Improved process for the preparation of esters of aromatic di-carboxylic acids, and esters so produced

摘要

Diesters of low-boiling alcohols and aromatic dicarboxylic acids are made by esterifying the desired acid or acids with mono- or polyhydric high-boiling alcohols or mixtures thereof and subjecting the product to ester interchange with the desired low-boiling alcohol in the presence of alkaline or acid ester interchange catalysts. Low-boiling alcohols are defined as methyl, ethyl and propyl alcohol. High-boiling alcohols are defined as monohydric alcohols of 4 or more carbon atoms, e.g. n-butyl, -hexyl, -heptyl and -octyl alcohol and polyhydric alcohols such as glycols or glycerol. Mixtures of high-boiling alcohols may be used. Tere- and iso-phthalic acids are specified. Solubility increasing substances such as butyrolactone may be present. Water, added before, or obtained in the reaction may be removed by azeotropic distillation with the mono- or poly-hydric alcohols. The initial esterification is effected in conventional fashion. Alkaline ester interchange catalysts mentioned are alkylates, e.g. a methylate, and alcoholic solutions of hydroxides the reaction being effected at pH8-10 and -10 DEG C.-+80 DEG C. Temperatures are generally higher when acid ester interchange catalysts are used and the desired ester is soluble in the reaction product so that the process has to be repeated preferably with the addition of fresh quantities of high boiling ester and the removal of a part of the residue, from which acid and high boiling alcohol may be recovered. Terephthalic acid containing isophthalic acid as impurity may be esterified with methanol and the diesters of each acid separately obtained since that of the former precipitates from the reaction product at about 30 DEG C., whereas that of the latter precipitates only on cooling to a lower temperature, e.g. 0 DEG to 10 DEG C. Examples are provided illustrating the preparation of dimethyl terephthalate by the methods given above.