A composite cellulose aerogel and its preparation method and application

Abstract

The present invention provides a composite cellulose aerogel, a preparation method, and an application thereof, belonging to the field of aerogel technology. The present invention comprises mixing nanocellulose, a silane coupling agent, and water, and performing a modification treatment to obtain 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, and performing a hydrolysis reaction to obtain an alumina sol; mixing the modified nanocellulose slurry with the alumina sol, and subjecting the obtained mixed slurry 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; and freeze-drying the frozen molded body to obtain the composite cellulose aerogel. The composite cellulose aerogel prepared using the method of the present invention is a hydrophobic, flame-resistant gel with a wide range of applications.

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 widespread attention for its exceptional properties, including extremely low thermal conductivity and low density. Silica aerogel and its composite aerogels have begun to be used in building insulation. Although costs are gradually decreasing, the production of aerogels, typically produced using supercritical drying technology, remains relatively expensive.

[0003] Cellulose is the world's most abundant renewable polymer. Its molecular structure is composed of repeating anhydrous glucose units. Cellulose aerogels are prepared using cellulose as the raw material, primarily through freeze-drying, which offers advantages over supercritical drying, such as lower cost and simpler operation. Cellulose aerogels also possess higher mechanical properties than pure silica aerogels. Their low density and high porosity make them suitable for direct use as thermal insulation materials. However, existing cellulose aerogel technologies suffer from water absorption and flammability issues, limiting their 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, gas-resistant, and flame-retardant gel with 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:

[0006] The present invention provides a method for preparing a composite cellulose aerogel, comprising the following steps:

[0007] The nanocellulose, silane coupling agent and water are mixed and modified to obtain a modified nanocellulose liquid;

[0008] 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 alumina sol;

[0009] The modified nanocellulose solution is mixed with an alumina sol, and the resulting mixed solution 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 solution to the mass of the organic aluminum source used to prepare the alumina sol is 1 to 3:1;

[0010] The frozen molded body is freeze-dried to obtain the composite cellulose aerogel.

[0011] Preferably, the nanocellulose comprises 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 0.3-4 wt%.

[0012] 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.

[0013] Preferably, the modification treatment is carried out at a temperature of 15-35° C. for a time of 0.5-2 h; and the modification treatment is carried out under ultrasonic conditions.

[0014] Preferably, the organoaluminum source includes aluminum sec-butoxide and / or aluminum isopropoxide; and the molar ratio of the organoaluminum source to water in the organoaluminum source solution is 1:30-120.

[0015] Preferably, the hydrolysis reaction temperature is 85-95° C., and the time is 6-9 hours.

[0016] Preferably, the unidirectional directional freezing includes: 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.

[0017] Preferably, the bottom of the mold is made of copper, and the surrounding material of the mold is polytetrafluoroethylene.

[0018] 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%.

[0019] The present invention provides the use of the composite cellulose aerogel described in the above technical solution as a flame retardant and thermal insulation material.

[0020] 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 to obtain 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 to obtain an alumina sol; mixing the modified nanocellulose slurry with the alumina sol, and performing unidirectional directionally freezing 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 uses biomass raw material nanocellulose as the structural framework of the aerogel, adopts a silane coupling agent and nanocellulose for chemical cross-linking to improve the mechanical properties and hydrophobicity of the aerogel, adopts an organic aluminum source as a flame retardant, and then prepares the 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

[0021] Figure 1 This is a photo of the appearance of the cellulose aerogel prepared in Example 1;

[0022] Figure 2 This is a microscopic electron microscope image of the cellulose aerogel prepared in Example 1;

[0023] Figure 3 : The stress-strain curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3;

[0024] Figure 4 This is a graph showing the hydrophobic angle test results of the cellulose aerogel prepared in Example 1;

[0025] Figure 5 Thermogravimetric curves of cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3;

[0026] Figure 6 Thermogravimetric derivative curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1-3. DETAILED DESCRIPTION

[0027] The present invention provides a method for preparing a composite cellulose aerogel, comprising the following steps:

[0028] The nanocellulose, silane coupling agent and water are mixed and modified to obtain a modified nanocellulose liquid;

[0029] 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 alumina sol;

[0030] The modified nanocellulose solution is mixed with an alumina sol, and the resulting mixed solution 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 solution to the mass of the organic aluminum source used to prepare the alumina sol is 1 to 3:1;

[0031] The frozen molded body is freeze-dried to obtain the composite cellulose aerogel.

[0032] 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 using methods well known to those skilled in the art.

[0033] The present invention comprises mixing nanocellulose, a silane coupling agent, and water, and performing a modification treatment to obtain a modified nanocellulose solution. As one 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; the concentration of the nanocellulose in the system during the modification treatment may be 0.3 to 4 wt%, specifically 0.3 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, or 4 wt%. 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.

[0034] As an embodiment of the present invention, the nanocellulose can be first stirred and mixed with water, and then a silane coupling agent is added to the resulting suspension for a second stirring and mixing, followed by a modification treatment. As an embodiment of the present invention, the temperature of the first stirring and mixing can be 15~35°C, 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°C, 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 stirring speed. 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°C, 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 is 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.

[0035] The present invention mixes an organoaluminum source with water, adjusts the pH value of the resulting organoaluminum source solution to 3-4, and performs a hydrolysis reaction to obtain an alumina sol. As one embodiment of the present invention, the organoaluminum source may include aluminum sec-butoxide and / or aluminum isopropoxide, specifically aluminum sec-butoxide or aluminum isopropoxide; and the molar ratio of the organoaluminum source to water in the organoaluminum source solution 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.

[0036] In one embodiment of the present invention, the temperature for mixing the organoaluminum source and water can be 85-95°C, specifically 85°C, 88°C, 90°C, 92°C, or 95°C; the mixing time can be 1-2 hours, specifically 1 hour, 1.5 hours, or 2 hours; and the mixing of the organoaluminum source and water can be carried out under stirring. In one embodiment of the present invention, the reagent used to adjust the pH of the organoaluminum source solution can be an organic acid reagent, specifically acetic acid. In the present invention, the pH of the organoaluminum source solution is adjusted to 3-4, specifically 3.5. Adjusting the pH of the organoaluminum source solution to the above range facilitates the hydrolysis of the organoaluminum source under acidic conditions. In one embodiment of the present invention, the hydrolysis reaction temperature can be 85-95°C, specifically 85°C, 88°C, 90°C, 92°C, or 95°C; the mixing time can be 6-9 hours, specifically 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, or 9 hours; and 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 alumina sol.

[0037] After obtaining the modified nanocellulose slurry and the alumina sol, the present invention mixes the modified nanocellulose slurry with the alumina sol, and performs unidirectional direction freezing on the obtained mixed slurry to obtain a frozen molded body. In the present invention, the ratio of the mass of nanocellulose used to prepare the modified nanocellulose slurry 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 slurry 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, a dehydration condensation reaction will further occur between the incompletely reacted silane coupling agent and the incompletely reacted pseudo-boehmite. As one 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 (at a temperature of -196°C) for 15 to 60 minutes, or more preferably 30 to 45 minutes. The thermal conductivity of the mold bottom material is higher than the thermal conductivity of the surrounding material of the mold. Specifically, the mold bottom material may be copper, and the surrounding material may be polytetrafluoroethylene. The mold size may be 5 cm × 5 cm × 5 cm. In this embodiment of the present invention, the mold bottom material is copper with high thermal conductivity, while the surrounding material is polytetrafluoroethylene with low thermal conductivity. Therefore, when the mold bottom is immersed in liquid nitrogen, heat in the mixed liquid is transferred from top to bottom. Water in the mixed liquid closer to the mold bottom preferentially forms ice crystals. The temperature gradient difference from top to bottom causes the ice crystals to grow vertically from bottom to top. As a result, the ice crystals form a vertical directional structure after growth. After subsequent freeze drying, i.e., after the ice crystals are eliminated, the resulting aerogel retains this directional structure. Therefore, the present invention does 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 exhibits anisotropy.

[0038] 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℃, specifically -60℃, -70℃, -80℃ or -90℃; the time can be 48~72h, 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℃, specifically 60℃; the time can be 2.5~3.5h, specifically 3h; in the embodiment of the present invention, drying is beneficial to ensure that the moisture is completely removed.

[0039] 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 within the material. This makes the composite cellulose aerogel have a low thermal conductivity (radial thermal conductivity of 0.0224 W / mK) and excellent thermal insulation performance. Nanocellulose, such as bacterial cellulose, is composed of dehydrated glucose, which contains three active hydroxyl groups, making the bacterial cellulose inherently water-absorbent. By adding a silane coupling agent, such as trimethylmethoxysilane, the bacterial cellulose is chemically cross-linked. The methoxy groups in the trimethylmethoxysilane undergo hydrolysis and condense with the hydroxyl groups on the bacterial cellulose, reducing the number of exposed hydroxyl groups in the bacterial cellulose. Furthermore, trimethylmethoxysilane contains a hydrophobic methyl group. The reduction of hydroxyl groups and the addition of methyl groups make the composite cellulose aerogel hydrophobic (with a hydrophobic angle of 138-145°). Furthermore, after hydrolysis and condensation of an organic aluminum source, such as aluminum sec-butoxide, the aluminum element is evenly distributed throughout the aerogel's interior and surface. Upon combustion, it forms a flame-retardant layer, making the composite cellulose aerogel flame-retardant. This layer does not affect the aerogel's microstructure, resulting in excellent mechanical properties. In addition, nanocellulose, a biomass raw material, is used as the structural framework of the aerogel. Rich chemical cross-links are generated between the nanocellulose and the silane coupling agent, and the aerogel microstructure is arranged in an orderly manner 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 its original height after undergoing 50 compressions at 80% strain.

[0040] The present invention provides the use of the composite cellulose aerogel described in the above technical solution as a flame retardant thermal insulation material. The present invention does not particularly limit the specific application of the composite cellulose aerogel, and any method familiar to those skilled in the art can be used.

[0041] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some 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 making creative efforts are within the scope of protection of the present invention.

[0042] Example 1

[0043] 0.3 g of bacterial cellulose was mixed with 60 g of deionized water, and the mixture was stirred at room temperature for 1 hour to obtain a bacterial cellulose suspension; 0.5 g of methyltrimethoxysilane was added to the bacterial cellulose suspension, and the mixture was stirred at room temperature for 3 hours, and then ultrasonicated at an ultrasonic power of 200 W for 1 hour to obtain a modified bacterial cellulose liquid;

[0044] 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.

[0045] 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 and freeze-dried at -80°C for 72 hours, and then placed in a 60°C oven for 3 hours to completely remove moisture to obtain a composite bacterial cellulose aerogel.

[0046] Example 2

[0047] 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.

[0048] Comparative Example 1

[0049] The method of Example 1 was followed, except that methyltrimethoxysilane and alumina sol were omitted in this comparative example, as follows:

[0050] 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. The bacterial cellulose suspension was poured into a mold, and then the method in Example 1 was followed to obtain a bacterial cellulose aerogel.

[0051] Comparative Example 2

[0052] The method of Example 1 was followed, except that methyltrimethoxysilane was omitted in this comparative example, as follows:

[0053] 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;

[0054] 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.

[0055] 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 the operation was performed according to the method in Example 1 to finally obtain a composite bacterial cellulose aerogel.

[0056] Comparative Example 3

[0057] The operation was carried out in accordance with the method of Example 1, except that the alumina sol was omitted in this comparative example, as follows:

[0058] 0.3 g of bacterial cellulose was mixed with 60 g of deionized water, and the mixture was stirred at room temperature for 1 hour to obtain a bacterial cellulose suspension; 0.5 g of methyltrimethoxysilane was added to the bacterial cellulose suspension, and the mixture was stirred at room temperature for 3 hours, and then ultrasonicated at an ultrasonic power of 200 W for 1 hour to obtain a modified bacterial cellulose liquid;

[0059] The modified bacterial cellulose solution was poured into a mold, and then the process was performed according to Example 1 to finally obtain a composite bacterial cellulose aerogel.

[0060] Comparative Example 4

[0061] The method of Example 1 was followed, except that the amount of aluminum sec-butoxide used in this example was 0.08 g.

[0062] Comparative Example 5

[0063] The method of Example 1 was followed, except that the preparation method of the alumina sol used in this comparative example was as follows:

[0064] 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.

[0065] Test Example 1

[0066] Figure 1 This is a photograph of the appearance of the cellulose aerogel prepared in Example 1. The results show that the cellulose aerogel is a white solid. In addition, the cellulose aerogels prepared in the other examples and comparative examples are also white solids.

[0067] Figure 2This is a microscopic electron microscope image of the cellulose aerogel prepared in Example 1, where the left side corresponds to the direction of ice crystal growth and the right side corresponds to the direction perpendicular to the ice crystal growth direction. The results show that an ordered pore structure is formed in the direction of ice crystal growth, with a pore size between 10 and 100 μm and a pore wall thickness of approximately 0.9 to 1.1 μm, while a sheet-like continuous fiber structure is presented perpendicular to the ice crystal growth direction, indicating the anisotropy of the cellulose aerogel in the microstructure.

[0068] Test Example 2

[0069] The performance tests of the cellulose aerogels prepared in the examples and comparative examples were conducted as follows:

[0070] Mechanical properties: Testing the stress of cellulose aerogel under 80% compressive strain;

[0071] Elastic properties: The ratio of the height of the cellulose aerogel after 50 cycles of 80% compression strain to the initial height of the sample is tested;

[0072] Hydrophobicity: Testing the hydrophobic angle of cellulose aerogel;

[0073] Flame retardant properties: The cellulose aerogel was tested according to the vertical combustion test in UL94.

[0074] The above performance test results are shown in Table 1. Figure 3 2 are stress-strain curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1-3. Figure 4 The hydrophobic angle test results of the cellulose aerogel prepared in Example 1 are shown in the figure. The results show that the cellulose aerogel provided by the present invention has excellent mechanical properties. As can be seen from 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-butoxide. The stress under 80% compression strain conditions increases from 25kPa to 81kPa. In addition, the cellulose aerogel provided by the present invention has excellent hydrophobic properties. Figure 4 The hydrophobic angle test result of the cellulose aerogel prepared in Example 1 shown in FIG shows that the hydrophobic angle of the cellulose aerogel is 138°, indicating that the cellulose aerogel is a hydrophobic material.

[0075] Table 1 Test results of cellulose aerogels prepared in Examples and Comparative Examples

[0076]

[0077] Figure 5 Thermogravimetric curves of cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3 are shown. Figure 6Thermogravimetric derivative curves of the cellulose aerogels prepared in Example 1 and Comparative Examples 1 to 3 are shown. The results show that the addition of flame retardants such as silane coupling agents and aluminum sec-butoxide improves the high-temperature resistance of the cellulose aerogels and reduces their decomposition rate under high-temperature conditions.

[0078] 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 principles of the present invention. These improvements and modifications should also be regarded as within 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: Nanocellulose, a silane coupling agent and water are mixed and modified to obtain a modified nanocellulose solution; during the modification, the concentration of nanocellulose in the system is 0.3-4 wt %; the mass ratio of the nanocellulose to the silane coupling agent is 1:1-2; the modification temperature is 15-35° C., the time is 0.5-2 h; and the modification is carried out under ultrasonic conditions with an ultrasonic power of 180-220 W; 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 alumina sol; The modified nanocellulose liquid and the alumina sol are mixed under stirring for 1 to 3 hours, and the resulting mixed liquid is subjected to unidirectional directional freezing to obtain a frozen molded body; wherein the ratio of the mass of the 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; 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 material of the bottom of the mold is higher than the thermal conductivity of the material surrounding the mold; 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.

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.

4. 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.

5. The preparation method according to claim 1 or 4, characterized in that The temperature of the hydrolysis reaction is 85-95° C., and the time is 6-9 hours.

6. The preparation method according to claim 1, characterized in that The bottom of the mold is made of copper, and the surroundings of the mold are made of polytetrafluoroethylene.

7. The composite cellulose aerogel prepared by the preparation method according to any one of claims 1 to 6, characterized in that: It has a porous structure with a pore diameter of 10~100μm, a pore wall thickness of 0.9~1.1μm, and a porosity ≥97%.

8. Use of the composite cellulose aerogel according to claim 7 as a flame retardant and thermal insulation material.

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