CC BY
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ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
AZERBAIJAN CHEMICAL JOURNAL № 1 2021
61
UDC 544.4;544.47:544.344
MECHANISM OF BIOMIMETIC OXIDATION OF CYCLOHEXANE TO CYCLOHEXANONE BY HYDROGEN PEROXIDE
S.A.Aghamammadova
M.Nagiev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
aghamammadova@yahoo.com
Received 22.07.2020 Accepted 12.10.2020
The process of gas-phase oxidation of cyclohexane was studied in the presence of a heterogeneous bio-mimetic catalyst (per-FTPhPFe(III)OH/Al2O3), at 130-2500C, in which high yields of cyclohexanone and cyclohexanol were obtained up to 25.2% with a selectivity of ~80% at a cyclohexane conversion of 34%. The mechanism of the conversion of cyclohexane to cyclohexanone has been studied in detail, and the coherently synchronized character of the reaction proceeding is shown.
Keywords: cyclohexane, oxidation, biomimetic catalysts, cyclohexanone, hydrogen peroxide.
doi.org/10.32737/0005-2531-2021-1-61-66 Introduction
The oxidation of cyclohexane to cyclohexanol and cyclohexanone is one among the important industrial processes. Both oxidation products are the main industrial precursors of respectively, caprolactam and adipic acid, the building blocks of polymers nylon-6 and nylon-6,6 [1, 2]. In addition, cyclohexanone is in particular also a starting material in the synthesis of insecticides, herbicides and pharmaceuticals [2].
The most promising raw material for the production of cyclohexanol, cyclohexanone and adipic acid is cyclohexane and its derivatives, which are isolated from the gasoline fractions of some oils. During the liquid-phase oxidation of cyclohexane homologues (methyl-, ethyl-, isopro-pyl-, and dimethylcyclohexanes) with air mixtures of mono- and dibasic acids C1-C4 are formed. Currently, the main industrial process for the oxidation of naphthenes is the oxidation of cyclohexane to cyclohexanone and adipic acid [3].
The initiation of the oxidation reaction of cyclohexane can be carried out by various methods with application catalysts - soluble salts of metals of variable valence (mainly cobalt salts) and initiating additives (hydrogen bromide, nitrogen dioxide, cyclohexanone), as well as thermally and photochemically [3].
The industrial method for the production of cyclohexanol, cyclohexanone and adipic acid is based on the liquid-phase oxidation of cyclo-
hexane with molecular oxygen in the presence of cobalt naphthenate at a temperature of 433453 K, air pressure of 9-13 atm [4-6]. However, under these industrial conditions, the maximum selectivity of the process does not exceed 70% at the conversion of cyclohexane up to 9%.
Most of the known oxidation processes of cyclohexane in the presence of various catalysts - MnO, FeZSM, copper pyrophosphate and in the biomimetic iron-porphyrin system FeP/O2/Zn/AcOH/CH3CN are also carried out in the liquid phase [7-9].
The creation of heterogeneous biomimet-ic catalysts made it possible to carry out some well-known oxidation processes, including the process of oxidation of cyclohexane to cyclo-hexanol and cyclohexanone in the gas phase, that led to the elimination of the disadvantages existing in liquid-phase oxidation.
Heterogenized biomimetic catalysts based on iron porphyrin complexes have been developed, on which the oxidation processes were carried out in the gas phase. One of the main properties of these catalysts is functional specificity, which is characteristic of enzymes associated with the special properties of protein molecules. The synthesized catalysts mimic the main physicochemical properties of the cyto-chrome P-450 monooxygenase enzyme, such as activity and selectivity [10, 11].
In contrast to the well-known studies on biomimetic oxidation of cyclohexane, carried
out mainly in the liquid phase, the gas-phase oxidation of cyclohexane on heterogeneous bioimitator, as was shown in [12, 13], has significant advantages: the process is carried out in a flow system with a fixed catalyst layer through more simplified technology, where reaction products are not mixed with the catalyst and after the reaction their separation is not required and no additional costs are required.
Experimental part
The process of monoxidation of cyclohex-ane was carried out on heterogeneous biomimetic catalysts such as hemin - protoporphyrin iron(III) (PPFe3+OH/Al2O3) and perfluorotetraphenylpor-phyrin iron(III) (per-FTPhPFe3+OH/Al2O3), which were synthesized in several stages [14]. The primary stage in the synthesis of bioimitators is the dissolution of iron porphyrin complexes in appropriate solvents. Immobilization of active centers on a solid support Al2O3 was carried out by extracting them from solvents.
Experimental study of the oxidation of cyclohexane on synthesized biomimetic catalysts was carried out in the gas phase in a flowing quartz reactor with reaction zone volume of 3.0 cm3 at the temperature of 130-2300C and atmospheric pressure in the presence of biomi-metic catalyst with the active part of perfluori-nated iron tetraphenylporphyrin backed on a solid Al2O3 support. As an oxidizing agent were used aqueous solutions of hydrogen peroxide of various concentrations (20-40%).
Hydrogen peroxide is an affordable and cheap "green oxidizer", according to the principles of the "Green Chemistry" concept and is increasingly used in hydrocarbons oxidation processes [15, 16].
Analysis of reaction products was carried out on chromium-mass spectrometer 5975 MSD + 7820 GC System and gas-liquid chromatograph 7820A GC System from Agilent Technology and on the chromatograph HXM-80.
Results and discussion
Experimental studies of the activity of per-FTPhPFe(III)OH/Al2O3 catalyst deponding on the temperature, concentration of hydrogen peroxide and contact time made it possible to
obtain data indicative of its high activity in the process of cyclohexane monooxidation.
Kinetic regularities of cyclohexane conversion over per-FTPhPFe(III)OH/Al2O3 bio-mimetic catalyst as a function of temperature are presented in Figure 1. As shown in figure, oxidation of cyclohexane to cyclohexanol and cyclohexanone at the temperature range of 150-1800C proceeds at the highest rate. With an increase in temperature, the yield of cyclohexene sharply increases and the yield of cyclohexadi-ene slightly increases, which is explained by the increase of the reaction rate of oxidative dehy-drogenation of cyclohexane under these conditions (200-2300C).
While the yield of cyclohexanol decreases with temperature, the yield of cyclohexanone at 1800C reaches its maximum (10.34%).
It can be concluded from the nature of the kinetic curves 2 and 3 (Figure 1) that the formation of cyclohexanone probably does not pass through the formation of an intermediate product of cyclohexanol. The temperature dependence of the kinetic curves shows that, starting from 2000C H2O2 decomposition into molecular oxygen stabilizes, that indicates full expenditure of hydrogen peroxide in the system for carrying out of two interrelated reactions. This undoubtedly affects the processes of monooxidation and hydroxylation of cyclohexane. This experimental fact indicates that the processes of monooxidation and hydroxylation are interrelated with the reaction of H2O2 decomposition and proceed in a coherent manner with it. It is shown in [17] that such reaction system is effectively realized in a coherently synchronized regime. In this context, the kinetic data obtained in Figure1 are consistent with the theory and practice of coherently synchronized reactions [18].
Since cyclohexanone has been the dominant (target) product in our study (from Figure!), and its maximum yield (10%) has been observed at 1800C, the effect of contact time on the cyclohexane oxidation has been investigated at this temperature.
Kinetic curves in Figure 2 show that the increase in the contact time (t) in cyclohexane monooxidation reaction up to 10 seconds the yield of cyclohexanone increases (up to 12.1%), and then the yield varies insignificantly.
biomimetic: c
ho,
25%,
V
HO,
1.41
ml/h,
Fig. 1. Temperature dependence of the products yields of cyclohexane oxidative conversion by hydrogen peroxide in a mixture with 6.25% of СбИцОИ and 2.53% of СИзСбИц on per-FTPhPFe(III)/Al2O3
VCHl2 = 0.9 ml/h, C6H12:H2O2 = 1: 1.5. 1 -
conversion of C6Hi2, 2 - cyclohexanol, 3 - cyclohexanone, 4 - cyclohexene, 5 - cyclohexadiene, 6 - O2.
The rate of cyclohexene production process also increases, while the yield of cyclohexanone increases insignificantly. In this case, conversion of cyclohexane increases with increasing contact time. Kinetic regularities from Figure 2 lead us to the conclusion that cyclo-hexanone is probably not formed from cyclo-hexanol. It should be noted that in the absence of cyclohexane, H2O2 at these contact times has been completely decomposed.
Based on previous studies on coherently synchronized oxidation reactions by hydrogen peroxide [16, 18] and the nature of the kinetic curves according too of the experimental results
Scheme
Fig. 2. Dependence of the product yields of cy-clohexane oxidation in a mixture with 6.25% of C6H„OH and 2.53% of CH3C6H11 on per-FTPhPFe(III)/Al2O3 biomimetic by hydrogen
peroxide on contact time: cH O = 25%, t =
1800C, C6Hj2:H2O2 = 1:1. 1 - conversion of C6Hi2, 2 - cyclohexanol, 3 - cyclohexanone, 4 -cyclohexene, 5 - O2.
of this process, we can say that the biomimetic oxidation of cyclohexane with hydrogen peroxide is a complex reaction consisting of two coherently synchronized reactions: 1) catalase and 2) monooxygenase. The coherently synchronized nature of the process is associated with the inducing ability of hydrogen peroxide, which contributes to the formation of a highly active intermediate complex, similar to the enzyme-substrate complex. The generalized scheme of coherently synchronized biomimetic oxidation of cyclohexane by hydrogen peroxide is described as follows [10, 16, 18-20]:
It follows from the scheme that a highly active intermediate (ImtOOH) obtained as a result of the interaction of hydrogen peroxide with a bioimitator at the first stage of the primary (catalase) reaction, interacts with the second H2O2 molecule and with cyclohexane in the secondary (monooxygenase) reaction leads to the formation of final reaction products [18 ].
The Al2O3 we used as a matrix, possessing acid-base properties, has Brensted and Lewis acid and basic centers, which play a key role in the formation of reaction products. Aluminum oxide acts as an electron donor and acceptor, thereby facilitating the cleavage and formation of bonds.
A detailed study of the characteristics of
these centers gives a complete understanding of the mechanism of the redox action of a biomimetic catalyst and the possibility of selective transformation of the substrate into target products [21].
Since, according to our experimental data, the peroxidase reaction does not proceed, the probable mechanism of the formation of cyclo-hexanone from cyclohexane can be presented in the following form according to Scheme I, which shows the electronic mechanism of the catalase reaction with the preparation of the active intermediate complex ImtOOH (1) and the mechanism of its interaction with cyclohexane, which leads to the formation of cyclohexanone on the surface of the biomimetic (2) and (3).
Mechanism of catalase reaction
(1)
Mechanism of formation of cyclohexanone
(2)
According to the results of studies and the presented schemes (2 and 3), the direct conversion of cyclohexane to cyclohexanone occurs through the formation of an intermediate bio-mimetic-substrate complex Imt-0-СбНц, which is formed by the interaction of the intermediate (ImtOOH) with cyclohexane (2). The interaction of this complex with another Н2О2 molecule leads to oxidation-reduction transformation of the complex into cyclohexanone, water and original biomimic (3). This mechanism agrees with the fact that cyclohexanone is formed precisely under usage the most concentrated H202 (30% or more) and a lower temperature (150-1800C).
References
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TSiKLOHEKSANIN TSiKLOHEKSANONA HiDROGEN PEROKSiDLO BiOMiMETiK OKSiDLO^MOSi
MEXANiZMi
S.O.Agamammadova
Heterogen biomimetik katalizator (per-FTPhPFe(III)OH/Al2O3) i§tirakinda 130-2500C temperaturda tsikloheksanin qaz fazada oksidla§masi prosesi öyrsnilmi^, tsikloheksanin 34% konversiyasinda tsikloheksanon va tsikloheksanolun ~80% selektivlikla 25.2%-э qadar yüksak giximi müayyan olunmu§dur. Tsikloheksanin tsikloheksanona gevrilma mexanizmi daqiqlikla öyranilmiij va reaksiyanin koherent-sinxronla§dirilmi§ xüsusiyyati göstarilmi§dir
Agar sözlzr: tsikloheksan, oksidh§m3, biomimetik katalizatorlar, tsikloheksanon, hidrogen peroksid.
МЕХАНИЗМ БИОМИМЕТИЧЕСКОГО ОКИСЛЕНИЯ ЦИКЛОГЕКСАНА В ЦИКЛОГЕКСАНОН
ПЕРОКСИДОМ ВОДОРОДА
С.А.Агамамедова
Изучен процесс газофазного окисления циклогексана в присутствии гетерогенного биомиметического катализатора (per-FTPhPFe(III)OH/Al2O3) при 130-2500С, в котором были получены высокие выхода циклогексанона и циклогексанола до 25.2% с селективностью ~80% при конверсии циклогексана 34%. Подробно изучен механизм превращения циклогексана в циклогексанон, показан когерентно-синхронизированный характер протекания реакции.
Ключевые слова: циклогексан, окисление, биомиметические катализаторы, циклогексанон, пероксид водорода.
