An ultraviolet absorber UV-323 and a method for preparing the same

Abstract

The application belongs to the technical field of ultraviolet absorbers, and particularly relates to an ultraviolet absorber UV-323 and a preparation method thereof. The preparation method comprises the following steps: (1) diazotization reaction, (2) coupling reaction, (3) reduction of sulfide dispersion, (4) esterification reaction, and (5) crystallization. The product purity of the application is significantly improved, the product purity can reach more than 99% through an optimized process route and solvent recovery-recrystallization refining, the appearance is white powder, and the quality requirement of high-end application fields is met. The total reaction yield of the application is significantly improved, the diazotization temperature control is optimized, the coupling solvent system is improved, the selective reduction of sulfide is realized, and the esterification conditions are accurately controlled, so that the yield of each step is effectively guaranteed, and an economically feasible technical scheme is provided for industrialized scale production.

Description

Technical Field

[0001] This invention belongs to the field of ultraviolet absorber technology, specifically relating to an ultraviolet absorber UV-323 and its preparation method. Background Technology

[0002] Benzotriazole UV absorbers are important light stabilizers. Since their introduction to the market in the 1960s, they have been widely used in the fields of plastics, coatings, and synthetic fibers. Their action is based on the intramolecular proton transfer mechanism in the excited state, enabling them to efficiently and selectively absorb harmful ultraviolet light in the 270-380nm range and convert it into harmless heat energy, while absorbing almost no visible light. Therefore, they are particularly suitable for colorless, transparent, and light-colored polymer products. Furthermore, due to their excellent compatibility with polymers such as PE, PVC, PP, PS, PC, ABS, and epoxy resins, and their non-blooming and non-exudation properties, they are widely used in automotive coatings, industrial coatings, food and pharmaceutical packaging, plastic containers, and polypropylene fibers. They are often used synergistically with hindered amine light stabilizers to enhance anti-aging effects.

[0003] UV-323, a benzotriazole absorber that has emerged in the last decade, exhibits strong absorption peaks in both UVB and UVA bands due to its reactivity, making it valuable for applications in high-end printing inks and photovoltaic films. Currently, the preparation of UV-323 faces several technical bottlenecks, hindering further improvements in product quality and the greening of industrial production. Its mainstream synthetic route follows a three-step process of diazotization-coupling-reduction, using o-nitroaniline and p-hydroxyphenylethanol as core raw materials, undergoing a series of reactions to ultimately close the ring and generate the target product. Although this process is relatively mature, several challenges remain in specific steps. In the traditional preparation process of UV-323, the diazotization reaction is extremely temperature-sensitive and is a strongly exothermic process. o-Nitroaniline reacts with sodium nitrite under strongly acidic conditions at 0-5°C to form an unstable diazonium salt. Even slightly elevated local temperatures can easily trigger diazonium salt decomposition and side reactions, reducing yield and posing safety hazards. The subsequent coupling reaction must be carried out in a weakly alkaline medium. Due to the limited water solubility of p-hydroxyphenylethanol, large amounts of organic solvents (such as methanol and ethanol) must be used. Furthermore, slight misjudgment of pH can lead to incomplete coupling or increased byproducts, increasing production costs and environmental burden, and making it difficult to guarantee the purity of the intermediate. The azo intermediate undergoes reduction and ring closure to form benzotriazole. In structural analysis, existing reduction methods generally suffer from insufficient selectivity. Zinc powder reduction generates a large amount of zinc-containing waste residue, polluting the environment. Hydrazine hydrate reduction is highly toxic and carries significant operational risks. While catalytic hydrogenation is relatively environmentally friendly, it has stringent requirements for catalyst activity and reaction selectivity, often resulting in over-reduction or insufficient reduction, directly limiting product purity and yield. Furthermore, the crystallization and purification stages often employ simple cooling crystallization, lacking precise control over crystal morphology and particle size distribution. This leads to poor dispersibility and insufficient thermal stability of the product in the polymer matrix. Moreover, as a small molecule additive, it exhibits certain volatility and migration limitations under high-temperature processing conditions, further affecting its application in high-performance materials.

[0004] To address the aforementioned issues, there is an urgent need to provide a safe and efficient method for preparing UV-323, an ultraviolet absorber. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides an ultraviolet absorber, UV-323, and its preparation method, overcoming the drawbacks of complex processes, high pollution, and high costs, while exhibiting outstanding advantages such as high raw material conversion rate and excellent product quality.

[0006] The specific technical solution is as follows: The first objective of this invention is to provide a method for preparing ultraviolet absorber UV-323, comprising the following steps: (1) Diazotization reaction: o-nitroaniline is dissolved in concentrated hydrochloric acid, heated to 60-70℃ to dissolve, deionized water is added, then the temperature is lowered to 0-5℃, and sodium nitrite solution is slowly added. The temperature is controlled at 0-5℃ to react and obtain a diazonium salt solution. (2) Coupling reaction: Dissolve p-hydroxyphenylethanol in a mixed solution of methanol and water, then add caustic soda and potassium carbonate, continue to cool down, slowly add the diazonium salt solution obtained in step (1), and after the addition is complete, slowly raise the temperature to room temperature to react and obtain the azo compound; (3) Reduction of sulfide dispersion: Dissolve the azo compound obtained in step (2) in a mixed solvent of ethanol and water, then add the sulfide-supported dispersion reducing agent and caustic soda flakes, heat up, and after the reaction is completed, slowly cool to 0°C. At this temperature, slowly add 36% (mass fraction) concentrated hydrochloric acid to adjust the pH to 1-2, and precipitate the solid. Filter and dry the solid to obtain the reduction product. (4) Esterification reaction: Under a nitrogen atmosphere, the reduction product obtained in step (3) was dissolved in toluene, heated and stirred until completely dissolved, then methanesulfonic acid and methacrylic acid were added, the temperature was slowly raised to initiate reflux, and after the reaction was completed, the mixture was slowly cooled to room temperature to obtain a crude reaction mixture. (5) Crystallization: The crude reaction mixture obtained in step (4) was extracted twice with sodium hydroxide solution, then washed twice with deionized water, and toluene was recovered by rotary evaporation. The mixture was then recrystallized with dichloromethane and ethanol, filtered and dried to obtain the white powdered target product UV absorber UV-323.

[0007] The specific reaction route is as follows: .

[0008] Furthermore, in step (1), the mass fraction of the concentrated hydrochloric acid is 36%, and the amount of concentrated hydrochloric acid used is 35-60 parts by weight, the amount of o-nitroaniline used is 10-20 parts, the amount of deionized water used is 20-30 parts, and the amount of sodium nitrite used is 8-15 parts.

[0009] Furthermore, in step (2), by weight, the amount of diazonium salt solution obtained in step (1) is 95-160 parts, the amount of methanol is 50-60 parts, the amount of p-hydroxyphenylethanol is 10-20 parts, the amount of caustic soda is 5-20 parts, the amount of potassium carbonate is 20-30 parts, and the water is deionized water, with the amount of deionized water being 60-90 parts.

[0010] Further, in step (3), by weight, the amount of the azo compound obtained in step (2) is 20-25 parts, the amount of the sulfide-supported dispersion reducing agent is 30-75 parts, and the amount of caustic soda is 20-45 parts; the ethanol is anhydrous ethanol, and the amount of anhydrous ethanol is 150-200 parts; the water is deionized water, and the amount of deionized water is 100-150 parts.

[0011] Furthermore, in step (4), the ratio of the amount of the reduction product, methacrylic acid, toluene, and methanesulfonic acid obtained in step (3) is 10-15 g: 5-20 ml: 100-300 ml: 2-4 ml.

[0012] Furthermore, in step (5), the equivalent concentration of the sodium hydroxide solution is 2N, the ethanol is anhydrous ethanol, and the ratio of the crude reaction mixture, sodium hydroxide solution, dichloromethane and anhydrous ethanol obtained in step (4) is 2.2-3.2 g: 9-15 g: 3-5 ml: 10-20 ml.

[0013] Furthermore, in step (1), after the diazotization reaction is completed, the reaction time at 0-5℃ is 1-2 hours.

[0014] Furthermore, in step (2), after adding caustic soda and potassium carbonate, the temperature is further reduced to 0-5℃, and the diazonium salt solution obtained in step (1) is added dropwise at 0-5℃. The reaction time at room temperature is 1-4 hours.

[0015] Furthermore, in step (3), after adding the sulfide-supported dispersion reducing agent and caustic soda flakes, the temperature is raised to 70-90℃ and the reaction time is 1-4 hours.

[0016] Furthermore, in step (4), the temperature is raised to 50-60℃ and stirred until completely dissolved, and the temperature is raised to 130-170℃ to initiate reflux. The reaction time is 3-12 hours.

[0017] Furthermore, in step (5), the drying temperature is 80-90℃.

[0018] Further, in step (3), the preparation method of the sulfide-supported dispersion reducing agent includes the following steps: using industrially generated sodium sulfide, sodium dithionite, sodium thiosulfate or sodium metabisulfite as sulfur-containing compound raw materials, and using silica gel, molecular sieve, diatomaceous earth, neutral alumina or basic alumina as dispersion carriers, the sulfur-containing compound raw materials are suspended in a solvent under stirring, and then the dispersion carrier is added, heated and stirred in an air atmosphere, and after the reaction is completed, filtered and dried to obtain the sulfide-supported dispersion reducing agent.

[0019] Furthermore, the mass ratio of the sulfur-containing compound raw material to the dispersion carrier is (1-10):1.

[0020] Furthermore, the mass ratio of sulfur-containing compound raw material to dispersion carrier is (1-5):1.

[0021] Furthermore, the mass ratio of the sulfur-containing compound raw material to the solvent is (0.1-1):1.

[0022] Furthermore, the mass ratio of sulfur-containing compound raw material to solvent is (0.6-1):1.

[0023] Furthermore, the mass ratio of sulfur-containing compound raw material to solvent is (0.8-1):1.

[0024] Furthermore, the silica gel has a mesh size of 300-500, and the molecular sieve is a 5A molecular sieve.

[0025] Furthermore, sodium thiosulfate is preferred as the sulfur-containing compound raw material, and neutral alumina is preferred as the dispersion carrier.

[0026] Furthermore, the solvent is selected from one or more of methanol, ethanol, and water.

[0027] Furthermore, the heating and stirring time is 1.5-3 hours, and the temperature is 40-60℃.

[0028] Furthermore, the drying time is 20-26 hours, and the temperature is 50-70℃.

[0029] A second objective of this invention is to provide an ultraviolet absorber, UV-323, obtained by the preparation method described above.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention optimizes the temperature gradient control of the diazotization reaction, significantly improving the stability and purity of the diazonium salt. Traditional processes typically employ a single low-temperature condition for diazotization, while this invention innovatively introduces a two-stage temperature control strategy of "heating and dissolving, followed by low-temperature reaction": first, o-nitroaniline is dissolved in 36% (mass fraction) concentrated hydrochloric acid, heated to 60-70℃ to fully dissolve and form a homogeneous slurry, and then cooled to 0-5℃ before slowly adding sodium nitrite solution for diazotization. This strategy stems from a deep understanding of the dissolution characteristics of o-nitroaniline and the diazotization reaction mechanism—the full dissolution of o-nitroaniline in concentrated hydrochloric acid ensures the homogeneous and efficient conduct of the subsequent diazotization reaction, while the low-temperature condition of 0-5℃ effectively inhibits the decomposition of the diazonium salt and the occurrence of side reactions.

[0031] (2) This invention innovatively uses a methanol-water mixed solvent system in the coupling reaction and introduces a combined alkaline control strategy of caustic soda and potassium carbonate. Traditional coupling reactions often use a large amount of a single organic solvent, which is not only costly but also puts pressure on the environment. Based on a systematic analysis of the polarity of the reaction system and the solubility parameters of p-hydroxyphenylethanol, this invention selects a mixed solvent of methanol and water as the coupling medium, which not only ensures the full dissolution and dispersion of p-hydroxyphenylethanol but also significantly reduces the amount of pure organic solvent used, reflecting the concept of green chemistry. At the same time, the synergistic use of caustic soda and potassium carbonate to provide an alkaline environment, compared with a single strong base system, the composite alkaline agent can achieve a milder and more stable pH control, effectively avoiding incomplete coupling or side reactions caused by drastic pH fluctuations, and improving the purity and yield of the azo intermediate.

[0032] (3) This invention innovatively employs a sulfide-supported dispersion reducing agent to achieve a highly efficient and environmentally friendly closed-loop reduction of azo intermediates. While there are many traditional reduction methods for benzotriazole UV absorbers, each has serious drawbacks: the zinc powder method generates a large amount of zinc-containing waste residue, the hydrazine hydrate method is highly toxic, and the catalytic hydrogenation method has difficulty controlling selectivity. This invention uses a sulfide-supported dispersion reducing agent to carry out the reduction reaction in an ethanol-water mixed solvent system at 70-80℃. The sulfide reduction method has the advantages of good reduction selectivity and mild reaction conditions, effectively avoiding the problems of over-reduction or insufficient reduction that often occur in one-step reduction. Studies have shown that the sulfide-supported dispersion reducing agent can successfully reduce azo compounds to the target benzotriazole molecule in an ethanol-water medium containing sodium hydroxide, and the product can be precipitated by adjusting the pH with concentrated hydrochloric acid after the reaction, making post-treatment simple. This reduction process avoids the large-scale use of zinc powder, significantly reduces the emission of zinc-containing waste residue, and fully embodies the concept of green chemistry.

[0033] (4) This invention designs an optimized purification and crystallization process, achieving high-purity refining of the product through solvent recovery and gradient recrystallization. After the esterification reaction, this invention uses 2N sodium hydroxide solution and water for sequential extraction and washing to remove unreacted methacrylic acid and the catalyst methanesulfonic acid. Subsequently, toluene solvent is recovered by rotary evaporation, which reduces production costs and volatile organic compound emissions. Finally, recrystallization is performed using dichloromethane and anhydrous ethanol. Utilizing the difference in solubility of the product and impurities in the two solvents, reaction residues and colored impurities are effectively removed, yielding a high-purity white powder target product. This crystallization process stems from a precise understanding of the product's solubility characteristics. By controlling the appropriate solvent ratio and crystallization conditions, the regularity of the product crystals and batch stability are ensured.

[0034] (5) The purity of the product of the present invention is significantly improved. Through the optimized process route and solvent recovery-recrystallization refining, the purity of the product can reach more than 99%, and the appearance is white powder, which meets the quality requirements of high-end application fields.

[0035] (6) The overall reaction yield of the present invention is significantly improved. Through the optimization of diazotization temperature control, the improvement of coupling solvent system, the selective reduction of sulfide and the precise control of esterification conditions, the yield of each step is effectively guaranteed, providing an economical and feasible technical solution for industrial-scale production.

[0036] (7) The present invention uses a sulfide dispersion reduction process to replace the traditional zinc powder method, which effectively avoids the discharge of zinc-containing waste residue. Combined with the use of methanol-water mixed solvent system and the recycling of toluene, the overall environmental friendliness of the process is fundamentally improved, which is in line with the development trend of green chemical industry. Attached Figure Description

[0037] Figure 1 The azo compound intermediate in this invention 1 H-NMR spectrum; Figure 2 The reduction product (benzotriazole compound intermediate) in this invention 1 H-NMR spectrum; Figure 3 UV-323 in this invention 1 H-NMR spectrum; Figure 4 This is the HPLC chromatogram of UV-323 in this invention. Detailed Implementation

[0038] The principles and features of the present invention are described below with reference to examples. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0039] Preparation Example A method for preparing a sulfide-supported dispersion reducing agent includes the following steps: Using industrially produced sodium thiosulfate as a sulfur-containing compound raw material and neutral alumina as a dispersion carrier, 90 g of sodium thiosulfate was suspended in a mixed solvent of 60 g of ethanol and 40 g of water under stirring. Then, 30 g of neutral alumina was added, and the mixture was heated and stirred at 50 °C in air for 2 hours. After the reaction was completed, the mixture was filtered and dried at 60 °C in a vacuum oven for 24 hours to obtain a sulfide-supported dispersion reducing agent. The mass ratio of sulfur-containing compound raw material to dispersion carrier is 3:1, and the mass ratio of sulfur-containing compound raw material to mixed solvent is 9:10.

[0040] Example 1 A method for preparing ultraviolet absorber UV-323 includes the following steps: (1) Diazotization reaction: First, 10 g of o-nitroaniline was added to 35 g of 36% (mass fraction) concentrated hydrochloric acid, and stirring was started. The temperature was raised to 60°C and kept at this temperature for 15 minutes to ensure complete dissolution of the raw materials. Then, 20 g of deionized water was added to the reaction system, and the reaction solution was cooled to 0°C. Next, a pre-prepared sodium nitrite solution (8 g of sodium nitrite dissolved in 21 g of water) was slowly added dropwise. After the addition was completed, the reaction temperature was controlled at 0°C, and the reaction was continued for 1 hour to obtain a yellow-brown transparent diazonium salt solution.

[0041] (2) Coupling reaction: Add 10 g of p-hydroxyphenylethanol to the flask, start stirring, then add 50 g of methanol and 60 g of deionized water, stir at room temperature until completely dissolved, then add 5 g of caustic soda and 20 g of potassium carbonate, stir thoroughly until completely dissolved, ensuring that all materials are completely dissolved. Then, cool the reaction solution to 0°C, and then slowly add 95 g of the diazonium salt solution obtained in step (1) at 0°C for about 1 hour. After the addition is complete, slowly raise the temperature to about 25°C and continue the reaction for 4 hours. Filter and wash several times with deionized water, and after drying, a dark red solid azo compound is obtained.

[0042] (3) Reduction of sulfide dispersion: 20 g of the azo compound obtained in step (2) was added to the flask and dissolved in a mixed solvent of 150 g of anhydrous ethanol and 100 g of deionized water. Then, 30 g of sulfide-supported dispersion reducing agent (prepared in the preparation example) and 20 g of caustic soda were added. After stirring, the reaction system was slowly heated to 80°C. The reaction was carried out at this temperature for 2 hours. The solution gradually changed from dark red to yellow. After the reaction was completed, it was slowly cooled to 0°C. At this temperature, 36% (mass fraction) concentrated hydrochloric acid was slowly added dropwise to adjust the pH to 1. A large amount of product was precipitated. Stirring was continued at 0°C for 20 minutes to ensure that the product was completely precipitated. The product was filtered and dried to obtain the reduced product.

[0043] (4) Esterification reaction: Under a nitrogen atmosphere, 10 g of the reduction product obtained in step (3) was added to the reaction flask, followed by 200 mL of toluene. The temperature was slowly raised to 50 °C and stirred until completely dissolved. Then, 2 mL of methanesulfonic acid and 5 mL of methacrylic acid were added, and the mixture was slowly heated to 130 °C to initiate reflux. The reaction was carried out at this temperature for 12 h. After the reaction was completed, the mixture was slowly cooled to room temperature to obtain the crude reaction mixture.

[0044] (5) Crystallization: First, 22 g of the crude reaction mixture obtained in step (4) was extracted twice with 90 g of 2N sodium hydroxide solution, then washed twice with deionized water, and toluene was recovered by rotary evaporation. Subsequently, it was recrystallized three times with 30 mL of dichloromethane and 100 mL of anhydrous ethanol. After filtration, a white powdery solid was obtained and dried at 80 °C to obtain the target product UV-323.

[0045] This embodiment effectively suppresses the decomposition of diazonium salts and side reactions by controlling the temperature gradient of the diazotization reaction (60℃→0℃). In the coupling reaction, the methanol-water mixed solvent system ensures the complete dissolution and dispersion of p-hydroxyphenylethanol while significantly reducing the amount of pure organic solvent used, embodying the concept of green chemistry. Simultaneously, the use of caustic soda and potassium carbonate to synergistically provide an alkaline environment effectively avoids incomplete coupling or side reactions caused by drastic pH fluctuations, improving the purity and yield of the azo intermediate. In the reduction step, an innovative use of a sulfide-supported dispersion reducing agent achieves a highly efficient and environmentally friendly closed-loop reduction of the azo intermediate, significantly reducing emissions and fully demonstrating the concept of green chemistry.

[0046] Example 2 A method for preparing ultraviolet absorber UV-323 includes the following steps: (1) Diazotization reaction: First, 12 g of o-nitroaniline was added to 41 g of 36% (mass fraction) concentrated hydrochloric acid, and stirring was started. The temperature was raised to 70°C and kept at this temperature for 15 minutes to ensure complete dissolution of the raw materials. Then, 23 g of deionized water was added to the reaction system, and the reaction solution was cooled to 0°C. Next, a pre-prepared sodium nitrite solution (9 g of sodium nitrite dissolved in 24 g of water) was slowly added dropwise. After the addition was complete, the reaction temperature was controlled at 0°C, and the reaction was continued for 1.5 hours to obtain a yellow-brown transparent diazonium salt solution.

[0047] (2) Coupling reaction: Add 11 g of p-hydroxyphenylethanol to the flask, start stirring, then add 55 g of methanol and 67 g of deionized water, stir at room temperature until completely dissolved, then add 7 g of caustic soda and 22 g of potassium carbonate, stir thoroughly until completely dissolved, ensuring that all materials are completely dissolved. Then, cool the reaction solution to 3°C, and then slowly add 109 g of the diazonium salt solution obtained in step (1) at 3°C ​​for about 1 hour. After the addition is complete, slowly raise the temperature to about 25°C and continue the reaction for 4 hours. Filter and wash several times with deionized water, and after drying, a dark red solid azo compound is obtained.

[0048] (3) Reduction of sulfide dispersion: Add 21 g of the azo compound obtained in step (2) to a round-bottom flask and dissolve it in a mixed solvent of 159 g of anhydrous ethanol and 110 g of deionized water. Then add 40 g of sulfide-supported dispersion reducing agent (prepared in the preparation example) and 25 g of caustic soda flakes. After stirring, slowly heat the reaction system to 80°C. React at this temperature for 3 hours. The solution gradually changes from dark red to yellow. After the reaction is completed, slowly cool to 0°C. At this temperature, slowly add 36% (mass fraction) concentrated hydrochloric acid to adjust the pH to 1. A large amount of product precipitates out. Continue stirring at 0°C for 20 minutes to ensure that the product is completely precipitated. Filter and dry to obtain the reduced product.

[0049] (4) Esterification reaction: Under a nitrogen atmosphere, 11 g of the reduction product obtained in step (3) was added to the reaction flask, followed by 180 mL of toluene. The temperature was slowly raised to 50 °C and stirred until completely dissolved. Then, 2.5 mL of methanesulfonic acid and 7 mL of methacrylic acid were added, and the mixture was slowly heated to 150 °C to initiate reflux. The reaction was carried out at this temperature for 10 h. After the reaction was completed, the mixture was slowly cooled to room temperature to obtain the crude reaction mixture.

[0050] (5) Crystallization: First, 26 g of the crude reaction mixture obtained in step (4) was extracted twice with 90 g of 2N sodium hydroxide solution, then washed twice with deionized water, and toluene was recovered by rotary evaporation. Subsequently, it was recrystallized three times with 30 mL of dichloromethane and 130 mL of anhydrous ethanol. After filtration, a white powdery solid was obtained and dried at 80 °C to obtain the target product UV-323.

[0051] In this embodiment, by adjusting the amount of concentrated hydrochloric acid, increasing the dissolution temperature, and extending the reaction time, complete dissolution and conversion of o-nitroaniline can be ensured, thereby improving the efficiency of subsequent reactions. In the coupling reaction, increasing the coupling reaction temperature and adjusting the amount of alkali helps to accelerate the reaction rate. In the reduction step, adjusting the amount of reducing agent and the reaction time allows for better control of the reduction degree and optimization of product quality. In the esterification reaction, adjusting the reaction temperature accelerates the reaction rate and reduces the overall reaction time.

[0052] Example 3 A method for preparing ultraviolet absorber UV-323 includes the following steps: (1) Diazotization reaction: First, 14 g of o-nitroaniline was added to 48 g of 36% (mass fraction) concentrated hydrochloric acid, and stirring was started. The temperature was raised to 70°C and kept at this temperature for 15 minutes to ensure complete dissolution of the raw materials. Then, 23 g of deionized water was added to the reaction system, and the reaction solution was cooled to 5°C. Next, a pre-prepared sodium nitrite solution (11 g of sodium nitrite dissolved in 29 g of water) was slowly added dropwise. After the addition was complete, the reaction temperature was controlled at 5°C, and the reaction was continued for 2 hours to obtain a yellow-brown transparent diazonium salt solution.

[0053] (2) Coupling reaction: Add 14 g of p-hydroxyphenylethanol to the flask, start stirring, then add 55 g of methanol and 73 g of deionized water, stir at room temperature until completely dissolved, then add 7 g of caustic soda and 23 g of potassium carbonate, stir thoroughly until completely dissolved, ensuring that all materials are completely dissolved. Then, cool the reaction solution to 5°C, and then slowly add 125 g of the diazonium salt solution obtained in step (1) at 5°C for about 1.5 hours. After the addition is complete, slowly raise the temperature to about 25°C and continue the reaction for 3 hours. Filter and wash several times with deionized water, and after drying, a dark red solid azo compound is obtained.

[0054] (3) Reduction of sulfide dispersion: Add 22 g of the azo compound obtained in step (2) to a round-bottom flask and dissolve it in a mixed solvent of 170 g of anhydrous ethanol and 120 g of deionized water. Then add 48 g of sulfide-supported dispersion reducing agent (prepared in the preparation example) and 30 g of caustic soda flakes. After stirring, slowly heat the reaction system to 70°C. React at this temperature for 4 hours. The solution gradually changes from dark red to yellow. After the reaction is completed, slowly cool to 0°C. At this temperature, slowly add 36% (mass fraction) concentrated hydrochloric acid to adjust the pH to 1. A large amount of product precipitates out. Continue stirring at 0°C for 20 minutes to ensure that the product is completely precipitated. Filter and dry to obtain the reduced product.

[0055] (4) Esterification reaction: Under a nitrogen atmosphere, 13 g of the reduction product obtained in step (3) was added to the reaction flask, followed by 180 mL of toluene. The temperature was slowly raised to 60 °C and stirred until completely dissolved. Then, 3 mL of methanesulfonic acid and 10 mL of methacrylic acid were added, and the mixture was slowly heated to 130 °C to initiate reflux. The reaction was carried out at this temperature for 12 h. After the reaction was completed, the mixture was slowly cooled to room temperature to obtain the crude reaction mixture.

[0056] (5) Crystallization: First, 28 g of the crude reaction mixture obtained in step (4) was extracted twice with 90 g of 2N sodium hydroxide solution, then washed twice with deionized water, and toluene was recovered by rotary evaporation. Subsequently, it was recrystallized three times with 38 mL of dichloromethane and 140 mL of anhydrous ethanol. After filtration, a white powdery solid was obtained and dried at 80 °C to obtain the target product UV-323.

[0057] In this embodiment, by adjusting the amount of concentrated hydrochloric acid, increasing the diazotization reaction temperature, and extending the dropping time of the diazonium salt solution, the forward diazotization coupling reaction is ensured. By fine-tuning the reaction temperature and time at each step, energy consumption and production efficiency can be optimized while ensuring complete reaction. Particularly in the reduction step, precise control of the reducing agent dosage and reaction conditions allows for a better balance between reaction rate and selectivity, thereby improving product quality and yield.

[0058] Example 4 A method for preparing ultraviolet absorber UV-323 includes the following steps: (1) Diazotization reaction: First, 18 g of o-nitroaniline was added to 54 g of 36% (mass fraction) concentrated hydrochloric acid, and stirring was started. The temperature was raised to 70°C and kept at this temperature for 15 minutes to ensure complete dissolution of the raw materials. Then, 28 g of deionized water was added to the reaction system, and the reaction solution was cooled to 0°C. Next, a pre-prepared sodium nitrite solution (13 g of sodium nitrite dissolved in 35 g of water) was slowly added dropwise. After the addition was complete, the reaction temperature was controlled at 0°C, and the reaction was continued for 1 hour to obtain a yellow-brown transparent diazonium salt solution.

[0059] (2) Coupling reaction: Add 20 g of p-hydroxyphenylethanol to the flask, start stirring, then add 57 g of methanol and 85 g of deionized water, stir at room temperature until completely dissolved, then add 17 g of caustic soda and 28 g of potassium carbonate, stir thoroughly until completely dissolved, ensuring that all materials are completely dissolved. Then, cool the reaction solution to 0°C, and then slowly add 120 g of the diazonium salt solution obtained in step (1) at 0°C for about 1 hour. After the addition is complete, slowly raise the temperature to about 25°C and continue the reaction for 4 hours. Filter and wash several times with deionized water, and after drying, obtain a dark red solid azo compound.

[0060] (3) Reduction of sulfide dispersion: Add 24 g of the azo compound obtained in step (2) to a round-bottom flask and dissolve it in a mixed solvent of 180 g of anhydrous ethanol and 140 g of deionized water. Then add 70 g of sulfide-supported dispersion reducing agent (prepared in the preparation example) and 43 g of caustic soda flakes. After stirring, slowly heat the reaction system to 90°C. React at this temperature for 1 hour. The solution gradually changes from dark red to yellow. After the reaction is completed, slowly cool to 0°C. At this temperature, slowly add 36% (mass fraction) concentrated hydrochloric acid to adjust the pH to 1. A large amount of product precipitates out. Continue stirring at 0°C for 20 minutes to ensure that the product is completely precipitated. Filter and dry to obtain the reduced product.

[0061] (4) Esterification reaction: Under a nitrogen atmosphere, 14 g of the reduction product obtained in step (3) was added to the reaction flask, followed by 100 mL of toluene. The temperature was slowly raised to 60 °C and stirred until completely dissolved. Then, 3.5 mL of methanesulfonic acid and 17 mL of methacrylic acid were added, and the mixture was slowly heated to 170 °C to initiate reflux. The reaction was carried out at this temperature for 3 h. After the reaction was completed, the mixture was slowly cooled to room temperature to obtain the crude reaction mixture.

[0062] (5) Crystallization: First, 24 g of the crude reaction mixture obtained in step (4) was extracted twice with 150 g of 2N sodium hydroxide solution, then washed twice with deionized water, and toluene was recovered by rotary evaporation. Subsequently, it was recrystallized three times with 45 mL of dichloromethane and 180 mL of anhydrous ethanol. After filtration, a white powdery solid was obtained and dried at 90 °C to obtain the target product UV-323.

[0063] In this embodiment, by using a higher concentration of diazonium salt aqueous solution and a larger esterification reaction temperature gradient, the reaction rate can be accelerated and production efficiency improved. Simultaneously, by increasing the amount of reducing agent and shortening the reduction reaction time, the completeness of the reaction can be ensured, thereby improving product purity. Furthermore, reducing the amount of toluene and increasing the concentration of the reaction solution during esterification helps to accelerate the reaction and further improve product quality.

[0064] Example 5 A method for preparing ultraviolet absorber UV-323 includes the following steps: (1) Diazotization reaction: First, 20 g of o-nitroaniline was added to 60 g of 36% (mass fraction) concentrated hydrochloric acid, and stirring was started. The temperature was raised to 70°C and kept at this temperature for 15 minutes to ensure complete dissolution of the raw materials. Then, 30 g of deionized water was added to the reaction system, and the reaction solution was cooled to 0°C. Next, a pre-prepared sodium nitrite solution (15 g of sodium nitrite dissolved in 40 g of water) was slowly added dropwise. After the addition was completed, the reaction temperature was controlled at 0°C, and the reaction was continued for 1 hour to obtain a yellow-brown transparent diazonium salt solution.

[0065] (2) Coupling reaction: Add 20 g of p-hydroxyphenylethanol to the flask, start stirring, then add 60 g of methanol and 90 g of deionized water, stir at room temperature until completely dissolved, then add 20 g of caustic soda and 30 g of potassium carbonate, stir thoroughly until completely dissolved, ensuring that all materials are completely dissolved. Then, cool the reaction solution to 0°C, and then slowly add 160 g of the diazonium salt solution obtained in step (1) at 0°C for about 1 hour. After the addition is complete, slowly raise the temperature to about 25°C and continue the reaction for 1 hour. Filter and wash several times with deionized water, and after drying, obtain a dark red solid azo compound.

[0066] (3) Reduction of sulfide dispersion: Add 25 g of the azo compound obtained in step (2) to the reaction flask and dissolve it in a mixed solvent of 200 g of anhydrous ethanol and 150 g of deionized water. Then add 75 g of sulfide-supported dispersion reducing agent (prepared in the preparation example) and 45 g of caustic soda flakes. After stirring, slowly heat the reaction system to 90°C. React at this temperature for 1 hour. The solution gradually changes from dark red to yellow. After the reaction is completed, slowly cool to 0°C. At this temperature, slowly add 36% (mass fraction) concentrated hydrochloric acid to adjust the pH to 1. A large amount of product precipitates out. Continue stirring at 0°C for 20 minutes to ensure that the product is completely precipitated. Filter and dry to obtain the reduced product.

[0067] (4) Esterification reaction: Under a nitrogen atmosphere, 15 g of the reduction product obtained in step (3) was added to the reaction flask, followed by 300 mL of toluene. The temperature was slowly raised to 50 °C and stirred until completely dissolved. Then, 4 mL of methanesulfonic acid and 20 mL of methacrylic acid were added. The mixture was slowly heated to 170 °C to initiate reflux. The reaction was carried out at this temperature for 3 h. After the reaction was completed, the mixture was slowly cooled to room temperature to obtain the crude reaction mixture.

[0068] (5) Crystallization: First, 32 g of the crude reaction mixture obtained in step (4) was extracted twice with 90 g of 2N sodium hydroxide solution, then washed twice with deionized water, and toluene was recovered by rotary evaporation. Subsequently, it was recrystallized three times with 50 mL of dichloromethane and 200 mL of anhydrous ethanol. After filtration, a white powdery solid was obtained and dried at 80 °C to obtain the target product UV-323.

[0069] In this embodiment, the reaction rate is accelerated by adjusting the amount of concentrated hydrochloric acid and the proportion of alkali in the coupling reaction. Simultaneously, the reduction reaction time is reduced by significantly increasing the amount of reducing agent and raising the reduction reaction temperature, thereby improving the reaction rate and product purity. Furthermore, adjusting the proportion of reactants during esterification increases the intermolecular collision rate, and raising the esterification reaction temperature further accelerates the reaction and improves product quality.

[0070] In each embodiment, the azo compound intermediate obtained in step (2) 1 H-NMR spectra, such as Figure 1 As shown, the specific data is as follows: 1 H NMR (500 MHz, CDCl3) δ 12.23 (s, 1H), 8.08 (dd, J = 8.2, 1.4 Hz, 1H), 7.95 (dd, J = 8.1, 1.4 Hz, 1H), 7.83 (d, J = 2.3 Hz, 1H), 7.74 (td, J =7.8, 1.4 Hz, 1H), 7.60 (ddd, J = 8.5, 7.4, 1.4 Hz, 1H), 7.30 (dd, J = 8.5, 2.3Hz, 1H), 7.02 (d, J = 8.4 Hz, 1H), 3.92 (t, J = 6.5 Hz, 2H), 2.92 (t, J = 6.5Hz, 2H). The reduction product (benzotriazole compound intermediate) obtained in step (3) 1 H-NMR spectra, such as Figure 2 As shown, the specific data is as follows: 1 H NMR (500 MHz, CDCl3) δ 11.21 (s, 1H), 8.28 (d, J = 2.3 Hz, 1H), 7.93(dt, J = 6.1, 3.1 Hz, 2H), 7.48 (dq, J = 5.9, 2.9 Hz, 2H), 7.22 (dd, J = 8.4, 2.1 Hz, 1H), 7.14 (d, J= 8.4 Hz, 1H), 3.96 – 3.91 (m, 2H), 2.93 (t, J = 6.5Hz, 2H). The UV-323 obtained in step (5) 1 H-NMR spectra, such as Figure 3 As shown, the specific data is as follows: 1 H NMR (500 MHz, CDCl3) δ 11.22 (s, 1H), 8.31 (d, J = 2.2 Hz, 1H), 7.93(dd, J = 6.5, 3.1 Hz, 2H), 7.48 (dd, J = 6.6, 3.1 Hz, 2H), 7.22 (dd, J = 8.4, 2.2 Hz, 1H), 7.14 (d, J = 8.4 Hz, 1H), 6.12 (s, 1H), 5.55 (s, 1H), 4.40 (t, J =6.8 Hz, 2H), 3.04 (t, J = 6.8 Hz, 2H), 1.94 (s, 3H). The HPLC chromatogram of UV-323 obtained in step (5) is as follows: Figure 4 As shown in Table 1, the analysis results are as follows.

[0071] Table 1 Analysis Results

[0072] Comparative Example 1 A method for preparing ultraviolet absorber UV-323, referring to Example 1, differs from Example 1 in that step (1) diazotization reaction: 10 g of o-nitroaniline is added to 35 g of 36% (mass fraction) concentrated hydrochloric acid, stirring is started, and the reaction is carried out directly at room temperature (about 25°C), while a pre-prepared sodium nitrite solution (8 g of sodium nitrite dissolved in 21 g of water) is slowly added dropwise. After the addition is complete, the reaction continues for 1 hour.

[0073] (2) Coupling reaction - (5) Crystallization: Same as in Example 1.

[0074] This comparative example aims to verify the importance of temperature control in the diazotization reaction. In this example, the failure to effectively control the diazotization reaction temperature led to instability and partial decomposition of the diazonium salt. This not only reduced the yield of the diazonium salt but also affected the effectiveness of subsequent coupling reactions. The yield and purity of the final product were both lower than those of Example 1, fully demonstrating the importance of precise temperature control for improving reaction selectivity and product quality.

[0075] Comparative Example 2 A method for preparing ultraviolet absorber UV-323, referring to Example 2, differs from Example 2 in that, in step (2) coupling reaction, 11 g of p-hydroxyphenylethanol is dissolved in 122 g of methanol (instead of a mixed solvent of water and methanol).

[0076] (1) Diazotization reaction, (3) Sulfide dispersion reduction - (5) Crystallization: Same as in Example 2.

[0077] This comparative example aims to verify the importance of solvent selection in the coupling reaction. While methanol was used as the solvent in this example to simplify the reaction, the low solubility of potassium carbonate in methanol led to incomplete reaction. Furthermore, the generated azo compounds dissolved in large quantities in methanol, resulting in a reduced product yield. This result demonstrates the superiority of using a mixed solvent of methanol and water, which not only improves reaction efficiency but also facilitates the subsequent handling of azo compounds.

[0078] Comparative Example 3 A method for preparing ultraviolet absorber UV-323, referring to Example 3, differs from Example 3 in that step (3) zinc powder is reduced in one step: 22 g of the azo compound obtained in step (2) of Example 3 is added to the reaction flask, followed by 23 g of 300 mesh zinc powder, 21 g of caustic soda flakes and 250 g of anhydrous ethanol. The reaction is carried out at 80°C for 4 hours. After the reaction is completed, the mixture is slowly cooled to room temperature, and excess zinc powder is removed by filtration. The mixture is washed multiple times with 200 g of 5 wt% sodium hydroxide solution, and the filtrates are combined. The filtrate is slowly cooled to 0°C, and 36 wt% concentrated hydrochloric acid is slowly added to adjust the pH to 1. The mixture is filtered to obtain a filter cake, which is dried at 80°C for 8 hours to obtain the reduced product.

[0079] (1) Diazotization, (2) Coupling, (4) Esterification and (5) Crystallization: Same as in Example 3.

[0080] This comparative example aims to verify the superiority of one-step reduction using sulfide dispersions. While zinc powder reduction can also achieve one-step reduction, it suffers from low selectivity, generates amino and nitrogen oxide byproducts, and produces a large amount of zinc waste, which is inconsistent with the trend of green chemistry. In contrast, the reduction using sulfide-supported dispersion reducing agents exhibits high efficiency, high selectivity, and low pollution, demonstrating the superiority of using sulfide-supported dispersion reducing agents.

[0081] Comparative Example 4 A method for preparing ultraviolet absorber UV-323, referring to Example 3, differs from Example 3 in that step (3) involves a two-step reduction method using hydrazine hydrate and zinc powder: 3.1 Reduction of hydrazine hydrate: 22 g of the azo compound obtained in step (2) of Example 3 was added to the reaction flask, followed by 35 mL of toluene, 70 g of deionized water and 4 g of caustic soda. The mixture was slowly heated to 65 °C. At this temperature, 4.2 g of hydrazine hydrate with a mass fraction of 60% was slowly added dropwise. The mixture was then heated to 75 °C and stirred for 3 hours. After the reaction was completed, the upper toluene layer was separated and 35 mL of isopropanol and 15 g of water were added. The pH of the system was adjusted to 6 by slowly adding concentrated sulfuric acid with a mass fraction of 62.5%. The lower aqueous phase was removed and 18 mL of isopropanol and 25 g of water were added. The system was slowly cooled to 5 °C to allow crystals to precipitate. The mixture was filtered and dried at 80 °C for 8 hours to obtain the nitrogen oxide intermediate. 3.2 Zinc powder reduction: Add 18 g of the nitrogen oxide intermediate obtained in step 3.1 to the reaction flask, followed by 53 g of 300 mesh zinc powder, 35 mL of toluene, and 25 g of deionized water. Slowly raise the temperature to 70 °C, then slowly add 93 g of 98% concentrated sulfuric acid. Keep the reaction at the temperature for 2 hours. After the reaction is complete, separate the aqueous phase, add 35 mL of isopropanol, and slowly lower the temperature to 5 °C to precipitate crystals. Filter and dry at 80 °C for 8 hours to obtain the reduction product.

[0082] (1) Diazotization, (2) Coupling, (4) Esterification and (5) Crystallization: Same as in Example 3.

[0083] This comparative example aims to verify the superiority of one-step reduction of sulfide dispersions. While the two-step reduction method using hydrazine hydrate and zinc powder can yield the target product, it is cumbersome and complex, and the target yield is lower than that of the one-step reduction method. Furthermore, the high toxicity of hydrazine hydrate has prevented its industrial application, which is inconsistent with the trend of green chemistry. In contrast, the reduction using a sulfide-supported dispersion reducing agent is highly efficient, selective, and low-polluting, demonstrating its superiority.

[0084] Comparative Example 5 A method for preparing an ultraviolet absorber UV-323, referring to Example 3, differs from Example 3 in that step (3) sulfide reduction: 48 grams of sodium thiosulfate is used instead of 48 grams of sulfide-supported dispersion reducing agent.

[0085] (1) Diazotization, (2) Coupling, (4) Esterification and (5) Crystallization: Same as in Example 3.

[0086] This comparative example aims to verify the importance of the dispersion medium in the reducing agent of the sulfide-supported dispersion. In this comparative example, the sulfide will release heat violently when dissolved in a large amount of water, while the dispersion carrier can slowly release the sulfide, avoiding excessive local concentration that would lead to violent heat release, and effectively suppressing the occurrence of excessive reduction side reactions, thereby achieving the goal of highly selective synthesis of benzotriazole ultraviolet absorbers.

[0087] Comparative Example 6 A method for preparing an ultraviolet absorber UV-323, referring to Example 4, differs from Example 4 in that step (4) esterification reaction: 3.5 ml of p-toluenesulfonic acid is used instead of 3.5 ml of methanesulfonic acid.

[0088] (1) Diazotization reaction, (2) Coupling reaction, (3) Reduction of sulfide dispersion and (5) Crystallization: Same as in Example 4.

[0089] This comparative example aims to verify the importance of catalyst selection in the esterification reaction. In this example, p-toluenesulfonic acid exhibits significantly lower catalytic activity, typically requiring a larger dosage to achieve a similar esterification rate; it also has poor thermal stability, easily decomposing at 170°C and causing the product to darken in color; furthermore, p-toluenesulfonic acid is highly corrosive to equipment, and its wastewater treatment is complex and environmentally burdensome. This result demonstrates the crucial role of catalyst selection in the reaction.

[0090] Comparative Example 7 A method for preparing an ultraviolet absorber UV-323, referring to Example 1, differs from Example 1 in that step (5) crystallization: 130 ml of ethanol is used instead of 30 ml of dichloromethane and 100 ml of anhydrous ethanol for recrystallization.

[0091] (1) Diazotization reaction - (4) Esterification reaction: Same as in Example 1.

[0092] This comparative example aims to verify the importance of solvent selection during crystallization. In this example, using ethanol as the sole solvent resulted in larger product particles that were prone to containing small amounts of residual impurities. In contrast, the presence of a small amount of dichloromethane dissolves low-polarity impurities and locks them in the mother liquor, forming more regular crystals. This improves product purity, facilitates drying, and reduces solvent residue. This result validates the rationality of the present invention in its choice of crystallization solvent.

[0093] Performance testing experiments: The UV absorber UV-323 obtained in each embodiment and comparative example was tested using the following methods: 1. Product transmittance test Experimental method: The transmittance of UV-323 was measured using a visible spectrophotometer. Instrument model: 722N 2. Product purity test Experimental method: The purity of UV-323 was determined by high performance liquid chromatography (HPLC). Instrument model: LC5s Injector: 7725 (7725 series injection valve) Injection volume: 20 μL Quantitative loop volume: 20 μL Column: C18, 5μm, 4.6mm ID x 250mm Column temperature: 30℃ Detector: UVD (ultraviolet / visible light) Detection wavelength: 300.0nm Infusion pump: binary gradient system (10504010) Mobile phase: Methanol: Water = 100: 0 Flow rate: 1 mL / min 3. Product melting point test Experimental method: The melting range of UV-323 was determined using a digital melting point apparatus. Instrument Model: WPS-2 Melting point measurement range: room temperature - 300℃ Minimum temperature reading: 0.1℃ Linear heating rate: Eight settings are available: 0.2, 0.5, 1, 1.5, 4, and 5 (°C / min). Capillary dimensions: inner diameter 0.9-1.1 mm, wall thickness 0.1-0.15 mm, length approximately 120 mm.

[0094] The test results of each embodiment and comparative example are shown in Table 2.

[0095] Table 2 Test results of each embodiment and comparative example

[0096] By comparing the test results in Table 2, we can see that: 1. Light transmittance: The average light transmittance of all embodiments reached over 96%, with a maximum of 97.8%, while the average light transmittance of the comparative examples was only 88.2-92.3%. The embodiments improved by more than 4 percentage points compared with the optimal comparative example (comparative example 2), which confirms that the technical solution of the present invention has brought unexpected technical effects on light transmittance.

[0097] 2. Purity: The purity of the products in Examples 1-5 of this invention is all above 98%, significantly higher than that of the comparative examples. This is mainly due to the innovations of this invention in the control of diazotization reaction temperature, selection of coupling reaction solvent, and reduction strategy of sulfide dispersion.

[0098] 3. Melting Range: The melting range width of Examples 1-5 shows a significant narrowing trend, with Example 5 having a melting range of only 97-99℃ (width 2℃). In contrast, the melting range width of Comparative Examples 1-7 is generally larger, and the starting temperature of the comparative examples is as low as 88℃. The melting range of the examples is generally concentrated in a narrow high-temperature band, while the melting range of the comparative examples is widely distributed, with obvious low-temperature tailing or high-temperature overflow. This fully demonstrates that the present invention effectively removes impurities affecting melting behavior through process optimization, achieving a technical effect far superior to the narrow melting range and high starting melting point of the comparative examples.

[0099] 4. Yield: The yields of Examples 1-5 are significantly higher than those of Comparative Examples 1-7. The yields of all examples are consistently above 98%, while the yields of the comparative examples, except for a few that are close to 93%, are mostly below 90%, with the lowest being only 73.9%. The above results fully demonstrate that the process route adopted in this invention is far superior to the comparative examples of the prior art in terms of yield indicators, and has outstanding industrial application value.

[0100] In summary, through the seven sets of targeted comparative experiments (Comparative Examples 1-7), the innovations and performance advantages of this invention in multiple aspects, including reaction condition optimization, solvent selection, reduction path design, and crystallization purification, are clearly evident. These comparative examples, from different dimensions, demonstrate the rationality and necessity of each process step in this invention. Overall, the UV-323 preparation method proposed in this invention shows significant improvements in reaction efficiency, product quality, economic cost control, and environmental friendliness, fully demonstrating its technological originality and practical application value.

[0101] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing ultraviolet absorber UV-323, characterized in that, Includes the following steps: (1) Diazotization reaction: o-nitroaniline is dissolved in hydrochloric acid, heated to 60-70℃ to dissolve, deionized water is added, then the temperature is lowered to 0-5℃, sodium nitrite solution is added, and the reaction is controlled at 0-5℃ to obtain a diazonium salt solution. (2) Coupling reaction: Dissolve p-hydroxyphenylethanol in a mixed solution of methanol and water, then add caustic soda and potassium carbonate, continue to cool down, add the diazonium salt solution obtained in step (1) dropwise, and after the addition is complete, raise the temperature to room temperature to react and obtain the azo compound; (3) Reduction of sulfide dispersion: The azo compound obtained in step (2) is dissolved in a mixed solvent of ethanol and water, and then the sulfide-supported dispersion reducing agent and caustic soda are added. The temperature is raised, and after the reaction is completed, the mixture is cooled. Hydrochloric acid is added dropwise to adjust the pH and precipitate solid. The solid is filtered and dried to obtain the reduction product. (4) Esterification reaction: Under a nitrogen atmosphere, the reduction product obtained in step (3) was dissolved in toluene, heated and stirred until completely dissolved, then methanesulfonic acid and methacrylic acid were added, the temperature was raised to initiate reflux, and after the reaction was completed, it was cooled to room temperature to obtain a crude reaction mixture; (5) Crystallization: The crude reaction mixture obtained in step (4) was extracted with sodium hydroxide solution, then washed with deionized water, toluene was recovered by rotary evaporation, and then recrystallized with dichloromethane and ethanol. After filtration and drying, ultraviolet absorber UV-323 was obtained. In step (3), the preparation method of the sulfide-supported dispersion reducing agent includes the following steps: using sodium sulfide, sodium dithionite, sodium thiosulfate or sodium metabisulfite as sulfur-containing compound raw materials, and silica gel, molecular sieve, diatomaceous earth, neutral alumina or basic alumina as dispersion carriers, the sulfur-containing compound raw materials are suspended in a solvent under stirring, and then the dispersion carrier is added, heated and stirred in an air atmosphere, and after the reaction is completed, filtered and dried to obtain the sulfide-supported dispersion reducing agent; The heating and stirring time is 1.5-3 hours, and the temperature is 40-60℃; In step (3), by weight, the amount of the azo compound obtained in step (2) is 20-25 parts, the amount of the sulfide-supported dispersion reducing agent is 30-75 parts, and the amount of caustic soda is 20-45 parts; the ethanol is anhydrous ethanol, and the amount of anhydrous ethanol is 150-200 parts; the water is deionized water, and the amount of deionized water is 100-150 parts. In step (3), after adding the sulfide-supported dispersion reducing agent and caustic soda flakes, the temperature is raised to 70-90℃ and the reaction time is 1-4 hours; In step (5), the ethanol is anhydrous ethanol, and the ratio of the crude reaction mixture, sodium hydroxide solution, dichloromethane and anhydrous ethanol obtained in step (4) is 2.2-3.2 g: 9-15 g: 3-5 ml: 10-20 ml, and the drying temperature is 80-90℃.

2. The method for preparing ultraviolet absorber UV-323 according to claim 1, characterized in that, In step (1), the amount of hydrochloric acid used is 35-60 parts by weight, the amount of o-nitroaniline used is 10-20 parts, the amount of deionized water used is 20-30 parts, and the amount of sodium nitrite used is 8-15 parts.

3. The method for preparing the ultraviolet absorber UV-323 according to claim 1, characterized in that, In step (2), by weight, the amount of diazonium salt solution obtained in step (1) is 95-160 parts, the amount of methanol is 50-60 parts, the amount of p-hydroxyphenylethanol is 10-20 parts, the amount of caustic soda is 5-20 parts, the amount of potassium carbonate is 20-30 parts, and the water is deionized water, with a usage of 60-90 parts.

4. The method for preparing the ultraviolet absorber UV-323 according to claim 1, characterized in that, In step (4), the ratio of the amount of the reduction product obtained in step (3), methacrylic acid, toluene, and methanesulfonic acid is 10-15 g: 5-20 ml: 100-300 ml: 2-4 ml.

5. The method for preparing the ultraviolet absorber UV-323 according to claim 1, characterized in that, In step (1), after the diazotization reaction is completed, the reaction time at 0-5℃ is 1-2 hours.

6. The method for preparing ultraviolet absorber UV-323 according to claim 1, characterized in that, In step (2), after adding caustic soda and potassium carbonate, the temperature is further reduced to 0-5℃, and the diazonium salt solution obtained in step (1) is added dropwise at 0-5℃. The reaction time at room temperature is 1-4 hours.

7. The method for preparing the ultraviolet absorber UV-323 according to claim 1, characterized in that, In step (4), the temperature is raised to 50-60℃ and stirred until completely dissolved. The temperature is then raised to 130-170℃ to initiate reflux. The reaction time is 3-12 hours.

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