Composite cellulose aerogel as well as preparation method and application thereof

Abstract

The invention provides composite cellulose aerogel as well as a preparation method and application thereof, and belongs to the technical field of aerogel. The preparation method comprises the following steps: mixing nanocellulose, a silane coupling agent and water, and carrying out modification treatment to obtain modified nanocellulose feed liquid; the method comprises the following steps: mixing an organic aluminum source with water, adjusting the pH value of the obtained organic aluminum source feed liquid to 3-4, and carrying out hydrolysis reaction to obtain alumina sol; mixing the modified nanocellulose feed liquid with alumina sol, and performing one-way directional freezing on the obtained mixed feed liquid to obtain a frozen molded body; wherein the mass ratio of the nano-crystalline cellulose used for preparing the modified nano-crystalline cellulose feed liquid to the organic aluminum source used for preparing the aluminum oxide sol is (1-3): 1; and carrying out freeze drying on the freeze-formed body to obtain the composite cellulose aerogel. The composite cellulose aerogel prepared by the method disclosed by the invention is hydrophobic flame-retardant aerogel and has a wide application range.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogels, and in particular to a composite cellulose aerogel and a preparation method and application thereof. Background Art

[0002] As the lightest solid material, aerogel has attracted wide attention for its excellent properties such as extremely low thermal conductivity and low density. Silica aerogel and its composite aerogel have begun to be used in the field of building insulation. Although the cost is gradually decreasing, the related technology usually uses supercritical drying technology to prepare aerogel, so its production cost is still relatively expensive.

[0003] Cellulose is the most abundant renewable polymer in the world. Cellulose is composed of repeated anhydrous glucose units in its molecular structure. Cellulose aerogel is prepared using cellulose as the raw material. The drying method is mainly freeze-drying, which has the advantages of low cost and simple operation compared with supercritical drying. The mechanical properties of cellulose aerogel are higher than those of pure silica aerogel. At the same time, cellulose aerogel has the characteristics of low density and high porosity, and can be used directly as a thermal insulation material. However, cellulose aerogel in related technologies has problems of water absorption and flammability, which limits its scope of application. Summary of the invention

[0004] The object of the present invention is to provide a composite cellulose aerogel and a preparation method and application thereof. The composite cellulose aerogel prepared by the method of the present invention is a hydrophobic, flame-retardant gel and has a wide range of applications.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions: The present invention provides a method for preparing a composite cellulose aerogel, comprising the following steps: The nanocellulose, the silane coupling agent and water are mixed and modified to obtain a modified nanocellulose liquid; Mixing an organic aluminum source with water, adjusting the pH value of the obtained organic aluminum source solution to 3-4, and performing a hydrolysis reaction to obtain an aluminum oxide sol; The modified nanocellulose slurry is mixed with an alumina sol, and the obtained mixed slurry is subjected to unidirectional freezing to obtain a frozen molded body; wherein the ratio of the mass of the nanocellulose used to prepare the modified nanocellulose slurry to the mass of the organic aluminum source used to prepare the alumina sol is 1-3:1; The frozen molded body is freeze-dried to obtain the composite cellulose aerogel.

[0006] Preferably, the nanocellulose includes one or more of bacterial cellulose, carboxylated cellulose and cellulose nanocrystals; and the concentration of the nanocellulose in the system during the modification treatment is 0.3-4wt%.

[0007] Preferably, the silane coupling agent includes one or more of methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, ethoxytrimethylsilane, diethoxydimethylsilane and triethoxymethylsilane; and the mass ratio of the nanocellulose to the silane coupling agent is 1:1-2.

[0008] Preferably, the temperature of the modification treatment is 15-35° C., and the time is 0.5-2 h; and the modification treatment is carried out under ultrasonic conditions.

[0009] Preferably, the organic aluminum source comprises aluminum sec-butoxide and / or aluminum isopropoxide; and the molar ratio of the organic aluminum source to water in the organic aluminum source solution is 1:30-120.

[0010] Preferably, the hydrolysis reaction temperature is 85-95° C. and the time is 6-9 hours.

[0011] Preferably, the unidirectional directional freezing comprises: placing the mixed liquid in a mold, immersing the bottom of the mold containing the mixed liquid in liquid nitrogen for 15 to 60 minutes; the thermal conductivity of the bottom material of the mold is higher than the thermal conductivity of the surrounding material of the mold.

[0012] Preferably, the bottom of the mold is made of copper, and the surroundings of the mold are made of polytetrafluoroethylene.

[0013] The present invention provides a composite cellulose aerogel prepared by the preparation method described in the above technical solution, which has a pore structure, a pore diameter of 10-100 μm, a pore wall thickness of 0.9-1.1 μm, and a porosity of ≥97%.

[0014] The present invention provides the use of the composite cellulose aerogel described in the above technical solution as a flame retardant and heat-insulating material.

[0015] The invention provides a method for preparing a composite cellulose aerogel, comprising the following steps: mixing nanocellulose, a silane coupling agent and water, performing a modification treatment, and obtaining a modified nanocellulose slurry; mixing an organic aluminum source with water, adjusting the pH value of the obtained organic aluminum source slurry to 3-4, performing a hydrolysis reaction, and obtaining an alumina sol; mixing the modified nanocellulose slurry with the alumina sol, and performing unidirectional directional freezing on the obtained mixed slurry to obtain a frozen molded body; wherein the ratio of the mass of the nanocellulose used to prepare the modified nanocellulose slurry to the mass of the organic aluminum source used to prepare the alumina sol is 1-3:1; and freeze-drying the frozen molded body to obtain the composite cellulose aerogel. The present invention adopts nanocellulose, a biomass raw material, as the structural framework of the aerogel, adopts a silane coupling agent and the nanocellulose for chemical cross-linking to improve the mechanical properties and hydrophobic properties of the aerogel, adopts an organic aluminum source as a flame retardant, and then prepares a composite cellulose aerogel based on unidirectional directional freezing and freeze-drying technology. The composite cellulose aerogel has good hydrophobicity and flame retardancy, high elasticity and low thermal conductivity, and can be used as a flame retardant thermal insulation material in the field of building insulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a photo of the appearance of the cellulose aerogel prepared in Example 1; Figure 2 This is a microscopic electron microscope image of the cellulose aerogel prepared in Example 1; Figure 3 : The stress-strain curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3; Figure 4 This is a graph showing the hydrophobic angle test results of the cellulose aerogel prepared in Example 1; Figure 5 Thermogravimetric curves of cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3; Figure 6 The thermogravimetric derivative curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1-3. DETAILED DESCRIPTION

[0017] The present invention provides a method for preparing a composite cellulose aerogel, comprising the following steps: The nanocellulose, the silane coupling agent and water are mixed and modified to obtain a modified nanocellulose liquid; Mixing an organic aluminum source with water, adjusting the pH value of the obtained organic aluminum source solution to 3-4, and performing a hydrolysis reaction to obtain an aluminum oxide sol; The modified nanocellulose slurry is mixed with an alumina sol, and the obtained mixed slurry is subjected to unidirectional freezing to obtain a frozen molded body; wherein the ratio of the mass of the nanocellulose used to prepare the modified nanocellulose slurry to the mass of the organic aluminum source used to prepare the alumina sol is 1-3:1; The frozen molded body is freeze-dried to obtain the composite cellulose aerogel.

[0018] In the present invention, unless otherwise specified, the raw materials used are commercially available products well known to those skilled in the art or are prepared by methods well known to those skilled in the art.

[0019] The present invention mixes nanocellulose, a silane coupling agent and water, performs a modification treatment, and obtains a modified nanocellulose liquid. As an embodiment of the present invention, the nanocellulose may include one or more of bacterial cellulose, carboxylated cellulose and cellulose nanocrystals, specifically bacterial cellulose, carboxylated cellulose or cellulose nanocrystals; when the modification treatment is performed, the concentration of the nanocellulose in the system may be 0.3-4wt%, specifically 0.3wt%, 0.5wt%, 0.8wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt% or 4wt%. As an embodiment of the present invention, the silane coupling agent may include one or more of methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, ethoxytrimethylsilane, diethoxydimethylsilane and triethoxymethylsilane, specifically methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, ethoxytrimethylsilane, diethoxydimethylsilane or triethoxymethylsilane; the mass ratio of the nanocellulose to the silane coupling agent may be 1:1~2, specifically 1:1, 1:1.3, 1:1.5, 1:1.8 or 1:2.

[0020] As an embodiment of the present invention, the nanocellulose and water can be first stirred and mixed, and then a silane coupling agent is added to the obtained suspension for second stirring and mixing, and then the modification treatment is performed. As an embodiment of the present invention, the temperature of the first stirring and mixing can be 15~35℃, specifically room temperature, and the time of the first stirring and mixing can be 0.5~2h, specifically 0.5h, 1h, 1.5h or 2h; the temperature of the second stirring and mixing can be 15~35℃, specifically room temperature, and the time of the second stirring and mixing can be 1~5h, specifically 1h, 2h, 3h, 4h or 5h; the present invention does not specifically limit the speed of the stirring. As an embodiment of the present invention, the modification treatment can be carried out under ultrasonic conditions, the temperature of the modification treatment can be 15~35℃, specifically room temperature; the time can be 0.5~2h, specifically 0.5h, 1h, 1.5h or 2h; the ultrasonic power can be 180~220W, specifically 180W, 190W, 200W, 210W or 220W. In the embodiment of the present invention, the modification treatment is carried out under the above conditions, so that the silane coupling agent can be hydrolyzed to generate hydroxyl groups, and the hydroxyl groups generated by the hydrolysis of the silane coupling agent undergo a dehydration condensation reaction with the hydroxyl groups on the nanocellulose, thereby modifying the nanocellulose.

[0021] The present invention mixes an organic aluminum source with water, adjusts the pH value of the obtained organic aluminum source liquid to 3-4, performs a hydrolysis reaction, and obtains an alumina sol. As an embodiment of the present invention, the organic aluminum source may include aluminum sec-butoxide and / or aluminum isopropoxide, specifically aluminum sec-butoxide or aluminum isopropoxide; the molar ratio of the organic aluminum source to water in the organic aluminum source liquid may be 1:30-120, specifically 1:30, 1:35, 1:40, 1:45, 1:50, 1:60, 1:80, 1:100 or 1:120.

[0022] As an embodiment of the present invention, the temperature of the organic aluminum source and water can be 85-95°C, specifically 85°C, 88°C, 90°C, 92°C or 95°C; the time can be 1-2h, specifically 1h, 1.5h or 2h; the mixing of the organic aluminum source and water can be carried out under stirring. As an embodiment of the present invention, the reagent used to adjust the pH value of the organic aluminum source solution can be an organic acid reagent, and the organic acid reagent can be acetic acid; the present invention adjusts the pH value of the organic aluminum source solution to 3-4, specifically 3.5. The present invention adjusts the pH value of the organic aluminum source solution to the above range, which is conducive to the hydrolysis reaction of the organic aluminum source under acidic conditions. As an embodiment of the present invention, the temperature of the hydrolysis reaction can be 85-95°C, specifically 85°C, 88°C, 90°C, 92°C or 95°C; the time can be 6-9h, specifically 6h, 6.5h, 7h, 7.5h, 8h, 8.5h or 9h; the hydrolysis reaction can be carried out under stirring. In the present invention, during the hydrolysis reaction, an organic aluminum source such as aluminum sec-butoxide is hydrolyzed to generate pseudo-boehmite, and the pseudo-boehmite is dehydrated to form an alumina sol.

[0023] After obtaining the modified nanocellulose liquid and the alumina sol, the present invention mixes the modified nanocellulose liquid with the alumina sol, and performs unidirectional directional freezing on the obtained mixed liquid to obtain a frozen molded body. In the present invention, the ratio of the mass of nanocellulose used to prepare the modified nanocellulose liquid to the mass of the organic aluminum source used to prepare the alumina sol is 1 to 3:1, specifically 1:1, 1.5:1, 2:1, 2.5:1 or 3:1. The amount of the modified nanocellulose liquid and the alumina sol used in the present invention is to ensure that the mass ratio of the nanocellulose to the organic aluminum source meets the above requirements. As an embodiment of the present invention, the mixing can be carried out under stirring conditions, and the mixing time can be 1 to 3 hours, specifically 2 hours; during the mixing process, the incompletely reacted silane coupling agent and the incompletely reacted pseudo-boehmite will further undergo a dehydration condensation reaction. As an embodiment of the present invention, the unidirectional directional freezing may include: placing the mixed liquid in a mold, immersing the bottom of the mold containing the mixed liquid in liquid nitrogen (temperature of -196°C) for 15 to 60 minutes, and further for 30 to 45 minutes; the thermal conductivity of the material of the bottom of the mold is higher than the thermal conductivity of the material around the mold, specifically, the material of the bottom of the mold may be copper, and the material around the mold may be polytetrafluoroethylene; the size of the mold may be 5cm×5cm×5cm. In an embodiment of the present invention, the material of the bottom of the mold is copper with high thermal conductivity, and the material around the mold is polytetrafluoroethylene with low thermal conductivity. Therefore, when the bottom of the mold is immersed in liquid nitrogen, the heat in the mixed liquid will be transferred from top to bottom, and the water in the mixed liquid closer to the bottom of the mold will preferentially form ice crystals. The temperature gradient difference from top to bottom will cause the ice crystals to grow vertically from bottom to top, so the ice crystals will form a vertical directional structure after growth, and the aerogel obtained after subsequent freeze drying, that is, after the ice crystals are eliminated, will also retain this directional structure. Therefore, the present invention will not affect the chemical composition of the mixed liquid or the internal pore distribution of the aerogel through the unidirectional directional freezing method, so that the aerogel presents anisotropy.

[0024] After obtaining the frozen molded body, the present invention freeze-dries the frozen molded body to obtain the composite cellulose aerogel. As one embodiment of the present invention, the freeze-drying temperature can be -60~-90°C, specifically -60°C, -70°C, -80°C or -90°C; the time can be 48~72h, and further can be 60~72h. The present invention removes moisture from the frozen molded body by freeze-drying. As one embodiment of the present invention, the freeze-drying can also include drying, and the drying temperature can be 55~65°C, specifically 60°C; the time can be 2.5~3.5h, specifically 3h; in the embodiment of the present invention, drying is beneficial to ensure complete removal of moisture.

[0025] The present invention provides a composite cellulose aerogel prepared by the preparation method described in the above technical solution. The composite cellulose aerogel provided by the present invention has good hydrophobicity and flame retardancy while also having high elasticity and low thermal conductivity. Specifically, the composite cellulose aerogel provided by the present invention has the characteristics of low density and high porosity. For example, the composite cellulose aerogel has a rich micron-scale pore structure (pore diameter of 10-100 μm, pore wall thickness of 0.9-1.1 μm), and a porosity of ≥97%, which hinders solid-phase heat transfer and gas-phase heat transfer inside the material, which makes the composite cellulose aerogel have a lower thermal conductivity (radial thermal conductivity of 0.0224 W / mK) and excellent thermal insulation performance. The constituent unit of nanocellulose such as bacterial cellulose is dehydrated glucose, which contains three active hydroxyl groups, making bacterial cellulose itself water-absorbent. By adding a silane coupling agent such as trimethylmethoxysilane to chemically crosslink with bacterial cellulose, the methoxy group in trimethylmethoxysilane undergoes a condensation reaction with the hydroxyl group on bacterial cellulose through hydrolysis, reducing the exposed hydroxyl groups in bacterial cellulose. At the same time, trimethylmethoxysilane contains a hydrophobic group methyl. The reduction of hydroxyl groups and the addition of methyl groups make the composite cellulose aerogel hydrophobic (the hydrophobic angle is 138~145°). Moreover, after the organic aluminum source such as aluminum sec-butylate undergoes hydrolysis and condensation reaction, the aluminum element is evenly distributed inside and on the surface of the aerogel, and it will form a flame retardant layer when burning, making the composite cellulose aerogel flame retardant, and it will not affect the microstructure of the aerogel, so the composite cellulose aerogel still has good mechanical properties. In addition, nanocellulose, a biomass raw material, is used as the structural framework of the aerogel, and abundant chemical cross-links are generated between the nanocellulose and the silane coupling agent. The aerogel microstructure is arranged in an orderly manner by using unidirectional directional freezing and freeze-drying methods. This makes the composite cellulose aerogel prepared by the present invention have high elasticity, and can still maintain 90-95% of the original height after 50 compressions under 80% strain.

[0026] The present invention provides the application of the composite cellulose aerogel described in the above technical solution as a flame retardant thermal insulation material. The present invention has no special limitation on the specific application of the composite cellulose aerogel, and any method familiar to those skilled in the art can be used.

[0027] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Example 1 0.3 g of bacterial cellulose was mixed with 60 g of deionized water, and stirred at room temperature for 1 h to obtain a bacterial cellulose suspension; 0.5 g of methyltrimethoxysilane was added to the bacterial cellulose suspension, and stirred at room temperature for 3 h, and then ultrasonicated at an ultrasonic power of 200 W for 1 h to obtain a modified bacterial cellulose liquid; 0.15 g of aluminum sec-butoxide was mixed with 1 g of deionized water, and the mixture was stirred at 90° C. for 1 h. The pH of the obtained aluminum sec-butoxide solution was adjusted to 3.5 with acetic acid, and the mixture was stirred at 90° C. for 6 h to obtain an alumina sol. The alumina sol is added to the modified bacterial cellulose liquid, and stirred at room temperature for 1 hour to obtain a mixed liquid; the mixed liquid is poured into a mold, and frozen in liquid nitrogen at -196°C for 0.5 hour, and then the mold is transferred to a freeze dryer, freeze-dried at -80°C for 72 hours, and then placed in an oven at 60°C for 3 hours to completely remove moisture, thereby obtaining a composite bacterial cellulose aerogel.

[0029] Example 2 The method of Example 1 was followed, except that the amount of bacterial cellulose used in this example was 0.6 g, the amount of methyltrimethoxysilane used was 1 g, the amount of aluminum sec-butoxide used was 0.3 g, and the mass of deionized water used to prepare the aluminum sec-butoxide solution was 2 g.

[0030] Comparative Example 1 The method of Example 1 was followed, except that methyltrimethoxysilane and alumina sol were omitted in this comparative example, as follows: 0.3 g of bacterial cellulose was mixed with 60 g of deionized water, and stirred at room temperature for 1 hour to obtain a bacterial cellulose suspension; the bacterial cellulose suspension was poured into a mold, and then operated according to the method in Example 1 to finally obtain a bacterial cellulose aerogel.

[0031] Comparative Example 2 The method of Example 1 was followed, except that methyltrimethoxysilane was omitted in this comparative example, as follows: 0.3 g of bacterial cellulose was mixed with 60 g of deionized water and stirred at room temperature for 1 h to obtain a bacterial cellulose suspension; 0.15 g of aluminum sec-butoxide was mixed with 1 g of deionized water, and the mixture was stirred at 90° C. for 1 h. The pH of the obtained aluminum sec-butoxide solution was adjusted to 3.5 with acetic acid, and the mixture was stirred at 90° C. for 6 h to obtain an alumina sol. The alumina sol was added to the bacterial cellulose suspension and stirred at room temperature for 1 hour to obtain a mixed liquid; the mixed liquid was poured into a mold, and then operated according to the method in Example 1 to finally obtain a composite bacterial cellulose aerogel.

[0032] Comparative Example 3 The method of Example 1 was followed, except that the alumina sol was omitted in this comparative example, as follows: 0.3 g of bacterial cellulose was mixed with 60 g of deionized water, and stirred at room temperature for 1 h to obtain a bacterial cellulose suspension; 0.5 g of methyltrimethoxysilane was added to the bacterial cellulose suspension, and stirred at room temperature for 3 h, and then ultrasonicated at an ultrasonic power of 200 W for 1 h to obtain a modified bacterial cellulose liquid; The modified bacterial cellulose liquid was poured into a mold, and then the operation was performed according to the method in Example 1 to finally obtain a composite bacterial cellulose aerogel.

[0033] Comparative Example 4 The method of Example 1 was followed, except that the amount of aluminum sec-butoxide used in this example was 0.08 g.

[0034] Comparative Example 5 The method of Example 1 was followed, except that the preparation method of the alumina sol used in this comparative example was as follows: 0.15 g of aluminum chloride hexahydrate was mixed with 1 g of deionized water, and after stirring for 30 minutes, 0.15 g of propylene oxide was added dropwise. After the addition was completed, stirring was continued for 5 minutes, and then the mixture was allowed to stand at room temperature for 10 minutes to obtain an alumina sol.

[0035] Test Example 1 Figure 1 This is a photograph of the appearance of the cellulose aerogel prepared in Example 1, and the results show that the cellulose aerogel is a white solid. In addition, the cellulose aerogels prepared in other examples and comparative examples are also white solids.

[0036] Figure 2 This is a microscopic electron microscope image of the cellulose aerogel prepared in Example 1, wherein the left side corresponds to the growth direction of the ice crystals, and the right side corresponds to the direction perpendicular to the growth direction of the ice crystals. The results show that an ordered pore structure is formed in the growth direction of the ice crystals, with a pore size between 10 and 100 μm and a pore wall thickness of about 0.9 to 1.1 μm, while a lamellar continuous fiber structure is presented perpendicular to the growth direction of the ice crystals, indicating the anisotropy of the cellulose aerogel in the microstructure.

[0037] Test Example 2 The performance tests of the cellulose aerogels prepared in the examples and comparative examples were carried out as follows: Mechanical properties: Testing the stress of cellulose aerogel under 80% compressive strain; Elastic properties: The ratio of the height of the cellulose aerogel after 50 times of 80% compression strain to the initial height of the sample; Hydrophobic properties: Testing the hydrophobic angle of cellulose aerogel; Flame retardant properties: The cellulose aerogel was tested according to the vertical combustion test in UL94.

[0038] The above performance test results are specifically shown in Table 1. Figure 3 1 and 2 are stress-strain curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1-3. Figure 4 The hydrophobic angle test result diagram of the cellulose aerogel prepared in Example 1 shows that the cellulose aerogel provided by the present invention has excellent mechanical properties. For example, according to the results of Example 1 and Comparative Example 1, the compression performance of the cellulose aerogel can be improved by adding a silane coupling agent and a flame retardant such as aluminum sec-butylate, and the stress under 80% compression strain conditions increases from 25 kPa to 81 kPa. In addition, the cellulose aerogel provided by the present invention has excellent hydrophobic properties. Figure 4 From the test results of the hydrophobic angle of the cellulose aerogel prepared in Example 1 shown in , it can be seen that the hydrophobic angle of the cellulose aerogel is 138°, indicating that the cellulose aerogel is a hydrophobic material.

[0039] Table 1 Test results of cellulose aerogels prepared in Examples and Comparative Examples

[0040] Figure 5 The thermogravimetric curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3 are shown in FIG. Figure 6 The thermogravimetric derivative curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3 show that the addition of flame retardants such as silane coupling agents and aluminum sec-butoxide can improve the high temperature resistance of the cellulose aerogel and reduce the decomposition rate of the cellulose aerogel under high temperature conditions.

[0041] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a composite cellulose aerogel, characterized in that: The following steps are involved: The nanocellulose, the silane coupling agent and water are mixed and modified to obtain a modified nanocellulose liquid; The organic aluminum source is mixed with water, the pH value of the obtained organic aluminum source solution is adjusted to 3-4, and a hydrolysis reaction is performed to obtain an aluminum oxide sol; The modified nanocellulose slurry is mixed with an alumina sol, and the obtained mixed slurry is subjected to unidirectional freezing to obtain a frozen molded body; wherein the ratio of the mass of the nanocellulose used to prepare the modified nanocellulose slurry to the mass of the organic aluminum source used to prepare the alumina sol is 1-3:1; The frozen molded body is freeze-dried to obtain the composite cellulose aerogel.

2. The preparation method according to claim 1, characterized in that: The nanocellulose includes one or more of bacterial cellulose, carboxylated cellulose and cellulose nanocrystals; when the modification treatment is performed, the concentration of the nanocellulose in the system is 0.3-4wt%.

3. The preparation method according to claim 1 or 2, characterized in that: The silane coupling agent includes one or more of methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, ethoxytrimethylsilane, diethoxydimethylsilane and triethoxymethylsilane; the mass ratio of the nanocellulose to the silane coupling agent is 1:1-2.

4. The preparation method according to claim 3, characterized in that: The temperature of the modification treatment is 15-35° C., and the time is 0.5-2 h; the modification treatment is carried out under ultrasonic conditions.

5. The preparation method according to claim 1, characterized in that: The organic aluminum source includes aluminum sec-butoxide and / or aluminum isopropoxide; the molar ratio of the organic aluminum source to water in the organic aluminum source liquid is 1:30-120.

6. The preparation method according to claim 1 or 5, characterized in that: The temperature of the hydrolysis reaction is 85-95° C. and the time is 6-9 hours.

7. The preparation method according to claim 1, characterized in that: The unidirectional directional freezing comprises: placing the mixed liquid in a mold, immersing the bottom of the mold containing the mixed liquid in liquid nitrogen for 15 to 60 minutes; the thermal conductivity of the bottom material of the mold is higher than the thermal conductivity of the surrounding material of the mold.

8. The preparation method according to claim 7, characterized in that: The bottom of the mold is made of copper, and the surroundings of the mold are made of polytetrafluoroethylene.

9. The composite cellulose aerogel prepared by the preparation method according to any one of claims 1 to 8, characterized in that: It has a pore structure with a pore diameter of 10~100μm, a pore wall thickness of 0.9~1.1μm, and a porosity of ≥97%.

10. Use of the composite cellulose aerogel according to claim 9 as a flame retardant thermal insulation material.

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