Molybdenum-vanadium bimetallic oxide catalyst and its application in chemical looping oxidative dehydrogenation of alkane

Abstract

A molybdenum-vanadium bimetal oxide catalyst having a molecular formula of Mo.sub.1V.sub.y, where y represents an atomic molar ratio of vanadium and molybdenum. An oxygen support Mo.sub.1V.sub.y is prepared by an impregnation method including impregnation, drying, calcination, and tablet pressing. In the dehydrogenation reaction of a light alkane to an alkene over the supported molybdenum-vanadium bimetal oxide, the reaction temperature is 450° C.-550° C. Propane can be oxidized and dehydrogenated to produce propylene with a high activity and high selectivity. A conversion rate of propane remains at 30%-40%, and a selectivity for propylene is 80%-90%. A fresh oxygen support changes from a high-valence state to a low-valence state after reacting with propane. A low-valence state oxygen support reacts with air or oxygen to be oxidized to a high-valence state, and recovers lattice oxygen and cycles again.

Claims

1. A method of chemical looping oxidative dehydrogenation of a light alkane over a molybdenum-vanadium bimetal oxide catalyst, comprising: uniformly mixing a molybdenum-vanadium bimetal oxide catalyst with quartz sand in a reactor, wherein, the molybdenum-vanadium bimetal oxide catalyst comprises: a solid solution composed of an oxide of molybdenum (Mo) and an oxide of vanadium (V), a molar ratio of Mo and V in the molybdenum-vanadium bimetal oxide catalyst is 1-(4-30), and Mo enters bulk phase lattice of V.sub.2O.sub.5, resulting in a lattice distortion of V.sub.2O.sub.5 and forming the solid solution, and a mass ratio of the molybdenum-vanadium bimetal oxide catalyst to the quartz sand is (0.2-1):1; introducing nitrogen to the reactor to remove oxygen and air, and then introducing a light alkane to the reactor, wherein, the light alkane is an alkane selected from the group consisting of ethane, propane, n-butane and iso-butane, a total flow of the light alkane and nitrogen is 20 ml/min-50 ml/min, and a volume percent of the light alkane to a total volume of the light alkane and the nitrogen is 10%-30%; and contacting the light alkane with the molybdenum-vanadium bimetal oxide catalyst at atmospheric pressure and a temperature from 450° C.-500° C. to produce a product comprising an alkene.

2. The method of chemical looping oxidative dehydrogenation of the light alkane according to claim 1, wherein, an oxidative dehydrogenation reaction is performed under an anaerobic condition, the molybdenum-vanadium bimetal oxide catalyst is used as an oxygen support, and reacts with the light alkane via the oxidative dehydrogenation reaction, lattice oxygen in the oxygen support reacts with a hydrogen atom in the light alkane to generate water, the oxygen support is reduced to a low-valence state, and the light alkane is oxidized to an alkene.

3. The method of chemical looping oxidative dehydrogenation of the light alkane according to claim 2, wherein, lattice oxygen in oxygen support participates in the oxidative dehydrogenation reaction, and as the oxidative dehydrogenation reaction proceeds, the lattice oxygen is gradually consumed and results in a decrease in an activity of the molybdenum-vanadium bimetal oxide catalyst and a low-valence state oxygen support; the low-valence state oxygen support is cycled and regenerated by reacting with air or oxygen to oxidize the low-valence state oxygen support to a high-valence state oxygen support and returning the high-valence state oxygen support to the reactor.

4. The method of chemical looping oxidative dehydrogenation of the light alkane according to claim 1, wherein, the contacting comprises a gas-solid contacting method selected from a gas-solid countercurrent contacting method and a gas-solid concurrent contacting method, and the reactor employed in the oxidative dehydrogenation reaction is one selected from the group consisting of a fixed bed reactor, a moving bed reactor, and a circulating fluidized bed reactor.

5. The method of chemical looping oxidative dehydrogenation of the light alkane according to claim 1, wherein, the mass ratio of the molybdenum-vanadium bimetal oxide catalyst to the quartz sand is (0.5-0.8):1.

6. The method of chemical looping oxidative dehydrogenation of the light alkane according to claim 1, wherein, the molar ratio of Mo and V in the molybdenum-vanadium bimetal oxide catalyst is 1:(6-18).

7. The method of chemical looping oxidative dehydrogenation of the light alkane according to claim 1, wherein, the molybdenum-vanadium bimetal oxide catalyst is a supported catalyst, a support is Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2 or a molecular sieve, a mass percent of the oxide of molybdenum over a mass of the support is 1%-30%, and a mass percent of the oxide of vanadium over the mass of the support is 4%-60%.

8. The method of chemical looping oxidative dehydrogenation of the light alkane according to claim 7, wherein, the mass percent of the oxide of molybdenum over the mass of the support is 10%-20% and the mass percent of the oxide of vanadium over the mass of the support is 40%-60%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram showing a device and a chemical looping oxidative dehydrogenation process of propane of the present invention.

(2) FIG. 2 is a schematic diagram showing a result of a catalyst activity test with different addition amounts of Mo in a chemical looping oxidative dehydrogenation process of propane.

(3) FIG. 3 is a schematic diagram showing a result of a catalyst activity test with different temperatures in a chemical looping oxidative dehydrogenation process of propane.

(4) FIG. 4 is a schematic diagram showing a result of a catalyst activity test at different weight hourly space velocities (WHSV) in a chemical looping oxidative dehydrogenation process of propane.

(5) FIG. 5 shows schematic diagrams of results of catalyst activity tests by respectively using VO.sub.x catalyst and Mo.sub.1V.sub.6 catalyst with different reaction times in the chemical looping oxidative dehydrogenation process of propane.

(6) FIG. 6 is a spectrum diagram showing a result of a H.sub.2 temperature-programmed reduction (H.sub.2-TPR) test of a fresh oxygen support (catalyst) prepared by the present invention.

(7) FIG. 7 is a spectrum diagram showing a result of a X-ray diffraction (XRD) test of a fresh oxygen support (catalyst) of the present invention.

(8) FIG. 8 shows schematic diagrams of results of cycling stability tests of a reaction regeneration cycle by using the catalyst Mo.sub.1V.sub.6 in a chemical looping oxidative dehydrogenation process of propane.

(9) FIG. 9 shows schematic diagrams of results of cycling stability tests of a reaction regeneration cycle of the oxygen support by using the catalyst Mo.sub.1V.sub.6 before and after a chemical looping oxidative dehydrogenation process of propane.

(10) FIG. 10 shows schematic diagrams of results of lattice oxygen consumption of VO.sub.x and MoV.sub.6 at different reaction times in the chemical looping oxidative dehydrogenation process of propane.

(11) FIG. 11 shows schematic diagrams of phase changes of VO.sub.x and Mo.sub.1V.sub.6 at different reaction times in the chemical looping oxidative dehydrogenation process of propane.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(12) The technical solution of the present disclosure is further described hereinafter with reference to the embodiments.

(13) Firstly, a Mo—V bimetal oxide catalyst is prepared. Each part by mass is 1 g. Meanwhile, the single metal oxide catalysts of V and Mo are prepared, which are used for the comparison and verification. The three metal oxide catalysts are prepared by the same preparation process parameters.

Embodiment 1

(14) Step 1, 1.8 parts by mass of ammonium metavanadate (NH.sub.4VO.sub.3) and 2.9 parts by mass of oxalic acid (C.sub.2H.sub.2O.sub.4) are weighed and dissolved in 3 mL of deionized water. After the reaction is completed, a certain mass of ammonium molybdate ((NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O) is added according to the atomic ratio of molybdenum and vanadium, and 2.0 parts by mass of Al.sub.2O.sub.3 are added in the above-mentioned solution.

(15) Step 2, the product obtained in step 1 is dried at a room temperature of 25° C. for 12 h, and then is dried at 70° C. for 12 h, and finally is calcined at 600° C. for 4 h in air atmosphere to obtain the molybdenum-vanadium bimetal composite oxide supported on alumina is obtained, wherein the molecular formula thereof is Mo.sub.1V.sub.y, and y represents an amount of V corresponding to 1 mol of Mo, y=4, 6, 9, 12, 18, 30, i.e. the molar ratio of V to Mo.

(16) Step 3: the Mo.sub.1V.sub.y solid powder is pressed into a granular catalyst with a size of 20-40 mesh.

Embodiment 2

(17) The reaction is performed by using the same method as in embodiment 1. The difference thereof is that the mass of ammonium molybdate in step 1 is 0, and the VO.sub.x catalyst is obtained.

Embodiment 3

(18) Step 1, 1.472 parts by mass of ammonium molybdate ((NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O) are weighed and dissolved in 3 mL of deionized water. 2.0 parts by mass of Al.sub.2O.sub.3 are impregnated in the above-mentioned solution, and are dried at a room temperature for 12 h and dried at 80° C. for 2 h.

(19) Step 2, the product obtained in step 1 is dried at a room temperature of 25° C., and is dried at 70° C. for 12 h, and is finally calcined at 600° C. for 4 h in air atmosphere to obtain molybdenum oxide supported on alumina, wherein the molecular formula of molybdenum oxide is MoO.sub.x.

(20) Step 3, the MoO.sub.x solid powder is pressed into a granular catalyst with a size of 20-40 mesh.

Embodiment 4

(21) 0.25 g-0.8 g of the VO.sub.x, Mo.sub.1V.sub.y, and MoO.sub.x oxygen supports (i.e., three kinds of oxide catalysts) obtained in embodiments 1-3 are respectively weighed and mixed with 2 mL of quartz sand (SiC), and are then added into a fixed bed tubular reactor. The experiment is performed at 450° C.-500° C. under a normal pressure condition. N.sub.2 is introduced to remove oxygen and air, then propane is introduced, wherein the total flow of propane and nitrogen is 21 ml/min, and the volume percent of propane is 20%. The product compositions are detected by gas chromatography.

(22) The conversion rate of propane is calculated according to the following formula:
X.sub.C.sub.3.sub.H.sub.6=F.sub.C.sub.3.sub.H.sub.6.sup.in−F.sub.C.sub.3.sub.H.sub.6.sup.out/F.sub.C.sub.3.sub.H.sub.6.sup.in,

(23) wherein: X.sub.C.sub.3.sub.H.sub.6—conversion rate of propane, %; F.sub.C.sub.3.sub.H.sub.6.sup.in—molar flow of propane in reactor inlet, mol/min; and F.sub.C.sub.3.sub.H.sub.6.sup.out—molar flow of propane in reactor outlet, mol/min.

(24) The selectivity of the gas phase product is calculated according to the following formula:
S.sub.product A=n.sub.product A/Σn.sub.product=x.sub.product A

(25) wherein: S.sub.product A—selectivity of gas phase product A, %; n.sub.product A—yield of gas phase product A, mol; Σn.sub.product—sum of amounts of all product materials of gas phase, mol; and x.sub.product A—content of gas phase product A in all gas phase products.

(26) The gas phase product A includes: C.sub.3H.sub.6, CO.sub.x (carbon oxide, i.e. carbon monoxide, carbon dioxide), CH.sub.4, C.sub.2H.sub.6, and C.sub.2H.sub.4.

(27) The catalyst activity of the above embodiments is determined at a reaction time of 5 min. As shown in FIG. 2, the histogram represents the conversion rate or selectivity of the product, and the stars correspond to the propylene yields. With the increase of the molybdenum content, the conversion rate of propane is increased, and the selectivity of propylene is enhanced, which are both maintained above 80%. The highest selectivity for propylene of Mo.sub.1V.sub.6 is 89%. The pure vanadium oxide VO.sub.x has a relatively high surface oxygen activity, resulting in the complete oxidation of propane into CO.sub.x. The pure molybdenum oxide MoO.sub.x has relatively low surface oxygen activity, resulting in a low conversion rate of propane, and propane or propylene is completely oxidized into CO.sub.x (compared with VO.sub.x, Mo.sub.1V.sub.y, and the combination of VO.sub.x and MoO.sub.x). The molybdenum-vanadium bimetal oxide can effectively enhance the selectivity for propylene while inhibiting the surface oxygen activity. However, an excessive addition of molybdenum leads to a decrease of the conversion rate of propane and the selectivity for propylene. Therefore, the optimum addition amount of Mo of the molybdenum-vanadium bimetal oxide is Mo.sub.1V.sub.6. (The products represented by the histogram in the drawings correspond to FIG. 2).

(28) Referring to the molybdenum-vanadium bimetal oxide Mo.sub.1V.sub.6 with an optimum addition amount of Mo, the results of the performance test of Mo.sub.1V.sub.6 with different reaction temperatures in FIG. 3 show that, as the reaction temperature increases, the conversion rate of propane increases, while the selectivity of CO.sub.x is gradually decreased. However, as the reaction temperature is further increased, the C—C bond of propane is more inclined to be broken to produce methane, resulting in a decrease in the selectivity for propylene. According to the schematic diagram showing the results of the catalyst activity tests at different weight hourly space velocities (WHSV) in the chemical looping oxidative dehydrogenation process of propane by using VO.sub.x and Mo.sub.1V.sub.6 in FIG. 4, where I, II and III correspond to the oxygen support of 0.25 g, 0.5 g and 0.8 g, respectively (the WHSV is a ratio of the gas flow to the mass of the catalyst, and the WHSV is adjusted by changing the mass of the catalyst with a constant gas flow), as the WHSV of the reaction decreases, the conversion rate of propane is increased, while the selectivity of propylene is reduced. The reason is that the decrease of residence time cause propane or propylene to be completely oxidized by an oxide with a strong surface activity to generate CO.sub.x.

(29) FIG. 5 shows schematic diagrams of the results of catalyst activity tests by respectively using VO.sub.x catalyst and Mo.sub.1V.sub.6 catalyst at different reaction times in the chemical looping oxidative dehydrogenation process of propane. The histogram corresponds to the propylene yield, and the curves 1-4 correspond to the curve using the metal V oxide catalyst. Curve 1 presents the selectivity of propylene. Curve 2 presents the conversion rate of propane. Curve 3 presents the selectivity for carbon oxide. Curve 4 presents the selectivity for methane, ethane, and ethylene, and others. Curves 5-8 correspond to the curve using the molybdenum-vanadium bimetal catalyst. Curve 5 presents the selectivity for propylene. Curve 6 presents the conversion rate of propane. Curve 7 presents the selectivity for carbon oxide. Curve 8 presents the selectivity for methane, ethane, and ethylene, and others. As the reaction time increases, the lattice oxygen in the oxygen support is gradually consumed. According to the results of the activity tests at different reaction times in the chemical looping oxidative dehydrogenation process of propane by respectively using VO.sub.x catalyst and Mo.sub.1V.sub.6 catalyst in FIG. 5, in the initial stage 0-3 min of the reaction, the activity of lattice oxygen and the conversion rate of propane are the highest, while the higher oxygen activity leads to complete oxidation to generate CO.sub.x. As the surface lattice oxygen is gradually consumed, the highest C.sub.3H.sub.6 yield is obtained at 3-5 min, which indicates that the bulk phase lattice oxygen is the main active oxygen species that activates propane to produce propylene. At the late stage of the reaction of 10-15 min, the lattice oxygen is exhausted. At this time, the main reaction is the non-oxidative dehydrogenation reaction of propane in V.sub.2O.sub.3.

(30) The fresh oxygen support (catalyst) prepared in the present invention is performed on a H.sub.2-TPR test, and the results are shown in FIG. 6. There are mainly two types of oxygen species in the oxygen support, i.e. OI and OIL The lattice oxygen OI releasing at a lower temperature has a higher activity, which belongs to the main oxygen species that completely oxidizes propane or propylene. The lattice oxygen OII releasing at a higher temperature has a moderate activity, and can selectively oxidatively dehydrogenate propane to produce propylene. In addition, with the increase of the Mo content, the reduction peak of the OI species gradually weakens, while the reduction peak of the OII species gradually increases, indicating that the addition of Mo effectively regulates the activity of the lattice oxygen species in the oxygen support, and inhibits the OI species having a stronger activity. The XRD experiment is performed on an X-ray diffractometer of model Rigaku C/mx-2500. As shown in FIG. 7, the fresh prepared catalyst mainly contains V.sub.2O.sub.5. With the addition of Mo content, the characteristic peak of V.sub.2O.sub.5 occurs a shift at a certain angle, which indicates that Mo enters the bulk phase lattice of V.sub.2O.sub.5, resulting in a lattice distortion of V.sub.2O.sub.5, and the lattice constant is changed, which provides an evidence for the formation of molybdenum-vanadium solid solution.

(31) After the reaction is completed, the lattice oxygen is gradually consumed, resulting in a decrease of the catalyst activity. The catalyst is regenerated (i.e., oxidized to a high-valence state) by using oxygen or air, and recovers lattice oxygen and returns back to the reactor for reaction. The result of the cycling stability test of the chemical looping oxidative dehydrogenation process of propane in FIG. 8 (each product corresponds to the mark in the drawing) shows that the performance basically remains unchanged during the redox cycle process, indicating that the molybdenum-vanadium bimetal oxide has a good oxidative regeneration performance. After 50 cycles (cycles of “reaction-regeneration-reaction-regeneration”), the results of XRD and H.sub.2-TPR of the oxygen support and the fresh oxygen support are shown in FIG. 9, where the left side is XRD, and curves 1 and 2 represent XRD line spectra of the fresh support and the cycled oxygen support, respectively; and the right side is H.sub.2-TPR, and the curves 3 and 4 represent the H.sub.2-TPR line spectra of the fresh support and the cycled oxygen support, respectively. In contrast, it shows that the phase structure of the oxygen support is not changed substantially, which well confirms the excellent stability performance of the molybdenum-vanadium solid solution.

(32) The schematic diagrams in FIG. 10 and FIG. 11 showing results of lattice oxygen consumption tests and phase changes of VO.sub.x catalyst and Mo.sub.1V.sub.6 catalyst at different reaction times in the chemical looping oxidative dehydrogenation process of propane, where the left side presents the catalyst VO.sub.x, and the right side presents the catalyst Mo.sub.1V.sub.6. With the increase of the reaction time, the lattice oxygen in the oxygen support is gradually consumed, and is remarkably consumed at the initial stage, in that propane and propylene are completely oxidized into CO.sub.x, and the lattice oxygen is greatly consumed. However, with the consumption of lattice oxygen, the oxygen activity is constantly weakened. The lattice oxygen selectively oxidatively dehydrogenate propane to generate propylene. The lattice oxygen is consumed slowly, and is exhausted at the late stage, and then the reaction enters a non-oxidative dehydrogenation stage. According to the lattice oxygen consumption comparison of VO.sub.x and Mo.sub.1V.sub.6 in the left diagram, the addition of Mo effectively reduces a large amount of lattice oxygen consumed by the side reaction that generates CO.sub.x and increases the lattice oxygen consumed by the oxidative dehydrogenation that selectively produces propylene. With the increase of the reaction time, VO.sub.x and Mo.sub.1V.sub.6 undergo a change in the phase structure, from V.sub.2O.sub.5 to V.sub.2O.sub.4 to V.sub.2O.sub.3, which mainly correspond to the complete oxidation stage, the selective oxidative dehydrogenation stage and the non-oxidative dehydrogenation stage, respectively, in view of the reaction time.

(33) The catalyst may be prepared according to an adjustment of the preparation parameters of the contents of the present disclosure, realizing an effective catalysis of propane. The illustrative description of the present disclosure is provided above. It should be noted that any simple variations, modifications or other equivalent replacements made by those skilled in the art without creative efforts should fall within the protection scope of the present disclosure.