Method for separating lignocellulose component by alcohol compound molten salt hydrate

Abstract

The invention provides a method for separating a lignocellulose component from an alcohol compound molten salt hydrate, and belongs to the technical field of biomass energy conversion and utilization. According to the method, an organic acid acidified molten salt hydrate and alcohol are combined for use, so that the three components of lignocellulose are efficiently separated into lignin, cellulose and hemicellulose, and the cellulose and hemicellulose are further directionally converted to obtain oligosaccharides, including xylooligosaccharide and dextrose oligosaccharide. The separation condition does not need high temperature and is relatively mild, the reaction steps are simplified, and meanwhile, the environmental friendliness of the reaction is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of biomass energy conversion and utilization, and more specifically, to a method for separating lignocellulose components by using alcohol-compounded molten salt hydrate. Background Art

[0002] The overuse of fossil resources has caused a series of serious environmental problems. Countries have reached a consensus on developing green renewable resources to replace fossil resources. Lignocellulose is the most abundant carbon-containing renewable resource in nature. It has the advantages of short regeneration cycle, cheap and easy to obtain. It is one of the potential substitutes for fossil resources. Lignocellulose is mainly composed of cellulose, hemicellulose and lignin. The constituent units, connection methods and physical and chemical properties of these three components are very different, making it difficult to convert and utilize them at the same time. Therefore, the efficient separation of the three components is the basis and prerequisite for realizing the high-value utilization of all components of lignocellulose. The separation methods of the three components of lignocellulose are divided into two categories: one is the "cellulose priority" strategy, the core of which is to protect the original structure of cellulose and fully dissolve and remove the lignin component through strong alkali, high temperature and long-term reaction. However, this strategy will lead to partial degradation of hemicellulose and severe condensation of lignin structure, resulting in waste of resources; the other is the "lignin priority strategy", the core of which is to protect the original structure of lignin, especially the β-O-4 bond content. By regulating the reaction system and reaction conditions, lignin is dissolved under mild conditions, and hemicellulose is hydrolyzed into monosaccharides mainly composed of xylose, while cellulose is retained in solid form. However, the yield of lignin separated by this strategy is usually below 60%, and the separation efficiency is low.

[0003] Molten salt hydrate (MSH) is a special type of inorganic salt solution that can selectively dissolve cellulose hemicellulose but completely insoluble in lignin. This property can be used to separate lignocellulose components. For example, the literature (Green Chem., 2023, 25, 9272) uses molten salt acidified with hydrochloric acid to separate lignocellulose raw materials pretreated with microwave acid hydrolysis, effectively separating lignin and cellulose. However, microwave treatment increases costs and equipment requirements, and the conversion products of cellulose are numerous and have poor selectivity, which is not conducive to industrial applications. By adding an organic solvent to the molten salt to form a mutually soluble two-phase reaction system, the structure of lignin can be protected and the separation efficiency can be improved.

[0004] The prior art discloses a method for separating lignocellulose by forming a mutually soluble two-phase system with molten salt and small molecule solvents such as methanol, and disassembling the lignocellulose components at a relatively low temperature. However, in this process, cellulose is converted into long-chain oligosaccharides, short-chain oligosaccharides, glucose, and hydroxymethylfurfural products, and hemicellulose is converted into short-chain oligoxylose, xylose, furfural and other products. The conversion products of cellulose and hemicellulose have poor selectivity, and separation and subsequent conversion and utilization are difficult. The prior art also discloses a method for separating the three components of lignocellulose by a mutually soluble two-phase reaction system of a hydrochloric acid-acidified molten salt and a compound of a binary / trivalent alcohol. The binary / trivalent alcohol effectively protects the uncondensed structure of lignin, but cellulose and hemicellulose are simultaneously converted into oligosaccharides and monosaccharides, resulting in poor product selectivity and difficulty in separation.

[0005] Therefore, in order to improve product selectivity and purity and ensure the separation effect of lignocellulose, it is of great application value to develop a method for separating the three components of lignocellulose into lignin, cellulose and hemicellulose with high efficiency, and further converting cellulose and hemicellulose into oligoxylose and oligoglucose by alcohol-compounded molten salt hydrate to separate lignocellulose components. Summary of the invention

[0006] The present invention aims to overcome the deficiencies of the prior art and provides a method for separating lignocellulose components by using an alcohol-compounded molten salt hydrate, so as to achieve efficient separation and directional conversion of the three components of lignocellulose, obtain lignin and oligosaccharides, wherein the oligosaccharides include xylose oligosaccharides and glucose oligosaccharides, and maintain the high purity of lignin.

[0007] Therefore, the object of the present invention is to provide a method for separating lignocellulose components by using alcohol-compounded molten salt hydrate.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for separating lignocellulose components by using an alcohol-compounded molten salt hydrate, comprising the following steps:

[0010] S1. Acidifying the molten salt hydrate solution;

[0011] S2. mixing the lignocellulosic raw material, the acidified post-treated molten salt hydrate solution and the alcohol at a temperature below 150° C. for reaction;

[0012] S3. After the reaction is completed, the upper organic phase and the lower aqueous phase are collected respectively;

[0013] Wherein, the alcohol is an alcohol having 6 to 12 carbon atoms;

[0014] The acid used in the acidification treatment is an organic acid.

[0015] The present invention discloses a method for separating lignocellulose components by using alcohol-compounded molten salt hydrate, deconstructing three components of lignocellulose and converting them into sugar compounds and uncondensed lignin. The method constructs a two-phase system of organic acidic molten salt / alcohol that is mutually immiscible. On the one hand, the molten salt hydrate is treated with an organic acid to prevent condensation from occurring during the subsequent lignin extraction process, strip the connection between the components, extract the amphiphilic lignin component in situ into the amphiphilic alcohol system, block its contact with the molten salt system, and inhibit its structural polycondensation. On the other hand, cellulose and hemicellulose can be degraded into oligosaccharides. The molten salt hydrate degrades cellulose into oligosaccharides of 4 to 11, and hemicellulose into oligosaccharides of polymerization degree of 4 to 15. If inorganic acids such as hydrochloric acid and sulfuric acid are used, the lignin structure will be more strongly destroyed, resulting in the breaking of the internal connection of cellulose and hemicellulose, thereby releasing monosaccharides. The organic acid used in the present invention is not enough to completely destroy these connections, and can maintain longer fragments, which is helpful for the generation of polysaccharides.

[0016] Molten salt hydrate (MSH) can effectively destroy the hydrogen bond network in cellulose and hemicellulose due to its strong polarity. This destructive effect causes the swelling of the cellulose structure, which in turn promotes its dissolution in molten salt hydrate. Under acidic conditions, molten salt hydrate can catalyze the selective cleavage of some glycosidic bonds in cellulose and hemicellulose, converting these polysaccharides into oligosaccharides with a specific degree of polymerization.

[0017] Lignin, as an aromatic polymer with high molecular weight and complex structure, remains relatively stable during this process and is finally separated as a solid residue with high purity and structural integrity.

[0018] At the same time, alcohols with 6 to 12 carbon atoms undergo alkoxylation reactions with the Cα-OH groups in the phenylpropane structural units of lignin, effectively preventing the condensation reaction of lignin. In addition, the weak polarity and amphiphilic properties of alcohols with 6 to 12 carbon atoms give them good affinity with lignin. This affinity enables lignin to separate from the acidic aqueous phase and transfer to an organic phase system dominated by alcohols with 6 to 12 carbon atoms. In the process of transferring lignin to the alcohol phase, the purity of lignin is improved due to the reduction of its contact and reaction with other components in the reaction system.

[0019] Lignocellulose is composed of three components: cellulose, hemicellulose and lignin. Preferably, the organic acid in step S1 is selected from at least one of citric acid, acetic acid, salicylic acid, p-toluenesulfonic acid or succinic acid.

[0020] Preferably, the concentration of the acid used in the acidification treatment in step S1 is 20-500 mM.

[0021] Preferably, the mass of the acid added for the acidification treatment in step S1 is 0.01 to 0.05 times the mass of the molten salt hydrate.

[0022] Specifically, the pH value of the solution after acidification is 3 to 3.5.

[0023] Specifically, the enriched carbohydrate compound is an oligosaccharide, including xylose oligosaccharide with a polymerization degree of 4 to 15 and glucose oligosaccharide with a polymerization degree of 4 to 11.

[0024] Preferably, the molten salt hydrate solution in step S1 is formed by dissolving molten salt hydrate in water.

[0025] Preferably, the molten salt hydrate in step S1 is selected from at least one of LiBr, LiCl, ZnBr2 or ZnCl2.

[0026] More preferably, the molten salt hydrate in step S1 is selected from at least one of LiBr and LiCl.

[0027] Preferably, the concentration of the molten salt hydrate solution in step S1 is 20-85 wt.%.

[0028] More preferably, the concentration of the molten salt hydrate solution in step S1 is 40-65 wt.%.

[0029] Preferably, the alcohol in step S2 is an alcohol having 6 to 8 carbon atoms.

[0030] Preferably, the amount of the acidified molten salt hydrate solution added in step S2 is 40 to 60 times the mass of the lignocellulosic raw material added.

[0031] Preferably, the lignocellulosic raw material in step S2 is at least one of pine, poplar, eucalyptus or corn straw.

[0032] Preferably, the amount of the lignocellulosic raw material added in step S2 is 0.1-1 g.

[0033] Preferably, the alcohol in step S2 is at least one of hexanol, octanol or 2-ethylbutanol.

[0034] Preferably, the amount of alcohol added in step S2 is 10 to 40 times the mass of the added lignocellulosic raw material.

[0035] Preferably, the reaction temperature in step S2 is 70-150° C. A temperature higher than 150° C. will cause severe condensation of lignin and darken the color.

[0036] More preferably, the reaction temperature in step S2 is 90-130°C.

[0037] Preferably, the reaction time in step S2 is 0.5 to 5 h.

[0038] More preferably, the reaction time in step S2 is 1 to 4 hours.

[0039] Preferably, the method further comprises the step of post-treating the upper organic phase to obtain a filter residue containing lignin.

[0040] Preferably, the method further comprises the steps of using an anti-solvent to precipitate the lower aqueous phase to separate the oligosaccharides, and then adding water to dissolve the oligosaccharides to separate the xylose oligosaccharides from the glucose oligosaccharides.

[0041] Specifically, the method for separating lignocellulose components by combining alcohol with molten salt hydrate further comprises the following steps:

[0042] The upper organic phase rich in lignin is subjected to rotary evaporation to remove alcohol. The solid is freeze-dried to constant weight to obtain a dry filter residue containing lignin; the lower aqueous phase rich in carbohydrate compounds is used for oligosaccharides to be separated by anti-solvent precipitation, and the polarity difference of the anti-solvent is used to form a stronger interaction with the molten salt hydrate, weakening the interaction between the oligosaccharides and the molten salt hydrate, promoting the precipitation of oligosaccharides, and then adding water to dissolve the oligoxylose and separate it from the oligoglucose.

[0043] Lignin in the alcohol phase can be obtained by rotary evaporation. Oligosaccharides and oligoxylose in the molten salt phase can be separated by precipitation by adding low-carbon alcohols, ketones and other small molecule anti-solvents, and then oligoxylose can be further separated from oligosaccharides by adding water to dissolve, so as to efficiently separate the three components of lignocellulose and convert them into specific products.

[0044] Preferably, the anti-solvent is one of methanol, ethanol, isopropanol, acetone or acetonitrile.

[0045] Anti-solvent precipitation is a process in which the oligosaccharide-enriched hydrolyzate is added to 10 times the volume of the anti-solvent. After being stirred evenly at room temperature, a white solid precipitates out, and the oligosaccharide is obtained by centrifugation.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The invention provides a method for separating lignocellulose components by using an alcohol-compounded molten salt hydrate. The method uses a molten salt hydrate acidified with an organic acid and an alcohol in combination, so as to achieve high-efficiency separation and directional conversion of three components of lignocellulose. The yield of uncondensed lignin obtained by separation is more than 87.6%, the purity of lignin is more than 80%, the yield of oligosaccharides is more than 75%, and the yield of oligoxylose is more than 60%, showing the high efficiency and high selectivity of the method in separating biomass components. The separation conditions of the invention are relatively mild, high temperature is not required, the reaction steps are simplified, and the environmental friendliness of the reaction is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Flow chart for the separation of various components of lignocellulose;

[0049] Figure 2 The figure is the effect of different citric acid concentrations on the yield of oligosaccharides and monosaccharide products in Example 1 and Example 3;

[0050] Figure 3 Graph showing the effect of different citric acid concentrations on the yield of xylan and xylose products. Other reaction conditions were the same as in Example 1. DETAILED DESCRIPTION

[0051] In order to more clearly and completely describe the technical solution of the present invention, the present invention is further described in detail through specific embodiments below. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention. Various changes can be made within the scope of the rights of the present invention.

[0052] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0053] Test indicators:

[0054] (1) Lignin yield (%): The upper organic phase rich in lignin is evaporated to remove alcohol and then the solid is freeze-dried to constant weight to obtain a dry filter residue containing lignin. The ratio of the obtained dry filter residue to the lignin content measured in the raw material components is the lignin yield;

[0055] (2) Purity of lignin (%): measured by cysteine ​​method:

[0056] 1. Solution preparation: First, dissolve 10 g of L-cysteine ​​in 100 mL of 72% sulfuric acid to prepare a cysteine ​​solution containing 72% sulfuric acid;

[0057] 2. Sample treatment: 5 mg of lignin sample was placed in a glass vial, and then 1.0 mL of the cysteine ​​solution prepared above was added;

[0058] 3. Mixing: Use a magnetic stirrer to stir the mixture at a speed of 400 r / min for 60 min to ensure that the sample is completely dissolved;

[0059] 4. Dilution: The solution was further diluted to 50 mL with deionized water to form a diluted solution with a concentration of less than 1 g / L for UV detection;

[0060] 5. Ultraviolet absorption measurement: measure the ultraviolet absorption of the diluted solution at λ = 283nm;

[0061] 6. Calculate the purity: Use the following formula to calculate the purity of lignin:

[0062] Lignin purity (%) = (A 283 ×V) / (ε×m)

[0063] A 283 is the UV absorption of the diluted solution at 283 nm; V is the total volume of the diluted solution (liters); m is the mass of the lignin sample (grams); ε is the absorption coefficient of the UV absorption of lignin at λ = 283 nm

[0064] (3) Yield of oligosaccharide (%): The yield was calculated by analyzing the lower aqueous phase solution enriched with sugar compounds using high performance liquid chromatography;

[0065] (4) Yield of xylooligosaccharides (%): The yield was calculated by analyzing the lower aqueous phase solution enriched with sugar compounds using high performance liquid chromatography.

[0066] Example 1 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0067] S1 Weigh 4 g of ultrapure water, 0.089 g of citric acid and 6 g of LiBr, and mix them to obtain a 60 wt.% LiBr solution containing 240 mM citric acid concentration.

[0068] S2 Put 0.2g of pine wood, 10g of 60wt.% LiBr solution containing 240mM citric acid concentration, and 5g of hexanol solution into a 100mL thick-walled glass reaction tube, pH=3, put it into an oil bath at 110℃, react at normal pressure for 1h, then take it out and cool to room temperature after the reaction is completed.

[0069] S3 uses a separatory funnel to separate the reaction solution, the upper layer is an organic phase rich in lignin, and the lower layer is an aqueous phase containing sugar compounds.

[0070] S4 The upper organic phase rich in lignin was subjected to rotary evaporation. The solid insoluble matter was freeze-dried to constant weight to obtain a dry filter residue containing lignin. The purity of lignin is shown in Table 1.

[0071] S5 collects the lower aqueous phase enriched with carbohydrate compounds, uses methanol for anti-solvent precipitation to separate oligosaccharides, and then oligoxylose is dissolved by adding water and further separated from oligoglucose.

[0072] (2) Take 1g of the liquid separated by the anti-solvent and mix it with 5g of 4wt.% H2SO4 in a 15mL thick-walled glass reactor. In a 130℃ oil bath, stir magnetically at 300rpm for 1h. After the reaction is completed, cool to room temperature. After the reaction is completed, cool to room temperature. The oligosaccharides and oligoxylose in the hydrolyzate are fully hydrolyzed into glucose and xylose, respectively. The solution is analyzed by high performance liquid chromatography, and the amount of oligosaccharides and oligoxylose is calculated using the additional amount of glucose and xylose formed by dilute acid hydrolysis. The product yield is shown in Table 1.

[0073] Example 2 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0074] The experimental method is the same as that of Example 1, except that the acid in S1 has a concentration of 240 mM hydrochloric acid.

[0075] Example 3 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0076] The experimental method is the same as that of Example 1, except that the acid in S1 is 30 mM citric acid solution.

[0077] Example 4 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0078] The experimental method is the same as that of Example 1, except that the reaction time in S2 is 40 min.

[0079] Example 5 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0080] The experimental method is the same as that of Example 1, except that the reaction time in S2 is 5 h.

[0081] Example 6 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0082] The experimental method is the same as that of Example 1, except that the concentration of the molten salt hydrate solution in S1 is 20 wt.%.

[0083] Example 7 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0084] The experimental method is the same as that of Example 1, except that the concentration of the molten salt hydrate solution in S1 is 85 wt.%.

[0085] Example 8 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0086] The experimental method is the same as that of Example 1, except that the alcohol in S2 is dodecanol.

[0087] Example 9 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0088] The experimental method is the same as that of Example 1, except that the alcohol in S2 is octanol.

[0089] Example 10 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0090] The experimental method is the same as that of Example 1, except that the alcohol in S2 is 2-ethylbutanol.

[0091] Example 11 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0092] The experimental method is the same as that of Example 1, except that the molten salt compound in S2 is LiCl.

[0093] Example 12 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0094] The experimental method is the same as that of Example 1, except that the molten salt compound in S2 is ZnBr2.

[0095] Comparative Example 1 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0096] A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate is different from that in Example 1 in that no alcohol is added in S2, and the corresponding reaction is terminated without performing a step of separating and removing the alcohol. The specific steps are as follows:

[0097] S1 Weigh 4 g of ultrapure water, 0.089 g of citric acid and 6 g of LiBr, and mix them to obtain a 60 wt.% LiBr solution containing 240 mM citric acid concentration.

[0098] S2: Put 0.2g of pine wood and 10g of 60wt.% LiBr solution containing 240mM citric acid into a 100mL thick-walled glass reaction tube, put it into an oil bath at 110°C, react at normal pressure for 1h, take it out, and cool it to room temperature after the reaction is completed.

[0099] S3: Filter the reaction solution and conduct quantitative analysis by high performance liquid chromatography.

[0100] (1) 1 g of the filtered reaction solution was diluted 10 times with water and quantitatively analyzed by high performance liquid chromatography. The yields of glucose and xylose were calculated and shown in Table 1.

[0101] (2) Take 1g of the filtered reaction solution and mix it with 5g of 4wt.% H2SO4 in a 15mL thick-walled glass reactor. In a 130℃ oil bath, stir magnetically at 300rpm for 1h. After the reaction is completed, cool to room temperature. The oligosaccharides and oligoxylose in the hydrolyzate are fully hydrolyzed into glucose and xylose, respectively. The solution is analyzed by high performance liquid chromatography, and the amount of oligosaccharides and oligoxylose is calculated by the additional amount of glucose and xylose formed by dilute acid hydrolysis. The product yield is shown in Table 1.

[0102] S4: Filter the reaction solution by suction, wash the solid insoluble matter with ultrapure water until the filtrate is transparent. Freeze-dry the solid insoluble matter to constant weight to obtain a dry filter residue containing lignin. The purity of lignin is shown in Table 1.

[0103] Comparative Example 2 A method for separating lignocellulose components by using an alcohol-compounded molten salt hydrate

[0104] The experimental method is the same as that of Example 1, except that the alcohol in S2 is butanol.

[0105] Comparative Example 3 A method for separating lignocellulose components by combining alcohol with molten salt hydrate

[0106] The experimental method is the same as that of Example 1, except that the alcohol in S2 is tetradecanol.

[0107] Comparative Example 4 A method for separating lignocellulose components by combining alcohol with molten salt hydrate

[0108] The experimental method is the same as that of Example 1, except that no molten salt compound is added to S2.

[0109] Comparative Example 5 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0110] The experimental method is the same as that of Example 1, except that the molten salt compound is not acidified in step S1.

[0111] Comparative Example 6 A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate

[0112] The experimental method is the same as that of Example 1, except that the molten salt compound is acidified with sulfuric acid in step S1 so that the pH value of the solution is 3, which is the same as the pH value of the solution in Example 1.

[0113] Table 1 Product yield and lignin purity

[0114]

[0115]

[0116] It can be seen from Table 1 that the methods for separating lignocellulose components by using alcohol-compounded molten salt hydrates in Examples 1 to 12 of the present invention have product yields of more than 75% oligosaccharide yield, more than 60% xylose yield, more than 87.6% lignin yield, and more than 80% lignin purity. Among them, the method in Example 1 has the best extraction product and purity for lignin.

[0117] In Comparative Example 1, no alcohol is added, the purity of lignin decreases, and the yields of oligosaccharides and oligoxylose decrease. In Comparative Example 2, butanol is used to replace hexanol, which makes it difficult to inhibit the condensation of lignin and the purity decreases significantly. In Comparative Example 3, tetradecanol is used, which cannot further improve the extraction purity of lignin, but decreases it, and there are hidden dangers in storage and use safety. In Comparative Example 4, no molten salt compound is added, which is difficult to convert into oligosaccharides, and the yields are significantly reduced. In Comparative Example 5, the molten salt compound is not acidified in step S1, and the comprehensive yield and purity are reduced, indicating that acidification is a necessary condition for the reaction. In Comparative Example 6, sulfuric acid is used to acidify the molten salt compound, cellulose and hemicellulose are converted into oligosaccharides and monosaccharides, and the yields of oligosaccharides are reduced.

[0118] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for separating lignocellulose components by using alcohol-compounded molten salt hydrate, characterized in that: The following steps are involved: S1. Acidifying the molten salt hydrate solution; S2. mixing the lignocellulosic raw material, the acidified post-treated molten salt hydrate solution and the alcohol at a temperature below 150° C. for reaction; S3. After the reaction is completed, the upper organic phase and the lower aqueous phase are collected respectively; Wherein, the alcohol is an alcohol having 6 to 12 carbon atoms; The acid used in the acidification treatment is an organic acid.

2. The method according to claim 1, characterized in that: The organic acid in step S1 is selected from at least one of citric acid, acetic acid, salicylic acid, p-toluenesulfonic acid or succinic acid.

3. The method according to claim 1, characterized in that The concentration of the acid used in the acidification treatment in step S1 is 20-500 mM.

4. The method according to claim 1, characterized in that The molten salt hydrate in step S1 is selected from at least one of LiBr, LiCl, ZnBr2 or ZnCl2.

5. The method according to claim 1, characterized in that The concentration of the molten salt hydrate solution in step S1 is 20-85 wt.%.

6. The method according to claim 1, characterized in that The alcohol in step S2 is an alcohol having 6 to 8 carbon atoms.

7. The method according to claim 1, characterized in that The amount of alcohol added in step S2 is 10 to 40 times the mass of the lignocellulose raw material added.

8. The method according to claim 1, characterized in that The reaction time in step S2 is 0.5 to 5 hours.

9. The method according to claim 1, characterized in that: The method also includes the step of post-treating the upper organic phase to obtain a filter residue containing lignin.

10. The method according to claim 1, characterized in that The method also includes the steps of using an anti-solvent to precipitate the lower aqueous phase to separate the oligosaccharides, and then adding water to dissolve the oligosaccharides to separate the xylose oligosaccharides from the glucose oligosaccharides.

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