Dimethyl Terephthalate and Terephthalic Acid

There are only two top 50 chemicals, terephthalic acid and dimethyl terephthalate, derived from /7-xylene and none from o- or m-xylene. But phthalic anhydride is made in large amounts from o-xylene. 

Terephthalic acid is commonly abbreviated TA or TPA. The abbreviation PTA (P = pure) is reserved for the product of 99% purity for polyester manufacture. For many years polyesters had to be made from dimethyl terephthalate (DMT) because the acid could not be made pure enough economically. Now either can be used. TA is made by air oxidation of /7-xylene in acetic acid as a solvent in the presence of cobalt, manganese, and bromide ions as catalysts at 200°C and 400 psi. TA of 99.6% purity is formed in 90% yield. This is called the Amoco process. 

A partial mechanism with some intermediates is given on the next page. Details are similar to the cyclohexane to cyclohexanoneicyclohexanol process discussed in this chapter. Section 4. 

The crude TA is cooled and crystallized. The acetic acid and xylene are evaporated and the TA is washed with hot water to remove traces of the catalyst and acetic acid. Some p-formylbenzoic acid is present as an impurity from incomplete oxidation. This is most easily removed by hydrogenation to /7-methylbenzoic acid and recrystallization of the TA to give 99.9% PTA, which is a polyester-grade product, mp 300°C. 

DMT can be made from crude TA or from / -xylene directly. Esterification of TA with methanol occurs under sulfiiric acid catalysis. Direct oxidation of /7-xylene with methanol present utilizes copper and manganese salt catalysis. 

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Purified terephthalic acid and dimethyl terephthalate are used as raw materials for the production of saturated polyesters. During 1993, the combined worldwide production of purified terephthafic acid plus dimethyl terephthalate exceeded 14 x 10 t (42), which is 80% of the total benzenepolycarboxyfic acid production. Terephthafic acid is also produced ia technical or cmde grades which are not pure enough for manufacture of poly(ethylene terephthalate). In almost all cases, the technical-grade material is immediately converted to purified terephthafic acid or dimethyl terephthalate, which together are the articles of commerce. 

Table 13. Physical Constants of Terephthalic Acid and Dimethyl Terephthalate...

Manufacture and Processing. Terephthalic acid and dimethyl terephthalate did not become large-volume industrial chemicals until after World War II. Imperial Chemical Industries in the United Kingdom in 1949 and Du Pont in the United States in 1953 commercialized fibers made from poly(ethylene terephthalate). Dimethyl terephthalate and ethylene glycol were the comonomers used by both companies (see Fibers, polyester). 

Mixed xylenes are used as an octane improver in gasoline and for commercial solvents, particularly in industrial cleaning operations. By far, most of the commercial activity is with the individual isomers. Para-xylene, the most important, is principally used in the manufacture of terephthalic acid and dimethyl terephthalate en route to polyester plastics and fibers (Dacron, films such as Mylar, and fabricated products such as PET plastic bottles). Ortho-xylene is used to make phthalic anhydride, which in turn is used to make polyester, alkyd resins, and PVC plasticizers. Meta-xylene is used to a limited extent to make isophthahc acid, a monomer used in making thermally stable polyimide, polyester, and alkyd resins. 

Reactions of the Methyl Groups. These reactions include oxidation, polycondensation, and ammoxidation. PX can be oxidized to both terephthalic acid and dimethyl terephthalate, which are then condensed with ethylene glycol to form polyesters. Oxidation of OX yields phthalic anhydride, which is used in the production of esters. These are used as plasticizers for synthetic polymers. MX is oxidized to isophthalic acid, which is also converted to esters and eventually used in plasticizers and resins (see Phthalic ACIDS and otherbenzenepolycarboxylic acids). 

Terephthalic acid and dimethyl terephthalate are used to produce polyester fibers, polyester resins, and polyester film. Terephthalic acid or dimethyl terephthalate is usually reacted with ethylene glycol to give polyethylene terephthalate) but sometimes it is combined with 1,4-butanediol to yield poly (butylene terephthalale). Polyester fibers are used in the textile industry. Films find applications as magnetic tapes, electrical insulation, photographic film, packaging, and polyester bottles. 

Process Economics Program Report SRI International. Menlo Park, CA, Isocyanates IE, Propylene Oxide 2E, Vinyl Chloride 5D, Terephthalic Acid and Dimethyl Terephthalate 9E, Phenol 22C, Xylene Separation 25C, BTX, Aromatics 30A, o-Xylene 34 A, m-Xylene 25 A, p-Xylene 93-3-4, Ethylbenzene/Styrene 33C, Phthalic Anhydride 34B, Glycerine and Intermediates 58, Aniline and Derivatives 76C, Bisphenol A and Phosgene 81, C1 Chlorinated Hydrocarbons 126, Chlorinated Solvent 48, Chlorofluorocarbon Alternatives 201, Reforming for BTX 129, Aromatics Processes 182 A, Propylene Oxide Derivatives 198, Acetaldehyde 24 A2, 91-1-3, Acetic Acid 37 B, Acetylene 16A, Adipic Acid 3 B, Ammonia 44 A, Caprolactam 7 C, Carbon Disulfide 171 A, Cumene 92-3-4, 22 B, 219, MDA 1 D, Ethanol 53 A, 85-2-4, Ethylene Dichloride/Vinyl Chloride 5 C, Formaldehyde 23 A, Hexamethylenediamine (HMDA) 31 B, Hydrogen Cyanide 76-3-4, Maleic Anhydride 46 C, Methane (Natural Gas) 191, Synthesis Gas 146, 148, 191 A, Methanol 148, 43 B, 93-2-2, Methyl Methacrylate 11 D, Nylon 6-41 B, Nylon 6,6-54 B, Ethylene/Propylene 29 A, Urea 56 A, Vinyl Acetate 15 A. 

Polyethylene terephthalate) in short PET is a polyester. It is mainly used in the garment industry with or without natural cotton and has trade names such as Terylene , Dacron , etc. As the name indicates, it is a polymer between terephthalic acid (PT) and ethylene glycol. Both terephthalic acid and dimethyl terephthalate (DMT) can be used to make the polymer. A majority of the modem plants tend to use PT as the starting material because of the availability of high-purity PT on a large scale. Both PT and DMT are first converted to bis(hydroxy ethyl) terephthalate 8.17 (see reaction 8.26). For PT this is effected by a straightforward esterification reaction. For DMT a transesterification reaction catalyzed by zinc and manganese acetate is used. 

About half of the benzene produced as a chemical feedstock is for styrene production, followed by large fractions for phenol and cyclohexane-based products. As much as half of the toluene produced is converted to benzene, depending on the price and demand differential. The largest use of toluene itself is as a component of gasoline. Much smaller amounts are used as a solvent, or in the manufacture of dinitrotoluene and trinitrotoluene for military applications. Xylenes are also used in gasoline formulations and function as octane improvers like toluene. para-Xylene and o-xylene are the dominant isomers of value as chemical feedstocks, for the production of terephthalic acid (and dimethyl terephthalate) and phthalic anhydride, respectively. Polyester and the synthetic resin markets, in turn, are major consumers of these products. meto-Xylene is oxidized on a much smaller scale to produce isophthalic acid, of value in the polyurethane and Nomex aramid (poly(m-phenylene isophthalamide)) technologies. 

Table 13 2 lists the average commercial specifications of terephthalic acid and dimethyl terephthalate used to manufacture fibers. Terephthalic acid and its methyl ester are utilized chiefly in the form of ethylene glycol polyester for fibers, films, and resins. They are also used in the manufacture of polyesters of butylene glycol (PBT). Among the various applications is the manufacture of additives for lubricants, plasticizers and adhesives. Uses in Western Europe, the United States and Japan in 1984 are given in Table 13.3. This table also gives the. production- capacities -of. terephthalic. acid and dimethyl terephthalate in Western Europe, the United States and Japan in 1984.

Toxicology ACGIH TLV 5 mg(Mn)/m LD50 (oral, rat) 2940 mg/kg mod. toxic by ingestion mutagenic data TSCA listed Hazardous Decomp. Prods. Heated to decomp., emits acrid smoke and irritating fumes Uses Textile dyeing oxidation catalyst catalyst in prod, of purified terephthalic acid and dimethyl terephthalate for polyester mfg. catalyst in nylon mfg. paint and varnish drier water treatment fertilizer feed additive in... 

Combined 1993 production figures for pure terephthalic acid and dimethyl terephthalate were about 3 5 Mt in the U.S.A., 2Mt in western Europe and l-5Mt in Japan. Over 95% is converted to polyethylene terephthalate by reaction with ethylene glycol. While fibre production remains the largest use, the major area of activity is the production of PET bottle resins, with world capacity scheduled to grow rapidly (presently over 2 Mt per annum, of which 0 9 Mt per annum is in the U.S.A.). 

Terephthalic acid and dimethyl terephthalate are used almost exclusively in the production of terephthalic acid diglycol ester and other esters, which are trans formed into polyeste ondensation and processed to fibers (e.g. Diolen,... 

Adipic acid dimethyi adipate (polyesters), acrylamide (acrylonitrile co-polymers), acrylonitrile (acrylamide co-polymers), acrylic acid (acrylic polymers), buta-1,3-dlene (PS co-polymers, elastomers), butan-1,4-diol (polyesters), 2,2-bis(4-hydroxyphenyl)butan-1-ol (polyesters), 1,1-bis(4-hydroxyphenyl)cyclohexane (polyesters), 4,4 -(propane-2,2-diyl)-dlphenol known as bisphenol A (polyesters), ethyleneglycol (polyestere), formaldehyde (phenol-formaldehyde resins), isophthalic acid (PET), caprolactame and C -C,2 aminocar-boxyllc acids and their lactames (polyamides), melamine (amino-formaldehyde resins), methacryllc acid and methyl methacrylate (acrylic polymers), methylstyrene (PS and co-polymers), propyleneglycol (polyesters), sebacic acid and dimethyl sebacate (polyesters), styrene (PS and co-polymers), terephthalic acid and dimethyl terephthalate (PET), vinyl acetate (vinyl acetate co-polymers), vinyl chloride (PVC and co-polymers), vinylldenechlorlde (PVdC and co-polymers)... 

Within the aromatics e.g. benzene, toluene and xylene) the xylenes are used as a feedstock for the production terephthalic acid and dimethyl terephthalate, both monomers that are used for the production of polyethylene terephthalate (PET), which is the main constituent of plastic bottles and polyester clothing. There are three very close boiling isomers of xylene ortho-, meta- and para-) and it is mainly p-xylene that is used for PET production. The fractionation processes for all existing xylene isomers (crystallization or simulated moving bed) are expensive technologies and it is of interest to study the potentials of membranes for such a separation. 

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