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CHEMICAL PROBLEMS 2021 no. 2 (19) ISSN 2221-8688
101
UDC 66.094.258.094.3/547.267
RESEARCH INTO REACTION ABILITY OF CYCLOHEXANOL AND METHYLCYCLOHEXANOLS IN THE OXIDATIVE DEHYDRATION REACTION OVER
MODIFIED ZEOLITE CATALYSTS
A.M. Aliyev, G.A. Ali-zade, M.Q. Aliyeva, A.R. Safarov, V.M. Yariyev, R.A. Ahmedov
Acad. M.F. Nagiyev Institute of Catalysis and Inorganic Chemistry 113, H.Cavidave., Baku, AZ1043; e-mail:agil_s@mail.ru
Received 08.04.2021 Accepted 17.06.2021
Abstract: The article studied and compared the reactivity of cyclohexanol and methylcyclohexanol isomers in the oxidative dehydrogenation reaction over modified zeolite catalysts. It found that rates of oxidative dehydrogenation of all methylcyclohexanol isomers are practically the same and exceed rates of oxidative dehydrogenation of cyclohexanol into cyclohexanone.
Keywords: cyclohexanol, methylcyclohexanol, zeolites, reactivity, reaction mechanism. DOI: 10.32737/2221-8688-2021-2-101-106
Introduction
Cyclohexanone and
methylcyclohexanones are valuable products of organic synthesis. They are used in organic synthesis as intermediates in the production of caprolactam from the polymer of which polyamide fiber is made and are also used as solvents for many substances. In addition, methylcyclohexanone is the main raw material in the asymmetric synthesis of optically active substances, plasticizers and enanthic acid [1].
The main industrial method for producing of cyclohexanone is the oxidation of cyclohexanol with atmospheric oxygen at high pressure, mainly in the presence of homogeneous catalysts — soluble cobalt salts [2]. Methylcyclohexanone is obtained by oxidation of 2-methylcyclohexanol in the liquid phase at relatively high pressures in the presence of homogeneous catalysts - soluble cobalt salts, and atmospheric oxygen is used as an oxidizing agent [3]. The main disadvantage of liquid-phase processes is the difficulty in separating the catalyst from the liquid catalyzate. Therefore, on an industrial scale, mainly gas-phase processes of oxidative dehydrogenation of cyclohexanol and methylcyclohexanol are used with using as catalysts metals such as nickel, cobalt, iron,
copper, zinc, chromium, ruthenium, rhodium, palladium, etc. on different carriers (silicates, aluminosilicates, aluminum oxides, zirconium and titanium dioxides or their mixtures). The process of oxidative dehydrogenation on these catalysts proceeds at relatively high temperatures [4].
Our previous works [5-6] dealt with the selection of active zeolite catalysts modified by ion exchange method for the reaction of oxidative dehydrogenation of cyclohexanol and 4-methylcyclohexanol. As a result of these studies, it revealed that natural clinoptilolite with a silicate modulus X = 8.68 modified by the ion exchange method with cations of Cu2+- 0.5 wt. %, Pd2+ - 0.15 wt. %, Sn2+ - 0.5 wt. % shows relatively high catalytic activity in the reaction of oxidative dehydrogenation of cyclohexanol to cyclohexanone, and clinoptilolite containing 0.5 wt. % of Cu2+ and 0.15 wt. % of Pd2+ is an active catalyst for the oxidative dehydrogenation of 4-methylcyclohexanol to 4-methylcyclohexanone. Using these ultradispersed catalysts, the kinetic regularities of the oxidative dehydrogenation of cyclohexanol to cyclohexanone and 4-methylcyclohexanol to 4-methylcyclohexanone were studied and a mechanism and theoretically
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CHEMICAL PROBLEMS 2021 no. 2 (19)
grounded kinetic models of these processes proposed [7-8].
For optimal design of the reactors of large unit power for the above processes, it becomes necessary to study and compare the reactivity of reagents in the oxidative dehydrogenation reaction. However, for a
complete comparison of the rate of formation of cyclohexanone and methylcyclohexanones, the kinetic regularities of the processes of obtaining two more isomers of methylcyclohexanol are remained unexplored. These are 2- and 3-methylcyclohexanol.
Results and discussion
We carried out additional experiments to study the kinetic regularities of the reactions of oxidative dehydrogenation of 2-methylcyclohexanol and 3-methylcyclohexanol
on CuPd-clinoptilolite. The experiments were carried out in keeping with the method described in [8]. The experimental results are presented in Tables 1, 2.
Table 1. Results of the study into the reaction kinetics of oxidative dehydrogenation of 2-
methylcyclohexanol
Concentration of cations, wt. % T,0C Volumetric speed, hour-1 Molar ratio, nalcohol:nair Yield of 2-methylcyclo-hexanone, %
Cu2+ Pd2+
0.5 - 210 1590 0.36:2.64 59.0
0.5 0.1 210 1590 0.36:2.64 94.9
0.5 0.15 210 1590 0.36:2.64 97.3
0.5 0.15 200 1590 0.36:2.64 95.8
0.5 0.15 180 1590 0.36:2.64 93.0
0.5 0.15 190 1590 0.36:2.64 94.0
0.5 0.15 210 3000 0.36:5.18 94.1
0.5 0.15 230 3000 0.36:5.18 97.4
0.5 0.15 200 2000 0.36:3.88 93.7
0.5 0.15 210 2000 0.36:3.88 93.6
0.5 0.15 210 3500 0.36:5.21 97.3
0.5 0.15 220 3500 0.36:5.21 95.2
0.5 0.15 210 1000 0.35:2.45 98.6
0.5 0.15 230 3500 0.35:5.21 95.5
0.5 0.15 210 1500 0.36:2.56 97.9
Table 2. Results of the study into reaction kinetics of oxidative dehydrogenation of 3-
methylcyclohexanol
Concentration of cations, wt. % T,0C Volumetric speed, hour-1 Molar ratio, nalcohol:nair Yield of 3-methylcyclo-hexanone, %
Cu2+ Pd2+
0.5 - 210 1590 0.36:2.64 60.0
05 0.1 210 1590 0.36:2.64 95.2
0.5 015 210 1590 0.36:2.64 99.0
0.5 0.15 200 1590 0.36:2.64 97.2
0.5 0.15 180 1590 0.36:2.64 91.8
0.5 0.15 190 1590 0.36:2.64 95.9
0.5 0.15 210 3000 0.36:5.18 94.3
0.5 0.15 230 3000 0.36:5.18 96.0
0.5 0.15 200 2000 0.36:3.88 97.0
0.5 0.15 210 2000 0.36:3.88 97.7
0.5 0.15 210 3500 0.36:5.21 97.0
0.5 0.15 220 3500 0.36:5.21 98.2
0.5 0.15 210 1000 0.35:2.45 99.2
0.5 0.15 230 3500 0.35:5.21 97.5
0.5 0.15 210 1500 0.36:2.56 98.2
Based on the literature analysis [4] and carried out the experimental studies, the following mechanism of the oxidative dehydrogenation of 2-, 3-methylcyclohexanol has been proposed: upon the adsorption of 2-, 3-methylcyclohexanol, its protonation occurs with the participation of Bronsted acid sites of the catalyst with further elimination and formation of surface alcoholate. The transformation of the
ki
^2ZO
O2 + 2Z
ZO + CH3C6H10OH-
ZO(CH3C6H10OH )-
surface alcoholate into 2-, 3-methylcyclohexanone occurs when it interacts with dissociatively adsorbed oxygen atoms through the formation of a ketone-like surface compound on 2-, 3-methylcyclohexanone. In so doing, the initial state of the catalyst is restored. A simplified stage scheme of the mechanism can be represented as follows:
ZO(CH3C6H10OH )
^Z + CH3C6H9O + H2O
1 2
1
2
O2 + CH3C6H10OH = CH3C6H9O + H2O
Here ki, k2, k2 are reaction rate constants.
It should be noted that the process of formation of cyclohexanone and 4-methylcyclohexanone occurs exactly in line with this mechanism [7, 8]. In general, according to the above mechanism, oxidative dehydrogenation processes occur on modified zeolite catalysts for all cyclic and aliphatic alcohols.
Considering the identity of the
mechanism of the above processes, their theoretically based kinetic models are the same as well. Thus, for parametric identification, i.e. for determining constants of the kinetic equation for the rate of formation of 2-, 3-methylcyclohexanone, there could be used the kinetic model of the rate of formation of cyclohexanone [7]:
rc6H12O = k1P2
1 + 4
2
r k1P2 , k^
^ 2k2P1 1 2k3
1k P u k^
1 2k2P1 2k3
where rC
C6H12O
cyclohexanone; P1
- rate of formation of P2 - partial pressures of
cyclohexanol and oxygen, respectively; ki, k2, k3 - rate constants of stages corresponding to the index equal to the Arrhenius dependences:
1
1
2
kj = k0je RT . Here Ej - activation energy of the
j-th reaction, k0j - pre-exponential factor of the
rate constant of the j-th reaction.
On the basis of kinetic studies (Table 1, 2), the constants of kinetic models for processes of oxidative dehydrogenation of 2-, 3-methylcyclohexanol to 2-, 3-methylcyclohexanones have been determined.
Calculations of numerical values of the pre-exponential factors lnk0 (lnK°) and activation energies (Ei) have been carried out by the "Sliding tolerance" and "Powell" methods using the program "Poisk" system [9]. The numerical values of the constants of kinetic models for the formation of cyclohexanone and methylcyclohexanones are presented in Table 3.
Table 3. Numerical values of constants of kinetic models
E
lnk0 E, kcal/mol
Cyclohexanone
lnk01 7.49 Ei 8.56
lnk02 0.68 E2 3.0
lnk03 59.29 e3 9.45
2-Methylcyclohexanone
lnk- 10.2 ^ ii 7.0
lnk0i2 1.15 E2 ^ ii 3.4
lnk0i3 65.3 E3 ^ ii 6.5
3-Methylcyclohexanone
lnk01 10.4 -^iii 7.1
lnk0i2 1.1 E2 -^iii 3.6
lnk0i3 66.5 E3 -^iii 6.7
4-Methylcyclohexanone
lnk0vV 10.5 iv 7.2
lnk0VV 1.2 E2 iv 3.6
lnk0vV 65.6 E3 iv 6.8
With these numerical values of the kinetic parameters, the relative error of the experimental and calculated data does not exceed 3%. This confirms the correctness of the proposed hypothesis in the course of the said processes.
The reactivity of reagents in the studied oxidative dehydrogenation reactions over modified zeolite catalysts can be revealed on the
_ EL
kI = k0ie RT
basis of comparative analysis of their kinetic models. Since the right-hand sides of their kinetic equations have the same form, for comparison, we can use their rate constants as follows: kI, kII, km, kIV - the rate constant for
the formation of cyclohexanone, 2-, 3-, and 4-methylcyclohexanones, respectively. They have the following Arrhenius dependencies:
kn - kiie
Ei
RT
_ Eiii
k - k0i e RT
kIIi - kiiie
_ Eiv
k - k0i e RT kiV - kive
Here the value of i varies from one to three,
according to the number of elementary stages.
Conclusion
Since the reactions under consideration proceed on modified zeolite catalysts of the same nature, the numerical values of activation energies differ insignificantly (Table 3). So, for the first
stage of the process in all the four processes of oxidative dehydrogenation reactions, we can write:
Ej=8.56;
En= 7.0;
F1 =7 1
Flv=7.2.
Hence it follows that to compare the rate of formation of target products, it is sufficient to compare pre-exponential factors of the rate constants. From the data shown in Table 3 this be concluded for all three isomers of
10.5, and for
lnk?1 = 7.49. Thus, methylcyclohexanol
can
methylcyclohexanol lnk01^
cyclohexanol isomers of
all three
are more
reactive in the oxidative dehydrogenation reaction than cyclohexanol which is consistent with literature data that substituents on the cyclohexane ring increase the rate of dehydrogenation. In this case, the methyl group accelerates the oxidative dehydrogenation process, and the isomer rates are practically the same, i.e.:
r2-methylcyclohexsanone~ r3-methylcyclohexsanone ~ r4- methylcyclohexsanone >rcyclohexsanone
Here r - rate of formation of cyclohexanone, 2-, 3-, 4- methylcyclo-hexanones, respectively. Therefore, 2-, 3-, 4-
methylcyclohexanols are more reactive in the oxidative dehydrogenation reaction than cyclohexanol.
References
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3. Mekhtiev S.J., Aliyev A.F., Samedov Z.J. Oxidation of methylcyclohexane with atmospheric oxygen in the liquid phase. Izv. AN Azerb. SSR Ser. Phys.-tech. and chem. 1958, no.6, pp. 137-149.
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MODiFiKASiYA OLUNMU§SEOLiTKATALiZATORLARI ÜZdRiNDd OKSiDLd$DiRiCi DEHiDROGENLд$Мд REAKSiYALARINDA TSiKLOHEKSANOLUN Yd METiLTSiKLOHEKSANOLLARINREAKSiYA QABiLiYYdTiNiN ÖYRdNiLMdSi
A.M. dliyev, G.д. dli-zad9, M.Q. dliyeva, A.R. Sdfdrov, Y.M. Yariyev, R.A. Экшэйоу
Akademik M.F.Nagiyev adina Kataliz va Qeyri Üzvi Kimya institutu AZ1043, Baki, H. Cavid pr.,113; e-mail:agil_s@mail.ru
Mdqalddd seolit katalizatorlari üzarinda oksidla^dirici dehidrogenla§ma reaksiyalarinda tsikloheksanolun va metiltsikloheksanolun izomerlarinin reaksiya qabiliyyatinin öyranilmasi va müqaisali tadqiqi göstarilib. Müayyan olunub ki, metiltsikloheksanolun bütün izomerlarinin oksidla^dirici dehidrogenla§ma reaksiyalarinin süratlari praktiki olaraq barabardirlar va tsikloheksanolun tsikloheksanona oksidla^dirici dehidrogenla§ma reaksiyasinin süratindan yüksakdirlar.
Agar sözlw. tsikloheksanol, metiltsikloheksanol, seolitlar, reaksiya qabiliyyati, reaksiyanin mexanizmi.
ИЗУЧЕНИЕ РЕАКЦИОННОЙ СПОСОБНОСТИЦИКЛОГЕКСАНОЛА И МЕТИЛЦИКЛОГЕКСАНОЛОВ В РЕАКЦИИ ОКИСЛИТЕЛЬНОГО ДЕГИДРИРОВАНИЯ НА МОДИФИЦИРОВАННЫХ ЦЕОЛИТНЫХ КАТАЛИЗАТОРАХ
А.М. Алиев, Г.А. Али-заде, М.К. Алиева, А.Р. Сафаров, В.М. Ярыев, Р.А. Ахмедов
Институт катализа и неорганической химии им. акад. М. Ф. Нагиева НАНА AZ 1043, Баку, пр. Г. Джавида,113; e-mail:agil_s@mail.ru
В работе проведено исследование по изучению и сравнению реакционной способности циклогексанола и изомеров метилциклогексанола в реакции окислительного дегидрирования на модифицированных цеолитных катализаторах. Было выявлено, что скорости процессов окислительного дегидрирования всех изомеров метилциклогексанола практически одинаковые и превышают скорость окислительного дегидрирования циклогексанола в циклогексанон.
Ключевые слова: циклогексанол, метилциклогексанол, цеолиты, реакционная способность, механизм реакции.
