Dimethyl terephthalate (DMT) is an organic compound with the formula C₆H₄(COOCH₃)₂. It is the diester formed from

terephthalic acid Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced annua ...

and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.

Production

Dimethyl terephthalate (DMT) has been produced in a number of ways. Conventionally, and still of commercial value, is the direct esterification of terephthalic acid. Alternatively, it can be prepared by alternating oxidation and methyl-

esterification In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glyceride ...

steps from

p-xylene ''p''-Xylene ( ''para''-xylene) is an aromatic hydrocarbon. It is one of the three isomers of dimethylbenzene known collectively as xylenes. The ''p-'' stands for ''para-'', indicating that the two methyl groups in ''p''-xylene occupy the diame ...

via methyl ''p''-toluate (PT).

Dimethyl terephthalate by the Witten process

The method for the production of DMT from ''p''-xylene (PX) and methanol consists of a multistep process involving both oxidation and esterification. A mixture of p-xylene (PX) and methyl ''p''-toluate is oxidized with air in the presence of cobalt and manganese catalysts. The acid mixture resulting from the oxidation is esterified with methanol to produce a mixture of esters. The crude ester mixture is then distilled to remove all the heavy boilers and residue that are produced; lighter esters such as monomethyl terephthalate are recycled to the oxidation section. The raw DMT is then sent to the crystallization section to remove DMT isomers, residual acids and aromatic aldehydes.

Oxidation of methyl p-toluate followed by esterification also yields dimethyl terephthalate (DMT) as shown in the below reaction: :

Dimethyl terephthalate by direct esterification

DMT can also be made by a different route, namely direct esterification of terephthalic acid with methanol. The dimethyl terephthalate that is formed is then purified by distillation. Even terephthalic acid of low purity may be used in this method. :C₈H₆O₄ + 2CH₃OH → C₁₀H₁₀O₄ + 2 H₂O in the presence of o-xylene at 250–300 °C.

Uses

DMT is used in the production of polyesters, including polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and

polybutylene terephthalate Polybutylene terephthalate (PBT) is a thermoplastic engineering polymer that is used as an insulator in the electrical and electronics industries. It is a thermoplastic (semi-)crystalline polymer, and a type of polyester. PBT resists solvents, ...

(PBT). Structurally, DMT consists of a

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen ato ...

ring substituted at the 1 and 4 positions with methyl carboxylate (-CO₂CH₃) groups. Because DMT is volatile, it is an intermediate in some schemes for the recycling of PET, e.g. from

plastic bottle A plastic bottle is a bottle constructed from high-density or low density plastic. Plastic bottles are typically used to store liquids such as water, soft drinks, motor oil, cooking oil, medicine, shampoo, milk, and ink. The size ranges from ...

s. Hydrogenation of DMT affords the diol cyclohexanedimethanol, which is a useful monomer.

References

External links

* Supplier:Teijin Limited https://web.archive.org/web/20150503041344/http://www.teijin.com/products/chemicals/dmt/ * http://www.inchem.org/documents/icsc/icsc/eics0262.htm *
U.S. National Library of Medicine: Hazardous Substances Databank – Dimethyl+terephthalate
{{DEFAULTSORT:Dimethyl Terephthalate Methyl esters Terephthalate esters Commodity chemicals