Method for synthesizing cellulose ester by using acidic ionic liquid as catalyst

Abstract

The invention discloses a method for synthesizing cellulose ester by taking acidic ionic liquid as a catalyst. The method comprises three steps of constant-temperature activation, esterification, hydrolysis and post-treatment. The 1, 8-diazabicyclo [5.4. 0] undec-7-ene-sulfuric acid [DBUH < + >] [HSO4 <->], triethylbenzyl-sulfuric acid [NB222H < + >] [HSO4 <->] and tetramethylguanidine-sulfuric acid [TMGH < + >] [HSO4 <->] acidic ionic liquid catalysts synthesized by the method are low in cost and easy to synthesize. The catalyst can be used for synthesis processes of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate at the same time, the synthesized cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate are excellent in performance, and in addition, the used catalyst has high thermal stability and low volatility, is easy to recycle, and is suitable for industrial production. The method provided by the invention reduces the generation and treatment cost of pollutants, overcomes the problems of serious corrosivity and difficult reuse of the traditional sulfuric acid catalyst, and has important significance for promoting the green and efficient production of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate.

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 using acidic ionic liquid as a catalyst. Background Art

[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 to expand its application field by improving its properties. Cellulose can react with different chemical substances, including acids, anhydrides or other substances, to produce cellulose products with high added value. Among them, cellulose acetate is the most important organic acid ester of cellulose and 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 is currently worth US$6.15 billion and is expected to grow at an annual rate of 8.9% to reach US$10.25 billion by 2028. Cellulose acetate propionate and cellulose acetate butyrate are both important cellulose organic acid ester derivatives and are widely used as additives in the coatings industry.

[0003] Cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate are all produced by the acylation of cellulose. On an industrial scale, cellulose is acylated with sulfuric acid as a catalyst, excess acetic acid and acetic anhydride, propionic acid and propionic anhydride, butyric acid and butyric anhydride, and complexes of different acids and complexes of different anhydrides as reaction medium and acylating agent to produce cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. 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 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 and may also lead to the formation of by-products. Therefore, it is crucial to find more efficient and environmentally friendly catalysts for the synthesis of 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 bonds and electrostatic interactions. Due to the asymmetry of their own structures and the adjustable denaturation of anions and cations, ionic liquids exhibit many unique physical and chemical properties, such as high stability, excellent designability, and easy directional functionalization. These properties have enabled ionic liquids to show significant advantages and good prospects in practical applications. In view of the specific requirements of different catalytic reactions for solvents and catalysts, ionic liquids can meet specific needs by simply adjusting the types of anions and cations, so that using ionic liquids as solvents or catalysts in reactions has become a hot topic in their application research. At present, acidic ionic liquids containing hydrogen sulfate have been widely used in a variety of esterification and hydrolysis reactions, and have achieved good catalytic effects.

[0005] Therefore, for the requirement of acidic catalysts for 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] At present, a variety of ionic liquids have been developed for the synthesis of cellulose esters. Patent (CN 103613672 A) discloses that N-methylimidazole hydrochloride, N-methylimidazole hydrogen sulfate and 1-vinyl-3-(3-sulfopropyl) imidazole hydrogen sulfate are used to catalyze the 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; 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 have shown good catalytic effects in the synthesis of cellulose esters, they are all imidazolyl ionic liquids, which are prone to deactivation. This is because imidazolyl salts are easily deprotonated at the C2 position to produce active nucleophiles, which will further react with electrophilic substances, such as aldehydes, alkyl halides, ethyl formate, acetic anhydride and isothiocyanate. Therefore, non-imidazole ionic liquid acidic catalysts need to be developed for the synthesis of cellulose esters. Summary of the invention

[0007] The object of the present invention is 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 simultaneously used in the synthesis process of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is as follows: A method for synthesizing cellulose ester using an acidic ionic liquid as a catalyst comprises the following steps: (1) Constant temperature activation: cellulose and glacial acetic acid or a mixture of glacial acetic acid and propionic acid or a mixture of glacial acetic acid and butyric acid are stirred at a constant temperature and at a constant speed 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 - ]; (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 filtered, washed and dried to obtain cellulose acetate, cellulose acetate propionate or cellulose acetate butyrate.

[0009] Furthermore, 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.

[0010] Furthermore, 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.

[0011] Furthermore, 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.

[0012] Furthermore, in step (2), 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 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 used is 0.5%-5% of the mass of the cellulose.

[0013] Furthermore, 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, triethylbenzyl ammonium chloride and tetramethylguanidine, heat them for reaction, add n-heptane and let them stand, and take the upper layer to obtain the acidic ionic liquid catalyst, which are [DBUH + ][HSO4 - ]、[N B222 H + ][HSO4 - ] and [TMGH + ][HSO4 - ], which has the following structure: 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.

[0014] The temperature of the heating reaction is 70-100°C, the 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.

[0015] The reaction temperature in step (2) is 40-60°C.

[0016] In step (3), the mass ratio of the desalted water to the cellulose ester solution is 10-30:1, the hydrolysis temperature is 20-50° C., and the hydrolysis time is 1-5 h.

[0017] Compared with the prior art, the advantages of the present invention are as follows: The present invention uses an acidic ionic liquid with low cost and easy synthesis to replace sulfuric acid as a catalyst for preparing cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. The catalyst has high thermal stability and low volatility, and the catalyst can be recycled, which reduces the impact on the environment and helps to overcome the problem that traditional sulfuric acid catalysts are seriously corrosive and difficult to reuse. 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 at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 For acidic ionic liquid [DBUH + ][HSO4 - ]、[N B222 H + ][HSO4 - ] and [TMGH + ][HSO4- ] structure diagram. DETAILED DESCRIPTION

[0019] 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.

[0020] Example 1 Sulfuric acid and 1,8-diazabicyclo[5.4.0]undec-7-ene were mixed in a molar ratio of 1:1 and reacted at 90 °C for 12 h. Then, 10 times the mass of n-heptane as that of 1,8-diazabicyclo[5.4.0]undec-7-ene was added. After standing for 0.5 h, the upper layer was separated to obtain 1,8-diazabicyclo[5.4.0]undec-7-ene-sulfuric acid ionic liquid ([DBUH + ][HSO4 - ]), its thermal stability and volatility were tested, and its decomposition temperature was 232-254℃, and its volatility was close to 0 at 100℃.

[0021] 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) 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 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 [DBUH + ][HSO4 - ] Acidic ionic liquid catalyst, react at 50 ℃ constant temperature for 5 hours, after the reaction is complete, cellulose acetate solution is obtained. Add 20 times the mass of desalted water (salt content 1-5 mg / L) of cellulose to the cellulose acetate solution, hydrolyze at 40 ℃ constant temperature for 3 hours, filter, wash and dry the precipitated white precipitate to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid is recovered by washing with ethyl acetate solvent. The obtained cellulose acetate has a degree of polymerization of 280 and a combined acid of 54%.

[0022] Example 2 Sulfuric acid and triethylbenzyl were mixed in a molar ratio of 1:1 and reacted at 90 °C for 12 h. Then, n-heptane (10 times the mass of triethylbenzyl) was added and allowed to stand for 0.5 h. The upper layer was separated to obtain triethylbenzyl-sulfuric acid ionic liquid ([N B222 H + ][HSO4 - ]), its thermal stability and volatility were tested, and its decomposition temperature was 232-254℃, and its volatility was close to 0 at 100℃.

[0023] 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) 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 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 [N B222 H + ][HSO4 - ] Acidic ionic liquid catalyst, react at 50 ℃ constant temperature for 5 hours, and after the reaction is complete, cellulose acetate solution is obtained. Add 20 times the mass of desalted water (salt content 1-5 mg / L) of cellulose to the cellulose acetate solution, and hydrolyze for 3 hours at 40 ℃ constant temperature. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid is washed with ethyl acetate solvent to recover the ionic liquid. The obtained cellulose acetate has a degree of polymerization of 260 and a combined acid of 54%.

[0024] Example 3 Sulfuric acid and tetramethylguanidine were mixed in a molar ratio of 1:1 and reacted at 90 °C for 12 h. Then, n-heptane with a mass 10 times that of tetramethylguanidine was added. After standing for 0.5 h, the upper layer was separated to obtain tetramethylguanidine-sulfuric acid ionic liquid ([TMGH + ][HSO4 - ]), its thermal stability and volatility were tested, and its decomposition temperature was 232-254℃, and its volatility was close to 0 at 100℃.

[0025] 1 H NMR (300 MHz, DMSO- d6 ), δ: 2.684 (s, 12H, CH3) 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 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 - ] Acidic ionic liquid catalyst, react at 50 ℃ constant temperature for 5 hours, and after the reaction is complete, cellulose acetate solution is obtained. Add 20 times the mass of desalted water (salt content 1-5 mg / L) of cellulose to the cellulose acetate solution, and hydrolyze for 3 hours at 40 ℃ constant temperature. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid is recovered by washing with ethyl acetate solvent. The obtained cellulose acetate has a degree of polymerization of 300 and a combined acid of 55%.

[0026] The structural properties of the cellulose acetate obtained in Examples 1-3 are shown in Table 1.

[0027] Table 1 Structure and properties of cellulose acetate As shown in Table 1, the cellulose acetate synthesized by the three ionic liquids has a high degree of polymerization. In addition, the total degree of substitution of acetyl groups in the three samples is similar. However, due to the different catalysts, the distribution uniformity of the substituents at C-2, C-3, and C-6 on the glucose ring is different. This may be due to the different solubility properties of the acidic ionic liquids, which directly leads to the difference in the distribution uniformity of the substituents at the C-2, C-3, and C-6 positions in cellulose acetate.

[0028] Example 4 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 stirring kettle 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 -] Acidic ionic liquid catalyst and acetic anhydride 5 times the mass of cellulose were reacted at a constant temperature of 50 ° C for 5 h. After the reaction was complete, a cellulose acetate solution was obtained. Desalted water (salt content 1-5 mg / L) 20 times the mass of cellulose was added to the cellulose acetate solution, and the hydrolysis reaction was carried out at a constant temperature of 40 ° C for 3 h. 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. The above process was repeated three times. The results are shown in Table 2. The reused acidic ionic liquid showed a catalytic effect similar to that of the fresh ionic liquid, indicating that the three acidic ionic liquids used have better stability in the catalytic preparation of cellulose acetate.

[0029] Table 2 Reusability of acidic ionic liquids in catalyzing cellulose acetylation to prepare cellulose acetate The prepared cellulose acetate was subjected to other performance analyses, and the results are shown in Table 3. Its moisture content, thermal stability, and free acid all met the quality standards (refer to ASTM D871-96 (2019)) and can be directly used for commercial sale.

[0030] Table 3 Quality indexes of cellulose acetate synthesized using acidic ionic liquid as catalyst Example 5 A mixture of cellulose and acetic acid and propionic acid (the molar ratio of acetic acid to propionic acid is 1:8) was added to an intermittent stirring kettle 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. A mixture of acetic anhydride and propionic anhydride (the molar ratio of acetic anhydride to propionic anhydride is 1:8) with a mass of 5 times that of cellulose and [DBUH + ][HSO4 - ] Acidic ionic liquid catalyst, react at 50 ℃ constant temperature for 5 hours, and after the reaction is complete, a cellulose ester solution is obtained. Add 20 times the mass of desalted water (salt content 1-5 mg / L) of cellulose to the cellulose ester solution, and hydrolyze for 3 hours at 40 ℃ constant temperature. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate propionate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid can be recovered by washing with ethyl acetate solvent. The total degree of substitution of the obtained cellulose acetate propionate is 2.61, the acetyl content is 8.2%, the propionyl content is 41.5%, and the viscosity is 152.3 mPa / s.

[0031] Example 6 A mixture of cellulose and acetic acid and butyric acid (the molar ratio of acetic acid to butyric acid is 1:4) was added to an intermittent stirring kettle 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. A mixture of acetic anhydride and propionic anhydride (the molar ratio of acetic anhydride to butyric anhydride is 1:4) with a mass of 5 times that of cellulose and [DBUH + ][HSO4 - ] Acidic ionic liquid catalyst, react at 50 ℃ constant temperature for 5 h, and after the reaction is complete, a cellulose ester solution is obtained. Add 20 times the mass of desalted water (salt content 1-5 mg / L) of cellulose to the cellulose ester solution, and hydrolyze for 3 h at 40 ℃ constant temperature. The precipitated white precipitate is filtered, washed and dried to obtain cellulose acetate butyrate. The filtered filtrate is evaporated and dehydrated, and the ionic liquid is recovered by washing with ethyl acetate solvent. The total degree of substitution of the obtained cellulose acetate butyrate is 2.11, the acetyl content is 13.5%, and the propionyl content is 27.5%.

[0032] 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 an acidic ionic liquid as a catalyst, characterized in that: The following steps are involved: (1) Constant temperature activation: cellulose and glacial acetic acid or a mixture of glacial acetic acid and propionic acid or a mixture of glacial acetic acid and butyric acid are stirred at a constant temperature and at a constant speed 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 - ]; (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 filtered, washed and dried to obtain cellulose acetate, cellulose acetate propionate or cellulose acetate butyrate.

2. The method according to claim 1, characterized in that: 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.

3. The method according to claim 1, characterized in that: 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, characterized in that: 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, characterized in that: In step (2), the mass ratio of acetic anhydride or the mixture of acetic anhydride and propionic anhydride or the 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 acidic ionic liquid catalyst used is 0.5%-5% of the mass of cellulose.

6. The method according to claim 1, characterized in that: 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, triethylbenzyl ammonium chloride and tetramethylguanidine, and heat them for reaction, add n-heptane and let it stand, and take the upper layer to obtain the acidic ionic liquid catalyst, which are [DBUH + ][HSO4 - ]、[N B222 H + ][HSO4 - ] and [TMGH + ][HSO4 - ], which has the following structure: 。 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 triethylbenzyl or tetramethylguanidine is 0.5:1-3:

1.

8. The method according to claim 6, characterized in that: The heating reaction temperature is 70-100°C, the 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, characterized in that: The reaction temperature in step (2) is 40-60°C.

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

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