A method for synthesizing cellulose ester using amino acid ionic liquid as catalyst

Abstract

The application discloses a method for synthesizing cellulose ester by using amino acid ionic liquid as a catalyst. The amino acid ionic liquid used is lysine trifluoromethane sulfonate [Lys][CF3SO3]2, lysine p-toluene sulfonate [Lys][Tos]2, tryptophan trifluoromethane sulfonate [Trp][CF3SO3], tryptophan p-toluene sulfonate [Trp][Tos] and phenylalanine trifluoromethane sulfonate [Phe][CF3SO3], which can be simultaneously used in the synthesis processes of cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate and cellulose acetate butyrate, and the synthesized cellulose ester has excellent performance. In addition, the amino acid ionic liquid used has the advantages of low toxicity, easy biodegradation and reproducibility, overcomes the problems of serious corrosion and difficult reuse of a traditional sulfuric acid catalyst, and has important significance for promoting the green and efficient production of cellulose ester.

Description

TECHNICAL FIELD

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

[0002] Cellulose is one of the most abundant natural organic polymers on earth. By chemical modification, it can be converted into cellulose ester, which can be endowed with new properties and uses. Cellulose ester is an organic polymer compound formed by the reaction of cellulose molecular chain and ester group. 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 manufacture filters, film and the like; cellulose propionate is generated by the reaction of cellulose and propionic anhydride, and has high water resistance and oil resistance, and is suitable for use in coatings and adhesives; cellulose butyrate is generated by the reaction of cellulose and butyric anhydride, and has better heat resistance and chemical resistance, and is often used to manufacture plastic products. Cellulose acetate propionate and cellulose acetate butyrate are generated by the reaction of cellulose and mixed organic acid anhydride, and are important cellulose organic acid ester derivatives, which are widely used as additives in the coating industry.

[0003] In the industrial production process of cellulose ester, sulfuric acid is often used as a catalyst, and excess organic acid and organic acid anhydride are used as reaction medium and acylating agent respectively to carry out acylation to obtain cellulose ester. 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 propionate, cellulose butyrate, 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, thereby affecting the overall quality of the product. Therefore, it is crucial to find a more efficient and more environmentally friendly catalyst for the synthesis of cellulose ester.

[0004] Ionic liquid is composed of only ions, and the ion categories include organic cations and organic or inorganic anions. Unlike the form of traditional ionic compounds, ionic liquid is in liquid state at room temperature, so it is also called room temperature molten salt. Compared with general salts and organic solvents, ionic liquid has the following advantages: 1) very wide liquid temperature range and excellent thermal stability; 2) almost no volatilization, which can avoid the loss and pollution in the process of using traditional organic solvents; 3) strong solubility, which can dissolve many organic and inorganic substances; 4) structural adjustability, which can be designed according to the actual use purpose. Based on the above advantages, ionic liquid is widely used in separation process, electrochemistry field and catalytic process.

[0005] At present, various ionic liquids have been developed for cellulose ester synthesis. Patent (CN 103613672 A) discloses N-methyl imidazole hydrochloride, N-methyl imidazole bisulfate and 1-vinyl-3-(3-sulfopropyl) imidazole bisulfate for catalyzing synthesis of cellulose acetate; patent (CN 104130332 A) discloses a method for synthesizing cellulose ester catalyzed by 1-butyl-3-methyl imidazole bisulfate ionic liquid; patent (CN 116606381 B) discloses a method for synthesizing triacetate cellulose catalyzed by 1-allyl-3-(3-sulfopropyl) imidazole bisulfate ionic liquid. Although the above ionic liquids all show good catalytic effect in cellulose ester synthesis, the above ionic liquids are all imidazole-based ionic liquids, which have the disadvantages of non-renewable raw materials, complex synthesis process, environmental unfriendliness, great toxicity, poor biodegradability and great water disturbance in pretreatment process. Therefore, it is necessary to develop suitable ionic liquid catalysts for cellulose ester synthesis process. SUMMARY

[0006] The present application aims to provide a method for synthesizing cellulose ester with amino acid ionic liquid as catalyst, which has the advantages of low toxicity, easy biodegradability and renewability, and can be used in synthesis processes of cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate and cellulose acetate butyrate.

[0007] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0008] A method for synthesizing cellulose ester with amino acid ionic liquid as catalyst, comprising the following steps:

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

[0010] (2) amino acid ionic liquid catalyst and organic anhydride are added to the suspension obtained in step (1), and the mixture is reacted at 40-60℃ for 2-15h; 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) 10-30 times the mass of desalted water is added to the reaction solution obtained in step (2), and the mixture is reacted at 20-50℃ for 1-5h, and then white precipitate is separated out, filtered, washed and dried to obtain cellulose ester.

[0012] Further, the organic acid in step (1) is one of acetic acid, propionic acid and 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; and the mass ratio of cellulose to organic acid is 1.5:1-1:10.

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

[0014] Further, the preparation method of the amino acid ionic liquid catalyst in step (2) is that an amino acid is dissolved in deionized water, a trifluoromethanesulfonic acid or p-toluenesulfonic acid aqueous solution is added dropwise, the reaction is completed at a certain temperature, the excess water is evaporated, and the amino acid ionic liquid catalyst is obtained after washing with a nonpolar solvent, which is [Lys][CF3SO3]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos] and [Phe][CF3SO3] respectively, and the structures are as follows:

[0015]

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

[0017] The molar ratio of deionized water to 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 DEG C, and the reaction time is 1-6 h.

[0020] The nonpolar solvent is one of ethyl acetate, benzene, carbon tetrachloride and dichloroethane.

[0021] Compared with the prior art, the application has the following advantages:

[0022] The present invention uses low-toxic, biodegradable, and renewable amino acid ionic liquids as catalysts for cellulose ester preparation, replacing sulfuric acid and imidazole ionic liquids. This helps overcome the severe corrosiveness and difficulty in reusing traditional sulfuric acid catalysts, as well as the non-renewable raw materials, complex and environmentally unfriendly synthesis processes, high toxicity, and poor biodegradability of traditional imidazole ionic liquids. Furthermore, the synthesized acidic ionic liquids have the advantages of high activity and strong solubility, and can be used in the synthesis 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 solutions of the present invention are 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 the 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] Cellulose and glacial acetic acid were added into a batch stirred tank at a mass ratio of 1:5, and stirred at 40°C for 80 min at a speed of 100 r / min to obtain a uniform suspension. Acetic anhydride in an amount of 5 times the mass of cellulose and [Lys][CF3SO3]2 amino acid ionic liquid catalyst in an amount of 1% of the mass of cellulose were added into the uniform suspension, and reacted at 50°C for 5 h. After the reaction was completed, desalination water (containing salt in an amount of 1-5 mg / L) in an amount of 20 times the mass of cellulose was added, and hydrolysis was carried out at 40°C for 3 h. The white precipitate that separated out was filtered, washed, and dried to obtain cellulose acetate. The filtrate after filtration was evaporated and washed with ethyl acetate to recover the ionic liquid. The cellulose acetate obtained had 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 water to lysine molar ratio of 30:1, and then p-toluenesulfonic acid aqueous solution (50 wt.%) was added dropwise into the solution at a p-toluenesulfonic acid to lysine molar ratio of 2:1, and reacted at 80°C for 3 h. 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 the lysine p-toluenesulfonic acid salt [Lys][Tos]2 was obtained after washing three times and removing the ethyl acetate.

[0031] 1 HNMR (300 MHz, 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] Cellulose and glacial acetic acid were added into a batch stirred tank at a mass ratio of 1:5, and stirred at 40°C for 80 min at a speed of 100 r / min to obtain a uniform suspension. Acetic anhydride in an amount of 5 times the mass of cellulose and [Lys][Tos]2 amino acid ionic liquid catalyst in an amount of 1% of the mass of cellulose were added into the uniform suspension, and reacted at 50°C for 5 h. After the reaction was completed, desalination water (containing salt in an amount of 1-5 mg / L) in an amount of 20 times the mass of cellulose was added, and hydrolysis was carried out at 40°C for 3 h. The white precipitate that separated out was filtered, washed, and dried to obtain cellulose acetate. The filtrate after filtration was evaporated and washed with ethyl acetate to recover the ionic liquid. The cellulose acetate obtained had a degree of polymerization of 277 and a combined acid of 55.2%.

[0033] Example 3

[0034] Trp] [CF3SO3] was prepared by dissolving Trp in deionized water at a molar ratio of water to Trp of 30:1, then adding trifluoromethanesulfonic acid to the solution at a molar ratio of 1:1, and reacting at 80°C for 3h. After the reaction was completed, excess solvent water was removed by high-temperature evaporation, then ethyl acetate was added for washing, the amount of ethyl acetate added was 5 times the mass of Trp, and after washing three times, the ethyl acetate was removed to obtain Trp trifluoromethanesulfonate [Trp] [CF3SO3].

[0035] 1 HNMR (300 MHz, DMSO-d6), δ: 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] Cellulose and glacial acetic acid were added to a batch stirred tank at a mass ratio of 1:5, and stirred at 100 r / min for 80 min at a constant temperature of 40°C to obtain a uniform suspension. Acetic anhydride in an amount of 5 times the mass of cellulose and [Trp] [CF3SO3] amino acid ionic liquid catalyst in an amount of 1% of 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 completed, 20 times the mass of desalting water (containing 1-5 mg / L of salt) was added to the cellulose, and hydrolysis was carried out at a constant temperature of 40°C for 3h. The white precipitate that separated out was filtered, washed, and dried to obtain cellulose acetate. The filtrate was evaporated to remove water, and washed with ethyl acetate to recover the ionic liquid. The cellulose acetate obtained had a degree of polymerization of 285, and the combined acid was 54.7%.

[0037] Example 4

[0038] Trp] [Tos] was prepared by dissolving Trp in deionized water at a molar ratio of water to Trp of 30:1, then adding p-toluenesulfonic acid aqueous solution (50 wt.%) to the solution at a molar ratio of 1:1, and reacting at 80°C for 3h. After the reaction was completed, excess solvent water was removed by high-temperature evaporation, then ethyl acetate was added for washing, the amount of ethyl acetate added was 5 times the mass of Trp, and after washing three times, the ethyl acetate was removed to obtain Trp p-toluenesulfonate [Trp] [Tos].

[0039] 1 HNMR (300 MHz, 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] Cellulose and glacial acetic acid were added to a batch stirred tank in a 1:5 mass ratio. The mixture was stirred at 100 rpm at 40°C for 80 minutes to obtain a uniform suspension. Acetic anhydride (5 times the mass of the cellulose) and a [Trp][Tos] amino acid ionic liquid catalyst (1% of the mass of the cellulose) were added to the homogeneous suspension. The mixture was reacted at 50°C for 5 hours. After the reaction was complete, desalted water (1-5 mg / L) (20 times the mass of the cellulose) was added. The mixture was hydrolyzed at 40°C for 3 hours. The precipitated white precipitate was filtered, washed, and dried to obtain cellulose acetate. The filtered filtrate was dehydrated and washed with ethyl acetate to recover the ionic liquid. The resulting cellulose acetate had a degree of polymerization of 295 and a bound acid content 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. 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 phenylalanine. After washing three times, the 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] Cellulose and glacial acetic acid were added to a batch stirred tank at a mass ratio of 1:5. The mixture was stirred at 100 rpm for 80 minutes at a constant temperature of 40°C to obtain a uniform suspension. Acetic anhydride (5 times the mass of the cellulose) and a [Phe][CF3SO3] amino acid ionic liquid catalyst (1% of the mass of the cellulose) were added to the uniform suspension. The mixture was reacted at 50°C for 5 hours. After the reaction was complete, desalted water (1-5 mg / L) (20 times the mass of the cellulose) was added. The mixture was hydrolyzed at 40°C for 3 hours. The precipitated white precipitate was filtered, washed, and dried to obtain cellulose acetate. The filtered filtrate was dehydrated 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 55.0%.

[0045] Example 6

[0046] The five amino acid ionic liquids recovered from Examples 1-5 were all subjected to repeated use performance testing. Specifically, cellulose and glacial acetic acid were added to a batch stirred tank at a mass ratio of 1:5, and stirred at 100 r / min for 80 min at a constant temperature of 40°C to obtain a uniform suspension. To the uniform suspension, 1% of the recovered [Lys][CF3SO3]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos] and [Phe][CF3SO3] amino acid ionic liquid catalysts were added, and 5 times the mass of cellulose of acetic anhydride was added, and the reaction was carried out at a constant temperature of 50°C for 5 h. After the reaction was complete, 20 times the mass of cellulose of desalting water (containing 1-5 mg / L of salt) was added, 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. The filtrate was evaporated and washed with ethyl acetate to recover the ionic liquid. The results of two repeated uses are shown in Table 1. The amino acid ionic liquids that were repeatedly used exhibited similar catalytic effects to the fresh ionic liquids, indicating that the five amino acid ionic liquids used had good stability in the process of catalyzing the preparation of cellulose acetate.

[0047] Table 1 Repeated use performance of amino acid ionic liquids in the reaction of catalyzing the preparation of cellulose acetate from cellulose

[0048]

[0049] Example 7

[0050] Cellulose and propionic acid were added to a batch stirred tank at a mass ratio of 1:6, and stirred at 100 r / min for 80 min at a constant temperature of 40°C to obtain a uniform suspension. To the uniform suspension, 6 times the mass of cellulose of propionic anhydride and 1% of the [Lys][CF3SO3]2 amino acid ionic liquid catalyst were added, and the reaction was carried out at a constant temperature of 50°C for 5 h. After the reaction was complete, 20 times the mass of cellulose of desalting water (containing 1-5 mg / L of salt) was added, 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 propionate, and the propionyl content was 49.5%.

[0051] Example 8

[0052] Cellulose and butyric acid were added into a stirred tank reactor at a mass ratio of 1:8, and stirred at 40°C for 80 min at a speed of 100 r / min to obtain a uniform suspension. To the uniform suspension, 8 times the mass of cellulose of acetic anhydride and propionic anhydride mixture (molar ratio of acetic anhydride to propionic anhydride was 1:8) and 1% of the mass of cellulose of [Lys][CF3SO3]2 amino acid ionic liquid catalyst were added, and reacted at 50°C for 5 h. After the reaction was completed, 20 times the mass of cellulose of desalting water (salt content was 1-5 mg / L) was added, and hydrolysis was carried out at 40°C for 3 h. The white precipitate was filtered, washed and dried to obtain cellulose acetate propionate. The total degree of substitution of the obtained cellulose acetate propionate was 2.58, the acetyl content was 8.2%, and the propionyl content was 42.5%.

[0053] Example 9

[0054] Cellulose and a mixture of glacial acetic acid and propionic acid (molar ratio of glacial acetic acid to propionic acid was 1:8) were added into a stirred tank reactor at a mass ratio of 1:7, and stirred at 40°C for 80 min at a speed of 100 r / min to obtain a uniform suspension. To the uniform suspension, 7 times the mass of cellulose of acetic anhydride and propionic anhydride mixture (molar ratio of acetic anhydride to propionic anhydride was 1:8) and 1% of the mass of cellulose of [Lys][CF3SO3]2 amino acid ionic liquid catalyst were added, and reacted at 50°C for 5 h. After the reaction was completed, 20 times the mass of cellulose of desalting water (salt content was 1-5 mg / L) was added, and hydrolysis was carried out at 40°C for 3 h. The white precipitate was filtered, washed and dried to obtain cellulose acetate propionate. The total degree of substitution of the obtained cellulose acetate propionate was 2.58, the acetyl content was 8.2%, and the propionyl content was 42.5%.

[0055] Example 10

[0056] Cellulose and a mixture of glacial acetic acid and butyric acid (molar ratio of glacial acetic acid to butyric acid was 1:8) were added into a stirred tank reactor at a mass ratio of 1:8, and stirred at 40°C for 80 min at a speed of 100 r / min to obtain a uniform suspension. To the uniform suspension, 8 times the mass of cellulose of acetic anhydride and butyric anhydride mixture (molar ratio of acetic anhydride to butyric anhydride was 1:8) and 1% of the mass of cellulose of [Lys][CF3SO3]2 amino acid ionic liquid catalyst were added, and reacted at 50°C for 5 h. After the reaction was completed, 20 times the mass of cellulose of desalting water (salt content was 1-5 mg / L) was added, and hydrolysis was carried out at 40°C for 3 h. The white precipitate was filtered, washed and dried to obtain cellulose acetate propionate. The total degree of substitution of the obtained cellulose acetate propionate was 2.58, the acetyl content was 8.2%, and the propionyl content was 42.5%.

[0057] The above merely describes preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the present application should be included in the scope of the present application.

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 constant 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]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos], and [Phe][CF3SO3]; (3) adding desalted water (10-30 times the mass) to 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; The structural formulas of [Lys][CF3SO3]2, [Lys][Tos]2, [Trp][CF3SO3], [Trp][Tos] and [Phe][CF3SO3] are as follows: 。 2. The method according to claim 1, wherein: 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; and the mass ratio of cellulose to organic acid is 1.5:1-1:

10.

3. The method according to claim 1, wherein: 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; and the mass ratio of organic anhydride to cellulose is 2-10:

1.

4. The method according to claim 1, wherein: 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, wherein: The preparation method of the amino acid ionic liquid catalyst in step (2) comprises: dissolving an 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; the amino acid is one of lysine, tryptophan, and phenylalanine.

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

7. The method according to claim 5, 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.

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

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

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