Silver catalysts, process for their production and their use in the production of ethylene oxide.

The invention relates to silver catalysts which consist of silver and a promoter on a heat-resistant, porous support material. It also relates to a process for the preparation of these catalysts and their use for the production of ethylene oxide by oxidation of ethylene with oxygen.

Ethylene oxide is produced on an industrial scale by direct oxidation of ethylene with oxygen over a silver catalyst. A general description of the process can be found in Kirk-Othmer: Encyclopedia of Chemical Technology, Volume 9, pages 432 to 471, John Wiley, London-NY, 1980.

An ethylene and oxygen-containing gas enters the top of the reactor. This consists of a bundle of several thousand pipes from 6 to over 10 m in length. The gas flows over the catalyst located in the tubes, preferably at a temperature of 200 to 300 ° C. and 1 to 2 MPa overpressure, with 5 to 20% of the ethylene used being converted.

In addition to the formation of ethylene oxide, a significant proportion of the ethylene, namely about 25%, is oxidized to carbon dioxide and water (total oxidation), which the following reaction equations are intended to illustrate:<ul id="ul0001" list-style="none"><li>Ethylene oxide formation<chemistry id="chem0001" num="0001"><img file="EP0097935A2_D0001.tif" /></chemistry></li><li>Total oxidation<chemistry id="chem0002" num="0002"><img file="EP0097935A2_D0002.tif" /></chemistry></li></ul>

To control the temperature and dissipate the heat, there is a heat transfer medium around the tubes, which transports the heat released out of the reactor. The reaction gas containing ethylene oxide and carbon dioxide leaves the reactor to enter the work-up section where ethylene oxide and carbon dioxide are separated. The gas, which has been freed of ethylene oxide and carbon dioxide, is again enriched with ethylene and oxygen and conveyed back to the reactor. It is a continuous cycle with heterogeneous catalysis.

A supported silver catalyst serves as the catalyst. The quality of such a catalyst is essentially characterized by its selectivity, its activity and its service life.

Selectivity is the molar percentage of converted ethylene that reacts to form ethylene oxide. The activity is characterized by the ethylene oxide concentration at the reactor outlet under otherwise constant conditions (eg temperature, pressure, amount of gas, amount of catalyst). The higher the ethylene oxide concentration, the higher the activity. In other words, the lower the temperature required to reach a certain ethylene oxide concentration, the higher the activity.

In addition to the activity and selectivity, the life of the catalyst is an important quality feature. The effectiveness of the catalysts deteriorates with the progress of use, that is, activity and selectivity decrease continuously, so that depending on the rate of aging, a catalyst change must be carried out sooner or later. However, such a catalyst replacement is cumbersome, expensive and involves loss of material. The old catalyst has to be emptied tube by tube from several thousand reactor tubes and then the new catalyst has to be refilled. This takes a few weeks and there is a corresponding loss of production. The old catalyst must also be refurbished to recover the silver. This inevitably leads to a loss of silver. The carrier material cannot be reused. Even when making a new one_SSilver catalyst losses occur. Large quantities of silver have to be transported, insured, interest and possibly duty paid. A good silver catalyst for the production of ethylene oxide by direct oxidation of ethylene should therefore have the highest possible activity and selectivity and also a long service life.

The production of supported silver catalysts per se has long been known. They are preferably produced using the following process:<ul id="ul0002" list-style="none"><li>The carrier material is soaked with a solution of a silver salt.</li><li>The impregnated carrier material is dried, the silver salt being deposited on the carrier material.</li><li>The support material thus impregnated with silver salt is subjected to conditions under which the silver salt decomposes, whereby elemental silver is formed, which is present in finely divided form on the support. This step can be done purely thermally by heating to about 170 to 400 ° C or by reducing agents such as formaldehyde.</li></ul>

It was recognized early on that the effectiveness of silver catalysts can be improved by so-called promoters. Such promoters are chemical elements that already contribute to an increase in effectiveness in an amount of 10 to 1000 mg / kg, based on the amount of silver. The elements potassium, rubidium and cesium have proven to be particularly effective promoters. With good activity, selectivities of around 80% can be achieved.

More recently, it has been found that not only the promoter itself, but also the manner in which it is applied to the substrate, namely the time of its application with respect to the application of silver, is important in the production of new supported silver catalysts Role play. It also seems to be of some importance when the reduction of the applied silver compound to metallic silver takes place.

According to German Offenlegungsschrift 24 48 449, it should be advantageous to first deposit the entire amount of promoter and only then the entire amount of silver on the support, the reduction of the deposited silver compound to metallic silver then being carried out. In contrast, in German Offenlegungsschriften 27 33 688 and 30 11 717 and in British Patent 20 02 252 it is recommended that, in the production of new silver supported catalysts with improved effectiveness, the total amount of silver in a first impregnation and in a subsequent second impregnation the total amount of promoter upset the reduction of the applied silver compound after the first impregnation is carried out under partly special conditions. In addition to these two procedures, the method of simultaneously applying silver and promoter to the carrier material is also known. This type of production of new silver catalysts with increased effectiveness is described in German Offenlegungsschriften 23 00 512, 27 34 912, 29 51 969 and 29 51 970.

The silver catalyst known from German Offenlegungsschrift 23 00 512 consists of silver in an amount of 1.5 to 20% by weight and potassium, rubidium and / or cesium as a promoter in an amount of 0.001 to 0.05% by weight a heat-resistant, porous support material, percentages by weight based in each case on the weight of the catalyst (total weight of the catalyst), the silver and the promoter being applied to the support at the same time. It is applied by impregnating the carrier material with appropriate impregnation solutions. The silver compound on the carrier material is thermally converted (reduced) into metallic silver.

This silver catalyst has a relatively good selectivity due to the potassium, rubidium and cesium promoters used. Nevertheless, further attempts have been made to use the simultaneous method_Aseparating these promoters with the silver on the support material to achieve a further increase in selectivity and an improvement in the other important properties, such as in particular activity and lifetime. For this purpose, a special impregnation solution is used in the aforementioned German Offenlegungsschriften 27 34 912 and 29 51 969 and the impregnated and activated catalyst is treated in a mill in accordance with the aforementioned German laid-open specification 29 51 970 in order to remove 1 to 10% by weight from its surface , which should also achieve a long service life.

Finally, in a number of publications, for example in German Offenlegungsschriften 24 54 972, 26 40 540, 27 23 918, 28 19 595 and 28 20 170, it is found that new silver-supported catalysts with increased effectiveness are obtained if, in addition to the Silver applies several special promoter metals to the carrier material, it being irrelevant in which order silver and promoter are deposited on the carrier material.

The prior art on the procedure for applying silver and promoter to the support material and in the reduction of the applied silver compound to metallic silver shows that several methods are already known which are said to lead to effective supported silver catalysts. However, there is still a need for new silver catalysts with good properties, in particular for silver catalysts which have high activity, selectivity and service life, especially since the known production processes sometimes require special impregnations, special reductions and / or subsequent special treatments of the finished catalyst.

It has now surprisingly been found that supported silver catalysts with high activity, selectivity and life span are obtained if the simultaneous application of silver and promoter in the required amounts to the support material does not take place at once, but in two stages and in each stage a very specific subset is applied to silver and promoter. It was not to be expected that such a high effect would be achieved with this variant of simultaneous application of silver and promoter, especially in German Offenlegungsschrift 29 51 970 under repeated impregnation of the support _Vno importance is attached to the use of appropriate impregnation solutions for the simultaneous application of the required amount of silver and promoter and an increase in the effectiveness of the catalyst is allegedly only achieved with the after-treatment of the catalyst described there to remove its surface skin.

It has also been found that silver catalysts with a high activity, selectivity and lifespan are surprisingly obtained even when the application of silver and potassium, rubidium and / or cesium as a promoter on the support material is carried out in such a way that the total amount of silver and only part of the total amount of promoter and then the remaining amount (to the total amount) of promoter is applied.

The silver catalyst according to the invention, consisting of silver in an amount of 3 to 20 wt .-% and potassium, rubidium and / or cesium as a promoter in an amount of 0.003 to 0.05 wt .-% on a heat-resistant, porous support material, weight percentages in each case based on the weight of the catalyst (weight of the finished catalyst or total weight of the catalyst), wherein the silver and the promoter have been applied to the carrier material using impregnation solutions and the applied silver compound has been reduced to metallic silver, characterized in that the application of silver and promoter and the reduction have been carried out according to one of the two methods below :<ul id="ul0003" list-style="none"><li>a) simultaneous application of 55 to 85% by weight of the total amount of silver and 15 to 45% by weight of the total amount of promoter in a first impregnation,</li><li>b) drying the product obtained in step a),</li><li>c) simultaneous application of the rest of the total amount of silver and amount of promoter to the product obtained in step b) in a second impregnation and</li><li>d) heating the product obtained in step c) to reduce the applied silver compound to metallic silver; or</li><li>a ') simultaneous or sequential application of the total amount of silver and 15 to 45% by weight of the total amount of promoter in one or in two impregnations,</li><li>b ') heating the product obtained in step a') to reduce the applied silver compound to metallic silver,</li><li>c ') applying the rest of the total amount of promoter to the product obtained in step b') in a further impregnation and</li><li>d ') drying the product obtained in step c').</li></ul>

The process according to the invention for producing a silver catalyst, wherein the silver and the promoter are applied to the support material using impregnation solutions and the applied silver compound is reduced to metallic silver, is characterized in that it is carried out by one of the two methods below:<ul id="ul0004" list-style="none"><li>a) simultaneous application of 55 to 85% by weight of the total amount of silver and 15 to 45% by weight of the total amount of promoter in a first impregnation,</li><li>b) drying the product obtained in step a),</li><li>c) simultaneous application of the rest of the total amount of silver and amount of promoter to the product obtained in step b) in a second impregnation and</li><li>d) heating the product obtained in step c) to reduce the applied silver compound to metallic silver; or</li><li>a ') simultaneous or sequential application of the total amount of silver and 15 to 45% by weight of the total amount of promoter in one or in two impregnations,</li><li>b ') heating the product obtained in step a') to reduce the applied silver compound to metallic silver,</li><li>c ') applying the rest of the total amount of promoter to the product obtained in step b') in a further impregnation and</li><li>d ') drying the product obtained in step c').</li></ul>

The amount of silver is preferably 7 to 14 wt .-% and the amount of promoter 0.008 to 0.035 wt .-%, each based on the weight of the catalyst.

The promoter is preferably cesium. In the first method according to the invention, preferably 60 to 75% by weight of the total amount of silver and 25 to 40% by weight of the total amount of promoter are applied in step a) and the rest of the total amount of silver and promoter in step c). In the second method according to the invention, 20 to 35% by weight of the total amount of promoter is preferably applied in step a ').

The usual, commercially available heat-resistant and porous materials are suitable as the support material for the silver catalysts according to the invention. These are inert even in the presence of the reaction conditions prevailing in the oxidation of ethylene and the chemical compounds present. The support material for producing the silver catalysts according to the invention is not critical; suitable supports are, for example, carbon, corundum, silicon carbide, silicon dioxide, aluminum oxide and mixtures of aluminum oxide and silicon dioxide. Α-Alumina is preferred because it has a largely uniform pore diameter. It has a specific surface area of ​​0.1 to 1 m^Z./ g, preferably 0.2 to 0.6 m²/ g (measured according to the known BET method), a specific pore volume of 0.1 to 1 cm³/ g, preferably 0.2 to 0.6 cm³/ g (measured by the known mercury or water adsorption method), an apparent porosity of 20 to 70% by volume, preferably 40 to 60% by volume (measured by the known mercury or water adsorption method), an average pore diameter of 0, 3 to 15 µm, preferably 1 to 10 µm, and a percentage of pores with a diameter of 0.03 to 10 μm of at least 50% by weight (pore diameter and pore diameter distribution are known to be determined from the specific surface area and the apparent porosity).

The carrier material is advantageously used in the form of granules, spheres, rings, pellets and the like. Examples of preferred support materials containing a-alumina or a-alumina are the types offered by Norton Company with the designations SA 5551 and SA 5552 or the SAHM types from United Catalyst.

The application of silver and promoter to the carrier material according to the invention takes place with the aid of the known impregnation process. The carrier material is then brought into contact with solutions, preferably soaked by immersion, which consist of a solvent and an amount of silver compound and / or promoter compound which is sufficient for the silver deposition and for the promoter deposition on the carrier material, whereupon the carrier material is separated from the excess solution and is dried.

According to the first method according to the invention, the carrier material is first impregnated in such a way that 55 to 85% by weight, preferably 60 to 75% by weight, of the total amount of silver and 15 to 45% by weight, preferably 25 to 40% by weight. % of the total amount of promoter applied. After this first impregnation, the rest (to the respective total amount) of silver and promoter is applied to the dried carrier material by a second impregnation. The solutions for the two impregnation steps according to the invention therefore essentially consist of a solvent and a sufficient amount of silver compound and promoter compound.

According to the second method according to the invention, the carrier material is impregnated in step a ') in such a way that the total amount of silver, but only 15 to 45% by weight, preferably 20 to 35% by weight, of the total amount of promoter is applied. This can be achieved by impregnating the support material first with a solution with at least one silver compound and then with a solution with at least one promoter compound (variant 1) or with a solution which contains at least one silver compound and at least one promoter compound at the same time (simultaneous deposition of silver and promoter compound, variant 2). The solutions for the impregnation step a ') accordingly consist essentially of a solvent and a sufficient amount of silver compound and promoter compound. Of the two variants of step a '), the simultaneous deposition of silver and promoter compound is preferred. Depending on the carrier material used, the expedient concentrations of silver compound and promoter compound in the solutions can be easily and quickly determined by preliminary tests and analytical determination of the amounts of the compounds actually deposited.

The impregnation can be carried out according to one of the usual methods. They are expediently carried out by soaking (immersing, pouring over) the support material with the impregnation solution in a vessel, the solution penetrating into the pores of the support material by absorption and / or by capillary action and separating the silver compound and promoter compound. The excess impregnation solution is then separated off (e.g. by pouring, draining,_Abfiltration or centrifugation), after which the soaked carrier material is dried. The amount of impregnation solution is generally chosen so that there is an excess in volume, based on the volume of substrate material to be impregnated. In general, 0.5 to 3 times, preferably 1 to 2 times the volume of impregnating liquid, based on the volume of carrier material, is used. The impregnation time, that is the time in which the carrier material remains in contact with the impregnation liquid, is clearly to be chosen so that the required amount of silver compound and promoter compound to be deposited is applied to the carrier. It is generally 5 to 60 minutes and depends in particular on the concentration of the impregnating solution of silver and promoter compound, on the carrier material used and on its respective absorbency. The temperature used during impregnation can vary within wide limits. It is generally impregnated at room temperature. To accelerate the impregnation process, higher temperatures can also be used. The impregnation temperature is therefore usually 15 to 80 ° C, preferably 20 to 50 ° C. The impregnation is usually carried out at atmospheric pressure.

The products (impregnated carrier materials) obtained from the impregnations are generally dried at a temperature of 20 to 150 ° C., preferably 50 to 120 ° C. Evaporation of the solvent can be carried out, for example, using tray dryers, rotary kilns or by passing hot inert gases, such as nitrogen and / or carbon dioxide. The temperature clearly depends on the boiling point of the solvent in the impregnation liquid.

The silver compound deposited on the carrier material is reduced to metallic silver by heating. A temperature of 170 to 400 ° C., preferably 200 to 350 ° C., is generally required for the thermal reduction of the silver compounds. The heating to these temperatures can be carried out, for example, in a tray dryer, rotary tube oven, electrically heated tube or by passing a correspondingly heated inert gas, such as air, nitrogen, carbon dioxide or a mixture thereof. The conversion of the silver compound into metallic silver can also be carried out using superheated steam. The heating time required at the temperatures mentioned is generally 0.2 to 5 hours, preferably this time should be 0.3 to 1 hour. The decomposition of the silver compound forms a firmly adhering precipitate of metallic silver particles on the carrier material (the promoter compounds are not reduced to the corresponding metal, the alkali metals potassium, rubidium and cesium are therefore essentially in the form of their cations and not as free alkali metal). The silver (the silver particles) is usually in the form of firmly adhering, essentially uniformly distributed, non-contiguous, discrete particles with a diameter of less than 1.5 μm. In general, the silver particles have a diameter of 0.1 to 1 μm and an average diameter of 0.2 to 0.7 μm.

Suitable silver compounds, silver complexing agents, promoter compounds and suitable solvents and suitable impregnation solutions are specified below.

Silver salts are preferably used as silver compounds. Suitable inorganic salts are, for example, silver nitrate and silver carbonate. Suitable organic salts are, for example, those of mono- or polybasic carboxylic acids and hydroxycarboxylic acids with up to 6 carbon atoms, such as silver acetate, silver lactate and silver oxalate. The silver compounds are water-soluble and can be decomposed to metallic silver in the heat.

It is advantageous to use a silver complexing agent in order to increase the solubility of the silver compound. Such complexing agents are known to be ammonia, amino acids and / or amines, such as alkylenediamines with 2 to 4 carbon atoms, for example ethylenediamine, alkanolamines with 2 to 4 carbon atoms, for example ethanolamine, mono-, di- and trialkylamines with 1 to 4 carbon atoms (in the alkyl group) , for example methylamine, isopropylamine, isobutylamine and secondary butylamine, and polyamines. Among the complexing agents mentioned, the alkylamines are preferred, preferably the monoalkylamines having 1 to 4 carbon atoms. The complexing agents are added in an amount sufficient to convert the silver compound quantitatively into the silver amino complex. A small molar excess of amino groups is usually necessary for this. Since a silver cation binds 2 amino groups, the use of at least 2 amino groups per mol of silver cation is necessary.

Salts, hydroxides and oxides of the metals mentioned potassium, rubidium and cesium are suitable as promoter compounds. The salts are preferred, although it is not critical which anion is present in the salt. Nitrate, chloride, carbonate and phosphate may be mentioned as inorganic salts. Organic salts which may be mentioned are those of mono- and polybasic carboxylic acids and hydroxycarboxylic acids having up to 6 carbon atoms, for example the formate, acetate, oxalate, citrate and lactate. Like the silver compounds, the promoter compounds are water-soluble.

Suitable solvents for the silver and promoter compounds are water, aliphatic alcohols with 1 to 5, preferably 1 to 3 carbon atoms, such as methanol, ethanol, propanol and isopropanol, aliphatic ketones with 3 to 5 carbon atoms, such as acetone, aliphatic and cyclic ethers, such as Diethyl ether, methyl ethyl ether, dipropyl ether and dioxane, esters, such as methyl acetate and ethyl acetate, acid amides, such as dimethylformamide, and nitriles, such as acetonitrile, and mixtures thereof. In the presence of silver compounds and silver and promoter compounds, water is preferred, and in the case of the presence of promoter compounds alone, the aliphatic alcohols having 1 to 3 carbon atoms (with possibly a small amount of water as solubilizer) are preferred as solvents.

For step a) of the catalyst preparation according to the invention, a suitable impregnation solution consists essentially of (1) a (water-soluble and heat-decomposable) silver salt in an amount of 30 to 40% by weight, (2) at least one (water-soluble) promoter salt, preferably a cesium salt , in an amount of 0.02 to 0.04% by weight, (3) water in an amount of 20 to 30% by weight, and (4) a monoalkylamine having 1 to 4 carbon atoms as a complexing agent for the silver salt in an amount of 30 to 40% by weight, percentages by weight based on the weight of the solution. For step c) it essentially consists of (1) a (water-soluble and heat-decomposable) silver salt in an amount of 30 to 40% by weight, (2) at least one (water-soluble) promoter salt, preferably a cesium salt, in an amount of 0.045 to 0.07% by weight, (3) water in an amount of 20 to 30% by weight and (4) a monoalkylamine with 1 to 4 carbon atoms as complexing agent for the silver salt in an amount of 30 to 40% by weight %, Percentages by weight based on the weight of the solution. (It goes without saying that the specific concentration of silver salt and promoter salt in the two impregnation solutions depends on the amount of silver and promoter that is to be applied to the support.) The product obtained in step c) is not necessary Before it is treated according to step d), dry it specially. The solvent is evaporated anyway when it is heated to the reduction temperature in step d).

The following impregnation solutions are expediently used for step a ') of the catalyst preparation according to the invention: The solution for applying the silver to the carrier material according to variant 1 essentially consists of (1) a (water-soluble and heat-decomposable) silver salt in an amount of 30 to 40% by weight, (2) water in an amount of 20 to 30% by weight .-%, and (3) a monoalkylamine having 1 to 4 carbon atoms as a complexing agent for the silver salt in an amount of 30 to 40 wt .-%; the solution for applying the promoter according to variant 1 essentially consists of (1) at least one (water-soluble and / or alcohol-soluble) promoter salt, preferably one. Cesium salt in an amount of 0.01 to 0.03% by weight, (2) water in an amount of 0 to 5% by weight, and (3) an aliphatic alcohol having 1 to 3 carbon atoms as a residual amount, that is Percentage to 100% by weight; the solution for the simultaneous application of silver and promoter compound according to variant 2 of step a ') essentially consists of (1) a (water-soluble and heat-decomposable) silver salt in an amount of 30 to 40% by weight, (2) at least one ( water-soluble) promoter salt, preferably a cesium salt, in an amount of 0.01 to 0.03% by weight, (3) water in an amount of 20 to 30% by weight, and (4) a monoalkylamine having 1 to 4 carbon atoms as a complexing agent for the silver salt in an amount of 30 to 40% by weight, percentages by weight based on the weight of the solution. A suitable impregnation solution for step c ') essentially consists of (1) at least one (water-soluble and / or alcohol-soluble) promoter salt, preferably a cesium salt, in an amount of 0.04 to 0.08% by weight, (2) Water in an amount of 0 to 5% by weight, and (3) an aliphatic alcohol with 1 to 3 carbon atoms as a residual amount, that is a percentage to 100% by weight, percentages by weight based on the weight of the solution. (It goes without saying that the specific concentration of silver salt and promoter salt in the impregnation solutions depends on the amount of silver and promoter that one wants to apply to the support.)

In variant 1 of step a '), intermediate drying is expediently carried out, that is to say that drying is carried out after the impregnation with the solution of the silver compound and before the impregnation with the promoter compound. It is not necessary to subject the product obtained in step a ') to its own drying before it is treated in step b'). The evaporation of the solvent from the support material can namely also take place in step b '), in which heating is carried out to higher temperatures anyway.

In the second method of the process according to the invention, it has proven advantageous in some cases to subject the silver catalyst obtained after steps a ') to d') to a laundry treatment in which the catalyst is contacted with a solvent in which the Carrier-applied promoter compound is soluble. The laundry treatment is carried out especially when the finished catalyst contains an excessive amount of promoter compound. The excess amount can be removed in a simple manner by washing. The laundry treatment can also be carried out in order to quickly and easily produce a certain optimal alkali metal concentration in the finished catalyst.

Water, aliphatic alcohols having 1 to 4 carbon atoms or mixtures thereof are preferably used as the washing liquid. A particularly preferred washing liquid consists of methanol, ethanol, propanol and / or isopropanol and water in an amount of 0 to 20% by weight, based on the weight of the washing liquid.

The washing itself can be carried out using the procedures known per se, the catalyst being brought into contact with the washing liquid and then separated from it and dried. According to a preferred method of operation, the washing is carried out by showering the catalyst with the washing liquid in a vessel and then separating it from it by centrifuging, filtering, suction filtering or simply pouring (decanting). The time that the catalyst is left in contact with the wash liquid is not critical per se. It depends primarily on the amount of promoter compound to be removed and is generally 0.2 to 20 minutes, preferably 0.5 to 5 minutes. As a rule, it is sufficient to carry out the washing process once. However, it may be advantageous to carry out batch washing by washing the catalyst 2 to 5 times, advantageously 2 to 3 times, preferably using a fresh (new, not yet used) washing liquid in each case. The amount of washing liquid (for one or more washes) depends on the amount of catalyst to be washed and is of course to be dimensioned such that the catalyst comes into contact with the liquid, if necessary with stirring. It is expediently (in parts by volume) at least about 1/3 of the amount of catalyst (in parts by volume), preferably about 1 to 3 times the amount of washing liquid is used. The temperature and pressure during washing is not critical; it can be washed without pressure or under pressure. In general, the washing is carried out at a temperature of 15 to 80 ° C, preferably 20 to 50 ° C.

Drying of the washed catalyst can be achieved, for example, with the aid of an inert gas such as nitrogen, carbon dioxide, air or mixtures thereof and / or by heating the catalyst, under which vacuum can also be used to accelerate the drying process. The temperature at which drying is carried out is not critical. It suitably depends on the boiling point of the washing liquid used. Appropriate drying temperatures are 20 to 150 ° C, preferably 50 to 120 ° C.

In the silver catalyst according to the invention, the silver is distributed essentially uniformly on the inner and outer surfaces of the support material. The alkali metal (the alkali metal compound), however, is unevenly (asymmetrically) distributed. A higher promoter concentration is present in the outer layers of the support body than in the inner layers. The promoter is thus in the form of a concentration gradient such that a higher promoter concentration is present on the outside in each support body than on the inside.

The silver catalyst according to the invention has high activity and selectivity and a long service life. Due to its long lifespan, it maintains its high effectiveness over a relatively long period of use. This has the further advantage that, in the silver catalyst according to the invention, an exchange or a regeneration is only necessary after relatively large time intervals.

The process according to the invention for the production of silver catalysts is simple and easy to carry out. It does not contain any complicated or time-consuming process steps, and no special impregnation solutions are required either.

The conditions to be used when using the new silver catalyst according to the invention, such as temperature, pressure, residence time, diluent, brake substances for controlling the catalytic oxidation of ethylene with oxygen, recycling, process engineering measures for increasing the ethylene oxide yield and the like, are known per se. The reaction temperature is generally 150 to 400 ° C, preferably 200 to 300 ° C, the reaction pressure 0.15 to 3 MPa, preferably 1 to 2 MPa. The feed mixture used generally contains 5 to 30 mol% ethylene, 3 to 15 Mcl% oxygen and the remainder inert gases such as nitrogen, carbon dioxide, water vapor, methane, ethane, argon and the like, and vinyl chloride, 1,2 dichloroethane and the like as brake substances. The ethylene oxide is isolated in the customary manner from the reaction product and the gas mixture is possibly purified as usual and recycled again.

According to a preferred embodiment of the use of the silver catalysts according to the invention, ethylene oxide is produced by oxidation of ethylene with a gas mixture containing about 8.5% by weight of oxygen at a temperature of 200 to 270 ° C. in the presence of the new silver catalyst.

The invention will now be explained in more detail using examples.

Examples 1 to 4 relate to the first method and Examples 5 to 8 relate to the second method of the catalyst preparation according to the invention.

example 1

A solution was prepared from a catalyst according to the invention<ul id="ul0005" list-style="none"><li>13.03 g (37.9441% by weight) of silver nitrate</li><li>9.10 g (26.4997 wt%) distilled water</li><li>0.01 g (0.0291% by weight) of cesium nitrate</li><li>12.20 g (35.5271% by weight) of isobutylamine</li></ul>prepares.

The carrier material SA 5552 from Norton Company, namely a-aluminum oxide in the form of cylinders with a specific surface area of ​​0.3 m, was added to this solution²/ g, fully immersed for 15 minutes at room temperature. After the excess impregnation solution had been drained off over a sieve, the moist carrier material was dried for 30 minutes at 105 ° C. in an air-nitrogen atmosphere (process steps a) and b).

After the semi-finished catalyst had cooled, it was mixed with a solution consisting of<ul id="ul0006" list-style="none"><li>13.03 g (36.6557% by weight) of silver nitrate</li><li>10.00 g (28.1318% by weight) of distilled water</li><li>0.017 g (0.0478% by weight) of cesium nitrate</li><li>12.50 g (35.1647% by weight) of isobutylamine</li></ul>again impregnated and dried as described above (process step c).

Drying was followed by a half-hour reduction in a rotary kiln preheated to 300 ° C; an air-nitrogen mixture was passed through the furnace (process step d). A silver catalyst with 11.4% by weight of silver and 0.015% by weight of cesium was obtained.

74% by weight of the total amount of silver and 33% by weight of the total amount of cesium were applied by the first impregnation and the respective remaining amount was applied by the second impregnation.

20th ml of the catalyst prepared in this way were introduced into a stainless steel pressure reactor and at 210 ° C. and 1.3 MPa gas pressure with an operating gas consisting of 30 vol.% ethylene, 50 vol.% methane, 8.5 vol.% Oxygen, 0.0003 vol .-% vinyl chloride and the rest nitrogen. The space-time speed was 3000 normal liters of gas per liter of catalyst per hour.

The gas leaving the reactor contained 1.5% by volume of ethylene oxide. From this, the selectivity (= moles of ethylene oxide formed per mole of converted ethylene) of 82.1% with 6% ethylene conversion was calculated.

In a long-term test under the above-mentioned conditions, the selectivity decreased over the course of 4 months by only 0.3 points (this is to 81.8%).

Example 2 (comparative example) -

In this comparative example, the entire amount of cesium and silver was applied in just one impregnation.

The support material of Example 1 was only once with a solution consisting of<ul id="ul0007" list-style="none"><li>31.5 g (49.4910% by weight) of silver nitrate</li><li>15.6 g (24.5098% by weight) of distilled water</li><li>0.048 g (0.0754% by weight) of cesium nitrate</li><li>16.5 g (25.9238 wt%) ethylenediamine</li></ul>soaked and dried and reduced as in Example 1. The finished catalyst contained 11.3% by weight of silver and 0.016% by weight of cesium.

In a long-term test with this catalyst over 4 months using the conditions of Example 1, the following was found:<ul id="ul0008" list-style="none"><li>required temperature for 1.5 vol .-%<img file="EP0097935A2_D0003.tif" /></li></ul>

Example 3

In this example according to the invention, a solution was found<ul id="ul0009" list-style="none"><li>12,000 g (37.4859% by weight) of silver nitrate</li><li>9,000 g (28.1145% by weight) of distilled water</li><li>0.012 g (0.0375% by weight) of cesium acetate</li><li>11,000 g (34.3621 wt%) of secondary butylamine.</li></ul>

The carrier material SAHM from United Catalyst, namely a-aluminum oxide in the form of spheres with a diameter of 8 mm and a specific surface area of ​​0.2 m, was added to this solution²/ g, immersed and dried for 10 minutes as described in Example 1.

After the semi-finished catalyst had cooled, it was mixed with a solution consisting of<ul id="ul0010" list-style="none"><li>12,000 g (37.4789% by weight) of silver nitrate</li><li>9,000 g (28.1092% by weight) of distilled water</li><li>0.018 g (0.0562% by weight) of cesium acetate</li><li>11,000 g (34.3557% by weight) of secondary butylamine</li></ul>impregnated and dried again as described above. The drying was followed by a 20 minute reduction in a glass tube preheated to 280 ° C. as an oven (40 liters of air and 20 liters of nitrogen were passed through the oven per hour).

A silver catalyst with 11.5% by weight of silver and 0.019% by weight of cesium was obtained.

68% by weight of the total silver and 37% by weight of the total cesium were applied by the first impregnation and the respective remaining amount was applied by the second impregnation.

20th As described in Example 1, ml of the finished catalyst was subjected to a test in the printing apparatus for 4 months, the following result being found:<ul id="ul0011" list-style="none"><li>required temperature for<img file="EP0097935A2_D0004.tif" /></li></ul>

Example 4

In this example, a cesium and rubidium salt was used as a promoter. Otherwise, the procedure was as in Example 3.

The solution for the first impregnation had the following composition:<ul id="ul0012" list-style="none"><li>12,000 g (37.4906% by weight) of silver nitrate</li><li>9,000 g (28.1180% by weight) of distilled water</li><li>0.005 g (0.0156% by weight) of cesium carbonate</li><li>0.003 g (0.0094 wt%) rubidium nitrate</li><li>11,000 g (34.3664% by weight) of secondary butylamine.</li></ul>

Solution for the second impregnation:<ul id="ul0013" list-style="none"><li>12,000 g (36.8992% by weight) of silver nitrate</li><li>9.500 g (29.2119% by weight) of water</li><li>0.012 g (0.0369% by weight) of cesium carbonate</li><li>0.009 g (0.0277 wt%) rubidium nitrate</li><li>11,000 g (33.8243% by weight) of secondary butylamine.</li></ul>

The finished catalyst contained 11.2% by weight of silver, 0.013% by weight of cesium and 0.008% by weight of rubidium. In the first impregnation step, 71% by weight of the total silver and 25% by weight of the entire promoter and the remaining amounts were applied with the second impregnation.

The test over 2 months, as described in Example 1, showed the following result:<ul id="ul0014" list-style="none"><li>required temperature for<img file="EP0097935A2_D0005.tif" /></li></ul>

Example 5

To produce a catalyst according to the invention, the carrier material SA 5552 from the Norton Company, which is a-aluminum oxide in the shape of a sphere with an 8 mm diameter and a specific surface of 0.3 m, was used²/ g, used. The carrier material evacuated in a glass flask was made up with a silver and cesium salt solution consisting of<ul id="ul0015" list-style="none"><li>30,000 g (38.4556% by weight) of silver nitrate</li><li>20,000 g (25.6371% by weight) of distilled water</li><li>28,000 g (35.8919% by weight) of isobutylamine</li><li>0.012 g (0.0154% by weight) of cesium nitrate</li></ul>completely poured over. After standing for 15 minutes, the excess solution was decanted off. It was dried for 45 minutes in a nitrogen-flushed drying cabinet at 110 ° C. (process step a ')

The dry sample was then heated in a glass tube through which an air-nitrogen mixture (40 liters of air and 20 liters of nitrogen per hour) flowed to 280 ° C for 30 minutes. After the reduction of the silver salt, a catalyst with 8.6% by weight of silver and 0.0038% by weight of cesium was obtained (process step b ').

The crude catalyst cooled to room temperature was then (process step c ') soaked with the following cesium salt solution:<tables id="tabl0001" num="0001"><img file="EP0097935A2_D0006.tif" /></tables>

For this purpose, it was completely covered with the solution in a vessel and left at room temperature for 10 minutes. After decanting and drying at 110 ° C. in a nitrogen-flushed drying cabinet (process step d '), a catalyst with 0.0125% by weight of cesium and 8.6% by weight of silver was obtained (the entire impregnation with the first Amount of silver and 30% by weight of the total amount of cesium and with the second impregnation the remaining amount of cesium).

25th ml of the catalyst thus prepared were in a reactor at 200 ° C and normal pressure with a gas mixture consisting of 30 vol .-% ethylene, 50 vol .-% methane, 8.5 vol .-% oxygen, 0.0003 vol .-% % Vinyl chloride, balance nitrogen, tested. The space-time speed was 400 normal liters of gas per liter of catalyst per hour. The gas leaving the reactor contained 1.3% by volume of ethylene oxide. From this, the selectivity (that is moles of ethylene oxide formed per mole of converted ethylene) of 81.5% with 5% ethylene conversion was calculated.

In a long-term test under the conditions mentioned, the selectivity decreased over the course of 3 months by only 0.3 points to 81.2%.

Example 6

In this example according to the invention, the carrier material of the SAHM type from United Catalyst, an α-aluminum oxide in the form of spheres with an 8 mm diameter and a specific surface area of ​​0.2 m²/ g, used. It was made in a beaker with a solution<ul id="ul0016" list-style="none"><li>30,000 g (39.4659% by weight) of silver nitrate</li><li>20,000 g (26.3106% by weight) of distilled water</li><li>22,000 g (28.9417% by weight) of secondary butylamine</li><li>4.000 g (5.2621% by weight) of ethylenediamine</li><li>0.015 g (0.0197% by weight) of cesium nitrate</li></ul>

completely poured over and left for 15 minutes. After decanting off the excess impregnation solution, drying was carried out for 30 minutes in a drying cabinet under a nitrogen atmosphere at 100 ° C. (process step a ').

To reduce the silver nitrate on the support to metallic silver, the oven was heated to 250 ° C. for 1 hour in a drying cabinet (which was flushed with 80 liters of air and 60 liters of nitrogen per hour) (process step b ').

The crude catalyst obtained contained 8.6% by weight of silver and 0.0045% by weight of cesium.

The crude catalyst was impregnated with the following cesium salt solution for 15 minutes (process step c '):

<tables id="tabl0002" num="0002"><img file="EP0097935A2_D0007.tif" /></tables>After decanting and drying for 30 minutes at 100 ° C (process step dl, a silver catalyst was obtained which contained 0.0107% by weight of cesium and 8.6% by weight of silver, 40% by weight of the total cesium in the first Impregnation step were applied.

As in Example 1, this catalyst was tested in a long-term test of 3 months, with the following result being obtained:<ul id="ul0017" list-style="none"><li>required temperature for<img file="EP0097935A2_D0008.tif" /></li></ul>

Example 7 (comparative example)

In this comparative example, the entire amount of silver and cesium was applied in one impregnation step (one-stage simultaneous application of the entire amount of silver and promoter). The carrier and the method of operation corresponded to Example 1.

The impregnation solution had the following composition:<ul id="ul0018" list-style="none"><li>30.00 g (38.4369% by weight) of silver nitrate</li><li>20.00 g (25.6246% by weight) of distilled water</li><li>28.00 g (35.8744% by weight) of secondary butylamine</li><li>0.05 g (0.0641% by weight) of cesium nitrate.</li></ul>

The finished catalyst contained 8.5% by weight of silver and 0.0130% by weight of cesium.

Testing the catalyst for 2 months under the conditions described in Example 1 showed the following result:<ul id="ul0019" list-style="none"><li>required temperature for<img file="EP0097935A2_D0009.tif" /></li></ul>

Example 8

In this comparative example, the entire amount of cesium was applied in the second impregnation step after the entire amount of silver had been applied in the first impregnation step. The carrier and the method of operation corresponded to Example 1.

The impregnation solution for the first impregnation step had the following composition:<ul id="ul0020" list-style="none"><li>30.0 g (38.46% by weight) of silver nitrate</li><li>20.0 g (25.64% by weight) of distilled water</li><li>28.0 g (35.90% by weight) of secondary butylamine.</li></ul>

The catalyst obtained after the reduction of the silver nitrate applied had 8.6% by weight of silver. The entire amount of cesium was now applied with the following solution:<tables id="tabl0003" num="0003"><img file="EP0097935A2_D0010.tif" /></tables>

The finished catalyst contained 8.6% by weight of silver and 0.0135% by weight of cesium.

Testing this catalyst for 1 month under the conditions described in Example 1 showed the following result: required temperature for<tables id="tabl0004" num="0004"><img file="EP0097935A2_D0011.tif" /></tables>