Method for preparing xylo-oligosaccharides based on a conjugate- deep eutectic solvent double system

Abstract

The application belongs to the technical field of agricultural and forestry biomass utilization, and discloses a method for preparing xylo-oligosaccharide based on a conjugate-Deep eutectic solvent double system, which comprises the following steps: after a lignocellulosic biomass raw material is crushed, the raw material is treated with dilute alkali, solid-liquid separation is performed, and an alkali extraction liquid is obtained, then acid is added to construct a conjugate acid-alkali buffer system, and then choline chloride is added and mixed to obtain a conjugate-Deep eutectic solvent double system with a strong hydrogen bond network, and finally, a thermal reaction treatment is performed at a temperature of no more than 150 DEG C to obtain a xylo-oligosaccharide liquid. Compared with the existing method, the method has higher efficiency in treating macromolecular polysaccharides, and the over-hydrolysis phenomenon of xylo-oligosaccharide is greatly reduced, and the yield of xylo-oligosaccharide is higher; the whole process does not use enzymes, and meets the low-cost requirement; and the method also has the advantages of simple operation, easy industrial production and the like, and is helpful to promote the high-value utilization of straw resources and realize the sustainable development of agricultural waste.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of agroforestry biomass utilization, and relates to a method for preparing xylooligosaccharide based on a conjugate- deep eutectic solvent double system. BACKGROUND

[0002] Xylooligosaccharide (XOS) is a functional oligosaccharide formed by 2-9 xylose units connected by β-1, 4 glycosidic bonds, which is mainly extracted from lignocellulosic biomass. As a new type of prebiotic, xylooligosaccharide has unique bifidogenic properties: its indigestibility in the human stomach ensures its direct access to the distal intestine, and through selective promotion of the proliferation and metabolic activity of probiotics such as Bifidobacterium and Lactobacillus, it plays a role in intestinal microecological regulation. Studies have shown that short-chain fatty acids and other metabolic products produced by the fermentation of xylooligosaccharide by probiotics can mediate a variety of physiological functions. This plant-derived green nutrient has been widely used in the fields of functional food, healthy beverage and feed additive, and has shown significant health promotion effect and sustainable development value.

[0003] Straw of Gramineae crops (such as rice, wheat, barley, corn and sorghum) is the main source of lignocellulosic biomass raw materials. At present, the preparation of xylooligosaccharide from lignocellulosic straw has become a research hotspot, and the existing production methods mainly include three categories, namely alkali-enzyme synergistic method, acid method and hydrothermal liquefaction method. The alkali-enzyme synergistic method is to dissolve the cell wall matrix in an alkaline medium, release xylan and then perform specific enzymatic hydrolysis using endoxylanase. The product polydispersity of this method is controllable, but there are difficulties in alkali recovery and high cost of enzyme preparation. The acid method is to use strong acid or high-concentration organic acid to catalyze the hydrolysis of hemicellulose, although the reaction speed is fast, but the strong acid system increases the corrosion rate of the equipment by 2-3 times, and generates a large amount of by-products, resulting in total organic carbon (TOC) exceeding the standard in wastewater. The combined treatment of acid and enzyme can effectively reduce the reaction intensity and reduce the generation of reaction by-products, but it is limited by the high cost of enzyme preparation, and this combined treatment method also cannot achieve the low-cost production of xylooligosaccharide.

[0004] Therefore, the existing technology has defects such as poor raw material adaptability (high-purity xylan is required), low yield (only a few percent), high cost (expensive enzyme preparation), high reaction energy consumption (high temperature and high pressure), strong product heterogeneity (over-hydrolysis of xylooligosaccharide), and environmental pollution (strong acid / alkali wastewater). It is imperative to further develop a new method for preparing xylooligosaccharide from lignocellulosic biomass, which is green, efficient and low-cost. SUMMARY

[0005] The present application is directed to the technical field of preparing xylo-oligosaccharides from lignocellulosic biomass, and a green, efficient and low-cost directional catalysis technology based on conjugate-Deep eutectic solvent double system is developed to break through the multiple technical barriers of traditional xylo-oligosaccharide preparation process.

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

[0007] A method for preparing xylo-oligosaccharides based on conjugate-Deep eutectic solvent double system, comprising the following steps:

[0008] (1) After the lignocellulosic biomass raw material is crushed, dilute alkali is used as an extractant for pretreatment, and after solid-liquid separation, an alkali extraction liquid and solid material are obtained;

[0009] (2) Acetic acid is added to the alkali extraction liquid to construct a conjugate acid-base buffer system;

[0010] (3) Choline chloride is further added to the conjugate acid-base buffer system, and after mixing, a conjugate-Deep eutectic solvent double system with strong hydrogen bond network is obtained;

[0011] (4) The conjugate-Deep eutectic solvent double system is subjected to thermal reaction treatment at a temperature not exceeding 150℃, and after solid-liquid separation, a xylo-oligosaccharide liquid is obtained.

[0012] The application integrates alkali-hydrogen peroxide pretreatment of lignocellulosic biomass raw materials (hemicellulose retention rate > 85%) and a conjugate- deep eutectic solvent double system to synergistically improve the yield of xylo-oligosaccharide and the conversion rate of xylan. First, alkali-hydrogen peroxide pretreatment is used to dissolve xylan in the raw material first, providing a better reaction raw material basis for further hydrolysis to form xylo-oligosaccharide; the peroxide generated by the reaction of hydrogen peroxide and alkali intensifies the ability of sodium hydroxide to remove lignin, improves the efficiency of xylan dissolution, and the decomposition of hydrogen peroxide releases a large amount of heat, which can greatly reduce the energy consumption of the reaction. Further, dilute alkali and acetic acid form a conjugate acid-base buffer system; the added choline chloride and acetic acid form a strong hydrogen-bonded network of conjugate- deep eutectic solvent system; the conjugate- deep eutectic solvent double system is subjected to thermal reaction treatment at a temperature not exceeding 150℃, which can effectively destroy the lignin-carbohydrate complex (LCC) and the ether bond and ester bond inside the lignin, realizing the depolymerization of LCC and lignin macromolecular chains, on the one hand, the xylan in the LCC component connected by chemical bonds is released; on the other hand, the lignin component in the depolymerized LCC is reduced in molecular weight due to the breaking of ether bonds and ester bonds, the three-dimensional network structure is destroyed, and the reactive sites are exposed, thereby improving the dissolution rate of lignin, and the xylan component physically wrapped but not directly connected in the lignin is dissolved; the above two kinds of dissolved xylan are hydrolyzed into xylo-oligosaccharide under the protection of the conjugate acid-base buffer system formed by dilute alkali and acetic acid, so that the yield of xylo-oligosaccharide is obviously improved and more stable. Further, through the use of sodium hydroxide, acetic acid and choline chloride, the final reaction liquid contains sodium acetate, choline chloride and dietary fiber, and by controlling the reaction pH, the reaction reagent can be changed from wastewater to high-quality animal feed required by animals, and has excellent preservative effect.

[0013] The above method, further, in step (1), the lignocellulosic biomass raw material is selected from any one of water straw, rice husk, corn cob, corn stalk, sugarcane residue, wheat straw, barley straw, sorghum straw, phyllostachys reticulatus, phyllostachys edulis, reed and bamboo, and the mass content of hemicellulose in the lignocellulosic biomass raw material is 10-40%, and the particle size of the crushed lignocellulosic biomass raw material is 1-200 mesh.

[0014] Further, in step (1), the dilute alkali is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide and lithium hydroxide, and the concentration of the dilute alkali is 0.04-10%.

[0015] Further, in step (1), the extractant further includes hydrogen peroxide, and the mass concentration of the hydrogen peroxide in the extractant is 0.5-1%. A peroxide catalytic system is formed, which further enhances the dissolution of lignin and hemicellulose by reacting with the deep lignin of the lignocellulosic raw material, and provides more xylan for the subsequent reaction.

[0016] Further, in step (1), the solid-liquid ratio of the lignocellulosic biomass raw material to the dilute alkali is 1 (g): 4-20 (mL); the temperature of the dilute alkali treatment is 80-120°C, and the reaction time is 0.5-10 h.

[0017] Further, in step (2), the amount of acetic acid added to the alkali extraction solution is controlled so that the pH value of the conjugate acid-base buffer system is controlled at 4-6. The self-balancing buffer system with a pH value of 4-6 is formed, the dynamic stability of the pH value of the reaction system is achieved, and the H + concentration is precisely controlled to selectively break the beta-1, 4-glycosidic bonds in xylan, inhibit the excessive decomposition of hydrolysis products xylo-oligosaccharides, and provide a stable and favorable reaction environment for the hydrolysis of xylan into xylo-oligosaccharides.

[0018] Further, in step (2), the amount of acetic acid added to the alkali extraction solution is controlled so that the pH value of the conjugate acid-base buffer system is controlled at 4-6. In this way, the formed complex solution can be recovered in the form of an animal feed additive.

[0019] Further, in step (2), the mass concentration of the acetic acid is 30-100%.

[0020] Under the above acid-base process conditions, the pH self-balancing catalytic microenvironment is constructed, which has the following effects: (1) a xylan efficient dissolution mechanism is formed, which allows a large amount of xylan in straw and other crude materials to be released into the alkali solution (xylan yield ≥ 16%); (2) a glycosidic bond site selective breaking mechanism is formed, which allows xylan in straw and other crude materials to be directionally converted into target products (xylo-oligosaccharide yield ≥ 15%); (3) a byproduct inhibition system is constructed, which reduces the amount of xylose and other byproducts by 70%; (4) compared to directly using a conjugate acid-base system, the reaction temperature is higher and the byproduct reaction is larger, and the present application reduces the reaction temperature by 40-60°C through the addition of alkali to dissolve xylan and then the addition of acetic acid to form a conjugate acid-base system, thereby greatly reducing the separation energy consumption and water consumption.

[0021] Further, in step (3), the H + concentration of the alkali extraction solution is 2-4:1 with respect to the molar ratio of choline chloride.

[0022] Further, in step (4), the temperature of the heat reaction treatment is controlled at 100-140°C.

[0023] Further, in step (4), the pressure of the heat reaction treatment is 0.1-0.4 MPa, and the time is 10-60 min.

[0024] By limiting the H +The lignin and lignin-carbohydrate complex in the raw material are dissolved, and macromolecular xylan produced by alkali pretreatment is released into the solution to further hydrolysis and efficient conversion by taking the molar ratio of choline chloride, the heat reaction condition of 120-140 DEG C, 0.1-0.4 MPa (medium temperature and low pressure) for 10-60 min.

[0025] Further, no reaction enzyme or enzyme preparation is added in the whole process of the method.

[0026] Compared with the prior art, the method has the beneficial effects that:

[0027] 1. The alkali pretreatment and conjugate- eutectic solvent double system combined treatment of biomass in the method are used to dissolve and degrade xylan, and the conjugate- eutectic solvent double system is used to improve the yield of xylo-oligosaccharide and the conversion rate of xylan.

[0028] 2. Compared with the existing alkali-enzyme synergistic method, the efficiency of treating macromolecular polysaccharide is higher, the over-hydrolysis of xylo-oligosaccharide is greatly reduced, and the yield of xylo-oligosaccharide is higher; compared with the existing hydrothermal method, the treatment temperature of the method is reduced to 100-140 DEG C (reduced by 40-80 DEG C), the pressure and energy consumption of the reaction kettle are greatly reduced, and the over-hydrolysis of xylo-oligosaccharide is also reduced; compared with the above-mentioned method of the prior art and the test without using the conjugate- eutectic solvent double system, the yield of xylo-oligosaccharide is greatly improved.

[0029] 3. The buffer system of the method realizes the reaction under near-neutral conditions (pH 4.0-6.0), the corrosion rate is lower, and the service life of the reaction kettle can be prolonged by more than 3 times; the soluble buffer components (dilute alkali and acetic acid) are used, and recycling is realized by evaporation crystallization or spray drying, so that the raw material cost is reduced; the reaction process of the method does not discharge waste liquid, and no strong acid / strong alkali hazardous waste is generated, so that energy saving and environmental protection are realized, and the clean production requirement is met.

[0030] 4. The method can also be used for hydrogen peroxide and dilute alkali synergistic pretreatment, which can intensify the ability of sodium hydroxide to remove lignin, improve the dissolution efficiency of xylan, and reduce the energy consumption of the reaction.

[0031] 5. The method selects acetic acid as the acid reagent, and compared with other types of acids, acetic acid can further react to generate sodium acetate without impurities, and can be used for producing animal feed additives.

[0032] 6. The method does not use enzyme in the whole process, meets the low-cost requirement, and has the advantages of simple operation and easy industrial production, and is helpful to promote the high-value utilization of straw resources and realize the sustainable development of agricultural wastes. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings described below are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0034] Figure 1 is a process flow diagram of the present application.

[0035] Figure 2 is the bacteriostatic effect of the sodium acetate composite animal feed additive obtained in Embodiment 2 of the present application; wherein, Figure A is the bacteriostatic experiment of Saccharomyces cerevisiae (A) Saccharomyces cerevisiae ), Figure B is the bacteriostatic experiment of Escherichia coli (B) Escherichia coli ), and Figure C is the bacteriostatic experiment of Staphylococcus aureus (C) Staphylococcus aureus ). DETAILED DESCRIPTION

[0036] In order to facilitate the understanding of the present application, the following will combine the drawings and the preferred embodiments to describe the present application more fully and in detail, but the protection scope of the present application is not limited to the following specific embodiments.

[0037] Unless otherwise defined, all the professional terms used in the following have the same meaning as generally understood by those skilled in the art. The professional terms used in the present application are only for the purpose of describing the specific embodiments, and are not intended to limit the protection scope of the present application.

[0038] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present application can be purchased from the market or can be prepared by the existing method.

[0039] A method for preparing xylo-oligosaccharides based on a conjugate- deep eutectic solvent double system, as shown in Figure 1 , includes three steps of dilute alkali pretreatment, construction of a conjugate- deep eutectic solvent double system, and controlled depolymerization (heat treatment) of xylan, and the preparation process is as follows:

[0040] (1) Dilute alkali pretreatment process:

[0041] The lignocellulosic biomass raw material is removed from impurities and crushed, and the lignocellulosic biomass raw material used is selected from any one of the following plants: rice straw, rice husk, sugarcane residue, corn cob, corn stalk, wheat straw, barley straw, sorghum straw, phyllostachys reticulatus, phyllostachys edulis, reed and bamboo, etc., or plant processing residue, and the hemicellulose mass content is between 10-40%, and it is crushed to a particle size of 1-200 mesh.

[0042] A certain mass of the above crushed raw material is placed in a reactor, and a base with a mass concentration of 0.04-10% and hydrogen peroxide with a mass concentration of 0.5-1% are added according to a solid-liquid ratio of 1:4-20 g / mL, and the base is any one of sodium hydroxide, potassium hydroxide, calcium hydroxide, and lithium hydroxide, and the reaction is carried out at a temperature of 50-160°C for 0.5-10 h to obtain a reaction mixture; the above mixture is subjected to solid-liquid separation by a filtering device to obtain an alkali extraction liquid, wherein the xylan concentration is 5-20 mg / mL.

[0043] (2) Construction of a conjugate-Deep eutectic solvent double system:

[0044] The alkali extraction liquid obtained in step (1) is added to acetic acid to adjust the pH to 4-6, and the mass concentration of acetic acid used is 30-100%; the above alkali extraction liquid is added with choline chloride according to a molar ratio of H + : choline chloride = 2-4:1, and the conjugate-Deep eutectic solvent double system is obtained after mixing.

[0045] (3) Controllable depolymerization (mesophilic hydrolysis) of xylan:

[0046] The conjugate-Deep eutectic solvent double system obtained in step (2) is kept at a temperature of 100-140°C and a pressure of 0.1-0.4 MPa for 10-60 min; the above conjugate-Deep eutectic solvent double system is subjected to solid-liquid separation by a filtering device, and the obtained liquid is an oligomeric xylose liquid, wherein the oligomeric xylose concentration is 1-15 mg / mL.

[0047] In the following examples, the detection standards for the concentration and yield of oligomeric xylose are as follows:

[0048] According to the method of NREL / TP-510-42623 of the U.S. Renewable Energy Laboratory, high-performance liquid chromatography is used to detect the monosaccharide and oligosaccharide concentrations in the sugar liquid, and the chromatographic conditions are as follows: a Japan Shimadzu Nexera LC-40 high-performance liquid chromatography system is configured with a Bio-Rad Aminex HPX-87H chromatographic column and a guard column, a differential refractive index detector, a column temperature of 65°C, a mobile phase of 0.005 mol / L sulfuric acid, and a sample injection volume of 10 μL; an external standard method is used for detection.

[0049] The water content is determined according to the Energy Industry Standard NB / T 34057.3-2017.

[0050] The yield calculation method of monosaccharides and oligosaccharides is as follows:

[0051] Yield%=sugar concentration mg / mL x sugar liquid volume mL / (biomass dry powder mass g x (100-water content%) / 100) x 100.

[0052] Example 1: Xylan extraction from rice straw

[0053] A method for preparing xylo-oligosaccharides based on a deep eutectic solvent double system, comprising the following steps:

[0054] (1) Fresh biomass samples taken from rice straw were washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0055] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, a 4% sodium hydroxide solution was added according to a solid-liquid ratio of 1:10 g / mL, the temperature was raised to 100°C and maintained for 1 h, and after the reaction, filtration was performed to obtain an alkali extraction liquid and solid materials, the concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0056] (3) Acetic acid was added to the alkali extraction liquid to a pH of 5.0, and then 0.0149 mol of choline chloride was added to obtain a deep eutectic solvent double system;

[0057] (4) The deep eutectic solvent double system was reacted at 140°C and 0.4 MPa for 30 min, and after the reaction was completed, filtration was performed to obtain a xylo-oligosaccharide liquid. The concentration and yield are shown in Table 1.

[0058] Table 1 Concentration and yield of each product in Example 1

[0059]

[0060] Example 2: Xylan extraction from rice straw

[0061] A method for preparing xylo-oligosaccharides based on a deep eutectic solvent double system, comprising the following steps:

[0062] (1) Fresh biomass samples taken from rice straw were washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0063] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, a 4% sodium hydroxide solution was added according to a solid-liquid ratio of 1:10 g / mL, the temperature was raised to 100°C and maintained for 1 h, and after the reaction, filtration was performed to obtain an alkali extraction liquid and solid materials, the concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0064] (3) Acetic acid was added to the alkali extraction liquid to a pH of 5.0, and then 0.0149 mol of choline chloride was added to obtain a deep eutectic solvent double system;

[0065] (4) The deep eutectic solvent double system was reacted at 140°C and 0.4 MPa for 30 min, and after the reaction was completed, filtration was performed to obtain a xylo-oligosaccharide liquid. The concentration and yield are shown in Table 1.

[0066] (5) The xylooligosaccharide solution was subjected to ultrafiltration (2000 Da), nanofiltration (200 Da), and spray drying to obtain sodium acetate compound animal feed additive. The antibacterial effect of the sodium acetate compound animal feed additive is shown in [reference needed]. Figure 2 .

[0067] Table 2. Concentration and yield of each product in Example 2

[0068]

[0069] Depend on Figure 2 Sodium acetate compound animal feed additive was used to sequentially treat brewer's yeast ( Saccharomyces cerevisiae ), Escherichia coli ( Escherichia coli ) and Staphylococcus aureus ( Staphylococcus aureus Antibacterial experiments were conducted. Sodium acetate compound animal feed additive was prepared as a 1% (w / w) aqueous solution, and 100 μL was added. Under pH 5.0 conditions, distinct inhibition zones appeared on each plate, indicating that the additive has a good antibacterial effect, with the best inhibitory effect against *Saccharomyces cerevisiae*.

[0070] Example 3: Xylan extraction from rice straw

[0071] A method for preparing xylooligosaccharides based on a conjugated-eutectic solvent dual system includes the following steps:

[0072] (1) Wash the fresh biomass sample taken from rice straw, dry it at 105℃ to constant weight, and then crush it to 60 mesh for later use;

[0073] (2) Weigh 5 g of the above-mentioned biomass dry powder into a reaction vessel, add 4% sodium hydroxide solution at a solid-liquid ratio of 1:10 g / mL, heat to 100℃ and maintain for 1 h, filter after reaction to obtain alkaline extract and solid. The concentration of xylan in the alkaline extract is 15 mg / mL and the yield is 15%.

[0074] (3) Add acetic acid to the reaction solution until the pH is 5.0, and then add 0.0149 mol of choline chloride to obtain a conjugate-eutectic solvent dual system;

[0075] (4) The conjugate-eutectic solvent dual system was reacted at 140℃ and 0.4 MPa for 10 min. After the reaction was completed, the xylooligosaccharide solution was obtained by filtration. The concentration and yield are shown in Table 3.

[0076] Table 3. Concentration and yield of each product in Example 3

[0077]

[0078] Example 4: Xylan extraction from rice straw

[0079] A method for preparing xylooligosaccharides based on a conjugate- deep eutectic solvent double system, comprising the following steps:

[0080] (1) The fresh biomass sample taken from rice straw was washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0081] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, 4% sodium hydroxide solution was added according to a solid-liquid ratio of 1:10 g / mL, the temperature was raised to 100°C and maintained for 1 h, and after the reaction, filtration was performed to obtain an alkali extraction liquid and solid materials. The concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0082] (3) Acetic acid was added to the reaction liquid to a pH of 5.0, and then 0.0149 mol of choline chloride was added to obtain a conjugate- deep eutectic solvent double system;

[0083] (4) The conjugate- deep eutectic solvent double system was reacted at 140°C and 0.4 MPa for 20 min, and after the reaction was completed, filtration was performed to obtain a xylooligosaccharide liquid. The concentration and yield are shown in Table 4.

[0084] Table 4 Concentration and yield of each product in Example 4

[0085]

[0086] Example 5: Xylan extraction from south reed

[0087] A method for preparing xylooligosaccharides based on a conjugate- deep eutectic solvent double system, comprising the following steps:

[0088] (1) The fresh biomass sample taken from south reed was washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0089] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, 4% sodium hydroxide solution was added according to a solid-liquid ratio of 1:10 g / mL, the temperature was raised to 100°C and maintained for 1 h, and after the reaction, filtration was performed to obtain an alkali extraction liquid and solid materials. The concentration of xylan in the alkali extraction liquid was 16 mg / mL, and the yield was 16%;

[0090] (3) Acetic acid was added to the reaction liquid to a pH of 5.0, and then 0.0149 mol of choline chloride was added to obtain a conjugate- deep eutectic solvent double system;

[0091] (4) The conjugate- deep eutectic solvent double system was reacted at 140°C and 0.4 MPa for 30 min, and after the reaction was completed, filtration was performed to obtain a xylooligosaccharide liquid. The concentration and yield are shown in Table 5.

[0092] Table 5 Product concentration and yield in Example 5

[0093]

[0094] Example 6: Xylan extraction from rice straw

[0095] A method for preparing xylooligosaccharides based on a conjugate- deep eutectic solvent double system, comprising the following steps:

[0096] (1) Fresh biomass samples taken from rice straw were washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0097] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, 4% sodium hydroxide solution was added according to a solid-liquid ratio of 1:10 g / mL, and the temperature was raised to 100°C for 1 h. After reaction, filtration was performed to obtain an alkali extraction liquid and solid materials. The concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0098] (3) Acetic acid was added to the reaction liquid to a pH of 5.0, and 0.0149 mol of choline chloride was added to obtain a conjugate- deep eutectic solvent double system;

[0099] (4) The conjugate- deep eutectic solvent double system was reacted at 130°C and 0.4 MPa for 30 min. After the reaction was completed, filtration was performed to obtain a xylooligosaccharide liquid. The concentration and yield are shown in Table 6.

[0100] Table 6 Product concentration and yield in Example 6

[0101]

[0102] Example 7: Xylan extraction from rice straw

[0103] A method for preparing xylooligosaccharides based on a conjugate- deep eutectic solvent double system, comprising the following steps:

[0104] (1) Fresh biomass samples taken from rice straw were washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0105] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, 4% sodium hydroxide solution was added according to a solid-liquid ratio of 1:10 g / mL, and the temperature was raised to 100°C for 1 h. After reaction, filtration was performed to obtain an alkali extraction liquid and solid materials. The concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0106] (3) adding acetic acid to the reaction solution to a pH of 5.0, and then adding 0.0149 mol of choline chloride to obtain a conjugate- deep eutectic solvent double system;

[0107] (4) reacting the conjugate- deep eutectic solvent double system at 130℃ and 0.4 MPa for 30 min, and then filtering to obtain an oligoxylan solution. The concentration and yield are shown in Table 7.

[0108] Table 7. Concentration and yield of each product in Example 7

[0109]

[0110] Example 8: Xylan extraction from rice straw

[0111] A method for preparing oligoxylan based on a conjugate- deep eutectic solvent double system, comprising the following steps:

[0112] (1) washing fresh biomass samples taken from rice straw, drying at 105℃ until constant weight, and then crushing to 60 mesh for use;

[0113] (2) taking 5 g of the above biomass dry powder into a reaction kettle, adding a 4% sodium hydroxide solution according to a solid-liquid ratio of 1:10 g / mL, heating to 100℃ and keeping for 1 h, filtering after reaction to obtain an alkali extraction solution and solid matter, the concentration of xylan in the alkali extraction solution being 15 mg / mL, and the yield being 15%;

[0114] (3) adding acetic acid to the alkali extraction solution to a pH of 4.0, and then adding 0.0149 mol of choline chloride to obtain a conjugate- deep eutectic solvent double system;

[0115] (4) reacting the conjugate- deep eutectic solvent double system at 140℃ and 0.4 MPa for 30 min, and then filtering to obtain an oligoxylan solution. The concentration and yield are shown in Table 8.

[0116] Table 8. Concentration and yield of each product in Example 8

[0117]

[0118] Comparative Example 1:

[0119] A method for preparing oligoxylan based on alkali treatment and enzymatic hydrolysis (alkali-enzyme synergistic method), comprising the following steps:

[0120] (1) washing fresh biomass samples taken from rice straw, drying at 105℃ until constant weight, and then crushing to 60 mesh for use;

[0121] (2) Take 5 g of the above biomass dry powder in a reaction kettle, add 4% sodium hydroxide solution with a solid-liquid ratio of 1:10 g / mL, and heat to 100°C for 1 h. After reaction, filter to obtain an alkali extraction liquid and a solid. The concentration of xylan in the alkali extraction liquid is 15 mg / mL, and the yield is 15%;

[0122] (3) The reaction liquid is adjusted to pH 5.0 with acetic acid, and 1‰ xylanase is added for enzymatic hydrolysis at 50°C for 24 h. After reaction, heat to 100°C for 10 min to inactivate the enzyme. After inactivation, separate the solid phase and the xylo-oligosaccharide liquid by filtration. The concentration and yield are shown in Table 9.

[0123] Table 9 Concentrations of various products in Comparative Example 1

[0124]

[0125] As shown in Table 9, the concentration and yield of oligosaccharides in Comparative Example 1 are both lower than those in Example 1, and the monosaccharide concentration in Comparative Example 1 is higher than that in Example 1. Compared with the alkali-enzyme combined treatment method of Comparative Example 1, the method of Example 1 has higher efficiency in treating macromolecular xylan, and the yield of xylo-oligosaccharide is increased by 55%. It is shown that compared with the alkali-enzyme combined treatment method, the co-ionic deep eutectic solvent double system of the present application has better effect in promoting the hydrolysis of xylan, so that the yield of xylo-oligosaccharide is further improved. And the whole process of the present application does not use enzyme, which can avoid the consumption of about 30 kg of xylanase per ton of raw material, and meets the low cost requirement.

[0126] Comparative Example 2:

[0127] A method for preparing xylo-oligosaccharide based on heat treatment (hydrothermal method) includes the following steps:

[0128] (1) Wash the fresh biomass sample taken from rice straw, dry at 105°C to constant weight, and crush to 60 mesh for use;

[0129] (2) Take 4 g of the above biomass dry powder in a reaction kettle, add an aqueous solution with a solid-liquid ratio of 1:10 g / mL, and heat to 180-200°C for 10 min. After reaction, filter to obtain a xylo-oligosaccharide liquid. The concentration and yield are shown in Table 10.

[0130] Table 10 Concentrations and yields of various products in Comparative Example 2

[0131]

[0132] As shown in Table 10, the concentration and yield of oligosaccharides in Comparative Example 2 are both lower than those in Example 1. Compared with the hydrothermal method in Comparative Example 2, the yield of xylo-oligosaccharides in Example 1 is increased by 105%; the optimal reaction temperature of the present application is reduced to 120-140°C (reduced by 40-60°C), the pressure of the reactor is reduced from 1.4 Mpa to 0.4 Mpa, and the over-hydrolysis of xylo-oligosaccharides is reduced by 48%. It is shown that compared with the hydrothermal method, the conjugate- deep eutectic solvent double system has higher effect of promoting the dissolution of xylo-oligosaccharides and the hydrolysis ability of xylan macromolecules, so that the yield of xylo-oligosaccharides is further improved; in addition, under the treatment conditions of 180-200°C in Table 10, the concentration of xylose obtained is much higher than that in Example 1, which shows that the hydrothermal method cannot effectively control the over-hydrolysis of xylo-oligosaccharides.

[0133] Comparative Example 3:

[0134] A method for preparing xylo-oligosaccharides based on acid addition, comprising the following steps:

[0135] (1) The fresh biomass sample taken from water rice straw is washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0136] (2) 4 g of the above biomass dry powder is weighed into a reaction kettle, 1 M sodium acetate solution with pH of 5.0 is added according to solid-liquid ratio of 1:10 g / mL, and the temperature is raised to 160°C and kept for 30 min. After reaction, filtration is performed to obtain xylo-oligosaccharide solution. The concentration and yield are shown in Table 11.

[0137] Table 11 Concentration of each product in Comparative Example 3

[0138]

[0139] As shown in Table 11, the concentration and yield of oligosaccharides in Comparative Example 3 are both lower than those in Example 1 under the condition of 140°C for 30 min, which shows that compared with the sodium acetate heat treatment of biomass, the conjugate- deep eutectic solvent double system has higher effect of promoting the dissolution of xylo-oligosaccharides and the hydrolysis ability of xylan macromolecules, so that the yield of xylo-oligosaccharides is increased by 318%.

[0140] Comparative Example 4:

[0141] A method for preparing xylo-oligosaccharides based on conjugate acid-base buffer system, comprising the following steps:

[0142] (1) The fresh biomass sample taken from water rice straw is washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0143] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, 4% sodium hydroxide solution was added according to the solid-liquid ratio of 1:10 g / mL, and the temperature was raised to 100°C and kept for 1 h. After reaction, filtration was performed to obtain an alkali extraction liquid and a solid. The concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0144] (3) Acetic acid was added to the reaction liquid to a pH of 5.0, and the reaction was carried out at 140°C for 30 min. After the reaction was completed, filtration was performed to obtain an oligomeric xylan liquid. The concentration and yield are shown in Table 12.

[0145] Table 12 Concentrations of various products in Comparative Example 4

[0146]

[0147] As shown in Table 12, compared with Example 1, less choline chloride was added in Comparative Example 4, and no conjugate- eutectic solvent double system was formed. LCC was not effectively degraded, and the yield of oligomeric xylan was lower. Compared with Comparative Example 4, the yield of oligomeric xylan of the present application was increased by 18%.

[0148] Comparative Example 5:

[0149] A method for preparing oligomeric xylan based on a conjugate acid-base buffer system, comprising the following steps:

[0150] (1) Fresh biomass samples taken from rice straw were washed, dried at 105°C to constant weight, and then crushed to 60 mesh for use;

[0151] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, 4% sodium hydroxide solution was added according to the solid-liquid ratio of 1:10 g / mL, and the temperature was raised to 100°C and kept for 1 h. After reaction, filtration was performed to obtain an alkali extraction liquid and a solid. The concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0152] (3) Acetic acid was added to the reaction liquid to a pH of 1.0, and the reaction was carried out at 140°C and 0.4 MPa for 30 min. After the reaction was completed, filtration was performed to obtain an oligomeric xylan liquid. The concentration and yield are shown in Table 13.

[0153] Table 13 Concentrations of various products in Comparative Example 5

[0154]

[0155] Comparative Example 6:

[0156] A method for preparing oligomeric xylan based on a conjugate acid-base buffer system, comprising the following steps:

[0157] (1) The fresh biomass sample taken from rice straw was washed, dried at 105°C to constant weight, and then ground to 60 mesh for use;

[0158] (2) 5 g of the above biomass dry powder was weighed into a reaction kettle, 4% sodium hydroxide solution was added according to the solid-liquid ratio of 1:10 g / mL, and the temperature was raised to 100°C for 1 h. After reaction, filtration was performed to obtain an alkali extraction liquid and a solid. The concentration of xylan in the alkali extraction liquid was 15 mg / mL, and the yield was 15%;

[0159] (3) Acetic acid was added to the reaction liquid to pH 8.0, and the reaction was carried out at 140°C and 0.4 MPa for 30 min. After the reaction was completed, filtration was performed to obtain an oligomeric xylan liquid. The concentration and yield are shown in Table 14.

[0160] Table 14 Concentrations of various products in Comparative Example 6

[0161]

[0162] As can be seen from Tables 13-14, neither a higher nor a lower pH is conducive to the equilibrium stability of the reaction. When the pH value of the conjugate acid-base buffer system is controlled at 5 (Examples 1-2), the β-1,4-glycosidic bond in xylan can be selectively broken, the over-decomposition of the hydrolysis product oligomeric xylan is inhibited, a stable and favorable reaction environment for the hydrolysis of xylan into oligomeric xylan is provided, and the purpose of improving the yield of oligomeric xylan is achieved.

Claims

1. A method for preparing xylooligosaccharides based on a conjugated-eutectic solvent dual system, characterized in that, The method does not involve the addition of reactive enzymes or enzyme preparations throughout its entire process, and includes the following steps: (1) After crushing the lignocellulosic biomass raw material, it is pretreated with dilute alkali as an extractant, and the alkali extract and solid are obtained after solid-liquid separation; (2) Add acetic acid to the alkaline extract to construct a conjugate acid-base buffer system; (3) Add choline chloride to the conjugate acid-base buffer system, and the H+ of the alkaline extract... + The molar ratio of choline chloride to choline chloride is 2~4:

1. After mixing, a conjugated-eutectic solvent dual system with a strong hydrogen bond network is obtained. (4) The conjugated-eutectic solvent dual system is subjected to thermal reaction treatment at a temperature of 100~140℃, and xylooligosaccharide solution is obtained after solid-liquid separation.

2. The method according to claim 1, characterized in that, In step (1), the lignocellulosic biomass raw material is selected from any one of rice straw, rice husk, corn cob, corn stalk, sugarcane bagasse, wheat straw, barley straw, sorghum straw, reed, reed, and alfalfa, and its hemicellulose content is 10-40% by mass. The particle size of the lignocellulosic biomass raw material after crushing is 1-200 mesh.

3. The method according to claim 1, characterized in that, In step (1), the dilute alkali is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide and lithium hydroxide, and the mass concentration of the dilute alkali is 0.04~10%; the solid-liquid ratio of the lignocellulosic biomass raw material to the extractant is 1 g: 4~20 mL; the pretreatment temperature is 80~120℃ and the pretreatment time is 0.5~10 h.

4. The method according to claim 1, characterized in that, In step (1), the extractant also includes hydrogen peroxide, and the mass concentration of hydrogen peroxide in the extractant is 0.5~1%.

5. The method according to claim 1, characterized in that, In step (2), the amount of acetic acid added to the alkaline extract is controlled so that the pH value of the conjugate acid-base buffer system is controlled at 4~6.

6. The method according to claim 1, characterized in that, In step (2), the mass concentration of the acetic acid is 30~100%.

7. The method according to claim 1, characterized in that, In step (4), the pressure of the thermal reaction treatment is 0.1~0.4 MPa and the time is 10~60 min.

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