Straw-based mesoporous biochar and preparation method thereof

Abstract

The application discloses a kind of straw-based mesoporous biochar and preparation method thereof, belong to the technical field of biomass carbon material preparation.The method first pulverizes straw to obtain straw powder, is activated by sodium sulfite-sodium hydroxide mixed aqueous solution, dried, and activated straw powder is obtained;Then the activated straw powder is dispersed in deionized water, polyacrylic acid is added, and anhydrous ethanol is slowly added, so that polyacrylic acid is precipitated in situ in straw particles and pores, and modified straw powder is obtained;Finally, porous biochar is prepared by pyrolysis under inert atmosphere.The application uses the in-situ precipitation method of polyacrylic acid in straw suspension system to modify activated straw, and can prepare porous biochar with pore size concentrated in 2-10nm, with abundant adsorption active sites.The raw material of the application is agricultural waste straw, which is low in cost and simple in process, and is suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of biochar material preparation technology, specifically relating to a straw-based mesoporous biochar and its preparation method. Background Technology

[0002] my country has a huge annual output of agricultural straw resources, including massive amounts of corn straw, rice straw, and soybean straw, among other agricultural and forestry waste. Large quantities of crop straw are indiscriminately burned, buried on-site, or piled up haphazardly in the fields, causing a serious waste of biomass resources and generating large amounts of smoke, particulate matter, and greenhouse gases, leading to air pollution, soil compaction, and increased environmental pressure. The resource utilization and high-value utilization of straw is an important research direction for modern agricultural circular economy and solid waste resource treatment.

[0003] Biochar is the final carbonization product of wood, straw, or other crop residues and wastes after high-temperature pyrolysis under limited or anaerobic conditions (Kätterer et al. 2019; Lehmann 2007). It possesses abundant pore structure, large specific surface area, and excellent adsorption and stability. It has broad application prospects in water treatment, heavy metal adsorption, organic pollutant removal, gas separation, and soil improvement.

[0004] Existing conventional biochar preparation processes mostly employ a single high-temperature pyrolysis method, which has significant drawbacks: the pore structure is uncontrollable, and the product is predominantly macropores. The number of mesopores is low, especially the proportion of 2-10 nm mesopores, resulting in insufficient adsorption active sites. The pores are prone to collapse during high-temperature carbonization, leading to poor material adsorption stability, low adsorption selectivity and saturation capacity, and poor cycle stability.

[0005] To address the technical problems of existing biochar, such as disordered pore size distribution, low proportion of mesopores, limited specific surface area, weak adsorption performance, and poor structural controllability, there is an urgent need to develop a low-cost straw-based biochar preparation technology that can directionally enrich mesopores, has a large specific surface area, and excellent adsorption performance. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a straw-based mesoporous biochar and its preparation method. The method produces porous biochar with a concentrated pore size of 2-10 nm, a very high mesopore ratio, and excellent adsorption performance. The process is simple, controllable, green, and low-cost, making it suitable for large-scale production.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing straw-based mesoporous biochar, comprising the following steps:

[0009] S1. The straw is repeatedly washed with deionized water, then dried, crushed, and sieved to obtain straw powder;

[0010] S2. Soak 10-20g of straw powder in 150-300mL of a mixed aqueous solution of Na2SO3 and NaOH, heat to 80-100℃, stir for 2-4h, after the reaction is complete, centrifuge, rinse with deionized water, dry, and obtain activated straw powder.

[0011] S3. Disperse the activated straw powder in 100-150mL of deionized water, then add 2.8-3.2g of polyacrylic acid (PAA), stir for 20-30min, then slowly add 80-100mL of anhydrous ethanol, stir for 10-20min; then centrifuge, wash with anhydrous ethanol, and dry to obtain modified straw powder.

[0012] S4. Spread the modified straw powder evenly on a ceramic boat and place it in a tube furnace; introduce inert gas, set the heating rate to 3-5℃ / min, heat to 800-1000℃, and keep it at that temperature for 2-3 hours for pyrolysis; after pyrolysis, allow it to cool naturally to room temperature, remove the solid product, grind it, and sieve it to obtain straw-based mesoporous biochar.

[0013] Further, the straw mentioned in step S1 is one of rice straw, corn straw, and soybean straw; the drying temperature is 60-80℃, and the sieving is pulverizing and then passing it through a 20-40 mesh sieve.

[0014] Furthermore, the concentration of Na2SO3 in step S2 is 0.1-0.15 g / mL;

[0015] Furthermore, the concentration of the NaOH is 0.1-0.3 g / mL;

[0016] Furthermore, the average molecular weight Mv of the polyacrylic acid described in step S3 is 3000;

[0017] Furthermore, the inert gas mentioned in step S4 is argon or nitrogen.

[0018] Through the above preparation method, the present invention obtains a straw-based mesoporous biochar.

[0019] This invention first activates straw using a sodium sulfite and sodium hydroxide composite system, thereby disrupting the dense structure of straw lignocellulose and initially constructing a porous precursor framework.

[0020] Next, PAA was dissolved in deionized water, and then anhydrous ethanol was added to reduce the solubility of polyacrylic acid, allowing the polyacrylic acid to precipitate in situ within the straw particles and their internal pores. Finally, carbonization was carried out at high temperature in an inert atmosphere to obtain porous biochar with concentrated mesopores, a large specific surface area, and abundant adsorption sites.

[0021] The beneficial effects of this invention are as follows:

[0022] (1) By combining the activation of composite alkali salt with the in-situ precipitation modification of polyacrylic acid in the straw system, it is beneficial to obtain a pore structure with pore size mainly distributed in 2-10 nm, increase the proportion of pores in this range, and improve the problems of uncontrollable pore size and mesopore scarcity in traditional biochar.

[0023] (2) The mesoporous structure provides a large number of adsorption active sites, has a large specific surface area, high pore utilization, and excellent isothermal adsorption performance;

[0024] (3) Simple process, green and low cost: using waste straw as raw material, the process conditions are mild, the parameters are controllable, and there are no toxic reagents. The whole process is green and environmentally friendly, suitable for industrial-scale production, and can be used for CO2 gas adsorption. Attached Figure Description

[0025] Figure 1 Here is a magnified SEM image of a portion of the biochar prepared in Example 1;

[0026] Figure 2 The BJH (adsorption) pore volume and pore size distribution curves of the biochar prepared in Example 1;

[0027] Figure 3 BJH (adsorption) pore volume and pore size distribution curves of the biochar prepared for Comparative Example 1.

[0028] Figure 4 The Tyndall effect of PAA micelles in the dissolved state of PAA in deionized water and in a mixed solution of PAA in deionized water and anhydrous ethanol is shown in (a) PAA dissolution and (b) PAA micelles.

[0029] Figure 5 The N2 adsorption-desorption isotherms of the biochar prepared in Example 1 and Comparative Examples 1-3 are shown.

[0030] Figure 6 The CO2 adsorption isotherms of Example 1 and Comparative Examples 1-3 are shown at 298 K. Detailed Implementation

[0031] To make the above-mentioned objectives, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to examples. The following content is merely illustrative and explanatory of the concept of the present invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, as long as they do not deviate from the inventive concept, all of which should fall within the protection scope of the present invention. The preparation method of the present invention will be described below through specific embodiments.

[0032] Example 1

[0033] A method for preparing straw-based mesoporous biochar includes the following steps:

[0034] S1. Select rice straw, remove surface dust and impurities, wash repeatedly with deionized water, dry at 70℃ for 18 hours, pulverize and pass through a 30-mesh sieve to obtain rice straw powder.

[0035] S2. Weigh 15g of rice straw powder and soak it in 220mL of mixed aqueous solution, where the concentration of Na2SO3 is 0.12g / mL and the concentration of NaOH is 0.20g / mL; heat to 90℃ and stir for 3h; after the reaction is completed, centrifuge, wash with deionized water until neutral, and dry at 65℃ to obtain activated straw powder.

[0036] S3. Disperse activated straw powder in 120 mL of deionized water, add 3.0 g of polyacrylic acid with a molecular weight of 3000, and stir for 25 min; then slowly add 90 mL of anhydrous ethanol over 10 min and continue stirring for 15 min; centrifuge, wash twice with anhydrous ethanol, and dry at 55 °C to obtain modified straw powder.

[0037] S4. Spread the modified straw powder evenly on a ceramic boat, place it in a tube furnace, and purge with high-purity nitrogen for 25 min. Heat to 900℃ at 4℃ / min and hold for 2.5 h. Cool to room temperature under nitrogen protection, grind through a 100-mesh sieve to obtain straw-based mesoporous biochar, denoted as S-1.

[0038] Example 2

[0039] A method for preparing straw-based mesoporous biochar includes the following steps:

[0040] S1. Select corn stalks, wash them, dry them at 60℃ for 24 hours, and then crush them through a 20-mesh sieve.

[0041] S2. Take 10g of corn stalk powder, soak it in 150mL of mixed solution (Na2SO3 0.1g / mL, NaOH 0.1g / mL), stir at 80℃ for 4h, centrifuge, wash with water and dry.

[0042] S3. Disperse in 100mL of deionized water, add 2.8g of polyacrylic acid (Mv=3000), stir for 20min, add 80mL of anhydrous ethanol dropwise, stir for 10min, centrifuge, wash with alcohol and dry.

[0043] S4. Under argon protection, the temperature was increased to 800℃ at 3℃ / min, held for 3h, cooled, ground and sieved to obtain straw-based mesoporous biochar, denoted as S-2.

[0044] Example 3

[0045] A method for preparing straw-based mesoporous biochar includes the following steps:

[0046] S1. Select soybean straw, wash it, dry it at 80℃ for 12 hours, and crush it through a 40-mesh sieve;

[0047] S2. Take 20g of soybean straw powder, soak it in 300mL of mixed solution (Na2SO3 0.15g / mL, NaOH 0.3g / mL), stir at 100℃ for 2h, centrifuge, wash with water and dry.

[0048] S3, dispersed in 150mL of deionized water, add 3.2g of polyacrylic acid (Mv=3000), stir for 30min, add 100mL of anhydrous ethanol dropwise, stir for 20min, centrifuge, wash with alcohol and dry;

[0049] S4. Under nitrogen protection, the temperature was increased to 1000℃ at 5℃ / min, held for 2 hours, cooled, ground, and sieved to obtain straw-based mesoporous biochar, denoted as S-3.

[0050] Example 4

[0051] A method for preparing straw-based mesoporous biochar includes the following steps:

[0052] S1. Select corn stalks, wash, dry, crush, and pass through a 30-mesh sieve;

[0053] S2. Take 18g of straw powder, place it in 250mL of mixed solution (Na2SO3 0.12g / mL, NaOH 0.2g / mL), stir at 95℃ for 2.5h, wash with water and dry.

[0054] S3, dispersed in 125 mL of deionized water, added 3.1 g of polyacrylic acid, stirred for 28 min, added dropwise 95 mL of anhydrous ethanol, stirred for 18 min, washed with ethanol and dried;

[0055] Under a nitrogen atmosphere, the temperature was increased to 950℃ at a rate of 4.5℃ / min, held for 2.2h, cooled, and ground to obtain straw-based mesoporous biochar, denoted as S-4.

[0056] Comparative Example 1

[0057] Traditional direct pyrolysis biochar

[0058] Rice straw was selected, washed, dried, and pulverized. The pulverized straw was then placed directly into a tube furnace and heated to 900℃ at a nitrogen atmosphere of 4℃ / min. The temperature was maintained for 2.5 hours, cooled, and ground to obtain unmodified raw biochar, denoted as D-1.

[0059] Comparative Example 2

[0060] A method for preparing biochar includes the following steps:

[0061] S1. Select rice straw, remove surface dust and impurities, wash repeatedly with deionized water, dry at 70℃ for 18 hours, pulverize and pass through a 30-mesh sieve to obtain rice straw powder.

[0062] S2. Weigh 15g of rice straw powder and soak it in 220mL of mixed aqueous solution, where the concentration of Na2SO3 is 0.12g / mL and the concentration of NaOH is 0.20g / mL; heat to 90℃ and stir for 3h; after the reaction is completed, centrifuge, wash with deionized water until neutral, and dry at 65℃ to obtain activated straw powder.

[0063] S3. Spread the activated straw powder evenly on a ceramic boat, place it in a tube furnace, and purge with high-purity nitrogen for 25 minutes. Increase the temperature to 900℃ at 4℃ / min and keep it at that temperature for 2.5 hours. Cool to room temperature under nitrogen protection, grind through a 100-mesh sieve to obtain biochar, denoted as D-2.

[0064] Comparative Example 3:

[0065] PAA micelles are first prepared and then mixed with activated straw (the micelles are added after pre-preparation, which is different from the in-situ precipitation method of this invention).

[0066] Operating steps:

[0067] S1. Select rice straw, remove surface dust and impurities, wash repeatedly with deionized water, dry at 70℃ for 18 h, pulverize and pass through a 30-mesh sieve to obtain rice straw powder.

[0068] S2. Weigh 15g of rice straw powder and soak it in 220mL of mixed aqueous solution, where the concentration of Na2SO3 is 0.12g / mL and the concentration of NaOH is 0.20g / mL; heat to 90 ℃ and stir for 3 h; after the reaction is completed, centrifuge, wash with deionized water until neutral, and dry at 65℃ to obtain activated straw powder.

[0069] S3 Pre-preparation of PAA micelles: 3.0 g of polyacrylic acid with a molecular weight of 3000 was completely dissolved in 120 mL of deionized water and stirred for 25 min; 90 mL of anhydrous ethanol was slowly added dropwise and stirred for 15 min to prepare a PAA micelle dispersion; the above micelle dispersion was directly added to activated straw powder and stirred thoroughly; centrifuged, washed twice with anhydrous ethanol, and dried at 55 °C to obtain pretreated straw powder;

[0070] S4. Spread the pretreated powder evenly on a ceramic boat, place it in a tube furnace, and purge with high-purity nitrogen for 25 min. Increase the temperature to 900℃ at 4℃ / min and hold for 2.5 h. Cool to room temperature under nitrogen protection, grind through a 100-mesh sieve to obtain biochar, denoted as D-3.

[0071] Test conditions: The nitrogen adsorption-desorption method was used to test the pore structure of the sample. The BET model was used to calculate the specific surface area, and the BJH adsorption branch was used to calculate the pore volume and pore size distribution.

[0072] Table 1. Structural properties of biochar in each embodiment and comparative example

[0073] S-1 726.3±18.5 76.2±1.8 S-2 661.7±15.2 71.5±2.1 S-3 684.5±16.8 73.8±1.9 S-4 702.8±17.1 74.7±1.7 D-1 447.2±12.3 32.6±2.5 D-2 554.9±14.6 46.2±2.2 D-3 528.6±13.9 43.1±2.0

[0074] As shown in Table 1, D-1, obtained by direct pyrolysis, has the lowest specific surface area and a low proportion of 2-10 nm pore volume. D-2, after activation with sodium sulfite and sodium hydroxide, shows a certain improvement in specific surface area and mesopore volume. D-3, produced by pre-preparing PAA micelles and then mixing them with activated straw, exhibits better performance than D-1 and D-2, but is still lower than S-1 to S-4 of this invention. This invention achieves in-situ precipitation of polyacrylic acid by adding anhydrous ethanol to a straw suspension system, resulting in porous biochar with a higher BET specific surface area and a significantly improved proportion of 2-10 nm pore volume.

[0075] Figure 1 The image shows a SEM image of the biochar prepared in Example 1. The high-magnification SEM image shows that the surface of the biochar is obviously wrinkled and has a large number of mesoporous pits.

[0076] Figure 2 The curves show the BJH (adsorption) pore volume and pore size distribution of the biochar prepared in Example 1. As can be seen from the curves, the mesopore volume of the sample in Example 1 is mainly concentrated in the 2-10 nm range, and the pore size distribution range is narrow.

[0077] Figure 3 BJH (adsorption) pore volume and pore size distribution curves of the biochar prepared for Comparative Example 1.

[0078] The biochar prepared in Comparative Example 1 has a wide mesopore size distribution, with a large number of mesopores larger than 10 nm, and the proportion of the 2-10 nm range is significantly lower than that in Example 1.

[0079] Figure 4 The Tyndall effect of PAA micelles in the dissolved state of PAA in deionized water and in a mixed solution of PAA in deionized water and anhydrous ethanol is shown in (a) PAA dissolution and (b) PAA micelles. Figure 4 (a) The polyacrylic acid is completely dissolved in deionized water, and the system is a homogeneous, transparent solution with no Tyndall effect. Anhydrous ethanol and deionized water are present in the system, such as... Figure 4 As shown in (b), a clear Tyndall effect pathway appears in the system, indicating that PAA nanomicelle particles have formed inside the system. In the process of this invention, ethanol is added dropwise to the straw powder suspension system, and PAA micelles are precipitated in situ in the straw particles and their internal pores.

[0080] Figure 5 The N2 adsorption-desorption isotherms of the biochar prepared in Example 1 and Comparative Examples 1-3 are shown.

[0081] The sample in Example 1 exhibits a typical Type IV adsorption isotherm. The nitrogen adsorption capacity of the sample in Example 1 is significantly higher than that of D-1, D-2, and D-3 in the low-pressure region, indicating that the sample in Example 1 has a richer mesoporous structure. The hysteresis loop in the medium- and high-pressure sections reflects the presence of a mesoporous structure in the sample. Comparison shows that: the D-1 directly pyrolyzed sample has the lowest overall adsorption capacity; only the D-2 composite alkali-activated sample shows an improved adsorption capacity, but its adsorption capacity in the low-pressure region is still lower than that of Example 1; the D-3 externally pre-prepared micelle blend sample has a better adsorption capacity than D-1 and D-2, but lower than that of Example 1, indicating that the present invention can achieve a higher nitrogen adsorption capacity.

[0082] The straw-based mesoporous biochar prepared by this invention has a mesoporous structure with pore sizes concentrated in the range of 2-10 nm and a large specific surface area, which can effectively reduce the diffusion resistance of CO2 gas and expose sufficient adsorption active sites, and can be applied to CO2 adsorption and capture.

[0083] Under simulated flue gas conditions at low pressure of 0-1 bar, the CO2 adsorption isotherm performance of Example S-1 and Comparative Examples D-1, D-2, and D-3 samples at 298 K was tested.

[0084] Figure 6 The CO2 adsorption isotherms of Example 1 and Comparative Examples 1-3 are shown at 298 K.

[0085] As shown in the figure, the adsorption capacity of all samples gradually increased with increasing pressure. Sample S-1 exhibited a significantly higher adsorption growth rate in the low-pressure range than the other comparative samples, demonstrating the strong CO2 capture capability of the controllable mesoporous structure of this invention at low pressure. Comparative samples D-1, D-2, and D-3 showed a gradual increase in adsorption capacity at low pressure. The test results indicate that the optimal sample of this invention, S-1 straw-based mesoporous biochar, can achieve a CO2 adsorption capacity of 1.22 mmol·g⁻¹ at 298 K. -1 .

Claims

1. A method for preparing straw-based mesoporous biochar, characterized in that, Includes the following steps: S1. The straw is repeatedly washed with deionized water, then dried, crushed, and sieved to obtain straw powder; S2. Soak 10-20g of straw powder in 150-300mL of a mixed aqueous solution of Na2SO3 and NaOH, heat to 80-100℃, stir for 2-4h, after the reaction is complete, centrifuge, rinse with deionized water, dry, and obtain activated straw powder. S3. Disperse the activated straw powder in 100-150mL of deionized water, then add 2.8-3.2g of polyacrylic acid (PAA), stir for 20-30min, then slowly add 80-100mL of anhydrous ethanol, and stir for 10-20min. Then, the straw was centrifuged, washed with anhydrous ethanol, and dried to obtain modified straw powder. S4. Spread the modified straw powder evenly on a ceramic boat and place it in a tube furnace; introduce inert gas, set the heating rate to 3-5℃ / min, heat to 800-1000℃, and keep it at that temperature for 2-3 hours for pyrolysis; after pyrolysis, allow it to cool naturally to room temperature, remove the solid product, grind it, and sieve it to obtain straw-based mesoporous biochar.

2. The preparation method according to claim 1, characterized in that, The straw mentioned in step S1 is one of rice straw, corn straw, or soybean straw; the drying temperature is 60-80℃; and the sieving is the process of crushing the straw and then passing it through a 20-40 mesh sieve.

3. The preparation method according to claim 1, characterized in that, The concentration of Na2SO3 in step S2 is 0.1-0.15 g / mL.

4. The preparation method according to claim 1, characterized in that, The concentration of NaOH in step S2 is 0.1-0.3 g / mL.

5. The preparation method according to claim 1, characterized in that, The average molecular weight Mv of the polyacrylic acid described in step S3 is 3000.

6. The preparation method according to claim 1, characterized in that, The inert gas mentioned in step S4 is argon or nitrogen.

7. A straw-based mesoporous biochar prepared by the preparation method according to any one of claims 1-7.

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