High efficiency silver catalysts for the production of ethylene oxide via ethylene oxidation

Abstract: This invention relates to a process of preparing silver-containing catalysts and their carriers for the production of ethylene oxide via ethylene oxidation and also to the applications of said catalysts in producing ethylene oxide . A commercial trihydrated α-alumina , boehmite , carbonaceous materials , a fluxing agent , fluoride and a binder are mixed with water , kneaded andextruded to form strips which are cut and shaped. The shaped bodies are then dried , calcined and converted to α-alumina bodies i.e.carriers. This process is characterized by using trihydrated α-alumina , boehmite alumina and carbonaceous materials which have a good matching of particle sizes and proportions in preparing alumina carriers with the following pore structure: specific surface area 0.2 - 2 m²/g pore volume > 0.5 ml/g pore radius > 30 µ, 25-10% of total volume < 30 µ, 75-90% of total volume Said alumina carriers are impregnated with silver compounds and promoters, and then dried , activated and used in ethylene oxidation for making ethylene oxide. The selectivity of the catalyst reaches from 83 to 84 percent.

Description: This invention relates to a process for preparing silver-containing catalysts and their carriers for the production of ethylene oxide via ethylene oxidation. While ethylene oxide is produced by ethylene oxidation, carbon dioxide is also formed by the side reaction. The reaction heat of the side reaction is twenty times that of the main reaction and the ethylene oxide produced in the main reaction will further be oxidised into carbon dioxide if it is not removed in time. In order to increase the activity of the catalysts, sufficient specific surface area of silver particles must be afforded. Therefore, the catalyst carriers are required to have enough specific surface area. However, oversized specific surface area will make the transfer of the reaction heat difficult, aggravate the side reaction and decrease the selectivity of the catalysts. In order to offer the catalysts a high selectivelty, an ideal pore structure which matches the surface of the catalyst is required so that suitable conditions for heat and mass transfer can be attained and the side reaction can be suppressed. Since the reaction takes place under nearly diffusion controlling conditions, searching for carriers with an optimum matching between pore structure and specific surface area has become an important subject in developing silver catalysts with a high selectivity. US-A-4,379,134 (& EP-A-0,058,419) discloses a process for the production of alpha-alumina bodies that are useful as catalyst supports having a specific surface area of less than one square meter per gram with at least 85 percent of the pore volume represented by pores having a diameter of from 1 to 20 micrometres (microns) (radius 0.5 to 10 micrometres), and not more than 10 percent of the pore volume represented by pores having a diameter from 20 to 100 micrometres (radius 10 to 50 micrometres) formed by incorporating carbonaceous material in the production of the alumina carriers. GB-A-1,465,523 discloses a method for producing shaped bodies of alumina for use in catalytic applications having a wide range of specific surface area from 0.1 to 60 m²/gm and a pore volume greater than 0.1 ml/gram represented by pores larger than 0.1 micrometres, exclusive of pores produced by burnout material. US-A-3,172,893 discloses a catalyst for the production of ethylene oxide via ethylene oxidation comprising silver coated pellets formed by bonding alumina particles of uniform relatively large diameter in the range 400-600 micrometres and pores having a diameter of from 80-200 micrometres (radius 40-100 micrometres). EP-A-0,244,895 discloses a silver catalyst for the oxidation of ethylene to ethylene oxide comprising alumina particles enriched with an alkali metal oxide or hydroxide promoter and a fluoride anion each present in an amount between 10 and 1000 parts by weight per million parts by weight of the total catalyst. EP-A-0,247,414 discloses a silver deposited catalyst for producing ethylene oxide by ethylene oxidation having an alumina carrier with a surface area of 0.6 to 2 m²/gm, a silica content of 0.5 to 12 percent by weight, and a sodium content of 0.08 to 2 percent by weight. The purpose of the present invention is to avoid the disadvantages arising from oversized or undersized pore structure and specific surface area and inadequate pore size distribution in the prior art, and to provide alumina carriers having a better matching specific surface area and pore structure, and to greatly increase the selectivity of the silver catalysts in commercial applications. The purpose of the invention is achieved by the process for producing a silver catalyst defined in Claim 1. According to the requirement of the pore formation, trihydrated α-alumina and boehmite alumina having different particle sizes are used as starting materials with carbonaceous material having matching particle sizes to provide the alumina carriers with the desired pore structure. Advantageously, the alumina carriers have a pore structure with a specific surface area of 0.8-1.3 m²/gm and a pore volume of 0.5-0.7 ml/gm. Unlike the process disclosed in US-A-4,379,134 which uses expensive boehmite alumina, the present invention uses a mixture of boehmite alumina and cheaper α-alumina, preferably commercially available trihydrated α-alumina having a low sodium content. The carbonaceous materials are used to make the large pores and include petroleum coke, carbon powder, graphite, polyethylene, rosin and mixtures thereof. The carbonaceous materials are carbonised and oxidised and become gas to escape during calcination. In this way, large pores are formed in the carriers which benefit the diffusion and heat transfer of the reaction gases in the catalysts. Although US-A-4,379,134 uses carbonaceous materials to make large pores with reference to US-A-3,726,811 and US-A-3,119,660, the fraction of the large pores having a diameter greater than 20 micrometres (radius 10 micrometres) does not exceed 10 percent of the total pore volume. The present invention on the other hand shows that, if the fraction of large pores having a radius greater than 30 micrometres (diameter 60 micrometres) is from 10 to 25 percent of the total pore volume, the selectivity of the catalyst will increase significantly. If excess carbonaceous materials are added, the strength of the alumina carriers will decrease and, according to a preferred feature of the invention, the added carbonaceous material ranges from 20 to 30 percent of the weight of the alumina. Fluxing agents can decrease the calcination temperature and make the alumina carriers have enough crush strength. The preferred fluxing agents used in the invention consist of magnesium nitrate, magnesium oxide and feldspar. The amount of the fluxing agent preferably ranges from 1.5 to 7 percent of the alumina by weight. The crush strength of the alumina carriers is preferably greater than 5 kg/particle based on a particle having an outer diameter of 6.5mm, an inner diameter of 2.5mm and a length of 6.5mm. Binders make crystal alumina disperse and bind during mixing to form an extrudable paste. The preferred binders used in the invention include nitric acid, alumina gel, propanoic acid, acetic acid and formic acid. The amount of added nitric acid or alumina gel preferably ranges from 25 to 60 percent of the alumina by weight. Fluorides make alumina easy to transform into crystals and the alumina is converted completely into α-alumina crystals during the calcination which benefits the elimination of unnecessary micropores. The preferred fluorides used in the invention consist of ammonium fluoride, hydrogen fluoride and aluminum fluoride. The amount of the added fluoride anions preferably ranges from 0.5 to 5.0 percent of the alumina by weight. After being extruded and shaped, the paste is preferably dried to a water content of less than 30 percent. The carrier bodies can take the shape of rings, balls, cylinders or perforated cylinders. The carrier bodies are preferably dried at 80°C to 120°C for a period of 1 to 24 hours which is controlled according to the water content. After being dried, the carrier bodies are preferably heated to a temperature from 1450°C to 1550°C and kept at the temperature for about 2 to 6 hours so that all the alumina is converted to α-alumina. Meanwhile, the fluoride anions can eliminate micropores to make the pores having the radius of less than 30 micrometres range mainly from 0.5 micrometres to 5 micrometres, and the pores having the radius of larger than 30 micrometres range from 25 to 10 percent of the total volume. Silver catalysts are prepared from the alumina bodies by impregnating them with solutions of silver compounds that are preferably sufficient to allow the silver to be supported on the carrier in an amount from 1-25% of the weight of the catalyst. The impregnated carriers are then separated from the solution and the silver compound is reduced to silver. When or before or after the silver compound is supported on the carriers, in addition to the silver compound, one or more promoters chosen from alkali metals such as potassium, rubidium and caesium or alkaline earth metals such as barium is/are supported on the carriers. According to a preferred method, silver oxalate is precipitated by mixing aqueous solutions of silver nitrate and ammonium oxalate. The precipitate is then filtered and washed to eliminate nitrate anions. The silver oxalate is then dissolved in an aqueous solution of ethyldiamine and diethanolamine and promoters are added to form an impregnation solution. The alumina carrier is impregnated with the impregnation solution and drained and heated in an air flow at a temperature from 550°C to 600°C for a duration to produce a silver catalyst containing 5 to 20 percent of silver by weight. Silver nitrate can be replaced by silver oxide in the above method. Alternatively, silver oxalate can directly form a complex with amines without filtration and then the alumina carriers are impregnated with the complex solution. Promoters of alkali metals such as potassium, rubidium and caesium are usually doped onto the catalyst in amounts (calculated on metal) of from 20, more preferably 40, to 1000 parts per million parts of the catalyst by weight, and promoters of alkaline earth metals such as barium are doped in amounts (calculated on metal) of less than 1000 parts per million parts of the catalyst by weight. The alkali metals deposited on the alumina carriers can be washed with absolute alcohol or absolute methanol. The concentration of the alkali metals can be controlled in a certain range because some of the alkali metals are cleaned away. All kinds of the silver catalysts prepared by the process of the invention are evaluated in a single tube reactor or micro reactor. The single tube reactor has a diameter of 21mm, a 7.07m catalyst bed height, and an effective volume of 2.5 litres. Constitution of the reacting Gas: C₂H₄ 12-25% (mole concentrations) O₂ 6.5%-7.5% CO₂ 6-1⊘% N₂ rest Dichloroethane ⊘.⊘5-1.⊘5 ppm Space Velocity 7⊘⊘⊘/hour Temperature 22⊘-245°C Pressure 21Kg/cm² Ethylene oxide in the Effluent 1.3-1.6% Time-Space Yield 17⊘-26⊘g Ethylene Oxide/l Cat. h The constitution of the inlet and exit gases of the reactor are measured continuously when the conditions described above are reached. The selectivity of the catalyst is calculated by the following equation after correcting the data for volume thrinkage. Wherein , ΔEO is the concentration difference of the ethylene oxide between exit and inlet gases. Once the termperature remains unchanged (between 22⊘° and 245°C ) with a constant conversion and the maximum selectity , the continous measurement is carried out again and the selectivity is calculated by averaging the data of more than 3⊘ groups. Compared with the prior arts , the present invention has the following advantage : the silver catalysts prepared by the process of the invention are particularly suitable for ethylene oxidization to produce ethylene oxide and has a selectivity of from 83 to 84 percent in the conditions disclosed above in industrial facilities. The invention will be further discribed in detail in the following examples: Example 1 77 kilograms of trihydrated α-alumina of less than 5⊘ mesh with low sodium content , 23 kilograms of boehmite alumina of less than 2⊘⊘ mesh , 1.7 kilograms of ammonium fluoride , 2⊘ kilograms of petroleum coke of 3⊘-2⊘⊘ mesh, 2 kilograms of magnesium nitrate were uniformly mixed in a mixer. 2⊘ kilograms of said mixture were placed into a kneading machine , to which 2.7 liters of dilute nitric acid (acid:water=1:3)and an adequate amount of water were added. The mixture was kneaded to form an extrudable blend. Said extrudable blend was then extruded to form a ring shaped bodies having an outside diameter of 6.5mm , a length of 6.5mm and a inner diameter of 2.5mm. Said ring shaped bodies were dried at a temperature from about 8⊘°C to 1⊘⊘°C for more than 2 hours to lower the water content of the bodies to below 25 percent and then calcined in a tunnel kiln, the temperature of which rose to 148⊘°C in 6 hours and remained constant for 6 hours. After the temperature of the tunnel kiln went down , pure alumina ring shaped bodies were obtained which had physical properties as follows: Crush strength kg/particle 5.5 Water absorption % 68 Bulk Density gm/cm³ ⊘.55 Specific surface area m²/g 1.1 pore volume ml/g ⊘.56 pore radius (micrometres) distribution 〈% of total volume〉 < ⊘.5 12.8 ⊘.5-1 36.3 1 -5 23.⊘ 5 -1⊘ 5.6 1⊘ -3⊘ 5.8 3⊘ -51 7.3 >51 9.2 A catalyst was prepared by the following steps: 2.8 kilograms of silver nitrate was dissolved into 3 litres of deionised water, and 1.12 kilograms of ammonium oxalate was dissolved into 11 litres of deionised water at 50°C. The two solutions were mixed and white silver oxalate was precipitated. Then the precipitate was filtered and washed with distilled water until the washings contained no nitrate anions. The filter cake contained about 50 percent of silver and about 30 percent of water. 1.2 kilograms of ethyldiamine, 0.4 kilograms of diethanolamine, 1.5 kilograms of deionised water were added into a container made of stainless steel with a stirrer to get a mixed solution. The filter cake of silver oxalate was added into the mixed solution with stirring at below 40°C to dissolve all silver oxalate. Then 4.5 grams of barium nitrate, 9.5 grams of caesium sulphate and about 1 kilogram of deionised water were added into the mixed solution to form an impregnating solution which had about 22 percent of silver, 300 ppm of barium and 900 ppm of caesium. 3 kilograms of the carriers were placed into a container which can be evacuated to below 10mm Hg. The impregnation solution was added, and the carriers immersed for 30 minutes and then the extra solution dripped away. The impregnated alumina carriers were heated in an air flow at a temperature of about 550°C for a minute and the resulting silver catalyst contained 15 percent of silver, 200 ppm of barium and 600 ppm of caesium. The catalysts were evaluated with a single tube reactor with an inner diameter of 21mm, a bed height of 7.07m and an effective volume of 2.45 litres. There is a jacket outside the reactor tube through which a heat conducting oil is passed to heat up the reactor or remove the heat of reaction. The conditions used for the evaluation were as follows: Constitution of the reaction gases: C₂H₄ 20% mole O₂ 7% mole CO₂ 8% mole N₂ rest Concentration of Ethylene Oxide in the effluent from the reactor 1.3-1.4% mole Reaction Pressure 21 Kg/cm² Space Velocity 7000/h Time-Space Yield 180-200g Ethylene Oxide/l Cat.h and controlling the dichloroethane content of the reaction gas and making the selectivity of the catalyst the highest. The selectivity of the catalysts prepared reaches 84.1% at a reaction temperature of 233°C. Example 2 Alumina ring shaped bodies, i.e. carriers, were prepared by means of the procedure similar to Example 1 except that 32 kilograms of graphite powder was substituted for petroleum coke and aluminium gel having about 10 percent of alumina was substituted for nitric acid for shaping. The alumina ring shaped bodies had physical properties as follows: Crush strength kg/particle 6.5 Water absorption rate % 72.0 Bulk density gm/cm² 0.5 Specific surface area m²/g 1.1 pore volume ml/g ⊘.66 pore radius (micrometres) distribution 〈% of total volume〉 < ⊘.5 7.3 ⊘.5-1 27.⊘ 1 -5 35.⊘ 5 -1⊘ 6.2 1⊘ -3⊘ 1.6 3⊘ -51 15.⊘ >51 7.9 A catalyst was prepared by means of the same procedure as in Example 1. The catalyst was evaluated by a single tube in the same conditions as Example 1. The temperature of the reaction remains 235°C and the selectivity of the catalyst is 83.9%. Example 4-8 Alumina ring shaped bodies and silver catalysts were prepared by the same procedure as Example 1. A variety of catalysts containing different amounts of barium were prepared by changing the barium content in the impregation solution. The catalysts were evaluated in microreactor with an inner diameter of 4mm , a 8cm height of the catalyst bed , an effective volume of 1 ml and catalyst sizes of from 11 to 18 mesh. The reaction gas mixtures were prepared in advance by mixing the concerned gases in a high pressure cylinder. The constitution of the reaction gas and the conditions for the evaluation were the same as Example 1. Dichloroethane, a restrainer in the reaction gas had a content of ⊘.4 ppm. The reaction temperature and the selectivity were measured. The results are shown in Table 1. Examples Ba in Impregnation solution ppm Ag in catalyst % Ba in catalyst ppm Evaluation Temperature °C Results ΔEO % S % 4 ⊘ 15.6 ⊘ 222 1.39 82.1 5 3⊘⊘ 15.9 13⊘ 227 1.38 83.9 6 5⊘⊘ 15.4 35⊘ 224 1.38 83.2 7 7⊘⊘ 15.3 49⊘ 226 1.38 83.⊘ 8 9⊘⊘ 14.9 61⊘ 226 1.38 82.7 Comparative Example 3 Alumina bodies were prepared with the same prescription as Example 2 and by the same method as Example 1 except that the amount of added graphite was 2/5 of the amount used in Example 2. The physical properties of the alumina bodies prepared were as follows: Crush strength kg/particle 9.3 Water absorption rate % 69.⊘ Bulk Density gm/cm³ ⊘.5⊘ specific surface area m²/g 1.⊘6 pore volume ml/g ⊘.63 pore radius (micrometres) distribution 〈% of total volume〉 < ⊘.5 8.5 ⊘.5-1 12.5 1 -5 68.⊘ 5 -1⊘ 7.5 1⊘ -3⊘ ⊘.2 3⊘ -51 1.2 >51 1.8 A catalyst was prepared by the same method as Example 1. The catalyst was evaluated in a single tube in the same conditions as Example 1. The reaction took place at a temperature of 237°C. The selectivity of the catalyst was 77.7 percent. Examples 9-11 Catalysts and alumina bodies were prepared in the same way as Example 1 except that the cesium content in impregnation solutions was changed and the evaluation of the catalysts was done in the same way as Example 4-8. the results are shown in Table 2. Examples Cs in Impregnation solution ppm Ag in catalyst % Cs in catalyst ppm Evaluation Temperature °C Results ΔEO % S % 9 5⊘⊘ 12.3 28⊘ 231 1.39 83.3 1⊘ 7⊘⊘ 12.7 4⊘⊘ 236 1.39 84.4 11 9⊘⊘ 13.3 54⊘ 243 1.37 83.9 While the invention has been described in connection with what is presently considered to be the most practical and perferred embodiments , it is understood that the invention is not to be limited to the disclosed embodiments , but on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims: A process for preparing a silver-containing catalyst for the production of ethylene oxide via ethylene oxidation, characterised by the steps of:- (a) preparing a mixture of:- (i) trihydrated α-alumina with a particle size of less than 50 mesh and boehmite alumina with a particle size of less than 200 mesh in proportions of from 1:1 to 9:1 based upon the weight of alumina, (ii) a carbonaceous material having a particle size of from 20 to 200 mesh in an amount of from 10 to 40 percent of alumina by weight, (iii)a fluxing agent, (iv) a fluoride, (v) a binder, and (vi) water; (b) extruding the mixture to form a shaped body; (c) drying and calcining the shaped body to convert it to an alumina carrier having the following pore structure:- Specific surface area 0.2-2m²/g Pore volume > 0.5 ml/g Pore radius Percent of the total volume < 30 micrometres 75-90 > 30 micrometres 25-10 provided that more than 10 percent of the total volume of the pore structure has a pore radius greater than 10 micrometres; (d) impregnating said alumina carrier with a solution of a silver compound and, before, during or after that impregnation, a promoter; and (e) reducing and activating said silver impregnated carrier. A process according to Claim 1 characterised in that the carrier has a specific surface area of 0.8-1.3 m²/g and a pore volume of 0.5-0.7 ml/g. A process according to Claim 1 or Claim 2 characterised in that the amount of carbonaceous material is 20 to 30 percent of alumina by weight. A process according to any one of the preceding Claims characterised in that the trihydrated α-alumina has a low sodium content. A process according to any one of the preceding Claims characterised in that the carbonaceous material is selected from the group consisting of petroleum coke, carbon powder, graphite, polyethylene, rosin and mixtures thereof. A process according to any one of the preceding Claims characterised in that the amount of fluxing agent is from 1 to 7 percent of alumina by weight. A process according to any one of the preceding Claims characterised in that the fluxing agent is selected from the group consisting of magnesium nitrate, magnesium oxide and feldspar. A process according to any one of the preceding Claims characterised in that the amount of fluoride is from 0.5 to 5 percent of the weight of the alumina calculated on the fluoride anions. A process according to any one of the preceding Claims characterised in that the fluoride is selected from the group consisting of aluminium fluoride, ammonium fluoride and hydrogen fluoride. A process according to any one of the preceding Claims characterised in that the amount of binder is from 25 to 60 percent of the alumina by weight. A process according to any one of the preceding Claims characterised in that the binder is selected from the group consisting of dilute nitric acid, alumina gel, propanoic acid, acetic acid and formic acid. A process according to any one of the preceding Claims characterised in that the shaped body is a ring, cylinder, ball or perforated cylinder. A process according to any one of the preceding Claims characterised in that the shaped body is dried to a water content of less than 30 percent prior to calcining. A process according to Claim 13 characterised in that the shaped body is dried at a temperature in the range 80°C to 120°C for a period of 1 to 24 hours. A process according to any one of the preceding Claims characterised in that the shaped body is calcined at a temperature in the range 1450°C to 1550°C for a period of 2 to 6 hours. A process according to any one of the preceding Claims characterised in that the amount of silver content is from 1 to 25 percent of the weight of the catalyst and, more preferably, from 5 to 20 percent of the weight of the catalyst. A process according to any one of the preceding Claims characterised in that the alumina carrier is impregnated by a solution of the silver compound and the promoter. A process according to any one of Claims 1 to 16 characterised in that the impregnation with the promoter is carried out after the reduction of the silver impregnated carrier. A process according to any one of the preceding Claims characterized in that the promoter is at least one alkali metal and/or alkaline earth metal. A process according to any one of the preceding Claims characterized in that the promoter is an alkali metal in an amount (calculated on metal) from 20, preferably 40, to 1000 parts per million parts of catalyst by weight. A process according to Claim 19 or Claim 20 characterised in that the alkali metal is selected from the group consisting of potassium, rubidium and caesium. A process according to any one of Claims 1 to 19 characterised in that the promoter is an alkaline earth metal in an amount (calculated on metal) less than 1000 parts per million parts of catalyst by weight. A process according to Claim 22 characterised in that the alkaline earth metal is barium. A silver catalyst made by the process according to any one of the preceding Claims. A silver catalyst according to Claim 24 characterised in that the ratio of the percent of the total volume of the pore structure having a pore radius not greater than 10 micrometres to that having a pore radius not less than 10 micrometres is approximately 3:1. Use of the silver catalyst according to Claim 24 or Claim 25 for the production of ethylene via ethylene oxidation.

Inventors: JIN GUOQUAN RESEARCH INSTITUTE, JIN JIQUAN RESEARCH INSTITUTE, LUO GUOCHUN TIANJIN CHEMICAL R, SHANG LIANDI TIANJIN CHEMICAL, XU YONG RESEARCH INSTITUTE OF, JIN, GUOQUAN RESEARCH INSTITUTE OF BEIJING YANSHAN, JIN, JIQUAN RESEARCH INSTITUTE OF BEIJING YANSHAN, LUO, GUOCHUN TIANJIN CHEMICAL RESEARCH INSTITUTE, SHANG, LIANDI TIANJIN CHEMICAL RESEARCH INSTITUTE, XU, YONG RESEARCH INSTITUTE OF BEIJING YANSHAN
Applicants: BEIJING RES INST YANSHAN PETRO, CHINA PETROCHEMICAL CORP, CHINA PETROCHEMICAL CORPORATION, RESEARCH INSTITUTE OF BEIJING YANSHAN PETROCHEMICAL CORPORATION
Application year: 1989
Main classification: C07D301
IPC codes: C07D303, B01J37, C07D301, B01J23, B01J27, C07B61