Multimetal Oxide Compositions

1. Field of the Invention

The present invention relates to multimetal oxide compositions of the formula I

where

A is Mo.sub.12 X.sub.a.sup.1 X.sub.b.sup.2 X.sub.c.sup.3 X.sub.d.sup.4 S.sub.e X.sub.f.sup.5 O.sub.x

B is X.sub.g.sup.6 X.sub.h.sup.7 O.sub.y

X.sup.1 is phosphorus, arsenic, boron, germanium and/or silicon,

X.sup.2 is vanadium, niobium and/or tungsten,

X.sup.3 is hydrogen, of which up to 97 mol % may have been replaced by potassium, rubidium, cesium and/or ammonium (NH.sub.4),

X.sup.4 is antimony and/or bismuth,

X.sup.5 is rhenium and/or rhodium,

X.sup.6 is copper, iron, cobalt, nickel, zinc, cadmium, manganese, magnesium, calcium, strontium and/or barium,

X.sup.7 is niobium, tantalum and/or antimony,

a is from 1 to 6, preferably from 1 to 3, particularly preferably from 1.5 to 2.5

b is from 0 to 6, preferably from 0.2 to 4, particularly preferably from 0.5 to 2

c is from 3 to 5,

d is from 0 to 6, preferably from 0 to 3, particularly preferably from 0.5 to 1.5

e is from 0 to 3, preferably from 0.01 to 1, particularly preferably from 0.01 to 0.2,

f is from 0 to 3, preferably from 0.01 to 1, particularly preferably from 0.01 to 0.5,

g is from 0.5 to 1.5, preferably from 0.9 to 1.1,

is from 2 to 4, preferably from 2 to 3, particularly preferably from 2 to 2.5,

x and y are numbers determined by the valency and frequency of the elements other than oxygen in I, and

p and q are numbers other than zero whose ratio p/q is from 12:0.1 to 12:48, preferably from 12:0.25 to 12:12, particularly preferably from 12:0.5 to 12:4,

which contain component [A].sub.p in the form of three-dimensionally extended regions A of the chemical composition

which are delimited from their local environment due to their chemical composition which is different from their local environment, and component [B].sub.q in the form of three-dimensionally extended regions B of the chemical composition

which are delimited from their local environment due to their chemical composition which is different from their local environment, where the regions A and B are distributed relative to one another as mixture of finely divided A and finely divided B.

The present invention also relates to a process for the preparation of these compositions, and to their use.

2. Description of the Prior Art

EP-A 442 517 and EP-A 424 900 relate to multimetal oxide compositions whose empirical elemental composition corresponds to that of the novel multimetal oxide compositions.

These multimetal oxide compositions are prepared by converting suitable sources of the constituents of the desired multimetal oxide compositions in the requisite amounts into an intimate dry mix, and subsequently calcining the latter at elevated temperature for several hours.

The resultant multimetal oxide compositions are recommended, inter alia, as catalysts for the preparation of methacrylic acid from methacrolein by gas-phase catalytic oxidation. However, the multimetal oxide compositions of this prior art have the disadvantage that both their activity and the selectivity in the formation of methacrylic acid for a given conversion are not entirely satisfactory. The same applies to the reproducibility of their preparation and to their service lives, which are particularly unsatisfactory if the reaction gases comprising methacrolein as the principal constituent contain organic acids as secondary constituents.

It is an object of the present invention to provide multimetal oxide compositions which do not have the disadvantages of the multimetal oxide compositions of the prior art.

We have found that this object is achieved by the compositions I defined at the outset. EP-A 835, DE-C 3 338 380, DE-A 42 20 859 and the earlier application DE-A 43 07 381 (O.Z. 0050/43890) likewise relate to multimetal oxide compositions which are suitable as catalysts for the preparation of methacrylic acid from methacrolein by gas-phase catalytic oxidation and which advantageously likewise have a two phase structure, but the elemental composition of the phases of these multimetal oxide compositions of the prior art differs from that of the novel multimetal oxide compositions I.

Advantageous compositions I are those in which X.sup.1 is phosphorus. Favorable compositions I are furthermore those in which X.sup.2 is vanadium. It is also advantageous if from 3 to 30 mol % of X.sup.3 is potassium, rubidium, cesium and/or ammonium. The preferred hydrogen substitute is cesium. X.sup.4 is preferably antimony and X.sup.5 is advantageously rhodium. X.sup.6 is advantageously copper and/or zinc, copper being particularly preferred, and X.sup.7 is advantageously antimony.

It is furthermore advantageous if at least one of the two components [A].sub.p and [B].sub.q in the novel multimetal oxide compositions is in the form of three-dimensionally extended regions having the chemical composition A and B respectively with a maximum diameter d.sub.A or d.sub.B (longest line connecting two points on the surface (interface) of the region and passing through the center of gravity of the region) of >0 to 200 .mu.m, preferably from 1 to 200 .mu.m. The maximum diameter range is very particularly from 1 to 50 .mu.m, particularly advantageously from 1 to 30 .mu.m. It is of course also possible for the maximum diameter to be from 50 to 150 .mu.m or from 75 to 125 .mu.m (the experimental determination of the maximum diameter allows, for example, the method of energy-dispersive X-ray analysis (EXDS), for example using a JEOL JCXA/733 electron beam microprobe).

It is preferred if both component [A].sub.p and component [B].sub.q in the novel multimetal oxide compositions are essentially in crystalline form, ie. in general both regions A and regions B essentially comprise small crystallites whose maximum extension is typically-from 0.1 to 1 .mu.m.

Of a particularly favorable nature are multimetal oxide compositions whose regions A essentially comprise crystallites whose structural type corresponds to that of the ammonium salt of molybdatophosphoric acid ((NH.sub.4).sub.3 PO.sub.4 (MoO.sub.3).sub.12 .multidot.4H.sub.2 O). The presence of this crystal structural type can be detected, for example, from the fact that the X-ray diffraction pattern of the novel multi-metal oxide composition contains the diffraction pattern of the ammonium salt of molybdatophosphoric acid (fingerprint), slight differences with respect to the intensity and position of the diffraction lines being possible depending on the elemental composition. The X-ray diffraction fingerprint of the ammonium salt of molybdatophosphoric acid is published, for example, in Card 9-412 of the JCPDS-ICDD Index (1991), which is known and generally accessible to the person skilled in the art. Another source is the National Bureau of Standards (U.S.), Circ. 539, 8 10 (1959). Any antimony in component [A].sub.p is not, in contrast to the other possible constituents of this component, incorporated into the crystallites containing the molybdatophosphoric acid ammonium salt structural type and is located on the surface of these crystallites or in their interstices. It is advantageous if from 85 to 95% by weight of the antimony is present in the form of senarmontite (a crystalline form of antimony trioxide) in spaces between crystallites of the molybdatophosphoric acid ammonium salt structural type formed essentially from the other elements, while from 5 to 15% by weight is in amorphous solution in the surface of such crystallites (the preparation of multimetal oxide compositions containing senarmontite is given in the earlier application DE-A 43 29 907 (O.Z. 0050/44276).

Preference is furthermore given to multimetal oxide compositions whose regions B essentially comprise crystallites having the copper antimonate (CuSb.sub.2 O.sub.6) structural type (card 17-284 of the JCPDS-ICDD Index (1989)), small differences in intensity and position of the diffraction lines also being possible here depending on the elemental composition. In addition to the copper antimonate structure, the trirutile (=tapiolite) structure may also occur as a major or minor constituent. This structural type is known to the person skilled in the art (for example card 23-1124 of the JCPDS-ICDD Index (1989)). Other possible structural types are those of columbite (card 39-1493 of the JCPDS-ICDD Index (1989) and that of niobite (card 39-980 of the JCPDS-ICDD Index (1989)).

The novel compositions I are obtainable in a simple manner, for example by first forming the oxometallates

separately in finely divided form (starting composition 1). The oxometallates B can be prepared by preparing a highly intimate, preferably finely divided, dry mix of suitable sources of their elemental constituents and calcining this dry mix for several hours at from 400.degree. to 1000.degree. C., preferably at from 500.degree. to 850.degree. C. The calcination can take place under inert gas, but also under a mixture of inert gas and oxygen, such as air. In general, the calcination time required decreases with increasing calcination temperature. It is essential that the element sources are either already oxides or are compounds which can be converted into oxides by heating, if necessary in the presence of oxygen. Apart from the oxides, suitable starting compounds are therefore in particular halides, nitrates, formates, oxalates, acetates, carbonates and hydroxides. The intimate mixing of the starting compounds can be carried out in dry or wet form. If it is carried out in dry form, the starting compounds are expediently employed as finely divided powders. However, the intimate mixing is preferably carried out in wet form. In this case, the starting compounds are usually mixed with one another in the form of an aqueous solution and/or suspension. After completion of the mixing operation, the fluid composition is dried and then calcined. The drying is preferably carried out by spray-drying (inlet temperature: 250.degree. to 600.degree. C., outlet temperature: 80.degree. to 130.degree. C.). After calcination, the composition can be comminuted again (for example by wet or dry grinding, for example in a ball mill or by jet grinding), giving a powder generally comprising essentially spherical particles, from which the particle class having a maximum particle diameter in the range desired for the composition I (in general >0 to 200 .mu.m, usually from 1 to 200 .mu.m, preferably from 1 to 50 .mu.m, particularly preferably from 1 to 30 .mu.m) can be separated off by classification in a manner known per se (for example wet or dry screening). The individual powder particle generally comprises crystallites with a maximum extension typically from 0.1 to 1 .mu.m. It has proven particularly favorable to start from an aqueous suspension of an X.sup.7 oxide (for example antimony trioxide) and to dissolve the X.sup.6 element therein as the nitrate or acetate, to spray-dry the resultant aqueous mixture as described, and then to calcine the resultant powder as described.

A finely divided intimate dry mix is produced in a corresponding manner from sources, suitable in the same way, of the elemental constituents of the oxometallates A

but this is generally not recalcined (starting composition 2). If the starting composition 2 is employed after precalcination, the calcination is expediently carried out at from 250.degree. to 450.degree. C. (inert gas, air).

Particularly suitable starting compounds are:

Mo: ammonium heptamolybdate,

V: ammonium metavanadate,

P: from 70 to 100%, preferably from 76 to 85%, phosphoric acid,

Sb: senarmontite,

S: ammonium sulfate,

Re: rhenium pentoxide or ammonium perrhenate,

B: boric acid,

As: arsenic trioxide,

Si: waterglass,

Nb: ammonium niobium oxalate or ammonium niobate,

Alkali metals: alkali metal nitrates,

NH.sub.4 : ammonium sulfate, nitrate or carbonate,

Bi: bismuth nitrate.

The starting composition 1 and the starting composition 2 are subsequently mixed with one another in the desired mixing ratio in wet or dry form (preferably in dry form), and the mixture is shaped and then calcined for several hours at from 250.degree. to 450.degree. C. The calcination can be carried out under an inert gas, but alternatively under a mixture of inert gas and oxygen, for example air. The shaping of the mixture of starting composition 1 and starting composition 2 can be carried out by compaction (for example tableting or extrusion), if necessary with addition of lubricants, such as graphite or stearic acid, and shaping auxiliaries and reinforcing materials, such as microfibers of glass, asbestos, silicon carbide or potassium titanate. In the case of unsupported catalysts, the compaction gives the desired catalyst geometry directly, hollow cylinders having an external diameter and a length of from 2 to 10 mm and a wall thickness of from 1 to 3 mm being preferred. Very generally, the mixture of starting composition 1 and starting composition 2 can be shaped either before or after the calcination. This can also be carried out, for example, by comminuting the mixture after the calcination and applying it to inert supports to produce coated catalysts. However, the application can also be carried out before the final calcination. In this case, the application is preferably carried out as described in EP-B 293 859. It is of course also possible to employ the novel compositions I in powder form.

The novel compositions I are particularly suitable as catalysts of increased selectivity for a given conversion, increased activity, extended service life and improved reproducibility in the preparation of methacrylic acid from methacrolein by gas-phase catalytic oxidation.

The catalytic gas-phase oxidation of methacrolein to methacrylic acid using the novel catalysts can be carried out in a manner known per se, for example in the manner described in DE-A 40 22 212.

The same applies to the removal of the methacrylic acid from the product-gas stream. The oxidant oxygen can be employed, for example, in the form of air, but also in pure form.

Owing to the high heat of reaction, the reactants are preferably diluted with an inert gas such as N.sub.2, CO.sub.2, saturated hydrocarbons and/or steam.

The methacrolein: oxygen: steam: inert gas ratio used is preferably from 1:(1 to 3):(2 to 20):(3 to 30), particularly preferably from 1:(1 to 3):(3 to 10):(7 to 18). The methacrolein employed can have been obtained in various ways, for example by gas-phase oxidation of isobutene, tert-butanol or the methyl ether of tert-butanol. It is advantageous to use methacrolein which is obtainable by condensation of propanol with formaldehyde in the presence of secondary amines and acids in the liquid phase by the processes described in German Patent 875 114 and in DE-B 28 55 514. The gas-phase oxidation can be carried out either in a fluidized-bed reactor or in a fixed-bed reactor. It is preferably carried out in a tube-bundle reactor whose tubes contain the catalyst composition, in the form of a fixed bed, preferably in the form of cylindrical particles. The reaction temperature is generally from 250.degree. to 350.degree. C., the reaction pressure is usually in the range from 1 to 3 bar, and the overall space-time yield is preferably from 800 to 1800 l (s.t.p.)/1/h. Under these conditions, the methacrolein conversion for a single pass through the reactor is usually from 60 to 90 mol %. Interestingly, the novel compositions retain their properties with virtually no change over an increased operating time.

However, the process described usually does not give pure methacrylic acid, but instead a product mixture from which the methacrylic acid must subsequently be separated off. This can be carried out in a manner known per se, for example by scrubbing the reaction gases at from 40.degree. to 80.degree. C. with water after indirect and/or direct cooling, giving an aqueous methacrylic acid solution, from which the methacrylic acid is usually removed by extraction with an organic solvent and separated therefrom by distillation.

In addition to gas-phase catalytic oxidation of methacrolein to methacrylic acid, the novel compositions I are also capable of catalyzing the gas-phase catalytic oxidation and ammonoxidation of other saturated, unsaturated and aromatic hydrocarbons, alcohols, aldehydes and amines.

Specific mention may be made of the gas-phase catalytic oxidation of other C.sub.3 -C.sub.6 -alkanes, alkanols, alkanals, alkenes and alkenols (for example propylene, acrolein, tert-butanol, the methyl ether of tert-butanol, isobutene, isobutane or isobutyraldehyde) to olefinically unsaturated aldehydes and/or carboxylic acids, and the corresponding nitriles (ammonoxidation, in particular of propene to acrylonitrile and of isobutene or tert-butanol to methacrylonitrile). Special mention may be made of the preparation of acrylic acid, acrolein and methacrolein, and the oxidation of n-butane to maleic anhydride and the oxidation of butadiene to furan. However, they are also suitable for the oxidative dehydrogenation of organic compounds.

Conversion, selectivity and residence time in this specification are defined as follows, unless stated otherwise: ##EQU1##

a) Preparation of novel multimetal oxide compositions M and multimetal oxide compositions MV for comparison (the contents of hydrogen, ammonium and oxygen in the resultant compositions are determined by the particular preparation process; the values were not determined regularly and are therefore not present regularly in the stated stoichiometries)

MV1: Example 4 of EP-A 442 517 was repeated. The catalyst geometry selected was hollow cylinders measuring 7 mm (height).times.5 mm (external diameter).times.3 mm (internal diameter). The resultant catalyst has the following stoichiometry:

MV2: Example 2 of EP-A 424 900 was repeated (catalyst geometry selected: 5 mm.times.3 mm.times.2 mm hollow cylinders). The antimony trioxide used was pure finely divided senarmontite having a number average particle diameter of 1.5 .mu.m. Resultant catalyst stoichiometry

M1: Starting composition 2: first 100 g of molybdenum trioxide, then 4.21 g of vanadium pentoxide, 2.81 g of antimony pentoxide, 2.70 g of bismuth trioxide and 8.21 g of 76% phosphoric acid were stirred successively into 800 g of water. The resultant mixture was stirred at 95.degree. C. for 12 hours. Small amounts of insoluble material were filtered off. First a solution of 3.37 g of cesium hydrogencarbonate and 4.06 g of potassium hydrogen-carbonate in 100 g of water and then a solution of 5.6 g of ammonium carbonate in 100 g of water were added continuously to the resultant solution, in each case over the course of 30 minutes. The resultant aqueous mixture was spray-dried at an outlet temperature of 120.degree. C. The resultant starting composition 2 had the following stoichiometry:

Starting composition 1: finely divided CuSb.sub.2 O.sub.6 was prepared as described by V. Propach, D. Reinen, Zeitschrift fur Anorganische und Allgemeine Chemie, Volume 369 (1969), pp. 278-294.

Starting composition 2 and starting composition 1 were dry-mixed in a molar ratio of 1:0.2. After addition of 3% by weight of graphite, the dry composition was tabletted to give hollow cylinders (7 mm.times.5 mm.times.3 mm) and calcined for 5 hours at 380.degree. C. in a stream of air.

The stoichiometry of the resultant composition M1 corresponded to that of MV1.

M2: Starting composition 2: 100 g of ammonium heptamolybdate, 1.66 g of ammonium metavanadate and 4.77 g of potassium nitrate were stirred successively into 250 g of water. The resultant aqueous mixture was warmed to from 30.degree. to 35.degree. C., and 9.13 g of 76% phosphoric acid were then added. The resultant aqueous mixture was warmed to from 40.degree. to 45.degree. C. 2.75 g of finely divided antimony trioxide (pure senarmontite) having a number average particle diameter of 1.5 .mu.m were then added, and the aqueous mixture was heated to 95.degree. C., kept at this temperature for 30 minutes and subsequently spray-dried at an outlet temperature of 120.degree. C.

The resultant starting composition 2 had the following stoichiometry:

Starting composition 1: as for M1, but the product was subsequently ground to a particle size of less than 50 .mu.m.

Starting composition 2 and starting composition 1 were dry-mixed in a molar ratio of 1:0.1. After addition of 3% by weight of graphite, the dry composition was tabletted to give hollow cylinders (5 mm.times.3 mm.times.2 mm) and calcined at 380.degree. C. for 5 hours in a stream of air. The stoichiometry of the resultant composition M2 correspond to that of MV2.

M3: Starting composition 2: 1,000 g of ammonium heptamolybdate, 55.21 g of ammonium metavanadate and 60% of a solution of 110.24 g of cesium nitrate in 250 g of water were dissolved successively in 1,200 g of water. The temperature of the solution was adjusted to from 37.degree. to 42.degree. C., and first 91.29 g of 76% phosphoric acid and then 2.49 g of ammonium sulfate were added. The resultant aqueous mixture was warmed to from 42.degree. to 45.degree. C. 96.31 g of finely divided antimony trioxide (25% of valentinite, 75% of senarmontite) having a number average particle diameter of 3.2 .mu.m were then added, and the mixture was warmed to 95.degree. C. During the heating phase, the remaining 40% of the aqueous cesium nitrate solution, divided into three equal portions, were added when temperatures of 80.degree., 90.degree. and 95.degree. C. had been reached, each portion all at once, and the mixture was subsequently spray-dried at an outlet temperature of 120.degree. C. The resultant starting composition 2 had the following stoichiometry:

Starting composition 1: as for M1.

The mixing and finishing of the catalyst were carried out as for M2, but the molar ratio between starting composition 2 and starting composition 1 was 1:4.

M4: Starting composition 2: 1,000 g of ammonium heptamolybdate, 55.21 g of ammonium metavanadate and 128.8 g of cesium nitrate were stirred successively into 1,200 g of water. The temperature of the aqueous mixture was raised to from 40.degree. to 45.degree. C. 91.29 g of 76% phosphoric acid and 2.49 g of ammonium sulfate were subsequently added, and the temperature of the resultant aqueous mixture was adjusted to from 45.degree. to 48.degree. C. 51.6 g of finely divided antimony trioxide (pure senamontite) having a number average particle diameter of 2.4 .mu.m were then added, and the aqueous mixture was warmed to 95.degree. C., kept at this temperature for 1 hour and subsequently spray-dried at an outlet temperature of 110.degree. C. The resultant starting composition 2 had the following stoichiometry:

Starting composition 1: as for M1:

The mixing and finishing of the catalyst were carried out as for M2, but the molar ratio between starting composition 2 and starting composition 1 was 1:0.2.

M5: Starting composition 2: as for M4.

Starting composition 1: finely divided NiSb.sub.2 O.sub.6 was prepared by the method of V. Propach, D. Reinen, Zeitschrift fur Anaorganische und Allgemeine Chemie, Vol. 369 (1969) pp. 278-294.

The mixing and finishing of the catalyst were carried out as for M2, but the molar ratio between starting composition 2 and starting composition 1 was 1:4.

M6: Starting composition 2: as for M3, but without addition of ammonium sulfate and with a modified stoichiometry:

Starting composition 1: as for M5.

The mixing and finishing of the catalyst were carried out as for M2, but the molar ratio between starting composition 2 and starting composition 1 was 1:0.2.

M7: Starting composition 2: as for M4.

Starting composition 1: finely divided ZnSb.sub.2 O.sub.6 was prepared by the method of V. Propach, D. Reinen, Zeitschrift fur Anorganische und Allgemeine Chemie, Vol. 369 (1969) pp. 278-294.

The mixing and finishing of the catalyst were carried out as for M2, but the molar ratio between starting composition 2 and starting composition 1 was 1:1.

MS: As for M7, but the molar ratio between starting composition 2 and starting composition 1 was 1:0.2.

M9: Starting composition 2: as for M4.

Starting composition 1: finely divided MnSb.sub.2 O.sub.6 was prepared by the method of V. Propach, D. Reinen, Zeitschrift fur Anorganische und Allgemeine Chemie, Vol. 369 (1969) pp. 278-294.

The mixing and finishing of the catalyst were carried out as for M2, but the molar ratio between starting composition 2 and starting composition 1 was 1:1.

M10: As for M9, but the molar ratio between starting composition 2 and starting composition 1 was 1:0.2.

M11: As for M9, but the finely divided MnSb.sub.2 O.sub.6 was replaced by CoSb.sub.2 O.sub.6 prepared as described in the same reference. The molar mixing ratio between starting composition 2 and starting composition 1 was 1:1.

M12: As for M11, but the molar mixing ratio between starting composition 2 and starting composition 1 was 1:0.2.

M13: As for M12, but the finely divided COSb.sub.2 O.sub.6 was replaced by CuNb.sub.2 O.sub.6 prepared as described in the same reference.

M14: As for M12, but the finely divided CoSb.sub.2 O.sub.6 was replaced by CuTa.sub.2 O.sub.6 prepared as described in the same reference.

M15: As for M12, but the finely divided CoSb.sub.2 O.sub.6 was replaced by CuSb.sub.1.8 Nb.sub.0.2 O.sub.6 prepared as described in the same reference.

M16: As for M4, but starting composition 1 and starting composition 2 were compounded in the same mixing ratio with addition of water, extruded to give solid cylinders (6 mm.times.6 mm), dried and calcined as in M2.

M17: Starting composition 2: 1,000 g of ammonium heptamolybdate, 55.21 g of ammonium metavanadate and 60% of a solution of 128.8 g of cesium nitrate in 250 g of water were dissolved successively in 1,000 g of water. The temperature of the solution was adjusted to from 37.degree. to 42.degree. C. First 91.29 g of 76% phosphoric acid and 2.49 g of ammonium sulfate were subsequently added. The temperature of the resultant aqueous mixture was adjusted to from 40.degree. to 45.degree. C. 51.6 g of finely divided antimony trioxide (25% of valentinite, 75% of senarmontite) having a number average particle diameter of 3.2 .mu.m were subsequently added, and the mixture was warmed to 95.degree. C. During the heating phase, the remaining 40% of the cesium nitrate solution, divided into three equal portions, were added when temperatures of 80.degree., 90.degree. and 95.degree. C. had been reached, each portion all at once. The aqueous mixture was subsequently stirred at 95.degree. C. for 30 minutes, and starting composition 1 was added just before the spray-drying (outlet temperature: 110.degree. C.). the molar mixing ratio between starting composition 2 and starting composition 1 was 1:0.2.

Starting composition 2 had the following stoichiometry:

Starting composition 1: as for M1.

M18: As for M9, but the finely divided MnSb.sub.2 O.sub.6 was replaced by Cu.sub.0.5 Ni.sub.0.5 Sb.sub.2 O.sub.6 prepared as described in the same reference. The molar mixing ratio between starting composition 2 and starting composition 1 was 1:0.2.

M19: Starting composition 2: 1,000 g of molybdenum trioxide and 52.65 g of vanadium pentoxide were suspended in 1,000 g of water. 112 g of 76% phosphoric acid were added to the resultant aqueous suspension. The mixture was subsequently stirred at 95.degree. C. for 12 hours, and small amounts of undissolved material were then filtered off. The resultant aqueous solution was cooled to 50.degree. C., 154.2 g of tetrapropylammonium bromide were added continuously over the course of 2 hours, and the mixture was subsequently stirred at 50.degree. C. for 1 hour, spray-dried at an outlet temperature of 110.degree. C. and then calcined at 390.degree. C. for 10 hours. The resultant starting composition 2 had the following stoichiometry:

Starting composition 1: As for M15.

Starting composition 2 and starting composition 1 were dry-mixed in a molar ratio of 1:0.2. After addition of 3% by weight of graphite, the dry composition was tabletted to give hollow cylinders (7 mm.times.5 mm.times.3 mm) and calcined for 5 hours at 380.degree. C. in a stream of air.

M20: Starting composition 2: as for M19, but 50 mol % of the phosphorus were replaced by arsenic, added as arsenic(III) oxide.

Starting composition 1: as for M15.

The mixing and finishing of the catalyst were carried out as for M19.

b) Use of the multimetal oxide compositions from a) as catalysts for the gas-phase oxidation of methacrolein to methacrylic acid

The catalysts were introduced into a tubular reactor (internal diameter 10 mm, 100 g of catalyst bed, temperature control by salt bath), and the reactor was charged with a gaseous mixture having the composition

5% by volume of methacrolein,

10% by volume of oxygen,

30% by volume of steam and

55% by volume of nitrogen

at a reaction temperature in the range from 270.degree. to 340.degree. C. using a residence time of 3.6 seconds. The salt-bath temperature was essentially set in all cases so that a uniform methacrolein conversion of about 80.5% was obtained. A lower salt-bath temperature indicates an increased catalyst activity. The product-gas mixture flowing out of the reactor was analyzed by gas chromatography. The results for the selectivity of the formation of methacrylic acid using the various catalysts are shown in the table below.

Finally, the X-ray diffraction patterns of all multimetal oxide compositions M1 to M20 show both the fingerprint of the ammonium salt of molybdatophosphoric acid ((NH.sub.4).sub.3 PO.sub.4 (MoO.sub.3).sub.12 .multidot.4H.sub.2 O) and a copper antimonate fingerprint.