Long-term operation of a heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid

A process for the long-term operation of a heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid, by conducting a starting reaction gas mixture which comprises acrolein, molecular oxygen and at least one inert diluent gas through a fixed catalyst bed at elevated temperature whose catalysts are such that their active composition is at least one multimetal oxide which contains the elements Mo and V, and by, in order to counteract the deactivation of the fixed catalyst bed, increasing the temperature of the fixed catalyst bed over time,

• which comprises
• interrupting the gas phase partial oxidation at least once before the temperature increase of the fixed catalyst bed is permanently ≧10° C., and, at a temperature of the fixed catalyst bed of from 200 to 450° C., conducting an acrolein-free, oxidizing gas mixture G comprising molecular oxygen, inert gas and optionally steam and also optionally CO through the fixed catalyst bed.