A method for synthesizing benzohydroxamic acid

Abstract

The present invention belongs to the field of organic synthesis technology and specifically relates to a method for synthesizing benzohydroxamic acid, comprising: 1) an oximation step: mixing methyl benzoate, hydroxylamine sulfate, a phase transfer catalyst, a defoaming agent, and water, adding a base, and conducting an oximation reaction at 25-42°C to obtain an oximation system; 2) an acidification step: adding concentrated sulfuric acid to the oximation system and conducting an acidification reaction to obtain an acidified system; and 3) a crystallization step: cooling the acidified system to 20-25°C, crystallizing, and filtering to obtain benzohydroxamic acid crystals. The benzohydroxamic acid product prepared by the present invention has a high yield and a purity of up to 99%, and can be directly used as a mineral flotation agent. Furthermore, the present invention has the advantages of stable production indicators, simple process control, and low cost. Furthermore, the use of water as a solvent makes it non-toxic and environmentally friendly.

Description

Technical Field

[0001] The invention belongs to the technical field of organic synthesis, and particularly relates to a method for synthesizing benzohydroxamic acid. Background Art

[0002] Alkyl hydroxamic acid is a collector for oxide ores and transition metal minerals. The oxygen and nitrogen atoms in the hydroxamic acid molecular structure contain lone pair electrons and can react with metal ions Cu 2+ 、Co 2+ 、Ni 2+ 、W 2+ 、Sn 2+ 、Fe 3+ and Zn 2+ Hydroxamic acid forms stable chelates with various metal ions. Hydroxamic acid has been widely used in hydrometallurgy, biomedicine, chemical separation, organic preparation and other fields. Especially in the flotation industry, hydroxamic acid is a highly efficient chelating oxide ore collector and is widely used in the recovery of various metal oxide ores and rare minerals.

[0003] Alkyl hydroxamic acids are synthesized by reacting the corresponding carboxylic acid esters with hydroxylamine. Most of these use organic solvents as dispersants, resulting in high costs for recovering the organic solvents. CN114797705A discloses a system and method for continuously preparing phenylhydroxamic acid using a microreactor. The raw materials, phenylhexanoic acid methyl ester and hydroxylamine hydrochloride or hydroxylamine sulfate, are mixed and reacted in the presence of sodium alkoxide within a microreactor to produce phenylhydroxamic acid sodium salt, which is then further acidified to obtain phenylhydroxamic acid. The method comprises multiple microreactors. The sodium alkoxide catalyst required for the reaction is expensive, and the equipment investment required for mass production is substantial. CN109810022B discloses a high-content liquid sodium benzohydroxamate, its preparation, and application. However, its application in polymer emulsions results in high wastewater treatment and production costs, and the sodium salt is less stable than hydroxamic acid. CN103922968B discloses a method for preparing hydroxamic acid or hydroxamate. The process has a high yield of benzohydroxamic acid and produces no wastewater. However, the process uses highly toxic and flammable methanol as a solvent. Furthermore, the purity of the product prepared by the process is only about 50%, which cannot meet the requirements of mineral flotation.

[0004] Zhang Faming et al. proposed a one-pot process for the synthesis of benzohydroxamic acid in the Journal of Chemical Minerals and Processing, Issue 4, 2020. This process involves esterifying an alkyl acid with methanol in the presence of sulfuric acid to form an alkyl carboxylate, which is then directly oximated to hydroxamic acid without separation. Because the sulfuric acid catalyst consumes some sodium hydroxide, the amount of sodium hydroxide added is greater than that of the alkyl ester. Furthermore, excess methanol is not separated during the esterification process, resulting in methanol loss. This process, therefore, is costly. Summary of the Invention

[0005] The present invention aims to provide a method for synthesizing benzohydroxamic acid, in order to provide a benzohydroxamic acid production process with low production cost and high product purity.

[0006] In order to achieve the above object, the present invention provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0007] Oximation step: methyl benzoate, hydroxylamine sulfate, a phase transfer catalyst, a defoamer and water are mixed, a base is added, and an oximation reaction is carried out at 25 to 42° C. to obtain an oximation system;

[0008] Acidification step: adding concentrated sulfuric acid to the oximation system to carry out an acidification reaction to obtain an acidified system;

[0009] Crystallization step: the acidified system is cooled to 20-25° C., crystallized, and filtered to obtain benzohydroxamic acid crystals.

[0010] Methyl benzoate is readily soluble in organic solvents but poorly soluble in water, whereas hydroxylamine sulfate and alkali, while readily soluble in water, are poorly soluble in the organic phase, making a two-phase reaction difficult. The inventors attempted to use methanol as the reaction solvent, dissolving the hydroxylamine sulfate and alkali in a small amount of water before reacting with methyl benzoate and methanol. As the reaction proceeds, the resulting sodium benzohydroxamate produces a large amount of foam, making the reaction difficult to proceed; furthermore, as the acidification reaction proceeds, excess sodium sulfate is produced, which precipitates during crystallization, resulting in reduced product purity.

[0011] The working principle and beneficial effects of this solution are as follows: Using water as the solvent in the present invention improves the solubility of sodium benzohydroxamate in water compared to methanol. This facilitates the release of sodium benzohydroxamate from the organic phase into the aqueous phase, promoting the forward progress of the oximation reaction. Furthermore, using water as the solvent is non-toxic, environmentally friendly, and low-cost.

[0012] Moreover, in the present invention, a phase transfer catalyst is added in the oximation step. The phase transfer catalyst can undergo ion exchange with the reactants in the aqueous phase to form ion pairs soluble in organic solvents, and the reactants in the aqueous phase are transferred to the organic phase. Then, ion exchange is performed again to promote the reaction and accelerate the reaction rate.

[0013] The inventors also discovered that the sodium benzohydroxamate generated during the reaction produces foam that envelops the reactant methyl benzoate, causing it to float on the upper surface of the reaction solution, making it difficult for it to come into contact with the aqueous phase, hindering the reaction. Therefore, in the present invention, a defoaming agent is added during the oximation step to facilitate the entry of the sodium benzohydroxamate generated during the reaction into the reaction system, thereby reducing foam generation, facilitating the reaction, and improving the yield of benzohydroxamic acid.

[0014] The inventors also discovered that after the acidification reaction, the crystallization temperature significantly affects the purity and yield of the benzohydroxamic acid product. Low crystallization temperatures cause the byproduct sodium sulfate to crystallize with the product, reducing its purity; high crystallization temperatures also result in low product yields. Therefore, the present invention controls the crystallization temperature at 20-25°C to ensure optimal product purity and yield.

[0015] The synthesis method of the present invention has the advantages of stable production indicators, simple process control, and low cost. In addition, the benzohydroxamic acid product prepared by the present invention has high purity and good yield. The purity of benzohydroxamic acid is as high as 99%, and it can be directly used as a mineral flotation agent without further purification.

[0016] Optionally, the molar ratio of the methyl benzoate, hydroxylamine sulfate, base, phase transfer catalyst and defoamer is 1:0.5-0.6:2-2.2:0.016-0.02:0.015-0.134. In this embodiment, the molar ratio of the methyl benzoate, hydroxylamine sulfate, base, phase transfer catalyst and defoamer is limited so that the proportion and amount of the methyl benzoate, hydroxylamine sulfate, base, phase transfer catalyst and defoamer are appropriate.

[0017] Optionally, the molar ratio of methyl benzoate, hydroxylamine sulfate, base, phase transfer catalyst and defoaming agent is 1:0.5:2:0.018:0.134. In this embodiment, under this molar ratio, the purity and yield of benzohydroxamic acid are both good.

[0018] Optionally, the molar ratio of methyl benzoate to concentrated sulfuric acid is 1:0.50 to 0.53. In this solution, under this molar ratio, the molar amount of concentrated sulfuric acid is appropriate to avoid excessive or insufficient use of concentrated sulfuric acid.

[0019] Optionally, the molar ratio of methyl benzoate to concentrated sulfuric acid is 1:0.51.

[0020] Optionally, the phase transfer catalyst is a quaternary ammonium salt, and the quaternary ammonium salt is selected from one or more of benzyltriethylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium hydrogen sulfate, trioctylmethylammonium chloride, dodecyltrimethylammonium chloride and tetradecyltrimethylammonium chloride.

[0021] Optionally, the phase transfer catalyst is tetraethylammonium chloride.

[0022] Optionally, the defoaming agent is a C4-C8 alcohol.

[0023] Optionally, the defoaming agent is butanol.

[0024] Optionally, the base is one or both of sodium hydroxide and potassium hydroxide.

[0025] Optionally, the base is sodium hydroxide.

[0026] Optionally, in the oximation step, the oximation reaction temperature is 38-42° C. Using this temperature range for the oximation reaction can ensure the reaction rate and avoid many side reactions: when the reaction temperature is too low, the reaction rate is slow; when the reaction temperature is too high, many side reactions occur, such as the hydrolysis reaction of methyl benzoate.

[0027] Optionally, in the acidification step, the temperature of the acidification reaction is 30-42° C. In this embodiment, since benzohydroxamic acid itself is not very stable, the temperature of the oximation reaction and the acidification reaction should not be too high to reduce product decomposition.

[0028] Optionally, in the crystallization step, the crystallization temperature is 22-25° C. In this embodiment, 22-25° C. is the preferred crystallization temperature range, and the obtained benzohydroxamic acid product has a good yield and high purity.

[0029] Optionally, in the crystallization step, the crystallization temperature is 22°C.

[0030] Alternatively, hydroxylamine sulfate is replaced by hydroxylamine hydrochloride in the oximation step.

[0031] Alternatively, in the acidification step, concentrated sulfuric acid is replaced by concentrated hydrochloric acid. DETAILED DESCRIPTION

[0032] The following is further described in detail through specific implementation methods:

[0033] Example 1

[0034] This embodiment provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0035] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.5 g (0.009 mol) of tetraethylammonium chloride, and 5 g (0.067 mol) of butanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 30 hours to obtain an oximation system.

[0036] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0037] 3) Crystallization step: The acidified system was cooled to 22° C. and crystallized at 22° C. to obtain 57 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 99% as determined by HPLC.

[0038] The oximation reaction equation is shown in formula (1):

[0039]

[0040] Example 2

[0041] This embodiment provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0042] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.9 g (0.009 mol) of tetraethylammonium bromide, and 5 g (0.067 mol) of butanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 34 hours to obtain an oximation system.

[0043] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0044] 3) Crystallization step: The acidified system was cooled to 22° C. and crystallized at 22° C. to obtain 56 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 98% as determined by HPLC.

[0045] Example 3

[0046] This embodiment provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0047] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.9 g (0.009 mol) of tetraethylammonium bromide, and 6 g (0.067 mol) of amyl alcohol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 34 hours to obtain an oximation system.

[0048] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0049] 3) Crystallization step: The acidified system was cooled to 22° C. and crystallized at 22° C. to obtain 59 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 93% as determined by HPLC.

[0050] Example 4

[0051] This embodiment provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0052] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.9 g (0.009 mol) of tetraethylammonium bromide, and 8 g (0.067 mol) of heptanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 34 hours to obtain an oximation system.

[0053] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0054] 3) Crystallization step: The acidified system was cooled to 20° C. and crystallized at 20° C. to obtain 58 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 95% as determined by HPLC.

[0055] Example 5

[0056] This embodiment provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0057] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.5 g (0.009 mol) of tetraethylammonium chloride, and 5 g (0.067 mol) of butanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 30 hours to obtain an oximation system.

[0058] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0059] 3) Crystallization step: The acidified system was cooled to 25° C. and crystallized at 25° C. to obtain 50 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 99% as determined by high performance liquid chromatography.

[0060] Comparative Example 1

[0061] This comparative example provides a method for synthesizing benzohydroxamic acid, comprising the following steps: placing 68 g (0.5 mol) of methyl benzoate and 150 g of methanol in a reaction flask and stirring them uniformly; mixing 30 g of water with 41 g (0.25 mol) of hydroxylamine sulfate and 40 g (1.0 mol) of sodium hydroxide to obtain a mixed solution; adding the mixed solution to the reaction flask in batches; controlling the reaction temperature below 42° C.; stirring at 42° C. for 168 hours; and then adding 25 g (0.255 mol) of concentrated sulfuric acid dropwise for acidification to obtain 319 g of a viscous solid. The purity of the benzohydroxamic acid, as measured by high-performance liquid chromatography, is 47%.

[0062] Comparative Example 2

[0063] This comparative example provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0064] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, and 1.5 g (0.009 mol) of tetraethylammonium chloride were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 30 hours to obtain an oximation system.

[0065] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0066] 3) Crystallization step: The acidified system was cooled to 22° C. and crystallized at 22° C. to obtain 53 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 95% as determined by HPLC.

[0067] Comparative Example 3

[0068] This comparative example provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0069] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, and 5 g (0.067 mol) of butanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was continued for 44 hours to obtain an oximation system.

[0070] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0071] 3) Crystallization step: The acidified system was cooled to 22° C. and crystallized at 22° C. to obtain 54 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 97% as determined by HPLC.

[0072] Comparative Example 4

[0073] This comparative example provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0074] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.5 g (0.009 mol) of tetraethylammonium chloride, and 5 g (0.067 mol) of butanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 30 hours to obtain an oximation system.

[0075] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the system into benzohydroxamic acid, thereby obtaining an acidified system.

[0076] 3) Crystallization step: The acidified system was cooled to 15° C. and crystallized at 15° C. to obtain 109 g of a pale yellowish-white powder. The purity of the benzohydroxamic acid was 60% as determined by HPLC.

[0077] Comparative Example 5

[0078] This comparative example provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0079] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.5 g of tetraethylammonium chloride, and 5 g (0.067 mol) of butanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 30 hours to obtain an oximation system.

[0080] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0081] 3) Crystallization step: The acidified system was cooled to 10° C. and crystallized at 10° C. to obtain 137 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 50% as determined by HPLC.

[0082] Comparative Example 6

[0083] This comparative example provides a method for synthesizing benzohydroxamic acid, comprising the following steps:

[0084] 1) Oximation Step: 68 g (0.5 mol) of methyl benzoate, 41 g (0.25 mol) of hydroxylamine sulfate, 350 g of water, 1.5 g (0.009 mol) of tetraethylammonium chloride, and 5 g (0.067 mol) of butanol were added to a four-necked flask and stirred uniformly. 40 g (1.0 mol) of solid sodium hydroxide was added portionwise. The reaction temperature was maintained at 42°C and the reaction was allowed to proceed for 30 hours to obtain an oximation system.

[0085] 2) Acidification step: 25 g (0.255 mol) of concentrated sulfuric acid was added dropwise to the oximation system to carry out an acidification reaction, with the reaction temperature being no higher than 42° C., to convert the sodium benzohydroxamate in the oximation system into benzohydroxamic acid, thereby obtaining an acidified system.

[0086] 3) Crystallization step: The acidified system was cooled to 18° C. and crystallized at 18° C. to obtain 98 g of pale yellowish-white flaky crystals. The purity of benzohydroxamic acid was 65% as determined by HPLC.

[0087] Summarizing the selection of phase transfer catalyst, defoaming agent, solvent, reaction time and crystallization temperature in Examples 1 to 5 and Comparative Examples 1 to 6, the results are shown in Table 1.

[0088] Table 1 Selection of parameters in each embodiment and comparative example and purity and yield of the product

[0089]

[0090]

[0091] In Table 1, “-” means no addition.

[0092] As can be seen from Table 1, the benzohydroxamic acid products prepared in Examples 1-5 have good purity and yield. In particular, Example 1 has a purity of 99% and a yield of 82%, which is superior to the purity of each comparative example and the yield is superior to that of Comparative Examples 2 and 3. Furthermore, the benzohydroxamic acid products prepared in Comparative Examples 1 to 6 cannot achieve a balance between purity and yield: higher purity leads to lower yield, and higher yield leads to lower purity. However, Examples 1-5 of the present invention achieve a good balance between purity and yield.

[0093] In summary, the present invention uses water as a solvent. Compared with methanol as a solvent, the solubility of sodium benzohydroxamate in water is better. The present invention is conducive to the freeing of sodium benzohydroxamate from the organic phase to the aqueous phase, promoting the positive oximation reaction. In addition, the present invention uses water as a solvent, which is non-toxic and environmentally friendly. In addition, the present invention adds a phase transfer catalyst and a defoamer to the reaction system, and controls the oximation reaction temperature, acidification temperature and crystallization temperature, thereby improving the purity and yield of the benzohydroxamic acid product, and having stable production indicators, simple process control and low cost.

[0094] The above are only embodiments of the present invention. The invention is not limited to the fields involved in this implementation case. Common knowledge such as the known specific structures and characteristics in the scheme is not described in detail here. Ordinary technicians in the relevant field are aware of all the common technical knowledge in the technical field to which the invention belongs before the application date or priority date, can obtain all the existing technologies in the field, and have the ability to apply conventional experimental means before that date. Ordinary technicians in the relevant field can improve and implement this scheme in combination with their own abilities under the inspiration given by this application. Some typical known structures or known methods should not become obstacles for ordinary technicians in the relevant field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the present invention. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for synthesizing benzohydroxamic acid, characterized in that: The following steps are involved: The oximation step comprises mixing methyl benzoate, hydroxylamine sulfate, a phase transfer catalyst, a defoaming agent and water, adding a base, and carrying out an oximation reaction at 25 to 42° C. to obtain an oximation system; wherein the phase transfer catalyst is a quaternary ammonium salt, and the quaternary ammonium salt is selected from one or more of benzyltriethylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium hydrogen sulfate, trioctylmethylammonium chloride, dodecyltrimethylammonium chloride and tetradecyltrimethylammonium chloride; Acidification step: adding concentrated sulfuric acid to the oximation system to carry out an acidification reaction to obtain an acidified system; Crystallization step: the acidified system is cooled to 20-25° C., crystallized, and filtered to obtain benzohydroxamic acid crystals.

2. The method for synthesizing benzohydroxamic acid according to claim 1, wherein: The molar ratio of the methyl benzoate, hydroxylamine sulfate, base, phase transfer catalyst and defoaming agent is 1:0.5-0.6:2-2.2:0.016-0.02:0.015-0.

134.

3. The method for synthesizing benzohydroxamic acid according to claim 2, wherein: The molar ratio of the methyl benzoate, hydroxylamine sulfate, base, phase transfer catalyst and defoaming agent is 1:0.5:2:0.018:0.

134.

4. The method for synthesizing benzohydroxamic acid according to claim 1, wherein: The molar ratio of the methyl benzoate to the concentrated sulfuric acid is 1:0.50-0.

53.

5. The method for synthesizing benzohydroxamic acid according to claim 1, wherein: The defoaming agent is a C4-C8 alcohol.

6. The method for synthesizing benzohydroxamic acid according to claim 5, wherein: The defoaming agent is butanol.

7. The method for synthesizing benzohydroxamic acid according to claim 1, wherein: In the acidification step, the temperature of the acidification reaction is 30-42°C.

8. The method for synthesizing benzohydroxamic acid according to claim 1, wherein: In the crystallization step, the crystallization temperature is 22-25°C.

9. The method for synthesizing benzohydroxamic acid according to any one of claims 1 to 8, characterized in that: In the oximation step, hydroxylamine sulfate was replaced with hydroxylamine hydrochloride; and / or, in the acidification step, concentrated sulfuric acid is replaced by concentrated hydrochloric acid.

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