DE19927624A1 - Gas phase oxidation of propylene to acrylic acid using ring-shaped multi-metal oxide catalyst of specified geometry to maximize selectivity and space-time yield - Google Patents

Abstract

Gas phase oxidation of propylene into acrylic acid in which the mixture of reaction gas is oxidized on a first fixed-bed catalyst and the mixture of product gas which contain acrolein oxidized on a second fixed-bed catalyst. The catalyst shaped bodies comprise an annular geometry in both reaction steps. Gas-phase oxidation of propylene in a one-stage reaction to acrolein and/or acrylic acid or in a two-stage reaction to acrylic acid is such that (i) the initial charge in both cases is \- 160 Nl propylene/l catalyst hour; (ii) the charge to the 2nd stage (if used) includes \- 140 NL acrolein/l catalyst hour ; and (iii) the catalyst or catalysts is/are of ring form with an outer diameter 2-11 mm, length 2-11 mm and a wall-thickness 1-5 mm. The process comprises (A) feeding a mixture of propylene, molecular oxygen and an inert gas with molar ratio O2 : C3H6 \- 1 at high temperature over a solid bed catalyst with characteristics (i) and (iii) above and comprising a multi-metal oxide containing Mo and/or W together with Bi, Te, Sb, Sn and/or Cu such that the propylene conversion and the combined selectivity for acrolein and the acrylic acid by-product are both \- 90 mol.%; and optionally (B) cooling the acrolein-containing gas product and optionally adding molecular O2 and/or inert gas prior to passing it at high temperature and with molar ratio O2 : acrolein \- 0.5 over a Mo- and V-containing multi-metal oxide solid bed catalyst such that the acrolein conversion is \- 90 mol.% and the combined two-stage acrylic acid selectivity (based on propylene conversion) is \- 80 mol.%.

Description

The present invention relates to a process for the catalytic gas-phase oxidation of propene to acrylic acid, in which a reaction gas starting mixture 1 containing the molecular oxygen and at least one inert gas which comprises 1 at least 20% of its volume of molecular nitrogen are present and the propene in a molar ratio of O ₂ : C ₃ H ₆ ≧ 1, initially in a first reaction stage at elevated temperature, over a first fixed bed catalyst, the active composition of which is at least one molybdenum and / or tungsten and bismuth, tellurium, antimony, tin and / or copper-containing multi-metal oxide, leads to the fact that the propene conversion with a single pass ≧ 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation taken together are ≧ 90 mol%, the temperature of the product gas mixture leaving the first reaction stage by indirect cooling if necessary reduced and the product gas mixture if appropriate molecular oxygen and / or inert gas, and since after the product gas mixture as acrolein, molecular oxygen and at least one inert gas, which consists of at least 20% of its volume of molecular nitrogen, containing reaction gas starting mixture 2, the molecular Contains oxygen and the acrolein in a molar ratio O ₂ : C ₃ H ₄ O ≧ 1, in a second reaction stage at elevated temperature over a second fixed bed catalyst, the active composition of which is at least one multimetal oxide containing lybdenum and vanadium, that the Acrolein conversion in a single pass ≧ 90 mol% and the selectivity of the acrylic acid formation balanced over both reaction stages, based on converted propene, is ≧ 80 mol%.

The aforementioned process of catalytic gas phase oxidation from propene to acrylic acid is generally known (see e.g. DE-A 30 02 829). In particular, the two reaction stages are for known (see e.g. EP-A 714700, EP-A 700893, EP-A 15565, DE-C 28 30 765, DE-C 33 38 380, JP-A 91/294239, EP-A 807465, WO 98/24746, EP-B 279374, DE-C 25 13 405, DE-A 33 00 044, EP-A 575897 and DE-A 198 55 913).

Acrylic acid is an important monomer which, as such or in Form of its alkyl ester for the production of z. B. ge as adhesives suitable polymers are used.  

The aim of every two-stage fixed-bed gas phase oxidation of propene to acrylic acid is basically to achieve the highest possible space-time yield of acrylic acid (RZA _AS ) (that is, in a continuous process, the hourly and total volume of the catalyst bed used in liters produced Total amount of acrylic acid).

There is therefore general interest in having two of these stage fixed bed gas phase oxidation of propene to acrylic acid on the other hand, under the highest possible load of the first festival bed catalyst bed with propene (the amount of Propene in standard liters (= Nl; the volume in liters that the ent speaking amount of propene under normal conditions, d. i.e., at 25 ° C and 1 bar, would take) understood as part of the Reak tion gas starting mixture 1 per hour by one liter of kata lysator fill 1 is performed) and on the other hand under one loading the second fixed bed catalyst bed as high as possible treatment with acrolein (the amount of acrolein in Normli tern (= Nl; the volume in liters that the corresponding acrolein amount under normal conditions, d. that is, at 25 ° C and 1 bar would) understood that as part of the reaction gas outlet mixture 2 per hour through a liter of catalyst bulk Tung 2 is carried out) without the passage of the reaction gas starting mixture through the the fixed bed catalyst beds sales of propene and acrolein and the Se. balanced over both reaction stages selectivity of the associated acrylic acid formation (related on converted propene).

The realization of the above is affected by the fact that both the fixed bed gas phase oxidation of propene to acrolein as well as the fixed bed gas phase oxidation of acrolein acrylic acid is strongly exothermic on the one hand and on the other hand of a variety of possible parallel and follow-up reactions is sliding.

With increasing propene or acrolein contamination of the respective Fixed bed catalyst bed must, when realizing the ans strived for an essentially constant pro pen or acrolein conversion, therefore it can be assumed that due to the increased heat production the selectivity of the value product formation decreases (see e.g. EP-B 450 596, Example 1 and Ex ample 2).

The conventional processes of the catalytic fixed bed gas phase Oxidation of propene to acrolein or acrolein to acrylic acid, which are characterized in that as a main name  component of the inert diluent gas nitrogen and also one in a reaction zone and along this reaction more homogeneous, d. i.e., chemically via the fixed bed catalyst bed uniformly composed, fixed bed catalyst used and the temperature of the reaction zone on one above the reaction zone uniform value is maintained (under the temperature of a reaction onszone is the temperature of the reaction zone fixed bed catalyst bed when performing the process understood in the absence of a chemical reaction; is this Temperature within the reaction zone is not constant, so means the term temperature of a reaction zone here is the number average value of the temperature of the catalyst bed along the reaction zone), therefore restrict the value of the propene or Acrolein loading of the fixed bed catalyst bed.

So the applied value of the propene load of the fixed bed is catalyst bed normally at values ≦ 155 Nl propene / l Ka talysatorschüttung.h (see e.g. EP-A 15565 (maximum propene last = 120 Nl propene / l.h), DE-C 28 30 765 (maximum propene last = 94.5 Nl propene / l.h), EP-A 804465 (maximum propene last = 128 Nl propene / l.h), EP-B 279374 (maximum propene last = 112 Nl propene / l.h), DE-C 25 13 405 (maximum propene last = 110 Nl propene / l.h), DE-A 33 00 044 (maximum propene last = 112 Nl propene / l.h), EP-A 575897 (maximum propene last = 120 Nl propene / l.h), DE-C 33 38 380 (in essentially all Examples the maximum propene load is 126 Nl propene / l.h; just in the case of a special catalyst composition, a Propene load of 162 Nl / l.h realized) and DE-A 198 55 913 (maximum Propene load = 155 Nl propene / l.h).

WO 98/24746 already considers it with a propene load of up to 148.8 Nl propene / l.h as required, the catalyst bulk structure so that their volume-specific activity gradually in the flow direction of the reaction gas mixture takes.

JP-A 91/294239 does disclose an exemplary embodiment tion form with essentially conventional procedure a propene load of the catalyst bed of 160 Nl propene / l.h for a catalytic gas phase oxidation of propene to acrolein as possible, but also only for the price of one in flow direction of the reaction gas mixture gradually increasing volu men-specific activity. Such a procedure is great technically but not very practical, the gas phase catalyst table oxidation of propene to acrolein usually in Tube bundle reactors with several thousand contact tubes  leads, each of which with the graded catalyst fill must be loaded.

In EP-B 450596 using a structured Ka catalyst filling with an otherwise conventional procedure a propene load of the catalyst bed of 202.5 Nl propene / l.h realized. However, this at the expense of a reduced value product selectivity.

EP-B 253409 and the associated equivalent, EP-B 257565, disclose that when using an inert diluent gas has a higher molar heat capacity than molecular nitrogen shows, the proportion of propene in the reaction gas starting mixture increased can be. Nevertheless, it is also present in the two mentioned fonts the maximum realized propene load of the Catalyst bed at 140 Nl propene / l.h.

In EP-A 293224 propene loads above 160 Nl Propen / l.h realized. However, this at the expense of a special inert diluent gas to be used, which is completely free of mo is molecular nitrogen. A disadvantage of this diluent gas in particular, that it is in all its components, in the sub switched to molecular nitrogen, is a product of value that if the process is carried out continuously, at least partially for reasons of economy have to be recycled into the gas phase oxidation.

The procedures of the prior art are correspondingly restricted Technique the applicable value of acrolein loading the hard Bed catalyst bed in a catalytic gas phase oxide tion of acrolein to acrylic acid normally to values ≦ 150 Nl Acrolein / l catalyst.h (see e.g. EP-B 714700 .; there the maximum applied acrolein load = 120 Nl acrolein / l.h).

Two-stage gas phase oxidation of propene to acrylic acid, at those in the two oxidation states both have a high propenbela as well as a high acrolein load of the respective festival bed catalyst are driven, are from the prior art almost unknown.

One of the few exceptions is the one already cited EP-B 253409 and the associated equivalent, EP-B 257565. Nevertheless, these two writings also contain the maximum realized propene load, despite using an in diluent gas with a higher molar heat capacity than nitrogen, and therefore essentially also the with direct passage of the propane oxidati product gas mixture  stage in the acrolein oxidation stage following acroleinbe loading of the catalyst bed with ≦ 140 Nl reactant (propene or acrolein) /l.h.

The object of the present invention was therefore a as the initially defined process of catalytic gas phaseso Provide oxidation of propene to acrylic acid ensures an increased space-time yield of acrylic acid without the disadvantages of the high-load modes of operation of the prior art to assign.

Accordingly, a process for the catalytic gas phase oxidation of propene to acrylic acid, in which a reaction gas starting mixture 1 containing the propene, molecular oxygen and at least one inert gas, which consists of at least 20% of its volume of molecular nitrogen, comprising the molecular oxygen and the propene in one contains molar ratio O ₂ : C ₃ H ₆ ≧ 1, first in a first reaction stage at elevated temperature over a first fixed bed catalyst, the active composition of which is at least a molybdenum and / or tungsten as well as bismuth, tellurium, anti mon, tin and / or Copper-containing multimetal oxide means that the propene conversion in a single pass is ≧ 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation combined is betragen 90 mol%, the temperature of the first reaction stage possibly leaving the product gas mixture by indirect cooling decreased and that Product gas mixture optionally adds molecular oxygen and / or inert gas, and then the product gas mixture as acrolein, molecular oxygen and at least one inert gas, which consists of at least 20% of its volume of molecular nitrogen, containing reaction gas starting mixture 2, which contains the molecular oxygen and the acrolein contains a molar ratio of O ₂ : C ₃ H ₄ O ≧ 1, in a second reaction stage at elevated temperature via a second fixed bed catalyst, the active composition of which contains at least one multimetal oxide containing molybdenum and vanadium, leads to acrolein conversion in one pass ≧ 90 mol% and the selectivity of the acrylic acid formation balanced over both reaction stages, based on converted propene, is ≧ 80 mol%, which is characterized in that

• a) the load of the first fixed bed catalyst with the in Re Action gas starting mixture 1 propene contained ≧ 160 Nl Pro pen / l catalyst bed. h,  
• b) the first fixed bed catalyst from one into two spatially successive reaction zones A, B arranged kata there is gate filling, the temperature of the reaction zone A 300 to 330 ° C and the temperature of reaction zone B 300 to 365 ° C and at the same time at least 50 ° C above the temperature of reaction zone A,
• c) the reaction gas starting mixture 1, the reaction zones A, B in flows through the chronological sequence "first A", "then B",
• d) the reaction zone A up to a conversion of the propene Extends from 40 to 80 mol%,
• e) the load of the second fixed bed catalyst with the in Re Action gas starting mixture 2 contain acrolein N 140 Nl Acrolein / l catalyst bed. H,
• f) the second fixed bed catalyst from one to two spatially successive reaction zones C, D arranged Kataly satorschüttung, the temperature of the reaction zone C 230 to 270 ° C and the temperature of reaction zone D Is 250 to 300 ° C and at the same time at least 10 ° C above is half the temperature of reaction zone A,
• g) the reaction gas starting mixture 2, the reaction zones C, D in the time sequence "first flows through C", "then D" and
• h) reaction zone C until conversion of acrolein extends from 55 to 85 mol%.

The reaction zone A preferably extends to a prop from 50 to 70 mol% and particularly preferably up to one Propene conversion from 65 to 75 mol%.

According to the invention, the temperature of reaction zone B is in advantageously 300 to 340 ° C. and particularly advantageously 310 up to 330 ° C. The temperature of reaction zone B is also present moves at least 10 ° C above the temperature of reaction zone A.

The higher the propene load on the catalyst bed 1 when it inventive method is chosen, the larger the Difference between the temperature of reaction zone A and the Temperature of reaction zone B can be selected. Usually it will the aforementioned temperature difference in the Ver but do not drive more than 50 ° C. That is, the difference between the temperature of reaction zone A and the temperature of the  According to the invention, reaction zone B can be up to 20 ° C, up to 25 ° C, up to up to 30 ° C, up to 40 ° C, up to 45 ° C or up to 50 ° C.

As a rule, the pro referring to the simple round Pen conversion in the process according to the invention in the first reaction level ≧ 92 mol% or ≧ 94 mol%. The one in the first Selectivity reaction stage resulting in a single pass the formation of acrolein and the formation of acrylic acid by-products regularly ≧ 92 mol% or ≧ 94 mol%, often ≧ 95 mol% or ≧ 96 mol% or ≧ 97 mol%.

Surprisingly, the above does not only apply to Pro Pen loads of the catalyst bed 1 of ≧ 165 Nl / l.h or from ≧ 170 Nl / l.h or ≧ 175 Nl / l.h or ≧ 180 Nl / l.h, but also with propene loads on the catalyst bed 1 of ≧ 185 Nl / l.h or ≧ 190 Nl / l.h or ≧ 200 Nl / l.h or ≧ 210 Nl / l.h as well as at Load values ≧ 220 Nl / l.h or ≧ 230 Nl / l.h or ≧ 240 Nl / l.h or ≧ 250 Nl / l.h.

It is surprising that the aforementioned values can be achieved even then are, if that for the reaction gas starting mixture 1 fiction According to the inert gas used to ≧ 30 vol%, or to ≧ 40 vol%, or ≧ 50% by volume, or ≧ 60% by volume, or ≧ 70% by volume, or ≧ 80 vol%, or ≧ 90 vol%, or ≧ 95 vol% from molecular Nitrogen exists.

In the case of propene loads on the catalyst bed 1 above 250 Nl / lh, the use of inert (inert diluent gases should generally be those for the process according to the invention which are less than 5%, preferably less than, during the single pass through the respective reaction stage Convert 2%) Diluent gases such as propane, ethane, methane, pentane, butane, CO _2, CO, water vapor and / or noble gases are recommended for the reaction gas starting mixture 1. Of course, these gases and their mixtures can also be used at lower propene loads on the catalyst bed 1 in the reaction gas outlet mixture 1 or be used as the sole dilution gases. It is surprising that the process according to the invention can be carried out with a homogeneous, ie chemically uniform, catalyst bed 1 viewed over the reaction zones A, B without suffering significant losses in sales and / or selectivity.

Normally, the propene in the process according to the invention load of the first fixed bed catalyst did not reach 600 Nl / l.h. exceed. The propene loads are typically of the first fixed bed catalyst in the process according to the invention  without significant loss of sales and selectivity in values ≦ 300 Nl / l.h, often with values ≦ 250 Nl / l.h.

The working pressure in the method according to the invention in which he most reaction stage both below normal pressure (e.g. up to 0.5 bar) and above normal pressure. Typically the working pressure in the first reaction stage at values of 1 to 5 bar, often 1.5 to 3.5 bar. Usually it will the reaction pressure in the first reaction stage is not 100 bar exceed.

According to the invention, the molar ratio of O ₂ : C ₃ H ₆ in the reaction gas starting mixture 1 must be ≧ 1. This ratio will usually be around ≦ 3. The molar ratio of O ₂ : C ₃ H ₆ in reaction gas starting mixture 1 according to the invention is frequently ≧ 1.5 and ≦ 2.0.

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

The proportion of propene in the reaction gas starting mixture 1 can be invented according to z. B. at values of 4 to 15 vol .-%, often 5 to 12 vol% or 5 to 8 vol .-% (each based on the Ge total volume).

Frequently, the method according to the invention is used in a pro pen: oxygen: indifferent gases (including water vapor) Vo lumen ratio in reaction gas starting mixture 1 of 1: (1.0 to 3.0) :( 5 to 25), preferably 1: (1.7 to 2, 3) :( 10 to 15) by to lead.

As a rule, the reaction gas starting mixture 1 contains in addition to the components mentioned essentially no further components ten.

As fixed bed catalysts 1 come for the inventive Ver consider all those whose active mass at least is a multimetal oxide containing Mo, Bi and Fe.

In principle, all those catalysts described in DE-C 33 38 380, DE-A 199 02 562, EP-A 15565, DE-C 23 80 765, EP-A 807465, EP-A 279374 , DE-A 33 00 044, EP-A 575897, US-A 4438217, DE-A 198 55 913, WO 98/24746, DE-A 197 46 210 (those of the general formula II), JP-A 91/294239 , EP-A 293224 and EP-A 700714, are used as fixed bed catalysts 1. This applies in particular to the exemplary embodiments in these documents, among which those of EP-A 15565, EP-A 575897, DE-A 197 46 210 and DE-A 198 55 913 are particularly preferred. Particularly noteworthy in this context are a catalyst according to example 1c 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 Ni _6.5 Zn ₂ Fe ₂ Bi ₁ P _0.0065 K _0.06 O _x .10SiO ₂ . Also to be emphasized is the example with the current No. 3 from DE-A 198 55 913 (stoichiometry: Mo ₁₂ Co ₇ Fe ₃ Bi _0.6 K _0.08 Si _1.6 O _x ) as a hollow cylinder full catalyst of geometry 5 mm × 3 mm × 2 mm (outer diameter × height × inner diameter) and the multimetal oxide II full catalyst according to Example 1 of DE-A 197 46 210. Furthermore, the multimetal oxide catalysts of US Pat. No. 4,438,217 should be mentioned. The latter applies in particular if these hollow cylinders have a geometry of 5 mm × 2 mm × 2 mm, or 5 mm × 3 mm × 2 mm, or 6 mm × 3 mm × 3 mm, or 7 mm × 3 mm × 4 mm (each Have outside diameter × height × inside diameter).

A large number of the multimetal active compounds suitable as fixed bed catalysts 1 can be obtained 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 = subtotal a number 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, the DE-A 40 23 239) and are usually made into spheres in bulk, Rings or cylinders shaped or in the form of Schalenka  analyzers, d. i.e., preformed coated with the active composition, inert support bodies used. Of course you can but can also be used in powder form as catalysts 1. Of course, according to the invention, catalyst 1 can also be used the multimetal oxide catalyst ACS-4 comprising Bi, Mo and Fe Fa. Nippon Shokubai can be used.

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 producing an intimate, preferably finely divided, dry mixture which is composed according to their stoichiometry, from suitable sources of their elemental constituents, and this calcined at temperatures from 350 to 650 ° C. The calcination can be done both under inert gas and under an oxidative atmosphere such as e.g. B. air (mixture of inert gas and oxygen) and under a reducing atmosphere (z. B. mixture of inert gas, NH ₃ , CO and / or H ₂ ). The calcination time 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 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 ₄ ) 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 to form completely gaseous compounds at the latest during the later calcination, can also be incorporated into the intimate dry mixture become).

The intimate mixing of the starting compounds for production of multimetal oxide masses I can be in dry or wet form respectively. If it is done in dry form, then the starting compounds expediently used as finely divided powders and after mixing and optionally compressing the calcine subject. The intimate mixing is preferably carried out in each case but in wet form. Usually the exit ver bindings in the form of an aqueous solution and / or suspension mixed together. Particularly intimate dry mixtures are used in the get the described mixing process if only from sources of the elementary constituent in a dissolved form tuenten is assumed. Water is preferred as the solvent used. The aqueous mass obtained is then ge  dries, the drying process preferably by spray drying of the aqueous mixture with exit temperatures of 100 up to 150 ° C.

The Mul suitable as fixed bed catalysts 1 according to the invention time oxide masses, in particular those of the general formula I, can be used for the process according to the invention both in powder form as well as shaped to certain catalyst geometries be, the shape before or after the final Calcination can be done. For example, from the powder form of the active composition or its uncalcined and / or partial calcined precursor mass by compression to the desired Ka Talysatorgeometrie (e.g. by tableting, extrusion or Extrusion) full catalysts are produced, against also aids such. B. graphite or stearic acid as Lubricants and / or molding aids and reinforcing agents such as Microfibers made of glass, asbestos, silicon carbide or potassium titanate can be added. Suitable all-catalyst geometries are e.g. B. solid 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 is appropriate. Of course the unsupported catalyst can also have spherical geometry, the Ball diameter can be 2 to 10 mm.

Of course, the shape of the powdered active mass or its powdery form, not yet and / or partially calcined, precursor mass also by applying on pre shaped inert catalyst support. The coating of the Support body for the production of the shell catalysts is in the Usually run in a suitable rotatable container like this e.g. B. from DE-A 29 09 671, EP-A 293859 or from EP-A 714700 is known. It is useful for coating the carrier body moistens the powder mass to be applied and after application, e.g. B. by means of hot air, dried again. The layer thickness of the powder applied to the carrier body mass is advantageously in the range of 10 to 1000 microns, preferred in the range 50 to 500 μm and particularly preferably in the range 150 down to 250 µm.

Usual porous or non-porous can be used as carrier materials Aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, Si licium carbide or silicates such as magnesium or aluminum silicate be used. The carrier bodies can be regular or irregular be shaped like a gel, with regularly shaped carrier bodies with clearly formed surface roughness, e.g. B. balls or Hollow cylinders are preferred. The use of essentially non-porous, rough surface, spherical Trä  preferably from steatite, the diameter of 1 to 8 mm, preferably 4 to Is 5 mm. However, the use of cylinders is also suitable as a carrier body, the length of which is 2 to 10 mm and the outside diameter knife is 4 to 10 mm. In the case of suitable according to the invention ten rings as a carrier body, the wall thickness is beyond usually 1 to 4 mm. According to the invention preferably to ver turning ring-shaped carrier bodies have a length of 3 to 6 mm, an outer diameter of 4 to 8 mm and a wall thickness from 1 to 2 mm. Above all, are also suitable according to the invention Geometry rings 7 mm × 3 mm × 4 mm (outer diameter ser × length × inner diameter) as a support body. The delicacy the kataly to be applied to the surface of the carrier body Table active oxide masses is of course the desired Adjusted shell thickness (see. EP-A 714 700).

Inexpensive 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 '} 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 = 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, gal lium, 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, comprising three-dimensionally extended areas of the chemical composition Y ¹ _{a '} Y ² _b, which are delimited from their local environment on account of their composition which is different from their local environment _' O _x' , the largest diameter (longest connecting section through the center of gravity of the area of two points located on the surface (interface) of the area) 1 nm to 100 µm, often 10 nm to 500 nm or 1 µm to 50 or 25 µm , is.

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

Among these, those are preferred which have the general formula III

[Bi _{a ''} Z ² _{b ''} O _{c ''} ] _{p ''} [Z ² ₁₂ Z ³ _{c ''} Z ⁴ _{d ''} Fe _{e ''} Z ⁵ _{f '''} Z ⁶ _g'' Z ⁷ _{h ''} O _{y ''} ] _{q ''} (III)

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 which are determined by the valency and frequency of the element other than oxygen in III,
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 , y ² _b , O _x ,] _p ([Bi _{a ''} Z ² _b'' 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 inventive multimetal oxide compositions II (multimetal oxide compositions III) in the form of three-dimensionally expanded form due to their local environment their chemical composition, different from their local environment, defined areas of the chemical composition y ¹ _a , y ² _b , O _x , [Bi _{a ''} Z ² _{b ''} O _{x ''} ], whose size diameter is in the area 1 nm is up to 100 µm.

With regard to the shape, multimetal oxide mas applies sen II catalysts that in the multimetal oxide masses I-cataly sators said.

The production of multimetal oxide II active materials is e.g. B. described in EP-A 575897 and in DE-A 198 55 913.

This is carried out in a manner which is expedient in terms of application technology tion of the first reaction stage of the process according to the invention in a two-zone tube bundle reactor. A preferred variant egg nes according to the invention usable two-zone tube bundle reactor fenbart DE-C 28 30 765. But also in DE-C 25 13 405, the US-A 3147084, DE-A 22 01 528, EP-A 383224 and DE-A 29 03 218 disclosed two-zone tube bundle reactors are for carrying out the first reaction stage of the invention Process suitable.

That is, in the simplest way, it is according to the invention using fixed bed catalyst 1 in the metal tubes of a tube bundle reactor and around the metal pipes are two of each other in the essentially spatially separated temperature control media, as a rule Melting salt, led. The pipe section over which ever extends salt bath, represents a Re according to the invention action zone. That is, a salt bath A flows around in the simplest way that section of the tubes (reaction zone A) in which the oxidative conversion of propene gang) until sales in the range of 40 to 80 mol% and a salt bath B flows around the section of Pipes (the reaction zone B), in which the oxidative An final implementation of the propene (in a single pass) by Achieving a sales value of at least 90 mol% (If necessary, can be applied to the Re Action zones A, B connect further reaction zones that are in individual temperatures).

Appropriately from an application point of view, the first reaction stage comprises of the process according to the invention no further reaction zones. That is, the salt bath B expediently flows around the section of the pipes, in which the oxidative subsequent conversion of propene (at  single pass) up to a sales value ≧ 92 mol% or ≧ 94 mol% or more.

The start of reaction zone B is usually behind that Hot spot maximum of reaction zone A. The hot spot maximum of Reaction zone B is normally below the hot spot maximum temperature of reaction zone A.

According to the invention, the two salt baths A, B can be relative to the stream direction of flow of the reaction flowing through the reaction tubes gas mixture in cocurrent or in countercurrent through which the Re action tubes surrounding space. Of course can, according to the invention, a direct current also in reaction zone A. tion and in reaction zone B a counterflow (or vice versa returns) can be applied.

Of course, you can in all of the above-mentioned cases ions within the respective reaction zone, relative to the Reaction tubes, parallel flow of the molten salt overlap a cross flow so that the individual reaction Zone one as in EP-A 700714 or in EP-A 700893 described tube bundle reactor corresponds and overall in Longitudinal section through the contact tube bundle a meandering Flow course of the heat exchange medium results.

The reaction gas starting mixture 1 of Ka to 1 preheated to the reaction temperature guided.

Usually are in the aforementioned tube bundle reactors Contact tubes made of ferritic steel and have typi Scher way on a wall thickness of 1 to 3 mm. Your inside diameter This is usually 20 to 30 mm, often 22 to 26 mm. Appropriately from an application point of view, this is in the tube bundle number of contact tubes accommodated at a minimum 5000, preferably to at least 10000. Often this is Number of contact tubes housed in the reaction vessel 15,000 to 30,000. Tube bundle reactors with an above 40,000 lying number of contact tubes are rather the exception. The contact tubes inside the container are normally homo gene arranged distributed, the distribution expediently so is chosen that the distance of the central inner axes from to contact tubes closest to each other (the so-called contact tube pitch) is 35 to 45 mm (see e.g. EP-B 468290).  

Fluid temperatures are particularly suitable as heat exchangers media. The use of melts is particularly favorable 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.

As a rule, in all the constellations mentioned above the flow in the two-zone tube bundle reactors speed within the two required heat levels Exchange medium circuits selected so that the temperature of the heat exchange medium from the entry point into the reaction zone up to the point of exit from the reaction zone at 0 to 15 ° C increases. That is, the aforementioned ΔT can be 1 to 10 ° C according to the invention, or 2 to 8 ° C or 3 to 6 ° C.

The temperature at which the heat exchange medium enters the reactor Onszone A is normally 300 to 330 ° C according to the invention. The Entry temperature of the heat exchange medium in the reaction zone B according to the invention is normally on the one hand 300 to 365 ° C and on the other hand is at least 5 ° C above the inlet temperature of that entering reaction zone A. Heat exchange medium.

The inlet temperature of the heat exchange medium is preferably into reaction zone B at least 10 ° C above the entry temperature temperature of the heat build-up entering reaction zone A. means. The difference between the inlet temperatures in reaction zone A or B can therefore be up to 20 ° C., up to 25 ° C, up to 30 ° C, up to 40 ° C, up to 45 ° C or up to 50 ° C. Usually the aforementioned temperature diff limit, however, should not exceed 50 ° C. The higher the propenbela Stung the catalyst bed 1 in the inventive method is chosen, the greater the difference between the one temperature of the heat exchange medium in reaction zone A and the inlet temperature of the heat exchange medium in the Re action zone B.

The entry temperature of the heat exchange is advantageous in the reaction zone B according to the invention at 300 to 340 ° C. and particularly advantageously 310 to 330 ° C.

Of course, in the method according to the invention two reaction zones A, B also spatially separated from each other tube bundle reactors. If necessary, between A heat exchanger is also attached to the two reaction zones A, B.  become. The two reaction zones A, B can of course also can be designed as a fluidized bed.

Catalysts can also be used in the process according to the invention fillings 1 are used, their volume-specific assets act in the flow direction of the reaction gas starting mixture 1 con increases continuously, abruptly or gradually (this can be done as in WO 98/24746 or as described in JP-A 91/294239 or can also be caused by dilution with inert material). As well can also for the described two-zone mode of operation in the EP-A 293224 and inert diluent recommended in EP-B 257565 tion gases (e.g. only propane or only methane etc.) the. The latter can also be combined with a current if required direction of the reaction gas mixture increasing volume-specific activity of the catalyst bed.

At this point it should be pointed out again that for carrying out reaction stage 1 of the invention Process in particular also described in DE-AS 22 01 528 The two-zone tube bundle reactor type that can be used Possibility includes from the hotter heat exchange medium of the Re action zone B to discharge a partial amount to reaction zone A, to optionally warm a cold reaction gas outlet mixture or a cold circulating gas. Furthermore, the tube bundle characteristics within an individual reac tion zone as described in EP-A 382098.

According to the invention, it has proven to be expedient that he product gas mixture leaving the reaction stage before entry cool in the second reaction stage in order to make burning parts of the formed in the first reaction stage Suppress acroleins. Usually between the an aftercooler switched in both reaction stages. This can be done in The simplest case is an indirect tube bundle heat exchanger. The product gas mixture is usually ge through the pipes leads and around the pipes a heat exchange medium is led, the Type of heat exchange recommended for the tube bundle reactors media can correspond. The inside of the pipe is an advantage inert packing (e.g. stainless steel spirals, stea tit, balls made of steatite etc.). The same improve it Heat exchange and catch from the fixed bed catalyst if necessary of the first reaction stage sublimating molybdenum trioxide before it enters the second reaction stage from. It is beneficial if the aftercooler is made with zinc silicate color coated stainless steel.  

Appropriately from an application point of view, the product gas mixture of the first reaction stage is cooled in the aftercooler described above to a temperature of 210 to 290 ° C., frequently 220 to 260 ° C. or 225 to 245 ° C. In other words, the product gas mixture from the first reaction stage can be cooled to temperatures below the temperature of reaction zone C. However, the after-cooling described is by no means compulsory and can in particular be dispensed with in particular if the route of the product gas mixture from the first reaction stage to the second reaction stage is kept short. Usually, the process according to the invention is further implemented in such a way that the oxygen requirement in the second reaction stage is not already covered by a correspondingly high oxygen content of the reaction gas starting mixture 1, but the required oxygen is added in the range between the first and second reaction stages. This can be done before, during and / or after cooling. As a source of the molecular oxygen required in the second reaction stage, both pure oxygen and mixtures of oxygen and inert gas, e.g. B. air or air de-oxygenated with molecular nitrogen (e.g. ≧ 90% by volume O _2, ≦ 10% by volume N ₂ ). The oxygen source is regularly added in a form compressed to the reaction pressure.

The acrolein content in the reaction gas starting mixture 2 thus produced can according to the invention, for. B. 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).

According to the invention, the molar ratio of O ₂ : acrolein in the reaction gas starting mixture must be 2 ≧ 1. This ratio will usually be around ≦ 3. According to the invention, the molar ratio of O ₂ : acrolein in the reaction gas starting mixture 2 will frequently be 1 to 2 or 1 to 1.5. The process according to the invention is frequently used with an acrolein: oxygen: water vapor: inert gas volume ratio (Nl) of 1: (1 to 3) :( 0 to 20) :( 3 to 30), preferably 3 to 30), preferably 1: Execute (1 to 3) :( 0.5 to 10) :( 7 to 10).

The working pressure can be both in the second reaction stage half of normal pressure (e.g. up to 0.5 bar) and above Normal pressure. Typically, the working pressure in the second reaction stage according to the invention at values from 1 to 5 bar, often 1 to 3 bar. Usually the reaction pressure in the second reaction stage does not exceed 100 bar.  

The reaction zone C preferably extends to an acro Line sales from 65 to 80 mol%. In addition, the temperature of the Reaction zone C advantageously at 245 to 260 ° C. The temperature of the Reaction zone D is preferably at least 20 ° C. above that Temperature of the reaction zone D and is advantageously 265 to 285 ° C.

The higher the acrolein load on the catalyst bed 2 at method chosen according to the invention, the greater should be Difference between the temperature of reaction zone C and the Temperature of the reaction zone D can be selected. Usually it will the aforementioned temperature difference in the Ver but do not drive more than 40 ° C. That is, the difference between the temperature of the reaction zone C and the temperature of the According to the invention, reaction zone D can be up to 15 ° C, up to 25 ° C, up to up to 30 ° C, up to 35 ° C or up to 40 ° C.

Otherwise, the simple pass of the second reak tion level related acrolein conversion in the inventive method ren ≧ 92 mol%, or ≧ 94 mol%, or ≧ 96 mol%, or ≧ 98 mol% and often even ≧ 99 mol%. The selectivity of the acrylic Acid formation, based on converted acrolein, becomes common moderate ≧ 92 mol%, or ≧ 94 mol%, often ≧ 95 mol% or ≧ 96 mol% or ≧ 97 mol%.

Surprisingly, the above does not only apply to Acro linse loads of the catalyst bed 2 of ≧ 140 Nl / l.h or ≧ 150 Nl / l.h or from ≧ 160 Nl / l.h or ≧ 170 Nl / l.h or ≧ 175 Nl / l.h or ≧ 180 Nl / l.h, but also with acrolein contamination Catalyst bed of ≧ 185 Nl / l.h or ≧ 190 Nl / l.h or ≧ 200 Nl / l.h or ≧ 210 Nl / l.h as well as with load values ≧ 220 Nl / l.h or ≧ 230 Nl / l.h or 240 Nl / l.h or ≧ 250 Nl / l.h.

It is surprising that the aforementioned values can be achieved even then are if the ver in the second reaction stage according to the invention turned inert gas to ≧ 30 vol%, or to ≧ 40 vol%, or to ≧ 50 vol%, or to 60 vol%, or to zu 70 vol%, or to ≧ 80 vol%, or 90 vol%, or 95 vol% from molecular Nitrogen exists.

In the process according to the invention, the inert diluent gas in the second reaction stage is expediently composed of 5 to 20% by weight of H ₂ O (formed in the first reaction stage) and 70 to 90% by volume of N ₂ .

In addition to the components mentioned in this document, this contains Reaction gas starting mixture 2 normally essentially none other components.

If the second fixed bed catalyst has acrolein loads above 250 Nl / lh, the use of inert diluent gases such as propane, ethane, methane, butane, pentane, CO ₂ , CO. Steam and / or noble gases recommended. Of course, these gases can also be used with lower acrolein loads. It is surprising that the process according to the invention can be carried out with a homogeneous, ie, chemically uniform, catalyst bed, viewed over both reaction zones C, D, without experiencing any appreciable loss in sales and / or selectivity.

The acrolein is normally used in the process according to the invention loading of the second fixed bed catalyst the value of 600 Nl / l.h do not exceed. The acroleinbela are typically located stungen of the catalyst bed 2 in the inventive method without significant loss of sales and selectivity Values ≦ 300 Nl / l.h, often with values ≦ 250 Nl / l.h.

As a rule, acrolein is used in the process according to the invention loading of the second catalyst bed about 10 Nl / l.h, often about 20 or 25 Nl / l.h below the propene load of the first Catalyst bed are. This is primarily due to this ren that in the first reaction stage both sales and Selectivity usually does not reach 100%. Furthermore, the Oxygen demand of the second reaction stage usually by Air covered.

Remarkably, in the method according to the invention Selectivity of the acrylic balanced over both reaction stages Acid formation, based on converted propene, even at the highest Propene and acrolein loads usually at values ≧ 83 mol%, often at ≧ 85 mol% or ≧ 88 mol%, often at ≧ 90 mol% or 93 mol%.

As fixed bed catalysts 2 come for the gas phase catalytic Acrolein oxidation in the second reaction stage all of those into consideration whose active composition contains at least one Mo and V. of the multimetal oxide.

Multimetal oxide catalysts which are suitable in this way can for example US-A 3 775 474, US-A 3 954 855, the US-A 3 893 951 and US-A 4 339 355. Further  the multimetal oxide materials are particularly suitable EP-A 427 508, DE-A 29 09 671, DE-C 31 51 805, the DE-AS 26 26 887, DE-A 43 02 991, EP-A 700 893, the EP-A 714 700 and DE-A 197 36 105. Particularly preferred are in In this connection, the exemplary embodiments of the EP-A 714 700 and DE-A 197 36 105.

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 = subsume a number that is determined by the valency and frequency of the elements other than oxygen in IV.

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 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 '= a number which is determined by the valency and frequency of the elements in V other than oxygen.

The multimetal oxide active compositions (IV) suitable according to the invention are known per se, e.g. B. in DE-A 43 35 973 or in EP-A 714700 disclosed way available.

In principle, 2 suitable multimetal oxide active compositions, in particular those of the general formula IV, can be prepared in a simple manner by using suitable sources of their elemental constituents, as intimate, preferably finely divided, their stoichiometry as appropriate, he dry mixture and this produces calcined at temperatures from 350 to 600 ° C. The calcination can be carried out both under inert gas and under an oxidative atmosphere such as e.g. B. air (mixture of inert gas and oxygen) and also under a reducing atmosphere (e.g. mixtures of inert gas and reducing gases such as H ₂ , NH ₃ , CO, methane and / or acrolein or the reducing gases mentioned per se) be performed. The calcination time can range from a few minutes to a few hours and usually decreases with temperature. As sources for the elemental constituents of the multimetal oxide active materials IV, those compounds are considered 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 production of multimetal oxide masses IV can be in dry or in wet Shape. If it is done in a dry form, the Starting compounds expediently as finely divided powders used and after mixing and optionally compressing the Subjected to calcination. The intimate vermi is preferred but in wet form.

The starting compounds are usually in the form of a aqueous solution and / or suspension mixed together. Especially intimate dry mixtures are in the described mixing process drive then received only from in dissolved form existing sources of elementary constituents becomes. Water is preferably used as the solvent. On finally the aqueous mass obtained is dried, the Drying process preferably by spray drying the aqueous Mixing with outlet temperatures of 100 to 150 ° C takes place.

The multi suitable according to the invention as fixed bed catalysts 2 metal oxide compositions, in particular those of the general formula IV, can be used for the process according to the invention both in powder form as well as shaped to certain catalyst geometries be, the shape before or after the final Calcination can be done. For example, from the powder form of the active mass or its uncalcined precursor mass by compression to the desired catalyst geometry (e.g. by Tableting, extrusion or extrusion) full catalysts are prepared, where appropriate tools such. B. Graphite or stearic acid as a lubricant and / or molding aid agents and reinforcing agents such as microfibers made of glass, asbestos, Silicon carbide or potassium titanate can be added. Yeah gnet full catalyst geometries are z. B. solid cylinder or hollow cylinders 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.

Of course, the shape of the powdered active mass or its powdery, not yet calcined, Precursor mass also by applying to preformed inert Catalyst carrier take place. The coating of the carrier body for Manufacture of the shell catalysts is usually in one suitable rotatable container executed, as z. B. from the  Known from DE-A 29 09 671, EP-A 293859 or from EP-A 714700 is.

The coating is expediently used to coat the carrier body powder mass to be applied is moistened and after application, e.g. B. by means of hot air, dried again. The layer thickness of the powder mass applied to the carrier body becomes more expedient example in the range 10 to 1000 microns, preferably in the range 50 to 500 µm and particularly preferably in the range 150 to 250 µm, chosen.

Usual porous or non-porous can be used as carrier materials Aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, Silicon carbide or silicates such as magnesium or aluminum silicate be used. The carrier bodies can be regular or irregular be shaped like a gel, with regularly shaped carrier bodies with clearly formed surface roughness, e.g. B. balls or Hollow cylinders are preferred. The use of essentially non-porous, rough surface, spherical Trä preferably from steatite, the diameter of 1 to 8 mm, preferably 4 to Is 5 mm. However, the use of cylinders is also suitable as a carrier body, the length of which is 2 to 10 mm and the outside diameter knife is 4 to 10 mm. In the case of suitable according to the invention ten rings as a carrier body, the wall thickness is beyond usually 1 to 4 mm. According to the invention preferably to ver turning ring-shaped carrier bodies have a length of 3 to 6 mm, an outer diameter of 4 to 8 mm and a wall thickness from 1 to 2 mm. Above all, are also suitable according to the invention Rings of geometry 7 mm × 3 mm × 4 mm (outside diameter knife × length × inside diameter) as a support body. The delicacy the kataly to be applied to the surface of the carrier body Table active oxide masses is of course the desired Adjusted shell thickness (see. EP-A 714 700).

Cheap 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 _{d ''} Z ⁴ _{e ''} Z ⁵ _{f ''} Z ⁶ _{g ''} O _{x ''} ,
E: = Z ⁷ ₁₂ Cu _{h ''} H _{i ''} O _{y ''} ,
Z ¹ = W, Mb, 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 = 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 determined by the valency and frequency of the element other than oxygen in VI and
p, q = non-zero numbers whose ratio p / q is 160: 1 to 1: 1,
and which are obtainable by making a multimetal oxide mass E

Z ⁷ ₁₂ Cu _{h '} H _i' O _{y '} (E),

separately in finely divided form (starting mass 1) and then the preformed 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 ^4th , 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 ''} Z ⁶ _{g ''} (D),

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

The multimetal oxide compositions VI in which the units are preferred processing of the preformed solid starting mass 1 into an aqueous Starting mass 2 takes place at a temperature ≦ 70 ° C. A detail gated description of the production of multimetal oxide materials VI Catalysts contain e.g. B. EP-A 668104, DE-A 197 36 105 and DE-A 19 52 86 46.  

With regard to the shape, multimetal oxide applies mass VI catalysts in the multimetal oxide masses IV-Ka talysators said.

This is carried out in a manner which is expedient in terms of application technology tion of the second reaction stage of the process according to the invention in a two-zone tube bundle reactor. A preferred variant one which can be used according to the invention for the second reaction stage DE-C 28 30 765 discloses a two-zone tube bundle reactor. But also those in DE-C 25 13 405, US-A 3147084, DE-A 22 01 528, the EP-A 383224 and DE-A 29 03 582 disclosed two-zone tube bundles reactors are for carrying out the second reaction stage of the method according to the invention.

That is, it is in a simple manner according to the invention using fixed bed catalyst in the metal pipes of a pipe bundle reactor and around the metal pipes are two of each other in the essentially spatially separated temperature control media, as a rule Melting salt, led. The pipe section over which ever sparse salt bath, according to the invention represents one Rekationszone.

That is, a salt bath C flows around those Ab in a simple manner sections of the tubes (reaction zone C) in which the oxidative conversion of acrolein (in a single pass) to to achieve a sales value in the range of 55 to 85 mol.% and a salt bath D flows around the section of the pipes (the Rekationszone D), in which the oxidative connection conversion of acrolein (in a single pass) until reaching one Sales value of at least 90 mol.% Accomplished (if necessary the reaction zones C, D to be used according to the invention Connect more reaction zones based on individual temperatu be held).

Appropriately from an application point of view, reaction stage 2 comprises no further reaction zones. That is, the salt bath D appropriately flows around the section of the pipes in which the oxidative subsequent conversion of acrolein (at single pass) up to a sales value of ≧ 92 mol%, or ≧ 94 mol%, or ≧ 96 mol%, or ≧ 98 mol%, and often even ≧ 99 mol% or more.

The start of reaction zone D is usually behind that Hot spot maximum of the reaction zone C. The temperature of the hot point maximums of reaction zone D is normally below the maximum hot spot temperature of reaction zone C.  

According to the invention, the two salt baths C, D can be relative to Flow direction of the reak flowing through the reaction tubes tion gas mixture in cocurrent or in countercurrent through the Reaction tubes surrounding space can be performed. Of course can, according to the invention, a direct current in reaction zone C. tion and in the reaction zone D a counterflow (or vice versa returns) can be applied.

Of course, you can in all of the above-mentioned cases ions within the respective reaction zone, relative to the Reaction tubes, parallel flow of the molten salt overlap a cross flow so that the individual reaction Zone one as in EP-A 700714 or in EP-A 700893 be corresponds to the tube bundle reactor described and overall in the longitudinal direction cut a meandering stream through the contact tube bundle The course of the heat exchange medium results.

Usually are in the aforementioned tube bundle reactors Contact tubes made of ferritic steel and have typi Scher way on a wall thickness of 1 to 3 mm. Your inside diameter This is usually 20 to 30 mm, often 22 to 26 mm. Appropriately from an application point of view, this is in the tube bundle number of contact tubes accommodated at a minimum 5000, preferably to at least 10000. Often the number is Number of contact tubes accommodated in the reaction container 15,000 to 30000. Tube bundle reactors with a lie above 40,000 The number of contact tubes is rather the exception. Inner The contact tubes are normally homogeneous half of the container distributed arranged, the distribution chosen appropriately is that the distance of the central inner axes from each other closest contact tubes (the so-called contact tube division) Is 35 to 45 mm (see EP-B 468290).

Fluids are particularly suitable as heat exchange medium Temperature control media. The use of melt is particularly favorable zen of salts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or of low melting metals like Sodium, mercury and alloys of various metals.

As a rule, in all the constellations mentioned above the flow of current in the two-zone tube bundle reactors speed within the two required heat levels Exchange medium circuits chosen so that the temperature of the heat exchange medium from the entry point into the reaction zone to the exit point from the reaction zone rises by 0 to 15 ° C.  

That is, the aforementioned ΔT can be 1 to 10 ° C, or 2 up to 8 ° C or 3 to 6 ° C.

The inlet temperature of the heat exchange medium in the According to the invention, reaction zone C is normally 230 to 270 ° C. The inlet temperature of the heat exchange medium in the According to the invention, reaction zone D is normally on the one hand 250 ° C to 300 ° C and on the other hand is at least at the same time 10 ° C above the entry temperature of the reaction zone C entering heat exchange medium.

The inlet temperature of the heat exchange medium is preferably into the reaction zone D at least 20 ° above the entry temperature rature of the heat exchange entering reaction zone C. by means of. The difference between the entry temperatures in the According to the invention, reaction zone C or D can thus be up to 15 ° C. up to up to 25 ° C, up to 30 ° C, up to 35 ° C or up to 40 ° C. Usually, the above temperature is not more than 50 ° C. The higher the acrolein load on the catalyst bed 2 is selected in the method according to the invention, so the difference between the inlet temperature of the Heat exchange medium in the reaction zone C and the entry temperature of the heat exchange medium in the reaction zone D his. The entry temperature into the reaction is preferably zone C at 245 to 260 ° C and the entry temperature into the Reaction zone D at 265 to 285 ° C.

Of course, in the method according to the invention two reaction zones C, D also spatially separated from each other tube bundle reactors. If necessary, between A heat exchanger is also attached to the two reaction zones C, D become. The two reaction zones C, D can of course also can be designed as a fluidized bed.

Catalysts can also be used in the process according to the invention fillings 2 are used, their volume-specific Continuous activity in the flow direction of the reaction gas mixture increases gradually, abruptly or in stages (this can be caused, for example, by Dilution with inert material or variation of the activity of the Multimetal oxide are caused).

Likewise, for the two-zone procedure described in the second reaction stage also in EP-A 293224 and in EP-B 257565 recommended inert diluent gases (e.g. only propane, or only methane etc.) can be used. The latter if necessary also combined with one in the flow direction of the reaction gas  mixture decreasing volume-specific activity of the catalytic converter goal filling 2.

At this point it should be pointed out again that for implementation in the second reaction stage of inventive method in particular also in DE-AS 22 01 528 described two-zone tube reactor type used the one that includes the possibility of hotter warmth exchange medium of reaction zone D a subset of the reaction zone C to evacuate if necessary to warm one Reaction gas starting mixture 2 or a cold cycle gas cause. Furthermore, the tube bundle characteristic, within a ner individual reaction zone as described in EP-A 382 098 ben be designed.

The method according to the invention is particularly suitable for a continuous implementation. It is surprising that at oncei an increased space-time yield of the product of value dung enables, without the selectivity of the value pro to significantly affect product formation. Rather, in the As a rule, even an increased selectivity of the Wertpro duct formation observed. The latter is probably due to this lead that the inventive method due to the in Be rich of increased propene or acrolein sales he present high temperatures a lower readsorption of formed Acrolein / acrylic acid conditional on the fixed bed catalyst.

It is also noteworthy that the catalyst life at inventive method despite the extreme catalyst can fully satisfy the burden of reactants.

Pure acrylic acid is not used in the process according to the invention but obtained a mixture, the secondary components of which Acrylic acid in a manner known per se (e.g. rectificative and / or crystallisatively) can be separated. Not implemented Acrolein, propene and used and / or in the course of the reak tion formed inert diluent gas can in the gas phases oxidation can be recycled. In the two-stage according to the invention The gas phase oxidation emanating from propene takes place Recycle expediently to the first oxidation stage. Well Of course, the procedure according to the invention can also if necessary in the case of conventional propene loads.

Incidentally, turnover, selectivity and residence time are defined as follows in this document, unless stated otherwise:

Examples and comparative examples a) Preparation of the fixed bed catalyst 1 1. Production of a starting mass 1

In 775 kg of an aqueous nitric acid bismuth nitrate solution (11.2% by weight Bi, free nitric acid 3 to 5% by weight; masses density: 1.22 to 1.27 g / ml), 209.3 kg were added in portions at 25 ° C. Tungsten acid (72.94 wt .-% W) stirred. The resulting aqueous mixture was then stirred at 25 ° C for 2 h and then spray dried.

The spray drying was carried out in a rotary disk spray tower in countercurrent at a gas inlet temperature of 300 ± 10 ° C and a gas outlet temperature of 100 ± 10 ° C. The spray powder obtained was then calcined at a temperature in the range from 780 to 810 ° C. (in an air-flow rotary kiln (1.54 m ³ internal volume, 200 Nm ³ air / h)). It is essential for the exact setting of the calcination temperature that it has to be based on the desired phase composition of the calcination product. The phases WO ₃ (monoclinic) and Bi ₂ W ₂ O ₉ are desired; the presence of γ-Bi ₂ WO ₆ (Russel lit) is undesirable. If the compound γ-Bi ₂ WO ₆ can therefore still be detected after the calcination using a reflection in the powder X-ray diffractogram at a reflection angle of 2⊖ = 28.4 ° (CuKα radiation), the preparation must be repeated and the calcination temperature within the specified temperature range until the reflex disappears. The preformed calcined mixed oxide thus obtained was ground so that the X ₅₀ value of the resulting grains was microns (see FIG. Ullmann's Encyclopedia of Industrial Chemistry, 6 ^th Edition (1998), Electronic Release, Chapter 3.1.4 or DIN 66141). 5 The ground material was then mixed with 1% by weight (based on the ground material) of finely divided SiO ₂ (vibrating weight 150 g / l; X ₅₀ value of the SiO ₂ particles was 10 μm, the BET surface area was 100 m ² / G).

2. Production of a starting mass 2

Solution A was prepared by stirring in at 60 ° C 600 l of water dissolved 213 kg of ammonium heptamolybdate and the resulting solution while maintaining the 60 ° C and stirring with 0.97 kg of an aqueous potassium hydroxide solution at 20 ° C. (46.8 wt% KOH).

A solution B was prepared by adding 116.25 kg of an aqueous iron nitrate solution (14.2% by weight of Fe) at 60 ° C. in 262.9 kg of an aqueous cobalt nitrate solution (12.4% by weight of Co). At closing, the solution B was pumped continuously into the pre-placed solution A over a period of 30 minutes while maintaining the 60 ° C. The mixture was then stirred at 60 ° C for 15 minutes. Then 19.16 kg of a silica gel (46.80% by weight SiO ₂ , density: 1.36 to 1.42 g / ml, pH 8.5 to 9.5, alkali content max. 0. 5 wt .-%) added and then stirred for a further 15 minutes at 60 ° C.

Subsequently, in a turntable spray tower in countercurrent spray dried (gas inlet temperature: 400 ± 10 ° C, gas outlet temperature: 140 ± 5 ° C). The resulting spray powder had one Loss on ignition of approx. 30% by weight (anneal for 3 hours at 600 ° C).

3. Production of the multimetal oxide active material

The starting mass 1 was compared with the starting mass 2 for a multimetal oxide active mass of stoichiometry

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

required amount homogeneously mixed. Based on the aforementioned total mass, an additional 1.5% by weight of fine-grained graphite (sieve analysis: min. 50% by weight ≦ 24 µm, max. 10% by weight ≧ 24 µm and ≦ 48 µm, max. 5 % By weight ≧ 48 µm, BET surface area: 6 to 13 m ² / g) homogeneously mixed. The resulting dry mixture was pressed into hollow cylinders with a length of 3 mm, an outer diameter of 5 mm and a wall thickness of 1.5 mm and then thermally treated as follows.

Muffle furnace with air flowing through it (60 l internal volume, 1 l / h air per gram of active mass precursor mass) was at a heating rate from 180 ° C / h initially from room temperature (25 ° C) to 190 ° C heats. This temperature was maintained for 1 h and then increased to 210 ° C with a heating rate of 60 ° C / h. The 210 ° C was which in turn should be maintained for 1 h before using a Heating rate of 60 ° C / h, were increased to 230 ° C. That temperature was also maintained 1 hour before, again with a heating rate of 60 ° C / h, was increased to 265 ° C. The 265 ° C were then also maintained for 1 h. There after was first cooled to room temperature and thus the Decomposition phase essentially completed. Then with a heating rate of 180 ° C / h to 465 ° C and this calcina maintenance temperature for 4 h. A pour out the resulting full catalyst rings formed the fixed bed Talysator 1.

b) Preparation of the fixed bed catalyst 2 1. Production of the catalytically active oxide mass Mo ₁₂ V ₃ W _1.2 Cu _2.4 O _n

190 g of copper (II) acetate monohydrate were dissolved in a solution I in 2700 g of water. In 5500 g of water, 860 g of ammonium heptamolybdate tetrahydrate, 143 g of ammonium metavanadate and 126 g of ammonium paratungstate heptahydrate were successively dissolved to a solution II at 95 ° C. Solution I was then stirred into solution II all at once and then enough of a 25% by weight aqueous NH ₃ solution was added until a solution was obtained again. This was spray dried at an outlet temperature of 110 ° C. The resulting spray powder was kneaded per kg powder with 0.25 kg of a 30% by weight aqueous acetic acid solution using a kneader from Werner & Pfleiderer of the type ZS1-80 and then at a temperature of 110 ° C. for 10 hours in a drying cabinet dries.

700 g of the catalyst precursor thus obtained (50 cm long, 12 cm internal diameter) in a rotary kiln in an air / nitrogen mixture [(200 l N _2/15 l air) / h] calcined. As part of the calcination, the modeling clay was first continuously heated from room temperature (approx. 25 ° C) to 325 ° C within one hour. It was then kept at this temperature for 4 h. The mixture was then heated to 400 ° C. within 15 min, held at this temperature for 1 h and then cooled to room temperature.

The calcined catalytically active material became one finely divided powder, of which 50% of the powder particles passed a sieve with a mesh size of 1 to 10 µm and its Proportion of particles with a longest expansion above 50 microns was less than 1%.

2. Shell catalyst production

28 kg ring-shaped carrier body (7 mm outer diameter, 3 mm length, 4 mm inner diameter, steatite, with a surface roughness Rz according to EP-B 714700 of 45 µm and with a total pore volume ≦ 1% by volume based on the volume of the carrier body, manufacturer : Caramtec DE) were filled in a coating pan (inclination angle 90 °; Hicoater from Lödige, DE) with an internal volume of 200 l. The coating pan was then set in rotation at 16 rpm. 2000 g of an aqueous solution consisting of 75% by weight of H ₂ O and 25% by weight of glycerol were sprayed onto the support bodies via a nozzle within 25 minutes. At the same time, 7 kg of the catalytically active oxide powder from a) were continuously metered in via a shaking channel outside the spray cone of the atomizing nozzle in the same period. During the coating process, the powder supplied was completely absorbed onto the surface of the carrier bodies, and no agglomeration of the finely divided oxidic active composition was observed. After the addition of powder and aqueous solution had ended, at a speed of 2 revolutions / min, 20 min. Hot air at 110 ° C was blown into the coating pan. The mixture was then dried in air for 2 h at 250 ° C. in a stationary bed (tray oven). Annular shell catalysts were obtained, the proportion of oxidic active mass, based on the total mass, of 20% by weight. The shell thickness, as viewed over the surface of a carrier body as well as over the surface of various carrier bodies, was 230 ± 25 µm. A bed of the resulting shell catalyst rings formed the fixed bed catalyst 2.

c) Gas phase catalytic oxidation of propene to acrylic acid 1. The first reaction stage

A first reaction tube (V2A steel; 30 mm outer diameter; 2 mm wall thickness; 26 mm inner diameter, length: 439 cm, as well a thermal tube centered in the center of the reaction tube (4 mm Outer diameter) to accommodate a thermocouple with the the temperature in the reaction tube can be determined) from the bottom up on a contact chair (44 cm long)  next to a length of 30 cm with a rough surface showing steatite balls (4 to 5 mm diameter; inert material for heating the reaction gas starting mixture 1) and then over a length of 300 cm with the in a) manufactured all-catalyst rings loaded before the Be Send on a length of 30 cm with the above Steatite balls as refill was completed. The ver the remaining 35 cm contact tube was left empty.

The part of the first reaction tube that be with solid was sent, was by means of 12 cylindrical around the tube cast aluminum blocks, each 30 cm long, through electric heating tapes were heated, thermostatted (Comparative experiments with a corresponding one using a nitrogen-bubbled salt bath heated reaction tube show that the aluminum block thermostat is a salt bath Thermostat can simulate). The first six Aluminum blocks in the flow direction defined one Defi reaction zone A and the remaining aluminum blocks a reaction zone B. The ends free of solids of the reaction tube were located under increased pressure chem steam kept at 220 ° C.

2. The second stage of the reaction

A second reaction tube (V2A steel; 30 mm outer diameter; 2 mm wall thickness; 26 mm inner diameter, length: 439 cm as well a thermo tube centered in the center of the reaction tube (4 mm Outer diameter) to accommodate a thermocouple with the the temperature in the reaction tube can be determined) from the bottom up on a contact chair (44 cm long) next to a length of 30 cm with a rough surface showing steatite balls (4 to 5 mm diameter; inert material for tempering the reaction gas starting mixture 2) and then over a length of 300 cm with the in b) Manufactured coated catalyst rings before the Loading over a length of 30 cm with the aforementioned Steatite balls as refill was completed. The ver the remaining 35 cm contact tube was left empty.

The part of the second reaction tube that be with solid was sent, was by means of 12 cylindrical around the tube cast aluminum blocks, each 30 cm long thermo statistically (comparative tests with a corresponding heated by means of a nitrogen-pearled salt bath 03773 00070 552 001000280000000200012000285910366200040 0002019927624 00004 03654en Reakti onsrohr showed that the aluminum block thermostat can simulate a salt bath thermostat). The  defined the first six aluminum blocks in the direction of flow a reaction zone C and the remaining six aluminum blocks defined a reaction zone D. The solid Free ends of the reaction tube were under pressure sensitive water vapor kept at 220 ° C.

3. The gas phase oxidation

The first reaction tube described above was continuously charged with a reaction gas starting mixture of the following composition, the load and the thermostatting of the first reaction tube being varied:
6 to 6.5 vol% propene,
3 to 3.5% by volume H ₂ O,
0.3 to 0.5 vol% CO,
0.8 to 1.2 vol% CO ₂ ,
0.025 to 0.04 vol% acrolein,
10.4 to 10.7% by volume of O ₂ and
as a residual amount of 100% molecular nitrogen.

The product gas mixture of the first reaction stage was on the end a small sample for a taken from gas chromatographic analysis. Otherwise it was Product gas mixture under the addition of a temperature of 25 ° C air directly into the subsequent acroleino oxidation stage (to acrylic acid) led (reaction stage 2). The product gas mixture of the acrolein oxidation stage was just if a small sample for a gas chromatographic Analysis taken. In addition, the acrylic acid from Product gas mixture of the second reaction stage in itself known separated and part of the remaining remainder Reuse gases to feed the propene oxidation level det (as a so-called cycle gas), which the acrolein content of aforementioned feed gas and the low variance of the Feed composition explained.

The pressure at the entrance to the first reaction tube varied in Depending on the selected propene load in the range of 3.0 to 0.9 bar. At the end of reaction zones A, C was also an analysis center. The pressure at the entrance of the two th reaction tube varied depending on the Acrolein exposure in the range of 2 to 0.5 bar.  

The depending on the selected loads and the ge chose aluminum thermostatization as well as the practiced Air addition (after the first reaction stage) yielded results nisse is shown in the table below.

T _A , T _B , T _C , T _D stand for the temperature of the aluminum blocks in reaction zones A, B, C and D.
U _PA is the propene conversion at the end of reaction zone A.
U _PB is the propene conversion at the end of reaction zone B.
S _WP is the selectivity of acrolein formation and acrylic acid by-product formation taken together after the first reaction stage and based on the propene converted.
U _AC is the acrolein conversion at the end of reaction zone C.
U _AD is the acrolein conversion at the end of reaction zone D.
U _PD is the propene conversion at the end of reaction zone D.
S _AS is the selectivity of the acrylic acid formation after the second reaction stage and based on the propene converted.
RZA _AS is the space-time yield of acrylic acid at the outlet of the second reaction tube.
V is the molar ratio of molecular oxygen: acrolein in the reaction gas starting mixture 2
M is the amount of air injected after the first reaction stage.

Claims (31)

1. A process for the catalytic gas phase oxidation of propene to acrylic acid, in which a reaction gas starting mixture 1 containing the propylene, molecular oxygen and at least one inert gas, which consists of at least 20% of its volume of molecular nitrogen, which contains the molecular oxygen and the propene in one molar ratio O ₂ : C ₃ H ₆ ≧ 1 ent, initially in a first reaction stage at elevated temperature via a first fixed bed catalyst whose active composition contains at least one molybdenum and / or tungsten and bismuth, tellurium, antimony, tin and / or copper Multimetal oxide is, that the propene conversion at a single pass ≧ 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation taken together are ≧ 90 mol%, the temperature of the product gas mixture leaving the first reaction stage may be reduced by indirect cooling and the product gas mixture optionally adding molecular oxygen and / or inert gas, and thereafter the product gas mixture as acrolein, molecular oxygen and at least one inert gas, which consists of at least 20% of its volume of molecular nitrogen, containing reaction gas starting mixture 2, which contains the molecular oxygen and acrolein in one mo laren ratio O ₂ : C ₃ H ₄ O ≧ 1 contains, in a second reaction stage at elevated temperature over a second fixed bed catalyst, the active composition of which is at least one multimetal oxide containing molybdenum and vanadium, leads to the acrolein conversion in one pass ≧ 90 mol% and the selectivity of the acrylic acid formation balanced over both reaction stages, based on converted propene, is ≧ 80 mol%, characterized in that

• a) the loading of the first fixed bed catalyst with the propene contained in the reaction gas starting mixture 1 ≧ 160 Nl propene / l catalyst bed.h,
• b) the first fixed bed catalyst consists of a catalyst bed arranged in two spatially successive reaction zones A, B, the temperature of the reaction zone A being 300 to 330 ° C and the temperature of the reaction zone B being 300 to 365 ° C and at the same time at least 5 ° C is above the temperature of reaction zone A,
• c) the reaction gas starting mixture 1 flows through the reaction zones A, B in the chronological sequence "first A", "then B",
• d) reaction zone A extends to a propene conversion of 40 to 80 mol%,
• e) the loading of the second fixed bed catalyst with the acrolein enthaltenen 140 Nl acrolein / l catalyst bed contained in the reaction gas starting mixture 2 is h,
• f) the second fixed bed catalyst consists of a catalyst bed arranged in two spatially successive reaction zones C, D, the temperature of the reaction zone C being 230 to 270 ° C and the temperature of the reaction zone D being 250 to 300 ° C and at the same time at least 10 ° C is above reaction zone C,
• g) the reaction gas starting mixture 2 flows through the reaction zones C, D in the chronological sequence "first C", "then D" and
• h) the reaction zone C extends to a conversion of the acro linse of 55 to 85 mol%.

2. The method according to claim 1, characterized in that reaction zone A up to a propene conversion of 50 extends to 70 mol%. 3. The method according to claim 1, characterized in that itself reaction zone A up to a propene conversion of 65 extends to 75 mol%. 4. The method according to any one of claims 1 to 3, characterized records that the reaction zone C up to a conversion of acrolein ranges from 65 to 80 mol%.   5. The method according to any one of claims 1 to 4, characterized records that the temperature of the reaction zone B at least 10 ° C above the temperature of reaction zone A. 6. The method according to any one of claims 1 to 5, characterized records that the temperature of the reaction zone D at least 20 ° C above the reaction zone C. 7. The method according to any one of claims 1 to 6, characterized records that the temperature of the reaction zone B 300 to Is 340 ° C. 8. The method according to any one of claims 1 to 6, characterized records that the temperature of reaction zone B 310 to Is 330 ° C. 9. The method according to any one of claims 1 to 8, characterized records that the temperature of the reaction zone C 245 to Is 260 ° C. 10. The method according to any one of claims 1 to 8, characterized records that the temperature of the reaction zone C 265 to Is 285 ° C. 11. The method according to any one of claims 1 to 10, characterized records that the propene conversion in the first reaction stage in one pass ≧ 94 mol%. 12. The method according to any one of claims 1 to 11, characterized records that the selectivity of acrolein formation and Acrylic acid by-product formation in the first reaction stage taken together in a single pass ≧ 94 mol%. 13. The method according to any one of claims 1 to 12, characterized records that the acrolein conversion in the second reaction step with a single pass is ≧ 94 mol%. 14. The method according to any one of claims 1 to 13, characterized records that the selectivity of the two reaction stages balanced acrylic acid formation, based on the implemented pro pen, ≧ 85 mol%. 15. The method according to any one of claims 1 to 14, characterized records that the propene loading of the first fixed bed catalyst sators ≧ 165 Nl / l.1.   16. The method according to any one of claims 1 to 15, characterized records that the propene loading of the first fixed bed catalyst sator ≧ 170 Nl / l.1. 17. The method according to any one of claims 1 to 16, characterized records that the at least one in the reaction gas output Mix 1 contained inert gas to ≧ 40 vol .-% from molecular Nitrogen exists. 18. The method according to any one of claims 1 to 16, characterized records that the at least one in the reaction gas output Mix 1 contained inert gas to ≧ 60 vol .-% from molecular Nitrogen exists. 19. The method according to any one of claims 1 to 18, characterized records that the at least one in the reaction gas output Mix 1 contained inert gas water vapor. 20. The method according to any one of claims 1 to 19, characterized in that the at least one inert gas contained in the reaction gas starting mixture 1 comprises CO ₂ and / or CO. 21. The method according to any one of claims 1 to 20, characterized records that the propene content of the reaction gas outlet gem is 14 to 15 vol .-%. 22. The method according to any one of claims 1 to 21, characterized in that the active composition of the first fixed bed catalyst at least one multimetal oxide of 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,
c = 0 to 10,
d = 0 to 2,
e = 0 to 8,
f = 0 to 10 and
n = a number which is determined by the valency and frequency of the elements in I other than oxygen. 23. The method according to any one of claims 1 to 21, characterized in that the active composition of the first fixed bed catalyst at least one multimetal oxide 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 ⁷ _y' ] 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, 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,
containing three-dimensionally extended areas of the chemical composition Y ¹ _{a '} Y ² _b' O _{x '} , the size of which are from 1 nm to 100 µm, different from their local surroundings due to their composition which is different from their local surroundings. 24. The method according to any one of claims 1 to 23, characterized records that the first fixed bed catalyst ring and / or spherical catalysts.   25. The method according to claim 24, characterized in that the ring geometry is the following:
Outside diameter: 2 to 10 mm,
Length: 2 to 10 mm,
Wall thickness: 1 to 3 mm. 26. The method according to claim 24, characterized in that the spherical catalyst consisting of a shell catalyst a spherical support (1 to 8 mm in diameter) and one on this applied active mass shell (10 to 1000 µm Thickness). 27. The method according to any one of claims 1 to 26, characterized records that the first and second reaction stages be carried out in a two-zone raw bundle reactor. 28. The method according to any one of claims 1 to 27, characterized in that the active composition of the second fixed bed catalyst at least one multimetal oxide of 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)
With
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 determined by the valency and frequency of the elements other than oxygen in IV. 29. The method according to any one of claims 1 to 27, characterized in that the active composition of the second fixed bed catalyst at least one multimetal oxide 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 _{d ''} Z ⁴ _{e ''} Z ⁵ _{f ''} Z ⁶ _{g ''} O _{x ''}
E: = Z ⁷ ₁₂ Cu _h H _{i ''} O _{y ''}
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, Co, 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 determined by the valency and frequency of the element other than oxygen in VI and
p, q = non-zero numbers whose ratio p / q is 160: 1 to 1: 1,
and which are obtainable by using a multimetal oxide mass (E)
Z ⁷ ₁₂ Cu _{h ''} H _{i ''} O _{y ''} (E),
separately formed in finely divided form (starting mass 1) and then the preformed 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 , the above elements in stoichiometry D
Mo ₁₂ V _{a ''} Z ¹ _{b ''} Z ² _{c ''} Z ³ _{d ''} Z ⁴ _{e ''} Z ⁵ _{f ''} Z ⁶ _{g ''} (D),
contains (starting mass 2), incorporated in the desired p: q ratio, the resulting aqueous mixture dries, and the resulting dry precursor mass is calcined before or after drying to the desired catalyst geometry at temperatures of 250 to 600 ° C. 30. The method according to any one of claims 1 to 29, characterized records that the second fixed bed catalyst annular Ka includes analyzers. 31. The method according to any one of claims 1 to 29, characterized records that the second fixed bed catalyst spherical Ka includes analyzers.