Method for preparing cationic starch through dual effects of eutectic solvent and mechanochemistry

Abstract

The invention belongs to the technical field of starch modification preparation, and relates to a method for preparing cationic starch through dual effects of a deep eutectic solvent and mechanochemistry. A novel reactive eutectic solvent is designed, mechanical force is introduced, the reaction with the overestimated content is achieved, etherification reaction is carried out in mechanical equipment, the conversion rate of 2, 3-epoxypropyltrimethylammonium chloride is greatly increased, and a prepared crude product is washed with an ethanol water solution to prepare a pure product. Compared with mature commercial reagents such as polyacrylamide, the method has the advantages that the optimal removal rate is similar, the risk of secondary pollution is avoided, compared with the current synthesis process, the reaction efficiency is high, and the prepared cationic starch is high in substitution degree, free of three wastes and environment-friendly, and has a wide application prospect in the future.

Description

Technical Field

[0001] The present invention belongs to the technical field of starch modification preparation, and relates to a method for preparing cationic starch by the dual action of deep eutectic solvent and mechanochemistry, which is a method for preparing modified cationic starch under dry conditions. Technical Background

[0002] Biomass is a material with rich resources and renewable properties, having many advantages such as non-toxic, inexpensive, green and pollution-free, biodegradable, etc. It can be continuously utilized through the natural carbon cycle and is an important material resource to support the sustainable development of humanity in the future. Starch, as a biomass material, exists very widely in nature, commonly found in grains, roots and tubers of leguminous plants, and is the main energy storage substance of plants. Due to its characteristics of low cost, biological renewability, biodegradability, and easy modification, it has been widely used in industries such as food, textile, papermaking, pharmaceutical, and construction. However, native starch has disadvantages such as insoluble in cold water, easy to retrograde and recrystallize, poor emulsification performance, and poor mechanical properties, which narrow the application range of starch. Therefore, in order to make full use of the advantages of starch, it is necessary to modify it to overcome the deficiencies of starch and expand its application range in industrial production. Starch is the second largest type of polysaccharide, and the presence of a large number of hydroxyl groups on its backbone allows modification through oxidation, cross-linking, graft copolymerization, etherification, etc. to introduce cationic groups and increase surface activity and solubility in cold water.

[0003] Under alkaline conditions, the product prepared by the reaction of starch with a cationic reagent containing groups such as sulfonium, ammonium, phosphonium, amino or imino groups is called cationic starch, which belongs to chemically modified starch. As early as 1930, there were reports on the synthesis of cationic starch. It was not until 1952 that the application value of cationic starch was realized. In 1955, the industrial-scale production of cationic starch began, and in 1957, the first patent related to cationic starch appeared. Subsequently, the types and quantities of cationic starch increased rapidly, gradually developing towards starch with unique functionality, amphoteric starch or composite starch, which further expanded the application value and scope of cationic starch.

[0004] At present, the preparation of cationic starch is mostly based on graft copolymerization. However, the main problem of "tailor-made" graft copolymers is that a large amount of water is required during the synthesis and at the end of the process to remove unreacted toxic chemicals and organic solvents, and the cost of cationic monomers is high, making it difficult to apply on a large scale. Therefore, the production of high-performance starch-based materials requires the development of a practical and inexpensive method to address these limitations. Solvents are crucial in starch processing. However, some conventional solvents for starch have several health and environmental risks. Therefore, it is very important to find greener solvents to replace them. According to the principles of green chemistry, green solvents should have the characteristics of less waste generation, less harmful chemical synthesis, high safety, and less environmental impact. The sustainability of solvents should also be related to recyclability, availability, stability, flammability, efficiency and cost. Based on these considerations, a low eutectic solvent is selected. After starch is treated with a low eutectic solvent, the crystalline region is destroyed, and the etherification reagent can penetrate well into the interior of the particles, making the reaction more sufficient. At the same time, the introduction of mechanical force can greatly reduce the amount of low eutectic solvent used and achieve efficient reaction with high solid content. The starch granules will be broken and refined by mechanical force, and the internal structure will be destroyed. The hydrogen bonds between the molecules will be opened, the molecular chain length will be broken, and the crystalline area will continue to break until it disappears. The amylose mixed in the crystalline area will be released, the amorphous area will continue to increase, the crystallinity of the starch granules will continue to decrease, and the activity of the starch molecules will continue to increase. Summary of the invention

[0005] In view of the problems of large environmental pollution, waste of resources, low production efficiency, etc. in the preparation process of cationic starch in the current market, the present invention provides a method for preparing quaternary ammonium cationic modified starch with simple production process, high reaction efficiency and excellent effect. The flocculant prepared by this method can be efficiently used in wastewater treatment. The present invention also designs a new reactive low eutectic solvent, and introduces mechanical force at the same time to achieve the reaction of overestimation content, and carries out etherification reaction in mechanical equipment, which greatly improves the conversion rate of 2,3-epoxypropyltrimethylammonium chloride, prepares a crude product and prepares a pure product by washing with ethanol aqueous solution. Compared with mature commercial reagents such as polyacrylamide, the optimal removal rate is similar, there is no risk of secondary pollution, and compared with the current synthesis process, the reaction efficiency is high, the prepared cationic starch has a high degree of substitution, no three wastes are generated, and it is environmentally friendly, and has broad application prospects in the future.

[0006] The technical solution of the present invention:

[0007] A method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry, the steps are as follows;

[0008] Step 1. Pretreatment of alkalization of starch: Spray sodium hydroxide alkaline solution onto starch, with the mass ratio of sodium hydroxide to starch being 1:80 - 1:160, stir evenly to obtain alkalized starch;

[0009] Step 2. Preparation of deep eutectic solvent: Mix according to the molar ratio of 2,3-epoxypropyltrimethylammonium chloride to urea of 1:1 - 1:3, heat to 50 - 90 °C to form a light yellow transparent liquid, and obtain the deep eutectic solvent;

[0010] Step 3. Mix and stir the alkalized starch and the deep eutectic solvent, then add them into mechanical equipment, raise the temperature for reaction. After the reaction ends, obtain the crude product; Wash the crude product several times with ethanol aqueous solution until the impurities are washed clean, filter and dry to obtain the final product, namely cationic starch.

[0011] The mass concentration of the sodium hydroxide solution is 5% - 20%;

[0012] The mechanical equipment is a mixer.

[0013] The mass concentration of the ethanol aqueous solution is 95%.

[0014] In the above Step 3, the molar ratio of alkalized starch to deep eutectic solvent is 1:0.5 - 1:2.

[0015] In the above Step 3, the stirring time is 20 minutes; the reaction temperature is 50 °C - 90 °C, and the reaction time is 0.5 h - 1.5 h.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) The preparation process is simple. Currently, the process for preparing cationic starch mainly uses the wet method, which leads to the generation of a large amount of waste liquid, seriously pollutes the environment, and is difficult to post-treat. However, under the dual action of mechanical force and deep eutectic solvent, a reaction with high solid content is achieved, and the preparation process flow is simple and can be completed under relatively mild conditions.

[0018] (2) The reaction efficiency is relatively high. The degree of substitution of the prepared cationic starch can reach 0.62, which is greatly improved compared with the degree of substitution of cationic starch prepared by the ordinary dry method between 0.2 - 0.35, and the conversion rate is also relatively high, greatly reducing the dosage of etherifying agent and reducing costs.

[0019] (3) Environmentally friendly. The addition amount of sodium hydroxide used in the alkalization process can be almost negligible and does not require additional treatment. The introduction of mechanical force realizes a reaction with high solid content and no generation of three wastes, while the reaction of starch and etherifying agent in the traditional method requires a large amount of alkali and solvent, resulting in serious pollution.

[0020] (4) Modified natural starch can be used as a flocculant, with a purification rate of up to over 98% for wastewater treatment and no problem of secondary pollution, having broad application prospects. Description of the Drawings

[0021] Figure 1 It is the infrared spectrogram of the raw starch in Examples 1 - 4 and cationic starches with different degrees of substitution of the products.

[0022] Figure 2 It is the XRD spectrogram of the raw starch in Examples 1 - 4 and cationic starches with different degrees of substitution of the products.

[0023] Figure 3 It is the influence of the feeding amounts of four cationic starches with different degrees of substitution in Examples 1 - 4 on the sewage removal rate. Detailed Embodiments

[0024] The following further illustrates the detailed embodiments of the present invention in combination with the drawings and technical solutions.

[0025] Example 1:

[0026] First, the starch is pretreated by alkalization. 0.2 g of NaOH is dispersed in 4 mL of water and mixed evenly with 16.2 g of starch. Then, 15.2 g of 2,3 - epoxypropyltrimethylammonium chloride and 6 g of urea are heated at 70 °C until it becomes a light yellow transparent liquid. The alkalized starch and the prepared deep eutectic solvent are mixed evenly, added to a mixer, heated to 70 °C and mechanically stirred for 1.5 h to obtain a crude product. The crude product is washed with 95% ethanol - aqueous solution until the presence of chloride ions cannot be detected by silver nitrate solution to remove the unreacted etherifying agent, and then dried to obtain the final product. The degree of substitution of the obtained product is 0.62. Through the flocculation experiment, it is found that its removal rate of suspended particles in kaolin simulated wastewater is as high as 98.5%. This example is the optimal reaction condition for preparing cationic starch, and the degree of substitution of the obtained product is 0.62.

[0027] Example 2:

[0028] First, the starch was pretreated by alkalization. 0.2 g of NaOH was dispersed in 1 mL of water and mixed evenly with 32.4 g of starch. Then, 15.2 g of 2,3-epoxypropyltrimethylammonium chloride and 6 g of urea were heated at 70 °C until they became a light yellow transparent liquid. The alkalized starch was mixed evenly with the prepared deep eutectic solvent and added to a mixer. The temperature was raised to 50 °C and mechanically stirred for 0.5 h to obtain a crude product. The crude product was washed with a 95% ethanol-aqueous solution until the presence of chloride ions could not be detected by silver nitrate solution, and the unreacted etherifying agent was removed. After drying, the final product was obtained. The degree of substitution of the prepared product was 0.21. The flocculation experiment found that the removal rate of suspended particles in the kaolin simulated wastewater was as high as 98.1%.

[0029] Example 3:

[0030] First, the starch was pretreated by alkalization. 0.15 g of NaOH was dispersed in 2 mL of water and mixed evenly with 16.2 g of starch. Then, 15.2 g of 2,3-epoxypropyltrimethylammonium chloride and 12 g of urea were heated at 70 °C until they became a light yellow transparent liquid. The alkalized starch was mixed evenly with the prepared deep eutectic solvent and added to a mixer. The temperature was raised to 70 °C and mechanically stirred for 1 h to obtain a crude product. The crude product was washed with a 95% ethanol-aqueous solution until the presence of chloride ions could not be detected by silver nitrate solution, and the unreacted etherifying agent was removed. After drying, the final product was obtained. The degree of substitution of the prepared product was 0.37. The flocculation experiment found that the removal rate of suspended particles in the kaolin simulated wastewater was as high as 98.4%.

[0031] Example 4:

[0032] First, the starch was pretreated by alkalization. 0.1 g of NaOH was dispersed in 2 mL of water and mixed evenly with 8.1 g of starch. Then, 15.2 g of 2,3-epoxypropyltrimethylammonium chloride and 18 g of urea were heated at 90 °C until they became a light yellow transparent liquid. The alkalized starch was mixed evenly with the prepared deep eutectic solvent and added to a mixer. The temperature was raised to 90 °C and mechanically stirred for 1.5 h to obtain a crude product. The crude product was washed with a 95% ethanol-aqueous solution until the presence of chloride ions could not be detected by silver nitrate solution, and the unreacted etherifying agent was removed. After drying, the final product was obtained. The degree of substitution of the prepared product was 0.49. The flocculation experiment found that the removal rate of suspended particles in the kaolin simulated wastewater was as high as 98.4%.

[0033] Figure 1 In it, (Ⅰ) is the raw starch, and (Ⅱ-Ⅴ) are the infrared spectra of cationic starches with degrees of substitution of 0.21, 0.37, 0.49, and 0.62. The structures of the four cationic starches were characterized by Fourier transform infrared spectroscopy, and the obtained spectra are as Figure 1As shown. A peak at 1480 cm appears in the Fourier transform infrared spectrum of the cationic starch. -1 This is because the vibration absorption of C-H in -N + (CH 3 ) 3 of 2,3-epoxypropyltrimethylammonium chloride appears in each sample. Figure 2 In (Ⅰ) is the raw starch, and (Ⅱ - Ⅴ) are the XRD patterns of cationic starches with substitution degrees of 0.21, 0.37, 0.49, and 0.62 respectively. According to the results of X-ray powder diffraction tests, all four special diffraction peaks of the four cationic starches disappear, indicating that the crystalline regions in the modified starch completely disappear. Under the dual action of mechanical force and deep eutectic solvent, the starch granules are continuously refined, the particle size is continuously decreasing, the molecular weight is decreasing, the crystalline structure inside the starch granules is destroyed, and the etherification reaction proceeds throughout the granules, and the reaction is very thorough. Figure 3 It is the relationship between the feeding amount of four cationic starches with different substitution degrees and the sewage removal rate. Samples 1 - 4 are cationic starches with substitution degrees of 0.21, 0.37, 0.49, and 0.62. With the increase of cationic starch, the removal rate of the cationic starch for kaolin suspension increases rapidly and reaches nearly 100% at the optimal dosage, indicating good flocculation performance. This is because the charge density distribution on the surface and inside of the flocs is not uniform. With the increase of the dosage, more and more colloidal particles are adsorbed by the flocs, and the non-uniformity of its charge density is getting higher. At this time, the electrostatic interaction will cause the flocs to break and the removal rate to decrease. This phenomenon is called redispersion, so the phenomenon that the flocculation effect first increases and then decreases with the increase of the dosage appears.

[0034] In summary, the present invention discloses a method for preparing cationic starch by the dual action of deep eutectic solvent and mechanochemistry, provides a preparation method of quaternary ammonium type cationic modified starch with a simple production process, green environmental protection, and high application efficiency, and through the flocculation experiment on kaolin simulated wastewater, it is found that the modified cationic starch has a very significant effect in water treatment. Moreover, the developed new process is simple and easy to operate, the raw material utilization rate is high, the prepared cationic starch has a high substitution degree and a high reaction efficiency, which conforms to the concept of green chemistry and has the potential for large-scale application in the future.

Claims

1. A method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry, characterized in that: The steps are as follows; Step 1: Alkalinization pretreatment of starch: spray sodium hydroxide alkaline solution into starch, the mass ratio of sodium hydroxide to starch is 1:80-1:160, stir evenly, and obtain alkalized starch; Step 2, preparing a low eutectic solvent: mixing 2,3-epoxypropyltrimethylammonium chloride and urea in a molar ratio of 1:1 to 1:3, heating to 50-90° C. to form a light yellow transparent liquid, and obtaining a low eutectic solvent; Step 3: Mix and stir the alkalized starch and the low eutectic solvent, then add them into a mechanical device, heat them for reaction, and obtain a crude product after the reaction is completed; wash the crude product repeatedly with ethanol aqueous solution several times until the impurities are washed away, filter and dry to obtain the final product, i.e., cationic starch.

2. The method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry according to claim 1, characterized in that: The mass concentration of the sodium hydroxide solution is 5%-20%.

3. The method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry according to claim 1, characterized in that: The mechanical equipment is an internal mixer.

4. The method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry according to claim 1, characterized in that: The mass concentration of the ethanol aqueous solution is 95%.

5. The method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry according to claim 1, characterized in that: In the step 3, the molar ratio of alkalized starch to low eutectic solvent is 1:0.5 to 1:

2.

6. The method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry according to claim 1, characterized in that: In the step 3, the reaction temperature is 50°C-90°C, and the reaction time is 0.5h-1.5h.

7. The method for preparing cationic starch by dual action of a deep eutectic solvent and mechanochemistry according to claim 1, characterized in that: In the step 3, the reaction temperature is 50°C-90°C, and the reaction time is 0.5h-1.5h.

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