Patent ID: 7687425

Claim:
A catalyst system for preparing phthalic anhydride, which comprises four catalyst layers which are arranged above one another in a 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 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 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 layers, and wherein the titanium dioxide in the upper middle layer has a BET surface area of from 10 to 35 m 2 /g and that in the lower middle layer has a BET surface area of from 15 to 40 m 2 /g, with the proviso that the BET surface area of the titanium dioxide in the upper middle layer is less than or equal to the BET surface area in the lower middle layer.