Production of liquid ortho-phthalic acid and its conversion to high purity phthalic anhydride

摘要

Phthalic anhydride of commercially acceptable quality can be recovered from the neat catalytic oxidation of liquid o-xylene with air in the presence of the catalysis system provided by cobalt, manganese and bromine conducted in a single step and in the presence of a small amount of benzoic acid to make miscible liquid o-phthalic acid and liquid oxylene conducted in a single step to an 80 to 90 mole percent yield of o-phthalic acid provided the resulting liquid oxidation effluent is subjected to rapid dehydration of o-phthalic acid to its anhydride and rapid evaporation of the anhydride, water and compounds boiling between water and said anhydride which evaporation entrains phthalide in the vapor mixture followed by contact of the vapor mixture with Pd/C and introducing the vapor mixture into a fractionating system whose reflux liquid is inert, boils at a temperature below the boiling temperature of phthalic anhydride and is a solvent therefor at a temperature below the freezing temperature of said anhydride whereby impure liquid anhydride is withdrawn as water-free liquid and charged to a step of heating with alkali metal hydroxide to remove phthalide and then fractionated to remove benzoic acid as a first fraction, high purity phthalic anhydride as a second fraction and materials boiling higher than the anhydride as a liquid third fraction.