Patent ID: 6982347

Claim:
A process for partially oxidizing acrolein to acrylic acid in the gas phase under heterogeneous catalysis, comprising: passing a starting gas mixture which comprises acrolein, molecular oxygen and at least one inert gas containing at least 20% by volume of molecular nitrogen and contains molecular oxygen and the acrolein in a molar O 2 :C 3 H 4 O ratio of ≧0.5 through one reaction stage over a fixed catalyst bed which is arranged in two spatially successive reaction zones A,B, the temperature of reaction zone A being a temperature in the range from 230 to 320° C. and the temperature of reaction zone B likewise being a temperature in the range from 230 to 320° C., whose active composition in each reaction zone is at least one multimetal oxide comprising the elements Mo and V, in such a way that reaction zone A extends to an acrolein conversion ranging from 45 to 85 mol % and, on single pass of the starting gas mixture through the fixed catalyst bed, the acrolein conversion is ≧90 mol % and the selectivity to acrylic acid, based on acrolein converted is ≧90 mol %, the chronological sequence in which the starting gas mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones, wherein a) the hourly space velocity of the acrolein contained in the starting gas mixture over the fixed catalyst bed is ≦145 l (STP) of acrolein/l of fixed catalyst bed·h and ≧70 l (STP) of acrolein/l of fixed catalyst bed·h, b) the volume-specific activity of the fixed catalyst bed is either constant or increases at least once in the flow direction of the reaction gas mixture over the fixed catalyst bed, and c) the difference T maxA −T maxB , determined from the highest temperature T maxA which the reaction gas mixture has within the reaction zone A and the highest temperature T maxB which the reaction gas mixture has within reaction zone B, is ≧0° C.