GB646968A - Improvements in the manufacture of esters of aromatic dicarboxylic acids - Google Patents

Abstract

In the preparation of a di-ester of an aromatic dicarboxylic acid with a distillable aliphatic mono-hydroxy compound by reaction between the anhydride of the acid and the hydroxy compound in presence of an esterification catalyst, the quantity of hydroxy compound is limited so as to avoid substantial reduction in conversion, based on the hydroxy compound, below the peak conversion obtainable and the di-ester and mono-ester produced are separately recovered, and the mono-ester is employed as part of a further batch together with fresh anhydride and hydroxy compound. Di-esters of phthalic acid and hydrogenated phthalic acids such as di-and tetra-hydrophthalic acids with methanol, ethanol, propanols, butanols and glycol ethers such as ethylene glycol mono-methyl ether are referred to. For dimethyl phthalate, the amount of methanol used is at most twice the theoretical, generally less than 50, e.g. 20-30 per cent excess over the theoretical. About 1-3 per cent of sulphuric acid, based on the weight of phthalic anhydride, is suitable. Other acid substances such as sodium bisulphate may be used. The anhydride may first be heated with the theoretical amount of methanol, e.g. at 90-100 DEG C. for up to 3 hours, unchanged methanol distilled off by raising the temperature 10-30 DEG C., and additional methanol then added slowly with distillation of water and some methanol. Recycled mono-methyl phthalate, e.g. 30-35 moles per cent of the anhydride, reduces the amount of methanol used after the first batch. The reaction product is washed with aqueous alkali such as 10-30 per cent caustic soda to separate dimethyl phthalate from an aqueous layer which is acidified to give monomethyl phthalate for recycling. Reduced pressure may be used for distilling water and excess hydroxy compound where the latter is glycol mono-methyl ether. Aqueous alcohols, e.g. 90-95 per cent ethanol, may be used. Examples describe the preparation of dimethyl and diethyl phthalates. Specification 625,203 is referred to.