Preparation method of lignin-based environment-friendly wood adhesive

Abstract

The application discloses a preparation method of a lignin-based environment-friendly wood adhesive, which comprises the following steps: firstly, adding cerium chloride hexahydrate, magnesium chloride and zinc chloride into deionized water and stirring and mixing; then, adding ammonia water, hydrogen peroxide and sodium hydroxide solution in sequence and heating to carry out reaction to obtain a precipitate; washing the precipitate and calcining the precipitate at high temperature to obtain a catalyst; adding the obtained catalyst into a reactor, adding lithium hydroxide, lignin and a solvent, and then carrying out reaction at a temperature of 180-220 DEG C for 20-60 min; cooling, filtering, removing the precipitate and obtaining a filtrate; adding the obtained filtrate into sodium hydroxide and heating to 93-95 DEG C; then, adding formaldehyde water solution at a constant speed to carry out reaction; and finally, cooling to obtain a viscous lignin-based environment-friendly wood adhesive. The application improves the reaction activity between lignin and formaldehyde and phenol through catalysis, obtains a green and environment-friendly lignin-based adhesive, and solves the problem that lignin is not effectively utilized.

Description

Technical Field

[0001] This invention belongs to the technical field of producing wood adhesives using biomass resources, and specifically relates to a method for preparing a lignin-based environmentally friendly wood adhesive. Background Technology

[0002] Lignin is the only renewable resource in nature that can provide aromatic rings. It is an amorphous network macromolecule composed of three basic structural units: guaiacol (G), syringylpropane (S), and p-hydroxyphenylpropane (H), linked by COC and CC bonds. In woody plants, lignin is almost entirely composed of G and S structural units, while in herbaceous plants, it possesses G, H, and S structural units, with H units typically comprising less than 40%. Due to its highly cross-linked and complex structure, lignin currently lacks widespread applications. A large amount of lignin from the pulp and paper industry is not effectively utilized, being concentrated for combustion or discharged directly into rivers as waste liquid, causing significant environmental pollution. To achieve full utilization of biomass and sustainable development, the catalytic conversion of lignin to produce high-value-added products is a current research hotspot.

[0003] The catalytic depolymerization of lignin to prepare phenolic compounds is one of the most common methods for its high-value utilization. However, the depolymerization products of lignin are exceptionally complex in composition, with most boiling points concentrated between 160℃ and 280℃. Furthermore, the high viscosity of the solvent system after depolymerization makes the separation and purification of the products extremely difficult, hindering their direct use as industrial raw materials. Currently, there are two common methods to achieve high-value applications for lignin depolymerization products without purification. One method involves hydrogenating and deoxygenating the lignin depolymerization products into hydrocarbon fuels under a hydrogen atmosphere. However, this process requires the destruction of stable aromatic rings under a hydrogen atmosphere, resulting in very high energy consumption.

[0004] Another possible application is to use the depolymerization products as a partial substitute for phenol in the production of phenolic resins. This is because the monophenolic compounds obtained from lignin depolymerization possess certain reactivity, especially the guaiacol and p-hydroxyphenyl compounds, which have structures similar to phenol. Their ortho-vacancy sites exhibit strong reactivity and can participate in the condensation reaction of phenol and formaldehyde under certain conditions. However, since lignin is a polyphenolic polymer, a large number of active groups are encapsulated within the molecule, resulting in strong intermolecular interactions and an interwoven aromatic ring structure, making it difficult to dissolve in water and common solvents. Furthermore, many active sites have been substituted, leading to significantly lower reactivity of lignin with formaldehyde compared to phenol. To increase the substitution rate of lignin for phenol, it is necessary to improve the reactivity of lignin. Chemical modification methods such as hydroxymethylation, demethylation, and phenolation can significantly increase the number of active sites in lignin, but these methods are complex and costly, and are currently under-researched. Moreover, regarding the directional depolymerization of lignin, in addition to catalyst design and development, selecting suitable solvent systems is equally crucial. Catalytic depolymerization of lignin in water, organic solvents, or a water-organic solvent biphase system can yield monophenolic compounds in yields ranging from 2 wt% to 61 wt%. However, due to the poor solubility of lignin in these solvents, as the concentration increases (>5 wt%), lignin particles tend to agglomerate and carbonize, and the depolymerization products also undergo significant repolymerization to form insoluble substances, resulting in a sharp decline in product selectivity and liquefaction efficiency. In fact, for the preparation of lignin-phenolic resins, catalytic depolymerization of lignin is not suitable for high-boiling-point organic solvents. If these organic solvents cannot be removed or recovered in the later stages of lignin-phenolic resin synthesis, the application of lignin-phenolic resins will be greatly limited due to environmental protection and cost considerations. Summary of the Invention

[0005] To address the aforementioned shortcomings, this invention discloses a method for preparing a lignin-based environmentally friendly wood adhesive. By improving the reactivity between lignin and formaldehyde and phenol through catalysis, the prepared lignin-based adhesive meets the performance requirements for adhesives used in engineered wood products and features green environmental protection and simple processing, thus solving the problem of ineffective utilization of lignin.

[0006] This invention is achieved using the following technical solution:

[0007] A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0008] (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then, continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. After the addition is completed, raise the temperature to 60℃ and react at a constant temperature for 30-40 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 800-850℃ for 2-3 h to obtain catalyst.

[0009] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hexahydrate solution is (0.20-0.25):(0.10-0.15):(0.08-0.1), the volume ratio of cerium chloride in the hexahydrate solution to deionized water is (0.20-0.25) mol:(300-400) mL, and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is (300-400):(50-60):(10-20):(90-100).

[0010] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly, and react at a temperature of 180-220°C for 20-60 minutes. After naturally cooling to 80°C, filter to remove the precipitate and obtain the filtrate.

[0011] The solvent is a mixture of deionized water, dimethyl carbonate and phenol, and the lignin is any one of sodium lignin sulfonate, calcium lignin sulfonate, alkali lignin, sulfate lignin, Klason lignin and organic solvent lignin.

[0012] The weight ratio of the catalyst, lithium hydroxide and lignin is (2-3):(0.2-1.2):(60-90), and the weight ratio of lignin and solvent is 1:(3-6).

[0013] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, heat it to 93-95°C under stirring. Then, add a formaldehyde aqueous solution with a mass fraction of 30%-40% at a uniform rate. After the addition is complete, continue the reaction for 60-90 minutes. Then, cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0014] The weight ratio of the added sodium hydroxide to the lignin in step (2) is (12-15):(60-90); the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is (250-350) mL:(60-90) g.

[0015] Analysis of the three structural units of lignin shows that the lignin depolymerization products composed of H-type structural units are most suitable for preparing phenolic resins. However, in G and S-type structural units, the ortho-position of the phenolic hydroxyl group is replaced by -OCH3, resulting in weak reactivity with formaldehyde, or even no possibility of condensation reaction with formaldehyde, posing a risk of slow formaldehyde release during the use of phenolic resins. Therefore, this invention designs and prepares a composite particle catalyst CeO2-1 / 2MgO-1 / 3ZnO, which can selectively catalyze the relatively unstable H-type bagasse lignin under mild conditions, yielding mainly H-type monophenol compounds. Furthermore, the addition of lithium hydroxide ensures that the lignin conversion reaction is carried out under alkaline conditions, which is beneficial for improving the catalytic effect. The H-type monophenol compounds can partially replace phenol as highly active mixed phenols in the preparation of phenolic resins. Additionally, the lignin macromolecular fragments composed of G and S-type structural units in the products, utilizing their structural compatibility with phenolic resins, can participate in the preparation of phenolic resins through blending, thereby enhancing the overall performance of lignin-based adhesives (see [link to product description]). Figure 1 and Figure 2 ).

[0016] Meanwhile, since quinone intermediates in the depolymerization process of lignin in alkaline systems are prone to repolymerization and the formation of insoluble precipitates, this invention adds phenol and dimethyl carbonate (DMC) to the reaction system. The formaldehyde generated during the cleavage of the CO bonds in lignin is captured through the condensation reaction of phenol and formaldehyde to form phenolic resin. This converts the quinone intermediates into enol ether structures or promotes the formation of methylated lignin through dimethyl carbonate, thereby indirectly reducing the repolymerization of quinone intermediates and solving the problem of their tendency to repolymerize and form insoluble precipitates (see [link to relevant documentation]). Figure 3 ).

[0017] Furthermore, in step (1), the mass fraction of the ammonia water is 20% to 30%, the mass fraction of the hydrogen peroxide is 25% to 35%, and the mass fraction of the sodium hydroxide solution is 25% to 35%.

[0018] Furthermore, in step (1), the total time for adding ammonia, hydrogen peroxide and sodium hydroxide solution is controlled to be 30 to 40 minutes.

[0019] Furthermore, in step (2), the stirring speed is 300-400 r / min.

[0020] Furthermore, in the solvent, the volume ratio of deionized water, dimethyl carbonate, and phenol is (4-5):(0.1-0.2):(94-95).

[0021] Furthermore, in step (2), the lignin is either sodium lignin sulfonate or Klason lignin; the weight ratio of the lignin to the solvent is 1:4.

[0022] Furthermore, in step (3), the filtrate obtained in step (2) is taken and sodium hydroxide is added, and then the temperature is raised to 93-95°C under stirring at a speed of 200-300 r / min.

[0023] Furthermore, in step (3), the time for adding the formaldehyde aqueous solution is controlled at 20-30 minutes. By controlling the time of adding the formaldehyde aqueous solution, the rate of formaldehyde addition is controlled, which is beneficial to the reaction of phenol and formaldehyde condensation to form phenolic resin. The formaldehyde generated during the breaking of the CO bond of lignin is captured, thereby effectively converting the quinone structure intermediate and reducing the repolymerization of the quinone structure intermediate.

[0024] Compared with existing technologies, this technical solution has the following advantages:

[0025] 1. The catalyst prepared by this invention has the advantages of good dispersibility and high activity. It can selectively depolymerize H-type lignin into H-type monophenol compounds under mild conditions, while lignin composed of G and S-type structural units can be retained as lignin molecular chain segments. The preparation process employs a stepwise precipitation method, first forming Mg(OH)2 and Zn(OH)2 precipitates under ammonia conditions, and then further adding H2O2-NaOH to precipitate Ce. 3+ The Ce(OH)4 precipitate is converted into Ce(OH)4, allowing it to be uniformly deposited on the surface of Mg(OH)2 and Zn(OH)2 particles, thus obtaining a highly dispersible catalyst precursor and effectively ensuring the catalyst's reactivity. Simultaneously, the addition rate of ammonia, hydrogen peroxide, and sodium hydroxide solutions can be controlled during the reaction, thereby regulating the reaction intensity and facilitating the acquisition of a highly dispersible catalyst precursor. Adding too quickly can lead to an excessively rapid reaction, preventing the large amount of Ce(OH)4 generated from uniformly depositing on the surface of Mg(OH)2 and Zn(OH)2 particles.

[0026] 2. Due to the complex and variable structure of lignin, differences in molecular weight, the connection mode between basic structural units, and the functional group structure exist depending on the source, processing method, and even the harvesting season. These structural differences in lignin also affect the stability of lignin-based products. The method of this invention can selectively depolymerize H-type lignin, while the other degradation products, G-type and S-type lignin molecular chain segments, are blended to participate in the preparation of adhesives. Therefore, the method of this invention has high adaptability and is not strict about the type of lignin raw material.

[0027] 3. The method of the present invention has the characteristics of being green, environmentally friendly and low-cost. The water-dimethyl carbonate-phenol mixture used can, on the one hand, serve as a reaction solvent to promote the dissolution of lignin and increase the initial concentration of lignin in the reaction; on the other hand, it can serve as an intermediate product capture agent to reduce the formation of useless products due to the re-aggregation of intermediates in quinone structures; in addition, it can also serve as a reaction raw material for preparing adhesives, and lignin-based adhesives can be directly prepared without separation and purification steps. Attached Figure Description

[0028] Figure 1 This is a schematic diagram illustrating the principle of lignin-directed depolymerization as described in this invention.

[0029] Figure 2 This is a schematic diagram illustrating the principle of preparing lignin-based adhesives using lignin depolymerization products as described in this invention.

[0030] Figure 3 This is a schematic diagram illustrating the principle of quinone-type intermediate transformation treatment during the lignin depolymerization process described in this invention. Implementation

[0031] The present invention is further illustrated by the following examples, but these are not intended to limit the invention. Specific experimental conditions and methods not specified in the following examples are generally conventional methods well known to those skilled in the art.

[0032] Example 1: A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0033] (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. The total time for adding ammonia water, hydrogen peroxide and sodium hydroxide solution is controlled at 33 min. After the addition is completed, the temperature is raised to 60℃ and then kept at a constant temperature for 30 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 800℃ for 2 h to obtain catalyst.

[0034] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hydrated solution is 0.24:0.12:0.08; the volume ratio of cerium chloride in the hydrated solution to deionized water is 0.24 mol:300 mL; and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is 300:55:15:95. The mass fraction of the ammonia is 25%, the mass fraction of the hydrogen peroxide is 30%, and the mass fraction of the sodium hydroxide solution is 30%.

[0035] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly at a speed of 300 r / min. Then react at a temperature of 208℃ for 35 min. After naturally cooling to 80℃, filter to remove the precipitate and obtain the filtrate.

[0036] The solvent is a mixture of deionized water, dimethyl carbonate, and phenol, wherein the volume ratio of deionized water, dimethyl carbonate, and phenol in the solvent is 5:0.2:94.8; the lignin is Klason lignin.

[0037] The weight ratio of the catalyst, lithium hydroxide and lignin is 3:1.0:75, and the weight ratio of lignin and solvent is 1:4.

[0038] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, under stirring at a speed of 250 r / min, heat it to 95°C. Then, add a 35% formaldehyde aqueous solution at a uniform rate. The time for adding the formaldehyde aqueous solution is controlled at 20 min. After the addition is completed, continue the reaction for 60 min. Then, cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0039] The weight ratio of the added sodium hydroxide to the lignin in step (2) is 13.0:75; the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is 300mL:75g.

[0040] Example 2: A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0041] (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. The total time for adding ammonia water, hydrogen peroxide and sodium hydroxide solution is controlled at 35 min. After the addition is completed, raise the temperature to 60℃ and react at a constant temperature for 35 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 820℃ for 2.5 h to obtain catalyst.

[0042] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hydrated solution is 0.22:0.13:0.09; the volume ratio of cerium chloride in the hydrated solution to deionized water is 0.22 mol:350 mL; and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is 350:52:18:95. The mass fraction of the ammonia is 28%, the mass fraction of the hydrogen peroxide is 32%, and the mass fraction of the sodium hydroxide solution is 28%.

[0043] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly at a speed of 350 r / min. Then react at a temperature of 215℃ for 25 min. After naturally cooling to 80℃, filter to remove the precipitate and obtain the filtrate.

[0044] The solvent is a mixture of deionized water, dimethyl carbonate, and phenol, wherein the volume ratio of deionized water, dimethyl carbonate, and phenol in the solvent is 4.5:0.1:94.5; the lignin is sodium lignin sulfonate.

[0045] The weight ratio of the catalyst, lithium hydroxide, and lignin is 2.5:0.5:85, and the weight ratio of lignin to solvent is 1:3.8.

[0046] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, under the stirring condition of 250 r / min, heat it to 94°C. Then, add a 32% formaldehyde aqueous solution at a uniform rate. The time for adding the formaldehyde aqueous solution is controlled at 22 min. After the addition is completed, continue the reaction for 70 min. Then, cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0047] The weight ratio of the added sodium hydroxide to the lignin in step (2) is 14:85; the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is 310mL:85g.

[0048] Example 3: A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0049] (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. The total time for adding ammonia water, hydrogen peroxide and sodium hydroxide solution is controlled at 38 min. After the addition is completed, raise the temperature to 60℃ and react at a constant temperature for 35 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 830℃ for 2.5 h to obtain catalyst.

[0050] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hydrated solution is 0.24:0.14:0.085; the volume ratio of cerium chloride in the hydrated solution to deionized water is 0.24 mol:380 mL; and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is 380:50:15:95. The mass fraction of the ammonia is 25%, the mass fraction of the hydrogen peroxide is 30%, and the mass fraction of the sodium hydroxide solution is 30%.

[0051] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly at a speed of 350 r / min. Then react at a temperature of 200℃ for 42 min. After naturally cooling to 80℃, filter to remove the precipitate and obtain the filtrate.

[0052] The solvent is a mixture of deionized water, dimethyl carbonate, and phenol, wherein the volume ratio of deionized water, dimethyl carbonate, and phenol in the solvent is 5:0.2:94.8; the lignin is alkali lignin.

[0053] The weight ratio of the catalyst, lithium hydroxide and lignin is 2.7:0.8:60, and the weight ratio of lignin and solvent is 1:5.

[0054] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, under stirring at a speed of 250 r / min, heat it to 93°C. Then, add a 35% formaldehyde aqueous solution at a uniform rate. The time for adding the formaldehyde aqueous solution is controlled at 25 min. After the addition is completed, continue the reaction for 60 min. Then, cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0055] The weight ratio of the added sodium hydroxide to the lignin in step (2) is 12.7:60; the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is 292mL:60g.

[0056] Example 4: A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0057] (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. The total time for adding ammonia water, hydrogen peroxide and sodium hydroxide solution is controlled at 38 min. After the addition is completed, raise the temperature to 60℃ and react at a constant temperature for 40 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 840℃ for 3 h to obtain catalyst.

[0058] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hydrated solution is 0.21:0.11:0.095; the volume ratio of cerium chloride in the hydrated solution to deionized water is 0.21 mol:380 mL; and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is 380:56:14:98. The mass fraction of the ammonia is 25%, the mass fraction of the hydrogen peroxide is 30%, and the mass fraction of the sodium hydroxide solution is 30%.

[0059] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly at a speed of 350 r / min. Then react at a temperature of 185°C for 55 min. After naturally cooling to 80°C, filter to remove the precipitate and obtain the filtrate.

[0060] The solvent is a mixture of deionized water, dimethyl carbonate, and phenol, wherein the volume ratio of deionized water, dimethyl carbonate, and phenol in the solvent is 4.2:0.15:94.5; the lignin is an organic solvent lignin.

[0061] The weight ratio of the catalyst, lithium hydroxide, and lignin is 2.3:0.75:95, and the weight ratio of lignin to solvent is 1:3.5.

[0062] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, heat it to 94.5°C under stirring at a speed of 250 r / min. Then, add a 35% formaldehyde aqueous solution at a uniform rate. The time for adding the formaldehyde aqueous solution is controlled at 25 min. After the addition is completed, continue the reaction for 85 min. Then, cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0063] The weight ratio of the added sodium hydroxide to the lignin in step (2) is 14.2:95; the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is 325mL:95g.

[0064] Example 5: A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0065] (1) Add cerium chloride hexahydrate, magnesium chloride and zinc chloride to deionized water and stir to mix evenly to obtain mixture A. Then continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. The total time for adding ammonia water, hydrogen peroxide and sodium hydroxide solution is controlled at 35 min. After the addition is completed, the temperature is raised to 60℃ and then kept at a constant temperature for 35 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 825℃ for 2.5 h to obtain catalyst.

[0066] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hydrated solution is 0.24:0.12:0.08; the volume ratio of cerium chloride in the hydrated solution to deionized water is 0.24 mol:300 mL; and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is 300:52.5:15:95. The mass fraction of ammonia is 25%, the mass fraction of hydrogen peroxide is 30%, and the mass fraction of sodium hydroxide solution is 30%.

[0067] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly at a speed of 300 r / min. Then react at a temperature of 195℃ for 48 min. After naturally cooling to 80℃, filter to remove the precipitate and obtain the filtrate.

[0068] The solvent is a mixture of deionized water, dimethyl carbonate, and phenol, wherein the volume ratio of deionized water, dimethyl carbonate, and phenol in the solvent is 5:0.15:94.5; the lignin is sulfate lignin.

[0069] The weight ratio of the catalyst, lithium hydroxide, and lignin is 2:0.85:80, and the weight ratio of lignin to solvent is 1:5.5.

[0070] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, under the stirring condition of 250 r / min, heat it to 95°C and then add a 35% formaldehyde aqueous solution at a uniform rate. The time for adding the formaldehyde aqueous solution is controlled at 25 min. After the addition is completed, continue the reaction for 85 min and then cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0071] The weight ratio of the added sodium hydroxide to the lignin in step (2) is 14.6:80; the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is 290mL:80g.

[0072] Example 6: A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0073] (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. The total time for adding ammonia water, hydrogen peroxide and sodium hydroxide solution is controlled at 30 min. After the addition is completed, raise the temperature to 60℃ and react at a constant temperature for 30 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 800℃ for 2 h to obtain catalyst.

[0074] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hydrated solution is 0.20:0.10:0.08; the volume ratio of cerium chloride in the hydrated solution to deionized water is 0.20 mol:300 mL; and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is 300:50:10:90. The mass fraction of the ammonia is 20%, the mass fraction of the hydrogen peroxide is 25%, and the mass fraction of the sodium hydroxide solution is 25%.

[0075] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly at a speed of 300 r / min. Then react at a temperature of 180℃ for 20 min. After naturally cooling to 80℃, filter to remove the precipitate and obtain the filtrate.

[0076] The solvent is a mixture of deionized water, dimethyl carbonate, and phenol, wherein the volume ratio of deionized water, dimethyl carbonate, and phenol in the solvent is 4:0.1:94; the lignin is an organic solvent lignin.

[0077] The weight ratio of the catalyst, lithium hydroxide and lignin is 2:0.2:60, and the weight ratio of lignin and solvent is 1:3.

[0078] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, under the stirring condition of 200-300 r / min, heat it to 93°C. Then, add a 30% formaldehyde aqueous solution at a uniform rate. The time for adding the formaldehyde aqueous solution is controlled at 20 min. After the addition is completed, continue the reaction for 60 min. Then, cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0079] The weight ratio of the added sodium hydroxide to the lignin in step (2) is 12:60; the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is 250mL:60g.

[0080] Example 7: A method for preparing a lignin-based environmentally friendly wood adhesive, comprising the following steps:

[0081] (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. The total time for adding ammonia water, hydrogen peroxide and sodium hydroxide solution is controlled at 40 min. After the addition is completed, the temperature is raised to 60℃ and then kept at a constant temperature for 40 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 850℃ for 3 h to obtain catalyst.

[0082] The molar ratio of cerium chloride, magnesium chloride, and zinc chloride in the hydrated solution is 0.25:0.15:0.1; the volume ratio of cerium chloride in the hydrated solution to deionized water is 0.25 mol:400 mL; and the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is 400:60:20:100. The mass fraction of the ammonia is 30%, the mass fraction of the hydrogen peroxide is 35%, and the mass fraction of the sodium hydroxide solution is 35%.

[0083] (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly at a speed of 400 r / min. Then react at a temperature of 220°C for 60 min. After naturally cooling to 80°C, filter to remove the precipitate and obtain the filtrate.

[0084] The solvent is a mixture of deionized water, dimethyl carbonate, and phenol, wherein the volume ratio of deionized water, dimethyl carbonate, and phenol in the solvent is 5:0.2:95; the lignin is sulfate lignin.

[0085] The weight ratio of the catalyst, lithium hydroxide and lignin is 3:1.2:90, and the weight ratio of lignin and solvent is 1:6.

[0086] (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, under the stirring condition of 300 r / min, heat it to 95°C and then add a 40% formaldehyde aqueous solution at a uniform rate. The time for adding the formaldehyde aqueous solution is controlled at 30 min. After the addition is completed, continue the reaction for 90 min and then cool to obtain a viscous lignin-based environmentally friendly wood adhesive.

[0087] The weight ratio of the added sodium hydroxide to the lignin in step (2) is 15:90; the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is 350mL:90g.

[0088] Comparative Example 1: The preparation method of the lignin-based environmentally friendly wood adhesive described in this comparative example differs from the method described in Example 1 only in that step (1) is omitted, and the lignin-based environmentally friendly wood adhesive is prepared directly according to the method of steps (2) to (3), without adding a catalyst.

[0089] Comparative Example 2: The preparation method of the lignin-based environmentally friendly wood adhesive described in this comparative example differs from the method described in Example 1 only in that, in step (1), cerium chloride hexahydrate is not used, but only magnesium chloride and zinc chloride are used to prepare the catalyst.

[0090] Comparative Example 3: The difference between the preparation method of the lignin-based environmentally friendly wood adhesive described in this comparative example and the method described in Example 1 is that, in step (1), the molar ratio of the hexahydrate cerium chloride, magnesium chloride and zinc chloride is 0.28:0.10:0.06.

[0091] Comparative Example 4: The preparation method of the lignin-based environmentally friendly wood adhesive described in this comparative example differs from the method described in Example 1 only in that, in step (1), the mixture A is stirred and hydrogen peroxide and sodium hydroxide solution are added dropwise at a uniform rate. The total time for adding hydrogen peroxide and sodium hydroxide solution is controlled at 30-40 minutes, and no ammonia is added.

[0092] Comparative Example 5: The preparation method of the lignin-based environmentally friendly wood adhesive described in this comparative example differs from the method described in Example 1 only in that, in step (2), the solvent is a mixture of deionized water and phenol, and the volume ratio of deionized water to phenol in the solvent is 5:95.

[0093] Experimental example: Pretreated lignin and lignin-based adhesives were prepared according to the methods described in Examples 1-7 and Comparative Examples 1-5, and then tested according to the following methods. The test results are shown in Table 1.

[0094] (I) Characterization of the structure of lignin and lignin pretreatment products

[0095] (1) The molecular weight and molecular weight distribution of lignin were determined by a Waters 1515 / 2414 gel permeation chromatograph. The test was conducted using Waters Styragel columns in series (HR1, HR3 and HR4), with DMF as the mobile phase, column temperature maintained at 40℃, and flow rate of 1 ml / min. Polystyrene was used as the standard.

[0096] (2) The content of H, G and S structural units was determined by two-dimensional nuclear magnetic resonance (NMR) according to the literature. (Gao Miao, Cai Jibao, Xu Zhenyu, et al. Two-dimensional NMR spectral characterization of tobacco lignin structure [J]. Journal of Tobacco Science, 2022, 28(2). 1~12).

[0097] (ii) Adhesive performance testing

[0098] (1) Determination of viscosity, solid content and free formaldehyde

[0099] The viscosity, solid content, and free formaldehyde content of adhesive products shall be determined in accordance with GB / T 14074-2006 Test Methods for Wood Adhesives and Resins.

[0100] (2) Determination of adhesive strength

[0101] Pine veneers with a width of 400 mm × 400 mm were heat-treated to a moisture content of approximately 6%–8%. A double-sided adhesive application rate of 175 g / m² was evenly applied to the veneer surface, and the veneers were aged in a 50°C oven for 30 minutes. Then, the veneers were pressed in a flat vulcanizing machine at 150°C for 4 minutes and 1.5 MPa to prepare a three-layer plywood. Finally, according to the Class I plywood standard GB / T17657-2013, the bond strength was tested using a universal testing machine.

[0102] Table 1. Detection results of the experimental cases

[0103]

[0104] As can be seen from Table 1, the present invention has good raw material adaptability. Whether it is sodium lignin sulfonate, calcium lignin sulfonate, alkali lignin, sulfate lignin, Klason lignin or organic solvent lignin, under the conditions of the embodiments of the present invention, the viscosity, solid content, free formaldehyde content and bonding strength of the obtained lignin-based adhesive products can meet the national standard requirements for adhesives for wood-based panels (GBT 14732-2006 Urea-formaldehyde, phenolic and melamine-formaldehyde resins for wood industry adhesives).

[0105] Meanwhile, the CeO2-1 / 2MgO-1 / 3ZnO catalyst effectively promotes the depolymerization of H units and forms low-molecular-weight lignin molecular fragments during lignin pretreatment. Compared to no catalyst (Comparative Example 1), the addition of CeO2-1 / 2MgO-1 / 3ZnO (Example 1) increased the degradation rate of H units from 10.9% to 85.7%, and decreased the number-average molecular weight of the degradation product fragments from 8474 g / mol to 2984. Furthermore, the lignin-based adhesives prepared from the degradation products showed improvements in viscosity, solid content, free formaldehyde content, and bonding strength. Changing the ratio of Ce, Mg, and Zn in the catalyst, the co-precipitation method, and the steps (Comparative Examples 2-4) significantly affected the performance of both the lignin pretreatment products and the lignin-based adhesive products. In addition, trace amounts of DMC in the solvent also have a significant impact on the performance of lignin-based adhesives. DMC can act as an intermediate product trapping agent in the reaction, reducing the formation of useless products due to the repolymerization of quinone-structured intermediates, thereby obtaining better performance of lignin-based adhesives.

[0106] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a lignin-based environmentally friendly wood adhesive, characterized in that: Includes the following steps: (1) Take cerium chloride hexahydrate, magnesium chloride and zinc chloride and add them to deionized water and stir to mix evenly to obtain mixture A. Then, continue stirring mixture A at a speed of 250 r / min and add ammonia water, hydrogen peroxide and sodium hydroxide solution dropwise in sequence at a uniform rate. After the addition is completed, raise the temperature to 60℃ and react at a constant temperature for 30-40 min. Then filter to obtain precipitate. Then wash the precipitate with deionized water until the washing solution is neutral. Then calcine the precipitate at 800-850℃ for 2-3 h to obtain catalyst. The molar ratio of cerium chloride hexahydrate, magnesium chloride, and zinc chloride is (0.20–0.25):(0.10–0.15):(0.08–0.1); the volume ratio of cerium chloride hexahydrate to deionized water is (0.20–0.25) mol:(300–400) mL; the volume ratio of deionized water, ammonia, hydrogen peroxide, and sodium hydroxide solution is (300–400):(50–60):(10–20):(90–100); the mass fraction of ammonia is 20%–30%; the mass fraction of hydrogen peroxide is 25%–35%; and the mass fraction of sodium hydroxide solution is 25%–35%. (2) Take the catalyst obtained in step (1) and add it to the reactor, along with lithium hydroxide, lignin and solvent. Then stir and mix evenly, and react at a temperature of 180-220°C for 20-60 minutes. After naturally cooling to 80°C, filter to remove the precipitate and obtain the filtrate. The solvent is a mixture of deionized water, dimethyl carbonate, and phenol; the lignin is any one of sodium lignin sulfonate, calcium lignin sulfonate, alkali lignin, sulfate lignin, Klason lignin, and organic solvent lignin; in the solvent, the volume ratio of deionized water, dimethyl carbonate, and phenol is (4-5):(0.1-0.2):(94-95). The weight ratio of the catalyst, lithium hydroxide and lignin is (2-3):(0.2-1.2):(60-90), and the weight ratio of lignin and solvent is 1:(3-6). (3) Take the filtrate obtained in step (2) and add sodium hydroxide. Then, heat it to 93-95°C under stirring. Then, add a formaldehyde aqueous solution with a mass fraction of 30%-40% at a uniform rate. After the addition is complete, continue the reaction for 60-90 minutes. Then, cool to obtain a viscous lignin-based environmentally friendly wood adhesive. The weight ratio of the added sodium hydroxide to the lignin in step (2) is (12-15):(60-90); the ratio of the added formaldehyde aqueous solution to the lignin in step (2) is (250-350) mL:(60-90) g.

2. The preparation method of the lignin-based environmentally friendly wood adhesive according to claim 1, characterized in that: In step (1), the total time for adding ammonia, hydrogen peroxide and sodium hydroxide solution is controlled within 30 to 40 minutes.

3. The preparation method of the lignin-based environmentally friendly wood adhesive according to claim 1, characterized in that: In step (2), the stirring speed is 300-400 r / min.

4. The preparation method of the lignin-based environmentally friendly wood adhesive according to claim 1, characterized in that: In step (2), the lignin is either sodium lignin sulfonate or Klason lignin; the weight ratio of the lignin to the solvent is 1:

4.

5. The method for preparing the lignin-based environmentally friendly wood adhesive according to claim 1, characterized in that: In step (3), the filtrate obtained in step (2) is taken and sodium hydroxide is added, and then the temperature is raised to 93-95°C under stirring at a speed of 200-300 r / min.

6. The method for preparing the lignin-based environmentally friendly wood adhesive according to claim 1, characterized in that: In step (3), the time for adding formaldehyde aqueous solution is controlled at 20-30 min.

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