High-purity submicron or nano lignin and preparation method thereof

Abstract

The invention discloses high-purity submicron or nano lignin and a preparation method thereof, and belongs to the technical field of agricultural and forestry biomass resource utilization and energy conservation and environmental protection. According to the method disclosed by the invention, the biomass raw material is subjected to the processes of pickling and deliming, microwave drying, microwave extraction by ethylene glycol, water, sulfuric acid and formic acid, and lignin separation by water and acetic acid, so that the high-purity submicron or nano lignin with the average particle size of 90-400nm, the ash content of less than 50mg / kg and the alkali metal impurity content of less than 5mg / kg can be obtained; and the used ethylene glycol solvent is subjected to distillation concentration, performance regeneration and cyclic utilization. The lignin has good dispersing performance and adsorption performance, can be applied to wastewater treatment and dispersants of dye, pesticide, coal water slurry and other systems, can also be carbonized to prepare submicron / nanoscale porous carbon materials, and is applied to the fields of biological medicine, micro-nano devices and the like.

Description

Technical Field

[0001] The present invention relates to a high-purity submicron or nano lignin and a preparation method thereof, belonging to the technical fields of resource utilization of agricultural and forestry biomass and energy conservation and environmental protection. Background Art

[0002] Lignin widely exists in lignocellulosic plant cells, accounting for up to 20-35 wt.% in biomass. It is the second-largest renewable resource after cellulose and the second most abundant biomass resource on the earth. Currently, the main way to utilize lignin globally is direct combustion for heating, and the high-value utilization rate is less than 5%. Submicron / nano materials are substances that are full of vitality, have diverse functions, and are widely used in the field of submicron / nano technology. Submicron materials refer to materials with sizes between 100 and 1000 nanometers, and nano materials refer to ultra-fine particles with sizes between 1 and 100 nm.

[0003] When the particle size of a substance enters the submicron / nano scale, it and the solid composed of it will possess various characteristics such as small-size effect, surface effect, macroscopic quantum tunneling effect, and quantum size effect. Similarly, when the particle size of lignin reaches the submicron / nano level, its utilization value will be greatly improved. Submicron / nano lignin has wide applications in the fields of advanced functional materials, biological materials, biomedicine, micro-nano devices, binders, dispersants, and renewable disposable products.

[0004] However, the current mainstream lignin production process is by-product alkali lignin from the paper industry. The content of alkali metal salt impurities in the lignin obtained by this process exceeds 20 wt.%, and the particle size is relatively large, which limits its high-value utilization. Compared with industrial lignin by-produced from alkaline papermaking, the organic solvent extraction process can extract high-purity lignin from lignocellulosic biomass. However, problems such as high prices of organic solvents, low lignin extraction rate, and difficulties in solvent recovery and recycling limit the large-scale application of this process in the preparation of high-purity lignin. Although low-boiling organic solvents such as ethanol can be recovered and recycled by evaporation, their low boiling points require the use of pressurized equipment to achieve a relatively high lignin extraction rate. The use of high-pressure equipment not only increases the cost but also poses risks such as explosion. In addition, in the process of organic solvent extraction, there are still great challenges in the formation and regulation technology of submicron / nano lignin particles. Therefore, developing a new type of low-cost high-purity and ultra-fine organic solvent lignin organic solvent extraction and solvent recycling and regeneration process to extract high-purity submicron / nano lignin from abundant agricultural and forestry biomass raw materials can not only effectively improve the performance of lignin and promote its high-value utilization but also is expected to promote the resource utilization of agricultural and forestry biomass. Summary of the Invention

[0005] In order to overcome the problems in the background art, the object of the present invention is to provide a high-purity submicron or nano lignin and a preparation method thereof.

[0006] In order to achieve the above object, the present invention is realized through the following technical solutions:

[0007] A preparation method of high-purity submicron or nano lignin, comprising the following steps:

[0008] (1) Dry the lignocellulose raw material, then break and sieve it after microwave heating to obtain biomass particles;

[0009] (2) Place the biomass particles in a hydrochloric acid solution for pickling, then wash and dry to obtain dried wood cellulose particles;

[0010] (3) Place the dried wood cellulose particles in a mixed solution for microwave heating, then separate the lignin precipitate, and freeze-dry the lignin precipitate to obtain high-purity submicron or nano lignin; the mixed solution comprises the following raw materials in parts by mass: 84-93 parts of ethylene glycol, 0.5-1 part of water, 0.5-1 part of sulfuric acid, and 1-3 parts of formic acid.

[0011] More preferably, in the step (1), the temperature of microwave heating is 105 °C and the time is 6-12 h.

[0012] More preferably, in the step (2), the concentration of the hydrochloric acid solution is 0.5-2 mol·L -1 , and the mass ratio of the hydrochloric acid solution to the biomass particles is (8-15):(0.5-2).

[0013] More preferably, the pickling specifically includes: placing the biomass particles in a hydrochloric acid solution and stirring for 10-36 h, then standing for 0.3-1 h.

[0014] More preferably, in the step (2), microwave drying technology is used for drying, the microwave frequency is 80-240 W, the temperature is 105 °C, and the time is 0.5-1 h.

[0015] More preferably, in the step (3), the mass ratio of the dried wood cellulose particles to ethylene glycol is (5-10):(84-93).

[0016] More preferably, in the step (3), the microwave frequency of microwave heating is 80-360 W, the temperature is 130-180 °C, and the time is 0.1-0.3 h.

[0017] More preferably, in the step (3), separating the lignin precipitate specifically includes: separating the liquid mixture and the solid component from the microwave-heated substance by filtration; adding water and acetic acid to the liquid mixture, then standing, and finally vacuum filtering to obtain the organic solvent lignin solid component.

[0018] More preferably, the freeze-drying temperature is -50°C and the time is 24 - 48 h.

[0019] More preferably, in step (3), the filtrate after vacuum filtration is placed in a vacuum filtration rotary evaporator, and water is distilled out under a water bath heating at 70 - 80°C in a water bath to obtain a concentrated ethylene glycol solvent. The pH value of the recovered ethylene glycol solvent is measured with a pH test paper, and sulfuric acid is added to the solvent until its pH value is the same as that of the mixed solution in step (3) to regenerate the performance of the recovered ethylene glycol solvent, and the recovered ethylene glycol solvent is recycled.

[0020] More preferably, the lignocellulosic raw material is one or a mixture of softwood (pine), hardwood (eucalyptus), agricultural waste (corn stalks, sugarcane bagasse, corn cobs), and herb (rice straw), all of which can be purchased from the market.

[0021] The present invention also claims the high-purity submicron / nano lignin prepared by the preparation method of the high-purity submicron / nano lignin, with an average particle size of 90 - 400 nm, an ash content of less than 50 mg / kg, and an alkali metal impurity content of less than 5 mg / kg.

[0022] Advantages of the present invention: Through the processes of pickling, microwave drying, microwave extraction with ethylene glycol, water, sulfuric acid and formic acid, and separating lignin with water and acetic acid, the present invention can obtain high-purity submicron or nano lignin with an average particle size of 90 - 400 nm, an ash content of less than 50 mg / kg, and an alkali metal impurity content of less than 5 mg / kg. This lignin has good dispersion performance and adsorption performance, can be applied as a dispersant in wastewater treatment and systems such as dyes, pesticides, and water coal slurries, and can also be used to prepare submicron / nano-scale porous carbon materials after carbonization, and be applied in fields such as biomedicine and micro-nano devices. Description of the Drawings

[0023] Figure 1 SEM and TEM images of the nano lignin particles obtained in Example 1, (a) is the SEM image, and (b) is the TEM image.

[0024] Figure 2 SEM and TEM images of the submicron lignin particles obtained in Example 2, (a) is the SEM image, and (b) is the TEM image. Detailed Embodiments

[0025] The following are further detailed descriptions of the present invention with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

[0026] A preparation method of high-purity submicron or nano lignin, comprising the following steps:

[0027] (1) Raw material drying and crushing: The lignocellulosic raw material is first preliminarily dried in the air, and then put into a microwave heating and drying device to be dried at 105 °C for 6 - 12 h. After that, it is crushed and sieved to select biomass particles with a mesh size of 120 - 200. In the present invention, the lignocellulosic raw material is dried by microwave drying, so that it has a fluffy structure, which is beneficial to subsequent pickling and ethylene glycol swelling, and promotes the formation of submicron / nano-scale lignin.

[0028] (2) Pickling pretreatment: The biomass particles with a mesh size of 120 - 200 are mixed evenly with a hydrochloric acid solution with a concentration of 0.5 - 2 mol·L -1 in a glassware according to the following weight parts: 0.5 - 2 parts of biomass particles, 8 - 15 parts of hydrochloric acid solution; a magnetic rotor is put into the mixture in the glassware, and then it is put into a water bath with magnetic stirring and stirred at a speed of 10 - 50 r / min at room temperature for 10 - 36 h; after stirring, it is left standing for 0.3 - 1 h. After the solid particles precipitate, the supernatant is poured off. Then deionized water is added and stirred, and after standing for 0.1 - 0.5 h and precipitation, the supernatant liquid is poured off. The above operation is repeated until the pH value of the solution becomes about 6 - 7. Finally, the solid particles are separated by 1 - 3 layers of slow filter paper and transferred to a microwave drying device to be dried at 105 °C for 0.5 - 1 h under a microwave frequency of 80 - 240 W. In the present invention, the pickling and ash removal process is used to remove the extractable ash in the biomass and the ash existing in the lignin cross-linked structure, further expanding the pore structure of the raw material and removing impurities, and also being beneficial to promoting the lignin to be submicron / nano-scale.

[0029] (3) Microwave heating ethylene glycol extraction: The dried lignocellulose particles in step (2) are mixed evenly according to the following weight parts: 5 - 10 parts of biomass raw material, 84 - 93 parts of ethylene glycol, 0.5 - 1 part of deionized water, 0.5 - 1 part of sulfuric acid, 1 - 3 parts of formic acid; the uniformly mixed material is transferred to a two-necked round-bottom flask with a reflux device and stirred evenly; then it is transferred to a microwave reactor and heated at a microwave frequency of 80 - 360 W and 130 - 180 °C for 0.1 - 0.3 h. After the reaction, it is naturally cooled to room temperature. In the present invention, during the lignin precipitation and separation process, by adding a specific proportion of deionized water, sulfuric acid and formic acid to the ethylene glycol solution, it is beneficial to the formation and uniform distribution of micro / nano lignin particles; during the lignin filtration process, a combination of a microporous filter membrane with a specific pore size and slow filter paper is used to avoid the breakage of the filter membrane during the lignin filtration process and filter out the lignin particles to the greatest extent.

[0030] (4) Lignin precipitation and separation: Filter the liquid mixture and solid components with 2 - 3 layers of slow filter paper with a pore size of 1 - 3 μm; then add 700 - 1200 parts by weight of deionized water and 10 - 30 parts of acetic acid to the filtered liquid mixture, and let it stand at room temperature for 6 - 12 h to precipitate the organic solvent lignin. Then, use 2 - 4 layers of microporous filter membranes with a pore size of 0.05 - 0.1 μm and 1 - 3 layers of slow filter paper with a pore size of 1 - 3 μm to filter out the solid component of the organic solvent lignin under vacuum filtration.

[0031] (5) Lignin freeze-drying: First, put the filtered lignin into a refrigerator at -20 °C to -25 °C for 6 - 12 h to freeze the water remaining on the lignin. Then, put the frozen lignin into a vacuum freeze-dryer and freeze-dry it at -50 °C for 24 - 48 h to obtain dry and uniform lignin particles.

[0032] (6) Solvent recycling and regeneration: Place the filtrate filtered in step (4) in a vacuum filtration rotary evaporator, distill out the water by heating in a water bath at 70 - 80 °C in a water bath, obtain the concentrated ethylene glycol solvent, measure the pH value of the recycled ethylene glycol solvent with a pH test paper, add sulfuric acid to the solvent until its pH value is the same as that of the solution in step (3) to regenerate the performance of the recycled ethylene glycol solvent, and recycle the recycled ethylene glycol solvent. Using a vacuum filtration rotary evaporation device to evaporate and dehydrate the ethylene glycol solution at a lower temperature and energy consumption, obtain the concentrated ethylene glycol solvent, add sulfuric acid to the solvent to make its pH value the same as that of the first solution, regenerate its performance, and repeatedly recycle the recycled and regenerated solvent, which can effectively reduce the energy consumption and cost of the process.

[0033] Example 1

[0034] A preparation method of high-purity nano-lignin, comprising the following steps:

[0035] (1) First, preliminarily dry the lignocellulose raw material in the air, and then put it into a microwave drying device to dry it at 105 °C for 6 h at a microwave frequency of 240 W. Then, crush and sieve it, and select the lignocellulose particles with a mesh size of 120 - 200 meshes as biomass particles.

[0036] (2) Mix the biomass particles with a concentration of 2 mol·L -1The hydrochloric acid solution is mixed evenly in a glassware according to the following parts by weight: 1 part of biomass particles and 8 parts of hydrochloric acid solution; a magnetic rotor is placed into the mixture in the glassware, and then it is placed into a water bath with magnetic stirring and stirred at a speed of 50 r / min at room temperature for 10 h; after the stirring ends, it is left to stand for 0.3 h. After the solid particles precipitate, the supernatant is poured off, and then deionized water is added and stirred. After standing for 0.5 h and precipitating, the supernatant liquid is poured off. The above operations are repeated until the pH value of the supernatant solution becomes about 6. Finally, the solid particles are separated by three layers of slow filter paper and transferred to a microwave drying device to be dried at 105 °C at a microwave frequency of 160 W for 1 h.

[0037] (3) The dried wood cellulose particles are mixed evenly according to the following parts by weight: 10 parts of dried wood cellulose particles, 85 parts of ethylene glycol, 1 part of deionized water, 1 part of sulfuric acid, and 3 parts of formic acid; the evenly mixed materials are transferred to a two-necked round-bottom flask with a reflux device and stirred evenly; then it is transferred to a microwave reactor and heated at a microwave frequency of 360 W and 180 °C for 0.3 h. After the reaction ends, it is naturally cooled to room temperature.

[0038] (4) The liquid mixture and solid components are filtered out with three layers of slow filter paper with a pore size of 1 μm; then 1200 parts by weight of deionized water and 30 parts of acetic acid are added to the filtered liquid mixture, and it is left to stand at room temperature for 12 h to precipitate organic solvent lignin. Then, the solid component of the organic solvent lignin is filtered out with four layers of microporous filter membranes with a pore size of 0.1 μm and three layers of slow filter paper with a pore size of 3 μm under vacuum filtration by a vacuum filter.

[0039] (5) The solid component of the filtered organic solvent lignin is first placed in a refrigerator at -25 °C and frozen for 12 h to freeze the water remaining on the lignin. Then, the frozen lignin is placed in a vacuum freeze dryer and freeze-dried at -50 °C for 48 h to obtain dry and uniform high-purity submicron / nano lignin.

[0040] (6) The filtrate after vacuum filtration by the vacuum filter in step (4) is placed in a vacuum filtration rotary evaporator, and the water is distilled out by heating in a water bath at 80 °C in a water bath to obtain a concentrated ethylene glycol solvent, and the pH value of the recovered ethylene glycol solvent is measured with a pH test paper. Sulfuric acid is added to the solvent until its pH value is consistent with the pH of the solution in step (3) to regenerate the performance of the recovered ethylene glycol solvent, and the recovered ethylene glycol solvent is recycled.

[0041] In this example, the yield of the high-purity submicron / nano lignin is 21%, as Figure 1As shown, the nano-lignin particles are evenly distributed, and nano-scale lignin can be obtained. The average particle size of high-purity submicron / nano lignin is 100 nm, the ash content is 20 mg / kg, and the alkali metal impurity content is 2 mg / kg.

[0042] Example 2

[0043] A method for preparing high-purity submicron lignin comprises the following steps:

[0044] (1) The lignocellulose raw material is first preliminarily dried in air, and then put into a microwave drying device to be dried at 105°C for 12 h at a microwave frequency of 240 W. After that, it is crushed and sieved, and the lignocellulose particles with a mesh size of 120 - 200 are selected as biomass particles.

[0045] (2) The biomass particles and a hydrochloric acid solution with a concentration of 1 mol·L -1 are mixed evenly in a glassware according to the following weight parts: 2 parts of biomass particles and 10 parts of hydrochloric acid solution; a magnetic rotor is put into the mixture in the glassware, and then it is put into a water bath with magnetic stirring and stirred at a rotation speed of 30 r / min at room temperature for 20 h; after the stirring is finished, it is left standing for 0.4 h. After the solid particles precipitate, the supernatant is poured out and removed. Then deionized water is added and stirred, and after standing for 0.3 h and precipitation, the supernatant liquid is poured out and removed. The above operation is repeated until the pH value of the supernatant solution becomes about 7. Finally, the solid particles are separated by two layers of slow filter paper and transferred to a microwave drying device to be dried at 105°C for 0.7 h at a microwave frequency of 150 W.

[0046] (3) The dried wood cellulose particles are mixed evenly according to the following weight parts: 7 parts of dried wood cellulose particles, 90 parts of ethylene glycol, 1 part of deionized water, 1 part of sulfuric acid, and 1 part of formic acid; the uniformly mixed materials are transferred to a two-necked round-bottom flask with a reflux device and stirred evenly; then it is transferred to a microwave reactor and heated at 150°C for 0.2 h at a microwave frequency of 150 W. After the reaction is finished, it is naturally cooled to room temperature.

[0047] (4) The liquid mixture and the solid components are filtered out with two layers of slow filter paper with a pore size of 2 μm; then 900 parts of deionized water and 20 parts of acetic acid are added to the filtered liquid mixture, and the organic solvent lignin is precipitated by standing at room temperature for 8 h. Then the solid component of the organic solvent lignin is filtered out by a three-layer microporous filter membrane with a pore size of 0.08 μm and two layers of slow filter paper with a pore size of 2 μm under vacuum filtration.

[0048] (5) The filtered organic solvent lignin solid component is first placed in a refrigerator at -23°C for 8 hours to freeze the water remaining on the lignin. Then, the frozen lignin is placed in a vacuum freeze dryer and freeze-dried at -50°C for 24 hours to obtain dry and uniform high-purity submicron / nano lignin.

[0049] (6) The filtrate after vacuum filtration in step (4) is placed in a vacuum filtration rotary evaporator, and water is distilled out under a water bath heating at 75°C in a water bath to obtain a concentrated ethylene glycol solvent. The pH value of the recovered ethylene glycol solvent is measured with a pH test paper, and sulfuric acid is added to the solvent until its pH value is the same as that of the solution in step (3) to regenerate the performance of the recovered ethylene glycol solvent, and the recovered ethylene glycol solvent is recycled.

[0050] In this example, the yield of high-purity submicron / nano lignin is 20%, as Figure 2 shown, submicron lignin is obtained, with an average particle size of 200 nm, an ash content of 30 mg / kg, and an alkali metal impurity content of 3 mg / kg.

[0051] Example 3

[0052] A preparation method of high-purity submicron lignin, comprising the following steps:

[0053] (1) The lignocellulose raw material is first preliminarily dried in air, and then put into a microwave drying device and dried at a microwave frequency of 240 W at 105°C for 6 hours. After that, it is crushed and sieved, and lignocellulose particles of 120 - 200 meshes are selected as biomass particles.

[0054] (2) The biomass particles and a hydrochloric acid solution with a concentration of 0.5 mol·L -1 are mixed evenly in a glassware according to the following weight parts: 0.5 parts of biomass particles and 8 parts of hydrochloric acid solution; a rotor with magnetism is put into the mixture in the glassware, and then it is put into a water bath with magnetic stirring and stirred at a rotation speed of 10 r / min at room temperature for 36 hours; after stirring, it is left standing for 1 hour. After the solid particles precipitate, the supernatant is poured off. Then, deionized water is added and stirred, and after standing for 0.1 hour and precipitation, the supernatant liquid is poured off. The above operation is repeated until the pH value of the supernatant solution becomes about 6. Finally, the solid particles are separated with a layer of slow filter paper and transferred to a microwave drying device and dried at a microwave frequency of 80 W at 105°C for 0.5 hour.

[0055] (3) Mix the dried wood cellulose particles evenly according to the following parts by weight: 5 parts of dried wood cellulose particles, 92 parts of ethylene glycol, 0.5 part of deionized water, 0.5 part of sulfuric acid, and 2 parts of formic acid; transfer the evenly mixed materials to a two-necked round-bottom flask equipped with a reflux device and stir evenly; then transfer it to a microwave reactor and heat it at a microwave frequency of 80 W and 130 °C for 0.1 h, and naturally cool it to room temperature after the reaction ends.

[0056] (4) Filter out the liquid mixture and solid components with two layers of slow filter paper with a pore size of 1 μm; then add 700 parts of deionized water and 10 parts of acetic acid to the filtered liquid mixture, and let it stand at room temperature for 6 h to precipitate organic solvent lignin. Then use two layers of microporous filter membranes with a pore size of 0.05 μm and one layer of slow filter paper with a pore size of 1 μm to filter out the solid component of organic solvent lignin under vacuum filtration.

[0057] (5) First, place the filtered solid component of organic solvent lignin in a refrigerator at -20 °C for 6 h to freeze the water remaining on the lignin. Then, place the frozen lignin in a vacuum freeze dryer and freeze-dry it at -50 °C for 24 h to obtain dry and uniform high-purity submicron / nano lignin.

[0058] (6) Place the filtrate after vacuum filtration in step (4) in a vacuum filtration rotary evaporator, distill out the water in a water bath at 70 °C in a water bath, obtain a concentrated ethylene glycol solvent, measure the pH value of the recovered ethylene glycol solvent with a pH test paper, and add sulfuric acid to the solvent until its pH value is the same as that of the solution in step (3) to regenerate the performance of the recovered ethylene glycol solvent, and recycle the recovered ethylene glycol solvent.

[0059] In this example, the yield of high-purity submicron / nano lignin is 16%, obtaining submicron lignin with an average particle size of 400 nm, an ash content of 40 mg / kg, and an alkali metal impurity content of 5 mg / kg.

[0060] Example 4

[0061] A method for preparing high-purity submicron / nano lignin, comprising the following steps:

[0062] (1) First, preliminarily dry the wood cellulose raw material in the air, and then put it into a microwave drying equipment and dry it at a microwave frequency of 240 W and 105 °C for 6 h, and then crush and screen it to select wood cellulose particles with a mesh size of 120-200 as biomass particles.

[0063] (2) Mix the biomass particles with a concentration of 2 mol·L -1The hydrochloric acid solution is mixed evenly in a glassware according to the following parts by weight: 1 part of biomass particles and 15 parts of hydrochloric acid solution; a rotor with magnetism is placed into the mixture in the glassware, and then it is placed into a water bath with magnetic stirring and stirred at a speed of 50 r / min at room temperature for 10 h; after the stirring is completed, it is allowed to stand for 0.3 h. After the solid particles precipitate, the supernatant is poured off, and then deionized water is added and stirred. After standing for 0.5 h and precipitation, the supernatant liquid is poured off. The above operations are repeated until the pH value of the supernatant solution becomes about 6. Finally, the solid particles are separated out with three layers of slow filter paper and transferred to a microwave drying device to be dried at a microwave frequency of 240 W at 105 °C for 1 h.

[0064] (3) The dried wood cellulose particles are mixed evenly according to the following parts by weight: 10 parts of dried wood cellulose particles, 85 parts of ethylene glycol, 1 part of deionized water, 1 part of sulfuric acid, and 3 parts of formic acid; the uniformly mixed materials are transferred to a two-necked round-bottom flask with a reflux device and stirred evenly; then it is transferred to a microwave reactor and heated at a microwave frequency of 360 W and 180 °C for 0.3 h. After the reaction is completed, it is naturally cooled to room temperature.

[0065] (4) The liquid mixture and solid components are filtered out with three layers of slow filter paper with a pore size of 1 μm; then 1200 parts by weight of deionized water and 30 parts of acetic acid are added to the filtered liquid mixture, and it is allowed to stand at room temperature for 12 h to precipitate organic solvent lignin. Then, the organic solvent lignin solid components are filtered out with four layers of microporous filter membranes with a pore size of 0.1 μm and three layers of slow filter paper with a pore size of 3 μm under vacuum filtration of a vacuum filter.

[0066] (5) The filtered organic solvent lignin solid components are first placed in a refrigerator at -25 °C and frozen for 12 h to freeze the water remaining on the lignin. Then, the frozen lignin is placed in a vacuum freeze dryer and freeze-dried at -50 °C for 48 h to obtain dry and uniform high-purity submicron / nano lignin.

[0067] (6) The filtrate after vacuum filtration in step (4) is placed in a vacuum filtration rotary evaporator, and the water is distilled out by heating in a water bath at 80 °C in a water bath to obtain a concentrated ethylene glycol solvent. The pH value of the recovered ethylene glycol solvent is measured with a pH test paper, and sulfuric acid is added to the solvent until its pH value is consistent with the pH of the solution in step (3) to regenerate the performance of the recovered ethylene glycol solvent, and the recovered ethylene glycol solvent is recycled.

[0068] In this example, the yield of high-purity submicron / nano lignin is 20%, high-purity nano lignin is obtained, the average particle size of the high-purity nano lignin is 90 nm, the ash content is 15 mg / kg, and the alkali metal impurity content is 1 mg / kg.

[0069] Comparative Example 1

[0070] A preparation method of submicron lignin, comprising the following steps:

[0071] (1) First, preliminarily dry the lignocellulosic raw material in air, then put it into a drying equipment and dry it at 105 °C for 6 h. After that, crush and sieve it, and select the lignocellulosic particles with a mesh size of 120 - 200 meshes as biomass particles.

[0072] (2) Mix the biomass particles evenly according to the following weight parts: 10 parts of dried lignocellulosic particles, 85 parts of ethylene glycol, 1 part of deionized water, 1 part of sulfuric acid, and 3 parts of formic acid; transfer the evenly mixed material to a two-necked round-bottom flask with a reflux device and stir evenly; then transfer it to a microwave reactor and heat it at a microwave frequency of 360 W and 180 °C for 0.3 h. After the reaction, naturally cool it to room temperature.

[0073] (3) Filter out the liquid mixture and solid components with a 3-layer slow filter paper with a pore size of 1 μm; then add 1200 parts of deionized water and 30 parts of acetic acid to the filtered liquid mixture, and let it stand at room temperature for 12 h to precipitate the organic solvent lignin. Then use a 4-layer microporous filter membrane with a pore size of 0.1 μm and a 3-layer slow filter paper with a pore size of 3 μm to filter out the solid component of the organic solvent lignin under vacuum filtration.

[0074] (4) First, put the filtered solid component of the organic solvent lignin into a refrigerator at -25 °C and freeze it for 12 h to freeze the water remaining on the lignin. Then, put the frozen lignin into a vacuum freeze dryer and freeze-dry it at -50 °C for 48 h to obtain dry and uniform high-purity submicron / nano lignin.

[0075] (5) Place the filtrate after vacuum filtration in step (3) in a vacuum filtration rotary evaporator, distill out the water in a water bath at 80 °C in a water bath, obtain a concentrated ethylene glycol solvent, measure the pH value of the recovered ethylene glycol solvent with a pH test paper, add sulfuric acid to the solvent until its pH value is consistent with the pH value of the solution in step (3) to regenerate the performance of the recovered ethylene glycol solvent, and recycle the recovered ethylene glycol solvent.

[0076] The only difference between the preparation method of a high-purity submicron lignin in this comparative example and Example 1 is that: the biomass raw material is not pretreated by acid washing and deashing; in this comparative example, submicron lignin is obtained, the yield of the submicron lignin is 19%, the average particle size is 1 μm, the ash content is 3000 mg / kg, and the alkali metal impurity content is 1000 mg / kg.

[0077] Comparative Example 2

[0078] The only difference between the preparation method of a high-purity submicron / nano lignin in this comparative example and Example 1 is that formic acid is not added in steps (3) and (4); the yield of the high-purity submicron / nano lignin in this comparative example is 18%, micron-sized lignin is obtained, the average particle size is 20 μm, the ash content is 1000 mg / kg, and the alkali metal impurity content is 500 mg / kg.

[0079] Comparative Example 3

[0080] The only difference between the preparation method of a high-purity submicron / nano lignin in this comparative example and Example 1 is that acetic acid in step (4) is replaced by hydrochloric acid.

[0081] The yield of lignin in this comparative example is 20%, submicron-sized lignin is obtained, the average particle size is 900 nm, the ash content is 900 mg / kg, and the alkali metal impurity content is 400 mg / kg.

[0082] Comparative Example 4

[0083] The only difference between the preparation method of a high-purity submicron / nano lignin in this comparative example and Example 1 is that microwave drying in step (1) is replaced by ordinary oven drying at a temperature of 105 °C for 12 h.

[0084] The yield of lignin in this comparative example is 20%, submicron-sized lignin is obtained, the average particle size is 700 nm, the ash content is 700 mg / kg, and the alkali metal impurity content is 300 mg / kg.

[0085] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made in form and details without departing from the scope defined by the claims of the present invention.

Claims

1. A preparation method of high-purity submicron or nano lignin, characterized in that: It includes the following steps: (1) Dry the lignocellulosic raw material, then microwave heat it, crush it, and sieve it to obtain biomass particles; (2) Place the biomass particles in a hydrochloric acid solution for pickling, then wash and dry them to obtain dried wood cellulose particles; (3) Place the dried wood cellulose particles in a mixed solution for microwave heating, then separate the lignin precipitate, and freeze-dry the lignin precipitate to obtain high-purity submicron or nano lignin; the mixed solution includes the following raw materials in parts by mass: 84-93 parts of ethylene glycol, 0.5-1 part of water, 0.5-1 part of sulfuric acid, and 1-3 parts of formic acid.

2. The preparation method of the high-purity sub-micron or nano lignin according to claim 1, characterized in that: In step (1), the temperature of microwave heating is 105°C and the time is 6-12 h.

3. The preparation method of high-purity submicron or nano lignin according to claim 1, characterized in that: In the step (2), the concentration of the hydrochloric acid solution is 0.5 - 2 mol·L -1 , and the mass ratio of the hydrochloric acid solution to the biomass particles is (8 - 15):(0.5 - 2).

4. The preparation method of the high-purity submicron or nano lignin according to claim 1, wherein: The pickling specifically includes: placing the biomass particles in a hydrochloric acid solution and stirring for 10-36 h, then standing for 0.3-1 h.

5. The preparation method of the high-purity submicron or nano lignin according to claim 1, wherein: In step (2), microwave drying technology is used for drying, the microwave frequency is 80-240 W, the temperature is 105°C, and the time is 0.5-1 h.

6. The preparation method of the high-purity submicron or nano lignin according to claim 1, characterized in that: In step (3), the mass ratio of the dried wood cellulose particles to ethylene glycol is (5-10):(84-93).

7. The preparation method of high-purity submicron or nano lignin according to claim 1, characterized in that: In step (3), the microwave frequency of microwave heating is 80-360 W, the temperature is 130-180°C, and the time is 0.1-0.3 h.

8. The preparation method of the high-purity submicron or nano lignin according to claim 1, characterized in that: In step (3), separating the lignin precipitate specifically includes: filtering the microwave-heated substance to separate the liquid mixture and the solid component; adding water and acetic acid to the liquid mixture, then standing, and finally vacuum filtering to obtain the organic solvent lignin solid component.

9. The preparation method of the high-purity sub-micron or nano lignin according to claim 1, characterized in that: The lignocellulosic raw material is one or a mixture of softwood, hardwood, agricultural waste, and herbs.

10. The high-purity submicron / nano lignin prepared by the method for preparing high-purity submicron or nano lignin according to claim 1, characterized in that: The average particle size is 90-400 nm, the ash content is less than 50 mg / kg, and the alkali metal impurity content is less than 5 mg / kg.

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