Patent ID: 7863463

Claim:
A method of preparing a purified dianhydride, said method comprising: (a) providing a first mixture comprising: an organic solvent, a dianhydride, and at least 10,000 parts per million of a metal halide salt relative to the total weight of the dianhydride; (b) combining the first mixture with water and at least one inorganic acid to form a second mixture; (c) heating said second mixture until substantially all of said dianhydride is converted to a tetraacid formed as an ingredient in a third mixture, said third mixture comprising an organic layer and an aqueous layer; (d) subjecting the third mixture to at least one operation selected from the group consisting of: (i) removing substantially all of the organic solvent to produce an aqueous slurry of the tetraacid; (ii) removing the organic layer from the aqueous layer to form a mixture comprising the tetraacid and the organic solvent; and (iii) filtering to form a tetraacid wet cake; and (e) subjecting the product formed in step (d) to at least one operation selected from the group consisting of: (iv) filtering to provide a tetraacid wet cake; (v) combining a tetraacid wet cake with at least one organic solvent to produce a mixture comprising a tetraacid and an organic solvent; and (vi) effecting ring closure of a tetraacid ingredient; to provide a purified dianhydride comprising less than 50 parts per million of said metal halide salt.