Chitin nanocrystal as well as green preparation method and application of recyclable ferric trichloride hexahydrate based on microwave assistance

Abstract

The invention discloses chitin nanocrystals as well as a green preparation method and application of recyclable ferric trichloride hexahydrate based on microwave assistance. The method comprises the following steps: by taking FeCl3. 6H2O as Lewis acid and a catalyst, hydrolyzing chitin under the assistance of microwaves, purifying to obtain chitin nanocrystals, and recovering FeCl3. 6H2O. The FeCl3. 6H2O plays a dual role of Lewis acid and coordination, the chitin nanocrystal with high crystallinity and high length-diameter ratio can be rapidly obtained under microwave irradiation, and meanwhile, the recovery rate of the FeCl3. 6H2O serving as a catalyst exceeds 90%. According to the method, the chitin nanocrystals with higher aspect ratio and crystallinity can be rapidly, environmentally and efficiently produced, meanwhile,-NH2 is introduced to improve surface activity, water dispersibility and compatibility with other materials, and the application range of the chitin nanocrystals in food and medical industries is widened.

Description

Technical Field

[0001] The invention belongs to the technical field of chitin nanocrystals, and particularly relates to a green preparation method and application of chitin nanocrystals and microwave-assisted recyclable ferric chloride hexahydrate. Background Art

[0002] Chitosan, typically extracted from the exoskeletons of crustaceans and the cell walls of certain algae and fungi, is the second-largest natural polysaccharide after cellulose. Chitosan nanocrystals, obtained by removing the amorphous regions and retaining the crystalline regions, are needle-shaped and have a high aspect ratio. They typically exhibit good dispersibility in water, compensating for chitosan's insolubility in water and most common organic solvents.

[0003] However, conventional production, such as concentrated inorganic acid preparation, usually involves hazardous chemicals and potential toxicity, resulting in large amounts of unrecoverable acid waste, high water consumption and equipment corrosion. Long reaction times can easily cause excessive hydrolysis and even carbonization of the product, resulting in reduced efficiency. Although mechanical methods for preparing nanocrystals do not use too many chemical reagents, they have high energy consumption, low efficiency, and poor product crystallinity; enzymatic hydrolysis has problems such as easy inactivation of enzyme preparations and high reaction costs. These shortcomings highlight the limitations of traditional methods in terms of economy, environment, and efficiency. Therefore, there is an urgent need to develop environmentally friendly, efficient, and high-yield alternatives to conveniently extract chitin nanocrystals.

[0004] Common Lewis acids, such as ferric chloride (FeCl3), aluminum chloride, and zinc chloride, exhibit stronger electrophilicity, solvent effect modulation, and catalytic ability after accepting electrons. They can hydrolyze in water and gradually generate hydrogen ions (H + ), establishing a low-pH reaction system. The interaction of iron ions with hydroxyl, carbonyl, and amino groups (-NH2) has long been demonstrated, promoting the disruption of hydrogen bond networks. Consequently, FeCl3 is widely used for the dissolution, catalytic depolymerization, and functional modification of chitin and cellulose. Furthermore, FeCl3·6H2O is inexpensive, non-toxic, abundant, and recyclable, making it a promising candidate for the sustainable production of chitin nanocrystals.

[0005] Microwave irradiation has attracted increasing attention due to its uniform heating, high energy efficiency and low by-product generation, and has gradually become a viable low-carbon heating alternative to high-emission equipment such as boilers and electric furnaces. Summary of the Invention

[0006] In order to address the shortcomings and deficiencies of the prior art, the primary purpose of the present invention is to provide a simple, green, microwave-assisted method for preparing chitin nanocrystals with recyclable FeCl3·6H2O.

[0007] The present invention uses the strong Lewis acid FeCl3·6H2O as a traditional acid substitute. FeCl3·6H2O plays the dual role of Lewis acid and coordination. The Fe in FeCl3·6H2O 3+ As a trivalent metal ion with high charge density, it has strong Lewis acidity and empty orbital electron-accepting ability, and can selectively coordinate and complex with the exposed amide and hydroxyl groups in the amorphous region of the chitin molecular chain. The coordination and complexation effect is more significant for the amorphous region with loose molecular chains and more accessible coordination sites. In addition, since microwave irradiation can enhance the dipole rotation of water molecules, when FeCl3·6H2O is a recyclable and non-toxic alternative to traditional acid, microwave assistance can accelerate the reaction of iron ions (Fe 3+ ) hydrolysis and complexation, rapidly increasing H + concentration and promote Fe 3+ Uniform diffusion in the amorphous region. At the same time, Fe 3+ The presence of FeCl3·6H2O promotes the cleavage of the amide C-N bond in chitin and accelerates the deacetylation process. Therefore, high-crystallinity, high-aspect-ratio chitin nanocrystals can be quickly obtained under microwave irradiation, while the recovery rate of FeCl3·6H2O as a catalyst exceeds 90%.

[0008] Compared with the traditional acid hydrolysis method, this method can quickly, environmentally friendly and efficiently produce chitin nanocrystals with higher aspect ratio and crystallinity, while introducing -NH2 to improve surface activity, water dispersibility and compatibility with other materials, expanding its application in the food and pharmaceutical industries, while being consistent with global efforts to promote environmental sustainability, resource efficiency and green technology advancement.

[0009] The preparation method of the present invention achieves a high chitin nanocrystal deacetylation rate exceeding that of the acid hydrolysis method through less waste generation and higher sample yield, giving it a higher -NH2 amount and ζ potential, confirming the low environmental factor coefficient of the method, complying with the principles of waste minimization and resource efficiency, and supporting a more sustainable production process.

[0010] Another object of the present invention is to provide chitosan nanocrystals obtained by the above preparation method. The obtained chitosan nanocrystals have high crystallinity, high aspect ratio and high deacetylation degree.

[0011] Another object of the present invention is to provide an application of the above-mentioned chitosan nanocrystal.

[0012] The purpose of the present invention is achieved through the following technical solutions:

[0013] A microwave-assisted, recyclable FeCl3·6H2O green method for preparing chitin nanocrystals comprises the following steps:

[0014] FeCl3·6H2O is mixed with water to prepare a FeCl3·6H2O solution, α-chitin is added, microwave hydrolysis is performed, the hydrolysis is terminated, and the obtained precipitate is purified after centrifugation to obtain chitosan nanocrystals; the liquid obtained by centrifugation is concentrated and recrystallized to recover FeCl3·6H2O.

[0015] Preferably, the FeCl3·6H2O is mixed with water at 60-100°C for 3-8 minutes to obtain a FeCl3·6H2O solution.

[0016] Preferably, the concentration of the FeCl3·6H2O solution is 0.5 to 1.5 g / mL, more preferably 0.9 to 1.2 g / mL.

[0017] Preferably, the material-liquid ratio of the α-chitin to water is 1 g:10 mL to 1 g:8 mL.

[0018] Preferably, the power of the microwave hydrolysis is 240 to 320 W, more preferably 280 to 320 W; the temperature is 80 to 110° C., more preferably 80 to 90° C.; and the time is 0.5 to 1 hour.

[0019] Preferably, the heating rate of the microwave hydrolysis is 5-15°C / min; more preferably 10°C / min.

[0020] Preferably, the terminating the hydrolysis means adding water to the hydrolysis system to terminate the reaction, and the volume ratio of the added water to the hydrolysis system is (150-250): (18-20).

[0021] Preferably, the rotation speed of the centrifugal separation is 8000-10000 rpm; more preferably 9000-10000 rpm.

[0022] Preferably, the method for purifying the precipitate to obtain chitosan nanocrystals is: washing the precipitate with a hydrochloric acid solution, then dispersing it in water, dialyzing it, and ultrasonically treating it to obtain a chitosan nanocrystal suspension.

[0023] More preferably, the concentration of the hydrochloric acid solution is 0.5 to 1.5 mol / L; more preferably 1 mol / L.

[0024] More preferably, the dialysis membrane used for the dialysis is 3500-5000 Da, and the dialysis time is 48-72 hours.

[0025] More preferably, the process parameters of the ultrasonic treatment are 400-500W, 20-30kHz, 1-3 seconds on / 1-3 seconds off pulse mode, ultrasonic temperature is 20-30°C, and ultrasonic time is 6-12 minutes.

[0026] More preferably, the chitosan nanocrystal suspension is stored below 4°C.

[0027] Preferably, the concentration is performed by rotary evaporation for 5 to 8 hours.

[0028] Preferably, the concentrated solution is refrigerated at 3-5° C. for 72-96 hours to achieve recrystallization recovery of FeCl 3 ·6H 2 O.

[0029] The present invention also provides chitin nanocrystals obtained by the microwave-assisted and recyclable FeCl3·6H2O green preparation method of chitin nanocrystals.

[0030] Preferably, the chitosan nanocrystals have an aspect ratio of 13 to 14, an average length of 217 to 225 nm, and a diameter of 15.6 to 16.8 nm.

[0031] Preferably, the deacetylation degree of the chitin nanocrystals is 20.5-22.2%.

[0032] The present invention also provides applications of the chitosan nanocrystals in the fields of food and medicine preparation.

[0033] Preferably, the application refers to application in degradable food packaging and drug delivery carriers.

[0034] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0035] (1) The preparation method of the present invention is simple, has a fast synthesis rate, and has a very high recovery rate of hydrolysis raw materials. It does not require the use of corrosive liquids and expensive and complex instruments and equipment. The reagents used are easily available and are basically non-toxic and harmless, conforming to the concept of green chemistry and reducing environmental pollution. It is easy to scale up industrially and use closed-loop processing, making it suitable for the construction of green factory production lines.

[0036] (2) The preparation method of the present invention introduces microwave digestion technology to achieve high energy efficiency transmission, so that Fe 3+ It is distributed more quickly in the reaction system, accelerating the dissociation of the amorphous region of chitin, and improving the yield and sample uniformity. 3+ Due to its slow-release acidity and large charge density, it is easier to be located in the loose / defective areas between chains, thereby achieving directional exfoliation rather than the comprehensive degradation caused by traditional inorganic acids.

[0037] (3) Due to Fe 3+As a trivalent metal ion with high charge density, it has strong Lewis acidity and empty orbital electron-accepting ability, and can selectively coordinate and complex with the amide groups and hydroxyl groups exposed in the amorphous regions of the chitin molecular chain. The coordination effect on the amorphous regions with loose molecular chains and more accessible coordination sites is particularly significant. Therefore, the chitin nanocrystals prepared by the present invention have high yield, high crystallinity and high aspect ratio, and are effective in enhancing tensile strength, elongation at break, and water resistance in low water activity applications such as films. It has good application potential in low water activity environments such as degradable food packaging and pharmaceutical carriers.

[0038] (4) Fe 3+ During the hydrolysis process, an unexpected accelerated deacetylation effect is exhibited, which makes the chitin nanocrystal particles have good positive charge and colloidal stability at low pH. The stable dispersion is better than that of conventional acid method products, and it is suitable for the preparation of nanocomposite materials with high dispersion requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a transmission electron microscope (TEM) image of chitin nanocrystals prepared greenly based on microwave-assisted recyclable FeCl3·6H2O in Example 1 of the present invention.

[0040] Figure 2 This is a scanning electron microscope (SEM) image of chitin nanocrystals prepared greenly based on microwave-assisted recyclable FeCl3·6H2O in Example 1 of the present invention.

[0041] Figure 3 These are optical photographs of chitosan nanocrystal suspensions prepared based on microwave-assisted recyclable FeCl3·6H2O green treatment for 0.25 hours, 0.75 hours, and 2 hours in Example 1 of the present invention.

[0042] Figure 4 The deacetylation degree of chitin nanocrystals prepared based on microwave-assisted recyclable FeCl3·6H2O green method (T2FM-Ch3) in Example 1 of the present invention is compared with the deacetylation degree of α-chitin (C-0) of chitin nanocrystals treated by the same method for 0.25 hours (T2FM-Ch1), 2 hours (T2FM-Ch5), hydrochloric acid treatment for 1 hour (T2H-Ch1), 4 hours (T2H-Ch3, Comparative Example 1), 12 hours (T2H-Ch5) and FeCl3·6H2O treatment for 1 hour (T2F-Ch1), 4 hours (T2F-Ch3, Comparative Example 3), and 12 hours (T2F-Ch5).

[0043] Figure 5 These are photos of four FeCl3·6H2O recovery experiments in Example 1 of the present invention. DETAILED DESCRIPTION

[0044] The present invention will be described in further detail below with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.

[0045] In the examples of the present invention, if the specific conditions are not specified, the experiments were carried out according to conventional conditions or the conditions recommended by the manufacturer. All raw materials and reagents used without specifying the manufacturer are conventional products that can be purchased from the market.

[0046] Example 1

[0047] A microwave-assisted, recyclable FeCl3·6H2O green method for preparing chitin nanocrystals T2FM-Ch3, comprising the following steps:

[0048] 20 mL of deionized water and 20 g of FeCl₃·6H₂O were stirred in an 80°C water bath for 5 minutes to obtain an FeCl₃ solution. This solution was then transferred to a microwave digestion tube, and 2 g of pure α-chitin raw material was added. Microwave hydrolysis was then performed at 280 W and 80°C at a heating rate of 10°C / min for 0.75 hours. 150 mL of deionized water was added to terminate the reaction, and the mixture was centrifuged at 9000 rpm. The resulting precipitate was then washed three times with 1 mol / L hydrochloric acid to remove iron ions. The washed precipitate was dispersed in water and the resulting suspension was dialyzed using a 5000 Da membrane for 72 hours. The suspension was then sonicated at 450 W, 25 kHz, and a 2-second on / 2-second off pulse mode for 10 minutes to obtain a suspension of chitin nanocrystals, T₂FM-Ch₃. The waste liquid containing FeCl₃ and hydrochloric acid after centrifugation and washing was collected and concentrated by rotary evaporation for 6 hours. The concentrated solution was refrigerated at 4°C for 72 hours to obtain recrystallized FeCl₃·6H₂O, which was dried in a desiccator for subsequent use.

[0049] According to Example 1, the whole process of synthesizing chitin nanocrystals takes only 45 minutes, and the crystallinity of the product chitin nanocrystal T2FM-Ch3 is as high as 89.73%, the yield is 83.8%, the aspect ratio is about 14 (average length of about 224.8nm, diameter of about 16nm), and the deacetylation degree is 21.96% (see Figure 4 , T2FM-Ch3), with a zeta potential of +25 mV at pH 6. After four cycles, the recovery rate of FeCl3·6H2O remained above 89%. This method is environmentally friendly and safe.

[0050] Chitosan nanocrystals T2FM-Ch1 were obtained by following the process of Example 1, except that the hydrolysis time was changed from 0.75 hours to 0.25 hours.

[0051] The product chitin nanocrystals T2FM-Ch1 have a crystallinity of 83.21%, a yield of 88.4%, a deacetylation degree of 14.97%, and a ζ potential of +13.9 mV at pH = 6.

[0052] Chitosan nanocrystals T2FM-Ch2 were obtained by following the process of Example 1, except that the hydrolysis time was changed from 0.75 hours to 0.5 hours.

[0053] The crystallinity of the product chitin nanocrystals T2FM-Ch2 is 86.28%, the yield is 86.2%, the deacetylation degree is 17.52%, and the ζ potential reaches +19.3 mV at pH = 6.

[0054] Chitosan nanocrystals T2FM-Ch4 were obtained by following the process of Example 1, except that the hydrolysis time was changed from 0.75 hours to 1 hour.

[0055] The product chitin nanocrystals T2FM-Ch4 have a crystallinity of 84.52%, a yield of 75.6%, a deacetylation degree of 25.95%, and a ζ potential of +28.4 mV at pH = 6.

[0056] Chitosan nanocrystals T2FM-Ch5 were obtained by following the process of Example 1, except that the hydrolysis time was changed from 0.75 hours to 2 hours.

[0057] The product chitin nanocrystals T2FM-Ch5 have a crystallinity of 78.87%, a yield of 68.2%, a deacetylation degree of 27.09%, and a ζ potential of +31.5 mV at pH = 6.

[0058] Figure 1 and Figure 2 TEM and SEM images of chitin nanocrystals (T2FM-Ch1, T2FM-Ch3, T2FM-Ch5) prepared greenly based on microwave-assisted recyclable FeCl3·6H2O; Figure 1 and Figure 2 As can be seen, the T2FM-Ch3 nanocrystals exhibited elongated rod-like structures with uniform size and sharp boundaries, demonstrating excellent hydrolysis exfoliation. T2FM-Ch1 exhibited incomplete hydrolysis, resulting in a coarser diameter. T2FM-Ch3 exhibited over-hydrolysis, resulting in small particles and elongated filaments. SEM images taken after drying revealed a dense, interwoven fiber network, demonstrating the excellent dispersibility of the chitin nanocrystals, which maintained a certain degree of stacking order during the drying process.

[0059] Figure 3 From left to right are optical photographs of chitin nanocrystals T2FM-Ch1, T2FM-Ch3, and T2FM-Ch5 suspensions obtained by changing the hydrolysis time according to the process of Example 1; Figure 3It can be seen that the chitin nanocrystal suspension treated for 0.75 hours is uniform and translucent, indicating that the particle size is small and the dispersion is good, while the hydrolysis is incomplete at 0.25 hours, resulting in a turbid and granular suspension. The transparency of the suspension after 2 hours of hydrolysis is enhanced, indicating excessive hydrolysis, accompanied by a decrease in yield.

[0060] Figure 4 This is a bar graph comparing the deacetylation degree of chitin nanocrystals (T2FM-Ch1, T2FM-Ch3, T2FM-Ch5) prepared by microwave-assisted recyclable FeCl3·6H2O green method in Example 1 and chitin nanocrystals prepared by hydrochloric acid alone in Comparative Examples 1-2 and FeCl3·6H2O alone in Comparative Example 3 for different time periods. Figure 4 It can be seen that the presence of FeCl3·6H2O significantly increased the deacetylation degree, indicating that Fe 3+ The participation of the microwaves not only promotes hydrolysis but also has a certain synergistic ability to promote N-deacetylation. Furthermore, microwaves enhance the reactivity of polar groups, further accelerating the reaction rate. Compared with Comparative Examples 1 and 3, the hydrolysis time was significantly shortened and the hydrolysis efficiency was improved. Furthermore, the degree of deacetylation increased with increasing treatment time. The preparation method of the present invention provides a feasible, green and efficient route for the preparation of amino-type chitin nanocrystals.

[0061] Figure 5 The following is a photo of the recycling experiment after FeCl3·6H2O was recycled and reused after four cycles according to the process of Example 1. Figure 5 The appearance of FeCl₃·6H₂O remained unchanged after four consecutive recycling cycles, indicating that even if iron oxide byproducts were produced through excessive hydrolysis and oxidation, they were filtered out during the initial removal of insoluble impurities and did not affect the quality of the subsequently recrystallized FeCl₃·6H₂O. After four recycling cycles, FeCl₃·6H₂O retained its strong hydrolysis catalytic activity, and the chitin nanocrystals prepared exhibited excellent structural stability and dispersion properties.

[0062] Example 2

[0063] A microwave-assisted, recyclable FeCl3·6H2O green method for preparing chitin nanocrystals comprises the following steps:

[0064] 18 mL of deionized water and 21.6 g of FeCl₃·6H₂O were stirred in an 80°C water bath for 5 minutes to obtain an FeCl₃ solution. This solution was then transferred to a microwave digestion tube, and 2.25 g of pure α-chitin raw material was added. Microwave hydrolysis was performed at 300 W and 90°C at a heating rate of 10°C / min for 0.75 hours. 180 mL of deionized water was added to terminate the reaction, and the mixture was centrifuged at 10,000 rpm. The resulting precipitate was then washed three times with 1 mol / L hydrochloric acid to remove iron ions. The washed precipitate was dispersed in water and the resulting suspension was dialyzed using a 3500 Da membrane for 60 hours. The suspension was then sonicated at 450 W, 25 kHz, and a 2-second on / 2-second off pulse mode for 8 minutes to obtain a chitin nanocrystal suspension. The waste liquid containing FeCl₃ and hydrochloric acid after centrifugation and washing was collected and concentrated by rotary evaporation for 8 hours. The concentrated solution was refrigerated at 4°C for 72 hours to obtain recrystallized FeCl₃·6H₂O, which was dried in a desiccator for subsequent use.

[0065] According to Example 2, the entire process of synthesizing chitin nanocrystals takes only 45 minutes, and the product crystallinity is as high as 88.53%, the yield is 83.14%, the aspect ratio is about 14 (average length of about 217.3 nm, diameter of about 15.6 nm), the degree of deacetylation is 22.16%, and the zeta potential reaches +26.1 mV at pH = 6. After four cycles of experiments, the recovery rate of FeCl3·6H2O is still higher than 88%.

[0066] Example 3

[0067] A microwave-assisted, recyclable FeCl3·6H2O green method for preparing chitin nanocrystals comprises the following steps:

[0068] 18 mL of deionized water and 16.2 g of FeCl₃·6H₂O were stirred in an 80°C water bath for 5 minutes to obtain an FeCl₃ solution. This solution was then transferred to a microwave digestion tube, and 2 g of pure α-chitin raw material was added. Microwave hydrolysis was performed at 320 W and 90°C at a heating rate of 10°C / min for 0.75 hours. The reaction was terminated by adding 250 mL of deionized water and centrifuged at 10,000 rpm. The resulting precipitate was then washed three times with 1 mol / L hydrochloric acid to remove iron ions. The washed precipitate was dispersed in water and the resulting suspension was dialyzed using a 4000 Da membrane for 48 hours. The suspension was then sonicated at 450 W, 25 kHz, and a 2-second on / 2-second off pulse mode for 12 minutes to obtain a chitin nanocrystal suspension. The waste liquid containing FeCl₃ and hydrochloric acid after centrifugation and washing was collected and concentrated by rotary evaporation for 5 hours. The concentrated solution was refrigerated at 4°C for 96 hours to obtain recrystallized FeCl₃·6H₂O, which was dried in a desiccator for subsequent use.

[0069] According to Example 3, the whole process of synthesizing chitin nanocrystals only takes 45 minutes, and the product crystallinity is as high as 87.22%, the yield is 85.31%, the aspect ratio is about 13 (average length is about 219.58nm, diameter is about 16.82nm), the deacetylation degree is 20.52%, and the zeta potential reaches +23.73mV at pH=6. After four cycles of experiments, the recovery rate of FeCl3·6H2O is still higher than 90%.

[0070] Comparative Example 1

[0071] A method for preparing chitin nanocrystals using hydrochloric acid, comprising the following steps:

[0072] 2 g of pure α-chitin raw material was added to 60 mL of 3 mol / L hydrochloric acid and hydrolyzed in an 80°C water bath for 4 hours. The reaction was terminated by adding 150 mL of deionized water and centrifuged at 9000 rpm. The supernatant was discarded and the precipitate was retained. The dispersed suspension was dialyzed using a 5000 Da membrane for 72 hours, followed by sonication at 450 W, 25 kHz, and a 2-second on / 2-second off pulse mode for 10 minutes to obtain a suspension of chitin nanocrystals, T2H-Ch3.

[0073] According to Comparative Example 1, the product chitin nanocrystals T2H-Ch3 have a crystallinity of 86.48%, a yield of 76.9%, an aspect ratio of about 10 (average length of about 288.1 nm, diameter of about 27.4 nm), and a deacetylation degree of 12.97% (see Figure 4 , T2H-Ch3), the zeta potential reached +16.1 mV at pH=6.

[0074] Chitosan nanocrystals T2H-Ch1 were obtained by following the process of Comparative Example 1, except that the heating and hydrolysis time was changed from 4 hours to 1 hour.

[0075] The product chitin nanocrystals T2H-Ch1 have a crystallinity of 82.53%, a yield of 86.3%, a deacetylation degree of 12.03%, and a zeta potential of +13.8 mV at pH = 6.

[0076] According to the process of comparative example 1, only the heating and hydrolysis time was changed from 4 hours to 2 hours to obtain chitin nanocrystals T2H-Ch2.

[0077] The crystallinity of the product chitin nanocrystals T2H-Ch2 is 84.45%, the yield is 84.4%, the deacetylation degree is 12.28%, and the ζ potential reaches +14.1 mV at pH = 6.

[0078] According to the process of comparative example 1, only the heating and hydrolysis time was changed from 4 hours to 8 hours to obtain chitin nanocrystals T2H-Ch4.

[0079] The crystallinity of the product chitin nanocrystals T2H-Ch4 is 82.69%, the yield is 63.2%, the deacetylation degree is 14.11%, and the ζ potential reaches +17.1 mV at pH = 6.

[0080] Chitosan nanocrystals T2H-Ch5 were obtained by following the process of Comparative Example 1, except that the heating and hydrolysis time was changed from 4 hours to 12 hours.

[0081] The product chitosan nanocrystals T2H-Ch5 have a crystallinity of 76.23%, a yield of 51.4%, a deacetylation degree of 16.03%, and a zeta potential of +20.0 mV at pH = 6.

[0082] Comparative Example 2

[0083] A method for preparing chitin nanocrystals using hydrochloric acid, comprising the following steps:

[0084] 2 g of pure α-chitin raw material was added to 60 mL of 3 mol / L hydrochloric acid and heated in an 80°C water bath for 0.75 hours. The reaction was terminated by adding 150 mL of deionized water and centrifuged at 9000 rpm. The supernatant was discarded and the precipitate was retained. The dispersed suspension was dialyzed using a 5000 Da membrane for 72 hours, followed by sonication at 450 W, 25 kHz, and a 2-second on / 2-second off pulse mode for 10 minutes to obtain a suspension of chitin nanocrystals, T2H-Ch6.

[0085] According to Comparative Example 2, the product has a crystallinity of 80.07%, a yield of 87.1%, an aspect ratio of approximately 7 (average length of approximately 500.56 nm, diameter of approximately 71.59 nm), a deacetylation degree of 11.28%, and a ζ potential of +13.1 mV at pH = 6.

[0086] Comparative Example 3

[0087] A method for preparing chitin nanocrystals using FeCl3·6H2O, comprising the following steps:

[0088] 20 mL of deionized water and 20 g of FeCl₃·6H₂O were stirred in an 80°C water bath for 5 minutes to obtain an FeCl₃ solution. 2 g of pure α-chitin raw material was added and heated for hydrolysis at 80°C for 4 hours. The reaction was terminated by adding 150 mL of deionized water and centrifuged at 9000 rpm. The supernatant was discarded and the precipitate was then washed three times with 1 mol / L hydrochloric acid to remove iron ions. The dispersed suspension was dialyzed with a 5000 Da membrane for 72 hours and then sonicated for 10 minutes at 450 W, 25 kHz, and a 2-second on / 2-second off pulse mode to obtain a suspension of chitin nanocrystals, T₂F-Ch₃.

[0089] According to Comparative Example 3, the product has a crystallinity of 89.34%, a yield of 74.1%, an aspect ratio of about 12 (average length of about 249.36 nm, diameter of about 21.19 nm), and a deacetylation degree of 20.25% (see Figure 4 , T2F-Ch3), the ζ potential reached +23.9 mV at pH=6.

[0090] Chitosan nanocrystals T2F-Ch1 were obtained by following the process of Comparative Example 3, except that the heating and hydrolysis time was changed from 4 hours to 1 hour.

[0091] The product chitin nanocrystals T2F-Ch1 have a crystallinity of 83.15%, a yield of 84.0%, a deacetylation degree of 13.39%, and a zeta potential of +11.1 mV at pH = 6.

[0092] According to the process of comparative example 3, only the heating and hydrolysis time was changed from 4 hours to 2 hours to obtain chitin nanocrystals T2F-Ch2.

[0093] The crystallinity of the product chitin nanocrystals T2F-Ch2 is 85.20%, the yield is 77.3%, the deacetylation degree is 17.26%, and the ζ potential reaches +15.1 mV at pH = 6.

[0094] According to the process of comparative example 3, only the heating and hydrolysis time was changed from 4 hours to 8 hours to obtain chitin nanocrystals T2F-Ch4.

[0095] The crystallinity of the product chitin nanocrystals T2F-Ch4 is 80.53%, the yield is 67.8%, the deacetylation degree is 25.84%, and the ζ potential reaches +24.1 mV at pH = 6.

[0096] Chitosan nanocrystals T2F-Ch5 were obtained by following the process of Comparative Example 3, except that the heating and hydrolysis time was changed from 4 hours to 12 hours.

[0097] The product chitin nanocrystals T2F-Ch5 have a crystallinity of 72.66%, a yield of 58.8%, a deacetylation degree of 28.41%, and a ζ potential of +32.8 mV at pH = 6.

[0098] Comparative Example 4

[0099] A method for preparing chitin nanocrystals using FeCl3·6H2O, comprising the following steps:

[0100] 20 mL of deionized water and 20 g of FeCl₃·6H₂O were stirred in an 80°C water bath for 5 minutes to obtain an FeCl₃ solution. 2 g of pure α-chitin raw material was added and hydrolyzed at 80°C for 0.75 hours. The reaction was terminated by adding 150 mL of deionized water and centrifuged at 9000 rpm. The supernatant was discarded and the precipitate was then washed three times with 1 mol / L hydrochloric acid to remove iron ions. The dispersed suspension was dialyzed with a 5000 Da membrane for 72 hours and then sonicated for 10 minutes at 450 W, 25 kHz, and a 2-second on / 2-second off pulse mode to obtain a suspension of chitin nanocrystals, T₂F-Ch₂.

[0101] According to Comparative Example 4, the product has a crystallinity of 82.83%, a yield of 85.2%, an aspect ratio of approximately 7.8 (average length of approximately 511.31 nm, diameter of approximately 65.72 nm), a degree of deacetylation of 11.15%, and a ζ potential of +10.78 mV at pH = 6.

[0102] Table 1 Hydrolysis results of Example 1 and Comparative Examples 1-4

[0103]

[0104] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A microwave-assisted, recyclable FeCl3·6H2O green method for preparing chitin nanocrystals, characterized in that: The following steps are involved: FeCl3·6H2O is mixed with water to prepare a FeCl3·6H2O solution, α-chitin is added, microwave hydrolysis is performed, the hydrolysis is terminated, and the obtained precipitate is purified after centrifugation to obtain chitosan nanocrystals; the liquid obtained by centrifugation is concentrated and recrystallized to recover FeCl3·6H2O.

2. The method according to claim 1, characterized in that The concentration of the FeCl3·6H2O solution is 0.5-1.5 g / mL; And / or, the material-liquid ratio of the α-chitin to water is 1 g:10 mL to 1 g:8 mL.

3. The method according to claim 2, characterized in that The concentration of the FeCl3·6H2O solution is 0.9-1.2 g / mL.

4. The method according to claim 1 or 2, characterized in that The microwave hydrolysis is performed at a power of 240 to 320 W, a temperature of 80 to 110° C., and a time of 0.5 to 1 hour. And / or, the heating rate of the microwave hydrolysis is 5-15°C / min.

5. The method according to claim 4, characterized in that: The power of the microwave hydrolysis is 280-320W; the temperature is 80-90°C.

6. The method according to claim 1 or 2, characterized in that: The termination of the hydrolysis refers to adding water to the hydrolysis system to terminate the reaction, wherein the volume ratio of the added water to the hydrolysis system is (150-250): (18-20); And / or, the rotation speed of the centrifugal separation is 8000-10000 rpm.

7. The method according to claim 1 or 2, characterized in that: The method for purifying the precipitate to obtain chitosan nanocrystals is as follows: washing the precipitate with a hydrochloric acid solution, dispersing it in water, dialyzing it, and ultrasonically treating it to obtain a chitosan nanocrystal suspension; and / or, the concentration of the hydrochloric acid solution is 0.5 to 1.5 mol / L; And / or, the dialysis membrane used for the dialysis is 3500-5000 Da, and the dialysis time is 48-72 hours; And / or, the process parameters of the ultrasonic treatment are 400-500W, 20-30kHz, 1-3 seconds on / 1-3 seconds off pulse mode, ultrasonic temperature is 20-30°C, and ultrasonic time is 6-12 minutes; And / or, the concentration is rotary evaporation concentration, the time is 5 to 8 hours; And / or, the concentrated solution is refrigerated at 3-5° C. for 72-96 hours to achieve recrystallization recovery of FeCl 3 ·6H 2 O.

8. A chitosan nanocrystal obtained by the method according to any one of claims 1 to 7.

9. The chitosan nanocrystal according to claim 8, characterized in that: The chitin nanocrystals have an aspect ratio of 13 to 14, an average length of 217 to 225 nm, and a diameter of 15.6 to 16.8 nm; And / or, the deacetylation degree of the chitin nanocrystals is 20.5-22.2%.

10. Use of the chitosan nanocrystals according to claim 9 in the fields of food and medicine preparation.

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