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

Abstract

The invention belongs to the field of silver catalysts, and relates to a composite alpha-alumina carrier, a preparation method thereof, a silver catalyst and application. The composite alpha-alumina carrier comprises an alpha-alumina carrier and a titanium-containing porous alumina film coating the alpha-alumina carrier, the porosity of pores contained in the titanium-containing porous alumina film is not less than 15%, the content of titanium is 0.05-5.0 wt%, and on the basis of the total weight of the composite alpha-alumina carrier, the porosity of the pores contained in the titanium-containing porous alumina film is not less than 15%. The content of the titanium-containing porous aluminum oxide film is 0.05 to 10.0 weight percent. The preparation method comprises the following steps: adding a proper amount of a pore-forming agent, a mineralizing agent and a titanate coupling agent into an aluminum-containing solution, carrying out ultrasonic oscillation, dipping and drying an alumina carrier in the aluminum-containing solution, and carrying out high-temperature heating so that the pore-forming agent is heated and decomposed to generate gas, thereby forming a layer of titanium-containing porous film on the surface of the alumina carrier. Finally, the silver catalyst prepared from the carrier has higher reaction activity and selectivity.

Description

Technical Field

[0001] This invention belongs to the field of silver catalysts, specifically relating to a composite α-alumina support, a method for preparing the composite α-alumina support, the composite α-alumina support obtained by the method, a silver catalyst prepared from the composite alumina support, and the application of the silver catalyst in the epoxidation of ethylene to prepare 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 a pore radius greater than 30 μm accounting for less than 25% 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. 2The 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. CN1511632 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. Summary of the Invention

[0006] In view of the above-mentioned state of the prior art, the inventors of the present invention have conducted extensive and in-depth research in the field of silver catalysts. As a result, they found that by ultrasonically dispersing a solution containing aluminum compounds, pore-forming agents, mineralizing agents, and titanate coupling agents and impregnating and heating it, a layer of titanium-containing porous alumina film is coated on the surface of the support. The silver catalyst prepared from this support can significantly improve the reaction activity and selectivity of the catalyst.

[0007] To achieve the objectives of this invention, a first aspect of this invention provides a composite α-alumina carrier, the composite α-alumina carrier comprising an α-alumina carrier and a titanium-containing porous alumina film covering the outside thereof.

[0008] A second aspect of the present invention provides a method for preparing a composite α-alumina support, comprising the following steps:

[0009] (1) Dissolve aluminum-containing compounds, pore-forming agents, mineralizing agents and titanate coupling agents in water, and prepare a solution by ultrasonic stirring;

[0010] (2) The α-Al2O3 support is immersed in the solution obtained in step (1), filtered, and then heated to obtain a composite α-alumina support with a titanium-containing porous alumina film coated on the outer surface.

[0011] A third aspect of the present invention provides a composite α-alumina support prepared by the method described above.

[0012] A fourth aspect of the present invention provides a silver catalyst for the oxidation of ethylene to produce ethylene oxide, the silver catalyst comprising:

[0013] a) the α-alumina support described above;

[0014] b) Silver deposited on the α-alumina support;

[0015] c) Alkali metals and / or alkali metal-based compounds;

[0016] d) Alkaline earth metals and / or compounds based on alkaline earth metals;

[0017] e) Rhenium metal and / or rhenium-based compounds;

[0018] f) Optional rhenium synergists.

[0019] The fifth aspect of the present invention provides the application of the composite alumina support or the silver catalyst described herein in the epoxidation of ethylene to prepare ethylene oxide.

[0020] This invention involves adding appropriate amounts of pore-forming agent, mineralizer, and titanate coupling agent to an aluminum-containing solution and then ultrasonically vibrating it. After the alumina support is impregnated and dried in the aluminum-containing solution, it is heated at high temperature. The pore-forming agent decomposes to generate gas, thereby forming a titanium-containing porous film on the surface of the alumina support. Finally, the silver catalyst made from this support achieves higher reactivity and selectivity.

[0021] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0022] 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.

[0023] To achieve the objectives of this invention, a first aspect of this invention provides a composite α-alumina carrier, the composite α-alumina carrier comprising an α-alumina carrier and a titanium-containing porous alumina film covering the outside thereof.

[0024] According to the present invention, preferably, the porosity of the pores contained in the titanium-containing porous alumina film is not less than 15%, and the titanium content is 0.05-5.0 wt%, preferably 0.1-1.5 wt%.

[0025] According to the present invention, preferably, the content of the titanium-containing porous alumina film is 0.05 to 10.0 wt%, more preferably 0.1 to 5.0 wt%, based on the total weight of the composite α-alumina carrier.

[0026] In this invention, the pore structure of the support was determined using a mercury porosimeter. The crystal phase composition of the alumina support was determined using XRD powder diffraction.

[0027] A second aspect of the present invention provides a method for preparing a composite α-alumina support, comprising the following steps:

[0028] (1) Dissolve aluminum-containing compounds, pore-forming agents, mineralizing agents and titanate coupling agents in water, and prepare a solution by ultrasonic stirring;

[0029] (2) The α-Al2O3 support is immersed in the solution obtained in step (1), filtered, and then heated to obtain a composite α-alumina support with a titanium-containing porous alumina film coated on the outer surface.

[0030] In this invention, an aluminum-containing compound, a pore-forming agent, a mineralizing agent, and a titanate coupling agent are first dissolved in deionized water and then ultrasonically vibrated to prepare a solution for later use. Next, an α-Al₂O₃ support is immersed in the above-mentioned solution of the aluminum-containing compound, pore-forming agent, mineralizing agent, and titanate coupling agent under reduced pressure for a certain period of time, then filtered, dried, and subsequently subjected to heat treatment. The drying temperature is 60-400℃, and the drying time is controlled between 0.01-24.00 hours depending on the moisture content. The dried sample is typically calcined at a temperature of 500-1600℃, preferably 800-1400℃, for at least 0.1 hours, preferably 0.2-8.0 hours. High-temperature heating causes the pore-forming agent to decompose and generate gas, forming a porous film. Under high temperature, the alumina film is partially or almost entirely converted to α-Al₂O₃, for example, more than 10% is converted to α-Al₂O₃, resulting in a composite α-Al₂O₃ support.

[0031] The α-alumina support obtained by the support preparation method according to the present invention can be in shapes common in the art, such as annular, spherical, cylindrical or porous cylindrical, or other shapes.

[0032] After obtaining the α-alumina support of the present invention, the silver catalyst can be prepared by means known to those skilled in the art or by conventional means, for example by impregnating the above-mentioned alumina support with a solution containing a silver compound and an organic amine.

[0033] According to the present invention, preferably, in step (1), the titanate coupling agent is selected from at least one of isopropyl triisostearate titanate, isopropyl trioleoyloxy titanate, isopropyloxy tris(dodecylbenzenesulfonyloxy) titanate and isopropyl tristearate titanate; based on the total weight of the solution obtained in step (1), the amount of titanate coupling agent added is 0.01-5.0 wt%, preferably 0.05-2.0 wt% by weight.

[0034] 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 at least one of aluminum sulfate, aluminum nitrate and aluminum isopropoxide; 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%.

[0035] According to the present invention, preferably, in step (1), the pore-forming agent is selected from at least one of ammonium bicarbonate, ammonium carbonate, urea, citric acid, sucrose, citrate and glucose; based on the total weight of the solution obtained in step (1), the amount of pore-forming agent added is 0.01-10.00 wt%, preferably 0.10-5.00 wt% by weight.

[0036] According to the present invention, preferably, in step (1), the mineralizing agent is selected from at least one of boric acid, boron oxide, hydrogen fluoride, aluminum fluoride, ammonium fluoride, calcium fluoride, magnesium fluoride and cryolite; based on the total weight of the solution obtained in step (1), the amount of mineralizing agent added is 0.01-10.0 wt%, preferably 0.1-2.0 wt% by weight.

[0037] According to the present invention, preferably, in step (2), the conditions for the heat treatment include: a temperature of 500-1600℃, preferably 800-1400℃, and a time of not less than 0.1 hours, preferably 0.2-8.0 hours.

[0038] A third aspect of the present invention provides a composite α-alumina support prepared by the method described above.

[0039] A fourth aspect of the present invention provides a silver catalyst for the oxidation of ethylene to produce ethylene oxide, the silver catalyst comprising:

[0040] a) the α-alumina support described above;

[0041] b) Silver deposited on the α-alumina support;

[0042] c) Alkali metals and / or alkali metal-based compounds;

[0043] d) Alkaline earth metals and / or compounds based on alkaline earth metals;

[0044] e) Rhenium metal and / or rhenium-based compounds;

[0045] f) Optional rhenium synergists.

[0046] According to the present invention, preferably, the preparation method of the silver catalyst includes the following steps:

[0047] S1. 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;

[0048] S2. The composite α-alumina support is immersed in the silver ammonia solution obtained in step S1 and drained, and then calcined and activated to prepare the silver catalyst.

[0049] According to the present invention, preferably, in step S1, the silver-containing compound is a silver-containing organic and / or inorganic compound, preferably a silver-containing organic acid and / or inorganic acid salt, more preferably silver nitrate and / or silver oxalate; the alkali metal auxiliary is selected from at least one of lithium, sodium, potassium, rubidium and cesium compounds; the rhenium-containing compound is selected from at least one of rhenium oxide, perrhenic acid, cesium perrhenate, methylrhenium trioxide (VII) and ammonium perrhenate.

[0050] According to the present invention, preferably, based on the total weight of the silver catalyst, the amount of silver-containing compound added is such that the content of silver in the silver catalyst on an atomic basis is 2-39 wt%, preferably 10-35 wt%; the amount of alkaline earth metal promoter added is such that the content of alkaline earth metal in the silver catalyst is 1-2000 ppm, preferably 5-1500 ppm; and the amount of rhenium-containing compound added is such that the content of rhenium metal in the silver catalyst on an atomic basis is 0.1-2000 ppm, preferably 100-1000 ppm.

[0051] According to the present invention, preferably, in step S2, 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.

[0052] The active component, silver, can be added via a traditional impregnation method, such as immersing the support in a silver amine complex solution containing ethylenediamine and ethanolamine, followed by leaching to remove excess solution and heat treatment. Silver compounds used can be silver nitrate, silver oxalate, or other silver precursor compounds, with silver oxalate being preferred. To ensure uniform and sufficient silver loading, the support is preferably pre-vacuumed, and after impregnation and leaching, it should be rapidly activated for at least 0.5 minutes in flowing air or an inert gas such as nitrogen or argon at 100-600°C. To ensure high catalyst activity, the heat treatment temperature should not exceed 600°C. To further improve the performance of the catalyst, silver catalysts can be prepared by adding compounds or mixtures of alkali metals such as lithium, sodium, potassium, rubidium, and cesium, compounds or mixtures of alkaline earth metals such as calcium, strontium, and barium, as well as rhenium compounds and rhenium synergists and other catalytic additives. These catalyst additives can be applied to the support before, during, or after impregnation of silver, or they can be impregnated on the support after the silver compound is reduced.

[0053] The fifth aspect of the present invention provides the application of the composite alumina support or the silver catalyst described herein in the epoxidation of ethylene to prepare ethylene oxide.

[0054] 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.

[0055] Carrier surface modification: Examples 1-9 and Comparative Example 1

[0056] For detailed specifications of the initial carrier, please refer to the detailed description of the invention. 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.

[0057] The carrier surface modification process in Example 1 is as follows:

[0058] Preparation of solutions containing aluminum compounds, pore-forming agents, and mineralizing agents.

[0059] Weigh 12 grams of aluminum sulfate octadeca and dissolve it in 191 grams of deionized water;

[0060] Add 2 grams of ammonium bicarbonate, 1 gram of boric acid, and 0.5 grams of isopropyl triisostearate titanate to the above solution and continue ultrasonic vibration.

[0061] Alumina thin film coating process on carrier surface

[0062] Place approximately 60 grams of the carrier sample (for details, please refer to CN88100400.6 and US5063195) into the solution prepared in step I, immerse the carrier, evacuate to a vacuum of 10 mmHg or higher, then let stand for about 20 minutes, filter to remove excess solution, and dry at 100°C for 12 hours.

[0063] Next, the dried carrier sample is placed in a high-temperature kiln and heated at 1050℃ for 2 hours to complete the surface coating modification of the carrier.

[0064] The carrier surface modification process in Example 2 is as follows:

[0065] The carrier modification process in Example 2 is similar to that in 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 ammonium bicarbonate added is 4 grams, the amount of boric acid added is 1 gram, and 0.5 grams of isopropyl triisostearate titanate is added.

[0066] The carrier surface modification process in Example 3 is as follows:

[0067] The carrier modification process in Example 3 is similar to that in 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 ammonium bicarbonate added is 4 grams, the amount of boric acid added is 2 grams, and 0.5 grams of isopropyl triisostearate titanate is added.

[0068] The carrier surface modification process in Example 4 is as follows:

[0069] The carrier modification process in Example 4 is similar to that in 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 citric acid added is 4 grams, the amount of calcium fluoride added is 2 grams, and 0.5 grams of isopropyl triisostearate titanate is added.

[0070] The carrier surface modification process in Example 5 is as follows:

[0071] The carrier modification process in Example 5 is similar to that in 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, the amount of calcium fluoride added is 2 grams, and 0.5 grams of isopropyl triisostearate titanate is added.

[0072] The carrier surface modification process in Example 6 is as follows:

[0073] The carrier modification process in Example 6 is similar to that in 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 ammonium bicarbonate added is 2 grams, the amount of boric acid added is 1 gram, and 0.5 grams of isopropyl triisostearate titanate is added.

[0074] The carrier surface modification process in Example 7 is as follows:

[0075] The carrier modification process in Example 7 is similar to that in 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 ammonium bicarbonate added is 4 grams, the amount of boric acid added is 2 grams, and 1.0 gram of isopropyl triisostearate titanate is added.

[0076] The carrier surface modification process in Example 8 is as follows:

[0077] The carrier modification process in Example 8 is similar to that in 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 sucrose added is 4 grams, the amount of calcium fluoride added is 2 grams, and 0.5 grams of isopropyltrioleoyloxytitanate is added.

[0078] The carrier surface modification process in Example 9 is as follows:

[0079] The only difference from Example 1 is that the mixed solution is not treated with ultrasound.

[0080] Comparative Example 1:

[0081] Comparative Example 1 used an untreated initial carrier sample.

[0082] Physical property tests of the carrier: Examples 1-9 and Comparative Example 1

[0083] Table 1. Physical property test results of carrier comparative example 1 and examples 1-9

[0084] Carrier number Porosity (%) of alumina film Comparative Example 1 of Carriers - Carrier Example 1 ≥15 Carrier Example 2 ≥15 Carrier Example 3 ≥15 Carrier Example 4 ≥15 Carrier Example 5 ≥15 Carrier Example 6 ≥15 Carrier Example 7 ≥15 Carrier Example 8 ≥15 Carrier Example 9 ≥15

[0085] As can be seen from Table 1, the porosity of the porous alumina films of the composite alumina carriers prepared by the surface coating process is not less than 15%.

[0086] Catalyst preparation: Examples 1-9 and Comparative Example 1

[0087] 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 perperurate aqueous solution (concentration of 0.0162g / ml, based on the atomic weight of rhenium) in sequence, mix well to prepare a 300g impregnation solution for later use.

[0088] Take 20g of each of the supports prepared in Examples 1-9 and Comparative Example 1, 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, heat the impregnated support sample in an air stream at 350°C for 2 minutes, and then cool it to obtain the silver catalyst.

[0089] The performance of the corresponding silver catalysts obtained from the supports in Examples 1-9 and Comparative Example 1 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.

[0090] Table 2 Results of catalyst performance testing

[0091] Catalyst number Reaction temperature ℃ Selectivity % Comparative Example 1 226.5 82.47 Example 1 221.3 84.13 Example 2 222.6 84.62 Example 3 223.6 85.49 Example 4 224.0 85.31 Example 5 225.1 85.66 Example 6 220.7 84.18 Example 7 224.0 85.24 Example 8 223.2 85.39 Example 9 224.2 83.72

[0092] As can be seen from Table 2, compared with Comparative Example 1, which was prepared by a support without surface modification, the silver catalyst prepared by the alumina support of the present invention, which was prepared by ultrasonically oscillating a solution of aluminum compound, pore-forming agent, mineralizing agent and titanate coupling agent and impregnating and heating to coat the surface of the support with a titanium-containing porous alumina film, all showed higher reactivity and selectivity.

[0093] 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.

[0094] 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, characterized by, The composite α-alumina carrier comprises an α-alumina carrier and a porous titanium-containing alumina film coated on the outer surface of the α-alumina carrier.

2. The composite alpha-alumina support of claim 1, wherein, The porosity of the pores contained in the porous titanium-containing alumina film is not less than 15%, and the content of titanium is 0.05-5.0 wt%, preferably 0.1-1.5 wt%.

3. The composite alpha-alumina support of claim 1, wherein, The content of the porous titanium-containing alumina film is 0.05-10.0 wt%, preferably 0.1-5.0 wt% based on the total weight of the composite α-alumina carrier.

4. A method for producing a composite α-alumina carrier, characterized by, The method comprises the following steps: (1) dissolving an aluminum-containing compound, a pore-forming agent, a mineralizer and a titanate coupling agent in water, and preparing a solution after ultrasonic stirring; (2) immersing the α-Al2O3 carrier in the solution obtained in step (1), and performing leaching and heating treatment to obtain a composite α-alumina carrier coated with a porous titanium-containing alumina film on the outer surface.

5. The method of claim 4, wherein the composite α-alumina support is prepared by the steps of: In step (1), the titanate coupling agent is selected from at least one of isopropyl triisostearate titanate, isopropyl trioleate acyloxy titanate, isopropoxy tridodecyl benzene sulfonyloxy titanate and isopropyl tri-stearate titanate; the addition amount of the titanate coupling agent is 0.01-5.0 wt%, preferably 0.05-2.0 wt% based on the total weight of the solution obtained in step (1).

6. The method of claim 4, wherein the composite α-alumina support is prepared by the steps of: In step (1), the aluminum-containing compound is at least one of inorganic and / or organic aluminum-containing compounds, preferably at least one of aluminum sulfate, aluminum nitrate, aluminum halide, aluminum isopropyl alcohol and potassium aluminum sulfate, more preferably at least one of aluminum sulfate, aluminum nitrate and aluminum isopropyl alcohol; the addition 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% based on the total weight of the solution obtained in step (1).

7. The method of claim 4, wherein the composite α-alumina support is prepared by the steps of: In step (1), the pore-forming agent is selected from at least one of ammonium bicarbonate, ammonium carbonate, urea, citric acid, sucrose, citrate and glucose; the addition 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).

8. The method of claim 4, wherein the composite α-alumina support is prepared by the steps of: In step (1), the mineralizer is selected from at least one of boric acid, boron oxide, hydrogen fluoride, aluminum fluoride, ammonium fluoride, calcium fluoride, magnesium fluoride and cryolite; the addition amount of the mineralizer is 0.01-10.0 wt%, preferably 0.1-2.0 wt% based on the total weight of the solution obtained in step (1).

9. The method of claim 4, wherein the composite α-alumina support is prepared by the steps of: In step (2), the heating treatment is performed under the conditions of a temperature of 500-1600 ℃, preferably 800-1400 ℃, and a time of not less than 0.1 hour, preferably 0.2-8.0 hours.

10. The composite α-alumina carrier prepared by the method of any one of claims 4-9.

11. A silver catalyst for the oxidation of ethylene to ethylene oxide, characterized in that The silver catalyst comprises: a) the composite α-alumina carrier of any one of claims 1-3, 10; b) silver deposited on the α-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 co-agent, optionally rhenium.

12. The silver catalyst for the oxidation of ethylene to ethylene oxide of claim 11, wherein, The preparation method of the silver catalyst comprises the following steps: S1. dissolving a silver-containing compound, an alkali metal co-agent, a rhenium-containing compound in an amine-containing solution or aqueous ammonia to prepare a silver-ammonia solution; S2. dipping the composite α-alumina carrier of any one of claims 1-3, 10 into the silver-ammonia solution obtained in step S1, draining, and then performing a calcination activation to prepare the silver catalyst.

13. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 12, wherein, In step S1, the silver-containing compound is an organic and / or inorganic silver-containing compound, preferably an organic acid and / or inorganic acid salt containing silver, more preferably silver nitrate and / or silver oxalate; the alkali metal co-agent is selected from at least one of the compounds of lithium, sodium, potassium, rubidium and cesium; and the rhenium-containing compound is selected from at least one of the oxides of rhenium, cesium perrhenate, methyltrioxorhenium (VII) and ammonium perrhenate. The silver-containing compound is added in an amount such that the content of silver in the silver catalyst is 2-39 wt%, preferably 10-35 wt%, based on the total weight of the silver catalyst; the alkali metal co-agent is added in an amount such that the content of the alkali metal in the silver catalyst is 1-2000 ppm, preferably 5-1500 ppm; and the rhenium-containing compound is added in an amount such that the content of rhenium metal in the silver catalyst is 0.1-2000 ppm, preferably 100-1000 ppm, based on the total weight of the silver catalyst.

14. The silver catalyst for the oxidation of ethylene to ethylene oxide according to claim 12, wherein, In step S2, the calcination is performed in air or a nitrogen-oxygen mixture gas with an oxygen content of not more than 21%, the temperature of the calcination is 100-600°C, preferably 150-500°C, and the time is 0.5-120 min, preferably 1-30 min.

15. Use of the silver catalyst of any one of claims 11-14 in the preparation of ethylene oxide by ethylene epoxidation.