Process for the long-term operation of a heterogeneously catalyzed partial gas phase oxidation of an organic starting compound

A process for the long-term operation of a heterogeneously catalyzed partial gas phase oxidation of an organic starting compound to an organic target compound, in which a reaction gas input mixture comprising the organic starting compound and molecular oxygen is first conducted through a freshly charged fixed catalyst bed which is installed into two spatially successive temperature zones A, B whose temperatures TA and TB are such that the difference ΔTBA between the temperature TB of the temperature zone B and the temperature TA of the temperature zone A and calculated with the higher of the two temperatures as the minuend >0° C., with the proviso that the reaction gas input mixture flows through temperature zones A, B in the time sequence “first A” and “then B”, temperature zone A extending up to a conversion of the organic starting compound of CA=from 15 to 85 mol % and the conversion of the organic starting compound increasing in temperature zone B to a value CB of ≧90 mol %, and in which, with increasing operating time, in order to counteract the reduction in the quality of the fixed catalyst bed, the temperature of temperature zones A, B is then changed, wherein, with increasing operating time, the temperature of that temperature zone which initially had the lower temperature is increased and the difference ΔTBA between the temperatures of the two temperature zones is reduced, the difference being formed by the temperature of that temperature zone which initially had the higher temperature retaining its position as the minuend.