Patent ID: 8153825

Claim:
A process for preparing phthalic anhydride by catalytic gas phase oxidation of o-xylene, wherein, in a main reactor, a gaseous mixture of o-xylene and an oxygenous gas is passed through at least two reaction zones whose temperature can be controlled independently, and converted to a gaseous intermediate reaction product which comprises unconverted o-xylene, phthalic anhydride underoxidation products and phthalic anhydride, and the intermediate reaction product is introduced into a postreactor, wherein the temperature of the reaction zones in the main reactor is regulated in such a way that the concentration of unconverted o-xylene in the intermediate reaction product is at least 0.5% by weight, based on the weight of the organic components in the intermediate reaction product, wherein the reaction zone furthest downstream is operated at lower temperature than the reaction zone adjacent upstream, and wherein the concentration of o-tolylaldehyde in the intermediate reaction product is at least 0.25% by weight.