Heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid

A process for partially oxidizing propene to acrylic acid in the gas phase under heterogeneous catalysis by 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 O2:C3H6 ratio of ≧1 in a first reaction stage over a fixed catalyst bed 1 which is arranged in two spatially successive reaction zones A, B, the temperature of reaction zone A being a temperature in the range from 290 to 380° C. and the temperature of reaction zone B likewise being a temperature in the range from 290 to 380° C., and whose active composition is at least one multimetal oxide comprising the elements Mo, Fe and Bi, in such a way that reaction zone A extends to a propene conversion of from 40 to 80 mol % and the propene conversion on single pass through the fixed catalyst bed 1 is ≧90 mol % and the accompanying selectivity of acrolein formation and also of acrylic acid by-production taken together is ≧90 mol %, the temperature of the product gas mixture leaving the first reaction stage is optionally reduced by cooling and molecular oxygen and/or inert gas are optionally added to the product gas mixture, and then the product gas mixture, as a starting reaction gas mixture 2 comprising acrolein, molecular oxygen and at least one inert gas and containing the molecular oxygen and the acrolein in a molar O2:C3H4O ratio of ≧0.5, is conducted in a second reaction stage over a fixed catalyst bed 2 which is arranged in two spatially successive reaction zones C, D, the temperature of reaction zone C being a temperature in the range from 230 to 320° C. and the temperature of reaction zone D likewise being a temperature in the range from 230 to 320° C., and whose active composition is at least one multimetal oxide comprising the elements Mo and V, in such a way that reaction zone C extends to an acrolein conversion of from 45 to 85 mol % and the acrolein conversion on single pass through the fixed catalyst bed 2 is ≧90 mol % and the selectivity of acrylic acid formation assessed over all reaction zones, based on converted propene, is ≧80 mol %, the sequence in time in which the reaction gas mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones, wherein

• a) the hourly space velocity of the propene contained in the starting reaction gas mixture 1 on the fixed catalyst bed 1 is <160 l (STP) of propene/l of fixed catalyst bed 1·h and ≧90 l (STP) of propene/l of fixed catalyst bed 1·h;
• b) the volume-specific activity of the fixed catalyst bed 1 is either constant or increases at least once in the flow direction of the reaction gas mixture over the fixed bed catalyst bed 1;
• c) the difference TmaxA−TmaxB, formed from the highest temperature TmaxA which the reaction gas mixture has within reaction zone A and the highest temperature TmaxB which the reaction gas mixture has within reaction zone B is ≧0° C.;
• d) the hourly space velocity of the acrolein contained in the starting reaction gas mixture 2 on the fixed catalyst bed 2 is <145 l (STP) of acrolein/l of fixed catalyst bed 2·h and ≧70 l (STP) of acrolein/l of fixed catalyst bed 2·h;
• e) the volume-specific activity of the fixed catalyst bed 2 increases at least once in the flow direction of the reaction gas mixture over the fixed bed catalyst bed 2;
• and
• f) the difference TmaxC−TmaxD formed from the highest temperature TmaxC which the reaction gas mixture has within reaction zone C and the highest temperature TmaxD which the reaction gas mixture has within reaction zone D, is ≧0° C.