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c© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a26 193

Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid 1

Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid

Richard J. Sheehan, Amoco Research Center, Amoco Chemical Company, Naperville, Illinois, United States

1. Introduction . . . . . . . . . . . . . . . . . 12. Physical Properties . . . . . . . . . . . . 13. Production . . . . . . . . . . . . . . . . . . 23.1. Amoco Oxidation . . . . . . . . . . . . . 33.2. Amoco Purification . . . . . . . . . . . . 43.3. Multistage Oxidation . . . . . . . . . . . 53.4. Dynamit-Nobel (Witten) Process . . . 53.5. Esterification of Terephthalic Acid . . 73.6. Hydrolysis of Dimethyl Terephthalate 7

3.7. Alternative and Past Technologies . . 84. Environmental Protection . . . . . . . . 95. Quality Specifications . . . . . . . . . . . 96. Storage and Transportation . . . . . . 107. Uses . . . . . . . . . . . . . . . . . . . . . . 108. Economic Aspects . . . . . . . . . . . . . 119. Toxicology and Occupational Health . 1210. References . . . . . . . . . . . . . . . . . . 12

1. Introduction

Terephthalic acid [100-21-0], and isophthalicacid [121-91-5], both C8H6O4, have the IUPACnames 1,4- and 1,3-benzenedicarboxylic acid.Dimethyl terephthalate [120-61-6], C10H10O4,is also known as 1,4-benzenedicarboxylic aciddimethyl ester. The acids are produced by oxi-dation of the methyl groups on the correspond-ingp-xylene [106-42-3] orm-xylene [108-38-3].After oxidation to a carboxylic acid, reactionwith methanol [67-56-1] gives the methyl ester,dimethyl terephthalate.

Terephthalic acid and dimethyl terephthal-ate are used to make saturated polyesters withaliphatic diols as the comonomer. Isophthalicacid is used as a feedstock for unsaturated poly-esters as well as a comonomer in some saturatedproducts. Structures are as follows:

Terephthalic acid came to prominencethrough the work of Whinfield and Dick-son in Britain around 1940 [4]. Earlier workby Carothers and coworkers in the UnitedStates established the feasibility of producinghigh molecular weight linear polyesters by re-acting diacids with diols, but they used aliphaticdiacids and diols. These made polyesters which

were unsuitable to be spun into fibers. Whin-field and Dickson found that symmetrical aro-matic diacids yield high-melting, crystalline,and fiber-forming materials; poly(ethylene tere-phthalate) has since become the largest volumesynthetic fiber. Worldwide, terephthalic acidplus dimethyl terephthalate ranked about 25thin tonnage of all chemicals produced in 1992,and about tenth in terms of organic chemicals.

2. Physical Properties

Terephthalic acid,Mr 166.13, is available acom-mercially as a free-flowing powder composed ofrounded crystals. It forms needles if recrystal-lized slowly. Vapor pressure is low: 0.097 kPa at250 ◦C, with sublimation at 402 ◦C and atmo-spheric pressure. Melting has been reported at427 ◦C.

Dimethyl terephthalate, Mr 194.19, melts at140.6 ◦C and has sufficient vapor pressure forvacuum distillation. The molten form is pre-ferred commercially, but flakes and briquettesare available when long transport distances arerequired.

Isophthalic acid,Mr 166.13, melts at 348 ◦C.Vapor pressure is 0.61 kPa at 250 ◦C.

Both terephthalic and isophthalic acid are sta-ble, intractable compounds with low solubil-ities in most solvents; isophthalic acid is 2 –5 times more soluble than terephthalic acid inthe same solvent. Terephthalic acid solubilities> 10 g per 100 g solvent at room temperature oc-cur with ammonium, potassium, or sodium hy-

2 Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid

Table 1. Physical properties of terephthalic acid and isophthalicacid

Terephthalic Isophthalic

Crystal density, g/cm3, 25 ◦C 1.58 1.53Specific heat, J g−1 K−1, to 100 ◦C 1.20 1.22Heat of formation, kJ/mol − 816 − 803Heat of combustion, kJ/mol, 25 ◦C 3189 3203First dissociation constant 2.9×10−4 2.4×10−4

Second dissociation constant 3.5×10−5 2.5×10−5

Ignition temperature in air, ◦C 680 700

Table 2. Solubility of terephthalic acid and isophthalic acid(g/100 g solvent)

25 ◦C 150 ◦C 200 ◦C 250 ◦C

Terephthalic acid solubility, g/100 g solventWater 0.0017 0.24 1.7 12.6Methanol 0.10 3.1Acetic acid 0.013 0.38 1.5 5.7

Isophthalic acid solubility, g/100 g solventWater 0.012 2.8 25.2Methanol 1.06 8.1Acetic acid 0.23 4.3 13.8 44.5

droxide, dimethylformamide, and dimethyl sulf-oxide. Tetramethylurea and pyridine each dis-solve ca. 7 g per 100 g.

Dimethyl terephthalate is also stable, andmore soluble in common organic solvents thanthe acids. Itsmain physical properties are amelt-ing point below the point of degradation, and avapor pressure which allows for purification bydistillation.

Other physical properties of terephthalic acidand isophthalic acid are listed in Tables 1 and2. Some physical properties of dimethyl tere-phthalate are listed below:

Solubility, g/100 g solventMethanol, 25 ◦C 1.0Methanol, 60 ◦C 5.7Ethyl acetate, 25 ◦C 3.5Ethyl acetate, 60 ◦C 16.0Trichloromethane, 25 ◦C 10.0Trichloromethane, 60 ◦C 23.0Benzene, 25 ◦C 2.0Benzene, 60 ◦C 14.0Toluene, 25 ◦C 4.3Toluene, 60 ◦C 10.4Dioxane, 25 ◦C 7.5Dioxane, 60 ◦C 26.5

Vapor pressure, kPa140 ◦C 1.58160 ◦C 2.62200 ◦C 10.88250 ◦C 42.60

Density at mp 1.07 g/cm3

Viscosity, 180 ◦C 0.0071 Pa · sViscosity, 200 ◦C 0.0060 Pa · sSpecific heat, solid 1.55 J g−1 K−1

Specific heat, liquid 1.94 J g−1 K−1

Heat of fusion 159.1 kJ/KgHeat of vaporization 355.5 kJ/KgHeat of formation −740.2 kJ/molHeat of combustion, 25 ◦C 4660 kJ/molFlash point, DIN 51 758 141 ◦C

3. Production

p-Xylene is the feedstock for all terephthalicacid and dimethyl terephthalate production; m-

xylene is used for all isophthalic acid. Oxida-tion catalysts and conditions have been devel-oped which give nearly quantitative oxidationof the methyl groups, leaving the benzene ringvirtually untouched. These catalysts are com-binations of cobalt, manganese, and bromine,or cobalt with a co-oxidant, e.g., acetaldehyde[75-07-0]. Oxygen is the oxidant in all pro-cesses. Acetic acid [64-19-7] is the reaction sol-vent in all but one process. Given these constantfactors, there is only one industrial oxidationprocess, with different variations, two separatepurification processes, and one process whichintermixes oxidation and esterification steps.

3.1. Amoco Oxidation

About 70% of the terephthalate feedstock usedworldwide is produced with a catalyst systemdiscovered by Scientific Design [5,6]. Almost100% of new plants use this reaction. A sep-arate company, Mid-Century Corporation, wasestablished tomarket this technology, and subse-quently purchased by Amoco Chemical. Amocodeveloped a commercial process, as did MitsuiPetrochemical, nowMitsui Sekka.Mitsuiwas anearly licensee ofMid-Century. Both Amoco andMitsui participate in joint-venture companies,and both have licensed the process. Licenseesare distributed around the world, and some haverelicensed the process to other companies.

A soluble cobalt –manganese – bromine cat-alyst system is the heart of the process. Thisyields nearly quantitative oxidation of the p-xylene methyl groups with small xylene losses[7]. Acetic acid is the solvent, and oxygen incompressed air is the oxidant. Various saltsof cobalt and manganese can be used, andthe bromine source can be HBr, NaBr, or


Figure 1. Catalytic, liquid-phase oxidation of p-xylene to terephthalic acid by the Amoco processa) Oxidation reactor; b) Surge vessel; c) Filter; d) Dryer; e) Residue still; f) Dehydration column

tetrabromoethane [79-27-6] among others. Thehighly corrosive bromine – acetic acid environ-ment requires the use of titanium-lined equip-ment in some parts of the process.

A feed mixture of p-xylene, acetic acid, andcatalyst is continuously fed to the oxidation re-actor (Fig. 1). The feed mixture also containswater, which is a byproduct of the reaction. Thereactor is operated at 175 – 225 ◦C and 1500 –3000 kPa. Compressed air is added to the re-actor in excess of stoichiometric requirementsto provide measurable oxygen partial pressureand to achieve high p-xylene conversion. The re-action is highly exothermic, releasing 2×108 Jper kilogram p-xylene reacted. Water is alsoreleased. The reaction of 1mol p-xylene with3mol dioxygen gives 1mol terephthalic acidand 2mol water. Only four hydrogen atoms, re-presenting slightly over 2wt% of the p-xylenemolecule, are not incorporated in the terephthal-ic acid.

Owing to the low solubility of terephthalicacid in the solvent, most of it precipitates as itforms. This yields a three-phase system: solidterephthalic acid crystals; solventwith some dis-solved terephthalic acid; and vapor consisting ofnitrogen, acetic acid, water, and a small amountof oxygen. The heat of reaction is removed bysolvent evaporation. A residence time up to 2 his used. Over 98% of the p-xylene is reacted,and the yield to terephthalic acid is > 95mol%.Small amounts of p-xylene and acetic acid arelost, owing to complete oxidation to carbon ox-ides, and impurities such as oxidation intermedi-ates are present in reactor effluent. The excellent

yield and low solvent loss in a single reactor passaccount for the near universal selection of thistechnology for new plants.

The oxidation of the methyl groups occursin steps, with two intermediates, p-toluic acid[99-94-5] and 4-formylbenzoic acid [619-66-9].While 4-formylbenzoic acid is the IUPAC nameof the intermediate, it is customarily referred toas 4-carboxybenzaldehyde (4-CBA).

4-Formylbenzoic acid is troublesome, owingto its structural similarity to terephthalic acid.It co-crystallizes with terephthalic acid and be-comes trapped and inaccessible for completionof the oxidation. Up to 5000 ppm 4-formylben-zoic acid can be present, and this necessitatesa purification step to make the terephthalic acidsuitable as a feedstock for polyester production.

The slurry is passed from the reactor to oneor more surge vessels where the pressure is re-duced. Solid terephthalic acid is then recoveredby centrifugation or filtration, and the cake isdried and stored prior to purification. This is typ-ically referred to as crude terephthalic acid, butis > 99% pure.

Vapor from the reactor is condensed in over-head heat exchangers, and the condensate is re-


Figure 2. Purification of terephthalic acid by the Amoco processa) Slurry drum; b) Hydrogenation reactor; c) Crystallizers; d) Centrifuge; e) Dryer

fluxed to the reactor. Steam is generated by thecondensation and is used as a heating source inother parts of the process. Oxygen-depleted gasfrom the condensers is scrubbed to remove mostuncondensed vapors. Similar to the reactor con-densate, liquid from centrifuges or filters is sentto solvent recovery. Since the centrifugate or fil-trate contains dissolved species, it is first sentto a residue still. Vapor from the still and othervents from throughout the oxidation process aresent to a solvent dehydration tower. The towerremoves the water formed in the reaction as theoverhead stream, and the acetic acid from thetower bottom is combined with fresh acetic acidto make up for process losses, and returned tothe process.

Isophthalic acid is also produced by this pro-cess from m-xylene. Because isophthalic acidis several times more soluble than terephthal-ic acid, much less precipitates in the reactor.Consequently, isophthalic acid from this processcontains much less 3-formylbenzoic acid, sinceit tends to stay in solution where complete ox-idation can occur. Further purification was notcarried out in the past, but a purified grade thatis nowbeing producedwill become the standard.

3.2. Amoco Purification

The purification process developed by AmocoChemical [8] and used on terephthalic acid fromtheAmoco oxidation process supplies over 60%of the terephthalate feedstock for polyester pro-duction (Fig. 2).

Crude terephthalic acid is unsuitable as afeedstock for polyester, primarily owing to the4-formylbenzoic acid impurity concentration.

There are also yellow impurities and resid-ual amounts of catalyst metals and bromine.The Amoco purification process removes 4-formylbenzoic acid to < 25 ppm, and also givesa white powder from the slightly yellow feed.

It is necessary to make all impurities accessi-ble to reaction, so the crude terephthalic acid isslurried with water and heated until it dissolvesentirely. A solution of at least 15wt% is ob-tained, and this requires a temperature≥ 260 ◦C.The solution passes to a reactor where hydro-gen is added and readily dissolves. The solutionis contacted with a carbon-supported palladiumcatalyst. Reactor pressure is held above the va-por pressure of water to maintain a liquid phase.

The 4-formylbenzoic acid is converted to p-toluic acid in the reactor, and some coloredimpurities are hydrogenated to colorless com-pounds. The reaction is highly selective; loss ofterephthalic acid by carboxylic acid reduction orring hydrogenation is < 1%.

After reaction, the solution passes to a seriesof crystallizers where the pressure is sequen-tially decreased [9]. This results in a steppedtemperature reduction, and crystallization of theterephthalic acid. Themore soluble p-toluic acidformed in the reactor, and other impurities, re-main in the mother liquor. After leaving the finalcrystallizer, the slurry undergoes centrifugationand/or filtration to yield a wet cake, and the cakeis dried to give a free-flowing terephthalic acidpowder as the product. Over 98wt% of the in-coming terephthalic acid is recovered as purifiedproduct.

As with the Amoco oxidation, this purifica-tion process is also used with isophthalic acid.


3.3. Multistage Oxidation

Several companies, mostly in Japan [10,11],have developed processes to reduce the 4-formylbenzoic acid content to 200 – 300 ppm bymore intensive oxidation. A separate purifica-tion step is eliminated; the concentration of 4-formylbenzoic acid is low enough for the tere-phthalic acid to be suitable as a feedstock forsome polyester products where high feedstockpurity is not critical. The product is often calledmedium-purity terephthalic acid, and accountsfor about 11%of the terephthalic acid produced.Most of these processes also use the catalyst sys-tem discovered by Scientific Design.

Most medium-purity terephthalic acid is pro-duced by Mitsubishi Kasei and its licensees.They have named this product Q-PTA, and ithas a typical 4-formylbenzoic acid level of ca.290 ppm [10]. Mitsubishi has also developed astill more intensive oxidation process where the4-formylbenzoic acid level is further reduced.The product is called S-QTA.

The oxidation of p-xylene in acetic acid witha cobalt –manganese – bromine catalyst is car-ried out as in the Amoco oxidation. The slurryis heated to 235 – 290 ◦C and oxidized further inanother reactor. More catalyst can be added inaddition to the temperature increase [12].

Heating gives increased terephthalic acidsolubility, and as crystals dissolve, some 4-formylbenzoic acid and colored impurities arereleased. Although the terephthalic acid is notcompletely soluble at the higher temperature,the crystals can digest. Digestion is a dynamicequilibrium process wherein crystals constantlydissolve and reform. This increases the releaseof 4-formylbenzoic acid into solution where ox-idation can be completed. While the need for aseparate purification process is eliminated, an-other reactor is needed in the oxidation process.Also, at higher temperature, acetic acid tends tobe oxidized to a larger extent to carbon oxidesand water [12].

The rest of the Mitsubishi process consistsof solid – liquid separation and drying to obtainthe powdered product. Acetic acid must be de-hydrated and recycled to the process.

Eastman Chemical has also developed amedium-purity terephthalic acid product. Theprocess does not employmanganese, only cobaltand bromine. Two oxidation stages are used,

both at 175 – 230 ◦C [13]. Instead of heating bet-ween stages to obtain increased solubility, asperformed byMitsubishi, the contents of the firstreactor are sent to hydroclones where hot, freshacetic acid displaces the mother liquor. Samar-ium may be added to the catalyst. A residencetime up to 2 h with air addition provides suf-ficient digestion of the crystals to yield a 40 –270 ppm level of 4-formylbenzoic acid [13].Downstream recovery of the terephthalic acidcrystals by solid – liquid separation and dryingmust again be performed.

3.4. Dynamit-Nobel (Witten) Process

Most dimethyl terephthalate is made by a pro-cess first developed by Chemische Werke Wit-ten, with work also being done at a divisionof Standard Oil of California. Modificationswere made by Hercules and Dynamit-Nobel.Huls Troisdorf licensed the process with fur-ther modifications [14]. Dimethyl terephthalateis formed in four steps. First, p-xylene is passedthrough an oxidation reactor,where p-toluic acidis formed. It then passes to an esterification reac-tor, the second step, where methanol is added toform methyl p-toluate [99-75-2]. The methyl p-toluate is isolated and returned to the oxidationreactor for oxidation to monomethyl terephthal-ate [1679-64-7], the third step, followed by thefourth step, esterification to dimethyl terephthal-ate. The sequence is as follows:

The process as licensed by Huls Troisdorf isillustrated in Figure 3. Fresh and recovered p-


Figure 3. Production of dimethyl terephthalate by the Dynamit Nobel processa) Oxidation reactor; b) Esterifier; c) Expansion vessel; d) Methanol recovery column; e, f) Methyl p-toluate and dimethylterephthalate columns; g, j) Dissolvers; h, k) Crystallizers; i, l) Centrifuges

xylene, along with catalyst (mostly cobalt withsome manganese) are combined with methyl p-toluate and fed to the liquid-phase oxidation re-actor. Because bromine and acetic acid are notused, vessels lined with titanium or other ex-pensive metals are not necessary. Oxygen sup-plied by compressed air is added at the bottom.Oxidation conditions are 140 – 180 ◦Cand 500 –800 kPa. The heat generated by oxidation is re-moved by vapors of unreacted p-xylene and thewater of reaction. Cooling coils in the reactor areused to generate steam. The steam and reactorvapors are condensed and combined to recoverp-xylene for recycle.

The oxidation effluent is then heated andsent to the esterification reactor, operated at250 ◦C and 2500 kPa. Excess vaporized meth-anol is sparged into the esterifier, where the p-toluic acid and monomethyl terephthalate areconverted noncatalytically to methyl p-toluateand dimethyl terephthalate, respectively. Over-head vapors from the esterification reactor arecondensed and fed to a distillation system,wherethe water from the esterification is separatedfrom methanol, which is recycled.

The remainder of the process separates the di-methyl terephthalate from methanol and methylp-toluate, which are recycled, and residue andwastewater, which go to waste treatment.

The product from the esterifier goes to an ex-pansion vessel. Vapor from this vessel feeds amethanol recovery column, where the metha-nol overhead goes to methanol recovery, and themethyl p-toluate bottoms are recycled to the ox-idation reactor. Liquid from the expansion ves-

sel feeds two vacuum distillation columns in se-ries, which yield crude dimethyl terephthalate.The first column recovers more methyl p-toluateoverhead for recycle to oxidation, and the bot-toms feeds the crude dimethyl terephthalate col-umn, where the product is taken overhead. Thebottoms from the dimethyl terephthalate col-umn, containing heavy byproducts and catalystmetals, can be mixed with water from the oxida-tion, which dissolves the catalyst. The resultingslurry is centrifuged; the catalyst solution is re-cycled, and the cake is sent to disposal.

The crude dimethyl terephthalate goesthrough two stages of crystallization. It is slur-ried with the methanol mother liquor from thesecond crystallization stage and dissolved byheating. Dimethyl terephthalate is crystallizedfrom this solution on cooling, by flashing themethanol. The dimethyl terephthalate cake isseparated by centrifugation, dissolved in freshmethanol, and crystallized in the same way. Thewet dimethyl terephthalate cake can be meltedand stored molten, or dried and flaked. Thereis usually a distillation column after the secondcentrifuge for further purification.

Mother liquor from the first centrifugation issent to the methanol recovery system. The cen-trifugation yields a cleaner mother liquor whichis combined with methanol recovered from themelting operation for use in dissolving the crudedimethyl terephthalate entering the crystalliza-tion section. Side-streams from throughout theprocess are recycled to appropriate points tomaximize yield and to minimize methanol andcatalyst use.


Figure 4. Esterification of terephthalic acid and separation of purified dimethyl terephthalatea) Esterifier; b) o-Xylene scrubber; c) Methanol column; d) o-Xylene recovery column; e) 4-Formylbenzoic ester stripper;f) Purification column

3.5. Esterification of Terephthalic Acid

Terephthalic acid can be produced and, in a sep-arate process, esterified with methanol to di-methyl terephthalate which is then purified bydistillation (Fig. 4) [15,16]. This process can beused on highly impure terephthalic acid, becauseof the purification achievable by distillation.

Crude terephthalic acid and excess methanolare mixed and pumped to the esterification reac-tor. In this example, o-xylene [95-47-6] is usedto enhance the subsequent separations. The tere-phthalic acid is rapidly esterified by the metha-nol at 250 – 300 ◦Cwithout catalysis, although acatalyst can be used. Methanol vapor carries di-methyl terephthalate and o-xylene from the reac-tor to a columnwhere o-xylene is added to scruboutmonomethyl terephthalate and return it to thereactor for completion of the esterification. Thevapor contains the dimethyl terephthalate prod-uct as well as methanol, o-xylene, water fromthe esterification, and esterified impurities in theterephthalic acid feed. Several distillation stepsare needed to separate out the dimethyl tere-phthalate and to process the separated streamsfor recovery of valuable components for recy-cle.

The reactor overhead vapor first goes to amethanol column where methanol is removedoverhead, water and somemethanol form a side-draw, and the bottoms contain the dimethyl tere-phthalate, o-xylene, and impurities.

Next, in the o-xylene recovery column, di-methyl terephthalate purification is started; itoperates at 10 – 20 kPa absolute so that a tem-perature of 200 – 230 ◦C can be used. o-Xyleneis removed overhead, the methyl esters of 4-formylbenzoic acid and p-toluic acid are re-moved in the middle, and dimethyl terephthal-ate forms the bottoms. A 4-formylbenzoic acidstripping column follows, where the middlestream from the previous column is sent, so that4-formylbenzoic ester canbe removedoverhead.Finally, the bottoms from both the o-xylene re-covery column and the 4-formylbenzoic acidstripper are sent to a purification column, wherethe dimethyl terephthalate product is taken over-head.

Methanol from the top of themethanol recov-ery column is sent to a purification columnwherethe overhead contains low boilers, and the meth-anol from the bottoms is recycled. Themethanoland water from the side-draw of the methanolrecovery column are sent to a methanol dehy-dration column, where the water is removed andthe dehydrated methanol recycled.

3.6. Hydrolysis of DimethylTerephthalate

High-purity terephthalic acid can be producedby hydrolysis of dimethyl terephthalate. Slightlyover 2% of terephthalate feedstock is producedby this route.


Dimethyl terephthalate and recoveredmonomethyl terephthalate are combined withwater from the methanol recovery tower of thedimethyl terephthalate process and heated to260 – 280 ◦C at 4500 – 5500 kPa in a hydroly-sis reactor, where the methyl esters are hydro-lyzed. The overhead methanol plus water vaporis returned to the methanol recovery tower forseparation. The hydrolysis reactor liquid is sentto a series of crystallizers and cyclones. Af-ter cooling to crystallize the terephthalic acidat ca. 200 ◦C, washing cyclones are used to re-movemother liquorwhich containsmonomethylterephthalate. The cyclone underflow is furthercooled to 100 ◦C, and final crystallization oc-curs. This slurry is centrifuged and dried to givethe final product.

Monomethyl terephthalate from the cycloneoverflow is recovered, again by cooling to100 ◦C and centrifuging, and the cake is recy-cled to the hydrolysis reactor.

3.7. Alternative and Past Technologies

Oxidation of p- orm-xylene is also possible withacetic acid solvent and a cobalt catalyst with anacetaldehyde activator. Reaction temperature is120 – 140 ◦C, and as a result the oxidation re-quires a residence time ≥ 2 h. Titanium-linedvessels are not required because bromine is notused. The acetaldehyde is converted to aceticacid as it promotes the reaction. After reaction,the terephthalic or isophthalic acid is recoveredby centrifugation or filtration and drying, muchlike the Amoco process, and acetic acid is reco-vered for recycle by distillation.

With isophthalic acid, which has a lower con-centration of 3-formylbenzoic acid, consider-able purification is possible by slurrying in wa-ter to ca. 20 – 25wt%, heating to 240 – 260 ◦Cto dissolve, and crystallizing by cooling in batchmode. Recovery of the crystals is again by cen-trifugation or filtration and drying.

Eastman Chemical in the United States alsoused an acetaldehyde activator with a cobalt cat-alyst to produce terephthalic acid. Bromine isnow being used in place of acetaldehyde [17].

Mobil Chemical in the United States had acommercial terephthalic acid operation whichhas since been abandoned [18]. As with theabove processes, a cobalt catalyst was used with

acetic acid solvent; the activator was 2-butanone[78-93-3]. After reaction, the crude terephthal-ic acid was leached by adding pure acetic acidand heating to achieve partial solubility; this re-moved gross impurities. Final purification wasby sublimation and catalytic treatment of the va-por. Impure terephthalic acid was vaporized ina steam carrier, and catalyst and hydrogen wereadded to convert undesirable organic impurities.Subsequent cooling of the stream condensed theterephthalic acid, leaving impurities in the vapor.Product crystals were removed by cyclones.

A process no longer practiced is basedon Henkel technology. Starting with phthalicanhydride [85-44-9], the monopotassium anddipotassium o-phthalate salts were formed in se-quence. The dipotassium salt was isolated fromsolution by spray drying, and isomerized todipotassium terephthalate under carbon dioxideat 1000 – 5000 kPa and 350 – 450 ◦C. This saltwas dissolved in water and recycled to the startof the process, where terephthalic acid crystalsformed during the production of the monopotas-sium salt. The crystals were recovered by filtra-tion.

Two proposed processes which were nevercommercialized were by Lummus, using a dini-trile route [19] and by Eastman, using the forma-tion of 1,4-diiodobenzene [624-38-4] with car-bonylation to aromatic acids [20].

Toluene [108-88-3] is a potential feedstockfor manufacture of terephthalic acid and ischeaper than p-xylene. Mitsubishi Gas Chem-ical has researched a process where a complexbetween toluene and hydrogen fluoride – borontrifluoride is formed, that can be carbonylatedwith carbon monoxide to form a p-tolualdehyde[104-87-0] complex [21]. After decompositionof the complex, p-tolualdehyde can be oxi-dized in water with a manganese – bromine cat-alyst system to terephthalic acid. While cheapertoluene is the feedstock and acetic acid is not re-quired, the complexities of handling hydrogenfluoride – boron trifluoride and the need for car-bon monoxide add other costs. This process hasnot been commercialized.

4. Environmental Protection

The three chemicals discussed here consist onlyof carbon, hydrogen, and oxygen, so carbon


dioxide andwater are the final effluents of oxida-tive degradation. There are traces of the cobalt –manganese – bromine catalyst in waste streams,however.

Effluents from the terephthalic acid processinclude water generated during oxidation andwater used as the purification solvent. Gaseousemissions are mostly the oxygen-depleted airfrom the oxidation step. Volumes are large, butthe chemicals in the streams can be effectivelydestroyed and removed.

Water effluents are subjected to aerobicwastewater treatment, where the dissolvedspecies, mostly terephthalic acid, acetic acid,and impurities such as p-toluic acid, are oxidizedto carbon dioxide andwater by the action of bac-teria which are acclimated to these chemicals.The bacterial growth is a sludge which can bedried andburned or spread on land.An anaerobicprocess has been developed to treat the waste-water [22]. Advantages include much less wastesludge production, less utility consumption, andthe generation of methane, which can be burnedfor energy recovery.

Waste gas is scrubbed in process equipmentto remove acetic acid vapors for recovery. Traceamounts of other compounds formed in the re-actor can be removed by catalytic oxidation, fol-lowed by scrubbing tomeet themost demandingregulations for process vents [23].

Waste streams from distillation in the di-methyl terephthalate process can be burned forenergy recovery.

Another aspect of environmental protectionis the recyclability of polyesters. Post-consumercontainers can be ground and cleaned, and thenextruded and spun into fiber to fill bedding, andquilted clothing. Textile fibers can also be made.The polyester can also be reacted to regener-ate the terephthalate feedstock. High-tempera-ture hydrolysis yields terephthalic acid. Reac-tion with methanol yields dimethyl terephthal-ate.

5. Quality Specifications

Feedstocks for polyester production must beextremely pure; they are among the puresthigh-volume chemicals sold by industry. Ifcertain impurities are present in high enoughconcentrations, harmful effects can be mea-

sured, e.g., monofunctional compounds cancap the polyester chain and limit molecularmass buildup. Trifunctional carboxylic acids cancause chain branching which leads to undesir-able rheological and spinning properties. Col-ored impurities can be incorporated into thepolyester. In particular, 4-formylbenzoic acidlimits polyester molecular mass and causes yel-lowness [24]. The particle size of terephthalicacid determines how the powder flows, and theviscosity of the slurry when mixed with 1,2-ethanediol [107-21-1].

Owing to the consistent high purity of theseproducts and the different effects of impurities,specifications put limits on specific impurities orcolor, rather than overall purity (Table 3). Trendsin quality specifications are away from maxi-mum or minimum values, toward a target valuewith an allowable range. This has been institutedto some extent by Amoco Chemical for the p-toluic acid content, for example.

Table 3. Specifications and typical analyses of purified terephthalicacid

Property Specification Typical value

Acid number, mg KOH/g 675± 2 673 – 675Ash, ppm ≤ 15 < 3Metals,∗ ppm ≤ 9 < 2Water, wt% ≤ 0.2 0.14-Formylbenzoic acid, ppm ≤ 25 15∗∗p-Toluic acid, ppm 125± 45 125∗∗∗Co+Mn+Fe +Cr +Ni +Mo+Ti.∗∗Medium-purity terephthalic acid typically has 250 ppm4-formylbenzoic acid and <50 ppm p-toluic acid.

In Table 3, acid number, determined by titra-tion, is an overall purity measurement. A per-fectly pure sample will have an acid number of675.5mg KOH/g, but the low impurity levelsmake the acid number meaningless as a quanti-tative indication of purity, and it is being phasedout.

Impurities specified include p-toluic acid and4-formylbenzoic acid, residual water, and tracemetals, the ash being trace metal oxides. Thereare also color and particle size measurements.Polyester producers often have specific testswhich they prefer for color or particle size.

Medium-purity terephthalic acid specifica-tions address the same impurities, except that


4-formylbenzoic acid concentrations are up to300 ppm, and residual acetic acid is present.

Typical dimethyl terephthalate specificationsare shown in Table 4. Freezing point is anextremely sensitive purity indicator. Since di-methyl terephthalate is often shipped and storedin molten form, color measurements are madeon the melt. A color stability test can determinethe sample’s resistance to degradation. This re-sistance is reduced by the presence of unester-ified acid groups, so these residual groups aremeasured.

Table 4. Specifications and typical analyses of dimethylterephthalate

Property Specification Typical value

Freezing point, ◦C ≥ 140.62 140.64Acid number, mg KOH/g 0.03 0.01Molten color, APHA ≤ 25 10Color stability,∗ APHA ≤ 25 15Water, wt% ≤ 0.02 0.01

∗ 4 h at 175 ◦C

Specifications for isophthalic acid are similarto those of terephthalic acid, m-toluic acid and3-formylbenzoic acid being the impurities.

6. Storage and Transportation

Terephthalic acid powder is stored in silos andtransported in bulk by rail hopper car, hoppertruck, or bulk container with a polyethyleneliner. Hopper cars and trucks should be dedi-cated to this service, because of the extreme pu-rity requirements. Shipment is also in 1 t bags,or paper bags containing ca. 25 kg. Isophthalicacid, being a much lower volume chemical, hasa higher proportion shipped in bags.

Dimethyl terephthalate is stored molten ininsulated heated tanks, and is preferentiallyshipped molten in insulated rail tank cars ortank trucks. It is also solidified into briquettesor flakes, and shipped in 1 t or 25 kg bags.

The huge production volume of terephthal-ic acid and dimethyl terephthalate favors theuse of bulk shipment with minimized packag-ing costs. Rail cars are preferred where possible,and they can accommodate up to 90 t. Containerswith plastic liners hold 20 t terephthalic acid, andthese are the preferred ocean shipping method.

7. Uses

Terephthalic acid and dimethyl terephthalate areused almost exclusively to produce saturatedpolyesters. Poly(ethylene terephthalate), the al-ternating copolymer of terephthalic acid and1,2-ethanediol, accounts for> 90% of this use, withaworldwidedemandof 12.6×106 t in 1992.Tex-tile and industrial fiber accounted for 75% ofthis demand. Polyester is the largest volume syn-thetic fiber. Food and beverage containers ac-counted for 13%, and constituted the fastestgrowing segment. Film for audio, video, andphotography took 7%.

Other uses are for poly(butylene terephthal-ate), a high-performance molding resin madeby reaction with 1,4-butanediol [74829-49-5],and for special industrial coatings, solvent-freecoatings, electrical insulating varnishes, aramidfibers, and adhesives. A small amount of bis(2-ethylhexyl) terephthalate [6422-86-2] is pro-duced as a plasticizer, and some dimethyl tere-phthalate is ring-hydrogenated to produce thecyclohexane analog, 1,4-cyclohexanedicarbox-ylic acid [1076-97-7], for specialty polyestersand coatings. Isophthalic acid is used as acomonomer with terephthalic acid in bottle andspecialty resins. It is also a feedstock for coat-ings and for high-performance unsaturated poly-esters, being first reacted with maleic anhydride[108-31-6] and then this resin is cross-linkedwith styrene [100-42-5].

8. Economic Aspects

Together, terephthalic acid and dimethyl tere-phthalate rank about 25th in tonnage of all chem-icals, and about tenth for organic chemicals. Therapid growthof polyester use has led to an above-average growth for these feedstocks, and this isprojected to continue as deeper penetration intofiber and container markets is made. The versa-tility of terephthalate and isophthalate polyestersis projected to drive growth in the foreseeablefuture.

Capacity for the three types of terephthalatefeedstocks since 1980 is given in Table 5. Pu-rified terephthalic acid has grown rapidly, andis projected to supply virtually all the growth


for polyesters, about 7% annually to the year2000. Medium-purity terephthalic acid capacityhas grown by construction of a few new plants,and by conversion of some dimethyl terephthal-ate capacity. Dimethyl terephthalate has shownlittle change in capacity since 1980, and essen-tially none is expected in the future.

Table 5. Capacity of terephthalate feedstocks (103 t/a)

Feedstock∗ 1980 1986 1992

PTA 2800 4000 8100MTA 420 580 1300DMT 3300 3600 3300

∗ PTA purified terephthalic acid; MTA medium-purityterephthalic acid; DMT dimethyl terephthalate.

The Far East contains most of the world’spopulation and most of the garment manufac-turing industry. As a result, it is experiencingmost of the growth in manufacturing capacityfor polyester fibers, and most of the growth forterephthalic acid production (Table 6). Approxi-mately 75% of the growth in terephthalate feed-stocks in 1986 – 1992 was due to purified tere-phthalic acid plants built in the Far East.

Table 6. Geographical distribution of terephthalate production(103 t/a)

1980 1986 1992

North America 3300 3400 3600Europe 1500 2100 2700Far East 1500 2300 5800Rest of World 220 380 600

Pricing is often determined in conjunctionwith long-term contracts, and is most influ-enced by the cost of p-xylene. In 1992, priceswere about $ 0.60/kg for terephthalic acid and$ 0.57/kg for dimethyl terephthalate [25]. Be-cause a givenmass of terephthalic acid produces17% more polyester than the same mass of di-methyl terephthalate, the dimethyl terephthalatepricemust be adjusteddownward, and allowanceis made for the value of the methanol generatedduring transesterification.

The major producer of purified terephthal-ic acid is Amoco Chemical, with over 17% ofthe world’s production in 1992. Joint ventures ofAmoco, the largest beingChinaAmericanPetro-chemical in Taiwan and Samsung Petrochemicalin Korea, also produce about 17%. Other major

producers are Mitsui Sekka in Japan and Impe-rial Chemical Industries in England and Taiwan.China is certain to become amajor producer, andlarge plants are being planned in Southeast Asia.Larger producers of medium-purity terephthalicacid areMitsubishi Kasei in Japan, and EastmanandDuPont in theUnited States. Some polyesterproducers have licensed technology to supplytheir own feedstock.All licensors are themselvesproducers of terephthalic acid, or have obtainedthe technology from a producer.

Major dimethyl terephthalate producers areDuPont, Eastman Chemical, and HoechstCelanese in the United States, Hoechst in Eu-rope, and Teijin Petrochemical in Japan. Thereare several small, older plants, mostly in Europeand China.

Isophthalic acid production capacity is250×103 t/a, ca. 2% of the terephthalate total.Amoco Chemical is the major producer.

9. Toxicology and OccupationalHealth

Terephthalic acid, dimethyl terephthalate, andisophthalic acid all have low toxicity and causeonly mild and reversible irritation to skin, eyes,and the respiratory system.

For oral ingestion of terephthalic acid, LD50values for rats have been reported as 18.8 g/kg[26], and for mice as 6.4 g/kg [27], or > 5 g/kg[28]. Dimethyl terephthalate LD50 values forrats are 6.5 g/kg [29] and 4.39 g/kg [26]. Somemortalitywas found at 5 g/kg oral ingestion [30],and with a 28-day oral uptake of 5% in the feed,ca. 2.5 g/kg. For isophthalic acid, LD50 for ratswas reported as 10.4 g/kg [26].

Interperitoneal administration of terephthal-ic acid in rats showed LD50 values > 1.43 g/kg[28] and 1.9 g/kg [31], with LD100 = 3.2 g/kg[31].Mortalitywas found at 0.8 g/kg inmice and1.6 g/kg in rats [27]. For dimethyl terephthalate,an LD50 of 3.9 g/kg has been reported [29], andfor isophthalic acid, 4.3 g/kg [26].

Ingestion of terephthalic acid or dimethylterephthalate results in rapid distribution and ex-cretion in unchanged form [32]. In rats, at highingestion rates of about 3% terephthalic acid ordimethyl terephthalate in the feed, bladder cal-culi are formed which are mostly calcium tere-phthalate; these injure the bladder wall and lead


to cancer.Calculi cannot formunless the calciumterephthalate solubility is exceeded (thresholdeffect) [33]. Neither compound has been foundto be genotoxic (Ames test) [34].

Inhalation of terephthalic acid dust appearsto pose no clear hazards. Inhalation by rats for6 h/d, 5 d/week, for 4 weeks at 25mg/m3 pro-duced no fatalities [35]. Dimethyl terephthalatedust at 16.5 and 86.4mg/m3 for 4 h/d caused notoxicological effects after 58 days [29]. The ratsintermittently reacted to the nuisance dust levelsat the higher concentration.

Irritation of the skin, eyes, and mucous mem-branes by any of these chemicals is mild andreversible. In rabbits, the FHSA test for tere-phthalic acid for eyes gave scores of 14.0/110.0and 3.5/110.0 at 1 and 24 h after exposure. TheFHSAskin test scorewas0.4/8.0 [35]. For isoph-thalic acid, eye and skin irritancy scores were5.3/110.0 and 0.2/8.0 [36]. Irritation due to di-methyl terephthalate has also been reported tobe mild [30].

Terephthalic acid, dimethyl terephthalate,and isophthalic acid can all form dust clouds.As with any flammable substance, an explosioncan occur, given proper dust and oxygen concen-trations. Reported limits on the explosive regionfor terephthalic acid dust clouds are minimumdust content of 40 g/m3 and minimum oxygencontent 12.4% at 20 ◦C. At 150 ◦C, minimumoxygen is 11.1% [37]. The maximum concen-tration of an explosible dust cloud has been cal-culated as 1400 g/m3 [38].

Dimethyl terephthalate and isophthalic acidhave a more stringent oxygen limit. In testswith carbon dioxide diluent, the minimum oxy-gen content was 12, 14, and 15% for dimethylterephthalate, isophthalic acid, and terephthalicacid [39]. The oxygen content is higher for car-bon dioxide diluent than for nitrogen, owing tothe higher heat capacity of carbon dioxide (the12.4%minimumoxygen content reported abovefor terephthalic acid was for nitrogen diluent).

For molten dimethyl terephthalate, the flashand fire points, determined by the Clevelandopen-cup method, are 146 and 155 ◦C [1].

10. References

General References1. Kirk-Othmer, 18, 732 – 777.

2. P. Raghavendrachar, S. Ramachandran, Ind.Eng. Chem. Prod. Res. Dev. 31 (1992)453 – 462.

3. J. E. McIntyre, Chem. Eng. Monogr. 15 (1982)400 – 444.

Specific References4. Calico Printers Assoc, GB 578 079, 1941 (J. R.

Whinfield, J. T. Dickson).5. Mid-Century Corp., US 2 833 816, 1955 (R. S.

Barker, S. A. Soffer).6. Mid-Century Corp., US 3 089 906, 1958 (R. S.

Barker, S. A. Soffer).7. W. Partenheimer in D.W. Blackburn (ed.):

Catalysis of Organic Reactions, MarcelDekker, New York 1990, pp. 321 – 346.

8. Standard Oil Company (Indiana), US3 584 039, 1967 (D.H. Meyer).

9. Standard Oil Company (Indiana), US3 931 305, 1973 (J. A. Fisher).

10. K. Matsuzawa, Chem. Econ. Eng. Rev. 8(1976) no. 8, 25 – 30.

11. M. Hizikata, Chem. Econ. Eng. Rev. 9 (1977)no. 9, 32 – 38.

12. Mitsubishi Chem. Ind. Ltd., US 4 877 900,1988 (A. Tamaru, Y. Izumisawa).

13. Eastman Kodak Company, US 4 447 646, 1983(G. I. Johnson, J. E. Kiefer).

14. H. J. Korte, H. Schroeder, A. Schoengen: “ThePTA Process of Huls Troisdorf AG,” AIChESummer National Meeting, Denver, Co. 1988.

15. E. I. DuPont de Nemours, US 2 491 660, 1949(W. F. Gresham).

16. Standard Oil Company (Indiana), US2 976 030, 1957 (D.H. Meyer).

17. Eastman News 43 (1988) no. 14.18. H. S. Bryant, C. A. Duval, L. E. McMakin, J. I.

Savoca, Chem. Eng. Prog. 67 (1971) no. 9,69 – 75.

19. A. P. Gelbein, M. C. Sze, R. T. Whitehead,CHEMTECH 3 (1973) 479 – 483.

20. Eastman Kodak Company, US 4 705 890, 1987(G. R. Steinmetz, M. Rule).

21. Chem. Eng. (N.Y.) 83 (1976) no. 17, 27 – 28.22. S. Shelley, Chem. Eng. (N.Y.) 98 (1991)

no. 12, 90 – 93.23. T. G. Otchy, K. J. Herbert: “First Large Scale

Catalytic Oxidation System for PTA Plant COand VOC Abatement,” Ann. Air WasteManagem. Assoc. Meet. 85th 1992.

24. W. Berger, D. Dornig, Faserforsch. Textiltech.29 (1978) 256 – 262.

25. Chem. Markt. Rep. 242 (1992) no. 4, 42.26. J. V. Marhold, Sb. Vys. Toxicologickeho

Vysetrien Latid A Priravku 1972, 52.


27. A. E. Moffitt et al., Am. Ind. Hyg. Assoc. J. 36(1975) 633 – 641.

28. A. Hoshi, R. Yanal, K. Kuretani, Chem.Pharm. Bull. 16 (1968) 1655 – 1660.

29. W. J. Krasavage, Am. Ind. Hyg. Assoc. J. 34(1973) 455 – 462.

30. Haskel Laboratories, unpublished data,Wilmington, Delaware, USA, 1979.

31. E. O. Grigas, R. Ruiz, D.M. Aviado, Toxicol.Appl. Pharmacol. 18 (1971) 469 – 486.

32. A. Hoshi, K. Kuretani, Chem. Pharm. Bull. 16(1968) 131 – 135.

33. H. d’A. Heck, Banbury Rep. 25 (1987)233 – 244.

34. E. Zeiger, S. Haworth, W. Speck, K.Mortelmans, EHP Environ. Health Perspect.45 (1982) 99 – 104.

35. Amoco Chemical Company, Material SafetyData Sheet, Amoco TA-33, Chicago, Ill., 1993.

36. Amoco Chemical Company, Material SafetyData Sheet, Amoco PIA, Chicago, Ill., 1993.

37. A.D. Craven, M.G. Foster, Combust. Flame11 (1967) 408 – 414.

38. S. Nomur, M. Torimoto, T. Tanake, Ind. Eng.Chem. Proc. Res. Dev. 23 (1984) 420 – 423.

39. M. Jacobson, J. Nagy, A. R. Cooper, Rep.Invest. U.S. Bur. Mines 5971 (1962) 25 – 26.

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