Method for reducing phthalide content of phthalic anhydride

摘要

Phthalide impurity is removed from crude phthalic anhydride (made, e.g., by vapour phase air oxidation of o-xylene) by heating with H 2 SO 4 in a mixture, which contains chemically combined H 2 O as a hydrate of H 2 SO 4 or as phthalic acid, under such conditions that sufficent chemically combined H 2 O is retained to effect the selective reaction of H 2 SO 4 with phthalide to form phthalic anhydride, SO 2 and H 2 O until a major portion of phthalide has reacted.