Article

Effect of Re and Al2O3 Promotion on the Working Stability of Cobalt Catalysts for the Fischer–Tropsch Synthesis

Authors:
  • Platov South-Russian State Polytechnic University (NPI)
  • Южно-Российский государственный политехнический университет (НПИ) имени М.И. Платова
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... To regulate the most important physicochemical and catalytic properties of cobalt FT synthesis systems, the introduction of promoters is traditionally used. The selective synthesis of C5+ hydrocarbons is carried out on catalysts promoted by metals (Ru, Re, etc.), oxides of alkali, refractory metals (Al2O3, MgO, etc.) and a number of transition metals [25][26][27][28][29][30][31]. Promoters perform functions related to structures, electronics, texture modifiers, stabilizers, etc., which can improve catalytic characteristics (particle size, dispersion and degree of cobalt reduction, determining the interaction of cobalt with the carrier, the number and structure of active centers). ...
Article
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New bifunctional cobalt catalysts for combined Fischer–Tropsch synthesis and hydroprocessing of hydrocarbons containing Pt were developed. To prepare catalysts in the form of a composite mixture, the FT synthesis catalyst Co-Al2O3/SiO2 and ZSM-5 zeolite in the H-form were used as metal and acid components, respectively, with boehmite as a binder. The catalysts were characterized by various methods, such as XRD using synchrotron radiation, SEM, EDS, TEM and TPR. The effect of the Pt introduction method on the particle size and conditions for cobalt reduction was studied. The testing of catalysts in Fischer–Tropsch synthesis was carried out at a pressure of 2.0 MPa, a temperature of 240 and 250 °C, an H2/CO ratio of 2 and a synthesis gas volumetric velocity of 1000 h−1. It is shown that the method of introducing a hydrogenating metal by adjusting the nano-sized spatial structure of the catalyst determined the activity in the synthesis and group and fractional composition of the resulting products. It is established that the presence of Pt intensified the processes of synthesis and hydrogenation, including isomeric products, and reduced the content of unsaturated hydrocarbons. The application of Pt by impregnation onto the surface of the metal component of the catalysts provided the highest productivity for C5+ hydrocarbons, and for the acidic component, it enabled maximum cracking and isomerizing abilities.
... The dependence of CO chemisorption on the concentration of Al 2 O 3 is of an extreme character with a maximum corresponding to the catalyst with the optimum content of the promoter. The relatively high chemisorption of CO changes the ratio of surface concentrations of hydrogen and carbon monoxide, promoting the formation of C 5+ hydrocarbons.The stability of the developed catalyst and the Co/γ-Al 2 O 3 reference catalyst promoted by Re in FT synthesis (pressure, 0.1 MPa; ratio H 2 : CO = 2; gas hourly space velocity (GHSV), 100 h −1 ) was studied during continuous tests lasting 200-300 h[110]. The prospects of using Co/SiO 2 catalysts promoted by Al 2 O 3 for the selective synthesis of C 5+ hydrocarbons (including long chain products of C 35+ ) were confirmed. ...
... The dependence of CO chemisorption on the concentration of Al 2 O 3 is of an extreme character with a maximum corresponding to the catalyst with the optimum content of the promoter. The relatively high chemisorption of CO changes the ratio of surface concentrations of hydrogen and carbon monoxide, promoting the formation of C 5+ hydrocarbons.The stability of the developed catalyst and the Co/γ-Al 2 O 3 reference catalyst promoted by Re in FT synthesis (pressure, 0.1 MPa; ratio H 2 : CO = 2; gas hourly space velocity (GHSV), 100 h −1 ) was studied during continuous tests lasting 200-300 h[110]. The prospects of using Co/SiO 2 catalysts promoted by Al 2 O 3 for the selective synthesis of C 5+ hydrocarbons (including long chain products of C 35+ ) were confirmed. ...
Article
The review considers modern approaches to the development of a technology for producing cobalt catalysts that are applied in the majority of known processes aimed to synthesize hydrocarbons from СО and Н2 by the Fischer – Tropsch method. The development of efficient catalysts makes it possible to replace the fuel produced from fossil oil by an alternative ultrapure fuel, thus reducing the detrimental effect on the environment. The results of R&D studies, particularly those obtained recently by the authors, were analyzed to reveal topical trends in the development of high-performance catalysts for the synthesis of hydrocarbons by the Fischer – Tropsch method within the classical and integrated GTL technology, including the creation of polyfunctional systems of new types. The review provides information mainly on the catalytic characteristics of the supported cobalt catalysts synthesized by impregnation and the related polyfunctional hybrid catalysts for the selective synthesis of fuel fraction hydrocarbons. The control of selectivity and productivity as well as changes in activity and physicochemical properties of the catalysts during a long-term operation are considered.
... The addition of rhenium to iron or cobalt catalysts promotes higher activity and selectivity towards alcohols and larger chain hydrocarbons, increases particle dispersibility and decreases nanoparticle average sizes while also lowering the reduction temperature of the first metal [30,88,89] . Since the patents of Mauldin, from Exxon [90] , and the Shell Middle Distillate processes [91] , the cobalt catalyst promoted by rhenium has been extensively studied in FTS, and it currently remains a topic of research interest since improvements are still needed in preventing catalyst deactivation and further improving control over selectivity [92][93][94][95][96] . ...
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Although rhenium may not be the most common choice of active species in catalysis, it has been reported as a highly active and selective catalyst over a wide range of reactions. Its applications include hydrogenation reactions of great relevance in the field of renewable materials and bio‐derived platform molecules, such as valorization of lignin, CO2, and carboxylic acids. Different from several transition metals, rhenium presents oxidation numbers varying from −3 to +7. Such diversity in the coordination chemistry of rhenium is reflected in the variety of known rhenium compounds, since this metal can form stable structures such as ligand‐bridged multinuclear and organometallic compounds as well as inorganic oxides, metal‐organic frameworks, and clusters. The exceptional flexibility in rhenium speciation yields numerous selective catalysts; however, it also makes the characterization of rhenium catalysts challenging, and its influence on the catalytic activity is not trivial. This review will outline the most established rhenium‐based materials used in hydrogenation catalysis and shed some light on the relation of rhenium species to catalyst selectivity based on advanced characterization techniques. Finally, our perspectives on the use of rhenium catalysts to produce value‐added products will be given.
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The effect exerted by the content of metallic (Co-Al 2 O 3 /SiO 2 catalyst) and acidic (ZSM-5 zeolite in the H-form) components on the properties of bifunctional catalyst for the integrated synthesis of waxy diesel fuel by the Fischer–Tropsch method was studied. Catalysts represented by a composite mixture with a boehmite binder were characterized by XRD, BET and TPR methods. The testing was performed in a flow reactor with a fixed catalyst bed at a pressure of 2.0 MPa, temperature 240 °С and gas hourly space velocity 1000 h –1 . Activity and selectivity of the catalysts as well as the fractional and hydrocarbon composition of the products were investigated in dependence on the ratio of components. It was found that productivity of the synthesis for С 5+ hydrocarbons and selectivity for the С 11 –С 18 diesel fraction products with a high content of isomeric products correlated with the ratio of metallic and acidic components in the catalysts. The composition of the catalyst recommended for the diesel fuel production has the 1.17 ratio of metallic and acidic components.
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Article
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The influence of cobalt particle size in the range 3 to 18 nm on the Fischer–Tropsch synthesis intrinsic activity and product distribution was investigated over 26 alumina (γ-Al2O3 and α-Al2O3) supported catalysts. A volcano-like curve was obtained for the γ-Al2O3 based catalysts when the C5+ selectivity was plotted as a function of the particle size. The maximum selectivity was located at 7–8 nm. This is the first time an optimum particle size has been identified. An apparent optimum size was also discovered for the series of α-Al2O3 based catalysts. Interestingly, the C5+ selectivity of the α-Al2O3 based catalysts was higher than the selectivity of the γ-Al2O3 based catalysts at all particle sizes. Thus, the selectivity is related both to the particle size and the support. No relation was observed between the cobalt particle size and the cobalt site-time yield.
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The deactivation of Co/SiO2 catalyst for Fischer–Tropsch synthesis (FTS) at different H2 / CO ratios was investigated by XRD, FTIR, BET, XPS, TPR and H2 chemisorption. It was found that the deactivation rate of the catalyst increased with the rise of the H2 / CO ratio. The generation of silicates and/or hydrosilicates species was evidenced by TPR and XPS, and their amounts were monotonously enhanced with increasing H2 / CO ratio, which suggested that the deactivation was caused by the transformation of metallic cobalt into inactive silicates and the high partial pressure of H2 facilitated the formation of the silicates. Moreover, the percentage loss of the surface cobalt was larger than that of bulk cobalt, suggesting that the cobalt silicates and/or hydrosilicates species were formed mainly on the surface of the catalyst or in the small crystallites. For the catalyst run at H2 / CO ratio of 1, it was observed that the sintering also contributed to the catalyst deactivation, but it was a less important factor for the deactivation.
Article
The effect of the measurement conditions on the hydrogen chemisorption on CoRe/γ-Al2O3 has been studied systematically. The measurement temperature is one of the most important factors. In the range from 298 K to 433 K the amount of strongly adsorbed hydrogen increases, and remains stable up to 473 K. The difference observed is significant, the estimated cobalt dispersion is up to 20% higher at the higher temperatures. Different γ-alumina samples were examined, and the behaviour was similar for all the samples as well as for samples with different apparent particle sizes. The most effective evacuation temperature to remove the adsorbed hydrogen on cobalt before hydrogen chemisorption analysis was found to be the temperature used for catalyst reduction.
Article
The impact of water on the deactivation of a 0.5% Pt-promoted 15% Co/Al2O3 catalyst was studied by XAFS. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under an inert gas blanket. Synthesis operating conditions were maintained constant while differing amounts of argon were replaced by added water. Below 25% added water (H2O/CO=1.2; H2O/H2=0.6), the slight negative effect on activity was reversible, and no changes were observed in the EXAFS or XANES spectra. This indicates that the effect of water in this range is most likely kinetic. However, XAFS results strongly suggest that, above 25% water addition, the sudden irreversible loss in activity is due to reaction of the cobalt clusters with the support, forming cobalt aluminate-like species. The XAFS and previously reported activity data indicate that there are two regions for the water effect: at lower H2O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H2O/CO ratios irreversible oxidation of cobalt occurs.
Article
An investigation of the CO hydrogenation of Pt- or Re-promoted 8.7 wt% Co/Al2O3 (1.0 wt% Pt or 1.0 wt% Re) has been carried out at two different conditions: 473 K, 5 bar, H2/CO = 2 and 493 K, 1 bar, H2/CO = 7.3. The addition of Pt or Re significantly increases the CO hydrogenation rate (based on weight of Co), but the selectivity was not changed by the presence of Pt or Re. The results show that the observed increases in the reaction rates are caused by increased reducibility and increased number of surface exposed Co-atoms. Steadystate isotopic transient kinetic analysis (SSITKA) with carbon tracing was used to decouple the effects of the concentration of active surface intermediates and the average site reactivity of intermediates during steady-state CO hydrogenation. The SSITKA results show that the concentration of active surface intermediates leading to CH4 increased as a result of the addition of a noble metal promoter. However, the average site activity was not significantly affected upon Re or Pt addition.
Article
A systematic study of the effect of γ-Al2O3 support variables on Fischer–Tropsch synthesis activity and selectivity was carried out at industrially relevant conditions (T=483 K, P=20 bar, H2/CO=2.1). A total of 13 catalysts were prepared by incipient wetness impregnation of γ-Al2O3 supports of varying pore characteristics and chemical purities with aqueous solutions containing the required amounts of cobalt and rhenium precursor to give nominal loadings of 20 and 0.5 wt%, respectively. The catalysts were analysed for cobalt, rhenium, nitrogen, and sodium and characterised by nitrogen adsorption/desorption, mercury intrusion, X-ray diffraction, hydrogen chemisorption, temperature-programmed reduction, and oxygen titration. The size of the Co3O4 cobalt particles was controlled by the support pore size, with small particles formed in narrow pores and large particles formed in wide pores. The degree of reduction increased with increasing catalyst pore size and Co3O4 particle size. The catalysts contained different amounts of sodium (20–113 ppm) and the site-time yield decreased with increasing sodium content. Positive correlations were found between cobalt particle size and C5+ selectivity and between catalyst pore size and C5+ selectivity. The results indicate that the C5+ selectivity is controlled by the size and appearance of cobalt particles.
Article
The unpromoted and promoted Fischer–Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer–Emmett–Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al2O3 catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.
Article
The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of γ-Al2O3 supported cobalt catalysts was investigated in fixed-bed reactors at T = 483 K, P = 20 bar, and H2/CO = 2.0. Catalysts containing 20 wt.% cobalt and 0 or 0.5 wt.% rhenium were prepared by incipient wetness impregnation of different γ-Al2O3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re-promoted catalysts, also perrhenic acid. The γ-Al2O3 supports had very different pore characteristics. The post-calcination Co3O4 crystallite size was predominantly controlled by the γ-Al2O3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co3O4 was reduced in two steps to Co0 with CoO as intermediate species. However, while reduction of Co3O4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co0 was dependent on the catalyst properties. Large particles present in wide pores were easier to reduce than small particles located in narrow pores. In addition, Re promoted the reduction of CoO. The effect of rhenium as a reduction promoter was less pronounced at increasing pore size and particle size. Re also had a similar positive impact on the cobalt dispersion of the catalysts. Although Re significantly increased the Fischer–Tropsch synthesis cobalt-time yield, it did not modify the site-time yield. The deactivation rates of all the catalysts were also similar up to 100 h on stream. Positive correlations were found between the catalyst pore diameter and the C5+ selectivity and between the cobalt particle size and the C5+ selectivity. Re had a consistent positive, albeit small, effect on the C5+ selectivity.
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