WO2024141928A1

WO2024141928A1 - A process for the preparation of 2,2'-dithiobisbenzothiazole

Info

Publication number: WO2024141928A1
Application number: PCT/IB2023/063241
Authority: WO
Inventors: Jayabalan LAKSHMANAN; Janardanan Kapyur NAMBOODIRI; Monojit CHAUDHURI
Original assignee: Nitrex Chemicals India Limited
Priority date: 2022-12-28
Filing date: 2023-12-26
Publication date: 2024-07-04

Abstract

A PROCESS FOR THE PREPARATION OF 2,2'-DITHIOBISBENZOTHIAZOLE The present disclosure relates to a process for the preparation of 2,2'-dithiobisbenzothiazole The process of the present disclosure uses less amount of base and avoids the formation of a huge amount of inorganic salt as a byproduct. The process of the present disclosure uses a minimal amount of base. The process is simple, economical, eco-friendly, and commercially scalable. The present disclosure provides 2,2'-dithiobisbenzothiazole with a comparatively high purity and high yield.

Description

A PROCESS FOR THE PREPARATION OF 2,2’-DITHIOBISBENZOTHIAZOLE

FIELD

The present disclosure relates to a process for the preparation of 2,2’-dithiobisbenzothiazole.

DEFINITIONS

As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicates otherwise.

Initial melting point refers to a temperature at which the solid starts melting and the first drop of liquid is observed.

Compounding refers to a process of blending different additives with raw rubber before vulcanization or curing.

BACKGROUND

The background information herein below relates to the present disclosure but is not necessarily prior art.

2,2’-dithiobisbenzothiazole, commonly known as MBTS, is an important disulfide compound of 2-mercaptobenzothiazole (MBT) family. 2,2’-dithiobisbenzothiazole is widely used as a vulcanization accelerator and also employed as an intermediate in the preparation of pharmaceuticals.

The structural representation of 2,2’-dithiobisbenzothiazole is given as formula (I) below:

2,2’-dithiobisbenzothiazole has a curing range at a temperature of above 140°C, hence it is considered as a safe curing agent. 2,2’-dithiobisbenzothiazole is used as a vulcanization accelerator in the compounding of natural rubber (polyisoprene) and synthetic rubbers (butadiene, styrene-butadiene, nitrile -butadiene rubbers). These rubber-based products have applications in tire treads, conveyor belts, transmission belts, v-belts, footwear, cables, molded or extruded rubber products, and the like.

Currently, the process for the preparation of 2,2’-dithiobisbenzothiazole involves the oxidation of sodium salt of 2-mercaptobenzothiazole. The oxidation is performed using elemental chlorine, chlorine based oxidizing agents such as hypochlorites, chloramines, and the like, alkali bromate solutions, in-situ generated nitrous acid, and hydrogen peroxide in the presence of an acidic species like mineral acids or carbon dioxide that are sufficient enough to neutralize the sodium salt of 2-mercaptobenzothiazole. The process thus produces a huge amount of inorganic salt as a byproduct. Such methods require purification treatments of effluents to remove inorganic salt which is a costlier, energy-intensive, and tedious process. Additionally, the presence of higher salt levels in the effluent will lead to the inefficient treatment of the effluents by secondary biological techniques and hence hazardous to the environment.

Further, the impurities in 2,2’-dithiobisbenzothiazole may affect the quality and storage stability of the final product produced by using 2,2’-dithiobisbenzothiazole.

Still further, the conventional processes produce 2,2’-dithiobisbenzothiazole with a low yield, a low purity due to the presence of over-oxidized acidic impurities, a lower initial melting point, requires energy-intensive operations due to excess usage of solvents and auxiliary raw materials, requires subsequent recoveries for recycling, resulting in final product with undesirable physical properties.

Therefore, there is felt a need to provide a process for the preparation of 2,2’- dithiobisbenzothiazole that mitigates the aforestated drawbacks or at least provide an alternative solution.

OBJECTS

Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:

It is an object of the present disclosure to ameliorate one or more problems of the background or to at least provide a useful alternative.

Another object of the present disclosure is to provide a process for the preparation of 2,2’- dithiobisbenzothiazole . Yet another object of the present disclosure is to provide a process for the preparation of 2,2’- dithiobisbenzothiazole with a comparatively better purity and yield.

Still another object of the present disclosure is to provide a simple and cost-effective process for the preparation of 2,2’-dithiobisbenzothiazole.

Another object of the present disclosure is to provide an environment-friendly and commercially scalable process for the preparation of 2,2’-dithiobisbenzothiazole.

Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.

SUMMARY

The present disclosure relates to a process for the preparation of 2,2’-dithiobisbenzothiazole. The process comprises reacting 2-mercaptobenzothiazole, a base, and optionally an agglomerating agent in a first fluid medium under stirring at a first predetermined temperature for a first predetermined time period to obtain a slurry. The slurry is oxidized with an oxidizing agent for a second predetermined time period at a second predetermined temperature to obtain a product mass comprising 2,2’-dithiobisbenzothiazole. The 2,2’- dithiobisbenzothiazole in the product mass is separated followed by drying at a third predetermined temperature for a third predetermined time period to obtain 2,2’- dithiobisbenzothiazole .

In an embodiment of the present disclosure, the base is at least one selected from the group consisting of ammonia, tertiary butylamine, cyclohexylamine, and sodium carbonate.

In an embodiment of the present disclosure, the agglomerating agent is at least one selected from the group consisting of toluene, o-xylene, m-xylene, p-xylene, and xylene mixture.

In an embodiment of the present disclosure, the first fluid medium is selected from the group consisting of water, and aqueous alcohol.

In an embodiment of the present disclosure, the aqueous alcohol is at least one selected from the group consisting of aqueous methanol, aqueous ethanol, and aqueous isopropanol; wherein the concentration of aqueous alcohol is in the range of 60% w/w to 95% w/w.

In an embodiment of the present disclosure, the mole ratio of the base to 2- mercaptobenzothiazole is in the range of 1 : 10 to 1: 18. In an embodiment of the present disclosure, the mass ratio of the agglomerating agent to 2- mercaptobenzothiazole is in the range of 1:5 to 1: 15.

In an embodiment of the present disclosure, the ratio of 2-mercaptobenzothiazole to the first fluid medium is in the range of 1:3 w/v to 1:5 w/v.

In an embodiment of the present disclosure, the first predetermined temperature is in the range of 40°C to 80°C.

In an embodiment of the present disclosure, the first predetermined time period is in the range of 10 min to 30 min.

In an embodiment of the present disclosure, the oxidizing agent is hydrogen peroxide.

In an embodiment of the present disclosure, the concentration of the oxidizing agent is in the range of 2 w/w % to 5 w/w %.

In an embodiment of the present disclosure, the oxidizing agent is added in at least two portions to obtain the product mass comprising 2,2’-dithiobisbenzothiazole.

In an embodiment of the present disclosure, the first portion of the oxidizing agent is in an amount in the range of 70% to 80% of the total weight of the oxidizing agent.

In an embodiment of the present disclosure, the second portion of the oxidizing agent is in an amount in the range of 20% to 30% of the total weight of the oxidizing agent.

In an embodiment of the present disclosure, the second predetermined temperature is in the range of 40°C to 80°C.

In an embodiment of the present disclosure, the second predetermined time period is in the range of 3 hours to 7 hours.

In an embodiment of the present disclosure, the mole ratio of the oxidizing agent to 2- mercaptobenzothiazole is in the range of 1: 1.5 to 1:2.5.

In an embodiment of the present disclosure, the third predetermined temperature is in the range of 90°C to 120°C.

In an embodiment of the present disclosure, the third predetermined time period is in the range of 5 hours to 10 hours.

In an embodiment of the present disclosure, the yield of 2,2’-dithiobisbenzothiazole is in the range of 96% to 99%. In an embodiment of the present disclosure, the purity of 2,2’-dithiobisbenzothiazole is in the range of 97% to 99%.

DETAILED DESCRIPTION

Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.

The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.

As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed elements.

The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer or section from another component, region, layer or section. Terms such as first, second, third etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.

Currently, the process for the preparation of 2,2’-dithiobisbenzothiazole involves the oxidation of sodium salt of 2-mercaptobenzothiazole. The oxidation is performed using elemental chlorine, chlorine based oxidizing agents such as hypochlorites, chloramines, and the like, alkali bromate solutions, in-situ generated nitrous acid, and hydrogen peroxide in the presence of an acidic species like mineral acids or carbon dioxide, that are sufficient enough to neutralize the sodium salt of 2-mercaptobenzothiazole. The process thus produces a huge amount of inorganic salt as a byproduct. Such methods require purification treatments to remove inorganic salt which is a costlier, energy-intensive, and tedious process. Additionally, the presence of higher salt levels in the effluent will lead to the inefficient treatment of the effluents by secondary biological techniques and hence hazardous to the environment.

Still further, the conventional processes produce 2,2’-dithiobisbenzothiazole with a low yield, a low purity due to the presence of over-oxidized acidic impurities, a lower initial melting point, requires energy-intensive operations due to excess usage of solvents and auxiliary raw materials, requires subsequent recoveries for recycling, and resulting in a final product with undesirable physical properties.

The present disclosure provides an improved process for the preparation of 2,2’- dithiobisbenzothiazole .

The process of the present disclosure is simple, environment friendly, economical, results in improved yield and higher purity of 2,2’-dithiobisbenzothiazole, and is commercially scalable.

In an aspect of the present disclosure, there is provided a process for the preparation of 2,2’- dithiobisbenzothiazole .

The process is described in detail.

In a first step, 2-mercaptobenzothiazole, a base, and optionally an agglomerating agent are reacted in a first fluid medium under stirring at a first predetermined temperature for a first predetermined time period to obtain a slurry.

In an embodiment of the present disclosure, a predetermined amount of 2- mercaptobenzothiazole is mixed with a predetermined amount of a first fluid medium followed by adding a base and an agglomerating agent under stirring at a first predetermined temperature for a first predetermined time period to obtain a slurry. In an embodiment of the present disclosure, the base is at least one selected from the group consisting of ammonia, tertiary butylamine, cyclohexylamine, and sodium carbonate. In an exemplary embodiment of the present disclosure, the base is ammonia.

In an embodiment of the present disclosure, the bases are water soluble and can be recovered after filtration from the mother liquor (filtrate) by distillation and/or concentration.

In an embodiment of the present disclosure, the agglomerating agent is at least one selected from the group consisting of toluene, o-xylene, m-xylene, p-xylene, and xylene mixture. In an exemplary embodiment of the present disclosure, the agglomerating agent is toluene. In another exemplary embodiment of the present disclosure, the agglomerating agent is o- xylene.

In an embodiment of the present disclosure, the xylene mixture is a combination of o-xylene, m-xylene, and p-xylene.

In an embodiment of the present disclosure, the first fluid medium is selected from the group consisting of water and aqueous alcohol. In an exemplary embodiment of the present disclosure, the first fluid medium is water. In another exemplary embodiment of the present disclosure, the first fluid medium is aqueous alcohol.

In an embodiment of the present disclosure, the aqueous alcohol is at least one selected from the group consisting of aqueous methanol, aqueous ethanol, and aqueous isopropanol; wherein the concentration of aqueous alcohol used is in the range of 60% w/w to 95% w/w.

In an exemplary embodiment of the present disclosure, the aqueous alcohol is aqueous isopropanol. In another exemplary embodiment of the present disclosure, the aqueous alcohol is aqueous methanol.

In an exemplary embodiment of the present disclosure, the concentration of aqueous alcohol is 75% w/w. In another exemplary embodiment of the present disclosure, the concentration of aqueous alcohol is 90% w/w.

In an embodiment of the present disclosure, the agglomerating agent is used when water is used as a first fluid medium.

In an embodiment of the present disclosure, the mole ratio of the base to 2- mercaptobenzothiazole is in the range of 1: 10 to 1: 18. In an exemplary embodiment of the present disclosure, the mole ratio of the base to 2-mercaptobenzothiazole is 1: 12.5. In another exemplary embodiment of the present disclosure, the mole ratio of the base to 2- mercaptobenzothiazole is 1: 14.4. In still another exemplary embodiment of the present disclosure, the mole ratio of the base to 2-mercaptobenzothiazole is 1: 14.5.

In an embodiment of the present disclosure, the mass ratio of the agglomerating agent to 2- mercaptobenzothiazole is in the range of 1:5 to 1: 15. In an exemplary embodiment of the present disclosure, the mass ratio of the agglomerating agent to 2-mercaptobenzothiazole is 1:9.6.

In an embodiment of the present disclosure, the ratio of the 2-mercaptobenzothiazole to the first fluid medium is in the range of 1:3 w/v to 1:5 w/v. In an exemplary embodiment of the present disclosure, the ratio of the 2-mercaptobenzothiazole to the first fluid medium is 1:4 w/v.

In an embodiment of the present disclosure, the first predetermined temperature is in the range of 40°C to 80°C. In an exemplary embodiment of the present disclosure, the first predetermined temperature is 60°C.

In an embodiment of the present disclosure, the first predetermined time period is in the range of 10 min to 30 min. In an exemplary embodiment of the present disclosure, the first predetermined time period is 15 min.

In a second step, the slurry is oxidized with an oxidizing agent at a second predetermined time period at a second predetermined temperature to obtain a product mass comprising 2,2’- dithiobisbenzothiazole .

In an embodiment of the present disclosure, the oxidizing agent is in aqueous form.

In an embodiment of the present disclosure, the second predetermined time period is in the range of 3 hours to 7 hours. In an exemplary embodiment of the present disclosure, the second predetermined time period is 4 hours. In another exemplary embodiment of the present disclosure, the second predetermined time period is 5 hours.

In an embodiment of the present disclosure, the second predetermined temperature is in the range of 40°C to 80°C. In an exemplary embodiment of the present disclosure, the second predetermined temperature is 65°C.

In an embodiment of the present disclosure, the oxidizing agent is slowly added at a uniform rate at a temperature in the range of 40°C to 80°C for a time period in the range of 3 hours to 7 hours. In another embodiment of the present disclosure, the oxidizing agent is added in at least two portions to obtain the product mass comprising 2,2’-dithiobisbenzothiazole. In an exemplary embodiment of the present disclosure, the oxidizing agent is added in two portions.

In an embodiment of the present disclosure, the concentration of an aqueous oxidizing agent is in the range of 2 w/w% to 5 w/w%. In an exemplary embodiment of the present disclosure, the concentration of an aqueous oxidizing agent is 3 w/w%.

In an embodiment of the present disclosure, the mole ratio of the oxidizing agent to 2- mercaptobenzothiazole is in the range of 1: 1.5 to 1:2.5. In an exemplary embodiment of the present disclosure, the mole ratio of the oxidizing agent to 2-mercaptobenzothiazole is 1: 1.74. In another exemplary embodiment of the present disclosure, the mole of the oxidizing agent to 2-mercaptobenzothiazole is 1: 1.9. In still another exemplary embodiment of the present disclosure, the mole of the oxidizing agent to 2-mercaptobenzothiazole is 1: 1.67.

In an embodiment of the present disclosure, the first portion of the oxidizing agent is added in an amount in the range of 70% to 80% of the total weight of the oxidizing agent. In an exemplary embodiment of the present disclosure, the first portion of the oxidizing agent is 75%.

In an embodiment of the present disclosure, the first portion of the oxidizing agent is slowly added for a time period in the range of 2 hours to 4 hours. In an exemplary embodiment of the present disclosure, the first portion of the oxidizing agent is slowly added for 3 hours.

In an embodiment of the present disclosure, the second portion of the oxidizing agent is added in an amount in the range of 20% to 30% of the total weight of the oxidizing agent. In an exemplary embodiment of the present disclosure, the second portion of the oxidizing agent is 25%.

In an embodiment of the present disclosure, the second portion of the oxidizing agent is added slowly in comparison to the first portion of the oxidizing agent.

In an embodiment of the present disclosure, the second portion of the oxidizing agent is slowly added for a time period in the range of 1 hour to 3 hours. In an exemplary embodiment of the present disclosure, the second portion of the oxidizing agent is slowly added for 2 hours. In a third step, 2,2’-dithiobisbenzothiazole in the product mass is separated followed by drying at a third predetermined temperature for a third predetermined time period to obtain 2,2’-dithiobisbenzothiazole.

In an embodiment of the present disclosure, the product mass comprising 2,2’ - dithiobisbenzothiazole is maintained at a third predetermined temperature, and the product mass is fdtered to obtain a wet cake and a filtrate (mother liquor). The wet cake is washed by using a second fluid medium followed by drying at the third predetermined temperature for a third predetermined time period to obtain 2,2 ’-dithiobisbenzothiazole.

In an embodiment of the present disclosure, the second fluid medium is selected from the group consisting of water, and aqueous alcohol. In an exemplary embodiment of the present disclosure, the second fluid medium is water.

In an embodiment of the present disclosure, the aqueous alcohol is at least one selected from the group consisting of aqueous methanol, aqueous ethanol, and aqueous isopropanol; wherein the concentration of aqueous alcohol is in the range of 60% w/w to 95 % w/w.

In an embodiment of the present disclosure, the third predetermined temperature is in the range of 90°C to 120°C. In an exemplary embodiment of the present disclosure, the third predetermined temperature is 105°C.

In an embodiment of the present disclosure, the third predetermined time period is in the range of 5 hours to 10 hours. In an exemplary embodiment of the present disclosure, the third predetermined time period is 8 hours.

In an embodiment of the present disclosure, the wet cake is washed till the pH of the washings is neutral.

A schematic representation for the preparation of 2,2 ’-dithiobisbenzothiazole is given below as scheme A.

2, 2'-D ith iobisbenzol hiaz ole

Scheme A In an embodiment of the present disclosure, 2,2’-dithiobisbenzothiazole has a yield in the range of 96% to 99%.

In an embodiment of the present disclosure, 2,2’-dithiobisbenzothiazole has a purity in the range of 97% to 99%.

In an embodiment of the present disclosure, the free 2-mercaptobenzothiazole in 2,2’- dithiobisbenzothiazole is below 1%.

In an embodiment of the present disclosure, 2,2’-dithiobisbenzothiazole is evaluated for initial melting point and bulk density, the critical parameters to evaluate the quality of the product.

In an embodiment of the present disclosure, the bulk density of 2,2’-dithiobisbenzothiazole is in the range of 0.4 g/cc to 0.5 g/cc.

In accordance with the present disclosure, the bulk density below 0.4 g/cc is undesirable because it leads to increased water retention in the wet cake before drying thereby making the process in-efficient due to the fact that the water retention in the wet cake creates issues during fdtration, milling, and drying, as well as challenges in end-use handling at industrial scale. A bulk density of above 0.5 g/cc is unfavorable in practical use as there is a consequent reduction in the initial melting point, in general, owing to the presence of higher unreacted 2- mercaptobenzothiazole in the given reaction conditions. Thus, the initial melting point and the bulk density are the critical parameters to ensure the quality of the final product.

In an embodiment of the present disclosure, the free MBT in 2,2’-dithiobisbenzothiazole is in the range of 0.35% to 0.5%.

The foregoing description of the embodiments has been provided for purposes of illustration and is not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.

The present disclosure is further described in light of the following experiments which are set forth for illustration purposes only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to an industrial/commercial scale and the results obtained can be extrapolated to industrial scale.

EXPERIMENTAL DETAILS

EXAMPLE 1: Preparation of 2,2’-dithiobisbenzothiazole in accordance with the present disclosure

41.75 g (0.25 Mole) of 2-mercaptobenzothiazole (titrimetric purity 98.4%, initial melting point of 174.5°C) and 170 ml of water were charged into a reactor, followed by adding 1.30 ml of 25 % w/w ammonia solution (6.96 mole % to 2-mercaptobenzothiazole) and 5 ml of toluene (10.42 weight% to 2-mercaptobenzothiazole) in about 15 min under stirring at 200 rpm at 60°C to obtain a slurry. Maintaining the slurry at 65°C, a first portion of 112 g (0.0988 mole) of the aqueous hydrogen peroxide (3.00% w/w) was slowly added under stirring for 3 hours to obtain a mass. To the obtained mass, a second portion of 37 g (0.0326 mole) of the aqueous hydrogen peroxide (3.00 % w/w) was slowly added over 2 hours at 65 °C to obtain a reaction mass. The reaction mass was stabilized/equilibrated by stirring at 65°C for 30 minutes to obtain a product mass comprising 2,2’-dithiobisbenzothiazole. Maintaining the temperature at 60°C, the product mass was filtered by using a Buchner funnel set-up under vacuum to obtain a wet cake and a filtrate (280 ml). The wet cake was washed with water until the washings achieved a neutral pH to obtain a washed wet cake. The washed wet cake was dried at 105°C for 480 minutes to obtain 40.7 g of 2,2’-dithiobisbenzothiazole. The physical properties of the product are provided below in Table 1.

Table 1: Physical Properties of 2, 2’-dithiobisbenzothiazole

EXAMPLE 2: Preparation of 2,2’-dithiobisbenzothiazole in accordance with the present disclosure

41.75 g (0.25 Mole) of 2-mercaptobenzothiazole (titrimetric purity 98.4%, initial melting point of 174.5°C) and 170 ml of water were charged into a reactor, followed by adding 1.30 ml of 25% w/w ammonia solution (6.96 mole% to 2-Mercaptobenzothiazole) and 5 ml of toluene (10.42 weight% to 2-Mercaptobenzothiazole) in about 15 min under stirring at 200 rpm at 60°C to obtain a slurry. Maintaining the slurry at 65°C, a first portion of 112 g (0.0988 mole) of the aqueous hydrogen peroxide (3.00% w/w) was slowly added under stirring for 3 hours to obtain a mass. To the so obtained mass, a second portion of 37 g (0.0326 mole) of the aqueous hydrogen peroxide (3.00% w/w) was slowly added over 2 hours at 65°C to obtain a reaction mass. The reaction mass was stabilized/equilibrated by stirring at 65 °C for 30 minutes to obtain a product mass comprising 2,2’-dithiobisbenzothiazole. Maintaining the temperature at 60°C, the product mass was filtered by using a Buchner funnel set-up under vacuum to obtain a wet cake and a filtrate (280 ml). The wet cake was washed with water until the washings achieved a neutral pH to obtain a washed wet cake. The washed wet cake was dried at 105°C for 480 minutes to obtain 40.75 g of 2,2’-dithiobisbenzothiazole. The physical properties of the product are provided below in Table 2.

Table 2: Physical properties of 2,2’-dithiobisbenzothiazole

EXAMPLE 3: Preparation of 2,2’-dithiobisbenzothiazole in accordance with the present disclosure

100 g (0.5988 Mole) of 2-mercaptobenzothiazole (titrimetric purity 98.4%, initial melting point of 174.5°C) and 407 ml of water were charged into a reactor, followed by adding 3.10 ml of 25% w/w ammonia solution (6.93 mole % to 2-Mercaptobenzothiazole) and 12 ml of toluene (10.44 weight% to 2-Mercaptobenzothiazole) in about 15 min under stirring at 200 rpm at 60°C to obtain a slurry. Maintaining the slurry at 65°C, a first portion of 292.5 g hydrogen peroxide (3.00% w/w) (0.2580 mole) was slowly added under stirring for 3 hours to obtain a mass. To the so obtained mass, a second portion of 97.5 g of the aqueous hydrogen peroxide (3.00 % w/w) (0.0860 mole) was slowly added over 2 hours at 65°C to obtain a reaction mass. The reaction mass was stabilized/equilibrated by stirring at 65 °C for 30 minutes to obtain the product mass comprising 2,2’-dithiobisbenzothiazole. Maintaining the temperature at 60°C, the product mass was filtered by using a Buchner funnel set-up under vacuum to obtain a wet cake and a filtrate (755 ml mother liquor). The wet cake was washed with water until the washings achieved a neutral pH to obtain a washed wet cake. The washed wet cake was dried at 105°C for 480 minutes to obtain 97.77 g of 2,2’- dithiobisbenzothiazole. The physical properties of the product are provided below in Table 3.

Table 3: Physical properties of 2,2’-dithiobisbenzothiazole

EXAMPLE 4: Preparation of 2,2’-dithiobisbenzothiazole (oxidizing agent added in one lot) in accordance with the present disclosure

41.75 g (0.25 Mole) of 2-mercaptobenzothiazole (titrimetric purity 98.4%, initial melting point of 174.5°C) and 170 ml of water were charged into a reactor, followed by adding 1.30 ml of 25% w/w ammonia solution (6.96 mole% to 2- Mercaptobenzothiazole) and 5 ml of toluene (10.42 weight% to 2-Mercaptobenzothiazole) in about 15 min under stirring at 200 rpm at 60°C to obtain a slurry. Maintaining the slurry at 65 °C 149 g (0.1315 Mole) of aqueous hydrogen peroxide (3.00% w/w) was slowly added under stirring for four hours at a uniform rate to obtain a reaction mass. The reaction mass was stabilized/equilibrated by stirring at 65°C for 30 minutes to obtain a product mass comprising 2,2’- dithiobisbenzothiazole. Maintaining the temperature at 60°C, the product mass was fdtered by using a Buchner funnel set-up under vacuum to obtain a wet cake and a filtrate (280 ml mother liquor). The wet cake was washed with water until the washings achieved a neutral pH to obtain a washed wet cake. The washed wet cake was dried at 105°C for 480 minutes to obtain 40.75 g of 2,2’-dithiobisbenzothiazole. The physical properties of the product are provided below in Table 4.

Table 4: Physical properties of 2,2’-dithiobisbenzothiazole

EXAMPLE 5: Preparation of 2,2’-dithiobisbenzothiazole by varying the amount of the base 2,2’-dithiobisbenzothiazole was prepared similarly to the process steps as disclosed in Example 1 by varying the quantity of the base (NH₃). The physical properties of the product were evaluated, the results are provided below in Table 5.

Table 5: Physical properties of 2,2’-dithiobisbenzothiazole

It is evident from the above table that the amount of base is important to achieve the desired physical properties of 2,2’-dithiobisbenzothiazole. The reaction in the absence of the base results in producing the MBTS with low yield, low purity, low initial melting point, and leaving a high amount of unreacted mercaptobenzothiazole leading to increased bulk density (refer to Example 5.1 in Table 5). The product (MBTS) with higher unreacted mercaptobenzothiazole and in turn a lower initial melting point is not desirable. Further, the use of a high amount of base (>8 Mole % to MBT) leaves a high amount of water in the cake, which requires a longer period for drying. Thus, the optimum amount of ammonia is essential to provide 2,2’-dithiobisbenzothiazole with desired physical properties, leaving a low amount of water in the wet cake (refer to examples 1 and 5.3 in Table 5).

EXAMPLE 6: Preparation of 2,2’-dithiobisbenzothiazole by varying the amount of the agglomerating agent

2,2’-dithiobisbenzothiazole was prepared similarly to the process steps as disclosed in Example 1 by varying the quantity of the agglomerating agent (toluene). The physical properties of the product were evaluated, the results are provided below in Table 6.

Table 6: Physical properties of 2,2’-dithiobisbenzothiazole

It is evident from the above table that the amount of agglomerating agent is important to achieve the desired physical properties of the 2,2’-dithiobisbenzothiazole. The reaction in the absence of the agglomerating agent results in producing the MBTS with low yield, low purity, low initial melting point, and leaving a high amount of unreacted mercaptobenzothiazole with low bulk density for the dried product (refer to Example 6. 1 in Table 6). Thus in Example 6.1, though the unreacted 2-mercaptobenzothiazole is higher, the bulk density is lower due to the absence of the agglomerating agent in the reaction. Further, the use of a low amount agglomerating agent leaves a high amount of water in the cake, which requires a longer period for drying. Thus, the optimum amount of agglomerating agent (Example 1) is essential to provide the 2,2’-dithiobisbenzothiazole with desired physical properties, leaving a low amount of water in the wet cake.

EXAMPLE 7: Preparation of 2,2’-dithiobisbenzothiazole in the absence of the base and the agglomerating agent The 2,2’-dithiobisbenzothiazole was prepared in the absence of the base (NH₃) and the agglomerating agent (toluene) by using the same process disclosed in Example 1. The physical properties of the product were evaluated, the results are provided below in Table 7.

Table 7: Physical properties of 2,2’-dithiobisbenzothiazole

It is evident from the above table that the base and the agglomerating agent are important to achieve the desired physical properties of the 2,2’-dithiobisbenzothiazole. The reaction in the absence of the base and agglomerating agent results in the production of 2, 2’- 5 dithiobisbenzothiazole (the final product) with low purity, reduced initial melting point, and leaving a high amount of unreacted mercaptobenzothiazole (refer Examples 7.1 in Table 7). Further, the absence of the base and the agglomerating agent leaves a high amount of water in the cake with low bulk density for the dried product, which requires a longer period for drying which is not economical at commercial scale production. Thus, the optimum amount 0 of the base and the agglomerating agent are essential to provide the 2,2’- dithiobisbenzothiazole with desired physical properties, leaving a low amount of water in the wet cake.

EXAMPLE 8: Preparation of 2,2’-dithiobisbenzothiazole by the addition of the oxidizing agent in one lot in less time period 5 2,2 ’-dithiobisbenzothiazole was prepared by the addition of an oxidizing agent in one lot by using the same process disclosed in Example 4, except that the complete oxidizing agent is added in 15 minutes. The physical properties of the product were evaluated, the results are provided below in Table 8.

Table 8: Physical properties of 2,2 ’-dithiobisbenzothiazole

It is evident from the above table that the addition of an oxidizing agent in respect of time and the manner by which it was added, is important to achieve the desired physical properties of the 2,2’-dithiobisbenzothiazole. The reaction in which the oxidizing agent is added in one lot

5 in less time results in producing the final product with low yield, low purity, reduced initial melting point, and acidic pH of the mass (pH 2.0) leaving a high amount of unreacted mercaptobenzothiazole (refer Examples 8.1, 8.2 and 8.3 in Table 7). Further, the oxidizing agent was added in one lot leaving a high amount of water in the cake which requires a longer period for drying (refer to examples 8.1 and 8.2 in Table 7).

It is evident from the above data that an increased quantity of the base (to counter the mass becoming acidic at the end of oxidation) decreases the yield of the product indicating the formation of more acidic impurities that are distributed in the mother liquor as soluble salt of the base and in the product. Further, the filtrate was evaluated for the presence of Total Dissolved Solids (TDS) and Chemical Oxygen Demand (COD). The results are provided5 below in Table 9.

Table 9: Estimation of solid content in 2,2’-dithiobisbenzothiazole

It is evident from the above table that as the quantity of base is increased in experiments 8.2 and 8.3, thereby having an increased amount of TDS and COD in the mother liquor.

EXAMPLE 9: Preparation of 2,2’-dithiobisbenzothiazole by using o-xylene as an agglomerating agent

2,2’-dithiobisbenzothiazole was prepared similarly to the process steps as disclosed in Example 1 with o-xylene as an agglomerating agent. The physical properties of the products were evaluated, the results are provided below in Table 10. 5 Table 10: Physical properties of 2,2’-dithiobisbenzothiazole

S

It is evident from the above table that the reaction in the presence of the o-xylene as an agglomerating agent results in producing the MBTS with less yield, and purity of MBTS with a comparatively low initial melting point when compared with the results obtained from

5 Toluene as an agglomerating agent.

EXAMPLE 10: Preparation of 2,2’-dithiobisbenzothiazole with bases other than ammonia 2,2’-dithiobisbenzothiazole was prepared similarly to the process steps as disclosed in Example 1 by using bases other than ammonia. The physical properties of the products were 10 evaluated, the results are provided below in Table 11.

Table 11: Physical properties of 2,2’-dithiobisbenzothiazole

*H₂O₂ added by lots at the end after 0.55 mole until free MBT tested is below 0.5 % It is evident from the above table that the base is important to achieve the desired physical properties of 2,2’-dithiobisbenzothiazole. The reaction in the presence of a base other than ammonia results in producing the final product (2,2’-dithiobisbenzothiazole) with either a low yield and/or leaving a high amount of unreacted mercaptobenzothiazole. Further, the use of bases other than ammonia leaves a high amount of water in the cake, which requires a longer period for drying.

EXAMPLE 11: Preparation of 2,2’-dithiobisbenzothiazole in accordance with the present disclosure using aqueous isopropanol

41.75 g (0.25 Mole) of 2-mercaptobenzothiazole (titrimetric purity 98.4%, initial melting point of 174.5°C) and 170 ml of 90% w/w aqueous isopropanol were charged into a reactor, followed by adding 1.30 ml of 25 % w/w ammonia solution (6.96 Mole % to 2- Mercaptobenzothiazole) in about 15 min under stirring at 200 rpm at 60°C to obtain a slurry. Maintaining the slurry at 65°C, a first portion of 127.5 g (0.1125 mole) of the aqueous hydrogen peroxide (3.00% w/w) was slowly added under stirring for 3 hours to obtain a mass. To the so obtained mass, a second portion of 42.5 g (0.0375 mole) of the aqueous hydrogen peroxide (3.00 % w/w) was slowly added for 2 hours at 65°C to obtain a reaction mass. The reaction mass was stabilized/equilibrated by stirring at 65°C for 30 minutes to obtain a product mass comprising 2,2’-dithiobisbenzothiazole. Maintaining the temperature at 60°C, the product mass was filtered by using a Buchner funnel set-up under vacuum to obtain a wet cake and a filtrate (282 ml of mother liquor). The wet cake was washed with water until the washings achieved a neutral pH to obtain a washed wet cake. The washed wet cake was dried at 105°C for 480 minutes to obtain 40.30 g of 2,2’-dithiobisbenzothiazole. The physical properties of the product are provided below in Table 12.

Table 12: Physical Properties of 2,2’-dithiobisbenzothiazole

EXAMPLE 12: Preparation of 2,2’-dithiobisbenzothiazole in accordance with the present disclosure using aqueous methyl alcohol

41.75 g (0.25 Mole) of 2-mercaptobenzothiazole (titrimetric purity 98.4%, initial melting point of 174.5°C) and 170 ml of 75% w/w aqueous methanol were charged into a reactor, followed by adding 1.30 ml of 25 % w/w ammonia solution (6.96 Mole % to 2- Mercaptobenzothiazole) in about 15 min under stirring at 200 rpm at 60°C to obtain a slurry. Maintaining the slurry at 65°C, a first portion of 127.5 g (0.1125 mole) of the aqueous hydrogen peroxide (3.00% w/w) was slowly added under stirring for 3 hours to obtain a mass. To the so obtained mass, a second portion of 42.5 g (0.0375 mole) of the aqueous hydrogen peroxide (3.00 % w/w) was slowly added for 2 hours at 65°C to obtain a reaction mass. The reaction mass was stabilized/equilibrated by stirring at 65°C for 30 minutes to obtain a product mass comprising 2,2’-dithiobisbenzothiazole. Maintaining the temperature at 60°C, the product mass was filtered by using a Buchner funnel set-up under vacuum to obtain a wet cake and a filtrate (270 ml of mother liquor). The wet cake was washed with water until the washings achieved a neutral pH to obtain a washed wet cake. The washed wet cake was dried at 105°C for 480 minutes to obtain 40.40 g of 2,2’-dithiobisbenzothiazole. The physical properties of the product are provided below in Table 13.

Table 13: Physical Properties of 2,2’-dithiobisbenzothiazole

The experiments above clearly demonstrate that substituting water with aqueous alcohols like aqueous isopropanol and aqueous methanol yields comparable results.

It is evident from the above experiments that substituting water with aqueous alcohols like aqueous isopropanol and aqueous methanol yields similar results.

TECHNICAL ADVANCEMENTS

The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a process for the preparation of 2,2’-dithiobisbenzothiazole that:

• avoids the use of sodium salt of 2 -mercaptobenzothiazole;

• uses a minimum amount of base;

• the organic bases used can be recovered and reused;

• avoids the formation of inorganic salts thus avoiding the tedious purification and effluent treatment processes;

• results in a low level of Total Dissolved Solids (TDS) and chemical oxygen demand (COD) in the effluent;

• provides 2,2’-dithiobisbenzothiazole with a higher initial melting point;

• is simple and economical; and

• gives a higher yield and purity of 2,2’-dithiobisbenzothiazole in comparison to the conventional processes.

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

The foregoing description of the specific embodiments so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.

While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

Claims

CLAIMS:

1. A process for the preparation of 2,2’-dithiobisbenzothiazole, said process comprising the following steps: a. reacting 2-mercaptobenzothiazole, a base, and optionally an agglomerating agent in a first fluid medium under stirring at a first predetermined temperature for a first predetermined time period to obtain a slurry; b. oxidizing said slurry with an oxidizing agent for a second predetermined time period at a second predetermined temperature to obtain a product mass comprising 2,2’-dithiobisbenzothiazole; and c. separating 2,2’-dithiobisbenzothiazole from said product mass followed by drying at a third predetermined temperature for a third predetermined time period to obtain 2,2’-dithiobisbenzothiazole.

2. The process as claimed in claim 1, wherein said base is at least one selected from the group consisting of ammonia, tertiary butylamine, cyclohexylamine, and sodium carbonate.

3. The process as claimed in claim 1, wherein said agglomerating agent is at least one selected from the group consisting of toluene, o-xylene, m-xylene, p-xylene, and xylene mixture.

4. The process as claimed in claim 1, wherein said first fluid medium is selected from the group consisting of water and aqueous alcohol.

5. The process as claimed in claim 4, wherein said aqueous alcohol is at least one selected from the group consisting of aqueous methanol, aqueous ethanol, and aqueous isopropanol; wherein the concentration of aqueous alcohol is in the range of 60% w/w to 95% w/w.

6. The process as claimed in claim 1, wherein said first predetermined temperature is in the range of 40°C to 80°C.

7. The process as claimed in claim 1, wherein said first predetermined time period is in the range of 10 min to 30 min.

8. The process as claimed in claim 1, wherein the ratio of said 2- mercaptobenzothiazole to said first fluid medium is in the range of 1:3 (w/v) to 1:5 (w/v).

9. The process as claimed in claim 1, wherein the mole ratio of said base to 2- mercaptobenzothiazole is in the range of 1 : 10 to 1: 18.

10. The process as claimed in claim 1, wherein the mass ratio of said agglomerating agent to said 2-mercaptobenzothiazole is in the range of 1:5 to 1: 15.

11. The process as claimed in claim 1, wherein said oxidizing agent is hydrogen peroxide.

12. The process as claimed in claim 1, wherein said oxidizing agent is added in at least two portions to obtain the product mass comprising 2,2’-dithiobisbenzothiazole.

13. The process as claimed in claim 1, the mole ratio of said oxidizing agent to said 2- mercaptobenzothiazole is in the range of 1: 1.5 to 1:2.5.

14. The process as claimed in claim 12, wherein a first portion of said oxidizing agent is in the range of 70 mass% to 80 mass% with respect to the total mass of oxidizing agent.

15. The process as claimed in claim 12, wherein a second portion of said oxidizing agent is in the range of 20 mass% to 30 mass% with respect to the total mass of oxidizing agent.

16. The process as claimed in claim 1, wherein said second predetermined temperature is in the range of 40°C to 80°C.

17. The process as claimed in claim 1, wherein said second predetermined time period is in the range of 3 hours to 7 hours.

18. The process as claimed in claim 1, wherein the concentration of said oxidizing agent is in the range of 2 w/w% to 5 w/w%.

19. The process as claimed in claim 1, wherein a yield of 2,2’-dithiobisbenzothiazole is in the range of 96% to 99%.

20. The process as claimed in claim 1, wherein a purity of 2,2’-dithiobisbenzothiazole is in the range of 97% to 99%.

21. The process as claimed in claim 1, wherein said third predetermined temperature is in the range of 90°C to 120 °C and said third predetermined time period is in the range of 5 hours to 10 hours to obtain 2,2’-dithiobisbenzothiazole.