Preparation method of oxidized lignin based on lignin priority strategy

Abstract

The invention discloses a preparation method of oxidized lignin based on a lignin priority strategy. The method comprises the following steps: by taking biomass as a raw material, imidazole type multi-acid site ionic liquid as a catalyst and a mixed solution of an organic solvent and water as a solvent, heating to 80-140 DEG C under the condition that the oxygen pressure is 0.1-1.5 MPa, and reacting for 1-5 hours; and separating the mixture obtained by the reaction to obtain an oxidized lignin solid. According to the method, the two processes of biomass grading and lignin oxidation modification are combined, so that repeated solid-liquid separation, lignin precipitation and other operations are avoided, the operation process is simplified, and the operation cost is saved.

Description

Technical Field

[0001] The invention relates to the technical field of lignocellulose pretreatment, and in particular to a preparation method of lignin based on lignin preferential oxidation. Background Art

[0002] At present, biomass is mainly used for pulping and papermaking and fermentation to produce bio-based chemicals such as bioethanol, sugar and lactic acid. In the biorefining process, due to its stubborn structure, it needs to be pretreated to remove the protective and preventive lignin components, aiming to improve the quality of paper or obtain the maximum sugar yield during the fermentation process. In addition, in addition to being used as a surfactant, water reducer and dispersant, lignin, as one of the few renewable aromatic polymers, can also replace the petroleum-based route to prepare aromatic compounds. However, biomass pretreatment requires the use of strong acids and alkalis, which causes the breakage of natural ether bonds in the lignin molecular structure and the irreversible formation of highly stable CC bonds, which leads to the characteristics of complex lignin structure, poor solubility and high chemical bond dissociation energy. Therefore, the development of a new biomass delignification process, that is, the lignin priority strategy, without affecting the subsequent utilization of cellulose, has received widespread attention.

[0003] The lignin priority strategy is an important way to achieve the utilization of all components of biomass. This strategy refers to the prevention of recombination reactions through catalytic or group protection chemical means during the biomass delignification process. It is not equivalent to the high value of lignin as a single component, but refers to the sustainable utilization of all components of biomass. At present, the lignin priority strategy mainly includes: ① coupling reduction reactions in the lignin organic solvent extraction process to transform and consume carbon cations or free radical intermediates to prevent the irreversible formation of CC bonds; ② using diols to capture and transform unstable aldehyde intermediates in the acid / base catalyzed delignification process, or cyclizing C through aldehyde reagents. α -OH and Cγ-OH to avoid the production of benzyl cations; ③ Utilize the good solubility of ionic liquids in biomass and the controllable chemical and physical properties to achieve the extraction and separation of lignin. Compared with the first two pretreatment methods, a major and widely emphasized advantage of the ionic liquid pretreatment process is that it can decrystallize the cellulose part of lignocellulose while destroying the lignin and hemicellulose network.

[0004] For example, Rodríguez et al. (H. Rodríguez, M. Francisco, M. Rahman, N. Sun, RD Rogers. Biphasic liquid mixtures of ionic liquids and polyethylene glycols. Physical Chemistry Chemical Physics, 2009, 11 (46), 10916-10922.) used polyethylene glycol and ionic liquid binary solvents (PEG-2000 and [C 2 mim]Cl) to separate a mixture of cellulose and lignin. It was found that the ionic liquid phase included cellulose and lignin, while the PEG phase only contained lignin; Xu et al. (JKXu, L.Dai, Y.Gui, L.Yuan, CTZhang, Y.Lei. Synergistic benefits from a lignin-first biorefinery of poplar via coupling acesulfamate ionic liquid followed by mildalkaline extraction. Bioresource Technology, 2020, 303: 122.) first used a food additive-type ionic liquid to dissolve the biomass, and then used a compound alkali method to separate the lignin. The results showed that the lignin removal rate reached 70.7%, and the subsequent cellulose bioconversion rate increased from 15.4% to 90.5%. However, this method includes two processes: dissolving biomass with ionic liquid and separating lignin with alkali. The process is cumbersome, and the ionic liquid is used as a solvent in a relatively large amount.

[0005] As we all know, lignin is a rigid, hyperbranched biomacromolecule composed of three different types of phenylpropane units, which contains a large number of -OH, -COOH, CH 3 O- and other active functional groups. Therefore, lignin can be combined and grafted with other materials to prepare new materials through electrostatic interactions, hydrogen bonding, and covalent interactions. In addition, the oxidative modification of lignin can introduce more active -OH groups and more negative charges, thereby improving its reactivity and dispersibility. For example, Kalliola et al. (A. Kalliola, T. Vehmas, T. T. Tamminen. Alkali-O 2Oxidized lignin-abio-based concrete plasticizer. Industrial Crops and Products, 2015, 74: 150-157) uses base-catalyzed oxidation to modify lignin. Compared with sodium lignin sulfonate, the oxidized lignin shows better plasticizing properties for cement paste, mortar and concrete. Mattinen et al. (ML Mattinena, JJ Valle-Delgado, T. Leskinen, T. Anttila, G. Riviere, M. Sipponen, A. Paananen, K. Lintinen, M. Kostiainen, M. Enzymatically and chemically oxidized ligninnanoparticles for biomaterial applications. Enzyme and Microbial Technology, 2018, 111: 48-56.) Using hydrogen peroxide as an oxidant, colloidal lignin nanoparticles are prepared by base-catalyzed oxidation of lignin under ultrasonic conditions. The material exhibits excellent stability in tetrahydrofuran organic solvent, making it have great application potential in the fields of pharmaceuticals, food and cosmetics. However, both methods use industrial lignin as raw material and prepare oxidized lignin by base catalysis, which is equivalent to extracting lignin and then oxidizing it. Not only is the process complicated, but it also significantly increases the preparation cost. Summary of the invention

[0006] The present invention aims to provide a one-pot method for preparing high-quality oxidized lignin based on a lignin priority strategy, which has a simple process, can avoid repeated solid-liquid separation and lignin precipitation operations, and has a low preparation cost.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for preparing oxidized lignin based on a lignin priority strategy, using biomass as a raw material, imidazole-type multi-acid site ionic liquid as a catalyst, and a mixed solution of an organic solvent and water as a solvent, heating to 80-140°C under an oxygen pressure of 0.1-1.5 MPa for reaction for 1-5 hours; the reaction mixture is separated to obtain an oxidized lignin solid.

[0009] To further achieve the purpose of the present invention, preferably, the imidazole type polyacid site ionic liquid is [HIm][HSO 4 ]、[C 3 H 6 SO 3HIm][H 2 PO 4 ]、[C 3 H 6 SO 3 HIm][Cl]、[C 3 H 6 SO 3 HIm][HSO 4 ]、[C 3 H 6 SO 3 HIm][FeCl 4 ]、[C 3 H 6 SO 3 HIm][ZnCl 3 ]、[C 3 H 6 SO 3 HIm][CoCl 3 ]、[C 3 H 6 SO 3 HIm][CuCl 3 ]、[C 3 H 6 SO 3 HIm][MnCl 3 ] or [C 3 H 6 SO 3 HIm][AlCl 4 ] or a combination of both.

[0010] Preferably, the [C 3 H 6 SO 3 HIm][H 2 PO 4 ]、[C 3 H 6 SO 3 HIm][Cl] or [C 3 H 6 SO 3 HIm][HSO 4 ] is prepared by the following method: imidazole is dissolved in acetonitrile or acetone solvent, an equimolar amount of 1,3-propyl sultone is added, and reflux reaction is carried out; the resulting mixed solution is filtered, washed, and dried to obtain a white solid intermediate; then, the intermediate is dissolved in water, an equimolar amount of H 2 SO 4 , H 3 PO 4 or HCl solution, react at 40-60°C for 5-10h; remove the solvent, wash and dry;

[0011] The [HIm][HSO 4 ] Obtained by reacting equimolar amounts of imidazole and sulfuric acid at 40-60°C for 5-10h.

[0012] The [C 3 H 6 SO 3 HIm][FeCl 4 ]、[C 3 H 6 SO 3 HIm][ZnCl 3 ]、[C 3 H 6 SO 3 HIm][CoCl 3 ]、[C 3 H 6 SO 3 HIm][CuCl 3 ]、[C 3 H 6 SO 3 HIm][MnCl 3 ] or [C 3 H 6 SO 3 HIm][AlCl 4 ] is prepared by the following method: imidazole is dissolved in acetonitrile or acetone solvent, an equimolar amount of 1,3-propyl sultone is added, and reflux reaction is carried out; the resulting mixed solution is filtered, washed, and dried to obtain a white solid intermediate; then, the intermediate is dissolved in water, an equimolar amount of FeCl is added 3 、ZnCl 2 、CoCl 2 , CuCl 2 、MnCl 2 or AlCl 3 Aqueous solution, react at room temperature for 5-10 hours; remove the solvent, wash and dry;

[0013] Preferably, the reflux reaction temperature is 60-120°C and the time is 6-12h;

[0014] Said removal of solvent is carried out by rotary evaporation;

[0015] The drying is vacuum drying, the drying temperature is 80-120°C, and the drying time is 8-16h;

[0016] In grams and milliliters respectively, the imidazole is dissolved in acetonitrile or acetone, and the mass volume ratio of imidazole to acetonitrile or acetone is (1-5):50;

[0017] The mass volume ratio of the intermediate to water is (1-5):20, with grams and milliliters as units respectively.

[0018] Preferably, the biomass is one or more of bagasse, corn cobs, miscanthus, poplar, bamboo and corn stalks.

[0019] Preferably, the organic solvent is one of methanol, ethanol, n-propanol, formic acid and acetic acid; and the reaction is carried out in a lined stainless steel reactor.

[0020] Preferably, the volume fraction of the organic solvent in the mixed liquid is 50-90%.

[0021] Preferably, the mass volume ratio of the biomass to the mixed liquid is (1-3):30 in grams and milliliters respectively; the molar mass ratio of the catalyst to the biomass is (0.37-2.22):2 in mmol and g respectively.

[0022] Preferably, the separation is to separate the solid and liquid of the reaction mixture by suction filtration, add water to the filtrate to precipitate, let it stand, and then separate the solid-liquid two phases by suction filtration at normal pressure.

[0023] Preferably, the standing time is 24-48 hours; and the amount of water added for the water precipitation is 2-4 times that of the filtrate.

[0024] Compared with the prior art, the invention has the following advantages:

[0025] The present invention utilizes the characteristics of acidic ionic liquids that have both catalytic hydrolysis and oxidation functions, and uses them in the catalytic biomass oxidation pretreatment delignification process. By selectively breaking the lignin-polysaccharide connecting bonds in bagasse biomass and oxidizing Cα benzyl alcohol in the lignin molecule, a one-pot in-situ separation of oxidatively modified lignin is achieved, and a new lignin priority strategy and technology is developed. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The figure shows the influence of different pretreatment temperatures on the delignification process in Examples 3 to 6 of the present invention.

[0027] Figure 2 This is the infrared spectra of oxidized lignin in Examples 3 to 6 of the present invention.

[0028] Figure 3 2D HSQC NMR analysis diagram of oxidized lignin in Examples 3 to 6 of the present invention.

[0029] Figure 4 This is the effect of different pretreatment times on the delignification process in Examples 16 to 19 of the present invention.

[0030] Figure 5 This is the infrared spectrum of bagasse, oxidized lignin and residue in Example 18 of the present invention.

[0031] Figure 6 This is the SEM image of the residue, oxidized lignin and bagasse in Example 18 of the present invention.

[0032] Figure 7 This is the effect of different solvents on the delignification process in Examples 20 to 23 of the present invention.

[0033] Figure 8 This is the effect of different oxygen pressures on the delignification process in Examples 24 to 26 of the present invention.

[0034] Fig. 9 This is the GC-MS graph of the oxidative lignin depolymerization product in Example 27 of the present invention.

[0035] Fig.10 This is the MS graph of 4-ethylphenol, the depolymerization product of oxidative lignin in Example 27 of the present invention. DETAILED DESCRIPTION

[0036] In order to better understand the technical features of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments, but the scope of protection claimed by the present invention is not limited thereto.

[0037] Aiming at the problems of the prior art related to the complex preparation process of oxidized lignin, the difficulty in separation and the high preparation cost, and most of them require the use of strong acids and alkalis and harsh conditions such as high temperature and high pressure. Most of the industrial lignin obtained by the prior art is difficult to convert and utilize due to the formation of a large number of stable CC bonds; the present invention utilizes the biomass under the catalytic action of acidic imidazole-type polyacid site ionic liquid to break the chemical bonds connecting the three main components of the biomass (lignin, cellulose and hemicellulose), mainly obtaining lignin and cellulose, while the hemicellulose is hydrolyzed into monosaccharides or oligosaccharides, which are further oxidized to form oxidized lignin and cellulose in a pure oxygen atmosphere.

[0038] The invention is characterized in that the imidazole-type multi-acid site ionic liquid has both catalytic hydrolysis function and oxidation function, and has good thermal stability. In addition, the multi-acid site characteristics significantly enhance the catalytic activity, selectively catalyze the breaking of the lignin-polysaccharide bond of wood fiber and oxidize the hydroxyl group on the α-C of the lignin molecule, thereby realizing a one-pot separation and preparation of oxidatively modified lignin.

[0039] The yield of oxidized lignin in the present invention is as high as 17.1wt.%, and the recovery rate of cellulose residue is 57.3wt.%. The yield of monophenol obtained by depolymerization of oxidized lignin is 245.0mg g -1, which are much higher than the two industrial lignins, alkali lignin and sodium lignin sulfonate, and the selectivity of 4-ethylphenol is 35.4%. The main monophenol products are 4-ethylphenol, 4-ethyl-2-methoxyphenol, 2-methoxy-4-propylphenol, 2.6-dimethoxy-4-ethylphenol and 2.6-dimethoxy-4-propylphenol. Among them, the proportions of H, G and S unit products are 38.1, 33.4 and 28.5% respectively. The oxidized lignin obtained by the present invention has great potential application value in the fields of concrete plasticization (cement diffuser), removal of heavy metals in sewage, preparation of aromatic chemicals and medical biomaterials.

[0040] The quality performance of the oxidized lignin of the present invention is mainly in the depolymerization performance; it is specifically achieved by the following operations: first, 100 mg of lignin, 50 mg of Pd / C catalyst (5wt%) and 10 mL of ethanol-water mixed solvent (8:2, v / v) are placed in a reactor, filled with 1.0 MPa argon gas, and reacted at 230°C for 3 hours; after the reaction is complete, solid-liquid separation is performed by suction filtration, the filtrate is added with internal calibration and volume gas chromatography-mass spectrometry, the liquid after volume adjustment is added to deionized water to regenerate lignin, and after solid-liquid separation, the lignin is washed and dried to obtain regenerated lignin, which is used for structural characterization and conversion rate calculation.

[0041] The calculation formulas for the fiber residue rate and the oxidized lignin yield in the present invention are shown in formulas (1-1) and (1-2).

[0042]

[0043] Among them, m, m OL and m RE are the mass of bagasse, oxidized lignin and fiber residue, g; Y OL and Y RE are the yields of oxidized lignin and fiber residue, respectively.

[0044] It should be noted that although the catalyst of the present invention is less publicized, it can also be purchased. 3 H 6 SO 3 HIm][H 2 PO 4 ]、[C 3 H 6 SO 3 HIm][Cl] or [C 3 H 6 SO 3 HIm][HSO 4] is preferably prepared by the following method: dissolving imidazole in acetonitrile or acetone solvent, adding an equimolar amount of 1,3-propyl sultone, and reflux reaction; after the reaction, the resulting mixed solution is filtered, washed, and dried to obtain a white solid intermediate; then, the intermediate is dissolved in water, and an equimolar amount of H 2 SO 4 , H 3 PO 4 or HCl solution, react at 40-60°C for 5-10h; remove the solvent, wash and dry.

[0045] [C] of the present invention 3 H 6 SO 3 HIm][FeCl 4 ]、[C 3 H 6 SO 3 HIm][ZnCl 3 ]、[C 3 H 6 SO 3 HIm][CoCl 3 ]、[C 3 H 6 SO 3 HIm][CuCl 3 ]、[C 3 H 6 SO 3 HIm][MnCl 3 ] and [C 3 H 6 SO 3 HIm][AlCl 4 ] Preferably, the preparation method is as follows: dissolving imidazole in acetonitrile or acetone solvent, adding an equimolar amount of 1,3-propyl sultone, and reflux reaction; after the reaction, the obtained mixed solution is filtered, washed, and dried to obtain a white solid intermediate; then, the intermediate is dissolved in water, and an equimolar amount of FeCl is added. 3 、ZnCl 2 、CoCl 2 , CuCl 2 、MnCl 2 or AlCl 3 Aqueous solution, react at room temperature for 5-10 hours; remove the solvent, wash and dry;

[0046] Based on the above [C 3 H 6 SO 3 HIm][H 2 PO 4 ]、[C 3 H 6 SO3 HIm][Cl]、[C 3 H 6 SO 3 HIm][HSO 4 ] and based on the above [C 3 H 6 SO 3 HIm][FeCl 4 ]、[C 3 H 6 SO 3 HIm][ZnCl 3 ]、[C 3 H 6 SO 3 HIm][CoCl 3 ]、[C 3 H 6 SO 3 HIm][CuCl 3 ]、[C 3 H 6 SO 3 HIm][MnCl 3 ] or [C 3 H 6 SO 3 HIm][AlCl 4 ] preparation method, further preferably, the reflux reaction temperature is 60-120°C and the time is 6-12h; the solvent removal is performed by rotary evaporation; the drying is vacuum drying, the drying temperature is 80-120°C, and the drying time is 8-16h; the imidazole is dissolved in acetonitrile or acetone, and the mass volume ratio of imidazole to acetonitrile or acetone is (1-5):50, respectively, in grams and milliliters. The mass volume ratio of the intermediate to water is (1-5):20, respectively, in grams and milliliters.

[0047] The biomass raw materials used in the examples were crushed by a plant crusher. The reaction was carried out in a lined stainless steel reactor.

[0048] There are many choices for biomass in the present invention. The biomass in the prior art can basically achieve the purpose of the present invention. Preferably, one or more of bagasse, corn cob, Miscanthus, poplar, bamboo and corn stalk are selected. The organic solvent can be selected according to solubility, preferably one of methanol, ethanol, n-propanol, formic acid and acetic acid; the volume fraction of the organic solvent in the mixed solution can be selected according to the dissolution requirements, preferably the volume fraction of the organic solvent in the mixed solution is 50-90%. As for the amount of biomass, solvent and catalyst, it can be selected according to the purpose of the invention; preferably, the mass volume ratio of biomass to mixed solution is (1-3):30 in grams and milliliters respectively; the molar mass ratio of catalyst to biomass is (0.37-2.22):2 in mmol and g respectively.

[0049] The classification of the present invention is to separate the solid and liquid of the reaction mixture by suction filtration, take the filtrate and add water to precipitate, let it stand, and then use normal pressure suction filtration method to achieve solid-liquid two-phase separation.

[0050] It should be noted that, in the oxygen pressure of the present invention, 0.1 MPa refers to one standard atmospheric pressure. The oxidant molecular oxygen of the present invention is a green oxidant.

[0051] Example 1: [C 3 H 6 SO 3 HIm][HSO 4 ] Preparation of ionic liquids

[0052] 0.1 mol of imidazole and 0.1 mol of 1.3-propane sultone were dissolved in acetonitrile and reacted at 60°C for 8 h to form a quaternary ammonium salt intermediate [C 3 H 6 SO 3 ][Im]; then, 0.01 mol [C 3 H 6 SO 3 ][Im] was dissolved in 50 mL of water, and a dilute solution of sulfuric acid, hydrochloric acid or phosphoric acid was slowly added dropwise, and the mixture was reacted at 50 °C for 5 h. After the water was removed by rotary evaporation, the mixture was dried at 80 °C in vacuum for 24 h to obtain the target ionic liquid [C 3 H 6 SO 3 HIm][HSO 4 ].

[0053] Example 2: [C 3 H 6 SO 3 HIm][FeCl 4 ] Preparation of ionic liquids

[0054] 0.1 mol of imidazole and 0.1 mol of 1.3-propane sultone were dissolved in acetonitrile and reacted at 60°C for 8 h to form a quaternary ammonium salt intermediate [C 3 H 6 SO 3 ][Im]; then, 0.01 mol [C 3 H 6 SO 3 ][Im] was dissolved in 50 mL of water and an equal amount of FeCl was slowly added dropwise 3 The solution was reacted at room temperature for 10 h. After removing the water by rotary evaporation, it was dried at 80 °C in vacuum for 24 h to obtain the target ionic liquid [C 3 H 6 SO 3 HIm][FeCl 4 ].

[0055] Example 3: A method for preparing oxidized lignin based on a lignin priority strategy

[0056] The extraction of oxidized lignin includes the following steps:

[0057] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 80°C and reacted for 4 h.

[0058] After the reaction is complete, the reaction mixture is filtered to separate the solid and liquid phases, and the filtrate is added to 3 times deionized water to precipitate the oxidized lignin, and then allowed to stand overnight.

[0059] After standing overnight, the solid-liquid two-phase separation was achieved again by normal pressure filtration method. The obtained lignin solid was washed 3 times with 100 mL of deionized water, and then dried and ground to finally obtain oxidized lignin solid.

[0060] After testing, the lignin yield obtained in this embodiment reached 7.6wt.%, while retaining 81.0wt.% of the residue rate. The test results are shown in the attached Figure 1 .

[0061] Example 4: A method for preparing oxidized lignin based on a lignin priority strategy

[0062] The difference between this embodiment and embodiment 3 is that:

[0063] 2g bagasse, 1.84mmol [C 3H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0064] The lignin yield obtained in this example reached 16.6wt.%, while retaining a residue rate of 57.3wt.%. The results are shown in the attached Figure 1 .

[0065] Example 5: A method for preparing oxidized lignin based on a lignin priority strategy

[0066] The difference between this embodiment and embodiment 3 is that:

[0067] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 120°C and reacted for 4 h.

[0068] The lignin yield obtained in this example reached 13.8wt.%, while retaining a 43.5wt.% residue rate. The results are shown in the attached Figure 1 .

[0069] Example 6: A method for preparing oxidized lignin based on a lignin priority strategy

[0070] The difference between this embodiment and embodiment 3 is that:

[0071] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 140°C and reacted for 4 h.

[0072] The lignin yield obtained in this example reached 14.6wt.%, while retaining a 37.2wt.% residue rate. The results are shown in the attached Figure 1 .

[0073] Figure 2 and 3They are respectively the FT-IR spectra of the oxidized lignin, residue and bagasse in Examples 3 to 6 of the present invention and the two-dimensional heteronuclear single quantum coherence spectrum (2D HSQC NMR) of the oxidized lignin. Figure 2 It can be seen that in the lignin sample, the wavelengths at 1600, 1516 and 1466 cm -1 The absorption at 1708cm belongs to the stretching vibration peak of the aromatic ring skeleton. The absorption intensity is greatly enhanced compared with bagasse, indicating that lignin is effectively separated from bagasse, while there is no absorption in the residue, indicating that lignin is completely separated. -1 The absorption peak at the conjugated C=O peak increases with the increase of temperature, which indicates that the aromatic C α The -OH unit is oxidized. -1 There is no obvious characteristic absorption peak belonging to carbohydrates, indicating that lignin contains less carbohydrate impurities. At the same time, the characteristic peaks of the three basic structural units of lignin were also observed in the lignin sample spectrum. For example, 1165cm -1 The H structural unit is at 1273, 1226 and 835 cm -1 belongs to the G structural unit, while 1332 and 1132 cm -1 In addition, the lignin sample spectrum also found a -1 The vibration peak of -OH at , its intensity decreases with the increase of temperature, suggesting that the side chain -OH is oxidized. Figure 3 In the spectrum, it is divided into three regions, including the aliphatic region, the oxygenated aliphatic region and the aromatic region. In the aliphatic region, there are a certain amount of aromatic rings α-CH, β-CH 2 and γ-CH 3 In the oxygen-containing fat region, there are a large number of methoxyl groups, β-O-4, β'-O-4, α-O-4, β-β and β-5, three structural unit connecting bonds of lignin molecules, and in the aromatic region, there are basic structural units such as ferulate (FA), p-hydroxyphenyl (H), lignin (G) and syringyl (S). In addition, with the increase of pretreatment temperature, the C of β-O-4 bond α The characteristic signal of -H is also significantly weakened, which is mainly attributed to the HC in the lignin molecule. α The -OH is oxidized to form a ketone.

[0074] Example 7: A method for preparing oxidized lignin based on a lignin priority strategy

[0075] The difference between this embodiment and embodiment 3 is that:

[0076] 2g bagasse, 1.00mmol [C 3 H 6 SO 3HIm][FeCl 4 ] Acidic ionic liquid and 30 mL of ethanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 0.5 MPa. The reactor was electrically heated to 100°C and reacted for 3 h.

[0077] The lignin yield obtained in this example reached 8.0 wt.%, while retaining a residue rate of 67.8 wt.%.

[0078] Example 8: A method for preparing oxidized lignin based on a lignin priority strategy

[0079] The difference between this embodiment and embodiment 3 is that:

[0080] 2g bagasse, 0.50mmol[C 3 H 6 SO 3 HIm][HSO 4 ] and 0.50mmol[C 3 H 6 SO 3 HIm][FeCl 4 ]Acidic ionic liquid and 30 mL of ethanol-water binary solvent (80:20, v / v) were mixed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 0.5 MPa. The reactor was electrically heated to 100 °C and reacted for 3 h.

[0081] The lignin yield obtained in this example reached 9.0 wt.%, while retaining a residue rate of 68.0 wt.%.

[0082] Example 9: A method for preparing oxidized lignin based on a lignin priority strategy

[0083] The difference between this embodiment and embodiment 3 is that:

[0084] 2g bamboo, 0.50mmol[C 3 H 6 SO 3 HIm][HSO 4 ] and 0.50mmol[C 3 H 6 SO 3 HIm][FeCl 4 ]Acidic ionic liquid and 30 mL of ethanol-water binary solvent (80:20, v / v) were mixed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 0.5 MPa. The reactor was electrically heated to 120°C and reacted for 4 h.

[0085] The lignin yield obtained in this example reached 13.4 wt.%, while retaining a residue rate of 55.3 wt.%.

[0086] Example 10: A method for preparing oxidized lignin based on a lignin priority strategy

[0087] The difference between this embodiment and embodiment 3 is that:

[0088] 2g Miscanthus sinensis, 0.50mmol [C 3 H 6 SO 3 HIm][HSO 4 ] and 0.50mmol[C 3 H 6 SO 3 HIm][FeCl 4 ]Acidic ionic liquid and 30 mL of ethanol-water binary solvent (80:20, v / v) were mixed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 0.5 MPa. The reactor was electrically heated to 120°C and reacted for 4 h.

[0089] The lignin yield obtained in this example reached 9.9 wt.%, while retaining a residue rate of 57.4 wt.%.

[0090] Example 11: A method for preparing oxidized lignin based on a lignin priority strategy

[0091] The difference between this embodiment and embodiment 3 is that:

[0092] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of methanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0093] The lignin yield obtained in this example reached 10.7 wt.%, while retaining a residue rate of 57.8 wt.%.

[0094] Example 12: A method for preparing oxidized lignin based on a lignin priority strategy

[0095] The difference between this embodiment and embodiment 3 is that:

[0096] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO4 ] Acidic ionic liquid and 30 mL of formic acid-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0097] The lignin yield obtained in this example reached 4.3 wt.%, while retaining a residue rate of 46.7 wt.%.

[0098] Example 13: A method for preparing oxidized lignin based on a lignin priority strategy

[0099] The difference between this embodiment and embodiment 3 is that:

[0100] 2 g bagasse, 1.84 mmol [HIm] [HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0101] The lignin yield obtained in this example reached 3.0 wt.%, while retaining 83.5 wt.% of the residue rate.

[0102] Example 14: A method for preparing oxidized lignin based on a lignin priority strategy

[0103] The difference between this embodiment and embodiment 3 is that:

[0104] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][H 2 PO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0105] The lignin yield obtained in this example reached 3.0 wt.%, while retaining 89.9 wt.% of the residue rate.

[0106] Example 15: A method for preparing oxidized lignin based on a lignin priority strategy

[0107] The difference between this embodiment and embodiment 3 is that:

[0108] 2g bagasse, 1.84mmol [C 3 H 6 SO 3HIm][Cl] acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100 °C and reacted for 4 h.

[0109] The lignin yield obtained in this example reached 2.5 wt.%, while retaining a residue rate of 91.6 wt.%.

[0110] Example 16: A method for preparing oxidized lignin based on a lignin priority strategy

[0111] The difference between this embodiment and embodiment 3 is that:

[0112] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 1 h.

[0113] The lignin yield obtained in this example reached 6.6wt.%, while retaining a 77.5wt.% residue rate. The results are shown in the attached Figure 4 .

[0114] Example 17: A method for preparing oxidized lignin based on a lignin priority strategy

[0115] The difference between this embodiment and embodiment 3 is that:

[0116] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 2 h.

[0117] The lignin yield obtained in this example reached 16.1wt.%, while retaining a residue rate of 56.5wt.%. The results are shown in the attached Figure 4 .

[0118] Example 18: A method for preparing oxidized lignin based on a lignin priority strategy

[0119] The difference between this embodiment and embodiment 3 is that:

[0120] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 3 h.

[0121] The lignin yield obtained in this example reached 17.1wt.%, while retaining a 57.3wt.% residue rate. The results are shown in the attached Figure 4 .

[0122] Attached Figure 5 and 6 The infrared spectra and SEM images of the residue, oxidized lignin and bagasse in Example 18 of the present invention are shown in the attached figure. Figure 5 The infrared spectrum of the residue is at 1368, 1062 and 890 cm -1 Obvious characteristic peaks appeared at the adjacent Figure 6 In the experiment, the surface of bagasse was smooth and full, but after the reaction, the surface became rough, loose and bubbly, mainly because most of the hemicellulose and lignin were dissolved and converted into soluble substances. The surface of the separated lignin was non-sheet-like and the particle size was less than 500nm. At the same time, the residue-based carbon material (calcined at 800℃ in a nitrogen atmosphere for 2h) was observed to have a lamellar structure and a macroporous structure similar to graphene, and most of the pores were about 50nm. This is mainly because the main component of the residue is cellulose, which is easily oxidized during the delignification process, and the oxygen-containing groups are lost during high-temperature calcination, forming a large number of macropores.

[0123] Example 19: A method for preparing oxidized lignin based on a lignin priority strategy

[0124] The difference between this embodiment and embodiment 3 is that:

[0125] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 5 h.

[0126] The lignin yield obtained in this example reached 15.4wt.%, while retaining a 56.9wt.% residue rate. The results are shown in the attached Figure 4 .

[0127] Example 20: A method for preparing oxidized lignin based on a lignin priority strategy

[0128] The difference between this embodiment and embodiment 3 is that:

[0129] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol solvent were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0130] The lignin yield obtained in this example reached 10.0wt.%, while retaining a residue rate of 54.5wt.%. The results are shown in the attached Figure 7 .

[0131] Example 21: A method for preparing oxidized lignin based on a lignin priority strategy

[0132] The difference between this embodiment and embodiment 3 is that:

[0133] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (90:10, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0134] The lignin yield obtained in this example reached 13.6 wt.%, while retaining a residue rate of 58.5 wt.%. The results are shown in the attached Figure 7 .

[0135] Example 22: A method for preparing oxidized lignin based on a lignin priority strategy

[0136] The difference between this embodiment and embodiment 3 is that:

[0137] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (60:40, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0138] The lignin yield obtained in this example reached 10.8wt.%, while retaining a residue rate of 55.5wt.%. The results are shown in the attached Figure 7 .

[0139] Example 23: A method for preparing oxidized lignin based on a lignin priority strategy

[0140] The difference between this embodiment and embodiment 3 is that:

[0141] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (50:50, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.0 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0142] The lignin yield obtained in this example reached 9.0wt.%, while retaining a residue rate of 57.2wt.%. The results are shown in the attached Figure 7 .

[0143] Example 24: A method for preparing oxidized lignin based on a lignin priority strategy

[0144] The difference between this embodiment and embodiment 3 is that:

[0145] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 0.1 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0146] The lignin yield obtained in this example reached 10.1wt.%, while retaining 63.0wt.% of the residue rate. The results are shown in the attached Figure 8 .

[0147] Example 25: A method for preparing oxidized lignin based on a lignin priority strategy

[0148] The difference between this embodiment and embodiment 3 is that:

[0149] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 0.5 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0150] The lignin yield obtained in this example reached 14.3wt.%, while retaining a residue rate of 57.7wt.%. The results are shown in the attached Figure 8 .

[0151] Example 26: A method for preparing oxidized lignin based on a lignin priority strategy

[0152] The difference between this embodiment and embodiment 3 is that:

[0153] 2g bagasse, 1.84mmol [C 3 H 6 SO 3 HIm][HSO 4 ] Acidic ionic liquid and 30 mL of n-propanol-water binary solvent (80:20, v / v) were placed in a 100 mL reactor, and the air was replaced by oxygen and the reactor was pressurized to 1.5 MPa. The reactor was electrically heated to 100°C and reacted for 4 h.

[0154] The lignin yield obtained in this example reached 15.5wt.%, while retaining a residue rate of 55.2wt.%. The results are shown in the attached Figure 8 .

[0155] Example 27: Study on the depolymerization properties of the obtained oxidized lignin

[0156] In order to evaluate the depolymerization performance of the separated oxidized lignin, the present invention uses it as a raw material for alcoholysis to prepare high value-added aromatic chemicals, and the specific steps are as follows: first, 100 mg of oxidized lignin, 50 mg of Pd / C (5wt%, purchased from Shanghai Aladdin Reagent Company) and 10 mL of ethanol-water mixed solvent (8:2, v / v) are placed in a 100 mL reactor, filled with 1.0 MPa argon gas, and reacted at 230°C for 3 hours; after the reaction is complete, the solid-liquid separation is performed by suction filtration, and the filtrate is added with internal calibration and volume gas analysis, and the liquid after volume is added to a certain amount of deionized water to regenerate lignin, and then after solid-liquid separation, it is washed and dried to obtain regenerated lignin, and it is used for structural characterization and conversion rate calculation. The calculation formulas for monophenol yield and lignin conversion rate are shown in formulas (1-3), (1-4) and (1-5):

[0157]

[0158] Among them, f i , Y i and A i are the correction factor, yield and peak area of ​​product i respectively; Y is the total monophenol yield; M, M relig and M OL represents the mass of internal standard solution, regenerated lignin and oxidized lignin, respectively (g); W and A are the mass concentration and peak area of ​​the internal standard; C OL is the conversion rate of oxidized lignin.

[0159] In this example, the lignin conversion rate was 82.8%, and the total yield of phenols was 245.0 mg g -1 The yield of 4-ethylphenol was 86.7 mg g -1 , with a selectivity of 35.4%.

[0160] Attached Fig. 9 This is the GC spectrum of the volatile products of the depolymerization of oxidized lignin in Example 27. As can be seen from the figure, the main products of the depolymerization of oxidized lignin are 4-ethylphenol, 4-ethyl-2-methoxyphenol, 2-methoxy-4-propylphenol, 2.6-dimethoxy-4-ethylphenol and 2.6-dimethoxy-4-propylphenol. Among them, the proportions of H, G and S unit products are 38.1, 33.4 and 28.5% respectively, which shows that the H unit of the oxidized lignin is easy to be depolymerized, followed by the G unit and finally the S unit. Fig.10 This is a mass spectrum (MS) of the main product 4-ethylphenol obtained by depolymerization of oxidized lignin in Example 27. As shown in the figure, the maximum mass-to-charge ratio (m / z) of the product is 122, which corresponds to the relative molecular mass (122) of 4-ethylphenol. In addition, by comparing with the Agilent NIST MS Search 2.4 database, it is determined that the product is 4-ethylphenol.

[0161] Comparative Example 1: Comparison of industrial lignin depolymerization performance

[0162] 100 mg of alkali lignin (Kraft lignin), 50 mg of Pd / C catalyst (5 wt.%) and 10 mL of ethanol-water mixed solvent (8:2, v / v) were weighed and placed in a high-pressure reactor. The air in the reactor was replaced with argon three times and then pressurized to 1.0 MPa (argon). At a speed of 600 rpm, the reactor was heated to 230°C at a rate of 5°C / min and maintained for 4 hours.

[0163] In this comparative example, the lignin conversion rate was 62.5%, and the total yield of phenols was 26.3 mg g -1.

[0164] Comparative Example 2: Study on the depolymerization performance of the obtained oxidized lignin

[0165] The difference between this comparative example and comparative example 1 is that:

[0166] 100 mg of sodium lignosulphonate, 50 mg of Pd / C catalyst (5 wt.%) and 10 mL of ethanol-water mixed solvent (8:2, v / v) were weighed and placed in a high-pressure reactor. The air in the reactor was replaced with argon three times and then pressurized to 1.0 MPa (argon). At a speed of 600 rpm, the reactor was heated to 230°C at a rate of 5°C / min and maintained for 4 hours. The total yield of phenols obtained in this comparative example was 44.0 mg g -1 .

[0167] As can be seen from the above examples, the present invention introduces oxidation reaction into the biomass delignification process, that is, the functionalized ionic liquid is used to selectively break the lignin-polysaccharide connection bond in the bagasse biomass and oxidize the C α Benzyl alcohol was used to achieve one-pot in-situ oxidation and separation of lignin, realizing a new lignin priority strategy. The yield of oxidized lignin obtained was as high as 3.0-17.1wt.%, and the recovery rate of cellulose residue was 46.7-89.9wt.%. Under the same depolymerization conditions, the monophenol yield of the oxidized lignin depolymerized in Example 7 was 245.0mg g -1 , which is much higher than the monophenol yields of two industrial lignins, alkali lignin and sodium lignin sulfonate (26.3 and 44.0 mg g -1 ), which are much higher than the two industrial lignins, alkali lignin and sodium lignin sulfonate, and the selectivity of 4-ethylphenol is as high as 35.4%. The main monophenol products are 4-ethylphenol, 4-ethyl-2-methoxyphenol, 2-methoxy-4-propylphenol, 2.6-dimethoxy-4-ethylphenol and 2.6-dimethoxy-4-propylphenol. These monophenol products are all high value-added chemicals. Among them, 4-ethylphenol is widely used in food additives, cosmetics, medicine and other fields. At the same time, the obtained oxidized modified lignin can not only be used as a cement diffuser, sewage heavy metal remover and for the preparation of aromatic chemicals, but also can be used as a biomaterial in the fields of pharmaceuticals, food and cosmetics due to the introduction of a large number of active groups such as -OH and more negative charges.

[0168] The implementation methods of the present invention are not limited to the above-mentioned embodiments, and any other changes, substitutions, and combinations made without departing from the spirit and principle of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for preparing oxidized lignin based on a lignin priority strategy, characterized in that: Biomass is used as raw material, imidazole-type multi-acid site ionic liquid is used as catalyst, and a mixed solution of organic solvent and water is used as solvent. The mixture is heated to 80-140° C. under an oxygen pressure of 0.1-1.5 MPa for reaction for 1-5 hours. The reaction mixture is separated to obtain oxidized lignin solid.

2. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 1, characterized in that: The imidazole-type multi-acid site ionic liquid is [HIm][HSO4], [C3H6SO3HIm][H2PO4], [C3H6SO3HIm][Cl], [C3H6SO3HIm][HSO4], [C3H6SO3HIm][FeCl4], [C3H6SO3HIm][ZnCl3], [C3H6SO3HIm][CoCl3], [C3H6SO3HIm][CuCl3], [C3H6SO3HIm][MnCl3] or [C3H6SO3HIm][AlCl4].

3. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 2, characterized in that: The [C3H6SO3HIm][H2PO4], [C3H6SO3HIm][Cl] or [C3H6SO3HIm][HSO4] is prepared by the following method: dissolving imidazole in acetonitrile or acetone solvent, adding an equimolar amount of 1,3-propyl sultone, and refluxing the mixture; filtering the mixture after the reaction, washing, and drying to obtain an intermediate; then, dissolving the intermediate in water, adding an equimolar amount of H2SO4, H3PO4 or HCl solution, and reacting at 40-60°C for 5-10h; removing the solvent, washing, and drying; The [HIm][HSO4] is obtained by reacting equimolar amounts of imidazole and sulfuric acid at 40-60° C. for 5-10 hours; The [C3H6SO3HIm][FeCl4], [C3H6SO3HIm][ZnCl3], [C3H6SO3HIm][CoCl3], [C3H6SO3HIm][CuCl3], [C3H6SO3HIm][MnCl3] or [C3H6SO3HIm][AlCl4] are prepared by the following method: dissolving imidazole in acetonitrile or acetone solvent, adding an equimolar amount of 1,3-propyl sultone, and refluxing the mixture; filtering the mixture after the reaction, washing, and drying to obtain an intermediate; then, dissolving the intermediate in water, adding an equimolar amount of FeCl3, ZnCl2, CoCl2, CuCl2, MnCl2 or AlCl3 aqueous solution, and reacting at room temperature for 5-10 hours; removing the solvent, washing, and drying.

4. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 3, characterized in that: The reflux reaction temperature is 60-120°C and the time is 6-12h; Said removal of solvent is carried out by rotary evaporation; The drying is vacuum drying, the drying temperature is 80-120°C, and the drying time is 8-16h; In grams and milliliters respectively, the imidazole is dissolved in acetonitrile or acetone, and the mass volume ratio of imidazole to acetonitrile or acetone is (1-5):50; The mass volume ratio of the intermediate to water is (1-5):20, with grams and milliliters as units respectively.

5. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 1, characterized in that: The biomass is one or more of bagasse, corn cobs, miscanthus, poplar, bamboo and corn stalks.

6. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 1, characterized in that: The organic solvent is one of methanol, ethanol, n-propanol, formic acid and acetic acid; the reaction is carried out in a stainless steel reactor with an inner lining.

7. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 1, characterized in that: The volume fraction of the organic solvent in the mixed liquid is 50-90%.

8. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 1, 5, 6 or 7, characterized in that: The mass volume ratio of the biomass to the mixed solution is (1-3):30 in grams and milliliters respectively; the molar mass ratio of the catalyst to the biomass is (0.37-2.22):2 in mmol and g respectively.

9. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 1, characterized in that: The classification is to separate the solid and liquid of the reaction mixture by suction filtration, add water to the filtrate for precipitation, let it stand, and then separate the solid-liquid two phases by normal pressure suction filtration.

10. The method for preparing oxidized lignin based on the lignin priority strategy according to claim 9, characterized in that: The standing time is 24-48 hours; the amount of water added for the water precipitation is 2-4 times that of the filtrate.

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