Method for efficiently separating lignin from straw

Abstract

The invention provides a method for efficiently separating lignin from straw, which comprises the following steps: a) adding a biomass derivative into benzyl alcohol to form a uniform solution, mixing a straw raw material with the obtained solution for reaction, and carrying out solid-liquid separation to obtain solid residues and a liquid-phase component; b) adding water into the liquid phase component, and standing for layering to obtain a water phase and an organic phase; and c) adding an anti-solvent into the organic phase, and carrying out centrifugal separation to obtain a supernatant and a precipitate which is lignin. According to the method, the biomass derivative is compounded with benzyl alcohol to pretreat the straw raw material, lignin in the straw can be efficiently separated under the mild condition, the separated lignin and xylose generated by hemicellulose hydrolysis can be recycled in a simple mode, and the method has the potential of being suitable for application and popularization.

Description

Technical Field

[0001] The present invention belongs to the technical field of lignin separation, and relates to a method for efficiently separating lignin from straw. Background Art

[0002] Biomass resources are a rich carbon-containing renewable resource and can be used as an ideal substitute for fossil energy. The high-value utilization of biomass resources is an effective way to alleviate the current energy and environmental pressures. Straw is a typical lignocellulosic biomass, and its efficient conversion into chemical raw materials is of great significance for promoting rural development and agricultural construction. Straw is mainly composed of three major components: cellulose, hemicellulose, and lignin. These components are entangled and cross-linked with each other to form a dense network structure. The conversion and utilization of any one of these components will inevitably be interfered by the other two components, resulting in low direct conversion and utilization efficiency of straw. Lignin plays the role of an "adhesive" among the three major components. Its efficient separation can deconstruct the dense network of biomass and is one of the necessary steps for the high-value utilization of straw. Therefore, it is necessary to find a mild and simple pretreatment method to efficiently separate lignin from straw, so as to realize the separation and utilization of the three major components of straw.

[0003] In recent years, methods for separating lignin by pretreating straw have been widely studied, and a variety of lignin separation methods have been developed, which can be mainly divided into three categories: physical pretreatment methods, chemical pretreatment methods, and biological pretreatment methods. Physical pretreatment methods mainly include mechanical crushing, microwave radiation method, steam explosion method, etc. Separating lignin by physical pretreatment methods usually requires high energy consumption, and high temperature and high pressure will be generated during the pretreatment process. Chemical pretreatment methods mainly rely on alkali and alkali-oxygen methods, which require high equipment requirements, and the alkaline substances generated after pretreatment are difficult to remove and are extremely likely to cause environmental pollution. Biological pretreatment methods have relatively mild reaction conditions and are environmentally friendly, but the reaction period often lasts for several weeks, and the pretreatment efficiency is low, making it impossible to be widely applied on a large scale.

[0004] How to develop a lignin separation method that can efficiently separate the three major components of straw under mild conditions is a problem to be solved at present. Summary of the Invention

[0005] In view of the above problems, the present invention proposes a method for efficiently separating lignin from straw based on the compounding of salicylic acid derivatives and benzyl alcohol. The salicylic acid derivatives have a structure similar to that of lignin, can approach lignin in straw directionally, selectively polarize and break the connection bonds at the LCC site. At the same time, the abundant hydroxyl and carboxyl groups in the salicylic acid derivatives can form a hydrogen bond network with benzyl alcohol, weakening the hydrogen bonds between lignin and carbohydrates, and synergistically promoting the release and dissolution of lignin in the solvent. Pretreating straw by this method can efficiently separate lignin in straw raw materials under mild conditions and in a short time.

[0006] The object of the present invention is to provide a method for efficiently separating lignin from straw.

[0007] The present invention provides a method for efficiently separating lignin from straw based on the compounding of salicylic acid derivatives and benzyl alcohol, comprising the following steps:

[0008] Step A: Add salicylic acid derivatives to benzyl alcohol to form a uniform solution, mix and react the straw raw material with the solution, and perform solid-liquid separation to obtain a solid residue and a liquid phase component;

[0009] Step B: Add water to the liquid phase component and let it stand for stratification to obtain an aqueous phase and an organic phase;

[0010] Step C: Add an anti-solvent to the organic phase, and perform centrifugal separation to obtain a supernatant and a precipitate, and the precipitate is lignin.

[0011] Step A:

[0012] Preferably, the salicylic acid derivatives are 5-sulfosalicylic acid and 5-nitrosalicylic acid.

[0013] Preferably, the concentration of the salicylic acid derivatives in the solution is 1 wt% to 20 wt%, specifically 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 16 wt%, 18 wt%, 20 wt%.

[0014] Preferably, the straw raw material is crushed crop straw, and more preferably, the crushed crop straw has a particle size ≤ 20 mesh.

[0015] Preferably, the solid-liquid ratio of the straw raw material to the solution is 1 g:(5 - 20) ml, specifically 1 g:5 ml, 1 g:10 ml, 1 g:15 ml or 1 g:20 ml.

[0016] Preferably, the reaction temperature is 80 - 120 °C and the reaction time is 40 - 120 min.

[0017] Benzyl alcohol is a common biodegradable organic solvent. Its structure contains both a benzene ring and a hydroxyl group, which has a high similarity with the structural units of lignin and can have a strong interaction with lignin. Therefore, compared with other organic solvents, benzyl alcohol has excellent lignin dissolution ability. At the same time, benzyl alcohol has stable chemical properties, low toxicity and corrosiveness, and is suitable as a solvent for separating lignin from straw raw materials. In addition, benzyl alcohol is hardly soluble in water, and a two-phase system can be formed by adding water to separate the soluble sugars in the reaction solution after pretreatment.

[0018] The biomass derivative used in the present invention is a salicylic acid derivative, such as 5-sulfosalicylic acid and 5-nitrosalicylic acid, preferably 5-sulfosalicylic acid. The salicylic acid derivative is characterized by having a hydrophobic benzene ring structure and a hydrophilic hydroxyl structure similar to lignin, which can be directionally adsorbed to the lignin in straw, selectively polarize and break the connecting bonds at the LCC site. At the same time, the abundant hydroxyl and carboxyl groups in 5-sulfosalicylic acid can form a hydrogen bond network with benzyl alcohol, weakening the hydrogen bonds between lignin and carbohydrates, and synergistically promoting the release and dissolution of lignin in the solvent.

[0019] In the present invention, first, the straw raw material is mixed and reacted with a salicylic acid derivative benzyl alcohol solution. Among them, the straw raw material is crushed crop straw, mainly composed of cellulose, hemicellulose and lignin. In order to make the straw raw material fully contact and react with the solution, it is preferred to first crush the straw raw material into powder, and the particle size of the crushed straw powder is preferably ≤20 mesh. The reaction temperature is preferably 80 - 120°C, specifically 80°C, 90°C, 100°C, 110°C or 120°C; the reaction time is preferably 40 - 120 min, specifically 40 min, 60 min, 80 min, 100 min or 120 min.

[0020] After the reaction is completed, when the reaction system is cooled to room temperature, the solid residue and the liquid phase component are separated, and the separated solid residue is washed with ethanol and water respectively to remove the residual solvent on the solid surface. The present invention has no special limitation on the method of solid-liquid separation, including but not limited to solid-liquid separation methods well known to those skilled in the art such as filtration, suction filtration, and centrifugation.

[0021] Step B:

[0022] Preferably, the volume ratio of the liquid phase component to the added water is 1:(1 - 5). After standing and separating, the upper layer is the aqueous phase and the lower layer is the organic phase. Preferably, a separating funnel is used to separate the aqueous phase and the organic phase. The aqueous phase contains xylose generated by the hydrolysis of hemicellulose in the straw raw material, which can be used for the recovery and further conversion and utilization of xylose; the organic phase contains lignin separated from the straw raw material, and lignin is recovered by adding an anti-solvent precipitation thereto.

[0023] Step C:

[0024] Preferably, the anti-solvent is one or more of ethyl acetate, isopropyl ether, dimethyl carbonate, and diethyl ether, preferably dimethyl carbonate.

[0025] Preferably, the volume ratio of the anti-solvent to the organic phase is 1:(8 - 50).

[0026] Let it stand for lignin precipitation. The precipitated lignin is separated by centrifugation. The supernatant is subjected to vacuum rotary evaporation to recover the anti-solvent, and the remaining benzyl alcohol part can be recycled without further purification.

[0027] The present invention provides a method for efficiently separating lignin from straw based on the compounding of salicylic acid derivatives and benzyl alcohol. It can efficiently separate lignin from straw raw materials under relatively mild conditions, while retaining a relatively complete cellulose component. Both the separated lignin and the xylose generated by the hydrolysis of hemicellulose can be recovered in a simple manner. In addition, the solvents involved in each reaction step can be recovered and recycled, without causing pollution to the environment. Compared with the existing straw lignin separation methods, the method adopted by the present invention is economical and environmentally friendly, with higher treatment efficiency, and is suitable for further popularization and application.

[0028] Using the present invention to pretreat corn straw, the lignin separation rate can reach more than 85%, and the cellulose retention rate is above 85%.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] The present invention uses a benzyl alcohol solution compounded with salicylic acid derivatives to pretreat straw raw materials, efficiently separating lignin from the raw materials under relatively mild conditions, while retaining a relatively complete cellulose component. In addition, both the separated lignin and the xylose generated by the hydrolysis of hemicellulose can be recovered in a simple manner. Moreover, this method is relatively friendly to the environment. The solvents involved in each reaction step can be recovered and recycled, without causing pollution to the environment. It can achieve a lignin separation efficiency of more than 85% at a relatively low temperature and within a relatively short reaction time, having significant advantages over the existing lignin separation methods, and is suitable for further popularization and application.

[0031] The present invention has been described in detail above, but the above embodiments are essentially illustrative only and are not intended to limit the present invention. In addition, the present invention is not limited by any theory described in the foregoing prior art or the invention content or the following examples.

[0032] Unless otherwise expressly stated, a numerical range in the entire application document includes any sub-range therein and any numerical value incremented by the smallest sub-unit of the given value therein. Unless otherwise expressly stated, a numerical value in the entire application document represents an approximate measure or limitation of a range of embodiments that includes a slight deviation from the given value, as well as embodiments having approximately the mentioned value and having the exact value mentioned. Except for the working examples provided at the end of the detailed description, all numerical values of parameters (such as quantities or conditions) in this application document (including the appended claims) should be understood to be modified by the term "about" in all cases, regardless of whether "about" actually appears before the numerical value. "About" means that the stated numerical value allows for some imprecision (somewhat close to the exact value; approximately or reasonably close to the value; approximate). If the imprecision provided by "about" is not understood in this ordinary meaning in the art, then "about" as used herein means at least the variations that can be produced by measuring and using ordinary methods for these parameters. For example, "about" can include variations of less than or equal to 10%, less than or equal to 5%, less than or equal to 4%, less than or equal to 3%, less than or equal to 2%, less than or equal to 1%, or less than or equal to 0.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a scanning electron microscope (SEM) image of corn stover before pretreatment;

[0034] Figure 2 is a scanning electron microscope (SEM) image of corn stover after pretreatment;

[0035] Figure 3 is the Fourier transform infrared (FTIR) spectrum of corn stover before and after pretreatment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The present invention will be further described below in conjunction with embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection claimed for the present invention.

[0037] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0038] Raw materials:

[0039] The corn stover is from Mengcheng area, Anhui Province;

[0040] 5-Sulfosalicylic acid is purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0041] Calculation method:

[0042] To obtain the structural and compositional changes of corn stover during the pretreatment process, it is necessary to analyze the components of the samples before and after pretreatment. According to the two-step acid hydrolysis method developed by the National Renewable Energy Laboratory (NERL) in the United States, the contents of sugars and lignin in the biomass composition can be measured. To ensure the universality of the experimental laws, each experiment is carried out three times, and the results are averaged. The specific procedure is as follows:

[0043] (1) Measurement of carbohydrate compounds

[0044] Take an appropriate amount of the sample to be tested (m1) and place it in a beaker. Add 72% concentrated sulfuric acid to submerge the sample. Stir the mixed liquid at room temperature for 1 h, then transfer it to a pressure-resistant glass bottle. Continue to add a certain amount of deionized water to dilute the acid concentration to 4%, and record the solution volume V0 at this time. Hydrothermal treatment is carried out in an autoclave for 1 h at a temperature of 121 °C. After completion, it is naturally cooled to room temperature, filtered and washed with deionized water, and dried to a constant weight (m2) in an oven at 105 °C. The filtrate is filtered through a 0.45-μm filter and analyzed by high-performance liquid chromatography. To accurately measure the content of monosaccharides in the filtrate, first, weigh the required monosaccharide standard and prepare a stock standard solution with a concentration of 10 mg / mL. Then, take an appropriate amount of the stock standard solution and prepare standard mixed solutions with concentrations of 60 μg / mL, 50 μg / mL, and 40 μg / mL.

[0045] Cellulose% = 0.9 * V0 * Cglucose * 100% / m1;

[0046] (2) Measurement of acid-insoluble lignin

[0047] Grind the solid obtained after two-step acid hydrolysis and add it to a porcelain boat, then put it into a muffle furnace and calcine it at 550 °C for 2 h, with the heating rate controlled at 5 °C / min. After the calcination is completed, wait for the muffle furnace to cool naturally to room temperature, and weigh the mass of the solid and record it as (m3).

[0048] Acid-insoluble lignin% = (m2 - m3) * 100% / m1;

[0049] (3) Determination of the content of acid-soluble lignin

[0050] First, take a certain volume of the filtrate obtained by filtering the sulfuric acid hydrolysis, and dilute it to the target concentration according to the set multiple. Then, measure the absorbance of ultraviolet light at a wavelength of 205 nm with a UV-visible spectrophotometer, using a 4% sulfuric acid solution as the blank control group. It should be noted that the absorbance values measured in the experiment are valid within the range of 0.7 - 1. The calculation formula for the content of acid-soluble lignin is as follows:

[0051] The content of acid-soluble lignin in the filtrate (g / L) = A * D / 110,

[0052] Percentage of acid-soluble lignin in the sample to be tested % = D * d * A * V0 * 100% / (110 * m1),

[0053] Where:

[0054] D represents the dilution factor of the filtrate,

[0055] A represents the ultraviolet absorbance value,

[0056] d is the optical path width of the quartz cuvette (mL),

[0057] V0 represents the total volume of the hydrolysis solution (mL),

[0058] 110 represents the molar extinction coefficient (L / g·cm) m 2 ,

[0059] m1 represents the dry weight of the sample to be tested (g),

[0060] The lignin content in the sample is the sum of the acid-insoluble lignin and acid-soluble lignin contents.

[0061]

[0062] Example 1

[0063] 1) Crush corn straw and pass it through a 20-mesh sieve. Weigh 4.0 g and measure 60 ml of a 5-sulfosalicylic acid compounded with benzyl alcohol solution with a 5-sulfosalicylic acid concentration of 4%. Place it in a 100-ml mechanical stirring reactor.

[0064] 2) Adjust the reactor rotation speed to 500 rmp, heat up to 100 °C and maintain for 1 h.

[0065] 3) After the reaction, turn off the power of the reactor and cool the reactor to room temperature in circulating water.

[0066] 4) Pour the reactants out of the reactor, vacuum filter to obtain the reaction solution and solid residue, and wash the solid residue with ethanol and water respectively.

[0067] 5) Add 2 volumes of water to the reaction solution, let it stand for stratification, and use a separating funnel to separate the aqueous phase and the organic phase.

[0068] 6) Add 10 volumes of dimethyl carbonate to the organic phase to precipitate lignin, centrifuge to recover the precipitate, and wash and dry to obtain solid lignin.

[0069] 7) Use a two-step acid hydrolysis method for the solid residue filtered in step 4) to measure the lignin separation rate and cellulose retention rate;

[0070] After measurement, under the above conditions, the lignin separation rate is 86.5% and the cellulose retention rate is 88.5%.

[0071] Figure 1 This is the scanning electron microscope image (SEM) of corn stover before pretreatment in this example; Figure 2 This is the scanning electron microscope image (SEM) of corn stover after pretreatment in this example; Figure 3 This is the Fourier transform infrared spectrum (FTIR) of corn stover before and after pretreatment in this example; By Figure 1 and Figure 2 Through comparison, it can be seen that the surface of the original corn stover presents a smooth and dense structure. The hemicellulose and lignin in the corn stover are entangled and combined with the cellulose fibers to form a dense three-dimensional network structure. This compact and highly ordered structure hinders the further development and utilization of corn stover. After pretreatment with 5-sulfosalicylic acid compounded with benzyl alcohol, the surface of the solid residue is rough and uneven, with many cracks and pores, which is related to the shedding of hemicellulose and lignin.

[0072] Through Figure 3 comparison of the FTIR spectra of the straw before and after pretreatment in 1 it can be seen that the stretching vibration peak of the lignin aromatic ring structure corresponding to the position of 1514 cm- 1 and the vibration peak of the C=O bond of hemicellulose corresponding to the position of 1256 cm-

[0073] Example 2

[0074] According to the steps in Example 1, with all other conditions unchanged, change the solid-liquid ratio of the reaction between corn stover and the 5-sulfosalicylic acid compounded with benzyl alcohol solution. The solid-liquid ratios are 4 g:20 ml, 4 g:40 ml, and 4 g:80 ml respectively. After measurement, the corresponding lignin separation rates are 77.1%, 81.4%, and 87.6% respectively, and the cellulose retention rates are 89.2%, 88.7%, and 87.8% respectively.

[0075] Example 3

[0076] According to the steps in Example 1, with all other conditions unchanged, change the concentration of 5-sulfosalicylic acid. The concentrations are 2%, 6%, and 8% respectively. After measurement, the corresponding lignin separation rates are 76.2%, 89.4%, and 88.7% respectively, and the cellulose retention rates are 94.2%, 81.1%, and 73.1% respectively.

[0077] Example 4

[0078] According to the steps in Example 1, with all other conditions remaining unchanged, the reaction temperature was changed. The reaction temperatures were 80°C, 90°C, 110°C, and 120°C respectively. After measurement, the corresponding lignin separation rates were 76.4%, 82.4%, 89.9%, and 92.7% respectively, and the cellulose retention rates were 92.20%, 90.24%, 84.20%, and 72.30% respectively.

[0079] Example 5

[0080] According to the steps in Example 1, with all other conditions remaining unchanged, the reaction time was changed. The reaction times were 40 min, 50 min, 70 min, and 80 min respectively. After measurement, the corresponding lignin separation rates were 83.23%, 84.50%, 85.72%, and 87.70% respectively, and the cellulose retention rates were 90.61%, 89.80%, 88.20%, and 87.60% respectively.

[0081] As can be seen from the above examples, by using the method provided by the present invention to separate lignin from straw, a relatively high lignin separation rate and cellulose retention rate can be obtained.

[0082] Comparative Experiment 1

[0083] According to the steps in Example 1, with all other conditions remaining unchanged, the type of 5-sulfosalicylic acid complexing solvent was changed. The solvents were water, methanol, ethanol, and benzaldehyde. After measurement, the corresponding lignin separation rates were 41.23%, 49.82%, 57.12%, and 78.70% respectively, and the cellulose retention rates were 71.36%, 92.17%, 89.20%, and 89.70% respectively.

[0084] Comparative Experiment 2

[0085] According to the steps in Example 1, with all other conditions remaining unchanged, the type of benzyl alcohol complexing solute was changed. The solutes were salicylic acid, 2-hydroxy-5-methylbenzenesulfonic acid, and p-toluenesulfonic acid. After measurement, the corresponding lignin separation rates were 39.23%, 60.72%, and 80.14% respectively, and the cellulose retention rates were 92.34%, 88.42%, and 85.67% respectively.

[0086] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for efficiently separating lignin from straw, characterized in that, It includes the following steps: Step A: Add a salicylic acid derivative to benzyl alcohol to form a homogeneous solution, mix the straw raw material with the solution for reaction, and perform solid-liquid separation to obtain a solid residue and a liquid-phase component; Step B: Add water to the liquid-phase component and let it stand for layering to obtain an aqueous phase and an organic phase; Step C: Add an anti-solvent to the organic phase, perform centrifugal separation to obtain a supernatant and a precipitate, and the precipitate is lignin.

2. The preparation method according to claim 1, characterized in that, In Step A, the salicylic acid derivative is 5-sulfosalicylic acid and 5-nitrosalicylic acid.

3. The preparation method according to claim 1, wherein In Step A, the concentration of the salicylic acid derivative in the solution is 1 wt% to 20 wt%.

4. The preparation method according to claim 1, characterized in that, In Step A, the straw raw material is crushed crop straw, preferably crop straw with a particle size ≤ 20 mesh after crushing.

5. The preparation method according to claim 1, characterized in that, In Step A, the solid-liquid ratio of the straw raw material to the solution is 1 g:(5 - 20) ml.

6. The preparation method according to claim 1, characterized in that, In Step A, the reaction temperature is 80 - 120 °C and the reaction time is 40 - 120 min.

7. The preparation method according to claim 1, wherein In Step B, the volume ratio of the liquid-phase component to the added water is 1:(1 - 5).

8. The preparation method according to claim 1, characterized in that, In Step C, the anti-solvent is one or more of ethyl acetate, isopropyl ether, dimethyl carbonate, and ether.

9. The preparation method according to claim 1, characterized in that, In Step C, the volume ratio of the anti-solvent to the organic phase is 1:(8 - 50).

10. The preparation method according to claim 1, characterized in that, In Step C, rotary evaporation is used to recover the anti-solvent in the supernatant, and the remaining benzyl alcohol is recycled.

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