CN113800500B - Method for preparing boron-doped carbon nano sheet from lignin and product - Google Patents

Method for preparing boron-doped carbon nano sheet from lignin and product Download PDF

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CN113800500B
CN113800500B CN202111138120.4A CN202111138120A CN113800500B CN 113800500 B CN113800500 B CN 113800500B CN 202111138120 A CN202111138120 A CN 202111138120A CN 113800500 B CN113800500 B CN 113800500B
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doped carbon
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CN113800500A (en
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杨海平
董志国
王贤华
陈培骜
陈应泉
邵敬爱
张�雄
曾阔
杨晴
张世红
陈汉平
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Huazhong University of Science and Technology
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Abstract

The invention discloses a method for preparing boron-doped carbon nano sheets by using lignin and a product thereof, belonging to the field of lignin utilization. Adding lignin powder, potassium phosphate and boron additives into water, performing ultrasonic treatment to obtain a dispersion liquid, then dropwise adding the dispersion liquid into liquid nitrogen to quickly freeze, quickly transferring the dispersion liquid to a freeze dryer for vacuum drying, then pyrolyzing the obtained lignin aerogel in an inert atmosphere, finally washing inorganic components in coke by using an ethanol solution, and drying to obtain a boron-doped carbon nano sheet product, wherein the boron-doped carbon nano sheet product has the characteristics of high specific surface area, low density, classification holes and the like, and has good performances on macromolecular dye adsorption and electrocatalytic hydrogen peroxide preparation. The method disclosed by the invention is simple and feasible in process, suitable for various lignin types, flexible in regulation and control and good in uniformity of the prepared product.

Description

Method for preparing boron-doped carbon nano sheet from lignin and product
Technical Field
The invention belongs to the field of lignin utilization, and in particular relates to a method for preparing boron-doped carbon nano sheets from lignin and a product.
Background
Lignin is the only natural aromatic polymer in nature, its content in biomass is inferior to cellulose, and is widely present in industrial byproducts such as papermaking black liquor and ethanol fermentation residues. The global paper industry has been counted to produce about 5000 ten thousand tons of industrial lignin per year, with alkali lignin accounting for about 85%. The alkali lignin has rich active functional groups, and the carbon content is up to more than 60%, so that the alkali lignin is a good carbon material precursor.
The coke yield obtained by high-temperature carbonization of alkali lignin is up to 60%, but due to the hot melting characteristic of lignin, the direct pyrolysis coke has small specific surface area, single pore structure, mainly micropores, narrow pore channels, poor transmission capacity, unstable pore structure and easy collapse. On the other hand, due to poor water solubility of alkali lignin and weak particle dispersion capability, coke prepared by direct carbonization is seriously agglomerated and has non-uniform physicochemical properties.
In view of the above problems, the following solutions currently exist: the patent application with publication number of CN 1061855920A discloses a preparation method and application of lignin porous carbon material, the method adopts alkali lignin as a carbon precursor and KOH as an activator, and the porous carbon material with high specific surface area is obtained through pretreatment, carbonization and activation, but the obtained porous carbon is still serious in agglomeration, KOH has serious corrosion to equipment, and the carbon yield is low, so that the method is not beneficial to industrial production.
The patent application with publication number of CN105817202A discloses a preparation method and application of three-dimensional lignin-based hierarchical pore activated carbon. The method has complicated steps and is not easy to remove the silicon dioxide template.
Compared with the traditional massive porous carbon, the two-dimensional carbon nano-sheet has higher mass transfer capability and can expose more active sites. The patent application with publication number of CN109485029B discloses a preparation method and application of lignin porous carbon nano-sheets, wherein the method adopts water-soluble sulfonated lignin as a carbon precursor and a dispersing agent, and oxalate as an activating agent to prepare the lignin porous carbon nano-sheets. The lignin porous carbon obtained by the method has higher specific surface area and structural regularity, and has low activation efficiency although oxalate corrosiveness, and the method is only suitable for water-soluble lignin.
The introduction of heteroatoms into the porous carbon material can regulate the pore structure and enhance the physicochemical activity of the material. The patent application with publication number of CN 108529591B discloses a preparation method and application of B, N co-doped porous carbon nano-sheets, wherein bis (2-chloroethyl) amine hydrochloride is taken as a raw material, and is combined with boric acid and pyrolyzed to obtain B and N co-doped carbon nano-sheets, so that the preparation method has higher capacitance performance. The carbon nano-sheet obtained by the method has higher cost and is not beneficial to industrial production.
In view of the foregoing, there is a need in the art to develop a novel method for preparing porous carbon materials, which is simple, effective, low-cost, environmentally friendly, harmless, and capable of continuously and efficiently preparing carbon nanoplatelets with uniform morphology, high specific surface area and multiple pores.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a method for preparing boron-doped carbon nano sheets from lignin and a product thereof, and the novel process method is designed to prepare the boron-doped carbon nano sheets with lamellar shape, low density and high porosity, so as to solve the problems of complex process, high cost, uneven quality and narrow application range of the process method in the preparation of the carbon nano sheets in the prior art.
To achieve the above object, according to one aspect of the present invention, there is provided a method for preparing boron doped carbon nanoplatelets from lignin, comprising the steps of:
s1: dissolving lignin, potassium phosphate and boron additives in water according to a preset mass ratio to obtain a mixed solution;
s2: carrying out ultrasonic treatment on the mixed solution in the step S1, fully dispersing lignin in the mixed solution, and carrying out crosslinking reaction on borate ions and hydroxyl functional groups on the surface of the lignin, so as to obtain lignin solution;
s3: dropwise adding the lignin solution obtained in the step S2 into liquid nitrogen for quick freezing, and then putting the obtained lignin solution frozen product into a freeze dryer for further freeze-drying treatment to obtain expanded lignin spherical particles;
s4: carrying out pyrolysis carbonization and cooling on the lignin spherical particles in the S3 in an inert atmosphere to obtain lignin coke;
s5: and (3) putting the lignin coke in the step S4 into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the boron-doped carbon nano sheet product.
In the invention, potassium phosphate is used as a template agent and a dispersing agent to improve the dispersibility of lignin in an aqueous solution, and simultaneously, lignin molecules are induced to self-assemble to obtain a nano lignin colloid solution; utilizing boron additives to carry out crosslinking chelation reaction with active hydroxyl groups in lignin or construct hydrogen bonds, enhancing the stability of nano lignin, and introducing boron atoms into a carbon matrix as active centers; and after overspeed freezing, superfine ice crystals can be grown in lignin molecules to construct a three-dimensional pore canal, and the lamellar, low-density and high-porosity boron-doped carbon nano-plate is finally obtained through high-temperature carbonization.
As a further preferred, the lignin is one or more of alkali lignin, kraft lignin, organosolv lignin and hydrolyzed lignin. The lignin is cheap and easy to obtain, and belongs to industrial byproducts.
As a further preferred, the boron-based additive is preferably one or more of boric acid, ammonium borate, potassium tetraborate, sodium tetraborate. The additive is cheap and easy to obtain, high in solubility, simple to operate and wide in application range.
As a further preferable mode, in the step S1, the mass of lignin added per milliliter of water is in the range of 1mg to 10mg. Under the above process, the drying energy consumption can be reduced while ensuring the full dispersion of lignin particles.
Further preferably, in step S1, the mass ratio of potassium phosphate to lignin is 1:20 to 2:1, preferably 1:10 to 1:1. Under the process, lignin can be ensured to be uniformly dispersed in the aqueous solution, the carbon nano-sheet is prepared, and meanwhile, the treatment energy consumption is reduced.
Further preferably, in step S1, the mass ratio of the boron-based additive to lignin is 1:50 to 1:5, preferably 1:20 to 1:10. Under the process, the lignin can be ensured to be kept stable in the carbonization process, and meanwhile, excessive crosslinking is avoided to increase the pyrolysis energy consumption.
More preferably, in step S2, the power of the ultrasonic treatment is 60W to 100W, the frequency is 40Hz to 60Hz, and the time is 10min to 60min. Under the process, the treatment energy consumption can be reduced while the dispersibility of lignin particles is ensured.
Further preferably, in step S3, the dropping speed of the lignin solution is 5ml/min to 20ml/min. Under the above process, the internal pore canal of lignin can be ensured to be uniformly distributed, and the drying time is reduced.
Further preferably, in step S3, the degree of vacuum of the freeze dryer is 1.0X10 -5 bar～2.0×10 -5 The bar is frozen at the temperature of minus 60 ℃ to minus 70 ℃, the freezing time is 1h, the drying temperature is 15 ℃ to 25 ℃, and the drying time is 24h to 48h. Under the above process, lignin can be ensured to be sufficiently dried, and secondary dissolution is avoided.
As a further preferable mode, in the step S4, the pyrolysis temperature is 700-900 ℃, the heating rate is 5-20 ℃/min, and the heat preservation time is 30-120 min. Under the process, the volatile components can be ensured to be fully released, and meanwhile, the collapse of the pore canal can be avoided.
As a further preferable mode, in the step S5, the volume ratio of the ethanol to the water is 1:1-1:3, the mass ratio of the solute to the solution is 1:500-1:1000, the stirring speed is 300-800 rad, the stirring time is 10-24 h, the drying temperature is 45-65 ℃ and the drying time is 24-48 h. Under the process, the inorganic impurities in the coke can be completely removed.
According to another aspect of the present invention, there is provided a boron doped carbon nanoplatelet prepared by the method. The average thickness of the boron doped carbon nano sheet is less than 100nm, and the specific surface area is higher than 1000m 2 And/g, belonging to the microporous-mesoporous composite material.
In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:
1. according to the invention, potassium phosphate is used as a dispersing agent and a template agent to induce lignin self-assembly, a boron-series additive and lignin are used for carrying out crosslinking chelation reaction, boron atoms are anchored on lignin, a lignin nano structure is kept stable in the carbonization process, ice crystals grow in lignin particles to construct a three-dimensional pore canal in combination with overspeed freezing, and the boron-doped carbon nano sheet with graded holes and low density characteristics is obtained through slow carbonization. The principle makes the preparation method suitable for various industrial lignin, and can flexibly regulate and control the thickness, pore structure and active site of the carbon nano sheet.
2. The invention also researches and designs the proportion of potassium phosphate, boron additives and lignin to obtain a better mixing proportion, specifically, the mass ratio of the potassium phosphate to the lignin is 1:20-2:1, preferably 1:10-1:1, the mass ratio of the boron additives to the lignin is 1:50-1:5, preferably 1:20-1:10, and the mass range of the lignin added into per milliliter of water is 1 mg-10 mg, so that the ideal lignin colloid solution is ensured to be obtained. Otherwise, when the boron additive is below the above range, the product is a bulk carbon material. Meanwhile, when the potassium phosphate is below the above range, lignin cannot be sufficiently dispersed.
3. The invention researches and designs the overspeed freeze drying process to obtainThe preferred process, specifically, the drop velocity of lignin solution is 5 ml/min-20 ml/min, the vacuum degree of the freeze dryer is 1.0X10 -5 bar～2.0×10 -5 The bar is frozen at the temperature of minus 60 ℃ to minus 70 ℃, the freezing time is 1h, the drying temperature is 15 ℃ to 25 ℃, and the drying time is 24h to 48h. Under the freeze drying process, fully dried and porous nano lignin aerosol can be obtained. The pretreatment method can keep the structure of nano lignin, and simultaneously forms three-dimensional pore channels in the nano lignin, compared with chemical activation, the pretreatment method is more environment-friendly, and the arrangement of the pore channels is controllable.
4. The invention also carries out research and design on the pyrolysis carbonization process, and obtains a better process, wherein the specific heating rate is 5-20 ℃/min, the carbonization temperature is 700-900 ℃, and the heat preservation time is 30-120 min. Under the carbonization process, the sufficient release of volatile matters can be ensured, and the stable pore structure is maintained. Otherwise, when the temperature rising rate is too high, the carbon nano sheet product cannot be obtained; when the carbonization temperature is too low or the heat preservation time is too short, the pore structure of the carbon nano-sheet is poor.
5. The invention also carries out research design on a carbon material purification and drying process to obtain a better process, wherein the volume ratio of specific ethanol to water is 1:1-1:3, the mass ratio of solute to solution is 1:500-1:1000, the stirring speed is 300-800 rad, the stirring time is 10-24 h, the drying temperature is 45-65 ℃ and the drying time is 24-48 h. Otherwise, when the concentration of the ethanol solution is too high or too low, it may result in the potassium phosphate or boron oxide not being completely removed from the carbon material.
6. The thickness of the prepared carbon nano-sheet is lower than 100nm, and the specific surface area is more than 1000m 2 And/g, belonging to the microporous-mesoporous composite material. Compared with the existing biomass-based carbon nano-sheet, the biomass-based carbon nano-sheet has the advantages of thinner thickness, higher ductility and better uniformity.
7. The boron doped carbon nano sheet prepared by the invention has good effects in adsorption and electrocatalysis. Specifically, the adsorption capacity of the carbon nano-sheet to macromolecular dye Congo red reaches 990mg/g and the adsorption capacity to methylene blue reaches 440mg/g, and meanwhile, the carbon nano-sheet is used as an oxygen reduction reaction catalyst to prepare H 2 O 2 The time selectivity is as high as 85 percent, which is superior to the prior artMost amorphous porous carbon materials.
Drawings
Fig. 1 is a flow chart of preparing a carbon nano sheet from lignin according to an embodiment of the present invention.
Fig. 2 (a) is a local micro-morphology of the lignin carbon nanoplatelets provided in example 1 of the present invention, and fig. 2 (b) is another local micro-morphology of the lignin carbon nanoplatelets provided in example 1 of the present invention;
FIG. 3 is a graph showing the nitrogen adsorption and desorption curves and pore size distribution of lignin carbon nanoplatelets provided in example 1 of the present invention;
FIG. 4 (a) is an adsorption curve of 500mg/L methylene blue to lignin carbon nanoplatelets provided in example 1 of the present invention, and FIG. 4 (b) is an adsorption curve of 1500mg/L Congo red dye to lignin carbon nanoplatelets provided in example 1 of the present invention;
FIG. 5 is a lignin carbon nanoplatelet micro-morphology provided in example 2 of the present invention;
fig. 6 (a) is a graph of hydrogen peroxide selectivity and voltage in the hydrogen peroxide production by the catalytic oxygen reduction reaction of the lignin carbon nanoplatelets provided in example 2 of the present invention, and fig. 6 (b) is a graph of electron transfer number and voltage in the hydrogen peroxide production by the catalytic oxygen reduction reaction of the lignin carbon nanoplatelets provided in example 2 of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
The invention provides a method for preparing boron-doped carbon nano-sheets by using lignin, which utilizes potassium phosphate as a template agent and a dispersing agent to improve the dispersibility of lignin in aqueous solution, induces lignin molecules to generate self-assembly to obtain lignin sol with a special structure, further utilizes boron additives to generate crosslinking chelation reaction with active hydroxyl groups in the lignin or construct hydrogen bonds to keep the molecular structure of the lignin stable, and can grow superfine ice crystals in lignin molecules to construct three-dimensional pore channels and finally obtain lamellar, low-density and high-porosity boron-doped carbon nano-sheets through high-temperature carbonization. Meanwhile, the carbon nano-sheet prepared by the method has wide application prospect in dye adsorption and oxygen reduction electrocatalysis.
The invention provides a method for preparing boron-doped carbon nano sheets by lignin, which comprises the following steps:
s1, dissolving lignin, potassium phosphate and boron additives in water according to a preset mass ratio to prepare a mixed solution. Wherein the lignin is one or more of alkali lignin, kraft lignin, organic solvent lignin and hydrolytic lignin, the mass range of lignin added into each milliliter of water is 1 mg-10 mg, and the mass ratio of potassium phosphate to lignin is 1:20-2:1, preferably 1:10-1:1. The boron-based additive is preferably one or more of boric acid, ammonium borate, potassium tetraborate and sodium tetraborate, and the mass ratio of the additive to lignin is 1:50-1:5, preferably 1:20-1:10.
S2, performing ultrasonic treatment on the mixed solution to fully dissolve the potassium phosphate and the boron additives, fully dispersing lignin particles, inducing lignin molecular self-assembly, and fully performing crosslinking chelation reaction with borate so as to obtain a lignin colloid solution. Specifically, the power of ultrasonic treatment is 60-100W, the frequency is 40-60 Hz, and the time is 10-60 min.
And S3, dropwise adding the lignin solution into liquid nitrogen at a constant speed for quick freezing, and then putting the lignin solution into a freeze dryer for freeze-drying to obtain lignin aerogel. Specifically, the drop velocity of lignin solution is 5 ml/min-20 ml/min, and the vacuum degree of the freeze dryer is 1.0X10 -5 bar～2.0×10 -5 bar, freezing temperature is minus 60 ℃ to minus 70 ℃, freezing time is 0.5h to 2h, drying temperature is 15 ℃ to 25 ℃, and drying time is 24h to 48h.
S4, performing high-temperature slow carbonization on the lignin aerogel in an inert atmosphere, and cooling to obtain lignin coke. Specifically, the carbonization temperature is 700-900 ℃, the heating rate is 5-20 ℃/min, and the heat preservation time is 30-120 min.
And S5, placing the lignin coke into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the carbon nano-sheet. Specifically, the volume ratio of ethanol to water is 1:1-1:3, the mass ratio of solute to solution is 1:500-1:1000, the stirring speed is 300-800 rad, the stirring time is 10-24 h, the drying temperature is 45-65 ℃, and the drying time is 24-48 h.
The carbon nano-sheet prepared by the method has low density (less than 100 mg/cm) 3 ) Thin (< 50 nm), high specific surface area (> 1000 m) 2 The characteristic of/g) is strong in mass transfer capability, and because the carbon skeleton contains doped boron atoms, lewis acid sites can be provided, charge transfer is promoted, and adsorption of dye molecules and catalytic oxygen reduction are carried out to prepare H 2 O 2 All have good performance and wide application prospect.
The following are specific examples:
example 1
S1, dissolving alkali lignin, potassium phosphate and ammonium borate in water according to a preset mass ratio to prepare a mixed solution. Wherein, the mass range of lignin added into each milliliter of water is 10mg, the mass ratio of potassium phosphate to lignin is 1:1, and the mass ratio of ammonium borate to lignin is 1:12.
S2, performing ultrasonic treatment on the mixed solution to fully dissolve the potassium phosphate and the boron additives, fully dispersing lignin particles, inducing lignin molecular self-assembly, and fully performing crosslinking chelation reaction with borate so as to obtain a lignin colloid solution. Specifically, the power of the ultrasonic treatment is 100W, the frequency is 60Hz, and the time is 30min.
And S3, dropwise adding the lignin solution into liquid nitrogen at a constant speed for quick freezing, and then putting the lignin solution into a freeze dryer for freeze-drying to obtain lignin aerogel. Specifically, the drop velocity of lignin solution is 5ml/min, and the vacuum degree of the freeze dryer is 2.0X10 - 5 bar, freezing temperature of-70 ℃, freezing time of 1h, drying temperature of 25 ℃ and drying time of 48h.
S4, performing high-temperature slow carbonization on the lignin aerogel in an inert atmosphere, and cooling to obtain lignin coke. Specifically, the carbonization temperature is 900 ℃, the heating rate is 10 ℃/min, and the heat preservation time is 60min.
And S5, placing the lignin coke into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the carbon nano-sheet. Specifically, the volume ratio of ethanol to water is 1:2, the mass ratio of solute to solution is 1:1000, the stirring speed is 800rad, the stirring time is 24 hours, the drying temperature is 65 ℃, and the drying time is 48 hours.
The lignin carbon nanoplatelets obtained in example 1 have microscopic morphology as shown in fig. 2 (a) and fig. 2 (b), and the thickness of the carbon nanoplatelets is about 87nm as seen in fig. 2 (a), and the surface of the carbon nanoplatelets extends over penetrating mesopores as seen in fig. 2 (b), so that the lignin carbon nanoplatelets have excellent mass transfer capability. As shown in FIG. 3, the pore diameter distribution of the carbon nanoplatelets is mainly concentrated near 5nm, and the specific surface area is 1012m 2 And/g, wherein the micropores and mesopores account for 40% and 60% of each other, and the total pore volume is 0.8cc/g. The adsorption performance curves of the carbon nano-sheet provided in example 1 on 500mg/L methylene blue and 1500mg/L Congo red are shown in fig. 4 (a) and 4 (b), wherein the adsorption capacity of the carbon nano-sheet on the methylene blue reaches 440mg/g, the adsorption capacity of the carbon nano-sheet on the Congo red reaches 990mg/g, and the adsorption capacity of the carbon nano-sheet reaches more than 80% of the saturated adsorption capacity in a short time, so that the adsorption efficiency is high.
Example 2
S1, dissolving alkali lignin, potassium phosphate and ammonium borate in water according to a preset mass ratio to prepare a mixed solution. Wherein, the mass range of lignin added into each milliliter of water is 10mg, the mass ratio of potassium phosphate to lignin is 1:10, and the mass ratio of ammonium borate to lignin is 1:12.
S2, performing ultrasonic treatment on the mixed solution to fully dissolve the potassium phosphate and the boron additives, fully dispersing lignin particles, inducing lignin molecular self-assembly, and fully performing crosslinking chelation reaction with borate so as to obtain a lignin colloid solution. Specifically, the power of the ultrasonic treatment is 100W, the frequency is 60Hz, and the time is 40min.
And S3, dropwise adding the lignin solution into liquid nitrogen at a constant speed for quick freezing, and then putting the lignin solution into a freeze dryer for freeze-drying to obtain lignin aerogel. Specifically, the drop velocity of lignin solution is 5ml/min, and the vacuum degree of the freeze dryer is 2.0X10 - 5 bar, freezing temperature of-70 ℃, freezing time of 1h, drying temperature of 25 ℃ and drying time of 48h.
S4, performing high-temperature slow carbonization on the lignin aerogel in an inert atmosphere, and cooling to obtain lignin coke. Specifically, the carbonization temperature is 900 ℃, the heating rate is 5 ℃/min, and the heat preservation time is 60min.
And S5, placing the lignin coke into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the carbon nano-sheet. Specifically, the volume ratio of ethanol to water is 1:1, the mass ratio of solute to solution is 1:500, the stirring speed is 500rad, the stirring time is 10h, the drying temperature is 65 ℃, and the drying time is 48h.
The lignin carbon nanoplatelets obtained in example 2 have a microscopic morphology as shown in fig. 5, and it can be seen that the thickness of the carbon nanoplatelets is about 66nm. The electrocatalytic performance curve of the lignin carbon nano-sheet obtained in the example 2 in the KOH electrolyte with oxygen saturation of 0.1M is shown in fig. 6, and it can be seen that the selectivity of the carbon nano-sheet to hydrogen peroxide is as high as 85%, the electron transfer number is between 2.3 and 2.7, and the electron transfer process is close to the two-electron transfer process.
Example 3
S1, dissolving lignin, potassium phosphate and boron additives in water according to a preset mass ratio to prepare a mixed solution. Wherein the lignin is alkali lignin, the mass range of the lignin added into each milliliter of water is 1mg, and the mass ratio of potassium phosphate to lignin is 1:20. The boron-based additive is preferably boric acid, and the mass ratio of the additive to lignin is 1:50.
S2, performing ultrasonic treatment on the mixed solution to fully dissolve the potassium phosphate and the boron additives, fully dispersing lignin particles, inducing lignin molecular self-assembly, and fully performing crosslinking chelation reaction with borate so as to obtain a lignin colloid solution. Specifically, the power of the ultrasonic treatment is 60W, the frequency is 40Hz, and the time is 10min.
And S3, dropwise adding the lignin solution into liquid nitrogen at a constant speed for quick freezing, and then putting the lignin solution into a freeze dryer for freeze-drying to obtain lignin aerogel. Specifically, the drop velocity of lignin solution is 5ml/min, and the freeze dryerVacuum degree of 1.0X10 - 5 bar, freezing temperature of-60 ℃, freezing time of 0.5h, drying temperature of 15 ℃ and drying time of 24h.
S4, performing high-temperature slow carbonization on the lignin aerogel in an inert atmosphere, and cooling to obtain lignin coke. Specifically, the carbonization temperature is 700 ℃, the heating rate is 5 ℃/min, and the heat preservation time is 30min.
And S5, placing the lignin coke into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the carbon nano-sheet. Specifically, the volume ratio of ethanol to water is 1:1, the mass ratio of solute to solution is 1:500, the stirring speed is 300rad, the stirring time is 10h, the drying temperature is 45 ℃, and the drying time is 24h.
Example 4
S1, dissolving lignin, potassium phosphate and boron additives in water according to a preset mass ratio to prepare a mixed solution. Wherein the lignin is Kraft lignin, the mass range of lignin added into each milliliter of water is 10mg, and the mass ratio of potassium phosphate to lignin is 2:1. The boron-based additive is preferably sodium tetraborate, and the mass ratio of the additive to lignin is 1:5.
S2, performing ultrasonic treatment on the mixed solution to fully dissolve the potassium phosphate and the boron additives, fully dispersing lignin particles, inducing lignin molecular self-assembly, and fully performing crosslinking chelation reaction with borate so as to obtain a lignin colloid solution. Specifically, the power of the ultrasonic treatment is 100W, the frequency is 60Hz, and the time is 60min.
And S3, dropwise adding the lignin solution into liquid nitrogen at a constant speed for quick freezing, and then putting the lignin solution into a freeze dryer for freeze-drying to obtain lignin aerogel. Specifically, the drop velocity of lignin solution is 20ml/min, and the vacuum degree of the freeze dryer is 2.0X10 - 5 bar, freezing temperature of-70 ℃, freezing time of 2h, drying temperature of 25 ℃ and drying time of 48h.
S4, performing high-temperature slow carbonization on the lignin aerogel in an inert atmosphere, and cooling to obtain lignin coke. Specifically, the carbonization temperature is 900 ℃, the heating rate is 20 ℃/min, and the heat preservation time is 120min.
And S5, placing the lignin coke into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the carbon nano-sheet. Specifically, the volume ratio of ethanol to water is 1:3, the mass ratio of solute to solution is 1:1000, the stirring speed is 800rad, the stirring time is 24 hours, the drying temperature is 65 ℃, and the drying time is 48 hours.
Example 5
S1, dissolving lignin, potassium phosphate and boron additives in water according to a preset mass ratio to prepare a mixed solution. Wherein the lignin is organic solvent lignin, the mass range of the lignin added into each milliliter of water is 5mg, and the mass ratio of potassium phosphate to lignin is 1:10. The boron-based additive is preferably potassium tetraborate, and the mass ratio of the additive to lignin is 1:20.
S2, performing ultrasonic treatment on the mixed solution to fully dissolve the potassium phosphate and the boron additives, fully dispersing lignin particles, inducing lignin molecular self-assembly, and fully performing crosslinking chelation reaction with borate so as to obtain a lignin colloid solution. Specifically, the power of the ultrasonic treatment is 80W, the frequency is 50Hz, and the time is 50min.
And S3, dropwise adding the lignin solution into liquid nitrogen at a constant speed for quick freezing, and then putting the lignin solution into a freeze dryer for freeze-drying to obtain lignin aerogel. Specifically, the drop velocity of lignin solution is 10ml/min, and the vacuum degree of the freeze dryer is 1.5X10 - 5 bar, freezing temperature of-65 ℃, freezing time of 1h, drying temperature of 20 ℃ and drying time of 36h.
S4, performing high-temperature slow carbonization on the lignin aerogel in an inert atmosphere, and cooling to obtain lignin coke. Specifically, the carbonization temperature is 800 ℃, the heating rate is 15 ℃/min, and the heat preservation time is 90min.
And S5, placing the lignin coke into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the carbon nano-sheet. Specifically, the volume ratio of ethanol to water is 1:2, the mass ratio of solute to solution is 1:800, the stirring speed is 600rad, the stirring time is 18h, the drying temperature is 55 ℃, and the drying time is 36h.
Example 6
S1, dissolving lignin, potassium phosphate and boron additives in water according to a preset mass ratio to prepare a mixed solution. Wherein the lignin is hydrolytic lignin, the mass range of the lignin added into each milliliter of water is 4mg, and the mass ratio of potassium phosphate to lignin is 1:1. The boron-based additive is preferably sodium tetraborate, and the mass ratio of the additive to lignin is 1:10.
S2, performing ultrasonic treatment on the mixed solution to fully dissolve the potassium phosphate and the boron additives, fully dispersing lignin particles, inducing lignin molecular self-assembly, and fully performing crosslinking chelation reaction with borate so as to obtain a lignin colloid solution. Specifically, the power of the ultrasonic treatment is 90W, the frequency is 45Hz, and the time is 40min.
And S3, dropwise adding the lignin solution into liquid nitrogen at a constant speed for quick freezing, and then putting the lignin solution into a freeze dryer for freeze-drying to obtain lignin aerogel. Specifically, the drop velocity of lignin solution is 9ml/min, and the vacuum degree of the freeze dryer is 1.6X10 - 5 bar, freezing temperature of-68 ℃, freezing time of 1.5h, drying temperature of 20 ℃ and drying time of 30h.
S4, performing high-temperature slow carbonization on the lignin aerogel in an inert atmosphere, and cooling to obtain lignin coke. Specifically, the carbonization temperature is 750 ℃, the heating rate is 14 ℃/min, and the heat preservation time is 100min.
And S5, placing the lignin coke into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the carbon nano-sheet. Specifically, the volume ratio of ethanol to water is 1:1.3, the mass ratio of solute to solution is 1:800, the stirring speed is 500rad, the stirring time is 20h, the drying temperature is 50 ℃, and the drying time is 42h.
According to the invention, potassium phosphate is used as a dispersing agent and a template agent, lignin is induced to self-assemble in a solution system to form nano micelles, a boron additive is used as a structural stabilizer, and ultra-fast freezing is combined to construct a multistage pore canal, so that the boron-doped carbon nano sheet with high porosity and low density is prepared, and the carbon material has strong adsorption capacity and electrocatalytic performance.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims (9)

1. The method for preparing the boron-doped carbon nano sheet by using lignin is characterized by comprising the following steps of:
s1: dissolving lignin, potassium phosphate and a boron additive in water according to a preset mass ratio to obtain a mixed solution, wherein the lignin is one or more selected from alkali lignin, kraft lignin and hydrolyzed lignin;
s2: carrying out ultrasonic treatment on the mixed solution in the step S1, using potassium phosphate as a template agent and a dispersing agent to improve the dispersibility of lignin in the aqueous solution, and simultaneously inducing lignin molecules to self-assemble to obtain a nano lignin colloid solution, so that lignin is fully dispersed in the mixed solution, and a boron additive and hydroxyl functional groups on the surface of the lignin are subjected to a crosslinking reaction, thereby obtaining a lignin solution;
s3: dropwise adding the lignin solution obtained in the step S2 into liquid nitrogen for quick freezing, and then putting the obtained lignin solution frozen product into a freeze dryer for further freeze-drying treatment to obtain expanded lignin spherical particles;
s4: carrying out pyrolysis carbonization and cooling on the lignin spherical particles in the S3 in an inert atmosphere to obtain lignin coke;
s5: and (3) putting the lignin coke in the step S4 into an ethanol solution, stirring, washing, filtering to remove inorganic components, and drying to obtain the boron-doped carbon nano sheet product.
2. The method for preparing boron-doped carbon nano-sheets from lignin according to claim 1, wherein in the step S1, lignin of 1 mg-10 mg is added to each milliliter of water.
3. The method for preparing boron-doped carbon nano-sheets from lignin according to any one of claims 1 to 2, wherein in step S1, the mass ratio of potassium phosphate to lignin is 1:20 to 2:1, and the mass ratio of boron-based additive to lignin is 1:50 to 1:5.
4. The method for preparing boron-doped carbon nano-sheets from lignin according to claim 3, wherein in the step S3, the dropping speed of the lignin solution is 5 ml/min-20 ml/min.
5. The method for preparing boron-doped carbon nanoplatelets from lignin according to claim 4, wherein in step S3, the degree of vacuum of the freeze dryer is 1.0X10 -5 bar ~ 2.0×10 -5 The bar is frozen at the temperature of minus 60 ℃ to minus 70 ℃, the freezing time is 0.5-h-2 h, the drying temperature is 15-25 ℃, and the drying time is 24 h-48 h.
6. The method for preparing boron-doped carbon nano sheets from lignin according to claim 5, wherein in the step S4, the pyrolysis temperature is 700-900 ℃, the heating rate is 5-20 ℃ per minute, and the heat preservation time is 30-120 minutes.
7. The method for preparing boron-doped carbon nano sheets from lignin according to claim 6, wherein in the step S5, the volume ratio of ethanol to water in the ethanol solution is 1:1-1:3, the mass ratio of solute to solution is 1:500-1:1000, the stirring time is 10-24 h, the drying temperature is 45-65 ℃ and the drying time is 24-48 h.
8. The method for preparing boron-doped carbon nano sheets from lignin according to claim 6, wherein in the step S2, the power of ultrasonic treatment is 60-100W, the frequency is 40-60 Hz, and the time is 10-60 min.
9. The method of preparing boron-doped carbon nanoplatelets from lignin according to claim 8, wherein the boron-based additive is selected from one or more of boric acid, ammonium borate, sodium tetraborate, and potassium tetraborate.
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