Method for synthesizing cellulose ester with acidic ionic liquid as catalyst

Abstract

The present invention discloses a method for synthesizing cellulose ester using acidic ionic liquid as catalyst, comprising three steps of constant temperature activation, esterification, hydrolysis and post-treatment. + ][HSO4 ‑ ]、Triethylbenzyl-sulfate[N B222 H + ][HSO4 ‑ ], tetramethylguanidine-sulfate [TMGH + ][HSO4 ‑ Acidic ionic liquid catalysts are low-cost and easy to synthesize. This catalyst can be used in the synthesis of cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. The resulting cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate exhibit excellent performance. Furthermore, the catalyst exhibits high thermal stability and low volatility, making it easily recyclable, reducing pollutant generation and disposal costs. This overcomes the severe corrosiveness and difficulty in reusing of traditional sulfuric acid catalysts, making it a valuable tool for promoting the green and efficient production of cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of cellulose ester preparation, and particularly relates to a method for synthesizing cellulose ester by using an acidic ionic liquid as a catalyst. BACKGROUND

[0002] Cellulose is the most abundant organic polymer provided by plants and other resources, but the characteristics of its natural form limit its application. Cellulose derivatization is one of the common methods for expanding its use by improving its performance. Cellulose can react with different chemicals, including acids, acid anhydrides or other substances, to generate cellulose products with high added value. Among them, cellulose acetate is the most important organic acid ester of cellulose, which has been widely used for about 150 years. It can be used to produce biodegradable films, membranes, plastic materials, biomedical applications, and raw materials for textiles or cigarette filters. The global cellulose acetate market value is 6.15 billion US dollars, and it is expected to grow at a rate of 8.9% per year, reaching 10.25 billion US dollars by 2028. Cellulose acetate propionate and cellulose acetate butyrate are also important cellulose organic acid ester derivatives, which are widely used as additives in the coating industry.

[0003] Cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate are all generated by acylation of cellulose. On an industrial scale, cellulose is catalyzed by sulfuric acid, and excess acetic acid and acetic anhydride, propionic acid and propionic anhydride, butyric acid and butyric anhydride, as well as different acid compound and different acid anhydride compound, are used as reaction medium and acylation agent, respectively, to prepare cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. Due to the uncontrollable nature of the acylation process, cellulose ester needs to be further processed, such as hydrolysis, to obtain partially substituted cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. Although inorganic strong acids such as sulfuric acid have good catalytic activity, they may cause structural damage to the cellulose chain during the reaction process, thereby affecting the overall quality of the product and may also lead to the formation of by-products. Therefore, it is crucial to find more efficient and environmentally friendly catalysts for synthesizing cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate.

[0004] Ionic liquids are molten salts that are liquid at relatively low temperatures and are composed of specific organic cations containing elements such as N and P and organic or inorganic anions through hydrogen bonding and electrostatic interactions. Due to their structural asymmetry and the adjustable nature of their anions and cations, ionic liquids exhibit many unique physicochemical properties, such as high stability, excellent designability, and easy directional functionalization. These properties have given ionic liquids significant advantages and promising prospects in practical applications. Ionic liquids can meet the specific requirements of different catalytic reactions for solvents and catalysts by simply adjusting the type of anions and cations, making their use as solvents or catalysts in reactions a hot topic in research. Currently, acidic ionic liquids containing bisulfate have been widely used in various esterification and hydrolysis reactions, achieving excellent catalytic results.

[0005] Therefore, in order to meet the requirement of acidic catalysts for the cellulose acylation and hydrolysis reactions involved in the production of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate, the design and development of acidic ionic liquid catalysts with low cost and easy synthesis will help overcome the problems of traditional sulfuric acid catalysts being highly corrosive and difficult to reuse, which is of great significance for promoting the green and efficient production of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate.

[0006] Currently, a variety of ionic liquids have been developed for the synthesis of cellulose esters. Patent (CN 103613672 A) discloses the use of N-methylimidazole hydrochloride, N-methylimidazole hydrogen sulfate, and 1-vinyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate for catalytic synthesis of cellulose acetate; Patent (CN 104130332 A) discloses a method for synthesizing cellulose esters catalyzed by 1-butyl-3-methylimidazole hydrogen sulfate ionic liquid; and Patent (CN116606381 B) discloses a method for synthesizing cellulose triacetate catalyzed by 1-allyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ionic liquid. Although the above ionic liquids all exhibited excellent catalytic effects in cellulose ester synthesis, they are all imidazolium-based ionic liquids, which are susceptible to deactivation. This is because the imidazolium salts are easily deprotonated at the C2 position to produce active nucleophiles, which then react with electrophiles such as aldehydes, alkyl halides, ethyl formate, acetic anhydride, and isothiocyanates. Therefore, the development of non-imidazole ionic liquid acidic catalysts for cellulose ester synthesis is needed. Summary of the Invention

[0007] The present invention aims to provide a method for synthesizing cellulose ester using an acidic ionic liquid as a catalyst. The catalyst has the advantages of being reusable and environmentally friendly, and can be used simultaneously in the synthesis process of cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate.

[0008] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0009] A method for synthesizing cellulose ester with an acidic ionic liquid as a catalyst, comprising the following steps:

[0010] (1) constant temperature activation: uniformly stirring cellulose with glacial acetic acid or a mixture of glacial acetic acid and propionic acid or a mixture of glacial acetic acid and butyric acid at a constant temperature to obtain a uniform suspension;

[0011] (2) esterification: adding an acidic ionic liquid catalyst and acetic anhydride or a mixture of acetic anhydride and propionic anhydride or a mixture of acetic anhydride and butyric anhydride to the suspension obtained in step (1) to obtain a cellulose ester solution; the acidic ionic liquid catalyst is one of [DBUH + ][HSO4 - ], [N B222 H + ][HSO4 - ] or [TMGH + ][HSO4 - ];

[0012] (3) hydrolysis and post-treatment: adding desalted water to the cellulose ester solution obtained in step (2) to perform a hydrolysis reaction, and after white precipitates are separated out, filtering, washing and drying to obtain cellulose acetate or cellulose acetate propionate or cellulose acetate butyrate.

[0013] Further, in step (1), the mass ratio of cellulose to glacial acetic acid or a mixture of glacial acetic acid and propionic acid or a mixture of glacial acetic acid and butyric acid is 1.5:1-1:10.

[0014] Further, in step (1), the molar ratio of glacial acetic acid to propionic acid in the mixture of glacial acetic acid and propionic acid is 1:1-1:12, and the molar ratio of glacial acetic acid to butyric acid in the mixture of glacial acetic acid and butyric acid is 1:1-1:8.

[0015] Further, in step (1), the temperature of the constant temperature is 30-50 ℃, the stirring speed is 80-200 r / min, and the stirring time is 40-100 min.

[0016] Further, in step (2), the mass ratio of acetic anhydride or a mixture of acetic anhydride and propionic anhydride or a mixture of acetic anhydride and butyric anhydride to cellulose is 2-10:1, the molar ratio of acetic anhydride to propionic anhydride in the mixture of acetic anhydride and propionic anhydride is 1:1-1:12, the molar ratio of acetic anhydride to butyric anhydride in the mixture of acetic anhydride and butyric anhydride is 1:1-1:8, and the amount of the acidic ionic liquid catalyst is 0.5%-5% of the mass of cellulose.

[0017] Further, the preparation method of the acidic ionic liquid catalyst in step (2) is that one of sulfuric acid, 1,8-diazabicyclo[5.4.0]undec-7-ene, triethylbenzylammonium chloride and tetramethylguanidine is mixed and heated to react, then n-heptane is added and placed, and the upper liquid is taken to obtain the acidic ionic liquid catalyst, which is [DBUH + ][HSO4 - ], [NH B222 H + ][HSO4 - ] and [TMGH + ][HSO4 - ] respectively, and the structures are as follows:

[0018]

[0019] The molar ratio of the sulfuric acid to 1,8-diazabicyclo[5.4.0]undec-7-ene or triethylbenzyl or tetramethylguanidine is 0.5:1-3:1.

[0020] The heating reaction temperature is 70-100 DEG C, the time is 10-13 h, the mass ratio of the n-heptane to 1,8-diazabicyclo[5.4.0]undec-7-ene or triethylbenzylammonium chloride or tetramethylguanidine is 10:1, and the standing time is 0.5 h.

[0021] The reaction temperature in step (2) is 40-60 DEG C.

[0022] The mass ratio of the desalting water to the cellulose ester solution in step (3) is 10-30:1, the hydrolysis temperature is 20-50 DEG C, and the hydrolysis time is 1-5 h.

[0023] Compared with the prior art, the application has the following advantages:

[0024] The acidic ionic liquid selected in the application has low cost and is easy to synthesize, and is used to replace sulfuric acid as the catalyst for preparing cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate, the catalyst has high thermal stability and low volatility, the catalyst can be recycled, the influence on the environment is reduced, the problem that the traditional sulfuric acid catalyst is corrosive and difficult to reuse can be overcome, and in addition, the synthesized acidic ionic liquid has the advantages of high activity and strong solubility, and can be used in the synthesis process of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The acidic ionic liquid [DBUH + ][HSO4 - ], [NH B222 H + ][HSO4- ] and [TMGH + ][HSO4 - ] are shown in the structural diagrams. DETAILED DESCRIPTION

[0026] In order to make the content of the present application more convenient to understand, the technical solutions of the present application are further described below in combination with specific embodiments, but the present application is not limited to this.

[0027] Example 1

[0028] Sulfuric acid and 1,8-diazabicyclo[5.4.0]undec-7-ene are mixed in a molar ratio of 1:1 and then reacted at 90℃ for 12 h, after which 10 times the mass of 1,8-diazabicyclo[5.4.0]undec-7-ene is added in the form of n-heptane, and after standing for 0.5 h, the upper liquid is separated, thereby obtaining 1,8-diazabicyclo[5.4.0]undec-7-ene-sulfuric acid ionic liquid ([DBUH + ][HSO4 - ]), which is subjected to thermal stability and volatility testing, and has a decomposition temperature of 232-254℃ and a volatility close to 0 at 100℃.

[0029] 1 H NMR (300 MHz, DMSO- d6 ), δ: 1.559 (m, 6H, Pyr and Cy), 1.653 (m, 2H, Cy), 2.446 (t, 2H, Cy), 2.499 (t, 2H, Pyr), 3.310 (t, 2H, Pyr), 3.379 (t, 2H, Cy)

[0030] Cellulose and glacial acetic acid are added to a batch stirred tank in a mass ratio of 1:5, and stirred at a constant temperature of 40℃ and a rotation speed of 100 r / min for 80 min, thereby obtaining a uniform suspension. To the uniform suspension, 5 times the mass of cellulose in the form of acetic anhydride and 1% of the mass of cellulose in the form of [DBUH + ][HSO4 - ] acidic ionic liquid catalyst are added, and reacted at a constant temperature of 50℃ for 5 h. After the reaction is complete, a cellulose acetate solution is obtained. To the cellulose acetate solution, 20 times the mass of cellulose in the form of desalinated water (containing 1-5 mg / L of salt) is added, and hydrolysis is carried out at a constant temperature of 40℃ for 3 h. The white precipitate that separates out is filtered, washed, and dried, thereby obtaining cellulose acetate. The filtrate after filtration is subjected to evaporation and dehydration, and washed with ethyl acetate solvent, thereby recovering the ionic liquid. The cellulose acetate obtained has a degree of polymerization of 280 and a combined acid of 54%.

[0031] Example 2

[0032] Sulfuric acid and triethylbenzene were mixed in a molar ratio of 1:1 and reacted at 90 ℃ for 12 h, then 10 times the mass of triethylbenzene in n-heptane was added, and after standing for 0.5 h, the upper liquid was separated, and triethylbenzene-sulfuric acid ionic liquid ([N B222 H + ][HSO4 - ]) was obtained. Its thermal stability and volatility were detected, and its decomposition temperature was 232-254 ℃, and its volatility was close to 0 at 100 ℃.

[0033] 1 H NMR (300 MHz, DMSO- d6 ), δ: 1.310 (t, 9H, CH3), 3.115 (q, 6H, CH2), 4.307 (s, 2H, CH2), 7.444 (m, 5H, Ph)

[0034] Cellulose and glacial acetic acid were added to a batch stirred tank in a mass ratio of 1:5, and stirred at a constant temperature of 40 ℃ and a speed of 100 r / min for 80 min to obtain a uniform suspension. To the uniform suspension, 5 times the mass of cellulose in acetic anhydride and 1% of the mass of cellulose in [N B222 H + ][HSO4 - ] acidic ionic liquid catalyst were added, and reacted at a constant temperature of 50 ℃ for 5 h. After the reaction was complete, a cellulose acetate solution was obtained. To the cellulose acetate solution, 20 times the mass of cellulose in desalted water (containing 1-5 mg / L of salt) was added, and hydrolysis was carried out at a constant temperature of 40 ℃ for 3 h. The white precipitate that separated was filtered, washed, and dried to obtain cellulose acetate. The filtrate was evaporated and dehydrated, and washed with ethyl acetate to recover the ionic liquid. The cellulose acetate obtained had a degree of polymerization of 260, and the combined acid was 54%.

[0035] Example 3

[0036] Sulfuric acid and tetramethylguanidine were mixed in a molar ratio of 1:1 and reacted at 90 ℃ for 12 h, then 10 times the mass of tetramethylguanidine in n-heptane was added, and after standing for 0.5 h, the upper liquid was separated, and tetramethylguanidine-sulfuric acid ionic liquid ([TMGH + ][HSO4 - ]) was obtained. Its thermal stability and volatility were detected, and its decomposition temperature was 232-254 ℃, and its volatility was close to 0 at 100 ℃.

[0037] 1 H NMR (300 MHz, DMSO- d6), δ: 2.684 (s, 12H, CH3)

[0038] Cellulose and glacial acetic acid were added to an intermittent stirred tank at a mass ratio of 1:5 and stirred at a speed of 100 r / min for 80 min at a constant temperature of 40 ℃ to obtain a uniform suspension. Acetic anhydride (5 times the mass of cellulose) and [TMGH (1% of the mass of cellulose)] were added to the uniform suspension. + ][HSO4 - A cellulose acetate solution was prepared by adding an acidic ionic liquid catalyst at 50°C for 5 hours. After the reaction was complete, a cellulose acetate solution was obtained. Desalted water (salt content 1-5 mg / L) was added to the cellulose acetate solution at a constant temperature of 40°C for 3 hours. The resulting white precipitate was filtered, washed, and dried to obtain cellulose acetate. The filtrate was evaporated and washed with ethyl acetate to recover the ionic liquid. The resulting cellulose acetate had a degree of polymerization of 300 and a bound acid content of 55%.

[0039] The structural properties of the cellulose acetate obtained in Examples 1-3 are shown in Table 1.

[0040] Table 1 Structure and properties of cellulose acetate

[0041]

[0042] Table 1 shows that the cellulose acetates synthesized using the three ionic liquids all exhibited high degrees of polymerization. Furthermore, the total degree of acetyl substitution in the three samples was similar. However, the differences in catalysts resulted in differences in the uniformity of the substituent distribution at C-2, C-3, and C-6 on the glucose ring. This is likely due to the varying solubility properties of the acidic ionic liquids, which directly led to the differences in the uniformity of the substituent distribution at the C-2, C-3, and C-6 positions in the cellulose acetate.

[0043] Example 4

[0044] The three ionic liquids recovered in Examples 1-3 were tested for their reusability. Specifically, cellulose and glacial acetic acid were added to an intermittent stirred tank at a mass ratio of 1:5, and stirred at a speed of 100 r / min for 80 min at a constant temperature of 40°C to obtain a uniform suspension. 1% of the mass of cellulose was added to the uniform suspension. + ][HSO4 - ]、[N B222 H + ][HSO4 - ] and [TMGH + ][HSO4 -The cellulose was added into a mixture of acetic acid and propionic acid (molar ratio of acetic acid to propionic acid was 1:8) at a mass ratio of 1:5 in a batch stirred tank, and stirred at a constant temperature of 40 °C at a speed of 100 r / min for 80 min to obtain a uniform suspension. A mixture of acetic anhydride and propionic anhydride (molar ratio of acetic anhydride to propionic anhydride was 1:8) in an amount of 5 times the mass of the cellulose and [DBUH + ][HSO4 - ] acidic ionic liquid catalyst in an amount of 1% of the mass of the cellulose was added to the uniform suspension, and the reaction was carried out at a constant temperature of 50 °C for 5 h. After the reaction was completed, a cellulose ester solution was obtained. Desalted water (salt content of 1-5 mg / L) in an amount of 20 times the mass of the cellulose was added to the cellulose ester solution, and the hydrolysis reaction was carried out at a constant temperature of 40 °C for 3 h. The white precipitate that separated out was filtered, washed, and dried to obtain cellulose acetate propionate. The filtrate after filtration was evaporated and dehydrated, and washed with ethyl acetate solvent to recover the ionic liquid. The total degree of substitution of the cellulose acetate propionate obtained was 2.61, the acetyl content was 8.2%, the propionyl content was 41.5%, and the viscosity was 152.3 mPa / s.

[0045] Table 2 Repeated use performance of acidic ionic liquid in the reaction of catalyzing the preparation of cellulose acetate by acetylation of cellulose

[0046]

[0047] Other performance analysis was carried out on the cellulose acetate prepared, and the results are shown in Table 3. The water content, thermal stability, and free acid all met the quality standards (refer to ASTM D871-96 (2019)), and the cellulose acetate can be directly used for commercial sale.

[0048] Table 3 Quality indicators of cellulose acetate synthesized using acidic ionic liquid as catalyst

[0049]

[0050] Example 5

[0051] The cellulose was added into a mixture of acetic acid and propionic acid (molar ratio of acetic acid to propionic acid was 1:8) at a mass ratio of 1:5 in a batch stirred tank, and stirred at a constant temperature of 40 °C at a speed of 100 r / min for 80 min to obtain a uniform suspension. A mixture of acetic anhydride and propionic anhydride (molar ratio of acetic anhydride to propionic anhydride was 1:8) in an amount of 5 times the mass of the cellulose and [DBUH + ][HSO4 - ] acidic ionic liquid catalyst in an amount of 1% of the mass of the cellulose was added to the uniform suspension, and the reaction was carried out at a constant temperature of 50 °C for 5 h. After the reaction was completed, a cellulose ester solution was obtained. Desalted water (salt content of 1-5 mg / L) in an amount of 20 times the mass of the cellulose was added to the cellulose ester solution, and the hydrolysis reaction was carried out at a constant temperature of 40 °C for 3 h. The white precipitate that separated out was filtered, washed, and dried to obtain cellulose acetate propionate. The filtrate after filtration was evaporated and dehydrated, and washed with ethyl acetate solvent to recover the ionic liquid. The total degree of substitution of the cellulose acetate propionate obtained was 2.61, the acetyl content was 8.2%, the propionyl content was 41.5%, and the viscosity was 152.3 mPa / s.

[0052] Example 6

[0053] Cellulose and a mixture of glacial acetic acid and butyric acid (molar ratio of glacial acetic acid and butyric acid is 1:4) were added into a batch stirred tank at a mass ratio of 1:5, and stirred at a constant temperature of 40 ℃ and a rotating speed of 100 r / min for 80 min to obtain a uniform suspension. A mixture of acetic anhydride and propionic anhydride (molar ratio of acetic anhydride and butyric anhydride is 1:4) with a mass of 5 times that of cellulose and [DBUH + ][HSO4 - ] acidic ionic liquid catalyst with a mass of 1% of cellulose was added into the uniform suspension, and reacted at a constant temperature of 50 ℃ for 5 h. After the reaction was completed, a cellulose ester solution was obtained. Desalted water (salt content of 1-5 mg / L) with a mass of 20 times that of cellulose was added into the cellulose ester solution, and hydrolysis reaction was carried out at a constant temperature of 40 ℃ for 3 h. A white precipitate was separated out, and after filtration, washing and drying, cellulose acetate butyrate was obtained. The filtrate was dehydrated by evaporation, and washed with ethyl acetate solvent to recover the ionic liquid. The total degree of substitution of the obtained cellulose acetate butyrate was 2.11, the acetyl content was 13.5%, and the propionyl content was 27.5%.

[0054] The above only describes the preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be covered by the present application.

Claims

1. A method for synthesizing cellulose ester using an acidic ionic liquid as a catalyst, characterized in that: The following steps are involved: (1) Constant temperature activation: cellulose is stirred at a constant temperature and uniform speed with glacial acetic acid, a mixture of glacial acetic acid and propionic acid, or a mixture of glacial acetic acid and butyric acid to obtain a uniform suspension; (2) Esterification: adding an acidic ionic liquid catalyst and acetic anhydride or a mixture of acetic anhydride and propionic anhydride or a mixture of acetic anhydride and butyric anhydride to the suspension obtained in step (1) to react to obtain a cellulose ester solution; the acidic ionic liquid catalyst is [DBUH + ][HSO4 - ]、[N B222 H + ][HSO4 - ] or [TMGH + ][HSO4 - ] one of the [DBUH + ][HSO4 - ]、[N B222 H + ][HSO4 - ] or [TMGH + ][HSO4 - ] is as follows: ； (3) Hydrolysis and post-treatment: desalted water is added to the cellulose ester solution obtained in step (2) to carry out a hydrolysis reaction, and a white precipitate is precipitated and filtered, washed, and dried to obtain cellulose acetate, cellulose acetate propionate, or cellulose acetate butyrate.

2. The method according to claim 1, wherein: The mass ratio of cellulose to glacial acetic acid or a mixture of glacial acetic acid and propionic acid or a mixture of glacial acetic acid and butyric acid in step (1) is 1.5:1-1:

10.

3. The method according to claim 1, wherein: The molar ratio of glacial acetic acid to propionic acid in the mixture of glacial acetic acid and propionic acid in step (1) is 1:1-1:12, and the molar ratio of glacial acetic acid to butyric acid in the mixture of glacial acetic acid and butyric acid is 1:1-1:

8.

4. The method according to claim 1, wherein: The constant temperature in step (1) is 30-50°C, the stirring speed is 80-200 r / min, and the stirring time is 40-100 min.

5. The method according to claim 1, wherein: The mass ratio of the acetic anhydride or the mixture of acetic anhydride and propionic anhydride or the mixture of acetic anhydride and butyric anhydride to the cellulose in step (2) is 2-10:1, the molar ratio of acetic anhydride to propionic anhydride in the mixture of acetic anhydride and propionic anhydride is 1:1-1:12, and the molar ratio of acetic anhydride to butyric anhydride in the mixture of acetic anhydride and butyric anhydride is 1:1-1:

8. The amount of the acidic ionic liquid catalyst used is 0.5%-5% of the mass of the cellulose.

6. The method according to claim 1, wherein: The preparation method of the acidic ionic liquid catalyst in step (2) is to mix sulfuric acid with one of 1,8-diazabicyclo[5.4.0]undec-7-ene, triethylbenzylammonium chloride and tetramethylguanidine, heat the mixture and react, add n-heptane and let it stand, and take the upper liquid to obtain the acidic ionic liquid catalyst, which are [DBUH + ][HSO4 - ]、[N B222 H + ][HSO4 - ] or [TMGH + ][HSO4 - ].

7. The method according to claim 6, characterized in that: The molar ratio of the sulfuric acid to 1,8-diazabicyclo[5.4.0]undec-7-ene or triethylbenzylammonium chloride or tetramethylguanidine is 0.5:1-3:

1.

8. The method according to claim 6, wherein: The heating reaction temperature is 70-100° C., the reaction time is 10-13 h, the mass ratio of n-heptane to 1,8-diazabicyclo[5.4.0]undec-7-ene or triethylbenzylammonium chloride or tetramethylguanidine is 10:1, and the standing time is 0.5 h.

9. The method according to claim 1, wherein: The reaction temperature in step (2) is 40-60°C.

10. The method according to claim 1, wherein: The mass ratio of the desalted water to the cellulose ester solution in step (3) is 10-30:1, the hydrolysis temperature is 20-50°C, and the hydrolysis time is 1-5 h.

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