Process for long-term operation of a heterogeneously catalyzed partial gas phase oxidation of propene to acrolein

A process for the long-term operation of a heterogeneously catalyzed partial gas phase oxidation of propene to acrolein, comprising conducting a reaction gas input mixture which comprises propene, molecular oxygen and at least one inert gas through a fixed catalyst bed whose active material is at least one multimetal oxide comprising the elements Mo, Fe and Bi, with the proviso that:

• the reaction gas input mixture comprises the molecular oxygen and the propene in a molar O2:C3H6 ratio of ≧1,
• the fixed catalyst bed is arranged in two spatially successive temperature zones A, B,
• both the temperature TA of temperature zone A and the temperature TB of temperature zone B are a temperature in the temperature range from 280 to 420° C.,
• 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 CA of the propene present in the reaction gas input mixture in the range from 45 to 85 mol %, and the conversion of the propene increasing in temperature zone B to a value CB of ≧90 mol %,
• in a single pass of the reaction gas input mixture through the overall fixed catalyst bed, the selectivity of acrolein formation, based on propene converted, is ≧80 mol %,
• the space velocity of propene present in the reaction gas input mixture on the fixed catalyst bed is ≧140 l (STP) of propene/l of fixed catalyst bed·h,
• the temperatures TA and TB on completion of fresh charging of the fixed catalyst bed are such that the difference ΔTBA=TB−TA>0° C.,
• then, with increasing operating time, in order to counteract the reduction in the quality of the fixed catalyst bed, at least one of the two temperatures TA, TB is increased, wherein
• the increasing of the at least one of the two temperatures TA, TB is undertaken such that the difference ΔTBA=TB−TA increases with increasing operating time.