A method for preparing aromatic monomers by preferential lignin dissolution and in-situ oxidative depolymerization.

Abstract

This invention discloses a method for preferentially dissolving lignin and preparing aromatic monomers through in-situ oxidative depolymerization. The method utilizes a eutectic solvent solution (DES) prepared with tetramethylammonium hydroxide, imidazole, and water, which has the function of preferentially dissolving lignin. This achieves the effect of removing a large amount of lignin while retaining a large amount of hemicellulose. Furthermore, it efficiently prepares aromatic monomers under mild reaction conditions and without external catalysts, avoiding the loss of a large amount of carbohydrates, improving the purity of lignin and the conversion rate of aromatic monomers. The method eliminates the need for lignin separation and purification steps, is simple, and allows for solvent recycling, making it easy to apply industrially.

Description

Technical fields:

[0001] This invention relates to the field of lignocellulose biomass refining technology, specifically to a method for preparing aromatic monomers by preferential lignin dissolution and in-situ oxidative depolymerization. Background technology:

[0002] Biorefining from lignocellulose biomass is an important approach to addressing the energy crisis and promoting sustainable development. Lignin, one of the three major components of lignocellulose, is the most abundant aromatic polymer in nature, possessing rich oxygen-containing functional groups and a high calorific value, making it a promising candidate for producing high-density fuels, functional materials, and chemicals. Demodified eutectic solvents (DES) are ionic liquid-like solvents synthesized from hydrogen bond donors and acceptors. They can selectively dissolve lignin, protect and swell cellulose components, and are widely used in the fractionation of lignocellulose due to their low toxicity, low cost, and ease of synthesis. However, in the existing DES process, the lignin ether bonds are easily broken, leading to lignin structural condensation and increasing the difficulty of subsequent depolymerization. Simultaneously, during lignin extraction, the lignin-containing liquid phase is accompanied by the dissolution of a large amount of hemicellulose. The hemicellulose-derived sugars dissolved in the liquid solvent are difficult to separate and utilize, affecting not only the conversion efficiency of lignin but also causing sugar loss. For example, Chinese patent (CN108660837 B) discloses a method for separating cellulose, hemicellulose, and lignin from plant fiber raw materials. The method involves pressurizing or heating the plant fiber raw material with a lactic acid / choline chloride eutectic solvent at a maximum temperature of 120–150°C for 1–4 hours, selectively dissolving lignin and hemicellulose. The remaining insoluble matter is washed to obtain cellulose. The eutectic solvent solution containing lignin and hemicellulose is then diluted with water to precipitate lignin, which is then washed and separated to obtain lignin. The remaining eutectic solvent aqueous solution containing hemicellulose is separated by nanofiltration to obtain hemicellulose. However, during the lactic acid / choline chloride eutectic solvent treatment, hemicellulose and lignin dissolve together, requiring further separation. This not only affects the conversion efficiency of lignin but also causes sugar loss. While the lignin yield is 64.9%, nearly 50% of the hemicellulose is removed, resulting in a low hemicellulose retention rate. For example, CN 113603899 B discloses a method for pretreating lignocellulose using a Lewis base-assisted neutral eutectic solvent. The air-dried and pulverized plant fiber raw material is placed in a prepared glycerol / choline chloride eutectic solvent, and then a Lewis base is added. This method can remove up to 88.1% of lignin, but hemicellulose is also removed simultaneously, with a removal rate as high as 80.4%. In CN 115160591B, the results of alkaline DES treatment using an alkaline eutectic solvent of choline chloride, ethylene glycol, and the alkaline auxiliary sodium sulfide hydrate, reacted at 100–150°C for 10–60 min, still showed a hemicellulose retention rate of less than 60%. Furthermore, although the eutectic solvent (DES) in the above system can be recycled multiple times, the gradual accumulation of hemicellulose-derived sugars increases the viscosity of the DES, thus weakening its lignin extraction effect. Therefore, it is necessary to find new eutectic solvents that can reduce the loss of hemicellulose components in the raw material while ensuring a high lignin removal rate. Summary of the Invention:

[0003] The purpose of this invention is to provide a method for the preferential dissolution of lignin and in-situ oxidative depolymerization to prepare aromatic monomers. This method solves the problem that existing eutectic solvents cannot simultaneously ensure high lignin removal rate and hemicellulose retention rate, and avoids the loss of hemicellulose-derived sugars caused by hemicellulose dissolving in the eutectic solvent liquid phase, as well as the problem of affecting downstream lignin conversion and utilization.

[0004] This invention is achieved through the following technical solutions:

[0005] A method for preferential lignin dissolution includes the following steps: mixing lignocellulose biomass with a eutectic solvent solution at a mass ratio of 1:5–1:15, preferably 1:10–1:15, reacting at 30–90℃, preferably 50–90℃ for 1–7 h, preferably 3–5 h, and separating the solid and liquid phases to obtain a cellulose-rich solid phase and a eutectic solvent liquid phase. The cellulose-rich solid phase contains cellulose and a large amount of hemicellulose, and the eutectic solvent liquid phase is rich in lignin with high β-O-4 bond retention, high purity, and high reactivity. The eutectic solvent solution is prepared as follows: mixing tetramethylammonium hydroxide and imidazole or urea at a molar ratio of 1:1–1:7, and then adding an appropriate amount of deionized water to prepare a 5wt%–40wt% eutectic solvent aqueous solution.

[0006] A method for preparing aromatic monomers by in-situ oxidative depolymerization includes the following steps: eutectic solvent liquid phase, rich in high-β-O-4 bond retention, high-purity, and highly reactive lignin obtained by the above-mentioned lignin preferential dissolution method, is subjected to in-situ oxidative depolymerization at 1 MPa oxygen atmosphere and 60℃–100℃ for 1–4 h to prepare aromatic monomers. The aromatic monomers are mainly benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, eugenol, vanillin ethyl ketone, and benzonitrile.

[0007] A method for preparing aromatic monomers through in-situ oxidative depolymerization is disclosed. This method involves a one-pot in-situ depolymerization of lignocellulosic biomass, comprising the following steps: mixing lignocellulosic biomass with a eutectic solvent aqueous solution at a solid-liquid ratio of 1:15–1:50, and performing an oxidative depolymerization reaction at 60℃–100℃ for 1–4 hours under an oxygen atmosphere of 1 MPa. After solid-liquid separation, hydrochloric acid is added to the liquid phase product to precipitate unconverted lignin. Aromatic monomers are then extracted using an organic solvent. The aromatic monomers are primarily benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, eugenol, vanillin ethyl ketone, and benzonitrile. The eutectic solvent solution is prepared as follows: tetramethylammonium hydroxide and imidazole are mixed at a molar ratio of 1:1–1:7, and then an appropriate amount of deionized water is added to prepare a 5wt%–40wt% eutectic solvent aqueous solution.

[0008] Preferably, the above-mentioned oxidative depolymerization process may be carried out by adding an exogenous metal oxide catalyst as needed, and the exogenous metal oxide is selected from MnO2 and MoO2.

[0009] Lignocellulose biomass includes one or more of the following: sawdust, fruit shells, and straw.

[0010] The beneficial effects of this invention are as follows:

[0011] (1) The eutectic solvent solution (DES) prepared by tetramethylammonium hydroxide, imidazole or urea and water of the present invention has the function of preferentially dissolving lignin. While efficiently dissolving lignin, it retains cellulose and hemicellulose in the solid phase, which can ensure a high recovery rate of cellulose and hemicellulose, avoid the loss of a large amount of carbohydrates, and improve the purity of lignin.

[0012] (2) The eutectic solvent solution (DES) prepared by tetramethylammonium hydroxide, imidazole or urea and water of the present invention can achieve preferential dissolution of lignin under mild conditions of low DES concentration and 30–90°C, reducing chemical reagents and energy consumption.

[0013] (3) The eutectic solvent solution prepared by tetramethylammonium hydroxide, imidazole and water of the present invention has a protective function for lignin, with high β-O-4 bond content and no significant increase in C-C bond content. It has high reactivity and can directly depolymerize lignocellulose raw materials in situ under mild reaction conditions and without external catalysts. The yield of aromatic monomers generated by lignin depolymerization is up to 58.88%, without the need for lignin separation and extraction steps.

[0014] (4) The eutectic solvent solution (DES) prepared by tetramethylammonium hydroxide, imidazole or urea and water of the present invention can be recycled by adjusting the pH value to 13-14, and has good recycling performance, achieving zero waste discharge, which is in line with the concept of environmental protection.

[0015] In summary, the eutectic solvent solution prepared by tetramethylammonium hydroxide, imidazole, and water of this invention preferentially dissolves lignin, efficiently preparing aromatic monomers under mild reaction conditions and without exogenous catalysts, while highly retaining holocellulose containing cellulose and a large amount of hemicellulose. This ensures a high recovery rate of cellulose and hemicellulose, avoids the loss of a large amount of carbohydrates, improves the purity of lignin, eliminates the need for lignin separation and purification steps, simplifies the process, allows for solvent recycling, and is easy to apply industrially. Attached image description:

[0016] Figure 1 This is a schematic diagram of the process flow of the present invention. Detailed implementation method:

[0017] The following is a further description of the invention, but not a limitation thereof.

[0018] Example 1: Preferential dissolution of lignin and stepwise in-situ preparation of aromatic monomers

[0019] Tetramethylammonium hydroxide and imidazole were prepared at a molar ratio of 1:7 to form a clear, transparent, and homogeneous solution. A certain amount of water was then added to prepare a 10wt% DES solution. Coconut shells and the DES solution were mixed at a solid-liquid ratio of 1:5, and the reaction was carried out at 30℃ for 7 hours. After the reaction, the mixture was separated into solid and liquid phases to obtain a cellulose-rich solid phase and a eutectic solvent liquid phase. The cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, while the eutectic solvent liquid phase was rich in lignin with high β-O-4 bond retention, high purity, and high reactivity. Under these conditions, the raw material composition changed as follows: cellulose retention rate was 93.13%, hemicellulose retention rate was 99.29%, and lignin removal rate was 57.77%. The lignin obtained from the liquid phase had a β-O-4 bond retention rate of 84.11%, an increase in C-C bonds of 1.21%, and a lignin purity of 89.83%. Aromatic monomers were prepared by direct liquid-phase depolymerization at 100℃ with oxygen (1 MPa O2) for 2 hours. The yield of aromatic monomers was 30.84% ​​without the addition of an external catalyst. The main aromatic monomers were benzaldehyde (14.13%), phenol (6.26%), benzyl alcohol (1.03%), guaiacol (1.09%), 4-hydroxybenzaldehyde (2.24%), vanillin (1.08%), eugenol (2.73%), vanillin ethyl ketone (1.14%), and benzonitrile (1.14%). After precipitating unconverted lignin with hydrochloric acid, the aromatic monomers were extracted with ethyl acetate, and the DES was recovered and recycled after the addition of potassium hydroxide.

[0020] Example 2: Preferential dissolution of lignin and stepwise in-situ preparation of aromatic monomers

[0021] Tetramethylammonium hydroxide and imidazole were prepared at a molar ratio of 3:7 to form a clear, transparent, and homogeneous solution. A certain amount of water was then added to prepare a 30wt% DES solution. Coconut shells and DES were mixed at a solid-liquid ratio of 1:10, and the reaction was carried out at 50℃ for 3 hours. After the reaction, the mixture was separated into solid and liquid phases to obtain a cellulose-rich solid phase and a eutectic solvent liquid phase. The cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, while the eutectic solvent liquid phase was rich in lignin with high β-O-4 bond retention, high purity, and high reactivity. Under these conditions, the raw material composition changed as follows: cellulose retention rate was 91.21%, hemicellulose retention rate was 95.14%, and lignin removal rate was 67.37%. The lignin obtained from the liquid phase had a β-O-4 bond retention rate of 80.11%, an increase in C-C bonds of 1.83%, and a lignin purity of 90.12%. Aromatic monomers were prepared by direct liquid-phase depolymerization at 100℃ with oxygen (1 MPa O2) for 2 hours. The yield of aromatic monomers was 45.22% without the addition of an external catalyst. The main aromatic monomers were benzaldehyde (16.68%), phenol (15.25%), benzyl alcohol (1.63%), guaiacol (1.88%), 4-hydroxybenzaldehyde (1.78%), vanillin (2.51%), eugenol (1.65%), vanillin ethyl ketone (2.33%), and benzonitrile (1.51%). After precipitating unconverted lignin with hydrochloric acid, the aromatic monomers were extracted with ethyl acetate, and the DES was recovered and recycled after the addition of potassium hydroxide.

[0022] Example 3: Preferential dissolution of lignin and stepwise in-situ preparation of aromatic monomers

[0023] Tetramethylammonium hydroxide and imidazole were prepared at a molar ratio of 5:7 to form a clear, transparent, and homogeneous solution. A certain amount of water was then added to prepare a 40wt% DES solution. Coconut shells and DES were mixed at a solid-liquid ratio of 1:15, and the reaction was carried out at 90℃ for 1 hour. After the reaction, the mixture was separated into solid and liquid phases to obtain a cellulose-rich solid phase and a eutectic solvent liquid phase. The cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, while the eutectic solvent liquid phase was rich in lignin with high β-O-4 bond retention, high purity, and high reactivity. Under these conditions, the raw material composition changed as follows: cellulose retention rate was 81.21%, hemicellulose retention rate was 75.14%, and lignin removal rate was 78.86%. The lignin obtained from the liquid phase had a β-O-4 bond retention rate of 78.11%, an increase in C-C bonds of 2.01%, and a lignin purity of 91.33%. Aromatic monomers were prepared by direct liquid-phase depolymerization at 100℃ with oxygen (1 MPa O2) for 2 hours. The yield of aromatic monomers was 57.33% without the addition of an external catalyst. The main aromatic monomers were benzaldehyde (18.16%), phenol (20.28%), benzyl alcohol (1.87%), guaiacol (2.08%), 4-hydroxybenzaldehyde (3.56%), vanillin (2.51%), eugenol (3.65%), vanillin ethyl ketone (3.67%), and benzonitrile (1.55%). After precipitating unconverted lignin with hydrochloric acid, the aromatic monomers were extracted with ethyl acetate, and the DES was recovered and recycled after the addition of potassium hydroxide.

[0024] Example 4: One-pot in-situ preparation of aromatic monomers

[0025] Tetramethylammonium hydroxide and imidazole were prepared at a molar ratio of 1:7 to form a clear, transparent, and homogeneous solution. A certain amount of water was then added to prepare a 30wt% DES solution. Coconut shells and DES were mixed at a solid-liquid ratio of 1:50 and reacted at 60°C for 4 hours under an oxygen atmosphere of 1 MPa. After the reaction, the mixture was separated into solid and liquid phases. The solid phase was rich in holocellulose (cellulose and a large amount of hemicellulose). Hydrochloric acid was added to the liquid phase to precipitate unconverted lignin. Aromatic monomers were then extracted using an organic solvent. Under these conditions, the changes in raw material composition were as follows: cellulose retention rate was 78.61%, hemicellulose retention rate was 58.58%, and lignin removal rate was 74.72%. Without the addition of an exogenous catalyst, the yield of aromatic monomers was 49.90%, and the main aromatic monomers were benzaldehyde (3.04%), phenol (36.20%), benzyl alcohol (3.56%), guaiacol (0.71%), 4-hydroxybenzaldehyde (1.71%), vanillin (0.88%), eugenol (1.06%), vanillin ethyl ketone (1.86%), benzonitrile (0.87%), etc.

[0026] Example 5: One-pot in-situ preparation of aromatic monomers

[0027] Tetramethylammonium hydroxide and imidazole were prepared at a molar ratio of 3:7 to form a clear, transparent, and homogeneous solution. A certain amount of water was then added to prepare a 30wt% DES solution. Coconut shells and DES were mixed at a solid-liquid ratio of 1:50 and reacted at 80℃ for 3 hours under an oxygen atmosphere of 1 MPa. After the reaction, the mixture was separated into solid and liquid phases. The solid phase was rich in holocellulose (cellulose and a large amount of hemicellulose). Hydrochloric acid was added to the liquid phase to precipitate unconverted lignin. Aromatic monomers were then extracted using an organic solvent. Under these conditions, the changes in raw material composition were as follows: cellulose retention rate was 83.72%, hemicellulose retention rate was 54.53%, and lignin removal rate was 81.79%. Without the addition of an exogenous catalyst, the yield of aromatic monomers was 56.47%, and the main aromatic monomers were benzaldehyde (18.16%), phenol (20.28%), benzyl alcohol (1.87%), guaiacol (2.08%), 4-hydroxybenzaldehyde (3.56%), vanillin (2.51%), eugenol (3.65%), vanillin ethyl ketone (3.67%), benzonitrile (1.55%), etc.

[0028] Example 6: One-pot in-situ preparation of aromatic monomers

[0029] Tetramethylammonium hydroxide and imidazole were prepared at a molar ratio of 5:7 to form a clear, transparent, and homogeneous solution. A certain amount of water was then added to prepare a 30wt% DES solution. Coconut shells and DES were mixed at a solid-liquid ratio of 1:50 and reacted at 100℃ for 2 hours under an oxygen atmosphere of 1 MPa. After the reaction, the mixture was separated into solid and liquid phases. The solid phase was rich in holocellulose (cellulose and a large amount of hemicellulose). Hydrochloric acid was added to the liquid phase to precipitate unconverted lignin. Aromatic monomers were then extracted using an organic solvent. Under these conditions, the changes in raw material composition were as follows: cellulose retention rate was 86.19%, hemicellulose retention rate was 54.43%, and lignin removal rate was 90.77%. Without the addition of a catalyst, the yield of aromatic monomers was 58.88%, and the main aromatic monomers were benzaldehyde (18.16%), phenol (20.28%), benzyl alcohol (1.87%), guaiacol (2.08%), 4-hydroxybenzaldehyde (3.56%), vanillin (2.51%), eugenol (3.65%), vanillin ethyl ketone (3.67%), benzonitrile (1.55%) and other substances.

[0030] Example 7: Preferential dissolution of lignin and stepwise in-situ preparation of aromatic monomers

[0031] Tetramethylammonium hydroxide and urea were prepared at a molar ratio of 1:2 to form a clear, transparent, and homogeneous solution. A certain amount of water was then added to prepare a 30wt% DES solution. Coconut shells and DES were mixed at a solid-liquid ratio of 1:15, and the reaction was carried out at 50℃ for 3 hours. After the reaction, the solid and liquid phases were separated to obtain a cellulose-rich solid phase and a eutectic solvent liquid phase. The cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, while the eutectic solvent liquid phase was rich in lignin with high β-O-4 bond retention, high purity, and high reactivity. Under these conditions, the raw material composition changed as follows: cellulose retention rate was 88.21%, hemicellulose retention rate was 91.54%, and lignin removal rate was 65.22%. The lignin obtained from the liquid phase had a β-O-4 bond retention rate of 83.84%, a C-C bond increase of 2.01%, and a lignin purity of 89.83%. Aromatic monomers were prepared by direct liquid-phase depolymerization at 100℃ with oxygen (1MPa O2) for 2 hours. Without the addition of an external catalyst, the yield of aromatic monomers was only 27.33%. The aromatic monomers were mainly benzaldehyde (5.61%), phenol (7.28%), benzyl alcohol (0.67%), 4-hydroxybenzaldehyde (1.06%), vanillin (1.01%), eugenol (0.33%), vanillin ethyl ketone (0.32%), benzonitrile (11.05%) and other substances.

[0032] Examples 8-12:

[0033] Different types of metal oxide catalysts (CuO, Fe2O3, MnO2, MoO2, CeO2) were added to the liquid phase after the reaction in Example 2 for oxidative depolymerization, while keeping other conditions constant. The yields of aromatic monomers with and without external catalysts were compared, and the results are shown in Table 2. Table 2 shows that adding MnO2 in Example 10 and MoO2 in Example 11 helped improve the yield of aromatic monomers, while adding CuO, Fe2O3, and CeO2 was detrimental to improving the yield of aromatic monomers. Regarding the yield of benzaldehyde, adding CuO, Fe2O3, MnO2, MoO2, and CeO2 metal oxide catalysts helped improve the yield of benzaldehyde.

[0034] Comparative Example 1:

[0035] Referring to Example 2, the difference lies in the type of eutectic solvent solution DES, which is a mixture of ethylene glycol and choline chloride, and also contains Lewis bases.

[0036] The eutectic solvent solution for this comparative example was prepared as follows: Choline chloride and ethylene glycol were mixed at a molar ratio of 1:2 according to patent CN 113603899 B, and sodium carbonate (20% of the oven-dried coconut shell) was added to prepare a transparent and homogeneous solvent. Maintaining the reaction conditions of Example 2, the prepared DES was mixed with coconut shell at a mass ratio of 1:10. The reaction was carried out at 50°C for 3 hours. After the reaction, solid-liquid separation was performed. Under these conditions, the changes in raw material components were as follows: cellulose retention rate was 86.86%, hemicellulose retention rate was 86.77%, and lignin removal rate was only 17.95%, failing to achieve the effect of high lignin removal while simultaneously retaining high hemicellulose.

[0037] Comparative Example 2:

[0038] Referring to Example 2, the difference lies in the type of eutectic solvent solution DES, which is a mixture of choline chloride and lactic acid.

[0039] The eutectic solvent solution for this comparative example was prepared as follows: Choline chloride and lactic acid were mixed at a molar ratio of 1:9 according to patent CN 108660837 B, and stirred at 60°C to prepare a transparent and homogeneous solvent. Maintaining the reaction conditions of Example 2, the prepared DES was mixed with coconut shell at a mass ratio of 1:10, and the reaction was carried out at 50°C for 3 hours. After the reaction, solid-liquid separation was performed. Under these conditions, the changes in raw material components were as follows: cellulose retention rate was 90.52%, hemicellulose retention rate was 91.84%, and lignin removal rate was only 10.19%, failing to achieve the effect of high lignin removal while simultaneously retaining high hemicellulose.

[0040] Comparative Example 3

[0041] Referring to Example 2, the difference lies in the type of eutectic solvent solution DES, which is a mixture of ethylene glycol and choline chloride, and also contains the alkaline additive sodium sulfide or its hydrate.

[0042] The eutectic solvent solution for this comparative example was prepared as follows: Following patent CN 115160591 B, choline chloride and ethylene glycol were mixed at a molar ratio of 1:2, and 10% sodium sulfide nonahydrate was added. The mixture was stirred at 80°C to prepare a transparent and homogeneous solvent. Maintaining the reaction conditions of Example 2, the prepared DES was mixed with coconut shell at a mass ratio of 1:10. The reaction was carried out at 50°C for 3 hours. After the reaction, solid-liquid separation was performed. Under these conditions, the changes in raw material components were as follows: cellulose retention rate was 87.14%, hemicellulose retention rate was 87.11%, and lignin removal rate was only 13.68%, failing to achieve the effect of high lignin removal while simultaneously retaining high hemicellulose.

[0043] Comparative Example 4

[0044] Referring to Example 2, the difference lies in the type of eutectic solvent solution DES, which is a mixture of imidazole and choline chloride.

[0045] The eutectic solvent solution for this comparative example was prepared as follows: Following Example 1 of patent CN 114806616 A, choline chloride and imidazole were mixed at a molar ratio of 1:2 and stirred at 125°C to prepare a transparent and homogeneous solvent. Maintaining the reaction conditions of Example 2, the prepared DES was mixed with coconut shell at a mass ratio of 1:10, and the reaction was carried out at 50°C for 3 hours. After the reaction, solid-liquid separation was performed. Under these conditions, the changes in raw material components were as follows: cellulose retention rate was 82.32%, hemicellulose retention rate was 81.97%, and lignin removal rate was only 17.69%, failing to achieve the effect of high lignin removal while simultaneously retaining high hemicellulose.

[0046] Comparative Example 5

[0047] Referring to Example 3, the difference lies in the type of eutectic solvent solution DES, which is a mixture of imidazole and choline chloride.

[0048] The eutectic solvent solution for this comparative example was prepared as follows: Following Example 1 of patent CN 114806616A, choline chloride and imidazole were mixed at a molar ratio of 1:2 and stirred at 125°C to prepare a transparent and homogeneous solvent. Maintaining the reaction conditions of Example 3, the prepared DES was mixed with coconut shell at a mass ratio of 1:15, and the reaction was carried out at 90°C for 1 hour. After the reaction, solid-liquid separation was performed. Under these conditions, the changes in raw material components were as follows: cellulose retention rate was 77.08%, hemicellulose retention rate was 65.18%, and lignin removal rate was 26.40%, failing to achieve the effect of high lignin removal while simultaneously retaining high hemicellulose.

[0049] Result detection

[0050] The effects of changes in the composition of the lignocellulose raw materials in the above embodiments and comparative examples, as well as the structure and application properties of the resulting lignin, were tested. The tests included cellulose retention rate, hemicellulose retention rate, lignin removal rate, lignin β-O-4 bond and C-C bond content, and aromatic monomer yield. Specific testing methods are as follows:

[0051] Cellulose retention: Cellulose recovery was determined using a two-step acid hydrolysis method and high-performance liquid chromatography (NREL-2011 "Determination of structural carbohydrates and lignin in biomass").

[0052] Hemicellulose retention and lignin removal: The methods for determining hemicellulose retention are the same as those for cellulose retention.

[0053] The content of lignin β-O-4 bonds and C-C bonds was determined using nuclear magnetic resonance (NMR) with DMSO-d6 as the solvent.

[0054] Aromatic monomer yield: determined using GC / MS and GC, with helium as the carrier gas.

[0055] The results of component variation analysis and lignin structure analysis are shown in Table 1 below:

[0056] Table 1

[0057]

[0058]

[0059] The results of lignin catalytic oxidation are shown in Table 2.

[0060] Table 2

[0061]

[0062] As can be seen from Tables 1 and 2 above, the tetramethylammonium hydroxide-imidazolium or urea DES solution of the present invention achieves high lignin removal while retaining high hemicellulose. Even under mild conditions of 50°C or less, the lignin removal rate reaches about 60%, and the hemicellulose retention rate reaches over 95%, truly achieving preferential lignin dissolution. Simultaneously, the lignin obtained by the present invention exhibits high β-O-4 bond retention, few newly formed C-C bonds, and high reactivity. Without the addition of an exogenous catalyst, the in-situ oxidative depolymerization yield of aromatic monomers reaches a maximum of 58.88%, which increases to a maximum of 76.96% after the addition of an exogenous metal catalyst. Although tetramethylammonium hydroxide-urea can also achieve preferential lignin dissolution, it cannot achieve a high aromatic monomer yield. Furthermore, the tetramethylammonium hydroxide-imidazolium DES solution can also achieve direct in-situ depolymerization of lignocellulose raw materials, with an aromatic monomer yield reaching up to about 60% without the addition of an exogenous catalyst. This process eliminates the need for a lignin separation and extraction step.

[0063] Meanwhile, the raw materials were treated with DES as a comparative example, as shown in Table 1. The results showed that the DES solution prepared with tetramethylammonium hydroxide-imidazolium or urea and water, as described in the embodiments of this invention, could not achieve the effect of high lignin removal while maintaining high hemicellulose retention. In the comparative example, under low lignin removal rates, high hemicellulose retention was meaningless for improving the refining efficiency of lignocellulosic biomass.

[0064] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preferential lignin dissolution, characterized in that, The method includes the following steps: mixing lignocellulose biomass with a low-eutectic solvent aqueous solution at a mass ratio of 1:5–1:15, reacting at 30–90℃ for 1–7 h, and separating the solid and liquid phases to obtain a cellulose-rich solid phase and a low-eutectic solvent liquid phase. The cellulose-rich solid phase contains cellulose and hemicellulose, and the low-eutectic solvent liquid phase is rich in lignin with high β-O-4 bond retention, high purity, and high reactivity. The low-eutectic solvent aqueous solution is prepared as follows: mixing tetramethylammonium hydroxide and imidazole at a molar ratio of 1:1–1:7, and then adding deionized water to prepare a 5wt%–40wt% low-eutectic solvent aqueous solution.

2. The method according to claim 1, characterized in that, Lignocellulose biomass and eutectic solvent aqueous solution are mixed at a mass ratio of 1:10-1:15 and reacted at 50-90℃ for 3-5 hours. Solid-liquid separation is performed to obtain cellulose-rich solid phase and eutectic solvent liquid phase.

3. A method for preparing aromatic monomers through in-situ oxidative depolymerization, characterized in that, The method includes the following steps: oxidizing and depolymerizing a eutectic solvent phase rich in high-β-O-4 bond retention, high-purity, and highly reactive lignin obtained by the lignin preferential leaching method according to claim 1 or 2 in situ at 60℃–100℃ for 1–4 h to prepare aromatic monomers. The aromatic monomers are mainly benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, eugenol, vanillin ethyl ketone, and benzonitrile.

4. The method according to claim 3, characterized in that, The oxidative depolymerization process may be carried out by adding an exogenous metal oxide catalyst as needed, which may be selected from MnO2 or MoO2.

5. A method for preparing aromatic monomers through in-situ oxidative depolymerization, characterized in that, This method is a one-pot in-situ depolymerization of lignocellulose biomass, comprising the following steps: mixing lignocellulose biomass with a eutectic solvent aqueous solution at a solid-liquid ratio of 1:15–1:50, and performing an oxidative depolymerization reaction at 60℃–100℃ for 1–4 hours under an oxygen atmosphere of 1 MPa. After solid-liquid separation, hydrochloric acid is added to the liquid phase product after the reaction to precipitate unconverted lignin. Aromatic monomers are then extracted using an organic solvent. The aromatic monomers are mainly benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, eugenol, vanillin ethyl ketone, and benzonitrile. The eutectic solvent aqueous solution is prepared as follows: tetramethylammonium hydroxide and imidazole are mixed at a molar ratio of 1:1–1:7, and then deionized water is added to prepare a 5wt%–40wt% eutectic solvent aqueous solution.

6. The method according to any one of claims 1-5, characterized in that, Lignocellulose biomass includes one or more of the following: sawdust, fruit shells, and straw.

Images