Preparation method of magnesium oxide aerogel

Abstract

The invention discloses a preparation method of magnesium oxide aerogel, and belongs to the technical field of aerogel, the preparation method comprises the following steps: preparing a first reinforcing agent, preparing a second reinforcing agent, preparing magnesium sol, preparing silica sol, and mixing; the mixing comprises the following steps: mixing the magnesium sol, the silica sol, the first reinforcing agent and the second reinforcing agent, performing ultrasonic stirring, standing and aging under a closed condition at 40-45 DEG C, completely dipping in absolute ethyl alcohol, standing at 30-35 DEG C, and performing supercritical drying in a carbon dioxide environment to obtain the magnesium oxide aerogel. The prepared magnesium oxide aerogel is high in hydrophobicity, high in specific surface area, low in heat conductivity and high in strength.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogels, and particularly relates to a preparation method of magnesium oxide aerogel. Background Art

[0002] An aerogel refers to a nano-porous solid material formed by replacing the liquid phase in a gel with a gas through a sol-gel method and using a certain drying method. It has a high porosity, a high specific surface area, and a low density, so it has excellent heat insulation performance and adsorption performance. Further, aerogels exhibit excellent performance in thermics, electricity, optics, acoustics, adsorption, catalysis, etc., and are widely used in fields such as aerospace, building energy conservation, chemical industry, electronics and electricity, and biomedicine.

[0003] Classified according to the skeleton composition, aerogels include silica aerogel, alumina aerogel, zirconia aerogel, zinc oxide aerogel, carbon aerogel, polyimide aerogel, polyurethane aerogel, etc. Different skeleton compositions result in different physical and chemical properties of aerogels.

[0004] Magnesium oxide has advantages such as non-toxicity and antibacterial properties. The specific surface area of magnesium oxide aerogel is nearly ten times that of powdered magnesium oxide. Magnesium oxide aerogel not only has obvious advantages in organic catalysis but also can adsorb toxic gases. In the preparation of magnesium oxide aerogel, the commonly used magnesium sources are magnesium alkoxides or inorganic magnesium salts. Among them, when the magnesium source is a magnesium alkoxide, it mainly hydrolyzes through the magnesium alkoxide to generate Mg-OH, and dehydration condensation occurs between Mg-OH to generate Mg-O-Mg; however, the hydrolysis rate of the magnesium alkoxide is fast, and a large amount of Mg-OH will be generated in a short time. Further, a gel with poor structural properties will be formed in a short time, resulting in poor mechanical properties of the prepared magnesium oxide aerogel; when the magnesium source is an inorganic magnesium salt, after adding the inorganic magnesium salt and an epoxide with proton absorption ability to water, the inorganic magnesium salt exists in the form of magnesium ions in water. Although the hydrolysis degree of magnesium ions is very small, still some magnesium ions will hydrolyze, making the aqueous solution acidic. The epoxide with proton absorption ability can consume the hydrogen ions in the solution, promote the hydrolysis of magnesium ions, increase the Mg-OH in the aqueous solution, and a small part of Mg-OH undergoes dehydration condensation to generate Mg-O-Mg, but most of the Mg-OH generates magnesium hydroxide precipitation, which affects the formation of magnesium oxide aerogel.

[0005] To solve the above problems, the commonly used solutions are as follows: The first solution is to add glycerol to extend the gel time for the problems existing in magnesium alkoxides; the second solution is to add polyvinyl alcohol-based water-soluble polymers as network structure guiding agents for the problems existing in inorganic magnesium salts.

[0006] However, in the above methods, the glycerol used in the first solution method will increase the hydrophilicity and decrease the hydrophobicity of the prepared magnesium oxide aerogel. Further, it will lead to poor moisture resistance of the prepared magnesium oxide aerogel. In addition, it will also affect the application effect of the magnesium oxide aerogel in organic catalysis. The polyvinyl alcohol-based water-soluble polymer used in the second solution method will not only increase the hydrophilicity and decrease the hydrophobicity of the prepared magnesium oxide aerogel, but also the dosage of the polyvinyl alcohol-based water-soluble polymer will affect the specific surface area, mechanical strength, and heat insulation performance of the magnesium oxide aerogel. When the dosage of the polyvinyl alcohol-based water-soluble polymer is large, it can effectively avoid the formation of magnesium hydroxide precipitation and increase the crosslinking density, thereby increasing the specific surface area and porosity of the magnesium oxide aerogel. Further, it can improve the heat insulation performance of the magnesium oxide aerogel. However, the mechanical strength of the hydrogel formed by the polyvinyl alcohol-based water-soluble polymer is low. With the increase of the polyvinyl alcohol dosage, it will lead to a decrease in the mechanical properties of the aerogel formed by the polyvinyl alcohol-based water-soluble polymer.

[0007] To address the above problems, a common solution is to add methyltrimethoxysilane during the preparation of magnesium oxide aerogel to improve the hydrophobicity of the magnesium oxide aerogel and enhance its mechanical properties. However, methyltrimethoxysilane will cause phase separation in the prepared magnesium oxide aerogel, affecting the specific surface area and porosity of the prepared magnesium oxide aerogel. Further, it will lead to a decrease in the heat insulation performance of the prepared magnesium oxide aerogel.

[0008] Chinese Patent CN108017073B discloses a method for preparing magnesium oxide aerogel using bischofite as a raw material. The preparation method includes the following steps: mixing bischofite, distilled water, alcohol solvent, and water-soluble polymer, stirring for 5 - 120 min to obtain a transparent solution, and then adding an epoxide to the solution and stirring for 5 - 60 min to obtain a sol; sealing the prepared sol for gelation, with the gelation temperature being 20 - 95°C and the gelation time being 2 - 20 h to obtain a wet magnesium oxide gel; soaking and washing the obtained wet magnesium oxide gel with distilled water, with the number of washing times being no less than three and the amount of distilled water used being no less than the volume of the wet gel, and then drying to prepare magnesium oxide aerogel. This method solves the problem that when using magnesium alkoxide to prepare wet magnesium oxide gel, magnesium alkoxide is extremely prone to hydrolysis and polycondensation, and the process is difficult to control. However, it can be seen from the examples of this patent that when the dosage of the water-soluble polymer increases, the specific surface area increases, the thermal conductivity decreases, and the strength decreases. It is difficult to simultaneously achieve a high specific surface area, low thermal conductivity, and high strength. Moreover, the hydrophobicity of the magnesium oxide aerogel prepared by this method is poor. Summary of the Invention

[0009] Aiming at the deficiencies of the existing technology, the present invention provides a method for preparing magnesium oxide aerogel. The prepared magnesium oxide aerogel has strong hydrophobicity, high specific surface area, low thermal conductivity, and high strength.

[0010] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A preparation method of magnesium oxide aerogel, comprising: preparing a first reinforcing agent, preparing a second reinforcing agent, preparing a magnesium sol, preparing a silica sol, and mixing; For the preparation of the first reinforcing agent, after mixing magnesium chloride hexahydrate and the first portion of deionized water, stir at a stirring speed of 50 - 100 rpm for 10 - 30 min at room temperature, add the first portion of polyvinyl alcohol 1788, continue stirring for 10 - 30 min, add ammonia water to adjust the pH to 11 - 11.5, control the dropping speed of ammonia water at 4 - 6 mL / min, continue stirring for 30 - 35 min, centrifuge at a centrifugal speed of 7000 - 8000 rpm for 10 - 12 min, discard the supernatant, and vacuum dry at 70 - 80 °C to obtain magnesium hydroxide powder; after mixing the second portion of polyvinyl alcohol 1788 and the second portion of deionized water, stir at a stirring speed of 50 - 100 rpm for 10 - 30 min at room temperature, add all the above magnesium hydroxide powder, continue stirring for 1.5 - 2 h, centrifuge at a centrifugal speed of 9000 - 10000 rpm for 10 - 12 min, perform 3 - 4 freeze-thaw cycles on the precipitate, and vacuum dry at 70 - 80 °C to obtain the first reinforcing agent; In the preparation of the first reinforcing agent, the dosage ratio of magnesium chloride hexahydrate, the first portion of deionized water, the first portion of polyvinyl alcohol 1788, the second portion of polyvinyl alcohol 1788, and the second portion of deionized water is 95 - 100 g:500 - 550 mL:1.4 - 1.5 g:9 - 10 g:500 - 550 mL; The concentration of the ammonia water is 2 mol / L; For the freeze-thaw cycle, place the precipitate at -25 °C to -20 °C for 8 - 9 h and at 24 - 27 °C for 4 - 5 h; For the preparation of the second reinforcing agent, after mixing magnesium chloride hexahydrate, absolute ethanol, and deionized water, stir at a stirring speed of 50 - 100 rpm for 10 - 15 min at room temperature, add propylene oxide, continue stirring for 10 - 15 min, let stand for 45 - 50 h, centrifuge at a centrifugal speed of 9000 - 10000 rpm for 10 - 12 min, discard the supernatant, wash the precipitate with deionized water 3 - 4 times, vacuum dry at 70 - 80 °C, and mix with cetyltrimethylammonium bromide and dimethyl sulfoxide, then stir at room temperature for 30 - 40 min to obtain the second reinforcing agent; In the preparation of the second reinforcing agent, the dosage ratio of magnesium chloride hexahydrate, absolute ethanol, deionized water, propylene oxide, cetyltrimethylammonium bromide, and dimethyl sulfoxide is 90 - 100 g:550 - 600 mL:500 - 550 mL:300 - 320 mL:1.8 - 2 g:100 - 110 mL; To prepare the magnesium sol, mix magnesium chloride hexahydrate, absolute ethanol, deionized water, and polyvinyl alcohol 2488, stir at room temperature for 20 - 25 min, add propylene oxide, and continue stirring for 10 - 15 min to obtain the magnesium sol; In the preparation of the magnesium sol, the dosage ratio of magnesium chloride hexahydrate, absolute ethanol, deionized water, polyvinyl alcohol 2488, and propylene oxide is 100 - 110 g: 1800 - 2200 mL: 1000 - 1200 mL: 5 - 6 g: 460 - 500 mL; To prepare the silica sol, mix methyltrimethoxysilane, absolute ethanol, and deionized water, stir at 40 - 45 °C at a stirring speed of 100 - 300 rpm for 10 - 20 min, add hydrochloric acid aqueous solution to adjust the pH to 3 - 3.5, continue stirring for 20 - 25 h, add ammonia water to adjust the pH to 7.5 - 8, and continue stirring for 10 - 15 min to obtain the silica sol; In the preparation of the silica sol, the volume ratio of methyltrimethoxysilane, absolute ethanol, and deionized water is 50 - 52: 200 - 250: 20 - 23; The concentration of the hydrochloric acid aqueous solution is 1 mol / L; The concentration of the ammonia water is 2 mol / L; For the mixing, mix the magnesium sol, silica sol, first reinforcing agent, and second reinforcing agent, stir ultrasonically, under airtight conditions at 40 - 45 °C, stand and age for 30 - 35 h, completely immerse in absolute ethanol, stand at 30 - 35 °C for 48 - 60 h, replace the absolute ethanol once every 12 h, and perform supercritical drying in a carbon dioxide environment to obtain the magnesium oxide aerogel; In the mixing, the dosage ratio of the magnesium sol, silica sol, first reinforcing agent, and second reinforcing agent is 2800 - 3000 mL: 240 - 270 mL: 14 - 16 g: 100 - 110 mL; When performing ultrasonic stirring, the frequency is 20 - 25 kHz, the stirring speed is 150 - 200 rpm, and the time is 30 - 40 min; The temperature of the supercritical drying is 50 °C, the pressure is 15 MPa, and the time is 20 - 25 h.

[0011] Compared with the prior art, the beneficial effects of the present invention are: (1)The preparation method of the magnesium oxide aerogel of the present invention uses a first reinforcing agent and a second reinforcing agent. When preparing the first reinforcing agent, magnesium hydroxide is first prepared in an aqueous solution of polyvinyl alcohol, and then the magnesium hydroxide is mixed with the aqueous solution of polyvinyl alcohol. Polyvinyl alcohol is adsorbed on the surface of the magnesium hydroxide, and then freeze-thaw cycles are carried out to obtain magnesium hydroxide coated with polyvinyl alcohol aerogel on the surface. After mixing the first reinforcing agent with magnesium sol and silica sol, the polyvinyl alcohol aerogel on the surface of the first reinforcing agent can act as a network structure guiding agent to promote the formation of magnesium oxide gel. At the same time, the magnesium hydroxide in the first reinforcing agent can improve the strength of the magnesium oxide gel. When preparing the second reinforcing agent, magnesium chloride hexahydrate is used as the magnesium source to obtain magnesium hydroxide with a high specific surface area. The surface of the magnesium hydroxide contains Mg-OH, and then it is mixed with cetyltrimethylammonium bromide and dimethyl sulfoxide to obtain a magnesium hydroxide dispersion, that is, the second reinforcing agent. The surface of the magnesium hydroxide in the second reinforcing agent is coated with cetyltrimethylammonium bromide. The hydrophilic end of the cetyltrimethylammonium bromide is connected to the magnesium hydroxide. After mixing the second reinforcing agent with magnesium sol and silica sol, the cetyltrimethylammonium bromide on the surface of the second reinforcing agent can promote the dispersion of magnesium hydroxide. At the same time, the hydrophobic end can also combine with the silica sol to prevent particle growth, inhibit phase separation, and can also promote the connection between the silica sol and the second reinforcing agent; (2)The magnesium oxide aerogel prepared by the present invention has strong hydrophobicity, and the water contact angle is 127-131°; (3)The magnesium oxide aerogel prepared by the present invention has a high specific surface area, low thermal conductivity, and high strength. The density is 0.07-0.08 g / cm 3 , the BET specific surface area is 725-758 m 2 / g, the compressive strength is 0.14-0.15 MPa, and the thermal conductivity is 0.020-0.023 W / (m•K). Detailed implementation manners

[0012] In order to have a clearer understanding of the technical features, purposes, and effects of the present invention, the detailed implementation manners of the present invention are now described.

[0013] The room temperature in the examples and comparative examples is 23°C.

[0014] Example 1 A preparation method of magnesium oxide aerogel includes the following steps: 1. Preparation of the first reinforcing agent: Mix 95 g of magnesium chloride hexahydrate and 500 mL of deionized water, stir at a stirring speed of 50 rpm at room temperature for 10 min, add 1.4 g of polyvinyl alcohol 1788, continue stirring for 10 min, add ammonia water to adjust the pH to 11, control the dropping rate of ammonia water at 4 mL / min, continue stirring for 30 min, centrifuge at a centrifugal speed of 7000 rpm for 10 min, discard the supernatant, and vacuum dry at 70 °C to obtain magnesium hydroxide powder; Mix 9 g of polyvinyl alcohol 1788 and 500 mL of deionized water, stir at a stirring speed of 50 rpm at room temperature for 10 min, add all the above-mentioned magnesium hydroxide powder, continue stirring for 1.5 h, centrifuge at a centrifugal speed of 9000 rpm for 10 min, discard the supernatant, place the precipitate at -25 °C for 8 h and at 24 °C for 4 h, take this as one freeze-thaw cycle, and perform a total of 3 freeze-thaw cycles, then vacuum dry at 70 °C to obtain the first reinforcing agent; The concentration of the ammonia water is 2 mol / L; 2. Preparation of the second reinforcing agent: Mix 90 g of magnesium chloride hexahydrate, 550 mL of absolute ethanol, and 500 mL of deionized water, stir at a stirring speed of 50 rpm at room temperature for 10 min, add 300 mL of propylene oxide, continue stirring for 10 min, let stand for 45 h, centrifuge at a centrifugal speed of 9000 rpm for 10 min, discard the supernatant, wash the precipitate with deionized water 3 times, vacuum dry at 70 °C, and then mix with 1.8 g of cetyltrimethylammonium bromide and 100 mL of dimethyl sulfoxide, and stir at room temperature for 30 min to obtain the second reinforcing agent; 3. Preparation of magnesium sol: Mix 100 g of magnesium chloride hexahydrate, 1800 mL of absolute ethanol, 1000 mL of deionized water, and 5 g of polyvinyl alcohol 2488, stir at room temperature for 20 min, add 460 mL of propylene oxide, and continue stirring for 10 min to obtain magnesium sol; 4. Preparation of silica sol: Mix 50 mL of methyltrimethoxysilane, 200 mL of absolute ethanol, and 20 mL of deionized water, stir at a stirring speed of 100 rpm at 40 °C for 10 min, add hydrochloric acid aqueous solution to adjust the pH to 3, continue stirring for 20 h, add ammonia water to adjust the pH to 7.5, and continue stirring for 10 min to obtain silica sol; The concentration of the hydrochloric acid aqueous solution is 1 mol / L; The concentration of the ammonia water is 2 mol / L; 5. Mixing: Mix 2800 mL of magnesium sol, 240 mL of silica sol, 14 g of the first reinforcing agent, and 100 mL of the second reinforcing agent. After that, stir ultrasonically for 30 min, control the ultrasonic frequency during stirring at 20 kHz, and the stirring speed at 150 rpm. Under airtight conditions at 40 °C, let it stand and age for 30 h, then fully immerse it in absolute ethanol, let it stand at 30 °C for 48 h, replace the absolute ethanol once every 12 h, and perform supercritical drying in a carbon dioxide environment. Control the temperature during supercritical drying at 50 °C, the pressure at 15 MPa, and the time at 20 h to obtain magnesium oxide aerogel.

[0015] The density of the magnesium oxide aerogel obtained in this example is 0.07 g / cm 3 , the BET specific surface area is 725 m 2 / g, the compressive strength is 0.14 MPa, the thermal conductivity is 0.022 W / (m•K), and the water contact angle is 127°.

[0016] Example 2 A preparation method of magnesium oxide aerogel, comprising the following steps: 1. Preparation of the first reinforcing agent: Mix 98 g of magnesium chloride hexahydrate and 520 mL of deionized water, then stir at a stirring speed of 80 rpm at room temperature for 20 min, add 1.4 g of polyvinyl alcohol 1788, continue stirring for 20 min, add ammonia water to adjust the pH to 11.2, control the dropping rate of ammonia water at 5 mL / min, continue stirring for 32 min, centrifuge at a centrifugal speed of 7500 rpm for 11 min, discard the supernatant, and vacuum dry at 75 °C to obtain magnesium hydroxide powder; Mix 9.5 g of polyvinyl alcohol 1788 and 520 mL of deionized water, then stir at a stirring speed of 80 rpm at room temperature for 20 min, add all the above magnesium hydroxide powder, continue stirring for 1.5 h, centrifuge at a centrifugal speed of 9500 rpm for 11 min, discard the supernatant, place the precipitate at -25 °C for 8.5 h and at 25 °C for 4.5 h, take this as 1 freeze-thaw cycle, and perform a total of 4 freeze-thaw cycles, then vacuum dry at 75 °C to obtain the first reinforcing agent; The concentration of the ammonia water is 2 mol / L; 2. Preparation of the second reinforcing agent: Mix 95 g of magnesium chloride hexahydrate, 580 mL of absolute ethanol, and 520 mL of deionized water, then stir at a stirring speed of 80 rpm at room temperature for 12 min, add 310 mL of propylene oxide, continue stirring for 12 min, let it stand for 48 h, centrifuge at a centrifugal speed of 9500 rpm for 11 min, discard the supernatant, wash the precipitate with deionized water 3 times, vacuum dry at 75 °C, and then mix with 1.9 g of cetyltrimethylammonium bromide and 105 mL of dimethyl sulfoxide, and stir at room temperature for 35 min to obtain the second reinforcing agent; 3. Preparation of magnesium sol: Mix 105 g of magnesium chloride hexahydrate, 2000 mL of absolute ethanol, 1100 mL of deionized water, and 5.5 g of polyvinyl alcohol 2488, stir at room temperature for 23 min, add 480 mL of propylene oxide, and continue stirring for 12 min to obtain magnesium sol; 4. Preparation of silica sol: Mix 51 mL of methyltrimethoxysilane, 230 mL of absolute ethanol, and 22 mL of deionized water, stir at 42 °C at a stirring speed of 200 rpm for 15 min, add hydrochloric acid aqueous solution to adjust the pH to 3.2, continue stirring for 22 h, add ammonia water to adjust the pH to 7.7, and continue stirring for 12 min to obtain silica sol; The concentration of the hydrochloric acid aqueous solution is 1 mol / L; The concentration of the ammonia water is 2 mol / L; 5. Mixing: Mix 2900 mL of magnesium sol, 250 mL of silica sol, 15 g of the first reinforcing agent, and 105 mL of the second reinforcing agent, stir ultrasonically for 35 min, control the frequency during ultrasonic stirring to be 22 kHz and the stirring speed to be 180 rpm, statically age for 32 h under sealed conditions at 42 °C, completely immerse in absolute ethanol, statically stand at 32 °C for 60 h, replace the absolute ethanol once every 12 h, and perform supercritical drying in a carbon dioxide environment, control the temperature during supercritical drying to be 50 °C, the pressure to be 15 MPa, and the time to be 22 h to obtain magnesium oxide aerogel.

[0017] The density of the magnesium oxide aerogel obtained in this example is 0.07 g / cm 3 , the BET specific surface area is 758 m 2 / g, the compressive strength is 0.14 MPa, the thermal conductivity is 0.020 W / (m•K), and the water contact angle is 130°.

[0018] Example 3 A preparation method of magnesium oxide aerogel, comprising the following steps: 1. Preparation of the first reinforcing agent: Mix 100 g of magnesium chloride hexahydrate and 550 mL of deionized water, and stir at a stirring speed of 100 rpm for 30 min at room temperature. Add 1.5 g of polyvinyl alcohol 1788, continue stirring for 30 min, add ammonia water to adjust the pH to 11.5, control the dropping speed of ammonia water at 6 mL / min, continue stirring for 35 min, centrifuge at a centrifugal speed of 8000 rpm for 12 min, discard the supernatant, and vacuum dry at 80 °C to obtain magnesium hydroxide powder. Mix 10 g of polyvinyl alcohol 1788 and 550 mL of deionized water, and stir at a stirring speed of 100 rpm for 30 min at room temperature. Add all the above-mentioned magnesium hydroxide powder, continue stirring for 2 h, centrifuge at a centrifugal speed of 10000 rpm for 12 min, discard the supernatant, place the precipitate at -20 °C for 9 h and at 27 °C for 5 h, which is regarded as one freeze-thaw cycle, and conduct a total of 4 freeze-thaw cycles, then vacuum dry at 80 °C to obtain the first reinforcing agent; The concentration of the ammonia water is 2 mol / L; 2. Preparation of the second reinforcing agent: Mix 100 g of magnesium chloride hexahydrate, 600 mL of absolute ethanol, and 550 mL of deionized water, and stir at a stirring speed of 100 rpm for 15 min at room temperature. Add 320 mL of propylene oxide, continue stirring for 15 min, let it stand for 50 h, centrifuge at a centrifugal speed of 10000 rpm for 12 min, discard the supernatant, wash the precipitate with deionized water 4 times, vacuum dry at 80 °C, and then mix with 2 g of cetyltrimethylammonium bromide and 110 mL of dimethyl sulfoxide, and stir at room temperature for 40 min to obtain the second reinforcing agent; 3. Preparation of magnesium sol: Mix 110 g of magnesium chloride hexahydrate, 2200 mL of absolute ethanol, 1200 mL of deionized water, and 6 g of polyvinyl alcohol 2488, stir at room temperature for 25 min, add 500 mL of propylene oxide, and continue stirring for 15 min to obtain magnesium sol; 4. Preparation of silica sol: Mix 52 mL of methyltrimethoxysilane, 250 mL of absolute ethanol, and 23 mL of deionized water, stir at a stirring speed of 300 rpm for 20 min at 45 °C, add hydrochloric acid aqueous solution to adjust the pH to 3.5, continue stirring for 25 h, add ammonia water to adjust the pH to 8, and continue stirring for 15 min to obtain silica sol; The concentration of the hydrochloric acid aqueous solution is 1 mol / L; The concentration of the ammonia water is 2 mol / L; 5. Mixing: After mixing 3000 mL of magnesium sol, 270 mL of silica sol, 16 g of the first reinforcing agent, and 110 mL of the second reinforcing agent, ultrasonically stir for 40 min. Control the frequency during ultrasonic stirring to be 25 kHz and the stirring speed to be 200 rpm. Under airtight conditions at 45 °C, let it stand and age for 35 h, then completely immerse it in absolute ethanol, let it stand at 35 °C for 60 h, replace the absolute ethanol once every 12 h, and perform supercritical drying in a carbon dioxide environment. Control the temperature during supercritical drying to be 50 °C, the pressure to be 15 MPa, and the time to be 25 h to obtain magnesium oxide aerogel.

[0019] The density of the magnesium oxide aerogel obtained in this example is 0.08 g / cm 3 , the BET specific surface area is 740 m 2 / g, the compressive strength is 0.15 MPa, the thermal conductivity is 0.023 W / (m•K), and the water contact angle is 131°.

[0020] Comparative Example 1 Based on the preparation method of the magnesium oxide aerogel in Example 2, omit the first step of preparing the first reinforcing agent, and use 3.8 g of polyvinyl alcohol 1788 and 11.2 g of magnesium hydroxide with an average particle size of 1 μm to replace the addition of 15 g of the first reinforcing agent in the fifth-step mixing; The remaining operations are the same as those in Example 2.

[0021] The density of the magnesium oxide aerogel obtained in this comparative example is 0.11 g / cm 3 , the BET specific surface area is 517 m 2 / g, the compressive strength is 0.08 MPa, the thermal conductivity is 0.029 W / (m•K), and the water contact angle is 115°.

[0022] Comparative Example 2 Based on the preparation method of the magnesium oxide aerogel in Example 2, omit the second step of preparing the second reinforcing agent, and use a mixture of 1.8 g of cetyltrimethoxysilane and 105 mL of dimethyl sulfoxide to replace the addition of 105 mL of the second reinforcing agent in the fifth-step mixing; The remaining operations are the same as those in Example 2.

[0023] The density of the magnesium oxide aerogel obtained in this comparative example is 0.16 g / cm 3 , the BET specific surface area is 481 m 2 / g, the compressive strength is 0.13 MPa, the thermal conductivity is 0.034 W / (m•K), and the water contact angle is 133°.

[0024] After comparing the density, BET specific surface area, compressive strength, thermal conductivity, and water contact angle of the magnesium oxide aerogels in Examples 1-3 and Comparative Examples 1-2, it was found that the results of the density, BET specific surface area, compressive strength, thermal conductivity, and water contact angle of Comparative Example 1 were worse than those of Example 2, and the results of the density, BET specific surface area, and thermal conductivity of Comparative Example 2 were worse than those of Example 2. This further illustrates that when polyvinyl alcohol is added alone, polyvinyl alcohol will cause a decrease in the mechanical properties of the aerogel. It also shows that when magnesium hydroxide is added alone, due to the poor dispersibility of magnesium hydroxide and the weak binding force with the sol, it will also affect the density, BET specific surface area, thermal conductivity, and water contact angle of the aerogel. It further illustrates that when cetyltrimethylammonium bromide is added alone, cetyltrimethoxysilane will cause phase separation in the aerogel, resulting in a decrease in the BET specific surface area and thermal conductivity of the aerogel and an increase in density.

[0025] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing magnesium oxide aerogel, characterized in that, Including: Preparing a first reinforcing agent, preparing a second reinforcing agent, preparing a magnesium sol, preparing a silica sol, and mixing; For the preparation of the first reinforcing agent, after mixing magnesium chloride hexahydrate and a first portion of deionized water, stirring at room temperature, adding a first portion of polyvinyl alcohol 1788, stirring, adjusting the pH to 11 - 11.5, stirring, centrifuging, discarding the supernatant, and vacuum drying to obtain magnesium hydroxide powder; after mixing a second portion of polyvinyl alcohol 1788 and a second portion of deionized water, stirring at room temperature, adding the magnesium hydroxide powder, stirring, centrifuging, performing 3 - 4 freeze-thaw cycles on the precipitate, and vacuum drying to obtain the first reinforcing agent; For the preparation of the second reinforcing agent, after mixing magnesium chloride hexahydrate, absolute ethanol, and deionized water, stirring at room temperature, adding propylene oxide, stirring, standing, centrifuging, washing the precipitate, and vacuum drying, then mixing with cetyltrimethylammonium bromide and dimethyl sulfoxide and stirring at room temperature to obtain the second reinforcing agent; For the mixing, after mixing the magnesium sol, silica sol, first reinforcing agent, and second reinforcing agent, performing ultrasonic stirring, standing and aging under closed conditions at 40 - 45 °C, completely immersing in absolute ethanol, standing at 30 - 35 °C, and performing supercritical drying in a carbon dioxide environment to obtain magnesium oxide aerogel.

2. The preparation method of the magnesium oxide aerogel according to claim 1, wherein, In the preparation of the first reinforcing agent, the dosage ratio of magnesium chloride hexahydrate, the first portion of deionized water, the first portion of polyvinyl alcohol 1788, the second portion of polyvinyl alcohol 1788, and the second portion of deionized water is 95 - 100 g : 500 - 550 mL : 1.4 - 1.5 g : 9 - 10 g : 500 - 550 mL.

3. The preparation method of the magnesium oxide aerogel according to claim 1, characterized in that, In the preparation of the first reinforcing agent, for the freeze-thaw cycle, the precipitate is placed at -25 °C to -20 °C for 8 - 9 h and at 24 - 27 °C for 4 - 5 h.

4. The preparation method of the magnesium oxide aerogel according to claim 1, characterized in that, In the preparation of the second reinforcing agent, the dosage ratio of magnesium chloride hexahydrate, absolute ethanol, deionized water, propylene oxide, cetyltrimethylammonium bromide, and dimethyl sulfoxide is 90 - 100 g : 550 - 600 mL : 500 - 550 mL : 300 - 320 mL : 1.8 - 2 g : 100 - 110 mL.

5. The preparation method of the magnesium oxide aerogel according to claim 1, characterized in that, For the preparation of the magnesium sol, after mixing magnesium chloride hexahydrate, absolute ethanol, deionized water, and polyvinyl alcohol 2488, stirring at room temperature, adding propylene oxide, and stirring to obtain the magnesium sol.

6. The preparation method of the magnesium oxide aerogel according to claim 5, characterized in that, In the preparation of the magnesium sol, the dosage ratio of magnesium chloride hexahydrate, absolute ethanol, deionized water, polyvinyl alcohol 2488, and propylene oxide is 100 - 110 g : 1800 - 2200 mL : 1000 - 1200 mL : 5 - 6 g : 460 - 500 mL.

7. The preparation method of the magnesium oxide aerogel according to claim 1, characterized in that, For the preparation of the silica sol, after mixing methyltrimethoxysilane, absolute ethanol, and deionized water, stirring at 40 - 45 °C, adjusting the pH to 3 - 3.5, stirring, and then adjusting the pH to 7.5 - 8 and stirring to obtain the silica sol.

8. The preparation method of the magnesium oxide aerogel according to claim 7, wherein, In the preparation of the silica sol, the volume ratio of methyltrimethoxysilane, absolute ethanol, and deionized water is 50 - 52 : 200 - 250 : 20 - 23.

9. The preparation method of the magnesium oxide aerogel according to claim 1, characterized in that, In the said mixing, the dosage ratio of magnesium sol, silica sol, first reinforcing agent, and second reinforcing agent is 2800 - 3000 mL: 240 - 270 mL: 14 - 16 g: 100 - 110 mL.

10. The preparation method of the magnesium oxide aerogel according to claim 1, characterized in that, In the said mixing, the frequency during ultrasonic stirring is 20 - 25 kHz, the stirring speed is 150 - 200 rpm, and the time is 30 - 40 min; The temperature of the said supercritical drying is 50 °C, the pressure is 15 MPa, and the time is 20 - 25 h.