Lignin-covalent organic framework based superhydrophobic composite material and preparation method and application thereof

Abstract

The application belongs to the technical field of biomass high-value utilization and functional material preparation, and particularly relates to a super-hydrophobic composite material based on lignin-covalent organic framework, and a preparation method and application thereof. Oxygen alkali pulping black liquor lignin is used as raw material, and after being modified by amination, a covalent organic framework is in-situ grown on the surface of the lignin through a Schiff base reaction, so that stable covalent bond connection is realized, thereby a super-hydrophobic composite material with a multi-level structure is constructed. The super-hydrophobic composite material is compounded with a polydimethylsiloxane prepolymer, a curing agent and an organic solvent, so that a super-hydrophobic coating material can be prepared. The obtained material has excellent hydrophobic performance (a contact angle is greater than or equal to 150°) and good chemical stability, the prepared coating layer has a self-cleaning performance, and can be applied to the field of oil-water separation.

Description

Technical Field

[0001] This invention belongs to the field of biomass high-value utilization and functional material preparation technology, specifically involving superhydrophobic composite materials based on lignin-covalent organic frameworks, their preparation methods and applications. Background Technology

[0002] Lignin is the second most abundant natural aromatic polymer after cellulose, and its resource utilization is of great significance to sustainable development. In the pulping industry, black liquor contains abundant lignin, which is currently mainly recovered through combustion, resulting in low added value and easy environmental pollution and resource waste. Therefore, promoting the high-value conversion of lignin from black liquor has become crucial in the field of biomass utilization.

[0003] Lignin molecules contain abundant active functional groups such as phenolic and alcoholic hydroxyl groups, exhibiting both good biocompatibility and biodegradability, demonstrating broad potential in the field of functional materials. In particular, lignin produced during oxy-alkali pulping processes exhibits higher reactivity and a richer number of active sites under oxidative-alkaline conditions, providing a favorable foundation for functional modification. However, current methods for hydrophobic modification of lignin mainly rely on long-chain fatty acid esterification, silanization grafting, or blending with hydrophobic polymers. These methods typically suffer from cumbersome reaction steps, reliance on harmful organic solvents or fluorine-containing reagents, and poor environmental compatibility. Furthermore, existing lignin-based hydrophobic materials are mostly constructed through physical blending or weak interactions, resulting in weak interfacial bonding, easy phase separation, or loss of functional components, leading to insufficient performance stability during long-term use.

[0004] Meanwhile, covalent organic frameworks (COFs) have shown great potential in the field of hydrophobic functional materials due to their advantages such as designable structure, regular pores, and high specific surface area. Hydrophobic COFs, in particular, have been preliminarily studied in areas such as separation and adsorption. However, current strategies for combining lignin and COFs are mostly limited to simple physical mixing or functional superposition, lacking systematic design at the level of covalent bonding and structural synergy. This results in insufficient interfacial bonding strength and difficulty in synergistically controlling multi-level structures, thus limiting further improvement in the hydrophobic properties and long-term stability of composite materials. Summary of the Invention

[0005] The purpose of this invention is to provide a superhydrophobic composite material based on lignin-covalent organic framework, its preparation method and application. A stable covalent bond between lignin and hydrophobic COF is achieved through a simple and controllable chemical reaction, thereby obtaining a composite material with stable structure, strong interfacial bonding and excellent hydrophobic properties, so as to solve the problems of complex lignin hydrophobic modification process, insufficient interfacial bonding and poor material stability in the prior art.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: On the one hand, the present invention provides a method for preparing a composite material based on a lignin-covalent organic framework, comprising the following steps: The process includes the following steps: dispersing aminated lignin and a portion of aldehyde-containing monomers in an organic solvent and stirring to react; then adding the remaining aldehyde-containing monomers, amino-containing monomers and catalyst to carry out a Schiff base reaction, thereby obtaining a composite material based on a lignin-covalent organic framework. The mass ratio of aminated lignin to aldehyde-containing monomers is (0.725-7.25):1.

[0007] This preparation method uses oxo-alkali pulping black liquor as raw material, efficiently extracting oxo-alkali lignin through acid precipitation and solvent purification steps. The process is green and achieves value-added utilization of waste resources. An amino group is introduced onto the lignin surface through an amination reaction, significantly enhancing its reactivity and laying the foundation for subsequent covalent bonding. Using a stepwise reaction strategy, some aldehyde-containing monomers are first anchored onto the surface of the aminated lignin, and then in-situ Schiff base polymerization is carried out with the remaining monomers and amino monomers, successfully constructing a lignin-covalent organic framework composite structure linked by covalent bonds. This method not only achieves strong interfacial bonding between lignin and the covalent organic framework, but also endows the composite material with excellent hydrophobicity, high stability, and controllable pore characteristics through multi-scale structural regulation. The entire process is mild, simple to operate, and considers both environmental friendliness and the feasibility of large-scale production.

[0008] Secondly, the present invention provides a method for preparing a lignin-covalent organic framework-based composite material, wherein the lignin-covalent organic framework composite material obtained by the method has a water contact angle greater than 150°, preferably ≥160.5°.

[0009] This composite material combines lignin with a covalent organic framework through covalent bonding, effectively solving the problems of weak interfacial bonding and easy phase separation in traditional physical blending, and exhibiting excellent stability and structural integrity.

[0010] Thirdly, the present invention provides the application of lignin-covalent organic framework composite materials in the preparation of superhydrophobic coatings and superhydrophobic coatings.

[0011] Lignin-covalent organic framework composites possess both hierarchical porous structures and hydrophobic functional groups, endowing the materials with excellent hydrophobic properties and tunable surface characteristics, and have broad application potential in fields such as oil-water separation and adsorption.

[0012] Fourthly, the present invention provides a superhydrophobic coating comprising an AL-COF composite material, a polydimethylsiloxane (PDMS) prepolymer, a curing agent, and an organic solvent.

[0013] Fifthly, the present invention provides a superhydrophobic coating, which is obtained by spraying the superhydrophobic coating onto the surface of a substrate and drying and curing it; the substrate is filter paper, used for oil-water separation.

[0014] The beneficial effects of this invention are: (1) This invention uses pulping black liquor as raw material, extracts and amides lignin, and then covalently bonds it with aldehyde-containing monomers in situ to construct a hydrophobic covalent organic framework, thereby achieving stable covalent bonding between lignin and the COF matrix and synergistic regulation of multi-scale structures. The resulting composite material has a strong interfacial bond and significantly improved hydrophobic properties. Moreover, the preparation process is simple and controllable, providing a new approach for the high-value utilization of lignin and the development of functional hydrophobic materials.

[0015] (2) All raw materials used in this invention are fluorine-free and non-toxic, and the preparation process does not involve fluorine-containing compounds or heavy metal catalysts, making it environmentally friendly. The prepared AL-COF composite material can be widely used in self-cleaning coatings, oil-water separation membranes, corrosion-resistant materials, microfluidic devices, and drug sustained-release carriers, and has broad application prospects. Attached Figure Description

[0016] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0017] Figure 1 Fourier transform infrared (FT-IR) spectra of the OL, AL, and AL-COF composite materials prepared in Example 1 and the OL prepared in Comparative Example 4. Figure 2 The X-ray photoelectron spectroscopy (XPS) spectra of the AL prepared in Examples 1 and 2. Figure 3 Including Examples 1 and 3, the step of anchoring BHpTA monomer to AL has a BHpTA monomer to AL mass ratio of (a) 0.725:1, (b) 1.45:1, (c) 2.9:1 and (d) 7.25:1, and the resulting AL-COF composite material is obtained by scanning electron microscopy (SEM). Figure 4 The phosphorus NMR spectra of OL and AL prepared in Example 1 ( 31 P NMR); Figure 5 The X-ray photoelectron energy C1s fine spectrum (C1sXPS) of the AL and AL-COF composite material prepared in Example 1. Figure 6 The static water contact angle test results of the AL-COF composite material prepared in Example 1 after immersion in solutions of different pH values ​​for 24 h are shown. Figure 7 This is a comparison of the self-cleaning properties of the filter paper covered with the superhydrophobic coating prepared in Example 4 and the original filter paper. Figure 8 To build a gravity-driven separation device; Figure 9 The efficiency and throughput of the filter paper coated with the superhydrophobic coating prepared in Example 4 for (a) separation of six oil-water mixtures; and (b) cyclic separation test of dichloromethane / water mixture. Figure 10 This is a reaction mechanism diagram of the lignin-covalent organic framework superhydrophobic composite material prepared in Example 1. Detailed Implementation

[0018] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Specific conditions not specified in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Components whose manufacturers are not specified are all commercially available conventional products.

[0019] This invention addresses the problems of weak interfacial bonding and poor structural synergy in existing technologies when lignin is combined with covalent organic frameworks (COFs), resulting in insufficient hydrophobicity and stability of the materials. It extracts lignin from black liquor, a waste product of the pulping industry, as a raw material. High-purity lignin can be obtained through a simple purification process, solving the environmental pollution problem caused by black liquor discharge and transforming low-value industrial byproducts into high-value-added functional material raw materials, aligning with the concepts of green chemistry and circular economy. Then, utilizing the bifunctional characteristics of silane coupling agents, amino active sites are simultaneously introduced during the reaction, significantly improving the hydrophobic properties of lignin. The ethoxy group of APTES undergoes hydrolytic condensation with the hydroxyl group on lignin, successfully grafting the amino group onto the lignin molecule surface, laying the structural foundation for subsequent covalent bonding with COFs. The propyl segment in APTES and the oligomeric siloxane structure formed by its self-condensation construct a stable hydrophobic layer on the lignin surface, significantly improving the hydrophobicity of the modified lignin. Then, an in-situ growth strategy is employed. First, the aldehyde monomer is anchored to the amino groups on the modified lignin surface. Then, the remaining monomers are introduced to undergo a Schiff base reaction, allowing the COF to nucleate and grow in situ on the lignin surface. Compared to traditional physical blending or non-covalent composites, covalent bonds have higher bond energies, effectively resisting solvent erosion, mechanical friction, and temperature changes. This ensures the structural integrity and performance stability of the composite material during use, avoiding the degradation of hydrophobic properties caused by component shedding. The specific technical solution adopted is as follows: On the one hand, embodiments of the present invention provide a method for preparing a composite material based on a lignin-covalent organic framework, comprising the following steps: dispersing aminated lignin and a portion of aldehyde-containing monomers in an organic solvent and stirring to react, then adding the remaining aldehyde-containing monomers, amino-containing monomers and catalysts, and performing a Schiff base reaction to obtain a composite material based on a lignin-covalent organic framework. This invention uses highly active lignin (which has a lower molecular weight and increased phenolic hydroxyl content compared to traditional lignin) as raw material, modifies it by amylation, and then adopts an in-situ growth strategy. First, the aldehyde monomer is anchored to the amino group on the surface of the modified lignin, and then the remaining monomers are introduced to carry out Schiff base reaction, so that COF can be nucleated and grown in situ on the lignin surface, thus preparing a composite material with stable structure, excellent hydrophobic properties and environmental friendliness, and further applying it to the preparation of superhydrophobic coatings.

[0020] In this study, aldehyde-containing monomers were anchored to aminated lignin, with a mass ratio of aminated lignin to aldehyde-containing monomers of (0.725-7.25):1. The surface of the aminated lignin was completely covered by a continuous and dense COF shell, further increasing the static water contact angle. This indicates that a relative excess of aldehyde-containing monomers is beneficial for improving anchoring efficiency and promoting uniform COF growth.

[0021] Preferably, the aldehyde-containing monomer is anchored to the aminated lignin, and the mass ratio of the aminated lignin to the aldehyde-containing monomer is (2.9-7.25):1. The amount of the aldehyde-containing monomer used for anchoring accounts for 40-60% of its total amount.

[0022] In some other embodiments, the stirring reaction is carried out at a temperature of 25-60°C for 1-3 hours. Preferably, the amount of aldehyde-containing monomer used for anchoring accounts for 50% of its total amount. The reaction temperature for anchoring the aldehyde-containing monomer to aminated lignin is 40°C, and the reaction time is 2 hours.

[0023] In some other embodiments, the total molar ratio of aldehyde-containing monomers to the molar ratio of amino-containing monomers is 1:(0.5-1). Preferably, the total molar ratio of aldehyde-containing monomers to amino-containing monomers is 1:0.5; the Schiff base reaction temperature is 40°C and the reaction time is 4 hours.

[0024] In some other embodiments, in step (3), the aldehyde-containing monomer includes 2,5-diheptoxy-1,4-terephthalaldehyde; and the amino-containing monomer includes 1,3,5-tris(4-aminophenyl)benzene.

[0025] In some other embodiments, the preparation method of aminated lignin is as follows: oxoalkali lignin is dispersed in an organic solvent, and an amination reaction is carried out by adding a silane coupling agent under a protective atmosphere to obtain aminated lignin. Silane coupling agents include one or more of 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane; The mass ratio of oxyalkali lignin to silane coupling agent is 1:(1.9-5.6). The amination reaction is carried out at a temperature of 65-75℃ for 20-30 hours.

[0026] Studies have found that the amount of silane coupling agent added has a significant impact on the degree of amination, hydrophobicity, and solubility of oxoalkali lignin in subsequent reactions. When the amount of silane coupling agent added is relatively low, the degree of amination on the lignin surface is low, the improvement in hydrophobicity is limited, and the subsequent bonding efficiency with COF is insufficient. When the amount of APTES is appropriately increased, the degree of amination is significantly improved, the hydrophobicity of lignin is significantly improved, and the modified product maintains good dispersibility in organic solvents, which is beneficial to the subsequent in-situ growth reaction. When the amount of silane coupling agent added is further increased, the improvement in the degree of amination and hydrophobicity slows down, and the solubility of the modified lignin in organic solvents decreases significantly, resulting in agglomeration or precipitation, which is not conducive to the subsequent reaction. Preferably, the mass ratio of oxoalkali lignin to silane coupling agent is 1:(2.8-4.7).

[0027] In some other embodiments, the preparation method of oxoalkali lignin is as follows: filtration of oxoalkali pulping black liquor to obtain filtrate, adjustment of the pH of the filtrate with acid to obtain crude lignin, and dispersion of the crude lignin in 1,4-dioxane solution to react and obtain oxoalkali lignin. The pH of the filtrate is 2-6, and the acidic solution includes sulfuric acid; The reaction temperature is 40-60℃, and the reaction time is 2-3 days.

[0028] Sulfuric acid can rapidly neutralize the alkali in black liquor from oxygen-alkali pulping, adjusting the pH to 2-6 to induce lignin cation and precipitation. High lignin precipitation yields can be obtained by adjusting the pH to the range of 2-4 using sulfuric acid. Preferably, adjusting the black liquor pH to 3-4 with sulfuric acid ensures both precipitation yield and avoids excessive impact on the lignin structure.

[0029] Secondly, embodiments of the present invention provide a method for preparing lignin-covalent organic framework composite materials, which yields lignin-covalent organic framework composite materials.

[0030] Thirdly, embodiments of the present invention provide the application of lignin-covalent organic framework composite materials in the preparation of superhydrophobic coatings and superhydrophobic coatings.

[0031] In other embodiments, the superhydrophobic coating comprises a lignin-covalent organic framework composite material, a polydimethylsiloxane prepolymer, a curing agent, and an organic solvent.

[0032] The mass ratio of the lignin-covalent organic framework composite material, polydimethylsiloxane prepolymer, and curing agent is (0.01-0.1):1:0.1.

[0033] Anhydrous ethanol and n-hexane were used as solvents to prepare PDMS solutions, and the concentration of the PDMS solutions was 5-10% (w / v).

[0034] Preferably, anhydrous ethanol is used as the solvent for preparing the PDMS solution, and the concentration of the PDMS solution is 10% (w / v). The mass of the AL-COF composite material added to the PDMS solution is 20-100 mg.

[0035] Preferably, the mass of the AL-COF composite material added to the PDMS solution is 50 mg.

[0036] In some other embodiments, the static water contact angle of the superhydrophobic coating is ≥160.5°.

[0037] The present invention will be further described below with reference to embodiments: Example 1 This embodiment provides a superhydrophobic composite material based on a lignin-covalent organic framework and its preparation method. The reaction mechanism diagram is shown below. Figure 10 As shown, the specific steps include: (1) The pH of the filtered oxoalkali pulping black liquor was adjusted to 4 using sulfuric acid. The solid was separated by centrifugation, and the collected solid was freeze-dried for 3 days to obtain crude lignin. 4 g of crude lignin was uniformly dispersed in 100 mL of 80% 1,4-dioxane solution and reacted at 50℃ for 2 days. After centrifugation, the liquid portion was concentrated by rotary evaporation, and water was added to allow the solid to precipitate. The solid was then centrifuged and freeze-dried to obtain oxoalkali lignin (abbreviated as OL). The static water contact angle of the obtained OL was 34.3°.

[0038] OL 31 p NMR spectra as follows Figure 1 As shown, the OL sample (Comparative Example 4), obtained by adjusting the pH of black liquor to 2 with sulfuric acid, has an FT-IR spectrum at 1715 cm⁻¹. -1 A distinct C=O stretching vibration peak appears at 2850 cm⁻¹. -1 2940 cm -1 The intensity of the stretching vibration peak of the aliphatic CH group was significantly lower than that of the sample at pH=4 (Example 1), indicating that the lignin precipitated under pH=2 conditions underwent partial sulfonation and condensation, affecting its subsequent modification activity.

[0039] (2) Take 0.5 g of OL obtained in step (1) and 30 mL of toluene and add it to a three-necked flask. Sonicate for 30 min. Under nitrogen protection, add 2 mL of 3-aminopropyltriethoxysilane (APTES) to the three-necked flask. Stir magnetically at 70 °C for 24 h. After the reaction is complete, centrifuge to separate the product, wash away unreacted APTES with toluene, and dry under vacuum at 60 °C to obtain amino-modified lignin (AL). The static water contact angle of the obtained AL is 137.7°.

[0040] The amount of APTES added has a significant impact on the degree of amination, hydrophobicity, and solubility of OL in subsequent reactions. For example... Figure 2 As shown, AL (APTES 2 mL) exhibited a distinct N1s characteristic peak at 399.8 eV, with an atomic percentage of N of 5.14%. When the amount of APTES added was relatively low, the degree of amination on the lignin surface was low, the improvement in hydrophobicity was limited, and the subsequent bonding efficiency with COF was insufficient. When the amount of APTES was appropriately increased, the degree of amination was significantly improved, the hydrophobicity of lignin was significantly improved, and the modified product maintained good dispersibility in organic solvents, which was beneficial to the subsequent in-situ growth reaction. When the amount of APTES added was further increased, the improvement in the degree of amination and hydrophobicity slowed down, and the solubility of the modified lignin in organic solvents decreased significantly, resulting in agglomeration or precipitation, which was not conducive to the subsequent reaction.

[0041] (3) Take 5 mg of AL, 0.04 mol (14.5 mg) of BHpTA monomer and 5 mL of ethyl acetate obtained in step (2) and add them to a beaker. Stir magnetically at 40 °C for 2 h to anchor the BHpTA monomer to AL. Then add 0.04 mol (14.5 mg) of BHpTA monomer, 0.04 mol (14.1 mg) of 1,3,5-tris(4-aminophenyl)benzene (TAPB) monomer and 1 mL of 12 M acetic acid. Stir magnetically at 40 °C for 4 h. After the reaction is complete, wash with anhydrous ethanol and dry under vacuum at 60 °C to obtain the lignin-covalent organic framework superhydrophobic composite material (abbreviated as AL-COF composite material). The static water contact angle of the obtained AL-COF composite material is 157.9°.

[0042] To anchor the BHpTA monomer in the AL step, the amount of BHpTA monomer added was fixed at 0.04 mmol (14.5 mg), while the amount of AL added was varied from 2 to 20 mg. All other steps were the same as in Example 1 to obtain the AL-COF composite material. Figure 3 The amounts of AL added in (a), (b), (c) and (d) were 20, 10, 5 and 2 mg, respectively; Figure 3 (a) The sample corresponds to the AL-COF composite material obtained in Example 3. Figure 3 (c) Samples correspond to the AL-COF composite material obtained in Example 1. Figure 3 Samples (b) and (d) correspond to the step of anchoring BHpTA monomers to AL, in which the amount of AL added is 10 mg and 2 mg, respectively.

[0043] like Figure 3 As shown, with a fixed BHpTA monomer addition of 14.5 mg, when the mass ratio of BHpTA monomer to AL was 0.725:1, COF was sporadically distributed on the AL surface, with incomplete encapsulation. When the mass ratio of BHpTA monomer to AL was 1.45:1, the COF coating density increased, and the static water contact angle improved. When the mass ratio of BHpTA monomer to AL was (2.9-7.25):1, the AL surface was completely encapsulated by a continuous and dense COF shell, and the static water contact angle further increased. This indicates that a relatively excessive amount of BHpTA monomer is beneficial for improving anchoring efficiency and promoting uniform COF growth.

[0044] Depend on Figure 4 It can be seen that the degree of substitution of the OL hydroxyl group reaches 92.7%.

[0045] like Figure 5 As shown, in the C1s spectrum of AL-COF, the intensity of the C=O peak at 288.0 eV is significantly reduced, while a new characteristic peak appears at 287.8 eV, which is attributed to the carbon atom of the imine bond (C=N).

[0046] Example 2 This embodiment provides a superhydrophobic composite material based on a lignin-covalent organic framework and its preparation method, specifically including the following steps: (1) Take 0.5 g of OL from step (1) of Example 1 and add 30 mL of toluene to a three-necked flask and sonicate for 30 min. Under nitrogen protection, add 1 mL of APTES to the three-necked flask. Stir magnetically at 70 °C for 24 h. After the reaction is complete, centrifuge to separate the product, wash away unreacted APTES with toluene, and dry under vacuum at 60 °C to obtain AL. The static water contact angle of the obtained AL (1 mL of APTES) is 116.8°; (2) Take 5 mg of AL, 0.04 mol (14.5 mg) of BHpTA monomer and 5 mL of ethyl acetate obtained in step (2) and add them to a beaker. Stir magnetically at 40 °C for 2 h to anchor the BHpTA monomer onto the AL. Then add 0.04 mol (14.5 mg) of BHpTA monomer, 0.04 mol (14.1 mg) of TAPB monomer and 1 mL of 12 M acetic acid. Stir magnetically at 40 °C for 4 h. After the reaction is complete, wash with anhydrous ethanol and dry under vacuum at 60 °C to obtain the AL-COF composite material. The static water contact angle of the obtained AL-COF composite material is 147.1°.

[0047] Example 3 This embodiment provides a superhydrophobic composite material based on a lignin-covalent organic framework and its preparation method, specifically including the following steps: 20 mg of AL, 0.04 mol (14.5 mg) of BHpTA monomer, and 5 mL of ethyl acetate obtained in step (2) of Example 1 were added to a beaker and magnetically stirred at 40 °C for 2 h to anchor the BHpTA monomer onto the AL. Then, 0.04 mol (14.5 mg) of BHpTA monomer, 0.04 mol (14.1 mg) of TAPB monomer, and 1 mL of 12 M acetic acid were added, and the mixture was magnetically stirred at 40 °C for 4 h. After the reaction was completed, the mixture was washed with anhydrous ethanol and vacuum dried at 60 °C to obtain the AL-COF composite material. The static water contact angle of the obtained AL-COF composite material was 152.5°.

[0048] Example 4 This embodiment provides a superhydrophobic coating and its preparation method, specifically including the following steps: (1) Immerse 9 cm medium-speed quantitative filter paper in anhydrous ethanol solution and sonicate for 20 min, then rinse with deionized water and dry in a 60℃ oven for later use; (2) Prepare a 10% (w / v) PDMS solution by mixing 1 g of polydimethylsiloxane (PDMS) prepolymer with 0.1 g of Sylgard 184 curing agent at a ratio of 10:1 (w / w) and adding 10 mL of anhydrous ethanol as a solvent. (3) Take 50 mg of the AL-COF composite material obtained in step (3) of Example 1 (taking this as an example) and add it to the PDMS solution. Mix vigorously for 1 h to ensure uniform dispersion and obtain a superhydrophobic coating. (4) The superhydrophobic coating was introduced into the spray gun, and the nozzle diameter was adjusted to 0.5 mm. The filter paper was placed on a 40℃ hot plate, with the distance between the spray gun and the filter paper 15 cm. The coating was sprayed onto the surface of the filter paper in multiple applications, and then placed in a 120℃ oven to cure for 2 h, resulting in a superhydrophobic coating on the surface of the filter paper. The static water contact angle of the obtained superhydrophobic coating was 160.5°.

[0049] Comparative Example 1 Unlike Example 1, BHpTA monomer (0.06 mmol, 21.8 mg) and TAPB monomer (0.04 mmol, 11.6 mg) were weighed and transferred to separate 10 mL centrifuge tubes. Then, 5 mL of ethyl acetate was added to the centrifuge tubes as the reaction solvent, and the two monomers were uniformly dispersed in the reaction medium by sonication. Finally, 1 mL of 12 M acetic acid was added to the centrifuge tubes as a catalyst, and the reactants were reacted at 40 °C for 4 h. After the reaction was complete, the material was washed three times with ethanol and dried under vacuum at 60 °C to obtain BHpTA-TAPB-COF. The static water contact angle of the obtained BHpTA-TAPB-COF was 136.2°.

[0050] Comparative Example 2 The AL obtained in step (2) of Example 1 and the BHpTA-TAPB-COF obtained in Comparative Example 1 (with a ratio equivalent to AL-COF in Example 1) were placed in a mortar and physically ground and mixed for 30 min to obtain a physically mixed sample. The static water contact angle of the obtained sample was 141.6°.

[0051] Comparative Example 3 5 mg of AL, 0.08 mol (29 mg) of BHpTA monomer, 0.04 mol (14.1 mg) of TAPB monomer, 5 mL of ethyl acetate, and 1 mL of 12 M acetic acid obtained in step (2) of Example 1 were added to a beaker and reacted with magnetic stirring at 40 °C for 6 h. After the reaction was completed, the mixture was washed with anhydrous ethanol and dried under vacuum at 60 °C to obtain the AL-COF composite material. The static water contact angle of the obtained AL-COF composite material was 148.3°.

[0052] Comparative Example 4 Unlike Example 1, in step (1), sulfuric acid was used to adjust the pH of the filtered oxygen-alkali pulping black liquor to 2, while the other preparation steps were the same as in Example 1. The static water contact angle of the obtained AL-COF composite material was 149.5°.

[0053] Performance testing (1) The static water contact angle test method is as follows: Take the samples prepared in Examples 1-3 and Comparative Examples 1-4 respectively, spread them evenly on a glass slide, and press them into a flat sheet using a tablet press. Cut the filter paper covered with the superhydrophobic coating prepared in Example 4 into 1 cm pieces. 2 Place the sample on a glass slide for testing. Perform the test using a contact angle meter at room temperature. Place a 3 μL drop of deionized water on the sample surface and measure the static water contact angle after the droplet has stabilized.

[0054] (2) The static water contact angle test method after soaking at different pH is as follows: Take the AL-COF composite material prepared in Example 1 and soak it in solutions with pH=1, 3, 5, 7, 9, 11 and 13 (adjust the pH with hydrochloric acid or sodium hydroxide) for 24 hours. After taking it out, wash it with deionized water 3 times, dry it under vacuum at 60℃, and test the static water contact angle according to the aforementioned method.

[0055] Test results are as follows Figure 6 As shown, the static water contact angle decreased most significantly after immersion in strong acid (pH=1) and strong alkali (pH=13) environments. This is mainly because, under extreme pH conditions, the imine bonds (C=N) in the COF backbone and the silicon-oxygen bonds (Si-O-Si) in the silane layer undergo varying degrees of hydrolysis or chemical erosion, leading to an increase in the material's surface energy. In a near-neutral environment with pH=5-9, the water contact angle remained above 155° without significant attenuation, indicating that the core structure of the AL-COF composite material is highly stable within this pH range.

[0056] (3) Self-cleaning performance test: such as Figure 7 As shown, the filter paper covered with the superhydrophobic coating prepared in Example 4 and the original filter paper were respectively glued onto a glass slide, set at a certain tilt angle, sprinkled with fine sand, and water droplets were added to the surface of the filter paper to compare the self-cleaning performance of the two filter papers.

[0057] (4) Oil-water separation performance test: To verify the oil-water separation performance of the filter paper covered with the superhydrophobic coating prepared in Example 4, a gravity-driven separation device was built. Figure 8 Its separation efficiency and throughput for various oil-water mixtures were tested. For example... Figure 8 As shown, the gravity-driven separation device consists of an iron frame, flask clamps, two filter cups of the same diameter, filter cup clamps, and a beaker.

[0058] Place the superhydrophobic coated filter paper between two filter cups, secure it with filter cup clamps, and then vertically mount it on an iron stand with flask clamps, placing the beakers below the filter cups. During measurement, slowly pour the oil-water mixture into the upper filter cup opening, and start timing when the oil phase contacts the filter paper surface.

[0059] like Figure 9As shown, the separation efficiencies of this filter paper for dichloromethane, o-dichlorobenzene, nitrobenzene, n-hexane, toluene, and mesitylene are 99.4%, 98.7%, 97.9%, 99.6%, 98.5%, and 98.8%, respectively, corresponding to fluxes of 2154.2, 493.8, 491.5, 448.9, 421.4, and 294.7 L·m⁻¹. -2 ·h -1 Its excellent cycle stability was demonstrated in 10 dichloromethane / water separation cycle experiments. Even after the 10th cycle, its separation efficiency remained above 99%, and the flux remained stable at 2123.1 L·m³. -2 ·h -1 .

[0060] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing composite materials based on lignin-covalent organic frameworks, characterized in that, Includes the following steps: Aminated lignin and some aldehyde-containing monomers are dispersed in an organic solvent and stirred to react. Then, the remaining aldehyde-containing monomers, amino-containing monomers and catalysts are added to carry out a Schiff base reaction to obtain a composite material based on a lignin-covalent organic framework. The aldehyde-containing monomer is anchored to aminated lignin, and the mass ratio of aminated lignin to aldehyde-containing monomer is (0.725-7.25):1; the amount of aldehyde-containing monomer used for anchoring accounts for 40-60% of its total amount.

2. The method for preparing the composite material based on lignin-covalent organic framework as described in claim 1, characterized in that, The stirring reaction is carried out at a temperature of 25-60℃ for 1-3 hours.

3. The method for preparing the composite material based on lignin-covalent organic framework as described in claim 1, characterized in that, The ratio of the total molar amount of aldehyde-containing monomers to the molar amount of amino-containing monomers is 1:(0.5-1). The catalyst includes acetic acid, and the Schiff base reaction is carried out at a temperature of 25-60°C for 4-6 hours.

4. The method for preparing the composite material based on lignin-covalent organic framework as described in claim 1, characterized in that, The aldehyde-containing monomer includes 2,5-diheptaoxy-1,4-terephthalaldehyde; The amino-containing monomer includes 1,3,5-tris(4-aminophenyl)benzene.

5. The method for preparing the composite material based on lignin-covalent organic framework as described in claim 1, characterized in that, The preparation method of the aminated lignin is as follows: oxyalkali lignin is dispersed in an organic solvent, and an amination reaction is carried out by adding a silane coupling agent under a protective atmosphere to obtain aminated lignin. The silane coupling agent includes one or more of 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane; The mass ratio of the oxoalkali lignin to the silane coupling agent is 1:(1.9-5.6). The amination reaction is carried out at a temperature of 65-75℃ for 20-30 hours.

6. The method for preparing the composite material based on lignin-covalent organic framework as described in claim 5, characterized in that, The preparation method of the oxoalkali lignin is as follows: filtration of oxoalkali pulping black liquor to obtain filtrate, adjustment of the pH of the filtrate with acid to obtain crude lignin, and dispersion of the crude lignin in 1,4-dioxane solution to obtain oxoalkali lignin. The pH of the filtrate is 2-6, and the acid solution includes sulfuric acid; The reaction is carried out at a temperature of 40-60℃ for 2-3 days.

7. A lignin-covalent organic framework composite material prepared by the method for preparing a lignin-covalent organic framework composite material according to any one of claims 1-6.

8. The application of the lignin-covalent organic framework composite material of claim 7 in the preparation of superhydrophobic coatings and superhydrophobic coatings.

9. The application as described in claim 8, characterized in that, The superhydrophobic coating comprises a lignin-covalent organic framework composite material, a polydimethylsiloxane prepolymer, a curing agent, and an organic solvent; The mass ratio of the lignin-covalent organic framework composite material, the polydimethylsiloxane prepolymer, and the curing agent is (0.01-0.1):1:0.

1.

10. The application as described in claim 8, characterized in that, The static water contact angle of the superhydrophobic coating is ≥150°.

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