Process for preparing 3,4,3,4-benzophenone-tetracarboxylic dianhydride

A process for preparing 3,4,3',4'-benzophenone-tetracarboxylic dianhydride which comprises condensing an impure ortho-xylene mixture, said impure ortho-xylene mixture containing at least one of meta-xylene, para-xylene, ethylbenzene and cumene, with acetaldehyde in contact with an acid catalyst to obtain an impure 1,1-bis(3,4-dimethylphenyl) ethane mixture, oxidizing the mixture with nitric acid to obtain a mixture of carboxylic acids, from which by crystallization and dehydration, substantially pure 3,4,3',4'-benzophenonetetracarboxylic dianhydride is recovered.