that physical properties of reaction medium were governed by RKS (Redlich–Kwong–Soave) equation. ATR (auto-thermal reforming) in synthesis gas production unit and slurry phase reaction over Co-based catalyst in FTS (Fischer–Tropsch synthesis) unit were considered as reaction models for GTL process. The effect of reaction temperature on CO conversion and C₅–C₂₀ hydrocarbon yield in FTS unit was mainly examined. Simulation and experimental results showed that optimum reaction temperature in FTS unit was 255 °C. Simulation results were also compared to experimental results to confirm the reliability of simulation model. Simulation results were reasonably well matched with experimental results.     

Study on an iron–manganese Fischer–Tropsch synthesis catalyst prepared from ferrous sulfate

A systematic study was undertaken to investigate the effects of the initial oxidation degree of iron on the bulk phase composition and reduction/carburization behaviors of a Fe–Mn–K/SiO₂ catalyst prepared from ferrous sulfate. The catalyst samples were characterized by powder X-ray diffraction (XRD), Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and H₂ (or CO) temperature-programmed reduction (TPR). The Fischer–Tropsch synthesis (FTS) performance of the catalysts was studied in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that the fresh catalysts are mainly composed of α-Fe₂O₃ and Fe₃O₄, and the crystallite size of iron oxides is decreased with the increase of the initial oxidation degree of iron. The catalyst with high content of α-Fe₂O₃ in its as-prepared state has high content of iron carbides after being reduced in syngas. However, the catalyst with high content of Fe₃O₄ in its as-prepared state cannot be easily carburized in CO and syngas. FTS reaction study indicates that Fe-05 (Fe³⁺/Fe_total = 1.0) has the highest CO conversion, whereas Fe-03 (Fe³⁺/Fe_total = 0.55) has the lowest activity. The catalyst with high CO conversion has a high selectivity to gaseous hydrocarbons (C₁–C₄) and low selectivity to heavy hydrocarbons (C₅₊).     

Effect of Ru addition to Co/SiO2/HZSM-5 catalysts on Fischer–Tropsch synthesis of gasoline-range hydrocarbons

Microporous HZSM-5 zeolite and mesoporous SiO₂ supported Ru–Co catalysts of various Ru adding amounts were prepared and evaluated for Fischer–Tropsch synthesis (FTS) of gasoline-range hydrocarbons (C₅–C₁₂). The tailor-made Ru–Co/SiO₂/HZSM-5 catalysts possessed both micro- and mesopores, which accelerated hydrocracking/hydroisomerization of long-chain products and provided quick mass transfer channels respectively during FTS. In the same time, Ru increased Co reduction degree by hydrogen spillover, thus CO conversion of 62.8% and gasoline-range hydrocarbon selectivity of 47%, including more than 14% isoparaffins, were achieved simultaneously when Ru content was optimized at 1 wt% in Ru–Co/SiO₂/HZSM-5 catalyst.     

An experimental investigation of Fischer–Tropsch synthesis over washcoated metallic structured supports

In principle, the application of monolithic catalysts to the Fischer–Tropsch synthesis can solve many of the problems related to the classical Fischer–Tropsch reactors, in particular concerning the necessity to operate with short diffusion distances and low pressure drops, preferably according to the ideal plug-flow behavior, while still maintaining a reasonable inventory of catalytic material in the reactor volume.The preparation of prototype cobalt-based catalysts, washcoated onto metallic structured supports with different geometries, is described herein, together with the evaluation of the catalytic properties of such systems in the Fischer–Tropsch synthesis at industrially relevant process conditions (220–235 °C, 20 bar, 2.1  mol_H2/mol_CO,  5000 cm³(STP)_CO+H2/h/g_cat). Comparative tests with the same catalyst in the powdered form were also carried out at the same process conditions.It was found that the structured catalysts maintained the activity and the selectivity of the original powdered catalyst, provided that the washcoat thickness is sufficiently thin.     

Effect of promotion with ruthenium on the structure and catalytic performance of mesoporous silica (smaller and larger pore) supported cobalt Fischer–Tropsch catalysts

The effects of promotion with ruthenium on the structure of cobalt catalysts and their performance in Fischer–Tropsch synthesis were studied using MCM-41 and SBA-15 as catalytic supports. The catalysts were characterized by N₂ physisorption, H₂-temperature programmed reduction, in situ magnetic measurements, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that monometallic cobalt catalysts supported by smaller pore mesoporous silicas (d_p = 3–4 nm) had much lower activity in Fischer–Tropsch synthesis than their larger pore counterparts (d_p = 5–6 nm). Promotion with ruthenium of smaller pore cobalt catalysts led to a considerable increase in Fischer–Tropsch reaction rate, while the effect of the promotion with ruthenium was less significant with the catalysts supported by larger pore silicas.Characterizations of smaller pore cobalt catalysts revealed strong impact of ruthenium promotion on the repartition of cobalt between reducible Co₃O₄ phase and barely reducible amorphous cobalt silicate in the calcined catalyst precursors. Smaller pore monometallic cobalt catalysts showed high fraction of barely reducible cobalt silicate. Promotion with ruthenium led to a significant increase in the fraction of reducible Co₃O₄ and in decrease in the amount of cobalt silicate. In both calcined monometallic and Ru-promoted cobalt catalysts supported by larger pore silicas, easy reducible Co₃O₄ was the dominant phase. Promotion with ruthenium of larger pore catalysts had smaller influence on cobalt dispersion, fraction of reducible cobalt phases and thus on catalytic performance.     

Effects of Zn, Cu, and K Promoters on the Structure and on the Reduction, Carburization, and Catalytic Behavior of Iron-Based Fischer–Tropsch Synthesis Catalysts

Zn, K, and Cu effects on the structure and surface area and on the reduction, carburization, and catalytic behavior of Fe–Zn and Fe oxides used as precursors to Fischer–Tropsch synthesis (FTS) catalysts, were examined using X-ray diffraction, kinetic studies of their reactions with H₂ or CO, and FTS reaction rate measurements. Fe₂O₃ precursors initially reduce to Fe₃O₄ and then to metallic Fe (in H₂) or to a mixture of Fe_2.5C and Fe₃C (in CO). Zn, present as ZnFe₂O₄, increases the surface area of precipitated oxide precursors by inhibiting sintering during thermal treatment and during activation in H₂/CO reactant mixtures, leading to higher FTS rates than on ZnO-free precursors. ZnFe₂O₄ species do not reduce to active FTS structures, but lead instead to the loss of active components; as a result, maximum FTS rates are achieved at intermediate Zn/Fe atomic ratios. Cu increases the rate of Fe₂O₃ reduction to Fe₃O₄ by providing H₂ dissociation sites. Potassium increases CO activation rates and increases the rate of carburization of Fe₃O₄. In this manner, Cu and K promote the nucleation of oxygen-deficient FeO _x species involved as intermediate inorganic structures in reduction and carburization of Fe₂O₃ and decrease the ultimate size of the Fe oxide and carbide structures formed during activation in synthesis gas. As a result, Cu and K increase FTS rates on catalysts formed from Fe–Zn oxide precursors. Cu increases CH₄ and the paraffin content in FTS products, but the additional presence of K inhibits these effects. Potassium titrates residual acid and hydrogenation sites and increases the olefin content and molecular weight of FTS products. K increases the rate of secondary water–gas shift reactions, while Cu increases the relative rate of oxygen removal as CO₂ instead of water after CO is dissociated in FTS elementary steps. Through these two different mechanisms, K and Cu both increase CO₂ selectivities during FTS reactions on catalysts based on Fe–Zn oxide precursors.     

Study on products distribution of iron and iron–zeolite catalysts in Fischer–Tropsch synthesis

The dependencies of hydrocarbon product distributions of iron and physical mixture of iron–zeolite catalyzed Fischer–Tropsch synthesis on reaction conditions include: reaction temperature, reaction pressure, H₂/CO feed ratios; space velocity and effect of zeolite presence have been investigated. The concept of two superimposed Anderson–Schulz–Flory distributions has been applied for the representation of the product distribution for both iron and iron–zeolite catalysts. Zeolite presence increased secondary reactions that include cracking of heavier products and light olefins oligomerization. Product distribution of iron–zeolite catalyst was comparable with iron catalyst at high space velocity, because the role of the zeolite on overall reaction declined.The results of the product distribution dependency on the reaction conditions over both iron and iron–zeolite catalysts showed that the average number of carbon decreases with H₂/CO ratio increasing and the reaction temperature in product.     

Effects of Zr and K Promoters on Precipitated Iron-Based Catalysts for Fischer–Tropsch Synthesis

Abstract  

The effects of Zr and K promoters on the structure, adsorption, reduction, carburization and catalytic behavior of precipitated iron-based Fischer–Tropsch synthesis (FTS) catalysts were investigated. The catalysts were characterized by N₂ physisorption, temperature-programmed reduction/desorption (TPR/TPD) and M?ssbauer effect spectroscopy (MES) techniques. As revealed by N₂ physisorption, Zr and/or K promoted catalysts showed lower surface area than Fe/SiO₂ catalyst. Zr promoter inhibited the reduction and carburization because of the interaction between Fe and Zr in Fe–Zr/SiO₂ catalysts. K promoter enhanced the reduction in CO and apparently facilitated the CO adsorption, thus promoted the carburization, but it retarded the reduction in H₂ and severely suppressed the H₂ adsorption. Compared with the singly promoted catalysts, the doubly promoted catalyst had the highest FTS activity. In addition, both Zr and K promoters suppressed the formation of methane and shifted the production distribution to heavy hydrocarbons.     

Cobalt species in promoted cobalt alumina-supported Fischer–Tropsch catalysts

The structure of cobalt species at different stages of the genesis of monometallic and Pt-promoted cobalt alumina-supported Fischer–Tropsch catalysts was studied using X-ray diffraction, UV–visible spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectron spectroscopy, and DSC–TGA thermal analysis. The catalysts were prepared by incipient wetness impregnation using solutions of cobalt nitrate and dihydrogen hexachloroplatinate. Both variation of catalyst calcination temperature between 473 and 773 K and promotion with 0.1 wt% of Pt had no significant affect on the size of supported Co₃O₄ crystallites. The size of cobalt oxide particles in the calcined catalysts seems to be influenced primarily by the pore diameter of the support. Cobalt reducibility was relatively low in monometallic cobalt alumina-supported catalysts and decreased as a function of catalyst calcination temperature. The effect was probably due to the formation of mixed surface compounds between Co₃O₄ and Al₂O₃ at higher calcination temperatures, which hinder cobalt reduction. Promotion with platinum spectacularly increased the rate of cobalt reduction; the promotion seemed to reduce the activation energy of the formation of cobalt metallic phases. Analysis of the magnetization data suggests that the presence of Pt led to the reduction of smaller cobalt oxide particles, which could not be reduced at the same conditions in the cobalt monometallic catalysts. Promotion of cobalt alumina-supported catalysts with small amounts of Pt resulted in a significant increase in Fischer–Tropsch cobalt time yield. The efficient control of cobalt reducibility through catalyst calcination and promotion seems to be one of the key issues in the design of efficient cobalt alumina-supported Fischer–Tropsch catalysts.     

Chemisorption of C3 hydrocarbons on cobalt silica supported Fischer–Tropsch catalysts

Chemisorption of propene and propane was studied in a pulse reactor over a series of cobalt silica-supported Fischer–Tropsch catalysts. It was shown that interaction of propene with cobalt metal particles resulted in its rapid autohydrogenation. The reaction consists in a part of the propene being dehydrogenated to surface carbon and CH_x chemisorbed species; hydrogen atoms released in the course of propene dehydrogenation are then involved in hydrogenation of remaining propene molecules to propane at 323–423 K or in propene hydrogenolysis to methane and ethane at temperatures higher than 423 K. The catalyst characterization suggests that propene chemisorption over cobalt catalysts is primarily a function of the density of cobalt surface metal sites. A correlation between propene chemisorption and Fischer–Tropsch reaction rate was observed over a series of cobalt silica-supported catalysts. No propane chemisorption was observed at 323–373 K over cobalt silica-supported catalysts. Propane autohydrogenolysis was found to proceed at higher temperatures, with methane being the major product of this reaction over cobalt catalysts. Hydrogen for propane autohydrogenolysis is probably provided by adsorbed CH_x species formed via propane dehydrogenation. Propene and propane chemisorption is dramatically reduced upon the catalyst exposure to synthesis gas (H₂/CO = 2) at 323–473 K. Our results suggest that cobalt metal particles are probably completely covered by carbon monoxide molecules under the conditions similar to Fischer–Tropsch synthesis and thus, most of cobalt surface sites are not available for propene and propane chemisorption.     

Effect of SiO2 content on iron-based catalysts for slurry Fischer–Tropsch synthesis

A study has been carried out to investigate the effects of binder SiO₂ content on catalytic behavior of spray-dried precipitated Fe/Cu/K/SiO₂ catalysts for Fischer–Tropsch synthesis (FTS). The catalysts were characterized by means of N₂ physisorption, H₂ temperature-programmed reduction (TPR), scanning electron microscopy (SEM), and Mössbauer effect spectroscopy (MES). The Fischer–Tropsch synthesis performances (activity, selectivity and stability) of the catalysts were studied in a slurry-phase continuously stirred tank reactor (CSTR). The results indicated that the increase of SiO₂ content stabilizes Fe₃O₄ phase and suppresses the further reduction and carburization of the catalysts in syngas. Long time on stream FTS performances showed that the catalyst with SiO₂ improves its reaction stability. The selectivities to light hydrocarbons (methane, C₂–C₄, C₅–C₁₁) are enhanced whereas those to heavy hydrocarbons (C₁₂⁺) are suppressed with increasing SiO₂ content. The results were explained to the interactions of Fe–SiO₂ and K–SiO₂. From the present study, it is found that a catalyst with composition of 100Fe/5Cu/4.2K/25SiO₂ on mass basis displays both better FTS performances and a good attrition resistance, which is suitable for the use in CSTRs or SBCRs (Slurry Bubble Column Reactors) for FTS reaction.     

Influence of Ru segregation on the activity of Ru–Co/γ-Al2O3 during FT synthesis: A comparison with that of Ru–Co/SiO2 catalysts

γ-Al₂O₃ and SiO₂ supported Co catalysts, with varying amounts of Ru, were prepared and evaluated for Fischer–Tropsch synthesis (FTS). The composition of Ru for optimum activity was found to be support-dependent. The reducible Co₃O₄ was high in the region of 0–1.64 wt.% of Ru in Co/SiO₂ catalysts. Co/γ-Al₂O₃ displayed a maximum for reducible Co species at 0.42 wt.% Ru. Segregation of Ru occurred beyond this composition decreasing the extent of reduction. Co/γ-Al₂O₃ catalysts showed lower activity and olefin selectivity, in spite of higher Co dispersion, than Co/SiO₂ catalysts. The catalytic performance depends on the amount of reducible Co species, which again depends upon the optimum content of Ru.     

Evidence of strong metal–oxide interactions in promoted Rh/SiO2 on CO hydrogenation: Analysis at the site level using SSITKA

It has been suggested that the behavior of Group VIII metal catalysts supported on transition metal oxides can be significantly affected by pretreatment conditions due to strong metal–oxide interactions (SMOI). However, the origins for the SMOI effect are still in debate. In this research, SMOI of Rh and vanadium oxide (as a promoter) supported on SiO₂ were studied at the site level for the first time, which provides an insight into the modification of surface properties after high temperature reduction. H₂ chemisorption, Fischer–Tropsch synthesis (FTS), and SSITKA (steady-state isotopic transient kinetic analysis) were used to probe the SMOI effects. The catalytic properties of the catalysts for CO hydrogenation were investigated using a differential fixed bed reactor at 230 °C and 1.8 atm, while for SSITKA, a reaction temperature of 280 °C and an excess of H₂ was used to maximize methane production. The addition of V to Rh/SiO₂ suppresses H₂ chemisorption, and high reduction temperature further decreases H₂ chemisorption on Rh/V/SiO₂ but has little effect on Rh/SiO₂. As reduction temperature increases, the activity for CO hydrogenation on Rh/SiO₂ remains essentially unchanged, but the activity of Rh/V/SiO₂ decreases significantly. SSITKA shows that the concentration of surface reaction intermediates decreases on Rh/V/SiO₂ as the reduction temperature increases, but the activities of the reaction sites increase. The results suggest that Rh being covered by VO_x species is probably the main reason for the decreased overall activity induced by high reduction temperature, but more active sites appear to be formed probably at the Rh–VO_x interface.     

Fischer–Tropsch synthesis over cobalt dispersed on carbon nanotubes-based supports and activated carbon

Carbon nanotubes (CNTs) and the ones grown on MgO and alumina are used as supports for cobalt catalyst in Fischer–Tropsch (FT) synthesis. Carbon nanotubes were synthesized by chemical vapor deposition of methane on 5.0 wt.% iron on MgO or alumina at 950 °C. The carbon nanotubes were characterized by SEM and TEM microscopy and Raman spectroscopy. Cobalt nitrate was impregnated onto the supports by impregnation, and the samples were dried and reduced in-situ at 400 °C for 12 h, and then FT synthesis was carried out in a fixed-bed reactor. The catalysts were characterized by BET surface area measurement, TPR and TPD. The effect of carbon nanotubes as cobalt support on CO conversion, product selectivity, and olefin to paraffin ratio of FT synthesis was investigated and compared with activated carbon as well as Al₂O₃, as a traditional support. The results revealed that the activity of the Co/CNT catalyst was improved by 22%, compared to the conventional Co/alumina catalysts. Also the cobalt supported on CNTs grown on MgO (Co/CNT–MgO) shows the highest selectivity to C₅₊ as the most desired FTS products. The C₅₊ selectivity enhancement was about 37, 34, 17, and 77% as compared to the Co/CNT, Co/alumina, Co/CNTs-alumina, and Co/activated carbon, respectively. Also the olefin/paraffin ratio on the Co/CNTs-MgO catalyst is about 7.7 times higher than the conventional cobalt catalysts. It seems that the degree of reduction of cobalt is higher when supported on CNTs than on alumina. This leads to higher FTS activity. Also, the particle size distribution of the cobalt is affected by the CNT–MgO support leading to higher C₅₊ selectivity.     

Silica-supported rhodium-cobalt catalysts for Fischer–Tropsch synthesis

The effects of the addition of Ag, Au, or Rh to a 15 wt% Co/SiO₂ catalyst on the Fischer–Tropsch (FT) synthesis were studied. Both Au and Rh showed a promoting effect on the FT activity, whereas the addition of Ag decreased the activity. The addition of a small amount of Rh (0.1–0.5 wt%) increased the CO conversion by 50% without affecting the selectivity. It was found that Rh catalyzed the reduction of cobalt oxides, but it did not change the number of surface cobalt atoms. It is proposed that the higher activity of Rh-promoted catalysts is due to the hydrogen spillover from Rh to Co during FT synthesis.     

Intensified Fischer–Tropsch synthesis process with microchannel catalytic reactors

A microchannel catalytic reactor with improved heat and mass transport has been used for Fischer–Tropsch synthesis. It was demonstrated that this microchannel reactor based process can be carried out at gas hourly space velocity (GHSV) as high as 60,000 h⁻¹ to achieve greater than 60% of single-pass CO conversion while maintaining relatively low methane selectivity (<10%) and high chain growth probability (>0.9). In this study, performance data were obtained over a wide range of pressure (10–35 atm) and hydrogen-to-carbon monoxide ratio (1–2.5). The catalytic materials were characterized using BET, scanning electron microcopy (SEM), transmission electron microcopy (TEM), and H₂ chemisorption. A three-dimensional pseudo-homogeneous model was used to simulate temperature profiles in the exothermic reaction system in order to optimize the reactor design. Intraparticle non-isothermal characteristics are also analyzed for the FT synthesis catalyst.     

Intrinsic kinetics of Fischer–Tropsch reactions over an industrial Co–Ru/γ-Al2O3 catalyst in slurry phase reactor

The rate of Fischer–Tropsch synthesis over an industrial well-characterized Co–Ru/γ-Al₂O₃ catalyst was studied in a laboratory well mixed, continuous flow, slurry reactor under the conditions relevant to industrial operations as follows: temperature of 200–240 °C, pressure of 20–35 bar, H₂/CO feed ratio of 1.0–2.5, gas hourly space velocity of 500–1500 N cm³ g_cat^− 1 h^− 1 and conversions of 10–84% of carbon monoxide and 13–89% of hydrogen. The ranges of partial pressures of CO and H₂ have been chosen as 5–15 and 10–25 bar respectively. Five kinetic models are considered: one empirical power law model and four variations of the Langmuir–Hinshelwood–Hougen–Watson representation. All models considered incorporate a strong inhibition due to CO adsorption. The data of this study are fitted fairly well by a simple LHHW form − R_H2 + CO = ap_H2^0.988p_CO^0.508 / (1 + bp_CO^0.508)² in comparison to fits of the same data by several other representative LHHW rate forms proposed in other works. The apparent activation energy was 94–103 kJ/mol. Kinetic parameters are determined using the genetic algorithm approach (GA), followed by the Levenberg–Marquardt (LM) method to make refined optimization, and are validated by means of statistical analysis. Also, the performance of the catalyst for Fischer–Tropsch synthesis and the hydrocarbon product distributions were investigated under different reaction conditions.     

Co/SiO2 Sol–Gel Catalysts for Fischer–Tropsch Synthesis

Six catalysts with a different cobalt content were prepared following the sol–gel method. The samples were tested by the Fischer–Tropsch synthesis performed both at low and high temperature and pressure. All the catalytic performances are well correlated with the characterization results which highlighted the good metal distribution and the presence of a large amount of metallic cobalt (but not the whole) on the surface of both 10 and 30% Co catalysts. As a result, high CO conversions were observed with a particular sensitivity to the reaction pressure and also the limitation of the produced hydrocarbons fraction to C₆ or C₉ at low or high pressure, respectively.     

Study of an iron-manganese Fischer–Tropsch synthesis catalyst promoted with copper

The metal–silica interaction and catalytic behavior of Cu-promoted Fe–Mn–K/SiO₂ catalysts were investigated by temperature-programmed reduction/desorption (TPR/TPD), differential thermogravimetric analysis, in situ diffuse reflectance infrared Fourier transform analysis, and Mössbauer spectroscopy. The Fischer–Tropsch synthesis (FTS) performance of the catalysts with or without copper was studied in a slurry-phase continuously stirred tank reactor. The characterization results indicate that several kinds of metal oxide–silica interactions are present on Fe–Mn–K/SiO₂ catalysts with or without copper, which include iron–silica, copper–silica, and potassium–silica interactions. In addition to the well-known effect of Cu promoter on easing the reduction of iron-based FTS catalysts, it is found that Cu promoter can increase the rate of carburization, but does not vary the extent of carburization during the steady-state FTS reaction. The basicity of the Cu and K co-promoted catalyst is greatly enhanced, as demonstrated by CO₂-TPD results. In the FTS reaction, Cu improves the rate of catalyst activation and shortens the induction period, whereas the addition of Cu has no apparent influence on the steady-state activity of the catalyst. Promotion of Cu strongly affects hydrocarbon selectivity. The product distribution shifts to heavy hydrocarbons, and the olefin/paraffin ratio is enhanced on the catalyst due to the indirect enhancement of surface basicity by the copper promotion effect.