GB623836A - Manufacture of terephthalic acid - Google Patents

Abstract

Terephthalic acid is manufactured by passing oxygen, air or other mixture of gases containing oxygen into liquid p-xylene at a temperature above 100 DEG C., said liquid containing as an oxidation catalyst a small proportion of an oil-soluble compound of cobalt, continuing the passage of gas until a substantial amount of solid separates from the reaction medium, and recovering this solid. The catalyst may be suspended or preferably dissolved in the p-xylene, advantageously in an amount corresponding to 0.002 to 0.02 per cent of metal, based on the weight of p-xylene. When working at atmospheric pressure, the temperature is conveniently 130-140 DEG C. at the outset and may later be raised to 145 DEG C. or higher. The process may be made continuous by continuously or periodically adding p-xylene and withdrawing the separated solid. The p-xylene should be free from antioxidants and catalyst poisons, but may contain minor proportions of hydrocarbon impurities such as benzene, toluene, ethylbenzene or o- or m-xylene, or inert diluents, e.g. chlorobenzenes. The oxidizing gas should be well distributed by injecting it through a sintered glass or porous ceramic disc, or preferably through high velocity jets, with or without the use of a high-speed agitator. The process is conveniently effected in plant constructed from glass, nickel, enamelled metal or anodized aluminium, or lined with tantalum. The presence of chromium, copper or alloys containing them is undesirable, though a measure of success can be achieved with stainless steel equipment if a small proportion of a lead or barium salt is present in the reaction medium. The product may be purified by washing with a hot lower aliphatic alcohol, e.g. methyl, ethyl or isopropyl alcohol, preferably with boiling methyl alcohol. Alternatively, the terephthalic acid may be isolated in the form of an ester by esterifying the separated solid with excess of a lower aliphatic alcohol, e.g. methyl (preferred), ethyl or isopropyl alcohol, with or without the aid of an esterification catalyst, and separating the terephthalic acid ester from the esterified solid. The residues from either method of purification may be returned to the reaction medium for further oxidation. Alternatively, partial oxidation products in the separated solid may be oxidized to terephthalic acid by the action of mild oxidative chemical reagents, e.g. dilute nitric acid, or permanganates in the presence of alkali. Examples illustrate various features of the invention.