Method for catalytic separation of lignin by metal salt-organic acid composite deep eutectic solvent

Abstract

The application discloses a method for catalytically separating lignin by using a metal salt-organic acid composite eutectic solvent, and belongs to the technical field of lignin separation. The method comprises the following steps: pretreating wood fiber cellulose-based biomass materials, including crushing and drying; preparing a metal salt-organic acid composite eutectic solvent formed by mixing a hydrogen bond donor, a hydrogen bond acceptor and a catalyst; stirring and reacting the pretreated biomass materials with the metal salt-organic acid composite eutectic solvent; diluting the obtained mixed solution with an ethanol solution, filtering, evaporating and concentrating the filtrate, dispersing the concentrated solution in a hydrochloric acid aqueous solution, centrifuging, washing and freeze-drying, and finally obtaining lignin powder. The method for catalytically separating lignin by using the metal salt-organic acid composite eutectic solvent can realize rapid and efficient separation of lignin by using a deep eutectic solvent (DES) and a double-catalyst synergistic system, and has the advantages of low preparation cost, simple equipment requirement and low pollution.

Description

Technical Field

[0001] This invention relates to the field of lignin separation and purification technology, and in particular to a method for separating lignin by catalysis using a metal salt-organic acid composite eutectic solvent. Background Technology

[0002] Lignin, one of the three major components of plant cell walls, is a highly cross-linked amorphous three-dimensional polymer composed of styrene-propylene structural units linked by ether and ester bonds. It covalently bonds with polysaccharide components such as hemicellulose and cellulose, forming lignin-carbohydrate complexes (LCCs) and constituting a complex three-dimensional supramolecular network structure. This complex cross-linking characteristic makes lignin difficult to separate effectively using conventional solvents or physical methods. Furthermore, during separation, lignin is prone to condensation, repolymerization, or partial degradation, further increasing the difficulty of separation and high-value utilization. Traditional lignin separation technologies include sulfate methods, acid methods, thermochemical methods, organic solvent methods, and ionic liquid methods, but these methods often suffer from harsh operating conditions, low separation purity, significant structural damage, and severe pollution, limiting their high-value utilization. Therefore, how to efficiently and selectively separate lignin, cellulose, and hemicellulose while maximizing the preservation of their structure and functionality is a significant challenge in the utilization of biomass resources.

[0003] With the advancement of green chemistry and sustainable processing concepts, eutectic solvents (DES) have emerged as novel green solvents in lignin separation due to their advantages such as low volatility, low toxicity, biodegradability, and low preparation cost. DES systems typically consist of hydrogen bond donors and acceptors, such as choline chloride / lactic acid or choline chloride / glycerol. Through hydrogen bond networks, they lower the melting point of reactants, effectively breaking insoluble bonds between lignin and other molecules or within lignin molecules (e.g.,...). β - O (e.g., -4 bonds, ester / ether bonds), thereby achieving efficient dissolution and separation of lignin solids. To further improve separation efficiency, catalysts such as sulfuric acid, oxalic acid, and aluminum chloride are usually introduced into the system. However, single-catalyst DES systems have limited capabilities in lignin dissolution and depolymerization, and there is a lack of dual-catalytic schemes that can achieve efficient lignin separation. Summary of the Invention

[0004] The purpose of this invention is to provide a method for the catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent. By utilizing a eutectic solvent and a dual-catalytic synergistic system to treat lignocellulose biomass, rapid and efficient separation of lignin is achieved. At the same time, the preparation process is low-cost, requires simple equipment, and produces minimal pollution.

[0005] To achieve the above objectives, the present invention provides a method for the catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent, comprising the following steps: S1. Pretreatment of lignocellulose-based biomass materials; S2. To prepare a metal salt-organic acid composite eutectic solvent, the hydrogen bond donor, hydrogen bond acceptor and dual catalyst are mixed, heated in a water bath and stirred to obtain a uniform and transparent metal salt-organic acid composite eutectic solvent. S3. Mix the pretreated lignocellulose-based biomass material from S1 with the metal salt-organic acid composite eutectic solvent prepared in S2, and stir to obtain a reaction solution. S4. Mix the reaction solution obtained in S3 with the ethanol solution and filter to obtain filtrate and biomass residue; S5. After evaporating and concentrating the filtrate obtained in S4, disperse it in hydrochloric acid aqueous solution, stir, centrifuge, wash, and freeze-dry to obtain lignin powder.

[0006] Preferably, in S1, the pretreatment includes: pulverizing the lignocellulose-based biomass material to 20-100 mesh, drying it at 90-110°C, and then sealing and storing it for later use.

[0007] Preferably, in S1, the lignocellulosic biomass material is one or more of the following: wheat straw, rice straw, rice husk, corn stalk, corn cob, wheat husk, peanut stalk, peanut shell, cotton stalk, rapeseed straw, soybean straw, and wood processing residues.

[0008] Preferably, in S2, the hydrogen bond donor is one or more of urea, formic acid, lactic acid, and glycerol, the hydrogen bond acceptor is choline chloride, and the catalyst includes a metal salt and an organic acid. The metal salt is one of ferric chloride, copper sulfate, aluminum chloride, and zinc chloride, and the organic acid is one of succinic acid, malic acid, citric acid, sorbic acid, oxalic acid, benzenesulfonic acid, and acetic acid.

[0009] Preferably, in S2, the molar ratio of hydrogen bond acceptor to hydrogen bond donor is 1:2-10, the amount of catalyst is 0.5-10% of the total mass of the metal salt-organic acid composite eutectic solvent, and the mass ratio of metal salt to organic acid is 1:1-10.

[0010] Preferably, in S2, the water bath heating temperature is 60-90℃, and the stirring time is 0.5-2h.

[0011] Preferably, in S3, the mass ratio of the pretreated lignocellulose-based biomass material to the metal salt-organic acid composite eutectic solvent is 1:5-12.

[0012] Preferably, in S3, the temperature of the stirring reaction is 110-180℃, the reaction time is 3-4h, and the stirring speed is 250-1000r / min.

[0013] Preferably, in S4, the volume ratio of the reaction solution to the ethanol solution is 2:1.

[0014] Preferably, in step S5, the filtrate is evaporated and concentrated to a solution free of ethanol, the pH of the hydrochloric acid aqueous solution is 2-7, the stirring time is 10-60 min, and the solution is washed 3-5 times.

[0015] The mechanism of this invention lies in constructing a synergistic separation system of DES deconstruction and dual catalytic bond breaking by metal salts and organic acids, thereby achieving highly efficient and selective separation of lignin from lignocellulosic biomass. First, the eutectic solvent, composed of choline chloride (a hydrogen bond acceptor) and hydrogen bond donors such as urea, formic acid, lactic acid, or glycerol, has strong hydrogen bonding ability and good swelling and permeation properties. It can enter the interior of lignocellulose, destroy the original hydrogen bond association and dense network structure between lignin, hemicellulose, and cellulose, and weaken the binding between lignin-carbohydrate complexes, thereby improving the solubility and migration ability of lignin.

[0016] Building upon this foundation, a metal salt and an organic acid are introduced into the system to form a dual-catalytic synergistic system. The organic acid provides Brønsted acid, which can donate protons and promote an acid-catalyzed reaction environment, thus facilitating the hydrolysis of hemicellulose and the formation of lignin. β - O -4 ether bonds break; metal ions in metal salts act as Lewis acids, which can accept electron pairs and activate chemical bonds, and coordinate with oxygen-containing functional groups such as hydroxyl, ether oxygen, and carbonyl groups in lignin and hemicellulose. That is, metal ions form coordination bonds with atoms containing lone pairs of electrons, which enhances the polarization of related chemical bonds, lowers the energy barrier required for bond breaking, and promotes the depolymerization and cleavage of lignin macromolecules.

[0017] Meanwhile, organic acids can also form dynamic complexes or coordination structures with metal ions, further regulating the acidity and catalytic activity of the system. This synergistically enhances the destructive effect of the DES system (i.e., disrupting the dense structure and intermolecular interactions of the three elements in the cell wall of biomass plants, transforming them from a stable, tightly aggregated state to a looser, more easily separable, or more reactive state) and the dual-catalytic bond-breaking effect. Furthermore, the oxygen atoms in the hydroxyl groups of organic acids have high electron affinity, especially in multi-component organic acid / metal salt systems, where electrostatic interactions are more likely to occur, forming more stable multi-molecular systems and combining into a more stable hydrogen bond network.

[0018] In addition, DES promotes the uniform dispersion of catalyst ions and reactant molecules, thereby improving the contact efficiency between the catalyst and the substrate and enhancing the mass transfer performance during the reaction. Simultaneously, the strong hydrogen bond donors and acceptors in DES interact with lignin through hydrogen bonds and π-π interactions, promoting the dissolution and depolymerization of lignin and improving the efficiency of lignin separation and catalytic conversion.

[0019] After the reaction is complete, the addition of ethanol reduces the viscosity of the system and promotes the separation of the dissolved lignin from the residual solids. The filtrate is then concentrated and dispersed in water. Because the solubility of lignin in the aqueous phase decreases significantly, while the eutectic solvent components and small molecule impurities are preferentially retained in the liquid phase, lignin precipitates. After centrifugation, washing, and freeze-drying, lignin powder is obtained.

[0020] Therefore, this invention employs the aforementioned method for separating lignin using a metal salt-organic acid composite eutectic solvent. This method leverages the destructive effect of the eutectic solvent on the biomass structure and the selective cleavage of key bonds in lignin via the dual catalysis of the metal salt and organic acid. This system is low-cost, stable, environmentally friendly, and simple to operate. Furthermore, the metal salt-organic acid composite eutectic solvent system contains no volatile organic compounds, enabling gentle and efficient separation of lignin in a short time. It also mitigates environmental pollution and high energy consumption to some extent, achieving rapid, efficient, and low-pollution separation and purification of lignin.

[0021] The technical solution of the present invention will be further described in detail below through embodiments. Detailed Implementation

[0022] This invention provides a method for the catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent, comprising the following steps: S1. Pretreatment of lignocellulose-based biomass materials; S2. To prepare a metal salt-organic acid composite eutectic solvent, the hydrogen bond donor, hydrogen bond acceptor and dual catalyst are mixed, heated in a water bath and stirred to obtain a uniform and transparent metal salt-organic acid composite eutectic solvent. S3. Mix the pretreated lignocellulose-based biomass material from S1 with the metal salt-organic acid composite eutectic solvent prepared in S2, and stir to obtain a reaction solution. S4. Mix the reaction solution obtained in S3 with the ethanol solution and filter to obtain filtrate and biomass residue; S5. After evaporating and concentrating the filtrate obtained in S4, disperse it in hydrochloric acid aqueous solution, stir, centrifuge, wash, and freeze-dry to obtain lignin powder.

[0023] In this invention, in S1, the pretreatment includes: pulverizing the lignocellulose-based biomass material to 20-100 mesh, drying it at 90-110°C, and then sealing and storing it for later use.

[0024] In this invention, in S1, the lignocellulose-based biomass material is one or more of the following: wheat straw, rice straw, rice husk, corn stalk, corn cob, wheat husk, peanut stalk, peanut shell, cotton stalk, rapeseed straw, soybean straw, and wood processing residues.

[0025] In this invention, in S2, the hydrogen bond donor is one or more of urea, formic acid, lactic acid, and glycerol, the hydrogen bond acceptor is choline chloride, and the catalyst includes a metal salt and an organic acid. The metal salt is one of ferric chloride, copper sulfate, aluminum chloride, and zinc chloride, and the organic acid is one of succinic acid, malic acid, citric acid, sorbic acid, oxalic acid, benzenesulfonic acid, and acetic acid.

[0026] In this invention, the molar ratio of hydrogen bond acceptor to hydrogen bond donor is 1:2-10, the amount of catalyst is 0.5-10% of the total mass of the metal salt-organic acid composite eutectic solvent, and the mass ratio of metal salt to organic acid is 1:1-10.

[0027] In this invention, in step S2, the water bath heating temperature is 60-90℃, and the stirring time is 0.5-2h.

[0028] In this invention, in S3, the mass ratio of the pretreated lignocellulose-based biomass material to the metal salt-organic acid composite eutectic solvent is 1:5-12.

[0029] In this invention, in step S3, the temperature of the stirring reaction is 100-180℃, the reaction time is 3-4h, and the stirring speed is 250-1000r / min.

[0030] In this invention, in step S4, the volume ratio of the reaction solution to the ethanol solution is 2:1.

[0031] In this invention, in step S5, the filtrate is evaporated and concentrated to a solution free of ethanol, the pH of the hydrochloric acid aqueous solution is 2-7, the stirring time is 10-60 min, and the solution is washed 3-5 times.

[0032] The technical solution of the present invention will be further illustrated by the following embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.

[0033] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0034] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0035] In this invention, unless otherwise specified, all other test materials and instruments are conventional test materials in the field and can be purchased through commercial channels.

[0036] Example 1 This invention provides a method for the catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent, comprising the following steps: S1. Pretreatment of lignocellulose-based biomass materials: Corn stalks are crushed to 60 mesh, dried at 105℃, and then sealed and stored for later use.

[0037] S2. Preparation of metal salt-organic acid composite eutectic solvent: Choline chloride and lactic acid are prepared at a molar ratio of 1:2. 1% ferric chloride and 3% citric acid of the total mass of the metal salt-organic acid composite eutectic solvent are added to the mixture in a flask. The mixture is then heated and stirred in an 80°C water bath for 1 hour until fully dissolved to form a uniform and transparent solution. After cooling, the metal salt-organic acid composite eutectic solvent is obtained.

[0038] S3. The lignocellulose-based biomass material pretreated in S1 is mixed with the metal salt-organic acid composite eutectic solvent prepared in S2 at a mass ratio of 1:10, and stirred at 110℃ and 500r / min for 3h to obtain a reaction solution.

[0039] S4. After mixing the reaction solution obtained in S3 with the ethanol solution, filter the mixture to obtain filtrate and biomass residue. The ratio of reaction solution to ethanol solution is 2:1.

[0040] S5. After evaporating and concentrating the filtrate obtained in S4 into an ethanol-free solution, disperse it in a hydrochloric acid aqueous solution with pH=2 and stir for 30 min. Centrifuge, wash 3 times, and freeze dry to obtain lignin powder.

[0041] Example 2 The only difference between this embodiment and Example 1 is that in S3, the reaction is stirred at 120°C, while all other conditions are the same.

[0042] Example 3 The only difference between this embodiment and Example 1 is that in S3, the reaction is stirred at 130°C, while all other conditions are the same.

[0043] Example 4 The only difference between this embodiment and Example 1 is that in S3, the reaction is stirred at 150°C, while all other conditions are the same.

[0044] Example 5 The only difference between this embodiment and Example 1 is that in S2, the hydrogen bond donor is glycerol and the metal salt in the catalyst is zinc chloride; in S3, the reaction is stirred at 120°C, and all other conditions are the same.

[0045] Comparative Example 1 The only difference between this comparative example and Example 1 is that no dual catalyst was added in S2; all other conditions are the same.

[0046] Comparative Example 2 The only difference between this comparative example and Example 1 is that no metal salt was added in S2; all other conditions are the same.

[0047] Comparative Example 3 The only difference between this comparative example and Example 1 is that in S3, the reaction was stirred at 100°C, while all other conditions were the same.

[0048] Comparative Example 4 The only difference between this comparative example and Example 1 is that in S3, the reaction solution was obtained after stirring for 2 hours, while all other conditions were the same.

[0049] Comparative Example 5 The only difference between this comparative example and Example 1 is that in S3, the reaction solution was obtained after stirring for 1 hour, while all other conditions were the same.

[0050] The lignin separation efficiency of Examples 1-5 and Comparative Examples 1-5 was investigated. The lignin separation efficiency was calculated based on the lignin content in corn straw, a lignocellulose-based biomass material. The results are shown in Table 1. The contents of lignin, cellulose and hemicellulose in corn straw are shown in Table 2.

[0051] Table 1. Lignin separation efficiency of Examples 1-5 and Comparative Examples 1-5

[0052] Table 2. Lignin, cellulose, and hemicellulose content in corn stalks

[0053] As shown in Tables 1 and 2, comparing Examples 1-5 and Comparative Examples 1-5 respectively, it can be seen that the extraction of lignin from lignocellulosic biomass materials by dual catalytic synergistic treatment based on DES (Example 1) improved by 13.08% compared with untreated (Comparative Example 1) and by 9.92% compared with single catalytic treatment (Comparative Example 2).

[0054] The reason is that although the DES system in Comparative Example 1 can break some hydrogen bonds, its ability to break ether and ester bonds in LCC is limited. It lacks metal salts to promote the breaking of ether bonds and organic acids to provide an acidic environment to promote the hydrolysis of ester bonds. In contrast, the organic acid in Comparative Example 2 can only provide an acidic environment. The ability of a single acidic condition to depolymerize lignin macromolecules is limited, and it can only dissolve some lignin components with loosely linked ester bonds.

[0055] Besides the catalyst system, reaction temperature and reaction time are also key conditions of this invention. When the reaction temperature was increased from 100℃ (Comparative Example 3) to 110℃ (Example 1), 120℃ (Example 2), 130℃ (Example 3), and 150℃ (Example 4), the lignin separation efficiency was increased by 16.53%, 34.28%, 54.39%, and 71.9%, respectively. When the reaction time was increased from 1h (Comparative Example 5) to 2h (Comparative Example 4) and 3h (Example 1), the lignin separation efficiency was increased by 5.62% and 14.87%, respectively.

[0056] The reasons are as follows: In Comparative Example 3, the reaction temperature was low, resulting in a significant decrease in the catalytic reaction rate. The reaction solvent had low activity at low temperatures, making it difficult to break the bonds between lignin and cellulose and hemicellulose. In Comparative Example 4, the reaction time did not reach the time threshold for complete lignin dissolution. Lignin dissolution requires sufficient time for DES to penetrate, for the catalytic reaction to occur, and for the dissolved lignin to diffuse into the solvent phase. The short reaction time meant the reaction had not yet reached equilibrium, and some lignin remained encapsulated within the cellulose-hemicellulose network structure. Comparative Example 5 was similar to Comparative Example 4; however, at the reaction time in Comparative Example 5, the reaction was in its initial stage, and a large amount of lignin remained in the biomass residue.

[0057] By comparing Comparative Example 3 and Comparative Example 5, it can be seen that even if the time is insufficient, increasing the temperature can still improve efficiency; conversely, even if the temperature is low, extending the time can slightly improve the separation efficiency.

[0058] Furthermore, the efficiency of Example 1 (29.75%) was significantly higher than that of Comparative Example 2 (19.83%) and Comparative Example 1 (16.67%), and was 9.92% higher than that of Comparative Example 2. This improvement could not be explained by the effect of the organic acid alone, indicating that the addition of the metal salt produced an additional synergistic effect in the presence of the organic acid.

[0059] From the perspective of the mechanism of action, the organic acid (citric acid) provides an acidic environment, mainly breaking the ester bonds in the lignin-carbohydrate complex; the metal salt (FeCl3), as a Lewis acid, can catalyze the breaking of the ether bonds in the lignin structure. β - O -4). The two catalysts act on different types of chemical bonds, forming complementary bonds, thereby synergistically disrupting the bonding network between lignin and carbohydrates, achieving efficient separation of lignin.

[0060] In summary, the reaction temperature in this invention should be no less than 110°C and the reaction time should be no less than 3 hours. Under these conditions, the DES-based dual-catalytic system significantly demonstrates the effectiveness and superiority of this method.

[0061] Therefore, the present invention adopts the above-mentioned method of separating lignin by catalytic eutectic solvent of metal salt-organic acid composite, and uses DES to treat lignocellulose biomass through a dual catalytic synergistic system to achieve rapid and efficient separation of lignin. At the same time, the preparation process has low cost, simple equipment requirements and low pollution.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for the catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent, characterized in that: Includes the following steps: S1. Pretreatment of lignocellulose-based biomass materials; S2. To prepare a metal salt-organic acid composite eutectic solvent, the hydrogen bond donor, hydrogen bond acceptor and dual catalyst are mixed, heated in a water bath and stirred to obtain a uniform and transparent metal salt-organic acid composite eutectic solvent. S3. Mix the pretreated lignocellulose-based biomass material from S1 with the metal salt-organic acid composite eutectic solvent prepared in S2, and stir to obtain a reaction solution. S4. Mix the reaction solution obtained in S3 with the ethanol solution and then filter to obtain the filtrate and biomass residue. S5. After evaporating and concentrating the filtrate obtained in S4, disperse it in hydrochloric acid aqueous solution, stir, centrifuge, wash, and freeze-dry to obtain lignin powder.

2. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S1, the pretreatment includes: pulverizing the lignocellulose-based biomass material to 20-100 mesh, drying it at 90-110℃, and then sealing and storing it for later use.

3. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S1, the lignocellulose-based biomass material is one or more of the following: wheat straw, rice straw, rice husk, corn stalk, corn cob, wheat husk, peanut stalk, peanut shell, cotton stalk, rapeseed straw, soybean straw, and wood processing residues.

4. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S2, the hydrogen bond donor is one or more of urea, formic acid, lactic acid, and glycerol, the hydrogen bond acceptor is choline chloride, and the catalyst includes a metal salt and an organic acid. The metal salt is one of ferric chloride, copper sulfate, aluminum chloride, and zinc chloride, and the organic acid is one of succinic acid, malic acid, citric acid, sorbic acid, oxalic acid, benzenesulfonic acid, and acetic acid.

5. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S2, the molar ratio of hydrogen bond acceptor to hydrogen bond donor is 1:2-10, the amount of catalyst is 0.5-10% of the total mass of the metal salt-organic acid composite eutectic solvent, and the mass ratio of metal salt to organic acid is 1:1-10.

6. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S2, the water bath heating temperature is 60-90℃, and the stirring time is 0.5-2h.

7. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S3, the mass ratio of the pretreated lignocellulose-based biomass material to the metal salt-organic acid composite eutectic solvent is 1:5-12.

8. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S3, the temperature of the stirring reaction is 110-180℃, the reaction time is 3-4h, and the stirring speed is 250-1000r / min.

9. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In S4, the volume ratio of the reaction solution to the ethanol solution is 2:

1.

10. The method for catalytic separation of lignin using a metal salt-organic acid composite eutectic solvent according to claim 1, characterized in that: In step S5, the filtrate is evaporated and concentrated to a solution free of ethanol. The pH of the hydrochloric acid aqueous solution is 2-7, the stirring time is 10-60 min, and the solution is washed 3-5 times.

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