MOLYBDENUM-VANADIUM BIMETALLIC OXIDE CATALYST AND ITS APPLICATION IN CHEMICAL LOOPING OXIDATIVE DEHYDROGENATION OF ALKANE

Abstract

A molybdenum-vanadium bimetallic oxide catalyst and its application in the chemical looping oxidative dehydrogenation of alkane. The molecular formula of molybdenum-vanadium bimetallic oxide catalyst is MoVy and y represents the atomic molar ratio of vanadium and molybdenum. The supported MoVy catalyst is prepared by impregnation method, following the drying, calcination and tablet pressing. The reaction temperature was 450-550 C., and propane could be oxidized and dehydrogenated to propylene with high activity and selectivity, with propane conversion rate remaining at 30-40% and propylene selectivity at 80-90%. The fresh catalysts were reduced to the lower valence states with the lattice oxygen diffusion to propane. After the dehydrogenation, the reduced samples were regenerated to recover to the initial state and regain the lattice oxygen. During the redox cycles, the reaction performance remains stable, which can be used in the fixed bed reactor, moving bed reactor or circulating fluidized bed.

Claims

1. A molybdenum-vanadium bimetallic oxide catalyst comprising a molar ratio of a metal Mo and a metal V is 1:(4-30) for a solid solution composed of a plurality of molybdenum oxides and a plurality of vanadium oxides, and the metal Mo enters a lattice of V.sub.2O.sub.5, resulting in a lattice distortion of V.sub.2O.sub.5 and forming a molybdenum-vanadium solid solution.

2. The molybdenum-vanadium bimetallic oxide catalyst according to claim 1, wherein the preferred molar ratio of the metal Mo and the metal V is 1:(6-18).

3. The molybdenum-vanadium bimetallic oxide catalyst according to claim 1, wherein the catalyst is a supported catalyst, and a support is Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2 or zeolites, a mass percentage of the molybdenum oxide is 1-30%, and a mass percentage of the vanadium oxide is 4-60%.

4. The molybdenum-vanadium bimetallic oxide catalyst according to claim 3, wherein the mass percentage of the molybdenum oxide is 10-20% and the mass percentage of the vanadium oxide is 40-60%.

5. A preparation method of a molybdenum-vanadium bimetallic oxide catalyst, the method comprising: step 1: evenly dispersing an ammonium metavadate and an oxalic acid in deionized water, and then adding an ammonium molybdate according to atom ratios of a vanadium and a molybdate to form a dipping solution; step 2, impregnating a support in the dipping solution prepared in the step 1 for an equal volume impregnation; step 3, after the step 2, drying the support at 20-25 C. for 8 to 12 h, then at 60-80 C. for 10-12 h and, finally, calcinating samples at 500-600 C. for 2-4 h under air atmosphere; wherein, a molecular formula of the molybdenum-vanadium bimetallic oxide catalyst is MoVy, where y represents a ratio of metal V and Mo.

6. The preparation method of molybdenum-vanadium bimetallic oxide catalyst according to claim 5, wherein in the step 1, a mass ratio of the oxalic acid and the ammonium metavanadate is (2.8-3):(1.5-2).

7. The preparation method of molybdenum-vanadium bimetallic oxide catalyst according to claim 5, wherein in the step 2, the support is Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2 or zeolites.

8. The preparation method of molybdenum-vanadium bimetallic oxide catalyst according to claim 5, wherein in the step 3, the drying at 20-25 C. is performed for 10-12 h, and further comprises drying for 10-12 h at 80-90 C. before calcinating, at 500-600 C. for 2-4 h under air atmosphere.

9. A method of chemical looping oxidative dehydrogenation of alkane, comprising: using the molybdenum-vanadium bimetallic oxide catalyst according to claim 1.

10. The method of chemical looping oxidative dehydrogenation according to claim 9, wherein a reaction is under a plurality of anaerobic conditions and the molybdenum-vanadium bimetallic oxide catalyst serves as an oxygen carrier; wherein, the oxygen carrier reacts with a propane to produce a propylene and water to reduce the molybdenum-vanadium bimetallic oxide catalyst to a lower valence state.

11. The method of chemical looping oxidative dehydrogenation according to claim 9, wherein the alkane is an ethane, a propane, an n-butane and/or an isobutene.

12. The method of chemical looping oxidative dehydrogenation according to claim 9, wherein a gas-solid two-phase contact comprises countercurrent and concurrent contacts and a plurality of reactors comprises a fixed bed reactor, a moving bed reactor or a circulating fluidized bed reactor.

13. The method of chemical looping oxidative dehydrogenation according to claim 10, wherein a lattice oxygen of the molybdenum-vanadium bimetallic oxide catalyst is involved in the reaction; and as the reaction progresses, the lattice oxygen is consumed gradually, reducing the catalyst activity; and a regeneration by air or oxygen to regain the lattice oxygen is provided.

14. The method of chemical looping oxidative dehydrogenation according to claim 9, wherein the reaction is carried out under the atmospheric pressure at a reaction temperature of 450-550 C. using the molybdenum-vanadium bimetallic oxide catalyst and quartz sand mixture; a weight hourly space velocity (WHSV) of propane is 0.5-2 h.sup.1, and a propane volume percentage is 10-30%.

15. The molybdenum-vanadium bimetallic oxide catalyst according to claim 2, wherein the catalyst is a supported catalyst, and a support is Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2 or zeolites, a mass percentage of the molybdenum oxide is 1-30%, and a mass percentage of the vanadium oxide is 4-60%.

16. The method of chemical looping oxidative dehydrogenation according to claim 10, wherein the alkane is an ethane, a propane, an n-butane and/or an isobutene.

17. The method of chemical looping oxidative dehydrogenation according to claim 10, wherein a gas-solid two-phase contact comprises countercurrent and concurrent contacts and a plurality of reactors comprises a fixed bed reactor, a moving bed reactor or a circulating fluidized bed reactor.

18. The method of chemical looping oxidative dehydrogenation according to claim 10, wherein the reaction is carried out under the atmospheric pressure at a reaction temperature of 450-550 C. using the molybdenum-vanadium bimetallic oxide catalyst and quartz sand mixture; a weight hourly space velocity (WHSV) of propane is 0.5-2 h.sup.1, and a propane volume percentage is 10-30%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 shows a diagram of the chemical looping oxidative dehydrogenation of propane.

[0026] FIG. 2 shows the activity test results of catalysts with different Mo additions.

[0027] FIG. 3 shows the activity test results of catalysts at different temperatures.

[0028] FIG. 4 shows the activity test results of catalysts at the different weight hourly space velocities (WHSV).

[0029] FIG. 5 shows the activity test results under different reaction times over VO.sub.x and MoV.sub.6 catalyst.

[0030] FIG. 6 shows the spectrum of H.sub.2-TPR results of the fresh oxygen carrier (catalyst).

[0031] FIG. 7 shows the XRD results of the fresh oxygen carrier (catalyst).

[0032] FIG. 8 shows the cyclic stability results over MoV6.

[0033] FIG. 9 shows the structure analysis during redox cycles over MoV.sub.6.

[0034] FIG. 10 shows the results of the lattice oxygen consumption over VO.sub.x and MoV.sub.6 under different reaction time. FIG. 11 shows the phase changes of VO.sub.x and MoV.sub.6 at different reaction time.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] Firstly, the preparation of MoV bimetallic oxide catalyst was carried out, with each having a mass of 1 g. Meanwhile, single metal oxide catalyst of V and Mo was prepared, which was used for the comparison. The same preparation process parameters were selected for the preparation of three kinds of metal oxide catalysts.

EXAMPLE 1

[0036] Step 1: We dissolved 1.8 parts in mass of ammonium metabanadate (NH.sub.4VO.sub.3) and 2.9 parts in mass of oxalic acid (C.sub.2H.sub.2O.sub.4) in 3 mL of deionized water. After the reaction was complete, we added a certain mass of ammonium molybdate ((NH.sub.4).sub.6Mo.sub.7O.sub.24. 4H.sub.2O) according to the atomic ratio between vanadium and molybdenum, and then, 2.0 parts in mass of Al.sub.2O.sub.3 were added in the above solution.

[0037] Step 2: the material obtained in step 1 was dried at 25 C. for 12 hours. Then the samples were dried at 70 C. for another 12 hours and finally calcined at 600 C. for 4 hours in air atmosphere. The molybdenum-vanadium bimetallic composite oxide supported on alumina was obtained, and its molecular formula was MoVy, where y is the moles of V relative to 1 mol Mo. y is equal to 6.

[0038] Step 3: The samples were grinded into the solid powder with a size of 20-40 mesh.

[0039] Step 4: Reactivity tests were performed in a quartz fixed-bed reactor with an internal diameter of 8 mm loaded with 500 mg catalysts (20-40 mesh) mixed with 1 mL of quartz particles with 20-40 meshes at atmospheric pressure. Switching between propane and air flows was employed during tests. The bed temperature was typically 500 C. and the samples were reduced using propane (4 mL/min) diluted in nitrogen (17 mL/min) at 1.4 atm. The weight hourly space velocity (WHSV) of propane was about 1 h.sup.1. The catalysts were then re-oxidized using air (15 mL/min). Between the reduction and re-oxidation reaction period, a purging period (17 mL/min of nitrogen) was introduced to prevent the mixing between propane and air. One redox cycle was completed. The stability test was carried out over MoV.sub.6 for 100 continuous redox cycles. The time for reduction, re-oxidation and purging was set to 10 min, 15 min and 10 min. Exhaust streams were analyzed using an online gas chromatography (GC) (2060) equipped with a flame ionization detector (Chromosorb 102 column) and a thermal conductivity detector (Al.sub.2O.sub.3 Plot column). The instantaneous propane conversion, product selectivity and propylene productivity were calculated from Eq. (1) and Eq. (2) respectively:

Con(%)=100([F.sub.C3H8].sub.inlet[F.sub.C3H8].sub.outlet)/[F.sub.C3H8].sub.inlet. (1)

Sel(%)=100n.sub.i[F.sub.i].sub.outlet/(n.sub.i[F.sub.i].sub.outlet) (2)

Productivity=Con(%)Sel(%)/10000n.sub.i/m (3)

[0040] where i stands for different hydrocarbon products in exhaust gases, n.sub.i is the number of carbon atoms of component i, and F.sub.i is the corresponding molar flow rate. m is the weight of the vanadium oxides.

[0041] The accumulative conversion, selectivity and C.sub.3H.sub.6 yield were calculated from the GC data normalized to the amount of vanadium.

Yield=(Productivity dt)/N (4)

Con(%)=(Con(%) dt) (5)

Sel(%)=(Sel(%) dt) (6)

[0042] where N is the amount of vanadium in vanadium redox oxides.

EXAMPLE 2

[0043] The reaction is carried out using the same method as in example 1. The difference is only that the mass of ammonium molybdate in step 1 is 0, and VO.sub.x catalyst is obtained.

EXAMPLE 3

[0044] The reaction is carried out using the same method as in example 1. The difference is only that y in step 2 is 0, and MoO.sub.x catalyst is obtained.

EXAMPLE 4

[0045] The reaction is carried out using the same method as in example 1. The difference is only that y in step 2 is 4.

EXAMPLE 5

[0046] The reaction is carried out using the same method as in example 1. The difference is only that y in step 2 is 9.

EXAMPLE 6

[0047] The reaction is carried out using the same method as in example 1. The difference is only that y in step 2 is 12.

EXAMPLE 7

[0048] The reaction is carried out using the same method as in example 1. The difference is only that y in step 2 is 18.

EXAMPLE 8

[0049] The reaction is carried out using the same method as in example 1. The difference is only that y in step 2 is 30.

EXAMPLE 9

[0050] The reaction is carried out using the same method as in example 1. The difference is only that the support in step 1 is SiO.sub.2.

EXAMPLE 10

[0051] The reaction is carried out using the same method as in example 1. The difference is only that the support in step 1 is TiO.sub.2.

EXAMPLE 11

[0052] The reaction is carried out using the same method as in example 1. The difference is only that the drying temperature in step 2 is 60 C.

EXAMPLE 12

[0053] The reaction is carried out using the same method as in example 1. The difference is only that the drying temperature in step 2 is 80 C.

EXAMPLE 13

[0054] The reaction is carried out using the same method as in example 1. The difference is only that the drying temperature in step 2 is 80 C.

EXAMPLE 14

[0055] The reaction is carried out using the same method as in example 1. The difference is only that the drying time in step 2 is 11 h.

EXAMPLE 15

[0056] The reaction is carried out using the same method as in example 1. The difference is only that the drying temperature in step 2 is 12 h.

EXAMPLE 16

[0057] The reaction is carried out using the same method as in example 1. The difference is only that the calcination temperature in step 2 is 500 C.

EXAMPLE 17

[0058] The reaction is carried out using the same method as in example 1. The difference is only that the calcination temperature in step 2 is 550 C.

EXAMPLE 18

[0059] The reaction is carried out using the same method as in example 1. The difference is only that the calcination time in step 2 is 3 h.

EXAMPLE 19

[0060] The reaction is carried out using the same method as in example 1. The difference is only that the calcination time in step 2 is 4 h.

EXAMPLE 20

[0061] The reaction is carried out using the same method as in example 1. The difference is only that the calcination temperature in step 4 is 450 C.

EXAMPLE 21

[0062] The reaction is carried out using the same method as in example 1. The difference is only that the calcination temperature in step 4 is 550 C.

EXAMPLE 22

[0063] The reaction is carried out using the same method as in example 1. The difference is only that the weight hourly space velocity (WHSV) of propane in step 4 is 0.5 h.sup.1.

EXAMPLE 23

[0064] The reaction is carried out using the same method as in example 1. The difference is only that the weight hourly space velocity (WHSV) of propane in step 4 is 2 h.sup.1.

[0065] The FIG. 2 shows that the addition of Mo improves the selectivity of propylene. However, excessive addition of Mo leads to the decreasing of propane conversion. As a result, the MoVO.sub.x oxides (V/Mo=6) reaches 6.9 mol C.sub.3H.sub.6/kg-cat/h with 36% C.sub.3H.sub.8 conversion and 89% C.sub.3H.sub.6 selectivity at 500 C. and 1 h.sup.1 WHSV C.sub.3H.sub.8. Non-doped VO.sub.x shows almost 17% CO.sub.x selectivity with inhibited C.sub.3H.sub.6 selectivity of 79% at 500 C. and 1 h.sup.1 WHSV C.sub.3H.sub.8. Notably, pre-reduced VO.sub.x shows lower C.sub.3H.sub.8 conversion (20%) via a PDH scheme. Pre-reduction by H.sub.2 consumes the lattice oxygen, leading to the decrease of C.sub.3H.sub.8 conversion.

[0066] The FIG. 3 shows that a higher temperature benefits C.sub.3H.sub.8 conversion, whereas a higher temperature favors the CC cleavage and the formation of CH.sub.4.

[0067] The FIG. 4 shows that a lower space velocity would contribute to the CO.sub.x selectivity and inhibiting propylene formation. The main reason is that the reduction of residence time will cause propane or propylene to be completely oxidized to CO.sub.x by surface lattice oxygen with strong activity.

[0068] FIG. 5 shows that the conversion, selectivity and yield as a function of reaction time over MoV.sub.6 at 500 C. and 1 h.sup.1 WHSV propane. With the increase of reaction time, the lattice oxygen is consumed gradually. In the initial reaction stage from 0 to 3 min, there is the highest lattice oxygen activity, leading to the highest propane conversion rate. However, the higher oxygen activity caused the overoxidation of propane or propylene to CO.sub.x. With the gradual depletion of surface lattice oxygen, the yield of C.sub.3H.sub.6 at 3-5 min reached the highest, indicating that the lattice oxygen from the bulk phase is the main reactive oxygen species that activates propane to produce propylene. At the end of the reaction range, from 10 to 15 min, the lattice oxygen is exhausted, and PDH dominated on the reduced VO.sub.x.

[0069] FIG. 6 shows that two obvious reduction peaks emerged in the H.sub.2-TPR results, one related with OI diffusing at low temperatures and the other originated from OII diffusing at high temperatures. The OI species are responsible for overoxidation to CO.sub.x and OII species are responsible for oxidative dehydrogenation for propylene. In addition, with the increase of Mo content, the reduction peak of OI species was gradually weakened, while the reduction peak of OII species was gradually increased, indicating that Mo addition indeed effectively regulated the activity of lattice oxygen species in oxygen carriers and inhibited the OI species with strong activity.

[0070] Moreover, with Mo addition, the FIG. 7 shows that the diffraction peaks of V.sub.2O.sub.5 (JCPDS 89-0611) become broader and shift, indicating the existence of lattice distortion or residual stress of VO bonds. These characterization results have already been a sign for the formation of MoV solid solutions and the variations of VO bonds with the doping of Mo.

[0071] The consumption of lattice oxygen leads to the decrease of propane conversion, and the regeneration is needed to regain the lattice oxygen. The redox stability test in FIG. 8 shows that the performance remains stable during the 100 redox cycles.

[0072] The FIG. 9 shows that the increasing diffraction intensity indicates the slight sintering of crystal particles and the favorable restoration of MoVO.sub.x solid solutions in EDS mappings shows the excellent redox stability. Therefore, the sintering of catalysts could be inhibited due to the higher energy barrier of particle sintering than that of forming MoV solid solution through interphase diffusion. OI shows increase in weight after 50 redox cycles, which contributes to the increasing selectivity of CO.sub.x, indicating that anti-suppression of non-selective OI species may lead to the deactivation of MoVO.sub.x solid solution. This also confirms that OI is responsible for over-oxidation, whereas the selective OII is positively related to the formation of propylene.

[0073] As shown in FIGS. 10 and 11, the left side is catalyst VO.sub.x, and the right side is catalyst MoV.sub.6. The addition of Mo effectively reduces a large amount of lattice oxygen consumed by CO.sub.x and increases the lattice oxygen consumed by oxidative dehydrogenation to propylene. With the increase of reaction time, the lattice oxygen diffusion from vanadium oxygen carriers to replenish the consumed oxygen leads to the submergence of crystalline V.sub.2O.sub.5 and gradually transforming to VO.sub.2 and V.sub.2O.sub.3. Compared with VO.sub.x, MoVO.sub.x shows a lower rate for the phase transformation. The reaction period follows overoxidation, oxidative dehydrogenation and non-oxidative dehydrogenation period.

[0074] The preparation parameters can be adjusted according to the contents of the invention, and the preparation of the catalyst and effective catalysis for propane can be realized. The above exemplary description of the invention should indicate that, without breaking away from the core of the invention, any simple deformation, modification or other equivalent replacement that can be made by technicians in the field without the cost of creative labor falls within the protection scope of the invention.