GB768133A - Improvements relating to the purification of crude phthalic anhydride - Google Patents

Abstract

Crude phthalic anhydride is purified by heating at reflux temperature (about 280-285 DEG C), and after dehydration if necessary, for about 3 to 12 hours in the presence of 0.02 to 0.2 per cent by weight of alkali metal phthalate which is formed in situ by adding 0.01 to 0.1 per cent of alkali metal hydroxide by weight of the crude phthalic anhydride, removing a heads cut relatively rich in benzoic acid during or after the heat treatment and then subjecting the resulting mixture to fractional distillation to withdraw commercially pure phthalic anhydride. The crude phthalic anhydride may be obtained by vapour phase oxidation of naphthalene or o-xylene in the presence of a vanadium catalyst at temperatures of 300-500 DEG C. If the product contains water it may be dehydrated before adding alkali metal hydroxide as an aqueous solution. The alkali metal hydroxide may be lithium, sodium or potassium hydroxide. Fractionation of the heat treated product may be carried out at atmospheric pressure or under partial vacuum. It is preferably effected at 30 mm. Hg, with a 1 : 1 reflux ratio using a column containing from 4 to 6 theoretical plates. In examples, a product obtained by oxidation of naphthalene over vanadium pentoxide at 425-500 DEG C. is dehydrated, aqueous sodium hydroxide is added and the mixture refluxed at 288 DEG C.; the reflux ratio is adjusted to 6 : 1 and a heads cut of benzoic acid is removed and the residue is (1) subjected to vacuum fractionation at 30 mm. Hg. with a 1 : 1 reflux ratio; or (2) fractionated at atmospheric pressure using a 1 : 1 reflux ratio. The product is almost colourless phthalic anhydride. Specification 670,429 is referred to.