Carbon dioxide catalytic modified silica aerogel and production method thereof

Abstract

The invention discloses carbon dioxide catalytic modified silica aerogel and a production method thereof, and relates to the technical field of production of building thermal insulation materials. The preparation method comprises the following steps: diluting water glass with water, dropwise adding the diluted water glass into a mixed solution of water and dilute nitric acid, dropwise adding dilute ammonia water to adjust the pH value, heating in a water bath, soaking in a sodium methylsilanolate aqueous solution, isopropanol and a dilute alkali solution, placing in a high-temperature and high-pressure reaction kettle, adding HMDZ, normal hexane and isopropanol, introducing carbon dioxide for modification, adding pentafluoropropane, heating to separate normal hexane, and cooling to room temperature. And drying to obtain the carbon dioxide catalytic modified silica aerogel. Carbon dioxide is introduced for modification, the situation that acid serves as a catalyst, the water content in a system is increased is avoided, excessive consumption of a modifier is reduced, meanwhile, the washing process is omitted, the acid and alkali resistance of equipment is reduced, the boiling point of pentafluoropropane is low, the flame retardant effect is achieved, and the preparation process is safer. The carbon dioxide catalytic modified silica aerogel disclosed by the invention has good hydrophobicity, low heat conductivity and low density.

Description

Technical Field

[0001] This invention relates to silica aerogels and their preparation methods, and in particular to a carbon dioxide catalytically modified silica aerogel and its production method, belonging to the field of building insulation material production technology. Background Technology

[0002] Silica aerogel is a type of aerogel with a density of 0.003~0.5 g / cm³. 3 Silica aerogels are novel mesoporous materials composed of solid SiO2, characterized by low density, high porosity, and high specific surface area. At the nanoscale, silica aerogels possess extremely small pores, and their network structure exhibits complex cross-linking and branching, resulting in low thermal conductivity, ultra-low dielectric constant, and low refractive index. Their large internal surface area and high specific surface area make them promising for applications in building window and door insulation materials, Cherenkov radiators, sound insulation materials, catalyst supports, lithium-ion batteries, laser lighting, and aerospace.

[0003] To produce silica aerogels at low cost, researchers have mostly used inexpensive water glass for preparation. For example, Liu Guangwu et al. used water glass as the silicon source to prepare silica aerogels, modified the aerogels by hydrolyzing hexamethyldisiloxane with hydrochloric acid as a catalytic modifier, and then dried them at atmospheric pressure to obtain a density of 80–200 mg / cm³. 3 Specific surface area is 568 m² 2 / g, pore volume is 2.9cm³ 3 A superhydrophobic SiO2 aerogel with a thermal conductivity of 0.026 mW / m·K at room temperature was prepared using this method. Although this method produces silica aerogels with good performance parameters at a relatively low cost, the use of acid as a catalyst for hydrolysis of the modifier inevitably introduces water into the system. This requires equipment with high acid and alkali resistance, leads to excessive waste of the modifier, and poses a flammability risk when drying at atmospheric pressure. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a carbon dioxide-catalyzed modified silica aerogel and its production method. By using carbon dioxide for catalytic modification, the problem of excessive consumption of the modifier due to the introduction of water into the system by adding acid as a modifier, as is common in traditional methods, is avoided. This also reduces the requirements for the equipment's acid and alkali resistance and avoids the need for washing to remove salt byproducts generated during modification. Furthermore, drying with pentafluoropropane avoids the flammability issue during drying. This invention is beneficial for the large-scale industrial production of silica aerogels and offers certain performance indicators and economic benefits.

[0005] According to one aspect of the present invention, a method for producing carbon dioxide catalytically modified silica aerogel is provided, comprising: S1, mixing water glass with water at room temperature, then titrating the mixture with a solution of water and dilute nitric acid under stirring, then adding dilute ammonia to adjust the pH value, waiting for gelation, and then heating in a water bath to obtain a wet gel; S2, immersing the obtained wet gel in a sodium methylsilanoate aqueous solution, isopropanol, and a dilute alkaline solution in sequence; S3, pouring the soaked wet gel into a high-pressure reactor, adding n-hexane, isopropanol, and hexamethyldisilazane, adjusting the pressure, introducing carbon dioxide, heating to modify the gel, then turning off the carbon dioxide, heating again, and continuing the reaction in a sealed environment; S4, after modification, removing the aqueous phase from the reactor, adding pentafluoropropane, heating to discharge n-hexane and pentafluoropropane in the form of steam, condensing the discharged steam to separate n-hexane, and returning the uncondensed pentafluoropropane steam to the reactor until no n-hexane is discharged, removing the gel block from the reactor and placing it at room temperature to obtain carbon dioxide catalytically modified silica aerogel.

[0006] Optionally, in S1, the dilute ammonia solution is prepared by diluting concentrated ammonia solution with a concentration of 25% to 28% with water at a volume ratio of 1:5, and the pH value adjusted by adding dilute ammonia solution is in the range of 4 to 8.

[0007] Optionally, in S1, the water bath heating temperature is 60℃ and the heating time is 20min.

[0008] Optionally, in S2, the sodium methylsilanolate aqueous solution is prepared by mixing a 30% sodium methylsilanolate stock solution with water and adding it dropwise to a 25% dilute nitric acid and water mixed solution to prepare an acidic sodium methylsilanolate aqueous solution with a concentration of 1%~5% and a pH value of 2.0~2.4; the soaking time of the sodium methylsilanolate aqueous solution is 0.5~2.5h.

[0009] Optionally, in S2, the dilute alkaline solution is prepared by adding ammonia to water and adjusting the pH to 8.3~8.7; the soaking time of the dilute alkaline solution is 0.5~2.5h.

[0010] Optionally, in S3, the amount of n-hexane added is 40 mL, the amount of isopropanol added is 1.3 mL, and the amounts of hexamethyldisilazane added are 0.72 mL, 2.14 mL, 3.6 mL, 5 mL, and 6.4 mL, respectively.

[0011] Optionally, in S3, the pressure is adjusted to 0.3 MPa; carbon dioxide is introduced, and the temperature for modification is raised to 50~60℃; the carbon dioxide is turned off, and the temperature is raised again to 60~70℃.

[0012] Optionally, after S3, a secondary modification is also included, that is, repeating the modification once. Specifically, after the first modification is completed, the aqueous phase is removed, and hexane, isopropanol, and hexamethyldisilazane are added. The pressure is adjusted, carbon dioxide is introduced, and the temperature is raised to carry out the modification. After the modification is completed, the carbon dioxide is turned off, the temperature is raised, and the reaction is carried out again in a closed environment.

[0013] Optionally, in S4, pentafluoropropane is added and the temperature is raised to 100°C; the discharged steam is condensed in a 30°C storage tank to separate n-hexane; the presence of n-hexane in the discharged steam is detected by gas chromatography.

[0014] According to one aspect of the present invention, a carbon dioxide catalytically modified silica aerogel is provided, which is prepared by the production method of carbon dioxide catalytically modified silica aerogel as described above.

[0015] The beneficial effects of this invention are: This invention avoids the problem of excessive acidity caused by adding strong acids (such as concentrated nitric acid or concentrated sulfuric acid) during modification, which necessitates the introduction of water into the system and leads to excessive waste of the modifier hexamethyldisilazane. This achieves the goal of avoiding the need for washing to produce salt byproducts due to the addition of acid during modification. At the same time, it avoids the problem of requiring equipment with strong acid and alkali resistance when strong acids are used as catalysts.

[0016] The present invention adopts a sodium methylsilanolate solution soaking method to reduce the consumption of hexamethyldisilazane, which is more stable than the gel network skeleton of water glass and sodium methylsilanolate as silicon source.

[0017] Soaking in isopropanol solution can better induce sodium methylsilanolate to graft onto the gel network backbone, thus playing a role in interface modification.

[0018] This invention utilizes pentafluoropropane for drying. Pentafluoropropane has a low boiling point and flame-retardant properties, reducing the risk of deflagration during the drying process and achieving a flammable-free drying operation. Furthermore, the pentafluoropropane can be easily separated from the mixed vapors of hexane or heptane, allowing for the recycling of the pentafluoropropane.

[0019] This invention optimizes the network framework structure of silica aerogel by changing the raw material ratio, gel pH value, and modification method, making the pore distribution of silica aerogel more uniform, further improving the hydrophobic properties of silica aerogel, reducing the shrinkage and collapse of the network structure during drying, and further obtaining silica aerogel products with excellent performance.

[0020] This invention helps reduce the cost of preparing silica aerogels, facilitates large-scale industrial production, and has good application prospects. Attached Figure Description

[0021] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 Scanning electron microscope images of carbon dioxide catalytically modified silica aerogels prepared with different silica contents (wet gels), where (a) 4% silica content, (b) 5% silica content, (c) 6% silica content, (d) 7% silica content, and (e) 8% silica content; Figure 2 Pore ​​size distribution of carbon dioxide catalytically modified silica aerogels prepared for different silicon contents (wet gels); Figure 3 Appearance and scanning electron microscope images of carbon dioxide catalytically modified silica aerogels prepared for different pH values, where (a) pH=4, (b) pH=5, (c) pH=6, (d) pH=7, and (e) pH=8. Figure 4 The hydrophobic angle diagram of the silica aerogel after immersion in sodium methylsiloxane solution; Figure 5 Scanning electron microscope images of carbon dioxide catalytically modified silica aerogels prepared with different HMDZ contents, where (a) 0.72 mL, (b) 2.14 mL, (c) 3.6 mL, (d) 5 mL, and (e) 6.4 mL; Figure 6 Hydrophobic angle diagrams of carbon dioxide catalytically modified silica aerogels prepared for different HMDZ contents, where (a) 0 mL, (b) 2.14 mL, (c) 3.6 mL, and (d) 5 mL; Figure 7 Scanning electron microscope (SEM) images of carbon dioxide catalytically modified silica aerogels prepared at different modification temperatures, including (a) 50℃+60℃, (b) 50℃+70℃, (c) 60℃+60℃, and (d) 60℃+70℃. Figure 8 Hydrophobic angle diagrams of carbon dioxide catalytic modified silica aerogels prepared at different modification temperatures are shown, where (a) 50℃+60℃, (b) 50℃+70℃, (c) 60℃+60℃, and (d) 60℃+70℃. Figure 9 This is a scanning electron microscope image of silica aerogel that has been soaked in sodium methylsiloxane solution and then dried with pentafluoropropane. Detailed Implementation

[0022] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0023] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0024] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0025] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0026] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0027] This invention provides a method for producing carbon dioxide catalytically modified silica aerogel (also referred to as silica aerogel), comprising the following steps: S1, at room temperature, 6 mL of water glass (sodium silicate, chemical formula R2O·nSiO2) with a modulus of 3.2 was mixed with 6 mL of water. Then, it was titrated in a mixed solution of 17 mL of water and 4 mL of dilute nitric acid (concentration of 25%) under stirring. The pH value during titration was controlled using a benchtop pH meter. The pH value at the end of titration was controlled at about 2.5. Then, dilute ammonia was added to adjust the pH value to 4~8 (such as pH=4, 5, 6, 7 or 8, preferably 5). After waiting for gelation, it was heated in a water bath at 60°C for 20 min to obtain a wet gel with a volume of 33 mL. The dilute ammonia solution is a diluted solution of concentrated ammonia (concentration 25%~28%) and water, with a dilution volume ratio of concentrated ammonia:water = 1:5.

[0028] The silicon content of the gel = density of water glass * silicon content of water glass * volume of water glass solution used / total volume of wet gel. The silicon content in the obtained wet gel is controlled to be 4%, 5%, 6%, 7%, or 8%, preferably 6%.

[0029] S2, after soaking the wet gel in sodium methylsiloxane aqueous solution for a period of time (0.5~2.5h), soak it in 100mL isopropanol for one hour, and then soak the wet gel in 100mL dilute alkaline solution for 0.5~2.5h; The concentration of sodium methylsilanolate aqueous solution can be 1%, 2%, 3%, 4%, or 5%, and its pH value is maintained at around 2.2 (e.g., 2.0~2.4) using nitric acid. The concentration of sodium methylsilanolate aqueous solution is equal to the volume of the sodium methylsilanolate stock solution * the density of sodium methylsilanolate * the content of the sodium methylsilanolate stock solution / the total volume of the sodium methylsilanolate aqueous solution (where the total volume includes the volume of the sodium methylsilanolate stock solution, water, and nitric acid).

[0030] A dilute alkaline solution is prepared by adding ammonia water dropwise to water, and the ammonia water is used to maintain the pH value of the dilute alkaline solution at around 8.5 (e.g., 8.3~8.7).

[0031] S3. Pour the wet gel soaked in S2 into a high-pressure reactor, add n-hexane, isopropanol, and hexamethyldisilazane (HMDZ), adjust the reactor pressure to 0.3 MPa, introduce carbon dioxide, and heat to 50~60℃ (preferably 50℃) for modification. Then turn off the carbon dioxide and heat again (60~70℃, preferably 60℃) to continue the reaction in a sealed environment. The amount of n-hexane added was 40 mL, and the amount of isopropanol added was 1.3 mL.

[0032] The total amount of hexamethyldisilazane (HMDZ) added was 0.72 mL, 2.14 mL, 3.6 mL, 5 mL, and 6.4 mL, with 3.6 mL being preferred.

[0033] Modification refers to hydrophobic modification using hexamethyldisilazane. The modification can be a single modification or a two-stage modification, preferably a two-stage modification.

[0034] The soaked wet gel was poured into a high-pressure reactor, and hexane, isopropanol, and hexamethyldisilazane were added. The pressure was adjusted, carbon dioxide was introduced, and the temperature was raised for modification. Then the carbon dioxide was turned off, the temperature was raised again, and the reaction continued in a closed environment. This is one modification (i.e., the steps of one modification include raising the temperature by introducing carbon dioxide and then raising the temperature again without introducing carbon dioxide).

[0035] The double modification involves repeating the modification process after the first modification is completed. Specifically, after the first modification reaction is finished, the lower aqueous phase is aspirated with a dropper to prevent excessive consumption of the modifying agent hexamethyldisilazane during the second modification. Then, hexane, isopropanol, and hexamethyldisilazane are added, the pressure is adjusted, carbon dioxide is introduced, and the temperature is raised for modification. The carbon dioxide is then turned off, the temperature is increased, and the reaction is carried out again under high temperature, high pressure, and a closed environment to complete the double modification process.

[0036] It should be noted that the higher the temperature, the more vigorous the molecular motion. The purpose of turning off carbon dioxide and raising the temperature is to accelerate the movement of n-hexane into the gel pores, so as to better replace the water in the pores.

[0037] S4. After removing the aqueous phase in the reactor, pentafluoropropane is added to the reactor and heated to 100°C to discharge n-hexane and pentafluoropropane in the form of vapor. A 30°C storage tank is connected outside the reactor to condense the n-hexane. The remaining pentafluoropropane vapor is then circulated back into the reactor to keep the amount of pentafluoropropane in the reactor constant. This cycle is repeated until no n-hexane is detected in the vapor discharged from the reactor using a gas chromatograph. The cycle ends when the gel block in the reactor is removed and placed at room temperature to obtain carbon dioxide catalytically modified silica aerogel.

[0038] The heating to 100℃ is to better allow pentafluoropropane to displace the n-hexane in the gel pores. After the pentafluoropropane displaces the n-hexane, liquid pentafluoropropane remains in the gel pores. Since pentafluoropropane has a low boiling point, placing the sample at room temperature to dry it will cause the liquid pentafluoropropane in the pores to turn into a gaseous state, thus allowing the pentafluoropropane to be discharged from the gel pores and achieving drying.

[0039] Furthermore, referring to Figure 1 and Figure 2 The images show scanning electron microscope (SEM) images and pore size distribution diagrams of carbon dioxide catalytically modified silica aerogels prepared with different silica contents (wet gels), respectively.

[0040] When the silicon content is <6%, the stability of the three-dimensional network structure of the silica aerogel is poor; when the silicon content is >6%, the stability of the three-dimensional network structure of the silica aerogel gradually stabilizes, and its pores are overfilled; when the silicon content is 6%, the three-dimensional network structure of the silica aerogel has the best effect.

[0041] Reference Figure 3 The images show the appearance and scanning electron microscope (SEM) images of carbon dioxide catalytically modified silica aerogels prepared at different pH values ​​(adjusted by adding dilute ammonia).

[0042] At pH 5, the modified gel showed almost no shrinkage and had a certain degree of elasticity. Compared with other pH values, the chain structure of the sample at pH 5 was relatively stable, the aerogel skeleton was robust, and it was less likely to shrink and collapse when drying.

[0043] Reference Figure 4 , Figure 4 The image shows the hydrophobic angle of the silica aerogel after immersion in sodium methylsiloxane solution.

[0044] Reference Figure 5 and Figure 6 Scanning electron microscope (SEM) images and schematic diagrams of hydrophobic angles of carbon dioxide-catalyzed modified silica aerogels prepared with different HMDZ contents are shown. Figure 6 (a) is untreated (HMDZ=0mL) without hexamethyldisilazane modification, with a contact angle of 70.6° and is non-hydrophobic.

[0045] A small amount of HMDZ cannot adequately modify the silica network structure. With the increase of the modifier content, when 3.6 mL of hexamethyldisilazane is added, a typical three-dimensional network nanoporous structure is exhibited, and the hydrophobic angle will increase. Further increasing the HMDZ content will be somewhat wasteful and will not significantly improve the performance.

[0046] Reference Figure 7 , Figure 8 The images show scanning electron microscope (SEM) images and hydrophobic angle diagrams of carbon dioxide-catalyzed modified silica aerogels prepared at different modification temperatures.

[0047] By exploring temperature combinations of ventilation (introducing carbon dioxide) and non-ventilation (closing carbon dioxide) (50℃+60℃, 50℃+70℃, 60℃+60℃, 60℃+70℃), the combination of 50℃+60℃ was found to be optimal.

[0048] With increasing modification temperature, HMDZ modifies the silica aerogel more thoroughly. When modified at 50℃ for 1 h and then at 60℃ for another 1 h, the silica aerogel powder network framework structure is more loosely distributed, the three-dimensional network structure is more uniform and complete, and the connections between pores are also tighter. However, increasing temperature can lead to the collapse of the aerogel's microstructure and excessive aggregation of nanoparticles. Furthermore, while HMDZ can better bind to the aerogel surface with increasing temperature, excessively high temperatures can cause HMDZ to decompose and volatilize.

[0049] Reference Figure 9 , Figure 9 This is a scanning electron microscope image of silica aerogel that has been soaked in sodium methylsiloxane solution and then dried with pentafluoropropane.

[0050] The silica aerogel samples that were soaked in sodium methylsilanolate solution and then dried with pentafluoropropane also exhibited a typical three-dimensional network nanoporous structure, but had a lower density and a larger specific surface area compared to the samples obtained after soaking in sodium methylsilanolate solution and then at atmospheric pressure.

[0051] Furthermore, the following examples demonstrate the performance of the carbon dioxide catalytically modified silica aerogel prepared using the method of the present invention.

[0052] Example 1: S1, water glass was mixed with water at room temperature, and then titrated into a mixed solution of water and dilute nitric acid under stirring to prepare a gel with a silicon content of 6%. Dilute ammonia was added dropwise to adjust the pH value and wait for gelation. Then, a wet gel was obtained by water bath at 60°C for 20 minutes.

[0053] S2. Pour the wet gel obtained in S1 into a high-pressure reactor, add 40 mL of n-hexane, 1.3 mL of isopropanol and 5 mL of hexamethyldisilazane (HMDZ), adjust the pressure to 0.3 MPa, introduce carbon dioxide, and heat to 50 °C for modification.

[0054] S3, after removing the aqueous phase in the reactor, is dried at 100℃ under normal pressure for 3.5 hours.

[0055] Example 2: The difference from Example 1 is that dilute ammonia was added dropwise to adjust the pH value to 6, while the other technical indicators and preparation methods are the same as in Example 1.

[0056] Example 3: The difference from Example 2 is that 3.6 mL of hexamethyldisilazane (HMDZ) was added. The other technical indicators and preparation methods are the same as in Example 2.

[0057] Example 4: The difference from Example 3 is that a secondary modification is used, that is, S2 is replaced with: The wet gel was poured into a high-pressure reactor, and 40 mL of n-hexane, 1.3 mL of isopropanol, and 3.6 mL of hexamethyldisilazane (HMDZ) were added. The pressure was adjusted to 0.3 MPa, carbon dioxide was introduced, and the mixture was modified at 50 °C for 1 h. After the modification was completed, the carbon dioxide was turned off and the temperature was raised to 70 °C. The reaction was then carried out again in a closed high-temperature and high-pressure environment.

[0058] After removing the aqueous phase, add 40 mL of n-hexane, 0.8 mL of isopropanol, and 3.6 mL of hexamethyldisilazane (HMDZ), adjust the pressure to 0.3 MPa, introduce carbon dioxide, and modify at 50 °C for 1 h. After modification, turn off the carbon dioxide and raise the temperature to 70 °C for further reaction in a closed high temperature and high pressure environment.

[0059] The remaining technical specifications and preparation methods are the same as in Example 3.

[0060] Example 5: The difference from Example 4 is that, after S1, the wet gel is further soaked in an aqueous solution of sodium methylsiloxane for 1 hour, then soaked in isopropanol for 1 hour, and then soaked in a dilute alkaline solution for 1 hour.

[0061] In addition, in S2, the amount of hexamethyldisilazane (HMDZ) added each time is 0.8 mL (the total amount added is 1.6 mL).

[0062] The remaining technical specifications and preparation methods are the same as in Example 4.

[0063] Example 6: The difference from Example 4 is that pentafluoropropane is added for drying, that is, S3 is replaced with: After removing the aqueous phase from the reactor, add 40 mL of pentafluoropropane to the reaction mixture and heat to 100 °C to allow hexane and pentafluoropropane to be discharged in vapor form. Connect a 30 °C storage tank to the outside of the reactor to condense the hexane. Then, recirculate the remaining pentafluoropropane vapor back into the reactor to keep the amount of pentafluoropropane in the reactor constant. Repeat this cycle until no hexane is detected in the vapor discharged from the reactor using a gas chromatograph. The cycle ends when the gel block in the reactor is removed and the reactor is placed at room temperature.

[0064] The remaining technical specifications and preparation methods are the same as in Example 4.

[0065] Example 7: The difference from Example 5 is that pentafluoropropane is added for drying, that is, S3 is replaced with: After removing the aqueous phase from the reactor, add 40 mL of pentafluoropropane to the reaction mixture and heat to 100 °C to allow hexane and pentafluoropropane to be discharged in vapor form. Connect a 30 °C storage tank to the outside of the reactor to condense the hexane. Then, recirculate the remaining pentafluoropropane vapor back into the reactor to keep the amount of pentafluoropropane in the reactor constant. Repeat this cycle until no hexane is detected in the vapor discharged from the reactor using a gas chromatograph. The cycle ends when the gel block in the reactor is removed and the reactor is placed at room temperature.

[0066] The remaining technical specifications and preparation methods are the same as in Example 5.

[0067] Performance testing: The density, porosity, particle size distribution, and hydrophobicity of the carbon dioxide catalytically modified silica aerogels prepared in Examples 1-7 were tested, and the results are as follows: The density of the product in Example 1 is 0.165 g / cm³. 3 It has a porosity of 92.5% and a specific surface area of ​​635 m². 2 / g, pore volume 2.78m³ 3 / g, pore size distribution 2~50nm (mainly concentrated in 2~2nm), hydrophobic angle 137.4°, thermal conductivity 0.0232W / (m·K).

[0068] The density of the product in Example 2 is 0.197 g / cm³. 3 It has a porosity of 91.6% and a specific surface area of ​​614 m². 2 / g, pore volume 2.33m³ 3 / g, pore size distribution 2~50nm (mainly concentrated in 2~2nm), hydrophobic angle 137.88°, thermal conductivity 0.0257W / (m·K).

[0069] The density of the product in Example 3 was 0.082 g / cm³. 3 It has a porosity of 96.27% and a specific surface area of ​​679 m². 2 / g, pore volume 3.34m³ 3 / g, pore size distribution 2~50nm (mainly concentrated in 2~2nm), hydrophobic angle 140.6°, thermal conductivity 0.0219 W / (m·K).

[0070] The density of the product in Example 4 is 0.116 g / cm³. 3 It has a porosity of 94.73% and a specific surface area of ​​627 m². 2 / g, pore volume 2.84m³ 3 / g, pore size distribution 2~50nm (mainly concentrated in 2~2nm), hydrophobic angle 137.81°, thermal conductivity 0.0207 W / (m·K).

[0071] The density of the product in Example 5 was 0.096 g / cm³. 3 It has a porosity of 95.94% and a specific surface area of ​​697 m². 2 / g, pore volume 3.23m³ 3 / g, pore size distribution 2~50nm (mainly concentrated in 2~2nm), hydrophobic angle 136.8°, thermal conductivity 0.0214W / (m·K).

[0072] The density of the product in Example 6 is 0.068 g / cm³. 3 The porosity is 96.91%, and the specific surface area is 837 m². 2 / g, pore volume 3.34m³ 3 / g, pore size distribution 2~50nm (mainly concentrated in 2~2nm), hydrophobic angle 146.4°, thermal conductivity 0.0199 W / (m·K).

[0073] The density of the product in Example 7 is 0.074 g / cm³. 3 The porosity is 95.64%, and the specific surface area is 797 m². 2 / g, pore volume 3.28m³ 3 / g, pore size distribution 2~50nm (mainly concentrated in 2~2nm), hydrophobic angle 144.7°, thermal conductivity 0.02207W / (m·K).

[0074] The test results above show that the silica aerogel prepared by carbon dioxide catalytic modification is lightweight (density 68-197 mg / cm3), has high porosity (91.6%-96.91%), and possesses high hydrophobicity, high specific surface area and excellent thermal insulation properties.

[0075] Furthermore, compared to primary modification, secondary modification resulted in a more uniform and complete three-dimensional network structure, exhibiting higher thermal stability. Immersion in sodium methylsilanolate aqueous solution, isopropanol, and dilute alkaline solution before placing the sample in the reactor, compared to the method without these additions, resulted in the same silica aerogel properties, although the amount of the modifier hexamethyldisilazane was reduced, although the hydrophobic angle was slightly decreased. Samples dried at room temperature with pentafluoropropane also exhibited silica aerogel properties with a lower thermal conductivity. Simultaneously, pentafluoropropane, as a flame retardant, further enhanced the safety of the entire reaction.

[0076] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0077] The steps in the method of this invention can be adjusted, combined, or deleted according to actual needs. The technical features can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the embodiments are described. However, as long as the combinations of these technical features do not contradict each other, they should all be considered within the scope of this invention.

[0078] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing carbon dioxide-catalyzed modified silica aerogel, characterized in that, include: S1, mix water glass with water at room temperature, then titrate the mixture with a solution of water and dilute nitric acid under stirring, then add dilute ammonia to adjust the pH value, wait for gelation, and then heat in a water bath to obtain a wet gel; S2, the obtained wet gel is soaked in sodium methylsilanoate aqueous solution, isopropanol and dilute alkaline solution in sequence; S3, after soaking, the wet gel is poured into a high-pressure reactor, and hexane, isopropanol and hexamethyldisilazane are added. The pressure is adjusted, carbon dioxide is introduced, and the temperature is raised for modification. Then the carbon dioxide is turned off, the temperature is raised again, and the reaction continues in a closed environment. S4. After modification, remove the aqueous phase in the reactor, add pentafluoropropane, and heat up to discharge n-hexane and pentafluoropropane in the form of steam. The discharged steam is condensed to separate n-hexane, and the uncondensed pentafluoropropane steam is returned to the reactor until there is no n-hexane in the discharged steam. Then, take out the gel block in the reactor and place it at room temperature to obtain carbon dioxide catalytic modified silica aerogel.

2. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, In S1, the dilute ammonia solution is prepared by diluting concentrated ammonia solution (25%~28%) with water at a volume ratio of 1:5, and the pH value adjusted by adding dilute ammonia solution is in the range of 4~8.

3. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, In S1, the water bath heating temperature is 60℃ and the heating time is 20min.

4. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, In S2, the sodium methylsilanolate aqueous solution is prepared by mixing a 30% sodium methylsilanolate stock solution with water and adding it dropwise to a 25% dilute nitric acid and water mixed solution to prepare an acidic sodium methylsilanolate aqueous solution with a concentration of 1%~5% and a pH value of 2.0~2.4; the soaking time of the sodium methylsilanolate aqueous solution is 0.5~2.5h.

5. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, In S2, the dilute alkaline solution is prepared by adding ammonia water dropwise to water and adjusting the pH to 8.3~8.7; the soaking time of the dilute alkaline solution is 0.5~2.5h.

6. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, In S3, the amount of n-hexane added is 40 mL, the amount of isopropanol added is 1.3 mL, and the amounts of hexamethyldisilazane added are 0.72 mL, 2.14 mL, 3.6 mL, 5 mL, and 6.4 mL, respectively.

7. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, In S3, the pressure is adjusted to 0.3 MPa; carbon dioxide is introduced, and the temperature for modification is raised to 50~60℃; the carbon dioxide is turned off, and the temperature is raised again to 60~70℃.

8. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, Following S3, a second modification is performed, that is, the modification is repeated once more, specifically: After the first modification is completed, the aqueous phase is removed, and hexane, isopropanol, and hexamethyldisilazane are added. The pressure is adjusted, carbon dioxide is introduced, and the temperature is raised for further modification. After the modification is completed, the carbon dioxide is turned off, the temperature is increased, and the reaction is carried out again in a closed environment.

9. The method for producing carbon dioxide catalytically modified silica aerogel according to claim 1, characterized in that, In S4, pentafluoropropane is added and the temperature is raised to 100°C; the discharged steam is condensed in a 30°C storage tank to separate n-hexane; the presence of n-hexane in the discharged steam is detected by gas chromatography.

10. A carbon dioxide-catalyzed modified silica aerogel, characterized in that, It is prepared by the production method of carbon dioxide catalytic modified silica aerogel according to any one of claims 1 to 10.

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