DE10219686A1 - Production of methacrolein from isobutane comprises catalyzed partial oxidation of isobutene with molecular oxygen in a predetermined molar ratio to molecular nitrogen - Google Patents

Abstract

Providing a process for the production of methacrolein from isobutane. A process for the production of methacrolein from isobutane comprises: (A) partial, selective, heterogeneously catalyzed dehydrogenation of isobutane in the gas phase in a reaction zone (A) to form a mixture of isobutene and unreacted isobutane; (B) transport of the gas mixture to a reaction zone (B) followed by a selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase to form a methacrolein containing product mixture; (C) the methacrolein in the product mixture from (B) is either used for the production of methacrylic acid or other purpose and the non-reacted isobutane is either recycled to reaction zone (A) or used for another purpose and is characterized in that the molar ratio of molecular oxygen to molecular nitrogen is 1:1-1:10.

Description

• A) in einer Reaktionszone A das iso-Butan einer partiellen selektiven heterogen katalysierten Dehydrierung in der Gasphase unter Bildung eines Produktgasgemisches A, das iso-Buten und nicht umgesetztes iso-Butan enthält, unterwirft,
• B) iso-Butan und iso-Buten enthaltendes Produktgasgemisch A in eine Reaktionszone B führt und in der Reaktionszone B das iso-Buten einer selektiven heterogen katalysierten Partialoxidation mit molekularem Sauerstoff in der Gasphase unter Bildung eines Methacrolein enthaltenden Produktgasgemisches B unterwirft und
• C) das im Produktgasgemisch B enthaltene Methacrolein entweder zur Herstellung von Methacrylsäure oder für einen sonstigen Zweck verwendet und wenigstens im Produktgasgemisch B enthaltenes nicht umgesetztes iso-Butan entweder in die Reaktionszone A zurückführt oder für einen sonstigen Zweck verwendet.

The present invention relates to a process for the preparation of methacrolein from isobutane, in which

• A) in a reaction zone A, the isobutane is subjected to a partially selective, heterogeneously catalyzed dehydrogenation in the gas phase to form a product gas mixture A which contains isobutene and unreacted isobutane,
• B) product gas mixture A containing isobutane and isobutene leads into reaction zone B and in reaction zone B the isobutene is subjected to selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase to form a product gas mixture B containing methacrolein and
• C) the methacrolein contained in the product gas mixture B is used either for the production of methacrylic acid or for another purpose and at least unreacted isobutane contained in the product gas mixture B is either returned to the reaction zone A or used for another purpose.

Methacrolein is an important intermediate that used for example for the production of methacrylic acid. Methacrylic acid is an important basic chemical that is considered such and / or in the form of their methyl ester for the preparation of Polymers are used, for example in disperse Distribution in aqueous medium as a binder be applied.

DE-A 33 13 573 describes a method as described at the beginning known for the production of methacrolein from isobutane. As Source for the molecular needed in reaction zone B. DE-A 33 13 573 on page 28 recommends the use of oxygen of practically pure molecular oxygen. You can also use less pure molecular oxygen, but be the use of pure molecular oxygen is preferred.

A disadvantage of using essentially pure molecular oxygen as an oxygen source for reaction zone B. however, that the isobutane accompanying isobutane is then present in the Reaction zone B is not completely inert, but in one noticeable extent also partially and / or completely oxidized becomes. While the latter in particular an undesirable loss due to isobutane, partial oxidation products of the isobutane such as B. iso-butyraldehyde or iso-butyric acid in the Usually unwanted companions of methacrolein, which in its Subsequent uses usually interfere with and are difficult to achieve Methacrolein are separable (even if only in small amounts be formed).

The object of the present invention was therefore to begin with Process described for the production of methacrolein from iso- To provide butane, which has the disadvantages described only to a reduced extent.

• A) in einer Reaktionszone A das iso-Butan einer partiellen selektiven heterogen katalysierten Dehydrierung in der Gasphase unter Bildung eines Produktgasgemisches A, das iso-Buten und nicht umgesetztes iso-Butan enthält, unterwirft,
• B) iso-Butan und iso-Buten enthaltendes Produktgasgemisch A in eine Reaktionszone B führt und in der Reaktionszone B das iso-Buten einer selektiven heterogen katalysierten Partialoxidation mit molekularem Sauerstoff in der Gasphase unter Bildung eines Methacrolein enthaltenden Produktgasgemisches B unterwirft und
• C) das im Produktgasgemisch B enthaltene Methacrolein entweder zur Herstellung von Methacrylsäure oder für einen sonstigen Zweck verwendet und wenigstens im Produktgasgemisch B enthaltenes nicht umgesetztes iso-Butan entweder in die Reaktionszone A zurückführt oder für einen sonstigen Zweck verwendet,

gefunden, das dadurch gekennzeichnet ist, daß der in der Reaktionszone B benötigte molekulare Sauerstoff der Reaktionszone B in Begleitung von molekularem Stickstoff zugeführt wird, wobei das molare Verhältnis V von molekularem Sauerstoff zu molekularem Stickstoff 1 : 1 bis 1 : 10 beträgt. Accordingly, a process for the production of methacrolein from isobutane, in which

• A) in a reaction zone A, the isobutane is subjected to a partially selective, heterogeneously catalyzed dehydrogenation in the gas phase to form a product gas mixture A which contains isobutene and unreacted isobutane,
• B) product gas mixture A containing isobutane and isobutene leads into reaction zone B and in reaction zone B the isobutene is subjected to selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase to form a product gas mixture B containing methacrolein and
• C) the methacrolein contained in the product gas mixture B is used either for the production of methacrylic acid or for another purpose and at least unreacted isobutane contained in the product gas mixture B is either returned to the reaction zone A or used for another purpose,

found, which is characterized in that the molecular oxygen required in reaction zone B is fed to reaction zone B accompanied by molecular nitrogen, the molar ratio V of molecular oxygen to molecular nitrogen being 1: 1 to 1:10.

In other words, V can be 1: 2, or 1: 3, or 1: 4, or 1: 5, or 1: 6, or 1: 7, or 1: 8, or 1: 9, or 1:10. A V in the range from 1: 2 to 1: 8 or in the range from 1: 3 is favorable up to 1: 8, or 1: 4 to 1: 7 inventive method, the aforementioned accompaniment of molecular oxygen realized by molecular nitrogen so that the reaction zone B the molecular oxygen as Part of a gas that supplies the molecular oxygen and the molecular nitrogen already mentioned in one Ratios V contains, or only from molecular oxygen and molecular nitrogen in such a ratio V. Prefers is in the method according to the invention as a source for the in Reaction zone B required at least molecular oxygen partially, particularly preferably predominantly or exclusively Air used. However, it can e.g. B. the reaction zone B also air and additional molecular oxygen or air and additional molecular nitrogen or air and also a mixture of molecular nitrogen and molecular oxygen that the two Contains gas components in a ratio other than air, be fed. It is only essential to the invention that overall Ratio V is observed. According to the Reaction zone B so molecular oxygen and molecular Nitrogen can also be supplied separately.

That is, while according to the teaching of DE-A 33 13 573 the selective heterogeneously catalyzed partial oxidation of isobutene in the Reaction zone B takes place in a reaction gas mixture, which is mainly isobutene, isobutane and molecular oxygen contains, contains the reaction gas mixture in reaction zone B in the process according to the invention, isobutene is necessary, iso-butane, molecular oxygen and molecular nitrogen. The additional presence of the latter is guaranteed at Method according to the invention surprisingly a decrease in undesired conversion of isobutane to an undesirable by-product.

Of course, the feed gas mixture (= the mixture of all gas streams supplied to the reaction zone) of reaction zone B in the procedure according to the invention in addition to the components already mentioned, other components such as. B. CO, CO ₂ , H ₂ O, noble gases such as He and / or Ar, hydrogen, methane, ethylene, ethane, butanes, butenes, butynes, pentanes, propyne, allenes, propane, propylene, acrolein and / or methacrolein.

As a rule, the proportion of the feed gas mixture should Reaction zone B of molecular nitrogen, based on the in amount of isobutene contained in this feed gas mixture, not less than 500 mol% according to the invention. That is, at inventive method, the proportion of Feed gas mixture of reaction zone B on molecular Nitrogen, based on the amount of isobutene contained, at least 500 mol%, or at least 600 mol%, or at least Amount to 900 mol%. Usually the ratio of im Feed gas mixture of reaction zone B contained molar Amount of molecular nitrogen in the feed gas mixture Reaction zone B amount of isobutene according to the invention however ≤ 20: 1, often ≤ 12: 1.

The molar ratio of the in the feed gas mixture Reaction zone B contained amount of molecular nitrogen to im Feed gas mixture of reaction zone B contained amount Isobutane is generally used in the process according to the invention not less than 1: 1. Usually this will be However, the ratio should not be above 20: 1.

In other words, the molar ratio of im Feed gas mixture of reaction zone B contained amount molecular nitrogen in the feed gas mixture Reaction zone B amount of isobutane 1: 1 to 20: 1, or 2: 1 to 16: 1, or 2: 1 to 6: 1.

In the process according to the invention, the composition of the feed gas mixture of reaction zone B will often be selected so that it fulfills the following molar ratios:
isobutane: isobutene: N ₂ : O ₂ : H ₂ O: H ₂ : other = 10-40: 4-8: 20-80: 5-20: 0-20: 0-10: 0-5 ,

The aforementioned molar ratios are advantageously according to the invention = 10-25: 4-8: 40-80: 10-15: 5-15: 2-6: 0.1-3.

An essential feature of the procedure according to the invention is that in reaction zone A, in contrast to the case of a heterogeneous and / or heterogeneously catalyzed partial oxide hydrogenation of isobutane, at least intermediate molecular hydrogen is formed, which is why the product gas mixture A usually contains molecular hydrogen. In addition, the catalytic dehydrogenation in reaction zone A without additional measures endothermic, whereas catalytic oxide hydrogenation is exothermic runs. As a rule, in the method according to the invention molar ratio of that contained in product gas mixture A. Isobutene to the molecular hydrogen contained in product gas mixture A. ≤ 10, usually ≤ 5, often ≤ 3, often ≤ 2.

Normally the reciprocal of the above Do not exceed ratio 2. That is, usually at inventive method the molar ratio of im Iso-butene contained in product gas mixture A in product gas mixture A contained molecular hydrogen ≥ 0.5, mostly ≥ 0.8, many times ≥ 1.2, or ≥ 1.5.

In order to achieve interesting conversions in reaction zone A in the partial heterogeneously catalyzed dehydrogenation to be carried out according to the invention, based on a single pass, it is generally necessary to work at relatively high reaction temperatures (these reaction temperatures are typically 300 to 700 ° C.). Since dehydrogenation (cleavage of CH) is kinetically disadvantageous compared to cracking (cleavage of CC), it takes place on selectively acting catalysts. One hydrogen molecule is usually generated as a by-product per isobutene molecule formed. As a result of the selectively acting catalysts, which are usually designed so that they develop significant dehydrogenation in the absence of oxygen at the above-mentioned temperatures (e.g. at 600 ° C.) (with isobutane loads on the catalysts from e.g. . 1000 h ^-1 (Nl / l Kat.h) iso-butene yield is usually at least 30 mol% in a single pass (based on iso-butane used), by-products such as methane, ethylene, propane, propene and ethane only in minor quantities.

Since the dehydrogenation reaction takes place with an increase in volume, the conversion can be increased by lowering the partial pressure of the products. This can be done in a simple manner, for. B. by dehydration at reduced pressure and / or by admixing substantially inert diluent gases such as. B. Reach water vapor, which is normally an inert gas for the dehydrogenation reaction. A further advantage of dilution with water vapor is generally reduced coking of the catalyst used, since the water vapor reacts with coke formed according to the principle of coal gasification. In addition, water vapor can also be used as the diluent gas in the subsequent reaction zone B. According to the invention, it is entirely possible to use the total amount or even only a partial amount of the molecular nitrogen to be used in reaction zone B according to the invention for dilution in reaction zone A. Other suitable diluents for reaction zone A are e.g. B. CO, CO ₂ and noble gases such as He, Ne and Ar (in principle, reaction zone A can also be used without a diluent, ie the feed gas mixture of reaction zone A can only consist of isobutane or only isobutane and molecular oxygen ). All of the diluents mentioned can be used in reaction zone A either alone or in the form of a wide variety of mixtures. It is advantageous according to the invention that the diluents suitable for reaction zone A are generally also suitable diluents for reaction zone B. In general, inert (ie, less than 5 mol%, preferably less than 3 mol% and even better less than 1 mol%) chemically changing diluents are preferred in the respective reaction zone.

In principle, all come for reaction zone A according to the invention dehydrogenation catalysts known in the prior art Consideration. They can be roughly divided into two groups. Namely into those that are oxidic in nature (e.g. chromium oxide and / or Aluminum oxide) and those consisting of at least one on one, usually oxidic, carrier-deposited, usually comparatively noble, metal (e.g. platinum).

Among other things, all of them can be used for stage A according to the invention Dehydrogenation catalysts are used in the DE-A 100 60 099 (the exemplary embodiment), WO 99/46039, the US-A 4,788,371, EP-A 705 136, WO 99/29420, the US-A 5 220 091, US-A 5 430 220, US-A 5 877 369, the DE-A 100 47 642, EP-A 117 146, DE-A 199 37 106, DE-A 199 37 105, DE-A 102 11 275 and DE-A 199 37 107 be recommended. In particular, this document can be used for everyone appropriately addressed for the reaction zone A according to the invention Dehydrogenation process variants according to both the catalyst Example 1, and also according to Example 2, and also according to Example 3, as well as according to Example 4 of DE-A 199 37 107.

These are dehydrogenation catalysts that 10 to 99.9% by weight of zirconium dioxide, 0 to 60% by weight of aluminum oxide, Silicon dioxide and / or titanium dioxide and 0.1 to 10% by weight at least one element of the first or second main group, one Elements of the third subgroup, an element of the eighth Subgroup of the Periodic Table of the Elements, lanthanum and / or Contain tin, with the proviso that the sum of the Weight percent 100 wt .-% results.

That required in the context of the method according to the invention at least one catalyst bed (e.g. fluidized bed, moving bed or Fixed bed) can cause the dehydrogenation catalyst to differ Shaped geometries included. For the method according to the invention suitable geometries are e.g. B. forms such as grit, tablets, Monoliths, spheres or extrudates (strands, wagon wheels, stars, Rings). The length in the case of extrudates is expedient 2 to 15 mm, often 2 to 10 mm, often 6 to 10 mm and the The diameter of the extrudate cross section is advantageously 1 to 5 mm, often 1 to 3 mm. The wall thickness is in the case of rings advantageous 0.3 to 2.5 mm, their length 2 to 15 mm, often 5 to 15 mm and the diameter of their cross section 3 to 10 mm. On suitable molding process discloses e.g. B. DE-A 100 47 642 and DE-A 199 37 107. It is based on the fact that oxidic Carrier materials with concentrated mineral acid (e.g. concentrated Nitric acid), knead relatively well and into an extruder or extruder have the corresponding molded article transferred.

The moldings are then dried, calcined and then pour salt solutions over it in a specific order. Finally, it is dried and calcined again.

The reaction zone A relevant for the process according to the invention can in principle be implemented in all reactor types known from the prior art for heterogeneously catalyzed partial dehydrogenations of hydrocarbons in the gas phase over fixed bed catalysts. Typical reaction temperatures are 200 to 800 ° C or 400 to 650 ° C. The working pressure is typically in the range of 0.5 to 10 bar. Typical catalyst loads with reaction gas are 300 to 16000 h ^-1 .

To carry out reaction zone A of the invention In principle, all methods known in the prior art come Reactor types and process variants into consideration. descriptions Such process variants contain z. B. all regarding Prior art documents called dehydrogenation catalysts.

A comparatively detailed description of Dehydrogenation processes suitable according to the invention also contain "Catalytica® Studies Division, Oxidative Dehydrogenation and Alternative Dehydrogenation Processes, Study Number 4192 OD, 1993, 430 Ferguson Drive, Mountain View, California, 94043-5272 U.S.A. ".

Characteristic of the partially heterogeneously catalyzed Dehydrogenation of isobutane is that it is endothermic. That is, the for setting the required reaction temperature required heat (energy) must either be the reaction gas beforehand and / or supplied in the course of the catalytic dehydrogenation or in Reaction gas mixture are generated.

It is also suitable for heterogeneously catalyzed dehydrogenation of iso- Butane due to the high reaction temperatures required typically that high-molecular high-boiling in small quantities organic compounds, right down to carbon that deposit on the catalyst surface and deactivate it. To this disadvantageous Minimizing side effects can, as already mentioned, lead to catalytic dehydrogenation at elevated temperature over the Iso-butane to be catalyzed with steam be diluted. Deposited carbon is among the so given conditions according to the principle of coal gasification partially or completely eliminated.

Another way to get deposited carbon compounds Eliminate is to remove the dehydrogenation catalyst from time to time Time at elevated temperature with an oxygen-containing To flow through gas and thus the deposited carbon practically burn off. Suppression of education Carbon deposits are also possible by the fact that Iso-butane to be catalytically dehydrated before being elevated Temperature is passed over the dehydrogenation catalyst, molecular hydrogen.

Of course there is also the possibility of water vapor and molecular hydrogen to be catalytically dehydrogenated Add isobutane in a mixture. An addition of molecular hydrogen for the catalytic dehydrogenation of isobutane also reduces the undesirable formation of all, acethylene and others Coal precursors as by-products.

In the majority of the known methods the heterogeneous catalyzed partial dehydrogenation of dehydrogenated Hydrocarbons such as isobutane will reduce the heat of dehydrogenation outside the Reactor generated and fed to the reaction gas from the outside. This however requires complex reactor and process concepts and leads to steep, especially with high sales Temperature gradients in the reactor, with the control disadvantage of one increased by-product formation.

Alternatively, the heat of dehydrogenation can also be directly in the Reaction gas itself can be generated by adding molecular oxygen either formed during dehydration or additionally supplied hydrogen exothermic to water vapor burns. For this purpose, the reaction gas before and / or after entering the reaction zone containing the dehydrogenation catalyst molecular oxygen-containing gas and optionally Added hydrogen. Either the dehydrogenation catalyst itself (this applies to most dehydrogenation catalysts) and / or additionally introduced oxidation catalysts generally facilitate the Desired hydrogen oxidation (cf. DE-A 100 28 582). Heat of reaction released in this way by means of hydrogen combustion enables the complete waiver of one in favorable cases indirect reactor heating and thus comparatively simple Process concepts as well as limited with high sales Temperature gradients in the reactor.

With the aforementioned procedure, one can also share of external molecular hydrogen e.g. B. waive if to apply the principle of DE-A 102 11 275.

According to this process principle, the catalytic Dehydrogenation zone (here the reaction zone A) and the at least one containing hydrocarbon to be dehydrogenated (here isobutane) Reaction gas fed continuously. The reaction gas is in the catalytic dehydrogenation zone over at least one Fixed catalyst bed, on which by catalytic dehydrogenation molecular hydrogen and partially at least one dehydrated Hydrocarbon (here iso-butene) are formed. Before and / or after entering the catalytic dehydrogenation zone Reaction gas containing at least one molecular oxygen gas added, which in the catalytic dehydrogenation zone in Reactive gas contained molecular hydrogen at least partially oxidized to water vapor. Then the catalytic Dehydrogenation zone taken from a product gas mixture, the molecular Hydrogen, water vapor, the at least one dehydrated Hydrocarbon and the at least one to be dehydrogenated Contains hydrocarbon in two subsets of identical composition split and one of the two subsets becomes the source for molecular hydrogen into the catalytic dehydrogenation zone recycled (cycle gas). The other subset would be the The inventive method as product gas mixture A Feed reaction zone B.

The reaction zone A can in the process according to the invention of course, be designed so that in the direction of flow of the Reaction gas still on the fixed bed of the dehydrogenation catalyst a fixed catalyst bed follows, on the one contained in the reaction gas molecular hydrogen with molecular oxygen selective heterogeneously catalyzed at least partially burned to water vapor is so that the product gas mixture A in the invention Process can be largely or completely free of hydrogen. Suitable catalysts in this regard, for example, disclose US-A 4788371, US-A 4886928, US-A 5430209, the US-A 55530171, US-A 5527979, EP-A 832056 and the US-A 5563314. By final cooling in the reaction zone A can condensate water vapor contained in the gas mixture (and if necessary, returned to reaction zone A) and so in product gas mixture A essentially free of water vapor is produced become. Separations going beyond that are to be on the way to Product gas mixture A in the process according to the invention not be made.

A suitable reactor shape for the reaction zone according to the invention A is the fixed bed tube or tube bundle reactor. That is, the Dehydrogenation catalyst and optionally more specific Hydrogen oxidation catalyst, such as. B. in the scriptures US-A 4788371, US-A 4886928, US-A 5430209, US-A 5550171, US-A 5527979, US-A 5563314 and EP-A 832056, is in a reaction tube or in a bundle of Reaction tubes as a fixed bed. The reaction tubes are usually indirectly heated in that in the surrounding the reaction tubes Space a gas, e.g. B. burned a hydrocarbon such as methane becomes. It is convenient to use this indirect form of heating only at first about 20 to 30% of the fixed bed apply and the remaining fill length by the in the frame the radiant heat released on combustion to the required reaction temperature to warm up. The indirect heating of the Reaction gas can advantageously be heated directly by combustion with molecular oxygen in the reaction gas be coupled. This way is comparatively easier Form an approximately isothermal reaction can be achieved. Suitable reaction tube inner diameters are about 10 to 15 cm. A typical tube bundle dehydrogenation reactor comprises 300 to 1000 Reaktionssrohre. The temperature inside the reaction tube is moving in the range of 300 to 700 ° C, preferably in the range of 400 to 700 ° C. The working pressure is usually in the range of 0.5 to 8 bar, often at 1 to 2 bar or also at 3 to 8 bar. With The reaction gas is advantageous to the tubular reactor Reaction temperature supplied preheated. Usually that leaves Product gas mixture the reaction tube with another (higher or lower) temperature than the inlet temperature (see also US-A 4902849, US-A 4996387 and US-A 5389342). As part of the The aforementioned procedure is the use of oxidic Dehydrogenation catalysts based on chromium and / or Aluminum oxide useful. Often you do not become a diluent gas use, but of essentially pure isobutane as Run out starting reaction gas. The dehydrogenation catalyst will also mostly applied undiluted.

Typical catalyst loads with isobutane are 500 to 2000 h ^-1 (= Nl / l catalyst.h).

On an industrial scale, you would have about three tube bundle reactors in parallel operate, two of these reactors would usually in Dehydration operation while in one of the reactors Catalyst feed is regenerated.

Of course, reaction zone A according to the invention can also be designed in a walking bed. For example, that Catalyst moving bed can be accommodated in a radial flow reactor. In the catalyst moves slowly from top to bottom while the reaction gas mixture flows radially. This The procedure is, for example, in the so-called UOP-Oleflex dehydrogenation process applied. Because the reactors in this process are quasi Operated adiabatically, it is advisable to use several reactors to be operated in series (typically up to four). This allows excessive temperature differences of the reaction gas mixture at the reactor inlet and at the reactor outlet avoid (this works in the adiabatic mode Reaction gas starting mixture as a heat transfer medium, from the Heat content the reaction temperature is dependent) and still achieve attractive overall sales.

When the catalyst bed has left the moving bed reactor, it is fed to the regeneration and then reused. As a dehydrogenation catalyst for this process, for. B. a spherical dehydrogenation catalyst can be used, which consists essentially of platinum on a spherical alumina support. In order to avoid premature catalyst aging, hydrogen is expediently added to the isobutane to be dehydrogenated. The working pressure is typically 1 to 5 bar. The hydrogen to iso-butane ratio (the molar) is expediently 0.1 to 1. The reaction temperatures are preferably 550 to 650 ° C. and the catalyst loading with the reaction gas mixture is selected to be about 200 to 1000 h ^-1 . The catalyst feed can also consist of a mixture of dehydrogenation and H ₂ oxidation catalysts, as recommended by EP-A 832056.

In the fixed bed process described, the Catalyst geometry also spherical, but also cylindrical (hollow or being full.

As a further process variant for the invention Reaction zone A describes Proceedings De Witt, Petrochem. Review, Houston Texas, 1992 a, N1 the possibility of a heterogeneous catalyzed fluidized bed dehydrogenation in which the iso-butane is not diluted.

Appropriately, two fluidized beds are next to each other operated, one of which is usually in the state of Regeneration located. Chromium oxide is used as the active substance Alumina for use. The working pressure is typically 1 to 1.5 bar and the dehydration temperature is usually 550 up to 600 ° C. The heat required for dehydration will introduced into the reaction system in that the Dehydrogenation catalyst is preheated to the reaction temperature. The Working pressure is regularly 1 to 2 bar and the The reaction temperature is typically 550 to 600 ° C. The above Dehydration is also known in the literature as Snamprogetti-Yarsintez Process known.

As an alternative to the procedures described above the reaction zone A according to the invention also according to one of ABB Process developed by Lummus Crest (cf. Proceedings De Witt, Petrochem. Review, Houston Texas, 1992, P1).

The heterogeneously catalyzed described so far The dehydrogenation process of isobutane is common in that it is used in isobutane conversions > 30 mol% (usually ≤ 70 mol%) are operated (related on a single reactor pass).

According to the invention it is advantageous that it for process according to the invention is sufficient in reaction zone A Isobutane conversion of ≥ 5 mol% to ≤ 30 mol% or ≤ 25 mol% achieve. In other words, according to the invention, reaction zone A can also be used for Isobutane conversions of 10 to 30 mol% are operated (the Revenues relate to a single reactor run). This is moving among other things, therefore, that the remaining amount of not implemented iso-butane in the subsequent reaction zone B with molecular nitrogen is diluted, which is the iso-butyraldehyde and / or Iso-butyric acid by-product formation is reduced.

It is for the realization of the aforementioned iso-butane conversions favorable, the iso-butane dehydrogenation according to the invention in the Reaction zone A at a working pressure of 0.3 to 3 bar perform. It is also favorable to use the isobutane to be dehydrogenated Dilute water vapor. So the heat capacity of the Water on the one hand is part of the impact of the endothermic energy Balancing dehydration and reducing the other hand Dilution with water vapor the educt and product partial pressure, which is has a favorable effect on the equilibrium position of the dehydration. Furthermore, the use of water vapor has the same effect as before mentioned, advantageously on the life of the dehydrogenation catalyst out. If necessary, it can also be a molecular component Hydrogen can be added. The molar ratio of Molecular hydrogen to iso-butane is usually ≤ 5 molar ratio of water vapor to iso-butane can therefore be in the Reaction zone A variant with comparatively little Isobutane conversion ≥ 0 to 30, expediently 0.1 to 2 and favorably 0.5 to 1 be. As favorable for a procedure with low Iso-butane conversion also proves that with a single Reactor passage of the reaction gas is only a comparatively low one Amount of heat is consumed and to achieve sales with a one-off Reactor passage comparatively low reaction temperatures are sufficient.

It is therefore expedient according to the invention in reaction zone-A- Variant with a comparatively low isobutane conversion to carry out iso-butane dehydrogenation (quasi) adiabatically. That is, you will Reaction gas starting mixture usually to a temperature of Heat 500 to 700 ° C (e.g. by direct firing the in a wall surrounding the preheater), or to 550 to 650 ° C. in the Normally, a single flexible passage through one Catalyst bed should be sufficient to achieve the desired conversion achieve, the reaction gas mixture to about 30 ° C to 200 ° C (depending on sales) will cool. A presence of water vapor as a heat transfer medium makes one from the point of view adiabatic driving style noticeably noticeable. The lower one Reaction temperature enables longer service life of the used Catalyst bed.

In principle, the invention is also Reaction zone A variant with comparatively low isobutane conversion, whether run adiabatically or isothermally, both in a fixed bed reactor and can also be carried out in a moving bed or fluidized bed reactor.

Remarkably, their realization, especially in the adiabatic operation, a single shaft furnace reactor as Fixed bed reactor sufficient, that of the reaction gas mixture axially and / or is flowed through radially.

In the simplest case, it is one Reaction tube whose inside diameter is 0.1 to 10 m, possibly also 0.5 is up to 5 m and in which the fixed catalyst bed on a Carrier device (z. B. a grating) is applied. That with Catalyst-fed reaction tube that is in adiabatic operation can be thermally insulated with the hot, iso-butane containing, reaction gas flows axially. The Catalyst geometry can be either spherical, strand-like or be circular. The catalyst is advantageously in the the aforementioned case can also be used in split form. For realization a radial flow of the reaction gas containing isobutane can the reactor z. B. from two in a jacket, Centrally nested cylindrical gratings exist and the catalyst bed is arranged in the annular gap his. In the adiabatic case, the jacket would again be thermal isolated.

As a catalyst feed for the invention Reaction zone A variant with a comparatively low iso-butane conversion single pass are particularly suitable in the DE-A 199 37 107, especially all of them as examples disclosed catalysts.

After a long period of operation, the aforementioned catalysts are e.g. B. can be regenerated in a simple manner by diluting air at a temperature of 300 to 900 ° C, often at 400 to 800 ° C, often at 500 to 700 ° C, first in the first regeneration stages with nitrogen and / or steam Catalyst bed conducts. The catalyst load with regeneration gas can, for. B. 50 to 10000 h ^-1 and the oxygen content of the regeneration gas 0.5 to 20 vol .-%.

In subsequent further regeneration stages, air can be used as the regeneration gas under otherwise identical regeneration conditions. Appropriately from an application point of view, it is advisable to purge the catalyst with inert gas (for example N ₂ ) before it is regenerated.

Afterwards it is usually recommended to use pure molecular hydrogen or with diluted by inert gas molecular hydrogen (the hydrogen content should be ≥ 1 vol .-% to regenerate) in the otherwise identical condition grid. It is often advantageous to use the regeneration procedure several times to perform one after the other.

The reaction zone A variant according to the invention with a comparatively low iso-butane conversion (30 30 mol%) can in all cases be operated with the same catalyst loads (both the reaction gas as a whole and also the isobutane contained in the same) as the variants with high isobutane conversion (> 30 mol%). This exposure to reaction gas can e.g. B. 100 to 10000 h ^-1 , often 100 to 3000 h ^-1 , that is, often about 100 to 2000 h ^-1 .

The inventive method can be used in a particularly elegant manner Reaction zone A variant with comparatively little Realize iso-butane conversion in a tray reactor.

This contains more than one die spatially in succession Catalyst bed catalyzing dehydrogenation. The The number of catalyst beds can be 1 to 20, advantageously 2 to 8, but also 4 to 6 be. The catalyst beds are preferably radial or axial arranged one behind the other. Appropriately from an application point of view such a fixed-bed type tray reactor applied.

In the simplest case, the fixed catalyst beds are in one Shaft furnace reactor axially or in the annular gaps from centric cylindrical grids placed one inside the other.

The reaction gas mixture is expediently on its way from one catalyst bed to the next catalyst bed, e.g. B. by passing over heated with hot gases Heat exchanger fins or by passing through pipes heated with hot fuel gases, subjected to reheating in the tray reactor.

If the tray reactor is operated adiabatically, it is for the desired iso-butane conversions (≤ 30 mol%) in particular Use of those described in DE-A 199 37 107 Catalysts, in particular of the exemplary embodiments, sufficient, the reaction gas mixture to a temperature of 450 to preheated to 550 ° C in the dehydrogenation reactor and to keep within the tray reactor in this temperature range. That is, the total iso-butane dehydrogenation is so extreme to realize low temperatures, which can affect the service life of the Fixed catalyst beds proved to be particularly cheap.

It is even more clever, the one described above Perform reheating in a direct way. To do this Reaction gas mixture either before flowing through the first Catalyst bed and / or between the subsequent catalyst beds to a limited extent molecular oxygen above one containing gas added. Depending on the used Dehydrogenation catalyst is limited combustion in the Reaction gas mixture contained hydrocarbons, if necessary already Coal deposited on the catalyst surface or coal-like compounds and / or from in the course of Isobutane dehydrogenation formed and / or added to the reaction gas mixture Hydrogen causes (it can also be useful from an application point of view be to insert catalyst beds in the tray reactor, which with Are charged catalyst, the specific (selective) Combustion of hydrogen (and / or hydrocarbon) catalyzed (As such catalysts come e.g. those of the writings US-A 4 788 371, US-A 4886928, US-A 5430209, US-A 55 530 171, US-A 5 527 979 and US-A 5 563 314); for example such catalyst beds in an alternating manner to the Beds containing dehydrogenation catalyst in the tray reactor be housed). The heat of reaction released thereby enables an almost isothermal mode of operation in a quasi-autothermal way the heterogeneously catalyzed isobutane dehydrogenation. With increasing selected residence time of the reaction gas in the catalyst bed is such an iso-butane dehydration with decreasing and in substantially constant temperature possible, which is particularly long Downtimes enabled.

As a rule, an oxygen feed as described above should be carried out in such a way that the oxygen content of the reaction gas mixture, based on the amount of isobutane and isobutene contained therein, is 0.5 to 10% by volume. Both pure molecular oxygen or with inert gas, e.g. B. CO, CO ₂ , N ₂ , noble gases, diluted oxygen, but especially air. The resulting combustion gases usually have an additional diluting effect and thereby promote the heterogeneously catalyzed isobutane dehydrogenation.

The dehydrogenation temperature in the tray reactor in the process according to the invention is generally 400 to 800 ° C., the pressure in general 0.2 to 10 bar, preferably 0.5 to 4 bar and particularly preferably 1 to 2 bar. The total gas load (GHSV) is usually 500 to 2000 h ^-1 , in high-load operation also up to 16000 h ^-1 , regularly 4000 to 16000 h ^-1 .

The isothermal of the heterogeneously catalyzed isobutane dehydrogenation can be further improved in that in the tray reactor in the spaces between the catalyst beds closed, in front of their Filling evacuated internals (e.g. tubular), attaches. Of course, such internals can also be used in the respective Be placed catalyst bed. These internals included suitable solids or liquids above one evaporate or melt at a certain temperature and thereby heat consume and where this temperature falls below again condense, releasing heat.

One possibility of heating the reaction gas mixture in reaction zone A of the process according to the invention to the required reaction temperature is also to burn part of the isobutane and / or H ₂ contained therein by means of molecular oxygen (e.g. on suitable combustion catalysts having a specific action, e.g. by simply passing it over and / or passing it through) and using the heat of combustion released in this way to bring it to the desired reaction temperature. The resulting combustion products such as CO ₂ , H ₂ O and the N ₂ which may accompany the molecular oxygen required for the combustion advantageously form inert diluent gases.

It goes without saying that reaction zone A according to the invention can also be used in a jet pump cycle reactor, like the one he did DE-A 102 11 275 describes. In general, everyone is in the DE-A 102 11 275 described dehydrogenation variants in the reaction zone A according to the invention applicable.

It is essential according to the invention that the isobutane used in reaction zone A need not be pure isobutane. Rather, the isobutane used can contain up to 50% by volume of other gases such as e.g. B. ethane, methane, ethylene, n-butanes, n-butenes, propyne, acetylene, propane, propene, H ₂ S, SO ₂ , pentanes, etc. contain. The crude isobutane to be used advantageously contains at least 60% by volume, advantageously at least 70% by volume, preferably at least 80% by volume, particularly preferably at least 90% by volume and very particularly preferably at least 95% by volume. on isobutane. In particular, a mixture of isobutane, isobutene and circulating gas originating from reaction zone B can also be used for reaction zone A according to the invention.

The product gas mixture A leaving reaction zone A in the process according to the invention contains at least the constituents isobutane and isobutene and, as a rule, molecular hydrogen. In addition, however, it will normally also contain gases from the group comprising N ₂ , H ₂ O, methane, ethane, ethylene, propane, propene, CO and CO ₂ and, if appropriate, O ₂ .

It will usually be at a pressure of 0.3 to 10 bar and often have a temperature of 450 to 500 ° C. In general, reactors with passivated inner walls are used for reaction zone A according to the invention. The passivation can e.g. B. done by applying sintered aluminum oxide to the inner wall before the dehydrogenation or a silicon-containing steel is used as the reactor material, which forms a passivating SiO ₂ layer on the surface under reaction conditions. However, it can also be effected in situ by adding small amounts of passivating auxiliaries (for example sulfides) to the feed gas mixture in reaction zone A.

Product gas mixture A leaving reaction zone A, which Contains isobutane and isobutene, is according to the invention Procedure used to feed reaction zone B to the same the iso-butylene of a selective heterogeneous catalyzed gas phase partial oxidation with molecular oxygen to subject a product gas mixture containing methacrolein. If necessary, the product gas mixture A is passed through beforehand indirect heat exchange to that required in reaction zone B. Bring reaction temperature.

Corresponding ones to be used in reaction zone B. Multimetal oxide catalysts have been described many times and are known to the person skilled in the art well known. As a rule, catalysts are placed on the Basis of the element combination containing Mo-Bi-Fe Multimetal oxides preferred according to the invention.

For example, US-A 4954650, US-A 5166119 disclose the DE-A 101 21 592 (multimetal oxide compositions of the formulas I, II and III in the same DE-A), DE-A 100 46 957 (Multimetalloxidmassen der Formulas I and II in the same DE-A), DE-A 101 01 695 (Multimetal oxide compositions of the formulas I, II and III in the same DE-A), DE-A 100 63 162 (multimetal oxide compositions of the formula I in same DE-A), DE-A 100 59 713 (multimetal oxide masses of Formula I in the same DE-A) and DE-A 100 49 873 (Multimetal oxide compositions of the formula I in the same DE-A) for reaction zone B suitable multimetal oxide catalysts.

A large number of the multimetal oxide materials suitable as catalysts for reaction zone B can be obtained under the general formula I.

Mo ₁₂ Bi _a Fe _b X ¹ _c X ² _d X ³ _e X ⁴ _f O _n (I),

in which the variables have the following meaning:
X ¹ = nickel and / or cobalt,
X ² = an alkali metal, thallium and / or an alkaline earth metal,
X ³ = zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and / or tungsten,
X ⁴ = silicon, aluminum, titanium and / or zirconium,
a = 0.5 to 5,
b = 0.01 to 5, preferably 2 to 4,
c = 0 to 10, preferably 3 to 10,
d = 0 to 2, preferably 0.02 to 2,
e = 0 to 8, preferably 0 to 5,
f = 0 to 10 and
n = a number which is determined by the valency and frequency of the elements in I other than oxygen,
subsumed.

They are available in a manner known per se (cf. e.g. DE-A 101 21 592) and are usually made into spheres, Rings or cylinders shaped or in the form of Shell catalysts, d. that is, coated with the active composition preformed, inert carrier bodies used.

Suitable full catalyst geometries are e.g. B. full cylinder or Hollow cylinder with an outer diameter and a length of 2 to 10 mm. In the case of the hollow cylinder, a wall thickness of 1 to 3 mm appropriate. Of course, the full catalyst can also Have spherical geometry, the ball diameter 2 to 10 mm can be. Suitable shell catalyst geometries are disclosed likewise DE-A 101 21 592.

Multimetal oxide compositions which are favorable according to the invention as catalysts for reaction zone B are also compositions of the general formula II

[Y ¹ _a 'Y ² _b ' O _x '] _p [Y ³ _c ' Y ⁴ _d 'Y ⁵ _e ' Y ⁶ _f 'Y ⁷ _g ' Y ² _h 'O _y '] _q (II),

in which the variables have the following meaning:
Y ¹ = only bismuth or bismuth and at least one of the elements tellurium, antimony, tin and copper,
Y ² = molybdenum and / or tungsten,
Y ³ = an alkali metal, thallium and / or samarium,
Y ⁴ = an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and / or mercury,
Y ⁵ = iron or iron and at least one of the elements chromium, cerium and vanadium,
Y ⁶ = phosphorus, arsenic, boron and / or antimony,
Y ⁷ = a rare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and / or uranium,
a '= 0.01 to 8,
b '= 0.1 to 30,
c '= 0 to 4,
d '= 0 to 20,
e '> 0 to 20,
f '= 0 to 6,
g '= 0 to 15,
h '= 8 to 16,
x ', y' = numbers which are determined by the valency and frequency of the elements in II other than oxygen and
p, q = numbers whose ratio p / q is 0.1 to 10.

In favorable cases, the multimetal oxide masses II contain three-dimensionally extended areas of the chemical composition Y ¹ _a 'Y ² _b ' O _x ', the size of which (longest going through the center of gravity of the area), separated from its local environment due to its composition which is different from its local environment Connection distance between two points on the surface (interface) of the area) 1 nm to 100 µm, often 10 nm to 500 nm or 1 µm to 50 or 25 µm.

With regard to the shape, multimetal oxide materials apply II catalysts in the multimetal oxide masses I catalysts said.

The production of multimetal oxide mass II catalysts is z. B. in EP-A 575897, DE-A 198 55 913, DE-A 100 46 957 and DE-A 101 21 592.

The simplest form of implementation of reaction zone B is a Tube bundle reactor, the contact tubes of which are charged with catalyst are. The design can be completely analogous to how it is EP-A 911313, EP-A 979813, EP-A 990636 and Teach DE-A 28 30 765 for the partial propylene oxidation to acrolein. Otherwise, reaction zone B can be designed as it is teach US-A 4954650 and US-A 5166119.

The reaction temperature is usually 250 to 450 ° C. The reaction pressure is advantageously 0.5 to 5, advantageously 1 to 3 bar. The load (Nl / lh) of the oxidation catalysts is often 1500 to 2500 h ^-1 or 4000 h ^-1 .

In principle, reaction zone B can also be designed how it z. B. in DE-A 198 37 517, DE-A 199 10 506, the DE-A 199 10 508 and DE-A 198 37 519 for analog reactions is described.

The external temperature control, if appropriate in Multi-zone reactor systems, in a manner known per se to the special reaction gas mixture composition and Adjusted catalyst feed.

The reaction zone B required in the invention total molecular oxygen required is the Feed gas mixture of reaction zone B normally in its total amount admitted in advance.

Normally there is a reaction zone B in the feed gas molar ratio of isobutene: molecular oxygen from 1: 1 to 3, often set 1: 1.5 to 2.

An excess (based on the stoichiometry of the Gas phase partial oxidation) of molecular oxygen affects in the Usually beneficial to the kinetics of gas phase oxidation in the Reaction zone B. In contrast to the conditions in the reaction zone A to be used according to the invention are the thermodynamic conditions in reaction zone B by the molar The reactant ratio is essentially unaffected since the heterogeneously catalyzed gas phase partial oxidation of isobutene Methacrolein is subject to kinetic control.

In principle, z. B. in reaction zone B also the iso- Butene compared to molecular oxygen in a molar excess be presented. In this case, the excess comes iso-butene actually plays the role of a diluent gas.

As a source for everything needed in reaction zone B. molecular oxygen, which is the product gas mixture A normally before using it to feed reaction zone B admixed comes in particular with molecular nitrogen diluted oxygen. In an expedient way you will at least to cover a partial need for molecular Oxygen Use air as the source of oxygen, because in this way the to be used in reaction zone B according to the invention Nitrogen in the simplest way into the reaction system can be introduced.

However, a partial amount of the total molecular oxygen required in reaction zone B can also already be contained in product gas mixture A fed to reaction zone B. The product gas mixture A preferably contains no oxygen. Molecular oxygen diluted with inert gases such as CO ₂ , CO, noble gases, N ₂ and / or saturated hydrocarbons can also be used as a further oxygen source that can be used in the reaction zone. Such an oxygen source can e.g. B. be a branch gas branched from the process according to the invention and recycled into reaction zone B.

With the method according to the invention, apart from Molecular already contained in product gas mixture A. Oxygen, as a source of molecular oxygen in the following Reaction zone B at least partially and preferably exclusively Air used.

By adding cold, room temperature, air hot product gas mixture A can be used in the context of the invention Cooling of the process also directly Product mixture A can be effected on its way into reaction zone B.

The one leaving reaction zone B to be used according to the invention Product gas mixture B is generally essentially composed of the target product methacrolein or its mixture with Methacrylic acid, unreacted molecular oxygen, isobutane, unreacted isobutene, molecular nitrogen, molecular hydrogen, a by-product and / or Dilution gas with water vapor used as a by-product and / or carbon oxides used as diluent gas, and small amounts of other lower aldehydes, hydrocarbons and other inert diluent gases. The content of iso-butyraldehyde and iso-butyric acid is however minimized according to the invention. is not methacrolein, but ultimately methacrylic acid Desired target product can be, as it is DE-A 33 13 573 recommends using the product gas mixture B containing methacrolein as such lead into a reaction zone C and in the reaction zone C the methacrolein of a selective catalyzed partial oxidation with molecular oxygen in the gas phase to form a Submit product gas mixture C containing methacrylic acid.

Reveal favorable catalysts for such a reaction zone C. z. B. DE-A 44 05 060, US-A 5 166 119, US-A 5 153 162, US-A 4 954 650, US-A 4 558 028 and DE-A 198 15 279. They also teach the manufacture of such catalysts. In the As a rule, catalysts are based on the Element combination Mo-P containing multimetal oxides preferred. Next They usually contain metallic and molybdenum and phosphorus transition metallic elements, in particular copper, vanadium, Arsenic, antimony, cesium and potassium (see DE-A 43 29 907, DE-A 26 10 249, JP-A 7/185354).

DE-A 199 22 113 proposes multimetal oxide compositions of the general formula III

[A] _p [B] _q (III),

in which the variables have the following meaning:
A: Mo ₁₂ X _a ¹ X _b ² X _c ³ X _d ⁴ X _e ⁵ O _x
B: Mo _f X ⁶ _g X ⁷ _h O _y
X ¹ = H, of which up to 97 mol% can be replaced by ammonium, K, Rb, and / or Cs,
X ² = V, Nb, Ta, W and / or Re,
X ³ = B, Si, P, Ge, As and / or Sb,
X ⁴ = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mg, Ca, Sr and / or Ba,
X ⁵ = S,
X ⁶ = Cu, Fe, Co, Ni, Zn, Cd, Mn, Mg, Ca, Sr and / or Ba,
X ⁷ = Nb, Ta and / or Sb,
a = 1 to 3,
b = 0.1 to 2,
c = 0 to 5,
d = 0 to 1,
e = 0 to 1,
f = 0 to 2,
g = 0.5 to 1.5,
h = 2 to 4,
x, y = numbers which are determined by the valency and frequency of the elements other than oxygen in (I),
p = 1 and q = 0 or
p, q = integers other than zero, the ratio p / q of which is from 160: 1 to 1: 1, the standard deviation of the stoichiometric coefficient a of the variable X ^{1 of} individual crystallites within component A of the multimetal oxide mass being less than 0.40 , preferably less than 0.20, in particular less than 0.11.

They preferably contain the portion [A] _p in the form of three-dimensionally extended areas A of the chemical composition A, which are differentiated from their local environment due to their different chemical composition, and the portion [B] _q in the form of three-dimensionally expanded area, based on their local environment areas B different from their local surroundings, areas B of chemical composition B, areas A, B being distributed relative to one another as in a finely divided mixture of A and B.

DE-A 44 05 060 recommends similar multimetal oxide materials as suitable for a reaction zone C. Like that Multimetal oxide catalysts for reaction zone B are also Multimetal oxide catalysts for reaction zone C usually in bulk Balls, rings or cylinders shaped or in the form of Shell catalysts, d. that is, coated with the active composition preformed, inert carrier bodies used.

Suitable full catalyst geometries are e.g. B. full cylinder or Hollow cylinder with an outer diameter and a length of 2 to 10 mm. In the case of the hollow cylinder, a wall thickness of 1 to 3 mm appropriate. Of course, the full catalyst can also Have spherical geometry, the ball diameter 2 to 10 mm can be. Suitable shell catalyst geometries are e.g. B. which is disclosed in DE-A 101 21 592.

The simplest form of implementation of reaction zone C is how in the case of reaction zone B a tube bundle reactor, which is why all made with respect to the tube bundle reactor of reaction zone B. Statements in analog form also for a tube bundle reactor Reaction zone C is valid.

What is the current flow of reaction gas and temperature control medium (e.g. Salt bath), both those for reaction zone B as well as those recommended for reaction zone C. Shell and tube reactors operated both in cocurrent and in countercurrent become. Of course, cross-flow guides can also be overlaid become. A meandering guidance of the Temperature medium around the contact tubes, which over the reactor considered again in cocurrent or in countercurrent to Reaction gas mixture can take place.

A particularly simple form of implementation of the reaction zones B, C therefore forms a tube bundle reactor within which the Catalyst feed along the individual contact tubes changes accordingly. If necessary, the loading of the Contact tubes with catalyst interrupted by an inert bed (EP-A 911313, EP-A 979813, EP-A 990636 and the DE-A 28 30 765 teach such a procedure in an equivalent manner Example using the propylene partial oxidation to acrylic acid). in the In the case of this form of implementation, the feed gas mixture must be for reaction zone B already requires that in reaction zone C. contain molecular oxygen.

However, the two reaction zones B, C are preferably in the form two tube bundle systems connected in series. If appropriate, these can be located in a reactor, where the transition from one tube bundle to the other tube bundle from one not accommodated in the contact tubes (expediently walkable) bed of inert material. While the contact tube is usually washed around by a heat transfer medium , it reaches one attached as described above Not pouring in. But the two will be advantageous Contact tube bundles in spatially separate reactors accommodated. There can be between the two tube bundle reactors an intercooler to make a possible Methacrolein afterburning in the product gas mixture, which the Reduce reaction zone B. Instead of tube bundle reactors can also plate heat exchanger reactors with salt and / or Evaporative cooling, as z. B. DE-A 199 29 487 and DE-A 199 52 964 describe, be used.

The reaction temperature in reaction zone C is usually 230 to 350 ° C, often 250 to 320 ° C. The reaction pressure in reaction zone C is advantageously 0.5 to 5, often 1 to 2 or 3 bar. The loading (Nl / lh) of the oxidation catalysts in reaction zone C with the feed gas mixture is frequently from 1000 to 2500 h ^-1 or up to 4000 h ^-1 .

As already mentioned, the in the reaction zone C as All in all, oxidizing agent required molecular oxygen Charge gas mixture of reaction zone B in its total amount be added in advance. Of course, but can also reaction zone B are supplemented with oxygen. From the latter You will make use of this option in particular if the two reaction zones B, C in the form of two in a row switched tube bundle systems can be realized.

Since the molecular nitrogen used in reaction zone B is also part of the feed gas mixture of reaction zone C, such an oxygen supplementation can be carried out using pure molecular oxygen. Molecular oxygen diluted with inert gases such as CO ₂ , CO, noble gases, N ₂ and / or saturated hydrocarbons can also be used as an oxygen source that can also be used for such supplementary purposes. Such an oxygen source can e.g. B. from the process according to the invention branched, recycled into the reaction zone C cycle gas. According to the invention, air is preferably used for such an oxygen supplement.

An excess (based on on the reaction stoichiometry) of molecular oxygen in the Usually advantageous based on the kinetics of gas phase oxidation. A molar ratio is preferred in reaction zone C. Methacrolein: molecular oxygen from 1: 1 to 3, common 1: 1.5 to 2.5 set.

The method according to the invention is often carried out in such a way that sat in the product gas mixture C at least 50 mol%, preferably at least 60 mol% of the total in the different reaction zones molecular oxygen supplied have been implemented.

Frequently in the process according to the invention in the reaction zone C for a molar methacrolein: molecular oxygen:  Water vapor: isobutane: molecular nitrogen: other Diluent ratio of 2-6: 2-20: 5-30: 0-40: 20-80: 0-6 worked.

In principle, reaction zones B and C can also be used in a single reaction zone. In this case the two reaction steps take place (isobutene → methacrolein; Methacrolein → methacrylic acid) in an oxidation reactor with a catalyst is charged, the implementation of both Can catalyze reaction steps. Of course, can also the catalyst feed within one summarized oxidation zone along the reaction coordinate continuously or change abruptly.

By adding cold (ambient temperature) Air to hot product gas mixture B can within the method according to the invention also directly cooling the Product gas mixture B are caused before it is fed to the Reaction zone C is used.

The product gas mixture C leaving the reaction zone C is in generally composed of methacrylic acid, Methacrolein, unreacted molecular oxygen, isobutane, molecular nitrogen, a by-product and / or Molecular steam used as diluent gas Hydrogen, as a by-product and / or as a diluent gas carbon oxides used, as well as small amounts of other lower Aldehydes, hydrocarbons and other inert diluent gases. Its iso-butyraldehyde and iso-butyric acid content is minimized according to the invention.

The methacrylic acid can in itself from the product gas mixture C in be separated in a known manner.

For example, the product gas mixture C (e.g., a Can have outlet temperature of 220 ° C) initially by means of a 10 wt .-% aqueous solution of methacrylic acid, the z. B. by Addition of small amounts of hydroquinone monomethyl ether (MEHQ) be polymerization-inhibited and have a temperature of 80 ° C. can be cooled by direct contact. For this, the aqueous methacrylic acid solution in a on internals in sprayed essential free device into the product gas mixture C and with this led in direct current. Mist that forms can be separated from the gas phase in two Venturi separators become. Then the cooled product gas mixture in the Bottom of an absorption column, e.g. B. a packed column, guided. At the top of the column is the absorption liquid Water that contains dissolved polymerization inhibitors in countercurrent abandoned to the rising gas. The head temperature can e.g. B. 63 ° C and the bottom temperature z. B. 70 ° C.

Together with medium and high-boiling by-products such as vinegar, Propionic, acrylic, maleic, fumaric, citraconic and formic acid and formaldehyde and optionally formed isobutyraldehyde as well as iso-butyric acid, the methacrylic acid is absorbed from the Gas phase separated into the aqueous phase.

From the 10 to the deducted from the sump of the absorber 20 wt .-% aqueous methacrylic acid solution Extract methacrylic acid using suitable extraction agents, e.g. B. Ethylhexanoic acid, separated off and subsequently isolated by rectification become.

The residual gas leaving the absorber at the top generally contains isobutane, isobutene, methacrolein, O ₂ , H ₂ , N ₂ , CO, CO ₂ , H ₂ O, noble gases and other lower aldehydes and hydrocarbons.

The methacrolein can be removed by washing with water separated and again from the wash water by stripping with air freed and returned to the reaction zone C with the air become.

In addition, from the residual gas remaining after the methacrylic acid has been separated off by absorption with subsequent desorption and / or stripping and reuse of the absorbent in a high-boiling hydrophobic organic solvent, the isobutane and isobutene contained in other gases such as O ₂ , H ₂ , N ₂ , CO, CO ₂ , noble gases, etc. are essentially separated off and returned to reaction zone A. If necessary, the mixture of the remaining other gases can be returned to reaction zones A, B and / or C as diluent gas.

Generally suitable for the aforementioned purpose as Absorbent relatively non-polar organic solvents, such as. B. aliphatic hydrocarbons, preferably none to the outside acting polar groups contain, but also aromatic Hydrocarbons. In general, it is desirable that Absorbent a boiling point as high as possible at the same time highest possible solubility for isobutane and / or isobutene and lowest possible solubility for the other residual gas components Has.

Examples of suitable absorbents are aliphatic hydrocarbons, for example C ₈ -C ₂₀ -alkanes or alkenes, or aromatic hydrocarbons, for example middle oil fractions from paraffin distillation or ethers with bulky groups on the O atom, or mixtures thereof, these being a polar one Solvents such as B. the 1,2-dimethylphthalate disclosed in DE-A 43 08 087 can be added. Also suitable are esters of benzoic acid and phthalic acid with straight-chain alkanols containing 1 to 8 carbon atoms, such as n-butyl benzoate, methyl benzoate, ethyl benzoate, dimethyl phthalate, diethyl phthalate, and so-called heat transfer oils, such as diphenyl, diphenyl ether and chlorides and mixtures thereof, and mixtures and triarylalkenes, for example 4-methyl-4'-benzyl-diphenylmethane and its isomers 2-methyl-2'-benzyl-diphenylmethane, 2-methyl-4'-benzyldiphenylmethane and 4-methyl-2'-benzyl-diphenylmethane and mixtures such isomers. A suitable absorbent is also a solvent mixture of diphenyl and diphenyl ether, preferably in the azeotropic composition, in particular from about 25% by weight of diphenyl (biphenyl) and about 75% by weight of diphenyl ether, for example the commercially available diphenyl. This solvent mixture often contains a solvent such as dimethyl phthalate in an amount of 0.1 to 25% by weight, based on the total solvent mixture. Particularly suitable absorbents are also octanes, nonanes, decanes, undecanes, dodecanes, tridecanes, tetradecanes, pentadecanes, hexadecanes, heptadecanes and octadecanes, with tetradecanes in particular having proven to be particularly suitable. It is favorable if the absorbent used on the one hand meets the boiling point mentioned above, but on the other hand has a not too high molecular weight. The molecular weight of the absorbent is advantageously 300 300 g / mol. The paraffin oils with 8 to 10 carbon atoms described in DE-A 33 13 573 are also suitable. Examples of suitable commercial products are e.g. B. the Halpasole®i products sold by Haltermann, such as B. Halpasol 250 / 340i and Halpasol 250 / 275i, and printing ink oils under the names PKWF and Printosol.

The implementation of the absorption is not subject to any particular Restrictions. All methods and processes customary in the art can be used Conditions are applied. preferably the gas mixture at a pressure of 1 to 50 bar, preferably 2 to 20 bar, particularly preferably 5 to 10 bar, and a temperature of 0 to 100 ° C, in particular from 30 to 50 ° C, in contact with the absorbent brought. You can in absorption columns, for. B. tray columns Bell and / or sieve trays, columns with structured Packs or packed columns, in trickle and spray towers, Graphite block absorbers, surface absorbers such as thick film and Thin film absorbers as well as plate washers, cross veil washers and Rotation washers are carried out. It can also be cheap absorption in a bubble column with or without internals to take place.

The separation of the isobutane and / or isobutene from the absorbate can by stripping, flash evaporation (flashing) and / or Distillation (rectification).

Gases suitable for stripping are, in particular, those which together with the isobutane and isobutene into reaction zone A can be returned.

As such gases come nitrogen, air, oxygen, Oxygen / nitrogen mixtures, isobutane and water vapor into consideration. at the use of air or oxygen / nitrogen Mixtures in which the oxygen content is above 10% by volume, it may make sense to enter one before or during the typing process Add gas that reduces the explosion area. Especially Gases with a heat capacity of. are suitable for this purpose ³ 29 J / mol-K (based on 250 C and 1 atm). For example, as such a gas can also be used iso-butane.

The separation of the isobutane and / or isobutene from the absorbate can also be done by distillation. Around Minimizing absorbent losses can be both when stripping resulting gas phase as well as a rising during distillation Gas phase can be washed in countercurrent with water. As Wash water can contain condensed water vapor contained in the residual gas be used.

Otherwise, as in WO-0196271 using the example of propane / propene described procedure.

Further possibilities for the separation of isobutane and / or isobutene from residual gas are adsorption, rectification and partial condensation. Fractional pressure distillation is preferably carried out at low temperatures. The pressure to be applied may e.g. B. 10 to 100 bar. Packing columns, tray columns or packing columns can be used as rectification columns. Tray columns with dual-flow trays, bubble trays or valve trays are suitable. The reflux ratio can e.g. B. 1 to 10. Other separation options are e.g. B. pressure extraction, pressure swing adsorption, pressure washing, partial condensation and pressure extraction. As part of a fractional distillation, the dividing line can e.g. B. be placed so that essentially all those components are separated at the top of the rectification column whose boiling point is lower than the boiling point of isobutene. These components will primarily be the carbon oxides CO and CO ₂ as well as unreacted oxygen and N ₂ and possibly H ₂ . Steam can be returned to reaction zone A together with isobutane and isobutene.

A more detailed description of the separation outlined above of methacrylic acid and methacrolein from a product gas mixture as the product gas mixture C can be found in EP-B 297445.

Of course, for this purpose, too Separation method of US-A 4925981 and US-A 4554054 applied become.

All of these methods have in common that after the Methacrylic acid separation an unreacted residual isobutane gas remains.

This can also be recycled as such into reaction zone A. In order to avoid an accumulation of gases such as nitrogen or CO ₂ in such a case over time, a partial amount of the residual gas can be discharged and z. B. burn for energy purposes. In the context of the method according to the invention, it is also possible to completely dispense with the return of residual gas to reaction zone A and to supply the entire residual gas for combustion for the purpose of generating energy.

Of course, according to the invention, the procedure can also be such that part of the residual gas unchanged into reaction zone A. returns and only from the remaining part of isobutane and isobutene separated in the mixture and also in reaction zone A and / or recycled into reaction zone B. In the latter case, one unites the remaining part of the residual gas expediently with the Product gas mixture A.

Separated isobutane and / or isobutene can also be used Recycling purposes as a return to reaction zone A are supplied (e.g. the production of isobutanol).

If gas containing iso-butane and iso-butene to be returned to reaction zone A still contains carbon monoxide, this can be catalytically selectively burned to CO ₂ before (or after) entry into reaction zone A. The heat of reaction released can be used to heat up to the dehydrogenation temperature.

A catalytic afterburning of CO contained in the residual gas to CO ₂ can also be recommended if the carbon oxides are to be separated from the residual gas before it is returned to the reaction zone A (or other), but CO ₂ can be removed comparatively easily (e.g. B. by washing with a basic liquid).

When using dehydrogenation catalysts, the opposite Oxygen or oxygen-containing compounds are sensitive, you will get these oxygenates before recycling residual gas into the Separate reaction zone A from the residual gas. With those in this Scripture particularly recommended for catalytic dehydration This is not necessary for catalysts.

The advantage of a reduced isobutyraldehyde and iso-butyric acid by-product formation occurs essentially regardless of those used in reaction zones B and C. Multimetal oxide catalysts. Preferably, those in this Writing explicitly named multimetal oxide catalysts used. It essentially occurs regardless of whether the volume-specific catalyst activity in reaction zones B, C kept constant or increasing along the reaction coordinate or decreasing.

An immediate use of the methacrolein containing Product gas mixture B for feeding a reaction zone C is however not the variant preferred according to the invention. incoming Research has shown that this is due to the fact that contained in the product gas mixture B, other than methacrolein, Components in a subsequent reaction zone C the Reduce selectivity of methacrylic acid formation by negatively affecting the catalyst performance, especially with regard to the Long-term behavior, impact. Is also an immediate Use of the product gas mixture B containing methacrolein for Feeding a reaction zone C is not recommended if the conversion of isobutene in reaction zone B is less than 95 mol% or less than 98 mol% (often the Conversion of isobutene in reaction zone B 60 to 95 mol%).

In other words, expediently according to the invention, the procedure is such that prior to using the product gas mixture B for charging a reaction zone C or a reaction zone for production of methacrylic acid esters such as methyl methacrylate from those Components operating at a lower atmospheric pressure Boil temperature as methacrolein, at least one, usually Iso-butane and isobutene-containing subset, preferred at least 80 mol%, particularly preferably at least 90 mol% and completely particularly preferably separates at least 95 mol% (the molar The basis of reference is the sum of those in product gas mixture B. contained molar amounts of the individual components; the separation must not done homogeneously across the various components; she Rather, z. B. quantitative components and others capture only partially; only in the sum must the named Percentage can be achieved). According to the invention is recommended it also, the methacrolein essentially as such from the Separate product gas mixture B and then to feed one Reaction zone C or for another use like that Preparation of alkyl esters of methacrylic acid, e.g. B. the Manufacture of methyl methacrylate to use. If necessary, for aforementioned uses of methacrolein also in essentially with the water vapor contained in product gas mixture B. separated.

The aforementioned partitions can be made using known methods be performed. For this, reference is made to DE-A 33 13 573, DE-A 24 00 260, GB-A 1199432 and US-A 4234519. For example, the product gas mixture can be a multi-stage fractional distillation, preferably under pressure become. In the first stage, the bottom of the column Methacrolein and constituents heavier than methacrolein separated. In the following, this bottom liquid can either be used as those for feeding a reaction zone C or for Used for esterification purposes or previously the methacrolein contained in it in a further fractional distillation stage overhead be separated purely. From the fractionated in the first Distillation stage overhead, lighter than methacrolein boiling components, which include isobutene and isobutane, can in a further fractional distillation stage contained isobutane and isobutene separated off at the bottom and be returned to reaction zone A. That over Upside down gas mixture can e.g. B. disposed of by incineration or if necessary as diluent gas in reaction zones A, B and / or C can also be used. One is satisfied with the aforementioned Separation steps with separations from a limited sharpness, can the fractionations can also be carried out by simple condensation replace.

The desired separation can also be achieved, for example achieve that the product gas mixture B is cooled and so condensed water-containing methacrolein. For complete Removal of methacrolein from the remaining gas phase you will wash the latter with a recirculating water stream. The resulting aqueous methacrolein solutions can then be stripped off, creating a vapor containing methacrolein contains that for esterification purposes or for loading one Reaction zone C can be used.

In practice, you will do this by doing this Product gas mixture B with water or with an aqueous methacrolein solution (those with hydroquinone or its methyl ether are polymerization-inhibited) by direct cooling (by spraying the Water or the aqueous methacrolein solution in it Product gas mixture B and subsequent guidance in direct current). to The gas stream can subsequently separate mist which forms through Venturi separators. The coolant will z. B. in a heat exchanger in a circle. Of course can cooling also takes place indirectly. The cooled gas mixture is then led to an absorption device (e.g. Packed column or tray columns) in which the methacrolein (in usually in countercurrent operation) in one stream recirculating water is absorbed (e.g. at temperatures in the range of 15 to 20 ° C).

An aqueous methacrolein solution is obtained as the absorbent z. B. may contain up to 2 mol% or more methacrolein. In a wiper can remove the methacrolein at lower pressure and stripped of water again using heat and as Steam flow are withdrawn (the water contained in it can Frozen out and / or condensed as required). The stripped water is e.g. B. for absorption purposes in the Recycled methacrolein absorber. The share of the in the process formed water (the contained in the product gas mixture B Water vapor condenses or absorbs with the methacrolein in the Absorption liquid) can either be disposed of (e.g. the Sewerage supplied) and / or as water vapor in reaction zone A be returned.

The gas stream leaving the methacrolein absorber contains isobutene, isobutane, noble gases, carbon oxides, hydrogen, oxygen, Nitrogen and other by-products such as ethane, propane or Methane. It can either as such into reaction zone A be recycled (using a subset to prevent By-product level is removed) or you can do it in it contained valuable products iso-butane and iso-butene by absorption in a non-polar organic solvent (see Scripture already described separation of isobutane and isobutene from residual gas; all methods given there can be found here be applied accordingly) and subsequent desorption separate and return to reaction zone A. The not absorbed Gas components can in turn be disposed of and / or as Dilution gases in reaction zones A, B and / or C use Find.

Instead of separated isobutane, isobutene or these components gases contained in the reaction zone A, can they can also be used for other purposes. exemplary Their incineration in a power plant for the purposes of Energy production or the production of isobutanol.

Of course, here too, the procedure can be such that you only a part of the containing iso-butane and iso-butene, of Methacrolein largely exempt gas as such into the Recycle reaction zone A and only from the remaining part iso-butane and separates isobutene in the mixture and also into the reaction zone A and / or in reaction zone B or another Feeds use.

It also applies here that in the event that gas to be returned to reaction zone A and containing isobutane and isobutene still contains CO, this gas catalytically selectively to CO ₂ before (and / or after) it enters reaction zone A. can be burned. The heat of reaction released can in turn be used to heat up to the dehydrogenation temperature.

Such a catalytic post-combustion of CO to CO ₂ can also be recommended if the carbon oxides are to be separated off before the gas stream is returned to reaction zone A (or into another), since CO ₂ can be separated off comparatively easily (e.g. by washing with a basic liquid; as such, for example, aqueous carbonate or amine solutions or organic amines come into consideration; since the presence of CO ₂ does not interfere in any of the different reaction zones, a buildup of the carbon oxides is generally achieved for reasons of economy allow in the recycle stream up to a concentration at which one can oxidize and / or separate them conveniently and economically).

When using dehydrogenation catalysts which are sensitive to oxygen or oxygen-containing compounds, these oxygenates will be removed from them before the gas streams are returned to reaction zone A. If the gas stream contains hydrogen at the same time, this can e.g. B. on the way of a selective catalytic combustion of H ₂ with O ₂ to H ₂ O. Likewise, hydrogen contained in H ₂ containing waste gases intended for disposal purposes may contain e.g. B. separated by means of membrane processes and fed to reaction zone A if necessary.

The separation of the methacrolein from the product gas mixture B can but also in other known ways. For example by extraction with a suitable organic solvent and subsequent rectificative work-up or by adsorption and subsequent desorption.

It is essential to the invention that this is as described by Secondary components partially or completely freed methacrolein same separation effort always less iso-butaraldehyde or iso- Butyric acid is contained as according to the method of The closest prior art obtained methacrolein.

So partially or completely of secondary components exempted methacrolein can now be used in an advantageous manner, for. B. for Preparation of alkyl esters of methacrylic acid, in particular Methyl methacrylate can be used. To do this, you become methacrolein directly in the presence of oxygen and suitable catalysts with the appropriate alcohol, e.g. B. implement methanol. To the methods of the prior art can be used, as they e.g. B. in DE-A 30 18 071, US-A 6 107 515 or GB-A 2070601 are described. Usually this is done Implementation in the liquid phase using precious metals such as Palladium-containing catalysts.

However, it can also be selective in a manner known per se Gas phase catalytic oxidative production of methacrylic acid be used.

The catalysts for reaction zone B can already be recommended multimetal oxide catalysts can be used. in the the gas phase oxidation of methacrolein may also increase Methacrylic acid can be carried out as recommended in the prior art (see e.g. US-A 5 166 119, US-A 5153162, US-A 4 954 650, US-A 4 558 028 and DE-A 198 15 279). The separation of the Methacrylic acid from the product gas mixture can be used as for the Reaction zone C already described (see. DE-A 198 36 477).

The reactors described as being suitable for reaction zone C can also be used here. Because of the high heat of reaction, the reactants methacrolein and O _{2 are} preferably diluted with inert gas such as N ₂ , CO ₂ , saturated hydrocarbons (e.g. isobutane) and / or with water vapor.

Pure molecular oxygen or with can be used as the oxygen source Inert gases dilute molecular oxygen, e.g. B. air (preferably) used.

As a rule, with a molar methacrolein: oxygen: Water vapor: inert gas ratio from 2 to 6: (2 to 20): (5th to 30): (0 to 6), particularly preferably from 3 to 5: (7 to 12)  : (15 to 25): (0 to 4) worked. The gas phase oxidation itself, as well as reaction zone B, can both in Fluidized bed reactors as well as in fixed bed reactors can be realized. The latter is preferred. The reaction temperature is usually in the range of 230 to 350 ° C and the reaction pressure in Range from 0.5 to 3 bar and the total space load is preferably 1000 to 4000 Nl / l.h. The (as always in this document) methacrolein conversion based on simple reactor run is usually 60 to 90 mol%. The separation of the As already mentioned, methacrylic acid from the product gas mixture can the reaction zone C described. That is, one can the product gas mixture according to direct and / or indirect Wash cooling at temperatures from 40 ° C to 80 ° C with water, whereby an aqueous methacrylic acid solution is obtained from which the Methacrylic acid e.g. B. by extraction with an organic Solvent removed and separated from the same rectificatively can. The residual gas leaving the absorber can be used as inert diluent gas in reaction zones A, B and / or C. shared and / or z. B. disposed of by incineration.

Iso-butane forms one of the diluent gases used the isobutane-containing residual gas in a convenient manner either as such or after absorption and desorption in the already recommended non-polar organic solvents in the Return reaction zone A.

In general, if gases contained in reaction zone A contain O ₂ , this oxygen in reaction zone A can be used to selectively burn combustible substances such as hydrocarbons, coke, CO or, preferably, H ₂ in reaction zone A, and so on to form the heat of dehydrogenation required in reaction zone A. The methacrolein oxidation is expediently carried out with a corresponding excess of oxygen, so that the aforementioned residual gas which is returned to reaction zone A has a sufficient amount of oxygen for this purpose.

Examples 1. Preparation of a dehydrogenation reactor

A solution of 11.993 g of SnCl ₂ .2H ₂ O and 7.886 g of H ₂ PtCl ₆ .6H ₂ O in 600 ml of ethanol is poured over 1000 g of a split ZrO ₂ .SiO ₂ mixed oxide.

The mixed oxide has a ZrO ₂ / SiO ₂ weight ratio of 95: 5. The manufacturer of the mixed oxide is Norton (USA).

The mixed oxide has the following specification:
Type AXZ 311070306, Lot-No. 2000160042, sieve fraction 1.6 to 2 mm, BET surface area: 86 m ² / g, pore volume: 0.28 ml / g (mercury porosimetry measurement).

The excess ethanol is drawn off on a Rotavapor while rotating in a water jet pump vacuum (20 mbar) at a water bath temperature of 40 ° C. The mixture is then first dried under standing air at 100 ° C. for 15 h and then calcined at 560 ° C. for 3 h. The dried solid is then poured with a solution of 7.71 g of CsNO ₃ , 13.559 g of KNO ₃ and 98.33 g of La (NO ₃ ) ₃ .6H ₂ O in 2500 ml of H ₂ O. The excess water is drawn off on the Rotavapor while rotating in a water jet pump vacuum (20 mbar) at a water temperature of 85 ° C. The mixture is then first dried under standing air at 100 ° C. for 15 h and then calcined at 560 ° C. for 3 h.

The resulting catalyst precursor has a composition of Pt _0.3 Sn _0.6 Cs _0.5 K _0.5 La _3.0 (stoichiometric coefficients represent weight ratios) (ZrO ₂ ) _95. (SiO ₂ ) ₅ (stoichiometric coefficients represent Weight ratios).

With 20 ml of the catalyst precursor obtained is a vertical vertical tube reactor loaded (reactor length: 800 nm; wall thickness: 2 mm, inner diameter: 20 mm; Reactor material: internally ionized (i.e., alumina coated) steel tube; heating: electrical (furnace from HTM Reetz, LOBA 1100-28-650-2) on one longitudinal length of 650 mm; Length of the catalyst bed: 75 mm; Position of the catalyst bed: on the longitudinal center of the Tubular reactor; Filling up the remaining reactor volume and below with steatite balls (inert material) with a diameter of 4-5 mm, sitting down on a contact chair).

Subsequently, the reaction tube is charged with 9.3 Nl / h of hydrogen for 30 minutes along the heating zone at 500 ° C. (based on a tube through which an identical inert gas stream flows). The hydrogen stream is then replaced with a constant wall temperature, first for 30 min by a 23.6 Nl / h stream of 80 vol.% Nitrogen and 20 vol. Air and then for 30 min by an identical stream of pure air. While maintaining the wall temperature, flushing is then carried out with an identical flow of N ₂ for 15 min and finally reduced again with 9.3 Nl / h of hydrogen for 30 min. This completes the activation of the catalyst precursor. A dehydrogenation reactor (reaction zone A reactor) charged with dehydrogenation catalyst A is present.

2. Preparation of a reaction zone B reactor a) Preparation of a starting mass B1

To prepare the starting material B1, 2000 g of (NH ₄ ) ₆ Mo ₇ O ₂₄ .4H ₂ O are dissolved in portions in 5.4 l of water at 60 ° C. and 9.2 g of a 47.5% by weight solution are added with stirring aqueous KOH solution at 20 ° C and then mixed with 387.8 g of a 47.5 wt .-% aqueous, 20 ° C having CsNO ₃ solution while maintaining the 60 ° C (= starting solution 1). A starting solution 2 is prepared by adding 1123.6 g of an aqueous iron nitrate solution (13.8% by weight) in 2449.5 g of an aqueous cobalt nitrate solution (12.5% by weight Co) at 60 ° C. while maintaining the 60 ° C. -% Fe) is stirred in.

The starting solution 2 at 60 ° C. is stirred into the starting solution 1 at 60 ° C. within a period of 30 minutes. 15 minutes after stirring has ended, 157.0 g of silica sol (density: 1.39 g / ml; ph = 8.8; alkali metal content: ≤ 0.5% by weight, 50.0% by weight) SiO ₂ ; manufacturer: Dupont; type: Ludox®TM) stirred in (at 60 ° C). The aqueous mixture is then stirred for a further 15 minutes. The aqueous suspension is then spray dried (outlet temperature: 110 ° C., spray dryer from Niro DK; type: Niro A / S Atomizer Transportable Minor System, centrifugal atomizer from Niro, DK), using a spray powder with a grain size of 20 µm to 25 µm occurs that has a loss on ignition (3 h at 600 ° C in air) of about 30 wt .-%. This spray powder forms the starting mass B1.

b) Preparation of a starting mass B2

To 6344.6 g of an aqueous nitric acid bismuth nitrate solution (free nitric acid: 4% by weight, density: 1.24 mg / l; 11.2% by weight Bismuth) at 20 ° C with stirring in portions 1715.6 g Tungsten acid (72.94 wt .-% W) given. It becomes an aqueous Get suspension, which is stirred at 20 ° C for 2 h. Then they are dried by spray drying (outlet temperature: 110 ° C, Manufacturer: Niro DK; Type: Niro A / S Atomizer Transportable Minor System, centrifugal atomizer from Niro, DK). In this way a spray powder with a grain size of 20 µm to 25 µm is obtained a loss on ignition (3 h at 600 ° C in air) of about 12% by weight having. 400 g of this powder are after adding 37 g of water using a LUK 075 kneader from Werner & Pfleiderer (Kneader has two counter-rotating sigma blades) Kneaded for 30 min. After kneading, the kneaded material becomes coarse divided and for 2 h in a drying cabinet (Binder, DE, Type: FD 53) dried at 120 ° C. The total amount of dry goods is made in a muffle furnace from Nabertherm, internal volume approx. 120 l, for 2 h at 800 ° C under an air flow of 1000 Nl / h calcined. The airflow is approx. 20 ° C when it is in the Muffle furnace is led. To the calcination temperature linearly heated from 25 ° C within 8 h.

The calcined product is then ground to a number-average particle size (narrow distribution, longest dimension) of approximately 5 μm and with 1% by weight (based on the SiO ₂ -free mass) of finely divided SiO ₂ (shaking weight: 150 g / l; number-average particle size: 10 µm (longest dimension, narrow distribution); BET surface area: 100 m ² / g) mixed.

This mixture forms the starting mass B2

c) Catalyst production

1096 g of the starting mass B1 and 200 g of the starting mass B2 are added with (based on the total mass of B1 and B2 used) 1.5% by weight of finely divided graphite (according to sieve analysis at least 50% by weight <24 μm; 24 µm <max. 10% by weight <48 µm; 5% by weight> 48 µm; BET surface area: 10 m ² / g) homogeneously mixed as a tabletting aid. A mixture is obtained which has the following molar element stoichiometry (after calcination):

[Bi ₂ W ₂ O ₉ .2WO ₃ ] _0.5 [Mo ₁₂ Co _5.5 Fe _2.94 Si _1.59 Cs ₁ K _0.08 O _x ] ₁ .C _y .

Circular full tablets become one from the mixture Diameter of 16 mm and a height of 3 mm. The pressure is 9 bar. The tablets are crushed and placed over a sieve (0.8 mm mesh size) sieved. The sieve passage becomes after Addition of 2% by weight of graphite (based on the weight of the Sieve pass) in a tablet press machine (type: Kilian S100, Pressing force: 15-20 N) to cylindrical rings with a geometry of 5 mm (Outer diameter) × 3 mm (height) × 2 mm (hole diameter) tableted.

• a) von Raumtemperatur wird innerhalb von 2 h linear auf 180°C aufgeheizt und dann während 1 h bei 180°C gehalten;
• b) anschließend wird innerhalb von 1 h linear von 180°C auf 210°C aufgeheizt und dann während 1 h bei 210°C gehalten;
• c) dann wird innerhalb von 1 h linear von 210°C auf 250°C aufgeheizt und dann während 1 h bei 250°C gehalten;
• d) danach wird innerhalb von 1,5 h linear von 250°C auf 450°C aufgeheizt und dann während 10 h bei 450°C gehalten;
• e) abschließend wird durch sich selbst Überlaßen auf Raumtemperatur (ca. 25°C) abgekühlt.

150 g of these rings are calcined in a convection oven (Nabertherm, internal volume: approx. 80 l) as follows:

• a) from room temperature is linearly heated to 180 ° C within 2 h and then kept at 180 ° C for 1 h;
• b) the mixture is then linearly heated from 180 ° C. to 210 ° C. within 1 h and then kept at 210 ° C. for 1 h;
• c) is then linearly heated from 210 ° C to 250 ° C within 1 h and then held at 250 ° C for 1 h;
• d) the mixture is then linearly heated from 250 ° C. to 450 ° C. within 1.5 h and then kept at 450 ° C. for 10 h;
• e) finally, it is left to cool to room temperature (approx. 25 ° C).

During the entire calcination, 150 Nl / h of air are passed through the Furnace led.

The end product forms the one to be used in reaction zone B. Multimetal oxide catalyst B.

d) Feeding the reaction zone B reactor

A vertical reactor tube (tube length: 1500 mm; Wall thickness: 2.5 mm; Inner diameter: 15 mm; Reactor material: V2A Steel; in a longitudinal center length of 1300 mm in an oven from HTM Reetz, the remaining pipe length at the pipe entrance and the remaining pipe length at the pipe outlet is with electrical Heated heating tapes) is charged with 100 g of catalyst B. The The length of the catalyst bed is 650 mm. The position of the The catalyst bed in the reaction tube is in the middle. Downward and upward is the residual reaction tube volume with steatite balls (Inert material; diameter: 2-3 mm) padded, the entire reaction tube loading down on a contact chair with a height of 10 cm.

3. Preparation of a reaction zone C reactor a) Preparation of a starting mass C1

4620 g (NH ₄ ) ₆ Mo ₇ O ₂₄ .4H ₂ O and 153.2 g NH ₄ VO ₃ are dissolved at 60 ° C in 5 l of water preheated to 60 ° C with constant stirring. 421.7 g of a 76% strength by weight aqueous H ₃ PO ₄ solution are added dropwise to this solution while maintaining the 60 ° C. within 1 min with stirring. Then 10.9 g of diammonium sulfate and then 317.8 g of powdered Sb ₂ O ₃ (senarmontite) are incorporated with stirring. The resulting mixture is then within 30 min. heated to 90 ° C (mixture 1). In parallel, 424.9 g of CsNO _{3 are dissolved} at 90 ° C. in 850 ml of water preheated to 90 ° C. to form a solution 1. Then, while maintaining 90 ° C., 446.5 g of an aqueous, 90 ° C. copper nitrate solution (15.5% by weight Cu) are added dropwise to mixture 1 with constant stirring within 4 minutes. Spray drying is then carried out at an initial temperature of 110 ° C (Niro DK, type: Niro A / S Atomizer Transportable Minor System, centrifugal atomizer from Niro, DK). A spray powder with a grain size of 20 to 30 µm is obtained, which forms the starting mass C1 and has the following molar element stoichiometry (after calcination):

Mo ₁₂ P _1.5 Vo _0.6 Cs ₁ Cu _0.5 Sb ₁ S _0.04 O _x .

b) Preparation of a starting mass C2

880 g of powdered Sb ₂ O ₃ (senarmontite) with an Sb content of 83% by weight at 20 ° C. are suspended in 4 l of water with stirring. A solution of 670.5 g of Cu (NO ₃ ) ₂ .2H ₂ O in 4 l of water is then stirred into the suspension while maintaining the 20 ° C. An aqueous suspension is obtained which is stirred for a further 2 hours at 80 ° C. and then spray-dried (outlet temperature: 110 ° C., company: Niro DK, type: Niro A / S atomizer portable minor system, centrifugal atomizer from Niro, DK). The spray powder has a grain size of 20-30 µm. 700 g of the spray powder are thermally treated in a cylindrical rotary kiln (length: the calcination chamber 0.50 m; inner diameter: 12.5 cm) while passing 200 Nl / h of air as follows: The temperature is raised linearly to 150 ° C. within 1 h , The mixture is then heated linearly to 200 ° C. within 4 h. The mixture is then heated linearly to 300 ° C. within 2 h and then linearly to 400 ° C. within 2 h. Finally, the mixture is heated linearly to 900 ° C. within 48 h.

After cooling to room temperature, a powder is obtained which has a specific BET surface area of 0.3 m ² / g. This powder forms the starting mass C2 and essentially shows the diffraction reflections of CuSb ₂ O ₆ (comparison spectrum 17-0284 from the JCPDS-ICDD file). The starting mass C2 has the following molar element stoichiometry:

CuSb ₂ O ₆ .

c) Catalyst production

From the starting mass C1 and from the starting mass C2, the mixture stoichiometry (after calcination) (Mo ₁₂ P _1.5 V _0.6 Cs ₁ Cu _0.5 Sb ₁ S _0.04 O _x ) _1. (Cu ₁ Sb ₂ O ₆ ) _0.5 corresponding amounts intimately mixed. Then, to 500 g of the aforementioned mixture, 2% by weight of finely divided graphite (according to sieve analysis, at least 50% by weight <24 µm; 24 µm <max. 10% by weight <48 µm; 5% by weight> 48 µm; BET surface area: 10 m ² / g) added as a tabletting aid.

In a tablet press machine (type: Kilian S 100) are made the mixture without the use of further additives cylindrical rings of geometry 7 mm (outer diameter) × 7 mm (height) × 3 mm (Hole diameter) shaped.

Then 500 g of the rings in a forced air oven (Fa. Nabertherm, approx. 80 l internal volume) under a constant air flow (500 Nl / h.kg solid) thermally treated as follows: With 4 ° C / min is heated from 25 ° C to 270 ° C, the Intermediate temperatures 180 ° C, 220 ° C and the final temperature 270 ° C each 30 min being held. Finally, the temperature with a Speed increased from 2 ° C / min to 370 ° C and this temperature maintained for 6 h.

Then it is cooled to room temperature and the hollow cylinder is closed Processed chippings with a length of 1.6-3 mm. This Grit forms to be used in reaction zone C. Multimetal oxide catalyst C.

d) charging the reaction zone C reactor

With 75 g of the multimetal oxide catalyst C a vertical standing reactor tube (tube length: 1800 mm; wall thickness: 1 mm; Inner diameter: 8 mm; Reactor material: V2A steel; on a longitudinal length of 1600 mm in an HTM Reetz oven located). The length of the catalyst bed is 1000 mm. The position of the catalyst bed in the reaction tube is longitudinal. It will go down and up Residual reaction tube volume with steatite balls (inert material; diameter: 2-3 mm) filled, with the entire reaction tube charge after sits on a contact chair 10 cm high. The Pipe remaining length at the pipe entrance and the remaining pipe length at the pipe exit is heated with electric heating tapes.

4. Implementation of the method according to the invention

• A) The reaction zone A reactor from 1. is with an outer wall temperature control along the heating zone of 500 ° C (based on a tube flowed through with an identical inert gas stream) with a mixture of 20 Nl / h isobutane, 10 Nl / h air and 8 g / h of steam are fed as a reaction gas mixture.
  The isobutane is metered in by means of a mass flow controller from Brooks, while the water is first metered liquid into an evaporator by means of a HPLC Pump 420 from Kontron, evaporated in the same and then mixed with the isobutane and the air. The temperature of the feed gas mixture for the feed is 150 ° C. The outlet pressure at the tubular reactor is set to 1.5 bar by means of a pressure control from REKO located at the reactor outlet.
  After the pressure control, the product gas mixture A is expanded to normal pressure and cooled, the water vapor contained condensing out. The remaining gas is analyzed by GC (HP 6890 with chemical station, detectors: FID; WLD, separation columns: Al ₂ O ₃ / KCI (chrome pack), carboxen 1010 (Supelco). The feed gas mixture is also analyzed accordingly.
  The following analysis results are obtained after three weeks of operation:
  
  
  These values correspond to an isobutane conversion of 25 mol% based on a single throughput and a selectivity of isobutene formation of 90 mol%.
  The reaction zone B reactor from 2 is regulated on its part located in the furnace to an outer wall temperature (based on a tube through which an identical inert gas stream flows) at which the isobutene conversion is 92 mol% with a single passage of the reaction gas mixture. The heating band at the reaction tube inlet (where the contact chair is located) is also set to this temperature and the heating band at the reaction tube outlet is set to a 50 ° C lower temperature.
  The feed consists of a mixture of 45 Nl / h air (temp. = 20 ° C) and the 46 Nl / h of the product gas mixture A (temp. = 500 ° C). The air is metered in using a Brooks mass flow controller. The temperature of the feed gas mixture is brought to the outer wall temperature of the reactor. The outlet pressure of the reactor is set to 1.3 bar by means of a pressure control located at the reaction tube outlet.
  After the pressure control, the product gas mixture B temperature = 300 ° C) is depressurized to normal pressure and analyzed by GC (HP 6890 with Chem-Station, detectors: TCD, FID, separation columns: Poraplot Q (chrome pack), carboxen 1010 (Supelco)). The feed gas mixture is also analyzed in an identical manner.
  The following results are obtained after 3 weeks of operation:
  
  
  These values correspond to a one-pass iso-butene conversion of 92 mol% and a selectivity of methacrolein formation of 84 mol%.
  The reaction zone C reactor is regulated on its part located in the furnace to an outer wall temperature of 290 ° C. (based on a tube through which an identical inert gas stream flows). The heating band at the reaction tube inlet (where the contact is located) is set to 290 ° C and the heating band at the reaction tube outlet is set to 200 ° C. The feed gas mixture consists of 40 Nl / h air (20 ° C), 25 Nl / h nitrogen (20 ° C) and the rest of the product gas mixture B, which remains after separation of all components with lower boiling points than methacrolein. This residue essentially contains 18 Nl / h water vapor, 3.5 Nl / h methacrolein and 0.1 Nl / h methacrylic acid. The air and nitrogen are metered in using mass flow controllers from Brooks. The temperature of the feed gas mixture is brought to 290 ° C.
  The pressure at the reaction tube outlet is set to 1.3 bar by means of a pressure regulator located at the reactor outlet.
  After the pressure control, the product gas mixture C (temperature: 20 ° C.) is expanded to normal pressure and analyzed by means of GC (HP 6890 with Chem-Station, detectors: TCD, FID, separation columns: Poraplot Q (chrome pack), carboxen 1010 (Supelco). In the feed gas is also analyzed accordingly.
  The following results are obtained after 3 weeks of operation:
  
  
  These values correspond to a single pass methacrolein conversion of 60 mol% and a selectivity of methacrylic acid formation of 83 mol%.
• B) The procedure of A) according to the invention is repeated, but no components are separated from the product gas mixture B. Rather, the product gas mixture B as such (92 Nl / h) in a mixture with 29 Nl / h air is used as the feed gas for the reaction zone C reactor.
  The loading of the catalyst feed in the reaction zone C reactor with methacrolein or oxygen is thus, as in A), 3.5 Nl / h (methacrolein) or 8.0 Nl / h (oxygen).
  After 3 weeks of operation, the following results are obtained:
  
  
  
  
  These values correspond to a single pass methacrolein conversion of 40 mol% and a selectivity of methacrylic acid formation of 75 mol%.

5. Comparative example

• A) Process 4.A) according to the invention is repeated. In the However, feed to the reaction zone B reactor instead of 45 Nl / h air 9 Nl / h pure oxygen is used. The total content of the product gas mixture B. iso-butyraldehyde + iso-butyric acid is compared to the experimental procedure 4.A) noticeably increased.
• B) The method 4.B) is repeated, however are for the feed gas for the reaction zone C reactor not 29 Nl / h air but 5.8 Nl / h pure oxygen used. The total content of the product gas mixture C in iso- Butyraldehyde + iso-butyric acid is opposite to that Test procedure 4.B) noticeably increased.

Claims (7)

1. A process for the preparation of methacrolein from isobutane, in which A) in a reaction zone A, the isobutane is subjected to a partially selective, heterogeneously catalyzed dehydrogenation in the gas phase to form a product gas mixture A which contains isobutene and unreacted isobutane, B) product gas mixture A containing isobutane and isobutene leads into reaction zone B and in reaction zone B the isobutene is subjected to selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase to form a product gas mixture B containing methacrolein and C) the methacrolein contained in the product gas mixture B is used either for the production of methacrylic acid or for another purpose and at least unreacted isobutane contained in the product gas mixture B is either returned to the reaction zone A or used for another purpose, characterized in that the molecular oxygen required in reaction zone B is fed to reaction zone B accompanied by molecular nitrogen, the molar ratio of molecular oxygen to molecular nitrogen being 1: 1 to 1:10. 2. The method according to claim 1, characterized in that the in reaction zone B required molecular oxygen Reaction zone B is at least partially supplied as air. 3. The method according to claim 1 or 2, characterized in that the methacrolein contained in the product gas mixture B for Production of methacrylic acid is used, from which Product gas mixture B of those contained in Normal pressure boiling at lower temperatures than methacrolein, Ingredients include at least one isobutane and isobutene separated subset separated and the remaining, Mixture containing methacrolein for feeding a Reaction zone is used in which the methacrolein one selective heterogeneously catalyzed gas phase partial oxidation is subjected to the formation of methacrylic acid. 4. The method according to claim 3, characterized in that one from the subset containing isobutane and isobutene, iso- Butane and iso-butene by absorption in a non-polar separates organic solvent, the isobutane and isobutene by subsequent desorption and / or stripping from the The absorbate is liberated and returned to reaction zone A. 5. The method according to claim 1 or 2, characterized in that the methacrolein contained in the product gas mixture B for Production of methyl methacrylate is used, from which Product gas mixture B of those contained in Normal pressure lower than components boiling methacrolein at least one subset containing isobutane and isobutene separated and the remaining methacrolein containing mixture used to feed a reaction zone in which the methacrolein in the liquid phase in Presence of oxygen and a catalyst with methanol Methyl methacrylate is implemented. 6. The method according to claim 5, characterized in that one from the subset containing isobutane and isobutene, iso- Butane and iso-butene by absorption in a non-polar separates organic solvent, the isobutane and isobutene by subsequent desorption and / or stripping from the The absorbate is liberated and returned to reaction zone A. 7. The method according to any one of claims 1 to 2, characterized characterized in that contained in the product gas mixture B. Methacrolein is used to make methacrylic acid the product gas mixture B containing methacrolein as is used to feed a reaction zone, in which the methacrolein is a selective heterogeneous catalyzed gas phase partial oxidation to form Is subjected to methacrylic acid.