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Article

The Influence of Platinum on the Catalytic Properties of Bifunctional Cobalt Catalysts for the Synthesis of Hydrocarbons from CO and H2

by
Roman E. Yakovenko
1,*,
Ivan N. Zubkov
1,*,
Ol’ga P. Papeta
1,
Yash V. Kataria
1,
Vera G. Bakun
1,
Roman D. Svetogorov
2 and
Alexander P. Savost’yanov
1
1
Research Institute “Nanotechnologies and New Materials”, Platov South-Russian State Polytechnic University (NPI), Prosveschenya 132, 346428 Novocherkassk, Russia
2
National Research Center “Kurchatov Institute”, 1 Akademika Kurchatova Square, 123098 Moscow, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(6), 351; https://doi.org/10.3390/catal14060351
Submission received: 12 April 2024 / Revised: 23 May 2024 / Accepted: 27 May 2024 / Published: 29 May 2024
(This article belongs to the Section Industrial Catalysis)
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Versions Notes

Abstract

:
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.

Graphical Abstract

1. Introduction

The modern economy is focused on the creation and implementation of new “green” technologies, and the bulk of the needs for energy and fuel is provided by various types of fossil raw materials and, as a result, is accompanied by a set of environmental problems (greenhouse gas emissions, anthropogenic air pollution, accumulation of waste oil products, etc.) [1,2]. A universal way to obtain environmentally friendly alternative fuels to oil are XTL (X-to-liquid) technologies using coal, biomass, shale, natural and associated petroleum gases, etc. [2,3,4,5,6]. GTL-version technology for the production of transport fuels with high added value from synthesis gas by catalytic Fischer–Tropsch synthesis (FT), combined with hydroprocessing of the resulting hydrocarbons (mainly n-alkanes and alkenes with different carbon chain lengths) in one reaction apparatus, is commercially promising.
The implementation of such technologies involves the creation of multifunctional catalytic systems that are effective in the conditions of FT synthesis and that have potential in the reactions of hydrocracking, isomerization, oligomerization, aromatization and hydrogenation of hydrocarbons [7,8,9,10,11,12,13,14,15,16]. The catalyst simultaneously implements the function of FT synthesis (hydrogenating metals) and hydrorefining products (zeolites or zeolite-like structures). A particularly difficult problem in the development of catalysts is obtaining a synergistic effect in the interaction between functioning metal and acid centers, mainly with the quantitative ratio and degree of proximity between these centers. The concept of proximity of active centers was subjected to detailed analysis in [17,18]. Moreover, an important aspect is the assessment and regulation of the state of metal centers and the factors influencing the manifestation of the relatively less studied metal function [19].
Group VIII metals are used as active metals for FT synthesis, but iron and cobalt, as a rule, find practical application in the high- and low-temperature FT process [20,21,22,23,24]. To prepare catalysts, a variety of methods are considered, including precipitation (co-precipitation), impregnation, the adsorption process, ion exchange, mixing, etc. The technological cycle of preparation is completed by heat treatment and activation of the catalysts. However, when preparing deposited systems, the most popular options remain the impregnation method, with careful control of the concentration and calcination temperature, and ion exchange, with a limitation of the concentration but obtaining a higher dispersion of metals in zeolites.
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).
Some promoters, including noble metals, can act as monometallic catalysts as well as structural and electron promoters when loaded in individual layers, apparatuses or mixed catalyst systems. The combination of these effects ensures maximization of the yield and quality of products of multifunctional systems in the combined synthesis of FT, including the cracking and isomerization stages. For example, it reduces the proportion of unsaturated hydrocarbons by introducing additional hydrogenating components into the catalysts, such as Pt, Pd and Ni [32,33,34,35,36,37,38,39], which have been studied little in the process of the conversion of C16+ hydrocarbons using systems containing Pt [35,37].
Thus, cobalt-based FT synthesis catalysts have many possibilities for use and the production of desired products. A detailed review of data published in articles devoted to the study of the process can provide a large set of materials for discussing trends in improving methods and formulations for the preparation of catalysts, including promoters [40] and the development of new and known production technologies, including the optimization of reaction apparatuses and parameters for subsequent industrial application. Moreover, as most authors note, scaling up a catalyst to commercial production requires a lot of effort, and publicly available information on industrial production processes in the presence of catalysts is limited and fragmentary. Data on the formulation of commercial-type cobalt catalysts from Sasol, Shell, Exxon Mobil, ConocoPhillips, Velocys and Compact G and their applications are provided in [41]. A well-known bifunctional catalyst from Chevron that is in demand was developed for the production of synthetic oil in the hybrid FT process [42]. The catalyst for a low-temperature process at medium pressure contains the following, in wt.%: 7.5 Co, 0.19 Ru, 75 ZSM-5 (or ZSM-12) and 17 aluminum oxide. Catalyst granules are prepared by impregnation with solutions of active components in an acid component (which makes it difficult to control the concentration and distribution of components in the crystalline structure of the zeolite), followed by mixing and molding with a binder.
Such studies were carried out in relation to the bifunctional system we previously developed for the one-stage synthesis of hydrocarbon fuel fractions [15,16,43,44]. A composite catalyst in the form of granules was obtained by mixing a metal component—the catalyst for the synthesis of FT Co-Al2O3/SiO2; an acid component—ZSM-5 zeolite in the H-form; and a boehmite binder. It is established that the implementation of catalyst technology in industrial conditions makes it possible to reproduce the characteristics of the catalyst obtained in laboratory conditions and can be recommended to produce industrial batches [45].
A promising option for expanding the scope of the application of the catalyst may be searching for the optimal hydrogenating metal, including Pt. The method of introducing Pt into the catalyst (using a precursor, which, according to [46], provides the highest dispersion of the metal and the best catalytic characteristics) is seen as a way to influence the state of cobalt centers as an active metal, in conjunction with the acid centers of the zeolite in the spatial structure of the catalyst. The presence of a hydrogenating agent, as a second metal in the composition of a bifunctional catalyst, should obviously be accompanied by an effect on the particle size, dispersion, crystalline orientation of cobalt and other components in the system and the occurrence of effects that depend not only on the number and proximity of metal and acid centers but also, possibly, on the number of acid sites between two metal centers.
The aim of the current work was to study the effect of the Pt introducing method on the catalytic properties of composite bifunctional catalysts based on cobalt for the combined process of Fischer–Tropsch synthesis and hydrocarbon hydroprocessing.

2. Results and Discussion

Studies were carried out on the properties and catalytic characteristics of bifunctional composite catalysts: K and one containing a Pt additive. Data on the method of introducing Pt into the catalyst structure are included in Table 1. The cobalt content in the catalysts, according to X-ray fluorescence analysis, was in the range of 7.2–7.4 wt.%.
SEM images of the oxide surface of PtHZ and Pt(Co-Al2O3/SiO2) catalysts with the addition of Pt in the structure of the acid and metal components introduced by impregnation are shown in Figure 1. Elemental mapping (Figure 2) indicates a uniform surface distribution of oxygen, silicon, aluminum and fragmentary cobalt; the presence of platinum was not recorded. The cobalt content, according to energy-dispersive micro spectral determination, was 5.1 and 5.8 wt.%.
Studies of the phase composition and crystal structure of bifunctional catalysts in oxide form were carried out using X-ray and synchrotron (Figure 3) radiation. The resulting diffraction patterns included elements typical of the patterns of individual catalyst components [47]. Thus, reflections in the region of small angles 2θ ≈ 5°–15° corresponded to ZSM-5 zeolite. The most intense recorded reflection of the zeolite catalyst K (hkl 0 5 1), with an intensity close to 100 arb. units, had a planar distance d of 0.383 nm 2θ ≈ 20°–30°, corresponding to the Al2O3 oxide phase formed during the thermal decomposition of the binder boehmite. The weak intensity of Al2O3 reflections indicated insufficient crystallization of the phase during heat treatment of the catalysts. SiO2 was X-ray amorphous. Cobalt was found in the form of Co3O4 oxide (cubic spinel structure), with the main reflections in the range 2θ ≈ 15°–30°. The diffraction pattern for the catalysts was similar. According to data [46,48], the presence of Pt has virtually no effect on the framework structure of the zeolite, including under ion exchange conditions. In this case, the metal was detected in the form of larger particles, in comparison with the ion exchange method [46], and one reflection in the range of 2θ ≈ 15°–20° near Co3O4 for monometallic catalysts sometimes leads to a strong metal–support interaction [46]. For the studied catalysts with Pt in the structure of the acid component and on the surface of the composite, they were accompanied by some shift of the Co3O4 reflection (hkl 3 1 1) from d = 0.244 nm at an intensity of 107 arb. units up to d = 0.247 nm. The intensity of the reflection for the PtHZ and PtK catalysts decreased by 1.2 and 1.35 times, which may have been a consequence of the interaction of metals; for the Pt(i)HZ catalyst, it increased by 1.35 times. Pt in the Pt(Co-Al2O3/SiO2) and PtSiO2 catalysts can be present in the form of nanoparticles with sizes below the detection limit [49]. During the subsequent formation of the active component during the reduction of Co3O4, as can be seen in the example of a sample without hydrogenating additive K (Figure 4), a CoO oxide phase appeared in the structure of the catalysts, recorded in the same part of the diffraction pattern. It is obvious that the process of the reduction of possible mixed cobalt compounds was also underway.
The average size of Co3O4 crystallites according to the Debye–Scherrer equation and metallic cobalt Co0, obtained by different XRD methods (see Table 2 below), showed that the cobalt was fixed in the structure of the metal component and remained stable during the preparation of the composite catalyst. The average particle size of Co3O4 catalysts was in the range of 12–14 nm, and for Co0, the range was 9–10 nm, which was close to the optimal one (8–10 nm) [22], corresponding to a high activity and selectivity of catalysts in the synthesis of C5+ hydrocarbons. The PtSiO2 catalyst with Pt in the SiO2 structure was characterized by minimal dispersion and large Co0 crystallites. Among the samples with the addition of Pt, the PtK catalyst with metal particles deposited on the surface of the composite K system had the maximum dispersion.
The surface of the reduced catalysts was illustrated by TEM micrographs and histograms of the size distribution of metallic cobalt particles (see Figure 5 below) and ensure that the first citation of each figure appears in numerical order.). For the K and PtK systems, the average particle size of Co0 had the smallest and similar values. As can be seen from the distribution of particles with a size of 4–15 nm, the presence of Pt stimulated an increase in the relative proportion of particles with a size of 8–10 nm, caused by the interaction of metals. When Pt was introduced by zeolite ion exchange into the Co-Al2O3/SiO2 catalyst, the average particle size of Co0 was 9.4 ± 1.2 nm. Moreover, Co0 particles with a size of 6–16 nm were found in the particle size distribution diagrams. In this case, the chosen method of applying platinum led to the formation of a narrower maximum particle size distribution of 8–11 nm.
The H2 TPR spectra change, depending on the method of introducing the additive into the composite system (Figure 6) and the process of catalyst reduction, is displayed by peaks with hydrogen absorption maxima corresponding to the Co3+→Co2+ transition in the region of 190–380 °C and the Co2+→Co0 transition in the region of 280–600 °C. The presence of Pt stimulated the reduction of Co3O4. In particular, when Pt was applied to the K catalyst surface, it shifted the temperature of the first maximum by ~100 °C. When introducing the Pt(i)HZ and PtHZ catalysts into the structure of the acidic component by the methods of ion exchange and impregnation, both maximums were at 20–25 °C (they had a similar configuration for the spectra, including the intensity of the processes occurring). The maximums of the metal component of the Pt(Co-Al2O3/SiO2) catalyst were at 165 and 60 °C. Note that the shift of the peaks to temperatures below 300 °C may have been due to the decomposition of Pt compounds, which are precursors of the hydrogenating metal. Moreover, according to [48], peaks in the range of 180–240 °C can be associated with the reduction of PtO2 in zeolite channels.
The ratio of the hydrogen absorption peak S2/S1 of the catalyst spectra, necessary for the implementation of the step-by-step transition of Co3+→Co0, differed from the theoretically expected value of 3, in accordance with the stoichiometry of reactions (Table 3), fluctuating in the range 2.7–3.6. The slight decrease in the ratio of S2/S1, in comparison with the stoichiometric value, indicated that part of the cobalt was apparently in the form of difficult-to-reduce compounds, for example, silicates; the increase was due to the reduction by Pt. The degree of cobalt reduction for the catalyst K according to XRD data was 50%. The combination of Pt with cobalt, when applied to the surface of the K and Co-Al2O3/SiO2 catalysts and the SiO2 carrier, due to the decrease in temperature, high flow rate for the Pt(Co-Al2O3/SiO2) catalyst and the relative difficulty of the reduction process for the Pt/SiO2 catalyst, led to a change in the shape of the TPR spectra, with the degree of cobalt reduction being 54, 48 and 58% respectively. The absence of an H2 absorption peak in the high-temperature region for the Pt(Co-Al2O3/SiO2) catalyst and other samples confirms that the main part of cobalt is bound in the metal structure and does not significantly interact with the acid and binding components of the composite catalyst with the formation of difficult-to-reduce compounds [50].
Experiments conducted at a temperature of 240 °C, pressure of 2.0 MPa, H2/CO ratio of 2 and a GHSV of 1000 h−1 showed that the catalysts were active in the process of FT synthesis (Table 4). The maximum selectivity and productivity for C5+ hydrocarbons with minimal formation of C1–C4 hydrocarbons were recorded for Pt(i)HZ and Pt(Co-Al2O3/SiO2) catalysts. It was obvious that the introduction of Pt by each of the above-mentioned methods, accompanied by the formation of combined FT synthesis centers and acid sites in nanoscale proximity to each other, resulted in similar catalytic properties [33]. In case of using another method for the impregnation of Pt or changing the nanoscale spatial structure of bifunctional catalysts [49], including proximity (a micrometer-length scale often leads to suboptimal catalytic characteristics [33]) and the ratio of metal and acid sites shows no promoting effect, causing a reduction in the degree of synthesis gas conversion and selectivity for C5+ hydrocarbons.
The process of the formation of the porous structure of bifunctional catalysts (including secondary) is reflected by the pore size distribution curves in Figure 7. These curves made it possible to estimate the spatial position of metal and acid centers for the observed pores in the range of 1–15 nm, including areas in the region of 1–4 nm with maxima of about 2 nm and undivided pores of 3 nm. These corresponded to the contribution to the total pore volume of the zeolite of 3–7 nm, with a maximum of about 5 nm for Co-Al2O3/SiO2 (Figure 7a) and 4–6 nm for boehmite.
C5+ hydrocarbons obtained in the presence of the FT synthesis catalyst Co-Al2O3/SiO2 were 97.4% alkanes for the normal structure, including 46.7% long-chain C19+. In bifunctional systems, the combination of Co/SiO2 and an acidic component containing, for example, Pd, significantly reduces the selectivity of the formation of long-chain hydrocarbons and stimulates the synthesis of iso-alkanes and short-chain alkenes, confirming that the cracking and isomerization of FT hydrocarbons is carried out by the acidic component of the catalyst [51]. In the case of the catalysts under study, it ensured the selective conversion of 63.9% of the primary products formed on metal active centers into hydrocarbons of predominantly fuel fractions (C5–C10 and C11–C18) with a total content of isomers ranging from around 35 to 55% (Table 5). It is known that, when converting C15+ n-alkanes during hydroisomerization on catalysts with HZSM-5-type MFI and a 10-membered ring size, a high density and strength of acid sites [19,33,51], fewer mono- and multi-branched isomers and more cracking products are usually obtained. This is associated with the selectivity of the zeolite shape (blocking the adsorption of large molecules of n-alkanes at the entrance to the pores), leading to the formation of multibranched alkanes and subsequent cracking, which is intensified when a highly active acid component is used. For example, with the introduction of Pt into the zeolite structure, preferably by impregnation, for the PtHZ catalyst, the n-alkanes obtained in the presence of the catalyst underwent secondary transformation to the greatest extent (the quantity of iso-alkanes in the C11–C18 diesel fraction reached 55%). The presence of a hydrogenating additive determined a change in the balance of the synthetic, cracking and isomerizing functions of the composite system, but for all catalysts except PtHZ, it stimulated the synthesis of FT (the quantity of n-alkanes in the synthesis products for the PtK and Pt(CoAl2O3/SiO2) samples was the maximum, and the isomerizing ability decreased). Due to the manifestation of the hydrogenation effect, the formation of alkenes decreased by 2.3–3.5 times, and the o/p index did not exceed 0.3.
The molecular weight distribution (MWD) of C5+ hydrocarbons for synthetic oil samples (mainly from C5 to C30) confirms these changes (Figure 8). The combination of cobalt and Pt in the structure of the acid component of the PtHZ and Pt(i)HZ catalysts shifted the distribution toward products with lower molecular weights (from C4 to C26), in comparison with other MWDs of a pronounced bimodal nature for the Pt(i)HZ catalyst. There was active formation of iso-alkanes. The presence of Pt on the surface of the PtK composite and in the structure of the metal component of the Pt(CoAl2O3/SiO2) catalyst caused an increase in the formation of C5–C30 n-alkanes from 36.1 to 53.5 and 59.2%, as well as their derivatives, for the PtK catalyst, mainly involving hydrocarbons of gasoline fractions. Moreover, the formation of branched alkenes for all samples decreased by more than 1.6 times.
Increasing the process temperature to 250 °C (as a rule, zeolite catalysts, including those with ZSM-5 [37], are active at higher temperatures) led to intensification of the synthesis, significantly leveling out differences in the activity of the catalysts. The degree of CO conversion for catalysts varied in the range of 85.9–92.5%, and selectivity for C5+ remained at a level of 70% (Table 6). In addition to Pt(i)HZ, the ion exchange method led to a decrease in conversion and selectivity, which could have been due to the higher dispersity of Pt in the structure of the acid component [52]. Moreover, the highly dispersed metal deposited on the surface of the metal component, as well as the close proximity of metal and acid centers provided by the selected preparation method, explains the better performance of FT synthesis for the Pt(CoAl2O3/SiO2) catalyst, reaching a productivity of 149.3 kg/m3cat∙h for C5+ hydrocarbons. As is known [32], the optimal combination of two functions of catalysts is determined not only by the kinetics of the formation of intermediate products on metal centers with subsequent transformation on zeolites but also by the mass transfer of intermediate products between the active areas of the surface and inside the catalyst particles.
The group and fractional composition of C5+ hydrocarbons obtained at 250 °C largely retained changes associated with the presence of Pt (Table 7). The process of the destruction of the formed n-alkanes increased. The cracking [49] of hydrocarbons on the Pt(Co-Al2O3/SiO2) catalyst accelerated most significantly (by 1.5 times) with the increasing temperature. For promoted systems, in addition to the PtK catalyst, the total content of iso-alkanes and branched alkenes in the composition of the products reached ~55%. The isomer ratio was close to 2:1, and that for the Pt(Co-Al2O3/SiO2) catalyst was 1.2:1. The iso/n indicator increased to 1.1–1.4. The formation of alkenes, which were mainly C5–C10 hydrocarbons, decreased by 1.5–2.8.
The molecular weight distribution (MWD) of the C5+ hydrocarbons (Figure 9) illustrates the process of changing the composition of the synthesized products at a temperature of 250 °C. The composition of hydrocarbons produced on composite K and on catalysts containing Pt in the structure of the acid component contained C5–C27 n-alkanes in approximately equal quantities. The synthesis was accompanied by an increase in isomeric processes, including the formation of branched alkenes. Similar processes on PtK and Pt(Co-Al2O3/SiO2) catalysts affected hydrocarbons with a molecular weight up to C30, which were predominantly groups above C15.
The dynamics of the changes in the composition of the obtained synthetic oil samples depending on the FT synthesis temperature is presented in Figure 9. An increase in temperature shifted the molecular weight distribution (MWD) of C5+ hydrocarbons toward products of lower molecular weight while maintaining the pronounced bimodal nature of the MWD for catalysts with a modified Pt acid component. An increase in temperature for all catalysts increased the selectivity with respect to the formation of hydrocarbons in the C5–C10 range of gasoline; the total content of iso-alkanes and branched alkenes in the fractions was close to 50% (slightly lower for PtK). Moreover, the hydrocarbon selectivity of the C11–C18 diesel fraction decreased but remained stable at 40% for the Pt(Co-Al2O3/SiO2) catalyst and increased 1.5 times due to C19+ products for the Pt(i)HZ catalyst. The isomers in the fraction of all catalysts were predominantly iso-alkanes, the maximum amount of which for the PtHZ catalyst exceeded 60%.
The authors of [53] studied hybrid catalysts prepared by co-precipitation in suspensions. It was shown that the chosen preparation method, composition and method of introducing platinum under process conditions similar to ours made it possible to obtain C5+ hydrocarbons with a selectivity of 81.6%. However, in this case, the degree of CO conversion did not exceed 42.1%. Lower values of the degree of CO conversion and higher values of selectivity for C5+, in comparison with the catalyst synthesized by us, were due to different preparation methods and cobalt concentrations. The authors of [54] studied the effect of platinum (platinum deposited on zeolite by impregnation) on activity in reactions to produce liquid C5+ hydrocarbons with layer-by-layer loading of the catalyst. Under similar experimental conditions, the selectivity for C5+ was 58.1%. The Fischer–Tropsch synthesis on commercial zeolite-containing catalysts with the addition of Ru was reported by the authors of [42,55]. The developed catalysts produced C5+ hydrocarbons with a selectivity of 68.1–75.8%, and the degree of CO conversion did not exceed 50–55%. On our catalyst, the degree of CO conversion exceeded the degree of CO conversion on known catalysts, and the selectivity for C5+ remained at a level of 70–72%, which is close to or, in some cases, higher than the values for similar catalysts (see Table 8).

3. Materials and Methods

3.1. Catalyst Preparation

Bifunctional catalysts were prepared in the form of a composite mixture. The FT synthesis catalyst Co-Al2O3/SiO2 (20 wt.% Co) was used as a metal component for the selective synthesis of long-chain hydrocarbons [56] with the silica gel carrier KSK (Salavat Catalyst Plant LLC, Bashkortostan, Russia), the acidic component zeolite ZSM-5 (zeolite modulus is 40) in the H-form (Ishimbay Specialized Chemical Plant of Catalysts LLC, Ishimbay, Russia) and the boehmite binder Plural Sasol SB Al(OH)O (“Sasol”, Sandton, South Africa, TH 80). The crushed (<0.1 mm) components were mixed with an aqueous alcohol solution of triethylene glycol with nitric acid with a concentration of 0.65 wt.%, and the volume ratio of nitric acid to triethylene glycol was 1:3. The catalyst granules were extruded and kept for 24 h at room temperature and were further subjected to heat treatment in the following order: 4 h at a temperature of 80 °C, 3 h at 100–150 °C and 4 h at 400 °C, after which the granules were ground and filtered to obtain catalyst particles 1–2 mm in size.
The mass ratio of components in the catalyst corresponded to 35 for Co-Al2O3/SiO2, 30 for HZSM-5 and 35 for Al2O3. The catalyst without additives, K, was labeled as a composite. The introduction of Pt into the catalyst composition as a promoter, with a wt.% of 0.3 in the final catalyst, was carried out by various methods, such as impregnation of the composite catalyst PtK, the FT synthesis catalyst Pt(Co-Al2O3/SiO2), the FT synthesis catalyst carrier PtSiO2 and the HZSM-5 zeolite PtHZ, and ion exchange for the HZSM-5 zeolite Pt (i) HZ.
The Pt(Co-Al2O3/SiO2) and PtSiO2 catalysts were prepared on a silica gel support and dried for 2–4 h at a temperature of 140–160 °C. When preparing the catalyst for FT synthesis, the stage of impregnation of carrier granules was carried out for 0.5 h at 70–80 °C with constant stirring. An aluminum additive in the form of aluminum nitrate was introduced into an aqueous solution of cobalt nitrate (50–55% conc.) to obtain a mass ratio of 20:1 (Co:Al2O3) in the impregnating solution. The catalyst granules were dried for 24 h at room temperature, after which they were subjected to heat treatment in the following order: 4 h at a temperature of 80 °C, 2–4 h at 100–150 °C and 4 h at 300 °C. For the FT catalyst granules and carrier at the stage of introducing Pt, an aqueous solution of hydrogen hexachloroplatinate (IV) H2[PtCl6] was used during impregnation for 0.5 h at 70–80°C with constant stirring. Drying and heat treatment of wet carrier granules, followed by impregnation with solutions of cobalt and aluminum nitrate, were carried out as described above. The obtained catalysts were subjected to a heat treatment cycle similar to the one previously mentioned.
When preparing the PtK and PtHZ catalysts, the composite catalyst and HZSM-5 zeolite were impregnated with an aqueous solution of hydrogen hexachloroplatinate (IV) H2[PtCl6] for 0.5 h at 70–80 °C with constant stirring, after which the excess solution was eliminated. Wet zeolite powder and catalyst granules were then subjected to heat treatment in the following order: 4 h at 80 °C, 4 h at 100–150 °C and 4 h at 550 °C.

3.2. Catalyst Characterization

The cobalt content in the catalysts was determined by X-ray fluorescence analysis (XRF) on an ARL QUANT’X X-ray energy dispersive spectrometer (Thermo Scientific, Emeryville, CA, USA) under the following conditions: an air medium, a Teflon substrate and an effective irradiation area of 48.9 mm2.
The study of the parameters of the porous structure of zeolites and catalysts was carried out by the nitrogen adsorption–desorption method using a Nova 1200e sorbometer gas sorption analyzer (Quantachrome, Boynton Beach, FL, USA). The pore volume was determined by the BJH method at a relative partial pressure P/P0 = 0.95, the pore size distribution was calculated from the BJH desorption curve (Barrett–Joyner–Halenda), and the pore volume was measured using the t-method (de Boer and Lippens). The samples were subjected to a vacuum treatment for 5 h at a temperature of 350 °C. The volume of meso- and macropores was calculated using the instrument software package.
X-ray phase analysis of oxide catalysts, as well as ZSM-5 zeolite, was carried out on an ARLX’TRA Powder Diffractometer (Thermo Fisher Scientific, Basel, Switzerland) with monochromatized CuKα radiation (wavelength λ = 1.5406 nm) using the point-by-point scanning method (step 0.01°, accumulation time at point 2 s) in the 2θ range from 10° to 90°, and it was carried out by precision X-ray diffraction using synchrotron radiation at the X-ray diffraction analysis station of the National Research Center “Kurchatov Institute” (wavelength λ = 0.074 nm, beam size on a sample of 400 μm) using transmission geometry and while recording scattered radiation with the two-dimensional detector Rayonix SX165 (Evanston, IL, USA), LaB6 standard (NIST SRM 660a). The catalyst sample was placed in a 300 μm cryostat and rotated around a horizontal axis during measurement, allowing diffraction patterns to be averaged according to sample orientations. X-ray diffraction patterns of the reduced catalysts using synchrotron radiation were obtained on the Swiss–Norwegian line (SNILESRF) with monochromatic radiation (λ = 0.07121 nm). Identification of the phase composition was carried out using the ICDD PDF-2 electronic database of diffraction standards in the Crystallographica software package, Version 3,1,0,2 with RDB support [47]. The diffraction patterns were processed in the FullProf program; the CSR sizes for cobalt oxide were calculated for the characteristic line with a 2θ value equal to 36.8°, according to the Debye–Scherrer equation [57], and an average particle size and dispersity of metallic cobalt in accordance with the methods from [58].
The surface morphology of the catalysts in the oxide form was studied by scanning electron microscopy (SEM) using a Quanta 200 scanning electron microscope (FEI, Hillsboro, OR, USA). Images were taken in the modes of recording secondary and reflected electrons at an accelerating voltage of up to 30 kV. Energy dispersive analysis (EDA) was carried out using an EDAX Genesis 2000 XMS 30 X-ray microanalysis system (EDAX, Mahwah, NJ, USA). The state of the catalyst surface of the optimal composition was studied using a JSM-6490LV scanning electron microscope (JEOL, Tokyo, Japan, accelerating voltage of 30 kV), which was equipped with an INCA Penta energy dispersive detector FET 3 (Oxford Instruments, Abingdon, UK).
The structure of the reduced catalysts was studied by transmission electron microscopy (TEM) on a Tecnai G2 Spirit BioTWIN microscope (Thermo Fisher Scientific, Eindhoven, The Netherlands) with an accelerating voltage of 120 kV. The samples were preliminarily reduced with a nitrogen–hydrogen mixture (10% hydrogen by volume) at a flow rate of 20 mL min−1 for 1 h in the temperature range of 20–500 °C with linear heating at a rate of 20 °C min−1.
The study of processes on the surface of catalysts using temperature-programmed reduction (TPR) methods was carried out on a Micromeritics ChemiSorb 2750 analyzer (Micromeritics, Norcross, GA, USA) with a thermal conductivity detector (TCD). The catalysts were kept in a helium flow (20 mL min−1) for 1 h at a temperature of 200 °C and cooled to room temperature, and a mixture of 10% hydrogen and 90% nitrogen (20 mL min−1) was supplied. TPR was carried out in the temperature range of 200–800 °C with a heating rate of 20 °C min−1.

3.3. Catalyst Characterization

Studies of the catalytic properties were carried out in an isothermal reactor with a diameter of 16 mm and a stationary catalyst bed. An amount of 10 cm3 of catalyst (fraction of 1–2 mm) was loaded into the reactor in a mixture along with 30 cm3 of quartz of the same granulometric composition. The catalyst was reduced with hydrogen for 1 h at a temperature of 400 °C and GHSV of 3000 h−1. The activation of samples with synthesis gas with a ratio of H2/CO = 2 and catalytic tests were carried out at a pressure of 2.0 MPa and GHSV of 1000 h−1, raising the temperature from 180 °C to a temperature corresponding to the degree of CO conversion of about 50%, or 240–250 °C at a rate of 2.5 °C h−1. Balance experiments were carried out for 40–90 h to analyze the composition and amount of gas at the outlet of the installation every 2 h. The catalytic properties of the samples were judged by the degree of CO conversion, selectivity and productivity of catalysts and the fractional and hydrocarbon composition of the synthesis products.
An analysis of the composition of the source gas and gaseous synthesis products was carried out using a Crystal 5000 gas chromatograph (CHROMATEC, Yoshkar-Ola, Russia) equipped with a thermal conductivity detector and two columns (Haysep R active phase and NaX molecular sieves). The analysis mode was temperature-programmed with a heating rate of 8 °C min−1. The condensed synthesis products were separated by distillation at atmospheric pressure. The isolating fuel fractions had the following boiling points: gasoline, up to 180 °C; diesel, 180–330 °C; and distillation residue, above 330 °C. The composition of C5+ hydrocarbons was determined using an Agilent 7890A gas chromatography–mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) equipped with an MSD 5975C detector and an HP-5MS capillary column.

4. Conclusions

Bifunctional cobalt catalysts containing Pt were developed for combined Fischer–Tropsch synthesis and hydroprocessing of hydrocarbons. Experiments conducted at a pressure of 2.0 MPa, a temperature of 240 and 250 °C, a H2/CO ratio of 2 and a GHSV of 1000 h−1 establish 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.
The presence of Pt activated the processes of hydrogenation and Fischer–Tropsch synthesis, including the formation of isomers of the C11–C18 diesel fraction, and reduced the selectivity for unsaturated hydrocarbons. The bifunctional system synthesized through the introduction of Pt by impregnation into the structure of the metal component of the Fischer–Tropsch synthesis catalyst Co-Al2O3/SiO2 showed the best results for C5+ hydrocarbons. The productivity of the catalyst was around 138.5–149.3 kg/m3cat·h. An increase in the synthesis temperature increased the activity and isomerizing ability of the catalysts, and the iso/n index reached 1.1–1.4. The catalyst with the addition of Pt into the structure of the acid component by impregnation had the greatest cracking and isomerizing abilities.

Author Contributions

Project administration, R.E.Y.; Writing—original draft preparation, I.N.Z.; Investigation, O.P.P.; Writing—review and editing, Y.V.K.; Formal analysis, V.G.B.; Methodology, R.D.S.; Conceptualization, A.P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Science Foundation, No. 23-73-10108.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM images of catalysts in oxide form B: (a) PtHZ; (b) Pt(Co-Al2O3/SiO2).
Figure 1. SEM images of catalysts in oxide form B: (a) PtHZ; (b) Pt(Co-Al2O3/SiO2).
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Catalysts 14 00351 g002aCatalysts 14 00351 g002b
Figure 2. Localization of elements on the surface of catalysts: (a) PtHZ; (b) Pt(Co-Al2O3/SiO2).
Figure 2. Localization of elements on the surface of catalysts: (a) PtHZ; (b) Pt(Co-Al2O3/SiO2).
Catalysts 14 00351 g002aCatalysts 14 00351 g002b
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Figure 3. X-ray diffraction patterns of HZSM-5 and catalysts: 1—HZSM-5; 2—K; 3—PtHZ; 4—Pt(i)HZ; 5—PtK; 6—Pt(Co-Al2O3/SiO2); 7—PtSiO2.
Figure 3. X-ray diffraction patterns of HZSM-5 and catalysts: 1—HZSM-5; 2—K; 3—PtHZ; 4—Pt(i)HZ; 5—PtK; 6—Pt(Co-Al2O3/SiO2); 7—PtSiO2.
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Figure 4. Diffraction patterns of Co-Al2O3/SiO2 and K catalysts: 1—Co-Al2O3/SiO2; 2—K (oxide form); 3—K (reduced form).
Figure 4. Diffraction patterns of Co-Al2O3/SiO2 and K catalysts: 1—Co-Al2O3/SiO2; 2—K (oxide form); 3—K (reduced form).
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Catalysts 14 00351 g005aCatalysts 14 00351 g005b
Figure 5. Results of TEM microscopy of catalysts: (a) K; (b) Pt(i)HZ; (c) PtK; (d) Pt(Co-Al2O3/SiO2).
Figure 5. Results of TEM microscopy of catalysts: (a) K; (b) Pt(i)HZ; (c) PtK; (d) Pt(Co-Al2O3/SiO2).
Catalysts 14 00351 g005aCatalysts 14 00351 g005b
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Figure 6. TPR spectra of catalysts: 1—K; 2—PtHZ; 3—Pt(i)HZ; 4—PtK; 5—Pt(Co-Al2O3/SiO2); 6—PtSiO2.
Figure 6. TPR spectra of catalysts: 1—K; 2—PtHZ; 3—Pt(i)HZ; 4—PtK; 5—Pt(Co-Al2O3/SiO2); 6—PtSiO2.
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Figure 7. Pore size distribution for catalysts: (a) Co-Al2O3/SiO2; (b) K.
Figure 7. Pore size distribution for catalysts: (a) Co-Al2O3/SiO2; (b) K.
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Catalysts 14 00351 g008
Figure 8. Molecular mass distribution of C5+ hydrocarbons obtained at FT synthesis temperatures of 240 °C for catalysts: (1) K; (2) PtHZ; (3) Pt(i)HZ; (4) PtK; (5) Pt(Co-Al2O3/SiO2).
Figure 8. Molecular mass distribution of C5+ hydrocarbons obtained at FT synthesis temperatures of 240 °C for catalysts: (1) K; (2) PtHZ; (3) Pt(i)HZ; (4) PtK; (5) Pt(Co-Al2O3/SiO2).
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Figure 9. Molecular mass distribution of C5+ hydrocarbons obtained at FT synthesis temperatures of 250 °C for catalysts: (1) K; (2) PtHZ; (3) Pt(i)HZ; (4) PtK; (5) Pt(Co-Al2O3/SiO2).
Figure 9. Molecular mass distribution of C5+ hydrocarbons obtained at FT synthesis temperatures of 250 °C for catalysts: (1) K; (2) PtHZ; (3) Pt(i)HZ; (4) PtK; (5) Pt(Co-Al2O3/SiO2).
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Table 1. Composition of bifunctional catalysts.
Table 1. Composition of bifunctional catalysts.
CatalystPt-Impregnated *
K-
PtHZHZSM-5
Pt(i)HZHZSM-5 **
PtKComposite
Pt(Co-Al2O3/SiO2)Co-Al2O3/SiO2
PtSiO2SiO2
Note: *—Pt content was determined to be 0.3 wt.% in the finished catalyst considering data on the total pore volume of the support and catalysts, cm3/g: HZSM-5—0.25, Co-Al2O3/SiO2—0.55, SiO2—0.53, composite—0.59. **—through ion-exchange.
Table 2. Results of XRD and TEM microscopy of catalysts.
Table 2. Results of XRD and TEM microscopy of catalysts.
CatalystParticle Size According to XRD, nm *D, %Particle Size According to XRD, nmD, %Co0 Particle Size by TEM, nm **
Co3O4Co0Co3O4Co0
K11.0 ± 0.68.211.714.510.88.98.8 ± 1.3
PtHZ13.0 ± 0.79.79.914.210.69.0-
Pt(i)HZ13.0 ± 0.710.29.413.710.39.39.4 ± 1.2
PtK12.2 ± 0.79.110.512.89.610.08.9 ± 1.2
Pt(Co-Al2O3/SiO2)13.1 ± 0.49.89.814.410.88.99.4 ± 1.2
PtSiO216.7 ± 0.812.57.718.013.57.1-
Note: *—according to XRD data using synchrotron radiation; **—according to TEM data.
Table 3. TPR H2 data for catalysts.
Table 3. TPR H2 data for catalysts.
CatalystPeak 1Peak 2S2/S1R, % *
Temperature, °CArea S1, %Temperature, °CArea S2, %
K35527.045173.02.750.0
PtHZ33426.143373.92.8-
Pt(i)HZ33226.243373.82.8-
PtK24125.544274.52.954.4
Pt(CoAl2O3/SiO2)18921.939078.73.648.0
PtSiO233622.141177.93.558.0
Note: *—degree of reduction of catalysts according to XRD data (catalyst weight—0.10 g; gas mixture of composition: 10% hydrogen + 90% nitrogen; flow rate—20 mL/min; temperature range—20–400 °C; heating rate—20 °C/min; duration—1 h at a temperature of 400 °C).
Table 4. Catalytic parameters of FT synthesis (240 °C).
Table 4. Catalytic parameters of FT synthesis (240 °C).
CatalystConversion CO, %Selectivity, %Productivity for Hydrocarbons C5+, kg/(m3cat·h)
CH4C2–C4C5+CO2
K75.618.711.967.12.3106.0
PtHZ66.317.311.268.43.1101.9
Pt(i)HZ84.014.48.274.43.0135.6
PtK74.718.612.865.72.9105.6
Pt(CoAl2O3/SiO2)87.216.59.470.63.5138.5
PtSiO261.120.116.960.22.884.2
Table 5. Hydrocarbon composition of FT synthesis products (240 °C).
Table 5. Hydrocarbon composition of FT synthesis products (240 °C).
CatalystHydrocarbonsGroup Composition of Hydrocarbons C5+, wt.%Totaliso/n **o/p ***
C5–C10C11–C18C19+
Kn-alkanes12.518.45.236.10.80.7
iso-alkanes9.510.81.722.0
alkenes18.32.3-20.6
alkenes *14.07.3-21.3
Total54.338.86.9100.0
PtHZn-alkanes14.113.66.133.81.20.3
iso-alkanes13.820.27.441.4
alkenes12.7--12.7
alkenes *11.60.5-12.1
Total52.234.313.5100.0
Pt(i)HZn-alkanes14.415.714.544.60.70.3
iso-alkanes9.112.412.133.6
alkenes13.6--13.6
alkenes *7.90.3-8.2
Total45.128.426.5100.0
PtKn-alkanes23.820.59.253.50.50.2
iso-alkanes8.015.63.927.5
alkenes10.11.5-11.6
alkenes *5.91.5-7.4
Total47.839.113.1100.0
Pt(CoAl2O3/SiO2)n-alkanes18.827.612.859.20.50.3
iso-alkanes7.810.41.719.9
alkenes6.70.7-7.4
alkenes *10.03.5-13.5
Total43.342.214.5100.0
PtSiO2n-alkanes16.919.08.944.80.90.3
iso-alkanes10.617.81.730.1
alkenes8.40.1-8.5
alkenes *12.54.1-16.6
Total48.441.010.6100.0
Note: *—branched alkenes; **—ratio of hydrocarbons of iso-structures to hydrocarbons of normal structure; ***—ratio of alkenes to alkanes (olefins to paraffins).
Table 6. Catalytic parameters of FT synthesis (250 °C).
Table 6. Catalytic parameters of FT synthesis (250 °C).
CatalystConversion CO, %Selectivity, %Productivity for Hydrocarbons C5+, kg/(m3cat·h)
CH4C2–C4C5+CO2
K85.915.88.272.83.3132.0
PtHZ77.415.59.171.44.0124.3
Pt(i)HZ90.316.29.070.34.4137.8
PtK90.615.88.370.35.6137.0
Pt(Co-Al2O3/SiO2)92.515.48.071.84.8149.3
Table 7. Hydrocarbon composition of FT synthesis products (250 °C).
Table 7. Hydrocarbon composition of FT synthesis products (250 °C).
CatalystHydrocarbonsGroup Composition of Hydrocarbons C5+, wt.%Totaliso/n ** o/p ***
C5–C10C11–C18C19+
Kn-alkanes9.413.13.726.21.11.0
iso-alkanes11.510.52.124.1
alkenes20.32.1-22.4
alkenes *20.96.4-27.3
Total62.132.15.8100.0
PtHZn-alkanes12.010.14.326.41.40.6
iso-alkanes11.019.75.936.6
alkenes15.2--15.2
alkenes *21.40.4-21.8
Total59.630.210.2100.0
Pt(i)HZn-alkanes10.916.36.133.31.10.4
iso-alkanes12.121.04.837.9
alkenes12.51.0-13.5
alkenes *12.23.1-15.3
Total47.741.410.9100.0
PtKn-alkanes22.914.46.543.80.80.3
iso-alkanes12.915.22.931.0
alkenes10.50.7-11.2
alkenes *11.82.2-14.0
Total58.132.59.4100.0
Pt(Co-Al2O3/SiO2)n-alkanes18.416.04.839.21.10.5
iso-alkanes11.715.91.429.0
alkenes7.30.8-8.1
alkenes *17.36.4-23.7
Total54.739.16.2100.0
Note: *—branched alkenes; **—ratio of hydrocarbons of iso-structures to hydrocarbons of normal structure; ***—ratio of alkenes to alkanes (olefins to paraffins).
Table 8. Comparison of synthesized catalysts.
Table 8. Comparison of synthesized catalysts.
CatalystLoading, wt.%ImpregnatedReaction ConditionsCatalysis PerformanceRef.
T,
°C		P, MPaH2/CO RatioGHSV, h−1Conversion CO, %C5+ Selectivity, %
Pt(Co-Al2O3/SiO2)7.4Co 0.3PtPt impregnated in Co-Al2O3/SiO22402.02.0100087.270.6this work
Pt(Co-Al2O3/SiO2)7.4Co 0.3PtPt impregnated in Co-Al2O3/SiO22502.02.0100092.571.8this work
Co-Pt/ZSM-513Co 0.3PtPt with Co impregnated in ZSM-52402.02.0n.d.42.181.6[53]
Co-Mn+Pt-ZSM-5
(layer-by-layer loading)		20.0 Co-3.0 Mn + 0.5 PtPt-impregnated ZSM-52502.02.0n.d.n.d.58.1[54]
Co-Ru/Al2O3
(Riogen, USA)		11.7Co
n.d. Ru		n.d.2402.02.0n.d.55.568.1[42]
Co-Ru/ZSM-5 Al2O37.5Co 0.19RuRu-impregnated ZSM-5+A2O32201.02.0150050.075.8[55]
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Yakovenko, R.E.; Zubkov, I.N.; Papeta, O.P.; Kataria, Y.V.; Bakun, V.G.; Svetogorov, R.D.; Savost’yanov, A.P. The Influence of Platinum on the Catalytic Properties of Bifunctional Cobalt Catalysts for the Synthesis of Hydrocarbons from CO and H2. Catalysts 2024, 14, 351. https://doi.org/10.3390/catal14060351

AMA Style

Yakovenko RE, Zubkov IN, Papeta OP, Kataria YV, Bakun VG, Svetogorov RD, Savost’yanov AP. The Influence of Platinum on the Catalytic Properties of Bifunctional Cobalt Catalysts for the Synthesis of Hydrocarbons from CO and H2. Catalysts. 2024; 14(6):351. https://doi.org/10.3390/catal14060351

Chicago/Turabian Style

Yakovenko, Roman E., Ivan N. Zubkov, Ol’ga P. Papeta, Yash V. Kataria, Vera G. Bakun, Roman D. Svetogorov, and Alexander P. Savost’yanov. 2024. "The Influence of Platinum on the Catalytic Properties of Bifunctional Cobalt Catalysts for the Synthesis of Hydrocarbons from CO and H2" Catalysts 14, no. 6: 351. https://doi.org/10.3390/catal14060351

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