Method for efficiently preparing polydextrose oligosaccharide from corn cob residues based on eutectic solvent synergistic microwave assistance

Abstract

The invention belongs to the field of biomass resource utilization, and particularly relates to a method for efficiently preparing glucose oligosaccharide from corn cob residues based on cooperation of a deep-eutectic solvent and microwave assistance. According to the method, lignin and hemicellulose in the corn cob residues are removed by utilizing the deep eutectic solvent (DES), the lignin is recovered, then the residues are treated by utilizing the advantages of lithium bromide and microwaves to efficiently prepare the oligosaccharides, the yield of the oligosaccharides is 72.6-77.1 wt% (based on the mass of cellulose in the corn cob residues), the distribution range of the polymerization degree of the oligosaccharides is 2-13, and the yield of the oligosaccharides is 72.6-77.1 wt% (based on the mass of cellulose in the corn cob residues). Wherein the glucose oligosaccharide with the polymerization degree of 3-9 accounts for most, resource utilization of the corn cob residues can be achieved, waste discharge can be reduced, the preparation cost of the glucose oligosaccharide can be reduced, wide application of the glucose oligosaccharide in the fields of food, medicine and the like is promoted, and important practical significance and wide market prospects are achieved.

Description

Technical Field

[0001] The invention belongs to the field of biomass resource utilization, and specifically relates to a method for efficiently preparing oligoglucose from xylose residue based on a low eutectic solvent and microwave assistance. Background Art

[0002] As the world's attention to sustainable development continues to increase, the efficient conversion of biomass resources has become a research hotspot. Xylose residue, as the main by-product produced in the xylose production process, is usually rich in a large amount of cellulose, lignin and a small amount of hemicellulose. However, most xylose residues are currently only treated as ordinary waste, which not only causes a huge waste of resources, but also may have a negative impact on the ecological environment such as soil and water bodies due to unreasonable disposal. How to convert these potential effective ingredients in xylose residues into high value-added products has become a key issue that needs to be broken through.

[0003] As a functional sugar, oligosaccharide has shown broad application prospects in many fields such as food and medicine. In the food field, it can be used as a sweetener to give food a unique flavor; at the same time, oligosaccharide has good moisture retention and stability, which can effectively improve the texture and shelf life of food. In the medical field, oligosaccharide can regulate the balance of intestinal flora, promote the growth of beneficial bacteria, and enhance the body's immunity; in addition, it also has potential application value in drug carriers, sustained-release preparations, etc.

[0004] There are many problems with the existing oligoglucose preparation methods. On the one hand, the cost of raw materials remains high, and some processes rely on expensive natural sugars or raw materials that have undergone complex purification, which greatly limits the large-scale production and application of oligoglucose; on the other hand, traditional processes are often cumbersome, involving multi-step reactions and complex separation and purification processes, which not only consumes a lot of time and energy, but also leads to a low yield of oligoglucose.

[0005] In summary, developing a method of using xylose residue as raw material, first using a deep eutectic solvent (DES) to remove lignin and hemicellulose and recover lignin, and then using the advantages of lithium bromide and microwaves to treat the residue to efficiently prepare oligosaccharides has important practical significance and broad market prospects. This can not only realize the resource utilization of xylose residue and reduce waste emissions, but also reduce the preparation cost of oligosaccharides and promote its wide application in food, medicine and other fields. Summary of the invention

[0006] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a method for efficiently preparing oligoglucose from xylose residue based on deep eutectic solvent and microwave-assisted. The present invention utilizes deep eutectic solvent (DES) to remove lignin and hemicellulose from xylose residue, recovers lignin, and then utilizes the advantages of lithium bromide and microwave to efficiently prepare oligoglucose from the residue, which can not only realize the resource utilization of xylose residue and reduce waste discharge, but also reduce the preparation cost of oligoglucose, promote its wide application in the fields of food, medicine, etc., and has important practical significance and broad market prospects.

[0007] In view of this, the present invention provides a method for efficiently preparing oligoglucose from xylose residues based on a deep eutectic solvent and microwave assistance, the method comprising the following steps:

[0008] (1) mixing xylose residue powder and DES, treating at 90-100° C. for 4-6 hours, and filtering to obtain a cellulose-containing residue and a solution containing hemicellulose and lignin;

[0009] (2) adding the residue obtained in step (1) to an acidified lithium bromide solution, placing the solution in a microwave heating device and heating the solution to 80-100° C. for reaction for 5-15 minutes; after the reaction is completed, rapidly cooling the reaction system to room temperature, filtering and separating to obtain a liquid phase containing glucose and oligosaccharides and an unreacted solid phase;

[0010] (3) adding an organic alcohol to the liquid phase obtained in step (2) to precipitate oligoglucose and separate the oligoglucose, wherein the oligoglucose has a degree of polymerization of 2-13.

[0011] Furthermore, in step (1), the particle size of the xylose residue powder is 0.1-1 mm.

[0012] Furthermore, in step (1), the DES is prepared by heating and stirring ethylamine hydrochloride, glycerol and aluminum chloride at 60-80° C. for 3 hours, wherein the molar ratio of ethylamine hydrochloride, glycerol and aluminum chloride is 1:2-3:0.01-0.03.

[0013] Furthermore, in step (1), the solution containing hemicellulose and lignin is treated with deionized water to obtain lignin.

[0014] Furthermore, in step (2), the mass volume ratio of the residue to the acidified lithium bromide is 1:15-30.

[0015] Furthermore, in step (2), the acid in the acidified lithium bromide solution is hydrochloric acid or sulfuric acid, and the acid concentration is 5-10 mol / L.

[0016] Furthermore, in step (2), the microwave power is 100-300W.

[0017] Furthermore, in step (3), the organic alcohol is one of methanol, ethanol, propanol or butanol, preferably ethanol.

[0018] Furthermore, in step (3), the liquid after the oligoglucose is precipitated is treated with a dialysis bag with a molecular weight of 100, the liquid outside the dialysis bag is collected, the organic alcohol and water therein are removed by rotary evaporation, and the lithium bromide is recovered for reuse.

[0019] Furthermore, the xylose residue powder consists of 59.9% cellulose, 9.6% hemicellulose and 21.5% lignin.

[0020] One or more of the above technical solutions have the following beneficial effects:

[0021] (1) The present invention utilizes a deep eutectic solvent (DES) to remove lignin and hemicellulose from xylose residue and recover lignin, and then utilizes the advantages of lithium bromide and microwaves to treat the residue to efficiently prepare oligoglucose, wherein the yield of oligoglucose is 72.6-77.1wt% (based on the mass of cellulose in xylose residue), and the distribution range of the degree of polymerization of oligosaccharides is 2-13, wherein oligoglucose with a degree of polymerization of 3-9 accounts for the majority, which can not only realize the resource utilization of xylose residue and reduce waste discharge, but also reduce the preparation cost of oligoglucose, and promote its wide application in the fields of food, medicine, etc., and has important practical significance and broad market prospects.

[0022] (2) It can be seen from the results of the examples of the present invention that the yield of oligoglucose after treatment with different DES ratios, microwave power and acidified lithium bromide concentrations is not significant. The combination of the three treatment methods can synergistically improve the yield of oligoglucose in xylose residue, mainly because the low eutectic solvent (DES) can selectively dissolve hemicellulose and lignin in xylose residue, thereby effectively removing these impurities, and can recover lignin by precipitation method, so that the raw materials for subsequent preparation of oligoglucose are purer, which is beneficial to improving the selectivity of the reaction and the quality of oligoglucose; lithium bromide can react with cellulose under specific conditions to promote the depolymerization of cellulose, and lithium bromide has good solubility and stability, and can be evenly dispersed in the reaction system, which helps to improve the uniformity and controllability of the reaction; microwaves can quickly penetrate the reaction materials, causing the internal temperature of the materials to rise rapidly, achieving rapid and uniform heating, effectively shortening the reaction time, and improving the reaction efficiency; at the same time, microwaves can also affect the activity and reaction path of molecules through non-thermal effects, promote the formation of target products, and reduce the occurrence of side reactions.

[0023] (3) The present invention converts the abundant, low-cost and renewable xylose residue waste raw material into high-value-added oligosaccharides. The separation time is short and the separation method is simple and easy. Lithium bromide is recovered and reused through dialysis rotation, realizing the high-value comprehensive utilization of biomass resources and having good economic and environmental benefits. DETAILED DESCRIPTION

[0024] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0025] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0026] The present invention is further described in conjunction with specific examples. The following examples are only for explaining the present invention and are not intended to limit the content thereof. If the specific experimental conditions are not specified in the examples, they are usually carried out under conventional conditions or under conditions recommended by the sales company; the materials and reagents used in the examples, unless otherwise specified, can be purchased through commercial channels.

[0027] In view of the low yield of preparing oligoglucose using xylose residue in the prior art, the present invention provides a method for efficiently preparing oligoglucose from xylose residue based on a deep eutectic solvent and microwave-assisted method. First, a deep eutectic solvent (DES) is used to remove lignin and hemicellulose and recover lignin. Then, the advantages of lithium bromide and microwave are used to treat the residue to efficiently prepare oligoglucose, thereby realizing efficient resource utilization of xylose residue.

[0028] Specifically, the embodiment of the present invention discloses a method for efficiently preparing oligoglucose from xylose residues based on a deep eutectic solvent and microwave assistance, comprising the following steps:

[0029] (1) mixing xylose residue powder and DES, treating at 90-100° C. for 4-6 hours, and filtering to obtain a cellulose-containing residue and a solution containing hemicellulose and lignin;

[0030] (2) adding the residue obtained in step (1) to an acidified lithium bromide solution, placing the solution in a microwave heating device and heating the solution to 80-100° C. for reaction for 5-15 minutes; after the reaction is completed, rapidly cooling the reaction system to room temperature, filtering and separating to obtain a liquid phase containing glucose and oligosaccharides and an unreacted solid phase;

[0031] (3) adding an organic alcohol to the liquid phase obtained in step (2) to precipitate oligoglucose and separate the oligoglucose, wherein the oligoglucose has a degree of polymerization of 2-13.

[0032] Among them, the deep eutectic solvent (DES) can selectively dissolve the hemicellulose and lignin in the xylose residue, thereby effectively removing these impurities, and can recover the lignin by precipitation, making the raw material for the subsequent preparation of oligosaccharides - cellulose purer, which is conducive to improving the selectivity of the reaction and the quality of oligosaccharides. At the same time, the synthesis process of the deep eutectic solvent is simple, without the need for complex separation and purification steps, and the cost is relatively low.

[0033] Lithium bromide has unique advantages in the preparation of oligoglucose. It can react with cellulose under specific conditions to promote the depolymerization of cellulose. Lithium bromide has good solubility and stability and can be evenly dispersed in the reaction system, which helps to improve the uniformity and controllability of the reaction. At the same time, lithium bromide has good recycling performance, which provides the possibility of reducing production costs. Microwaves, as an efficient heating method, play an important role in the preparation of oligoglucose. Microwaves can quickly penetrate the reaction materials, causing the internal temperature of the materials to rise rapidly, achieving rapid and uniform heating, effectively shortening the reaction time, and improving the reaction efficiency. In addition, microwaves can also affect the activity and reaction path of molecules through non-thermal effects, promote the formation of target products, and reduce the occurrence of side reactions.

[0034] In one or more embodiments of the present invention, in step (1), the processing process of the xylose residue powder is specifically as follows: the xylose residue is washed and dried, and the dried xylose residue is crushed into a particle size of 0.1-1 mm by a crushing device to obtain a pretreated xylose residue powder.

[0035] In one or more embodiments of the present invention, in step (1), the DES is prepared by heating and stirring ethylamine hydrochloride, glycerol and aluminum chloride at 60-80° C. for 3 hours until the DES is completely dissolved into a transparent liquid, wherein the molar ratio of ethylamine hydrochloride, glycerol and aluminum chloride is 1:2-3:0.01-0.03.

[0036] In one or more embodiments of the present invention, in step (1), the solution containing hemicellulose and lignin is treated with deionized water to obtain lignin, specifically, the solution containing hemicellulose and lignin is added to water to precipitate lignin, and the lignin is filtered, recovered, and dried to obtain a lignin product.

[0037] In one or more embodiments of the present invention, in step (2), the mass volume ratio of the residue to the acidified lithium bromide solution is 1:15-30.

[0038] In one or more embodiments of the present invention, in step (2), the acidified lithium bromide solution has a lithium bromide solution concentration of 50-65 wt%, and the acid is hydrochloric acid or sulfuric acid, and the acid concentration is 5-10 mol / L.

[0039] In one or more embodiments of the present invention, in step (2), the microwave power is 100-300W.

[0040] In one or more embodiments of the present invention, in step (3), the organic alcohol is one of methanol, ethanol, propanol or butanol. Preferably, the organic alcohol is ethanol.

[0041] The mass volume ratio of the liquid phase to the organic alcohol is 1:15.

[0042] In one or more embodiments of the present invention, in step (3), the liquid after the oligoglucose is precipitated is treated with a dialysis bag with a molecular weight of 100, the liquid outside the dialysis bag is collected, the organic alcohol and water therein are removed by rotary evaporation, and the lithium bromide is recovered for reuse.

[0043] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

[0044] Example 1

[0045] The xylose residue was washed with water for multiple times (to remove impurities), dried at 60°C to constant weight, and the dried xylose residue was crushed to a particle size of 0.1-1 mm by a crushing equipment to obtain pretreated xylose residue powder. The biomass component determination method (NREL / TP-510-42618) was used to determine that the mass proportion of cellulose in the xylose residue powder was 59.9%, the mass proportion of hemicellulose was 9.6%, and the mass proportion of lignin was 21.5%.

[0046] Example 2

[0047] Preparation of DES: Mix ethylamine hydrochloride-glycerol-aluminum chloride in a molar ratio of 1:2:0.01, heat and stir at 60°C for 3 hours until all dissolved into a transparent liquid.

[0048] The xylose residue powder prepared in Example 1 was mixed with DES in a mass ratio of 1:10, treated at 90°C for 4 hours to remove most of the hemicellulose and lignin in the raw material, and filtered to obtain a cellulose-rich residue and a solution containing hemicellulose and lignin. The filtrate was added to water to precipitate lignin, which was filtered, recovered, and dried to obtain a lignin product, with a lignin yield of 91.3wt% (based on the lignin content in the xylose residue).

[0049] The cellulose-rich residue is added to an acidified lithium bromide solution at a solid-liquid mass volume ratio of 1:15, and the reaction is carried out at 80°C under a microwave power of 100W for 5 minutes. In this process, the acidified lithium bromide solution serves as a reaction medium and catalyst to promote the hydrolysis of cellulose to generate oligoglucose. The concentration of the lithium bromide solution is 50wt%, and the acid is sulfuric acid with an acid concentration of 5mol / L.

[0050] Separation and purification of products: After the reaction is completed, the reaction system is quickly cooled to room temperature, and filtered to separate the liquid phase containing glucose and oligoglucose and the unreacted solid phase.

[0051] For the liquid product, ethanol is added to precipitate oligoglucose, the mass volume ratio of the liquid phase to ethanol is 1:15, and then the oligoglucose is separated by filtration; the precipitated part is oligoglucose, and the yield of oligoglucose is 72.6wt% (based on the mass of cellulose in xylose residue). The distribution of oligoglucose is detected by ion chromatography. The distribution range of the polymerization degree of oligoglucose is 2-13, among which the proportion of oligoglucose with a polymerization degree of 3-9 accounts for 81% of all oligoglucose.

[0052] The separated liquid is treated with a dialysis bag with a molecular weight of 100, and the liquid outside the dialysis bag is collected, and the ethanol and water therein are removed by rotary evaporation, and the lithium bromide is recovered for repeated use.

[0053] Example 3

[0054] Preparation of DES: Mix ethylamine hydrochloride-glycerol-aluminum chloride in a molar ratio of 1:3:0.02 and heat and stir at 70°C for 3 hours until they are completely dissolved into a transparent liquid.

[0055] The xylose residue powder prepared in Example 1 was mixed with DES in a mass ratio of 1:10, treated at 90°C for 6 hours to remove most of the hemicellulose and lignin in the raw material, and filtered to obtain a cellulose-rich residue and a solution containing hemicellulose and lignin. The filtrate was added to water to precipitate lignin, which was filtered, recovered, and dried to obtain a lignin product, with a lignin yield of 93.6wt% (based on the lignin content in the xylose residue).

[0056] The cellulose-rich residue is added to an acidified lithium bromide solution at a solid-liquid mass volume ratio of 1:20, and the reaction is carried out at 80°C under a microwave power of 200W for 8 minutes. In this process, the acidified lithium bromide solution serves as a reaction medium and catalyst to promote the hydrolysis of cellulose to generate oligoglucose. The concentration of the lithium bromide solution is 60wt%, and the acid is hydrochloric acid with an acid concentration of 5mol / L.

[0057] Separation and purification of products: After the reaction is completed, the reaction system is quickly cooled to room temperature, and filtered to separate the liquid phase containing glucose and oligoglucose and the unreacted solid phase.

[0058] For the liquid product, ethanol is added to precipitate oligoglucose, the mass volume ratio of the liquid phase to ethanol is 1:15, and then the oligoglucose is separated by filtration; the precipitated part is oligoglucose, and the yield of oligoglucose is 77.1wt% (based on the mass of cellulose in xylose residue). The distribution of oligoglucose is detected by ion chromatography. The distribution range of the polymerization degree of oligoglucose is 2-13, among which the proportion of oligoglucose with a polymerization degree of 3-9 accounts for 84% of all oligoglucose.

[0059] The separated liquid is treated with a dialysis bag with a molecular weight of 100, and the liquid outside the dialysis bag is collected, and the ethanol and water therein are removed by rotary evaporation, and the lithium bromide is recovered for repeated use.

[0060] Example 4

[0061] Preparation of DES: Mix ethylamine hydrochloride-glycerol-aluminum chloride in a molar ratio of 1:2:0.03 and heat and stir at 70°C for 3 hours until they are completely dissolved into a transparent liquid.

[0062] The xylose residue powder prepared in Example 1 and DES were mixed in a mass volume ratio of 1:10, treated at 100° C. for 4 hours to remove most of the hemicellulose and lignin in the raw material, and filtered to obtain a cellulose-rich residue and a solution containing hemicellulose and lignin. The filtrate was added to water to precipitate lignin, which was filtered, recovered, and dried to obtain a lignin product, with a lignin yield of 92.7 wt% (based on the lignin content in the xylose residue).

[0063] The cellulose-rich residue is added to an acidified lithium bromide solution at a solid-liquid mass volume ratio of 1:30, and the reaction is carried out at 80°C under a microwave power of 300W for 10 minutes. In this process, the acidified lithium bromide solution serves as a reaction medium and catalyst to promote the hydrolysis of cellulose to generate oligoglucose. The concentration of the lithium bromide solution is 55wt%, and the acid is sulfuric acid with an acid concentration of 7mol / L.

[0064] Separation and purification of products: After the reaction is completed, the reaction system is quickly cooled to room temperature, and filtered to separate the liquid phase containing glucose and oligoglucose and the unreacted solid phase.

[0065] For the liquid product, ethanol is added to precipitate oligoglucose, the mass volume ratio of the liquid phase to ethanol is 1:15, and then the product is filtered and separated; the precipitated part is oligoglucose, the yield of oligoglucose is 75.8wt%, and the distribution of oligoglucose is detected by ion chromatography. The distribution range of the polymerization degree of oligoglucose is 2-13, among which the proportion of oligoglucose with a polymerization degree of 3-9 accounts for 80.7% of all oligoglucose.

[0066] The separated liquid is treated with a dialysis bag with a molecular weight of 100, and the liquid outside the dialysis bag is collected, and the ethanol and water therein are removed by rotary evaporation, and the lithium bromide is recovered for repeated use.

[0067] Comparative Example 1:

[0068] Preparation of DES: Mix ethylamine hydrochloride-glycerol-aluminum chloride in a molar ratio of 1:3:0.02 and heat and stir at 70°C for 3 hours until they are completely dissolved into a transparent liquid.

[0069] The xylose residue powder prepared in Example 1 was mixed with DES in a mass volume ratio of 1:10, treated at 90°C for 6 hours to remove most of the hemicellulose and lignin in the raw material, and filtered to obtain a cellulose-rich residue and a solution containing hemicellulose and lignin. The filtrate was added to water to precipitate lignin, which was filtered, recovered and dried to obtain a lignin product, and the yield of lignin was 93.6wt% (based on the content of lignin in the xylose residue).

[0070] The above cellulose-rich residue is added to an acidified lithium bromide solution at a solid-liquid mass volume ratio of 1:20, and the reaction is carried out under conventional heating at 80°C for 8 minutes. In this process, the acidified lithium bromide solution serves as a reaction medium and catalyst to promote the hydrolysis of cellulose to produce oligoglucose. The concentration of the lithium bromide solution is 60wt%, and the acid is hydrochloric acid with an acid concentration of 5mol / L.

[0071] Separation and purification of products: After the reaction is completed, the reaction system is quickly cooled to room temperature, and filtered to separate the liquid phase containing glucose and oligoglucose and the unreacted solid phase.

[0072] For the liquid product, ethanol was added to precipitate oligoglucose, and the mass volume ratio of the liquid phase to ethanol was 1:15, and then the products were filtered and separated. The precipitated part was oligoglucose, and the yield of glucose was 7.8% and the yield of oligoglucose was 10.9%, indicating that the cellulose residue was not effectively degraded under conventional heating.

[0073] Comparative Example 2:

[0074] The xylose residue powder prepared in Example 1 was added to an acidified lithium bromide solution, with a solid-liquid mass volume ratio of 1:20, and the reaction was carried out at 80° C. under a microwave power of 200 W for 8 minutes. In this process, the acidified lithium bromide solution was used as a reaction medium and catalyst to promote the hydrolysis of cellulose to generate oligoglucose. The concentration of the lithium bromide solution was 60wt%, and the acid was hydrochloric acid with an acid concentration of 5mol / L.

[0075] Separation and purification of products: After the reaction is completed, the reaction system is quickly cooled to room temperature, and filtered to separate the liquid phase containing glucose and oligoglucose and the unreacted solid phase.

[0076] For the liquid product, ethanol was added to precipitate oligoglucose, and the mass volume ratio of the liquid phase to ethanol was 1:15, and then the product was filtered and separated; the precipitated part was oligoglucose, and the yield of oligoglucose was 16.2wt%, indicating that the failure to remove lignin and hemicellulose reduced the contact opportunity between cellulose and the solution, limiting the degradation of cellulose.

[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for efficiently preparing oligoglucose based on deep eutectic solvent and microwave-assisted xylose residue, characterized in that: The method comprises the following steps: (1) mixing xylose residue powder and DES, treating at 90-100° C. for 4-6 hours, and filtering to obtain a cellulose-containing residue and a solution containing hemicellulose and lignin; (2) adding the residue obtained in step (1) to an acidified lithium bromide solution, placing the solution in a microwave heating device and heating the solution to 80-100° C. for reaction for 5-15 minutes; after the reaction is completed, rapidly cooling the reaction system to room temperature, filtering and separating to obtain a liquid phase containing glucose and oligosaccharides and an unreacted solid phase; (3) adding an organic alcohol to the liquid phase obtained in step (2) to precipitate oligoglucose and separate the oligoglucose, wherein the oligoglucose has a degree of polymerization of 2-13.

2. The method according to claim 1, characterized in that In step (1), the particle size of the xylose residue powder is 0.1-1 mm.

3. The method according to claim 1, characterized in that In step (1), the DES is prepared by heating and stirring ethylamine hydrochloride, glycerol and aluminum chloride at 60-80° C. for 3 hours, wherein the molar ratio of ethylamine hydrochloride, glycerol and aluminum chloride is 1:2-3:0.01-0.

03.

4. The method according to claim 1, characterized in that In step (1), the solution containing hemicellulose and lignin is treated with deionized water to obtain lignin.

5. The method according to claim 1, characterized in that In step (2), the mass volume ratio of the residue to the acidified lithium bromide is 1:15-30.

6. The method according to claim 1, characterized in that In step (2), the acid in the acidified lithium bromide solution is hydrochloric acid or sulfuric acid, and the acid concentration is 5-10 mol / L.

7. The method according to claim 1, characterized in that In step (2), the microwave power is 100-300W.

8. The method according to claim 1, characterized in that In step (3), the organic alcohol is one of methanol, ethanol, propanol or butanol. Preferably, the organic alcohol is ethanol.

9. The method according to claim 1, characterized in that In step (3), the liquid after the oligoglucose is precipitated is treated with a dialysis bag with a molecular weight of 100, the liquid outside the dialysis bag is collected, the organic alcohol and water therein are removed by rotary evaporation, and the lithium bromide is recovered for reuse.

10. The method according to claim 1, characterized in that The xylose residue powder consists of 59.9% cellulose, 9.6% hemicellulose and 21.5% lignin.