Method for preparing bamboo lignin with high antioxidant activity through one-pot two-step method

Abstract

The invention belongs to the technical field of lignin, and particularly relates to a method for preparing bamboo lignin with high antioxidant activity through a one-pot two-step method. The method provided by the invention comprises the following four steps: constructing a deep-eutectic solvent, extracting lignin, obtaining a bamboo cellulose-rich substance, and obtaining the degraded bamboo lignin. The eutectic solvent used in the method provided by the invention has the advantages of low vapor pressure, good thermal stability, biodegradability, recyclability and the like, has good environmental friendliness, and conforms to the concept of sustainable development. The bamboo wood lignin is extracted and degraded through a one-pot two-step method, the lignin in the bamboo wood can be efficiently dissolved and separated, meanwhile, damage to cellulose and hemicellulose is small, and the antioxidant activity of the extracted lignin can be remarkably improved. The lignin with high antioxidant activity prepared by the method can be applied to the fields of food, cosmetics, medicines and the like.

Description

[0001] The invention belongs to the technical field of lignin, and particularly relates to a method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step process. Background Art

[0002] Lignin is the third most abundant natural renewable polymer in nature, after cellulose and chitin. It provides structural support for plant cell walls and protects against microbial and insect damage. Lignin's aromatic structure and active groups, such as phenolic hydroxyl groups, impart excellent biocompatibility, photostability, UV absorption, and biodegradability. It has found applications in food packaging, cosmetics, and polymer materials.

[0003] Lignin structures vary among plants. Herbaceous plant lignin is primarily composed of syringyl and guaiacyl structures, with a high content of phenolic hydroxyl groups and strong antioxidant activity. Woody plant lignin is primarily composed of guaiacyl and p-hydroxyphenyl structures, with a high content of methoxy groups and relatively low antioxidant activity. Currently, most methods for extracting lignin result in high molecular weights, and their antioxidant activity is insufficient for practical use. Physical, chemical, or biological modification methods can be used to reduce lignin's molecular weight and increase its antioxidant activity, potentially expanding its potential as a natural antioxidant.

[0004] Deep eutectic solvents (DESs), as a green and efficient pretreatment system for lignocellulosic materials, have shown significant potential in the field of lignin extraction and degradation in recent years. DESs can be used to effectively extract lignin from plant raw materials.

[0005] However, the pretreatment system for lignocellulosic materials constructed with DESs is highly acidic, which can over-degrade carbohydrates when treating bamboo, leading to cellulose loss. Currently, there is a lack of a method that can retain bamboo cellulose while obtaining bamboo lignin with high purity and good antioxidant properties. Summary of the Invention

[0006] To address the current problem of the lack of a method for obtaining bamboo lignin with high purity and good antioxidant properties while retaining bamboo cellulose, the present invention provides a one-pot two-step method for preparing bamboo lignin with high antioxidant activity. Bamboo is selected as the raw material, and a low eutectic solvent is used to extract and degrade the bamboo lignin to prepare bamboo lignin with high antioxidant activity.

[0007] This is achieved specifically through the following technical solutions:

[0008] A one-pot two-step method for preparing bamboo lignin with high antioxidant activity comprises the following steps:

[0009] (1) Constructing a deep eutectic solvent: Weigh choline chloride and an organic acid, place them in a container, and stir to form a deep eutectic solvent;

[0010] (2) Extracting lignin: mixing the deep eutectic solvent obtained in (1) with bamboo powder, stirring, and then filtering to obtain a solid component and a liquid component;

[0011] (3) Obtaining bamboo cellulose-rich material: soaking the solid component obtained in (2) in an ethanol solution, removing the ethanol after soaking, and washing with distilled water for multiple times to obtain the bamboo cellulose-rich material;

[0012] (4) Obtaining degraded bamboo lignin: The liquid component obtained in (2) is placed in a container for heating treatment, and then excess distilled water is added to precipitate lignin. The lignin is separated by filtration, and then the lignin obtained by filtration is placed in an oven for drying to obtain degraded bamboo lignin.

[0013] Furthermore, in step (1), the organic acid is lactic acid, acetic acid, formic acid, malic acid, citric acid, or p-toluenesulfonic acid.

[0014] In step (1), the molar ratio of choline chloride to organic acid is 1:1 to 1:15.

[0015] In step (1), the stirring temperature is 80 to 120° C., the stirring speed is 100 to 800 rpm, and the stirring time is 0.5 to 3 h.

[0016] In step (2), the mass volume ratio of the bamboo powder to the deep eutectic solvent is 1:5 to 1:40; the stirring temperature is 80 to 140° C., the stirring speed is 100 to 800 rpm, and the stirring time is 1 min to 5 h.

[0017] In step (3), the volume concentration of the ethanol solution is 50% to 90%, and is specifically prepared from ethanol and water.

[0018] In step (4), the heating method is oil bath heating, the heating temperature is 80 to 140° C., and the heating time is 1 to 10 hours.

[0019] In step (4), the drying temperature is 80 to 100° C., and the drying time is 2 to 10 hours.

[0020] Beneficial effects

[0021] The present invention provides a one-pot two-step method for preparing bamboo lignin with high antioxidant activity, which has the following effects:

[0022] 1. The deep eutectic solvent used in the method provided by the present invention has the advantages of low vapor pressure, good thermal stability, biodegradability and recyclability, is environmentally friendly, and conforms to the concept of sustainable development.

[0023] 2. The present invention extracts and degrades bamboo lignin through a one-pot two-step method, which can efficiently dissolve and separate the lignin in bamboo while causing less damage to cellulose and hemicellulose.

[0024] 3. The present invention extracts and degrades bamboo lignin through a one-pot two-step method, which can significantly improve the antioxidant activity of the extracted lignin.

[0025] 4. The high antioxidant activity lignin prepared by the present invention can be used in the fields of food, cosmetics, medicine, etc., expanding the application range of lignin. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Antioxidant activity of lignin and BHT;

[0027] Figure 2 Total phenolic hydroxyl content of lignin. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further defined below in conjunction with specific implementation methods, but the scope of protection required is not limited to the description.

[0029] Example 1

[0030] A one-pot two-step method for preparing bamboo lignin with high antioxidant activity comprises the following steps:

[0031] (1) Constructing a deep eutectic solvent: Choline chloride and p-toluenesulfonic acid were weighed, placed in a container, and stirred at 400 rpm at 100 °C for 2 h to form a deep eutectic solvent with a total volume of 20 mL. The molar ratio of choline chloride to p-toluenesulfonic acid was 1:1.

[0032] (2) Lignin extraction: About 1 g of bamboo powder was added to the prepared deep eutectic solvent and stirred at 120°C and 400 rpm for 5 min using a magnetic stirrer. Subsequently, solid-liquid separation was performed using a vacuum filtration device to obtain solid and liquid components.

[0033] (3) Obtaining bamboo cellulose-rich material: Slowly adding 130 mL of 70% (V / V) ethanol in batches to the solid component separated in (2), stirring for 30 min, and then washing with distilled water several times to remove the deep eutectic solvent on the surface to obtain a solid which is the extracted bamboo cellulose-rich material;

[0034] (4) Obtaining degraded bamboo lignin: The liquid component obtained in (2) was placed in an oil bath and continued to be stirred at 120°C for 1 hour. After the treatment, 200 mL of distilled water was added to precipitate the lignin. After filtration, the filtered solid was placed in an oven at 80°C and dried for 6 hours to finally obtain the extracted-degraded bamboo lignin.

[0035] Example 2

[0036] About 1 g of bamboo powder was taken and the same operation as in Example 1 was performed, except that in step (4), the stirring time was 3 h.

[0037] Example 3

[0038] About 1 g of bamboo powder was taken and the same operation as in Example 1 was performed, except that in step (4), the stirring time was 5 h.

[0039] Example 4

[0040] About 1 g of bamboo powder was taken and the same operation as in Example 1 was performed, except that in step (4), the stirring time was 7 h.

[0041] Example 5

[0042] About 1 g of bamboo powder was taken and the same operation as in Example 1 was performed, except that in step (4), the stirring time was 9 h.

[0043] Example 6

[0044] About 1 g of bamboo powder was taken and the same operation as in Example 1 was performed, except that in step (4), the temperature of the oil bath was 140° C. and the stirring time was 5 h.

[0045] Based on the experience gained during implementation, it was found that the process that had the greatest impact on the yield of cellulose-rich material was (2) extracting the lignin portion. The stirring treatment time in (2) was adjusted, and at the same time, the lignin heating degradation process was not performed to obtain Comparative Examples 1 to 3. Furthermore, it was found that the process that had the greatest impact on the antioxidant activity of lignin was (1) constructing a deep eutectic solvent. The type of deep eutectic solvent in (1) was adjusted to obtain Comparative Examples 4 to 8.

[0046] Comparative Example 1

[0047] (1) Constructing a deep eutectic solvent: Choline chloride and p-toluenesulfonic acid were weighed, placed in a container, and stirred at 400 rpm for 2 h at 100 °C to form a deep eutectic solvent with a total volume of 20 mL. The molar ratio of choline chloride to p-toluenesulfonic acid was 1:1.

[0048] (2) Lignin extraction: About 1 g of bamboo powder was added to the prepared deep eutectic solvent and stirred at 120°C and 400 rpm for 1 min using a magnetic stirrer. Subsequently, solid-liquid separation was performed using a vacuum filtration device to obtain solid and liquid components.

[0049] (3) Obtaining bamboo cellulose-rich material: 130 mL of 70% ethanol was slowly added in batches to the solid component separated in (2), stirred for 30 min, and then washed with distilled water several times to remove the low eutectic solvent on its surface to obtain a solid which is the extracted bamboo cellulose-rich material.

[0050] (4) Obtaining bamboo lignin: The liquid component obtained in (2) was added to 200 mL of distilled water to precipitate lignin. After filtration, the filtered solid was placed in an oven at 80° C. and dried for 6 h to finally obtain extracted-degraded bamboo lignin.

[0051] Comparative Example 2

[0052] Take about 1g of bamboo powder and perform the same operation as in Comparative Example 1. The difference from Comparative Example 1 is that the lignin part is extracted in step (2) and the stirring treatment time is 5 minutes.

[0053] Comparative Example 3

[0054] Take about 1g of bamboo powder and perform the same operation as in Comparative Example 1. The difference from Comparative Example 1 is that the lignin part is extracted in step (2) and the stirring treatment time is 3h.

[0055] Comparative Example 4

[0056] Take about 1g of bamboo powder and perform the same operation as in Comparative Example 1. The difference from Comparative Example 1 is that choline chloride and lactic acid are used to construct a low eutectic solvent in step (1), and the lignin part is extracted in step (2). The stirring treatment time is 3h.

[0057] Comparative Example 5

[0058] About 1 g of bamboo powder was taken and the same operation as in Comparative Example 1 was performed. The difference from Comparative Example 1 was that choline chloride and acetic acid were used to construct a low eutectic solvent in step (1), and the lignin part was extracted in step (2). The stirring treatment time was 3 h.

[0059] Comparative Example 6

[0060] About 1 g of bamboo powder was taken and the same operation as in Comparative Example 1 was performed. The difference from Comparative Example 1 was that choline chloride and formic acid were used to construct a low eutectic solvent in step (1), and the lignin part was extracted in step (2). The stirring treatment time was 3 h.

[0061] Comparative Example 7

[0062] About 1 g of bamboo powder was taken and the same operation as in Comparative Example 1 was performed. The difference from Comparative Example 1 was that choline chloride and malic acid were used to construct a low eutectic solvent in step (1), and the lignin part was extracted in step (2). The stirring treatment time was 3 h.

[0063] Comparative Example 8

[0064] Take about 1g of bamboo powder and perform the same operation as in Comparative Example 1. The difference from Comparative Example 1 is that choline chloride and citric acid are used to construct a low eutectic solvent in step (1), and the lignin part is extracted in step (2). The stirring treatment time is 3h.

[0065] The yields of fiber-rich materials and lignin obtained in the above examples and comparative examples were calculated, and the results are shown in Table 1. The results of Comparative Examples 1 to 3 show that when the choline chloride-p-toluenesulfonic acid treatment time for bamboo powder was extended from 1 minute (Comparative Example 1) to 5 minutes (Comparative Example 2) and 3 hours (Comparative Example 3), the yield of cellulose-rich material decreased from 72.25% to 48.30% and 11.03%, respectively, while the lignin yield increased from 6.32% to 19.90% and 51.45%, respectively. The lignin purity increased from 87.52% to 88.16% and decreased to 66.33%, respectively. It can be seen that excessively long treatment times can lead to reduced lignin purity. This shows that by adjusting the stirring time during lignin extraction, efficient dissolution and separation of bamboo lignin can be achieved; using choline chloride-p-toluenesulfonic acid as a deep eutectic solvent to treat bamboo powder can reduce damage to cellulose and hemicellulose by controlling the treatment time.

[0066] Table 1 Fiber-rich material yield and lignin yield of each embodiment and comparative example

[0067]

[0068]

[0069] IC 50DPPH The value is a key indicator for measuring the antioxidant activity of a compound and is often used for horizontal comparison of the antioxidant capacity of different samples. The lower the value, the stronger the antioxidant capacity of the compound.

[0070] The antioxidant activity of the bamboo lignin prepared in the above examples and comparative examples was tested, and the results are shown in Table 2. As can be seen from Table 2, in Comparative Examples 1 to 3, the time for treating bamboo powder with choline chloride-p-toluenesulfonic acid was extended from 1 min to 5 min and 3 h, and the IC of the extracted lignin was 1.5447 W / m. 50DPPH The values decreased from 797.55 μg / mL to 558.55 and 417.69 μg / mL, respectively. It was found that prolonging the stirring treatment time can improve the antioxidant activity of bamboo lignin.

[0071] In addition, it can be seen from the results of Comparative Example 2 and Example 3 that under the same stirring treatment time, when the lignin degradation time is extended from 0h to 5h, the IC 50DPPHThe value decreased from 558.55 μg / mL to 380.47 μg / mL, indicating that extending the degradation treatment time can improve the antioxidant activity of bamboo lignin.

[0072] On the other hand, the degradation time of fixed lignin is 5h. According to the results of Examples 3 and 6, when the degradation temperature is increased from 120℃ to 140℃, the IC 50DPPH The value decreased from 380.47 μg / mL to 276.89 μg / mL, indicating that increasing the degradation temperature can further improve the antioxidant activity of bamboo lignin.

[0073] From the results of Comparative Examples 3 to 8, it can be seen that the lignin yield of Comparative Example 3 is 51.45%, far exceeding that of the other examples. At the same time, the product of yield and purity, that is, the yield of pure lignin, is also the highest in the p-toluenesulfonic acid combination, indicating that the deep eutectic solvent constructed with choline chloride and p-toluenesulfonic acid has the best extraction performance.

[0074] By exploring the method, process, reaction time and degradation temperature of lignin extraction, the present invention provides a one-pot two-step method for preparing bamboo lignin with high antioxidant activity, which can prepare bamboo lignin with high antioxidant activity.

[0075] Table 2 Antioxidant activity of bamboo lignin prepared in various examples and comparative examples

[0076]

[0077]

[0078] As shown in Table 2, the difference between Comparative Example 2, Example 3 and Example 6 lies in the treatment time and temperature of the liquid components. 50DPPH The values were 558.55 μg / mL, 380.47 μg / mL and 276.89 μg / mL, respectively, indicating that the present invention improved the antioxidant activity of the obtained lignin by adjusting the stirring treatment process and the degradation process.

[0079] IC from Comparative Examples 3 to 8 50DPPH It can be seen that when choline chloride is used to construct a deep eutectic solvent with different organic acids, the use of p-toluenesulfonic acid (Comparative Example 3) and citric acid (Comparative Example 8) can significantly improve the antioxidant property of the obtained lignin. 50DPPH They were 417.69 μg / mL and 582.90 μg / mL respectively.

[0080] The antioxidant activity of the bamboo lignin prepared in Example 6 and Comparative Example 2 was measured using commercial antioxidant butylated hydroxytoluene (BHT) as a control. Figure 1 As shown. Figure 1It can be seen that at low concentrations (≤800 μg / L), the antioxidant activity of the extracted lignin prepared in control group 2 is lower than that of BHT, while under all test concentration conditions, the antioxidant activity of the extracted-degraded lignin prepared in Example 6 is higher than that of BHT, which indicates that the antioxidant activity of the lignin extracted in the present invention is better.

[0081] Phenolic hydroxyl group is the core structural unit of many natural antioxidants, and its content directly affects the antioxidant activity of the compound. The total phenolic hydroxyl group content of the bamboo lignin prepared in Example 6 and Comparative Example 2 was tested, and the results are as follows: Figure 2 As shown by Figure 2 It can be seen that the lignin extraction method proposed in the present invention can increase its phenolic hydroxyl content. Figure 1 The results in Figure 3 indicate that the antioxidant activity of the extracted-degraded lignin of the present invention is mainly due to the increase in the phenolic hydroxyl content of the final lignin.

[0082] Lignin's molecular weight is generally negatively correlated with its antioxidant activity. Low-molecular-weight lignin has a higher ratio of end groups to side chain functional groups, resulting in higher chemical reactivity and a higher likelihood of modification through grafting, esterification, and epoxidation. The polydispersity index (PDI) can be used to characterize the degree of molecular weight dispersion of a polymer. A low PDI generally ensures more uniform lignin properties.

[0083] The molecular weight and polydispersity index of the bamboo lignin prepared in Example 6 and Comparative Example 2 were tested, and the results are shown in Table 3. It can be seen from Table 3 that the molecular weight of the bamboo lignin extracted by the method proposed in the present invention is low, and its PDI is low.

[0084] Table 3 Molecular weight and polydispersity index of lignin prepared in Example 6 and Comparative Example 2

[0085] sample <![CDATA[M W (g / mol)]]> <![CDATA[M n (g / mol)]]> <![CDATA[PDI(M W / M n )]]> Example 6 1352 511 2.65 Comparative Example 2 1891 739 2.56

[0086] In summary, the present invention uses bamboo as a lignin source and a deep eutectic solvent composed of choline chloride and an organic acid as a solvent and catalyst. Through a one-pot, two-step reaction method, the method effectively retains the cellulose-rich components in bamboo powder while producing bamboo lignin with excellent antioxidant properties and more uniform dispersion. The method provided by the present invention helps improve the comprehensive utilization rate of bamboo resources and promote the sustainable development of the bamboo industry.

[0087] Although the present invention has been disclosed above with reference to preferred embodiments, this is not intended to limit the present invention. Any researcher in this field may, without departing from the spirit and scope of the present invention, adopt the design parameters and contents of the above-disclosed embodiments to change and modify the research scheme of the present invention. Therefore, any simple modifications, parameter changes, and modifications made to the above-disclosed embodiments based on the research essence of the present invention without departing from the content of the present invention are within the scope of protection of the present invention.

Claims

1. A method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step process, characterized in that: The method comprises the following steps: (1) Constructing a deep eutectic solvent: Weigh choline chloride and an organic acid, place them in a container, and stir to form a deep eutectic solvent; (2) Extracting lignin: The deep eutectic solvent obtained in (1) was mixed with bamboo powder, stirred, and then vacuum filtered to obtain a solid component and a liquid component; (3) Obtaining a bamboo-rich cellulose material: washing the solid component obtained in (2) with an ethanol solution, removing the ethanol after washing, and washing with distilled water for multiple times to obtain a bamboo-rich cellulose material; (4) Obtaining degraded bamboo lignin: The liquid component obtained in (2) is placed in a container for heating treatment, and then excess distilled water is added to precipitate lignin. The lignin is separated by filtration, and then the lignin obtained by filtration is placed in an oven for drying to obtain degraded bamboo lignin.

2. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The organic acid in (1) is any one of lactic acid, acetic acid, formic acid, malic acid, citric acid, and p-toluenesulfonic acid.

3. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The molar ratio of choline chloride to organic acid in (1) is 1:1 to 1:

15.

4. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The stirring temperature in (1) is 80 to 120° C., the stirring speed is 100 to 800 rpm, and the stirring time is 0.5 to 3 h.

5. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The mass volume ratio of the bamboo powder to the deep eutectic solvent in (2) is 1:5 to 1:

40.

6. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The stirring temperature in (2) is 80 to 140° C., the stirring speed is 100 to 800 rpm, and the stirring time is 1 min to 5 h.

7. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The volume concentration of the ethanol solution in (3) is 50% to 90%.

8. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The heating method described in (4) is oil bath heating, the heating temperature is 80 to 140° C., and the heating time is 1 to 10 hours.

9. The method for preparing bamboo lignin with high antioxidant activity by a one-pot two-step method according to claim 1, wherein: The drying temperature in (4) is 80 to 100° C., and the drying time is 2 to 10 hours.

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