Synthesis of a MoVNbTe catalyst having a reduced niobium and tellurium content and higher activity for the oxidative dehydrogenation of ethane

Abstract

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

Claims

1. A mixed oxide material comprising the elements molybdenum, vanadium, niobium and tellurium which in the XRD using Cu-Ka 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° (I), said mixed oxide material having the following stoichiometry:
Mo.sub.1Va.sub.aNb.sub.bTe.sub.cO.sub.n   (I) a=0.2 to 0.35, b=greater than 0 to 0.08, c=greater than 0 to 0.08, n=an integer determined by the valence and abundance of the elements other than oxygen in (I).

2. The mixed oxide material as claimed in claim 1, wherein said mixed oxide material has a BET surface area which is greater than 15 m.sup.2/g.

3. A process for producing a mixed oxide material as claimed in claim 1, comprising the steps: a) production of a mixture of starting compounds containing molybdenum, vanadium, niobium and a tellurium-containing starting compound in which tellurium is present in the oxidation state +4, oxalic acid and at least one further 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 mixed oxide material present in the suspension resulting from step b).

4. The process as claimed in claim 3, wherein the tellurium-containing starting compound is tellurium dioxide or a compound of the formula Mx.sup.n+TeO.sub.3 where n=1 or 2 and x=2/n, where M is an alkali metal or alkaline earth metal.

5. The process as claimed in claim 3, wherein the mixture of starting compounds is present as aqueous suspension.

6. The process as claimed in claim 3, wherein the mixture of starting compounds contains a dicarboxylic acid, a dial or another compound having two hydroxy groups in adjacent positions as further oxo ligand.

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

8. The process as claimed in claim 3, wherein the mixture of starting compounds contains vanadium pentoxide.

9. The process as claimed in claim 3, wherein the mixture of starting compounds contains citric acid as further oxo ligand.

10. The process as claimed in claim 3, wherein the mixture of starting compounds contains citric acid and glycol as further oxo ligands.

11. The process as claimed in claim 3, wherein the drying in step c) is carried out at from 50° C. to 400° C.

12. The process as claimed in claim 3, wherein the drying in step c) is carried out in two steps, firstly at from 50° C. to 150° C. and then at from 350° C. to 400° C.

13. The process as claimed in claim 3, wherein drying is followed by activation at from 500° C. to 650° C. under inert gas.

14. The process as claimed in claim 3, wherein the mixed oxide material present in the suspension resulting from step b) has, in the XRD using Cu-Ka radiation, 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. A process for the oxidative dehydrogenation of ethane to ethene, the method comprising contacting ethane with the mixed oxide material as claimed in claim 1.

16. A process for the oxidation of propane to acrylic acid, the method comprising oxidizing the propane while in contact with the mixed oxide material as claimed in claim 1.

17. A process for the ammoxidation of propane by means of ammonia to acrylonitrile, the method comprising contacting the propane and the ammonia with the mixed oxide material as claimed in claim 1.

Description

(1) The MoVNbTe mixed oxide of the invention is used as catalyst material in the examples and will therefore sometimes be referred to as catalyst in the experimental part.

(2) FIG. 1: X-ray diffraction pattern of the catalyst of example 1.

(3) FIG. 2: X-ray diffraction pattern of the catalyst of example 2.

(4) FIG. 3: X-ray diffraction pattern of the catalyst of comparative example 1.

(5) FIG. 4: X-ray diffraction pattern of the catalyst of comparative example 2.

(6) FIG. 5: X-ray diffraction pattern of the catalyst of example 4.

(7) FIG. 6: STEM image of the catalyst of example 1, in which the crystal structure of the M1 phase can be seen.

(8) FIG. 7: SEM image of the catalyst of example 1, in which the acicular crystal shape of the M1 phase can be seen.

(9) FIG. 8: N.sub.2 pore distribution of the catalyst of example 1.

(10) FIG. 9: N.sub.2 pore distribution of the catalyst of example 2.

(11) FIG. 10: N.sub.2 pore distribution of the catalyst of example 3.

(12) FIG. 11: comparison of the catalytic activity of the catalysts of examples 1 and 2 in the oxidative dehydrogenation of ethane.

(13) FIG. 12: ethane ODH activity of examples 4 and 5.

(14) FIG. 13: X-ray diffraction pattern of the catalyst of example 5.

(15) It can clearly be seen that the XRD of the catalyst according to the invention in FIG. 2 has the typical reflections of the M1 phase at (2θ=) 26.2°±0.5° (h), 27.0°±0.5° (i), 7.8°±0.5° (k) and 28.0°±0.5° (l) (when using Cu—Kα radiation), even though an Mo/Nb ratio of only 1:0.05 and an Mo/Te ratio of 0.05 are present. The reflections are somewhat broader than in the comparative examples in which a high-temperature treatment has taken place (FIG. 3). FIG. 4 shows that in comparative examples 1 and 2 without the high-temperature treatment, only the reflection at 22.5°, which indicates the plane spacing, can be clearly identified. Only after the high-temperature treatment (FIG. 3) does this catalyst display the typical reflections of the M1 phase.

(16) FIG. 11 shows that the catalyst according to the invention of example 1 displays a higher activity in the oxidative dehydrogenation of ethane than those of the comparative examples.

(17) It can clearly be seen that the uncalcined catalyst according to the invention of example 1 is significantly more active with only half as much niobium and tellurium. The calcined catalyst according to the invention having only half as much niobium and tellurium of example 2 is just as active as the prior art catalyst which has likewise been treated at high temperature in comparative example 1. However, it is significantly cheaper since less of the expensive metals niobium and tellurium are required.

(18) Methods of Characterization:

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

(20) 1. BET Surface Area:

(21) 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).

(22) 2. Chemical Analysis (ICP) with Digestion Method

(23) Apparatus Used:

(24) Multiwave GO microwave

(25) Reaction vessel made of PTFE

(26) Plastic tube 50 ml

(27) ICP Spectro Arcos

(28) Chemicals Used:

(29) HF 40% AR

(30) HCl 37% AR

(31) Sulfuric acid 98% AR

(32) Sulfuric acid 1:1

(33) The sample was in each case finely milled before the analysis.

(34) 50 mg of sample were weighed into a reaction vessel and admixed with 2 ml of twice-distilled water, 2 ml of hydrofluoric acid, 2 ml of hydrochloric acid and the vessel was closed. The sample was subsequently subjected to the following microwave program:

(35) step 1 10 min. to 100° C., 1 min. hold,

(36) step 2 5 min. to 180° C., 20 min. hold.

(37) 0.1 ml of scandium standard are placed in a plastic tube and the digestion solution is then transferred and subsequently heated, made up to the mark and shaken.

(38) All elements were detected on the Arcos ICP; the following basic settings were used:

(39) plasma power: 1400 watt

(40) cooling gas flow: 14 l/min

(41) auxiliary gas flow: 1.4 l/min

(42) atomizing gas flow: 0.8 l/min

(43) The standards are all adapted with acid and the concentration by mass of scandium is 2 mg/l.

(44) Standards:

(45) Mo 300/400/500 mg/l

(46) Nb 100/50/20 mg/l

(47) Te 150/100/50 mg/l

(48) V 100/50/20 mg/l

(49) Wavelengths:

(50) TABLE-US-00001 Mo 287.151 nm corr. Sc 424.683 nm 202.095 nm corr. Sc 424.683 nm 204.664 nm corr. Sc 424.683 nm 202.095 nm Nb 269.706 nm corr. Sc 424.683 nm 316.240 nm corr. Sc 424.683 nm 316.340 nm Te 225.902 nm corr. Sc 335.373 nm 170.000 nm corr. Sc 335.373 nm 170.000 nm V 292.402 nm corr. Sc 424.683 nm 292.402 nm 311.071 nm corr. Sc 424.683 nm

(51) $w\left(E^{*}\mathrm{in}\mathrm{percent}\right)=\frac{\begin{array}{c}\beta \left(E^{*}-\mathrm{measured}\mathrm{value}\mathrm{in}\mathrm{mg}/l\right)\times \\ V\left(\mathrm{volumetric}\mathrm{flask}\mathrm{in}l\right)\times 100\end{array}}{m\left(\mathrm{weight}\mathrm{used}\mathrm{in}\mathrm{mg}\right)}$

(52) E*=respective element

(53) 3. X-Ray Powder Diffraction (XRD)

(54) The X-ray diffraction pattern was produced by X-ray powder diffraction (XRD) and evaluation according to the Scherrer formula.

(55) The diffraction patterns were recorded on a PANalytical Empyrean, equipped with a Medipix PIXcel 3D detector, in θ-θ geometry in an angle range of 2θ=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. 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.

(56) 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.

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

(58) 4. STEM

(59) Scanning transmission electron microscopy was carried out on an FEI Titan 80/300 TEM/STEM electron microscope using an acceleration voltage of 300 keV. The spherical aberration was compensated for by means of illumination correction. All high-angle annular dark field (HAADF) images were recorded using a convergence half angle of 17.4 mrad and annular dark field detector half angles of 70-200 mrad. The crystal samples were prepared by means of the microtome technique.

(60) 5. SEM

(61) Scanning electron micrographs were recorded on a JEOL JSM-7500F using a secondary electron detector. The acceleration voltage was 2.0 kV and the emission current was 10 μA. The working spacing was about 8 mm.

WORKING EXAMPLES

Example 1

MoV.SUB.0.3.Nb.SUB.0.05.Te.SUB.0.05

(62) 75 ml of twice-distilled water were placed in a 100 ml PTFE beaker, 175.8 mg of monoethylene glycol were added dropwise and 5397.5 mg of MoO.sub.3, 1023.3 mg of V.sub.2.sub.5, 299.2 mg of TeO.sub.2, 274.4 mg of Nb.sub.2O.sub.5.Math.xH.sub.2O (Nb=63.45% by weight), 540.3 mg of citric acid and 168.5 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 horizontal 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.

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

(64) 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.

(65) A yield of solid of 6.8 g was achieved, and the elemental composition of the metals in the product normalized to molybdenum was MoV.sub.0.30Te.sub.0.05Nb.sub.0.05O.sub.x, which corresponds to a mass-based composition of 53.0% by weight of Mo, 8.4% by weight of V, 2.9% by weight of Te and 2.3% by weight of Nb.

(66) Scanning transmission electron micrographs of the product are shown in FIGS. 6 and 7.

(67) The BET surface area of the product is 66.4 m.sup.2/g, and the product has a pore volume of 0.11 cm.sup.3/g and a pore distribution shown in FIG. 8.

Example 2

(68) The catalyst described in example 1 was subjected to a heat treatment in a tube furnace. For this purpose, 1 g of the dried solid was transferred to a porcelain boat so that the bottom of the boat was covered with powder to a height of about 2 mm. Activation was carried out at 600° C. for 2 hours, at a heating rate of 10° C./min in an N.sub.2 stream of 100 ml/min. The elemental composition of the metals in the product normalized to molybdenum was: MoV.sub.0.30Te.sub.0.04Nb.sub.0.04O.sub.x.

(69) The BET surface area of the product was 25.0 m.sup.2/g, and the product had a pore volume of 0.04 cm.sup.3/g and a pore distribution shown in FIG. 9.

(70) The XRD of the product is shown in FIG. 2.

(71) Comparative Example 1: (MoV.sub.0.3Nb.sub.0.1Te.sub.0.1 from Soluble Precursors)

(72) 3.3 l of distilled H.sub.2O were placed in an autoclave (40 l) and heated to 80° C. while stirring. Meanwhile, 725.58 g of ammonium heptamolybdate tetrahydrate (from HC Starck) was introduced and dissolved (AHM solution). In each of three 5 l glass beakers, 1.65 l of distilled H.sub.2O was likewise heated to 80° C. while stirring on a magnetic stirrer with temperature regulation. 405.10 g of vanadyl sulfate hydrate (from 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).

(73) 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).

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

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

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

(77) 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 yield of solid achieved was 0.8 kg, and the product was calcined at 280° C. for 4 hours in air (heating rate 5° C./min, air: 1 l/min ).

(78) Activation was carried out in an N.sub.2 gas atmosphere in a retort in the furnace at 600° C. for 2 hours (heating rate 5° C./min, N.sub.2: 0.5 l/min ). After this treatment, the BET surface area was 13 m.sup.2/g.

(79) This gave a catalyst having the stoichiometry Mo.sub.1V.sub.0.3Nb.sub.0.10Te.sub.0.10O.sub.x, corresponding to a proportion by weight of the metals based on the total weight of the catalyst of Mo=49% by weight; V=7.9% by weight; Te=6.5% by weight; Nb=4.9% by weight.

(80) The mother liquor after the filtration still contained 0.23% by weight of vanadium and 0.1% by weight of molybdenum.

Comparative Example 2

(81) The catalyst from comparative example 1 was used immediately after the calcination at 280° C. for 4 hours. The calcination at 600° C. under nitrogen for 2 hours was not carried out.

Example 3

(82) The catalytic activity of the catalysts of example 1 and comparative examples 1 and 2 in the oxidative dehydrogenation of ethane was examined in the temperature range from 330° C. to 420° C. at atmospheric pressure in a tube reactor. For this purpose, 25 mg (example 1 and comparative example 1) or 200 mg (comparative example 2) of catalyst (particle size 150-212 μm) were in each case 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 silica wool plugs.

(83) 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 1 hour, the gas fed in was switched over to the reaction gas mixture.

(84) 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.

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

(86) The ethylene formation rates under the above-described conditions are shown in FIG. 11. In the measurement of comparative example 1, 200 mg of catalyst instead of 25 mg were used because the catalyst of the invention was so much more active that the activity could not be measured using the same mass flow regulators and the same amount of catalyst. However, the graph in FIG. 11 is normalized to the space velocity so that the values are comparable.

Example 4

MoV.SUB.0.30.Nb.SUB.0.03.Te.SUB.0.03

(87) 75 ml of twice-distilled water were placed in a 100 ml PTFE beaker, 180.3 mg of monoethylene glycol were added dropwise and 5399.9 mg of MoO.sub.3, 1024.0 mg of V.sub.2O.sub.5, 180.2 mg of TeO.sub.2, 166.8 mg of Nb.sub.2O.sub.5.Math.xH.sub.2O (Nb=63.45% by weight), 542.4 mg of citric acid and 101.3 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 on a horizontal 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.

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

(89) 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.

Example 5

MoV.SUB.0.30.Nb.SUB.0.06.Te.SUB.0.03

(90) 75 ml of twice-distilled water were placed in a 100 ml PTFE beaker, 182.3 mg of monoethylene glycol were added dropwise and 5406.7 mg of MoO.sub.3, 1023.1 mg of V.sub.2O.sub.5, 177.7 mg of TeO.sub.2, 329.9 mg of Nb.sub.2O.sub.5.Math.xH.sub.2O (Nb=63.45% by weight), 543.4 mg of citric acid and 204.5 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 on a horizontal 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.

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

(92) 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.

Example 6

(93) The catalytic activity of the catalysts of examples 4 and 5 in the oxidative dehydrogenation of ethane was examined in the temperature range 330-420° C. at atmospheric pressure in a tube reactor. For this purpose, 50 mg of catalyst (particle size 150-212 μm) were in each case diluted with silicon carbide (particle size 150-212 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 silica wool plugs.

(94) 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 1 hour, the gas fed in was switched over to the reaction gas mixture.

(95) 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.

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

(97) The ethylene formation rates under the above-described conditions are shown in FIG. 12.

(98) TABLE-US-00002 TABLE 1 Pore Calc. BET volume 600° C./N.sub.2 Composition [m.sup.2/g] [cm.sup.3/g] Example 1 No MoV.sub.0.3Nb.sub.0.05Te.sub.0.05O.sub.x 60.4 0.11 Example 2 Yes MoV.sub.0.3Nb.sub.0.05Te.sub.0.05O.sub.x 25 0.04 Example 4 No MoV.sub.0.3Nb.sub.0.03Te.sub.0.03O.sub.x 38.1 0.11 Example 5 No MoV.sub.0.3Nb.sub.0. 06Te.sub.0.03O.sub.x 69.4 0.13 Comp. Yes MoV.sub.0.3Nb.sub.0.1Te.sub.0.1O.sub.x 13 0.03 example 1 Comp. No No M1 phase example 2

(99) Table 1 shows the stoichiometries and the BET surface areas of the catalysts according to the invention together with comparative examples.