Preparation method and application of high-transparency hydrophilic aerogel

Abstract

The invention provides a preparation method and application of high-transparency hydrophilic aerogel. The preparation method of the high-transparency hydrophilic aerogel comprises the following steps: 1) mixing a silicon source, water and an acid catalyst, and carrying out condensation reaction to obtain hydrolysate; and (2) adding a base catalyst and n-heptane into the hydrolysate obtained in the step (1), carrying out polycondensation reaction, gelling after the reaction is finished to obtain wet gel, and sequentially aging, replacing and drying, thereby obtaining the product. The silicon dioxide aerogel with high transparency and good hydrophilicity is prepared on the premise of not using a surfactant and an alcohol solvent, and meanwhile, the silicon dioxide aerogel is relatively low in density and relatively low in heat conductivity coefficient.

Description

Technical Field

[0001] This invention relates to the field of silica aerogels, and more specifically, to a method for preparing a highly transparent hydrophilic aerogel and its application. Background Technology

[0002] Aerogels have become an important area of ​​research in materials science in recent years. As an ultralight, porous material with a three-dimensional network structure, aerogels exhibit excellent properties in thermal, mechanical, acoustic, and optical fields due to their extremely low density, high porosity, and large specific surface area, and are widely used, especially in thermal insulation materials. However, traditional aerogel preparation processes typically use organic solvents, which lead to problems such as high cost, high toxicity, and environmental pollution, severely restricting their large-scale application. Against this backdrop, developing green, environmentally friendly, and low-cost aerogel preparation methods has become a current research hotspot. Research on aerogel preparation using water as the main solvent has attracted much attention. This not only effectively avoids the environmental safety hazards caused by traditional water and alcohol mixed solvents, but also significantly simplifies the production process and reduces production costs, which has important practical significance and application value.

[0003] However, the preparation of silica-based aerogels in a pure water system (without alcohol solvents) usually requires the assistance of surfactants, such as hexadecyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). While the use of these surfactants can promote aerogel formation, it also brings challenges to the subsequent wastewater treatment, increasing the environmental burden. Therefore, how to prepare highly transparent monolithic aerogels using water as the main solvent without the use of surfactants and without the addition of alcohol solvents is a scientific problem worthy of investigation. Summary of the Invention

[0004] The primary objective of this invention is to provide a method for preparing a highly transparent hydrophilic aerogel. The aerogel obtained by this method has low density, low thermal conductivity, and high transparency.

[0005] The preparation method of the highly transparent aerogel (also known as highly transparent silica aerogel) provided by the present invention includes the following steps: 1) Mix silicon source, water, and acid catalyst to carry out a condensation reaction to obtain hydrolysate; 2) Add an alkaline catalyst and n-heptane to the hydrolysate obtained in step S1 to carry out a polycondensation reaction. After the reaction is completed, gel is formed to obtain a wet gel. After aging, displacement and drying, the product is obtained.

[0006] In a preferred embodiment of the present invention, alcohol solvents such as ethanol are not used in step 1). Phase transfer catalysts and surfactants are also not required in the scheme of the present invention.

[0007] In a preferred embodiment of the present invention, in step 1), the silicon source can be a commonly used organosilicon source in the art, preferably tetramethoxysilane, tetraethoxysilane Si28 or Si40.

[0008] In a preferred embodiment of the present invention, in step 1), in order to obtain a highly transparent and well-shaped aerogel, the molar ratio of silicon to water in the silicon source is 1:(13~26). The amount of acid catalyst used is 0.03%-0.1% of the total amount of hydrolysate.

[0009] In a specific embodiment of the present invention, in step 1), the acid catalyst can be hydrochloric acid, phosphoric acid, or nitric acid, preferably hydrochloric acid with a concentration of 1-3 mol / L. In the scheme of the present invention, when the concentration of the acid catalyst is too low, it will affect the transparency of the obtained silica aerogel.

[0010] In a specific embodiment of the present invention, in step 1), the temperature of the condensation reaction can be 50~60℃, and the reaction time is 7~8h. In the scheme of the present invention, if the temperature is too high, the loss of hydrolysate will be large; if the temperature is too low, the transparency of the aerogel will decrease; if the reaction time is too long, it will not have much impact on the transparency of the aerogel; if the reaction time is too short, the hydrolysate gels for too long, and the transparency will decrease.

[0011] In step 1) of the present invention, after the condensation reaction is completed, the system is usually cooled to room temperature and then stored at low temperature.

[0012] In this invention, the addition of n-heptane is one of the core inventive points. When attempting to replace n-heptane with solvents such as n-hexane, isooctane, n-octane, and toluene, the effects of this invention could not be achieved: the aerogel obtained with the addition of n-heptane was less prone to collapse, formed a solid block, and was transparent, while also exhibiting low density and high light transmittance; however, when n-hexane, isooctane, or n-octane were used instead, the resulting samples had lower transparency and were difficult to form a solid block; and when toluene was used instead of n-heptane, the resulting aerogel was prone to collapse, and the drying temperature was too high, making it unusable.

[0013] In a preferred embodiment of the present invention, the mass ratio of n-heptane to water in step 1) is 1:1 to 1:3, and in a specific embodiment it can be 1:1.5, 1:2, 1:2.5, etc. In the solution of the present invention, if too much n-heptane is used, the aerogel gelation time will be too long; while if too little n-heptane is used, the transparency of the obtained aerogel will be reduced.

[0014] In a preferred embodiment of the present invention, in step 2), the alkaline catalyst can be one or more of ammonia, tetramethylammonium hydroxide, sodium hydroxide, lithium hydroxide, and ammonium fluoride, preferably ammonia. The amount of alkaline catalyst used is to adjust the pH of the system to 7-8. Meanwhile, the applicant of this invention discovered during the research process that when the alkaline catalyst is ammonia, the concentration of ammonia is preferably 1-3 mol / L. Too high ammonia concentration will lead to a decrease in the transparency of the aerogel; while too low ammonia concentration will lead to a longer gelation time and failure to gel.

[0015] In a specific embodiment of the present invention, the polycondensation reaction in step 2) is carried out at room temperature and pressure, and the reaction time is relatively fast, usually 1-2 minutes. In a specific embodiment of the present invention, the aging in step 2) can be a commonly used aging step in the art, such as aging in a sealed system at a certain temperature (60℃-65℃) for 20-24 hours. The displacement step in step 2) of the present invention can also be a commonly used displacement step in the art, such as changing the displacement solution every 24 hours (the displacement solution can be anhydrous ethanol, approximately 100 mL each time), and the number of displacements is 4-5 times. The drying step in step 2) of the present invention can use supercritical drying, and the drying time is sufficient to ensure the aerogel is dry, which can be 8-9 hours.

[0016] The aerogel prepared using the method provided in this invention exhibits high transparency (the aerogel obtained by this invention has a thickness of approximately 10 mm and a light transmittance of over 90% at 550 nm, preferably over 95%), good hydrophilicity, low density, and low thermal conductivity. The high transparency and hydrophilicity of the aerogel obtained by this invention make its application possible in fields such as optical devices and environmental remediation.

[0017] Another object of the present invention is to provide a highly transparent aerogel obtained by the above preparation method.

[0018] Another object of the present invention is to provide the above-described preparation method and the application of the highly transparent aerogel obtained by the above-described preparation method in the field of optical device fabrication or environmental remediation.

[0019] This invention yields highly transparent and hydrophilic silica aerogels without the use of surfactants or alcohol solvents. The silica aerogels obtained by this invention are not easily collapsed, remaining as monolithic and transparent blocks. While maintaining low density and low thermal conductivity, the silica aerogels obtained by this invention exhibit more uniform pores, a larger specific surface area, and higher transparency. Compared with existing technologies, the silica aerogel blocks prepared using the method provided by this invention show higher transmittance at the same wavelength (0~1000nm). Under illumination at wavelengths of 500nm~1000nm, the transmittance of the silica aerogels obtained by this invention reaches over 95%, approaching 100%. The aerogel preparation method provided by this invention is more environmentally friendly, containing no metal ions. Attached Figure Description

[0020] Figure 1 These are sample images of silica aerogels obtained in Example 1 (left) and Comparative Example 1 (right).

[0021] Figure 2 The transmittance of the silica aerogel prepared in Example 1 at different wavelengths (0-1000 nm).

[0022] Figure 3 The transmittance of the silica aerogel prepared for Comparative Example 1 at different wavelengths (0-1000 nm). Detailed Implementation

[0023] The specific embodiments of the present invention will be further described in detail below with reference to examples. The following typical examples and comparative examples are used to illustrate the present invention, but are not intended to limit the scope of the invention. Unless otherwise specified, the conditions in the embodiments of the present invention are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products. In the present invention, unless otherwise specified, "%" refers to a percentage by mass.

[0024] Example 1 S1, under normal temperature and pressure, 2.08g TEOS, 4.6g ultrapure water and 0.05ml 1mol / L HCl were placed in a reactor and subjected to hydrolysis and condensation reaction at 50℃ for 8h. After the reaction was completed, the solution was stored at low temperature to obtain the hydrolysate.

[0025] S2, at room temperature and pressure, the hydrolysate obtained in step S1, 0.27 ml of 1 mol / L ammonia water, and 2.3 g of n-heptane are mixed (the pH of the system is 7.5), and stirred for 1 min (to carry out the polycondensation reaction). After the reaction is completed, the wet gel is obtained after it gels.

[0026] S3. The wet gel obtained in step S2 is aged in a sealed system at 60°C for 24 hours. After aging, the replacement solution is replaced every 24 hours (the replacement solution is anhydrous ethanol, about 100 mL each time). The replacement is completed after 4 replacements.

[0027] S4. The wet gel obtained from step S3 is placed in a supercritical drying apparatus for supercritical drying for 8 hours to obtain a 10mm thick highly transparent silica aerogel block. The specific method of supercritical drying is as follows: the temperature of the supercritical drying vessel is raised from room temperature to 60℃, the pressure of the drying vessel is maintained at 20MPa, supercritical carbon dioxide is continuously circulated for 8 hours, and finally the pressure is released to atmospheric pressure at a rate of 0.8MPa / h.

[0028] Comparative Example 1 S1, under normal temperature and pressure, 2.08g TEOS, 4.60g ethanol, and 0.05ml 1mol / L HCl were placed in a reaction vessel and subjected to a hydrolysis-condensation reaction at 50℃ for 8h. After the reaction was completed, the solution was stored at low temperature to obtain the hydrolysate.

[0029] S2, at room temperature and pressure, the hydrolysate obtained in step S1 and 0.27 ml of 1 mol / L ammonia water are stirred in the reaction system for 1 min (to carry out polycondensation reaction). After the reaction is completed, the wet gel is obtained after gelation.

[0030] S3. The wet gel obtained in step S2 is aged in a sealed system at 60°C for 24 hours. After aging, the replacement solution is replaced every 24 hours (the replacement solution is anhydrous ethanol, about 100 ml each time). The replacement is completed after 4 replacements.

[0031] S4. The wet gel that was replaced in step S3 is placed in a supercritical drying device for supercritical drying for 8 hours to obtain a silica aerogel block with a thickness of 10 mm. The specific method of supercritical drying is as follows: the temperature of the supercritical drying vessel is raised from room temperature to 60°C, the pressure of the drying vessel is maintained at 20 MPa, supercritical carbon dioxide is continuously circulated for 8 hours, and finally the pressure is released to atmospheric pressure at a rate of 0.8 MPa / h.

[0032] Performance Characterization The silica aerogel blocks obtained in the examples and comparative examples were subjected to performance tests. The density was tested according to GB / T 5211.4, the thermal conductivity according to ISO 22007-2, the specific surface area according to GB / T 19587, the pore volume according to GB / T 19587, the pore size according to GB / T 19587, the water contact angle according to GB / T30693, and the transmittance according to a UV-Vis spectrophotometer. The test results are shown in Table 1. Figure 1The images show sample images of silica aerogels obtained in Example 1 and Comparative Example 1 (under visible light (wavelength around 550 nm)). Figure 2 and Figure 3 The transmittance diagrams of the silica aerogel prepared in Example 1 at different wavelengths (0~1000nm) and the transmittance diagrams of the silica aerogel prepared in Comparative Example 1 at different wavelengths (0~1000nm) are shown respectively.

[0033] Table 1 ; From Table 1 and Figure 1 As can be seen, the silica aerogel block obtained by this invention has uniform aerogel channels, a large specific surface area, and good light transmittance. From... Figure 2 and Figure 3 As can be seen, compared with the prior art, the silica aerogel block obtained by the preparation method provided by the present invention has higher transmittance at the same wavelength (0~1000nm). Under light illumination with wavelengths of 500nm~1000nm, the transmittance is as high as 95% or more, which is infinitely close to 100%.

[0034] Finally, the method of this invention is merely a preferred embodiment and is not intended to limit the scope of protection of this invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a highly transparent aerogel, characterized in that, Includes the following steps: 1) Mix silicon source, water, and acid catalyst to carry out a condensation reaction to obtain hydrolysate; 2) Add an alkaline catalyst and n-heptane to the hydrolysate obtained in step 1) to carry out a polycondensation reaction. After the reaction is completed, gel is formed to obtain a wet gel. After aging, displacement and drying, the product is obtained.

2. The preparation method according to claim 1, characterized in that, No alcohol solvents are used in step 1).

3. The preparation method according to claim 1, characterized in that, In step 1), the molar ratio of the silicon source to water is 1:(13~26).

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step 1), the silicon source is tetramethoxysilane, tetraethoxysilane Si28, or Si40.

5. The preparation method according to any one of claims 1 to 3, characterized in that, In step 1), the amount of acid catalyst used is 0.03% to 0.1% of the total amount of the hydrolysate.

6. The preparation method according to claim 5, characterized in that, In step 1), the acid catalyst is hydrochloric acid with a concentration of 1~3 mol / L.

7. The preparation method according to any one of claims 1 to 3, characterized in that, In step 2), the mass ratio of n-heptane to water in step 1) is 1:1 to 1:

3.

8. The preparation method according to any one of claims 1 to 3, characterized in that, In step 2), the alkaline catalyst is ammonia water, and the concentration of the ammonia water is 1~3 mol / L.

9. The highly transparent aerogel obtained by the preparation method according to any one of claims 1 to 8.

10. The preparation method according to any one of claims 1 to 7 or the application of the highly transparent aerogel according to claim 8 in the field of optical device fabrication or environmental remediation.

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