CC BY
24
ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 3 2023 39
ISSN 0005-2531 (Print)
UDC 547.057; 547.21
THE SCIENTIFIC BASIS OF INDUSTRIAL APPLICATION OF THE PROCESS OF SELECTIVE HYDROGENATION OF BENZENE IN DIFFERENT CATALYTIC
SYSTEMS
N.T.Shikhverdieva, U.A.Mammedova, N.A.Zeynalov
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, Ministry of Science and Education
of the Republic of Azerbaijan
Received 24.01.2023 Accepted 16.02.2023
Pollution of the environment with substances obtained in the production of oil and other products in various industries forces us to take measures to solve environmental problems that have become relevant in recent years. Arenes, belonging to the class of petroleum-derived aromatic hydrocarbons, are very life-threatening substances due to their carcinogenic effects. In this regard, one of the important issues is the release of benzene, a special representative of arenes, from its carcinogenic effect as a toxic and volatile compound, and its use in the production of cyclohexane, which is the main raw material for the production of caprolactam and nylon. The selectivity of the hydrogenation of benzene and the high yield of cy-clohexane, the target product, depend on the reaction conditions and the choice of catalysts. In recent years, the application of polymer and zeolite nanocomposite catalysts, as well as modified metal polymer-based catalysts, in obtaining highly active and selective catalysts for the purpose of effective implementation of aromatic hydrocarbon hydrogenation processes is of great interest. Thus, naphthenic hydrocarbons are considered the main starting materials for the synthesis of various classes of compounds. Derivatives of these compounds are used in the pharmaceutical industry as special-purpose ca-prolactam, adipic acid, cyclohexanone, as a solvent for essential oils, varnishes, paints, as an extractant, etc. are used in synthesis. As a result of investigating the catalytic properties of polymer-mineral catalysts in the benzene hydrogenation reaction, it was determined that these reactions show selectivity in the direction of cyclohexane formation in narrow-pore zeolites (mordenite, clinoptilolite)In the presented work, the main information was collected by analyzing the literature on the work carried out in this direction.
Keywords: benzene, hydrogenation, cyclohexane, cyclohexadiene, cyclohexene, nickel nanoparticles, ruthenium particles, platin nanoparticles, mesoporous silica, selectivity.
doi.org/10.32737/0005-2531-2023-3-39-52
is a colorless liquid with a special sweet smell, which is used in industry as the main raw material for the production of pharmaceuticals, various plastics, synthetic rubbers, and paints. Although it is contained in crude oil, it is more appropriate to synthesize it from other components from a commercial point of view [2]. At high concentrations, benzene is an ecologically dangerous toxic substance and one of the most common anthropogenic xenobiotics. The main sources of benzene released into the environment through waste water or air emissions are petrochemical, coke industry, fuel production and transport. So, it easily evaporate from water reservoirs, passes from soil to plants and poses a serious threat to the ecosystem. Benzene has the property of accumulation, due to its
Introduction
Hydrocarbons with one or more benzene rings in their molecules are called arenes. Aromatic hydrocarbons are considered the main starting materials for the synthesis of various classes of compounds. Due to their active single and double bonds, the functional derivatives of these compounds are used in the targeted production of special purpose polymer and composite materials, physiologically active substances, and at the same time analogs of natural compounds and medicinal preparations. It is widely used in obtaining cyclohexane, cyclohexene and many physiologically active substances from aromatic hydrocarbons [1]. The simplest representative of arenes is benzene. Benzene (C6 He)
lipophilicity, it can accumulate in the cells of adipose tissue of animals, thereby poisoning them. However, the hydrogenation of this dangerous substance leads to obtaining economically very valuable products.
Hydrogenation of benzene
Hydrogenation is the addition of hydrogen to organic compounds containing multiple bonds. Since the benzene ring has three double bonds, it can bind three hydrogen molecules, and as a result of the hydrogenation reaction, its concentration drops below 1% by volume. It is very necessary to carry out this process in accordance with environmental norms against the carcinogenicity of benzene and its negative effects on the environment. As a result of the reaction, cyclohexane is obtained, an intermediate product in the production of nylon, adipic acid, which is environmentally safe and used as an industrial solvent [3-5]. Based on this, the hydrogenation reaction for benzene saturation has an important role in the oil industry [6]. The targeted hydrogenation of benzene in a mixture of aromatics (BTK (benzene, toluene, xylene fraction) presents some difficulties. Because the stabilization of aromatic rings by resonance is unsaturation complicates the process [7]. During the selective hydrogenation of benzene to cyclohex-ane, the octane number decreases, this in turn is
very important for the production of less volatile and less toxic fuels [8].
Due to the increasing demand for the hydrogenation of aromatic compounds, the development of effective hydrogenation catalysts is one of the priority issues [9].
The results of the research conducted on the hydrogenation of benzene over various catalysts have shown that, depending on various factors - the properties of the polymer, the structural properties, as well as the type of zeolite, the structural properties, and the reaction conditions, as a result of the reaction, in addition to the target product - cyclohexane, intermediate compounds such as cyclohexadiene and cyclohexene are formed [10]. Cycloalkanes and cycloalkenes are obtained in industry mainly in the liquid phase and indirectly rather than directly from primary reagents. However, the main shortcomings of this method are the degree of conversion of raw materials, the low selectivity of the process according to the target products, and the use of an aggressive reaction medium.
The hydrogenation of benzene to cyclo-hexane takes place in stages, depending on the amount of combined hydrogen, 1,3-cyclohexa-diene, cyclohexene and cyclohexane are obtained, each of which has its own application areas.
The incomplete hydrogenation products of benzene are 1,3-cyclohexadiene and cyclohexene. The high reactivity of unsaturated cyclic hydrocarbons creates wide opportunities for obtaining synthetic rubbers, fibers, pharmaceuticals, plasti-cizers, emulsifiers, solvents [11]. Thus, oligomers of 1,3-cyclohexadiene (mol mass 270-750) are
used for the preparation of stabilizing additives to polyethylene-based composites. These stabilizers aim to increase the resistance of catalysts against thermal aging. Polymers and copolymers of 1,3-cyclohexadiene are added to light-resistant materials to increase sensitivity to radiation [12].
The selective hydrogénation of benzene to cyclohexane is an economically interesting and technically challenging process. Cyclohexane is an important intermediate product for the production of adipic acid and key end products such as nylon 6 and nylon 66 in a highly efficient and less harmful form [13]. By using a complex tetraphase system (g/m/m/b) including hydrogen (g), water (m), an organic phase, benzene (m) and a ruthenium-bearing catalyst (b), it is possible to obtain cyclohexane, a desired benzene-based intermediate prduct [14]. Cyclohexane is used in the production of a number of compounds: caprolactam, adipic acid, hexamethylenediamine, cyclohexa-none, etc. From this point of view, the synthesis of highly active composites and the hydrogenation of benzene to cyclohexane on a polymer-zeolite-based selective catalyst are of great practical importance.
Intermediate products (cyclohexadiene, cyclohexene) cannot be separated from the medium because they hydrogenate faster than benzene. When the reaction temperature rises above 250°C, isomerization of cyclohexane to methylcyclopentane occurs.
Primary benzene contains n-heptane, methylcyclohexane, toluene, etc. the presence of a mixture of compounds may affect the quality of the finished product (caprolactam) in the future. N-heptane and methylcyclohexane do not change during the hydrogenation reaction, while toluene is converted to methylcyclohexane [15].
Cyclohexane has become a highly industrially important substance in the modern world. From this point of view, the main goal in the hydrogenation of benzene is the synthesis of cyclohexane. Cyclohexane is considered a commercial product for industrial processes used to dissolve fats, oils, resins and remove
dyes. It is a relatively stable colorless volatile liquid present in crude oil at concentrations ranging from 0.1 to 1.0%, widely used as an intermediate ring in nylon production. Only a small part of it is used as a solvent and diluent for polymer reactions. For the first time, it was obtained directly from the corresponding products of crude oil processing by fractional distillation. Currently, most cyclohexane is obtained by direct hydrogenation of benzene. Usually, the reaction is carried out in the vapor or mixed phase using a fixed bed reaction. The temperature of the reactor is controlled so that it is between 176.7 and 2600C [16]. Higher temperatures can lead to thermodynamic limitations in benzene conversion, thermal cracking, and increased byproducts. It is possible to obtain cyclohexane by the method of rectification of petroleum products. A number of methods for obtaining cyclohexane from high-boiling oil distillates have been developed. For example, 80% cyclohexane fractions can be obtained by refining gasoline. These fractions are used as solvents in many cases, and high-purity cyclo-hexane is usually required for chemical processing [17]. This cannot be achieved by the rectification method due to the large amount of near-boiling components. Therefore, the extractive distillation method is used, and phenol is used as an extractant. Through this method, cyclohexane of high concentration is obtained.
The process of hydrogenation of aromatic compounds is mainly carried out during the production of non-aromatic fuels and solvents. Aromatic compounds reduce fuel quality. The aim of the researches is to optimize the design of reactors used during hydrogenation and to develop kinetic and deactivation models of hydrogenation of aromatic substances based on the results of experiments conducted in the liquid phase. For this, the hydrogenation of benzene, toluene, tetraleine, naphthalene and their mixtures over a nickel catalyst was studied in a continuous three-phase reactor. This model was chosen for monoaromatic (toluene, benzene), partially hydrogenated polyaromatic (tetralein) and polyaromatic (naphthalene) compounds [18]. The kinetics and mechanism of hydro-
genation of benzene and toluene over Pt/TiO2 catalyst were studied under strong metal-support interaction (SMS) and in the absence of metal support. Both processes were investigated and the kinetics and mechanism of the reactions were found to occur independent of SMS. In the presence of SMS, a decrease in catalytic activity is observed, which is explained by structural changes occurring in the active centers of the catalyst and the presence of strong hydrogen bonds to the surface [19].
Hydrogenation of benzene can be carried out in the vapor phase at a pressure of 18-20 atm, at a temperature of 150-2500C with the presence of a nickel-chromium catalyst, first in a tube reactor, and then in a column type reactor (filled). In order to keep benzene in the vapor phase, the gas/benzene ratio must be strictly maintained. Conversion of benzene to cyclohex-ane is usually 99.6-99.9%. The yield of cyclo-hexane in the reacting benzene reaches almost 100%, that is, complete conversion of benzene to cyclohexane is achieved. Therefore, cyclohexane is cleaned from n-heptane and other high-
temperature boiling compounds in the rectification column [20].
The rectification column installed in the hydrogenation section ensures the purification of cyclohexane obtained from benzene of the above quality to the required degree of purity (the amount of n-heptane is not more than 0.01% - 100 ppm).
The picture shows a simple scheme of the process of obtaining cyclohexane by the method of hydrogenation of benzene.
The technological scheme consists of three main sections: the reaction section; separation and stabilization of products. Considering that the removal of cyclohexane from oil is considered a very difficult and expensive process, the process of obtaining cyclohexane by hydrogenation of benzene is purposeful as a research object. The hydrogenation and dehydrogenation reaction of benzene is in chemical equilibrium. Reducing the temperature, increasing the pressure, and with properly selected catalysts, the hydrogenation reaction of benzene can be directed in the direction of obtaining cyclohexane [21].
Fig. 1. 1 - primary reactor, 2 - secondary reactor, 3 - separator, 4 - recirculation compressor, 5 - stabilization column.
In general, obtained cyclohexane is mainly used for the production of 3 industrial products: caprolactam, adipic acid and hexameth-ylenediamine. Hexamethylenediamine, in turn, is used in the production of raw materials for the production of synthetic fibers (polyamide, nylon), resins, technical plastics and plasticiz-ers. Hydrogenation of benzene is one of the initial stages in the production of caprolactam, the starting product for the production of polyamide plastics, without which it is impossible to imagine the development of modern society. Valuable properties of polyamide fibers provide the basis for their wide application in technical products and consumer goods. The most important field of application of polyamide fiber is the wheel and tire industry [22].
The nature of catalysts
Although obtaining modified metal catalysts for the conversion of environmentally toxic raw materials into valuable products is a difficult process, it is important for understanding the reaction mechanism and, accordingly, for the development of new highly efficient catalytic materials.
Important properties of the catalyst required for hydrogenation of benzene include high activity, selectivity, stability and resistance to coking. Among the various metal-based catalysts used for this reaction, nickel-modified catalysts have been considered more than other catalysts due to their high activity in the hydrogenation reaction and, in particular, their low economic cost. As early as 1897, Sabate and Sanderan discovered that benzene hydrogenates to cyclohexane in the presence of finely ground nickel (Sabate-Sanderan reaction). Later, it was determined that it is possible to successfully use ordinary nickel, carrier-impregnated nickel, and mixed nickel composites for the same purpose. In 1922, the famous scientist and Nobel laureate Paul Sabatier reviewed the working state of the catalyst in organic chemistry and emphasized the special abilities of the nickel catalyst [23]. The interesting part of his observation is that nickel has high activity with maximum variability. Considering the unusual properties of nickel, he said of the nickel catalyst: "It can be compared to a spirited horse, delicate, difficult to control, and incapable of sustainable work." [24]. However,
Sabatier described the formation of another type of nickel by changing the catalyst preparation conditions, and suggested that "this type of nickel catalyst maintains its long-term activity in all processes." Nearly a century ago, Sabatier noted that nickel's incredible catalytic properties combined with its advantages and disadvantages. This dual nature was identified in the nickel catalyst and influenced the development of this field. Since then, the development of organo-nickel chemistry has facilitated the development of numerous catalytic systems and research. Nickel polymerization catalysts are suitable for both theoretical and industrial fields of materials science. It involves understanding the role of ligands in building a homogeneous nickel catalyst and the role of the nanoscale in creating a nickel catalyst. In organic synthesis, the main effect is the cross-linking of the elec-trophilic atoms of the nickel catalyst with the nucleophiles of carbon (organic-metallic substances). Various organic-metallic substances such as magnesium, boron, zinc and silicon are used as catalysts in reactions. The correct selection of ligands enables the use of various reagents with simple and inexpensive catalysts. The use of simple catalysts such as Ni in various processes requires special attention. Catalysts containing Ni exhibit high activity in reactions of obtaining unsaturated compounds (alkenes, dienes, alkynes, etc.). Indeed, coordination of unsaturated molecules to nickel active centers "activates" them against anomalous reactivity that would not otherwise be possible. In comparison with other catalysts, the number of processes carried out with the participation of new Ni-containing catalysts has increased in recent years. In the last ten years, great progress has been observed in the field of nickel catalyst use [25]. These properties of nickel as an oxidizing agent and the ability to easily pass through several oxidation states enable the development of innovative reactions.
Samples of nickel-containing catalysts on various carriers were prepared and tested. It has been established that the nickel-containing catalyst supported by a-Al2O3 exhibits high activity in the benzene hydrogenation reaction. The area of nickel metal and its crystal size are determined. The catalyst containing 40% nickel showed the highest activity during the process [26].
Finely ground platinum also shows good results. Hydrogenation of benzene to cyclohex-ene was carried out at low temperature (273-343 K) and atmospheric pressure on PtCo and PtNi bimetal and Co, Ni, Pt monometal, y-Al2O3 catalyst as carriers. As a result of Fourier transform IR spectroscopy and flow reactor studies, the Pt-supported catalyst showed higher activity for the hydrogenation of benzene over both bimetallic catalysts PtCo, PtNi and monometallic Co, Ni, and Pt catalysts [27]. It is possible to use palladium, molybdenum, tungsten, rhenium and their compounds in carriers or as part of complex oxide systems, as well as skeleton and metal catalysts. There is also a special group of sulfide catalysts, such as mixed sulfides of nickel, molybdenum, tungsten, and other metals.
Many parameters affecting the catalytic activity of metal-based catalysts in the hydrogenation reaction have been reported in various published studies: the nature of the modifier, interaction with the active metal, acidity, metal particle size, etc. reported [28, 29].
It is possible to significantly improve the catalytic properties of catalysts by modifying a number of metals into carriers. In this regard, interest in mesostructured silica materials has increased in recent years, because they have a number of useful properties specific to carriers and adsorbents. Thus, they have adjustable size (from 2 to 50 nm), regular cylindrical pores, high chemical and thermal stability, large surface area (700-1500 m 2 /g) [30-35].
Hydrogenation of benzene to cyclohex-ane in liquid phase, Ru / SiO2 catalyst prepared with ruthenium (III) chloride impregnated on hydrophilic non-porous silica was investigated. At 423 K and 5 MPa hydrogen pressure, 60% of benzene was converted to 14% of cyclohexene in a biphasic water/benzene system. The selectivity of cyclohexene was mainly observed in the presence of ethylene glycol/water and glycerin / water mixture, which consists of aqueous organic molecules that cause an increase in hy-drophilicity around the ruthenium particles during the desorption of cyclohexene. Over the past forty years, a number of studies have been conducted to develop an exploitable process and various results have been obtained. However, there are a number of challenges for conducting
research in the industry, which in turn indicates the need for more research in this area. The catalytic activity of the suspended Ru-based catalyst in the process of selective hydrogenation of benzene to cyclohexane was studied. The reaction was carried out in a liquid phase reactor at a temperature of 423 K under a pressure of 5 MPa, with a mole ratio of benzene:ZnS04=0.6. No correlation was found between the porosity of benzene and hydrogen and the selectivity of the catalyst. It was determined that there is no strong dependence between their efficiency and selectivity at the stage of catalyst preparation. In order to study the effect of Ru on the selectivity, the effect of alkali or alkaline-earth hydroxide used was studied [36].
It was determined that the effect of the received polymer-mineral hybrid nanocatalysts on the benzene conversion reaction is in the following direction: hydrogenation of benzene; formation of hydrocarbon compounds. The yield of the reaction product is affected by the properties of the polymer and the porous structure of the zeolite, physical and chemical composition and the reaction conditions.
It was determined that the modification of mesoporous silica containing nickel, copper or silver with lanthanum in the reaction of hydrogenation of benzene and its derivatives leads to a decrease in the catalytic activity of the catalyst in these processes. It has been shown that benzene hydrogenation with La-Ni/MC catalyst reaches 100% conversion after five minutes of starting the reaction, but with Ni/MC catalyst this indicator is reached after 20 minutes [37].
Nickel-filled composites wet-impregnation method is prepared. The catalytic properties of the catalyst depend on the nature of the zeolite, the reaction temperature, and the relative pressure of the reagents. The obtained results confirm that nickel nanoparticles modified on Al-MCF-zeolite hybrids have favorable properties. High activity and selectivity for the Ni/Al-M-MOR catalyst was in the temperature range of 150-2100C, and for Ni/AlM13X this indicator was about 83% at 170°C.
Nickel-containing catalysts are more profitable than catalysts based on noble metals (Pt, Pd, Ru, Rh, Ir, Os). Catalysts based on these noble metals have high catalytic activity
and it is possible to carry out the processes based on them at relatively low temperature and pressure. The lifetime of these catalysts is often estimated to be years. From recent studies, it can be concluded that more attention is paid to Pd-, Pt-, Ru- and Rh-catalysts and bimetallic systems modified on carriers. In this regard, a zeolite/Pt(NH3)4Cl2-based catalyst was proposed for the hydrogenation of benzene [38]. At the same time, a Ru-Zn/ZnO2 catalyst was developed for the hydrogenation of benzene [39]. The optimal amount of ruthenium in this catalyst is 10%, and zinc is 2.78%.
Kinetics, thermodynamics of the process of selective hydrogenation of benzene, methods of conducting and obtained patents
The hydrogenation reaction of benzene is a typical exothermic, heterogeneous catalytic reaction. This reaction goes through a series of successive stages:
1. External diffusion of benzene molecules to the catalyst surface;
2. Adsorption of hydrogen and benzene on the surface of the catalyst;
3. Internal diffusion of reagents in the core of the catalyst;
45
4. Chemical effect of benzene and hydrogen on the active surface of the catalyst;
5. Desorption of reaction products (cyclo-hexane, etc.).
Kinetics and thermodynamics of the selective hydrogenation of benzene
In 1934 Horiuti and Polanyi first proposed a stepwise mechanism for benzene hydrogenation over metallic surfaces [40]. A more complex model considering a single-stage and multi-stage route was tested under high pressure (about 2 MPa) [41] using nickel as a cata-lyst.The latter, based on empirical observations of the interaction of aromatic molecules with metal surfaces and chemisorption phenomena, is generally accepted, although some researchers report selectivity for cyclohexene approaching unity at very low benzene conversions, which is in principle not consistent with the existence of a one-step route [42].
In other words, the mechanism claims that benzene interacts with active sites through s, p and p/s bonding, giving rise to, in the case of p and p/s interactions, two different active species and therefore two parallel routes, whereas the s-bonded species is unreactive, if not inhibitory (Figure 2) [43].
Fig.2. Mechanisms for one-step (Rideal) [44], [45]. and multi-step (Rooney) [46] benzene hydrogenation as proposed by Prasad. S and s represent benzene and hydrogen coordinating active sites, respectively, and the wide dashed line van der Waals interactions.
It was determined that, in general, desorption of cyclohexene from the catalytic surface ismuch easier to achieve than desorption of cyclohexadiene, since the latter is very unstable when adsorbed due to the loss of resonance [47] 1,3-cyclohexadiene was only unequivocally detected as an intermediate during the dehydro-genation of cyclohexene by Pt catalysts using sum frequency generation (SFG) surface vibrational spectroscopy [48].
It is necessary to pay attention to some points. It has been observed for a number of different catalysts that a pressure of about 4-6 MPa is considered to be the optimum pressure in the partial hydrogenation reaction of benzene, while at higher pressure values the production of cyclohexene decreases [49-50]. In the stepwise hydrogenation route, high pressures are preferred (1) [51] and in one-step hydrogenation (2), pressure changes at certain intervals are considered. Thus, at very high values of hydrogen pressure, the selectivity of the reaction in the partially hydrogen-ated product decreases [52].
Hydrogen stays in the status of adsorption and disassociation. If there are enough disassociated hydrogen atoms around the activated benzene molecules, the benzene is easily hydrogen-ated to cyclohexane. On the other hand, benzene molecules adsorb on the Ru catalyst surface in a suitable angle through o/n bonds. The activated benzene molecules occur reaction with the closed hydrogen atoms to form cyclohexadiene, cyclohexene, and finally convert to cyclohexane via sub-step hydrogenation. Or benzene molecules adsorb vertically on the active sits of Ru, and it is considered inactive. The bond types of benzene with Ru catalyst surface are determined by the number, geometry structure, and electronic property of Ru active sites. And it provides the reference for utilizing additives and surface modification to increase the yield and selectivity of cyclohexene [53].
The most difficult stage is the join of the first pair of hydrogen atoms, which leads to the disruption of the aromatic structure. Subsequent reactions proceed rapidly with increasing temperature and pressure. The heat effect of the reaction is 50 Kcal/mol. The presence of sulfur
compounds (hydrogen sulfide, carbon disulfide, mercaptans, thiophene) in the raw material causes a small amount of irreversible poisoning of the catalyst. In the hydrogenation reaction, only cyclohexane, can be separated as the final product.
This can be clearly imagined if we look at the thermodynamics of the process. From a thermodynamic point of view, partial hydrogenation of benzene is undesirable because cycloal-kane is at least 75 kJmol-1 more stable in Gibbs free energy (AG) than cyclohexene [54]. Figure 3 shows the heat energies of hydrogenation of benzene to its possible products at 250C. It can also be seen that entropy pushes the direction of the reaction towards the fully hydrogenated product, as the stability difference between cy-clohexene and cyclohexane is 120 kJ mol-1 in terms of AH°, which is considered the more stable state. Real benzene is much more stable than the Kekule structure would suggest. Every time we do a thermochemical calculation based on the Kekule structure, we get an answer that is wrong by about 150 kJ mol-1. This is most easily shown by changing the enthalpy of hydrogenation. When hydrogen is added to benzene, cyclohexane, C6H12, is formed. The "CH" groups become CH2 and the double bond is replaced by a single one. The enthalpy change during the first step of reaction is -120 kJ mol-1. In other words, when 1 mole of cyclohexene reacts, 120 kJ of heat energy is evolved [55].
When the reaction happens, bonds are broken (C=C and H-H) and this costs energy. Other bonds have to be made, and this releases energy. Because the bonds made are stronger than those broken, more energy is released than was used to break the original bonds and so there is a net evolution of heat energy [57]. If the ring had two double bonds in it initially (cyclohexa-1,3-diene), exactly twice as many bonds would have to be broken and exactly twice as many made [58]. In other words, you would expect the enthalpy change of hydrogenation of cyclohexa-1,3-diene to be exactly twice that of cyclohexene - that is, -240 kJ mol-1. In fact, the enthalpy change is -232 kJ mol-1 - which isn't far off what we are predicting.
Fig.3. Heats of hydrogénation for benzene and its derivatives [56].
Applying the same argument to the Kekule structure for benzene (what might be called cyclohexa-1,3,5-triene), you would expect an enthalpy change of -360 kJ mol-1, because there are exactly three times as many bonds being broken and made as in the cyclohexene case. In fact what we get is -208 kJ mol-1 - not even within distance of the predicted value [59].
The most important point to notice is that real benzene is much lower down the diagram than the Kekule form predicts. The lower down a substance is, the more energetically stable it is. This means that real benzene is about 150 kJ mol-1 more stable than the Kekule structure gives it credit for. This increase in stability of benzene is known as the delocalisation energy or resonance energy of benzene. The first term (delocalisation energy) is the more commonly used [60].
Process methods and acquired patents
Selective hydrogenation of benzene to cyclohexene is a very difficult process, but by implementing this process, cost reduction in the production of adipic acid or s -caprolactam can be achieved. In industry, these products are obtained by hydrogenation of benzene to cyc-lohexanone and cyclohexanol over nickel or platinum catalysts and subsequent oxidation of the obtained products [61]. Benzene to cyclohexene with the aid of modified ruthenium-bearing acid catalysts includes an important step such as hydration of cyclohexanol, an intermediate product obtained during selective
hydrogenation. Compared to the solubility of benzene in water, the low solubility of cyclo-hexene in water (at 1500C, 50 bar, coefficient 6) creates difficulties in the re-adsorption of cy-clohexene and subsequent hydrogenation to cyclohexane [62].
Hydrogenation methods in heterogeneous systems are widely used on an industrial scale. Typical hydrogenation reactions are carried out at pressures from 100 kPa to 30,000 kPa and temperatures ranging from 40 to 3500 C. Hydrogenation of benzene is usually carried out with nickel and platinum metals on the carrier. Previously, in industrial processes carried out under high temperature and pressure, side reactions also occurred with the main reaction; for example, cracking with the formation of n-hexane, isomerization of cyclohexane to methylcyclo-pentane, isomerization with separation of lighter hydrocarbons and cracking [63]. The production of high-purity cyclohexane follows a two-stage scheme: in the 1st stage, used Ni on the carrier (30-45% Ni/Al2O3); In the 2nd stage, Pt catalysts are used on the carrier (0.2-1.0% Pt/ AhO3). As a result, cyclohexane is obtained with a purity of 99.6-99.9% [64].
Hydrogenation can be carried out as a liquid or vapor phase process. In the first option, it is possible to carry out the process in a plant, but often high operating pressure, as a factor that ensures the acceleration of the process, usually reduces the solubility of hydrogen in the organic liquid phase, which increases the construction and operation costs of the plant.
These methods differ from each other according to the technologies used to neutralize the reaction components or mixtures obtained on the basis of hydration.
For example, US Patent No. 3,711,566 (Estes et al.) discloses the hydrogenation of a sulfur-containing aromatic hydrocarbon feedstock over a fluorinated platinum catalyst. Sulfur is a strong poison for platinum catalysts and causes rapid deactivation of the catalyst during hydrogenation. Addition of fluorine to the catalyst prevents sulfur poisoning and increases the activity of hydrocracking [65].
The process of hydrogenation of benzene to cyclohexane with catalyst grains in a stationary phase reactor is of industrial importance and is also a typical example of exothermic hydrogenation reactions of unsaturated hydrocarbons. When the reaction is carried out in the gas phase, in the presence of highly active catalysts, internal and external diffusion retardation of the reaction rate is observed, resulting in significant heating of the catalyst relative to the reaction flow. In this regard, keeping such temperature increases under control in order to avoid undesirable side effects such as reduction of selectivity, reversible reaction effects, coking and poisoning of catalysts.
US patent No. 5,730,843 describes the process of hydrogenation of benzene in 2 alternating reactors at a temperature of 2000C and a pressure of 4 MPa in the liquid phase over a nickel catalyst to obtain cyclohexane. Hydrogen is supplied to the bottom of the first reactor, bubbling in the liquid and helping to stabilize the catalyst in the environment. The heat of the reaction is removed due to evaporation of a part of the reaction mixture and recirculation of a part of the liquid together with the catalyst through the heat exchanger [66]. The disadvantage of the method is the complexity of controlling the processes that require the organization of liquid recirculation together with the catalyst, which actually increases the cost of the technological scheme and requires the connection of additional machines and devices to the production process.
US Patent No. 4,626,604 (Hiles et al.) discloses a multi-step hydrogenation process involving at least 3 adiabatic reaction reservoirs. Since
hydrogenation occurs sequentially, lower operating temperatures are more likely to be used, which in turn reduces the formation of complex esters as byproducts that poison the catalyst and lower catalytic activity. In this invention, the main goal of Hiles et al was to achieve vaporization of unsaturated liquid aromatic hydrocarbons before mixing with gaseous hydrogen [67].
USA patent No. 4,731,496 describes the process of hydrogenation of benzene to cyclohexane in the gas phase with the presence of a special nickel catalyst coated on a mixture of titanium dioxide and zirconium dioxide [68].
US 6,750,374 [69] contains up to 15 moles of carbon monoxide, hydrogen containing mixtures such as light hydrocarbons and 15% to 35% Ni and 1% to 15% Cu as carrier. The process of hydrogenation of benzene over aluminum oxide is given. The catalyst has the ability to store auxiliary components such as Mo, Zn, Co, Fe. The most optimal scheme of hydrogenation of benzene over a nickel-chromium catalyst in the vapor phase using a combination of 2 reactors with a mobile and stationary phase catalyst was selected for the calculations. Hydrogenation in the vapor phase also allows obtaining energy vapor that is cost-effective in terms of energy. When performing the process in the liquid phase, difficulties arise due to the use of a mobile phase catalyst. The use of two reactors makes it possible to increase the productivity and conversion rate of raw ma-terials.The hydrogenation of benzene is carried out in a rack-type adiabatic reactor in which the catalysts are placed in contact with the benzene. The process is carried out by recirculation of gas containing a large amount of hydrogen at high temperature and pressure in a platinum catalyst. This gas enters the reactor partly with raw materials, partly into the layer between the catalysts, and at the same time into the cyclohexane recirculation process [70]. A disadvantage of the method is the need to recycle the resulting cyclohexane, which actually reduces the yield of the process for fresh raw materials. Comparative characteristics of benzene hydrogenation processes over different catalysts are given in Table.
Comparative characteristics of benzene hydrogénation processes
No Catalyst Carrier agent P, MPa Temperature, 0 C Usage duration (year) Conversion, % Advantages Missing aspects
1 Nickel Kiselgur, aluminum oxide <3 150-250 2 < 99.9 Being cheap Hypersensitivity to sulfur compounds
2 Nickel-chromium Chromium oxide 3 160-170 2 95 Resistance to catalytic poison Sensitivity to sulfur compounds
2-6 120-250 2 99.9
3 Platinum Aluminum oxide < 3 150-250 3 100 Low sensitivity to sulfur compounds, regeneration possibilities High cost, sensitivity to moisture in benzene
4 Sulfide 30 250-380 2 The possibility of using low-grade benzene without the need to -remove sulfur -compounds High cost (5 times more expensive than nickel), formation of methyl cyclo- pentane, importance of cleaning cyclohexane from sulfur and byproducts
5 Tungsten-nickel-sulphide, 2NiS, WS2 Glinosem 30 300 2 99.5
Without carrier 27-30 280-380 2 99.0
Recent research has focused on finding more efficient and cheaper catalysts. Of more interest are nickel catalysts composed of copper-modified Ag, Ru, Re, Zn, Mo, and Pd or one or more components of the ferromolyb-denum group.
Thus, during the review of literature materials, it was determined that conducting the process of selective hydrogenation of aromatic hydrocarbons over various catalysts is one of the important areas that always attract the attention of researchers. Among the many different catalysts that have been tested and proposed as catalysts, choosing the most effective and efficient one is one of the priority issues of researchers. Selection of both the optimal metal and the optimal carrier for these catalysts requires high sensitivity. However, many unsolved questions about this topic remain open. In this regard, taking into account the fact that there are few existing studies in the literature on the study of the above-mentioned arene hydrogenation reaction on polymer-based Ni, Ru, Pt, Pd-retaining catalysts and that the obtained results are not satisfactory, it is one of the important issues to carry out scientific research studies in the mentioned direction in the future. It is also necessary to take into account environmental restrictions, which play an important role today. This trend, demonstrated by the transition from classical inorganic additives to "green" addi-
tives or modifiers directly incorporated into catalysts, has changed approaches to the structure of catalysts and should be improved in the future.
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MÜXTOLiF KATALiTiK SiSTEMLORDO BENZOLUN SELEKTiV fflDROGENLO§MOSi PROSESiNiN
SONAYEDO TOTBiQiNiN ELMi OSASLARI
N.T.§ixverdiyeva, Ü.O.Mammadova, N.A.Zeynalov
Sanayenin müxtalif sahalarinda neft va digar mahsullarin istehsali zamani alinan maddalar asasinda ba§ veran straf mühitin girklanmasi son illarda aktual olan ekoloji problemlarin halli istiqamatinda tadbirlar görmaya macbur edir. Neftdan alda olunan aromatik karbohidrogenlar sinifina aid olan arenlar kanserogen tasirlarina göra hayat ügün gox tahlükali materiallardandir. Bu baximdan arenlarin xüsusi nümayandasi olan benzolun zaharli va ugucu birla§ma kimi kanserogen tasirlarindan azad edilarak kaprolaktam va neylonun istehsalinda asas xammal olan tsikloheksanin alinmasinda istifada edilmasi mühüm masalalardan biridir. Benzolun hidrogenla§masi prosesinin selektivliyi va alinan maqsadli mahsul olan tsikloheksanin giximinin yüksak olmasi reaksiyanin getma §araiti va katalizatorlarin segimindan asilidir. Son illarda aromatik karbohidrogenlarinin hidrogenla§masi proseslarinin effektiv hayata kegirilmasi maqsadi ila yüksak aktiv va selektiv katalizatorlarin alinmasinda polimer va seolit nanokompozit katalizatorlarinin, elaca da modi-fikasiya olunmu§ metal polimer asasli katalizatorlarin tatbiqi böyük maraq dogurur. Bela ki, naften karbohidrogenlar müxtalif sinif birla§malarin sintezi ügün asas ilkin maddalar hesab olunur. Bu birla§malarin töramalari xüsusi tayinatli kaprolaktamin, adipin tur§usunun, tsikloheksanonun, eyni zamanda efir yaglarinin, laklarin, boyalarin hall edicisi kimi, ekstraqent kimi aczagiliq sanayesinda va s. sintezinda istifada olunurlar. Benzolun hidrogenla§masi reaksiyasinda po-limer-mineral katalizatorlarinin katalitik xassalarinin ara§dinlmasi naticasinda müayyan olunmu§dur ki, bu reaksiyalar dar masamali seolitlarda (mordenit, klinoptilolit) tsikloheksanin amala galmasi istiqamatinda selektivlik göstarir.Taqdim edilan i§da bu istiqamatda aparilan i§lara dair adabiyyat materiallari tahlil edilarak asas malumatlar toplanmi§dir.
Agar sözlar: benzol, hidrogenh§m3, tsikloheksan, tsikloheksadien, tsikloheksen, nikel nanohissaciklar, rutenium hiss3cikl3ri, platin nanohissaciklari, mezomasamali silisium, selektivlik.
НАУЧНЫЕ ОСНОВЫ ПРОМЫШЛЕННОГО ПРИМЕНЕНИЯ ПРОЦЕССА СЕЛЕКТИВНОГО ГИДРИРОВАНИЯ БЕНЗОЛА В РАЗЛИЧНЫХ КАТАЛИТИЧЕСКИХ СИСТЕМАХ
Н.Т.Шихвердиева, У.А.Мамедова, Н.А.Зейналов
Загрязнение окружающей среды веществами, полученными при производстве нефти и других продуктов в различных областях промышленности, заставляет принимать меры по решению экологических проблем, ставших актуальными в последние годы. Арены, принадлежащие к классу ароматических углеводородов нефтяного происхождения, являются очень опасными для жизни веществами из-за их канцерогенного действия. В связи с этим одним из важных вопросов является освобождение бензола, специального представителя аренов, от его канцерогенного действия как токсичного и летучего соединения, и использование его в производстве циклогексана, являющихся основным сырьем в производство капролактама и нейлона. Селективность процесса гидрирования бензола и высокий выход циклогексана, целевого продукта, зависят от условий реакции и выбора катализаторов. В последние годы большой интерес вызывает применение полимерных и цеолитных нанокомпозитных катализаторов, а также модифицированных металлополимерных катализаторов для получения высокоактивных и селективных катализаторов с целью эффективной реализации процессов гидрирования ароматических углеводородов. Нафтеновые углеводороды считаются основным исходным сырьем для синтеза различных классов соединений. Производные этих соединений используются в качестве капролактама специального назначения, в фармацевтической промышленности ,используются в синтезе адипиновой кислоты, циклогексанона, в качестве растворителя эфирных масел, лаков, красок, в качестве экстрагента и др. В результате исследования каталитических свойств полимерминеральных катализаторов в реакции гидрирования бензола установлено, что эти реакции проявляют селективность в направлении образования циклогексана в узкопористых цеолитах (мордените, клиноптилолите). В представленной работе основная информация была собрана путем анализа литературных материалов о работах, проводимых в этом направлении.
Ключевые слова: бензол, гидрирование, циклогексан, циклогексадиен, циклогексен, наночастицы никеля, частицы рутения, наночастицы платины, мезопористый кремнезем, селективность.
