Dimethyl terephthalate


Dimethyl terephthalate is an organic compound with the formula C6H42. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.

Production

Dimethyl terephthalate 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 steps from p-xylene via methyl p-toluate.

Dimethyl terephthalate (DMT) (Witten Process)

The common method for the production of DMT from p-xylene and methanol consists of a multistep process involving both oxidation and esterification. A mixture of p-xylene and methyl p-toluate is oxidized with air in the presence of a 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 distilled to remove all the heavy boilers and residue produced; the lighter esters 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 as shown in the below reaction:
Dimethyl terephthalate production through direct esterification
If highly impure terephthalic acid is available, DMT can be made in a separate process by esterification with methanol to dimethyl terephthalate, which is then purified by distillation:
at presence of o-xylene at 250–300 °C.

Use

DMT is used in the production of polyesters, including polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate. It consists of benzene substituted with carboxymethyl groups at the 1 and 4 positions. Because DMT is volatile, it is an intermediate in some schemes for the recycling of PET, e.g. from plastic bottles.
Hydrogenation of DMT affords the diol cyclohexanedimethanol, which is a useful monomer.