A method for preparing hard carbon material by using crop straw

Abstract

The application discloses a method for preparing hard carbon material by using crop straw, and belongs to the technical field of carbon material preparation. The method takes crop straw as raw material, and obtains pretreated straw powder through cleaning, crushing and sieving; the pretreated straw powder is mixed with a transition metal salt-biomass composite modifier, stirred and dried to prepare a supported precursor; the precursor is heated and catalytically carbonized under a nitrogen protection atmosphere, and heat preservation is carried out to obtain a carbonized product; finally, the carbonized product is subjected to acid pickling, water washing until neutral and drying to obtain the hard carbon material. The hydrogen bond effect of the transition metal salt-biomass composite modifier and the straw realizes the loading of transition metal ions, the transition metal has the dual effects of catalysis and templating, no additional activator is needed, the directional conversion of the straw into the hard carbon material is realized, the raw material advantage of high hydroxyl content of the crop straw is fully utilized, and the prepared hard carbon material is stable in structure and high in purity.

Description

Technical Field

[0001] This application belongs to the field of carbon material preparation technology, specifically relating to a method for preparing hard carbon materials using crop straw. Background Technology

[0002] As a major agricultural waste, crop straw's core components are cellulose, hemicellulose, and lignin. Examples include wheat straw, sorghum straw, millet straw, oat straw, and cotton straw. Due to their dry growing environment and abundant sunshine, these materials have higher cellulose and lignin content, denser hydroxyl groups on their molecular chains, rich carbon content, and significant structural advantages, making them ideal precursors for hard carbon materials. They also have broad application prospects in energy storage fields such as sodium-ion batteries. However, existing technologies for preparing hard carbon from straw have many drawbacks: the traditional stepwise carbonization-activation method is lengthy and requires the addition of activators such as KOH and CO2, resulting in high energy consumption, high cost, and environmental risks; the direct carbonization method produces products with loose pore structure and low specific surface area, easily exceeding the graphitization limit, making it difficult to meet adsorption or electrochemical performance requirements; some catalytic carbonization technologies rely on precious metal catalysts or complex templates, and suffer from problems such as long carbonization time and uneven temperature field distribution, leading to poor batch stability and limited large-scale production.

[0003] Therefore, developing a simple, low-cost preparation method that requires no additional activators and can precisely control the pore structure and purity of hard carbon is of great practical significance for promoting the resource utilization of agricultural waste and the industrialization of hard carbon materials. Summary of the Invention

[0004] The purpose of this application is to address the problems of complex processes, the need for additional activators, uneven product porosity, and high costs in the existing preparation of hard carbon from crop straw, and to provide a short-process, low-cost preparation method. This method achieves efficient resource utilization of agricultural waste while obtaining hard carbon materials with high specific surface area, high purity, and stable structure, meeting the application needs of adsorption, electrochemistry, and other fields, and balancing environmental protection and industrial feasibility.

[0005] To achieve the above objectives, this application adopts the following technical solution:

[0006] This application provides a method for preparing hard carbon materials using crop straw, comprising the following steps:

[0007] S1. Take crop straw, wash, crush and sieve it to obtain pretreated straw powder;

[0008] S2. The pretreated straw powder is mixed with a transition metal salt-biomass composite modifier, stirred and dried to obtain a supported straw precursor;

[0009] S3. The loaded straw precursor is placed in a protective atmosphere and subjected to a heat-preserving reaction to obtain carbonized products;

[0010] S4. After acid washing, water washing and drying, the carbonization product is used to obtain hard carbon material;

[0011] Among them, the transition metal salt-biomass composite modifier is prepared from transition metal salt and biomass components.

[0012] Furthermore, in step S1, the crop straw includes any one of wheat straw, cotton straw, or reed straw; the crushing conditions are: crushing with a crusher for 10-30 minutes; and sieving with a sieve mesh of 80-120 mesh.

[0013] Furthermore, in S2, the preparation steps of the transition metal salt-biomass composite modifier include: taking the transition metal salt and biomass components, mixing them at a mass ratio of 1:(0.3~0.8), adding deionized water and stirring to prepare a mixed solution with a solute mass fraction of 5%~10%, thereby obtaining the transition metal salt-biomass composite modifier.

[0014] Furthermore, in step S2, the transition metal salt includes either ferric chloride or nickel chloride; the biomass component includes either chitosan or sodium lignin sulfonate.

[0015] Furthermore, in step S2, the solid-liquid ratio of the pretreated straw powder to the transition metal salt-biomass composite modifier is 1g:(5~10)ml;

[0016] The stirring conditions are: temperature 40~60℃, time 2~3h, speed 300-400rpm;

[0017] The drying conditions are: temperature 70~90℃, time 1~3h.

[0018] Furthermore, in S3, the protective atmosphere is nitrogen, and the nitrogen flow rate is 150~250 mL / min;

[0019] The reaction conditions for the heat preservation reaction are: heating rate 10~15℃ / min, temperature 700~900℃, and time 2~3h.

[0020] Furthermore, in S4, the pickling uses an acid reagent with a mass fraction of 3% to 5%, and the pickling time is 0.5 to 1 hour.

[0021] Furthermore, in S4, the acid reagent includes either hydrochloric acid or sulfuric acid.

[0022] Furthermore, in S4, the washing conditions are as follows: wash with deionized water until the pH of the filtrate is 6.5~7.5;

[0023] The drying temperature is 90~110℃, and the drying time is 10~14h.

[0024] Furthermore, the specific surface area of ​​hard carbon materials is 800~1200m². 2 / g.

[0025] This application discloses a method for preparing hard carbon materials using crop straw. Crop straw is used as a natural carbon source precursor. The main components of crop straw are cellulose, hemicellulose, and lignin. A large number of hydroxyl groups (-OH) are densely distributed on the molecular chains of these three macromolecules. Cotton straw, wheat straw, sorghum straw, millet straw, and oat straw grow in dry environments with sufficient sunlight, resulting in higher cellulose and lignin content and a denser distribution of hydroxyl groups (-OH) on their molecular chains. These hydroxyl groups, as strongly polar functional groups, possess good coordination ability and reactivity, providing natural sites for loading transition metal salt-biomass composite modifiers.

[0026] In the catalytic stage, the transition metal salts selected in this application, such as ferric chloride or nickel chloride, dissociate into metal ions after dissolving in water. These are then combined with biomass components, such as chitosan or sodium lignosulfonate. Chitosan molecules contain abundant amino and hydroxyl groups, while sodium lignosulfonate contains sulfonic acid and hydroxyl groups. The metal ions first form stable coordination bonds with the functional amino or sulfonic acid groups of the biomass components, constructing a transition metal ion-biomass component composite unit and preventing metal ion aggregation. Subsequently, the transition metal salt-biomass composite modifier, through the hydroxyl groups contained in the biomass components, forms hydrogen bonds with the hydroxyl groups on the straw molecular chain, allowing the transition metal ions to be loaded onto the surface and internal pores of the straw fibers, enhancing the bonding strength between the transition metal ions and the straw fibers.

[0027] After entering the carbonization stage under a nitrogen protective atmosphere, as the temperature gradually increases, transition metal ions decompose on the straw fibers to form elemental metal particles. On the one hand, these elemental metal particles act as highly efficient catalytic centers, reducing the activation energy of C-C bonds and CO bonds on the main chains of cellulose and lignin macromolecules in crop straw through electron transfer. This breaks down the energy barrier of macromolecular chain cleavage during traditional carbonization, promoting the directional and efficient cleavage of cellulose and lignin macromolecular chains. This allows the carbon elements in the straw to rapidly remove heteroatoms such as hydrogen and oxygen, escaping in the form of small molecules such as H2O and CO2, and then re-aggregating and reconstructing to form an amorphous, disordered carbon skeleton. The high carbon content in crop straw provides ample raw materials for the formation of the carbon skeleton. This catalytic effect not only shortens the time and temperature required for the carbonization reaction, but also inhibits the orderly stacking of carbon layers through directional pyrolysis, avoiding the degradation of the electrochemical or adsorption performance of hard carbon materials due to graphitization. On the other hand, the metal elemental particles simultaneously act as physical templates during the formation of the carbon skeleton. The carbon layers grow around the surface of the metal particles in a coating manner during the growth process. The spatial occupancy effect of the metal particles effectively prevents the excessive aggregation and pore collapse of the carbon layers, so that the microstructure of the carbon skeleton is spatially restricted by the template, and will not cause pore collapse or uneven distribution due to irregular growth.

[0028] After the carbonization reaction is completed, an acid washing process is used. The acid reagent dissolves the metal particles in the carbon skeleton, removing the metal template. The space originally occupied by the metal particles forms channels that match the template size, ultimately constructing a porous structure in the carbon skeleton. The entire process does not require the addition of traditional activators such as KOH or CO2. Through the continuous reaction of catalytic carbonization of metal particles, template pore formation, and acid washing to remove the template, the efficient conversion of straw into high specific surface area and high purity hard carbon material is achieved. The functional groups and reaction mechanisms at each stage work together to ensure the stability of the product's structure and performance.

[0029] Beneficial technical effects:

[0030] This application discloses a method for preparing hard carbon materials using crop straw. Crop straw is used as a natural carbon source precursor. The dry, sunny growing environment results in higher cellulose and lignin content and a denser distribution of hydroxyl groups in the straw raw material, with the abundant hydroxyl groups serving as active centers for subsequent reactions. By introducing a biomass composite modifier, the functional groups it contains form stable coordination bonds with transition metal ions, thereby preventing metal ion aggregation. This yields a transition metal salt-biomass composite modifier. Then, through hydrogen bonding between the biomass components and the hydroxyl groups in the straw fibers, the transition metal ions are loaded onto the surface and interior of the straw fibers. During carbonization, the metal particles formed by the decomposition of transition metal ions lower the activation energy of C-C and CO bonds on the main chains of cellulose and lignin macromolecules through electron transfer, promoting the directional cleavage of macromolecular chains and facilitating the removal of heteroatoms from carbon elements and their reconstruction into an amorphous, disordered carbon skeleton. This fundamentally inhibits graphitization caused by the ordered stacking of carbon layers, avoiding performance degradation due to structural transformation. Furthermore, the metal particles act as physical templates, guiding the carbon layers to grow around them through spatial occupancy effects, effectively preventing excessive carbon layer aggregation and pore collapse. The metal template is then removed during subsequent acid leaching, resulting in a hierarchical porous structure. The entire process requires no additional traditional activators, simplifying the preparation process and establishing a low-cost, environmentally friendly, and efficient pathway for preparing hard carbon materials. Attached Figure Description

[0031] Figure 1 This is a flowchart of a method for preparing hard carbon materials using crop straw according to this application. Detailed Implementation

[0032] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, a detailed description of specific embodiments of this application will be provided below.

[0033] Many specific details are set forth in the following description in order to provide a full understanding of this application. However, this application may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0034] Example 1

[0035] like Figure 1 As shown, this embodiment provides a method for preparing hard carbon materials using crop straw, including the following steps:

[0036] S1. Take cotton stalks, wash them with clean water to remove surface impurities, then crush them in a pulverizer for 10 minutes and pass them through an 80-mesh sieve to obtain pretreated stalk powder;

[0037] S2. Weigh ferric chloride and chitosan at a mass ratio of 1:0.3, add deionized water and stir to dissolve, and prepare a 5% mass fraction of transition metal salt-biomass composite modifier mixed solution; mix the pretreated straw powder with the transition metal salt-biomass composite modifier mixed solution at a solid-liquid ratio of 1g:5ml, stir at 40℃ and 300rpm for 2h, and then dry at 70℃ for 1h to obtain the loaded straw precursor;

[0038] S3. The loaded straw precursor was placed in a tube furnace, and nitrogen was introduced as a protective atmosphere at a flow rate of 150 mL / min. The temperature was increased to 700℃ at a rate of 10℃ / min, and the reaction was maintained at this temperature for 2 hours to obtain the carbonized product.

[0039] S4. Using 3% hydrochloric acid as the acid reagent, mix the carbonization product with the hydrochloric acid and acid wash for 0.5 h. Then wash repeatedly with deionized water until the pH of the filtrate is 6.5. Dry at 90℃ for 10 h to obtain a specific surface area of ​​800 m². 2 / g of hard carbon material.

[0040] Example 2

[0041] like Figure 1 As shown, this embodiment provides a method for preparing hard carbon materials using crop straw, including the following steps:

[0042] S1. Take wheat straw, wash it with clean water to remove surface impurities, then crush it in a pulverizer for 30 minutes and pass it through a 120-mesh sieve to obtain pretreated straw powder;

[0043] S2. Weigh nickel chloride and sodium lignosulfonate at a mass ratio of 1:0.8, add deionized water and stir to dissolve, preparing a 10% mass fraction of transition metal salt-biomass composite modifier mixed aqueous solution; mix the pretreated straw powder with the transition metal salt-biomass composite modifier mixed solution at a solid-liquid ratio of 1g:10ml, stir at 60℃ and 400rpm for 3h, and then dry at 90℃ for 3h to obtain the loaded straw precursor;

[0044] S3. Place the precursor in a tube furnace, introduce nitrogen as a protective atmosphere at a flow rate of 250 mL / min, heat to 900℃ at a heating rate of 15℃ / min, and hold the reaction for 3 hours to obtain the carbonized product.

[0045] S4. Using 5% sulfuric acid as the acid reagent, the carbonization product was mixed with sulfuric acid and acid-washed for 1 hour. Then, it was repeatedly washed with deionized water until the pH of the filtrate reached 7.5. The filtrate was dried at 110℃ for 14 hours to obtain a specific surface area of ​​1200 m². 2 / g of hard carbon material.

[0046] Example 3

[0047] like Figure 1 As shown, this embodiment provides a method for preparing hard carbon materials using crop straw, including the following steps:

[0048] S1. Take reed stalks, wash them with clean water to remove surface impurities, then crush them in a pulverizer for 20 minutes and pass them through a 100-mesh sieve to obtain pretreated stalk powder;

[0049] S2. Weigh ferric chloride and chitosan at a mass ratio of 1:0.5, add deionized water and stir to dissolve, and prepare a mixed aqueous solution of transition metal salt-biomass composite modifier with a mass fraction of 7.5% solute; mix the pretreated straw powder with the mixed solution of transition metal salt-biomass composite modifier at a solid-liquid ratio of 1g:7.5ml, stir at 50℃ and 350rpm for 2.5h, and then dry at 80℃ for 2h to obtain the loaded straw precursor;

[0050] S3. Place the precursor in a tube furnace, introduce nitrogen as a protective atmosphere at a flow rate of 200 mL / min, heat to 800℃ at a rate of 12.5℃ / min, and hold the reaction for 2.5 h to obtain the carbonized product.

[0051] S4. Using 4% hydrochloric acid as the acid reagent, mix the carbonization product with the hydrochloric acid and acid wash for 0.75 h. Then wash repeatedly with deionized water until the pH of the filtrate is 7.0. Dry at 100℃ for 12 h to obtain a specific surface area of ​​1000 m². 2 / g hard carbon material.

[0052] Example 4

[0053] like Figure 1 As shown, this embodiment provides a method for preparing hard carbon materials using crop straw, including the following steps:

[0054] S1. Take cotton stalks, wash them with clean water to remove surface impurities, then crush them in a pulverizer for 18 minutes and pass them through a 90-mesh sieve to obtain pretreated stalk powder;

[0055] S2. Weigh nickel chloride and sodium lignosulfonate at a mass ratio of 1:0.6, add deionized water and stir to dissolve, preparing a mixed aqueous solution of transition metal salt-biomass composite modifier with a solute mass fraction of 8%; mix the pretreated straw powder with the mixed solution of transition metal salt-biomass composite modifier at a solid-liquid ratio of 1g:8ml, stir at 55℃ and 380rpm for 2.8h, and then dry at 85℃ for 2.5h to obtain the loaded straw precursor;

[0056] S3. Place the precursor in a tube furnace, introduce nitrogen as a protective atmosphere at a flow rate of 220 mL / min, heat to 850 °C at a heating rate of 12 °C / min, and hold the reaction for 2.2 h to obtain the carbonized product.

[0057] S4. Using 4.5% sulfuric acid as the acid reagent, the carbonization product was mixed with sulfuric acid and acid-washed for 0.9 h. Then, it was repeatedly washed with deionized water until the pH of the filtrate reached 7.2. The filtrate was dried at 105 °C for 13 h to obtain a specific surface area of ​​1100 m². 2 / g hard carbon material.

[0058] Comparative Example 1

[0059] This comparative example provides a method for preparing hard carbon materials using crop straw. The difference between this comparative example and Example 1 is that no biomass components are added, but the other process parameters and operating steps are exactly the same as in Example 1.

[0060] Comparative Example 2

[0061] This comparative example provides a method for preparing hard carbon materials using crop straw. The difference between this comparative example and Example 1 is that there is no acid washing step in this comparative example, but the other process parameters and operating steps are exactly the same as in Example 1.

[0062] The performance of the hard carbon materials prepared from crop straw in Examples 1-4 and Comparative Examples 1-2 was tested, and the results are shown in Table 1.

[0063] Table 1. Performance test results of hard carbon materials prepared in Examples 1-4 and Comparative Examples 1-2

[0064]

[0065] Examples 1-4 used crop straw as a carbon source precursor and, relying on the dual catalytic and templating effects of transition metal salts, prepared high-performance hard carbon materials through a continuous process of coordination, catalytic carbonization-in-situ pore formation, and acid elution to remove the template. The results showed that the specific surface area of ​​the products was all between 800 and 1200 m². 2 Within the range of / g, the residual metal content is <0.5%, the degree of graphitization is <10%, and the electrochemical sodium intercalation capacity reaches 286~312mAh / g. This excellent performance stems from the formation of stable coordination units between biomass components and transition metal ions, which avoids metal ion aggregation. Furthermore, through hydrogen bonding and loading with straw hydroxyl groups, the metal particles are ensured to be uniformly dispersed and exert their catalytic effect, reducing the activation energy of straw macromolecular chain pyrolysis, constructing an amorphous carbon skeleton and inhibiting graphitization. At the same time, the metal particles act as physical templates, guiding the formation of uniform hierarchical pores. After acid leaching, the pore structure is further optimized, achieving a synergistic effect of catalysis and pore formation.

[0066] Comparative Example 1 did not use transition metal salts; hard carbon materials were prepared solely through the direct carbonization of straw, resulting in a product with a specific surface area of ​​only 412 m². 2 The graphitization degree is as high as 15.3%, but the electrochemical sodium intercalation capacity is only 165 mAh / g. This is because the lack of coordination and loading of biomass components makes metal ions prone to agglomeration and unable to be uniformly dispersed in the straw matrix, resulting in insufficient catalytic activity and incomplete cleavage of straw macromolecular chains. During carbonization, the carbon layers tend to stack in an orderly manner to form a graphitized structure. Moreover, without template-guided pore formation, the pore structure formed solely by the natural pyrolysis of straw is loose and unevenly distributed, leading to a significant decrease in adsorption and electrochemical performance. This fully demonstrates that the dual role of catalysis and template is the core of this scheme for achieving high-performance hard carbon preparation.

[0067] Comparative Example 2 did not undergo an acid washing step, although the product had a specific surface area of ​​986 m². 2 The product exhibits a high degree of graphitization (8.1%), but a high metal residue (8.6%), resulting in a significant decrease in electrochemical sodium intercalation capacity to 210 mAh / g. This is because the unremoved metal particles occupy the pore space within the carbon framework, blocking ion transport channels. Simultaneously, the presence of metal impurities disrupts the structural stability of the carbon framework, leading to a substantial decline in electrochemical performance. This verifies the necessity of the acid washing step for removing the template, purifying the pore structure, and ensuring product performance. It also demonstrates the technical logic of the complementary and indispensable processes in this scheme.

[0068] It should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application, and all such modifications and substitutions should be covered within the scope of the claims of this application.

Claims

1. A method for preparing hard carbon materials using crop straw, characterized in that, Includes the following steps: S1. Take crop straw, wash, crush and sieve it to obtain pretreated straw powder; S2. The pretreated straw powder is mixed with a transition metal salt-biomass composite modifier, stirred and dried to obtain a supported straw precursor; S3. The loaded straw precursor is placed in a protective atmosphere and subjected to a heat-preserving reaction to obtain carbonized products; S4. After acid washing, water washing and drying, the carbonization product is used to obtain hard carbon material; The transition metal salt-biomass composite modifier is prepared from transition metal salts and biomass components.

2. The method for preparing hard carbon materials using crop straw according to claim 1, characterized in that, In S1, the crop straw includes any one of wheat straw, cotton straw, or reed straw; the crushing conditions are: crushing with a crusher for 10-30 minutes; and the sieve mesh size is 80-120 mesh.

3. The method for preparing hard carbon materials using crop straw according to claim 1, characterized in that, In S2, the preparation steps of the transition metal salt-biomass composite modifier include: taking the transition metal salt and biomass components, mixing them at a mass ratio of 1:(0.3~0.8), adding deionized water and stirring to prepare a mixed solution with a solute mass fraction of 5%~10%, thereby obtaining the transition metal salt-biomass composite modifier.

4. The method for preparing hard carbon materials using crop straw according to claim 1, characterized in that, In S2, the transition metal salt includes either ferric chloride or nickel chloride; the biomass component includes either chitosan or sodium lignin sulfonate.

5. The method for preparing hard carbon materials using crop straw according to claim 1, characterized in that, In S2, the solid-liquid ratio of the pretreated straw powder to the transition metal salt-biomass composite modifier is 1g:(5~10)ml; The stirring conditions are: temperature 40~60℃, time 2~3h, speed 300-400rpm; The drying conditions are: temperature 70~90℃, time 1~3h.

6. The method for preparing hard carbon materials using crop straw according to claim 1, characterized in that, In S3, the protective atmosphere is nitrogen, and the nitrogen flow rate is 150~250 mL / min; The reaction conditions for the heat preservation reaction are: heating rate 10~15℃ / min, temperature 700~900℃, and time 2~3h.

7. The method for preparing hard carbon materials using crop straw according to claim 1, characterized in that, In S4, the pickling uses an acid reagent with a mass fraction of 3% to 5% and the pickling time is 0.5 to 1 hour.

8. A method for preparing hard carbon materials using crop straw according to claim 7, characterized in that, In S4, the acid reagent includes either hydrochloric acid or sulfuric acid.

9. A method for preparing hard carbon materials using crop straw according to claim 1, characterized in that, In S4, the washing conditions are: washing with deionized water until the pH of the filtrate is 6.5~7.5; The drying temperature is 90~110℃, and the drying time is 10~14h.

10. A method for preparing hard carbon materials using crop straw according to any one of claims 1 to 9, characterized in that, The hard carbon material has a specific surface area of ​​800~1200 m². 2 / g.

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