Catalytic Hydrogenation of Anthracene on Binary (Bimetallic) Composite Catalysts

: The catalytic activity of the binary composite catalysts of Fe 2 O 3 -CoO/CaA and Fe 2 O 3 - CoO/ZSM-5 was studied. They were obtained by impregnation of CaA and ZSM-5 zeolites with aqueous solutions of sulfates of iron (FeSO 4 · 7H 2 O) and cobalt (CoSO 4 · 7H 2 O). The total metal content was no more than 5%. Then, oxidizing burning at 720 ◦ C for 60 min was performed to produce the metal oxides. It was found that the obtained Fe-Co/CaA catalyst contains iron and cobalt as CoFe 2 O 4 compound, and the Fe-Co/ZSM-5 catalyst includes CoFe 2 O 4 and CoFe. The phase composition of the obtained catalysts was detected by the X-ray diffraction analysis. The surface morphology was investigated by the electron microscopy. The elemental composition of the obtained catalysts was determined by energy dispersive spectroscopy with mapping and inductively coupled plasma atomic emission spectroscopy. The atomic absorption analysis by the IR-spectroscopy showed the shifts of absorption bands in the infrared spectra of the pure zeolites and with added Fe and Co. The catalytic hydrogenation of anthracene was performed to determine the catalytic properties of the obtained catalysts. It is one of the most common model compounds applied to investigate the efﬁciency of catalytic systems. The result of hydrogenation found that conversion of anthracene at 400 ◦ C, initial pressure of 6 MPa and duration of 60 min using the Fe-Co/CaA catalytic system equaled to ~87%. However, hydrogenation products equaled to ~84%. Anthracene conversion using the Fe-Co/ZSM-5 catalytic system and the same conditions was ~91%; among them, hydrogenated derivatives were ~71%. The proposed method is characterized by its simple execution. The obtained catalysts are be slightly inferior to platinum and rhodium catalysts in the catalytic activity.


Introduction
Development of the new heterogeneous catalysts is an important direction, i.e., the progress of the chemical industry is impossible without it. At the same time, the availability of materials and simplicity of the synthesis method is a significant point. Catalysts with deposited metals of the iron subgroup (Fe, Co, Ni) or their compounds on various carriers have highly catalytic activity. Thus, they can increase the efficiency of processing various organic raw materials into valuable products [1]. The choice in favor of the iron subgroup metals is due to their prevalence in nature and affordability [2,3] in comparison with the noble metals.
Among the variety of catalysts, the bimetallic catalysts have a special place, i.e., their high activity and stability attracts attention. The unique properties of the bimetallic catalysts are related to presence of a synergistic effect [4]. This class of substances is of the scientific and practical interest to use them in the processing of the organic compounds.
It should be stated that for each case, the nature of a catalyst support and precursor as well as the conditions for the obtaining the catalyst play an important role for its properties [5]. So in the work of Xie et al. [6], it was found that the use of non-calcined SBA-15, containing an abundance of Si-OH groups, has an effect on the dispersion of Co and Fe particles, which contributes to the production of an effective catalyst for the processing of volatile organic compounds, besides having high stability. The dependence of physicochemical properties on the SiO 2 content in the NiMo/SiO 2 -Al 2 O 3 catalyst obtained by wet impregnation was studied in Leyva et al. [7], and it was found that with a change in the SiO 2 content, the yields of products during the hydrotreating of heavy oil change.
The study of Przydacz et al. [8] shows that the higher the value of the specific surface area of the Ni-Fe/TiO 2 catalyst substrate is, the higher the yield of biofuels during the hydroxygenation of 5-hydroxymethylfurfural is.
The ratio of metals in bimetallic catalysts is also an equally significant factor influencing their effectiveness. For example, Huang et al. [9] found that a bimetallic Ni-Co/CZY catalyst with a ratio of Ni:Co = 1:9 exhibits high catalytic activity in ammonia decomposition reactions, providing a conversion rate of 100%, which can be used to produce hydrogen. Eswaramoorthi et al. [10] determined that at a content of 3 wt.% Ni and 12 wt.% Mo in the NiMo/MWCNTs catalyst, its activity in the process of hydrotreating gas oil is higher than when using a catalyst with the same Ni and Mo content but with a more traditional Al 2 O 3 substrate.
Zeolites are most commonly used as a catalyst support material in the manufacture of catalysts. Zeolites are one of the best materials due to their crystalline structure, the acidic properties and thermal stability. They provide high catalytic activity, selective adsorption, the ability to regulate acid properties and regeneration. Kantarellis et al. [11] showed that pyrolysis of biomass in the presence of a Ni-V/HZSM-5 catalyst obtained by impregnation, but with preliminary physicochemical treatment of the substrate, contributes to the production of deoxygenated liquid products saturated with hydrogen. Hu et al. [12] achieved a conversion of 100% by hydrogenation of acetylene in the presence of a CuNi/ZSM-12 catalyst obtained by impregnation at initial humidity prepared by incipient wetness impregnation method, while ethylene (82.48%) accounted for a significant proportion of the products obtained.
However, the properties of catalysts with this type of carriers also depend on the production conditions. So Aziz et al. [13] revealed that the bimetallic catalyst Fe-Co/ZSM-5, obtained in situ by hydrothermal method, demonstrates high efficiency in the adsorption and catalytic decomposition of volatile organic compounds than obtained by impregnation.
The structure of zeolite is an assembly of tetrahedral building blocks, i.e., Al and Si atoms are bound together by oxygen. The Al/Si ratio is an essential feature impacting on the thermal stability of zeolite [14]. The spherical pores connected by channels and the large specific surface area develop good conditions for adsorption, the ion exchange and catalytic reactions. The microporous structure of zeolites with the prescribed size pores can be used as the molecular sieves to extract the certain chemical substances with the molecular sizes relevant to size of zeolite pores [15,16]. The ability to regulate the structures and acidities of zeolites is their key advantage to select the support material, i.e., the presence of Bronsted and Lewis acid sites is a significant factor in the catalytic processes [17]. Zeolites modified by adding different metals to them, removing aluminum or silicon from the framework structure or by replacing them with other elements [18] open the broad prospects to apply them in the processing of various organic raw materials [19][20][21][22][23].
It is known that zeolite itself can act as a catalyst due to the presence of active sites in micropores. However, the efficiency of the catalytic properties of zeolite decreases due to the steric factor caused by the pore sizes. Therefore, it is necessary that the synthesis method of zeolites provides the possibility to regulate their properties. Modification of zeolite with various agents forming the additional mesopores enhances the probability of access of larger molecules to the active site [24]. Wang et al. [25] reported that mesoporous zeolite CaA obtained by hydrothermal method demonstrates high activity in the catalytic Scheconversion of methanol to dimethyl ether. Sahoob et al. [26] determined the optimal conditions for obtaining crystalline zeolite ZSM-5, which has high selectivity for ethylene and propylene in the process of conversion of methanol to olefins.
In order to study the efficiency of catalysts, the polyaromatic compounds are used as the model objects. The anthracene is one of the most common polycyclic aromatic compounds used as a model compound in various studies [27][28][29][30]. Anthracene as other polyaromatic hydrocarbons is part of heavy organic raw materials, e.g., the petroleum products, coal tar and organic waste [31]. It is examined as a model compound to study various catalytic systems and to develop the processing methods of different organic raw materials [32][33][34][35][36][37][38][39][40][41][42][43].
The current development trend of directions to involve the raw residues of the oil and coal industry in the production processes of the valuable fuels is related to the increasing demand of energy and environmental requirements. Accumulation of huge amounts of waste from coal and oil production constitutes a danger due to the high content of polyaromatic compounds, i.e., they are toxic [44]. The toxic, carcinogenic and mutagenic properties of polyaromatic compounds can cause the severe consequences for flora and fauna [45]. Ingress of these compounds in organisms increases the risk of cancer, various tumors and mutations [46]. The issue is relevant for countries with a developed coal and petrochemical industry. Therefore, development of the effective catalysts for the catalytic hydrotreatment of the polyaromatic substances is an urgent direction.
The previous studies have shown the high catalytic activity of bimetallic Fe-Co catalyst added on a carbon sorbent in hydrogenation of phenanthrene [47]. Conversion of phenanthrene was higher than for iron oxide added on a carbon sorbent and an industrial catalyst.
This study is devoted to research of the catalytic activity of iron and cobalt compounds added on aluminosilicate zeolites of ZSM-5 and CaA, which are common and widely used in various industrial processes [48].
ZSM-5 zeolite is a system of 14 tetrahedrons of AlO 4 and SiO 4 , and their spatial arrangement forms a pentasil. Pentasil has eight faces, all pentagonal in shape. The possibility to synthesize ZSM-5 zeolite with different Si/Al ratio obtains catalysts for the specific tasks. Therefore, this zeolite has an extensive field of commercial application [49], and it attracts the attention of researchers as a promising material to develop new catalysts with the unique properties.
CaA zeolite has a wide commercial application due to its specific properties [50]. CaA structure consists of sodalite blocks connected to each other by four-member rings [51].
The bimetallic catalysts of Fe-Co/CaA and Fe-Co/ZSM-5 were synthesized by the impregnation method. The impregnation method is one of the most common methods of adding active phases on supports. As a result, the monometallic and mixed catalytic systems can be obtained. The ZSM-5 and CaA zeolites are impregnated with aqueous solutions of FeSO 4 ·7H 2 O and CoSO 4 sulfates under the intensive stirring, then the obtained catalyst systems are evaporated and calcined at 720 • C to transfer the active phases into the oxide forms. The method proposed in this study does not require pre-preparation of the support and initial reagents. The catalytic activity of the obtained bimetallic catalysts is comparable with catalysts of platinum and rhodium.
The surface morphology of the obtained catalysts was studied by scanning electron microscopy (SEM). The qualitative chemical composition was determined by energy dispersive spectroscopy (EDS) and infrared spectroscopy (IR). The phase composition was studied by X-ray phase analysis (XRD). Quantification of applied metal content was performed using the inductively coupled plasma atomic emission spectroscopy (ICP-AES).
The catalytic activity of the obtained composite catalysts of Fe-Co/CaA and Fe-Co/ZSM-5 was assessed using the process of anthracene hydrogenation in an autoclave in a hydrogen atmosphere. Hydrogenation products were identified by gas chromatography (GC).

Results
Diffractogram of the X-ray phase analysis in Figure 1 illustrates that CaA and ZSM-5 zeolites contain oxides and oxide-hydroxide compounds of iron and cobalt after impregnation and calcination procedures.

Results
Diffractogram of the X-ray phase analysis in Figure 1 illustrates that CaA and ZSM-5 zeolites contain oxides and oxide-hydroxide compounds of iron and cobalt after impregnation and calcination procedures. Absence of the distinct peaks of iron and cobalt and their compounds on a diffractogram ( Figure 1) may be due to the fact that the catalysts were prepared using sulfate solutions of iron and cobalt with a total concentration of iron and cobalt not more than 5%, which is within the background. Iron and cobalt can be precipitated on the zeolite surface in a non-crystalline form [52], or their ions can be incorporated in the zeolite crystal lattice [53]. It is known that iron and cobalt added on ZSM-5 are detected by X-ray phase analysis at their concentration of 50% [54]. Absence of the distinct peaks of iron and cobalt and their compounds on a diffractogram ( Figure 1) may be due to the fact that the catalysts were prepared using sulfate solutions of iron and cobalt with a total concentration of iron and cobalt not more than 5%, which is within the background. Iron and cobalt can be precipitated on the zeolite surface in a non-crystalline form [52], or their ions can be incorporated in the zeolite crystal lattice [53]. It is known that iron and cobalt added on ZSM-5 are detected by X-ray phase analysis at their concentration of 50% [54].
After calcination of the impregnated zeolites to 720 • C, the gas-phase evaporation of sulfates and sulfides of iron and cobalt created conditions for incorporation of iron and cobalt ions into the zeolite lattice. It is known that incorporation of a metal ion into the zeolite lattice is possible by the gas-phase method [54]. In our case, the calcination could cause gas-phase evaporation of sulfates and sulfides of metals.
The X-ray phase analysis (Figure 1)  For the Fe-Co/ZSM-5 catalyst (Figure 1 After calcination of the impregnated zeolites to 720 °C, the gas-phase evaporation of sulfates and sulfides of iron and cobalt created conditions for incorporation of iron and cobalt ions into the zeolite lattice. It is known that incorporation of a metal ion into the zeolite lattice is possible by the gas-phase method [54]. In our case, the calcination could cause gas-phase evaporation of sulfates and sulfides of metals. The X-ray phase analysis (Figure 1) showed the presence of small peaks in the Fe-Co/CaA catalyst, mainly CoFe2O4, identified at angles of 30 For the Fe-Co/ZSM-5 catalyst (Figure 1    Inductively coupled plasma atomic emission (ICP-AES) analysis showed that the Fe-Co/CaA catalyst has Fe ~2.2 and Co ~1.7. The Fe-Co/ZSM-5 catalyst has Fe ~2.2 and Co ~1.8.
Based on the IR-spectroscopy (Figure 4), it was determined that spectra show shifts of some absorption bands associated with effects of the precipitated metals of iron and cobalt.  Inductively coupled plasma atomic emission (ICP-AES) analysis showed that the Fe-Co/CaA catalyst has Fe~2.2 and Co~1.7. The Fe-Co/ZSM-5 catalyst has Fe~2.2 and Co~1.8.
Based on the IR-spectroscopy (Figure 4), it was determined that spectra show shifts of some absorption bands associated with effects of the precipitated metals of iron and cobalt.  The energy dispersive spectroscopy detected the presence of Fe, Co, Na, Al, Si, S and O elements in the obtained Fe-Co/CaA catalyst (Figure 2). Sulfur can be formed during the calcination of Fe-Co/CaA catalyst after impregnation with sulfate solutions; however, it is not observed for Fe-Co/ZSM-5 catalyst. The energy dispersive spectra obtained for the Fe-Co/ZSM-5 catalyst (Figure 3) determine the presence of Fe, Co, Al, Si and O.
Based on the IR-spectroscopy (Figure 4), it was determined that spectra show shifts of some absorption bands associated with effects of the precipitated metals of iron and cobalt.  Values of the wave numbers for absorption bands of the original zeolites and with added metals of iron and cobalt are presented in Table 1. Absorption bands for pure CaA zeolite are observed in the range of 460-3500 cm −1 . In order to obtain Fe-Co/CaA catalyst (Figure 4a), the small shifts in values of absorption bands compared to pure CaA zeolite and appearance of a band of weak intensity at 3142 cm −1 are detected ( Table 1). The results of IR spectroscopy (Figure 4a A shift in values of the absorption bands may be due to the influence of added metals on zeolite structure [55].
The results of anthracene hydrogenation with the obtained catalysts of Fe-Co/CaA and Fe-Co/ZSM-5 are presented in Table 2. Table 2 demonstrates that after hydrogenation of anthracene with the obtained catalysts of Fe-Co/CaA and Fe-Co/ZSM-5, the products consist mainly of di-and tetrahydroanthracene.
Anthracene conversion is slightly higher with Fe-Co/ZSM-5 catalyst than with Fe-Co/CaA. However, Fe-Co/CaA has a greater hydrogenation capacity, i.e., the total content of degradation products is about 2%, but their total fraction was~15% for Fe-Co/ZSM-5. The appearance of phenanthrene in products of anthracene hydrogenation using Fe-Co/CaA and Fe-Co/ZSM-5 may be related to isomerization of 2-ethylbiphenyl under the influence of hydrogen [56]. Table 2 describes that using the Fe-Co/ZSM-5, the degradation products consist mostly of 2-ethylbiphenyl, which is probably a source of phenanthrene formation. Table 2. Composition of anthracene hydrogenation products in the presence of Fe-Co/CaA and Fe-Co/ZSM-5.  The probable mechanism of anthracene hydrogenation with obtained catalysts is as follows (Scheme 1): Catalysts 2023, 13, 957 9 of 14 Scheme 1. The probable mechanism of anthracene hydrogenation with Fe-Co/CaA and Fe-Co/ZSM-5 catalysts. Table 3 compares values of anthracene conversions and yields of dihydroanthracene, tetrahydroanthracene and octahydroanthracene during anthracene hydrogenation with catalysts of platinum and rhodium added on Al2O3 [36] with the obtained values in this study.  Table 3 compares values of anthracene conversions and yields of dihydroanthracene, tetrahydroanthracene and octahydroanthracene during anthracene hydrogenation with catalysts of platinum and rhodium added on Al 2 O 3 [36] with the obtained values in this study.  Table 3 demonstrates that the catalysts obtained in this study are inferior to catalysts of platinum and rhodium in total anthracene conversion and octahydroanthracene yield. Otherwise, the difference is not significant.

Materials
In order to synthesize catalysts, we used iron sulfate (FeSO 4 ·7H 2 O) and cobalt sulfate (CoSO 4 ·7H 2 O) purchased in Karagandareactivsbyt LLP (Karaganda, Kazakhstan), CaA zeolite produced by TATSORB (Tatarstan, Russia) and ZSM-5 zeolite made by Xi'an Lvneng Purification Technology Co., Ltd. (Xi'an, Shanxi, China). ZSM-5 zeolite with the molar ratio of SiO 2 /Al 2 O 3 = 50 and crystallinity degree of about 90% was chosen for the catalyst synthesis. The catalytic properties of the obtained catalysts were evaluated during the hydrogenation of anthracene produced by Merck (Darmstadt, Germany, European Union).

Synthesis of Fe-Co/CaA and Fe-Co/ZSM-5 Catalysts
The Fe-Co/CaA and Fe-Co/ZSM-5 composite binary catalysts were synthesized under the same conditions and component ratios. The ratio of zeolites and sulfates of iron and cobalt was used so that the total content of iron and cobalt in the obtained catalysts was no more than 5% (based on 2.5% of Fe and 2.5% of Co). To

Hydrogenation of Anthracene
Hydrogenation process was performed in a CJF-0.05 reactor manufactured by Zhengzhou Keda Machinery and Instrument (Zhengzhou, China), 0.05 l capacity, equipped with a stirrer, temperature and pressure sensors and an emergency pressure relief system. The initial pressure was 6 MPa. The autoclave temperature of 400 • C was the beginning of a reaction.
The premixed initial components were placed in a reactor, purged with hydrogen and pressurized. The autoclave heating rate was 10 • C/min. After cooling the reactor to room temperature, the reaction mixture was dissolved in benzene and analyzed by gas-liquid chromatography.

Physical and Chemical Studies
The phase composition of the synthesized composite catalysts of Fe-Co/CaA and Fe-Co/ZSM-5 was performed on a D8 ADVANCE ECO X-ray diffractometer (Bruker, Karlsruhe, Germany) using CuKα-radiation (λ = 1.5406 Å) in the angle (2θ) range of 15-100 • and step of 0.02 • . Processing of diffractograms to identify compounds was performed using the POW_COD database.
The surface morphology and energy-dispersive spectra of the obtained catalysts were studied using a TESCAN MIRA 3 LMU scanning electron microscope (TESCAN, Brno, Czech Republic) at an accelerating voltage of 20 kV. Sample preparation for SEM was as follows: a small amount of sample was added to a double-sided conductive carbon tape and sprayed as a thin layer of carbon.
The detectors used were a secondary electron detector (SE detector) and a backscattered electron detector (BSE detector). The SE detector showed the topographic contrast, and the BSE detector demonstrated the compositional contrast.
Inductively coupled plasma atomic emission analysis (ICP-AES) was performed on a Profile Plus instrument (Teledyne Leeman Labs, Hudson, NH, USA). The method is based on actuation of the spectrum by inductively coupled plasma with subsequent registration of spectral line emission by photoelectric method. The analysis uses dependence of the spectral line intensities of the elements on their mass fractions in the sample. For ICP-AES analysis, a 0.1 g sample weight was weighed with an accuracy of no more than ±0.0003, dissolved in 10 cm 3 of nitric acid (1:1) at heating. Solutions were heated to remove nitrogen oxides without boiling, cooled, transferred to 100 cm 3 volumetric flasks, diluted to the mark with water and stirred. Spectrometer was prepared for operation and operated according to the spectrometer operating instructions.
Preparation of samples for IR-spectrometry was performed by tablet pressing. Pure KBr, previously crushed and dried, was used as a substance of the matrix.
Tablet pressing was made in the following sequence: 2 mg of the test substance was weighed on analytical scales, and KBr powder was added. The total weight of the charge was adjusted to 300 mg. The prepared charge was placed in an agate mortar for grinding and mixing. The resulting mixture of the test substance and KBr was evenly poured into a mold and pressed at 8. The products of anthracene hydrogenation were identified using a Crystalux 4000 M chromatograph with a plasma ionization detector on a Zebron ZB-5 column (Phenomenex, Torrance, CA, USA): column type-capillary; length-30 m; inner diameter-0.32 mm; stationary phase thickness-0.50 µm. Temperature range from 60 to 360 • C. Temperature program was held from 120 to 260 • C by 12 • C per min.
The quantitative calculation of chromatographic information was performed using the program "NetChrom V 2.1" by the percentage normalization method. The concentration (wt. %) of hydrogenation products was calculated from the area of peaks. The anthracene conversion was calculated based on the obtained gas chromatography analysis data on the concentrations of hydrogenation products (wt. %) and the concentration of anthracene remaining after hydrogenation. The yield was calculated by the following formula: where Q is the conversion, and X n is the concentration (wt. %) of anthracene remaining after hydrogenation. Using the Fe-Co/CaA catalyst, the average value of the anthracene remaining after hydrogenation ( Table 2) was 13.21%, and the conversion was calculated using the following formula: 100% − 13.21% = 86.70~87%. Using the Fe-Co/ZSM-5 catalyst, the amount of anthracene remaining after hydrogenation was 9.21%; hence, the conversion was 100% − 9.21% = 90.79~91%. A method was developed for gas chromatography to analyze hydrogenation products of anthracene and phenanthrene. This method used a database of the component composition based on analysis of the standard samples.

Conclusions
The physical and chemical properties of bimetallic composite catalysts of Fe-Co/CaA and Fe-Co/ZSM-5 obtained by impregnation of zeolites in sulfate solutions with subsequent calcination were studied. It was established that distribution of iron and cobalt in the obtained catalysts had a heterogeneous character. Despite the nonuniform distribution of iron and cobalt on surface of the obtained catalysts, their catalytic activity was quite high.
Hydrogenation of anthracene with Fe-Co/CaA composite catalyst forms hydrogenation products mostly. The Fe-Co/ZSM-5 catalyst has cracking properties and hydrogenation ability. Anthracene conversion with the Fe-Co/ZSM-5 catalyst was slightly higher than with the Fe-Co/CaA catalyst. Formation of phenanthrene in hydrogenation products is probably due to isomerization of 2-ethylbiphenyl. The Fe-Co/ZSM-5 catalyst also has a higher isomerizing ability than Fe-Co/CaA.
Thus, impregnation of zeolites with sulfates of iron and cobalt with their subsequent calcination can be used to obtain the composite bimetallic catalysts with high catalytic activity.