Terephthalic acid

[ter-ef-thal-ik, ter-]

Terephthalic acid is one isomer of the three phthalic acids. It finds important use as a commodity chemical, principally as a starting compound for the manufacture of polyester (specifically PET), used in clothing and to make plastic bottles. It is also known as 1,4-benzenedicarboxylic acid, and it has the chemical formula C₆H₄(COOH)₂. It has recently become an important component in the development of hybrid framework materials.

History

Phthalic acid (the ortho isomer of terephthalic acid) was obtained by French chemist Auguste Laurent in 1836 by oxidizing naphthalene tetrachloride, and, believing the resulting substance to be a naphthalene derivative, he named it naphthalenic acid. Swiss chemist Jean Charles Galissard de Marignac determined its formula and showed Laurent’s supposition to be incorrect, upon which Laurent gave it its present name, with teres meaning well-turned, refined, elegant in latin (symmetry increased over ortho and meta isomers).

Properties

It is almost insoluble in water, alcohol and ether; it sublimes rather than melting when heated. This insolubility makes it relatively awkward to work with, and up until around 1970 most crude terephthalic acid was converted to the dimethyl ester for purification.

Production

Terephthalic acid can be formed in the laboratory by oxidizing para-diderivatives of benzene, or best by oxidizing caraway oil, a mixture of cymene and cuminol, with chromic acid.

On an industrial scale, terephthalic acid is produced, similar to benzoic acid, by oxidation of p-xylene by oxygen from air. This is done using acetic acid as solvent, in the presence of a catalyst such as cobalt-manganese, using a bromide promoter. The yield is close to 100%. The crude product is purified by hydrogenation while in a water solution to convert trace amounts of impurities to less harmful species. The solution is then cooled in a stepwise manner to crystallize out a highly pure terephthalic acid. Over 90% of the terephalate feedstock for the polyester industry uses this process, and all plants built within the last few decades are of this technology. Alternatively, but not commercially, it can be made, via the Henkel process (company) also known as Raecke process (patent holder), which involves the rearrangement of phthalic acid to terephthalic acid via the corresponding potassium salts. Terephthalic acid and dimethyl terephthalate are essentially all used as a monomer component in the production of polymers, principally polyethylene terephthalate (polyester or PET). World production in 1970 was around 1.75 million tonnes.^[2]. By 2006, global PTA demand had substantially exceeded 30 million tonnes.

References

1911 Encyclopedia
Basic Organic Chemistry: Part 5, Industrial Products, J.M. Tedder, A. Nechvatal, A.H. Tubb (editors), John Wiley & Sons, Chichester, UK (1975).