Patent ID: 7807853

Claim:
A process for heterogeneously catalyzed partial gas phase oxidation of propylene to acrylic acid, in which, in a first reaction zone, a starting reaction gas mixture 1 which comprises propylene and molecular oxygen as reactants and at least propane as an inert diluent gas and comprises the molecular oxygen and the propylene in a molar O 2 :C 3 H 6 ratio of ≧1 is first, in a first reaction stage at elevated temperature, conducted through at least one first catalyst bed whose catalysts have at least one multimetal oxide comprising Mo, Fe and Bi as the active composition in such a way that the propylene conversion in single pass through the catalyst bed is ≧90 mol % and the accompanying selectivity S AC of acrolein formation and of acrylic acid by-product formation together is ≧80 mol %, optionally the temperature of the product gas mixture 1 leaving the first reaction stage is reduced by direct cooling or by indirect cooling or by direct and indirect cooling, and, optionally secondary gas in the form of molecular oxygen or inert gas or molecular oxygen and inert gas is added to product gas mixture 1, and then product gas mixture 1, as a starting reaction gas mixture 2 which comprises acrolein and molecular oxygen as reactants and at least propane as an inert diluent gas and comprises 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 and with formation of a product gas mixture 2, is conducted through at least one second catalyst bed whose catalysts have at least one multimetal oxide comprising Mo and V as the active composition in such a way that the acrolein conversion in single pass through the catalyst bed is ≧95 mol % and the selectivity S AA of acrylic acid formation assessed over both reaction stages, based on propylene converted, is ≧70 mol %, wherein starting reaction gas mixture 1, based on the molar amount of propane present therein, comprises ≦3 mol % of cyclopropane and has been obtained by adding the propylene required for the process as a constituent of crude propylene which consists of propylene to an extent of ≧90% by weight and of propane and propylene to an extent of ≧97% by weight.