Modified ethylene oxide catalyst and a process for its preparation
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The instant invention relates to a modified silver catalyst useful in the epoxidation of ethylene to ethylene oxide and more particularly to a cesium promoted silver catalyst which shows a greatly improved selectivity in the process for producing ethylene oxide by direct oxidation of ethylene with molecular oxygen. 
     2. Description of the Prior Art 
     Supported silver catalysts have long been used for the air oxidation of ethylene and more recently in a so-called &#34;oxygen process.&#34; Although the first reference to the use of silver as such a catalyst was made by Walter in British Pat. No. 21,941 (1905), it was not until some thirty years later that the first disclosures were made of the use of silver as a catalyst in the vapor phase oxidation of ethylene to ethylene oxide. See Societe Francaise De Catalyse Generalisee, French Pat. No. 729,952 (1932); and Lefort, U.S. Pat. No. 1,998,878 (1935). 
     A variety of techniques have been developed for the depositing of relatively small, but highly active amounts of silver on surfaces of non-silver support such as alumina. For example, McKim and Cambron in Canadian Journal of Research, Volume 27, Section B (1949) at 813-817, describe a method for depositing particulate silver on a support by decomposing silver oxalate in aqueous ethanolamine at 60° C and forming a paste which is applied to the surface of the support. In U.S. Pat. No. 3,043,854 issued July 10, 1962,  to Endler, a silver coating formed by decomposition of a silver carbonate slurry is applied to a catalyst support surface. 
     It has been disclosed that supported silver catalysts can be prepared by impregnating a porous substrate with certain silver containing solutions and evaporating or decomposing the solutions to deposit silver on the substrate. U.S. Pat. No. 3,702,259 to Nielsen describes the use of an aqueous silver oxalate impregnating solution which employs a solubilizing/reducing agent of ethylenediamine, a mixture of ethylenediamine or ethanolamine and ammonia or a mixture of ethylenediamine and ethanolamine. Van Bylandtlaan, in Belgium Pat. No. 808,278 (1974) employs an aqueous solution of hexamethylenetetramine with an ethylenediamine silver complex to deposit silver on an alumina support by decomposition. Additionally, it has been disclosed in Japanese Pat. No. 71/19,606 to Fujii et al that impregnation of inorganic supports with aqueous silver nitrate/alkanolamine complexes with subsequent thermal decomposition gives supported silver catalysts for ethylene epoxidation. 
     Recently it has been disclosed in British Pat. No. 1,413,251 to Neilsen and Rochelle that certain alkali metals can be deposited on a refractory support coincidently with the silver metal (U.S. Pat. No. 3,962,136). 
     Surprisingly, it has now been discovered that certain cesium promoted silver catalysts are extremely stable, physically durable and more selective in specific ethylene oxidation processes than heretofore known promoted catalysts. 
     The promoted silver catalyst is easily prepared by impregnating a porous, inorganic substrate with an impregnating solution comprising a solvent, a silver carboxylate, certain amines and a salt of cesium. The impregnated support is heated to decompose the complex thus depositing the silver on the substrate and activating the catalyst. 
     It has been found that the instant promoted catalysts provide outstanding selectivity in air oxidation processes or oxygen processes using nitrogen or inert gas ballast. This is particularly important in that such processes are not closed systems and some proportion of the unreacted ethylene is lost by venting excess gas. Additionally, the instant catalysts show high attrition resistance and surprisingly high mechanical strength. 
     Further, contrary to the prior art, the concentration of promoter on the catalyst surface is not solely dependent on the physical characteristics of that surface (Belg. Pat. No. 828,932). Moreover, the promoter need not be deposited on the substrate prior to the silver surface. 
     SUMMARY OF THE INVENTION 
     According to the broad aspect of the instant invention, a cesium modified silver catalyst comprises a porous, inorganic support containing from about 5 ppm to about 300 ppm cesium and elemental silver. The cesium and the silver are simultaneously deposited on the support by impregnating the support with a liquid phase comprising an effective amount of a cesium cation dispersed in a silver carboxylate/amine complex. The impregnated support is then heated at temperatures of from 50° C to 300° C to evaporate volatiles, decompose the complex, and activate the catalyst. 
     The complex is formed by dissolving a silver carboxylate in a solubilizing amount of an amine containing complexing agent selected from: 
     a) diamines wherein at least one amine moiety is primary or secondary, but no more than one is primary; 
     b) polyamines containing at least three amino moieties wherein at least one is primary or secondary; or 
     c) amino ethers containing at least one ether linkage and at least one amino moiety which is primary or secondary. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with a preferred embodiment, the supported cesium promoted silver catalyst of the instant invention is prepared in four steps. In a first step, an impregnating solution comprising a solvent, a salt of cesium, and a silver carboxylate/amine complex, as more fully described hereinafter, is prepared by dissolving a silver carboxylate in an excess of a polyalkylene polyamine having terminal primary nitrogen moieties at temperatures sufficient to dissolve the silver carboxylate to form the silver carboxylate/amine complex. To the formed complex is added a solution of the salt of cesium dissolved in the solvent. 
     In a second step, an inorganic porous support, as more fully described hereinafter, and preferably a high-purity α-alumina support, is impregnated by immersing the support in the impregnating solution at about atmospheric pressure and then subjecting the immersed support to vacuum at temperatures of from about 20° C to about 40° C. After the vacuum is broken, the excess solution is drained. In a third step, the drained support is heated to evaporate volatiles at temperatures of from about 50° C to 150° C in a forced-air heater for a time from about 1 to about 12 hours. In a final step, the dried, impregnated support is heated in the presence of forced air at temperatures of from about 200° C to about 300° C to decompose the silver carboxylate/amine complex and activate the supported, promoted silver catalyst material. 
     The Impregnating Solution 
     The impregnating solution of the instant invention comprises a salt of cesium and a silver carboxylate/amine complex. The impregnating solution can best be characterized as a homogeneous liquid at impregnating temperatures. The silver carboxylate/amine complex is formed by solubilizing a silver salt of an organic acid in a solubilizing amount of certain amine-containing complexing agents. Surprisingly, these silver carboxylate/amine complexes are stable in high solution concentrations at impregnating temperatures, and contain large amounts of silver which are carried to the support. Additionally, these complexes yield a solution which is compatible with most cesium salts and are of a viscosity which is suitable for impregnation of porous, inorganic supports. 
     The silver carboxylate/amine complex containing impregnating solutions of the instant invention can best be described in terms of their method of preparation. Specifically, a silver salt of an organic acid is dissolved in a solubilizing amount of certain amine-containing complexing agents at temperatures in the range of from about 0° to about 50° C. 
     The useful silver salts of organic acids can be generally described as silver carboxylates which readily thermally decompose. Such compounds can be carboxylates of mono-carboxylic or poly-carboxylic acids. Preferably, the silver salt is of a mono-carboxylic or di-carboxylic acid, wherein the organic moiety contains less than about 10 carbon atoms. Those carboxylates of less than about 10 carbon atoms are preferred in order to obtain a favorable concentration of silver in the organic acid salt, and ultimately thus in the complex solution, while providing for facile thermal decomposition. It should be noted that while silver salts of organic acids containing more than about 10 carbon atoms are useful, they produce a silver amine complex which become increasingly difficult to decompose as the molecular weight increases and will reduce the amount of silver ultimately available for deposition on the support. 
     Examples of suitable silver carboxylates include silver carbonate, silver acetate, silver malonate, silver glycolate, silver oxalate, silver formate, silver citrate, silver lactate, silver pyruvate, and the like. The most preferred silver carboxylates are silver oxalate and silver acetate because of availability and solubility. 
     The useful amine containing complexing agents of the instant invention can be generically described as: 
     (a) diamines wherein at least one amino moiety is primary or secondary provided no more than one amino moiety is primary; 
     (b) polyamines containing at least three amino moieties wherein at least one is primary or secondary; and, 
     (c) amino ethers containing at least one ether (oxy) linkage wherein at least one amino moiety is primary or secondary. 
     Although all aliphatic diamines meeting the above criteria are useful as complexing agents, a preferred group of diamines are those compounds of the formula ##STR1## wherein R 1  is a straight or branched chain alkylene radical of from 2 to 8 carbon atoms; wherein R 2  and R 3  and R 4  are, independently, hydrogen or an alkyl radical of from 1 to 5 carbon atoms, provided that R 4  be hydrogen only when R 2  or R 3  is alkyl. Exemplary of such compounds are N-methyl-ethylenediamine, N-ethyl-ethylenediamine, N,N&#39;-dimethyl-ethylenediamine, N,N&#39;-diethyl propylenediamine, N-ethyl-N&#39;-methyl ethylenediamine and the like. Another class of useful diamines is piperazine, the N-alkyl substituted piperazines and the C-alkyl substituted piperazines. It should be noted that the symmetrical, lower alkylene primary diamines are not useful or desired complexing agents in accordance with the invention. These compounds alone do not readily form suitable complex solutions with silver salts of carboxylic acids. 
     While all aliphatic polyamines containing at least three amino moieties wherein at least one is primary are useful as complexing agents, a preferred group is the polyalkylene polyamines of the formula ##STR2## wherein R is a straight or branched chain alkylene radical having from 2 to about 4 carbon atoms, R 1 , R 2 , R 3 , R 4  and R 5  are, independently, hydrogen or an alkyl radical of from 1 to 5 carbon atoms provided at least one of R 1 , R 2 , R 3 , R 4  or R 5  is hydrogen; or R 1  and R 2  or R 3  and R 4  with the nitrogen to which they are attached form a piperazine ring and n is an integer of from 1 to about 4. Examples include N(aminoethyl)-piperazine, N,N&#39;-bis(2-aminoethyl)piperazine, diethylenetriamine, N-methyldiethylenetriamine, triethylenetetramine and the like. The most preferred polyalkylene polyamine compounds are diethylenetriamine and triethylenetetramine. 
     The amino ethers that are useful within the scope of the instant invention are the saturated and unsaturated, substituted and unsubstituted aliphatic amino ethers. These compounds may be straight or branched chain, acyclic, alicyclic, heterocyclic, or cyclic. Examples of such compounds include morpholine, the C-substituted morpholines, etc.; the bis(aminoalkyl) ethers, the N-alkyl bis(aminoalkyl) ethers, etc.; the polyoxyalkylene amines; the polyoxyalkylene polyamines, etc.; the alkoxyalkyl amines; aminocontaining ethers derived from furan; and the like. 
     One preferred class of amino ethers is morpholine and the C-alkyl substituted morpholines. Another preferred class is the polyoxyalkyleneamines of molecular weight less than 1,000 and more preferably less than 500. Examples include the polyoxypropylenediamines of molecular weight less than about 400, and polyoxypropylenetriamine of molecular weight about 400. Both of the above polyoxypropyleneamine compounds contain terminal primary amino groups. 
     The amount of a particular amine containing complexing agent utilized in forming the silver carboxylate/amine complex impregnating solution is somewhat empirical. Generally that amount of amine complexing agent sufficient to completely dissolve the silver salts, i.e., a solubilizing amount, is utilized. Normally, a solubilizing amount will be a slight molar excess of the amine containing agent. This amount can be readily determined by the skilled artisan in that it is generally that amount sufficient to completely dissolve the required amount of silver salt, which can be determined by observation. The amount of silver salt required is somewhat empirical and generally determined by the amount of silver ion required in solution and the porosity of the support. 
     As hereinbefore mentioned, it is desirable to have the complex as &#34;rich&#34; as possible in silver. Generally, the impregnating solution should contain an amount of about twice that desired in the finished catalyst on a wt. % basis with a support having about a 50% porosity. It is preferable, therefore, to obtain a complex which contains more than about 10 wt. % silver in the impregnating solution and more preferably from about 12 to about 25 wt. % silver. 
     When the preferred polyalkylene polyamines are utilized, it is desirable to have from about 1 to about 6 amine equivalents of the polyalkylene polyamine for each equivalent of silver in order to form the optimum complex containing solution. 
     The silver salt is preferably solubilized in the amine containing agent at temperatures in the range of about 20° C to about 40° C. Temperatures in excess of 50° C are not preferred, since higher temperatures tend to cause accelerated decomposition of the complex. 
     If desired, solubilizers can be added in order to facilitate dissolution of the silver salt in the amine complexing agent. Examples of such solubilizers include water, aqueous ammonia, and the like. In accordance with a preferred embodiment, water is utilized as the solubilizing agent. Water not only reduces the viscosity of the impregnating solution, reduces the amount of amine required to solubilize the silver salt, and reduces potential hazards of handling the solution, but also acts as a solvent for the silver salt/amine complex, as well as the cesium salt thus preventing premature precipitation. 
     Examples of suitable soluilizers include aqueous methylamine, ethylamine, diethylamine, triethylamine, and pyridine. It is, however, recognized that the marginal advantages of such solubilizers may be outweighed by the fact that certain lower molecular weight amines or ammonia can form explosive solids with silver. In addition, although not necessary, small amounts of hydrogen peroxide or other suitable oxidizing agents may be added to minimize premature reduction of the silver in the complex. 
     The cesium metal salts which can be utilized are generally those which are soluble in the silver/amine complex. Since the metal is deposited on the surface of the support in the form of the cation rather than the free alkali metal no particular effectiveness is observed with the use of any specific anion. For example, nitrates, nitrites, hydroxide, carbonates, bicarbonates, oxalates, acetates, tartrates, lactates, and the like may be used. It should be noted, however, that those cesium metal salts which react with or cause the silver present in the amine complex to precipitate prematurely should be avoided. A cesium salt of an organic carboxylic acid or hydroxide is conveniently used. Cesium hydroxide is preferred. 
     The amount of the cesium metal salt present in the impregnating solution will depend upon that amount desired in the activated catalyst, the solubility of the salt, the stability of the complex, the porosity of the support, etc. Generally that amount of cesium salt sufficient to deposit from about 4×10 -5  gew to about 3×10 -3  gew cesium per kilogram of finished catalyst is effective. Suitable impregnating solutions contain from about 10 ppm to about 600 ppm cesium cation. The amount of cesium cation required in solution is capable of determination by conventional analysis of the amount of material actually deposited. Generally, the impregnating solution should contain an amount about twice that desired in the finished catalyst on a ppm basis with a support having about 50% porosity. 
     The Support 
     The support utilized to form the novel promoted silver catalyst of the instant invention can be generally described as a porous, inorganic substrate having those characteristics which are well known in the art and particularly known in the ethylene epoxidation art. Suitable supports which can be used in accordance with the instant invention are glass, alumina, silica, silicaalumina, inert metals, silicon carbide and zirconia. It is essential that the support chosen have a high porosity (i.e., high solvent absorption), low surface area and a controlled pore size. Preferably, from about 70 to 100% of the pore diameters are between about 1 and 30μ and more preferably between about 1 and about 10μ. The advantages of the instant catalyst are particularly evident when α-alumina supports are utilized. 
     A preferred support media has an average pore diameter of from about 4 to about 6μ with a pore volume of from about 0.3 to about 0.6 cc/g and has a surface area less than about 1 m 2  /g. A particularly preferred support is high purity α-alumina having the above characteristics. 
     Preparation of the Supported Silver Catalyst 
     In preparing the stable, promoted silver catalyst of the instant invention, a suitable support is first contacted with the impregnating solution and subsequently heated at elevated temperatures to first evaporate the volatiles and finally to decompose the silver carboxylate/amine complex and activate the catalyst material. Although the preparation of the supported catalyst can be accomplished in two steps; i.e., an immersion step and an evaporation, activating, and decomposition step at incrementally increasingly elevated temperatures, it is preferable to prepare the catalyst of the instant invention in three distinct steps. 
     After the impregnating solution has been prepared, as described hereinabove, the substrate to be impregnated is contacted with the solution in a first step. This is preferably accomplished by immersion of the substrate in a suitably large body of impregnating solution to completely cover the substrate. The immersed substrate is then subjected to an evacuated atmosphere for a time period sufficient to remove entrapped air from the support pores at temperatures of from about 0° C to about 50° C and more preferably from about 20° C to about 40° C. 
     The impregnation time will depend on the characteristics of the substrate and the viscosity of the impregnating solution and can be readily determined by the skilled artisan. Although somewhat empirical, it is generally sufficient to contact the porous substrate with the impregnating solution for a time from about five minutes to several hours. When utilizing impregnating solutions containing silver salts of polyalkylene polyamines, a time from about ten minutes to two hours is sufficient. After the substrate has been contacted for sufficient time under vacuum, the vacuum is broken to return the pressure to atmosphere and then the excess solution is physically drained from the substrate. 
     In a second step the drained substrate is dried in the presence of a heated flowing gas stream. The stream may comprise air or air diluted with sufficient inert gas to render the admixture substantially inert. The gas stream is heated to temperatures of from about 50° C to 150° C for a period sufficient to evaporate the volatiles. Generally the time required to dry the impregnated substrate is somewhat empirical and can be readily determined by the skilled artisan for a particular substrate and impregnating solution. Time periods of from about one to about twelve hours have been found sufficient. It should be noted that during the drying step temperatures in excess of about 150° C should be avoided as the complex may tend to decompose too rapidly and/or cause the volatiles to evaporate so readily as to disturb the uniformity of the catalyst material. Although not required, it is found that first thoroughly drying the impregnated substrate prior to thermal decomposition yields a more uniform catalyst. 
     In the third step the dried impregnated substrate is heated in the presence of flowing air, or a flowing inert atmosphere to temperatures in excess of about 180° C and preferably from about 200° C to about 300° C to decompose the complexing agent and activate the supported silver catalyst materials. The time required to thoroughly decompose the silver salt/amine complex and activate the catalyst is somewhat empirical but generally times in the range from about one to twelve hours have been found sufficient. 
     It will be realized by the skilled artisan that when other solvents such as water, or aqueous solubilizing agents such as aqueous ammonia, aqueous alkylamines, and the like are present in the complexing agent, the time required for drying may be somewhat variable. The specific times required are generally within the above broad limits and can be determined by the skilled artisan without undue experimentation. Additionally, when high molecular weight amines are utilized, washing of the dried substrate may be advantageous to remove excess organic material prior to activation. The washing may be accomplished in a conventional manner with lower alkanols. 
     The Oxidation Reaction 
     The cesium promoted silver catalyst of the instant invention has been shown to be a particularly selective catalyst in the direct oxidation of ethylene with molecular oxygen to ethylene oxide. The epoxidation of ethylene to ethylene oxide can best be described as a controlled oxidation. It is important to minimize complete oxidation in accordance with the epoxidation process while maximizing the selectivity and conversion to the desired epoxidized products. 
     The conditions for carrying out such an oxidation reaction in the presence of silver catalyst generally and more particularly the novel, promoted catalyst of the instant invention are broadly described in the prior art. Such methods and manner of production are well known to the skilled artisan. For examle, those methods which appear and are described in U.S. Pat. No. 3,119,837, British Pat. Nos. 1,314,613 and 1,132,095. These teachings apply to a number of conditions including suitable temperatures, pressures, residence times, diluents, inhibitors and the like. Additionally, the desirability of recycling unreacted feed or use of successive conversion processes such as by employing series reactors, can be readily determined by the skilled artisan. 
     It has been found that the promoted silver catalyst of the instant invention is surprisingly stable under a broad spectrum of reaction conditions, while maintaining a high degree of selectivity and productivity. 
     Regardless of the character of the support utilized, the catalyst is preferably shaped into particles, pellets, spheres or the like of a suitable size for employment in fixed bed application. It will be realized that conventional commercial fixed bed ethylene oxidation reactors may be utilized. Such reactors typically take the form of a plurality of parallel elongated tubes packed with a catalyst material. 
     Generally, it has been found that the promoted catalyst of the instant invention is particularly useful for so-called air oxidation process or oxygen processes wherein ballast &#34;gases&#34; are primarily comprised of nitrogen, argon or helium rather than the hydrocarbon type component such as methane, ethane or large excesses of ethylene. Surprisingly, it has been shown in such air processes (as will be seen in the following examples) that the promoted catalyst of the instant invention shows an unexpected increase in selectivity when compared with unpromoted catalysts; or, prior art promoted catalysts. 
     Generally, the process is carried out in vapor phase wherein a single gaseous feed stream is continuously charged to a suitable catalyst containing reactor. The reaction is carried out at temperature from about 200° C to about 300° C, and preferably in the range of about 220° C to about 260° C. The pressures are not critical and may vary from about atmospheric to about 35 atm. with about 13 atm. to about 20 atm. being preferred at the preferred temperature range. The feed admixture is preferably fed in a single stream to the reactor in order that the constituents be thoroughly admixed. 
    
    
     The invention will be further illustrated by the following specific examples, which are given by way of illustration and not as limitations on the scope of this invention. 
     EXAMPLE I 
     This example illustrates preparation of the stable supported silver catalyst of the instant invention. In a first step, silver oxalate was prepared. To an appropriate clean, dry vessel equipped with stirring apparatus were charged a solution of 18.4 g potassium oxalate dissolved in 150 cc deionized water and a solution of 34.0 g silver nitrate in 150 cc deionized water. The two solutions were admixed at 60° C and atmospheric pressure by stirring for several minutes. The mixture was then filtered and the residue washed with four aliquots of hot, deionized water totaling 50 cc. The residue was then further washed with two 25 cc aliquots of absolute methanol. The residue was then partially air dried by evacuating the lower portion of the filter surface. 
     In a second step, the slightly moist filter cake was slurried in 30 ml deionized water. A solution of 0.066 g of 50% aqueous cesium hydroxide was added, followed by a mixture of 30 g diethylenetriamine and 10 g deionized water. The latter was added slowly with stirring to prevent overheating. The resulting solution was dark and contained traces of suspended solids. 
     In a third step, the solution prepared in step two was used as an impregnating solution. The solution was drawn into a 500 ml stainless steel sampling cylinder containing 50 g of alumina support having a pore volume of 0.41 cc/g, a surface area of less than about 1 m 2  /g and an average pore diameter of 5.9 μ. Full pump vacuum was maintained on the cylinder for about one hour. The cylinder was returned to atmospheric pressure, and allowed to stand about 10 minutes. The cylinder was then subjected to 200 psig nitrogen pressure for about 30 minutes to assure that the support pores were completely filled with impregnating solution. The excess solution was then drained from the support and the cylinder purged with nitrogen several times. 
     In a fourth step, the cylinder was attached to a forced-air heater and dried for one hour at 120° ± 5° C, then raised to about 250° C. This temperature was maintained for one hour using a 50 scfm stream of air at 40 psig. After cooling, the catalyst material weighed 56 g and contained 12.2 wt. % silver. The estimated cesium concentration on the catalyst was in the range of 130-180 ppm based on the total weight of catalyst. 
     EXAMPLE II 
     This example illustrates the preparation of a non-promoted silver catalyst. In a first step a silver oxalate was prepared by admixing 102 g of reagent grade silver nitrate and 44 g of reagent grade ammonium oxalate. The admixture was filtered and washed, first with small portions of hot water, then with small portions of anhydrous methanol. The residue was aspirated nearly dry under vacuum. 
     In a second step, the moist silver oxalate was added to a beaker containing 110 ml of deionized water, then stirred to form a slurry. The slurry was then chilled below room temperature. Keeping the solution below 60° C, a solution containing 90 g diethylenetriamine (DETA) and 30 ml deionzed water was slowly added. The resulting solution was dark brown and contained traces of suspended black solids. 
     In a third step the support was impregnated with the impregnating solution prepared in step two. About 325 g of low surface area, high porosity α-alumina support of controlled pore size was immersed in the impregnating solution. The immersed support was then subjected to vacuum for 10 minutes and then returned to atmospheric pressure for 30 minutes. 
     In a fourth step, the impregnated support was drained, then placed in a forced air dryer and heated at about 125° C in a rapid stream of dry air for 1 hour. The support was then heated to about 250° C over a period of about 1 hour and held at that temperature for one hour. 
     The catalyst material weighed 359 g and contained 9.7 wt. % silver by atomic absorption analysis. 
     EXAMPLE III 
     In this example a cesium promoted catalyst was prepared, using the procedures and apparatus of Example II. Prior to the formation of silver oxalate slurry, as described in the second step of Example II, 0.23 g of 50% aqueous CsOH solution was added to the deionized water. The amount of support material impregnated in accordance with the procedure as described in step three of Example II was 280 g. 
     The prepared catalyst material showed 10.0 wt. % silver by atomic absorption. 
     EXAMPLE IV 
     In this example, a cesium promoted catalyst was prepared. In a first step, 150 g of reagent grade ammonium oxalate and 340 g of reagent grade silver nitrate were admixed in solution, then washed and dried essentially by the manner of Example II. The silver oxalate slurry was stirred for 30 minutes, then filtered under aspirator vacuum. The residue was washed first with 400 ml of hot, deionized water in small portions then with small portions of anhydrous methanol, using a total of 400 ml of methanol. The solid was then aspirated nearly dry under vacuum. 
     In the second step, the silver oxalate was slurred in a solution of 0.66 g of 50% aqueous cesium hydroxide in 300 ml deionized water. The slurry was stirred with cooling in an ice bath. A cold solution of 300 g DETA and 100 g deionized water was then added to the slurry with stirring, and the resultant admixture maintained at about 35° C. The solids of the slurry dissolved to give 1041 g of a dark solution containing traces of suspended, very fine black solids. 
     In the third step, a high porosity α-alumina support which had been previously screened to pass a 1/4 inches mesh screen but was retained on a No. 31/2 ASTM sieve was impregnated. The support was immersed in the solution at atmospheric pressure and then evacuated to about 1 mm Hg for a few minutes, and again allowed to attain atmospheric pressure. 
     In the fourth step, the immersed material was allowed to stand at atmospheric pressure and then the liquid was decanted. The wet solids were placed on 14 inches × 16 inches stainless steel trays which in turn were placed in a forced air production oven. Employing maximum intake and exhaust settings, the material was heated at 120° C for 2 hours. The temperature was then increased to 250° C for one hour at maximum air intake with exhaust 75% closed. The oven was then cooled to 50° C with full air intake and exhaust. 
     The prepared catalyst material weighed 1880 g. The material was divided into two separate samples and analyzed by atomic absorption. The analysis showed 10.2 and 10.0 wt. % silver. 
     EXAMPLE V 
     An umpromoted silver catalyst was then prepared using the procedure, apparatus and reactants of Experiment IV except that the cesium hydroxide was eliminated in step two. 
     EXAMPLE VI 
     In this example the promoted catalyst prepared in Example I was compared with a nearly identical unpromoted catalyst. The difference in preparation between the two resides in the absence of the 0.066 g 50% aqueous cesium hydroxide which was added in step two of Example I. For the comparison, a miniature ethylene oxide reactor was employed utilizing 3.5 g of 30-40 mesh catalyst in a 0.2 × 5 inches reaction zone. The test was run at a reactor temperature of 220° to 250° C, mass velocity of about 5 g feed/g of catalyst per hour, and reactor pressure of 200 psig. The feed gas composition was 7% ethylene (99.8 minimum mol % purity), 6% oxygen and the balance nitrogen. The performance results at comparable ethylene conversions are shown in Table I at different reactor temperatures. Ethylene dichloride inhibitor was added in amounts necessary to optimize selectivity. 
     
                       TABLE I                                                     
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Run              Promoted      Unpromoted                                 
No.    Temp.     Selec.   Conv.  Selec. Conv.                             
______________________________________                                    
1      220° C                                                      
                 77       28     78     28                                
2      230° C                                                      
                 76       36     75     35                                
3      240° C                                                      
                 75       45     73     43                                
4      250° C                                                      
                 75       52     68     50                                
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     This example demonstrates that the promoted catalyst gives higher selectivities under reaction conditions wherein the conversion rates are most favorable. 
     EXAMPLE VII 
     In this example the catalysts in Examples II and III were performance tested as in Example VI. Included for comparison are standard nonpromoted catalysts used in a commercial ethylene oxide process plant. In each run, the reactor operating conditions were adjusted so that the selectivities, conversions and ethylene oxide outlet concentrations matched those for a typical commercial catalyst in a large pilot plant reactor. As before, ethylene feed concentration was 7 mol %, and oxygen 6 mol %. The CO and ethane levels were 8% and 200-300 ppm, respectively, whereas in Example VI these constituents were negligible. The results are shown in Table II. 
     
                       TABLE II                                                    
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       Silver Plant                                                       
                   Promoted  Unpromoted                                   
       Catalyst    Catalyst  Catalyst                                     
       A      B        Ex. III   Ex. II                                   
______________________________________                                    
Temp., ° C                                                         
         260      257      251     244                                    
Selectivity                                                               
         70.5     70.3     77.7    71.2                                   
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     As can be seen, the unpromoted catalyst of Example II showed superior activity to the plant catalyst and the promoted catalyst of the instant invention showed a marked increase in selectivity over the unpromoted catalyst material. 
     EXAMPLE VIII 
     This example shows selectivity enhancement of the promoted catalysts is related to the method of preparation of the catalyst material. The following experiments were performed in an ethylene oxide reactor as described in Example VI at constant reactor productivity and constant conversion. All catalysts contained essentially the same amounts of silver and all promoted catalysts essentially the same amount of cesium. 
     
                       TABLE III                                                   
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Run                  Promoted    Unpromoted                               
No.    Amine.sup.1   Selectivity Selectivity                              
______________________________________                                    
1     DETA.sup.2     79.0        71.4                                     
2     DETA-Morpholine                                                     
                     76.8        72.3                                     
3     EDA-MEA.sup.3  74.4        72.2                                     
4     MEA-NH.sub.3.sup.4                                                  
                     76.8        74.2                                     
______________________________________                                    
 .sup.1 Amine used in impregnating solution inpreparation of catalyst     
 substantially as in Example I.                                           
 .sup.2 Diethylenetriamine                                                
 .sup.3 Ethylenediamine-Monoethanolamine--prior art catalyst.             
 .sup.4 Monoethanolamine-ammonia--prior art catalyst.                     
 
    
     This example shows the degree of selectivity enhancement obtained using a cesium promoter is effected by the specific amines used to make the silver impregnating solution. The preferred catalyst of the instant invention prepared from a silver-DETA complex showed the greatest selectivity enhancement when a promotor was used. 
     EXAMPLE IX 
     In this example a catalyst was prepared from a silver-DETA complex in a manner substantially similar to that of Example I. In each preparation, the amount of cesium promoter employed was varied. Each catalyst was then employed in an epoxidation reaction wherein a constant ethylene oxide concentration in the reactor effluent was maintained. The feed stream for each run comprised about 7 mol % ethylene, 7 to 8 mol % CO 2  and 6 mol % oxygen with promoter and inhibitor concentrations selected to provide maximum selectivity. The reactor system was substantially that described in Example VI. The results are shown in Table IV. 
     
                       TABLE IV                                                    
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     Cesium conc.,                                                        
Run  ppm on total  Reactor      Selectivity                               
No.  catalyst      temp., ° C                                      
                                mol %                                     
______________________________________                                    
1     0            240          72.0                                      
2     12           240          73.8                                      
3     29           240          74.9                                      
4     41           240          76.2                                      
5    111           247          78.1                                      
6    122           250          79.0                                      
7    162           253          77.7                                      
8    190           257          78.2                                      
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     While the invention has been explained in relation to its preferred embodiment, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification and is intended to cover such modifications as fall within the scope of the appended claims.