FIELD

The present disclosure relates to a process for producing aromatic dicarboxylic acids. Particularly, the present disclosure relates to a process for purification of aromatic dicarboxylic acids.

More particularly, the present disclosure relates to a process for purification of Terephthalic acid.

BACKGROUND

Aromatic dicarboxylic acid such as Terephthalic acid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. There is also a demand for terephthalic acid in the production of polybutylene terephthalate and several other engineering polymers. In the research laboratory, terephthalic acid has been popularized as a component for the synthesis of metal- organic frameworks. Due to the wider applications of terephthalic acid, many methods for its manufacturing and purification have been developed in the recent years.

Some of the prior art documents which disclose the processes for the synthesis of terephthalic acids are as follows.

US patent 2833816 discloses a process for producing a polycarboxylic aromatic acid such as terephthalic acid by oxidation of aromatic compounds such as xylene at 120 to 275°C in the presence of acetic acid, metal catalyst and a bromine source. During the oxidation reaction, major intermediates that are formed are p-toulic acid and 4- carboxy-benzaldehyde (4-CBA). P-toulic acid remains in soluble state in the solvent. However, 4-CBA co-crystallizes with product due to its similar crystal structure. Terephthalic acid which contains 4-CBA is typically termed as crude terephthalic acid. The crude terephthalic acid (CTA) containing 4-CBA is highly undesirable in producing polyester as it as act as a chain terminating agent during the polymerization. Hence CTA is required to be further purified by additional steps. l US patent 7094925 discloses a process for the oxidation of an alkyl-aromatic compound which comprises admixing the aromatic compound with an oxidising agent or sulfur compound in the presence of an ionic liquid and a nitrogen oxyacid species. Ionic liquid used in the process comprises an organic anion selected from the group consisting of trifluoroacetate, acetate, methanesulfonate, and combinations thereof or an anion based on sulfur, nitrogen, phosphorous, silicon, selenium, tellurium, arsenic, antimony, bismuth, or oxoanions of a metal. The process disclosed in US7094925 is carried out under Bronsted acidic conditions.

US2009/0326265 discloses the use of l-ethyl-3-methylimidazolium bromide as a bromine source during the oxidation. It is disclosed that the use of molecular bromine typically releases free bromine which causes corrosion to the equipment, whereas Ionic liquid based bromine source does not release the free bromine and hence advantageous during the manufacturing.

In US patent 6355835, the use of methylethlyketone (MEK) as a promoter to replace the bromine source is disclosed. However, the process disclosed in US6355835 requires high amount of catalysts. Another disadvantage of the process is that the organic promoter may undergo oxidation at the process operating conditions, further complicating purification steps.

US Patent 6153790 discloses utilization of a catalyst system which is a combination of cobalt and zirconium species for the preparation of aromatic carboxylic acids. The process disclosed in US6153790 avoids utilization of a halide promoter.

US2010/0174111 discloses a process for providing crystalline terephthalic acid comprising: a) providing a composition comprising terephthalic acid and one or more ionic liquids; and b) combining the composition of step (a) with a non-solvent (water), thereby crystallizing terephthalic acid. US20120004449 discloses a process for oxidizing an alkyl-aromatic compound which comprises forming a mixture comprising the alkyl-aromatic compound, a solvent, a bromine source, and a catalyst; and contacting the mixture with an oxidizing agent to produce a solid oxidation product comprising at least one of an aromatic aldehyde, an aromatic alcohol, an aromatic ketone, and an aromatic carboxylic acid; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms and an ionic liquid selected from the group consisting of an imidazolium ionic liquid, a pyridinium ionic liquid, a phosphonium ionic liquid, a tetra alkyl ammonium ionic liquid, and combinations thereof.

US2012/0004450 discloses a process for producing terephthalic acid from para-xylene which comprises forming a mixture comprising the para-xylene, a solvent, a bromine source, a catalyst and optionally ammonium acetate; and oxidizing the para-xylene by contacting the mixture with an oxidizing agent to produce a solid oxidation product comprising terephthalic acid, 4-carboxybenzaldehyde, and para-toluic acid; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms, a dialkyl imidazolium ionic liquid, and optionally water.

US2012/0004451 discloses a process for producing terephthalic acid from para-xylene which comprises forming a mixture comprising the para-xylene, a solvent, a bromine source, a catalyst and ammonium acetate; and oxidizing the para-xylene by contacting the mixture with an oxidizing agent to produce a solid oxidation product comprising terephthalic acid, 4-carboxybenzaldehyde, and para-toluic acid; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms.

US 2012/0004454 discloses a mixture for oxidizing an alkyl-aromatic compound comprising: the alkyl-aromatic compound, a solvent, a bromine source, and a catalyst; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms and an ionic liquid selected from the group consisting of aft imidazolium ionic liquid, a pyridinium ionic liquid, a phosphonium ionic liquid, a tetra alkyl ammonium ionic liquid, and combinations thereof. US2012/0004455A1 discloses a solid terephthalic acid composition comprising terephthalic acid, para-toluic acid, and 4-carboxybenzaldehyde; wherein the composition has a 4-carboxybenzaldehyde content of less than about 4,000 ppm, and a para-toluic acid content of more than about 2,000 ppm.

US2012/0004456A1 discloses a process for purifying crude terephthalic acid comprising a contaminant at a first concentration, the process comprising: contacting the crude terephthalic acid with a solvent comprising an ionic liquid to produce a solid terephthalic acid product having a second concentration of the contaminant lower than the first concentration. The ionic liquid utilized is selected from the group consisting of an imidazolium ionic liquid, a pyridinium ionic liquid, a phosphonium ionic liquid, a terra alkyl ammonium ionic liquid, and combinations thereof.

The processes disclosed in the above mentioned patent documents do not discuss the various other intermediates and side products that are formed during the manufacturing of terephthalic acids. Further, these processes utilizes bromine source in an excess amount as a promoter. Also these known disclosures use different ionic liquids, as solvents in the oxidation step and purification steps. Accordingly, it is desired to develop a simple and economic method for the preparation of terphthalic acid which can reduce utilization of number of chemicals as well as which can yield highly pure terephthalic acid.

OBJECTS

Some of the objects of the present disclosure are as follows:

An object of the present disclosure is to provide a feasible process for producing pure terephthalic acid.

Another object of the present disclosure is to provide a process for producing terephthalic acid, wherein the impurity content of 4-carboxy-benzaldehyde (4-CBA) in the purified terephthalic acid is less than 4000 ppm. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the process for producing terephthalic acid in accordance with one exemplary embodiment of the present disclosure.

SUMMARY:

In accordance with the present disclosure there is provided a process for preparing an aromatic dicarboxylic acid, particularly terephthalic acid; said process comprising the following steps:

- oxidizing p-xylene with an oxidizing agent in the presence of a carboxylic acid solvent, at least one catalyst and at least one ionic liquid to obtain a mixture containing crude terephthalic acid;

- first separating said mixture to obtain crude terephthalic acid and first mother liquor containing ionic liquid, residual solvent, catalyst and impurities;

- dissolving the crude terephthalic acid in the at least one ionic liquid followed by recrystallizing terephthalic acid; and

second separating and purifying terephthalic acid by treating terephthalic acid with the at least one ionic liquid to obtain purified terephthalic acid and second mother liquor containing ionic liquid and impurities,

said process being characterized by :

a. the formation of 4-carboxy-benzaldehyde (4-CBA) being less than 4000 ppm;

b. the ionic liquids used in the oxidation step, the dissolution step and the purification step are same; and

c. the ionic liquid is proportional to p-xylene.

In one embodiment the process further comprises a step of recycling of the second mother liquor containing ionic liquid from the purification step to the oxidation step. In another embodiment the process further comprises the step of recycling of the first mother liquor containing ionic liquid from the first separating step to the oxidation step or the dissolution step or the purification step.

Typically, the ionic liquid used in the oxidation step, the dissolution step and the purification step are fresh ionic liquid or the ionic liquid present in the first or second mother liquor.

In one embodiment of the present disclosure the process further comprises the steps of:

- separating ionic liquid, carboxylic acid solvent and catalyst from said first mother liquor using at least one technique selected from the group consisting of distillation, extraction and membrane filtration; and

- recycling the separated ionic liquid, carboxylic acid solvent and catalyst independently.

In one embodiment of the present disclosure the process further comprising a step of incorporating of at least one bromine source selected from the group consisting of HBr, NaBr, KBr, Nt^Br, benzylbromide, monobromoacetic acid, di-bromo acetic acid, bromoacetyl bromide, tetrabromomethane and ethylene di-bromide.

Typically, the ionic liquid is at least one selected from the group consisting of alkyl ionic liquids and aryl alkyl ionic liquids.

Typically, the ionic liquid comprises an organic cation selected from the group consisting of quaternary ammonium, cholinium, sulfonium, phosphonium, guanidinium, imidazolium, pyridinium, pyrrolidinium, morpholinium, quinolinium, isoquinolium, pyrazolium and piperidinium; and an anion selected from the group consisting of chloride, bromide, fluoride, iodide, mesylate, tosylate, hydrogen sulfate, sulfate, alkyl sulfonate, phosphates, phosphonates, akyl phosphates, nitrates, nitrites, carbonates, acetates, bicarbonates, hydroxides and oxides. In one of the embodiments, the ionic liquid comprises an organic cation selected from the group consisting of quaternary ammonium, cholinium, sulfonium, phosphonium, guanidinium, imidazolium, pyridinium, pyrrolidinium, morpholinium, quinolinium, isoquinolium, pyrazolium and piperidinium; and bromide as an anion.

In one embodiment of the present disclosure the ionic liquid is at least one aryl alkyl ionic liquid selected from the group consisting of 1 -benzyl, 3 -methyl imidazolium bromide; 1 -benzyl, 3 -methyl imidazolium chloride; 1 -benzyl, 3 -methyl imidazolium acetate; 1 -benzyl, 3 -methyl imidazolium methane sulfonate; 1 -benzyl, 3 -methyl imidazolium phosphate; Benzyl tributyl phosphonium bromide, Benzyl tributyl ammonium bromide; 1-phenyl, 3-methyl imidazolium chloride; 1-phenyl, 3-methyl imidazolium bromide; 1-phenyl, 3-methyl imidazolium acetate; l-phenyl _> 3-methyl imidazolium methane sulfonate; 1-phenyl, 3-methyl imidazolium phosphate; Phenyl tributyl phosphonium bromide; Phenyl tributyl ammonium bromide; 1,3-dibenzyl imidazolium chloride; 1,3-dibenzyl imidazolium bromide; 1,3-dibenzyl imidazolium acetate; 1,3-dibenzyl imidazolium methane sulfonate; and 1,3-dibenzyl imidazolium phosphate.

In another embodiment of the present disclosure the ionic liquid is at least one alkyl ionic liquid selected from the group consisting of 1 -butyl, 3-methyl imidazolium bromide; 1 -butyl, 3-methyl imidazolium chloride; 1 -butyl, 3-methyl imidazolium acetate; 1-butyl, 3-methyl imidazolium methane sulfonate; 1-butyl, 3-methyl imidazolium phosphate; 1-ethyl, 3-methyl imidazolium chloride; 1-ethyl, 3-methyl imidazolium bromide; 1-ethyl, 3-methyl imidazolium acetate; 1-ethyl, 3-methyl imidazolium methane sulfonate; 1-ethyl, 3-methyl imidazolium phosphate; Tetrabutyl phosphonium chloride; Tetrabutyl phosphonium bromide; Tetrabutyl phosphonium acetate; Tetrabutyl phosphonium methane sulfonate; Tetrabutyl phosphonium phosphate; Trihexyl Tetradecyl phosphonium chloride; Trihexyl Tetradecyl phosphonium bromide; Trihexyl Tetradecyl phosphonium acetate; Trihexyl Tetradecyl phosphonium decanoate; Tetrabutyl ammonium chloride; Tetrabutyl ammonium bromide; Tetrabutyl ammonium acetate; Tetrabutyl ammonium methane sulfonate; Tetrabutyl ammonium phosphate; Choline chloride; Choline bromide; choline acetate; and choline methane sulfonate.

Typically, the oxidation step is carried out at a temperature of 100 to 2500C at a pressure of 10 to 60 bar.

Typically, the step of dissolving terephthalic acid in an ionic liquid is carried out at a temperature of 25 to 225°C.

Typically, the concentration of ionic liquid used in the oxidation step ranges between 0.04 to 50 % with respect to total reaction mass.

Typically, the oxidizing agent is oxygen or air.

Typically, the carboxylic acid solvent is acetic acid.

Typically, the catalyst comprises at least one metal compound, the metal being selected from the group consisting of cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium and titanium.

Typically, the catalyst is at least one selected from the group consisting of cobalt acetate, manganese acetate cerium acetate, potassium acetate, cesium acetate, zirconium acetate, copper acetate, cobalt oxalate, manganese oxalate,cerium oxalate, potassium oxalate, cesium oxalate, zirconium oxalate, copper oxalate.

Typically, the proportion of the ionic liquid to the carboxylic acid solvent ranges between 1:1 to 1:20.

Typically, the concentration of the catalyst is 0.02 to 2.5 % with respect to total reaction mass.

DETAILED DESCREPTION Conventionally tefephthalic acid is produced by wet oxidation of para-xylene. In the process of wet oxidation, acetic acid is used as solvent, cobalt and manganese acetates are used as catalyst and hydrogen bromide used as a promoter.

Molecular species that are detected during the wet oxidation process of para-xylene are terephthalic acid as a main product (crude terephthalic acid), intermediates such as para-tolualdehyde, para-toluic acid,4-carboxybenzhaldehyde and side products such as isophthalic acid, phthalic acid, meta or ortho-tolualdehyde, metaor ortho-toluic acid, 2 or 3-carboxybenzhaldehyde, 3 or 4-Bromo methyl benzoic acid, benzoic acid, trimelliticacidSjtrimesic acid,benzaldehyde, phthalaldehyde, ethylbenzaldehyde, methylstyrene, diphenic acid, 2- biphenyl carboxylic acid, hemi melitic acid, dimethyl terephthalate, methyl p-toulate, 3-hydroxy 4-methyl benzoic acid, terephthal aldehyde, styrene, phenol, toluene, benzene,ethylbenzene, methylethylbenzene, formaldehyde, 1,3-cyclopentadiene, indene, methylnaphthalene, anthracene, phenantrene, phenylacetylene, methylbiphenyl, diphenylbutane, naphthalene, and 4,4- dimethylbibenzyl, vinylacetylene. The intermediates form in large amount and eventually convert into crude terephthalic acid during the wet oxidation of para- xylene. However, the side products are formed in minor quantities. It is important that the product, intermediates and side-products remain in soluble state during the reaction for complete conversion. In the conventional process, terephthalic acid crystals formed during the oxidation traps some of intermediates such as 4-CBA. To produce PET from terephthalic acid, it is essential to have 4-CBA content as low as 100 ppm in terephthalic acid. Therefore, in the conventional process, the impure Terephthalic acid is again needs to be subjected to hydrogenation to convert 4-CBA into p-toulic acid. Subsequently, p-toulic acid needs to be separated to obtain pure Terephthalic acid.

To overcome the drawbacks associated with the prior art process, the present disclosure provides a simple, economic and efficient process for the synthesis of pure terephthalic acid which involves utilization of ionic liquids. The inventors of the present disclosure found that the ionic liquids can be used as a promoter as well as co- solvent during the manufacturing of terephthalic acid. The ionic compounds used in the process of the present disclosure are capable of solubilizing the intermediates and side products during the both oxidation and purification stage. It is advantageous that these intermediates to be remained in dissolved form in ionic compounds, as they can be further oxidized into the desired product during the manufacturing of terephthalic acid. Thus the concentration of intermediates can be reduced during the oxidation stage itself, thereby eliminating hydrogenation stage as against conventional manufacturing process. However, complete elimination of purification stage is not possible since co-crystallization of 4- CBA with terephthalic acid will takes place in oxidation step due to similar crystal structure of these two compounds. Extracting or controlled crystallization of 4-CBA from the dissolved mixture of crude terephthalic acid is required.

In accordance with the present disclosure the ionic liquid employed in the process of preparing terephthalic acid is at least one selected from the group consisting of alkyl ionic liquids and aryl alkyl ionic liquids. Typically, the ionic liquid comprises an organic cation selected from the group consisting of quaternary ammonium, cholinium, sulfonium, phosphonium, guanidinium, imidazolium, pyridinium, pyrrolidinium, morpholinium, quinolinium, isoquinolium, pyrazolium and piperidinium; and an anion selected from the group consisting of chloride, bromide, fluoride, iodide, mesylate, tosylate, hydrogen sulfate, sulfate, alkyl sulfonate, phosphates, phosphonates, akyl phosphates, nitrates, nitrites, carbonates, acetates, bicarbonates, hydroxides and oxides.

In one of the preferred embodiment, the ionic liquid comprises an organic cation selected from the group consisting of quaternary ammonium, cholinium, sulfonium, phosphonium, guanidinium, imidazolium, pyridinium, pyrrolidinium, morpholinium, quinolinium, isoquinolium, pyrazolium and piperidinium; and bromide as an anion. It is found that the use of ionic compound having bromide anion in the oxidation step as well as in the purification step eliminates need of incorporation of bromine source such as HBr, KBr and the like.

In one embodiment of the present disclosure the ionic liquid employed is at least one aryl alkyl ionic liquid selected from the group consisting of 1 -benzyl, 3 -methyl imidazolium bromide; 1 -benzyl, 3 -methyl imidazolium chloride; 1 -benzyl, 3 -methyl imidazolium acetate; 1 -benzyl, 3 -methyl imidazolium methane sulfonate; 1 -benzyl, 3- methyl imidazolium phosphate; Benzyl tributyl phosphonium bromide, Benzyl tributyl ammonium bromide; 1 -phenyl, 3-methyl imidazolium chloride; 1 -phenyl, 3- methyl imidazolium bromide; 1-phenyl, 3-methyl imidazolium acetate; 1-phenyl, 3- methyl imidazolium methane sulfonate; 1-phenyl, 3-methyl imidazolium phosphate; Phenyl tributyl phosphonium bromide; Phenyl tributyl ammonium bromide; 1,3- dibenzyl imidazolium chloride; 1,3-dibenzyl imidazolium bromide; 1,3-dibenzyl imidazolium acetate; 1,3-dibenzyl imidazolium methane sulfonate; and 1,3-dibenzyl imidazolium phosphate.

In another embodimeiit of the present disclosure the ionic liquid employed is at least one alkyl ionic liquid selected from the group consisting of 1-butyl, 3-methyl imidazolium bromide;, 1-butyl, 3-methyl imidazolium chloride; 1-butyl, 3-methyl imidazolium acetate; 1-butyl, 3-methyl imidazolium methane sulfonate; 1-butyl, 3- methyl phosphate; 1 -ethyl, 3-methyl imidazolium chloride; 1 -ethyl, 3- methyl imidazolium bromide; 1 -ethyl, 3-methyl imidazolium acetate; 1 -ethyl, 3- methyl imidazolium methane sulfonate; 1-ethyl, 3-methyl imidazolium phosphate; Tetrabutyl phosphonium chloride; Tetrabutyl phosphonium bromide; Tetrabutyl phosphonium acetate; Tetrabutyl phosphonium methane sulfonate; Tetrabutyl phosphonium phosphate; Trihexyl Tetradecyl phosphonium chloride; Trihexyl Tetradecyl phosphonium bromide; Trihexyl Tetradecyl phosphonium acetate; Trihexyl Tetradecyl phosphonium decanoate; Tetrabutyl ammonium chloride; Tetrabutyl ammonium bromide; Tetrabutyl ammonium acetate; Tetrabutyl ammonium methane sulfonate; Tetrabutyl ammonium phosphate; Choline chloride; Choline bromide; choline acetate; and choline methane sulfonate.

The present disclosure provides an integrated process for the synthesis of pure terephthalic acid comprising the steps of oxidation, purification, crystallization of product, and recovery and reuse/recycling of solvents, ionic compounds, and catalyst. Particularly, the integrated process of the present disclosure involves the following steps:

In the first step, p-xylene is oxidized with an oxidizing agent in the presence of a carboxylic acid solvent, at least one catalyst and at least one ionic liquid to obtain a mixture containing crude terephthalic acid. Typically, the oxidation step is carried out at a temperature of 100 to 250°C at a pressure of 10 to 60 bar. The concentration of ionic liquid in the oxidation step is maintained between 0.04 and 50 % with respect to total reaction mass. Typically, the oxidizing agent is oxygen or air. Typically, the carboxylic acid solvent is acetic acid. Typically, the proportion of the ionic liquid to the carboxylic acid solvent is maintained between 1:1 and 1:20.

In accordance with the present disclosure, the catalyst employed in the oxidation step comprises at least one metal compound, the metal being selected from the group consisting of cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium and titanium. Typically, the catalyst is at least one selected from the group consisting of cobalt acetate, manganese acetate, , cerium acetate, potassium acetate, cesium acetate, zirconium acetate, copper acetate, cobalt oxalate, manganese oxalate, cerium oxalate, potassium oxalate, cesium oxalate, zirconium oxalate, copper oxalate.

Typically, the catalyst is used at a concentration of 0.02 to 2.5 % with respect to total reaction mass.

The obtained mixture is then separated first time to obtain crude terephthalic acid and first mother liquor containing ionic liquid, residual solvent, catalyst and impurities. In the second step, the crude terephthalic acid is dissolved in the at least one ionic liquid followed by recrystallizing terephthalic acid. Typically, the step of dissolving crude terephthalic acid in the ionic liquid is carried out at a temperature of 25 to 225°C.

Optionally, the dissolution process is also carried out in staged manner. The dissolution of crude terephthalic acid is done with ionic compounds or mixture of ionic compounds. Optionally, the dissolution step as disclosed in the present disclosure is followed by extraction of impurities in the same vessel or another vessel. The extraction of impurities is carried out with ionic compounds, mixture of ionic compounds or mixture of ionic compounds and molecular solvents. The molecular solvents that are used in the present disclosure are selected from the group comprising of acetic acid, aliphatic or aromatic alcohols or water or combination of these solvents.

In the next step, terephthalic acid is separated second time and purified by treating terephthalic acid with the at least one ionic liquid to obtain purified terephthalic acid and second mother liquor containing ionic liquid and impurities. The remained mother liquors of each steps are then recycled. Solid pure terephthalic acid obtained by the process of the present disclosure, if required, is further subjected to filtration, washing and drying.

The process of the present disclosure is characterized by the following features:

• the formation of 4-carboxy-benzaldehyde (4-CBA) being less than 4000 ppm;

• the ionic liquids used in the oxidation step, the dissolution step and the purification step are same; and

• the ionic liquid is proportional to p-xylene.

The process further involves a step of recycling of the second mother liquor containing ionic liquid from the purification step to the oxidation step.

The process still further comprises the step of recycling of the first mother liquor containing ionic liquid from the first separating step to the oxidation step or the dissolution step or the purification step.

Typically, the the ionic liquid used in the oxidation step, the dissolution step and the purification step are fresh ionic liquid or the ionic liquid present in the first or second mother liquor. The process described above also involves the steps such as separation of ionic liquid, carboxylic acid solvent and catalyst from the first mother liquor using at least one technique selected from the group consisting of distillation, extraction and membrane filtration; and recycling of the separated ionic liquid, carboxylic acid solvent and catalyst independently.

The process of the present disclosure further comprises a step of incorporating at least one bromine source as a promoter during the oxidation step. Typically, the bromine source is selected from the group consisting of HBr, NaBr, KBr, HjBr, benzylbromide, monobromoacetic acid, di-bromo acetic acid, bromoacetyl bromide, tetrabromomethane and ethylene di-bromide.

The process for producing terephthalic acid in accordance with one exemplary embodiment of the present disclosure is illustrated in Figure 1.

Raw materials for producing terephthalic acid, namely Para- xylene, acetic acid and catalyst are introduced to oxidation section (20) from lines 1, 2 and 3 respectively. Ionic liquid is introduced to an oxidation section (20) via line 4. Line 5 having reaction slurry is connected to a crystallization section (30), wherein crude terephthalic acid is separated. The separated CTA is sent to a purification section (40) for purification via line 6. Mother liquor from crystallization section (30) is recycled to said oxidation section (20) via line 7. Or said mother liquor from crystallization section (30) is transferred to an ionic liquid separation section (50) via line 7A, where acetic acid and catalyst are separated by distillation or extraction or membrane filtration or combinations thereof. Recovered acetic acid and catalyst are recycled to the oxidation section (20) via line 8 and separated ionic liquids are sent to the purification section (40) via line 9 for CTA purification and optionally, fresh ionic liquid can be supplied via line 12. From CTA purification section (40), ionic liquids having 4-CBA and other intermediates in dissolved state are recycled to the oxidation section (20) via line 10. Purified terephthalic acid is separated via line 11 from the purification-section (40). The process of the present disclosure is further illustrated herein below with the help of the following examples. 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.

Examples:

Example 1:

Oxidation of p-xylene (40.91 g) was carried out at 215 °C temperature and 40 bar pressure using a mixture of acetic acid, 1 -butyl, 3 -methyl imidazolium acetate and 1- benzyl, 3 -methyl imidazolium bromide as a solvent in a ratio of 76: 20: 4 respectively. Cobalt acetate and manganese acetate (300 ppm and 400 ppm respectively) were used as catalyst. After 3 hours reaction, crude terephthalic acid (CTA) from mother liquor was separated by filtration. CTA (61 g) was then sent to a purification section. 321 g of acetic acid was separated from the mother liquor by distillation, whereas organics and catalyst were separated by precipitation method. The obtained purified 1 -butyl, 3- methyl imidazolium acetate and 1 -benzyl, 3 -methyl imidazolium bromide complex (105 g) was then sent to CTA purification. After purification of CTA, 1-butyl, 3- methyl imidazolium acetate and 1 -benzyl, 3 -methyl imidazolium bromide complex was further recycled to oxidation for next run. Final 4-CBA content in the purified terephthalic acid was 150 ppm, whereas in CTA was 252 ppm.

Example 2:

Oxidation of p-xylene (82 g) was carried out at 215 °C temperature and 40 bar pressure using a mixture of acetic acid and Tetrabutyl phosphonium bromide as a solvent in a ratio of 80: 20 respectively. Cobalt acetate and manganese acetate (1650 ppm and 1120 ppm respectively) were used as catalyst. After 3 hours reaction, crude terephthalic acid (CTA) from mother liquor was separated by filtration. The obtained CTA (120 g) was sent to purification section. 300 g of acetic acid was separated from the mother liquor by distillation, whereas organics and catalyst were separated by precipitation method. Purified Tetrabutyl phosphonium bromide was sent to a CTA purification section along with fresh Tetrabutyl phosphonium bromide. After purification of CTA, Tetrabutyl phosphonium bromide was recycled to oxidation for next run. Final 4-CBA content in purified terephthalic acid was 1%, whereas in CTA was 1.9 %.

Example 3:

Oxidation of p-xylene (82 g) was carried out at 215 °C temperature and 40 bar pressure using a mixture of acetic acid and 1 -benzyl, 3 -methyl imidazolium bromide as a solvent in a ratio of 70:30 respectively. Cobalt acetate and manganese acetate (300 ppm and 400 ppm respectively) were used as catalyst. After 3 hours reaction, crude terephthalic acid (CTA) from mother liquor was separated by filtration. The obtained CTA (109 g) was sent to purification section. 260 g of acetic acid was separated from the mother liquor by distillation, whereas organics and catalyst were separated by precipitation method. Purified 1 -benzyl, 3 -methyl imidazolium bromide was sent to a CTA purification section along with fresh 1 -benzyl, 3-methyl imidazolium bromide. After purification of CTA, 1 -benzyl, 3-methyl imidazolium bromide was recycled to oxidation for next run. Final 4-CBA content in purified terephthalic acid was 1000 ppm, whereas in CTA was 3100 ppm.

Technical advance and economic significance:

- The process of the present disclosure is capable of reducing the formation of impurities such as 4-carboxy-benzaldehyde (4-CBA) during the manufacturing of terephthalic acid, thereby producing terephthalic acid in a highly pure form.

- The process of the present disclosure eliminates additional steps such as hydrogenation.

- The process of the present disclosure utilizes ionic liquid as a promoter as well as co-solvent. - The process of present disclosure in one embodiment utilizes the same ionic liquid/s in both the steps i.e. oxidation and purification, which in turn keep the terephthalic acid , 4-CBA, other intermediates and side products in solubilized form.

- The process of the present disclosure utilize ionic compound having bromide anion in oxidation as well as in purification stage, which eliminates incorporation of additional bromine source such as HBr, NaBr and KBr.

- The process of the present disclosure is economic as it reuse/ recycle the recovered solvent, ionic liquid and catalyst.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "a", "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.

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

While certain embodiments of the disclosure have been described, these embodiments have been presented by way of examples only, and are not intended to limit the scope of the disclosure. Variations or modifications in the composition of this disclosure, within the scope of the disclosure, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this disclosure.