Method for reducing lignin color by combining acidic deep eutectic solvent with monohydric alcohol
By treating lignin under mild conditions with a mixture of acidic eutectic solvent and monohydric alcohol, the problem of dark lignin color in existing technologies has been solved, achieving efficient and environmentally friendly lignin lightening and expanding its application areas.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SOUTH CHINA UNIV OF TECH
- Filing Date
- 2023-12-08
- Publication Date
- 2026-05-26
AI Technical Summary
Existing technologies for reducing lignin color often require harsh reaction conditions, which can lead to the destruction of the lignin structure or the generation of dark-colored groups by side reactions, limiting their application in fields with high color requirements. Furthermore, existing methods are time-consuming or cause significant pollution.
A mixture of acidic eutectic solvent and monohydric alcohol is used to treat lignin under mild reaction conditions to achieve decolorization in one step while preserving the structural integrity of lignin. The molar ratio of acidic eutectic solvent (composed of hydrogen bond acceptors and donors) to monohydric alcohol is 1:0.1–8, the reaction temperature is 80–190℃, and the reaction time is 0.5–10 h.
It achieves efficient lightening of lignin, reduces chemical usage, minimizes pollution, expands the application range of lignin, and is simple to operate and recyclable, suitable for a variety of lignin raw materials.
Smart Images
Figure CN117534846B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of high-value utilization technology of lignin, and specifically relates to a method for reducing the color of lignin by combining an acidic eutectic solvent with a monohydric alcohol. Background Technology
[0002] Lignin is the second largest renewable resource on Earth after cellulose and a major source of aromatic compounds in nature. In cellulose-based industries such as pulp and paper making and biofuels, lignin is a byproduct, with an effective utilization rate of less than 10%. This is because, under cellulose-first strategies, such as chemical pulping, lignin removal processes need to be carried out under harsh conditions to achieve as complete a removal of lignin as possible. However, under harsh conditions of high temperature, high pressure, and high-concentration solutions, lignin undergoes numerous side reactions, leading to the formation of a large number of chromophores and auxochromes. These side reactions cause lignin to darken in color, thus limiting its application in fields where color accuracy is critical, such as sunscreens. Eutectic solvents are novel green solvents composed of hydrogen bond donors and acceptors, with acidic eutectic solvents showing the best lignin removal effect. Compared to traditional chemical pulping processes, lignin can be removed under relatively mild reaction conditions to obtain pulp fibers, reducing energy consumption in the cooking process. Taking a eutectic solvent composed of benzyltriethylammonium chloride and formic acid as an example, it can remove more than 90% of lignin by reacting at atmospheric pressure and 130°C for 2 hours (Green Chem., 2023, 25, 3256). However, acidic eutectic solvent systems still experience a large number of side reactions during lignin extraction, and the resulting lignin byproducts are still relatively dark in color.
[0003] Currently, light-colored lignin is generally extracted from lignocellulose raw materials under mild reaction conditions. For example, lignin extracted using a mild dioxane aqueous solution is very light in color, but it is mainly used for structural studies. Industrial lignin separated from the pulp and paper industry and the cellulosic ethanol industry, however, is very dark in color. Chinese patent CN105199119B discloses a method for lightening industrial lignin. This method uses tetrahydrofuran as a lignin solvent and employs ultraviolet irradiation to reduce the color of lignin. This method is mild and can be carried out under sunlight. However, the process is slow and extremely time-consuming. The core of this method is to destroy the benzene ring structure of lignin through an oxidation reaction, thereby weakening the conjugated structure in the lignin macromolecule and achieving lignin decolorization. This weakens the amphiphilicity of lignin. Chinese patent CN111315802B achieves decolorization of lignin sulfonate and / or sulfonated lignin through a combined treatment of hypochlorite and hydrogen peroxide. This process is similar to pulp bleaching technology; however, the strong oxidizing conditions also severely damage the benzene ring structure of lignin. In the previous Chinese patent authorization text CN115678036B, a novel ternary eutectic solvent was used to extract light-colored lignin from woody biomass raw materials under low temperature and short time conditions, but its purpose was not to decolorize the lignin. Summary of the Invention
[0004] In order to overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide a method for reducing lignin color by combining an acidic eutectic solvent with a monohydric alcohol.
[0005] The lignin lightening method provided by this invention is environmentally friendly and simple, fully utilizes the advantages of wood biomass resources, and promotes the high-value utilization of lignin.
[0006] The objective of this invention is achieved through the following solution:
[0007] A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol includes the following steps:
[0008] (1) Acidic eutectic solvent and monohydric alcohol are mixed to obtain a mixed solvent;
[0009] (2) Add lignin to the mixed solvent obtained in step (1), react, and obtain a mixed solution;
[0010] (3) Add water to the mixed solution obtained in step (2) to precipitate light-colored lignin.
[0011] The acidic eutectic solvent in step (1) consists of hydrogen bond acceptors and hydrogen bond donors; the molar ratio of hydrogen bond acceptors to hydrogen bond donors is 1:0.1 to 8.
[0012] The hydrogen bond acceptor is at least one of benzyltrimethylammonium chloride, benzyltriethylammonium chloride, and choline chloride.
[0013] The hydrogen bond donor is at least one of formic acid, acetic acid, and lactic acid.
[0014] The monohydric alcohol in step (1) is at least one of methanol, ethanol, n-propanol, isopropanol, and n-butanol.
[0015] The molar ratio of the acidic eutectic solvent and the monohydric alcohol in step (1) is 1.1-9:1-20.
[0016] The lignin in step (2) is at least one of eutectic solvent lignin, organic solvent lignin, alkali lignin, sulfate lignin, and sulfonated lignin.
[0017] The lignin in step (2) has a mass percentage of 0.5 wt% to 5 wt% in the mixed solution.
[0018] The reaction in step (2) involves placing the mixed solution in a reaction vessel and reacting it; the gaseous atmosphere of the reaction is air or nitrogen.
[0019] The reaction in step (2) is carried out at a temperature of 80–190°C for 0.5–10 h.
[0020] The mass ratio of the mixed solution to water in step (3) is 1:4 to 50.
[0021] Mechanism of the invention:
[0022] This invention utilizes a mixed solvent consisting of an acidic eutectic solvent (composed of carboxylic acid and quaternary ammonium salt) and a monohydric alcohol to treat dark lignin. This method achieves efficient one-step decolorization without damaging the lignin's structure, resulting in lignin with a brightness similar to that of polished lignin. The method is applicable to various lignin raw materials, is simple to operate, and allows for solvent reuse. The resulting lighter-colored lignin has a wider range of applications and higher added value.
[0023] Compared with the prior art, the present invention has the following advantages and beneficial effects:
[0024] (1) The lignin decolorization method provided by the present invention uses low cost, non-toxic or low toxicity low cost low eutectic solvent and monohydric alcohol, which can be recycled and reused. This reduces the amount of chemicals used and also reduces the burden of subsequent pollution treatment, making it economical and environmentally friendly.
[0025] (2) While achieving lignin light color, it can retain the benzene ring structure of the lignin core, thus expanding the application scenarios of lignin.
[0026] (3) It has a wide range of applications for raw materials and can reduce the color of lignin or lignin derivatives under various processes. This method can also be directly used in the eutectic solvent black liquor after cooking, directly realizing the upgrading and transformation of lignin in the black liquor. There is no need to extract lignin from the black liquor in advance, simplifying the process and realizing the continuous production of high-value-added cellulose and lignin products in the eutectic solvent. Attached Figure Description
[0027] Figure 1 The images show the actual products of alkali lignin before and after color reduction in Example 1.
[0028] Figure 2 The images show the infrared spectra of alkali lignin before and after color reduction in Example 1. Detailed Implementation
[0029] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.
[0030] Unless otherwise specified, all reagents used in the examples are commercially available.
[0031] Alkali lignin (CAS No.: 8068-05-1) and sodium lignin sulfonate (CAS No.: 8061-51-6) were both purchased from Shanghai Maclean Biochemical Technology Co., Ltd.
[0032] The eutectic solvent lignin, sulfate lignin, and eucalyptus lignin were all prepared in the laboratory. The specific preparation steps are described in detail in the published work (Green Chem., 2023, 25, 3256).
[0033] Example 1
[0034] A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol includes the following steps:
[0035] Benzyltriethylammonium chloride, formic acid, and isopropanol were mixed thoroughly in a molar ratio of 1:1:11 and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of alkali lignin and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air, the reaction time was 2.0 h, and the reaction temperature was 150 °C. After the reaction was complete, 50.0 g of water was added to the resulting solution to precipitate light-colored lignin.
[0036] Example 2
[0037] A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol includes the following steps:
[0038] Benzyltriethylammonium chloride, formic acid, and isopropanol were mixed thoroughly in a molar ratio of 1:1.3:1.7 and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of the eutectic solvent lignin and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air, the reaction time was 5.0 h, and the reaction temperature was 130 °C. After the reaction was complete, 50.0 g of water was added to the resulting solution to precipitate light-colored lignin.
[0039] Example 3
[0040] A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol includes the following steps:
[0041] First, lignin in eucalyptus wood was dissolved using a eutectic solvent with a molar ratio of benzyltriethylammonium chloride to formic acid of 1:5 at 130°C for 1.5 hours. 275 g of the eutectic solvent black liquor and 117.75 g of isopropanol were weighed and added to a 1 L medium-low pressure reactor, resulting in a molar ratio of benzyltriethylammonium chloride, formic acid, and isopropanol of 1:5:3.3. The mass percentage of lignin in the mixed solution was 0.95 wt%. The reaction atmosphere was 0.5 MPa nitrogen, the reaction time was 4.0 hours, and the reaction temperature was 190°C. After the reaction, 10 g of the sample was mixed with 50.0 g of water, and light-colored lignin precipitated.
[0042] Example 4
[0043] A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol includes the following steps:
[0044] Choline chloride, lactic acid, and n-propanol were mixed thoroughly in a molar ratio of 1:1:11 and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of alkali lignin and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air, the reaction time was 2.5 h, and the reaction temperature was 150 °C. After the reaction was complete, 50.0 g of water was added to the resulting solution to precipitate light-colored lignin.
[0045] Example 5
[0046] A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol includes the following steps:
[0047] Choline chloride, acetic acid, and ethanol were mixed thoroughly in a molar ratio of 1:1:11 and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of sodium lignosulfonate and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air, the reaction time was 3.0 h, and the reaction temperature was 140 °C. After the reaction was complete, 50.0 g of water was added to the resulting solution to precipitate light-colored lignin.
[0048] Example 6
[0049] A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol includes the following steps:
[0050] Benzyltrimethylammonium chloride, acetic acid, and methanol were mixed thoroughly in a molar ratio of 1:1:11 and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of sulfate lignin and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air, the reaction time was 5.0 h, and the reaction temperature was 130 °C. After the reaction was complete, 50.0 g of water was added to the resulting solution to precipitate light-colored lignin.
[0051] Example 7
[0052] First, a eutectic solvent with a molar ratio of benzyltriethylammonium chloride to formic acid of 1:5 was used to dissolve lignin in eucalyptus wood at 130°C for 2 hours. 6.56 g of the reacted eutectic solvent black liquor and 3.44 g of isopropanol were mixed thoroughly and placed in a 20 mL reaction vessel. The molar ratio of benzyltriethylammonium chloride, formic acid, and isopropanol was 1:5:4.2; the mass percentage of lignin in the mixed solution was 0.89 wt%. The reaction atmosphere was air, the reaction temperature was 150°C, and the reaction time was 2.0 h. After the reaction was complete, 50.0 g of water was added to the solvent to precipitate the lignin.
[0053] The following comparative cases demonstrate that both acidic eutectic solvents and monohydric alcohols play an indispensable role in the decolorization of lignin. Comparative Examples 1 and 2 demonstrate the role of acidic eutectic solvents, while Comparative Examples 3 and 4 demonstrate the role of monohydric alcohols.
[0054] Comparative Example 1
[0055] Benzyltriethylammonium chloride, water, and isopropanol were mixed thoroughly at a molar ratio of 1:2.1:8.8, and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of alkali lignin and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air. The reaction temperature was 150 °C, and the reaction time was 2.0 h. After the reaction was complete, 50.0 g of water was added to the resulting solution to precipitate lignin.
[0056] Comparative Example 2
[0057] Formic acid and isopropanol were mixed thoroughly at a molar ratio of 1:15 and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of alkali lignin and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air, the reaction temperature was 150 °C, and the reaction time was 2.0 h. After the reaction was completed, the lignin was essentially undissolved.
[0058] Comparative Example 3
[0059] First, lignin in eucalyptus wood was dissolved using a eutectic solvent with a molar ratio of benzyltriethylammonium chloride to formic acid of 1:5 at 130°C for 2 hours. 10g of the resulting eutectic solvent black liquor was placed in a 20mL reaction vessel. The mass percentage of lignin in the mixed solution was 1.36wt%. The reaction atmosphere was air, the reaction temperature was 150°C, and the reaction time was 2.0 hours. After the reaction was complete, 50.0g of water was added to the solvent to precipitate the lignin.
[0060] Comparative Example 4
[0061] Benzyltriethylammonium chloride, formic acid, and water were mixed thoroughly in a molar ratio of 1:1.1:5.5, and stirred at room temperature to form a homogeneous and transparent liquid, yielding a mixed solvent. 0.1 g of alkali lignin and 10.0 g of the mixed solvent were weighed and added to a 20 mL reaction vessel. The reaction atmosphere was air, the reaction temperature was 150 °C, and the reaction time was 2.0 h. After the reaction was complete, 50.0 g of water was added to the resulting solution to precipitate lignin.
[0062] The color of lignin is represented by the Lab value measured by a colorimeter. L: represents the lightness of the color; positive values indicate a whitish hue, and negative values indicate a darker hue. a: represents the red-green value; positive values indicate a reddish hue, and negative values indicate a greenish hue. b: represents the yellow-blue value; positive values indicate a yellowish hue, and negative values indicate a bluish hue. Table 1 lists the Lab values of the lignin raw materials and the lignin in each example. The light-colored lignin produced by some processes is close to the level of ground wood lignin. Figure 1 The images show the actual products of alkali lignin before and after color reduction in Example 1. Figure 2 The infrared spectra of alkali lignin before and after color reduction in Example 1 are shown at 1600, 1510, and 1460 cm⁻¹. -1 The peaks at the positions represent the characteristic peaks of the benzene rings in lignin. The presence of these characteristic peaks before and after color fading indicates that the basic benzene ring structure in lignin has not been destroyed.
[0063] Table 1. Lab values of lignin in different examples
[0064] L a b Eucalyptus wood lignin 60.54 16.91 24.06 Alkali lignin 21.34 4.77 8.22 Eutectic Solvent Lignin 22.38 4.17 6.61 Example 1 57.29 9.53 28.06 Example 2 48.64 9.82 23.79 Example 3 47.31 10.28 21.28 Example 4 51.02 10.51 30.38 Example 5 51.18 6.61 22.48 Example 6 56.78 9.53 28.06 Example 7 43.89 8.39 19.89 Comparative Example 1 18.25 3.28 7.88 Comparative Example 2 20.25 6.56 9.22 Comparative Example 3 17.78 7.93 10.36 Comparative Example 4 25.78 7.93 10.36
[0065] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.
Claims
1. A method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol, characterized in that, Includes the following steps: (1) Acidic eutectic solvent and monohydric alcohol are mixed to obtain a mixed solvent; The acidic eutectic solvent is composed of hydrogen bond acceptors and hydrogen bond donors; the molar ratio of hydrogen bond acceptors to hydrogen bond donors is 1:0.1~8; the hydrogen bond acceptor is at least one of benzyltrimethylammonium chloride, benzyltriethylammonium chloride, and choline chloride; the hydrogen bond donor is at least one of formic acid, acetic acid, and lactic acid. The monohydric alcohol is at least one of methanol, ethanol, n-propanol, isopropanol, and n-butanol; (2) Add lignin to the mixed solvent obtained in step (1) and react to obtain a mixed solution; the reaction temperature is 80~190℃; the time is 0.5~10 h; (3) Add water to the mixed solution obtained in step (2), and light-colored lignin will precipitate; The molar ratio of the acidic eutectic solvent and the monohydric alcohol in step (1) is 1.1~9:1~20.
2. The method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol according to claim 1, characterized in that: The lignin in step (2) is at least one of eutectic solvent lignin, organic solvent lignin, alkali lignin, sulfate lignin, and sulfonated lignin.
3. The method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol according to claim 1, characterized in that: The lignin in step (2) has a mass percentage of 0.5 wt% to 5 wt% in the mixed solution.
4. The method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol according to claim 1, characterized in that: The gaseous atmosphere for the reaction in step (2) is air or nitrogen.
5. The method for reducing lignin color using an acidic eutectic solvent in combination with a monohydric alcohol according to claim 1, characterized in that: The mass ratio of the mixed solution to water in step (3) is 1:4~50.