Efficient preparation method of nano cellulose

Abstract

The invention discloses an efficient preparation method of nanocellulose, and belongs to the field of biological material preparation. The preparation method comprises the following steps: (1) pretreating straws to obtain small straw blocks; (2) putting the small straw blocks into a deep eutectic solvent composed of polybasic acid and polyhydric alcohol, uniformly stirring and mixing, and heating to obtain a mixed solution; (3) adding an initiation treatment reagent into the mixed solution to perform purification treatment, and then performing solid-liquid separation and decoloration to obtain nano cellulose; and (4) recovering the deep eutectic solvent, and initiating the treatment reagent to be reused. Compared with the prior art, the method has the advantages that the high conversion efficiency of 72.3%-98.1% is achieved, the solvent recovery rate is high, the cost effectiveness of the method is proved through further economic analysis, and the method is suitable for sustainable large-scale nano cellulose production. In general, the green, efficient, economical and feasible deep eutectic solvent route provides a promising solution for high-value utilization of biomass resources.

Description

Technical Field

[0001] The present invention belongs to the field of biocompatible material preparation, and particularly relates to an efficient preparation method of nanocellulose. Background Art

[0002] Cellulose is the most abundant renewable and biodegradable resource on earth, with an annual output reaching approximately 180 billion tons. Nanocellulose mainly refers to nanocrystalline cellulose (NCC) and cellulose nanofibers (CNF), which are nanoscale cellulose crystals and cellulose fibers obtained by treating cellulose. It is a kind of cellulose with a diameter of dozens of nanometers and a length of dozens to hundreds of nanometers. Nanocellulose has advantages such as a large specific surface area, extremely high strength and modulus, and a low coefficient of thermal expansion. Therefore, separating and preparing nanocellulose from renewable resources is one of the current research hotspots. However, at present, most of the materials used to prepare nanocellulose are cotton fibers, hemp fibers, etc., and natural raw materials such as straw are used less. The main reason is that the removal efficiency of substances such as lignin and hemicellulose in straw is low. This results in a complex and costly straw pretreatment process, restricting its wide application in the field of nanocellulose preparation. If a technology for efficiently and low-costly removing lignin and hemicellulose from straw can be developed to reduce the pretreatment difficulty and cost, straw is expected to become a high-quality natural raw material for preparing nanocellulose, greatly improving the resource utilization rate.

[0003] Currently, there are many methods for preparing nanocellulose, but these methods all have disadvantages and limited applications. For example, the acid hydrolysis method has difficulties in treating waste acid, requires high requirements for reaction equipment, and is prone to environmental pollution; the bacterial method has a complex process and long time consumption; the nanocellulose prepared by the enzyme hydrolysis method has large size differences; the TEMPO oxidation method has expensive reagents; the mechanical method has high energy consumption. In addition, the nanocellulose prepared by these methods generally cannot be directly used and needs to be modified, which often changes or destroys the original nanocellulose structure, increases the reaction steps, and raises the production cost.

[0004] Therefore, it is of great significance to develop an environmentally friendly, efficient, and low-energy-consuming method for preparing nanocellulose. Summary of the Invention

[0005] Object of the Invention: To solve the problems existing in the prior art, the object of the present invention is to provide an environmentally friendly, efficient, and low-energy-consuming method for efficiently preparing nanocellulose.

[0006] Technical Solution: The efficient preparation method of nanocellulose provided by the present invention includes the following steps:

[0007] (1) Pretreat the straw to obtain small straw pieces;

[0008] (2) Place the small pieces of straw in a deep eutectic solvent composed of a polybasic acid and a polyhydric alcohol, stir and mix evenly, and obtain a mixed solution after heat treatment; wherein the molecular weights and carbon atom numbers of the polybasic acid and the polyhydric alcohol in the deep eutectic solvent are determined according to the cellulose content of the straw;

[0009] (3) Add an initiating treatment reagent to the mixed solution for purification treatment, and then perform solid-liquid separation and decolorization to obtain nanocellulose;

[0010] (4) Recover the deep eutectic solvent and the initiating treatment reagent for reuse.

[0011] Further, in step (1), the straw includes corn, wheat, rice, sorghum, and mulberry branches; the pretreatment includes: washing, drying, pulverizing, and sieving, and the particle size range of the sieving is 0.2 - 0.4 cm.

[0012] Further, in step (2), in the deep eutectic solvent, the molar ratio of the polybasic acid to the polyhydric alcohol is 1 - 3:2 - 7.

[0013] Further, in step (2), the polybasic acid is citric acid, oxalic acid, maleic acid, succinic acid, glutaric acid, adipic acid, or fumaric acid; the polyhydric alcohol is ethylene glycol, glycerol, sorbitol, butanediol, octanediol, hexanediol, or propylene glycol.

[0014] Further, in step (2), the cellulose content of the straw > 38%, and the carbon atom number of the polybasic acid or / and the polyhydric alcohol in the deep eutectic solvent ≤ 3; the cellulose content of the straw is 30 - 38 wt%, and the carbon atom number of the polybasic acid or / and the polyhydric alcohol in the deep eutectic solvent is 3 - 4; the cellulose content of the straw < 30 wt%, and the carbon atom number of the polybasic acid or / and the polyhydric alcohol in the deep eutectic solvent ≥ 5.

[0015] Further, in step (2), the mass ratio of the small pieces of straw to the deep eutectic solvent is 1 - 28:100, and the conditions of the heat treatment are: using a homogenizer, heating and stirring at 60 - 100 °C for 20 - 60 min.

[0016] Further, in step (3), the initiating treatment reagent is one of the combination of 2,2,6,6 - tetramethylpiperidine - 1 - oxyl - sodium hypochlorite and sodium bromide, the combination of N,N'-dihydroxy pyromellitimide and copper oxide, or the hydrogen peroxide - disodium ethylenediaminetetraacetate - organic sodium salt system. Among them, the mass ratio of 2,2,6,6 - tetramethylpiperidine - 1 - oxyl - sodium hypochlorite to sodium bromide is 16:10 - 15:1, the mass ratio of N,N'-dihydroxy pyromellitimide to copper oxide is 10:11 - 12, and the mass ratio of hydrogen peroxide - disodium ethylenediaminetetraacetate - organic sodium salt is 1:11 - 12:5; the mass ratio of the deep eutectic solvent, water and the initiating treatment reagent is 20 - 25:70 - 80:0.01 - 0.05.

[0017] Preferably, the initiating treatment reagent is the combination of 2,2,6,6 - tetramethylpiperidine - 1 - oxyl - sodium hypochlorite and sodium bromide, and the mass ratio is 16:10:1.

[0018] Further, in step (3), the parameters of the purification treatment are: the treatment temperature is 60 - 100 °C, and the treatment time is 1 - 20 min.

[0019] Further, in step (3), the steps of decolorization are: collecting the product obtained after solid - liquid separation, adding hydrogen peroxide for decolorization, waiting for complete decolorization, standing for precipitation overnight, and filtering and centrifuging to obtain the precipitate, which is nano - cellulose; among them, the dosage of hydrogen peroxide is 1 - 10 wt% of the nano - cellulose.

[0020] Principle of the invention: In the present invention, a deep eutectic solvent is designed according to the straw substrate, which is different from traditional general solvents. This customized method can more precisely adapt to the characteristics of biomass such as straw, improve the treatment efficiency and effect, achieve more effective conversion of biomass, and open up a new path for the preparation of nano - cellulose; in multiple links such as deep eutectic solvent treatment and initiator - initiated treatment, emphasis is placed on recycling and reusing relevant substances. This not only reduces production costs, but also reduces resource waste and waste emissions, conforms to the concept of green chemistry, and makes the entire preparation process more sustainable.

[0021] The present invention orderly combines multiple operations such as mechanical crushing, deep eutectic solvent treatment and washing, initiator treatment, stirring and decolorization into a complete process, achieving a seamless conversion from biomass to nano - cellulose. This integrated process improves the continuity and stability of production, which is conducive to the large - scale preparation of nano - cellulose.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following remarkable effects: (1) The present invention designs and prepares deep eutectic solvents for the first time according to the components of natural biomass, and these deep eutectic solvents can efficiently separate the components of natural biomass: cellulose, hemicellulose and lignin; (2) Compared with the prior art, the method of the present invention achieves a relatively high conversion efficiency of 72.3-98.1%, and has a high solvent recovery rate; (3) The cost-effectiveness of the method of the present invention makes it applicable to sustainable large-scale production of nanocellulose. Description of the Drawings

[0023] Figure 1 It is the preparation flow chart of nanocellulose in Example 1;

[0024] Figure 2 It is the suspension diagram of nanocellulose in water in Example 1;

[0025] Figure 3 It is the scanning electron microscope image of nanocellulose in Example 1. Detailed Embodiments

[0026] The present invention will be further described in detail below in conjunction with the embodiments and the drawings.

[0027] Example 1: As Figure 1 shown, the efficient preparation method of nanocellulose provided in this embodiment includes the following steps:

[0028] (1) Pretreat the corn straw: Take the corn straw, wash it to remove impurities, dry, crush and sieve it to obtain small pieces of corn straw with a particle size of about 0.2 cm;

[0029] (2) Detect that the cellulose content of the corn straw is 37.5 wt%, and select the composition of the deep eutectic solvent as: ethylene glycol and maleic acid, with a molar ratio of 1:2;

[0030] In a 500 ml three-necked flask, add 100 g of the above deep eutectic solvent, 350 g of water and 1 g of small pieces of corn straw, stir and mix evenly, and use a homogenizer for heat treatment, treat at 80 °C for 10 min to obtain a mixed solution;

[0031] (3) Prepare the initiating treatment reagent: 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) (0.016 g per gram of cellulose) and NaBr (0.01 g per gram of cellulose), and then gradually add dropwise the NaClO2 solution (the mass of sodium hypochlorite is 0.001 g) under continuous stirring;

[0032] Add 0.1 g of the above-mentioned initiating treatment reagent to the mixed solution for purification treatment, treat at 60 °C for 20 min, then carry out solid-liquid separation and decolorization: collect the product obtained after solid-liquid separation, add hydrogen peroxide for decolorization, wait until complete decolorization, let it stand for precipitation overnight, filter and centrifuge to obtain the precipitate, which is nano-cellulose; among them, the dosage of hydrogen peroxide is 5 wt% of nano-cellulose; Obtain nano-cellulose, and its suspension diagram and electron microscope diagram in water are as Figure 2 and Figure 3 shown. It can be seen from the figure that the prepared nano-cellulose particles are small, disperse well in water and are not easy to precipitate.

[0033] (4) Recover the deep eutectic solvent and the initiating treatment reagent for reuse.

[0034] Example 2: The difference from Example 1 is that the cellulose content of the straw is 30 wt%, and the composition of the deep eutectic solvent used is glycerol and glutaric acid.

[0035] Comparative Example 1: The difference from Example 1 is that no initiating treatment reagent is used for purification, and the obtained nano-cellulose is yellowish in color, indicating that the lignin is not completely removed.

[0036] Comparative Example 2: The difference from Example 1 is that the composition of the deep eutectic solvent is citric acid and ethylene glycol, and the molar ratio is 1:2.

[0037] Comparative Example 3: The difference from Example 2 is that the composition of the deep eutectic solvent is fumaric acid and glycerol, and the molar ratio is 1:2.

[0038] Detect the preparation results of Examples 1-2 and Comparative Examples 1-3, and the results are shown in Table 1 below. It can be seen from Table 1 that when the cellulose content in Example 1 is 37.5 wt%, the deep eutectic solvent is selected as ethylene glycol and maleic acid, and the number of carbon atoms is 2 and 4, while in Comparative Example 2, the deep eutectic solvent is selected as citric acid and ethylene glycol, and the number of carbon atoms is 2 and 6. The increase in the number of carbon atoms leads to a low conversion rate of nano-cellulose; in Example 2, the cellulose content is 30 wt%, the deep eutectic solvent is selected as glycerol and glutaric acid, and the number of carbon atoms is 3 and 5, while in Comparative Example 3, the deep eutectic solvent is selected as fumaric acid and glycerol, and the number of carbon atoms is 4 and 3. The decrease in the number of carbon atoms leads to a decrease in the conversion rate of nano-fibers. Thus, it can be seen that for straw with a cellulose content ≥ 37.5 wt%, the number of carbon atoms of the deep eutectic solvent needs to be ≤ 4, and for straw with a cellulose content ≤ 30 wt%, the number of carbon atoms of the deep eutectic solvent needs to be ≥ 5.

[0039] Table 1 Preparation results of nano-cellulose in Examples 1-2 and Comparative Examples 1-3

[0040]

[0041] The nanocellulose obtained by adding an initiating treatment reagent in Examples 1 - 2 is white and has a relatively high yield. After treatment with a deep eutectic solvent composed of ethylene glycol and maleic acid, the cellulose conversion rate of the obtained nanocellulose reached 96.2%. Function of the initiating treatment reagent: After adding the initiating treatment reagent in Examples 1, 2, Comparative Example 2, and Comparative Example 3, the products are all white, and the cellulose conversion rate is relatively high (80.5% - 96.2%), indicating that the initiating treatment reagent can effectively promote the reaction, help remove impurities such as lignin, and improve the purity and yield of nanocellulose. In Comparative Example 1 where no initiating treatment reagent was added, the product was light yellow, indicating that the lignin was not completely removed, and the cellulose conversion rate was only 66.3%, confirming that the initiating treatment reagent is indispensable in the preparation of nanocellulose and can significantly improve the product quality.

[0042] As can be seen from the above, the advantages of the present invention using a combination of polyols and polyacids (deep eutectic solvent):

[0043] 1. High conversion rate: Taking Example 1 as an example, the deep eutectic solvent composed of ethylene glycol and maleic acid enables the cellulose conversion rate to be as high as 96.2%. The unique intermolecular interaction of the deep eutectic solvent can effectively swell cellulose, break its internal hydrogen bond network, promote the depolymerization of cellulose into nanocellulose, and improve the conversion efficiency.

[0044] 2. Good selectivity: Different combinations of polyols and polyacids show certain selectivity. For example, in Example 2, the combination of glycerol and glutaric acid, although the conversion rate (86.3%) is slightly lower than that in Example 1, can also effectively prepare nanocellulose with high product purity. This selectivity provides the possibility for precisely regulating the preparation of nanocellulose according to different application requirements.

[0045] 3. Environmental friendliness: Compared with traditional organic solvents, the deep eutectic solvents composed of polyols and polyacids usually have advantages such as low volatility and biodegradability, and have obvious advantages in the green chemical preparation of nanocellulose, meeting the concept of sustainable development. Reasons for the differences in the effects of different combinations: There are differences between Comparative Example 2 (citric acid and ethylene glycol), Comparative Example 3 (fumaric acid and glycerol) and the results of Examples 1 and 2. This may be due to the different carbon atom numbers of different polyols and polyacids, resulting in different molecular structures and functional group properties, affecting the physical and chemical properties of the deep eutectic solvent, such as melting point, viscosity, polarity, etc., and then changing its ability to dissolve and convert cellulose, as well as the synergistic effect with the initiating treatment reagent, ultimately leading to differences in the yield and quality of the prepared nanocellulose.

Claims

1. An efficient preparation method of nanocellulose, characterized in that: The following steps are involved: (1) pre-treating the straw to obtain straw pieces; (2) placing a small piece of straw in a deep eutectic solvent composed of a polyacid and a polyol, adding water, stirring and mixing, and heating to obtain a mixed solution; The carbon atom numbers of the polyacid and polyol in the deep eutectic solvent are determined according to the cellulose content of the straw; (3) adding an initiating treatment reagent to the mixed solution for purification, followed by solid-liquid separation and decolorization to obtain nanocellulose; (4) Recover the deep eutectic solvent and initiation treatment reagents for reuse.

2. The efficient preparation method according to claim 1, characterized in that: In step (1), the straw includes corn, wheat, rice, sorghum, and mulberry branches.

3. The efficient preparation method according to claim 1, characterized in that: In step (1), the pretreatment includes: washing, drying, crushing and sieving, and the particle size range of the sieving is 0.2-0.4 cm.

4. The efficient preparation method according to claim 1, characterized in that: In step (2), in the deep eutectic solvent, the molar ratio of the polyacid to the polyol is 1-3:2-7.

5. The efficient preparation method according to claim 1, characterized in that: In step (2), the polyacid is citric acid, oxalic acid, maleic acid, succinic acid, glutaric acid, adipic acid or fumaric acid; and the polyol is ethylene glycol, glycerol, sorbitol, butylene glycol, octanediol, hexylene glycol or propylene glycol.

6. The efficient preparation method according to claim 1, characterized in that: In step (2), the mass ratio of the straw pieces to the deep eutectic solvent is 1-28:

100.

7. The efficient preparation method according to claim 1, characterized in that: In step (2), the conditions for the heating treatment are: using a homogenizer, heating and stirring at 60-100° C. for 20-60 minutes.

8. The efficient preparation method according to claim 1, characterized in that: In step (3), the initiation treatment reagent is one of a combination of 2,2,6,6-tetramethylpiperidine-1-oxy-sodium hypochlorite and sodium bromide, a combination of N,N'-dihydroxypyromellitic acid imide and copper oxide, or a hydrogen peroxide-ethylenediaminetetraacetic acid disodium-organic acid sodium salt system, wherein the mass ratio of 2,2,6,6-tetramethylpiperidine-1-oxy-sodium hypochlorite and sodium bromide is 16:10-15:1, the mass ratio of N,N'-dihydroxypyromellitic acid imide and copper oxide is 10:11-12, and the mass ratio of hydrogen peroxide-ethylenediaminetetraacetic acid disodium-organic acid sodium salt is 1:11-12:5; the mass ratio of the deep eutectic solvent, water and the initiation treatment reagent is 20-25:70-80:0.01-0.

05.

9. The efficient preparation method according to claim 1, characterized in that: In step (3), the parameters of the purification treatment are: treatment temperature is 60-100°C, and treatment time is 1-20min.

10. The efficient preparation method according to claim 1, characterized in that: In step (3), the decolorization step is: collecting the product obtained after solid-liquid separation, adding hydrogen peroxide for decolorization, and after complete decolorization, standing for precipitation overnight, filtering and centrifuging to obtain a precipitate, which is nanocellulose.

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