Patent ID: 7985705

Claim:
A process for preparing phthalic anhydride by gas-phase oxidation of xylene, naphthalene or mixtures thereof in a shell-and-tube reactor, wherein the starting materials are passed over a catalyst system for preparing phthalic anhydride, which comprises three, four, or five catalyst layers which are arranged above one another in the reaction tube and whose active composition comprises from 70 to 99% by weight of titanium dioxide in the anatase modification, wherein (i) in the uppermost layer the catalyst comprises from 7 to 10% by weight, based on the total catalyst, of active composition comprising from 6 to 11% by weight of V 2 O 5 , from 0 to 3% by weight of Sb 2 O 3 , from 0.1 to 1% by weight of alkali (calculated as alkali metal) and TiO 2 in anatase form having a BET surface area of from 5 to 30 m 2 /g as balance on support material, (ii) in the middle layer or layers the catalyst comprises from 7 to 12% by weight, based on the total catalyst, of active composition comprising from 5 to 13% by weight of V 2 O 5 , from 0 to 3% by weight of Sb 2 O 3 , from 0 to 0.4% by weight of P, from 0 to 0.4% by weight of alkali (calculated as alkali metal) and TiO 2 in anatase form having a BET surface area of from 10 to 40 m 2 /g as balance on support material, (iii) in the bottom-most layer the catalyst comprises from 8 to 12% by weight, based on the total catalyst, of active composition comprising from 5 to 30% by weight of V 2 O 5 , from 0 to 3% by weight of Sb 2 O 3 , from 0.05 to 0.4% by weight of P, from 0 to 0.3% by weight of alkali (calculated as alkali metal) and TiO 2 in anatase form having a BET surface area of from 15 to 50 m 2 /g as balance on support material, with the proviso that the BET surface area of the titanium dioxide in the uppermost layer (i) is less than the BET surface area of the titanium dioxide in the middle layer or layers (ii) and the BET surface area of the titanium dioxide in the bottom-most layer (iii) is greater than the BET surface area of the titanium dioxide in the middle layer or layers, wherein the titanium dioxide in at least one catalyst layer consists of a mixture of titanium dioxides having different BET surface areas.