A method for extracting light-colored lignin

Abstract

The application discloses a method for extracting light-colored lignin and belongs to the technical field of high-value lignin. The method comprises the following steps: S1. mixing a eutectic solvent with a polyol to obtain an extraction solution; S2. mixing the extraction solution with lignocellulose, reacting, and performing solid-liquid separation; and S3. mixing the separated solution with water, precipitating a solid, and obtaining light-colored lignin. In the application, benzyltriethylammonium chloride is used as a hydrogen bond acceptor, anhydrous oxalic acid is used as a hydrogen bond donor, and the light-colored lignin is extracted from lignocellulose after mixing with the polyol under specific temperature and time, so that the L* value of the lignin is improved while the yield of the lignin is ensured. In the application, the hydroxyl structure of the polyol is used to form a weak hydrogen bond with lignin molecules, so that the oxidation and condensation of the lignin are inhibited, the dissolution capacity of the system for the lignin is improved, the lignin is efficiently extracted under mild conditions, and the light-colored lignin with a high L* value and high purity is obtained.

Description

Technical Field

[0001] This invention relates to the field of lignin high-value technology, specifically to a method for extracting light-colored lignin. Background Technology

[0002] Lignin is a natural aromatic polymer with broad application prospects in adhesives, resins, dyes, bio-based materials, and pharmaceutical intermediates. Traditional lignin extraction methods mainly include alkaline methods, acid methods, and organic solvent methods. Alkaline and acid methods are highly corrosive, require sophisticated equipment, and cause serious environmental pollution. Conventional organic solvent methods are highly volatile, toxic, and have high recovery costs, limiting their industrial application. Moreover, to improve lignin yield, reactions are usually carried out under harsh conditions, but strong acids, alkalis, or high temperatures can easily cause deep degradation, condensation, and discoloration of lignin, resulting in dark-colored, low-quality lignin, severely limiting its application in fields where product color is critical (such as sunscreen). Therefore, efficiently extracting light-colored lignin remains a significant challenge.

[0003] As a new type of green solvent, eutectic solvents are often used in the extraction of lignin in existing single or binary eutectic solvent processes. These processes are prone to lignin oxidation and intermolecular condensation reactions, resulting in products with darker colors and lower L* values. This greatly limits the application of light-colored lignin in high-end materials, daily chemicals, and fine chemicals.

[0004] Existing technology CN117534846A discloses a method for reducing lignin color using an acidic eutectic solvent combined with a monohydric alcohol, which extracts lignin by adding a monohydric alcohol to the eutectic solvent. However, the lignin extracted by this prior art has a low L* value and does not focus on the lignin yield. Therefore, how to further improve the L* value of lignin while ensuring the lignin yield has become an urgent problem to be solved. Summary of the Invention

[0005] To address the problems existing in the prior art, the primary objective of this invention is to provide a method for extracting light-colored lignin.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for extracting light-colored lignin includes the following steps: S1. Mix the eutectic solvent with the polyol to obtain the extract; S2. Mix the extract with lignocellulose, react, and then separate the solid and liquid components; S3. Mix the separated solution with water to precipitate a solid, yielding light-colored lignin; The eutectic solvent includes benzyltriethylammonium chloride and anhydrous oxalic acid; the reaction temperature is ≥85℃ and the reaction time is ≥4h.

[0007] This invention uses benzyltriethylammonium chloride as a hydrogen bond acceptor and anhydrous oxalic acid as a hydrogen bond donor. After mixing with a polyol, light-colored lignin is extracted from lignin cellulose under specific temperature and time conditions, ensuring lignin yield while increasing the L* value of the lignin. This invention utilizes the polyhydroxy structure of the polyol to form weak hydrogen bonds with lignin molecules, inhibiting lignin oxidative condensation and simultaneously enhancing the system's solubility for lignin. This achieves efficient lignin extraction under mild conditions, yielding light-colored lignin with a high L* value and high purity.

[0008] Among them, benzyltriethylammonium chloride contains a hydrophobic benzyl aromatic ring structure, which can generate a strong π-π stacking interaction with the numerous benzene ring structures inside lignin. This allows it to effectively penetrate the dense cell wall network of biomass, disrupt the aromatic skeleton stacking structure of lignin, and disintegrate the stable polymerization system of lignin at the microscopic level, thus achieving efficient lignin dissolution. Secondly, benzyltriethylammonium chloride has a larger volume and stronger steric hindrance quaternary ammonium cation structure. Compared with small molecule quaternary ammonium salts, its cation spatial extension range is larger, which can more efficiently insert into the cross-linking gaps between cellulose, hemicellulose and lignin, breaking the hydrogen bonds and ether bonds between lignin and polysaccharide components, achieving selective lignin exfoliation without excessively damaging the cellulose skeleton, ensuring a high cellulose retention rate.

[0009] Anhydrous oxalic acid, as a hydrogen bond donor, can form a stable binary eutectic solvent with benzyltriethylammonium chloride, while providing suitable acidity to efficiently break the internal bonds of lignocellulose, promote lignin dissolution, and enhance the solubility of lignin through hydrogen bonding. It is the core component for achieving efficient lignin extraction.

[0010] As an adjuvant, polyols can disrupt the hydrogen bonds of the lignin-carbohydrate complex (LCC) with their multiple hydroxyl groups, swell plant cell walls, and gently dissociate lignin. Furthermore, their hydroxyl groups have weak reducing power and are more inert than small molecule alcohols, which can effectively inhibit the condensation of phenolic hydroxyl groups and oxidative discoloration of lignin during the extraction process, reduce the generation of conjugated chromophores, avoid lignin browning, and significantly increase the L* value of lignin. At the same time, they can reduce the viscosity of the eutectic solvent system, enhance the solid-liquid mass transfer efficiency, promote the rapid and complete dissolution of lignin, and significantly improve the extraction rate.

[0011] In this invention, light-colored lignin refers to lignin with an L* value ≥ 47.

[0012] Preferably, the polyol is at least one selected from ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, or glycerol.

[0013] More preferably, the polyol is 1,4-butanediol.

[0014] Preferably, the molar ratio of benzyltriethylammonium chloride to anhydrous oxalic acid is (1~6):(1~6).

[0015] More preferably, the molar ratio of benzyltriethylammonium chloride to anhydrous oxalic acid is 1:(1~1.5).

[0016] Preferably, the method for preparing the eutectic solvent includes the following steps: mixing raw materials for the eutectic solvent and heating them to obtain the eutectic solvent.

[0017] More preferably, the heating temperature is 70~110°C.

[0018] Preferably, the mass of the polyol is 10-50 wt% of the mass of the eutectic solvent.

[0019] In this invention, the mass of the polyol is any value among 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, 29wt%, 30wt%, 31wt%, 32wt%, 33wt%, 34wt%, 35wt%, 36wt%, 37wt%, 38wt%, 39wt%, 40wt%, 41wt%, 42wt%, 43wt%, 44wt%, 45wt%, 46wt%, 47wt%, 48wt%, 49wt%, and 50wt% of the mass of the eutectic solvent.

[0020] More preferably, the mass of the polyol is 20-45 wt% of the mass of the eutectic solvent.

[0021] More preferably, the mass of the polyol is 38-42 wt% of the mass of the eutectic solvent.

[0022] Preferably, in S1, the mixing temperature is 60~90℃.

[0023] Preferably, in S1, the mixing time is 20-60 minutes.

[0024] Preferably, the solid-liquid mass ratio of the extract to lignocellulose is (8~10):1.

[0025] Preferably, the lignocellulose is bamboo powder and / or wood powder.

[0026] More preferably, the lignocellulose needs to be pretreated.

[0027] More preferably, the pretreatment includes drying, pulverizing, and sieving.

[0028] More preferably, the drying temperature is 70~110°C.

[0029] More preferably, the lignocellulose has a mesh size of 40 to 80 mesh.

[0030] Preferably, the reaction temperature is 85~110℃.

[0031] In this invention, the reaction temperature in S2 is any value among 85℃, 86℃, 87℃, 88℃, 89℃, 90℃, 91℃, 92℃, 93℃, 94℃, 95℃, 96℃, 97℃, 98℃, 99℃, 100℃, 101℃, 102℃, 103℃, 104℃, 105℃, 106℃, 107℃, 108℃, 109℃, and 110℃.

[0032] More preferably, the reaction temperature is 88~102°C.

[0033] More preferably, the reaction temperature is 88~92°C.

[0034] Preferably, the reaction time is 4 to 10 hours. The reaction time can be any value among 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, and 10 hours.

[0035] More preferably, the reaction time is 6 to 9 hours.

[0036] More preferably, the reaction time is 7 to 9 hours.

[0037] Preferably, in step S2, the solid-liquid separation further includes adding ethanol to the system after the reaction and mixing.

[0038] Ethanol is added to the system after the reaction because the extract has a high viscosity. Washing with ethanol can break down the structure of the extract, allowing the solid residue to be completely separated from the liquid, which is beneficial for further extraction of lignin.

[0039] More preferably, the mass of the ethanol is 2 to 3 times the total mass of the system after the reaction.

[0040] More preferably, the mixing method is low-temperature stirring.

[0041] More preferably, the temperature of the low-temperature stirring is 50~80°C.

[0042] More preferably, the low-temperature stirring time is 1 to 2 hours.

[0043] Preferably, the solid-liquid separation method is vacuum filtration.

[0044] More preferably, in S3, the separated solution is the solution after ethanol removal, and the volume ratio of the solution after ethanol removal to water is 1:(10~15).

[0045] Preferably, in step S3, the mixture further includes separation and freeze-drying.

[0046] More preferably, the separation is centrifugation.

[0047] More preferably, the centrifugation speed is 8000-10000 r / min and the time is 5-10 min.

[0048] More preferably, the freeze-drying temperature is -60 to -40°C and the time is 8 to 16 hours.

[0049] Compared with the prior art, the present invention has the following beneficial effects: This invention uses benzyltriethylammonium chloride as a hydrogen bond acceptor and anhydrous oxalic acid as a hydrogen bond donor. After mixing with a polyol, light-colored lignin is extracted from lignin cellulose at specific temperatures and times, ensuring lignin yield while increasing the L* value of the lignin. This invention utilizes the hydroxyl structure of the polyol to form weak hydrogen bonds with lignin molecules, inhibiting lignin oxidative condensation and simultaneously enhancing the system's solubility for lignin. This achieves efficient lignin extraction under mild conditions, yielding light-colored lignin with a high L* value and high purity. Attached Figure Description

[0050] Figure 1 This is the technical solution route of the present invention.

[0051] Figure 2 Images of lignin samples extracted in Example 1, Comparative Example 3, and Comparative Example 5.

[0052] Figure 3 The graph shows the data on cellulose retention rate, hemicellulose removal rate, and lignin removal rate in the solid residues obtained in Examples 1-4.

[0053] Figure 4 The graph shows the data on cellulose retention rate, hemicellulose removal rate, and lignin removal rate in the solid residues obtained in Examples 1, 6, and Comparative Example 2. Detailed Implementation

[0054] The present invention is further illustrated below with reference to specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions in the art or as recommended by the manufacturer; the raw materials and reagents used, unless otherwise specified, are all commercially available from the conventional market. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention are within the scope of protection claimed by the present invention.

[0055] Bamboo powder: Manufacturer: Guangde Buqiucao Bamboo Industry Co., Ltd.

[0056] In this invention, the lignin removal rate is determined by high performance liquid chromatography.

[0057] In this invention, the method for calculating the solid residue recovery rate is as follows: Solid residue recovery rate = (Oven-dry mass of solid residue / Oven-dry mass of bamboo powder) × 100%; In this invention, the method for calculating the lignin yield is as follows: Lignin yield = [Oven-dry weight of lignin (g) / (Oven-dry weight of bamboo powder (g) × Mass fraction of lignin in bamboo powder)] × 100%; In this invention, the method for calculating the lignin removal rate is as follows: Lignin removal rate = [1 - (lignin content in residue × solid residue recovery rate) / lignin content in bamboo powder] × 100%; In this invention, the method for calculating the lignin L* value is as follows: The L* value of lignin was tested using a spectrophotometer (data color200 SCI mode).

[0058] Example 1 This embodiment provides a method for extracting light-colored lignin, including the following steps: S0. Mix triethylbenzylammonium chloride and anhydrous oxalic acid at a molar ratio of 1:1 until homogeneous, heat to 90°C and stir at 600 rpm until clear to obtain a eutectic solvent.

[0059] S1. Mix 1,4-butanediol with the prepared eutectic solvent, wherein the mass of 1,4-butanediol is 40 wt% of the mass of the eutectic solvent. Mix in a sealed container at 90°C for 20 min to obtain the extract.

[0060] S2. The bamboo powder was dried at 105℃, pulverized, and sieved to obtain bamboo powder with a mesh size of 60 mesh. The solid-liquid mass ratio of the extract to the bamboo powder was 10:1. Each raw material was weighed and placed in a constant-temperature oil bath at 90℃ for 8 hours. After the reaction was complete, ethanol was added to the system, with a mass twice the total mass of the system. The mixture was stirred at 60℃ for more than 1 hour, and then vacuum filtered to obtain the solid residue and filtrate.

[0061] S3. After evaporating the ethanol from the filtrate by heating, deionized water is added. The volume ratio of the filtrate to water after removing the ethanol is 1:10. The solid precipitates out and is centrifuged at 10000 r / min for 5 min. Then, it is freeze-dried at -60℃ for 10 h to obtain light-colored lignin.

[0062] Example 2 This embodiment provides a method for extracting light-colored lignin, which differs from Example 1 in that 1,4-butanediol in S1 is replaced with ethylene glycol.

[0063] Example 3 This embodiment provides a method for extracting light-colored lignin, which differs from Example 1 in that 1,4-butanediol in S1 is replaced with 1,3-propanediol.

[0064] Example 4 This embodiment provides a method for extracting light-colored lignin, which differs from Example 1 in that 1,4-butanediol in S1 is replaced with 1,2-pentanediol.

[0065] Example 5 This embodiment provides a method for extracting light-colored lignin, which differs from Example 1 in that the mass of 1,4-butanediol in S1 is 20 wt% of the mass of the eutectic solvent.

[0066] Example 6 This embodiment provides a method for extracting light-colored lignin, which differs from Embodiment 1 in that the reaction temperature in S2 is 100°C.

[0067] Example 7 This embodiment provides a method for extracting light-colored lignin, which differs from Embodiment 1 in that the reaction time in S2 is 6 hours.

[0068] Example 8 This embodiment provides a method for extracting lignin, which differs from Example 1 in that the mass of 1,4-butanediol in S1 is 10 wt% of the mass of the eutectic solvent.

[0069] Example 9 This embodiment provides a method for extracting lignin, which differs from Example 1 in that the mass of 1,4-butanediol in S1 is 50 wt% of the mass of the eutectic solvent.

[0070] Comparative Example 1 This comparative example provides a method for extracting lignin, which differs from Example 1 in that it does not contain 1,4-butanediol.

[0071] Comparative Example 2 This comparative example provides a method for extracting lignin, which differs from Example 1 in that the reaction temperature in S2 is 80°C.

[0072] Comparative Example 3 This comparative example provides a method for extracting lignin, which differs from Example 1 in that it does not contain 1,4-butanediol and the reaction time in S2 is 2 hours.

[0073] Comparative Example 4 This comparative example provides a method for extracting lignin, which differs from Example 1 in that anhydrous oxalic acid in S0 is replaced with anhydrous formic acid, 1,4-butanediol is absent in S1, and the reaction time in S2 is 2 hours.

[0074] Comparative Example 5 This comparative example provides a method for extracting lignin, which differs from Example 1 in that anhydrous oxalic acid in S0 is replaced with anhydrous malonic acid, 1,4-butanediol is absent in S1, and the reaction time in S2 is 2 hours.

[0075] Comparative Example 6 This comparative example provides a method for extracting lignin, which differs from Example 1 in that the reaction time in S2 is 2 hours.

[0076] Comparative Example 7 This comparative example provides a method for extracting lignin, including the following steps: S1. Mix choline chloride, glycerol, and maleic acid in a molar ratio of 1:3:4, mix and heat at 130°C until a homogeneous, transparent, and clear eutectic solvent is formed.

[0077] S2. Add bamboo powder and the eutectic solvent in S1 to the reactor at a solid-liquid mass ratio of 1:15. The reaction temperature is 130℃ and the reaction time is 2h. After the reaction is completed, add 45mL of ethanol to the system, stir, and then filter to obtain the filtrate.

[0078] S3. Water was added to the filtrate to precipitate lignin. After centrifugation and freeze-drying, lignin was obtained with a yield of 38.4%. The L* value was 36.3.

[0079] Comparative Example 8 This comparative example provides a method for extracting lignin, including the following steps: S1. Mix choline chloride, ethylene glycol, and p-hydroxybenzenesulfonic acid in a molar ratio of 1:3:4, mix at 130°C and heat and stir until a homogeneous, transparent, and clear eutectic solvent is formed.

[0080] S2. Add bamboo powder and the eutectic solvent in S1 to the reactor at a solid-liquid mass ratio of 1:15. Set the reaction temperature to 130℃ and the reaction time to 2h. After the reaction is complete, add 45mL of ethanol to the system, stir, and then filter to obtain the filtrate.

[0081] S3. Water was added to the filtrate to precipitate lignin. After centrifugation and freeze-drying, lignin was obtained with a yield of 34.6%. The L* value was 42.1.

[0082] The pretreated residue prepared in this embodiment was experimentally measured, and the solid residue recovery rate and lignin removal rate were calculated. The results are shown in Table 1. The L* value of the extracted lignin was tested using the following methods: the color of lignin was quantified by the brightness CIEL*a*b* values, where the L* value better reflects the color performance of lignin; a larger L* value indicates a lighter color of the lignin sample. The reflected light from the powder surface was detected using a colorimeter, and the L* value was calculated, representing the lightness of the color. a* represents the red-green value, and b* represents the yellow-blue value. Measurements must be performed under a uniform light source, and the instrument must be calibrated to ensure accuracy. The test results are shown in Table 1.

[0083] Table 1 Test Results

[0084] Table 1 shows that the solid residue recovery rate of Examples 1-9 is ≤65.60%, the lignin removal rate is ≥50.31%, and the lignin yield is ≥46.82%. Among them, the L* value of Examples 1-6 and Example 8 is ≥48.5, and the L* value of Examples 7 and 9 is similar to that of Example 1. The table shows that Example 1 has the best lignin removal efficiency, lignin yield, and L* value of the obtained lignin. This is because, on the one hand, the benzyl quaternary ammonium salt of the hydrogen bond acceptor has a benzyl aromatic ring that can bind to the phenylpropane structure of lignin through π-π interaction. Combined with its excellent phase transfer ability, it can efficiently transfer lignin from the solid phase of the raw material to the liquid phase, selectively breaking the lignin-carbohydrate complex (LCC complex) and the β-O-4 ether bond of lignin. Under mild reaction conditions, it can significantly inhibit the oxidation of lignin phenolic hydroxyl groups and the cross-linking condensation of benzene rings, thus preventing the product from turning brown and black at the source. On the other hand, the polyhydroxyl groups of the polyol can destroy the hydrogen bonds of the lignin-carbohydrate complex (LCC), swell the plant cell wall, and gently dissociate lignin, effectively inhibiting the oxidation condensation of phenolic hydroxyl groups and preventing lignin browning. Moreover, its hydroxyl reducing power and inertness are better than those of small molecule alcohols, reducing the oxidation polymerization of lignin phenolic hydroxyl groups during the extraction process, effectively increasing the L* value of lignin and reducing the color of lignin.

[0085] Comparative Example 1 did not introduce 1,4-butanediol. Compared with Example 1, the solid residue recovery rate was increased, the lignin removal rate was decreased, and the lignin L* value and yield were significantly reduced.

[0086] The reaction temperature of Comparative Example 2 was 80℃. Although the extracted lignin had a high L* value, the lignin yield was low.

[0087] Comparative Example 3 did not introduce 1,4-butanediol and the reaction time was 2 hours. The solid residue recovery rate was high, the lignin removal rate was low, and the lignin yield was low.

[0088] Comparative Examples 4 and 5 replaced the hydrogen bond donor in the eutectic solvent with that in Comparative Example 3, resulting in extremely low lignin yields. As can be seen from Comparative Examples 3 to 5, anhydrous oxalic acid, as a hydrogen bond donor, can extract more lignin from lignocellulose and maintain the L* value of lignin.

[0089] The reaction time of Comparative Example 6 was 2 hours. The solid residue recovery rate was relatively high, the lignin removal rate was relatively low, and the lignin L* value and yield were significantly reduced.

[0090] Comparative Example 7 used glycerol instead of 1,4-butanediol, maleic acid instead of anhydrous oxalic acid, and acetylcholine as the hydrogen bond acceptor. The L* value of the extracted lignin decreased significantly, and it turned dark brown. The lignin extraction rate was 45%. Comparative Example 8 used p-hydroxybenzenesulfonic acid instead of anhydrous oxalic acid, ethylene glycol instead of 1,4-butanediol, and acetylcholine as the hydrogen bond acceptor. The lignin extraction rate was higher, but the L* value was lower, and the lignin color was darker.

[0091] Figure 3 The graphs show the data on cellulose retention rate, hemicellulose removal rate, and lignin removal rate in the solid residues obtained in Examples 1-4. The comparison shows that for the eutectic solvent system composed of triethylbenzylammonium chloride and anhydrous oxalic acid, using 1,4-butanediol as an adjuvant is more conducive to lignin removal, thereby increasing the lignin yield.

[0092] Figure 4 The graphs show the cellulose retention rate, hemicellulose removal rate, and lignin removal rate in the solid residues obtained in Examples 1, 6, and Comparative Example 2. The comparison shows that, under the reaction system of this invention, controlling the reaction temperature at 90°C can significantly improve the lignin removal rate, thereby increasing the lignin yield.

[0093] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for extracting light-colored lignin, characterized in that, Includes the following steps: S1. Mix the eutectic solvent with the polyol to obtain the extract; S2. Mix the extract with lignocellulose, react, and then separate the solid and liquid components; S3. Mix the separated solution with water to precipitate a solid, yielding light-colored lignin; The eutectic solvent includes benzyltriethylammonium chloride and anhydrous oxalic acid; the reaction temperature is ≥85℃ and the reaction time is ≥4h.

2. The method according to claim 1, characterized in that, The polyol is at least one of ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, or glycerol.

3. The method according to claim 1, characterized in that, The molar ratio of benzyltriethylammonium chloride to anhydrous oxalic acid is (1~6):(1~6).

4. The method according to claim 1, characterized in that, The mass of the polyol is 10-50 wt% of the mass of the eutectic solvent.

5. The method according to claim 1, characterized in that, The solid-liquid mass ratio of the extract to lignocellulose is (8~10):

1.

6. The method according to claim 1, characterized in that, The lignocellulose is bamboo powder and / or wood powder.

7. The method according to claim 1, characterized in that, The reaction temperature is 85~110℃.

8. The method according to claim 1, characterized in that, The reaction time is 4 to 10 hours.

9. The method according to claim 1, characterized in that, In S2, the solid-liquid separation includes adding ethanol to the system after the reaction and mixing.

10. The method according to claim 9, characterized in that, In S3, the separated solution is the solution after ethanol removal, and the volume ratio of the solution after ethanol removal to water is 1:(10~15).

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