Patent ID: 7956200

Claim:
A process for preparing carboxylic acids and/or carboxylic anhydrides by partial oxidation of aromatic hydrocarbons in the gas phase by means of a gas comprising molecular oxygen at elevated temperature over a catalyst whose catalytically active composition is applied in the form of a shell to a catalyst support comprising an inert, nonporous support material, wherein the catalyst used is a coated catalyst whose catalytically active composition, based on its total weight, comprises from 30 to 100% by weight of one or more silver-vanadium oxide bronzes having an Ag:V atomic ratio of from 0.15 to 0.95 and has a BET surface area of from 2 to 100 m 2 /g, which coated catalyst has been produced from a precatalyst comprising the inert, nonporous support material and one or more layers applied thereto in the form of a shell or shells, wherein this/these shell layer or layers comprises/comprise a multimetal oxide of formula I Ag a-b M b V 2 O x ×c H 2 O I where M is a metal selected from the group consisting of Li, Na, K, Rb, Cs, Tl, Mg, Ca, Sr, Ba, Cu, Zn, Cd, Pb, Cr, Au, Al, Fe, Co, Ni and/or Mo, a is from 0.3 to 1.9 and b is from 0 to 0.5, with the proviso that the difference (a−b)≧0.1 and c is from 0 to 20 and x is a number determined by the valence and amount of elements different from oxygen in the formula I, which has a crystal structure giving an X-ray powder diffraction pattern which displays reflections at the lattice spacings d of 15.23+0.6, 12.16+0.4, 10.68+0.3, 3.41+0.04, 3.09+0.04, 3.02+0.04, 2.36+0.04 and 1.80+0.04 Å, and said precatalyst comprises from 30 to 100% by weight of the multimetal oxide, based on the total weight of the shell layer or layers, and optionally in the presence of at least one further coated catalyst for the oxidation of aromatic hydrocarbons to carboxylic acids and/or carboxylic anhydrides which is different from the above-described coated catalyst and whose catalytically active composition comprises vanadium pentoxide and anatase as significant catalytically active constituents and, if such a further coated catalyst is present, it is used in a combined catalyst bed with the coated catalyst of the above composition in the oxidation reactor.