Patent ID: 7211691

Claim:
A process for partially oxidizing propene to acrylic acid in the gas phase under heterogeneous catalysis by conducting a starting reaction gas mixture comprising propene, molecular oxygen and at least one inert gas, and containing the molecular oxygen and the propene in a molar O 2 :C 3 H 6 ratio of ≧1, in a reaction stage over a fixed catalyst bed whose active composition is at least one multimetal oxide comprising the elements Mo, Fe and Bi, in such a way that the fixed catalyst bed is arranged in two spatially successive temperature zones A, B, both the temperature of temperature zone A and the temperature of temperature zone B are a temperature in the range from 290 to 380° C., the fixed catalyst bed consists of at least two spatially successive fixed catalyst bed zones, and the volume-specific activity within one fixed catalyst bed zone is substantially constant and increases sharply in the flow direction of the reaction gas mixture at the transition from one fixed catalyst bed zone to another fixed catalyst bed zone, the temperature zone A extends up to a conversion of the propene of from 40 to 80 mol %, on single pass of the starting reaction gas mixture through the entire fixed catalyst bed, the propene conversion is ≧90 mol % and the selectivity of acrolein formation based on converted propene is ≧90 mol %, the sequence in time in which the reaction gas mixture flows through the temperature zones A, B corresponds to the alphabetic sequence of the temperature zones, the hourly space velocity of the propene contained in the starting reaction gas mixture on the fixed catalyst bed is ≧90 l (STP) of propene/l of fixed bed catalyst·h the difference T maxA −T maxB , formed from the highest temperature T maxA which the reaction gas mixture has within temperature zone A, and the highest temperature T maxB which the reaction gas mixture has within temperature zone B is ≧0° C., wherein the transition from temperature zone A to temperature zone B in the fixed catalyst bed does not coincide with a transition from one fixed catalyst bed zone to another fixed catalyst bed zone.