GB670434A - Improvements in or relating to methods of purifying crude phthalic anhydride - Google Patents

Abstract

Phthalic anhydride is obtained from crude phthalic anhydride by heating a mixture of the crude anhydride and at least 0.0035 mol. per mol. of phthalic anhydride in the crude anhydride of an inorganic acid oxidizing agent or sulphur trioxide or hydrogen peroxide at a temperature above 270 DEG C., adding thereto at least 0.001 mol. of an alkali metal nitrate per mol. of phthalic anhydride in the crude material, heating the resulting mixture at a temperature above 270 DEG C. and for a period of time sufficient to permit separation of phthalic anhydride from the impurities therein by distillation, and then separating phthalic anhydride from the mixture in purified form. The crude phthalic anhydride may be obtained from the vapour phase catalytic oxidation of naphthalene. The crude anhydride can be given one or more distillations prior to the above process. The amount of oxidizing agent employed may be substantially equivalent to that required to oxidize the reducing impurities in the crude material, e.g. from 0.0035 to 0.04 mol. per mol. of phthalic anhydride in the crude material. The inorganic acid oxidizing agent may be nitric or sulphuric acid. The heating with the oxidizing agent and the crude phthalic anhydride may be carried out at a p temperature above 270 DEG C., but not substantially in excess of the boiling-point of phthalic anhydride at atmospheric pressure and without substantial loss of phthalic anhydride vapours, and may be continued for a period of time sufficient to substantially complete the reaction between the oxidizing agent and the reducing impurities in the crude anhydride, e.g. for a period of from 1 to 3 hours. The amount of alkali metal nitrate employed may be from 0.0015 to 0.01 mol. of alkali metal nitrate per mol. of phthalic anhydride in the crude. The alkali metal nitrate may be lithium, sodium, potassium or caesium nitrate and may contain water of crystallization. The heating with the alkali metal nitrate may be conducted at a temperature above 270 DEG C. but not substantially in excess of the boiling-point of phthalic anhydride at atmospheric pressure and without substantial loss of phthalic anhydride vapour, e.g. for a period of 1 to 3 hours. The phthalic anhydride may be separated from the mixture by distillation at sub-atmospheric pressure, e.g. at an absolute pressure below 400 mm. of mercury. A number of detailed examples are given.