Method for preparing aromatic monomer by preferential dissolution and in-situ oxidative depolymerization of lignin

Abstract

The invention discloses a method for preparing an aromatic monomer by preferential dissolution of lignin and in-situ oxidative depolymerization, which adopts a deep eutectic solvent solution (DES) prepared from tetramethylammonium hydroxide, imidazole and water to have the function of preferentially dissolving out lignin, and can achieve the effect of high hemicellulose retention while high lignin is removed. The aromatic monomer is efficiently prepared under mild reaction conditions and without an exogenous catalyst, loss of a large amount of carbohydrates is avoided, the purity of lignin and the conversion rate of the aromatic monomer are improved, lignin separation and purification steps are not needed, the process is simple, the solvent can be recycled, and industrial application is easy.

Description

Technical field:

[0001] The invention relates to the technical field of lignocellulose biomass refining, and in particular to a method for preparing aromatic monomers by preferentially dissolving lignin and in-situ oxidative depolymerization. Background technology:

[0002] Using lignocellulosic biomass for biorefining is an important way to cope with the energy crisis and promote sustainable development. Lignin, as one of the three major components of lignocellulose, is the most abundant aromatic polymer in nature. It has rich oxygen-containing functional groups and high calorific value, and has great potential in the production of high-density fuels, functional materials and chemicals. Deep eutectic solvent (DES) is an ionic liquid-like solvent synthesized from hydrogen bond donors and hydrogen bond acceptors. It can selectively dissolve lignin, protect and swell cellulose components. Due to its low toxicity, low cost and easy synthesis, it is widely used in the fractionation of lignocellulose. However, in the process of existing deep eutectic solvent DES treatment, it is easy to cause the lignin ether bond to break, resulting in lignin structure condensation, which increases the difficulty of subsequent depolymerization. At the same time, in the process of DES extraction of lignin, the liquid phase containing lignin will be accompanied by a large amount of hemicellulose dissolving. The hemicellulose-derived sugar dissolved in the liquid phase solvent is difficult to be separated and utilized, which not only affects the conversion and utilization efficiency of lignin, but also causes sugar loss. For example, a Chinese patent (CN108660837 B) discloses a method for separating the three components of cellulose, hemicellulose and lignin in a plant fiber raw material. The plant fiber raw material is subjected to pressurization or normal pressure heating and cooking treatment by using a lactic acid / choline chloride low eutectic solvent, the maximum temperature is 120-150°C, the insulation time is 1-4h, lignin and hemicellulose are selectively dissolved, and the remaining insoluble matter is washed to obtain cellulose; the low eutectic solvent solution containing lignin and hemicellulose is added with water to precipitate lignin, and lignin is obtained by washing and separation; the remaining low eutectic solvent aqueous solution containing hemicellulose is separated by nanofiltration to obtain hemicellulose. However, during the treatment with the lactic acid / choline chloride low eutectic solvent, hemicellulose and lignin will be dissolved together, and they will continue to be separated later, which not only affects the conversion and utilization efficiency of lignin, but also causes sugar loss. While the lignin yield is 64.9%, nearly 50% of the hemicellulose is removed, and the hemicellulose retention rate is low. For example, CN 113603899 B discloses a method for pretreating lignocellulose using a Lewis base-assisted neutral low eutectic solvent. The air-dried and crushed plant fiber raw material is placed in a prepared glycerol / choline chloride low eutectic solvent, and then a Lewis base is added. Up to 88.1% of lignin can be removed, but hemicellulose is also removed at the same time, with a removal rate of up to 80.4%. In the treatment results of alkaline DES reported in CN 115160591B, an alkaline low eutectic solvent of choline chloride, ethylene glycol and alkaline auxiliary agent sodium sulfide hydrate is used to react at 100-150° C. for 10-60 minutes, and the retention rate of hemicellulose is still less than 60%. In addition, although the low eutectic solvent (DES) in the above system can be recycled for many times, the gradual accumulation of hemicellulose-derived sugars will increase the viscosity of DES, thereby weakening its extraction effect on lignin. Therefore, it is necessary to find a new low eutectic solvent to reduce the loss of raw material hemicellulose components while ensuring a high lignin removal rate. Summary of the invention:

[0003] The purpose of the present invention is to provide a method for preferentially dissolving lignin and in-situ oxidative depolymerization to prepare aromatic monomers, which solves the problem that the low eutectic solvent in the prior art is difficult to simultaneously ensure a high lignin removal rate and a hemicellulose retention rate, and avoids the loss of hemicellulose-derived sugars caused by the dissolution of hemicellulose in the liquid phase of the low eutectic solvent and the problem of affecting the downstream conversion and utilization of lignin.

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

[0005] A method for preferentially dissolving lignin, the method comprising the following steps: mixing lignocellulosic biomass with a low eutectic solvent solution at a mass ratio of 1:5-1:15, preferably 1:10-1:15, reacting at 30-90°C, preferably 50-90°C, for 1-7h, preferably 3-5h, and performing solid-liquid separation to obtain a holographic cellulose-rich solid phase and a low eutectic solvent liquid phase, wherein the holographic cellulose-rich solid phase contains cellulose and a large amount of 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 preparation method of the low eutectic solvent solution is 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% low eutectic solvent aqueous solution.

[0006] A method for preparing aromatic monomers by in-situ oxidative depolymerization, the method comprising the following steps: subjecting a low eutectic solvent liquid phase obtained by the above-mentioned lignin preferential dissolution method and rich in high β-O-4 bond retention, high purity and high reactivity lignin to in-situ oxidative depolymerization for 1-4 hours under 1MPa oxygen atmosphere and 60°C-100°C conditions to prepare aromatic monomers, wherein the aromatic monomers are mainly benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, syringaldehyde, vanillyl acetone and benzonitrile.

[0007] A method for preparing aromatic monomers by in-situ oxidative depolymerization, which is a one-pot method for in-situ depolymerization of lignocellulosic biomass, comprising the following steps: mixing lignocellulosic biomass with a low eutectic solvent aqueous solution at a solid-liquid ratio of 1:15-1:50, performing oxidative depolymerization reaction at 60°C-100°C for 1-4h in an oxygen atmosphere of 1MPa, separating the solid and the liquid, adding hydrochloric acid to the liquid phase product after the reaction to precipitate unconverted lignin, and extracting aromatic monomers with an organic solvent, wherein the aromatic monomers are mainly benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, syringaldehyde, vanillyl ethyl ketone, and benzonitrile; the low 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% low eutectic solvent aqueous solution.

[0008] Preferably, an exogenous metal oxide catalyst is added in the above-mentioned oxidative depolymerization process as required, and the exogenous metal oxide is selected from one of MnO2 and MoO2.

[0009] Lignocellulosic biomass includes one or more of sawdust, fruit shells, and straw.

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

[0011] (1) The low eutectic solvent solution (DES) prepared by tetramethylammonium hydroxide, imidazole or urea and water of the present invention has the function of preferentially dissolving lignin, and can efficiently dissolve lignin while retaining cellulose and hemicellulose in the solid phase, thereby ensuring a high recovery rate of cellulose and hemicellulose, avoiding the loss of a large amount of carbohydrates, and improving the purity of lignin.

[0012] (2) The low 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, thereby reducing the consumption of chemical agents and energy.

[0013] (3) The low eutectic solvent solution prepared by tetramethylammonium hydroxide, imidazole and water of the present invention has a protective function for lignin, has a high β-O-4 bond content, has no significant increase in CC bond content, has high reaction activity, and can directly depolymerize lignocellulose raw materials in situ under mild reaction conditions and without exogenous catalysts. The yield of aromatic monomers produced by lignin depolymerization is as high as 58.88%, and no lignin separation and extraction step is required.

[0014] (4) The low 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, achieves zero waste discharge, and complies with the concept of environmental protection.

[0015] In summary, the low eutectic solvent solution prepared by tetramethylammonium hydroxide, imidazole and water of the present invention preferentially dissolves lignin, and efficiently prepares aromatic monomers under mild reaction conditions and without exogenous catalysts. At the same time, the holocellulose containing cellulose and a large amount of hemicellulose is highly retained, and a high recovery rate of cellulose and hemicellulose can be ensured, and the loss of a large amount of carbohydrates is avoided, and the purity of lignin is improved. No lignin separation and purification steps are required, the process is simple, the solvent can be recycled, and it is easy to be industrially applied. Description of the drawings:

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

[0017] The following is a further description of the present invention, rather than a limitation of the present invention.

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

[0019] Tetramethylammonium hydroxide and imidazole were prepared in a molar ratio of 1:7 to form a clear, transparent and uniform solution, and then a certain amount of water was added to prepare a DES solution with a concentration of 10wt%. Coconut shell and DES solution were mixed in a solid-liquid ratio of 1:5, and the reaction conditions were 30℃ for 7h. After the reaction, the mixture was separated into solid and liquid to obtain a holographic cellulose-rich solid phase and a low eutectic solvent liquid phase. The holographic cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, and the low eutectic solvent liquid phase was rich in high β-O-4 bond retention, high purity and high reactivity lignin. Under this condition, the changes in the raw material components were as follows: cellulose retention rate was 93.13%, hemicellulose retention rate was 99.29%, and lignin removal rate was 57.77%. The β-O-4 bond retention rate of the lignin obtained in the liquid phase was 84.11%, the CC bond increase was 1.21%, and the lignin purity was 89.83%. Liquid phase direct oxygenation (1MPa O2) at 100℃ for 2h to catalyze depolymerization to prepare aromatic monomers. The yield of aromatic monomers is 30.84% ​​without adding exogenous catalysts. The types of aromatic monomers are mainly benzaldehyde (14.13%), phenol (6.26%), benzyl alcohol (1.03), guaiacol (1.09), 4-hydroxybenzaldehyde (2.24%), vanillin (1.08%), syringaldehyde (2.73%), vanillyl acetone (1.14%), benzonitrile (1.14%), etc. After adding hydrochloric acid to precipitate the unconverted lignin, the aromatic monomers are extracted with ethyl acetate, and DES is recovered and recycled after adding potassium hydroxide.

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

[0021] Tetramethylammonium hydroxide and imidazole were prepared in a molar ratio of 3:7 to form a clear, transparent and uniform solution, and then a certain amount of water was added to prepare a DES solution with a concentration of 30wt%. Coconut shell and DES were mixed in a solid-liquid ratio of 1:10, and the reaction conditions were 50℃ for 3h. After the reaction, the mixture was separated into solid and liquid to obtain a holographic cellulose-rich solid phase and a low eutectic solvent liquid phase. The holographic cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, and the low eutectic solvent liquid phase was rich in high β-O-4 bond retention, high purity and high reactivity lignin. Under this condition, the changes in the raw material components were as follows: the cellulose retention rate was 91.21%, the hemicellulose retention rate was 95.14%, and the lignin removal rate was 67.37%. The β-O-4 bond retention rate of the lignin obtained in the liquid phase was 80.11%, the CC bond increase was 1.83%, and the lignin purity was 90.12%. Liquid phase direct oxygenation (1MPa O2) at 100℃ for 2h to catalyze depolymerization to prepare aromatic monomers. The yield of aromatic monomers is 45.22% without adding exogenous catalysts. The types of aromatic monomers are mainly benzaldehyde (16.68%), phenol (15.25%), benzyl alcohol (1.63), guaiacol (1.88), 4-hydroxybenzaldehyde (1.78%), vanillin (2.51%), syringaldehyde (1.65%), vanillyl acetone (2.33%), benzonitrile (1.51%), etc. After adding hydrochloric acid to precipitate the unconverted lignin, the aromatic monomers are extracted with ethyl acetate, and DES is recovered and recycled after adding potassium hydroxide.

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

[0023] Tetramethylammonium hydroxide and imidazole were prepared in a molar ratio of 5:7 to form a clear, transparent and uniform solution, and then a certain amount of water was added to prepare a DES solution with a concentration of 40wt%. Coconut shell and DES were mixed in a solid-liquid ratio of 1:15, and the reaction conditions were 90℃ for 1h. After the reaction, the mixture was separated into solid and liquid to obtain a holographic cellulose-rich solid phase and a low eutectic solvent liquid phase. The holographic cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, and the low eutectic solvent liquid phase was rich in high β-O-4 bond retention, high purity and high reactivity lignin. Under this condition, the changes in the raw material components were as follows: the cellulose retention rate was 81.21%, the hemicellulose retention rate was 75.14%, and the lignin removal rate was 78.86%. The β-O-4 bond retention rate of the lignin obtained in the liquid phase was 78.11%, the CC bond increase was 2.01%, and the lignin purity was 91.33%. Liquid phase direct oxygenation (1MPa O2) at 100℃ for 2h to catalyze depolymerization to prepare aromatic monomers. The yield of aromatic monomers is 57.33% without adding exogenous catalysts. The types of aromatic monomers are mainly benzaldehyde (18.16%), phenol (20.28%), benzyl alcohol (1.87), guaiacol (2.08), 4-hydroxybenzaldehyde (3.56%), vanillin (2.51%), syringaldehyde (3.65%), vanillyl acetone (3.67%), benzonitrile (1.55%), etc. After adding hydrochloric acid to precipitate the unconverted lignin, the aromatic monomers are extracted with ethyl acetate, and DES is recovered and recycled after adding potassium hydroxide.

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

[0025] Tetramethylammonium hydroxide and imidazole were prepared in a molar ratio of 1:7 to form a clear, transparent and uniform solution, and then a certain amount of water was added to prepare a DES solution with a concentration of 30wt%. Coconut shell and DES were mixed in a solid-liquid ratio of 1:50, and reacted at 60°C for 4h under an oxygen atmosphere of 1MPa. After the reaction, the mixture was separated into solid and liquid, and the solid phase was rich in holocellulose (cellulose and a large amount of hemicellulose). After hydrochloric acid was added to the liquid phase to precipitate the unconverted lignin, the aromatic monomers were extracted using an organic solvent. Under this condition, the changes in the raw material components were as follows: the cellulose retention rate was 78.61%, the hemicellulose retention rate was 58.58%, and the lignin removal rate was 74.72%. Without adding exogenous catalysts, the yield of aromatic monomers is 49.90%. The types of aromatic monomers are mainly benzaldehyde (3.04%), phenol (36.20%), benzyl alcohol (3.56%), guaiacol (0.71%), 4-hydroxybenzaldehyde (1.71%), vanillin (0.88%), syringaldehyde (1.06%), vanillyl ethyl ketone (1.86%), benzonitrile (0.87%) and other substances.

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

[0027] Tetramethylammonium hydroxide and imidazole were prepared in a molar ratio of 3:7 to form a clear, transparent and uniform solution, and then a certain amount of water was added to prepare a DES solution with a concentration of 30wt%. Coconut shell and DES were mixed in a solid-liquid ratio of 1:50, and reacted at 80°C in an oxygen atmosphere of 1MPa for 3h. After the reaction, the mixture was separated from the solid and liquid, and the solid phase was rich in holocellulose (cellulose and a large amount of hemicellulose). After hydrochloric acid was added to the liquid phase to precipitate the unconverted lignin, the aromatic monomers were extracted using an organic solvent. The changes in the raw material components under this condition were: the cellulose retention rate was 83.72%, the hemicellulose retention rate was 54.53%, and the lignin removal rate was 81.79%. Without adding exogenous catalysts, the yield of aromatic monomers is 56.47%. The types of aromatic monomers are mainly benzaldehyde (18.16%), phenol (20.28%), benzyl alcohol (1.87%), guaiacol (2.08), 4-hydroxybenzaldehyde (3.56%), vanillin (2.51%), syringaldehyde (3.65%), vanillyl ethyl ketone (3.67%), benzonitrile (1.55%) and other substances.

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

[0029] Tetramethylammonium hydroxide and imidazole are prepared in a molar ratio of 5:7 to form a clear, transparent and uniform solution, and then a certain amount of water is added to prepare a DES solution with a concentration of 30wt%. Coconut shell and DES are mixed in a solid-liquid ratio of 1:50, and reacted at 100°C for 2h in an oxygen atmosphere of 1MPa. After the reaction, the mixture is separated into solid and liquid, and the solid phase is rich in holocellulose (cellulose and a large amount of hemicellulose). After hydrochloric acid is added to the liquid phase to precipitate the unconverted lignin, the aromatic monomers are extracted using an organic solvent. Under this condition, the changes in the raw material components are as follows: the cellulose retention rate is 86.19%, the hemicellulose retention rate is 54.43%, and the lignin removal rate is 90.77%. Without adding a catalyst, the yield of aromatic monomers is 58.88%. The types of aromatic monomers are mainly benzaldehyde (18.16%), phenol (20.28%), benzyl alcohol (1.87%), guaiacol (2.08), 4-hydroxybenzaldehyde (3.56%), vanillin (2.51%), syringaldehyde (3.65%), vanillyl ethyl ketone (3.67%), benzonitrile (1.55%) and other substances.

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

[0031] Tetramethylammonium hydroxide and urea were prepared in a molar ratio of 1:2 to form a clear, transparent and uniform solution, and then a certain amount of water was added to prepare a DES solution with a concentration of 30wt%. Coconut shell and DES were mixed in a solid-liquid ratio of 1:15, and the reaction conditions were 50℃ for 3h. After the reaction, the solid and liquid were separated to obtain a holographic cellulose-rich solid phase and a low eutectic solvent liquid phase. The holographic cellulose-rich solid phase contained cellulose and a large amount of hemicellulose, and the low eutectic solvent liquid phase was rich in high β-O-4 bond retention, high purity and high reactivity lignin. Under this condition, the changes in the raw material components were as follows: the cellulose retention rate was 88.21%, the hemicellulose retention rate was 91.54%, and the lignin removal rate was 65.22%. The β-O-4 bond retention rate of the lignin obtained in the liquid phase was 83.84%, the CC bond increase was 2.01%, and the lignin purity was 89.83%. Aromatic monomers were prepared by direct liquid phase oxygenation (1MPa O2) catalytic depolymerization at 100°C for 2h. The yield of aromatic monomers was only 27.33% without adding exogenous catalysts. The types of aromatic monomers were mainly benzaldehyde (5.61%), phenol (7.28%), benzyl alcohol (0.67%), 4-hydroxybenzaldehyde (1.06%), vanillin (1.01%), syringaldehyde (0.33%), vanillyl ethyl ketone (0.32%), benzonitrile (11.05%) and other substances.

[0032] Embodiment 8-12:

[0033] By adding different types of metal oxide catalysts (CuO, Fe2O3, MnO2, MoO2, CeO2) to the liquid phase after the reaction in Example 2 for oxidative depolymerization, other conditions remain the same, and the aromatic monomer yields with and without exogenous catalyst addition are compared. The results are shown in Table 2. As can be seen from Table 2, the addition of MnO2 in Example 10 and the addition of MoO2 in Example 11 are helpful to improve the aromatic monomer yield, while the addition of CuO, Fe2O3, and CeO2 is not conducive to improving the aromatic monomer yield. From the yield of benzaldehyde, the addition of CuO, Fe2O3, MnO2, MoO2, and CeO2 metal oxide catalysts is helpful to improve the yield of benzaldehyde.

[0034] Comparative Example 1:

[0035] Refer to Example 2, the difference is that the deep eutectic solvent solution DES is of a different type, which is a mixture of ethylene glycol and choline chloride, and a Lewis base is also added.

[0036] The low eutectic solvent solution of this comparative example is prepared as follows: according to patent CN 113603899 B, choline chloride and ethylene glycol are mixed at a molar ratio of 1:2, and sodium carbonate (20% of absolute dry coconut shell) is added to prepare a transparent and uniform solvent. Maintaining the reaction conditions of Example 2, the prepared DES and coconut shell are mixed at a mass ratio of 1:10, the reaction conditions are 50°C for 3h, and the solid-liquid separation is performed after the reaction. Under this condition, the changes in the raw material components are as follows: the cellulose retention rate is 86.86%, the hemicellulose retention rate is 86.77%, and the lignin removal rate is only 17.95%, which does not achieve the effect of high lignin removal and high hemicellulose retention.

[0037] Comparative Example 2:

[0038] Refer to Example 2, the difference is that the deep eutectic solvent solution DES is of a different type, which is a mixture of choline chloride and lactic acid.

[0039] The low eutectic solvent solution of this comparative example was prepared as follows: according to patent CN 108660837 B, choline chloride and lactic acid were mixed at a molar ratio of 1:9, and stirred at 60°C to prepare a transparent and uniform solvent. Maintaining the reaction conditions of Example 2, the prepared DES and coconut shell were mixed at a mass ratio of 1:10, and the reaction conditions were 50°C for 3h. After the reaction, the solid-liquid separation was performed. Under this condition, the changes in the raw material components were as follows: the cellulose retention rate was 90.52%, the hemicellulose retention rate was 91.84%, and the lignin removal rate was only 10.19%, which did not achieve the effect of high lignin removal and high hemicellulose retention.

[0040] Comparative Example 3

[0041] Refer to Example 2, the difference is that the low eutectic solvent solution DES is of a different type, which is a mixture of ethylene glycol and choline chloride, and an alkaline auxiliary agent sodium sulfide or its hydrate is also added.

[0042] The low eutectic solvent solution of this comparative example is prepared as follows: according to patent CN 115160591 B, choline chloride and ethylene glycol are mixed at a molar ratio of 1:2, 10% sodium sulfide nonahydrate is added, and stirred at 80°C to prepare a transparent and uniform solvent. Maintaining the reaction conditions of Example 2, the prepared DES and coconut shell are mixed at a mass ratio of 1:10, the reaction conditions are 50°C for 3h, and the solid-liquid separation is performed after the reaction. Under this condition, the changes in the raw material components are as follows: the cellulose retention rate is 87.14%, the hemicellulose retention rate is 87.11%, and the lignin removal rate is only 13.68%, which does not achieve the effect of high lignin removal and high hemicellulose retention.

[0043] Comparative Example 4

[0044] Refer to Example 2, the difference is that the deep eutectic solvent solution DES is of a different type, which is a mixture of imidazole and choline chloride.

[0045] The low eutectic solvent solution of this comparative example is prepared as follows: according to Example 1 of patent CN 114806616 A, choline chloride and imidazole are mixed in a molar ratio of 1:2, stirred at 125°C, and prepared into a transparent and uniform solvent. Maintaining the reaction conditions of Example 2, the prepared DES and coconut shell are mixed in a mass ratio of 1:10, the reaction conditions are 50°C for 3h, and the solid-liquid separation is performed after the reaction. Under this condition, the changes in the raw material components are as follows: the cellulose retention rate is 82.32%, the hemicellulose retention rate is 81.97%, and the lignin removal rate is only 17.69%, which does not achieve the effect of high lignin removal and high hemicellulose retention.

[0046] Comparative Example 5

[0047] Refer to Example 3, the difference is that the deep eutectic solvent solution DES is of a different type, which is a mixture of imidazole and choline chloride.

[0048] The low eutectic solvent solution of this comparative example is prepared as follows: according to Example 1 of patent CN 114806616A, choline chloride and imidazole are mixed in a molar ratio of 1:2, stirred at 125°C, and prepared into a transparent and uniform solvent. Maintaining the reaction conditions of Example 3, the prepared DES and coconut shell are mixed in a mass ratio of 1:15, the reaction conditions are 90°C for 1h, and the solid-liquid separation is performed after the reaction. Under this condition, the changes in the raw material components are: cellulose retention rate is 77.08%, hemicellulose retention rate is 65.18%, and lignin removal rate is 26.40%, which does not achieve the effect of high lignin removal and high hemicellulose retention.

[0049] Results

[0050] The effects of the changes in the components of the lignocellulose raw materials in the above examples and comparative examples and the structure and application performance of the obtained lignin were tested, including the cellulose retention rate, hemicellulose retention rate, lignin removal rate, lignin β-O-4 bond and CC bond content, and aromatic monomer yield. The specific testing methods are as follows:

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

[0052] Hemicellulose retention rate and lignin removal rate: The determination method is the same as that of cellulose retention rate.

[0053] The content of β-O-4 bond and CC bond of lignin was measured by nuclear magnetic resonance spectroscopy, and the solvent used was DMSO-d6.

[0054] Aromatic monomer yield: GC / MS and GC were used for determination, and helium was used as carrier gas.

[0055] The results of component change analysis and lignin structure analysis are described 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-imidazole or urea DES solution of the present invention achieves the effect of high hemicellulose retention while high lignin removal, and even under mild conditions of less than or equal to 50°C, the lignin removal rate reaches about 60%, and the hemicellulose retention rate reaches more than 95%, truly realizing the preferential dissolution of lignin. At the same time, the lignin obtained by the present invention has a high retention rate of β-O-4 bonds, few newly added CC bonds, and high reaction activity. Without the addition of exogenous catalysts, the in-situ oxidative depolymerization aromatic monomer yield is as high as 58.88%, and after adding exogenous metal catalysts, it is increased to 76.96%. Although tetramethylammonium hydroxide-urea can also achieve preferential dissolution of lignin, it cannot achieve a higher aromatic monomer yield. And the tetramethylammonium hydroxide-imidazole DES solution can also achieve direct in-situ depolymerization of lignocellulose raw materials. Without the addition of exogenous catalysts, the aromatic monomer yield can reach up to about 60%, and the process does not require lignin separation and extraction steps.

[0063] At the same time, the raw material was treated with DES disclosed in the existing literature as a comparative example. The results are shown in Table 1. The DES solution composed of tetramethylammonium hydroxide-imidazole or urea and water in the embodiments of the present invention cannot achieve the effect of high lignin removal and high hemicellulose retention. In the comparative example, under the condition of low lignin removal rate, the high retention of hemicellulose is 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 embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for preferentially dissolving lignin, characterized in that: The method comprises the following steps: mixing lignocellulosic biomass with a low eutectic solvent solution in a mass ratio of 1:5-1:15, reacting at 30-90°C for 1-7h, and separating the solid and liquid to obtain a holographic cellulose-rich solid phase and a low eutectic solvent liquid phase, wherein the holographic 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 preparation method of the low eutectic solvent solution is as follows: mixing tetramethylammonium hydroxide and imidazole in 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 The lignocellulosic biomass is mixed with a low eutectic solvent solution in a mass ratio of 1:10-1:15, reacted at 50-90° C. for 3-5 hours, and solid-liquid separation is performed to obtain a cellulose-rich solid phase and a low eutectic solvent liquid phase.

3. A method for preparing aromatic monomers by in-situ oxidative depolymerization, characterized in that: The method comprises the following steps: subjecting a low eutectic solvent liquid phase rich in high β-O-4 bond retention, high purity and high reactivity lignin obtained by the method for preferentially dissolving lignin according to claim 1 or 2 to in-situ oxidative depolymerization for 1-4 hours under 1MPa oxygen atmosphere and 60°C-100°C conditions to prepare aromatic monomers, wherein the aromatic monomers are mainly benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, syringaldehyde, vanillyl acetone and benzonitrile.

4. The method according to claim 3, characterized in that In the oxidative depolymerization process, an exogenous metal oxide catalyst is added as needed, and the exogenous metal oxide is selected from one of MnO2 and MoO2.

5. A method for preparing aromatic monomers by in-situ oxidative depolymerization, characterized in that: The method is a one-pot method for in-situ depolymerization of lignocellulosic biomass, comprising the following steps: mixing lignocellulosic biomass with a low eutectic solvent aqueous solution at a solid-liquid ratio of 1:15-1:50, performing an oxidative depolymerization reaction at 60°C-100°C for 1-4 hours in a 1MPa oxygen atmosphere, separating the solid and the liquid, adding hydrochloric acid to the liquid phase product after the reaction to precipitate unconverted lignin, and then extracting aromatic monomers using an organic solvent, wherein the aromatic monomers are mainly benzaldehyde and phenol, and also include benzyl alcohol, guaiacol, 4-hydroxybenzaldehyde, vanillin, syringaldehyde, vanillyl acetone, and benzonitrile; the low eutectic solvent 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% low eutectic solvent aqueous solution.

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

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