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UDC 544.4;544.47:544.344
COHERENTLY SYNCHRONIZED OXIDATION AND DEHYDROGENATION OF CYCLOHEXANE BY HYDROGEN PEROXIDE
S.A.Aghamammadova, I.T.Nagieva, L.M.Gasanova, T.M.Nagiev
Nagiev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan [email protected]; [email protected] Received 08.07.2016
Inducing effect of hydrogen peroxide on synchronous oxidation reaction is accompanied by the occurrence of two interconnected and interacting reactions. The reaction of H202 decomposition (primary) generates the leading active * OH and HO* free radicals in a system. During the interaction of active and free radicals with the substrate, the conversion of the substrate occurs in the secondary reaction, coherently-synchronized with the primary one. The mechanism of such coherently synchronized reactions is being examined in the process of cyclohexane oxidation with hydrogen peroxide in homogeneous and heterogeneous systems. The process of coupling dehydrogenation of cyclohexane to cyclohexene and cyclohexadiene with hydrogen peroxide was carried out without catalyst. This process was also carried out at a fairly low temperature in a heterogeneous catalytic system. Biomimetic catalyst, which simulates the basic functions of the enzyme group of oxidoreductase - cytochrome P-450, was used as the catalyst here. Characterized by its highly active and selective action these biomimetic catalysts are synthesized based on iron-porphyrin complexes. Based on this experimental researches, the complex reaction, consisting of parallel-sequential oxidation and dehydrogenation reactions, which are coherently synchronized, proceeds during the process of cyclohexane oxidation with biomimetic catalyst.
Keywords: hydrogen peroxide, cyclohexane,
cyclohexanol.
Introduction
Inducing effect of hydrogen peroxide on synchronous oxidation reaction is accompanied by the occurrence of two interconnected and interacting reactions [1, 2]. The reaction of H202 decomposition (primary) generates the leading active * OH and HO* free radicals in a system.
During the interaction of active and free radicals with the substrate, the conversion of the substrate occurs in the secondary reaction, coherently-synchronized with the primary one [3].
The mechanism of proceeding such coherently synchronized reactions is being considered in the process of cyclohexane oxidation with hydrogen peroxide in homogeneous and heterogeneous systems. The cyclohexane oxidation is the most commonly studied process, where the valuable for organic synthesis compounds are achieved during oxidative dehydrogenation or at oxidation [4-7]. The implementation of thermo-dynamically hindered reactions is of great interest for petrochemical and organic synthesis. Among them the reactions of partial dehydrogenation of
oxidation, biomimetic catalysts, cyclohexanone,
cyclohexane to cyclohexene and cyclohexadiene should be mentioned.
AG, kJ/mol 39.88
C6H12 ^ C6H10 + H2
C6H10 ^ C6H8 + H2
115.37
It is of course more beneficial in terms of energy to carry out such reactions in conjunction with other reactions, in this case, with the reaction of hydrogen peroxide decomposition.
Experimental part
Experimental studies of the biomimetic monooxidation of cyclohexane were performed in a quartz flow reactor of the integral type with the volume of the reaction zone of 3 cm (d =1.8 cm), whose design ensured the introduction of H2O2 into the reaction zone in an undecomposed form. The cyclohexane taken as feedstock had 88.85% C6H12, 6.25% C6H11OH and 2.53% CH3C6Hn. The biomimetic catalysts were per-FTPhPFe(III)OH/Al2O3 and PPFe(III)OH/Al2O3,
which were synthesized by adsorption of ironporphyrin complexes on the active carrier of Al2O3 from the corresponding solvents. The concentrations of the active complexes per-FTPhPFe(m)OH and PPFe(m)OH were 1.58 mg/g and 0.66 mg/g, respectively. The process was carried out at a temperature of 130-2300C and atmospheric pressure, using as an oxidizer an aqueous solution of hydrogen peroxide of various concentrations. The reaction products were analyzed on a chrom-mass spectrometer 5975MSD+7820GC System and on a gas-liquid chromatograph Agilent-7820A GC system of the company Agilent Technologies
Results and discussion
The process of coupling dehydrogenation of cyclohexane to cyclohexene and cyclo-hexadiene with hydrogen peroxide was carried out without catalyst [8].
The reaction was examined at a temperature range of 450-6500C at various flow rates and ratios of starting reactants. The process was of sequential and autocatalytic nature with the period of self-acceleration (kinetic curves for cyclohexadiene and benzene), as shown in Figure 1. Under optimum conditions, the yield of cyclohexene was up to 19.4%, of cyclohexadiene was 3.4% and benzene 2.4%.
Fig.1. Kinetic curves for coupling dehydrogenation of cyclohexane: 1 - cyclohexane, 2 -cyclohexene, 3 - benzene, 4 - cyclohexadiene.
Kinetic curves show that the accumulation of cyclohexene as an intermediate product at the initial stage of the process increases up until the rate of its consumption and the
rate of accumulation at the conventional contact time equal to t = 4.2 s, becomes equal, and cyclohexene concentration reaches a maximum value. Cyclohexene yield decreases with the further increase in contact time. The rate of formation of cyclohexene and cyclohexadiene initially increases, and reaches its maximum at the inflection point, which is typical for sequential reactions.
At a temperature range of 450-5000C the cyclohexene is formed, with the increase in temperature range between 5600C and 6500C dehydrogenation products contain cyclohexa-diene and benzene along with cyclohexene. These data also suggest that with further increase in temperature the ring-opening decomposition reaction of the cyclohexane occurs along with the process of the dehydrogenation. The consequence is the formation of gaseous products such as H2, CO2, CH4, C2H4.
These experimental data aggregate shows that in the studied range of variations by reaction parameters under the best conditions reaction proceeds in the direction of the oxidative dehydrogenation of cyclohexane achieving the yield of desired products C6H10, C6H8 u C6H6 - 19.4, 3.4 and 2.4% respectively.
It is necessary to identify the nature of the interaction of H202 with the hydrocarbon, resulting with the formation of unsaturated compound under high-temperature oxidation conditions in order to identify the mechanism of dehydrogenation of cyclohexane.
A prerequisite for the coherently synchronized reactions to proceed in the reaction system is its quantitative characteristic defined by the determinant equation [1]:
D = ^(flI rac + r2 1 rac ^ >
where r1 and r2 - consumption rates of an actor (H202) in the primary and secondary reactions respectively, rac - consumption rate of an acceptor (C6H12), v - stoichiometric factor (in our case v=1). The value of the determinants calculated by using experimental data D-0.1 indicates the induced nature of dehydrogenation of cyclohexane.
Under the conditions of the high-temperature cyclohexane gas-phase oxidation
with hydrogen peroxide, the reaction proceeds by a free radical mechanism, where active * OH and HO* radicals are formed in the primary reaction - in the reaction of H202 decomposition, and they are consumed in the secondary one - in the dehydrogenation of cyclohexane. Considering that at a high temperature the concentration of HO* radicals in the reaction system is much higher than the concentration of * OH radicals [9], the HO* radicals play a key role in the mechanism of the dehydrogenation:
chain initiation H2O2 OH - 217.9 kJ/mol chain propagation H2O2 +* OH ^ HO*+ HO +107.7 kJ/mol
C6H12 + HO* ^ C6H10 + H2O+*OH + 40.17 kJ/mol
C6H10 + HO* ^ CH + H2O+*OH +104.6 kJ/mol
CH + HO* ^ C6H6 + H2O+*OH + 66.9 kJ/mol
chain termination J*OH + wall^* OH (ads) [ HO* + wall ^ HO* (ads)
Each successive reaction of oxidative dehydrogenation of cyclohexane is a combination of chain initiation, propagation and termination stages. The system of kinetic
equations, adequately describing the experimental data, is as follows [10]:
-d[C6HJ / dx = kfJCHJiHA], -d[C6H1o]/dl= kff^HJ^] - k^HJ^] , -d[C6H]/dT = ^^HJ^] -^^H]^] , where
24 9 exp(—264000RT) cm3mol- 1s-1,
keff1 - 110
' exp(-222000RT) cm3moL V1,
keff2 - 10
kffs = 10228 exp(-205000RT) cm3mol- 1s-1.
Examination of the process of coherently synchronized cyclohexane oxidation with hydrogen peroxide was carried out also at a fairly low temperature in a heterogeneous catalytic system. Biomimetic catalyst [11], which simulates the basic functions of the enzyme group of oxidoreductase - catalase and monooxygenase, was used as the catalyst here. Characterized by its highly active and selective action these biomimetic catalysts are synthesized based on iron-porphyrin complexes simulating the active component of cytochrome P-450 [12].
The process of cyclohexane gas phase oxidation was carried out with heterogeneous biomimetic catalysts per-FTPhPFe(III)/Al2O3 and PPFe(III)/Al2O3 at a temperature of 130-2500C and a molar ratio C6H12: H2O2 = 1:1.7 with 25% aqueous H202 solution. The results of these studies are shown in Figure 2.
yield.?*
90 ■
80 ■
70 ■
60 ■ 40 ■
Fig.2. The dependences of the products yields of coherently synchronized oxidation reaction of cyclohexane with biomimetic catalyst per-FTPhPFe(III)/Al2O3 on the temperature: 1 - conversion, 2 - cyclo-hexanol, 3 - cyclohexanone, 4 - cyclo-hexene, 5 - cyclohexadiene, 6 - O2;
[H2O2]=25%, ^ =1.41 mL/h, V^ = 0.9 mL/h, C6H12:H2O2=1:1.7.
180 200
230 t, «С
6
As shown in Figure 2 at the temperature up to 150-1600C the reaction that mainly proceeds is cyclohexane oxidation reaction, forming cyclohexanol and cyclohexanone. At the temperature above 1800C the reaction of oxidative dehydrogenation of cyclohexane to cyclohexene (curve 4, Figure 2) begins to accelerate, respectively, the yield of cyclo-hexanol and cyclohexanone decreases.
The sharp increase in the yield of cyclohexene along with the decrease in the yield of formed cyclohexanol indicates that at the temperature of 2300C or higher cyclo-hexanol can also be converted into cyclohexene at lower concentrations of H202 in the reaction
system. This conclusion is confirmed by the results of experimental studies of the process of cyclohexanol oxidation with hydrogen peroxide at a lower concentration (20%) with the biomimetic catalyst, where at elevated temperature (200-2300C) the cyclohexene yield is high (16-30%).
The experiments on the effect of the H202 concentration at a constant temperature (2000C) and contact time were conducted. As shown in Figure 3, increased H202 concentration in the reaction zone contributes to the increase of oxidation reaction rate, which results with the increase of cyclohexanone yield and decrease of cyclohexene yield.
The dependence of the reaction yield of cyclohexanol coherently synchronized oxidation with bio simulator per-FTPhPFe(III)/Al2O3 on the temperature: [H2O2]=20%, Vho =1.41 mL/h, V^2=0.9 mL/h, C6H„OH:H2O2=1:1
Composition of feedstock, % Composition of obtained reaction yield, %
u o a o u 2-methyl-cyclohexanol hexanoic acid hexanoic acid cyclohexyl ester a o u O U 1,2-cyclo-hexanediol 1,3-cyclo-hexadiene 2-methyl-cyclohexanol other oxygenates conversion
150 97.524 0.481 0.712 0.845 83.921 2.321 2.248 0.105 0.093 9.405 14.079
180 97.524 0.481 0.712 0.845 78.186 7.841 2.875 0.145 0.583 10.753 19.814
200 97.524 0.481 0.712 0.845 67.494 16.688 2.887 0.279 0.295 10.652 30.506
230 97.524 0.481 0.712 0.845 55.255 30.374 2.0 0.1 0.641 10.271 42.745
40 %,CHO,
Fig.3. The dependences of the products yields of coherently synchronized oxidation reaction of cyclohexane with biomimetic catalyst per-FTPhPFe(III)/Al2O3 on concentration H2O2: 1 - conversion, 2 - cyclohexanol, 3 - cyclohexanone, 4 - cyclohexene, 5 - cyclohexa-
diene, 6 - O2; i=200°C, VH O =1.41 mL/h,
=0.9 mL/h.
Thus, the complex reaction, consisting of parallel-sequential oxidation and dehydroge-nation reactions, which are coherently synchronized, proceeds during the process of cyclohexane oxidation with biomimetic catalyst. Depending on the reaction parameters it is possible to deliberately adjust the direction of oxidation reaction and reaction rate.
It should be noted that in terms of the mechanism of each stage of this complex reaction of cyclohexane parallel-sequential oxidation with hydrogen peroxide with biomi-
metics, each oxidation reaction is carried out under inducing effect of hydrogen peroxide intermediates and consists of two coherently synchronized reactions - 1) catalase (H202 decomposition reaction) and 2) monooxygenase (cyclohexane oxidation reaction) with biome-metic catalyst.
The probable mechanism of cyclohexane coherently synchronized oxidation, considering the parallel-sequential nature of oxidation and dehydrogenation, can be expressed by the general scheme [3]:
H2O2
H2O
tOH
tOOH
H2O + O2
H2O2
react. prod. (C6H11OH; C6HioO; C<Hio; СбН; СбНб)
•6Н12; CHio; C<Hs; СбНцОН
Conclusion
Thus, the formed active intermediate complex of hydroperoxide (ImtOOH), as in the case of free radical HO*, is the key active centre for coherent synchronization of catalase (primary) and monooxygenase (secondary) reactions. Coherently synchronized nature of the cyclohexane oxidation reaction follows both from the experimental results and from the mechanism of the reaction.
References
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herent Synchronized Ethylene Monooxidation by 2012. V. 2. No 4. P. 306-312.
TSIKLOHEKSANIN HIDROGEN PEROKSIDLO KOHERENT-SINXRONLA§DIRILMI§ OKSiDLO§MOSi VO DEHiDROGENLO§MOSi
S.O.Agamammadova, i.T.Nagiyeva, L.M.Hasanova, T.M.Nagiyev
Hidrogen peroksidin onunla sinxron olan oksidlaçma reaksiyasina induksiyaedici tasiri sistemda iki qarçiliqli alaqa va tasirda olan reaksiyanin getmasi ila naticalanir. H202-in parçalanmasi (birinci reaksiya) ila sistema aktiv * OH va HO* sarbast radikallari genera olunur ki, bu radikallarin substrat ila qarçiliqli tasiri naticasinda birinci ila koherent-sinxronlaçdmlmiç ikinci reaksiya baç verir. Bela koherent-sinxronlaçdirilmiç oksidlaçma reaksiyalarina nümuna olaraq, tsikloheksanin hidrogen peroksidla homogen va heterogen çaraitda oksidlaçmasi reaksiyasinin mexanizmi ôyranilmiçdir. Tsikloheksanin hidrogen peroksidla katalizator tatbiq etmadan tsikloheksen va tsikloheksadiena qoçulmuç dehidrogenlaçmasi va hamçinin onun kifayat qadar kiçik temperaturlarda heterogen katalitik sistemlarda oksidlaçmasi prosesi hayata keçirilmiçdir. Katalizator kimi oksireduktaza fermentlar qrupuna daxil olan sitoxrom P-450-in asas funksiyalarini imitasiya edan biomimetik katalizator istifada olunmuçdur. Seçicilik va yüksak aktivliyina göra farqlanan bu biomimetik katalizatorlar damirporfirin komplekslari asasinda sintez olunmuçdur. Tacrübi tadqiqatlar asasinda müayyan olunmuçdur ki, biomimetik katalizator üzarinda tsikloheksanin oksidlaçmasi zamani sistemda paralel-ardicil oksidlaçma va dehidrogenlaçma reaksiyalarindan ibarat mürakkab proses baç verir. Bu reaksiyalarin har biri iki koherent-sinxronlaçdirilmiç reaksiyadan: 1) hidrogen peroksidin parçalanmasi va 2) tsikloheksanin oksidlaçmasi va ya dehidrogenlaçmasi reaksiyalarindan ibaratdir.
Açar sözlar: hidrogen peroksid, tsikloheksan, oksidhçma, biomimetik katalizatorlar, tsikloheksanon, tsikloheksanol.
КОГЕРЕНТНО-СИНХРОНИЗИРОВАННОЕ ОКИСЛЕНИЕ И ДЕГИДРИРОВАНИЕ ЦИКЛОГЕКСАНА
ПЕРОКСИДОМ ВОДОРОДА
С.А.Агамамедова, И.Т.Нагиева, Л.М.Гасанова, Т.М.Нагиев
Индуцирующее действие пероксида водорода на синхронную с ней реакцию окисления сопровождается протеканием двух взаимосвязанных и взаимодействующих реакций. Реакция разложения Н2О2 (первичная) генерирует в систему ведущие активные * OH и HO* свободные радикалы, при взаимодействии которых с субстратом происходит превращение его во вторичной, когерентно-синхронизированной с первичной реакцией. Механизм протекания таких когерентно-синхронизированных реакций рассматривается в процессе окисления циклогексана пероксидом водорода в гомогенной и гетерогенной системах. Осуществлен процесс сопряженного дегидрирования циклогексана в циклогексен и циклогексадиен пероксидом водорода без применения катализатора, и также его окисление при довольно низкой температуре в гетерогенно-каталитической системе. При этом использовали биомиметический катализатор, имитирующий основные функции фермента группы оксиредуктазы - цитохрома Р-450. Отличающиеся своим избирательным и высокоактивным действием такие биомиметические катализаторы синтезированы на основе железопорфириновых комплексов. На основе экспериментальных исследований установлено, что в процессе окисления циклогексана на биомиметическом катализаторе происходит сложная реакция, состоящая из параллельно-последовательных реакций окисления и дегидрирования. Каждая из этих реакций состоит из двух когерентно-синхронизированных реакций: 1) реакция разложения пероксида водорода и 2) реакция окисления или дегидрирования.
Ключевые слова: пероксид водорода, циклогексан, окисление, биомиметические катализаторы, циклогексанон, циклогексанол.
