Extraction and purification method and application of high-purity and high-activity lignin

Abstract

The invention provides an extraction and purification method and application of high-purity and high-activity lignin, and belongs to the technical field of lignin extraction. According to the method provided by the invention, the high-purity lignin can be efficiently and mildly separated from the wood fiber raw material, and the lignin product is low in impurity content, complete in active group retention and excellent in dispersity and reaction activity. The obtained lignin can be directly used as a high-performance additive or a reaction monomer for preparing epoxy resin, phenolic resin and other composite materials, the thermal mechanical performance, mechanical strength and thermal stability of the materials can be remarkably improved, the application prospect of the lignin in the field of high-added-value materials is expanded, meanwhile, the whole process conditions are mild, the solvent is green and recyclable, and the method is suitable for industrial production. And the method has good environmental benefits and industrialization potential.

Description

Technical Field

[0001] This invention relates to the field of lignin extraction technology, and in particular to a method for extracting and purifying high-purity, high-activity lignin and its application. Background Technology

[0002] Lignin is the second most abundant natural polymer after cellulose, widely found in plant cell walls, and together with cellulose and hemicellulose, it forms the basic framework of lignocellulose raw materials. It is a complex amorphous three-dimensional network polymer composed of phenylpropane units linked by ether bonds and carbon-carbon bonds, containing abundant active functional groups such as phenolic hydroxyl groups, alcoholic hydroxyl groups, and methoxy groups.

[0003] However, lignin is tightly cross-linked with carbohydrates in its natural state, making separation difficult. Currently, the main industrial sources of lignin are byproducts of pulp and paper production, such as Kraft lignin and sulfonated lignin. These traditional methods typically employ harsh conditions such as strong alkalis, strong acids, or high temperatures and pressures, leading to severe condensation, degradation, and alterations in the chemical structure of lignin during separation. This results in a wide molecular weight distribution, high impurity content (such as sugars and ash), and significant loss of active groups. The resulting lignin products exhibit low chemical reactivity, poor uniformity, and dark color, often limiting their use to inexpensive fuels and failing to meet the stringent requirements for raw material purity, structural integrity, and reactivity in the synthesis of high-performance materials. Although some research has focused on developing gentler separation techniques, such as organic solvent methods, ionic liquid methods, and enzymatic methods, achieving a good balance between separation efficiency, product purity, structural protection, and process cost and complexity often limits their practical application.

[0004] Therefore, developing a separation and purification technology that can efficiently, cleanly, and at low cost obtain high-purity, highly reactive lignin is key to realizing the high-value utilization of lignin resources. Summary of the Invention

[0005] The purpose of this invention is to provide a method for extracting and purifying high-purity, high-activity lignin and its application, which solves the problems of low product purity, severe structural damage, complex processes, and environmental unfriendliness in existing lignin extraction methods, making them difficult to use in the field of high-performance materials.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing high-purity lignin, comprising the following steps: S1. Pretreatment of wood fiber raw materials using ethanol aqueous solution; S2. Enzymatic hydrolysis of the pretreated solid was performed using a complex enzyme system containing ferulic acid esterase, xylanase, cellulase and glucose oxidase. S3. The enzymatically hydrolyzed slurry is subjected to solid-liquid separation to obtain a supernatant containing lignin; S4. Polyethylene glycol and sodium citrate are added to the supernatant to form a mixed system. The pH value of the mixed system is adjusted to separate the phases and the lignin-rich upper phase is collected. This upper phase is then mixed with a blank lower phase solution and the pH value is adjusted for back-extraction. Finally, the purified lignin upper phase is obtained. S5. The purified lignin upper phase is subjected to acid precipitation treatment, the precipitate is collected and dried to obtain high-purity lignin.

[0008] Preferably, in step S1, the volume concentration of the ethanol aqueous solution is 30% to 50%, the pretreatment temperature is 85°C to 100°C, the pretreatment time is 45 minutes to 90 minutes, and the liquid-solid ratio of the ethanol aqueous solution to the wood fiber raw material is 10:1 to 20:1 mL / g.

[0009] Preferably, in step S2, the amount of the complex enzyme system added is based on the following per gram of pretreated solid: 5 to 15 U ferulic acid esterase, 300 to 800 U xylanase, 3 to 8 FPU cellulase, and 30 to 80 U glucose oxidase. The enzymatic hydrolysis treatment is performed at a pH of 4.8 to 5.2, a temperature of 48°C to 55°C, and a reaction time of 12 to 24 hours.

[0010] Preferably, in step S2, the concentration of hydrogen peroxide in the reaction system is monitored and maintained at 0.1 to 1.0 mM during the enzymatic hydrolysis process; In step S2, after the enzymatic hydrolysis is completed, the slurry is further heated to 75°C to 85°C and maintained for 5 to 15 minutes to inactivate the enzyme.

[0011] Preferably, in step S4, the polyethylene glycol is PEG4000, and its addition amount is 0.3 to 0.5 times the volume of the supernatant; the addition amount of sodium citrate is 0.2 to 0.4 times the volume of the supernatant. In step S4, the mixed system is first separated by adjusting the pH to 4.4 to 4.6 to obtain the first upper phase; in the back-extraction step, the first upper phase is mixed with the blank lower phase solution, and the pH is adjusted to 4.2 to 4.4 for a second phase separation. The volume of the blank lower phase solution is 40% to 60% of the volume of the first upper phase.

[0012] Preferably, in step S5, 1 to 5 times the volume of water is added to the purified lignin upper phase for dilution, and then the pH is adjusted to 2.0 to 3.0, and the mixture is allowed to stand for 1 to 4 hours to precipitate the lignin.

[0013] The present invention also provides a high-purity lignin, which is prepared by the above-described preparation method.

[0014] This invention also provides the application of high-purity lignin in the preparation of polymer material modifiers, surfactants, dispersants or carbon material precursors.

[0015] The present invention also provides an epoxy resin composite material comprising the above-mentioned high-purity lignin and epoxy resin.

[0016] Preferably, the high-purity lignin in the composite material is filled in an amount of 1% to 10% of the mass of the epoxy resin prepolymer; The preparation method of the epoxy resin composite material includes: dispersing the high-purity lignin in a solvent and mixing it with the epoxy resin prepolymer, removing the solvent, adding a curing agent, and curing.

[0017] The technical effects and advantages of this invention are as follows: The method provided by this invention can efficiently and gently separate high-purity lignin from wood fiber raw materials. The resulting lignin product has low impurity content, retains intact active groups, and exhibits excellent dispersibility and reactivity. The obtained lignin can be directly used as a high-performance additive or reactive monomer in the preparation of composite materials such as epoxy resins and phenolic resins, significantly improving the thermomechanical properties, mechanical strength, and thermal stability of the materials. This expands the application prospects of lignin in high-value-added materials. Furthermore, the entire process is mild, and the solvent is green and recyclable, demonstrating good environmental benefits and industrialization potential. Attached Figure Description

[0018] Figure 1 Photograph of the high-purity lignin sample obtained by this invention; Figure 2 The results are from infrared spectroscopy of lignin. Detailed Implementation

[0019] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0020] Example 1 This embodiment provides a precise lignin separation scheme: Experimental materials Poplar wood powder (40-60 mesh).

[0021] Chemicals: Ethanol, polyethylene glycol (PEG4000), sodium citrate, citric acid, sodium hydroxide, hydrochloric acid, pH buffer (acetic acid-sodium acetate).

[0022] Enzyme preparations: ferulic acid esterase (FAE), xylanase (XYN), cellulase (CEL, such as filter paper enzyme FPase), glucose oxidase (GOx).

[0023] The steps are as follows: Step 1: Gentle Pretreatment Weigh 10.0 g of oven-dried wood powder and place it in a pressure-resistant bottle. Add 150 mL of 40% (v / v) ethanol aqueous solution (liquid-solid ratio 15:1), and place the bottle in a 90°C water bath. Stir gently for 60 minutes. After the reaction is complete, filter under vacuum, wash the solid residue with 50°C deionized water until neutral, and drain.

[0024] Step 2: Complex enzyme treatment 1. Transfer the pretreated solid to an Erlenmeyer flask and prepare a 10% (w / v) suspension using 0.1M pH 5.0 acetate-sodium acetate buffer.

[0025] 2. Preheat the suspension in a 50°C constant temperature water bath shaker for 10 minutes.

[0026] 3. Add the compound enzyme solution simultaneously in the following proportions (based on gram of pretreated solid): ferulic acid esterase (FAE): 10 U / g; xylanase (XYN): 500 U / g; cellulase (CEL): 5 FPU / g; glucose oxidase (GOx): 50 U / g (starting amount).

[0027] 4. React at 50°C and pH 5.0 with gentle shaking (150 rpm) for 18 hours.

[0028] 5. At the 2nd, 6th and 12th hour of the reaction, the H2O2 concentration in the supernatant was detected using H2O2 test strips, with the target being to maintain it at 0.2-0.8 mM.

[0029] 6. After the reaction is complete, immediately heat the slurry to 80°C and maintain it for 10 minutes to completely inactivate all enzymes.

[0030] Step 3: Solid-liquid separation 1. Transfer the inactivated slurry to a centrifuge tube and centrifuge at 8000 rpm for 15 minutes.

[0031] 2. Carefully collect the supernatant; this supernatant contains activated lignin, soluble sugars, enzymes, proteins, etc.

[0032] 3. Solid residue (mainly cellulose) can be collected separately. All subsequent operations should be performed on the supernatant.

[0033] Step 4: pH-responsive aqueous two-phase extraction and back-extraction purification 1. Construct the phase, accurately measuring the volume of the supernatant obtained in step 3 (denoted as V mL). Construct the system according to the following proportions: supernatant: V mL; 50% (w / w) PEG4000 solution: 0.4V mL; 30% (w / w) sodium citrate solution: 0.3V mL 2. Slight oscillations completely mix the system, forming a homogeneous phase.

[0034] 3. Slowly adjust the pH of the mixture to 4.5 using 1M dilute citric acid solution. The system will then rapidly separate into phases. Allow it to stand to allow the phase separation to become clear.

[0035] 4. Separation yielded: Upper phase (U1): rich in PEG and target activated lignin; Lower phase (L1): rich in citrate, most pigments, oligosaccharides, salts, and residual enzymes and proteins, etc. 5. Purification: Take all the upper phase (U1) and prepare a fresh blank lower phase solution (L2) with the same composition as L1, with a volume of 50% of U1. Remix U1 and L2 in a clean container, adjust the pH of this new mixture to 4.3 with dilute acid, shake briefly for 10 seconds, and centrifuge at low speed to ensure complete phase separation. The final upper phase (U2) is obtained, containing high-purity lignin, and the waste lower phase (L2') contains a small amount of hydrophilic impurities eluted from U1.

[0036] Step 5: Product Recovery and Solvent Recycling Add 3 times the volume of deionized water to the final upper phase (U2) and adjust the pH to 2.5 with 1M HCl. Let it stand at room temperature for at least 2 hours to allow lignin to precipitate completely. Collect the precipitate by centrifugation. Wash the precipitate twice with a small amount of acidic water (pH 3), and then wash it once with deionized water. Dry the washed precipitate under vacuum at 40°C to constant weight to obtain high-purity lignin powder. (See photo below.) Figure 1 As shown.

[0037] After all the lower phases are combined, PEG and citrate can be recovered and recycled through crystallization, ultrafiltration or dialysis.

[0038] Example 2 This embodiment provides a precise lignin separation scheme: Experimental materials Poplar wood powder (40-60 mesh).

[0039] Chemicals: Ethanol, polyethylene glycol (PEG4000), sodium citrate, citric acid, sodium hydroxide, hydrochloric acid, pH buffer (citric acid-sodium citrate).

[0040] Enzyme preparations: ferulic acid esterase (FAE), xylanase (XYN), cellulase (CEL, such as filter paper enzyme FPase), glucose oxidase (GOx).

[0041] The steps are as follows: Step 1: Gentle Pretreatment Weigh 10.0 g of oven-dried wood powder and place it in a pressure-resistant bottle. Add 100 mL of 50% (v / v) ethanol aqueous solution (liquid-solid ratio 10:1), and place the bottle in a 100°C water bath. Stir gently for 45 minutes. After the reaction is complete, filter under vacuum, wash the solid residue with 50°C deionized water until neutral, and drain.

[0042] Step 2: Complex enzyme treatment 1. Transfer the pretreated solid to an Erlenmeyer flask and prepare a 12% (w / v) suspension using 0.1M pH 5.0 acetate-sodium acetate buffer.

[0043] 2. Preheat the suspension in a 55°C constant temperature water bath shaker for 10 minutes.

[0044] 3. Add the compound enzyme solution simultaneously according to the following proportions (based on each gram of pretreated solid): ferulic acid esterase (FAE): 15 U / g; xylanase (XYN): 800 U / g; cellulase (CEL): 8 FPU / g; glucose oxidase (GOx): 80 U / g (starting amount).

[0045] 4. React at 55°C and pH 5.0 with gentle shaking (150 rpm) for 12 hours.

[0046] 5. At the 2nd, 4th and 8th hour of the reaction, the concentration of H2O2 in the supernatant was detected using H2O2 test strips, with the target being to maintain it at 0.2-0.8 mM.

[0047] 6. After the reaction is complete, immediately heat the slurry to 85°C and maintain it for 10 minutes to completely inactivate all enzymes.

[0048] Step 3: Solid-liquid separation 1. Transfer the inactivated slurry to a centrifuge tube and centrifuge at 10,000 rpm for 10 minutes.

[0049] 2. Carefully collect the supernatant; this supernatant contains activated lignin, soluble sugars, enzymes, proteins, etc.

[0050] 3. Solid residue (mainly cellulose) can be collected separately. All subsequent operations should be performed on the supernatant.

[0051] Step 4: pH-responsive aqueous two-phase extraction and back-extraction purification 1. Construct the phase, accurately measuring the volume of the supernatant obtained in step 3 (denoted as V mL). Construct the system according to the following proportions: supernatant: V mL; 50% (w / w) PEG4000 solution: 0.35V mL; 30% (w / w) sodium citrate solution: 0.35V mL 2. Slight oscillations completely mix the system, forming a homogeneous phase.

[0052] 3. Slowly adjust the pH of the mixture to 4.5 using 1M dilute citric acid solution. The system will then rapidly separate into phases. Centrifuge at low speed to ensure clear phase separation.

[0053] 4. The upper phase (U1) is obtained by separation: rich in PEG and target activated lignin; the lower phase (L1) is rich in citrate, most pigments, oligosaccharides, salts and residual enzymes and proteins, etc. 5. Purification: Take all the upper phase (U1) and prepare a fresh blank lower phase solution (L2) with the same composition as L1, with a volume of 40% of U1. Remix U1 and L2 in a clean container, finely adjust the pH of this new mixture to 4.3, briefly shake to mix for 10 seconds, and centrifuge at low speed to allow for complete phase separation. The final upper phase (U2) is obtained, containing high-purity lignin, and the waste lower phase (L2') contains a small amount of hydrophilic impurities eluted from U1.

[0054] Step 5: Product Recovery and Solvent Recycling Add twice the volume of deionized water to the final upper phase (U2) for dilution, and adjust the pH to 2.5 with 1M HCl. Let it stand at room temperature for at least 1.5 hours to allow lignin to fully precipitate. Collect the precipitate by centrifugation. Wash the precipitate twice with a small amount of acidic water (pH 3), and then wash it once with deionized water. Freeze-dry the washed precipitate to constant weight to obtain high-purity lignin powder.

[0055] After all the lower phases are combined, PEG and citrate can be recovered and recycled through crystallization, ultrafiltration or dialysis.

[0056] Example 3 This embodiment provides a precise lignin separation scheme: Experimental materials Poplar wood powder (40-60 mesh), or other wood fiber raw materials.

[0057] Chemicals: Ethanol, polyethylene glycol (PEG4000), sodium citrate, citric acid, sodium hydroxide, hydrochloric acid, pH buffer (acetic acid-sodium acetate).

[0058] Enzyme preparations: ferulic acid esterase (FAE), xylanase (XYN), cellulase (CEL, such as filter paper enzyme FPase), glucose oxidase (GOx).

[0059] The steps are as follows: Step 1: Gentle Pretreatment Weigh 10.0 g of oven-dried wood powder and place it in a pressure-resistant bottle. Add 200 mL of 30% (v / v) ethanol aqueous solution (liquid-solid ratio 20:1), and place the bottle in an 85°C water bath. Stir gently for 90 minutes. After the reaction is complete, filter under vacuum, wash the solid residue with 50°C deionized water until neutral, and drain.

[0060] Step 2: Complex enzyme treatment 1. Transfer the pretreated solid to an Erlenmeyer flask and prepare an 8% (w / v) suspension using 0.1M pH 5.0 acetate-sodium acetate buffer.

[0061] 2. Preheat the suspension in a 48°C constant temperature water bath shaker for 10 minutes.

[0062] 3. Add the compound enzyme solution simultaneously according to the following proportions (based on each gram of pretreated solid): ferulic acid esterase (FAE): 5 U / g; xylanase (XYN): 300 U / g; cellulase (CEL): 3 FPU / g; glucose oxidase (GOx): 30 U / g (starting amount).

[0063] 4. React at 48°C and pH 5.0 with gentle shaking (150 rpm) for 24 hours.

[0064] 5. At the 3rd, 8th and 18th hours of the reaction, the concentration of H2O2 in the supernatant was detected using H2O2 test strips, with the target being to maintain it at 0.2-0.8 mM.

[0065] 6. After the reaction is complete, immediately heat the slurry to 75°C and maintain it for 10 minutes to completely inactivate all enzymes.

[0066] Step 3: Solid-liquid separation 1. Transfer the inactivated slurry to a centrifuge tube and centrifuge at 6000 rpm for 20 minutes.

[0067] 2. Carefully collect the supernatant; this supernatant contains activated lignin, soluble sugars, enzymes, proteins, etc.

[0068] 3. Solid residue (mainly cellulose) can be collected separately. All subsequent operations should be performed on the supernatant.

[0069] Step 4: pH-responsive aqueous two-phase extraction and back-extraction purification 1. Construct the phase, accurately measuring the volume of the supernatant obtained in step 3 (denoted as V mL). Construct the system according to the following proportions: supernatant: V mL; 50% (w / w) PEG4000 solution: 0.45 V mL; 30% (w / w) sodium citrate solution: 0.25 V mL 2. Slight oscillations completely mix the system, forming a homogeneous phase.

[0070] 3. Slowly adjust the pH of the mixture to 4.5 using 1M dilute citric acid solution. The system will then rapidly separate into phases. Allow it to stand to allow the phase separation to become clear.

[0071] 4. Separation yielded: Upper phase (U1): Rich in PEG and target activated lignin; Lower phase (L1): Rich in citrate, most pigments, oligosaccharides, salts, and residual enzyme proteins and other impurities. 5. Purification: Take all the upper phase (U1) and prepare a fresh blank lower phase solution (L2) with the same composition as L1, with a volume of 60% of U1. Remix U1 and L2 in a clean container, adjust the pH of this new mixture to 4.3 with dilute acid, shake briefly for 10 seconds, and centrifuge at low speed to ensure complete phase separation. The final upper phase (U2) is obtained, containing high-purity lignin, and the waste lower phase (L2') contains a small amount of hydrophilic impurities eluted from U1.

[0072] Step 5: Product Recovery and Solvent Recycling Add 4 times the volume of deionized water to the final upper phase (U2) for dilution, and adjust the pH to 2.5 with 1M HCl. Let it stand at room temperature for more than 3 hours to allow lignin to fully precipitate, and collect the precipitate by centrifugation. Wash the precipitate twice with a small amount of acidic water (pH 3), and then wash it once with deionized water. Dry the washed precipitate under vacuum at 40°C to constant weight to obtain high-purity lignin powder.

[0073] After all the lower phases are combined, PEG and citrate can be recovered and recycled through crystallization, ultrafiltration or dialysis.

[0074] Experimental Example Lignin sample to be tested: Experimental group (Exp-Lig): Lignin powder prepared strictly according to the mild pretreatment, compound enzyme treatment and pH-responsive aqueous two-phase extraction-back-extraction process described in Example 1.

[0075] Control group 1 (Ctrl-Lig-A): The process was basically the same as in "Example 1", but in step two (compound enzyme treatment), glucose oxidase (GOx) was replaced with an equal volume of pH 5.0 buffer, that is, the endogenous H2O2 in situ generation step was cancelled, and other conditions remained unchanged to prepare lignin powder (the lignin powder in this sample is only a pseudonym and may contain more impurities).

[0076] Control group 2 (Ctrl-Lig-B): Conventional industrial alkali lignin (in this experiment, the sample was derived from the sulfate pulping process) was used as a representative of conventional process lignin.

[0077] (I) Characterization of basic properties of lignin products 1. Determination of acid-insoluble lignin content (purity) This experiment uses the content of acid-insoluble lignin as a direct characterization of the purity of lignin products. Approximately 0.1 g (accurate to 0.0001 g) of dried lignin samples were accurately weighed into clean, acid-resistant conical flasks. 15 mL of a 72% sulfuric acid solution was slowly added, and the samples were continuously mechanically stirred in a 15-20°C water bath for 2 hours to ensure complete sulfonation. Subsequently, the mixture was diluted with deionized water to a sulfuric acid concentration of 3%, transferred to a heat-resistant glass bottle, and hydrolyzed in an autoclave at 121°C for 1 hour. After hydrolysis, the mixture was cooled to room temperature and vacuum filtered using a pre-weighed sintered sand crucible (G3). The residue was repeatedly washed with hot water until the filtrate was neutral and free of sulfate ions (tested with barium chloride solution). The crucible and residue were dried in a 105°C oven to constant weight, cooled in a desiccator, and then accurately weighed. The acid-insoluble lignin content was calculated using the formula (mass of acid-insoluble residue / initial sample mass) × 100%. Each sample was measured in triplicate, and the average value was taken.

[0078] 2. Ash content determination Weigh approximately 1.00 g (accurate to 0.0001 g) of each lignin sample into a pre-weighed porcelain crucible. Place the crucible in a muffle furnace and heat according to the following procedure: increase the temperature from room temperature to 105 °C at a rate of 10 °C / min and hold for 1 hour; then continue to increase the temperature at the same rate to 575 ± 25 °C and calcine at this temperature for 4 hours until constant weight. After calcination, remove the crucible after the furnace temperature drops below 150 °C, immediately transfer it to a desiccator to cool to room temperature, and weigh accurately. The ash content (%) is calculated using the formula (mass of residue after calcination / initial mass of sample) × 100%. Each sample is measured in triplicate.

[0079] 3. Determination of carboxyl and phenolic hydroxyl content (potentiometric titration method) A non-aqueous potentiometric titration method was used. 50.00 mg of dried sample was accurately weighed and dissolved in 10 mL of a dioxane / water (9:1, v / v) mixed solvent. Under magnetic stirring and nitrogen protection, titration was performed using 0.05 mol / L sodium hydroxide standard solution at a rate of 0.05 mL / min, and the pH-volume curve was recorded. By analyzing the inflection point of the titration curve, the base consumption corresponding to the carboxyl group and the total acidic groups (carboxyl + phenolic hydroxyl) was calculated. The phenolic hydroxyl content was calculated from the difference. Each sample was titrated independently twice.

[0080] 4. Surface charge assessment (Zeta potential measurement) Aqueous dispersions of lignin samples with a concentration of 0.1 mg / mL were prepared and thoroughly dispersed using an ultrasonic cell disruptor (200 W, 2-second operation, 2-second interval, ice-water bath, 10 minutes) before being transferred to the sample cell. Zeta potential values ​​were measured at 25 °C using a Zeta potential analyzer, with five consecutive measurements taken for each sample and the average value recorded.

[0081] 5. Commissioned chemical structure analysis The following tests were commissioned to a third-party organization: 1) Fourier transform infrared spectroscopy analysis (see results in the image below). Figure 2 ); 2) Quantitative 31 P-NMR spectroscopy analysis accurately determines the content of carboxyl groups, various phenolic hydroxyl groups, and aliphatic hydroxyl groups (unit: mmol / g) to assess the retention of β-O-4 and other linkage bonds.

[0082] (II) Preparation and performance testing of lignin / epoxy resin composite materials 1. Preparation of composite materials A composite material with a lignin filler content of 5% by mass of epoxy resin prepolymer was prepared. Lignin was dispersed in acetone and sonicated for 30 minutes. It was then mixed with the epoxy resin prepolymer (bisphenol A type) at 60°C and stirred for 2 hours, followed by solvent removal. After cooling, polyetheramine D230 curing agent was added stoichiometrically, stirred until homogeneous, and then vacuum degassed before being poured into a mold. The mixture was cured using a program of 80°C / 2h + 120°C / 2h. Standard test strips were then fabricated. A pure epoxy resin control group was also prepared simultaneously.

[0083] 2. Composite material performance testing Thermomechanical property analysis (DMTA): A dynamic thermomechanical analyzer was used in three-point bending mode, with a temperature range of 30-200°C, a heating rate of 3°C / min, and a frequency of 1 Hz. The peak value of the loss factor was taken as the glass transition temperature (Tg). Three parallel samples were tested in each group.

[0084] Tensile property testing: Use a universal testing machine to test dumbbell-shaped specimens at a rate of 2 mm / min, and calculate the tensile strength and elastic modulus. Each test should consist of no fewer than 5 specimens.

[0085] Thermal stability analysis (TGA): Take 5-10 mg of sample and heat it from 50°C to 800°C at 10°C / min under a nitrogen flow (50 mL / min). Record the TG / DTG curve and take the temperature at which the weight loss is 5% as the initial decomposition temperature (Td5%).

[0086] The experimental results are shown in Tables 1 and 2 below: Table 1 Characterization data of lignin products

[0087] Table 2 Composite Material Performance Data

[0088] Experimental results show that the lignin (Exp-Lig) prepared using the complete process of this invention exhibits excellent performance in several key indicators: it has the highest acid-insoluble lignin content (>98%), extremely low ash content (approximately 0.2%), significantly increased carboxyl content, enhanced surface negative charge, and effectively preserved β-O-4 structure. This structural integrity is clearly reflected in the Fourier transform infrared spectrum of the product: at 2936 cm⁻¹... - The distinct aliphatic CH absorption peak at ¹ indicates that the side chain structure was preserved under mild processing; while at 3411 cm⁻¹ - ¹ Strong hydroxyl peak and 1513 cm⁻¹ - The typical characteristic peaks of the lignin benzene ring at point ¹ confirm that the process of this invention successfully avoids the destruction of the active structure of lignin while achieving a high extraction rate. Compared with the control group lacking glucose oxidase (GOx) and traditional industrial alkali lignin, this product has significant advantages in purity, ash control, and structural integrity, fully demonstrating the significant synergistic effect of the GOx-mediated in-situ oxidation and pH-responsive aqueous two-phase extraction technology used in this invention in the efficient extraction and structural protection of lignin.

[0089] As demonstrated by the above embodiments, this invention provides a highly efficient lignin separation technology integrating mild pretreatment, complex enzymatic hydrolysis, and selective extraction purification. This technology works synergistically to successfully achieve efficient dissolution, activation, and deep purification of lignin, yielding a high-purity lignin product with excellent structural integrity. When this product is used directly as a functional additive in polymer composites, it exhibits excellent reinforcing and modifying effects, verifying the feasibility and significant advantages of this technology in obtaining high-performance bio-based material raw materials, and demonstrating broad application prospects.

[0090] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity lignin, characterized in that, Includes the following steps: S1. Pretreatment of wood fiber raw materials using ethanol aqueous solution; S2. Enzymatic hydrolysis of the pretreated solid was performed using a complex enzyme system containing ferulic acid esterase, xylanase, cellulase and glucose oxidase. S3. The enzymatically hydrolyzed slurry is subjected to solid-liquid separation to obtain a supernatant containing lignin; S4. Polyethylene glycol and sodium citrate are added to the supernatant to form a mixed system. The pH value of the mixed system is adjusted to separate the phases and the lignin-rich upper phase is collected. This upper phase is then mixed with a blank lower phase solution and the pH value is adjusted for back-extraction. Finally, the purified lignin upper phase is obtained. S5. The purified lignin upper phase is subjected to acid precipitation treatment, the precipitate is collected and dried to obtain high-purity lignin.

2. The preparation method according to claim 1, characterized in that, In step S1, the volume concentration of the ethanol aqueous solution is 30% to 50%, the pretreatment temperature is 85°C to 100°C, the pretreatment time is 45 minutes to 90 minutes, and the liquid-solid ratio of the ethanol aqueous solution to the wood fiber raw material is 10:1 to 20:1 mL / g.

3. The preparation method according to claim 1, characterized in that, In step S2, the amount of the complex enzyme system added per gram of pretreated solid is: 5 to 15 U ferulic acid esterase, 300 to 800 U xylanase, 3 to 8 FPU cellulase, and 30 to 80 U glucose oxidase. The enzymatic hydrolysis treatment is performed at a pH of 4.8 to 5.2, a temperature of 48°C to 55°C, and a reaction time of 12 to 24 hours.

4. The preparation method according to claim 3, characterized in that, In step S2, the concentration of hydrogen peroxide in the reaction system is monitored and maintained at 0.1 to 1.0 mM during the enzymatic hydrolysis process; In step S2, after the enzymatic hydrolysis is completed, the slurry is further heated to 75°C to 85°C and maintained for 5 to 15 minutes to inactivate the enzyme.

5. The preparation method according to claim 1, characterized in that, In step S4, the polyethylene glycol is PEG4000, and its addition amount is 0.3 to 0.5 times the volume of the supernatant; the addition amount of sodium citrate is 0.2 to 0.4 times the volume of the supernatant. In step S4, the mixed system is first separated by adjusting the pH to 4.4 to 4.6 to obtain the first upper phase; in the back-extraction step, the first upper phase is mixed with the blank lower phase solution, and the pH is adjusted to 4.2 to 4.4 for a second phase separation. The volume of the blank lower phase solution is 40% to 60% of the volume of the first upper phase.

6. The preparation method according to claim 1, characterized in that, In step S5, 1 to 5 times the volume of water is added to the purified lignin upper phase for dilution, and then the pH is adjusted to 2.0 to 3.

0. The mixture is then allowed to stand for 1 to 4 hours to precipitate the lignin.

7. A high-purity lignin, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 6.

8. The use of the high-purity lignin according to claim 7 in the preparation of polymer material modifiers, surfactants, dispersants or carbon material precursors.

9. An epoxy resin composite material, characterized in that, It comprises the high-purity lignin and epoxy resin as described in claim 7.

10. The epoxy resin composite material according to claim 9, characterized in that, The high-purity lignin in the composite material is filled at a rate of 1% to 10% of the mass of the epoxy resin prepolymer. The preparation method of the epoxy resin composite material includes: dispersing the high-purity lignin in a solvent and mixing it with the epoxy resin prepolymer, removing the solvent, adding a curing agent, and curing.

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