Preparation and Property Analysis of Antioxidant of Carbazole Derivatives

doi:10.21203/rs.3.rs-3285542/v1

Carbazole derivatives can be used as antioxidants in the lubricating- oil industry. The alkylation of carbazole with bromoisopropane and chlorotert butane catalyzed by anhydrous aluminum chloride was studied. 3,6-di-iso-propylcarbazole and 3,6-di-tert-butylcarbazole were synthesized from dichloromethane and dibromomethane at ,25 ℃ respectively. The effects of reaction time, catalyst dosage, and the molar ratio of carbazole to chloroalkanes were investigated, and orthogonal experiments were carried out. The structures of the carbazole derivatives were characterized by IR and MS. The thermal stability of the synthesized carbazole derivatives was investigated using differential scanning calorimetry (DSC). The carbazole derivatives were added to the lubricating oil with a mass fraction of 0.8%, and the miscibility, stability and oxidation resistance of the mixed system were evaluated using the mechanical stirring method and rotary pressure vessel oxidation test. The DSC results showed good thermal stability for the carbazole derivatives. The mechanical stirring method also proved that the mixture of oil and carbazole derivatives has good solubility and stability. The results of the rotary pressure vessel oxidation test showed that isopropyl and tertbutyl carbazole derivatives increased the oxidation-induction period of the lubricating oil by 1.39 and 1.91 times, respectively.

carbazole

carbazole derivatives

lubricating oil

antioxidants

Mineral lubricating oils inevitably come in contact with light, air, high temperatures, and metals. Oxidation reactions occur, and aldehydes, ketones, and acids are generated under the action of the above factors, which further condense into carbonaceous materials and asphalt. Thus, mechanical equipment is blocked, and the service cycle of lubricating oils is greatly shortened [1–2]. Friction in industrial machinery can easily cause mechanical wear and destruction during machine-industry development. Approximately 85% of the reasons leading to machine failure are caused by forms of friction and wear. Lubrication is the most effective measure to reduce friction and wear [3–4]. Lubricating oils play an important role in the long-term and reliable operation of equipment, implementation of intelligent and green manufacturing, industrial foundations, and other major engineering systems. Various additives are typically added to lubricating oil to prolong its service life and greatly improve its thermal stability and antioxidant properties [5–10]. Among these, antioxidant properties are indispensable; thus, antioxidants must be added to lubricating oils to slow down the rate of oxidation deterioration. The oxidation of lubricating oil is essentially a hydrocarbon oxidation process, and its mechanism is a free-radical chain reaction. Both local and international researchers have studied the antioxidant properties of some additives; however, there are relatively few studies on new carbazole lubricating-oil antioxidants. Carbazole is an aromatic amine, and its atoms are in the same plane; therefore, it has strong electron conjugation and high thermal stability. Carbazole derivatives are promising high-temperature antioxidants. In this study, carbazole-derived lubricant antioxidants were prepared and used for the development and utilization of aromatic amine antioxidants in lubricating oils.

Carbazole,, dichloromethane, and anhydrous aluminum chloride were used as the reaction materials. 3,6-ditert-butylcarbazole was synthesized by introducing tert-butylcarbazole, whereas 3,6-diisopropyl carbazole was synthesized by introducing isopropyl. The lipophilicity of carbazole was improved by the introduction of alkyl groups. The resulting products were quenched and subjected to salt washing, water washing, extraction, water removal, filtration, rotary evaporation, and other related chemical processes to obtain the crude product, which was then separated and purified. The structures of the synthesized products were characterized using Fourier-transform infrared (FT-IR) spectroscopy and mass spectrometry (MS). The thermal stability of the carbazole-derived antioxidant was determined by thermogravimetric analysis. The antioxidant properties of the carbazole derivatives were determined using the rotary pressure vessel oxidation test (RPVOT) and differential scanning calorimetry (DSC).

2.1. Single-factor experimental results and analysis

2.1.1. Investigation of reaction time

The reaction time was changed from 0.5 h, to 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 h to form 10 groups of comparative tests to investigate the effect of reaction time on the yield of 3, 6-di-tert-butylcarbazole. The experimental results are shown in Figure 1.

Figure 1 shows that the yield of 3,6-di-tert-butylcarbazole increased over time. When the reaction time was approximately 3 h, the yield reached a high value, and subsequently, the increase in the yield of the reaction was not significant.

2.1.2. Investigation of the ratio of carbazole to anhydrous aluminum chloride

The ratio of n(carbazole) to n(anhydrous aluminum chloride) was changed, and the quantitative relationships of 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, and 1:3(mol:mol) were used to conduct six groups of comparative tests; the results are shown in Figure

As shown in Figure 2, the yield of 3,6-di-tert-butylcarbazole increased with an increasing mass of anhydrous aluminum chloride. When the molar ratio of carbazole to anhydrous aluminum chloride was 1:1, the product yield reached a high value, the yield of the reaction increased slightly, and the increasing trend was weak.

2.1.3. Investigation of the molar ratio of carbazole to chloro-tert-butane

The ratio of n(carbazole) to n(chloro-tert-butane) was varied, and quantitative relationships of 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, and 1:5(mol:mol) were used for ten groups of comparative tests. The results are shown in Figure 3.

As shown in Figure 3, the yield of 3,6-di-tert-butylcarbazole first increased and then decreased with an increase in the ratio of carbazole to chloro-tert-butane. When n(carbazole):n(chloro-tert-butane) was between 1:2(mol:mol) and 1:2.5(mol:mol), the highest yield was achieved, and the yield of the reaction decreased gradually with an increase in the content of chlorinated tert-butane.

2.2 Orthogonal experiment results and analysis

Based on the single-factor experiment, an orthogonal experiment with three factors and three levels was adopted. The factor levels are listed in Table 1, and the experimental results are listed in Tables 2 and 3.

Table 1. Experimental conditions

No.	A Time/h	B n_carbazole:n_{chloro-tert-butane}	C n_carbazole:n_{anhydrous aluminium chloride}
1	2.5	1:2	1:1
2	3	1:2.5	1:1.5
3	3.5	1:3	1:2

Table 2. The orthogonal L₉ (3³) test results analysis

No.	factors			The yield (%)
No.	A	B	C	The yield (%)
1	1	1	1	66.24
2	1	2	2	70.77
3	1	3	3	58.93
4	2	1	2	70.52
5	2	2	1	74.72
6	2	3	3	62.47
7	3	1	3	76.21
8	3	2	1	78.77
9	3	3	2	67.13
K₁	195.94	212.97	219.73
K₂	207.71	224.26	208.42
K₃	212.11	188.53	197.61
k₁	65.31	70.99	73.24
k₂	69.24	74.75	69.47
k₃	74.04	62.84	65.87
R	8.73	11.91	7.37
the order	B>A>C
optimal levels	A₃	B₂	C₁
optimal conditions	A₃B₂C₁

Table 3. Variance analysis of the orthogonal test

Variation sources	Sum of squares of deviations from the mean	df	Mean square	F
A	0.68	2	0.34	1.37
B	1.18	2	0.59	3.12
C	0.47	2	0.235	1.28

Table 2 shows that the most important factor affecting the yield of carbazole derivatives is the molar ratio of carbazole to chlorinated tert-butane. This is followed by the reaction time and the molar ratio of carbazole to anhydrous aluminum chloride. The variance analysis also indicates that the effect of the molar ratio of carbazole to chlorinated tert-butane is prominent. The overall trend of the influence of n(carbazole):n(chlorinated tert-butane substances) on the yield shows that the initial rate clearly increases as the amount of tert-butyl chloride increases. The yield of 3,6-di-tert-butylcarbazole reaches its maximum when the molar ratio of carbazole to chloro-tert-butane reaches 1:2.5(mol:mol). Then, the yield of 3,6-di-tert-butylcarbazole decreases with an increase in the tert-butane chloride content. According to the calculation of the static charge distribution on carbazole molecule carbon by Bonesi et al. [11] using the PM3 method, the value of C3=C6 (Figure 4) is -0.126, that of C1=C8 is -0.107, that of C2=C7 is -0.082, and that of C4=C5 is -0.049. The electron cloud densities of C1, C3, C6, and C8 are relatively high and are prone to the electrophilic substitution reaction. Polysubstituted carbazole may be obtained if the molar ratio of carbazole to tert-butane chloride exceeds 1:2. The optimal molar ratio of carbazole to chlorinated tert-butane was 1:2(mol:mol).The optimal reaction time was 3.5 h, and the molar ratio of carbazole to anhydrous aluminum chloride was 1:1(mol:mol). The yield of the carbazole derivatives was at its highest under the above reaction conditions.

The optimization process of isopropyl-substituted carbazole is the same as above.

2.3 Structure characterization and performance analysis

The structure of the product was analyzed by FT-IR spectroscopy and MS. The application performance of the product was tested using DSC and RPVOT.

2.3.1FT-IR analysis

Taking 3,6-di-tert-butylcarbazole as an example, the IR spectrum of 3,6-di-tert-butylcarbazole is shown in Figure 5. The IR absorption peaks are at 3450 cm^-1, 1650 cm^-1, and 1390 cm^-1, among which 3450 cm^-1 is the stretching vibration peak of -NH-, 1650 cm^-1 is the absorption peak of five-membered heterocyclics, and 1390 cm^-1 is the IR-absorption peak of the tert-butyl group. The synthesized carbazole derivative was determined to be 3,6-di-tert-butylcarbazole according to the results of the IR spectrum analysis.

2.3.2 MS analysis

Figure 5 shows the mass spectrum of the target product. The maximum mass/nucleus ratio (M/z) of the unknown molecule is 284. The relative molecular weight of the unknown substance is observed to be 284, and the synthesized carbazole derivative should be 3,6-di-tert-butylcarbazole according to the molecular ion peak and several fragment ion peaks with high abundance, such as M/z=198, M/z=183 and M/z=155.

The location of the first fragment ion peak is M/z =198, and the difference between the fragment ion peak and relative molecular mass of 284 is M/z =86. This indicates that 3,6-ditert-butylcarbazole was rearranged after tert-butylcarbazole was crushed by the ion source. The position of the second fragment ion peak is M/z =183, and the difference in the mass/nucleus ratio between the fragment ion peak and first fragment ion peak is M/z =15, which is the fragment ion peak generated when -NH- was shot down after 3,6-dimethylcarbazole was bombarded by an ion source. The position of the third fragment ion peak is M/z =155, and the difference in the mass/nucleus ratio between the fragment ion peak and second fragment ion peak is M/z =28, which is the fragment ion peak generated after the two methyl groups were smashed by the ion-source bombardment. The last position with high abundance is m/z=99, which is the fragment peak formed after the benzene ring was rearranged to make its structure relatively stable.

The synthesized carbazole derivative was determined to be 3,6-di-tert-butylcarbazole based on its infrared spectrum, mass spectrum, and electron cloud density.

2.3.3 Solubility and stability test

Taking the compatibility and stability of 3,6-ditert-butylcarbazole as an example, the experimental results showed that carbazole-derived antioxidants dissolved well in lubricating oil and kept the lubricating oil transparent. After 48 h, the prepared oil samples remained clear and transparent. The lubricating oil sample became turbid when carbazole was directly added to the it. The solubility of the carbazole derivatives in lubricating oil was increased by the introduction of tert-butyl groups in the benzene ring of carbazole, and the stability of the synthesized carbazole derivatives and oil was good.

2.3.4 Differential scanning calorimetry experiment result analysis

Differential scanning calorimetry (DSC) curves of the carbazole, isopropyl carbazole, and tert-butylcarbazole derivatives were obtained using a thermal analyzer after the products were dried and ground. The DSC curves are shown in Figure 6. As shown in Figure 6, the DSC curves of the carbazole derivatives are all on the right side of that of carbazole, and the curve slopes of the carbazole derivatives are higher than those of carbazole. Therefore, the oxidation stabilities of the two carbazole derivatives improved.

2.3.5. RPVOT analysis

Carbazole-derived antioxidants were added to the lubricating oil according to the 1.5.3 method to prepare an oil sample, and the oxidation-induction period of the oil samples was determined by the RPVOT method according to the SH/T 0193-2008 standard; the results are shown in Figure 7.

Figure 7 shows that the oxidation-induction period of the lubricating oil was 133 min. The oxidation-induction period of the lubricating oil was 168 min after carbazole was added, which was approximately 1.26 times that of the crude oil. The oxidation-induction period of the lubricating oil was 185 min after the isopropyl carbazole derivatives were added, which was approximately 1.39 times that of crude oil. The oxidation-induction period of the lubricating oil was 254 min after the addition of tert-butylcarbazole derivatives, which was approximately 1.91 times that of crude oil. Carbazole derivatives can be used as free radical scavengers because of the relatively active H on N. It can combine with the free radicals generated in the oxidation process of lubricating oil, terminate the growth of free radicals, and cut off the chain reaction. Carbazole is also a hydrogen donor; however, the free radical formed after carbazole donates a H atom is stable under strong conjugation and cannot easily continue to react. Therefore, the tert-butyl carbazole derivative exhibited better oxidation resistance as a lubricating oil additive. When tert-butyl was added to the benzene ring of carbazole, all the atoms were in the same plane, and the structure of the carbazole derivatives was extremely stable. The results showed that using carbazole derivatives as antioxidants, especially tert-butylcarbazole derivatives, can prolong the antioxidant time and service life of lubricating oils.

3.1. Materials and methods

Chemical-grade carbazole, anhydrous aluminum trichloride, tert-butane chloride, bromoisopropane, dibromomethane, dichloromethane, ammonia, an iodine-free refined salt, sodium hydroxide, ethyl acetate, thin-layer chromatography (TLC) silica gel, and anhydrous magnesium sulfate were purchased from Shanghai Macklin Chemicals (China) and used without further purification. Potassium hydroxide (spectral grade), mineral oil (medical grade), and all other chemicals, including petroleum ether (reagent grade), were obtained from Tianjin Reagent Chemicals (China). The lubricating oil was obtained from Jiangsu Kuorun Chemical Co., Ltd.

The Nexus470 (Thermo Nicolet Corp.), SHO193C automatic rotary pressure vessel oxidation test instrument (Shandong Shengtai Instrument Co., Ltd., China), DSC-Q20 differential scanning calorimeter (Waters Technology (Shanghai) Co., Ltd., China), and 6460 mass spectrometer (Agilent Technology Co., Ltd., USA) were used.

3.2 The reaction principle

Under the catalysis of anhydrous aluminum chloride, carbazole reacts with tert-butyl chloride to form tert-butyl-substituted carbazole derivatives through Friedel–Crafts alkylation at room temperature, according to the following reaction:

Under the catalysis of anhydrous aluminum chloride, isopropyl-substituted carbazole derivatives were generated by the reaction of carbazole and bromoisopropane through Friedel–Crafts alkylation at room temperature using the following reaction formula:

3.3 Preparation method

Synthesis of carbazole derivatives

Certain amounts of carbazole and anhydrous aluminum chloride were added to a single-port round-bottom flask, and anhydrous dichloromethane was used as the solvent.

The carbazole and catalyst were thoroughly mixed under anhydrous conditions. Carbazole and anhydrous aluminum chloride were completely mixed and dissolved under agitation, and cooled to 0 ℃. The chlorinated tert-butane was dropped into the mixture, and the mixture was stirred quickly at room temperature to ensure that it was completely mixed when the temperature of the mixing system reached 0 ℃. The reaction ended, and the tert-butylcarbazole derivatives were obtained after 3 h.

The synthesis of the propyl carbazole derivatives was similar to that of the tert-butylcarbazole derivatives, which is not detailed here.

3.4 Separation and purification of carbazole derivatives

After the reaction, the carbazole derivatives were quenched with a high concentration of ammonia water. They were then washed with distilled water and then with sodium chloride. The quenched, washed, and salt-washed solutions were then extracted with dichloromethane. The organic layer in the lower layer of the separating funnel was collected and the water layer was removed. Anhydrous magnesium sulfate was added to the extracted solution to remove residual water, and the gray-brown solid sample was filtered and dried. The yields of the carbazole derivatives were calculated as follows:

The yield=(experimental value/theoretical value)×100%(1)

3.5 Structure analysis of carbazole derivatives

3.5.1. FT-IR analysis

The structures of the carbazole derivatives were characterized by FT-IR spectroscopy. An IR spectrometer with KBr pellets was used.

3.5.2. MS analysis

The samples were dissolved in acetone and analyzed using MS. The ion source was electrospray ionization, the mode was set to MS2 scan, and the polarity was set to positive.

3.6 Properties analysis of carbazole derivatives

3.6.1. Solubility and stability

The synthesized carbazole-derivative lubricating oil antioxidant was added to the lubricating oil with a mass fraction of 0.8%, and the prepared oil sample was fully stirred using an electric stirrer for 30 min. The mixture was stirred and left to sit for a while before being heated in a water bath. The mixed oil sample and carbazole derivatives were heated to 50–60 ℃ in a water bath and then stirred for 30 min. The sample was put into a bottle, and was left standing for 48 h to observe the solubility of the carbazole derivatives and oil sample and stability of the mixed system after cooling.

3.6.2. Thermal stability

DSC oxidation tests were performed to determine the thermal stabilities of the carbazole derivatives. Both static and dynamic methods may be used for differential scanning calorimetry. In this study, a dynamic DSC method was used to determine the oxidation stability of the carbazole derivatives. The selected gas atmosphere was nitrogen, the programmed temperature was set to 10 ℃/min, and the measured temperature range was set to 0–350 ℃. The experimental procedure was performed after the setting.

3.6.3. The oxidation resistance

The RPVOT method was used for antioxidant testing. The synthetic carbazole-derived antioxidant and 50 mL of lubricating oil were added to a quartz cup with a 5 m-long copper coil, and 5 mL of pure water was added to the quartz cup to provide a high-humidity environment. The oxygen cylinder was opened, and the automatic oxygenation mode was selected in the RPVOT. Oxygen was filled in the reaction chamber at a pressure of 615 KPa. The reaction temperature was set to 140 ℃, and the rotary pressure vessel oxidation test was carried out in an oil-bath environment. The oxidation conditions were as follows: the quartz cup was at an angle of 30° to the horizontal plane in the reaction chamber, and the rotation speed was 120 r/min. The procedure was performed according to the national standard calibration method (SH/T 0193–2008). The experiment was terminated when a specified pressure decrease was reached. The oxidation-induction period of the sample spanned from the beginning to the end of the oxidation [12].

Two carbazole derivatives were prepared via orthogonal experiments. The most influential factor in the synthesis of tert-butylcarbazole derivatives was the mass ratio of carbazole to chlorinated tert-butane, followed by the reaction time and mass ratio of carbazole to anhydrous aluminum chloride, as determined by the mean and range analysis of the orthogonal experiment. The optimum reaction conditions for the synthesis of 3, 6-ditert-butylcarbazole were as follows: reaction time of 3.5 h, mass ratio of carbazole to chlorinated tert-butane of 1:2.5(mol:mol), and mass ratio of carbazole to anhydrous aluminum chloride of 1:1(mol:mol). The synthesis conditions for 3, 6-diisopropyl carbazole were as follows: reaction time of 3 h, mass ratio of carbazole to bromoisopropane of 1:2.5(mol:mol), and mass ratio of carbazole to anhydrous aluminum chloride of 1:1.5(mol:mol). The synthesized carbazole derivatives were characterized using IR spectroscopy and MS.

The properties of the carbazole derivatives were determined using mechanical agitation, differential scanning calorimetry, and rotary pressure vessel oxidation. The solubilities of the carbazole derivatives and lubricating oil mixtures were satisfactory. The prepared oil samples were transparent, clear, and stable. The DSC experimental results showed good thermal stability, and the RPVOT results showed good performance for carbazole derivatives as antioxidant additives in lubricating oil.

Author Contributions: Experiment, Z.Z. and C.H.; product analysis, Z.J.and Z.B.; writing—original draft preparation, C.H.; writing—review and editing, C.H.; project administration, C.H.; funding acquisition, C.H. All authors have read and agreed to the published version of the manuscript. All authors reviewed the manuscript.

Funding: This research was funded by the Foundation of Liaoning Key Laboratory of Chemical Additive Synthesis and Separation (ZJNK2004), Education Department of Liaoning Province Scientific Research Projects (LJKZ1201), and Education Department of Liaoning Province Scientific Research Projects (LJKMZ20221857).

Acknowledgments: We are grateful to the Foundation of the Liaoning Key Laboratory of Chemical Additive Synthesis and Separation (ZJNK2004) and the Education Department of Liaoning Province Scientific Research Projects (LJKZ1201).

Conflicts of Interest:The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results”.

Availability of data and materials:All data generated or analyzed during this study are included in this article and the raw data is available from the corresponding author if it requested.

Ethics approval and consent to participate: Research is not involving human participants or animals.

Consent for publication: not applicable

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No competing interests reported.

Preparation and Property Analysis of Antioxidant of Carbazole Derivatives

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Journal Publication

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Abstract

Figures

1. Introduction

2. Results

3. Experiment