Amphiphilic nitrogen-doped coal-based carbon dots and preparation method thereof

Abstract

The invention provides an amphiphilic nitrogen-doped coal-based carbon dot and a preparation method thereof, and belongs to the technical field of carbon nanomaterials. The method comprises the following steps: performing oxidation-depolymerization treatment on coal by using a mixed solution of formic acid and hydrogen peroxide to obtain coal-based carbon dots; uniformly dispersing the coal-based carbon dots and polyamine in a solvent 1, and then carrying out hydrothermal reaction 1 to obtain nitrogen-doped coal-based carbon dots; dissolving the nitrogen-doped coal-based carbon dots in a solvent 2, and then adding halogenated alkane for reaction 2 to obtain an intermediate product; the intermediate product is dissolved in a solvent 3, then a catalyst and a sulfonating agent are added for a reaction 3, and the amphiphilic nitrogen-doped coal-based carbon dots are obtained. The coal-based carbon dots have controllable hydrophilic and hydrophobic structures, excellent surface activity and interfacial activity, excellent dispersion stability and ultralow interfacial tension in a high-salinity oil-water system, and can meet the actual application requirements in the fields of oil-water interface regulation and control, solid-liquid interface wetting regulation and control and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon nanomaterials, and in particular to amphiphilic nitrogen-doped coal-based carbon dots and a preparation method thereof. Background Art

[0002] Carbon dots (Cdots) are a class of carbon-based nanomaterials smaller than 10 nm in size that exhibit excellent photoluminescence, good chemical stability, and low biotoxicity. Due to their simple preparation methods, abundant raw materials, and environmental friendliness, Cdots hold great promise for applications in bioimaging, sensing, photocatalysis, energy storage, and environmental remediation. In recent years, coal, as an inexpensive and abundant carbon source, has been widely used to prepare low-cost Cdots. Bituminous coal, in particular, possesses a rich aromatic carbon skeleton and naturally occurring oxygen-containing functional groups, providing an excellent structural foundation for the preparation of high-performance Cdots.

[0003] However, traditional coal-based carbon dot preparation methods usually rely on strong acid oxidation systems (such as nitric acid, sulfuric acid, etc.), which have problems such as harsh reaction conditions, high energy consumption, and serious environmental pollution. The resulting coal-based carbon dots are mainly hydrophilic surfaces, with a single functional structure, limited interfacial activity and dispersibility, and are difficult to meet the application requirements in complex oil-water environments. In order to improve the functionality of coal-based carbon dots, the existing technology mainly regulates the optical properties and reactivity of carbon dots by doping with heterogeneous elements (such as nitrogen, sulfur, phosphorus) or modifying surface functional groups (such as carboxyl and hydroxyl groups). However, these modification methods often only improve the hydrophilicity of carbon dots, and have limited improvement in the stable dispersion ability in non-polar media or oil-water interface environments. At present, there is no literature reporting the use of coal resources to prepare carbon dots with precise amphiphilic control, and can achieve ultra-low interfacial tension in high-salt oil-water environments, while having excellent solid-liquid interface wettability control. The development of this new material is of great significance for expanding the application field of coal-based carbon dots and realizing high-value-added green transformation of coal resources.

[0004] Therefore, there is an urgent need to develop an amphiphilic coal-based carbon dot material with adjustable surface structure, excellent interfacial activity and good dispersion stability to meet the practical application needs in fields such as oil-water interface regulation and solid-liquid interface wetting regulation. Summary of the Invention

[0005] The present invention aims to overcome the shortcomings of existing coal-based carbon dots, such as their limited functionality, insufficient interfacial activity, and poor salt tolerance and stability, by providing amphiphilic nitrogen-doped coal-based carbon dots and a method for preparing them. The coal-based carbon dots provided by the present invention possess a controllable hydrophilic-hydrophobic structure, excellent surface and interfacial activity, exhibit excellent dispersion stability, and exhibit ultra-low interfacial tension in high-salinity oil-water systems, meeting practical application requirements in fields such as oil-water interface regulation and solid-liquid interface wettability control.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention is a method for preparing amphiphilic nitrogen-doped coal-based carbon dots, comprising the following steps:

[0008] Coal was oxidized and depolymerized using a mixed solution of formic acid and hydrogen peroxide to obtain coal-based carbon dots.

[0009] The coal-based carbon dots and polyamine are uniformly dispersed in a solvent 1, and then a hydrothermal reaction 1 is performed to obtain nitrogen-doped coal-based carbon dots;

[0010] Dissolving the nitrogen-doped coal-based carbon dots in solvent 2, and then adding a halogenated alkane to carry out reaction 2 to obtain an intermediate product;

[0011] The intermediate product is dissolved in solvent 3, and then a catalyst and a sulfonating agent are added to carry out reaction 3 to obtain the amphiphilic nitrogen-doped coal-based carbon dots.

[0012] In a preferred embodiment of the present invention, the coal is anthracite, coking coal, lignite or bituminous coal; before the coal is oxidized and depolymerized using a mixed solution of formic acid and hydrogen peroxide, the process further includes a step of crushing and screening the coal; the screening is through a 200-mesh sieve.

[0013] In a preferred embodiment of the present invention, the volume ratio of formic acid to hydrogen peroxide is 5:1. The oxidation-depolymerization treatment of coal using the formic acid and hydrogen peroxide mixed solution specifically comprises uniformly dispersing the coal in the formic acid and hydrogen peroxide mixed solution and stirring the mixture at 400-800 rpm at room temperature for 16-20 hours. The present invention does not impose any particular limitation on the method for uniformly dispersing the coal in the formic acid and hydrogen peroxide mixed solution; methods well known to those skilled in the art, such as ultrasound, may be used.

[0014] In a preferred embodiment of the present invention, the mass volume ratio of the coal to the mixed solution of formic acid and hydrogen peroxide is 0.2-0.3 g:30-45 mL.

[0015] In a preferred embodiment of the present invention, after the reaction is completed by stirring at room temperature, the step of centrifuging the obtained reaction solution, collecting the supernatant, and concentrating the supernatant to remove the solvent is further included.

[0016] In a preferred embodiment of the present invention, the polyamine is ethylenediamine, diethylenetriamine or triethylenetetramine; the mass ratio of the coal-based carbon dots to the polyamine is 1:10; the temperature of the hydrothermal reaction 1 is 180-220° C., and the time is 8-14 h.

[0017] In a preferred embodiment of the present invention, the solvent 1 is water. The present invention does not impose any particular limitation on the amount of the solvent 1, as long as the amount is sufficient for the hydrothermal reaction 1 to proceed.

[0018] In a preferred embodiment of the present invention, after the hydrothermal reaction 1 is completed, the step of dialyzing the resulting reaction solution and evaporating the solvent is further included. The dialysis is specifically performed through a dialysis bag with a molecular weight cut-off of 500-1000 Da for 24-48 hours.

[0019] In a preferred embodiment of the present invention, the halogenated alkane is a halogenated dodecane, a halogenated tetradecane, a halogenated hexadecane or a halogenated octadecane, and the halogen element in the halogenated alkane is a chlorine element or a bromine element; the molar ratio of the halogenated alkane to the polyamine is 1:1; the temperature of the reaction 2 is 80-100° C., and the time is 5-10 hours.

[0020] In a preferred embodiment of the present invention, the halogenated alkane is hexadecane chloride, octadecane chloride, hexadecane bromide or octadecane bromide.

[0021] In a preferred embodiment of the present invention, the solvent 2 is anhydrous ethanol. The present invention does not impose any particular limitation on the amount of the solvent 2, as long as the amount is sufficient for reaction 2 to proceed.

[0022] In a preferred embodiment of the present invention, after the reaction 2 is completed, the steps of collecting the organic phase from the reaction solution, purifying the organic phase, and removing the solvent are further included. The steps of collecting the organic phase from the reaction solution and purifying the organic phase specifically include: mixing the reaction solution with water, allowing the mixture to stand for separation, collecting the organic phase, and then adding n-hexane to the organic phase for extraction.

[0023] In a preferred embodiment of the present invention, the catalyst is sodium hydroxide or urea; the amount of the catalyst is 3% to 5% of the total mass of the intermediate product and the sulfonating agent; the sulfonating agent is 1,3-propane sultone or aminosulfonic acid; the mass ratio of the sulfonating agent to the intermediate product is 3:1 to 5:1; the temperature of reaction 3 is 90 to 100° C., and the time is 6 to 8 hours.

[0024] In the present invention, when the sulfonating agent is 1,3-propane sultone, the catalyst is sodium hydroxide; when the sulfonating agent is aminosulfonic acid, the catalyst is urea.

[0025] In a preferred embodiment of the present invention, the intermediate product is dissolved in solvent 3, the reaction system is first heated to 75-85° C., and then a catalyst and a sulfonating agent are added to carry out reaction 3.

[0026] In a preferred embodiment of the present invention, the solvent 3 is methanol. The present invention does not impose any particular limitation on the amount of the solvent 3, as long as the amount is sufficient for reaction 3 to proceed.

[0027] In a preferred embodiment of the present invention, after the reaction 3 is completed, the process further includes neutralizing the obtained reaction solution, and then dialyzing and drying the neutralized reaction solution in sequence; the dialysis is performed in a 1000-1500Da dialysis bag for 12-36 hours.

[0028] The second technical solution of the present invention is an amphiphilic nitrogen-doped coal-based carbon dot prepared by the above preparation method.

[0029] The present invention discloses the following technical effects:

[0030] The amphiphilic nitrogen-doped coal-based carbon dots prepared by the present invention have excellent surface activity, interfacial activity and dispersion stability, and are suitable for the following fields:

[0031] (1) Oil-water interface control

[0032] The amphiphilic nitrogen-doped coal-based carbon dot material of the present invention can be rapidly adsorbed and arranged at the oil-water interface, significantly reducing the interfacial tension, especially maintaining ultra-low interfacial tension in a high-salt environment. It is suitable for use in the fields of emulsification, interface stabilization, and enhanced recovery in oil and gas field exploitation and oil field injection and production projects.

[0033] (2) Solid-liquid interface wetting control

[0034] The amphiphilic nitrogen-doped coal-based carbon dot material of the present invention can significantly reduce the contact angle of hydrophobic surfaces and improve the wetting performance of water phase on oily or hydrophobic surfaces. It is suitable for use in fields such as antifouling coatings, interface modifiers, and oil-water separation membrane materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] Figure 1 TEM image of the amphiphilic nitrogen-doped coal-based carbon dots prepared in Example 4;

[0037] Figure 2 is the particle size distribution of the amphiphilic nitrogen-doped coal-based carbon dots prepared in Example 4;

[0038] Figure 3 This is the UV spectrum of the amphiphilic nitrogen-doped coal-based carbon dots prepared in Example 4;

[0039] Figure 4 This is the fluorescence spectrum of the amphiphilic nitrogen-doped coal-based carbon dots prepared in Example 4. DETAILED DESCRIPTION

[0040] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0041] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0042] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0043] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0044] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0045] The present invention is based on the three-dimensional aromatic skeleton and oxygen-containing functional groups naturally present in coal. Through mild and green formic acid / hydrogen peroxide oxidation-depolymerization, sp-rich coal-based skeleton is converted into 2The advantages of the conjugated structure and multiple defect sites are fully preserved. Subsequently, nitrogen heterocycles and amino functional groups are introduced under hydrothermal conditions, significantly improving the polarity and surface reactivity of the carbon dots. Through controlled long-chain alkylation and sulfonate group modification, an interface structure with both hydrophilic and hydrophobic properties is constructed at the nanoscale. Thanks to the π-electron conjugated system provided by the inherent aromatic core of the coal-based carbon dots and the multiple active sites after doping, these amphiphilic nitrogen-doped coal-based carbon dots can rapidly self-assemble into stable micelles in aqueous solution under the synergistic action of multiple mechanisms such as strong hydrophobic chain anchoring, nitrogen heterocycle-induced π-π stacking, and electrostatic repulsion formed by sulfonate groups. This significantly reduces surface tension and oil-water interfacial tension, demonstrating super salt-resistant interfacial stability. At the same time, they have the characteristics of rapid wetting and spreading on solid hydrophobic surfaces, making them show great potential in fields such as antifouling coatings and oil-water interface treatment.

[0046] The preparation method of amphiphilic nitrogen-doped coal-based carbon dots in the present invention comprises the following steps:

[0047] Step 1: Preparation of coal-based carbon dots

[0048] Coal was pulverized and passed through a 200-mesh sieve. 0.2-0.3 g of coal powder was weighed and added to 30-45 mL of a mixture of formic acid and hydrogen peroxide (5:1 by volume). The mixture was ultrasonically shaken (40 kHz) for 30-60 minutes to fully disperse the coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 400-800 rpm for 16-20 hours at room temperature. After the reaction, unreacted coal powder was removed by centrifugation (10,000-12,000 rpm for 10-20 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene membrane, and the resulting solution was rotary evaporated to remove the solvent, yielding coal-based carbon dots.

[0049] Step 2: Preparation of nitrogen-doped coal-based carbon dots

[0050] The coal-based carbon dots and polyamine were mixed at a mass ratio of 1:10, ultrasonically dispersed in 50-70 mL of deionized water, and then transferred to a polytetrafluoroethylene-lined autoclave. The mixture was hydrothermally reacted at 180-220°C for 8-14 hours. The resulting solution was dialyzed through a dialysis bag with a molecular weight cutoff of 500-1000 Da for 24-48 hours, and then rotary evaporated to remove water, yielding nitrogen-doped coal-based carbon dots.

[0051] Step 3: Preparation of amphiphilic nitrogen-doped coal-based carbon dots

[0052] Dissolve nitrogen-doped coal-based carbon dots in 30-45 mL of anhydrous ethanol, add an equimolar amount of halogenated alkane to the polyamine, and stir at 80-100°C for 5-10 hours. After the reaction, transfer the mixture to a separatory funnel, add 20-30 mL of distilled water, shake thoroughly, and allow the mixture to separate. The lower organic phase is retained. For further purification, add 5-10 mL of n-hexane to the organic phase, repeat the extraction process two to four times, and then remove the solvent under reduced pressure at 60-70°C to obtain the intermediate product.

[0053] The resulting intermediate product is dissolved in 30-45 mL of methanol and heated to 75-85°C. A sulfonating agent and catalyst are then added (the mass ratio of sulfonating agent to intermediate product is 3:1-5:1, and the catalyst dosage is 3%-5% of the total mass of the intermediate product and sulfonating agent). The mixture is then heated to 90-100°C and stirred for 6-8 hours. After the reaction is complete, the system is cooled naturally and neutralized by adding sodium hydroxide in an amount equimolar to the sulfonating agent. The product is dialyzed in a 1000-1500 Da dialysis bag for 12-36 hours and then dried in a vacuum desiccator to obtain the target amphiphilic nitrogen-doped coal-based carbon dots.

[0054] The polyamine is ethylenediamine, diethylenetriamine or triethylenetetramine;

[0055] The halogenated alkane is a halogenated dodecane, a halogenated tetradecane, a halogenated hexadecane or a halogenated octadecane, and the halogen element in the halogenated alkane is a chlorine element or a bromine element;

[0056] The catalyst is sodium hydroxide or urea; the sulfonating agent is 1,3-propane sultone or aminosulfonic acid; when the sulfonating agent is 1,3-propane sultone, the catalyst must be sodium hydroxide; when the sulfonating agent is aminosulfonic acid, the catalyst must be urea.

[0057] Unless otherwise specified, the technical solutions described in the present invention are all conventional solutions in the field, and the reagents or raw materials used, unless otherwise specified, are purchased from commercial channels or have been disclosed.

[0058] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with the examples, but the content of the present invention is not limited to the following examples.

[0059] Example 1

[0060] Step 1: Preparation of coal-based carbon dots

[0061] Coal was pulverized and passed through a 200-mesh sieve. 0.2 g of coal powder was weighed and added to 30 mL of a mixture of formic acid and hydrogen peroxide (5:1 by volume). The mixture was ultrasonically shaken (40 kHz) for 30 minutes to fully disperse the coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 400 rpm for 16 hours at room temperature. After the reaction, unreacted coal powder was removed by centrifugation (10,000 rpm for 10 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene membrane, and the resulting solution was rotary evaporated to remove the solvent, yielding coal-based carbon dots.

[0062] Step 2: Preparation of nitrogen-doped coal-based carbon dots

[0063] The coal-based carbon dots and polyamine were mixed at a mass ratio of 1:10, ultrasonically dispersed in 50 mL of deionized water, and then transferred to a polytetrafluoroethylene-lined autoclave. The mixture was hydrothermally reacted at 180°C for 8 hours. The resulting solution was dialyzed for 24 hours using a 500 Da molecular weight cutoff (MWCO) tubing. The water was then removed by rotary evaporation to yield nitrogen-doped coal-based carbon dots.

[0064] Step 3: Preparation of amphiphilic nitrogen-doped coal-based carbon dots

[0065] Nitrogen-doped coal-based carbon dots were dissolved in 30 mL of anhydrous ethanol, and an equimolar amount of halogenated alkane was added to the polyamine. The mixture was stirred at 80°C for 5 h. After completion of the reaction, the mixture was transferred to a separatory funnel, 20 mL of distilled water was added, and the mixture was thoroughly shaken and allowed to stand for separation. The lower organic phase was retained. For further purification, 5 mL of n-hexane was added to the organic phase, and the extraction was repeated twice. The solvent was then removed by vacuum distillation at 60°C to obtain the intermediate product.

[0066] The resulting intermediate product was dissolved in 30 mL of methanol and heated to 75°C. A sulfonating agent and catalyst were added (the mass ratio of sulfonating agent to intermediate product was 3:1, and the catalyst dosage was 3% of the total mass of the intermediate and sulfonating agent). The reaction was continued by stirring at 90°C for 6 hours. After the reaction was completed, the system was allowed to cool naturally, and sodium hydroxide (equimolar to the sulfonating agent) was added for neutralization. The product was dialyzed in a 1000 Da dialysis bag for 12 hours and then dried in a vacuum dessicator to obtain the target product, amphiphilic nitrogen-doped coal-based carbon dots.

[0067] The coal in step 1 is anthracite;

[0068] The polyamine in step 2 is ethylenediamine;

[0069] The halogenated alkane in step 3 is hexachlorodecane;

[0070] The sulfonating agent and catalyst in the step three are 1,3-propane sultone and sodium hydroxide respectively.

[0071] Example 2

[0072] Step 1: Preparation of coal-based carbon dots

[0073] Coal was pulverized and passed through a 200-mesh sieve. 0.3 g of coal powder was weighed and added to 30 mL of a mixture of formic acid and hydrogen peroxide (5:1 by volume). The mixture was ultrasonically shaken (40 kHz) for 60 minutes to fully disperse the coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 800 rpm for 20 hours at room temperature. After the reaction, unreacted coal powder was removed by centrifugation (12,000 rpm for 20 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene membrane, and the resulting solution was rotary evaporated to remove the solvent, yielding coal-based carbon dots.

[0074] Step 2: Preparation of nitrogen-doped coal-based carbon dots

[0075] The coal-based carbon dots and polyamine were mixed at a mass ratio of 1:10, ultrasonically dispersed in 70 mL of deionized water, and then transferred to a polytetrafluoroethylene-lined autoclave. The mixture was hydrothermally reacted at 220°C for 14 hours. The resulting solution was dialyzed for 48 hours using a 1000 Da molecular weight cutoff (MWCO) dialysis bag. The water was then removed by rotary evaporation to yield nitrogen-doped coal-based carbon dots.

[0076] Step 3: Preparation of amphiphilic nitrogen-doped coal-based carbon dots

[0077] Nitrogen-doped coal-based carbon dots were dissolved in 45 mL of anhydrous ethanol, and an equimolar amount of halogenated alkane was added to the polyamine. The mixture was stirred at 100°C for 10 hours. After completion of the reaction, the mixture was transferred to a separatory funnel, 30 mL of distilled water was added, and the mixture was thoroughly shaken and allowed to stand for separation. The lower organic phase was retained. For further purification, 10 mL of n-hexane was added to the organic phase, and the extraction was repeated four times. The solvent was then removed by vacuum distillation at 70°C to obtain the intermediate product.

[0078] The resulting intermediate product was dissolved in 45 mL of methanol and heated to 85°C. A sulfonating agent and catalyst were added (the mass ratio of sulfonating agent to intermediate product was 5:1, and the catalyst dosage was 5% of the total mass of the intermediate and sulfonating agent). The reaction was continued by stirring at 100°C for 8 hours. After the reaction was completed, the system was allowed to cool naturally, and sodium hydroxide was added in an amount equimolar to the sulfonating agent for neutralization. The product was dialyzed in a 1500 Da dialysis bag for 36 hours and then dried in a vacuum dessicator to obtain the target product, amphiphilic nitrogen-doped coal-based carbon dots.

[0079] The coal in step 1 is coking coal;

[0080] The polyamine in step 2 is diethylenetriamine;

[0081] The halogenated alkane in step 3 is octadecane bromide;

[0082] The sulfonating agent and catalyst in the step 3 are aminosulfonic acid and urea respectively.

[0083] Example 3

[0084] Step 1: Preparation of coal-based carbon dots

[0085] Coal was pulverized and passed through a 200-mesh sieve. 0.25 g of coal powder was weighed and added to 37.5 mL of a mixture of formic acid and hydrogen peroxide (5:1 by volume). The mixture was ultrasonically shaken (40 kHz) for 45 minutes to fully disperse the coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 600 rpm for 18 hours at room temperature. After the reaction, unreacted coal powder was removed by centrifugation (11,000 rpm for 15 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene membrane, and the resulting solution was rotary evaporated to remove the solvent, yielding coal-based carbon dots.

[0086] Step 2: Preparation of nitrogen-doped coal-based carbon dots

[0087] The coal-based carbon dots and polyamine were mixed at a mass ratio of 1:10, ultrasonically dispersed in 60 mL of deionized water, and then transferred to a polytetrafluoroethylene-lined autoclave. The mixture was hydrothermally reacted at 200°C for 11 hours. The resulting solution was dialyzed for 36 hours using a 500 Da molecular weight cutoff (MWCO) dialysis bag. The water was then removed by rotary evaporation to yield nitrogen-doped coal-based carbon dots.

[0088] Step 3: Preparation of amphiphilic nitrogen-doped coal-based carbon dots

[0089] Nitrogen-doped coal-based carbon dots were dissolved in 37.5 mL of anhydrous ethanol, and an equimolar amount of halogenated alkane was added to the polyamine. The mixture was stirred at 90°C for 7.5 hours. After the reaction, the mixture was transferred to a separatory funnel, 25 mL of distilled water was added, and the mixture was thoroughly shaken and allowed to stand for separation. The lower organic phase was retained. For further purification, 7.5 mL of n-hexane was added to the organic phase, and the extraction was repeated three times. The solvent was then removed by vacuum distillation at 65°C to obtain the intermediate product.

[0090] The resulting intermediate product was dissolved in 37.5 mL of methanol and heated to 80°C. A sulfonating agent and catalyst were added (the mass ratio of sulfonating agent to intermediate product was 4:1, and the catalyst dosage was 4% of the total mass of the intermediate and sulfonating agent). The mixture was heated to 95°C and stirred for 7 hours. After the reaction, the system was allowed to cool naturally, and sodium hydroxide (equimolar to the sulfonating agent) was added for neutralization. The product was dialyzed in a 1000 Da dialysis bag for 24 hours and then dried in a vacuum dessicator to obtain the target product, amphiphilic nitrogen-doped coal-based carbon dots.

[0091] The coal in step 1 is lignite;

[0092] The polyamine in step 2 is triethylenetetramine;

[0093] The halogenated alkane in step 3 is hexadecane bromide;

[0094] The sulfonating agent and catalyst in the step three are 1,3-propane sultone and sodium hydroxide respectively.

[0095] Example 4

[0096] Step 1: Preparation of coal-based carbon dots

[0097] Coal was pulverized and passed through a 200-mesh sieve. 0.2 g of coal powder was weighed and added to 30 mL of a mixture of formic acid and hydrogen peroxide (5:1 by volume). The mixture was ultrasonically shaken (40 kHz) for 30 minutes to fully disperse the coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 600 rpm for 18 hours at room temperature. After the reaction, unreacted coal powder was removed by centrifugation (10,000 rpm for 15 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene membrane, and the resulting solution was rotary evaporated to remove the solvent, yielding coal-based carbon dots.

[0098] Step 2: Preparation of nitrogen-doped coal-based carbon dots

[0099] The coal-based carbon dots and polyamine were mixed at a mass ratio of 1:10, ultrasonically dispersed in 65 mL of deionized water, and then transferred to a polytetrafluoroethylene-lined autoclave. The mixture was hydrothermally reacted at 200°C for 12 hours. The resulting solution was dialyzed for 24 hours using a 1000 Da molecular weight cutoff (MWCO) dialysis bag. The water was then removed by rotary evaporation to yield nitrogen-doped coal-based carbon dots.

[0100] Step 3: Preparation of amphiphilic nitrogen-doped coal-based carbon dots

[0101] Nitrogen-doped coal-based carbon dots were dissolved in 30 mL of anhydrous ethanol, and an equimolar amount of halogenated alkane was added to the polyamine. The mixture was stirred at 80°C for 5 h. After completion of the reaction, the mixture was transferred to a separatory funnel, 20 mL of distilled water was added, and the mixture was thoroughly shaken and allowed to stand for separation. The lower organic phase was retained. For further purification, 5 mL of n-hexane was added to the organic phase, and the extraction was repeated three times. The solvent was then removed by vacuum distillation at 60°C to obtain the intermediate product.

[0102] The resulting intermediate product was dissolved in 30 mL of methanol and heated to 80°C. A sulfonating agent and catalyst were added (the mass ratio of sulfonating agent to intermediate product was 5:1, and the catalyst dosage was 4% of the total mass of the intermediate and sulfonating agent). The reaction was continued by stirring at 90°C for 6 hours. After the reaction was completed, the system was allowed to cool naturally, and sodium hydroxide (equimolar to the sulfonating agent) was added for neutralization. The product was dialyzed in a 1000 Da dialysis bag for 12 hours and then dried in a vacuum dessicator to obtain the target product, amphiphilic nitrogen-doped coal-based carbon dots.

[0103] The coal in step 1 is bituminous coal;

[0104] The polyamine in step 2 is diethylenetriamine;

[0105] The halogenated alkane in step 3 is octadecane chloride;

[0106] The sulfonating agent and catalyst in the step 3 are aminosulfonic acid and urea respectively.

[0107] Comparative Example 1

[0108] The bituminous coal was crushed and passed through a 200-mesh sieve. 0.2 g of bituminous coal powder was weighed and added to a 30 mL mixed solution of formic acid and hydrogen peroxide (formic acid to hydrogen peroxide volume ratio of 5:1). The mixture was shaken under ultrasonic treatment (frequency 40 kHz) for 30 minutes to fully disperse the bituminous coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 600 rpm for 18 hours at room temperature. After the reaction was completed, the unreacted bituminous coal powder was removed by centrifugation (10,000 rpm, 15 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene filter membrane, and the resulting solution was rotary evaporated to remove the solvent to obtain coal-based carbon dots. (That is, this comparative example is the coal-based carbon dots prepared in step 1 of Example 4)

[0109] Comparative Example 2

[0110] Step 1: Preparation of coal-based carbon dots

[0111] Bituminous coal was pulverized and passed through a 200-mesh sieve. 0.2 g of bituminous coal powder was weighed and added to 30 mL of a mixture of formic acid and hydrogen peroxide (5:1 by volume). The mixture was ultrasonically shaken (40 kHz) for 30 minutes to fully disperse the bituminous coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 600 rpm for 18 hours at room temperature. After the reaction, unreacted bituminous coal powder was removed by centrifugation (10,000 rpm for 15 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene membrane, and the resulting solution was rotary evaporated to remove the solvent, yielding coal-based carbon dots.

[0112] Step 2: Preparation of nitrogen-doped coal-based carbon dots

[0113] The coal-based carbon dots were mixed with a polyamine (diethylenetriamine) at a mass ratio of 1:10, ultrasonically dispersed in 65 mL of deionized water, and then transferred to a polytetrafluoroethylene-lined autoclave. The mixture was hydrothermally reacted at 200°C for 12 hours. The resulting solution was dialyzed for 24 hours using a 1000 Da molecular weight cutoff dialysis bag. The water was then removed by rotary evaporation to obtain nitrogen-doped coal-based carbon dots. (This comparative example is based on the nitrogen-doped coal-based carbon dots prepared in step 2 of Example 4.)

[0114] Comparative Example 3

[0115] Step 1: Preparation of coal-based carbon dots

[0116] Coal was pulverized and passed through a 200-mesh sieve. 0.2 g of coal powder was weighed and added to 30 mL of a mixture of formic acid and hydrogen peroxide (5:1 by volume). The mixture was ultrasonically shaken (40 kHz) for 30 minutes to fully disperse the coal powder. The mixture was then transferred to a magnetic stirrer and stirred at 600 rpm for 18 hours at room temperature. After the reaction, unreacted coal powder was removed by centrifugation (10,000 rpm for 15 minutes). The supernatant was filtered through a 0.22 μm polytetrafluoroethylene membrane, and the resulting solution was rotary evaporated to remove the solvent, yielding coal-based carbon dots.

[0117] Step 2: Preparation of amphiphilic coal-based carbon dots

[0118] Coal-based carbon dots were dissolved in 30 mL of anhydrous ethanol, and a halogenated alkane (the same molar amount as the halogenated alkane (chlorooctadecane) in step 3 of Example 4) was added. The mixture was stirred at 80°C for 5 h. After completion of the reaction, the mixture was transferred to a separatory funnel, 20 mL of distilled water was added, and the mixture was shaken thoroughly before being allowed to stand and separate. The lower organic phase was retained. For further purification, 5 mL of n-hexane was added to the organic phase, and the extraction was repeated three times. The solvent was then removed by vacuum distillation at 60°C to obtain the intermediate product.

[0119] The obtained intermediate product was dissolved in 30 mL of methanol and heated to 80 ° C. Then, a sulfonating agent and a catalyst were added (the mass ratio of the sulfonating agent to the intermediate product was 5:1, and the amount of the catalyst was 4% of the total mass of the intermediate product and the sulfonating agent). The reaction was continued to be heated to 90 ° C and stirred for 6 hours. After the reaction was completed, the system was cooled naturally and sodium hydroxide was added in an amount equimolar to the sulfonating agent for neutralization. The product was dialyzed in a 1000 Da dialysis bag for 12 hours and then dried in a vacuum dryer to obtain amphiphilic coal-based carbon dots. (That is, the difference between this comparative example and Example 4 is that nitrogen doping is omitted)

[0120] The coal in step 1 is bituminous coal;

[0121] The halogenated alkane in step 2 is octadecane chloride;

[0122] The sulfonating agent and catalyst in the step 2 are aminosulfonic acid and urea respectively.

[0123] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the surface tension, interfacial tension, contact angle and Zeta potential of the carbon dot materials obtained in Examples 1-4 and Comparative Examples 1-3 were systematically tested. The amphiphilic nitrogen-doped coal-based carbon dots prepared in Example 4 were used as an example to perform TEM morphology observation, particle size distribution analysis, and UV absorption and fluorescence emission performance characterization. Specifically, the surface tension test was performed using a QBZY fully automatic surface tensiometer (Shanghai Fangrui Instrument Co., Ltd., China), the interfacial tension test was performed using a TX-500C interfacial tensiometer (CNG Enterprises Ltd., USA), the contact angle was measured using a JC2000C contact angle meter (Shanghai Zhongchen Digital Technology Equipment Co., Ltd., China), and the Zeta potential was measured using a nanoparticle size potentiometer (Malvern Panalytic, UK); the TEM test was performed using a JEM-2100 transmission electron microscope (JEOL, Japan), the UV-visible absorption spectrum was performed using a TU-1900 UV-visible spectrophotometer (Beijing Puxi General Instrument Co., Ltd., China), and the fluorescence spectrum was measured using a F-4000 fluorescence spectrophotometer (Hitachi, Japan). The results are shown in Tables 1-4 and Figure 1-4 As shown:

[0124] Table 1 Surface tension test data

[0125]

[0126]

[0127]

[0128] Table 2 Interfacial tension test data

[0129]

[0130]

[0131] Table 3 Contact angle test data

[0132] sample Sample dosage (mg / L) Experimental conditions Contact angle (°) Example 1 1000 25℃, let stand on paraffin film surface for 100s 47.01 Example 2 1000 25℃, let stand on paraffin film surface for 100s 46.56 Example 3 1000 25℃, let stand on paraffin film surface for 100s 45.89 Example 4 1000 25℃, let stand on paraffin film surface for 100s 44.63 Comparative Example 1 1000 25℃, let stand on paraffin film surface for 100s 96.53 Comparative Example 2 1000 25℃, let stand on paraffin film surface for 100s 92.01 Comparative Example 3 1000 25℃, let stand on paraffin film surface for 100s 80.62

[0133] Table 4 Zeta potential test data

[0134] sample Sample dosage (mg / L) Experimental conditions Zeta potential (mV) Example 1 1000 25℃ -39.2 Example 2 1000 25℃ -40.9 Example 3 1000 25℃ -41.6 Example 4 1000 25℃ -42.4 Comparative Example 1 1000 25℃ -10.9 Comparative Example 2 1000 25℃ -15.2 Comparative Example 3 1000 25℃ -26.5

[0135] From the test data of Examples 1-4 and Comparative Examples 1-3 in Table 1, it can be seen that the amphiphilic nitrogen-doped coal-based carbon dots prepared by the present invention can significantly reduce the surface tension of aqueous solutions as the concentration increases, and eventually tend to stabilize. This indicates that they have obvious self-assembly behavior in water and excellent surface activity. In contrast, the coal-based carbon dots prepared in Comparative Example 1 have less effect on surface tension and are mainly hydrophilic. Although the nitrogen-doped coal-based carbon dots prepared in Comparative Example 2 have improved surface activity, their ability to reduce surface tension is still limited. The surface activity of the amphiphilic coal-based carbon dots prepared in Comparative Example 3 is better than that of Comparative Examples 1 and 2, but there is still a large gap compared with Examples 1-4. The main reason for this is that the amphiphilic coal-based carbon dots in Comparative Example 3 are prepared by directly introducing hydrophobic alkyl chains onto the surface of the coal-based carbon dots. Due to the small number of native amino groups on the surface of the coal-based carbon dots, the number of grafted alkyl chains is limited, which affects their surface activity.

[0136] Furthermore, the test data from Examples 1-4 and Comparative Examples 1-3 in Tables 2-4 show that the amphiphilic nitrogen-doped coal-based carbon dots prepared in the present invention have negative zeta potentials, indicating that they are anionic surfactants, and the absolute values are all greater than 30 mV, demonstrating excellent dispersion stability. Furthermore, the samples from Examples 1-4 also possess excellent oil-water interface control capabilities, significantly reducing the oil-water interfacial tension and achieving ultra-low interfacial tension (<0.004 mN / m) at sodium chloride concentrations as high as 10%, demonstrating excellent salt tolerance and stability. Furthermore, the amphiphilic nitrogen-doped coal-based carbon dots prepared in Examples 1-4 exhibit excellent hydrophobic surface wetting properties, enabling rapid wetting and spreading of droplets on hydrophobic surfaces, with a minimum wetting angle as low as 44.63° after 100 seconds of standing. In comparison, the carbon dot samples prepared in Comparative Examples 1-3 are significantly inferior in terms of aqueous solution stability, oil-water interface control capabilities, and wetting properties.

[0137] from Figure 1 and Figure 2 It can be seen that the particle size distribution of the amphiphilic nitrogen-doped coal-based carbon dots prepared in Example 4 is between 1.75–5.25 nm, with an average particle size of 3.58±0.62 nm, which is significantly smaller than 10 nm. The overall distribution is uniform and there is no obvious aggregation phenomenon. Figure 3 and Figure 4 The test results further show that the sample of Example 4 has a strong absorption peak at 205–206 nm, corresponding to sp 2 The carbon dot nanoparticles exhibit π→π transitions, while the weak absorption bands at 236–238 nm and 275–276 nm are attributed to n→π transitions of surface oxygen / nitrogen functional groups. Furthermore, within the excitation wavelength range of 320–400 nm, the sample exhibits typical excitation-dependent fluorescence properties, with the emission peak position gradually red-shifting with increasing excitation wavelength, consistent with an emission mechanism dominated by surface states of the carbon dots.

[0138] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing amphiphilic nitrogen-doped coal-based carbon dots, characterized in that: The following steps are involved: Coal was oxidized and depolymerized using a mixed solution of formic acid and hydrogen peroxide to obtain coal-based carbon dots. The coal-based carbon dots and polyamine are uniformly dispersed in a solvent 1, and then a hydrothermal reaction 1 is performed to obtain nitrogen-doped coal-based carbon dots; Dissolving the nitrogen-doped coal-based carbon dots in solvent 2, and then adding a halogenated alkane to carry out reaction 2 to obtain an intermediate product; The intermediate product is dissolved in solvent 3, and then a catalyst and a sulfonating agent are added to carry out reaction 3 to obtain the amphiphilic nitrogen-doped coal-based carbon dots.

2. The preparation method according to claim 1, characterized in that The volume ratio of formic acid to hydrogen peroxide is 5:

1. The oxidation-depolymerization treatment of coal using the mixed solution of formic acid and hydrogen peroxide is specifically as follows: the coal is evenly dispersed in the mixed solution of formic acid and hydrogen peroxide, and the reaction is stirred at room temperature at 400-800 rpm for 16-20 hours.

3. The preparation method according to claim 2, characterized in that After the reaction is completed by stirring at room temperature, the method further comprises the steps of centrifuging the obtained reaction solution, collecting the supernatant, and concentrating the supernatant to remove the solvent.

4. The preparation method according to claim 1, characterized in that The polyamine is ethylenediamine, diethylenetriamine or triethylenetetramine; the mass ratio of the coal-based carbon dots to the polyamine is 1:10; the temperature of the hydrothermal reaction 1 is 180-220° C., and the time is 8-14 hours.

5. The preparation method according to claim 1, characterized in that After the hydrothermal reaction 1 is completed, the steps of dialyzing the obtained reaction solution and evaporating to remove the solvent are also included.

6. The preparation method according to claim 1, characterized in that The halogenated alkane is a halogenated dodecane, a halogenated tetradecane, a halogenated hexadecane or a halogenated octadecane, and the halogen element in the halogenated alkane is a chlorine element or a bromine element; the molar ratio of the halogenated alkane to the polyamine is 1:1; the temperature of the reaction 2 is 80-100° C., and the time is 5-10 hours.

7. The preparation method according to claim 1, characterized in that After the reaction 2 is completed, the steps of collecting the organic phase in the reaction solution, purifying the organic phase and removing the solvent are also included.

8. The preparation method according to claim 1, characterized in that The catalyst is sodium hydroxide or urea; the amount of the catalyst is 3% to 5% of the total mass of the intermediate product and the sulfonating agent; the sulfonating agent is 1,3-propane sultone or aminosulfonic acid; the mass ratio of the sulfonating agent to the intermediate product is 3:1 to 5:1; the temperature of reaction 3 is 90 to 100° C., and the time is 6 to 8 hours.

9. The preparation method according to claim 1, characterized in that After the reaction 3 is completed, the steps of neutralizing the obtained reaction solution and then dialyzing and drying the neutralized reaction solution are further included.

10. Amphiphilic nitrogen-doped coal-based carbon dots prepared by the preparation method according to any one of claims 1 to 9.

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