A method for preparing lignin nanoparticles and levulinic acid by using wood chip pre-hydrolysate
By subjecting the wood chip pre-hydrolysate to hydrothermal treatment and centrifugal washing and drying, stable lignin nanoparticles and high-yield levulinic acid were prepared, solving the problems of low resource utilization and low levulinic acid yield of wood chip pulping pre-hydrolysate and achieving environmentally friendly and efficient utilization.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- QILU UNIVERSITY OF TECHNOLOGY (SHANDONG ACADEMY OF SCIENCES)
- Filing Date
- 2024-01-05
- Publication Date
- 2026-07-31
AI Technical Summary
In existing technologies, the resource utilization rate of wood chip pulp prehydrolysate is low, the yield of acetylpropionic acid is low, lignin is not fully utilized, and there are environmental pollution problems.
Lignin nanoparticles were prepared by adding p-toluenesulfonic acid to the pre-hydrolyzed wood chip solution after hot water cooking and then performing hydrothermal treatment, followed by centrifugation, washing, and drying. Aleucopropionic acid was then obtained through a second hydrothermal treatment.
It achieves high stability and high yield of lignin nanoparticles and high yield of levulinic acid, and is simple to operate, has high resource utilization, and is environmentally friendly.
Smart Images
Figure CN117946414B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of biomass refining technology, and in particular to a method for preparing lignin nanoparticles and acetylpropionic acid using wood chip pre-hydrolyzed solution. Background Technology
[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.
[0003] In the pulp and paper and biomass refining industries, pre-hydrolyzing wood chips via hot water cooking is a crucial pretreatment method for producing high-purity cellulose-based products and bio-based chemicals. This process reduces the hemicellulose and lignin content in the wood chips, increasing cellulose purity and thus improving subsequent production efficiency. Furthermore, pre-hydrolysis facilitates the penetration and impregnation of chemicals, reducing the amount of chemicals used. During the hot water cooking pre-hydrolysis process, a small portion of the hemicellulose and lignin in the lignocellulose raw material undergoes hydrolysis and degradation, dissolving in the pre-hydrolysis solution to form a pre-hydrolyzed liquid rich in sugars and low molecular weight lignin. The pre-treated wood chips then proceed to the subsequent pulping process, while the pre-hydrolyzed liquid is typically discharged as wastewater, causing wastewater pollution and resource waste.
[0004] Hemicellulose is an important raw material for the production of levulinic acid, which, as a crucial bio-based platform chemical, is widely used in pharmaceuticals, biochemicals, food additives, and energy. Lignin, with its non-toxic, thermally stable, biocompatible, and biodegradable properties, has found widespread application as a carrier for precious metal nanomaterials, an emulsion stabilizer, a surfactant, a heavy metal adsorbent, and a drug delivery carrier. However, due to the complex composition of poplar wood chip pre-hydrolyzed solutions and the complex separation and purification steps, the yield of levulinic acid is low; lignin has also not been fully utilized.
[0005] Therefore, how to make full use of wood chip pulp pre-hydrolysate in a simple and convenient way to prepare lignin nanoparticles with good stability and levulinic acid with high yield is an urgent problem to be solved. Summary of the Invention
[0006] In view of this, the present invention provides a method for preparing lignin nanoparticles and levulinic acid using wood chip pre-hydrolysate. The method obtains lignin nanoparticles with a stable structure through a one-step reaction and obtains levulinic acid with a high yield through a two-step hydrothermal reaction. The operation process is simple and convenient, and can fully utilize the wood chip pulping pre-hydrolysate for high-value purposes.
[0007] This invention provides a method for preparing lignin nanoparticles and levulinic acid using a wood chip pre-hydrolyzed solution, comprising the following steps:
[0008] Add 10-30 wt% p-toluenesulfonic acid to the pre-hydrolyzed wood chip solution after hot water cooking, mix thoroughly, and then perform a first hydrothermal treatment at a temperature of 100-110℃ for 0.5-2 hours. Then centrifuge, wash and dry the precipitate to obtain lignin nanoparticles. Heat the centrifuged solution to 140-190℃ for a second hydrothermal treatment for 0.5-4 hours to obtain levulinic acid.
[0009] Preferably, the hot water cooking step is as follows: placing the wood chips in a cooking pot at 160-190°C for 70-100 minutes to obtain a pre-hydrolyzed wood chip solution.
[0010] Preferably, the lignin content in the wood chip pre-hydrolyzed solution is 8-15 g / L, and the hemicellulose content is 15-25 g / L.
[0011] Preferably, the lignin nanoparticles have a particle size of 120–450 nm and a sugar content of 8–12 wt%.
[0012] Preferably, the concentration of p-toluenesulfonic acid added is 18-22 wt%.
[0013] Preferably, the centrifugation speed is 8000-10000 rpm and the centrifugation time is 5-15 min.
[0014] Preferably, the solvent used for washing is distilled water.
[0015] Preferably, the drying temperature is 50-70°C and the time is 8-20 hours.
[0016] Preferably, the centrifuged solution is heated to 160-180°C for a second hydrothermal treatment, which lasts for 1-2 hours, to obtain levulinic acid.
[0017] Preferably, after the second hydrothermal treatment, the step further includes extracting levulinic acid using an organic solvent.
[0018] Furthermore, the organic solvent includes one of tri-n-octylphosphine oxide, tributyl phosphate, ethyl acetate, or dichloromethane.
[0019] Preferably, after the step of obtaining levulinic acid, the method further includes a step of concentrating and crystallizing to recover p-toluenesulfonic acid, wherein the recovered p-toluenesulfonic acid is added to the wood chip pre-hydrolysate after hot water cooking for reuse.
[0020] Compared with the prior art, the present invention has achieved the following beneficial effects:
[0021] (1) Wood chip pre-hydrolyzed liquid, as the raw material of the present invention, is itself an industrial waste that pollutes the environment; therefore, the raw material required by the present invention is low in cost and universal, and is a green, environmentally friendly, and easy-to-scale production method.
[0022] (2) The present invention can simultaneously utilize the low content of hemicellulose and lignin in the wood chip pre-hydrolysate, and p-toluenesulfonic acid can be recycled throughout the process, which is a green preparation process.
[0023] (3) The lignin nanoparticles prepared by the present invention can be directly precipitated after one-step hydrothermal treatment without the need for the addition of anti-solvent, and the operation method is simple.
[0024] (4) This invention utilizes the characteristics of sugar and lignin components in the pretreatment solution as a key step in the preparation of lignin nanoparticles by precisely controlling the hydrothermal reaction temperature and the addition ratio of p-toluenesulfonic acid. The first hydrothermal treatment effectively degrades some hemicellulose while retaining some hemicellulose / polysaccharide, allowing the nanoparticles to maintain their shape while increasing their stability. Therefore, due to the abundance of phenolic hydroxyl groups and appropriate amounts of sugar components in the lignin nanoparticles, the surface of the lignin nanoparticles exhibits high hydroxyl ionization, resulting in a higher specific surface charge in the solution system and demonstrating excellent solution dispersibility and stability. In addition, the lignin nanoparticles exhibit good monodispersity. Attached Figure Description
[0025] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.
[0026] Figure 1 This is a technical roadmap of an embodiment of the present invention;
[0027] Figure 2 This describes the formation mechanism of the lignin nanoparticles in this invention.
[0028] Figure 3 These are scanning electron microscope images and size distribution diagrams of the lignin nanoparticles prepared in Examples 1 (a, e), 2 (b, f), 3 (c, g), and 4 (d, h) of the present invention.
[0029] Figure 4These are laser irradiation images of the lignin nanoparticle aqueous dispersions prepared in Examples 1, 2, 3, and 4 of this invention, from left to right: Examples 1, 2, 3, and 4.
[0030] Figure 5 These are photographs of the dispersion stability of the lignin nanoparticle aqueous dispersions prepared in Examples 1, 2, 3, and 4 of this invention before and after standing. From left to right, they are Examples 1, 2, 3, and 4.
[0031] Figure 6 These are scanning electron microscope images and size distribution diagrams of the lignin nanoparticles prepared in Examples 8(a, c) and 9(b, d) of the present invention. Detailed Implementation
[0032] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.
[0033] This invention provides a method for preparing lignin nanoparticles and levulinic acid using a wood chip pre-hydrolyzed solution, comprising the following steps:
[0034] Add 10-30 wt% p-toluenesulfonic acid to the pre-hydrolyzed wood chip solution after hot water cooking, mix thoroughly, and then perform a first hydrothermal treatment at a temperature of 100-110℃ for 0.5-2 hours. Then centrifuge, wash and dry the precipitate to obtain lignin nanoparticles. Heat the centrifuged solution to 140-190℃ for a second hydrothermal treatment for 0.5-4 hours to obtain levulinic acid.
[0035] In existing processes, pre-hydrolyzed liquid is often treated as waste, resulting in low resource utilization and environmental pollution. Because hot water cooking is a relatively mild pre-hydrolyzed method, the resulting wood chip pre-hydrolyzed liquid has low lignin and hemicellulose content, making it difficult to utilize. However, this invention discovers that by adding an appropriate proportion of p-toluenesulfonic acid and precisely controlling the temperature, lignin nanoparticles with uniform particle size and high stability can be obtained, which are easily dispersed and do not easily settle in solution systems. This is because the regulation of temperature and p-toluenesulfonic acid affects the degradation of hemicellulose. A suitable temperature ensures that the lignin nanoparticles contain appropriate amounts of oligosaccharides and monosaccharides, which carry a negative charge in the aqueous medium. Too low a temperature will lead to excessively high sugar content, resulting in a significant increase in electrostatic repulsion and steric hindrance, making it difficult for lignin molecules to cluster and form nanoparticles; or the formed nanoparticles will have an excessively large particle size and poor stability. Excessive temperature will cause a significant decrease in sugar content. Although the electrostatic repulsion and steric hindrance forces in the system decrease, the hydrogen bonds and π-π stacking forces between lignin molecules increase, which is conducive to the aggregation of lignin into nanoparticles with smaller particle size. However, the surface charge of the generated lignin nanoparticles is too low, making them unstable. The formed nanoparticles are severely cross-linked, making it impossible to obtain monodisperse lignin nanoparticles.
[0036] Lignin is a polymer with a three-dimensional network structure containing phenylpropane structural units linked by ether and carbon-carbon bonds. In recent years, research on its preparation into lignin nanoparticles has developed rapidly. Lignin nanoparticles possess characteristics such as non-toxicity, good thermal stability, biocompatibility, and biodegradability, and have found wide applications in fields such as noble metal nanomaterial carriers, emulsion stabilizers, surfactants, heavy metal adsorbents, and drug delivery carriers.
[0037] The hot water cooking step described in this invention involves placing wood chips in a cooking pot at 160–190°C for 70–100 minutes to obtain a pre-hydrolyzed wood chip liquor. This step is a common pretreatment step in existing pulping processes. Hot water cooking pre-hydrolyzation is relatively mild, ensuring an appropriate reduction in the hemicellulose and lignin content of the wood chips without affecting the overall yield of the wood chips. It also results in a more porous structure in the wood chips, which helps reduce the amount of chemicals used in subsequent processing. The pre-hydrolyzed wood chip liquor obtained after hot water cooking pretreatment contains 8–15 g / L of lignin and 15–25 g / L of hemicellulose.
[0038] In this invention, the lignin nanoparticles have a particle size of 120–450 nm and a sugar content of 8–12 wt%. Lignin nanoparticles within the above particle size and sugar content ranges maintain good stable dispersion in solution or emulsion systems and are not prone to sedimentation.
[0039] In this invention, the concentration of p-toluenesulfonic acid is 18–22 wt%. The concentration of p-toluenesulfonic acid affects the yield of lignin nanoparticles.
[0040] This invention does not impose special limitations on the centrifugation process; it only requires successful separation of the solid and liquid. The centrifugation speed described in this invention is 8000–10000 rpm, and the centrifugation time is 5–15 min.
[0041] In this invention, the solvent used for washing is distilled water. Using distilled water to wash the lignin nanoparticles until they are neutral facilitates their subsequent utilization.
[0042] The present invention does not impose any special restrictions on the drying temperature. The drying temperature described in the present invention is 50-70°C and the time is 8-20 hours.
[0043] To obtain a higher yield of levulinic acid, the present invention heats the centrifuged solution to 160-180°C for a second hydrothermal treatment, which lasts for 1-2 hours, to obtain levulinic acid.
[0044] In this invention, after the second hydrothermal treatment, the method further includes a step of extracting levulinic acid using an organic solvent. This invention does not impose any particular limitation on the organic solvent; any commonly used organic extractant in the art can be used. Preferably, the solvent is selected from tri-n-octylphosphine oxide, tributyl phosphate, ethyl acetate, or dichloromethane.
[0045] In this invention, after the step of obtaining levulinic acid, the method further includes a step of concentrating and crystallizing to recover p-toluenesulfonic acid. The recovered p-toluenesulfonic acid is added to the wood chip pre-hydrolyzed solution after hot water cooking for reuse. Experimental verification by the inventors revealed that lignin nanoparticles prepared from recycled p-toluenesulfonic acid have essentially the same effect as those prepared from fresh p-toluenesulfonic acid.
[0046] The technical solution of the present invention will be further described below with reference to specific embodiments. In the following embodiments, the preparation process of poplar veneer pre-hydrolyzed solution is as follows: poplar wood chips are placed in a cooking pot at 170°C for hydrothermal treatment for 90 minutes to obtain poplar wood chip pre-hydrolyzed solution, wherein the lignin content is 9.4 g / L and the hemicellulose content is 21.3 g / L.
[0047] Example 1
[0048] Preparation of lignin nanoparticles:
[0049] Measure 50 ml of poplar wood chip pre-hydrolysate and mix it with 20 wt% p-toluenesulfonic acid. Stir continuously for 10 minutes in a constant temperature water bath. Then transfer the mixture to a sealed stainless steel autoclave lined with polytetrafluoroethylene and heat at 90°C for 1 hour, allowing it to cool naturally to room temperature. Centrifuge the pre-hydrolysate at 9500 rpm, repeatedly wash the solid with distilled water until neutral, and dry in a vacuum drying oven to obtain lignin nanoparticles, such as... Figure 3 As shown in a, the sugar content in the lignin nanoparticles is 15.3 wt%. The high sugar content results in high electrostatic repulsion and steric hindrance during the assembly process of the lignin nanoparticles. The particle size of the lignin nanoparticles is about 750 nm.
[0050] Example 2
[0051] Preparation of lignin nanoparticles:
[0052] Measure 50 ml of poplar wood chip pre-hydrolysate and mix it with 20 wt% p-toluenesulfonic acid. Stir continuously for 10 minutes in a constant temperature water bath. Then transfer the mixture to a sealed stainless steel autoclave lined with polytetrafluoroethylene and heat at 100°C for 1 hour, allowing it to cool naturally to room temperature. Centrifuge the pre-hydrolysate at 9500 rpm, repeatedly wash the solid with distilled water until neutral, and dry it in a vacuum drying oven to obtain lignin nanoparticles, such as... Figure 3 As shown in b, the sugar content in the lignin nanoparticles is 11.1 wt%. The high sugar content results in high electrostatic repulsion and steric hindrance during the assembly process of the lignin nanoparticles. The particle size of the lignin nanoparticles is about 400 nm.
[0053] Example 3
[0054] Preparation of lignin nanoparticles:
[0055] Measure 50 ml of poplar wood chip pre-hydrolysate and mix it with 20 wt% p-toluenesulfonic acid. Stir continuously for 10 minutes in a constant temperature water bath. Then transfer the mixture to a sealed stainless steel autoclave lined with polytetrafluoroethylene and heat at 110°C for 1 hour, allowing it to cool naturally to room temperature. Centrifuge the pre-hydrolysate at 9500 rpm, repeatedly wash the solid with distilled water until neutral, and dry it in a vacuum drying oven to obtain lignin nanoparticles, such as... Figure 3 As shown in c, the sugar content in the lignin nanoparticles is 9.0 wt%. The decrease in sugar content leads to a decrease in electrostatic repulsion and steric hindrance, while the hydrogen bonds and π-π stacking forces between lignin molecules increase, resulting in a reduction in the nanoparticle size to approximately 150 nm.
[0056] Example 4
[0057] Preparation of lignin nanoparticles:
[0058] Measure 50 ml of poplar wood chip pre-hydrolysate and mix it with 20 wt% p-toluenesulfonic acid. Stir continuously for 10 minutes in a constant temperature water bath. Then transfer the mixture to a sealed stainless steel autoclave lined with polytetrafluoroethylene and heat at 120°C for 1 hour, allowing it to cool naturally to room temperature. Centrifuge the pre-hydrolysate at 9500 rpm, repeatedly wash the solid with distilled water until neutral, and dry in a vacuum drying oven to obtain lignin nanoparticles, such as... Figure 3 As shown in d, the sugar content in the lignin nanoparticles is 4.8 wt%. The decrease in sugar content leads to a decrease in electrostatic repulsion and steric hindrance, while the hydrogen bonds and π-π stacking forces between lignin molecules increase, resulting in a reduction in the nanoparticle size to approximately 90 nm. It can also be clearly seen from the figure that the formed nanoparticles are severely cross-linked, making it impossible to obtain monodisperse lignin nanoparticles. This is because the surface charge of the lignin nanoparticles prepared in this embodiment is too low and unstable, leading to cross-linking.
[0059] 30 mg of the lignin nanoparticles prepared in Examples 1-4 were dispersed in 30 ml of deionized water. Figure 4 A significant Tyndall effect can be observed. The stability of the lignin nanoparticles in water was observed after 14 days of standing. Figure 5 As shown, the lignin nanoparticles in Example 1 settled, indicating poor stability, while the lignin nanoparticles in Examples 2-4 remained uniformly dispersed in water.
[0060] The performance of nanoparticles is usually closely related to their size and surface properties. The dispersion stability of lignin nanoparticles can prevent aggregation or precipitation during use, which is of great significance for their widespread application. The lignin nanoparticles from Example 3 were used to prepare a Pickering emulsion. 12 mL of olive oil was added to 28 mL of a diluted 0.2 wt% lignin nanoparticle suspension. After homogenization at 9000 rpm for 5 min, a stable Pickering emulsion was formed with latex particle size of approximately 20 μm. It exhibited good long-term storage stability, remaining intact after 10 days of storage without stratification. The Pickering emulsion prepared from the lignin nanoparticles of Example 2 had latex particle size of approximately 40 μm and remained intact after 5 days of storage. In the preparation of the Pickering emulsion, lignin nanoparticles with excessively large particle sizes (Example 1) tended to settle easily, exhibiting poor emulsification properties and stratifying within 3 days; nanoparticles with excessively small particle sizes (Example 4) had low hydrophilicity and poor long-term storage stability, stratifying within 5 days.
[0061] Example 5
[0062] Preparation of levulinic acid:
[0063] 50 ml of the supernatant remaining after centrifugation of lignin in Example 3 was used to prepare levulinic acid. Without an additional catalyst, p-toluenesulfonic acid in the supernatant catalyzed the conversion of xylose and glucose into levulinic acid. The reaction was carried out in a sealed stainless steel autoclave lined with polytetrafluoroethylene and heated at 140°C for 1 hour to prepare levulinic acid. The supernatant was tested, and the results showed that the yield of levulinic acid after the above steps was 41%.
[0064] Example 6
[0065] Preparation of levulinic acid:
[0066] 50 ml of the supernatant remaining after centrifugation of lignin in Example 3 was used to prepare levulinic acid. Without an additional catalyst, p-toluenesulfonic acid in the supernatant catalyzed the conversion of xylose and glucose into levulinic acid. The reaction was carried out in a sealed stainless steel autoclave lined with polytetrafluoroethylene and heated at 180°C for 1 hour to prepare levulinic acid. The supernatant was tested, and the results showed that the yield of levulinic acid after the above steps was 49%.
[0067] Example 7
[0068] Preparation of levulinic acid:
[0069] 50 ml of the supernatant remaining after centrifugation of lignin in Example 3 was used to prepare levulinic acid. Without an additional catalyst, p-toluenesulfonic acid in the supernatant catalyzed the conversion of xylose and glucose into levulinic acid. The reaction was carried out in a sealed stainless steel autoclave lined with polytetrafluoroethylene and heated at 160°C for 0.5 h to prepare levulinic acid. The supernatant was tested, and the results showed that the yield of levulinic acid after the above steps was 33%.
[0070] Example 8
[0071] Preparation of levulinic acid and recycling of p-toluenesulfonic acid:
[0072] 50 ml of the supernatant remaining after centrifugation of lignin in Example 3 was used to prepare levulinic acid. Without an additional catalyst, p-toluenesulfonic acid in the supernatant catalyzed the conversion of xylose and glucose into levulinic acid. The reaction was carried out in a sealed stainless steel autoclave lined with polytetrafluoroethylene and heated at 160°C for 1 hour to prepare levulinic acid. The supernatant was tested, and the results showed that the yield of levulinic acid after the above steps was 57%. P-toluenesulfonic acid was recovered and reused by extraction with organic solvents such as tri-n-octylphosphine oxide and rotary evaporation, allowing for the preparation of lignin nanoparticles with similar morphology and size. Figure 6a and c are scanning electron microscope images and size distribution diagrams of the lignin nanoparticles prepared in this embodiment, respectively. It can be seen that the morphology and size are similar to those of the lignin nanoparticles prepared from fresh p-toluenesulfonic acid.
[0073] Example 9
[0074] Preparation of levulinic acid and recycling of p-toluenesulfonic acid:
[0075] 50 ml of the supernatant remaining after centrifugation of lignin in Example 8 was used to prepare levulinic acid. Without an additional catalyst, p-toluenesulfonic acid in the supernatant catalyzed the conversion of xylose and glucose into levulinic acid. The reaction was carried out in a sealed stainless steel autoclave lined with polytetrafluoroethylene and heated at 160°C for 1 hour to prepare levulinic acid. The supernatant was tested, and the results showed that the yield of levulinic acid after the above steps was 57%. P-toluenesulfonic acid was recovered and reused by extraction with organic solvents such as tri-n-octylphosphine oxide. Lignin nanoparticles with similar morphology and size could then be prepared. Figure 6 Figures b and d are scanning electron microscope images and size distribution diagrams of the lignin nanoparticles prepared in this embodiment, respectively. It can be seen that the morphology and size are similar to those of the lignin nanoparticles prepared from fresh p-toluenesulfonic acid.
[0076] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for producing lignin nanoparticles and levulinic acid using wood chip prehydrolysate, characterized by, Includes the following steps: Add 10-30 wt% p-toluenesulfonic acid to the pre-hydrolyzed wood chip solution after hot water cooking, mix thoroughly, and then perform a first hydrothermal treatment at a temperature of 100-110℃ for 0.5-2 hours. Then centrifuge, wash and dry the precipitate to obtain lignin nanoparticles. Heat the centrifuged solution to 140-190℃ for a second hydrothermal treatment for 0.5-4 hours to obtain levulinic acid. The hot water cooking step is as follows: place the wood chips in a cooking pot at 160-190℃ for 70-100 minutes to obtain the pre-hydrolyzed wood chip solution.
2. The method of claim 1, wherein, The wood chip pre-hydrolyzed solution contains 8-15 g / L of lignin and 15-25 g / L of hemicellulose.
3. The method of claim 1, wherein, The lignin nanoparticles have a particle size of 120~450nm; the sugar content of the lignin particles is 8~12wt%.
4. The method of claim 1, wherein, The concentration of p-toluenesulfonic acid added is 18~22wt%.
5. The method of claim 1, wherein, The centrifugation speed is 8000~10000 rpm, and the centrifugation time is 5~15 min.
6. The method of claim 1, wherein, The solvent used for washing is distilled water, and the drying temperature is 50~70℃, and the time is 8~20h.
7. The method of claim 1, wherein, The centrifuged solution is heated to 160-180℃ for a second hydrothermal treatment, which lasts for 1-2 hours, to obtain levulinic acid.
8. The method of claim 1, wherein, The second hydrothermal treatment also includes a step of extracting levulinic acid using an organic solvent.
9. The method of claim 8, wherein, The organic solvent includes one of tri-n-octylphosphine oxide, tributyl phosphate, ethyl acetate, or dichloromethane.
10. The method of claim 1, wherein, Following the step of obtaining levulinic acid, the process further includes a step of concentrating and crystallizing to recover p-toluenesulfonic acid, which is then added to the wood chip pre-hydrolysate after hot water cooking for reuse.