Method for preparing graphite nanocapsules from lignin and graphite nanocapsules

Abstract

The present invention belongs to the technical field of nano-graphite carbon material preparation, relates to the application of lignin, and specifically to a method for preparing graphite nanocapsules from lignin and graphite nanocapsules. The steps of its method include: S1, mixing lignin, transition metal salt and water, uniformly dispersing, and obtaining a lignin-metal salt mixed solution; S2, pre-freezing the lignin-metal salt mixed solution using liquid nitrogen, and then freeze-drying to obtain a lignin-metal salt complex; S3, heating the lignin-metal salt complex to 600-900°C at a heating rate of 5-20°C / min for slow pyrolysis to obtain metal-loaded lignin pyrolytic carbon; S4, flash Joule heating of the metal-loaded lignin pyrolytic carbon to obtain hollow graphite nanocapsules. The method solves the problems of high cost, complex process, uneven quality and narrow applicability of graphite nanocapsules in the prior art.

Description

Technical Field

[0001] The invention belongs to the technical field of nano-graphite carbon material preparation, relates to the application of lignin, and specifically relates to a method for preparing graphite nanocapsules from lignin and the graphite nanocapsules. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance understanding of the overall background of the invention and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to those skilled in the art.

[0003] Graphite nanocapsules are hollow, spherical nanostructures encapsulated by graphene layers. They possess excellent electrical conductivity, high surface area, large pore volume, and chemical stability, and have numerous applications in drug delivery, electronic devices, biosensors, energy storage, and other fields. Currently, the main methods for preparing graphite nanocapsules include template-based methods, chemical vapor deposition, and solvent-assisted self-assembly. However, these methods generally suffer from high raw material costs, complex processes, high energy consumption, and poor biocompatibility. Therefore, there is an urgent need for a green, low-cost, and rapid method for preparing high-quality graphite nanocapsules.

[0004] Lignin is the only renewable aromatic polymer in nature. It contains up to 60% carbon, is abundant in industrial reserves, and is inexpensive. It also possesses a rich array of hydrophilic and hydrophobic functional groups and can self-assemble in solution, making it an ideal precursor for preparing carbon nanocapsules. However, existing lignin-based carbon nanocapsules are mostly amorphous carbon, making them difficult to graphitize, resulting in poor activity, stability, and conductivity. Ultrahigh-temperature heat treatment (>2000°C) can increase the degree of graphitization of carbon nanocapsules, but this can lead to structural collapse.

[0005] Patent application publication number CN111285349A discloses a method for preparing highly graphitized boron-doped carbon nanocapsules. This method involves hydrothermally treating natural halloysite powder with a sugar compound to produce hydrothermal carbon. The hydrothermal carbon is then treated with a mixed acid solution to form an etched template. The resulting material is then graphitized at high temperature under the protection of an external boron source and protective gas, ultimately yielding a highly graphitized boron-doped carbon nanocapsule material. This method uses high raw material costs, has low production efficiency, and is not suitable for lignin.

[0006] Patent publication number CN107754793A discloses a method for preparing a catalyst comprising porous graphite nanocapsules coated with active metals. This method first synthesizes a carbon-metal precursor from an active metal precursor, an auxiliary metal precursor, and a carbon precursor including lignin. This precursor is then carbonized in a carbon-containing atmosphere to obtain a porous graphite-coated metal nanoparticle composite material. The porous graphite nanocapsules are then obtained by acid washing. This method requires a carbon-containing atmosphere as a second carbon source, resulting in a low degree of graphitization of the resulting graphite nanocapsules. The preparation process is complex and costly, and the acid washing process is prone to causing equipment corrosion and environmental pollution.

[0007] Patent publication number CN115799499A discloses a method for preparing hollow spherical graphite particles. This method involves pre-carbonizing a carbon source, including lignin, to produce carbon powder. This is then mixed with a metal catalyst to produce composite particles. The composite particles are then isostatically pressed, carbonized at high temperature, and acid-washed to produce hollow spherical graphite. While the hollow spherical graphite produced by this method has a high degree of graphitization, it is micron-sized and, similarly, suffers from complex production processes, high costs, and environmental concerns.

[0008] In summary, it is difficult to prepare graphite nanocapsules using lignin as the sole carbon source with high efficiency and low cost in the existing technology, which cannot meet the high-value utilization of lignin and the green, low-cost and rapid preparation of graphite nanocapsules. Summary of the Invention

[0009] To address the shortcomings of the prior art, the present invention provides a method for preparing graphite nanocapsules from lignin and the graphite nanocapsules themselves. The present method enables the preparation of graphite nanocapsules with regular and uniform morphology, resolving the problems of prior art graphite nanocapsules, such as high production costs, complex processes, uneven quality, and limited applicability.

[0010] In order to achieve the above object, the present invention provides the following technical solutions:

[0011] In a first aspect, a method for preparing graphite nanocapsules from lignin comprises the following steps:

[0012] S1, mixing lignin, transition metal salt and water, and dispersing them evenly to obtain a lignin-metal salt mixed solution;

[0013] S2. pre-freezing the lignin-metal salt mixed solution with liquid nitrogen, and then freeze-drying to obtain a lignin-metal salt complex;

[0014] S3, heating the lignin-metal salt complex to 600-900°C at a heating rate of 5-20°C / min for slow pyrolysis to obtain metal-loaded lignin pyrolysis charcoal;

[0015] S4. Flash Joule heating is performed on the metal-loaded lignin pyrolysis carbon to obtain hollow graphite nanocapsules.

[0016] In a second aspect, a graphite nanocapsule is obtained by the method described in the first aspect of the present invention.

[0017] In a third aspect, a use of the graphite nanocapsule described in the second aspect of the present invention in drug delivery, electronic devices, biosensors or energy storage.

[0018] The beneficial effects of the present invention are:

[0019] 1. The present invention utilizes liquid nitrogen freeze-drying to rapidly obtain a well-dispersed nano-lignin-metal salt complex; placing the lignin-metal salt complex in a tubular furnace for slow pyrolysis can convert the metal salt into metal nanoparticles, yielding a metal-loaded lignin pyrolytic carbon with strong electrical conductivity; flash Joule heating can instantly heat the metal-loaded lignin pyrolytic carbon to an ultra-high temperature (3000K) and rapidly cool it, causing the pyrolytic carbon to rapidly and deeply carbonize; at this extremely high temperature, the metal nanoparticles can undergo a dissolution-precipitation reaction with carbon atoms, forming a graphite carbon layer on the surface, while the metal particles rapidly sublime to form hollow graphite nanocapsules. In addition, the extremely rapid heating and cooling rates can prevent the growth of metal particles, ensuring that the carbon product is at the nanoscale. The above principles make this preparation method applicable to a variety of industrial lignins, and the size and number of graphite layers of the carbon product can be flexibly controlled.

[0020] 2. The graphite nanocapsules produced by this invention have a regular hollow spherical structure with adjustable size and wall thickness, a high degree of graphitization, a high specific surface area, and a high pore volume. Compared to existing graphite nanocapsule products, this method offers higher production efficiency, lower costs, and better conformity to the structural characteristics of graphite nanocapsules. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0022] Figure 1 This is a flow chart of a method for preparing graphite nanocapsules from lignin according to an embodiment of the present invention;

[0023] Figure 2 This is a microscopic morphology of the graphite nanocapsules prepared in Example 1 of the present invention;

[0024] Figure 3 This is the XRD pattern of the graphite nanocapsules prepared in Example 1 of the present invention;

[0025] Figure 4FIG2 is a diagram of the N2 isothermal adsorption and desorption of the graphite nanocapsules prepared in Example 1 of the present invention;

[0026] Figure 5 This is a microscopic morphology of the graphite nanocapsules prepared in Example 2 of the present invention;

[0027] Figure 6 This is a microscopic morphology of the graphite nanocapsules prepared in Comparative Example 1 of the present invention;

[0028] Figure 7 This is a microscopic morphology of the graphite nanocapsules prepared in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0029] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0030] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0031] Flash Joule heating (FJH), as described in this invention, involves heating a quartz tube by discharging a high voltage (above 60V) in a fraction of a second to a temperature exceeding 3000K. After heating, most of the heat is dissipated through blackbody radiation, keeping the tube cool and enabling rapid cooling.

[0032] Since it is difficult to prepare graphite nanocapsules using lignin as the sole carbon source with high efficiency and low cost using existing methods, the present invention provides a method for preparing graphite nanocapsules from lignin and graphite nanocapsules.

[0033] A typical embodiment of the present invention provides a method for preparing graphite nanocapsules from lignin, comprising the following steps:

[0034] S1. Mix lignin, transition metal salt and water and disperse them evenly to obtain a lignin-metal salt mixed solution.

[0035] S2. pre-freezing the lignin-metal salt mixed solution with liquid nitrogen, and then freeze-drying to obtain a lignin-metal salt complex;

[0036] S3, heating the lignin-metal salt complex to 600-900°C at a heating rate of 5-20°C / min for slow pyrolysis to obtain metal-loaded lignin pyrolysis charcoal;

[0037] S4. Flash Joule heating is performed on the metal-loaded lignin pyrolysis carbon to obtain hollow graphite nanocapsules.

[0038] In the present invention, liquid nitrogen freeze-drying can prevent lignin particles and transition metal salts from agglomerating, thereby producing a nano-lignin-metal composite; slow pyrolysis in a tubular furnace can convert the lignin-metal composite into lignin pyrolysis carbon loaded with nano-metal particles having strong conductivity; flash Joule heating can heat the carbon nanosheets to a temperature of approximately 3000K in a very short time, within which the transition metal can be converted into metal nanoparticles, dissolve amorphous carbon, and precipitate graphite carbon layers; and simultaneously, the transition metal is rapidly sublimated and removed at ultra-high temperatures, ultimately obtaining graphite nanocapsules.

[0039] The lignin described in the present invention may be at least one of groundwood lignin, alkali lignin, Kraft lignin, organosolv lignin, lignin sulfonate and hydrolyzed lignin.

[0040] In the transition metal salt of the present invention, the transition metal is a transition metal that can catalyze the production of graphene, such as iron, cobalt, nickel, copper, molybdenum, etc.; the salt can be an inorganic salt, such as chloride, nitrate, sulfate, phosphate, etc., or an organic salt, such as acetate, oxalate, citrate, etc. In some embodiments, the transition metal salt includes at least one of nickel acetate, nickel nitrate, nickel sulfate, nickel chloride, ferric acetate, ferric nitrate, ferric sulfate, ferric chloride, cobalt acetate, cobalt nitrate, cobalt sulfate, and cobalt chloride. The transition metal salt used in the present invention is cheap and readily available, has high solubility, is simple to operate, and has a wide range of applications.

[0041] In order to avoid the introduction of other impurities, the water used in the process of preparing the lignin-metal salt mixed solution of the present invention is deionized water.

[0042] To ensure uniform dispersion of the lignin and transition metal salt in water, in some embodiments, the uniform dispersion process in step S1 is: first, stirring, then ultrasonic dispersion. Specifically, the ultrasonic dispersion is performed at a power of 60 to 100 W, a frequency of 40 to 60 Hz, and a duration of 10 to 60 minutes. By controlling the ultrasonic treatment parameters, the lignin and metal salt are fully dispersed and dissolved in the aqueous solution.

[0043] In some embodiments, the concentration of lignin in the lignin-metal salt mixed solution is 5 to 20 mg / mL, and the amount of transition metal salt added is 0.2 to 5 mmol per gram of lignin, preferably 1 to 3 mmol per gram of lignin. The present invention has studied and designed the amount of lignin and transition metal salt added to obtain a better mixing ratio. The above-mentioned lignin concentration can ensure that the lignin is fully dispersed in the aqueous solution to obtain nano-lignin spherical particles and reduce the drying time. The above-mentioned amount of metal salt added can ensure that the metal salt is fully dissolved in the aqueous solution and uniformly dispersed on the surface and inside of the lignin particles. When the lignin concentration is higher than the above range, the lignin particles agglomerate and nano-scale spherical lignin particles cannot be obtained. When the metal salt concentration is lower than the above range, the yield of graphite nanocapsules is low; when the metal salt concentration is higher than the above range, the metal particles aggregate and nano-scale graphite capsules cannot be formed.

[0044] When liquid nitrogen is used to pre-freeze the lignin-metal salt mixed solution, the lignin metal salt can be directly placed in the liquid nitrogen for pre-freezing. In some embodiments, the lignin-metal salt mixed solution is dripped into the liquid nitrogen for pre-freezing. The dripping method can accelerate the pre-freezing speed of the liquid nitrogen on the lignin-metal salt mixed solution. At the same time, after pre-freezing, the specific surface area increases, which increases the speed of freeze-drying and reduces the freeze-drying time, thereby saving costs. Specifically, the dripping rate is 5 to 20 mL / min.

[0045] In some embodiments, the freeze-drying parameters are: vacuum degree of 1.0×10 -5 ~2.0×10 -5 bar, the cold trap temperature is -60 to -70°C, the drying temperature is 15 to 25°C, and the drying time is 48 to 72 hours. By controlling the freeze-drying parameters, the pretreated lignin-metal salt composite powder is ensured to be fully dried.

[0046] The slow pyrolysis described in the present invention is achieved through a relatively low heating rate. Its main purpose is to convert metal salts into metal nanoparticles. The size of the metal nanoparticles and the resistivity of the pyrolytic carbon can be adjusted by the heating parameters of the slow pyrolysis. When the heating rate is higher than 20°C / min or the pyrolysis temperature is lower than 600°C, the crystallinity of the metal is low, the volatile matter is not fully released, and the conductivity of the lignin pyrolytic carbon is poor; when the heating rate is lower than 5°C / min or the pyrolysis temperature is higher than 900°C, the metal particle size is too large and nanoscale graphite capsules cannot be formed. In some embodiments, the holding time of the slow pyrolysis is 1 to 5 hours. By adjusting the holding time of the slow pyrolysis, the present invention can better prepare the lignin-metal salt complex into metal-loaded lignin pyrolytic carbon that meets the requirements for preparing graphite nanocapsules by flash Joule heating. Specifically, the slow pyrolysis is carried out in a tubular furnace.

[0047] In some embodiments, in step S4, the loading amount of the metal-loaded lignin pyrolytic carbon is 50 to 200 mg, the flash Joule heating process is performed at a voltage of 70 to 150 V, the initial resistance is 5 to 20 Ω, and the power-on time is 0.5 to 2 s. The present invention studies and designs the flash Joule heating process to obtain optimal process parameters to ensure that the lignin pyrolytic carbon is heated to the temperature required to form graphite nanocapsules at a sufficiently high heating rate. Otherwise, when the mixture loading amount and resistance are higher than this range, or the voltage and power-on time are lower than this range, the temperature or heating rate will be lower, thereby reducing the success rate of obtaining the graphite nanocapsule product. The adjustment of the initial resistance is mainly achieved by the compression density of the metal-loaded lignin pyrolytic carbon. The voltage of the present invention is directly provided by a DC power supply after voltage regulation.

[0048] A second embodiment of the present invention provides a graphite nanocapsule obtained by the above method.

[0049] A third embodiment of the present invention provides a use of the above-mentioned graphite nanocapsules in drug delivery, electronic devices, biosensors or energy storage.

[0050] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples.

[0051] Example 1

[0052] A method for preparing graphite nanocapsules from lignin, such as Figure 1 As shown, the following steps are included:

[0053] S1: Kraft lignin and ferric acetate were dissolved in deionized water at a predetermined mass ratio, stirred, and ultrasonically treated to obtain a mixed solution. Specifically, 5 mg of lignin and 3 mmol of ferric acetate were added per milliliter of water. The ultrasonic treatment was performed at a power of 100 W, a frequency of 60 Hz, and a duration of 60 minutes.

[0054] S2: Drop the lignin-metal salt mixed solution into liquid nitrogen and freeze it quickly, then place it in a freeze dryer and dry it in vacuum to obtain a lignin-metal salt complex. Specifically, the drop rate of the lignin-metal salt mixed solution is 5 mL / min, and the vacuum degree of the freeze dryer is 1.0×10 -5 bar, cold trap temperature is -70℃, drying temperature is 15℃, and drying time is 72h.

[0055] S3: Slowly pyrolyzing the lignin-metal salt complex in a tubular furnace to obtain metal-loaded lignin pyrolysis charcoal. Specifically, the pyrolysis temperature is 900° C., the heating rate is 5° C. / min, and the holding time is 5 hours.

[0056] S4: Flash Joule heating the metal-loaded lignin pyrolytic carbon to obtain graphite nanocapsules coated with a small amount of metal impurities. Specifically, the metal-loaded lignin pyrolytic carbon has a loading of 50 mg, the flash Joule heating voltage is 150 V, the initial resistance is 5 Ω, and the power-on time is 2 seconds.

[0057] The microscopic morphology of the graphite nanocapsules prepared in this embodiment is as follows: Figure 2 As shown. It can be seen that the graphite nanocapsules present a regular hollow spherical structure with an average diameter of 140.31nm. The outer wall is a multilayer graphene structure with an average wall thickness of 23.36nm. The internal space of the capsule is large and no obvious metal particles are found. The XRD and N2 adsorption-desorption curves of the graphite nanocapsules prepared in this embodiment are shown in FIG. Figure 3 and Figure 4 As shown in Figure 2, the graphitization degree of graphite nanocapsules is as high as 90.1%, and the specific surface area reaches 294.23 m 2 / g, and the pore volume reaches 0.58cc / g.

[0058] Example 2

[0059] A method for preparing graphite nanocapsules from lignin, such as Figure 1 As shown, the following steps are included:

[0060] S1: Alkali lignin and nickel acetate were dissolved in deionized water at a predetermined mass ratio, stirred, and ultrasonically treated to obtain a mixed solution. Specifically, 10 mg of lignin and 2 mmol of nickel acetate were added per milliliter of water. The ultrasonic treatment was performed at a power of 90 W, a frequency of 50 Hz, and a duration of 30 minutes.

[0061] S2: Drop the lignin-metal salt mixed solution into liquid nitrogen and freeze it quickly, then place it in a freeze dryer and dry it in vacuum to obtain a lignin-metal salt complex. Specifically, the drop rate of the lignin-metal salt mixed solution is 10 mL / min, and the vacuum degree of the freeze dryer is 1.5×10 -5 bar, cold trap temperature is -60℃, drying temperature is 25℃, and drying time is 72h.

[0062] S3: Slowly pyrolyzing the lignin-metal salt complex in a tubular furnace to obtain metal-loaded lignin pyrolysis charcoal. Specifically, the pyrolysis temperature is 800° C., the heating rate is 10° C. / min, and the holding time is 3 hours.

[0063] S4: Flash Joule heating the metal-loaded lignin pyrolytic carbon to obtain graphite nanocapsules coated with a small amount of metal impurities. Specifically, the metal-loaded lignin pyrolytic carbon has a loading of 100 mg, the flash Joule heating voltage is 100 V, the initial resistance is 10 Ω, and the power-on time is 0.5 s.

[0064] The microscopic morphology of the graphite nanocapsules obtained in Example 2 is as follows: Figure 5 The corresponding structural parameters are listed in Table 1. It can be seen that the graphite nanocapsules present a regular hollow spherical structure with an average diameter of 86.47nm. The outer wall is a multilayer graphene structure with an average wall thickness of 15.78nm. The internal space of the capsule is small and no obvious metal particles are found. The degree of graphitization is 83.2% and the specific surface area is 306.38m 2 / g, and the pore volume is 0.49cc / g.

[0065] Example 3

[0066] A method for preparing graphite nanocapsules from lignin, such as Figure 1 As shown, the following steps are included:

[0067] S1: Dissolve lignin (an organic solvent) and cobalt acetate in deionized water at a predetermined mass ratio, stir, and ultrasonicate to obtain a mixed solution. Specifically, 20 mg of lignin and 1 mmol of nickel acetate were added per milliliter of water. The ultrasonication was performed at a power of 60 W, a frequency of 40 Hz, and a duration of 10 minutes.

[0068] S2: Drop the lignin-metal salt mixed solution into liquid nitrogen and freeze it quickly, then place it in a freeze dryer and dry it in vacuum to obtain a lignin-metal salt complex. Specifically, the drop rate of the lignin-metal salt mixed solution is 20 mL / min, and the vacuum degree of the freeze dryer is 2.0×10 -5 bar, cold trap temperature is -60℃, drying temperature is 25℃, and drying time is 48h.

[0069] S3: Slowly pyrolyzing the lignin-metal salt complex in a tubular furnace to obtain metal-loaded lignin pyrolysis charcoal. Specifically, the pyrolysis temperature is 600° C., the heating rate is 20° C. / min, and the holding time is 1 hour.

[0070] S4: Flash Joule heating the metal-loaded lignin pyrolytic carbon to obtain graphite nanocapsules coated with a small amount of metal impurities. Specifically, the metal-loaded lignin pyrolytic carbon has a loading of 200 mg, the flash Joule heating voltage is 70 V, the initial resistance is 20 Ω, and the power-on time is 0.5 s.

[0071] The structural parameters of the graphite nanocapsules prepared in Example 3 are listed in Table 1. The average diameter is 42.84 nm, the outer wall is a multilayer graphene structure, the average wall thickness is 6.01 nm, the internal space of the capsule is small, and no obvious metal particles are found. The degree of graphitization is 74.3%, and the specific surface area is 425.82 m 2 / g, and the pore volume is 0.33cc / g.

[0072] Example 4

[0073] A method for preparing graphite nanocapsules from lignin, such as Figure 1 As shown, the following steps are included:

[0074] S1: Dissolve lignin sulfonate and ferric nitrate in deionized water at a predetermined mass ratio, stir thoroughly, and ultrasonicate to obtain a mixed solution. Specifically, 15 mg of lignin and 2.5 mmol of nickel acetate were added per milliliter of water. The ultrasonic treatment was performed at a power of 60 W, a frequency of 40 Hz, and a duration of 30 minutes.

[0075] S2: Drop the lignin-metal salt mixed solution into liquid nitrogen and freeze it quickly, then place it in a freeze dryer and dry it in vacuum to obtain a lignin-metal salt complex. Specifically, the drop rate of the lignin-metal salt mixed solution is 20 mL / min, and the vacuum degree of the freeze dryer is 2.0×10 -5 bar, cold trap temperature is -70℃, drying temperature is 25℃, and drying time is 48h.

[0076] S3: Slowly pyrolyzing the lignin-metal salt complex in a tubular furnace to obtain metal-loaded lignin pyrolysis charcoal. Specifically, the pyrolysis temperature is 700° C., the heating rate is 10° C. / min, and the holding time is 4 hours.

[0077] S4: Flash Joule heating the metal-loaded lignin pyrolytic carbon to obtain graphite nanocapsules coated with a small amount of metal impurities. Specifically, the metal-loaded lignin pyrolytic carbon has a loading of 150 mg, the flash Joule heating voltage is 120 V, the initial resistance is 10 Ω, and the power-on time is 1 second.

[0078] The structural parameters of the graphite nanocapsules prepared in Example 4 are listed in Table 1. The average diameter is 53.54 nm, the outer wall is a multilayer graphene structure, the average wall thickness is 10.39 nm, the internal space of the capsule is small, and no obvious metal particles are found. The degree of graphitization is 84.6%, and the specific surface area is 321.73 m 2 / g, and the pore volume is 0.41cc / g.

[0079] Table 1 Structural parameters of graphite nanocapsules prepared in Examples 1 to 4

[0080]

[0081]

[0082] Comparative Example 1

[0083] The difference between this comparative example and Example 1 is that the metal-loaded lignin pyrolysis carbon is not subjected to flash Joule heat treatment.

[0084] S1: Kraft lignin and ferric acetate were dissolved in deionized water at a predetermined mass ratio, stirred, and ultrasonically treated to obtain a mixed solution. Specifically, 5 mg of lignin and 3 mmol of ferric acetate were added per milliliter of water. The ultrasonic treatment was performed at a power of 100 W, a frequency of 60 Hz, and a duration of 60 minutes.

[0085] S2: Drop the lignin-metal salt mixed solution into liquid nitrogen and freeze it quickly, then place it in a freeze dryer and dry it in vacuum to obtain a lignin-metal salt complex. Specifically, the drop rate of the lignin-metal salt mixed solution is 5 mL / min, and the vacuum degree of the freeze dryer is 1.0×10 -5 bar, cold trap temperature is -70℃, drying temperature is 15℃, and drying time is 72h.

[0086] S3: Slowly pyrolyzing the lignin-metal salt complex in a tubular furnace to obtain metal-loaded lignin pyrolysis charcoal. Specifically, the pyrolysis temperature is 900° C., the heating rate is 5° C. / min, and the holding time is 5 hours.

[0087] The microscopic morphology of the carbon material obtained in Comparative Example 1 is as follows Figure 6 As shown in Figure 3, no graphite nanocapsule structure is formed, the metal particles are wrapped by amorphous carbon, and no graphene layer is formed, which indicates that flash Joule heating treatment is crucial for the formation of graphite nanocapsule structure.

[0088] Comparative Example 2

[0089] The difference between this comparative example and Example 2 is that the flash Joule heating voltage and the initial resistance are lower than the range of the claims.

[0090] S1: Alkali lignin and nickel acetate were dissolved in deionized water at a predetermined mass ratio, stirred, and ultrasonically treated to obtain a mixed solution. Specifically, 10 mg of lignin and 2 mmol of nickel acetate were added per milliliter of water. The ultrasonic treatment was performed at a power of 90 W, a frequency of 50 Hz, and a duration of 30 minutes.

[0091] S2: Drop the lignin-metal salt mixed solution into liquid nitrogen and freeze it quickly, then place it in a freeze dryer and dry it in vacuum to obtain a lignin-metal salt complex. Specifically, the drop rate of the lignin-metal salt mixed solution is 10 mL / min, and the vacuum degree of the freeze dryer is 1.5×10 -5 bar, cold trap temperature is -60℃, drying temperature is 25℃, and drying time is 72h.

[0092] S3: Slowly pyrolyzing the lignin-metal salt complex in a tubular furnace to obtain metal-loaded lignin pyrolysis charcoal. Specifically, the pyrolysis temperature is 800° C., the heating rate is 10° C. / min, and the holding time is 3 hours.

[0093] S4: Flash Joule heating the metal-loaded lignin pyrolytic carbon to obtain graphite nanocapsules coated with a small amount of metal impurities. Specifically, the metal-loaded lignin pyrolytic carbon has a loading of 100 mg, the flash Joule heating voltage is 60 V, the initial resistance is 30 Ω, and the power-on time is 0.5 s.

[0094] The microscopic morphology of the graphite nanocapsules obtained in Comparative Example 2 is as follows: Figure 7 As shown in the figure, the graphite nanocapsules exhibit a regular hollow spherical structure with a multilayer graphene outer wall, but significant metal particles remain inside the capsules. This indicates that flash Joule heating conditions are crucial for the sublimation removal of metal impurities.

[0095] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing graphite nanocapsules from lignin, characterized in that: The following steps are involved: S1, mixing lignin, transition metal salt and water, and dispersing them evenly to obtain a lignin-metal salt mixed solution; S2. pre-freezing the lignin-metal salt mixed solution with liquid nitrogen, and then freeze-drying to obtain a lignin-metal salt complex; S3, heating the lignin-metal salt complex to 600-900°C at a heating rate of 5-20°C / min for slow pyrolysis to obtain metal-loaded lignin pyrolysis charcoal; S4, flash Joule heating the metal-loaded lignin pyrolysis carbon to obtain hollow graphite nanocapsules; The transition metal salt comprises at least one of nickel acetate, nickel nitrate, nickel sulfate, nickel chloride, ferric acetate, ferric nitrate, ferric sulfate, ferric chloride, cobalt acetate, cobalt nitrate, cobalt sulfate, and cobalt chloride; The concentration of lignin in the lignin-metal salt mixed solution is 5-20 mg / mL, and the amount of transition metal salt added is 0.2-5 mmol per gram of lignin; During the flash Joule heating process, the voltage was 70–150 V, the initial resistance was 5–20 Ω, and the power-on time was 0.5–2 s.

2. The method for preparing graphite nanocapsules from lignin as claimed in claim 1, wherein: The process of uniform dispersion in step S1 is: first stirring uniformly, and then ultrasonic dispersion.

3. The method for preparing graphite nanocapsules from lignin as claimed in claim 2, wherein: During ultrasonic dispersion, the power is 60~100 W, the frequency is 40~60 Hz, and the time is 10~60 min.

4. The method for preparing graphite nanocapsules from lignin as claimed in claim 1, wherein: The amount of transition metal salt added is 1~3 mmol per gram of lignin.

5. The method for preparing graphite nanocapsules from lignin as claimed in claim 1, wherein: The lignin-metal salt mixed solution was dropped into liquid nitrogen for pre-freezing.

6. The method for preparing graphite nanocapsules from lignin as claimed in claim 5, wherein: The infusion rate is 5~20mL / min.

7. The method for preparing graphite nanocapsules from lignin as claimed in claim 1, wherein: The freeze drying parameters were: vacuum degree 1.0×10 -5 ~2.0×10 -5 bar, cold trap temperature is -60~-70 ℃, drying temperature is 15~25 ℃, and drying time is 48~72 h.

8. The method for preparing graphite nanocapsules from lignin as claimed in claim 1, wherein: The holding time for slow pyrolysis is 1 to 5 h.

9. The method for preparing graphite nanocapsules from lignin as claimed in claim 1, wherein: In step S4, the loading amount of the metal-loaded lignin pyrolysis carbon is 50-200 mg.

10. A graphite nanocapsule, characterized in that: Obtained by the method according to any one of claims 1 to 9.

11. Use of the graphite nanocapsule according to claim 10 in drug delivery, electronic devices, biosensing or energy storage.

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