Preparation method of composite aerogel with flame retardancy and capable of photocatalytically degrading dyes

Abstract

The present invention belongs to the field of composite aerogel functional materials, and discloses a preparation method of a composite aerogel with flame retardancy and photocatalytic degradation of dyes. Chitosan is dissolved in an acidic ionic liquid of 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]), and a TiO2@MXene composite is added under ultrasonic action for electrostatic self-assembly. Then, low molecular weight carboxymethyl chitosan and a small molecule cross-linking agent are added under microwave radiation to enhance the cross-linking reaction and electrostatic force. Finally, a chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel material is prepared by microwave vacuum freeze-drying. The method of the present invention is simple and easy to implement. The obtained composite aerogel has the advantages of strong structural stability, low dissolution rate, high porosity, high flame retardancy, good photocatalytic activity, etc., and has broad application prospects in the fields of photocatalytic degradation of pollutants, high-temperature heat insulation, flame retardancy and fire prevention, etc.

Description

Technical Field

[0001] The present invention relates to a preparation method and use of a composite aerogel with flame retardancy and photocatalytic degradation of dyes, and specifically relates to a preparation method of a chitosan / carboxylated chitosan / TiO2@MXene composite aerogel and its application in flame retardancy and fire prevention and photocatalytic degradation of dyes, belonging to the field of composite aerogel functional materials. Background Art

[0002] Aerogels are widely used in the fields of fire resistance and thermal insulation due to their high porosity, low thermal conductivity, ultra-light structure and high thermal insulation performance [Chen J, Xie H, Lai X, et al. An ultrasensitive fire-warning chitosan / montmorillonite / carbon nanotube composite aerogel with high fire-resistance[J]. Chemical Engineering Journal, 2020, 399: 125729]. In recent years, biomaterial-derived flame-retardant aerogels have attracted much attention due to their advantages such as renewability, environmental friendliness, low economic cost, light weight and high thermal insulation performance. Natural polysaccharide polymers are one of the most abundant renewable biomass materials with better biodegradability and environmental friendliness than fossil-based materials, making them an excellent candidate for the production of sustainable aerogels in the field of flame retardant materials. Therefore, people are committed to the development of polysaccharide-based aerogel products due to their high porosity, low density, non-toxicity, biosustainability and biodegradability. Chitosan is a biopolymer with amino polysaccharides and is considered a promising alternative to traditional flame retardants because it contains abundant hydroxyl and amino groups, which give chitosan aerogel excellent carbonization and thermal insulation capabilities [ChenH Q, Xu YJ, Jiang ZM, et al. The thermal degradation property and flame-retardant mechanism of coated knitted cotton fabric with chitosan and APP byLBL assembly[J]. Journal of Thermal Analysis and Calorimetry, 2020, 140: 591-602]. Although chitosan has many advantages and applications, its flame retardant properties are limited. Many studies have shown that the addition of materials such as montmorillonite, carbon nanotubes, and graphene oxide can significantly improve the flame retardant properties of organic aerogels. However, inorganic substances such as montmorillonite and graphene oxide may have poor compatibility with the chitosan aerogel matrix, resulting in the destruction of the aerogel network structure and seriously affecting the mechanical properties of the aerogel.

[0003] MXene is an emerging two-dimensional layered transition metal carbide / carbonitride, where M represents early transition metals and X represents carbon and / or nitrogen. Due to its outstanding physical and chemical properties, MXene shows potential applications in many fields, including catalysis, batteries, thermal energy storage, and electromagnetic interference. MXene has also been explored as a flame retardant due to its excellent thermal stability, high specific surface area, good catalytic activity, excellent mechanical properties, and tunable chemical properties [Yu B, Yuen A C Y, Xu X, et al. Engineering MXene surface with POSS for reducing fire hazards of polystyrene with enhanced thermal stability[J]. Journal of Hazardous Materials, 2021, 401: 123342], and its advantages are also beneficial to the development of high-performance polymer composites. Recent studies have demonstrated the catalytic effect of titanium-containing metal compounds in MXene, which can effectively inhibit the release of smoke and promote the formation of a carbon layer [Liu C, Wu W, Shi Y, et al. Creating MXene / reduced graphene oxide hybrid towards highly fire safe thermoplastic polyurethane nanocomposites[J]. Composites Part B: Engineering, 2020, 203: 108486]. The incorporation of MXene nanosheets can significantly improve the thermal insulation and flame retardancy of aerogels. Meanwhile, the groups on the surface of MXene can form a stable hydrogen bond structure with the aerogel matrix. Introducing MXene into chitosan aerogels can enhance the mechanical strength of the matrix and improve the flame retardancy of the aerogels. At the same time, the traditional method to improve the flame retardancy of chitosan aerogels is to phosphorylate chitosan. However, the molecular weight of chitosan decreases significantly in strong acids. Therefore, the phosphorylation reaction is difficult to control, and there are smoke and toxicity during the combustion process. MXene can form strong hydrogen bond interactions with chitosan, improving the mechanical strength of chitosan composite aerogels without damaging the properties of chitosan and also having good flame retardancy. MXene can be well mixed with chitosan aerogels to form an MXene / chitosan complex, thereby improving the flame retardancy of chitosan aerogels.

[0004] The treatment and recycling of printing and dyeing wastewater have become urgent problems to be solved in society. Titanium dioxide (TiO2) is a new type of photocatalytic material that can convert organic pollutants such as dyes, surfactants, polymers, and aromatics in printing and dyeing wastewater into small molecule non-toxic compounds under ultraviolet light irradiation. Most of the energy in the reaction process comes from solar radiation, and there is no secondary pollution in the photocatalytic reaction process. The photocatalytic efficiency of TiO2 is mainly limited by its large band gap and the rapid recombination of photo-generated electrons / hole pairs. Powdered TiO2 photocatalyst has low adsorption capacity, easy agglomeration, easy recombination of photo-generated electrons and holes, low utilization rate of visible light, and difficult separation from the solution, which is not conducive to recycling and not suitable for large-scale industrial use. At present, chitosan / TiO2 composite aerogel can efficiently treat wastewater and is easy to recycle. The abundant amino and hydroxyl groups in chitosan can effectively enrich pollutants in water, and the three-dimensional network formed by chitosan can evenly load TiO2, which is beneficial to the adsorption and photocatalytic degradation of pollutants. However, the existing chitosan / TiO2 composite aerogel has insufficient structural strength, and the dye degradation rate decreases significantly after being reused a certain number of times. Moreover, the separation efficiency of photo-generated electrons and holes in TiO2 in the aerogel is limited, resulting in low photocatalytic activity and dye degradation rate, and a long time for dye degradation. Summary of the Invention

[0005] In view of the above problems existing in the prior art, the purpose of the present invention is to provide a preparation method and application of a composite aerogel with flame retardancy and photocatalytic dye degradation. By compounding chitosan / carboxymethyl chitosan / TiO2@MXene, a composite aerogel material with strong structural stability, low dissolution rate, high porosity, high flame retardancy, and good photocatalytic activity is obtained, which has broad application prospects in the fields of photocatalytic degradation of pollutants in wastewater, high-temperature heat insulation, and flame retardant and fire prevention.

[0006] To achieve the above purpose, the technical solutions adopted by the present invention are as follows:

[0007] A composite aerogel with flame retardancy and photocatalytic dye degradation is prepared by dissolving chitosan in 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid, adding TiO2@MXene composite under ultrasonic action for electrostatic self-assembly, then adding low molecular weight carboxymethyl chitosan and small molecule cross-linking agent under microwave radiation to enhance cross-linking reaction and electrostatic force, and finally obtaining the product by microwave vacuum freeze-drying.

[0008] Preferably, the carboxyl group content at the C6 position of the low molecular weight carboxymethyl chitosan is 50.64-71.26%, the degree of deacetylation is 89.17-96.48%, the viscosity-average molecular weight is 0.35-0.89×10^4, the solubility in water is 17.83-28.02 g / 100 mL, and the isoelectric point pH = 4.7-5.1. Its structural formula is as follows:

[0009]

[0010] The dissolution rate of the flame-retardant and photocatalytic dye-degrading composite aerogel is 7.06 - 14.51%, the specific surface area is 13.72 - 32.96 m 2 / g, the average pore size is 60.3 - 96.9 μm, the porosity is 81.42 - 90.68%, the limiting oxygen index is 30.5 - 40.1%, the photocatalytic degradation rate of methylene blue (MB) dye after 2 h is 80.79 - 100%, and the photocatalytic degradation rate of rhodamine B (RhB) dye is 82.65 - 98.26%.

[0011] The preparation method of the flame-retardant and photocatalytic dye-degrading composite aerogel of the present invention is carried out according to the following steps:

[0012] (1) Dissolve 1 - 2 g of LiF in 20 - 40 mL of 9 mol / L HCl solution, stir at room temperature for 30 - 60 min to obtain a mixed solution, then slowly add 1 - 2 g of Ti3AlC2 powder to the LiF / HCl mixed solution, and slowly stir (rotation speed 40 - 100 rpm) at 45 °C for 48 - 60 h; filter and collect the solid, wash it with deionized water 3 - 5 times, then centrifuge at 3500 - 5000 rpm for 10 - 20 min until the pH of the solid is 6.0 - 6.4, treat it with a 500 - 680 W high-power ultrasonic machine for 60 - 80 min, and then centrifuge at a high speed (rotation speed 8200 - 12000 rpm) for 6 - 10 min to obtain uniformly dispersed MXene nanosheets.

[0013] (2) Ultrasonically disperse anatase TiO2 sol with a mass concentration of 0.1 - 0.2% for 20 - 30 min, add the MXene nanosheets and stir for 10 - 20 min, then add 10 mL of absolute ethanol and perform ultrasonic treatment for 1 - 2 h. Transfer the mixed solution to a reaction kettle and carry out hydrothermal reaction at 100 - 120 °C for 3 - 5 h. After cooling to room temperature, centrifuge to separate the product, wash it with an ethanol solution with a mass concentration of 60%, and then freeze-dry to obtain a TiO2@MXene composite; the mass ratio of anatase TiO2 to MXene nanosheets is 1 - 6:1.

[0014] (3) Dissolve chitosan in 1-ethyl-3-methylimidazolium acetate ionic liquid at 60-80 °C to prepare a solution with a mass concentration of 1-4%, add the TiO2@MXene composite, and carry out an ultrasonic chemical reaction at 50-70 °C for 1-3 h. Let it stand for 20-40 min, then add low-molecular-weight carboxymethyl chitosan and a small-molecule crosslinking agent and carry out a crosslinking reaction under microwave radiation for 45-140 min to form multi-site chemical crosslinking and electrostatic interaction among chitosan, low-molecular-weight carboxymethyl chitosan, and the TiO2@MXene composite; the mass ratio of chitosan to the TiO2@MXene composite is 1:1; the mass ratio of chitosan to low-molecular-weight carboxymethyl chitosan is 3-10:1; the mass ratio of chitosan to the small-molecule crosslinking agent is 4-10:1; the small-molecule crosslinking agent is selected from tartaric acid, malic acid, oxalic acid, glutamic acid, maleic acid, or maleic anhydride.

[0015] (4) After the reaction, let the composite hydrogel stand and age for 20-45 min, use an ultrasonic defoamer to treat it for 30-90 min to remove the internal bubbles of the composite hydrogel, then place it in a cryogenic refrigerator at -80 to -65 °C for pre-freezing for 6-10 h, and then carry out microwave vacuum freeze-drying for 18-24 h to obtain a composite aerogel with flame retardancy and photocatalytic dye degradation (i.e., chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel).

[0016] Preferably, in step (3), the 1-ethyl-3-methylimidazolium acetate ionic liquid is acidic with a pH of 4.2-5.0.

[0017] Preferably, in step (3), the power of the ultrasonic wave in the ultrasonic chemical reaction is 240-360 W.

[0018] Preferably, in step (3), the power of the microwave radiation is 620-800 W, and the microwave radiation temperature is 50-65 °C.

[0019] Preferably, in step (4), the power of the ultrasonic defoamer is 1400-2100 W.

[0020] Preferably, in step (4), the temperature of the microwave vacuum freeze-drying is -86 to -70 °C, the microwave power is 1800-2400 W, and the vacuum degree is 6-15 Pa.

[0021] By optimizing the mass ratio of chitosan to low-molecular-weight carboxymethyl chitosan and the TiO2@MXene composite, the ultrasonic reaction time, the microwave radiation reaction temperature and time, the dosage of the small-molecule crosslinking agent, and the microwave vacuum freeze-drying conditions, a series of composite aerogel materials with different porosities can be obtained.

[0022] The chitosan / carboxylated chitosan / TiO2@MXene composite aerogel of the present invention can be used for flame retardancy and photocatalytic degradation of dyes in wastewater. After the photocatalytic degradation of dyes, the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel can be regenerated by decomposing the dyes under ultraviolet irradiation in clear water and can be reused.

[0023] Compared with the prior art, the preparation principle and beneficial effects of the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel in the present invention are as follows:

[0024] 1. The present invention uses 1-ethyl-3-methylimidazolium acetate ionic liquid as a solvent and reaction medium, which has a high hydrogen bond basicity, low viscosity, small vapor pressure, is non-volatile, can be recycled, and is green and environmentally friendly; 1-ethyl-3-methylimidazolium acetate is an environmentally friendly imidazole acidic ionic liquid miscible with water, which is more soluble in chitosan and makes the amino group of chitosan carry a positive charge, increasing the hydrogen bond and electrostatic interaction between the amino group and the negatively charged groups such as -OH, -F, -O on the surface of the TiO2@MXene complex, forming a stable aerogel structure; at the same time, 1-ethyl-3-methylimidazolium acetate ionic liquid can absorb the water generated by the reaction of chitosan, low molecular weight carboxylated chitosan and small molecule cross-linking agent, promoting the forward cross-linking reaction and further enhancing the three-dimensional skeleton structure of the composite aerogel.

[0025] 2. The low molecular weight carboxylated chitosan used in the present invention (with a C6 carboxyl content of 50.64 - 71.26%, a deacetylation degree of 89.17 - 96.48%, a viscosity-average molecular weight of 0.35 - 0.89×10^4, a water solubility of 17.83 - 28.02 g / 100 mL, and an isoelectric point pH = 4.7 - 5.1), compared with ordinary chitosan, has good water solubility, can avoid the use of acids and organic solvents and environmental pollution, and the carboxylated chitosan molecule contains both carboxyl and amino groups, is an amphoteric polyelectrolyte, similar to the protein structure, and has better biocompatibility and human affinity. The carboxyl group at the C6 position of the low molecular weight carboxylated chitosan molecule can cross-link with the amino group of chitosan through amide bonds, and the amino and hydroxyl groups in its molecule can form multi-site hydrogen bond binding with the abundant -OH, -F, -O and other groups on the surface of the TiO2@MXene complex. At the same time, the low molecular weight carboxylated chitosan has a small molecular weight (viscosity-average molecular weight of 0.35 - 0.89×10^4), low solution viscosity, and is easy to be uniformly mixed with macromolecular chitosan, promoting the formation of multi-site cross-linking of low molecular weight carboxylated chitosan between macromolecular chitosans, significantly increasing the skeleton strength and structural stability of the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel matrix.

[0026] 3. The present invention utilizes a small molecule crosslinking agent to carry out a crosslinking reaction with chitosan, carboxylated chitosan, and TiO2@MXene in 1-ethyl-3-methylimidazolium acetate ionic liquid. The carboxyl groups and anhydrides in the small molecule crosslinking agent can respectively crosslink with the amino groups and hydroxyl groups of chitosan and carboxylated chitosan to form covalent bond bindings, and generate hydrogen bond and electrostatic interactions with groups such as -OH, -F, and -O in the TiO2@MXene composite, and further form a composite aerogel with a three-dimensional framework structure through electrostatic self-assembly. Without adding surfactants or emulsifiers, it has the advantages of simple process and controllability, can enhance the three-dimensional network structure of the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel, has a small dissolution loss rate, high mechanical properties, and good flame retardant and photocatalytic properties.

[0027] 4. The present invention dissolves chitosan into 1-ethyl-3-methylimidazolium acetate ionic liquid, adds the TiO2@MXene composite and conducts sonochemical reactions. The multiple effects of ultrasonic waves such as dispersion, pulverization, and activation can promote the dispersion of chitosan macromolecular chains and the hydrogen bond binding between chitosan and TiO2@MXene; at the same time, the ultrasonic waves generate cavitation effects, and the formation, growth, and collapse of cavitation bubbles will occur in the reaction solution. When the cavitation bubbles collapse, high temperature and high pressure will be generated in an extremely short time and within an extremely small space of the cavitation bubbles, which provides an energy source for sonochemical synthesis and greatly accelerates the electrostatic assembly of chitosan and the TiO2@MXene composite. Ultrasonic waves have an obvious dispersion effect on chitosan macromolecules. Through the strong impact and cavitation effect of ultrasonic waves, the entangled chitosan macromolecular chains are evenly separated, enabling the chitosan and the TiO2@MXene composite sheets to be fully contacted, promoting the hydrogen bond binding between the positively charged amino groups in chitosan and the -OH, -F, -O and other groups on the surface of TiO2@MXene, thereby forming a three-dimensional mesh structure with a stable framework.

[0028] 5. The present invention uses microwave radiation to catalyze the crosslinking reaction of chitosan with TiO2@MXene, low molecular weight carboxylated chitosan, and a small molecule crosslinking agent. Microwave has the advantages of rapid heating, precise temperature control, and energy saving; in the microwave radiation reaction, the effective action of microwave radiation on polar substances can be utilized to accelerate the reaction. The radiation effect of microwave can increase the kinetic energy of chitosan, TiO2@MXene, low molecular weight carboxylated chitosan, and the small molecule crosslinking agent, promote the movement between molecular chain segments, accelerate the multi-site crosslinking reaction rate of chitosan and the TiO2@MXene composite, low molecular weight carboxylated chitosan, and the small molecule crosslinking agent, save reaction time and reduce energy consumption, and enhance the aerogel framework structure.

[0029] 6. After the composite hydrogel of the present invention is left standing for aging, it is treated with an ultrasonic defoamer to remove the internal bubbles in the composite hydrogel. The ultrasonic defoamer mainly utilizes the cavitation effect generated by ultrasonic waves in the liquid, causing the gas dissolved in the liquid to continuously aggregate into very small bubbles, and finally becoming spherical bubbles that break away from the liquid surface, thereby achieving the purpose of liquid degassing and defoaming. After being treated with the ultrasonic defoamer, the components in the chitosan / carboxychitosan / TiO2@MXene composite aerogel can be combined more tightly, which is beneficial to increasing the skeletal stability and mechanical properties of the composite aerogel.

[0030] 7. The chitosan / carboxychitosan / TiO2@MXene composite hydrogel of the present invention is pre-frozen in an ultra-low temperature refrigerator and then freeze-dried by microwave vacuum to obtain a composite aerogel. By pre-freezing, the freezing rate of the water inside the chitosan / carboxychitosan / TiO2@MXene composite hydrogel is slowed down, thereby reducing the crystallization risk of water during freezing, which helps to form a stable three-dimensional skeletal structure. At the same time, pre-freezing can also reduce the crystallization and deformation during the freeze-drying process of the composite aerogel, effectively avoiding the structural collapse of the composite aerogel caused by the rapid removal of water, and significantly improving the skeletal strength of the chitosan / carboxychitosan / TiO2@MXene composite aerogel. The present invention uses microwave vacuum freeze-drying to obtain the chitosan / carboxychitosan / TiO2@MXene composite aerogel. By combining microwave drying and vacuum drying, the respective advantages can be fully exerted. The ice crystals generated inside the chitosan / carboxychitosan / TiO2@MXene hydrogel during pre-freezing are sublimated and dried under vacuum and below the eutectic temperature, and the sublimation latent heat is provided to the material to be dried in the frozen state through a microwave generator to quickly remove the water in the composite aerogel. Compared with the disadvantage of slow conventional heat conduction rate in a vacuum state, the drying time is shortened and the drying efficiency is improved. The temperature of microwave vacuum freeze-drying is uniform, the water molecules in the composite aerogel are evenly distributed, and the water drying rate is consistent, maintaining the network porous structure of the composite aerogel and enhancing the stability of the composite aerogel skeletal structure.

[0031] 8. The TiO2@MXene composite material is synthesized by a hydrothermal reaction in the present invention. Since the Fermi level of the MXene nanosheets is lower than that of TiO2, electrons are photo-induced to transfer from the conduction band (CB) of TiO2 to the metal MXene, leaving holes in the valence band (VB) of TiO2. A Schottky barrier is generated at the interface, which prevents the diffusion of electrons to TiO2, effectively suppressing the recombination of electrons and holes on the surface of TiO2, thereby improving the photocatalytic activity of the TiO2@MXene composite. At the same time, defects such as a large band gap of TiO2, easy recombination of carriers, easy aggregation of microparticles, and limited photocatalytic performance are avoided. Adding the TiO2@MXene composite into a chitosan / carboxymethyl chitosan matrix to construct a stable composite aerogel can effectively improve the removal rate of dye pollutants. At the same time, it has high photocatalytic activity, a fast dye degradation rate, and good recyclability, showing great potential for practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 and Figure 2 are the schematic diagrams for the preparation of the chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel of the present invention.

[0033] Figure 3 is the scanning electron microscope image of the chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel in Test Item 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] For a better understanding of the technical features, objectives, and beneficial effects of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but the present invention is not limited to the following embodiments.

[0035] The test method for the photocatalytic degradation performance of the aerogel obtained in the following examples is as follows: the initial concentration of methylene blue (MB) dye is 10 mg / L, and a certain amount of composite aerogel (TiO2 mass concentration is 0.005%) is taken. Mix the samples in the dark, set the stirring speed to 300 rpm and start timing. At this time, the sample time is recorded as "-60 min". Use a pipette to take 3 mL and transfer it to a centrifuge tube. Use a disposable syringe with a Nylon needle filter to filter insoluble substances such as composite aerogel, and transfer it to a cuvette. Use deionized water as a reference sample for measurement; perform absorbance tests on the samples in the wavelength range of 400 - 800 nm. After the test is completed, wash the cuvette 3 times with dilute hydrochloric acid, then wash it with anhydrous ethanol, and finally dry it with a hair dryer. Take samples for absorbance tests every 20 min; when sampling for the fourth time, that is, when the time is recorded as 0 min; at the same time, turn on the LED ultraviolet lamp with a wavelength of 365 nm and a power of 75 W and perform the same operation. After 20 min, 40 min, 60 min, 80 min, 100 min, and 120 min from the start of recording, stop using the ultraviolet LED lamp. When performing photocatalytic degradation of rhodamine B (RhB) dye, replace the Nylon filter with a glass fiber filter, and keep other steps unchanged. Convert the absorbance at the highest point of the characteristic peak of the absorbance curve to the corresponding dye concentration through the standard curve. The initial dye concentration is represented by C0 mg / L, and the dye concentration at a certain moment is represented by C t mg / L. The photocatalytic degradation rate of the dye in the dye solution is as shown in the formula: .

[0036] I. Preparation of chitosan / carboxychitosan / TiO2@MXene composite aerogel

[0037] Example 1

[0038] (1) Dissolve 1 g of LiF in 25 mL of 9 mol / L HCl solution, stir at room temperature for 40 min to obtain a mixed solution, then slowly add 1 g of Ti3AlC2 powder to the LiF / HCl mixed solution, and slowly stir (rotation speed 45 rpm) at 45 °C for 48 h; filter and collect the solid, wash it 3 times with deionized water, then centrifuge at 3800 rpm for 12 min until the pH of the solid is 6.0, treat it with a 540 W high-power ultrasonic machine for 65 min, and then centrifuge at high speed (rotation speed 8500 rpm) for 6 min to obtain uniformly dispersed MXene nanosheets.

[0039] (2) The anatase TiO₂ sol with a mass concentration of 0.18% was ultrasonically dispersed for 25 min, then MXene nanosheets were added (the mass ratio of anatase TiO₂ to MXene was 1:1), and stirred and dispersed for 15 min. Then 10 mL of absolute ethanol was added and ultrasonically treated for 1 h. The mixed solution was transferred to a reaction kettle and hydrothermally reacted at 100 °C for 3 h. After cooling to room temperature, the product was centrifuged, washed with an ethanol solution with a mass concentration of 60%, and then freeze-dried to obtain the TiO₂@MXene composite.

[0040] (3) Chitosan was dissolved in 1-ethyl-3-methylimidazolium acetate ionic liquid with a pH of 4.8 at 70 °C to prepare a solution with a mass concentration of 1.5%. The TiO₂@MXene composite was added (the mass ratio of chitosan to the TiO₂@MXene composite was 1:1), and ultrasonic (ultrasonic power was 260 W) chemical reaction was carried out at 50 °C for 1.5 h, and then left standing for 25 min. Then low-molecular-weight carboxymethyl chitosan (the carboxyl group content at the C6 position of low-molecular-weight carboxymethyl chitosan was 53.85%, the degree of deacetylation was 90.22%, the viscosity-average molecular weight was 0.72×10⁴, the solubility in water was 20.70 g / 100 mL, and the isoelectric point pH = 5.0) and small-molecule cross-linking agent malic acid (the mass ratio of chitosan to malic acid was 4:1) were added according to the mass ratio of chitosan to low-molecular-weight carboxymethyl chitosan of 4:1, and cross-linking reaction was carried out under microwave radiation (microwave radiation power was 640 W, microwave radiation temperature was 50 °C) for 60 min, so that multi-site chemical cross-linking and electrostatic interaction were formed among chitosan, low-molecular-weight carboxymethyl chitosan and the TiO₂@MXene composite.

[0041] (4) After the reaction was completed, the composite hydrogel was left standing and aged for 25 min, and the ultrasonic defoamer (ultrasonic defoamer power was 1500 W) was used to treat for 45 min to remove the internal bubbles of the composite hydrogel. Then it was placed in a -65 °C ultra-low temperature refrigerator for pre-freezing for 7 h, and then microwave vacuum freeze-drying (microwave vacuum freeze-drying temperature was -70 °C, microwave power was 2000 W, and vacuum degree was 12 Pa) was carried out for 20 h to obtain the chitosan / carboxymethyl chitosan / TiO₂@MXene composite aerogel with flame retardancy and photocatalytic dye degradation performance.

[0042] After testing, the dissolution rate of the chitosan / carboxymethyl chitosan / TiO₂@MXene composite aerogel prepared in this example was 12.09%, the specific surface area was 16.83 m 2 / g, the average pore diameter was 88.26 μm, the porosity was 83.67%, and the limiting oxygen index was 33.2%.

[0043] After testing the performance of photocatalytic degradation of dyes, the composite aerogel obtained in this example had a photocatalytic degradation rate of 85.96% for methylene blue (MB) dye and 88.37% for rhodamine B (RhB) dye after 2 h; after 7 cycles of use, the photocatalytic degradation rate of the composite aerogel for methylene blue was 64.51% and for rhodamine B was 62.85% after 2 h.

[0044] Example 2

[0045] (1) Dissolve 1.5 g of LiF in 30 mL of 9 mol / L HCl solution, stir for 45 min at room temperature to obtain a mixed solution, then slowly add 1.5 g of Ti3AlC2 powder to the LiF / HCl mixed solution, and slowly stir (rotation speed 60 rpm) at 45 °C for 52 h; filter and collect the solid, wash it 4 times with deionized water, then centrifuge at 4500 rpm for 15 min until the pH of the solid is 6.0, treat it with a 580 W high-power ultrasonic machine for 70 min, and then centrifuge at a high speed (rotation speed 9600 rpm) for 8 min to obtain uniformly dispersed MXene nanosheets.

[0046] (2) Ultrasonically disperse anatase TiO2 sol with a mass concentration of 0.15% for 25 min, then add MXene nanosheets (the mass ratio of anatase TiO2 to MXene is 2:1), stir and disperse for 18 min, then add 10 mL of absolute ethanol and ultrasonically treat for 1.5 h. Transfer the mixed solution to a reaction kettle and carry out hydrothermal reaction at 110 °C for 3 h. After cooling to room temperature, centrifuge to separate the product, wash it with an ethanol solution with a mass concentration of 60%, and then freeze-dry to obtain TiO2@MXene composite.

[0047] (3) Dissolve chitosan in 1-ethyl-3-methylimidazolium acetate ionic liquid with a pH of 4.6 at 70 °C to prepare a solution with a mass concentration of 2%. Add TiO2@MXene composite (the mass ratio of chitosan to TiO2@MXene composite is 1:1), and carry out ultrasonic (ultrasonic power is 300 W) chemical reaction at 60 °C for 2 h, let it stand for 30 min, then add low-molecular-weight carboxymethyl chitosan (the carboxyl group content at C6 of low-molecular-weight carboxymethyl chitosan is 61.54%, the degree of deacetylation is 93.11%, the viscosity-average molecular weight is 0.58×10^4, the solubility in water is 22.36 g / 100 mL, and the isoelectric point pH = 4.9) and small-molecule cross-linking agent oxalic acid (the mass ratio of chitosan to oxalic acid is 6:1) according to the mass ratio of chitosan to low-molecular-weight carboxymethyl chitosan of 6:1, and carry out cross-linking reaction under microwave radiation (microwave radiation power is 720 W, microwave radiation temperature is 60 °C) for 80 min to form multi-site chemical cross-linking and electrostatic interaction among chitosan, low-molecular-weight carboxymethyl chitosan and TiO2@MXene composite.

[0048] After the reaction, the composite hydrogel was allowed to stand and age for 30 min, and then treated with an ultrasonic defoamer (the power of the ultrasonic defoamer was 1800 W) for 60 min to remove the internal bubbles of the composite hydrogel. Then it was placed in a -70 °C ultra-low temperature refrigerator for pre-freezing for 8 h, and then subjected to microwave vacuum freeze-drying (the temperature of microwave vacuum freeze-drying was -80 °C, the microwave power was 2100 W, and the vacuum degree was 10 Pa) for 20 h to obtain a chitosan / carboxychitosan / TiO2@MXene composite aerogel with flame retardancy and photocatalytic dye degradation properties.

[0049] After testing, the dissolution rate of the chitosan / carboxychitosan / TiO2@MXene composite aerogel prepared in this example was 10.52%, the specific surface area was 21.39 m 2 / g, the average pore diameter was 80.17 μm, the porosity was 86.04%, and the limiting oxygen index was 35.7%.

[0050] After testing the photocatalytic dye degradation performance, the photocatalytic degradation rate of the composite aerogel obtained in this example for methylene blue (MB) dye was 92.65% after 2 h, and the photocatalytic degradation rate for rhodamine B (RhB) dye was 93.41%; after 7 cycles of use, the photocatalytic degradation rate of the composite aerogel for methylene blue was 68.55% after 2 h, and the photocatalytic degradation rate for rhodamine B was 70.89%.

[0051] Example 3

[0052] (1) Dissolve 2 g of LiF in 40 mL of 9 mol / L HCl solution, stir at room temperature for 50 min to obtain a mixed solution, and then slowly add 2 g of Ti3AlC2 powder to the LiF / HCl mixed solution, and slowly stir (rotation speed 82 rpm) at 45 °C for 60 h; filter and collect the solid, wash it 5 times with deionized water, and then centrifuge at 4800 rpm for 18 min until the pH of the solid is 6.2. Treat it with a 620 W high-power ultrasonic machine for 75 min, and then centrifuge at a high speed (rotation speed 11000 rpm) for 8 min to obtain uniformly dispersed MXene nanosheets.

[0053] (2) Ultrasonically disperse anatase TiO2 sol with a mass concentration of 0.12% for 30 min, then add MXene nanosheets (the mass ratio of anatase TiO2 to MXene is 3:1), stir and disperse for 18 min, then add 10 mL of absolute ethanol and perform ultrasonic treatment for 2 h. Transfer the mixed solution to a reaction kettle and carry out hydrothermal reaction at 120 °C for 4 h. After cooling to room temperature, centrifuge to separate the product, wash it with an ethanol solution with a mass concentration of 60%, and then obtain the TiO2@MXene composite by freeze-drying.

[0054] (3) Dissolve chitosan in 1-ethyl-3-methylimidazolium acetate ionic liquid with a pH of 4.4 at 75 °C to prepare a solution with a mass concentration of 3%. Add the TiO₂@MXene composite (the mass ratio of chitosan to the TiO₂@MXene composite is 1:1), and carry out an ultrasonic chemical reaction (the power of the ultrasonic wave is 300 W) at 60 °C for 2 h. Let it stand for 35 min, then add low-molecular-weight carboxymethyl chitosan (the carboxyl content at the C6 position of low-molecular-weight carboxymethyl chitosan is 70.13%, the degree of deacetylation is 95.08%, the viscosity-average molecular weight is 0.42×10⁴, the solubility in water is 26.31 g / 100 mL, and the isoelectric point pH = 4.7) and the small-molecule cross-linking agent tartaric acid (the mass ratio of chitosan to tartaric acid is 8:1) according to the mass ratio of chitosan to low-molecular-weight carboxymethyl chitosan of 8:1, and carry out a cross-linking reaction under microwave radiation (the power of microwave radiation is 720 W, the microwave radiation temperature is 60 °C) for 120 min, so that multi-site chemical cross-linking and electrostatic interaction are formed among chitosan, low-molecular-weight carboxymethyl chitosan, and the TiO₂@MXene composite.

[0055] (4) After the reaction, let the composite hydrogel stand and age for 40 min, use an ultrasonic defoamer (the power of the ultrasonic defoamer is 2000 W) to treat it for 70 min to remove the internal bubbles of the composite hydrogel, then put it into a -75 °C ultra-low temperature refrigerator for pre-freezing for 10 h, and then carry out microwave vacuum freeze-drying (the temperature of microwave vacuum freeze-drying is -84 °C, the microwave power is 2200 W, and the vacuum degree is 8 Pa) for 22 h to obtain a chitosan / carboxymethyl chitosan / TiO₂@MXene composite aerogel with flame retardancy and photocatalytic dye degradation properties.

[0056] After testing, the loss rate of the chitosan / carboxymethyl chitosan / TiO₂@MXene composite aerogel prepared in this example is 7.11%, the specific surface area is 32.94 m 2 ² / g, the average pore diameter is 60.8 μm, the porosity is 90.59%, and the limiting oxygen index is 40.1%.

[0057] After testing the photocatalytic dye degradation performance, the photocatalytic degradation rate of the composite aerogel obtained in this example for methylene blue (MB) dye is 100% after 2 h, and the photocatalytic degradation rate for rhodamine B (RhB) dye is 98.18%; after 7 cycles of use, the photocatalytic degradation rate of the composite aerogel for methylene blue is 77.56% after 2 h, and the photocatalytic degradation rate for rhodamine B is 73.60%.

[0058] Example 4

[0059] (1) Dissolve 2 g of LiF in 40 mL of 9 mol / L HCl solution, stir at room temperature for 55 min to obtain a mixed solution, and then slowly add 2 g of Ti3AlC2 powder to the above LiF / HCl mixed solution. Stir slowly (rotation speed 95 rpm) at 45 °C for 60 h; filter to collect the solid and wash it 5 times with deionized water, then centrifuge at 4800 rpm for 20 min until the pH of the solid is 6.2. Treat it with a 650 W high-power ultrasonic machine for 75 min, and then centrifuge at high speed (rotation speed 11,800 rpm) for 10 min to obtain uniformly dispersed MXene nanosheets.

[0060] (2) Ultrasonically disperse anatase TiO2 sol with a mass concentration of 0.12% for 30 min, then add MXene nanosheets (the mass ratio of anatase TiO2 to MXene is 4:1), stir and disperse for 18 min, then add 10 mL of absolute ethanol and ultrasonically treat for 2 h. Transfer the mixed solution to a reaction kettle and carry out hydrothermal reaction at 120 °C for 4.5 h. After cooling to room temperature, centrifuge to separate the product, wash it with an ethanol solution with a mass concentration of 60%, and then obtain the TiO2@MXene composite by freeze-drying.

[0061] (3) Dissolve chitosan in 1-ethyl-3-methylimidazolium acetate ionic liquid with a pH of 4.4 at 75 °C to prepare a solution with a mass concentration of 3%. Add the TiO2@MXene composite (the mass ratio of chitosan to TiO2@MXene composite is 1:1), and carry out ultrasonic (ultrasonic power is 340 W) chemical reaction at 65 °C for 3 h, let it stand for 38 min, and then add low-molecular-weight carboxymethyl chitosan (the carboxyl content at the C6 position of low-molecular-weight carboxymethyl chitosan is 70.13%, the degree of deacetylation is 95.08%, the viscosity-average molecular weight is 0.42×10^4, the solubility in water is 26.31 g / 100 mL, and the isoelectric point pH = 4.7) and small-molecule crosslinking agent tartaric acid (the mass ratio of chitosan to tartaric acid is 9:1) according to the mass ratio of chitosan to low-molecular-weight carboxymethyl chitosan of 9:1. Carry out crosslinking reaction under microwave radiation (microwave radiation power is 760 W, microwave radiation temperature is 65 °C) for 130 min to form multi-site chemical crosslinking and electrostatic interaction among chitosan, low-molecular-weight carboxymethyl chitosan and the TiO2@MXene composite.

[0062] After the reaction, the composite hydrogel was allowed to stand and age for 40 min, and then treated with an ultrasonic defoamer (the power of the ultrasonic defoamer was 2000 W) for 80 min to remove the internal bubbles in the composite hydrogel. Then it was placed in a -78 °C ultra-low temperature refrigerator for pre-freezing for 10 h, and then subjected to microwave vacuum freeze-drying (the temperature of microwave vacuum freeze-drying was -84 °C, the microwave power was 2200 W, and the vacuum degree was 8 Pa) for 22 h to obtain a chitosan / carboxychitosan / TiO2@MXene composite aerogel with flame retardancy and photocatalytic dye degradation properties.

[0063] After testing, the dissolution rate of the chitosan / carboxychitosan / TiO2@MXene composite aerogel prepared in this example was 9.06%, the specific surface area was 30.17 m 2 / g, the average pore diameter was 66.4 μm, the porosity was 89.02%, and the limiting oxygen index was 38.7%.

[0064] After testing the photocatalytic dye degradation performance, the photocatalytic degradation rate of the composite aerogel obtained in this example for methylene blue (MB) dye was 99.73% after 2 h, and the photocatalytic degradation rate for rhodamine B (RhB) dye was 97.05%; after 7 cycles of use, the photocatalytic degradation rate of the composite aerogel for methylene blue was 75.02% after 2 h, and the photocatalytic degradation rate for rhodamine B was 72.18%.

[0065] Comparative Example 1

[0066] Preparation of chitosan / carboxychitosan composite aerogel (without adding TiO2@MXene complex):

[0067] (1) Chitosan was dissolved in 1-ethyl-3-methylimidazolium acetate ionic liquid with a pH of 4.4 at 75 °C to prepare a solution with a mass concentration of 3%. Then, low-molecular-weight carboxychitosan (the carboxyl group content at the C6 position of low-molecular-weight carboxychitosan was 70.13%, the degree of deacetylation was 95.08%, the viscosity-average molecular weight was 0.42×10^4, the solubility in water was 26.31 g / 100 mL, and the isoelectric point pH = 4.7) and small-molecule crosslinking agent tartaric acid (the mass ratio of chitosan to tartaric acid was 8:1) were added according to the mass ratio of chitosan to low-molecular-weight carboxychitosan of 8:1, and crosslinking reaction was carried out under microwave radiation (the power of microwave radiation was 720 W, and the microwave radiation temperature was 60 °C) for 120 min to form multi-site chemical crosslinking and electrostatic interaction between chitosan, low-molecular-weight carboxychitosan and small-molecule crosslinking agent tartaric acid.

[0068] (2)After the reaction, the composite hydrogel was allowed to stand and age for 40 min, and then treated with an ultrasonic defoamer (the power of the ultrasonic defoamer was 2000 W) for 70 min to remove the internal bubbles of the composite hydrogel. Then it was placed in a -75 °C ultra-low temperature refrigerator for pre-freezing for 10 h, and then subjected to microwave vacuum freeze-drying (the temperature of microwave vacuum freeze-drying was -84 °C, the microwave power was 2200 W, and the vacuum degree was 8 Pa) for 22 h to obtain a chitosan / carboxylated chitosan composite aerogel.

[0069] After testing, the dissolution rate of the chitosan / carboxylated chitosan composite aerogel prepared in this comparative example was 16.51%, the specific surface area was 30.24 m 2 / g, the average pore size was 72.6 μm, the porosity was 89.17%, and the limiting oxygen index was 24.5%. After testing the photocatalytic degradation performance of dyes, the composite aerogel obtained in this comparative example had basically no photocatalytic degradation effect on methylene blue (MB) dye and rhodamine B (RhB) dye.

[0070] Comparative Example 2

[0071] Preparation of chitosan / carboxylated chitosan / TiO2@MXene composite aerogel (without adding small molecule crosslinking agent):

[0072] (1)Dissolve 2 g of LiF in 40 mL of 9 mol / L HCl solution, stir at room temperature for 50 min to obtain a mixed solution, then slowly add 2 g of Ti3AlC2 powder to the above LiF / HCl mixed solution, and slowly stir (rotation speed 82 rpm) at 45 °C for 60 h; filter and collect the solid, wash it 5 times with deionized water, then centrifuge at 4800 rpm for 18 min until the pH of the solid is 6.2, treat it with a 620 W high-power ultrasonic machine for 75 min, and then centrifuge at a high speed (rotation speed 11000 rpm) for 8 min to obtain uniformly dispersed MXene nanosheets.

[0073] (2)Ultrasonically disperse anatase TiO2 sol with a mass concentration of 0.12% for 30 min, then add MXene nanosheets (the mass ratio of anatase TiO2 to MXene is 3:1), stir and disperse for 18 min, then add 10 mL of absolute ethanol and perform ultrasonic treatment for 2 h. Transfer the mixed solution to a reaction kettle and carry out hydrothermal reaction at 120 °C for 4 h. After cooling to room temperature, centrifuge to separate the product, wash it with an ethanol solution with a mass concentration of 60%, and then obtain the TiO2@MXene composite by freeze-drying.

[0074] (3) Dissolve chitosan in 1-ethyl-3-methylimidazolium acetate ionic liquid with a pH of 4.4 at 75 °C to prepare a solution with a mass concentration of 3%. Add the TiO2@MXene composite (the mass ratio of chitosan to the TiO2@MXene composite is 1:1), and perform ultrasonic (ultrasonic power is 300 W) chemical reaction at 60 °C for 2 h. Let it stand for 35 min, and then add low-molecular-weight carboxymethyl chitosan according to the mass ratio of chitosan to low-molecular-weight carboxymethyl chitosan of 8:1 (the carboxyl content at the C6 position of low-molecular-weight carboxymethyl chitosan is 70.13%, the degree of deacetylation is 95.08%, the viscosity-average molecular weight is 0.42×10^4, the solubility in water is 26.31 g / 100 mL, and the isoelectric point pH = 4.7) for cross-linking reaction under microwave radiation (microwave radiation power is 720 W, microwave radiation temperature is 60 °C) for 120 min, so that chitosan, low-molecular-weight carboxymethyl chitosan and the TiO2@MXene composite form multi-site chemical cross-linking and electrostatic interaction.

[0075] (4) After the reaction, let the composite hydrogel stand and age for 40 min. Use an ultrasonic defoamer (ultrasonic defoamer power is 2000 W) to treat it for 70 min to remove the internal bubbles of the composite hydrogel, and then place it in a -75 °C ultra-low temperature refrigerator for pre-freezing for 10 h. Then, perform microwave vacuum freeze-drying (microwave vacuum freeze-drying temperature is -84 °C, microwave power is 2200 W, vacuum degree is 8 Pa) for 22 h to obtain the chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel.

[0076] After testing, the dissolution rate of the chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel prepared in this comparative example is 20.14%, the specific surface area is 13.36 m 2 / g, the average pore diameter is 129.4 μm, the porosity is 70.46%, and the limiting oxygen index is 30.9%.

[0077] After testing the photocatalytic degradation performance of dyes, the photocatalytic degradation rate of the composite aerogel obtained in this comparative example for methylene blue (MB) dye is 88.47% after 2 h, and the photocatalytic degradation rate for rhodamine B (RhB) dye is 87.15%; after 7 cycles of use, the photocatalytic degradation rate of the composite aerogel for methylene blue is 51.07% after 2 h, and the photocatalytic degradation rate for rhodamine B is 48.21%.

[0078] II. Detection tests are carried out on the samples obtained in the above examples

[0079] Test item 1: Dissolution rate, specific surface area, average pore diameter, porosity, and mechanical property tests of composite aerogels formed by adding different masses of small molecule cross-linking agents

[0080] According to the method in Example 3, chitosan / carboxylated chitosan / TiO2@MXene composite aerogels were prepared by changing the mass ratio of chitosan to the small molecule cross-linking agent. The test results of indexes such as the dissolution rate, specific surface area, average pore size, porosity, and mechanical properties of the composite aerogels are shown in Table 1. The mechanical properties of the composite aerogels were tested by compression on a CMT6104 universal testing machine according to the ASTM D3575-14 standard. The maximum pressure that the composite aerogel per square meter area can withstand without collapsing was measured, and the constant compression speed was 2 mm / min.

[0081] Table 1 Test results of the physical and chemical property indexes of the composite aerogels

[0082]

[0083] In the present invention, chitosan was dissolved in 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) ionic liquid, and the TiO2@MXene composite material was added under ultrasonic action for electrostatic self-assembly. Then, low molecular weight carboxylated chitosan and a small molecule cross-linking agent were added under microwave radiation to enhance the cross-linking reaction and electrostatic force. After that, microwave vacuum freeze-drying was carried out to obtain a chitosan / carboxylated chitosan / TiO2@MXene composite aerogel with a stable structure, high porosity, and a three-dimensional network skeleton. The data in Table 1 show that by optimizing the addition amount of the small molecule cross-linking agent, the carboxyl groups and anhydrides in the small molecule cross-linking agent can covalently cross-link with the amino groups and hydroxyl groups in the chitosan and carboxylated chitosan molecules dissolved in the 1-ethyl-3-methylimidazolium acetate ionic liquid. At the same time, the positively charged amino groups in the chitosan and carboxylated chitosan molecules can also form hydrogen bonds and electrostatic binding with groups such as -OH, -F, and -O in the TiO2@MXene composite, and further electrostatic self-assembly occurs under the connection of the small molecule cross-linking agent to form a stable three-dimensional skeleton structure, enhancing the mechanical properties of the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel. Compared with the composite aerogel samples without adding the small molecule cross-linking agent, the composite aerogels with different mass ratios of the small molecule cross-linking agent added have smaller average pore sizes and dissolution rates, higher specific surface areas and porosities, and significantly improved mechanical properties.

[0084] Test item 2: Cross-sectional scanning electron microscopy analysis of the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel

[0085] The micropore distribution on the cross-section of the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel was observed using a scanning electron microscope (200×). Four samples of the composite aerogel were taken. The first sample was the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel sample obtained by the method of Comparative Example 2. The second sample was the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel obtained by the method of Example 1. The third sample was the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel obtained by the method of Example 2. The fourth sample was the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel obtained by the method of Example 3. The test results are shown in Figure 3 Figures (a) to (d) in turn.

[0086] It can be seen from Figure 3 that the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel presents a typical porous three-dimensional network structure. This indicates that the amino group of chitosan dissolved in the acidic ionic liquid 1-ethyl-3-methylimidazolium acetate carries a positive charge and can interact with the negatively charged groups such as -OH, -F, and -O on the surface of the TiO2@MXene complex through hydrogen bonding and electrostatic forces. At the same time, the carboxyl group in the low-molecular-weight carboxylated chitosan molecule can form an amide bond cross-linking with the amino group of chitosan, and the amino and hydroxyl groups in its molecule can form multi-site hydrogen bonding with the abundant negatively charged groups on the surface of TiO2@MXene, thus constructing the framework structure of the composite aerogel. For the composite aerogel prepared without adding a small molecule cross-linking agent, due to the lack of cross-linking bonds and electrostatic interactions generated by the small molecule cross-linking agent between chitosan, low-molecular-weight carboxylated chitosan, and TiO2@MXene nanosheets, the cross-linking degree between the components of the composite aerogel is reduced, and a relatively tight pore structure cannot be formed, and the structural stability of the aerogel is relatively loose (see Figure 3 Figure a), and at the same time, it is not conducive to the electrostatic assembly and uniform distribution of the TiO2@MXene composite nanosheets, resulting in a decrease in the photocatalytic dye degradation efficiency and reusability of the composite aerogel. However, adding a small molecule cross-linking agent can further enhance the covalent bond binding between chitosan and low-molecular-weight carboxylated chitosan, as well as the hydrogen bonding and electrostatic self-assembly with the -OH, -F, -O and other groups on the surface of the TiO2@MXene complex, thereby forming an interconnected and dense three-dimensional porous structure with smaller pore sizes and an increased specific surface area (see Figure 3 Figures b to d); at the same time, the TiO2@MXene composite nanosheets are tightly encapsulated by chitosan and low-molecular-weight carboxylated chitosan and evenly distributed on the surface of the aerogel, and the surface of the composite aerogel becomes rough, which helps to improve the flame retardancy and photocatalytic performance of the composite aerogel.

[0087] Test item 3: Cone calorimetry analysis of the chitosan / carboxylated chitosan / TiO2@MXene composite aerogel

[0088] The fire behavior of the composite aerogel was analyzed using a cone calorimeter. Two samples of the composite aerogel were taken. The first was a chitosan / carboxymethyl chitosan composite aerogel sample obtained by the method of Comparative Example 1, and the second was a chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel obtained by the method of Example 3. The test results are shown in Table 2.

[0089] Table 2 Cone calorimeter test results of the composite aerogel

[0090]

[0091] The ignition time (TTI), peak heat release rate (PHRR), and total heat release (THR) are several important parameters of the combustion behavior during the combustion process of the composite material. The chitosan / carboxymethyl chitosan composite aerogel without the addition of the TiO2@MXene complex reached the PHRR at 9 s and burned out at 39 s. The TTI, PHRR, and THR values were 10 s, 76.966 KW / m 2 and 4.351 MJ / m 2 , obviously, the chitosan / carboxymethyl chitosan composite aerogel is a highly flammable composite material. When the chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel reached the PHRR at 14 s and burned out at 61 s, the combustion time increased by 56.4% compared with the chitosan / carboxymethyl chitosan composite aerogel without the addition of the TiO2@MXene complex. The TTI, PHRR, and THR values were 28 s, 39.499 KW / m 2 and 3.269 MJ / m 2 , and the PHRR and THR values decreased by 48.67% and 24.86% respectively compared with the chitosan / carboxymethyl chitosan composite aerogel. This indicates that the addition of the TiO2@MXene complex can reduce the PHRR and THR during the combustion process of the aerogel and prevent the rapid heat release of the aerogel. At the same time, Table 2 shows that the total smoke production (TSP) value of the chitosan / carboxymethyl chitosan / TiO2@MXene composite aerogel is 0.062 m 2 , which is 77.62% lower than that of the chitosan / carboxymethyl chitosan aerogel (0.277 m 2 ); meanwhile, the addition of the TiO2@MXene complex also reduces the release of CO and CO2. This shows that the layered blocking effect of the TiO2@MXene complex nanosheets and the synergistic carbonization effect of MXene with chitosan and carboxymethyl chitosan can strengthen the generation of an integrated and dense protective carbon layer, thereby inhibiting the emission of smoke during the combustion process. It can be seen that the addition of the TiO2@MXene complex can effectively improve the flame retardant performance of the composite aerogel.

[0092] In summary, in the present invention, chitosan is dissolved in 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) acidic ionic liquid, and the TiO2@MXene composite material is added under ultrasonic action for electrostatic self-assembly. Then, low-molecular-weight carboxymethyl chitosan and small-molecule crosslinking agent are added under microwave radiation to enhance the crosslinking reaction and electrostatic force. Finally, the composite aerogel material of chitosan / carboxymethyl chitosan / TiO2@MXene with stable structure, good flame retardancy, high photocatalytic activity and three-dimensional network is prepared by microwave vacuum freeze-drying. The method of the present invention is simple, easy to operate, has mild reaction conditions, is green and environmentally friendly. The obtained composite aerogel has the advantages of good mechanical properties, low dissolution rate, high porosity, high-efficiency flame retardancy and strong photocatalytic activity, and has great application potential in the fields of photocatalytic degradation of pollutants, high-temperature heat insulation, flame retardancy and fire prevention, etc.

[0093] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A composite aerogel having both flame retardancy and photocatalytic degradation of dyes, characterized in that: The composite aerogel is prepared by dissolving chitosan in 1-ethyl-3-methylimidazolium acetate ionic liquid, adding TiO2@MXene complex under ultrasonic action for electrostatic self-assembly, then adding low molecular carboxyl chitosan and small molecule cross-linking agent under microwave radiation to enhance cross-linking reaction and electrostatic force, and then performing microwave vacuum freeze drying; The preparation method of the TiO2@MXene composite is: Dissolve 1-2 g of LiF in 20-40 mL of 9 mol / L HCl solution, stir at room temperature for 30-60 min to obtain a mixed solution, then add 1-2 g of Ti3AlC2 powder, and stir at 45 °C for 48-60 h; collect the solid by filtration and wash with deionized water 3-5 times, then centrifuge at 3500-5000 rpm for 10-20 min until the pH of the solid is 6.0-6.4, treat with a 500-680 W high-power ultrasonic machine for 60-80 min, and then centrifuge at high speed for 6-10 min to obtain uniformly dispersed MXene nanosheets; Ultrasonic dispersion of anatase TiO2 sol with a mass concentration of 0.1-0.2% for 20-30 minutes, then adding the MXene nanosheets and stirring and dispersing for 10-20 minutes, then adding 10 mL of anhydrous ethanol for ultrasonic treatment for 1-2 hours, transferring the mixed solution to a reactor for hydrothermal reaction at 100-120° C. for 3-5 hours, cooling to room temperature, centrifuging and separating the product, washing with an ethanol solution with a mass concentration of 60%, and then freeze-drying to obtain a TiO2@MXene composite; the mass ratio of anatase TiO2 to MXene nanosheets is 1-6:1; The small molecule cross-linking agent is selected from tartaric acid, malic acid, oxalic acid, glutamic acid, maleic acid or maleic anhydride.

2. The flame-retardant composite aerogel having photocatalytic degradation of dyes according to claim 1, characterized in that: The low molecular weight carboxyl chitosan has a C6 carboxyl content of 50.64-71.26%, a deacetylation degree of 89.17-96.48%, a viscosity average molecular weight of 0.35-0.89 thousand, a solubility in water of 17.83-28.02 g / 100 mL, an isoelectric point pH of 4.7-5.1, and a structural formula as follows: 。 3. The flame-retardant composite aerogel having photocatalytic degradation of dyes according to claim 1, characterized in that: The composite aerogel has a dissolution rate of 7.06-14.51% and a specific surface area of ​​13.72-32.96 m 2 / g, the average pore size is 60.3~96.9μm, the porosity is 81.42~90.68%, and the limiting oxygen index is 30.5~40.1%.

4. A method for preparing the flame-retardant composite aerogel having both photocatalytic degradation of dyes as claimed in any one of claims 1 to 3, characterized in that: The steps include: (1) Dissolve 1-2 g of LiF in 20-40 mL of 9 mol / L HCl solution, stir at room temperature for 30-60 min to obtain a mixed solution, then add 1-2 g of Ti3AlC2 powder, and stir at 45 °C for 48-60 h; collect the solid by filtration and wash it with deionized water for 3-5 times, then centrifuge at 3500-5000 rpm for 10-20 min until the pH of the solid is 6.0-6.4, treat it with a 500-680 W high-power ultrasonic machine for 60-80 min, and then centrifuge it at high speed for 6-10 min to obtain uniformly dispersed MXene nanosheets; (2) ultrasonically dispersing anatase TiO2 sol with a mass concentration of 0.1 to 0.2% for 20 to 30 minutes, adding the MXene nanosheets and stirring and dispersing them for 10 to 20 minutes, then adding 10 mL of anhydrous ethanol and ultrasonically treating them for 1 to 2 hours, transferring the mixed solution to a reactor and performing a hydrothermal reaction at 100 to 120° C. for 3 to 5 hours, cooling to room temperature, centrifuging and separating the product, washing it with an ethanol solution with a mass concentration of 60%, and then freeze-drying it to obtain a TiO2@MXene composite; the mass ratio of anatase TiO2 to MXene nanosheets is 1 to 6:1; (3) Dissolving chitosan in 1-ethyl-3-methylimidazolium acetate ionic liquid at 60-80° C. to prepare a solution with a mass concentration of 1-4%, adding TiO2@MXene composite to carry out ultrasonic chemical reaction at 50-70° C. for 1-3 hours, standing for 20-40 minutes, and then adding low molecular weight carboxyl chitosan and small molecule crosslinking agent to carry out crosslinking reaction under microwave radiation for 45-140 minutes, so that chitosan, low molecular weight carboxyl chitosan and TiO2@MXene composite form multi-site chemical crosslinking and electrostatic force; the mass ratio of chitosan to TiO2@MXene composite is 1:1; the mass ratio of chitosan to low molecular weight carboxyl chitosan is 3-10:1; the mass ratio of chitosan to small molecule crosslinking agent is 4-10:1; the small molecule crosslinking agent is selected from tartaric acid, malic acid, oxalic acid, glutamic acid, maleic acid or maleic anhydride; (4) After the reaction is completed, the composite hydrogel is allowed to stand for 20 to 45 minutes, and is treated with an ultrasonic defoamer for 30 to 90 minutes to remove bubbles inside the composite hydrogel. The composite hydrogel is then placed in an ultra-low temperature refrigerator at -80 to -65°C for pre-freezing for 6 to 10 hours, and then subjected to microwave vacuum freeze drying for 18 to 24 hours to obtain a composite aerogel that is flame retardant and has photocatalytic properties for degrading dyes.

5. The preparation method according to claim 4, characterized in that: In step (3), the pH of the 1-ethyl-3-methylimidazolium acetate ionic liquid is 4.2 to 5.

0.

6. The preparation method according to claim 4, characterized in that: In step (3), the ultrasonic power of the ultrasonic chemical reaction is 240-360 W, the power of the microwave radiation is 620-800 W, and the microwave radiation temperature is 50-65° C.

7. The preparation method according to claim 4, characterized in that: In step (4), the power of the ultrasonic defoamer is 1400-2100W.

8. The preparation method according to claim 4, characterized in that: In step (4), the temperature of the microwave vacuum freeze-drying is -86 to -70°C, the microwave power is 1800 to 2400W, and the vacuum degree is 6 to 15Pa.

9. An application of the flame-retardant composite aerogel having both photocatalytic degradation of dyes according to any one of claims 1 to 3, characterized in that: Used for flame retardancy and / or photocatalytic degradation of dyes in wastewater.

10. The use according to claim 9, characterized in that: The chitosan / carboxyl chitosan / TiO2@MXene composite aerogel after photodegradation of the dye was regenerated by decomposing the dye under ultraviolet irradiation in clean water.

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