Patent ID: 6700000
Filing Date: 2004-03-02
Classification: B01J,C07D

Abstract:
A process for preparing phthalic anhydride by catalytic gas-phase oxidation of xylene and/or naphthalene by a gas comprising molecular oxygen in a fixed bed at elevated temperature, which process comprises conducting the oxidation in the presence of at least three coated catalysts arranged in superposed zones, which catalysts have a layer of a catalytically active composition of metal oxides applied to a core of nonporous support material, the active composition comprising from 3 to 8% by weight of V2O5, from 0 to 3.5% by weight of Sb2O3, from 0 to 0.3% by weight of P, alkali metal selected from the group consisting of potassium, rubidium and cesium as a dopant, and as balance TiO2 in anatase form having a BET surface area of from 18 to 22 m2/g, wherein the catalyst activity rises from zone to zone from the gas inlet end to the gas outlet end and the activity of the catalysts of the individual zones is set such thata) the least active catalyst comprises from 5 to 9% by weight, based on the total catalyst, of the active composition and the active composition comprises from 0.1 to 0.5% by weight of alkali metal (calculated as metal); b) the next more active catalyst comprises from 1 to 5% by weight (absolute) more of the active composition than the catalyst (a), and the active composition comprises from b to 0.25% by weight (absolute) less of alkali metal than the active composition of the catalyst (a); and c) the most active catalyst comprises from 1 to 5% by weight (absolute) more of the active composition than the catalyst (a), and the active composition comprises from 0.15 to 0.4% by weight (absolute) less of alkali metal than the active composition of the catalyst (a).