A method of modifying lignin by a ball milling method

Abstract

The application provides a method for modifying lignin by a ball milling method, in which ball milling CuAAc is used to modify natural lignin for the first time, and the degradation performance of lignin in the ball milling is studied; meanwhile, the reaction conditions such as the particle size of the grinding ball, the reaction time and the reagent ratio are investigated, and the optimal reaction conditions of the ball milling CuAAc for lignin are determined. The use of the ball milling CuAAc to modify natural lignin can greatly reduce the use of solvents, thereby reducing the production cost, avoiding the use of toxic solvents, and having great economic benefits and social values, and also provides a path for the application of solvent-free green chemistry to sustainable biomass.

Description

Technical Field

[0001] This invention relates to the field of polymer technology, and in particular to a method for modifying lignin by ball milling. Background Technology

[0002] Mechanochemistry combines mechanical processing and chemical reactions at the molecular level. At the chemical reaction level, mechanochemistry primarily refers to applying mechanical energy to condensed matter such as solids and liquids through shearing, friction, impact, and compression, inducing changes in their structure and physicochemical properties, and triggering chemical reactions. Unlike ordinary thermochemical reactions, mechanochemistry, due to its advantages such as being solvent-free, energy-efficient, having high conversion rates, and operating at room temperature, has been nominated by IUPAC as one of the ten technologies that will change the world.

[0003] Ball milling is one of the most effective tools in mechanochemical synthesis. During ball milling, mechanical forces cause high-energy collisions between atoms, mixing particles and activating the breaking and formation of surface chemical bonds. In stirred ball mills, this mechanical force is impact force, while in planetary ball mills it is shear force. Ball milling mechanochemistry has many advantages, including: 1) Because the amount of solvent used in ball milling is zero or minimal, the selection of reactants does not need to consider miscibility; there is no need to over-add other reactants to ensure sufficient reaction of certain reactants, thus achieving relatively accurate stoichiometry. Simultaneously, solvent-free chemical reactions can be achieved, saving costs and being environmentally friendly. 2) Mechanochemistry can synthesize products that cannot be synthesized through solution reactions. 3) Room temperature conditions are sufficient to complete the synthesis, while in some cases, solution reactions require high temperatures. 4) Ball milling reactions can synthesize some compounds that cannot be synthesized or have very low yields in traditional chemical synthesis methods. 5) Ball milling simplifies reaction conditions; for example, condensation polymerization requires high temperatures in solution reactions, while in ball milling, the reaction can be rapidly initiated at room temperature.

[0004] Lignin is a complex organic polymer that forms an important structural material in the supporting tissues of vascular plants and some algae, and is the second most abundant biomass resource. Lignin possesses advantages such as high mechanical hardness, good thermal stability, and low cost. Its main application is as a combustion energy source, and it is also used in the synthesis of functional polymer materials, such as self-healing materials, shape memory materials, and organic polymer dispersants. Due to its limited solubility, lignin is only soluble in polar and toxic organic solvents, such as DMF (N,N-Dimethylformamide), DMSO (Dimethyl sulfoxide), and ethylene glycol. Therefore, these solvents are commonly used in lignin research. However, while the use of these solvents promotes green biomass, it also increases the cost of producing lignin-derived materials. To achieve sustainable development goals and minimize the cost of lignin-derived materials, the use of solvents must be avoided.

[0005] In the synthesis of lignin-based polymers, the CuAAc reaction is frequently used on lignin. The CuAAc reaction is a click chemistry reaction that occurs between azide groups and triple bonds. Ball milling of CuAAc has been applied to one-pot CuAAc reactions of alkynes, bromides, and sodium azides, but its application in the modification of lignin has not been reported. Summary of the Invention

[0006] Based on the above-mentioned technical problems to be solved, the present invention provides a method for modifying lignin by ball milling.

[0007] This invention provides a method for modifying lignin by ball milling, the method comprising:

[0008] S01: The lignin-alkynyl group, ethyl azide, Cu(OAc)2 and ZrO2 balls are mixed and placed in a planetary ball mill for ball milling to obtain a ball-milled mixture;

[0009] The lignin-alkynyl group, ethyl azide, Cu(OAc)₂, and ZrO₂ balls were mixed and placed in a planetary ball mill. The mixture was ball-milled for 10-30 minutes at a speed of 550-650 rpm to obtain a ball-milled mixture. The molar ratio of lignin-alkynyl group, ethyl azide, and Cu(OAc)₂ was 1:1.2:0.1. The ethyl azide was vacuum-dried before use. In this application, 40 ZrO₂ balls with a diameter of 3 mm were used to ball-mill the lignin-alkynyl group, ethyl azide, and Cu(OAc)₂. Preferably, the ball milling speed was 600 rpm and the milling time was 30 minutes.

[0010] In this application, the method for preparing lignin-alkynyl groups includes:

[0011] S011: Mix lignin, bromopropyne, potassium carbonate and N,N-dimethylformamide and reflux at 75°C with stirring.

[0012] The lignin, bromopropyne, potassium carbonate, and N,N-dimethylformamide are mixed and refluxed at 75°C for 24-50 hours. In this application, lignin includes, but is not limited to, enzymatically hydrolyzed lignin and dealkalized lignin; the molar ratio of lignin, bromopropyne, and potassium carbonate is 1:(2-2.5):(2-2.5). The amount of N,N-dimethylformamide added is sufficient to dissolve the lignin, bromopropyne, and potassium carbonate; preferably, the amount of N,N-dimethylformamide added is 50-100 mL.

[0013] S012: After cooling to room temperature, filter and concentrate to obtain a concentrated solution;

[0014] The product after reflux and stirring was cooled to room temperature. After removing potassium carbonate by filtration, it was concentrated using a rotary evaporator to obtain a concentrated solution.

[0015] S013: The concentrated solution is precipitated in cold water and diethyl ether, and the precipitate is vacuum dried at 50°C to obtain lignin-alkynyl;

[0016] The concentrated solution was precipitated in cold water and diethyl ether to remove unreacted bromopropyne, yielding a precipitate. The precipitate was then vacuum dried at 50°C to obtain lignin-alkynyl groups.

[0017] S02: After removing Cu(OAc)2 from the ball-milled mixture, the mixture is concentrated to obtain a concentrated solution;

[0018] After removing Cu(OAc)₂ from the ball-milled mixture, it is concentrated using a rotary evaporator to obtain a concentrated solution. In this application, the method for removing Cu(OAc)₂ varies depending on the type of lignin used. When enzymatically hydrolyzed lignin is used, the ball-milled mixture is purified by flash column chromatography using dichloromethane as the eluent to remove Cu(OAc)₂. When alkali-degraded lignin is used, the ball-milled mixture is dissolved in N,N-dimethylformamide and precipitated with cold water to remove Cu(OAc)₂.

[0019] S03: The concentrated liquid is precipitated in cold diethyl ether, and the precipitate is vacuum dried at 50°C to obtain modified lignin.

[0020] This application provides a method for ball milling to modify lignin, which for the first time uses CuAAc to modify natural lignin, and investigates the degradation performance of lignin during ball milling. Simultaneously, it examines reaction conditions such as ball size, reaction time, and reagent ratio, determining the optimal reaction conditions for CuAAc ball milling on lignin. Using CuAAc ball milling to modify natural lignin can significantly reduce solvent usage, thereby lowering production costs and avoiding the use of toxic solvents, resulting in significant economic and social benefits. It also provides a pathway for solvent-free green chemistry to be applied to sustainable biomass.

[0021] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0022] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 The structural formulas of coumarin, coniferyl alcohol and sinigrin provided in the embodiments of the present invention are, from left to right, coumarin, coniferyl alcohol and sinigrin;

[0024] Figure 2 The structural formulas of o-vanillin, 2-methoxy-4-methylphenol and 2-methoxy-4-propylphenol provided in the embodiments of the present invention are, from left to right, o-vanillin, 2-methoxy-4-methylphenol and 2-methoxy-4-propylphenol;

[0025] Figure 3 The reaction principle diagrams of the three lignin model compounds provided in the embodiments of the present invention are shown below;

[0026] Figure 4 The o-vanillin-alkyne, lignin model compound A-triazole provided in the embodiments of the present invention 1 H NMR spectrum;

[0027] Figure 5 The 2-methoxy-4-methylphenol-alkyne and lignin model compound β-triazole provided in the embodiments of the present invention 1 HNMR spectrum;

[0028] Figure 6 The 2-methoxy-4-propylphenol-alkyne and lignin model compound C-triazole provided in the embodiments of the present invention 1 HNMR spectrum;

[0029] Figure 7A schematic diagram illustrating the reaction principle of ball milling for modifying lignin, provided in an embodiment of the present invention.

[0030] Figure 8 The enzymatic hydrolysis of lignin-alkynyl groups and the modified enzymatic hydrolysis of lignin provided in the embodiments of the present invention 1 H NMR characterization diagram;

[0031] Figure 9 The alkali-dealkali lignin-alkynyl and modified alkali-dealkali lignin provided in the embodiments of the present invention 1 H NMR characterization diagram. Detailed Implementation

[0032] This application is the first to apply ball-milled CuAAc to the modification of natural lignin. Before modifying natural lignin with ball-milled CuAAc, this application first modifies the monomeric lignin model compound using ball-milled CuAAc. Only when the monomeric lignin model compound is successfully modified can the ball-milled CuAAc modification of natural lignin be practically applied.

[0033] The main monomers of lignin are p-coumarol, coniferyl alcohol, and sinapyl alcohol, all of which have hydroxyl, carbonyl, and methoxy groups attached to their benzene rings. Their specific structural formulas are shown below. Figure 1 Currently, research on lignin monomers commonly uses three p-hydroxycinnamon alcohol compounds to study the properties of lignin, known as lignin model compounds. Besides being produced through the decomposition of natural lignin, these lignin model compounds also contain functional groups similar to the three p-hydroxycinnamon alcohols, such as o-vanillin, 2-methoxy-4-methylphenol, and 2-methoxy-4-propylphenol, with specific structural formulas as follows: Figure 2 In this application, the reactivity of o-vanillin, 2-methoxy-4-methylphenol, and 2-methoxy-4-propylphenol in ball-milled CuAAc was studied, and the properties of natural lignin were investigated using these three lignin model compounds. These are described in detail below.

[0034] I. About Experimental Materials

[0035] Alkali-degraded lignin (L0045) and 2-methoxy-4-methylphenol (M0114) were purchased from TCI. 2-methoxy-4-propylphenol (W359807) was purchased from Sigma-Aldrich. Enzymatically hydrolyzed lignin was purchased from Jinzhou Lingyu Co., Ltd. All other chemicals were purchased from Aladdin and, unless otherwise specified, were ready for use without further purification.

[0036] II. Regarding testing and experimental instruments

[0037] 1. FTIR-ATR spectra (Fourier Transform infrared spectroscopy) were obtained using a Nicolet iS50 instrument (Thermo Nicolet Co., Madison). Data acquisition was conducted in the wavenumber range of 4000–500 cm⁻¹.

[0038] 2. Nuclear magnetic resonance spectra were obtained using a Bruker Avance 400MHz spectrometer in CDCl3 or DMSO-d6 solvent. 1 HNMR (English name: Nuclear Magnetic Resonance Spectroscopy).

[0039] 3. The ball mill process is carried out on Fritsch GmbH's advanced planetary micro-mill P7 production line. Both the grinding jar and the grinding balls are made of ZrO2 material.

[0040] 4. GPC (Gel Permeation Chromatography) was performed on an Agilent 1260 Infinity instrument, using Waters Styragel HR SE and Styragel HR 2 columns in series. The mobile phase was DMF, and the flow rate was 1 ml / min. Enzymatically hydrolyzed lignin and dealkalized lignin were directly used for GPC detection without further purification. Lignin-alkynyl and lignin-triazole were synthesized and purified, and then analyzed for GPC. All test samples dissolved well in DMF. Polystyrene was used as a standard for GPC calibration.

[0041] III. Ball-milled CuAAc-modified three lignin model compounds

[0042] 1. Ball-milled CuAAc modified o-vanillin

[0043] 1.1 Synthesis of o-vanillin-alkynes

[0044] In a 500 mL round-bottom flask, 4 g of 26.29 mmol of o-vanillin, 2.72 mL of 31.55 mmol of bromopropyne, 4.36 g of 31.55 mmol of potassium carbonate, 200 mL of acetone, and a stir bar were added to form a mixture. The mixture was refluxed and stirred for 14 hours, then cooled to room temperature. The refluxed mixture was then evaporated using a rotary evaporator to remove acetone and concentrate the product. The concentrate was dissolved in diethyl ether. The concentrate dissolved in diethyl ether was washed twice with deionized water and saturated sodium chloride solution. The organic phase was dried over MgSO4, filtered, and the diethyl ether was removed by rotary evaporation. The product was purified by column chromatography using an ethyl acetate:n-hexane eluent of 1:3 and dried overnight in a vacuum oven at 25 °C to obtain pale yellow crystalline 3-methoxy-2-(prop-2-yne-1-ethoxy)benzaldehyde, i.e., o-vanillin-alkyne. The specific reaction principle is shown in the attached figure. Figure 3 (a).

[0045] 1.2 Ball milling of CuAAc to synthesize lignin model compound A-triazole

[0046] 0.2 g of 1.05 mmol of o-vanillin-alkyne, 0.13 mL of 1.15 mmol of ethyl azide, 0.0095 g of 0.052 mmol of Cu(OAc)₂, and 40 ZrO₂ balls with a diameter of 3 mm were added to a 45 mL ball mill jar. The ethyl azide solution was vacuum dried before use. The mixture was ball milled at 600 rpm for 30 minutes. After purification by column chromatography using an ethyl acetate:n-hexane eluent of 3:2, the eluent was removed by rotary evaporation to obtain the purified product. The purified product was dried overnight in a vacuum oven at 25 °C to obtain a white powder, ethyl 2-(4-((2-formyl-6-methoxyphenoxy)methyl)-1H-1,2,3-triazol-1-yl)ethyl acetate, i.e., the lignin model compound A-triazole.

[0047] 2. Ball milling CuAAc to modify 2-methoxy-4-methylphenol

[0048] 2.1 Synthesis of 2-methoxy-4-methylphenol-alkyne

[0049] In a 250 mL round-bottom flask, add 2 g of 14.47 mmol of 2-methoxy-4-methylphenol, 1.49 mL of 17.36 mmol of bromopropyne, 2.4 g of 17.36 mmol of potassium carbonate, 100 mL of acetone, and a stir bar to form a mixture. Reflux the mixture for 14 hours and then cool to room temperature. Use a rotary evaporator to evaporate the stirred refluxed mixture to remove acetone and concentrate it. The concentrate is dissolved in diethyl ether. Wash the concentrate dissolved in diethyl ether twice with deionized water and saturated sodium chloride solution. Dry the organic phase with MgSO4, filter, and remove the diethyl ether using a rotary evaporator. Purify the product by column chromatography with an eluent of ethyl acetate:petroleum ether = 1:15. Dry overnight in a vacuum oven at 25 °C to obtain a colorless, viscous liquid, 2-methoxy-4-methyl-1-(propyl-2-yne-1-hydroxy)benzene, i.e., 2-methoxy-4-methylphenol-alkyne. The specific reaction principle is shown in the attached figure. Figure 3 (b)

[0050] 2.2 Ball milling of CuAAc to synthesize lignin model compound β-triazole

[0051] In a 45 mL ball mill jar, 0.2 g of 1.13 mmol of 2-methoxy-4-methylphenol-alkyne, 0.16 mL of 1.36 mmol of ethyl azide, 0.01 g of 0.056 mmol of Cu(OAc)₂, and 40 ZrO₂ balls with a diameter of 3 mm were added. The ethyl azide solution was vacuum dried before use. The mixture was ball milled at 600 rpm for 30 minutes. The product was then purified by flash column chromatography using dichloromethane as the eluent to remove the copper catalyst. The mixture was concentrated using a rotary evaporator. The concentrate was precipitated in cold diethyl ether to remove unreacted ethyl azide. The precipitate was dried overnight in a vacuum oven at 35 °C to obtain white flocculent 2-(4-(2-methoxy-4-methylphenoxy)methyl)-1H-1,2,3-triazol-1-yl)ethyl acetate, i.e., the lignin model compound β-triazole.

[0052] 3. Ball milling CuAAc to modify 2-methoxy-4-propylphenol

[0053] 3.1 Synthesis of 2-methoxy-4-propylphenol-alkyne

[0054] In a 500 mL round-bottom flask, 2 g of 12.03 mmol of 2-methoxy-4-propylphenol, 1.55 mL of 18.05 mmol of bromopropyne, 2.49 g of 18.05 mmol of potassium carbonate, 100 mL of acetone, and a stir bar were added to form a mixture. The mixture was refluxed and stirred for 24 hours, then cooled to room temperature. The refluxed mixture was then evaporated using a rotary evaporator to remove acetone and concentrate the solution. The concentrate was dissolved in diethyl ether. The concentrate dissolved in diethyl ether was washed twice with deionized water and saturated sodium chloride solution. The organic phase was dried over MgSO4, filtered, and the diethyl ether was removed by rotary evaporation. The solution was purified by column chromatography using an eluent of ethyl acetate:petroleum ether = 1:10 and dried overnight in a vacuum oven at 25 °C to obtain a pale yellow viscous liquid, 2-methoxy-1-(propyl-2-yne-1-hydroxy)-4-propylbenzene, i.e., 2-methoxy-4-propylphenol-alkyne. The specific reaction principle is shown in the attached figure. Figure 3 (c)

[0055] 3.2 Ball milling of CuAAc to synthesize the lignin model compound C-triazole

[0056] 0.2 g (0.98 mmol) of 2-methoxy-4-propylphenol-alkyne, 0.13 mL (1.18 mmol) of ethyl azide, 0.0090 g (0.0049 mmol) of Cu(OAc)₂, and 40 ZrO₂ balls with a diameter of 3 mm were added to a 45 mL ball mill jar. The ethyl azide solution was vacuum dried before use. The mixture was ball milled at 600 rpm for 30 minutes. After purification by column chromatography using an ethyl acetate:n-hexane eluent of 1:1, the eluent was removed by rotary evaporation to obtain the purified product. The purified product was dried overnight in a vacuum oven at 25 °C to obtain white crystals of 2-(4-(2-methoxy-4-propylphenoxy)methyl)-1H-1,2,3-triazol-1-yl)ethyl acetate, i.e., the lignin model compound C-triazole.

[0057] In this embodiment, the prepared o-vanillin-alkyne, lignin model compound A-triazole, 2-methoxy-4-methylphenol-alkyne, lignin model compound B-triazole, 2-methoxy-4-propylphenol-alkyne, and lignin model compound C-triazole were subjected to chloroform treatment. 1 H NMR characterization yielded the following results: Figure 4-6 Among them, attached Figure 4 The middle left image shows o-vanillin-alkynes. 1 The right image shows the ¹H NMR spectrum of the lignin model compound A-triazole. 1 H NMR spectrum; attached Figure 5 The middle left image shows 2-methoxy-4-methylphenol-alkyne. 1The right image shows the 1H NMR spectrum of the lignin model compound β-triazole. 1 H NMR spectrum; attached Figure 6 The middle left image shows 2-methoxy-4-propylphenol-alkyne. 1 The right image shows the 1H NMR spectrum of the lignin model compound C-triazole. 1 HNMR image.

[0058] From the appendix Figure 4-6 It is evident that for the three lignin-alkynyl groups, the single proton at the alkynyl terminus appears at 2.5 ppm, while the two CH2 protons adjacent to the alkynyl group appear at 4.8 ppm. For the three lignin model compounds—triazoles—the single proton on the alkynyl group, originally at 2.5 ppm, is consumed; the two CH2 protons adjacent to the alkynyl group now shift to the triazole group at 5.4 ppm; the single proton on the triazole group appears at 7.8 ppm; and the two CH2 protons between the triazole and ether group appear at 5.2 ppm. This indicates that the three lignin model compounds—triazoles—perform well in ball-milled CuAAc. In this case, ball-milled CuAAc may be applicable to natural lignin.

[0059] IV. Ball milling CuAAc modified natural lignin

[0060] Based on the above research and discussion on ball milling CuAAc in lignin model compounds, this application applies the ball milling CuAAc reaction to natural lignin and provides a method for modifying lignin by ball milling, the method comprising:

[0061] S01: After mixing lignin, bromopropyne, potassium carbonate, and N,N-dimethylformamide, the mixture is refluxed and stirred at 75°C for 24-50 h. In this application, the lignin includes either enzymatically hydrolyzed lignin or dealkalized lignin; the molar ratio of lignin, bromopropyne, and potassium carbonate is 1:(2-2.5):(2-2.5), and the amount of N,N-dimethylformamide added is 50-100 mL.

[0062] S02: Cool the product after reflux and stirring to room temperature. After removing potassium carbonate by filtration, concentrate the solution using a rotary evaporator to obtain a concentrated solution.

[0063] S03: The concentrated solution was precipitated in cold water and diethyl ether to remove unreacted bromopropyne, yielding a precipitate. The precipitate was then vacuum dried at 50°C to obtain lignin-alkynyl groups.

[0064] S04: Mix lignin-alkynyl, ethyl azide, Cu(OAc)2, and ZrO2 balls and place them in a planetary ball mill. Mill the mixture at 550-650 rpm for 10-30 min to obtain a ball-milled mixture. The molar ratio of lignin-alkynyl, ethyl azide, and Cu(OAc)2 is 1:1.2:0.1. Ethyl azide is vacuum dried before use.

[0065] S05: After removing Cu(OAc)2 from the ball-milled mixture, the mixture is concentrated using a rotary evaporator to obtain a concentrated solution.

[0066] S06: The concentrated liquid is precipitated in cold diethyl ether, and the precipitate is vacuum dried at 50°C to obtain modified lignin.

[0067] The following describes the ball milling method for modifying lignin provided in this application, taking enzymatic hydrolysis of lignin and dealkalization of lignin as examples respectively. The reaction principle is shown in the appendix. Figure 7 As shown. The lignin used in the embodiments of this application is not limited to enzymatically hydrolyzed lignin and dealkalized lignin; other lignins can also be modified using the methods provided in the embodiments of this application.

[0068] Example 1

[0069] This application provides a method for modifying lignin by ball milling, the method comprising:

[0070] S101: Add 2g (9.09mmol) of enzymatically hydrolyzed lignin, 1.95mL (22.72mmol) of bromopropyne, 3.14g (22.72mmol) of potassium carbonate, 50mL of N,N-dimethylformamide, and a stir bar to a 100mL round-bottom flask. Reflux and stir at 75°C for 24 hours.

[0071] S102: Cool the product after reflux and stirring to room temperature. After removing potassium carbonate by filtration, concentrate the solution using a rotary evaporator to obtain a concentrated solution.

[0072] S103: The concentrated solution was precipitated in cold water and ether to remove unreacted bromopropyne, yielding a brown powder precipitate. The brown powder precipitate was then vacuum-dried at 50°C to obtain a light brown enzymatically hydrolyzed lignin-alkynyl group.

[0073] S104: In a 45 mL ball mill jar, add 0.2 g of enzymatically hydrolyzed lignin-alkynyl group, 57 μL of ethyl azide, 0.41 mmol of ethyl azide, 6 mg of Cu(OAc)₂, and 40 ZrO₂ balls with a diameter of 3 mm and mix. Ethyl azide is vacuum dried before use. The mixture is ball-milled at 600 rpm for 30 min to obtain the ball-milled mixture.

[0074] S105: The ball-milled mixture was purified by flash column chromatography using dichloromethane as the eluent to remove Cu(OAc)2. The concentrate was then obtained by rotary evaporation.

[0075] S106: The concentrate was precipitated in cold diethyl ether to remove unreacted ethyl azide, yielding a brown powder precipitate. The brown powder precipitate was then vacuum-dried at 50°C to obtain brown modified enzymatically hydrolyzed lignin.

[0076] Example 2

[0077] This application provides a method for modifying lignin by ball milling, the method comprising:

[0078] S201: Add 2g of 9.09mmol of dealkalized lignin, 1.56mL of 18.18mmol of bromopropyne, 2.51g of 18.18mmol of potassium carbonate, 50mL of N,N-dimethylformamide, and a stir bar to a 100mL round-bottom flask. Reflux and stir at 75°C for 50h.

[0079] S202: Cool the product after reflux and stirring to room temperature. After removing potassium carbonate by filtration, concentrate the solution using a rotary evaporator to obtain a concentrated solution.

[0080] S203: The concentrated solution was precipitated in cold water and diethyl ether to remove unreacted bromopropyne, yielding a light brown precipitate. The light brown precipitate was then vacuum dried at 50°C to obtain a light brown dealkalized lignin-alkynyl group.

[0081] S204: 0.2 g of 0.06 mmol of dealkalized lignin-alkynyl group, 10 μl of 0.072 mmol of ethyl azide, 1.08 mg of 0.006 mmol of Cu(OAc)₂, and 40 ZrO₂ balls with a diameter of 3 mm were mixed. The ethyl azide was vacuum dried before use. The mixture was ball-milled at 600 rpm for 30 min to obtain the ball-milled mixture.

[0082] S205: The ball-milled mixture was dissolved in N,N-dimethylformamide and precipitated with cold water to remove Cu(OAc)2. The concentrate was obtained by rotary evaporation.

[0083] S206: The concentrated solution was precipitated in cold diethyl ether to remove unreacted ethyl azide, yielding a brown powder precipitate. The brown powder precipitate was then vacuum dried at 50°C to obtain brown modified alkali-free lignin.

[0084] Example 3

[0085] This application provides a method for modifying lignin by ball milling, the method comprising:

[0086] S301: Add 2g (9.09mmol) of enzymatically hydrolyzed lignin, 1.95mL (22.72mmol) of bromopropyne, 3.14g (22.72mmol) of potassium carbonate, 50mL of N,N-dimethylformamide, and a stir bar to a 100mL round-bottom flask. Reflux and stir at 75°C for 24 hours.

[0087] S302: Cool the product after reflux and stirring to room temperature. After removing potassium carbonate by filtration, concentrate the solution using a rotary evaporator to obtain a concentrated solution.

[0088] S303: The concentrated solution was precipitated in cold water and ether to remove unreacted bromopropyne, yielding a brown powder precipitate. The brown powder precipitate was then vacuum-dried at 50°C to obtain a light brown enzymatically hydrolyzed lignin-alkynyl group.

[0089] S304: In a 45 mL ball mill jar, add 0.2 g of enzymatically hydrolyzed lignin-alkynyl group, 57 μL of ethyl azide, 0.41 mmol of ethyl azide, 6 mg of Cu(OAc)₂, and 40 ZrO₂ balls with a diameter of 3 mm and mix. Ethyl azide is vacuum dried before use. The mixture is ball-milled at 550 rpm for 10 min to obtain the ball-milled mixture.

[0090] S305: The ball-milled mixture was purified by flash column chromatography using dichloromethane as the eluent to remove Cu(OAc)2. The concentrate was then obtained by rotary evaporation.

[0091] S306: The concentrated solution was precipitated in cold diethyl ether to remove unreacted ethyl azide, yielding a brown powder precipitate. The brown powder precipitate was then vacuum-dried at 50°C to obtain brown modified enzymatically hydrolyzed lignin.

[0092] Example 4

[0093] This application provides a method for modifying lignin by ball milling, the method comprising:

[0094] S401: Add 2g of 9.09mmol of dealkalized lignin, 1.56mL of 18.18mmol of bromopropyne, 2.51g of 18.18mmol of potassium carbonate, 50mL of N,N-dimethylformamide, and a stir bar to a 100mL round-bottom flask. Reflux and stir at 75°C for 50h.

[0095] S402: Cool the product after reflux and stirring to room temperature. After removing potassium carbonate by filtration, concentrate the solution using a rotary evaporator to obtain a concentrated solution.

[0096] S403: The concentrated solution was precipitated in cold water and diethyl ether to remove unreacted bromopropyne, yielding a light brown precipitate. The light brown precipitate was then vacuum dried at 50°C to obtain a light brown dealkalized lignin-alkynyl group.

[0097] S404: 0.2 g of 0.06 mmol of dealkalized lignin-alkynyl group, 10 μl of 0.072 mmol of ethyl azide, 1.08 mg of 0.006 mmol of Cu(OAc)₂, and 40 ZrO₂ balls with a diameter of 3 mm were mixed. The ethyl azide was vacuum dried before use. The mixture was ball-milled at 650 rpm for 20 min to obtain the ball-milled mixture.

[0098] S405: The ball-milled mixture was dissolved in N,N-dimethylformamide and precipitated with cold water to remove Cu(OAc)2. The concentrate was obtained by rotary evaporation.

[0099] S406: The concentrated solution was precipitated in cold diethyl ether to remove unreacted ethyl azide, yielding a brown powder precipitate. The brown powder precipitate was then vacuum dried at 50°C to obtain brown modified alkali-free lignin.

[0100] In the embodiments of this application, the prepared enzymatically hydrolyzed lignin-alkynyl, modified enzymatically hydrolyzed lignin, dealkalized lignin-alkynyl, and modified dealkalized lignin were respectively processed in DMSO-d6. 1 H NMR characterization yielded the following results: Figure 8 , 9 .

[0101] From the appendix Figure 8 , 9 It is evident that the alkynyl group exhibits a single proton (g) at 4.1 ppm and two protons (f) at 4.4 ppm. After ball milling CuAAc, the proton g is consumed, and the f is transferred to f' at 5.4 ppm. The chemical shift of the proton g' on the triazole is 8.2 ppm. The conversion rates of both ball-milled CuAAc methods exceeded 99%. These successful reactions validate that ball-milled CuAAc can be successfully applied to three-dimensional polyphenol lignin and modify natural lignin.

[0102] V. Does lignin degrade during ball milling?

[0103] Ball mills possess high energy output; planetary ball mills generate shear energy, and stirred ball mills generate impact energy. These energies can disrupt polymer structures and lead to polymer degradation. Therefore, the milling process influences the energy required for lignin degradation. Lignin is a polymer with a complex three-dimensional structure composed of strong chemical bonds such as aryl ethers (α-O-4′ and β-O-4′), resin alcohols (β-β′), benzocoumarins (β-5′), biphenyls (5-5′), and 1,2-diarylpropane (β-1′), making it susceptible to disruption by ball milling. Lignin is known to be robust and potentially robust enough to maintain the integrity of its polyphenolic structure during ball milling. Whether lignin degrades during ball milling is twofold. If lignin degrades, ball milling may be a potential method to break down the macromolecular structure of lignin to obtain smaller molecules. Alternatively, if the macromolecular structure of lignin remains intact, ball milling may represent a novel approach for synthesizing lignin-based polymers.

[0104] To investigate whether lignin degrades during ball milling, the molecular weights of enzymatically hydrolyzed lignin, dealkalized lignin, enzymatically hydrolyzed lignin-alkynyl group, modified enzymatically hydrolyzed lignin, dealkalized lignin-alkynyl group, and modified dealkalized lignin were determined. The results are shown in Table 1.

[0105] Table 1: Molecular weight of lignin and its derivatives

[0106]

[0107] As shown in Table 1, the molecular weight of the modified lignin did not decrease after ball milling, meaning there was no significant degradation. This indicates that lignin exhibits high strength and is not easily degraded during ball milling, suggesting that ball milling may be a novel approach for lignin modification.

[0108] VI. Determination of Optimal Reaction Conditions

[0109] While maintaining a high conversion rate, this application further optimizes the reaction conditions for ball milling CuAAc and lignin to conform to the principles of green chemistry. The determination of the optimal conditions for the molar ratio of alkyne to ethyl azide, the type of milling balls, and the milling time are described below.

[0110] 1. Molar ratio of alkyne to ethyl azide

[0111] This application investigated the ratio of alkyne to ethyl azide to incorporate as many materials as possible into the final product. Specifically, the molar ratio of alkyne to ethyl azide was set to 1:1, 1:1.2, and 1:2, respectively, and modified enzymatically hydrolyzed lignin and modified dealkalized lignin were prepared. The conversion rates of the modified enzymatically hydrolyzed lignin and modified dealkalized lignin were measured, and the results are shown in Table 2.

[0112] Table 2: Conversion rates of modified enzymatically hydrolyzed lignin and modified dealkalized lignin at different molar ratios of alkyne to ethyl azide.

[0113]

[0114] a Conversion rate 1 HNMR monitoring; ball milling conditions: Cu(OAc)2, 40*3mm ZrO2 balls, 600rpm, 30min.

[0115] As shown in Table 2, when the molar ratio of acetylene to ethyl azide is 1:2, the conversion rates of both modified enzymatically hydrolyzed lignin and modified dealkalized lignin are >99%, indicating that the materials have been completely converted. According to "atom economy," the amount of azide needs to be reduced to incorporate as much azide as possible into the product. When the molar ratio of acetylene to ethyl azide is 1:1, the conversion rates of modified enzymatically hydrolyzed lignin and modified dealkalized lignin are 91% and 93%, respectively, and the acetylene conversion rate cannot reach its optimal level. When the molar ratio of acetylene to ethyl azide is 1:1.2, the conversion rates of both modified enzymatically hydrolyzed lignin and modified dealkalized lignin are >99%, achieving complete acetylene conversion. Therefore, the optimal ratio is obtained when the molar ratio of acetylene to ethyl azide is 1:1.2.

[0116] 2. Types of grinding balls

[0117] The total mass of the grinding balls reflects the impact energy, which affects the effective energy transfer during the collision between the grinding balls and the jar wall, as well as the abrasive and conversion rates. With the same total grinding ball mass, more small-sized grinding balls can be used, and fewer large-sized grinding balls can be used. More small-sized balls result in more contact points between the balls and the jar wall, improving milling efficiency, providing greater maneuverability, and a longer trajectory. Large balls have less maneuverability and a shorter trajectory. Smaller, larger balls can circumvent these characteristics. In the embodiments of this application, in a 45 ml ZrO2 grinding jar, nine 5 mm diameter grinding balls and forty 3 mm diameter grinding balls with the same total mass were used to study the conversion rates of enzymatically hydrolyzed lignin and dealkalized lignin, and the results are shown in Table 3.

[0118] Table 3: Effect of milling ball type on the conversion rate of enzymatically hydrolyzed lignin and dealkalized lignin

[0119]

[0120] a The total mass of the two types of balls is equal. b Use the surface area of ​​the sphere. c The transformation process is from 1 H NMR monitoring; ball milling conditions: alkyne to azide ratio = 1:1.2, Cu(OAc)2, 600 rpm, 30 minutes.

[0121] As shown in Table 3, compared to milling balls with a diameter of 5 mm, milling balls with a diameter of 3 mm have a larger specific surface area, and the conversion rates of enzymatically hydrolyzed lignin and dealkalized lignin both reach over 99%, which is far greater than the conversion rates when using milling balls with a diameter of 5 mm. This indicates that, for the same total ball mass, more small balls are beneficial for improving conversion. Therefore, in the embodiments of this application, it is preferable to use 40 milling balls with a diameter of 3 mm to modify natural lignin.

[0122] 3. Ball grinding time

[0123] According to the principles of "energy-efficient design" in green chemistry, energy demand should be recognized and minimized. This means that ball milling time needs to be reduced to minimize milling energy. In the embodiments of this application, ball milling was performed for 10 min, 20 min, and 30 min respectively to study the conversion rate of enzymatically hydrolyzed lignin and dealkalized lignin, and the results are shown in Table 4.

[0124] Table 4: Effects of different ball milling times on the conversion rates of enzymatically hydrolyzed lignin and dealkalized lignin.

[0125]

[0126] a Conversion rate 1 HNMR monitoring; ball milling conditions: alkyne to azide ratio = 1:1.2, Cu(OAc)2, 40*3mm ZrO2 balls, 600rpm.

[0127] As shown in Table 4, when the ball milling time is 30 min, the conversion rate of enzymatically hydrolyzed lignin and dealkalized lignin can reach 99%. When the ball milling time is 20 min, the conversion rate of enzymatically hydrolyzed lignin is 99%, and the conversion rate of dealkalized lignin is 97%. The difference in conversion may be because the higher molecular weight and more complex structure of dealkalized lignin hinders the conversion rate.

[0128] In summary, the optimal reaction conditions for ball milling CuAAc-modified enzymatically hydrolyzed lignin are: a molar ratio of alkyne to ethyl azide acetate of 1:1.2, Cu(OAc)₂ catalysis, ball milling with 40*3mm ZrO₂ at 600 rpm for 20 min. The optimal reaction conditions for ball milling CuAAc-modified dealkalized lignin are: a molar ratio of alkyne to ethyl azide acetate of 1:1.2, Cu(OAc)₂ catalysis, ball milling with 40*3mm ZrO₂ at 600 rpm for 30 min.

[0129] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0130] It should be understood that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. The invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method of modifying lignin by a ball milling process, characterized by, The method comprises the following steps: mixing lignin, propargyl bromide, potassium carbonate and N,N-dimethylformamide, and stirring under reflux at 75℃ for 24-50h; cooling to room temperature, filtering and concentrating to obtain a concentrated solution; precipitating the concentrated solution in cold water and ether, and vacuum drying the precipitate at 50℃ to obtain the lignin-propargyl. The lignin comprises one of enzymatic hydrolysis lignin and delignified lignin. The molar ratio of the lignin-propargyl, the ethyl azidoacetate and the Cu(OAc)2 is 1:1.2:0.

1. The lignin-propargyl, the ethyl azidoacetate and the Cu(OAc)2 are ball-milled by using 40 ZrO2 balls with a diameter of 3mm. The ball-milling speed is 550-650rpm, and the ball-milling time is 10-30min.

2. The method of modifying lignin by ball milling according to claim 1, characterized in that, The ball-milling speed is 600rpm, and the ball-milling time is 30min.

3. The method of modifying lignin by ball milling according to claim 1, characterized in that, The removal of the Cu(OAc)2 from the ball-milling mixture comprises purifying the ball-milling mixture by flash column chromatography with dichloromethane as eluent.

4. The method of modifying lignin by ball milling according to claim 1, characterized in that, The removal of the Cu(OAc)2 from the ball-milling mixture comprises dissolving the ball-milling mixture in N,N-dimethylformamide and precipitating in cold water.

5. The method of modifying lignin by ball milling according to claim 1, characterized in that, The molar ratio of the lignin, the propargyl bromide and the potassium carbonate is 1:(2-2.5):(2-2.5).

6. The method of modifying lignin by a ball milling process according to claim 1, characterized by, ​ 7. The method of modifying lignin by ball milling according to claim 1, characterized in that, ​ 8. The method of modifying lignin by a ball milling process according to claim 1, characterized by, ​ 9. The method of modifying lignin by a ball milling process according to claim 1, characterized by, ​

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