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77Section 3. Petrochemistry
Section 3. Petrochemistry
Frolov Aleksandr Sergeevich, Yaroslavl state technical university postgraduate student, chemical engineering faculty E-mail: [email protected] Kurganova Ekaterina Anatolievna, Yaroslavl state technical university, Associate professor, chemical engineering faculty E-mail: [email protected]
Koshel’ Georgiy Nikolaevitch, Yaroslavl state technical university Professor, chemical engineering faculty E-mail: [email protected] Nesterova Tatiana Nikolaevna, Samara state technical university Associate professor, chemical engineering faculty
Aerobic Oxidation of 2-isopropyl-1,4-dimethylbenzene
to tertiary hydroperoxide
Abstract: The regularities of aerobic oxidation of isopropyl-para-xylene to tertiary hydroperoxide in the environment of standard industrial initiators, as well as N-hydroxyphthalimide as catalyst are studied. A mechanism of liquid-phase catalyzed oxidation of isopropyl-para-xylene is discussed.
Keywords: oxidation, 2,5-xylenol, hydroperoxide, N-hydroxyphthalimide.
Introduction
Liquid-phase oxidation of alkylaromatic hydrocarbons forms the basis for promising methods of obtaining various oxygen compounds — (alkyl) phenols and aliphatic ketones, which are widely used in synthesis of polymeric substances with a range of useful properties.
The “cumene” method of phenol and acetone production and the styrene and propylene oxide cosynthesis (Halcon process) have been thoroughly researched and are widely used in industry. The key stage in these processes is the liquid-phase initiated
oxidation ofisopropylbenzene (IPB) or ethylbenzene to respective hydroperoxides (HP) [1, 2].
This important direction of organic synthesis can be broadened by using other alkylaromatic hydrocarbons and their HPs.
This paper discusses the study of liquidphase selective oxidation of 2-isopropyl-
1,4-dimethylbenzene (IPX) to tertiary HP, which forms the basis of synthesis of 2,5-xylenol, a large volume oil synthesis product, which is applied in production of vitamin E, plasticizers, paint and coatings, and disinfectants, according to the scheme:
CHyCH-CH3
CH
CH
CHyC—CH3
OH
O,
CH
CH
3
O II
CH—C—CH
CH
3
3
16
Aerobic Oxidation of 2-isopropyl-1,4-dimethylbenzene to tertiary hydroperoxide
At present, environmentally friendly and economically efficient methods of synthesis of dimethylphenol derivatives are non-existent. The known methods of xylenol synthesis involve multiple stages, have low outcomes of target product, high production costs and great expenditure of assistant substances. All this considerably limits their industrial potential and, therefore, keeps the industry from producing substances which have many valuable properties and are important for the economy. The main source of industrial production of xylenols are the cresol (phenol) fractions which result from thermal treatment of fuels. This method is very complicated in terms of end product separation, and does not allow to obtain individual xylenols.
It is impossible to simply project the «cumene» method patterns onto IPX oxidation, for a number of reasons. The liquid phase oxidation of the isopropyl xylene derivatives progresses less rapidly than that of IPBs, and with production of both the primary and the tertiary IPX HP. The selectivity of emergence of the tertiary HP, as a rule, does not exceed 60-65%. Up to now, these complications have not been overcome, which hinders the practical implementation of the «oxidation» method of producing many valuable organic synthesis products based on IPX and its HP.
At present, a trend ofusing N-hydroxyphthalimide and its analogues as catalyst for radical chain reactions is gaining popularity [3, 4]. For instance, as refers to oxidation processes, it was suggested to use similar catalyst systems to intensify both the “cumene” process [5, 6], and for various isopropylarenes [79], cyclohexane [10], cymenes [11-13] and other substances.
At present, literary data on 2-isopropyl-
1,4-dimethylbenzene oxidation with phthalimide catalysts are absent. Therefore, carrying out a complex study of the intensification of selective oxidation of IPX to tertiary HP and syntheses possible on this basis presents a considerable scientific and practical interest.
Materials and methods
IPX was obtained by alkylation of p-xylene by isopropyl alcohol in presence of concentrated
sulfuric acid (at temperature 20 °C, molar proportion of p-xylene: IPA: acid 3:1:3, time of reaction 4 hours). At the end of the reaction, the hydrocarbon layer was separated from water solution of sulfuric acid, rinsed in water until neutral reaction, and dehydrated over calcium chloride. The obtained alkylate was rectified in vacuum. The composition of IPX was confirmed by gasliquid chromatography, infrared spectral analysis and nuclear magnetic resonance testing. The IR-spectroscopic analysis confirmed the structure of the substance, including the presence of the isopropyl group (absorption band 1363 cm-1 and 1382 cm-1) and substitution to positions 1, 2, 4 (187 cm-1 and 879 cm-1). The IPX had the following constants: boiling temperature 195 °C, nD 20 1.4955, d420 0.865.
The chromatographic analysis was carried out with the help of chromatograph “Chromatech — Crystal 5000.2” with flame-ionizing detector. A capillary column SK-5, 30 m. long, 0.32 mm in diameter, was filled with 5% phenyl 95% dimeth-ylpolysiloxane. The carrier gas was nitrogen, with consumption 2 cm 3/min. The programmed temperature rise was from 80 to 200 °C at 8 °C per minute. The IR-spectroscopic analysis was carried out with the help of the device Fourier RX-1. The specter processing was carried out with the help of «Spectrum» software provided by the company PerkinElmer. The specters were recorded within the range of 4000-400 cm-1, as a micro layer between the glasses of potassium bromide and with a cuvet with d= 0.0011 cm, made of potassium bromide. The nuclear magnetic resonance specter was registered on spectrometer model Bruker DRX 400 (400.4 MHz) [14].
The IPX was oxidized by air oxygen in a glass reactor 10 cm 3 in volume, on a closed circuit flow through system (Fig. 1) with constant intensive mixing in presence of N-hydroxyphthalimide catalyst, 2-2.5% by mass of hydrocarbon (0.1656 mol per kg of hydrocarbon). The progress of the reaction was controlled by the oxygen absorption, oxidation was performed in a kinetic mode, when the reaction time does not depend on the intensity of mixing.
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Section 3. Petrochemistry
Fig 1. Scheme of kinetic apparatus for liquid phase oxidation of the IPX.
1. The measuring cylinder; 2, 12, 15 — valves; 3, 10 — the gas burette; 4 — connecting tube; 5 — the reactor holder; 6 — reactor; 7 — the electric motor sheave; 8 — rocker; 9 — refrigerator; 11 — calcium chloride pipe; 13 — three-way valve; 14 — pressure vessel.
The apparatus was employed in the following way: 3 cm 3 of hydrocarbon, with pre-calculated amount of catalyst or initiator, were inserted through a drop funnel into the oxidation reactor 3. The reactor was connected by a vacuum hose with a system of gas burettes (all valves closed), and cooled water was supplied. Then, via valve 13, the apparatus was repeatedly connected to the vacuum pump and to the oxygen vessel, in order to remove the air, and the burette was filled with oxygen by opening the valves. After the system was filled with oxygen, the valve 13 was turned to isolate it from the atmosphere, and water of the required temperature was supplied to the pipework of the oxidation reactor from a thermostat. With the help of pressure vessel 14 the system was put under excess pressure (20-40 mm of mercury). The removal of the excess gas from the reaction zone was performed by opening the valve 2 to a necessary degree. The speed of exhaust was set so that its further increase did not affect the oxygen absorption. The volume of oxygen in burette 10 and measuring cylinder 1 were measured simultaneously, and the shaking device was turned on. The volume of excess gas in burette 13 was determined by the measure of the liquid forced into the measuring cylinder 1. After certain intervals of time, with the
help of pressure vessel 14 the pressure in the system was leveled with atmospheric, and the quantity of gases in both burettes was measured. After absorption of the required quantity of oxygen, the valves 12, 2 and 15 were closed, and the reactor disconnected from the system. After each experiment, the oxidate was tested for content ofhydroperoxide and organic acids, and the selectivity of the process, as the relation of the quantity of oxygen required to obtain a specified volume of HP to total oxygen absorption, was calculated.
The balance experiments for IPX oxidation were performed in a three-necked flask with a mixer. The oxidate was rinsed with water until neutral. From the rinse water, xylene acid (melting point 130 °C) was extracted. By rectification in vacuum, it was possible to extract out of the oxidate IPX hydroperoxide (boiling point 90-93 °C at 1 mm mercury, nD 20 1,5211, d420 1,017); dimethylacetophenone, (b. p. 110-112 °C at 13 mm mercury, nD 20 1,5301, d420 1,006) and dimethylxylilcarbinol (b. p. 80-82 °C at 1 mm mercury, nD 20 1,5212, d420 0,980).
The acid decomposition of IPX hydroperoxide was performed with concentrated sulfuric acid in acetone (0,5% H2SO4 of HP content) in a threenecked flask, equipped with a thermometer and a
18
Aerobic Oxidation of 2-isopropyl-1,4-dimethylbenzene to tertiary hydroperoxide
mixer. The oxidate containing IPX HP was heated to 65 °C and then the calculated quantity of 98% H2SO4 was added to the reaction mass. The process was carried out until complete decomposition of the HP. At the end of the decomposition, the reaction mass was tested by gas-liquid chromatography and infrared spectrographic analysis, as described above. Out of the products of acid decomposition of the IPX hydroperoxide, it was possible to extract, with the output of 90-95%, 2,5-dimethylphenol (m. p. 73 °C) and acetone.
N-HPI was synthesized by reaction of hydroxi-lamine muriate in pyridine with phthalic anhydride. A flask, equipped with a mixer, reverse refrigerator and thermometer, was loaded with 0.11 mol NH2OH*HCl and 150 cm 3 pyridine. At 30 °C,
0.1 mol of phthalic anhydride was quickly inserted, and mixing was continued until the solution became transparent (at 42 °C). On a rotary evaporator in vacuum of the water-jet pump the pyridine was removed of the solution, and the hot viscous residue was quickly poured into 200 ml of acetic acid. The residue of N-HPI was separated, washed on the filter with a solution of 0.01 N acetic acid and dried in vacuum. The melting temperature of the
obtained N-HPI was 231.5 °C (233 °C according to literary data). The structure of N-HPI was confirmed by IR-spectroscopic analysis.
The IR specter of N-HPI clearly shows the bands of stretching vibrations of the C-H bonds, and C=C bonds of the benzene ring in the frequency domain 3030, 1606 and 1080 cm-1. The most intensity is observed in absorption bands in the areas of 1789, 1736 и 1710 cm-1, which is characteristic to С=О groups in imides. The presence of hydroxo group is confirmed by a band at 3134 cm-1, and the frequency 975 cm-1 indicate the presence of the N-O-H bonds.
Results and discussion
Liquid-phase oxidation of IPX was performed by air oxygen at 120-140 °C, catalyzed by IPB HP, at atmospheric pressure and constant mixing on the kinetic laboratory setup flow- closed type. The progress of the reaction was controlled by oxygen absorption. The oxidation of IPX was carried out in kinetic mode. The oxidate was tested for tertiary IPX HP by iodometric method, the content of organic acids was determined by potentiometric titration. The results of the experiments are represented in Table 1.
Table 1. - Impact of concentration of IPB HP and temperature on selectivity of tertiary IPX HP and IPX conversion during liquid phase oxidation. Reaction time — 90 minutes.
Temperature, °C Concentration, wt% Selectivity of tertiary IPX HP formation,% IPX conversion,%
IPB HP initiator tertiary IPX HP Organic acid *
100 2.5 0.9 88.3 1.0
110 2.5 1.4 87.9 1.6
120 2.5 2.7 0.1 86.2 3.1
130 1.2 2.8 0.1 85.2 3.2
2.5 3.7 0.2 85.0 4.4
5.0 5.9 0.3 83.9 7.0
140 2.5 5.5 0.6 83.3 5.8
* calculated for xylenic acid.
The data from Table 1 shows that with the increase in the temperature ofthe reaction from 100 to 140 °C the hydrocarbon oxidation rate also increases. Under these conditions, it is possible to accumulate about 4-5% of tertiary IPX HP in 1.5 hours, with selectivity of its formation not higher than 88%.
The experimental data, and the current views on the mechanism of alkylaromatic hydrocarbons
oxidation, allowed us to represent the transformation of IPX during its oxidation in the following way (Scheme).
The main product of IPX oxidation (I) under the chosen conditions, is its tertiary hydroperoxide
(II) , and, simultaneously, dimethylacetophenone
(III) . Further transformation of tertiary IPX hydroperoxide (II) leads to the formation of
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Section 3. Petrochemistry
dimethylxylenecarbinol (IV). On deeper stages, further oxidation of (III) to xylylic acid (V) and formaldehyde (VI) is possible. The latter oxidizes to formic acid (VII) which catalyzes the
dissipation of tertiary IPX HP, with emergence of
2,5-dimetilphenol (VIII), which is one of the major agents of slowing and even complete inhibition of IPX and acetone (IX) oxidation.
O,
O O
II CH3C 3 1 =o- 1 I о
ArCH3 - ifVH:
J| Л
chVY t о
V
CHyCH-CH3
OOH
I
CHyC-CH3
CH
HCOH
VI
OH
I
ch3c-ch3
HCOOH
VII
CH
3
CH
3 II
HCOOH
CH
CH
3
IV
OH
CH
CH
3
O
3
+ CH—C—CH3 IX
VIII
Therefore, the analysis of main and secondary shown on Figure 1, has a considerable impact on the
products of IPX oxidation shows that the main group oxidation rate of the aromatic hydrocarbon isopropyl
within IPX which oxidizes is the isopropyl group. derivative.
At the same time, the presence of methyl groups, as
Fig 2. Comparative Data on Liquid Phase Oxidation of Isopropylbenzenes. Temperature 120 °C, concentration of the initiator (IPB HP) 2.5 wt%
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3
Aerobic Oxidation of 2-isopropyl-1,4-dimethylbenzene to tertiary hydroperoxide
The aforementioned difference in the oxidation reactive ability of the isopropylphenil radical (R/) speed of isopropyl derivatives of benzene, toluene which significantly depends on the number of methyl
and xylenes, is, apparently, connected with variations in groups in the aromatic nucleus.
ch,-c-ch
'3
(R-)
(CH3)n n = 0,1,2
To improve the conversion of the IPX successfully applied before to intensify the liquid-and selectivity of tertiary IPX HP formation, phase oxidation of alkyl and cyclohexyl-aromatic N-hydroxyphthalimide (N-HPI), which had been hydrocarbons [11,15,16].
Table 2. - The impact of N-HPI concentration and temperature on selectivity of IPX HP formation and IPX conversion during liquid-phase oxidation. The reaction time is 90 min.
Temperature, °C Concentration, wt% Selectivity of IPX HP formation,% IPX conversion^
N-HPI Catalyst Tertiary IPX HP
110 2.0 1.2 98.3 1.3
120 2.0 3.0 98.1 3.1
130 1.0 3.1 98.0 3,1
1.5 4.1 98.2 4.2
2.0 6.6 98.4 6.7
3.0 6.9 97.5 7.1
4.0 5.9 97.7 6.0
140 2.0 8.8 96.8 9.1
As seen from Table 2, with oxidation of IPX in the temperature range of 110-140 °C, in presence of N-HPI, the speed of IPX oxidation increases by 1.52 times, as compared to the process initiated by IPB HP. At that, the speed of IPX HP accumulation also increases from 5 to 9% in 1.5 hours, and selectivity of its formation rises from 83-88 to 97-98%, as compared to reaction initiated by IPB HP. The
oxidation rate of IPX depends on concentration of N-HPI, and its reuse does not lead to decrease in the speed or selectivity of the HP formation, which signifies that N-HPI is a catalyst of the process [17].
The catalytic role of N-hydroxyphthalimide in the process of selective liquid phase IPX oxidation to peroxide can be described in the form of the following catalytic cycle:
/
CH3
COOH
CH3
CH3
COO»
CH3
3
3
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Section 3. Petrochemistry
The synthesized tertiary IPX hydroperoxide was tested by acid dissolution with concentrated sulfuric acid as catalyst. In the reaction products,
2,5-xylenol and acetone were discovered. The output of 2,5-xylenol and acetone was 90-95% and 80-85% respectively, which indirectly confirms the composition of the substance.
Conclusions
It is determined, that liquid phase oxidation of 2-isopropyl-1,4-dimethylbenzene in presence of N-hydroxyphthalimide results in production of tertiary IPX hydroperoxide with 97-98% selectivity and 5-9% conversion.
In liquid phase oxidation of 2-isopropyl-1,4-dimethylbenzene the methyl groups connected with the aromatic nucleus are not able to react, however, their presence in the molecule significantly reduces the speed of IPX oxidation.
The mechanism of liquid phase oxidation of IPX in presence of N-HPI is discussed.
Acknowledgements
This workwas financially supported by The Ministry ofEducation and Science ofRussian Federation within the framework of the basic part ofgovernmental tasks of Samara State Technical University (project code 1708) and the basic part of governmental tasks of Yaroslavl State Technical University (Job # 2014/259).
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