Dimethyl terephthalate (DMT) is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the formula . 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 tons are produced annuall ...

and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

. It is a white solid that melts to give a distillable colourless liquid.

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 acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

steps from ''para''-xylene via methyl ''para''-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. : in the presence of o-xylene at .

Uses

DMT is used in the production of

polyester Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include some natura ...

s, including

polyethylene terephthalate Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in synthetic fibre, fibres for clothing, packaging, conta ...

(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, s ...

(PBT). Structurally, DMT consists of a

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

ring substituted at the 1 and 4 positions with methyl carboxylate 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 or milk. They range in sizes, from very ...

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

Hazards

DMT is not directly dangerous to humans, but it is harmful to aquatic organisms, so environmental releases are avoided. It is flammable and the dust may form explosive mixtures with air.

References

External links

* http://www.inchem.org/documents/icsc/icsc/eics0262.htm {{Authority control Methyl esters Terephthalate esters Commodity chemicals