Nitrogen-containing heterocyclic borate-modified black phosphorus nanosheets for lubricating oil additives, preparation method and application

Abstract

The application discloses nitrogen-containing heterocyclic borate modified black phosphorus nanosheet for lubricating oil additives, a preparation method and application. The nitrogen-containing heterocyclic borate modified black phosphorus nanosheet provided by the application can significantly improve the extreme pressure bearing capacity of base oil as an additive, has excellent dispersibility and chemical stability in an oil-based system, and can effectively inhibit oxidative degradation and agglomeration settlement. Compared with unmodified black phosphorus, the structure stability of the nitrogen-containing heterocyclic borate modified black phosphorus nanosheet is significantly improved, and the nitrogen-containing heterocyclic borate modified black phosphorus nanosheet is more suitable for long-term use as a lubricating oil additive. The additive of the application adopts a green lubricating system constructed by low-toxicity nitrogen-containing heterocyclic borate and degradable black phosphorus nanosheets, does not contain common pollutants of traditional heavy metal salts, phosphorus and chlorine type extreme pressure agents, can significantly reduce potential hazards to equipment, oil and environment, and meets the requirements of green lubrication and sustainable application.

Description

Technical Field

[0001] This invention belongs to the field of lubricating materials technology, specifically relating to a nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheet for use as a lubricating oil additive, its preparation method and application. Background Technology

[0002] Lubricating oil additives are key active components that enhance the overall performance of base oils, encompassing functions such as extreme pressure anti-wear, anti-oxidation, corrosion inhibition, anti-corrosion, and detergency and dispersion. Traditional lubrication systems typically rely on the synergistic effect of multiple additives to achieve excellent performance; however, different types of additives often exhibit chemical or physical incompatibility, easily leading to antagonistic effects and decreased lubrication performance. Furthermore, to achieve the desired performance, existing additives usually require high dosages, which may result in increased oil viscosity or higher operating costs.

[0003] Two-dimensional nanomaterials are used in lubrication systems due to their unique layered structure and interfacial properties. Black phosphorus nanosheets, as an anisotropic two-dimensional material, have layers held together by van der Waals forces, facilitating interlayer slip under shear conditions, which helps reduce the coefficient of friction. However, black phosphorus nanosheets are prone to aggregation or sedimentation in oils, exhibiting limited dispersion stability and are easily oxidized. In existing technologies, the dispersion stability of black phosphorus nanosheets in oils can be improved through surface modification; however, the compatibility issues between black phosphorus nanosheets and other additives remain unresolved. Nitrogen-containing heterocyclic borate esters possess both a borate ester framework and nitrogen-containing heterocyclic groups, exhibiting good base oil compatibility, oxidation resistance, and hydrolytic stability. They can construct a protective film at the friction interface composed of boron oxides, nitrogen-containing coordination films, and organic layers. In existing technologies, they can be used in various lubrication systems, such as transmission fluids, industrial gear oils, and insulating oils. Compared to the traditional additive ZDDP, nitrogen-containing heterocyclic borate esters offer lower ash content, are free of heavy metals, and are more environmentally friendly. However, the existing technology lacks a lubricant additive that can achieve synergistic effects between black phosphorus nanosheets and nitrogen-containing heterocyclic borate esters at low addition levels, thereby taking into account dispersibility, stability and multifunctionality. Summary of the Invention

[0004] The first objective of this invention is to provide a nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheet for use as a lubricating oil additive, and the second objective of this invention is to provide a method for preparing the nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheet and its application.

[0005] The first objective of this invention is achieved by providing a nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheet for use as a lubricating oil additive. The nitrogen-containing heterocyclic borate ester is grafted onto the surface of the black phosphorus nanosheet via POB coordination bonds in an organic-inorganic hybrid manner. The structural formula of the nitrogen-containing heterocyclic borate ester is shown in Formula I. .

[0006] The second objective of this invention is achieved by the method for preparing the nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets for use as a lubricating oil additive, which is carried out according to the following steps: 1) Black phosphorus powder was added to N-methylpyrrolidone solvent to prepare a dispersion with a concentration of 0.8-2.0 mg / mL. Then, nitrogen-containing heterocyclic borate ester was added to make the mass-volume ratio of nitrogen-containing heterocyclic borate ester to black phosphorus nanosheets 1.0-1.8 mg / mL. The dispersion was obtained by ultrasonic reaction at 10-30℃. 2) The nitrogen-containing heterocyclic borate modified black phosphorus dispersion was replaced with tert-butanol and then freeze-dried to obtain nitrogen-containing heterocyclic borate modified black phosphorus nanosheet powder.

[0007] In step 1), the ultrasonic treatment duration is 4-8 hours and the ultrasonic frequency is 35-45 kHz.

[0008] In step 2), the freeze-drying conditions are as follows: after rapid pre-freezing at -80℃ or liquid nitrogen, the freeze dryer is placed in the freeze dryer and freeze-dried for 24-48 hours under vacuum conditions of gradually increasing the temperature to -20℃ to -10℃ and below 0.1 mbar.

[0009] The application of the nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheets is as an additive in the preparation of lubricating oil.

[0010] This invention prepares a nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheet additive via an organic-inorganic hybrid approach. The black phosphorus nanosheets and the nitrogen-containing heterocyclic borate ester form a stable structure in the lubricating oil system through physical adsorption and coordination. Physical adsorption (van der Waals forces, π-π stacking) achieves initial interface coverage and low shear slip. The benzothiazole group in the nitrogen-containing heterocyclic borate ester molecule, with its N and S atoms, enhances adsorption capacity. The OBO structure in the nitrogen-boron heterocyclic structure facilitates the formation of a POB coordination structure in the black phosphorus nanosheets, making the additive more stable in the oil. The long-chain fatty acid side chains can form an ordered, oriented layer, which helps to form an organic-inorganic film on the surface of the black phosphorus nanosheets.

[0011] The beneficial effects of this invention are as follows: 1. The nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets provided by this invention can significantly improve the extreme pressure carrying capacity of base oils, and exhibit excellent dispersibility and chemical stability in oil-based systems, effectively inhibiting oxidative degradation and agglomeration sedimentation. Compared with unmodified black phosphorus, the nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets of this invention have significantly improved structural stability, making them more suitable for long-term use as lubricating oil additives. Lubricating oils with these black phosphorus nanosheets added exhibit excellent friction-reducing effects and operational stability during long-term friction. 2. The additive of this invention adopts a green lubrication system synergistically constructed with low-toxicity nitrogen-containing heterocyclic borate esters and biodegradable black phosphorus nanosheets. It does not contain pollutants commonly found in traditional heavy metal salts, phosphorus, and chlorine-type extreme pressure agents, which can significantly reduce potential hazards to equipment, oils, and the environment, and meet the needs of green lubrication and sustainable applications. Attached Figure Description

[0012] Figure 1 The structural characterization diagrams of BTPP in the nitrogen-containing heterocyclic borate ester structure prepared in Example 1 of this invention are shown in (a) Fourier transform infrared (FTIR) spectrum and (b) nuclear magnetic resonance (NMR) spectrum. 1 HNMR spectrum; Figure 2 The following are structural characterization diagrams of DOBC in the nitrogen-containing heterocyclic borate ester structure prepared in Example 1 of this invention: (a) is the FTIR image, and (b) is... 1 HNMR spectrum; Figure 3 Here are the structural characterization diagrams of NB-BTA in the nitrogen-containing heterocyclic borate ester structure prepared in Example 1 of this invention: (a) is the FTIR spectrum, and (b) is... 1 HNMR spectrum; Figure 4 This is a schematic diagram of the microstructure of black phosphorus powder prepared by high-energy ball milling in Example 1 of the present invention; Figure 5 This is a schematic diagram of the microstructure of black phosphorus nanosheets exfoliated by NMP in Example 1 of the present invention; Figure 6 This is a schematic diagram of the microstructure of the nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheet powder prepared in Example 1 of the present invention; Figure 7 The figure shows the results of the dispersion stability test of lubricating oils with different additives. Figure 8 The graph shows the extreme pressure performance test results of the additive prepared in Example 1 of this invention and the additive in the comparative example. Figure 9 The graph shows the friction coefficient test results of the additive prepared in Example 1 of this invention and the additive in the comparative example. Detailed Implementation

[0013] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0014] This invention discloses a nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheet for use as a lubricating oil additive. The nitrogen-containing heterocyclic borate ester is grafted onto the surface of the black phosphorus nanosheet via POB coordination bonds in an organic-inorganic hybrid manner. The structural formula of the nitrogen-containing heterocyclic borate ester is shown in Formula I. .

[0015] The black phosphorus nanosheets have a diameter of 50-100 nm and a thickness of 10-20 nm.

[0016] The method for preparing the nitrogen-containing heterocyclic borate ester is as follows: BTPP and DOBC are dissolved in xylene, heated to 110-130 °C under nitrogen protection and refluxed for 3-5 h. After the reaction is completed, the mixture is cooled to room temperature, the solvent is removed by rotary evaporation, and the mixture is washed with an organic solvent. Then, it is purified by silica gel column chromatography to obtain the target product. The product is dried in a vacuum drying oven at 40-60 °C to obtain the nitrogen-containing heterocyclic borate ester shown in Formula I. The molar ratio of BTPP to DOBC is 1:1.

[0017] Silica gel column chromatography was performed using gradient elution with petroleum ether / ethyl acetate at a volume ratio of 40:1 to 10:1.

[0018] This invention also provides a method for preparing nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets for use as lubricating oil additives, which is carried out according to the following steps: 1) Black phosphorus powder was added to N-methylpyrrolidone solvent to prepare a dispersion with a concentration of 0.8-2.0 mg / mL. Then, nitrogen-containing heterocyclic borate ester was added to make the mass-volume ratio of nitrogen-containing heterocyclic borate ester to black phosphorus nanosheets 1.0-1.8 mg / mL. The dispersion was obtained by ultrasonic reaction at 10-30℃. 2) The nitrogen-containing heterocyclic borate modified black phosphorus dispersion was replaced with tert-butanol and then freeze-dried to obtain nitrogen-containing heterocyclic borate modified black phosphorus nanosheet powder.

[0019] In step 1), the ultrasonic treatment duration is 4-8 hours and the ultrasonic frequency is 35-45 kHz.

[0020] In step 2), the freeze-drying conditions are as follows: after rapid pre-freezing at -80 ℃ or liquid nitrogen, the freeze dryer is placed in the freeze dryer and freeze-dried for 24-48 h under vacuum conditions of gradually increasing the temperature to -20℃ to -10℃ and below 0.1 mbar.

[0021] The present invention further provides the application of the nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheets as an additive in the preparation of lubricating oil.

[0022] The base oil of the lubricating oil is transformer oil or mineral oil.

[0023] The nitrogen-containing heterocyclic borate modified black phosphorus nanosheets have a mass percentage of 0.5-1.5% in the lubricating oil. Example

[0024] 1. Preparation of nitrogen-containing heterocyclic borate ester NB-BTA (1) Preparation of BTPP 0.1 mol of 2-mercaptobenzothiazole and 60 mL of 10 wt.% NaOH aqueous solution were added to a 250 mL three-necked flask. The mixture was heated to 65 °C and stirred for 30 min, followed by slow dropwise addition of 0.1 mol of 3-chloro-1-propanol. The mixture was refluxed for 8 h. After the reaction was complete, impurities were removed by filtration, and the organic phase was extracted, washed, and dried. The solvent was removed by rotary evaporation to obtain the crude product. The target compound was obtained by silica gel column chromatography.

[0025] (2) Preparation of DOBC Oleic acid (0.1 mol) was added to a 250 mL flask and heated to 110 °C. Diethanolamine (0.04 mol) was slowly added with stirring, and the reaction mixture was heated to 150 °C and reacted for 4 h. The temperature was then lowered to 80 °C, and diethanolamine (0.06 mol) and potassium hydroxide (0.86 g) were added. The reaction mixture was kept at this temperature for 3 h, then further cooled to 50 °C. Under nitrogen protection, 0.1 mol of boron trichloride was added, and the reaction was kept at this temperature for 4 h until no more water evaporated. The reaction was then complete. The residual solvent was removed by rotary evaporation, and the product was dried in a vacuum drying oven at 50 °C for 12 h to obtain the target product.

[0026] (3) Preparation method of NB-BTA: BTPP (0.1 mol) and DOBC (0.1 mol) were added to a 250 mL flask, along with 40 mL of xylene as a solvent. The mixture was heated to 120 °C and refluxed for 4 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, the solvent was removed by rotary evaporation, and the product was washed with petroleum ether. The product was then purified by silica gel column chromatography (using a gradient elution of petroleum ether / ethyl acetate at a volume ratio of 40:1-10:1) to obtain the target product. The product was dried in a vacuum drying oven at 50 °C to obtain the yellow oily target compound NB-BTA, i.e., the nitrogen-containing heterocyclic borate ester of Formula I.

[0027] 2. Preparation of black phosphorus powder Weigh 2.0 g of 95% pure lumpy red phosphorus and place it in a stainless steel ball mill jar. Select stainless steel grinding balls with diameters of 10 mm and 6 mm, with a ball diameter ratio of 10:20 and a ball-to-material ratio of 20:1. After sealing the ball mill jar, install it on a high-speed vibrating ball mill according to the equipment operating requirements. Set the ball milling program as follows: vibration frequency 50 Hz, run for 0.5 h, then stop and cool for 1 h, then run in the same direction for 0.5 h, stop for 1 h, and repeat this cycle 10 times (total effective ball milling time 5 h). After ball milling, let the ball mill jar stand for 48 h to reduce internal activity, and then safely open the ball mill jar in the glove box to obtain black phosphorus powder.

[0028] 3. Preparation of Nitrogen-Containing Heterocyclic Borate Modified Black Phosphorus Nanosheet Dispersion 150 mg of black phosphorus powder was weighed and added to 110 mL of NMP. Under a nitrogen atmosphere, the mixture was magnetically stirred for 30 min, then dispersed and exfoliated using a cell disruptor for 180 h. The cell disruptor settings were: 450 W, 3 s operation, and 3 s sonication. Stable black phosphorus nanosheets were obtained. Subsequently, 50 mL of nitrogen-containing heterocyclic borate ester was added, and the mixture was sonicated at a frequency of 35-45 kHz for 6 h to obtain a suspension, maintaining the water temperature below 30 °C. The suspension was divided into four centrifuge tubes and centrifuged at 11000 rpm for 15 min (40 mL per tube) to obtain the precipitate. After removing the supernatant, 10 mL of anhydrous ethanol was added to each of the four centrifuge tubes for washing. The precipitate was then collected into two new centrifuge tubes (20 mL of nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheet dispersion per tube), and centrifuged again for 10 min to remove residual NMP. The nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheet dispersion was obtained.

[0029] 4. Preparation of nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheet lubricating oil additive 20 mL of anhydrous tert-butanol was added to the dispersion of nitrogen-containing heterocyclic borate-modified black phosphorus nanosheets in a glove box to resuspend the sample. The sample was then centrifuged under the same conditions, and this process was repeated twice. The final suspension was transferred to a lyophilization bottle, rapidly pre-frozen at -80 °C or liquid nitrogen, and then placed in a freeze dryer. The sample was freeze-dried for 48 h under a vacuum condition of -20 °C to -10 °C and below 0.1 mbar, until the solvent completely sublimated. Immediately after freeze-drying, the obtained nitrogen-containing heterocyclic borate-modified black phosphorus nanosheet powder was collected under an inert atmosphere and stored in a sealed drying tube sealed with an inert gas.

[0030] Combination Figures 1-3 It can analyze and characterize the structure of intermediates and target products. Figure 1 FT-IR (a) of intermediate BTPP and 1 1H NMR (b) data spectrum: located at 3389 cm⁻¹ in FT-IR -1 With 1243 cm -1 The absorption peaks at these locations correspond to the stretching and bending vibrations of the OH functional group, respectively. The chemical shift in the 1H NMR spectrum is located at δ 2.60–2.66 ppm, indicating the introduction of a hydroxyl functional group into the structure. The absorption peaks at 1607 cm⁻¹ correspond to the stretching and bending vibrations of the OH functional group, respectively. -1 With 747 cm -1 The absorption peaks correspond to the stretching vibrations of the aromatic ring CH and C=N in the benzothiazole group, respectively.

[0031] Figure 2Data map of intermediate DOBC: located at 3011 cm -1 With 1615 cm -1 The absorption peaks correspond to the CH stretching vibration peak and the C=C bending vibration peak in the -C=C- structure, respectively, which correspond to the chemical shifts in the 1H NMR spectrum located at δ 5.30-5.38 ppm. (1716 cm⁻¹) -1 The absorption peak at 1265 cm⁻¹ corresponds to the stretching vibration of C=O in the heterocyclic structure. -1 The absorption peak at the specified location corresponds to the stretching vibration of the CO bond and also matches the CO chemical shift on the heterocycle in the 1H NMR spectrum. In the structural characterization of NB-BTA, the absorption peaks and NMR signals corresponding to the aforementioned characteristic functional groups are all preserved and reflected, indicating that the intermediate BTPP reacts with DOBC and successfully generates the target nitrogen-containing heterocyclic borate ester structure. Example

[0032] The difference between this embodiment and Example 1 is that the volume of NMP in step 3 is 120 mL and the volume of nitrogen-containing heterocyclic borate ester is 40 mL. The other steps are the same as in Example 1, and nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheets are prepared. Example

[0033] The difference between this embodiment and Example 1 is that the volume of NMP in step 3 is 100 mL and the volume of nitrogen-containing heterocyclic borate ester is 60 mL. The other steps are the same as in Example 1, and nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheets are prepared. Example

[0034] The NB-BTA / BP powder obtained in Example 1 was added to transformer oil at a mass fraction of 0.5 wt.%, and after magnetic stirring for 30 min, it was ultrasonically treated for 30 min to make it uniformly and stably distributed to obtain a lubricating oil sample. Example

[0035] The NB-BTA / BP powder obtained in Example 1 was added to transformer oil at a mass fraction of 1.0 wt.%, and after magnetic stirring for 30 min, it was ultrasonically treated for 30 min to make it uniformly and stably distributed to obtain a lubricating oil sample. Example

[0036] The NB-BTA / BP powder obtained in Example 1 was added to transformer oil at a mass fraction of 1.5 wt.%, and after magnetic stirring for 30 min, it was ultrasonically treated for 30 min to make it uniformly and stably distributed to obtain a lubricating oil sample.

[0037] Comparative Example 1 Transformer oil was used as a separate lubricant sample, without the addition of any nanomaterials or functional additives.

[0038] Comparative Example 2 Only 1.0 wt.% NB-BTA was added to the transformer oil, and the oil was fully dispersed and homogeneous by magnetic stirring and ultrasonic treatment to obtain a lubricating oil sample.

[0039] Comparative Example 3 Only 1.0 wt.% black phosphorus nanosheets were added to transformer oil, and the oil was fully dispersed and uniformly obtained by magnetic stirring and ultrasonic treatment.

[0040] Comparative Example 4 Black phosphorus nanosheets and nitrogen-containing heterocyclic borate esters were added to transformer oil, with a total addition amount of 1.0 wt.% and a mass ratio of 1:1. No surface modification or chemical bonding was performed; the oil was dispersed only through physical mixing to obtain the lubricating oil sample.

[0041] Comparative Example 5 A sulfur-phosphorus type extreme pressure anti-wear additive (1.0 wt.% ZDDP) was added to transformer oil, and a lubricating oil sample was obtained by magnetic stirring and ultrasonic treatment. Experimental Example 1: Determination of the dispersion stability of the lubricating oil sample prepared in Example 4 For the embodiments 4 The dispersion stability of the lubricating oil samples prepared in Examples 1, 3, and 4 was tested using the following method: The lubricating oil sample containing the additive was prepared as described in Example 4. The sample was placed in a transparent glass bottle, and the system was uniformly dispersed by ultrasonic and magnetic stirring. Then, it was allowed to stand at room temperature for a certain period (e.g., 48-196 h). During the standing period, the dispersion stability of the additive in the lubricating oil was determined by observing whether obvious stratification, precipitation, or aggregation of suspended particles occurred.

[0042] Result: As Figure 7 As shown in Example 4, the nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets of the present invention exhibit excellent dispersion stability in base oil: they remained uniform even after prolonged standing without stratification or sedimentation. In Comparative Example 2, the sample with only 1.0 wt.% nitrogen-containing heterocyclic borate ester added still showed good oil solubility and dispersion stability; in Comparative Example 3, the sample with only 1.0 wt.% black phosphorus nanosheets added showed obvious agglomeration and sedimentation after standing, indicating poor dispersion stability; in Comparative Example 4, the dispersibility was improved after physical compounding of black phosphorus nanosheets with nitrogen-containing heterocyclic borate ester, but slight stratification was still observed, indicating that physical compounding can only partially improve the dispersion stability of black phosphorus nanosheets in oil, while the composite material modified by the present invention showed the best effect.

[0043] Experimental Example 2: Lubrication Performance Testing of Nitrogen-Containing Heterocyclic Borate Modified Black Phosphorus Nanosheets (Examples 4-6) The frictional properties of the lubricating oil samples prepared in Examples 4-6 and Comparative Examples 1-5 were tested. The maximum non-seize load (PB value) was tested using a four-ball friction testing machine (MS-10J type). The test conditions were as follows: the load was increased stepwise according to the procedure until obvious seizing or welding of the steel balls occurred; the rotation speed was 1760 r / min; the test time was 10 s; and the test temperature was room temperature (or 25℃). The standard test steel ball was a GCr15 bearing steel ball with a specification of 12.700 mm ± 0 (-0.2) μm, and the standard followed was GB / T3142-2019 "Determination of Lubricant Load Capacity - Four-Ball Method". Before the experiment, the steel balls, oil cups, and other components were ultrasonically cleaned using a mixture of anhydrous ethanol and petroleum ether. The samples were installed according to the operating procedure, and each group of tests was completed sequentially.

[0044] Results Analysis: The extreme pressure test results of the four-ball experiment show that ( Figure 8 The extreme pressure values ​​of the experimental groups showed significant differences: the base oil (Comparative Example 1) had the lowest extreme pressure value, at only 353 N; the addition of nitrogen-containing heterocyclic borate ester alone (Comparative Example 2) increased it to 667 N, and the addition of black phosphorus nanosheets alone (Comparative Example 3) increased it to 883 N, indicating that the extreme pressure properties of black phosphorus nanosheets are higher than those of nitrogen-containing heterocyclic borate ester additives. The extreme pressure value of black phosphorus nanosheets and nitrogen-containing heterocyclic borate esters, which were only dispersed by physical mixing (Comparative Example 4) without surface modification or chemical bonding, was 932 N when added to the base oil. Comparative Example 5 was the commercially available sulfur-phosphorus type extreme pressure anti-wear additive ZDDP, whose extreme pressure anti-wear ability was at a medium level, showing a significant difference compared to the extreme pressure anti-wear performance of other comparative examples and examples. In contrast, the additives NB-BTA / BP exhibited superior extreme pressure performance, achieving extreme pressure values ​​of 1167 N, 1294 N, and 1096 N at addition amounts of 0.5 wt.%, 1.0 wt.%, and 1.5 wt.% (Examples 4-6), respectively, all significantly higher than those in Comparative Examples 1-5. The highest extreme pressure value was observed at an addition amount of 1.0 wt.%, representing an improvement of approximately 72.7% compared to the base oil, and approximately 48.5% and 31.7% compared to NB-BTA and BP, respectively. These results demonstrate that the nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets of this invention can significantly enhance the extreme pressure carrying capacity of base oils.

[0045] Experimental Example 3: Tribological property testing of nitrogen-containing heterocyclic borate modified black phosphorus nanosheets (Examples 4-6) Long-term friction performance tests were conducted on the lubricating oil samples prepared in Examples 4-6 and Comparative Examples 1-5. The friction coefficient was tested using the same friction testing machine as in Experimental Example 2. The test conditions were: load 392 N, rotation speed 1200 r / min, time 60 min, and test temperature 75 ℃. After the test, the change in friction coefficient over time was recorded to evaluate the frictional stability of the lubricating oil. The standard followed was SH / T 0202-2014 "Determination of Anti-wear Properties of Lubricating Oils - Four-ball Method".

[0046] Results analysis: Examples 4-6 consistently exhibited significantly lower coefficients of friction than the comparative examples throughout the entire testing process. Figure 9 Among them, the steady-state friction coefficients of Examples 4, 5, and 6 were approximately 0.52, 0.55, and 0.59, respectively (corresponding to curves generally falling within the low friction range of approximately 0.05-0.06), and the friction curves showed small fluctuations and gradual changes over time, without obvious frictional instability or sudden increases, indicating good friction reduction effect and operational stability during long-term friction. In contrast, the friction coefficients of the comparative samples were generally at a higher level, with average friction coefficients of approximately 0.101, 0.82, 0.89, 0.80, and 0.87, respectively. The curve changes show that the comparative samples generally exhibited large fluctuations during friction, with some samples showing an increase or significant fluctuation in friction coefficient in the middle and later stages, reflecting insufficient lubricant film stability and difficulty in forming a durable and effective friction-reducing protective layer at the friction interface.

Claims

1. A nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheet for use as a lubricating oil additive, characterized in that, Nitrogen-containing heterocyclic borate esters are grafted onto the surface of black phosphorus nanosheets via POB coordination bonds in an organic-inorganic hybrid manner. The structural formula of the nitrogen-containing heterocyclic borate ester is shown in Formula I: .

2. The nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheets according to claim 1, characterized in that, The black phosphorus nanosheets have a diameter of 50-100 nm and a thickness of 10-20 nm.

3. The nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets for use as a lubricating oil additive according to claim 1, characterized in that, The method for preparing the nitrogen-containing heterocyclic borate ester is as follows: BTPP and DOBC are dissolved in xylene, heated to 110-130 °C under nitrogen protection and refluxed for 3-5 h. After the reaction is completed, the mixture is cooled to room temperature, the solvent is removed by rotary evaporation, and the mixture is washed with an organic solvent. Then, it is purified by silica gel column chromatography to obtain the target product. The product is dried in a vacuum drying oven at 40-60 °C to obtain the nitrogen-containing heterocyclic borate ester shown in Formula I. The molar ratio of BTPP to DOBC is 1:

1.

4. The nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets for use as a lubricating oil additive according to claim 3, characterized in that, Silica gel column chromatography was performed using gradient elution with petroleum ether / ethyl acetate at a volume ratio of 40:1 to 10:

1.

5. The method for preparing nitrogen-containing heterocyclic borate ester-modified black phosphorus nanosheets for use as a lubricating oil additive according to claim 1, characterized in that, Follow these steps to achieve the following: 1) Black phosphorus powder was added to N-methylpyrrolidone solvent to prepare a dispersion with a concentration of 0.8-2.0 mg / mL. Then, nitrogen-containing heterocyclic borate ester was added to make the mass-volume ratio of nitrogen-containing heterocyclic borate ester to black phosphorus nanosheets 1.0-1.8 mg / mL. The dispersion was obtained by ultrasonic reaction at 10-30℃. 2) The nitrogen-containing heterocyclic borate modified black phosphorus dispersion was replaced with tert-butanol and then freeze-dried to obtain nitrogen-containing heterocyclic borate modified black phosphorus nanosheet powder.

6. The preparation method according to claim 5, characterized in that, In step 1), the ultrasonic treatment duration is 4-8 hours and the ultrasonic frequency is 35-45 kHz.

7. The preparation method according to claim 5, characterized in that, In step 2), the freeze-drying conditions are as follows: after rapid pre-freezing at -80℃ or liquid nitrogen, the freeze dryer is placed in the freeze dryer and freeze-dried for 24-48 hours under vacuum conditions of gradually increasing the temperature to -20℃ to -10℃ and below 0.1 mbar.

8. The application of the nitrogen-containing heterocyclic borate ester modified black phosphorus nanosheets as an additive in the preparation of lubricating oils according to claim 1.

9. The application according to claim 8, characterized in that, The base oil of the lubricating oil is transformer oil or mineral oil.

10. The application according to claim 8, characterized in that, The nitrogen-containing heterocyclic borate modified black phosphorus nanosheets have a mass percentage of 0.5-1.5% in the lubricating oil.

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