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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2021 ISSN 0005-2531 (Print)
UDC 547.534.1: 542.941.8
EFFECT OF CARBON DIOXIDE ON OXIDATIVE ETHYLBENZENE DEHYDROGENATION IN THE PRESENCE OF ALUMINUM-CHROMIUM CATALYSTS
M.T.Mamedova
Yu.Mammadaliyev Institute of Petrochemical Processes, NAS of Azerbaijan
Received 23.09.2020 Accepted 21.12.2020
In order to create a more efficient process for the production of styrene from ethylbenzene by selective oxidation of the resulting hydrogen, model aluminum-chromium catalysts with different modifier (Cu) contents were prepared, and O2, CO2 and a mixture of CO2:air were used as an oxidant. It has been established that the process of obtaining of styrene from ethylbenzene on alumochromic catalysts is characterized by a high intensity. Alumochromic catalysts modified by Cu and promoted by 15 mass.% K2CO3 in the presence of carbon dioxide increase the conversion of ethylbenzene by 14%, and the selectivity of the formation of styrene by 10% as a result of the transferring dehydrogenation of compaction products to the products of oxidative compaction. The presence of oxygen in the oxidizing mixture CO2:air blocks the participation of carbon dioxide in the selective oxidation of hydrogen isolating at the dehydrogenation of ethylbenzene. It has been established that the most selective alumochromic catalysts in the oxidative dehy-drogenation of ethylbenzene to styrene contain 1.5-2.5 mass.% of the modifying component (Cu).
Keywords: ethylbenzene, styrene, oxidative dehydrogenation, model alumochromic catalysts, modification by copper, the oxidizing mixture of CO2:air, CO2, products of compaction, products of oxidative compaction.
doi.org/10.32737/0005-2531-2021-2-69-79
Introduction
Industrial processes based on dehydrogenation are accompanied by high energy costs. This, in its turn, limits the amount of the initial reagent, and therefore, industrial dehydrogena-tion processes are implemented at low volume rates (v.r.). For the dehydrogenation of ethylbenzene (EB) to styrene (St), the maximum space velocity (in liquid) does not exceed 0.5 h-1 [1]. As a result, the production of styrene on an industrial scale requires the use of reactors a large capacity that, in turn, increases the material consumption of the process. Thus, the intensification of the process for the production of styrene from ethylbenzene with reduced energy and material costs is an urgent problem in petrochemistry and requires finding effective solutions to remove these shortcomings. The oxidative dehydrogenation of ethylbenzene to styrene by oxidizing the hydrogen isolated in this process is one of the possible solutions to the problems which arise, since in this case occurs an effective shift of the reaction towards styrene formation and ensuring the energy costs
of the reaction. With this important to note that the water molecules formed can promote the desorption of styrene product molecules from the catalyst surface and increase the selectivity of the process. In this regard, there is an intensive search for oxidants for the catalytic oxidation of EB in St. To the number of like oxidant -refers O2, SO2, N2O, CO2 [2-7]. The latter compound, as a mild oxidizing agent, is the subject of ever-expanding research. In 1991, Matsui and his colleagues first reported the positive effect of CO2 in relation to deactivation of an industrial catalyst for the dehydrogenation of ethylbenzene to styrene [8]. After that, interest in the presence of CO2 in the processes of hydrocarbon conversion has increased intensively [9, 10]. It was determined that in the reaction of converting EB to St, CO2 acts as a diluent and oxidizing agent, shifts the equilibrium of EB conversion towards styrene formation, increases EB conversion, improves selectivity styrene [11], maintains the active phase of oxide catalysts in the required oxidation grade, provides better heat supply due to its high heat capacity,
suppresses undesirable deep oxidation products, removes coke from the catalyst surface by the reverse Boudoir reaction [12]. Bartholomew C.H. determined a number of oxidizing abilit of various gases at coke gasification: O2 (105)> H2O (3)> CO2 (1)> H2 (0.003). The numbers in parentheses clearly indicate that CO2 can be considered an oxidizing agent [13]. Typical characteristics of these gases were summarized in tabular form in a review by Park and Han [14]. According to these data, C02 has the highest heat capacity among other typical gases and, due to this, C02 prevents hot spots on the catalyst surface. Taking into account all these unique features, CO2 can be considered as a mild oxidizing agent for use in the oxidative conversion of hydrocarbons [15].
One of the first studies on the use of CO2 as a diluent and oxidizing agent in the dehydrogenation of EB in St was carried out by Sugino M.O. and co-workers. They used lithium promoted iron catalyst supported on activated carbon. At 500-700°C, the styrene yield reached 40-45% with a styrene selectivity of more than 90%. In addition to styrene, CO and water formed. This proved that the reaction proceeds according to the oxidative dehydrogenation mechanism [16].
The mechanism of conversion of EB to St in the presence of CO2 and the role of the latter in this process are extensively discussed in the literature. Mimura and Saito showed that the conversion of EB to St in the presence of C02 onto an iron oxide catalyst supported on alumina proceeds in two ways: the one-step path by reaction 1 - the oxidative dehydrogenation of EB in the presence of oxygen formed from the dissociation of CO2 and the two-step path by reaction 2 and 3 - first, the usual dehydrogenation of EB occurs by 2, and then the oxidation of the formed hydrogen with CO2 to water (reverse reaction of the conversion of water gas and CO) (3) [17]; Ventaka R.M. and Kamaraju S.R.R. showed that the process on Co3O4/Al2O3 proceeds in a two-step way (equations 2 and 3) [18]; In [4], the authors suggest that the process on KFePMo11W proceeds in a one-step way (1); Pak and Wislov-sky [19], as well as Shuwei [12] showed that EB
dehydrogenation in the presence of CO2 on VSbOx/Al2O3 and V2O5/SiO2 catalysts, respectively, proceeds according to the redox mechanism, where oxidative dehydrogenation occurs due to lattice oxygen of the catalyst (equation 4 and 5):
C6H5CH2-CH3+C02^C6H5CH=CH2+C0+H20 (1) C6H5CH2-CH3^C6H5CH=CH2+H2 (2)
H2+C02^C0+H20 (3) C,,H,CH2-CHi+M0,-^C,1H,CH=CH2+H20+
+MO,, (4)
MO^+CO^MCK.+CO: (5)
here MOx - metal oxide included in the catalyst composition.
As a result of investigation, carried out by dissefied scientists, it was found that the mechanism of conversion of EB to St in the presence of CO2 depends on the nature of the catalyst.
Many researchers have compared the activity of catalysts in the presence of CO2 and other oxidizing agents.
D. Gong and co-workers on comparing the activity of the V0.43Sb0.57Ox/Al2O3 catalyst in the presence of O2, N2, H2O and CO2 in the conversion of EB to St. [20] they found that the greatest yield of styrene and selectivity for styrene in the presence of CO2.
Based on the thermodynamic calculation, Mimura and Saito comparing the equilibrium yield of styrene in direct EB dehydrogenation in the presence of water vapor and in oxidative dehydrogenation in the presence of CO2 revealed that at the same temperatures, the equilibrium yield of St in the presence of CO2 rather higher. They estimating the energy costs of obtaining St from EB in the presence of C02 (commercial process) and showed that this cost is 6.3x108 kal/t St and 1.5x108 kal/t St respectively [17].
Javani F. and Trifiro F. proved that, in the presence of CO2, the dehydrogenation equilibrium of EB shifts toward styrene formation and leads to an increase in ethylbenzene conversion and styrene selectivity [21].
Since the 90s intensive research has been continuing on the search for optimal catalysts
for the oxidative dehydrogenation of EB in St in the presence of CO2. Studies have shown that catalysts with an optimal balance of acid-base and redox properties, have high activity and selectivity for styrene [22] in the presence of CO2. Oxides of some metals deposited on alumina; catalysts with of high activity in the reverse water-gas shift reaction, metal oxides supported on mesoporous materials are highly active in the conversion of EB to styrene in the presence of CO2 [7, 19]. These studies found that the formation of carbon deposits on the surface of the catalysts during the reaction, usually lead to de-activation of the catalyst by coking. However, there exist are a number of cases whene such deposits can improve catalytic properties. It was shown in [24-26] that carbon deposits, which initially form on the surface of catalysts, are responsible for the increased activity and selectivity of the reaction. However, increasing the amount of carbon deposits value more than the monolayer reduces the activity of the catalyst due to the loss of available surface area and oxygen groups (mainly the -C = O group) on these deposits. It is shown that defects and edges of carbon deposits play an important role in the reaction mechanism.
Despite the wide range of catalysts studied and the selection of various oxidizing agents that make it possible to subject EB to oxidative dehydrogenation in St, and the presence of such catalytic process parameters as achieving 100% EB conversion and 97.7% St selectivity at 3800C and a contact time of 3.6 sec [27], oxidative dehydrogenation of EB in St is still not industrially implemented. Therefore, new research is required related to the search for new, more stable catalytic systems, and clarification of the features of this reaction.
In this regard, we have synthesized several variants of catalysts and studied their activity in the oxidative dehydrogenation of ethylbenzene.
In the investigation we carried out earlier for the oxidative dehydrogenation of ethylben-zene to styrene based on y-Al2O3, catalysts containing oxides of Mg, Zr and phosphorus, as well as mixtures of these modifiers, were pre-
pared [28]. It was established the activities of these samples depend on the amount and ratio of the components deposited, and the optimal values are 1.0% by mass of ZrO2 and 2.0-4.0% by mass of MgO. The synthesized samples have a high activity in the oxidative dehydrogenation of ethylbenzene to styrene (EB conversion reaches 60% with a selectivity of 90%). The drowing of MgO and ZrO2 on y-Al2O3 separately and together contribute to a decrease in the process temperature by 20 and 500C, respectively. The introduction of phosphorus into MgO and ZrO2/Al2O3 catalysts helps to reduce the catalyst activation time and increases the stability of its operation. The development of the catalyst is accompanied by the accumulation of carbon deposits, leading to an increase in the yield of styrene, and further to a decrease in its activity.
The catalysts were synthesized based on the spent industrial aluminum-chromium catalyst used in the hydrocarbon dehydrogenation by SABIC Co., and the catalysts were modified with copper and potassium carbonate [29]. The catalysts were tested in the oxidative dehydrogenation of ethylbenzene to styrene. It was found that the prepared and modified 1 wt % Cu catalyst, upon the conversion of ethylbenzene to styrene in the presence of O2, exhibits an activity of 53% with a selectivity for styrene of 85%. It was shown that the introduction of carbon dioxide into the reaction zone leads to an increase in activity to 60-63%, and selectivity to 89-91%. The stability of the functioning of a copper-promoted catalyst and the effect of water vapor on the process were studied. It was found that under conditions close to industrial, within 100 h, a stable conversion of the mixture (ethylbenzene:O2 = 9:1):H2O = 4:1 with a selectivity for styrene of at least 90% at a space velocity of 2 h-1 (by liquid ethylbenzene).
Continuing the our work in the present study, we synthesized model aluminum-chromium catalysts modified with copper and promoted with potassium carbonate and studied their activity in the conversion of ethylbenzene to styrene in the presence of O2, CO2 and their mixture under conditions close to the conditions
for the industrial dehydrogenation of this hydrocarbon to St (6000C, atmospheric pressure, lowering the partial pressure of the hydrocarbon by dilution), but characterized by an increased volumetric rate of the reactant feed.
Experimental part
As an aluminum component in the synthesis of an aluminum-chromium catalyst, an industrial paste-like aluminum gel was used. The chromium component was implemented following from potassium bichromate. The initial potassium bichromate was thoroughly mixed with ammonium chloride and calcined at temperatures of 550-6000C. Ammonium bichromate formed as a result of the exchange decomposes to chromium(III) oxide. Obtained dark green mixture was washed with distilled water to remove potassium chloride, filtered, and then the precipitate was treated with a concentrated KOH solution. After vigorous stirring for 5 hours, the solution was diluted with distilled water and left overnight. After this, a gellike precipitate formed was decanted, which was additionally dehydrated in air at room temperature for one day. Then, the paste-like gels of aluminum and chromium oxides were thoroughly mixed until a homogeneous consistent mass was obtained, which was extruded by passing it through a 3 mm diameter, dried under an electric incandescent lamp (24 hours), were crushed and placed into 5-6 mm cylinders, and dried at 800C (3 h), 1200C (3 h), and stepwise raising the temperature (1000C/1.5 h), calcined at 7000C for 3 hours.
The finished sample contained 30 mass.% Cr2O3, the rest was Al2O3. Part of the sample was promoted with potassium carbonate, keeping in solution of given concentration for 24 hours, and then evaporated and subjected to drying and calcination according to the method described above. The content of K2CO3 in the finished catalyst was 15%.
The rest of the sample was modified with copper, keeping the sample in a porcelain dish with a given concentration of copper nitrate solution (48 h). After that, the sample was dried and treated with saturated sodium hydrocar-
bonate solution, decanted with distilled water, dried at 800C (3 h) and 1200C (3 h). The obtained samples were placed in a reactor and reduced in a stream of hydrogen at 1850C and the linear rate of hydrogen supply to the reactor was 20 cm3/min. The recovery process was continued until the complete release of water vapor in the trap placed at the outlet of the reactor. The copper-modified samples were further promoted with K2CO3 as described above.
The specific surface area of the catalysts was determined by benzene adsorption and calculated by the BET equation.
Ethylbenzene transformations were studied in a flow-type catalytic unit equipped with a quartz reactor with a catalyst loading of 3-10 cm3.
Before carrying out the experiments, the catalysts were subjected to standard processing: in a stream of air (20 cm3/mm), the temperature of the reactor was raised to 6500C and the sample was kept at this temperature for 1 h. Further, the temperature was lowered to 6000C, and after its stabilization ethylbenzene was introduced into the reactor by a liquid volume velocity of 2 h-1, and air speed was regulated so as to adhere to the ratio of EB:O2 = 4:1 (25 cm3/min). Ethylbenzene was fed into the reactor by a mechanical syringe into the mixer-evaporator, where air also entered, and the reactants were mixed. It should be noted that the evaporator-mixer also served as a heater. The air used in the studied reaction was pre-humidified by moistened by bubbling through water at a temperature of 700C.
CO2 was supplied to the reactor from the gas meter by displacing it at a certain rate, controlled by the supply of a solution from a measuring tank (20-40 ml/min). Carbon dioxide also entered the evaporator-mixer, and then into the reactor as part of the mixture.
Gaseous and liquid products were analyzed simultaneously using an online GC (Younlin Instrument, Acme 6000 series, Korea) equipped with a katharometer and flame ionization detector. To analyze gaseous products (hydrogen, carbon monoxide, methane and carbon dioxide), a Poropak Q column (1.83 mx3.2 mm) was used, and liquid products were ana-
lyzed in a column (length 30 meters, inner diameter 0.32 mm) with Innowax (film thickness
0.25 pm).
Results and discussion
Preliminary studies of the conversion of EB were carried out in the absence of oxidizing agents. In order to maintain the hydrodynamic parameters of the process, the oxidizing agent was replaced with an inert gas (nitrogen), and the volumetric flow rate of the liquid was 0.5 h-
1. As can be seen from figure 1 a and b (curves 1 and 1 '), at the initial moment of the reaction, a certain time interval is observed corresponding to the activation of the catalyst, the activity of which reaches 30% by 30 min of experiment and remains almost constant.
Analysis of the reaction products showed their composition: benzene - 14%, toluene -4% and the target product styrene - 80%. The remaining unidentified products (~2-4%) are likely to be degradation products.
Carrying out the reaction in the presence of an oxidizing agent - oxygen of the air shows (Figure 1 a and b, cr. 2, 2 ') that the main laws of the conversion of ethylbenzene to styrene remain practically unchanged and selectivity by styrene remains at the level of 80%. At the same time, the introduction of oxygen into the process increases the degree of conversion of ethylbenzene, ceteris paribus, by almost 20%.
In this case, due to the action of the oxidizing agent, the volumetric rate of the process increased approximately four times (2 h-1 by liquid) and, consequently, the intensity of the process also increased.
The copper modification of the model catalyst has a significant effect on the conversion of ethylbenzene. From the data in Figure 1 a and b (curves 4 and 4') it is seen that in the presence of oxygen, modification with copper not only increases the degree of conversion of ethylbenzene, but also increases the selectivity for the target product by 4-6%. From the data in Figure 1 a and b, it also follws that during the dehydrogenation of ethylbenzene on a copper-modified aluminum-chromium catalyst without an oxidizing agent, curves 1 and 3; 1' and 3' almost coincide. This suggests that the modification of the aluminum chromium catalyst with copper does not affect the "pure" de-hydrogenation ability of the model catalyst. Therefore, an increase in the activity and a shift in the reaction of dehydrogenation of ethylben-zene on an aluminum-chromium catalyst towards the formation of the target product may be associated with an increase in the ability of the catalyst to selectively oxidize the resulting hydrogen. An additional argument to this is the results of previous studies on the oxidative de-hydrogenation of ethylbenzene on copper-
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20
40
60 80 time, min
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100 120
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60 80 time, min b)
100 120
Fig. 1a and b. Dehydrogenation of ethylbenzene to styrene on model aluminum-chromium catalysts: T = 6000C,WHSV = 2h-1. Al2O3^Cr2O3 15% K2CO3 -1,1' - non-oxidative dehydrogenation, 2,2' - oxidative dehydrogenation; 1.5%Cu/Al2O3^Cr2O3+15%K2CO3 - 3, 3' - non-oxidative dehydrogenation; 4, 4' - oxidative dehydrogenation 1, 2, 3, 4 - conversion; 1', 2', 3', 4' - selectivity.
aluminum catalysts [30]. By the results of these studies, the process proceeds only with a catalyst sample, the copper content of which is 2530 mass.%. A sample containing 1.0-1.5 mass.% copper is not active in the process under study.
The replacement of oxygen by carbon dioxide during the oxidative dehydrogenation of ethylbenzene to styrene leads to a noticeable increase in the selectivity of the process for styrene (Table 1).
As noted above, of particular interest is the study of the conversion of EB to St in the presence of mixtures of two oxidizing agents: CO2: air. These studies were carried out under the composition of the mixture components changed from zero to 100%. It has been established, that the yield of products and the overall conversion of EB depend on the ratio of the components of the oxidant.
The results obtained show (Figure 2) that with an increase in air concentration, above 40 vol. %, the activity of the catalyst increases,
reaching a limit value of 75%. With an air ratio of less than 40 vol. %, the activity of the catalyst increases again and, when using individual CO2, reaches a limit value of 42%. It can be seen from the data in Figure 2 that the stabilized EB conversion (i.e., after the catalyst reaches a stationary level) has a complex dependence on the composition of the CO2: air mixture and is characterized by the presence of a certain minimum value, within the ratio of CO2: air = 3:2.
The complex nature of the dependence on the CO2: air ratio is also observed for the selectivity of the formation of St. From the data in Figure 2 it can be seen that with an increase in the air content in the mixture used up to almost 80%, the selectivity (for St) of the oxidation process exceeds the similar selectivity of the catalyst in direct dehydrogenation of EB. At higher concentrations of oxygen (air) in a mixture with carbon dioxide, the selectivity of oxi-dative dehydrogenation decreases quite sharply to 62%.
Inertqas and oxydant EB:Oxy. WHSV, h-1 Conv. of EB, % Select. for St, % Hydrocarbon products, % CD*, wt%
СН4 С2 СбН6 С7Н8
Nitroqen EB:N2 0.5 28 80 0.9 2.0 5.0 3.5 0.83
Air EB:02 2.0 75 62 6.0 3.0 3.0 2.5 0.92
CO2 EB:C02 2.0 42 90 0.3 0.7 2.2 2.8 0.88
Table 1. The effect of air and CO2 on the conversion of ethylbenzene and the composition of the resulting hydrocarbon products. Catalyst - LSroCu/A^Os^Os + 15% K2CO3; T = 600°C; t = 2 h
: - Accumulation of carbon deposits (CD) on the catalyst for 1 h.
T3
Ф 90
80-
70-
60-
50-
o <л
ш 40
30-
■ — конверсия этиблензола л—выход стирола
селективность по стиролу
20
40 60 air, vol.%
80
100
Fig. 2. The dependence of the oxidative de-hydrogenation of ethylbenzene to styrene on the composition of the oxidant CO2: air. Catalyst - 1.5% Cu/Al2O3Cr2O3 +15% K2CO3; T = 6000C; WSHV = 2 h-1, t = 2 h.
0
From the data in Figure 2 also implies that under the influence of carbon dioxide, a selective oxidation of part of the formed hydrogen occurs. Due to this, the conversion of EB increases by 12% with approximately the same increase in selectivity for St. Taking into account that the yield of low molecular weight carbons C1-C2 decreases by 14%, i.e. the contribution of hydrodealkylation is reduced in comparison with conventional dehydrogenation of EB, it can be assumed that part of the generated hydrogen is spent on interaction with C02 (reaction 3).
The effect of air on the EB transformation is characterized by a high conversion of the latter and a relatively low yield of St (selectivity ~ 60%). In particular, with decreasing contact time, conversion decreases, and selectivity increases due to a reducing contribution of deeper EB oxidation. A similar effect is exerted by a decrease in the concentration of oxygen (air) in the oxidizing mixture used, leading to an increase in the selectivity of St formation (Figure 2). At the same time, the oxidative interaction of CO2 with hydrogen formed as a result of dehydrogenation of EB is suppressed by oxygen. The observed decrease in catalyst activity can be explained by this fact.
Naturally, the process of oxidative dehy-drogenation is accompanied by the formation and accumulation of products of oxidative compaction (POC) and, as a result of which, the development of a low-activity catalyst takes place [31, 32]. It is assumed that in the initial non-stationary stage of the process, St is formed at the Lewis acid centers and at POC, which is formed with the participation of these centers and oxygen. Responsible for oxidative dehy-drogenation of EB is POC; its accumulation to a monolayer coating is the beginning of a steady-state mode of catalyst functioning [31]. A possible reason for the catalytic activity of POC may be a large geometric correspondence between its structure and structure EB, due to which on the surface are created high concentrations of carbon-oxygen centers responsible for the formation of St. Moreover, it is assumed that oxygen from oxygen-containing surface
groups is sufficiently reactive to participate in oxidative dehydrogenation of EB in St [31]. Such a scheme does not imply the formation of any EB intermediates with oxygen. Therefore, the formation of compaction products (CP) in direct EB dehydrogenation is similar to CP preceding the formation of POC. In this regard, it can be assumed that similar forms of POC arise under the influence of CO2 with preceding CP, formed as a result of the direct dehydrogenation of EB. To confirm this assumption, comparative studies of the conversion of EB to St in the presence of carbon dioxide were carried out both on a pure catalyst sample (after regeneration) and on a sample previously brought to a stationary level by direct EB dehydrogenation. The results of these studies are shown in Figure 3, from the data of which it is seen that the conversion of EB to St in the presence of CO2 increases with the duration of the experiment. In the case of preliminary brought of the catalyst to a stationary level, subsequent activation of the process occurs less intensively compared to a fresh catalyst sample. In this case, it can be noted that the exit of the catalyst to the stationary mode in both studied cases coincides. As mentioned above, for the development of the process of oxidative dehydrogenation of EB is responsible POC. Their formation under the influence of carbon dioxide on the stabilized during direct dehydrogenation of EB in St sample of copper-modified aluminum chromium catalyst can be caused by the interaction of CO2 molecules with the resulting CP with transformation of them to the POC.
Identical POC are also formed on a fresh catalyst. Moreover, as can be seen from the coincidence of the time for the catalyst to reach steady-state condition, it is supposed that the process is limited by the interaction of CO2 with the CP formed from the interaction of EB with Lewis acid centers. An additional confirmation of the proposal made on the transformation of CP into POC in the presence of CO2 can serve as the data presented in Figure 4.
According to this figure, the data of curve 1 of Figure 3 can be straightened by representing the time of the experiment in a se-
quence known as Fibonacci series, i.e. in the sequence in which each element is the sum of 2 previous terms, i.e. n3 = n1 + n2, n4 = n2 + n3 ... etc. Such a linear dependence of unsteady conversion, which describes the activation of oxidative dehydrogenation of EB in St, when representing the experiment time in the sequence of the Fibonacci series, can mean the occurrence of two successive processes: the formation of CP, which is primary and its transition to POC as a result of interaction with CO2.
Thus, it can suggest the scheme for the participation of CO2 in the oxidative dehydro-genation of EB in St:
□+ C02^[0...C0-^| [O...CO.^| + + H2^CO+ H20+^,n
where □□ - carbon defects contained in PC; [O.CO-^] [O...CO... □□] show the interaction of CO2 with carbon centers (defects) containing CP, and its dissociation into an oxygen atom and CO.
Thus, CO2 is involved in the selective oxidation of hydrogen generated during the dehydrogenation of EB. Due to this, the conversion of EB and the yield of St exceed the equilibrium
yield of St, which occurs during the direct dehydrogenation of EB. The decrease in the activity of the oxidative dehydrogenation catalyst when using CO2:air mixtures <3:2 is apparently due to the interaction of the □ centers with oxygen and the formation of new centers according to the scheme:
2^ + 02 ^ 2CO + 2aDD
An increase in oxygen concentration leads to the activation of these centers
+ O2 = O
and the course of oxidative dehydrogenation of EB in St according to the mechanism described in [31].
The effect of the copper content in aluminium-chromium contacts on the oxidative dehydrogenation of ethylbenzene to styrene was also studied. Testing of copper-modified aluminum-chromium samples in the oxidative dehydrogenation of EB in St was carried out with an oxidizing mixture of CO2: air = 85:15, i.e. enriched with carbon dioxide. The results of these tests are given in Table 2.
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**t
0
20
-■-1
-*- 2
40 60 80 100 120 140
time, min
Fig. 3. The dependence of the oxidative dehydrogenation of ethylbenzene to styrene in the presence of CO2 on the duration of the experiment: catalyst 1.5%Cu/Al2O3-Cr2O3 + 15% K2CO3; T = 6000C, WSHV = 2 h-1: 1 - fresh catalyst, 2 - catalyst stabilized by direct dehydrogenation of ethylbenzene.
£
40
30
20
10
1 2 3 5 8 13 21 34 55 89 144 lime, min
Fig. 4. The dependence of the oxidative dehydrogenation of ethylbenzene to styrene on the duration of the experiment the change of which is presented in the form of a Fibonacci sequence (n3 = n1 + n2; n4 = n2 + n3, ...): 1 - fresh catalyst,
2 - catalyst stabilized by direct dehydrogenation of ethylbenzene.
Table 2. The effect of the copper content in aluminium-chromium contacts on the oxidative dehydrogenation of
ethylbenzene to styrene: T = 6000C; WSHV= 2 h CO2: air = 85:15 (vol.); t = 2 h
Content of Cu, mass.% Specific surface area, m2/q Pore volume, cm3/g Conversion of EB, % Selectivity for St, % The yield of carbon by-products, mass.%
CH4 C2 № C7H8
0.5 65 0.21 33 89 0.14 0.5 1.9 2.4
1.5 67 0.23 35 91 0.15 0.5 2.0 2.4
2.5 69 0.24 37 91 0.15 0.5 1.9 2.5
3.5 71 0.27 40 86 0.21 2.7 7.6 2.5
5.0 70 0.25 39 82 0.20 2.6 8.0 2.6
From the data obtained it follows that with an increase in the copper concentration from 0.5 to 3.5%, a steady increase in the activity of the catalyst from 33 to 40% occurs. As for the selectivity of the formation of St, only on a sample with a content of 0.5% copper a slight increase in selectivity from 89 to 91% is observed. With an increase in the copper concentration to 2.5 mass%, this parameter does not change, but a further increase in the copper content leads to a noticeable decrease in the selectivity of oxidative dehydrogenation of EB in St.
The composition of hydrocarbon products formed during oxidative dehydrogenation of EB in St, are given in table. 2 as an example, shows that with small changes in the activity of the catalyst with an increase in the amount of Cu from 2.5 to 5.0%, significant changes are observed in the yields of benzene and C2 hydrocarbons. A four- and five-fold increase in the yields of these products at almost constant degrees of EB conversion indicates that, with an increase in the copper content in the contacts, the contribution to the process reaction of hydrogenolysis of the alkyl group increases. Therefore, the optimal copper content in the aluminium-chromium contacts of oxidative dehydrogenation of EB in St should be limited to 1-2.5 mass.%.
Conclusion
Copper modified aluminum chromium catalysts, similar in composition to the hydrocarbon dehydrogenation catalysts, have high catalytic activities in the oxidative dehydro-genation of ethylbenzene to styrene in the presence of O2 and CO2. The process of producing styrene from ethylbenzene on these catalysts is characterized by a high intensity (2 h-1), which
is 2 to 3 times higher than the intensity of direct dehydrogenation (0.5 h-1).
Aluminium-chromium catalysts modified with 1.5 mass.% Cu and promoted with 15 mass.% K2CO3 in the presence of carbon dioxide increase the conversion of ethylbenzene and the selectivity of styrene formation by 14 and 10%, respectively, as a result of the conversion of compaction products into oxidative compaction products. The presence of oxygen in the CO2:air oxidizing mixture blocks the participation of carbon dioxide in the selective oxidation of hydrogen released during the dehydrogena-tion of ethylbenzene.
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ETiLBENZOLUN ALÜMOXROM KATALiZATORLARININ i§TiRAKI iLO OKSiDLO§DiRiCi DEHiDROGENLO§MOSiNO KARBON DiOKSiDiN TOSiRi
M.T.Mammadova
Omala galan hidrogenin selektiv oksidla§dirilmasi yolu ila etilbenzoldan stirolun daha samarali alinma prosesinin yaradilmasi maqsadila misla modifikasiya olunmu§ alümoxrom katalizatorlari hazirlanmi§ va onlarin aktivliyi O 2 (hava), CO2 va CO2: hava qan§iginin i§tirakibda öyronilmiijdir. Malum olmu§dur ki, sintez olunmu§ alümoxrom katalizatorlari üzarinda etilbenzoldan stirolun alinmasi prosesi yüksak intensivlikla gedir. Misla modifikasiya olunmu§ va 15 küt.% К2С03 ila promotorla§dirilmi§ alümoxrom katalizatorlari CO2-nin i§tirakinda üzarlarinda amala galan sixla§ma mahsullarimn oksidla§dirici sixla§ma mahsullarlna gevrilmasi hesabina etilbenzolun konversiyasini 14, stirola göra selektivliyi isa 10% yüksaldirlar. CO2:hava oksidla§dirici qari§igindaki oksigen etilbenzolun birba§a dehidro-genla§masindan amala galan hidrogenin selektiv oksidla§dirilmasinda CO2-ya mane olur. Malum olmu§dur ki, tarkibinda 1.5-2.5% modifikator (Cu) olan alümoxrom katalizatoru etilbenzolun stirola oksidla§dirici dehidrogenla§-masinda daha yüksak selektivliya malikdir.
Agar sözlzr: Etilbenzol, stirol, oksidla§dirici dehidrogenla§ma, model alümoxrom katalizatorlari, misla modifikasiya, CO2:hava oksidla§dirici qari§igi, CO2, sixla§ma mahsullari, oksidla§mi§ sixla§ma mahsullari.
ВЛИЯНИЕ ДИОКСИДА УГЛЕРОДА НА ОКИСЛИТЕЛЬНОЕ ДЕГИДРИРОВАНИЕ ЭТИЛБЕНЗОЛА В ПРИСУТСТВИИ АЛЮМОХРОМОВЫХ КАТАЛИЗАТОРОВ
М.Т.Мамедова
С целью создания более эффективного процесса получения стирола из этилбензола путем селективного окисления образующегося водорода приготовлены модельные алюмохромовые катализаторы с различным содержанием модификатора (Си), а в качестве оксиданта использованы О2, СО2 и смесь СО2:воздух. Установлено, что процесс получения стирола из этилбензола на алюмохромовых катализаторах харак-теризуется высокой интенсивностью. Алюмохромовые катализаторы, модифицированные Си и промоти-рованные 15 мас% К2С03, в присутствии диоксида углерода повышают конверсию этилбензола на 14%, а селективность образования стирола на 10% в результате перехода продуктов уплотнения в продукты окислительного уплотнения. Наличие кислорода, в окислительной смеси С02: воздух, блокирует участие диоксида углерода в селективном окислении водорода, выделяющегося при дегидрировании этилбензола. Установлено, что наиболее селективные в окислительном дегидрировании этилбензола в стирол алюмохромовые катализаторы, содержат 1.5-2.5 мас. % модифицирующего компонента (Си).
Ключевые слова: этилбензол, стирол, окислительное дегидрирование, модельные алюмохромные катализаторы, модификация медью, окислительная смесь СО2:воздух, СО2, продукты уплотнения, продукты окислительного уплотнения.
