Preparation method of green and efficient lignin solvent

Abstract

The application belongs to the technical field of biomass resource high-value utilization and green solvent, and particularly relates to a preparation method of a green and efficient lignin solvent. By constructing a eutectic solvent system taking ethanolamine as a hydrogen bond donor and a specific nitrogen-containing compound as a hydrogen bond acceptor, efficient dissolution of lignin is realized under relatively mild conditions, so as to overcome the problems of low dissolution efficiency or high environmental load in the prior art. The application has the following beneficial effects: the eutectic solvent preparation method is simple, raw materials are easy to obtain, the system is green and environmentally friendly, conforms to the development direction of green chemistry, and exhibits excellent dissolution capacity for lignin, thereby providing a new idea for dissolution and high-value utilization of lignin.

Description

Technical Field

[0001] This invention belongs to the field of high-value utilization of biomass resources and green solvent technology, specifically relating to a method for preparing a green and efficient lignin solvent. Background Technology

[0002] Lignin is an important component of plant cell walls and the only naturally renewable aromatic polymer. Industrial lignin mainly comes from the pulp and paper industry and biorefining, characterized by its wide availability, large output, and low cost. However, industrial lignin has a complex molecular structure, a wide molecular weight distribution, and strong intermolecular hydrogen bonds and hydrophobic interactions, resulting in poor solubility in common solvents, which severely restricts its further application in materials, chemicals, and other fields.

[0003] Existing lignin solvents mainly include strong bases, organic solvents, and ionic liquid systems. Among them, strong bases and some organic solvents can easily cause damage to the lignin structure, affecting its structural integrity; although ionic liquids have good dissolving ability, they have problems such as high cost, complex preparation, and insufficient environmental friendliness.

[0004] In recent years, deep eutectic solvents (DES) have attracted widespread attention in the field of biomass treatment due to their advantages such as simple preparation, low volatility, high designability, and environmental friendliness. However, existing DES systems for lignin dissolution are mostly based on choline chloride DES, and their dissolution efficiency still needs to be improved. Therefore, developing a green and efficient deep eutectic solvent for dissolving lignin is of great significance. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by constructing a eutectic solvent system with ethanolamine as a hydrogen bond donor and specific nitrogen-containing compounds as hydrogen bond acceptors, thereby achieving efficient dissolution of lignin under relatively mild conditions and overcoming the problems of low dissolution efficiency or high environmental impact in existing technologies.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing regenerated lignin includes the following steps:

[0008] S1 mixes hydrogen bond acceptors and ethanolamine, which serves as a hydrogen bond donor, in a certain molar ratio to form a homogeneous, transparent eutectic solvent under heating and stirring conditions.

[0009] S2. Add lignin in batches to the eutectic solvent obtained in step S1, and heat and stir until the alkali lignin added within the set time no longer completely dissolves.

[0010] In some embodiments, to verify the dissolution effect of the eutectic solvent on lignin and its reversible separation performance, the lignin can be dissolved at a mass ratio of 1:5 to DES, and the mixture can be added dropwise to ethanol-water under stirring, followed by centrifugation to obtain regenerated lignin.

[0011] Optionally, the regeneration process further includes centrifuging, discarding the supernatant, adding ethanol and water again, centrifuging again to precipitate lignin, and freeze-drying to obtain regenerated lignin.

[0012] The hydrogen bond acceptor is selected from at least one of pyrazole, choline chloride, DBU, or proline.

[0013] The molar ratio of the hydrogen bond acceptor to ethanolamine is 1:(3-8), and the heating and stirring conditions are 80℃ and 300rpm, respectively.

[0014] Preferably, the lignin is alkali lignin, sulfate lignin, organic solvent lignin, etc., produced by pulping or biorefining enterprises;

[0015] Preferably, the heating temperature in S2 is 80-140℃ and the stirring speed is 200-400 rpm.

[0016] Preferably, the set time in S2 is within 2-3 hours.

[0017] Preferably, the volume ratio of ethanol to water during the regeneration process is 9:1 to 7:3, specifically, the volume is 50-60 ml.

[0018] Preferably, the stirring rate during the regeneration process is 400-600 rpm.

[0019] Preferably, the centrifugation rate during regeneration is 8000 rpm to 10000 rpm, and the centrifugation time is 10 min to 15 min.

[0020] The inventors first used DBU as a hydrogen bond acceptor and acetic acid, ethylene glycol, and ethanolamine (three substances with different pH ranges) as hydrogen bond donors to form a diethyl ether solvent (DES) for dissolving lignin. They found that the ethanolamine system was stronger, thus determining ethanolamine as the hydrogen bond donor and pyrazole, choline chloride, and proline (which have similar nitrogen-containing structures to DBU) as hydrogen bond acceptors to complete this invention. Therefore, compared with existing technologies, this invention has the following advantages: the method for preparing the eutectic solvent is simple, the raw materials are readily available, and the system is green and environmentally friendly, conforming to the development direction of green chemistry; in particular, the prepared eutectic solvent exhibits excellent dissolving ability for lignin, providing a new approach for the dissolution and high-value utilization of lignin. Detailed Implementation

[0021] To better understand the technical content of this invention, specific embodiments are provided below to further illustrate the invention.

[0022] Example 1: Determination of Hydrogen Bond Donors

[0023] In this embodiment, DBU was selected as the hydrogen bond acceptor, and acetic acid, ethylene glycol, and ethanolamine were used as representative acidic, neutral, and basic hydrogen bond donors, respectively, to compare the effects of hydrogen bond donors with different acid-base properties on lignin dissolution behavior.

[0024] In previous experiments, the inventors conducted exploratory studies on hydrogen bond acceptors and different hydrogen bond donors under various molar ratios. The results showed that for neutral and basic hydrogen bond donor systems, the dissolution capacity of the eutectic solvent system for lignin gradually increased with the increase of the amount of hydrogen bond donor. Among them, the dissolution effect was better when the molar ratio of hydrogen bond acceptor to hydrogen bond donor was 1:6.

[0025] For acidic hydrogen bond donor systems, if the proportion of hydrogen bond donors is too high, the acidity of the system is significantly enhanced, which can easily lead to unfavorable structural changes in lignin. Furthermore, relevant literature typically uses a lower proportion of acidic hydrogen bond donors to construct eutectic solvent systems. Therefore, in this embodiment, a relatively low molar ratio of 1:2 for the acidic hydrogen bond donor acetic acid is selected as a representative condition.

[0026] It should be noted that the purpose of this embodiment is to compare the effects of hydrogen bond donors with different acid-base properties on the lignin solubility, rather than to systematically optimize the molar ratio of each system. The selected molar ratio is a representative condition that can stably form a eutectic solvent and reflect the differences in solubility.

[0027] (1) Weigh the corresponding mass of reagents according to the molar ratios of DBU:acetic acid = 1:2, DBU:ethylene glycol = 1:6 and DBU:ethanolamine = 1:6 respectively, place them in blue-capped glass bottles, heat and stir at 80 ℃ and 300 rpm until the system changes from a solid-liquid mixture to a homogeneous, clear and transparent liquid, and obtain the corresponding eutectic solvent.

[0028] In this invention, solubility experiments were conducted on other systems, using molar ratios of 1:2, 1:4, and 1:6. It was found that the 1:6 system was more suitable for lignin solubility. However, in acidic systems, the proportion of hydrogen bond donors should not be too high. A review of existing literature indicates that a molar ratio of 1:2 is generally used. The initial purpose was to determine the effect of hydrogen bond donors at different pH ranges on lignin solubility; therefore, a molar ratio of 1:6, which is favorable for solubility, was chosen for non-acidic systems, while a molar ratio of 1:2 was used for acidic systems.

[0029] (2) Weigh 5 g of the eutectic solvent prepared above and place it in a reaction vessel. Heat and stir at 120 °C and 300 rpm. Add alkali lignin to the system stepwise, adding 15 mg each time. Continue adding lignin after the previous addition is completely dissolved until there are still visible undissolved solids within 2 h.

[0030] Based on the total mass of completely dissolved alkali lignin, calculate the solubility of alkali lignin in each eutectic solvent. The solubility S is calculated using the following formula:

[0031] ;

[0032] Experimental results show that under the above conditions,

[0033] The lignin solubility in the DBU:acetic acid system is 71.75%.

[0034] The lignin solubility in the DBU: ethylene glycol system is 105.89%.

[0035] The lignin solubility of the DBU:ethanolamine system was significantly increased to 140.97%.

[0036] (3) As can be seen from the above comparison results, under the same hydrogen bond acceptor conditions, the eutectic solvent formed by using basic ethanolamine as a hydrogen bond donor exhibits significantly higher solubility for alkali lignin than the systems using acidic and neutral hydrogen bond donors. Therefore, this invention preferably uses ethanolamine as a hydrogen bond donor to construct a highly efficient eutectic solvent system for dissolving lignin.

[0037] Example 2

[0038] (1) Based on the molar ratio of pyrazole to ethanolamine of 1:6, calculate and accurately weigh the corresponding mass of hydrogen bond acceptor (HBA, pyrazole) and hydrogen bond donor (HBD, ethanolamine), and place them in a blue-capped glass bottle. Heat and stir at 80 °C and 300 rpm until the system changes from a solid-liquid mixture to a homogeneous, clear, and transparent liquid, thus obtaining the desired eutectic solvent (DES).

[0039] (2) The dissolution experiment of lignin in eutectic solvent and the method for calculating solubility were carried out in accordance with Example 1.

[0040] Under the above conditions, the solubility of alkali lignin in this eutectic solvent is 163.46%.

[0041] (3) To verify the dissolution effect of the prepared eutectic solvent on lignin and its regeneration feasibility, a mass ratio of lignin to eutectic solvent of approximately 1:5 (i.e., lignin content of approximately 20 wt%) was selected as the regeneration experimental condition. On the one hand, this loading ratio is in the medium to high concentration range commonly used in existing studies on the dissolution of lignin by eutectic solvents and ionic liquid systems, which can effectively reflect the dissolution capacity of the solvent system; on the other hand, under this ratio condition, the system can still maintain good fluidity and homogeneity, which is conducive to the subsequent controllable regeneration and separation of lignin through non-solvent induction. Therefore, 5 g of eutectic solvent was weighed, and 1 g of alkali lignin was added at once. The mixture was stirred and dissolved for 1 h at 120 ℃ and 300 rpm to form a homogeneous dissolution system. Then, the mixed solution was slowly added dropwise to 50 mL of ethanol-water (9:1 / v:v), and stirred for 30 min at 500 rpm to allow lignin to regenerate and precipitate. The precipitate was obtained by centrifugation and washed repeatedly with ethanol and water (9:1 / v:v) until neutral. Finally, the regenerated lignin sample was obtained by freeze drying.

[0042] Based on preliminary exploratory experimental results and common practices in the field of eutectic solvents, in the remaining embodiments, only the type of DES was changed (Table 1), while the ratio of hydrogen bond acceptor to hydrogen bond donor remained at 1:6. To demonstrate the significant decrease in the lignin-dissolving performance of the eutectic solvent system when the amount of hydrogen bond donor is significantly insufficient, deviating from the concept of this invention, a molar ratio of hydrogen bond acceptor to ethanolamine of 1:2 was selected in the comparative examples. The remaining experimental procedures were consistent with Example 2. The DES used in different embodiments and comparative examples, along with the experimental results, are listed in Table 1.

[0043] Table 1

[0044]

[0045] Based on the experimental results of Examples 1 to 5 and the comparative examples, it can be clearly seen that the lignin solubility of the selected eutectic solvent system is closely related to the type of hydrogen bond acceptor and the molar ratio of hydrogen bond donor.

[0046] First, in Examples 1 to 5, all eutectic solvent systems exhibited excellent lignin solubility, especially the ethanolamine-based eutectic solvent. Specifically, the eutectic solvent system formed by ethanolamine and nitrogen-containing hydrogen bond acceptors (such as DBU, proline, pyrazole, etc.) at a molar ratio of 1:6 generally showed high lignin solubility, ranging from 135.29% to 163.46%. This indicates that ethanolamine, as a hydrogen bond donor, can effectively synergize with hydrogen bond acceptors to form a strong hydrogen bond network, thereby significantly improving the lignin solubility.

[0047] Secondly, the results of Comparative Examples 1 and 2 show that when the ratio of hydrogen bond donors to hydrogen bond acceptors is unreasonable (e.g., DBU:ethanolamine = 1:2, betaine:ethanolamine = 1:2), the solubility of the eutectic solvent system is significantly reduced, with solubilities of 76.87% and 48.66%, respectively. This phenomenon indicates that when the proportion of hydrogen bond donors is too low, the stability of the hydrogen bond network is insufficient, making it unable to effectively dissolve lignin and thus affecting the solubility efficiency of lignin.

[0048] Therefore, the eutectic solvent system designed in this invention can provide excellent lignin dissolution performance within a suitable ratio range between hydrogen bond donors and hydrogen bond acceptors. In particular, the ethanolamine system not only has good dissolving ability but also effectively regulates the strength and stability of the hydrogen bond network, providing a reliable technical foundation for the efficient dissolution and subsequent high-value utilization of lignin.

Claims

1. A method for preparing regenerated lignin, characterized in that, Includes the following steps: S1 mixes hydrogen bond acceptors with ethanolamine as a hydrogen bond donor to form a homogeneous, transparent eutectic solvent under heating and stirring conditions. S2. Add lignin in batches to the eutectic solvent obtained in step S1, and heat and stir until the alkali lignin added at the set time no longer completely dissolves. S3. The mixture obtained in step S2 is added dropwise to ethanol-water under stirring conditions, and then centrifuged to obtain regenerated lignin.

2. The preparation method according to claim 1, characterized in that, S3 also includes centrifuging, discarding the supernatant, adding ethanol and water, centrifuging again to precipitate lignin, and freeze-drying to obtain regenerated lignin.

3. The preparation method according to claim 1, characterized in that, The hydrogen bond acceptor is selected from at least one of pyrazole, choline chloride, DBU, or proline.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the hydrogen bond acceptor to ethanolamine is 1:(3-8); the heating temperature is 70-90℃, and the stirring speed is 200-400 rpm, preferably 80℃ and 300 rpm.

5. The preparation method according to claim 1, characterized in that, The lignin is alkali lignin, sulfate lignin, or organic solvent lignin produced by pulping or biorefining.

6. The preparation method according to claim 1, characterized in that, The heating temperature in S2 is 80-140℃, and the stirring speed is 200-400 rpm; preferably, the set time in S2 is 2-3 h.

7. The preparation method according to claim 1, characterized in that, The volume ratio of ethanol to water in S3 is 9:1 to 7:3; the stirring speed in S3 is 400-600 rpm.

8. The preparation method according to claim 1, characterized in that, In S3, the centrifugation rate is 8000 rpm to 10000 rpm; the centrifugation time is 10 min to 15 min.

9. The green and efficient lignin solvent obtained by the preparation method according to any one of claims 1 to 8.

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