CN108298525B - Graphene microcrystal and preparation method thereof - Google Patents
Graphene microcrystal and preparation method thereof Download PDFInfo
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- CN108298525B CN108298525B CN201810223709.6A CN201810223709A CN108298525B CN 108298525 B CN108298525 B CN 108298525B CN 201810223709 A CN201810223709 A CN 201810223709A CN 108298525 B CN108298525 B CN 108298525B
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Abstract
The invention provides a graphene microcrystal and a preparation method thereof, wherein the graphene microcrystal is novel glassy carbon and is a three-dimensional network structure of chemically bonded graphene microcrystal, the graphene microcrystal is a graphene segment formed by sp2 state carbon atoms, and the three-dimensional network structure is a long-range disordered short-range ordered three-dimensional microcrystal structure formed by bonding the sp3 state carbon atoms with the graphene segment. The raw material for preparing the graphene microcrystal is lignin separated from biomass, including lignin extracted from black liquor generated by papermaking and cellulosic ethanol, crop straws, bagasse and wood. The lignin can be used as a raw material alone or mixed with certain high molecular substances, such as phenolic resin, furfural resin, epoxy resin and the like. The preparation method uses the green renewable resource lignin to prepare the graphene microcrystal, so that the production cost of the graphene microcrystal is reduced, and ecological and environmental protection is facilitated.
Description
Technical Field
The invention belongs to a carbon material branch in the field of material science and technology, and particularly relates to a graphene microcrystal and a preparation method thereof.
Background
The carbon material is an important branch of the material field, and novel allotropes of carbon and novel carbon materials are continuously emerged in the scientific theory and the technical field from the end of the last century to the present century. Only 3 carbon allotropes of graphite, diamond and amorphous carbon were known before the 80 s of the 20 th century, and fullerene (carbon 60, C) which is an allotrope of carbon was successively found after the 80 s of the 20 th century60) Carbon nanotubes and graphene. Particularly, Graphene (Graphene) discovered in 2004 is a two-dimensional planar crystal on the atomic scale of carbon elements, breaks through the concept of the traditional physics, forms a new scientific theory and a new technology, and also brings a miraculous new material for the field of material science. Vitreous carbon is an allotrope of carbon discovered by scientists in the uk and japan in the 1950 s and 1960 s,the glass-ceramic composite material has a microcrystalline structure like ceramic and glass, integrates the properties of glass, ceramic and graphite, has high strength, high elasticity, heat resistance and chemical corrosion resistance like glass and ceramic, has broken slag with sharp edges and rounded sections, has hardness far higher than that of graphite, and has electrical conductivity higher than that of graphite.
With the continuous development of carbon science and technology, people's understanding of the structure and properties of glassy carbon is further deepened. Scientists in the last 60, 70 s recognized that glassy carbon is a microcrystalline structure of carbon similar to glass and ceramics, which is different from amorphous carbon such as carbon black and activated carbon; and also different from crystalline carbon such as graphite and diamond. Glassy carbon does not graphitize and diamondize at any high temperature and pressure. Fullerenes and carbon nanotubes were discovered in succession after the last 80 s, and scientists made new explanations of the structure of glassy carbon, which is believed to be a microcrystal of fullerenes and/or carbon nanotubes, and still scientists believed glassy carbon to be a large fullerene structure. The scientific community at present generally considers that glassy carbon is a microcrystal structure of sp2 state carbon atoms bonded with sp3 state carbon atoms, and comprises fullerene microcrystals and carbon nanotube microcrystals, so that a three-dimensional microcrystal network system of short-range ordered and long-range disordered carbon is formed. This peculiar structure brings a number of magical properties to the glassy carbon: low density, high strength, high elasticity, high electrical and thermal conductivity, abnormally high strength at ultra-high temperatures (above 2400 ℃), high permeation resistance to non-oxygen gases and liquids, and chemical stability. These superior properties have led to the widespread use of glassy carbon in many extreme conditions and harsh environments, particularly in the military and aerospace fields. The traditional raw materials for preparing glassy carbon are phenolic resin, furfural resin and epoxy resin, which are called precursors of glassy carbon. The precursors are slowly heated to the high temperature of 800 ℃ to 2400 ℃ according to a certain temperature control program under the normal pressure or high pressure in an inert gas environment, and then are subjected to pyrolysis and carbon atom recombination to generate the glassy carbon. To obtain high quality bulk glassy carbon, commercial production heats up to 120 hours, even 400 hours, and pressures up to 40 GPa.
Some researchers believe that graphene discovered in 2004 is a crystal structure composed of sp2 carbon atoms like fullerene and carbon nanotube, and graphene fragments may also form a microcrystal of carbon glass, which is called graphene microcrystal, and is a new type of glassy carbon. The traditional raw materials for preparing the glassy carbon are prepared from stone raw materials such as petroleum, coal, natural gas and the like. The method takes green, renewable and cheap lignin as a raw material to prepare the graphene microcrystal. The lignin is separated and purified from the biomass waste, so that the waste is changed into valuable, the production cost is reduced, the consumption of coal and petroleum is reduced, and the ecology and the environment are protected. The lignin is a random macromolecular structure consisting of three phenolic monomers. The lignin has an elemental composition of about 63.4% carbon, 30% oxygen, 5.9% hydrogen, and 0.7% ash, and is a natural organic polymer substance with the highest carbon content, wherein the natural organic polymer substance has a large number of sp2 carbon atoms and a certain number of sp3 carbon atoms, which form a benzene ring structure. Upon pyrolysis, the sp2 state carbon atoms of lignin combine into graphene segments, and the sp3 state carbon atoms bond these graphene segments to form a graphene crystallite structure. Thus lignin is the best raw material for the preparation of graphene crystallites.
Disclosure of Invention
The invention discloses a 'graphene microcrystal' which particularly refers to a three-dimensional network microcrystal structure formed by graphene segments consisting of sp2 carbon atoms bonded by sp3 carbon atoms, is a novel type of glassy carbon and is different from glassy carbon taking fullerene and/or carbon nano tubes as microcrystals. The structure of the graphene microcrystal is shown in fig. 1, wherein parallel short lines are graphene segments, and sp3 carbon atoms are chemically bonded to each graphene segment at the intersection points of the graphene segments to form a three-dimensional network structure. The invention relates to a method for preparing graphene microcrystal by using green renewable resource lignin, which comprises lignin separated from biomass such as papermaking black liquor, cellulosic ethanol black liquor, crop straws, bagasse, waste wood and the like.
In contrast, the technical scheme of the invention is as follows:
a graphene microcrystal is a three-dimensional network structure of chemically bonded graphene microcrystals, wherein the graphene microcrystals are composed of graphene segments formed by sp2 state carbon atoms, and the graphene segments formed by sp2 state carbon atoms are connected through sp3 state carbon atoms in a covalent bond mode to form the three-dimensional network structure. The graphene crystallite of the present invention is distinguished from other types of glassy carbon by being a crystallite of carbon. The graphene microcrystal is a graphene segment formed by sp2 state carbon atoms, and the three-dimensional network structure is a microcrystal structure formed by graphene segments bonded by sp3 state carbon atoms and having long-range disorder and short-range order.
The graphene microcrystal is a novel type of glassy carbon and is a three-dimensional microcrystal network structure of carbon elements, wherein microcrystal units are graphene segments formed by sp2 state carbon atoms, and the graphene segments are connected with each other through sp3 state carbon atoms in a covalent bond mode. The microcrystal in the existing glassy carbon is fullerene and/or carbon nanotube fragments consisting of sp2 state carbon atoms, and the graphene microcrystal in the technical scheme of the invention is a graphene crystal discovered after 2004, so the glass is novel carbon glass.
As a further improvement of the invention, the graphene material comprises a three-dimensional network structure formed by connecting 4 chemically-bonded graphene microcrystal fragments with 4-face-oriented carbon atoms in sp3 states.
The preparation method of the graphene microcrystal adopts lignin melting, thermal cracking, carbonization at the temperature of more than 800 ℃, high-temperature graphitization, annealing cooling and the like.
As a further improvement of the invention, the preparation method of the graphene microcrystal comprises the following steps:
step S1, extracting lignin;
step S2, melting of lignin: heating the lignin to above 160 ℃ and keeping the temperature for above 180 minutes; preferably, the temperature is increased to 180 ℃;
step S3, thermal cracking of lignin: heating lignin to above 400 ℃ in an inert gas environment and keeping the temperature for more than 180 minutes; preferably, the lignin is heated to more than 450 ℃ and kept for more than 180 minutes;
step S4, carbonization of lignin: continuously heating the thermally cracked lignin obtained in the step S3 to above 800 ℃ in an inert gas environment, and keeping the temperature for above 240 minutes, wherein the gas pressure is 0.1MPa to 40 GPa;
step S5, further graphitization of the raw material: graphitizing the carbonized raw material in the step S4 at the temperature of 1000-2400 ℃, keeping the temperature for more than 240 minutes and keeping the air pressure of 0.1 MPa-40 GPa;
and step S6, annealing and cooling to room temperature.
The atmosphere in step S2 is an air atmosphere or an inert gas atmosphere.
As a further improvement of the invention, the temperature is raised to 1000 ℃ or higher in step S4.
As a further improvement of the present invention, the inert gas environment is nitrogen, argon, helium, water vapor, carbon dioxide or a combination thereof.
As a further improvement of the present invention, in step S1, the extracting of lignin comprises: adding an ethanol-water solvent into a lignin-containing raw material, heating to 180-220 ℃ in a sealed reaction kettle in an environment with the pH value of 3-4, keeping the temperature for more than 60 minutes, cooling, separating out insoluble cellulose, evaporating the solvent of the residual solution to obtain lignin containing hemicellulose and hydrolysis sugar, putting the lignin containing the hemicellulose and the hydrolysis sugar into a dilute sulfuric acid solution, heating at 80-100 ℃ for 80-150 minutes, filtering, and washing to obtain pure lignin.
The preparation of the graphene microcrystal requires specific pure lignin as a raw material, and most of lignin sold in the market is non-pure lignin such as lignosulfonate and the like, so that the preparation of the graphene microcrystal is not suitable. By adopting the technical scheme, pure lignin can be obtained.
As a further improvement of the invention, the added ethanol-water solvent has the ethanol content of 55 percent by mass and the dilute sulfuric acid concentration of 10 percent by mass.
As a further improvement of the present invention, the raw material of the lignin comprises at least one of papermaking black liquor, cellulosic ethanol black liquor, crop straw, bagasse or wood.
Preferably, bagasse is used as a raw material, 55% by mass of ethanol-water is used as a solvent, the mixture is heated to 200 ℃ in a sealed reaction kettle under a slightly acidic environment with the pH value of 3 to 4 and is kept for 90 minutes, lignin and part of hemicellulose are dissolved in an ethanol-water solution, insoluble cellulose is separated after cooling, and the solvent of the remaining solution is evaporated to obtain solid lignin, wherein a small amount of hemicellulose and hydrolysis sugar are contained. Putting lignin containing a small amount of hemicellulose into a 10% dilute sulfuric acid solution, heating at 90 deg.C for 120 min, wherein the hemicellulose is hydrolyzed into soluble sugar, filtering insoluble lignin, and washing with deionized water for 3 times to neutrality to obtain pure lignin. Pure lignin can also be obtained by other methods.
By adopting the technical scheme, the graphene microcrystal is prepared by taking the green, renewable and cheap lignin as a raw material, so that the preparation method is more environment-friendly. The lignin is separated and purified from the biomass waste, so that the waste is changed into valuable, the production cost is reduced, the consumption of coal and petroleum is reduced, and the ecology and the environment are protected.
The lignin is a random macromolecular structure consisting of three phenolic monomers. The lignin has an elemental composition of about 63.4% carbon, 30% oxygen, 5.9% hydrogen, and 0.7% ash, and is a natural organic polymer substance with the highest carbon content, wherein the natural organic polymer substance has a large number of sp2 carbon atoms and a certain number of sp3 carbon atoms, which form a benzene ring structure. Upon pyrolysis, the sp2 state carbon atoms of lignin combine into graphene segments, and the sp3 state carbon atoms bond these graphene segments to form a graphene crystallite structure. Thus lignin is the best raw material for the preparation of graphene crystallites.
As a further improvement of the invention, in order to improve the quality of the graphene microcrystal, lignin can be copolymerized with phenolic resin, epoxy resin and the like to form a compact larger block, and then the compact larger block is used for preparing the graphene microcrystal.
Compared with the prior art, the invention has the beneficial effects that:
firstly, with the technical solution of the present invention, the graphene microcrystal is different from crystalline carbon, such as graphite and diamond; and different from amorphous carbon, such as carbon black and activated carbon, the carbon material is a long-range disordered short-range ordered microcrystal structure, wherein microcrystals are fragments of graphene consisting of sp2 state carbon atoms, and the graphene fragments are combined with each other by sp3 state carbon atoms through covalent bonds to form a three-dimensional network structure which has special physical properties, chemical properties and electronic properties.
Secondly, the technical scheme of the invention replaces phenolic resin, furfural resin and epoxy resin which take petroleum and coal as raw materials with green renewable resources lignin to prepare the graphene microcrystal, the graphene microcrystal comprises lignin separated from biomass such as papermaking black liquor, cellulosic ethanol black liquor, crop straws, bagasse, waste wood and the like, and the raw materials for producing the lignin are waste and pollutants of paper making industry, agriculture and forestry, so that the production cost of the graphene microcrystal is reduced, the ecological and environmental protection is facilitated, and the graphene microcrystal has important social benefits, environmental protection and ecological benefits.
Thirdly, the invention has high economic benefit. The phenolic resin, the furfural resin and the epoxy resin are prepared by taking petroleum, natural gas and coal as raw materials through a chemical method, and the cost is far higher than that of lignin prepared from wastes. The graphene microcrystal is prepared by taking lignin as a raw material, so that the cost of the graphene microcrystal can be greatly reduced.
Fourth, lignin is the best raw material for preparing graphene crystallites from a chemical structure and a chemical principle. The lignin consists of three elements of carbon, oxygen and hydrogen, the carbon content is up to more than 63%, hydrogen and oxygen escape in the form of water molecules in thermal cracking, and the obtained glassy carbon does not contain any heteroatom. Particularly, the lignin contains a benzene ring structure consisting of a large number of carbon atoms in sp2 states, and the ratio of the carbon atoms in sp2 and sp3 states is about 24:9, so that the generation of graphene microcrystals is facilitated.
Drawings
Figure 1 is a schematic structural view of a graphene crystallite.
FIG. 2 chemical structure diagram of lignin.
Figure 3 comparison of graphene crystallite samples prepared with lignin at different temperatures; FIG. 3a shows the temperature at 800 ℃ and FIG. 3b shows the temperature at 1200 ℃.
FIG. 4 is a SEM photograph of a graphene microcrystal powder prepared at 1000 ℃.
FIG. 5 is a schematic diagram of a preparation of a graphene microcrystal using a tube furnace.
Figure 6 is a schematic of the preparation of graphene crystallites using a hydrothermal kettle.
Fig. 7 is a scanning electron microscope SEM image of a graphene microcrystal sample prepared by the one-step method according to embodiment 1 of the present invention.
Figure 8 Raman spectra of the one-step method prepared graphene crystallite samples of example 1 of the present invention.
Figure 9 XRD pattern of the one-step method prepared graphene microcrystal sample of example 1 of the present invention.
Figure 10 is a photograph of a sample of graphene crystallites prepared by the two-step process of example 2 of the present invention.
Figure 11 XPS spectra of graphene crystallite samples prepared by the two-step method of example 2 of the present invention.
The reference numerals include:
1-nitrogen cylinder, 2-tube furnace heater, 3-temperature controller, 4-quartz glass tube, 5-gas washing cylinder, 6-graphite crucible, 7-lignin, 8-stainless steel tank, 9-spiral steel cover, 10-small beaker; 11-graphite tank, 12-red copper sealing cover, 13-deionized water and 14-lignin.
Detailed Description
Preferred embodiments of the present invention are described in further detail below.
A graphene microcrystal, a microcrystal structure formed by graphene crystal fragments composed of sp2 state carbon atoms bonded by sp3 state carbon atoms, is a new type of glassy carbon, and is different from other types of glassy carbon, and the structure of the microcrystal is shown in FIG. 1. The parallel short lines in fig. 1 are graphene segments, and sp3 carbon atoms are chemically bonded to each graphene segment at the intersection points of the graphene segments to form a three-dimensional network structure. The invention relates to a method for preparing graphene microcrystal by using green renewable resource lignin, which comprises lignin separated from biomass such as papermaking black liquor, cellulosic ethanol black liquor, crop straws, bagasse, waste wood and the like.
The preparation method comprises the following steps:
1. extraction of pure lignin
The method for extracting lignin is not the main point of the invention, but the preparation of the graphene microcrystal requires specific pure lignin as a raw material, and most of commercially available lignin is non-pure lignin such as lignosulfonate, and is not suitable for preparing the graphene microcrystal. The method comprises the steps of taking bagasse as a raw material, taking 55% by mass of ethanol-water as a solvent, heating to 200 ℃ in a sealed reaction kettle under a slightly acidic environment with the pH value of 3-4, keeping the temperature for 90 minutes, dissolving lignin and part of hemicellulose in an ethanol-water solution, separating insoluble cellulose after cooling, and evaporating the solvent in the residual solution to obtain solid lignin, wherein the solid lignin contains a small amount of hemicellulose and hydrolysis sugar. Putting lignin containing a small amount of hemicellulose into a 10% dilute sulfuric acid solution, heating at 90 deg.C for 120 min, wherein the hemicellulose is hydrolyzed into soluble sugar, filtering insoluble lignin, and washing with deionized water for 3 times to neutrality to obtain pure lignin. Pure lignin can also be obtained by other methods. The chemical structure of lignin and a photograph of lignin powder extracted from bagasse are shown in fig. 2.
2. Preparation of graphene microcrystals
The preparation process of the graphene microcrystal may be divided into the following stages.
(1) Uniform melting of lignin powder
The lignin has no fixed melting point, and the glass point is between 160 ℃ and 180 ℃. Slowly raising the temperature from room temperature to 180 ℃, keeping the temperature for more than 180 minutes, and enabling water molecules and other small molecules to escape sufficiently without causing cavities in the molten lignin. This process can be carried out in air.
(2) Thermal cracking of lignin
Thermal cracking of lignin is carried out in an inert gas atmosphere, and the lignin is slowly heated from 180 ℃ to 450 ℃ in an inert gas flow and is kept for more than 180 minutes. In the process, water molecules and organic small molecules which are split off flow out along with the inert gas, and sp 2-state carbon atoms are recombined into graphene fragments. The process is sufficiently slow, preferably to maintain a certain pressure.
(3) High temperature carbonization of lignin
The primarily carbonized lignin is slowly raised from 450 c to a specified high temperature, e.g. 800 c (or 1000 c, 1200 c, 2400 c, etc.) in a stream of inert gas and held for over 240 minutes. In the process, the sp3 state carbon atoms bond graphene fragments consisting of sp2 state carbon atoms through firm chemical bonds to form a stable three-dimensional network.
(4) Annealing and cooling of graphene microcrystals
And (3) slowly annealing the graphene microcrystal generated in the steps, gradually cooling, reducing the defects formed by the dangling bonds and free carbon atoms of the graphene microcrystal as much as possible, and cooling to normal temperature to obtain a sample of the graphene microcrystal.
Stage (2) (thermal cracking of lignin) is a solid-gas reaction, and stage (3) (high temperature carbonization of lignin) is a solid-phase reaction, requiring a sufficiently long reaction time.
The quality of the graphene microcrystal prepared by taking lignin as a raw material according to the process, including the size, porosity, uniformity, density, hardness, elasticity, conductivity and the like of a block, has a great relationship with a temperature control program, the slower the temperature rise is, the longer the heat preservation time is, the temperature and pressure are high enough, the better the quality is, the commercial production needs 120 to 400 hours, the temperature is up to 2400 ℃, and certain high pressure is needed. Glassy carbons prepared at atmospheric pressure, in relatively short time and at relatively low temperatures, such as atmospheric pressure, 20 hours and 1200 ℃, although having relatively high porosity, the milled powders still possess the characteristics and properties of graphene crystallites. Figure 3 is a graph of graphene crystallite samples prepared with lignin at 800 ℃ (up) and 1200 ℃ (down), the former being more porous and the latter being more dense. Fig. 4 is a Scanning Electron Microscope (SEM) photograph of a graphene microcrystal powder prepared at 1000 ℃, and the ground graphene microcrystal particles are like cullet, have sharp edges and rounded sections, and have high hardness and elasticity, unlike graphene of the prior art.
After pure lignin is prepared, the quality control of graphene crystallite preparation mainly depends on a well-designed temperature control program. For convenience of explanation, the temperature control procedure and embodiment will be described below in terms of a preparation process at normal pressure, 1000 ℃ and 20 hours.
According to the tube furnace device shown in FIG. 4, 10 g of lignin powder was placed in a graphite crucible, the crucible cover was closed, the graphite crucible was placed in the middle of a quartz tube, vacuum-pumping was performed with a vacuum pump and nitrogen gas was filled, after repeating the operation twice, a nitrogen gas flow of 50ml/min was maintained, the power was turned on, and the temperature control program was set as follows.
Step S1: lignin melting stage
Heating from room temperature 20 deg.C to 180 deg.C at a heating rate of 1 deg.C/min, maintaining the temperature at 180 deg.C for 60 min to melt lignin and allow water molecules and other small molecules to escape.
Step S2: thermal decomposition stage of lignin
Heating from 180 ℃ to 450 ℃ at the heating rate of 1 ℃/minute, keeping the temperature of 450 ℃ for 120 minutes, carrying out thermal cracking reaction on the wood macromolecular structure, and enabling the generated volatile micromolecules to escape along with nitrogen. At this stage partial carbonization of lignin has occurred. To avoid the occurrence of voids, a certain high pressure is required at this stage.
Step S3: lignin carbonization and graphitization stage
Raising the temperature from 450 ℃ to 1000 ℃ at the temperature raising rate of 2 ℃/min, and keeping the temperature at 1000 ℃ for more than 180 minutes to further carbonize and graphitize the lignin. At this stage, the sp2 state carbon atoms start to graphitize to form graphene fragments, and the free sp3 state carbon atoms are combined with the dangling bonds at the edges of the graphene fragments to form a three-dimensional network structure.
Step S4: annealing and cooling stage of graphene microcrystal
Cooling from 1000 ℃ to 500 ℃ at a cooling rate of 3 ℃/min, and then, starting to naturally cool until the room temperature.
The above procedure took 1172 minutes, about 19.5 hours.
Example 1
In the embodiment, pure lignin extracted from bagasse by an ethanol-water organic solvent method is used as a raw material, a one-step preparation scheme is adopted, the reaction is carried out in a quartz tube type electric furnace, the reaction is completed in a nitrogen atmosphere, and the device is shown in fig. 5.
The preparation process comprises the following operations:
step S1: experimental apparatus and raw Material preparation
According to the tube furnace device shown in FIG. 5, 10 g of lignin powder was placed in a graphite crucible, the crucible cover was closed, the graphite crucible was placed in the middle of a quartz tube, vacuum-pumping was performed with a vacuum pump and nitrogen gas was filled, after repeating the operation twice, nitrogen gas flow of 50ml/min was maintained, and the power was turned on.
Step S2: setting temperature control program
The temperature control program of this example is shown in table 1, and has 8 operation stages in total, and the total operation time of the first 7 stages is 1172 minutes, about 19.5 hours, excluding the time of the natural cooling stage 8. The temperature control program was set as in table 1. And stored in the temperature controller.
Table 1 example 1 temperature control procedure for the "one-step" preparation protocol
Step S3: running a temperature control program
And starting a button for operating the program and a button for heating the power supply, starting the preparation process of the graphene microcrystal to automatically operate, operating for about 19.5 hours, opening the tube furnace after the furnace temperature is cooled to the room temperature, and taking out the graphene microcrystal sample. The reactions occurring in the above 8 stages are explained below.
Stage 1: 20 ℃ to 180 DEG C
The temperature was raised to the glass point of lignin and lignin began to melt.
And (2) stage: 180 ℃ to 180 DEG C
The temperature is kept at the glass point of the lignin, the lignin is fully melted, and water molecules and other small molecules are fully escaped.
And (3) stage: 180 ℃ to 450 DEG C
Slowly raising the temperature to the thermal cracking temperature of lignin, and starting cracking and partial carbonization of lignin macromolecules.
And (4) stage: 450 to 450 DEG C
The temperature is kept at the thermal cracking temperature of lignin, the lignin macromolecules are fully cracked and carbonized, and volatile micromolecules generated in the process slowly escape.
And (5) stage: 450 to 1000 DEG C
Slowly raising the temperature to 1000 ℃, and starting graphitization by sp2 state carbon atoms at the thermal cracking temperature of lignin to form small graphene crystals.
And 6: 1000 ℃ to 1000 DEG C
And keeping the temperature at 1000 ℃, fully combining sp2 carbon atoms into graphene crystal fragments, and combining free sp3 carbon atoms with dangling bonds at the edges of the graphene fragments to form a three-dimensional network structure.
And (7) stage: 1000 ℃ to 500 DEG C
The temperature is slowly reduced to 500 ℃, so that the defects caused by free carbon atoms and dangling bonds are reduced as much as possible, and a stable glassy carbon three-dimensional network structure is formed.
A Scanning Electron Microscope (SEM) photograph of the microcrystalline graphene powder produced in this example is shown in fig. 4; the Raman spectrum of the graphene crystallite is shown in figure 8; an XRD pattern of the graphene microcrystal is shown in fig. 9.
Example 2
The preparation reaction of the graphene microcrystal in example 1 was always carried out under normal pressure,
if a certain pressure is kept in the carbonization and graphitization stages, the method is very beneficial to improving the quality of the graphene microcrystal. However, the reaction conditions of high temperature and high pressure are very strict for the equipment. To create a high pressure reaction environment, this example used a "two-step" protocol to produce glassy carbon. The first step of the "two-step process" was carried out in a hydrothermal reaction kettle, 50ml of deionized water and 10 g of lignin were added to a 100ml hydrothermal reaction kettle, and the pressure of water vapor was about 130mPa at 350 ℃, at which temperature and pressure the lignin was primarily carbonized. And secondly, transferring the primarily carbonized lignin into a tubular furnace, and further carbonizing and graphitizing the primarily carbonized lignin in a nitrogen environment at normal pressure to prepare the graphene microcrystal.
The specific operation steps are described below.
The first step is as follows: preliminary carbonization in a hydrothermal kettle
Step S1: installation of hydrothermal kettle device and raw material addition
The preliminary carbonization of lignin is carried out in a hydrothermal kettle, and the structural schematic diagram of the hydrothermal kettle is shown in FIG. 6. A small beaker is placed in a steel barrel of a 100ml stainless steel water heating kettle, 50ml deionized water and 10 g lignin are added, a red copper sealing ring is placed between the steel barrel and a steel barrel cover, and the steel barrel cover is screwed by a large-size wrench to ensure reliable sealing.
Step S2: preliminary carbonization in hydrothermal reactor
And (3) putting the hydrothermal kettle into a muffle furnace, arranging a temperature controller, heating to 350 ℃ at the heating rate of 1 ℃/min, keeping the temperature for 120 min, and naturally cooling to room temperature.
Step S3: preliminary drying of carbonized lignin
Taking out the small beaker in the hydrothermal kettle, pouring out the water solution, taking out the primary carbonized lignin, and drying in a constant temperature oven at 120 ℃ for 2 hours.
The dried carbonized lignin is subjected to a second operation in a tube furnace for further carbonization and graphitization.
The second step is that: further carbonization and graphitization in a tube furnace
The process was carried out in a tube furnace, the specific operation was exactly the same as in the "one-step" protocol of example 1 and is not described further. A photograph of a sample of the graphene crystallites prepared in example 2 is shown in FIG. 10. the "two-step" graphene crystallites are mirror-like shiny and reflective flakes having a length and width of about 3 to 4 mm due to the tumbling and flushing action of the high pressure steam in the hydrothermal kettle. Due to the high pressure effect in the hydrothermal kettle, the quality of the graphene microcrystal prepared by the two-step method is obviously better than that of the graphene microcrystal prepared by the one-step method, but the size of the graphene microcrystal sample is smaller. An XPS spectrum of the graphene microcrystal generated in this example is shown in fig. 11.
The effectiveness of the technical scheme of the present invention is fully demonstrated from Raman spectra (fig. 8), XRD spectra (fig. 9) and XPS spectra (fig. 11) of the prepared graphene microcrystals. The D peak and the G peak in the Raman spectrum of the graphene microcrystal are completely consistent with those of reduced graphene oxide reported in the literature at the upper right corner (Nano Res (2008)1: 273291); the presence of graphene crystals in the graphene crystallite sample was confirmed by the complete agreement of the diffraction peak at 25.5 ° 2 θ in the XRD pattern of the graphene crystallites with that of the reduced graphene oxide reported in the upper right-hand literature (Materials research.2017; 20(1): 53-61). The XPS spectrum of fig. 11 has a large peak of sp2 carbon atoms and a small peak of sp3 carbon atoms, and the peak area of sp2 carbon atoms is much larger than that of sp3 carbon atoms, which is consistent with the composition of the graphene microcrystal. The graphene crystallite samples were felt to be very hard when crushed with an agate mortar, exhibiting a hardness and stiffness similar to glass and ceramics, quite unlike graphite and other carbon materials of low hardness.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.
Claims (8)
1. A graphene microcrystal, characterized by: the graphene microcrystalline is of a three-dimensional network structure of chemically bonded graphene microcrystals, the graphene microcrystals are composed of graphene segments formed by sp2 state carbon atoms, and the graphene segments formed by sp2 state carbon atoms are connected through sp3 state carbon atoms in a covalent bond mode to form the three-dimensional network structure.
2. The graphene microcrystal according to claim 1, characterized in that: the graphene microcrystalline structure comprises a three-dimensional network structure formed by connecting 4 chemical bonds with 4 oriented surface bodies of sp3 carbon atoms and graphene microcrystalline fragments.
3. The method for producing a graphene microcrystal according to claim 1, characterized in that: which comprises the following steps:
step S1, extracting lignin;
step S2, melting of lignin: heating the lignin to above 160 ℃ and keeping the temperature for above 180 minutes;
step S3, thermal cracking of lignin: heating lignin to above 400 ℃ in an inert gas environment and keeping the temperature for more than 180 minutes;
step S4, carbonization of lignin: continuously heating the thermally cracked lignin obtained in the step S3 to above 800 ℃ in an inert gas environment, and keeping the temperature for above 240 minutes, wherein the gas pressure is 0.1MPa to 40 GPa;
step S5, graphitizing the carbonized raw material in step S4 at the temperature of 1000-2400 ℃, keeping the graphitized raw material for over 240 minutes and keeping the air pressure of 0.1 MPa-40 GPa;
and step S6, annealing and cooling to room temperature.
4. The method for producing a graphene microcrystal according to claim 3, characterized in that: in step S4, the temperature is raised to 1000 ℃ or higher.
5. The method for producing a graphene microcrystal according to claim 3, characterized in that: the inert gas environment is one gas or a mixed gas environment of nitrogen, argon, helium, water vapor and carbon dioxide.
6. The method for producing a graphene microcrystal according to claim 3, characterized in that: in step S1, the extracting of lignin includes: adding an ethanol-water solvent into a lignin-containing raw material, heating to 180-220 ℃ in a sealed reaction kettle in an environment with the pH value of 3-4, keeping the temperature for more than 60 minutes, cooling, separating out insoluble cellulose, evaporating the solvent of the residual solution to obtain lignin containing hemicellulose and hydrolysis sugar, putting the lignin containing the hemicellulose and the hydrolysis sugar into a dilute sulfuric acid solution, heating at 80-100 ℃ for 80-150 minutes, filtering, and washing to obtain pure lignin.
7. The method for producing a graphene microcrystal according to claim 6, characterized in that: the ethanol-water solvent comprises 55% of ethanol by mass and 10% of dilute sulfuric acid by mass.
8. The method for producing a graphene microcrystal according to claim 6, characterized in that: the raw material of the lignin comprises at least one of papermaking black liquor, cellulosic ethanol black liquor, crop straw, bagasse or wood.
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