GB828919A - Preparation of dimethyl terephthalate - Google Patents

Abstract

The invention comprises a process for the preparation of dimethyl terephthalate by reacting methanol with terephthalic acid in the presence of a weak esterification catalyst to give about 88-92% conversion to the dimethyl ester, cooling the esterification reaction mixture to crystallize crude dimethyl terephthalate, separating the latter from the liquid portion of the reaction mixture, and then fractionally distilling it, to obtain dimethyl terephthalate of high purity as a heart cut. Any partially esterified terephthalic acid remaining in the mother liquor may be further reacted with methanol in a similar manner. The crystallized material may be washed with methanol and/or further recrystallized from a hydrocarbon solvent, e.g. para-xylene, benzene, alkybenzenes and paraffins. The final product may also be recrystallized from methanol. Any of the mother liquors may be recycled. Catalysts specified are zinc oxide or mixtures thereof with zinc dust and oxides of aluminium, lead, magnesium, and calcium, finely divided tin, manganese, bismuth, lead, silver, and copper, aluminium, cobalt, lead, magnesium, tin and manganese stearates, or amine resins. The reaction may be carried out at 185-210 DEG C. at 400-850 p.s.i.g. and the ratio of methanol to terephthalic acid is preferably 3-20 : 1. The catalyst concentration is preferably about 0.1-5 parts by weight (calculated as zinc oxide) per 100 parts of terephthalic acid. The process may be carried out batchwise, continuously or intermittently. Examples are given.