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In a process for the preparation of acrolein or acrylic acid or a mixture thereof from propane, the propane is subjected, in a first reaction stage, to a partial dehydrogenation under heterogeneous catalysis to give propene, of the components contained in the product gas mixture formed in the first

In a process for the preparation of acrolein or acrylic acid or a mixture thereof from propane, the propane is subjected, in a first reaction stage, to a partial dehydrogenation under heterogeneous catalysis to give propene, of the components contained in the product gas mixture formed in the first reaction stage other than propene and propane at least a portion of the molecular hydrogen present is then separated off from said mixture and the product gas mixture is then used for the preparation of acrolein and/or acrylic acid by gas-phase catalytic propylene oxidation, molecular nitrogen being present for diluting the reaction gas mixture during the propylene oxidation.

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1. A process for the preparation of acrolein or acrylic acid or a mixture thereof from propane, in whichA) in a first stage A, the propane is subjected to a partial dehydrogenation under heterogeneous catalysis in the gas phase with formation of a product gas mixture A which contains molecular hydro

1. A process for the preparation of acrolein or acrylic acid or a mixture thereof from propane, in whichA) in a first stage A, the propane is subjected to a partial dehydrogenation under heterogeneous catalysis in the gas phase with formation of a product gas mixture A which contains molecular hydrogen, propylene and unconverted propane,B) of the components contained in the product gas mixture A and differing from propane and propylene, at least a portion of the molecular hydrogen is separated off from said mixture A of stage A, containing molecular hydrogen, propylene and unconverted propane, the mixture is then used as product gas mixture A′ in a second stage B for feeding at least one oxidation reactor and, in the at least one oxidation reactor, the propylene is subjected to a selective partial gas-phase oxidation with molecular oxygen under heterogeneous catalysis to give a product gas mixture B which contains acrolein or acrylic acid or a mixture thereof as the desired product, andC) in a third stage C, the desired product is separated off from the product gas mixture B obtained in the partial oxidation of the propylene in stage B and at least unconverted propane contained in the product gas mixture of stage B is recycled to the dehydrogenation stage A,wherein molecular nitrogen is present as diluent gas in the partial oxidation of the propylene in stage B. 2. A process as claimed in claim 1, wherein the feed gas mixture of the at least one oxidation reactor in the second stage B contains at least 5 mol %, based on propylene contained therein, of molecular nitrogen. 3. A process as claimed in claim 1, wherein the feed gas mixture of the at least one oxidation reactor in the second stage B contains at least 50 mol %, based on propylene contained therein, of molecular nitrogen. 4. A process as claimed in claim 1, wherein the feed gas mixture of the at least one oxidation reactor in the second stage B contains at least 100 mol %, based on propylene contained therein, of molecular nitrogen. 5. A process as claimed in claim 1, wherein the molar ratio of the amount of molecular nitrogen contained in the feed gas mixture of the at least one oxidation reactor in the second stage B to the amount of propane contained in the same feed gas mixture is at least 0.05. 6. A process as claimed in claim 1, wherein the molar ratio of the amount of molecular nitrogen contained in the feed gas mixture of the at least one oxidation reactor in the second stage B to the amount of propane contained in the same feed gas mixture is at least from 0.05 to 5. 7. A process as claimed in claim 1, wherein the molar ratio of the amount of molecular nitrogen contained in the feed gas mixture of the at least one oxidation reactor in the second stage B to the amount of propane contained in the same feed gas mixture is at least from 0.5 to 3. 8. A process as claimed in claim 1, wherein the composition of the feed gas mixture of the at least one oxidation reactor in the second stage B fulfills the following molar ratios:propane:propene:N 2 :O 2 :H 2 O: others=from 0.5 to 20: 1: from 0.1 to 40: from 0.1 to 10: from 0 to 20: from 0 to 1. 9. A process as claimed in claim 1, wherein the composition of the feed gas mixture of the at least one oxidation reactor in the second stage B fulfills the following molar ratios:propane:propene:N 2 :O 2 :H 2 O: others=from 2 to 10:1: from 0.5 to 20: from 0.5 to 5: from 0.01 to 10: from 0 to 1. 10. A process as claimed in claim 1, wherein the composition of the feed gas mixture of the at least one oxidation reactor in the second stage B fulfills the following molar ratios:propane:propene:N 2 :O 2 :H 2 O: others=from 3 to 6:1: from 1 to 10: from 1 to 3: from 0.1 to 2: from 0 to 0.5. 11. A process as claimed in claim 1, wherein the molar ratio of propylene contained in the product gas mixture A to molecular hydrogen contained in the product gas mixture A is ≦100. 12. A process as claimed in claim 1, wherei n the molar ratio of propylene contained in the product gas mixture A to molecular hydrogen contained in the product gas mixture A is ≦50. 13. A process as claimed in claim 1, wherein the molar ratio of propylene contained in the product gas mixture A to molecular hydrogen contained in the product gas mixture A is ≦10. 14. A process as claimed in claim 1, wherein the molar ratio of propylene contained in the product gas mixture A to molecular hydrogen contained in the product gas mixture A is ≧0.05. 15. A process as claimed in claim 1, wherein the propane conversion achieved in stage A is from 5 to 25 mol %, based on a single pass. 16. A process as claimed in claim 1, wherein the propane conversion achieved in stage A is from 10 to 20 mol %, based on a single pass. 17. A process as claimed in claim 15, wherein the propane to be dehydrogenated is diluted in stage A with steam. 18. A process as claimed in claim 17, wherein the molar ratio of steam to propane to be dehydrogenated is from 0.1 to 2. 19. A process as claimed in claim 15, wherein molecular hydrogen is added to the propane to be dehydrogenated. 20. A process as claimed in claim 19, wherein the molar ratio of molecular hydrogen to propane to be dehydrogenated is >0 and ≦5. 21. A process as claimed in claim 15, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out adiabatically. 22. A process as carried out in claim 15, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out in a fixed-bed reactor through which the flow is axial or radial. 23. A process as claimed in claim 15, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out in a tray reactor. 24. A process as claimed in claim 23, wherein the temperature of the reaction gas mixture throughout the tray reactor is 450 to 550° C. 25. A process as claimed in claim 23, wherein the tray reactor contains from 2 to 8 catalyst beds spatially in succession. 26. A process as claimed in claim 23, wherein molecular oxygen is added to the reaction gas mixture during the partial dehydrogenation of the propane under heterogeneous catalysis in stage A. 27. A process as claimed in claim 1, wherein the amount of molecular hydrogen present is separated off from product gas mixture A before it is further used as product gas mixture A′. 28. A process as claimed in claim 1, wherein the components other than propane and propylene are separated off from product gas mixture A before it is further used as product gas mixture A′. 29. A process as claimed in claim 1, wherein the components other than propane and propylene are separated off from the product gas mixture A by bringing the product gas mixture A into contact with an organic solvent, adsorbing the propane and propylene selectively therein, liberating them again by subsequent desorption, stripping or desorption and stripping, and using them as product gas mixture A′ for feeding the at least one oxidation reactor in stage B. 30. A process as claimed in claim 1, wherein air is concomitantly used for feeding the at least one oxidation reactor of stage B. 31. A process as claimed in claim 1, wherein acrolein and/or acrylic acid contained in the product gas mixture B are separated off from said mixture, and at least a portion of the remaining residual gas is recycled to the dehydrogenation stage A. 32. A process as claimed in claim 28, wherein acrolein and/or acrylic acid contained in the product gas mixture B are separated off from said mixture, and the total amount of the remaining residual gas is recycled to the dehydrogenation stage A. 33. A process as claimed in claim 1, wherein acrolein and/or acrylic acid contained in the product gas mixture B are first separated off from said mixture, the components other than propane and propylene are separated off from the remaining residual gas, and the remaining propane and propene are recycled to the dehydrogenation stage A. 34. A process as claimed in claim 1, wherein stage B consists of two tube-bundle reactors connected in series. 35. A process as claimed in claim 23, wherein the tray reactor comprises at least one catalyst bed which catalyzes the combustion of hydrogen. 36. A process as claimed in claim 1, which is carried out as a process for the preparation of acrolein. 37. A process as claimed in claim 1, which is carried out as a process for the preparation of acrylic acid. 38. A process as claimed in claim 16, wherein the propane to be dehydrogenated is diluted in stage A with steam. 39. A process as claimed in claim 16, wherein molecular hydrogen is added to the propane to be dehydrogenated. 40. A process as claimed in claim 16, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out adiabatically. 41. A process as carried out in claim 16, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out in a fixed-bed reactor through which the flow is axial or radial. 42. A process as claimed in claim 16, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out in a tray reactor. 43. A process as claimed in claim 42, wherein the temperature of the reaction gas mixture throughout the tray reactor is 450 to 550 C. 44. A process as claimed in claim 42, wherein the tray reactor contains from 2 to 8 catalyst beds spatially in succession. 45. A process as claimed in claim 42, wherein molecular oxygen is added to the reaction gas mixture during the partial dehydrogenation of the propane under heterogeneous catalysis in stage A. 46. A process as claimed in claim 16, wherein the tray reactor comprises at least one catalyst bed which catalyzes the combustion of hydrogen. 47. A process as claimed in claim 1, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out adiabatically. 48. A process as carried out in claim 1, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out in a fixed-bed reactor through which the flow is axial or radial. 49. A process as claimed in claim 1, wherein the partial dehydrogenation of the propane under heterogeneous catalysis in stage A is carried out in a tray reactor.