GB789809A - Process for the manufacture of terephthalic acid - Google Patents

Abstract

Terephthalic acid is manufactured by a process wherein a terpene-containing by-product of the wood processing and allied industries is first subjected to isomerization by means of sulphuric acid, phosphoric acid, or a sulphonic acid, such as benzene sulphonic acid, having about the same acidity as phosphoric acid or sulphuric acid, at a temperature between about 50 DEG and 150 DEG C., and then, after the aqueous phase is removed, the organic phase is catalytically dehydrogenated at a temperature of about 250 DEG to about 500 DEG C., and the resulting dehydrogenation product consisting mainly of para-substituted benzene hydrocarbons is oxidized by a known method to terephthalic acid. The product of the dehydrogenation is mainly p-cymene with a minor amount of m-cymene. The yield of p-cymene is increased by intensive mixing during the isomerization and recycling the acid used. Specified dehydrogenation catalysts are chromium oxide-aluminium oxide, molybdenum oxide-aluminium oxide and vanadium oxide-aluminium oxide and mixtures thereof which may contain the oxides of metals in Groups I-IV such as the oxides of sodium, potassium, magnesium, calcium, aluminium when not otherwise present), the rare earth metals, thorium, zirconium and silicon. Compounds of other metals such as iron, nickel and copper, or metals, such as the noble metals, may be added to the catalysts. Aluminium oxide may be replaced by aluminium silicate or silicic acid. The oxidation of the para-substituted benzenes to terephthalic acid may be carried out with chromic or nitric acids, air or oxygen. In examples, the process of the invention is carried out batchwise or continuously using sulphuric acid and phosphoric acid as isomerizing acids, chromium oxide or molybdenum oxide-alumina dehydrogenation catalysts and oxidizing the para-cymene so obtained to terephthalic acid, an oxidation using chromic acid being detailed.