Process for preparing a supported silver catalyst

This invention relates to a supported silver catalyst for the manufacture of ethylene oxide prepared by a process comprising impregnating a porous catalyst support with a solvent containing a silver salt and treating the impregnated support to effect deposition of silver on the support surface. Following silver deposition, the support is impregnated with a liquid containing more than 30%, by volume, of an organic solvent capable of forming a complex with silver ion, and a compound of at least one metal promoter in an amount sufficient to deposit the desired amount of metal cation on said support. The impregnated support is then treated to effect deposition of the promoter. There is also described herein a process of making such catalyst and a process for producing ethylene oxide.

This invention relates to supported silver catalysts for the manufacture of 
ethylene oxide, their preparation, and their use in ethylene oxide 
processes. More specifically, the invention is concerned with preparing a 
metal cation promoted silver catalyst capable of oxidizing ethylene with 
an oxygen-containing gas in the vapor phase to produce ethylene oxide at 
high efficiencies. 
In characterizing catalysts useful for the manufacture of ethylene oxide, 
the term "selectivity" is employed herein as defined in U.S. Pat. No. 
3,420,784, patented Jan. 7, 1969, at column 3. The terms "efficiency" and 
"selectivity" as used throughout the specification with regard to the 
aforesaid catalysts are intended to be synonymous. 
Processes for preparing metal cation-promoted silver catalysts for the 
production of ethylene oxide are extensively described in the patent 
literature. The vast majority of these processes employ impregnation 
techniques wherein solutions containing solubilized compounds of silver 
and metal cation promoters are used to impregnate a porous carrier or 
support followed by heat treatment of the impregnated support to effect 
deposition of the silver and metal cation on the support. Processes for 
making coated catalysts employ techniques wherein silver and metal cations 
are coated onto a catalyst support from an emulsion or slurry followed by 
a heating step to remove the liquid present from the carrier and effect 
deposition of the silver and metal promoter. Coated catalysts are 
generally considered today to be less satisfactory than impregnated 
catalysts in commercial practice because it is generally believed that 
coating methods are unable to accomplish substantial deposition of silver 
into the interior surfaces of the carrier and consequently, the coated 
catalysts are more susceptible to silver loss by mechanical abrasion. 
The impregnation methods described in the art for preparing ethylene oxide 
catalysts include a wide variety of methods of depositing silver and metal 
cations onto a carrier. These methods are generally distinguished by the 
process conditions they employ such as low-temperature impregnation, high 
temperature impregnation, activation in an inert gas atmosphere and/or 
choice of solvent for the silver impregnating solution. 
Criticality is often taught to reside in the order of addition of the metal 
cation and silver to the carrier. Such processes are characterized by 
their employing either a coincidental (or simultaneous) method of 
depositing silver and metal cation onto the carrier or a sequential method 
of addition wherein silver is added either before or after the metal 
cation. The addition of silver to a carrier subsequent to the addition of 
metal cation is referred to herein as a "metal-first" sequential process 
of preparation, while the addition of silver to the carrier prior to the 
addition of the metal cation is referred to herein as a "silver-first" 
method of preparation. The coincidental (or simultaneous) addition of 
silver and metal cation to a carrier is referred to herein as a 
"coincidental method" of preparation. The use of the term "addition" of a 
metal cation and/or silver to a carrier is meant to include the steps of 
impregnating the porous carrier with a solution containing silver and/or 
metal cation, as the case may be, followed by deposition of same upon the 
carrier, usually by heat treatment. 
The comparative performance of catalysts produced by coincidental and 
sequential methods of impregnation has been reported in the art. For 
example, U.S. Pat. No. 3,563,914 to Wattimena, in Table III, compares the 
effect of the order of addition of alkali metal promoter and silver to a 
catalyst support on catalyst efficiency. The data in Table III is said to 
illustrate the advantage of adding an alkali metal promoter to the support 
before the silver compound. Specifically, the catalysts prepared by an 
alkali metal-first preparation procedure are shown to have an efficiency 
of 4-5 percent higher than catalysts prepared by a coincidental deposition 
of alkali metal and silver. Further, a catalyst prepared by the addition 
of silver to the carrier prior to alkali metal addition was by far the 
least efficient in that the selectivity was about 12 percent below that of 
a similar catalyst prepared by a coincidental method of deposition. In 
contrast with Wattimena's conclusion regarding the superiority of an 
alkali metal-first sequential order of addition, Belgian Pat. No. 793,658 
and U.S. Pat. Nos. 3,962,136, 4,101,115 and 4,012,425 to Nielsen et al 
indicate that the coincidental deposition of silver and alkali metal is 
the preferred method of catalyst preparation insofar as it results in the 
highest catalyst efficiencies. The aforementioned Belgian patent also 
provides a direct comparison of catalysts prepared by a method of 
coincidental deposition of silver and potassium with catalysts of similar 
composition prepared by a sequential process wherein silver is deposited 
prior to potassium. Specifically, Example III of the Belgian patent 
indicates that the maximum efficiency achieved with catalysts containing 
7.8 weight percent silver and varying amounts of co-deposited potassium 
was 76.3% under the stated test conditions whereas in Example VII of the 
patent the maximum selectivity achieved under the same conditions with 
catalysts containing the same amount of silver and similar amounts of 
potassium but prepared by a silver-first process was 73-74%, thus 
confirming the data in Wattimena concerning the inherent inefficiency of 
catalysts prepared by a silver-first sequential order of addition. 
U.S. Pat. No. 4,207,210 to Kilty, based upon British specification No. 1 
489 335, describes an alkali metal-first process for preparing ethylene 
oxide catalysts which is said to provide catalysts equivalent or even 
superior to those produced by coincidental methods of deposition such as 
set forth in the aforementioned U.S. patents to Nielsen et al. According 
to the described procedure of Kilty, an aqueous solution containing alkali 
metal is used to impregnate the porous carrier which is then dried to fix 
the alkali metal and thereafter the silver is supplied to the support. 
Tables A through E of the Kilty U.S. patent provide comparisons of 
catalysts prepared in accordance with the disclosed alkali metal-first 
method of addition with catalysts of similar composition prepared by the 
simultaneous addition of alkali metal and silver. The criticality of the 
alkali metal-first method of addition is, however, called into question by 
the reported data which fails to indicate any discernible difference 
between either method of preparation based on the measured catalyst 
efficiencies. Indeed, the alkali metal-first method of addition appears to 
be inherently identical to the coincidental deposition method used in the 
Nielsen et al patents as evidenced by the fact that both Kilty and Nielsen 
et al disclose that the alkali metal which is added to the carrier can be 
subsequently removed, if desired, using an alkanol solvent. This suggests 
that in the preparation procedure of Kilty, the alkali metal which is 
initially deposited on the carrier is resolubilized in the 
silver-containing impregnating solution thereby inherently effecting a 
coincidental deposition of silver and alkali metal. (Compare this to 
Wattimena, U.S. Pat. No. 3,563,914, discussed above.) This is further 
evidenced by a comparison of the curve shown in Kilty's British Patent 
Specification No. 1 489 335 wherein selectivity is plotted as a function 
of cesium content for a carrier having a surface area of 0.19 m.sup.2 /g, 
and the curve shown in Ser. No. 216,188, filed Jan. 7, 1972, now abandoned 
(the application from which the Nielsen et al U.S. patents were derived), 
wherein curve C represents as a function of cesium content the 
selectivities achieved with catalysts having an essentially similar silver 
content and alumina carrier to that used in the examples of Kilty but 
prepared by a coincidental method of deposition. The similarity of the two 
curves confirms the fact that the efficiencies produced with catalysts 
prepared by the coincidental method of Nielsen et al and the sequential 
method of Kilty are essentially equivalent. 
As noted in the prior art, processes for preparing catalysts by the 
silver-first method have obvious drawbacks with regard to the resulting 
catalyst efficiencies. The prior art has documented the markedly lower 
efficiencies of catalysts produced by the silver-first method relative to 
similar catalysts prepared by a coincidental method, the latter appearing 
to be essentially equivalent to an alkali metal-first order of addition. 
Thus, as discussed above, U.S. Pat. No. 3,563,914 to Wattimena and Belgian 
Pat. No. 793,658 contain comparative data clearly illustrating the 
relative inefficiency of catalysts produced by a silver-first sequential 
method of addition relative to a coincidental method of addition. While 
other patents in the art directed to silver-first methods of preparation 
do not provide sufficient data to allow such side-by-side comparisons to 
be made, nevertheless, the data which is provided appears to indicate that 
silver-first methods are the less preferred methods. U.S. Pat. No. 
4,033,903 to Maxwell, for example, discloses a silver-first method of 
addition wherein used ethylene oxide catalysts are reactivated by the 
addition of an alkali metal promoter to the aged catalyst. The process of 
the patent is said to be equally effective for enhancing the efficiency of 
freshly prepared catalysts by employing a heat treatment step intermediate 
to the steps of silver addition and alkali metal addition to the carrier. 
The effectiveness of this method of preparation seems somewhat doubtful, 
however, in view of the data shown in Table III of the patent wherein 
catalysts R and T, catalysts prepared by a silver-first method are shown 
to be inferior to catalyst Q, a silver catalyst containing no alkali metal 
promoter. Accordingly, based upon the data in the aforementioned patents 
there appears to be an obvious need in the art for a silver-first 
sequential method of catalyst preparation capable of providing catalysts 
which are no less efficient than those produced by the coincidental or 
metal-first methods. 
A common characteristic of the various silver-first methods of preparation 
described in the literature is their use of the same solvents for metal 
cation addition. That is, the methods disclosed in this literature suggest 
using water or a lower alcohol, such as, methanol or ethanol, as the 
solvent for effecting metal cation impregnation. Thus, for example, the 
aforementioned patent to Wattimena describes in Example III a silver-first 
addition wherein water is employed as the solvent for the alkali metal 
impregnation step. Belgian Pat. No. 793,658 which discloses a silver-first 
method of addition in Example VII thereof states that aqueous solutions of 
potassium were used as the impregnating medium for the promoter. U.S. Pat. 
No. 4,066,575 to Winnick describes a process of catalyst preparation 
characterized by an activation step wherein the carrier is heated in an 
inert gas atmosphere following its impregnation with a silver solution. An 
alkali metal promoter is thereafter deposited on the carrier employing as 
a solvent for the alkali metal, water or a lower alkanol such as, 
methanol, ethanol or propanol. Great Britain Patent Application No. 
2,045,636A attempts to distinguish itself from the prior art processes by 
its low-temperature deposition technique whereby the carrier impregnated 
with a silver-containing solution is maintained at temperatures below 
200.degree. C. prior to the so-called post deposition of alkali metal. The 
suggested solvents for such post-deposition of alkali metal are water and 
ethanol. German Offenlegungsschrift No. 2,914,640 discloses a sequential 
order of impregnation wherein silver is initially applied to the carrier 
from a suspension and the carrier thereafter immediately dried. Alkali 
metal is then added to the carrier from a solution using water as the 
solvent. U.S. Pat. No. 4,248,740 to Mitsuhata et al describes a catalyst 
preparation procedure employing a silver-first order of addition. The 
patentees recommend impregnating the carrier with an alkali metal solution 
containing water or a lower alcohol, such as methanol, ethanol or 
propanol. The solvent is then evaporated, care being taken to prevent 
heating of the catalyst to above 200.degree. C., a critical feature of the 
described process. In U.S. Pat. No. 4,168,247 to Hayden et al, there is 
described a preparation procedure for catalysts identified by the numbers 
34-37 which consists of a silver-first order of addition. The alkali metal 
promoters were dissolved in water with further addition of methanol, and 
the resulting solution used to impregnate the carrier. 
Japanese Patent Application No. 142,421/78 (Kokai No. 79,193/79) discloses 
a "post-treatment" of a used or stabilized silver catalyst by impregnating 
such catalyst with a solution containing an alkali metal promoter, an 
organic compound capable of forming a complex salt with silver ion and an 
alcohol of 1 to 4 carbon atoms. No alcohol other than methanol was used in 
the impregnating solution described in the examples. A further distinction 
between the process of the reference and the present invention resides in 
the fact that the improved efficiencies achieved in the examples of the 
reference can be attributable to the presence of an oxide of nitrogen in 
the catalyst (see, for example, GB No. 2,014,133A which discloses the 
beneficial effects of nitrates or nitrite forming substances in the 
manufacture of ethylene oxide), rather than, the promoting effect of 
alkali metal in accordance with the present invention. 
SUMMARY OF THE INVENTION 
The invention describes a process for preparing a supported silver catalyst 
for the production of ethylene oxide by the vapor phase oxidation of 
ethylene with an oxygen-containing gas, the catalyst produced by such 
process and the use of such silver catalyst for ethylene oxide 
manufacture. The process comprises impregnating a porous catalyst support 
with a solution comprising a solvent or a solubilizing agent, and silver 
salt in an amount sufficient to deposit the desired amount of silver on 
said support. The impregnated support is then treated to convert at least 
a fraction of the silver salt to silver metal and effect deposition of 
silver on the surface of said support. Following silver deposition, the 
support is impregnated with a liquid containing more than 30%, by volume, 
of an organic solvent capable of forming a complex with silver ion, and a 
compound of at least one metal cation promoter in an amount sufficient to 
deposit the desired amount of metal cation on said support. In accordance 
with another embodiment of the invention the impregnating solution 
containing the metal promoters is kept substantially free of lower 
alcohols, which as used herein refers to alcohols containing from 1 to 4 
carbon atoms. The impregnated support is thereafter treated to effect 
deposition of the promoter on the surface of said support. 
The catalyst preparation process of the invention, in its broadest aspect, 
concerns a process wherein silver and a metal promoter are sequentially 
deposited on the surfaces of a porous carrier by a silver-first method. 
The particular metal promoter employed is not critical to the invention 
and may include one or more alkali metals, such as lithium, sodium, 
potassium, rubidium and/or cesium; one or more alkaline earth metals, such 
as, barium, magnesium and strontium; or one or more of the other known 
promoters, such as thallium, gold, tin, antimony and rare earths; and the 
like. For purposes of convenience, the catalyst preparation process of the 
invention is described below in terms of a silver-first method of 
preparation wherein the promoter is selected from among alkali metals, it 
being recognized that other promoters of silver catalysts, such as those 
mentioned above, may optionally be substituted for or added to alkali 
metals in such process. 
The process of the invention is predicated on the discovery that a catalyst 
preparation procedure employing a silver-first addition of silver and 
metal cation to a porous carrier can provide catalysts as efficient as 
those produced by the coincidental deposition of the same onto the same or 
similiar carrier provided that the solvent for the metal cation 
impregnating solution is selected in accordance with the invention. That 
is, contrary to prior art experiences with silver-first methods of 
preparation wherein the resulting catalysts invariably are less efficient, 
even at their optimum, than corresponding catalysts prepared by a method 
of coincidental deposition, the catalysts of the invention provide 
improved selectivities to ethylene oxide and are equally as efficient as 
catalysts produced by coincidental methods of preparation. 
As used herein with reference to the silver catalyst and process of the 
invention, the term "optimum" efficiency is defined as the highest 
efficiency obtainable at any concentration of promoter for a given silver 
content, catalyst carrier, and method of preparation when tested at fixed 
operating conditions. 
The solvent employed in the metal cation impregnating solution is an 
essential feature of the present invention. The organic solvents useful 
for the invention are characterized by their capability of forming a 
silver complex in the presence of silver ion. Contrary to the disclosure 
in the aforementioned Japanese Patent Application No. 142,421/78, it has 
been found that such silver complex forming solvents can be effectively 
used in a silver-first sequential process of preparation in amounts in 
excess of 30%, by volume, of the impregnating solution. Indeed, as 
hereinafter described, depending upon the solubility of the metal promoter 
in such solvents, they are advantageously employed in the impregnating 
solution in concentrations as high as possible, generally above 50 weight 
%, and preferably about 80% or higher by weight of solution. Further, 
unlike the process of the aforementioned Japanese Application, the 
presence of a lower alcohol in the impregnating solution is not required 
in the method of the present invention. The term "lower alcohol" as used 
in this specification and the claims means an alcohol of not more than 
four carbon atoms. 
Suitable solvents used for impregnating the metal promoter in accordance 
with the invention include, among others, amino alcohols such as 
monoethanolamine; alkylene diamines such as ethylenediamine; alkyl amines 
such as isopropylamine; amino ethers such as bis(2-amino) ethyl ether; and 
amides such as formamide. 
In addition to the aforementioned improved catalyst efficiencies, another 
important characteristic of the process of the invention and one which 
provides an unexpected advantage over conventional methods of catalyst 
preparation relates to the fact that the amount of alkali metal promoter 
deposited upon the carrier need not be as narrowly controlled as in the 
prior art to achieve an optimum catalyst efficiency. It is known in the 
art that the coincidental method of producing ethylene oxide catalysts 
requires strict control of the amount of promoter added to the carrier in 
order to maximize the catalyst efficiency for the given carrier and silver 
content. The effect of promoter concentration on catalyst efficiency is 
graphically demonstrated by the drawing presented in the above-mentioned 
U.S. Ser. No. 216,188 (the parent application of the Nielsen et al U.S. 
patent ) which depicts the relative effects of cesium, rubidium and 
potassium as respective promoters in enhancing the efficiency of a silver 
catalyst to make ethylene oxide. Curves A, B and C of the drawing show the 
appropriate concentration ranges in which potassium, rubidium and cesium, 
respectively, provide the greatest degree of selectivity enhancement. From 
the curves it is evident that the amount of alkali metal which must be 
added to the carrier is critical if the maximum catalyst efficiency is to 
be realized. By way of comparison, in the present process the promoter 
concentration required to produce catalysts having optimum selectivities 
to ethylene oxide is not as narrowly critical. For example, the range of 
alkali metal concentrations capable of providing the optimum efficiency is 
far broader than the corresponding range for catalysts produced by 
coincidental methods of preparation in which alkali metals are the 
promoters. Thus, an important advantage of the present process resides in 
the fact that commercial-scale batches of ethylene oxide catalysts can be 
manufactured within a relatively broad specification of the metal content 
and still achieve optimum efficiency. 
When alkali metals are the promoters, the amount of alkali metal needed on 
the catalyst support according to the process of this invention to achieve 
an optimum efficiency is typically at least 10% greater than that amount 
of like alkali metal which provides the maximum enhancement of efficiency 
when used in a coincidental method of preparation with the same amount of 
silver and the same catalyst support. Even though this is the case, the 
amount of alkali metal to achieve optimum efficiency is still not as 
narrowly critical and will vary depending upon silver content, the 
catalyst support employed, the solvent for the alkali metal impregnating 
solution, and other catalyst preparation variables. 
CATALYST PREATION 
The catalyst preparation method of the invention concerns a silver-first 
sequential addition of silver and metal cation promoter to a porous 
carrier. Stated simply, the process involves a sequence of steps carried 
out in the following order: 
First, impregnating a porous catalyst support by immersing same in a 
silver-containing impregnating solution; 
Second, treating the impregnated support to effect deposition of silver on 
the surface of said support; 
Third, impregnating the product of step two by immersing same in a metal 
cation-containing impregnating solution as defined herein; and 
Fourth, treating the impregnated support to effect deposition of the metal 
promoter on the surface of said support. 
Silver deposition is generally accomplished by heating the impregnated 
carrier at elevated temperatures to evaporate the liquid within the 
support and effect deposition of the silver onto the interior and exterior 
carrier surfaces. Alternatively, a coating of silver may be formed on the 
carrier from an emulsion or slurry containing the same followed by heating 
the carrier as described above. Impregnation of the carrier is generally 
the preferred technique for silver deposition because it utilizes silver 
more efficiently than coating procedures, the latter being generally 
unable to effect substantial silver deposition onto the interior surfaces 
of the carrier. 
The silver solution used to impregnate the carrier is comprised of a silver 
salt or compound in a solvent or complexing/solubilizing agent such as the 
silver solutions disclosed in the art. The particular silver salt employed 
is not critical and may be chosen, for example, from among silver nitrate, 
silver oxide or silver carboxylates, such as, silver acetate, oxalate, 
citrate, phthalate, lactate, propionate, butyrate and higher fatty acid 
salts. 
A wide variety of solvents or complexing/solubilizing agents may be 
employed to solubilize silver to the desired concentation in the 
impregnating medium. Generally, the silver concentration in the 
impregnating medium should be sufficient to deposit on the support from 
about 2 to about 20 wt. % of silver based on the total weight of the 
catalyst. Among solvents disclosed in the art as being suitable for this 
purpose are lactic acid (U.S. Pat. Nos. 2,477,435 to Aries; and 3,501,417 
to DeMaio); ammonia (U.S. Pat. No. 2,463,228 to West et al); alcohols, 
such as ethylene glycol (U.S. Pat. Nos. 2,825,701 to Endler et al; and 
3,563,914 to Wattimena); and amines and aqueous mixtures of amines (U.S. 
Pat. Nos. 2,459,896 to Schwartz; 3,563,914 to Wattimena, 3,702,259 to 
Nielsen; and 4,097,414 to Cavitt). 
Following impregnation of the catalyst carrier with silver, the impregnated 
carrier particles are separated from any remaining non-absorbed solution 
or slurry. This is conveniently accomplished by draining the excess 
impregnating medium or alternatively by using separation techniques, such 
as, filtration or centrifugation. The impregnated carrier is then 
generally heat treated (e.g., roasted) to effect decomposition and 
reduction of the silver metal salt to metallic silver. Such roasting may 
be carried out at a temperature of from about 100.degree. C. to 
900.degree. C., preferably from 200.degree. C. to 700.degree. C., for a 
period of time sufficient to convert substantially all of the silver salt 
to silver metal. In general, the higher the temperature, the shorter the 
required reduction period. For example, at a temperature of from about 
400.degree. C. to 900.degree. C., reduction may be accomplished in about 1 
to 5 minutes. Although a wide range of heating periods have been 
suggested in the art to thermally treat the impregnated support, (e.g., 
U.S. Pat. No. 3,563,914 suggests heating for less than 300 seconds to dry 
but not roast reduce the catalyst; U.S. Pat. No. 3,702,259 discloses 
heating from 2 to 8 hours at a temperature of from 100.degree. C. to 
375.degree. C. to reduce the silver salt in the catalyst; and U.S. Pat. 
No. 3,962,136 suggests 1/2 to 8 hours for the same temperature range) it 
is only important that the reduction time be correlated with temperature 
such that substantially complete reduction of the silver salt to metal is 
accomplished. A continuous or step-wise heating program may be used for 
this purpose. 
Impregnation of the carrier with a solution containing a promoter salt or 
compound is carried out after silver deposition has been effected. The 
impregnating solution is prepared using one or more solvents as herein 
defined and contains an amount of promoter sufficient to achieve the 
desired concentration of promoter in the finished catalyst. The 
impregnated carrier particles are conveniently separated from any 
remaining non-absorbed solution by draining the excess impregnating 
solution or alternatively by using separation techniques, such as, 
filtration and centrifugation. The impregnated carrier is then generally 
heat treated at ambient or sub-atmospheric pressure to remove the solvent 
(or solvents) present and deposit (with or without decomposition) the 
alkali metal ions on to the silver and carrier surfaces. Such heating may 
be carried out at a temperature of from about 50.degree. C. to 900.degree. 
C., preferably from about 100.degree. C. to 700.degree. C. and most 
preferably from about 200.degree. C. to about 600.degree. C. 
Suitable alkali metal promoter compounds include all those soluble in the 
particular solvent or solubilizing agent employed. Accordingly, inorganic 
and organic compounds of alkali metals, such as, nitrates, halides, 
hydroxides, sulfates and carboxylates may be used. An inherent advantage 
of the process of the invention is that it allows the use of certain 
promoter compounds which could not ordinarily be used in conjunction with 
known coincidental methods of preparation because of the incompatibility 
of such salts with the impregnating solution used in the latter processes. 
As an illustration, alkaline earth salts such as salts of barium, calcium 
and magnesium can readily be solubilized in an impregnating solution and 
deposited upon the carrier in accordance with the process of the 
invention, but can not be added to an impregnating solution containing, 
for example, oxalic acid or carboxylic acid, solutions commonly employed 
in conventional coincidental methods of preparation for purposes of silver 
solubilization. 
The types of solvents useful for preparing the promoter impregnating 
solution are set forth above. Such solvents may be employed individually 
or in various combinations with each other provided that the salt of the 
desired promoter is sufficiently soluble therein. In the event that the 
promoter salt is not sufficiently soluble in the organic solvent to 
provide the desired concentration in the resulting impregnating solution, 
water may be added as a co-solvent for the promoter salt. Generally, 
organic solvent concentrations of 50 wt. % and higher are commonly 
employed for impregnation. In general, it is preferred that the 
concentration of organic solvent in the impregnating solution be as high 
as possible. 
Heat treatment of the impregnated carriers is preferably carried out in 
air, but a nitrogen, carbon dioxide or hydrogen atmosphere may also be 
employed. The equipment used for such heat treatment may use a static or 
flowing atmosphere of such gases to effect reduction. 
The particle size of silver metal deposited upon the carrier is a function 
of the catalyst preparation procedure employed. Thus, the particular 
choice of solvent and/or complexing agent, silver salt, heat treatment 
conditions and catalyst carrier may affect, to varying degrees, the size 
of the resulting silver particles. For carriers of general interest for 
the production of ethylene oxide, a distribution of silver particle sizes 
in the range of 0.05 to 2.0 microns is typically obtained. 
CARRIER SELECTION 
The catalyst carrier employed in practicing the invention may be selected 
from conventional, porous, refractory materials which are essentially 
inert to ethylene, ethylene oxide and other reactants and products at 
reaction conditions. These materials are generally labelled as 
"macroporous" and consist of porous materials having surface areas less 
than 10 m.sup.2 /g (square meters per gram of carrier) and preferably less 
than 1 m.sup.2 /g. The surface area is measured by the conventional B.E.T. 
method described by Brunauer, S., Emmet, P., and Teller, E., in J. Am. 
Chem. Soc. Vol. 60, pp 309-316, (1928). They typically possess pore 
volumes in the range of about 0.15-0.8 cc/g. A more preferred range is 
about 0.2-0.6 cc/g. Pore volumes may be measured by conventional mercury 
porosimetry or water absorption techniques. Median pore diameters for the 
above-described carriers range from about 0.01 to 100 microns, a more 
preferred range being from about 0.5 to 50 microns. 
Preferably, the carrier should not contain ions which are exchangeable with 
the metal cations supplied to the catalyst, either in the preparation or 
use of the catalyst. If the carrier contains such ion, the ion should be 
removed by standard chemical techniques such as leaching. 
The chemical composition of the carrier is not narrowly critical. Carriers 
may contain fused or bonded particles of, for example, of alpha-alumina, 
silicon carbide, silicon dioxide, zirconias, magnesia and various clays. 
In general, alpha-alumina based materials are preferred. These 
alpha-alumina based materials may be of very high purity, i.e., 98+weight 
% alpha-alumina, the remaining components being silica, alkali metal 
oxides (e.g., sodium oxide) and trace amounts of other metal and non-metal 
impurities; or they may be of lower purity, i.e., about 80 weight % 
alpha-alumina, the balance being a mixture of silicon dioxide, various 
alkali oxides, alkaline earth oxides, iron oxide, and other metal and 
non-metal oxides. The lower purity carriers are formulated so as to be 
inert under catalyst preparation and reaction conditions. A wide variety 
of such carriers are commercially available. The carriers are preferably 
shaped, typically in the form of pellets, extruded particles, spheres, 
rings and the like, for use in commercial reactors. The size of the 
carriers may vary from about 1/16" to 1/1". The carrier size and shape is 
chosen to be consistent with the type of reactor employed. In general, for 
fixed bed reactor applications, sizes in the range of 1/8" to 3/8" have 
been found to be most suitable in the typical tubular reactor used in 
commercial operations. 
ETHYLENE OXIDE PRODUCTION 
The silver catalysts of the invention are particularly suitable for use in 
the production of ethylene oxide by the vapor phase oxidation of ethylene 
with molecular oxygen. The products of the reactions are ethylene oxide 
and CO.sub.2 as a consequence of the following two competing reactions: 
EQU C.sub.2 H.sub.4 +1/2O.sub.2 .fwdarw.C.sub.2 H.sub.4 O (1) 
EQU C.sub.2 H.sub.4 +3O.sub.2 .fwdarw.2CO.sub.2 +2H.sub.2 O (2) 
The success in making reaction (1) more favored results in higher process 
efficiencies to ethylene oxide. The reaction conditions for carrying out 
the oxidation reaction are wellknown and extensively described in the 
literature. This applies to reaction conditions, such as, temperature, 
pressure, residence time, concentration of reactants, diluents (e.g., 
nitrogen, methane and recycled CO.sub.2), inhibitors (e.g., ethylene 
dichloride) and the like. In addition, the desirability of recycling 
unreacted feed, or employing a single-pass system, or using successive 
reactions to increase ethylene conversion by employing reactors in series 
arrangement can be readily determined by those skilled in the art. The 
particular mode of operation selected will usually be dictated by process 
economics. 
Generally, the process is carried out by continuously introducing a feed 
stream containing ethylene and oxygen to a catalyst-containing reactor at 
a temperature of from about 200.degree. to 300.degree. C., and a pressure 
which may vary from one atmosphere to about 30 atmospheres depending upon 
the mass velocity and productivity desired. Residence times in large-scale 
reactors are generally on the order of about 1-5 seconds. Oxygen may be 
supplied to the reaction in an oxygen-containing stream, such as, air or 
as commercial oxygen. The resulting ethylene oxide is separated and 
recovered from the reaction products using conventional methods. Byproduct 
CO.sub.2 is usually recycled in part with the unreacted ethylene to the 
reaction in commercial operations. 
CATALYST TESTING 
The catalysts cited in the Tables of the Examples below were all evaluated 
under standard test conditions using backmixed, bottom-agitated 
"magnedrive" autoclaves as described in FIG. 2 of the paper by J.M. Berty 
entitled "Reactor For Vapor Phase-Catalytic Studies", in Chemical 
Engineering Progress, Vol. 70, No. 5, pages 78-84, 1974. The reactor was 
operated at 1.0 mole % ethylene oxide in the outlet gas under the 
following standard inlet conditions: 
______________________________________ 
Component Mole % 
______________________________________ 
Oxygen 6.0 
Ethylene 8.0 
Ethane 0.50 
Carbon Dioxide 6.5 
Nitrogen Balance of Gas 
Parts per millon 
7.5 
Ethylene Chloride 
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The pressure was maintained constant at 275 psig and the total outlet flow 
maintained at 22.6 SCFH..sup.(1) The outlet ethylene oxide concentration 
was maintained at 1.0% by adjusting the reaction temperature. Thus, 
temperature (.degree.C.) and catalyst efficiency are obtained as the 
responses describing the catalyst performance. 
FNT (1) SCHF refers to cubic feet per hour at standard temperature and 
pressure, namely, 0.degree. C. and one atmosphere. 
A typical catalyst test procedure is comprised of the following steps: 
1. 80 cc of catalyst is charged to a backmixed autoclave. The volume of 
catalyst is measured in a 1" I.D. graduated cylinder after tapping the 
cylinder several times to thoroughly pack the catalyst. The weight of the 
catalyst is noted. 
2. The backmixed autoclave is heated to about reaction temperature in a 
nitrogen flow of 20 SCFH with the fan operating at 1500 rpm. The nitrogen 
flow is then discontinued and the above-described feed stream is 
introduced into the reactor. The total gas outlet flow is adjusted to 22.6 
SCFH. The temperature is adjusted over the next few hours so that the 
ethylene oxide concentration in the outlet gas is approximately 1.0%. 
3. The outlet oxide concentration is monitored over the next 4-6 days to 
make certain that the catalyst has reached its peak steady state 
performance. The temperature is periodically adjusted to achieve 1% outlet 
oxide. The selectivity of the catalyst to ethylene oxide and the 
temperature are thus obtained. 
The standard deviation of a single test result reporting catalyst 
efficiency in accordance with the procedure described above is 0.7% 
efficiency units. 
It should be noted that the above-described back mixed autoclave generates 
lower efficiencies than tubular reactors, hence, the efficiencies 
described herein are not directly comparable with those obtained in a 
tubular reactor. In addition, the catalyst particles tested in the 
following examples are shaped for use in commercial sized tubular 
reactors. Such particles are known to yield lower efficiencies than 
crushed catalyst or catalyst made on a crushed support, but they have a 
significant advantage for operation in a commercial reactor in that they 
do not create an undesirable pressure drop across the catalyst bed as 
would crushed catalyst or catalyst made on a crushed support.

EXAMPLE 1 
A catalyst containing 13 weight % Ag was prepared as hereinafter described 
on an alpha-alumina carrier "A" shaped as a ring having a diameter of 
5/16", a length of 5/16" and 1/8" diameter hole. The carrier had the 
following chemical composition and physical properties. 
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Chemical Composition of Carrier "A" 
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Alpha-Alumina 98.6 wt. % 
Silicon Dioxide 0.74 wt. % 
Calcium Oxide 0.22 wt. % 
Sodium Oxide 0.16 wt. % 
Ferric Oxide 0.14 wt. % 
Potassium Oxide 0.03 wt. % 
Magnesium Oxide 0.03 wt. % 
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Physical Properties of Carrier "A" 
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Surface Area.sup.(1) 0.3 m.sup.2 /g 
Pore Volume.sup.(2) 0.50 cc/g 
(or water absorption) 
Packing Density.sup.(3) 
0.70 g/ml 
Median Pore Diameter 21 microns 
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Pore Size Distribution, % Total Pore Volume (% TPV).sup.(4) 
Pore Size, Microns 
% TPV 
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0.1-1.0 1.5 
1.0-10.0 38.5 
10.0-30.0 20.0 
30-100 32.0 
&gt;100 8.0 
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.sup.(1) Method of measurement described in "Adsorption, Surface Area and 
Porosity", S. J. Gregg and K. S. W. Sing, Academic Press (1967), pages 
316-321. 
.sup.(2) Method of Measurement as described in ASTM C2046 
.sup.(3) Calculated value based on conventional measurement of the weight 
of the carrier in a known volume container. 
.sup.(4) Method of measurement described in "Application of Mercury 
Penetration to Materials Analysis", C. Orr Jr., Powder Technology, Vol. 3 
pp. 177-123 (1970). The carrier "A" was impregnated under vacuum as 
hereinafter described with a solution of silver salts which was prepared 
at a concentration such that the finished catalyst contained the desired 
amount of silver. The required concentration of silver in solution for the 
given carrier is calculated from the packing density (grams/cc) and the 
pore volume of the carrier which are either known or readily determined. 
Assuming that all of the silver in the impregnating solution contained in 
the pores of carrier "A" is deposited upon the carrier, approximately 23.6 
weight % silver in solution is needed to prepare a catalyst containing 
about 13 weight % silver. 
Preparation Of Silver Impregnating Solution 
774.9 gms of ethylenediamine (high purity grade) was mixed with 1600 g of 
distilled water with continuous stirring in a 7 liter stainless steel 
beaker containing a three inch stirring bar, the vessel being mounted on a 
6".times.6" magnetic stirrer-hot plate. The ingredients were added to the 
vessel in the order described with constant stirring. The resulting 
solution was cooled to 25.degree. C. and 812 g of oxalic acid dihydrate 
(reagent grade) was added in small portions, with continuous stirring, at 
a rate which maintained the temperature below 50.degree. C. Silver oxide 
powder, 1423.5 g, (Handy and Harmon, 850 Third Avenue, New York, N.Y. 
10022) was then added intermittently to the aqueous ethylenediamine oxalic 
acid solution while maintaining the temperature of the solution below 
50.degree. C. Finally, 283 g of monoethanolamine and 703 g of distilled 
water were added to bring the total volume of the impregnating solution to 
4000 cc. The specific gravity of the resulting solution was about 1.385. 
Catalyst Preparation 
A 2636 grams charge of carrier "A" was placed in a 5 liter, round bottomed 
vessel equipped with a side arm fitted with a stopcock connected to a 
three foot long, 1/4" O.D. tubing for the introducion of the impregnating 
solution which was contained in the above-described 7 liter stainless 
steel beaker located adjacent to the vessel. The vessel containing the 
carrier was evacuated to approximately 2 inches of mercury pressure for 
about 20 minutes after which the impregnating solution was slowly added to 
the carrier by opening the stopcock between the vessel and the beaker 
containing the impregnating solution until the carrier was completely 
immersed in solution. The vessel was then opened to the atmosphere to 
achieve atmospheric pressure, the carrier remaining immersed in the 
impregnating solution at ambient conditions for about one hour and 
thereafter drained of excess solution for about 30 minutes. 
The impregnating carrier was removed from the vessel and heat treated as 
follows to effect reduction of the silver salt. The impregnated carrier 
was spread out in a single layer of pellets on a 25/8" wide endless 
stainless steel belt (spiral weave) and transported through a 2".times.2" 
square heating zone for 2.5 minutes, the heating zone being maintained at 
500.degree. C. by passing hot air upward through the belt and about the 
catalyst particles at the rate of 266 SCFH. The hot air was generated by 
passing it through a 5 ft. long .times.2" I.D. stainless steel pipe which 
was externally heated by an electric furnace (Lindberg.TM. tubular 
furnace: 21/2" I.D., 3 feet long heating zone) capable of delivering 5400 
watts. The heated air in the pipe was discharged from a square 2".times.2" 
discharge port located immediately beneath the moving belt carrying the 
catalyst carrier. After being roasted in the heating zone, the silver 
impregnated catalyst was weighed, and based upon the weight gain of the 
carrier, was calculated to contain 13.1 weight % silver. The 
silver-containing catalyst is referred to as catalyst 1. 
Addition of Promoters 
To demonstrate the effect which the solvent in the alkali metal 
impregnating solution has on the efficiency of the finished catalyst, five 
(5) catalysts of similar composition were prepared (1A-1E) using a 
different impregnating solution for each one, each solution containing a 
different solvent as described below. Catalysts containing 13.1 wt. % 
silver, 0.00906 wt. % cesium and 0.00268 wt. % potassium were prepared 
from the above described catalyst 1 by the sequential addition of cesium 
and potassium promoters in accordance with the following general 
procedure. 
Each of the impregnating solutions used to prepare catalysts 1A through 1E 
was prepared by adding (a) 5.825 ml of an aqueous cesium hydroxide 
solution containing 0.0566 g of cesium and (b) 4.456 ml of an aqueous 
potassium carbonate solution containing 0.0167 g of potassium, to a 250 ml 
graduated cylinder. To each of the graduated cylinders there was added one 
of solvents A through E, identified below, in an amount sufficient to 
provide 250 ml of total solution. 
SOLVENTS 
A. Water 
B. A solution of 80 wt. % water, 20 wt. % monoethanolamine. 
C. A solution of 50 wt. % water, 50 wt. % monoethanolamine. 
D. Monoethanolamine. 
E. A solution of water-ethylenediamine-oxalic acid-monoethanolamine as 
described above in the preparation of catalyst 1, but containing no 
silver. 
For the preparation of each catalyst, a 100 g sample of catalyst 1 was 
placed in a 12" long.times.1.5" I.D. glass cylindrical vessel equipped 
with a side arm fitted with a stopcock so as to allow the evacuation of 
the vessel using a vacuum pump. A 500 ml separatory funnel containing one 
of the impregnating solutions described above was inserted through a 
rubber stopper in the top of the vessel. The impregnating vessel 
containing catalyst 1 was evacuated to approximately 2 inches of mercury 
pressure for about 20 minutes after which the impregnating solution was 
slowly added to the carrier by slowly opening the stopcock between the 
separatory funnel and the impregnating vessel until catalyst 1 was 
completely immersed. Following the addition of solution, the system was 
opened to the atmosphere, catalyst 1 remaining immersed in the 
impregnating solution at ambient conditions for about 30 minutes. The 
impregnated carrier was drained of excess solution and heat treated to 
effect deposition of alkali metal on the carrier in the same manner as 
described above with regard to the preparation of catalyst 1. The finished 
catalysts prepared from the impregnating solutions containing one of the 
solvents A-E are designated 1A, 1B, 1C, 1D and 1E, respectively. 
Table I below summarizes the test results for catalysts 1A through 1E when 
used for the oxidation of ethylene in accordance with the procedure 
detailed above. 
TABLE I 
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Catalyst Selectivity, % 
Temperature .degree.C. 
______________________________________ 
1A 70.6 261.4 
1B 71.8 257.5 
1C 73.7 252.4 
1D 74.0 256.8 
1E 75.3 251.9 
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As noted from Table I, catalyst 1A, which was prepared in accordance with 
the method of the prior art, provided the lowest selectivity in comparison 
with catalysts 1B-1E, prepared by the method of the invention.