There are two well-known rearrangement processes, designated the Henkel Ⅰ and the Henkel Ⅱ process after the firm responsible for their discovery. The classical processes were recently developed further by various firms - Henkel Ⅰ by Teijin and Kawasaki, Henkel Ⅱby Mitsubishi Chemical and Phillips /RhÔne-Poulenc. Teijin, as already mentioned, was the first to manufacture fiber grade terephthalic acid by this route.

The first step in the Henkel Ⅰ process is the manufacture of dipotassium phthalate from phthalie anhydride, which is then rearranged in an isomerization step to dipotassium terephthalate at 430-440°C and 5-20 bar CO₂ in the presence of a Zn-Cd catalyst:

Thc potassium recycle is conducted expediently with a potassium exchange between K terephthalate and phthalic anhydride. All plants using the process have been closed down since they could not compete with other processcs.

In the HenkelⅡ process, potassium benzoate is disproportionated at 430-440°C in the presence of CO₂ (50 bar) and Cd or Zn benzoate to dipotassium terephthalate and benzene with 95% selectivity:

The purification in both processes is conducted at the salt stage by decolorizing the aqueous solution with adsorbents and by recrystallization. In 1971, Mitsubishi Chemical was still using the Henkel Ⅱ process in a 23000 tonne-per-year plant in Japan; it was shut down in 1975.

Recently. an improved Henkel Ⅱ process has aroused interest. Phillips Petrolcum and Rhône-- Poulenc jointly developcd a continuous process for the manufacture of fiber grade terephthalic acid using the Henkel Ⅱ principle. It is characterized by a complete potassium recycle (exchange of potassium between potassium tercphthalate and benzoic acid) which prevents the formation of potassium salts. The disproportionation of potassium benzoate is carried out in suspension in a terphenyl mixture in the presence of zinc oxide.

The chain of terephthalic acid process has been extended with a newly revised route from Lummus. It is based, like several older processes (e.g., Allied, Distillers, and Showa Denko), on the ammoxidation of p- to terephthalonitrile:

No free oxygen is used in the ammoxidation step of the Lummus process. p-Xylene and NH₃ react more readily at 400-450°C with a fluidized-bed metal catalyst at a higher oxidation state, generally V₂O₅/Al₂O₃. The catalyst, which becomes reduced during the process, is then reoxidized in a separate reaction at 500°C with O₂ or air. The selectivity to terephthalonitrile and its precursor p-tolylnitrile, which is recycled to the amrnoxidation, is reported to be over 90%. The p-xylcnc conversion is adjusted to about 50%.

Terephthalonitrile is converted into pure terephthalic acid in three steps. The dinitrile is first hydrolyzed with steam to monoammonium terephthalate. In the second step, this ammonium salt is thermally cleaved into tcrephthalic acid and NH₃.

In the third step, hydrolysis is used to convert traces of the semiamide into acid. Pure terephthalic acid must have less than 10 ppm nitrogen-containing substances, since they cause a yellow coloration in the polyester:



This process has not, however, been used commercially.

On the other hand, Showa Denko (Japan) practices the commercial ammoxidation of p- or m-xylene to the corresponding dinitriles, terephthalonitrile and isophthalonitrile, which are generally used to manufacture the diamines:

Several other firms have developed processes for ammoxidation of alkylated aromatics; for example, Mitsubishi Gas Chemical has developed aprocess for the manufacture of isophthalonitrile from rn-xylene which is used in two commercial plants (USA, Japan). ation of isophthalonitrile yields m-xylylene diamine, which is converted to diisocyanate used in polyurethanes.

Japan Catalytic Chem. Ind. and BASF also produce phthalonitrile by the ammoxidation of o-xylene in their own processes. Phthalonitrile is an important precursor for the manufacture of phthalocyanine dyes.

Mitsubishi Gas Chemical has recently developed a terephthalic acid process to the pilot plant stage. The first step is the reaction of toluene with CO at 30-40°C in the presence of HF/BF, to give p-tolylaldehyde. Aldehyde yields of 96% (based on toluene) and 98% (based on CO) are obtained. After catalyst separation, the p-tolylaldehyde is purified and then oxidized to terephthalic acid: