DE19948523A1 - 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 one contains a propene, molecular oxygen and at least one inert gas containing reaction gas starting mixture 1 which contains the molecular oxygen and the propene in a molar ratio O ₂ : C ₃ H ₆ ≧ 1, contains, first in a first reaction stage at elevated temperature over a first fixed bed catalyst, the active composition of which contains at least one multimetal oxide containing molybdenum and / or tungsten and bismuth, tellurium, antimony, tin and / or copper, which leads to propene conversion one pass ≧ 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation taken together ≧ 90 mol%, the temperature of the product gas mixture leaving the first reaction stage may be reduced by direct and / or indirect cooling and the product gas mixture may be molecular Adds oxygen and / or inert gas, and then the product gas mixture as acrolein, molecular oxygen and reaction gas starting mixture 2 containing at least one inert gas, which contains the molecular oxygen and acrolein in a molar ratio of 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 molybdenum and vanadium, leads to the fact 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 the conversion Propene, ≧ 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. as adhesives suitable polymers is 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 one hand under the highest possible load the first hard bed catalyst bed with propene (below this, 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 Reaction gas starting mixture 1 per hour by one liter Catalyst bed 1 is performed) and on the other hand under a 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 the reaction mixture 2 passed per hour through a liter of catalyst bed 2 will be carried out without doing so during the single pass of the two reaction gas starting mixtures 1, 2 through the two solid bed catalyst bulk sales of propene and Acrolein as well as the 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 accompanied.

In principle, with the fixed bed gas phase described at the beginning Senoxidation of propene to acrylic acid as a catalytic active suitable multimetal oxides in powder form the. However, they usually become a specific catalyst Shaped geometries used because active mass powder because of the poor gas passage for large-scale use is suitable.

The object of the present invention was now that Geometry of the two fixed catalyst beds at the beginning to use the described catalytic fixed bed gas phase oxidation  to choose the shaped catalyst body so that at high educt load of the fixed bed catalyst beds and predetermined Educt conversion with a single pass through the fixed bed catalyst goal selectivity the target selectivity as high as possible compound acrylic acid results.

The following state of the art could be assumed the.

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 located in and along a reaction zone Reaction zone more homogeneous, d. i.e. over the fixed bed catalyst fix chemically composed, fixed bed catalyst used and the temperature of the reaction zone on a over the Reaction zone is kept uniform value (under temperature In a reaction zone, the temperature in the Fixed bed catalyst bed located in the reaction zone at Aus practice of the process in the absence of a chemical reaction Roger that; is this temperature within the reaction zone not constant, that is the term temperature of a reaction zone here the number average of the temperature of the catalyst bed along the reaction zone), limit the one to be used Value of the propene or acrolein pollution of the fixed bed catalyst fill usually to comparatively low values.

So the applied value of the propene load of the fixed bed is catalyst bed normally at values ≦ 155 Nl propene / l Catalyst bed.h (see e.g. EP-A 15565 (maximum propene last = 120 Nl propene / l.h), DE-C 28 30 765 (maximum propene load = 94.5 Nl propene / l.h), EP-A 804465 (maximum propene load = 128 Nl Propene / l.h), EP-B 279374 (maximum propene load = 112 Nl propene / l.h), DE-C 25 13 405 (maximum propene load = 110 Nl propene / l.h), DE-A 33 00 044 (maximum propene load = 112 Nl propene / l.h) and EP-A 575897 (maximum propene load = 120 Nl propene / l.h).

Likewise, in essentially all examples DE-C 33 38 380 the maximum propene load 126 Nl propene / l.h; single In example 3 of this document, a propene load of 162 Nl / l.h realized, being as a shaped catalyst body Solid cylinders consisting exclusively of active compound with a Length of 7 mm and a diameter of 5 mm can be used.  

EP-B 450596 uses a structured Catalyst bed in an otherwise conventional process have a propene load of the catalyst bed of 202.5 Nl / Pro pen / l.h. The shaped catalyst bodies are spherical Shell catalysts used.

In EP-A 293224 propene loads are also above of 160 Nl propene / l.h, one of molecular sticks completely free inert diluent gas is used. The catalyst geometry used is not disclosed.

EP-A 253409 and the associated equivalent, EP-A 257565, disclose that when using an inert diluent gas, which has a higher molar heat capacity than molecular nitrogen has, the proportion of propene in the reaction gas starting mixture can be increased. Nevertheless, is also included in the the aforementioned writings the maximum realized propenbela the catalyst bed at 140 Nl propene / l.h. To start Both writings make no difference to the catalytic converter geometry legend.

In the same way as the conventional methods of Catalytic fixed bed gas phase oxidation of propene to acrolein also limit the conventional methods of catalytic Fixed bed gas phase oxidation of acrolein to acrylic acid Fixed bed catalyst bed acrolein loading normally to values ≦ 150 Nl acrolein / l catalyst. h (see e.g. EP-B 700893; as shaped catalyst bodies are spherical Cup catalysts used).

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-A 253409 and the associated equivalent, EP-A 257565. A Another exception is the already cited EP-A 293224, wherein spherical shells in the oxidation of acrolein catalysts are used.

The basic usability of rings as geometry for the to be used in the two relevant oxidation states Shaped catalyst bodies are known from the prior art.  

For example, the ring geometry is described in DE-A 31 13 179 the aspect "lowest possible pressure loss" in general for exothermic fixed bed gas phase oxidations are suggested. Indeed this document indicates that the influence of geometry the selectivity of the product formation from reaction to reaction un can be different.

Otherwise, the use of ring geometry is Shaped catalyst bodies for catalytic gas phase oxidation of propene and / or acrolein e.g. B. from EP-A 575897, the DE-A 198 55 913 and EP-A 700893 previously known. In all cases the use of the ring shape having a catalyst shape body, however, limited to low educt loads.

The present invention therefore relates to a process for the catalytic gas phase oxidation of propene to acrylic acid, in which a starting gas mixture 1 containing propene, molecular oxygen and at least one inert gas, the molecular oxygen and the propene in a molar ratio O ₂ : C ₃ H ₆ ≧ 1 contains, initially in a first reaction stage at elevated temperature over a first fixed bed catalyst, the active composition of which contains at least one multi-metal oxide containing molybdenum and / or tungsten and bismuth, tellurium, antimony, tin and / or copper, which leads to propene conversion single pass ≧ 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation taken together ≧ 90 mol%, the temperature of the product gas mixture leaving the first reaction stage may be reduced by direct and / or indirect cooling and the product gas mixture added if necessary adding molecular oxygen and / or inert gas, and then the product gas mixture as acrolein, molecular oxygen and at least one inert gas-containing reaction gas starting mixture 2, which contains the molecular oxygen and 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 molybdenum and vanadium, leads to the fact 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, ≧ 80 mol%, which is characterized in that

• a) the load of the first fixed bed catalyst with the im Reaction gas starting mixture 1 contained propene ≧ 160 Nl Propene / l catalyst bed. H,  
• b) the load on the second fixed bed catalyst with the im Reaction gas starting mixture 2 contain acrolein N 140 Nl Acrolein / l catalyst bed.h is and
• c) both the geometry of the shaped catalyst bodies of the first fixed bed catalyst and the geometry of the shaped catalyst bodies of the second fixed bed catalyst is annular with the proviso that
  • - the outer ring diameter 2 to 11 mm,
  • - The ring length 2 to 11 mm and
  • - The wall thickness of the ring is 1 to 5 mm.

Ring-shaped geometries of the catalyst which are suitable according to the invention Shaped bodies of the two fixed catalyst beds are u. a. those, those in EP-A 184790, DE-A 31 13 179, DE-A 33 00 044 and EP-A 714700.

Furthermore, the ring-shaped suitable according to the invention Shaped catalyst bodies of both reaction stages, both fully catalyzed sators (consist exclusively of the catalytically active Mul active time oxide mass), shell catalysts (a ring-shaped The carrier body contains a shell on its outer surface by adsorption the catalytically active multimetal oxide mass applied) as also supported catalysts (contains an annular support body an absorbate of the catalytically active multimetal oxide mass).

Preferably, in the method according to the invention in the first reaction stage annular unsupported catalysts and in the second reaction stage ring-shaped coated catalysts puts. Of course, the combinations can "Shell catalytic converter / full catalytic converter" or "full catalytic converter / full catalyst "or" shell catalyst / shell catalyst "in the two successive reaction stages are used.

In the case of coated catalysts, such catalysts are used according to the invention preferred, the carrier rings a length of 2 to 10 mm (or 3rd up to 6 mm), an outside diameter of 2 to 10 mm (or 4 to 8 mm) and a wall thickness of 1 to 4 mm (or 1 to 2 mm) point. The carrier rings very particularly preferably have the Geometry 7 mm × 3 mm × 4 mm (outside diameter × length × inside diameter). The thickness of the on the annular support body as Shell applied catalytically active oxide mass is usually in general at 10 to 1000 µm. 50 are preferred up to 500 µm, particularly preferably 100 to 500 µm and very particularly preferably 150 to 250 microns.  

The above for the preferred geometry of the carrier rings of Shell catalysts suitable according to the invention The said applies in in the same way also for the carriers suitable according to the invention catalysts.

In the case of full catalyst rings suitable according to the invention, com especially those for which the interior applies diameter 0.1 to 0.7 times the outer diameter and Length is 0.5 to 2 times the outer diameter.

Have cheap all-catalyst rings usable according to the invention an outer diameter of 2 to 10 mm (or 3 to 7 mm), one Inner ring diameter of at least 1.0 mm, a wall thickness of 1 up to 2 mm (or at most 1.5 mm) and a length of 2 to 10 mm (or 3 to 6 mm). Often, suitable in accordance with the invention Fully catalytic converter rings with an outside diameter of 4 to 5 mm, the inside diameter 1.5 to 2.5 mm, the wall thickness 1.0 to 1.5 mm and the Length 3 to 6 mm.

That is, hollow cylinder solid catalyst geome suitable according to the invention trien (each outer diameter × height × inner diameter) the geometries 5 mm × 3 mm × 2 mm, 5 mm × 2 mm × 2 mm, 5 mm × 3 mm × 3 mm, 6 mm × 3 mm × 3 mm or 7 mm × 3 mm × 4 mm.

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 multimetal oxides 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 active compositions for 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 the multimetal oxide active composition according to Example 1c from EP-A 15565 and an active composition to be prepared in a corresponding manner, but the composition

Mo ₁₂ Ni _6.5 Zn ₂ Fe ₂ Bi ₁ P _0.0065 K _0.06 O _x .10SiO ₂ .

As a suitable according to the invention fixed bed catalysts are to be emphasized 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 (Ring) full catalyst of the geometry 5 mm × 3 mm × 2 mm (outer diameter × length × inner diameter) and the multi-metal oxide II full catalyst according to Example 1 of DE-A 197 46 210. Furthermore, the multi-metal oxide full catalysts of US Pat. No. 4,438,217 to call. 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 × length × inside diameter).

A large number of the multimetal oxide active compositions suitable for fixed bed catalysts 1 can be obtained using the general formula I

Mo ₁₂ Bi _a Fe _b X _c ¹ X _d ² X _e ³ X _f ⁴ 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,
subsume.

They are available in a manner known per se (see, for example, the DE-A 40 23 239) and are used according to the invention, for. B. either in Sub stamped into rings or in the form of ring-shaped shells catalysts, d. that is, annularly coated with the active composition preformed, inert carrier bodies 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 a dry mixture which is as inni ges, preferably finely divided, of their stoichiometry as possible, from suitable sources of their elemental constituents, and this at Temperatures calcined from 350 to 650 ° 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 (z. B. mixture of inert gas, NH ₃ , CO and / or H ₂ ). The calcination time can be from a few minutes to a few hours and usually decreases with temperature. Suitable sources for the elemental constituents of the multi-metal 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 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 however in wet form. Usually the output compounds in the form of an aqueous solution and / or suspension mixed together. Particularly intimate dry mixes obtained in the mixing process described if from finally from sources of the elemen available in a dissolved form tare constituents. As a solvent preferably water used. Then the obtained one dried aqueous mass, the drying process preferably by spray drying the aqueous mixture with outlet temperature tures from 100 to 150 ° C takes place.

The Mul suitable for fixed bed catalysts 1 according to the invention time oxide masses, in particular those of the general formula I, become more ring-shaped for the method according to the invention Catalyst geometry molded used, the shaping before or after the final calcination. At for example, from the powder form of the active material or its uncalcined and / or partially calcined precursor mass by compression to the desired catalyst geometry (e.g. by  Extrusion) circular unsupported catalysts are produced, where appropriate tools such. B. graphite or stearin acid as a lubricant and / or molding aid and reinforcement agents such as microfibers made of glass, asbestos, silicon carbide or Potassium titanate can be added.

Of course, the shape of the powdered active mass or its powdery form, not yet and / or partially calcined, precursor mass also by applying to a ring preformed inert catalyst supports. The coating the ring-shaped carrier body for the production of the shell catalyst sators is usually in a suitable rotatable container executed how it z. B. from DE-A 29 09 671, EP-A 293859 or is known from EP-A 714700. Expediently becomes Coating the ring-shaped carrier body to be applied Powder mass or the carrier body moistened and after opening bring, e.g. B. by means of hot air, dried again. The Layer thickness of the applied to the annular support body Powder mass is advantageously in the range 10 to 1000 microns, be preferably in the range from 50 to 500 μm and particularly preferably in the range range from 150 to 250 µm lying.

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. Carrier body with a clearly formed upper surface roughness is preferred. The use is suitable of essentially non-porous, rough-surface, ring-shaped Straps made of steatite. The use of ring-shaped is expedient against cylinders as carrier bodies, their length 2 to 10 mm and their Outside diameter is 4 to 10 mm. The wall thickness is 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).

Alternatively, for the purpose of shaping the ring-shaped carrier body also with one the starting connections of the elementary Constituents of the relevant multimetal oxide active substance ent holding solution and / or suspension soaked, dried and  finally, as described, while receiving the carrier catalysts, calcined.

Inexpensive multimetal oxide active compositions to be used according to the invention for 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),

1 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,
containing three-dimensionally extended areas of the chemical composition, delimited from their local environment on the basis of their composition which is different from their local environment, Y ¹ _{a '} Y ² _b' O _{x '} , the largest diameter of which (longest connecting section going through the center of gravity of the area) on the surface (interface) of the area) is 1 nm to 100 μm, often 10 nm to 500 nm or 1 μm to 50 or 25 μm.

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 _{x ''} ] _{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 2,
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, which is different from their local environment, is present in areas of the chemical composition y ¹ _{a '} y ² _b' O _x , [Bi _{a ''} Z ² _{b ''} O _{x ''} ], the size of which 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.

The first reaction stage is usually carried out of the inventive method in one with the annular Catalysts fed tube bundle reactor such as. B. in the EP-A 700714.

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 becomes a temperature medium (Single zone mode), usually a salt melt. The molten salt and reaction gas mixture can be simple Coincurrent or countercurrent can be performed. The molten salt (the Temperature control medium) can also be viewed through the reactor derform around the tube bundle, so that only about the entire reactor is viewed in cocurrent or countercurrent Direction of flow of the reaction gas mixture exists. The currents temperature of the temperature control medium (heat exchange medium) is usually dimensioned so that the temperature rise (due to the exothermic nature of the reaction) the heat exchange by means of from the entry point into the reactor to the exit put out of the reactor ≧ 0 to 10 ° C, often ≧ 2 to 8 ° C, often ≧ 3 is up to 6 ° C. The inlet temperature of the heat exchange medium in the tube bundle reactor is usually 310 to 360 ° C, often fig 320 to 340 ° C.

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.

The reaction gas starting mixture 1 is expediently the Catalyst feed 1 preheated to the reaction temperature fed. This amounts to the invention described in the same according to the variant of the first reaction stage often 310 to 360 ° C, often 320 to 340 ° C.

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 two-zone tube bundle reactor usable according to the invention discloses DE-C 28 30 765. But also those in DE-C 25 13 405, 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 the extends respective salt bath, according to the invention represents one Reaction 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 Follow-up implementation of the propene (in the single pass) by Achieving a sales value of at least 90 mol% (If necessary, you can contact those to be used according to the invention Reaction zones A, B connect further reaction zones that open 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 the Reaction tubes surrounding space can be performed. 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 is expediently the Catalyst feed 1 preheated to the reaction temperature fed.

Usually in the aforementioned tube bundle reactors (this also applies to the single-zone tube reactor) the contact tubes Made from ferritic steel and typically exhibit a wall thickness of 1 to 3 mm. Your inside diameter is usually 20 to 30 mm, often 21 to 26 mm. Application tech nisch expedient amounts to in the tube bundle container brought number of contact tubes to at least 5000, preferably to at least 10,000. Often the number in the reaction is Contact tubes 15000 to 30000 housed in containers delreactors with a number above 40,000 Contact tubes are rather the exception. Inside the container the contact tubes are normally distributed homogeneously, the distribution is expediently chosen so that the distance the central inner axes of cones closest to each other tact tubes (the so-called contact tube pitch) 35 to 45 mm is (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 to the point of exit from the reaction zone (due to the Exothermic reaction) increases by 0 to 15 ° C. That is, the pre according to the invention, ΔT can be 1 to 10 ° C, or 2 to 8 ° C or 3 up to 6 ° C.

The temperature at which the heat exchange medium enters the reactor Onszone A is normally 310 to 340 ° 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 315 to 380 ° 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 Reaction zone B.

The entry temperature of the heat exchange is advantageously by means of reaction zone B according to the invention at 330 to 370 ° C. and 1 particularly advantageous 340 to 370 ° 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.

Furthermore, in the method according to the invention (both in the Single-zone as well as in the two-zone variant) also catalyst 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 step-wise (this can e.g. how in WO 98/24746 or as described in JP-A 91/294239 or also by dilution with inert material). In addition to nitrogen, water vapor and / or carbon oxides the inert recommended in EP-A 293224 and in EP-B 257565 Dilution gases (e.g. only propane or only methane etc.) are used become. The latter can also be combined with one in Strö if required direction of the reaction gas mixture increasing volume spec fishing 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  To discharge reaction zone B to the 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 through the pipes and a heat exchange medium is led around the pipes, its type of heat build-up recommended for the tube bundle reactors media can correspond. The inside of the pipe is an advantage inert packing (e.g. spirals made of stainless steel, rings made of 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.

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%.

The method according to the invention is suitable for propene loads the catalyst bed 1 of ≧ 165 Nl / l.h or of ≧ 170 Nl / l.h or ≧ 175 Nl / l.h or ≧ 180 Nl / l.h, but also for propene pollution to 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 for load values of ≧ 220 Nl / l.h or ≧ 230 Nl / l.h or ≧ 240 Nl / l.h or N 250 Nl / l.h.

It can fiction for the starting reaction gas mixture 1 According to the inert gas to be used, ≧ 20% by volume or ≧ 30% by volume, or ≧ 40 vol%, or ≧ 50 vol%, or ≧ 60 vol%, or ≧ 70% by volume, or ≧ 80% by volume, or ≧ 90% by volume, or ≧ 95 vol% consist of molecular nitrogen.  

At propene loads on the catalyst bed 1 above 250 Nl / lh, however, the use of inert (inert diluent gases should generally be those which are less than 5%, preferably less, in the case of a single pass through the respective reaction stage) convert as 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 in the reaction gas starting mixture 1 even at lower propene loads according to the invention in the reaction gas starting mixture 1 or can be used as sole dilution gases. It is surprising that the method according to the invention can generally be carried out with good selectivities even with a homogeneous, ie chemically uniform, catalyst bed 1.

With increasing propene load, the described two zones driving style compared to the single zone mode described in the preferred first reaction stage.

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 at values ≦ 300 Nl / l.h, often at values ≦ 250 Nl / l.h.

The working pressure can in the process according to the invention in the first reaction stage both below normal pressure (e.g. to to 0.5 bar) and above normal pressure. More typical The working pressure in the first reaction stage at Wer from 1 to 5 bar, often 1.5 to 3.5 bar. Usually the reaction pressure in the first reaction stage is 100 bar do not 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 Total volume).

Frequently, the method according to the invention is used in a pro pen: oxygen: indifferent gases (including water vapor) Volume ratio in reaction gas starting mixture 1 of 1: (1.0 to 3.0): (5 to 25), preferably 1: (1.5 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.

Appropriately from an application point of view, the product gas mixture of the first reaction stage is cooled to a temperature of 210 to 290 ° C., frequently 220 to 260 ° C. or 225 to 245 ° C. in the aftercooler already mentioned. The product gas mixture of the first reaction stage can be cooled to temperatures below the temperature of the second reaction stage. However, the after-cooling described is by no means compulsory and can generally be omitted 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 often 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: 30, preferably 3: 1 in the reaction gas starting mixture. (1 to 3): (0.5 to 10): Execute (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.

Like the first reaction stage, the second reaction can stage of the method according to the invention in a simple manner a tube bundle charged with the annular catalysts reactor as he z. B. is described in EP-A 700893, by be performed.

That is, in the simplest way, it is according to the invention using fixed bed catalyst 2 in the metal tubes of a tube bundle reactor and around the metal pipes becomes a temperature control medium (Single-zone mode of operation), usually a salt melt. The molten salt and reaction gas mixture can be simple Coincurrent or countercurrent can be performed. The temperature control medium can but also viewed across the reactor in a meandering shape around the pipe be carried out bundles, so that only over the entire reactor considers a cocurrent or countercurrent to the flow direction of the Reaction gas mixture exists. The volume flow of the temperature control diums (heat exchange medium) is usually measured in this way sen that the temperature rise (due to the exothermic of Reaction) of the heat exchange medium from the entry point into the Reactor to the point of exit from the reactor 0 to 10 ° C, often fig is 2 to 8 ° C, often 3 to 6 ° C. The entry temperature of the heat exchange medium in the tube bundle reactor is in the Rule 230 to 300 ° C, often 245 to 285 ° C or 245 to 265 ° C. As Heat exchangers are suitable for the same fluids Temperature control media, as already for the first reaction stage have been described.

The reaction gas starting mixture 2 is expediently the Catalyst feed 2 preheated to the reaction temperature fed. This amounts to the invention described in the same according to the variant of the second reaction stage often 230 to 300 ° C, often 245 to 285 ° C or 245 to 265 ° C.  

Usually a single-zone procedure is the first reaction stage with a single-zone procedure of the second reaction stage com biniert, the relative current flow from the reaction gas mixture and temperature control medium is selected identically in both stages.

Of course, the second reaction stage of the inventive method in a manner corresponding to that he most reaction stage as two spatially successive reactions onszones C, D can be realized, the temperature of the Reaction zone C suitably 230 to 270 ° C and the temperature of the Reaction zone D is 250 to 300 ° C and at the same time at least 10 ° C above the temperature of reaction zone C.

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.

In general, in the method according to the invention, the simpl Chen passage of the second reaction stage related acrolein conversion in the process according to the invention ≧ 92 mol%, or ≧ 94 mol%, or ≧ 96 mol%, or ≧ 98 mol% and often even ≧ 99 mol% be. The selectivity of acrylic acid formation, based on um set acrolein, can regularly ≧ 92 mol%, or ≧ 94 mol%, frequently ≧ 95 mol% or ≧ 96 mol% or ≧ 97 mol% gene.

The process according to the invention is suitable for acrolein pollution gene 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 the catalyst when exposed to acrolein fill 2 of ≧ 185 Nl / l.h or of ≧ 190 Nl / l.h or ≧ 200 Nl / l.h or ≧ 210 Nl / l.h and with load values ≧ 220 Nl / l.h or ≧ 230 Nl / l.h or 240 Nl / l.h or ≧ 250 Nl / l.h.  

This can be done according to the invention in the second reaction stage Inert gas to be used to ≧ 30% by volume, or to ≧ 40% by volume, or to ≧ 50% by volume, or ≧ 60% by volume, or ≧ 70% by volume, or to ≧ 80 vol%, or zu 90 vol%, or ≧ 95 vol% from molecules there is nitrogen.

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 is exposed to acrolein 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 is recommended for reaction gas starting mixture 2. Of course, these gases can also be used with lower acrolein loads. In general, the process according to the invention can be carried out with good selectivities using a homogeneous, ie, a chemically uniform, catalyst bed 2.

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 to acrolein usually does not reach 100%. Fer ner the oxygen demand of the second reaction stage usually covered by air. With increasing acrolein is the described two-zone procedure compared to single zone procedure carried out in the second reaction stage prefers.

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 an annular fixed bed catalyst to be used according to the invention Sators 2 come for gas phase catalytic acrolein oxidation in the second stage of the reaction, consider all those whose Active composition containing at least one Mo and V multimetal oxide is.

Suitable multimetal oxide active materials 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 active 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 active compounds suitable for 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,
subsume.

Preferred embodiments within the active multimetal oxide 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, according to the invention, suitable multimetal oxide active compositions for fixed-bed catalysts 2, in particular those of the general formula IV, can be prepared in a simple manner by producing from intimate sources of their elemental constituents the most intimate, preferably finely divided, dry compound according to their stoichiometry, and this at Calcined 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 multimetal oxide materials suitable for fixed bed catalysts 2, in particular those of the general formula IV, are for the Process according to the invention for annular catalyst geometries molded used, the shape in completely corresponding Way as with the fixed bed catalysts 1 before or after closing calcination can take place. For example quite analogously from the powder form of the active material or its uncalci ned precursor mass by compression to the desired Catalyst geometry (e.g. by extrusion) annular solid catalysts are prepared, where appropriate auxiliary medium such as B. graphite or stearic acid as a lubricant and / or molding aids and reinforcing agents such as microfibers  Glass, asbestos, silicon carbide or potassium titanate can be added can. Suitable full catalyst geometries are, as already called, hollow cylinder with an outer diameter and a length from 2 to 10 mm. A wall thickness of 1 to 3 mm is appropriate moderate.

Of course, the shape of the powdered active mass or its powdery, not yet calcined, Precursor mass also by applying to preformed rings inert catalyst supports. The coating of the carrier Body for the production of the shell catalysts is usually executed in a suitable rotatable container, as z. B. from DE-A 29 09 671, EP-A 293859 or from EP-A 714700 is known.

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 body with clearly trained Surface roughness is preferred. It is suitable Use of hollow cylinders as a support body, the length of 2 to 10 mm and their outer diameter is 4 to 10 mm. The wall In addition, the thickness is 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 up to 8 mm and a wall thickness of 1 to 2 mm. According to the invention Also particularly suitable are rings with a geometry of 7 mm × 3 mm × 4 mm (Outer diameter × length × inner diameter) as a carrier body. To add the delicacy to the surface of the carrier body bringing catalytically active oxide materials is of course adapted the desired shell thickness (see. EP-A 714 700).

Of course, the multimetal oxide active materials can be the solid bed catalysts 2 also to ring-shaped supported catalysts be shaped.  

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, 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 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 they are formed into the desired catalyst geometry calcined at temperatures from 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 lated description of the preparation of multimetal oxide VI-active masses contain z. B. EP-A 668104, DE-A 197 36 105 and DE-A 195 28 646.

With regard to the shape, the following applies to multimetal oxide VI- Active compounds what has been said for the multimetal oxide IV active compounds.

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 the extends respective salt bath, according to the invention represents one Rekationszone.

That is, a salt bath C flows around them in a simple manner Sections of the tubes (the 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 aforementioned constellations 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 (just like in the tube bundle reactors of the single-zone mode) 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 Reaction zone C is in a two-zone according to the invention usually in the second reaction stage 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 lies at the same time at least 10 ° C above the inlet 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.  

Furthermore, in the method according to the invention in general, too Catalyst beds 2 are used, the volume speci fish activity in the flow direction of the reaction gas mixture increases continuously, abruptly or in stages (this can e.g. by dilution with inert material or variation of the activity of the multimetal oxide).

Likewise, for the procedure according to the invention, the two th 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 also if necessary combined with one in the flow direction of the reaction gas mixture decreasing volume-specific activity of the catalyst filling 2.

At this point it should be pointed out again that for carrying out the second reaction stage of the Invention according to the method in particular also in DE-AS 22 01 528 described two-zone tube bundle reactor type can be used, which includes the possibility of the hotter heat exchange medium the reaction zone D from the reaction zone C. lead to possibly warming up a reaction that is too cold to cause gas output mixture 2 or a cold cycle gas. Furthermore, the tube bundle characteristic, within an individual dual reaction zone as described in EP-A 382 098 be tested.

It goes without saying that the process according to the invention can also be carried out in a single two-zone tube bundle reactor, as z. B. in the DE-C 28 30 765, EP-A 911313 and EP-A 383224 ben is to realize that the first reaction stage in which he most reaction zone and the second reaction stage in the second Reaction zone of the two-zone tube bundle reactor is realized.

The method according to the invention is particularly suitable for a continuous implementation. It is surprising that at oncei according to the passage at high educt loading of the fixed bed catalysts enables good selectivities in the formation of valuable products.

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. Of course, the procedure according to the invention can if necessary can also be used 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) Production of a fixed bed catalyst 1 according to the invention 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 disc spray tower in cocurrent 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 mill was then ground with 1% by weight (based on the millbase) 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) mixed.

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). Solution B was then continuously pumped into solution A over a period of 30 minutes while maintaining 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.

Then was in a rotary disc spray tower in direct current 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. Then it 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 were invented fixed bed catalyst according to the invention 1.

b) Production of a fixed bed catalyst 2 according to the invention 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 dissolved in succession at 95 ° C. to form a solution II. Solution I was then stirred all at once into solution II and then enough of a 25% strength 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 with 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 dried.

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. The mixture 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 metered in continuously via a shaking channel outside the spray cone of the atomizer nozzle in the same period. During the coating process, the powder supplied was completely absorbed onto the surface of the support body, 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 under air for a further 2 h at 250 ° C. in a stationary bed (tray oven). There were ring-shaped coated catalysts, the proportion of active oxide mass, based on the total mass, was 20% by weight. The shell thickness, viewed both over the surface of a carrier body and over the surface of various carrier bodies, was 230 ± 25 µm. A bed of the resulting shell catalysts formed a fixed bed catalyst 2 according to the invention.

c) Production of a spherical comparative fixed bed shell talysators 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 disc spray tower in cocurrent 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 oriented towards 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 (Rus sellit) 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) mixed.

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) were added to the resulting aqueous mixture , 5% by weight) and then stirred at 60 ° C. for a further 15 minutes.

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.

The calcined catalytically active material became fine partial powder ground, of which 50% of the powder particles Sieve with a mesh size of 1 to 10 µm and its proportion  Particles with a longest dimension above 50 µm less than Was 1%.

4. Shell catalyst production

30 kg spherical carrier bodies (4-5 mm in diameter with a surface roughness Rz according to EP-B 714700 of 45 µm and with a total pore volume based on the volume of the carrier bodies ≦ 1% by volume, manufacturer: Ceramtec DE) were placed in a coating pan ( Tilt angle 90 °; Hicoater from Lödige, DE) filled with 200 l internal volume. The coating pan was then set in rotation at 16 rpm. 2500 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, 13 kg of the catalytically active oxide powder from a) were metered in continuously via a shaking channel outside the spray cone of the atomizer nozzle. During the coating, the powder supplied was completely absorbed onto the surface of the carrier body, 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). Spherical shell catalysts were obtained, the proportion of oxidic active mass, based on the total mass, of 30% by weight. The shell thickness, viewed both over the surface of a carrier body and over the surface of various carrier bodies, was 280 ± 25 µm. A bed of the resulting coated catalyst spheres formed the spherical comparative fixed bed catalyst 1.

d) Production of a spherical comparative fixed bed shell talysators 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 dissolved in succession at 95 ° C. to form a solution II. Solution I was then stirred all at once into solution II and then enough of a 25% strength 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 with 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 dried.

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 plasticine was first continuously heated to 325 ° C from one hour from room temperature (approx. 25 ° C) within 17906 00070 552 001000280000000200012000285911779500040 0002019948523 00004 17787. The mixture was then kept at this temperature for 4 h. The mixture was then heated to 400 ° C. in the course of 15 minutes, held at this temperature for 1 hour 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

30 kg spherical carrier body (4-5 mm diameter, stea tit, with a surface roughness Rz according to EP-B 714700 from 45 µm and related to the volume of the carrier body total pore volume ≦ 1% by volume, manufacturer: Caramtec DE) were in a coating pan (inclination angle 90 °; hicoater from Lödige, DE) filled with an internal volume of 200 l. The coating pan was then rotated at 16 rpm transferred. 1600 g were added via a nozzle within 25 min sprayed onto the carrier body in an aqueous solution. At the same time, 5.3 kg of the catalytic were in the same period active oxide powder from a) via a shaking channel outside of the spray cone is continuously metered into the atomizer nozzle. During the coating process, the powder supplied became full constantly added to the surface of the carrier body, a Agglomeration of the finely divided oxidic active mass was not observed. After the addition of powder and aq ger solution was at a rotation speed of 2 revolutions hours / min 20 min. 110 ° C hot air in the coating pan blown. Spherical shell catalysts were obtained ten, whose proportion of oxidic active mass, based on the Total mass, 15 wt .-% was. The shell thickness was both over the surface of a support body as well as over the  Viewed surface of different carrier bodies, at 210 ± 5 µm. A filling from the resulting shell catalyst satork balls formed the spherical comparison fixed bed steering catalyst 2.

e) Production of a comparative fixed bed full cylinder catalytic converter tors 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 disc spray tower in cocurrent 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) mixed.

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). Solution B was then continuously pumped into solution A over a period of 30 minutes while maintaining 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.

Then was in a rotary disc spray tower in direct current 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 finely divided 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 solid cylinders with a length of 3 mm and an outer diameter of 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 150 ° C / h initially from room temperature (25 ° C) to 180 ° C heats. This temperature was maintained for 1.5 hours and then increased to 200 ° C at a heating rate of 60 ° C / h. The 200 ° C was maintain it again for 1.5 hours before using a Heating rate increased from 60 ° C / h to 220 ° C. That temperature was also maintained for 1.5 hours before, again with a heating rate of 60 ° C / h, was increased to 250 ° C. The 250 ° C were then also maintained for 1.5 hours. Then it 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 formed the benchmark full-bed catalytic converter 1.

f) Production of a comparative fixed bed full cylinder catalytic converter tors 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 9,500 g of water, 860 g of ammonium heptamolybdate tetrahydrate, 143 g of ammonium metavanadate and 126 g of ammonium paratungstate heptahydrate were successively dissolved in 5500 g of water to give solution II. Solution I was then stirred all at once into solution II and then enough of a 25% strength 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 with 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 dried.

700 g of the so obtained. Catalyst precursor were mixed in a air / nitrogen mixture [(200 l N _2/15 l air) / h] in a rotary kiln (50 cm long, 12 cm internal diameter) calcined. In the course of the calcination, the modeling clay was first continuously heated from room temperature (approx. 25 ° C.) to 325 ° C. within one hour. The mixture was then kept at this temperature for 4 h. The mixture was then heated to 400 ° C. in the course of 15 minutes, held at this temperature for 1 hour 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%.

The catalytically active material thus obtained was according to Zumi addition of 3% by weight (based on the active composition) of graphite Solid cylinders with 3 mm length and 5 mm outside diameter ver presses.  

A bed of the resulting full catalysts bil extended the comparative fixed bed full cylinder catalyst 2.

g) 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) initially on 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 that in a) (or in c) or in e)) produced fixed bed catalyst 1 loaded before loading the length of 30 cm abge with the aforementioned steatite balls as a refill was closed. The remaining 35 cm contact tube were leave 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) initially on 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 one in b) (or in d) or f)) produced fixed bed catalyst 2 loaded before loading the length of 30 cm abge with the aforementioned steatite balls as a pre-fill was closed. The remaining 35 cm contact tube were leave 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 Reacti heated by means of a nitrogen pearled salt bath 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 water vapor held 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 manner separated and part of the remaining Residual gas to feed the propene oxidation stage again used (as a so-called cycle gas), which is the acrolein content of the 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 results obtained for the various catalyst coatings as a function of the selected loads and the selected aluminum thermostatting and the practice of adding air (after the first reaction stage) are shown in the tables below (the above examples can be used according to the invention in a corresponding manner (ie, for example with the same load) also carried out in single-zone tubular reactors; the temperature in the first stage should then be 320 to 360 ° C and in the second stage 245 to 275 ° C; in this regard, it is also recommended to use 300 in the first reaction stage cm feed with fixed bed catalyst 1 to be replaced by the following feed, which only extends to a length of 270 cm:
in the direction of flow, first a mixture of 65 vol.% fixed bed catalyst 1 and 35 vol.% steatite rings (outer diameter × inner diameter × length = 5 mm × 3 mm × 2 mm) and then over a length of 170 cm cm a mixture of 90% by volume of fixed bed catalyst 1 and 10% by volume of the above steatite rings;
In addition, it is advisable to replace the 300 cm loading with fixed bed catalyst 2 in the second reaction stage with the following loading of the appropriate length:
in the direction of flow, first a mixture of 70 vol.% fixed bed catalyst 2 and 30 vol.% steatite rings (outer diameter × inner diameter × length = 7 mm × 3 mm × 4 mm) and then 200 cm pure fixed bed catalyst 2 ).

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 (2)

1. Process of the catalytic gas phase oxidation of propene to acrylic acid, in which a starting gas 1 containing propene, molecular oxygen and at least one inert gas, which contains the molecular oxygen and the propene in a molar ratio O ₂ : C ₃ H ₆ ≧ 1, first in a first reaction stage at elevated temperature so over a first fixed bed catalyst, the active mass of which contains at least one molybdenum and / or tungsten and bismuth, Tel lur, antimony, tin and / or copper, leads to the propene conversion with a single pass gear ≧ 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation together betragen 90 mol%, the temperature of the product gas mixture leaving the first reaction stage may be reduced by direct and / or indirect cooling and the product gas mixture may have molecular oxygen and / or inertga s adds, and then the product gas mixture as acrolein, molecular oxygen and at least one inert gas-containing reaction gas starting mixture 2, which contains the molecular oxygen and acrolein in a molar ratio O ₂ : C ₃ H ₄ O ≧ 1, in a second reaction stage 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 fact that 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, ≧ 80 mol%, which is 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 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 and
• c) both the geometry of the shaped catalyst bodies of the first fixed bed catalyst and the geometry of the shaped catalyst bodies of the second fixed bed catalyst is annular with the proviso that

• - the outer ring diameter 2 to 11 mm,
• - The ring length 2 to 11 mm and
• - The wall thickness of the ring is 1 to 5 mm.

2. Process for the catalytic gas phase oxidation of propene to acrolein and / or acrylic acid, in which a reaction gas starting mixture 1 containing propene, molecular oxygen and at least one inert gas, the molecular oxygen and the propene in a molar ratio O ₂ : C ₃ H ₆ ≧ 1 ent, in a reaction stage at elevated temperature over a first fixed bed catalyst, the active mass of which contains at least a molybdenum and / or tungsten as well as bismuth, tellurium, timon, tin and / or copper containing multimetal oxide, leads to propene conversion a single pass ≧ 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation together ≧ 90 mol .-%, which is characterized in that

• a) the loading of the fixed bed catalyst with the propene contained in the reaction gas starting mixture 1 ≧ 160 Nl propene / l catalyst bed.h and
• b) the geometry of the shaped catalyst bodies of the fixed bed catalyst is annular with the proviso that

• - the outer ring diameter 2 to 11 mm,
• - The ring length 2 to 11 mm and
• - The wall thickness of the ring is 1 to 5 mm.