Supported cluster molybdenum oxide catalyst, preparation method thereof, and application thereof in propane dehydrogenation

Abstract

The present invention discloses a supported cluster molybdenum oxide catalyst, whose chemical formula is expressed as X-MoO x / ZT, where MoO x is the active component, ZT is the carrier, and X represents the theoretical loading of the active component. The theoretical loading of the active component is 1-15%, and the average particle size of the active component MoO x is 0.8-1.5 nm. The present invention also discloses a preparation method of the supported cluster molybdenum oxide catalyst and its application in propane dehydrogenation. The present invention can effectively avoid the growth and aggregation of active component particles during the heat treatment of the catalyst and the reaction process; the loading of the active component can be adjusted within the range of 1-15%, and the size of the active component nanoparticles can be controlled within the range of 0.8-1.5 nm; it shows excellent dehydrogenation activity, olefin selectivity and catalyst regeneration stability in the propane dehydrogenation reaction. After the cluster molybdenum oxide catalyst is repeatedly reacted and regenerated 4 times, the initial conversion rate of the catalyst only decreases by 0.8%.

Description

Technical Field

[0001] The invention relates to the field of catalysts and preparation thereof, in particular to a supported cluster molybdenum oxide catalyst and a preparation method thereof and application thereof in propane dehydrogenation. Background Art

[0002] Propylene is an extremely important basic chemical in the chemical industry and one of the basic raw materials for the production of plastics, rubber and fibers. The rapid development of my country's economy has increased the demand for propylene year by year. In recent years, relying on the strong downstream demand for propylene and the widespread application of natural gas and shale gas worldwide, direct dehydrogenation of propane to produce olefins has stood out from many propylene production processes and has become an important channel for increasing propylene production in my country. Propane dehydrogenation is a strongly endothermic reaction controlled by thermodynamics, and the reaction temperature is usually between 550-620°C. In the prior art, although the Cr-based oxide catalysts used in industry can obtain a higher propylene yield, the Cr element has a large environmental pollution, and its use prospects are greatly limited. Therefore, the development of environmentally friendly oxide catalysts with high selectivity and stability under the condition of considerable propane conversion rate is an important direction for the development of propane dehydrogenation industrial catalysts. Among them, supported Mo-based oxide catalysts have potential industrial application value due to their stable structure, not easy to be reduced, cheap and environmentally friendly, and have been widely studied.

[0003] It is generally believed that the unsaturated coordinated Mo-O bonds in molybdenum oxide species act as Lewis acid sites to activate the C-H bonds in propane molecules. Therefore, preparing small-sized molybdenum oxide can fully expose the reactive sites, improve the atomic economy, and is an important means to enhance the performance of supported molybdenum oxide catalysts. However, the active component nanoparticles of the supported catalysts prepared by the existing impregnation method are large, and they are prone to aggregation and growth under reaction conditions, resulting in catalyst deactivation. Therefore, the controllable preparation of supported cluster molybdenum oxide catalysts is the main challenge currently encountered. In recent years, reports on the preparation methods of supported cluster catalysts have been published. The literature by Chang Zhipeng, Song Zhen, Liu Gang, Rodriguez José A, Hrbek Jan, Surface Science, 2002, 512, 353 used Au(111) as the substrate and Mo(CO)6 as the precursor under ultra-high vacuum conditions to prepare supported molybdenum oxide cluster catalysts by atomic layer deposition. The size of molybdenum oxide can be regulated by changing the deposition conditions and coverage. The literature by Zhang Haifeng, Stender Matthias, Zhang Rui, Wang Chongmin, Li Jun, Wang Laisheng, J. Phys. Chem. B, 2004, 108, 12259 used triphenylphosphine as the stabilizer and prepared Au 20 clusters in a system with surfactants by liquid-phase synthesis. Whether it is gas-phase chemical deposition or liquid-phase synthesis technology, there are problems such as cumbersome preparation methods, high costs, poor structural stability of materials, and difficulty in realizing the large-scale preparation of solid catalysts. Therefore, developing a simple and scalable synthesis method for supported cluster catalysts is a hot and difficult point in the research field of catalyst materials.

[0004] In summary, supported cluster molybdenum oxide catalysts show high research value in propane dehydrogenation reactions due to their unique structural characteristics and excellent catalytic properties. However, the preparation methods of this type of structure reported in the existing literature have disadvantages such as cumbersome reaction conditions, difficult operation, high preparation costs, poor material practicality, and inability to be prepared on a large scale. Therefore, the controllable preparation of supported molybdenum oxide cluster catalysts and the research on the application of this type of catalyst in important petrochemical processes still need to be further carried out. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned existing technologies and provide a supported cluster molybdenum oxide catalyst with an adjustable active component loading in the range of 1-15% and an average particle size of the active component nanoparticles less than 1.5 nm.

[0006] The second object of the present invention is to provide a preparation method for the supported cluster molybdenum oxide catalyst.

[0007] The third object of the present invention is to provide the application of the supported cluster molybdenum oxide catalyst in propane dehydrogenation.

[0008] To achieve the above object, the technical solution adopted by the present invention to solve the above technical problems is: a supported cluster molybdenum oxide catalyst, the chemical formula is expressed as X-MoO x / ZT, where MoO x is the active component, ZT is the carrier, X represents the theoretical loading of the active component, where the theoretical loading of the active component is 1-15%, and the average particle size of the active component MoO x is 0.8-1.5 nm.

[0009] The preparation method of the supported cluster molybdenum oxide catalyst includes the following steps:

[0010] (1) Dissolve the Mo salt and the complexing agent in water to obtain a mixed salt solution; wherein, the concentration of Mo ions in the mixed salt solution is 1-200 mmol / L; the molar ratio of the complexing agent to Mo ions is 5-20:1;

[0011] (2) Transfer the mixed salt solution to a reaction kettle, seal it and heat it to 140-200 °C for reaction for 3-8 h, drop the reacted solution into the carrier precursor sol, and then dry the slurry at 60-120 °C to obtain the X-MoO x / ZT catalyst precursor;

[0012] (3) Place the X-MoO x / ZT catalyst precursor in a muffle furnace and calcine it at 400-900 °C for 1-10 h, place the calcined solid powder in a tube furnace, and reduce it at a temperature of 500-1000 °C for 2-8 h to obtain the X-MoO x / ZT catalyst.

[0013] The Mo salt can be ammonium molybdate.

[0014] The complexing agent is one or a combination of two of acetonitrile, glucose, ethylenediamine, citric acid, isopropylamine or ascorbic acid.

[0015] The carrier precursor sol is one of silica sol and alumina sol, and the solid content is 5-20%.

[0016] In the step (2), during the process of dropping the solution into the carrier precursor sol, keep stirring to ensure uniform mixing, preferably magnetic stirring, and the stirring speed range is 300-800 r / min.

[0017] In the step (3), the heating rate of calcination is 10 °C / min.

[0018] In the step (3), a mixed gas is passed through the tubular furnace to form an atmosphere containing hydrogen. The mixed gas is composed of hydrogen and argon, and the volume fraction of hydrogen is 10% H2 / Ar.

[0019] The beneficial effects of the present invention are as follows: Molybdenum ions are uniformly embedded in the supramolecular network framework generated by the coordination of organic small molecules through metal ion complexation and coordination assembly. Then, a supported cluster molybdenum oxide catalyst is synthesized by carrier encapsulation. This method can effectively avoid the growth and aggregation of active component particles during the heat treatment of the catalyst and the reaction process; the loading of the active component can be adjusted within the range of 1-15%, and the size of the active component nanoparticles can be controlled within the range of 0.8-1.5 nm; by fine-tuning the ratio of the complexing small molecule to the metal ion, the size of the molybdenum oxide particles can be regulated; the obtained supported molybdenum oxide cluster catalyst exhibits excellent dehydrogenation activity, olefin selectivity, and catalyst regeneration stability in the propane dehydrogenation reaction. After the cluster molybdenum oxide catalyst is repeatedly reacted and regenerated 4 times, the initial conversion rate of the catalyst only decreases by 0.8%; this preparation method has strong versatility, is green and environmentally friendly, and can greatly improve the performance of the catalyst by regulating the size of the active component without changing the catalyst composition. Description of the Drawings

[0020] Figure 1 XRD pattern of 5-MoO x / Al2O3 in Example 1;

[0021] Figure 2 HRTEM image of 5-MoO x / Al2O3 in Example 1;

[0022] Figure 3 TEM image of 2-MoO x / SiO2 in Example 2;

[0023] Figure 4 SEM image of 10-MoO x / Al2O3 in Example 3;

[0024] Figure 5 Graph of the change of conversion rate and selectivity with time of 2-MoO x / SiO2 catalyst in the propane dehydrogenation to propylene reaction in Example 2;

[0025] Figure 6 Regeneration data graph of 10-MoO x / Al2O3 catalyst in the propane dehydrogenation to propylene reaction in the preparation of Example 3. Detailed Embodiments

[0026] The present invention will be further described below in conjunction with specific embodiments. It should be noted that the embodiments do not constitute a limitation on the scope of protection required by the present invention.

[0027] Example 1

[0028] A preparation method of a supported cluster molybdenum oxide catalyst is as follows: 1.0 g of ammonium molybdate and 10 g of glucose are added to 100 mL of deionized water, and stirred until completely dissolved to obtain a mixed salt solution; the mixed salt solution is transferred to a 150 mL hydrothermal reaction kettle, sealed, and placed in an oven at a temperature of 160 °C for 5 h; after the hydrothermal reaction kettle cools down, it is taken out, and the solution is added dropwise to a beaker containing 100 g of aluminum sol, and the solid content of the aluminum sol is 10%. During this period, continuous stirring is carried out to ensure that the reaction solution and the aluminum sol are mixed evenly; after the dropping is completed, the slurry is poured into a watch glass and transferred to an oven at a temperature of 100 °C for dehydration and drying to obtain the X-MoO x / ZT catalyst precursor; then the dried solid X-MoO x / ZT catalyst precursor is loaded into a crucible and placed in a muffle furnace for calcination. It is heated to 700 °C at a heating rate of 10 °C / min and kept warm for 3 h; then the calcined sample is transferred to a tubular furnace, and a 10% H2 / Ar mixed gas is introduced with a flow rate of 50 mL / min, heated to 600 °C, and kept warm for 3 h; after the tubular furnace cools down, a 5% loading of 5-MoO x / Al2O3 catalyst is prepared.

[0029] The carrier of the 5-MoO x / Al2O3 catalyst prepared in this example is Al2O3, the loading of the active component MoO x is 5%, and the average particle size of the active component is less than 1.5 nm. As Figure 1 shown, it can be seen that the carrier is mainly in the γ crystal form after calcination and reduction, and no peaks of the active component are observed, indicating that the Mo species are highly dispersed on the alumina carrier and there are no large particles. As Figure 2 shown, in the HRTEM image, it can be known by measuring the lattice fringes that the active component is dispersed on the alumina carrier in the form of MoO2 species.

[0030] Example 2

[0031] A preparation method of a supported cluster molybdenum oxide catalyst is as follows: Add 0.4 g of ammonium molybdate, 1.6 g of citric acid, and 1.2 g of isopropylamine into 100 mL of deionized water, and stir until completely dissolved to obtain a mixed salt solution; Transfer the mixed salt solution to a 150 mL hydrothermal reaction kettle, seal it, and place it in an oven at 180 °C for 3 h; After the hydrothermal reaction kettle cools down, take out the solution and add it dropwise to a beaker containing 50 g of silica sol. The solid content of the aluminum sol is 20%. During this period, continuously stir to ensure that the reaction solution and the silica sol are mixed evenly; After the dropping is completed, pour the slurry into a watch glass and transfer it to an oven at 90 °C for dehydration and drying to obtain the X-MoO x / ZT catalyst precursor; Then place the dried solid X-MoO x / ZT catalyst precursor in a crucible and place it in a muffle furnace for roasting. Heat it up to 600 °C at a heating rate of 10 °C / min and keep it at this temperature for 5 h; Then transfer the roasted sample to a tubular furnace, introduce a 10% H2 / Ar mixed gas with a flow rate of 50 mL / min, heat it up to 700 °C, and keep it at this temperature for 4 h; After the tubular furnace cools down, obtain a 2% loading of 2-MoO x / SiO2 catalyst.

[0032] The 2-MoO x / SiO2 catalyst prepared in this example has SiO2 as the carrier and the active component MoO x with a loading of 2%. As Figure 3 shown, it can be seen that the active component nanoparticles in the catalyst are evenly dispersed on the silica carrier, with a narrow particle size distribution and an average particle size of 1.1 nm.

[0033] Example 3

[0034] A preparation method of a supported cluster molybdenum oxide catalyst is as follows: Add 2.0 g of ammonium molybdate, 2.0 g of ascorbic acid, and 0.5 g of acetonitrile into 100 mL of deionized water, and stir until completely dissolved to obtain a mixed salt solution; Transfer the mixed salt solution to a 150 mL hydrothermal reaction kettle, seal it, and place it in an oven at 160 °C for 3 h; After the hydrothermal reaction kettle cools down, take out the solution and add it dropwise to a beaker containing 200 g of aluminum sol. The solid content of the aluminum sol is 5%. During this period, continuously stir to ensure that the reaction solution and the aluminum sol are mixed evenly; After the dropping is completed, pour the slurry into a watch glass and transfer it to an oven at 95 °C for dehydration and drying to obtain the X-MoO x / ZT catalyst precursor; Then place the dried solid X-MoO xThe precursor of the ZT catalyst was placed in a crucible and calcined in a muffle furnace. It was heated to 500 °C at a heating rate of 10 °C / min and held for 8 h. Then, the calcined sample was transferred to a tubular furnace, and a 10% H2 / Ar mixed gas was introduced at a flow rate of 50 mL / min. It was heated to 600 °C and held for 5 h. After the tubular furnace cooled down, a 10-MoO x / Al2O3 catalyst with a 10% loading was prepared.

[0035] The carrier of the 10-MoO x / Al2O3 catalyst prepared in this example is Al2O3, and the active component MoO x has a loading of 10%. As Figure 4 shown, it can be seen that even when the loading of the active component is increased, there is no agglomeration or particle growth of the active component, and the average particle size of the active component nanoparticles is 1.4 nm.

[0036] For the compositional analysis of the supported molybdenum oxide cluster catalysts prepared in Examples 1-3 above, the results of ICP-AES analysis are shown in the following table:

[0037] Sample <![CDATA[5-MoO x / Al2O3]]> <![CDATA[2-MoO x / SiO2]]> <![CDATA[10-MoO x / Al2O3]]> Theoretical loading (%) 5.0 2.0 10.0 Actual loading (%) 4.9 2.0 9.8

[0038] It can be seen from this that the actual Mo loading of the catalyst is basically the same as the theoretical feeding amount, and the active component does not lose during the catalyst preparation process.

[0039] Example 4

[0040] A preparation method of a supported cluster molybdenum oxide catalyst is as follows: 0.2 g of ammonium molybdate, 1.0 g of citric acid, and 0.8 g of ethylenediamine were added to 100 mL of deionized water, and stirred until completely dissolved to obtain a mixed salt solution; the mixed salt solution was transferred to a 150 mL hydrothermal reaction kettle, sealed, and placed in an oven at 160 °C for 6 h; after the hydrothermal reaction kettle cooled down, the solution was taken out and added dropwise to a beaker containing 100 g of silica sol, and the solid content of the aluminosilicate sol was 10%. During this period, continuous stirring was carried out to ensure uniform mixing of the reaction solution and the silica sol. After the dropping was completed, the slurry was poured into a petri dish and transferred to an oven at 110 °C for dehydration and drying to obtain the X-MoO x / ZT catalyst precursor; then the dried solid X-MoO x / ZT catalyst precursor was placed in a crucible and calcined in a muffle furnace. It was heated to 650 °C at a heating rate of 10 °C / min and held for 3 h; then, the calcined sample was transferred to a tubular furnace, and a 10% H2 / Ar mixed gas was introduced at a flow rate of 50 mL / min. It was heated to 600 °C and held for 4 h; after the tubular furnace cooled down, a 1-MoO x / SiO2 catalyst with a 1% loading was prepared.

[0041] The carrier of the 1-MoO x / SiO2 catalyst prepared in this example is SiO2, and the active component is MoO x with a loading of 1% and an average particle size of the active component of 0.9 nm.

[0042] Example 5

[0043] A preparation method of a supported cluster molybdenum oxide catalyst is as follows: Add 3.1 g of ammonium molybdate, 3.5 g of glucose, and 1.5 g of isopropylamine to 100 mL of deionized water, and stir until completely dissolved to obtain a mixed salt solution; Transfer the mixed salt solution to a 150 mL hydrothermal reactor, seal it, and place it in an oven at 170 °C for 4 h; After the hydrothermal reactor cools down, take out the solution and add it dropwise to a beaker containing 100 g of aluminum sol with a solid content of 10%, and continuously stir during the process to ensure that the reaction solution and the silica sol are mixed evenly; After the addition is completed, pour the slurry into a watch glass and transfer it to an oven at 90 °C for dehydration and drying to obtain the X-MoO x / ZT catalyst precursor; Then place the dried solid X-MoO x / ZT catalyst precursor in a crucible and place it in a muffle furnace for roasting, heating up to 750 °C at a heating rate of 10 °C / min, and holding for 4 h; Then transfer the roasted sample to a tubular furnace, introduce a 10% H2 / Ar mixed gas with a flow rate of 50 mL / min, heat up to 600 °C, and hold for 6 h; After the tubular furnace cools down, obtain a 15-MoO x / Al2O3 catalyst.

[0044] The 15-MoO x / Al2O3 catalyst prepared in this example has a carrier of Al2O3 and an active component of MoO x with a loading of 15% and an average particle size of the active component of 1.5 nm.

[0045] Application Example 1

[0046] Apply the supported cluster molybdenum oxide catalysts prepared in Examples 1-5 to the propane dehydrogenation to propylene reaction for an activity and selectivity evaluation experiment: The propane dehydrogenation reaction is carried out on a P&ID-A type micro-reaction fixed bed of Beijing Kaimenuo Technology Co., Ltd. The reaction temperature range is 560 - 600 °C, the reaction raw material is propane, and the reaction mass space velocity is 1.2 h -1The above-mentioned catalyst powder was tableted, and particles with a mesh size of 20 - 40 were screened for reaction evaluation. For different catalyst samples, to ensure a consistent Mo content during filling, 0.1 g - 1.5 g of catalyst particles were weighed and mixed with quartz sand to a volume of 2 mL. The mixed particles were then loaded into a quartz reaction tube. The concentrations of reactants and products were analyzed by on-line gas chromatography using a flame ionization detector, and the data was processed by the normalization method. The obtained data is summarized in the following table:

[0047] Catalyst sample Example 1 Example 2 Example 3 Example 4 Example 5 Temperature (°C) 580 600 580 600 600 Propane conversion rate (%) 32.6 38.9 42.2 27.8 34.2 Propylene selectivity (%) 89.6 91.9 92.2 89.8 90.6

[0048] In addition, as Figure 5 shown, the curves of the conversion rate and selectivity of the 2-MoO x / SiO2 catalyst prepared in Example 2 versus time in the propane dehydrogenation to propylene reaction indicate that the activity and propylene selectivity of this catalyst did not show a significant decline during the 200-min reaction process.

[0049] As Figure 6 shown, the regeneration data of the 10-MoO x / Al2O3 catalyst prepared in Example 3 in the propane dehydrogenation to propylene reaction show that the initial conversion rate and selectivity did not significantly decline after 4 regenerations.

[0050] The above are only the preferred embodiments of the present invention, and the scope of implementation of the present invention cannot be limited thereby. That is, any simple equivalent changes and modifications made in accordance with the scope of the patent application of the present invention and the content of the invention description still fall within the scope covered by the patent of the present invention.

Claims

1. A supported cluster molybdenum oxide catalyst, characterized in that: The chemical formula is expressed as X-MoO x / ZT, where MoO x is the active component, ZT is the carrier, X represents the theoretical loading amount of the active component, where the theoretical loading amount of the active component is 1-15%, and the average particle size of the active component MoO x is 0.8-1.5 nm; The preparation method comprises the following steps: (1) Dissolve the Mo salt and the complexing agent in water to obtain a mixed salt solution; wherein, the Mo ion concentration in the mixed salt solution is 1-200 mmol / L; the molar ratio of the complexing agent to the Mo ion is 5-20:1; (2) Transfer the mixed salt solution to a reaction kettle, seal it, heat it up to 140 - 200 °C and react for 3 - 8 h. Then, drop the reacted solution into the carrier precursor sol, and dry the slurry at 60 - 120 °C to obtain the precursor of X-MoO x / ZT catalyst; (3) Place the X-MoO x / ZT catalyst precursor in a muffle furnace and calcine it at 400 - 900 °C for 1 - 10 h. Place the calcined solid powder in a tubular furnace and reduce it at a temperature of 500 - 1000 °C for 2 - 8 h.

2. The preparation method of a supported cluster molybdenum oxide catalyst according to claim 1, characterized in that: The steps included are as follows: (1) Dissolve the Mo salt and the complexing agent in water to obtain a mixed salt solution; wherein, the Mo ion concentration in the mixed salt solution is 1-200 mmol / L; the molar ratio of the complexing agent to the Mo ion is 5-20:1; (2) Transfer the mixed salt solution to a reaction kettle, seal it, heat it up to 140 - 200 °C and react for 3 - 8 h. Then, drop the reacted solution into the carrier precursor sol, and then dry the slurry at 60 - 120 °C to obtain the X-MoO x / ZT catalyst precursor; (3) Place the X-MoO x / ZT catalyst precursor in a muffle furnace and calcine it at 400 - 900 °C for 1 - 10 h. Place the calcined solid powder in a tubular furnace and reduce it at a temperature of 500 - 1000 °C for 2 - 8 h.

3. The preparation method of a supported cluster molybdenum oxide catalyst according to claim 2, characterized in that: The Mo salt is ammonium molybdate.

4. The preparation method of a supported cluster molybdenum oxide catalyst according to claim 2, characterized in that: The complexing agent is one or a combination of two of acetonitrile, glucose, ethylenediamine, citric acid, isopropylamine or ascorbic acid.

5. The preparation method of a supported cluster molybdenum oxide catalyst according to claim 2, characterized in that: The carrier precursor sol is one of silica sol and alumina sol, and the solid content is 5-20%.

6. The preparation method of a supported cluster molybdenum oxide catalyst according to claim 2, characterized in that: In the step (2), stirring is maintained during the process of dropping the solution into the carrier precursor sol.

7. The preparation method of a supported cluster molybdenum oxide catalyst according to claim 2, characterized in that: In the step (3), the heating rate of the calcination is 10 °C / min.

8. The preparation method of a supported cluster molybdenum oxide catalyst according to claim 2, characterized in that: In the step (3), a mixed gas is passed through the tubular furnace to form an atmosphere with hydrogen. The mixed gas is composed of hydrogen and argon, and the volume fraction of hydrogen is 10%.

9. The application of a supported cluster molybdenum oxide catalyst according to claim 1 in propane dehydrogenation.

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