Method for preparing vanillin by depolymerizing lignin

Abstract

The invention belongs to the technical field of carbocyclic compounds, and particularly relates to a method for preparing vanillin by depolymerizing lignin, which comprises the following steps: (1) mixing a surfactant, a reinforcing agent, a buffering agent and water to prepare a depolymerized aqueous solution; (2) mixing lignin with a depolymerization aqueous solution, carrying out a depolymerization reaction, and filtering to obtain a mixed solution; (3) extracting and separating the mixed solution to obtain an oil phase; and (4) distilling the oil phase to obtain vanillin. The lignin is used as a raw material, water is used as a solvent, various surfactants are used as catalysts, the depolymerization effect and conversion rate of the lignin are improved, the reaction condition is mild, the preparation process is green and environment-friendly, the difficulty of the depolymerization process of the lignin is remarkably reduced, and the method is low in energy consumption, low in cost, environment-friendly and safe.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of carbon ring compounds, and particularly relates to a method for preparing vanillin by depolymerizing lignin. BACKGROUND

[0002] Lignin is the most abundant renewable aromatic polymer in nature, accounting for 15%-30% of the dry weight of plants, and the global annual output is about 30-48 billion tons. Alkaline lignin is one of the main types of industrial lignin, which is a by-product of the alkaline pulping process in the pulp and paper industry. It is rich in phenylpropane units (such as guaiacyl, syringyl and p-hydroxyphenyl) in its structure, and is an ideal raw material for producing high-value aromatic compounds (such as vanillin, eugenol and guaiacol). However, more than 95% of industrial lignin is directly burned or discarded, and the resource utilization rate is less than 5%, causing huge economic and environmental waste. Therefore, developing lignin directed conversion technology has become one of the core challenges in the field of biomass refining.

[0003] Vanillin has a molecular formula of C8H8O3, and is the most widely used food flavor additive in the world. It is also widely used in the pharmaceutical and cosmetic industries, and has a high annual demand. Currently, vanillin production mainly relies on the following two methods: one is chemical synthesis method: using petroleum-based guaiacol or natural eugenol and ferulic acid as raw materials, and synthesizing by chemical synthesis method. This method has low cost but relies on non-renewable resources, and the product contains catalyst residues, which is difficult to meet the demand of food production; the other is natural extraction method: directly extracting from vanilla bean pods, which accounts for only 1-2% of global supply. Due to the long growth cycle of plants and low extraction efficiency, the production effect is greatly discounted.

[0004] Currently, the methods for converting lignin to produce vanillin mainly include bacterial fermentation, catalytic oxidation, catalytic hydrogenolysis, thermal cracking and electrocatalytic cracking.

[0005] Chinese patent CN112251390A discloses a genetically engineered bacteria for synthesizing vanillin from lignin-containing biomass and its application. The method of knocking out the coding gene of vanillic acid dehydrogenase of ligninophilic Bacillus to inactivate the gene to accumulate vanillin. Vanillin has cytotoxicity to the bacterial cells at high concentrations, which can inhibit the growth and metabolic activity of the strain. In addition, long-term subculture may lead to loss of plasmid or decrease of gene expression level of the genetically engineered bacteria, and the strain stability needs to be maintained by antibiotic selection or chromosome integration strategy, which increases the operation cost.

[0006] Chinese patent CN107857696A discloses a method for preparing high-purity vanillin by catalytic oxidation of lignin. Lignin is mixed with cesium-based oxide catalyst, phenol is added and stirred to dissolve, and lignin is depolymerized under the reaction conditions of oxygen pressure 1-3 MPa and temperature 80-120℃ to obtain decomposition products mainly containing vanillin. The patent uses phenol as a solvent, which is a highly toxic organic substance. Its vapor can be absorbed through the respiratory tract and skin, causing central nervous system inhibition, liver and kidney damage, etc. In addition, phenolic wastewater needs to be treated by additional oxidation or adsorption, increasing the cost by about 20-30%.

[0007] Chinese patent CN118527148A discloses a preparation method of a carbon-oxygen double defect driven Ni / CeO2 / C catalyst. Cerium nitrate hexahydrate and nickel nitrate hexahydrate are dissolved in N,N-dimethylformamide solvent to obtain a first mixed solution. Terephthalic acid is dissolved in N,N-dimethylformamide solvent to obtain a second mixed solution. The first mixed solution is slowly added into the second mixed solution and stirred uniformly to obtain a pre-prepared solution. The pre-prepared solution is loaded into a reaction kettle for hydrothermal reaction. After the hydrothermal reaction is completed, the precipitate is separated and dried after being washed with ethanol. The Ni-doped Ce-based MOF self-sacrificial template is subjected to high-temperature calcination to obtain the carbon-oxygen double defect driven Ni / CeO2 / C catalyst. The preparation of the catalyst will use a large amount of DMF and other polluting solvents, and high-temperature calcination is required, which consumes a lot of energy. In addition, the reaction needs to be carried out at a very high temperature of 250-300℃, which requires a high-performance equipment. The catalytic hydrogenation cracking method depends on hydrogen source and needs to use high-pressure hydrogen gas (2-5 MPa), which has safety hazards and the catalyst is easy to be poisoned and deactivated. The patent has problems such as excessive hydrogenation of aromatic rings, which is not conducive to industrial development.

[0008] Chinese patent CN117486680A discloses a method for preparing aromatic oxygen-containing compounds by catalyzing lignin pyrolysis using a ternary perovskite. Alkaline earth metal nitrate A, rare earth metal nitrate R, and transition metal potassium cyanide T are used to prepare a ternary metal MOF precursor. The prepared ternary metal MOF precursor is calcined to prepare a ternary metal perovskite with adjustable morphology, specific surface area, and pore structure. The ternary metal MOF-based perovskite catalyzes lignin pyrolysis to prepare aromatic oxygen-containing compounds, including phenol, guaiacol, syringol, aldehyde, ester, and ether products. The preparation process of the catalyst uses the highly toxic compound potassium cyanide, which has a great safety hazard. The catalyst is prone to coking during pyrolysis, reducing the reaction activity. In addition, a large amount of flue gas is generated during pyrolysis, which pollutes the environment. The product types are complex, the selectivity is low, and the products are not easy to separate and purify.

[0009] Chinese patent CN119824470A discloses a method for electrocatalytic hydrogenation of lignin using Bi-(Et3N) catalyst, which belongs to the technical field of electrocatalytic hydrogenation of lignin. The method is to add Bi-(Et3N) catalyst and lignin model compound into an "H" type electrolytic cell, and to carry out electrocatalytic hydrogenation of the model compound under a constant current system. The preparation method of the catalyst is relatively complex, and in the reaction process, not only a large amount of electric energy is relied on, but also hydrogen is consumed, which is high in cost and complex in operation, and is not conducive to industrial production. In addition, the method is currently only applicable to 2-phenoxy-1-phenylethanol, a simple beta-O-4 type dimer model compound, and there are still certain difficulties in depolymerizing real lignin with complex structure.

[0010] In summary, the current production method of vanillin needs to solve the problems of environmental pollution, high reaction temperature, harsh conditions, complicated operation, high cost, low selectivity and difficult separation of products. SUMMARY

[0011] According to the deficiencies in the prior art, the technical problem to be solved by the present application is to provide a method for preparing vanillin from depolymerized lignin, which depolymerizes lignin under mild conditions and realizes efficient production of vanillin, and the production process is green and environmentally friendly.

[0012] The technical scheme adopted by the present application to solve the technical problem is: The method for preparing vanillin from depolymerized lignin comprises the following steps: (1) mixing a surfactant, a reinforcing agent, a buffer and water to prepare a depolymerization aqueous solution; (2) mixing lignin with the depolymerization aqueous solution and carrying out a depolymerization reaction to obtain a mixed solution by filtration; (3) carrying out extraction separation on the mixed solution to obtain an oil phase; (4) distilling the oil phase to obtain vanillin.

[0013] In step (1), the surfactant includes active agent A and active agent B, the active agent A is one or more of sodium dodecyl sulfate, sodium dodecyl polyoxyethylene ether sulfate, sodium dodecyl carbonate, sodium dodecyl benzene sulfonate or sodium dodecyl sulfonate, the active agent B is one or more of fatty alcohol polyoxyethylene ether, dodecyl phenol polyoxyethylene ether, alkyl alcohol amide or fatty acid polyoxyethylene ester, the reinforcing agent is disodium EDTA salt, and the buffer is one or more of sodium carbonate, sodium bicarbonate or sodium hydroxide.

[0014] The mass ratio of the active agent A, the active agent B, the reinforcing agent and the buffer is 1-100.9:1-40.5:0.1-16.2:0.1-12.1.

[0015] The concentration of the aqueous solution in step (1) is 0.01-35wt%.

[0016] The mass ratio of lignin to the aqueous solution in step (2) is 1:9-1500.

[0017] The temperature of the depolymerization reaction in step (2) is 25-180℃.

[0018] The pressure of the depolymerization reaction in step (2) is 0-10bar.

[0019] The time of the depolymerization reaction in step (2) is 10-1440min.

[0020] The extraction in step (3) is carried out by using an extractant, and the extractant is one of ethyl acetate, butyl acetate, ethyl butyrate, butanol, amyl alcohol or hexanol.

[0021] The distillation temperature in step (4) is 25-250℃, the distillation pressure is 1-3kPa, and the distillation fraction temperature is 100-200℃.

[0022] The active agent A is an anionic surfactant, and the active agent B is a nonionic and / or amphoteric surfactant.

[0023] The anionic surfactant used in the active agent A in the application can self-assemble to form micelles in water under the driving of thermodynamics, the micelles wrap and catalyze the depolymerization raw materials, that is, micellar microreactors are formed; the core driving force of thermodynamic driving is the hydrophobic effect in the aqueous solution, when the solution reaches the critical micelle concentration, the hydrophobic effect drives the molecules to aggregate into micelles, the head group faces outward to contact water, and the tail chain avoids contact with water. The disodium salt of EDTA can compress the double electric layer on the surface of the micelles, reduce the charge repulsion, promote the micelles to change from rod-like to spherical, significantly improve the reaction contact area and improve the reaction speed. The hydrophobic tail chain is close to each other to form a nonpolar core to escape from the water environment; the hydrophilic head group faces the water phase to form a shell and performs hydration with water molecules. The micelles are not rigid solids, but dynamic aggregates: lignin depolymerization monomers constantly enter and exit the micelles, and the residence time of each monomer is about 10 -3 -10 -8 seconds, and a typical micelle contains 50-100 monomers with a size of about 3-6nm.

[0024] The micelle surface formed by anionic surfactants has abundant negative charges, which can effectively enrich protons, and the hydrophilic groups of the micelles have electrophilic action with C atoms in the linkage (C-O bond) in lignin, thereby activating the beta-O-4 bond through electrostatic attraction, and the synergistic effect between the negative charges on the micelle surface and the electrophilic action promotes the electrophilic attack of protons to break the beta-O-4 bond, so that the reaction system has high catalytic activity; the nonionic and / or amphoteric surfactant used in the active agent B in the application can form a complex micelle with the anionic surfactant in the aqueous solution through electrostatic force and van der Waals force, thereby showing a strong confinement and isolation effect on lignin and reaction products, and effectively isolating the reaction products at the molecular level, so that the reaction products are not excessively reacted, and therefore the application has excellent selectivity.

[0025] Due to the unique amphiphilic structure and micro-separation environment of the complex micelles, the complex micelles can effectively isolate the reaction products; this isolation mechanism has important applications in the fields of catalysis, synthesis and drug delivery, and the complex micelles have a nanoscale independent space structure, including a hydrophobic core and a hydrophilic shell, the nonpolar environment of the hydrophobic core can accommodate hydrophobic vanillin molecules, the hydrophilic shell of the complex micelles is composed of negatively charged head groups and water molecules, forming an ionic barrier, the vanillin product is limited inside the micelles and separated from the external water phase, avoiding side reactions; the micelle surface has a high density of negative charges through electrostatic repulsion effect, forming a Stern layer and a diffuse double layer, repelling other anionic substances (such as negatively charged byproducts and nucleophilic reagents), preventing them from approaching the product in the core; the complex micelles are in dynamic equilibrium with monomers, and the rate of product entering and leaving the complex micelles is controlled by the hydrophobicity of the product; the stronger the hydrophobicity, the longer the residence time, and the better the stability of the complex micelle structure; the hydrophobic vanillin is "retained" in the micelles, prolonging the isolation time.

[0026] The enhancer EDTA disodium salt can effectively chelate calcium, magnesium and iron ions in water, and can significantly improve the stability and durability of the complex micelles in the reaction system. The buffer can stabilize the pH at 8.5-9.5, so that the complex micelles can work more stably. Through excellent synergistic effect among the surfactants, enhancers and buffers, a high-efficiency lignin catalysis and product protection system is established, and vanillin is obtained. Moreover, in subsequent separation, the surfactants do not enter the organic phase of the extractant, and can be recycled, the production process of the application is environmentally friendly, without secondary pollution, and the economy is significantly improved.

[0027] The application has the following beneficial effects: The application uses lignin as a raw material, water as a solvent, and various surfactants as catalysts to improve the depolymerization effect and conversion rate of lignin, and has mild reaction conditions, green and environmentally friendly preparation process, significantly reduces the difficulty of lignin depolymerization process, consumes less energy, has low cost, and is environmentally friendly and safe.

[0028] The application catalyzes lignin depolymerization to prepare vanillin, improves the selectivity of vanillin, obtains vanillin with high purity, greatly improves the production effect of vanillin, reduces the process requirement of vanillin separation and purification in the product, and solves the problem of difficult vanillin follow-up separation.

[0029] The application constructs a composite catalytic system through the synergistic effect between different types of surfactants, reduces the production difficulty of vanillin, has low raw material price, and has a green and environmentally-friendly production process, and the surfactants can be reused after separation. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a GC-MS detection diagram of the vanillin solution prepared in Example 1 of the application; Figure 2 is a process flow diagram of preparing vanillin from depolymerized lignin in Example 1 of the application. DETAILED DESCRIPTION

[0031] The embodiments of the application will be further described below in combination with the drawings.

[0032] Example 1 The method for preparing vanillin from depolymerized lignin comprises the following steps: (1) 3.5 g of sodium dodecyl benzene sulfonate, 1 g of fatty alcohol polyoxyethylene ether, 0.1 g of EDTA disodium salt, 0.1 g of sodium carbonate and 300 ml of water are mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution is 1.5 wt%; (2) 300 ml of the depolymerization aqueous solution is mixed with 10 g of lignin, and the mixture is subjected to a depolymerization reaction at 80 DEG C, 0 bar pressure and a stirring speed of 800 rpm for 240 min, and a mixed solution is obtained by filtration, and the conversion rate is 68%; (3) the mixed solution is extracted with 300 ml of ethyl acetate for three times to obtain an oil phase; (4) the oil phase is subjected to distillation at 2 kPa and 200 DEG C, and the fraction of 100-200 DEG C is collected to obtain vanillin.

[0033] A certain amount of vanillin product is diluted, and the solution composition is detected by GC-MS, and the detection result is shown in Figure 1 , wherein peak 1 in the figure is ethanol needle washing liquid, 2 is ethyl acetate solvent, 3 is vanillin product, and 4 is vanillic acid product, the vanillin is quantified by using an internal standard method, the yield of vanillin is 2.38%, the selectivity of vanillin is 95%, and the process flow of preparing vanillin from depolymerized lignin is shown in Figure 2 .

[0034] Example 2 A method for preparing vanillin from depolymerized lignin, comprising the following steps: (1) 1 g of sodium dodecyl sulfate, 3 g of fatty alcohol polyoxyethylene ether, 0.1 g of EDTA disodium salt, 0.1 g of sodium carbonate, and 42 L of water are mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution is 0.01 wt%; (2) 300 ml of the depolymerization aqueous solution is mixed with 100 g of lignin, and a depolymerization reaction is carried out at 25°C, 0 bar pressure, and a stirring speed of 800 rpm for 200 min, and a mixed solution is obtained by filtration, and the conversion rate is measured to be 46%; (3) The mixed solution is extracted with 300 ml of ethyl acetate in three times to obtain an oil phase; (4) The oil phase is distilled at 2 kPa and 200°C, and a fraction of 100-200°C is collected to obtain vanillin.

[0035] A certain amount of vanillin product is diluted, the solution composition is detected by GC-MS, the vanillin is quantified by an internal standard method, the yield of vanillin is 0.84%, and the selectivity of vanillin is 91%.

[0036] Example 3 A method for preparing vanillin from depolymerized lignin, comprising the following steps: (1) 2 g of sodium dodecyl polyoxyethylene ether sulfate, 3 g of dodecyl phenol polyoxyethylene ether, 0.3 g of EDTA disodium salt, 0.2 g of sodium bicarbonate, and 550 ml of water are mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution is 1 wt%; (2) 300 ml of the depolymerization aqueous solution is mixed with 60 g of lignin, and a depolymerization reaction is carried out at 60°C, 0 bar pressure, and a stirring speed of 800 rpm for 1440 min, and a mixed solution is obtained by filtration, and the conversion rate is measured to be 63%; (3) The mixed solution is extracted with 300 ml of butyl acetate in three times to obtain an oil phase; (4) The oil phase is distilled at 1 kPa and 205°C, and a fraction of 100-200°C is collected to obtain vanillin.

[0037] A certain amount of vanillin product is diluted, the solution composition is detected by GC-MS, the vanillin is quantified by an internal standard method, the yield of vanillin is 1.12%, and the selectivity of vanillin is 90%.

[0038] Example 4 A method for preparing vanillin from depolymerized lignin, comprising the following steps: (1) 19 g of sodium dodecyl benzene sulfonate, 3 g of fatty alcohol polyoxyethylene ether, 1 g of EDTA disodium salt, 1 g of sodium carbonate and 300 ml of water were mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution was 7.4 wt %; (2) 300 ml of the depolymerization aqueous solution was mixed with 30 g of lignin, and a depolymerization reaction was carried out at 120°C, 4 bar pressure and a stirring speed of 800 rpm for 500 min, and a mixed solution was obtained by filtration; the conversion rate was 72%.

[0039] (3) The mixed solution was extracted with 300 ml of ethyl acetate three times to obtain an oil phase; (4) The oil phase was distilled at 3 kPa and 150°C, and the fraction of 100-200°C was collected to obtain vanillin.

[0040] A certain amount of vanillin solution product was diluted, the solution composition was detected by GC-MS, the vanillin was quantified by internal standard method, the yield of vanillin was 3.67%, and the selectivity of vanillin was 92%.

[0041] Example 5 A method for preparing vanillin from depolymerized lignin, comprising the following steps: (1) 32.6 g of sodium laurate, 6.5 g of alkylolamide, 2.6 g of EDTA disodium salt, 3.3 g of sodium hydroxide and 300 ml of water were mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution was 13 wt %; (2) 300 ml of the depolymerization aqueous solution was mixed with 10 g of lignin at 80°C, 0 bar pressure and a stirring speed of 800 rpm for 100 min to carry out a depolymerization reaction, and a mixed solution was obtained by filtration, and the conversion rate was measured to be 63%; (3) The mixed solution was extracted with 300 ml of ethyl butyrate three times to obtain an oil phase; (4) The oil phase was distilled at 2 kPa and 200°C, and the fraction of 100-200°C was collected to obtain vanillin.

[0042] A certain amount of vanillin solution product was diluted, the solution composition was detected by GC-MS, the vanillin was quantified by internal standard method, the yield of vanillin was 2.15%, and the selectivity of vanillin was 98%.

[0043] Example 6 A method for preparing vanillin from depolymerized lignin, comprising the following steps: (1) 57.2 g of sodium dodecyl benzene sulfonate, 16.3 g of fatty acid polyoxyethylene ester, 8.2 g of EDTA disodium salt, 8.2 g of sodium bicarbonate and 300 ml of water were mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution was 23.10 wt %; (2) 300 ml of the depolymerization aqueous solution was mixed with 5 g of the lignin, and a depolymerization reaction was performed at 180°C, 10 bar, and a stirring speed of 800 rpm for 10 min, and a mixed solution was obtained by filtration, and the conversion rate was measured to be 81%; (3) The mixed solution was extracted with 300 ml of butanol in three times, and an oil phase was obtained; (4) The oil phase was distilled at 2 kPa and 200°C, and a fraction of 100-200°C was collected, and vanillin was obtained.

[0044] A certain amount of the vanillin solution product was diluted, the solution components were detected by GC-MS, the vanillin was quantified by an internal standard method, the yield of vanillin was 8.4%, and the selectivity of vanillin was 95%.

[0045] Example 7 A method for preparing vanillin from depolymerized lignin comprises the following steps: (1) 100.9 g of sodium dodecyl sulfonate, 11.2 g of fatty acid polyoxyethylene ester, 6.7 g of EDTA disodium salt, 1.12 g of sodium hydroxide, and 300 ml of water were mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution was 28.6 wt%; (2) 300 ml of the depolymerization aqueous solution was mixed with 2 g of the lignin, and a depolymerization reaction was performed at 100°C, 1.1 bar, and a stirring speed of 800 rpm for 1000 min, and a mixed solution was obtained by filtration, and the conversion rate was measured to be 68%; (3) The mixed solution was extracted with 300 ml of amyl alcohol in three times, and an oil phase was obtained; (4) The oil phase was distilled at 2 kPa and 200°C, and a fraction of 100-200°C was collected, and vanillin was obtained.

[0046] A certain amount of the vanillin solution product was diluted, the solution components were detected by GC-MS, the vanillin was quantified by an internal standard method, the yield of vanillin was 5.66%, and the selectivity of vanillin was 90%.

[0047] Example 8 A method for preparing vanillin from depolymerized lignin comprises the following steps: (1) 81 g of sodium dodecyl benzene sulfonate, 40.5 g of alkyl alcohol amide, 16.2 g of EDTA disodium salt, 12.1 g of sodium carbonate, and 300 ml of water were mixed to prepare a depolymerization aqueous solution, and the concentration of the depolymerization aqueous solution was 33.3 wt%; (2) 300 ml of the depolymerization aqueous solution was mixed with 0.3 g of the lignin, and a depolymerization reaction was performed at 140°C, 3 bar, and a stirring speed of 800 rpm for 800 min, and a mixed solution was obtained by filtration, and the conversion rate was measured to be 76%; (3) The mixed solution was extracted with 300 ml of pentanol three times to obtain an oil phase; (4) The oil phase was distilled at 2 kPa and 200°C, and a fraction of 100-200°C was collected to obtain vanillin.

[0048] A certain amount of vanillin solution product was diluted, and the solution composition was detected by GC-MS. The vanillin was quantified by an internal standard method. The yield of vanillin was 6.79%, and the selectivity of vanillin was 93%.

[0049] Comparative Example 1 In step (1), no sodium dodecyl benzene sulfonate was added, and the remaining steps were the same as in Example 1. The yield of vanillin was 0%, and the selectivity of vanillin was 0%.

[0050] Comparative Example 2 In step (1), no fatty alcohol polyoxyethylene ether was added, and the remaining steps were the same as in Example 1. The yield of vanillin was 0.87%, and the selectivity of vanillin was 74%.

[0051] Comparative Example 3 In step (1), no disodium EDTA was added, and the remaining steps were the same as in Example 1. The yield of vanillin was 1.01%, and the selectivity of vanillin was 66%.

[0052] Comparative Example 4 In step (1), no sodium carbonate was added, and the remaining steps were the same as in Example 1. The yield of vanillin was 0.83%, and the selectivity of vanillin was 81%.

Claims

1. A method for preparing vanillin by depolymerizing lignin, characterized in that: The following steps are involved: (1) Mixing a surfactant, an enhancer, a buffer and water to prepare a depolymerization aqueous solution; (2) mixing lignin with a depolymerization aqueous solution, performing a depolymerization reaction, and filtering to obtain a mixed solution; (3) extracting and separating the mixed solution to obtain an oil phase; (4) The oil phase is distilled to obtain vanillin.

2. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: In step (1), the surfactant includes active agent A and active agent B, active agent A is one or more of sodium lauryl sulfate, sodium lauryl polyoxyethylene ether sulfate, sodium lauryl carbonate, sodium dodecylbenzene sulfonate or sodium lauryl sulfonate, active agent B is one or more of fatty alcohol polyoxyethylene ether, dodecylphenol polyoxyethylene ether, alkyl alcohol amide or fatty acid polyoxyethylene ester, and the enhancer is EDTA disodium salt; the buffer is one or more of sodium carbonate, sodium bicarbonate or sodium hydroxide.

3. The method for preparing vanillin by depolymerizing lignin according to claim 2, characterized in that: The mass ratio of active agent A, active agent B, enhancer and buffer is 1-100.9:1-40.5:0.1-16.2:0.1-12.

1.

4. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: The concentration of the depolymerization aqueous solution in step (1) is 0.01-35 wt%.

5. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: In step (2), the mass ratio of lignin to the depolymerization aqueous solution is 1:9-1500.

6. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: The depolymerization reaction temperature in step (2) is 25-180°C.

7. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: The depolymerization reaction pressure in step (2) is 0-10 bar.

8. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: The depolymerization reaction time in step (2) is 10-1440 min.

9. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: The extraction in step (3) is performed using an extractant, which is one of ethyl acetate, butyl acetate, ethyl butyrate, butanol, pentanol or hexanol.

10. The method for preparing vanillin by depolymerizing lignin according to claim 1, characterized in that: In step (4), the distillation temperature is 25-250°C, the distillation pressure is 1-3 kPa, and the distillation fraction temperature is 100-200°C.

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