A method for preparing terephthalic acid and terephthalic acid prepared thereby

Abstract

The present invention relates to a method for producing terephthalic acid and terephthalic acid produced thereby. Specifically, according to one embodiment of the present invention, by including a step of hydrolyzing a compound produced by depolymerization of waste polyester using a specific hydrolysis catalyst, terephthalic acid can be produced in an environmentally friendly manner and high-purity terephthalic acid can be provided.

Description

Technical Field

[0001] The present invention relates to a process for preparing terephthalic acid in an environmentally friendly manner using waste polyester and recycled terephthalic acid prepared thereby. Background Art

[0002] Polyester, due to its excellent mechanical strength, heat resistance, transparency, and gas barrier properties, is widely used in materials such as beverage containers, packaging films, and audiovisual films, as well as industrial materials such as medical fibers and tire cord. Polyester sheets and plates, in particular, offer excellent transparency and mechanical strength, making them widely used as raw materials for boxes, cases, partitions, shelves, panels, packaging materials, construction materials, and interior and exterior materials.

[0003] As the amount of plastic waste, including polyester, generated globally each year becomes difficult to control, there is growing interest in recycling or reusing waste polyester. Furthermore, countries around the world have established regulations and programs to include recycling of waste plastic resources. For example, regulations are currently under discussion requiring the use of recycled resins at a certain percentage or higher in packaging materials used in various fields.

[0004] In particular, polyethylene terephthalate (PET) is widely used to manufacture a variety of products, including films, fibers, bottles, and containers, due to its excellent properties in terms of heat resistance, processability, transparency, and non-toxicity. However, most of them are landfilled or incinerated after use; therefore, research on recycling or regeneration processes using them is still ongoing.

[0005] For example, Korean Patent Publication No. 1997-0042469 discloses a technology for producing terephthalic acid by hydrolyzing waste polyethylene terephthalate with an alkaline aqueous solution to produce a slurry of alkali metal and alkaline earth metal salts of terephthalic acid, which is then neutralized with acid. The hydrolysis reaction produces terephthalate salts, rather than terephthalic acid, and requires a neutralization step involving the addition of acid to convert them into terephthalic acid. However, the resulting byproducts can pollute the environment and generate large amounts of acid treatment wastewater.

[0006]

Prior art literature

[0007] [Patent Literature]

[0008] (Patent Document 1) Korean Laid-Open Patent Publication No. 1997-0042469. Summary of the Invention

[0009] Technical issues

[0010] Therefore, an object of the present invention is to provide a process for producing high-purity terephthalic acid in an environmentally friendly manner by hydrolyzing a compound produced by depolymerization of waste polyester using a specific hydrolysis catalyst, and recycled terephthalic acid produced thereby.

[0011] Solution to the problem

[0012] According to one embodiment of the present invention, the process for preparing terephthalic acid comprises hydrolyzing a compound prepared by depolymerization of waste polyester, wherein the hydrolysis catalyst used in the hydrolysis step comprises at least one cation selected from the group consisting of Li + 、Na + , K + and Cs + Alkali metal ions, Be 2+ Mg 2+ , Ca 2+ and Ba 2+ Alkaline earth metal ions, NH 4+ and NR 4+ (where R is an alkyl group) ammonium ions and Zn 2+ or comprising at least one anion selected from the group consisting of OH - , OR - (where R is an alkyl group), HCO3 - 、CO3 2- , benzoate ion (C7H5O2 - ), a group consisting of 4-alkoxycarbonylbenzoate ion, acetate ion and terephthalate ion.

[0013] According to another embodiment of the present invention, the recovered terephthalic acid is produced according to the above-described process for producing terephthalic acid and has a total metal content of less than 100 ppm when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0014] According to another embodiment of the present invention, the polyester resin comprises recycled terephthalic acid.

[0015] Advantageous Effects of the Invention

[0016] According to one embodiment of the present invention, the process for preparing terephthalic acid comprises hydrolyzing a compound prepared by depolymerization of waste polyester using a hydrolysis catalyst, wherein the hydrolysis catalyst comprises at least one cation selected from the group consisting of alkali metal ions such as Li + 、Na + , K + and Cs + , alkaline earth metal ions such as Be 2+ Mg 2+ , Ca 2+ and Ba2+ , ammonium ions such as NH 4+ and NR 4+ (wherein R is an alkyl group), and Zn 2+ or comprising at least one anion selected from the group consisting of OH - , OR - (where R is an alkyl group), HCO3 - 、CO3 2- , benzoate ion (C7H5O2 - ), a group consisting of 4-alkoxycarbonylbenzoate ions, acetate ions and terephthalate ions, which not only can produce terephthalic acid in an environmentally friendly manner, but also can produce high-purity terephthalic acid.

[0017] Specifically, in conventional alkaline hydrolysis reactions, solid terephthalic acid is not immediately produced, but rather terephthalate salts are produced, requiring an additional neutralization step. Furthermore, there is the problem that the acid used to neutralize the terephthalate salts, such as sulfuric acid or hydrochloric acid, produces environmentally polluting byproducts such as NaSO and NaCl, or a large amount of acid treatment wastewater.

[0018] In contrast, according to one embodiment of the present invention, in the process for preparing terephthalic acid, at least one cation is included as a hydrolysis catalyst, the cation being selected from the group consisting of alkali metal ions such as Li + 、Na + , K + and Cs + , alkaline earth metal ions such as Be 2+ Mg 2+ , Ca 2+ and Ba 2+ , ammonium ions such as NH 4+ and NR 4+ (wherein R is an alkyl group), and Zn 2+ or comprising at least one anion selected from the group consisting of OH - , OR - (where R is an alkyl group), HCO3 - 、CO3 2- , benzoate ion (C7H5O2 - ), 4-alkoxycarbonylbenzoate ions, acetate ions, and terephthalate ions. Unlike conventional processes, this process uses a small amount of terephthalate ions, allowing direct production of solid terephthalic acid without additional steps. This process is not only easy to operate but also reduces process costs. Furthermore, since no environmental pollutants are generated, terephthalic acid can be produced in a very environmentally friendly manner.

[0019] In addition, a process for producing terephthalic acid has been used, in which dimethyl terephthalate (DMT) or bis(2-hydroxyethyl)terephthalate (BHET) is produced from waste polyester and then hydrolyzed. In this case, a metal catalyst such as iron, cobalt, manganese, nickel, etc. is used in the hydrolysis. Because some of the produced terephthalic acid acts as an acid catalyst, the alcohol produced as a by-product in the hydrolysis reaction may decompose. Therefore, there is a problem of increased processing costs or the generation of non-recyclable waste.

[0020] Specifically, when a metal catalyst such as iron, cobalt, manganese, nickel, etc. is used in the hydrolysis, some of the terephthalic acid produced during the hydrolysis reaction dissolves in water and acts as an acid catalyst, which may cause the alcohol produced as a by-product in the hydrolysis reaction to be dehydrated to produce additional by-products such as dialkyl ethers and olefins. Therefore, not only is the recovery rate of the alcohol reduced, but also non-recyclable waste is generated, or a process for removing the additional by-products is required to recover the alcohol, resulting in increased process costs.

[0021]

[0022] On the contrary, according to one embodiment of the present invention, in the process for preparing terephthalic acid, at least one cation is used as a hydrolysis catalyst, and the cation is selected from the group consisting of alkali metal ions such as Li + 、Na + , K + and Cs + , alkaline earth metal ions such as Be 2+ Mg 2+ , Ca 2+ and Ba 2+ , ammonium ions such as NH 4+ and NR 4+ (wherein R is an alkyl group), and Zn 2+ or comprising at least one anion selected from the group consisting of OH - , OR - (where R is an alkyl group), HCO3 - 、CO3 2- , benzoate ion (C7H5O2 - ), a group consisting of 4-alkoxycarbonylbenzoate ions, acetate ions and terephthalate ions, some of the terephthalic acid produced during the hydrolysis reaction does not act as an acid catalyst; therefore, the decomposition rate of the alcohol can be reduced.

[0023] Best Mode for Carrying Out the Invention

[0024] The present invention will be described in detail below. The present invention is not limited to the following disclosure, and the present invention can be modified in various forms without changing the gist of the present invention.

[0025] Throughout this specification, when a component is referred to as “comprising” an element, it should be understood that other elements may be included rather than excluded unless specifically stated otherwise.

[0026] Unless otherwise indicated, all numbers and expressions relating to quantities of ingredients, reaction conditions, and so forth used herein are to be understood as modified by the term "about."

[0027] Throughout this specification, the terms first, second, etc. are used to describe various components. However, these components should not be limited by these terms. These terms are only used to distinguish one component from another.

[0028] Process for preparing terephthalic acid

[0029] According to one embodiment of the present invention, the process for preparing terephthalic acid comprises hydrolyzing a compound prepared by depolymerization of waste polyester, wherein the hydrolysis catalyst used in the hydrolysis step comprises at least one cation selected from the group consisting of Li + 、Na + , K + and Cs + Alkali metal ions, Be 2+ Mg 2+ , Ca 2+ and Ba 2+ Alkaline earth metal ions, NH 4+ and NR 4+ (where R is an alkyl group) ammonium ions and Zn 2+ or comprising at least one anion selected from the group consisting of OH - , OR - (where R is an alkyl group), HCO3 - 、CO3 2- , benzoate ion (C7H5O2 - ), a group consisting of 4-alkoxycarbonylbenzoate ion, acetate ion and terephthalate ion.

[0030] R may be an alkyl group having 1 to 10 carbon atoms or an alkyl group having 1 to 5 carbon atoms.

[0031] Waste polyester can be obtained by crushing or melting waste polyester products. For example, waste polyester can be obtained by crushing the used of Polyester products are recovered and separated, or obtained by converting them into pellets (post-consumer recycled material; PCR) or polyester waste (post-industrial recycled material; PIR), such as, but not limited to, defective products or waste that may be formed during the molding process of polyester films, fibers, containers, etc.

[0032] Specifically, the hydrolysis catalyst may include at least one catalyst selected from the group consisting of NaOH, NaHCO3, Na2CO3, NaOMe, KOH, K2CO3, KOtBu, CsOH, Ca(OH)2, LiOH, and NH4OH.

[0033] Because the specific hydrolysis catalysts listed above are used, some of the terephthalic acid produced does not act as an acid catalyst; therefore, the decomposition rate of the alcohol can be reduced. For example, when using a hydrolysis catalyst such as NaOH, unlike the prior art where some of the terephthalic acid produced acts as an acid catalyst, it is converted into TPA-Na salt, which dissolves in the water used in the hydrolysis reaction. As a result, the decomposition of the alcohol, a by-product of the hydrolysis reaction, can be effectively prevented.

[0034] According to one embodiment of the present invention, even in the process for producing terephthalic acid, alcohols such as R1-OH (wherein R is an alkyl group having 2 or more carbon atoms) can be formed as byproducts of the hydrolysis reaction. For dimethyl terephthalate, methanol can be formed. For bis(2-hydroxyethyl)terephthalate, ethylene glycol can be formed. However, according to one embodiment of the present invention, due to the use of a specific hydrolysis catalyst in the process for producing terephthalic acid, the decomposition rate of alcohols, methanol, and ethylene glycol is very low compared to the prior art; therefore, the generation rate of alcohol byproducts is low.

[0035] In addition, the amount of the hydrolysis catalyst used may be 0.01 to 1.0 mol based on 1 mol of the compound. For example, the amount of the hydrolysis catalyst used may be 0.02 to 1.0 mol, 0.02 to 0.8 mol, 0.03 to 0.5 mol, 0.05 to 0.4 mol, or 0.05 to 0.2 mol based on 1 mol of the compound prepared by depolymerization of waste polyester.

[0036] Additionally, water may be added during the hydrolysis step. Specifically, water may be added during the hydrolysis step in an amount of 1 to 500 times the weight of the compound. For example, the amount of water added during the hydrolysis step may be 1 to 450 times, 1 to 400 times, 1 to 250 times, 1 to 100 times, 1.2 to 50 times, or 1.5 to 30 times the weight of the compound.

[0037] Furthermore, the hydrolysis may be performed at 180° C. to 280° C. for 0.5 to 24 hours. For example, the hydrolysis may be performed at a temperature of 185° C. to 280° C., 200° C. to 275° C., 220° C. to 270° C., or 240° C. to 265° C. for 1 to 20 hours, 2.5 to 12 hours, or 3 to 8 hours.

[0038] The traditional method involves adding a metal catalyst, such as iron, cobalt, manganese, or nickel, to waste polyester, followed by direct hydrolysis with water. This method is very environmentally friendly. However, due to the required high temperatures of 300°C or higher, and the need for the reactor to withstand high pressures, processability is limited. In contrast, according to one embodiment of the present invention, the process for producing terephthalic acid has improved process conditions compared to existing technologies, resulting in excellent processability.

[0039] According to one embodiment of the present invention, solid terephthalic acid can be produced through a hydrolysis reaction. Specifically, the process can further include filtering, washing, and drying the hydrolysis reaction product produced by the hydrolysis reaction after the hydrolysis step. In other words, the hydrolysis reaction product produced by the hydrolysis reaction can be filtered, washed, and dried to produce solid terephthalic acid.

[0040] For example, the hydrolysis reaction product can be cooled to a suitable temperature, for example, from room temperature to below 100° C. where water does not evaporate, to obtain a slurry-like solution, which is then filtered to obtain a solid, which is then washed and vacuum-dried to obtain solid terephthalic acid.

[0041] Washing can be performed using a mixture of an alcohol having 4 or more carbon atoms and / or water, a protic solvent such as isopropyl alcohol and acetic acid, or an aprotic solvent such as acetone, dichloromethane, chloroform, tetrahydrofuran (THF), and toluene.

[0042] Washing effectively removes residual pigments or impurities produced during the hydrolysis process, particularly yellow impurities, thereby enhancing the yellowness index or color characteristics. Furthermore, since water is used for washing, mineral salts are removed, thereby improving quality.

[0043] For example, the solid obtained by filtration may be washed with a mixture of alcohol at 80°C to 150°C and / or water at 70°C to 95°C.

[0044] According to one embodiment of the present invention, the compound prepared by depolymerization of waste polyester may be the compound shown in Formula 1.

[0045] [Formula 1]

[0046]

[0047] In Formula 1, R1 is a substituted or unsubstituted alkyl group.

[0048] Specifically, R1 can be an unsubstituted alkyl group or an alkyl group substituted with a hydroxyl group. For example, the compound can be dimethyl terephthalate (DMT), dibutyl terephthalate (DBTP), diisooctyl terephthalate (DOTP) or bis(2-hydroxyethyl)terephthalate (BHET).

[0049] For example, dimethyl terephthalate can be produced by methanolysis of waste polyester. Bis(2-hydroxyethyl)terephthalate can be produced by glycolysis of waste polyester. Commercially available dimethyl terephthalate and bis(2-hydroxyethyl)terephthalate can also be used.

[0050] Alternatively, in Formula 1, R1 may be a substituted or unsubstituted alkyl group having 2 or more carbon atoms.

[0051] For example, R1 can be methyl, ethyl, propyl, n-propyl, isopropyl, butyl, n-butyl, isobutyl, tert-butyl, sec-butyl, 1-methyl-butyl, 1-ethyl-butyl, pentyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, hexyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 4-methyl-2-pentyl, 3,3-dimethylbutyl, 2-ethylbutyl, hexyl, 1-methylhexyl, 2-ethyl-1-hexyl, heptyl, n-heptyl, 1-methylheptyl, octyl, n-octyl, isooctyl, tert-octyl, 2-propylpentyl, n-nonyl, 2,2-dimethylheptyl, 1-ethyl-propyl, 1,1-dimethyl-propyl, isohexyl, 2-methylpentyl, 4-methylhexyl, 5-methylhexyl, decyl, undecyl, dodecyl, tridecyl, or tetradecyl.

[0052] According to one embodiment of the present invention, alcohol can be used as the by-product production in the hydrolysis step.Particularly, the decomposition rate of alcohol can be less than 10%.For example, by using gas chromatography analysis to filter the composition in the filtrate that hydrolysis reaction product obtains in slurry form, the decomposition rate (%) of alcohol can be calculated.The decomposition rate of alcohol can be 9% or lower, 6% or lower, 5% or lower, 4.5% or lower, 4% or lower, 3% or lower or 2% or lower.According to one embodiment of the present invention, in the technology of preparing terephthalic acid, not only the decomposition rate of alcohol is low, and because alcohol can be recovered and reused, therefore also is environmentally friendly, can also reduce process cost simultaneously.

[0053] According to another embodiment of the present invention, the depolymerization process may include alcoholysis.

[0054] Specifically, alcoholysis may be performed using an alcohol having 4 or more carbon atoms. In particular, when the compound is prepared by alcoholysis of waste polyester using an alcohol having 4 or more carbon atoms, the compound may be liquid at room temperature.

[0055] When a compound is produced by alcoholyzing waste polyester with an alcohol having four or more carbon atoms, the compound is liquid at room temperature. Therefore, insoluble impurities and additives such as colorants and pigments that may be present in the waste polyester can be easily removed. Consequently, the purity and yield of the produced terephthalic acid can be increased.

[0056] Furthermore, unlike conventional processes, solid terephthalic acid can be directly produced without the need for an additional neutralization step, resulting in excellent processability and environmental friendliness. Furthermore, ethylene glycol, a byproduct that may be formed during the production process, can be easily separated and recovered, and the alcohols containing four or more carbon atoms used in the alcoholysis reaction can also be easily separated and reused, significantly reducing process costs. Furthermore, since the purification and transfer steps outside of the reaction can be performed at room or low temperatures, processability and economic benefits can be further improved.

[0057] For example, alcoholysis can be carried out by first adding waste polyester, alcohol having 4 or more carbon atoms and a very small amount of alcoholysis catalyst into a first high-pressure reactor. The by-product ethylene glycol and unreacted alcohol (excess alcohol) formed during the alcoholysis reaction can be recovered and reused by a separate fractionation device after the reaction is completed.

[0058] In addition, the ethylene glycol that forms in the reaction process and unreacted alcohol can be discharged in real time with the form of gaseous mixture in the reaction process, then use external cooling device to condense and reclaim.In this case, alcohol can be fed to high-pressure reactor continuously, and its feeding rate and volume are identical with the volume and emission rate of the gaseous mixture of discharge.Unreacted alcohol can be separated from the gaseous mixture of discharge by simple technology, for example fractionation or layer separation.Isolated unreacted alcohol can be fed to the first high-pressure reactor again thus, and can therefrom reclaim ethylene glycol.

[0059] The liquid alcoholysis product obtained from the alcoholysis reaction can then be purified by cooling, adsorption, and filtration. The purified alcoholysis reaction mixture is fed into a second high-pressure reactor along with water for a hydrolysis reaction. A slurry solution is then obtained and filtered to yield solid terephthalic acid. In this case, a small amount of a hydrolysis catalyst may be added along with the water prior to the hydrolysis reaction. The hydrolysis catalyst may be the same as or different from the alcoholysis catalyst.

[0060] In addition, after hydrolysis reaction is completed, unreacted components are recovered and reintroduced into alcoholysis or hydrolysis reaction to react once or multiple times, thereby improved the productive rate of the terephthalic acid that finally generates. In addition, the generation amount of waste can also be reduced, and it is environmentally friendly. For example, the filtrate obtained after using a filter or similar device to filter unnecessary unreacted alcohol (having 4 or more carbon atoms) and byproduct ethylene glycol can be reused in hydrolysis reaction.

[0061] The alcoholysis reaction can be smoothly carried out as a non-catalytic reaction without using an alcoholysis catalyst, making it environmentally friendly. In particular, when the insoluble metal content in the waste polyester is high, the non-catalytic reaction may be conducive to effectively treating and removing impurities. Alternatively, an alcoholysis catalyst can be added to the alcoholysis reaction. Specifically, from an energy perspective, an alcoholysis catalyst can be added to enhance processability by increasing reactivity.

[0062] The alcoholysis catalyst may be a metal acetate, an alkali metal salt or a hydroxyl salt.

[0063] More specifically, the alcoholysis catalyst may comprise at least one cation selected from the group consisting of alkali metal ions such as Li + 、Na + , K + and Cs + , alkaline earth metal ions such as Be 2+ Mg 2+ , Ca 2+ and Ba 2+ , ammonium ions such as NH 4+ and NR 4+ (wherein R is an alkyl group), and Zn 2+ or comprising at least one anion selected from the group consisting of OH - , OR - (where R is an alkyl group), HCO3 - 、CO3 2- , benzoate ion (C7H5O2 - ), a group consisting of 4-alkoxycarbonylbenzoate ion, acetate ion and terephthalate ion. R may be an alkyl group having 1 to 10 carbon atoms or an alkyl group having 1 to 5 carbon atoms.

[0064] For example, the alcoholysis catalyst may include at least one selected from the group consisting of Zn(OAC)2, Co(OAc)2, Mn(OAc)2, Mg(OAc)2, Ca(OAc)2, Ba(OAc)2, LiOAc, NaOAc, KOAc, Zn(OAC)2·2H2O, Co(OAc)2·4H2O, Pb(OAc)2, Mn(OAc)2·4H2O, Mg(OAc)2·4H2O, Pd(OAc)2, Ti(OBu)4, Ti(OiPr)4, GeO2, Al(OiPr)3, Na2CO3, K2CO3, di-n-butyltin(IV) oxide, stannous octoate, titanium phosphate, and terephthalic acid.

[0065] Furthermore, the alcoholysis catalyst may be added in an amount of 10 ppm to 10,000 ppm based on the total weight of the waste polyester. For example, the alcoholysis catalyst may be added in an amount of 10 ppm to 9,000 ppm, 15 ppm to 8,000 ppm, 20 ppm to 6,000 ppm, 50 ppm to 3,500 ppm, 100 ppm to 1,500 ppm, 150 ppm to 1,000 ppm, 180 ppm to 500 ppm, or 200 ppm to 450 ppm based on the total weight of the waste polyester.

[0066] According to another embodiment of the present invention, the alcoholysis may include discharging unreacted alcohol and ethylene glycol as by-products.

[0067] Particularly, alcohol separates from the mixture of the alcohol and ethylene glycol of discharge, and the alcohol of separation can be used as the raw material recycling of alcoholysis.For example, during alcoholysis reaction and / or when alcoholysis reaction is completed, ethylene glycol and unreacted alcohol (excessive alcohol) as the by product formed in alcoholysis are carried out fractionation or layer separation to separate alcohol.Isolated alcohol can be introduced in alcoholysis and reused.In this case, the volume and feed rate of the alcohol of introducing reuse can be identical with the volume and discharge rate of the mixture of the alcohol and ethylene glycol of discharge.

[0068] Furthermore, the number of carbon atoms of the alcohol used in the alcoholysis may be 4 or more, 6 or more, 8 or more, 10 or more or 12 or more, and may also be 4 to 14, 4 to 13, 4 to 10, 4 to 8, 6 to 12, 8 to 14 or 8 to 13.

[0069] Since the alcoholysis of waste polyester is carried out using an alcohol having the aforementioned number of carbon atoms, the alcoholysis can be carried out at lower temperatures and pressures compared to conventional processes using waste polyester, which are carried out at high temperatures and high pressures. Consequently, the alcoholysis product, i.e., the aforementioned compound, can be produced in liquid form. Furthermore, when the number of carbon atoms of the alcohol falls within the aforementioned range, the alcoholysis reaction rate can be increased.

[0070] Additionally, the boiling point of the alcohol can be between 100°C and 290°C. For example, the boiling point of the alcohol can be between 100°C and 280°C, 100°C and 260°C, 100°C and 230°C, 110°C and 190°C, or 180°C and 290°C. When the boiling point of the alcohol meets the above range, the ethylene glycol formed as a byproduct during alcoholysis can be more easily removed or recovered in subsequent processes, thereby further improving processability. In recent years, in particular, the use of various monomer materials has become a trend in the field of polyester as a raw material; therefore, it can be conveniently used to remove various diol-type monomers used in waste polyester products, such as waste plastic products.

[0071] The weight ratio of the waste polyester to the alcohol may be 1:1 to 10. For example, the weight ratio of the waste polyester to the alcohol may be 1:1 to 8, 1:1 to 6, 1:1 to 4, 1:1 to 3.5, 1:1.1 to 3.3, 1:2 to 4, or 1:2 to 3.5.

[0072] In addition, the alcoholysis reaction can be carried out at a temperature of 160° C. to 280° C. and a pressure of 1 bar to 40 bar for 0.5 to 24 hours. For example, the alcoholysis reaction can be carried out at a temperature of 165° C. to 280° C., 165° C. to 270° C., 180° C. to 270° C., 190° C. to 250° C., 200° C. to 265° C., 220° C. to 265° C., 240° C. to 260° C., or 245° C. to 260° C., and a pressure of 1 bar to 38 bar, 1 bar to 33 bar, 1 bar to 28 bar, 1 bar to 24 bar, 2 bar to 40 bar, 3 bar to 35 bar, or 5 bar to 30 bar, for a duration of 0.5 to 22 hours, 1 hour to 15 hours, 1.5 hours to 10 hours, 2 hours to 8 hours, or 2 hours to 6 hours.

[0073] An alcoholysis catalyst may be added to the alcoholysis reaction. Specifically, before the alcoholysis reaction, from an energy perspective, an alcoholysis catalyst may be added to the mixture of waste polyester and alcohol to enhance processability by increasing reactivity.

[0074] The alcoholysis catalyst may be a metal acetate, an alkali metal salt or a hydroxyl salt.

[0075] More specifically, the alcoholysis catalyst may comprise at least one cation selected from the group consisting of alkali metal ions such as Li + 、Na + , K + and Cs + , alkaline earth metal ions such as Be 2+ Mg 2+ , Ca 2+ and Ba 2+ , ammonium ions such as NH 4+ and NR 4+ (wherein R is an alkyl group), and Zn 2+ or comprising at least one anion selected from the group consisting of OH - , OR - (where R is an alkyl group), HCO3 - 、CO3 2- , benzoate ion (C7H5O2 - ), a group consisting of 4-alkoxycarbonylbenzoate ion, acetate ion and terephthalate ion. R may be an alkyl group having 1 to 10 carbon atoms or an alkyl group having 1 to 5 carbon atoms.

[0076] For example, the alcoholysis catalyst may include at least one selected from the group consisting of Zn(OAC)2, Co(OAc)2, Mn(OAc)2, Mg(OAc)2, Ca(OAc)2, Ba(OAc)2, LiOAc, NaOAc, KOAc, Zn(OAC)2·2H2O, Co(OAc)2·4H2O, Pb(OAc)2, Mn(OAc)2·4H2O, Mg(OAc)2·4H2O, Pd(OAc)2, Ti(OBu)4, Ti(OiPr)4, GeO2, Al(OiPr)3, Na2CO3, K2CO3, di-n-butyltin(IV) oxide, stannous octoate, titanium phosphate, and terephthalic acid.

[0077] Furthermore, the alcoholysis catalyst may be added in an amount of 10 ppm to 10,000 ppm based on the total weight of the waste polyester. For example, the alcoholysis catalyst may be added in an amount of 10 ppm to 9,000 ppm, 15 ppm to 8,000 ppm, 20 ppm to 6,000 ppm, 50 ppm to 3,500 ppm, 100 ppm to 1,500 ppm, 150 ppm to 1,000 ppm, 180 ppm to 500 ppm, or 200 ppm to 450 ppm based on the total weight of the waste polyester.

[0078] The alcoholysis reaction can be smoothly carried out as a non-catalytic reaction without the use of an alcoholysis catalyst, making it environmentally friendly. In particular, when the insoluble metal content in waste polyester is high, the non-catalytic reaction may be beneficial for effectively treating and removing impurities.

[0079] According to another embodiment of the present invention, the process may further comprise purifying the compound prior to the hydrolysis step.

[0080] Specifically, the purification step may include adding at least one adsorbent selected from the group consisting of activated carbon, silica gel, alumina, zeolite, and activated clay, or performing adsorption by bed adsorption. More specifically, the adsorbent may be activated carbon or a mixture of activated carbon and silica gel. For example, the adsorbent may be a mixture of activated carbon and silica gel in a weight ratio of 1:0.5 to 1.5 or 1:0.8 to 1.2, but is not limited thereto.

[0081] The adsorbent may be added in an amount of 0.1 to 20 wt % based on the total weight of the compound. For example, the adsorbent may be added in an amount of 0.1 to 18 wt %, 0.1 to 15 wt %, 0.1 to 10 wt %, 0.1 to 5 wt %, or 0.1 to 2 wt %, based on the total weight of the compound.

[0082] As the step of purifying the compound using an adsorbent is further performed, particularly when the adsorbent is fed in an amount within the aforementioned numerical range, purity and yield can be further improved. Specifically, as the step of purifying the compound is further performed, insoluble impurities, such as metals that may be present in the compound or additives such as colorants and pigments that may be present in waste polyester, can be more effectively removed. Consequently, the purity and yield of the ultimately produced terephthalic acid can be further improved.

[0083] Furthermore, after the purification step, a concentration step may be further performed.

[0084] The concentration can be carried out at a temperature of 50° C. to 120° C. for a time of 0.5 to 6 hours. For example, the concentration can be carried out at a temperature of 55° C. to 115° C., 60° C. to 110° C., 65° C. to 105° C., or 75° C. to 100° C., by stirring the purified compound, for 1 to 5 hours, 1.5 to 4 hours, or 2 to 4 hours.

[0085] In addition, the purified compound may contain less insoluble impurities, such as metals. Specifically, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), the total metal content of the purified compound may be 100 ppm or less relative to the total weight of the purified compound.

[0086] For example, the purified compound may contain insoluble impurities, such as metals. The total metal content in the purified compound may be 80 ppm or less, 70 ppm or less, 60 ppm or less, 50 ppm or less, 40 ppm or less, or less than 30 ppm, relative to the total weight of the purified compound, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). In particular, the total content of Sb, Ti, and Zn may be less than 30 ppm, 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 5 ppm or less, 3 ppm or less, or 1 ppm or less.

[0087] As is well known, Sb is a widely used catalyst in common polyester polymerization due to its excellent stability, reaction rate, and cost. However, regulations are being strengthened due to its impact on human health and the environment, and therefore, it is a substance that must be removed during chemical recycling processes.

[0088] In addition, Ti can be used as a catalyst for polyester polymerization or as an additive in polyester processing in the form of TiO2. If it contains a certain amount or more, the quality of the recycled terephthalic acid prepared therefrom or the polyester resin using it may be reduced, thereby limiting its use.

[0089] Zn is also a component used as a PET polymerization catalyst. If it remains, it may affect the reactivity control in the production process of the recycled terephthalic acid or the polyester resin using it. Therefore, it is best to remove it. In particular, because it is widely used in chemical recycling processes, it is necessary to fully remove this substance contained in waste plastics (which are the raw materials of this process) and this substance added separately as a catalyst in the recycling process.

[0090] According to one embodiment of the present invention, as purification proceeds further, the total content of metals, particularly Sb, Ti and Zn as described above, in the purified compound is very low, at 30 ppm or less.

[0091] For example, the Sb content may be 30 ppm or less, 20 ppm or less, 10 ppm or less, or 1 ppm or less, relative to the total weight of the purified compound, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0092] The Ti content may be 30 ppm or less, 20 ppm or less, 10 ppm or less, or 1 ppm or less, relative to the total weight of the purified compound, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0093] The content of Zn may be 30 ppm or less, 20 ppm or less, 10 ppm or less, or 1 ppm or less, relative to the total weight of the purified compound, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0094] Furthermore, the yield of terephthalic acid based on the total weight of the compounds prepared by depolymerization of the waste polyester can be 80% or higher. For example, the yield of terephthalic acid based on the total weight of the compounds prepared by depolymerization of the waste polyester can be 82% or higher, 85% or higher, 87% or higher, 88% or higher, 89% or higher, 90% or higher, 91% or higher, 92% or higher, 94% or higher, 95% or higher, or 96% or higher.

[0095] Recycled terephthalic acid

[0096] According to another embodiment of the present invention, the recovered terephthalic acid is produced according to the above-described process for producing terephthalic acid and has a total metal content of less than 100 ppm when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0097] Specifically, the recovered terephthalic acid can be prepared according to the process for preparing terephthalic acid.

[0098] The total metal content of the recovered terephthalic acid may be less than 100 ppm, 90 ppm or less, 80 ppm or less, 65 ppm or less, 50 ppm or less, 35 ppm or less, less than 30 ppm, less than 20 ppm, 15 ppm or less, 9 ppm or less, 7 ppm or less, 5 ppm or less, or 1 ppm or less, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0099] Furthermore, the total content of Sb, Ti, and Zn in the recovered terephthalic acid may be less than 30 ppm when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). For example, the total content of Sb, Ti, and Zn in the recovered terephthalic acid, which may be harmful to humans or may act as a catalyst for reactions or side reactions in subsequent polymerization processes, may be 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 5 ppm or less, 3 ppm or less, or 1 ppm or less.

[0100] For example, the Sb content in the recovered terephthalic acid may be 30 ppm or less, 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 6 ppm or less, 5 ppm or less, 4 ppm or less, 3 ppm or less, 2 ppm or less, or 1 ppm or less, relative to the total weight of the recovered terephthalic acid, when measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0101] The Ti content in the recovered terephthalic acid may be 30 ppm or less, 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 5 ppm or less, 3 ppm or less, or 1 ppm or less, relative to the total weight of the recovered terephthalic acid, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0102] The Zn content in the recovered terephthalic acid may be 30 ppm or less, 25 ppm or less, 15 ppm or less, 10 ppm or less, 6 ppm or less, 5 ppm or less, 4 ppm or less, 3 ppm or less, 2 ppm or less, or 1 ppm or less, relative to the total weight of the recovered terephthalic acid, when measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0103] As measured by a colorimeter, the Chroma-b of the recovered terephthalic acid may be less than 2, 1.6 or less, 1.4 or less, 1.3 or less, or 1 or less. Since the above Chroma-b numerical ranges correspond to the Chroma-b numerical ranges of crude terephthalic acid typically produced in petrochemical processes, the recovered terephthalic acid meeting the above Chroma-b numerical ranges not only has a low yellowness index but also has excellent quality due to its high monomer purity.

[0104] Chromaticity-b (Col-b) is a color coordinate system developed by the International Commission on Illumination (CIE), where color is represented by L (lightness), a (the complementary color from green to red), and b (the complementary color from yellow to blue). It can be measured using a colorimeter.

[0105] In addition, when the recovered terephthalic acid is diluted to a concentration of 5% in dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and methylpyrrolidone (NMP), respectively, and measured, the yellowness index (YI) may be less than 2, 1.8 or less, or 1.7 or less.

[0106] Polyester resin and its preparation process

[0107] According to another embodiment of the present invention, the polyester resin comprises recycled terephthalic acid.

[0108] Specifically, the polyester resin may include recycled terephthalic acid, a diol compound or a derivative thereof, and optionally a dicarboxylic acid compound or a derivative thereof.

[0109] For example, the diol component or its derivative may include at least one selected from the group consisting of ethylene glycol, monoethylene glycol, diethylene glycol, 1,4-butanediol, 1,3-propylene glycol, 1,4-cyclohexanedimethanol, and neopentyl glycol. The dicarboxylic acid component or its derivative may include at least one selected from the group consisting of terephthalic acid (TPA), isophthalic acid (IPA), 2,6-naphthalene dicarboxylic acid (2,6-NDA), dimethyl terephthalate (DMT), dimethyl isophthalate (DMI), and dimethyl 2,6-naphthalene dicarboxylate (2,6-NDC). However, these are not limited thereto.

[0110] According to another embodiment of the present invention, a process for preparing a polyester resin comprises mixing recovered terephthalic acid with a diol compound or a derivative thereof and optionally a dicarboxylic acid compound or a derivative thereof, and then performing an esterification reaction; and performing a polycondensation reaction on the esterification reaction product.

[0111] The esterification reaction may be carried out at a temperature of 200° C. to 350° C., 220° C. to 320° C., or 250° C. to 290° C. In addition, the esterification reaction may be carried out under a pressure higher than normal pressure, which may be 0 kg / cm 2 Up to 10kg / cm2 (0mmHg to 7,355.6mmHg), 0kg / cm 2 Up to 5kg / cm 2 (0mmHg to 3,677.8mmHg) or 0kg / cm 2 Up to 2.0kg / cm 2 (0 mmHg to 1,471.1 mmHg) For example, the esterification reaction may be performed for 1 hour to 24 hours, 1 hour to 10 hours, or 1 hour to 6 hours, but is not limited thereto.

[0112] In addition, the polycondensation reaction can be carried out at a temperature of 150°C to 400°C, 200°C to 370°C, 250°C to 350°C, or 270°C to 300°C. In addition, the polycondensation reaction can be carried out under a reduced pressure of 0.01mmHg to 400mmHg, 0.05mmHg to 100mmHg, or 0.1mmHg to 100mmHg. In addition, the polycondensation reaction can be carried out for a desired time until the desired intrinsic viscosity is reached. For example, it can be carried out for 1 hour to 24 hours, 1 hour to 10 hours, or 1 hour to 4 hours.

[0113] Furthermore, a catalyst and / or a stabilizer may be further added during the esterification reaction and the polycondensation reaction.

[0114] For example, the catalyst for the esterification reaction can be sodium and magnesium formates; acetates, borates, fatty acid salts and carbonates of zinc (Zn), cadmium (Cd), manganese (Mn), cobalt (Co), calcium (Ca) and barium (Ba); and metallic magnesium (Mg); and oxides of lead (Pb), Zn, antimony (Sb) and germanium (Ge).

[0115] In addition, the catalyst for the polycondensation reaction can be, for example, a titanium-based catalyst such as tetraethyl titanate, acetyl tripropyl titanate, tetrapropyl titanate, tetrabutyl titanate, polybutyl titanate, 2-ethylhexyl titanate, octanediol titanate, lactic acid titanate, triethanolamine titanate, acetylacetonate titanate, ethyl acetoacetate titanate, isostearyl titanate, titanium dioxide, titanium dioxide / silicon dioxide copolymer, titanium dioxide / zirconium dioxide copolymer; a germanium-based catalyst such as germanium dioxide and a copolymer using germanium dioxide; or a tin-based catalyst such as monobutyl tin oxide, dibutyl tin oxide and monobutyl hydroxytin oxide.

[0116] In addition, the stabilizer may be a phosphorus-based compound such as phosphoric acid, trimethyl phosphate, and triethyl phosphate, but is not limited thereto.

[0117] According to another embodiment of the present invention, the process for preparing the polyester resin may further include performing a solid-state polymerization reaction. For example, after the polycondensation reaction, solid-state polymerization may be performed under a vacuum condition of 0.2 to 2.0 torr or in a nitrogen environment at a temperature of 190° C. to 230° C. DETAILED DESCRIPTION

[0118] Invention Mode

[0119] The present invention will be described in more detail below with reference to the following examples. However, these examples are for the purpose of illustrating the present invention, and the scope of the present invention is not limited thereto.

[0120] [Example]

[0121] Preparation Example 1-1

[0122] 1 kg of waste polyethylene terephthalate (waste PET) and 3.3 kg of 1-butanol were charged into a first high-pressure reactor having a capacity of 7 liters, and then 200 mg of Zn(OAC)2.2H2O as an alcoholysis catalyst (200 ppm relative to the total weight of the waste PET) was added thereto.

[0123] After all connections of the first high-pressure reactor were tightened and sealed, the temperature was raised to 250° C. over 1 hour. The alcoholysis reaction was carried out for 4 hours under stirring while the temperature was maintained at 250° C. After the alcoholysis reaction was completed, it was cooled to room temperature.

[0124] Then, 5 g of activated carbon as an adsorbent was added to 500 g of the alcoholysis reaction product, and the mixture was stirred at 100° C. for 3 hours, and then filtered and concentrated using a Buchner funnel to obtain liquid dibutyl terephthalate (DBTP).

[0125] Preparation Example 1-2

[0126] Liquid diisooctyl terephthalate (DOTP) was prepared in the same manner as in Preparation Example 1-1, except that 2-ethyl-1-hexanol was used instead of 1-butanol.

[0127] Preparation of terephthalic acid

[0128] Example 1

[0129] In a second high-pressure reactor having a capacity of 600 ml, 72.4 g (0.26 mol) of liquid dibutyl terephthalate (DBTP) prepared in Preparation Example 1-1 and 200 g (11.10 mol) of water were added, and then 1.02 g (0.026 mol, 0.1 mol based on 1 mol of DBTP) of NaOH was added as a hydrolysis catalyst.

[0130] The temperature of the second high-pressure reactor was then raised to 260°C, and a hydrolysis reaction was carried out for 4 hours while maintaining the temperature at 260°C. The mixture was then cooled to room temperature to obtain a hydrolysis reaction product in the form of a slurry. The slurry hydrolysis reaction product was filtered to obtain a solid, which was then washed with butanol (about 90°C) and water (about 90°C) and dried under vacuum to obtain 40.6 g of solid terephthalic acid (TPA) (yield: 94%). The decomposition rate (%) was calculated by analyzing the components of the filtrate using gas chromatography.

[0131] Example 2

[0132] 36.7 g (yield: 85%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 100 g (0.26 mol) of diisooctyl terephthalate (DOTP) prepared in Preparation Example 1-2 was used instead of liquid dibutyl terephthalate (DBTP) prepared in Preparation Example 1-1, and octanol (about 80° C.) was used instead of butanol (about 80° C.) during washing.

[0133] Example 3

[0134] 39.7 g (yield: 92%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 2.04 g (0.052 mol) of NaOH was used.

[0135] Example 4

[0136] 38.4 g (yield: 89%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 2.18 g (0.026 mol) of NaHCO 3 was used instead of NaOH.

[0137] Example 5

[0138] 38.9 g (yield: 90%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 2.76 g (0.026 mol) of Na 2 CO 3 was used instead of NaOH.

[0139] Example 6

[0140] 35.4 g (yield: 82%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 1.40 g (0.026 mol) of NaOMe was used instead of NaOH.

[0141] Example 7

[0142] 40.6 g (yield: 94%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 1.46 g (0.026 mol) of KOH was used instead of NaOH.

[0143] Example 8

[0144] 38.4 g (yield: 89%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 3.59 g (0.026 mol) of K 2 CO 3 was used instead of NaOH.

[0145] Example 9

[0146] 35.4 g (yield: 82%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that In other words, 2.92 g (0.026 mol) of KOtBu were used instead of NaOH.

[0147] Example 10

[0148] 39.7 g (yield: 92%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 2.73 g (0.013 mol) of Na2TPA (disodium terephthalate) was used instead of NaOH.

[0149] Example 11

[0150] 37.1 g (yield: 86%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 6.35 g (0.026 mol) of sodium monobutyl terephthalate was used instead of NaOH.

[0151] Example 12

[0152] 37.4 g (yield: 88%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 45.7 g (0.26 mol) of dimethyl terephthalate (DMT, manufacturer: Sigma Aldrich) was used instead of the liquid dibutyl terephthalate (DBTP) prepared in Preparation Example 1-1, and methanol (hot, about 110° C.) was used instead of butanol in washing.

[0153] Example 13

[0154] 40.8 g (yield: 96%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 65.1 g (0.26 mol) of bis(2-hydroxyethyl)terephthalate (BHET, manufacturer: Sigma Aldrich) was used instead of the liquid dibutyl terephthalate (DBTP) prepared in Preparation Example 1-1, and ethylene glycol (hot) was used instead of butanol (hot) in washing.

[0155] Example 14

[0156] In a second high-pressure reactor with a capacity of 600 ml, 100 g (0.26 mole) of liquid diisooctyl terephthalate (DOTP) prepared in Preparation Example 1-2 and 200 g (11.10 moles) of water were added, and then 1.02 g (0.026 mole, 0.1 mole based on 1 mole of DOTP) of NaOH was added as a hydrolysis catalyst.

[0157] The temperature of the second high-pressure reactor was then raised to 260°C, and a hydrolysis reaction was carried out for 4 hours while maintaining the temperature at 260°C. The mixture was then cooled to room temperature to obtain a hydrolysis reaction product in the form of a slurry. The filtrate obtained after filtering the hydrolysis reaction product in the form of a slurry was separated into layers to obtain an aqueous layer (190g). This layer was then fed back into the second high-pressure reactor, and 95g (0.24 mol) of liquid diisooctyl terephthalate (DOTP) prepared in Preparation Example 1-2 was added thereto, followed by another hydrolysis reaction at 260°C. The mixture was cooled again to room temperature to obtain a hydrolysis reaction product in the form of a slurry. The solid was filtered, washed with octanol (about 150°C) and water (about 90°C), and vacuum dried to obtain 36.8g (yield: 91%) of solid terephthalic acid (TPA). Here, the decomposition rate (%) was calculated by analyzing the components of the filtrate using gas chromatography.

[0158] Example 15

[0159] In a second high-pressure reactor with a capacity of 600 ml, 20 g (0.051 mol) of liquid dioctyl terephthalate (DOTP) prepared in Preparation Example 1-2 and 400 g (22.20 mol) of water were added, and then 0.20 g (0.0051 mol, 0.1 mol based on 1 mol of DOTP) of NaOH was added as a hydrolysis catalyst.

[0160] The temperature of the second high-pressure reactor was then raised to 260°C, and a hydrolysis reaction was carried out for 4 hours while maintaining the temperature at 260°C. The reaction was then cooled to room temperature to obtain a hydrolysis reaction product in the form of a slurry. The slurry hydrolysis reaction product was filtered to obtain a solid, which was then washed with octanol (about 120°C) and water (about 80°C) and dried under vacuum to obtain 8.4 g of solid terephthalic acid (TPA) (yield: 99%). The decomposition rate (%) was calculated by analyzing the components of the filtrate using gas chromatography.

[0161] Comparative Example 1

[0162] 39.7 g (yield: 92%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 14 mg of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0163] Comparative Example 2

[0164] 34.1 g (yield: 79%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 2, except that 20 mg of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0165] Comparative Example 3

[0166] 36.2 g (yield: 84%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 13, except that 9.1 mg of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0167] Comparative Example 4

[0168] 34.6 g (yield: 80%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 14, except that 13 mg of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0169] Comparative Example 5

[0170] 39.3 g (yield: 91%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 1, except that 5.7 g of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0171] Comparative Example 6

[0172] 35.9 g (yield: 83%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 2, except that 5.7 g of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0173] Comparative Example 7

[0174] 37.2 g (yield: 86%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 13, except that 5.7 g of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0175] Comparative Example 8

[0176] 34.1 g (yield: 79%) of solid terephthalic acid (TPA) was obtained in the same manner as in Example 14, except that 5.7 g of Zn(OAC)2.2H2O was used as a hydrolysis catalyst.

[0177] Test example: Metal content

[0178] Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to measure the metal content (ppm) in terephthalic acid in Examples 1 to 15 and Comparative Examples 1 to 8. ND means that the content is too low, less than 1 part per million (1 ppm), and a specific value cannot be measured.

[0179] [Table 1]

[0180]

[0181] As can be seen from Table 1 above, the recovered terephthalic acid prepared in Examples 1 to 15 is environmentally friendly because it is prepared by adding the hydrolysis catalyst according to the present invention and water and then hydrolyzing. Compared with Comparative Examples 1 to 8, the alcohol decomposition rate is low and the metal impurity content is very low, resulting in a high yield. Furthermore, the process for preparing terephthalic acid according to the present invention is environmentally friendly because the alcohol can be recovered and reused, which can reduce process costs.

Claims

1. A process for preparing terephthalic acid, comprising: Hydrolyzed bis(2-hydroxyethyl)terephthalate or a compound which is liquid at room temperature and is prepared by depolymerization of waste polyester, The compound is shown in Formula 1: [Formula 1] In Formula 1, R1 is an alkyl group substituted with a hydroxyl group, or a substituted or unsubstituted alkyl group having 2 or more carbon atoms; wherein the depolymerization comprises alcoholysis, and the alcoholysis is carried out using an alcohol having 4 or more carbon atoms; Wherein, the hydrolysis catalyst used in the hydrolysis step includes at least one selected from the group consisting of NaOH, NaHCO3, Na2CO3, NaOMe, KOH, K2CO3, KOtBu, CsOH, Ca(OH)2, LiOH and NH4OH.

2. The process for preparing terephthalic acid according to claim 1, wherein: The hydrolysis catalyst is used in an amount of 0.01 mol to 1.0 mol based on 1 mol of the compound.

3. The process for preparing terephthalic acid according to claim 1, wherein: The hydrolysis is performed at a temperature of 180° C. to 280° C. for 0.5 to 24 hours.

4. The process for preparing terephthalic acid according to claim 1, wherein: Water is added in the hydrolysis step, and the amount of water added is 1 to 500 times the weight of the compound.

5. The process for preparing terephthalic acid according to claim 1, wherein: The compound is dibutyl terephthalate or diisooctyl terephthalate.

6. The process for preparing terephthalic acid according to claim 1, wherein: The hydrolysis step produces alcohol as a by-product, and the decomposition rate of the alcohol is less than 10%.

7. The process for preparing terephthalic acid according to claim 1, wherein: In the alcoholysis, a weight ratio of waste polyester to alcohol is 1:1 to 10, and the alcoholysis is performed at a temperature of 160° C. to 280° C. and a pressure of 1 bar to 40 bar for 0.5 to 24 hours.

8. The process for preparing terephthalic acid according to claim 1, wherein: The process further comprises purifying the compound prior to the hydrolysis step.

9. The process for preparing terephthalic acid according to claim 8, wherein: The purification step comprises adding at least one adsorbent selected from the group consisting of activated carbon, silica gel, alumina, zeolite and activated clay, or performing adsorption by bed adsorption, and the content of the added adsorbent is 0.1 wt% to 20 wt% based on the total weight of the compound.

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