GB727184A - Separation of terephthalic acid - Google Patents

Abstract

Terephthalic acid is separated from admixture with isophthalic acid, by subjecting the mixture to partial esterification with a primary alkanol in the presence of a strong acid esterification catalyst, and at a temperature between 80 DEG and 150 DEG C., and separating the unesterified acid from the reaction mixture. The primary alkanols used in the esterification contain from 4 to 10 carbon atoms, and preferably are those containing 7 to 9 carbon atoms whose iso. phthalic esters are useful as plasticizers, e.g. 1-octanol, 2 : 2 : 4-trimethyl pentanol, 2-ethyl hexanol and 2 : 2 : 4 : 4-tetra-methylpentanol. With the higher alkanols, inert hydrocarbon or halogenated hydrocarbon diluents of boiling-point above 70 DEG C. may be used, and serve to remove the water formed in the reaction by entrainment. The catalyst, present in a concentration of 0.1 to 1.0 per cent by weight, may be sulphuric, hydrochloric, phosphoric, or p-toluene sulphonic acid but sulphuric acid is preferred. The esterification is carried out by refluxing the reactants and removing the water which is formed, either as described above, or if lower alkanols are used, by entrainment in the alkanol itself. When the amount of water formed diminishes the esterification is assumed complete, and the residue, enriched in terephthalic acid, removed by filtering or centrifuging. In examples: (1) a mixture containing 28.1 per cent terephthalic acid gives a residue containing 89.5 per cent terephthalic acid after 21 hours reflux, and (2) a mixture containing 40 per cent terephthalic acid gives a residue containing 96.1 per cent terephthalic acid after 12.25 hours reflux.