Method for preparing high-purity porous silicon carbide ceramic from biomass aerogel precursor

Abstract

The invention relates to a method for preparing high-purity porous silicon carbide ceramic from a biomass aerogel precursor. The method comprises the following specific steps: (1) preparing precursor sol; (2) gelatinizing, aging and forming a three-dimensional network structure; (3) solvent replacement; (4) performing supercritical drying to obtain biomass carbon-silicon dioxide composite aerogel; (5) carrying out carbon thermal reduction reaction to generate porous silicon carbide ceramic; and (6) decarbonizing and purifying to obtain the high-purity porous silicon carbide ceramic. The prepared porous silicon carbide ceramic material has high specific surface area, uniform pore size distribution and good mechanical strength, can effectively filter impurities such as particulate matters and harmful gases in gas, and shows excellent gas filtering performance. The material can be used in the fields of air purification, industrial waste gas treatment and the like, and has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of porous ceramics, and particularly to a method for preparing high-purity porous silicon carbide ceramics from biomass aerogel precursors. BACKGROUND

[0002] Porous silicon carbide ceramics, as a new type of high-temperature flue gas purification material, have a skeleton composed of a strong covalent Si-C network, which endows them with high specific surface area, high porosity, low thermal expansion coefficient, high thermal conductivity, and high-temperature strength. The unique porous network significantly enhances the mass transfer and reaction efficiency of pollutants (such as NO x , SO2, VOCs, PM) by prolonging the turbulent flow path of flue gas in the pores, thereby improving removal efficiency. Meanwhile, the high thermal conductivity of the silicon carbide skeleton can quickly disperse local high-temperature hot spots and inhibit the generation of thermal stress cracks. With the aid of controllable pore-forming agents and precursor conversion processes, the pore size, porosity, and pore connectivity of porous silicon carbide ceramics can be precisely adjusted on the nanometer to millimeter scale, and further functionalization can be achieved by in-situ growth of carbon nanotubes or loading of molecular sieves on the surface to realize multifunctional coupling. Therefore, porous silicon carbide ceramics are considered as ideal carrier / catalyst integrated materials for the next generation of high-temperature flue gas deep purification, with an operating temperature window of up to 1000°C or above and purification efficiency far exceeding traditional ceramics or metal carriers, earning them the reputation of "ultimate porous engine for flue gas treatment".

[0003] Currently, there are several patents related to the preparation of porous silicon carbide ceramics, but these patents still have some shortcomings. For example, some patents use the method of adding pore-forming agents, such as invention patent CN102659447A, which adds silicon powder, pore-forming agents, and organic resins, etc., and then removes the pore-forming additives at 800-1200°C to prepare porous silicon carbide ceramics. However, this method may cause the residue of pore-forming agents during sintering, affecting the purity and performance of the ceramics, and the type and amount of added pore-forming agents are difficult to accurately control, which may cause uneven pore structure. Invention patent CN1769241A uses graphite as a pore-forming agent to prepare porous ceramics by in-situ reaction of silicon carbide surface to generate mullite as a binding phase. However, this method has strict requirements on process conditions, and graphite is easily oxidized at high temperatures, which may lead to the collapse of pore structure and the decline of performance. Patents using template methods, such as invention patent CN101164655A, use polyurethane foam plastic as a template, immerse the slurry made of ceramic powder in the template, and then dry and sinter to obtain foam ceramics. This method produces ceramics with low strength, which is difficult to meet the requirements of some high-strength applications, and the preparation and processing of the template are relatively complex, with high cost.

[0004] Therefore, there is an urgent need to develop a new type of aerogel precursor method to prepare porous silicon carbide ceramic materials, effectively control the pore structure of silicon carbide ceramics through aerogel precursors, and select appropriate raw materials to reduce the cost of porous silicon carbide ceramics. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing high-purity porous silicon carbide ceramics from biomass aerogel precursors, addressing the shortcomings of existing technologies. This material is designed for high-temperature flue gas filtration and deep purification applications, meeting the quadruple performance requirements of high porosity, high specific surface area, high mechanical strength, and high thermal conductivity. It can operate continuously at temperatures above 1000℃ and effectively resists NO. x This invention aims to achieve efficient and synergistic removal of pollutants such as SO2, VOCs, and PM. It addresses the following shortcomings of existing technologies: (1) traditional pore-forming agent methods are prone to leaving impurities and exhibiting uneven pore structures; (2) template methods are complex and have low strength; and (3) existing silicon carbide carriers have insufficient specific surface area (<200m²). 2 / g), with limited active sites; (4) frequent thermal stress cracks and rapid increase in pressure drop under high temperature conditions. To this end, the present invention proposes an integrated route of "sol-gel-supercritical drying-precursor conversion", which achieves precise customization of silicon carbide pore structure at the 2-100nm scale through nanoscale control of aerogel framework, while maintaining high purity, high continuity and high thermal conductivity of framework.

[0006] The technical solution of this invention is: a method for preparing high-purity porous silicon carbide ceramics from biomass aerogel precursors, characterized by the following specific steps:

[0007] (1) Preparation of precursor sol: Disperse biomass source carbon in deionized water, add acid to adjust the pH value of the solution and heat and stir to obtain a uniform carbon-containing solution, then add pore-forming-crosslinking synergist and silicon source, mix evenly and stir to obtain precursor sol; (2) Gelation and aging: Pour the precursor sol obtained in step (1) into a mold and place it in a constant temperature oven at 60-80℃ for 6-48h to allow it to fully gel and form a three-dimensional network structure; (3) Solvent replacement: Immerse the gel obtained in step (3) in an alcoholic organic solvent for solvent replacement; (4) Drying: Place the gel obtained in step (3) in a constant temperature oven at 60-80℃ for 6-48h to allow it to fully gel and form a three-dimensional network structure; The gel after replacement is dried by supercritical drying to obtain biomass carbon-silica composite aerogel; (5) Carbothermic reduction reaction: Under the protection of a protective atmosphere, the biomass carbon-silica composite aerogel obtained in step (4) is heated to 1200-1600℃ and kept at the temperature for 1-6h to allow the biomass carbon and silica to undergo a carbothermic reduction reaction to generate porous silicon carbide ceramics; (6) Carbon removal and purification: The silicon carbide ceramics obtained in step (5) are heat-treated in an air atmosphere at 700-900℃ for 1-4h to remove the residual free carbon in the material and obtain high-purity porous silicon carbide ceramics.

[0008] The preferred biomass carbon source in step (1) is one or more of chitosan, cellulose, starch or lignin; the mass ratio of the biomass carbon source to the volume of deionized water is 0.02 to 0.06 g / mL.

[0009] The preferred acid in step (1) is one of acetic acid, oxalic acid, hydrochloric acid or sulfuric acid; the pH of the solution is adjusted to 1 to 3.

[0010] The preferred silicon source in step (1) is one or more of methyltrimethoxysilane (MTMS), tetraethyl orthosilicate (TEOS), or silica sol; the amount of silicon source added is controlled to be 2.0 to 4.0 (molar ratio of C to Si) of biomass carbon source to silicon source.

[0011] The preferred pore-forming crosslinking synergist in step (1) is urea or ammonia; the ratio of its added mass to the volume of deionized water is 0.3 to 0.4 g / mL.

[0012] The preferred step (1) is to heat and stir at a temperature of 70-90°C for 24-48 hours.

[0013] The preferred solvent used for solvent replacement in step (3) is one of ethanol, tert-butanol or acetone; the replacement solvent is replaced every 6 to 12 hours, and the replacement time is 1 to 3 days.

[0014] In the preferred step (4), the supercritical drying is CO2 supercritical drying, with the following specific parameters: drying time of 12-24h, drying pressure of 9-10MPa, and drying temperature of 40-50℃.

[0015] The preferred protective atmosphere in step (5) is nitrogen or argon; the gas flow is controlled at 50-200 mL / min; the programmed temperature rise process is as follows: from room temperature, the temperature is increased to 600-800℃ at 1-3℃ / min and held for 0.5-2h for carbonization, and then the temperature is increased to 1200-1600℃ at 2-5℃ / min and held for 1-6h.

[0016] The preferred step (6) requires a carbon removal and purification temperature of 750-850°C for 2-4 hours to completely remove impurity carbon.

[0017] The advantages of this invention compared to the prior art are as follows:

[0018] (1) High efficiency and low cost: Using biomass carbon sources (such as chitosan) to replace traditional expensive raw materials, combined with atmospheric pressure or supercritical drying process, significantly reduces production costs and equipment requirements, and is suitable for large-scale production.

[0019] (2) High purity and excellent performance: Through optimized carbothermal reduction and carbon removal purification steps (such as oxidizing atmosphere treatment), free carbon impurities are effectively removed, and the resulting silicon carbide ceramics have a purity of over 99%, large porosity, and high specific surface area, making them suitable for high-temperature and harsh environments.

[0020] (3) Flexible and environmentally friendly process: The method steps are simple and controllable (such as solvent replacement and heating program), the adjustable parameter range is wide (such as C / Si ratio and temperature), and the biomass raw materials are renewable, reducing environmental pollution and meeting the needs of sustainable development. Attached Figure Description

[0021] Figure 1 Image of a porous ceramic sample prepared in Example 1;

[0022] Figure 2 XRD pattern of porous ceramics prepared in Example 1;

[0023] Figure 3 Scanning electron microscope image of the porous ceramic prepared in Example 1;

[0024] Figure 4 BET test pattern of porous ceramics prepared in Example 1. Detailed Implementation

[0025] Example 1: 2g of chitosan powder was weighed and added to 100mL of deionized water. 1mL of acetic acid was added to adjust the pH to 3. The solution was stirred at 70℃ for 24h to obtain a clear and transparent chitosan solution. Subsequently, 30g of urea and 6.53g of methyltrimethoxysilane (MTMS) were added to this solution, and the mixture was stirred to obtain a homogeneous sol system with a C / Si molar ratio of 2. The resulting sol was poured into a mold and placed in a 60℃ constant temperature oven for 12h to complete gelation and aging. The gel block was then immersed in anhydrous ethanol for solvent replacement, with fresh ethanol replaced every 8h for 2 days. The replaced wet gel was placed in a CO2 supercritical drying apparatus and dried at 9MPa and 40℃ for 12h to obtain chitosan composite silica aerogel. The aerogel was placed in a tube furnace and heated to 600°C at a rate of 2°C / min under an argon atmosphere of 100 mL / min, and held for 1 hour for carbonization. Then, the temperature was further increased to 1350°C at a rate of 3°C / min and held for 3 hours for high-temperature treatment. After natural cooling to room temperature, it was heat-treated in air at 750°C for 2 hours to remove free carbon, finally obtaining pure porous silicon carbide ceramic. The ceramic sample of this embodiment is shown below. Figure 1 As shown. The XRD pattern of the porous ceramic prepared in this embodiment is shown below. Figure 2 As shown in the figure, the porous ceramic conforms to the diffraction peaks of silicon carbide, proving the successful synthesis of silicon carbide ceramic; the SEM image of the porous ceramic prepared in this embodiment is shown below. Figure 3As shown in the figure, its internal structure is porous; the BET test results of the porous ceramic prepared in this embodiment are as follows. Figure 4 As shown in the figure, the pores of this material are mainly mesopores, exhibiting a large specific surface area of ​​580 m². 2 / g, with an average pore size of approximately 10nm.

[0026] Example 2: 4g of cellulose powder was weighed and added to 100mL of deionized water. 1.5mL of oxalic acid was added to adjust the pH to 2. The solution was stirred at 80℃ for 36h to obtain a uniform and transparent cellulose solution. Subsequently, 35g of urea and 9.21g of tetraethyl orthosilicate (TEOS) were added to this solution, and stirring continued to obtain a uniform sol with a C / Si molar ratio of 3. The sol was poured into a mold and allowed to stand in a 70℃ oven for 24h to complete gelation and aging. The gel block was then immersed in tert-butanol for solvent replacement, changing the solvent every 12h for 3 days. The replaced wet gel was dried in a CO2 supercritical drying apparatus at 9.5MPa and 45℃ for 18h to obtain a cellulose / silicon composite aerogel. The aerogel was placed in a tube furnace and carbonized at 700℃ / min under a nitrogen atmosphere (150 mL / min) with a heating rate of 1℃ / min, and held for 1.5 h. Then, a high-temperature reaction was carried out by heating to 1400℃ at 4℃ / min and holding for 4 h. After cooling to room temperature, it was heat-treated in air at 800℃ for 3 h to remove free carbon, finally obtaining high-purity porous silicon carbide ceramic. XRD results of Example 2 showed successful synthesis of β-SiC with no impurity peaks. SEM images of Example 2 showed a relatively uniform pore size distribution, with an average pore size of approximately 15 nm. BET tests of Example 2 indicated a specific surface area of ​​520 m². 2 / g, slightly lower than in Example 1, possibly due to the slower TEOS hydrolysis-condensation rate, resulting in a slightly denser pore structure.

[0027] Example 3: Weigh 4g of soluble starch and add it to 100mL of deionized water. Add 1mL of hydrochloric acid to adjust the pH of the solution to 1.5. Stir at 90℃ for 48h to form a clear starch solution. Add 35g of ammonia and 4.15g of silica sol, and stir evenly to form a sol. At this time, the C / Si molar ratio is 3. Pour the sol into a mold and let it stand in a constant temperature oven at 75℃ for 18h to complete gelation and aging. Immerse the gel block in acetone for solvent replacement, changing the solvent every 10h for 2 days. The wet gel after replacement is dried in a CO2 supercritical drying device at 10MPa and 50℃ for 24h to obtain starch / silica composite aerogel. Place the aerogel in a tube furnace and carbonize it by heating at 3℃ / min under an argon atmosphere at 200mL / min for 2h, then heating at 5℃ / min for 5h for high-temperature reaction. After cooling to room temperature, the free carbon was removed by heat treatment at 850℃ in air for 2 hours to obtain high-purity porous silicon carbide ceramic. XRD analysis of Example 3 showed β-SiC as the main crystalline phase with trace amounts of α-SiC, indicating that high temperature promoted the crystal transformation. SEM images of Example 3 showed a honeycomb-like pore structure with relatively large pore sizes (approximately 50 nm). BET analysis showed a specific surface area of ​​460 m² for Example 3. 2 / g, with a relatively large pore volume, making it suitable for applications requiring large pore sizes, such as catalyst supports.

[0028] Example 4: Weigh 4g of lignin powder and add it to 100mL of deionized water. Add 0.8mL of concentrated sulfuric acid to adjust the pH of the solution to 1. Stir at 85℃ for 30h to form a homogeneous lignin solution. Add 35g of urea and 8.70g of MTMS, and stir until homogeneous to form a sol. At this point, the C / Si molar ratio is 3. Pour the sol into a mold and let it stand in an 80℃ constant temperature oven for 6h to complete gelation and aging. Immerse the gel block in ethanol for solvent replacement, changing the solvent every 6h for 1 day. The wet gel after replacement is dried in a CO2 supercritical drying device at 9MPa and 40℃ for 12h to obtain a lignin / silicon composite aerogel. Place the aerogel in a tube furnace and carbonize it by heating at 2℃ / min under a nitrogen atmosphere at 50mL / min to 600℃ and holding for 0.5h. Then, heat it at 3℃ / min to 1300℃ and hold for 3h for high-temperature reaction. After cooling to room temperature, the free carbon was removed by heat treatment at 750℃ in air for 2 hours to obtain high-purity porous silicon carbide ceramic. The XRD pattern of Example 4 shows a complete β-SiC crystal phase with no obvious impurity peaks. The SEM image of Example 4 shows a relatively dense pore structure with pore size distribution concentrated below 10 nm. The BET test specific surface area of ​​Example 4 is 610 m². 2 / g, the highest among all examples, indicates that the combination of lignin and MTMS is beneficial for forming aerogel structures with high specific surface area, which are suitable for adsorption or sensing applications.

[0029] Example 5: Weigh 4g of chitosan powder and add it to 100mL of deionized water. Add 1.2mL of acetic acid to adjust the pH of the solution to 2.5. Stir at 70℃ for 24h to form a chitosan solution. Add 35g of urea and 9.21g of TEOS, and stir evenly to form a sol. At this point, the C / Si molar ratio is 3. Pour the sol into a mold and let it stand in a 60℃ constant temperature oven for 12h to complete gelation and aging. Immerse the gel block in ethanol for solvent replacement, changing the solvent every 8h for 2 days. The replaced wet gel is dried in a CO2 supercritical drying device at 9MPa and 40℃ for 12h to obtain chitosan / silicon composite aerogel. Place the aerogel in a tube furnace and rapidly heat it to 800℃ at 5℃ / min under an argon atmosphere at 100mL / min and hold for 1h for carbonization. Then, continue heating at 5℃ / min to 1600℃ and hold for 6h for high-temperature reaction. After cooling to room temperature, the free carbon was removed by heat treatment at 900℃ in air for 1 hour to obtain high-purity porous silicon carbide ceramic. XRD analysis of Example 5 showed a complete β-SiC crystal phase, but a small amount of SiO2 residue, indicating that excessively rapid heating may have led to incomplete reaction. SEM images of Example 5 showed a coarse pore structure with sintering densification in some areas. The BET surface area of ​​Example 5 was 380 m² / s. 2 / g, significantly lower than in other embodiments, indicating that excessively rapid heating rates are detrimental to preserving the porous structure.

[0030] Example 6: Weigh 3g of chitosan and 3g of cellulose and mix them in 100mL of deionized water. Add 1mL of oxalic acid to adjust the pH of the solution to 2. Stir at 75℃ for 40h to form a mixed carbon source solution. Add 40g of ammonia and 9.79g of MTMS and stir until homogeneous to form a sol. At this point, the C / Si molar ratio is 4. Pour the sol into a mold and let it stand in a 65℃ constant temperature oven for 20h to complete gelation and aging. Immerse the gel block in tert-butanol for solvent replacement, changing the solvent every 9h for 2 days. The replaced wet gel is dried in a CO2 supercritical drying device at 9.5MPa and 42℃ for 15h to obtain a composite carbon source / silicon aerogel. Place the aerogel in a tube furnace and carbonize it by heating at 120mL / min nitrogen atmosphere to 700℃ at 2℃ / min and holding for 1h. Then, heat it at 3℃ / min to 1350℃ and hold for 4h for high-temperature reaction. After cooling to room temperature, the free carbon was removed by heat treatment at 800℃ for 2.5 h in air, yielding high-purity porous silicon carbide ceramic. XRD analysis of Example 6 showed a pure β-SiC crystal phase with no impurity peaks. SEM images of Example 6 showed a three-dimensional interconnected pore structure with a wide pore size distribution (10–40 nm). BET analysis of Example 6 showed a specific surface area of ​​550 m². 2 / g, with moderate pore volume and good overall performance, indicating that the mixed carbon source helps to form a more balanced pore structure.

Claims

1. A method for preparing high-purity porous silicon carbide ceramics from biomass aerogel precursors, characterized in that, The specific steps are as follows: (1) Preparation of precursor sol: Disperse biomass source carbon in deionized water, add acid to adjust the pH value of the solution and heat and stir to obtain a uniform carbon-containing solution, then add pore-forming-crosslinking synergist and silicon source, mix evenly and stir to obtain precursor sol; (2) Gelation and aging: Pour the precursor sol obtained in step (1) into a mold and place it in a constant temperature oven at 60-80℃ for 6-48h to gel and form a three-dimensional network structure; (3) Solvent replacement: Immerse the gel obtained in step (3) in an alcohol organic solvent for solvent replacement; (4) Drying: Dry the gel obtained in step (3) (3) The gel after replacement is dried by supercritical drying to obtain biomass carbon-silica composite aerogel; (5) Carbothermic reduction reaction: Under the protection of a protective atmosphere, the biomass carbon-silica composite aerogel obtained in step (4) is heated to 1200-1600℃ and kept at the temperature for 1-6h to allow the biomass carbon and silica to undergo a carbothermic reduction reaction to generate porous silicon carbide ceramics; (6) Carbon removal and purification: The silicon carbide ceramics obtained in step (5) are heat-treated in an air atmosphere at 700-900℃ for 1-4h to obtain high-purity porous silicon carbide ceramics.

2. The method according to claim 1, characterized in that, The biomass carbon source in step (1) is one or more of chitosan, cellulose, starch or lignin; the mass ratio of the biomass carbon source to the volume of deionized water is 0.02 to 0.06 g / mL.

3. The method according to claim 1, characterized in that, The acid in step (1) is one of acetic acid, oxalic acid, hydrochloric acid or sulfuric acid; the pH of the solution is adjusted to 1 to 3.

4. The method according to claim 1, characterized in that, The silicon source in step (1) is one or more of methyltrimethoxysilane (MTMS), tetraethyl orthosilicate (TEOS), or silica sol; the amount of silicon source added is controlled so that the molar ratio of biomass carbon source to silicon source is 2.0 to 4.

0.

5. The method according to claim 1, characterized in that, The pore-forming crosslinking synergist in step (1) is urea or ammonia; its added mass to deionized water volume ratio is 0.3-0.4 g / mL.

6. The method according to claim 1, characterized in that, The heating and stirring temperature in step (1) is 70-90℃, and the heating and stirring time is 24-48h.

7. The method according to claim 1, characterized in that, The solvent used for solvent replacement in step (3) is one of ethanol, tert-butanol or acetone; the replacement solvent is changed every 6 to 12 hours, and the replacement time is 1 to 3 days.

8. The method according to claim 1, characterized in that, In step (4), the supercritical drying is CO2 supercritical drying, with the following specific parameters: drying time of 12-24h, drying pressure of 9-10MPa, and drying temperature of 40-50℃.

9. The method according to claim 1, characterized in that, The protective atmosphere in step (5) is nitrogen or argon; the gas flow is controlled at 50-200 mL / min; the programmed temperature rise process is as follows: the temperature is increased to 600-800℃ at 1-3℃ / min and held for 0.5-2h for carbonization, and then the temperature is increased to 1200-1600℃ at 2-5℃ / min and held for 1-6h.

10. The method according to claim 1, characterized in that, The temperature required for carbon removal and purification in step (6) is 750-850℃, and the temperature is maintained for 2-4 hours.

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