Method for synthesizing cellulose ester by taking amino acid ionic liquid as catalyst

Abstract

The invention discloses a method for synthesizing cellulose ester by taking amino acid ionic liquid as a catalyst. The used amino acid ionic liquid is lysine trifluoromethane sulfonate [Lys] [CF3SO3] 2, lysine p-toluenesulfonate [Lys] [Tos] 2, tryptophan trifluoromethane sulfonate [Trp] [CF3SO3], tryptophan p-toluenesulfonate [Trp] [Tos] and phenylalanine trifluoromethane sulfonate [Phe] [CF3SO3]; the method can be used for synthesis processes of cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate and cellulose acetate butyrate at the same time, and the synthesized cellulose ester is excellent in performance. Besides, the used amino acid ionic liquid has the advantages of low toxicity, easiness in biodegradation and reproducibility, the problems that a traditional sulfuric acid catalyst is serious in corrosivity and difficult to reuse are solved, and the method has important significance on promoting green and efficient production of cellulose ester.

Description

Technical Field

[0001] The invention belongs to the technical field of cellulose ester preparation, and in particular relates to a method for synthesizing cellulose ester by using amino acid ionic liquid as catalyst. Background Art

[0002] Cellulose is one of the most abundant natural organic polymers on earth. By chemically modifying it and converting it into cellulose esters, it can be given new properties and uses. Cellulose ester is an organic polymer compound formed by the reaction of cellulose molecular chains with ester groups. It has good biodegradability, biocompatibility and plasticity, and is widely used in medicine, food, cosmetics, textiles, papermaking and other industries. Among them, cellulose acetate is generated by the reaction of cellulose and acetic anhydride, and is often used to make filters, films, etc.; cellulose propionate is generated by the reaction of cellulose and propionic anhydride, has high water resistance and oil resistance, and is suitable for coatings and adhesives; cellulose butyrate is generated by the reaction of cellulose and butyric anhydride, has better heat resistance and chemical resistance, and is often used to make plastic products. Cellulose acetate propionate and cellulose acetate butyrate are generated by the reaction of cellulose and mixed organic anhydrides. They are both important cellulose organic acid ester derivatives and are widely used as additives in the coating industry.

[0003] In the industrial production process of cellulose esters, sulfuric acid is often used as a catalyst, with excess organic acid and organic anhydride as the reaction medium and acylating agent, respectively, to carry out acylation to produce cellulose esters. Due to the uncontrollable nature of the acylation process, cellulose esters need to be further processed, such as hydrolysis, to obtain partially substituted cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate and cellulose acetate butyrate. Although strong inorganic 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. Therefore, it is crucial to find more efficient and environmentally friendly catalysts for the synthesis of cellulose esters.

[0004] Ionic liquids are composed only of ions, and the ion categories include organic cations and organic or inorganic anions. Unlike the morphology of traditional ionic compounds, ionic liquids are liquid at room temperature, so they are also called room temperature molten salts. Compared with general salts and organic solvents, ionic liquids have the following advantages: 1) They have a wide liquid temperature range and excellent thermal stability; 2) They are almost non-volatile, which can avoid the loss and pollution of traditional organic solvents during use; 3) They have strong solubility and can dissolve many organic and inorganic substances; 4) They have structural adjustability and can be designed according to the actual purpose of use. Based on the above advantages, ionic liquids are widely used in separation processes, electrochemical fields and catalytic processes.

[0005] At present, a variety of ionic liquids have been developed for the synthesis of cellulose esters. Patent (CN 103613672 A) discloses N-methylimidazole hydrochloride, N-methylimidazole hydrogen sulfate and 1-vinyl-3-(3-sulfopropyl) imidazole hydrogen sulfate for catalytic synthesis of cellulose acetate; Patent (CN 104130332 A) discloses a method for catalytic synthesis of cellulose esters by 1-butyl-3-methylimidazole hydrogen sulfate ionic liquid; Patent (CN 116606381 B) discloses a method for catalytic synthesis of cellulose triacetate by 1-allyl-3-(3-sulfopropyl) imidazolium hydrogen sulfate ionic liquid. Although the above ionic liquids all show good catalytic effects in the synthesis of cellulose esters, the above ionic liquids are all imidazolyl ionic liquids, which have the disadvantages of non-renewable synthetic raw materials, complex and environmentally unfriendly synthetic processes, high toxicity, poor biodegradability and large water interference in the pretreatment process. Therefore, it is necessary to develop suitable ionic liquid catalysts for the cellulose ester synthesis process. Summary of the invention

[0006] The object of the present invention is to provide a method for synthesizing cellulose ester using amino acid ionic liquid as a catalyst. The catalyst has the advantages of low toxicity, easy biodegradability and renewability, and can be simultaneously used in the synthesis process of cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate and cellulose acetate butyrate.

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

[0008] A method for synthesizing cellulose ester using amino acid ionic liquid as a catalyst comprises the following steps:

[0009] (1) cellulose and organic acid are activated at a temperature of 30-50° C. and a stirring speed of 80-200 r / min for 40-100 min to obtain a uniform suspension;

[0010] (2) adding an amino acid ionic liquid catalyst and an organic acid anhydride to the suspension obtained in step (1), and reacting at 40-60° C. for 2-15 hours; the amino acid ionic liquid catalyst is one or two of lysine trifluoromethanesulfonate [Lys][CF3SO3]2, lysine p-toluenesulfonate [Lys][Tos]2, tryptophan trifluoromethanesulfonate [Trp][CF3SO3], tryptophan p-toluenesulfonate [Trp][Tos], and phenylalanine trifluoromethanesulfonate [Phe][CF3SO3];

[0011] (3) adding desalted water in an amount 10-30 times the mass of the reaction solution obtained in step (2), reacting at 20-50° C. for 1-5 hours, filtering, washing and drying a white precipitate to obtain cellulose ester.

[0012] Furthermore, the organic acid in step (1) is one of acetic acid, propionic acid, butyric acid, or a mixed organic acid of acetic acid and propionic acid, or a mixed organic acid of acetic acid and butyric acid, wherein the molar ratio of acetic acid to propionic acid is 1:1-1:12, and the molar ratio of acetic acid to butyric acid is 1:1-1:15; the mass ratio of cellulose to organic acid is 1.5:1-1:10.

[0013] Furthermore, the organic anhydride in step (2) is one of acetic anhydride, propionic anhydride, butyric anhydride, or a mixed organic anhydride of acetic anhydride and propionic anhydride, or a mixed organic anhydride of acetic anhydride and butyric anhydride, wherein the molar ratio of acetic anhydride to propionic anhydride is 1:1-1:12, and the molar ratio of acetic anhydride to butyric anhydride is 1:1-1:15; the mass ratio of organic anhydride to cellulose is 2-10:1, and the amount of amino acid ionic liquid catalyst used is 0.5%-5% of the mass of cellulose.

[0014] Furthermore, the preparation method of the amino acid ionic liquid catalyst in step (2) is to dissolve the amino acid in deionized water, add trifluoromethanesulfonic acid or p-toluenesulfonic acid aqueous solution dropwise, and after the reaction is completed at a certain temperature, evaporate the excess water, and wash with a non-polar solvent to obtain the amino acid ionic liquid catalyst, which are [Lys][CF3SO3]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos] and [Phe][CF3SO3], respectively, and their structures are as follows:

[0015]

[0016] The amino acid is one of lysine, tryptophan and phenylalanine.

[0017] The molar ratio of the deionized water to the amino acid is 10-100.

[0018] The molar ratio of lysine to trifluoromethanesulfonic acid or p-toluenesulfonic acid is 1.8:1-3:1, and the molar ratio of tryptophan or phenylalanine to trifluoromethanesulfonic acid or p-toluenesulfonic acid is 0.9:1-1.5:1.

[0019] The reaction temperature is 50-90° C., and the reaction time is 1-6 h.

[0020] The non-polar solvent is one of ethyl acetate, benzene, carbon tetrachloride and ethylene dichloride.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] The present invention selects low-toxic, biodegradable, renewable amino acid ionic liquids to replace sulfuric acid and imidazole ionic liquids as catalysts for preparing cellulose esters, which will help overcome the problems of serious corrosiveness and difficulty in reuse of traditional sulfuric acid catalysts, as well as the problems of non-renewable synthetic raw materials, complex synthetic processes, unfriendly environment, high toxicity, and poor biodegradability of traditional imidazole ionic liquids. In addition, the synthesized acidic ionic liquids have the advantages of high activity and strong solubility, and can be used in the synthesis processes of cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, and cellulose acetate butyrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The structural diagrams of acidic ionic liquids [Lys][CF3SO3]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos] and [Phe][CF3SO3]. DETAILED DESCRIPTION

[0024] In order to make the contents of the present invention easier to understand, the technical solution of the present invention is further described below in conjunction with specific implementation methods, but the present invention is not limited thereto.

[0025] Example 1

[0026] Lysine was dissolved in deionized water at a molar ratio of 30:1 between water and lysine, and trifluoromethanesulfonic acid was then added dropwise to the solution at a molar ratio of 2:1 to lysine. The mixture was reacted at 80°C for 3 hours. After the reaction, excess solvent water was removed by high-temperature evaporation, and ethyl acetate was added for washing. The amount of ethyl acetate added was 5 times the mass of lysine. After washing three times, the ethyl acetate was removed to obtain lysine trifluoromethanesulfonate ionic liquid [Lys][CF3SO3]2.

[0027] 1 HNMR (300MHz, DMSO-d6), δ: 1.25 (m, 2H, CH2), 2.11 (m, 4H, CH2), 3.31 (t, 2H, CH2), 4.35 (t, H, CH);

[0028] Add cellulose and glacial acetic acid in a mass ratio of 1:5 to an intermittent stirring kettle, stir at a speed of 100r / min for 80min at a constant temperature of 40℃ to obtain a uniform suspension. Add acetic anhydride 5 times the mass of cellulose and [Lys][CF3SO3]2 amino acid ionic liquid catalyst 1% of the mass of cellulose to the uniform suspension, react at a constant temperature of 50℃ for 5h. After the reaction is complete, add 20 times the mass of desalted water (salt content 1-5 mg / L) to the cellulose, and hydrolyze at a constant temperature of 40℃ for 3h. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid can be recovered by washing with ethyl acetate solvent. The obtained cellulose acetate has a degree of polymerization of 275 and a combined acid of 55.1%.

[0029] Example 2

[0030] Lysine was dissolved in deionized water at a molar ratio of 30:1 between water and lysine, and then an aqueous p-toluenesulfonic acid solution (50 wt.%) was added dropwise to the solution, the molar ratio of p-toluenesulfonic acid to lysine was 2:1, and the reaction was carried out at 80°C for 3 hours. After the reaction was completed, excess solvent water was removed by high-temperature evaporation, and then ethyl acetate was added for washing, the amount of ethyl acetate added was 5 times the mass of lysine, and after washing three times, the ethyl acetate was removed to obtain lysine p-toluenesulfonate [Lys][Tos]2.

[0031] 1 HNMR (300MHz, DMSO-d6), δ: 1.25 (m, 2H, CH2), 2.16 (m, 4H, CH2), 3.31 (t, 2H, CH2), 4.35 (t, H, CH), 7.44 (m, 4H, Ph), 7.75 (m, 4H, Ph)

[0032] Add cellulose and glacial acetic acid in a mass ratio of 1:5 to an intermittent stirring kettle, stir at a speed of 100r / min for 80min at a constant temperature of 40℃ to obtain a uniform suspension. Add acetic anhydride 5 times the mass of cellulose and [Lys][Tos]2 amino acid ionic liquid catalyst 1% of the mass of cellulose to the uniform suspension, react at a constant temperature of 50℃ for 5h. After the reaction is complete, add 20 times the mass of desalted water (salt content 1-5 mg / L) to the cellulose, and hydrolyze at a constant temperature of 40℃ for 3h. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid can be recovered by washing with ethyl acetate solvent. The obtained cellulose acetate has a degree of polymerization of 277 and a combined acid of 55.2%.

[0033] Example 3

[0034] Tryptophan was dissolved in deionized water at a molar ratio of 30:1 between water and tryptophan, and trifluoromethanesulfonic acid was then added dropwise to the solution at a molar ratio of 1:1 to tryptophan. The mixture was reacted at 80°C for 3 hours. After the reaction, excess solvent water was removed by high-temperature evaporation, and then ethyl acetate was added for washing. The amount of ethyl acetate added was 5 times the mass of tryptophan. After washing three times, the ethyl acetate was removed to obtain tryptophan trifluoromethanesulfonate [Trp][CF3SO3].

[0035] 1 HNMR (300MHz, DMSO-d6), 3.52 (q, 2H, CH2), 4.85 (t, H, CH), 7.02 (m, 2H, Ph), 7.18 (s, H, Im), 7.33 (d, H, Ph), 7.57 (d, H, Ph)

[0036] Add cellulose and glacial acetic acid in a mass ratio of 1:5 to an intermittent stirring kettle, stir at a speed of 100r / min for 80min at a constant temperature of 40℃ to obtain a uniform suspension. Add acetic anhydride 5 times the mass of cellulose and [Trp][CF3SO3] amino acid ionic liquid catalyst 1% of the mass of cellulose to the uniform suspension, react at a constant temperature of 50℃ for 5h. After the reaction is complete, add 20 times the mass of desalted water (salt content 1-5 mg / L) to the cellulose, and hydrolyze at a constant temperature of 40℃ for 3h. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid can be recovered by washing with ethyl acetate solvent. The obtained cellulose acetate has a degree of polymerization of 285 and a combined acid of 54.7%.

[0037] Example 4

[0038] Tryptophan was dissolved in deionized water at a molar ratio of 30:1 between water and tryptophan, and then an aqueous p-toluenesulfonic acid solution (50 wt.%) was added dropwise to the solution, and the molar ratio of p-toluenesulfonic acid to tryptophan was 1:1. The mixture was reacted at 80°C for 3 hours. After the reaction was completed, excess solvent water was removed by high-temperature evaporation, and then ethyl acetate was added for washing, and the amount of ethyl acetate added was 5 times the mass of tryptophan. After washing three times, the ethyl acetate was removed to obtain tryptophan p-toluenesulfonate [Trp][Tos].

[0039] 1 H NMR (300MHz, DMSO-d6), δ: 2.43 (s, 3H, CH3), 3.52 (q, 2H, CH2), 4.85 (t, H, CH), 7.02 (m, 2H, Ph), 7.18 (s, H, Im), 7.33 (d, H, Ph), 7.44 (d, 2H, Ph), 7.57 (d, H, Ph), 7.75 (d, 2H, Ph)

[0040] Add cellulose and glacial acetic acid in a mass ratio of 1:5 to an intermittent stirring kettle, stir at a speed of 100r / min for 80min at a constant temperature of 40℃ to obtain a uniform suspension. Add acetic anhydride 5 times the mass of cellulose and [Trp][Tos] amino acid ionic liquid catalyst 1% of the mass of cellulose to the uniform suspension, react at a constant temperature of 50℃ for 5h. After the reaction is complete, add 20 times the mass of desalted water (salt content 1-5 mg / L) to the cellulose, and hydrolyze at a constant temperature of 40℃ for 3h. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid can be recovered by washing with ethyl acetate solvent. The obtained cellulose acetate has a degree of polymerization of 295 and a combined acid of 54.8%.

[0041] Example 5

[0042] Phenylalanine was dissolved in deionized water at a molar ratio of 30:1 between water and phenylalanine, and trifluoromethanesulfonic acid was then added dropwise to the solution at a molar ratio of 1:1 to phenylalanine, and the reaction was carried out at 80°C for 3 hours. After the reaction was completed, excess solvent water was removed by high-temperature evaporation, and then ethyl acetate was added for washing, wherein the amount of ethyl acetate added was 5 times the mass of phenylalanine. After washing three times, ethyl acetate was removed to obtain phenylalanine trifluoromethanesulfonate [Phe][CF3SO3].

[0043] 1 H NMR (300MHz, DMSO-d6), 3.65 (d, 2H, CH2), 4.83 (t, H, CH), 7.21 (m, 5H, Ph)

[0044] Add cellulose and glacial acetic acid in a mass ratio of 1:5 to an intermittent stirring kettle, stir at a speed of 100r / min for 80min at a constant temperature of 40℃ to obtain a uniform suspension. Add acetic anhydride 5 times the mass of cellulose and [Phe][CF3SO3] amino acid ionic liquid catalyst 1% of the mass of cellulose to the uniform suspension, react at a constant temperature of 50℃ for 5h. After the reaction is complete, add 20 times the mass of desalted water (salt content 1-5 mg / L) to the cellulose, and hydrolyze at a constant temperature of 40℃ for 3h. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid can be recovered by washing with ethyl acetate solvent. The obtained cellulose acetate has a degree of polymerization of 300 and a combined acid of 55.0%.

[0045] Example 6

[0046] The five kinds of amino acid ionic liquids recovered in Examples 1-5 were tested for their reusability. Specifically, cellulose and glacial acetic acid were added to an intermittent stirring kettle at a mass ratio of 1:5, and stirred at a speed of 100r / min for 80min at a constant temperature of 40°C to obtain a uniform suspension. 1% of the mass of cellulose was added to the uniform suspension, and [Lys][CF3SO3]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos] and [Phe][CF3SO3] amino acid ionic liquid catalysts and acetic anhydride 5 times the mass of cellulose were added to the uniform suspension, and reacted at a constant temperature of 50°C for 5h. After the reaction was complete, 20 times the mass of desalted water (salt content 1-5 mg / L) of cellulose was added, and the reaction was hydrolyzed at a constant temperature of 40°C for 3h. The precipitated white precipitate was filtered, washed and dried to obtain cellulose acetate. The filtered filtrate was evaporated and dehydrated, and the ionic liquid was recovered by washing with ethyl acetate solvent. It was reused twice, and the results are shown in Table 1. The reused amino acid ionic liquid showed a catalytic effect similar to that of the fresh ionic liquid, indicating that the five amino acid ionic liquids used had better stability in the catalytic preparation of cellulose acetate.

[0047] Table 1 Reusability of amino acid ionic liquids in catalyzing the preparation of cellulose acetate from cellulose

[0048]

[0049] Example 7

[0050] Cellulose and propionic acid were added to an intermittent stirring tank at a mass ratio of 1:6, and stirred at a speed of 100 r / min for 80 minutes at a constant temperature of 40°C to obtain a uniform suspension. Propionic anhydride 6 times the mass of cellulose and [Lys][CF3SO3]2 amino acid ionic liquid catalyst 1% of the mass of cellulose were added to the uniform suspension, and reacted at a constant temperature of 50°C for 5 hours. After the reaction was complete, desalted water 20 times the mass of cellulose (salt content 1-5 mg / L) was added, and hydrolyzed at a constant temperature of 40°C for 3 hours. The precipitated white precipitate was filtered, washed, and dried to obtain cellulose propionate with a propionyl content of 49.5%.

[0051] Example 8

[0052] Cellulose and butyric acid were added to an intermittent stirring tank at a mass ratio of 1:8, and stirred at a speed of 100 r / min for 80 minutes at a constant temperature of 40°C to obtain a uniform suspension. Butyric anhydride 8 times the mass of cellulose and [Lys][CF3SO3]2 amino acid ionic liquid catalyst 1% of the mass of cellulose were added to the uniform suspension, and reacted at a constant temperature of 50°C for 5 hours. After the reaction was complete, desalted water 20 times the mass of cellulose (salt content 1-5 mg / L) was added, and hydrolyzed at a constant temperature of 40°C for 3 hours. The precipitated white precipitate was filtered, washed, and dried to obtain cellulose butyrate with a butyryl content of 31.5%.

[0053] Example 9

[0054] A mixture of cellulose and glacial acetic acid and propionic acid (the molar ratio of glacial acetic acid to propionic acid is 1:8) is added to an intermittent stirring kettle at a mass ratio of 1:7, and stirred at a speed of 100r / min for 80min at a constant temperature of 40°C to obtain a uniform suspension. A mixture of acetic anhydride and propionic anhydride (the molar ratio of acetic anhydride to propionic anhydride is 1:8) with a mass of 7 times the mass of cellulose and a [Lys][CF3SO3]2 amino acid ionic liquid catalyst with a mass of 1% of the mass of cellulose are added to the uniform suspension, and the mixture is reacted at a constant temperature of 50°C for 5h. After the reaction is complete, 20 times the mass of desalted water (salt content of 1 to 5 mg / L) of cellulose is added, and the mixture is hydrolyzed at a constant temperature of 40°C for 3h. The precipitated white precipitate is filtered, washed, and dried to obtain cellulose acetate propionate. The total degree of substitution of the obtained cellulose acetate propionate is 2.58, the acetyl content is 8.2%, and the propionyl content is 42.5%.

[0055] Example 10

[0056] A mixture of cellulose and glacial acetic acid and butyric acid (the molar ratio of glacial acetic acid to butyric acid is 1:8) is added to an intermittent stirring tank at a mass ratio of 1:8, and stirred at a speed of 100r / min for 80min at a constant temperature of 40°C to obtain a uniform suspension. A mixture of acetic anhydride and butyric anhydride (the molar ratio of acetic anhydride to butyric anhydride is 1:8) with a mass of 8 times that of cellulose and a [Lys][CF3SO3]2 amino acid ionic liquid catalyst with a mass of 1% of cellulose are added to the uniform suspension, and the mixture is reacted at a constant temperature of 50°C for 5h. After the reaction is complete, 20 times the mass of desalted water (salt content of 1 to 5 mg / L) of cellulose is added, and the mixture is hydrolyzed at a constant temperature of 40°C for 3h. The precipitated white precipitate is filtered, washed, and dried to obtain cellulose acetate butyrate. The total degree of substitution of the obtained cellulose acetate butyrate is 2.21, the acetyl content is 15.5%, and the butyryl content is 26.3%.

[0057] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for synthesizing cellulose ester using amino acid ionic liquid as a catalyst, characterized in that: The following steps are involved: (1) Cellulose and organic acid are activated at a temperature of 30-50 °C and a stirring speed of 80-200 r / min for 40-100 min to obtain a uniform suspension; (2) adding an amino acid ionic liquid catalyst and an organic acid anhydride to the suspension obtained in step (1), and reacting at 40-60°C for 2-15 hours; the amino acid ionic liquid catalyst is one or two of [Lys][CF3SO3], [Lys][Tos], [Trp][CF3SO3], [Trp][Tos], and [Phe][CF3SO3]; (3) Add 10-30 times the mass of desalted water to the reaction solution obtained in step (2), react at 20-50°C for 1-5h, filter, wash and dry the precipitated white precipitate to obtain cellulose ester.

2. The method according to claim 1, characterized in that: The organic acid in step (1) is one of acetic acid, propionic acid, butyric acid, or a mixed organic acid of acetic acid and propionic acid, or a mixed organic acid of acetic acid and butyric acid, wherein the molar ratio of acetic acid to propionic acid is 1:1-1:12, and the molar ratio of acetic acid to butyric acid is 1:1-1:15; the mass ratio of cellulose to organic acid is 1.5:1-1:

10.

3. The method according to claim 1, characterized in that: The organic anhydride in step (2) is one of acetic anhydride, propionic anhydride, butyric anhydride, or a mixed organic anhydride of acetic anhydride and propionic anhydride, or a mixed organic anhydride of acetic anhydride and butyric anhydride, wherein the molar ratio of acetic anhydride to propionic anhydride is 1:1-1:12, and the molar ratio of acetic anhydride to butyric anhydride is 1:1-1:15; the mass ratio of organic anhydride to cellulose is 2-10:

1.

4. The method according to claim 1, characterized in that: The amount of amino acid ionic liquid catalyst used in step (2) is 0.5%-5% of the mass of cellulose.

5. The method according to claim 1, characterized in that: The preparation method of the amino acid ionic liquid catalyst in step (2) comprises: dissolving the amino acid in deionized water, adding trifluoromethanesulfonic acid or p-toluenesulfonic acid aqueous solution dropwise, reacting completely at a certain temperature, evaporating excess water, and washing with a non-polar solvent to obtain the amino acid ionic liquid catalyst, which are [Lys][CF3SO3]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos] and [Phe][CF3SO3], respectively, and their structures are as follows: 。 6. The method according to claim 5, characterized in that: The amino acid is one of lysine, tryptophan and phenylalanine.

7. The method according to claim 5, characterized in that: The molar ratio of deionized water to amino acid is 10-100.

8. The method according to claim 6, characterized in that: The molar ratio of lysine to trifluoromethanesulfonic acid or p-toluenesulfonic acid is 1.8:1-3:1, and the molar ratio of tryptophan or phenylalanine to trifluoromethanesulfonic acid or p-toluenesulfonic acid is 0.9:1-1.5:

1.

9. The method according to claim 5, characterized in that: The reaction temperature is 50-90 °C and the reaction time is 1-6 h.

10. The method according to claim 5, characterized in that: The non-polar solvent is one of ethyl acetate, benzene, carbon tetrachloride and ethylene dichloride.

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