Process for the preparation of 2-nitro-2'-hydroxy-5'-tert-octylazobenzene in aqueous medium

Abstract

The application discloses a method for preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in a water medium system. The method controls the molar dosage ratio of tert-octyl phenol and NaOH in an emulsification process, controls the mass concentration of tert-octyl phenol in an emulsion, controls the molar dosage ratio of o-nitroaniline and tert-octyl phenol, and controls the mass dosage ratio of an anionic high polymer surfactant and a non-ionic surfactant, so that 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene with high purity is prepared. Compared with a traditional synthesis process, the method uses water as a reaction medium, avoids the use of an organic solvent, effectively reduces the difficulty in treating subsequent wastewater, and achieves a target product yield of more than 95% and a purity of 91.79%-94.52%.

Description

Technical Field

[0001] This invention relates to the field of production of intermediates for ultraviolet absorbers in chemical products, and in particular to a method for preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium system. Background Technology

[0002] Ultraviolet (UV) absorbers, due to their good thermal, chemical, and photochemical stability, can effectively absorb UV rays from sunlight and fluorescent light sources and release the absorbed energy as heat. They have wide applications in the field of polymer materials, effectively extending their lifespan. Benzotriazole compounds are an important type of UV absorber with low toxicity (LD50). 50 With a concentration up to 5000 mg / kg, good polymer compatibility, strong UV absorption capacity, and good durability, it is widely used in synthetic materials such as food packaging, polyester resins, coatings, and automotive coatings. UV-329 (2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole) is a typical product among benzotriazole UV absorbers. Its synthesis process mainly involves preparing the intermediate 2-nitro-2'-hydroxy-5'-tert-octylazobenzene by diazo and coupling reaction of o-nitroaniline and p-tert-octylphenol, followed by reduction and ring closure to obtain the target product.

[0003] In the synthesis of the intermediate 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene, organic solvents (methanol, ethanol, etc.) are often used as coupling reaction media because the raw materials have extremely low water solubility for tert-octylphenol. This results in wastewater containing a large amount of organic solvents, which is difficult to treat and poses a significant environmental pollution problem. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium system. The method involves preparing a p-tert-octylphenol emulsion through emulsification, and then preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene by coupling reaction with a diazonium salt prepared from o-nitroaniline in an aqueous medium system. This method avoids the use of organic solvents and effectively reduces the difficulty of subsequent wastewater treatment.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium system. Under the emulsifying and dispersing action of an emulsifier, the raw material p-tert-octylphenol is dispersed in water, and then 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene is prepared by diazotization and coupling reaction with o-nitroaniline in an aqueous medium. This process includes the following steps:

[0006] (1) Preparation of p-tert-octylphenol emulsion: p-tert-octylphenol solid and NaOH solid particles were weighed in sequence according to a molar ratio of 1:1-1:2.5. The NaOH particles were prepared into a NaOH aqueous solution with a mass concentration of 30%. p-tert-octylphenol solid, NaOH solution and water were added to the reaction vessel. After heating until p-tert-octylphenol was completely melted, an emulsifier was added. After stirring for 15 min, emulsification and dispersion were carried out using a high shear homogenizer for 5-10 min. Then, the mixture was naturally cooled to room temperature under stirring to obtain p-tert-octylphenol emulsion. The mass percentage of p-tert-octylphenol in the p-tert-octylphenol emulsion was 10%-20%.

[0007] (2) Preparation of o-nitroaniline diazonium salt solution: Weigh o-nitroaniline and concentrated hydrochloric acid solution according to a molar ratio of 1:2.8, mix them, and then slurry them for 6-8 hours to obtain a slurry. Then add crushed ice to cool it down to 0-5℃. Weigh sodium nitrite according to a molar ratio of o-nitroaniline:sodium nitrite = 1:1.02 and prepare a sodium nitrite aqueous solution with a mass concentration of 30%. Add it to the slurry to carry out the diazotization reaction. The excess sodium nitrite in the reaction is removed with aminosulfonic acid. The reaction endpoint is when the reaction solution appears colorless when tested with starch potassium iodide test paper, which is the o-nitroaniline diazonium salt solution.

[0008] (3) Coupling reaction: Weigh the p-tert-octylphenol emulsion prepared in step (1) according to the molar ratio of o-nitroaniline: p-tert-octylphenol = 1:0.95-1:1 and add it to the coupling reaction vessel. Then, under mechanical stirring and reaction conditions of 0-5℃, add the o-nitroaniline diazonium salt solution prepared in step (2) dropwise to the coupling reaction vessel. After the addition is complete, adjust the pH to 3-4 with sodium acetate and continue the reaction until the diazonium salt reaction is complete. Then filter, wash and dry the reaction solution to obtain 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene.

[0009] The emulsifier is composed of anionic polymeric surfactant and nonionic surfactant. The amount of anionic polymeric surfactant accounts for 1%-1.5% of the mass of the p-tert-octylphenol emulsion, and the amount of nonionic surfactant accounts for 0.5%-1.5% of the mass of the p-tert-octylphenol emulsion.

[0010] The anionic polymeric surfactant is one or both of sodium salt of styrene-maleic anhydride copolymer and sodium polystyrene sulfonate. When used, the anionic polymeric surfactant is dissolved in NaOH solution to prepare a solution with a mass concentration of 10% and the pH value of the solution controlled at 7-8.

[0011] The nonionic surfactant is one or more of alkylphenol polyoxyethylene ethers with a degree of polymerization of ethylene oxide of 15-21.

[0012] The beneficial effects of this invention are:

[0013] (1) Water is used as the reaction medium during the preparation process, which avoids the use of organic solvents and effectively reduces the difficulty of subsequent wastewater treatment.

[0014] (2) The synthesis of 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium was achieved by emulsifying tert-octylphenol with raw materials. By controlling the ratio of tert-octylphenol to NaOH, the ratio of emulsifier, and the ratio of tert-octylphenol to o-nitroaniline during the emulsification process, the yield and purity of the target product were improved, with a yield >95% and a purity of 91.79%-94.52%. Attached Figure Description

[0015] Figure 1 This is a high-performance liquid chromatogram of 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene prepared in Example 1 of the present invention.

[0016] Figure 2 This is a high-performance liquid chromatogram of 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene prepared in Example 3 of the present invention.

[0017] Figure 3 This is a high-performance liquid chromatogram of 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene prepared in Example 5 of the present invention.

[0018] Figure 4 This is a high-performance liquid chromatogram of 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene prepared in Example 7 of the present invention. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0020] The method for preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium according to the present invention utilizes the emulsifying and dispersing effect of an emulsifier to disperse the raw material tert-octylphenol in an aqueous medium, followed by a diazo and coupling reaction with o-nitroaniline to prepare 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium, comprising the following steps:

[0021] (1) Preparation of p-tert-octylphenol emulsion: p-tert-octylphenol solid and NaOH solid particles were weighed in sequence according to a molar ratio of 1:1-1:2.5. The NaOH particles were prepared into a NaOH aqueous solution with a mass concentration of 30%. p-tert-octylphenol solid, NaOH solution and water were added to the reaction vessel. After heating until p-tert-octylphenol was completely melted, an emulsifier was added. After stirring for 15 min, emulsification and dispersion were carried out using a high shear homogenizer for 5-10 min. Then, the mixture was naturally cooled to room temperature under stirring to obtain p-tert-octylphenol emulsion. The mass percentage of p-tert-octylphenol in the p-tert-octylphenol emulsion was 10%-20%.

[0022] (2) Preparation of o-nitroaniline diazonium salt solution: Weigh o-nitroaniline and concentrated hydrochloric acid solution according to a molar ratio of 1:2.8, mix them, and then slurry them for 6-8 hours to obtain a slurry. Then add crushed ice to cool it down to 0-5℃. Weigh sodium nitrite according to a molar ratio of o-nitroaniline:sodium nitrite = 1:1.02 and prepare a sodium nitrite aqueous solution with a mass concentration of 30%. Add it to the slurry to carry out the diazotization reaction. The excess sodium nitrite in the reaction is removed with aminosulfonic acid. The reaction endpoint is when the reaction solution appears colorless when tested with starch potassium iodide test paper, which is the o-nitroaniline diazonium salt solution.

[0023] (3) Coupling reaction: Weigh the p-tert-octylphenol emulsion prepared in step (1) according to the molar ratio of o-nitroaniline: p-tert-octylphenol = 1:0.95-1:1 and add it to the coupling reaction vessel. Then, under mechanical stirring and reaction conditions of 0-5℃, add the o-nitroaniline diazonium salt solution prepared in step (2) dropwise to the coupling reaction vessel. After the addition is complete, adjust the pH to 3-4 with sodium acetate and continue the reaction until the diazonium salt reaction is complete. Then filter, wash and dry the reaction solution to obtain 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene.

[0024] The emulsifier is composed of anionic polymeric surfactant and nonionic surfactant. The amount of anionic polymeric surfactant accounts for 1%-1.5% of the mass of the p-tert-octylphenol emulsion, and the amount of nonionic surfactant accounts for 0.5%-1.5% of the mass of the p-tert-octylphenol emulsion.

[0025] The anionic polymeric surfactant is one or both of sodium salt of styrene-maleic anhydride copolymer and sodium polystyrene sulfonate. When used, the anionic polymeric surfactant is dissolved in NaOH solution to prepare a solution with a mass concentration of 10% and the pH value of the solution controlled at 7-8.

[0026] The nonionic surfactant is one or more of alkylphenol polyoxyethylene ethers with a degree of polymerization of ethylene oxide of 15-21.

[0027] This invention addresses the environmental pollution problem caused by using organic solvents as the coupling reaction medium in the preparation of 2-nitro-2'-hydroxy-5'-tert-octylazobenzene, an intermediate of the ultraviolet absorber UV-329. It provides a method for preparing 2-nitro-2'-hydroxy-5'-tert-octylazobenzene in an aqueous medium. This method prepares a p-tert-octylphenol emulsion through the emulsifying and dispersing effect of an emulsifier, and then couples it with a diazonium salt prepared from o-nitroaniline in an aqueous medium to obtain 2-nitro-2'-hydroxy-5'-tert-octylazobenzene. During the reaction, by selecting the emulsifier, controlling the emulsification process conditions, and controlling the ratio of p-tert-octylphenol to o-nitroaniline, high-purity 2-nitro-2'-hydroxy-5'-tert-octylazobenzene can be obtained. This method avoids the use of organic solvents, effectively reduces the difficulty of subsequent wastewater treatment, and improves the yield and purity of the target product, with a yield of >95% and a purity of 91.79%-94.52%.

[0028] Example 1

[0029] Taking a p-tert-octylphenol emulsion as an example, where the mass percentage of p-tert-octylphenol is 10%, emulsifier 1 is a styrene-maleic anhydride copolymer (sodium salt) at a dosage ratio of 1%, emulsifier 2 is OP-15 at a dosage ratio of 0.5%, the molar ratio of p-tert-octylphenol to NaOH is 1:1, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:0.95, the following is a possible interpretation.

[0030] (1) First, styrene-maleic anhydride copolymer (sodium salt) (SMA) is mixed with water and a 30% NaOH solution, wherein the amount of NaOH is 30% of the amount of SMA. The reaction solution is heated until the SMA dissolves to obtain a 10% SMA solution with a pH of 7-8.

[0031] (2) Weigh 6.93 parts by mass of p-tert-octylphenol and place it in a reaction vessel. Then add 4.48 parts by mass of 30% NaOH solution and 50.59 parts by mass of water. Heat the reaction solution and stir until the p-tert-octylphenol is completely melted. Then add 6.93 parts by mass of 10% SMA solution and 0.35 parts by mass of OP-15 to the reaction vessel. After the reaction solution is heated to the point where the p-tert-octylphenol melts, continue stirring for 5 minutes. Then emulsify the reaction solution with a high-shear emulsifier for 10 minutes. Then cool it naturally to room temperature under mechanical stirring to obtain the p-tert-octylphenol emulsion.

[0032] (3) Weigh 3.2 parts by mass of o-amine and 6.66 parts by mass of concentrated hydrochloric acid (36%-38%), mix them, and then beat them for 6-8 hours to obtain a pulping solution. Then add ice to the pulping solution to cool it down to 0°C. Weigh 1.63 parts by mass of sodium nitrite to prepare a sodium nitrite solution, and then add it dropwise to the pulping solution to carry out the diazotization reaction. After reacting for 90 minutes, use aminosulfonic acid to remove excess sodium nitrite. The reaction endpoint is determined by colorless detection of the solution with starch-potassium iodide test paper, and o-nitroaniline diazonium salt solution is obtained.

[0033] (4) Weigh 45.4 parts by mass of p-tert-octylphenol emulsion and add it to the coupling reaction vessel. Under mechanical stirring, add the prepared o-nitroaniline diazonium salt solution dropwise to the reaction vessel. After the addition is complete, adjust the pH of the reaction solution to pH=3-4 with sodium acetate. The coupling reaction temperature is 0-5℃. Continue stirring until the diazonium salt reaction is complete as detected by the permeation test, which is the reaction endpoint. Filter, wash, and dry the reaction solution to obtain the product. The yield of the product is calculated to be 96.78%. After dissolving the product in methanol, its purity is determined by high performance liquid chromatography to be 91.79%. Figure 1 As shown.

[0034] Example 2

[0035] Taking a p-tert-octylphenol emulsion as an example, where the mass percentage of p-tert-octylphenol is 20%, emulsifier 1 is styrene-maleic anhydride copolymer (sodium salt) at a dosage ratio of 1%, emulsifier 2 is OP-18 at a dosage ratio of 1%, the molar ratio of p-tert-octylphenol to NaOH is 1:1, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:0.95, the following is a suggested example.

[0036] In Example 1, the amount of p-tert-octylphenol in step (2) was adjusted to 13.87 parts by mass, the amount of 30% NaOH solution was adjusted to 8.96 parts by mass, and the amount of water was adjusted to 15.69 parts by mass. OP-15 was replaced with OP-18 and its amount was adjusted to 0.70 parts by mass. In step (4), the amount of p-tert-octylphenol emulsion was adjusted to 22.7 parts by mass. Other reaction conditions remained unchanged. The yield of the obtained product was 97.68%, and the purity of the product was 93.47% as determined by high performance liquid chromatography.

[0037] Example 3

[0038] Taking a p-tert-octylphenol emulsion as an example, where the mass percentage of p-tert-octylphenol is 15%, emulsifier 1 is styrene-maleic anhydride copolymer (sodium salt) at a dosage ratio of 1.5%, emulsifier 2 is OP-20 at a dosage ratio of 0.5%, the molar ratio of p-tert-octylphenol to NaOH is 1:1, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:0.95, the following is a possible interpretation.

[0039] In Example 1, the amount of p-tert-octylphenol in step (2) was adjusted to 10.39 parts by mass, the amount of 30% NaOH solution was adjusted to 6.72 parts by mass, the amount of water was adjusted to 41.78 parts by mass, the amount of SMA solution was adjusted to 10.39 parts by mass, and emulsifier 2 was replaced with OP-20; the amount of p-tert-octylphenol emulsion in step (4) was adjusted to 30.27 parts by mass, and other reaction conditions remained unchanged. The yield of the obtained product was 97.32%, and the purity of the product detected by high performance liquid chromatography was 92.09%. Figure 2 As shown.

[0040] Example 4

[0041] Taking a p-tert-octylphenol emulsion as an example, the mass percentage of p-tert-octylphenol is 11.87%, emulsifier 1 is styrene-maleic anhydride copolymer (sodium salt) at a dosage ratio of 1%, emulsifier 2 is OP-21 at a dosage ratio of 1%, the molar ratio of p-tert-octylphenol to NaOH is 1:1, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:0.95.

[0042] In Example 1, the amount of p-tert-octylphenol in step (2) was adjusted to 8.22 parts by mass, the amount of 30% NaOH solution was adjusted to 5.31 parts by mass, and the amount of water was adjusted to 48.47 parts by mass. OP-15 was replaced with OP-21 and its amount was adjusted to 0.70 parts by mass. The amount of p-tert-octylphenol emulsion in step (4) was adjusted to 38.25 parts by mass. Other reaction conditions remained unchanged. The yield of the obtained product was 96.95%, and the purity of the product was 92.18% as determined by high performance liquid chromatography.

[0043] Example 5

[0044] Taking a p-tert-octylphenol emulsion as an example, where the mass percentage of p-tert-octylphenol is 15%, emulsifier 1 is styrene-maleic anhydride copolymer (sodium salt) at a dosage ratio of 1.5%, emulsifier 2 is OP-18 at a dosage ratio of 1.5%, the molar ratio of p-tert-octylphenol to NaOH is 1:2, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:0.98, the following is a suggested example.

[0045] In Example 1, the amount of p-tert-octylphenol in step (2) was adjusted to 10.39 parts by mass, the amount of 30% NaOH solution was adjusted to 13.43 parts by mass, the amount of water was adjusted to 35.08 parts by mass, and the amount of SMA solution was adjusted to 10.39 parts by mass. OP-15 was replaced with OP-18, and the amount was adjusted to 1.05 parts by mass. In step (4), the amount of p-tert-octylphenol emulsion was adjusted to 31.22 parts by mass. Other reaction conditions remained unchanged. The yield of the obtained product was 96.35%, and the purity of the product detected by high performance liquid chromatography was 94.21%. Figure 3As shown.

[0046] Example 6

[0047] Taking a p-tert-octylphenol emulsion as an example, the mass percentage of p-tert-octylphenol is 15%, emulsifier 1 is styrene-maleic anhydride copolymer (sodium salt) with a dosage ratio of 1.5%, emulsifier 2 is OP-15 with a dosage of 0.5%, the molar ratio of p-tert-octylphenol to NaOH is 1:2, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:1.

[0048] In Example 1, the amount of p-tert-octylphenol in step (2) was adjusted to 10.39 parts by mass, the amount of 30% NaOH solution was adjusted to 13.43 parts by mass, the amount of water was adjusted to 35.08 parts by mass, and the amount of SMA solution was adjusted to 10.39 parts by mass. In step (4), the amount of p-tert-octylphenol emulsion was adjusted to 31.87 parts by mass. Other reaction conditions remained unchanged. The yield of the obtained product was 95.49%, and the purity of the product was 92.67% as determined by high performance liquid chromatography.

[0049] Example 7

[0050] Taking a p-tert-octylphenol emulsion as an example, where the mass percentage of p-tert-octylphenol is 15%, emulsifier 1 is styrene-maleic anhydride copolymer (sodium salt) at a ratio of 1.5%, emulsifier OP-15 at a ratio of 0.5%, the molar ratio of p-tert-octylphenol to NaOH is 1:2.5, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:0.98, the following is an example.

[0051] In Example 1, the amount of p-tert-octylphenol in step (2) was adjusted to 10.39 parts by mass, the amount of 30% NaOH solution was adjusted to 16.78 parts by mass, the amount of water was adjusted to 31.72 parts by mass, and the amount of SMA solution was adjusted to 10.39 parts by mass; the amount of p-tert-octylphenol emulsion in step (4) was adjusted to 31.22 parts by mass, with other reaction conditions remaining unchanged. The yield of the obtained product was 98.43%, and the purity of the product detected by high performance liquid chromatography was 94.52%. Figure 4 As shown.

[0052] Example 8

[0053] Taking a p-tert-octylphenol emulsion as an example, where the mass percentage of p-tert-octylphenol is 20%, emulsifier 1 is sodium polystyrene sulfonate at a dosage ratio of 1.5%, emulsifier 2 is OP-18 at a dosage ratio of 1.5%, the molar ratio of p-tert-octylphenol to NaOH is 1:1, and the molar ratio of o-nitroaniline to p-tert-octylphenol is 1:0.95, the following is a possible interpretation.

[0054] In Example 1, the styrene-maleic anhydride copolymer (sodium salt) in step (1) was replaced with sodium polystyrene sulfonate. The amount of p-tert-octylphenol in step (2) was adjusted to 13.86 parts by mass, the amount of 30% NaOH solution was adjusted to 8.95 parts by mass, the amount of water was adjusted to 36.08 parts by mass, and the amount of SMA solution was adjusted to 10.39 parts by mass. OP-15 was replaced with OP-18, and its amount was adjusted to 1.05 parts by mass. The amount of p-tert-octylphenol emulsion in step (4) was adjusted to 22.7 parts by mass. Other reaction conditions remained unchanged. The yield of the obtained product was 97.89%, and the purity of the product was 94.06% as determined by high performance liquid chromatography.

[0055] In summary, the content of this invention is not limited to the above-described embodiments. Those skilled in the art can easily propose other embodiments within the technical guiding principles of this invention, but such embodiments are all included within the scope of this invention.

Claims

1. A method for preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium system, characterized in that, Includes the following steps: (1) Preparation of p-tert-octylphenol emulsion: p-tert-octylphenol solid and NaOH solid particles were weighed in sequence according to a molar ratio of 1:1-1:2.

5. The NaOH particles were prepared into a NaOH aqueous solution with a mass concentration of 30%. p-tert-octylphenol solid, NaOH solution and water were added to the reaction vessel. After heating until p-tert-octylphenol was completely melted, an emulsifier was added. After stirring for 15 min, emulsification and dispersion were carried out using a high shear homogenizer for 5-10 min. Then, the mixture was naturally cooled to room temperature under stirring to obtain p-tert-octylphenol emulsion. The mass percentage of p-tert-octylphenol in the p-tert-octylphenol emulsion was 10%-20%. (2) Preparation of o-nitroaniline diazonium salt solution: Weigh o-nitroaniline and concentrated hydrochloric acid solution according to a molar ratio of 1:2.8, mix them, and then slurry them for 6-8 hours to obtain a slurry. Then add crushed ice to cool it down to 0-5℃. Sodium nitrite was weighed according to the molar ratio of o-nitroaniline:sodium nitrite = 1:1.02 and prepared into a sodium nitrite aqueous solution with a mass concentration of 30%. It was added to the pulping solution to carry out the diazotization reaction. Excess sodium nitrite in the reaction was removed with aminosulfonic acid. The reaction endpoint was when the reaction solution showed a colorless result when tested with starch potassium iodide test paper, which is the o-nitroaniline diazonium salt solution. (3) Coupling reaction: Weigh the p-tert-octylphenol emulsion prepared in step (1) according to the molar ratio of o-nitroaniline: p-tert-octylphenol = 1:0.95-1:1 and add it to the coupling reaction vessel. Then, under mechanical stirring and reaction conditions of 0-5℃, add the o-nitroaniline diazonium salt solution prepared in step (2) dropwise to the coupling reaction vessel. After the addition is complete, adjust the pH to 3-4 with sodium acetate and continue the reaction until the diazonium salt reaction is complete. Then filter, wash and dry the reaction solution to obtain 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene. The emulsifier is composed of anionic polymeric surfactant and nonionic surfactant. The amount of anionic polymeric surfactant accounts for 1%-1.5% of the mass of the p-tert-octylphenol emulsion, and the amount of nonionic surfactant accounts for 0.5%-1.5% of the mass of the p-tert-octylphenol emulsion. The anionic polymeric surfactant is one or two of sodium salt of styrene-maleic anhydride copolymer and sodium polystyrene sulfonate. The nonionic surfactant is one or more of alkylphenol polyoxyethylene ethers with a degree of polymerization of ethylene oxide of 15-21.

2. The method for preparing 2-nitro-2'-hydroxy-5'-tert-octyl azobenzene in an aqueous medium system according to claim 1, characterized in that, When using anionic polymeric surfactants, they are dissolved in NaOH solution to prepare a solution with a mass concentration of 10% and the pH value of the solution is controlled at 7-8.

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