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The present invention relates to a process for preparing unsaturated aldehydes or unsaturated carboxylic acids by heterogeneous catalytic gas phase oxidation of unsaturated or saturated hydrocarbons, comprising the process steps of: i) providing a gas mixture comprising a saturated hydrocarbon and c

The present invention relates to a process for preparing unsaturated aldehydes or unsaturated carboxylic acids by heterogeneous catalytic gas phase oxidation of unsaturated or saturated hydrocarbons, comprising the process steps of: i) providing a gas mixture comprising a saturated hydrocarbon and catalytically dehydrogenating the saturated hydrocarbon in the gas phase to obtain a gas mixture comprising an unsaturated hydrocarbon in a dehydrogenation reactor having a dehydrogenation catalyst material; or ii) providing a gas mixture comprising oxygen and an unsaturated hydrocarbon; iii) catalytically oxidizing the unsaturated hydrocarbon obtained in process step i) or provided in process step ii) in the gas phase to obtain a gas mixture comprising an unsaturated aldehyde in a first oxidation reactor having a first oxidation catalyst material; wherein at least one of the reactors selected from the dehydrogenation reactor, the first oxidation reactor and the second oxidation reactor comprises at least one foam body.

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1. A process for preparing unsaturated aldehydes or unsaturated carboxylic acids by heterogeneous catalytic gas phase oxidation of unsaturated or saturated hydrocarbons, comprising the process steps of: i) providing a gas mixture at least comprising a saturated hydrocarbon and catalytically dehydrog

1. A process for preparing unsaturated aldehydes or unsaturated carboxylic acids by heterogeneous catalytic gas phase oxidation of unsaturated or saturated hydrocarbons, comprising the process steps of: i) providing a gas mixture at least comprising a saturated hydrocarbon and catalytically dehydrogenating the at least one saturated hydrocarbon in the gas phase to obtain a gas mixture comprising an unsaturated hydrocarbon in a dehydrogenation reactor having a dehydrogenation catalyst material; orii) providing a gas mixture at least comprising oxygen and at least one unsaturated hydrocarbon wherein unsaturated hydrocarbon has a space velocity of at least 150 liters/hr;iii) catalytically oxidizing the unsaturated hydrocarbon obtained in process step i) or provided in process step ii) in the gas phase to obtain a gas mixture comprising an unsaturated aldehyde in a first oxidation reactor having a first oxidation catalyst material;iv) catalytically oxidizing the unsaturated aldehyde obtained in process step iii) in the gas phase to obtain a gas mixture comprising an unsaturated carboxylic acid in a second oxidation reactor having a second oxidation catalyst material;wherein at least one of the reactors selected from the dehydrogenation reactor, the first oxidation reactor and the second oxidation reactor comprises at least one open-cell foam body and the foam body is characterized by a cell width within a range of from 10 to 40 ppi, and wherein the foam bodies are present in the form of random packings, such random packings are characterized in that they have a diameter which is at most 50% and at least 5% of the diameter of the reactor, in the case of a tube bundle reactor, of the diameter of the particular reaction tubes, and, in the case of a wall reactor, of the distance between two adjacent plates, or in the form of blocks, such blocks are characterized in that they have a diameter which is at least 90% of the diameter of the reactor, in the case of a tube bundle reactor, of the diameter of the particular reaction tubes, and, in the case of a wall reactor, of the distance between two adjacent plates. 2. The process according to claim 1, wherein the process comprises process steps ii), iii) and iv), and the unsaturated hydrocarbon is propene, the unsaturated aldehyde acrolein and the unsaturated carboxylic acid acrylic acid. 3. The process according to claim 1, wherein the process comprises process steps ii), iii) and iv), and the unsaturated hydrocarbon is isobutene, the unsaturated aldehyde methacrolein and the unsaturated carboxylic acid methacrylic acid. 4. The process according to claim 1, wherein the process comprises process steps i), ii), iii) and iv), and the saturated hydrocarbon is propane, the unsaturated hydrocarbon propene, the unsaturated aldehyde acrolein and the unsaturated carboxylic acid acrylic acid. 5. The process according to claim 1, wherein the process comprises process steps i), ii), iii) and iv), and the saturated hydrocarbon is isobutane, the unsaturated hydrocarbon isobutene, the unsaturated aldehyde methacrolein and the unsaturated carboxylic acid methacrylic acid. 6. The process according to claim 1, wherein unsaturated hydrocarbon has a space velocity of at least 300 liters/hr. 7. The process according to claim 1, wherein the dehydrogenation reactor comprises the foam body. 8. The process according to claim 1, wherein the first oxidation reactor comprises the foam body. 9. The process according to claim 1, wherein the second oxidation reactor comprises the foam body. 10. The process according to claim 1, wherein the first and the second oxidation reactors comprise the foam body. 11. The process according to claim 1, wherein at least part of the surface of the open-cell foam body is coated with the dehydrogenation catalyst material, the first oxidation catalyst material, or the second oxidation catalyst material. 12. The process according to claim 10, wherein the open-cell foam body which has been coated with the catalyst material and has a cell width within a range from 10 to 40 ppi is obtainable by a process comprising the process steps of: A1) preparing a suspension composed of the catalyst material, a solvent or a solvent mixture and optionally further additives;A2) contacting at least part of the surface of the foam body with the suspension;A3) if appropriate discharging an excess of suspension;A4) if appropriate drying the foam body;A5) calcining the catalyst material. 13. The process according to claim 1, wherein the foam body itself is formed from a material with catalytic activity. 14. The process according to claim 1, wherein at least one of the reactors selected from the dehydrogenation reactor, the first oxidation reactor, the second oxidation reactor or a reactor for direct oxidation of saturated hydrocarbon to unsaturated carboxylic acids is a tube bundle reactor comprising at least two reaction tubes, and the dehydrogenation catalyst, the first oxidation catalyst, or the second oxidation catalyst, and the foam body or the foam body coated with the dehydrogenation catalyst, the first oxidation catalyst, or the second oxidation catalyst which catalyses the direct oxidation of a saturated hydrocarbon to an unsaturated carboxylic acid, or that formed from this catalyst material, is localized in the interior of the reaction tubes. 15. The process according to claim 14, wherein at least two tube bundle reactors are operated in parallel to one another. 16. The process according to claim 1, wherein at least one of the reactors selected from the dehydrogenation reactor, the first oxidation reactor, or the second oxidation reactor for direct oxidation of saturated hydrocarbon to unsaturated carboxylic acids is a wall reactor, and the dehydrogenation catalyst, the first oxidation catalyst, or the second oxidation catalyst, and the foam body or the foam body coated with the dehydrogenation catalyst, the first oxidation catalyst, the second oxidation catalyst or the catalyst material which catalyses the direct oxidation of a saturated hydrocarbon to an unsaturated carboxylic acid, or that formed from this catalyst material, is localized in the reaction chamber between two adjacent plates. 17. The process according to claim 1, wherein at least two separable foam bodies coated with a catalyst material or formed from this catalyst material are present in at least one of the reactors selected from the dehydrogenation reactor, the first oxidation reactor, the second oxidation reactor or a reactor for direct oxidation of saturated hydrocarbon to unsaturated carboxylic acids. 18. The process according to claim 17, wherein the at least two separable foam bodies coated with the catalyst material or formed from the catalyst material are present as random packings in a fixed bed or as foam body blocks. 19. The process according to claim 17, wherein at least two of the separable foam bodies of one reactor differ by their catalytic activity. 20. The process according to claim 19, wherein the dehydrogenation reactor, the first oxidation reactor, the second oxidation reactor or the reactor for direct oxidation comprise(s) the at least two separable foam bodies coated with the catalyst material or formed from the catalyst material, and wherein the at least two separable foam bodies coated with the catalyst material or formed from the catalyst material are arranged within the dehydrogenation reactor, the first oxidation reactor, the second oxidation reactor or the reactor for direct oxidation such that the catalytic activity per volume element increases in the direction from the reactant inlet to the product outlet. 21. A process for preparing purified unsaturated carboxylic acids, comprising the steps of: (I) preparing a gas mixture comprising an unsaturated carboxylic acid by a process according to claim 1;(II) absorbing the unsaturated carboxylic acid in a solvent to obtain a solution comprising an unsaturated carboxylic acid;(III) removing the unsaturated carboxylic acid from the solution comprising the unsaturated carboxylic acid by means of distillation, extraction, crystallization or a combination of these removal methods. 22. A process for preparing a polymer based on an unsaturated carboxylic acid, comprising the process steps of: (α1) preparing a purified unsaturated carboxylic acid by the process according to claim 21;(α2) free-radically polymerizing the unsaturated carboxylic acid.