Patent ID: 7211692

Claim:
A process for the long-term operation of a heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid, by initially conducting a starting reaction gas mixture 1 which comprises propene, molecular oxygen and at least one inert gas and contains the molecular oxygen and the propene in a molar O 2 :C 3 H 6 ratio of ≧1, in a first reaction stage, at elevated temperature, over a first fixed catalyst bed 1 whose catalysts are such that their active composition is at least one multimetal oxide containing molybdenum and/or tungsten and also at least one of the elements bismuth, tellurium, antimony, tin and copper, in such a way that the propene conversion in single pass is 93 mol % and the accompanying selectivity of acrolein formation and of acrylic acid by-production taken together are 90 mol %, optionally reducing the temperature of the product gas mixture 1 leaving the first reaction stage by direct and/or indirect cooling and optionally adding molecular oxygen and/or inert gas to the product gas mixture 1 , and then conducting the product gas mixture 1 , as a starting reaction gas mixture 2 which comprises acrolein, molecular oxygen and at least one inert gas and contains the molecular oxygen and the acrolein in a molar O 2 :C 3 H 4 O ratio of ≧0.5, in a second reaction stage, at elevated temperature, over a second fixed catalyst bed 2 whose catalysts are such that their active composition is at least one multimetal oxide containing the elements molybdenum and vanadium, in such a way that the acrolein conversion in single pass is ≧90 mol % and the selectivity of acrylic acid formation assessed over both reaction stages, based on propene converted, is ≧80 mol %, and by, in order to counteract the deactivation both of the fixed catalyst bed 1 and of the fixed catalyst bed 2 , independently increasing the temperature of the particular 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 2 is permanently ≧10° C., and, at a temperature of the fixed catalyst bed 1 of from 250 to 550° C. and a temperature of the fixed catalyst bed 2 of from 200 to 450° C. initially conducting a gas mixture G consisting of molecular oxygen, inert gas and optionally steam through the fixed catalyst bed 1 , and subsequently optionally through an intermediate cooler and finally through the fixed catalyst bed 2 , wherein said temperature increase is first based on the temperature of the fixed catalyst bed 2 set at the beginning of the process and subsequently based on said temperature after each said interruption and conducting of gas mixture G through said catalyst beds.