A bifunctional ionic liquid, its preparation method and application, and synthetic ester lubricating oil

Abstract

This invention provides a bifunctional ionic liquid, its preparation method, and its application in the synthesis of ester lubricating oils, belonging to the field of ionic liquid technology. The bifunctional ionic liquid provided by this invention can function as both a catalyst and a lubricating oil additive. After the catalytic synthesis of ester lubricating oils, there is no need to separate the ionic liquid. This not only significantly improves the performance of ester lubricating oils and maximizes the synergistic lubrication effect between the synthesized ester lubricating oil and the ionic liquid, but also solves the scientific and technological problem of the negative impact of residual catalysts on synthetic ester base oils. Furthermore, it eliminates the catalyst separation step, shortens the ester synthesis process, and facilitates industrial-scale mass production and promotes its widespread application. This invention offers a dual-purpose solution, providing a new approach for the green synthesis of ester lubricating oils.

Description

Technical Field

[0001] This invention relates to the field of ionic liquid technology, and in particular to a bifunctional ionic liquid, its preparation method and application, and synthetic ester lubricating oil. Background Technology

[0002] With the advancement of modern science and technology, mechanical equipment is increasingly developing towards high speed, heavy load, and high precision, placing increasingly higher demands on lubricating oils. The research and application of polyol ester lubricating oils, which possess excellent thermal stability, oxidation stability, good high and low temperature performance, good biodegradability, and excellent tribological properties, have received increasing attention. Ester oils are the most widely used synthetic oils besides synthetic hydrocarbon oils. Among ester oils, polyol ester lubricating oils exhibit the best performance. Polyol esters are obtained by esterifying polyols such as neopentyl glycol, pentaerythritol, and trimethylolpropane with long-chain carboxylic acids (generally C8-C12 or oleic acid). They possess good heat resistance, oxidation resistance, lubricity, viscosity-temperature characteristics, and good volatility. Furthermore, the lubrication performance of polyol esters is superior to diesters and mineral oils, and far superior to polyalphaolefins (PAO). Moreover, although the viscosity of mixed long and short chain esters is low, they have the lowest coefficient of friction. Finally, polyol esters have good viscosity-temperature characteristics and volatility. Due to these advantages, polyol esters have been widely used in the lubrication field.

[0003] In industrial production, the synthesis of polyol ester lubricating oils is currently generally carried out using concentrated sulfuric acid and p-toluenesulfonic acid as catalysts, employing traditional high-temperature heating methods. Because the catalysts used simultaneously perform oxidation, sulfonation, dehydration, and isomerization, a series of side reactions occur; the reaction products are complex, subsequent treatment is cumbersome, and a large amount of waste liquid is generated, polluting the environment; at the same time, the concentrated sulfuric acid catalyst severely corrodes equipment. These factors directly result in synthesized polyol esters with a darker color and lower purity, failing to yield high-quality products and directly affecting their application in industries with high product quality requirements. With increasing environmental awareness and the gradual improvement of environmental regulations, finding new, efficient, and environmentally friendly catalysts to replace traditional concentrated sulfuric acid and researching and developing new environmentally friendly synthesis methods have always been hot topics in esterification reaction research. Current research on the synthesis of polyol esters mainly focuses on the synthesis and selection of catalysts, including solid acid catalysts, functional carbon nanotube catalysts, ionic liquid catalysts, enzyme catalysts, and traditional catalysts such as sodium bisulfate. Synthetic methods include direct esterification and transesterification. Regarding heating methods, existing technologies have disclosed the use of microwave radiation to replace common heating methods, achieving good results. For the needs of environmental protection and sustainable energy development, it is essential to research and develop green synthesis methods for polyol esters.

[0004] Ionic liquids have demonstrated excellent catalytic performance in organic synthesis and catalysis, while also being environmentally friendly. In recent years, they have been widely used as novel catalysts and reaction media. Furthermore, the lubricating properties of ionic liquids have also attracted considerable attention: their wide liquid temperature range and extremely low vapor pressure suggest that they may possess superior comprehensive lubrication performance unmatched by other lubricating materials. For example, ionic liquids can solve problems such as easy solidification and volatilization loss of lubricants under harsh conditions. However, compared to other hydrocarbon synthetic oils, the cost of using ionic liquids as pure lubricants is relatively high. Therefore, using ionic liquids as high-efficiency additives for lubricating oils is a cost-effective approach with the potential for widespread industrial application.

[0005] Currently, after the synthesis reaction of ionic liquid catalysts used in the synthesis of ester oils is completed, they need to be separated from the ester oils, which causes considerable inconvenience to industrial production. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a bifunctional ionic liquid, its preparation method and application, and its use in the synthesis of ester lubricating oils. The bifunctional ionic liquid provided by this invention can be used as both a catalyst and a lubricating oil additive, and there is no need to separate the ionic liquid after the catalytic synthesis of ester lubricating oils.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] This invention provides a bifunctionalized ionic liquid having the structure shown in Formula I:

[0009]

[0010] This invention also provides a method for preparing the bifunctional ionic liquid described above, comprising the following steps:

[0011] 2,6-Di-tert-butyl-4-methylphenol, N-bromosuccinimide, azobisisobutyronitrile, and chloroform were mixed and subjected to a first substitution reaction to give 2,6-di-tert-butyl-4-bromomethylphenol.

[0012] The 2,6-di-tert-butyl-4-bromomethylphenol, N-methylimidazole and an organic solvent were mixed and subjected to a second substitution reaction to obtain 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole;

[0013] The bifunctionalized ionic liquid is obtained by mixing 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium, di(2-ethylhexyl) phosphate, an inorganic basic substance, and an organic solvent and carrying out a displacement reaction.

[0014] Preferably, the molar ratio of 2,6-di-tert-butyl-4-methylphenol to N-bromosuccinimide is 1:1.

[0015] Preferably, the temperature of the first substitution reaction is 65–85°C and the time is 8–12 h.

[0016] Preferably, the molar ratio of 2,6-di-tert-butyl-4-bromomethylphenol to N-methylimidazole is 1:1.

[0017] Preferably, the molar ratio of 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium to di(2-ethylhexyl)phosphate is 1:1.

[0018] Preferably, the temperature of the second substitution reaction is 20–30°C and the time is 5–10 h.

[0019] The present invention also provides the application of the bifunctional ionic liquid described in the above technical solution or the bifunctional ionic liquid prepared by the above technical solution in synthetic ester lubricating oil.

[0020] The present invention also provides a synthetic ester lubricating oil, which is prepared by esterification reaction of the following raw materials: organic alcohols, organic acids, dehydrating agents and catalysts, wherein the catalyst is the bifunctional ionic liquid described in the above technical solution or the bifunctional ionic liquid prepared by the preparation method described in the above technical solution.

[0021] Preferably, the organic alcohol includes pentaerythritol, the organic acid includes hexanoic acid, and the dehydrating agent includes petroleum ether.

[0022] This invention provides a bifunctional ionic liquid, and compared with the prior art, the advantages of this invention are as follows:

[0023] The bifunctional ionic liquid provided by this invention can be used as both a catalyst and a lubricant additive. After the catalytic synthesis of ester-based lubricating oils, there is no need to separate the ionic liquid. This not only significantly improves the performance of ester-based lubricating oils and maximizes the synergistic lubrication effect between the synthesized ester-based lubricating oil and the ionic liquid, but also solves the scientific and technological problem of the negative impact of residual catalysts on synthetic ester base oils. Furthermore, it eliminates the catalyst separation step, shortens the ester synthesis process, and facilitates industrial-scale mass production and promotes its widespread application. This invention offers a dual-purpose solution, providing a new approach for the green synthesis of ester-based lubricating oils. Attached Figure Description

[0024] Figure 1 Thermogravimetric curve of the bifunctional ionic liquid [(BHT-1)MIM][DEPH];

[0025] Figure 2 An optical micrograph of the wear scar diameter of the upper sample under PETH lubrication;

[0026] Figure 3 An optical micrograph of the wear scar diameter of the upper sample under PETH+[(BHT-1)MIM][DEPH] lubrication. Detailed Implementation

[0027] This invention provides a bifunctionalized ionic liquid having the structure shown in Formula I:

[0028]

[0029] In this invention, the chemical name of the bifunctional ionic liquid is 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium di(2-ethylhexyl)phosphate ionic liquid ([(BHT-1)MIM][DEPH]). The bifunctional ionic liquid is used as a catalyst in the preparation process of synthetic ester lubricating oil, and no separation is required after the esterification reaction is completed, so it can be used as an additive for synthetic ester lubricating oil.

[0030] This invention also provides a method for preparing the bifunctional ionic liquid described above, comprising the following steps:

[0031] 2,6-Di-tert-butyl-4-methylphenol, N-bromosuccinimide, azobisisobutyronitrile, and chloroform were mixed and subjected to a first substitution reaction to give 2,6-di-tert-butyl-4-bromomethylphenol.

[0032] The 2,6-di-tert-butyl-4-bromomethylphenol, N-methylimidazole and an organic solvent were mixed and subjected to a second substitution reaction to obtain 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole;

[0033] The bifunctionalized ionic liquid is obtained by mixing 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium, di(2-ethylhexyl) phosphate, an inorganic basic substance, and an organic solvent and carrying out a displacement reaction.

[0034] Unless otherwise specified, all raw materials used in this invention are commercially available products in the field.

[0035] In this invention, 2,6-di-tert-butyl-4-methylphenol, N-bromosuccinimide, azobisisobutyronitrile, and chloroform are mixed and subjected to a first substitution reaction to obtain 2,6-di-tert-butyl-4-bromomethylphenol.

[0036] In this invention, the molar ratio of 2,6-di-tert-butyl-4-methylphenol to N-bromosuccinimide is preferably 1:1.

[0037] In this invention, the temperature of the first substitution reaction is preferably 65-85°C, and the time is preferably 8-12 hours, more preferably 9-10 hours.

[0038] In this invention, the mixture is preferably subjected to degassing and nitrogen purging in sequence, and the nitrogen purging is preferably performed 3 times.

[0039] After the first substitution reaction is completed, the present invention preferably filters the obtained product and concentrates the obtained filtrate under reduced pressure to obtain the 2,6-di-tert-butyl-4-bromomethylphenol.

[0040] After obtaining 2,6-di-tert-butyl-4-bromomethylphenol, the present invention mixes the 2,6-di-tert-butyl-4-bromomethylphenol, N-methylimidazole and an organic solvent to carry out a second substitution reaction to obtain 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole.

[0041] In this invention, the molar ratio of 2,6-di-tert-butyl-4-bromomethylphenol to N-methylimidazole is preferably 1:1.

[0042] In this invention, the organic solvent is preferably propionitrile. There is no special limitation on the amount of the organic solvent used, as long as it can ensure that the raw materials are mixed evenly.

[0043] In this invention, the temperature of the second substitution reaction is preferably 20-30°C, more preferably 25°C, and the time is preferably 5-10 hours, more preferably 6-8 hours.

[0044] In this invention, the 2,6-di-tert-butyl-4-bromomethylphenol is dissolved in propionitrile, and an equimolar amount of N-methylimidazole is added dropwise to the above solution to carry out the second substitution reaction.

[0045] After the second substitution reaction is completed, the present invention preferably pours the obtained product into methyl tert-butyl ether for filtration, and dries the obtained filter cake in a vacuum oven to obtain 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole. The role of methyl tert-butyl ether is to purify the product 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole.

[0046] After obtaining 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole, the present invention mixes the 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole, di(2-ethylhexyl) phosphate, an inorganic basic substance and an organic solvent to carry out a displacement reaction to obtain the bifunctionalized ionic liquid.

[0047] In this invention, the molar ratio of 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium to di(2-ethylhexyl)phosphate is preferably 1:1.

[0048] In this invention, the inorganic alkaline substance is preferably potassium hydroxide, and the potassium hydroxide is preferably added in the form of an aqueous solution of potassium hydroxide.

[0049] In this invention, the organic solvent is preferably dichloromethane.

[0050] In this invention, the temperature of the displacement reaction is preferably room temperature, and the time is preferably 12 hours.

[0051] In this invention, the di(2-ethylhexyl) phosphate is added to a potassium hydroxide solution and stirred at 25°C for 2 hours to obtain a mixture. The 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium is dissolved in dichloromethane and added dropwise to the mixture to carry out the displacement reaction.

[0052] After the displacement reaction is completed, the present invention preferably extracts the obtained product three times with dichloromethane, dries the obtained organic layer with magnesium sulfate, filters and then distills under reduced pressure to obtain a crude product. The crude product is stirred with methyl tert-butyl ether at 25°C for 60 min and then filtered. The filtrate is distilled under reduced pressure and then stirred again with 2-isopropoxypropane at 25°C for 60 min and then filtered. The filtrate is then distilled under reduced pressure to obtain the 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium di(2-ethylhexyl)phosphate ionic liquid.

[0053] In this invention, the purpose of pulping and re-pulping is purification.

[0054] This invention also provides the application of the bifunctional ionic liquid described in the above-mentioned technical solution or the bifunctional ionic liquid prepared by the above-mentioned technical solution in synthetic ester lubricating oil. The bifunctional ionic liquid can be used as both a catalyst and a lubricating oil additive. After the catalytic synthesis of ester lubricating oil is completed, there is no need to separate the ionic liquid. This not only helps to significantly improve the performance of ester lubricating oil and maximizes the synergistic lubrication effect of synthetic ester lubricating oil and ionic liquid, but also solves the scientific and technological problem of the negative impact of residual catalyst on synthetic ester base oil. Furthermore, it omits the catalyst separation step, shortens the synthetic ester preparation process, and is conducive to industrial mass production and promotes its widespread application.

[0055] The present invention also provides a synthetic ester lubricating oil, which is prepared by esterification reaction of the following raw materials: organic alcohols, organic acids, dehydrating agents and catalysts, wherein the catalyst is the bifunctional ionic liquid described in the above technical solution or the bifunctional ionic liquid prepared by the preparation method described in the above technical solution.

[0056] In this invention, the organic alcohol preferably includes pentaerythritol, the organic acid preferably includes hexanoic acid, and the dehydrating agent preferably includes petroleum ether.

[0057] In this invention, the boiling range of the petroleum ether is preferably 90–120°C.

[0058] In this invention, the molar ratio of the hydroxyl group in the organic alcohol to the carboxyl group in the organic acid is preferably 1:1.

[0059] In this invention, the molar ratio of the hydroxyl group in the organic alcohol to the catalyst is preferably 40 to 200:1, and when the organic alcohol is preferably pentaerythritol, the molar ratio of pentaerythritol to the catalyst is preferably 10 to 50:1.

[0060] This invention also provides a method for preparing the synthetic ester lubricating oil described in the above technical solution, comprising the following steps:

[0061] The organic alcohols, organic acids, dehydrating agents, and catalysts are mixed and subjected to an esterification reaction to obtain the synthetic ester lubricating oil.

[0062] In this invention, the temperature of the esterification reaction is preferably 150-180°C, more preferably 160-170°C, and the time is preferably 5-10 hours, more preferably 7-9 hours.

[0063] In this invention, the organic alcohol and organic acid are placed in a three-necked flask, and then the bifunctionalized ionic liquid and a dehydrating agent are added to carry out the esterification reaction.

[0064] After the esterification reaction is completed, the present invention preferably involves rotary evaporation of the obtained esterification product to obtain the synthetic ester lubricating oil.

[0065] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0066] Example 1

[0067] 2,6-Di-tert-butyl-4-methylphenol (1 mol), N-bromosuccinimide (1 mol), azobisisobutyronitrile, and chloroform were mixed, degassed, and purged three times with nitrogen. The mixture was then subjected to a first substitution reaction at 65°C under a nitrogen atmosphere for 10 h. After the reaction, the product was filtered, and the filtrate was concentrated under reduced pressure to obtain 2,6-di-tert-butyl-4-bromomethylphenol.

[0068] 1 mol of 2,6-di-tert-butyl-4-bromomethylphenol was dissolved in propionitrile, and an equimolar amount of N-methylimidazole was added dropwise to the solution. The mixture was stirred at 25°C for 6 h to carry out the second substitution reaction. The reaction product was poured into methyl tert-butyl ether and filtered. The filter cake was dried in a vacuum oven to obtain 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole.

[0069] Di(2-ethylhexyl) phosphate (1 mol) was added to a potassium hydroxide solution and stirred at 25°C for 2 h. 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium (1 mol) was dissolved in dichloromethane and added dropwise to the above solution. The mixture was stirred at 25°C for 12 h to carry out a displacement reaction. The reaction product was extracted three times with dichloromethane, and the resulting organic layer was dried with magnesium sulfate, filtered, and then rotary distilled under reduced pressure to obtain the crude product. The crude product was stirred with methyl tert-butyl ether at 25°C for 60 min to form a slurry, then filtered. The filtrate was rotary distilled under reduced pressure, stirred with 2-isopropoxypropane at 25°C for 60 min, filtered, and then rotary distilled under reduced pressure to obtain the product 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium di(2-ethylhexyl) phosphate salt ionic liquid ([(BHT-1)MIM][DEPH]), with a yield of 95%.

[0070] 1 H NMR: EB9867-94-P1B (400MHz, CHLOROFORM-d) δ = 10.90 (s, 1H), 7.14 (s, 2H), 7.07 (s, 1H), 6.93 (s, 1H), 5.43-5.33 (m, 3 H),4.09(s,3H),3.78(brd,J=5.8Hz,4H),1.59-1.47(m,3H),1.42(s,18H),1.38-1.26(m,16H),0.86(t,J=7.4Hz,12H)

[0071] Catalytic reaction: Using the esterification reaction of pentaerythritol and hexanoic acid to obtain pentaerythritol tetrahexanoate (PETH) as a model reaction, pentaerythritol and hexanoic acid were weighed into a three-necked flask, and the prepared bifunctionalized ionic liquid and petroleum ether (boiling range: 90-120℃, dehydrating agent) were added to carry out the esterification reaction. The product was rotary evaporated to obtain pentaerythritol tetrahexanoate (PETH+[(BHT-1)MIM][DEPH]) containing the bifunctionalized ionic liquid [(BHT-1)MIM][DEPH].

[0072] The acid value of the product was measured according to the national standard GB / T4945-2002, and the degree of esterification of the product was analyzed by the acid value. The formula for calculating the esterification rate is as follows:

[0073] Esterification rate (%) = [(1 - product acid value / initial acid value) × acid-alcohol molar ratio ÷ 4] × 100%

[0074] The catalytic performance results of the bifunctional ionic liquid are shown in Table 1. Table 1 provides the screening data for the catalytic reaction conditions. As can be seen from Table 1, the bifunctional ionic liquid [(BHT-1)MIM][DEPH] has good catalytic activity, and the esterification reaction can reach an esterification rate of 97% in 7 hours.

[0075] Table 1 Catalytic performance of bifunctional ionic liquids

[0076]

[0077]

[0078] Thermal stability evaluation: Thermal stability was determined using a STA 449C Jupiter simultaneous TG-DSC. 10 mg of the bifunctionalized ionic liquid [(BHT-1)MIM][DEPH] was placed in the sample cell, and the test temperature ranged from 25 to 600 °C, with a temperature increase rate of 10 °C / min, under a nitrogen atmosphere.

[0079] Figure 1 Thermogravimetric curve of the bifunctional ionic liquid [(BHT-1)MIM][DEPH] is shown below. Figure 1 It can be seen that the bifunctionalized ionic liquid [(BHT-1)MIM][DEPH] did not show any mass loss below 200℃ and the thermal decomposition temperature was 200.8℃, indicating that the functionalized ionic liquid has very good stability under the reaction temperature conditions.

[0080] Tribological property evaluation of the product:

[0081] The friction coefficient f of pentaerythritol tetrahexanoate and the pentaerythritol tetrahexanoate containing the bifunctional ionic liquid [(BHT-1)MIM][DEPH] prepared in this example were tested using an SRV-IV micro-vibration friction and wear testing machine manufactured by Optimol Grease GmbH, Germany, under the conditions of 100℃, 25Hz frequency, 1mm amplitude, and 50N load for 30 minutes. The steel balls used in the test were GCr15 bearing steel with F=10mm, and the test specimens were GCr15 steel blocks with F24×7.9mm. The results are shown in Table 2. It can be seen that under the same experimental conditions, the bifunctional ionic liquid [(BHT-1)MIM][DEPH] as an additive can effectively lubricate the steel-steel friction pair, reduce the friction coefficient, and have a significant friction-reducing effect.

[0082] The wear scar diameters of pentaerythritol tetrahexanoate (Bailingwei) and the pentaerythritol tetrahexanoate SRV-IV product containing bifunctional ionic liquid prepared in this example were measured using an OLYMPUS X41 metallurgical microscope manufactured by PREDIT. The results are shown in [Figure number missing]. Figures 2-3 And Table 2, Figure 2 An optical micrograph of the wear scar diameter of the upper sample under PETH lubrication. Figure 3 An optical micrograph of the wear scar diameter of the upper sample lubricated with PETH+[(BHT-1)MIM][DEPH]. Figures 2-3 As can be seen from the results, under the same experimental conditions, pentaerythritol tetrahexanoate containing the bifunctional ionic liquid [(BHT-1)MIM][DEPH] can effectively lubricate steel-steel friction pairs, reduce the wear scar diameter, and improve friction reduction performance.

[0083] Table 2. Friction coefficients and wear scar diameters of PETH and PETH lubricated with bifunctional ionic liquid [(BHT-1)MIM][DEPH].

[0084]

[0085]

[0086] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A synthetic ester lubricating oil characterized in that, It is prepared by esterification of the following raw materials: organic alcohols, organic acids, dehydrating agents, and catalysts, wherein the catalyst is a bifunctional ionic liquid with the structure shown in Formula I: Formula I; The esterification reaction was carried out at a temperature of 160°C. The organic alcohol is pentaerythritol, the organic acid is hexanoic acid, and the dehydrating agent is petroleum ether. The molar ratio of hydroxyl groups in the organic alcohol to the catalyst is 40:1; There is no need to separate the bifunctional ionic liquid after the catalytic synthesis of ester lubricating oils is completed.

2. The synthetic ester lubricating oil according to claim 1, characterized in that, The preparation method of the bifunctionalized ionic liquid includes the following steps: 2,6-Di-tert-butyl-4-methylphenol, N-bromosuccinimide, azobisisobutyronitrile, and chloroform were mixed and subjected to a first substitution reaction to give 2,6-di-tert-butyl-4-bromomethylphenol. The 2,6-di-tert-butyl-4-bromomethylphenol, N-methylimidazole and an organic solvent were mixed to carry out a second substitution reaction to obtain 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole; The bifunctionalized ionic liquid is obtained by mixing 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium, di(2-ethylhexyl) phosphate, an inorganic basic substance, and an organic solvent and carrying out a displacement reaction.

3. The synthetic ester lubricating oil according to claim 2, characterized in that, The molar ratio of 2,6-di-tert-butyl-4-methylphenol to N-bromosuccinimide is 1:

1.

4. The synthetic ester lubricating oil according to claim 2, characterized in that, The temperature of the first substitution reaction is 65~85℃ and the time is 8~12h.

5. The synthetic ester lubricating oil according to claim 2, characterized in that, The molar ratio of 2,6-di-tert-butyl-4-bromomethylphenol to N-methylimidazole is 1:

1.

6. The synthetic ester lubricating oil according to claim 2, characterized in that, The molar ratio of 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazolium to di(2-ethylhexyl)phosphate is 1:

1.

7. The synthetic ester lubricating oil according to claim 2, characterized in that, The second substitution reaction is carried out at a temperature of 20-30°C for 5-10 hours.

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