Method for extracting lignin through acidic eutectic system

Abstract

The invention belongs to the field of biomass lignin separation and purification, and discloses a method for extracting lignin through an acidic eutectic system, and the method comprises the following steps: adding an acidic catalyst and a biomass raw material into the eutectic system, reacting at 100-140 DEG C for 1-3 hours, and extracting the lignin, the eutectic system is a eutectic system composed of choline chloride and polyhydric alcohol. According to the method, under the action of the acid catalyst, the biomass raw material is pretreated in the specific eutectic system, so that the lignin with moderate molecular weight distribution, small polydispersity coefficient and good thermal stability is obtained, and further high-value conversion and utilization of the lignin are promoted; in addition, the preparation method is simple, easy to implement, green, safe and convenient for subsequent large-scale production and application.

Description

Technical Field

[0001] The invention belongs to the field of separation and purification of biomass lignin, and relates to a method for extracting lignin through an acidic low eutectic system, and in particular to a method for extracting high-quality lignin through a specific acidic low eutectic system. Background Art

[0002] Lignin is an amorphous polymer composed of phenylpropane units connected by carbon-carbon bonds and ether bonds. It is the second largest biomass resource in the plant kingdom after cellulose. The global annual output is about 50 million tons, of which about 10% to 20% comes from agricultural residues, and about 20% to 30% comes from forest biomass materials. The sources are very wide and the output is huge. Lignin has a complex structure, and the structure and properties of lignin obtained by different pretreatment methods vary greatly. For example, the lignin separated by the papermaking industry mainly exists in the form of sulfonates and sulfates. This type of lignin contains sulfur, which has problems such as wide molecular weight distribution, high dispersion, and poor thermal stability, which seriously affects its subsequent high-value utilization.

[0003] Patent 201610262776.X discloses a method for obtaining lignin by treating crushed eucalyptus powder with alcohol organic solvents combined with acid precipitation. However, the weight-average molecular weight of the lignin extracted by this method is less than 3000 g / mol, and the polydispersity coefficient is close to 3.0.

[0004] Patent 202111366876.4 discloses a method for pretreating eucalyptus graded lignin using a two-phase system based on a low eutectic system, wherein the selected low eutectic system is composed of choline chloride and lactic acid, and the lignin extraction phase is an acetone / water system. The obtained lignin has a weight-average molecular weight of no more than 2500 g / mol, and the polydispersity coefficient is close to 2.0.

[0005] The above findings show that the lignin obtained by the existing separation and extraction methods not only has a low molecular weight, but also a wide molecular weight distribution. The uniformity of the overall physical and chemical properties of the lignin may be poor, which is not conducive to the further conversion and utilization of the subsequent separated lignin.

[0006] Therefore, in response to the above-mentioned problems, it is urgent to develop efficient biomass pretreatment methods to obtain lignin with moderate molecular weight distribution, small polydispersity coefficient and good thermal stability, so as to promote further high-value conversion and utilization of lignin. Summary of the invention

[0007] Based on the above-mentioned problems, the purpose of the present invention is to provide a method for extracting lignin through an acidic low eutectic system. Under the action of acid, the biomass raw material is pretreated in the low eutectic system to obtain lignin with moderate molecular weight distribution, small polydispersity coefficient and good thermal stability, thereby promoting further high-value conversion and utilization of lignin; in addition, the preparation method is simple, easy to implement, green and safe, and convenient for subsequent large-scale production and application.

[0008] One of the objects of the present invention is to provide a method for extracting lignin by an acidic eutectic system, the method comprising:

[0009] Adding an acidic catalyst and a biomass raw material into a eutectic system, reacting at 100-140° C. (e.g., 100° C., 105° C., 110° C., 115° C., 120° C., 125° C., 130° C., 135° C., 140° C., etc.) for 1-3 h (e.g., 1 h, 1.5 h, 2 h, 2.5 h, 3 h, etc.) to extract lignin;

[0010] The eutectic system is a eutectic system composed of choline chloride and polyols.

[0011] The present invention provides a method for extracting lignin, which is to pretreat biomass raw materials in a low eutectic system under the action of acid, so as to obtain lignin with moderate molecular weight distribution, small polydispersity coefficient and good thermal stability, thereby promoting further high-value conversion and utilization of lignin; in addition, the preparation method is simple, easy to implement, green and safe, and convenient for subsequent large-scale production and application.

[0012] In the present invention, the eutectic system is a eutectic system mixture composed of choline chloride as a hydrogen bond acceptor and a polyol as a hydrogen bond donor. The freezing point is significantly lower than the melting point of a single component, so that the biomass raw material is dissolved in the eutectic system. Under the action of an anti-solvent, lignin can be gradually separated, thereby facilitating the acquisition of high-quality lignin. In addition, the eutectic system selected in the present application has low toxicity, so that it can become a green solvent for separating lignin.

[0013] In this application, acid is selected as a catalyst. On the one hand, it can enhance the interaction between lignin and the components of the low eutectic system, thereby promoting the efficient removal of lignin components; on the other hand, the acid catalyst can protonate the α-OH group in the lignin, resulting in dehydration and the generation of a benzyl carbon cation intermediate, which further depolymerizes to form a monomer. In addition, the depolymerized lignin monomers will undergo further polycondensation under the action of the acid catalyst. In summary, the presence of an acid catalyst promotes the depolymerization and re-polycondensation of lignin to form a dynamic balance, and promotes the acquisition of lignin with a moderate molecular weight, a small polydispersity coefficient, and good thermal stability.

[0014] Preferably, the preparation method of the eutectic system comprises: mixing choline chloride and a polyol, and heating at 50-70° C. (e.g., 50° C., 52° C., 55° C., 57° C., 60° C., 62° C., 65° C., 67° C., 70° C., etc.) to obtain a eutectic system;

[0015] The polyol includes any one of ethylene glycol, propylene glycol or glycerol, or a combination of at least two thereof;

[0016] The molar ratio of the polyol to choline chloride is (2-4):1, such as 2:1, 2.5:1, 3:1, 3.5:1, 4:1, etc.

[0017] In the present invention, a specific type of polyol and choline chloride are selected, and within a specific molar ratio range, the low eutectic system formed has a better effect in extracting lignin. When other hydrogen bond donors or hydrogen bond acceptors are used, or the molar ratio is not within the range specified in the present application, the extraction effect of lignin is poor, and it is not conducive to obtaining high-quality lignin.

[0018] Preferably, the acidic catalyst comprises any one of hydrochloric acid, sulfuric acid or p-toluenesulfonic acid or a combination of at least two thereof;

[0019] The amount of the acidic catalyst added is 2-6% of the molar amount of choline chloride in the eutectic system, such as 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, etc.

[0020] In the present invention, the specific addition amount of the acid catalyst can promote the depolymerization and re-condensation of lignin to form a dynamic balance, thereby promoting the acquisition of lignin with a moderate molecular weight, a small polydispersity coefficient and good thermal stability.

[0021] Preferably, the biomass raw material includes any one of corn straw, wheat straw or bamboo powder, or a combination of at least two thereof;

[0022] Based on the added molar amount of choline chloride in the low eutectic system being 1 mol, the added amount of the biomass raw material is 4-6 g, for example, 4 g, 4.5 g, 5 g, 5.5 g, 6 g, etc.

[0023] In the present invention, in order to further improve the extraction efficiency, the biomass raw material is crushed before adding to obtain a powdered substance with a smaller particle size (such as 10-100 mesh, for example 10 mesh, 20 mesh, 30 mesh, 40 mesh, 50 mesh, 60 mesh, 70 mesh, 80 mesh, 90 mesh, 100 mesh, etc.).

[0024] Preferably, the method further comprises performing a solid-liquid separation on the mixture after the reaction, and then precipitating lignin from the liquid obtained by the solid-liquid separation by an anti-solvent precipitation method, and then performing a second solid-liquid separation to obtain lignin.

[0025] Preferably, when performing the first solid-liquid separation, deionized water is added to the mixture after the reaction, and the amount of deionized water added is 8-12 g, such as 8 g, 9 g, 10 g, 11 g, 12 g, etc., based on the added molar amount of choline chloride in the eutectic system being 1 mol;

[0026] The method of the primary solid-liquid separation is filtration;

[0027] The solvent used in the anti-solvent precipitation method is deionized water. Based on the added molar amount of choline chloride in the low eutectic system being 1 mol, the added amount of deionized water is 200-350 g, for example, 200 g, 210 g, 220 g, 230 g, 240 g, 250 g, 260 g, 270 g, 280 g, 290 g, 300 g, 310 g, 320 g, 330 g, 340 g, 350 g, etc.;

[0028] The secondary solid-liquid separation is carried out by centrifugation.

[0029] As a preferred technical solution of the present invention, the method comprises the following steps:

[0030] (1) mixing a polyol and choline chloride in a molar ratio of (2-4):1, and heating at 50-70° C. to obtain a eutectic system;

[0031] (2) adding an acidic catalyst having a molar amount of choline chloride of 2-6% and 4-6 g of a biomass raw material (the amount of the biomass raw material added is based on the molar amount of choline chloride added in the low eutectic system being 1 mol) to the low eutectic system in step (1), reacting at 100-140° C. for 1-3 h to obtain a mixture;

[0032] (3) Add 8-12 g of deionized water to the mixture obtained in step (2) for a solid-liquid separation, add 200-350 g of anti-solvent to the liquid obtained by the solid-liquid separation to precipitate lignin, and then perform a secondary solid-liquid separation to obtain lignin.

[0033] The second object of the present invention is to provide a method for extracting lignin according to the first object.

[0034] Preferably, the weight average molecular weight of the lignin is 6300-7800 g / mol, for example 6300 g / mol, 6400 g / mol, 6500 g / mol, 6600 g / mol, 6700 g / mol, 6800 g / mol, 6900 g / mol, 7000 g / mol, 7100 g / mol, 7200 g / mol, 7300 g / mol, 7400 g / mol, 7500 g / mol, 7600 g / mol, 7700 g / mol, 7800 g / mol, etc.;

[0035] The number average molecular weight of the lignin is 4400-5700 g / mol, for example, 4400 g / mol, 4500 g / mol, 4600 g / mol, 4700 g / mol, 4800 g / mol, 4900 g / mol, 5000 g / mol, 5100 g / mol, 5200 g / mol, 5300 g / mol, 5400 g / mol, 5500 g / mol, 5600 g / mol, 5700 g / mol, etc.;

[0036] The polydispersity coefficient of the lignin is 1.3-1.52, such as 1.3, 1.35, 1.4, 1.45, 1.5, 1.52, etc.;

[0037] The thermal decomposition starting temperature of the lignin is 200-210°C (eg, 200°C, 202°C, 205°C, 207°C, 210°C, etc.), and the maximum weight loss rate temperature is 250-275°C.

[0038] The third object of the present invention is to provide a use of the lignin according to the second object in the preparation of fine chemicals.

[0039] Technical features and beneficial effects of the present invention:

[0040] The present invention provides a new method for extracting lignin, which uses acid as a catalyst to extract lignin from biomass raw materials in a low eutectic system, thereby obtaining lignin with moderate molecular weight distribution, small polydispersity coefficient and good thermal stability, thereby promoting further high-value conversion and utilization of lignin; in addition, the preparation method is simple, easy to implement, green and safe, and convenient for subsequent large-scale production and application. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0042] In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the technical solution of the present application will be described in detail below in conjunction with specific examples. If the specific experimental conditions are not specified in the examples, they are usually in accordance with conventional conditions or the conditions recommended by the reagent company; the raw materials, reagents, consumables, etc. used in the following examples, unless otherwise specified, can be obtained through commercial channels.

[0043] Example 1

[0044] This embodiment provides a method for extracting lignin, comprising:

[0045] After mixing glycerol and choline chloride in a molar ratio of 2:1, the mixture is heated to 60°C under stirring until a uniform transparent fluid is obtained, i.e., a eutectic system, and then sulfuric acid with a molar amount of 2% of choline chloride is added as a catalyst. 5g of bamboo powder crushed to 20 mesh is added to the above 100g eutectic system containing an acid catalyst, heated to 100 degrees Celsius and stirred for reaction for 1 hour, and then 10g of deionized water is immediately added to the flask, and the solid residue is filtered while hot. For the liquid obtained after filtration, 250g of deionized water is added to precipitate lignin, and then a lignin product is obtained by centrifugation.

[0046] Example 2

[0047] The difference from Example 1 is that the molar amount of sulfuric acid added as a catalyst is 4% of the molar amount of choline chloride added.

[0048] Example 3

[0049] The difference from Example 1 is that the added molar amount of the catalyst sulfuric acid is 6% of the added molar amount of the choline chloride.

[0050] Example 4

[0051] After mixing glycerol and choline chloride in a molar ratio of 3:1, the mixture was heated to 60°C under stirring until a uniform transparent fluid was obtained, i.e., a eutectic system, and then sulfuric acid with a molar amount of 2% of choline chloride was added as a catalyst. 5 g of bamboo powder crushed to 20 mesh was added to the above 100 g eutectic system containing an acid catalyst, heated to 110 degrees Celsius and stirred for reaction for 2 hours, and then 8 g of deionized water was immediately added to the flask, and the solid residue was filtered while hot. For the liquid obtained after filtration, 200 g of deionized water was added to precipitate lignin, and then a lignin product was obtained by centrifugation.

[0052] Example 5

[0053] The only difference from Example 4 is that the molar ratio of glycerol to choline chloride is 4:1.

[0054] Example 6

[0055] Ethylene glycol and choline chloride were mixed in a molar ratio of 2:1 and heated to 60°C under stirring until a uniform transparent fluid was obtained, i.e., a eutectic system, and then sulfuric acid with a molar amount of 2% of choline chloride was added as a catalyst. 5 g of bamboo powder crushed to 20 mesh was added to the above 100 g of the eutectic system containing an acid catalyst, heated to 110 degrees Celsius and stirred for reaction for 2 hours, and then 12 g of deionized water was immediately added to the flask, and the solid residue was filtered while hot. For the liquid obtained after filtration, 350 g of deionized water was added to precipitate lignin, and then a lignin product was obtained by centrifugation.

[0056] Example 7

[0057] The only difference from Example 6 is that the same molar amount of ethylene glycol is replaced by propylene glycol.

[0058] Example 8

[0059] After mixing glycerol and choline chloride in a molar ratio of 2:1, the mixture is heated to 60°C under stirring until a uniform transparent fluid is obtained, i.e., a eutectic system, and then 2% hydrochloric acid with a molar amount of choline chloride is added as a catalyst. 5g of bamboo powder crushed to 20 mesh is added to the above 100g eutectic system containing an acid catalyst, heated to 110 degrees Celsius and stirred for reaction for 2 hours, and then 10g of deionized water is immediately added to the flask, and the solid residue is filtered while hot. For the liquid obtained after filtration, 300g of deionized water is added to precipitate lignin, and then a lignin product is obtained by centrifugation.

[0060] Example 9

[0061] The only difference from Example 8 is that the same amount of hydrochloric acid added is replaced by p-toluenesulfonic acid.

[0062] Example 10

[0063] After mixing glycerol and choline chloride in a molar ratio of 2:1, the mixture is heated to 60°C under stirring until a uniform transparent fluid is obtained, i.e., a eutectic system, and then p-toluenesulfonic acid with a molar amount of 2% of choline chloride is added as a catalyst. 5g of bamboo powder crushed to 20 mesh is added to the above 100g eutectic system containing an acid catalyst, heated to 140 degrees Celsius and stirred for reaction for 3 hours, and then 10g of deionized water is immediately added to the flask, and the solid residue is filtered while hot. For the liquid obtained after filtration, 250g of deionized water is added to precipitate lignin, and then a lignin product is obtained by centrifugation.

[0064] Embodiment 11

[0065] After mixing glycerol and choline chloride in a molar ratio of 2:1, the mixture is heated to 60°C under stirring until a uniform transparent fluid is obtained, i.e., a eutectic system, and then sulfuric acid with a molar amount of 2% of choline chloride is added as a catalyst. 5g of corn stalks crushed to 20 mesh are added to the above 100g eutectic system containing an acid catalyst, heated to 110 degrees Celsius and stirred for reaction for 2 hours, and then 10g of deionized water is immediately added to the flask, and the solid residue is filtered while hot. For the liquid obtained after filtration, 250g of deionized water is added to precipitate lignin, and then the lignin product is obtained by centrifugation.

[0066] Example 12

[0067] The only difference from Example 11 is that the corn straw is replaced by wheat straw of equal mass and equal mesh size.

[0068] After drying, the lignin products obtained in Examples 1-12 were subjected to the following performance tests:

[0069] (1) analyzing its weight average molecular weight and number average molecular weight by gel permeation chromatography and calculating the polydispersity coefficient;

[0070] (2) The thermal decomposition starting temperature and the maximum weight loss rate temperature were analyzed by thermogravimetric analyzer.

[0071] See the table below for specific test data.

[0072] Table 1 Molecular weight and thermal properties analysis of lignin extracted from acidic eutectic system

[0073]

[0074]

[0075] In order to compare and illustrate the performance of lignin extracted by the method of the present application, ground wood lignin MWL of corn straw, wheat straw and bamboo powder was separated and extracted according to the method described in the literature (ACSSustain.Chem.Eng.2017,5(12):11618-11627). The molecular weight distribution, polydispersity coefficient and thermal properties are shown in Table 2:

[0076] Table 2 Properties of ground wood lignin from different sources

[0077]

[0078] By comparing Table 1 and Table 2, it can be seen that: compared with the ground wood lignin of different raw materials in Table 2, the molecular weight of the lignin extracted by the acidic low eutectic system in this application has decreased to a certain extent, indicating that the original natural lignin has undergone a certain degree of fracture and decomposition during the pretreatment process. However, it should be noted that the polydispersity coefficient of the lignin extracted by the acidic low eutectic system is significantly lower than that of the ground wood lignin, indicating that the molecular weight distribution of the former is more uniform. In addition, the polydispersity coefficient of the lignin extracted by the acidic low eutectic system is significantly higher than that of the ground wood lignin in terms of thermal decomposition starting temperature and maximum weight loss rate temperature, indicating that the lignin extracted by the acidic low eutectic system has better thermal stability. A more uniform molecular weight distribution and better thermal stability are conducive to the downstream high-value conversion and utilization of lignin.

[0079] Comparative Example 1

[0080] After mixing glycerol and choline chloride in a molar ratio of 2:1, the mixture was heated to 60°C under stirring until a uniform transparent fluid was obtained, i.e., a eutectic system, without adding an acid catalyst. 5 g of bamboo powder crushed to 20 mesh was added to the above 100 g of the eutectic system containing an acid catalyst, heated to 110 degrees Celsius and stirred for reaction for 2 hours, and then 10 g of deionized water was immediately added to the flask, and the solid residue was filtered while hot. For the liquid obtained after filtration, 250 g of deionized water was added to precipitate lignin, and then the lignin product was obtained by centrifugation.

[0081] Comparative Example 2

[0082] The only difference from Comparative Example 1 is that the bamboo powder is replaced by corn stalks of the same mass and the same mesh size.

[0083] Comparative Example 3

[0084] The only difference from Comparative Example 1 is that the bamboo powder is replaced by wheat straw of the same mass and the same mesh size.

[0085] After drying, the lignin samples extracted from Comparative Examples 1-3 were analyzed for weight average molecular weight and number average molecular weight by gel permeation chromatography, and the polydispersity coefficient was calculated; the thermal decomposition starting temperature and the maximum weight loss rate temperature were analyzed by thermogravimetric analyzer. Specific test data are shown in Table 3.

[0086] Table 3 Molecular weight and thermal properties analysis of lignin extracted from neutral eutectic system

[0087]

[0088] It can be seen from the comparative example in Table 3 that in the absence of an acid catalyst, the eutectic system can only extract a lignin component with a smaller molecular weight and a larger polydispersity coefficient. In the presence of an acid catalyst, on the one hand, the interaction between lignin and the components of the eutectic system can be enhanced, thereby promoting the efficient removal of the lignin component. On the other hand, the acid catalyst can protonate the α-OH group in the lignin, resulting in dehydration and the generation of a benzyl carbon cation intermediate, which further depolymerizes to form a monomer. In addition, the depolymerized lignin monomer will undergo further polycondensation under the action of an acid catalyst. In summary, the presence of a certain amount of acid catalyst promotes the depolymerization and re-polycondensation of lignin to form a dynamic balance, and promotes the acquisition of lignin with a moderate molecular weight, a small polydispersity coefficient, and good thermal stability.

[0089] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and intent of the present invention.

Claims

1. A method for extracting lignin by an acidic eutectic system, characterized in that: The method comprises: Adding an acidic catalyst and biomass raw materials into a eutectic system, reacting at 100-140°C for 1-3 hours, and extracting lignin; The eutectic system is a eutectic system composed of choline chloride and polyols.

2. The method according to claim 1, characterized in that The preparation method of the eutectic system comprises: mixing choline chloride and polyol, and heating at 50-70° C. to obtain a eutectic system; The polyol includes any one of ethylene glycol, propylene glycol or glycerol, or a combination of at least two thereof; The molar ratio of the polyol to choline chloride is (2-4):

1.

3. The method according to claim 1, characterized in that The acidic catalyst includes any one of hydrochloric acid, sulfuric acid or p-toluenesulfonic acid or a combination of at least two thereof; The amount of the acid catalyst added is 2-6% of the molar amount of choline chloride in the eutectic system.

4. The method according to claim 1, characterized in that The biomass raw material includes any one of corn straw, wheat straw or bamboo powder, or a combination of at least two of them; Based on the added molar amount of choline chloride in the low eutectic system being 1 mol, the added amount of the biomass raw material is 4-6 g.

5. The method according to claim 1, characterized in that The method further comprises performing a solid-liquid separation on the reaction mixture, and then precipitating lignin from the liquid obtained by the solid-liquid separation by an anti-solvent precipitation method, and then performing a second solid-liquid separation to obtain lignin.

6. The method according to claim 5, characterized in that When performing the first solid-liquid separation, deionized water is added to the mixture after the reaction, and the amount of deionized water added is 8-12 g, based on the added molar amount of choline chloride in the low eutectic system being 1 mol; The method of the primary solid-liquid separation is filtration; The solvent used in the anti-solvent precipitation method is deionized water. Based on the molar amount of choline chloride added in the eutectic system being 1 mol, the amount of deionized water added is 200-350 g. The secondary solid-liquid separation is carried out by centrifugation.

7. The method according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: (1) mixing a polyol and choline chloride in a molar ratio of (2-4):1, and heating at 50-70° C. to obtain a eutectic system; (2) adding an acidic catalyst having a molar amount of choline chloride of 2-6% and a biomass raw material to the eutectic system in step (1), reacting at 100-140° C. for 1-3 hours to obtain a mixture; (3) Add 8-12 g of deionized water to the mixture obtained in step (2) for a solid-liquid separation, add 200-350 g of anti-solvent to the liquid obtained by the solid-liquid separation to precipitate lignin, and then perform a secondary solid-liquid separation to obtain lignin.

8. Lignin is extracted according to the method according to any one of claims 1 to 7.

9. The lignin according to claim 8, characterized in that The weight average molecular weight of the lignin is 6300-7800 g / mol, and the number average molecular weight of the lignin is 4400-5700 g / mol; The polydispersity coefficient of the lignin is 1.3-1.52; The thermal decomposition starting temperature of the lignin is 200-210°C, and the maximum weight loss rate temperature of the lignin is 250-275°C.

10. Use of the lignin according to claim 8 or 9 in the preparation of fine chemicals.

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