Patent ID: 7985705
Filing Date: 2011-07-26
Classification: B01J,C07C

Abstract:
1. 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 (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 (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 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,