Preparation method and application of high-purity natural cellulose

Abstract

The invention discloses a preparation method and application of high-purity natural cellulose. According to the method, a pre-cooled phosphoric acid / hydrogen peroxide mixed aqueous solution system is adopted, phosphoric acid is a cellulose intercrystalline swelling reagent, and hydrogen peroxide is an oxidation reagent precursor. The invention provides a cellulose intercrystalline swelling strategy to promote full exposure of hemicellulose embedded in a cellulose crystalline region and further realize deep removal of hemicellulose through acidic hydrolysis dissolution; meanwhile, peroxyphosphoric acid is used for promoting oxidative degradation and dissolution of residual lignin under mild conditions, and the purity of natural cellulose is further improved. The natural cellulose prepared by the method has high purity, and can be applied to the field of manufacturing of high-end cellulose of lyocell grade, acetification grade, nitration grade and the like. The method is mild in condition, simple in process and controllable in process, the cellulose industry is promoted to extend to the high end of a value chain, and the method has a wide application prospect.

Description

Technical Field

[0001] The invention relates to the technical field of natural cellulose extraction, and in particular to a method for preparing high-purity natural cellulose. Background Art

[0002] Cellulose is one of the largest environmentally friendly, biodegradable natural organic polymer materials on earth. It is used in important fields such as papermaking, textiles, construction, military industry, food, packaging, and medicine. It has penetrated into all aspects of people's lives and is an important part of the national economy. Dissolving pulp is a high-purity cellulose product with an α-cellulose content of more than 90%. It is an important raw material for the production of cellulose esters (nitrocellulose, cellulose acetate), cellulose ethers (carboxymethyl cellulose), lyocell fibers, and nanocellulose. It has important utilization value. However, my country's natural cellulose sources are heavily dependent on imports, with an import dependence of up to 89.81%, especially high-end natural cellulose such as lyocell fiber special and acetified grade. my country is currently unable to produce it independently and is completely dependent on imports. In recent years, with the continuous escalation and increase of international trade tensions, and the continuous evolution of "decoupling and supply cuts", how to achieve independent and controllable production of high-end cellulose has become an important strategic choice for my country at present, which will be of great significance to the sustainable development of my country's cellulose industry and national security.

[0003] High-end cellulose has extremely high requirements for cellulose purity. Taking acetified and nitrocellulose as examples, the α-cellulose content is usually higher than 96%. However, due to the complex multi-level entanglement structure of plant fiber cells and the embedding characteristics of hemicellulose in the crystalline region of cellulose, a very stubborn biological barrier is formed, resulting in low accessibility of hemicellulose and difficulty in deep removal. If the amount of hemicellulose removed is increased by traditional process measures such as extending the pretreatment time, increasing the treatment temperature or increasing the amount of chemicals, it will inevitably cause strong degradation of cellulose, resulting in a decrease in cellulose yield and degree of polymerization. At present, the traditional single purification method dissolves cellulose to fully expose and efficiently remove hemicellulose, but often leads to a change in the cellulose crystal structure (cellulose I→cellulose II), resulting in a decrease in the reactivity of cellulose, affecting subsequent processing performance. How to remove hemicellulose in depth while reducing damage to cellulose to obtain high-purity and high-yield natural cellulose has become a bottleneck problem that needs to be solved in the field of high-end cellulose manufacturing, and it has a very prominent urgency. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing high-purity natural cellulose in view of the problems existing in the prior art. The method of the present invention prepares high-purity natural cellulose through the intercrystalline swelling technology of cellulose. The cellulose content of the high-purity natural cellulose prepared by the method reaches more than 98%, and the crystal structure of the cellulose does not change and is still cellulose I.

[0005] The technical problems solved by the present invention and the technical solutions thereof are as follows:

[0006] (1) Due to the complex multi-level entanglement structure of plant fiber cells and the fact that hemicellulose is embedded in the crystalline region of cellulose, a very stubborn biological barrier is formed, resulting in low accessibility of hemicellulose and difficulty in deep removal. If the amount of hemicellulose removed is increased by traditional process measures such as extending the pretreatment time, increasing the pretreatment temperature or increasing the amount of chemicals, it will inevitably cause strong degradation of cellulose, resulting in a decrease in cellulose yield and degree of polymerization. In addition, the method of dissolving cellulose to fully expose and efficiently remove hemicellulose involves a cellulose dissolution and regeneration process, which often leads to a transformation of the cellulose crystal structure (cellulose I→cellulose II), resulting in a decrease in the reactivity of cellulose and affecting its processing properties. To this end, the present invention innovatively proposes a cellulose intercrystalline swelling technology, which intends to fully expose the hemicellulose embedded in the crystalline region of cellulose under mild conditions, and then remove it through acidic hydrolysis to achieve the purpose of deep removal of hemicellulose.

[0007] Considering that the effects of phosphoric acid reagent on cellulose include: hydrolysis, swelling, dissolution and esterification. Here, the purpose is to utilize the swelling effect of phosphoric acid reagent on cellulose, and by swelling the cellulose intercrystalline, the hemicellulose embedded in the cellulose crystalline region is fully exposed, and dissolved by hydrolysis in an acidic environment. Objectively, the hydrolysis, dissolution and esterification of cellulose by phosphoric acid reagent should also be avoided or reduced as much as possible. ① Hydrolysis will cause cellulose degradation, resulting in a decrease in the yield and degree of polymerization of natural cellulose. Studies have shown that hydrolysis is mainly affected by temperature and intensifies with increasing temperature. Therefore, the present invention adopts a low-temperature precooling process to alleviate the hydrolysis of cellulose. ② Dissolution triggers the cellulose dissolution and regeneration process, and the crystalline structure of cellulose is converted from cellulose I to cellulose II, resulting in a decrease in cellulose reactivity. The swelling and dissolution of cellulose are essentially different degrees of swelling of cellulose by phosphoric acid reagent. The swelling of cellulose is mainly concentrated in the amorphous region of cellulose, while the dissolution of cellulose penetrates into the crystalline region of cellulose. The degree of cellulose swelling is mainly affected by the concentration of phosphoric acid reagent. The higher the concentration, the deeper the swelling goes into the crystalline region of cellulose. Therefore, the present invention avoids or alleviates the cellulose dissolution by regulating the concentration of the phosphoric acid reagent to satisfy the cellulose intercrystalline swelling effect. ③ Phosphation will cause the consumption of the phosphoric acid reagent and cellulose modification. The phosphation effect is mainly positively correlated with the reaction temperature and the treatment time. The present invention adopts a low-temperature / short-sequence process to alleviate the phosphation effect on the premise of satisfying the cellulose intercrystalline swelling effect. Including combined intermittent ultrasonic auxiliary measures through mechanical effects and cavitation effects, it can effectively promote the swelling of cellulose. Appropriate intermittent time allows cellulose fibers to have time to recover and rearrange after ultrasonic action, which can avoid excessive damage to the cellulose structure while maintaining an efficient reaction rate.

[0008] (2) The residual lignin content in the dissolving pulp is about 0.05%, but lignin is highly hydrophobic and difficult to swell or dissolve; at the same time, the unique spatial network structure of lignin and the chemical bond connection between it and carbohydrates (lignin-carbohydrate complex, LCC) hinder the solvent from penetrating into the fiber during the dissolving pulp processing, causing fiber swelling or uneven reaction. To this end, the present invention proposes an oxidation synergistic strategy, that is, in the process of cellulose pretreatment, a green oxidizing agent is appropriately added to further treat the residual lignin and dissolve it through oxidative degradation, so as to further improve the purity of natural cellulose.

[0009] Considering that hydrogen peroxide reagent is green and environmentally friendly, it will not produce secondary chemical pollution. However, hydrogen peroxide is very unstable as an oxidant and is easily decomposed and ineffective. The combination of phosphoric acid reagent and hydrogen peroxide reagent stabilizes the oxidation efficiency of hydrogen peroxide by converting hydrogen peroxide into a more stable form of peroxyphosphoric acid. Peroxyphosphoric acid produces an electrophilic reagent (HO +). It can attack the electron-rich functional groups (olefins, carbonyl groups and aromatic ring structures) in the lignin macromolecules, causing the lignin to be oxidized and degraded into small molecules and dissolved.

[0010] In this process, hydrogen peroxide is used as an oxidizing agent and is consumed during the treatment process. Phosphoric acid is used as a cellulose swelling agent and hydrogen peroxide stabilizing agent, and there is no chemical consumption during the treatment process, and the filtrate after treatment can be reused.

[0011] A method for preparing high-purity natural cellulose of the present invention comprises the following steps:

[0012] S1. Phosphoric acid, hydrogen peroxide and water are mixed and placed in a constant temperature cold water area, mixed evenly under mechanical stirring, and the temperature is kept constant to obtain a pre-cooled phosphoric acid / hydrogen peroxide / aqueous solution system;

[0013] S2. adding the bleached pulp to the pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step S1, and treating it under mechanical stirring and ultrasonic assistance to promote cellulose intercrystalline swelling, hemicellulose hydrolysis and dissolution, and residual lignin oxidation and degradation and dissolution, while keeping the cellulose crystal structure unchanged;

[0014] S3. Filter and wash the pulp suspension obtained after the treatment in step S2 to remove the dissolved hemicellulose, lignin fragments and chemical reagents, thereby preparing the high-purity natural cellulose.

[0015] Furthermore, the pre-cooled phosphoric acid / hydrogen peroxide / water solution system in step S1 comprises phosphoric acid, hydrogen peroxide and water in a mass ratio of 75-80:1:12-13.

[0016] Furthermore, the precooling temperature of the precooled phosphoric acid / hydrogen peroxide / water solution system in step S1 is -12 to -5°C.

[0017] Furthermore, the bleached pulp in step S2 is bleached kraft pine pulp with a brightness ≥ 90% ISO, bleached kraft eucalyptus pulp with a brightness ≥ 88% ISO, or bleached kraft bamboo pulp with a brightness ≥ 85% ISO.

[0018] Furthermore, in step S2, the dosage ratio of the bleached pulp to the pre-cooled phosphoric acid / hydrogen peroxide / water solution, that is, the ratio of pulp mass / solution mass is 1:10-11.

[0019] Furthermore, in step S2, the mechanical stirring treatment time is 120 min, and the rotation speed of the mechanical stirring is 400 rpm.

[0020] Furthermore, in step S2, the ultrasonic assisted treatment is from 90 minutes of mechanical stirring treatment to 120 minutes of mechanical stirring treatment, and the ultrasonic assisted treatment process is: power 650W, ultrasound 2s or 3s, stop 2s or 5s, and the ultrasonic assisted treatment time is 30 minutes.

[0021] Furthermore, the high-purity natural cellulose prepared in step S3 has a crystal structure of cellulose I, a cellulose content of ≥98%, a hemicellulose content of <2%, and a residual lignin content of ≤0.01%.

[0022] The high-purity natural cellulose prepared by the preparation method of the present invention can be applied to the production of high-end cellulose such as lyocell grade, acetification grade, and nitration grade.

[0023] Compared with the prior art, the method of the present invention has the following beneficial effects:

[0024] (1) The natural cellulose prepared by the present invention has a crystalline structure of cellulose I and high purity, and has broad application prospects in the fields of high-end cellulose manufacturing such as lyocell grade, acetification grade, and nitration grade.

[0025] (2) Phosphoric acid is used as a cellulose swelling agent and hydrogen peroxide stabilizing agent. It is cheap and readily available, and there is no chemical consumption during the treatment process. The treated filtrate can be reused, which helps to reduce the overall treatment cost.

[0026] (3) The preparation process of the present invention is simple, the conditions are mild, the process is controllable, and it is suitable for large-scale industrial production. This is not only expected to promote the development of natural cellulose preparation technology, but also to meet the market's urgent demand for high-end natural cellulose. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0028] Example 1

[0029] This embodiment provides a method for preparing high-purity natural cellulose as follows:

[0030] (1) Preparing a precooled phosphoric acid / hydrogen peroxide / water solution system: 529.41 g of 85% concentrated phosphoric acid, 20 g of 30% hydrogen peroxide and 73.41 g of deionized water were added to a 1000 mL beaker, and placed in a constant temperature water bath at -12°C, and stirred at a speed of 400 r / min for 30 min to make the phosphoric acid / hydrogen peroxide / water mixed evenly and keep the temperature constant, thereby obtaining a precooled phosphoric acid / hydrogen peroxide / water solution system.

[0031] (2) 60 g of ISO bleached pine pulp with a whiteness of ≥90% by absolute dry weight is added to the pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step (1) and mixed. The mechanical agitator is adjusted to stir mechanically at 400 rpm for 90 min to make it evenly dispersed. From the 90 min to the 120 min of mechanical stirring treatment, an ultrasonic cell disruptor is used for auxiliary treatment at a power of 650 W for 30 min (ultrasonication for 3 s, stop for 5 s) to obtain a pulp suspension.

[0032] (3) The pulp suspension obtained after the treatment in step (2) is filtered and washed with a large amount of deionized water to remove the dissolved hemicellulose, lignin fragments and chemical reagents, and tested with pH test paper until it is neutral. The pulp suspension is dehydrated at a speed of 5000 r / min using a centrifuge for 5 minutes, and the dehydrated pulp is placed in a 60° C. oven and dried for 48 hours to obtain high-purity natural cellulose.

[0033] The natural cellulose obtained in this Example 1 had a cellulose content of 99.12%, a hemicellulose content of 0.82%, and a residual lignin content of 0.004%.

[0034] The X-ray diffraction spectrum analysis results of the natural cellulose obtained in this Example 1 showed that the cellulose crystal structure was cellulose I.

[0035] Example 2

[0036] This embodiment provides a method for preparing high-purity natural cellulose as follows:

[0037] (1) Preparing a precooled phosphoric acid / hydrogen peroxide / water solution system: 564.71 g of 85% concentrated phosphoric acid, 20 g of 30% hydrogen peroxide and 78.71 g of deionized water were added to a 1000 mL beaker, and placed in a constant temperature water bath at -5°C, and stirred at a speed of 400 r / min for 30 min to make the phosphoric acid / hydrogen peroxide / water mixed evenly and keep the temperature constant, thereby obtaining a precooled phosphoric acid / hydrogen peroxide / water solution system.

[0038] (2) 60 g of ISO bleached eucalyptus kraft pulp with a brightness of ≥88% is taken by absolute dry weight, and added to the entire pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step (1) and mixed, and the mechanical stirrer is adjusted to stir mechanically at 400 rpm for 90 min to make it evenly dispersed, and an ultrasonic cell disruptor is used for auxiliary treatment at a power of 650 W for 30 min (ultrasonication for 3 seconds, stop for 5 seconds) from the 90 min of mechanical stirring treatment to the 120 min of mechanical stirring treatment to obtain a pulp suspension.

[0039] (3) The pulp suspension obtained after the treatment in step (2) is filtered and washed with a large amount of deionized water to remove the dissolved hemicellulose, lignin fragments and chemical reagents, and tested with pH test paper until it is neutral. The pulp suspension is dehydrated by a centrifuge at a speed of 5000 r / min for 5 minutes, and the dehydrated pulp is placed in a 60°C oven and dried for 48 hours to obtain high-purity natural cellulose.

[0040] The natural cellulose obtained in Example 2 had a cellulose content of 99.19%, a hemicellulose content of 0.73%, and a residual lignin content of 0.01%.

[0041] The X-ray diffraction spectrum analysis results of the natural cellulose obtained in this Example 2 showed that the cellulose crystal structure was cellulose I.

[0042] Example 3

[0043] This embodiment provides a method for preparing high-purity natural cellulose as follows:

[0044] (1) Preparing a precooled phosphoric acid / hydrogen peroxide / water solution system: 557.65 g of 85% concentrated phosphoric acid, 20 g of 30% hydrogen peroxide and 77.65 g of deionized water were added to a 1000 mL beaker, and placed in a constant temperature water bath at -7°C, and stirred at a speed of 400 r / min for 30 min to make the phosphoric acid / hydrogen peroxide / water mix evenly and keep the temperature constant, thereby obtaining a precooled phosphoric acid / hydrogen peroxide / water solution system.

[0045] (2) 60 g of ISO bleached kraft bamboo pulp with a whiteness of ≥85% is taken by absolute dry weight, and added to the pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step (1) and mixed, and the mechanical stirrer is adjusted to stir mechanically at 400 rpm for 90 min to make it evenly dispersed, and an ultrasonic cell disruptor is used for auxiliary treatment at a power of 650 W for 30 min (ultrasonication for 2 s, stop for 2 s) from the 90 min of mechanical stirring treatment to the 120 min of mechanical stirring treatment to obtain a pulp suspension, and the pH value is measured to be 1.5.

[0046] (3) The pulp suspension obtained after the treatment in step (2) is filtered and washed with a large amount of deionized water to remove the dissolved hemicellulose, lignin fragments and chemical reagents, and tested with pH test paper until it is neutral. The pulp suspension is dehydrated at a speed of 5000 r / min using a centrifuge for 5 minutes, and the dehydrated pulp is placed in a 60°C oven and dried for 48 hours to obtain high-purity natural cellulose.

[0047] The natural cellulose obtained in Example 3 had a cellulose content of 98.85%, a hemicellulose content of 0.96%, and a residual lignin content of 0.008%.

[0048] The X-ray diffraction spectrum analysis results of the natural cellulose obtained in this Example 3 showed that the cellulose crystal structure was cellulose I.

Claims

1. A method for preparing high-purity natural cellulose, characterized in that: The following steps are involved: S1. Phosphoric acid, hydrogen peroxide and water are mixed and placed in a constant temperature cold water area, mixed evenly under mechanical stirring, and the temperature is kept constant to obtain a pre-cooled phosphoric acid / hydrogen peroxide / aqueous solution system; S2. adding the bleached pulp to the pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step S1, and treating it under mechanical stirring and ultrasonic assistance to promote cellulose intercrystalline swelling, hemicellulose hydrolysis and dissolution, and residual lignin oxidation and degradation and dissolution, while keeping the cellulose crystal structure unchanged; S3. Filter and wash the pulp suspension obtained after the treatment in step S2 to remove the dissolved hemicellulose, lignin fragments and chemical reagents, thereby preparing the high-purity natural cellulose.

2. The preparation method according to claim 1, characterized in that: The precooled phosphoric acid / hydrogen peroxide / water solution system in step S1 comprises phosphoric acid, hydrogen peroxide and water in a mass ratio of 75-80:1:12-13.

3. The preparation method according to claim 1, characterized in that: The precooling temperature of the phosphoric acid / hydrogen peroxide / water solution system in step S1 is -12 to -5°C.

4. The preparation method according to claim 1, characterized in that: The bleached pulp in step S2 is bleached kraft pine pulp with a whiteness of ≥90% ISO, bleached kraft eucalyptus pulp with a whiteness of ≥88% ISO, or bleached kraft bamboo pulp with a whiteness of ≥85% ISO.

5. The preparation method according to claim 1, characterized in that: The dosage ratio of the bleached pulp to the pre-cooled phosphoric acid / hydrogen peroxide / water solution in step S2, that is, the ratio of pulp mass to solution mass, is 1:10-11.

6. The preparation method according to claim 1, characterized in that: In step S2, the mechanical stirring treatment time is 120 min, and the rotation speed of the mechanical stirring is 400 rpm.

7. The preparation method according to claim 1, characterized in that: The ultrasonic assisted treatment in step S2 is from 90 minutes of mechanical stirring treatment to 120 minutes of mechanical stirring treatment, and the ultrasonic assisted treatment process is: power 650W, ultrasonic 2s or 3s, stop 2s or 5s, and the ultrasonic assisted treatment time is 30min.

8. The preparation method according to claim 1, characterized in that: The high-purity natural cellulose prepared in step S3 has a crystal structure of cellulose I, a cellulose content of ≥98%, a hemicellulose content of <2%, and a residual lignin content of ≤0.01%.

9. Application of the high-purity natural cellulose prepared by the preparation method according to any one of claims 1 to 8 in the production of lyocell-grade, acetified-grade and nitrated-grade high-end cellulose.