PREPARATION OF PYROMELLITIC ANHYDRIDE

The invention relates to a process for the preparation of pyromellitic dianhydride (PMDA) by heterogeneously catalysed oxidation in the gas phase by means of a gas containing molecular oxygen, characterised in that the substances which are oxidised are benzaldehydes which are 2,4,5-trialkylated with C1- to C3-alkyl groups or mixtures of benzaldehydes which are 2,4,5-trialkylated with C1- to C3-alkyl groups and benzenes which are 1,2,4,5-tetralkylated with C1- to C3-alkyl groups, in the presence of a catalyst which contains, as active components, 5 to 95 % by weight of one or more transition metal oxides of sub-group IV of the Periodic Table, 1 to 50 % by weight of one or more transition metal oxides of sub-group V of the Periodic Table, 0 to 10 % by weight of one or more oxides of elements of main group I of the Periodic Table and/or 0 to 50 % by weight of one or more oxides of elements of main groups III, IV and V of the Periodic Table and of elements of sub-groups VI and VII of the Periodic Table, and the percentages in each case being based on the total weight of the active components and totalling 100 % by weight.