CA1292975C - Silver catalyst - Google Patents

Abstract

A B S T R A C T

IMPROVED SILVER CATALYST

The invention relates to an improved silver catalyst, suitable for use in the oxidation of ethylene to ethylene oxide, characterized by a) a calcined, alkalimetal-enriched alumina carrier and b) from 1 to 25 per cent by weight of metallic silver, based on the weight of the total catalyst, c) an alkali metal of the group consisting of potassium, rubidium and cesium, in the form of their oxide or hydroxide as a promoter and d) a fluoride-anion, the latter two under c) and d) each being in an amount between-10 and 1000 parts by weight per million parts by weight of the total catalyst.

Description

' - ~
~29~9~7S

IMPROVED SILVER C~LYST

The invention relates to an improved silver catalyst and to a process for preparing such silver catalyst, suitable for use in the oxidation of ethylene to ethylene oxide, to a process for preparing ethylene oxide~by the use of such catalyst and to ethylene oxide so prepared.
It is generally known that silver catalysts are applied in the oxidation of ethylene to ethylene oxide. It is referred to the British Patent Specification 1,413,251, in which such silver catalysts are disclosed. Moreover there is disclosed in the appli-cation that small am~unts of one or more pramoters are present, such as oe sium cc~pounds, rubidium conpounds and potassium co~r po~nds.
Applicant has now found silver catalysts with improved ~electivity and stability.
The invention relates to an improved silver catalyst, suitable for use in the oxidation of ethylene to ethylene oKide, characterized by a) a calcined, alkali metal enriched alumina carrier and b) from 1 to 25 per cent by weight of metallic silver, based on the weight of the total catalyst, c) an alkali metal of the group consisting of potassium, r~bidium and oe sium, in the form of their oxide or hydroxide as a promoter and d) a fluoride-anion, the latter two under c) and d) each being present in an amcunt between 10 and 1000 parts by weight per million parts by weight of the total catalyst.
The invention further relates to a pro oe ss for preparing a silver catalyst, suitable for use in the oxidation of ethylene to ethylene oxide characterized in that an alkali metal enriched -`` lZ9Z97S

alumina carrier, which has been calcined, is impregnated with a solution of a silver compound, sufficient to cause precipitation on the carrier of from 1 to 25 per cent by weight, on the total catalyst, of silver, and before, during or after that impregnation also with one or more dissolved potassium-, rubidium- or cesium compounds as promoter and with an additional source of fluoride-anions, and after precipitation the silver cowpcund on the impregnated carrier is reduced to metallic silver.
The carrier used in the inventive process for the preparation of silver catalysts, is an alkali metal enriched alumina carrier, which has been calcined, preferably to a temperature of between 1200 C and 1700 C. A large part of the calcined material will be alpha-alumina, but the existence of spinels or other configurations can not be excluded, since the calcined material is enriched with alkali metal. Salts or hydroxide of an alkali metal have been mixed with the original alumina. Suitable salts include fluorides, nitrates, chlorides and sulphates. Suitable metals are lithium, sodium, potassium, rubidium and cesium. Preferred oompounds are cesium fluoride, cesium chloride, lithium fluoride, lithium nitrate and cesium hydroxide. Preferably the aIkali metal compound is mixed with the alumina in such quantity that the atamic ratio of alkali/-aluminium is between 0.0005 and 0.1. If desired silicium dioxide is additionally mixed with the alumina in such quantity that the atomic ratio of silicium/aluminium is between 0.1 and 0.5.
m e aluminas may be modifications which by calcination provide alpha-alumina, such like gamma-alumina. Hydrated aluminas may also be suitable, such as boehmite, which latter by calcining via gamma-alumina provides alpha-alumina.
Preferably the carrier is prepared by mixing the alumina with water and alkali metal salt or hydroxide, extruding the obtained mixture to shaped particles and calcining the shaped particles, preferably to a temperature between 1200 C and 1700 C. The calcination m~y be carried out in one or more steps, dbpendiD~ on the choice of alumina modification. Generally a sufficient am3unt of water is added to form a paste suitable for extrusion. m e `-" 129Z9~75 obtained extrudable paste is then extruded and shaped to particles.
The shaped particles are heated in order to evaporate the water.
me solid particles are then calcined, preferably to a temperature between 1200 C and 1700 C.
Suitable aluminas are powders of gamma-alumina, alpha-alumina monohydrate, alpha-alumina trihydrate or beta-alumina manohydrate, which powder during calcination are sintered. At the calcination temperature the crystal structure may be mcdified. The cubic structure of gamma-alumina is converted into the hexagonal structure of alpha-alumina, depending on the amount and nature of the additive used. m e catalytically active surface of the enriched alumina may be between 0.1 and 5 m2/g, preferably between 0.2 and 2 m2/g. me shaped alumina particles comprise i.a. bars, rings, pellets, tablets and triangles. mey are especially suitable in fixed bed applications in ethylene oxide preparation.
In order to prepare a suitable catalyst the calcined, aikali metal enrich~ alumina carrier is impregnated with a solution of a ~ilver compound sufficient to cause precipitation on the carrier of from 1 to 25 per cent by weight, on the total catalyst, of silver, the so impregnated carrier is separated from the solution and the precipitated silver compound is reduced to metallic silver. Herein-after several detailed methods will be disclosed.
As a promoter is added to the silver solution, one or more of the alkali metals potassium, rubidium and cesium, preferably in the form of their salts or hydroxides. Although the metals potassium, rubidium and cesium in pure metallic form exist, they are in that form not suitable for use. m erefore they are administered in a solution of their salts or hydroxide. m e alumuna carrier is impregnated with the prcmDter before, during or after the impregnation of the silver salt has taken place. The promoter may even be brought on the carrier after reduction to metallic silver has taken place. The amount of promoter generally lies between 10 and 1000 parts by weight of potassium, rubidium or oe sium metal per million parts by weight of total catalyst. Preferably a~unts between 250 and 750 parts by weight are present on the total catalyst.

~Z9Z975 m e alumina carrier is also impregnated with a source of fluoride-anions. miS may be done the same time that the pro~oter is added, before or later. m e function of the F ions is not quite understood. The amount of fluoride-anions present on the alumina carrier generally is between 10 and 1000 pæts by weight, preferably between 100 and 400 parts by weight, per million parts by weight of the total catalyst. Suitable sources of fluoride-anions are ammonium fluoride (NH4F), ammoniumhydrogen fluoride (NH4HF2), lithium fluoride, sodium fluoride and silver fluoride.
Generally the alumina carrier is mlxed with a silver salt or a silver salt-oomplex containing aqueous solution, so that the alumina carrier is impregnated with said aqueous solution, there-after the impregnated carrier is separated from the aqueous solution, e.g. by filtration and then dried. The thus obtained impregnated alumina carrier is heated to a temperature in the range of from 100 C to 400 C, during a period sufficient to cause reduction of thé silver salt (complex) to metallic silver and to form a layer of finely divided silver, which is bound to the surface of the alumina carrier. A reducing gas or an inert gas may be conducted over the alumina carrier during this heating step.
~here are known several methods to add the silver to the alumina carrier. m e carrier may be impregnated with an aqueous silver nitrate containing solution, and then dried after which - drying step the silver nitrate is reduced with hydrogen or hydra-zine. m e alumina carrier may also be impregnated with an ammoniacal solution of silver oxalate or silver carbonate, and then dried, after which drying step the silver oxalate or silver carbonate is reduced to metallic silver by heating to e.g. up to 400 C. Specific solutions of silver salts with solubilizing and reducing agents may be employed as well, e.g. combinations of vicinal alkanolamines, alkyldiamines and ammonia.
The amount of prom~ter generally lies between 10 and 1000 ppm of alkali metal calculated on the total carrier material. Pre~er-ably am~unts between 250 and 750 ppm are especially suitable.
Suitable compounds of potassium, rubidium and cesium are, for lZ9Z9'7~

example, the nitrates, oxalates, carboxylic acid salts or hydrox-ides. The most preferred promoter is cesium among the alkali metals, preferably applied in an aqueous solution of cesium hydrox-ide or oe sium nitrate.
mere are known excellent methods of applying the promoters ooincidentally with the silver on the carrier. Suitable alkali metal salts are generally those which are soluble in the silver-precipitating liquid phase. Besides the above-mentioned compounds may be mentioned the nitrites, chlorides, iodides, brcmides, bicarbonates, acetates, tartrates, lactates and isopropoxides. m e use of aLkali metal salts which react with the silver salt in solution must be avoided, e.g. the use of potassium chloride together with silver nitrate in an aqueous solution, since then silver chloride is prematurely precipitated. The use of potassium nitrate is recommended instead of potassium chloride. However potassium chloride may be used together with a silversalt-amine-complex in aqueous solution, since then silver chloride is not precipitated prematurely from the solution.
The amount of promoter on the alumina carrier may also be regulated within certain limits by washing out the surplus of aLkali material with methanol or ethanol. Temperatures, contact times and drying with gases may be regulated. Traces of alcohol in the pores of the carrier must be avoided.
A preferred process of impregnating the alumina carrier consists of impregnating the carrier with an aquRous solution containing a silver salt of a carboxylic acid, an organic amine, a salt of potassium, rubidium or cesium. A potassium containing silver oxalate solution may be prepared. Silver oxide (slurry in water) is reacted with a ~ixture of ethylene diamine and oxalic acid, so that an aqueous solution of silver oxalate-ethylene diamine-complex is obtained, to which solution is added a certain amount of potassium compound. Other amunes, such as ethanolamine~
may be added as well. A potassium contaLning silver oxalate solution may also be prepared by precipitating silver oxalate from a solution of potassium oxalate and silver nitrate and rinsing with 129Z~S

water or alcohol the abtained silver oxalate in order to remove the adhering potassium salt until the desired potassium content is abtained. The potassium containing silver oxalate is then solubilized with ammonia and/or an amine in water. ~ubidium and cesium containing solution may be prepared also in these ways. The impregnated alumina carriers are then heated to a temperature between 100 C and 400 C, preferably between 125 C and 325 C.
It is observed that independent of the form in which the silver is present in the solution before precipitation on the carrier, the term "reduction to metallic silver" is used, while in the meantime often decomposition by heating occurs. We prefer to use the term ~reduction", since the positively charged Ag+ ion is converted into metallic Ag atom. Reduction times may generally vary from 5 min to 8 hcurs, depending on the circumstances.
The promoter on the alumina surface is preferably present in the form of oxide potassium, rubidium or cesium. Mixtures of oxides are not excluded.
The silver catalysts according to the present invention have been shown to be particularly selective and stable catalysts in the direct axidation of ethylene with molecular oxygen to ethylene oxide. The conditions for carrying out such an oxidation reaction in the presence of the silver catalysts according to the pres nt invention broadly aomprise those already described in the prior art. This applies, for example, to suitable temperatures, pressures, residenae times, diluent materials, such as nitrogen, carbon dioKide, steam, argon, methane or other saturated hydro-carbons, presence or absence of moderating agen*s to control the catalytic action, for example, 1-2-dichloroethane, vinyl chloride or chlorinated polyphenyl ccmF1unds, the desirability of employing recycle operations or applying successive aonversions in different reactors to increase the yields of ethylene oxide, and any other special aonditions which may be selected in processes for preparing ethylene oxide. Pressures in the range of from atm~spheric to 35 bar are generally emplayed. Higher pressures are, hcwever, by no means excluded. Mblecular oxygen employed as reactant can be abtained from conventional sources. m e suitable oxygen charge may consist essentially of relatively pure oxygen, a concentrated oxygen stream oo~prising oxygen in major amount with lesser amounts of one or more diluents, such as nitrogen and argon, or another oxygen-containing stream, such as air~ It is therefore evident that the use of the present silver catalysts in ethylene oxidation reactions is in no way limited to the use of specific conditions among those which are known to be effective.
In a preferred application of the silver catalysts according to the present invention, ethylene oxide is produced when an oxygen-containing gas is contacted with ethylene in the presence of the present catalysts at a temperature in the range of from l90 C
to 285 C and preferably 200 C to 270 C.
Generally in the reaction of ethylene with oxygen to ethylene oxide, the ethylene present is at least a double amount (on a mol basis) compared with the oxygen, but the applied amount of ethylene is often much higher. Therefore the conversion is calculated accordLng to the mol percentage of oxygen, which has been used.
The oxygen conversion is dependent on the reaction temperature, which latter is a measure for the activity of the catalyst employed. The values T30, T40 and T50 indicate the temperatures at 30 mol%, 40 mol% and 50 mol% conversion of the oxygen respectively in the reactor, and the values T are expressed in C. These temperatures are higher when the conversion of the oxygen is higher. Moreover these temperatures are strongly dependent on the employed catalyst and reaction conditions. The ælectivities (to ethylene oxide) indicate the molar percentage of ethylene oxide in - the reaction mixture oompared with the total molar amount of converted matter. The æ lectivity is indicated e.g. as S30, S40 and S50, which means the ælectivity at 30, 40 and 50 mol% oxygen conversion respectively.
The stability of the silver catalyst cannot be expressed directly. Tb measure the stability experiments during a con-siderable time, e.g. a year would be ne oe ssary. Applicant has now found that the æ time consuming tests can be simulated by carrying 1;~9Z975 out the experiments during about one month under the extreme high velocity of thirty thousand litres gas.litre catalyst l.h 1 also indicated as GHSV). This velocity is much higher than that used in commercial ethylene oxide processes (the latter GHSV = 2800-8000).
During the whole test period the above defined S and T values are measured regularly. After the reaction has finished, the total amcunt of produced ethylene oxide per ml of catalyst is determlned.
The selectivity and the activity of the catalyst are extrapolated on the basis that one ml of catalyst would have produced 1000 g of ethylene oxide. The new catalyst is considered to be more stable than a standard catalyst, if the differences in T- and S-values, measured on the new catalyst (preferably at the beginning and at the end of the reaction) are smaller than those measured on the standard catalyst, which in every experiment is present. The stability tests are carried out at constant oxygen conversion of 35~.
Example 1 A. 8 g of cesium fluoride dissolved in 832 ml water was mixed with 800 g of Kaiser alumina (26405) ~hl~03.H20) by addition of the cesium fluoride solution to the alumina, and the mixture was kneaded during 30 min. The obtained paste was extruded. The obtained shaped pieces w~re dried at 120 C and then calcined at periodically increased temperature. Up to 700 C was calcined firstly at an increase in temperature of 200 C/h, then was calcined for one hour at 700 C, whereafter the te~perature in two hours reached 1600 C. Finally was calcined further for one hour at 1600 C. The pore volume of the alpha-alumina shaped pie oe s was 0.45 ml/g and the average pore diameter was 1.6 ~m. m e abtained ring-shaped pieces were impregnated with an aqueous solution of silver oxalate, to which oe sium hydroxide and a D nium fluoride was added. m e impregnation was carried out for 10 min under vacuum, whereafter the shaped pieces were separated from the aqueous solution, and then placed in a heat air stream at a temperature of 250-270 C during 10 ~ln, in order to oonvert the silver oxalate into metallic silver. m e aqueous solution of silver ~ Z92975 oxalate oontained 28 per oe nt by weight of Ag (calculated on the total weight of the solution), wherein the silver oxalate was oo~plexed with ethylene diamine and to which solution was added cesium hydroxide and amm~nium fluoride. The impregnated shaped pieces before heat treatment contained 17.1 per cent by weight (calculated one the weight of the total catalyst) of silver and 280 ppm of oe sium and 200 ppm of F ~calculated on one million parts by weight of total catalyst).

B. A second catalyst was prepared in the same manner as above described, except that the amount of cesium as promDter was 330 ppm.
Both silver catalysts were employed in the preparation of ethylene oxide from ethylene and oxygen. A cylindric steel reactor with a length of 40 cm and a diameter of 5 mm was ,pletely filled with crushed catalyst particles of about 1 mm. The reactor was pla oe d in a bath of silica and alumina particles in fluid bed state. A gas mixture of the following oomposition was introduced into the reactor: 30 mol% ethylene, 8.5 mol% oxygen, 7 mol% carbon dioxide and 54.5 1% nitrogen and 5.5 ppm vinyl chloride as moderator. The GHSV was 3300 h 1 The pressure was maintained at 15 bar and the temperature aepenaent on the oxygen oonversion.
Measuring- mstruments were ccnnectcd to the reactor and to a oom-puter, such that oonversion and reaction temperature could be precisely regulated. With the aid of gaschrcmatography and mass-spectrosoopy the amounts of reaction products were determined. The oxygen conversion was 40%.
A third catalyst was prepared according to Example lA, withthe exception that ammonium fluoride was not added.
All three catalysts were tested on their selectivity:
The selectivity values (S~0) of the first and the seoond catalyst were 81.2 and 81.3, while the selectivity (S40) of the third catalyst was 79.9.
All three catalysts did not differ substantially in activity.
It proved that the addition of fluoride anions oonsiderably -" ~29Z9~5 improved the selectivity of the catalyst.
Example 2 5.34 g of cesium fluoride was dissolved in 1070 ml water.
800 g of Kaiser alumina (26405) (A12O3.H2O) and 166.8 g of silicium dioxide (150 g of dry compound) were muxed and the muxture was kneaded for 15 min. In one ninute the CsF solution was added to the mixture and the muxture was again kneaded. The obtained paste was then extruded. The obtained shaped pieces were dried for one hour at 120 C and then calcined at periodically increased temperature.
Up to 500 C was firstly calcined at an increase in temperature of 200 C/h, then was calcined for one hour at 500 C, whereafter the temperature in two hours reached 1600 C. Finally was calcined for six hours at 1600 C. The pore volume of the shaped pieces was 0.25 ml.g 1 and the average pore diameter 1.3 ~m.
The ring-shaped pieces were impregnated with an aqueous solution of silver oxalate, to which solution oe sium hydroxide and ammonium fluoride was added. The impregnation was carried but for 10 min in vacuum, whereafter the shaped pieces were separated from the solution and then placed in a stream of heat air for 10 mun at a temperature of 250-270 C, in order to convert the silver oxalate in metallic silver. The aqueous solution of silver oxalate was a 28 per cent by weight containing silver solution, wherein the silver oxalate was complexed with ethylene diamine and to which solution the necessary additives were added. The impregnated shaped pieces before heat treatment contained 13.4 per cent by weight of silver (calculated on the total weight of the catalyst), 660 ppm of cesium and 200 ppm of fluorine (calculated on one million parts by weight of total catalyst).
The silver catalyst was employed in the preparation of ethylene oxide from ethylene and oxygen. A cylindric steel reactor with a length of 40 cm and a diameter of 5 mm was oompletely filled with crushed catalyst particles of about 1 mm. The reactor was then placed in a bath of silica- and alumina particles maintained in fluid bed. A gas muxture of the following oomposition was intro-duced into the reactor: 30 mol% ethylene, 8.5 mol~ oxygen, 7 mo1%

129;~97S

carbon dioxide and 54,5 mol% nitrogen and 5.5 ppm vinylchloride as moderator. me GHSV was 3300 h 1 The pressure was maintained at 15 bar and the temperature was dependent on the oxygen conversion, the latter being 40%. Measuring-instruments were connected to the reactor and to a computer, such that conversion and reaction temperature could be precisely regulated. With the aid of gaschroma-tography and mass-spectroscopy the amounts of reaction products could be determined.
The selectivity (S40) of the above-mentioned silver catalyst was 81.5, while the selectivity (S40) of a non-amm~nium fluoride doped silver catalyst was 80.1.
Example 3 1.79 g of cesium fluoride dissolved in 861 ml water was mixed with 810 g of Kaiser alumina (26405) (A1203.H2G) by addition of the cesium fluoride solution to the alumina, and the mLxture was kneaded during 30 min. m e obtained paste was extruded. The obtained shaped pieces were dried at 120 C and then calcined at periodically increased temperature. Up to 500 C wa8 calcLned firstly at an increase in temperature of 200 C/h, then was calcined for one hour at 500 C, whereafter the temperature in tw~
hours reached 1600 C. Finally was calcined further for six hours at 1600 C. The pore volume of the alpha-alumina shaped pieces was 0.50 ml/g and the average pore diameter was 1.2 ~m.
The obtained ring-shaped pieces were impregnated with an aqueous solution of silver oxalate, to which oe sium hydroxide and ammonium fluoride was added. The impregnation was carried out for 10 min under vacuum, whereafter the shaped pieoe s were separated from the aqueous solution, and then placed in a heat air stream at a temperature of 250-270 C during 10 min, in order to convert the silver oxalate into metallic silver. The aqueous solution of silver oxalate contained 28 per cent by weight of Ag (calculated on the total weight of the solution), wherein the silver oKalate was complexed with ethylene diamine and to which solution was added oe sium hydroxide and am~onium fluoride. The impregnated shaped pieces before heat treabment contained 16.9 per oe nt by weight lZ5~Z9~5 (calculated on the weight of the total catalyst) of silver and 600 ppm of oe sium and 200 ppm of F (calculated on one million parts by weight of total catalyst).
The silver catalyst was employed in the preparation of ethylene o~ide from ethylene and oxygen. A cylindric steel reactor with a length of 40 cm and a diameter of 5 mm was oompletely filled with crushed catalyst particles of about 1 mm. me reactor was placed in a bath of silica and alumina particles in fluid ~ed state. A gas mLxture of the following composition was introdu oe d into the reactor: 30 mol% ethylene, 8.5 mol% oxygen, 7 mol% carbon dioxide and 54.5 mol% nitrogen and 5.5 ppm vinyl chloride as moderator. The GHSV was 3300 h 1. The pressure was maintained at 15 bar and the temperature dependent on the oxygen conversion.
Measuring-instruments were connected to the reactor and to a computer, such that conversion and reaction temperature could be precisely regulated. With the aid of gaschromatography and mass spectrosoopy the amounts of reaction products were determined. m e oxygen conversion was 40%.
The selectivity (S40) of the abcve-mentioned silver catalyst was 82,5 Example 4 The silver catalyst prepared according to the process dis-closed in Example lA and the silver catalyst prepared by the pro oe ss disclosed in E~ample 3 were both tested on their stability in the reaction of ethylene to ethylene oxide.
A steel cylindric reactor with a length of 15 cm and a diameter of 3 mm was filled oompletely with catalyst particles of about 0.3 mm. The reactor was placed in a bath, which oonsisted of silicium¦aluminium particles in a fluidized state. A gas mixture with the followlng oo~position was conducted through the reactor:
30 mol% ethylene, 8.5 mol% oxygen, 7 mol% carbon dioxide and 54.5 mol% nitrogen and 7 parts, per million parts of gas, of vinyl-chloride as moderator. The GHSV was 30,000 l. il.h 1 The pressure was 15 bar and the temperature was dependent of the oxygen con-version. The measuring instruments were oonnec*ed to the reactor lZ~?29~75 and to a computer, in such a way that conversion and temperature could be regulated precisely. With the aid of gaschromatography or mass spectroscopy the conter.t of each reaction component was determined. The stability test was carried out at a constant oxygen conversion of 35%. During the test, at regular intervals, the reaction temperature at 35% oxygen conversion was determined. Also the selectivity to ethylene oxide was determined at regular inter-vals. After 40 days the tests were discontinued and the total amount of produced ethylene oxide per ml catalyst was determined.
lOFrom the measured reaction temperatures, starting at the beginning of the reaction, the increase in reaction temperature was calculated in C for the moment at which lO00 g ethylene oxide per ml catalyst would have been produced (~T1000). From the measured selectivities the decrease in selectivity in mol% was calculated for the moment at which 1000 g ethylene oxide per ml catalyst would have been produced (~S1000).
The same measurements and calculations were carried out with a third silver catalyst which did not contain fluorine, but which - still contained oe sium fluoride in it8 carrier and which further in all aspects was pr~pared in the same way as the inventive catalysts.
In the Table the ~S1000 and aT1000 are given in the percentage of the ~S1000 and ~ of the third catalyst.

CATALYST
. . _ CARRIER NH~F QSl ~
enriched appl ed CsF YES (ex.1~) 81 100 CsF YES (ex.3) 43 91 CsF NO 100 100

Claims (16)

1. An improved silver catalyst, suitable for use in the oxidation of ethylene to ethylene oxide, characterized by a) a calcined, alkali metal-enriched alumina carrier and b) from 1 to 25 per cent by weight of metallic silver, based on the weight of the total catalyst, c) an alkali metal of the group consisting of potassium, rubidium and cesium, in the form of their oxide or hydroxide as a promoter and d) a fluoride-anion, the latter two under c) and d) each being present in an amount between 10 and 1000 parts by weight per million parts by weight of the total catalyst.

2. A silver catalyst according to claim 1, characterized in that the amount of alkali metal as defined under c) is between 250 and 750 parts by weight.

3. A silver catalyst according to claim 1, characterized in that the amount of fluoride-anion is between 100 and 400 parts by weight per million parts by weight of total catalyst.

4. A silver catalyst according to claim 1, 2 or 3, characterized in that the alkali metal-enriched alumina carrier contains cesium as the alkali metal.

5. A process for preparing a silver catalyst according to claim 1, suitable for use in the oxidation of ethylene to ethylene oxide characterized in that an alkali metal-enriched alumina carrier, which has been calcined, is impregnated with a solution of a silver compound, sufficient to cause precipitation on the carrier of from 1 to 25 per cent by weight, on the total catalyst, of silver, and before, during or after that impregnation also with one or more dissolved potassium-, rubidium- or cesium -compounds as promoter and with an additional source of fluoride-anions, and after precipitation the silver compound on the impregnated carrier is reduced to metallic silver.

6. A process according to claim 5, characterized in that the alumina carrier is calcined at a temperature of between 1200°C
and 1700 °C.

7. A process according to claim 5, characterized in that the alumina carrier is mixed with cesium fluoride, cesium chloride, lithium fluoride, lithium nitrite or cesium hydroxide.

8. A process according to claim 5, 6 or 7, characterized in that an alkali metal compound is mixed with the alumina in such quantity that the atomic ratio of alkali/aluminium is between 0.0005 and 0.1.

9. A process according to claim 8, characterized in that silicium dioxide is additionally mixed with the alumina in such quantity that the atomic ratio of silicium/aluminium is between 0.1 and 0.5.

10. A process according to claim 5, 6, 7 or 9, characterized in that the enriched alumina carrier is extruded and shaped to particles, which are calcined to a temperature of between 1200 °C
and 1700 °C.

11 A process according to claim 5, 6, 7 or 9, characterized in that the promoter is present on the alumina carrier in an amount between 10 and 1000 parts by weight of potassium, rubidium or cesium metal per million parts by weight of total catalyst.

12. A process according to claim 11, characterized in that the promoter is present in an amount between 250 and 750 parts by weight,

13. A process according to claim 5, 6, 7, 9 or 12, characterized in that the source of fluoride-anions is ammonium fluoride, ammonium hydrogen fluoride, lithium fluoride, sodium fluoride or silver fluoride.

14. A process according to claim 13, characterized in that the amount of fluoride-anions present on the alumina carrier is between 10 and 1000 parts by weight per million parts by weight of total catalyst.

15. A process according to claim 14, characterized in that the amount of fluoride-anions present on the alumina carrier is between 100 and 400 parts by weight per million of total catalyst.

16. A process for preparing ethylene oxide by oxidation of ethylene in the presence of a silver catalyst according to claim 1, 2 or 3 or prepared by means of a process according to claim 5, 6, 7 or 9.