DE10219685A1 - Production of methacrylic acid from isobutane comprises formation of methacrolein using a specified molar ratio of oxygen to nitrogen followed by further oxidation to methacrylic acid. - Google Patents

Abstract

Production of methacrylic acid from isobutane comprises partial dehydrogenation of isobutane, separation of isobutane and isobutene and catalyzed partial oxidation of isobutene with molecular oxygen in the gas phase to form a methacrolein containing product mixture. A process for the production of methacrylic acid 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 whereby the molar conversion of isobutene is at least 95 mol.% (C) the methacrolein containing product mixture from (B) is transferred to a reaction zone (C), without separation of the components, to undergo a selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase to form a methacrylic acid containing product mixture and is characterized in that components in the reaction mixture from (A) other than isobutane and isobutene are separated off to at least 80 mol.% and that the molar ratio of molecular oxygen to molecular nitrogen in reaction zone (B) is 1:1-1:10.

Description

• A) in einer Reaktionszone A das iso-Butan einer partiellen selektiven heterogen katalytisierten Dehydrierung in der Gasphase unter Bildung eines Poduktgemisches A, das iso-Buten und nicht umgesetztes iso-Butan enthält, unterwirft,
• B) iso-Butan und iso-Buten enthaltendes Produktgasgemisch A zur Beschickung einer Reaktionszone B verwendet 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 mit der Maßgabe unterwirft, dass der molare Umsatz an iso-Buten ≥ 95 mol-% beträgt und
• C) Methacrolein enthaltendes Produktgasgemisch B ohne vorherige Abtrennung von darin enthaltenen Bestandteilen zur Beschickung einer Reaktionszone C verwendet und in der Reaktionszone C das Methacrolein einer selektiven heterogen katalysierten Partialoxidation mit molekularem Sauerstoff in der Gasphase unter Bildung eines Methacrylsäure enthaltenden Produktgasgemisches C unterwirft.

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

• A) in a reaction zone A, the isobutane is subjected to partial selective heterogeneously catalyzed dehydrogenation in the gas phase to form a product mixture A which contains isobutene and unreacted isobutane,
• B) product gas mixture A containing isobutane and isobutene is used to feed a 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, with the proviso that that the molar conversion of isobutene is ≥ 95 mol% and
• C) Methacrolein-containing product gas mixture B is used to feed a reaction zone C without prior removal of constituents contained therein, and in reaction zone C the methacrolein is subjected to selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase to form a product gas mixture C containing 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 used as a binder become.

DE-A 33 13 573 describes a method as described at the beginning known for the production of methacrylic acid from isobutane. As DE-A 33 13 573 considers one particularly advantageous Procedure in which the product gas mixture A without prior separation of components contained therein Feeding reaction zone B used and as a source for the in the Reaction zone B required molecular oxygen purer Oxygen is used.

However, a disadvantage of such a procedure is that on the one hand the selectivity of the methacrylic acid formation per se and on the other hand, the amount of formation of even the smallest amounts particularly undesirable since it is difficult to separate from the target product and by-products that are particularly disruptive in subsequent uses Butyraldehyde and iso-butyric acid are not fully satisfactory can.

The object of the present invention was therefore to begin with Process described for the preparation of methacrylic acid To provide iso-butane that the described Disadvantages 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 Produktgemische A, das iso-Buten und nicht umgesetztes iso-Butan enthält, unterwirft,
• B) iso-Butan und iso-Buten enthaltendes Produktgasgemisch A zur Beschickung einer Reaktionszone B verwendet 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 mit der Maßgabe unterwirft, dass der molare Umsatz an iso-Buten ≥ 95 mol-% beträgt und
• C) Methacrolein enthaltendes Produktgasgemisch B ohne vorherige Abtrennung von darin enthaltenen Bestandteilen zur Beschickung einer Reaktionszone C verwendet und in der Reaktionszone C des Methacrolein einer selektiven heterogen katalysierten Partialoxidation mit molekularem Sauerstoff in der Gasphase unter Bildung eines Methacrylsäure enthaltenden Produktgasgemisches C unterwirft,

gefunden, das dadurch gekennzeichnet ist, dass
man aus dem iso-Butan und iso-Buten enthaltenden Produktgasgemisch A vor seiner Verwendung zur Beschickung der Reaktionszone B von den darin enthaltenen, von iso-Butan und iso-Buten verschiedenen, Bestandteilen wenigstens 80 mol-% abtrennt und 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 preparation of methacrylic acid from isobutane, in which

• A) in a reaction zone A, the isobutane is subjected to a partial selective, heterogeneously catalyzed dehydrogenation in the gas phase to form a product mixture A which contains isobutene and unreacted isobutane,
• B) product gas mixture A containing isobutane and isobutene is used to feed a 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, with the proviso that that the molar conversion of isobutene is ≥ 95 mol% and
• C) product gas mixture B containing methacrolein is used to feed a reaction zone C without prior separation of constituents contained therein and is subjected to selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase in reaction zone C of methacrolein to form a product gas mixture C containing methacrylic acid,

found, which is characterized in that
at least 80 mol% is separated off from the product gas mixture A containing isobutane and isobutene before it is used to feed reaction zone B from the constituents contained therein, different from isobutane and isobutene, and that required in reaction zone B. Molecular oxygen 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 ratio V in the range from 1: 2 to 1: 5 or in is favorable Range from 1: 3 to 1: 5, or 1: 3.5 to 1: 4.5. In a more appropriate way The way in the method according to the invention is the aforementioned Accompanying molecular oxygen with molecular nitrogen realized so that the reaction zone B the molecular Oxygen as part of a gas that supplies the molecular Oxygen and molecular nitrogen in one contains the ratio V mentioned, or only from molecular oxygen and molecular nitrogen in such a ratio V. In the method according to the invention, preference is given as a source for the molecular oxygen required in reaction zone B. at least partially, particularly preferably predominantly or only air is 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 an additional one Mixture of molecular nitrogen and molecular oxygen, that the two gas components in a different ratio than Contains air to be supplied. It is only essential to the invention that overall the ratio V is maintained. According to the invention reaction zone B could be such 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.

Another feature essential to the invention is that one from the containing isobutane and isobutene Product gas mixture A before it is used to feed the reaction zone B of the contained therein, isobutane and isobutene various constituents separates at least 80 mol%. The The molar reference base is the sum of the B in the product gas mixture contained molar amounts of the various individual components. According to the invention, the aforementioned separation does not have to be homogeneous the different components are done. Rather, it can Capture individual components quantitatively and others only partially. The sum of the above-mentioned percentages of 80 mol% can be achieved. According to the invention are preferred from the Product gas mixture A before it is used to feed the Reaction zone B from those contained therein, from isobutane and iso- Butene various, constituents at least 85 mol%, especially preferably at least 90 mol%, very particularly preferably at least 95 mol%, better at least 97 mol% and best at least 99 mol% or 100 mol% separated.

It goes without saying that the feed gas mixture (= the mixture of all gas streams fed to the reaction zone) of reaction zone B in the procedure according to the invention in addition to the constituents already mentioned, other constituents such as, for. 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 700 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 16: 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 16: 1, or 2: 1 to 10: 1, or 2: 1 to 4: 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: 20-70: 5-20: 0-20: 0-5: 0-5 ,

The aforementioned molar ratios are advantageously according to the invention = 15-25: 4-8: 40-60: 10-15: 5-15: 0-1: 0.1-3. Frequently they will be about 20: 6: 50: 12: 10: 0: 2.

An essential feature of the procedure according to the invention is that in reaction zone A, in contrast to the case of a homogeneous 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.

Before using product gas mixture A to load the According to the invention, reaction zone B becomes one of the therein amount of molecular hydrogen usually at least 80 mol%, preferably at least 85 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%, or at least 97 mol%, or at least 99 mol% and often that Separate the total amount of molecular hydrogen.

As a rule, the molar in the process according to the invention Ratio of isobutene contained in product gas mixture A to im Molecular hydrogen product gas mixture A containing ≤ 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 contrast, in the method according to the invention molar ratio of in the feed gas mixture of reaction zone B (i.e., molecular weight contained in the feed gas mixture B) Hydrogen to isobutene contained in the feed mixture B in usually ≤ 1: 10, usually ≤ 1: 50, often ≤ 1: 100.

In addition to molecular hydrogen, the product gas mixture A, as constituents other than isobutane and isobutene, generally also contains gases from the group comprising N ₂ , CO, CO ₂ , H ₂ O, methane, ethane, ethylene, propane, propylene and, if appropriate, O ₂ and others.

It goes without saying that in the process according to the invention Product gas mixture A before it is used to feed the Reaction zone B, the above-mentioned components usually in Composite, d. that is, usually together with the im Product gas mixture A contained molecular hydrogen, from in Product gas mixture A largely contains isobutane and isobutene (e.g. to at least 80 mol%, or to at least 85 mol%, or to at least 90 mol% or at least 95 mol%, or at least 97 mol% or at least 99 mol%) or completely separate. However, such a composite separation is not in accordance with the invention mandatory. Rather, according to the invention, it can also be used by an Product gas mixture A contained more and one component other component contained in the product gas mixture A less be separated. According to the invention, however, one is preferred the product gas mixture A, before it is used to feed the Reaction zone B, of all contained therein, of isobutane and Isobutene different, constituents at least a subset split off.

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) the isobutene yield is usually at least 30 mol% in a single pass (based on the isobutane used)), by-products such as methane, ethylene, propane, propene and Ethan 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 dilution gas in the subsequent oxidation stage B. However, water vapor can also be easily partially or completely removed from the product gas mixture A of the process A according to the invention (for example by condensation), which opens up the possibility of using the product gas mixture A ′ obtainable in the reaction zone B as essential to the invention to be used to increase diluent gas N ₂ . 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 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; that is, the feed gas mixture of reaction zone A can consist only 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, which is why their quantitative separation from product gas mixture A is not essential according to the invention. 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 dehydrogenation catalysts known in the prior art are suitable for stage A according to the invention. They can be roughly divided into two groups. Namely in those which are oxidic in nature (e.g. chromium oxide and / or aluminum oxide) and in those which consist of at least one, usually comparatively noble, metal (e.g. Platinum).

Among other things, for stage A according to the invention all dehydrogenation catalysts 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, the DE-A 199 37 105, DE-A 102 11 275 and DE-A 199 37 107 be recommended. In particular, for everyone in this document as suitable for reaction zone A according to the invention mentioned 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 Shaping geometries. 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 expediently 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) mixes kneading and comparatively well 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 basically be used in all of the state of the art for heterogeneously catalyzed, partial dehydrogenation of Known hydrocarbons in the gas phase on fixed bed catalysts Reactor types can be realized.

Typical reaction temperatures range from 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 be either in advance of the reaction gas and / or fed in the course of catalytic dehydrogenation.

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 used in the process according to the invention take as product gas mixture A and according to the invention Feed off 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.

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 cold analyzer, such as. B. in the scriptures US-A 4 788 372, US-A 4 88 928, US-A-5 430 209, US-A 5 550 171, US-A 5 527 979, US-A 5 563 314 and EP-A 832 056, is in a reaction tube or in a bundle of Reaction tubes as a fixed bed. The reaction tubes are Usually heated indirectly in that the reaction tubes surrounding space a gas, e.g. B. a hydrocarbon such as methane, is burned. 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 bed length by the in Radiation heat released during combustion to the required reaction temperature to warm up. Indirect heating of the reaction gas can advantageously with direct Heating by combustion with molecular oxygen in the Reaction gas can be coupled. This way is comparatively simple form an approximately isothermal reaction can be achieved.

Suitable reaction tube inner diameters are about 10 to 15 cm. A typical bundle dehydrogenation tube reactor comprises 300 to 1000 reaction tubes. The temperature in the interior of the reaction tube is in the range from 300 to 700 ° C., preferably in the range from 400 to 700 ° C. The working pressure is usually in the range of 0.5 to 8 bar, often 1 to 2 bar or 3 to 8 bar. The reaction gas is advantageously fed to the tubular reactor preheated to the reaction temperature. As a rule, the product gas mixture leaves the reaction tube at a different (higher or lower) temperature than the inlet temperature (cf. also US Pat. Nos. 4,902849, 4,996,387 and 5,389,342). In the context of the aforementioned procedure, the use of oxidic dehydrogenation catalysts based on chromium and / or aluminum oxide is expedient. Often you will not use a diluent gas, but will start with essentially pure isobutane as the starting reaction gas. The dehydrogenation catalyst is also usually used 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 regeneration and then reused. As a dehydrogenation catalyst for this process, for. B. a spherical dehydrogenation catalyst can be used in the essentially made of platinum on a spherical aluminum oxide support consists. 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 832 056.

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 has in common that it is used in isobutane conversions of> 30 mol% (usually ≤ 70 mol%) are operated (based on 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.

The nitrogen also achieves essentially the same effect in reaction zone C.

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 one-off Reactor passage of the reaction gas is only a comparative one low amount of heat is consumed and contributes to sales comparatively low one-time reactor run 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 heater), 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 reaction zone according to the invention is also 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 reaction zone according to the invention A variant with comparatively low isobutane 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 repeat 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 or the same 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 operations 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, all of them are in DE-A 102 11 275 described dehydration 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 separations from product gas mixture A 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 generally molecular hydrogen. In addition, however, it will generally 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 is usually at a pressure of 0.3 to 10 bar are located 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 prior to 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.

The essential separation of iso-butane and Isobutene various components from product mixture A can be done in different ways. So can successively u. a. Separation methods are applied from each of which is just individual components abzutrennvermag.

For example, you can separate the hydrogen in that the product gas mixture A, if necessary after using it beforehand cooled in an indirect heat exchanger (more appropriate Way, the heat extracted thereby becomes one for heating feed gas used according to the invention), passes over a membrane, usually designed as a tube, which is only permeable to the molecular hydrogen. The individually separated molecular hydrogen can Requires at least partially in reactor zone A or one other evaluation. In the simplest case, it can be in Fuel cells are burned.

Water vapor contained in the reaction gas mixture A can in one Condensation stage individually separated in a simple manner if necessary, at least partially into reaction zone A be returned.

A separation of hydrogen and water vapor is e.g. B. thereby possible that the hydrogen contained in the reaction gas mixture by means of oxygen through selective heterogeneously catalyzed Combustion on suitable catalysts converted into water vapor and subsequently separated by condensation. Will in Reaction gas mixture A optionally contained oxygen for the aforementioned selective combustion can be used on the aforementioned Way at the same time that contained in the product gas mixture A. molecular oxygen to be separated with. Regarding one disclose suitable catalysts for such selective combustion for example US-A 4 788 371, US-A 4 886 928 which US-A 5 430 209, US-A 55 530 171, US-A 55 279 979 and US US-A 5 563 314.

In a corresponding manner, CO contained in reaction gas mixture A can be selectively burned to CO ₂ using suitable catalysts and the same can be separated off, together with CO ₂ already contained in reaction gas mixture A, by washing with a basic liquid. As such basic liquids come e.g. B. aqueous alkali hydroxide solutions, aqueous ammonia solutions or organic amines. Nitrogen remaining in a mixture with isobutane and isobutene can, if necessary, be separated from it by condensation of the hydrocarbons.

An easy way to do essentially all of isobutane and Iso-butene different components the product gas mixture A to separate is to cool, preferably cool (preferably at temperatures of 10 to 70 ° C), product gas mixture A, e.g. B. at a pressure of 0.1 to 50 bar and a temperature from 0 to 100 ° C, with a (preferably high-boiling) organic solvent (preferably a hydrophobic) in which Isobutane and isobutene are preferably absorbed in contact to bring (e.g. by simply passing through). By following Desorption (e.g. by heating, flash evaporation (Flashing) and / or distillation (rectification)) or stripping with a gas which is inert with respect to reaction zone B. (e.g. nitrogen and / or water vapor) and / or molecular Oxygen or mixtures of inert gases and molecular Oxygen (e.g. air) becomes the isobutane and isobutene in a mixture recovered and used to feed reaction zone B.

When using air respectively Oxygen / nitrogen mixtures in which the oxygen content exceeds 10% by volume , as stripping gas, it can be useful before or during the Add a gas to the stripping process that covers the explosion area reduced. Gases with a. Are particularly suitable for this purpose Heat capacity of ≥ 29 J / mol.K (based on 25 ° C and 1 atm). To the For example, isobutane can also be used as such a gas become.

The molecular hydrogen containing exhaust gas of the absorption can, for. B. again subject to a membrane separation and then, if necessary, use the separated hydrogen in reaction zone A. The residual gas remaining after the hydrogen separation can also be used as a diluent gas in reaction zones A, B and / or C. The boiling point of the organic absorbent should preferably be 100 100 ° C, particularly preferably 180 180 ° C. The absorption can be carried out in columns as well as in rotary absorbers. You can work in cocurrent or in countercurrent. Suitable absorption columns are e.g. B. tray columns (with bell, centrifugal and / or sieve trays), columns with structured packing (e.g. sheet metal packing with a specific surface area of 100 to 500 m ² / m ³ , e.g. Mellapak® 250 Y) and Packed columns (e.g. filled with Raschig packings). Of course, trickle and spray towers, graphite block absorbers, surface absorbers such as thick-film and thin-film absorbers as well as rotary columns, plate washers, cross-curtain washers and rotary washers can also be used.

It is favorable according to the invention if the one to be used organic absorbent on the one hand the already given Recommendation for the boiling point fulfilled, on the other hand at the same time but has a not too high molecular weight. With advantage the molecular weight of the absorbent is ≤ 300 g / mol.

Suitable absorbents according to the invention are e.g. B. relatively non-polar organic solvents, which preferably do not contain any polar group acting outwards. Examples include aliphatic (e.g. C ₈ - to C ₁₈ -alkenes) or aromatic hydrocarbons, e.g. B. middle oil fractions from paraffin distillation, or ethers with bulky groups on the O atom, or mixtures thereof, these a polar solvent 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, e.g. B. 4-methyl-4'-benzyl-diphenylmethane and its isomers 2-methyl-2'-benzyl-diphenyl-methyl, 2-methyl-4'-benzyl-diphenylethane and 4-methyl-2'-benzyl-diphenylmethane and Mixtures of such isomers. A suitable absorbent is 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 diphyl. 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. Octane, nonane, decane, undecane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, heptadecane and octadecane should also be mentioned as possible absorbents.

Also suitable are those described in DE-A 33 13 573 Paraffin oils with 8 to 10 carbon atoms. Examples of suitable ones Commercial products are e.g. B. from the company Haltermann distributed products Halpasole®i, such as. B. Halpasol 250/340 i and Halpasol 250/275 i, as well as printing ink oils under the names PKWF and Printosol.

In order to separate the isobutene from the absorbate Minimizing absorbent losses can be both when stripping resulting gas phase and also one during distillation or Rectify the rising gas phase using water in countercurrent getting washed. As wash water can be in the exhaust gas or absorption contained water vapor used after its condensation become. Otherwise, in the case of absorptive separation as in the Procedure described in WO-0196271 using the example of propane / propene become.

An alternative option iso-butane and iso-butene various components from the product gas mixture A separating offers fractional distillation (rectification). In A fractional pressure distillation is expediently used performed at low temperatures. The pressure to be applied can z. B. 10 to 100 bar. Can be used as rectification columns Packed columns, tray columns or packed columns be used. Bottom columns are those with dual flow Bottoms, bell bottoms or valve bottoms. In a simple way z. B. two rectification columns connected in series become. In the first column they can be heavier than isobutane and Iso-butene boiling constituents are separated off at the bottom. In The second column can then use isobutane and isobutene from the lighter-boiling constituents can be separated off at the bottom.

After the separation of at least required according to the invention 80 mol% of iso-butane contained in product gas mixture A. and iso-butene different, components can do it Product gas mixture A 'obtained for feeding the reaction zone B can be used. If necessary, the product gas mixture A 'in advance by indirect heat exchange to that in the Reaction zone A 'brought required reaction temperature.

The methacrolein-containing methoxide formed in reaction zone B Product gas mixture B is then without prior separation from it contained components for feeding a reaction zone C used.

The basis for the design of the second part of the method according to the invention in two spatial successive reaction zones B, C forms the fact that the heterogeneous catalyzed gas phase partial oxidation of isobutene with molecular oxygen to methacrylic acid in two along the Reaction coordinate runs consecutive steps, of which the first to methacrolein and the second from methacrolein to Methacrylic acid leads.

The one to be used can be used in each of the two reaction zones B, C oxidic catalyst also adapted in an optimizing manner become like the reaction conditions. So for the first one Odxidationszone, the reaction zone B (iso-butene → methacrolein), in the Usually a catalyst based on the element combination Mo-Bi-Fe containing multimetal oxides preferred, while for the second oxidation zone, reaction zone C (Methacrolein → methacrylic acid) normally catalysts on the Basis of the Mo-P element combination Multimetal oxides (especially the so-called heteropolyacids) are preferred become.

For example, US-A 4 954 650, US-A 5 166 119, 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 (multimetal oxide materials of the 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 turned 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 , which are different from their local surroundings due to their composition which is different from their local surroundings, and their maximum diameter (longest connecting section going through the center of gravity of the area of two points on the surface (interface) of the area) is 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 2.5.

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 it is used 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 'supplied to reaction zone B. However, oxygen is preferably no longer contained therein. 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 any already contained in the product gas mixture A ' molecular oxygen, as a source of molecular oxygen in the subsequent reaction zone B at least partially and preferably only air is used.

By adding cold air at room temperature hot product gas mixture A 'can be used in the context of the invention If necessary, the process cools down the product mixture A ' its way into reaction zone B can be effected.

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 (the molar conversion of isobutene in reaction zone B according to the invention is preferably 96 96 or ≥ 97 mol%, particularly preferably ≥ 98 mol% and entirely particularly preferably ≥ 99 mol%), molecular nitrogen (optionally molecular hydrogen), as a by-product and / or water vapor used as diluent gas, as By-product and / or used as a diluent Carbon oxides, as well as small amounts of other lower aldehydes, Hydrocarbons and other inert diluent gases. The salary of iso-butyraldehyde and iso-butyric acid, however minimized according to the invention. It is essential according to the invention that the molar Conversion of isobutene in reaction zone B is ≥ 95 mol%.

Cheap catalysts for the reaction zone C disclose z. B. DE-A 44 05 060, US-A 5 166 119, US-A 5 153 162, the US-A 4 954 650, US-A 4 558 028 and DE-A 198 15 279. You also teach the manufacture of such catalysts. In addition to molybdenum and phosphorus, they usually contain metallic and transition metal 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 = 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 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 coefficients a of the variable X ^{1 of} individual crystallites within component A of the multimetal oxide mass being less than 0.40, is 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. those 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 tubes are 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 3 or 2 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 considered as a further oxygen source which can 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 process according to the invention is often carried out in such a way that 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 Dilution gases ratio of 2-5: 5-15: 0-20: 5-25: 20-80: 0-6 worked.

In principle, however, reaction zones B and C can be formally can also be combined into a single reaction zone. In this In this case, the two reaction steps take place (iso-butene → methacrolein; Methacrolein → methacrylic acid) in an oxidation reactor which is charged with a catalyst, the implementation of both can catalyze successive reaction steps.

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 steam used as diluent gas, (if necessary molecular hydrogen), as a by-product and / or as Diluent gas with used carbon oxides, as well as small amounts other lower aldehydes, hydrocarbons and other inert Diluent gases. Its content of iso-butyraldehyde and According to the invention, isobutyric acid is minimized.

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 8000 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 deducted from the sump of the absorber, 10 to 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 , (optionally 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.

According to the invention, the residual gas remaining after the methacrylic acid separation, which normally contains unreacted isobutane and isobutene, will be recycled as such into reaction zone A. However, in the process according to the invention, the iso-butane and isobutene which are generally present from other gases such as O ₂ can also be obtained from the residual gas which remains after the methacrylic acid has been separated off by absorption with subsequent desorption and / or stripping and the use of absorbent in a high-boiling hydrophobic organic solvent (optionally H ₂ ), N ₂ , CO, CO ₂ , noble gases, etc. are essentially separated off first and then returned to reaction zone A. If necessary, the mixture of the remaining other gases can be recycled as the diluent gas into reaction zones B and / or C. But you can also be removed, e.g. B. burned.

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 ≥ 29 J / mol-K (based on 25 ° 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 . 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 is an unreacted isobutane and usually Residual gas containing iso-butene remains. Preferred according to the invention is, as already mentioned, as such in the Reaction zone A recycled.

Of course, according to the invention, the procedure can also be such that only part of the residual gas remains unchanged in reaction zone A. recycles and only from the remaining part iso-butane and separates isobutene in a mixture and also into reaction zone A. recirculates.

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.

Of course, separated isobutane and / or isobutene or residual gas containing these gases also for other recycling purposes be fed as a return to reaction zone A. (e.g. the production of iso-butanol or the combustion for Purpose of energy production).

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.

As a rule, in reaction zone C in the invention Molar methacrolein procedure: Oxygen:  Water vapor: inert gas ratio of (2-5): (5-15): (0-20): (5-25):  (20-80): (0-6), particularly preferred from (3-4): (6-10): (10-20):  (10-20): (40-70): (0-4) worked. Just like that Reaction zone B can not only in reaction zone C. Fixed bed reactors but also implemented in fluidized bed reactors become.

The (as always in this document) on a simple reactor run related methacrolein conversion in the reaction zone is C. usually about 60 to 90 mol%.

It is generally true that when gases returned to reaction zone A contain O ₂ , this oxygen in the reaction zone can be used to selectively burn combustible substances such as hydrocarbon, 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 returned to reaction zone A has a sufficient amount of oxygen for this purpose.

The process according to the invention is often carried out in such a way that 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.

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® ™) 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 V _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 mixed intimately. 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 at a External wall temperature control along the heating zone of 500 ° C (based on a pipe through which an identical inert gas stream flows) 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 by means of a mass flow controller Brooks metered in, while the water using a HPLC Pump 420 from Kontron initially liquid into one Evaporator dosed, evaporated in the same and then with the iso- Butane and the air is mixed. The temperature of the Feed gas mixture is 150 ° C when loading. By means of one located at the reactor outlet Pressure control from REKO becomes the outlet pressure at the tubular reactor set to 1.5 bar.

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 means of GC (HP 6890 with chemical station, detectors: FID; WLD, separation columns: Al ₂ O ₃ / KCl (chrome pack), carboxen 1010 (Supelco). The feed gas mixture is also analyzed accordingly.

The following analysis results are obtained after three weeks of operation:

A throughput corresponds to these values related iso-butane conversion of 25 mol .-% and a selectivity of Isobutene formation of 90 mol%.

The reaction zone B reactor from 2nd is on its in the furnace part to an outside wall temperature (based on a pipe through which an identical inert gas stream flows) regulated, in which the iso-butene turnover with a single Passage of reaction mixture is 98 mol%. The heating tape on Reaction tube entrance (where the contact chair is located) is also set to this temperature and that Heating tape at the reaction tube outlet is reduced to a 50 ° C Temperature set. The product gas mixture A become all components different from isobutane and isobutene separated.

The remaining mixture of 15 Nl / h isobutane and 4.5 Nl / h Together with 45 Nl / h, iso-butene forms air and 7 Nl / h Steam the feed gas mixture for reaction zone B- Reactor. The temperature of the feed gas mixture is raised to Reactor outer wall temperature brought. By means of a Reaction tube outlet pressure control is the Outlet pressure of the reactor set to 1.3 bar.

The product gas mixture B (Temperature = 300 ° C) relaxed to normal pressure and using GC (HP 6890 with Chem-Station, detectors: TCD, FID, separation columns: Poraplot Q (chrome pack), carboxen 1010 (Supelco)) analyzed. The feed gas mixture is also identical analyzed.

After 3 weeks of operation, the following results are obtained:

A single pass corresponds to these values related iso-butene conversion of 98 mol% and a selectivity of Methacrolein formation of 84 mol%.

The reaction zone C reactor is on its in the furnace part to an outside wall temperature of 290 ° C (based on a pipe through which an identical inert gas stream flows) regulated.

The heating tape at the reaction tube entrance (where the Contact) is set to 290 ° C and the heating tape on The reaction tube outlet is set to 200 ° C.

The feed gas mixture of the reaction zone C reactor consists of 29 Nl / h air and the product gas mixture B (73 Nl / h). The temperature of the feed gas is brought to 290 ° C.

By means of one located at the reactor outlet Pressure control, the pressure at the reaction tube outlet is 1.3 bar set.

The product gas mixture C (Temperature: 200 ° C) relaxed to normal pressure and using GC (HP 6890 with Chem-Station, detectors: TCD, FID, separation columns: Poraplot Q (chrome pack), Carboxen 1010 (Supelco) analyzed. The feed gas is also produced in a corresponding manner analyzed.

After 3 weeks of operation, the following results are obtained:

A single pass corresponds to these values related methacrolein conversion of 60 mol .-% and a selectivity the methacrylic acid formation of 81 mol%.

5. Comparative examples

• A) The method according to 4. is repeated. When charging the reaction zone B reactor, however, a mixture of the entire product gas mixture A (41 Nl / h, of which 11 Nl / h is water vapor) and 45 Nl / h air is used as the feed gas mixture.
  The loading of the catalyst feed in the reaction zone B reactor with isobutene or oxygen is thus, as in 4. 4.5 Nl / h isobutene and 9 Nl / h oxygen, and the loading of the catalyst feed in the reaction zone C reactor with methacrolein or oxygen, as in 4. 3.8 Nl / h methacrolein and 8 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%.
• B) The method according to 4. is repeated, however, for the feed gas for reaction zone B- Reactor not 45 Nl / h air, but 9 Nl / h pure molecular Oxygen used. The total salary of the Product gas mixture C is isobutyraldehyde + isobutyric acid noticeably increased compared to the experiment in 4th.

Claims (11)

1. A process for the preparation of methacrylic acid 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 mixture A which contains isobutene and unreacted isobutane, B) product gas mixture A containing isobutane and isobutene is used to feed a 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, with the proviso that that the molar conversion of isobutene is ≥ 95 mol% and C) product gas mixture B containing methacrolein is used to feed a reaction zone C without prior separation of constituents contained therein and is subjected to selective heterogeneously catalyzed partial oxidation with molecular oxygen in the gas phase in reaction zone C of methacrolein to form a product gas mixture C containing methacrylic acid, characterized in that at least 80 mol% is separated from the product gas mixture A containing isobutane and isobutene before it is used to feed reaction zone B from the constituents contained therein, other than isobutane and isobutene, and the in Molecular oxygen required in reaction zone B is fed to reaction zone B accompanied by molecular nitrogen, the molecular ratio V of molecular oxygen to molecular nitrogen being 1: 1 to 1:10. 2. The method according to claim 1, characterized in that V 1: 3 to 1:10. 3. The method according to claim 1, characterized in that V 1: 3 to 1: 6. 4. The method according to any one of claims 1 to 3, characterized characterized in that as a source for the in reaction zone B oxygen is at least partially used. 5. The method according to any one of claims 1 to 4, characterized characterized that one is from the isobutane and isobutene containing product gas mixture A before its use for Feeding reaction zone B from those contained therein Isobutane and isobutene various components separates at least 90 mol%. 6. The method according to any one of claims 1 to 4, characterized characterized that one is from the isobutane and isobutene containing product gas mixture A before its use for Feeding reaction zone B from those contained therein Isobutane and isobutene various components separates at least 95 mol%. 7. The method according to any one of claims 1 to 5, characterized characterized in that in reaction zone A the conversion of Isobutane is ≥ 5 mol% to ≤ 30 mol%. 8. The method according to any one of claims 1 to 7, characterized characterized in that the separation of isobutane and iso- Butene different components from the product gas mixture A carried out so that the product gas mixture A with a organic solvent in contact, in which isobutane and isobutene are preferably absorbed and absorbed by subsequent desorption and / or stripping the isobutane and iso- Butene freed from the absorbate and used to feed the Reaction zone B used. 9. The method according to any one of claims 1 to 8, characterized characterized that the molar conversion of iso-butene in the Reaction zone B is ≥ 97 mol%. 10. The method according to any one of claims 1 to 8, characterized characterized that the molar conversion of iso-butene in the Reaction zone B is ≥ 98 mol%. 11. The method according to any one of claims 1 to 10, characterized characterized in that the product gas mixture B for the purpose of Feeding of reaction zone C supplemented with air.