A method for preparing a silica aerogel

Abstract

The application provides a preparation method of silica aerogel, which comprises the following steps: preparing a eutectic solvent by using ethylene glycol and an organic acid, mixing tetraethyl orthosilicate, water and the eutectic solvent for hydrolysis, adjusting pH by using ammonia water, aging, replacing and modifying, and finally grading drying under normal pressure to obtain silica aerogel. The polycondensation process of tetraethyl orthosilicate in the eutectic solvent is very uniform, a firm skeleton network is formed, and the void collapse and skeleton structure damage of the silica aerogel will not be caused in the normal pressure drying process. The prepared silica aerogel has a porosity of more than 90%, and a specific surface area of 1094 m 2 / g.

Description

Technical Field

[0001] This invention belongs to the field of nanoporous material preparation technology, specifically relating to a method for preparing silica aerogel. Background Technology

[0002] Silica aerogel is a nanomaterial with a three-dimensional porous network structure, possessing advantages such as large specific surface area, low thermal conductivity, high porosity, and excellent mechanical properties. Its excellent thermal insulation properties and high porosity make it widely used in new energy vehicles, building insulation, industrial insulation, catalysis, and adsorption. Silica aerogel also exhibits unique physical and chemical properties in acoustic, optical, and electrical aspects, and it holds broad application potential in military and aerospace fields.

[0003] Traditional silica aerogels are prepared by using organosilicones such as tetraethyl orthosilicate and tetramethyl orthosilicate as raw materials, and wet gels are prepared by the sol-gel method. Then, silica aerogels are obtained by supercritical drying. However, the preparation of silica aerogels still has the following problems: (1) The silicon source needs to be hydrolyzed under acidic conditions first. The acidity of the solution during the hydrolysis process is generally controlled by using a certain concentration of hydrochloric acid or sulfuric acid. Hydrochloric acid has strong volatility and corrosiveness. Improper operation can cause burns to the human body. According to the "Regulations on the Administration of Precursor Chemicals", hydrochloric acid and sulfuric acid are both classified as Class III precursor chemicals, which pose certain safety hazards. In addition, the traditional silicon source hydrolysis process takes a long time and has low production efficiency. (2) The commonly used drying processes are supercritical drying and freeze drying. Supercritical drying requires adjusting the drying process to a supercritical state. The high temperature and pressure result in expensive equipment, difficult operation, high cost, and the risk of explosion and leakage. Freeze drying has the disadvantages of high cost and long cycle.

[0004] Therefore, developing new preparation methods and drying processes is of great practical significance for the low-cost production of aerogels. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing silica aerogel in view of the shortcomings of the prior art, which effectively solves the problems of long silicon source hydrolysis time, safety hazards of inorganic acid and high cost of supercritical drying in the existing silica aerogel preparation technology.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing silica aerogel, the method comprising the following steps:

[0007] S1. Mix ethylene glycol and organic acid, heat and stir at 80°C for 2 hours to obtain a clear and transparent eutectic solvent;

[0008] S2. Tetraethyl orthosilicate, water and the eutectic solvent obtained in S1 are mixed and stirred. The mixture is heated under reflux at 100°C for 2 hours to hydrolyze the solution and obtain the hydrolyzed solution. Then, ammonia is added to the hydrolyzed solution to adjust the pH to 8-10. After stirring, the mixture is allowed to stand to obtain a wet gel.

[0009] S3. The wet gel obtained in S2 is immersed in anhydrous ethanol for aging, and the anhydrous ethanol is replaced every 12 hours for a total of 2 times; then it is replaced and modified in a modifier / n-hexane mixed solution, and the modifier / n-hexane mixed solution is replaced every 12 hours for a total of 2 times; take it out to obtain the modified wet gel.

[0010] S4. The modified wet gel obtained in S3 is subjected to fractional drying under normal pressure to obtain silica aerogel.

[0011] Preferably, the organic acid in S1 includes one or more of citric acid, oxalic acid, and malic acid; the molar ratio of ethylene glycol to organic acid is 4:(1.5 to 0.5).

[0012] Preferably, the volume ratio of tetraethyl orthosilicate, water and eutectic solvent in S2 is 5:(0.4-1):(8-15).

[0013] Preferably, the modifier in the modifier / n-hexane mixed solution in S3 is one or more of trimethylchlorosilane, methyltrichlorosilane, dimethyldichlorosilane, and silicon tetrachloride; the volume ratio of the modifier to n-hexane is 1:(5-10).

[0014] Preferably, the graded drying process described in S4 is as follows: drying at 60°C for 2 hours, then drying at 80°C for 2 hours, and finally drying at 120°C for 2 hours.

[0015] Preferably, the specific surface area of ​​the silica aerogel in S4 is 750.4–1094 m². 2 / g, with an average pore size of 6.7–10.8 nm.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] 1. This invention uses a eutectic solvent system as the solvent. The polycondensation process of tetraethyl orthosilicate in this solvent is very uniform, forming a strong skeletal network. During the drying process at normal pressure, it will not cause the voids of the silica aerogel to collapse or the skeletal structure to be destroyed. The prepared silica aerogel has the characteristics of low density, low thermal conductivity, large specific surface area and high porosity. At the same time, the use of eutectic solvent also avoids the safety hazards of inorganic acids such as hydrochloric acid and sulfuric acid in experimental research and industrial production in conventional methods.

[0018] 2. In existing silica aerogel preparation technologies, the silicon source hydrolysis process takes a long time, generally 12 hours; while in the present invention, the eutectic solvent reflux reaction can achieve hydrolysis in 2 hours, which is much shorter.

[0019] 3. Most existing silica aerogel preparation technologies use supercritical drying, which involves expensive equipment and high operating costs. This invention uses atmospheric pressure drying, which is simple to operate and has mild conditions. It can greatly save on the investment in drying equipment, solve the problems of long drying time and high cost, and is conducive to large-scale promotion. It has important practical significance for the low-cost production of silica aerogels.

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope image of the silica aerogel prepared in Example 1 of the present invention.

[0022] Figure 2 This is the N2 isothermal adsorption-desorption curve of the silica aerogel prepared in Example 1 of this invention.

[0023] Figure 3 This is a contact angle test diagram of the silica aerogel prepared in Example 1 of the present invention.

[0024] Figure 4 This is a TG-DSC image of the silica aerogel prepared in Example 1 of this invention.

[0025] Figure 5 This is an X-ray diffraction pattern of the silica aerogel prepared in Example 1 of this invention. Detailed Implementation

[0026] Example 1

[0027] This embodiment describes a method for preparing silica aerogel, which is as follows:

[0028] 4 mol of ethylene glycol and 1 mol of oxalic acid were mixed and heated at 80°C for 2 hours until a clear and transparent eutectic solvent was formed. 5 mL of tetraethyl orthosilicate, 1 mL of water, and 8 mL of the above eutectic solvent were mixed and stirred, and the mixture was heated under reflux at 100°C for 2 hours to hydrolyze the solution. The pH of the solution was adjusted to 8 with ammonia water, and the mixture was stirred vigorously for 5 minutes. The solution was then allowed to stand to form a wet gel. The wet gel was immersed in anhydrous ethanol for 12 hours of aging, the solution was changed, and the aging was repeated for another 12 hours. A trimethylchlorosilane / n-hexane mixed solution was prepared at a volume ratio of 1:7. The wet gel was immersed in the trimethylchlorosilane / n-hexane mixed solution for displacement and modification. The mixed solution was changed every 12 hours for a total of 2 times. The wet gel was removed and dried at 60°C for 2 hours, then at 80°C for 2 hours, and finally at 120°C for 2 hours to obtain silica aerogel.

[0029] The density of the silica aerogel prepared in this embodiment is 0.26 g / cm³. 3 It has a contact angle of 140.1°, a thermal conductivity of 0.031 W / (m·K), a porosity of 92%, and a specific surface area of ​​1094 m². 2 / g, with an average pore size of 6.7nm.

[0030] Figure 1 This is a scanning electron microscope image of the silica aerogel prepared in this embodiment. As can be seen from the image, the network skeleton of the silica aerogel is beaded, with a loose and complete structure, a particle size range of 10.1 to 30.3 nm, and an average particle size of 21.5 nm.

[0031] Figure 2 This is the N2 isotherm adsorption-desorption curve of the silica aerogel prepared in this embodiment. As can be seen from the figure, the adsorption of nitrogen by the silica aerogel is a typical type IV adsorption, belonging to the mesoporous adsorption-desorption isotherm. The specific surface area of ​​the silica aerogel is 1094 m². 2 / g, with an average pore size of 6.7nm and a pore volume of 2.6cm³. 3 / g.

[0032] Figure 3 This is a contact angle test diagram of the silica aerogel prepared in this embodiment. As can be seen from the figure, the contact angle of the silica aerogel is 140.1°, which proves that the silica aerogel has good hydrophobicity.

[0033] Figure 4 The figure shows the TG-DSC curve of the silica aerogel prepared in this embodiment. As can be seen from the figure, the mass loss of the silica aerogel mainly occurs after 430°C, and the heat flux peak appears at 525°C. The mass loss rate at 800°C under nitrogen atmosphere is 17.5%, indicating that the silica aerogel has good thermal stability.

[0034] Figure 5The figure shows the X-ray diffraction pattern of the silica aerogel prepared in this embodiment. As can be seen from the figure, a strong characteristic diffraction peak appears in the range of 20-25°, while there are no significant characteristic diffraction peaks at other positions, indicating that the silica aerogel has an amorphous (non-crystalline) structure.

[0035] Example 2

[0036] 4 mol of ethylene glycol and 1 mol of citric acid were mixed and heated at 80°C for 2 hours until a clear and transparent eutectic solvent was formed. 5 mL of tetraethyl orthosilicate, 1 mL of water, and 9 mL of the above eutectic solvent were mixed and stirred, and then heated under reflux at 100°C for 2 hours to hydrolyze the solution. The pH of the solution was adjusted to 9 with ammonia water, and the mixture was stirred vigorously for 5 minutes. The solution was then allowed to stand to form a wet gel. The wet gel was immersed in anhydrous ethanol for 12 hours of aging, the solution was changed, and the aging was repeated for another 12 hours. A trimethylchlorosilane / n-hexane mixed solution was prepared at a volume ratio of 1:10. The wet gel was immersed in the trimethylchlorosilane / n-hexane mixed solution for displacement and modification. The mixed solution was changed every 12 hours for a total of 2 times. The wet gel was then dried at 60°C for 2 hours, then at 80°C for 2 hours, and finally at 120°C for 2 hours to obtain silica aerogel.

[0037] The density of the silica aerogel prepared in this embodiment is 0.34 g / cm³. 3 It has a contact angle of 135.7°, a thermal conductivity of 0.052 W / (m·K), a porosity of 89%, and a specific surface area of ​​896 m². 2 / g, with an average pore size of 8.3nm.

[0038] Example 3

[0039] 4 mol of ethylene glycol and 1 mol of malic acid were mixed and heated at 80°C for 2 hours until a clear and transparent eutectic solvent was formed. 5 mL of tetraethyl orthosilicate, 1 mL of water, and 9 mL of the above eutectic solvent were mixed and stirred, and then heated under reflux at 100°C for 2 hours for hydrolysis. The pH of the solution was adjusted to 8.5 with ammonia water, and the mixture was stirred vigorously for 5 minutes. The mixture was then allowed to stand to form a wet gel. The wet gel was immersed in anhydrous ethanol for 12 hours of aging, the solution was changed, and the aging was repeated for another 12 hours. A methyltrichlorosilane / n-hexane mixed solution was prepared at a volume ratio of 1:7. The wet gel was immersed in the methyltrichlorosilane / n-hexane mixed solution for displacement and modification. The mixed solution was changed every 12 hours for a total of 2 times. The wet gel was removed and dried at 60°C for 2 hours, then at 80°C for 2 hours, and finally at 120°C for 2 hours to obtain silica aerogel.

[0040] The density of the silica aerogel prepared in this embodiment is 0.31 g / cm³. 3 It has a contact angle of 138.7°, a thermal conductivity of 0.045 W / (m·K), a porosity of 91%, and a specific surface area of ​​924 m².2 / g, with an average pore size of 7.9nm.

[0041] Example 4

[0042] 4 mol of ethylene glycol and 0.8 mol of oxalic acid were mixed and heated at 60°C for 2 hours until a clear and transparent eutectic solvent was formed. 5 mL of tetraethyl orthosilicate, 0.4 mL of water, and 10 mL of the above eutectic solvent were mixed and stirred, and then heated under reflux at 120°C for 2 hours for hydrolysis. The pH of the solution was adjusted to 8 with ammonia water, and the mixture was stirred vigorously for 5 minutes. The mixture was then allowed to stand to form a wet gel. The wet gel was immersed in anhydrous ethanol for 12 hours of aging, the solution was changed, and the aging was repeated for another 12 hours. A silicon tetrachloride / n-hexane mixed solution was prepared at a volume ratio of 1:7. The wet gel was immersed in the silicon tetrachloride / n-hexane mixed solution for displacement and modification. The mixed solution was changed every 12 hours for a total of 2 times. The wet gel was removed and dried at 60°C for 2 hours, then at 80°C for 2 hours, and finally at 120°C for 2 hours to obtain silica aerogel.

[0043] The density of the silica aerogel prepared in this embodiment is 0.48 g / cm³. 3 It has a contact angle of 134.3°, a thermal conductivity of 0.054 W / (m·K), a porosity of 87%, and a specific surface area of ​​914 m². 2 / g, with an average pore size of 8.5nm.

[0044] Example 5

[0045] 4 mol of ethylene glycol and 1.5 mol of malic acid were mixed and heated at 80°C for 2 hours until a clear and transparent eutectic solvent was formed. 5 mL of tetraethyl orthosilicate, 1 mL of water, and 15 mL of the above eutectic solvent were mixed and stirred, and the mixture was heated under reflux at 100°C for 2 hours to hydrolyze the solution. The pH of the solution was adjusted to 10 with ammonia water, and the mixture was stirred vigorously for 5 minutes. The solution was then allowed to stand to form a wet gel. The wet gel was immersed in anhydrous ethanol for 12 hours of aging, the solution was changed, and the aging was repeated for another 12 hours. A dimethyl dichlorosilane / n-hexane mixed solution was prepared at a volume ratio of 1:8. The wet gel was immersed in the dimethyl dichlorosilane / n-hexane mixed solution for displacement and modification. The mixed solution was changed every 12 hours for a total of 2 times. The wet gel was removed and dried at 60°C for 2 hours, then at 80°C for 2 hours, and finally at 120°C for 2 hours to obtain silica aerogel.

[0046] The density of the silica aerogel prepared in this embodiment is 0.34 g / cm³. 3 It has a contact angle of 137.5°, a thermal conductivity of 0.049 W / (m·K), a porosity of 89%, and a specific surface area of ​​935 m². 2 / g, with an average pore size of 7.8nm.

[0047] Example 6

[0048] 4 mol of ethylene glycol and 0.5 mol of citric acid were mixed and heated at 100°C for 2 hours until a clear and transparent eutectic solvent was formed. 5 mL of tetraethyl orthosilicate, 0.6 mL of water, and 12 mL of the above eutectic solvent were mixed and stirred, and then heated under reflux at 100°C for 2 hours for hydrolysis. The pH of the solution was adjusted to 9 with ammonia water, and the mixture was stirred vigorously for 5 minutes. The mixture was then allowed to stand to form a wet gel. The wet gel was immersed in anhydrous ethanol for 12 hours of aging, the solution was changed, and the aging was repeated for another 12 hours. A trimethylchlorosilane / n-hexane mixed solution was prepared at a volume ratio of 1:5. The wet gel was immersed in the trimethylchlorosilane / n-hexane mixed solution for displacement and modification. The mixed solution was changed every 12 hours for a total of 2 times. The wet gel was then dried at 60°C for 2 hours, then at 80°C for 2 hours, and finally at 120°C for 2 hours to obtain silica aerogel.

[0049] The density of the silica aerogel prepared in this embodiment is 0.45 g / cm³. 3 It has a contact angle of 139.5°, a thermal conductivity of 0.046 W / (m·K), a porosity of 88%, and a specific surface area of ​​974 m². 2 / g, with an average pore size of 7.9nm.

[0050] Comparative Example 1

[0051] Mix 5 mL of tetraethyl orthosilicate, 1 mL of water, 7 mL of ethylene glycol, and 2.5 g of oxalic acid thoroughly. Heat under reflux at 100 °C for 2 hours to hydrolyze the mixture. Adjust the pH of the solution to 8 with ammonia, stir vigorously for 5 minutes, and allow to stand to form a wet gel. Immerse the wet gel in anhydrous ethanol for 12 hours, change the solution, and age it again for 12 hours. Prepare a trimethylchlorosilane / n-hexane mixed solution at a volume ratio of 1:7. Immerse the wet gel in the trimethylchlorosilane / n-hexane mixed solution for displacement and modification, changing the mixed solution every 12 hours for a total of 2 times. Remove the wet gel and dry it at 60 °C for 2 hours, then at 80 °C for 2 hours, and finally at 120 °C for 2 hours to obtain silica aerogel.

[0052] The density of the prepared silica aerogel was 0.67 g / cm³. 3 It has a contact angle of 136°, a thermal conductivity of 0.062 W / (m·K), a porosity of 81%, and a specific surface area of ​​750.4 m². 2 / g, with an average pore size of 10.8nm.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a silica aerogel, characterized by, The method comprises the following steps: S1, mixing ethylene glycol and organic acid, heating and stirring at 80 DEG C for 2h to obtain a clear transparent eutectic solvent; S2, mixing tetraethyl orthosilicate, water and the eutectic solvent obtained in S1, stirring and heating reflux hydrolysis at 100 DEG C for 2h to obtain a hydrolyzed solution; then adding ammonia water to the hydrolyzed solution to adjust the pH to 8-10, stirring and standing to obtain a wet gel; S3, immersing the wet gel obtained in S2 in anhydrous ethanol for aging, replacing the anhydrous ethanol every 12h for a total of 2 times; then replacing and modifying in a modifier / n-hexane mixed solution, replacing the modifier / n-hexane mixed solution every 12h for a total of 2 times; taking out to obtain a modified wet gel; S4, performing fractional drying on the modified wet gel obtained in S3 under normal pressure to obtain a silica aerogel.

2. The method of claim 1, wherein, The organic acid in S1 comprises one or more of citric acid, oxalic acid and malic acid; the molar ratio of ethylene glycol to organic acid is 4:(1.5-0.5).

3. The method of claim 1, wherein, The volume ratio of tetraethyl orthosilicate, water and eutectic solvent in S2 is 5:(0.4-1):(8-15).

4. The method of claim 1, wherein, The modifier in the modifier / n-hexane mixed solution in S3 is one or more of trimethylchlorosilane, methyltrichlorosilane, dimethyldichlorosilane and silicon tetrachloride; the volume ratio of the modifier to n-hexane is 1:(5-10).

5. The method of claim 1, wherein, The fractional drying process in S4 is: drying at 60 DEG C for 2h, then drying at 80 DEG C for 2h, and finally drying at 120 DEG C for 2h.

6. The method of claim 1, wherein, The specific surface area of the silica aerogel of S4 is 750.4-1094 m 2 / g, and the average pore size is 6.7-10.8 nm.

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