Straw biochar material with multi-stage gradient pore structure and preparation method and application thereof

Abstract

This invention belongs to the field of biochar technology, specifically relating to a straw biochar material with a multi-level gradient pore structure, its preparation method, and its application. In this invention, straw is placed in a buffer solution containing hydrolytic enzymes, and the resulting mixture undergoes a hydrolysis reaction. The resulting pre-etched biomass template is placed in a solution containing magnesium salts, aluminum salts, and organic ligands, and subjected to alternating vacuum-pressure impregnation. Following this, an in-situ solvothermal reaction is carried out, and the resulting biomass template loaded with MOF precursors is pyrolyzed and carbonized in a nitrogen-containing atmosphere, simultaneously undergoing plasma treatment to obtain a straw biochar material with a multi-level gradient pore structure. The straw biochar material prepared by this invention not only possesses a multi-level gradient pore structure but also exhibits high selective capture capacity for low-concentration CO2, achieving high-capacity, fast-kinetic, and high-stability CO2 adsorption.

Description

Technical Field

[0001] This invention belongs to the field of biochar technology, specifically relating to a straw biochar material with a multi-level gradient pore structure, its preparation method, and its application. Background Technology

[0002] With the acceleration of industrialization, excessive emissions of greenhouse gases such as carbon dioxide (CO2) have led to significant global warming and extreme weather events. Among the many carbon capture, utilization, and storage (CCUS) technologies, adsorption separation has attracted much attention due to its high energy efficiency, simple equipment, and strong operational flexibility.

[0003] Crop straw, as a widely available, inexpensive, and renewable biomass resource, is an ideal precursor for the preparation of porous carbon materials. Converting straw into biochar through pyrolysis not only enables the resource utilization of agricultural waste but also transforms it into a CO2 adsorbent with high specific surface area and stable structure.

[0004] Despite extensive research on CO2 adsorption using modified biochar, several bottlenecks remain in practical applications. First, uneven pore structure distribution leads to high mass transfer resistance. Traditional biochar preparation relies heavily on physical activation (e.g., steam, prolonged pyrolysis time) or chemical activation (e.g., KOH, ZnCl2 impregnation). These methods often result in random pore size distribution and a lack of an ordered hierarchical pore system. In particular, the natural vascular bundle structure of straw is prone to local collapse during high-temperature activation, causing significant diffusion resistance for gas molecules entering deep micropores and resulting in slow adsorption kinetics. Second, low utilization of active sites and easy deactivation. Existing modification methods often employ surface-loaded organic amines or physically loaded nano-metal oxides. This "exogenous loading" approach often results in active components accumulating only on the surface of the carbon material or within macropores, easily clogging micropore channels. Furthermore, active components are prone to thermal degradation or detachment during repeated adsorption-desorption cycles, leading to poor cycle stability. Finally, the surface chemical environment is poorly regulated. Existing nitrogen doping techniques typically require the introduction of ammonia gas at extremely high temperatures (>800℃), which not only consumes a large amount of energy but also easily damages the mechanical strength of the carbon substrate. Meanwhile, how to precisely construct "trapping cage" structures with specific polarities at the nanoscale to achieve highly selective capture of low-concentration CO2 remains a pressing problem in this field. Finally, the utilization of biological templates is insufficient. Current technologies often treat straw as a simple carbon source, neglecting its biological structural advantages as a natural micro / nanofluidic channel. There is a lack of a process that can utilize biological enzymes to "modify" the template at the microscopic level, thereby directionally guiding the formation of nanostructures during subsequent pyrolysis.

[0005] Therefore, developing a CO2 adsorption composite material that can maintain the advantages of the natural skeleton of straw and achieve high capacity, fast kinetics, and high stability through micro-nano structure recombination, as well as its preparation method, is of great scientific significance and practical value for promoting the industrial application of CCUS technology. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a straw biochar material with a multi-level gradient pore structure, its preparation method and application. The straw biochar material prepared by the present invention not only has a multi-level gradient pore structure, but also has a high selective capture capacity for low concentration CO2, realizing high capacity, fast kinetics and high stability CO2 adsorption.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing straw biochar material with a multi-level gradient pore structure, comprising the following steps: Straw was placed in a buffer solution containing hydrolytic enzymes, and the resulting mixture was subjected to a hydrolysis reaction to obtain a pre-etched biomass template. The pre-etched biomass template was placed in a solution containing magnesium salt, aluminum salt and organic ligand, and after vacuum-pressure alternating impregnation, an in-situ solvothermal reaction was carried out to obtain a biomass template loaded with MOF precursor. The biomass template with the MOF precursor was pyrolyzed and carbonized in an atmosphere containing nitrogen, and plasma treatment was performed simultaneously to obtain straw biochar material with a multi-level gradient pore structure.

[0008] Preferably, the hydrolytic enzyme includes cellulase, hemicellulase and pectinase; the mass ratio of cellulase, hemicellulase and pectinase is 2.5~4:1.5~2.5:0.5~1.5.

[0009] Preferably, the hydrolysis reaction is carried out at a temperature of 48±2℃ for 3~5 hours.

[0010] Preferably, the organic ligand includes one or more of 2-aminoterephthalic acid, terephthalic acid, and trimesic acid.

[0011] Preferably, the in-situ solvothermal reaction is carried out at a temperature of 100~160℃ for a time of 12~48h.

[0012] Preferably, the temperature of the pyrolysis carbonization is 550~650℃.

[0013] Preferably, the plasma treatment is cold plasma radio frequency treatment; the power of the plasma treatment is 200~350W, and the time is 1.5~3h.

[0014] Preferably, the buffer solution is a sodium acetate buffer, a citrate-sodium citrate buffer, or a phosphate buffer; the pH value of the buffer solution is 4.5~5.5.

[0015] The present invention also provides a straw biochar material with a multi-level gradient pore structure prepared by the preparation method described above, comprising straw biochar, Mg and Al anchored in the pores of the straw biochar and N doped in the carbon lattice of the straw biochar; The straw biochar material with a multi-level gradient pore structure has micropores and mesopores; the average pore size of the micropores and mesopores is 2~10nm, and the volume of micropores and mesopores accounts for ≥75% of the total pore volume.

[0016] The present invention also provides the application of the straw biochar material with multi-level gradient pore structure described in the above technical solution in CO2 treatment.

[0017] This invention provides a method for preparing straw biochar material with a multi-level gradient pore structure, comprising the following steps: Straw was placed in a buffer solution containing hydrolytic enzymes, and the resulting mixture was subjected to a hydrolysis reaction to obtain a pre-etched biomass template. The pre-etched biomass template was placed in a solution containing magnesium salt, aluminum salt and organic ligand, and after vacuum-pressure alternating impregnation, an in-situ solvothermal reaction was carried out to obtain a biomass template loaded with MOF precursor. The biomass template with the MOF precursor was pyrolyzed and carbonized in an atmosphere containing nitrogen, and plasma treatment was performed simultaneously to obtain straw biochar material with a multi-level gradient pore structure.

[0018] Beneficial effects: This method abandons the traditional "carbonization followed by loading" approach and proposes a "biotemplate-induced in-situ nanoconstruction" scheme. Nanopre-etching technology: Hydrolytic enzymes (cellulase / hemicellulase) are used to perform limited "micro-cutting" of straw, reserving nanoscale diffusion channels on the cell wall through enzymatic hydrolysis without damaging the fiber skeleton. Self-assembled "nanocages": Precursors of metal-organic frameworks (MOFs) are grown in situ inside the straw, anchoring them within the nanopores using the capillary effect of the straw. Plasma modification: Plasma treatment is introduced during carbonization to achieve in-situ doping of nitrogen heteroatoms into the carbon lattice at lower temperatures, forming basic functional groups such as pyrrole nitrogen and pyridine nitrogen. This avoids pore collapse caused by high-temperature annealing, thus enabling the straw biochar material prepared by this invention to not only retain a multi-level gradient pore structure but also form a very strong CO2 dipole moment attraction region on the surface, greatly improving the selective capture capacity for low-concentration CO2 and achieving high-capacity, fast-kinetic, and high-stability CO2 adsorption. Under conditions of 25℃ and 1 bar, the straw biochar material prepared by this invention has an adsorption capacity of ≥4.5 mmol / g for CO2, and after 20 cycles of use, the adsorption capacity retention rate is ≥95%. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the preparation process of straw biochar material with multi-level gradient pore structure in an embodiment of the present invention; Figure 2 The image shows a SEM image of the straw biochar material with a multi-level gradient pore structure prepared in Example 1. Detailed Implementation

[0020] This invention provides a method for preparing straw biochar material with a multi-level gradient pore structure, comprising the following steps: Straw was placed in a buffer solution containing hydrolytic enzymes, and the resulting mixture was subjected to a hydrolysis reaction to obtain a pre-etched biomass template. The pre-etched biomass template was placed in a solution containing magnesium salt, aluminum salt and organic ligand, and after vacuum-pressure alternating impregnation, an in-situ solvothermal reaction was carried out to obtain a biomass template loaded with MOF precursor. The biomass template with the MOF precursor was pyrolyzed and carbonized in an atmosphere containing nitrogen, and plasma treatment was performed simultaneously to obtain straw biochar material with a multi-level gradient pore structure.

[0021] Unless otherwise specified, the present invention does not have special requirements on the source of raw materials used, and commercially available products well known to those skilled in the art can be used.

[0022] This invention places straw in a buffer solution containing hydrolytic enzymes, and the resulting mixture undergoes a hydrolysis reaction to obtain a pre-etched biomass template.

[0023] In one implementation, the straw is agricultural straw, specifically corn straw; the straw is straw powder pulverized to 100-200 mesh, specifically 100 mesh, 150 mesh, or 200 mesh.

[0024] In one embodiment, the hydrolytic enzyme (biological enzyme) includes cellulase, hemicellulase and pectinase; the mass ratio of cellulase, hemicellulase and pectinase is 2.5~4:1.5~2.5:0.5~1.5, and in a specific embodiment it is 3:2:1.

[0025] The core framework of straw is cellulose. Cellulase is responsible for degrading the amorphous and partially crystalline regions of cellulose, and is the main factor in the formation of nanopores. Hemicellulose tightly encapsulates cellulose and lignin. The role of hemicellulosease is to break down these encapsulation layers, opening pathways for cellulase. Pectin exists in the intercellular layer of plant cells, playing a role in binding cells. Pre-penetration by pectinase can soften the tissue, making the distribution of nanopores more uniform.

[0026] In one embodiment, the buffer solution is sodium acetate buffer, citrate-sodium citrate buffer, or phosphate buffer, with sodium acetate buffer being used in a specific embodiment; the pH value of the buffer solution is 4.5~5.5, with 4.8, 5.0, or 5.2 being used in a specific embodiment; the solid-liquid ratio of the straw and the buffer solution containing hydrolytic enzyme is 1:15~40, with 1:20 being used in a specific embodiment; the solid-liquid ratio refers to the mass ratio of the straw and the buffer solution containing hydrolytic enzyme.

[0027] In one embodiment, the hydrolysis reaction is carried out at a temperature of 48±2℃, specifically 48℃, for a time of 3~5 hours, specifically 3 hours, 4 hours, or 5 hours; the hydrolysis reaction is conducted under stirring conditions; the stirring speed is 100~300 rpm, specifically 150 rpm, 200 rpm, or 300 rpm; the pH value of the mixed solution during the hydrolysis reaction is 4.8~5.2, specifically 4.8, 5.0, or 5.2. When the pH value of the mixed solution during the hydrolysis reaction is not within the range of 4.8~5.2, the pH value is adjusted to 4.8~5.2. This invention does not have specific limitations on the process of adjusting the pH value; it is sufficient to keep the pH value within the range of 4.8~5.2.

[0028] In the buffer solution, the straw powder transitions from sedimentation to uniform suspension, ensuring that the surface of each powder particle is in full contact with the hydrolytic enzyme solution. The weak shear force generated by stirring helps to peel off reaction products (such as reducing sugars), preventing product inhibition effects and helping enzyme molecules penetrate deeper into the xylem vessel walls. This maintains the integrity of the vessel skeleton, avoids fiber breakage caused by high shear forces, and ensures "etching" rather than "crushing."

[0029] This invention utilizes the selective degradation characteristics of biological enzymes to directionally etch nanoscale diffusion channels with a diameter of 5-20 nm onto the tube wall by microscopically "modifying" the structure of lignocellulose without damaging the natural fiber vascular skeleton of straw.

[0030] In one embodiment, after the hydrolysis reaction, the present invention further includes: subjecting the product obtained from the hydrolysis reaction to instantaneous high-temperature inactivation, followed by sequential washing and drying to obtain a pre-etched biomass template; the instantaneous high-temperature inactivation temperature is 80~95℃, specifically 85℃ in this embodiment, and the time is 5~15min, specifically 8min, 10min or 15min in this embodiment; the washing is performed 2~5 times, specifically 2 times, 3 times or 5 times in this embodiment; the washing continues until the washing solution is neutral and colorless and transparent; the washing solution used is deionized water; for the last washing, the washing solution is a mixed solution of anhydrous ethanol and deionized water; the drying temperature is 50~70℃, specifically 60℃ in this embodiment, and the drying time is 10~24h, specifically 10h, 12h or 15h in this embodiment.

[0031] After obtaining the pre-etched biomass template, the present invention places the pre-etched biomass template in a solution containing magnesium salt, aluminum salt and organic ligand, performs vacuum-pressure alternating impregnation, and then performs an in-situ solvothermal reaction to obtain a biomass template loaded with MOF precursor.

[0032] In one embodiment, the magnesium salt includes one or more of magnesium nitrate hexahydrate, magnesium chloride, and magnesium acetate, with magnesium nitrate hexahydrate being a specific example; the aluminum salt includes one or more of aluminum nitrate nonahydrate, aluminum chloride hexahydrate, and aluminum isopropoxide, with aluminum chloride hexahydrate being a specific example; the concentrations of the magnesium salt and aluminum salt in the solution containing the magnesium salt, aluminum salt, and organic ligand are independently 0.01~0.35 mol / L, with the magnesium salt concentration being 0.12 mol / L, 0.2 mol / L, or 0.32 mol / L, and the aluminum salt concentration being 0.06 mol / L, 0.1 mol / L, or 0.16 mol / L, respectively; the organic ligand includes one or more of 2-aminoterephthalic acid, terephthalic acid, and trimesic acid, with the organic ligand being a specific example. The organic ligand is 2-aminoterephthalic acid; the concentration of the organic ligand in the solution containing magnesium salt, aluminum salt, and organic ligand is 0.005~0.6 mol / L, specifically 0.22 mol / L, 0.35 mol / L, or 0.55 mol / L in the specific embodiments; when the organic ligand is terephthalic acid, the concentration of terephthalic acid in the solution containing magnesium salt, aluminum salt, and organic ligand is 0.01~0.5 mol / L; when the organic ligand is trimesic acid, the concentration of trimesic acid in the solution containing magnesium salt, aluminum salt, and organic ligand is 0.005~0.5 mol / L; the concentration of the pre-etched biomass template in the solution containing magnesium salt, aluminum salt, and organic ligand is 5~30 g / L, specifically 20 g / L, 25 g / L, or 30 g / L in the specific embodiments.

[0033] In one embodiment, the solvent in the solution containing magnesium salt, aluminum salt and organic ligand is one or more of N,N-dimethylformamide (DMF), N,N-diethylformamide and ethanol, with N,N-dimethylformamide being used in a specific embodiment.

[0034] In one implementation method, the vacuum-pressure alternating impregnation involves first evacuating and then pressurizing the impregnation process. The vacuum level is -0.06 to -0.1 MPa, specifically -0.09 MPa in this embodiment. The holding time after vacuuming is 20 to 120 minutes, specifically 30 minutes in this embodiment. The pressurization level is 0.5 to 0.8 MPa, specifically 0.5 MPa, 0.6 MPa, or 0.8 MPa in this embodiment. The holding time under pressure is 0.5 to 2 hours, specifically 0.5 hours, 1 hour, or 2 hours in this embodiment. The pressurization rate is 0.05 to 0.2 MPa / s, specifically 0.05 MPa / s, 0.1 MPa / s, or 0.2 MPa / s in this embodiment. This invention first evacuates the straw template wall and deep pores of the tube bundle to remove air, then rapidly pressurizes to force the precursor solution to penetrate the nano-diffusion channels formed by bio-enzyme etching and the micropores of the straw inner wall.

[0035] In one embodiment, the temperature of the in-situ solvothermal reaction is 100~160℃, and in a specific embodiment it is 120℃, and the time is 12~48h, and in a specific embodiment it is 24h.

[0036] This invention utilizes the confinement effect of straw nanochannels to guide the precise growth of MOF crystals on the inner wall of the channels. This "inside-out" growth mode effectively avoids the aggregation of active components and ensures a high-density, uniform distribution of nano-adsorption sites within the biochar framework.

[0037] After obtaining the biomass template of the supported MOF precursor, the present invention pyrolyzes and carbonizes the biomass template of the supported MOF precursor in an atmosphere containing N, and simultaneously performs plasma treatment to obtain straw biochar material with a multi-level gradient pore structure.

[0038] As one implementation method, before the pyrolysis and carbonization, the method further includes: drying the biomass template of the MOF precursor; the drying temperature is 70~100℃, specifically 70℃, 80℃ or 100℃ in the embodiment, and the drying time is 10~15h, specifically 10h, 12h or 15h in the embodiment.

[0039] In one embodiment, the nitrogen-containing atmosphere includes ammonia and argon; the volume ratio of ammonia to argon is 1:3~9, specifically 1:4 in this embodiment; the pyrolysis carbonization temperature is 550~650℃, specifically 550℃, 600℃, or 650℃ in this embodiment; the rate of heating to the pyrolysis carbonization temperature is 3~5℃ / min, specifically 3℃ / min, 4℃ / min, or 5℃ / min in this embodiment; the pyrolysis carbonization time is the same as the plasma treatment time; the plasma treatment is cold plasma radio frequency treatment; the plasma treatment power is 200~350W, specifically 200W, 250W, or 350W in this embodiment, and the time is 1.5~3h, specifically 2h in this embodiment.

[0040] High-energy nitrogen free radicals generated by plasma intensely bombard and embeded into the biochar lattice during carbonization, achieving efficient formation of basic functional groups such as pyrrole nitrogen and pyridine nitrogen at relatively low temperatures. The resulting composite material not only retains the multi-level gradient pore structure but also forms a very strong CO2 dipole moment attraction region on its surface, greatly enhancing its selective capture capability for low-concentration CO2.

[0041] Figure 1 This is a schematic diagram of the preparation process of straw biochar material with multi-level gradient pore structure in an embodiment of the present invention, as shown below. Figure 1As shown, this invention involves treating crop straw in a sodium acetate buffer system containing a complex hydrolytic enzyme, then placing it in a high-pressure reactor containing a mixture of metal salt solution and organic ligands for alternating vacuum and pressure treatment, and finally pyrolyzing and carbonizing it in a tubular furnace under a mixed atmosphere of ammonia / argon, with simultaneous plasma treatment, to obtain nanoscale biochar CO2 adsorbent material.

[0042] The present invention also provides a straw biochar material with a multi-level gradient pore structure prepared by the preparation method described above, comprising straw biochar, Mg and Al anchored in the pores of the straw biochar and N doped in the carbon lattice of the straw biochar; The straw biochar material with a multi-level gradient pore structure has micropores and mesopores; the average pore size of the micropores and mesopores is 2~10nm, and the volume of micropores and mesopores accounts for ≥75% of the total pore volume.

[0043] In one embodiment, the average pore size of the micropores and mesopores is 2.1 nm, 2.4 nm, or 2.8 nm, and the volume of the micropores and mesopores accounts for 82%, 88%, or 92% of the total pore volume.

[0044] In one embodiment, the specific surface area of ​​the straw biochar material with a multi-level gradient pore structure is 800~1800 m². 2 / g, specifically 1140m in the embodiment. 2 / g、1310m 2 / g or 1625m 2 / g; the mass fraction of Mg in the straw biochar material with multi-level gradient pore structure is 5~15%, specifically 5.1%, 8.2% or 12.5% ​​in the embodiments; the mass fraction of Al is 2~8%, specifically 2.8%, 4.1% or 6.3% in the embodiments; and the mass fraction of N is 5~8%, specifically 5.5%, 6.2% or 7.8% in the embodiments.

[0045] In one embodiment, N in the straw biochar material with a multi-level gradient pore structure is pyrrole nitrogen and / or pyridine nitrogen, and in a specific embodiment, it is pyrrole nitrogen and pyridine nitrogen.

[0046] The present invention also provides the application of the straw biochar material with multi-level gradient pore structure described in the above technical solution in CO2 treatment.

[0047] In one embodiment, the application involves using the straw biochar material with the multi-level gradient pore structure to adsorb CO2; during adsorption, the CO2 concentration is 10-20%, specifically 15% in this embodiment. The present invention does not impose any particular limitation on the adsorption process; any adsorption process well-known in the art can be used.

[0048] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0049] Example 1 The specific preparation steps are as follows: ① Nanopore pre-etching: Take corn stalk powder crushed to 150 mesh, put it into sodium acetate buffer (pH=4.8) containing cellulase, hemicellulase and pectinase (mass ratio 3:2:1), set the solid-liquid ratio to 1:20, adjust the pH of the resulting mixture to 5.0, stir at 200 rpm for 4 h at 48℃, after the reaction is completed, inactivate by instantaneous high temperature at 85℃ for 10 min, wash 3 times with deionized water and dry at 60℃ for 12 h to obtain pre-etched biomass template; ② In-situ construction of nanosites: A vacuum-pressure alternating impregnation process was used. The above template was placed in a DMF solution containing magnesium nitrate hexahydrate, aluminum trichloride (aluminum chloride hexahydrate), and 2-aminoterephthalic acid (ligand). The concentration of magnesium nitrate hexahydrate was 0.2 mol / L, the concentration of aluminum trichloride was 0.1 mol / L, the concentration of 2-aminoterephthalic acid was 0.35 mol / L, and the concentration of the template in the DMF solution was 25 g / L. First, the vacuum was evacuated to -0.09 MPa and maintained for 30 min. Then, the pressure was rapidly increased to 0.6 MPa at a rate of 0.1 MPa / s and maintained for 1 h. Then, it was transferred to a reaction vessel and subjected to an in-situ solvothermal reaction at 120 °C for 24 h. ③ Gradient confined carbonization and in-situ doping: The biomass template of the MOF precursor, dried at 80℃ for 12h, was placed in a tube furnace with a mixed atmosphere of ammonia and argon (volume ratio 1:4) and heated to 600℃ at a rate of 4℃ / min. At the same time, cold plasma radio frequency treatment with a power of 250W was turned on for 2h. After cooling, straw biochar material with a multi-level gradient pore structure was obtained, in which the mass fraction of Mg was 8.2%, the mass fraction of Al was 4.1%, and the mass fraction of N was 5.5%.

[0050] The biochar sample to be tested was placed in a fully automated gas adsorption analyzer and degassed at vacuum and 150°C for 6 hours to thoroughly remove moisture and impurities from the pores. In a controlled environment of 25°C, 90% pure CO2 gas was gradually introduced into the test chamber. The amount of CO2 adsorbed was calculated using the ideal gas law.

[0051] The CO2 / N2 adsorption selectivity was determined using the Ideal Adsorption Solution Theory (IAST), with the selectivity coefficient S = q1y2 / q2y1, where q1 and y1 represent the adsorption volume and partial pressure of CO2, respectively; and q2 and y2 represent the adsorption volume and partial pressure of N2, respectively. The CO2 / N2 volume ratio used was 15 / 85.

[0052] The static adsorption capacity of CO2 is 4.85 mmol / g, the CO2 / N2 selectivity coefficient is 88, and the specific surface area is 1310 m². 2 / g, the average pore size of micropores and mesopores is 2.4nm, accounting for 88% of the total pore volume (the volume of micropores and mesopores).

[0053] Example 2 Compared with Example 1, the etching process is prolonged, increasing channel density, improving the permeability of high-viscosity solutions, enhancing the degree of carbonization, and strengthening the nitrogen free radical impaction intensity, thus forming a biochar composite material with high-density adsorption sites. The specific preparation steps are as follows: ① Nanopore pre-etching: Take rice straw powder pulverized to 100 mesh, put it into sodium acetate buffer (pH=4.8) containing cellulase, hemicellulase and pectinase (mass ratio 3:2:1), the solid-liquid ratio is 1:20, adjust the pH of the resulting system to 4.8, stir at 300 rpm for 5 h at 48℃, after the reaction is completed, inactivate by instantaneous high temperature at 85℃ for 15 min, wash 5 times with deionized water and dry at 60℃ for 15 h to obtain pre-etched biomass template; ② In-situ construction of nanosites: The template was placed in a DMF / ethanol solution containing magnesium nitrate hexahydrate, aluminum trichloride, and 2-aminoterephthalic acid (ligand). The concentration of magnesium nitrate hexahydrate was 0.32 mol / L, the concentration of aluminum trichloride was 0.16 mol / L, the concentration of 2-aminoterephthalic acid was 0.55 mol / L, and the concentration of the template in the DMF solution was 30 g / L. First, the vacuum was evacuated to -0.09 MPa and held for 30 min. Then, the pressure was rapidly increased to 0.8 MPa at a rate of 0.2 MPa / s and held for 2 h to enhance the penetration depth of the precursor. Then, it was transferred to a reaction vessel and subjected to an in-situ solvothermal reaction at 120 °C for 24 h. ③ Gradient confined carbonization and in-situ doping: The biomass template of the MOF precursor, dried at 100℃ for 15h, was placed in a tube furnace with a mixed atmosphere of ammonia and argon (volume ratio 1:4). The temperature was increased to 650℃ at a rate of 5℃ / min, and a cold plasma radio frequency treatment with a power of 350W was turned on for 2h. After cooling, a straw biochar material with a multi-level gradient pore structure was obtained, in which the mass fraction of Mg was 12.5%, the mass fraction of Al was 6.3%, and the mass fraction of N was 6.2%.

[0054] This embodiment achieves a higher specific surface area and a deeper distribution of active sites by extending the enzymatic hydrolysis time, increasing the impregnation pressure and plasma power, making it suitable for CO2 capture under high pressure.

[0055] The static adsorption capacity of CO2 is 5.62 mmol / g, the CO2 / N2 selectivity coefficient is 78, and the specific surface area is 1625 m². 2 / g, the average pore size of micropores and mesopores is 2.8nm, accounting for 92% of the total pore volume (the volume of micropores and mesopores).

[0056] Example 3 Compared with Example 1, moderate shearing, maintaining skeletal strength, and conventional pressure infiltration help retain more surface functional groups, reduce lattice damage, and form a biochar composite material with high nitrogen content. The specific preparation steps are as follows: ① Nanopore pre-etching: Take wheat straw powder pulverized to 200 mesh and put it into sodium acetate buffer (pH=5.2) containing cellulase, hemicellulase and pectinase (mass ratio 3:2:1). The solid-liquid ratio is 1:20. Adjust the pH of the system to 5.2 and stir at 150 rpm for 3 h at 48℃. After the reaction, inactivate it by instantaneous high temperature at 85℃ for 8 min. Wash it twice with deionized water and dry it at 60℃ for 10 h to obtain the pre-etched biomass template. ② In-situ construction of nanosites: The template was placed in a DMF / ethanol solution containing magnesium nitrate hexahydrate, aluminum trichloride, and 2-aminoterephthalic acid (ligand). The concentration of magnesium nitrate hexahydrate was 0.12 mol / L, the concentration of aluminum trichloride was 0.06 mol / L, the concentration of 2-aminoterephthalic acid was 0.22 mol / L, and the concentration of the template in the DMF solution was 20 g / L. First, the vacuum was evacuated to -0.09 MPa and held for 30 min. Then, the pressure was rapidly increased to 0.5 MPa at a rate of 0.05 MPa / s and held for 0.5 h. Then, it was transferred to a reaction vessel and subjected to an in-situ solvothermal reaction at 120 °C for 24 h. ③ Gradient confined carbonization and in-situ doping: The biomass template of the MOF precursor, dried at 70℃ for 10h, was placed in a tube furnace with a mixed atmosphere of ammonia and argon (volume ratio 1:4) and heated to 550℃ at a rate of 3℃ / min. At the same time, a cold plasma radio frequency treatment with a power of 200W was turned on for 2h. After cooling, a straw biochar material with a multi-level gradient pore structure was obtained, in which the mass fraction of Mg was 5.1%, the mass fraction of Al was 2.8%, and the mass fraction of N was 7.8%.

[0057] This embodiment employs a lower carbonization temperature and power, focusing on maximizing the retention of basic functional groups (such as pyrrole nitrogen) on the material surface, thereby improving selectivity for CO2 in extremely low concentrations (such as atmospheric environments).

[0058] The static adsorption capacity of CO2 is 4.5 mmol / g, the CO2 / N2 selectivity coefficient is 112, and the specific surface area is 1140 m². 2 / g, the average pore size of micropores and mesopores is 2.1nm, accounting for 82% of the total pore volume (the volume of micropores and mesopores).

[0059] Comparative Example 1 (Missing Pre-etching Process) The specific preparation steps are as follows: Take corn stalk powder pulverized to 150 mesh, without performing a biological enzyme soaking reaction, simply wash with deionized water at 48°C for 4 hours to remove surface dust and impurities, and then dry for later use. Following the method of step ② in Example 1, perform vacuum-pressure alternating impregnation and in-situ solvothermal reaction on the unenzymatically etched stalk powder to load magnesium / aluminum metal salts and ligands; following the method of step ③ in Example 1, heat to 600°C at 4°C / min under an ammonia / argon atmosphere, and treat with 250W cold plasma for 2 hours.

[0060] The difference from Example 1 is that the nanopore pre-etching step based on enzyme-directed shearing was completely omitted. Due to the lack of enzyme-directed shearing of cellulose / hemicellulose, nanoscale diffusion channels of 5-20 nm were not formed on the straw tube wall. This resulted in the subsequent MOF precursor only accumulating in the existing natural macropores of the straw, unable to penetrate deep into the cell wall, reducing the dispersion and contact area of ​​active sites. The static CO2 adsorption capacity was 2.80 mmol / g, the CO2 / N2 selectivity coefficient was 48, and the specific surface area was 720 m². 2 / g, the average pore size of micropores and mesopores is 6.5nm, accounting for 42%.

[0061] Comparative Example 2 (Conventional high-temperature carbonization as an alternative to cold plasma modification) This comparative example is used to verify the role of "cold plasma-assisted" in in-situ nitrogen doping and structure preservation.

[0062] The specific preparation steps are as follows: Following the method in step ① of Example 1, a biomass template with nanopores was obtained through directional shearing using bioenzymes. Following the method in step ② of Example 1, MOF crystals were precisely grown on the inner wall of straw. The loaded material was placed in a mixed atmosphere of ammonia and argon (volume ratio 1:4), heated to 600℃ at a rate of 4℃ / min, and maintained at this high temperature for 2 hours without activating cold plasma radio frequency treatment.

[0063] The difference from Example 1: Example 1 uses a dual mode of "thermal excitation + high-energy plasma bombardment"; while Comparative Example 2 relies only on conventional thermochemical reactions. Without the activation of cold plasma, the nitrogen species produced by ammonia decomposition have low kinetic energy, making it difficult to effectively embed into the deep layers of the carbon lattice at a relatively low temperature of 600°C. This results in significantly lower pyrrole nitrogen and pyridine nitrogen content in the final product compared to Example 1, and a lack of surface micro-defects formed by plasma bombardment. The static CO2 adsorption capacity is 3.50 mmol / g, the CO2 / N2 selectivity coefficient is 38, and the specific surface area is 1020 m². 2 / g, the average pore size of micropores and mesopores is 3.2nm, accounting for 75%.

[0064] Performance testing Figure 2 SEM image of the straw biochar material with a multi-level gradient pore structure prepared in Example 1. From... Figure 2 As can be seen, the nanopores formed by bio-enzyme etching provide in-situ confined and uniformly dispersed Mg / Al adsorption sites. The effective synergy of the multi-level gradient structure of macropores, mesopores, and micropores enables rapid diffusion and efficient capture of CO2.

[0065] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a straw biochar material having a multi-stage gradient pore structure, characterized in that, Includes the following steps: Straw was placed in a buffer solution containing hydrolytic enzymes, and the resulting mixture was subjected to a hydrolysis reaction to obtain a pre-etched biomass template. The pre-etched biomass template was placed in a solution containing magnesium salt, aluminum salt and organic ligand, and after vacuum-pressure alternating impregnation, an in-situ solvothermal reaction was carried out to obtain a biomass template loaded with MOF precursor. The biomass template with the MOF precursor was pyrolyzed and carbonized in an atmosphere containing nitrogen, and plasma treatment was performed simultaneously to obtain straw biochar material with a multi-level gradient pore structure.

2. The preparation method according to claim 1, characterized in that, The hydrolytic enzymes include cellulase, hemicellulase, and pectinase; the mass ratio of cellulase, hemicellulase, and pectinase is 2.5~4:1.5~2.5:0.5~1.

5.

3. The preparation method according to claim 1 or 2, characterized in that, The hydrolysis reaction was carried out at a temperature of 48±2℃ for 3~5 hours.

4. The preparation method according to claim 1, characterized in that, The organic ligand includes one or more of 2-aminoterephthalic acid, terephthalic acid, and trimesic acid.

5. The preparation method according to claim 1 or 4, characterized in that, The in-situ solvothermal reaction is carried out at a temperature of 100~160℃ for a time of 12~48h.

6. The preparation method according to claim 1, characterized in that, The temperature for pyrolysis and carbonization is 550~650℃.

7. The preparation method according to claim 1 or 6, characterized in that, The plasma treatment is a cold plasma radio frequency treatment; the power of the plasma treatment is 200~350W, and the time is 1.5~3h.

8. The preparation method according to claim 1, characterized in that, The buffer solution is a sodium acetate buffer, a citrate-sodium citrate buffer, or a phosphate buffer; the pH value of the buffer solution is 4.5~5.

5.

9. The straw biochar material with a multi-level gradient pore structure prepared by the preparation method according to any one of claims 1 to 8, characterized in that, Includes straw biochar, Mg and Al anchored in the pores of the straw biochar, and N doped in the carbon lattice of the straw biochar; The straw biochar material with a multi-level gradient pore structure has micropores and mesopores; the average pore size of the micropores and mesopores is 2~10nm, and the volume of micropores and mesopores accounts for ≥75% of the total pore volume.

10. The application of the straw biochar material with a multi-level gradient pore structure as described in claim 9 in CO2 treatment.

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