A catalyst for selective hydrogenation of acetylene and preparation method thereof

Abstract

The invention discloses an acetylene selective hydrogenation catalyst and a preparation method thereof, and belongs to the technical field of catalysts. The preparation method of the catalyst comprises the following steps: (1) impregnating a molybdenum source onto an alumina carrier; (2) calcining the impregnated product in air; (3) dissolving palladium acetate and adding the calcined product, heating and refluxing, separating and drying the solid, and obtaining a palladium-based catalyst. The benefits of the invention are as follows: (1) the entire process operation is simple and controllable, and does not require the use of special equipment. In addition, the alumina with good stability and low cost is used as a raw material, which is very suitable for large-scale industrial production; (2) molybdenum oxide and palladium elements are introduced, and molybdenum oxide can improve the hydrogen overflow performance of the catalyst and inhibit the adsorption and hydrogenation of ethylene; palladium and molybdenum oxide interact with each other, and the dispersion state and electronic structure of palladium in the catalyst are re-optimized, the atomic utilization rate of palladium is improved, the dosage is reduced, and the cost is reduced.

Description

Technical Field

[0001] The invention relates to a catalyst and a preparation method thereof, in particular to an acetylene selective hydrogenation catalyst and a preparation method thereof, belonging to the technical field of catalysts. Background Art

[0002] Selective hydrogenation of acetylene to ethylene is an important reaction, mainly used for ethylene purification in the petrochemical industry. In the petrochemical industry, ethylene is mainly obtained from petroleum cracking, which usually contains a small amount of acetylene gas (about 1%). For polymerization-grade ethylene, the acetylene content is required to be less than 1ppm, because acetylene can poison ethylene polymerization catalysts and significantly reduce the quality of downstream polyethylene products. For the production of high-quality polyethylene, the acetylene content in ethylene needs to be further reduced.

[0003] The selective hydrogenation of acetylene will have the following side reactions:

[0004] 1. Ethylene in the mixed gas is hydrogenated to produce ethane, which is a low value-added by-product;

[0005] 2. The ethylene in the mixed gas polymerizes to produce hydrocarbons, also known as green oil, which leads to a decline in economic benefits and accelerates the deactivation of the catalyst.

[0006] Therefore, the acetylene selective hydrogenation catalyst needs to have high catalytic selectivity to avoid side reactions such as over-hydrogenation and polymerization.

[0007] Currently, a variety of catalysts have been used for the selective hydrogenation of acetylene, among which palladium-based catalysts have attracted widespread attention due to their excellent catalytic performance. However, traditional palladium-based catalysts have problems such as insufficient catalytic activity, poor selectivity, and high cost. How to improve the atomic utilization of palladium in the catalyst, improve catalytic activity and selectivity, and reduce the amount of palladium used is one of the focuses of the industry. Summary of the invention

[0008] To overcome the deficiencies of the prior art, the present invention aims to provide an acetylene selective hydrogenation catalyst having high catalytic activity and selectivity and low palladium dosage and a preparation method thereof.

[0009] In order to achieve the above object, the present invention adopts the following technical solution:

[0010] A method for preparing an acetylene selective hydrogenation catalyst comprises the following steps:

[0011] (1) impregnating a molybdenum source onto an alumina carrier to obtain an impregnated product, wherein the molybdenum source is selected from any one of ammonium molybdate, phosphomolybdic acid, ammonium phosphomolybdate, silicomolybdic acid and ammonium silicomolybdate;

[0012] (2) calcining the impregnated product in air to obtain porous alumina loaded with molybdenum oxide;

[0013] (3) dissolving palladium acetate in a mixed solvent of acetonitrile and water, adding porous alumina loaded with molybdenum oxide, heating and refluxing for 8-12 hours under stirring, separating the solid and liquid after the reflux, and drying the solid to obtain a palladium-based catalyst, wherein the mixed solvent of acetonitrile and water is prepared by mixing acetonitrile and water in a mass ratio of 300:1.

[0014] Preferably, in step (1), the method for impregnating the molybdenum source onto the alumina carrier is: dissolving the molybdenum source in water, adding porous alumina, impregnating at 50-80°C for 2-3h, stirring continuously during the impregnation process, heating to 95°C after the impregnation, slowly evaporating the water under stirring, and grinding to obtain a solid powder, i.e., the impregnated product, wherein the molybdenum source, water, and porous alumina are used in a ratio of 1g:150g:10g.

[0015] Preferably, in step (2), the calcination temperature is 300-500° C. and the calcination time is 2-3 h.

[0016] Preferably, in step (3), the ratio of palladium acetate, acetonitrile and water mixed solvent, and molybdenum oxide-loaded porous alumina is 2 mg: 30.1 g: 2.0 g.

[0017] An acetylene selective hydrogenation catalyst is prepared by the above method.

[0018] The present invention is beneficial in that:

[0019] (1) The acetylene selective hydrogenation catalyst provided by the present invention uses alumina as a carrier. Alumina has good stability and low cost, and is very suitable for large-scale industrial production;

[0020] (2) The acetylene selective hydrogenation catalyst provided by the present invention introduces molybdenum oxide and palladium elements, wherein the molybdenum oxide can not only improve the hydrogen overflow performance of the catalyst and the catalytic activity of the catalyst, but also inhibit the adsorption and hydrogenation of ethylene, thereby improving the selectivity of the catalyst; the palladium and the molybdenum oxide interact with each other, the dispersion state and electronic structure of palladium in the catalyst are re-optimized, the atomic utilization of palladium is improved (achieving 100% utilization of precious metal atoms), the amount used is reduced, and the cost of the catalyst is reduced;

[0021] (3) The preparation method of the acetylene selective hydrogenation catalyst provided by the present invention can prepare the catalyst through a simple impregnation method (introducing molybdenum oxide) and a heat treatment step (introducing palladium element through surface reaction). The operation is simple and controllable, and no special equipment is required. It is very suitable for large-scale industrial production. In addition, by optimizing the preparation process, a catalyst with stable performance can be obtained, ensuring the long-term stable operation of the catalytic reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the TEM image of molybdenum-loaded alumina A;

[0023] Figure 2 is the energy spectrum scan of molybdenum-loaded aluminum oxide A, where A is the HAADF map, B is the oxygen element distribution map, C is the aluminum element distribution map, and D is the molybdenum element distribution map;

[0024] Figure 3 TEM images of palladium-based catalysts prepared using palladium acetate as a palladium source, wherein A is a TEM image of palladium-based catalyst A, B is a TEM image of palladium-based catalyst B, C is a TEM image of palladium-based catalyst C, D is a TEM image of palladium-based catalyst D, and E is a TEM image of palladium-based catalyst E;

[0025] Figure 4 is the energy spectrum scan of palladium-based catalyst A, wherein A is the HAADF map, B is the oxygen element distribution map, C is the aluminum element distribution map, D is the molybdenum element distribution map, and E is the palladium element distribution map;

[0026] Figure 5 TEM images of palladium-based catalysts prepared using palladium chloride as a palladium source, wherein A is a TEM image of palladium-based catalyst a, B is a TEM image of palladium-based catalyst b, C is a TEM image of palladium-based catalyst c, D is a TEM image of palladium-based catalyst d, and E is a TEM image of palladium-based catalyst e;

[0027] Figure 6 is the energy spectrum scan of palladium-based catalyst a, wherein A is the HAADF map, B is the oxygen element distribution map, C is the aluminum element distribution map, D is the molybdenum element distribution map, and E is the palladium element distribution map;

[0028] Figure 7 is the TEM image of palladium-based catalyst X;

[0029] Figure 8 It is the Pd element fine spectrum of palladium-based catalyst A, palladium-based catalyst a and palladium-based catalyst X. DETAILED DESCRIPTION

[0030] The present invention is based on the principle of single-atom catalysts, and by introducing molybdenum oxide and regulating the dispersion state and electronic structure of metal palladium, the catalytic performance (catalytic activity and selectivity) and atomic utilization of the catalyst are improved. The present invention is specifically introduced below in conjunction with the accompanying drawings and specific embodiments.

[0031] 1. Preparation of acetylene selective hydrogenation catalyst

[0032] The acetylene selective hydrogenation catalyst provided by the present invention uses aluminum oxide as a carrier and introduces molybdenum oxide and palladium elements. The preparation method thereof is as follows:

[0033] 1. Modify molybdenum oxide on the surface of aluminum oxide by impregnation

[0034] First, a molybdenum source is impregnated onto an alumina carrier. The molybdenum source is selected from any one of ammonium molybdate, phosphomolybdic acid, ammonium phosphomolybdate, silicomolybdic acid and ammonium silicomolybdate. The specific operation process is: 0.2g of the molybdenum source is dissolved in 30g of water, 2.0g of porous alumina is added, and it is immersed at 50-80°C for 2-3h. Stirring continuously during the impregnation process, heating to 95°C after the impregnation is completed, and the water is slowly evaporated under stirring. Grinding to obtain a solid powder, i.e., the product after impregnation.

[0035] Then, the impregnated product is calcined in air at a calcination temperature of 300-500° C. for a calcination time of 2-3 hours.

[0036] Table 1 Raw materials and process for modifying molybdenum oxide on the surface of aluminum oxide

[0037]

[0038] After testing, it was found that after the impregnated product was calcined in air, the molybdenum sources such as ammonium molybdate, phosphomolybdic acid, ammonium phosphomolybdate, silicomolybdic acid and ammonium silicomolybdate were all oxidized to molybdenum oxide, and these molybdenum oxides were all modified on the surface of the porous alumina, that is, porous alumina loaded with molybdenum oxide (referred to as molybdenum-loaded alumina) was obtained.

[0039] 2. Introduce palladium element on the surface of aluminum oxide through surface reaction between palladium source and molybdenum oxide

[0040] Taking palladium acetate (palladium source) as an example, palladium acetate reacts with molybdenum oxide to form palladium molybdate. The reaction equation is as follows:

[0041]

[0042]

[0043]

[0044]

[0045] The overall reaction equation for this process is:

[0046]

[0047] The reaction process of palladium acetate and molybdenum oxide to form palladium molybdate is very rapid and can be completed in an instant.

[0048] The specific operation process of introducing palladium element on the surface of alumina carrier is as follows: dissolve 2 mg of palladium acetate or palladium chloride (palladium source) into 30.1 g of acetonitrile and water mixed solvent (30 g acetonitrile + 0.1 g water), add 2.0 g of molybdenum-loaded alumina (carrier) prepared above, heat and reflux for 8-12 hours under stirring, separate the solid and liquid after reflux and dry the solid to obtain a palladium-based catalyst.

[0049] Table 2 Raw materials and processes for introducing palladium elements on the surface of alumina

[0050]

[0051] As a control, palladium acetate was directly loaded onto the surface of porous alumina using porous alumina as a carrier without loading molybdenum oxide to obtain a palladium-based catalyst X, the preparation method of which is as follows: 2 mg of palladium acetate was dissolved in 30.1 g of a mixed solvent of acetonitrile and water (30 g of acetonitrile + 0.1 g of water), 2.0 g of porous alumina was added, and the mixture was heated to reflux for 8 h under stirring. After the reflux, the solid and liquid were separated and the solid was dried to obtain a palladium-based catalyst X.

[0052] 2. Characterization of the structure of the catalyst for selective hydrogenation of acetylene

[0053] 1. Transmission electron microscopy (TEM) observation

[0054] TEM image of molybdenum-loaded alumina A is shown in Figure 1 .from Figure 1 It can be observed that the molybdenum-loaded alumina A presents a porous structure.

[0055] The energy spectrum scan of molybdenum-loaded aluminum oxide A is shown in Figure 2 .from Figure 2 It can be observed that Al, O, and Mo elements are evenly distributed without agglomeration.

[0056] The above results indicate that molybdenum oxide was successfully introduced onto the surface of the alumina carrier.

[0057] TEM images of palladium-based catalyst A, palladium-based catalyst B, palladium-based catalyst C, palladium-based catalyst D and palladium-based catalyst E are shown in Figure 3 .from Figure 3 No palladium particles were observed.

[0058] The energy spectrum scan of palladium-based catalyst A is shown in Figure 4.from Figure 4 It can be observed that Al, O, Mo and Pd elements are evenly distributed without agglomeration.

[0059] The above results indicate that molybdenum oxide and palladium are successfully introduced onto the surface of the alumina carrier, and the introduced palladium is in an atomically dispersed state.

[0060] TEM images of palladium-based catalyst a, palladium-based catalyst b, palladium-based catalyst c, palladium-based catalyst d and palladium-based catalyst e are shown in Figure 5 .from Figure 5 Palladium particles can be observed in the

[0061] The energy spectrum scan of palladium-based catalyst a is shown in Figure 6 .from Figure 6 It can be observed that Al, O and Mo elements are evenly distributed, while Pd is agglomerated.

[0062] The above results indicate that molybdenum oxide and palladium were successfully introduced onto the surface of the alumina carrier, but the introduced palladium was in a granular state and did not form an atomically dispersed state.

[0063] The TEM image of palladium-based catalyst X is shown in Figure 7 .from Figure 7 Palladium particles can be observed in the sample, but palladium does not form single atoms.

[0064] 2. X-ray Photoelectron Spectroscopy (XPS) Analysis

[0065] After the palladium-based catalyst A, palladium-based catalyst a and palladium-based catalyst X were reduced with hydrogen at 200° C., the fine spectra of the Pd element were determined by XPS.

[0066] The Pd element fine spectra of palladium-based catalyst A, palladium-based catalyst a and palladium-based catalyst X are shown in Figure 8 .from Figure 8 It can be observed that palladium-based catalyst A, palladium-based catalyst a and palladium-based catalyst X all present a single valence state, among which palladium-based catalyst a and palladium-based catalyst X are 0 valence, and palladium-based catalyst A presents a special valence state between 0 valence and +2 valence.

[0067] The above results indicate that Pd in ​​palladium-based catalyst A is dispersed at the atomic level and its electronic structure is directly affected by the surrounding chemical environment.

[0068] 3. Palladium loading detection

[0069] After detection by inductively coupled plasma optical emission spectrometer (ICP-OES), the palladium loadings of palladium-based catalyst A, palladium-based catalyst B, palladium-based catalyst C, palladium-based catalyst D and palladium-based catalyst E were 0.0635wt%, 0.0621wt%, 0.0644wt%, 0.0638wt% and 0.0647wt%, respectively; the palladium loadings of palladium-based catalyst a, palladium-based catalyst b, palladium-based catalyst c, palladium-based catalyst d and palladium-based catalyst e were 0.0715wt%, 0.0722wt%, 0.0707wt%, 0.0729wt% and 0.0730wt%, respectively; the palladium loading of palladium-based catalyst X was 0.0654wt%.

[0070] 3. Testing the catalytic performance of acetylene selective hydrogenation catalyst

[0071] The catalytic performance (catalytic activity and selectivity) of the acetylene selective hydrogenation catalyst was tested using a fixed bed reactor. Specifically:

[0072] 0.3 g of palladium-based catalyst A, palladium-based catalyst a or palladium-based catalyst X was weighed and loaded into a fixed bed reactor. Hydrogen was introduced and reduced at 300 ° C for 1 h (to obtain Pb single atoms), followed by nitrogen purge for 1 h. After that, the reaction pressure was adjusted to normal pressure, and a mixed gas of acetylene, ethylene, hydrogen and nitrogen was introduced. The volume percentages of acetylene, ethylene, hydrogen and nitrogen in the mixed gas were 1.0%, 20%, 20% and 59%, respectively. The volume space velocity was adjusted to 5000 h -1 The reaction was carried out at 30°C, 50°C and 70°C respectively. After the reaction, the conversion rate and selectivity of acetylene were calculated.

[0073] Table 3 Calculation results of acetylene conversion and selectivity

[0074]

[0075] From Table 3, we can see that:

[0076] (1) The palladium-based catalyst prepared by using palladium chloride instead of palladium acetate has a significantly lower selectivity for catalytic acetylene hydrogenation to ethylene because the palladium does not form a single atom;

[0077] (2) The palladium-based catalyst prepared by directly loading palladium acetate without loading molybdenum oxide lacks the hydrogen overflow promotion effect of molybdenum oxide, and palladium does not form single atoms, so the conversion rate and selectivity of catalytic acetylene hydrogenation to produce ethylene are significantly reduced.

[0078] It should be noted that the above embodiments are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for preparing an acetylene selective hydrogenation catalyst, characterized in that: The following steps are involved: (1) dissolving a molybdenum source in water, adding porous alumina, and impregnating at 50-80°C for 2-3h, stirring continuously during the impregnation process, heating to 95°C after the impregnation, slowly evaporating the water under stirring, and grinding to obtain a solid powder, i.e., the impregnated product, wherein the molybdenum source is selected from any one of ammonium molybdate, phosphomolybdic acid, ammonium phosphomolybdate, silicomolybdic acid and ammonium silicomolybdate, and the amount ratio of the molybdenum source, water and porous alumina is 1g:150g:10g; (2) calcining the impregnated product in air to obtain porous alumina loaded with molybdenum oxide; (3) dissolving palladium acetate in a mixed solvent of acetonitrile and water, adding porous alumina loaded with molybdenum oxide, heating and reflux for 8-12 hours under stirring, separating the solid and liquid after the reflux, and drying the solid to obtain a palladium-based catalyst, wherein the mixed solvent of acetonitrile and water is prepared by mixing acetonitrile and water in a mass ratio of 300:1, and the amount ratio of palladium acetate, acetonitrile and water mixed solvent, and porous alumina loaded with molybdenum oxide is 2 mg:30.1 g:2.0 g.

2. The preparation method according to claim 1, characterized in that: In step (2), the calcination temperature is 300-500°C and the calcination time is 2-3h.

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