A method for co-production of high quality lignin and fermentable sugars from lignocellulose by glyoxylic acid assisted deep eutectic solvent pretreatment

Abstract

The application discloses a method for co-producing high-quality lignin and fermentable sugar by glyoxylic acid-assisted eutectic solvent pretreatment of wood fiber, and belongs to the technical field of clean separation and high-value utilization of wood fiber raw materials. In the application, a glyoxylic acid-assisted acidic eutectic solvent system is used to pretreat wood fiber raw materials, the pretreatment liquid is subjected to rotary evaporation, and proper deionized water is added to precipitate lignin, which is subjected to water washing and freeze drying to obtain recovered lignin. Compared with a system without the addition of glyoxylic acid, the application can significantly improve the dissociation effect of the acidic eutectic solvent on the wood fiber raw materials at a lower glyoxylic acid addition amount, greatly improves the lignin removal rate and the quality of the lignin, and the introduction of glyoxylic acid helps to retain the carbohydrate retention rate of the wood fiber raw materials, and more than 90% of the carbohydrates are converted into corresponding monosaccharides through subsequent enzymatic saccharification.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of clean separation and high-value utilization of lignocellulosic raw materials, and more particularly relates to a method for cogeneration of high-quality lignin and fermentable sugar by glyoxylic acid-assisted deep eutectic solvent pretreatment of lignocellulosic materials. BACKGROUND

[0002] The energy crisis of fossil energy and the environmental problems caused by its carbon emissions have become important factors restricting human development. The proposal of carbon peak and carbon neutrality forces people to seek clean and renewable carbon sources. Lignocellulosic biomass is considered an ideal choice for achieving carbon balance by synthesizing basic constituent ingredients, cellulose, hemicellulose and lignin, from atmospheric carbon dioxide. Among them, lignin and hemicellulose act as a binder and reinforcing agent between cellulose, and their complex structure seriously hinders the dissociation and utilization of wood fiber raw materials by chemicals, enzymes and microorganisms, etc. Pretreatment can effectively break down the anti-depolymerization barrier (lignin) in lignocellulosic biomass, and achieve effective utilization of carbohydrates (cellulose and hemicellulose) in lignocellulosic raw materials.

[0003] In recent years, ionic liquid dissociation of lignocellulosic raw materials has been widely reported and is considered a new type of solvent that can replace traditional pretreatment solvents, but the problems of high toxicity, high cost and poor biodegradability limit its application in lignocellulosic raw material pretreatment. Compared with ionic liquids, deep eutectic solvents have the advantages of low cost, non-toxicity, easy biodegradability and easy preparation, and are considered the most potential solvent to replace traditional solvents and achieve green refining of lignocellulosic raw materials. Deep eutectic solvents (DES) are composed of hydrogen bond donors and hydrogen bond acceptors. Among them, deep eutectic solvents based on organic acids are the most widely reported, such as choline chloride / oxalic acid and choline chloride / p-toluenesulfonic acid systems, which can achieve effective removal of hemicellulose in lignocellulosic raw materials, but such systems generally have the contradictory phenomenon that low-temperature short-time pretreatment is difficult to achieve dissociation in lignocellulosic raw materials, while high-temperature long-time pretreatment causes severe lignin condensation, hemicellulose degradation and isomerization into pseudo-lignin, which is difficult to remove.

[0004] To solve this problem, relevant research introduces organic solvents such as polyhydric alcohols such as ethylene glycol, glycerol and butanediol into the system to inhibit the polycondensation reaction of lignin at high temperature, which can effectively improve the lignin removal rate and enzymatic hydrolysis yield. However, the amount of organic acid is usually large, and there is a serious esterification reaction between organic acid and polyhydric alcohol, especially under high temperature and long time pretreatment, which leads to the failure of part of the eutectic solvent, reduces the pretreatment effect and seriously limits its recycling. Glyoxylic acid assisted organic solvent pretreatment has been reported, and glyoxylic acid can undergo acetal reaction with lignin during pretreatment to inhibit excessive degradation and polycondensation of lignin, but its synergistic effect with eutectic solvent has never been reported. Therefore, the present application aims to develop an organic acid-based eutectic solvent protective additive-glyoxylic acid, which can greatly improve the effect of eutectic solvent on lignocellulosic raw materials and inhibit lignin polycondensation with a small amount of addition, and obtain high-quality lignin and greatly improve the enzymatic conversion rate of pretreated substrate. SUMMARY

[0005] In view of the above problems existing in the prior art, the technical problem to be solved by the present application is to provide a method for co-producing high-quality lignin and fermentable sugar by glyoxylic acid assisted eutectic solvent pretreatment of lignocellulose, which has the characteristics of simple process, green, safe and environmentally friendly, can significantly increase the lignin removal rate and reduce the polycondensation reaction.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0007] A method for co-producing high-quality lignin and fermentable sugar by glyoxylic acid assisted eutectic solvent pretreatment of lignocellulose, which adopts glyoxylic acid assisted eutectic solvent system to pretreat lignocellulosic raw materials, obtains high-quality lignin and pretreated materials, and performs enzymatic hydrolysis on the pretreated materials to obtain fermentable sugar; wherein the purity of the high-quality lignin is not less than 90%, the relative content of beta-O-4 aryl ether bond is not less than 15 / 100Ar, and the glucanase hydrolysis yield of the fermentable sugar is not less than 90%.

[0008] The glyoxylic acid assisted eutectic solvent system is a homogeneous and transparent liquid composed of a hydrogen bond acceptor, a hydrogen bond donor and an acidic additive; wherein the hydrogen bond acceptor is selected from any one of choline chloride, benzyltrimethylammonium chloride and betaine; the hydrogen bond donor is selected from any one of lactic acid and oxalic acid; and the acidic additive is selected from glyoxylic acid or its hydrate; as a preferred, the hydrogen bond acceptor is choline chloride; the hydrogen bond donor is selected from any one of lactic acid and oxalic acid; and the acidic additive is glyoxylic acid.

[0009] As a preferred, in the eutectic solvent, the molar ratio of hydrogen bond acceptor and hydrogen bond donor is 1:1-2, and the effective amount of glyoxylic acid accounts for 0.5%-8% of the mass of the eutectic solvent.

[0010] As preferred, the pretreatment temperature is 110-140℃, and the pretreatment time is 30-120 min.

[0011] As preferred, in the eutectic solvent, the molar ratio of hydrogen bond acceptor and hydrogen bond donor is 1:1-2, and the effective addition amount of glyoxylic acid accounts for 1%-6% of the mass of the eutectic solvent.

[0012] As preferred, in the eutectic solvent, the molar ratio of hydrogen bond acceptor and hydrogen bond donor is 1:2, and the effective addition amount of glyoxylic acid accounts for 2% of the mass of the eutectic solvent.

[0013] The method for preparing high-quality lignin and fermentable sugar by pretreating lignocellulose with glyoxylic acid-assisted eutectic solvent comprises the following steps:

[0014] 1) mixing hydrogen bond acceptor and hydrogen bond donor in a molar ratio of 1:1-2 to obtain a eutectic solvent, then mixing the eutectic solvent with glyoxylic acid, wherein the effective addition amount of glyoxylic acid accounts for 0.5%-8% of the mass of the eutectic solvent, heating and stirring, heating temperature is 70-110℃, and heating time is 30-120 min, to obtain a glyoxylic acid-assisted eutectic solvent system;

[0015] 2) mixing the glyoxylic acid-assisted eutectic solvent system prepared above with lignocellulose raw material in a mass ratio of 4-20:1, then performing heating pretreatment, pretreatment temperature is 110-140℃, and pretreatment time is 30-120 min, after the reaction is completed, adding an ethanol solution with a mass fraction of 10%-70% to separate the components, the addition amount of ethanol is 4-10 times the volume of the glyoxylic acid-assisted eutectic solvent system, after sufficient stirring, filtering to separate the solid and liquid, to obtain pretreated material and pretreated liquid; the lignocellulose raw material is moso bamboo that has been simply treated by a double-screw extruder, the moso bamboo contains 43.2% cellulose, 20.3% hemicellulose and 25.0% lignin, and the aryl ether bond content in the lignin is 59.2 / 100Ar;

[0016] 3) performing rotary evaporation on the pretreated liquid, adding appropriate deionized water to precipitate lignin, after water washing, performing freeze drying to obtain recovered lignin; after washing the pretreated material to neutral, balancing the moisture content, performing enzymatic hydrolysis to obtain fermentable sugar.

[0017] As preferred, in step 1), the heating temperature is 80℃, and the heating time is 60 min.

[0018] As preferred, in step 2), the mass ratio of the prepared glyoxylic acid-assisted eutectic solvent system to lignocellulose raw material is 4-10:1.

[0019] Preferably, in step 2), the pretreatment temperature is 120-140℃, and the pretreatment time is 60-120min.

[0020] Preferably, in step 2), after the reaction is completed, a 50% ethanol solution is added to separate the components, and the amount of addition is 5 times the volume of the glyoxylic acid-assisted eutectic solvent system.

[0021] Compared with the prior art, the present application has the following advantages:

[0022] 1) Compared with the system without adding glyoxylic acid, the present application can significantly improve the dissociation effect of the acidic eutectic solvent on the lignocellulosic raw material at a lower amount of glyoxylic acid addition, and greatly improve the lignin removal rate in the lignocellulosic raw material. A part of the lignin can be recovered by a simple method, and is rich in β-O-4 bonds and has high quality. This pretreatment helps to retain most of the carbohydrates in the lignocellulosic raw material, which can be converted to corresponding monosaccharides at a rate of nearly 100% after subsequent enzymatic saccharification;

[0023] 2) Under the same pretreatment conditions, the lignin removal rate of the choline chloride / oxalic acid system without adding glyoxylic acid is significantly lower than that of the glyoxylic acid-assisted system of the present application, and the polycondensation reaction of lignin is serious and the quality is low. After adding glyoxylic acid, the lignin removal is significantly increased, and more than half of the β-O-4 bonds in the lignin are retained, and the polycondensation reaction is obviously reduced;

[0024] 3) Compared with the prior art, the present application uses glyoxylic acid as a protective additive to synergize with the eutectic solvent, which can greatly improve the effect of the eutectic solvent on dissociating the lignocellulosic raw material at a small amount of addition, reduce the polycondensation reaction of lignin, and make a large amount of lignin removed and obtain high-quality lignin, while improving the enzymatic hydrolysis yield of carbohydrates to realize the high-value utilization of each component;

[0025] 4) In the pretreatment process of the existing eutectic solvent pretreatment system such as choline chloride / lactic acid, choline chloride / oxalic acid, etc., a large number of aryl ether bonds in lignin are broken, resulting in serious fragmentation of lignin. The fragmented lignin will re-polycondense. However, after adding glyoxylic acid in the present system, the formation of hemiacetal structure can effectively inhibit the breakage of aryl ether bonds in lignin and the occurrence of polycondensation reaction, and high-quality lignin is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The figure is the pretreatment material yield, glucan recovery rate, and lignin and xylan removal rate of Example 1;

[0027] Figure 2 The figure is the purity of the recovered lignin after pretreatment of Example 2;

[0028] Figure 3Figure 2 is a two-dimensional NMR spectrum of the recovered lignin from Example 1, 2, 3, 4; wherein enzymatic groundwood lignin (CEL) is used as a comparison;

[0029] Figure 4 Figure 6 is a plot of lignin reactivity in conventional deep eutectic solvent systems and the protective mechanism of glyoxylic acid addition to lignin from Example 2;

[0030] Figure 5 Figure 7 is a plot of pretreatment material yield, glucan recovery, and xylan, lignin removal from Example 2;

[0031] Figure 6 Figure 8 is a plot of pretreatment material glucan enzymatic hydrolysis yield from Example 2;

[0032] Figure 7 Figure 9 is a plot of recovered lignin purity from Example 2;

[0033] Figure 8 Figure 10 is a plot of pretreatment material glucan recovery and xylan, lignin removal from Example 3;

[0034] Figure 9 Figure 11 is a plot of recovered lignin purity from Example 3;

[0035] Figure 10 Figure 12 is a plot of pretreatment material glucan recovery and xylan, lignin removal from Example 4;

[0036] Figure 11 Figure 13 is a plot of recovered lignin purity from Example 4;

[0037] Figure 12 Figure 14 is a plot of recovered material glucan enzymatic hydrolysis yield from Example 4;

[0038] Figure 13 Figure 15 is a plot of pretreatment material glucan recovery and xylan, lignin removal from Comparative Example 1;

[0039] Figure 14 Figure 16 is a plot of pretreatment material glucan enzymatic hydrolysis yield from Comparative Example 1;

[0040] Figure 15 Figure 17 is a plot of recovered lignin NMR spectrum from Comparative Example 1; wherein enzymatic groundwood lignin (CEL) is used as a comparison. DETAILED DESCRIPTION

[0041] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described below with specific examples. Unless otherwise specified, the technical means used in the following examples are all conventional means well known to those skilled in the art. If no specific conditions are indicated in the examples, the conventional conditions or the conditions recommended by the manufacturer are used. If no manufacturer of the reagent or instrument is indicated, it is a conventional product that can be obtained by commercial purchase.

[0042] The bamboo used in the following examples is bamboo that has been simply treated by a double-screw extruder. The bamboo contains 43.2% cellulose, 20.3% hemicellulose and 25.0% lignin, and the content of aryl ether bonds in the lignin is 59.2 / 100 Ar.

[0043] Test method

[0044] 1. Lignin removal rate

[0045] The lignin content before and after pretreatment of the material is determined by the standard method of the U.S. Renewable Energy Laboratory, and the lignin removal rate is the content of lignin in the raw material minus the residual lignin content.

[0046] 2. Lignin purity

[0047] The purity of lignin is determined by the standard method of the U.S. Renewable Energy Laboratory, wherein the impurities are mainly sugar substances, i.e. the sugar content in lignin is deducted to obtain the purity of lignin.

[0048] 3. Relative content of lignin β-O-4 aryl ether bonds

[0049] The determination of the content of lignin β-O-4 aryl ether bonds uses the integral method, i.e. the content of β-O-4 signal peak accounts for the entire phenylpropane structural unit.

[0050] 4. Enzymatic hydrolysis experiment

[0051] The enzymatic hydrolysis experiment is carried out in a 150 mL enzyme hydrolysis bottle. 2.5 g of absolute dry pretreated material is weighed into a 150 mL hydrolysis bottle, 1 mol / L acetic acid-sodium acetate buffer 2.5 mL is added to adjust the pH value of the system to 4.8, 25 FPU / g (based on the mass of glucan, the same below) of cellulase, 20 U / g (based on the mass of xylan, the same below) of 6-glucosidase and 120 U / g (based on the mass of xylan, the same below) of xylanase are added, and an appropriate amount of deionized water is added to make the volume of the reaction system 50 mL. The enzyme hydrolysis bottle is placed in a 50°C, 150 / min shaking bed for hydrolysis for 48 h. After the enzymatic hydrolysis is completed, the hydrolysis product is centrifuged at 10,000 rpm for 10 min, the supernatant is taken to determine the concentration of fermentable sugar (glucose) therein, and the enzymatic hydrolysis yield is calculated.

[0052] Example 1

[0053] 1) Choline chloride and oxalic acid were mixed at a molar ratio of 1:2, and then mixed with effective amounts of glyoxylic acid monohydrate accounting for 0.5%, 1%, 1.5%, 2%, 4%, 6% and 8% of the mass of the solvent, respectively, and stirred at 80°C until a uniform and transparent liquid was formed;

[0054] 2) The moso bamboo was mixed with the deep eutectic solvent at a mass ratio of 1:10, and reacted at 130°C for 60 min. After the pretreatment was completed, an ethanol solution (50wt%) with a volume of 5 times that of the pretreatment liquid was added, and stirred for 2 h;

[0055] 3) Solid-liquid separation was performed to obtain lignin-enriched pretreatment liquid and carbohydrate-enriched pretreatment material. The ethanol was removed by rotary evaporation, and an appropriate amount of deionized water was added to cause the lignin to precipitate;

[0056] 4) After the pretreatment material was washed to neutral with distilled water, the moisture content was balanced at 4°C, and then chemical component content determination and enzyme hydrolysis experiments were performed;

[0057] 5) The lignin was washed with water, freeze-dried, and subjected to two-dimensional nuclear magnetic characterization.

[0058] As shown in Figure 1 , after the choline chloride / oxalic acid deep eutectic solvent pretreatment alone, the material yield was 68.3%. Among them, the hemicellulose was completely removed, about 20% of the cellulose was degraded, but the lignin removal rate was only 5.4%, indicating that the system had poor lignin removal effect. With the increase of the amount of glyoxylic acid from 0.5wt% to 2wt%, the removal rate of hemicellulose was always 100%, the removal rate of lignin increased from 18.7% to 79.3%, and the recovery rate of cellulose increased from 82.7% without adding glyoxylic acid to 95.2%, indicating that the addition of glyoxylic acid promoted the removal of lignin and increased the retention rate of cellulose. With the continuous increase of the amount of glyoxylic acid, the material yield increased from 51.5% to 55.1%, the removal rate of hemicellulose decreased slightly, the removal rate of lignin decreased significantly to 58.5%, and the recovery rate of cellulose remained basically unchanged, indicating that the excessive addition of glyoxylic acid was not conducive to the deconstruction of lignocellulosic raw materials, i.e. a small amount of glyoxylic acid (2wt%) could achieve effective deconstruction of the deep eutectic solvent for moso bamboo.

[0059] As shown in Figure 2 , the purity of the lignin (L 130℃-2％ ) recovered after pretreatment could reach 93.5%. The two-dimensional nuclear magnetic spectrum is shown in Figure 3As shown, enzymatic hydrolysis of milled wood lignin (CEL) was used as a control. The content of β-0-4 in CEL was 59.2 / 100 Ar, and the content of β-β and β-5 was 4.1 and 5.4 / 100 Ar, respectively, and there was no polycondensation structure in S and G units in the aromatic region. After 2 wt% glyoxylic acid assisted DES pretreatment at 130°C, the content of β-0-4 aryl ether bond was 17.3 / 100 Ar, which was much higher than that of the choline chloride / oxalic acid system without the addition of glyoxylic acid (0 / 100 Ar), and the polycondensed S and G in lignin accounted for only 19.6 and 15.9 / 100 Ar, which was much lower than that of the system without the addition (28.87 / 100 Ar and 34.1 / 100 Ar), indicating that the addition of glyoxylic acid can effectively protect the β-0-4 linkages in lignin and inhibit the polycondensation reaction of lignin.

[0060] As shown in Figure 4 , in the conventional deep eutectic solvent, the hydrogen proton attacks the α position of the lignin side chain to form an unstable carbocation, which further undergoes polycondensation reaction with lignin fragments (Route 1). After the addition of glyoxylic acid, it can react with the α and β position hydroxyl groups in lignin to form hemiacetal structures, thereby effectively inhibiting the breakage of lignin aryl ether bonds and polycondensation reaction (Route 2), and obtaining high-quality lignin.

[0061] Example 2

[0062] 1) Choline chloride and oxalic acid were mixed at a molar ratio of 1:2, and then mixed with an effective amount of 2% glyoxylic acid monohydrate based on the mass of the solvent, stirred at 80°C until a uniform and transparent liquid was formed;

[0063] 2) Bamboo was mixed with glyoxylic acid assisted deep eutectic solvent at a mass ratio of 1:10 at 110-140°C for 60 min, and then 5 times the volume of ethanol solution (50 wt%) was added and stirred for 2 h after the pretreatment was completed;

[0064] 3) Solid-liquid separation was performed to obtain lignin-rich pretreatment liquid and carbohydrate-rich pretreated material. After removing ethanol by rotary evaporation, lignin was precipitated by adding an appropriate amount of deionized water, and then centrifuged and freeze-dried to obtain lignin;

[0065] 4) After washing with distilled water, the pretreated material was equilibrated at 4°C, and then the chemical component content was determined and enzyme hydrolysis experiment was performed;

[0066] 5) Lignin was washed with water and freeze-dried, and then two-dimensional nuclear magnetic resonance characterization was performed.

[0067] As shown in Figure 5As shown, with the increase of temperature, the material yield decreased from 59.2% (110℃) to 50.3% (140℃). Among them, the removal rate of hemicellulose increased from 72.2% (110℃) to 100% (140℃), and the removal rate of lignin increased from 67.3% (110℃) to 79.3% (130℃), and the removal rate of lignin decreased to 75.0% when the temperature continued to rise to 140℃. The cellulose recovery rate decreased slightly with the increase of temperature, from 98.3% (110℃) to 91.4% (140℃).

[0068] As shown in Figure 6 , the glucanase hydrolysis yield after pretreatment increased from 63.6% to 94.9% as the pretreatment temperature increased from 110℃ to 140℃, indicating that the acetic acid assisted choline chloride / oxalic acid system had a significant effect on the deconstruction of bamboo.

[0069] As shown in Figure 7 , the purity of recovered lignin under different pretreatment temperatures can reach 91.2%~94.6%, and the two-dimensional nuclear magnetic spectrum is shown in Figure 3 , and enzyme-milled wood lignin (CEL) is used as a comparison. The content of β-O-4 in CEL is 59.2 / 100Ar, and the content of β-β and β-5 is 4.1 and 5.4 / 100Ar respectively, and there is no polycondensation structure in the S and G units in the aromatic region. After pretreatment at 110℃ with acetic acid assisted DES, more than half of the β-O-4 aryl ether bond (30.1 / 100Ar) is retained, which is much higher than that of choline chloride / oxalic acid system without adding acetic acid (0 / 100Ar), and the polycondensed S and G in lignin only account for 17.6 and 10.9 / 100Ar, which is much lower than that without adding system, indicating that the addition of acetic acid can effectively protect the β-O-4 linkage in lignin and inhibit the polycondensation reaction of lignin.

[0070] Example 3

[0071] 1) Mix choline chloride and oxalic acid at a molar ratio of 1:1, then mix with an effective amount of 0.5%~8% of monohydrate glyoxylic acid based on the mass of the solvent, stir at 110℃ until a uniform and transparent liquid is formed;

[0072] 2) Mix bamboo with acetic acid assisted eutectic solvent at a mass ratio of 1:50 at 110℃ for 120min, then add 4 times the volume of ethanol solution (50wt%) after pretreatment, stir for 2h;

[0073] 3) Solid-liquid separation to obtain lignin-rich pretreatment liquid and carbohydrate-rich pretreatment material. After removing ethanol by rotary evaporation, add an appropriate amount of deionized water to precipitate lignin, centrifuge and freeze-dry to obtain lignin;

[0074] 4) After washing the material with distilled water to neutral, equilibrate the moisture at 4℃, then determine the chemical composition and conduct the enzymatic hydrolysis experiment;

[0075] 5) After washing with water, freeze-dry the lignin and conduct the 2D NMR characterization.

[0076] The clear and transparent liquid of the deep eutectic solvent with glyoxylic acid can be formed faster at higher preparation temperature, indicating that the increase of temperature can significantly shorten the preparation time of the deep eutectic solvent. As shown in Figure 8 As shown in Fig. 2, similar to the choline chloride / oxalic acid = 1:2, the recovery rate of glucan gradually increases from 90.9% (0.5wt%) to 98.7% (8wt%) with the increase of the amount of glyoxylic acid, indicating that the addition of glyoxylic acid can significantly inhibit the degradation of glucan and improve the recovery rate of glucan. At the same time, the removal rate of xylan decreases from 98.3% to 61.7%. The removal rate of lignin shows an increasing trend (20.4%~76.8%) and then a decreasing trend (76.8%~51.0%), indicating that the addition of a small amount of glyoxylic acid can effectively inhibit the polycondensation reaction of lignin and its deposition on the fiber, thereby improving the removal rate of lignin. However, the addition of too much glyoxylic acid can inhibit the dissociation effect of the deep eutectic solvent on the lignocellulosic raw material, resulting in a decrease in the removal rate of lignin. Compared with Example 1, the decrease in the pretreatment effect caused by the decrease in the amount of hydrogen donor can be compensated by appropriately extending the pretreatment time, so that the pretreatment still maintains a high removal rate of lignin and xylan.

[0077] As shown in Fig. 2, similar to the choline chloride / oxalic acid = 1:2, the recovery rate of glucan gradually increases from 90.9% (0.5wt%) to 98.7% (8wt%) with the increase of the amount of glyoxylic acid, indicating that the addition of glyoxylic acid can significantly inhibit the degradation of glucan and improve the recovery rate of glucan. At the same time, the removal rate of xylan decreases from 98.3% to 61.7%. The removal rate of lignin shows an increasing trend (20.4%~76.8%) and then a decreasing trend (76.8%~51.0%), indicating that the addition of a small amount of glyoxylic acid can effectively inhibit the polycondensation reaction of lignin and its deposition on the fiber, thereby improving the removal rate of lignin. However, the addition of too much glyoxylic acid can inhibit the dissociation effect of the deep eutectic solvent on the lignocellulosic raw material, resulting in a decrease in the removal rate of lignin. Compared with Example 1, the decrease in the pretreatment effect caused by the decrease in the amount of hydrogen donor can be compensated by appropriately extending the pretreatment time, so that the pretreatment still maintains a high removal rate of lignin and xylan. Figure 9 As shown in Fig. 2, similar to the choline chloride / oxalic acid = 1:2, the recovery rate of glucan gradually increases from 90.9% (0.5wt%) to 98.7% (8wt%) with the increase of the amount of glyoxylic acid, indicating that the addition of glyoxylic acid can significantly inhibit the degradation of glucan and improve the recovery rate of glucan. At the same time, the removal rate of xylan decreases from 98.3% to 61.7%. The removal rate of lignin shows an increasing trend (20.4%~76.8%) and then a decreasing trend (76.8%~51.0%), indicating that the addition of a small amount of glyoxylic acid can effectively inhibit the polycondensation reaction of lignin and its deposition on the fiber, thereby improving the removal rate of lignin. However, the addition of too much glyoxylic acid can inhibit the dissociation effect of the deep eutectic solvent on the lignocellulosic raw material, resulting in a decrease in the removal rate of lignin. Compared with Example 1, the decrease in the pretreatment effect caused by the decrease in the amount of hydrogen donor can be compensated by appropriately extending the pretreatment time, so that the pretreatment still maintains a high removal rate of lignin and xylan. Figure 3 As shown in Fig. 2, similar to the choline chloride / oxalic acid = 1:2, the recovery rate of glucan gradually increases from 90.9% (0.5wt%) to 98.7% (8wt%) with the increase of the amount of glyoxylic acid, indicating that the addition of glyoxylic acid can significantly inhibit the degradation of glucan and improve the recovery rate of glucan. At the same time, the removal rate of xylan decreases from 98.3% to 61.7%. The removal rate of lignin shows an increasing trend (20.4%~76.8%) and then a decreasing trend (76.8%~51.0%), indicating that the addition of a small amount of glyoxylic acid can effectively inhibit the polycondensation reaction of lignin and its deposition on the fiber, thereby improving the removal rate of lignin. However, the addition of too much glyoxylic acid can inhibit the dissociation effect of the deep eutectic solvent on the lignocellulosic raw material, resulting in a decrease in the removal rate of lignin. Compared with Example 1, the decrease in the pretreatment effect caused by the decrease in the amount of hydrogen donor can be compensated by appropriately extending the pretreatment time, so that the pretreatment still maintains a high removal rate of lignin and xylan.

[0078] Example 4

[0079] 1) Mix choline chloride and oxalic acid at a molar ratio of 1:1, then mix with an effective amount of 2% of glyoxylic acid monohydrate based on the mass of the solvent, and stir at 70℃ until a uniform and transparent liquid is formed;

[0080] 2) Mix the bamboo with the deep eutectic solvent assisted by glyoxylic acid at a mass ratio of 1:4 at 110~140℃ for 30min, then add 10 times the volume of ethanol solution (50wt%) after the pretreatment, and stir for 2h;

[0081] 3) Solid-liquid separation to obtain a pretreatment liquid rich in lignin and a pretreatment material rich in carbohydrates. After removing ethanol by rotary evaporation, an appropriate amount of deionized water is added to precipitate lignin. The lignin is then separated by centrifugation and freeze-dried.

[0082] 4) The pretreated material was washed with distilled water until neutral and then equilibrated at 4°C. After that, the chemical composition content was determined and an enzyme hydrolysis experiment was performed.

[0083] The high-quality lignin obtained in Examples 1-4 were measured, and the following test data were obtained. The specific data are shown in Table 1.

[0084] Table 1 High-quality lignin obtained in Examples 1-4

[0085]

[0086]

[0087] like Figure 10 As shown, the dextran recovery rate after pretreatment was 95.8%–99.7%, decreasing slightly with increasing temperature, indicating that the system can recover almost all dextran. The xylan removal rate increased from 65.2% (110℃) to 94.6% (140℃) with increasing temperature. Similar to Example 2, increasing temperature facilitates xylan degradation and removal, reaching a maximum of 94.6% at 140℃. The lignin removal rate increased from 56.2% (110℃) to 75.0% (140℃), close to the highest value of 79.3% in Example 2. These results indicate that appropriately reducing the pretreatment time and hydrogen donor dosage has little effect on the removal rates of lignin and xylan. Figure 11 As shown, the purity of the recovered lignin was 90.2%–93.4%. The hydrolysis yield of dextranase in the pretreated material increased from 59.1% to 92.1%. Figure 12 This indicates that efficient conversion of dextran in materials can be achieved under these conditions.

[0088] Comparative Example 1

[0089] The pretreatment efficiency and lignin removal rate in the blank system without glyoxylic acid were determined using the following steps:

[0090] 1) Mix choline chloride and oxalic acid in a molar ratio of 1:2 and stir at 80°C until a homogeneous, transparent liquid is formed;

[0091] 2) Bamboo was mixed with deep eutectic solvent at a mass ratio of 1:10, and reacted at 110, 120, 130 and 140℃ for 60 min, respectively. After the pretreatment, 5 times liquid volume of ethanol / water solution (50% ethanol volume fraction) was added and stirred for 1 h;

[0092] 3) Solid-liquid separation was performed to obtain lignin-rich pretreatment liquid and carbohydrate-rich pretreatment material. The pretreatment liquid was rotary evaporated at 70℃ to remove ethanol, and appropriate distilled water was added to precipitate lignin;

[0093] 4) After the pretreatment material was washed to neutral with distilled water, the moisture content was balanced at 4℃, and then the chemical component content was determined and enzyme hydrolysis experiment was performed;

[0094] 5) Lignin was washed with water, freeze-dried and subjected to two-dimensional nuclear magnetic characterization.

[0095] As shown in Figure 13 , the temperature was increased from 100℃ to 110℃, the material yield was decreased from 67.4% to 64.2%, the hemicellulose removal rate was increased from 87.9% to 97.6%, and the lignin removal rate was increased from 35.2% to 39.7%, indicating that appropriate increase of pretreatment temperature helps to dissociate bamboo. Continue to increase the pretreatment temperature, the material yield gradually increases to 71.9%, indicating that continue to increase the temperature is not conducive to the deconstruction of bamboo. Among them, the lignin removal rate is decreased from 39.72% (110℃) to 9.9% (130℃), and the cellulose recovery rate is decreased to 84.6%, indicating that the deconstruction of bamboo is inhibited at high temperature. The result is because the lignin has undergone condensation reaction at high temperature, and the degraded hemicellulose is partially converted into pseudolignin, thereby limiting the deconstruction of lignocellulosic raw materials.

[0096] As shown in Figure 14 , similar to the lignin removal rate, the glucanase hydrolysis yield after pretreatment first increases and then decreases, from 22.8% (100℃) to 50.5% (110℃), and then decreases to 10.4% (130℃), thus it is difficult to realize the deconstruction and utilization of lignocellulosic raw materials in the choline chloride / oxalic acid system without glyoxylic acid assistance.

[0097] As shown in Figure 15 , compared with CEL, the β-O-4, β-β, β-5 and other linkages in the recovered lignin (L OADES-110℃ ) after pretreatment at 110℃ are completely broken, and the condensation of G and S accounts for 34.1 and 28.9 / 100 Ar, respectively, which is much higher than that in the glyoxylic acid assisted DES system, indicating that the lignin in the choline chloride / oxalic acid system is severely broken, and the fragmented lignin is severely condensed. The addition of glyoxylic acid in the present application protects the β-O-4 bond in lignin and inhibits its condensation reaction, which is a new type of clean system for obtaining high-quality lignin.

[0098] The above description is only the preferred embodiment of the present application, it should be pointed out that for those skilled in the art, without departing from the principles of the present application, can make several improvements and refinements, these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method of glyoxylate-assisted deep eutectic solvent pretreatment of lignocellulose for co-production of high-quality lignin and fermentable sugars, characterized in that, The application discloses a method for preparing high-quality lignin and fermentable sugar from lignocellulosic materials by using glyoxylic acid assisted deep eutectic solvent system, and the method comprises the following steps: 1) mixing choline chloride and oxalic acid in a molar ratio of 1:1-2 to obtain a deep eutectic solvent, and then mixing the deep eutectic solvent with glyoxylic acid, wherein the effective amount of glyoxylic acid accounts for 2% of the mass of the deep eutectic solvent, and heating and stirring are carried out at a heating temperature of 70-110 DEG C for a heating time of 30-120 min to obtain a glyoxylic acid assisted deep eutectic solvent system; 2) mixing the glyoxylic acid assisted deep eutectic solvent system prepared in the step 1) with lignocellulosic materials in a mass ratio of 4-20:1, and then carrying out heating pretreatment at a pretreatment temperature of 110-140 DEG C for a pretreatment time of 30-120 min, and then adding an ethanol solution with a mass fraction of 10%-70% to separate the components after the reaction is completed, wherein the amount of the ethanol added accounts for 4-10 times of the volume of the glyoxylic acid assisted deep eutectic solvent system, and then carrying out sufficient stirring, and then filtering to separate the solid and the liquid to obtain pretreated materials and a pretreated liquid; 3) carrying out rotary evaporation on the pretreated liquid, and then adding appropriate deionized water to make lignin precipitate, and then carrying out water washing and freeze drying to obtain recovered high-quality lignin, and then washing the pretreated materials to neutral, and then balancing the moisture, and then carrying out enzyme hydrolysis to obtain fermentable sugar. In the step 1), the heating temperature is 80 DEG C, and the heating time is 60 min.

2. The method of glyoxylate-assisted deep eutectic solvent pretreatment of lignocellulose for co-production of high quality lignin and fermentable sugars according to claim 1, characterized in that, In the step 2), the mass ratio of the prepared glyoxylic acid assisted deep eutectic solvent system to the lignocellulosic materials is 4-10:

1.

3. The method of glyoxylate-assisted deep eutectic solvent pretreatment of lignocellulose for co-production of high quality lignin and fermentable sugars according to claim 1, characterized in that, In the step 2), the pretreatment temperature is 120-140 DEG C, and the pretreatment time is 60-120 min.

4. The method of glyoxylate-assisted deep eutectic solvent pretreatment of lignocellulose for co-production of high quality lignin and fermentable sugars according to claim 1, characterized in that, In the step 2), after the reaction is completed, an ethanol solution with a mass fraction of 50% is added to separate the components, and the amount of the added ethanol accounts for 5 times of the volume of the glyoxylic acid assisted deep eutectic solvent system.

5. The method of glyoxylate-assisted deep eutectic solvent pretreatment of lignocellulose for co-production of high quality lignin and fermentable sugars according to claim 1, characterized in that, ​

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