Patent ID: 8859459

Claim:
A multilayer catalyst for the oxidation of naphthalene or o-xylene/naphthalene mixtures to phthalic anhydride, which comprises at least three catalyst layers which each comprise vanadium oxide and titanium dioxide and have alkali metal contents selected so that a) the alkali metal content of one catalyst layer A, which has a bed length in the range from 15 to 30% of the total catalyst fill height in the reactor, is the highest, b) a catalyst layer Z which follows the catalyst layer A in the flow direction has an alkali metal content of from 0 to 10% of the alkali metal content of the catalyst layer A and c) the catalyst layers located between the catalyst layers A and Z have an alkali metal content of from 30 to 90% of the alkali metal content of the catalyst layer A, with the alkali metal content of each catalyst layer being higher than the alkali metal content of the next catalyst layer in the flow direction and wherein at least one layer of said multilayer catalyst further comprises a promoter and said promoter is niobium oxide and the niobium oxide is present in an amount from 0.01 to 0.5% by weight, based on the catalytically active composition.