GB2297043A - Catalyst and its use in the unsteady-state oxidative dehydrogenation of alkylaromatics and paraffins by means of heterogeneous catalysis - Google Patents

Abstract

A novel catalyst which is carrier-free or applied to certain carriers comprises (a) at least one oxide of one of the elements Bi, Ce, Co, Cr, Cu, Fe, In, Mn, Mo, Nb, Sb, Sn and V which assume a plurality of oxidation states (b) at least one metal or noble metal which is selected from Ag, Au, Co, Cu, Ir, Ni, Os, Pd, Pt, Re, Rh and Ru and which differs from the above elements and optionally (c) an alkali metal, alkaline earth metal or rare earth metal in the form of an oxide or of a compound with another catalyst component, as is obtained when the metal or noble metal (b) has been applied as a sol to the oxide catalyst (a) which is carrier-free or applied to a carrier. The novel catalyst can be used for the catalytic oxidative dehydrogenation of a dehydrogenatable organic compound to an olefinically unsaturated compound by means of oxygen transfer by an oxygen carrier which has been oxidized beforehand and acts as a catalyst, in the absence of molecular oxygen, the catalyst being regenerated after use.

Description

Catalyst and its use in the unsteady-state oxidative dehydrogena tion of alkylaromatics and paraffins by means cf heterogeneous catalysis The present invention relates to a catalyst and a process for the preparation of olefinically unsaturated compounds by catalytic oxidation, ie. oxidative dehydrogenation by oxygen transfer by a previously oxidized oxygen carrier acting as a catalyst, in the absence of molecular oxygen. The present invention preferably relates to the catalytic oxidative dehydrogenation of alkylaromatics and paraffins to the corresponding alkenylaromatics and olefines, in particular the dehydrogenation of ethylbenzene to styrene.

Styrene and divinylbenzene are important monomers for industrial plastics and are used in large amounts.

Styrene is prepared predominantly by nonoxidative dehydrogenation of ethylbenzene over modified iron oxide catalysts, one mole of hydrogen being formed per mole of styrene. Unfortunately, this reaction is an equilibrium reaction which takes place at, typically, from 600 to 700 C and with a conversion of about 60% and a styrene selectivity of about 90%, the reverse reaction occurring with increasing conversion and increasing concentration of the target product and limiting the conversion.

On the other hand, a virtually quantitative conversion can be achieved by oxidative dehydrogenation in which the hydrocarbon to be converted is reacted with molecular oxygen, ie. in general air, since water is formed in this case. Furthermore, this reaction takes place at a lower temperature than the nonoxidative dehydrogenation. The disadvantage of the oxidative dehydrogenation with molecular oxygen is the total oxidation which takes place as a secondary reaction, carbon dioxide and further water thus occurring in the product stream. This phenomenon is frequently referred to as gasification.

It has therefore been proposed to use, instead of molecular oxygen, an oxygen carrier which acts as a catalyst, ie. controls the reaction, and consists of a reducible metal oxide. The oxygen carrier gradually becomes exhausted and must be regenerated in a second step, thus restoring the initial activity. This procedure frequently used in the traditional process engineering is referred to as the regenerative process. In the regeneration phase, for example, it is also possible to burn off coke deposits. The regeneration is highly exothermic, so that the waste heat evolved can be used, for example, for generating steam.

By dzouling the reduction step and the oxidation step, fle se- lectivity can be substantially increased.

Technically, there are two methods for decoupling, ie. seoarat- of the two steps in space and in time.

In the case of the separation of the two steps in space, a migrating bed or a circulating fluidized bed is used, the catalyst particles from the dehydrogenation zone being transported, after removal of the reaction products, to a separate regeneration reactor in which the reoxidation takes place. The regenerated catalyst is recycled to the dehydrogenation zone. Such a process can be set up as a continuous, ie. cyclic, process. The catalyst is subjected to high mechanical stresses and must therefore have sufficient hardness. Separation in time can be realized with a fixed-bed oxygen carrier by periodically switching between the useful reaction and, possibly after a flushing phase with inert gas, the regeneration.

The regenerative principle with the use of a reducible and regeneratable catalyst was first described for the oxidation or ammoxidation of propene to acrolein and acrylic acid or acrylonitrile (GB 885 422; GB 999 629; K. Aykan, J. Catal. 12 (1968) 281-290), arsenate and molybdate catalysts being used. The use of the process in the oxidative dehydrogenation of aliphatic alkanes to mono- and diolefins with ferrite catalysts (for example US 3 440 299, DE 21 18 344, DE 17 93 499) is likewise known, as is the use for the oxidative coupling of methane to give higher hydrocarbons, different catalyst classes being employed (for example US 4 795 849, DE 35 86 769 with Mn/Mg/Si oxides; US 4 568 789 with Ru oxide; EP 254 423 with Mn/B oxides on MgO; GB 2 156 842 with Mn304 spinels).The dehydrodimerization of toluene to stilbene in the absence of free oxygen by means of reducible catalysts, such as Bi/In/Ag oxides (EP 30 837) is also known.

Finally, the principle is also applied to the dehydrogenation, dehydrocyclization and dehydroaromatization of paraffin hydrocarbons for gasoline refinement (US 4 396 537 with Co/P oxide catalysts).

EP 397 37 and 403 462 disclose the use of the principle of the process of the oxidative dehydrogenation of paraffin hydrocarbons and alkylaromatics. These procedures use reducible oxides of metals selected from the group consisting o' V, Cr, Mn, -e, 3, Pb, Bi, Mo, U and Sn, applied to carriers comprising clays, zeolites and oxides of Ti, Zr, Zn, Th, Mg, Ca, 3a, Si anc Although a high yield is said to be achieved with these cata - lysts, a very high degree of gasification (total cmbusti) cc- curs in the initial phase of the dehydrogenation when the ;;v-=o- carbon comes into contact with the freshly regenerated and the-- fore particularly active catalyst. Apart from the loss of raw materials, the amount of oxygen consumed is also considerably m--0 than that consumed for the straighforward dehydrogenation, so that the oxygen carrier is prematurely exhausted and the cycle time unnecessarily shortened.

According to US 4 067 924, the unsteady-state dehydrogenarion of ethylbenzene takes place with magnesium chromite catalysts. The unsteady-state oxidative dehydrogenation of ethylbenzene and butene with Bi-Cr vanadates is described in US 3 842 132.

A process in which a dehydrogenation catalyst (V, Cr, Mo/A1203) and a redox-active YBCO perovskite, as a hydrogen acceptor, are connected in series is described in EP 558 148. According to a proposal in WO 94/05608, a dehydrogenation catalyst and a zeolite covered with a porous membrane, as a hydrogen absorber, are connected in series. US 3 488 402 proposes connecting a dehydrogenation catalyst (A1-Mg-O, Al-Cr-O) and a reducible oxidation catalyst (bismuth vanadate) in series.

In EP 556 489, propane and/or isobutane are subjected to unsteady-state oxidative dehydrogenation using a catalyst which consists of alkali-doped vanadium oxide on a carrier and may contain Pd and/or Pt and Ce.

Au/Ce oxide catalysts for the unsteady-state oxidative dehydrogenation of alkanes and alcohols are used in EP 543535.

The present invention relates to a novel redox-active catalyst containing noble metal and its preparation starting from a noble metal sol and the use of the catalyst for unsteady-state oxidative dehydrogenation reactions.

The use of noble metal sols as carrier-free catalysts has in principle long been known. As is evident from W. Hückel, Katalyse mit kolloiden Metallen, Leipzig, 1927, colloidal metals were used in particular for the catalytic hydrogenation of multiple bonds.

A more recent document, JA 03/215 607, discloses the preparatior: cf an organometallic sol by decomposition of an organometallic complex in an organic solvent and the use of said sol as a catalyst. The use of noble metal sols on carriers, prerajly y cew z materials, as catalysts is likewise known. For example, JA 03/086 240 describes the preparation of a rhodium or pla~ num organometallic sol and its application to a ceramic cari-.

These catalysts are suitable for removing hydrocarbons, CD and NOx from waste gases. A 53/058 621 describes the preparation cf a hydrogenaticn catalyst from a polymeric anion exchanger and a rhodium sol.

High reaction temperatures often result in losses in selectivity during oxidation and dehydrogenation due to increased formation of byproducts.

It is an object of the present invention to provide a highly reactive catalyst for oxidation and dehydrogenation reactions, which permits a reaction temperature substantially lower than was possible in the past, the particular object to be achieved being to provide a catalyst of this type which achieves a conversion which is at least as high as that achieved in the past and, by reducing total oxidation and formation of crack products as a result of the lower reaction temperature, produces an improvement in the selectivity. In addition, lower reaction temperatures reduce energy costs and advantageously lead to lower material stresses.

We have found that this object is achieved and that redox catalysts comprising at least one noble metal which is applied in the form of a sol to the carrier and at least one redox-active metal oxide have substantially higher activity than the noble metalfree redox catalysts known to date and permit high conversion and high selectivity even at relatively low temperature.

It is assumed - although this should not restrict the present invention - that the noble metal functions as a highly active dehydrogenation center and the redox system as a hydrogen acceptor, since hydrogen is a better reducing agent than ethylbenzene and is still capable of reacting even at low temperatures.

At a high temperature, the reverse process would in principle take place, ie. the noble metal would hydrogenate styrene to ethylbenzene, so that the styrene yield would even have to decrease in the case of doping with noble metal.

The present invention directly relates to a redox catalyst which contains noble metal and is of the abovementioned type and its use, ie. a process for the oxidative dehydrogenation of hydrocarbons.

The novel catalyst consists of at least one oxide selected from the croup consisting of the oxides of the elements Bi, Ce, Cc, Cr, Cu, Fe, In, Mn, Mo, Nb, Ni, Sb, Sn and V which assume a plurality of oxidation states, and said catalyst may be carrier free or may be applied to a carrier. Examples of suitable carriers are clays, PILC, zeolites, aluminum phosphate, silicon carbide, silicon nitride, boron nitride, carbon in any form and oxides of metals selected from the group consisting of Al, Ba, Ca, Mg, Th, Ti, Si, Zn or Zr, as well as mixtures and reaction products thereof. The catalyst may furthermore contain promoters, in particular alkaline earth or alkali metals and/or rare earths.

According to the invention, the catalyst furthermore contains at least one metal or noble metal which differs from the above elements and is selected from the elements Ag, Au, Co, Cu, Ir, Ni, Os, Pd, Pt, Re, Rh, and Ru, the catalyst having the properties which it acquires when the metal or noble metal has been applied as a sol to the oxide catalyst which is carrier-free or applied to a carrier.

A suitable catalyst preferably contains up to 30% by weight of (alkali metal, alkaline earth metal, rare earth) oxide, from 90 to 99.9% by weight of reducible metal oxide and from 0.1 to 10% by weight of (noble) metal, any carrier present not being included in the calculations.

The oxidative dehydrogenation to be operated with the catalyst may take place at from 200 to 700 C, preferably from 300 to 500 C, and at from 100 mbar to 10 bar, preferably from 500 mbar to 2 bar, at a liquid hourly space velocity (LHSV) of from 0.01 to 20 h-l, preferably from 0.1 to 5 h-1. In addition to the hydrocarbon to be oxidized/dehydrogeqated, diluents, for example CO2, N2, noble gases or steam, may be present in the feed. The regeneration of the reduced catalyst is carried out at from 100 to 800 C, preferably from 250 to 600 C, with a free oxidizing agent, preferably with N20 or an oxygen-containing gas, including pure oxygen.

Here too, diluents may be present in the reactive feed. For example, air or air having a low oxygen content is also suitable. The regeneration can be operated at reduced, atmospheric or superatmospheric pressure. Pressures of from 500 mbar to 10 bar are preferred here too.

The novel catalysts are prepared by mixing all components in the usual manner and then heating the mixture to a sufficiently high temperature in order to obtain an active, uniform catalyst. The process used is based on that described in detail below. The noble metal sol is added during the preparation of the catalyst.

In this case, the active component is in any case homogeneously distributed over the catalyst extrudate.

Dry blending, suspension, impregnation, precipitation, coprecl- pitation, spray drying and subsequently calcination, ie. heating to 300-1000 C, which can be carried out uniformly or stepwise, a-e suitable for this purpose. The required raw materials may be in the form of, for example, oxides, hydroxides, carbonates, acetates, nitrates or generally water-soluble salts of the particular metal atoms with inorganic or organic anions. Transition metal complexes may also be used. The calcination is then carried out at temperatures at which the particular raw materials form the catalyst, ie. are incorporated therein, said raw materials initially being converted as a rule directly into the active phase, generally oxides or oxide mixtures or mixed oxides.The state of the noble metal or noble metals used is not specially known and may correspond both to the elemental state and to a chemical compound with the other components. Typical calcination temperatures are from 300 to 1000 C, preferably from 400 to 800 C.

If a rare earth metal is to be used to support the action, particularly when lanthanum is employed, the oxide La2O3 should not be used as a starting material since the activity is then low.

Instead, basic lanthanum carbonate, La(OH) La3(CO3)2 or organic lanthanum compounds, such as lanthanum acetate, lanthanum formate or lanthanum oxalate, which lead to a finely divided and surfacerich active lanthanum phase in the calcination, should be employed.

A preferred calcination temperature for the decomposition of La(Ac)3 into the active lanthanum phase is, for example, from 600 to 800 C.

However, the catalyst is preferably prepared in a two-stage process, in which a noble metal sol is subsequently applied, by spraying or impregnation, to the catalyst which is initially prepared without noble metal as described above and already has redox activity.

For example, the catalyst described in the published DE-OS 4,423,975 can be used as a redox-active catalyst. A process in which the noble metal sol is mixed with the remaining raw materials cf the catalyst by kneading and the cata- lyst is then mold an =-rther processed In a zonven~ o TLan- ner is also recommended.

A noble metal sol can be prepared starting from nozzle :r.'eta salt, by reduction of the latter in aqueous solution. For axc - ple, aqueous solutions of the chlorides, acetates or nitrates of the noble metal may be used, but other noble metal salts are also possible and there is no restriction with regard to the amnions.

Organic compounds, such as ethanol, methanol, carboxylic acids and their alkali metal salts, and inorganic compounds, such as hydroxylamine or NaBH4, may serve as reducing agents. The size of the metal particles in the sol depends in general on the strength of the reducing agent used and on the metal salt used. In general, the formation of smaller metal particles is observed T.th stronger reducing agents. The sols can be stabilized by adding organic polymers, such as polyamines, polyvinylpyrrolidone or polyacrylates. However, the noble metal sols can also be prepared by other methods to be found as a rule in the relevant literature. Bnnemann et al. (Angew.Chemie, 103 (1991) 1344) describe, for example, the preparation of stable metal sols by reaction of metal salts with tetraoctylammonium boronate (C8H17)4N[BEtH3). The catalysts prepared in this manner have the advantage that the noble metal is present in highly disperse form. Since the noble metal component is already present in reduced form in the sol, an additional reduction step can be dispensed with. Since this reduction step often has to be carried out at a high temperature, the result may be agglomeration of the noble metal, which is associated with losses of activity.

The novel catalysts have a noble metal content of from 0.001 to 5, advantageously from 0.005 to 2, in particular from 0.1 to 1, % by weight.

Catalyst preparation: A previously unpublished proposal relates to the preparation of the catalyst which is based on K/La/Bi/TiO2, is active and selective per se and permits a maximum styrene yield of more than 90% at below 500 C, but whose activity rapidly decreases at below 450 C (referred to below as comparative catalyst).

First stage: 107.35 g of lanthanum acetate (La content 41.75% by weight) and 297.5 g of Tio2 of type DT-51 (Rhône-Poulenc) are dry-blended. 3% by weight of extrusion assistant and 159 ml of water are then added and the mixture is compacted in a kneader for two and a half hours. Drying is carried out for two hours at '20 ^ and calcining fcr five hours at 600 C. The material obtained is comminuted in an analytical m-'.l.

Second stage: 312 g of the material of the =irst stage are dryblended to i38.22 g of basic bismuth carbonate (3i content 818 by weight) and 91.55 g of K2C03 for one hour, 3% by weight of extru- sion assistant and water are added and compacting is carried out for two and a half hours in a kneader. The kneaded material is shaped in an extruder to give 3 mm solid extrudates. Drying is carried out for two hours at 120 C and calcining for two hours 500C.

The catalyst contains 12.5% of K2O, 25% of Bi2O3, 9.4% of Lay03 and 53.1% TiO2.

The cutting hardness of the extrudates is 7 N/extrudate, and the BET surface area is 16 m2/g.

For the reactor tests in a pulsed reactor, the extrudates are broken and a particle size fraction of from 50 to 100 Cun is separated off by sieving.

The result obtained with this catalyst as stated below for comparison is not a comparison with the prior art but is intended to illustrate the effect of adding a noble metal. The designations STV .. are random laboratory codes.

Example 1 (STV 33; 0.1t of Pd) 1. Preparation of a palladium sol: 5.482 g of an 11% strength palladium nitrate solution are dissolved in a mixture of 700 ml of distilled water and 300 ml of ethanol, and 5 g of polyvinylpyrrolidone are added The solution is first stirred for half an hour at room temperature, and then refluxed for four hours. After cooling, a stable sol is obtained.

2. Preparation of the novel catalyst: 12.0 g of the catalyst prepared for comparison (see above) are initially taken in a heatable pelletizing table. 20 ml of the sol are diluted to 200 ml and sprayed through a binary nozzle onto the catalyst. The pelletizing table is heated by a hot air stream during the spray process. After the end of the spray process, the catalyst is dried for sixteen hours at 120 C.

-xa--:e 2 (SV 35; .56 of Pd) As in Example 1, excerpt that 100 ml of the so were cil-eå 200 ml and then sprayed onto 12.0 g of the comparative catalyst.

Exarple 3 (STV 34; 1% of Pd) As in Example 1, except that 200 ml of the sol were sprayed undi- fuzed onto i2.0 g of the oomparatve catalyst.

Example 4 (STV 59; 0.43% of platinum) 1. Preparation of a platinum sol: 7.311 g of Pt(NO3)2 were dissolved in 1000 ml of distilled water.

For this purpose, 5.0 g of polyvinylpyrrolidone as a stabilizer and 300 ml of ethanol were added with vigorous stirring, stirring was continued for half an hour at room temperature and the mixture was then ref fluxed for four hours. After cooling, a stable sol (3 g of platinum per 1) was obtained.

First stage: 107.35 g of lanthanum acetate (La content: 41.75% by weight) and 297.5 g of TiO2 of type DT-51, produced by Rhône- Poulenc, are dry-blended for one hour. Thereafter, 3% by weight of an extrusion assistant and 134 ml of the platinum sol are added and compaction is carried out for two and a half hours in a kneader. Drying is carried out for two hours at 120 C and calcination for five hours at 600 C. The material obtained is comminuted in an analytical mill.

Second stage: 312 g of the material of the first stage are dryblended with 138.22 g of basic bismuth carbonate (Bi content 81% by weight) and 91.55 g of K2CO3 for one hour, 3% by weight of extrusion assistant and 105 ml of platinum sol are added and compaction is carried out for sixteen hours in a kneader. The kneaded material is shaped in an extruder to give 3 mm solid extrudates. Drying is carried out for two hours at 120 C and calcination for two hours at 500 C.

The prepared catalyst contains 0.43% of platinum, and the BET surface area is 21 m2/g.

Example 5 (STV 63; 0.43% of palladium i34 m: of the palladium so described in Example i and having a concentration of 3 g/l wer used in the first sa stage and - - .he second stage, as described in Example 4.

The prepared catalyst contains 0.43% cf palladium, and -e surface area is 20.87 m2/c.

Example 6 (STV 6;; 0.1% of platinum) 8.3 ml of the platinum sol having a concentration of 3 g/l are diluted to 100 ml. The dilute sol is sprayed onto 25 g of the comparative catalyst, as described in Example 1. The prepared catalyst contains 0.1% of platinum.

Example 7 (STV 62; 0.5% of platinum) 42 ml of platinum sol having a concentration of 3 g/l are diluted to 100 ml. The dilute sol is sprayed onto 25 g of the comparative catalyst, as described in Example 1. The prepared catalyst contains 0.5% of platinum.

Example 8 (STV 63; 1% of platinum) 84 ml of the platinum sol having a concentration of 3 g/l are diluted to 100 ml. The dilute sol is sprayed onto 25 g of the comparative catalyst, as described in Example 1. The prepared catalyst contains 1% of platinum.

Test results The catalytic oxidative dehydrogenation of ethylbenzene to styrene is investigated on the microscale in a pulsed reactor at from 400 to 500 C. The fixedbed catalyst (weight from 0.3 to 0.6 g) is brought into contact with a pulsed stream of ethylbenzene in the absence of free oxygen, and the reaction products are quantitatively analyzed (on-line GC) for each pulse. In the period between two successive pulses (about 1.5 min), helium carrier gas flows through the reactor. An individual pulse contains 380 g of ethylbenzene. The flow rate of the carrier gas is 21.5 ml/min.

In this way, the deactivation behavior of the catalyst can be monitored from the beginning with high time resolution and without dead time.

Initially, the catalyst is highly active and the reaction rate is correspondingly hIgh. However, the high initial activity results in a certain increase in the level of by-products (for example, in gasification to oxides of carbon) in conjunction with a theoretically lower selectivity. In the urther course, ~;-e formation of by-products declines and the selectivity then im- proves steadily until it reaches its final value. As the test progresses, however, the catalyst becomes increasingly deactI- vated in proportion to the consumption of its lattice oxygen, so that the ethylbenzene conversion decreases.Depending on = deactivation behavior of the catalyst, regeneration must be effected after from 90 te 200 pulses. It is found that the styrene yield, as a product of selectivity and conversion, generally passes through a flat maximum. The results listed in Table 1 (styrene yield and conversion) relate precisely to this maximum value.

After the end of a dehydrogenation phase, the system is switched over to an air stream of 25 ml/min and the catalyst is regenerated for about two hours at the particular reaction temperature.

This is followed by the next cycle. A plurality of cycles is passed through. The reoxidation of the deactivated reduced catalysts made it possible completely to restore the catalytic activity of both the comparative catalyst and the novel catalysts: no loss of activity with the passage of operating time was found over the measured cycles. The numerical values stated in Table 1 are the mean values of a plurality of cycles.

On the industrial scale, the catalyst will not be used until complete deactivation occurs; instead, the regeneration will be initiated while the conversion is still economically acceptable, ie.

has fallen from about 10 to 20% below the maximum value achieved.

In this case, the regeneration time can also be substantially shorter since the catalyst is only partly reduced.

Table 1 - Results

Catalyst Temperature Conversion Selectivity Yield [ C] [%] [%] [%] K/La/Bi/TiO2 400 53.1 92.7 49.2 (compar- 420 82.0 3.5 7.7 ison) 430 89.3 3.1 83.1 40 93.4 92.7 80.ó Example 1 400 8.0 68.8 5.5 0.1% of Pd 450 96.4 91.4 88.1 470 9.6 90.9 90.5 Example 2 390 63.1 95.1 60.0 0.5% of Pd 400 74.0 94.3 69.8 410 82.2 93.9 77.2 420 92.7 93.2 86.4 430 98.0 91.7 89.9 440 98.5 92.5 91.1 450 99.9 90.5 90.4 Example 3 400 63.7 94.2 60.0 1% of Pd 410 75.3 94.6 71.2 420 83.5 94.7 79.1 430 90.8 91.2 82.8 450 91.1 94.1 85.7 Example 4 420 93.3 97.5 91.0 0.43% of Pt Example 5 420 96.5 97.7 94.3 0.43% of Pd Example 6 420 92.9 96.4 89.6 0.1% of Pt Example 7 420 80.6 98.3 79.2 0.5% of Pt Example 8 420 70.9 99.0 70.2 1% of Pt The test results may be interpreted as follows: - Compared with a noble metal-free catalyst, the novel catalyst permits a considerably higher styrene yield at the same reac tor temperature; - A yield of 90% with virtually quantitative conversion at be low 450 C is achieved for the first time, ie. identical cata lytic performance at a substantially lower temperature.

Claims (17)

CLAN

1. A catalyst which is carrier-free or applied to a carrier selected from the clays, PILC, zeolites, aluminum phosphate, silicon carbide, silicon nitride, boron nitride, carbon and oxides of metals selected from Al, Ba, Ca, Mg, Th, Ti, Si, Zn and Zr, said catalyst comprising at least one oxide selected from oxides of the elements Bi, Ce, Co, Cr, Cu, Fe, In, Mn, Mo, Nb, Ni, Sb, Sn and V which assume a plurality of oxidation states and at least one metal or noble metal which differs from the above elements and is selected from the elements Ag, Au, Co, Cu, Ir, Ni, Os, Pd, Pt, Re, Rh and Ru, as obtained when the metal or noble metal has been applied as a sol to the oxide catalyst which is carrier-free or applied to a carrier.

2. A catalyst as claimed in claim 1, additionally containing an alkali metal, alkaline earth metal or rare earth metal, optionally in the form of an oxide or of a compound with another catalyst component.

3. A catalyst as claimed in claim 1 or 2, containing from 10 to 99.9% by weight of at least one oxide of one of the elements assuming a plurality of oxidation states, from 0.1 to 10% by weight of at least one of the metals or noble metals and 0 to 30% by weight of at least one alkali metal oxide,alkaline earth metal oxide or rare earth metal oxide, the percentages in each case being based on the composition in the highest oxidation state, and any carrier present not being included in the calculations.

4. A catalyst as claimed in any of claims 1 to 3, containing bismuth oxide on a titanium dioxide carrier, at least one alkali metal, alkaline earth metal or rare earth metal and at least one noble metal selected from platinum, palladium, ruthenium and rhodium, the noble metal having been applied in the form of a sol to the oxide catalyst.

5. A catalyst comprising a metal oxide and a noble metal or metal applied as sol to the metal oxide and substantially as hereinbefore described or illustrated in any of the foregoing Examples 1 to 8.

6. A process for the preparation of an olefinically unsaturated compound by catalytic oxidative dehydrogenation of a dehydrogenatable organic compound by means of oxygen transfer by an oxygen carrier which has been oxidized beforehand and acts as a catalyst, in the absence of molecular oxygen, the catalyst being regenerated after use, wherein the compound to be dehydrogenated is reacted with a catalyst as claimed in any of claims 1 to 5.

7. A process as claimed in claim 6, wherein a fixed-bed catalyst is used and the dehydrogenation and regeneration steps are decoupled with respect to time in such a way that periodic switching of the reactor inlet steam between starting material and regenerating gas takes place.

8. A process as claimed in claim 7, wherein a flushing operation in which a flushing gas flows through the fixed-bed reactor is introduced between the dehydrogenation step and the regeneration step.

9. A process as claimed in claim 5, wherein a circulating fluidized bed of catalyst is used and the dehydrogenation and regeneration steps are decoupled with respect to space in such a way that catalyst particles are cycled between a dehydrogenation reactor and a separate regeneration reactor.

10. A process as claimed in any of claims 6 to 9, wherein a dehydrogenatable alkylaromatic hydrocarbon is dehydrogenated to the corresponding alkenylaromatic hydrocarbon.

11. A process as claimed in claim 10, wherein ethylbenzene is dehydrogenated to styrene.

12. A process as claimed in any of claims6 to 9, wherein a dehydrogenatable paraffin is dehydrogenated to the corresponding olefin.

13. A process as claimed in any of claims 6 to 12, wherein the used catalyst is regenerated with an oxygen-containing gas or pure oxygen.

14. A process as claimed in any of claims 6 to 12,wherein the used catalyst is regenerated with N20 as an oxidizing agent.

15. A process as claimed in any of claims 6 to 14, wherein the oxidative dehydrogenation is carried out at from 200 to 8000C, at from 100 mbar to 10 bar and with an LHSV of from 0.01 to 20 h1

16. A process for the preparation of an olefinically unsaturated compound carried out substantially as hereinbefore described or exemplified.

17. Olefinically unsaturated compounds when prepared by a process as claimed in any of claims 6 to 16.