Method for oxidative depolymerization of lignin by using eutectic solvent system containing reversible soluble alkali

Abstract

The invention discloses a method for oxidative depolymerization of lignin through a deep eutectic solvent system containing reversible soluble alkali, and belongs to the technical field of efficient utilization of biomass resources, the method comprises the following steps: mixing a lignin raw material with the deep eutectic solvent system containing the reversible soluble alkali, adding hydrogen peroxide to carry out an oxidation reaction, after the reaction is finished, solid residues are centrifugally separated, CO2 is introduced into supernate obtained through centrifugation to remove reversible soluble alkali in reaction liquid, the lignin oligomer is precipitated through a gradient acid precipitation method, and the content of phenolic hydroxyl groups and carboxyl groups in the lignin oligomer can be adjusted by adjusting the dosage of the reversible soluble alkali and the dosage of hydrogen peroxide. The method can realize mild and efficient depolymerization of lignin, avoids a complex separation process for extracting solid lignin components, improves the separation purity of lignin, and is low in reaction solvent cost, simple in recovery process, convenient to operate, high in practicability and easy for industrial popularization.

Description

Technical Field

[0001] The invention belongs to the technical field of efficient utilization of biomass resources, and in particular relates to a method for oxidatively depolymerizing lignin by utilizing a low eutectic solvent system containing a reversible soluble alkali. Background Art

[0002] Lignin is a natural aromatic polymer widely found in plants. It is composed of phenylpropane structural units connected by ether bonds and carbon-carbon bonds, of which the ether bonds account for about 60-75% and the carbon-carbon bonds account for about 25-30%. Lignin is the most abundant non-petrochemical aromatic polymer resource in nature and an important hydrocarbon precursor, but lignin is difficult to be effectively utilized due to its high molecular weight, polydisperse molecular weight distribution, low reactivity and structural heterogeneity.

[0003] Oxidative depolymerization is a method of achieving lignin depolymerization through the combined action of oxidants, solvents and catalysts. Compared with the pyrolysis and hydrogenolysis of lignin, the outstanding advantages of oxidative depolymerization are mild reaction temperature and fast reaction speed, low cost and green environmental protection. The product obtained by depolymerization is a mixture of polydispersed lignin oligomers and phenolic monomers, which has the value or potential as an energy material chemical. However, the solvent system used for oxidative depolymerization in the prior art includes acid / alkaline aqueous solution, organic solvent, ionic liquid, etc., which has the disadvantages of complex process conditions and difficult product treatment. Low eutectic solvents are widely used because of their advantages of green environmental protection, biodegradability, recyclability, low cost and easy preparation.

[0004] The deep eutectic solvent (DES) can diffuse into the dense structure of lignin by hydrogen bonding, overcome the forces between and within lignin molecules, and has a good dissolving effect on lignin. DES is mainly composed of hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA), and can achieve selective dissolution and extraction separation of lignin by adjusting the composition and molar ratio of HBD and HBA. Among them, the alkaline deep eutectic solvent has the effect of degradation and solubilization on lignin, while reducing the occurrence of condensation reactions. Patent CN118085326A discloses a method for extracting lignin from grape vines and a lignin carboxymethyl cellulose composite film, and proposes to use alkaline low eutectic solvent potassium carbonate / ethylene glycol to extract lignin with high purity, low molecular weight and high hydroxyl. Patent CN115058020B discloses a method for preparing nano-lignin from industrial waste using alkaline low eutectic solvents at room temperature, and proposes to use alkaline low eutectic solvents such as choline chloride / N-methylethylenediamine, choline chloride / dimethylaminopropylamine to prepare nano-lignin with excellent antioxidant properties. However, after dissolving lignin, the low eutectic solvent system in the prior art usually requires a complex separation process to precipitate the lignin, resulting in low separation purity and high loss rate of lignin, which is not conducive to further high value of lignin; at the same time, the alkaline low eutectic solvent often needs to adjust the pH during the recovery of lignin, resulting in the alkaline components being reacted to form salts that are difficult to separate and accumulate in the low eutectic solvent system, reducing the recovery effect of the low eutectic solvent.

[0005] Therefore, how to provide an oxidative depolymerization solvent system with a simple solvent recovery process and high lignin separation purity is a technical problem that technical personnel in this field urgently need to solve. Summary of the invention

[0006] Technical problem: To solve the above problems, the present invention proposes a method for oxidatively depolymerizing lignin using a low eutectic solvent system containing a reversible soluble alkali, wherein lignin is oxidatively depolymerized into oligomers using a low eutectic solvent system containing a reversible soluble alkali and hydrogen peroxide.

[0007] Technical solution: To achieve the above object, the present invention provides a method for oxidatively depolymerizing lignin using a low eutectic solvent system containing a reversible soluble alkali, comprising the following steps:

[0008] Step 1. A low eutectic solvent is mixed with a reversibly soluble alkaline solution to obtain a mixed solvent;

[0009] Step 2. Adding lignin raw material and hydrogen peroxide to the mixed solvent obtained in step 1 to carry out oxidation reaction to obtain a reaction mixture;

[0010] Step 3. Adjust the pH of the reaction mixture obtained in step 2 to 1-4, separate the solid residue and the supernatant by centrifugation, and pass CO into the supernatant. 2The reversible soluble alkali is removed, and the depolymerization product is collected by gradient acid precipitation method and dried for later use.

[0011] in,

[0012] The low eutectic solvent is obtained by mixing and heating a hydrogen bond acceptor and a hydrogen bond donor.

[0013] The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:0.5 to 1:5.

[0014] The reversible soluble base is one of magnesium hydroxide, calcium hydroxide, strontium hydroxide or barium hydroxide.

[0015] The mass ratio of the reversibly soluble alkaline solution to the low eutectic solvent is 0.1:1 to 1:1.

[0016] The lignin raw material is enzymatically hydrolyzed lignin.

[0017] The mass ratio of the lignin raw material to the low eutectic solvent is 1:50 to 1:5.

[0018] The mass ratio of the hydrogen peroxide to the lignin raw material is 1:10 to 10:1.

[0019] The temperature of the oxidation reaction is 10-150° C. and the time is 1-10 hours.

[0020] The gradient acid precipitation method is used to adjust the pH of the supernatant to 6, 4, and 2 to obtain lignin by precipitation.

[0021] Beneficial effect: The present invention adds a reversible soluble alkali to the low eutectic solvent, which can not only enhance the alkalinity of the solvent system and efficiently dissolve lignin, but also the reversible soluble alkali can react with CO 2 Insoluble carbonate precipitation is formed and removed. Therefore, the recovery process of the low eutectic solvent containing a reversible soluble base is simpler than that of the conventional alkaline low eutectic solvent. Lignin and hydrogen peroxide are directly added to the low eutectic solvent system containing a reversible soluble base for oxidative depolymerization reaction to generate lignin oligomers, which greatly simplifies the process of converting lignin into phenolic oligomers. In addition, in the low eutectic solvent system, the cost of the reaction process is greatly reduced.

[0022] The present invention provides a method for oxidative depolymerization of lignin using a low eutectic solvent system containing a reversible soluble alkali. The present invention uses a low eutectic solvent system to solubilize lignin, and further directly oxidatively depolymerizes lignin into low molecular weight compounds in the low eutectic solvent system, thereby avoiding the complicated extraction and separation process of solid lignin components and simplifying the process flow. The present invention does not require the use of high-pressure equipment or hydrogen, and the low eutectic solvent can be recycled, significantly reducing the cost of the conversion process. In addition, the low eutectic solvent system used in the present invention contains a reversible soluble alkali, which provides alkaline conditions for the oxidative depolymerization of lignin, and the reversible soluble alkali can react with CO 2 The generated precipitate is separated from the low eutectic solvent, which is convenient for recycling the low eutectic solvent. The obtained lignin oligomer is rich in phenolic hydroxyl and carboxyl active functional groups, which is conducive to the development and utilization of high value-added lignin products. Finally, the reaction process of the present invention is simple to operate, highly practical, low in cost, and easy to promote industrialization. DETAILED DESCRIPTION

[0023] The lignin raw material is mixed with a low eutectic solvent containing a reversible soluble alkali, and then hydrogen peroxide is added to react. The resulting reaction pH is adjusted to 1-4, and the solid residue and the supernatant are separated by centrifugation. The solid residue is washed with distilled water and dried. The supernatant is subjected to a gradient acid precipitation method to collect the depolymerization product and dry it for later use.

[0024] Preferably, the low eutectic solvent is obtained by heating and mixing a hydrogen bond donor and a hydrogen bond acceptor.

[0025] Preferably, the hydrogen bond donors include carboxylic acids, polyols and amines.

[0026] The carboxylic acids include one or any of lactic acid, formic acid, and propionic acid;

[0027] The polyols include one or any of ethylene glycol, propylene glycol, and 1,4-butanediol;

[0028] The amines include one or any combination of urea, thiourea and ethanolamine.

[0029] Preferably, the hydrogen bond acceptors include quaternary ammonium salts, metal salts and zwitterions;

[0030] The quaternary ammonium salts include one or any of choline chloride and tetrabutylammonium chloride;

[0031] The metal salts include one or any combination of aluminum chloride, ferric chloride, and potassium carbonate;

[0032] The zwitterions include betaine.

[0033] Preferably, the molar ratio of the hydrogen bond donor to the hydrogen bond acceptor is 1:0.5 to 1:5.

[0034] Preferably, the reversible soluble base includes one or any combination of magnesium hydroxide, calcium hydroxide, strontium hydroxide and barium hydroxide.

[0035] The hydrogen bond donors selected for the low eutectic solvent in the present invention are mainly polyols and amines, and the hydrogen bond acceptors are mainly quaternary ammonium salts and metal salts. The first step is to dissolve the lignin component in a low eutectic solvent system containing a reversible soluble base. The polyols in the selected hydrogen bond donors can provide hydroxyl groups, interact with the ether bonds present in the lignin structure, enhance the dissolution effect of lignin, and the addition of polyols can prevent the occurrence of polymerization reactions. The addition of amides in the hydrogen bond donor provides alkaline conditions for the low eutectic solvent, which is conducive to the solubilization of lignin. Hydrogen bond acceptors choline chloride and potassium carbonate provide anions to dissolve lignin better, among which chloride ions have been shown to promote the cleavage of β-O-4 bonds and hinder lignin condensation. The second step is to add hydrogen peroxide to the low eutectic solvent system containing a reversible soluble base, and oxidative depolymerization of lignin macromolecules is low molecular weight phenolic compounds. The quaternary ammonium salt hydrogen bond acceptors in the deep eutectic solvent can interact with the hydrogen bond donor polyols or amides through strong hydrogen bonds, breaking the β-O-4 bonds in the lignin structure and promoting the catalytic conversion of macromolecular lignin. The alkaline environment provided by the deep eutectic solvent system can both dissolve lignin and promote the decomposition of hydrogen peroxide, which is beneficial to the oxidative depolymerization of lignin.

[0036] Preferably, the mass ratio of the reversibly soluble base to the low eutectic solvent is 0.1:1 to 1:1.

[0037] More preferably, the mass ratio of the reversibly soluble base to the low eutectic solvent is 0.1:1 to 0.6:1.

[0038] Preferably, the lignin raw material is one or any combination of alkali lignin, enzymatic lignin, and organic solvent lignin.

[0039] Preferably, the mass ratio of the lignin raw material to the low eutectic solvent is 1:50 to 1:5.

[0040] More preferably, the mass ratio of the lignin raw material to the low eutectic solvent is 1:40 to 1:10.

[0041] Preferably, the mass ratio of the oxidant hydrogen peroxide to the lignin raw material is 1:10 to 10:1.

[0042] More preferably, the mass ratio of the oxidant hydrogen peroxide to the lignin raw material is 1:5 to 5:1.

[0043] Preferably, the reaction temperature is 10-150° C. and the reaction time is 1-10 h.

[0044] More preferably, the reaction temperature is 20-100° C. and the reaction time is 1-5 h.

[0045] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with specific implementation methods.

[0047] The raw materials in the examples of the present invention are all purchased from commercial sources.

[0048] Example 1

[0049] The method of the present invention for oxidatively depolymerizing lignin using a low eutectic solvent system containing a reversible soluble alkali specifically comprises the following steps:

[0050] (1) adding choline chloride and urea in a molar ratio of 1:2 into a reaction container, respectively, heating and stirring the reaction container to form a uniform and stable low eutectic solvent, adding a 10 wt % saturated calcium hydroxide solution thereto, and stirring to obtain a low eutectic solvent system containing a reversible soluble base;

[0051] (2) 2 g of lignin and 40 g of a low eutectic solvent were added to a sealed reaction vessel, heated to 80° C. in a constant temperature oil bath and stirred for a period of time, and then 3 ml of hydrogen peroxide was added to the reactor for oxidation reaction for 2 h. After the reaction was completed, the mixture was quickly cooled to room temperature, and then the reaction mixture was separated by centrifugation to obtain a solid residue and a supernatant ①. The solid residue was washed with acidic water and dried;

[0052] (3) First, CO was introduced into the supernatant ①. 2 The calcium hydroxide in the precipitated reaction solution was centrifuged to precipitate calcium carbonate and obtain the supernatant ②. The lignin depolymerization product was extracted by gradient acid precipitation. The pH of the supernatant ② was adjusted to 6, 4, and 2 in sequence. After centrifugation, L pH＝6 , L pH＝4 , L pH＝2 , washed, dried to constant weight and ground for later use. The aqueous phase of the mixed low eutectic solvent is concentrated by rotary evaporation to recover the low eutectic solvent and deionized water.

[0053] For L pH＝6 , L pH＝4 , L pH＝2Conduct aqueous phase potentiometric titration to calculate the contents of phenolic hydroxyl and carboxyl groups, where L pH＝6 The phenolic hydroxyl content is 1.54mmol / g, and the carboxyl content is 1.99mmol / g; pH＝4 The phenolic hydroxyl content is 2.18mmol / g, and the carboxyl content is 2.85mmol / g; pH＝2 The phenolic hydroxyl content is 2.49 mmol / g, and the carboxyl content is 3.09 mmol / g.

[0054] Example 2

[0055] The method of the present invention for oxidatively depolymerizing lignin using a low eutectic solvent system containing a reversible soluble alkali specifically comprises the following steps:

[0056] (1) adding choline chloride and urea in a molar ratio of 1:2 into a reaction container, respectively, heating and stirring the reaction container to form a uniform and stable low eutectic solvent, adding a 20 wt % saturated calcium hydroxide solution thereto, and stirring to obtain a low eutectic solvent system containing a reversible soluble base;

[0057] (2) 2 g of lignin and 40 g of a low eutectic solvent were added to a sealed reaction vessel, heated to 80° C. in a constant temperature oil bath and stirred for a period of time, and then 3 ml of hydrogen peroxide was added to the reactor for oxidation reaction for 2 h. After the reaction was completed, the mixture was quickly cooled to room temperature, and then the reaction mixture was separated by centrifugation to obtain a solid residue and a supernatant ①. The solid residue was washed with acidic water and dried;

[0058] (3) First, CO was introduced into the supernatant ①. 2 The calcium hydroxide in the precipitated reaction solution was centrifuged to precipitate calcium carbonate and obtain the supernatant ②. The lignin depolymerization product was extracted by gradient acid precipitation. The pH of the supernatant ② was adjusted to 6, 4, and 2 in sequence. After centrifugation, L pH＝6 , L pH＝4 , L pH＝2 , washed, dried to constant weight and ground for later use. The aqueous phase of the mixed low eutectic solvent is concentrated by rotary evaporation to recover the low eutectic solvent and deionized water.

[0059] For L pH＝6 , L pH＝4 , L pH＝2 Conduct aqueous phase potentiometric titration to calculate the contents of phenolic hydroxyl and carboxyl groups, where L pH＝6 The phenolic hydroxyl content is 1.71mmol / g, and the carboxyl content is 2.05mmol / g; pH＝4 The phenolic hydroxyl content is 2.21mmol / g, and the carboxyl content is 3.16mmol / g; pH＝2 The phenolic hydroxyl content is 2.56 mmol / g, and the carboxyl content is 3.57 mmol / g.

[0060] Example 3

[0061] The method of the present invention for oxidatively depolymerizing lignin using a low eutectic solvent system containing a reversible soluble alkali specifically comprises the following steps:

[0062] (1) adding choline chloride and urea in a molar ratio of 1:2 into a reaction container, respectively, heating and stirring the reaction container to form a uniform and stable low eutectic solvent, adding a 30 wt % saturated calcium hydroxide solution thereto, and stirring to obtain a low eutectic solvent system containing a reversible soluble base;

[0063] (2) 2 g of lignin and 40 g of a low eutectic solvent were added to a sealed reaction vessel, heated to 80° C. in a constant temperature oil bath and stirred for a period of time, and then 3 ml of hydrogen peroxide was added to the reactor for oxidation reaction for 2 h. After the reaction was completed, the mixture was quickly cooled to room temperature, and then the reaction mixture was separated by centrifugation to obtain a solid residue and a supernatant ①. The solid residue was washed with acidic water and dried;

[0064] (3) First, CO was introduced into the supernatant ①. 2 The calcium hydroxide in the precipitated reaction solution was centrifuged to precipitate calcium carbonate and obtain the supernatant ②. The lignin depolymerization product was extracted by gradient acid precipitation. The pH of the supernatant ② was adjusted to 6, 4, and 2 in sequence. After centrifugation, L pH＝6 , L pH＝4 , L pH＝2 , washed, dried to constant weight and ground for later use. The aqueous phase of the mixed low eutectic solvent is concentrated by rotary evaporation to recover the low eutectic solvent and deionized water.

[0065] For L pH＝6 , L pH＝4 , L pH＝2 Conduct aqueous phase potentiometric titration to calculate the contents of phenolic hydroxyl and carboxyl groups, where L pH＝6 The phenolic hydroxyl content is 1.67mmol / g, and the carboxyl content is 2.01mmol / g; pH＝4 The phenolic hydroxyl content is 2.15mmol / g, and the carboxyl content is 3.07mmol / g; pH＝2 The phenolic hydroxyl content is 2.55 mmol / g, and the carboxyl content is 3.38 mmol / g.

[0066] Example 4

[0067] The method of the present invention for oxidatively depolymerizing lignin using a low eutectic solvent system containing a reversible soluble alkali specifically comprises the following steps:

[0068] (1) adding choline chloride and urea in a molar ratio of 1:2 into a reaction container, respectively, heating and stirring the reaction container to form a uniform and stable low eutectic solvent, adding a 20 wt % saturated calcium hydroxide solution thereto, and stirring to obtain a low eutectic solvent system containing a reversible soluble base;

[0069] (2) 2 g of lignin and 40 g of a low eutectic solvent were added to a sealed reaction vessel, heated to 80° C. in a constant temperature oil bath and stirred for a period of time, and then 5 ml of hydrogen peroxide was added to the reactor for oxidation reaction for 2 h. After the reaction was completed, the mixture was quickly cooled to room temperature, and then the reaction mixture was separated by centrifugation to obtain a solid residue and a supernatant ①. The solid residue was washed with acidic water and dried;

[0070] (3) First, CO was introduced into the supernatant ①. 2 The calcium hydroxide in the precipitated reaction solution was centrifuged to precipitate calcium carbonate and obtain the supernatant ②. The lignin depolymerization product was extracted by gradient acid precipitation. The pH of the supernatant ② was adjusted to 6, 4, and 2 in sequence. After centrifugation, L pH＝6 , L pH＝4 , L pH＝2 , washed, dried to constant weight and ground for later use. The aqueous phase of the mixed low eutectic solvent is concentrated by rotary evaporation to recover the low eutectic solvent and deionized water.

[0071] For L pH＝6 , L pH＝4 , L pH＝2 Conduct aqueous phase potentiometric titration to calculate the contents of phenolic hydroxyl and carboxyl groups, where L pH＝6 The phenolic hydroxyl content is 1.87mmol / g, and the carboxyl content is 2.42mmol / g; pH＝4 The phenolic hydroxyl content is 2.39mmol / g, and the carboxyl content is 3.46mmol / g; pH＝2 The phenolic hydroxyl content is 2.58 mmol / g, and the carboxyl content is 3.84 mmol / g.

[0072] Example 5

[0073] The method of the present invention for oxidatively depolymerizing lignin using a low eutectic solvent system containing a reversible soluble alkali specifically comprises the following steps:

[0074] (1) adding choline chloride and urea in a molar ratio of 1:2 into a reaction container, respectively, heating and stirring the reaction container to form a uniform and stable low eutectic solvent, adding a 20 wt % saturated calcium hydroxide solution thereto, and stirring to obtain a low eutectic solvent system containing a reversible soluble base;

[0075] (2) 2 g of lignin and 40 g of a low eutectic solvent were added to a sealed reaction vessel, heated to 80° C. in a constant temperature oil bath and stirred for a period of time, and then 10 ml of hydrogen peroxide was added to the reactor for oxidation reaction for 2 h. After the reaction was completed, the mixture was quickly cooled to room temperature, and then the reaction mixture was separated by centrifugation to obtain a solid residue and a supernatant ①. The solid residue was washed with acidic water and dried;

[0076] (3) First, CO was introduced into the supernatant ①. 2 The calcium hydroxide in the precipitated reaction solution was centrifuged to precipitate calcium carbonate and obtain the supernatant ②. The lignin depolymerization product was extracted by gradient acid precipitation. The pH of the supernatant ② was adjusted to 6, 4, and 2 in sequence. After centrifugation, L pH＝6 , L pH＝4 , L pH＝2 , washed, dried to constant weight and ground for later use. The aqueous phase of the mixed low eutectic solvent is concentrated by rotary evaporation to recover the low eutectic solvent and deionized water.

[0077] For L pH＝6 , L pH＝4 , L pH＝2 Conduct aqueous phase potentiometric titration to calculate the contents of phenolic hydroxyl and carboxyl groups, where L pH＝6 The phenolic hydroxyl content is 1.96mmol / g, and the carboxyl content is 3.07mmol / g; pH＝4 The phenolic hydroxyl content is 2.57mmol / g, and the carboxyl content is 3.79mmol / g; pH＝2 The phenolic hydroxyl content is 2.62 mmol / g, and the carboxyl content is 4.05 mmol / g.

[0078] Comparative Example

[0079] The reaction was carried out without adding hydrogen peroxide:

[0080] (1) adding choline chloride and urea in a molar ratio of 1:2 into a reaction container, respectively, heating and stirring the reaction container to form a uniform and stable low eutectic solvent, adding a 20 wt % saturated calcium hydroxide solution thereto, and stirring to obtain a low eutectic solvent system containing a reversible soluble base;

[0081] (2) 2 g of lignin and 40 g of a low eutectic solvent were added to a sealed reaction vessel, respectively, and heated to 80° C. in a constant temperature oil bath and stirred for a period of time. After the mixture was cooled to room temperature, the reaction mixture was separated by centrifugation to obtain a solid residue and a supernatant ①. The solid residue was washed with acidic water and dried.

[0082] (3) First, CO was introduced into the supernatant ①. 2The calcium hydroxide in the precipitated reaction solution was centrifuged to precipitate calcium carbonate and obtain the supernatant ②. The lignin depolymerization product was extracted by gradient acid precipitation. The pH of the supernatant ② was adjusted to 6, 4, and 2 in sequence. After centrifugation, L pH＝6 , L pH＝4 , L pH＝2 , washed, dried to constant weight and ground for later use. The aqueous phase of the mixed low eutectic solvent is concentrated by rotary evaporation to recover the low eutectic solvent and deionized water.

[0083] For L pH＝6 , L pH＝4 , L pH＝2 Conduct aqueous phase potentiometric titration to calculate the contents of phenolic hydroxyl and carboxyl groups, where L pH＝6 The phenolic hydroxyl content is 1.68mmol / g, and the carboxyl content is 1.96mmol / g; pH＝4 The phenolic hydroxyl content is 2.30mmol / g, and the carboxyl content is 3.03mmol / g; pH＝2 The phenolic hydroxyl content is 2.51 mmol / g, and the carboxyl content is 3.23 mmol / g.

[0084] It can be seen from the above examples that the method provided by the present invention can achieve mild depolymerization of lignin, increase the content of active functional groups phenolic hydroxyl and carboxyl in the lignin depolymerization product, and at the same time achieve the recovery and reuse of the alkaline low eutectic solvent by adding a reversible soluble base.

[0085] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

[0086] The depolymerization products obtained in Examples 1-5 and the comparative example were subjected to potentiometric titration test to measure the phenolic hydroxyl and carboxyl content in the products. The results are shown in Table 1 below.

[0087] Table 1 Phenolic hydroxyl and carboxyl content of depolymerization products

[0088]

[0089]

Claims

1. A method for oxidative depolymerization of lignin in a deep eutectic solvent system containing a reversible soluble alkali, characterized in that: The steps include: Step 1. A low eutectic solvent is mixed with a reversibly soluble alkaline solution to obtain a mixed solvent; Step 2. Adding lignin raw material and hydrogen peroxide to the mixed solvent obtained in step 1 to carry out oxidation reaction to obtain a reaction mixture; Step 3. Adjust the pH of the reaction mixture obtained in step 2 to 1-4, separate the solid residue and the supernatant by centrifugation, introduce CO2 into the supernatant to remove the reversible soluble alkali, collect the depolymerization product by gradient acid precipitation method and dry it for later use.

2. The method for oxidative depolymerization of lignin in a deep eutectic solvent system containing a reversible soluble alkali according to claim 1, characterized in that: The low eutectic solvent is obtained by mixing and heating a hydrogen bond acceptor and a hydrogen bond donor.

3. The method for oxidative depolymerization of lignin using a deep eutectic solvent system containing a reversible soluble alkali according to claim 2, characterized in that: The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:0.5 to 1:

5.

4. The method for oxidative depolymerization of lignin in a deep eutectic solvent system containing a reversible soluble alkali according to claim 1, characterized in that: The reversible soluble base is one of magnesium hydroxide, calcium hydroxide, strontium hydroxide or barium hydroxide.

5. The method for oxidative depolymerization of lignin using a deep eutectic solvent system containing a reversible soluble alkali according to claim 4, characterized in that: The mass ratio of the reversibly soluble alkaline solution to the low eutectic solvent is 0.1:1 to 1:

1.

6. The method for oxidative depolymerization of lignin using a deep eutectic solvent system containing a reversible soluble alkali according to claim 1, characterized in that: The lignin raw material is enzymatically hydrolyzed lignin.

7. The method for oxidative depolymerization of lignin using a deep eutectic solvent system containing a reversible soluble alkali according to claim 6, characterized in that: The mass ratio of the lignin raw material to the low eutectic solvent is 1:50 to 1:

5.

8. The method for oxidative depolymerization of lignin using a deep eutectic solvent system containing a reversible soluble alkali according to claim 6, characterized in that: The mass ratio of the hydrogen peroxide to the lignin raw material is 1:10 to 10:

1.

9. The method for oxidative depolymerization of lignin using a deep eutectic solvent system containing a reversible soluble alkali according to claim 1, characterized in that: The temperature of the oxidation reaction is 10-150° C. and the time is 1-10 hours.

10. The method for oxidative depolymerization of lignin using a deep eutectic solvent system containing a reversible soluble alkali according to claim 1, characterized in that: The gradient acid precipitation method is used to adjust the pH of the supernatant to 6, 4, and 2 to obtain lignin by precipitation.

Images