Preparation method and application of high-purity natural cellulose

Abstract

The application discloses a preparation method and application of high-purity natural cellulose. The method adopts a pre-cooled phosphoric acid / hydrogen peroxide mixed aqueous solution system, the phosphoric acid is an inter-crystalline swelling reagent for cellulose, and the hydrogen peroxide is an oxidizing reagent precursor. The application proposes an inter-crystalline swelling strategy for cellulose, promotes the full exposure of hemicellulose embedded in the crystalline region of cellulose, and then realizes the deep removal of hemicellulose through acid hydrolysis and dissolution; meanwhile, the residual lignin is promoted to be oxidized and degraded under mild conditions by using peroxophosphoric acid, so that the purity of the natural cellulose is further improved. The natural cellulose prepared by the method has high purity and can be applied to high-end cellulose manufacturing fields such as lyocell grade, acetylation grade and nitration grade. The method has mild conditions, simple process and controllable process, and will promote the extension of the cellulose industry to the high end of the value chain, and has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of natural cellulose extraction, and in particular, the present application relates to a preparation method of high-purity natural cellulose. BACKGROUND

[0002] Cellulose is one of the most abundant, environmentally friendly and biodegradable natural organic polymer materials on earth, which is applied to important fields such as papermaking, textile, construction, military, food, packaging, medicine and so on, and penetrates into all aspects of people's life, and is an important part of the national economy. Dissolving pulp is a high-purity cellulose product, and the content of alpha cellulose can reach more than 90%, which is an important raw material for producing cellulose ester (nitrocellulose, cellulose acetate), cellulose ether (carboxymethyl cellulose), lyocell fiber and nanocellulose products, and has important utilization value. However, the source of natural cellulose in China is highly dependent on imports, and the import dependence is as high as 89.81%, especially the high-end natural cellulose such as lyocell fiber special and acetylation grade.

[0003] High-end cellulose has very high requirements for cellulose purity. For example, the alpha-cellulose content of acetylation grade and nitration grade cellulose is usually higher than 96%. However, due to the complex multi-level winding structure of plant fiber cells and the embedding characteristics of hemicellulose by cellulose crystalline region, a very stubborn biological barrier is formed, which leads to low accessibility of hemicellulose and difficulty in deep removal. If the amount of hemicellulose is increased by prolonging the pretreatment time, increasing the treatment temperature or increasing the amount of chemical medicine, etc., the cellulose will be inevitably degraded, resulting in a decrease in cellulose yield and degree of polymerization. The traditional single purification method is to dissolve cellulose to fully expose and efficiently remove hemicellulose, but it often leads to the transformation of cellulose crystal structure (cellulose I→cellulose II), which results in the decrease of reactivity of cellulose and affects the subsequent processing performance. How to deeply remove hemicellulose while reducing the damage to cellulose and obtain high-purity and high-yield natural cellulose has become a bottleneck problem in the field of high-end cellulose manufacturing, and has very prominent urgency. SUMMARY

[0004] The present application aims at the problems existing in the prior art, and provides a preparation method of high-purity natural cellulose. The method of the present application prepares high-purity natural cellulose through intercrystalline swelling technology of cellulose. The cellulose content of the high-purity natural cellulose prepared by the method is more than 98%, and the crystal structure of the cellulose is not changed, and is still cellulose I.

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

[0006] (1) Due to the complex multi-level entangled structure of plant fiber cells and the fact that hemicellulose is embedded in the cellulose crystal region, 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 cellulose crystal region under mild conditions, and then remove it through acidic hydrolysis to achieve the purpose of deep removal of hemicellulose.

[0007] Considering that phosphoric acid reagents have effects on cellulose, including hydrolysis, swelling, dissolution, and esterification, the goal here is to utilize the swelling effect of phosphoric acid reagents on cellulose. By swelling the cellulose intercrystalline regions, the hemicellulose embedded in the cellulose crystalline regions is fully exposed, allowing for hydrolysis and dissolution in an acidic environment. Objectively, the hydrolysis, dissolution, and esterification effects of phosphoric acid reagents on cellulose should be avoided or minimized. Hydrolysis can cause cellulose degradation, leading to a decrease in native cellulose yield and degree of polymerization. Studies have shown that hydrolysis is primarily affected by temperature, increasing with increasing temperature. Therefore, the present invention employs a low-temperature precooling process to mitigate cellulose hydrolysis. The dissolution action 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 the phosphoric acid reagent. Cellulose swelling is mainly concentrated in the cellulose amorphous region, while cellulose dissolution penetrates into the cellulose crystalline region. The degree of cellulose swelling is mainly affected by the concentration of the phosphoric acid reagent. The higher the concentration, the deeper the swelling is into the cellulose crystalline region. Therefore, the present invention avoids or alleviates the cellulose dissolution effect by regulating the concentration of the phosphoric acid reagent to meet the cellulose intercrystalline swelling effect. Phosphation will lead to the consumption of phosphoric acid reagent and cellulose modification. Phosphation is mainly positively correlated with reaction temperature and treatment time. The present invention adopts a low-temperature / short-sequence process to alleviate phosphation while satisfying the intercrystalline swelling of cellulose. Including combined intermittent ultrasonic auxiliary measures through mechanical effect and cavitation effect, 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 structure of cellulose while maintaining an efficient reaction rate.

[0008] (2) The residual lignin content in dissolving pulp is approximately 0.05%, but lignin is highly hydrophobic and difficult to swell or dissolve. Furthermore, the unique spatial network structure of lignin and the chemical bonds between it and carbohydrates (lignin-carbohydrate complex, LCC) hinder the penetration of solvent into the fiber during dissolving pulp processing, resulting in fiber swelling or uneven reaction. Therefore, the present invention proposes an oxidative synergistic strategy, namely, during the cellulose pretreatment process, an appropriate green oxidizing agent is added to further treat the residual lignin, dissolving it through oxidative degradation, thereby further improving 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) through heterolytic cleavage reaction in acidic medium. + 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. There is no chemical consumption during the treatment process, and the treated filtrate 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 solution, 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 obtained in step S1, and treating with mechanical stirring and ultrasonic assistance to promote cellulose intercrystalline swelling, hemicellulose hydrolysis and dissolution, and residual lignin oxidative degradation and dissolution, while maintaining the cellulose crystal structure;

[0014] S3. The pulp suspension obtained after the treatment in step S2 is filtered and washed 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] Further, the pre-cooled phosphoric acid / hydrogen peroxide / water solution system in step S1 has a pre-cooling temperature of -12 to -5℃.

[0017] Further, 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.

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

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

[0020] Further, the ultrasonic-assisted treatment in step S2 is performed from 90 min to 120 min of the mechanical stirring treatment, and the ultrasonic-assisted treatment process is as follows: power 650 W, ultrasonic 2 s or 3 s, stop 2 s or 5 s, and ultrasonic-assisted treatment time 30 min.

[0021] Further, 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 application can be applied in the fields of high-end cellulose manufacturing such as lyocell grade, acetylation grade, and nitration grade.

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

[0024] (1) The natural cellulose prepared by the present application has a crystal structure of cellulose I and high purity, and has a wide application prospect in the fields of high-end cellulose manufacturing such as lyocell grade, acetylation grade, and nitration grade.

[0025] (2) Phosphoric acid is a cheap and readily available cellulose swelling agent and hydrogen peroxide stabilizing agent, and there is no chemical consumption during the treatment process. The filtrate after treatment can be reused, which is conducive to reducing the comprehensive treatment cost.

[0026] (3) The preparation process of the present application is simple, mild, and controllable, and is suitable for large-scale industrial production. This not only expects to promote the development of natural cellulose preparation technology, but also meets the urgent needs of the market for high-end natural cellulose. DETAILED DESCRIPTION

[0027] ​Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, 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 equally within the scope limited by the appended claims of the application.

[0028] Example 1

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

[0030] (1) Preparation of 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. The mixture was stirred at a speed of 400 r / min for 30 min to ensure that the phosphoric acid / hydrogen peroxide / water were evenly mixed and the temperature was kept constant to obtain a precooled phosphoric acid / hydrogen peroxide / water solution system.

[0031] (2) 60 g of ISO bleached pine pulp with a whiteness of ≥90% was taken on an absolute dry weight basis and added to the entire pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step (1) and mixed. The mechanical stirrer was adjusted and mechanically stirred at 400 rpm for 90 min to uniformly disperse the pulp. From the 90 min to the 120 min of mechanical stirring treatment, an ultrasonic cell disruptor was used for auxiliary treatment at a power of 650 W for 30 min (ultrasonication for 3 s, on-off 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 dissolved hemicellulose, lignin fragments, and chemical reagents, and then tested with pH paper until it is neutral. The pulp suspension is dehydrated using a centrifuge at a speed of 5000 r / min for 5 minutes. 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) Preparation of 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. The mixture was stirred at a speed of 400 r / min for 30 min to ensure that the phosphoric acid / hydrogen peroxide / water were evenly mixed and the temperature was kept constant to obtain a precooled phosphoric acid / hydrogen peroxide / water solution system.

[0038] (2) 60 g of ISO bleached eucalyptus sulfate pulp with a brightness of ≥88% was taken on an absolute dry weight basis and added to the entire pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step (1). The mechanical stirrer was adjusted and mechanically stirred at 400 rpm for 90 min to uniformly disperse the pulp. From the 90 min to the 120 min of mechanical stirring treatment, an ultrasonic cell disruptor was used for auxiliary treatment at a power of 650 W for 30 min (ultrasound for 3 s, on-off for 5 s) 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 then tested with pH paper until it is neutral. The pulp suspension is dehydrated using a centrifuge at a speed of 5000 r / min for 5 minutes. 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) Preparation of 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. The mixture was stirred at a speed of 400 r / min for 30 min to ensure that the phosphoric acid / hydrogen peroxide / water were evenly mixed and the temperature was kept constant to obtain a precooled phosphoric acid / hydrogen peroxide / water solution system.

[0045] (2) 60 g of ISO bleached kraft bamboo pulp with a whiteness ≥85% was taken on an absolute dry weight basis and added to the entire pre-cooled phosphoric acid / hydrogen peroxide / water solution system obtained in step (1) and mixed. The mechanical stirrer was adjusted and mechanically stirred at 400 rpm for 90 min to uniformly disperse the pulp. From the 90 min to the 120 min of mechanical stirring treatment, an ultrasonic cell disruptor was used for auxiliary treatment at a power of 650 W for 30 min (ultrasonication for 2 s, on-off for 2 s) to obtain a pulp suspension, and the pH value was 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 the pH is tested with pH 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 this 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 solution, 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 obtained in step S1, and treating with mechanical stirring and ultrasonic assistance to promote cellulose intercrystalline swelling, hemicellulose hydrolysis and dissolution, and residual lignin oxidative degradation and dissolution, while maintaining the cellulose crystal structure; S3. The pulp suspension obtained after treatment in step S2 is filtered and washed to remove dissolved hemicellulose, lignin fragments and chemical reagents, thereby preparing high-purity natural cellulose; 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; The pre-cooled phosphoric acid / hydrogen peroxide / water solution system in step S1 has a pre-cooling temperature of -12 to -5°C; The ratio of the bleached pulp to the pre-cooled phosphoric acid / hydrogen peroxide / water solution in step S2, i.e., the ratio of pulp mass to solution mass, is 1:10-11; In step S2, the mechanical stirring treatment time is 120 min, and the mechanical stirring speed is 400 rpm; In step S2, the ultrasonic-assisted treatment is carried out from 90 minutes of mechanical stirring to 120 minutes of mechanical stirring, and the ultrasonic-assisted treatment process is as follows: power 650W, ultrasonic treatment for 2 seconds or 3 seconds, and rest for 2 seconds or 5 seconds, and the ultrasonic-assisted treatment time is 30 minutes; The high-purity natural cellulose prepared in step S3 has a crystalline structure of cellulose , cellulose content ≥98%, hemicellulose content <2%, residual lignin content ≤0.01%.

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

3. Use of the high-purity natural cellulose prepared by the preparation method according to claim 1 or 2 in the production of lyocell-grade, acetified-grade, and nitrocellulose-grade cellulose.