EP1656335A1

EP1656335A1 - Method for the production of (meth)acrolein and/or (meth)acrylic acid

Info

Publication number: EP1656335A1
Application number: EP04741050A
Authority: EP
Inventors: Jochen Petzoldt; Signe Unverricht; Heiko Arnold; Klaus Joachim MÜLLER-ENGEL; Martin Dieterle
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2003-08-14
Filing date: 2004-07-15
Publication date: 2006-05-17
Also published as: TW200523245A; US20050038291A1; KR20060061357A; WO2005016861A1; RU2361853C2; RU2006107501A; BRPI0413440A; US7015354B2; JP2007502254A; MY138925A

Abstract

A method for the production of (meth)acrolein and/or (meth)acrylic acid by heterogeneously catalyzed gas phase partial oxidation of an organic precursor compound on a fresh catalyst fixed bed, wherein the method is initiated with a reduced charge of the catalyst fixed bed with a charging gas mixture.

Description

Process for the preparation of (meth) acrolein and / or (meth) acrylic acid

description

The present invention relates to a process for the preparation of (meth) acrolein and / or (eth) acrylic acid by heterogeneously catalyzed gas phase partial oxidation, in which a fresh fixed catalyst bed in a reactor is charged at elevated temperature with a feed gas mixture which, in addition to at least one, is partially added oxidizing organic precursor compound and molecular oxygen as the oxidizing agent comprises at least one diluent gas which is essentially inert under the conditions of the heterogeneously catalyzed gas phase partial oxidation.

In this document, the notation (meth) acrolein stands for methacrolein or acrolein.

In this document, (meth) acrylic acid stands for methacrylic acid or acrylic acid.

(eth) acrolein and (meth) acrylic acid form reactive monomers which e.g. for the production of polymers that include can be used as adhesives are suitable.

Industrially, (meth) acrolein and (meth) acrylic acid mainly by heterogeneously catalyzed gas-phase partial oxidation of suitable C ₃ - / C -Vorläufer- compounds, in particular of propene and propane in the case of acrolein and acrylic acid or of iso-butene and iso-butane in the case of methacrylic acid and methacrolein. In addition to propene, propane, isobutene and isobutane, other compounds containing 3 or 4 carbon atoms, such as isobutanol, n-propanol or the methyl ether (as a precursor of a C ₄ precursor) from iso, are also suitable as starting materials butanol. (Meth) acrylic acid can also be generated from (meth) acrolein.

The catalysts to be used for such gas phase partial oxidations are normally multi-element oxides in the solid state.

The heterogeneously catalyzed gas-phase partial oxidation of C / C precursors to (meth) acrolein and / or (meth) acrylic acid is therefore usually carried out by charging a fixed catalyst bed at elevated temperature with a Feed mixture which, in addition to the at least one organic precursor compound to be partially oxidized, contains molecular oxygen as the oxidizing agent.

The fixed catalyst bed is usually surrounded by an envelope (e.g. it can be in the contact tubes of a tube bundle reactor). On this side of the envelope, the exothermic partial oxidation takes place during the dwell time on the catalyst surface, and on the other side of the envelope a heat transfer medium (e.g. a salt bath) is conducted to absorb and remove the heat of reaction.

In addition, the reactants are generally diluted with a gas which is essentially inert under the conditions of the gas phase partial oxidation and which, with its heat capacity, is able to absorb additionally released heat of reaction and, in most cases, can at the same time have a favorable influence on the explosion behavior of the feed gas mixture. In addition, it usually has an advantageous influence on the reaction rate. Non-combustible gases are typically used as the inert diluent gases.

One of the most common inert diluent gases used is molecular nitrogen, which is automatically used whenever air is used as the oxygen source for the heterogeneously catalyzed gas phase partial oxidation.

Another common diluent gas is water vapor because of its general availability. In many cases, cycle gas is also used as an inert diluent gas (see e.g. EP-A 1180508). Circular gas is the residual gas that remains in the heterogeneously catalyzed gas phase partial oxidation of the at least one organic precursor compound when the target product ((meth) acrolein and / or (meth) acrylic acid) is more or less selectively (e.g. through Absorption in a suitable solvent) has separated. As a rule, it consists predominantly of the inert diluent gases used for the heterogeneously catalyzed gas phase partial oxidation as well as of water vapor usually formed as a by-product in the gas phase partial oxidation and of carbon oxides formed by undesired complete secondary oxidation. In some cases, it contains small amounts of oxygen (residual oxygen) not consumed in the gas phase partial oxidation and / or of unreacted organic starting compounds. Usually only a subset of the residual gas is used as circulating gas. The remaining amount of residual gas is usually burned.

Depending on the chosen catalyst feed and reaction conditions, the gas phase partial oxidation of the precursor compound can predominantly to (meth) acrolein, or lead to a mixture of (meth) acrolein and (meth) acrylic acid, or predominantly to (meth) acrylic acid.

The reason for this is that the gas phase partial oxidation of suitable C ₃ - / C precursor compounds to (meth) acrylic acid normally takes place in two successive steps. The first step leads to (meth) acrolein and the second step to (meth) acrylic acid.

As a rule, the two steps are carried out on catalyst feeds which are different from one another and are arranged spatially one behind the other, the individual catalyst feed being tailor-made for the respective reaction step to be catalyzed. One then speaks of a multi-stage gas phase partial oxidation. In the first stage (meth) acrolein is predominantly formed. The product gas mixture leaving the first stage is then passed directly into the second stage, where appropriate after intermediate cooling and / or supplementation with molecular oxygen (for example in the form of air), where the (meth) acrolein formed in the first stage leads to (meth ) acrylic acid is further oxidized.

The temperature in the respective reaction stage is usually also adapted in an optimizing manner to the respective reaction step.

The respective reaction stage is useful in a separate application

Reactor (e.g. in a tube bundle reactor) realized (see e.g. EP-A 700 893 and

EP-A 700 714).

However, both reaction stages can also be carried out in a single reactor, which then generally has more than one temperature zone (cf. e.g.

EP-A 1 106 598 and EP-A 990 636).

However, multielement oxide active materials are also known which can catalyze more than just one step (cf., for example, EP-A 962 253, EP-A 1 260 495, DE-A 10 122 027, EP-A 1 192 987 and EP-A 962 253).

In such cases, depending on the reaction conditions chosen, either essentially only (meth) acrolein, or a mixture of (meth) acrolein and (meth) acrylic acid, or essentially only (meth) acrylic acid can be produced in one reaction step. Such a reaction stage is normally carried out in a reactor.

Of course, however, a single reaction step can also be carried out in a reactor which more than one to improve the target product selectivity Has temperature zone, as is recommended, for example, in EP-A 1 106598, in WO 00/53556, in WO 00/53559, in WO 00/53557 and in WO 00/53558.

For the production of (meth) acrolein and / or (meth) acrylic acid by a process of heterogeneously catalyzed gas phase partial oxidation, therefore, at least one precursor compound to be partially oxidized, molecular oxygen as the oxidizing agent and at least one under the conditions of the heterogeneously catalyzed gas phase Reaction gas starting mixture comprising the partial oxidation of essentially inert diluent gas (the feed gas mixture) is passed through a fixed catalyst bed feed at elevated temperature (generally a few hundred ° C., usually 100 to 600 ° C.). The chemical conversion takes place during the contact time on the catalyst surface, and the heat of reaction is given off primarily by indirect heat exchange to a flowing heat transfer medium.

A disadvantage of such a heterogeneously catalyzed gas phase partial oxidation is that the heat of reaction has to be dissipated at a sufficient rate on the one hand in order to avoid overheating of the system. On the other hand, the heat must not be dissipated too quickly, otherwise the reaction may fall asleep. Conversely, especially at the beginning, the reaction must develop sufficient heat to start. This balance is made more difficult by the fact that the reactant concentration during passage through the catalyst feed is not constant, but rather decreases.

In the exit area of the reaction gas mixture from the fixed catalyst bed, this has a reducing effect on the reaction rate and the associated heat development, while in the entry area of the reaction gas mixture into the catalyst feed, the high reactant concentration accelerates the exothermic heat development.

The situation described above is further complicated by the fact that a fresh fixed catalyst bed does not have a stationary activity behavior, but instead goes through a so-called formation phase.

In order to avoid excessive, possibly uncontrolled, local heat developments when starting up a fresh catalyst feed, WO 02/098827 recommends changing the composition of the feed gas mixture over time such that a feed gas mixture with a very low content of the partially oxidized organic substances is initially used for at least one hour Compound (typically 0 to ≤ 0.5 vol .-%) is used. The content of the reactants is then Gaseous gas mixture increased in stages. As the reactant concentration in the feed gas mixture increases, the reactant ratio is also varied. Finally, an essentially stationary feed gas mixture is passed over the fixed catalyst bed.

As soon as the feed gas mixture contains the organic compound to be partially oxidized, the loading of the catalyst feed with the feed gas mixture is kept constant.

However, the procedure described in WO 02/098827 is disadvantageous insofar as there is no essentially stable charge gas mixture composition when exercised over several hours of operation. This is disadvantageous in that, depending on its composition, the feed gas mixture can have explosive and non-explosive states (cf. DE-A 10232482). Frequent changes in its composition should therefore be avoided.

In addition, it is not advantageous to start the gas phase partial oxidation under full load (final loading of the fixed catalyst bed with feed gas mixture), since a high loading of the fixed catalyst bed is synonymous with a shorter mean residence time in the fixed catalyst bed. However, a short residence time limits the time available for reaction on the catalyst.

The object of the present invention was therefore to largely remedy the disadvantages of the procedure according to the recognized prior art.

As a solution to the problem, a process for the production of (meth) acrolein and / or (meth) acrylic acid by heterogeneously catalyzed gas phase partial oxidation, in which a fresh fixed catalyst bed in a reactor is charged at elevated temperature with a feed gas mixture, the In addition to at least one organic precursor compound to be partially oxidized and molecular oxygen as the oxidizing agent, at least one diluent gas which is essentially inert under the conditions of the heterogeneously catalyzed gas phase partial oxidation is found, which is characterized in that the process after adjusting the composition of the feed gas mixture at im substantially constant conversion of the organic precursor compound and with essentially unchanged composition of the feed gas mixture initially over a running-in period of 3 days to 10 days at a low load and then carried out at a higher load on the catalyst feed with feed gas mixture. The loading of a fixed catalyst bed of a reaction stage with feed gas mixture is understood to mean the amount of feed gas mixture in standard liters (= Nl; the volume in liters which the corresponding amount of feed gas mixture would take under normal conditions, i.e. at 25 ° C. and 1 bar), by one hour Liters of fixed catalyst bed is guided (pre-fillings and backfills made of pure inert material are not included in the fixed catalyst bed; homogeneous mixtures of inert material shaped bodies and shaped catalyst bodies are, however, included in the fixed catalyst bed.

The advantage of the process according to the invention over the process of the prior art lies in the fact that it trims excessive heat development not by reducing the reactant content of the feed gas mixture but, in the case of full reactant content, by reducing the load on the fixed catalyst bed with feed gas mixture.

The conversion of the organic precursor compound (based on the single passage of the feed gas mixture through the fixed catalyst bed) is adjusted essentially constant to the target conversion. Essentially constant means that the maximum deviation from the sales arithmetically averaged over time is not more than ± 10%, preferably not more than ± 5% (reference basis is the sales arithmetically averaged over time).

Likewise, "with the composition of the feed gas mixture remaining essentially the same" means that the maximum deviation of the volume fraction of one of the components molecular oxygen, organic precursor compound and inert diluent gas in the feed gas mixture from the respective volume fraction, arithmetically averaged over time, of the respective component in the feed gas mixture does not exceed ± 10%, preferably not more than ± 5% (reference base is the respective volume fraction of the respective component in the feed gas mixture, arithmetically averaged over time).

The setting of the composition of the feed gas mixture and the temperature of the fixed catalyst bed for the process according to the invention can in principle be carried out according to the procedure described in WO 02/098827. The time frame required for this, however, is usually well under an hour. However, it can also be carried out in such a way that in a line leading via a static mixer, the reactor containing the fixed bed of catalyst initially only (optionally with a content of 2 to 4% by volume oxygen) inert gas (including water vapor), then the at least one organic one Vorläuferverbin- and finally the oxygen source (normally air). The fixed catalyst bed is already brought to the temperature required by the heat transfer medium during the inert gas supply, which is required at low loads in order to achieve the target conversion in a single pass through the catalyst feed.

A low loading of the fixed catalyst bed with feed gas mixture, while the composition of the feed gas mixture remains essentially the same, means a lower loading of the fixed catalyst bed with rectand.

If the loading of the fixed catalyst bed with feed gas mixture is increased in the further course of the process according to the invention, this reduces the average residence time of the rectands in the catalyst feed. In order to achieve a substantially constant conversion of the at least one organic precursor compound with a short residence time, it is therefore normally necessary to increase the temperature of the heat transfer medium used for the indirect heat exchange.

Lower loading of the fixed catalyst bed with feed gas mixture in the process according to the invention means that the lower loading is typically 40 to 80%, preferably 50 to 70% of the higher target (end) loading for which the reactor, including its catalyst loading, is designed.

That is, the reactor and fixed catalyst bed are designed for end loading with e.g. 150 NI propene / l fixed catalyst bed • h designed (the propene content in the feed gas mixture of a propene partial oxidation to acrolein and / or acrylic acid is typically 4 to 12 vol .-%), the 3 to 10 days run-in according to the invention is typically under a load of 100 Nl propene / l • h carried out. The above run-in could then also be carried out with appropriate loads of 80 to 120 Nl propene / l • h.

If a final load of 180 to 190 Nl propene / l • h is envisaged, the 3 to 10 day run-in according to the invention is typically carried out with a load of 120 Nl propene / l • h etc. However, the above run-in could then also with appropriate loads of 100 to 140 Nl propene / l • h. As a rule, the desired final load with an organic precursor compound is values ≥ 80 Nl / I • h, mostly values ≥ 100 Nl / I • h or ≥ 120 Nl / I • h. Final loads of 600 Nl / I • h or often 300 Nl / I • h are usually not exceeded.

After the break-in period, the load can be increased suddenly, continuously or step by step to the desired final load. The advantage of the procedure according to the invention is that after the running-in phase of 3 to 10 days, often 4 to 9 or 5 to 8 days, the process can be continued under higher loads with a comparatively increased target product selectivity and at the same time a comparatively lower heat transfer medium temperature. The basis for comparison in this regard is a shortened or disappearing break-in phase with less load. It also enables minimum hot spot temperatures (= designation for the highest temperature within the fixed catalyst bed through which the reaction gas mixture flows).

Suitable fixed bed catalysts for the process according to the invention for the production of (meth) acrolein, in particular for the production of acrolein from propene, are all those whose active composition is at least one multimetal oxide containing Mo, Bi and Fe. They are to be referred to here as fixed bed catalysts 1.

In principle, all those catalysts which are described in DE-C 3338380, DE-A 19902562, EP-A 15565, DE-C 2380765, EP-A 807465, EP-A 279374, DE-A 3300044, EP-A 575897, US-A 4438217, DE-A 19855913, WO 98/24746, DE-A 19746210 (those of general formula II), JP-A 91/294239, EP-A 293224 and EP-A 700714 are disclosed as fixed bed catalysts 1 can be used. This applies in particular to the exemplary embodiments in these documents, among which those of EP-A 15565, EP-A 575897, DE-A 19746210 and DE-A 19855913 are particularly preferred. Particularly noteworthy in this context are a catalyst according to Example 1 c from EP-A 15565 and a catalyst to be produced in a corresponding manner, the active composition of which, however, has the composition Mo ₁₂ i _{6 5 5} Zn ₂ Fe ₂ BiιPo, oo65 o, o ₆ θx • 10SiO ₂ having. Also to be emphasized is the example with the current No. 3 from DE-A 19855913 (stoichiometry: Mo ₁₂ Co ₇ Fe ₃ Bio _ι6 Ko, o ₈ Siι, ₆ Ox) as a hollow cylinder full catalyst of geometry 5 mm x 3 mm x 2 mm (outer diameter x height x inner diameter) and the multimetal oxide II - full catalyst according to Example 1 of DE-A 19746210. The multimetal oxide catalysts of US Pat. No. 4,438,217 should also be mentioned. The latter applies in particular if these hollow cylinders have a geometry of 5 mm x 2 mm x 2 mm, or 5 mm x 3 mm x 2 mm, or 6 mm x 3 mm x 3 mm, or 7 mm x 3 mm x 4 mm (each Outside diameter x height x inside diameter).

A large number of the multimetail oxide active compositions suitable as fixed bed catalysts 1 can be described using 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 = thallium, an alkali metal 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.

They are available in a manner known per se (see, for example, DE-A 4023239) and are usually formed in bulk into spheres, rings or cylinders or else in the form of coated catalysts, i.e. preformed, inert support bodies coated with the active composition. In principle, however, they can of course also be used in powder form as fixed bed catalysts 1. Of course, the multimetal oxide catalyst ACS-4 from Nippon Shokubai comprising Bi, Mo and Fe can also be used as fixed bed catalyst 1 according to the invention.

In principle, suitable active compositions for the fixed bed catalysts 1, in particular those of the general formula I, can be prepared in a simple manner by generating an intimate, preferably finely divided, dry mixture of suitable stoichiometry from suitable sources of their elemental constituents and this at temperatures of Caicinated at 350 to 650 ° C. The calcination can take place both under inert gas and under an oxidative atmosphere such as air (mixture of inert gas and oxygen) and also under a reducing atmosphere (eg mixture of inert gas, NH ₃ , CO and / or H ₂ ). The duration of the calculation can range from a few minutes to a few hours and usually decreases with temperature. Suitable sources for the elementary constituents of the multimetal oxide active materials I are those compounds which are already oxides and / or those compounds which can be converted into oxides by heating, at least in the presence of oxygen. In addition to the oxides, such starting compounds are in particular halides, nitrates, formates, oxalates, citrates, acetates, carbonates, amine complexes, ammonium salts and / or hydroxides (compounds such as NH ₄ OH, (NH) ₂ CO ₃ , NH ₄ NO ₃ , NH ₄ CHO ₂ , CH ₃ COOH, NH ₄ CH ₃ CO ₂ and / or ammonium oxalate, which can decompose and / or decompose into fully gaseous compounds at the latest during later calcination, can also be incorporated into the intimate dry mixture) ,

The intimate mixing of the starting compounds for the production of multimetal oxide masses I can take place in dry or in wet form. If it is carried out in dry form, the starting compounds are expediently used as finely divided powders and, after mixing and optionally compacting, are subjected to the calcination. However, the intimate mixing is preferably carried out in wet form. Usually, the starting compounds are mixed together in the form of an aqueous solution and / or suspension. Particularly intimate dry mixtures are obtained in the mixing process described if only sources of the elementary constituents present in dissolved form are used. Water is preferably used as the solvent. The aqueous mass obtained is then dried, the drying process preferably being carried out by spray drying the aqueous mixture at exit temperatures of 100 to 150 ° C.

The multimetal oxide compositions suitable as fixed bed catalysts 1 according to the invention, in particular those of the general formula I, can be used for the process according to the invention both in powder form and in the form of certain catalyst geometries, it being possible for the shaping to take place before or after the final calculation. For example, full catalysts can be produced from the powder form of the active composition or its uncalcined and / or partially caicinated precursor composition by compressing it to the desired catalyst geometry (e.g. by tableting, extruding or extruding), where appropriate auxiliaries such as e.g. Graphite or stearic acid can be added as a lubricant and / or molding aid and reinforcing agent such as microfibers made of glass, asbestos, silicon carbide or potassium titanate. Suitable unsupported catalyst geometries are e.g. Solid cylinder or hollow cylinder with an outer diameter and a length of 2 to 10 mm. In the case of the hollow cylinders, a wall thickness of 1 to 3 mm is appropriate. Of course, the full catalyst can also have a spherical geometry, the spherical diameter being 2 to 10 mm.

Of course, the shape of the powdery active composition or its powdery, not yet and / or partially caicinated, precursor composition can also be achieved by Applied to preformed inert catalyst supports. The coating of the support bodies for producing the coated catalysts is generally carried out in a suitable rotatable container, as is known, for example, from DE-A 2909671, EP-A 293859 or from EP-A 714700. To coat the carrier bodies, the powder mass to be applied is expediently moistened and dried again after application, for example by means of hot air. The layer thickness of the powder composition applied to the carrier body is expediently selected in the range from 10 to 1000 mm, preferably in the range from 50 to 500 mm and particularly preferably in the range from 150 to 250 mm.

Conventional porous or non-porous aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium or aluminum silicate can be used as carrier materials. The carrier bodies can have a regular or irregular shape, with regularly shaped carrier bodies with a clearly formed surface roughness, e.g. Balls or hollow cylinders are preferred. It is suitable to use essentially non-porous, rough-surface, spherical supports made of steatite, the diameter of which is 1 to 8 mm, preferably 4 to 5 mm. However, it is also suitable to use cylinders as carrier bodies, the length of which is 2 to 10 mm and the outside diameter is 4 to 10 mm. In the case of rings suitable as support bodies according to the invention, the wall thickness is moreover usually 1 to 4 mm. Annular support bodies to be used preferably according to the invention have a length of 3 to 6 mm, an outer diameter of 4 to 8 mm and a wall thickness of 1 to 2 mm. According to the invention, rings of geometry 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) are particularly suitable as carrier bodies. The fineness of the catalytically active oxide compositions to be applied to the surface of the carrier body is of course adapted to the desired shell thickness (cf. EP-A 714700).

Favorable multimetal oxide active compositions to be used according to the invention as fixed bed catalysts 1 are furthermore compositions of the general formula II

[Y ¹ _a Υ ² _bx .] P [Y ³ _c .Y ⁴ _d .Y ⁵ _e Υ ⁶ _f .Y ⁷ _g Y ² _h ] _q (II),

in which the variables have the following meaning:

Y ¹ = bismuth, tellurium, antimony, tin and / or 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, chromium, cerium and / or 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 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 .

containing three-dimensionally extended areas of the chemical composition Y ¹ _a Υ ² _b χ _' , which are separated from their local surroundings due to their composition which is different from their local surroundings, the size of which (longest connecting section through the center of gravity of the area) of two on the surface (interface) of the Range of points) is 1 nm to 100 μm, often 10 nm to 500 nm or 1 μm to 50 or 25 μm.

Particularly advantageous multimetal oxide compositions II according to the invention are those in which Y ^{1 is} bismuth.

Of these, those which have the general formula III

[Bi _a .Z ² _b O _J ..] _p n [Z ² ₁₂ Z ³ _c .Z ⁴ _rf .Fe _e .Z ⁵ _f ZVZ ⁷ _h »O] _q .. (IM)

in which the variants have the following meaning:

Z ² = molybdenum and / or tungsten,

Z ³ = nickel and / or cobalt,

Z ⁴ = thallium, an alkali metal and / or an alkaline earth metal,

Z ⁵ = phosphorus, arsenic, boron, antimony, tin, cerium and / or lead,

Z ⁶ = silicon, aluminum, titanium and / or zirconium, Z ⁷ = copper, silver and / or gold, a "= 0.1 to 1, b" = 0.2 to 2, c "= 3 to 10, d" = 0.02 to 2, e "= 0.01 to 5, preferably 0.1 to 3, f "= 0 to 5, g" = 0 to 10, h "= 0 to 1, x", y "= numbers determined by the valency and frequency of the element in III other than oxygen, p", q " = Numbers whose ratio p "/ q" is 0.1 to 5, preferably 0.5 to 2,

correspond, those masses III being very particularly preferred in which Z ² _b = (tungsten) b "and Z ² ₁₂ = (molybdenum) ₁₂ .

It is also advantageous if at least 25 mol% (preferably at least 50 mol% and particularly preferably at least 100 mol%) of the total fraction [Y ¹ _a Υ ² _b O _{X '} ] _p ([Bi _a »Z ² O _x »] _p ») of the multimetal oxide compositions II (multimetal oxide compositions III) which are suitable according to the invention as fixed bed catalysts 1 in the multimetal oxide compositions II (multimetal oxide compositions III) suitable according to the invention in the form of three-dimensionally expanded form, differentiated from their local surroundings due to their chemical composition which is different from their local surroundings , Areas of the chemical composition Y1 _a 'Y2 O _X' [Bi _a »Z ² » Oχ _" ) are present, the largest diameter of which is in the range 1 nm to 100 μm.

With regard to the shape, what has been said about the multimetal oxide mass I catalysts applies to multimetal oxide mass II catalysts.

The production of multimetal oxide materials II active materials is e.g. described in EP-A 575897 and in DE-A 19855913.

Appropriately for the application, the heterogeneously catalyzed gas-phase partial oxidation of an organic precursor compound to (meth) acrolein according to the invention is carried out in a tube bundle reactor charged with fixed-bed catalysts 1, as described e.g. is described in EP-A 700714.

This means that the fixed-bed catalyst 1 to be used is in the simplest way in the metal tubes of a tube bundle reactor and a temperature medium (single-zone mode of operation), usually a molten salt, is passed around the metal tubes. Salzschmel- ze and reaction gas mixture can be carried out in simple cocurrent or countercurrent. However, the molten salt (the temperature control medium) can also be passed through the reactor, viewed in a meandering manner, around the tube bundle, so that, viewed across the entire reactor, there is only a cocurrent or countercurrent to the direction of flow of the reaction gas mixture. The flow rate of the temperature control medium (heat exchange medium) is usually dimensioned such that the temperature rise (due to the exothermic nature of the reaction) of the heat exchange medium from the point of entry into the reactor to the point of exit from the reactor is ≥ 0 to 10 ° C, often ≥ 2 to 8 ° C, often ≥ 3 to 6 ° C. The entry temperature of the heat exchange medium into the tube bundle reactor, in particular in the case of the conversion of propene to acrolein, is generally 310 to 360 ° C., frequently 320 to 340 ° C.

Fluid heat transfer media are particularly suitable as heat exchange medium. The use of melts of salts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or of low-melting metals such as sodium, mercury and alloys of various metals is particularly favorable.

The feed gas mixture of the feed with fixed bed catalyst 1 is expediently preheated to the desired reaction temperature.

Especially in the case of the desired high (eg ≥ 160 Nl / I • h, but usually ≤ 600 NI / I • h) final loads of the feed with fixed bed catalyst 1 with the at least one organic precursor compound to be partially oxidized (eg the propene) the process according to the invention is expediently carried out in a two-zone tube bundle reactor. DE-C 2830765 discloses a preferred variant of a two-zone tube bundle reactor which can be used according to the invention. However, the two-zone tube bundle reactors disclosed in DE-C 2513405, US-A 3147084, DE-A 2201528, EP-A 383224 and DE-A 2903218 are also suitable.

That is, in the simplest manner, the fixed bed catalyst 1 to be used is located in the metal tubes of a tube bundle reactor, and two temperature-regulating media, generally salt melts, which are essentially spatially separated from one another, are guided around the metal tubes. The pipe section over which the respective salt bath extends represents a reaction zone. Preferably, for example, a salt bath A flows around that section of the tubes (reaction zone A) in which, for example, the oxidative conversion of the propene (in a single pass) takes place until conversion in the range from 40 to 80 mol% is achieved, and a salt bath B flows around, for example, the section of the tubes (reaction zone B), in which, for example, the oxidative subsequent conversion of the propene (in a single pass) until a Sales value of at least 90 mol .-% is usually carried out (if necessary, reaction zones A, B can be followed by further reaction zones which are kept at individual temperatures).

In principle, the salt bath can be conducted within the respective temperature zone as with the single-zone procedure. The temperature of salt bath B is normally at least 5 ° C above the temperature of salt bath A.

Incidentally, the two-zone high-load mode, e.g. as in DE-A 19948523 or as described in DE-A 19948248.

The example of the gas phase partial oxidation of propene to acrolein will be used to explain this in more detail. The other gas phase partial oxidations according to the invention of organic precursor compounds can be carried out analogously.

According to this, the process according to the invention is suitable for propene loads on the fixed bed catalyst bed 1 of> 70 Nl / I • h, ≥ 130 Nl / I • h, ≥ 180 Nl / I • h, ≥ 240 Nl / I • h, ≥ 300 Nl / I • h, but usually <600 Nl / l • h.

The inert gas to be used for the feed gas mixture can be added

≥ 20% by volume, or> 30% by volume, or ≥ 40% by volume, or ≥ 50% by volume, or ≥ 60% by volume, or ≥ 70% by volume %, or> 80 vol .-%, or ≥ 90 vol .-%, or ≥ 95 vol .-% consist of molecular nitrogen.

In the case of propene loads on the fixed bed catalyst bed 1 above 250 Nl / l • h, however, the use of inert (inert diluent gases should generally be those used here for the single passage through the respective fixed bed catalyst bed to less than 5%, preferably less, for the process according to the invention convert as 2%) Diluent gases such as propane, ethane, methane, pentane, butane, CO ₂ , CO, steam and / or noble gases are recommended for the feed gas mixture.

With increasing propene loading, the two-zone procedure described, as already mentioned, is preferred over the single-zone procedure described.

The working pressure in the process according to the invention in the propene partial oxidation can be both below normal pressure (for example up to 0.5 bar) and above normal pressure. Typically, the working pressure will be between 1 and 5 bar, often between 1.5 and 3.5 bar. The reaction pressure during the propene partial oxidation will normally not exceed 100 bar. The molar ratio of O ₂ : C ₃ H ₆ in the feed gas mixture is normally ≥ 1. Usually this ratio will be values ≤ 3. Often the molar ratio of O ₂ : C ₃ H ₆ in the feed gas mixture is ≥ 1, 5 and <2.0.

Both air and, for example, air depleted in molecular nitrogen (for example 90 90% by volume O ₂ , 10 10% by volume N ₂ ) can be considered as the source of the required molecular oxygen.

The propene content in the feed gas mixture can e.g. at values from 4 to

15% by volume, often 5 to 12% by volume or 5 to 8% by volume (in each case based on the total volume).

The process according to the invention is often used in the case of a propene: oxygen: indifferent gases (including water vapor) volume ratio in the reaction gas starting mixture (feed gas mixture) of 1: (1.0 to 3.0) :( 5 to 25), preferably 1: (1st , 5 to 2.3) :( 10 to 15). However, the feed gas mixture compositions of DE-A 10313209 can also be used according to the invention.

For the production of acrylic acid from acrolein, all those whose active composition is at least one Mo and V-containing multimetal oxide are suitable for the process according to the invention. They are to be referred to here as fixed bed catalysts 2.

Fixed bed catalysts 2 which are suitable in this way can be found, for example, in US Pat. No. 3,775,474, US Pat. No. 3,954,855, US Pat. No. 3,893,951 and US Pat. No. 4,339,355. Furthermore, the multimetal oxide compositions of EP-A 427 508, DE-A 2 909 671, DE-C 31 51 805, DE-AS 2 626 887, DE-A 43 02 991, EP- A 700 893, EP-A 714 700 and DE-A 19 73 6105 for fixed bed catalysts 2. Particularly preferred in this context are the exemplary embodiments of EP-A 714 700 and DE-A 19 73 6105.

A large number of the multimetal oxide active compositions suitable as fixed bed catalysts 2 can be obtained using the general formula IV

Mo ₁₂ V _a X ¹ _b X ² _c X ³ _d X ⁴ _e X ⁵ _f X ⁶ _g O _n (IV),

in which the variables have the following meaning: X ¹ = W, Nb, Ta, Cr and / or Ce,

X ² = Cu, Ni, Co, Fe, Mn and / or Zn,

X ³ = Sb and / or Bi, X ⁴ = one or more alkali metals,

X ⁵ = one or more alkaline earth metals,

X ⁶ = Si, Al, Ti and / or Zr, a = 1 to 6, b = 0.2 to 4, c = 0.5 to 18, d = 0 to 40, e = 0 to 2, f = 0 to 4, g = 0 to 40 and n = a number which is determined by the valency and frequency of the elements other than oxygen in IV,

subsumed.

Preferred embodiments within the active multimetal oxides IV are those which are covered by the following meanings of the variables of the general formula IV:

X ¹ = W, Nb, and / or Cr, X ² = Cu, Ni, Co, and / or Fe,

X ³ = Sb,

X ⁴ = Na and / or K,

X ⁵ = Ca, Sr and / or Ba,

X ⁶ = Si, Al, and / or Ti, a = 1.5 to 5, b = 0.5 to 2, c = 0.5 to 3, d = 0 to 2, e = 0 to 0.2, f = 0 to 1 and n = a number which is determined by the valency and frequency of the elements other than oxygen in IV.

However, very particularly preferred multimetal oxides IV are those of the general formula V Mo ₁₂ V _a .Y ¹ _b .Y ² _c .Y ⁵ _f Y ⁶ _g .O _n (V),

With

Y ¹ = W and / or Nb,

Y ² = Cu and / or Ni,

Y ⁵ = Ca and / or Sr,

Y ⁶ = Si and / or Al, a '= 2 to 4, b' = 1 to 1, 5, c '= 1 to 3, f = 0 to 0.5 g' = 0 to 8 and n '= one Number determined by the valency and frequency of elements other than oxygen in V.

The multimetal oxide active compositions (IV) which are suitable according to the invention are known per se, e.g. available in DE-A 4335973 or in EP-A 714700.

In principle, according to the invention, suitable multimetal oxide active compositions, in particular those of the general formula IV, can be prepared as fixed bed catalysts 2 in a simple manner by generating an intimate, preferably finely divided, dry mixture of stoichiometry in accordance with their stoichiometry from suitable sources of their elemental constituents and this Temperatures from 350 to 600 ° C caicinated. The caicination can be carried out both under inert gas and under an oxidative atmosphere such as air (mixture of inert gas and oxygen) and under a reducing atmosphere (eg mixtures of inert gas and reducing gases such as H ₂ , NH ₃ , CO, methane and / or acrolein or the reducing gases mentioned are carried out by themselves). The caicination time can last from a few minutes to a few hours and usually decreases with temperature. Suitable sources for the elementary constituents of the multimetal oxide active compositions IV are those compounds which are already oxides and / or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.

The intimate mixing of the starting compounds for the production of multimetal oxide compositions IV can take place in dry or in wet form. If it is carried out in dry form, the starting compounds are expediently in the form of finely divided powders used and subjected to calcination after mixing and optionally compressing. However, the intimate mixing is preferably carried out in wet form.

Usually, the starting compounds are mixed with one another in the form of an aqueous solution and / or suspension. Particularly intimate dry mixtures are obtained in the mixing process described if only sources of the elementary constituents present in dissolved form are used. Water is preferably used as the solvent. The aqueous mass obtained is then dried, the drying process preferably being carried out by spray drying the aqueous mixture at exit temperatures of 100 to 150 ° C.

The multimetal oxide compositions suitable according to the invention as fixed bed catalysts 2, in particular those of the general formula IV, can be used for the process according to the invention both in powder form and in the form of certain catalyst geometries, it being possible for the shaping to take place before or after the final caicination. For example, full catalysts can be produced from the powder form of the active composition or its uncalcined precursor composition by compression to the desired catalyst geometry (e.g. by tableting, extrusion or extrusion), where appropriate auxiliaries such as e.g. Graphite or stearic acid can be added as a lubricant and / or molding aid and reinforcing agent such as microfibers made of glass, asbestos, silicon carbide or potassium titanate. Suitable unsupported catalyst geometries are e.g. Solid cylinder or hollow cylinder with an outer diameter and a length of 2 to 10 mm. In the case of the hollow cylinders, a wall thickness of 1 to 3 mm is appropriate. Of course, the full catalyst can also have a spherical geometry, the spherical diameter being 2 to 10 mm.

Of course, the powdery active composition or its powdery, not yet caicinated, precursor composition can also be shaped by application to preformed inert catalyst supports. The coating of the support bodies for the production of the coated catalysts is usually carried out in a suitable rotatable container, as is e.g. is known from DE-A 2909671, EP-A 293859 or from EP-A 714700.

To coat the carrier bodies, the powder mass to be applied is expediently moistened and dried again after application, for example by means of hot air. The layer thickness of the powder composition applied to the carrier body is expediently selected in the range from 10 to 1000 μm, preferably in the range from 50 to 500 μm and particularly preferably in the range from 150 to 250 μm. Conventional porous or non-porous aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium or aluminum silicate can be used as carrier materials. The carrier bodies can have a regular or irregular shape, preference being given to regularly shaped carrier bodies with a clearly formed surface roughness, for example balls or hollow cylinders. It is suitable to use essentially non-porous, rough-surface, spherical supports made of steatite, the diameter of which is 1 to 8 mm, preferably 4 to 5 mm. However, it is also suitable to use cylinders as carrier bodies, the length of which is 2 to 10 mm and the outside diameter is 4 to 10 mm. In the case of rings suitable as support bodies according to the invention, the wall thickness is moreover usually 1 to 4 mm. Annular support bodies to be used preferably according to the invention have a length of 3 to 6 mm, an outer diameter of 4 to 8 mm and a wall thickness of 1 to 2 mm. Particularly suitable according to the invention are rings of geometry 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) as the carrier body. The fineness of the catalytically active oxide materials to be applied to the surface of the carrier body is of course adapted to the desired shell thickness (cf. EP-A 714700).

Favorable multimetal oxide active compositions to be used according to the invention as fixed bed catalysts 2 are furthermore compositions of the general formula VI,

[D] _P [E] _q (VI),

in which the variables have the following meaning:

D = Mo ₁₂ V _a .Z ¹ _b .Z ² _c .Z ³ _tf .Z ⁴ _e .Z ⁵ _f ZVθ ₎ .,

Z ¹ = W, Nb, Ta, Cr and / or Ce,

Z ² = Cu, Ni, Co, Fe, Mn and / or Zn, z ³ = Sb and / or Bi, z ⁴ = Li, Na, K, Rb, Cs and / or H z ⁵ = Mg, Ca, Sr and / or Ba, z ⁶ = Si, Al, Ti and / or Zr, z ⁷ = Mo, W, V, Nb and / or Ta,

a "1 to 8, b" = 0.2 to 5, c "= 0 to 23, d" = 0 to 50, e "= 0 to 2, f" = 0 to 5, g "= 0 to 50, h" = 4 to 30, i "= 0 to 20 and x", y "= numbers by the value and frequency of the element other than oxygen are determined in VI and p, q = non-zero numbers, the ratio p / q of which is 160: 1 to 1: 1,

and which are obtainable by using a multimetal oxide mass E

Z ⁷ ₁₂ Cu _h ..H, O _y .. (E),

separately formed in f one-piece form (starting mass 1) and then the pre-formed solid starting mass 1 in an aqueous solution, an aqueous suspension or in a finely divided dry mixture of sources of the elements Mo, V, Z ¹ , Z ² , Z ³ , Z ⁴ , Z ⁵ , Z ⁶ , which the aforementioned elements in the stoichiometry D

Mo ₁₂ V _a .Z ¹ _b .Z ² _c .Z ³ _d ^» Z ⁴ _e .Z ⁵ _f ZV (D),

contains (starting mass 2), incorporated in the desired ratio p: q, the resulting aqueous mixture dries, and the given dry precursor mass before or after drying to the desired catalyst geometry at temperatures of 250 to 600 ° C.

The multimetal oxide compositions VI are preferred, in which the preformed solid starting composition 1 is incorporated into an aqueous starting composition 2 at a temperature <70 ° C. A detailed description of the production of multimetal oxide materials VI catalysts contain e.g. EP-A 668104, DE-A 19736105 and DE-A 19528646.

With regard to the shaping, what has been said for the multimetal oxide compositions IV catalysts applies to multimetal oxide compositions VI catalysts.

Appropriately from an application point of view, the heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid according to the invention will be carried out in a tube bundle reactor charged with the fixed bed catalysts 2, as described, for example, in EP-A 700893. That is, in the simplest way, the fixed bed catalyst 2 to be used is located in the metal tubes of a tube bundle reactor and a temperature control medium (single-zone mode), usually a molten salt, is passed around the metal tubes. The molten salt and reaction gas mixture can be carried out in simple cocurrent or countercurrent. The temperature control medium (the molten salt) can, however, also be viewed through the reactor in a meandering manner around the tube bundle, so that viewed only across the entire reactor, there is a cocurrent or countercurrent to the direction of flow of the reaction gas mixture. The volume flow of the temperature control medium (heat exchange medium) is usually dimensioned such that the temperature rise (due to the exothermic nature of the reaction) of the heat exchange medium from the entry point into the reactor to the exit point from the reactor 0 to 10 ° C., frequently w 2 to 8 ° C, often 3 to 6 ° C. The inlet temperature of the heat exchange medium in the tube bundle reactor is usually 230 to 300 ° C, often 245 to 285 ° C or 245 to 265 ° C. Suitable heat exchangers are the same fluid tempering media as have already been described for the heterogeneously catalyzed gas-phase partial oxidation according to the invention of an organic precursor compound to (meth) acrolein.

The feed gas mixture is expediently fed to the feed with fixed bed catalyst 2 preheated to the desired reaction temperature.

Particularly in the case of the desired high (e.g. ≥ 140 Nl / I • h, but usually ≤ 600 Nl / I • h) final loads of the feed with fixed-bed catalyst 2 with acrolein, the process according to the invention is expediently carried out in a two-zone tube bundle reactor. A preferred variant of a two-zone tube bundle reactor which can be used according to the invention for this purpose is disclosed in DE-C 2830765. But also those in DE-C 2513405, US-A 3147084, DE-A 2201528, EP-A 383224 and DE-A 2903582 disclosed two-zone tube bundle reactors are suitable.

In other words, the fixed bed catalyst 2 to be used according to the invention is in a simple manner in the metal tubes of a tube bundle reactor, and two tempering media which are essentially spatially separated from one another, usually molten salts, are passed around the metal tubes. The pipe section over which the respective salt bath extends represents a reaction zone.

For example, a salt bath C preferably flows around those sections of the tubes (reaction zone C) in which the oxidative conversion of acrolein (in a single pass) takes place until a conversion value in the range of 55 to 85 mol% is reached and a salt bath D flows around the Section of pipes (the recession zone D), in which the oxidative subsequent conversion of acrolein (in a single pass) takes place until a conversion value of generally at least 90 mol% is reached (if necessary, further reaction zones can follow the reaction zones C, D to be used according to the invention, which are kept at individual temperatures).

In principle, the salt bath can be conducted within the respective temperature zone as with the single-zone procedure. The temperature of the salt bath D is normally at least 5 to 10 ° C above the temperature of the salt bath C.

Otherwise, the two-zone high-load mode can e.g. as in DE-A 19948523 or as described in DE-A 19948248.

According to this, the process according to the invention is suitable for acrolein loads in the fixed bed catalyst bed 2 of ≥ 70 Nl / I • h,> 130 Nl / I • h, ≥ 180 Nl / I • h, ≥ 240 Nl / I • h,> 300 Nl / I • h, but usually ≤ 600 Nl / l • h.

The inert gas to be used for the feed gas mixture can be ≥ 20 vol.%, Or> 30 vol.%, Or ≥ 40 vol.%, Or ≥ 50 vol.%, Or> 60 vol. -%, or> 70 vol .-%, or ≥ 80 vol .-%, or ≥ 90 vol .-%, or ≥ 95 vol .-% of molecular nitrogen.

If the gas phase partial oxidation of acrolein is the second reaction stage of a two-stage gas phase partial oxidation of propene to acrylic acid, the inert diluent gas is often 5 to 20% by weight H ₂ O (formed in the first reaction stage) and 70 up to 90 vol .-% consist of N ₂ .

If acrolein loads in the fixed bed catalyst bed 2 are above 250 Nl / l • h, however, the use of inert diluent gases such as propane, ethane, methane, butane, pentane, CO, CO, water vapor and / or noble gases is recommended for the process according to the invention. Of course, these gases can also be used at lower acrolein loads.

The working pressure in the gas phase partial oxidation of acrolein can be both below normal pressure (e.g. up to 0.5 bar) and above normal pressure. Typically, the working pressure in the gas phase partial oxidation of acrolein will be from 1 to 5 bar, often 1 to 3 bar.

The reaction pressure in the partial oxidation of acrolein will normally not exceed 100 bar. The molar ratio of O: acrolein in the feed gas mixture of the fixed bed catalyst bed 2 will normally be ≥ 1. This ratio will usually be at values ≤ 3. The molar ratio of O: acrolein in the above-mentioned feed gas mixture will frequently be 1 to 2 or 1 to 1.5 according to the invention. In the case of acrolein partial oxidation, the process according to the invention is often used with an acrolein: oxygen. Water vapor: inert gas - volume ratio (Nl) of 1: (1 to 3): (0 to 20): (3 to 30) present in the feed gas mixture ), preferably from 1: (1 to 3): (0.5 to 10): (7 to 10).

The acrolein content in the feed gas mixture can e.g. at values of 3 to 15% by volume, often 4 to 10% by volume or 5 to 8% by volume (in each case based on the total volume).

The heterogeneously catalyzed gas phase partial oxidation of methacrolein to methacrylic acid can be carried out analogously to that of acrolein to acrylic acid. However, those of EP-A 668103 are preferably used as catalysts. The other reaction conditions are also advantageously determined in accordance with EP-A 668103.

For the gas phase partial oxidation of propane to acrylic acid or from isobutane to methacrylic acid, the multimetal oxide catalysts, such as those used e.g. the documents DE-A 10248584, DE-A 10029338, DE-A 10033121, DE-A 10261186, DE-A 10254278, DE-A 10034825, EP-A 962253, EP-A 1260495, DE-A 10122027, EP-A 192987 and DE-A 10254279 recommend.

The reaction conditions can also be selected in accordance with these documents.

A single-zone reactor will usually be used as the reactor.

Finally, it should be noted that in a two-stage heterogeneously catalyzed gas phase partial oxidation for the production of (meth) acrylic acid (e.g. from propene to acrolein (1st stage) and then acrolein to acrylic acid (2nd stage)), in which the product gas mixture of the first Stage, optionally after cooling and metering in air as an oxygen source, is led into the second stage, with an application of the method according to the invention to the first stage automatically also an application of the method according to the invention to the second stage.

It should also be noted that in the method according to the invention it can be expedient for the transition from the lower load to the higher load Slightly reduce circulating gas content in the feed gas mixture (the reactant content in the feed gas mixture then increases slightly). The above is appropriate if the design of the cycle gas compressor limits the maximum compressible gas quantity. In conclusion, it should be noted that the process according to the invention can also be applied to freshly regenerated (eg according to EP-A 169449, EP-A 614872, EP-A 339119, DE-A 10249797 or DE-A 10350822) catalyst beds located in reactors.

Example and comparative examples

a) Experimental setup

A reaction tube (V2A steel; 30 mm outside diameter, 2 mm wall thickness, 26 mm inside diameter, length: 350 cm) as well as a thermotube centered in the middle of the reaction tube (4 mm outside diameter) for receiving a thermocouple with which the temperature in the reaction tube is determined over its entire length was freshly loaded from top to bottom as follows:

Section 1: 80 cm length steatite rings of geometry 7 mm x 7 mm x 4 mm (outer diameter x length x inner diameter) as a pre-fill.

Section 2: 100 cm length of catalyst feed with a homogeneous mixture of 30% by weight of steatite rings of geometry 5 mm x 3 mm x 2 mm (outside diameter x length x inside diameter) and 70% by weight of full catalyst from section 3.

Section 3: 170 cm length of catalyst feed with an annular (5 mm x 3 mm x 2 mm = outer diameter x length x inner diameter) full catalyst according to Example 1 of DE-A 10046957 (stoichiometry: [Bi ₂ W ₂ O ₉ x 2Wθ ₃ ] o, ₅ [Mθ ₁₂ Cθ ₅ , ₅ Fe ₂ , g ₄ Si ₁ , ₅ gKo, ₀₈ θ _x ] ₁ ).

The temperature of the reaction tube was controlled by means of a salt bath pumped in countercurrent. b) Execution of the experiment

The described, in each case freshly prepared, test arrangement was in each case continuously with a feed gas mixture (mixture of air, polymer grade propylene and cycle gas) of the composition

5.4% by volume of propene, 10.5% by volume of oxygen, 1.2% by volume of CO _x , 81.3% by volume of N ₂ , and 1.6% by volume of H ₂ O are charged , whereby the load and the thermostatting of the reaction tube were varied over time. The reaction tube was thermostatted in such a way that the propene conversion U (mol%) was about 95.0 mol% continuously when the feed gas mixture passed through the reaction tube once.

The following tables show the product selectivities S ^w (mol%) (sum of the selectivity of the acrolein formation and the.) Achieved depending on the load on the fixed catalyst bed (expressed as propene load in Nl / l • h) and the salt bath temperature T _s (° C) Selectivity of acrylic acid formation) and the maximum temperatures Tmax measured in ° C along the reaction tube. The target final load was 150 Nl / l • h. The results given always refer to the end of the respective operating period.

example

Comparative example

A comparison of the example and the comparative example shows that an immediate start-up of the fresh catalyst feed under the desired final load results in a catalyst feed that requires significantly higher salt bath temperatures for the same conversion. The higher maximum temperatures also cause the catalyst feed to age prematurely.

US Provisional Patent Application No. 60/494814, filed August 14, 2003, is incorporated by reference into this application.

In view of the above teachings, numerous changes and deviations from the present invention are possible. It is therefore believed that the invention, within the scope of the appended claims, may be practiced otherwise than as specifically described herein.

Claims

__öPatentansprüche

1. A process for the preparation of (meth) acrolein and / or (meth) acrylic acid by heterogeneously catalyzed gas phase partial oxidation, in which a fresh fixed catalyst bed in a reactor is charged at elevated temperature with a feed gas mixture which, in addition to at least one organic to be partially oxidized Precursor compound and molecular oxygen as the oxidizing agent comprises at least one diluent gas which is essentially inert under the conditions of the heterogeneously catalyzed gas phase partial oxidation, characterized in that the method after adjusting the composition of the feed gas mixture with essentially constant conversion of the organic precursor compound and with im essentially constant composition of the feed gas mixture initially over an entry period of 3 days to 10 days with a lower load and then with a higher load on the catalytic converter is charged with a feed gas mixture.

2. The method according to claim 1, characterized in that the lower load during the run-in period is 40 to 80% of the desired higher final load.

3. The method according to claim 2, characterized in that the desired final load with the feed gas mixture, expressed as the final load with organic precursor compound, is ≥ 80 Nl / l • h.

Method according to one of claims 1 to 3, characterized in that it is a method of heterogeneously catalyzed gas phase partial oxidation of propene to acrolein and / or acrylic acid.