Silica-based aerogel composite material, preparation method and application thereof

Abstract

This invention belongs to the field of aerogel technology, and relates to a silicon-based aerogel composite material, its preparation method, and its application. The preparation method of the silicon-based aerogel composite material is as follows: An active component precursor is added to a solvent and stirred until homogeneous, forming an active component precursor solution; ethanol and acid are added to the silicon-based precursor, stirred until homogeneous, and cooled to room temperature to form a silicon-based precursor solution; the active component precursor solution is added to form a precursor-silica sol mixed solution; an alkaline solution is added and stirred until homogeneous, forming a mixed solution; a stepped hot melt-gel treatment is performed, followed by standing to form a silicon-based composite wet gel; solvent replacement and drying yield silicon; the stepped hot melt-gel reaction includes 2-5 temperature stages, each lasting 10-80 minutes at 50-100°C. This invention employs a controllable stepped hot melt-gel process to obtain a silicon-based aerogel composite material with both high specific surface area and excellent mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of aerogel technology, specifically relating to a silicon-based aerogel composite material, its preparation method, and its application. Background Technology

[0002] SiO2 aerogel is a solid material with a three-dimensional nanoporous network structure, attracting much attention due to its excellent properties such as low density, high porosity, low thermal conductivity, and high specific surface area. During its preparation, the solvent is removed from the gel to maintain the intact porous framework structure. This unique nanoporous structure can restrict the movement of air molecules and prevent gas heat exchange, thus playing an important role in thermal insulation. However, the framework structure formed by the interconnected nanoparticles and the high porosity also lead to the brittle and fragile nature of aerogel materials. To overcome this drawback, researchers have enhanced the structure and mechanical properties of aerogels through process optimization, the introduction of reinforcing components, or cross-linking modification. However, the improvement in mechanical strength is often accompanied by a decrease in specific surface area, making it difficult to simultaneously achieve high porosity, high specific surface area, and good mechanical properties in silica aerogels.

[0003] Composite functional aerogel materials refer to novel composite materials with specific functions formed by introducing functional components such as metal nanoparticles, carbon materials, polymers, and metal-organic frameworks into the three-dimensional nanoporous network of aerogel using physical or chemical methods. These materials retain the inherent structural characteristics of aerogels while acquiring special functions such as photocatalysis, conductivity, magnetism, adsorption, and sensing through the introduction of functional components, showing broad application prospects in optics, sensing, catalysis, and supercapacitors. However, the development of composite functional aerogel materials still faces multiple challenges: the introduction of functional components often leads to a significant decrease in the inherent high porosity and specific surface area of ​​the aerogel; the uniformity of dispersion of active components in the three-dimensional network and the control of interfacial bonding strength are difficult, easily leading to aggregation and phase separation; and there are inherent contradictions among mechanical enhancement, electrical and thermal conductivity, and catalytic activity, making the synergistic optimization of multiple properties extremely challenging. Summary of the Invention

[0004] The purpose of this invention is to provide a silicon-based aerogel composite material, its preparation method, and its applications, thereby overcoming the shortcomings of existing technologies. A stepped hot melt-gel process significantly improves the uniformity of the gel network structure and the overall mechanical strength. During heating, stirring, and reflux, the stepped hot melt-gel process promotes the full reaction and self-assembly of the silicon precursor, forming a more uniform and stable three-dimensional nanoporous framework. Simultaneously, it promotes the high dispersion and stable binding of active components in the silica sol. This method yields a silicon-based aerogel composite material with both high specific surface area and excellent mechanical properties.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] In a first aspect, the present invention provides a method for preparing a silicon-based aerogel composite material, comprising the following steps: (1) Add the active component precursor to ethanol and solvent, stir and mix well to prepare an active component precursor solution; (2) Add the silicon-based precursor to ethanol and stir to mix well. Add acid solution, stir to mix well, and cool to room temperature to prepare silicon-based precursor solution. (3) Add the active component precursor solution to the silicon-based precursor solution to prepare a precursor-silica sol mixed solution; (4) Add an alkaline solution to the precursor-silica sol mixture and stir to mix well to prepare a mixed solution; (5) A stepped hot melt gel-gel process is adopted, in which the mixed solution is heated, stirred and refluxed in a stepwise manner, and then gelled and allowed to stand to form a silicon-based composite wet gel. After solvent replacement and drying, a silicon-based aerogel composite material is obtained. The stepped hot melt gel-gel process includes 2 to 5 temperature stages, using stepped cooling control. The processing time for each temperature stage is 10 to 80 minutes, and the processing temperature is 50 to 100℃.

[0007] The inventors discovered that a stepped hot melt-gel process can yield aerogel composites with both high specific surface area and high mechanical strength during the preparation of aerogel materials. The stepped heating process provides energy to the reaction system, accelerating reaction kinetics, making it easier to overcome reaction energy barriers, and promoting precursor hydrolysis and condensation reactions. Continuous stirring and thermal convection ensure more uniform mixing of reactants, reducing local concentration differences and thus improving the uniformity of the gel structure. Simultaneously, it allows for the condensation and recovery of volatile solvents, reducing raw material consumption, preventing reaction system imbalances caused by solvent evaporation, maintaining a stable liquid phase environment, and avoiding gel structure defects caused by solvent reduction. The stepped hot melt-gel process results in a more uniform microstructure, reduces stress concentration points, and thus improves the compressive strength of the material.

[0008] In some other embodiments, the stepped hot melt gel-gel process includes 2-3 temperature stages. Specifically, when the stepped hot melt gel-gel process uses 2 temperature stages: the temperature of the first temperature stage is 80-100°C, and the heating is applied for 20-30 min; the temperature of the second temperature stage is 50-60°C, and the heating is applied for 60-80 min.

[0009] When the stepped hot melt gel-gel process uses three temperature stages: the temperature of the first temperature stage is 85-95℃, and the heating is applied for 10-15 min; the temperature of the second temperature stage is 75-85℃, and the heating is applied for 15-25 min; the temperature of the third temperature stage is 50-60℃, and the heating is applied for 30-40 min.

[0010] Temperature gradients are achieved by controlling the heating source, with the mixed sol being allowed to naturally heat up or cool down between stages. A higher temperature is used in the initial stage to accelerate the hydrolysis of the silica sol precursor. Subsequent gradual cooling effectively suppresses the gelation rate of the sol, preventing excessively rapid reaction. Continuous application of heat throughout the process ensures the reaction proceeds fully, promoting the formation of a uniform and structurally stable nanoframework.

[0011] In some other embodiments, in step (1), the active component precursor is one or more of the following: ferric nitrate nonahydrate, ferric chloride, ferrous sulfate, ferrous oxalate, phosphate, molybdic acid, ammonium molybdate, molybdenum chloride, and molybdenum oxide powder. The solvent is one or more selected from N,N-dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, and acetonitrile; The molar ratio of the active component precursor, solvent and anhydrous ethanol is 1:(20~80):(10~50).

[0012] Specifically, the molar ratio of the active component precursor, solvent, and anhydrous ethanol is any one of the following values ​​or ranges: 1:20:10, 1:40:20, 1:60:30, or 1:80:50.

[0013] In some other embodiments, in step (2), the silicon-based precursor is one or more of tetraethyl orthosilicate, sodium silicate, methyltrimethoxysilane, methyltriethoxysilane, industrial silicon powder, rice husk ash, or fly ash. The acid solution is nitric acid or hydrochloric acid; The molar ratio of Si, ethanol and acid in the silicon-based precursor is (1~5):(2~10):1; The stirring temperature is 30~80℃, and the stirring time is 10~50 min.

[0014] Specifically, the molar ratio of Si, ethanol and acid in the silicon-based precursor is any one or a range of values ​​from 1:2:1, 1:5:1, 1:10:1, 2:2:1, 3:2:1, 4:2:1, 5:2:1 or 5:10:1.

[0015] The stirring temperature is any value or range of 30, 40, 50, 60, 70 or 80°C, and the stirring time is any value or range of 10, 20, 30, 40 or 50 min.

[0016] In some other embodiments, in step (3), the molar ratio of the active component in the active component precursor solution to Si in the silicon-based precursor solution is 1:(20-100), and the stirring time is 20-60 min.

[0017] Specifically, the molar ratio of the active component in the active component precursor solution to Si in the silicon-based precursor solution is any one or a range of 1:20, 1:30, 1:40, 1:50, 1:60, 1:70, 1:80 or 100, and the stirring time is any one or a range of 20, 30, 40, 50 or 60 min.

[0018] In some other embodiments, in step (4), the alkaline solution is urea or ammonia, the concentration of the alkaline solution is 10% to 80%, and the stirring time is 20 to 60 min.

[0019] Specifically, the concentration of the alkaline solution is any value or range of 10, 20, 40, 60 or 80%, and the stirring time is any value or range of 20, 30, 40, 50 or 60 min.

[0020] In some other embodiments, in step (5), in the stepped hot melt gel-gel reaction system, the temperature of the condensate water introduced through the condenser is 5~15℃, the flow rate of the condensate water is 0.5~1 m / s, and the heating and stirring speed is 600~800 r / min.

[0021] Specifically, the temperature of the condensate introduced into the stepped hot melt gel reaction system is any value or range of 5, 10 or 15°C, the flow rate of the condensate is any value or range of 0.5, 0.8 or 1 m / s, and the stirring speed is any value or range of 600 r / min, 650 r / min, 700 r / min, 750 r / min or 800 r / min.

[0022] In some other embodiments, in step (5), the standing temperature is 25~80℃ and the time is 3~72 h; the solvent replacement solution is anhydrous ethanol, deionized water solution, or alcohol-water mixture, the temperature is 25~80℃, the solvent replacement solution replacement cycle is 1~24 h, and the solvent replacement lasts for 1~5 days; Specifically, the settling temperature is any value or range of 25, 30, 40, 50, 60, 70 or 80°C, and the settling time is any value or range of 3, 12, 24, 36, 48 or 72 h.

[0023] The solvent replacement temperature is any value or range of 25, 30, 40, 50, 60, 70 or 80°C, the solvent replacement fluid replacement cycle is any value or range of 1, 3, 5, 10, 12, 16, 18, 20 or 24 h, and the solvent replacement duration is any value or range of 1, 2, 3, 4 or 5 days. The drying method is one or more of the following: supercritical ethanol drying, supercritical CO2 drying, freeze drying, atmospheric pressure drying, or vacuum drying.

[0024] In some other embodiments, in step (5), the molding method can be one or more of the following: block gel molding, solidified droplet molding, powder molding, film / coating molding, fiber molding, microsphere molding, or 3D printing molding. The solvent replacement solution is anhydrous ethanol, deionized water solution, or alcohol-water mixture.

[0025] In a second aspect, the present invention provides a silicon-based aerogel composite material prepared by the method described in the first aspect, characterized in that it comprises a SiO2 aerogel support and active components loaded therein and on its surface. The high dispersion of the active components on the SiO2 aerogel support exhibits excellent mechanical properties and a high specific surface area structural characteristics.

[0026] In some other embodiments, the loading of the active ingredient is 0.1~40 wt%; Specifically, the loading of the active ingredient is any one or a range of values ​​from 0.1, 0.5, 1, 2, 5, 10, 15, 20, 25, 30, 35 or 40 wt%. The active component is Fe2O3, MoO3, or Fe-MoO3. x One or more of them.

[0027] Thirdly, this invention provides the application of the silicon-based aerogel composite material described in the second aspect in methane conversion. The high dispersion of the active component on the SiO2 aerogel support, and the independently dispersed active centers, are key to improving the selectivity of the target product, formaldehyde, in the selective oxidation of methane to formaldehyde.

[0028] The beneficial effects of this invention are: (1) Based on the traditional method for preparing SiO2 aerogel, this invention employs a stepped hot melt-gel process to prepare a silicon-based aerogel composite material with both high specific surface area and excellent mechanical properties. During the stepped hot melt-gel process, continuous stirring and thermal convection promote the hydrolysis and condensation reactions of the precursor, improving the uniformity of the gel structure. Cooling and reflux reduce raw material consumption, maintain a stable liquid phase environment, and obtain a more uniform microstructure, which reduces stress concentration points and thus improves the compressive strength of the material.

[0029] (2) The preparation method in this invention enables the doped active components to be fully and uniformly combined with silica sol during the step-sol gel process, achieving high dispersion of the active components on the SiO2 aerogel support. The synthesized functional silica-based aerogel composite material can be applied in multiple fields. For example, in the field of selective methane oxidation, as a catalyst material, independently dispersed active centers are the key to improving the selectivity of the target product.

[0030] (3) Compared with traditional SiO2 aerogel materials, the silicon-based aerogel composite material prepared by this invention exhibits high specific surface area structural characteristics and mechanical strength that is tens of times higher than that of traditional aerogels. This invention effectively improves the brittleness limitation of common SiO2 aerogels, which are prone to powder shedding, and breaks through the technical bottleneck that it is difficult to achieve both high specific surface area and high mechanical strength. It provides a reliable material basis for silicon-based aerogel composite materials in practical applications that require high specific surface area and need to withstand certain mechanical stress. Attached Figure Description

[0031] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0032] Figure 1 This is the XRD pattern of the Fe2O3 / SiO2 aerogel composite material prepared in Example 1 of this invention; Figure 2 It is the Fe-MoO prepared in Example 2 of this invention. x Scanning electron microscope image of SiO2 aerogel composite material; Figure 3 These are the specific surface area and pore structure analysis diagrams of the SiO2 aerogel material prepared in Comparative Example 1 of the present invention, wherein (a) is the N2 adsorption-desorption isotherm and (b) is the pore size distribution diagram; Figure 4 It is the Fe-MoO prepared in Example 2 of this invention. x Structural model diagram of SiO2 aerogel composite material. Detailed Implementation

[0033] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Specific conditions not specified in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Components whose manufacturers are not specified are all commercially available conventional products.

[0034] Example 1 This embodiment provides a Fe2O3 / SiO2 aerogel material and its preparation method, specifically including the following steps: (1) Dissolve ferric nitrate nonahydrate in anhydrous ethanol, add N-methylpyrrolidone solvent and stir evenly at a certain temperature to obtain a precursor solution, wherein the molar ratio of ferric nitrate nonahydrate, N-methylpyrrolidone and anhydrous ethanol is 1:20:50.

[0035] (2) Sodium silicate was dissolved in anhydrous ethanol solution and stirred until homogeneous with a Fe to Si molar ratio of 1:100. Then, 3% hydrochloric acid aqueous solution was added dropwise. The volume ratio of sodium silicate, anhydrous ethanol and hydrochloric acid aqueous solution was 1:2:1. The mixed solution was stirred at 30°C for 30 min.

[0036] (3) After cooling at room temperature, start stirring and add the precursor solution dropwise. After stirring evenly for 60 min, a precursor-silica sol mixed solution is obtained. Add a 50% urea aqueous solution dropwise to the precursor-silica sol mixed solution. After stirring at room temperature for 60 min, transfer it to a stepped hot melt gel process device.

[0037] (4) The temperature of the condensate was set to 5℃, the flow rate to 0.8 m / s, the stirring speed to 600 r / min, the temperature of the heating mantle to 90℃, and the heating and stirring to 10 min. Then the temperature was set to 80℃ and the heating and stirring to 20 min was continued. Subsequently, the temperature of the heating mantle was set to 50℃ and the heating and stirring to 40 min was continued. The obtained mixed sol was passed through a coagulation bath droplet and placed at 60℃ for 12 h to obtain a silicon-based composite wet gel.

[0038] (5) The wet gel was immersed in an alcohol-water mixture and aged at 60°C. The alcohol-water mixture was replaced every 4 hours, and the solvent replacement lasted for 2 days. After solvent replacement, the silicon-based wet gel was dried under normal pressure to obtain the Fe2O3 / SiO2 aerogel composite material.

[0039] Figure 1 This is the XRD pattern of the Fe2O3 / SiO2 aerogel composite material prepared in Example 1. A broad amorphous diffraction peak is shown at 2θ of 20-30°, corresponding to amorphous SiO2. Characteristic peaks of Fe2O3 can be seen in the range of larger 2θ values, corresponding to the (104), (110), (113), (024), (116) and (300) crystal planes of Fe2O3, respectively.

[0040] Example 2 This embodiment provides a Fe-MoO x The SiO2 aerogel material and its preparation method specifically include the following steps: (1) The active component precursors, ferric nitrate nonahydrate and ammonium molybdate, are dissolved in anhydrous ethanol at a molar ratio of Fe:Mo = 1:1. N,N-dimethylformamide is added and stirred evenly at a certain temperature to obtain a precursor solution. The molar ratio of the active component precursor, N,N-dimethylformamide and anhydrous ethanol is 1:30:40.

[0041] (2) Tetraethyl orthosilicate was dissolved in anhydrous ethanol solution with a Fe-Mo to Si molar ratio of 1:60 and stirred evenly. A 5% concentration of nitric acid aqueous solution was added dropwise. The volume ratio of tetraethyl orthosilicate, anhydrous ethanol and nitric acid aqueous solution was 0.8:3:1. The mixed solution was stirred at 50°C for 20 min.

[0042] (3) After cooling at room temperature, start stirring and add the precursor solution dropwise. After stirring evenly for 40 min, a precursor-silica sol mixed solution is obtained. Add a 30% ammonia aqueous solution dropwise to the precursor-silica sol mixed solution. After stirring at room temperature for 40 min, transfer it to a stepped hot melt gel process device.

[0043] (4) The temperature of the condensate was set to 10℃, the flow rate to 0.8 m / s, the stirring speed to 700 r / min, and the oil bath temperature to 80℃. After heating and stirring for 30 min, the oil bath temperature was set to 60℃ and the stirring was continued for another 60 min. The resulting mixed sol was gelled by drop ball formation and placed at 40℃ for 12 h to obtain a silicon-based composite wet gel.

[0044] (5) The wet gel was immersed in anhydrous ethanol replacement solution and aged at 40°C. The replacement solvent was replaced every 4 hours, and the solvent replacement was carried out for 4 days. After solvent replacement, the silica-based wet gel was dried by supercritical ethanol to obtain Fe-MoO. x / SiO2 aerogel composite material.

[0045] Figure 2 It is the Fe-MoO prepared in Example 2 x Scanning electron microscope image of the Fe-MoO₂ aerogel composite material. The image shows that the material has a three-dimensional network framework structure composed of nanoscale particles, with abundant nanoscale pores, primarily mesoporous. x The loading of active components did not alter the three-dimensional network structure of the SiO2-based aerogel.

[0046] Figure 4 It is the Fe-MoO prepared in Example 2 x Structural model diagram of SiO2 aerogel composite material. SiO2 particles are linked together to form a nanonetwork structure, and FeO... x and MoOx The active oxide components are dispersed and loaded on the SiO2 framework.

[0047] Example 3 This embodiment provides a Fe2O3 / SiO2 aerogel composite material and its preparation method, specifically including the following steps: (1) The active component precursor ferric chloride is dissolved in anhydrous ethanol, acetonitrile is added and stirred evenly at a certain temperature to obtain a precursor solution, wherein the molar ratio of ferric chloride, acetonitrile and anhydrous ethanol is 1:50:30.

[0048] (2) Sodium silicate was dissolved in anhydrous ethanol solution with a Fe to Si molar ratio of 1:40 and stirred evenly. 8% hydrochloric acid aqueous solution was added dropwise. The volume ratio of sodium silicate, anhydrous ethanol and hydrochloric acid aqueous solution was 0.5:4:1. The mixed solution was stirred at 70°C for 10 min.

[0049] (3) After cooling at room temperature, start stirring and add the precursor solution dropwise. After stirring evenly for 30 min, a precursor-silica sol mixed solution is obtained. Add a 20% ammonia aqueous solution dropwise to the precursor-silica sol mixed solution. After stirring at room temperature for 30 min, transfer it to a stepped hot melt gel process device.

[0050] (4) The temperature of the condensate was set to 10℃, the flow rate to 0.8 m / s, the stirring speed to 750 r / min, and the water bath temperature to 100℃. After heating and stirring for 20 min, the water bath temperature was set to 60℃ and the stirring was continued for 80 min. The obtained mixed sol was then formed into a film and placed at 25℃ for 72 h to obtain a silicon-based composite wet gel.

[0051] (5) The wet gel was immersed in deionized water replacement solution and aged at 25°C. The deionized water was replaced every 4 hours, and the solvent replacement was carried out for 5 days. After solvent replacement, the silicon-based wet gel was dried by supercritical CO2 to obtain the Fe2O3 / SiO2 aerogel composite material.

[0052] Example 4 This embodiment provides a MoO3 / SiO2 aerogel composite material and its preparation method, specifically including the following steps: (1) Dissolve the active component precursor molybdenum chloride in anhydrous ethanol, add N,N-dimethylformamide and stir evenly at a certain temperature to obtain a precursor solution, wherein the molar ratio of molybdenum chloride, N,N-dimethylformamide and anhydrous ethanol is 1:80:10.

[0053] (2) Tetraethyl orthosilicate was dissolved in anhydrous ethanol solution with a molar ratio of Mo to Si of 1:20 and stirred evenly. A 10% concentration of nitric acid aqueous solution was added dropwise. The volume ratio of tetraethyl orthosilicate, anhydrous ethanol and nitric acid aqueous solution was 0.1:5:1. The mixed solution was stirred at 80°C for 10 min.

[0054] (3) After cooling at room temperature, start stirring and add the precursor solution dropwise. After stirring evenly for 20 min, a precursor-silica sol mixed solution is obtained. Add a 10% ammonia aqueous solution dropwise to the precursor-silica sol mixed solution. After stirring at room temperature for 20 min, transfer it to a stepped hot melt gel process device.

[0055] (4) The temperature of the condensate was set to 15℃, the flow rate to 0.5 m / s, the stirring speed to 600 r / min, and the water bath temperature to 90℃. After heating and stirring for 30 min, the water bath temperature was set to 50℃ and the stirring was continued for 60 min. The obtained mixed sol was then microsphere-formed and placed at 80℃ for 3 h to obtain a silicon-based composite wet gel.

[0056] (5) The wet gel was immersed in anhydrous ethanol replacement solution and aged at 80°C. The anhydrous ethanol was replaced in 1-hour cycles, and the solvent replacement lasted for 1 day. After solvent replacement, the silicon-based wet gel was dried by supercritical CO2 to obtain the MoO3 / SiO2 aerogel composite material.

[0057] Comparative Example 1 This comparative example provides a SiO2 aerogel material and its preparation method, specifically including the following steps: (1) Dissolve tetraethyl orthosilicate in anhydrous ethanol solution and stir until homogeneous. Add 1wt% nitric acid aqueous solution dropwise. The volume ratio of tetraethyl orthosilicate, anhydrous ethanol and nitric acid aqueous solution is 2:1:1. Stir the mixed solution at 40°C for 20 min to prepare silica sol mixed solution.

[0058] (2) After cooling at room temperature, add 80% urea aqueous solution dropwise to silica sol mixture, stir at room temperature for 50 min and then transfer to a stepped hot melt gel process device.

[0059] (3) Set the temperature of the condensate to 5℃, the flow rate to 0.5 m / s, the stirring speed to 600 r / min, set the temperature of the electric heating mantle to 90℃, heat and stir for 10 min; set the temperature to 80℃ and continue heating and stirring for 20 min; then set the temperature of the electric heating mantle to 50℃ and heat and stir for 40 min.

[0060] (4) The obtained mixed sol was poured into a mold for block gel molding and placed at 40°C for 18 h to obtain a silicon-based composite wet gel.

[0061] (5) The wet gel was immersed in ethanol replacement solution and aged at 40°C. The replacement solvent was replaced in 6-hour cycles, and the solvent replacement lasted for 3 days. After solvent replacement, the wet gel was dried by supercritical ethanol to obtain SiO2 aerogel material.

[0062] Figure 3 This is a diagram showing the specific surface area and pore structure analysis of the SiO2 aerogel material prepared in Comparative Example 1. The specific surface area of ​​the untreated SiO2 aerogel is 1208.4 m². 2 / g, the specific surface area of ​​SiO2 aerogel after heat treatment at 750℃ is 810.4 m² / g. 2 / g ( Figure 3 In (a), the isotherms are all typical type IV and have a type H1 hysteresis loop ( Figure 3 Figure (b) illustrates that SiO2 aerogel is a mesoporous material composed of aggregates of relatively uniform spherical particles. When SiO2 aerogel is used as a catalyst support, its high specific surface area can provide a good dispersion bed for the active components, increase the contact area between the catalyst and the reactant gas, and thus promote the occurrence of catalytic reactions.

[0063] Comparative Example 2 This comparative example provides a Fe-MoO x The SiO2 particle composite material and its preparation method include the following steps: (1) The active component precursors, ferric nitrate nonahydrate and ammonium molybdate, are dissolved in anhydrous ethanol at a molar ratio of Fe:Mo = 1:1. N,N-dimethylformamide is added and stirred evenly at a certain temperature to obtain a precursor solution. The molar ratio of the active component precursor, N,N-dimethylformamide and anhydrous ethanol is 1:30:40.

[0064] (2) Tetraethyl orthosilicate was dissolved in anhydrous ethanol solution with a Fe-Mo to Si molar ratio of 1:60 and stirred evenly. A 5% concentration of nitric acid aqueous solution was added dropwise. The volume ratio of tetraethyl orthosilicate, anhydrous ethanol and nitric acid aqueous solution was 0.8:3:1. The mixed solution was stirred at 50°C for 20 min.

[0065] (3) After cooling at room temperature, start stirring and add the precursor solution dropwise. After stirring evenly for 40 min, a precursor-silica sol mixed solution is obtained. Add a 30% ammonia aqueous solution dropwise to the precursor-silica sol mixed solution and stir at room temperature for 40 min.

[0066] (4) The well-stirred precursor-silica sol mixture is heated at 70°C to evaporate completely, and block particles are obtained.

[0067] (5) The bulk particle sample was heat-treated at 270℃, and the resulting bulk Fe-MoO was obtained. x / SiO2 particle composite material.

[0068] Unlike Example 2, the step-by-step hot melt gel-gel and supercritical drying aerogel preparation process after step (3) in Example 2 is omitted. Instead, the precursor-silica sol mixture is evaporated, dried, and heat-treated to obtain Fe-MoO. x / SiO2 particle composite material. Other preparation methods are the same as in Example 2.

[0069] Comparative Example 3 This embodiment provides a Fe-MoO x The SiO2 aerogel material and its preparation method specifically include the following steps: (1) The active component precursors, ferric nitrate nonahydrate and ammonium molybdate, are dissolved in anhydrous ethanol at a molar ratio of Fe:Mo = 1:1. N,N-dimethylformamide is added and stirred evenly at a certain temperature to obtain a precursor solution. The molar ratio of the active component precursor, N,N-dimethylformamide and anhydrous ethanol is 1:30:40.

[0070] (2) Tetraethyl orthosilicate was dissolved in anhydrous ethanol solution with a Fe-Mo to Si molar ratio of 1:60 and stirred evenly. A 5% concentration of nitric acid aqueous solution was added dropwise. The volume ratio of tetraethyl orthosilicate, anhydrous ethanol and nitric acid aqueous solution was 0.8:3:1. The mixed solution was stirred at 50°C for 20 min.

[0071] (3) After cooling at room temperature, stirring was started and the precursor solution was added dropwise. After stirring for 40 min to obtain a homogeneous precursor-silica sol mixture, a 30% ammonia aqueous solution was added dropwise to the precursor-silica sol mixture. After stirring at room temperature for 40 min, the mixture was transferred to a heating device. After heating and stirring for 90 min, the resulting mixed sol was gelled by drop ball formation and placed at 40℃ for 12 h to obtain a silica-based composite wet gel.

[0072] (4) The wet gel was immersed in anhydrous ethanol replacement solution and aged at 40°C. The replacement solvent was replaced every 4 hours, and the solvent replacement was carried out for 4 days. After solvent replacement, the silica-based wet gel was dried by supercritical ethanol to obtain Fe-MoO. x / SiO2 aerogel composite material.

[0073] Unlike Example 2, the step-by-step hot melt-gel process following step (3) in Example 2 was omitted, and the mixed sol was directly transferred to a conventional heating device for heat treatment. Other preparation methods were the same as in Example 2.

[0074] According to GB / T13480, the compressive modulus test is performed on a DECCA-1 universal testing machine. The composite material is placed stably at the center of the upper and lower compression plates of the testing machine to ensure uniform load distribution. The test parameters are set, and a compressive load is applied at a constant displacement rate of 0.2 mm / min. During the elastic deformation stage of the material, load and displacement data are continuously and synchronously recorded. Based on the obtained stress-strain curve, the ratio of stress increment to corresponding strain increment is calculated within the linear elastic range, which yields the compressive modulus of the material.

[0075] The selective oxidation of methane to formaldehyde was carried out in a self-made fixed-bed reactor. 0.6 g of aerogel composite catalyst was loaded into a fixed-bed reactor with a diameter of 1 cm. The reactor heating rate was set to 5 °C / min, and a temperature-controlled electric furnace was used to heat the reactor to the catalytic reaction temperature of 650 °C. When the temperature of the thermocouple-transferred catalytic zone in the reaction bed reached the reaction temperature, an ice bath containing liquid nitrogen and salt was prepared, and the condenser at the reactor outlet was placed in the ice bath environment for pre-cooling. After the reactor and bed temperatures reached the operating temperature and stabilized, and the condenser had been pre-cooled in the ice bath, CH4 and O2 were introduced into the reactor at a CH4:O2 ratio of 3:1 and a flow rate of 162.5 mL / min.

[0076] After the reaction experiment began, the gas exiting the reactor was analyzed online in real time to determine the composition of the gaseous reaction products at different time points during the experiment. Simultaneously, a bubble meter was used to monitor the gas flow rate at the outlet in real time. After the experiment, the condenser was removed, and the products in the condenser were weighed. The results relating methane CH4 conversion and formaldehyde HCHO selectivity were calculated based on the online monitoring results of the gaseous products and the identification and quantification of the products in the condenser. The formaldehyde HCHO yield was calculated by multiplying the methane CH4 conversion by the formaldehyde HCHO selectivity.

[0077] Table 1 shows the mechanical properties and specific surface area of ​​Examples 1-4 and Comparative Examples 1-2, and Table 2 shows the formaldehyde selectivity and formaldehyde yield of Example 2 and Comparative Example 2.

[0078] Table 1 Mechanical properties and specific surface area of ​​the examples and comparative examples

[0079] As shown in Table 1, compared with the pure SiO2 aerogel in Comparative Example 1, the mechanical properties of the SiO2 aerogel composites in Examples 2-4 after incorporating the active components were improved to varying degrees. It is precisely because of the introduction of the active components that the specific surface area in Examples 2-4, although slightly decreased, remained at a high level. The results indicate that the introduction of active components improved the mechanical properties of the aerogel composites.

[0080] Compared to the oxide particle composite material prepared by evaporation and drying in Comparative Example 2, the aerogel composite materials prepared by the stepped hot melt gel-gel and ethanol supercritical aerogel preparation processes in Examples 2-4 exhibit superior mechanical properties and specific surface area. These results demonstrate that the aerogel material preparation process can achieve better mechanical properties and a higher specific surface area. Furthermore, the preparation processes in Examples 2, Comparative Example 1, and Comparative Example 3 involved supercritical ethanol drying, and the specific surface area of ​​the samples obtained by supercritical ethanol drying was significantly greater than that obtained by other drying methods.

[0081] Compared to the aerogel composite material in Comparative Example 3 without the stepped hot melt-gel process, the aerogel composite material in Example 2 treated with the stepped hot melt-gel process exhibits superior mechanical properties and a higher specific surface area. This is because the stepped hot melt-gel process promotes rapid hydrolysis of the silica sol precursor and alkaline catalyst at a relatively high initial temperature, while the subsequent temperature reduction prevents rapid gelation of the sol, and the maintained heating state promotes complete reaction of the mixed sol. The introduction of the stepped hot melt-gel process makes the aerogel nanoframework structure more uniform and stable, thereby effectively improving the mechanical properties and specific surface area of ​​the aerogel composite material.

[0082] In summary, the results in Table 1 show that both the introduction of active components and the use of a step-by-step hot melt gel process can effectively improve the mechanical strength of composite materials. In particular, the latter can not only enhance mechanical properties but also significantly increase their specific surface area.

[0083] Table 2 Formaldehyde selectivity and formaldehyde yield of examples and comparative examples

[0084] As shown in Table 2, under the same experimental conditions, the porous Fe-MoO2 prepared by the stepped hot melt-gel process... x The SiO2 aerogel catalyst (Example 2) is significantly superior to the bulk Fe-MoO2 obtained by evaporation and drying in both formaldehyde selectivity and formaldehyde yield. x / SiO2 particle catalyst (Comparative Example 2). The former showed 10.3 times the formaldehyde selectivity and 2.3 times the formaldehyde yield of the former.

[0085] This result further verifies the conclusion in Table 1: the step-type hot melt gel-gel process can form aerogel composite materials with high specific surface area, thereby making the distribution of active components more uniform, which is beneficial to improving formaldehyde selectivity and ultimately increasing formaldehyde yield.

[0086] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a silica-based aerogel composite material, characterized by, Includes the following steps: (1) Add the active component precursor to ethanol and solvent, stir and mix well to prepare an active component precursor solution; (2) Add the silicon-based precursor to ethanol and stir to mix well. Add acid solution, stir to mix well, and cool to room temperature to prepare silicon-based precursor solution. (3) Add the active component precursor solution to the silicon-based precursor solution to prepare a precursor-silica sol mixed solution; (4) Add an alkaline solution to the precursor-silica sol mixture and stir to mix well to prepare a mixed solution; (5) A stepped hot melt gel-gel process is adopted, in which the mixed solution is heated, stirred and refluxed in a stepwise manner, and then gelled and allowed to stand to form a silicon-based composite wet gel. After solvent replacement and drying, a silicon-based aerogel composite material is obtained. The stepped hot melt gel-gel process includes 2 to 5 temperature stages, with each temperature stage having a processing time of 10 to 80 minutes and a processing temperature of 50 to 100°C.

2. The method for preparing the silicon-based aerogel composite material according to claim 1, characterized in that, In step (1), the active component precursor is one or more of the following: ferric nitrate nonahydrate, ferric chloride, ferrous sulfate, ferrous oxalate, phosphate, molybdic acid, ammonium molybdate, molybdenum chloride, and molybdenum oxide powder. The solvent is one or more selected from N,N-dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, and acetonitrile; The molar ratio of the active component precursor, solvent and anhydrous ethanol is 1:(20~80):(10~50).

3. The method for preparing the silicon-based aerogel composite material according to claim 1, characterized in that, In step (2), the silicon-based precursor is one or more of tetraethyl orthosilicate, sodium silicate, methyltrimethoxysilane, methyltriethoxysilane, industrial silicon powder, rice husk ash, or fly ash. The acid solution is nitric acid or hydrochloric acid; The molar ratio of Si, ethanol and acid in the silicon-based precursor is (1~5):(2~10):1; The stirring temperature is 30~80℃, and the stirring time is 10~50 min.

4. The method for preparing the silicon-based aerogel composite material according to claim 1, characterized in that, In step (3), the molar ratio of the active component in the active component precursor solution to Si in the silicon-based precursor solution is 1:(20-100), and the stirring time is 20-60 min.

5. The method for preparing the silicon-based aerogel composite material according to claim 1, characterized in that, In step (4), the alkaline solution is urea or ammonia, the concentration of the alkaline solution is 10% to 80%, and the stirring time is 20 to 60 minutes.

6. The method for preparing the silicon-based aerogel composite material according to claim 1, characterized in that, In step (5), the temperature of the condensate water introduced in the stepped hot melt gel-gel process is 5~15℃, the flow rate of the condensate water is 0.5~1 m / s, and the heating and stirring speed is 600~800 r / min.

7. The method for preparing the silicon-based aerogel composite material according to claim 1, characterized in that, In step (5), the temperature for standing is 25~80℃ and the time is 3~72 h. The solvent replacement solution is anhydrous ethanol, deionized water solution, or a mixture of alcohol and water. The temperature is 25~80℃. The solvent replacement solution replacement cycle is 1~24 h. The solvent replacement lasts for 1~5 days. The drying method is one or more of the following: supercritical ethanol drying, supercritical CO2 drying, freeze drying, atmospheric pressure drying, or vacuum drying.

8. A silicon-based aerogel composite material prepared by the method of any one of claims 1-7, characterized in that, It includes SiO2 aerogel carrier and active components loaded inside and on its surface.

9. The silicon-based aerogel composite material according to claim 8, characterized in that, The loading of the active ingredient is 0.1~40 wt%; The active ingredient is one or more of Fe2O3, MoO3, or Fe-MoO x The active ingredient is one or more of Fe2O3, MoO3, or Fe-MoO 10. The application of the silicon-based aerogel composite material according to claim 8 or 9 in methane conversion.

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