AU2021282481B2

AU2021282481B2 - A method for preparing betaine-modified cellulose ester capable of adsorbing dyes, and application of the cellulose ester

Info

Publication number: AU2021282481B2
Application number: AU2021282481A
Authority: AU
Inventors: Yu Liu; Yangyang Sun
Original assignee: Qilu University of Technology
Current assignee: Qilu University of Technology
Priority date: 2021-03-08
Filing date: 2021-12-09
Publication date: 2023-11-09
Anticipated expiration: 2041-12-09
Also published as: CN113019333A; AU2021282481A1; CN113019333B

Abstract

The present invention belongs to the field of polymer functional materials, and relates to a method for preparing a betaine-modified cellulose ester and its application in adsorbing anionic dyes. A quaternary ammonium reagent, trimethylglycine (betaine for short), is introduced into cellulose by esterification, under the action of a co-reactant p-toluenesulfonyl chloride (TsCl) to obtain the betaine-modified cellulose ester. The cellulose ester has a positive charge under weakly acidic conditions and can be used as an adsorbent for adsorbing anionic dye. The cellulose ester adsorbent is recovered by using a solvent method and then recycled. The present invention is convenient to operate, mild in conditions and high in practicability. 23 DRAWINGS (a) DS=1.6 ea (b) DS=1.0 DS=0. Cellulose I ' I ' I ' I ' I 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-) FIG. 1 1

Description

DRAWINGS

(a) DS=1.6 ea

(b) DS=1.0

DS=0.

Cellulose

I ' I ' I ' I ' I

4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-)

FIG. 1

A METHOD FOR PREPARING BETAINE-MODIFIED CELLULOSE ESTER CAPABLE OF ADSORBING DYES, AND APPLICATION OF THECELLULOSEESTER TECHNICAL FIELD

The present invention belongs to the field of polymer functional materials, and specifically relates to preparation and dye adsorption methods of a cellulose ester.

BACKGROUND

The information in the background is only intended to facilitate understanding of the overall background of the present invention, and is not necessarily regarded as an acknowledgment or suggest, in any form, that the information constitutes the prior art known to a person of ordinary skill in the art. Confronted with the shortage of fossil energy and environmental pollution, natural polymers have become a research hotspot at present. As one of the most abundant natural polymers in the world, cellulose has attracted much attention. It is estimated that more than 100 million tons of the cellulose are produced through photosynthesis each year. As a polymer connected by glucose via P-1,4 glycosidic bonds, cellulose has excellent characteristics such as abundant sources, sustainability, biodegradability, biocompatibility, and high mechanical strength. These advantages promote the wide application of cellulose in many fields, such as pulping and papermaking, energy development, textile industry, and environmental governance. However, cellulose is insoluble and has no chemical corrosion resistance and thermoplastic performance. Nowadays, cellulose derivatization has been conducted to overcome the drawbacks of cellulose or to give it new capabilities. Due to the addition of new groups, some new performance, such as higher dissolution ability, adsorption ability, chemical corrosion resistance and aging resistance, could be achieved. Therefore, application of the cellulose is greatly expanded by the derivatization of the cellulose. Due to alcoholic hydroxyl groups on an anhydroglucose unit of the cellulose, the cellulose is endowed with an excellent property of derivatization, which is important for high-value utilization of the cellulose. Through the derivatization of the cellulose, a variety of cellulose materials can be prepared. Through esterification, etherification, oxidation, graft copolymerization, and other reactions, active groups are introduced into the cellulose to

I change physical and chemical properties of the cellulose. Due to the addition of new groups, the shortcomings of the cellulose are overcome, and new performance, such as higher dissolution ability, adsorption ability, chemical corrosion resistance and aging resistance, can be achieved. According to the methods above, an esterification method has a relatively clear a reaction mechanism, and has the advantages of mild reaction conditions, high reaction efficiency and few by-products. An inorganic acid, an organic acid, an acid anhydride or an acyl halide can be introduced into the cellulose through an esterification reaction. At present, a series of cellulose ester derivatives have been successfully synthesized by scholars, and can be prepared into high-value materials such as thin films, medical dialysis membranes, cigarette filter membranes and coatings. However, the inventor found that the stability and adsorbability of existing cellulose esters still need to be improved to meet requirements for treatment of dye wastewater.

SUMMARY

In order to solve the problem above, the present invention provides a preparation method of a quaternary ammonium cellulose ester and a method for adsorbing dye by the ester. A cationic cellulose ester with high stability and certain adsorption ability for dyes is prepared. An esterifying agent in the present invention is trimethylglycine (betaine for short) and a co-reactant is p-toluenesulfonyl chloride. To achieve the foregoing technical objective, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a preparation method of a betaine-modified cellulose ester that is capable of adsorbing a dye, including: dissolving the cellulose in a mixed N,N-dimethylacetamide (DMAc)/lithium chloride (LiCl) homogenous solution; and adding betaine into the cellulose solution for an esterification reaction under the presence of a co-reactant p-toluenesulfonyl chloride (TsCl) to obtain the betaine-modified cellulose ester. In the present invention, a quaternary ammonium compound betaine is used as an esterifying agent to react with the cellulose to synthesize a cationic cellulose ester. Betaine exists in a variety of plants, animals and some microorganisms. It is highly soluble in water and has great cell compatibility with no toxicity effect on cells at high concentration. The cellulose ester of the present invention has a certain adsorption ability for dyes with anionic properties in a solution. In view of people's general concern about treatment of dye wastewater, it is of great significance to develop a dye adsorption reagent with strong stability. In a second aspect, the present invention provides a betaine-modified cellulose ester prepared by the method. Principles of the present invention: During the esterification reaction, the co-reactant TsCl and betaine form a highly active mixed acid anhydride first, and then the intermediate compound reacts with hydroxyl groups on the cellulose to obtain a cellulose ester. The cellulose ester having a positive charge under weakly acidic conditions and has the effect of adsorbing anionic dye molecules. In a third aspect, the present invention provides application of the betaine-modified cellulose ester above in adsorption of a dye. The cellulose ester in the present invention has a certain adsorption effect on a methyl orange dye and direct bordeaux B, and is expected to be widely used in treatment of dye wastewater. In a fourth aspect, the present invention provides adsorption and desorption methods of the betaine-modified cellulose ester. The adsorption method: adding the cellulose ester into a dye solution, adjusting pH value and performing mechanical stirring for 30-120 min; the desorption method includes: washing an adsorbent with water, adjusting the pH to 9.0 and repeating the steps above several times until a solution is basically colorless. The adsorption method of the present invention has a simple process and avoids excessive use of chemicals and consumption of resources; both the cellulose ester and the dyes are easy to desorb and be recycled, which facilitates environmental protection. Beneficial effects of the present invention are as follows. (1) The esterifying agent in the present invention is easy to obtain and is low-cost. (2) Water, methanol and DMAc for activating the cellulose in the present invention can be recycled. (3) A homogeneous synthesis reaction of the cellulose ester in the present invention has high efficiency, high degree of substitution (DS) is and mild reaction conditions. (4) The cellulose ester in the present invention has a certain adsorption effect on the methyl orange dye and direct bordeaux B. (5) The adsorption method of the present invention has a simple process, which avoids excessive use of chemicals and consumption of resources; the cellulose ester and the dyes are easy to desorb and both can be recycled, which is conducive to environmental protection.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings constituting of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and descriptions thereof are used to explain the present invention, and do not constitute an improper limitation to the present invention. FIG. 1 is an infrared spectrogram of neat cellulose and a cellulose ester in Example 1.

DETAILED DESCRIPTION

It should be noted that, the following detailed descriptions are all exemplary, and are intended to provide further descriptions of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those usually understood by a person of ordinary skill in the art to which the present invention belongs. It should be noted that the terms used herein are merely used for describing specific implementations, and are not intended to limit exemplary implementations of the present invention. As used herein, the singular form is also intended to include the plural form unless the context clearly dictates otherwise. In addition, it should further be understood that, terms "comprise" and/or "include" used in this specification indicate that there are features, steps, operations, devices, components, and/or combinations thereof. A method for preparing a cellulose ester capable of adsorbing dyes includes: activating cellulose with water, methanol and N,N-dimethylacetamide (DMAc) as exchange solvents in the process of solvent exchange method; dissolving lithium chloride (LiCl) in an N,N-dimethylacetamide (DMAc) solution at an appropriate temperature; dissolving the activated cellulose in a DMAc/LiCl system at an appropriate temperature to obtain a homogeneous and transparent cellulose solution; dissolving betaine in deionized water; dissolving the co-reactantp-toluenesulfonyl chloride (TsCl) in DMAc; adding TsCl dropwise into the cellulose solution; adding an esterifying agent betaine dropwise into the reaction system; performing a reaction for a period of time at an appropriate temperature; adding anhydrous ethanol into the reaction system to achieve the purposes of terminating the reaction and obtaining precipitate; washing the precipitate with 95% ethanol and removing a supernatant by suction filtration; washing the precipitate with deionized water and removing a supernatant by suction filtration; freeze-drying the product and grinding it into powder to obtain powdered cellulose ester; detecting the DS of the cellulose ester by using a heterogeneous saponification method; dissolving dyes in deionized water; adding a certain amount of the cellulose ester into a dye solution of a methyl orange dye and direct bordeaux B and performing stirring repeatedly for a period of time under certain temperature and pH conditions; adding a same mass of the neat cellulose into the dye solution to serve as a control under same adsorption conditions; separating the precipitate and the supernatant by centrifugation; measuring absorbance values of the dye solution before and after adsorption by using an ultraviolet spectrophotometer to measure the adsorption ability of the cellulose ester; and regenerating the adsorbent by using deionized water and a sodium hydroxide (NaOH) solution, and measuring a loss rate. In this application, the neat cellulose and the prepared cellulose ester are insoluble in the dye solution and can be precipitated by centrifugation after adsorbing the dyes. In this application, an esterification process refers to homogeneous esterification. In some embodiments, a mass fraction of LiCl in the DMAc/LiCl system is 8% (mass/volume). In some embodiments, a mass fraction of the cellulose in the cellulose solution is 2.0% (mass/volume). In some embodiments, a mass ratio of the cellulose to betaine to TsCl is 1:1:1, 1:2:1, 1:1:2, 1:2:2 or 1:2:3. In some embodiments, a reaction takes 16, 24 or 32 hours. In some embodiments, a reaction temperature is 80, 85 or 90°C. In some embodiments, a dye adsorption step includes: preparing a 20 mg/L dye solution and adding the cellulose ester into the dye solution according to a ratio of 2-5 g/L. A pH of the solution is adjusted to 4.0, 5.0, 6.0, 7.0 or 8.0, and the solution is stirred at 200 rpm for -120 min at room temperature. After the solution is centrifuged, an absorbance of the dye solution is measured by using an ultraviolet spectrophotometer.

In some embodiments, a dye desorption method includes: washing an adsorbent with distilled water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent, and washing the precipitate with a NaOH solution of pH 9.0 and repeating several times until the solution is basically colorless, which indicates that the dyes desorbed from the adsorbent are basically removed. Since esters are degraded to a certain extent in acidic or alkaline solutions, the loss rate of the adsorbent needs to be measured. Homogeneous esterification in the present invention refers to an esterification reaction in which the materials participating in the reaction are in a liquid phase; a mass fraction of LiCl in the DMAc/LiCl system is 8% (mass/volume), and the neat cellulose dissolved well at this ratio. The mass fraction of the cellulose in the cellulose solution is 2.0% (mass/volume). If the ratio is too low, the reaction efficiency is low, and if the ratio is too high, a viscosity of the cellulose solution is increased, which is not conducive to the reaction. The mass ratio of the cellulose to betaine to TsCl is 1:1:1, 1:2:1, 1:1:2, 1:2:2 or 1:2:3. Since a cellulose glucose residue contains 3 alcoholic hydroxyl groups, theoretically the mass ratio of the esterification reagent to the neat cellulose should be greater than 3. However, in this reaction, betaine is only soluble in deionized water. If the mass of betaine added is too high, the content of water in the reaction system is too high. This may affect the esterification reaction. In addition, during the esterification reaction, the co-reactant TsCl and betaine form a highly active mixed acid anhydride first, and then this intermediate compound reacts with hydroxyl groups on the cellulose to obtain a cellulose ester. Therefore, it is necessary to increase the amount of betaine and TsCl according to a ratio to promote the esterification reaction. The esterification reaction takes 16, 24 or 32 hours. Too short time leads to incomplete esterification, and too long time increases time cost. The esterification reaction temperature is 80, 85 or 90°C. Too low temperature leads to low reaction efficiency and too high temperature may lead to cellulose degradation. The concentration of the dye solution is 20 mg/L, and the addition amount of the cellulose ester is 2-5 g/L. If the addition amount is too low, the adsorption effect is poor, and if the addition amount is too high, the cost is increased. The pH of the dye solution is 4.0-8.0. The stirring time is 30-120 min. Esters are likely to be hydrolyzed under acidic or alkaline conditions, therefore, the pH should be appropriate. A solvent method is used for desorption of dyes, which is simple and efficient. The cellulose ester of the present invention is used for adsorbing the dyes. The cellulose ester and the dyes can be desorbed, making both recyclable. In the present invention: The cellulose and betaine are cheap and easy to obtain; and the dyes of methyl orange and direct bordeaux B are commonly used in textile, silk and leather manufacturing. In a preferred preparation method of the cellulose ester: The mass ratio of the cellulose to betaine to TsCl is 1:2:3; the esterification reaction takes 32 h; the esterification reaction temperature is 90°C; and it is measured by using a titration method that DS of the cellulose ester is 1.7. In the dye adsorption method of the present invention: The concentration of the dye solution is 20 mg/L, the added amount of the cellulose ester is 2-5 g/L, and the mixture is stirred at 200 rpm for 30-120 min at room temperature. The adsorption experiment is performed at pH of 4.0-8.0. The adsorbent and the supernatant are separated by centrifugation, and absorbance of the dyes in the supernatant is measured, followed by desorption experiment. The present invention is further described in detail below in conjunction with specific embodiments, and it should be pointed out that the specific embodiments are used for explaining rather than limiting the present invention. Example 1 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 h. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. A precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. An obtained cellulose ester was freeze-dried. The DS and the infrared spectrum (FIG. 1) of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 2 g/L, and the solution was stirring at 200 rpm for 30 min at room temperature. The methyl orange dye was adsorbed at pH of 4.0, and the direct bordeaux B was adsorbed at pH of 7.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 41.2%, and an adsorption capacity was 4.12 mg/g; and a removal rate of direct bordeaux B was 39.7%, and an adsorption capacity was 3.97 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 10.7%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption. Example 2 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 5 g/L, and the solution was stirring at 200 rpm for 30 min at room temperature. The methyl orange dye was adsorbed at the pH of 4.0, and the direct bordeaux B was adsorbed at the pH of 7.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 72.9%, and an adsorption capacity was 2.92 mg/g; and a removal rate of direct bordeaux B was 63.1%, and an adsorption capacity was 2.52 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 9.4%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption.

Example 3 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 2 g/L, and the solution was stirring at 200 rpm for 120 min at room temperature. The methyl orange dye was adsorbed at the pH of 4.0, and the direct bordeaux B was adsorbed at the pH of 7.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 56.3%, and an adsorption capacity was 5.63 mg/g; and a removal rate of direct bordeaux B was 45.7%, and an adsorption capacity was 4.57 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 11.0%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption. Example 4 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 2 g/L, and the solution was stirring at 200 rpm for 30 min at room temperature. The methyl orange dye was adsorbed at the pH of 5.0, and the direct bordeaux B was adsorbed at the pH of 5.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 38.9%, and an adsorption capacity was 3.89 mg/g; and a removal rate of direct bordeaux B was 49.1%, and an adsorption capacity was 4.91 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 11.7%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption. Example 5 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 2 g/L, and the solution was stirring at 200 rpm for 120 min at room temperature. The methyl orange dye was adsorbed at the pH of 5.0, and the direct bordeaux B was adsorbed at the pH of 5.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 43.3%, and an adsorption capacity was 4.33 mg/g; and a removal rate of direct bordeaux B was 47.0%, and an adsorption capacity was 4.70 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 10.3%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption.

Example 6 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 5 g/L, and the solution was stirring at 200 rpm for 30 min at room temperature. The methyl orange dye was adsorbed at the pH of 5.0, and the direct bordeaux B was adsorbed at the pH of 5.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 64.1%, and an adsorption capacity was 2.56 mg/g; and a removal rate of direct bordeaux B was 53.8%, and an adsorption capacity was 2.15 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 12.4%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption.

Example 7 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 5 g/L, and the solution was stirring at 200 rpm for 120 min at room temperature. The methyl orange dye was adsorbed at the pH of 5.0, and the direct bordeaux B was adsorbed at the pH of 5.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 67.5%, and an adsorption capacity was 2.70 mg/g; and a removal rate of direct bordeaux B was 60.9%, and an adsorption capacity was 2.436 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 11.2%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption.

Example 8 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 2 g/L, and the solution was stirring at 200 rpm for 30 min at room temperature. The methyl orange dye was adsorbed at the pH of 7.0, and the direct bordeaux B was adsorbed at the pH of 4.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 48.4%, and an adsorption capacity was 4.84 mg/g; and a removal rate of direct bordeaux B was 42.2%, and an adsorption capacity was 4.22 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 12.6%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption.

Example 9 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50°C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90°C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 2 g/L, and the solution was stirring at 200 rpm for 120 min at room temperature. The methyl orange dye was adsorbed at the pH of 7.0, and the direct bordeaux B was adsorbed at the pH of 4.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 50.9%, and an adsorption capacity was 5.49 mg/g; and a removal rate of direct bordeaux B was 43.7%, and an adsorption capacity was 4.22 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 40°C to constant weight and then weighed to calculate the adsorbent loss rate, which was 11.1%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption.

Example 10 Water, and methanol and DMAc were used for activating cellulose in the solvent exchange step at room temperature. 20 g of the cellulose was soaked in 1 L of deionized water for 24 hours. The deionized water was removed by suction filtration. The cellulose was soaked in methanol 4 times and DMAc 3 times for 30 min each time to realize solvent exchange, and the solvents were removed by using a suction filtration method. The cellulose was placed in a clean and dry beaker and dried at 40°C. 50 ml of a DMAc/LiC solution with a concentration of 8% (mass fraction) was prepared at 50 0C to quickly dissolve LiCl. 2 g of the cellulose activated in the step above was added into the 50 ml of solution and continuously stirred for about 20 min to obtain a homogeneous and transparent cellulose dispersion solution. The mass ratio of the cellulose to betaine to TsCl was adjusted to 1:2:3. Betaine was dissolved in deionized water, and TsCl was dissolved in DMAc. TsCl and betaine were added dropwise into the cellulose solution sequentially by using a constant pressure funnel. The solution was stirring continuously as the reagents were added, and a temperature of the system was gradually increased to 90 0 C. After the reaction was performed for 32 hours, 200 mL of anhydrous ethanol was added into the system to terminate the reaction. The precipitate was washed 3 times with 95% ethanol and finally washed 2 times with deionized water. The obtained cellulose ester was freeze-dried, and the DS and the infrared spectrum changes of the cellulose ester were measured. 20 mL of a methyl orange and direct bordeaux B dye solution with a concentration of 20 mg/L was prepared. The cellulose ester was added to the dye solution to make its final concentration of 2 g/L, and the solution was stirring at 200 rpm for 30 min at room temperature; and methyl orange and direct bordeaux B were adsorbed at the pH of 8.0. A standard solution was prepared under the same temperature and pH conditions. The neat cellulose was used as a control adsorbent under the same treatment conditions. The adsorbent and the supernatant were separated by centrifugation. The absorbance of the dye solution was measured by using an ultraviolet-visible spectrophotometer. Full-spectrum scanning was performed first to determine the maximum absorption wavelength, and then the absorbance value of the solution was measured. According to the results, it was calculated that a removal rate of methyl orange was 17.5%, and an adsorption capacity was 1.75 mg/g; and a removal rate of direct bordeaux B was 26.3%, and an adsorption capacity was 2.63 mg/g. The adsorbent was washed with deionized water to wash away the dyes that are not firmly adsorbed on the surface of the adsorbent. After centrifugation, 20 mL of deionized water with pH 9.0 was added into the tube, followed by stirring for 5 min. The step was repeated until the solution was clear and colorless, indicating that the dyes on the adsorbent were basically removed. After desorption, the residual NaOH solution on the adsorbent was washed with deionized water to neutral. The adsorbent was dried at 400 C to constant weight and then weighed to calculate the adsorbent loss rate, which was 9.4%. It can be seen that the cellulose ester has low loss rate, suggesting the high stability during adsorption and desorption.

Comparative Example 1 The difference with Example 1 lies in that a cellulose ester cannot be successfully synthesized by using H2SO4 as a catalyst without TsCl. It should be finally noted that the foregoing descriptions are merely preferred embodiments of the present invention, but are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for a person of ordinary skill in the art, modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent replacements can be made to some technical features in the technical solutions. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention. The specific implementations of the present invention are described above, but are not intended to limit the protection scope of the present invention. A person skilled in the art should understand that various modifications or deformations may be made without creative efforts based on the technical solutions of the present invention, and such modifications or deformations shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a betaine-modified cellulose ester capable of adsorbing dyes, comprising: activating cellulose and then dissolving the cellulose in a N,N-dimethylacetamide/ lithium chloride (DMAc/LiCl) system to obtain a cellulose solution; and adding betaine into the cellulose solution for an esterification reaction under the presence of a co-reactant p-toluenesulfonyl chloride (TsCl) to obtain the betaine modified cellulose ester; wherein in the cellulose solution, a molar ratio of the cellulose to betaine to TsCl is 1:(1-2):(1-3); wherein a mass-volume fraction of the cellulose in the cellulose solution is 2%.

2. The method for preparing the betaine-modified cellulose ester capable of adsorbing dyes according to claim 1, wherein the esterification reaction is performed at -90°C for 16-32 hours.

3. The method for preparing the betaine-modified cellulose ester capable of adsorbing dyes according to claim 1, wherein a mass-volume fraction of LiCl in the DMAc/LiCl system is 8%.

4. The method for preparing the betaine-modified cellulose ester capable of adsorbing dyes according to claim 1, wherein the step of activating cellulose specifically comprises activating cellulose with water, methanol and N,N dimethylacetamide (DMAc) as exchange solvents by using a solvent exchange method.

5. A betaine-modified cellulose ester prepared by the method according to any one of claims I to 4.

6. Use of the betaine-modified cellulose ester according to claim 5 in adsorption of a dye.

7. The use according to claim 6, wherein the dye is a methyl orange dye and/or a

direct bordeaux B.

8. A method for adsorption and desorption methods using the betaine-modified

cellulose ester according to claim 5, comprising the steps of adsorption and desorption,

wherein

the adsorption comprises adding the betaine-modified cellulose ester into a dye

solution, adjusting the pH value and performing mechanical stirring for 30-120 min;

and

the desorption comprises washing an adsorbent with water, adjusting a pH to 9.0

and repeating the steps above several times until a solution is basically colorless.