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CHEMICAL PROBLEMS 2021 no. 1 (19) ISSN 2221-8688
41
UDC 549.67: 544.47: 547.59: 542.941.8
RESEARCH INTO KINETIC REGULARITIES OF THE REACTION OF OXIDATIVE DEHYDROGENATION OF METHYLCYCLOHEXANE OVER MODIFIED ZEOLITES
A.I. Karimov
M.F. Nagiyev Institute of Catalysis and Inorganic Chemistry, National Academy of Sciences of Azerbaijan H. Javid ave., 113, Baku AZ1143, e-mail: kerimov.alibala@mail. ru
Received 09.03.2021 Accepted 05.05.2021
Abstract: Kinetic regularities of the oxidative dehydrogenation reaction of methylcyclohexane on the CoCr-clinoptilolite catalyst were investigated. Absence of internal and external diffusion inhibition was established, the reaction proceeds in the kinetic area, in which all diffusion stages proceed much faster than all chemical stages that make up the mechanism of this reaction. The influence of partial pressures of reagents, the reaction temperature and the space velocity of the reaction mixture on the course of the reaction was studied and optimal conditions for obtaining the intentional reaction product determined. Keywords: methylcyclohexane, methylcyclohexadiene, oxidative dehydrogenation, clinoptilolite, naphthenic hydrocarbons.
DOI: 10.32737/2221-8688-2021-1-41-46
Introduction
The kinetic modeling of experimental data and the development of a model equation can be applied in the design of a chemical reactor. Knowledge of the reaction mechanism in describing what actually happens during a chemical reaction makes it possible to perform the safe extrapolation and optimization of reaction variables and thus assist in better development and design of a new catalyst and catalyst system [1].
The mechanism of formation of diene hydrocarbons in terms of heterogeneous oxidative dehydrogenation of naphthenic hydrocarbons has for long been a subject of
discussion [2-5].
There are many schools of thought and disagreements in the literature regarding the kinetic mechanism of the oxidative dehydrogenation of naphthenic hydrocarbons to the corresponding diene hydrocarbons. Therefore, the detailed and rigorous kinetic analysis of extensive experimental datais necessitated.
The article depicts the outcomes of studying the kinetic regularities of the oxidative dehydrogenation of methyl-cyclohexane in order to clarify the possible mechanism of the reaction.
Experimental part
The results of the experimental investigation over selection of an active catalyst for the oxidative dehydrogenation of methyl cyclohexane showed that a metal zeolite catalyst synthesized on the basis of natural zeolite clinoptilolite by ion exchange and containing cations (Co2 + - 0.5%; Cr3 + - 0.25%)
exhibits the highest activity in reactions of oxidative dehydrogenation of
methylcyclohexane to methylcyclohexadiene -1,3. On this basis, the kinetic regularities of the course of the reaction were studied with the participation of this catalyst [6].
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CHEMICAL PROBLEMS 2021 no. 1 (19)
Table 1. Influence of catalyst particle sizes and linear velocity of the initial reaction mixture in the course of the reaction at a molar ratio of C7H14: O2: N2 = 1: 1: 5.3, Vh = 1.026 h-1, T = 3800 C
№ Catalyst particle size, mm Linear velocity of the reaction mixture, m/h Methylcyclohexadiene-1,3 yield, %
1 0.23-0.40 73.56 15.7
2 0.23-0.40 35.72 15.8
3 0.23-0.40 24.46 16.1
4 0.23-0.40 18.34 15.7
5 0.40-0.63 73.56 15.9
6 0.40-0.63 35.72 15.7
7 0.40-0.63 24.46 15.8
8 0.40-0.63 18.34 16.1
9 0.63-1.25 73.56 15.8
10 0.63-1.25 35.72 15.7
11 0.63-1.25 24.46 15.6
12 0.63-1.25 18.34 15.9
13 1.25-1.75 73.56 15.8
14 1.25-1.75 35.72 16.1
15 1.25-1.75 24.46 15.8
16 1.25-1.75 18.34 15.9
17 1.75-2.00 73.56 15.7
18 1.75-2.00 35.72 15.6
19 1.75-2.00 24.46 15.8
20 1.75-2.00 18.34 15.9
Preceding to studying the kinetic laws of the reaction, the region of occurrence was determined. For this purpose, a series of experiments was carried out with various sizes of catalyst grains: 0.25-0.40 mm, 0.40-0.63 mm, 0.63-1.25 mm, 1.25-1.60 mm, 1.60-2.00 mm and various linear velocities of the initial reaction mixture. The linear velocity varied by changing the volume of the catalyst at equal volumetric velocities [Table 1]. As follows from data in Table 1, a change in the size and linear velocity of the initial reaction mixture does not have a significant effect on the main parameters of the process; therefore, there are no internal and external diffusion inhibitions, i.e. the reaction proceeds in the kinetic region, in which all diffusion stages proceed much faster than all the chemical stages constituting the mechanism of this reaction. Kinetic experiments were carried out on a flow-through laboratory setup
in the temperature range 320-380 °C, volumetric velocities 500-3000 h-1, partial pressures of reagents PC7H14 = 0.04-0.14 atm; Po2 = 0.07-0.25 atm. The results of experimental studies of the kinetic laws in the process of oxidative conversion of methylcyclohexane are presented in Table 2-4. The reaction produces methylcyclohexene (A1), methylcyclohexadiene (A2), toluene (A3), and carbon dioxide (A4). As you can see from Table 2, an increase in PO2 from 0.04 to 0.14 atm leads to an increase in the yield of methicyclohexadiene-1,3 which is explained as being due to an increase in the concentration of surface oxygen, and with a further increase in PO2 to 0.2 atm. decreases slightly. In the entire studied range, the conversion of methylcyclohexane (X) is continuously increasing.
Table 2. Influence of oxygen partial pressure on oxidative dehydrogenation of methylcyclohexane on CoCr-clinoptilolite catalyst Vo = 2500 h-1; Vc7H14 = 0.69 l/h; = 0.02255mol / h; = 0.11 atm.;
Gcat = 1.78 g
T,0C c7 H14 < < P CaHjfiHs PO2 pn2 X,% A1 A2 A3 A4
320 0.02255 0.008 58 0.16768 0.11 0.04 0.84 4.25 1.5 0.6 2.1 0.05
0.02255 0.01799 0.15828 0.11 0.09 0.79 9.2 1.9 2.8 4.2 0.3
0.02255 0.02822 0.14805 0.11 0.14 0.75 10.7 2.2 3.1 4.6 0.8
0.02255 0.03926 0.13701 0.11 0.20 0.69 13.9 2.0 2.9 6.8 2.2
340 0.02255 0.00858 0.16768 0.11 0.04 0.84 11.4 3.2 2.6 3.8 1.8
0.02255 0.01799 0.15828 0.11 0.09 0.79 19.4 3.9 6.2 6.8 2.5
0.02255 0.02822 0.14805 0.11 0.14 0.75 21.2 4.4 6.5 7.0 3.3
0.02255 0.03926 0.13701 0.11 0.20 0.69 24.2 3.7 5.9 9.5 5.1
360 0.02255 0.00858 0.16768 0.11 0.04 0.84 23.7 5.8 6.6 6.4 4.9
0.02255 0.01799 0.15828 0.11 0.09 0.79 31.1 6.2 10.5 8.6 5.8
0.02255 0.02822 0.14805 0.11 0.14 0.75 33.8 6.7 11.1 9.3 6.7
0.02255 0.03926 0.13701 0.11 0.20 0.69 38.3 6.4 10.7 12.3 8.9
380 0.02255 0.00858 0.16768 0.11 0.04 0.84 36.3 6.9 10.5 10.1 8.8
0.02255 0.01799 0.15828 0.11 0.09 0.79 44.8 7.1 15.8 11.6 10.3
0.02255 0.02822 0.14805 0.11 0.14 0.75 46.6 7.2 15.7 12.2 11.5
0.02255 0.03926 0.13701 0.11 0.20 0.69 49.9 7.0 14.4 15.8 12.7
400 0.02255 0.00858 0.16768 0.11 0.04 0.84 38.7 6.3 9.2 13.0 10.2
0.02255 0.01799 0.15828 0.11 0.09 0.79 47.2 6.5 13.5 14.9 12.3
0.02255 0.02822 0.14805 0.11 0.14 0.75 49.9 7.2 14.4 15.6 12.7
0.02255 0.03926 0.13701 0.11 0.20 0.69 55.1 6.5 14.0 18.8 15.8
Table 3 shows that in the studied temperature range at a volumetric velocity of 2000 h-1, constant Po2 (0.14 atm.) And Pc7H14 variation from 0.06 to 0.25 atm., the dependence of the yield of methylcyclohexadiene-1,3 has an extreme feature and passes through a maximum. The maximum output is achieved when PC6H12 = 0.11 atm. Further increase in PC7H14 to 0.25 atm. leads to a decrease in the yield of
methylcyclohexadiene-1,3 and conversion of methylcyclohexane. The decrease in the conversion of methylcyclohexane is explained as being due to the fact that at a given partial pressure of oxygen, the relatively high partial pressures of methylcyclohexane prevent the coordination of oxygen with active sites of the metal zeolite catalyst.
Table 3. Effect of the partial pressure of methylcyclohexane on the oxidative dehydrogenation of methylcyclohexane on the CoCr-clinoptilolite catalyst Vo = 2000 h-1; VO2 = 0.69 l / h; = 0.02822
mol / hour; = 0.14 atm ; Gcat = 178 g
y 0 C n° c7h14 nl2 P 1 c7h14 P02 pn2 X,% A1 A2 A3 A4
320 0.01143 0.02822 0.16196 0.06 0.14 0.80 11.4 0.8 1.9 7.5 1.2
0.02255 0.02822 0.14805 0.11 0.14 0.75 10.7 2.2 3.1 4.6 0.8
0.03382 0.02822 0.13395 0.17 0.14 0.68 9.9 3.4 2.9 3.1 0.5
0.04509 0.02822 0.11983 0.25 0.14 0.61 8.5 4.1 2.0 2.2 0.2
340 0.01143 0.02822 0.16196 0.06 0.14 0.80 22.6 3.6 5.2 9.3 4.5
0.02255 0.02822 0.14805 0.11 0.14 0.75 21.2 4.4 6.5 7.0 3.3
0.03382 0.02822 0.13395 0.17 0.14 0.68 18.4 5.2 6.0 5.4 1.8
0.04509 0.02822 0.11983 0.25 0.14 0.61 15.4 6.0 3.9 4.5 1.0
360 0.01143 0.02822 0.16196 0.06 0.14 0.80 35.1 6.2 9.8 11.5 7.6
0.02255 0.02822 0.14805 0.11 0.14 0.75 33.8 6.7 11.1 9.3 6.7
0.03382 0.02822 0.13395 0.17 0.14 0.68 30.8 8.0 10.8 7.8 4.2
0.04509 0.02822 0.11983 0.25 0.14 0.61 27.4 8.7 8.5 6.9 3.3
380 0.01143 0.02822 0.16196 0.06 0.14 0.80 48.4 7.4 13.9 15.0 13. 1
0.02255 0.02822 0.14805 0.11 0.14 0.75 46.6 7.2 15.7 12.2 11. 5
0.03382 0.02822 0.13395 0.17 0.14 0.68 43.2 9.1 15.5 10.3 8.2
0.04509 0.02822 0.11983 0.25 0.14 0.61 39.1 9.9 12.8 10.2 6.2
400 0.01143 0.02822 0.16196 0.06 0.14 0.80 51.1 6.9 11.0 19.1 14. 1
0.02255 0.02822 0.14805 0.11 0.14 0.75 49.9 7.2 14.4 15.6 12. 7
0.03382 0.02822 0.13395 0.17 0.14 0.68 47.1 8.5 14.2 14.2 10. 2
0.04509 0.02822 0.11983 0.25 0.14 0.61 40.5 9.2 10.1 13.3 7.9
It follows from results above that optimal partial pressures of the reagents at which the highest yield of methylcyclohexadiene-1,3 is achieved are: PC7Hi4 = 0.11 atm. and PO2 = 0.14 atm. The effect of temperature and space velocity in the course of the reaction was
studied at optimal PC7H14 and PO2; the results of these studies are presented in Table 4. From Table 4 it follows that as temperature rises from 3200C to 4000C, the yield of methylcyclohexadiene-1,3 grows continuously.
№ nC6HuCH3 < < Vh, h-1 T,0C X,% A1 A2 A3 A4
1 320 16.45 0.05 0.6 11.9 3.9
2 340 26.59 0.09 1.9 16.8 7.8
3 0.00902 0.01129 0.05922 1000 360 35.2 1.1 3.4 20.9 9.8
4 380 47.2 1.5 5.0 23.8 16.9
5 400 51.7 1.3 4.8 25.9 19.7
6 320 12.9 0.9 1.5 8.7 1.8
7 340 23.1 1.9 3.6 12.7 4.9
8 0.01804 0.022576 0.14806 2000 360 34.9 3.7 5.7 17.2 8.3
9 380 46.2 4.6 8.2 20.3 13.1
10 400 50.8 4.2 7.8 23.5 15.3
11 320 10.7 2.2 3.1 4.6 0.8
12 340 21.2 4.4 6.5 7.0 3.3
13 0.02255 0.028226 0.14806 2500 360 33.8 6.7 11.1 9.3 6.7
14 380 46.6 7.2 15.7 12.2 11.5
15 400 49.9 7.2 14.4 15.6 12.7
Table 4. Influence of temperature and space velocity on the process of oxidative dehydrogenation of cyclohexane on a CoCr - clinoptilolite catalyst at a molar ratio of CeHi2: N2
= 1.00: 1.00: 5.3; Gca, = 1.78 q; Vca, = 2 cm3
16 320 10.3 5.2 3.2 1.6 0.3
17 340 19.9 6.9 6.9 3.7 2.4
18 0.02706 0.03386 0.77661 3000 360 31.7 10.1 11.3 6.1 4.2
19 380 43.7 12.0 15.9 9.6 6.2
20 400 47.1 11.2 14.3 12.8 8.8
from
With an upsurge in the space velocity 1000 to 3000 h-1, the conversion of
methylcyclohexane decreases due to a decrease in the contact time (Fig. 1). A decrease in the
contact time prevents the preoxidative dehydrogenation of these products into toluene and deep oxidation to CO2.
Fig. 1. Dependences of the conversion (X) of methylcyclohexane (1) and the yields (A) of the reaction products of methylcyclohexene (2), methylcyclohexadiene (3), and toluene (4) on the conditional contact time at a molar ratio of C6H12: O2: N2 = 1: 1: 5.3 and T = 3800C
If we assume that this reaction proceeds according to a sequential mechanism with the formation of toluene, respectively, then for the reaction of oxidative dehydrogenation of methylcyclohexane - methylcyclohexene and methylcyclohexadiene-1,3 these are intermediate products Fig. 1. It can be seen that the nature of curves of the dependences of the yields of intermediate and final products on the
conditional contact time does not correspond to the sequential mechanism of the reaction. Thus, on the basis of the experimental data obtained, it can be concluded that on the surface of the catalysts there are different active centers consisting of their components, which are responsible for the formation of reaction products, which is consistent with [7].
References
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MODiFiKASiYA OLUNMU§SEOLiTLdR ÜZdRINDdMETiLTSiKLOHEKSANIN OKSiDLd^DiRJCi DEHiDROGENLd$Md REAKSiYASININ KiNETiK QANUNAUYGUNL UQLARININ ÖYRdNiLMdSi
d.i. Krimov
AMEA-nin akad. M.Nagiyev adina Kataliz vd Qeyri-üzvi Kimya institutu AZ1143, Baki, H.Cavidpr., 113; e-mail: kerimov. alibala@mail. ru
Metilsikloheksanin CoCr-klinoptilolit katalizatoru üzdrindd oksidld^dirici dehidrogenld§md reaksiyasinin kinetic qanunauygunluqlari ara§dirilmi§dir. Daxili vd xarici diffuziya tormozlanmanin olmamasi tdsbit edildi, ydni reaksiya kinetic bölgddd davam edir, burada bütün diffuziya mdrhdldldri bu reaksiyanin mexanizmini td§kil eddn bütün kimydvi mdrhdldldrd nisbdtdn daha sürdtli gedir. Reagentldrin parsial tdzyiqldrinin, reaksiya temperaturu vd reaksiya qari§iginin hdcmi sürdtinin reaksiya gedipnd tdsiri öyrdnilmi§dir. Mdqsddli reaksiya mdhsulunu almaq ügün optimal §drtldr müdyydn edilmi§dir.
Agar sözlw. metiltsikloheksan ,metiltsikloheksadien, oksidld^dirici dehidrogenld§md, klinoptilolit, naften karbohidrogenldri.
ИССЛЕДОВАНИЕ КИНЕТИЧЕСКИХ ЗАКОНОМЕРНОСТЕЙ ПРОТЕКАНИЯ РЕАКЦИИ ОКИСЛИТЕЛЬНОГО ДЕГИДРИРОВАНИЯ МЕТИЛЦИКЛОГЕКСАНА НА
МОДИФИЦИРОВАННЫХ ЦЕОЛИТАХ
А.И. Керимов
Институт катализа и неорганической химии им. акад. М.Нагиева Национальной АН Азербайджана AZ1143 Баку, пр.Г.Джавида, 113; e-mail: kerimov.alibala@mail. ru
Исследованы кинетические закономерности протекания реакции окислительного дегидрирования метилциклогексана на катализаторе CoCr-клиноптилолит. Установлено отсутствие внутренне- и внешне-диффузионного торможения, т.е. реакция протекает в кинетической области, в которой все диффузионные этапы протекают значительно быстрее всех химических стадий, составляющих механизм этой реакции. Изучено влияние парциальных давлений реагентов, температуры реакции и объёмной скорости реакционной смеси на протекание реакции. Определены оптимальные условия получения целевого продукта реакции.
Ключевые слова: метилциклогексан, метилциклогексадиен, окислительное дегидрирование, клиноптилолит, нафтеновые углеводороды.
