Composite alpha-alumina carrier, preparation method thereof, and silver catalyst and application thereof

Abstract

The application belongs to the field of silver catalysts, and relates to a composite alpha-alumina carrier for a silver catalyst for ethylene oxidation to produce ethylene oxide, a preparation method of the composite alpha-alumina carrier, the silver catalyst and application. The composite alpha-alumina carrier comprises an alpha-alumina carrier and a porous alumina film containing zirconium coated outside the alpha-alumina carrier. The preparation method of the composite alpha-alumina carrier comprises the following steps: (1) dissolving an aluminum-containing compound, a pore-forming agent and a zirconium-containing compound in water, and preparing a solution after stirring; the pore-forming agent is urea, hexanediamine, sucrose or starch; (2) immersing the alpha-Al2O3 carrier in the solution obtained in step (1), and performing drying and calcination after leaching, to obtain the composite alpha-alumina carrier coated with a porous alumina film containing zirconium on the outer surface. The silver catalyst prepared from the alpha-alumina carrier of the application shows good reactivity and selectivity in the process of ethylene oxidation to produce ethylene oxide.

Description

Technical Field

[0001] This invention belongs to the field of silver catalysts, specifically relating to a composite α-alumina support for a silver catalyst used in the oxidation of ethylene to produce ethylene oxide, a method for preparing the composite α-alumina support, the composite α-alumina support obtained by the method, the silver catalyst prepared from the composite alumina support, and the application of the silver catalyst in the epoxidation of ethylene to produce ethylene oxide. Background Technology

[0002] In the oxidation of ethylene to ethylene oxide using a silver catalyst, side reactions occur, producing carbon dioxide and water. Activity, selectivity, and stability are the main performance indicators of silver catalysts. Activity refers to the reaction temperature required to reach a certain reaction load in the ethylene oxide production process. The lower the reaction temperature, the higher the catalyst activity. Selectivity is the ratio of the number of moles of ethylene converted to ethylene oxide to the total number of moles of ethylene reacted. Stability is represented by the rate of decrease in activity and selectivity; the smaller the rate of decrease, the better the catalyst stability. Using highly active, highly selective, and stable silver catalysts in the ethylene oxidation process to produce ethylene oxide can significantly improve economic efficiency. Therefore, manufacturing highly active, highly selective, and stable silver catalysts is a major research direction for silver catalysts. The performance of silver catalysts is not only significantly related to the catalyst composition and preparation method but also to the performance and preparation method of the catalyst support.

[0003] Existing methods for preparing silver catalysts include two processes: preparing a porous support (such as alumina) and applying an active component and additives to the support. In the preparation of silver catalysts, for supports with α-Al₂O₃ as the main component, a suitable specific surface area and pore structure are necessary. This is to provide sufficient space for the ethylene epoxidation reaction to allow the heat of reaction to dissipate, and also to facilitate the timely desorption of the reaction product ethylene oxide, avoiding deep oxidation and the generation of the byproduct carbon dioxide. Chinese patent CN1009437B uses a suitable ratio of alumina trihydrate to prepare a catalyst with a specific surface area of ​​0.2–2 μm. 2 Alumina support with a pore volume greater than 0.5 mL / g and less than 25% of which have a pore radius greater than 30 μm can achieve a selectivity of 83-84% for ethylene epoxidation reaction.

[0004] Improving the performance of silver catalysts by adding other components to the alumina support is also an important research direction. Furthermore, chemical treatment of the alumina support can also enhance the performance of silver catalysts. European patent EP0150238 uses a small amount of barium aluminate or barium silicate binder in the manufacturing process of high-purity, low-surface-area alumina supports, claiming to improve the support's shatter resistance and wear resistance; the patented support has a specific surface area of ​​less than 0.3 μm. 2 The catalysts prepared using this method have relatively low activity and selectivity. US4740493, US4829043, and EP0501317 patents use alumina supports containing certain amounts of soluble Ca, Al, K, and Na salts, claiming to reduce the rate of decrease in catalyst selectivity during use. US5384302 claims that pretreatment of α-Al₂O₃ reduces the Na, K, Ca, and Al ion content in the support, improving the support's crush resistance and wear resistance. EP0712334 prepares a silver catalyst by supporting an effective amount of silver, an alkali metal additive, an alkali metal additive, and an rhenium additive on a support containing at least 85% alumina and 0.001–2% magnesium in oxide form, thereby improving the catalyst's stability. US5100859, US5145824, EP0900126, US5801259, and US5733842 involve adding alkaline earth metals, silicon, and zirconium to α-Al₂O₃ to create a support, followed by impregnation with silver, alkali metal additives, rhenium additives, and their facilitators to produce a silver catalyst. These patents specify that calcium is the preferred alkaline earth metal, and that strontium and barium salts are used in compounds with zirconium. However, the effect of adding these two additives on the catalyst's performance is unknown. US5739075 involves pre-depositing a rare earth metal and another metal salt (alkaline earth metal or Group VIII transition metal) onto an alumina support, followed by calcination. The resulting silver catalyst is evaluated, and the results show that the selectivity of this catalyst decreases at a lower rate than that of the untreated catalyst sample. CN1511632C discovered that silver catalysts prepared by adding heavy alkaline earth metal compounds to alumina raw materials to form a support, impregnating a solution with silver compounds, organic amines and specific additives, and then heat-treating it in an oxygen-containing mixed gas showed improved activity and selectivity in the ethylene oxidation reaction.

[0005] Although the aforementioned patent documents employ various methods to improve alumina supports, resulting in varying degrees of improvement in the activity, stability, and selectivity of the catalysts, the requirements for support performance are constantly increasing with the large-scale industrial application of Re-containing high-selectivity silver catalysts. There is still room for improvement in silver catalysts for the preparation of ethylene oxide. Summary of the Invention

[0006] In view of the aforementioned state of the prior art, the inventors of this invention have conducted extensive and in-depth experimental research in the field of silver catalysts and their alumina supports. The results show that by coating the surface of an alumina support with a zirconium-containing porous alumina film, the silver catalyst prepared from this support can significantly improve the catalyst's reactivity and selectivity. Specifically, by adding an appropriate amount of a pore-forming agent and a zirconium-containing compound to an aluminum-containing solution, the alumina support is impregnated and dried in the aluminum-containing solution, followed by calcination. The pore-forming agent decomposes to generate gas, thereby forming a zirconium-containing porous film on the surface of the alumina support. Ultimately, the silver catalyst prepared from this support achieves higher reactivity and selectivity.

[0007] To achieve the objectives of this invention, a first aspect of this invention provides a composite α-alumina support for a silver catalyst used in the oxidation of ethylene to produce ethylene oxide, the composite α-alumina support comprising an α-alumina support and a zirconium-containing porous alumina film covering the outside.

[0008] According to the present invention, preferably, the porosity of the pores contained in the zirconium-containing porous alumina film is not less than 15%, and the zirconium content is 0.01 to 10.0 wt%, preferably 0.1 to 1 wt%.

[0009] A second aspect of the present invention provides a method for preparing a composite α-alumina support, comprising the following steps:

[0010] (1) Dissolve the aluminum-containing compound, the pore-forming agent and the zirconium-containing compound in deionized water and prepare a solution by stirring; the pore-forming agent is urea, hexamethylenediamine, sucrose and starch;

[0011] (2) The α-Al2O3 support is immersed in the solution obtained in step (1), drained, dried and calcined to obtain a composite alumina support with a zirconium-containing porous alumina film coated on the outer surface.

[0012] In this invention, the pore-forming agent is decomposed by calcination to generate gas and form a porous film. Under high temperature, the alumina film is partially or almost entirely converted into α-Al2O3, for example, more than 10% is converted into α-Al2O3, to obtain a composite α-Al2O3 support.

[0013] In this invention, the pore structure of the support was determined using a mercury porosimeter and a scanning electron microscope. The crystal phase composition of the alumina support was determined using XRD powder diffraction.

[0014] According to the present invention, preferably, in step (1), the aluminum-containing compound is an aluminum-containing inorganic compound and / or organic compound, preferably at least one of aluminum sulfate, aluminum nitrate, aluminum halide, aluminum isopropoxide and potassium aluminum sulfate, more preferably aluminum sulfate and / or aluminum nitrate; based on the total weight of the solution obtained in step (1), the amount of the aluminum-containing compound is 0.01-50.00 wt%, preferably 0.05-40.00 wt%, more preferably 0.10-30.00 wt%.

[0015] According to the present invention, preferably, in step (1), the amount of the pore-forming agent is 0.01-10.00 wt%, preferably 0.10-5.00 wt%, based on the total weight of the solution obtained in step (1).

[0016] According to the present invention, preferably, in step (1), the zirconium-containing compound is selected from at least one of zirconium nitrate, zirconium sulfate, zirconium oxynitrate, zirconium carbonate, zirconium hydroxide, zirconium silicate, zirconium citrate and zirconium fluoride, and the amount of the zirconium-containing compound is 0.01-10.0 wt%, preferably 0.1-2.0 wt%, based on the total weight of the solution obtained in step (1).

[0017] According to the present invention, preferably, in step (2), the drying temperature is 60-400℃ and the time is 0.01-24.00h; the calcination temperature is 500-1600℃, preferably 800-1400℃, and the time is 0.1h, preferably 0.5-6.5h.

[0018] A third aspect of the present invention provides a composite α-alumina support prepared by the method described above.

[0019] A fourth aspect of the present invention provides a silver catalyst for the oxidation of ethylene to produce ethylene oxide, the silver catalyst comprising:

[0020] a) the α-alumina support described above;

[0021] b) Silver deposited on the α-alumina support;

[0022] c) Alkali metals and / or alkali metal-based compounds;

[0023] d) Alkaline earth metals and / or compounds based on alkaline earth metals;

[0024] e) Rhenium metal and / or rhenium-based compounds;

[0025] f) Optional rhenium synergists.

[0026] According to the present invention, preferably, the method for preparing the silver catalyst includes the following steps:

[0027] (1) Dissolve the silver-containing compound, alkali metal auxiliaries, and rhenium-containing compound in an amine-containing solution or ammonia water to prepare a silver ammonia solution;

[0028] (2) The composite α-alumina support is immersed in the silver ammonia solution obtained in step (1) and drained, and then calcined and activated to produce a silver catalyst.

[0029] According to the present invention, preferably, in step (1), the silver-containing compound is a silver-containing organic and / or inorganic compound, preferably an organic acid and / or inorganic acid salt of silver, more preferably silver nitrate and / or silver oxalate, and the amount of silver-containing compound added is such that the content of silver in the silver catalyst on an atomic basis is 2 to 39 wt%, preferably 10 to 35 wt%, based on the total weight of the silver catalyst.

[0030] According to the present invention, preferably, in step (1), the alkali metal auxiliary is selected from at least one of lithium, sodium, potassium, rubidium and cesium compounds, and the amount of alkaline earth metal auxiliary added is such that the content of alkaline earth metal in the silver catalyst is 1 to 2000 ppm, preferably 5 to 1500 ppm, based on the total weight of the silver catalyst.

[0031] According to the present invention, preferably, in step (1), the rhenium-containing compound is selected from at least one of rhenium oxide, perrhenic acid, cesium perrhenate, methyl rhenium trioxide (VII), and ammonium perrhenate. Based on the total weight of the silver catalyst, the amount of the rhenium-containing compound added is such that the content of rhenium metal in the silver catalyst on an atomic basis is 0.1 to 2000 ppm, preferably 100 to 1000 ppm.

[0032] According to the present invention, preferably, in step (2), the calcination is carried out in air or a nitrogen-oxygen mixture with an oxygen content of not more than 21%, the calcination temperature is 100-600°C, preferably 150-500°C, and the time is 0.5-120 min, preferably 1-30 min.

[0033] A fourth aspect of the present invention provides the application of the aforementioned composite alumina support or the aforementioned silver catalyst in the epoxidation of ethylene to prepare ethylene oxide.

[0034] The silver catalyst prepared from the α-alumina support of the present invention exhibits good reactivity and selectivity in the oxidation of ethylene to produce ethylene oxide.

[0035] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0036] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0037] The present invention will be further described below with reference to the embodiments, but the scope of the present invention is not limited to these embodiments.

[0038] Carrier surface modification: Examples 1-9 and Comparative Examples 1-3

[0039] For detailed specifications of the initial carrier, please refer to the invention description. The initial carrier samples used in the examples and comparative examples were all prepared with the same carrier formulation. For details, please refer to CN88100400.6 and US5063195, which will not be elaborated here.

[0040] Example 1

[0041] The carrier surface modification process is as follows:

[0042] To prepare a solution containing aluminum, urea, and zirconium: Weigh 12 grams of aluminum sulfate octadecahydrate and dissolve it in 191 grams of deionized water; add 2 grams of urea and 4 grams of zirconium nitrate pentahydrate to the above solution and stir continuously.

[0043] Alumina film coating process on carrier surface:

[0044] Approximately 60 grams of the carrier sample was placed in the prepared solution, immersing the carrier completely. A vacuum of 10 mmHg or higher was applied, and the sample was allowed to stand for approximately 20 minutes. After leaching away excess solution, the impregnated carrier sample was dried at 80°C for 20 hours.

[0045] The dried carrier sample was placed in a muffle furnace and calcined at 1050℃ for 2 hours to complete the surface coating modification test of the carrier.

[0046] Example 2

[0047] The carrier modification process is similar to that of Example 1, except that the mass of aluminum sulfate octadecahydrate in the impregnation solution is 25 grams, the amount of deionized water added is 182 grams, the amount of urea added is 4 grams, and the amount of zirconium nitrate pentahydrate added is 4 grams.

[0048] Example 3

[0049] The carrier modification process is similar to that of Example 1, except that the mass of aluminum sulfate octadecahydrate in the impregnation solution is 25 grams, the amount of deionized water added is 182 grams, the amount of urea added is 4 grams, and the amount of zirconium nitrate pentahydrate added is 8 grams.

[0050] Example 4

[0051] The carrier modification process is similar to that of Example 1, except that the aluminum-containing compound used is replaced with 25 grams of aluminum nitrate nonahydrate, the amount of deionized water added is 182 grams, the amount of urea added is 4 grams, and the amount of zirconium sulfate added is 2 grams.

[0052] Example 5

[0053] The carrier modification process is similar to that of Example 1, except that the aluminum-containing compound used is replaced with 25 grams of aluminum nitrate nonahydrate, the amount of deionized water added is 182 grams, the amount of urea added is 4 grams, and the amount of zirconium sulfate added is 4 grams.

[0054] Example 6

[0055] The carrier modification process is similar to that of Example 1, except that the aluminum-containing compound used is replaced with 12 grams of aluminum isopropoxide, the amount of deionized water added is 191 grams, the amount of urea added is 2 grams, and the amount of zirconium nitrate pentahydrate added is 4 grams.

[0056] Example 7

[0057] The carrier modification process is similar to that of Example 1, except that the aluminum-containing compound used is replaced with 25 grams of aluminum isopropoxide, the amount of deionized water added is 182 grams, the amount of urea added is 4 grams, and the amount of zirconium nitrate pentahydrate added is 8 grams.

[0058] Example 8

[0059] The carrier modification process is similar to that of Example 1, except that the aluminum-containing compound used is replaced with 25 grams of aluminum isopropoxide, the amount of deionized water added is 182 grams, the amount of urea added is 4 grams, and the amount of zirconium sulfate added is 4 grams.

[0060] Example 9

[0061] The carrier modification process is similar to that of Example 1, except that 2g of urea is replaced with hexamethylenediamine.

[0062] Comparative Example 1

[0063] Comparative Example 1 uses the initial carrier sample that has not undergone any treatment.

[0064] Comparative Example 2

[0065] The carrier surface modification process is as follows:

[0066] The carrier modification process is similar to that of Example 1, except that urea is not added to the impregnation solution.

[0067] Comparative Example 3

[0068] The carrier modification process is similar to that of Example 1, except that zirconium nitrate pentahydrate is not added to the impregnation solution.

[0069] Carrier physical property testing:

[0070] The physical properties of the supports obtained in the examples and comparative examples were tested, and the results are shown in Table 1. The zirconium content was characterized by chemical analysis, and the porosity of the alumina film was statistically estimated by scanning electron microscopy.

[0071] Table 1. Physical property test results of the carriers in Comparative Examples 1-3 and Examples 1-9

[0072]

[0073]

[0074] As can be seen from Table 1, the porosity of the zirconium-containing porous alumina films prepared by the surface coating process is not less than 15% for the composite alumina carriers.

[0075] Catalyst preparation

[0076] Add 32.1g of ethylenediamine, 10.8g of ethanolamine, and 179.8g of deionized water to a stirred glass beaker to obtain a mixture. Slowly add 72.2g of silver oxalate to the mixture while maintaining the temperature below 40℃ and stirring continuously until the silver oxalate is completely dissolved. Then add 2.25mL of cesium nitrate aqueous solution (concentration of 0.03995g / mL, based on the atomic weight of cesium) and 2.78mL of ammonium perrhenate aqueous solution (concentration of 0.0162g / mL, based on the atomic weight of rhenium) in sequence, mix well to prepare 300g of impregnation solution for later use.

[0077] Take 20g of each of the supports prepared in Examples 1-9 and Comparative Examples 1-3, and place them in a vacuum-sealed glass container. Vacuum the container to below 10mmHg, pour in the impregnation solution to completely submerge the support, and keep it submerged for about 15 minutes. Then, filter to remove excess solution. Finally, place the impregnated support sample in a 350°C air stream and heat for 2 minutes, then cool to obtain the silver catalyst.

[0078] The performance of the corresponding silver catalysts obtained from the supports in Examples 1-9 and Comparative Examples 1-3 was determined using a microreactor evaluation device under the aforementioned process conditions. The data results on the 7th day after the start of the reaction are shown in Table 2.

[0079] Table 2 Results of catalyst performance testing

[0080] Catalyst number Reaction temperature ℃ Selectivity % Comparative Example 1 226.5 82.47 Comparative Example 2 226.2 82.63 Comparative Example 3 225.3 82.79 Example 1 221.3 84.13 Example 2 220.6 84.55 Example 3 222.1 85.52 Example 4 223.4 84.61 Example 5 224.5 85.33 Example 6 220.6 84.21 Example 7 221.9 85.14 Example 8 219.3 84.83 Example 9 220.5 84.02

[0081] As can be seen from Table 2, compared with the silver catalyst prepared by the support without surface modification in Comparative Example 1, the silver catalyst prepared by the alumina support with a zirconium-containing porous alumina film coated on the surface of the support according to the present invention has higher reactivity and selectivity.

[0082] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0083] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

Claims

1. A composite α-alumina support for a silver catalyst for the oxidation of ethylene to ethylene oxide, characterized in that, The composite α-alumina carrier comprises an α-alumina carrier and a porous zirconium-containing alumina film coated on the outer surface of the α-alumina carrier. The porosity of the pores contained in the porous zirconium-containing alumina film is not less than 15%, and the content of zirconium is 0.1-1 wt%. The composite α-alumina carrier is obtained by a preparation method comprising the following steps: (1) dissolving an aluminum-containing compound, a pore-forming agent and a zirconium-containing compound in water, and preparing a solution after stirring; the pore-forming agent is urea, hexanediamine, sucrose or starch; (2) immersing the α-Al2O3 carrier in the solution obtained in step (1), and performing drying and calcination after leaching to obtain a composite α-alumina carrier coated with a porous zirconium-containing alumina film on the outer surface.

2. A preparation method of a composite α-alumina carrier, comprising the following steps: (1) dissolving an aluminum-containing compound, a pore-forming agent and a zirconium-containing compound in water, and preparing a solution after stirring; the pore-forming agent is urea, hexanediamine, sucrose or starch; the zirconium-containing compound is at least one selected from zirconium nitrate, zirconium sulfate, zirconium oxynitrate, zirconium carbonate, zirconium hydroxide, zirconium silicate, zirconium citrate and zirconium fluoride; the amount of the zirconium-containing compound is 0.1-2.0 wt% based on the total weight of the solution obtained in step (1); (2) immersing the α-Al2O3 carrier in the solution obtained in step (1), and performing drying and calcination after leaching to obtain a composite α-alumina carrier coated with a porous zirconium-containing alumina film on the outer surface. The porosity of the pores contained in the porous zirconium-containing alumina film is not less than 15%, and the content of zirconium is 0.1-1 wt%.

3. The method of claim 2, wherein the composite α-alumina support is prepared by the steps of: In step (1), the aluminum-containing compound is an inorganic compound and / or an organic compound; the amount of the aluminum-containing compound is 0.01-50.00 wt% based on the total weight of the solution obtained in step (1).

4. The method of claim 3, wherein the composite α-alumina support is prepared by the steps of: The aluminum-containing compound is at least one selected from aluminum sulfate, aluminum nitrate, aluminum halide, aluminum isopropyl alcohol and potassium aluminum sulfate.

5. The method of claim 4, wherein the composite α-alumina support is prepared by the steps of: The aluminum-containing compound is aluminum sulfate and / or aluminum nitrate.

6. The method of claim 2, wherein the composite α-alumina support is prepared by the steps of: The amount of the aluminum-containing compound is 0.05-40.00 wt% based on the total weight of the solution obtained in step (1).

7. The method of claim 6, wherein the composite α-alumina support is prepared by the steps of: The amount of the aluminum-containing compound is 0.10-30.00 wt% based on the total weight of the solution obtained in step (1).

8. The method of claim 2, wherein the composite α-alumina support is prepared by the steps of: In step (1), the amount of the pore-forming agent is 0.01-10.00 wt% based on the total weight of the solution obtained in step (1).

9. The method of claim 8, wherein the composite α-alumina support is prepared by the steps of: The amount of the pore-forming agent is 0.10-5.00 wt% based on the total weight of the solution obtained in step (1).

10. The method for preparing the composite α-alumina support according to claim 2, wherein, In step (2), the drying temperature is 60-400℃, and the time is 0.01-24.00 h; the calcination temperature is 500-1600℃, and the time is 0.1 h.

11. The method of claim 10, wherein the composite α-alumina support is prepared by the steps of: The calcination temperature is 800-1400℃, and the time is 0.5-6.5 h.

12. A composite α-alumina carrier obtained by the preparation method of the composite α-alumina carrier according to any one of claims 2-11.

13. A silver catalyst for the oxidation of ethylene to ethylene oxide, characterized in that The silver catalyst comprises: a) an alpha-alumina carrier as claimed in claim 12; b) silver deposited on the alpha-alumina carrier; c) an alkali metal and / or an alkali metal-based compound; d) an alkaline earth metal and / or an alkaline earth metal-based compound; e) rhenium metal and / or a rhenium-based compound; f) an optional synergistic adjuvant of rhenium.

14. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 13, wherein, The preparation method of the silver catalyst comprises the following steps: (1) dissolving a silver-containing compound, an alkali metal adjuvant, a rhenium-containing compound in an amine-containing solution or aqueous ammonia to prepare a silver-ammonia solution; (2) immersing the composite alpha-alumina carrier as claimed in claim 12 in the silver-ammonia solution obtained in step (1) and draining, and then performing a calcination activation to prepare the silver catalyst.

15. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 14, wherein, In step (1), the silver-containing compound is an organic and / or inorganic silver-containing compound, and the amount of the silver-containing compound added is such that the content of silver in terms of atoms in the silver catalyst is 2-39 wt% based on the total weight of the silver catalyst.

16. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 15, wherein, The silver-containing compound is an organic acid and / or inorganic acid salt containing silver.

17. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 16, wherein, The silver-containing compound is silver nitrate and / or silver oxalate.

18. The silver catalyst for the oxidation of ethylene to ethylene oxide of claim 15, wherein, The amount of the silver-containing compound added is such that the content of silver in terms of atoms in the silver catalyst is 10-35 wt% based on the total weight of the silver catalyst.

19. The silver catalyst for the oxidation of ethylene to ethylene oxide of claim 14, wherein, In step (1), the alkali metal adjuvant is selected from at least one of the compounds of lithium, sodium, potassium, rubidium and cesium, and the amount of the alkali metal adjuvant added is such that the content of the alkali metal in the silver catalyst is 1-2000 ppm based on the total weight of the silver catalyst.

20. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 19, wherein, The amount of the alkali metal adjuvant added is such that the content of the alkali metal in the silver catalyst is 5-1500 ppm based on the total weight of the silver catalyst.

21. The silver catalyst for the oxidation of ethylene to ethylene oxide of claim 14, wherein, In step (1), the rhenium-containing compound is selected from at least one of the oxides of rhenium, perrhenic acid, cesium perrhenate, methyltrioxorhenium (VII) and ammonium perrhenate, and the amount of the rhenium-containing compound added is such that the content of rhenium metal in terms of atoms in the silver catalyst is 0.1-2000 ppm based on the total weight of the silver catalyst.

22. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 21, wherein, The amount of the rhenium-containing compound added is such that the content of rhenium metal in terms of atoms in the silver catalyst is 100-1000 ppm based on the total weight of the silver catalyst.

23. The silver catalyst for the oxidation of ethylene to ethylene oxide of claim 14, wherein, In step (2), the calcination is performed in air or a nitrogen-oxygen mixture gas with an oxygen content of not more than 21%, and the temperature of the calcination is 100-600°C and the time is 0.5-120 min.

24. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 23, wherein, The temperature of the calcination is 150-500°C and the time is 1-30 min.

25. Use of the silver catalyst as claimed in any one of claims 13-24 in the preparation of ethylene oxide by ethylene epoxidation.

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