A method for extracting and separating corn stalk lignin

Abstract

The application discloses a method for extracting and separating corn stalk lignin and relates to the technical field of lignin extraction and separation. The method comprises the following steps: synthesizing a deep eutectic solvent (DES) by using 1,4-butanediol aqueous solution and ionic liquid [Emim][OAc]; mixing the DES and corn stalk residues extracted by anhydrous ethanol, heating, and filtering under reduced pressure to obtain first solid residues and first filtrate; and extracting DES lignin from the first filtrate by using anhydrous ethanol. The extraction rate of the DES lignin is 29.3-61.4%, the purity reaches 89.8-99.3%, the IC 50 The highest reaches 66.8 μ g.mL ‑1 The first solid residues are rich in 77.1% of cellulose in mass fraction and can be used for further production of sugar platform products or preparation of cellulose-based functional materials. The application can be used in the field of corn stalk bio-refining.

Description

Technical Field

[0001] This invention relates to the field of clean separation technology of biomass components, and in particular to a method for extracting and separating lignin from corn stalks. Background Technology

[0002] Lignin is an amorphous polymer composed of three phenylpropane structural units—p-hydroxyphenyl, guaiacol, and syringyl—linked by carbon-carbon and ether bonds. It contains active functional groups such as carboxyl and hydroxyl groups, exhibiting certain antioxidant activity. Industrial lignin mainly originates as a byproduct of pulp and paper making and biorefining industries. Due to complex processing, industrial lignin undergoes condensation, degradation, and repolymerization reactions, significantly reducing its chemical reactivity and antioxidant activity, creating technical barriers to its high-value utilization. Groundwood lignin has a structure closest to natural lignin, with a higher content of active functional groups such as phenolic and alcoholic hydroxyl groups, resulting in superior chemical reactivity and antioxidant activity compared to industrial lignin. However, the extraction rate of groundwood lignin is low, typically only separating groundwood lignin with a lignin mass fraction of 15%–20%. In recent years, the "lignin-preferential conversion and utilization" strategy has attracted widespread attention. This strategy improves the overall utilization efficiency of biomass by preferentially converting lignin in lignocellulose, but balancing the high-value utilization of lignin with carbohydrates remains a technical bottleneck.

[0003] The article "Extraction of Lignin from Moso Bamboo Using Microwave-Assisted Ionic Liquid EmimOAc" published in the 2019 issue of *Chemical Industry and Engineering Progress* (Vol. 38, No. 9) extracted 14.7% lignin from moso bamboo using the microwave-assisted ionic liquid [Emim][OAc] (1-ethyl-3-methylimidazolium acetate), achieving an extraction rate of approximately 56.8%. However, the purity of the lignin was not determined using this method. A 2023 paper from Qingdao University of Science and Technology, "Extraction of Lignin from Agricultural and Forestry Biomass and Preparation of Nanofiber Materials Using a Choline Chloride Eutectic Solvent System," separated lignin from corn stalks using an alkaline eutectic solvent system. However, the extraction rate and purity of the lignin were low, at 43.88% and 38.11%, respectively. Summary of the Invention

[0004] This invention addresses the technical problems of low extraction rate and low purity of existing lignin by proposing a new method for the extraction and separation of lignin from corn stalks.

[0005] This invention discloses a method for extracting and separating lignin from corn stalks, comprising the following steps: 1. Crush, sieve and dry the air-dried corn stalks. After passing through 40-100 mesh, collect the stalk powder. Soxhlet extract the stalk powder with anhydrous ethanol and then dry it to obtain solid residue. 2. Transfer 150 mL of 70% (w / w) 1,4-butanediol aqueous solution to a round-bottom flask, add 2.5–7.5 g of ionic liquid [Emim][OAc], place the flask on a magnetic stirrer, heat to 60 °C at 800 rpm, and continue stirring and heating for 0.5 hours to obtain a series of eutectic solvents (DES) of [Emim][OAc] and 1,4-butanediol aqueous solution. 3. Add the solid residue extracted with anhydrous ethanol in step 1 and the DES obtained in step 2 to a micro reactor, tighten the reactor lid and start heating. Set the heating temperature to 100-140℃ and the reaction time to 0.5-1.5 hours. After the reaction is completed, wait for the mixture to cool to room temperature, remove the material from the reactor and filter it under reduced pressure. At the same time, wash the solid residue with 100mL of distilled water until pH=7. Collect the filtrate and solid residue separately. Transfer the solid residue to a forced-air drying oven and dry it at 105℃ for 6 hours to obtain the first-stage filtrate and the first-stage solid residue. 4. Concentrate the primary filtrate using a rotary evaporator until the volume of the concentrate is approximately 3-5 mL. Transfer the concentrate and 100 mL of anhydrous ethanol to a separatory funnel. Gently shake the separatory funnel to mix the mixture thoroughly. Let it stand for 2 hours to allow the mixture to separate into layers. Collect the bottom layer containing the precipitate. Centrifuge the mixture and then vacuum dry the solid precipitate to obtain DES lignin. Furthermore, in step one, the drying temperature is 105℃, and the sample is dried until the moisture content is less than 1%. Furthermore, in step three, the solid-liquid ratio of the solid residue extracted with anhydrous ethanol to DES is 1 g: (20-30 mL). Furthermore, in step four, the vacuum drying temperature is 55°C, and the sample is dried until the moisture content is less than 1%.

[0006] This invention utilizes an ionic liquid, 1-ethyl-3-methyl-imidazolium acetate, and an aqueous solution of 1,4-butanediol to synthesize a eutectic solvent. Then, lignin is dissociated from corn stalks using this eutectic solvent system under relatively mild conditions. Subsequently, solvent extraction and centrifugation are used to extract and separate the lignin from the mixture of the eutectic solvent and the dissociation products. This combined extraction process improves both the extraction rate and purity of lignin while enhancing its antioxidant activity.

[0007] The advantages of this invention are as follows: high extraction rate and purity of lignin, capable of extracting lignin with a theoretical lignin content of 61.4% from corn stalks, achieving a purity of 99.2%; high antioxidant activity of DES lignin, with a DPPH free radical scavenging rate 26% lower than that of alkali lignin extracted from papermaking black liquor, allowing for further development into a natural high-molecular-weight antioxidant; while achieving efficient separation and extraction of lignin, it also ensures the high-value utilization of cellulose, with the obtained primary solid residue having a cellulose content of 73.3%, which can be used for further production of sugar platform products or for further preparation of cellulose-based functional materials.

[0008] This invention can be used in the field of biorefining of corn stalks. Detailed Implementation

[0009] The beneficial effects of the present invention are verified using the following examples: Example 1: The extraction and separation method of corn stalk lignin in this example is carried out according to the following steps: 1. Cut the air-dried corn stalks into 1-2 cm long pieces, transfer them to a Tester FW-100 pulverizer and pulverize for 20 seconds, then sieve to collect 40-100 mesh straw powder. Transfer the straw powder to a forced-air drying oven and dry at 105°C until the moisture content is less than 1%, obtaining dried straw powder. Then transfer the dried straw powder to a Soxhlet extractor, add an appropriate amount of anhydrous ethanol for Soxhlet extraction. After the solution in the Soxhlet extractor is colorless and transparent, take out the straw sample and transfer the sample to a forced-air drying oven and dry at 105°C until the moisture content is less than 1%, obtaining the solid residue extracted with anhydrous ethanol. 2. Transfer 150 mL of 70% (w / w) 1,4-butanediol aqueous solution to a round-bottom flask, add 7.5 g of ionic liquid [Emim][OAc], then transfer the round-bottom flask containing the mixture to a magnetic stirrer heater, heat to 60 °C at 800 rpm, and continue stirring and heating for 0.5 hours to obtain a eutectic solvent (DES) of [Emim][OAc] and 1,4-butanediol aqueous solution. 3. Add 5g of the solid residue extracted with anhydrous ethanol in step 1 and 125mL of DES in step 2 to the Huotong HT-250FC micro reactor and mix well. Tighten the lid, set the heating temperature to 120℃ and the heating time to 0.5 hours and start the reaction. After the reaction is completed, wait for the mixture to cool to room temperature, take out the material in the reactor and filter it under reduced pressure. At the same time, wash the solid residue with 100mL of distilled water until pH=7. Collect the filtrate and solid residue separately. Transfer the solid residue to a forced-air drying oven and dry it at 105℃ for 6 hours to obtain the first-grade filtrate and the first-grade solid residue. IV. Concentrate the primary filtrate to approximately 3–5 mL using a rotary evaporator to obtain a concentrated solution. Add the concentrated solution and 100 mL of anhydrous ethanol to a separatory funnel, gently shake the funnel to thoroughly mix the mixture, and then let it stand for 2 hours to allow the mixture to separate into layers. Collect the bottom layer containing the precipitate and transfer it to a centrifuge tube. Centrifuge at 8000 rpm for 10 minutes, and transfer the solids to a 0.22 mL plate. μ The solid material was filtered under reduced pressure in a glass Buchner funnel using a microporous membrane, and was washed with about 20 mL of distilled water until neutral. The solid material was then transferred to a vacuum drying oven and dried at 55 °C until the moisture content was less than 1%, yielding DES lignin.

[0010] Example 2: This example differs from Example 1 in that the amount of ionic liquid [Emim][OAc] added in step two is 5g, while the other steps and parameters are the same as in Example 1.

[0011] Example 3: This example differs from Example 1 in that: in step two, the added ionic liquid [Emim][OAc] is 2.5g, while the other steps and parameters are the same as in Example 1.

[0012] Example 4: This example differs from Example 1 in that: in step three, 100 mL of DES is added to the Huotong HT-250FC micro reactor, while the other steps and parameters are the same as in Example 1.

[0013] Example 5: This example differs from Example 1 in that: in step three, the amount of DES added to the Huotong HT-250FC micro reactor is 150 mL, while the other steps and parameters are the same as in Example 1.

[0014] Example 6: This example differs from Example 1 in that the heating temperature of the reactor is set to 100°C in step three, while the other steps and parameters are the same as in Example 1.

[0015] Example 7: This example differs from Example 1 in that the heating temperature of the reactor is set to 140°C in step three, while the other steps and parameters are the same as in Example 1.

[0016] Example 8: This example differs from Example 1 in that the heating time of the reactor in step three is set to 0.5 hours, while the other steps and parameters are the same as in Example 1.

[0017] Example 9: This example differs from Example 1 in that the heating time of the reactor in step three is set to 1.5 hours, while the other steps and parameters are the same as in Example 1.

[0018] Example 10: This example differs from Example 1 in that: in step three, 5g of the solid residue extracted with anhydrous ethanol in step one, 6.25g of ionic liquid [Emim][OAc], and 125mL of distilled water are added to the Huotong HT-250FC micro reactor and mixed evenly. In step four, [Emim][OAc] lignin is obtained. Other steps and parameters are the same as in Example 1.

[0019] Example 11: This example differs from Example 1 in that: in step three, 5g of the solid residue extracted with anhydrous ethanol in step one and 125mL of 1,4-butanediol aqueous solution with a mass percentage concentration of 70% are added to the Huotong HT-250FC micro reactor and mixed evenly. In step four, 1,4-butanediol lignin is obtained. Other steps and parameters are the same as in Example 1.

[0020] Example 12: This example differs from Example 1 in that: in step three, 5g of the solid residue extracted with anhydrous ethanol in step one and 125mL of distilled water are added to the Huotong HT-250FC micro reactor and mixed evenly; in step four, liquid hot water lignin is obtained; other steps and parameters are the same as in Example 1.

[0021] The extraction rates, purity, molecular weight, molecular weight distribution, and antioxidant activity of DES lignin, [Emim][OAc] lignin, 1,4-butanediol lignin, and liquid hot water lignin obtained in step four of Examples 1 to 12 were tested using the following methods: The lignin content of the solid residues extracted with anhydrous ethanol in step one of Examples 1 to 12 was determined according to the People's Republic of China National Standard GB / T35818-2018, "Analytical Methods for Forestry Biomass Raw Materials: Determination of Polysaccharide and Lignin Content". The results are expressed as a percentage of the mass of the solid residues extracted with anhydrous ethanol. The theoretical content of the total added lignin was obtained by multiplying the added mass of the solid residues extracted with anhydrous ethanol in step three of Examples 1 to 12 by its lignin content. Then, the extraction rate of each extracted lignin was obtained by dividing the mass of the different lignins extracted in step four by the theoretical content of the total added lignin. The results are expressed as a percentage. The purity of each lignin was recorded by multiplying the sum of the masses of acid-insoluble lignin and acid-soluble lignin by the mass of the extracted lignin and multiplying the result by 100%. The molecular weight and molecular weight distribution of lignin were determined using gel permeation chromatography (GPC) software integrated into a high-performance liquid chromatograph (HPLC). The chromatographic columns were 79911GP-101 and 79911GP-104 in series, with tetrahydrofuran as the mobile phase. The column temperature was set to 30℃, the flow rate to 1.0 mL / min, and the injection volume to 40 mL / min. μ m, the standard is selected from polystyrene standard with a molecular weight range of 580 g / mol to 504500 g / mol; The paper "Utilizing Pd / SO4" published in the international journal *Polymers*, Volume 11, Article No. 1218, in 2019, was adopted. 2− / ZrO2 catalyst for the degradation of alkali lignin: improving its reactivity and antioxidant activity (The Effect of Degradation of Soda Lignin Using Pd / SO4) 2− The antioxidant activity of various lignin samples was evaluated using the DPPH (diphenyliodonium hexafluorophosphate) free radical scavenging method described in "ZrO2 as a Catalyst: Improved Reactivity and Antioxidant Activity". 4 mL of 0.04 mg / mL DPPH solution and 1 mL of lignin-dioxane aqueous solutions of different concentrations were mixed and allowed to stand for 30 minutes. The absorbance at 517 nm was recorded using a UV-Vis spectrophotometer. The DPPH free radical scavenging rate was calculated according to the formula. The concentration of the lignin-dioxane aqueous solution corresponding to a scavenging rate of 50% was recorded as the half-maximum scavenging rate (IC50). 50 IC of the sample 50 The smaller the value, the stronger its antioxidant activity; alkali lignin extracted from antioxidant BHT and papermaking black liquor was used as a reference for the lignin extracted in Examples 1 to 12.

[0022] In Examples 1 to 12, the contents of cellulose, hemicellulose, lignin, ash, moisture, and ethanol extract of the air-dried corn stalks in Step 1 were 39.6%, 22.7%, 14.2%, 5.2%, 8.1%, and 10.2%, respectively; the contents of cellulose, hemicellulose, lignin, and ash of the solid residues extracted with anhydrous ethanol in Steps 1 and 3 were 48.5%, 27.8%, 17.4%, and 6.3%, respectively; the extraction rates and purities of various extracted lignins are shown in Table 1.

[0023] Table 1. Extraction rate and purity of various lignins extracted in Examples 1 to 12. 1 DES Lignin-1 61.4 99.2 2 DES lignin-2 56.7 99.1 3 DES Lignin-3 29.3 98.9 4 DES Lignin-4 50.3 98.9 5 DES Lignin-5 60.9 99.1 6 DES Lignin-6 33.7 99.3 7 DES Lignin-7 60.3 98.9 8 DES Lignin-8 43.3 99.2 9 DES Lignin-9 61.3 89.8 10 [Emim][OAc]Lignin 50.7 98.2 11 1,4-Butanediol lignin 4.1 90.3 12 Liquid hot water lignin 6.3 92.5 As can be seen from the data in Table 1, in Examples 1-9, the extraction rate of lignin from corn straw synthesized using the ionic liquid [Emim][OAc] and 1,4-butanediol was 29.3%-61.4%, and the purity of DES lignin was 89.8%-99.3%. Among them, the extraction rate and purity of DES lignin in Example 1 were 61.4% and 99.2%, respectively, which were higher than those of the DES lignin samples in Examples 2-9. The extraction effect (extraction rate and purity) of the DES solvent system on corn straw lignin was better than that of the ionic liquid [Emim][OAc] solvent system, the 1,4-butanediol solvent system, or the distilled aqueous solution alone.

[0024] IC50 values ​​for molecular weight, molecular weight distribution, and DPPH free radical scavenging rate of DES lignin-1 (corresponding to Example 1), alkali lignin, and BHT 50 The values ​​are shown in Table 2.

[0025] Table 2. Molecular weights and IC50 values ​​of DPPH free radical scavenging rates for DES lignin-1, alkali lignin, and BHT 50 value DES Lignin-1 66.8 3700 1600 2.31 Alkali lignin 90.3 7700 4900 1.57 BHT 31.6 220.36 220.36 1 Table 2 shows the IC50 of the DPPH radical scavenging rate of DES lignin-1. 50 It is 66.8 μ g·mL -1 IC50 of DPPH radical scavenging rate compared to alkali lignin extracted from industrial papermaking black liquor 50 (90.3 μg·mL) -1 The IC of the sample was 26% lower. 50 A smaller value indicates stronger antioxidant activity; DES lignin-1 exhibits higher antioxidant activity. The weight-average molecular weight of DES lignin-1 is 3700 g·mol⁻¹. -1 The weight-average molecular weight of alkali lignin is lower than that of alkali lignin (7700 g·mol⁻¹). -1 This can be attributed to the changes in the macromolecular structure of lignin during extraction using the DES solvent system. β The -O-4 bond breaks, and lignin degrades into smaller molecules. This depolymerization exposes more active groups, leading to increased antioxidant activity, consistent with the superior antioxidant activity of DES lignin-1 compared to alkali lignin. While the antioxidant activity of DES lignin is weaker than that of the antioxidant BHT, it still holds the potential for development and utilization as a natural high-molecular-weight antioxidant.

[0026] Furthermore, the cellulose, hemicellulose, lignin, and ash contents of the primary solid residue described in step three of Example 1 are 77.1%, 3.7%, 7.8%, and 11.4%, respectively. This primary solid residue is a high-quality biomass material rich in cellulose, which can be used for further production of sugar platform products or for the further preparation of cellulose-based functional materials. Therefore, this invention achieves efficient separation and extraction of lignin while also ensuring the high-value utilization of cellulose, which is of great significance for realizing the high-value utilization of all components of corn straw lignocellulose resources.

Claims

1. A method for extracting and separating lignin from corn stalks, characterized in that... This method is performed in the following steps:

1. Crush, sieve and dry the air-dried corn stalks to collect 40-100 mesh stalk powder. Then, use anhydrous ethanol to perform Soxhlet extraction on the stalk powder and dry it to obtain the solid residue extracted with anhydrous ethanol.

2. Mix 1,4-butanediol aqueous solution and ionic liquid [Emim][OAc] evenly, then heat to 60°C using a magnetic stirrer at 800 rpm, and continue stirring and heating for 0.5 hours to obtain a eutectic solvent DES of [Emim][OAc] and 1,4-butanediol aqueous solution; wherein the mass percentage concentration of the 1,4-butanediol aqueous solution is 70%, and the volume ratio of the 1,4-butanediol aqueous solution to the mass ratio of the ionic liquid [Emim][OAc] is 150 mL: (2.5~7.5) g; 3. Add the solid residue extracted by anhydrous ethanol in step 1 and the DES in step 2 to a micro reactor, heat the reaction for a certain time, and after the reaction is completed, wait for the mixture to cool to room temperature, take out the material in the reactor and filter it under reduced pressure, collect the filtrate and solid residue respectively, transfer the solid residue to a forced-air drying oven and dry it at 105°C for 6 hours to obtain primary filtrate and primary solid residue.

4. Concentrate the primary filtrate using a rotary evaporator until the volume of the concentrate is 3-5 mL. Transfer the concentrate and 100 mL of anhydrous ethanol to a separatory funnel. Gently shake the separatory funnel to mix the mixture thoroughly. Let it stand for 2 hours to allow the mixture to separate into layers. Collect the bottom layer containing the precipitate. Centrifuge the mixture and vacuum dry the solid precipitate to obtain DES lignin.

2. The method for extracting and separating lignin from corn stalks according to claim 1, characterized in that... In step three, the mass ratio of the solid residue extracted by anhydrous ethanol to the volume of DES added to the micro reactor is 5 g: (100-150) mL, the heating time is 0.5-1.5 hours, and the heating temperature is 100℃-140℃.