Method for efficiently purifying lignin by using compound alkali system

Abstract

The invention provides a method for efficiently purifying lignin by using a composite alkali system, and relates to the technical field of lignin extraction and purification, wood fiber biomass is used as a raw material, KOH / thiourea composite alkali liquor with a specific mass ratio is adopted, and directional dissociation reaction is carried out under the conditions that the solid-to-liquid ratio is 1: (5-15), the temperature is 60-150 DEG C and inert gas protection is adopted; and after the reaction is ended, through a gradient acidification process (controlling the pH to be 10-12 and the pH to be 3-5 in stages) and by combining centrifugal filtration and water washing, ash content and carbohydrate residues are remarkably reduced, and high-quality lignin with the purity being greater than or equal to 91.16% is obtained. The method solves the problems of low purity and high impurity residue in the traditional process, and provides core raw material guarantee for lignin-based capacitor carbon.

Description

Technical Field

[0001] The present invention relates to the technical field of lignin extraction and purification, and in particular to a method for efficiently purifying lignin by utilizing a composite alkali system. Background Art

[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 primarily composed of three phenylpropane units (p-hydroxyphenyl H, guaiacyl G, and syringyl S) cross-linked through ether 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 engineering, and its high-value utilization is of great significance. Currently, industrial lignin is primarily derived from papermaking black liquor and bioethanol residues, but traditional extraction methods (such as the sulfate process and organic solvent method) present the following challenges.

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

[0004] (2) Structural destruction: Strong acid / strong alkali or high temperature treatment can easily lead to the breakage of lignin β-O-4 bonds 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 their 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 the present 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 residues in traditional processes, and provides a core raw material guarantee for lignin-based capacitor carbon.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions.

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

[0010] (1) Mixing: mixing the lignocellulosic biomass with a composite alkali solution at a solid-liquid ratio of 1:5-15, wherein the composite alkali solution comprises 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 reactor, and after the temperature is raised from room temperature to 60-150°C, the reaction is kept warm for 1-3 hours while stirring at a rate of 800-1000 rpm.

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

[0013] (4) Gradient acid precipitation: Acid precipitation solution was added to the lignin alkaline solution in stages for gradient acid precipitation. After adding 80-100 mL of acid precipitation solution, the mixture was allowed to stand for 6-8 hours and the precipitate was collected by centrifugation.

[0014] (5) Washing the precipitate: Use sufficient deionized water to wash the precipitate until it is neutral, and then dry it to obtain high-purity lignin.

[0015] Furthermore, the lignocellulosic biomass is bagasse.

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

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

[0018] Furthermore, the solid-liquid ratio of the lignocellulosic biomass to the composite alkali solution is 1:8, the purification reaction temperature is 120° C., and the insulation time is 2.5 h.

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

[0020] Furthermore, the gradient acid precipitation includes: (a) adding acid precipitation liquid to a pH value of 10-12 in the first stage to precipitate protein and hemicellulose residues; (b) adding acid precipitation liquid 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, and the lignin is used in the field of capacitor carbon materials or heavy metal adsorbents.

[0022] The present invention also provides a use of the lignin prepared by the above method in preparing capacitor carbon materials or heavy metal adsorbents.

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

[0024] The present invention provides a method for efficiently purifying lignin using a composite alkali system, and a high-quality lignin product is obtained by synergistically obtaining the following schemes.

[0025] (1) Directed bond cleavage by composite alkali synergistic dissociation mechanism: This application first precisely breaks the α-O-4 / β-O-4 ether bond and γ-ester bond between lignin and hemicellulose through the OH⁻ ionized by KOH, thereby disintegrating the rigid network of lignin-carbohydrate complex (LCC); in addition, the thiol (-SH) of thiourea provides electron transfer to protect the phenolic hydroxyl group, the amine group (-NH2) and the C=S bond form a three-dimensional hydrogen bond cage with the phenylpropane unit of lignin, enhancing the solubility of lignin, thereby achieving efficient separation.

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

[0027] (3) Gradient acid precipitation targeted purification: The lignin alkali solution was treated with a gradient acid precipitation mode. In the first stage (pH 10-12), impurities were precipitated above the isoelectric point of the protein (pI = 4-6), and the negatively charged protein / hemicellulose was flocculated and precipitated by the electrostatic repulsion effect. In the second stage (pH 3-5), the isoelectric point of lignin was approached to trigger the π-π stacking precipitation of lignin, avoiding the low pH condensation side reaction.

[0028] (4) Structural adaptability of bagasse raw materials: In addition, the application is limited to bagasse in the selection of raw materials. In this application, the natural structure of bagasse gives it an advantage in purification. First, the chemical composition of bagasse is a high proportion of G-type lignin (>80%): the G unit is rich in methoxy groups, forming a strong hydrogen bond network with thiourea, and the dissolution efficiency is improved; acetylated hemicellulose makes the acetyl group easily 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 pore structure (porosity 65%), which can promote the penetration of alkali solution and shorten the reaction time. Its low silicon content can reduce ash at the source and avoid silicate encapsulation of lignin.

[0029] In summary, this application breaks through the industry bottleneck of the inability to achieve both lignin purity and structural integrity through the three-dimensional synergy of raw material structure adaptation (sugarcane bagasse) - reaction system innovation (KOH / thiourea) - precise control of the purification process (gradient acid precipitation), providing an irreplaceable solution for the high value of biomass. DETAILED DESCRIPTION

[0030] The following examples may help those skilled in the art to more fully understand the present invention, but shall not limit 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 preparation method of the lignin includes the following steps.

[0033] (1) Mixing: The lignocellulosic biomass was mixed with a composite alkali solution at a solid-liquid ratio of 1:5, wherein the composite alkali solution contained KOH and thiourea in a mass ratio of 2:1 and a concentration of 5 wt %; the lignocellulosic biomass was bagasse.

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

[0035] (3) Reaction termination: After the insulation reaction is completed, argon gas is introduced at 30 mL / min to terminate the purification reaction, followed by centrifugal filtration to obtain lignin alkali solution; centrifugal filtration is as follows: centrifuge at 5000 rpm for 10 min in a high-speed centrifuge, and the supernatant is taken after centrifugation and filtered through 0.45 µm and 0.22 µm aqueous filter membranes in turn.

[0036] (4) Gradient acid precipitation: Acid precipitation solution (5% hydrochloric acid acid precipitation solution) was added to the lignin alkaline solution in stages for gradient acid precipitation. The gradient acid precipitation included: (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 was monitored in real time by an online pH meter; each time 100 mL of acid precipitation solution was added, the solution was allowed to stand for 6 h and the precipitate was collected by centrifugation.

[0037] (5) Washing the precipitate: Use sufficient deionized water to wash the precipitate until it is neutral, and dry it at 60 °C 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 preparation method of the lignin includes the following steps.

[0040] (1) Mixing: The lignocellulosic biomass was mixed with a composite alkali solution at a solid-liquid ratio of 1:8. The composite alkali solution contained KOH and thiourea in a mass ratio of 2:1, and the concentration of the composite alkali solution was 3 wt%. The lignocellulosic biomass was bagasse.

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

[0042] (3) Reaction termination: After the insulation reaction is completed, nitrogen gas is introduced at 40 mL / min to terminate the purification reaction, followed by centrifugal filtration to obtain lignin alkali solution; centrifugal filtration is as follows: centrifuge at 5000 rpm for 10 min in a high-speed centrifuge, and the supernatant is taken after centrifugation and filtered through 0.45 µm and 0.22 µm aqueous filter membranes in turn.

[0043] (4) Gradient acid precipitation: Acid precipitation solution (15% acetic acid acid precipitation solution) was added to the lignin alkaline solution in stages for gradient acid precipitation. The gradient acid precipitation included: (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 was monitored in real time by an online pH meter; each time 90 mL of acid precipitation solution was added, the solution was allowed to stand for 7 h and the precipitate was collected by centrifugation.

[0044] (5) Washing the precipitate: Use sufficient deionized water to wash the precipitate until it is 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 preparation method of the lignin includes the following steps.

[0047] (1) Mixing: The lignocellulosic biomass was mixed with a composite alkali solution at a solid-liquid ratio of 1:15, wherein the composite alkali solution contained KOH and thiourea in a mass ratio of 2:1 and the concentration of the composite alkali solution was 1 wt %. The lignocellulosic biomass was bagasse.

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

[0049] (3) Reaction termination: After the insulation reaction is completed, nitrogen gas is introduced at 50 mL / min to terminate the purification reaction, followed by centrifugal filtration to obtain lignin alkali solution; centrifugal filtration is as follows: centrifuge at 5000 rpm for 10 min in a high-speed centrifuge, and the supernatant is taken after centrifugation 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 acid precipitation solution) was added to the lignin alkaline solution in stages for gradient acid precipitation. The gradient acid precipitation included: (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 was monitored in real time by an online pH meter; each time 80 mL of acid precipitation solution was added, the solution was allowed to stand for 8 h, and the precipitate was collected by centrifugation.

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

[0052] Test example:

[0053] In order to illustrate the effect of the present application, the applicant conducted the following comparative test.

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

[0055] This study compared the effects of different biomass raw materials on the purity, ash content and carbohydrate residue of the finished lignin. The fixation conditions for each group were: KOH / thiourea = 2:1, solid-to-liquid ratio 1:8, 120°C for 2.5 h, and gradient acid precipitation (15% acetic acid). Apart from the different biomass feedstocks, all other procedures were identical, as described in Example 2. The results are shown in Table 1.

[0056]

[0057] The results in Table 1 confirm the specificity of the raw materials: sugarcane bagasse has a significantly improved purity due to its high G-type lignin + acetylated hemicellulose, and the ash and carbohydrate residues are reduced, resulting in the best purification effect.

[0058] Experiment 2: Comparison of the performance of different alkali solution systems.

[0059] This experiment compared the effects of different biomass feedstocks on the purity and ash content of the finished lignin. The fixed conditions for each group were: bagasse, a solid-to-liquid ratio of 1:8, 120°C for 2.5 h, and gradient acid precipitation (acetic acid). Apart from the different alkali solutions, all other procedures were identical, following the method described in Example 2. The results are shown in Table 2.

[0060]

[0061] According to the results in Table 2, the lignin in the KOH / thiourea system of the present application has high purity and low ash content. After replacing thiourea with urea, the purity is reduced by 3.86% (>3% is considered a significant difference) and the ash content is reduced, indicating that the composite alkali system of the present application has outstanding beneficial effects.

[0062] Experiment 3: Optimization and comparison of acid precipitation operation.

[0063] This test example compares the quality of the lignin products obtained under different acid precipitation operations, including the comparison of acid precipitation liquid type selection, gradient acid precipitation and single acid precipitation, and the impact of pH selection range deviation.

[0064] 1. Comparison of acid precipitation fluid types.

[0065] Fixation conditions: bagasse, KOH / thiourea (2:1), gradient acid precipitation (pH 10-12→3-5), that is, except for the different acid precipitation solutions, all other methods are the same as those in Example 2. The test results are shown in Table 3.

[0066]

[0067] 2. Gradient acid precipitation vs. single acid precipitation.

[0068] Fixed conditions: bagasse, KOH / thiourea (2:1), acid precipitation solution is 15% acetic acid, that is, except for the different acid precipitation operation, other methods are the same as those in Example 2. The test results are shown in Table 4.

[0069]

[0070] 3. The impact of pH range deviation.

[0071] Fixed conditions: bagasse, KOH / thiourea (2:1), gradient acid precipitation, acid precipitation solution is 15% acetic acid, that is, except for the different pH during the acid precipitation operation, other methods are the same as those in Example 2. The test results are shown in Table 5.

[0072]

[0073] Combined with the above results, it is proved that the acid precipitation operation of the present application can only achieve a purity of >91% in the pH range of 10-12→3-5 and with an acid precipitation liquid of 15% acetic acid, proving that the range accuracy is irreplaceable and that the selection of acetic acid as the acid precipitation liquid has the best overall performance.

[0074] Application example 1: Preparation of carbon nanotube / carbon composite electrodes.

[0075] Brief description of the method: The lignin obtained in Example 2 was used as a carbon source to grow carbon nanotubes by chemical vapor deposition and activated with KOH.

[0076] Performance: The resulting electrode exhibited a specific capacitance of 234.04 F / g (0.2 A / g) and a cycling stability (3,000 cycles) of 95.2%. Pure lignin-based carbon exhibited a specific capacitance of 61.22 F / g (0.2 A / g) and a cycling stability (3,000 cycles) of 78%.

[0077] Advantages: Low-ash lignin (2.49%) can prevent impurities from clogging the conductive channels and improve the uniformity of carbon nanotube growth.

[0078] Application example 2: heavy metal adsorbent.

[0079] Method description: The lignin obtained in Example 2 was grafted with acrylic acid to adsorb 100 mg / L Pb 2+ solution (pH 5.0).

[0080] Performance: 2 h adsorption rate 98.2%, maximum adsorption capacity 185.7 mg / g (the maximum adsorption capacity of commercial lignin is 112.4 mg / g, and the 2 h adsorption rate is 82.5%).

[0081] Advantages: The high-purity lignin of this application can expose more phenolic hydroxyl groups / C=O groups, which is comparable to Pb 2+ Chelate bonds were formed (FT-IR confirmed the displacement of the carboxyl groups).

[0082] The present invention uses wood fiber biomass (bagasse) as raw material and can significantly improve the purity of lignin by optimizing experimental factors such as solid-liquid ratio, reaction temperature, holding time, and acid precipitation operation, providing core raw material guarantee for the high-value application of lignin in the field of lignin-based composite electrodes / heavy metal adsorbents.

[0083] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made based on the present invention. Therefore, such modifications and improvements, which do not depart from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A method for efficiently purifying lignin using a composite alkali system, characterized in that: The method comprises the following steps: (1) Mixing: mixing the lignocellulosic biomass with a composite alkali solution at a solid-liquid ratio of 1:5-15, wherein the composite alkali solution comprises KOH and thiourea, and the concentration of the composite alkali solution is 1-5 wt%; (2) Purification reaction: Under inert gas protection, the mixture obtained in step (1) was placed in a reactor, and after the temperature was raised from room temperature to 60-150°C, the reaction was kept warm for 1-3 hours while stirring at a rate of 800-1000 rpm; (3) Reaction termination: After the heat preservation reaction is completed, the purification reaction is terminated by passing inert gas at 30-50 mL / min, followed by centrifugal filtration to obtain lignin alkali solution; (4) Gradient acid precipitation: Add acid precipitation solution to the lignin alkaline solution in stages for gradient acid precipitation. After adding 80-100 mL of acid precipitation solution, let it stand for 6-8 hours and collect the precipitate by centrifugation. (5) Washing the precipitate: Use sufficient deionized water to wash the precipitate until it is neutral, and then dry it to obtain high-purity lignin.

2. The method according to claim 1, characterized in that The lignocellulosic biomass is bagasse.

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

4. The method according to claim 1, wherein The composite alkali solution is a KOH / thiourea system, and the mass ratio of KOH to thiourea is 2:

1.

5. The method according to claim 1, wherein The solid-liquid ratio of the lignocellulosic biomass to the composite alkali solution is 1:8, the purification reaction temperature is 120° C., and the insulation time is 2.5 h.

6. The method according to claim 1, characterized in that The acid precipitation liquid is selected from one of acetic acid, hydrochloric acid and citric acid.

7. The method according to claim 1, characterized in that The gradient acid precipitation includes: (a) adding acid precipitation liquid to a pH value of 10-12 in the first stage to precipitate protein and hemicellulose residues; (b) adding acid precipitation liquid to a pH value of 3-5 in the second stage to precipitate high-purity lignin.

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