Synthesis of a moVNbTe catalyst from low-cost metal oxides

Abstract

A novel catalyst and process for producing a mixed oxide material containing molybdenum, vanadium, tellurium and niobium is disclosed. The material can be used as a catalyst for the oxidative dehydrogenation of ethane to ethene or the oxidation of propane to acrylic acid.

Claims

1. A process for producing a mixed oxide material, comprising the steps: a) production of a mixture of starting compounds, which contains starting compounds comprising molybdenum, vanadium, niobium, and tellurium, and also contains oxalic acid, a first chelating oxo ligand, and a second chelating oxo ligand, b) hydrothermal treatment of the mixture of starting compounds at a temperature of from 100° C. to 300° C. to give a product suspension, c) isolation and drying of the solid present in the product suspension resulting from step b), and d) activation of the solid obtained from step c) in an inert gas, wherein the mixture of starting compounds contains tellurium dioxide or a compound of the formula M.sub.x.sup.n+TeO.sub.3 where n=1 or 2 and x=2/n, where M is an alkali metal or alkaline earth metal, and at least one of molybdenum trioxide, vanadium pentoxide, and niobium pentoxide.

2. The process as claimed in claim 1, wherein the activation in step d) is carried out at a temperature in the range from 450° C. to 700° C.

3. The process as claimed in claim 1, wherein the mixture of starting compounds contains molybdenum trioxide.

4. The process as claimed in claim 1, wherein the mixture of starting compounds contains vanadium pentoxide.

5. The process as claimed in claim 1, wherein the mixture of starting compounds contains niobium pentoxide.

6. The process as claimed in claim 5, wherein the niobium pentoxide has a particle size D.sub.90 of less than 100 μm.

7. The process as claimed in claim 1, wherein the mixture of starting compounds contains tellurium dioxide having a particle size D.sub.90 of less than 100 μm.

8. The process as claimed in claim 1, wherein the first chelating oxo ligand or the second chelating oxo ligand is ethylene glycol, present in a molar ratio to molybdenum of 0.01:1 to 1:1.

9. The process as claimed in claim 1, wherein the first chelating oxo ligand or the second chelating oxo ligand is citric acid.

10. A mixed oxide material for the oxidation of ethane, which comprises the elements molybdenum, vanadium, niobium and tellurium and has the following stoichiometry:
Mo.sub.1V.sub.aNb.sub.bTe.sub.cO.sub.x where 0.27<a<0.31; 0.08<b<0.12; 0.08<c<0.12, and which in the XRD, when using Cu-Kα radiation, has diffraction reflections h, i, k and l whose peaks are approximately at the diffraction angles (2θ) 26.2°±0.5° (h), 27.0°±0.5° (i), 7.8°±0.5° (k) and 28.0°±0.5° (l).

11. The use of the mixed oxide material as claimed in claim 10 for the oxidative dehydrogenation of ethane to ethene.

12. The use of the mixed oxide material as claimed in claim 10 for the oxidation of propane.

13. The process as claimed in claim 1, wherein each chelating oxo ligand is independently present in the mixture of starting compounds in a molar ratio to molybdenum of 0.08:1 to 0.4:1.

14. The process as claimed in claim 1, wherein the product suspension resulting from step b) includes the mixed oxide material, having the following stoichiometry:
Mo.sub.1V.sub.aNb.sub.bTe.sub.cO.sub.x where 0.27<a<0.31; 0.08<b<0.12; 0.08<c<0.12, and which in the XRD, when using Cu-Kα radiation, has diffraction reflections h, i, k and l whose peaks are approximately at the diffraction angles (2θ) 26.2°±0.5° (h), 27.0°±0.5° (i), 7.8°±0.5° (k) and 28.0°±0.5° (l).

15. The process as claimed in claim 1, wherein the product suspension resulting from step b) comprises a mother liquor including less than 100 ppm of a combined amount of molybdenum, vanadium, niobium, and tellurium.

16. The process as claimed in claim 1, wherein each chelating oxo ligand is independently present in the mixture of starting compounds in a molar ratio to molybdenum of 0.08:1 to 0.4:1.

17. A process for producing a mixed oxide material, comprising the steps: a) production of a mixture of starting compounds, which contains starting compounds comprising molybdenum, vanadium, niobium, and tellurium, and also contains oxalic acid a first chelating oxo ligand and a second chelating oxo ligand, wherein molybdenum is present in the mixture in an atomic ratio to vanadium of 1:0.22 to 1:0.3, in an atomic ratio to niobium of 1:0.1 to 1:0.17, and in an atomic ratio to tellurium of 1:0.1 to 1:0.17, b) hydrothermal treatment of the mixture of starting compounds at a temperature of from 100° C. to 300° C. to give a product suspension, c) isolation and drying of the solid present in the product suspension resulting from step b), and d) activation of the solid obtained from step c) in an inert gas, wherein the mixture contains tellurium dioxide or a compound of the formula M.sub.x.sup.n+TeO.sub.3 where n=1 or 2 and x=2/n, where M is an alkali metal or alkaline earth metal.

18. The process as claimed in claim 17, wherein the mixture of starting compounds includes at least one of molybdenum trioxide, vanadium pentoxide, and niobium pentoxide.

19. The process as claimed in claim 17, wherein the product suspension resulting from step b) includes the mixed oxide material, having the following stoichiometry:
Mo.sub.1V.sub.aNb.sub.bTe.sub.cO.sub.x where 0.27<a<0.31; 0.08<b<0.12; 0.08<c<0.12, and which in the XRD, when using Cu-Kα radiation, has diffraction reflections h, i, k and l whose peaks are approximately at the diffraction angles (2θ) 26.2°±0.5° (h), 27.0°±0.5° (i), 7.8°±0.5° (k) and 28.0°±0.5° (l).

20. The process as claimed in claim 17, wherein the product suspension resulting from step b) comprises a mother liquor including less than 100 ppm of a combined amount of molybdenum, vanadium, niobium, and tellurium.

Description

(1) FIG. 1: XRD of the MoVTeNb mixed oxide of example 1.

(2) FIG. 2: XRD of the MoVTeNb mixed oxide of example 2.

(3) FIG. 3: XRD of the MoVTeNb mixed oxide of comparative example 1.

(4) FIG. 4: Pore distribution of the MoVTeNb mixed oxide of example 1.

(5) FIG. 5: Pore distribution of the MoVTeNb mixed oxide of example 2.

(6) FIG. 6: Pore distribution of the MoVTeNb mixed oxide of comparative example 1.

(7) FIG. 7: comparison of the catalytic activity of the MoVTeNb mixed oxides of example 2 and comparative example 1 in the ODH of ethane.

(8) The MoVTeNb mixed oxide produced by the novel process according to the invention is clearly more active. It achieves, normalized to 1 g of catalyst, a higher activity than the catalyst according to the prior art as per the comparative example (FIG. 7). This demonstrates that a mixed oxide material having new properties is obtained by the process of the invention. However, the new properties of the novel mixed oxide material cannot readily be measured using conventional characterization methods.

(9) Methods of Characterization:

(10) To determine the parameters of the catalysts according to the invention, the following methods are used:

(11) 1. BET Surface Area

(12) The determination is carried out by the BET method of DIN 66131; a publication of the BET method may also be found in J. Am. Chem. Soc. 60, 309 (1938). The measurements were carried out at 77 K on a Sorptomatic 1990 instrument. The sample was evacuated for 2 hours at 523 K before the measurement. The linear regression of the isotherms according to the BET method was carried out in a pressure range of p/p.sub.0=0.01−0.3 (P.sub.0=730 torr).

(13) 2. X-Ray Powder Diffraction (XRD)

(14) The X-ray diffraction patterns were recorded on a PANalytical Empyrean, equipped with a Medipix PIXcel 3D detector, in θ-θ geometry in an angle range of 20=5−70°. The X-ray tube produced Cu—K radiation. The Cu-Kβ radiation was suppressed by use of an Ni filter in the beam path of the incident X-ray beam, so that only Cu-Kα radiation having a wavelength of 15.4 nm (E=8.04778 keV) was diffracted by the sample.

(15) The height of the source-side beam path was adapted by means of an automatic divergence slit (programmable divergence slit—PDS) in such a way that the sample was irradiated over a length of 12 mm over the entire angle range. The width of the detector-side X-ray beam was restricted to 10 mm by means of a fixed orifice plate. Horizontal divergence was minimized by use of a 0.4 rad Soller slit.

(16) The height of the detector-side beam path was adapted in a manner analogous to the source-side beam path by means of an automatic anti-scatter slit (programmable anti-scatter slit—PASS) in such a way that the X-ray beam reflected by the sample over a length of 12 mm was detected over the entire angle range.

(17) The samples, depending on the amount available, were prepared either on an amorphous silicon sample plate or tableted as flat-bed samples.

(18) 3. Pore Distribution

(19) The pore size distribution was measured by means of nitrogen sorption measurements at 77 K on a Sorptomatic instrument or a TriStar 3000 instrument. Before the measurement, the sample was evacuated for 2 h at 523 K. Both adsorption and desorption isotherms were determined and employed for the evaluation by the Barrett-Joyner-Halenda method (BJH).

(20) The invention will now be illustrated with the aid of the following working examples, which are not to be construed as a restriction.

WORKING EXAMPLES

Example 1

(21) TeO.sub.2 (Alfa Aesar) was slurried in 200 g of distilled H.sub.2O and milled in a planetary ball mill using 1 cm balls (ZrO.sub.2). The portion was subsequently transferred with the aid of 500 ml of distilled H.sub.2O into a glass beaker. The Nb.sub.2O.sub.5 was slurried in 200 g of distilled H.sub.2O and milled in the same ball mill. The portion was subsequently transferred with the aid of 500 ml of distilled H.sub.2O into a glass beaker. On the next morning the Nb.sub.2O.sub.5 suspension was heated to 80° C. and 107.8 g of oxalic acid dihydrate were added and the mixture was stirred for about 1 hour. 6 l of distilled H.sub.2O were placed in an autoclave (40 l) and heated to 80° C. while stirring (speed of the stirrer: 90 rpm). When the water had reached this temperature, 61.58 g of citric acid, 19.9 g of ethylene glycol, 615.5 g of MoO.sub.3 (Sigma Aldrich), 124.5 g of V.sub.2O.sub.5, the milled TeO.sub.2 and the milled Nb.sub.2O.sub.5 in oxalic acid were added in succession. 850 ml of distilled H.sub.2O were used for transferring and rinsing the vessels. The total amount of water in the autoclave is 8.25 l. The contents of the autoclave were subsequently blanketed with nitrogen. A hydrothermal synthesis at 190° C./48 hours was carried out in the 40 l autoclave. After the synthesis, the mixture was filtered on a blue band filter with the aid of a vacuum pump and the filter cake was washed with 5 l of distilled H.sub.2O.

(22) Drying was carried out at 80° C. for 3 days in a drying oven and the product was subsequently milled in an impact mill. A solids yield of 0.8 kg was achieved.

(23) The subsequent calcination was carried out at 280° C. for 4 hours in air (heating rate 5° C./min, air: 1 l/min).

(24) Activation was carried out in a retort at 600° C. for 2 hours (heating rate 5° C./min, N.sub.2: 0.5 l/min).

(25) The product had a BET surface area of 9 m.sup.2/g and a pore volume=0.04 cm.sup.3/g.

Example 2

(26) 75 ml of twice-distilled water were placed in a 100 ml PTFE beaker, 177.8 mg of (mono)ethylene glycol were added dropwise and 5397.9 mg of MoO.sub.3, 1023.9 mg of V.sub.2O.sub.5, 599.1 mg of TeO.sub.2, 549.5 mg of Nb.sub.2O.sub.5.xH.sub.2O (Nb=63.45% by weight), 540.9 mg of citric acid and 338.3 mg of oxalic acid were subsequently slurried in. The Teflon beaker was closed and transferred into a stainless steel autoclave bomb. This was closed in a pressure-tight manner and clamped onto a horizontally rotating shaft in an oven which had been preheated to 190° C. After 48 hours, the autoclave bomb was taken from the oven and immediately quenched under running water and subsequently cooled in an ice bath for 45 minutes.

(27) The product suspension formed was filtered through filter paper (pore width 3 μm) and the solid was washed with 200 ml of twice-distilled water.

(28) The product obtained in this way was dried at 80° C. for 16 hours in a drying oven and then ground in a hand mortar.

(29) The solids yield was 6.2 g. The activation was carried out at 600° C. for 2 hours (heating rate 10° C./min, N.sub.2: 100 ml/min). The XRD diffraction pattern of the product is shown in FIG. 2, the BET surface area was 7.3 m.sup.2/g and the pore volume was less than 0.012 cm.sup.3/g.

Comparative Example 1

(30) 3.3 l of distilled H.sub.2O are placed in an autoclave (40 l) and heated to 80° C. while stirring. Meanwhile, 725.58 g of ammonium heptamolybdate tetrahydrate (from HC Starck) were introduced and dissolved (AHM solution). In each of three 5 l glass beakers, 1.65 l of distilled H.sub.2O were likewise heated to 80° C. while stirring on a magnetic stirrer with temperature regulation. 405.10 g of vanadyl sulfate hydrate (GfE, V content: 21.2%), 185.59 g of ammonium niobium oxalate (HC Starck, Nb content: 20.6%) and 94.14 g of telluric acid, respectively, were then introduced into these glass beakers and dissolved (V solution, Nb solution and Te solution).

(31) The V solution, the Te solution and finally the Nb solution were then pumped by means of a peristaltic pump into the AHM solution; pumping time: V solution: 4.5 min at 190 rpm (tube diameter: 8×5 mm), Nb solution: 6 min at 130 rpm (tube diameter: 8×5 mm).

(32) The suspension formed was stirred further at 80° C. for 10 minutes. The speed of the stirrer during the precipitation was 90 rpm.

(33) The suspension was subsequently blanketed with nitrogen by building up a pressure up to about 6 bar in the autoclave by means of nitrogen and opening the discharge valve to such an extent that flow under a pressure of N.sub.2 occurred through the autoclave (5 minutes). At the end, the pressure was released again to a residual pressure of 1 bar via the venting valve.

(34) The hydrothermal synthesis was carried out at 175° C. for 20 hours (heating time: 3 hours) in the 40 l autoclave using an anchor stirrer at a stirrer speed of 90 rpm.

(35) After the synthesis, the suspension was filtered on a blue band filter by means of a vacuum pump and the filter cake was washed with 5 l of distilled H.sub.2O.

(36) Drying was carried out at 80° C. for 3 days in a drying oven and the solid was subsequently milled in an impact mill; the solids yield was 0.8 kg.

(37) Calcination was carried out at 280° C. for 4 hours (heating rate of 5° C./min, air: 1 l/min). Activation was carried out at 600° C. for 2 hours in the retort (heating rate 5° C./min, N.sub.2: 0.5 l/min).

(38) The BET surface area of the product was 9 m.sup.2/g, and the pore volume=0.055 cm.sup.3/g.

Example 3

(39) The catalytic activity of the catalysts of example 2 and the comparative example in the oxidative dehydrogenation of ethane was examined in a tubular reactor under atmospheric pressure in the temperature range from 330 to 420° C. For this purpose, 25 mg (example 2) or 200 mg (comparative example 1) of catalyst (particle size from 150 to 212 μm) were diluted with silicon carbide (particle size from 150 to 212 μm) in a mass ratio of 1:5. A layer of 250 mg of silicon carbide of the same particle size was introduced both below and above the catalyst bed and the ends of the tube reactor were closed by means of quartz wool plugs.

(40) The reactor was flushed with inert gas before commencement of the experiment and subsequently heated to 330° C. under a helium flow of 50 sccm. After the desired temperature had been reached and was stable for one hour, the gas fed in was switched over to the reaction gas mixture.

(41) The inlet gas composition was C.sub.2H.sub.6/O.sub.2/He=9.1/9.1/81.8 (v/v) at a total volume flow of 50 sccm.

(42) Analysis of the product gas stream was carried out in a gas chromatograph equipped with Haysep N and Haysep Q columns, a 5 A molecular sieve column and a thermal conductivity detector.

(43) The ethylene formation rates under the above-described conditions are shown in FIG. 7.