Dimethyl terephthalate

Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.[1]

Dimethyl terephthalate
Names
Preferred IUPAC name
Dimethyl benzene-1,4-dicarboxylate
Other names
Dimethyl terephthalate
1,4-Benzenedicarboxylic acid dimethyl ester
Dimethyl 4-phthalate
Dimethyl p-phthalate
Di-Me terephthalate
Methyl 4-carbomethoxybenzoate
Methyl-p-(methoxycarbonyl)benzoate
Methyl terephthalate
Terephthalic acid methyl ester
Identifiers
3D model (JSmol)
Abbreviations DMT
1107185
ChemSpider
ECHA InfoCard 100.004.011
EC Number
  • 204-411-8
MeSH Dimethyl+4-phthalate
RTECS number
  • WZ1225000
UNII
Properties
C10H10O4
Molar mass 194.186 g·mol−1
Appearance white solid
Density 1.2 g/cm3, ?
Melting point 142 °C (288 °F; 415 K)
Boiling point 288 °C (550 °F; 561 K)
Acidity (pKa) -7.21
Basicity (pKb) -6.60
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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 steps from p-xylene via methyl p-toluate (PT).[1]

Dimethyl terephthalate (DMT) (Witten Process)

The common 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 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.[2]

Structure of monomethyl terephthalate.

Oxidation of methyl p-toluate followed by esterification also yields dimethyl terephthalate (DMT) as shown in the below reaction:[1]

Dimethyl terephthalate (DMT) 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:

C8H6O4 + 2CH3OH → C10H10O4 + 2 H2O

at presence of o-xylene at 250–300 °C.

Use

DMT is used in the production of polyesters, including polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutylene terephthalate (PBT). It consists of benzene substituted with carboxymethyl groups (CO2CH3) 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.

References
  1. Richard J. Sheehan "Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.doi:10.1002/14356007.a26_193
  2. "Dimethyl Terephthalate (DMT)". 2013-05-02.
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