Synthesis of a MoVNbTe catalyst having an increased specific surface and higher activity for the oxidative dehydrogenation of ethane to ethylene

Abstract

A novel mixed oxide material is disclosed which comprises molybdenum, vanadium, tellurium and niobium and the use of the 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-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), wherein the mixed oxide material has a pore volume of greater than 0.1 cm.sup.3/g.

2. The mixed oxide material as claimed in claim 1, wherein it has a BET surface area of more than 30 m.sup.2/g.

3. The mixed oxide material as claimed in claim 1, wherein it has a volume of the pores smaller than 10 nm of more than 0.2 cm.sup.3/g.

4. The mixed oxide material as claimed in claim 1, wherein the molar Mo:Te ratio is ≤11 and the molar Mo:Nb ratio is ≤11.

5. The mixed oxide material as claimed in claim 1, including an M1 crystalline phase having the formula
Mo.sub.1V.sub.aNb.sub.bTe.sub.cO.sub.x wherein a is 0.2-0.3, b is 0.1-0.2, c is 0.1-0.25, and x is selected such that the overall charge of the empirical formula is zero.

6. The mixed oxide material as claimed in claim 5, wherein it has a BET surface area of more than 30 m.sup.2/g.

7. The mixed oxide material as claimed in claim 5, wherein it has a volume of the pores smaller than 10 nm of more than 0.2 cm.sup.3/g.

Description

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

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

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

(4) FIG. 4: X-ray diffraction pattern of the comparative catalyst of comparative example 2, prior to activation.

(5) FIG. 5: pore distribution of the catalyst of example 1.

(6) FIG. 6: pore distribution of the catalyst of comparative example 1.

(7) FIG. 7: pore distribution of the catalysts of comparative example 2.

(8) FIG. 8: activity of the catalysts in the ODH reaction of ethane.

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

(10) FIG. 10: activity of the catalyst of example 2 in the ODH reaction.

(11) It can be seen that the X-ray diffractogram (XRD) of the catalyst according to the invention in FIG. 1 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). These reflections are broader than in the comparative examples treated by activation (FIGS. 2 and 3). The greater width is explained in that the crystallite size is smaller, which is associated with the greater specific surface area. It can be seen in FIG. 4 that without the activation, 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.

(12) Methods of Characterization:

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

(14) 1. BET Surface Area:

(15) 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).

(16) 2. N.sub.2 Pore Distribution

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

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

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

(20) 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.

(21) 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.

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

WORKING EXAMPLES

Example 1

(23) 75 ml of twice-distilled water were placed in a 100 ml PTFE beaker, 177.8 mg of monoethylene 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 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.

(24) 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.

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

(26) A yield of solid of 6.2 g was achieved. The BET surface area of the product was 83.3 m.sup.2/g, and the product had a pore volume of 0.2 cm.sup.3/g and a pore distribution shown in FIG. 5.

Example 2

(27) The synthesis was conducted as described in example 1, except that, after drying at 80° C. for 16 h, there was a further drying step at 400° C. for 3 h. The BET surface area of the product was 59.0 m.sup.2/g; the product had a pore volume of 0.176 cm.sup.3/g and a pore distribution which is shown in FIG. 9.

(28) It can be seen from FIG. 10 that, at 420° C., the catalyst, with an ethylene formation rate of 9×10.sup.−6 mol g.sup.−1.sub.cat s.sup.−1, had about the same activity as the catalyst from example 1 that had merely been dried at 80° C. (FIG. 8). The loss of activity thus does not occur until within the temperature range above 400° C.

Comparative Example 1

(29) The catalyst described in example 1 was subjected to a heat treatment (activation) 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 is covered with powder to a height of about 2 mm.

(30) Activation was carried out at 600° C. for 2 hours (heating rate: 10° C./min; N.sub.2: 100 ml/min). After this treatment the BET surface area was 7.3 m.sup.2/g, and the product had a pore volume of 0.013 cm.sup.3/g and a pore distribution shown in FIG. 6.

Comparative Example 2

(31) 3.3 1 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 (V solution, Nb solution and Te solution), respectively, were then introduced into these glass beakers and dissolved.

(32) The V solution, then 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).

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

(34) 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 occurs through the autoclave (5 minutes). At the end, the pressure was released again to a residual pressure of 1 bar via the venting valve.

(35) 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.

(36) 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.

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

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

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

(40) The product had a BET surface area of 13 m.sup.2/g and a pore volume of 0.055 cm.sup.3/g with a pore distribution shown in FIG. 7.

Comparative Example 3

(41) 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

(42) The catalytic activity of the catalysts of example 1 and comparative examples 1 and 2 in the oxidative dehydrogenation (“ODH”) 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.

(43) 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.

(44) 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.

(45) 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.

(46) The ethylene formation rates under the above-described conditions are shown in FIG. 8.

(47) The catalyst activity was normalized to the catalyst mass; the catalyst according to the prior art made from the soluble precursor compounds (comparative example 2) shows the lowest activity. Comparative example 1 has been prepared by the novel process of this patent, but was still calcined at 600° C. The highest catalytic activity is shown by the inventive catalysts without final high-temperature treatment.

(48) TABLE-US-00001 TABLE 1 BET [m.sup.2/g] Pore volume [cm.sup.3/g] Example 1 83.3 0.2 Example 2 59.0 0.176 Comparative example 1 7.3 0.013 Comparative example 2 13 0.055 Comparative example 3 (69) almost amorphous)

(49) Table 1 compares the BET surface areas and the pore volume of the catalyst according to the invention together with comparative examples.