DESC:FIELD
The present disclosure relates to a process for the preparation of polyethylene terephthalate.
BACKGROUND
Polyethylene terephthalate (PET) is a polymer resin of the polyester family. PET is typically used for making fibers that may be used in clothing, for making containers for liquids and food and engineering resins in combination with glass fiber.
Terephthalic acid is used as a precursor for the preparation of polyethylene terephthalate. Terephthalic acid is produced by the wet oxidation of p-xylene using oxygen as an oxidant in the presence of a catalyst and a promoter employing acetic acid as a solvent. Upon the wet oxidation process of p-xylene, a solid oxidation product is obtained, which is termed as crude terephthalic acid (CTA). CTA contains terephthalic acid as main product along with intermediate oxidation products of xylene such as p-tolualdehyde, p-toluic acid, 4-carboxybenzaldehyde (4-CBA) along with several side products and byproducts.
In order to use crude terephthalic acid as a starting material, for example, in the preparation of polyethylene terephthalate and polyester, the combined content of 4-carboxybenzaldehyde (4-CBA) and p-toluic acid should be below 190 ppm. When 4-CBA is present in large quantities in the crude terephthalic acid, it acts as a chain terminator during the PET polymerization process. Hence, the desired PET molecular weight may not be achieved.
During the production of terephthalic acid by oxidation of xylene, the typical amount of 4-CBA produced is from 4000 ppm to 10000 ppm and the typical amount of para-toluic acid in CTA is in the range of 150 ppm to 5000 ppm. Therefore, reducing the amount of 4-CBA impurity in crude terephthalic acid is very important, so as to use terephthalic acid for the preparation of polyethylene terephthalate. Conventionally, the crude terephthalic acid is purified by subjecting it to hydrogenation in the presence of a catalyst to convert 4-CBA into p-toluic acid, and separating p-toluic acid. The catalyst used for the hydrogenation of CBA is expensive, thereby making the process commercially non-viable.
There is, therefore, felt a need to provide simple and economical alternative process for the preparation of polyethylene terephthalate.
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 prior art or to at least provide a useful alternative.
An object of the present disclosure is to provide a process for the preparation of polyethylene terephthalate.
Another object of the present disclosure is to provide an economical and efficient process for the preparation of polyethylene terephthalate.
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 polyethylene terephthalate from crude terephthalic acid. The process comprises adducting a Lewis base and crude terephthalic acid under stirring to obtain a first mixture comprising a Lewis base-terephthalic acid adduct. The crude terephthalic acid comprises at least one organic aromatic acid selected from the group consisting of 4-carboxy benzaldehyde, p-toluic acid, isophthalic acid, orthophthalic acid, benzoic acid, and trimellitic acid. The Lewis base reacts with the organic aromatic acid to form adduct of Lewis base and the corresponding organic aromatic acid. The first mixture is subjected to selective crystallization to obtain Lewis base-terephthalic acid adduct in the form of crystals. The so formed crystals are separated from the first mixture by filtration. The crystals of Lewis base-terephthalic acid adduct are reacted with monoethylene glycol in the presence of a catalyst under stirring to obtain a second mixture comprising polyethylene terephthalate, followed by separating polyethylene terephthalate from the second mixture.
The yield of polyethylene terephthalate is in the range of 50% to 99% and the purity of polyethylene terephthalate is in the range of 80% to 100%.
The Lewis base of the present disclosure is at least one selected from the group consisting of substituted and non-substituted linear Lewis base, branched Lewis base, cyclic Lewis base, linear Lewis bases, polycyclic Lewis bases, heterocyclic Lewis bases, and aromatic Lewis bases.
In accordance with the embodiments of the present disclosure, the Lewis base is at least one selected from the group consisting of alkyl amines, aryl amines, alkyl phosphines, aryl aphosphines, alkyl sulphides and aryl sulphides. Lewis base can be selected from the group consisting of 1-methyl imidazole, N-methyl pyrrolidone, 1,5-dimethylpyrrolidone, N- methylpiperidone, and N-methylcaprolactam.
In accordance with the embodiments of the present disclosure, the step of adducting the crude terephthalic acid is carried out at a temperature is in the range of 50 to 200 °C, under the stirring in the range of 5 to 800 rpm.
The adducting of the Lewis base and crude terephthalic acid is carried out in the presence of at least one fluid medium selected from the group consisting of tetrahydrofuran, substituted tetrahydrofuran, 1,4-dioxane and substituted 1,4-dioxane. Optionally, the fluid medium comprises water in the range of 0 to 10% (w/w) of the fluid medium.
In accordance with the embodiments of the present disclosure, the step of reacting the crystals of Lewis base-terephthalic acid with monoethylene glycol is carried out at a temperature is in the range of 100 to 200 °C.

In accordance with one embodiment of the present disclosure, the step of selective crystallization comprises seeding the first mixture with the crystals of an adduct of pure terephthalic acid and Lewis base to facilitate selective crystallization of the Lewis base-terephthalic acid adduct.

In accordance with the embodiments of the present disclosure, the catalyst used is at least one selected from the group consisting of metal oxides, metal halides, organometallic complexes and mineral acids. The catalyst is at least one selected from the group consisting of antimony trioxide, antimony trichloride, and arsenic oxide.
The crude terephthalic acid also includes at least one metallic impurity selected from the group consisting of compounds of cobalt, manganese, iron and chromium, which can be removed by adding at least one chelating agent to the first mixture.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The process of the present disclosure will now be described with the help of the accompanying drawing, in which:
Figure 1 illustrates the DSC analysis graph for the final product formed in the experiment 1.
Figure 2 illustrates the DSC analysis graph for the final product formed in the experiment 2.

DETAILED DESCRIPTION
The present disclosure relates to a process for the preparation of polyethylene terephthalate. The process of the present disclosure uses crude terephthalic acid as starting material. The process involves purification of terephthalic acid by the selective crystallization of an adduct of terephthalic acid and a Lewis base, which is directly reacted with monoethylene glycol, without deadducting to obtain polyethylene terephthalate.
In accordance with one aspect of the present disclosure, there is provided a process for the preparation of polyethylene terephthalate from crude terephthalic acid.
The process of the present disclosure involves adducting a Lewis base with crude terephthalic acid under stirring to obtain a first mixture comprising Lewis base-terephthalic acid adduct.
In accordance with the embodiments of the present disclosure, the crude terephthalic acid comprises at least one organic aromatic acid selected from the group consisting of 4-carboxy benzaldehyde, p-toluic acid, isophthalic acid, orthophthalic acid and trimellitic acid. The Lewis base reacts with terephthalic acid and different aromatic organic acids present in the crude terephthalic acid to form corresponding adducts. Thus, the first mixture also contain adducts of the Lewis base and organic aromatic acids present in the crude terephthalic acid.
The Lewis base-terephthalic acid adduct is selectively crystallized while keeping other adducts and impurities in dissolved form. In accordance with one embodiment of the present disclosure, the selective crystallization is facilitated by seeding the crystals of pure terephthalic acid and the Lewis base. In accordance with another embodiment of the present disclosure, the selective crystallization is facilitated by decreasing the temperature.
The so formed Lewis base- terephthalic acid adduct in the form of crystals is separated from the first mixture by filtration.
In the next step, the Lewis base-terephthalic acid adduct is reacted with monoethylene glycol in the presence of a catalyst to obtain a second mixture comprising polyethylene terephthalate, followed by separating polyethylene terephthalate from the second mixture.
The yield of polyethylene terephthalate is in the range of 50% to 99% and the purity of polyethylene terephthalate is in the range of 80% to 100%.
In accordance with the embodiments of the present disclosure, the Lewis base is at least one selected from the group consisting of substituted linear Lewis bases, non-substituted linear Lewis bases, branched Lewis bases, cyclic Lewis bases, polycyclic Lewis bases, heterocyclic Lewis bases, and aromatic Lewis bases.
The non-limiting examples of the Lewis base include linear amines, branched substituted amines, non-substituted amines, alkyl amines, dialkyl amines and trialkyl amines, alkyl phosphines and aryl phosphines, linear and branched substituted and non-substituted mono and dialkyl sulfide, substituted and non-substituted imidazole, pyrazole, thiazole, isothiazole, azathiozole, oxothiazole, oxazine, oxazoline, oxazaborole, dithiozole, triazole, selenozole, oxahosphole, pyrrole, borole, furan, thiphene, phosphole, pentazole, indole, indoline, oxazole, isothirazole, tetrazole, benzofuran, dibenzofuran, benzothiophene, dibenzothoiphene, thiadiazole, pyridine, pyrimidine, pyrazine, pyridazine, piperazine, piperidine, morpholine, pyran, annoline, phthalazine, quinazoline, quinoxaline, tromethamine, triethanoamine, 1-(2-hydroxyethyl) pyrrolidine, 4-(2-hydroxyethyl)morpholine, L-Lysine, hydrbamine, N-methyl glucamine, ethylene diamine, ethanoamine, 2-dimethylamino ethanol, diethnolamine, dimethylethanolamine, choline, benzathine, benethamine, L-arginine, and ammonia.
In accordance with the embodiments of the present disclosure, the Lewis base is at least one selected from the group consisting of 1-methyl imidazole, N-methyl pyrrolidone, 1,5-dimethylpyrrolidone, N-methylpiperidone, and N-methylcaprolactam.
In accordance with the embodiments of the present disclosure, adducting of the Lewis base the crude terephthalic acid is carried out at a temperature in the range of 50 to 200 °C.
In accordance with the embodiments of the embodiments of the present disclosure, the step of adducting the Lewis base and the crude terephthalic acid is carried out at a stirring speed in the range of 5 to 800 rpm.
In accordance with the embodiments of the present disclosure, the selective crystallization of the adduct of Lewis base-terephthalic acid is facilitated by seeding the first mixture with the crystals of pure terephthalic acid and Lewis base.
In accordance with the embodiments of the present disclosure, the step of reacting the Lewis base and crude terephthalic acid is carried out in at least one fluid medium selected from the group consisting of tetrahydrofuran, substituted tetrahydrofuran, 1-4,dioxane, and substituted 1-4,dioxane.
In accordance with the embodiments of the present disclosure, the fluid medium comprises water in the range of 0 to 10% (w/w) of the fluid medium.
The catalyst used while reacting the crystals of Lewis base-terephthalic acid with monoethylene glycol is at least one selected from the group consisting of metal oxides, metal halides, organometallic complexes, and mineral acids.
In accordance with one embodiment of the present disclosure, the catalyst used while reacting the second mixture with monoethylene glycol is at least one selected from the group consisting of antimony trioxide, antimony trichloride, and arsenic oxide.
In accordance with the embodiments of the present disclosure, the crude terephthalic acid includes metallic impurity from the group consisting of cobalt, manganese, iron, and chromium.
In accordance with one embodiment of the present disclosure, the step of crystallization comprises seeding the crystals of pure terephthalic acid and the Lewis base in the first mixture to facilitate selective crystallization of the Lewis base- terephthalic acid adduct.
In accordance with one embodiment of the present disclosure, the step of reacting a Lewis base with the crude terephthalic acid comprises adding a chelating agent, wherein the chelating agent facilitates the removal of the metallic impurities.
In accordance with one embodiment of the present disclosure, the chelating agent is ethylenediaminetetraacetic acid (EDTA).
In accordance with the embodiments of the present disclosure, the adduct of Lewis base-terephthalic acid is washed with water, prior to reacting with monoethylene glycol.
In accordance with the embodiments of the present disclosure, the process does not require a separate step of purification of the crude terephthalic acid and operates using the adduct obtained from crude terephthalic acid, thereby providing an economical and efficient process for preparation of polyethylene terephthalate.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.
Experiment 1: Preparation of polyethylene terephthalate
49.8 g of crude terephthalic acid was mixed with 147.78 g of 1-methyl imidazole at a temperature of 105 °C. After cooling to 27°C, a solid adduct was obtained, which was filtered out and washed with fresh 1-methyl imidazole. 90 g of the adduct of terephthalic acid and 1-methyl imidazole obtained after washing was mixed with 170 g of monoethylene glycol and 0.13 g of antimony trioxide catalyst to obtain a reaction mass. The reaction mass was heated to a temperature of 170 °C. At this temperature a clear homogeneous reaction mass was obtained. 1-methyl imidazole and excess of monoethylene glycol was removed. A solid product obtained, was washed with water and dried at 70 °C for 6 hours. The final product was characterized by acid number and differential scanning calorimetry (DSC).
Figure 1 shows the DSC analysis graph for the final product formed. % Total carboxylic acid group (% TCAG) for the final product formed was 19.97, which was calculated by following procedure:
0.4 g of the final product was dissolved in 400 mL of distilled water to obtain a solution. 1 g of KCl was added to the solution with continuous stirring to obtain a dispersion. A pH electrode was inserted into the dispersion. The dispersion was titrated with 0.25 N sodium hydroxide, adding 20 mL of titrant rapidly. The pH value was allowed to stabilize. At pH 10.4, there was no further consumption of NaOH, which confirmed the formation of PET and no free carboxylic group was available. TCAG was calculated using formula given below:
% TCAG = {V×N× (45.02/1000) ×100}/ Weight of sample
Wherein, V= Volume of NaOH, N= Normality of NaOH, 45.02= equivalent weight of COOH group, 100= conversion to percentage, 1000= conversion factor
For purified terephthalic acid %TCAG was 59.088.
Yield of the polyethylene terephthalate was 60 %
Experiment 2: Preparation of polyethylene terephthalate
The procedure was similar to experiment 1, except that the quantity of ethylene glycol used was 51 g. After the completion of the reaction, the solid product obtained, was washed with water and dried at 70 °C for 6 hours. The product was characterized by acid number and differential scanning calorimetry (DSC).
Figure 2 shows the DSC analysis graph for the final product formed. % Total carboxylic acid group (% TCAG) for the final product formed was 9.0.
Yield of the polyethylene terephthalate was 80%
Experiment 3: Preparation of terephthalic acid-1-methyl imidazole adduct by seeding crystals of pure terephthalic acid-1-methyl imidazole adduct
49.8 g of crude terephthalic acid was mixed with 99.6 g of 1-methyl imidazole and 300 g of tetrahydrofuran. The mixture was stirred at a temperature of 72 °C and at atmospheric pressure for 2 hours to obtain a homogenous mixture comprising an adduct of terephthalic acid and 1-methyl imidazole. 100 mg crystals of pure terephthalic acid-1-methyl imidazole adduct were added to the homogeneous mixture to grow uniform needle type crystals of terephthalic acid-1-methyl imidazole adduct, when the cooling was started. The homogeneous mixture was then cooled to 30 °C and allowed to stand for 50 minutes to obtain crystals of the adduct of terephthalic acid and 1-methyl imidazole. The crystals obtained from the homogeneous mixture after crystallization were separated by filtration under vacuum.
Experiments 4 and 5: Preparation of polyethylene terephthalate using arsenic oxide / antimony trichloride
Experiment 4 was performed using procedure similar to experiment 1 except that the catalyst antimony trioxide was replaced with 0.25 g of arsenic oxide.
Experiment 5 was performed using procedure similar to experiment 1 except that the catalyst antimony trioxide was replaced with 0. 5 g of antimony trichloride
Experiment 6: Effect of chelating agent on metal reduction in purified terephthalic acid
25 g of crude terephthalic acid, 49.5 g of 1-methyl imidazole, 148.5 g of tetrahydrofuran, 2 g demineralized water, and 0.47 grams ethylenediamine tetraacetate (EDTA) as chelating agent were charged in a round bottom flask to obtain a mixture. The mixture was stirred at a temperature of 72 °C and at atmospheric pressure for 2 hours to obtain a homogenous mixture having an adduct of terephthalic acid and 1-methyl imidazole. The homogeneous mixture was then cooled to 30 °C and allowed to stand for 50 minutes to obtain crystals of the adduct of terephthalic acid and 1-methyl imidazole.
The crystals obtained from the homogeneous mixture after crystallization were separated by filtration under vacuum. The separated crystals were washed with 100 g of tetrahydrofuran. The crystals obtained from the above step were further taken in a round bottom flask and 400 mL of Methanol was added. The mixture was stirred at a speed of 400 rpm at 30 °C for 20 minutes to obtain pure terephthalic acid.
The removal of the metallic impurities was analyzed by Atomic absorption spectroscopy instrument. The metallic impurity in terms of cobalt and manganese content of crude terephthalic acid was 24 and 25 ppm respectively, which was reduced to 0.3 and 0.2 ppm respectively.
Thus, the addition of the chelating agent was helpful in reducing the metallic impurity of the crude terephthalic acid.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of:
• an economic and efficient process for preparing polyethylene terephthalate; and
• a process for preparing polyethylene terephthalate using crude terephthalic acid.
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 “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 invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.
The numerical values given for various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention unless there is a statement in the specification to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment 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:WE CLAIM:
1. A process for preparing polyethylene terephthalate from crude terephthalic acid, said process comprising the following steps:
a. adducting a Lewis base and crude terephthalic acid under stirring to obtain a first mixture comprising a Lewis base-terephthalic acid adduct;
b. subjecting said first mixture to selective crystallization to obtain Lewis base-terephthalic acid adduct in the form of crystals;
c. separating said crystals of said Lewis base-terephthalic acid adduct by filtration;
d. reacting said crystals of said Lewis base-terephthalic acid adduct with monoethylene glycol in the presence of a catalyst under stirring to obtain a second mixture comprising polyethylene terephthalate; and
e. separating said polyethylene terephthalate from said second mixture;
wherein the yield of said polyethylene terephthalate is in the range of 50% to 99% and the purity of said polyethylene terephthalate is in the range of 80% to 100%.

2. The process as claimed in claim 1, wherein said crude terephthalic acid comprises at least one organic aromatic acid selected from the group consisting of 4-carboxybenzaldehyde, p-toluic acid, isophthalic acid, orthophthalic acid, benzoic acid, and trimellitic acid.
3. The process as claimed in claim 2, wherein said first mixture comprises adducts of Lewis base and organic aromatic acids present in said crude terephthalic acid.

4. The process as claimed in claim 1, wherein said Lewis base is at least one selected from the group consisting of substituted linear Lewis bases, non-substituted linear Lewis bases, branched Lewis bases, cyclic Lewis bases, polycyclic Lewis bases, heterocyclic Lewis bases, and aromatic Lewis bases.

5. The process as claimed in claim 1, wherein said Lewis base is at least one selected from the group consisting of alkyl amines, aryl amines, alkyl phosphines, aryl phosphines, alkyl sulphide, and aryl sulphide.
6. The process as claimed in claim 1, wherein said Lewis base is at least one selected from a group consisting of 1-methyl imidazole, N-methyl pyrrolidone, 1,5-dimethylpyrrolidone, N-methylpiperidone and, N-methylcaprolactam.
7. The process as claimed in claim 1, wherein said step (a) is carried out at a temperature in the range of 50 to 200 °C.
8. The process as claimed in claim 1, wherein adducting of said Lewis base and said crude terephthalic acid in step (a) is carried out at a stirring speed in the range of 5 to 800 rpm.
9. The process as claimed in claim 1, wherein mixing of said Lewis base and said crude terephthalic acid in step (a) is carried out in at least one fluid medium selected from the group consisting of tetrahydrofuran, substituted tetrahydrofuran, 1,- 4-dioxane, and substituted 1,4-dioxane.
10. The process as claimed in claim 9, wherein said fluid medium comprises water in the range of 0 to 10% (w/w) of the fluid medium.
11. The process as claimed in claim 1, wherein the step (d) is carried out at a temperature in the range of 100 to 200 °C.
12. The process as claimed in claim 1, wherein step (b) comprises seeding the first mixture with crystals of an adduct of pure terephthalic acid and Lewis base to facilitate selective crystallization of said Lewis base- terephthalic acid adduct.
13. The process as claimed in claim 1, wherein said catalyst used in step (d) is at least one selected from the group consisting of metal oxides, metal halides, organometallic complexes, and mineral acids.
14. The process as claimed in claim 1, wherein said catalyst used in step (d) is at least one selected from the group consisting of antimony trioxide, antimony trichloride, and arsenic oxide.
15. The process as claimed in claim 1, wherein said crude terephthalic acid comprises at least one metallic impurity selected from the group consisting of compounds of cobalt, manganese, iron, and chromium.
16. The process as claimed in claim 15, wherein step (b) further involves a sub-step of adding at least one chelating agent to said first mixture to facilitate the removal of the metallic impurities.