Preparation method of phenothiazine and composite catalyst for preparation of phenothiazine

Abstract

The invention provides a preparation method of phenothiazine and a composite catalyst for preparation of phenothiazine. The preparation method of phenothiazine comprises the following steps: reacting diphenylamine with sulfur under the action of a composite catalyst, the composite catalyst comprises an iodine elementary substance and an organic acid, and the mass ratio of the organic acid to the iodine elementary substance is 1: (0.01-0.95); the organic acid is selected from one or more of methanesulfonic acid, trifluoromethanesulfonic acid, ethyl sulfonic acid and p-toluenesulfonic acid. The iodine elementary substance and the specific organic acid in a specific mass ratio are selected as the composite catalyst, in the phenothiazine preparation process, higher reaction temperature can be adapted, the catalytic effect is good, the conversion rate is high, aftertreatment is simple, and the prepared phenothiazine product is high in yield and purity.

Description

Technical Field

[0001] The present invention relates to the field of organic synthesis, and in particular to a method for preparing phenothiazine and a composite catalyst used for the preparation thereof. Background Art

[0002] Phenothiazine is an organic aromatic compound composed of two benzene rings linked by sulfur and nitrogen atoms. It is also known as thioxanthine, thiodiphenylamine, and dibenzothiazine. The unique nitrogen-hydrogen bonded amine structure in its molecular structure enables phenothiazine to act as both a free radical terminator and a peroxide decomposer, effectively preventing the polymerization of olefin monomers. It is widely used in the production of acrylic acid, acrylates, methacrylates, and vinyl acetate. Phenothiazine is also used as a pharmaceutical, dye, polyether, antioxidant, and rubber antioxidant. It is also used as an anthelmintic for livestock and an insecticide for fruit trees.

[0003] Phenothiazine is initially prepared by reacting diphenylamine and sulfur in the presence of a catalyst at high temperature. Currently, elemental iodine, aluminum chloride, and activated clay are the primary catalysts used in the preparation of phenothiazine. Under the catalytic action of the catalyst, diphenylamine and sulfur undergo a ring-closure reaction, removing a molecule of hydrogen sulfide to form a phenothiazine ring structure that combines both phenazine and thiazine structures.

[0004] Elemental iodine is currently the mainstream catalyst used in the preparation of phenothiazines. Despite its high catalytic activity, iodine readily sublimes and is expensive. Post-processing, including recrystallization, distillation, and decolorization, requires high energy consumption, significantly hindering the development of phenothiazines.

[0005] Aluminum chloride, a catalyst, has lower catalytic activity than elemental iodine. Since the reaction temperature is 170-180°C, while the sublimation temperature of aluminum chloride is 178°C, aluminum chloride is prone to sublimation, affecting the catalytic effect. Furthermore, the use of aluminum chloride as a catalyst requires the use of large amounts of alkali to neutralize the aluminum chloride in post-processing, resulting in a large amount of wastewater and waste salt, polluting the environment.

[0006] Compared to aluminum chloride and iodine, activated clay is a solid, recyclable catalyst with less stringent reaction temperature requirements. Due to its unique acidic active sites, activated clay's catalytic activity is lower than that of aluminum chloride and iodine, resulting in lower yields and product purity. Furthermore, large catalyst quantities are required, making post-reaction separation difficult. Due to the poor solubility of phenothiazine, a large amount of solvent is required to dissolve and filter the phenothiazine after the reaction, followed by vacuum distillation and recrystallization, resulting in high energy consumption.

[0007] Therefore, there is an urgent need to adopt a new method for preparing phenothiazine, so that the phenothiazine product obtained has high yield and purity, simple post-processing, and is green and environmentally friendly. Summary of the Invention

[0008] In order to overcome the technical defects such as low yield and purity and complex post-treatment existing in the preparation method of phenothiazine in the prior art, the present invention provides a preparation method of phenothiazine and a composite catalyst for its preparation. By selecting iodine and methanesulfonic acid with a specific mass ratio as the composite catalyst, in the process of preparing phenothiazine, the reaction temperature can be increased, the catalytic effect is good, the conversion rate is high, the post-treatment is simple, and the yield and purity of the prepared phenothiazine product are high.

[0009] The first aspect of the present invention provides a composite catalyst for preparing phenothiazine, wherein the composite catalyst comprises iodine and an organic acid, and the mass ratio of the organic acid to the iodine is 1:(0.01-0.95); the organic acid is selected from one or more of methanesulfonic acid, trifluoromethanesulfonic acid, ethanesulfonic acid and p-toluenesulfonic acid.

[0010] In one or more embodiments, the mass ratio of the organic acid to the iodine is 1:(0.025-0.75), preferably 1:(0.05-0.5).

[0011] In one or more embodiments, the organic acid exists in the form of an aqueous solution, and the mass concentration of the organic acid in the aqueous solution is 50%-99%, preferably 80%-99%, more preferably 95%-99%.

[0012] The second aspect of the present invention provides a preparation method of phenothiazine, which comprises reacting diphenylamine and sulfur under the action of a composite catalyst;

[0013] The composite catalyst comprises iodine and an organic acid, and the mass ratio of the organic acid to the iodine is 1:(0.01-0.95); the organic acid is selected from one or more of methanesulfonic acid, trifluoromethanesulfonic acid, ethanesulfonic acid and p-toluenesulfonic acid.

[0014] In one or more embodiments, the mass ratio of the methanesulfonic acid to the iodine is 1:(0.025-0.75), preferably 1:(0.05-0.5).

[0015] In one or more embodiments, the molar ratio of the diphenylamine to the iodine is (1-6000):1, preferably (100-6000):1, preferably (200-3000):1.

[0016] In one or more embodiments, the organic acid is added to the reaction system in the form of an aqueous solution of the organic acid, and the mass concentration of the organic acid in the aqueous solution is preferably 50%-99%, more preferably 80%-99%, particularly 95%-99%.

[0017] In one or more embodiments, the molar ratio of the diphenylamine to the sulfur is 1:(2 - 3), preferably 1:(2 - 2.5).

[0018] In one or more embodiments, the temperature of the reaction is 170 - 220 °C, preferably 180 - 210 °C, more preferably 190 - 200 °C.

[0019] In one or more embodiments, the reaction time is 1 - 8 h, preferably 1 - 5 h, more preferably 1 - 3 h.

[0020] In one or more embodiments, after the reaction is completed, post-treatment is further included, and the post-treatment includes: mixing the reaction solution after the reaction is completed with an organic solvent, followed by vacuum filtration and drying to obtain high-quality phenothiazine.

[0021] In one or more embodiments, the organic solvent is one or more of ethanol, toluene, ether, dichloromethane, ethylene glycol, ethyl acetate, and tetrahydrofuran.

[0022] In one or more embodiments, the temperature for mixing the reaction solution and the organic solvent is 105 - 120 °C. Detailed Description

[0023] To enable those skilled in the art to understand the features and effects of the present invention, the following provides a general description and definition of the terms and expressions mentioned herein. Unless otherwise specified, all technical and scientific terms used herein shall have the ordinary meaning understood by those skilled in the art regarding the present invention. In case of conflict, the definitions herein shall prevail.

[0024] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall not limit the scope of the present invention in any way, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0025] In this article, terms such as "comprising", "including", "containing" and similar terms encompass the meanings of "consisting essentially of" and "consisting of". For example, when it is disclosed herein that "A comprises B and C", it should be considered that "A consists essentially of B and C" and "A consists of B and C" have been disclosed herein.

[0026] In this article, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are only for the sake of brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values within the range (including integers and fractions).

[0027] In this article, unless otherwise specified, percentages refer to mass percentages and ratios refer to mass ratios.

[0028] In this article, when describing embodiments or examples, it should be understood that it is not intended to limit the present invention to these embodiments or examples. On the contrary, all alternatives, modifications, and equivalents of the methods and materials described in the present invention can be covered within the scope defined by the claims.

[0029] In this article, for the sake of brevity of description, not all possible combinations of the technical features in each embodiment or example are described. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered to be within the scope described in this specification.

[0030] During the research process of preparing phenothiazine, the inventors found that when a specific type of organic acid and iodine are used in combination as a composite catalyst, by utilizing the strong catalytic effect of iodine and the acidity of the organic acid; on the one hand, the amount of iodine used can be reduced; on the other hand, iodine mainly acts as an initiator and the amount used is small, so in the subsequent high-temperature reaction, the sublimation has little impact on the reaction. Therefore, the reaction temperature can be increased, the reaction conversion rate is high, and the yield and purity of the prepared phenothiazine product are high.

[0031] The present invention provides a method for preparing phenothiazine, which includes the following steps: reacting diphenylamine and sulfur under the action of a composite catalyst; wherein, the composite catalyst includes iodine and an organic acid, and the organic acid is selected from one or more of methanesulfonic acid, trifluoromethanesulfonic acid, ethanesulfonic acid, and p-toluenesulfonic acid; the mass ratio of the organic acid to iodine is 1:(0.01 - 0.95).

[0032] The mass ratio of the organic acid to iodine is preferably 1:(0.025 - 0.75), more preferably 1:(0.05 - 0.5), for example, 1:0.2, 1:0.08, 1:0.1, 1:0.3, 1:0.5, 1:0.6, 1:0.8, 1:0.9.

[0033] The organic acid and iodine of the present invention are used in combination, and most importantly, the reaction temperature is increased. When iodine and aluminum trichloride are used as catalysts, in order to avoid the sublimation of iodine and aluminum trichloride, the reaction temperature is mostly controlled below 180 °C, while the melting point of phenothiazine is greater than 180 °C, which makes the reaction a heterogeneous reaction and the reaction is not sufficient. Using an organic acid as the main catalyst for synthesizing phenothiazine and iodine as the catalytic initiator of the reaction, iodine plays a role in initiating catalysis during the initial heat preservation process. During the subsequent high-temperature reaction, the sublimation of iodine has no effect on the reaction, and thus the reaction temperature for preparing phenothiazine can be increased, the reaction is sufficient, the side reactions are few, the conversion rate is high, and the yield and purity of the prepared phenothiazine product are high.

[0034] In some preferred embodiments, the molar ratio of diphenylamine to iodine is (1 - 6000):1, preferably (100 - 6000):1, more preferably (200 - 3000):1, such as 5500:1, 5076:1, 4000:1, 3000:1, 2800:1, 2538:1, 2000:1, 1500:1, 1000:1, 800:1, 300:1, 100:1, 60:1, 20:1, 5:1.

[0035] In the present invention, the molar ratio of diphenylamine to sulfur can be 1:(2 - 3), preferably 1:(2 - 2.5), such as 1:2.8, 1:2.6, 1:2.4, 1:2.2, 1:2.1.

[0036] The temperature suitable for the reaction of the present invention can be 170 - 220 °C, preferably 180 - 210 °C, more preferably 190 - 200 °C.

[0037] In the present invention, the organic acid is preferably added to the reaction system in the form of an aqueous solution. The mass concentration of the organic acid in the aqueous solution is preferably 50% - 99%, more preferably 80% - 99%, especially 95% - 99%. In some preferred embodiments, the organic acid is selected from methanesulfonic acid or p-toluenesulfonic acid.

[0038] In the present invention, the reaction completion time can be until no hydrogen sulfide gas is produced. In some embodiments, the reaction time can be 1 - 8 h, preferably 1 - 5 h, especially 1 - 3 h.

[0039] In the present invention, the reaction formula for diphenylamine and sulfur to react to form phenothiazine under the action of a catalyst is as follows:

[0040]

[0041] In the present invention, the hydrogen sulfide tail gas generated during the reaction can be absorbed with a strong base solution for the preparation of sodium sulfide. The strong base solution can be a strong base solution conventionally used in the art for absorbing hydrogen sulfide tail gas, such as sodium hydroxide solution. The mass concentration of the sodium hydroxide solution is preferably 20 - 40 wt%, for example, 25 wt%, 30 wt%, 35 wt%.

[0042] In some preferred embodiments, the preparation method of phenothiazine comprises the following steps:

[0043] Mix diphenylamine and sulfur, heat up to the first temperature, and keep warm; then add a composite catalyst for reaction.

[0044] Among them, the first temperature is preferably 140 - 150 °C, such as 142 °C, 145 °C, 148 °C. The holding time is preferably 0.5 - 1 h.

[0045] Among them, the reaction temperature can be 170 - 220 °C, preferably 180 - 210 °C, more preferably 190 - 200 °C, such as 175 °C, 185 °C, 195 °C.

[0046] In some preferred embodiments, the preparation method of the present invention further comprises, after the reaction is completed, performing post-treatment; the post-treatment includes: mixing the reaction solution after the reaction is completed with an organic solvent, then performing vacuum filtration to obtain a filter cake, and drying to obtain high-quality phenothiazine.

[0047] Among them, the temperature for mixing the reaction solution and the organic solvent is preferably 105 - 120 °C, such as 110 °C, 120 °C. The mixing method can be stirring.

[0048] Among them, a suitable organic solvent can be one or more of ethanol, toluene, ether, dichloromethane, ethylene glycol, ethyl acetate, tetrahydrofuran, such as toluene. After vacuum filtration, the organic solvent in the filtrate can be removed and recycled for further use. In some preferred embodiments, the mass ratio of diphenylamine to the organic solvent is 1:(0.5 - 10), preferably 1:(1 - 5), more preferably 1:(1 - 3), such as 1:0.8, 1:2, 1:2.36, 1:4, 1:6, 1:8.

[0049] Among them, drying can be carried out in an oven. The drying temperature can be 70 - 90 °C, such as 80 °C. The drying time can be 7 - 9 h, such as 8 h.

[0050] The present invention includes phenothiazine prepared by using the preparation method of phenothiazine according to any one of the embodiments herein. The phenothiazine prepared by the present invention is a light yellow powder with high purity and short melting range. The purity is preferably greater than 98%, such as 98.5%, 99.1%, 99.2%, 99.3%.

[0051] The phenothiazine prepared by the preparation method of phenothiazine described in any embodiment of the present invention can be applied to the production of acrylic acid, acrylate, methacrylate, and vinyl acetate; it can also be used in the synthesis of drugs and dyes, polyethers, antioxidants, and rubber antioxidants, and as livestock anthelmintics and fruit tree insecticides, and all have good effects.

[0052] Advantages of the present invention:

[0053] In the present invention, by selecting iodine and a specific organic acid with a specific mass ratio as a composite catalyst, the total price of the catalyst is low. During the preparation of phenothiazine, the reaction temperature can be increased, the catalytic effect is good, the conversion rate is high, and the yield and purity of the prepared phenothiazine product are high. Moreover, by using the composite catalyst of the present invention to prepare phenothiazine, the post-treatment process is simple, the amount of solvent required is small, the resource utilization rate is high, the energy consumption is low, and the three wastes are few. Without rectification and recrystallization, high-purity phenothiazine products can be obtained. The organic solvent used in the post-treatment can be reused.

[0054] The preparation method of phenothiazine of the present invention is to prepare phenothiazine by a one-step method. The process is simple, the reaction conditions are broad, the cost is low, the operability is strong, and it can be applied to industrial scale-up production; the conversion rate and yield of the main product phenothiazine are high, and the side reactions are few.

[0055] In the preparation method of the present invention, hydrogen sulfide tail gas is absorbed by strong base to prepare sodium sulfide, which can improve the added value of by-products.

[0056] The present invention will be described below by way of specific examples. It should be understood that these examples are merely illustrative and are not intended to limit the scope of the present invention. The methods, reagents, and materials used in the examples are conventional methods, reagents, and materials in the art unless otherwise specified. The raw material compounds in the examples can be obtained through commercial channels.

[0057] Example 1

[0058] S1. Under an inert gas atmosphere, 84.6 g (0.5 mol) of diphenylamine and 34.6 g (1.08 mol) of sulfur are put into a 500 mL four-necked flask, heated to 140 °C, and kept warm for 0.5 h. Then, 0.5 g of methylsulfonic acid aqueous solution (wherein the mass concentration of methylsulfonic acid is 99%) and 0.05 g of iodine (0.000197 mol) are added to the reaction system as a composite catalyst, and the temperature is raised to 190 °C. The reaction is carried out for 1 - 3 h until no hydrogen sulfide gas is produced, and the hydrogen sulfide tail gas is absorbed by 30 wt% sodium hydroxide solution, and the reaction is completed.

[0059] S2. Post-treatment

[0060] After the reaction solution in step S1 is completed, the temperature is lowered to 120 °C, 200 g of toluene is added, and after refluxing and stirring for 1 h, the temperature is lowered to room temperature, and vacuum filtration is carried out to obtain the filter cake, which is the crude product of phenothiazine. The solvent in the filtrate is removed, and the solvent is recovered and reused. The filter residue after solvent removal is treated as solid waste. The filter cake is placed in a far-infrared vacuum oven at 80 °C and dried for 8 h to obtain 97.21 g of the fine product of phenothiazine, with a yield of 96.88%, a product purity of 99.3%, and a melting range of 184.5 - 185.2 °C.

[0061] Example 2

[0062] S1. Under an inert gas atmosphere, 126.9 g (0.75 mol) of diphenylamine and 51.9 g (1.62 mol) of sulfur are put into a 500 mL four-necked flask, heated to 150 °C, and kept warm for 1 h. Then, 0.75 g of an aqueous solution of methanesulfonic acid (where the mass concentration of methanesulfonic acid is 99%) and 0.076 g of iodine (0.000299 mol) are added to the reaction system as a composite catalyst, and the temperature is raised to 190 °C. The reaction is carried out for 1 - 3 h until no hydrogen sulfide gas is produced. The hydrogen sulfide tail gas is absorbed by a 30 wt% sodium hydroxide solution, and the reaction is completed.

[0063] S2. Post-treatment

[0064] After the reaction solution in step S1 is completed, the temperature is lowered to 110 °C, 300 g of toluene is added, and after refluxing and stirring for 1 h, the temperature is lowered to room temperature, and vacuum filtration is carried out to obtain the filter cake, which is the crude product of phenothiazine. The solvent in the filtrate is removed, and the solvent is recovered and reused. The filter residue after solvent removal is treated as solid waste. The filter cake is placed in a far-infrared vacuum oven at 80 °C and dried for 8 h to obtain 146.3 g of the fine product of phenothiazine, with a yield of 97.01%, a product purity of 99.1%, and a melting range of 184.3 - 185.2 °C.

[0065] Example 3

[0066] S1. Under an inert gas atmosphere, 84.6 g (0.5 mol) of diphenylamine and 34.6 g (1.08 mol) of sulfur are put into a 500 mL four-necked flask, heated to 140 °C, and kept warm for 0.5 h. Then, 0.5 g of methanesulfonic acid (where the mass concentration of methanesulfonic acid is 99%) and 0.025 g of iodine (0.0000985 mol) are added to the reaction system as a composite catalyst, and the temperature is raised to 190 °C. After reacting for 3 h, gas-phase detection shows that diphenylamine remains in the reaction solution, and the reaction time is continued for 4.5 h. Gas-phase detection shows that diphenylamine in the reaction solution has no remaining reaction, and the reaction ends. The total reaction time is 7.5 h. The hydrogen sulfide tail gas is absorbed by a 30 wt% sodium hydroxide solution.

[0067] S2. Post-treatment

[0068] After the reaction in step S1 is completed, cool the reaction solution to 120 °C, add 200 g of toluene, reflux and stir for 1 h, then cool to room temperature, carry out vacuum filtration under reduced pressure, and the obtained filter cake is the crude product of phenothiazine. The filtrate is stripped of the solvent, and the solvent is recovered and reused. The residue after solvent stripping is treated as solid waste. Place the filter cake in a far-infrared vacuum oven at 80 °C and dry for 8 h to obtain 96.81 g of the fine product of phenothiazine, with a yield of 96.4%, a product purity of 99.2%, and a melting range of 184.1 - 185.6 °C.

[0069] Example 4

[0070] S1. Under an inert gas atmosphere, put 84.6 g (0.5 mol) of diphenylamine and 34.6 g (1.08 mol) of sulfur into a 500 mL four-necked flask, heat up to 140 °C, keep warm for 0.5 h, then add 0.5 g of p-toluenesulfonic acid (the purity of p-toluenesulfonic acid is 98%) and 0.45 g of iodine (0.00177 mol) as a composite catalyst to the reaction system, heat up to 190 °C, and react for 1 - 3 h until no hydrogen sulfide gas is produced. The hydrogen sulfide tail gas is absorbed with 30 wt% sodium hydroxide solution, and the reaction is completed.

[0071] S2. Post-treatment

[0072] After the reaction in step S1 is completed, cool the reaction solution to 120 °C, add 200 g of toluene, reflux and stir for 1 h, then cool to room temperature, carry out vacuum filtration under reduced pressure, and the obtained filter cake is the crude product of phenothiazine. The filtrate is stripped of the solvent, and the solvent is recovered and reused. The residue after solvent stripping is treated as solid waste. Place the filter cake in a far-infrared vacuum oven at 80 °C and dry for 8 h to obtain 93.3 g of the fine product of phenothiazine, with a yield of 92.24%, a product purity of 98.5%, a conversion rate of 99.5%, and a melting range of 183.5 - 185.8 °C.

[0073] Comparative Example 1

[0074] The difference between Comparative Example 1 and Example 1 is that only 0.864 g of iodine is added as a catalyst, and methanesulfonic acid is not added, and the other operating conditions are the same as those in Example 1.

[0075] Place the obtained filter cake in a far-infrared vacuum oven at 80 °C and dry for 8 h to obtain 85.1 g of the fine product of phenothiazine, with a yield of 77.3%, a product purity of 90.5%, and a melting range of 183.4 - 185.5 °C.

[0076] Comparative Example 2

[0077] The difference between Comparative Example 2 and Example 1 is only that only 2.592 g of aluminum trichloride is used as a catalyst, iodine and methanesulfonic acid are not used, and the other operating conditions are the same as those in Example 1.

[0078] The prepared filter cake was placed in a far-infrared vacuum oven at 80 °C and dried for 8 h to obtain 82.1 g of high-quality phenothiazine, with a yield of 75.1%, a product purity of 91.2%, and a melting range of 183.2 - 185.5 °C.

[0079] Comparative Example 3

[0080] The difference between Comparative Example 3 and Example 1 is that only 5 g of activated clay (Macklin, purity 98%) was used as the catalyst, and iodine and methanesulfonic acid were not used, while the other operating conditions were the same as those in Example 1.

[0081] The prepared filter cake was placed in a far-infrared vacuum oven at 80 °C and dried for 8 h to obtain 63.2 g of high-quality phenothiazine, with a yield of 55.7%, a product purity of 87.5%, and a melting range of 180.4 - 188.5 °C.

[0082] Comparative Example 4

[0083] The difference between Comparative Example 4 and Example 1 is that only 0.55 g of methanesulfonic acid was used as the catalyst, and iodine was not added, while the other operating conditions were the same as those in Example 1.

[0084] No phenothiazine product was prepared, and the remaining amount of diphenylamine was 99.5%, without participating in the reaction. This is because iodine is the catalytic initiator of the reaction, and when iodine is not added, the reaction cannot occur.

[0085] Comparative Example 5

[0086] S1. Under an inert gas atmosphere, 84.6 g (0.5 mol) of diphenylamine and 34.6 g (‎1.08 mol) of sulfur were added to a 500 mL four-necked flask, heated to 140 °C, and kept warm for 0.5 h. Then, 0.5 g of an aqueous methanesulfonic acid solution (where the mass concentration of methanesulfonic acid was 99%) and 0.5 g of iodine (0.00197 mol) were added to the reaction system as a composite catalyst, and the temperature was raised to 190 °C and the reaction continued for 1 - 3 h until no hydrogen sulfide gas was produced. The hydrogen sulfide tail gas was absorbed with a 30 wt% sodium hydroxide solution, and the reaction was completed.

[0087] S2. Post-treatment

[0088] After the reaction in step S1 was completed, the reaction solution was cooled to 120 °C, 200 g of toluene was added, and after refluxing and stirring for 1 h, it was cooled to room temperature. Then, vacuum filtration was carried out under reduced pressure to obtain a filter cake, which was the crude product of phenothiazine. The solvent in the filtrate was removed, and the recovered solvent was recycled. The filter residue after solvent removal was treated as solid waste. The filter cake was dried in a far-infrared vacuum oven at 80 °C for 8 h to obtain 80.21 g of phenothiazine, with a yield of 64.6% and a product purity of 80.3% (detected by gas chromatography. When dissolved in ether, there were insoluble substances, and it was tested after filtering with a filter membrane). The melting range was 179.3 - 187.4 °C. The obtained product purity and yield both decreased, possibly because of over-catalysis by the dual catalyst, resulting in the formation of high-melting-point insoluble substances.

[0089] Comparative Example 6

[0090] Under an inert gas atmosphere, 84.6 g (0.5 mol) of diphenylamine and 34.6 g (1.08 mol) of sulfur were put into a 500 mL four-necked flask, heated to 140 °C, and kept warm for 0.5 h. Then, 0.5 g of methanesulfonic acid (where the mass concentration of methanesulfonic acid was 99%) and 0.005 g of iodine (0.0000197 mol) were added to the reaction system as a composite catalyst, and the temperature was raised to 190 °C and the reaction continued for 1 - 3 h until almost no hydrogen sulfide gas was produced. The residual amount of diphenylamine in the reaction solution detected by gas chromatography was 99.1%, indicating that the reaction did not occur and there was no further post-treatment.

[0091] Comparative Example 7

[0092] Under an inert gas atmosphere, 84.6 g (0.5 mol) of diphenylamine and 34.6 g (1.08 mol) of sulfur were put into a 500 mL four-necked flask, heated to 140 °C, and kept warm for 0.5 h. Then, 0.5 g of p-toluenesulfonic acid (the purity of p-toluenesulfonic acid was 98%) was added to the reaction system, and the temperature was raised to 190 °C and the reaction was carried out for 3 h. No hydrogen sulfide gas was observed during the reaction, and the residual amount of diphenylamine in the reaction solution detected by gas chromatography was 98.8%, indicating that the reaction did not occur and there was no further post-treatment.

[0093] Test Example 1

[0094] The purity and melting range of the high-quality phenothiazine products prepared in Examples 1 - 4, Comparative Examples 1 - 3, and Comparative Example 5 were tested, and the results are shown in Table 1.

[0095] 1. The purity test adopted a gas chromatography detection method. The analytical instrument used was an Agilent 6820 gas chromatograph; the chromatographic column used was an Agilent HP-5 gas chromatograph; the detector used was an FID (hydrogen flame ionization detector); the injection port mode was split; and the injection port temperature was 280 °C.

[0096] The operating conditions are as shown in Table 1 below:

[0097] Table 1. Operating Conditions of Gas Phase Detection Method

[0098]

[0099]

[0100] 2. The melting range was tested using a melting point instrument, and the analytical instrument was a digital melting point instrument; the heating rate was 20 °C / min; the temperature resolution was 0.1 °C; the accuracy was set to ±0.3 °C (<250 °C) and ±0.5 °C (>250 °C).

[0101] The conversion rates, yields, and purity results of Examples 1-4 and Comparative Examples 1-7 are shown in Table 2 below.

[0102] Table 2. Test Results of Each Example and Comparative Example

[0103] Serial number Conversion rate Yield Purity Melting range (°C) Example 1 99.8％ 96.88％ 99.3％ 184.5-185.2 Example 2 99.6％ 97.01％ 99.1％ 184.3-185.2 Example 3 99.5％ 94.60％ 99.2％ 184.1-185.6 Example 4 99.5％ 92.24％ 98.5％ 183.5-185.8 Comparative example 1 99.6％ 77.30％ 90.5％ 183.4-185.5 Comparative example 2 99.5％ 75.10％ 91.2％ 183.2-185.5 Comparative example 3 99.1％ 55.7％ 87.5％ 180.4-188.5 Comparative example 4 / / / / Comparative example 5 99.5％ 64.60％ 80.3％ 179.3-187.4 Comparative example 6 0.9％ / / / Comparative example 7 / / / /

[0104] The conversion rate was calculated based on diphenylamine. For example, in Example 1, before the reaction, diphenylamine was counted as 100%, and after the reaction, 0.2% of diphenylamine remained in the gas phase detection, indicating that 99.8% of the diphenylamine had reacted. Therefore, the conversion rate was 99.8%.

[0105] As can be seen from Table 2, compared with Comparative Examples 1-3 that only used iodine, aluminum trichloride, or activated clay as catalysts, Examples 1-4 used iodine and methylsulfonic acid or p-toluenesulfonic acid as composite catalysts for the preparation of phenothiazine, with higher conversion rates. The yields and purities of the prepared phenothiazine were much higher than those of Comparative Examples 1-3. Specifically, the purity ≥ 98.5% and the yield ≥ 92.24%; and the melting range of the phenothiazine prepared in Examples 1-4 was narrower. In Comparative Examples 4 and 7, when only methylsulfonic acid or p-toluenesulfonic acid was used as the catalyst, the reaction did not occur. In Comparative Example 5, when the mass ratio of methanesulfonic acid to iodine was 1:1, the purity and yield of the obtained product both decreased. In Comparative Example 6, when the mass ratio of methanesulfonic acid to iodine was 1:0.005, diphenylamine was not formed.

Claims

1. A composite catalyst for preparing phenothiazine, characterized in that, The composite catalyst includes iodine and organic acid, and the mass ratio of the organic acid to the iodine is 1:(0.01-0.95); the organic acid is selected from one or more of methanesulfonic acid, trifluoromethanesulfonic acid, ethanesulfonic acid, and p-toluenesulfonic acid.

2. The composite catalyst for preparing phenothiazine according to claim 1, characterized in that, The mass ratio of the organic acid to the iodine is 1:(0.025-0.75), preferably 1:(0.05-0.5).

3. The composite catalyst for preparing phenothiazine according to claim 1, characterized in that, The organic acid exists in the form of an aqueous solution, and the mass concentration of the organic acid in the aqueous solution is 50%-99%, preferably 80%-99%, more preferably 95%-99%.

4. A method for preparing phenothiazine, characterized in that, It includes reacting diphenylamine and sulfur under the action of a composite catalyst; The composite catalyst includes iodine and organic acid, and the mass ratio of the organic acid to the iodine is 1:(0.01-0.95); the organic acid is selected from one or more of methanesulfonic acid, trifluoromethanesulfonic acid, ethanesulfonic acid, and p-toluenesulfonic acid.

5. The preparation method of phenothiazine according to claim 4, wherein, The method has one or more of the following features: The mass ratio of the organic acid to the iodine is 1:(0.025-0.75), preferably 1:(0.05-0.5); The molar ratio of diphenylamine to iodine is (1-6000):1, preferably (100-6000):1, preferably (200-3000):

1.

6. The preparation method of phenothiazine according to claim 4, wherein, The method has one or more of the following features: The organic acid is added to the reaction system in the form of an aqueous solution of the organic acid, and the mass concentration of the organic acid in the aqueous solution is preferably 50%-99%, more preferably 80%-99%, particularly 95%-99%; The molar ratio of diphenylamine to sulfur is 1:(2-3), preferably 1:(2-2.5).

7. The preparation method of phenothiazine according to claim 4, characterized in that, The method has one or more of the following features: The temperature of the reaction is 170-220°C, preferably 180-210°C, more preferably 190-200°C; The reaction time is 1-8 h, preferably 1-5 h, more preferably 1-3 h.

8. The preparation method of phenothiazine according to claim 4, characterized in that, After the reaction is completed, post-treatment is further included, and the post-treatment includes: mixing the reaction solution after the reaction is completed with an organic solvent, and then performing vacuum filtration and drying to obtain high-quality phenothiazine.

9. The preparation method of phenothiazine according to claim 8, characterized in that, The organic solvent is one or more of ethanol, toluene, ether, dichloromethane, ethylene glycol, ethyl acetate, and tetrahydrofuran.

10. The preparation method of phenothiazine according to claim 8, characterized in that, In the post-treatment, the temperature for mixing the reaction solution and the organic solvent is 105-120°C.

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