A method for extracting lignin with high content of phenolic hydroxyl groups

Abstract

This invention discloses a method for extracting lignin with high phenolic hydroxyl content, belonging to the fields of biomass refining and green chemistry. The method involves pretreating plant fiber raw materials with bio-enzymes to obtain a solid residue; then, the solid residue is extracted and purified using a catalytic eutectic solvent to obtain the lignin with high phenolic hydroxyl content. A two-step strategy of "enzyme pretreatment exposure + catalytic DES directional bond breaking" is employed to extract lignin with high phenolic hydroxyl content from plant fibers. Enzyme pretreatment effectively breaks down the biomass degradation barrier, removing most of the carbohydrates and fully exposing the lignin. This not only results in a lignin yield of over 90% for subsequent DES extraction, but more importantly, it creates an optimal interaction interface for the DES catalyst to selectively break the β-O-4 ether bonds of lignin molecules, thereby generating a large number of phenolic hydroxyl groups in situ, significantly increasing the lignin content to far exceed that of lignin extracted by traditional methods and conventional DES, and producing a product with excellent reactivity.

Description

Technical Field

[0001] This invention belongs to the field of biomass refining and green chemistry technology, and in particular relates to a method for extracting lignin with high phenolic hydroxyl content. Background Technology

[0002] Lignin is one of the most abundant aromatic natural polymers in nature. As a renewable aromatic carbon resource, it shows broad application prospects in replacing fossil raw materials to prepare polymer materials such as phenolic resins, polyurethanes, and epoxy resins, as well as in developing high-value-added chemicals such as asphalt emulsifiers and antioxidants. However, the high-value utilization of lignin has long been limited by the quality of its source and extraction process.

[0003] Traditional industrial lignins (such as alkali lignin and sulfate lignin) are mainly derived from pulping or biorefining processes, whose extraction often relies on harsh conditions such as high temperatures, strong alkalis, or strong acids. These harsh conditions are not only energy-intensive and polluting, but more importantly, they trigger irreversible condensation reactions in lignin molecules, leading to the breakage of numerous key ether bonds such as β-O-4 in its natural structure and the formation of stable C-C bonds. This process increases the molecular weight and structural complexity of lignin, and significantly reduces the content of its most reactive functional group, the phenolic hydroxyl group. The phenolic hydroxyl group is the core active site for lignin to participate in chemical reactions, provide antioxidant properties, and replace phenol. The loss of its content directly results in low reactivity, poor solubility, and poor uniformity of traditional lignin, severely limiting its application potential in the fine chemical industry.

[0004] From the perspective of raw material structure, lignin, cellulose, and hemicellulose are tightly intertwined with each other through chemical bonds and physical entanglement in plant cell walls, forming a dense "lignin-carbohydrate complex." This natural barrier not only hinders the effective dissolution of lignin but also severely limits the accessibility of cellulase to cellulose, posing a significant challenge to the clean and efficient separation and full utilization of biomass components. In recent years, eutectic solvents (DES), as a green and designable novel solvent system, have received widespread attention in the field of lignin extraction. However, existing research mainly focuses on using DES to efficiently dissolve and separate lignin, with insufficient attention paid to how to selectively break the β-O-4 ether bonds within lignin during dissolution, thereby creating new phenolic hydroxyl groups in situ at the molecular level and enhancing lignin activity. Furthermore, most methods fail to fully consider the synergistic effect between pretreatment steps and DES extraction to achieve the linkage between cellulose saccharification and high-value lignin extraction, limiting the overall economic efficiency and effectiveness of the process. Traditional methods (such as the alkaline method and the sulfate method) extract lignin under high temperature and strong acid and alkali conditions, which easily triggers irreversible condensation reactions, resulting in low phenolic hydroxyl content and poor reactivity of the product, thus limiting its application in the field of fine chemicals.

[0005] Therefore, developing a green integrated process that can synergistically achieve efficient depolymerization of biomass, high-yield extraction of lignin, and simultaneous improvement of its phenolic hydroxyl content under mild conditions is of great scientific significance and application value for breaking through the technical bottleneck of high-value utilization of lignin and improving the overall economic benefits of biorefining. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a method for extracting lignin with high phenolic hydroxyl content. The method first selectively converts cellulose and hemicellulose in plant fiber raw materials into soluble sugars through enzymatic treatment, followed by solid-liquid separation to obtain a sugar solution and a lignin-rich solid residue. Then, extraction is performed using a eutectic solvent containing a metal salt catalyst, followed by precipitation purification to obtain lignin with high phenolic hydroxyl content. This invention significantly improves the extraction efficiency and phenolic hydroxyl content of lignin through the synergistic effect of enzyme treatment and a catalytic eutectic solvent, while simultaneously obtaining a high-purity sugar solution, achieving the graded and high-value utilization of all components of biomass. This method is mild, environmentally friendly, and provides a new technical pathway for the high-value utilization and biorefining of lignin.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A method for extracting lignin with high phenolic hydroxyl content includes the following steps: pretreating plant fiber raw materials with bio-enzymes to obtain solid residue; then extracting and purifying the solid residue with a catalytic eutectic solvent to obtain the lignin with high phenolic hydroxyl content.

[0008] Furthermore, the extraction method for lignin with high phenolic hydroxyl content specifically includes the following steps: S1. After mixing plant fiber raw materials, buffer solution and compound enzyme, the mixture is subjected to biological enzyme pretreatment, and the supernatant and solid residue are obtained by solid-liquid separation. S2. Wash the solid residue until it is neutral and dry it, then add it to a catalytic eutectic solvent and heat it to react. S3. The reaction mixture obtained in S2 is subjected to precipitation and purification treatment to obtain the lignin with high phenolic hydroxyl content.

[0009] Further, in step S1, the solid-liquid ratio of the plant fiber raw material and the buffer solution is 1g:15mL; the complex enzyme contains cellulase and hemicellulase, the mass ratio of cellulase and hemicellulase is 1:1, and the total enzyme activity of the complex enzyme is 15-30 FPU / g based on cellulase activity.

[0010] Furthermore, in step S1, the temperature of the bio-enzyme pretreatment is 45-50℃, and the time is 48-72 hours.

[0011] Further, in step S1, the buffer solution is a citrate-sodium citrate buffer solution with a pH of 4.5-5.5; the plant fiber raw material is a pulverized and dried plant fiber raw material powder with a particle size of 40-80 mesh. Further, in step S1, after the bio-enzyme pretreatment is completed, solid-liquid separation is performed to obtain a supernatant rich in fermentable sugars and a solid residue rich in lignin.

[0012] Further, in step S2, the preparation method of the catalytic eutectic solvent includes the following steps: mixing hydrogen bond acceptor and hydrogen bond donor in a molar ratio of 1:(3-8) to obtain a eutectic solvent, adding a catalyst accounting for 1-3% of the total mass of the eutectic solvent, heating and stirring to obtain the catalytic eutectic solvent.

[0013] Furthermore, the hydrogen bond acceptor is selected from choline chloride, the hydrogen bond donor is selected from lactic acid, ethylene glycol or glycerol; and the catalyst is selected from copper chloride, aluminum chloride or ferric chloride.

[0014] Furthermore, the hydrogen bond donor is selected from lactic acid; the catalyst is selected from copper chloride.

[0015] Further, in step S2, the dried solid residue is mixed with the catalytic eutectic solvent at a solid-liquid ratio of 1:20 (g / mL); the heating reaction is carried out at a temperature of 120-140 °C for 2-4 hours.

[0016] Furthermore, in step S2, the temperature at which the solid residue is washed to neutral and dried is 60-80 ℃.

[0017] The specific principle behind the efficient dissolution and selective bond breaking of lignin using a metal salt-modified eutectic solvent in this invention is as follows: the catalyst (such as a metal salt like copper chloride) acts as a Lewis acid site, preferentially coordinating with the ether oxygen atom of the β-O-4 ether bond in the lignin molecule, thus weakening the CO bond energy. Simultaneously, the strongly polar environment constructed by the hydrogen bond donor and acceptor in the DES further stabilizes the transition state during bond breaking, lowering the activation energy. Under mild heating conditions, this synergistic effect induces selective cleavage of the β-O-4 ether bond, thereby generating new phenolic hydroxyl groups in situ at the cleavage site. Furthermore, the DES exhibits excellent solubility for lignin, rapidly encapsulating and removing the cleaved lignin fragments from the reaction interface, effectively inhibiting the secondary condensation reaction that easily occurs in lignin macromolecules under acidic or high-temperature conditions, thus preserving a high content of phenolic hydroxyl active sites.

[0018] Further, in step S3, the specific operation of the precipitation and purification process is as follows: add water to dilute the reaction mixture obtained in S2, then add a precipitant, collect the precipitate, and wash and dry the precipitate to obtain the lignin with high phenolic hydroxyl content.

[0019] Furthermore, the precipitant is selected from ethanol, acetone, methanol, an ethanol-water mixture, an acetone-water mixture, or a methanol-water mixture. Compared with the prior art, the present invention has the following advantages and technical effects: This invention achieves a synergistic improvement in lignin activity and yield: It employs a two-step strategy of "enzyme pretreatment exposure + catalytic DES-directed bond cleavage" to extract lignin with high phenolic hydroxyl content from plant fiber raw materials. Enzyme pretreatment effectively breaks down the biomass' resistance to degradation, removing most of the carbohydrates and fully exposing the lignin. This not only results in a lignin yield of over 90% for subsequent DES extraction, but more importantly, it creates an optimal interaction interface for the DES catalyst to selectively cleave the β-O-4 ether bonds of lignin molecules, thereby generating a large number of phenolic hydroxyl groups in situ, significantly increasing their content—far exceeding that of lignin extracted by traditional methods and conventional DES—and producing products with excellent reactivity.

[0020] Full-component utilization of biomass: This invention constructs a graded refining technology route. The first step involves enzymatic hydrolysis, producing a high-purity sugar solution rich in glucose and xylose, which can be directly used for fermentation; the second step extracts highly active lignin from the residue. This process simultaneously achieves cellulose saccharification and lignin high-value utilization, transforming the three major components of lignocellulose into high-value-added products, significantly improving the economic efficiency and resource utilization rate of the biorefining process.

[0021] The process is green and environmentally friendly, with mild and controllable conditions: the entire process avoids the strong acids, strong alkalis, and toxic organic solvents used in traditional methods. The enzymatic hydrolysis conditions are mild, DES is biodegradable and easily recycled, and the extraction temperature is relatively low, effectively inhibiting secondary condensation of lignin. This process conforms to green chemistry principles, is environmentally friendly, and has significant advantages in energy consumption and cost. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 The infrared spectra of lignin extracted in Example 1 and Comparative Example 1 are shown. Detailed Implementation

[0024] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0025] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0026] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0027] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0028] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0029] Existing DES extraction technologies primarily focus on lignin dissolution, with insufficient attention paid to selectively breaking the key β-O-4 ether bonds in lignin molecules to in-situ increase the content of phenolic hydroxyl groups. Furthermore, they often neglect synergy with biomass pretreatment processes and the direct extraction of lignin from raw biomass. Due to the dense structure of the "lignin-carbohydrate complex," it is difficult to efficiently recover high-purity cellulose sugars while obtaining highly active lignin, resulting in low efficiency in the fractional utilization of all biomass components. This invention provides a green extraction method for lignin with high phenolic hydroxyl content. Through coupled enzyme pretreatment and catalytic DES treatment, it solves the key problems of severe loss of active functional groups and difficulty in achieving high-value utilization of all biomass components in traditional lignin extraction processes. It provides a simple, environmentally friendly, and highly efficient new process for high-value extraction of lignin.

[0030] This invention provides a method for extracting lignin with high phenolic hydroxyl content, specifically including the following steps: S1. After mixing plant fiber raw materials, buffer solution and compound enzyme, the mixture is subjected to biological enzyme pretreatment, and the supernatant and solid residue are obtained by solid-liquid separation. S2. Wash the solid residue until it is neutral and dry it, then add it to a catalytic eutectic solvent and heat it to react. S3. The reaction mixture obtained in S2 is subjected to precipitation and purification treatment to obtain the lignin with high phenolic hydroxyl content.

[0031] In some preferred embodiments of the present invention, in step S1, the solid-liquid ratio of the plant fiber raw material and the buffer solution is 1:15 (g / mL); the complex enzyme comprises cellulase and hemicellulase, the mass ratio of the cellulase and hemicellulase is 1:1, and the total enzyme activity of the complex enzyme is 15-30 FPU / g based on cellulase activity. For example, the total enzyme activity of the complex enzyme is 30 or 15 FPU / g.

[0032] In some preferred embodiments of the present invention, in step S1, the temperature of the bio-enzyme pretreatment is 45-50°C (exemplary: 50°C, 45°C), and the time is 48-72 hours (exemplary: 48 hours, 72 hours).

[0033] In some preferred embodiments of the present invention, in step S2, the method for preparing the catalytic eutectic solvent includes the following steps: mixing hydrogen bond acceptors and hydrogen bond donors in a molar ratio of 1:(3-8) to obtain a eutectic solvent (exemplary: 1:3, 1:5, 1:8), adding a catalyst accounting for 1-3% of the total mass of the eutectic solvent (exemplary: 1%, 2%, 3%), heating and stirring to obtain the catalytic eutectic solvent.

[0034] In some preferred embodiments of the present invention, the hydrogen bond acceptor is selected from choline chloride, the hydrogen bond donor is selected from lactic acid, ethylene glycol or glycerol; and the catalyst is selected from copper chloride, aluminum chloride or ferric chloride.

[0035] In some preferred embodiments of the present invention, in step S2, the dried solid residue is mixed with the catalytic eutectic solvent at a solid-liquid ratio of 1g:20mL; the heating reaction is carried out at a temperature of 120-140℃ (exemplary: 120℃, 140℃) for 2-4 hours (2 hours, 4 hours).

[0036] In some preferred embodiments of the present invention, the specific operation of the precipitation and purification process is as follows: water is added to dilute the reaction mixture obtained in S2, then a precipitant is added, the precipitate is collected, and the precipitate is washed and dried to obtain the lignin with high phenolic hydroxyl content.

[0037] In some preferred embodiments of the present invention, the precipitant is selected from ethanol, acetone, methanol, ethanol-water mixed solvent, acetone-water mixed solvent, or methanol-water mixed solvent.

[0038] The room temperature in this invention refers to 25±2℃.

[0039] Example 1: A method for extracting lignin with high phenolic hydroxyl content S1. Bioenzyme Pretreatment: Take 100g of corn stalk raw material, crush it using a pulverizer and pass it through a 60-mesh standard sieve. Then, dry it to constant weight at 60℃ in a forced-air drying oven. Mix the crushed and dried plant fiber raw material with 1.5 L of 0.05 M citrate-sodium citrate buffer solution (pH 5.0) in a 3L Erlenmeyer flask (solid-liquid ratio 1:15, g / mL). Add a compound enzyme preparation to the resulting mixture, wherein the mass ratio of cellulase to hemicellulase is 1:1. The total enzyme activity added is 30 FPU / g of oven-dry raw material, calculated based on cellulase activity. Place the mixture in a constant temperature shaking incubator and react for 48 hours at 50℃ and 150 rpm to obtain an enzymatic suspension. Centrifuge the enzymatic suspension at 8000 rpm. Centrifuge at rpm for 15 minutes, collect the sugar-rich supernatant and the lignin-rich enzymatic hydrolysis solid residue, transfer the obtained enzymatic hydrolysis solid residue to a beaker, add 1L of deionized water (mass ratio of enzymatic hydrolysis solid residue to deionized water is 1:10), magnetically stir and wash for 30 minutes, then centrifuge at 4000rpm for 10 minutes, discard the supernatant, repeat this washing process 4 times until the pH value of the last washing solution is stable at about 7.0, place the neutralized solid residue in a vacuum drying oven and dry (50℃) to constant weight; S2. Preparation of catalytic DES: Choline chloride and anhydrous lactic acid are mixed in a beaker at a molar ratio of 1:5 to obtain DES (eutectic solvent). Anhydrous copper chloride, accounting for 2% of the total mass of DES, is added to the DES as a catalyst. The flask is placed in an oil bath at 90°C and heated and stirred continuously for 2 hours under magnetic stirring until a homogeneous, transparent liquid without solid particles is formed, which is the catalytic DES. S3. Catalytic DES extraction: Accurately weigh 10.0g of the enzymatic hydrolysis solid residue prepared in S1 and add it together with 200 mL of catalytic DES prepared in S2 (solid-liquid ratio 1:20, g / mL) into a high-pressure reactor with a polytetrafluoroethylene liner. After sealing, place the reactor in an oven preheated to 140℃ and react for 2 hours. S4. Purification and recovery of lignin: After the reaction in S3 was completed, the reaction solution was cooled to room temperature. The reaction mixture was transferred to a beaker, and 600 mL of deionized water was added to dilute it and stirred for 30 minutes. Then, under continuous stirring, 400 mL of anhydrous ethanol was slowly added dropwise as a precipitant. After stirring evenly, the mixture was allowed to stand for 2 hours to obtain a suspension. The suspension was centrifuged at 8000 rpm for 15 minutes, and the supernatant was discarded (the DES-water-ethanol mixture can be recycled). 200 mL of ethanol-water mixture with a volume ratio of 1:1 was added to the obtained precipitate. After thorough vortexing and dispersion, the mixture was centrifuged again. This washing process was repeated 3 times to completely remove residual DES components and catalyst ions. The washed lignin was placed in a vacuum drying oven at 55℃ (-0.09 MPa) and dried to constant weight to prepare lignin with high phenolic hydroxyl content.

[0040] Example 2: A method for extracting lignin with high phenolic hydroxyl content S1. Bioenzyme pretreatment: Same as in Example 1; S2. Preparation of catalytic DES: In a round-bottom flask, choline chloride and ethylene glycol are mixed at a molar ratio of 1:3 to obtain DES (eutectic solvent). Anhydrous aluminum chloride (AlCl3) accounting for 3% of the total mass of DES is added to the DES as a catalyst. The round-bottom flask is placed in an oil bath at 80°C and magnetically stirred for 3 hours until a homogeneous, transparent liquid without solid particles is formed, which is the catalytic DES. S3. Catalytic DES extraction: Accurately weigh 10.0g of the enzymatic hydrolysis solid residue prepared in S1 and add it together with 200 mL of catalytic DES prepared in S2 (solid-liquid ratio 1:20, g / mL) into a high-pressure reactor with a polytetrafluoroethylene liner. After sealing, place the reactor in an oven preheated to 140℃ and react for 2 hours. S4. Purification and recovery of lignin: After the reaction in S3 was completed, the reaction solution was cooled to room temperature. The reaction mixture was transferred to a beaker, diluted with 600 mL of deionized water, and stirred for 30 minutes. Then, under continuous stirring, 400 mL of acetone was slowly added dropwise as a precipitant. After stirring evenly, the mixture was allowed to stand for 12 hours to obtain a suspension. The suspension was centrifuged at 8000 rpm for 15 minutes, and the supernatant was discarded. 200 mL of a 1:1 volume ratio ethanol-water mixture was added to the obtained precipitate. After thorough vortexing and dispersion, the mixture was centrifuged again. This washing process was repeated 3 times to completely remove residual DES components and catalyst ions. The washed lignin was placed in a vacuum drying oven at 55℃ (-0.09 MPa) and dried to constant weight to obtain lignin with high phenolic hydroxyl content.

[0041] Example 3: A method for extracting lignin with high phenolic hydroxyl content S1. Bioenzyme Pretreatment: Take 100g of poplar raw material, crush it using a pulverizer and pass it through a 60-mesh standard sieve. Then, dry it to constant weight at 60℃ in a forced-air drying oven. Mix the crushed and dried plant fiber raw material with 1.5 L of 0.05M citrate-sodium citrate buffer solution (pH 5.0) in a 3L Erlenmeyer flask (solid-liquid ratio 1:15, g / mL). Add a compound enzyme preparation to the resulting mixture, wherein the mass ratio of cellulase to hemicellulase is 1:1. The total enzyme activity added is 15 FPU / g of oven-dried raw material, calculated based on cellulase activity. Place the mixture in a constant temperature shaking incubator and react for 72 hours at 45℃ and 150 rpm to obtain an enzymatic suspension. Centrifuge the enzymatic suspension at 8000 rpm. Centrifuge at rpm for 15 minutes, collect the sugar-rich supernatant and the lignin-rich enzymatic hydrolysis solid residue, transfer the obtained enzymatic hydrolysis solid residue to a beaker, add 1L of deionized water (solid-liquid ratio 1:10), wash with magnetic stirring for 30 minutes, centrifuge at 4000 rpm for 10 minutes, discard the supernatant, repeat this washing process 4 times until the pH value of the last washing solution is stable at about 7.0, place the neutralized solid residue in a vacuum drying oven to dry (60℃) to constant weight; S2. Preparation of catalytic DES: Same as in Example 1; S3. Catalytic DES extraction: Accurately weigh 10.0g of the enzymatic hydrolysis solid residue prepared in S1 and add it together with 200 mL of catalytic DES prepared in S2 (solid-liquid ratio 1:20, g / mL) into a high-pressure reactor with a polytetrafluoroethylene liner. After sealing, place the reactor in an oven preheated to 120℃ and react for 4 hours. S4. Purification and recovery of lignin: Same as in Example 1, lignin with high phenolic hydroxyl content was prepared.

[0042] Example 4: A method for extracting lignin with high phenolic hydroxyl content S1 Bio-enzyme Pretreatment: Take 100g of pine wood raw material, crush it using a pulverizer and pass it through a 60-mesh standard sieve, and then dry it at 60℃ in a forced-air drying oven until constant weight. The pulverized and dried plant fiber raw material was mixed with 1.5 L of 0.05 M citrate-sodium citrate buffer (pH 5.0) in a 3 L Erlenmeyer flask (solid-liquid ratio 1:15, g / mL). A compound enzyme preparation was added to the resulting mixture, with a cellulase to hemicellulase mass ratio of 1:1. The total enzyme activity added was 30 FPU / g of oven-dry raw material, calculated based on cellulase activity. The mixture was placed in a constant temperature shaking incubator and reacted at 50℃ and 150 rpm for 48 hours to obtain an enzymatic hydrolysis suspension. The suspension was centrifuged at 8000 rpm for 15 minutes, and the sugar-rich supernatant and lignin-rich enzymatic hydrolysis solid residue were collected. The resulting solid residue was transferred to a beaker, and 1 L of deionized water (solid-liquid ratio 1:10) was added. After magnetic stirring and washing for 30 minutes, the mixture was further centrifuged at 4000 mL / min. Centrifuge at rpm for 10 minutes, discard the supernatant, and repeat this washing process 4 times until the pH of the last washing solution stabilizes at around 7.0. Place the neutralized solid residue in a vacuum drying oven and dry (50°C) to constant weight. S2. Preparation of catalytic DES: Choline chloride and glycerol are mixed in a beaker at a molar ratio of 1:8 to obtain DES (eutectic solvent). Ferric chloride (FeCl3) is added to the DES as a catalyst at a mass of 1% of the total mass of DES. The flask is placed in an oil bath at 100°C and heated and stirred continuously for 2 hours under magnetic stirring until a homogeneous, transparent liquid without solid particles is formed, which is the catalytic DES. S3. Catalytic DES extraction: Accurately weigh 10.0g of the enzymatic hydrolysis solid residue prepared in S1 and add it together with 200 mL of catalytic DES prepared in S2 (solid-liquid ratio 1:20, g / mL) into a high-pressure reactor with a polytetrafluoroethylene liner. After sealing, place the reactor in an oven preheated to 140℃ and react for 2 hours. S4. Purification and recovery of lignin: Same as in Example 1, lignin with high phenolic hydroxyl content was prepared.

[0043] Comparative Example 1 This comparative example is the same as Example 1, except that the pretreatment step of the biological enzyme is omitted, and the raw material is directly treated with catalytic DES. The specific preparation process is as follows: S1. Raw material preparation: Take 100g of corn stalk powder (60 mesh, dried at 60℃) from the same batch as in Example 1, without biological enzyme pretreatment; S2. Preparation of catalytic DES: Same as in Example 1; S3. Catalytic DES extraction: Accurately weigh 10.0g of corn stalk powder from S1 and add it together with 200 mL of catalytic DES prepared in S2 (solid-liquid ratio 1:20, g / mL) into a high-pressure reactor with a polytetrafluoroethylene liner. After sealing, place the reactor in an oven preheated to 140℃ and react for 2 hours. S4. Purification and recovery of lignin: Same as in Example 1, to obtain lignin product.

[0044] Figure 1 The infrared spectra of lignin extracted in Example 1 and Comparative Example 1 are shown below. Figure 1 As can be seen from the data, the lignin content of both Example 1 and Comparative Example 1 is 1510 cm⁻¹. -1 and 1600 cm -1 A distinct aromatic ring skeletal vibration peak appeared at 1215 cm⁻¹, indicating that the extraction process effectively preserved the core structure of lignin. However, in Example 1, the peak at 1215 cm⁻¹ was not observed. -1 (Phenolic CO stretching vibration) and 1325 cm -1 The absorption peak intensity at (syringyl ring and condensed phenolic hydroxyl vibration) is significantly higher than that of Comparative Example 1, and is at 3400 cm⁻¹ -1 The absorption peaks of the hydroxyl (-OH) groups around the lignin are much broader. This directly demonstrates that enzyme pretreatment combined with catalytic DES extraction can effectively promote the breaking of the β-O-4 bond in lignin and its conversion into more phenolic hydroxyl structures, verifying the significant advantages of the method of this invention in enhancing the chemical activity of lignin.

[0045] Comparative Example 2 Same as Example 1, except that anhydrous copper chloride was not added in step S2 when preparing the catalytic DES.

[0046] Comparative Example 3 This comparative example uses the industrially common sulfate method to prepare lignin. The specific preparation process is as follows: S1. Cooking reaction: Take 100g of corn stalk raw material, crush it using a pulverizer and pass it through a 60-mesh standard sieve, and then dry it to constant weight at 60℃ in a forced-air drying oven. Mix the crushed and dried plant fiber raw material with a cooking liquor composed of NaOH and Na2S (calculated as Na2O, effective alkali dosage is 18%, and sulfurization degree is 25%), with a solid-liquid ratio of 1:5 (g / mL). Put it into a stainless steel high-pressure reactor and cook it at 170℃ for 2 hours. S2. Black liquor separation and acidification precipitation: After the cooking reaction in S1 is completed, the liquor is cooled to room temperature and filtered to obtain black liquor. Under vigorous stirring, the black liquor is gradually acidified with 20% sulfuric acid solution to a pH of about 3.0. At this time, a large amount of lignin precipitates out. S3. Washing and drying: Collect the precipitate of S2 by centrifugation, wash it repeatedly with deionized water until the wash water is neutral and free of sulfate ions (tested by BaCl2 solution), and dry the washed precipitate in a vacuum drying oven at 60℃ to constant weight to obtain sulfate lignin.

[0047] Comparative Example 4 This comparative example uses a common organic solvent method to extract lignin, in order to compare the lignin products under relatively mild chemical conditions. The specific preparation process is as follows: S1. Organic solvent cooking: Take 100g of corn stalk raw material, crush it using a pulverizer and pass it through a 60-mesh standard sieve, and then dry it in a forced-air drying oven at 60℃ to constant weight. Mix the crushed and dried plant fiber raw material with a volume fraction of 60% ethanol aqueous solution at a solid-liquid ratio of 1:10 (g / mL), add a small amount of sulfuric acid to adjust the pH of the system to 3.5, put it into a high-pressure reactor, and react at 180℃ for 2 hours; S2. Solvent recovery and lignin precipitation: After the reaction in S1 is completed, the mixture is cooled, filtered, and the filtrate is rotary evaporated at 50°C to recover most of the ethanol. Two volumes of deionized water are added to the concentrated aqueous phase, and lignin precipitates out. S3. Washing and drying: Centrifuge to collect the precipitate, wash twice with acidified water (hydrochloric acid, pH=2), then wash with deionized water until neutral, and dry in a vacuum drying oven at 60℃ to constant weight to obtain organic solvent lignin.

[0048] The lignin yield and phenolic hydroxyl content in the obtained lignin were calculated in Examples 1-4 and Comparative Examples 1-4. The testing methods used in this invention are all conventional methods in the art, and the reagents and instruments used are commercially available. The mass of the extracted lignin was obtained by directly weighing the dried products collected at the end of each example and comparative example. The theoretical mass of lignin in the raw material was determined by quantitatively measuring the total content of acid-insoluble and acid-soluble lignin in the raw material using acid hydrolysis. The lignin yield was calculated using the following formula: Lignin yield (%) = (Mass of extracted lignin / Theoretical mass of lignin in raw material) × 100% The phenolic hydroxyl content in lignin was determined using a modified Folin-Ciocalteu method. The results are expressed as mmol / g lignin and are shown in Table 1. The specific operation of the modified Folin-Ciocalteu method is as follows: a standard curve was plotted using vanillin as a standard. The lignin to be tested was dissolved in dimethyl sulfoxide to prepare a sample solution of 1.0 mg / mL. 0.5 mL of the sample solution was mixed with 2.5 mL of Folin-Ciocalteu reagent (0.2 N) and 2.0 mL of sodium carbonate solution (20% w / v). After reacting at room temperature in the dark for 2 h, the absorbance at 760 nm was measured. The phenolic hydroxyl content (mmol / g) was calculated according to the standard curve.

[0049] Table 1. Lignin yield and phenolic hydroxyl content in the examples and comparative examples Based on the measurement results of Examples 1-4 and Comparative Examples 1-4 in Table 1, it can be seen that the synergistic process of bio-enzyme pretreatment-catalytic eutectic solvent extraction provided by the present invention is significantly superior to existing methods in terms of lignin yield and phenolic hydroxyl content.

[0050] Regarding lignin yield, the lignin yield of the extraction method for high phenolic hydroxyl content lignin provided by the present invention is consistently above 85% (Examples 1-4), which is significantly higher than that of the traditional alkaline method and organic solvent method. Among them, the lignin yield of Comparative 1 without enzyme pretreatment is only 68.4%, which proves the key role of enzyme pretreatment in improving extraction efficiency.

[0051] Regarding the phenolic hydroxyl content, the lignin obtained by this invention has a phenolic hydroxyl content of 2.83-3.05 mmol / g. The traditional alkaline method, due to severe condensation, has the lowest content (Comparative Example 3: 1.65); the organic solvent method also yields only about 2.05 mmol / g. Particularly noteworthy is Comparative Example 2, which only underwent enzyme pretreatment but did not contain a catalyst in the DES, exhibited a phenolic hydroxyl content comparable to that of the traditional method and significantly lower than that of Examples 1-4. This demonstrates that the catalyst in the DES is the decisive factor in selectively cleaving ether bonds and increasing the phenolic hydroxyl content.

[0052] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for extracting lignin with high phenolic hydroxyl content, characterized in that, The process includes the following steps: pretreating plant fiber raw materials with bio-enzymes to obtain solid residue; then extracting and purifying the solid residue with a catalytic eutectic solvent to obtain lignin with high phenolic hydroxyl content.

2. The method for extracting lignin with high phenolic hydroxyl content according to claim 1, characterized in that, Specifically, the following steps are included: S1. After mixing plant fiber raw materials, buffer solution and compound enzyme, the mixture is subjected to biological enzyme pretreatment, and the supernatant and solid residue are obtained by solid-liquid separation. S2. Wash the solid residue until it is neutral and dry it, then add it to a catalytic eutectic solvent and heat it to react. S3. The reaction mixture obtained in S2 is subjected to precipitation and purification treatment to obtain the lignin with high phenolic hydroxyl content.

3. The method for extracting lignin with high phenolic hydroxyl content according to claim 2, characterized in that, In step S1, the solid-liquid ratio of the plant fiber raw material and the buffer solution is 1g:15mL; the complex enzyme contains cellulase and hemicellulase, the mass ratio of cellulase and hemicellulase is 1:1, and the total enzyme activity of the complex enzyme is 15-30 FPU / g based on cellulase activity.

4. The method for extracting lignin with high phenolic hydroxyl content according to claim 2, characterized in that, In step S1, the temperature of the bio-enzyme pretreatment is 45-50℃, and the time is 48-72 hours.

5. The method for extracting lignin with high phenolic hydroxyl content according to claim 2, characterized in that, In step S2, the preparation method of the catalytic eutectic solvent includes the following steps: mixing hydrogen bond acceptor and hydrogen bond donor in a molar ratio of 1:(3-8) to obtain a eutectic solvent, adding a catalyst accounting for 1-3% of the total mass of the eutectic solvent, heating and stirring to obtain the catalytic eutectic solvent.

6. The method for extracting lignin with high phenolic hydroxyl content according to claim 5, characterized in that, The hydrogen bond acceptor is selected from choline chloride, and the hydrogen bond donor is selected from lactic acid, ethylene glycol, or glycerol; the catalyst is selected from copper chloride, aluminum chloride, or ferric chloride.

7. The method for extracting lignin with high phenolic hydroxyl content according to claim 6, characterized in that, The hydrogen bond donor is selected from lactic acid; the catalyst is selected from copper chloride.

8. The method for extracting lignin with high phenolic hydroxyl content according to claim 2, characterized in that, In step S2, the dried solid residue is mixed with the catalytic eutectic solvent at a solid-liquid ratio of 1g:20mL; the heating reaction is carried out at a temperature of 120-140℃ for 2-4 hours.

9. The method for extracting lignin with high phenolic hydroxyl content according to claim 2, characterized in that, In step S3, the specific operation of the precipitation and purification process is as follows: add water to dilute the reaction mixture obtained in S2, then add a precipitant, collect the precipitate, and wash and dry the precipitate to obtain the lignin with high phenolic hydroxyl content.

10. The method for extracting lignin with high phenolic hydroxyl content according to claim 9, characterized in that, The precipitant is selected from ethanol, acetone, methanol, ethanol-water mixed solvent, acetone-water mixed solvent, or methanol-water mixed solvent.

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