Improved dimethyl thio toluenediamine synthesis and separation method

Abstract

The invention relates to an improved synthesis and separation method of dimethyl thio toluenediamine, which comprises the following steps: by taking toluenediamine and dimethyl disulfide as raw materials, cuprous iodide as a catalyst and a phosphorus-containing additive as a protective auxiliary agent, reacting at 130-150 DEG C under normal pressure to synthesize the dimethyl thio toluenediamine, and carrying out extraction separation, alkali neutralization and saturated saline solution washing on the crude product by using dilute phosphoric acid containing phosphorous acid and a hydrochloric acid aqueous solution containing phosphorous acid to finally obtain the dimethyl thio toluenediamine with light color. When the method is used for preparing the dimethyl thio toluenediamine, the color of the diamine can be improved, and the purity of the diamine can be improved.

Description

Technical Field

[0001] The invention relates to a method for synthesizing and separating dimethylthiotoluenediamine in the field of fine chemicals, in particular to an improved method for synthesizing and separating dimethylthiotoluenediamine. Background Art

[0002] 3,5-dimethylthiotoluenediamine, abbreviated as DMTDA or DADMT, is a liquid diamine chain extender with relatively mild reactivity and excellent operating performance that has been popular in the market for forty years. The industrial product is a mixture of two isomers of 3,5-dimethylthio-2,4-toluenediamine and 3,5-dimethylthio-2,6-toluenediamine, with a mass ratio of 80 / 20. The trade name is Ethacure 300, abbreviated as E300. It is mainly used as a chain extender or curing agent for polyurethane elastomer materials. The classic synthesis process of DMTDA is that excess dimethyl disulfide and toluenediamine react with methylthio under the action of a catalyst at normal pressure and high temperature and then undergo post-treatment to obtain it.

[0003] In industry, DMTDA is mostly produced with a mixture of 2,4- / 2,6-diaminotoluene (80 / 20) as raw material. Common catalysts include Lewis acids such as aluminum chloride, zinc chloride, zinc bromide, zinc iodide, ferric chloride, boron trifluoride, hydrogen iodide, ammonium iodide, cuprous iodide-CuI, metal and halogen mixtures, etc. Different catalysts have different selectivities, among which cuprous iodide is a catalyst with good selectivity. In terms of post-treatment, the general treatment method for the crude product is to filter or distill the catalyst and leave it in the residual liquid. If the catalyst used is aluminum chloride that is soluble in the system, add alkali to precipitate it and then filter it.

[0004] The crude liquid product obtained after the reaction of dimethyl disulfide and toluenediamine is a mixture of aromatic amines with dimethylthiotoluenediamine as the main component, and the hydrophobicity order is trimethylthiotoluenediamine>dimethylthiotoluenediamine>monomethylthiotoluenediamine>toluenediamine. 2,4-Diaminotoluene in toluenediamine has a certain degree of water solubility, but its isomer 2,6-diaminotoluene is not highly water-soluble, and it is not suitable to remove the unreacted toluenediamine by washing with water at room temperature. The early synthesis method of DMTDA was not very mature, and high-temperature and long-term reaction would produce tar-like by-products. It was necessary to extract the target diamine with ether and then distill under reduced pressure; later it developed into direct segmented distillation of the mixture after the reaction, first distilling out the unreacted raw materials under reduced pressure, and then increasing the vacuum degree and temperature to distill to obtain a product with DMTDA as the main component.

[0005] The disadvantage of the vacuum distillation separation method is that DMTDA has a high boiling point and generally needs to be distilled under high vacuum conditions of about 180°C / 0.5mmHg, which requires a very high degree of vacuum. At the same time, aromatic diamines are prone to discoloration at high temperatures, and a small amount of methylthio groups will dissociate from the substitution position of the benzene ring, resulting in reduced product purity and yield. Based on the difference in hydrophobicity of diamines substituted with different amounts of methylthio groups, U.S. Patent No. 4727191A proposed a method for extracting and separating diamines substituted with different amounts of methylthio groups using a dilute acid solution. The patent used 1% and 10% phosphoric acid aqueous solutions to extract unreacted toluenediamine and monomethylthiotoluenediamine from monomethyl HDP240343CN Page 2 / 9

[0006] However, 10% phosphoric acid will bring out a small amount of the main product DMTDA and even a small amount of trimethylthiotoluenediamine, reducing the yield of DMTDA.

[0007] In summary, the current mainstream DMTDA production process still has problems such as dark product color and low purity, and the separation method of dimethylthiotoluene diamine still needs to be improved.

[0008] In view of the above-mentioned defects in the existing technology, the inventor has finally created the present invention which is of practical value after continuous research, design, repeated trials and improvements. Summary of the invention

[0009] The main purpose of the present invention is to overcome the defects of the existing technology and provide a new and improved method for synthesizing and separating dimethylthiotoluenediamine, so as to solve the problems of low product purity and dark color in the existing DMTDA production process, so that the obtained DMTDA product has high purity and light color, is very suitable for practical use and has industrial utilization value.

[0010] The invention is conceived to add a small amount of phosphite or aryl phosphine to the reaction system of dimethylthiotoluenediamine and obtain beneficial effects. Such phosphorus-containing additives not only have a certain antioxidant effect to reduce the color of the product, but also have a complexing effect on cuprous ions, thereby reducing the content of free cuprous ions and avoiding oxidation of cuprous ions, thereby improving the reaction efficiency.

[0011] In terms of post-treatment, the present invention separates the diamine mixture by acid extraction. Phosphoric acid is a medium-strong acid. A 4% to 6% dilute phosphoric acid aqueous solution can "extract" and separate the relatively hydrophilic monomethylthiotoluenediamine and toluenediamine by salting, so that the DMTDA content in the final product can be increased. The concentration selection of the phosphoric acid aqueous solution is relatively important. If the concentration is too high, a small amount of DMTDA or even trimethylthiotoluenediamine will be extracted, reducing the yield of DMTDA. Hydrochloric acid is a strong acid with a strong salt-forming solubility for aromatic diamines. If low-concentration dilute hydrochloric acid is used instead of dilute phosphoric acid to separate monomethylthiotoluenediamine and toluenediamine first, the yield of the target product is easily reduced. At the same time, if the hydrochloric acid concentration is high, the system viscosity is large and the separation effect is poor, and if the concentration is low, the equipment utilization efficiency is low. The present invention separates the reaction mixture by first extracting with a phosphoric acid solution and then extracting with hydrochloric acid, and at the same time, phosphorous acid is added to the inorganic acid to inhibit the deepening of the color of the aromatic amine during the pickling process.

[0012] The purpose of the present invention and the solution to the technical problem are achieved by adopting the following technical solutions.

[0013] An improved method for synthesizing and separating dimethylthiotoluenediamine proposed by the present invention comprises the following steps:

[0014] Step 1: under nitrogen protection, add cuprous iodide catalyst, one or two of phosphite or aryl phosphine to molten toluene diamine in sequence and stir evenly, then slowly add dimethyl disulfide dropwise to the reaction solution, reflux reaction at 130-150° C. for 5-8 hours, adjust the nitrogen flow rate to remove the methyl mercaptan generated by the reaction, and promote the reaction to proceed in the forward direction;

[0015] Step 2: After the reaction is completed, the temperature of the reaction liquid is lowered to 60-80° C., and unreacted dimethyl disulfide is distilled out under vacuum conditions to obtain a crude product; the temperature of the crude product is further lowered to less than 50° C. and a solvent of toluene is added to dilute it, and then the catalyst is separated by centrifugation or filtration to obtain a mixed solution diluted with toluene;

[0016] Step 3: adding an aqueous phosphoric acid solution containing phosphorous acid to the mixed solution diluted with toluene and stirring evenly, phosphoric acid and unreacted toluenediamine and byproduct monomethylthiotoluenediamine form ammonium salts and enter the aqueous phase, and after standing and stratifying, a first aqueous phase and a first organic phase are obtained; adding an aqueous hydrochloric acid solution containing phosphorous acid to the first organic phase twice and stirring evenly, hydrochloric acid and dimethylthiotoluenediamine form ammonium salts and enter the aqueous phase, and after standing and stratifying, a second aqueous phase and a second organic phase are obtained; adding alkali to the second aqueous phase to dissociate dimethylthiotoluenediamine from its water-soluble ammonium salt and standing and stratifying to obtain a third aqueous phase and a third organic phase, washing the third organic phase with a saturated sodium chloride solution, distilling and removing water, and filtering to obtain light-colored dimethylthiotoluenediamine with a purity of ≥98.5%.

[0017] Furthermore, the toluenediamine includes a mixture of 2,4-diaminotoluene:2,6-diaminotoluene in a mass ratio of 80:20 and pure 2,4-diaminotoluene.

[0018] Furthermore, the molar ratio of dimethyl disulfide to toluenediamine is 2.3-2.8:1, and the molar ratio of cuprous iodide to toluenediamine is 0.03-0.05:1.

[0019] Furthermore, the phosphites include but are not limited to tributyl phosphite, triethyl phosphite, and triphenyl phosphite; the aryl phosphine includes but is not limited to triphenyl phosphine, tri-p-tolyl phosphine, tri-m-tolyl phosphine, and dimethylphenyl phosphine; and the molar ratio of the phosphite or aryl phosphine to the cuprous iodide catalyst is 0.5 to 1:1.

[0020] Furthermore, the molar ratio of phosphoric acid to toluenediamine and monomethylthiotoluenediamine is 2-2.2:1, and the concentration of the phosphoric acid aqueous solution is 4%-6%; the molar ratio of hydrochloric acid to dimethylthiotoluenediamine is 2.1-2.3:1, and the concentration of the hydrochloric acid aqueous solution is 5%-20%, preferably 7%-15%; and the amount of phosphorous acid used is 1%-2% of the mass of phosphoric acid and hydrochloric acid.

[0021] The by-product methyl mercaptan produced by the reaction of toluenediamine and dimethyl disulfide is absorbed by alkaline solution and then converted into dimethyl disulfide by sulfur oxidation process for recycling.

[0022] The synthesized catalyst cuprous iodide can be mixed with fresh cuprous iodide for use after filtering and washing.

[0023] By means of the above technical solution, the present invention has at least the following advantages:

[0024] 1. The present invention adds a small amount of phosphite or aryl phosphine to the reaction system of dimethylthiotoluenediamine and achieves beneficial effects, thereby improving the reaction efficiency.

[0025] 2. The product produced in the DMTDA production process of the present invention has high purity and light color, is very suitable for practical use and has industrial utilization value.

[0026] 3. The present invention improves the yield of dimethylthiotoluenediamine;

[0027] 4. The thermal decomposition of the product of the present invention is reduced and the product odor is low;

[0028] 5. The byproducts and impurities of the present invention are fully utilized and are environmentally friendly.

[0029] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the preferred embodiments of the present invention are described in detail as follows.

[0030] The specific manufacturing method and processing method of the present invention are given in detail by the following examples.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS (NO DRAWINGS FOR THIS PRESENT PATENT) DETAILED DESCRIPTION

[0032] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation method, method, steps, characteristics and effects of an improved dimethylthiotoluenediamine synthesis and separation method proposed by the present invention are described in detail below in combination with preferred embodiments.

[0033] An improved method for synthesizing and separating dimethylthiotoluenediamine according to a preferred embodiment of the present invention comprises the following steps: firstly extracting the reaction mixture with a phosphoric acid solution and then extracting it with hydrochloric acid, and simultaneously adding phosphorous acid to the inorganic acid to inhibit the deepening of the color of the aromatic amine during the pickling process.

[0034] Step 1: under nitrogen protection, add cuprous iodide catalyst, one or two of phosphite or aryl phosphine to molten toluene diamine in sequence and stir evenly, then slowly add dimethyl disulfide dropwise to the reaction solution, reflux reaction at 130-150° C. for 5-8 hours, adjust the nitrogen flow rate to remove the methyl mercaptan generated by the reaction, and promote the reaction to proceed in the forward direction;

[0035] Step 2: After the reaction is completed, the temperature of the reaction liquid is lowered to 60-80° C., and unreacted dimethyl disulfide is distilled out under vacuum conditions to obtain a crude product; the temperature of the crude product is further lowered to less than 50° C. and a solvent toluene is added to dilute it, and then the catalyst is separated by centrifugation or filtration to obtain a mixed solution diluted with toluene;

[0036] Step 3: add an aqueous phosphoric acid solution containing phosphorous acid to the mixed solution diluted with toluene and stir evenly, phosphoric acid forms ammonium salt with unreacted toluenediamine and byproduct monomethylthiotoluenediamine and enters the aqueous phase, and after standing and stratifying, a first aqueous phase and a first organic phase are obtained; add an aqueous hydrochloric acid solution containing phosphorous acid to the first organic phase twice and stir evenly, hydrochloric acid forms ammonium salt with dimethylthiotoluenediamine and enters the aqueous phase, and after standing and stratifying, a second aqueous phase and a second organic phase are obtained; add alkali to the second aqueous phase to dissociate dimethylthiotoluenediamine from its water-soluble ammonium salt and stand and stratify to obtain a third aqueous phase and a third organic phase, wash the third organic phase with a saturated sodium chloride solution, distill and remove water, and filter to obtain light-colored dimethylthiotoluenediamine with a purity of ≥98.5%.

[0037] The toluenediamine includes a mixture of 2,4-diaminotoluene / 2,6-diaminotoluene in a mass ratio of 80 / 20, pure 2,4-diaminotoluene, etc. The molar ratio of dimethyl disulfide to toluenediamine is 2.3-2.8:1, and the molar ratio of cuprous iodide to toluenediamine is 0.03-0.05:1.

[0038] Furthermore, the phosphites include but are not limited to tributyl phosphite, triethyl phosphite, and triphenyl phosphite; the aryl phosphine includes but is not limited to triphenyl phosphine, tri-p-tolyl phosphine, tri-m-tolyl phosphine, and dimethylphenyl phosphine; and the molar ratio of the phosphite or aryl phosphine to the cuprous iodide catalyst is 0.5 to 1:1.

[0039] Furthermore, the molar ratio of phosphoric acid to toluenediamine and monomethylthiotoluenediamine is 2 to 2.2:1, the concentration of the phosphoric acid aqueous solution is 4% to 6%; the molar ratio of hydrochloric acid to dimethylthiotoluenediamine is 2.1 to 2.3

[0040] ∶1, the concentration of the hydrochloric acid aqueous solution is 5% to 20%, preferably 7% to 15%; the amount of phosphorous acid used is 1% to 2% of the mass of phosphoric acid and hydrochloric acid;

[0041] Some of the materials generated during the synthesis and post-treatment of DMTDA can be effectively recycled after further treatment: the filtered catalyst can be recycled after washing with a small amount of toluene, water washing, purification and drying, and the toluene washing liquid is incorporated into the crude product toluene mixed liquid; the first aqueous phase formed after extracting toluenediamine and monomethylthiotoluenediamine with an aqueous phosphoric acid solution is neutralized to pH=8-9 by adding alkali solution to separate the organic phase, and the organic phase is washed with a small amount of saturated sodium chloride aqueous solution to obtain a mixture of toluenediamine and monomethylthiotoluenediamine, which is accumulated and used as a raw material for synthesizing DMTDA; the organic phase remaining after the hydrochloric acid extraction of DMTDA is a mixture of phosphorus-containing compounds and TMTDA, which is collected and used as a curing agent component for paving materials.

[0042] Synthesis Specific Example of Dimethylthiotoluenediamine

[0043] Embodiment 1: In a four-mouth reaction flask equipped with stirring, a thermometer, a reflux condenser, and a nitrogen pipe, under nitrogen protection, 122.2 grams of toluenediamine were added and heated to 125°C, then 9.5 grams of cuprous iodide were added, 7.9 grams of triphenylphosphine were added after stirring at 130-140°C for 0.5 hours, 236 grams of dimethyl disulfide were slowly added dropwise within 5 hours, refluxed at 130-150°C, and stirring was continued for 1.5 hours, and sampling was performed at intervals during the period. When the raw material TDA chromatographic peak integral area accounted for less than 1%, the reaction was stopped, and the methyl mercaptan produced during the reaction was taken out by the nitrogen flow and oxidized and recovered. After the reaction was completed, the reaction liquid temperature was lowered to 60-80°C, and the excess dimethyl disulfide was decompressed and distilled off, then cooled to below 50°C, 100 grams of toluene were added to reduce viscosity, and a total of 216.5 grams of slightly yellow crude product (toluene was not included) was obtained after separating the precipitate by a centrifuge. According to GC analysis, assuming that the total area peak of the raw material toluenediamine and its derivatives is 1, the area proportions of toluenediamine, monomethylthiotoluenediamine, dimethylthiotoluenediamine, and trimethylthiotoluenediamine are 0.6%, 2.3%, 96.2%, and 0.9%, respectively.

[0044] Embodiment 2:

[0045] The operation steps are basically the same as those in Example 1, except that 122.2 g of toluenediamine, 7.7 g of cuprous iodide, 5.3 g of triphenylphosphine, 6.2 g of triphenyl phosphite, and 264 g of dimethyl disulfide are used. After the reaction, 218.3 g of a pale yellow liquid crude product is obtained. The composition of the product is shown in Table 1.

[0046] Embodiment 3:

[0047] The operation steps are basically the same as those in Example 1, except that 122.2 g of toluenediamine, 9.5 g of cuprous iodide, 6.2 g of triphenyl phosphite, 2.5 g of tributyl phosphite, and 236 g of dimethyl disulfide are used. After the reaction, 214.7 g of a pale yellow liquid crude product is obtained. The composition of the product is shown in Table 1.

[0048] Comparative Example 1:

[0049] The operation steps are basically the same as those in Example 1, except that the amount of toluenediamine used is 122.2 g, cuprous iodide is 9.5 g, the amount of phosphorus-containing organic matter added is 0, and dimethyl disulfide is 236 g. After the reaction, 210.4 g of a yellow liquid crude product is obtained. The total reaction time is about 9.5 hours. The product composition is shown in Table 1.

[0050] Table 1 Synthesis of dimethylthiotoluenediamine

[0051]

[0052] Separation Example of Dimethylthiotoluenediamine

[0053] Embodiment 4:

[0054] Take half of the toluene solution of the crude product obtained in Example 1, add 74 g of an aqueous solution with a phosphoric acid concentration of 4% and a phosphorous acid concentration of 0.06%, stir vigorously for 20 minutes, and separate into a first aqueous phase and a first organic phase after standing.

[0055] Add 325 g of an aqueous solution with a hydrochloric acid concentration of 10% and a phosphorous acid concentration of 0.15% to the first organic phase, stir vigorously and mix for 20 minutes, separate the aqueous phase after standing, wash and extract once again with 50 g of the above-mentioned 10% dilute hydrochloric acid solution, combine the aqueous phases obtained by the two extractions into the second aqueous phase, and the remaining organic phase is the second organic phase. Add 105 g of a 40% sodium hydroxide solution to the second aqueous phase and mix evenly, stand and separate to obtain a third aqueous phase and a third organic phase. The third organic phase is washed once with 30 g of a saturated sodium chloride solution, then vacuum dehydrated and filtered to obtain 101.5 g of a slightly yellow final product.

[0056] For the first aqueous phase, 6 grams of 40% sodium hydroxide solution was added thereto and mixed evenly. After cooling, water was separated. The main components of the remaining organic matter were unreacted toluenediamine and monomethylthiotoluenediamine.

[0057] Embodiment 5:

[0058] Take half of the toluene solution of the crude product prepared in Example 1, add 48 g of an aqueous solution with a phosphorous acid concentration of 0.12% and a phosphoric acid concentration of 6% to wash and remove toluenediamine and monomethylthiotoluenediamine. The remaining organic phase is then washed and extracted with 375 g of an aqueous solution with a hydrochloric acid concentration of 10% and a phosphorous acid concentration of 0.2%. The subsequent steps and reagent amounts are the same as in Example 4, and finally 101.8 g of a slightly yellow product is obtained.

[0059] Embodiment 6:

[0060] Take the crude product toluene solution obtained in Example 2, add 100 g of an aqueous solution having a phosphoric acid concentration of 5% and a phosphorous acid concentration of 0.075% to wash and remove toluenediamine and monomethylthiotoluenediamine, then use 968 g of an aqueous solution having a hydrochloric acid concentration of 8% and a phosphorous acid concentration of 0.08% to dissolve the DMTDA in the organic phase twice and allow to stand for stratification, add 222 g of a 40% concentration of sodium hydroxide solution to the aqueous phase after stratification and mix evenly, allow to stand to separate the oil phase, wash the oil phase with 50 g of a saturated sodium chloride solution, remove the moisture by vacuum and filter, and finally obtain 206.7 g of a slightly yellow product.

[0061] Embodiment 7:

[0062] The crude product toluene solution prepared in Comparative Example 1 was taken, and the concentration and amount of the acid and alkali aqueous solution used for post-purification treatment were the same as those in Example 6 to obtain 195.6 g of a yellow product.

[0063] Comparative Example 2:

[0064] The crude product toluene solution prepared in Example 3 was taken, and the concentrations and amounts of phosphoric acid, hydrochloric acid and sodium hydroxide aqueous solution used for post-purification treatment were the same as those in Example 6, but the acid aqueous solution did not contain phosphorous acid, and 203.3 g of a light yellow product was obtained, the color of which was slightly darker than before separation.

[0065] Comparative Example 3:

[0066] The toluene solution of the crude product synthesized according to Comparative Example 1 was taken, and the concentration and amount of the acid and alkali aqueous solution used for post-treatment were the same as those in Example 6, but the acid aqueous solution did not contain phosphorous acid, and 196.1 g of a yellow-brown product was obtained, and the color was significantly darker than before separation.

[0067] Comparative Example 4:

[0068] Take the toluene solution of the crude product synthesized according to Example 2. The difference from Example 6 is that the phosphoric acid solution used is 50 grams of an aqueous solution with a mass concentration of 10% and a phosphorous acid content of 0.15%. Other post-treatment conditions are the same, and finally 202.8 g of a slightly yellow product is obtained.

[0069] Table 2 Separation of methylthio substituted aromatic amines

[0070]

[0071]

[0072] Note: The yield is the total amount of the purified product divided by the theoretical yield calculated based on the amount of TDA used.

[0073] Table 3 Effect of phosphoric acid concentration on the yield of separated methylthio substituted aromatic amines

[0074]

[0075]

[0076] It can be seen from Table 1 that when synthesizing dimethylthiotoluenediamine, adding phosphite and aryl phosphorus to the reaction solution is beneficial to reducing the color of dimethylthiotoluenediamine.

[0077] It can be seen from Table 2 that when the methylthio-substituted aromatic amines are separated by extraction using only dilute acid aqueous solution and alkali solution, the color of the product is deepened. However, adding phosphorous acid with antioxidant properties to the inorganic acid solution used for extraction can effectively reduce the degree of deepening of the product color.

[0078] As can be seen from Table 3: selecting an appropriate concentration of an aqueous solution of an inorganic acid is helpful to improve the yield of the product.

[0079] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. An improved method for synthesizing and separating dimethylthiotoluenediamine, characterized in that: The following steps are involved: Step 1: under nitrogen protection, add cuprous iodide catalyst, one or two of phosphite or aryl phosphine to molten toluene diamine in sequence and stir evenly, then slowly add dimethyl disulfide dropwise to the reaction solution, reflux reaction at 130-150° C. for 5-8 hours, adjust the nitrogen flow rate to remove the methyl mercaptan generated by the reaction, and promote the reaction to proceed in the forward direction; Step 2: After the reaction is completed, the temperature of the reaction liquid is lowered to 60-80° C., and unreacted dimethyl disulfide is distilled out under vacuum conditions to obtain a crude product; the temperature of the crude product is further lowered to less than 50° C. and a solvent of toluene is added to dilute it, and then the catalyst is separated by centrifugation or filtration to obtain a mixed solution diluted with toluene; Step 3: adding an aqueous phosphoric acid solution containing phosphorous acid to the mixed solution diluted with toluene and stirring evenly, phosphoric acid and unreacted toluenediamine and byproduct monomethylthiotoluenediamine form ammonium salts and enter the aqueous phase, and after standing and stratifying, a first aqueous phase and a first organic phase are obtained; adding an aqueous hydrochloric acid solution containing phosphorous acid to the first organic phase twice and stirring evenly, hydrochloric acid and dimethylthiotoluenediamine form ammonium salts and enter the aqueous phase, and after standing and stratifying, a second aqueous phase and a second organic phase are obtained; adding alkali to the second aqueous phase to dissociate dimethylthiotoluenediamine from its water-soluble ammonium salt and standing and stratifying to obtain a third aqueous phase and a third organic phase, washing the third organic phase with a saturated sodium chloride solution, distilling and removing water, and filtering to obtain light-colored dimethylthiotoluenediamine with a purity of ≥98.5%.

2. An improved method for synthesizing and separating dimethylthiotoluenediamine according to claim 1, characterized in that: The toluenediamine includes a mixture of 2,4-diaminotoluene:2,6-diaminotoluene in a mass ratio of 80:20 and pure 2,4-diaminotoluene.

3. An improved method for synthesizing and separating dimethylthiotoluenediamine according to claim 1, characterized in that: The molar ratio of dimethyl disulfide to toluenediamine is 2.3-2.8:1, and the molar ratio of cuprous iodide to toluenediamine is 0.03-0.05:

1.

4. An improved method for synthesizing and separating dimethylthiotoluenediamine according to claim 1, characterized in that: The phosphites include but are not limited to tributyl phosphite, triethyl phosphite, and triphenyl phosphite; the aryl phosphine includes but is not limited to triphenyl phosphine, tri-p-tolyl phosphine, tri-m-tolyl phosphine, and dimethylphenyl phosphine; the molar ratio of the phosphite or aryl phosphine to the cuprous iodide catalyst is 0.5 to 1:

1.

5. The improved method for synthesizing and separating dimethylthiotoluenediamine according to claim 1, characterized in that: The molar ratio of phosphoric acid to toluenediamine and monomethylthiotoluenediamine is 2-2.2:1, and the concentration of the phosphoric acid aqueous solution is 4%-6%; the molar ratio of hydrochloric acid to dimethylthiotoluenediamine is 2.1-2.3:1, and the concentration of the hydrochloric acid aqueous solution is 5%-20%, preferably 7%-15%; the amount of phosphorous acid used is 1%-2% of the mass of phosphoric acid and hydrochloric acid.

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