Lewis acid-diol eutectic solvent and method for extracting lignin with complete structure

Abstract

The invention discloses a Lewis acid-diol eutectic solvent and a method for extracting lignin with a complete structure, and belongs to the technical field of biomass refining. Due to the advantages of renewability, low volatility, low cost, good biocompatibility, simple preparation and the like, the solvent is regarded as a green solvent. In the prior art, a certain application value is achieved in the aspect of extracting the complete structure of lignin by adding a strong acid substance into a deep eutectic solvent, but the practical application popularization of the deep eutectic solvent is limited due to the problems of high corrosivity, complex process, high cost and the like. The Lewis acid-diol eutectic solvent provided by the invention can effectively retain cellulose and remove hemicelluloses and lignin under mild conditions, and realizes efficient extraction of lignin. The extracted lignin has a high extraction rate and a complete structure. The method is easy to operate, remarkable in effect, high in practicability, small in pollution and easy to popularize, and technical support is provided for efficient utilization of biomass resources.

Description

Technical Field

[0001] The present invention relates to the technical field of biomass refining, and particularly to a Lewis acid-diol deep eutectic solvent and a method for extracting intact lignin. Background Art

[0002] Forest biomass, as a globally abundant renewable resource, its efficient utilization is of great significance for replacing non-renewable energy, reducing environmental pollution, and promoting the development of green and low-carbon economy. Lignin is one of the main components in forest biomass. As the only renewable aromatic compound, it can be used to prepare value-added chemicals, biofuels, and functional materials, and has the potential to replace traditional fossil resources. Traditional lignin extraction methods include chemical methods, physical methods, physicochemical methods, and biological methods. Among them, chemical treatment (such as acid-base pretreatment) has high extraction efficiency, but the lignin structure is severely damaged during the extraction process, reducing its chemical or biological reactivity and making it difficult to utilize the extracted lignin subsequently. Ionic liquids are a type of green solvent that can effectively extract lignin, but their industrial application is limited due to problems such as difficult synthesis and recovery, high cost, poor biotoxicity and stability.

[0003] Deep eutectic solvents are mixtures driven by hydrogen bonds formed by hydrogen bond acceptors and hydrogen bond donors, and have the advantages of renewable, low volatility, low cost, good biocompatibility, and simple preparation, and are regarded as a type of green solvent. In recent years, acidic deep eutectic solvents have been widely used in the extraction of lignin from lignocellulose due to the protons and active sites they provide. However, acidic deep eutectic solvents are extremely prone to cause excessive depolymerization and recondensation of lignin under high temperature and high pressure conditions, forming stubborn C-C bonds, increasing its structural heterogeneity and the difficulty of further depolymerization, severely limiting the subsequent high-value utilization of lignin. At the same time, the condensed lignin is easily adsorbed on the surface of cellulose, reducing the accessibility of cellulase, thereby significantly reducing the cellulase hydrolysis conversion efficiency and being unfavorable for the efficient utilization of each component of lignocellulosic biomass. Diol compounds, as nucleophiles, can be incorporated into β-aryl ether units through α-alkoxylation to form new β-O-4 bonds, thereby inhibiting the recondensation of lignin. However, the existing alcohol-based deep eutectic solvents have poor delignification effects. To solve this problem, strong acidic compounds (sulfuric acid, p-toluenesulfonic acid, and oxalic acid) are usually added to the alcohol-based deep eutectic solvents to form ternary deep eutectic solvents. Although the ternary solvents have certain delignification ability, they have problems such as strong corrosiveness, complex processes, and high costs, which limit the popularization of their practical applications.

[0004] The Chinese patent application document with the publication number CN110540508A discloses a deep eutectic solvent and its application in lignin extraction, belonging to the technical field of biomass refining, specifically relating to a deep eutectic solvent and its application in lignin extraction. The deep eutectic solvent is composed of a certain amount of hydrogen bond donor and hydrogen bond acceptor, and is an economical, environmentally friendly green solvent. Among them, choline-based deep eutectic solvents have extensive applications in lignocellulose extraction. In the deep eutectic solvents of choline chloride, due to the presence of chloride ions, the negative charge distribution is concentrated, resulting in a reduction in extraction efficiency and accelerating the aging of instruments. This method can effectively extract high-purity lignin, with simple operation, obvious effects, strong practicability, little pollution, and easy to promote. However, there is still a problem of low lignin extraction rate, and the structural integrity of the extracted lignin lacks a clear explanation. Summary of the Invention

[0005] The technical problem to be solved by the present invention is how to provide a mild binary deep eutectic solvent that can efficiently dissolve and extract structurally intact lignin.

[0006] The present invention solves the above technical problems through the following technical means:

[0007] In the first aspect of the present invention, a Lewis acid-diol deep eutectic solvent is proposed. The deep eutectic solvent includes a hydrogen bond acceptor and a hydrogen bond donor; the hydrogen bond acceptor is a Lewis acid; the hydrogen bond donor includes one or any several of ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol.

[0008] Preferably, the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is (0.5 - 2):(1.2 - 3), and more preferably 1:1, 1:2.

[0009] Preferably, the Lewis acid includes FeCl 3 , ZnCl 2 , SnCl 2 any one or more of them.

[0010] In the second aspect of the present invention, a preparation method of the above-mentioned Lewis acid-diol deep eutectic solvent is proposed, including the following steps: Mix the hydrogen bond donor and the hydrogen bond acceptor, and stir in a water bath at 70 - 90 °C for 1 - 2 h until a homogeneous transparent solution is obtained, and then dry it in a blast dryer to obtain it.

[0011] Preferably, the drying temperature is 50 - 70 °C, and the drying time is 10 - 15 h; more preferably 60 °C, 12 h.

[0012] In the third aspect of the present invention, the above-mentioned Lewis acid-diol deep eutectic solvent is proposed for application in dissolving industrial lignin.

[0013] Weigh 2 g of the eutectic solvent and place it in a magnetic stirrer. Heat it to the preset temperature and keep it at a constant temperature. Weigh 10 mg of industrial lignin and gradually add it to the eutectic solvent while performing magnetic stirring until the lignin is completely dissolved. After the lignin is completely dissolved, continue to add 10 mg of industrial lignin and repeat this operation until the lignin cannot be completely dissolved within 1 h. Record the mass of all the added industrial lignin and calculate its solubility in the eutectic solvent.

[0014] Specifically and preferably, the industrial lignin includes any one of enzymatic hydrolysis lignin, alkali lignin, and kraft lignin, but is not limited thereto.

[0015] Preferably, the heating temperature of the mixture is 30 - 90 °C, and more preferably 30 °C, 50 °C, 70 °C, and 90 °C.

[0016] In the fourth aspect of the present invention, a method for efficiently extracting structurally intact lignin is proposed. After pretreatment, lignocellulose is mixed and heated with the above-mentioned Lewis acid - diol eutectic solvent, and then anhydrous ethanol is added for suction filtration. After adding an aqueous hydrochloric acid solution to the filtrate and standing, after separation, the dried precipitate obtained is the extracted lignin.

[0017] Preferably, the mass ratio of lignocellulose to the Lewis acid - diol eutectic solvent is 1:5 - 1:50.

[0018] Preferably, the heating temperature is 60 - 110 °C, the heating time is 0.5 - 3 h, and more preferably 1 h.

[0019] Preferably, the concentration of the added aqueous hydrochloric acid solution is 3 M, and the volume is 10 times that of the filtrate.

[0020] After a large number of experimental verifications and analyses, to obtain the best results, the mass ratio is 1:5, the reaction temperature is 110 °C, and the reaction time is 1 h.

[0021] Preferably, the lignocellulose is corn straw powder.

[0022] The beneficial effects of the present invention are as follows:

[0023] 1. The present invention proposes a method based on a Lewis acid - diol eutectic solvent for dissolving and extracting structurally intact lignin. By optimizing the solvent composition and operating conditions, the efficient separation of lignin is achieved, meeting the technical requirements of greening and efficient utilization.

[0024] 2. The binary deep eutectic solvent used in the present invention is simple to prepare, low in cost, stable in properties, and has little environmental pollution. The present invention avoids the use of strong acidic substances and reduces the corrosion of equipment.

[0025] 3. The present invention efficiently extracts lignin with high purity, high β-O-4 content, and low molecular weight, which is beneficial to the subsequent high-value utilization of lignin. The treatment process of the present invention is simple and low in cost, and has high potential for industrial application.

[0026] 4. In view of the problems commonly faced by traditional methods, such as low lignin extraction rate, severe structural damage, and high cost, the Lewis acid-diol binary deep eutectic solvent constructed in the present invention reduces the use of solvents, improves the lignin extraction rate, and effectively retains its structure, providing an innovative solution for the efficient extraction and high-value utilization of lignin. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a comparison chart of the solubility of different industrial lignins obtained in Examples 1-15, where (a-c) are the solubilities of enzymatic hydrolysis lignin in SnCl 2 -based, FeCl 3 -based, and ZnCl 2 -based DESs systems; (d-f) are the solubilities of alkali lignin in SnCl 2 -based, FeCl 3 -based, and ZnCl 2 -based DESs systems; (g-i) are the solubilities of kraft lignin in SnCl 2 -based, FeCl 3 -based, and ZnCl 2 -based DESs systems.

[0028] Figure 2 It is the NMR spectrum of the lignin extracted in Example 16; where a) and b) are the side chain regions of the lignin hydrolyzed by cellulase and the lignin extracted by the deep eutectic solvent respectively, and c) and d) are the aromatic regions of the lignin hydrolyzed by cellulase and the lignin extracted by the deep eutectic solvent respectively; the content unit of the β-O-4 bond marked in the figure is per 100 aromatic units (100Ar). DETAILED DESCRIPTION OF THE INVENTION

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0030] The test materials, reagents, etc. used in the following examples can be obtained from commercial sources without special instructions.

[0031] For those without specific technical or conditions noted in the examples, they can all be carried out according to the techniques or conditions described in the literature in this field or according to the product instructions. Without special instructions, the quantitative tests in the following examples are all set with more than three repeated experiments, and the results are averaged.

[0032] The analysis methods adopted in the following examples are as follows:

[0033] Lignin solubility S 1 (wt%) = m 1 / m 2 ×100

[0034] Where m 1 is the total mass of the added lignin, and m 2 is the mass of the deep eutectic solvent.

[0035] Residue yield Y 1 (wt%) = m 3 / m 4 ×100

[0036] Cellulose retention rate R C (%) = m 5 / m 6 ×100

[0037] Hemicellulose removal rate R h (%) = (m 7 -m 8 / m 9 )×100

[0038] Lignin removal rate R l (%) = (m 10 -m l1 / m l2 )×100

[0039] Lignin yield Y 2 = m 13 / m 14 ×100

[0040] The β-O-4 content Y(%) of lignin = (I l +I 2 ) / I 3 ×100

[0041] Where m 3 is the mass of the residue, m 4 is the mass of the added corn straw, m 5 is the mass of cellulose in the residue, m6 is the mass of straw cellulose, m 7 is the mass of hemicellulose in the raw material, m 8 is the mass of hemicellulose in the residue, m 9 is the mass of hemicellulose in the straw, m 10 is the mass of lignin in the raw material, m 11 is the mass of lignin in the residue, m 12 is the mass of xylocellulose in the straw, m 13 is the mass of recovered lignin, m 14 is the mass of total lignin (acid-soluble and acid-insoluble) in corn straw. I 1 is the integral value of the Cα position (β-O-4 α ) signal in the 2D-HSQC NMR spectrum, that is, the content of uncondensed or unmodified β-O-4 bonds, I 2 is the integral value of the Cα position signal of α-alkylated β-O-4 bonds, that is, the new β-O-4 bonds generated by chemical treatment (such as reaction with diol compounds), I 3 is the integral value of the total aromatic ring signal, that is, the total number of lignin monomer units.

[0042] In the examples, the propylene glycol used is 1,3-propanediol, the butylene glycol is 1,4-butanediol, the pentylene glycol is 1,5-pentanediol, and the hexylene glycol is 1,6-hexanediol.

[0043] Example 1:

[0044] The prepared different Lewis acid-diol eutectic solvents were used to dissolve industrial lignin

[0045] A Lewis acid-diol eutectic solvent with a hydrogen bond acceptor of FeCl 3 , a hydrogen bond donor of ethylene glycol, was mixed in a molar ratio of hydrogen bond acceptor to hydrogen bond donor of 1:2 and stirred in a water bath at 80 °C for 1.5 h to obtain a homogeneous transparent solution, and then dried in a forced-air dryer at 60 °C for 12 h to obtain the FeCl 3 -ethylene glycol eutectic solvent.

[0046] The eutectic solvent of this example was used to dissolve industrial lignin:

[0047] Take 2.0 g of the FeCl 3 -ethylene glycol eutectic solvent of this example and put it into a 10 mL glass bottle, place it on a magnetic stirrer, set the rotation speed to 300 r / min, at room temperature, add about 10 mg of industrial lignin, and continuously stir to make the solid-liquid mixture evenly mixed until the industrial lignin is completely dissolved to form a stable system without precipitation, and calculate the solubility of the added industrial lignin.

[0048] Example 2:

[0049] The difference between this example and Example 1 is that the hydrogen bond donor is propylene glycol, and the rest is the same as in Example 1.

[0050] Example 3:

[0051] The difference between this example and Example 1 is that the hydrogen bond donor is butylene glycol, and the rest is the same as in Example 1.

[0052] Example 4:

[0053] The difference between this example and Example 1 is that the hydrogen bond donor is pentylene glycol, and the rest is the same as in Example 1.

[0054] Example 5:

[0055] The difference between this example and Example 1 is that the hydrogen bond donor is hexylene glycol, and the rest is the same as in Example 1.

[0056] Example 6:

[0057] The difference between this example and Example 1 is that the hydrogen bond acceptor is SnCl 2 , and the rest is the same as in Example 1.

[0058] Example 7:

[0059] The difference between this example and Example 6 is that the hydrogen bond donor is propylene glycol, and the rest is the same as in Example 6.

[0060] Example 8:

[0061] The difference between this example and Example 6 is that the hydrogen bond donor is butylene glycol, and the rest is the same as in Example 6.

[0062] Example 9:

[0063] The difference between this example and Example 6 is that the hydrogen bond donor is pentylene glycol, and the rest is the same as in Example 6.

[0064] Example 10:

[0065] The difference between this example and Example 6 is that the hydrogen bond donor is hexylene glycol, and the rest is the same as in Example 6.

[0066] Example 11:

[0067] The difference between this example and Example 1 is that the hydrogen bond acceptor is ZnCl 2 , and the rest is the same as in Example 1.

[0068] Example 12:

[0069] The difference between this example and Example 11 is that the hydrogen bond donor is propylene glycol, and the rest is the same as in Example 11.

[0070] Example 13:

[0071] The difference between this example and Example 11 is that the hydrogen bond donor is butanediol, and the rest is the same as in Example 11.

[0072] Example 14:

[0073] The difference between this example and Example 11 is that the hydrogen bond donor is pentanediol, and the rest is the same as in Example 11.

[0074] Example 15:

[0075] The difference between this example and Example 11 is that the hydrogen bond donor is hexanediol, and the rest is the same as in Example 11.

[0076] Example 16:

[0077] Using the ZnCl 2 -butanediol deep eutectic solvent pretreated corn straw powder to extract lignin, including the following steps:

[0078] Put the corn straw powder and ZnCl 2 -butanediol deep eutectic solvent into a high-temperature resistant glass bottle according to a mass ratio of 1:10, place it in an oil bath at 110 °C and heat and stir for 1 h. Then, add a small amount of absolute ethanol, filter by suction to obtain a filtrate and a residue rich in cellulose. Add the hydrochloric acid aqueous solution to the filtrate, let it stand for 12 h, and the obtained precipitate, after washing and drying, is the extracted lignin.

[0079] Measure the cellulose, hemicellulose and lignin contents of corn straw / residue rich in cellulose:

[0080] (1) Weigh about 0.3 g of dry corn straw / residue rich in cellulose and put it into a 100 mL high-temperature resistant glass bottle, add 3.0 mL of the prepared 72% sulfuric acid and mix well. Hydrolyze and react in a magnetic stirring water bath at 30 °C for 1 h. During this period, stir and mix with a glass rod to ensure that the sulfuric acid and the sample in the glass bottle can be fully mixed and hydrolyzed.

[0081] (2) Add 84 mL of deionized water to the glass bottle, put it into a high-temperature sterilizer, and keep it at 121 °C for 1 h. Then, cool the reaction mixture to room temperature, filter by suction through a pre-weighed G3 sintered glass funnel, collect the hydrolysate, wash it with deionized water until neutral, put it in an oven at 105 °C to dry, and weigh the mass of the filter residue.

[0082] (3) The filter residue was calcined in a muffle furnace at 575 °C for 6 h, dried, cooled to room temperature and weighed to calculate the mass of lignin and ash. The contents of glucose and xylose were determined by high performance liquid chromatography and used to calculate the cellulose content and hemicellulose content in the biomass respectively.

[0083] Each sample was repeated three times and the average value was taken.

[0084] The components in the corn straw / residue used in the examples of the present invention measured by the above detection method are shown in the following table:

[0085]

[0086]

[0087] Result: As Figure 1 shown in, the binary deep eutectic solvents prepared in Examples 1-15 can efficiently dissolve different industrial lignins under mild conditions. The solubility of enzymatic lignin, alkali lignin and kraft lignin reached 78.8 wt%, 45.5 wt% and 58.4 wt% respectively. Among them, the structure of enzymatic lignin is closest to the natural lignin in forest biomass. The zinc chloride-butylene glycol deep eutectic solvent with the best solubility of enzymatic lignin was selected as the solvent for extracting lignin from corn straw.

[0088] The results of lignin removal rate, solid yield and retention rate of cellulose and hemicellulose obtained by the above determination and calculation of Example 16. Experimental data prove that the binary deep eutectic solvent of the present invention significantly removes hemicellulose, effectively retains cellulose at the same time, and efficiently extracts lignin, providing technical support for the high-value utilization of biomass resources and having broad industrial application potential.

[0089] As Figure 2 shown, with the help of 2D HSQC NMR technology, the structures of the lignin extracted by the deep eutectic solvent and the lignin hydrolyzed by cellulase (CEL, using corn straw as raw material) were analyzed. ZnCl 2 -butylene glycol extracted lignin disappeared the signal at δC / δH 71.9 / 4.86, while new signals appeared at δC / δH 82.5 / 4.65 (A′α) and 70.4 / 3.32 (A′-PDO). This signal is a polyol alkylated β-O-4 (β′-O-4) substructure formed by the grafting of butylene glycol. This result indicates that the structure of the lignin extracted by ZnCl 2 -butylene glycol is more complete, which is conducive to the efficient extraction of lignin. Compared with CEL (50.5 / 100Ar), ZnCl 2The β-O-4 content (30.6 / 100Ar) of lignin extracted by 1,4-butanediol slightly decreased, accompanied by a slight condensation signal of S structural units in the aromatic region, indicating that β-O-4 cleavage cannot be avoided at 110°C, but its condensation degree is significantly lower than the pretreatment results of traditional deep eutectic solvents.

[0090] In summary, the pretreatment of Lewis acid-diol binary deep eutectic solvents effectively dissociates lignin and retains the β-O-4 structure of lignin. The binary deep eutectic solvents described in the present invention show significant cost-effectiveness in the mild and efficient extraction of lignin and have the potential for large-scale industrial applications. The lignin extracted by the method of the present invention has a high yield and good structural integrity, laying a foundation for the subsequent high-value utilization of lignin. For example, it can be used in the hydrogenolysis reaction of lignin to produce aromatic compounds or for the synthesis of new materials, providing strong support for the whole-chain development of biomass resources.

[0091] Example 17:

[0092] The difference between this example and Example 1 is that the molar ratio of hydrogen bond acceptor to hydrogen bond donor is 2:3, and the rest is the same as Example 1.

[0093] Example 18:

[0094] The difference between this example and Example 1 is that the molar ratio of hydrogen bond acceptor to hydrogen bond donor is 0.5:1.2, and the rest is the same as Example 1.

[0095] The above examples are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing examples, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various examples of the present invention.

Claims

1. A Lewis acid-diol deep eutectic solvent, characterized in that: The low eutectic solvent includes a hydrogen bond acceptor and a hydrogen bond donor; the hydrogen bond acceptor is a Lewis acid; the hydrogen bond donor includes one or any several of ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol.

2. The Lewis acid-diol deep eutectic solvent according to claim 1, characterized in that The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is (0.5-2):(1.2-3); the Lewis acid includes any one or more of FeCl3, ZnCl2, and SnCl2.

3. The method for preparing the Lewis acid-diol deep eutectic solvent according to claim 1, characterized in that: The hydrogen bond donor and the hydrogen bond acceptor are mixed and stirred in a water bath at 70-90° C. for 1-2 hours until a uniform transparent solution is obtained, and then dried in a forced air drying oven at 50-70° C. for 10-15 hours.

4. Use of the Lewis acid-diol deep eutectic solvent according to any one of claims 1 to 3 in dissolving industrial lignin.

5. The use according to claim 4, characterized in that: The industrial lignin includes any one of enzymatic lignin, alkali lignin and kraft lignin.

6. A method for efficiently extracting structurally intact lignin, characterized in that: The pretreated lignocellulose is mixed with the Lewis acid-diol low eutectic solvent according to any one of claims 1 to 3 and heated, and then anhydrous ethanol is added for suction filtration, and a hydrochloric acid aqueous solution is added to the filtrate, and the filtrate is allowed to stand for separation, and the obtained drying precipitate is the extracted lignin.

7. The method according to claim 6, characterized in that The mass ratio of lignocellulose to Lewis acid-diol low eutectic solvent is 1:5 to 1:

50.

8. The method according to claim 6, characterized in that The heating temperature is 60-110°C and the heating time is 0.5-3h.

9. The method according to claim 6, characterized in that The concentration of the added aqueous hydrochloric acid solution was 3 M, and the volume was 10 times that of the filtrate.

10. The method according to claim 6, characterized in that The wood cellulose is corn stalk powder.

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