A modified alpha-alumina carrier and its preparation method and silver catalyst and its application

Abstract

The present invention belongs to the field of silver catalysts, and relates to a modified α-alumina carrier and a preparation method thereof, and a silver catalyst and application thereof. The modified α-alumina carrier is an α-alumina carrier coated with a porous alumina film on the surface, the porosity of the pores contained in the porous alumina film is not less than 30%, and the specific surface area of ​​the modified α-alumina carrier is 0.5-4.5 m higher than the specific surface area of ​​the α-alumina carrier. 2 / g. Compared with the reference sample, the composite alumina carrier prepared by the surface coating process of the present invention has a significantly increased specific surface area, and the reaction activity of the silver catalyst made of the carrier is significantly improved, and the selectivity is also improved to a certain extent.

Description

Technical Field

[0001] The present invention belongs to the field of silver catalysts, and specifically relates to a modified α-alumina carrier, a preparation method of the modified α-alumina carrier, a silver catalyst comprising the modified α-alumina carrier, and application of the modified α-alumina carrier and the silver catalyst in preparing ethylene oxide by epoxidation of ethylene. Background Art

[0002] Under the action of silver catalyst, ethylene oxidation mainly produces ethylene oxide, while side reactions produce carbon dioxide and water. Among them, activity, selectivity and stability are the main performance indicators of silver catalysts. The so-called activity refers to the reaction temperature required when the ethylene oxide production process reaches a certain reaction load. The lower the reaction temperature, the higher the activity of the catalyst. The so-called selectivity refers to the ratio of the number of moles of ethylene converted into ethylene oxide to the total number of moles of ethylene in the reaction. The so-called stability is expressed as the rate of decrease of activity and selectivity. The smaller the rate of decrease, the better the stability of the catalyst. In the process of ethylene oxidation to produce ethylene oxide, the use of high-activity, high-selectivity and good-stability silver catalysts can greatly improve economic benefits. Therefore, the manufacture of high-activity, high-selectivity and good-stability silver catalysts is the main direction of silver catalyst research. In addition to the composition and preparation method of the catalyst, the performance of the silver catalyst is also closely related to the performance and preparation method of the carrier used in the catalyst.

[0003] The preparation method of silver catalyst in the prior art includes two processes: preparation of porous carrier (such as alumina) and application of active components and additives to the carrier. 2 O 3 For the carrier as the main component, a suitable specific surface area and pore structure are required. On the one hand, it is necessary to provide enough space for the ethylene epoxidation reaction to diffuse the reaction heat, and on the other hand, it is also conducive to the timely desorption of the reaction product ethylene oxide to avoid deep oxidation to generate byproduct carbon dioxide. Chinese patent CN1009437B uses alumina trihydrate with a suitable proportion to prepare a specific surface area of ​​0.2-2m 2 / g, an alumina carrier with a pore volume greater than 0.5ml / g, wherein the pores with a pore radius greater than 30μm account for less than 25%, and can achieve a selectivity of 83-84% for ethylene epoxidation reaction.

[0004] Adding other components to the alumina carrier to improve the carrier and thus improve the performance of the silver catalyst is also an important research direction. In addition, the performance of the silver catalyst can also be improved by chemically treating the alumina carrier. European patent EP0150238B1 uses a small amount of barium aluminate or barium silicate binder in the manufacturing process of high-purity, low-surface alumina carrier, claiming to improve the carrier's crushing strength and wear resistance. The specific surface area of ​​the carrier manufactured by this patent is less than 0.3m 2 / g, the activity and selectivity of the catalyst prepared are relatively low. The alumina carrier used in US4740493A, US4829043A and EP0501317A1 contains a certain amount of Ca, Al, K, and Na soluble salts, claiming to reduce the rate of decrease of catalyst selectivity during use. US5384302A claims that by pre-treating α-Al 2 O 3 Reducing the content of Na, K, Ca, and Al ions in the carrier improves the crushing strength and wear resistance of the carrier. EP0712334B1 loads an effective amount of silver, an alkali metal as an additive, a magnesium as an additive, and a rhenium as an additive on a carrier containing at least 85% alumina and 0.001-2% magnesium in the form of oxide to prepare a silver catalyst, thereby improving the stability of the catalyst. US5100859A, US5145824A, EP0900126B1, US5801259A, and US5733842A add alkaline earth metals, silicon, and zirconium to α-Al 2 O 3 The carrier is manufactured, and then silver, alkali metal additives, rhenium additives and their auxiliary agents are impregnated to form a silver catalyst. The patent points out that alkaline earth metals are preferably calcium, strontium and barium salts are used together with zirconium compounds, and the effect of adding the two on the catalyst performance is unknown. US5739075A pre-deposit an auxiliary amount of rare earth metal and another auxiliary amount of metal salt (alkaline earth metal or VIII group transition metal) on the surface of an alumina carrier, and then calcination treatment is performed, and finally the treated carrier is made into a silver catalyst. The evaluation results show that the selectivity decrease rate of the catalyst is less than that of the catalyst sample without pre-deposition treatment. CN1511632A found that the activity and selectivity of the silver catalyst prepared by adding a heavy alkaline earth metal compound to an alumina raw material to form a carrier, impregnating a solution prepared by a silver compound, an organic amine and a specific auxiliary agent, and heat-treating it in an oxygen-containing mixed gas in the ethylene oxidation reaction are improved.

[0005] Although the above patent documents use various methods to improve the alumina carrier, which improves the activity, stability and selectivity of the catalyst to varying degrees, the requirements for carrier performance are also increasing with the large-scale industrial application of Re-containing high-selectivity silver catalysts. Therefore, it is necessary to continuously improve the carrier performance. 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, and found that a silver catalyst prepared by coating a porous alumina film on the surface of an alumina carrier can significantly improve the reaction activity and selectivity of the catalyst.

[0007] The first aspect of the present invention provides a modified α-alumina carrier, wherein the modified α-alumina carrier is an α-alumina carrier coated with a porous alumina film on the surface, the porosity of the pores contained in the porous alumina film is not less than 30%, and the specific surface area of ​​the modified α-alumina carrier is 0.5-4.5m higher than the specific surface area of ​​the α-alumina carrier. 2 / g, preferably 0.6-2.5m 2 / g.

[0008] The second aspect of the present invention provides a method for preparing a modified α-alumina carrier, comprising the following steps:

[0009] Ⅰ) obtaining a membrane-forming solution, wherein the membrane-forming solution contains a water-soluble aluminum-containing compound, a pore-forming agent, water and optionally a citrate-containing compound;

[0010] II) placing the α-alumina carrier into the film-forming solution and impregnating it under reduced pressure, and performing a calcination treatment after solid-liquid separation to obtain an α-alumina carrier with a porous alumina film coated on its surface.

[0011] The third aspect of the present invention provides a modified α-alumina carrier prepared by the above method.

[0012] The fourth aspect of the present invention provides a silver catalyst, which comprises a modified α-alumina carrier and a catalytically effective amount of silver loaded thereon, wherein the modified α-alumina carrier is the above-mentioned modified α-alumina carrier.

[0013] The fifth aspect of the present invention provides the use of the modified α-alumina carrier and / or the silver catalyst in the epoxidation of ethylene to prepare ethylene oxide.

[0014] Compared with the reference sample, the composite alumina carrier prepared by the surface coating process of the present invention has a significantly increased specific surface area, the reaction activity of the silver catalyst made from the carrier is significantly improved, and the selectivity is also improved to a certain extent.

[0015] Other features and advantages of the present invention will be described in detail in the following detailed description. DETAILED DESCRIPTION

[0016] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0017] The present invention provides a modified α-alumina carrier, wherein the modified α-alumina carrier is an α-alumina carrier coated with a porous alumina film on the surface, the porosity of the pores contained in the porous alumina film is not less than 30%, and the specific surface area of ​​the modified α-alumina carrier is 0.5-4.5m higher than the specific surface area of ​​the α-alumina carrier. 2 / g, preferably 0.6-2.5m 2 / g.

[0018] In the present invention, the term "α-alumina support" refers to a support before modification, and the terms "modified α-alumina support" and "composite α-alumina support" refer to a support after modification.

[0019] The present invention has no particular limitation on the carrier before modification, and the carrier may have the following properties: the crushing strength of the α-alumina carrier is 35-500N / grain, preferably 50-300N / grain; the specific surface area is 0.6-2.6m 2 / g, preferably 1.0-2.2m 2 / g; the pore volume is 0.3-1.2 ml / g, preferably 0.4-0.8 ml / g.

[0020] The modified carrier of the present invention has an increased specific surface area. Specifically, the specific surface area of ​​the modified α-alumina carrier is 1.2-5.0 m 2 / g.

[0021] The present invention also provides a method for preparing a modified α-alumina carrier, which is also a surface modification method for an alumina carrier, comprising the following steps:

[0022] Ⅰ) obtaining a membrane-forming solution, wherein the membrane-forming solution contains a water-soluble aluminum-containing compound, a pore-forming agent, water and optionally a citrate-containing compound;

[0023] II) placing the α-alumina carrier into the film-forming solution and impregnating it under reduced pressure, and performing a calcination treatment after solid-liquid separation to obtain an α-alumina carrier with a porous alumina film coated on its surface.

[0024] By adding an appropriate amount of pore-forming agent to the aluminum-containing solution, the alumina carrier is immersed in the aluminum-containing solution and dried, and then calcined to decompose the pore-forming agent to produce gas, thereby forming a porous film on the surface of the alumina carrier. Finally, the silver catalyst made from the carrier obtains higher reaction activity.

[0025] In order to prepare the composite α-alumina carrier of the present invention, a water-soluble aluminum-containing compound is required. The water-soluble aluminum-containing compound can be various water-soluble inorganic or organic compounds containing aluminum, such as aluminum sulfate, aluminum nitrate, aluminum halide, aluminum isopropoxide, potassium aluminum sulfate or other aluminum-containing salts, preferably at least one of aluminum sulfate, aluminum nitrate and aluminum isopropoxide. Based on the total weight of the membrane-forming solution, the content of the water-soluble aluminum-containing compound is generally 0.01-50.00wt%, preferably 0.05-40.00wt%, and more preferably 0.10-30.00wt%.

[0026] When preparing the composite α-alumina carrier of the present invention, a pore former is required in step I). The pore former may be at least one of a thermally decomposable carbonate and a bicarbonate, preferably at least one of sodium carbonate, potassium carbonate, calcium carbonate, sodium bicarbonate, and ammonium bicarbonate. The amount of the pore former may be determined according to the expected pore porosity. According to a preferred embodiment of the present invention, the content of the pore former is 0.01-10.00wt%, preferably 0.10-5.00wt%, based on the total weight of the membrane-forming solution.

[0027] When preparing the composite α-alumina carrier of the present invention, a citrate-containing compound may be optionally added in step I), the purpose of which is to improve the dispersibility of various substances in the solution, thereby improving the selectivity of the obtained silver catalyst. Specifically, the citrate-containing compound may be citric acid and / or a citrate salt, preferably at least one of citric acid, ammonium citrate, potassium citrate, sodium citrate and calcium citrate. It is particularly preferred to use citric acid and ammonium citrate as the citrate-containing compound. Based on the total weight of the film-forming solution, the content of the citrate-containing compound is preferably 0.0-10.0wt%, and more preferably 0.1-5.0wt%.

[0028] In the present invention, the aluminum compound and the pore-forming agent are first dissolved in deionized water, and citric acid or its compound is added, and after stirring, a solution is prepared for use. 2 O 3 The carrier is placed in the above-mentioned solution containing Al and pore-forming agent and impregnated under reduced pressure for a certain period of time, and then calcined after solid-liquid separation. The impregnation is carried out at a pressure below 100 mmHg, and the impregnation time is 10 to 300 minutes. The solid-liquid separation is usually leaching and drying, and the drying temperature is 60-400°C, and the drying time is controlled at 0.01-24.00 hours according to the moisture content. The dried sample is usually calcined at a temperature of 500-1600°C, preferably 800-1400°C, for not less than 0.1 hour, preferably 0.5-6.5 hours. The pore-forming agent is decomposed by calcination to produce gas to form a porous film. Under the action of high temperature, the aluminum oxide film is partially or basically completely converted into α-A1 2O 3 , for example, more than 10% is converted into α-A1 2 O 3 , to obtain the composite α-A1 2 O 3 carrier.

[0029] The properties of the unmodified α-alumina carrier used to prepare the composite α-alumina carrier are as described above and will not be repeated here.

[0030] According to another aspect of the present invention, there is also provided a modified α-alumina carrier prepared by the above method, wherein the outer surface of the modified carrier is coated with a porous alumina film, the porosity of the pores contained in the film is not less than 30%, and the specific surface area of ​​the alumina carrier is preferably 1.2-5.0 m 2 / g.

[0031] In the present invention, the specific surface area and pore structure of the carrier are measured by nitrogen physical adsorption BET method and mercury intrusion. The crystal phase composition of the alumina carrier is measured by XRD powder diffraction method. The porosity of the pores in the film is measured by mercury intrusion method.

[0032] The modified α-alumina carrier obtained by the carrier preparation method of the present invention can be in a shape commonly found in the art, such as a ring shape, a sphere shape, a column shape or a porous column shape, or other shapes.

[0033] The present invention also provides a silver catalyst, which comprises a modified α-alumina carrier and a catalytically effective amount of silver loaded thereon, wherein the modified α-alumina carrier is the above-mentioned modified α-alumina carrier.

[0034] After the modified α-alumina carrier of the present invention is prepared, the silver catalyst can be prepared by a method known to those skilled in the art or by a conventional method, for example, the silver catalyst is obtained by impregnating the modified α-alumina carrier with a silver-containing impregnation solution, specifically, by impregnating the modified α-alumina carrier with a solution containing a silver compound and an organic amine.

[0035] More specifically, the silver catalyst is prepared by a method comprising the following steps:

[0036] i) obtaining an impregnation solution, wherein the impregnation solution comprises a silver-containing compound, an amine compound, water, an alkali metal auxiliary, an optional rhenium auxiliary and a co-auxiliary thereof;

[0037] ii) impregnating the modified α-alumina carrier in the impregnation solution of step i), and performing calcination and activation after solid-liquid separation to obtain the silver catalyst.

[0038] According to a specific embodiment of the present invention, the preparation method of the silver catalyst comprises the following steps:

[0039] 1) firstly dissolving a water-soluble aluminum-containing compound and ammonium bicarbonate in deionized water, and adding an optional citrate-containing compound, and stirring to prepare a solution for use;

[0040] II) α-Al 2 O 3 The carrier is placed in the above-mentioned solution containing aluminum compound and ammonium bicarbonate and impregnated under reduced pressure, and then drained and calcined to obtain a composite alumina carrier with a porous alumina film coated on the surface;

[0041] III) then dissolving appropriate amounts of a silver-containing compound, an alkali metal auxiliary agent, and a rhenium-containing compound in an amine solution or ammonia water to prepare a silver-ammonia solution;

[0042] IV) Finally, the carrier prepared in step II is immersed in the silver ammonia solution prepared in step III and drained, and then calcined and activated to prepare a silver catalyst.

[0043] The silver-containing compound is selected from organic or inorganic compounds containing silver, preferably organic acids or inorganic acid salts of silver, particularly preferably silver nitrate and silver oxalate, and the amount of silver added should be such that the content of silver in the silver catalyst is 2-39wt%, preferably 10-35wt%, based on the total weight of the silver catalyst. The addition of the active component silver can be achieved by conventional impregnation methods, for example, by immersing the carrier in a silver amine complex solution containing ethylenediamine and ethanolamine, then leaching to remove excess solution and heat treating. To ensure uniform and sufficient loading of silver, the carrier is preferably pre-evacuated, and after impregnation and solid-liquid separation (such as draining), it is quickly activated in a preferably flowing air or inert gas such as nitrogen, argon, etc. at 100-600°C, 150-500°C for more than 0.5 minutes, for example 0.5-120 minutes, preferably 1-30 minutes.

[0044] In order to further improve the performance of the catalyst, an auxiliary agent may be added to the silver catalyst during preparation. The alkali metal auxiliary agent is selected from lithium, sodium, potassium, rubidium or cesium compounds or any combination of two or more thereof and the amount of the auxiliary agent added is such that the content of the alkali metal in the silver catalyst is 1-2000 ppm, preferably 5-1500 ppm, based on the total weight of the silver catalyst. The rhenium auxiliary agent is selected from one or more of rhenium oxide, perrhenic acid, cesium perrhenate, methyltrioxyrhenium (VII) and ammonium perrhenate, and the amount of the auxiliary agent added to the impregnation solution is such that the total content of rhenium metal in the silver catalyst is 0-2000 ppm, preferably 100-1000 ppm, based on the total weight of the silver catalyst. The optional rhenium co-promoter can be selected from one or more of the salts or acids containing manganese, chromium, sulfur, cobalt, molybdenum, and nickel; the content of the rhenium co-promoter in the impregnation solution can be 0 to 2000 ppm based on the weight of the impregnation solution. These catalyst promoters can be applied to the support before, at the same time as, or after the silver is impregnated, or can be impregnated on the support after the silver compound is reduced.

[0045] The modified α-alumina carrier and the silver catalyst of the present invention can be used in the production of ethylene oxide by epoxidation of ethylene. Specifically, in the presence of the silver catalyst, a mixed gas of ethylene and oxygen or the like is reacted in a fixed bed micro-tubular reactor.

[0046] The present invention will be further described below in conjunction with embodiments, but the scope of the present invention is not limited to these embodiments.

[0047] Carrier Surface Modification: Examples 1 to 8 and Comparative Example 1

[0048] The initial carrier samples used in the examples and comparative examples are all made from the same carrier formula. For details, please refer to CN88100400.6 and US5063195, which will not be described in detail in this article. The physical properties of the initial carrier are as follows: crushing strength is 83N / grain, water absorption is 45%, and specific surface area is 1.161m 2 / g, and the pore volume is 0.46ml / g.

[0049] Example 1

[0050] This example is used to illustrate the modified carrier and preparation method of the present invention.

[0051] The surface modification process is as follows:

[0052] I. Preparation of a solution containing aluminum and ammonium bicarbonate

[0053] 1. Weigh 12 grams of aluminum sulfate 18hydrate and dissolve it in 191 grams of deionized water;

[0054] 2. Add 2 grams of ammonium bicarbonate to the above solution and continue stirring;

[0055] II. Alumina film coating process on the carrier surface

[0056] 1. Place about 60 grams of the initial carrier sample into the solution prepared in step Ⅰ, immerse the carrier, evacuate to above 10 mmHg, and then let it stand for about 20 minutes. After leaching away the excess solution, dry the impregnated carrier sample at 80°C for 20 hours;

[0057] 2. The dried carrier sample was placed in a muffle furnace and calcined at 1050° C. for 2 hours to complete the surface coating modification of the carrier and obtain a modified α-alumina carrier.

[0058] Example 2

[0059] The carrier modification process of Example 2 is similar to that of Example 1, except that the mass of aluminum sulfate 18hydrate in the impregnation solution is 25 grams, the amount of deionized water added is 182 grams, and the amount of ammonium bicarbonate added is 4 grams.

[0060] Example 3

[0061] The carrier modification process of Example 3 is similar to that of Example 1, except that the mass of aluminum sulfate 18hydrate in the impregnation solution is 25 grams, the amount of deionized water added is 182 grams, and the amount of ammonium bicarbonate added is 8 grams.

[0062] Example 4

[0063] The carrier modification process of Example 4 is similar to that of Example 1, except that the aluminum-containing compound used is changed to 25 grams of aluminum nitrate nonahydrate, the amount of deionized water added is 182 grams, and the amount of ammonium bicarbonate added is 4 grams.

[0064] Example 5

[0065] The carrier modification process of Example 5 is similar to that of Example 1, except that the aluminum-containing compound used is changed to 25 grams of aluminum nitrate nonahydrate, the amount of deionized water added is 182 grams, the amount of ammonium bicarbonate added is 4 grams, and in step 2 of step I, 1 gram of citric acid monohydrate is added.

[0066] Example 6

[0067] The carrier modification process of Example 6 is similar to that of Example 1, except that the aluminum-containing compound used is changed to 12 grams of aluminum isopropoxide, the amount of deionized water added is 191 grams, and the amount of ammonium bicarbonate added is 2 grams.

[0068] Example 7

[0069] The carrier modification process of Example 7 is similar to that of Example 1, except that the aluminum-containing compound used is changed to 25 grams of aluminum isopropoxide, the amount of deionized water added is 182 grams, and the amount of ammonium bicarbonate added is 4 grams.

[0070] Example 8

[0071] The carrier modification process of Example 8 is similar to that of Example 1, except that the aluminum-containing compound used is changed to 25 grams of aluminum isopropoxide, the amount of deionized water added is 182 grams, and the amount of ammonium bicarbonate added is 8 grams.

[0072] Comparative Example 1

[0073] The original carrier sample used in Comparative Example 1 was not subjected to any treatment.

[0074] Physical property test of carrier

[0075] Examples 1 to 8 and Comparative Example 1

[0076] Table 1 Physical property test results of carrier Comparative Example 1 and Examples 1 to 8

[0077] Carrier number <![CDATA[BET specific surface area (m 2 / g)]]> Alumina film porosity (%) Carrier Comparative Example 1 1.161 - Carrier Example 1 2.432 ≥30 Carrier Example 2 2.675 ≥30 Carrier Example 3 2.713 ≥30 Carrier Example 4 1.754 ≥30 Carrier Example 5 1.827 ≥30 Carrier Example 6 2.548 ≥30 Carrier Example 7 2.769 ≥30 Carrier Example 8 2.821 ≥30

[0078] It can be found from Table 1 that the specific surface area of ​​the composite alumina carrier prepared by the surface coating process is significantly larger than that of the reference sample, and the porosity of the alumina film is not less than 30%.

[0079] Catalyst preparation: Examples 1 to 8 and Comparative Example 1

[0080] Add 32.1g of ethylenediamine, 10.8g of ethanolamine and 179.8g of deionized water into a stirred glass beaker to obtain a mixed solution; slowly add 72.2g of silver oxalate into the mixed solution, keep the temperature below 40°C and continue stirring to completely dissolve the silver oxalate; then add 2.25ml of cesium nitrate aqueous solution (concentration of 0.03995g / ml, based on cesium atomic weight) and 2.78ml of ammonium perrhenate aqueous solution (concentration of 0.0162g / ml, based on rhenium atomic weight) in sequence, mix well to prepare 300g of impregnation solution for standby use.

[0081] Take 20g of the carriers prepared in Examples 1 to 8 and Comparative Example 1, respectively, and place them in a vacuum glass container; evacuate to below 10mmHg, pour the impregnation solution, completely immerse the carrier, keep for about 15 minutes, and then remove the excess solution by leaching. Finally, place the impregnated carrier sample in an air flow at 350℃ and heat for 2 minutes, and cool it to obtain a silver catalyst.

[0082] The performance of the catalyst samples was measured using a microreactor evaluation device under the aforementioned process conditions for the corresponding silver catalysts obtained from the carriers in Examples 1 to 8 and Comparative Example 1. The data results on the 7th day after the start of the reaction are shown in Table 2.

[0083] Table 2 Catalyst performance test results

[0084]

[0085]

[0086] As can be seen from Table 2, compared with the silver catalyst prepared in Comparative Example 1 using a carrier without surface modification treatment, the silver catalyst prepared using an alumina carrier with a porous alumina film coated on the carrier surface according to the present invention has significantly higher reaction activity and higher selectivity.

[0087] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0088] The endpoints and any values ​​of the ranges disclosed in this article 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

Claims

1. A method for preparing a modified α-alumina carrier, comprising the following steps: Ⅰ) obtaining a membrane-forming solution, wherein the membrane-forming solution contains a water-soluble aluminum-containing compound, a pore-forming agent, water, and an optional citrate-containing compound; based on the total weight of the membrane-forming solution, the content of the pore-forming agent is 0.10-5.00wt%; II) placing the α-alumina carrier into the film-forming solution under reduced pressure, and performing calcination after solid-liquid separation to obtain an α-alumina carrier with a porous alumina film coated on the surface; The pore-forming agent is at least one of sodium carbonate, potassium carbonate, calcium carbonate, sodium bicarbonate, and ammonium bicarbonate.

2. The method according to claim 1, wherein: The specific surface area of ​​the modified α-alumina carrier is 0.6-2.5 m higher than the specific surface area of ​​the α-alumina carrier. 2 / g.

3. The method according to claim 1, wherein: The α-alumina carrier has a crushing strength of 35-500 N / grain and a specific surface area of ​​0.6-2.6 m 2 / g; pore volume is 0.3-1.2ml / g; The specific surface area of ​​the modified α-alumina carrier is 1.2-5.0 m 2 / g.

4. The method according to claim 3, wherein: The α-alumina carrier has a crushing strength of 50-300 N / grain and a specific surface area of ​​1.0-2.2 m 2 / g; pore volume is 0.4-0.8ml / g.

5. The method according to claim 1, wherein: In step I), the water-soluble aluminum-containing compound is an inorganic aluminum compound and / or an organic aluminum compound; based on the total weight of the membrane-forming solution, the content of the water-soluble aluminum-containing compound is 0.01-50.00 wt%.

6. The method according to claim 5, wherein: The water-soluble aluminum-containing compound is at least one of aluminum sulfate, aluminum nitrate, aluminum halide, aluminum isopropoxide and potassium aluminum sulfate.

7. The method according to claim 6, wherein: The water-soluble aluminum-containing compound is at least one of aluminum sulfate, aluminum nitrate and aluminum isopropoxide.

8. The method according to claim 5, wherein: Based on the total weight of the membrane-forming solution, the content of the water-soluble aluminum-containing compound is 0.05-40.00 wt %.

9. The method according to claim 8, wherein: Based on the total weight of the membrane-forming solution, the content of the water-soluble aluminum-containing compound is 0.10-30.00 wt %.

10. The method according to claim 1, wherein: In step I), the citrate-containing compound is citric acid and / or citrate; based on the total weight of the membrane-forming solution, the content of the citrate-containing compound is 0.0-10.0 wt%.

11. The method according to claim 10, wherein: The citrate-containing compound is at least one of citric acid, ammonium citrate, potassium citrate, sodium citrate and calcium citrate.

12. The method according to claim 11, wherein: The citrate-containing compound is citric acid and / or ammonium citrate.

13. The method according to claim 10, wherein: Based on the total weight of the membrane-forming solution, the content of the citrate-containing compound is 0.1-5.0 wt %.

14. The method according to any one of claims 1 to 13, wherein: In step II), the immersion is carried out at a pressure below 100 mmHg, and the immersion time is 10 to 300 minutes.

15. The method according to any one of claims 1 to 13, wherein: In step II), the solid-liquid separation is leaching and drying, the drying temperature is 60-400° C., and the drying time is 0.01-24.00 hours.

16. The method according to any one of claims 1 to 13, wherein: In step II), the calcination conditions include: a temperature of 500-1600° C. and a calcination time of not less than 0.1 hour.

17. The method according to claim 16, wherein: The calcination conditions include: a temperature of 800-1400° C. and a calcination time of 0.5-6.5 hours.

18. The method according to any one of claims 1 to 13, wherein: The α-alumina carrier has a crushing strength of 35-500 N / grain and a specific surface area of ​​0.6-2.6 m 2 / g; pore volume is 0.3-1.2ml / g.

19. The method according to claim 18, wherein: The α-alumina carrier has a crushing strength of 50-300 N / grain and a specific surface area of ​​1.0-2.2 m 2 / g; pore volume is 0.4-0.8ml / g.

20. A modified α-alumina carrier obtained by the method according to any one of claims 1 to 19.

21. A silver catalyst, comprising a modified α-alumina carrier and a catalytically effective amount of silver loaded thereon, wherein the modified α-alumina carrier is the modified α-alumina carrier according to claim 20.

22. The silver catalyst according to claim 21, wherein The silver catalyst is obtained by impregnating the modified α-alumina carrier with a silver-containing impregnation solution.

23. The silver catalyst according to claim 22, wherein The silver catalyst is prepared by a method comprising the following steps: i) obtaining an impregnation solution, wherein the impregnation solution comprises a silver-containing compound, an amine compound, water, an alkali metal auxiliary, an optional rhenium auxiliary and a co-auxiliary thereof; ii) impregnating the modified α-alumina carrier in the impregnation solution of step i), and performing calcination and activation after solid-liquid separation to obtain the silver catalyst.

24. Use of the modified α-alumina carrier according to claim 20 and / or the silver catalyst according to any one of claims 21 to 23 in the preparation of ethylene oxide by epoxidation of ethylene.

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