Method for extracting and separating corn straw lignin by using eutectic solvent

Abstract

The invention relates to a method for extracting and separating corn straw lignin by using a deep eutectic solvent, which comprises the following steps: (1) carrying out Soxhlet extraction on air-dried corn straw fine powder by using absolute ethyl alcohol, and drying to obtain corn straw solid residues; (2) synthesizing a eutectic solvent by using cetyltrimethylammonium bromide and an ethylene glycol aqueous solution; and (3) carrying out reaction on the corn straw solid residues and the eutectic solvent under certain conditions, and then preparing the corn straw lignin extracted by the eutectic solvent by adopting the methods of reduced pressure filtration, rotary evaporation concentration, diethyl ether extraction, reduced pressure drying and the like. The extraction rate of the lignin is 50.73%-64.58%, and the purity of the lignin is 89.5%-99.4%. The method has the characteristics of high lignin extraction rate and high purity, and is of great significance to conversion and utilization of straw biomass lignin.

Description

Technical Field

[0001] The invention relates to the technical field of clean separation of biomass components, and in particular to a method for extracting and separating corn stalk lignin with a low eutectic solvent. Background Art

[0002] Corn stalks are rich in cellulose, hemicellulose and lignin, among which lignin is the only natural aromatic compound. Lignin can be used to produce products such as surfactants, flocculants and basic chemicals, and is a valuable renewable resource. Lignin, cellulose and hemicellulose in the stalks are connected by hydrogen bonds and chemical bonds to form a complex polymer with a three-dimensional network structure. This structural characteristic hinders the extraction and processing of lignin. At present, the extraction and separation methods of lignin mainly include acid extraction, alkaline extraction, organic solvent extraction, etc., but there are generally problems such as low lignin extraction rate, low lignin purity, complex extraction process and equipment corrosion. Low eutectic solvents are composed of a certain proportion of hydrogen bond acceptors and hydrogen bond donors, and are a new type of green "ionic liquid". Due to the advantages of wide sources, easy preparation and low chemical corrosion, low eutectic solvents have great application potential in the fields of extraction separation and chemical synthesis. If appropriate methods are used to extract and separate lignin from corn stalks using low eutectic solvents, it is not only expected to improve the extraction rate of corn stalk lignin, but also reduce the equipment corrosion problem caused by traditional acid and alkali solvents.

[0003] In 2021, the article "Low-temperature pretreatment of poplar wood with deep eutectic solvents and the evolution of three-component structures" published in the Journal of Fuel Chemistry and Technology, Vol. 49, No. 12, used 10 different carboxylic acid-choline chloride solvents to extract lignin from poplar wood, with the lignin extraction rate and purity reaching 63% and 90%, respectively. In 2020, the article "Effect of hydrogen bond donor on the choline chloride-based deep eutectic solvent-mediated extraction of lignin from pine wood" published in the International Journal of Biological Macromolecules, Vol. 165, No. 12, used a ternary deep eutectic solvent system synthesized by choline chloride, lactic acid and formic acid to extract pine wood lignin, with the lignin extraction rate and purity reaching 60% and 90%, respectively. The above studies used deep eutectic solvents to extract lignin from biomass, which improved the utilization rate of lignocellulosic biomass, but the extraction rate and purity of lignin were low. The present invention proposes a method for extracting and separating corn straw lignin by using a low eutectic solvent, which provides a theoretical basis for promoting the advancement of clean separation technology for biomass components. Summary of the invention

[0004] The present invention aims to solve the technical problems of low lignin extraction rate and low purity, and provide a new method for efficiently extracting and separating corn straw lignin with a low eutectic solvent. The lignin extracted by this method has an extraction rate of 50.73% to 64.58% and a purity of 89.5% to 99.4%.

[0005] The method for extracting corn straw lignin using a deep eutectic solvent provided by the present invention has the following technical scheme: 1. crushing and sieving the air-dried corn stalks to collect 60-80 mesh air-dried corn stalk fine powder, then using anhydrous ethanol to perform Soxhlet extraction on the air-dried corn stalk fine powder, and drying to obtain a corn stalk solid residue; 2. Mix 5-10 g of 97% hexadecyltrimethylammonium bromide and 150 mL of 70%-90% ethylene glycol, heat and stir to obtain a hexadecyltrimethylammonium bromide-ethylene glycol deep eutectic solvent; 3. Add the corn stalk solid residue described in step 1 and the low eutectic solvent described in step 2 to a microreactor and heat for a certain period of time. After the reaction is completed, wait for the material to cool to room temperature and perform vacuum filtration. At the same time, use 50 mL of distilled water to wash the material residue to pH = 7 to obtain a first-level solid residue and a first-level filtrate; use a rotary evaporator to concentrate the first-level filtrate to 3-5 mL to obtain a concentrated solution; transfer the concentrated solution to a separatory funnel, add 100 mL of anhydrous ether, shake the separatory funnel to mix the concentrated solution and anhydrous ether evenly, then let the separatory funnel stand for 2 hours, and collect the bottom layer of the mixture; use 0.22 μ m microporous filter membrane to reduce the pressure and filter the bottom layer mixture, transfer the solid residue to a vacuum drying oven, and dry it at 55 ° C and 0.02 MPa until the moisture content is less than 0.5%, thereby obtaining the secondary solid residue, i.e., the corn straw lignin extracted by the low eutectic solvent; Furthermore, in step 1, the drying temperature is 105° C., and the corn stalk solid residue is dried until the moisture content is less than 1.5%; Furthermore, in step three, the solid-liquid ratio of the corn straw solid residue and the low eutectic solvent is 1g: (10-30mL), the reaction time is 1-3 hours, and the reaction temperature is 100-140°C.

[0006] Compared with the current technical methods for extracting corn straw lignin, the advantages of the present invention are as follows: the present method uses a low eutectic solvent synthesized from hexadecyltrimethylammonium bromide and ethylene glycol solution to extract corn straw lignin, and the extracted corn straw lignin has high purity and high extraction rate, with the purity and extraction rate reaching 99.4% and 64.58% respectively. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 It is the corn stover lignin extracted by the low eutectic solvent in Example 1. DETAILED DESCRIPTION

[0008] The beneficial effects of the present invention are demonstrated by the following specific examples.

[0009] Embodiment 1: The method for extracting and separating corn straw lignin in this embodiment is carried out according to the following steps: 1. Cut the air-dried corn stalks into small pieces of 2 to 3 cm, and then use a Tester FW-100 grinder to grind for 30 seconds and then sieve to collect 60-80 mesh corn stalk fine powder, use anhydrous ethanol to perform Soxhlet extraction on the corn stalk fine powder, and then transfer the extracted corn stalk fine powder to a blast drying oven, and dry it at 105°C until the moisture content is less than 1.5%, to obtain corn stalk solid residue; 2. Add 150 mL of 80% by mass ethylene glycol aqueous solution and 7.5 g of 97% by mass cetyltrimethylammonium bromide into a round-bottom flask, heat and stir for 0.5 hours at a temperature of 65°C and a stirring speed of 600 rpm to obtain a low eutectic solvent of cetyltrimethylammonium bromide-ethylene glycol; 3. Add 5 g of the corn stalk solid residue described in step 1 and 100 mL of the low eutectic solvent described in step 2 to a microreactor, heat at 120°C for 2 hours, and after the reaction is completed, wait for the material to cool to room temperature and perform vacuum filtration, and use 50 mL of distilled water to wash the material residue to pH = 7 to obtain a first-level solid residue and a first-level filtrate; use a rotary evaporator to concentrate the first-level filtrate to 3-5 mL to obtain a concentrated solution; transfer the concentrated solution to a separatory funnel, add 100 mL of anhydrous ether, shake the separatory funnel to mix the concentrated solution and anhydrous ether evenly, and then let the separatory funnel stand for 2 hours to collect the bottom layer of the mixture; use 0.22 μ The bottom layer mixture was filtered under reduced pressure through a microporous filter membrane, and the solid residue was washed with 30 mL of anhydrous ether to a pH of 7. The solid residue was transferred to a vacuum drying oven and dried at 55 °C and 0.02 MPa to a moisture content of less than 0.5%, thereby obtaining a secondary solid residue, namely, corn straw lignin extracted by a low eutectic solvent.

[0010] Example 2: The difference between this example and Example 1 is that in step 2, 5 g of hexadecyltrimethylammonium bromide with a mass fraction of 97% is added, and the other steps and parameters are the same as those in Example 1.

[0011] Example 3: The difference between this example and Example 1 is that in step 2, 10 g of hexadecyltrimethylammonium bromide with a mass fraction of 97% is added, and the other steps and parameters are the same as those in Example 1.

[0012] Example 4: The difference between this example and Example 1 is that in step 3, 50 mL of the low eutectic solvent is added to the reactor, and the other steps and parameters are the same as those in Example 1.

[0013] Example 5: The difference between this example and Example 1 is that in step 3, 150 mL of the low eutectic solvent is added to the reactor, and the other steps and parameters are the same as those in Example 1.

[0014] Example 6: The difference between this example and Example 1 is that in step 3, the heating temperature of the reactor is set to 100°C, and the other steps and parameters are the same as those in Example 1.

[0015] Example 7: The difference between this example and Example 1 is that in step 3, the heating temperature of the reactor is set to 140°C, and the other steps and parameters are the same as those in Example 1.

[0016] Example 8: The difference between this example and Example 1 is that in step 3, the heating time of the reactor is set to 1 hour, and the other steps and parameters are the same as those in Example 1.

[0017] Example 9: The difference between this example and Example 1 is that in step 3, the heating time of the reactor is set to 3 hours, and the other steps and parameters are the same as those in Example 1.

[0018] Example 10: The difference between this example and Example 1 is that in step 2, the mass fraction of 150 mL of ethylene glycol aqueous solution for preparing the low eutectic solvent is 70%, and the other steps and parameters are the same as those in Example 1.

[0019] Example 11: The difference between this example and Example 1 is that in step 2, the mass fraction of 150 mL of ethylene glycol aqueous solution for preparing the low eutectic solvent is 90%, and the other steps and parameters are the same as those in Example 1.

[0020] Example 12: The difference between this example and Example 1 is that in step three, 5 g of the corn straw solid residue described in step one, 7.5 g of 97% by mass hexadecyltrimethylammonium bromide and 100 mL of distilled water are added to a microreactor, and then the heating reaction is started. The other steps and parameters are the same as in Example 1, and finally the extracted corn straw lignin is obtained.

[0021] Example 13: The difference between this example and Example 1 is that in step three, 5 g of the corn straw solid residue described in step one and 100 mL of 80% by mass ethylene glycol aqueous solution are added to the microreactor, and then the heating reaction is started. The other steps and parameters are the same as in Example 1, and finally the extracted corn straw lignin is obtained.

[0022] Example 14: This example differs from Example 1 in that in step three, 5 g of the corn straw solid residue described in step one and 100 mL of distilled water are added to a microreactor, and then the heating reaction is started. The other steps and parameters are the same as in Example 1, and finally the extracted corn straw lignin is obtained.

[0023] The method of the National Standard of the People's Republic of China "GB / T35818-2018 Analysis Method for Forestry Biomass Raw Materials - Determination of Polysaccharide and Lignin Content" was used to determine the lignin content of the air-dried corn straw described in step 1 of Examples 1 to 14, that is, the theoretical lignin content, and the results were expressed as a percentage based on the dry weight of the air-dried corn straw; the masses of the corn straw lignin extracted by the low eutectic solvent of Examples 1 to 11 and the corn straw lignin extracted in Examples 12 to 14 were respectively divided by the theoretical lignin content of the air-dried corn straw raw material to obtain the extraction rate of various extracted lignins, and the results were expressed as a percentage; the sum of the masses of the acid-insoluble lignin and the acid-soluble lignin of the various lignins extracted in Examples 1 to 14 was divided by their mass, and the result was multiplied by 100%, which was recorded as the purity of each lignin; the lignin extraction rate and purity of each example are shown in Table 1.

[0024] Table 1 The extraction rate and purity of various lignins in Examples 1 to 14.

[0025] Example Lignin sample name Lignin extraction rate (%) Lignin purity (%) 1 Sample 1 64.58 99.4 2 Sample 2 63.29 99.2 3 Sample 3 62.28 98.3 4 Sample 4 61.34 98.7 5 Sample 5 61.94 99.1 6 Sample 6 60.52 99.2 7 Sample 7 64.18 98.7 8 Sample 8 50.73 99.1 9 Sample 9 60.89 89.5 10 Sample 10 63.11 99.3 11 Sample 11 62.98 99.2 12 Sample 12 10.19 98.1 13 Sample 13 52.81 90.2 14 Sample 14 3.32 92.8

[0026] From the data in Table 1, it can be seen that the extraction rates of lignin samples 1 to 11 are 50.73% to 64.58%, which are higher than those of lignin samples 12 to 14. This shows that the extraction rate of lignin extracted using a low eutectic solvent is higher than that of lignin extracted using hexadecyltrimethylammonium bromide, ethylene glycol or distilled water alone; compared with lignin samples 2 to 14, lignin sample 1 has the highest extraction rate and purity, reaching 64.58% and 99.4%, respectively.

[0027] Figure 1 Middle 3370.5cm -1 The absorption peak at 2939.1cm represents the stretching vibration of lignin hydroxyl group OH; -1 The absorption peak at 1592.1cm represents the antisymmetric vibration of CH in the methoxy group on the benzene ring; -1 、1513.3cm -1 The absorption peak at 1446.6cm is the vibration of the aromatic ring skeleton of lignin; -1 The absorption peak at 1103.3cm is the asymmetric bending vibration of CH in methyl and methylene; -1 The absorption peak at is attributed to the COC anti-stretching vibration in lignin; Figure 1 It can be seen that the phenylpropane structural unit of the lignin of sample 1 is stable; the lignin extracted by the present invention has the characteristics of high extraction rate and high purity, which is of great significance to the conversion and utilization of straw biomass lignin.

Claims

1. A method for extracting and separating corn straw lignin by using a deep eutectic solvent, characterized in that Perform the extraction separation according to the following steps:

1. Using anhydrous ethanol to Soxhlet extract the air-dried corn stalk fine powder, and drying to obtain a solid residue of corn stalk; 2. Mix 5-10 g of 97% hexadecyltrimethylammonium bromide and 150 mL of 70%-90% ethylene glycol, heat and stir to obtain a hexadecyltrimethylammonium bromide-ethylene glycol deep eutectic solvent; 3. Add the corn straw solid residue described in step 1 and the low eutectic solvent described in step 2 to a micro reactor and heat them for a certain period of time. After the reaction is completed, wait for the material to cool to room temperature, filter under reduced pressure to obtain a first-level solid residue and a first-level filtrate; use a rotary evaporator to concentrate the first-level filtrate, add anhydrous ether to the concentrated liquid for extraction, and collect the bottom layer of the mixture; then filter the mixture under reduced pressure, and dry under reduced pressure to obtain a second-level solid residue, i.e., corn straw lignin extracted with the low eutectic solvent.

2. The method for extracting corn straw lignin by a deep eutectic solvent according to claim 1, characterized in that In step 1, the drying temperature is 105° C., and the corn stalk solid residue is dried until the moisture content is less than 1.5%.

3. A method for extracting corn straw lignin with a deep eutectic solvent according to claim 1 or 2, characterized in that In step 3, the volume ratio of the concentrated liquid and anhydrous ether is (3-5) mL:100 mL.

4. A method for extracting corn straw lignin with a deep eutectic solvent according to claim 1 or 2, characterized in that In step three, the solid-liquid ratio of the corn straw solid residue and the low eutectic solvent is 1 g: (10-30 mL), the reaction time is 1-3 hours, and the reaction temperature is 100-140°C.

Images