A method for preparing bamboo cellulose nanocrystals with microwave assistance

Abstract

The invention discloses a method for preparing bamboo cellulose nanocrystals assisted by microwaves. The method comprises the following steps: S1, adding bamboo powder to an alkaline solution, stirring and reacting in an oil bath at 80-100 DEG C to remove hemicellulose, filtering and washing until the pH value of the filtrate is nearly neutral, collecting samples and drying to constant weight; S2, pre-grinding the product sample of S1 to make the bamboo powder mesh number be 80-100 meshes; S3, adding the ground bamboo powder to a sodium chlorite solution, stirring and reacting in an oil bath at 70-85 DEG C to remove wood; the invention overcomes the bottleneck problems of low yield and high-concentration waste acid treatment in preparing cellulose nanocrystals by a traditional sulfuric acid hydrolysis method, and can improve the yield and crystallinity of cellulose nanocrystals and effectively reduce the generation of waste liquid through reasonable selection of process parameters of low-concentration acid hydrolysis, microwave, dialysis and ultrasonic treatment, obtain a dispersed and stable cellulose nanocrystal suspension, and the cellulose nanocrystal presents a rod-like structure, which is expected to promote the industrialization process of nanocellulose.

Description

Technical Field

[0001] The invention relates to the technical field of cellulose, and in particular to a method for preparing bamboo cellulose nanocrystals with the assistance of microwaves. Background Art

[0002] Cellulose is a recyclable natural polysaccharide polymer. Due to its low cost, non-toxicity, and biodegradability, it is often designed and developed into various functional and intelligent materials. Compared with natural cellulose, nanocellulose has many extraordinary advantages, such as low density, high specific surface area, high strength, and high reactivity. Nanocellulose has been isolated from a variety of sources, such as cotton, ramie, sisal, wheat straw, wood, bamboo, etc. Among them, bamboo is one of the fastest growing plants on earth, with abundant reserves and spindle shape. Bamboo fiber macromolecules are orderly arranged, highly oriented, and highly crystalline. It has been identified as a wood substitute and can be considered as a suitable source for the manufacture of nanocellulose.

[0003] At present, nanocellulose mainly includes cellulose nanocrystals, cellulose nanofibers and bacterial cellulose. Among them, cellulose nanocrystals have a rigid rod-like structure with high crystallinity and can be obtained by acid hydrolysis, TEMPO oxidation, enzymatic hydrolysis, ionic liquid and low eutectic solvent methods. Acid hydrolysis is a classic method for preparing cellulose nanocrystals, especially sulfuric acid, which removes the amorphous regions in natural cellulose and makes the sulfate groups react with the hydroxyl groups on the surface of cellulose nanocrystals to attach to the cellulose nanocrystals by esterification. Therefore, the surface of sulfated cellulose nanocrystals is negatively charged to form a stable suspension in water.

[0004] The Chinese invention patent with patent number CN117887102A discloses a method for optimizing cellulose nanocrystal suspension to prepare structural color film. The preparation steps of cellulose nanocrystal dispersion involved in the method include heating and hydrolyzing pure cellulose powder in 60wt% sulfuric acid solution, adding water, centrifuging to collect supernatant and dialysis steps; the Chinese invention patent with patent number CN116622268B discloses a cellulose nanocrystal superhydrophilic coating, preparation method and application. The technology adds bleached cellulose to sulfuric acid solution for hydrolysis, removes the sulfuric acid remaining on the surface after the reaction is terminated, and centrifuges to obtain a cellulose nanocrystal suspension.

[0005] Although the above technology obtains cellulose nanocrystal suspension based on sulfuric acid hydrolysis and is used for the preparation of functional materials, the sulfuric acid hydrolysis method mostly uses high-concentration sulfuric acid. Poor control of reaction conditions can easily lead to insufficient cellulose hydrolysis or excessive carbonization, which also causes corrosion to the reaction equipment. In addition, due to the use of relatively high-concentration sulfuric acid, the above technology requires multiple water washing and centrifugal acid removal to collect the supernatant during the preparation of the cellulose nanocrystal suspension, or the use of a high-molecular-weight dialysis bag to accelerate the acid removal after the supernatant is collected, resulting in the problem of large water consumption and difficult wastewater treatment in the product separation and purification process. In addition, cellulose is usually a cellulose microfibril aggregate after acid hydrolysis, and the dissociation of microfibrils to nano-scale cellulose requires certain mechanical treatment (such as high-pressure homogenization or high-intensity ultrasound). The cellulose nanocrystal suspension directly obtained by collecting the supernatant by the above technology is prone to reduce the yield of cellulose nanocrystals.

[0006] Therefore, it is necessary to develop a method based on the sulfuric acid hydrolysis method that can efficiently prepare high-yield and stably dispersed bamboo cellulose nanocrystals and reduce the generation of acid waste liquid. Summary of the invention

[0007] The present invention overcomes the shortcomings of the prior art and provides a method for preparing bamboo cellulose nanocrystals with the assistance of microwaves. The present invention uses low-concentration sulfuric acid under the assistance of microwaves, optimizes the molecular weight of the dialysis bag and uses ultrasonic dissociation of microfibrils to obtain bamboo cellulose nanocrystals with high yield and high crystallinity.

[0008] To achieve the above object, the technical solution adopted by the present invention is: a method for preparing bamboo cellulose nanocrystals with microwave assistance, comprising the following steps:

[0009] S1. Add bamboo powder to alkaline solution, stir in an oil bath at 80-100°C to remove hemicellulose, filter and wash with water until the pH of the filtrate is close to neutral, collect the sample and dry to constant weight;

[0010] S2, pre-grinding the product sample of S1 to make the bamboo powder mesh number between 80 and 100 meshes;

[0011] S3, adding the ground bamboo powder to a sodium chlorite solution, stirring the reaction in an oil bath at 70-85° C. to remove lignin, maintaining the reaction pH at 4.5 by adding glacial acetic acid, filtering and washing with water until the pH of the filtrate is close to neutral, collecting the sample and drying it to constant weight, and obtaining cellulose extracted from the bamboo powder;

[0012] S4, adding the bamboo powder cellulose product of S3 to a low concentration sulfuric acid solution, stirring and hydrolyzing in an oil bath at 40-60°C, adding water, taking out and placing in a microwave catalytic synthesis / extraction instrument for auxiliary reaction, and obtaining a milky white suspension;

[0013] S5. The milky white suspension obtained in S4 is centrifuged and dialyzed until it becomes neutral, and then ultrasonically treated with an ultrasonic cell disruptor to obtain a cellulose nanocrystal dispersion.

[0014] In a preferred embodiment of the present invention, in step S1, the bamboo powder is cleaned and dried before use, the drying temperature is 100-110° C., and the drying time is 6-12 hours.

[0015] In a preferred embodiment of the present invention, in step S1, the alkaline solution is one of potassium hydroxide or sodium hydroxide; the concentration of the alkaline solution is 2 to 8 wt %, and the reaction time is 2 to 3 h.

[0016] In a preferred embodiment of the present invention, in step S2, the concentration of the sodium chlorite solution is 0.5-2wt%, the pH is adjusted to 4.5 with glacial acetic acid before the reaction, and the reaction time is 20-24h.

[0017] In a preferred embodiment of the present invention, in step S3, the concentration of the low-concentration sulfuric acid solution is 35-50wt%, the hydrolysis time is 10-50min, and water twice the volume of the reaction solution is added after hydrolysis.

[0018] In a preferred embodiment of the present invention, in the step S3, the microwave catalysis conditions are: microwave temperature is 40-60° C., microwave power is 100-500 W, and microwave time is 10-50 min.

[0019] In a preferred embodiment of the present invention, in step S4, the centrifugal conditions are: centrifugal speed is 8000-10000 r / min, and centrifugation time is 8-15 min.

[0020] In a preferred embodiment of the present invention, in step S4, the dialysis uses a dialysis bag with a molecular weight cutoff of 1000Da or 12000-14000Da, and the dialysis process is assisted by magnetic stirring to accelerate acid removal.

[0021] In a preferred embodiment of the present invention, in step S4, the processing power of the ultrasonic cell disruptor is 2-6%, the ultrasonic processing time is 5-15 minutes, and the ultrasonic on and off time is 5 seconds.

[0022] The invention provides a bamboo fiber nanocrystal. The bamboo fiber nanocrystal is obtained by the above-mentioned preparation method. The yield of the bamboo cellulose nanocrystal is 82.02%, the crystallinity of the bamboo cellulose nanocrystal is 75.42%, the average length of the bamboo cellulose nanocrystal is 120.23nm, the average diameter is 3nm, the average aspect ratio is 40, the Zeta potential of the dispersion liquid is -51.7mV, the hydrodynamic diameter is 97.65nm, the polydispersity index PDI is lower than 0.3, and the bamboo cellulose nanocrystal has excellent dispersion stability.

[0023] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0024] (1) The present invention provides a method for preparing bamboo cellulose nanocrystals with the assistance of microwaves, which overcomes the bottleneck problems of low yield and high-concentration waste acid treatment in preparing cellulose nanocrystals by the traditional sulfuric acid hydrolysis method. By rationally selecting the process parameters of low-concentration acid hydrolysis, microwave, dialysis and ultrasonic treatment, the yield and crystallinity of cellulose nanocrystals can be improved, and the generation of waste liquid can be effectively reduced. A dispersed and stable cellulose nanocrystal suspension is obtained, and the cellulose nanocrystals present a rod-like structure, which is expected to promote the industrialization process of nanocellulose.

[0025] (2) In the present invention, the preparation process of bamboo cellulose nanocrystals is carried out by microwave-assisted method. Microwaves can penetrate the material and generate heat inside the material to assist the acid hydrolysis process. Compared with traditional external heating, it is more uniform and helps to uniformly hydrolyze the bamboo cellulose molecular chains. Microwave energy can be absorbed by polar molecules, such as sulfuric acid molecules, thereby increasing the vibration and collision frequency of molecules, accelerating the chemical reaction rate, shortening the reaction time, and reducing the side reactions that may occur under long-term high temperature, thereby maintaining the purity and crystallinity of bamboo cellulose nanocrystals.

[0026] (3) In the present invention, by preparing bamboo cellulose nanocrystals with the assistance of microwaves under low concentration of sulfuric acid, uniform heating can be effectively generated inside the sample to assist the acid hydrolysis process, so that the low concentration of sulfuric acid can evenly penetrate into the bamboo cellulose molecular chain, significantly improving the yield and crystallinity of bamboo cellulose nanocrystals; at the same time, bamboo cellulose is more effectively decomposed into a cellulose nanocrystal dispersion with uniform morphology and stable dispersion, which helps to maintain the uniformity of the nanocrystals and prevent them from re-polymerization.

[0027] (4) In the present invention, the sample after filtration and water washing is pre-grinded to 80-100 meshes to break the bamboo powder cellulose into smaller particles, which helps to increase the surface area of ​​the bamboo powder cellulose and then increase the contact area with the reagent, making the subsequent chemical treatment more uniform and efficient, thereby improving the efficiency of the subsequent reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 This is a physical picture of the bamboo cellulose nanocrystal dispersion prepared in Example 1 of the present invention;

[0030] Figure 2 is an infrared spectrum of bamboo cellulose nanocrystals prepared in Example 1 of the present invention;

[0031] Figure 3 is an X-ray diffraction pattern of bamboo cellulose nanocrystals prepared in Example 1 of the present invention;

[0032] Figure 4 is a transmission electron microscope image of bamboo cellulose nanocrystals prepared in Example 1 of the present invention;

[0033] Figure 5 is a particle size distribution diagram of the bamboo cellulose nanocrystal dispersion prepared in Example 1 of the present invention;

[0034] Figure 6 This is the Zeta potential distribution diagram of the bamboo cellulose nanocrystal dispersion prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0035] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0037] It should be noted that the raw materials, equipment and reagents used in the present invention can be purchased from the market or obtained through the preparation methods of existing technologies.

[0038] A method for preparing bamboo cellulose nanocrystals with microwave assistance comprises the following steps:

[0039] S1. Add bamboo powder to alkaline solution, stir in an oil bath at 80-100°C to remove hemicellulose, filter and wash with water until the pH of the filtrate is close to neutral, collect the sample and dry to constant weight;

[0040] S2, pre-grinding the product sample of S1 to make the bamboo powder mesh number between 80 and 100 meshes;

[0041] S3, adding the ground bamboo powder to a sodium chlorite solution, stirring the reaction in an oil bath at 70-85° C. to remove lignin, maintaining the reaction pH at 4.5 by adding glacial acetic acid, filtering and washing with water until the pH of the filtrate is close to neutral, collecting the sample and drying it to constant weight, and obtaining cellulose extracted from the bamboo powder;

[0042] S4, adding the bamboo powder cellulose product of S3 to a low concentration sulfuric acid solution, stirring and hydrolyzing in an oil bath at 40-60°C, adding water, taking out and placing in a microwave catalytic synthesis / extraction instrument for auxiliary reaction, and obtaining a milky white suspension;

[0043] S5. The milky white suspension obtained in S4 is centrifuged and dialyzed until it becomes neutral, and then ultrasonically treated with an ultrasonic cell disruptor to obtain a cellulose nanocrystal dispersion.

[0044] In some specific embodiments, in step S1, the bamboo powder is cleaned and dried before use, the drying temperature is 100-110° C., and the drying time is 6-12 hours; the alkaline solution is one of potassium hydroxide or sodium hydroxide; the concentration of the alkaline solution is 2-8wt%, and the reaction time is 2-3 hours.

[0045] In some specific embodiments, in step S3, the concentration of the sodium chlorite solution is 0.5-2 wt %, the pH is adjusted to 4.5 with glacial acetic acid before the reaction, and the reaction time is 20-24 h.

[0046] In some specific embodiments, in step S4, the concentration of the low-concentration sulfuric acid solution is 35-50wt%, the hydrolysis time is 10-50min, and water twice the volume of the reaction solution is added after hydrolysis; the microwave catalysis conditions are: microwave temperature is 40-60°C, microwave power is 100-500W, and microwave time is 10-50min;

[0047] In some specific embodiments, in step S5, the centrifugation conditions are: centrifugal speed of 8000-10000r / min, centrifugation for 8-15min; dialysis uses a dialysis bag with a molecular weight cutoff of 1000Da or 12000-14000Da, and the dialysis process is assisted by magnetic stirring to accelerate acid removal; the processing power of the ultrasonic cell disruptor is 2-6%, the ultrasonic treatment time is 5-15min, and the ultrasonic on and off time is 5s.

[0048] It should be noted that the present invention overcomes the bottleneck problems of low yield and high-concentration waste acid treatment of cellulose nanocrystals in the traditional sulfuric acid hydrolysis method. Through the reasonable selection of low-concentration acid hydrolysis, microwave, dialysis and ultrasonic treatment process parameters, the yield and crystallinity of cellulose nanocrystals can be improved, the generation of waste liquid can be effectively reduced, and a dispersed and stable cellulose nanocrystal suspension is obtained. The cellulose nanocrystals present a rod-like structure, which is expected to promote the industrialization process of nanocellulose.

[0049] Example 1

[0050] A method for preparing bamboo cellulose nanocrystals with microwave assistance comprises the following steps:

[0051] S1. Dry the raw bamboo powder in an oven at 105°C for 12 hours, add 20g of bamboo powder to 800mL of 5wt% NaOH solution, stir magnetically in an oil bath at 90°C for 3 hours, filter and wash with water until the pH is close to neutral, collect the sample and dry to constant weight;

[0052] S2, the dried and collected samples were pre-grinded by a ball mill to grind the bamboo powder to 80 mesh;

[0053] S3, adding the ground bamboo powder to 800 mL of 5 wt% sodium chlorite solution (adjusted to pH 4.5 with glacial acetic acid), stirring magnetically in a 75°C oil bath for 24 h, and maintaining the reaction pH at about 4.5 by adding glacial acetic acid, and finally filtering and washing with water until the pH of the filtrate is nearly neutral, collecting and drying to constant weight to obtain bamboo cellulose;

[0054] S4, adding bamboo cellulose to a 40wt% sulfuric acid solution, stirring and hydrolyzing in a 45°C oil bath for 40 minutes, adding 2 times the volume of water of the reaction solution, taking out and placing in a microwave catalytic synthesis / extraction instrument, reacting at 50°C with a microwave power of 300W for 30 minutes, and obtaining a milky white suspension after the reaction;

[0055] S5. Centrifuge the milky white suspension at 8000 r / min for 10 min, discard part of the supernatant, dialyze with a 1000Da dialysis bag until neutral, and use an ultrasonic cell disruptor to ultrasonically treat for 10 min at 5% power to obtain a bamboo cellulose nanocrystal dispersion.

[0056] Example 2

[0057] This embodiment is basically the same as embodiment 1, except that: step S4 is as follows: adding bamboo cellulose to a 50wt% sulfuric acid solution, stirring and hydrolyzing in a 45°C oil bath for 40 minutes, adding water twice the volume of the reaction solution, taking out and placing in a microwave catalytic synthesis / extraction instrument, reacting at 50°C with a microwave power of 300W for 30 minutes, and obtaining a milky white suspension after the reaction.

[0058] Example 3

[0059] This embodiment is basically the same as embodiment 1, except that: step S4 is as follows: adding bamboo cellulose to a 35wt% sulfuric acid solution, stirring and hydrolyzing in a 45°C oil bath for 40 minutes, adding water twice the volume of the reaction solution, taking out and placing in a microwave catalytic synthesis / extraction instrument, reacting at 50°C with a microwave power of 300W for 30 minutes, and obtaining a milky white suspension after the reaction.

[0060] Comparative Example 1

[0061] This comparative example is basically the same as Example 1, except that: Step S4 is as follows: adding bamboo cellulose to a 64wt% sulfuric acid solution, stirring and hydrolyzing in a 45°C oil bath for 40min, adding water twice the volume of the reaction solution, taking out and placing in a microwave catalytic synthesis / extraction instrument, reacting at 50°C with a microwave power of 300W for 30min, and obtaining a milky white suspension after the reaction.

[0062] Comparative Example 2

[0063] This comparative example is basically the same as Example 1, except that: Step S4 is as follows: adding bamboo cellulose to a 30wt% sulfuric acid solution, stirring and hydrolyzing in a 45°C oil bath for 40 minutes, adding water twice the volume of the reaction solution, taking out and placing in a microwave catalytic synthesis / extraction instrument, reacting at 50°C with a microwave power of 300W for 30 minutes, and obtaining a milky white suspension after the reaction.

[0064] Comparative Example 3

[0065] This comparative example is basically the same as Example 1, except that: Step S4 is: adding bamboo cellulose to a 40wt% sulfuric acid solution, stirring and hydrolyzing in a 45°C oil bath for 40 minutes, and adding water twice the volume of the reaction solution to obtain a milky white suspension.

[0066] Comparative Example 4

[0067] This comparative example is basically the same as Example 1, except that: Step S5 is as follows: centrifuge the milky white suspension at 8000 r / min for 10 min, discard part of the supernatant, dialyze with a 12000-14000 Da dialysis bag until neutral, and ultrasonically treat with an ultrasonic cell disruptor at 5% power for 10 min to obtain a bamboo cellulose nanocrystal dispersion.

[0068] In order to evaluate the specific technical effects of the microwave-assisted preparation method of bamboo nanocellulose of Examples 1-3 and Comparative Examples 1-4, statistical calculations were performed on the differences in the preparation methods of Examples 1-3 and Comparative Examples 1-4, as well as the yield and crystallinity of the prepared bamboo nanocellulose. The results are shown in Table 1.

[0069] Table 1:

[0070]

[0071] As shown in Table 1, by comparing Example 1 with Comparative Examples 1 and 2, it is known that when sulfuric acid is used as a reactant, a concentration that is too high or too low will slow down the hydrolysis reaction rate, thereby reducing the efficiency of converting bamboo cellulose into nanocrystals. Too low a sulfuric acid concentration cannot effectively remove the amorphous region in bamboo cellulose, resulting in a decrease in yield, because the hydrolysis is insufficient, the amount of cellulose nanocrystals generated is reduced, and the growth of cellulose crystals cannot be fully promoted, thereby affecting the crystallinity of the final product.

[0072] By comparing Example 1 with Comparative Example 3, it is known that by adopting a microwave-assisted method to prepare bamboo cellulose nanocrystals, microwaves can penetrate the material and generate heat inside the material, which is more uniform than traditional external heating, and is conducive to the uniform hydrolysis of bamboo cellulose molecular chains. Microwave energy can be absorbed by polar molecules, such as sulfuric acid molecules, thereby increasing the vibration and collision frequency of molecules, accelerating the chemical reaction rate, shortening the reaction time, and reducing possible side reactions under long-term high temperature, thereby maintaining the purity and crystallinity of bamboo cellulose nanocrystals.

[0073] By comparing Example 1 with Comparative Example 4, it is known that when a dialysis bag with a lower molecular weight cutoff (1000Da) is used, the yield of bamboo nanocellulose is higher, because a smaller molecular weight cutoff can intercept microfibrils with a wider molecular weight range, thereby more effectively removing acidic substances and other small molecular impurities in the reaction medium, which helps to improve the yield of bamboo nanocellulose. When a dialysis bag with a higher molecular weight cutoff (12000-14000Da) is used, only microfibrils with a narrower molecular weight range are intercepted, so the yield is lower, but the crystallinity is maintained at a relatively high level, indicating that a smaller molecular weight cutoff can more effectively retain cellulose microfibrils, reduce their loss during the dialysis process, and help maintain or improve the crystallinity of cellulose nanocrystals.

[0074] In order to verify the successful acquisition of bamboo cellulose nanocrystals in Example 1, Figure 1 The figure shows the bamboo cellulose nanocrystal dispersion prepared in Example 1; Figure 2 The Fourier transform infrared spectrum of the bamboo cellulose nanocrystals prepared in Example 1 is shown in FIG. Figure 3 The X-ray diffraction pattern of the bamboo cellulose nanocrystals prepared in Example 1 is shown; Figure 4 Table 2 shows the transmission electron microscopy images of the bamboo cellulose nanocrystals prepared in Example 1 and the statistical results of their short rod-like morphology length and diameter; Figure 5 The nanoparticle size distribution diagram of the bamboo cellulose nanocrystal dispersion prepared by the present invention is shown in FIG. Figure 6 The Zeta potential distribution diagram of the bamboo cellulose nanocrystal dispersion prepared in Example 1 is shown in FIG.

[0075] Table 2:

[0076]

[0077] As shown in Table 1-2 and Figure 1-6 It can be seen that by preparing bamboo cellulose nanocrystals with microwave assistance under low concentration sulfuric acid, uniform heating can be effectively generated inside the sample to assist the acid hydrolysis process, so that low concentration sulfuric acid can evenly penetrate into the bamboo cellulose molecular chain. At the same time, due to the centrifugal removal of the supernatant acid solution during the preparation process, the reaction precipitate at the bottom is almost completely retained. In addition, the use of a low molecular weight dialysis bag can intercept microfibrils with a wider molecular weight range, and then ultrasonic crushing is applied to efficiently dissociate the microfibrils to obtain bamboo cellulose nanocrystals with excellent performance. Compared with bamboo cellulose nanocrystals obtained using 64wt% classic sulfuric acid concentration, the method provided by the present invention significantly improves the yield and crystallinity of bamboo cellulose nanocrystals through microwave-assisted low concentration sulfuric acid and the coordinated optimization of preparation steps and corresponding parameters; at the same time, bamboo cellulose is more effectively decomposed into a cellulose nanocrystal dispersion with uniform morphology and stable dispersion, with a Zeta potential of -51.7mV, a hydrodynamic diameter of 97.65nm, a polydispersity coefficient PDI of less than 0.3, an average length of bamboo cellulose nanocrystals of 120.23nm, an average diameter of 3nm, and an aspect ratio of about 40.

[0078] The above is based on the ideal embodiment of the present invention. Through the above description, it is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is limited by the attached claims rather than the above description, and it is intended to include all changes within the meaning and scope of the equivalent elements of the claims. Any figure mark in the claims should not be regarded as limiting the claims involved.

[0079] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for preparing bamboo cellulose nanocrystals with microwave assistance, characterized in that: The following steps are involved: S1. Add bamboo powder to alkaline solution, stir in an oil bath at 80-100°C to remove hemicellulose, filter and wash with water until the pH of the filtrate is close to neutral, collect the sample and dry to constant weight; S2, pre-grinding the product sample of S1 to make the bamboo powder mesh number between 80 and 100 meshes; S3, adding the ground bamboo powder to a sodium chlorite solution, stirring the reaction in an oil bath at 70-85° C. to remove lignin, maintaining the reaction pH at 4.5 by adding glacial acetic acid, filtering and washing with water until the pH of the filtrate is close to neutral, collecting the sample and drying it to constant weight, and obtaining cellulose extracted from the bamboo powder; S4, adding the bamboo powder cellulose product of S3 to a 40wt% sulfuric acid solution, stirring and hydrolyzing in a 45°C oil bath for 40 minutes, adding 2 times the volume of water of the reaction solution, taking out and placing in a microwave catalytic synthesis / extraction instrument for auxiliary reaction, and obtaining a milky white suspension; S5, centrifuging the milky white suspension obtained in S4, dialyzing it with a dialysis bag having a molecular weight cutoff of 1000 Da until it becomes neutral, and ultrasonically treating it with an ultrasonic cell disruptor to obtain a cellulose nanocrystal dispersion; In the step S4, the microwave catalysis conditions are: microwave temperature of 40-60° C., microwave power of 100-500 W, and microwave time of 10-50 min.

2. The method for preparing bamboo cellulose nanocrystals with microwave assistance according to claim 1, characterized in that: In step S1, the bamboo powder is cleaned and dried before use, the drying temperature is 100-110° C., and the drying time is 6-12 hours.

3. The method for preparing bamboo cellulose nanocrystals with microwave assistance according to claim 1, characterized in that: In step S1, the alkaline solution is one of potassium hydroxide and sodium hydroxide; the concentration of the alkaline solution is 2-8 wt %, and the reaction time is 2-3 h.

4. The method for preparing bamboo cellulose nanocrystals with microwave assistance according to claim 1, characterized in that: In step S3, the concentration of the sodium chlorite solution is 0.5-2 wt %, the pH is adjusted to 4.5 with glacial acetic acid before the reaction, and the reaction time is 20-24 h.

5. The method for preparing bamboo cellulose nanocrystals with microwave assistance according to claim 1, characterized in that: In the step S5, the centrifugal conditions are: the centrifugal speed is 8000-10000 r / min, and the centrifugation time is 8-15 min.

6. The method for preparing bamboo cellulose nanocrystals with microwave assistance according to claim 1, characterized in that: In step S5, the dialysis process is assisted by magnetic stirring to accelerate acid removal.

7. The method for preparing bamboo cellulose nanocrystals with microwave assistance according to claim 1, characterized in that: In the step S5, the processing power of the ultrasonic cell disruptor is 2-6%, the ultrasonic processing time is 5-15 minutes, and the ultrasonic on and off time is 5 seconds.

8. A bamboo fiber nanocrystal, characterized in that: The bamboo fiber nanocrystals are obtained using the preparation method described in any one of claims 1 to 7, the bamboo cellulose nanocrystal yield is 82.02%, the bamboo cellulose nanocrystal crystallinity is 75.42%, the bamboo cellulose nanocrystals have an average length of 120.23 nm, an average diameter of 3 nm, an average aspect ratio of 40, a dispersion Zeta potential of -51.7 mV, a hydrodynamic diameter of 97.65 nm, and a polydispersity coefficient PDI of less than 0.3.

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