PREPARATION OF HIGH PURITY PHTHALIC ANHYDRIDE

PURPOSE:To prepare a high purity phthalic anhydride free from naphthoquinone, by adding an alkali metal compound to crude phthalic anhydride obtained by the vapor-phase catalytic oxidation of naphthalene, heat-treating the mixture introducing an oxygen-containing gas, and distilling the product. CONSTITUTION:Crude phthalic anhydride is incorporated with 0.0001-0.5wt% alkali metal compund (preferably carbonate or bicarbonate of Na, K, etc.), and heat-treated at 150-300 deg.C for 1-15hr while introducing a gas containing molecular oxygen at a rate of at least 1X10 mole/hour per 1kg of the crude raw material. Naphthoquiinone existing in the crude raw material as impurity is polymetized to a high-boiling material by the heat treatment. A high purity phthalic anhydride can be prepared by removing the polumerized naphthoqinone by distillation. The gas containing molecular oxygen is prepared usually by adding N2 gas to air to reduce the oxygen concentration to 1-10vol% (preferably 3-8vol%).