Alternative Routes for Terephthalic Acid Production

Other syntheses for terephthahc acid have been developed, although none has yet been used extensively. As with most chemical processes, the capital cost of new equipment usually means that modifications to existing processes are more easily adopted than completely different routes to the same product. 

In the Henkel process, dipotassium phthalate can be isomerized to dipotassium terephthalate at about 430°C and 5-20 bar using caibon dioxide with a soluble organic zinc or cadmium salt. An alternative approach is the disproportionation of potassium benzoate to give dipotassium terephthalate and benzene using zinc or cadmium benzoates at up to 50 bar pressure of caibon dioxide. In both operations the potassium is recycled by reaction with either phthalic anhydride or benzoic acid 

In a process developed by Lummus, / -xylene and ammonia are converted to terephthalonitrile in a fluidized bed at 450°C. A supported vanadium pentoxide catalyst is used and gives about 50% conversioa The vanadium pentoxide is reduced during the reaction and can be reoxidized with air in a separate reactor. The dinitrile is then hydrolyzed to terephthalic acid. The process has not been used in polyester production, although a similar process used in Japan produces diamines from terephthalonitrile or isophthalonitriles. 

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