Method for pretreating lignocellulose through cooperation of ternary eutectic solvent and dilute alkali

Abstract

The invention discloses a method for pretreating lignocellulose by using a ternary eutectic solvent in cooperation with dilute alkali, and belongs to the technical field of biomass pretreatment, the method comprises the following steps: carrying out alkali treatment on a wood-based biomass raw material by using a dilute alkali solution, and obtaining biomass residues subjected to alkali treatment after the reaction is completed; mixing the biomass residues subjected to alkali treatment with an acidic ternary eutectic solvent, and carrying out DES treatment, so as to obtain biomass residues subjected to combined pretreatment after the reaction is completed; the acidic ternary eutectic solvent comprises a hydrogen bond acceptor and a hydrogen bond donor, the hydrogen bond acceptor is choline chloride, and the hydrogen bond donor is ethylene glycol and oxalic acid. According to the combined treatment method disclosed by the invention, most cellulose is selectively reserved while lignin and part of hemicellulose which are crosslinked outside the lignocellulose are efficiently removed, the crystal structure of the cellulose is not obviously changed, meanwhile, the treatment condition is mild, the method is safe and green, and the production and recovery cost is low.

Description

Technical Field

[0001] The invention relates to the technical field of biomass pretreatment, and in particular to a method for pretreating lignocellulose using a ternary low eutectic solvent in coordination with a dilute alkali. Background Art

[0002] In the past few decades, fossil fuels have widely promoted the development of various fields of society. However, its combustion emits a large amount of carbon dioxide, which intensifies the greenhouse effect and causes many environmental problems. In addition, it is non-renewable, and the energy supply faces severe challenges. Therefore, it is urgent to seek alternative energy sources, and renewable biomass resources have attracted much attention. Wood-based biomass, whose main components are cellulose, hemicellulose and lignin, is considered to be the most abundant renewable resource on the earth. It is estimated that the plant biomass produced by photosynthesis each year is about 17 billion to 20 billion tons. Its cellulose and hemicellulose can be degraded into reducing sugars such as glucose and xylose by enzymatic hydrolysis, respectively, and then converted into a variety of platform compounds by microorganisms. However, the structural characteristics of wood-based biomass hinder its efficient utilization. Microscopically, its cellulose is wrapped by hemicellulose and lignin, which makes it difficult for cellulose to fully interact with cellulase and inhibits enzymatic hydrolysis. In addition, surface cross-linked lignin has high durability, stability, heterogeneity and stubbornness. During the bioconversion process, it will also reduce the activity of cellulase through non-productive binding. Secondly, hemicellulose is cross-linked to the surface of cellulose, which improves its strength while hindering the contact between cellulase and cellulose, reducing the efficiency of enzymatic hydrolysis. Therefore, pretreatment of lignocellulose to overcome its inherent resistance and thus improve the efficiency of enzymatic hydrolysis is a key step in achieving high-value conversion of wood-based biomass into fermentation substrate sugars.

[0003] At present, there are many physical, chemical, biological and other pretreatment methods for lignocellulose in the prior art. However, although chemical pretreatment such as concentrated acid and concentrated alkali can promote enzymatic hydrolysis and increase the yield of fermentable sugar, its effect needs to be improved on the one hand, and there are still problems such as corrosion of equipment and difficult treatment of waste liquid pollution on the other hand. In recent years, deep eutectic solvent (DES) has attracted much research attention. It is formed by intermolecular hydrogen bonds between hydrogen bond acceptors and donors. It is a unique green solvent with a melting point lower than the melting point of the component. Because of its advantages of wide raw material sources, simple preparation method, low toxicity, biodegradability and the ability to selectively dissolve non-cellulose components in wood-based biomass, it can better adapt to the pretreatment needs of various wood-based biomass. However, studies have shown that the effect of a single DES on the pretreatment of lignocellulose is limited. The cross-linked lignin of wood-based biomass includes acid-soluble lignin and acid-insoluble lignin. A single acid-base DES is difficult to remove most of the lignin, but the use of two DESs with different acid-bases increases the difficulty and cost of treatment.

[0004] Therefore, there is an urgent need for a method for treating lignocellulose with good pretreatment effect, safety, greenness, and low production and recycling costs. Summary of the invention

[0005] The purpose of the present application is to provide a method for pretreating lignocellulose by using a ternary low eutectic solvent in conjunction with a dilute alkali, wherein the lignocellulose is initially pretreated under alkaline conditions and then further pretreated under acidic ternary DES conditions, which greatly removes biological resistance and retains most of the cellulose, thereby increasing the yield of reducing sugars after enzymatic hydrolysis.

[0006] The embodiments of the present application can be implemented through the following technical solutions:

[0007] A method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali comprises the following steps:

[0008] S1) treating the wood-based biomass raw material with a dilute alkali solution, and obtaining an alkali-treated biomass residue after the reaction is completed;

[0009] S2) mixing the alkali-treated biomass residue with an acidic ternary low eutectic solvent to perform DES treatment, and obtaining a combined pretreated biomass residue after the reaction is completed;

[0010] The acidic ternary deep eutectic solvent comprises a hydrogen bond acceptor and a hydrogen bond donor, wherein the hydrogen bond acceptor is choline chloride, and the hydrogen bond donors are ethylene glycol and oxalic acid.

[0011] Preferably, the particle size of the wood-based biomass raw material in S1 is 40 to 80 meshes.

[0012] Preferably, the wood-based biomass raw material in S1 is sunflower straw.

[0013] Preferably, the dilute alkaline solution in S1 is a calcium hydroxide solution with a concentration of 1 to 20 g / L.

[0014] Preferably, the reaction temperature of the alkali treatment in S1 is 70-95° C., and the reaction time is 1-6 h.

[0015] Preferably, the molar ratio of choline chloride, ethylene glycol and oxalic acid in the acidic ternary deep eutectic solvent in S2 is 1:2:(0.5-2.5).

[0016] Preferably, the reaction temperature of the DES treatment in S2 is 110-140° C., and the reaction time is 20-60 min.

[0017] Preferably, the DES treatment in S2 is carried out under stirring conditions at a stirring speed of 100 to 200 rpm.

[0018] Preferably, the solid-liquid ratio of the wood-based biomass raw material to the dilute alkali solution in S1 is 1 g: (15-20) mL, and / or the solid-liquid ratio of the alkali-treated biomass residue to the acidic ternary low eutectic solvent in S2 is 1 g: (15-20) mL.

[0019] Preferably, after the reaction in S1 and / or S2 is completed, vacuum filtration, washing and drying are further performed.

[0020] The embodiment of the present application provides a method for pretreating lignocellulose with a ternary deep eutectic solvent and a dilute alkali, which has at least the following characteristics:

[0021] Beneficial effects:

[0022] The present invention uses wood-based biomass as raw material, and treats it with a combined treatment of first dilute alkali treatment and then acidic ternary DES treatment, while efficiently removing the cross-linked lignin and part of the hemicellulose outside the wood cellulose, selectively retaining most of the cellulose, and at the same time, the crystalline structure of the cellulose does not change significantly. This method effectively removes the biological barrier that hinders enzymatic hydrolysis, greatly improving the subsequent enzymatic hydrolysis efficiency and reducing sugar yield. In addition, the present invention uses a new acidic ternary low eutectic solvent synthesized from choline chloride, ethylene glycol, and oxalic acid. The processing solvent has the advantages of low cost, green recyclability, and good biocompatibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The component composition and ideal reducing sugar yield of the sunflower straw wood-based biomass raw material used in the present invention;

[0024] Figure 2 The reducing sugar release and yield of sunflower straw wood-based biomass under different pretreatment methods after 72 hours of enzymatic hydrolysis;

[0025] Figure 3 The scanning electron microscope images are of the untreated sample D1 (a, e), the sample D2 (b, f) treated with calcium hydroxide, the sample D3 (c, g) treated with a ternary deep eutectic solvent, and the sample of Example 1 (d, h), wherein a, b, c, d are at the same magnification, and e, f, g, h are at the same magnification;

[0026] Figure 4 X-ray diffraction patterns and cellulose crystallinity of each sample after different pretreatment methods;

[0027] Figure 5 The amount of reducing sugar released during the enzymatic hydrolysis of the sample in Example 1 at different time periods and the average enzymatic hydrolysis rate per 12 hours of enzymatic hydrolysis; DETAILED DESCRIPTION

[0028] The specific implementation methods of the present invention are described in detail below through specific examples. The examples are only used to explain the present invention and are not used to limit the scope of the present invention. The raw materials used in the examples can be purchased commercially or prepared by conventional methods.

[0029] The present invention provides a pretreatment method for wood-based biomass (LB). As described in the background technology, LB is a macromolecular substance, and its externally cross-linked lignin has a large molecular weight and is very stubborn. Traditional pretreatment methods, such as acid-base pretreatment, require extremely high temperatures or require a long pretreatment time, which is more corrosive to the reaction container, has high energy costs, and is more strictly controlled. At the same time, the dissociated lignin is not ideal in the traditional inorganic pretreatment solvent, and is easily left in the recovered solid phase and affects the subsequent enzymatic hydrolysis effect. Although the DES pretreatment method can more effectively remove and dissolve the LB externally cross-linked lignin, while making the treatment conditions milder, shortening the pretreatment time, increasing the yield of sugars, and reducing the corrosion of the bioreactor container, a single DES pretreatment method also has its limitations. Since the LB externally cross-linked lignin includes acid-soluble lignin and acid-insoluble lignin, a single acid-base DES is difficult to remove most of the lignin, and the use of two different acid-base DESs increases the difficulty and cost of the treatment.

[0030] Therefore, the inventors provide a pretreatment method combining dilute alkali and ternary low eutectic solvent. By first treating LB with dilute alkali, since the outermost layer of LB is mainly composed of lignin, the main component of which is phenylpropane. The aromatic structure gives it extraordinary stability and resistance to oxidation caused by external factors. According to research, the thermal decomposition of lignin often occurs in the temperature range of 450°C to 700°C; however, the oxidation of benzene ring side chains and other structural units will also trigger the chain depolymerization reaction of lignin, resulting in the degradation of the lignin structure. Therefore, a milder method for removing lignin is to break the benzene ring side chains to depolymerize lignin. Natural lignin is mainly composed of acid-insoluble lignin, so OH as a nucleophilic reagent - The alkali solution can chemically react with the phenolic hydroxyl groups in lignin to break the ether bonds in lignin and degrade the lignin macromolecules into alkaline lignin with smaller molecular weight. In addition, the alkali solution can also destroy the ester bonds between lignin and hemicellulose in the lignin-carbohydrate complex (LCC), thereby enhancing the separation rate of lignin. At the same time, cellulose and hemicellulose are mainly bound together by hydrogen bonds and internal van der Waals forces. - The presence of further weakens the strength of these hydrogen bonds, causing swelling and reducing polymerization. Therefore, the initial alkali pretreatment can effectively enhance the removal of LB outer layer lignin and weaken the internal cross-linking between lignin and cellulose and hemicellulose.

[0031] Furthermore, since the inner layer of LB is composed of cellulose and hemicellulose cross-linked therewith, compared with cellulose, hemicellulose can be dissolved in alkaline solution and is more easily hydrolyzed by acid. Therefore, the present invention further performs acidic ternary DES treatment after alkali pretreatment, which can better achieve the hydrolysis of residual hemicellulose. In addition, the benzyl ether bond in LCC is formed between the phenolic hydroxyl group of lignin and the free hydroxyl group of hemicellulose, which is easily hydrolyzed by acid but resistant to alkali. The subsequent acidic DES can effectively dissociate this part of the ether bond. It can be seen that the acidic ternary DES pretreatment creates an acidic environment, which can effectively eliminate the residual hemicellulose and acid-soluble lignin after alkali treatment.

[0032] The present invention provides a method for pretreating lignocellulose using a ternary deep eutectic solvent in collaboration with a dilute alkali, which specifically comprises the following steps:

[0033] Preparation of wood-based biomass raw materials, dilute alkaline solution and ternary deep eutectic solvent:

[0034] Preferably, the raw material is sunflower straw; preferably, the sunflower straw raw material is pre-treated by washing, drying and crushing; preferably, the particle size of the sunflower straw is 40-80 mesh.

[0035] The solute alkali in the dilute alkali solution can be selected from one or more of sodium hydroxide, potassium hydroxide and calcium hydroxide; preferably, the dilute alkali solution is a calcium hydroxide solution; preferably, the concentration of the calcium hydroxide solution is 1 to 20 g / L, more preferably 5 to 17.5 g / L, and more preferably 17.5 g / L.

[0036] The ternary deep eutectic solvent is a mixed solvent of choline chloride (ChCl), ethylene glycol (EG) and oxalic acid (OA), wherein choline chloride is a hydrogen bond acceptor, and ethylene glycol and oxalic acid are hydrogen bond donors; preferably, the molar ratio of choline chloride, ethylene glycol and oxalic acid is 1:2:(0.5-2.5), more preferably 1:2:(1-2.5), and more preferably 1:2:2.

[0037] The specific preparation operation of the ternary low eutectic solvent of the present invention is: choline chloride, ethylene glycol and oxalic acid are mixed in a molar ratio of 1:2:(0.5-2.5), and synthesized at 70-90°C for 1-2h until a transparent uniform solution is formed. It is worth noting that the synthesis of the above-mentioned ternary low eutectic solvent can adopt a low eutectic solvent synthesis method well known to those skilled in the art, and is not specifically limited here.

[0038] Alkali treatment:

[0039] The wood-based biomass raw material is mixed with a dilute alkali solution at a solid-liquid ratio of 1g:(15-20)mL, preferably, the solid-liquid ratio is 1g:20mL; the reaction temperature is 70-95°C, preferably 80-95°C, and more preferably 85°C; the reaction time is 1-6h, preferably 2-4h, and more preferably 2h.

[0040] After the reaction is completed, the biomass residue treated with alkali is obtained by post-treatment such as filtration, washing, and drying. Preferably, the washing condition is to wash the solid residue with deionized water until it is neutral, and the drying condition is to dry at 50-60° C. for 24-48 hours.

[0041] Ternary eutectic solvent treatment:

[0042] The biomass residue obtained after alkali treatment is mixed with the ternary low eutectic solvent at a solid-liquid ratio of 1g:(15-20)mL, preferably, the solid-liquid ratio is 1g:20mL; the reaction temperature is 110-140°C, preferably 120°C; the reaction time is 20-60min, preferably 30min; preferably, the above reaction is carried out under stirring conditions, and the stirring speed is 100-200rpm, more preferably 150rpm.

[0043] After the reaction is completed, the combined treated lignocellulose residue is obtained by post-treatment such as filtration, washing, and drying for subsequent enzymatic hydrolysis reaction. Preferably, the filtration is vacuum filtration, the washing condition is washing the solid residue with deionized water until it is neutral, and the drying condition is drying at 50-60°C for 24-48h.

[0044] The specific process of the enzymatic hydrolysis reaction of the present invention is: the wood-based biomass residue is mixed with 1FPU-1.25FPU / mL composite cellulase diluted with citric acid-sodium citrate at pH=4.5-5.5 at a solid-liquid ratio of 1g:(15-20)mL, and then the enzymatic hydrolysis is carried out for 36-72h at a constant temperature shaker at 100-200rpm to release reducing sugars in the wood-based biomass, wherein the composite cellulase includes cellulase and hemicellulase. The present invention has no particular limitation on the source of the cellulase, for example, commercially available Cellic CTec3 composite cellulase can be used.

[0045] Determination method: The determination method of lignin, cellulose and hemicellulose in the samples before and after pretreatment refers to the analysis method of the US Renewable Energy Laboratory (NREL). The specific calculation formula is as follows:

[0046]

[0047] Solid recovery rate (%) = solid recovery × 100%

[0048]

[0049] Reducing sugar yield (mg / g) = reducing sugar release × solid recovery

[0050]

[0051]

[0052] Wherein: the reducing sugar release reflects the amount of reducing sugar that can be released after enzymatic hydrolysis of a unit mass of wood-based biomass residue after pretreatment (mg / g); the reducing sugar yield reflects the amount of reducing sugar that can be released after enzymatic hydrolysis of a unit mass of wood-based biomass residue recovered after pretreatment of wood-based biomass (mg / g); the ideal reducing sugar yield reflects the amount of reducing sugar that can be released after enzymatic hydrolysis of the recovered wood-based biomass residue under the premise that no loss of cellulose and hemicellulose occurs after pretreatment of wood-based biomass and the saccharification rate is 100% (mg / g); the average enzymatic hydrolysis rate is the reducing sugar release rate (mg / h) during the enzymatic hydrolysis process (36-72h) calculated with a time period of 12h; 1.11 is the conversion coefficient of cellulose to glucose, and 1.14 is the conversion coefficient of hemicellulose to xylose.

[0053] Example 1

[0054] (1) Sunflower straw was selected as the wood-based biomass raw material. The sunflower straw was cleaned, air-dried, cut into short pieces, and dried in an oven at 60°C for 24 h. The raw materials were further crushed by a pulverizer and passed through a 40-mesh sieve. The unretained raw materials were further passed through an 80-mesh sieve, and the retained raw material particles were retained. Finally, raw material particles with a particle size of 40-80 mesh were obtained, which were then transferred to an oven at 60°C for further drying for 24 h and sealed for later use.

[0055] (2) Preparation of calcium hydroxide solution and synthesis of ternary low eutectic solvent: Calcium hydroxide and deionized water were mixed thoroughly at a ratio of 17.5 g:1 L, sealed and set aside; choline chloride, ethylene glycol and oxalic acid were reacted in an oil bath at a molar ratio of 1 mol:2 mol:2 mol for 2 h at 80° C. and 150 rpm until a transparent uniform solution was formed, which was set aside.

[0056] (3) Preliminary pretreatment with calcium hydroxide: 1 g of the wood-based raw material obtained in step (1) was mixed with the alkali solution prepared in step (2) in a 50 mL beaker at a ratio of 1 g:20 mL. The mixture was placed in a water bath at 85° C. for 2 h to fully soak the wood-based raw material. The mixture was shaken every 10 minutes to ensure that the wood-based raw material was fully soaked.

[0057] (4) Washing and Drying: After the reaction is completed, the conical flask is taken out and placed in a room temperature environment to cool. The solid and liquid phases are separated by vacuum filtration. The solid wood-based residue is washed with deionized water several times until the washing liquid becomes neutral. The solid filter residue is then transferred to an oven and dried at 60°C for 24 hours.

[0058] (5) Pretreatment with ternary deep eutectic solvent: 0.5 g of the wood-based residue obtained in step (4) and 10 g of the deep eutectic solvent synthesized in step (2) were mixed in a 50 mL flask and placed in an oil bath at 120° C. and 150 rpm for 30 minutes.

[0059] (6) Washing and drying: After the reaction is completed, the flask is taken out and placed in a room temperature environment to cool. The solid and liquid phases are separated by vacuum filtration. The solid wood-based residue is washed with deionized water several times until the washing liquid becomes neutral. The filter cake is then transferred to an oven and dried at 60°C for 24 hours.

[0060] (7) Enzymatic hydrolysis: 1 ml of 100 FPU / mL cellulase was diluted to 1 FPU / mL using a citric acid-sodium citrate buffer solution at pH 5.0. 2 ml of the diluted cellulase solution was taken and evenly mixed with 100 mg of the wood-based residue obtained in step (6). After sealing, the mixture was shaken at 150 rpm and 50° C. for 72 h in a constant temperature shaker to release reducing sugars in the wood-based residue.

[0061] It was determined that the reducing sugar yield after enzymatic hydrolysis in Example 1 above reached 446.88 mg / g biomass, the solid recovery rate was 53.36%, the xylan removal rate was 94.74%, the delignification rate was 75.38%, and the glucan recovery rate was 97.65%. Figure 5 The results further show the amount of reducing sugar released by the sample in Example 1 at different time periods and the average enzymatic hydrolysis rate per 12 hours of enzymatic hydrolysis. After 72 hours of enzymatic hydrolysis, the reducing sugar yield of the sample reached 446.88 mg / g. Figure 1 The ideal reducing sugar yield (449 mg / g) for cellulose hydrolysis in the sunflower biomass shown in the figure indicates that during the 72-hour enzymatic hydrolysis period, the cellulose in the sample was almost completely hydrolyzed, reaching 71.64% of the ideal reducing sugar yield (623.76 mg / g) for whole cellulose hydrolysis; and, the amount of reducing sugar released in the first 36 hours reached 885.45±2.32 mg / g, and the reducing sugar yield reached 410.68±1.07 mg / g, releasing more than 92% of the total amount (963.50±3.02 mg / g) of reducing sugar released in 72 hours, reaching 65.84% of the ideal reducing sugar yield (623.76 mg / g) for whole cellulose hydrolysis.

[0062] Example 2-Example 10

[0063] The steps are the same as those in Example 1, except that the concentration of calcium hydroxide, the reaction temperature of the preliminary pretreatment of calcium hydroxide, the ratio of the ternary deep eutectic solvent, and the reaction temperature of the pretreatment of the ternary deep eutectic solvent are changed accordingly, and the optimal experimental conditions are further explored by orthogonal experiments, as shown in Table 1 below.

[0064] Table 1 Reaction conditions and reducing sugar yield determination results of Examples 1 to 10

[0065]

[0066] Comparative Example 1

[0067] The wood-based biomass raw materials are directly enzymatically hydrolyzed without pretreatment. The specific steps are as follows:

[0068] (1) Sunflower straw was selected as the wood-based biomass raw material. The sunflower straw was cleaned, air-dried, cut into short pieces, and dried in an oven at 60°C for 24 h. The raw materials were further crushed by a grinder and passed through a 40-mesh sieve. The unretained raw materials were further passed through an 80-mesh sieve, and the retained raw material particles were retained. Finally, wood-based raw material particles with a particle size of 40-80 mesh were obtained. The particles were then transferred to an oven at 60°C for further drying for 24 h to obtain a wood-based residue sample D1.

[0069] (2) Enzymolysis: 1 ml of 100 FPU / mL cellulase was diluted to 1 FPU / mL using a citric acid-sodium citrate buffer solution at pH 5.0. 2 ml of the diluted cellulase solution was taken and evenly mixed with 100 mg of the wood-based residue sample D1 obtained in step (1). After sealing, the mixture was shaken at 150 rpm and 50° C. for 72 h in a constant temperature shaker to release reducing sugars in the wood-based residue.

[0070] Comparative Example 2

[0071] The wood-based biomass raw materials are only pretreated with calcium hydroxide and then enzymatically hydrolyzed. The specific steps are as follows:

[0072] (1) Sunflower straw was selected as the wood-based biomass raw material. The sunflower straw was cleaned, air-dried, cut into short pieces, and dried in an oven at 60°C for 24 h. The raw materials were further crushed by a grinder and passed through a 40-mesh sieve. The unretained raw materials were further passed through an 80-mesh sieve, and the retained raw material particles were retained. Finally, wood-based raw material particles with a particle size of 40-80 mesh were obtained. The raw materials were then transferred to an oven at 60°C for further drying for 24 h and sealed for later use.

[0073] (2) Preparation of calcium hydroxide solution: Mix calcium hydroxide and deionized water in a ratio of 17.5 g:1 L and seal the mixture for later use.

[0074] (3) Calcium hydroxide pretreatment: 1 g of the wood-based raw material obtained in step (1) was mixed with the alkali solution prepared in step (2) in a 50 mL beaker at a ratio of 1 g:20 mL, and the mixture was placed in a water bath at 85° C. for 2 h to fully soak the wood-based raw material. The mixture was shaken every 10 minutes to ensure that the wood-based raw material was fully soaked.

[0075] (4) Washing and drying: After the reaction is completed, the conical flask is taken out and placed in a room temperature environment to cool. The solid and liquid phases are separated by vacuum filtration. The solid wood-based residue is washed with deionized water several times until the washing liquid becomes neutral. The solid filter residue is then transferred to an oven and dried at 60°C for 24 h to obtain a wood-based residue sample D2.

[0076] (5) Enzymatic hydrolysis: 1 ml of 100 FPU / mL cellulase was diluted to 1 FPU / mL using a citric acid-sodium citrate buffer solution at pH 5.0. 2 ml of the diluted cellulase solution was taken and evenly mixed with 100 mg of the wood-based residue sample D2 obtained in step (4). After sealing, the mixture was shaken at 150 rpm and 50° C. for 72 h in a constant temperature shaker to release reducing sugars in the wood-based residue.

[0077] Comparative Example 3

[0078] The wood-based biomass raw materials are only pretreated with a ternary deep eutectic solvent and then enzymatically hydrolyzed. The specific steps are as follows:

[0079] (1) Sunflower straw was selected as the wood-based biomass raw material. The sunflower straw was cleaned, air-dried, cut into short pieces, and dried in an oven at 60°C for 24 h. The raw materials were further crushed by a grinder and passed through a 40-mesh sieve. The unretained raw materials were further passed through an 80-mesh sieve, and the retained raw material particles were retained. Finally, wood-based raw material particles with a particle size of 40-80 mesh were obtained. The raw materials were then transferred to an oven at 60°C for further drying for 24 h and sealed for later use.

[0080] (2) Synthesis of ternary deep eutectic solvent: Choline chloride, ethylene glycol and oxalic acid were reacted in a molar ratio of 1 mol:2 mol:2 mol in an oil bath at 80° C. and 150 rpm for 2 h until a transparent uniform solution was formed, which was then set aside.

[0081] (3) Pretreatment with ternary deep eutectic solvent: 2.5 g of the wood-based residue obtained in step (1) and 50 g of the deep eutectic solvent synthesized in step (2) were mixed in a 50 mL flask and placed in an oil bath at 120° C. and 150 rpm for 30 minutes.

[0082] (4) Washing and Drying: After the reaction is completed, the conical flask is taken out and placed in a room temperature environment to cool. The solid and liquid phases are separated by vacuum filtration. The solid wood-based residue is washed with deionized water several times until the washing liquid becomes neutral. The solid filter residue is then transferred to an oven and dried at 60°C for 24 h to obtain a wood-based residue sample D3.

[0083] (5) Enzymolysis: 1 ml of 100 FPU / mL cellulase was diluted to 1 FPU / mL using a citric acid-sodium citrate buffer solution at pH 5.0. 2 ml of the diluted cellulase solution was taken and evenly mixed with 100 mg of the wood-based residue sample D3 obtained in step (4). After sealing, the mixture was shaken at 150 rpm and 50° C. for 72 h in a constant temperature shaker to release reducing sugars in the wood-based residue.

[0084] Comparative Example 4

[0085] The wood-based biomass raw materials are first pretreated with a ternary low eutectic solvent and then treated with calcium hydroxide, and then enzymatically hydrolyzed. The specific steps are as follows:

[0086] (1) Sunflower straw was selected as the wood-based biomass raw material. The sunflower straw was cleaned, air-dried, cut into short pieces, and dried in an oven at 60°C for 24 h. The raw materials were further crushed by a grinder and passed through a 40-mesh sieve. The unretained raw materials were further passed through an 80-mesh sieve, and the retained raw material particles were retained. Finally, wood-based raw material particles with a particle size of 40-80 mesh were obtained. The raw materials were then transferred to an oven at 60°C for further drying for 24 h and sealed for later use.

[0087] (2) Preparation of calcium hydroxide solution and synthesis of ternary low eutectic solvent: Calcium hydroxide and deionized water were mixed thoroughly at a ratio of 17.5 g:1 L, sealed and set aside; choline chloride, ethylene glycol and oxalic acid were reacted in an oil bath at a molar ratio of 1 mol:2 mol:2 mol for 2 h at 80° C. and 150 rpm until a transparent uniform solution was formed, which was set aside.

[0088] (3) Ternary low eutectic solvent pretreatment: 2.5 g of the wood-based biomass raw material obtained in step (1) and 50 g of the low eutectic solvent synthesized in step (2) were mixed in a 100 mL flask and placed in an oil bath at 120° C. and 150 rpm for 30 minutes.

[0089] (4) Washing and drying: After the reaction is completed, the flask is taken out and placed in a room temperature environment to cool. The solid and liquid phases are separated by vacuum filtration. The solid wood-based residue is washed with deionized water several times until the washing liquid becomes neutral. The solid filter residue is then transferred to an oven and dried at 60°C for 24 hours.

[0090] (5) Further pretreatment with calcium hydroxide: 0.5 g of the biomass residue obtained in step (3) was mixed with the alkali solution prepared in step (2) in a ratio of 0.5 g:10 mL in a 50 mL conical flask, and the mixture was placed in a water bath at 85° C. for 2 h to fully soak the wood base, and then shaken every 10 minutes to ensure that the wood base raw material was fully soaked.

[0091] (6) Washing and drying: After the reaction is completed, the conical flask is taken out and placed in a room temperature environment to cool. The solid and liquid phases are separated by vacuum filtration. The solid wood-based residue is washed with deionized water several times until the washing liquid becomes neutral. The solid filter residue is then transferred to an oven and dried at 60°C for 24 h to obtain a wood-based residue sample D4.

[0092] (7) Enzymatic hydrolysis: 1 ml of 100 FPU / mL cellulase was diluted to 1 FPU / mL using a citric acid-sodium citrate buffer solution at pH 5.0. 2 ml of the diluted cellulase solution was taken and evenly mixed with 100 mg of the wood-based residue sample D4 obtained in step (6). After sealing, the mixture was shaken at 150 rpm in a constant temperature shaker at 50° C. for 72 h to release reducing sugars in the wood-based residue.

[0093] Figure 2 The comparison of reducing sugar release and yield after enzymatic hydrolysis of untreated sample D1, sample D2 pretreated with calcium hydroxide alone, sample D3 pretreated with ternary DES alone, sample D4 treated with ternary DES first and then calcium hydroxide, and sample treated with calcium hydroxide first and then ternary DES (Example 1) is shown. It can be seen that the reducing sugar release and yield of the sample treated with calcium hydroxide first and then ternary DES are significantly higher than those of other samples.

[0094] Figure 3 Scanning electron microscopy analysis of the untreated sample D1, the sample D2 pretreated with calcium hydroxide alone, the sample D3 pretreated with ternary DES alone, and the sample treated with calcium hydroxide first and then ternary DES (Example 1) is shown. It can be seen that the surface structure of the untreated sample D1 is compact and orderly arranged, the fibers are tightly interwoven, and the fiber surface has excellent smoothness, which helps to form a natural dense structure as a solid barrier against enzymatic hydrolysis; the complete and orderly appearance of the sample D2 pretreated with calcium hydroxide alone begins to be destroyed and decomposed into separate fiber bundles, cracks and peeling layers appear on the fiber surface, and the pores are enlarged or destroyed; the damage of the sample D3 pretreated with ternary DES alone is more obvious, and a large area of ​​the fiber surface is peeled off; the sample treated with calcium hydroxide first and then ternary DES (Example 1) has the most obvious damage on the fiber surface, with a large number of tiny particles appearing, which are small molecular lignin decomposed after pretreatment, and has a higher porosity, and most of the cellulose is separated and exposed.

[0095] Figure 4X-ray diffraction analysis of the untreated sample D1, the sample D2 pretreated with calcium hydroxide alone, the sample D3 pretreated with ternary DES alone, and the sample treated with calcium hydroxide first and then ternary DES (Example 1) are shown. It can be seen that all samples have two peaks near 22.4° and 16.1° in the 2θ range, indicating that each pretreatment method did not significantly destroy the crystalline structure of cellulose in the sample. Among them, the crystallinity index (CrI) of the sample treated with calcium hydroxide first and then ternary DES is 56.35%, which is significantly higher than 49.12% of calcium hydroxide alone, 51.94% of ternary DES alone, and 49.12% of the untreated sample. This indicates that the removal effect of calcium hydroxide followed by ternary DES treatment on the amorphous region containing hemicellulose and lignin is significantly higher than that of other treatment methods, and the selective retention effect on cellulose is better than that of other treatment methods.

[0096] The above is a detailed introduction to the specific implementation methods of the present application. For those skilled in the art, several improvements and modifications may be made to the present application without departing from the principles of the present application. These improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali, characterized in that: The following steps are involved: S1) treating the wood-based biomass raw material with a dilute alkali solution, and obtaining an alkali-treated biomass residue after the reaction is completed; S2) mixing the alkali-treated biomass residue with an acidic ternary low eutectic solvent to perform DES treatment, and obtaining a combined pretreated biomass residue after the reaction is completed; The acidic ternary deep eutectic solvent comprises a hydrogen bond acceptor and a hydrogen bond donor, wherein the hydrogen bond acceptor is choline chloride, and the hydrogen bond donors are ethylene glycol and oxalic acid.

2. The method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali according to claim 1, characterized in that: The particle size of the wood-based biomass raw material in S1 is 40-80 mesh.

3. The method for pretreating lignocellulose using a ternary deep eutectic solvent in cooperation with a dilute alkali according to claim 1, characterized in that: The wood-based biomass raw material in S1 is sunflower straw.

4. The method for pretreating lignocellulose using a ternary deep eutectic solvent in cooperation with a dilute alkali according to claim 1, characterized in that: The dilute alkaline solution in S1 is a calcium hydroxide solution with a concentration of 1 to 20 g / L.

5. The method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali according to claim 1 or 4, characterized in that: The reaction temperature of the alkali treatment in S1 is 70-95°C, and the reaction time is 1-6h.

6. The method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali according to claim 1, characterized in that: The molar ratio of choline chloride, ethylene glycol and oxalic acid in the acidic ternary low eutectic solvent S2 is 1:2:(0.5-2.5).

7. The method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali according to claim 1 or 6, characterized in that: The reaction temperature of DES treatment in S2 is 110-140°C, and the reaction time is 20-60 min.

8. The method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali according to claim 1 or 6, characterized in that: The DES treatment in S2 was carried out under stirring conditions with a stirring speed of 100 to 200 rpm.

9. The method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali according to claim 1, characterized in that: The solid-liquid ratio of the wood-based biomass raw material to the dilute alkali solution in S1 is 1 g: (15-20) mL, and / or the solid-liquid ratio of the alkali-treated biomass residue to the acidic ternary low eutectic solvent in S2 is 1 g: (15-20) mL.

10. The method for pretreating lignocellulose using a ternary deep eutectic solvent in conjunction with a dilute alkali according to claim 1, characterized in that: After the reaction in S1 and / or S2 is completed, vacuum filtration, washing and drying are also performed.

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