A method for separating lignocellulose components and directionally preparing monosaccharides based on molten salt systems.

Abstract

This invention discloses a method for separating lignocellulose components and directionally preparing monosaccharides based on a molten salt system. Using lignocellulose as raw material, a high-concentration LiBr molten salt hydrate system is constructed, achieving efficient decomposition under mild conditions. Hemicellulose and cellulose are hydrolyzed to produce xylose and glucose, respectively, while lignin is efficiently recovered through filtration, thus forming a biomass refining process that utilizes three components synergistically. This invention features mild reaction conditions, high sugar yield, and effectively inhibits C-C bond condensation, resulting in lignin with extremely high β-O-4 aryl ether bond retention, maximizing the preservation of its original structure and chemical activity. This allows it to serve as a high-quality precursor for efficient conversion into high-value-added aryl monomers in subsequent catalytic depolymerization, providing a new pathway for the high-value utilization of all components of biomass.

Description

Technical Field

[0001] This invention belongs to the field of lignocellulose biomass resource utilization, specifically relating to a method for separating lignocellulose biomass components and directing monosaccharide conversion based on a molten salt hydrate system. Background Technology

[0002] Biomass, as one of the most abundant renewable resources in nature, boasts significant advantages such as wide availability and strong sustainability. Lignocellulose is the most abundant component of biomass. Through effective conversion of lignocellulose, hemicellulose and cellulose can be depolymerized to generate carbohydrate platform compounds, which can then be further converted into high-value-added chemicals or liquid fuels. Simultaneously, lignin can be used to prepare phenolic compounds, thus achieving high-value utilization of all three components of lignocellulose. This process not only helps reduce carbon dioxide emissions but also decreases dependence on fossil resources, playing a crucial role in building a multi-energy complementary system and promoting the development of green chemical industry.

[0003] Lignocellulose is mainly composed of three basic components: cellulose, hemicellulose, and lignin. Cellulose typically accounts for 30-60%, hemicellulose 15-30%, and lignin 15-30%, with their proportions influenced by factors such as the type of raw material, plant growth environment, and tissue location. In addition, lignocellulose contains small amounts of solvent extracts and ash. Cellulose is a linear polymer structure formed by D-glucose units linked by β-1,4-glycosidic bonds. Numerous intra- and inter-chain hydrogen bonds contribute to its high crystallinity and dense structure, significantly reducing its accessibility to reactants and enzymes. Hemicellulose is a branched heteropolysaccharide that coexists with cellulose microfibers and is linked by hydrogen bonds, serving as an important source of pentose sugars. Lignin is a natural aromatic polymer with phenylpropane as its basic structural unit. It possesses a certain linear and three-dimensional network structure and can be used as an important raw material for the preparation of phenolic chemicals. Meanwhile, lignin and hemicellulose are connected by covalent bonds, acting as a natural binder in plant cell walls. The three components form a complex and dense structure through intermolecular forces, chemical bonds, and physical coating, which severely limits the accessibility and conversion efficiency of carbohydrate components, thus becoming a major obstacle to the efficient utilization of lignocellulose.

[0004] Therefore, effective pretreatment of lignocellulose to achieve tri-component decomposition and separation is of great significance. However, traditional processes that prioritize the utilization of hemicellulose and cellulose typically require high temperature and high acidity conditions. These harsh conditions easily lead to the degradation or isomerization of sugar products, reducing sugar yield, and also trigger the breaking and condensation of β-O-4 bonds in the lignin structure, increasing the difficulty of subsequent lignin depolymerization. Conversely, when a lignin-preferred utilization strategy is adopted, the high temperature and high pressure reaction conditions inevitably cause the degradation of cellulose and its hydrolysis products, reducing cellulose utilization efficiency and product purity. Although organic solvent systems can achieve selective extraction of lignin and avoid cellulose loss to some extent, they have problems such as environmental burden and high cost, limiting practical application. Therefore, there is an urgent need to develop a green and efficient fractionation technology that can achieve lignocellulose structural decomposition under mild conditions while retaining all three components.

[0005] Currently, several technical routes have been developed for the separation and conversion of lignocellulose. One is the alkaline treatment strategy, which dissolves lignin through alkaline cooking and separates it from carbohydrate components. A second is the hydrolysis strategy, which converts cellulose and hemicellulose into monosaccharides under acidic or enzymatic catalysis, while lignin is separated as residue. A third is the special solvent dissolution strategy, which selectively dissolves a component using organic solvents, ionic liquids, or eutectic solvents under relatively mild conditions. A fourth is the lignin-preferred strategy, which preferentially degrades lignin to generate monophenols or cycloalkanes, and then separates the carbohydrate components. While these methods have achieved some degree of structural disruption and component separation of lignocellulose, they still suffer from problems such as demanding conditions, insufficient selectivity, or high environmental costs.

[0006] In terms of component conversion, cellulose and hemicellulose obtained from lignocellulose separation can be further hydrolyzed to produce monosaccharides such as glucose and xylose, and then converted into high-value-added products such as fuel ethanol, ethylene glycol, 2,5-dimethylfuran, 5-hydroxymethylfurfural, and xylitol through biological or chemical pathways. In contrast, lignin has a lower oxygen content and a higher calorific value, making it highly promising for the catalytic preparation of aromatic phenolic compounds and high-quality fuels. The β-O-4 bond is the most abundant and easily broken linker in natural lignin macromolecules, and its high retention rate is significant. On the one hand, it characterizes the separated lignin as having a nearly natural, complete skeleton; on the other hand, the abundant β-O-4 bonds endow the lignin with extremely high chemical reactivity, effectively overcoming the bottleneck of traditional industrial lignin being difficult to utilize due to high condensation, and providing an ideal high-quality precursor for subsequent catalytic depolymerization to prepare high-value-added aromatic monomer chemicals. However, in conventional separation processes, the β-O-4 bonds in lignin are prone to breakage and condensation reactions, forming difficult-to-break C-C bonds, significantly reducing its subsequent conversion efficiency. Therefore, achieving the deconstruction and separation of lignocellulose while preserving the integrity of its three functional components as much as possible is of great significance for promoting its comprehensive and high-value utilization.

[0007] In conclusion, developing a new process that can not only achieve efficient separation of the three components of lignocellulose under moderate and mild conditions, but also maximize the preservation of the natural active structure of lignin and simultaneously promote the directional decomposition of carbohydrates into high-value-added sugars is of great significance for promoting the comprehensive utilization of lignocellulose. Summary of the Invention

[0008] To address the shortcomings and deficiencies of existing lignocellulose separation and conversion technologies, such as harsh reaction conditions, easy degradation of sugar products, severe lignin structural condensation, and difficulty in simultaneously achieving high-value utilization of the three components, the present invention aims to provide a method for separating the three components of lignocellulose and simultaneously promoting the efficient hydrolysis of carbohydrates under relatively mild conditions, namely, a method for separating lignocellulose components and directionally preparing monosaccharides based on a molten salt system.

[0009] This invention uses lignocellulose as raw material to construct a LiBr molten salt hydrate reaction system. Utilizing the synergistic effect of its anions and cations, the system swells, dissolves, and deconstructs the structure of lignocellulose. While disrupting the hydrogen bond network of hemicellulose and cellulose, and some lignin-carbohydrate complex structures, the system's water content and reaction conditions are controlled to preferentially hydrolyze hemicellulose to produce xylose and other products. Furthermore, it promotes the deconstruction and hydrolysis of cellulose to produce glucose. Simultaneously, by controlling the reaction system conditions, the depolymerization and separation of lignin structures are achieved, thus establishing a novel process for the three-component stepwise separation and conversion of lignocellulose.

[0010] The specific molten salt hydrate system constructed in this invention has precise multi-faceted regulatory effects on the structure of lignocellulose. Firstly, based on achieving efficient swelling and decomposition of lignocellulose, the reaction system of this invention can finely control the hydrolysis process of hemicellulose and cellulose, achieving high-yield directional conversion of sugars. Secondly, it overcomes the technical bottleneck of traditional biomass separation methods, such as traditional hydrothermal methods, strong acid organic solvent methods, or traditional industrial sulfate methods, which inevitably lead to a large number of β-O-4 aryl ether bonds breaking and severe carbon-carbon (CC) bond condensation during lignin removal, resulting in a β-O-4 bond retention rate generally below 30% or even complete decomposition. The process of this invention can effectively inhibit the breaking and condensation reactions of β-O-4 bonds in the lignin structure, obtaining lignin products with extremely high structural preservation, which is greatly beneficial for their subsequent high-value utilization.

[0011] To achieve this objective, the present invention adopts the following technical solution:

[0012] This invention provides a method for separating lignocellulose components and directionally preparing monosaccharides based on a molten salt system, comprising the following steps:

[0013] (1) Mix the lignocellulose raw material with the molten salt hydrate solution evenly and heat to react. The hemicellulose in the lignocellulose raw material is hydrolyzed to produce xylose and the cellulose is hydrolyzed to produce glucose. The solid and liquid are separated to obtain a liquid product containing xylose and glucose and a solid residue containing lignin.

[0014] (2) Add the liquid product from step (1) into a weak acid solution and heat the reaction to carry out secondary hydrolysis to obtain a solution containing xylose and glucose;

[0015] (3) Mix the solid residue from step (1) with the acetylation reagent, heat to react, and purify to obtain acetylated lignin.

[0016] Preferably, in the molten salt hydrate solution of step (1), the molten salt includes at least one of LiBr, LiCl, ZnBr2, and ZnCl2; more preferably, it is LiBr.

[0017] Preferably, the mass fraction of molten salt in the molten salt hydrate solution in step (1) is 50-70 wt.%; more preferably, the mass fraction of molten salt is 55-65 wt.%.

[0018] Preferably, the lignocellulose raw material in step (1) includes at least one of corn stalks, pine sawdust, poplar, and birch; more preferably, it is birch.

[0019] Preferably, the mass ratio of the lignocellulose raw material and the molten salt in step (1) is 1:30-1:70; more preferably, it is 1:30-1:60.

[0020] Preferably, the heating reaction in step (1) is carried out at a temperature of 90-150°C, more preferably at 120-140°C, and for a time of 0.5-7h, more preferably 1-6h.

[0021] As one preferred embodiment, the lignocellulose raw material is birch, the reaction temperature is 120-140℃ (e.g., 120℃, 130℃, 140℃, etc.), and the reaction time is 1-6h (e.g., 1h, 2h, 3h, 4h, 5h, 6h, etc.).

[0022] Preferably, the weak acid solution in step (2) includes at least one of citric acid solution, acetic acid solution, salicylic acid solution, and lactic acid solution; the concentration of the weak acid solution is 200-500 mmol / L.

[0023] Preferably, the heating reaction in step (2) is carried out at a temperature of 130-160°C, more preferably at 150°C, and for a time of 30-90 min, more preferably at 45 min.

[0024] Preferably, the mass ratio of the liquid product to the weak acid solution in step (2) is 1:3 to 1:8, more preferably 1:5.

[0025] Preferably, the acetylation agent in step (3) includes at least one of acetyl bromide and acetic anhydride.

[0026] Preferably, the heating reaction in step (3) is carried out at a temperature of 30-60°C, more preferably at 40°C, and for a time of 2-10 hours, more preferably at 6 hours.

[0027] Preferably, the mass-to-volume ratio of the solid residue to the acetylation reagent in step (3) is 0.25g:0.5mL-0.25g:2mL, more preferably 0.25g:1mL.

[0028] Preferably, the solvent for the heating reaction in step (3) is glacial acetic acid; the mass-volume ratio of the solid residue to glacial acetic acid is 0.25g:2.5mL-0.25g:5mL, more preferably 0.25g:4mL.

[0029] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0030] The method for separating lignocellulose components and directional conversion of monosaccharides based on a molten salt hydrate system described in this invention has the advantages of simple system composition, selective conversion of hemicellulose and cellulose, high monosaccharide yield, effective inhibition of lignin condensation reaction and obtaining lignin products with high β-O-4 structure retention, thereby enhancing their subsequent utilization value. Attached Figure Description

[0031] Figure 1The distribution of hemicellulose conversion products after LiBr molten salt pretreatment of birch in Examples 1-3 is shown in (a) 120℃, (b) 130℃, and (c) 140℃.

[0032] Figure 2 The distribution of cellulose conversion products after LiBr molten salt pretreatment of birch in Examples 1-3 is shown in (a) 120℃, (b) 130℃, and (c) 140℃.

[0033] Figure 3 The distribution of products from the further hydrolysis of the liquid products of Examples 2-3 into monosaccharides under weakly acidic conditions was shown. The reaction conditions were: citric acid solution of 0.3 mol / L, reaction temperature and time of 150℃ and 45 min.

[0034] Figure 4 The images show the 2D-HSQC NMR spectra of lignin samples from Examples 1-2, with the left image corresponding to Example 1 and the right image corresponding to Example 2.

[0035] Figure 5 The diagram shows the main bonds and structural units of the lignin samples in Examples 1-2 using 2D-HSQC NMR. Detailed Implementation

[0036] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0037] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0038] Example 1

[0039] S1 Weigh 30g of LiBr and dissolve it in 20g of deionized water. Shake and sonicate until the solid is completely dissolved to prepare a 60wt.% LiBr solution.

[0040] S2. Add 0.5g of birch wood, a magnetic slab, and 30g of 60wt.% LiBr solution to a 48mL thick-walled pressure-resistant bottle. Place the bottle in an oil bath at 120℃ and 500rpm and react at atmospheric pressure for 1, 2, 3, 4, 5, and 6 hours. After the reaction is complete, cool to room temperature.

[0041] S3 separates the reaction mixture by filtration to obtain liquid product 1 and solid residue 1.

[0042] S4. Add 1g of liquid product 1 obtained from S3 and 5g of 0.3mol / L citric acid solution to a 15mL thick-walled pressure-resistant bottle, place it in an oil bath at 150℃ for further hydrolysis for 45min, and cool to room temperature after the reaction is complete.

[0043] In step S5, 0.25 g of the solid residue obtained in step S3, 1 mL of glacial acetic acid, and 1 mL of acetyl bromide were added to a 20 mL sample vial, and the mixture was reacted at 40 °C with a magnetic stirrer for 6 h. After the reaction was completed, the reaction solvent was evaporated and recovered using a rotary evaporator (40 °C), and the lignin sample precipitated. The precipitate was washed with deionized water, separated by vacuum filtration, and dried to obtain acetylated lignin.

[0044] (1) The concentrations of xylose and glucose in the liquid product 1 obtained from S3 were determined by high performance liquid chromatography, and the yields of xylose and glucose were calculated. The results are as follows: Figure 1-2 As shown.

[0045] (2) Add 1g of the liquid product obtained from the separation of S3 and 5g of 4wt.% H2SO4 to a 15mL thick-walled pressure-resistant bottle. Hydrolyze the product in an oil bath at 130℃ and 500rpm for 1h. After the reaction is complete, cool to room temperature. Analyze the solution using high-performance liquid chromatography. Calculate the yield of xylooligosaccharides and oligodextrose by the additional amount of xylose and glucose formed by dilute acid hydrolysis. The results are as follows: Figure 1-2 As shown.

[0046] (3) The content of xylose and glucose in S4 hydrolysate was detected by high performance liquid chromatography, and the yield of xylose and glucose after secondary hydrolysis was calculated. The results are shown in Table 2.

[0047] (4) Add 0.3g of solid residue 1 obtained from S3 and 3mL of 72wt% H2SO4 solution to a 100mL thick-walled pressure-resistant bottle, and react in a constant temperature shaker at 30℃ for 60min. After the reaction, add 84mL of deionized water and place in a high-pressure steam sterilizer at 121℃ for complete hydrolysis for 60min. After the reaction, cool to room temperature, filter to obtain liquid product 2 and solid residue 2. The content of xylose and glucose in the hydrolysate (liquid product 2) is detected by high performance liquid chromatography. Combined with the content of hemicellulose, cellulose and lignin in birch, the removal rate of hemicellulose and cellulose and the retention rate of lignin of biomass (birch) after treatment in the S1 molten salt system are calculated. The results are shown in Table 1.

[0048] (5) The acetylated lignin obtained from S5 was characterized by two-dimensional nuclear magnetic resonance and the retention rate of β-O-4 bonds was calculated. The results are shown in Table 3.

[0049] Example 2

[0050] The difference between this embodiment and Embodiment 1 is that the reaction temperature in S2 is 130°C, while the other process parameters are the same as in Embodiment 1.

[0051] Example 3

[0052] The difference between this embodiment and Embodiment 1 is that the reaction temperature in S2 is 140°C, while the other process parameters are the same as in Embodiment 1.

[0053] Comparative Example 1

[0054] The difference between this comparative example and Example 1 is that citric acid is added in S1. The specific steps are as follows:

[0055] S1 Weigh 30g LiBr and 1.417g citric acid and dissolve them in 20g deionized water. Shake and sonicate until the solid is completely dissolved to prepare a 60wt.% LiBr solution containing 0.3mol / L citric acid.

[0056] S2 Add 0.5g of birch wood, magnetic material, and 30g of 60wt.% LiBr solution containing 0.3mol / L citric acid to a 48mL thick-walled pressure-resistant bottle. Place the bottle in an oil bath at 120℃ and 500rpm and react at normal pressure for 1, 2, 3, 4, 5, and 6 hours. After the reaction is complete, cool to room temperature.

[0057] S3 separates the reaction mixture by filtration to obtain liquid product 1 and solid residue 1.

[0058] S4: Add 0.25g of the solid residue obtained in S3, 1.4mL of glacial acetic acid, and 1mL of acetyl bromide to a 20mL sample vial, and react in a magnetically heated stirrer at 40℃ for 6h. After the reaction is complete, the reaction solvent is evaporated and recovered using a rotary evaporator (40℃), and the lignin sample precipitates. Deionized water is added to wash and precipitate the solid, which is then separated by suction filtration and dried to obtain acetylated lignin.

[0059] (1) Add 1g of the liquid product obtained from the separation of S3 and 1.5g of 4wt.% H2SO4 to a 15mL thick-walled pressure-resistant bottle. Hydrolyze the product in an oil bath at 130℃ and 500rpm for 1h. After the reaction is complete, cool to room temperature. Analyze the solution using high-performance liquid chromatography. Calculate the yield of xylooligosaccharides and oligodextrose by the additional amount of xylose and glucose formed by dilute acid hydrolysis. The results are as follows: Figure 1-2 As shown.

[0060] (2) Add 0.3g of solid residue 1 obtained from S3 and 3mL of 72wt% H2SO4 solution to a 100mL thick-walled pressure-resistant bottle, and react in a constant temperature shaker at 30℃ for 60min. After the reaction, add 84mL of deionized water and place in a high-pressure steam sterilizer at 121℃ for complete hydrolysis for 60min. After the reaction, cool to room temperature, filter to obtain liquid product 2 and solid residue 2. The content of xylose and glucose in the hydrolysate (liquid product 2) is detected by high performance liquid chromatography. Combined with the content of hemicellulose, cellulose and lignin in birch, the removal rate of hemicellulose and cellulose and the retention rate of lignin in the biomass (birch) after treatment in the S1 citric acid molten salt system are calculated. The results are shown in Table 1.

[0061] (3) The acetylated lignin obtained from S4 was characterized by two-dimensional nuclear magnetic resonance and the retention rate of β-O-4 bonds was calculated. The results are shown in Table 3.

[0062] Comparative Example 2

[0063] The difference between this comparative example and comparative example 1 is that the reaction temperature in S2 is 130°C, while other process parameters are the same as in example 1.

[0064] Table 1 Separation results of raw material components

[0065]

[0066] Table 2 Product yield in S4 hydrolysate

[0067]

[0068] Table 3. Retention rate of acetylated lignin ether bonds obtained from S5

[0069]

[0070] The effects of different reaction temperatures and times on component separation and monosaccharide-oriented conversion of lignocellulose in the LiBr molten salt hydrate system are shown in Table 1-2. When birch was used as raw material, the highest monosaccharide yield was obtained under the following conditions: hemicellulose and cellulose removal rates reached 99.41% and 99.8%, respectively, and lignin retention rate was 71.61%. The yields of xylose and glucose were 90.54% and 93.58%, respectively.

[0071] The ether bond content of lignocellulose treated with the LiBr molten salt system is shown in Table 3. Under reaction conditions of 120℃ for 3 h, the hemicellulose removal rate reached 100%, and the lignin retention rate was 90.84%, with a β-O-4 retention rate of 83.79% in the lignin sample. Under reaction conditions of 130℃ for 1 h, the hemicellulose removal rate was 92.93%, and the lignin retention rate was 97.55%, with a β-O-4 retention rate of 80.89% in the lignin sample.

[0072] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for separating lignocellulose components and directionally preparing monosaccharides based on a molten salt system, characterized in that, Includes the following steps: (1) Mix the lignocellulose raw material with the molten salt hydrate solution evenly and heat to react. The hemicellulose in the lignocellulose raw material is hydrolyzed to produce xylose and the cellulose is hydrolyzed to produce glucose. The solid and liquid are separated to obtain a liquid product containing xylose and glucose and a solid residue containing lignin. (2) Add the liquid product from step (1) into a weak acid solution and heat the reaction to carry out secondary hydrolysis to obtain a solution containing xylose and glucose; (3) Mix the solid residue from step (1) with the acetylation reagent, heat to react, and purify to obtain acetylated lignin.

2. The method according to claim 1, characterized in that, The heating reaction in step (1) is carried out at a temperature of 90-150°C, more preferably 120-140°C, and for a time of 0.5-7h, more preferably 1-6h.

3. The method according to claim 1 or 2, characterized in that, The weak acid solution in step (2) includes at least one of citric acid solution, acetic acid solution, salicylic acid solution, and lactic acid solution; And / or, the concentration of the weak acid solution in step (2) is 200-500 mmol / L; And / or, the mass ratio of the liquid product and the weak acid solution in step (2) is 1:3 to 1:8, more preferably 1:

5.

4. The method according to claim 1 or 2, characterized in that, The heating reaction in step (2) is carried out at a temperature of 130-160°C, more preferably 150°C, for a time of 30-90 min, more preferably 45 min.

5. The method according to claim 1 or 2, characterized in that, The mass ratio of the lignocellulose raw material and molten salt in step (1) is 1:30-1:70; more preferably 1:30-1:

60. And / or, the mass fraction of molten salt in the molten salt hydrate solution of step (1) is 50-70 wt.%; more preferably, the mass fraction of molten salt is 55-65 wt.%.

6. The method according to claim 1 or 2, characterized in that, In step (1), the molten salt in the molten salt hydrate solution includes at least one of LiBr, LiCl, ZnBr2, and ZnCl2; more preferably, it is LiBr. And / or, the lignocellulose raw material in step (1) includes at least one of corn stalks, pine sawdust, poplar, and birch; more preferably birch.

7. The method according to claim 1 or 2, characterized in that, The acetylation reagent in step (3) includes at least one of acetyl bromide and acetic anhydride.

8. The method according to claim 1 or 2, characterized in that, The heating reaction in step (3) is carried out at a temperature of 30-60°C, more preferably 40°C, for a time of 2-10 hours, more preferably 6 hours.

9. The method according to claim 1 or 2, characterized in that, The mass-volume ratio of the solid residue to the acetylation reagent in step (3) is 0.25g:0.5mL-0.25g:2mL, more preferably 0.25g:1mL.

10. The method according to claim 1 or 2, characterized in that, The solvent for the heating reaction in step (3) is glacial acetic acid; the mass-volume ratio of the solid residue to glacial acetic acid is 0.25g:2.5mL-0.25g:5mL, more preferably 0.25g:4mL.

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