Method for efficiently purifying lignin by using composite alkali system

Abstract

This invention provides a method for efficiently purifying lignin using a composite alkali system, belonging to the field of lignin extraction and purification technology. Using lignocellulosic biomass as raw material, this invention employs a KOH / thiourea composite alkali solution at a specific mass ratio, conducting a directional dissociation reaction under a solid-liquid ratio of 1:5-15, a temperature of 60-150℃, and inert gas protection. After the reaction is terminated, a gradient acid precipitation process (staged control of pH 10-12 and pH 3-5), combined with centrifugal filtration and water washing, significantly reduces ash and carbohydrate residues, obtaining high-quality lignin with a purity ≥91.16%. This method solves the problems of low purity and high impurity residues in traditional processes, providing a core raw material guarantee for lignin-based capacitor carbon.

Description

Technical Field

[0001] This invention relates to the field of lignin extraction and purification technology, specifically to a method for efficiently purifying lignin using a composite alkali system. Background Technology

[0002] Lignin is the second largest renewable organic resource in the plant kingdom after cellulose, accounting for 15%–30% of the dry weight of plant cell walls. It is mainly composed of three phenylpropane units (p-hydroxyphenyl H, guaiacol G, and syringyl S) cross-linked through ether bonds and carbon-carbon bonds to form a three-dimensional network structure. As an important biomass resource, lignin has broad application prospects in energy, materials, and chemical industries, and its high-value utilization is of great significance. Currently, industrial lignin mainly comes from papermaking black liquor and bioethanol residues, but traditional extraction methods (such as the sulfate method and organic solvent method) have the following problems.

[0003] (1) Low purity: Lignin co-precipitates with impurities such as hemicellulose and protein, resulting in ash (>8%) and carbohydrate residue (>5%), which affects subsequent high-value utilization.

[0004] (2) Structural damage: Strong acid / strong alkali or high temperature treatment can easily lead to the breakage of β-O-4 bonds in lignin and oxidation of phenolic hydroxyl groups, reducing its reactivity.

[0005] (3) Complex process: Some existing preparation methods (such as ionic liquid extraction) are costly and difficult to scale up, which limits industrial application.

[0006] Therefore, there is an urgent need to develop an efficient and simple method for lignin purification. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a method for efficiently purifying lignin using a composite alkali system. This method solves the problems of low purity and high impurity residue in traditional processes, and provides a core raw material guarantee for lignin-based capacitor carbon.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0009] A method for efficiently purifying lignin using a composite alkali system, comprising the following steps.

[0010] (1) Mixing: The wood fiber biomass and the composite alkali solution are mixed at a solid-liquid ratio of 1:5-15. The composite alkali solution contains KOH and thiourea, and the concentration of the composite alkali solution is 1-5 wt%.

[0011] (2) Purification reaction: Under the protection of inert gas, the mixture obtained in step (1) is placed in a reaction vessel, and after the temperature is raised from room temperature to 60-150℃, the reaction is kept at the temperature for 1-3 h, while stirring at a rate of 800-1000 rpm.

[0012] (3) Reaction termination: After the heat preservation reaction is completed, inert gas is introduced at 30-50 mL / min to terminate the purification reaction, followed by centrifugation and filtration to obtain lignin alkaline solution; the inert gas includes nitrogen, helium, neon, argon, xenon, etc.

[0013] (4) Gradient acid precipitation: Add acid precipitation solution to the lignin alkaline solution in stages for gradient acid precipitation. After each addition of 80-100 mL of acid precipitation solution, let stand for 6-8 h, and then centrifuge to collect the precipitate.

[0014] (5) Washing the precipitate: Wash the precipitate with sufficient deionized water until neutral, and dry to obtain high-purity lignin.

[0015] Furthermore, the lignocellulosic biomass is sugarcane bagasse.

[0016] Furthermore, the centrifugal filtration step (3) is as follows: centrifuge at 5000 rpm for 10 min in a high-speed centrifuge, and after centrifugation, take the supernatant and filter it through 0.45 µm and 0.22 µm aqueous filter membranes in sequence.

[0017] Furthermore, the composite alkaline solution is a KOH / thiourea system, wherein the mass ratio of KOH to thiourea is 2:1.

[0018] Furthermore, the lignocellulosic biomass and the composite alkaline solution have a solid-liquid ratio of 1:8, the purification reaction temperature is 120℃, and the holding time is 2.5 h.

[0019] Furthermore, the acid precipitation solution is selected from one of acetic acid, hydrochloric acid, and citric acid.

[0020] Furthermore, the gradient acid precipitation includes: (a) adding acid precipitation solution to a pH value of 10-12 in the first stage to precipitate protein and hemicellulose residues; (b) adding acid precipitation solution to a pH value of 3-5 in the second stage to precipitate high-purity lignin; wherein the pH range is monitored in real time by an online pH meter.

[0021] The present invention also provides lignin prepared according to the above method, wherein the lignin is used in the fields of capacitive carbon materials or heavy metal adsorbents.

[0022] The present invention also provides the application of lignin prepared by the above method in the preparation of capacitive carbon materials or heavy metal adsorbents.

[0023] In summary, due to the adoption of the above technical solutions, the present invention has at least the following beneficial effects.

[0024] This invention provides a method for efficiently purifying lignin using a composite alkali system, thereby obtaining high-quality lignin products through the following synergistic approach.

[0025] (1) Synergistic dissociation mechanism of compound base for directional bond cleavage: This application firstly uses the OH⁻ of KOH ionization to precisely break the α-O-4 / β-O-4 ether bond and γ-ester bond between lignin and hemicellulose, thus disintegrating the rigid network of lignin-carbohydrate complex (LCC); in addition, the thiol group (-SH) of thiourea provides electron transfer protection for phenolic hydroxyl groups, and the amino group (-NH2) and C=S bond form a three-dimensional hydrogen bond cage with the lignin phenylpropane unit, enhancing the solubility of lignin, thereby achieving efficient separation.

[0026] (2) Dynamic protection with inert gas: Nitrogen / argon is introduced during the reaction termination stage to form a gas phase oxygen barrier: inhibiting the oxidation of phenolic hydroxyl groups to quinones (quinone content <0.5%); blocking free radical chain reaction to maintain lignin reactivity.

[0027] (3) Gradient acid precipitation targeted purification: The lignin alkaline solution is treated by gradient acid precipitation. The first stage (pH 10-12): impurities are precipitated above the isoelectric point of protein (pI=4~6), and negatively charged protein / hemicellulose is flocculated and precipitated by electrostatic repulsion effect. The second stage (pH 3-5): approaching the isoelectric point of lignin, triggering lignin π-π stacking precipitation, and avoiding low pH condensation side reaction.

[0028] (4) Structural compatibility of bagasse raw material: In addition, this application limits the selection of raw materials to bagasse. In this application, the natural structure of bagasse gives it purification advantages. First, the chemical composition of bagasse is high in G-type lignin (>80%): G units are rich in methoxy groups, which form a strong hydrogen bond network with thiourea, thus improving the dissolution efficiency; acetylated hemicellulose makes the acetyl groups easy to be hydrolyzed by KOH, weakening the LCC binding force and reducing the carbohydrate residue to 1.32% (corn cob reaches 3.17%). In addition, bagasse has a multi-level porous structure (porosity 65%), which can promote alkali penetration and shorten the reaction time. Its low silicon content can reduce ash at the source and avoid silicate coating of lignin.

[0029] In summary, this application overcomes the industry bottleneck of the incompatibility between lignin purity and structural integrity through a three-dimensional synergy of raw material structure adaptation (bagasse), reaction system innovation (KOH / thiourea), and precise control of purification process (gradient acid precipitation), providing an irreplaceable solution for the high-value utilization of biomass. Detailed Implementation

[0030] The following embodiments can help those skilled in the art to more fully understand the present invention, but should not be construed as limiting the present invention in any way.

[0031] Example 1

[0032] This embodiment provides a method for efficiently purifying lignin using a composite alkali system. The method for preparing lignin includes the following steps.

[0033] (1) Mixing: The wood fiber biomass and the composite alkali solution are mixed at a solid-liquid ratio of 1:5. The composite alkali solution contains KOH and thiourea in a mass ratio of 2:1 and the concentration of the composite alkali solution is 5wt%. The wood fiber biomass is sugarcane bagasse.

[0034] (2) Purification reaction: Under argon protection, the mixture obtained in step (1) was placed in a reaction vessel, and after the temperature was raised from room temperature to 150°C, the reaction was kept at the temperature for 1 hour while stirring at a rate of 800 rpm.

[0035] (3) Reaction termination: After the heat preservation reaction is completed, argon gas is introduced at 30 mL / min to terminate the purification reaction. Then, centrifugation and filtration are performed to obtain lignin alkaline solution. The centrifugation and filtration are performed by centrifuging at 5000 rpm for 10 min in a high-speed centrifuge. After centrifugation, the supernatant is filtered through 0.45 µm and 0.22 µm aqueous filter membranes in sequence.

[0036] (4) Gradient acid precipitation: Acid precipitation solution (5% hydrochloric acid precipitation solution) is added to the lignin alkaline solution in stages to carry out gradient acid precipitation. The gradient acid precipitation includes: (a) adding acid precipitation solution to the pH value of 10-12 in the first stage to precipitate protein and hemicellulose residues; (b) adding acid precipitation solution to the pH value of 3-5 in the second stage to precipitate high-purity lignin; wherein, the pH range is monitored in real time by an online pH meter; after each 100 mL of acid precipitation solution is added, the mixture is allowed to stand for 6 h, and the precipitate is collected by centrifugation.

[0037] (5) Washing the precipitate: Wash the precipitate with sufficient deionized water until neutral, and dry it at 60 ℃ for 24 h to obtain high-purity lignin.

[0038] Example 2

[0039] This embodiment provides a method for efficiently purifying lignin using a composite alkali system. The method for preparing lignin includes the following steps.

[0040] (1) Mixing: The wood fiber biomass and the composite alkali solution are mixed at a solid-liquid ratio of 1:8. The composite alkali solution contains KOH and thiourea in a mass ratio of 2:1 and the concentration of the composite alkali solution is 3 wt%. The wood fiber biomass is sugarcane bagasse.

[0041] (2) Purification reaction: Under nitrogen protection, the mixture obtained in step (1) was placed in a reaction vessel, and the temperature was raised from room temperature to 120°C and kept at that temperature for 2.5 h while stirring at a rate of 900 rpm.

[0042] (3) Reaction termination: After the heat preservation reaction is completed, nitrogen gas is introduced at 40 mL / min to terminate the purification reaction. Then, centrifugation and filtration are performed to obtain lignin alkaline solution. The centrifugation and filtration are performed by centrifuging at 5000 rpm for 10 min in a high-speed centrifuge. After centrifugation, the supernatant is taken and filtered through 0.45 µm and 0.22 µm aqueous filter membranes in sequence.

[0043] (4) Gradient acid precipitation: Acid precipitation solution (15% acetic acid precipitation solution) is added to the lignin alkaline solution in stages for gradient acid precipitation. The gradient acid precipitation includes: (a) adding acid precipitation solution to the pH value of 10-12 in the first stage to precipitate protein and hemicellulose residues; (b) adding acid precipitation solution to the pH value of 3-5 in the second stage to precipitate high-purity lignin. The pH range is monitored in real time by an online pH meter. After each addition of 90 mL of acid precipitation solution, the mixture is allowed to stand for 7 h and then centrifuged to collect the precipitate.

[0044] (5) Washing the precipitate: Wash the precipitate with sufficient deionized water until neutral, and dry it at 60°C for 24 h to obtain high-purity lignin.

[0045] Example 3

[0046] This embodiment provides a method for efficiently purifying lignin using a composite alkali system. The method for preparing lignin includes the following steps.

[0047] (1) Mixing: The wood fiber biomass and the composite alkali solution are mixed at a solid-liquid ratio of 1:15. The composite alkali solution contains KOH and thiourea in a mass ratio of 2:1 and the concentration of the composite alkali solution is 1wt%. The wood fiber biomass is sugarcane bagasse.

[0048] (2) Purification reaction: Under nitrogen protection, the mixture obtained in step (1) was placed in a reaction vessel, and after the temperature was raised from room temperature to 60°C, the reaction was kept at the temperature for 3 h while stirring at a rate of 1000 rpm.

[0049] (3) Reaction termination: After the heat preservation reaction is completed, nitrogen gas is introduced at 50 mL / min to terminate the purification reaction. Then, centrifugation and filtration are performed to obtain lignin alkaline solution. The centrifugation and filtration are performed by centrifuging at 5000 rpm for 10 min in a high-speed centrifuge. After centrifugation, the supernatant is taken and filtered through 0.45 µm and 0.22 µm aqueous filter membranes in sequence.

[0050] (4) Gradient acid precipitation: Acid precipitation solution (8% citric acid precipitation solution) is added to the lignin alkaline solution in stages for gradient acid precipitation. The gradient acid precipitation includes: (a) adding acid precipitation solution to the pH value of 10-12 in the first stage to precipitate protein and hemicellulose residues; (b) adding acid precipitation solution to the pH value of 3-5 in the second stage to precipitate high-purity lignin. The pH range is monitored in real time by an online pH meter. After each addition of 80 mL of acid precipitation solution, the mixture is allowed to stand for 8 h and then centrifuged to collect the precipitate.

[0051] (5) Washing the precipitate: Wash the precipitate with sufficient deionized water until neutral, and dry it at 60°C for 24 h to obtain high-purity lignin.

[0052] Experimental example:

[0053] To illustrate the effectiveness of this application, the applicant conducted the following comparative experiment.

[0054] Experiment 1: Comparing the effects of different raw materials on the finished lignin.

[0055] This experiment compared the effects of selecting different biomass raw materials on the purity of lignin, ash content, and residual carbohydrate content of the finished product.

[0056] The fixed conditions for each group were: KOH / thiourea = 2:1, solid-liquid ratio 1:8, 120℃ × 2.5 h, and gradient acid precipitation (15% acetic acid). That is, except for the different biomass raw materials, all other operating methods were the same, and all were as described in Example 2. The results are shown in Table 1.

[0057]

[0058] The results in Table 1 confirm the specificity of the raw materials: sugarcane bagasse, due to its high G-type lignin and acetylated hemicellulose, significantly improves purity, reduces ash and carbohydrate residues, and achieves the best purification effect.

[0059] Experiment 2: Comparison of the performance of different alkaline solution systems.

[0060] This experiment compared the effects of different biomass raw materials on the purity and ash content of the finished product. The fixed conditions for each group were: sugarcane bagasse, solid-liquid ratio 1:8, 120℃ × 2.5 h, gradient acid precipitation (acetic acid). That is, except for the alkaline solution system, all other operating methods were the same, and all were as described in Example 2. The results are shown in Table 2.

[0061]

[0062] According to the results in Table 2, the KOH / thiourea system of this application has high lignin purity and low ash content. After replacing thiourea with urea, the purity decreased by 3.86% (>3% is considered a significant difference) and the ash content decreased, indicating that the composite alkali system of this application has outstanding beneficial effects.

[0063] Experiment 3: Optimization and comparison of acid precipitation operation.

[0064] This experiment compares the quality of lignin products obtained under different acid precipitation operations, including a comparison of acid precipitation solution type selection, gradient acid precipitation and single acid precipitation, and the effect of pH selection range deviation.

[0065] 1. Comparison of acid precipitation solution types.

[0066] Fixed conditions: sugarcane bagasse, KOH / thiourea (2:1), gradient acid precipitation (pH 10-12→3-5), that is, except for the acid precipitation solution, all other methods are the same as in Example 2. The test results are shown in Table 3.

[0067]

[0068] 2. Gradient acid precipitation vs. single acid precipitation.

[0069] Fixed conditions: sugarcane bagasse, KOH / thiourea (2:1), acid precipitation solution was 15% acetic acid. That is, except for the acid precipitation operation, all other methods were the same as in Example 2. The test results are shown in Table 4.

[0070]

[0071] 3. The effect of pH range deviation.

[0072] Fixed conditions: sugarcane bagasse, KOH / thiourea (2:1), gradient acid precipitation, the acid precipitation solution is 15% acetic acid, that is, except for the pH difference during the acid precipitation operation, all other methods are the same as in Example 2, and the test results are shown in Table 5.

[0073]

[0074] Based on the above results, it is proven that the acid precipitation operation of this application can achieve a purity of >91% only in the pH range of 10-12→3-5 and with 15% acetic acid as the precipitate. This proves that the accuracy of the range is irreplaceable and that acetic acid is the best choice for the overall performance of the precipitate.

[0075] Application Example 1: Preparation of carbon nanotube / carbon composite electrode.

[0076] Method Summary: The lignin obtained in Example 2 was used as a carbon source to grow carbon nanotubes by chemical vapor deposition and activated with KOH.

[0077] Performance: The tested electrodes showed that the specific capacitance of the capacitive carbon electrode was 234.04 F / g (0.2 A / g), and its cycle stability (3000 cycles) was 95.2%. In contrast, the specific capacitance of pure lignin carbon electrode was 61.22 F / g (0.2 A / g), and its cycle stability (3000 cycles) was 78%.

[0078] Advantages: Low ash lignin (2.49%) can prevent impurities from clogging the conductive channels and improve the uniformity of carbon nanotube growth.

[0079] Application Example 2: Heavy Metal Adsorbent.

[0080] Method Summary: The lignin obtained in Example 2 was grafted with acrylic acid to adsorb 100 mg / L Pb. 2+ Solution (pH 5.0).

[0081] Performance: 98.2% adsorption rate at 2 h, maximum adsorption capacity of 185.7 mg / g (the maximum adsorption capacity of commercial lignin is 112.4 mg / g, and the adsorption rate at 2 h is 82.5%).

[0082] Advantages: The high-purity lignin of this application can expose more phenolic hydroxyl / C=O groups, which is beneficial to Pb. 2+ A chelate bond is formed (FT-IR confirms carboxyl shift).

[0083] This invention uses lignocellulose biomass (sugarcane bagasse) as raw material and optimizes experimental factors such as solid-liquid ratio, reaction temperature, heat preservation time, and acid precipitation operation to significantly improve the purity of lignin, providing a core raw material guarantee for the high-value application of lignin in the field of lignin-based composite electrodes / heavy metal adsorbents.

[0084] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for efficiently purifying lignin using a composite alkali system, characterized in that, The method includes the following steps: (1) Mixing: The lignocellulosic biomass and the composite alkali solution are mixed at a solid-liquid ratio of 1:5-15. The composite alkali solution contains KOH and thiourea, and the concentration of the composite alkali solution is 1-5 wt%. (2) Purification reaction: Under the protection of inert gas, the mixture obtained in step (1) is placed in a reaction vessel, and after the temperature is raised from room temperature to 60-150℃, the reaction is kept at the temperature for 1-3 h, while stirring at a rate of 800-1000 rpm. (3) Reaction termination: After the heat preservation reaction is completed, inert gas is introduced at 30-50 mL / min to terminate the purification reaction, followed by centrifugation and filtration to obtain lignin alkaline solution; (4) Gradient acid precipitation: Add acid precipitation solution to the lignin alkaline solution in stages for gradient acid precipitation. After each addition of 80-100 mL of acid precipitation solution, let stand for 6-8 h, and then centrifuge to collect the precipitate. (5) Washing the precipitate: Wash the precipitate with sufficient deionized water until neutral, and dry to obtain high-purity lignin; The lignocellulose biomass is sugarcane bagasse; The acid precipitation solution is selected from one of acetic acid, hydrochloric acid, and citric acid; The gradient acid precipitation includes: (a) adding acid precipitation solution to a pH of 10-12 in the first stage to precipitate protein and hemicellulose residues; (b) In the next stage, acid precipitation solution is added until the pH value is 3-5 to precipitate high-purity lignin.

2. The method according to claim 1, characterized in that, The centrifugation filtration step (3) is as follows: centrifuge at 5000 rpm for 10 min in a high-speed centrifuge, and then take the supernatant and filter it through 0.45 µm and 0.22 µm aqueous filter membranes in sequence.

3. The method according to claim 1, characterized in that, The composite alkaline solution is a KOH / thiourea system, and the mass ratio of KOH to thiourea is 2:

1.

4. The method according to claim 1, characterized in that, The lignocellulosic biomass and the composite alkaline solution have a solid-liquid ratio of 1:8, the purification reaction temperature is 120℃, and the holding time is 2.5 h.

5. The lignin prepared by the method according to any one of claims 1-4, characterized in that, The lignin is used in the fields of capacitive carbon materials or heavy metal adsorbents.

6. The use of lignin prepared by the method according to any one of claims 1-4 in the preparation of capacitive carbon materials or heavy metal adsorbents.

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