Preparation method of high-regularity hydroxypropyl chitin

Abstract

The application discloses a preparation method of high-regularity hydroxypropyl chitin. The method comprises the following steps: dissolving chitin in an alkali / urea aqueous solution after purification, starting a gas-liquid dispersion system to atomize propylene oxide and then passing the propylene oxide into the chitin solution under low-temperature conditions, and starting intermittent microwave irradiation, so that the chitin and the propylene oxide are subjected to nucleophilic ring-opening reaction, and high-regularity hydroxypropyl chitin is obtained through post-treatment. Through the synergistic effect of atomization dispersion and intermittent microwave irradiation, the reaction time is significantly shortened, the regularity of the product is improved, and the technical defects of low chitin reaction efficiency and uneven product structure in the prior art are overcome.

Description

Technical Field

[0001] This invention relates to the field of chitin modification preparation process, specifically to a method for preparing highly regular hydroxypropyl chitin. Background Technology

[0002] Chitin, the second most abundant natural polysaccharide after cellulose, possesses excellent biocompatibility, biodegradability, and low toxicity, making it irreplaceable in biomedicine, cosmetics, and environmental protection. However, the strong hydrogen bonds between chitin molecules and its high crystallinity make it insoluble in water and most common organic solvents, greatly limiting its processing and application scope.

[0003] To overcome this challenge, chemical modification of chitin is a key approach, with the introduction of functional side chains being crucial. Hydroxypropylation is one of the most mature modification routes, introducing functional groups into the chitin backbone to reduce its crystallinity and thus improve its water solubility. For example, modified hydroxypropyl chitin not only retains the biocompatibility advantages of chitin but also endows it with good temperature sensitivity.

[0004] Natural chitin typically has a viscosity-average molecular weight of 50–1000 kDa, and its molecular chains are densely packed due to a large network of strong hydrogen bonds. The long and easily entangled chitin chains prevent the hydroxyl reaction sites from directly contacting propylene oxide during the preparation of hydroxypropyl chitin, hindering the reaction. Although the low-temperature NaOH / urea aqueous solution method can partially disrupt the crystal structure of chitin, allowing it to dissolve in aqueous solution, some chitin molecular chains remain entangled, preventing the full exposure of some hydroxyl reaction sites. This forces propylene oxide to penetrate gradually from the surface to the core during the reaction, attacking the hydroxyl reaction sites step by step. Ultimately, many hydroxyl sites in the system cannot fully contact propylene oxide and participate in the reaction. Consequently, hydroxypropyl chitin exhibits limited regularity, and its structural disorder weakens its crystallinity and leads to unstable mechanical and other physical properties, limiting its further widespread application. Summary of the Invention

[0005] The present invention aims to provide a method for preparing highly regular hydroxypropyl chitin, so as to solve the problems of low reaction efficiency and poor product regularity in the prior art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a method for preparing highly regular hydroxypropyl chitin, comprising the following steps: (1) Purification of crude chitin Chitin was pulverized and ground, then subjected to dilute acid treatment, alkali treatment, washing, and drying to obtain purified chitin.

[0007] (2) Preparation of chitin solution The purified chitin was dissolved in an alkaline / urea aqueous solution to prepare a chitin solution.

[0008] (3) Construction of the reaction system The prepared chitin solution was added to the reaction vessel, and the low-temperature circulation system was started to control the reaction system under low-temperature conditions.

[0009] (4) Atomization dispersion and microwave-assisted reaction The gas-liquid dispersion system is activated to atomize propylene oxide into micron-sized droplets, which are then uniformly dispersed and introduced into the chitin solution. Simultaneously, the microwave-assisted reaction device is turned on, and an intermittent microwave irradiation mode is adopted to enable the chitin and propylene oxide to undergo a nucleophilic ring-opening reaction. During the microwave intervals, the low-temperature circulation system maintains the reaction system at a low temperature.

[0010] (5) Post-processing After the reaction was completed, the microwave-assisted reaction device and the gas-liquid dispersion system were turned off, and the reaction solution was post-processed to obtain highly regular hydroxypropyl chitosan.

[0011] The "regularity" mentioned in this invention refers to the degree of order in the crystallization behavior of hydroxypropyl chitin molecular chains. Higher regularity means the molecular chains are more easily and regularly stacked, resulting in stronger crystallization ability. This manifests as a denser and more uniform distribution of crystal nuclei under a polarizing microscope and a higher degree of peak retention in X-ray diffraction patterns. The structural regularity of the product can be comprehensively evaluated through combined characterization using POM and XRD.

[0012] In step (1), the present invention does not have any special restrictions on the source of chitin, and it can be chitin from sources such as shrimp shells, crab shells, squid bones, etc.

[0013] Preferably, the chitin has a viscosity-average molecular weight of 50-1000 kDa and a degree of deacetylation of ≤10%.

[0014] More preferably, the viscosity-average molecular weight is 200~500 kDa and the degree of deacetylation is ≤5%.

[0015] In step (2), the alkali / urea aqueous solution is a NaOH / urea aqueous solution.

[0016] Preferably, the NaOH / urea aqueous solution is an 8 wt% NaOH / 4 wt% urea aqueous solution.

[0017] In step (2), the concentration of the chitin solution is 1~10 wt%.

[0018] Preferably, the concentration of the chitin solution is 5-8 wt%.

[0019] In step (3), the low temperature condition is -10℃ to 10℃. Preferably, the low temperature condition is -5°C to 5°C.

[0020] In step (4), the propylene oxide is atomized into micron-sized droplets during the atomization process. Preferably, the particle size of propylene oxide after atomization is 10~100 μm. More preferably, the particle size of propylene oxide after atomization is 20~50 μm.

[0021] In step (4), the flow rate of the propylene oxide is 5~50 mL / min.

[0022] Preferably, the flow rate of the propylene oxide is 20 mL / min.

[0023] In step (4), the power of the intermittent microwave irradiation is 400~1000 W.

[0024] Preferably, the power of the intermittent microwave irradiation is 500 W.

[0025] In step (4), the intermittent microwave irradiation is carried out in a manner that alternates between irradiation and intermittent irradiation, with the ratio of irradiation time to intermittent time being 1:5 to 5:1.

[0026] Preferably, the intermittent microwave irradiation method is: irradiation for 30 seconds, followed by an interval of 60 seconds.

[0027] In step (4), the equivalence ratio of propylene oxide to sugar units is (5~20):1.

[0028] Preferably, the equivalence ratio of propylene oxide to sugar units is 10:1.

[0029] In step (5), the post-processing includes at least one of neutralization, dialysis, concentration and drying.

[0030] In a second aspect, the present invention also provides a microwave-assisted atomization epoxidation reaction system for implementing the method, comprising: Reactor; The stirring motor, located at the top of the reactor, is connected to the stirring paddle inside the reactor. Microwave-assisted reaction device installed outside the reactor; Atomizing nozzles installed inside the reactor; And a low-temperature circulation system for controlling the reaction system at low temperatures.

[0031] Furthermore, the outlet of the atomizing nozzle is located below the liquid level inside the reactor.

[0032] Furthermore, the system also includes a gas-liquid dispersion system, which includes the nozzle.

[0033] The system also includes a control system, which is electrically connected to the cryogenic circulation system, the gas-liquid dispersion system, the microwave-assisted reaction device, and the stirring paddle, respectively, and is used to control the start-up and shutdown, operating parameters, and coordinated working sequence of each system.

[0034] This invention achieves efficient preparation of highly regular hydroxypropyl chitin through the synergistic effect of the following three aspects: (1) Activation effect of microwaves on molecular chains Uniform microwave irradiation disturbs the polar groups on the chitin molecular chains, weakening intermolecular hydrogen bonds and causing the partially entangled chitin molecular chains to rapidly untangle, thus exposing more hydroxyl reaction sites. Exposing more reaction sites simultaneously makes the reaction probabilities at different sites on each molecular chain more similar, resulting in a more regular final product structure.

[0035] (2) Mass transfer enhancement effect of gas-liquid dispersion system In the process of generating hydroxypropyl chitin through a nucleophilic ring-opening reaction between chitin and propylene oxide, this invention introduces a gas-liquid dispersion system to atomize propylene oxide and introduce it into the reaction system. This increases the contact area between propylene oxide and chitin, avoids excessive cross-linking caused by excessively high local concentrations of propylene oxide, and enhances the mass transfer efficiency of the reaction system.

[0036] (3) Synergistic effect of low-temperature intermittent microwave Microwave-assisted reaction at low temperatures accelerates molecular diffusion and ring-opening reactions, significantly increasing the reaction rate. Simultaneously, the intermittent microwave method maintains the overall low-temperature environment of the reaction system, which is beneficial for the stable dispersion of chitin after swelling, inhibits propylene oxide self-polymerization (the self-polymerization rate of propylene oxide increases significantly at ≥25℃), and avoids the degradation of chitin molecular chains.

[0037] The synergistic effect of the above three aspects enables the present invention to achieve a highly efficient and uniform hydroxypropylation reaction under the condition of maintaining a low-temperature stable system, thereby obtaining highly regular hydroxypropyl chitosan.

[0038] Compared with the prior art, the present invention has the following beneficial effects: (1) Significantly shorten reaction time and achieve a leap in production efficiency. Reaction time is one of the core indicators for evaluating the economic efficiency of a process. As shown in Table 1, conventional stirred reactions require more than 24 hours to achieve the target degree of substitution of 0.8 or higher. This is highly consistent with the 10-30 hour reaction time reported in existing technical literature, which is difficult to meet the actual needs of efficient production.

[0039] When this invention combines atomization dispersion with intermittent microwave irradiation, the reaction process is significantly accelerated, reaching the target degree of substitution in just 5 hours. This result means that, compared to conventional stirred reactions (>24 hours), the reaction time of this invention is reduced by more than 79%. This synergistic effect stems from the complementary advantages of both methods: atomization dispersion transforms propylene oxide into micron-sized droplets, increasing its specific surface area by hundreds of times, fundamentally solving the limitations of microscopic mixing and mass transfer of propylene oxide in chitin solutions; intermittent microwave irradiation provides a rapid and uniform energy supply to the main reaction through high-frequency oscillation of polar molecules. The synergistic effect of these two methods overcomes the long-standing technical prejudice in the field that "chitin's high crystallinity inevitably leads to a slow reaction."

[0040] (2) Significantly improves product regularity and enhances the stability of material physical properties. In addition to reaction efficiency and yield, this invention demonstrates significant advantages in regulating the molecular regularity of hydroxypropyl chitin.

[0041] Characterization by polarized light microscopy (POM) revealed that the hydroxypropyl chitin prepared in this invention not only exhibits higher substitution efficiency but also demonstrates more stable crystallization behavior and better optical anisotropy. Uniform microwave irradiation perturbs the polar groups on the chitin molecular chains, weakening intermolecular hydrogen bonds and causing the partially entangled chitin molecular chains to rapidly disperse, thereby exposing more hydroxyl reaction sites. The simultaneous exposure of more reaction sites also makes the reaction probabilities at different sites on each molecular chain more similar, resulting in a more regular final product structure. The results further demonstrate at the structural level that this invention not only has a "faster reaction" but also a "superior structure," exhibiting significant technological advancement.

[0042] (3) Increase yield Under the same reaction time (5 hours) and similar degree of substitution, the yield of the method of the present invention reaches 70%, while the yield of conventional stirred reaction is only 56% at 24 hours. The method of the present invention has a higher yield.

[0043] (4) Maintain the stability of the reaction system and suppress side reactions. This invention utilizes intermittent microwave irradiation under low-temperature conditions to maintain the overall low-temperature environment of the reaction system, which is beneficial to the stable dispersion state of chitin after swelling, inhibits the self-polymerization of propylene oxide (the self-polymerization rate of propylene oxide is significantly increased at ≥25℃), and avoids the degradation of chitin molecular chains. Attached Figure Description

[0044] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0045] Figure 1 Process flow diagram.

[0046] Figure 2 Polarized light microscope images (POM) of hydroxypropyl chitin obtained in Example 1 and Comparative Examples 1, 2, and 3 of this invention; A and C are Comparative Examples 1, 2, and 3, respectively, and D is Example 1.

[0047] Figure 3 X-ray diffraction (XRD) patterns of hydroxypropyl chitin obtained from chitin raw material (A), Example (B) of the present invention, and Comparative Examples 1, 2, and 3 (C, D, and E).

[0048] Figure 4 Fourier transform infrared (FTIR) spectra of hydroxypropyl chitin obtained in Example (A) of the present invention and Comparative Examples 1, 2, 3 (B, C, D).

[0049] Figure 5 The proton nuclear magnetic resonance spectra of hydroxypropyl chitin obtained in Example (A) and Comparative Examples 1, 2, and 3 (B, C, and D) of this invention ( 1 H-NMR).

[0050] Figure 6 A schematic diagram of a microwave-assisted atomization epoxidation reaction system for implementing the method described in this invention. Detailed Implementation

[0051] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0052] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0053] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0054] The chitin raw material used in this embodiment is shrimp shells and crab shells from which meat scraps and dirt have been removed. Its viscosity-average molecular weight is 300 kDa and its degree of deacetylation is ≤5%.

[0055] Example 1 according to Figure 1 The process flow shown in this embodiment, and the specific steps are as follows: Step 1: Purification of crude chitin Chitosan was pulverized and ground, then passed through a 100-mesh sieve to obtain a powdered raw material. The ground chitosan powder was added to a dilute hydrochloric acid aqueous solution and stirred in an ice bath for 2 hours. The mixture was then filtered through a silk cloth, and the solids were washed with deionized water until the pH reached approximately 6. The washed chitosan powder was then added to a sodium hydroxide aqueous solution and stirred in a 40°C water bath for 1.5 hours. The mixture was filtered through a silk cloth, and the solids were washed with distilled water until the pH reached approximately 8. The purified chitosan was dried to obtain a white powdery product.

[0056] Step 2: Preparation of chitin solution The purified chitin powder was dispersed in an 8wt% NaOH / 4wt% urea mixed aqueous solution pre-cooled to approximately -30°C. The solution was then thoroughly dissolved by vigorous stirring, and the process was repeated three times via freeze-thaw cycles to obtain a clear and transparent chitin solution. In this example, the concentration of the chitin solution was 5wt%.

[0057] Step 3: Construction of the reaction system The chitin solution obtained in step 2 is added to the reactor through the chitin solution inlet, and the reactor is sealed. The cryogenic circulation system is then started to lower the temperature of the reaction system to and stabilize it at the target reaction temperature of 5±2℃.

[0058] Step 4: Atomization and Microwave-Assisted Reaction The gas-liquid dispersion system is activated. Propylene oxide flows from the storage tank into the atomizing nozzle under high-pressure nitrogen. At the nozzle, the liquid propylene oxide is sheared and atomized into micron-sized droplets by a high-speed gas flow at a flow rate of 20 mL / min. The atomized propylene oxide droplet size is 20-50 μm, and it is uniformly dispersed into the chitin solution. The equivalence ratio of propylene oxide to chitin sugar units is 10:1.

[0059] Simultaneously with the commencement of atomization dispersion, the microwave-assisted reaction device was activated, employing an intermittent microwave irradiation mode. The microwave power was 500 W, with an irradiation time of 30 seconds followed by a 60-second interval. During microwave irradiation, polar molecules in the reaction system (such as water molecules and hydroxyl groups on the chitin molecular chain) oscillated at high frequency, generating a localized, instantaneous high-temperature effect, effectively accelerating the nucleophilic ring-opening reaction between propylene oxide and chitin hydroxyl groups. During the intervals, the reaction system rapidly returned to the preset low-temperature state via a continuously operating low-temperature circulation system. This helped maintain the stable swelling and dispersion of chitin and prevented propylene oxide self-polymerization and thermal degradation of the chitin molecular chain due to heat accumulation. Throughout the reaction, stirring was continuously operated at a low speed (approximately 100 rpm) to assist in the macroscopic mixing of the materials. The total reaction time was 5 hours.

[0060] Step 5: Post-processing After the reaction reaches the preset time, the microwave-assisted reaction device and the gas-liquid dispersion system are turned off, and the reaction liquid is discharged from the outlet. Next, by determining the reaction endpoint, the reaction liquid is treated using conventional post-processing techniques, including neutralization, dialysis, concentration, and drying, ultimately yielding a pure hydroxypropyl chitosan product.

[0061] Testing showed that the yield of hydroxypropyl chitin obtained in this embodiment was 70%, and the degree of substitution (DS) reached over 0.8.

[0062] Example 2 This embodiment is basically the same as Example 1, except that the concentration of the chitin solution is 8 wt%. All other conditions (including chitin raw material, propylene oxide to chitin sugar unit equivalent ratio of 10:1, reaction temperature of 5±2℃, microwave power of 500 W, irradiation mode of 30 seconds / intermittent 60 seconds, total reaction time of 5 hours, etc.) are the same as in Example 1.

[0063] Testing showed that the hydroxypropyl chitin obtained in this embodiment also exhibited good structural regularity.

[0064] Example 3 This embodiment is basically the same as Embodiment 1, except that the microwave power is 600 W and the irradiation mode is 30 seconds / intermittent 60 seconds. All other conditions are the same as in Embodiment 1.

[0065] Testing showed that the hydroxypropyl chitosan obtained in this embodiment also exhibited good regularity.

[0066] Example 4 This embodiment provides a microwave-assisted atomization epoxidation reaction system for implementing the method described in this invention, such as... Figure 6 As shown, the system includes: Reactor 1: Used to contain chitin solution and provide reaction space.

[0067] Stirring motor 2: Located at the top of the reactor and connected to the stirring paddle 3 inside the reactor, it is used to drive the stirring paddle 3 to assist in the macroscopic mixing of materials.

[0068] Microwave-assisted reaction device 4: Located outside the reactor, it is used to intermittently irradiate the reaction system inside the reactor with microwaves.

[0069] Atomizing nozzle 5: Located inside the reactor 1, its inlet is connected to the propylene oxide supply pipeline, and is used to atomize propylene oxide and spray it into the chitin solution.

[0070] Low-temperature circulation system (not shown in the figure): connected to the jacket or coil of the reactor, used to control the reaction system under low-temperature conditions.

[0071] Comparative Example 1 (Conventional stirring reaction) This comparative example uses the same chitin raw material (viscosity-average molecular weight 300 kDa, degree of deacetylation ≤5%) and the same reaction ratio as Example 1 (chitin solution concentration 5 wt%, solvent is 8 wt% NaOH / 4 wt% urea aqueous solution, equivalence ratio of propylene oxide to chitin sugar units 10:1, reaction temperature 5±2℃).

[0072] Unlike Example 1, this comparative example uses a conventional reactor, with propylene oxide added to the chitin solution in liquid form in one go, and the stirring speed is 300 rpm. Microwave assistance and atomization dispersion are not used.

[0073] The results showed that the required reaction time for this comparative example to achieve a degree of substitution (DS) ≥ 0.8 exceeded 24 hours. Samples taken 24 hours after the reaction were tested, and the yield was only 56%.

[0074] Comparative Example 2 (atomization dispersion only, no microwave) This comparative example is basically the same as Example 1, except that the gas-liquid dispersion system is turned on for atomization dispersion, but the microwave-assisted reaction device is not turned on.

[0075] The results showed that the reaction time required for this comparative example to achieve a degree of substitution (DS) ≥ 0.8 was significantly longer (10 h) than that in Example 1. Polarizing microscopy revealed that the regularity of the obtained product was lower than that in Example 1.

[0076] Comparative Example 3 (intermittent microwave only, without atomization dispersion) This comparative example is basically the same as Example 1, except that: intermittent microwave irradiation is turned on, but the gas-liquid dispersion system is not started, and propylene oxide is added in liquid form all at once.

[0077] The results showed that the reaction time required for this comparative example to achieve a degree of substitution (DS) ≥ 0.8 was significantly longer (12 h) than that in Example 1. Polarizing microscopy revealed that the regularity of the obtained product was lower than that in Example 1.

[0078] Table 1 Summary of the effects of the examples and comparative examples

[0079] Characterization of product regularity To verify the regularity of the hydroxypropyl chitin obtained in this invention, the samples obtained in Example 1 and Comparative Example 1 were characterized by polarized light microscopy (POM).

[0080] The obtained sample was prepared into an aqueous solution with a mass fraction of 2 wt%, dropped onto a glass slide, and allowed to evaporate naturally at room temperature to form a film. The film was then observed under cross-polarized light.

[0081] The results showed that the product obtained by the combined effects of atomization dispersion and intermittent microwave in Example 1 exhibited a denser and more uniform distribution of crystal nuclei under a polarizing microscope, and showed a higher degree of retention of diffraction peaks in the X-ray diffraction pattern, indicating that it has higher regularity.

[0082] exist Figure 2 As can be seen, discrete, flickering bright spots exist in the AD field of view—namely, crystal nuclei (and the spherulites growing from them). Crystal nuclei are the sole prerequisite for crystallization initiation; the number and morphology of crystal nuclei directly determine the rate of polymer crystallization, crystal appearance, and all mechanical and performance properties of the finished product. And only in… Figure 2 Numerous crystal nuclei were observed in D, and their distribution was uniform. This indicates that the hydroxypropyl chitin prepared in Example 1 has good crystallinity, which may also be related to the high regularity of the product. The hydroxypropyl chitin molecular chain segments prepared in Comparative Examples 1 and 2 and 3 are arranged more randomly, which also indicates that the regularity of the comparative examples is lower than that of the present invention.

[0083] contrast Figure 2 As can be seen from A and D, the present invention significantly improves the regularity of hydroxypropyl chitin through the synergistic effect of atomization dispersion and intermittent microwave, giving it more stable crystallization behavior and better optical anisotropy.

[0084] Depend on Figure 3As can be seen, the XRD patterns of the chitin raw material are compared with those of the samples obtained in the examples and comparative examples. The chitin raw material exhibits six characteristic diffraction peaks at 2θ = 9.3°, 12.7°, 19.2°, 20.8°, 23.3°, and 26.3°, corresponding to the (020), (021), (110), (120), (130), and (013) crystal planes, respectively, confirming that the raw material is α-crystalline chitin. A large number of hydroxyl and amino groups form a dense hydrogen bond network, binding the molecular chains and resulting in a well-organized, stable crystalline structure. Solvent molecules have difficulty penetrating and disrupting the crystal lattice, which is one of the main reasons why chitin is difficult to dissolve. After hydroxypropyl substitution, the diffraction peaks significantly weaken, and the material gradually transforms into an amorphous structure. This indicates that the homogeneous hydroxypropylation reaction disrupts the original hydrogen bond network of chitin, freeing the hydroxyl groups of the molecular chains from hydrogen bond binding, thereby enhancing their interaction with water molecules. Meanwhile, it was found that when the degree of substitution of the hydroxypropyl chitin obtained in Example 1 was not significantly different, the diffraction peak of Example 1 was more significant than that of Comparative Examples 1, 2, and 3. This indicates that the highly substituted hydroxypropyl chitin obtained in Example 1 has better crystallization properties, which may also be related to the higher material regularity.

[0085] Depend on Figure 4 As can be seen from the FTIR spectra of the samples obtained in the examples and comparative examples, all hydroxypropyl chitin samples showed a peak value at 1736 cm⁻¹. -1 The characteristic absorption peak of the hydroxyl group is observed at [location missing]. Compared with similar literature (Liu Xiaowei. [D]. Qingdao University, 2023), amide 1 exhibits an absorption peak (1662 cm⁻¹). -1 and 1625 cm -1 ) and amide band absorption peak (1559 cm⁻¹) -1 No significant changes were observed, indicating that no significant deacetylation reaction occurred during the reaction. Comparison of the infrared spectral characteristics of different samples revealed that, under the premise of no significant difference in the degree of substitution, the characteristic absorption peaks of the modified chitin obtained in the examples were clearer and narrower than those in the comparative example. The hydrogen bond absorption peak at 3440 cm⁻¹ showed a lower degree of broadening, and the retention of the amide band and sugar ring backbone peaks was higher. This indicates that the modified chitin obtained in the examples still retains some ordered molecular stacking structure and has relatively better crystallization performance, which may be related to the lower degree of molecular chain degradation and higher regularity during the reaction.

[0086] Depend on Figure 5As can be seen, the ¹H-NMR spectra of the samples obtained in the examples and comparative examples are not significantly different from those in relevant literature (Liu Xiaowei. [D]. Qingdao University, 2023), proving that the obtained product is hydroxypropyl chitin with the expected structure. The degree of substitution of hydroxypropyl chitin in each example and comparative example is calculated to be around 0.8–0.9, with no significant difference in the degree of substitution among the cases. Comparison of the ¹H-NMR spectra of different samples reveals that, under the premise that there is no significant difference in the degree of hydroxypropyl substitution, the ¹H-NMR signal of the sample obtained in the examples is more regular than that of the comparative example: the proton peak of the sugar ring skeleton is less broadened, the terminal proton signal still maintains a relatively clear peak shape, and the intensity distribution of the hydroxypropyl characteristic peak is more uniform; while the proton peak of the sugar ring in the comparative example sample is severely broadened, the baseline noise is higher, and some characteristic peaks overlap significantly. This indicates that under the reaction conditions of the examples, the chitin molecular chains are less degraded, the modification reaction is milder, and the molecular chains still retain some ordered stacking structure. This is consistent with the conclusion in the previous XRD and FT-IR analyses that the crystallization performance of the sample in the examples is better, further proving that the highly substituted hydroxypropyl chitin obtained under these conditions has higher regularity.

[0087] It should be noted that those skilled in the art may conceive of increasing the reaction rate by introducing a catalyst or intensifying stirring. However, the above methods are unlikely to achieve the same reaction rate as the present invention while simultaneously shortening the reaction time. Figure 2 The results show a highly uniform and continuous anisotropic crystalline structure. In contrast, this invention achieves uniform dispersion of reactants and simultaneous activation of molecular chains in chitin solution through the synergistic effect of atomization dispersion and intermittent microwave irradiation. Therefore, this invention not only achieves highly efficient reactions (reducing reaction time by more than 79%), but also yields products with higher regularity, a key advantage that is difficult to achieve simultaneously by introducing catalysts or strengthening stirring.

[0088] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing highly regular hydroxypropyl chitin, characterized in that, Includes the following steps: (1) Purification of crude chitin Chitin was pulverized and ground, then subjected to dilute acid treatment, alkali treatment, washing, and drying to obtain purified chitin. (2) Preparation of chitin solution The purified chitin was dissolved in an alkaline / urea aqueous solution to prepare a chitin solution; (3) Construction of the reaction system The prepared chitin solution was added to the reaction vessel, and the low-temperature circulation system was started to control the reaction system under low-temperature conditions. (4) Atomization dispersion and microwave-assisted reaction The gas-liquid dispersion system is activated to atomize propylene oxide into micron-sized droplets, which are then uniformly dispersed and introduced into the chitin solution. Simultaneously, the microwave-assisted reaction device is activated, and an intermittent microwave irradiation mode is adopted to enable the chitin and propylene oxide to undergo a nucleophilic ring-opening reaction. During the microwave intervals, the low-temperature circulation system maintains the reaction system at a low temperature. (5) Post-processing After the reaction was completed, the microwave-assisted reaction device and the gas-liquid dispersion system were turned off, and the reaction solution was post-processed to obtain highly regular hydroxypropyl chitosan.

2. The method according to claim 1, characterized in that, In step (1), the chitin has a viscosity-average molecular weight of 50~1000 kDa and a degree of deacetylation of ≤10%; the alkali / urea aqueous solution is a NaOH / urea aqueous solution.

3. The method according to claim 1, characterized in that, In step (2), the concentration of the chitin solution is 1~10wt%.

4. The method according to claim 1, characterized in that, In step (3), the low temperature condition is -10℃ to 10℃.

5. The method according to claim 1, characterized in that, In step (4), the power of the intermittent microwave irradiation is 400~1000 W, and the irradiation and intermittent irradiation are carried out alternately, with the ratio of irradiation time to intermittent time being 1:5 to 5:

1.

6. The method according to claim 1, characterized in that, In step (4), the particle size of the propylene oxide after atomization is 10~100 μm; the flow rate of the propylene oxide is 5~50 mL / min.

7. The method according to claim 6, characterized in that, In step (4), the particle size of the propylene oxide after atomization is 20~50 μm; the flow rate of the propylene oxide is 20 mL / min.

8. The method according to claim 1, characterized in that, In step (4), the equivalence ratio of propylene oxide to chitin sugar unit is (5~20):

1.

9. The method according to claim 1, characterized in that, In step (5), the post-processing includes at least one of neutralization, dialysis, concentration and drying.

10. A microwave-assisted atomization epoxidation reaction system for implementing the method according to any one of claims 1-9, characterized in that, include: Reactor; The stirring motor, located at the top of the reactor, is connected to the stirring paddle inside the reactor. Microwave-assisted reaction device installed outside the reactor; Atomizing nozzles installed inside the reactor; And a low-temperature circulation system for controlling the reaction system at low temperatures.

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