High-purity cellulose and preparation method thereof

Abstract

According to the high-purity cellulose and the preparation method thereof, a lignocellulose raw material and a green ionic solvent are mixed and heated for a certain time under the stirring condition to obtain slurry, and the green ionic solvent is prepared by heating a hydrogen bond donor and a hydrogen bond acceptor at the temperature of 30-100 DEG C for 0.5-3 h and fully stirring the hydrogen bond donor and the hydrogen bond acceptor; adding a viscosity reduction solvent into the slurry, stirring and filtering to obtain a primary extraction filtrate and residues, washing the residues with the viscosity reduction solvent until a washing solution is colorless, naturally airing or drying the residues to obtain high-purity cellulose, and synthesizing a novel functional green ionic solvent by regulating and controlling chemical structures of hydrogen bond donors and acceptors. A green ionic solvent is utilized to effectively break through the anti-degradation barrier of lignocellulose, and mild and efficient preparation of high-purity cellulose is realized.

Description

Technical Field

[0001] The present application relates to the technical field of lignocellulose utilization, and particularly relates to a method for preparing high-purity cellulose from lignocellulose in a mild, efficient and simple manner by using a green ionic solvent. Background Art

[0002] Cellulose is one of the main components of the lignocellulose cell wall, and exists in wood, cotton, hemp, Pennisetum giganteum, Arundo donax and agricultural straws, etc., and is widely used in fields such as papermaking, green textiles, bio-based fuels and food additives. Cellulose has significant advantages such as being renewable, completely biodegradable, non-toxic and harmless, and has high crystallinity, high strength and chemical stability. It is a high-quality and low-cost biomass resource that can replace non-renewable fossil resources to produce renewable energy and materials. However, cellulose has complex interactions with other main components (hemicellulose, lignin) of the cell wall, and it is difficult to separate and the purity is not high, resulting in high purification costs, and there are great challenges in the preparation of high-purity cellulose.

[0003] Existing high-purity cellulose preparation methods such as the sulfate method, the sulfite method and the ionic liquid method have problems such as long time consumption, complex post-treatment, and the chemical reagents used are difficult to biodegrade. Moreover, the sulfate method and the sulfite method require high-temperature and high-pressure conditions, with high energy consumption and a large amount of waste liquid, resulting in great environmental protection pressure. In comparison, the chemical reagents used in the ionic liquid method can be recycled, and the waste liquid treatment load is relatively small, and the reaction conditions are mild. For example, Chinese invention patent CN106702802B uses sulfamic acid to catalyze an imidazolium ionic liquid (1-methyl-3-butylimidazolium chloride) to obtain cellulose with a purity exceeding 64% from corn straw; Chinese invention patent CN115748281BN develops pyridine ring-based ionic liquids (ethylpyridinium bromide and N-allylpyridinium chloride), and under the catalysis of sulfamic acid, the cellulose purity is increased to more than 70%. However, the chemical reagents used in ionic liquids are costly, not completely biodegradable, and the separation and purification processes in the synthesis process are complex, resulting in poor economic and environmental feasibility, which limits the industrialized popularization and application.

[0004] In other words, the existing sulfate method and sulfite method have severe reaction conditions, large amount of waste liquid, and high risk of environmental pollution, while the ionic liquid method has high cost, complex reagent synthesis, and limited application range. The use of more green, environmentally friendly, simple to synthesize and low-cost solvent technology is the development trend of preparing high-purity cellulose, but there are few patents and research reports. Green ionic solvents contain anions, cations and molecules, and have rich non-covalent interactions such as hydrogen bonds, electrostatic effects and van der Waals forces. They can effectively break the anti-degradation barrier formed between cellulose and other main components (hemicellulose and lignin) in the cell wall, but they cannot completely remove the residual lignin components, resulting in the difficulty in significantly improving the purity of the obtained cellulose. Chinese invention patent CN112144309B uses a two-step hydrothermal-low eutectic solvent method to jointly treat wood cellulose to prepare cellulose with a purity of more than 80%, but the cumbersome preparation process limits its promotion and application.

[0005] Based on this, constructing a method for efficiently preparing high-purity cellulose based on green ionic solvents has important practical significance for the high-value utilization of cellulose, and will provide new ideas for companies related to biomass refining. Summary of the invention

[0006] The embodiments of the present application provide a high-purity cellulose and a method for preparing the same. By regulating the chemical structure of hydrogen bond donors and acceptors, a new functional green ionic solvent is synthesized. The green ionic solvent is used to effectively break the anti-degradation barrier of wood cellulose, thereby achieving the gentle and efficient preparation of high-purity cellulose.

[0007] In a first aspect, the present invention provides a method for preparing high-purity cellulose, the method comprising: (1) mixing a lignocellulosic raw material with a green ionic solvent and heating them under stirring conditions to obtain a slurry, wherein the green ionic solvent is prepared by heating a hydrogen bond donor and a hydrogen bond acceptor at 30° C. to 100° C. for 0.5 to 3 h and fully stirring; (2) Adding a viscosity reducing solvent to the slurry, stirring and filtering to obtain a primary filtrate and a residue, washing the residue with the viscosity reducing solvent until the washing liquid is colorless, and then naturally air-drying or drying the residue to obtain high-purity cellulose.

[0008] In some embodiments, the method for preparing high-purity cellulose additionally comprises the following steps: (3) The primary extraction filtrate and the washing liquid are combined to obtain a primary extraction mixed liquid, the primary extraction mixed liquid is evaporated and concentrated to obtain a concentrated liquid, the concentrated liquid is dropped into water, stirred, and then filtered to obtain lignin and a filtrate, and the water in the filtrate is removed to obtain a reusable green ionic solvent.

[0009] It should be emphasized that in this solution, new functionalized green ionic solvents are synthesized by regulating the chemical structures of hydrogen bond donors and hydrogen bond acceptors. This new functionalized green ionic solvent can well break the anti-degradation barrier of lignocellulose, and then realize the preparation of high-purity cellulose.

[0010] Regarding step (1): Furthermore, the green ionic solvent is prepared by heating a first hydrogen bond donor, a second hydrogen bond donor, and a hydrogen bond acceptor at 30°C to 100°C for 0.5 to 3 hours and stirring well. Among them, the first hydrogen bond donor is an organic acid, and the second hydrogen bond donor is a polyhydroxy alcohol. In the green ionic solvent, the proton dissociation of the first hydrogen bond donor provides an acidic environment. A chemical reaction occurs between the double hydroxyl structure of the second hydrogen bond donor and lignin, and the cations of the hydrogen bond acceptor form a stable solvent system through electrostatic interaction. Moreover, the first hydrogen bond donor can also cut the glycosidic bond inside hemicellulose, promoting the destruction of the dense structure of biomass, facilitating the selective dissolution of lignin by the second hydrogen bond donor, breaking macromolecules into small molecules. With the assistance of the first hydrogen bond donor, the second hydrogen bond donor can inhibit the re-adsorption of lignin degradation products on cellulose, which is beneficial to improving the purity of cellulose and accelerating the rate of preparing high-purity cellulose.

[0011] Furthermore, the molar ratio of the first hydrogen bond donor, the second hydrogen bond donor, and the hydrogen bond acceptor is 1 to 50:1:1 to 50. It should be noted that when the content of the first hydrogen bond donor is lower than 1:1:50, the solvent cannot be synthesized, and there is not enough hydrogen bond interaction to prepare high-purity cellulose from biomass. When the content of the hydrogen bond acceptor is lower than 50:1:1, not enough hydrogen bonds can be formed in the solvent, making it difficult to break the complex interactions between cellulose, hemicellulose, and lignin in biomass, and it is difficult to achieve the efficient removal of hemicellulose and lignin, so high-purity cellulose cannot be obtained.

[0012] Furthermore, the hydrogen bond acceptor is one or a combination of benzyltrimethylammonium chloride, benzyltriethylammonium chloride, ethyltrimethylammonium chloride, butyltrimethylammonium chloride, methyltriethylammonium chloride, butyltriethylammonium chloride, betaine hydrochloride, and choline hydroxide. The hydrogen bond acceptor in this solution interacts with the first hydrogen bond donor and the second hydrogen bond donor to enable sufficient hydrogen bonds to be formed between the components for the ionic solvent to prepare a liquid solvent.

[0013] Preferably, the hydrogen bond acceptor is ethyltrimethylammonium chloride, methyltriethylammonium chloride, and butyltriethylammonium chloride.

[0014] Furthermore, the first hydrogen bond donor is one or a combination of methanesulfonic acid, benzenesulfonic acid, pyruvic acid, and maleic acid; the second hydrogen bond donor is one or a combination of mannitol, 1,3-butanediol, 1,2-butanediol, and 1,3-propanediol.

[0015] Furthermore, the green ionic solvent of this solution is a homogeneous transparent solvent.

[0016] Furthermore, the lignocellulosic raw material is mixed with the green ionic solvent and heated for a certain time under stirring conditions at 500 revolutions per minute to obtain a slurry, where the green ionic solvent is prepared by heating a hydrogen bond donor and a hydrogen bond acceptor at 30°C to 100°C for 0.5 to 3 h under stirring conditions at 500 revolutions per minute.

[0017] Furthermore, the lignocellulosic raw material is selected from one or more of spruce, poplar, birch, eucalyptus, bamboo, corn stover, corn cob, wheat straw, sugarcane bagasse, rice straw, giant reed, Pennisetum giganteum, and other industrial and agricultural wastes rich in cellulose, and the size is less than 10 mesh.

[0018] Furthermore, the solid-liquid ratio of the lignocellulosic raw material to the green ionic solvent is 1:5 to 50. Preferably, the solid-liquid ratio of the lignocellulosic raw material to the green ionic solvent is 1:10 to 30. This is because when the solid-liquid ratio is too small, the green ionic solvent cannot effectively destroy the dense structure of the lignocellulosic raw material and is difficult to effectively dissolve hemicellulose and lignin; while when the solid-liquid ratio is too large, it will cause excessive waste of the green ionic solvent.

[0019] Furthermore, the heating temperature for mixing the lignocellulosic raw material with the green ionic solvent and heating under stirring conditions is 60 to 150°C, and the reaction time is 0.5 to 3 h. If the reaction temperature is lower than 60°C or the reaction time is less than 0.5 h, the dissolved lignin and hemicellulose are limited, and it is difficult to prepare high-purity cellulose; while when the reaction temperature is higher than 150°C or the reaction time is more than 3 h, the degree of polymerization of the prepared high-purity cellulose is too low, which is not conducive to subsequent utilization.

[0020] Regarding step (2): Furthermore, the viscosity-reducing solvent of this solution is any one of water, ethanol, and a 50 vol% ethanol aqueous solution. The hydroxyl groups of water and ethanol can form competitive adsorption with the hydrogen bond sites in the green ionic solvent, selectively weakening the hydrophobic interaction between lignin and cellulose, and thus promoting the precipitation of lignin.

[0021] Furthermore, the addition amount of the viscosity-reducing solvent is 0.5 to 10 times the volume of the green ionic solvent.

[0022] Regarding step (3): Furthermore, the initially extracted mixed solution is evaporated and concentrated to obtain a concentrated solution, the concentrated solution is dropped into water and stirred for 0.5 to 5 h, then filtered to obtain lignin and a filtrate, and the water in the filtrate is removed to obtain the reusable green ionic solvent.

[0023] Further, the initial extraction mixture is evaporated and concentrated until the water content is 0-60%.

[0024] In a second aspect, an embodiment of the present application provides a high-purity cellulose prepared by the above preparation method.

[0025] The main contributions and innovations of the present invention are as follows: An embodiment of the present application provides a preparation method for gently and efficiently preparing high-purity cellulose based on a green ionic solvent. The method is green and environmentally friendly, and has low costs. Polyols as the second hydrogen bond donors can effectively dissolve lignin and inhibit the adsorption of impurities such as lignin on the surface of cellulose, effectively break the ether bonds and glycosidic bonds inside hemicellulose and between hemicellulose and lignin, and break the dense structure of the lignocellulose cell wall. Moreover, polyols can only play these roles under acidic conditions and in the presence of hydrogen bond acceptors. The hydrogen bond acceptor forms a hydrogen bond interaction with the donor acid, making it easier for hydrogen ions to be released and accelerating the reaction. The hydrogen bond acceptor also attacks the hydrogen bond interactions between cellulose, hemicellulose, and lignin in biomass, and can better promote the removal of hemicellulose and lignin. Therefore, the green ionic solvent prepared by the present invention has excellent hemicellulose and lignin removal capabilities, and the prepared cellulose has high purity and a moderate degree of polymerization, which is very beneficial for subsequent high-value utilization. The process of the present invention is simple to operate, does not use toxic, harmful, or difficult-to-recycle reagents, and has environmental and economic feasibility. Detailed Description of the Embodiments

[0026] Here, exemplary embodiments will be described in detail. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with one or more embodiments of this specification. On the contrary, they are merely examples of devices and methods consistent with some aspects of one or more embodiments of this specification as detailed in the appended claims.

[0027] It should be noted that: in other embodiments, the steps of the corresponding methods are not necessarily executed in the order shown and described in this specification. In some other embodiments, the steps included in the method may be more or less than those described in this specification. In addition, a single step described in this specification may be decomposed into multiple steps for description in other embodiments; and multiple steps described in this specification may also be combined into a single step for description in other embodiments.

[0028] Example 1 Weigh ethyltrimethylammonium chloride, methanesulfonic acid and mannitol in a molar ratio of 1:0.5:4 into a three-necked flask, heat and stir at 60 °C to obtain a homogeneous transparent liquid, which is the green ionic solvent. After cooling to room temperature, store it in a desiccator. Weigh 2.00 g of Arundo donax powder (10 - 60 mesh) and 20 mL of the green ionic solvent and add them to the three-necked flask. React at 100 °C for 2.5 hours. After completion, pour the slurry into 60 mL of 50 vol% ethanol aqueous solution, stir at room temperature for 2 hours, and centrifuge to obtain the residue and the supernatant. Wash the residue until the washing liquid is colorless to obtain high-purity cellulose; combine the supernatant and the washing liquid and concentrate to 30 mL. Add the concentrated solution to 300 mL of deionized water to precipitate crude lignin. After testing, the cellulose content in the high-purity cellulose prepared by this method is 96.5%, and the degree of polymerization is 1200. The components of Arundo donax are 45.2% cellulose, 23.0% hemicellulose, and 24.0% lignin. The calculation formula for cellulose purity is: cellulose purity (%) = cellulose mass (g) / residue mass (g) * 100.

[0029] Example 2 Weigh methyltriethylammonium chloride, benzenesulfonic acid and 1,3-butanediol in a molar ratio of 1:0.3:6 into a three-necked flask, heat and stir at 60 °C to obtain a homogeneous transparent liquid, which is the green ionic solvent. After cooling to room temperature, store it in a desiccator. Weigh 2.00 g of wheat straw (10 - 60 mesh) and 30 mL of the green ionic solvent and add them to the three-necked flask. React at 120 °C for 1 hour, then pour the slurry into 60 mL of 50 vol% ethanol aqueous solution, stir at room temperature for 2 hours, and centrifuge to obtain the residue and the supernatant. Wash the residue until the washing liquid is colorless to obtain high-purity cellulose; combine the supernatant and the washing liquid and concentrate to 30 mL. Add the concentrated solution to 300 mL of deionized water to precipitate a solid, which is crude lignin. After testing, the cellulose content in the high-purity cellulose prepared by this method is 97.5%, and the degree of polymerization is 970. The components of wheat straw are 37.2% cellulose, 25.1% hemicellulose, and 24.2% lignin. The calculation formula for cellulose purity is: cellulose purity (%) = cellulose mass (g) / residue mass (g) * 100.

[0030] Example 3 Weigh butyltriethylammonium chloride, maleic acid and 1,2-butanediol in a molar ratio of 1:0.5:5 into a flask, heat and stir at 60 °C to obtain a homogeneous transparent liquid, which is the green ionic solvent. After cooling to room temperature, store it in a desiccator. Weigh 2.00 g of birch wood powder (10-60 mesh) and 30 mL of the green ionic solvent and add them to a three-necked flask. After reacting at 110 °C for 2 hours, pour the slurry into 60 mL of ethanol and stir at room temperature for 2 hours. Centrifuge to obtain the residue and the supernatant. Wash the residue until the washing liquid is colorless, which is high-purity cellulose; after combining the supernatant and the washing liquid, concentrate it to 30 mL. Add the concentrated solution to 300 mL of deionized water, and the precipitated solid is crude lignin. After testing, the cellulose content in the high-purity cellulose prepared by this method is 96.6%, and the degree of polymerization is 1300. The composition of birch wood powder is 46.2% cellulose, 20.1% hemicellulose, and 23.0% lignin. The formula for calculating cellulose purity is: cellulose purity (%) = cellulose mass (g) / residue mass (g) * 100.

[0031] Example 4 Weigh methyltriethylammonium chloride, pyruvic acid and 1,3-propanediol in a molar ratio of 1:2:5 into a flask, heat and stir at 60 °C to obtain a homogeneous transparent liquid, which is the green ionic solvent. After cooling to room temperature, store it in a desiccator. Weigh 2.00 g of poplar wood powder (10-60 mesh) and 30 mL of the green ionic solvent and add them to a three-necked flask. After reacting at 120 °C for 3 hours, pour the slurry into 60 mL of water and stir at room temperature for 3 hours. Centrifuge to obtain the residue and the supernatant. Wash the residue until the washing liquid is colorless, which is high-purity cellulose; after combining the supernatant and the washing liquid, concentrate it to 40 mL. Add the concentrated solution to 200 mL of deionized water, and the precipitated solid is crude lignin. After testing, the cellulose content in the high-purity cellulose prepared by this method is 97.2%, and the degree of polymerization is 1050. The composition of poplar wood powder is 43.2% cellulose, 22.8% hemicellulose, and 24.0% lignin. The formula for calculating cellulose purity is: cellulose purity (%) = cellulose mass (g) / residue mass (g) * 100.

[0032] Example 5 Weigh ethyltrimethylammonium chloride, methanesulfonic acid, and mannitol in a molar ratio of 1:1:2 into a flask. Heat and stir at 100 °C to obtain a homogeneous transparent liquid, which is the green ionic solvent. After cooling to room temperature, store it in a desiccator. Weigh 2.00 grams of Pennisetum giganteum (10 - 60 mesh) and 60 milliliters of the green ionic solvent and add them to a three-necked flask. After reacting at 120 °C for 2.5 hours, pour the slurry into 60 milliliters of water and stir at room temperature for 3 hours. Centrifuge to obtain the residue and the supernatant. Wash the residue until the washing liquid is colorless, which is high-purity cellulose; after combining the supernatant and the washing liquid, concentrate it to 70 milliliters. Add the concentrated liquid to 400 milliliters of deionized water, and the precipitated solid is crude lignin. Through experiments, the cellulose content in the high-purity cellulose prepared by this method is 96.2%, and the degree of polymerization is 840. The components of Pennisetum giganteum are 41.2% cellulose, 22.4% hemicellulose, and 23.1% lignin. The calculation formula for cellulose purity is: cellulose purity (%) = cellulose mass (g) / residue mass (g) * 100.

[0033] Comparative Example Prepare high-purity cellulose from poplar by the prehydrolysis-kraft method. 2.00 grams of raw materials (10 - 60 mesh) are subjected to high-temperature hydrothermal treatment at 170 °C for 0.5 hours, and then cooked at 165 °C by the kraft method for 2 hours. After solid-liquid separation, the cellulose pulp is obtained, and after drying, it is high-purity cellulose. The cellulose content in the high-purity cellulose prepared by this method is 85.7%, and the degree of polymerization is 1380. The components of poplar powder are 45.8% cellulose, 18.9% hemicellulose, and 26.1% lignin.

[0034] The comparison results between the comparative example and the example show that the prehydrolysis-kraft method has a complex treatment process and low purity of the prepared cellulose. However, the novel green ionic solvent technology described in the present invention has a short time-consuming, mild reaction conditions, simple subsequent treatment, and high purity of the prepared cellulose, which will greatly expand the application field of cellulose. In the comparative example, the non-covalent bond interactions such as internal hydrogen bonds in the reagents used in the prehydrolysis-kraft method are weak, and the complex interactions between cellulose and other main components are limitedly damaged, and only part of the lignin and hemicellulose can be separated, and higher-purity cellulose cannot be prepared. The novel green ionic solvent in the present invention effectively breaks the anti-degradation barrier of the lignocellulose cell wall through strong hydrogen bond interactions, removes the hemicellulose and lignin components, that is, realizes the efficient preparation of high-purity cellulose.

[0035] The above embodiments only represent several implementation manners of the present application, and their descriptions are relatively specific and detailed, but should not be construed as limiting the scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A method for preparing high-purity cellulose, characterized in that: Place include: (1) mixing a lignocellulosic raw material with a green ionic solvent and heating them under stirring conditions to obtain a slurry, wherein the green ionic solvent is prepared by heating a hydrogen bond donor and a hydrogen bond acceptor at 30° C. to 100° C. for 0.5 to 3 h and fully stirring; (2) Adding a viscosity reducing solvent to the slurry, stirring and filtering to obtain a primary filtrate and a residue, washing the residue with the viscosity reducing solvent until the washing liquid is colorless, and then naturally air-drying or drying the residue to obtain high-purity cellulose.

2. The method for preparing high-purity cellulose according to claim 1, characterized in that: include: (3) The primary extraction filtrate and the washing liquid are combined to obtain a primary extraction mixed liquid, the primary extraction mixed liquid is evaporated and concentrated to obtain a concentrated liquid, the concentrated liquid is dropped into water, stirred, and then filtered to obtain lignin and a filtrate, and the water in the filtrate is removed to obtain a reusable green ionic solvent.

3. The method for preparing high-purity cellulose according to claim 1, characterized in that: The green ionic solvent is prepared by heating a first hydrogen bond donor, a second hydrogen bond donor and a hydrogen bond acceptor at 30° C. to 100° C. for 0.5 to 3 hours and fully stirring, wherein the first hydrogen bond donor is an organic acid and the second hydrogen bond donor is a polyhydroxy alcohol.

4. The method for preparing high-purity cellulose according to claim 3, characterized in that: The molar ratio of the first hydrogen bond donor, the second hydrogen bond donor and the hydrogen bond acceptor is 1~50:1:1~50.

5. The method for preparing high-purity cellulose according to claim 3, characterized in that: The hydrogen bond acceptor is one or a combination of benzyltrimethylammonium chloride, benzyltriethylammonium chloride, ethyltrimethylammonium chloride, butyltrimethylammonium chloride, methyltriethylammonium chloride, butyltriethylammonium chloride, betaine hydrochloride and choline hydroxide; the first hydrogen bond donor is one or a combination of methanesulfonic acid, benzenesulfonic acid, pyruvic acid and maleic acid; the second hydrogen bond donor is one or a combination of mannitol, 1,3-butanediol, 1,2-butanediol and 1,3-propylene glycol.

6. The method for preparing high-purity cellulose according to claim 1, characterized in that: The solid-to-liquid ratio of the lignocellulosic raw material and the green ionic solvent is 1:5~50.

7. The method for preparing high-purity cellulose according to claim 1, characterized in that: The lignocellulosic raw material is mixed with a green ionic solvent and heated under stirring conditions at a heating temperature of 60 to 150° C. and a reaction time of 0.5 to 3 hours.

8. The method for preparing high-purity cellulose according to claim 1, characterized in that: The viscosity reducing solvent is any one of water, ethanol and 50 vol% ethanol aqueous solution.

9. The method for preparing high-purity cellulose according to claim 1, characterized in that: The amount of viscosity reducing solvent added is 0.5 to 10 times the volume of the green ionic solvent.

10. A high-purity cellulose, characterized in that: The high-purity cellulose is prepared according to the method for preparing high-purity cellulose according to any one of claims 1 to 9.