Process for preparing a catalyst, catalyst and process for the oxidative dehydrogenation of hydrocarbons

Abstract

A process for preparing a catalyst provided in the form of a metal oxide catalyst having at least one element selected from Mo, Te, Nb, V, Cr, Dy, Ga, Sb, Ni, Co, Pt and Ce. The catalyst is subjected to an aftertreatment to increase the proportion of the M1 phase, by contacting the catalyst with steam at a pressure below 100 bar or by contacting the catalyst with oxygen to obtain an aftertreated catalyst. The aftertreated catalyst may be used for oxidative dehydrogenation processes.

Claims

1. A process for oxidative dehydrogenation, which comprises using a catalyst in the form of a metal oxide catalyst which comprises at least the elements Mo, Te, Nb, and V, and contains M1 phase, said process comprising: preparing an aftertreated catalyst by calcining a catalyst precursor mixture to obtain a catalyst, and subjecting said catalyst to an aftertreatment to increase the fraction of the M1 phase of the catalyst, wherein in said aftertreatment said catalyst is contacted with a first gas consisting of steam at a pressure below 80 bar, or contacted with a second gas, wherein said second gas is pure oxygen, air, oxygen-enriched air, oxygen-depleted air, or a mixture consisting of at least one gas selected from helium, argon and nitrogen, and 20% oxygen, or contacted with a gas mixture consisting of the first gas and the second gas, and feeding a feed stream containing an alkane into a reactor appliance containing said aftertreated catalyst, wherein, by oxidative dehydrogenation of the alkane with oxygen in the presence of said aftertreated catalyst, an alkene-containing product stream is generated.

2. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said first gas and/or said second gas at a temperature of at least 200 C.

3. The process as claimed in claim 1, wherein said catalyst is a catalyst of the type MoV.sub.aTe.sub.bNb.sub.cO.sub.x, wherein a is in the range from 0.05 to 0.4, b is in the range from 0.02 to 0.2, c is in the range from 0.05 to 0.3, and x is the molar number to satisfy the valence state of the catalyst.

4. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said first gas for a time period of at least one hour.

5. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said second gas for a time period of at least one hour.

6. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said first gas and/or said second gas at a pressure in the range from 0.5 bar to below 80 bar.

7. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said gas mixture consisting of said first gas and said second gas.

8. A process for preparing a catalyst in the form of a metal oxide catalyst which comprises at least the elements Mo, Te, Nb, and V, and contains a M1 phase, said process comprising: calcining a catalyst precursor mixture to obtain a catalyst, and subjecting said catalyst to an aftertreatment to increase the fraction of the M1 phase of the catalyst, and wherein in said aftertreatment said catalyst is a) contacted with a first gas consisting of steam at a pressure below 80 bar, or b) contacted with a second gas, wherein said second gas is pure oxygen, air, oxygen-enriched air, oxygen-depleted air, or a mixture consisting of oxygen and at least one gas selected from helium, argon and nitrogen, wherein said mixture contains at least 20% oxygen, or c) contacted with a gas mixture consisting of the first gas and the second gas; to obtain an aftertreated catalyst.

9. The process as claimed in claim 8, wherein oxygen for the aftertreatment of the catalyst is generated by means of pressure-swing adsorption.

10. The process as claimed in claim 1, wherein the catalyst is subjected to the aftertreatment outside the reactor appliance and is then brought into the reactor appliance.

11. The process as claimed in claim 1, wherein the catalyst is diluted with an inert material, and wherein the catalyst is diluted with the inert material before or after the aftertreatment.

12. The process as claimed in claim 1, wherein a diluent is introduced into the reactor appliance, to control the heat of reaction in the oxidative dehydrogenation of the alkane, wherein said diluent is steam, nitrogen, air, or combination thereof.

13. The process as claimed in claim 1, wherein the first gas consists of steam at a pressure below 50 bar.

14. The process as claimed in claim 12, wherein the diluent is introduced into the reactor appliance in order to prevent an explosion in the oxidative dehydrogenation of the alkane.

15. The process as claimed in claim 1, wherein the catalyst is subjected to the aftertreatment in the reactor appliance.

16. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said first gas and/or said second gas at a temperature in the range from 200 C. to 650 C.

17. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said first gas and/or said second gas at a temperature in the range from 400 C. to 500 C.

18. The process as claimed in claim 1, wherein said catalyst is a catalyst of the type MoV.sub.aTe.sub.bNb.sub.cO.sub.x, wherein a is in the range from 0.12 to 0.25, b is in the range from 0.04 to 0.1, c is in the range from 0.1 to 0.18, and x is the molar number to satisfy the valence state of the catalyst.

19. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said first gas for a time period in the range from one hour to 24 hours.

20. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said second gas for a time period in the range from one to two hours.

21. The process as claimed in claim 1, wherein, during the aftertreatment, said catalyst is contacted with said first gas and/or said second gas at a pressure in the range from 2 bar to 10 bar.

22. The process as claimed in claim 8, wherein, during the aftertreatment, said catalyst is contacted with said second gas.

Description

(1) In the figures:

(2) FIG. 1 shows a diagram, in which, on the x-axis A, the rate constant k.sub.1 (molg.sup.1s.sup.1bar.sup.1) for the ODH C.sub.2H.sub.6.fwdarw.C.sub.2H.sub.4 at 370 C. is given, and on the y-axis B, the M1 concentration of the MoVTeNbO.sub.x catalyst used respectively;

(3) FIG. 2 shows a diagram, in which, on the x-axis A, the concentration of V(V/(Mo+V+Te++Nb)) at the surface of the MoVTeNbO.sub.x catalyst is stated, and on the y-axis B, the M1 concentration of the catalyst;

(4) FIG. 3 shows a block diagram of an appliance for carrying out the process according to the invention for the oxidative dehydrogenation of alkanes;

(5) FIG. 4 shows a diagram, in which, on the bottom x-axis, D, the time in hours is plotted, and on the top x-axis, G, the O.sub.2 concentration at the intake of the reactor appliance (mol %) is plotted, wherein, on the y-axis, the conversion of O.sub.2 or C.sub.2H.sub.6 in % is plotted;

(6) FIG. 5 shows a diagram in which, on the bottom x-axis, D, the time in hours is plotted, and on the top x-axis, G, the O.sub.2 concentration at the intake of the reactor appliance (mol %) is plotted, wherein, on the y-axis, the yield of CO, CO.sub.2 and C.sub.2H.sub.4 in % is plotted;

(7) FIG. 6 shows a diagram in which, on the x-axis, the sample number of 12 different catalyst patterns K is plotted, and on the y-axis, the respective fraction of M1, M2 and amorphous phase is plotted; and

(8) FIG. 7 shows two diagrams in which, on the x-axis, the temperature in C. during passage through a temperature profile is plotted, and on the y-axis, the respective fraction of M1 and M2 phase is plotted (in this division, the percentages only relate to the M1 to M2 phase ratio, and the amorphous fraction remains out of consideration here). In the top diagram, the catalyst was aftertreated only under helium atmosphere, whereas, in the bottom diagram, it was treated under synthetic air tin each case at a pressure of 1 bar).

(9) For the aftertreatment according to the invention, according to one embodiment of the invention, preferably a catalyst K of the composition

(10) MoV.sub.0.05-0.4Te.sub.0.02-0.2Nb.sub.0.05-0.30O.sub.x,

(11) in particular MoVo.sub.0.12-0.25Te.sub.0.04-0.10Nb.sub.0.10-0.18O.sub.x,

(12) comes into consideration (variants having additional dopings with other metals, e.g. Sb, are also possible). However, in principle, the use of other suitable catalysts, e.g. based on the metals V, Cr, Dy, Ga, Sb, Mo, Ni, Nb, Co, Pt, or Ce, and/or oxides thereof or else mixtures, in particular vanadium oxides, NiNbOx is also conceivable. The catalyst can also be diluted by a suitable inert material or be present diluted in the catalyst body.

(13) Maximizing activity and selectivity is then of great importance for practical implementation.

(14) In the case of the preferred above described catalyst K, this maximization is promoted, inter alia, by the fraction of the M1 phase. The fraction of this M1 phase is critical for the selective oxidation of hydrocarbons and a ratio of M1:M2 as high as possible should be sought after.

(15) FIG. 6 shows the distribution between M1, M2 and amorphous phase for various catalyst patterns K aftertreated according to the invention. In this case, the fraction of M1 phase varies between 20% by weight and 90% by weight, whereas the fraction of M2 phase is below 10% by weight. The remaining fraction is in each case an amorphous phase. Via the aftertreatment, M1 fractions between 20% by weight and at least 90% by weight, preferably of more than 70% by weight, can be achieved. Preferably, here, fractions of M2 phase of less than 5% by weight and a maximum of 30% by weight of amorphous phase are achieved. To that end, the catalyst K can first be prepared by a suitable synthesis. In the case of the present invention, hydrothermal synthesis, e.g., can be used (cf. example 1).

(16) Surprisingly, it has been found that as a result of the treatment steps according to the invention, the fraction of M1 phase was able to be increased further, wherein M1 fractions of above 90% by weight were achieved.

(17) As is shown in FIG. 1, experiments have found that the M1 phase is the sole active phase in ODH. Although the M2 phase can oxidize the alkene further, it does not activate the underlying alkane. This may be seen readily with reference to FIG. 1 which shows the rate constant k.sub.1 (in units of molg.sup.1s.sup.1bar.sup.1) for the ODH C.sub.2H.sub.6.fwdarw.C.sub.2H.sub.4 at 370 C. on the x-axis A and the M1 concentration (in % by weight) of the MoVTeNbO.sub.x catalyst respectively used on the y-axis B. Thereafter, the rate constant increases in proportion to the concentration of the M1 phase.

(18) It has been found that for the abovementioned catalysts the M1 concentration can be increased if the catalyst, e.g. in accordance with examples 2 and 3 is treated with steam (termed steaming) and also is treated with oxygen or air in accordance with examples 4 and 5.

(19) The air used in this case can also be prepared synthetically, or be oxygen-enriched or nitrogen-enriched. For the provision, in particular the use of pressure-swing adsorption processes comes into consideration or use may be made of an existing air separation plant, provided that corresponding infrastructure is present. In addition, such a treatment step can also proceed via the infeed of a further inert medium or diluent medium, or else a mixture can be used (e.g. a mixture of steam and (e.g. synthetic) air or oxygen).

(20) In addition, it has been found that the V fraction on the surface of the catalyst is a relevant factor. In this regard, it has been found that, apart from the fraction of the crystalline M1 phase, the amount of the vanadium on the surface, which has been measured by means of LEIS spectroscopy (this is what is termed low-energy ion scattering, a spectroscopic process that can determine the chemical composition of the outermost layer of a solid), not only correlates with the ethene yield, but also with the M1 fraction, as shown in FIG. 2, in which on the x-axis A, the concentration of vanadium (V/(Mo+V+Te+Nb)) on the surface of the MoVTeNbO.sub.x catalyst is plotted, and on the y-axis B, the M1 concentration of the catalyst is plotted.

EXAMPLE 1

(21) For carrying out the aftertreatment according to the invention, a plurality of MoV.sub.yTe.sub.0.1Nb.sub.0.1O.sub.x catalysts with y from the range 0.25 to 0.45 were prepared by a hydrothermal synthesis. For preparing 10 g of MoV.sub.yTe.sub.0.1Nb.sub.0.1O.sub.x catalyst, a corresponding amount of ammonium heptamolybdate (NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O was dissolved in 40 ml of twice-distilled water and heated to 80 C. Te, V and Nb precursorstelluric acid Te(OH).sub.6, vanadyl sulfate VOSO.sub.4 and ammonium nioboxalate C.sub.4H.sub.4NNbO.sub.9.xH.sub.2Owere each dissolved in 10 ml of twice-distilled H.sub.2O. First, the Te solution was added to the Mo at 80 C. After stirring for 20 minutes, the V solution was added dropwise over 20 minutes. After stirring for 15 minutes, to the MoVTe solution was added the Nb solution and the four-element mixture was stirred for a further 10 minutes. The synthesis temperature was kept above 80 C. for the entire mixing procedure of the reactants. The solution was then placed in an autoclave and made up to a volume of 280 ml using twice-distilled water.

(22) The remaining gas volume was purged with N.sub.2 before the synthesis. The hydrothermal treatment was carried out at temperatures in the range from 175 C. to 185 C. and the synthesis time was 24 to 120 hours. Thereafter, the catalyst was filtered, washed with twice-distilled water and dried overnight at 80 C. The calcination was performed in two steps: 2 hours at 250 C. in synthetic air followed by a thermal treatment at 600 C. (heating rate 10 C./min) for a further 2 hours at an inert gas (e.g. N.sub.2, Ar or He) flow rate of 100 ml/min.

EXAMPLE 2

(23) It has been found that exposing the catalysts K to steam at temperatures between 400 C. and 500 C. and a pressure of 1 bar for a time period of 1 hour to 24 hours (1 week at 400 C. gave similar results) increased the catalytic performance in relation to activity. By analyzing the catalysts by means of XRD before and after the steam treatment, it was able to be observed that this increase is due to an increased fraction of the M1 phase (XRD is X-ray diffraction, wherein the diffractograms obtained by this technique were subjected to a Rietveld lattice refinement in order to calculate the fraction of the different crystalline phases in % by weight. The amorphous contribution was likewise quantified, more precisely by calibration on the basis of an amorphous and a highly crystalline standard).

(24) Thus, e.g. a sample having nominal formula MoVo.sub.0.25Te.sub.0.1Nb.sub.0.1O.sub.x (chemical composition determined by inductively coupled plasma optical emission spectrometry (ICP-OES)): MoV.sub.0.13Te.sub.0.06Nb.sub.0.10O.sub.x was contacted with steam at 500 C. at 1 bar for 2 hours. In this case an increase of the M1 content by approximately 5% by weight (from 45% by weight to 51% by weight) was observed. In agreement with this increase in the active M1 phase, according to table 1 an increase of the ethene yield was observed in the activity test (temperature in the range from 370 C. to 430 C., 300 mg of catalyst, total flow rate in the range from 33 to 74 ml/min, gas composition: molar ratio of C.sub.2H.sub.6:O.sub.2:He=1:1:9).

(25) TABLE-US-00001 TABLE 1 Ethene yield Ethene yield M1 (400 C. (400 C. Ethene yield (% by 66 ml/min) 74 ml/min) (430 C. weight) (%) (%) 74 ml/min) (%) Before the 45 2.25 1.87 3.80 aftertreatment After the 51 2.59 2.25 4.45 aftertreatment

EXAMPLE 3

(26) In addition, a sample having the nominal formula MoV.sub.0.40Te.sub.0.10Nb.sub.0.10O.sub.x (chemical composition determined by ICP-OES: MoV.sub.0.20Te.sub.0.05Nb.sub.0.10O.sub.x) was contacted with steam at 400 C. and 1 bar for 2 hours. In this case, an increase in the M1 content as per table 2 by approximately 5% by weight (from 84% by weight to 89% by weight) was observed.

(27) TABLE-US-00002 TABLE 2 Ethene yield Ethene yield M1 (% (370 C. 68 ml/min) (400 C. 68 ml/min) by weight) (%) (%) Before the 84 12.2 23.3 aftertreatment After the 89 16.9 27.9 aftertreatment

EXAMPLE 4

(28) The aftertreatment of the MoVTeNbOx catalysts for 1 to 2 hours at 400 C. and a pressure of 1 bar under a stream of 10% by volume O.sub.2 and 90% by volume He or a synthetic air stream likewise increased the ethane conversion. This increase again was able to be assigned to an increase in the M1 concentration, more precisely, as before due to further crystallization of the amorphous component and by conversion of the M2 phase to the M1 phase.

(29) Thus. e.g. a sample having nominal formula MoV.sub.0.45Te.sub.0.1Nb.sub.0.1O.sub.x (chemical composition determined by ICP-OES: MoV.sub.0.25Te.sub.0.07Nb.sub.0.10O.sub.x) was contacted with O.sub.2 (synthetic air having 21% by volume O.sub.2) for 2 hours at 400 C. and a pressure of 1 bar. In this case it was observed that the M1 content is increased by the aftertreatment by 5% by weight (from 20% by weight to 25% by weight).

(30) The fresh catalyst K contained about 3.5% by weight M2 phase, but only 0.05% by weight M2 phase after the aftertreatment with O.sub.2. By in-situ XRD, it was observed that the aftertreatment with 02 permitted a recrystallization of the inactive M2 phase to the active M1 phase (cf. FIG. 7). This phenomenon is not observed when the same thermal treatment is carried out under inert gas (cf. FIG. 7). As a consequence of the higher M1 concentration of the aftertreated catalyst K, an increase of the ethene yield in the activity test was able to be observed (temperature 370 C. to 430 C., 300 mg to 315 mg of catalyst, flow rate 33 ml/min to 74 ml/min). These results are summarized in table 3:

(31) TABLE-US-00003 TABLE 3 Ethene yield Ethene yield M1 (370 C. (370 C. Ethene yield (% by 33 ml/min) 74 ml/min) (400 C. weight) (%) (%) 74 ml/min) (%) Before the 20 5.27 2.84 6.07 aftertreatment After the 25 6.44 3.31 6.56 aftertreatment

EXAMPLE 5

(32) In addition, a sample having nominal formula MoV.sub.0.40Te.sub.0.1Nb.sub.0.1O.sub.x (chemical composition determined by ICP-OES: MoVo.sub.0.27Te.sub.0.09Nb.sub.0.10O.sub.x) was contacted with O.sub.2 for 2 hours at 400 C. and a pressure of 1 bar. In this case, it was observed as per table 4 that the M1 content is increased by 1% by weight as a result of the aftertreatment (from 49% by weight to 50% by weight).

(33) TABLE-US-00004 TABLE 4 Ethene yield M1 Ethene yield Ethene yield (400 C., (% by (370 C., (400 C., 60 ml/min) weight) 33 ml/min) (%) 33 ml/min) (%) (%) Before the 49 22.0 39.2 27.1 aftertreatment After the 50 22.7 40.0 30.2 aftertreatment

(34) FIG. 3 shows an embodiment of the invention for the oxidative dehydrogenation of an alkane to form the corresponding alkene, e.g. ethane to ethene, using a catalyst K according to the invention.

(35) According thereto, as feed gases (feed stream E), an alkane, in the present case ethane, and also oxygen and/or air were supplied to a catalyst K as oxidizing agent 10 in a reactor appliance 1, which catalyst K is a MoVTeNbO.sub.x catalyst that is aftertreated according to the invention.

(36) In this case, the catalyst K can be introduced into the reactor appliance 1 in a form that is already aftertreated, or first subjected to an aftertreatment there by being exposed to steam and/or oxygen (K.fwdarw.K) by which the M1 content is increased.

(37) In the presence of the catalyst K, the ethane is oxidatively dehydrogenated with the formation of an ethylene-containing product stream P (instead of ethane, propane and/or butane also come into consideration as feed). In this case, it is a highly exothermic procedure. In particular in the formation of byproducts by superoxidation to CO and CO.sub.2, a disproportional amount of heat is released. For the controlled reaction outside of explosion ranges, therefore, an inert diluent V is introduced into the reactor appliance 1, which diluent can comprise, e.g., steam 11.

(38) The ethylene-containing stream P is taken off from the reactor appliance 1 and cooled 12 against the feed E, then further cooled 9, 8 and separated in a separator 2 into a liquid phase and a gaseous phase. The liquid phase substantially comprises water and is discarded 7 or as required vaporized 9 against the product stream P to generate the steam 1.

(39) In a CO.sub.2-removal unit 3, CO.sub.2 present in the product stream P is removed 5.

(40) After the CO.sub.2 removal unit 3, the product stream P passes through a separation part 3, in which inert substances 4 (e.g. N.sub.2, Ar, He) and unreacted ethane E are removed from the product stream P and are recirculated into the reactor appliance 1 or the feed E, wherein inert substances 4 can be recirculated into the reactor appliance 1 as diluents V, or are optionally passed out 6 of the process.

(41) The reactor appliance 1 can be constructed to be either isothermic or adiabatic.

(42) As process data for the reactor appliance 1 in the form of an isothermal reactor, e.g. constructed as a molten salt reactor, for example the following parameters can be used:

(43) Pressure in the reactor appliance 1 from 0.5 bar to 35 bar, preferably 1 bar to 15 bar, particularly preferably 2 bar to 10 bar.

(44) Temperature in the reactor appliance 1 between 250 C. to 650 C., preferably 280 C. to 550 C., particularly preferably 350 C. to 480 C.

(45) Feed compositions (feed stream E):

(46) preferably 5% by volume to 60% by volume ethane, 1% by volume to 40% by volume 02, 0% by volume to 70% by volume H.sub.2O, remainder N.sub.2,

(47) preferably 10% by volume to 55% by volume ethane, 5% by volume to 35% by volume O.sub.2, 0% by volume to 60% by volume H.sub.2O, remainder N.sub.2,

(48) particularly preferably 30% by volume to 50% by volume ethane, 10% by volume to 30% by volume O.sub.2, 0% by volume to 50% by volume H.sub.2O, remainder N.sub.2.

(49) The weight hourly space velocity (WHSV) is preferably in the range from 1.0 kg to 40 kg C.sub.2H.sub.6/h/kgCat, preferably in the range from 2 kg to 25 kg C.sub.2H/h/kgCat, particularly preferably in the range from 5 kg to 20 kg C.sub.2H.sub.6/h/kgCat.

(50) As process data for the reactor appliance 1 in the form of an adiabatic reactor, e.g. the following parameters can be used:

(51) Pressure in the reactor appliance 1 from 0.5 bar to 35 bar, preferably 1 bar to 15 bar, particularly preferably 2 bar to 10 bar.

(52) Temperature in the reactor appliance 1 between 250 C. to 650 C., preferably 280 C. to 550 C., particularly preferably 350 C. to 480 C.

(53) Feed compositions (feed stream E):

(54) preferably 1% by volume to 20% by volume ethane, 1% by volume to 15% by volume O.sub.2, 10% by volume to 95% by volume H.sub.2O, remainder N.sub.2,

(55) preferably 1% by volume to 15% by volume ethane, 1% by volume to 10% by volume O.sub.2, 20% by volume to 90% by volume H.sub.2O, remainder N.sub.2,

(56) particularly preferably 2% by volume to 8% by volume ethane, 1% by volume to 5% by volume O.sub.2, 25% by volume to 80% by volume H.sub.2O, remainder N.sub.2.

(57) The WHSV is preferably in the range from 2.0 kg to 50 kg C.sub.2H.sub.6/h/kgCat, preferably in the range from 5 kg to 30 kg C.sub.2H.sub.6/h/kgCat, particularly preferably in the range from 10 kg to 25 kg C.sub.2Hd h/kgCat.

(58) The fraction of the inert material in the fixed bed or catalyst K, K is preferably 30% by volume to 90% by volume, preferably 50% by volume to 85% by volume, particularly preferably 60% by volume to 80% by volume. A following optional second or further fixed bed can be constructed without inert material.

(59) A further aspect is avoiding explosive atmospheres, in order to exclude hazards to people, plant and environment.

(60) In the separation part 3, by partial cleavage of the product stream P, an enrichment of unreacted oxygen in substreams can occur, and so, again a critical composition can result. Such a composition should be avoided. According to the prior art, this is, e.g. owing to the use of scrubbers, adsorbents or else a targeted reaction to exhaustion of unreacted O.sub.2 (cf e.g. US20100256432). However, this means additional capital and operating costs and pollution of the environment.

(61) In the case of the catalyst K according to the invention, such additional apparatuses can be dispensed with, however, by operating the reactor appliance 1 in such a manner that at the reactor exit in each case only minimal O.sub.2 concentrations are achieved.

(62) This can also be utilized in order to operate a multistage reactor design, in which, in each stage, only small amounts of O.sub.2 are added, and so here also, safe operation is possible outside the relevant explosion ranges. This in addition promotes the selective formation of ethylene and suppresses the further oxidation to CO and CO.sub.2. In addition, the heat development can be safely controlled, since heat is only released in oxidation, that is to say in the presence of a corresponding amount of O.sub.2. In each further reactor stage, then, again a corresponding amount of O.sub.2 is fed in. Optionally, in each case, an intercooling can be performed between the reaction stages. In the limiting case, it can even be a reaction apparatus which comprises corresponding stepwise O.sub.2 infeed. Such a process procedure is only possible with a suitable robust catalyst K, as is provided by the present invention.

EXAMPLE 6

(63) In order to optimize the oxidative dehydrogenation (e.g. of ethane to ethene), it is desirable to achieve a very low concentration of O.sub.2 at the reactor outlet. This means a low concentration of oxygen in the feed E for the reactor appliance 1. However, this endangers the stability of the MoVTeNbO.sub.x catalysts to the extent that this material is subject to a reduction in the absence of O.sub.2 at the reaction temperature, which is accompanied by a loss of the M1 structure and an irreversible deactivation.

(64) Therefore, differing oxygen concentrations were introduced into the reactor appliance 1 at 430 C. and a pressure of 1 bar in order to determine a minimal O.sub.2 concentration which can be used in the ODH without impairing the stability of the catalyst K too greatly. In the experiment shown in FIG. 3, measurements were performed for 2 hours at 430 C. with a falling O.sub.2 concentration, starting from an initial value of 9.1% mol (molar ratio 1:1 with respect to ethene) to 1% mol.

(65) FIG. 4 shows in this case, on the bottom x-axis D, the time in hours, and on the top x-axis G the O.sub.2 concentration at the intake of the reactor appliance (mol %), wherein the conversion of O.sub.2 or C.sub.2H.sub.6 is plotted in % on the y-axis F. The overall flow rate was kept at 33 ml/min for 300 mg of catalyst K, and the feed composition (E) was 9.1 mol % C.sub.2H.sub.6, x mol % O.sub.2, and the remainder Hie (100-9.1-x % mol). After each measurement, the O.sub.2 concentration was again increased for 2 hours to the initial value (9.1 mol % O.sub.2) in order to check that no deactivation of the MoVTeNbO.sub.x catalyst had taken place. Under these conditions, it was observed that the conversion of O.sub.2 can reach a maximum of 90% without effecting an irreversible catalyst reduction.

(66) FIG. 5, on the bottom x-axis D, shows the time in hours, and on the top x-axis G, the O.sub.2 concentration at the intake of the reactor appliance (mol %), wherein the yield of CO, CO.sub.2 and C.sub.2H.sub.4 in % is plotted on the y-axis F. In this case, it becomes clear that, in particular even at 4 mol % O.sub.2 in the feed E, at which the O.sub.2 content at the outlet of the reactor appliance 1 is 0.5% mol, no significant deactivation of the catalyst K was able to be observed after 80 hours at 90% O.sub.2 conversion.

(67) This shows ultimately the robustness of a catalyst K optimized by the aftertreatment according to the invention, which advantageously permits operation under low oxygen concentrations at the outlet of the reactor appliance 1.

EXAMPLE 7

(68) A gas stream of 24.63 Nl/h consisting of 81.8% by volume N.sub.2, 9.1% by volume O.sub.2 and 9.1% by volume ethane is passed through a catalyst bed (length 72 mm) consisting of 4.0 g of a MoV.sub.aTe.sub.bNb.sub.cO.sub.x catalyst K according to the invention which has been aftertreated with steam, and of 22.7 g of inert material (beads of glass scrap, diameter approximately 2 mm), which is situated in an electrically temperature-controlled tubular reactor. The pressure is varied between 1 and 5 bar at a temperature of 370 C. and 400 C. The product gas is cooled by means of a heat exchanger using water cooling and the composition is then analyzed by means of gas chromatography. In this case the conversion rates and selectivities are seen in the following table 5 and determined by calculation result.

(69) TABLE-US-00005 TABLE 5 Temperature 1.0 barg 2.5 barg 5.0 barg 370 C. Ethane conversion rate [%] 20.94 32.99 37.91 Ethene selectivity [%] >99% 96.23 89.12 Ethene yield [%] 21.61 31.74 37.91 400 C. Ethane conversion rate [%] 37.18 50.87 61.95 Ethene selectivity [%] 92.86 87.22 77.72 Ethene yield [%] 34.53 44.37 48.14

(70) TABLE-US-00006 Reference signs 1 reactor appliance 2 separator 3 CO.sub.2 removal 3 separation part 4 inert substances 5, 6, 7 purge 8, 9, 12 heat exchangers 10 oxidizing agent 11 steam V diluent, e.g. steam E feed E ethane P product stream K catalyst (untreated) K aftertreated catalyst