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What is claimed is: 1. 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 O2:C3H6 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...
What is claimed is: 1. 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 O2:C3H6 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 SAC 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 O2:C3H4O 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 SAA 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. 2. The process according to claim 1, wherein the acrolein conversion in single pass through the catalyst bed is ≧96 mol %. 3. The process according to claim 1, wherein the acrolein conversion in single pass through the catalyst bed is ≧97 mol %. 4. The process according to claim 1, wherein the acrolein conversion in single pass through the catalyst bed is ≧98 mol %. 5. The process according to claim 1, wherein the acrolein conversion in single pass through the catalyst bed is ≧99 mol %. 6. The process according to claim 1, wherein starting reaction gas mixture 1, based on the amount of propane present therein, comprises ≦2 mol % of cyclopropane. 7. The process according to claim 1, wherein starting reaction gas mixture 1, based on the amount of propane present therein, comprises ≦1 mol % of cyclopropane. 8. The process according to claim 1, wherein starting reaction gas mixture 1, based on the amount of propane present therein, comprises ≦0.2 mol % of cyclopropane. 9. The process according to claim 1, wherein starting reaction gas mixture 1, based on the amount of propane present therein, comprises ≦0.15 mol % of cyclopropane. 10. The process according to claim 1, wherein the propylene conversion in single pass through the catalyst bed is ≧92 mol %. 11. The process according to claim 1, wherein the propylene conversion in single pass through the catalyst bed is ≧94 mol %. 12. The process according to claim 1, wherein starting reaction gas mixture 1, based on the amount of propane present therein, comprises ≧10 molppb of cyclopropane. 13. The process according to claim 1, wherein starting reaction gas mixture 1, based on the amount of propane present therein, comprises ≧50 molppb of cyclopropane. 14. The process according to claim 1, wherein starting reaction gas mixture 1, based on the amount of propane present therein, comprises ≧1 molppm of cyclopropane. 15. The process according to claim 1, wherein the at least one multimetal oxide comprising Mo, Fe and Bi is one of the formula IV: Mo12BiaFebXc1Xd2Xe3Xf4On (IV) where X1=nickel and/or cobalt, X2=thallium, an alkali metal and/or an alkaline earth metal, X3=zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and/or tungsten, X4=silicon, aluminum, titanium and/or zirconium, a=from 0.5 to 5, b=from 0.01 to 5, c=from 0 to 10, d=from 0 to 2, e=from 0 to 8, f=from 0 to 10, and n=a number which is determined by the valency and frequency of the elements in IV other than oxygen. 16. The process according to claim 1, wherein the at least one multimetal oxide comprising Mo and V is one of the formula VII: Mo12VaXb1Xc2Xd3Xc4Xf5Xg6On (VII) where X1=W, Nb, Ta, Cr and/or Ce, X2=Cu, Ni, Co, Fe, Mn and/or Zn, X3=Sb and/or Bi, X4=one or more alkali metals, X5=one or more alkaline earth metals, X6=Si, Al, Ti and/or Zr, a=from 1 to 6, b=from 0.2 to 4, c=from 0.5 to 18, d=from 0 to 40, e=from 0 to 2, f=from 0 to 4, g=from 0 to 40, and n=a number which is determined by the valency and frequency of the elements in VII other than oxygen. 17. The process according to claim 1, wherein the volume-specific activity of the at least one first catalyst bed increases at least once over the length of the flow path in flow direction of starting reaction gas mixture 1. 18. The process according to claim 1, wherein the volume-specific activity of the at least one second catalyst bed increases at least once over the length of the flow path in flow direction of starting reaction gas mixture 2. 19. The process according to claim 1, wherein the at least one first catalyst bed is a fixed bed and its propene loading is ≧120 l (STP)/l·h and ≦250 l (STP)/l·h. 20. The process according to claim 1, wherein starting reaction gas mixture 1 comprises from 6 to 13% by volume of propylene. 21. The process according to claim 1, wherein starting reaction gas mixture 1 comprises from >0 to 35% by volume of H2O. 22. The process according to claim 1, wherein starting reaction gas mixture 1 comprises from ≧0.01% by volume of propane. 23. The process according to claim 1, wherein starting reaction gas mixture 1 comprises from ≧1% by volume of propane. 24. The process according to claim 1, wherein starting reaction gas mixture 1 comprises from ≧5 to ≦70% by volume of propane. 25. The process according to claim 1, wherein starting reaction gas mixture 1 comprises from ≧0.01% by volume of CO2. 26. The process according to claim 1, wherein starting reaction gas mixture 1 comprises from ≧1% by volume of N2. 27. The process according to claim 1, wherein the acrylic acid is removed in a separation zone 1 from product gas mixture 2 by conversion to the condensed phase. 28. The process according to claim 27, wherein the acrylic acid is converted from product gas mixture 2 into the condensed phase by absorptive measures. 29. The process according to claim 27, wherein the acrylic acid is converted from product gas mixture 2 into the condensed phase by condensative measures. 30. The process according to claim 27, wherein the acrylic acid is converted from product gas mixture 2 into the condensed phase by absorptive and condensative measures. 31. The process according to claim 28, wherein the absorbent used is water or an aqueous solution. 32. The process according to claim 27, wherein the acrylic acid is removed in a separation zone 2 using at least one thermal separation process from the condensed phase obtained in separation zone 1. 33. The process according to claim 27, wherein at least a portion of the residual gas remaining in the conversion of the acrylic acid from product gas mixture 2 into the condensed phase is recycled into the first reaction stage and/or into the second reaction stage. 34. The process according to claim 1, wherein the propylene present in starting reaction gas mixture 1 is fed to starting reaction gas mixture 1 at least partly from a partial dehydrogenation of propane. 35. The process according to claim 34, wherein at least a portion of the residual gas remaining in the conversion of the acrylic acid from product gas mixture 2 into the condensed phase is recycled into the partial dehydrogenation of propane. 36. The process according to claim 1, which is followed by a process for preparing polymers in which acrylic acid prepared by the process according to claim 1 is polymerized. 37. The process according to claim 1, which is followed by a process for preparing acrylic esters in which acrylic acid prepared by the process according to claim 1 is esterified with an alcohol. 38. The process according to claim 37, which is followed by a process for preparing polymers in which acrylic ester prepared by the process according to claim 37 is polymerized.
