A preparation method of low-cost SiO2 aerogel

Abstract

The present invention relates to a method for preparing a low-cost SiO2 aerogel, which uses a low-cost inorganic silicon source as a raw material and deionized water as a solvent, and obtains a hydrophobic SiO2 aerogel through the sol-gel method in combination with supercritical drying or atmospheric drying, realizing the preparation of an inorganic silicon source SiO2 aerogel without hydrochloric acid and nitric acid catalysis. The material has a three-dimensional nanoporous network structure. Compared with the SiO2 aerogel prepared from an organic silicon source and the traditional atmospheric drying, the preparation period is greatly shortened. The process flow is green and environmentally friendly, and also avoids the use of an organic silicon source, reduces the cost, and improves the industrialization efficiency. The density of the prepared SiO2 aerogel is 43-115 kg / m 3 , the volume shrinkage rate is 2.6%-32.3%, the porosity is 94%-98%, the thermal conductivity is 0.018-0.025 W / (m·K), the water contact angle is 110°-152°, and the specific surface area is 265-736 m 2 / g, and the average pore diameter is 15-60 nm.

Description

Technical Field

[0001] The present invention belongs to the field of preparation processes of nanoporous materials, and relates to a method for preparing low-cost SiO2 aerogel, and particularly to a low-cost SiO2 aerogel prepared from an inorganic silicon source. Background Art

[0002] After years of research and improvement, SiO2 aerogel has now become a high-performance material with wide applications.

[0003] The main raw materials for preparing SiO2 aerogel are divided into organic silicon sources and inorganic silicon sources. Organic silicon sources include functional silanes such as methyl orthosilicate and ethyl orthosilicate. Currently, both in research and industrialization, organic silicon sources are used as raw materials, and CO2 supercritical drying or ethanol supercritical drying is used as the drying method. Inorganic silicon sources refer to silicates, silica sols, etc., which can be dissolved and dispersed in an aqueous solvent system to prepare silica sol. Silicate refers to a class of compounds composed of silicate ions and cations, such as sodium silicate. Inorganic silicon sources are widely available and relatively low in price, and atmospheric pressure drying is used as the drying method. However, the route of combining an inorganic silicon source aqueous system with atmospheric pressure drying has not yet been popularized to the industrialization stage. The main reason is that in the atmospheric pressure drying stage, the modification of functional silanes and the solvent replacement with low surface tension solvents such as n-hexane are inevitable, which results in a long and cumbersome preparation cycle. In addition, there is no advantage in cost reduction, and the cost is even higher than that of the traditional organic silicon source route. This is also the difficulty and great challenge in preparing aerogel from inorganic silicon sources.

[0004] In 2017, Xi'an University of Science and Technology used water glass as the raw material and hydrochloric acid as the catalyst, and prepared silica aerogel powder by sol-gel method, surface modification with hexamethyldisilazane / trimethylchlorosilane and atmospheric pressure drying in an alcohol / water system. The density of this material is 150 kg / m 3 , the porosity is as high as 90%, and the preparation cycle is more than 146 h. In 2018, KU Leuven in Belgium used water glass as the raw material and hydrochloric acid as the catalyst, and prepared SiO2 aerogel powder by sol-gel method, surface modification with n-hexane / trimethylchlorosilane / isopropanol solution and atmospheric pressure drying in an alcohol / water system. The density of this material is 132 kg / m 3 , the thermal conductivity is 0.0254 W / (m·K), and the preparation cycle is more than 45 h. In 2019, Wuhan University of Technology used water glass as the raw material and hydrochloric acid as the catalyst, and prepared SiO2 aerogel powder by sol-gel method, surface modification with hexamethyldisilazane / trimethylchlorosilane and atmospheric pressure drying in an alcohol / water system. The density of this material is 121 kg / m 3, the thermal conductivity is 0.0245 W / (m·K), and the preparation period is more than 50 h. From the above examples, it can be seen that the long preparation period is the main shortcoming. In addition, the catalysts involved in the experiment are controlled products such as hydrochloric acid, which seriously hinders the further promotion of the industrialization of aerogels. Therefore, it is necessary to explore and develop a low-cost and environmentally friendly preparation process for silicon-based aerogels to solve the current problems of high cost and long cycle. Summary of the Invention

[0005] The purpose of the present invention is to solve the limitations in the preparation process of SiO2 aerogels that rely on hydrochloric acid, nitric acid, etc., and at the same time solve the technical problems such as long and cumbersome material preparation cycles in the inorganic silicon source preparation technology of SiO2 aerogels. A low-cost preparation method for SiO2 aerogels is developed, and a green and environmentally friendly process route is formed to obtain a low-cost inorganic silicon source SiO2 aerogel whose adiabatic performance can be comparable to that of SiO2 aerogels prepared from organosilicon sources.

[0006] To solve the above technical problems, the technical solution of the present invention is as follows: Using a low-cost inorganic silicon source as the raw material and deionized water as the solvent, hydrophobic SiO2 aerogel is obtained through the sol-gel method in combination with supercritical drying or atmospheric drying, realizing the preparation of non-hydrochloric acid, nitric acid-catalyzed, inorganic silicon source SiO2 aerogel. This material has a three-dimensional nano-porous network structure. Compared with SiO2 aerogels prepared from organosilicon sources and traditional atmospheric drying, the preparation period is greatly shortened. This process is green and environmentally friendly, and also avoids the use of organosilicon sources, reducing costs and improving industrialization efficiency. The density of the prepared SiO2 aerogel is 43 - 115 kg / m 3 , the volume shrinkage rate is 2.6% - 32.3%, the porosity is 94% - 98%, the thermal conductivity is 0.018 - 0.025 W / (m·K), the water contact angle is 110° - 152°, the specific surface area is 265 - 736 m 2 / g, the average pore diameter is 15 - 60 nm, and it has a uniform three-dimensional network porous structure.

[0007] The specific technical solution of the present invention is as follows: A preparation method for low-cost SiO2 aerogel, and its specific steps are as follows:

[0008] (1) Preparation of SiO2 wet gel

[0009] Measure a certain amount of inorganic silicon source reagent and pour it into a container, add deionized water, and stir evenly to obtain an inorganic silicon source solution; adjust the rotation speed of the magnetic stirrer, and introduce CO2 gas into the above transparent solution at a certain rate. After maintaining the gas introduction for a certain time, stop the gas introduction and let it stand to gel to obtain SiO2 wet gel;

[0010] (2) Preparation of SiO2 aerogel

[0011] (a) Add absolute ethanol to the prepared SiO2 wet gel obtained above, and perform solvent replacement at a certain temperature; after completion, take out the SiO2 wet gel and perform supercritical drying with ethanol to obtain low-cost SiO2 aerogel;

[0012] (b) Break the prepared SiO2 wet gel into small pieces, add acidic aging solution thereto, and perform solvent replacement for a certain period of time; after this step is completed, take out the SiO2 wet gel, then add absolute ethanol thereto and perform solvent replacement for a certain period of time, and then dry to obtain low-cost SiO2 aerogel.

[0013] Preferably, the inorganic silicon source reagent described in step (1) is one of sodium silicate, potassium silicate, silica sol, sodium methyl silicate or potassium methyl silicate; among them, sodium silicate, potassium silicate, sodium methyl silicate and potassium methyl silicate are raw materials of analytical purity.

[0014] Preferably, the molar concentration of the inorganic silicon source solution described in step (1) is 0.7 - 1.5 mol / L.

[0015] Preferably, adjust the rotation speed of the magnetic stirrer in step (1) to 100 - 300 r / min.

[0016] Preferably, the method of introducing CO2 gas in step (1) is to directly insert a rubber tube into the bottom of the transparent solution, the rate is 50 - 300 mL / min, keep the ventilation time for 25 - 90 min, and immediately remove the rubber tube after the ventilation is completed.

[0017] Preferably, the replacement time of the absolute ethanol described in step (a) is 10 - 17 h, and the temperature is 60 - 80 °C.

[0018] Preferably, the SiO2 wet gel described in step (b) is broken into small pieces, and generally the small pieces are irregular three-dimensional blocks, and the largest size in the three-dimensional blocks is 0.5 - 1 cm.

[0019] Preferably, the acidic aging solution described in step (b) is a mixed solution of one of acetic acid, propionic acid or trimethylchlorosilane and absolute ethanol; among them, the volume fraction of acetic acid, propionic acid or trimethylchlorosilane in the mixed solution is 3.8% - 6.3%; the replacement time is 24 - 36 h.

[0020] Preferably, the replacement time of the absolute ethanol described in step (b) is 24 - 48 h.

[0021] Preferably, the drying regime in step (b) is: first dry at 50 - 70 °C for 15 - 20 h, and then dry at 100 - 120 °C for 6 - 11 h.

[0022] Beneficial effects:

[0023] (1) Compared with the current methods for preparing SiO2 aerogel using organosilicon sources and inorganic silicon sources, the present invention prepares SiO2 aerogel by catalyzing with CO2 gas, and the gel time and gel network structure can be regulated by controlling the flow rate of CO2 gas. The preparation method described in the present invention avoids the use of controlled products such as hydrochloric acid and nitric acid, and has the properties of low cost and environmental friendliness.

[0024] (2) Compared with the process of preparing SiO2 aerogel by the traditional atmospheric pressure drying method, the preparation method described in the present invention strengthens and stabilizes the pore structure of the wet gel with CO2, rather than through traditional functional silane chemical grafting modification and low surface tension organic solvent replacement. Therefore, the large amount of organic solvents is avoided, and industrialization is expected to be realized. And CO2 is generated by the reaction of sodium carbonate or sodium bicarbonate generated during the gelation process with the acidic aging solution. Sodium carbonate or sodium bicarbonate generated during the gelation process is evenly dispersed in the gel body, which also makes the pore structure of the wet gel uniformly strengthened and stabilized by CO2. Description of the Drawings

[0025] Figure 1 Macrophotograph of the SiO2 aerogel prepared in Example 1;

[0026] Figure 2 Pore size distribution diagram of the SiO2 aerogel prepared in Example 1;

[0027] Figure 3 Hydrophobic angle test diagram of the SiO2 aerogel prepared in Example 1;

[0028] Figure 4 Microscopic morphology diagram of the SiO2 aerogel prepared in Example 1. Detailed Embodiments

[0029] The present invention will be further described below in conjunction with examples, but the scope of protection is not limited thereto.

[0030] Example 1

[0031] Measure 0.0365 mL of sodium silicate and pour it into a container, add 50 mL of deionized water, and stir until the sodium silicate is dissolved to obtain an inorganic silicon source solution with a concentration of 0.73 mol / L. Adjust the rotation speed of the magnetic stirrer to 100 r / min. While continuously stirring, insert a rubber tube into the bottom of the transparent solution, and at the same time adjust the rate of CO2 gas introduced to 50 mL / min, keep the ventilation time for 90 min, and let it stand for gelation to obtain SiO2 wet gel.

[0032] Add absolute ethanol to the as-prepared SiO2 wet gel and perform solvent replacement at 60 °C for 17 h. After this step is completed, take out the SiO2 wet gel and perform ethanol supercritical drying to obtain low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 44 kg / m 3 , the volume shrinkage rate is 16.7%, the porosity is 94.6%, the thermal conductivity is 0.0185 W / (m·K), the water contact angle is 142°, the specific surface area is 312 m 2 / g, and the average pore diameter is 36 nm.

[0033] In addition, the as-prepared SiO2 wet gel can be broken into small blocks with a size of 0.5 cm, and a mixed solution of 200 ml of trimethylchlorosilane and absolute ethanol with a volume fraction of 3.8% is added thereto for solvent replacement for 24 h. After this step is completed, take out the SiO2 wet gel, add absolute ethanol for solvent replacement for 24 h, then dry at 50 °C for 20 h and at 100 °C for 11 h to obtain low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 95 kg / m 3 , the volume shrinkage rate is 32.3%, the porosity is 94%, the thermal conductivity is 0.025 W / (m·K), the water contact angle is 152°, the specific surface area is 736 m 2 / g, and the average pore diameter is 40 nm.

[0034] The macroscopic sample diagram, pore size distribution diagram, contact angle test diagram, and microscopic structure diagram of the hydrophobic inorganic silicon-based SiO2 aerogel prepared by atmospheric pressure drying are respectively as Figure 1 , Figure 2 , Figure 3 , Figure 4 shown. It can be intuitively seen from Figure 1 that the material has the unique "blue smoke" effect of SiO2 aerogel, indicating that SiO2 aerogel has been successfully prepared; Figure 2 It can be obtained from that the pore size distribution range of the SiO2 aerogel prepared by this method is wide, but mainly mesopores, and the pore size is mainly distributed around 40 nm, with a uniform pore structure, which also ensures its low thermal conductivity and low density properties; Figure 3 verifies that the aerogel has superhydrophobic properties; Figure 4 proves that the aerogel has a uniform three-dimensional network porous structure.

[0035] Example 2

[0036] Measure 0.0874 mol of potassium silicate and pour it into a container. Add 95 mL of deionized water and stir until the potassium silicate dissolves to obtain an inorganic silicon source solution with a concentration of 0.92 mol / L. Adjust the rotation speed of the magnetic stirrer to 200 r / min. While continuously stirring, insert a rubber tube to the bottom of the transparent solution, and at the same time adjust the rate of CO₂ gas introduction to 100 mL / min. Keep the gas introduction time for 75 min, then let it stand to gel to obtain a SiO₂ wet gel.

[0037] Add absolute ethanol to the above-prepared SiO₂ wet gel and perform solvent replacement at 70 °C for 10 h. After this step is completed, take out the SiO₂ wet gel and perform ethanol supercritical drying to obtain a low-cost SiO₂ aerogel. The density of the SiO₂ aerogel prepared by this process is 56 kg / m 3 , the volume shrinkage rate is 13.3%, the porosity is 95.8%, the thermal conductivity is 0.018 W / (m·K), the water contact angle is 138°, the specific surface area is 285 m 2 / g, the average pore diameter is 21 nm, and it has a uniform three-dimensional network mesoporous structure.

[0038] In addition, the above-prepared SiO₂ wet gel can be broken into small pieces with a size of 0.7 cm, and a mixed solution of 200 mL of trimethylchlorosilane and absolute ethanol with a volume fraction of 4.3% is added to it for solvent replacement for 28 h. After this step is completed, take out the SiO₂ wet gel, then add absolute ethanol for solvent replacement for 30 h, and then dry it at 60 °C for 17.5 h and at 110 °C for 6.5 h to obtain a low-cost SiO₂ aerogel. The density of the SiO₂ aerogel prepared by this process is 102 kg / m 3 , the volume shrinkage rate is 28.6%, the porosity is 95.7%, the thermal conductivity is 0.0242 W / (m·K), the water contact angle is 150°, the specific surface area is 696 m 2 / g, the average pore diameter is 34 nm, and it has a uniform three-dimensional network mesoporous structure.

[0039] Example 3

[0040] Measure 0.23 mol of sodium methyl silicate and pour it into a container. Add 200 mL of deionized water and stir until the sodium methyl silicate dissolves to obtain an inorganic silicon source solution with a concentration of 1.15 mol / L. Adjust the rotation speed of the magnetic stirrer to 300 r / min. While continuously stirring, insert a rubber tube to the bottom of the transparent solution, and at the same time adjust the rate of CO₂ gas introduction to 200 mL / min. Keep the gas introduction time for 45 min, then let it stand to gel to obtain a SiO₂ wet gel.

[0041] Add absolute ethanol to the as-prepared SiO2 wet gel and conduct solvent replacement at 80 °C for 12.5 h. After this step is completed, take out the SiO2 wet gel and conduct ethanol supercritical drying to obtain low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 69 kg / m 3 , the volume shrinkage rate is 7.72%, the porosity is 96.1%, the thermal conductivity is 0.018 W / (m·K), the water contact angle is 127°, the specific surface area is 366.8 m 2 / g, the average pore diameter is 47 nm, and it has a uniform three-dimensional network mesoporous structure.

[0042] In addition, the as-prepared SiO2 wet gel can be broken into small pieces with a size of 1 cm, and a mixed solution of 300 mL of propionic acid and absolute ethanol with a volume fraction of 4.9% is added thereto for solvent replacement for 30 h. After this step is completed, take out the SiO2 wet gel, add absolute ethanol thereto for solvent replacement for 36 h, then dry at 70 °C for 15 h and dry at 120 °C for 8.9 h to obtain low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 108 kg / m 3 , the volume shrinkage rate is 24.7%, the porosity is 96.7%, the thermal conductivity is 0.0235 W / (m·K), the water contact angle is 112°, the specific surface area is 533.8 m 2 / g, the average pore diameter is 27 nm, and it has a uniform three-dimensional network mesoporous structure.

[0043] Example 4

[0044] Measure 0.3168 mol of potassium methyl silicate and pour it into a container, add 240 mL of deionized water, stir until the potassium methyl silicate dissolves to obtain an inorganic silicon source solution with a concentration of 1.32 mol / L. Adjust the rotation speed of the magnetic stirrer to 150 r / min. While continuously stirring, insert a rubber tube into the bottom of the transparent solution, and at the same time adjust the rate of introducing CO2 gas to 300 mL / min, keep the ventilation time for 25 min, and let it stand to gel to obtain SiO2 wet gel.

[0045] Add absolute ethanol to the as-prepared SiO2 wet gel and conduct solvent replacement at 65 °C for 14 h. After this step is completed, take out the SiO2 wet gel and conduct ethanol supercritical drying to obtain low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 75.6 kg / m 3 , the volume shrinkage rate is 5.3%, the porosity is 97%, the thermal conductivity is 0.0193 W / (m·K), the water contact angle is 146.2°, the specific surface area is 658.7 m 2 / g, the average pore diameter is 23 nm, and it has a uniform three-dimensional network mesoporous structure.

[0046] In addition, the SiO2 wet gel prepared above can be broken into small pieces with a size of 0.8 cm, and 400 ml of a mixed solution of acetic acid and absolute ethanol with a volume fraction of 5.4% is added thereto, and solvent replacement is carried out for 35 h; after this step is completed, the SiO2 wet gel is taken out, and then absolute ethanol is added thereto for solvent replacement for 42 h, and then dried at 65 °C for 18 h and at 105 °C for 10 h, and a low-cost SiO2 aerogel is obtained. The density of the SiO2 aerogel prepared by this process is 112.6 kg / m 3 , the volume shrinkage rate is 20.3%, the porosity is 95.4%, the thermal conductivity is 0.022 W / (m·K), the water contact angle is 120.8°, the specific surface area is 493.7 m 2 / g, the average pore diameter is 15 nm, and it has a uniform three-dimensional network mesoporous structure.

[0047] Example 5

[0048] Measure 0.735 mol of potassium methyl silicate and pour it into a container, add 500 mL of deionized water, stir until the potassium methyl silicate is dissolved, and obtain an inorganic silicon source solution with a concentration of 1.47 mol / L. Adjust the rotation speed of the magnetic stirrer to 300 r / min. While continuously stirring, insert a rubber tube into the bottom of the transparent solution, and at the same time adjust the rate of introducing CO2 gas to 300 mL / min, keep the ventilation time for 35 min, and let it stand to gel to obtain a SiO2 wet gel.

[0049] Add absolute ethanol to the SiO2 wet gel prepared above, and carry out solvent replacement at 75 °C for 15.3 h; after this step is completed, take out the SiO2 wet gel and carry out ethanol supercritical drying to obtain a low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 84.3 kg / m 3 , the volume shrinkage rate is 4.19%, the porosity is 95.2%, the thermal conductivity is 0.020 W / (m·K), the water contact angle is 132°, the specific surface area is 609.6 m 2 / g, the average pore diameter is 18 nm, and it has a uniform three-dimensional network mesoporous structure.

[0050] In addition, the SiO2 wet gel prepared above can be broken into small pieces with a size of 0.9 cm, and 400 mL of a mixed solution of propionic acid and absolute ethanol with a volume fraction of 6.3% is added thereto, and solvent replacement is carried out for 36 h; after this step is completed, the SiO2 wet gel is taken out, and then absolute ethanol is added thereto for solvent replacement for 48 h, and then dried at 70 °C for 19 h and at 115 °C for 6 h, and a low-cost SiO2 aerogel is obtained. The density of the SiO2 aerogel prepared by this process is 115 kg / m 3, the volume shrinkage rate is 12.5%, the porosity is 97%, the thermal conductivity is 0.024 W / (m·K), the water contact angle is 119°, the specific surface area is 273 m 2 / g, the average pore size is 54 nm, and it has a uniform three-dimensional network porous structure.

[0051] Example 6

[0052] Measure 300.45 g of silica sol (SiO2 mass fraction is 30%, 1.5 mol of silicon source) and pour it into a container. Add 1000 mL of deionized water and stir until evenly mixed to obtain a 1.5 mol / L inorganic silicon source solution. Adjust the rotation speed of the magnetic stirrer to 300 r / min. While continuously stirring, insert a rubber tube into the bottom of the transparent solution, and at the same time adjust the rate of CO2 gas introduced to 300 mL / min. Keep the ventilation time for 85 min and let it stand to gel to obtain SiO2 wet gel.

[0053] Add absolute ethanol to the above-prepared SiO2 wet gel and perform solvent replacement at 65°C for 16 h; after this step is completed, take out the SiO2 wet gel and perform ethanol supercritical drying to obtain low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 97.5 kg / m 3 , the volume shrinkage rate is 10.3%, the porosity is 97%, the thermal conductivity is 0.025 W / (m·K), the water contact angle is 110°, the specific surface area is 699.8 m 2 / g, the average pore size is 20.3 nm, and it has a uniform three-dimensional network mesoporous structure.

[0054] In addition, the above-prepared SiO2 wet gel can be broken into small pieces with a size of 1 cm, and 1000 mL of a mixed solution of trimethylchlorosilane and absolute ethanol with a volume fraction of 5.0% is added thereto for solvent replacement for 36 h; after this step is completed, take out the SiO2 wet gel, and then add absolute ethanol for solvent replacement for 48 h, and then dry it at 65°C for 16.2 h and at 120°C for 7.5 h to obtain low-cost SiO2 aerogel. The density of the SiO2 aerogel prepared by this process is 115 kg / m 3 , the volume shrinkage rate is 22.6%, the porosity is 94%, the thermal conductivity is 0.024 W / (m·K), the water contact angle is 126.7°, the specific surface area is 265 m 2 / g, the average pore size is 60 nm, and it has a uniform three-dimensional network porous structure.

Claims

1. A preparation method of low-cost SiO2 aerogel, and the specific steps are as follows: (1) Preparation of SiO2 wet gel Measure a certain amount of inorganic silicon source reagent and pour it into a container, add deionized water, and stir evenly to obtain an inorganic silicon source solution; adjust the rotation speed of the magnetic stirrer, and introduce CO2 gas into the inorganic silicon source solution at a certain rate. After maintaining the gas introduction for a certain time, stop the gas introduction and let it stand for gelation to obtain SiO2 wet gel; wherein the rotation speed of the magnetic stirrer is adjusted to 100 - 300 r / min; the rate of introducing CO2 gas is 50 - 300 mL / min, and the gas introduction time is maintained for 25 - 90 min; (2) Preparation of SiO2 aerogel (a) Add anhydrous ethanol to the above-prepared SiO2 wet gel and perform solvent replacement at a certain temperature; after completion, take out the SiO2 wet gel and perform ethanol supercritical drying to obtain low-cost SiO2 aerogel; (b) Break the above-prepared SiO2 wet gel into small pieces, add an acidic aging solution thereto, and perform solvent replacement for a certain time; after this step is completed, take out the SiO2 wet gel, then add anhydrous ethanol thereto and perform solvent replacement for a certain time, and then dry to obtain low-cost SiO2 aerogel; wherein the acidic aging solution is a mixed solution of one of acetic acid, propionic acid or trimethylchlorosilane and anhydrous ethanol, and the volume fraction of acetic acid, propionic acid or trimethylchlorosilane in the mixed solution is 3.8% - 6.3%; the replacement time is 24 - 36 h.

2. The preparation method according to claim 1, wherein The inorganic silicon source reagent described in step (1) is one of sodium silicate, potassium silicate, silica sol, sodium methyl silicate or potassium methyl silicate.

3. The preparation method according to claim 1, characterized in that The molar concentration of the inorganic silicon source solution described in step (1) is 0.7 - 1.5 mol / L.

4. The preparation method according to claim 1, characterized in that The time for anhydrous ethanol replacement described in step (a) is 10 - 17 h, and the temperature is 60 - 80 °C.

5. The preparation method according to claim 1, characterized in that The anhydrous ethanol replacement time described in step (b) is 24 - 48 h.

6. The preparation method according to claim 1, characterized in that The drying regime in step (b) is: first dry at 50 - 70 °C for 15 - 20 h, and then dry at 100 - 120 °C for 6 - 11 h.

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