Chitosan / low-molecular-weight oxidized chitosan / titanium dioxide composite aerogel and application thereof

Abstract

This invention discloses a chitosan / low-molecular-weight chitosan oxide / titanium dioxide composite aerogel and its applications. The process involves dissolving chitosan and low-molecular-weight chitosan oxide in dilute nitric acid solution, adding a 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst, and conducting a nucleophilic reaction between chitosan and low-molecular-weight chitosan oxide under microwave irradiation. A solution of citric acid and tetraisopropyl titanate in isopropanol is added to the mixture in a vortex mixer and stirred until homogeneous. Then, the mixed solution is dropwise added to the chitosan / low-molecular-weight chitosan oxide solution under ultrasonic irradiation to carry out a sonic chemical reaction and cross-linking self-assembly. Finally, the composite aerogel with high porosity, stable structure, and a three-dimensional network framework is prepared by microwave vacuum freeze-drying. The composite aerogel obtained by this invention has advantages such as good mechanical properties, low solubility, low density, large specific surface area, and strong photocatalytic activity, showing great potential for application in the photodegradation of dyes, pesticides, and pollutants in wastewater.

Description

Technical Field

[0001] This invention relates to a method for preparing chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel and its application in photocatalytic degradation of dyes, belonging to the field of composite aerogel functional materials. Background Technology

[0002] The textile industry is a vital sector for national welfare and people's livelihood, but it is also a major consumer of water and energy resources. The annual wastewater discharge from the textile industry reaches 2 billion tons, ranking third among 41 industries nationwide. Among these, dyeing and printing, as a key link in enhancing the added value of textiles, accounts for 70-80% of the total wastewater and pollutant discharge from the entire industry. Dyeing and printing wastewater has a complex composition, containing various organic dyes, sizing agents, salt and alkali auxiliaries, and additives. It is highly toxic, difficult to treat, and easily produces substances toxic and harmful to animal and human health through oxidation, hydrolysis, and other chemical reactions in water bodies. Therefore, the treatment and recycling of dyeing and printing wastewater has become an urgent problem to be solved.

[0003] Chitosan (CS) is a product of the deacetylation of chitin. It is a green, natural, abundant, and widely used high-molecular-weight polymer polysaccharide. Due to its high viscosity, excellent adhesion properties, unique structural characteristics, polyelectrolyte properties, heavy metal chelation, and optical properties [Keshvardoostchokami M, Majidi M, Zamani A, et al. A review on the use of chitosan and chitosan derivatives as the bio-adsorbents for the water treatment: Removal of nitrogen-containing pollutants[J]. Carbohydrate Polymers, 2021, 273: 118625], chitosan has great potential in the treatment of dyeing and printing wastewater. Chitosan aerogels possess porous structures, large specific surface areas, and excellent adsorption properties, making them promising candidates for water treatment. Furthermore, the CS molecular chains of the aerogel contain more primary amine (-NH2) and hydroxyl (-OH) groups than ordinary polymers, providing numerous active sites for effectively adsorbing organic pollutants [Alves D, Healy B, Pinto L, et al. Recent developments inchitosan-Based adsorbents for the removal of pollutants from aqueous environments[J]. Molecules, 2021, 26(3):594]. Under acidic conditions, they can undergo ion exchange with anionic dyes; under neutral conditions, the amino groups undergo weak protonation, enabling chelation with metal cations, thus offering greater advantages in adsorbing dyes from dyeing and printing wastewater. However, the skeletal structure of chitosan aerogels is relatively fragile, and their mechanical stiffness, strength, and durability are also relatively weak, making them prone to dissolution and swelling in solution. This significantly limits the practical use and reusability of chitosan aerogels.

[0004] Titanium dioxide (TiO2) is a novel photocatalytic material that can convert organic pollutants such as dyes, surfactants, polymers, and aromatics in dyeing and printing wastewater into small-molecule, non-toxic compounds under ultraviolet light irradiation. The energy in the reaction process is largely derived from solar radiation, and the photocatalytic reaction produces no secondary pollution. The photocatalytic efficiency of TiO2 is mainly limited by its large band gap and the rapid recombination of photogenerated electron / hole pairs. Powdered TiO2 photocatalysts have low adsorption capacity, are prone to agglomeration, and photogenerated electrons and holes easily recombine, resulting in low utilization of visible light. Furthermore, separation from solution is difficult, hindering recovery and making it unsuitable for large-scale industrial use. Surface modification, doping, and composite materials can be used to improve the photocatalytic performance of TiO2. Currently, TiO2 / SiO2 aerogels are commonly used for wastewater treatment. The addition of SiO2 increases the specific surface area of ​​the aerogel and the number of reaction sites. However, studies have shown that the TiO2 and SiO2 phases are not sufficiently bonded at the atomic size level, resulting in poor photocatalytic activity and limiting its application. Chitosan / TiO2 aerogel can efficiently treat wastewater and is easy to recycle and reuse. The abundant amino and hydroxyl groups in chitosan can effectively enrich pollutants in water. The three-dimensional network formed by chitosan can uniformly load TiO2, which is beneficial to the adsorption and photocatalytic degradation of pollutants.

[0005] Tetraisopropyl titanate, as a common "titanium source," can form TiO2 through hydrolysis. However, the rapid hydrolysis rate of tetraisopropyl titanate affects the particle size of TiO2. Particle size is a decisive factor in heterogeneous catalysis. A smaller particle size increases the specific surface area, thereby increasing the number of active surface sites per square meter, increasing the adsorption capacity, and giving TiO2 higher catalytic activity [QiH U, Baoshun L, Zhengzhong Z, et al. Temperature effect on the photocatalytic degradation of methyl orange under UV-vis light irradiation[J]. Journal of Wuhan University of Technology, 2010, 25(2):210-213]. Nitrate ions can inhibit the rapid hydrolysis of tetraisopropyl titanate, effectively control the particle size of TiO2, and enhance its photocatalytic performance. The surface of low-molecular-weight oxidized chitosan has abundant active groups such as carboxyl and aldehyde groups, which can effectively adsorb TiO2, avoid rapid recombination of TiO2 and chitosan, and increase the stability of the aerogel. The chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel of the present invention has high photocatalytic activity and good mechanical properties, and has great potential for application in the photodegradation of dyes, pesticides, pollutants and other fields in wastewater. Summary of the Invention

[0006] In view of the above-mentioned problems existing in the prior art, the purpose of this invention is to provide a chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel, its preparation method and application, to obtain a composite aerogel material with good mechanical properties, low solubility, low density, large specific surface area and strong photocatalytic activity, which has broad application prospects in the field of photodegradation of dyes, pesticides and pollutants in wastewater.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A chitosan / low molecular weight chitosan oxide / titanium dioxide composite aerogel is prepared by dissolving chitosan and low molecular weight chitosan oxide in dilute nitric acid solution, adding 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst, and conducting a nucleophilic reaction between chitosan and low molecular weight chitosan oxide under microwave irradiation. A solution of citric acid and tetraisopropyl titanate in isopropanol is added to a vortex mixer and mixed thoroughly. Then, the mixed solution is dropwise added to the chitosan / low molecular weight chitosan oxide solution under ultrasonic irradiation to carry out ultrasonic chemical reaction and cross-linking self-assembly. Finally, it is obtained by microwave vacuum freeze-drying.

[0009] The chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel has a solubility loss rate of 8.51–15.93%, a specific surface area of ​​17.92–40.15 m² / g, an average pore size of 41.3–90.6 μm, a porosity of 82.67–93.26%, and a density of 32.18–51.07 mg / cm³. 3 .

[0010] Preferably, the low-molecular-weight oxidized chitosan has an aldehyde content of 41.39–60.57% at the C6 position, a carboxyl content of 23.64–45.12% at the C2 and C3 positions, a degree of deacetylation of 85.22–92.86%, a viscosity-average molecular weight of 0.7–1.3 million, a water solubility of 12.17–23.54 g / 100 mL, and an isoelectric point pH of 5.1–5.4. Its structural formula is as follows:

[0011]

[0012] The preparation method of the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel of the present invention is carried out according to the following steps:

[0013] (1) Chitosan and low-molecular-weight oxidized chitosan are dissolved in a 0.02-0.06 mol / L dilute nitric acid solution to prepare a solution with a total mass concentration of 1-3%. A 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst is added and the cross-linking reaction is catalyzed under microwave irradiation for 0.5-2 h to form multi-site chemical cross-links between chitosan and low-molecular-weight oxidized chitosan. The mass ratio of chitosan to low-molecular-weight oxidized chitosan is 4-12:1. The mass concentration of the 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst in the mixed solution is 2-8%.

[0014] (2) Add 5-10 mL of citric acid solution with a mass concentration of 10-15% and 100 mL of tetraisopropyl titanate isopropanol solution to a vortex mixer and stir for 20-40 min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound, add the mixed solution dropwise to the reaction solution of step (1) over 10-20 min to carry out ultrasonic chemical reaction for 2-4 h to obtain a composite hydrogel; the volume ratio of tetraisopropyl titanate to isopropanol is 1:8-12.

[0015] (3) After the reaction is completed, the composite hydrogel is allowed to stand for 60-90 minutes, then placed in an ultra-low temperature freezer at -60 to -40℃ for 2-5 hours, and then microwave vacuum freeze-dried for 24-36 hours to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

[0016] Preferably, in step (1) of the present invention, the power of microwave radiation is 380-600W and the temperature of microwave radiation is 30-45℃.

[0017] Preferably, in step (2) of the present invention, the power of the vortex mixer is 40-80W and the stirring speed is 500-700rpm.

[0018] Preferably, in step (2) of the present invention, the ultrasonic reaction is intermittent, with a 2-minute pause after every 30 minutes of ultrasonic reaction, and the power of ultrasonic oscillation is 160-220W.

[0019] Preferably, in step (3) of the present invention, the microwave vacuum freeze-drying temperature is -65 to -40°C, the microwave power is 840 to 1600W, and the vacuum degree is 20 to 35Pa.

[0020] By optimizing the mass ratio of chitosan to low-molecular-weight oxidized chitosan, the microwave radiation reaction temperature and time, the ultrasonic treatment time, the concentration of 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst, and the volume ratio of citric acid to tetraisopropyl titanate in isopropanol solution, a series of composite aerogel materials with different porosities and densities can be obtained.

[0021] The chitosan / low-molecular-weight oxidized chitosan / titanium dioxide composite aerogel of this invention can be used for photodegradation of dyes in wastewater, achieving a photodegradation rate of over 93.61% for methyl orange. Furthermore, the chitosan / low-molecular-weight oxidized chitosan / titanium dioxide composite aerogel, after photodegradation of the dye, can be regenerated and reused by decomposing the dye in clean water under ultraviolet light. Even after five cycles of use, the photodegradation rate of methyl orange can still reach 72.56–83.49% after a 2-hour period of degradation.

[0022] Compared with existing technologies, the preparation principle and beneficial effects of the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel in this invention are reflected in the following aspects:

[0023] 1. This invention involves adding a mixed solution of citric acid and tetraisopropyl titanate in isopropanol dropwise to a dilute nitric acid solution of chitosan and low-molecular-weight oxidized chitosan. Anatase TiO2 nanoparticles are prepared using a tetraisopropyl titanate hydrolysis method. During the hydrolysis of tetraisopropyl titanate, the addition of nitric acid readily reacts to generate Ti(NO3)4, which is extremely unstable and rapidly hydrolyzes in water to form TiO2. Simultaneously, nitrate ions can react with TiOH generated during the hydrolysis of tetraisopropyl titanate. 3+ The formation of a bridging structure restricts the orientation of these hydrolysis products after nucleation. Simultaneously, citric acid acts as a chelating agent to control the hydrolysis rate of tetraisopropyl titanate, which is beneficial for obtaining smaller, more uniform, and stable anatase-phase nano-titanium dioxide particles. Therefore, the method of this invention avoids the problems of easy agglomeration and uneven dispersion that occur with the direct addition of nano-TiO2. TiO2 nanoparticles are generated in situ through the hydrolysis of tetraisopropyl titanate in a nitric acid solution of chitosan and low-molecular-weight oxidized chitosan, resulting in a uniform and stable distribution of nano-TiO2 within the chitosan / low-molecular-weight oxidized chitosan composite aerogel matrix.

[0024] 2. The low-molecular-weight oxidized chitosan used in this invention has an aldehyde content of 41.39–60.57% at the C6 position, a carboxyl content of 23.64–45.12% at the C2 and C3 positions, a degree of deacetylation of 85.22–92.86%, a water solubility of 12.17–23.54 g / 100 mL, and an isoelectric point pH of 5.1–5.4. Compared with ordinary chitosan, low-molecular-weight oxidized chitosan has good water solubility, avoiding the use of acids and organic solvents and environmental pollution. Furthermore, the oxidized chitosan molecule contains both carboxyl and amino groups, making it an amphoteric polyelectrolyte with a structure similar to proteins, resulting in superior biocompatibility and affinity. The aldehyde group introduced at the C6 position of the low molecular weight oxidized chitosan molecule can undergo Schiff base crosslinking with the amino group of chitosan, and the carboxyl groups introduced at the C2 and C3 positions of its molecular chain can form amide bonds with the amino group of chitosan. At the same time, the low molecular weight oxidized chitosan has a small molecular weight (viscosity average molecular weight of 0.7-1.3 million) and low solution viscosity, which makes it easy to mix uniformly with macromolecular chitosan. This promotes the formation of multi-site crosslinking between chitosan macromolecules and significantly increases the skeletal strength and structural stability of the chitosan / low molecular weight oxidized chitosan aerogel matrix.

[0025] 3. This invention utilizes 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid as a catalyst. It has a wide liquid range, from room temperature to above 300°C, exhibits high thermal and chemical stability, and has a low vapor pressure. It is non-volatile, recyclable, and environmentally friendly. 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid is an acidic ionic liquid, which can positively charge the amino groups of chitosan, increasing the probability of affinity addition reactions between the amino groups and the aldehyde groups at the C6 position and the carboxyl groups at the C2 and C3 positions of low molecular weight oxidized chitosan. This is beneficial for accelerating the catalytic cross-linking reaction between chitosan and low molecular weight oxidized chitosan.

[0026] 4. This invention utilizes microwave radiation to catalyze the cross-linking reaction between chitosan and low-molecular-weight oxidized chitosan. Microwaves offer advantages such as rapid heating, precise temperature control, and energy efficiency. In the microwave radiation reaction, microwave radiation can effectively act on polar substances to accelerate the reaction. The radiation effect of microwaves enables the molecules of chitosan and low-molecular-weight oxidized chitosan to absorb energy, promoting the movement between molecular chain segments and accelerating the rate of multi-site cross-linking reaction between chitosan and low-molecular-weight oxidized chitosan under the catalysis of 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid, thus saving reaction time and reducing energy consumption.

[0027] 5. In this invention, a mixed solution of citric acid and tetraisopropyl titanate in isopropanol is added dropwise to a dilute nitric acid solution of chitosan and low-molecular-weight oxidized chitosan to undergo an ultrasonic chemical reaction, thereby crosslinking into a composite aerogel. The ultrasonic waves promote the homogeneous reaction of the substances and the dispersion of nano-TiO2 particles and macromolecular chitosan generated by the hydrolysis of tetraisopropyl titanate in nitric acid. At the same time, the ultrasonic waves generate a cavitation effect, in which cavitation bubbles form, grow, and collapse in the reaction solution. When the cavitation bubbles collapse, high temperature and high pressure are generated in a very short time and in a very small space, which provides an energy source for ultrasonic chemical synthesis and greatly accelerates the crosslinking reaction of chitosan and low-molecular-weight oxidized chitosan with TiO2 particles. Ultrasound has a significant dispersing effect on chitosan macromolecules. Through the strong impact and cavitation of ultrasound, the entangled chitosan macromolecular chains are uniformly separated, allowing low molecular weight oxidized chitosan and TiO2 nanoparticles to fully contact the cationic amino groups in the chitosan molecules. This promotes the ultrasonic chemical cross-linking reaction between chitosan, low molecular weight oxidized chitosan, and TiO2 nanoparticles, thereby forming a composite aerogel material with a stable three-dimensional network structure.

[0028] 6. This invention involves pre-freezing a chitosan / low-molecular-weight chitosan oxide / nano-TiO2 hydrogel in an ultra-low temperature freezer, followed by microwave vacuum freeze-drying to obtain a composite aerogel. Pre-freezing rapidly solidifies the water inside the chitosan / low-molecular-weight chitosan oxide / titanium dioxide hydrogel, forming ice crystals and creating a stable three-dimensional framework structure within the chitosan / low-molecular-weight chitosan oxide / titanium dioxide matrix. Freeze-drying then removes the solidified ice crystals, effectively preventing the collapse of the chitosan / low-molecular-weight chitosan oxide / titanium dioxide composite aerogel caused by rapid water removal. This invention uses microwave vacuum freeze-drying to obtain the chitosan / low-molecular-weight chitosan oxide / titanium dioxide composite aerogel, combining microwave drying and vacuum drying to fully utilize their respective advantages. The ice crystals generated inside the chitosan / low-molecular-weight chitosan oxide / titanium dioxide hydrogel during pre-freezing are sublimated and dried under vacuum and below the eutectic temperature. A microwave generator provides the latent heat of sublimation to the frozen material to be dried, rapidly removing moisture from the composite aerogel. Compared to the slow heat conduction rate under conventional vacuum conditions, this method shortens the drying time and improves drying efficiency. Microwave vacuum freeze drying provides uniform temperature, ensures uniform distribution of water molecules in the composite aerogel, and maintains a consistent water drying rate. This preserves the porous network structure of the composite aerogel, which is beneficial to the stability of the composite aerogel skeleton.

[0029] 7. This invention uses a vortex mixer to uniformly mix citric acid and the isopropanol solution of tetraisopropyl titanate. In the vortex mixer, citric acid and the isopropanol solution of tetraisopropyl titanate form a vortex mixing, which is fast and can mix all the test liquids adhering to the cup wall uniformly. It does not require magnetic stirring, is not affected by external contamination and magnetic fields, shortens the material mixing time and reduces energy consumption, enhances the chelation effect of citric acid on tetraisopropyl titanate and effectively controls the hydrolysis rate, improves the efficiency of tetraisopropyl titanate hydrolysis into small-particle TiO2, and helps to obtain uniform and stable small-sized anatase phase nano-titanium dioxide. Attached Figure Description

[0030] Figure 1 This is a diagram illustrating the preparation mechanism of the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel of the present invention.

[0031] Figure 2 These are scanning electron microscope (SEM) images of the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel in test item 2 of this invention. A, b, and c correspond to the following three samples in sequence: the first sample is the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel sample obtained according to the method of Comparative Example 1; the second sample is the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel obtained according to the method of Example 2; and the third sample is the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel obtained according to the method of Example 3. Detailed Implementation

[0032] To provide 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.

[0033] I. Preparation of Chitosan / Low Molecular Weight Oxidized Chitosan / Titanium Dioxide Composite Aerogel

[0034] Example 1

[0035] (1) Chitosan and low molecular weight oxidized chitosan (C6 aldehyde content 43.98%, C2 and C3 carboxyl content 27.04%, degree of deacetylation 91.17%, viscosity-average molecular weight 12,000, water solubility 15.28 g / 100 mL, isoelectric point pH = 5.3) were dissolved in 0.03 mol / L dilute nitric acid solution at a mass ratio of 10:1 to prepare a solution with a total mass concentration of 1.5%. 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst was added to make its mass concentration in the mixed solution 3%. Then, the cross-linking reaction was catalyzed under microwave radiation (microwave radiation power 420 W, microwave radiation temperature 30 °C) for 1 h to form multi-site chemical cross-links between chitosan and low molecular weight oxidized chitosan.

[0036] (2) Add 6 mL of 10% citric acid solution and 100 mL of tetraisopropyl titanate isopropanol solution (the volume ratio of tetraisopropyl titanate to isopropanol is 1:8) to a vortex mixer (vortex mixer power is 50W, stirring speed is 580rpm) and stir for 25min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound (the ultrasound reaction is intermittent, with a 2min pause after every 30min ultrasound reaction, and the power of ultrasound oscillation is 180W), add the mixed solution dropwise to the reaction solution of step (1) within 12min to carry out the ultrasound chemical reaction for 2h to obtain the composite hydrogel.

[0037] (3) After the reaction is completed, the composite hydrogel is left to stand for 70 minutes, then placed in a -45℃ ultra-low temperature freezer for 3 hours, and then subjected to microwave vacuum freeze drying (microwave vacuum freeze drying temperature is -50℃, microwave power is 960W, vacuum degree is 30Pa) for 24 hours to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

[0038] Testing showed that the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel prepared in this embodiment had a solubility loss rate of 13.29% and a specific surface area of ​​24.21 m². 2 The average pore size is 78.46 μm, the porosity is 84.13%, and the density is 46.59 mg / cm³. 3 The photodegradation rate of methyl orange after 2 hours was 95.22%; after five cycles of use, the photodegradation rate of methyl orange after 2 hours was 76.19% using the composite aerogel.

[0039] Example 2

[0040] (1) Chitosan and low molecular weight oxidized chitosan (C6 aldehyde content 51.08%, C2 and C3 carboxyl content 34.17%, degree of deacetylation 89.46%, viscosity-average molecular weight 10,000, water solubility 18.38 g / 100 mL, isoelectric point pH = 5.2) were dissolved in 0.04 mol / L dilute nitric acid solution at a mass ratio of 6:1 to prepare a solution with a total mass concentration of 2.0%. 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst was added to make its mass concentration in the mixed solution 5%. Then, the cross-linking reaction was catalyzed under microwave radiation (microwave radiation power 500 W, microwave radiation temperature 35 °C) for 1.5 h to form multi-site chemical cross-links between chitosan and low molecular weight oxidized chitosan.

[0041] (2) Add 8 mL of 10% citric acid solution and 100 mL of tetraisopropyl titanate isopropanol solution (the volume ratio of tetraisopropyl titanate to isopropanol is 1:9) to a vortex mixer (vortex mixer power is 68W, stirring speed is 600rpm) and stir for 30min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound (the ultrasound reaction is intermittent, with a 2min pause after every 30min ultrasound reaction, and the power of ultrasound oscillation is 200W), add the mixed solution dropwise to the reaction solution of step (1) within 15min to carry out the ultrasound chemical reaction for 3h to obtain the composite hydrogel.

[0042] (3) After the reaction is completed, the composite hydrogel is left to stand for 80 minutes, then placed in a -50℃ ultra-low temperature freezer for 3 hours to pre-freeze, and then subjected to microwave vacuum freeze drying (microwave vacuum freeze drying temperature is -55℃, microwave power is 1250W, vacuum degree is 26Pa) for 30 hours to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

[0043] Testing showed that the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel prepared in this embodiment had a solubility loss rate of 11.41% and a specific surface area of ​​32.71 m². 2 The average pore size is 65.09 μm, the porosity is 88.02%, and the density is 37.26 mg / cm³. 3 The photodegradation rate of methyl orange after 2 hours was 96.41%; after five cycles of use, the photodegradation rate of methyl orange after 2 hours was 78.63% using the composite aerogel.

[0044] Example 3

[0045] (1) Chitosan and low molecular weight oxidized chitosan (C6 aldehyde content 58.34%, C2 and C3 carboxyl content 42.50%, degree of deacetylation 86.94%, viscosity-average molecular weight 0.80,000, water solubility 21.45 g / 100 mL, isoelectric point pH = 5.1) were dissolved in 0.05 mol / L dilute nitric acid solution at a mass ratio of 8:1 to prepare a solution with a total mass concentration of 2.5%. 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst was added to make its mass concentration in the mixed solution 6%. Then, the cross-linking reaction was catalyzed under microwave radiation (microwave radiation power 540 W, microwave radiation temperature 40 °C) for 2 h to form multi-site chemical cross-links between chitosan and low molecular weight oxidized chitosan.

[0046] (2) Add 10 mL of 12% citric acid solution and 100 mL of tetraisopropyl titanate isopropanol solution (the volume ratio of tetraisopropyl titanate to isopropanol is 1:10) to a vortex mixer (vortex mixer power is 75W, stirring speed is 650rpm) and stir for 35min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound (the ultrasound reaction is intermittent, with a 2min pause after every 30min ultrasound reaction, and the power of ultrasound oscillation is 210W), add the mixed solution dropwise to the reaction solution of step (1) within 18min to carry out the ultrasound chemical reaction for 3h to obtain the composite hydrogel.

[0047] (3) After the reaction is completed, the composite hydrogel is left to stand for 90 minutes, then placed in a -55℃ ultra-low temperature freezer for 4 hours, and then subjected to microwave vacuum freeze drying (microwave vacuum freeze drying temperature is -60℃, microwave power is 1400W, vacuum degree is 22Pa) for 34 hours to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

[0048] The chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel prepared in this embodiment was tested and found to have a solubility loss rate of 8.93% and a specific surface area of ​​39.57 m². 2 The average pore size is 45.27 μm, the porosity is 91.38%, and the density is 40.01 mg / cm³. 3 The photodegradation rate of methyl orange after 2 hours was 98.95%; after five cycles of use, the photodegradation rate of methyl orange after 2 hours was 82.12%.

[0049] Example 4

[0050] (1) Chitosan and low molecular weight oxidized chitosan (C6 aldehyde content 58.34%, C2 and C3 carboxyl content 42.50%, degree of deacetylation 86.94%, viscosity-average molecular weight 0.80,000, water solubility 21.45 g / 100 mL, isoelectric point pH = 5.1) were dissolved in 0.05 mol / L dilute nitric acid solution at a mass ratio of 12:1 to prepare a solution with a total mass concentration of 3.0%. 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst was added to make its mass concentration in the mixed solution 8%. Then, the cross-linking reaction was catalyzed under microwave radiation (microwave radiation power 580 W, microwave radiation temperature 45 °C) for 2 h to form multi-site chemical cross-links between chitosan and low molecular weight oxidized chitosan.

[0051] (2) Add 10 mL of 14% citric acid solution and 100 mL of tetraisopropyl titanate isopropanol solution (the volume ratio of tetraisopropyl titanate to isopropanol is 1:12) to a vortex mixer (vortex mixer power is 75W, stirring speed is 650rpm) and stir for 40 min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound (the ultrasound reaction is intermittent, with a 2 min pause after every 30 min of ultrasound reaction, and the power of ultrasound oscillation is 210W), add the mixed solution dropwise to the reaction solution of step (1) within 18 min to carry out the ultrasound chemical reaction for 4 h to obtain the composite hydrogel.

[0052] (3) After the reaction is completed, the composite hydrogel is allowed to stand for 90 min, then placed in a -58℃ ultra-low temperature freezer for 4.5 h, and then microwave vacuum freeze-drying (microwave vacuum freeze-drying temperature is -62℃, microwave power is 1450W, vacuum degree is 20Pa) for 36 h to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

[0053] Testing showed that the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel prepared in this embodiment had a solubility loss rate of 10.14% and a specific surface area of ​​35.32 m². 2 The average pore size is 52.51 μm, the porosity is 89.23%, and the density is 44.19 mg / cm³. 3 The photodegradation rate of methyl orange after 2 hours was 97.13%; after five cycles of use, the photodegradation rate of methyl orange after 2 hours was 80.06% using the composite aerogel.

[0054] Comparative Example 1

[0055] Preparation of chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel (without adding 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst)

[0056] (1) Chitosan and low molecular weight oxidized chitosan (C6 aldehyde content 58.34%, C2 and C3 carboxyl content 42.50%, degree of deacetylation 86.94%, viscosity-average molecular weight 0.80,000, water solubility 21.45 g / 100 mL, isoelectric point pH = 5.1) were dissolved in 0.05 mol / L dilute nitric acid solution at a mass ratio of 8:1 to prepare a solution with a total mass concentration of 2.5%. Then, a cross-linking reaction was catalyzed under microwave radiation (microwave radiation power 540 W, microwave radiation temperature 40 °C) for 2 h to form multi-site chemical cross-links between chitosan and low molecular weight oxidized chitosan.

[0057] (2) Add 10 mL of 12% citric acid solution and 100 mL of tetraisopropyl titanate isopropanol solution (the volume ratio of tetraisopropyl titanate to isopropanol is 1:10) to a vortex mixer (vortex mixer power is 75W, stirring speed is 650rpm) and stir for 35min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound (the ultrasound reaction is intermittent, with a 2min pause after every 30min ultrasound reaction, and the power of ultrasound oscillation is 210W), add the mixed solution dropwise to the reaction solution of step (1) within 18min to carry out the ultrasound chemical reaction for 3h to obtain the composite hydrogel.

[0058] (3) After the reaction is completed, the composite hydrogel is left to stand for 90 minutes, then placed in a -55℃ ultra-low temperature freezer for 4 hours, and then subjected to microwave vacuum freeze drying (microwave vacuum freeze drying temperature is -60℃, microwave power is 1400W, vacuum degree is 22Pa) for 34 hours to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

[0059] Testing showed that the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel prepared in this embodiment had a solubility loss rate of 19.41% and a specific surface area of ​​18.74 m². 2 The average pore size is 94.33 μm, the porosity is 76.37%, and the density is 57.69 mg / cm³. 3 The photodegradation rate of methyl orange after 2 hours was 81.02%; after five cycles of use, the photodegradation rate of methyl orange after 2 hours was 57.35% using the composite aerogel.

[0060] II. Testing the samples obtained in the above embodiments.

[0061] Test Item 1: Test of the solubility, specific surface area, average pore size, porosity, mechanical properties, and photodegradation rate of composite aerogels formed by adding dilute nitric acid solutions of different molar concentrations.

[0062] Following the method in Example 3, chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogels were prepared by changing the molar concentration of dilute nitric acid solution. The test results of the composite aerogels, including dissolution rate, specific surface area, average pore size, porosity, mechanical properties, and methyl orange photodegradation rate, are shown in Table 1.

[0063] Table 1. Test results of the physicochemical properties of the composite aerogel.

[0064]

[0065] In Table 1, mechanical properties refer to the maximum pressure that the composite aerogel can withstand per square meter of area without collapsing during compression testing on a CMT6104 universal testing machine, with a constant compression rate of 2 mm / min according to ASTM D3575-14 standard.

[0066] In Table 1, the test method for the photodegradation rate of methyl orange is as follows: the absorbance at the highest point of the characteristic peak of the methyl orange solution (463 nm) is converted into the corresponding dye concentration using a standard curve. The initial dye concentration is expressed as C0 (mg / L), and the dye concentration after photodegradation at a certain moment is expressed as C... t (mg / L) indicates the photodegradation rate of the dye in the dyeing solution, as shown in the formula:

[0067]

[0068] The method for testing the photodegradation rate after repeated use was as follows: Chitosan / low-molecular-weight oxidized chitosan / titanium dioxide composite aerogel was added to a methyl orange dye solution and stirred in the dark for 60 min to reach adsorption equilibrium. Then, a UV lamp was turned on for 120 min of illumination. Every 20 min, 3 mL of the reaction solution was collected, centrifuged, and the absorbance was measured using a UV spectrophotometer. After photodegradation, the composite aerogel was removed and repeatedly washed with deionized water until the pH was neutral. Finally, the composite aerogel was freeze-dried under vacuum for the next photocatalytic degradation experiment. The above steps were repeated for 5 cycles, and the photodegradation rate of methyl orange dye after 5 cycles was calculated.

[0069] The data in Table 1 show that, in this invention, chitosan and low-molecular-weight oxidized chitosan are dissolved in dilute nitric acid solution, and a nucleophilic addition reaction between chitosan and low-molecular-weight oxidized chitosan is carried out under microwave irradiation by adding 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst. Then, a mixed solution of citric acid and tetraisopropyl titanate in isopropanol is added dropwise to the chitosan / low-molecular-weight oxidized chitosan solution under ultrasonic irradiation to carry out ultrasonic chemical reaction and cross-linking self-assembly. Finally, a chitosan / low-molecular-weight oxidized chitosan / titanium dioxide composite aerogel with high porosity, stable structure and three-dimensional network framework is prepared by microwave vacuum freeze drying. Using tetraisopropyl titanate as the "titanium source," the hydrolysis rate was controlled by limiting the orientation of the hydrolysis products after nucleation with nitric acid and by controlling the chelation effect of citric acid, resulting in nano-titanium dioxide with uniform particle size and small dimensions. This significantly improved the porosity, mechanical strength, and structural stability of the chitosan / low molecular weight chitosan oxide / titanium dioxide composite aerogel, and also significantly affected the average pore size, specific surface area, solubility loss rate, and photodegradation efficiency of the composite aerogel. However, when chitosan and low molecular weight chitosan oxide were dissolved in acetic acid solution, and then a mixture of citric acid and isopropanol solution of tetraisopropyl titanate was added for ultrasonic chemical reaction, the acetic acid could not effectively limit the orientation of the hydrolysis products after nucleation. This led to the aggregation of the nano-titanium dioxide generated by hydrolysis, resulting in larger particle size, poorer compatibility with the chitosan and low molecular weight chitosan oxide matrix, and weakened the strength of the composite aerogel framework. At the same time, the photocatalytic degradation rate and reusability of dyes decreased.

[0070] Test Item 2: Cross-sectional scanning electron microscopy analysis of chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel

[0071] The morphology of the cross-section of the chitosan / low molecular weight chitosan oxide / titanium dioxide composite aerogel was observed using a scanning electron microscope (400×). Three composite aerogel samples were taken: the first sample was the chitosan / low molecular weight chitosan oxide / titanium dioxide composite aerogel obtained according to the method of Comparative Example 1; the second sample was the chitosan / low molecular weight chitosan oxide / titanium dioxide composite aerogel obtained according to the method of Example 2; and the third sample was the chitosan / low molecular weight chitosan oxide / titanium dioxide composite aerogel obtained according to the method of Example 3. The test results are detailed below. Figure 2 (a)~(c).

[0072] Depend on Figure 2It can be seen that the chitosan / low-molecular-weight oxidized chitosan / titanium dioxide composite aerogel has a certain three-dimensional network structure. This indicates that the aldehyde and carboxyl groups of low-molecular-weight oxidized chitosan can form chemical bonds with the amino groups in chitosan to crosslink and construct the framework structure of the aerogel. The composite aerogel obtained without the addition of 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst, due to the lack of positive charge in the amino groups of chitosan molecules, has a slow crosslinking reaction rate with the aldehyde and carboxyl groups on the low-molecular-weight oxidized chitosan molecular chains. This reduces the degree of crosslinking between chitosan and low-molecular-weight oxidized chitosan, resulting in only an irregular lamellar network structure and poor aerogel structural stability (see...). Figure 2 a) This also hinders the electrostatic adsorption and uniform distribution of nano-TiO2 particles, leading to a decrease in the photodegradation efficiency of the composite aerogel. However, the addition of 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst accelerates the affinity addition reaction between chitosan and low-molecular-weight oxidized chitosan, resulting in a composite aerogel with a regular, interconnected pore structure, smaller three-dimensional pore size, and increased specific surface area (see...). Figure 2 b) and c), and promote the electrostatic assembly and uniform dispersion of TiO2 nanoparticles in the aerogel matrix, which to some extent enhances the three-dimensional framework stability and photocatalytic degradation activity of the composite aerogel.

[0073] In summary, this invention involves dissolving chitosan and low-molecular-weight oxidized chitosan in dilute nitric acid solution, adding 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst, and conducting a nucleophilic addition reaction between chitosan and low-molecular-weight oxidized chitosan under microwave irradiation. A solution of citric acid and tetraisopropyl titanate in isopropanol is added to the mixture in a vortex mixer and mixed thoroughly. Then, the mixed solution is added dropwise to the chitosan / low-molecular-weight oxidized chitosan solution under ultrasonic irradiation to perform ultrasonic chemical reaction and electrostatic self-assembly. Finally, a chitosan / low-molecular-weight oxidized chitosan / titanium dioxide composite aerogel with high porosity, stable structure, and a three-dimensional network framework is prepared by microwave vacuum freeze-drying. The composite aerogel obtained by this invention has advantages such as good mechanical properties, low solubility, low density, large specific surface area, and strong photocatalytic activity. It has broad application prospects in the field of photodegradation of dyes, pesticides, and pollutants in wastewater. Furthermore, by using isopropanol solution as the solvent for tetraisopropyl titanate and dilute nitric acid and citric acid as inhibitors and chelating agents for the hydrolysis of tetraisopropyl titanate, respectively, small-sized anatase phase titanium dioxide can be obtained, resulting in stronger photocatalytic activity.

[0074] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel, characterized in that: The chitosan / low-molecular-weight chitosan oxide / titanium dioxide composite aerogel is produced by dissolving chitosan and low-molecular-weight chitosan oxide in dilute nitric acid solution, adding 1-ethyl-1-methylpiperidine dihydrogen phosphate ionic liquid catalyst, and conducting a nucleophilic reaction between chitosan and low-molecular-weight chitosan oxide under microwave irradiation. A solution of citric acid and tetraisopropyl titanate in isopropanol is added to a vortex mixer and mixed thoroughly. Then, the mixture is added dropwise to the chitosan / low-molecular-weight chitosan oxide solution under ultrasonic treatment. The process involves ultrasonic chemical reaction and cross-linking self-assembly, followed by microwave vacuum freeze-drying to obtain the low-molecular-weight oxidized chitosan. The chitosan exhibits a C6 aldehyde group content of 41.39–60.57%, a C2 and C3 carboxyl group content of 23.64–45.12%, a degree of deacetylation of 85.22–92.86%, a viscosity-average molecular weight of 0.7–1.3 million, a water solubility of 12.17–23.54 g / 100 mL, and an isoelectric point pH of 5.1–5.

4. Its structural formula is as follows: ; The preparation method of the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel includes the following steps: (1) Chitosan and low-molecular-weight oxidized chitosan are dissolved in a 0.02-0.06 mol / L dilute nitric acid solution to prepare a solution with a total mass concentration of 1-3%. A 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst is added and the cross-linking reaction is catalyzed under microwave radiation for 0.5-2 h to form multi-site chemical cross-links between chitosan and low-molecular-weight oxidized chitosan. The mass ratio of chitosan to low-molecular-weight oxidized chitosan is 4-12:

1. The mass concentration of the 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst in the mixed solution is 2-8%. (2) Add 5-10 mL of citric acid solution with a mass concentration of 10-15% and 100 mL of tetraisopropyl titanate isopropanol solution to a vortex mixer and stir for 20-40 min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound, add the mixed solution dropwise to the reaction solution of step (1) within 10-20 min to carry out ultrasonic chemical reaction for 2-4 h to obtain composite hydrogel; the volume ratio of tetraisopropyl titanate to isopropanol is 1:8-12. (3) After the reaction is completed, the composite hydrogel is allowed to stand for 60-90 minutes, then placed in an ultra-low temperature freezer at -60 to -40℃ for 2-5 hours, and then microwave vacuum freeze-dried for 24-36 hours to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

2. The chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel according to claim 1, characterized in that: The chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel has a solubility loss rate of 8.51%–15.93% and a specific surface area of ​​17.92–40.15 m². 2 The average pore size is 41.3–90.6 μm, the porosity is 82.67–93.26%, and the density is 32.18–51.07 mg / cm³. 3 .

3. A method for preparing the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel according to any one of claims 1 to 2, characterized in that, Includes the following steps: (1) Chitosan and low-molecular-weight oxidized chitosan are dissolved in a 0.02-0.06 mol / L dilute nitric acid solution to prepare a solution with a total mass concentration of 1-3%. A 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst is added and the cross-linking reaction is catalyzed under microwave radiation for 0.5-2 h to form multi-site chemical cross-links between chitosan and low-molecular-weight oxidized chitosan. The mass ratio of chitosan to low-molecular-weight oxidized chitosan is 4-12:

1. The mass concentration of the 1-ethyl-1-methylpiperidine phosphate dihydrogen ionic liquid catalyst in the mixed solution is 2-8%. (2) Add 5-10 mL of citric acid solution with a mass concentration of 10-15% and 100 mL of tetraisopropyl titanate isopropanol solution to a vortex mixer and stir for 20-40 min to form a uniformly dispersed mixed solution. Then, under the action of ultrasound, add the mixed solution dropwise to the reaction solution of step (1) within 10-20 min to carry out ultrasonic chemical reaction for 2-4 h to obtain composite hydrogel; the volume ratio of tetraisopropyl titanate to isopropanol is 1:8-12. (3) After the reaction is completed, the composite hydrogel is allowed to stand for 60-90 minutes, then placed in an ultra-low temperature freezer at -60 to -40℃ for 2-5 hours, and then microwave vacuum freeze-dried for 24-36 hours to obtain chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel.

4. The preparation method according to claim 3, characterized in that: In step (1), the power of the microwave radiation is 380-600W and the temperature of the microwave radiation is 30-45℃.

5. The preparation method according to claim 4, characterized in that: In step (2), the power of the vortex mixer is 40-80W and the stirring speed is 500-700rpm.

6. The preparation method according to claim 3, characterized in that: In step (2), the ultrasonic reaction is intermittent, with a 2-minute pause after every 30 minutes of ultrasonic reaction, and the power of the ultrasonic oscillation is 160-220W.

7. The preparation method according to claim 3, characterized in that: In step (3), the microwave vacuum freeze-drying temperature is -65 to -50°C, the microwave power is 840 to 1600W, and the vacuum degree is 20 to 35Pa.

8. The application of the chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel according to any one of claims 1 to 2, characterized in that: Used for photodegradation of dyes in wastewater.

9. The application according to claim 8, characterized in that: The chitosan / low molecular weight oxidized chitosan / titanium dioxide composite aerogel, after photodegradation of dyes, can be regenerated and reused by decomposing the dyes in water under ultraviolet light.

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