Porous carbon with controllable pores and preparation method thereof

Abstract

The invention discloses porous carbon with controllable pores and a preparation method thereof, and the preparation method specifically comprises the following steps: (1) pretreatment: crushing a biochar raw material and then drying the crushed biochar raw material; (2) chemical activation: mixing the pretreated raw materials with an activating agent, heating to 600-900 DEG C in an inert atmosphere, performing heat treatment, cleaning with deionized water to be neutral after heat treatment, and drying; (3) pickling and purifying: firstly soaking and pickling the activated product with a compound mixed pickling solution, then repeatedly cleaning with deionized water until the product is neutral, drying and removing moisture; (4) grading is conducted, specifically, passivated products are screened through a grading wheel; wherein pickling purification is the core step of the method, the compound mixed pickling solution is formed by mixing hydrochloric acid, nitric acid and hydrofluoric acid with different concentrations and different proportions, the characteristics and advantages of various pickling solutions are fully utilized, the synergistic effect of the pickling solutions is exerted, the pore structure of the porous carbon is regulated and controlled through mutual matching of the concentrations and the proportions, and the porous carbon is prepared. And a new scheme is provided for accurate application of the porous carbon.

Description

Technical Field

[0001] The present invention relates to a porous carbon material, specifically to a pore-controllable porous carbon based on biochar as a raw material and its preparation method, belonging to the technical field of biomass porous carbon materials. Background Art

[0002] Porous carbon materials are carbon materials with special structures and properties, having excellent characteristics such as high specific surface area, high pore volume, and adjustable pore size structure. At the same time, their unique micropore (<2 nm), mesopore (2 - 50 nm), and macropore (>50 nm) synergistic system endows the materials with differentiated application potential. These excellent characteristics and differentiated capabilities make porous carbon materials have broad application prospects in various fields, such as electrochemical energy storage, catalysis, adsorption separation, biomedicine, etc.

[0003] In the field of electrochemical energy storage, the high specific surface area (up to 3000 m² / g) of the microporous structure provides abundant active sites for charge storage, and its electric double layer capacitance effect exhibits excellent power density in supercapacitors. When the pore size expands to the mesopore scale, its three-dimensional connected pore channels significantly improve the ion transport kinetics. For example, in lithium batteries, more micropores can provide more lithium storage active sites, increasing the lithium storage capacity; in addition, it can increase the diffusion coefficient of lithium ions in soft carbon and increase the contribution of the adsorption reaction capacity with fast kinetic characteristics, thereby achieving a higher reversible capacity and rate performance, and at the same time avoiding the passivation phenomenon of the electrode; the presence of mesopores is beneficial to shortening the ion diffusion path, promoting the rate charge and discharge performance, providing a fast channel for the transport of electrolyte ions, and improving electrolyte penetration. In short, micropores provide capacitance, and mesopores and macropores improve current.

[0004] The application in the field of catalysis fully reflects the spatial confinement effect of the pore structure. When loading platinum-based catalysts in mesoporous carbon, its regular 3 - 8 nm pore channels not only provide a uniform dispersion space for metal nanoparticles, but also can have a strong interaction with the metal through the pore wall defect sites, increasing the CO oxidation reaction activity by 40%. For microporous carbon materials, its sub-nanometer window pore size can act as a molecular sieve to achieve selective separation of reactants and products in the ethylbenzene dehydrogenation reaction. Recent research has further found that carbon-based materials with 0.5 - 0.7 nm ultra-micropores can achieve a separation ratio of up to 200:1 for the CO2 / N2 mixed gas.

[0005] In the biomedical field, the synergistic effect of multi-level pore structures is particularly prominent. The three-dimensional interconnected network constructed by the macroporous skeleton (50-200μm) provides a scaffold for cell proliferation, the antibiotics loaded in the mesoporous channels (10-30nm) can achieve sustained release, and the functional group modification of the microporous surface (1-2nm) can specifically capture various toxin molecules. Clinical trials have shown that the clearance rate of β2-microglobulin by this multi-level pore carbon hemoperfusion device can reach 85%, which is significantly better than traditional resin materials.

[0006] With the rapid development of the above fields, modern science and technology have put forward new generation functional requirements for porous carbon materials, such as intelligent response and multi-field coupling coordination. Traditional single-level pore structure materials are difficult to meet these composite performance requirements due to their single function. Based on the design concept of hierarchical porous structure, by constructing a multi-scale pore synergistic system with different pore structures and contents, it provides new possibilities for the realization of the third generation of intelligent carbon materials. Although this direction has broad prospects, how to accurately control the pore level and achieve the unity of surface functionalization and structural stability is still a key technical bottleneck that needs to be broken through in the current field of material preparation. Summary of the invention

[0007] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art, propose a porous carbon with controllable pores and a preparation method thereof, ensure the precise application of the porous carbon and improve its application performance. The preparation method is simple and easy, and the prepared porous carbon material has a high specific surface area, a controllable pore structure, and significantly improves its electrochemical performance.

[0008] In order to solve the above technical problems, the present invention provides a method for preparing porous carbon with controllable pores, the process comprising pretreatment - activation - acid washing purification - classification, wherein: (1) Preprocessing The biochar raw material is crushed to 80-200 mesh to increase the reaction contact area, and then dried at 80-120℃ for 12-24h to ensure complete removal of moisture in the carbon material; (2) Chemical activation The raw material pretreated in step (1) is mixed with an activator, and the temperature is raised to 600-900°C for heat treatment in an inert atmosphere, and the temperature is kept for 1-2 hours to expand the pores and adjust the pore distribution. The proportion of the activator and the temperature determine the final specific surface area. After the heat treatment, the raw material is washed with deionized water until it is neutral, and then dried; The activator is at least one of potassium hydroxide (KOH) and sodium hydroxide (NaOH), and the mass ratio of activator to pretreated raw material is 1:1-4:1; (3) Acid washing and purification First, a compound mixed acid cleaning solution is customized according to the required pore structure of the porous carbon. The compound mixed acid cleaning solution is composed of hydrochloric acid, nitric acid, and hydrofluoric acid with different concentrations and proportions. Subsequently, the product after activation is pickled in an ultrasonic water bath at 60°C for 15 minutes to remove potassium metal in the biochar. After potassium is dissolved out, pores can be left. Then, it is repeatedly washed with deionized water until neutral to remove the acid solution and impurities and stabilize the material structure. Subsequently, it is dried at 105°C to remove moisture; (4)Classification The passivated product is screened through a classification wheel. Preferably, all materials with D00 less than 1μm are screened out, those with D10 less than 3μm are screened out, and all those with D50 greater than 10μm are screened out; The further limited technical solution of the present invention: Further, in the preparation method of the pore-controllable porous carbon described above, in step (1), the raw material is a biochar raw material, and the biochar raw material is at least one of rice husk biochar, coconut shell biochar, or straw biochar.

[0009] Technical effect: The carbon source of the present invention is biomass biochar, which is renewable, has a wide source, is sustainable, has a low cost, and is green and environmentally friendly.

[0010] In the preparation method of the pore-controllable porous carbon described above, in step (2), the inert atmosphere is nitrogen or argon.

[0011] In the preparation method of the pore-controllable porous carbon described above, in step (2), the mass ratio of the pretreated raw material to the activator is 1:3.

[0012] In the preparation method of the pore-controllable porous carbon described above, in step (2), the activator is a solid mixture of KOH and NaOH.

[0013] Technical effect: The porous carbon material prepared by KOH activation usually has a specific surface area of more than 2000 m 2 / g. Its pore distribution is mainly micropores. However, too many micropores have a negative effect on the capacitance performance of the porous carbon material, restricting the transfer of electrolyte ions during rapid charging. Compared with KOH, the mesopore ratio of the porous carbon material prepared by NaOH activation is higher and it has better conductivity, which all have a promoting effect on the capacitance performance. Due to the different activation reaction mechanisms, the activated carbon prepared by NaOH activation usually has a lower specific surface area, which is not conducive to improving the capacitance performance; the activator used in the present invention is a mixture of KOH and NaOH, taking into account their advantages and disadvantages and making the best use of them.

[0014] In the preparation method of the pore-controllable porous carbon described above, according to the required characteristics of the porous carbon material, such as specific surface area, porosity, ash content, etc., the compound mixed acid cleaning solution in step (3) is at least one of hydrochloric acid, nitric acid, and hydrofluoric acid.

[0015] In the preparation method of the aforementioned pore-controllable porous carbon, in step (3), according to the pore structure required for the porous carbon, it is preferably a mixture of hydrochloric acid (95% concentration), nitric acid (75% concentration), and hydrofluoric acid (35% concentration) as the compounded mixed acid cleaning solution, and the volume ratio is 3:3:1.

[0016] Technical effect: The present invention uses a compounded mixed acid solution as the acid cleaning solution. Among them, hydrochloric acid can effectively remove potassium metal in the carbon to achieve the effect of creating pores, while nitric acid can better remove the remaining activators and ash in the carbon material. Hydrofluoric acid can directionally remove silicon elements in the carbon material and etch and create pores in the carbon skeleton. That is, hydrochloric acid and nitric acid are beneficial to the formation of mesoporous structures, and hydrofluoric acid is beneficial to increasing the specific surface area and forming microporous structures. The mixed acid of the present invention makes full use of the synergistic effect, better cleans the activated product, removes alkali metals and ash in the alkali activation process, and at the same time can further regulate the pore structure, adjust the ratio of micropores and mesopores, and adjust the surface chemical properties by the ratio and concentration of the compounded acid solution.

[0017] The present invention also designs a pore-controllable porous carbon.

[0018] The beneficial effects of the present invention are: In step (1) of the present invention, during pretreatment, the raw materials are dried after being crushed. The drying step depends on the raw materials. In principle, it should ensure that the moisture of the raw materials is completely removed.

[0019] The commonly used chemical activation method in modern industry has the advantages of low reaction temperature, short reaction time, high yield of porous carbon materials, and well-developed pore structures of the obtained porous carbon materials. However, at the same time, parameters such as its specific surface area and pores are difficult to control due to the microscopic nature of chemical reactions, and the pore structure parameters of the final product are difficult to be regulated in detail. Therefore, it is difficult to give full play to the full efficacy of various pores in the porous carbon materials in applications. The alkali activation and acid washing carried out in the present invention are the key steps for preparing porous carbon, mainly forming the target pores through the chemical corrosion of the alkali activator and the synergistic etching effect of various acid solutions. The present invention strictly limits the addition amounts of the alkali activator and the acid cleaning solution, adjusts the addition amounts and ratios according to the pore structure required for the target product, and obtains a porous carbon material with rich micropores and mesopores and a controllable content ratio.

[0020] The present invention performs a grading treatment after acid washing to make the material reach the optimal specific surface area. Brief Description of the Drawings

[0021] Figure 1 It is a flow chart of the preparation method of the pore-controllable porous carbon in the embodiment of the present invention. Detailed Embodiments

[0022] To make the present invention more clearly understood, the following further describes a preparation method of a porous carbon material of the present invention with reference to the accompanying drawings. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Example 1

[0023] This example provides a preparation method of porous carbon with controllable pores. As shown in the Figure 1 process flow includes pretreatment - activation - pickling purification - classification, where: (1) Pretreatment: The biochar raw material, rice husk biochar, is crushed to 80 mesh to improve the reaction activity of the raw material and increase the specific surface area. Subsequently, the crushed raw material is dried at 80 °C for 24 h; (2) Chemical activation: The raw material pretreated in step (1) is mixed with an activator and heated to 600 °C for heat treatment in an inert atmosphere of nitrogen. The heating rate is 3 °C / min -1 , and kept warm for 1 h. After heat treatment, it is washed with deionized water until neutral and then dried; The activator is a mixture of KOH and NaOH. By molar ratio, KOH:NaOH = 3:2. By mass ratio, the pretreated raw material:activator = 1:3; (3) Pickling purification: The activated product is first soaked in a compound mixed pickling solution (1 g of carbon uses 1 ml of pickling solution). The pickling solution is a mixture of hydrochloric acid (95% concentration), nitric acid (75% concentration) and hydrofluoric acid (35% concentration). By volume ratio, hydrochloric acid:nitric acid:hydrofluoric acid = 3:3:1. It is pickled at 60 °C for 15 min to remove residual activators and impurities, and then repeatedly washed with deionized water until the neutral pH = 7, and then dried at 105 °C to remove moisture; The impurities are effectively removed during the pretreatment of the raw material in step (1), improving the reaction activity of the biochar and qualitatively modifying the biochar raw material from the source. In addition, the additional advantage of this process is that the outer surface of the original rice husk is composed of a layer of dense silica small protrusion structures, and the inner surface structure is smooth. A small amount of fine dust particles are attached to both the inner and outer surfaces. During the pyrolysis process to produce carbon, the silicon element therein is retained because it is difficult to remove, and the pickling process containing hydrofluoric acid can perfectly remove the excess silicon element in the porous carbon product. The pores left after the removal of the silicon element can form microporous structures.

[0024] (4) Classification: The passivated product is screened through a classification wheel according to needs to obtain porous carbon. All materials with D00 less than 1 μm are screened out, those with D10 less than 3 μm are screened out, and all those with D50 greater than 10 μm are screened out.

[0025] In this embodiment, the ratio of the activator to the pretreated raw material is strictly controlled. Under the same process, the activator and the pretreated rice husk are mixed evenly at a mass ratio of 1:1, 2:1, 3:1, and 4:1, and then the activation process is carried out. The activated porous carbon materials are sequentially recorded as product - 1, product - 2, product - 3, and product - 4 according to the activation amount ratio. The pore structures of the unactivated porous carbon material and the four products. The surface area of the unactivated product is only 1254.6 m 2 / g, and the total pore volume is 0.254 m 3 / g. As the amount of the activator increases, the specific surface area of the prepared product does not always increase, but first gradually increases and then decreases. This is mainly because as the amount of the activator increases, the micropores originally formed on the surface of the carbon material will gradually expand to mesopores. When the amount of the activator continues to increase, the originally formed good pore structure will collapse slightly, resulting in a decrease in the specific surface area and the total pore volume. It can be seen that among the four products, product - 3 has the highest specific surface area and total pore area, indicating that the activated porous carbon material has a relatively high specific surface area and a good pore structure. When the ratio is 3:1, the specific surface area and pore volume of the prepared product are the largest and the performance is the best. Example 2

[0026] This embodiment provides a method for preparing pore - controllable porous carbon. As Figure 1 shown, the process includes pretreatment - activation - pickling purification - classification, where: (1) Pretreatment: The biochar raw material, coconut shell biochar, is crushed to 200 mesh and then dried at 120 °C for 12 h; (2) Chemical activation: The raw material pretreated in step (1) is mixed with the activator. KOH forms micro / mesopores by etching the carbon layer and is heat - treated in an inert atmosphere of argon to 900 °C with a heating rate of 3 °C / min -1 , keep warm for 1 h, and after heat - treatment, wash with deionized water until neutral and then dry; The activator is potassium hydroxide (KOH), and the mass ratio of the pretreated raw material to the activator is 1:2; (3) Pickling purification: The activated product is first soaked in a compound mixed acid pickling solution (1 g of carbon uses 1 ml of pickling solution). The pickling solution is a mixture of hydrochloric acid (95% concentration) and nitric acid (75% concentration), and the volume ratio of hydrochloric acid to nitric acid is 1:1. Ultrasonic pickling is carried out at 60 °C for 15 min to remove the residual active agent and impurities, and then repeatedly washed with deionized water until the neutral pH = 7, and then dried at 105 °C to remove moisture; (4) Classification: The passivated product is screened through a classification wheel to obtain porous carbon. All D00 are screened out, those with D10 less than 3 μm are screened out, and all those with D50 greater than 10 μm are screened out. Example 3

[0027] This embodiment provides a method for preparing porous carbon with controllable pores. As shown in Figure 1 the process includes pretreatment - activation - pickling purification - classification, where: (1) Pretreatment: Crush the biochar raw material, tobacco stalk biochar, to 120 mesh to improve the reaction activity of the raw material. Subsequently, dry the washed raw material at 100 °C for 18 h; (2) Chemical activation: Mix the raw material pretreated in step (1) with an activator, heat-treat it to 800 °C in an inert atmosphere of argon, and the heating rate is 3 °C / min -1 , keep it warm for 1.5 h, wash it with deionized water to neutral after heat treatment, and dry it; The activator is solid potassium hydroxide (KOH). By mass ratio, the pretreated raw material: activator = 1:2; (3) Pickling purification: Soak the activated product in a compound mixed pickling solution first (1 g of carbon uses 1 ml of pickling solution). The pickling solution is a mixture of hydrochloric acid (80% concentration), nitric acid (50% concentration), and hydrofluoric acid (20% concentration). By volume ratio, hydrochloric acid: nitric acid: hydrofluoric acid = 1:1:1. Ultrasonically pickle it at 60 °C for 15 min to remove residual activators and impurities, then repeatedly wash it with deionized water to neutral pH = 7, and then dry it at 105 °C to remove moisture; (4) Classification: Screen the passivated product through a classification wheel to obtain porous carbon. Screen out all D00, screen out those with D10 less than 3 μm, and screen out all those with D50 greater than 10 μm.

[0028] Apply the prepared porous carbon material to the fields of supercapacitors and batteries. Use the porous carbon materials prepared in Examples 1 - 3 to prepare silicon-carbon anode materials by the same existing process. The silicon-carbon anode materials corresponding to the porous carbon materials prepared in Examples 1 - 3 are silicon-carbon anode material 1, silicon-carbon anode material 2, and silicon-carbon anode material 3. Detect their respective performance parameters by existing technologies. The processing method of the present invention prepares materials with a high specific surface area, adjustable pore structure, and high electrochemical performance through the high-value utilization of biomass resources and the combination of synergistic effects. Moreover, dangerous chemicals such as concentrated sulfuric acid are not used in the process, greatly reducing the potential safety risks in the preparation process.

[0029] In addition to the above embodiments, the present invention may have other implementation manners. All technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A method for preparing porous carbon with controllable pores, characterized in that: The process includes pretreatment - activation - acid washing and purification - classification, among which: (1) Preprocessing The raw materials were crushed to 80-200 mesh and dried at 80-120°C for 12-24h; (2) Chemical activation The raw material pretreated in step (1) is mixed with an activator, heated to 600-900°C in an inert atmosphere for heat treatment, and kept warm for 1-2 hours. After the heat treatment, it is washed with deionized water until neutral, and dried; The activator is at least one of potassium hydroxide and sodium hydroxide, and the mass ratio of the activator to the pretreated raw material is 1:1-4:1; (3) Acid washing and purification A set of compound mixed acid washing liquid is customized according to the pore structure required by the porous carbon. The activated product is acid washed with the compound mixed acid washing liquid in an ultrasonic water bath at 60°C for 15 minutes to remove the residual active agent and impurities, and then repeatedly washed with deionized water until neutral, and then dried at 105°C to remove moisture; (4) Classification The passivated product is passed through a classification wheel to screen the material.

2. The method for preparing porous carbon with controllable pores according to claim 1, characterized in that: The raw material in step (1) is a biochar raw material, and the biochar raw material is at least one of rice husk biochar, coconut shell biochar or straw biochar.

3. The method for preparing porous carbon with controllable pores according to claim 1, characterized in that: The inert atmosphere in step (2) is nitrogen or argon.

4. The method for preparing porous carbon with controllable pores according to claim 1, characterized in that: In the step (2), the mass ratio of the pretreated biochar raw material to the activating agent is 1:

3.

5. The method for preparing porous carbon with controllable pores according to claim 1, characterized in that: The active agent in step (2) is a mixture of solid KOH and solid NaOH.

6. The method for preparing porous carbon with controllable pores according to claim 1, characterized in that: The compound mixed pickling solution in step (3) is at least one of hydrochloric acid, nitric acid and hydrofluoric acid.

7. The method for preparing porous carbon with controllable pores according to claim 6, characterized in that: The compound mixed pickling solution is a mixture of hydrochloric acid, nitric acid and hydrofluoric acid, with a volume ratio of hydrochloric acid: nitric acid: hydrofluoric acid = 2-4:1-3:1, the concentration of the hydrochloric acid is 50-95%, the concentration of the nitric acid is 50-75%, and the concentration of the hydrofluoric acid is 10-35%.

8. The method for preparing porous carbon with controllable pores according to claim 7, characterized in that: The concentrations of hydrochloric acid, nitric acid and hydrofluoric acid in the compound mixed pickling solution are 95%, 75% and 35% respectively, and the volume ratio is 3:3:

1.

9. A porous carbon with controllable pores obtained by the preparation method as claimed in any one of claims 1 to 8.

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