Propylene oxide process using alkaline earth metal compound-supported silver catalysts

Propylene is oxidized to propylene oxide in the vapor phase using an oxygen-containing gas and a supported silver catalyst comprising silver and a support comprised in whole or in substantial part of certain alkaline earth metal compounds. The alkaline earth metal compound may, for example, be a calcium compound such as calcium titanate, tribasic calcium phosphate, calcium molybdate, or calcium fluoride, a magnesium compound such as magnesium aluminate, or a strontium compound such as strontium titanate. Such supports provide significantly higher selectivity to the desired epoxide than would be expected from the performance of related materials. Propylene oxide selectivity may be further enhanced through the introduction of nitrogen oxide species such as NO, alkyl halides such as ethyl chloride, and carbon dioxide into the oxygen-containing gas.

FIELD OF THE INVENTION 
This invention relates to a process for the direct oxidation of propylene 
to propylene oxide in the vapor phase using molecular oxygen. In 
particular, the invention pertains to the use of catalysts comprised of 
silver supported on certain alkaline earth metal-containing compounds to 
selectively form the epoxide. 
BACKGROUND OF THE INVENTION 
The direct oxidation of ethylene to ethylene oxide by molecular oxygen is 
well-known and is, in fact, the method used currently for commercial 
production of ethylene oxide. The typical catalyst for such purpose 
contains metallic or ionic silver, optionally modified with various 
promoters and activators. Most such catalysts contain a porous, inert 
support or carrier such as alpha alumina upon which the silver and 
promoters are deposited. A review of the direct oxidation of ethylene in 
the presence of supported silver catalysts is provided by Sachtler et al. 
in Catalyst Reviews: Science and Engineering, 23 (1& 2), 127-149 (1981). 
It is also well-known, however, that the catalysts and reaction conditions 
which are best suited for ethylene oxide production do not give comparable 
results in the direct oxidation of higher olefins such as propylene. The 
discovery of processes capable of providing propylene oxide by vapor phase 
direct oxidation in higher yields than are presently attainable thus would 
be most desirable. 
New support materials are continuously being tried. However, many of those 
which were employed in the early development of the silver-bearing 
catalysts are, with some modifications, still being used. Materials which 
have found most widespread use are typically inorganic and generally are 
of a mineral nature. 
Alumina, in its various forms, particularly alpha-alumina, has been 
preferred as a support material for silver-containing catalysts in the 
preparation of epoxides. Numerous variations of surface area, pore 
dimensions, pore volume and particle size have been suggested as providing 
the ideal physical property or combination of properties for improving 
efficiency, activity or useful life of the catalyst. 
In seeking the ideal support material, there has been some departure from 
the commonly employed substances. For example, some use has been made of 
alkali metal and alkaline earth metal carbonates, both as the sole support 
material and in combination with other materials as the carrier for 
processes such as direct oxidation of alkenes to epoxides. For example, 
Canadian Pat. No. 1, 282,772 teaches the use of alkaline earth metal 
carbonates as supports for silver catalysts in olefin epoxidation systems. 
The development of alternative supports which provide equivalent or 
improved performance in epoxidation process as compared to known materials 
would be highly advantageous, as such alternative supports may be of lower 
cost or provide other practical benefits such as higher strength or 
structural integrity. Selecting materials which will be suitable for such 
purpose is not straightforward, however. For example, as will be 
subsequently demonstrated, not all alkaline earth metal-containing 
compounds perform equivalently as supports for silver epoxidation 
catalysts. Structurally analogous substances often exhibit radically 
different behavior in an epoxidation process. Predicting in advance which 
substances will provide the high degree of selectivity to epoxide which is 
required in a commercial process thus is nearly impossible. 
European Pat. No. 393,785 teaches a catalyst for the manufacture of 
alkylene oxide containing an impregnated silver metal on an inert 
refractory solid support, at least one promoter to enhance the efficiency 
of the catalyst and a manganese component. The efficiency promoter may be 
a compound comprising at least one alkali metal or oxyanion of an element 
other than manganese or oxygen selected from group 3b through 7b and 3a 
through 7a of the Periodic Table; titanates and phosphates are listed as 
being suitable oxyanions for such purpose. A maximum of 2 weight % of the 
anion in the catalyst is taught. A cationic promoter such as an alkaline 
earth metal may also be present up to a concentration of 1 weight percent 
in the finished catalyst. This publication thus does not contemplate the 
use of alkaline earth metal titanates or phosphates as the inert 
refractory solid support. 
SUMMARY OF THE INVENTION 
This invention provides a process for propylene epoxidation wherein a 
feedstream comprising oxygen and propylene is contacted in the vapor phase 
at a temperature of 180.degree. C. to 350.degree. C. with a supported 
silver catalyst comprising silver and a support comprising an alkaline 
earth metal-containing compound selected from the group consisting of 
alkaline earth metal titanates, tribasic calcium phosphate, magnesium 
aluminate, calcium molybdate, calcium fluoride, and mixtures thereof. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to a process for the vapor phase 
oxidation of propylene to propylene oxide, i.e., an epoxidation process 
performed in the presence of molecular oxygen and a particular class of 
supported silver catalysts. 
The support material used in the present invention is selected from one of 
several alkaline earth metal compound-containing carrier materials. The 
alkaline earth metal compound employed is an inorganic compound containing 
one or more alkaline earth metals, particularly calcium, strontium, 
magnesium or barium with calcium, strontium and barium being most 
preferred. Depending upon the alkaline earth metal selected, the alkaline 
earth metal compound may additionally contain titanate, phosphate, 
aluminate, molybdate, fluoride, or some combination thereof. Specifically, 
the alkaline earth metal compound is selected from the group consisting of 
alkaline earth metal titanates (e.g., calcium titanate, strontium 
titanate), tribasic calcium phosphate, magnesium aluminate, calcium 
molybdate, calcium fluoride and mixtures thereof. 
Tribasic calcium phosphate is an inorganic substance corresponding to the 
approximate empirical formula Ca.sub.10 (OH).sub.2 (PO.sub.4), containing 
34-40% Ca, and having the CAS Registry number CAS 12167-74-7. As will be 
demonstrated subsequently in the examples, tribasic calcium phosphate has 
unexpectedly been found to be far superior as a support material than 
related substances such as tricalcium phosphate (CAS 7758-87-4) and 
hydroxyapatite (CAS 1306-06-5). 
Calcium molybdate is the calcium salt of molybdic acid and has the chemical 
composition CaMoO.sub.4. 
Calcium fluoride has the chemical composition CaF.sub.2 and is found in 
nature as fluorite (pure form) or fluorspar (mineral), but may also be 
prepared synthetically by the reaction of a soluble calcium salt and 
sodium fluoride. 
Magnesium aluminate is an oxide of magnesium and aluminum corresponding 
approximately to the empirical formula MgO.Al.sub.2 O.sub.3. 
Alkaline earth metal titanates comprise the class of inorganic substances 
containing an alkaline earth metal such as barium, strontium, calcium or 
magnesium and a titanate species. Suitable alkaline earth metal titanates 
thus may correspond to the empirical formula MTiO.sub.3, M.sub.2 
TiO.sub.4, and MTi.sub.2 O.sub.5 wherein M preferably =Ba, Sr, Ca, or Mg. 
Any of the conventional methods for preparing such substances may be 
utilized. Barium titanate, for example, may be prepared by heating a 
mixture of the correct proportions of barium carbonate and titanium 
dioxide at 1300.degree. C. until the reaction is complete. Strontium 
titanate may be obtained in pure form by calcining the double strontium 
titanium oxalate precipitate from titanium tetrachloride solution. The 
calcium titanate can correspond to the compound CaTio.sub.3 (12049-50-2), 
which occurs naturally as the mineral perovskite, but which can also be 
synthesized by heating equimolar amounts of the oxide to 1350.degree. C. 
The term "calcium titanate" as used herein also embraces the substances 
having the formula 3CaO.2TiO.sub.2 (CAS 12013-80-8) and 3CaO.TiO (CAS 
12013-70-6). Magnesium titanates include the metatitanate MgTiO.sub.3, the 
orthotitanate Mg.sub.2 TiO.sub.4, and the dititanate MgTi.sub.2 O.sub.5. 
Such support materials are capable of providing exceptionally high 
propylene oxide selectivities and have been found to be surprisingly 
superior to other support materials in this respect. The carriers of the 
present invention may exist in various forms. In one embodiment, the 
carrier is one in which the alkaline earth metal compound is the 
predominate (i.e., at least 50% by weight) or, preferably, substantially 
the exclusive component of the support (i.e., the support consists 
essentially of one or more alkaline earth metal compounds). In other 
embodiments of the invention, the inorganic support material is used in 
conjunction with a solid substrate, i.e., a subsupport or substructure 
composed of a more conventional support material, such as alumina 
(preferably, alpha-alumina). This latter type of support may employ the 
alkaline earth metal compound material coated on individual, relatively 
small particles of substructure or subsupport or on a larger unit such as 
a three-dimensional framework having a honeycomb-type of structure. 
However, the alkaline earth metal compound support material will comprise 
at least 25 weight percent (in some embodiments, at least 35 weight 
percent) of the finished catalyst. The concentrations of alkaline earth 
metal compounds in the catalysts of the present invention thus are 
considerably greater than the amounts of compounds typically utilized by 
prior workers as promoters in supported silver catalysts. 
A granular form of the alkaline earth metal carbonate support material is 
preferred in the present invention, particularly when used as the 
exclusive or predominant component of the support. Alkaline earth metal 
carbonate materials suitable for use in the present invention may be 
commercially obtained as powders which can be converted to the preferred 
granular form by conventional methods. As described in greater detail 
below, the granular support may then be impregnated, or coated, with a 
solution containing a silver compound and thereafter reduced to elemental 
silver. 
Alternatively, as described below, the powdered granular support material 
may be combined with an appropriate silver-containing solution, such as 
that used conventionally to impregnate solid supports to form a slurry or 
paste. This material may then be spread on a suitable surface and dried 
and calcined at an appropriate temperature, such as about 500.degree. C. 
This results in an alkaline earth metal compound support with silver being 
supported thereon in its elemental state. The catalyst may then be 
impregnated with solutions of promoters, modifiers, co-catalysts or other 
additives of the types well known in the supported silver oxidation 
catalyst art (hereinafter referred to collectively as "promoters"), if so 
desired and thereafter dried. As an alternative, promoters may be 
dissolved in the same silver-containing impregnation solution used to form 
the coating paste or slurry with the alkaline earth metal compound 
material. 
The support material, before or after incorporation of the silver and 
optional promoter(s), can be formed into shaped composites suitable for 
use in propylene oxide manufacture. The composites may be formed by any 
suitable technique. For instance, it is possible to form the composites by 
compressing the support materials into a mold having a desired 
configuration. The size of the particles may be selected to be appropriate 
for the formation of the composite and are often in the range of about 
0.001 to about 5 millimeters in major dimension. 
When coated catalysts, i.e., those catalysts in which the alkaline earth 
metal compound material is coated on a substructure are employed, a slurry 
of said material, in either powder or granular form, may be mixed with the 
particles of substructure support material and thereafter dried. As with 
the predominant or exclusive alkaline earth metal compound support 
materials described above, the coated catalysts may also be prepared by 
using a solution of a silver compound and any promoter or the like which 
may be desired or separate solutions of silver compound and promoter(s) to 
form the slurry, followed by suitable drying and calcination. 
The surface area of the alkaline earth metal compound support material 
generally is at least 0.6 m.sup.2 /g, preferably at least 1.5 m.sup.2 /g. 
However, alkaline earth metal compound support materials having relatively 
high surface areas are also effective for the purposes of this invention. 
For instance, tribasic calcium phosphate support materials having surface 
areas of 50 to 100 m.sup.2 /g have been found to function quite 
effectively in the present invention. This finding was unexpected in view 
of the fact that support materials such as alpha alumina which are 
conventionally used for silver vapor phase oxidation catalysts preferably 
have much lower surface areas. The surface area is measured by the 
conventional B. E. T. method using nitrogen or krypton described by 
Brunauer, Emmett and Teller in J. Am. Chem. Soc. 60, 309-16 (1938). 
The support materials used in the present invention may generally be 
described as porous or microporous and typically have water pore volumes 
of about 0.05 to 0.80 cc/g. 
The supported silver catalysts are typically used as individual particles 
of irregular shape and size. This is true both for the predominate or 
exclusive alkaline earth metal compound supports as well as the alkaline 
earth metal compound-coated supports. However, in some instances the 
supports, particularly the coated supports, may have a particular shape 
and size and this is especially true of the subsupports used with the 
alkaline earth metal compound. Typically the subsupports are formed into 
aggregates or "pills" of a size and configuration to be usable in tubular 
reactors. These pills may be formed by conventional extrusion and firing 
techniques. The pills generally range in size from about 2 mm to about 15 
mm, preferably about 3 mm to about 12 mm. The size is chosen to be 
consistent with the type of reactor employed. For example, in fixed bed 
reactor applications, sizes ranging from about 3 mm to about 10 mm have 
been found to be most suitable in the tubular reactors commonly utilized. 
The shapes of the carrier aggregates useful for purposes of the present 
invention can vary widely and can be any of the forms conventionally used 
in the heterogeneous catalyst art. 
The alkaline earth metal compound- and alkaline earth metal compound-coated 
supports may be prepared as indicated above or obtained commercially. The 
carbonate-supported catalyst of the present invention may be prepared by 
any known method of introducing silver and/or a promoter in soluble form, 
to a support. A preferred method of introducing silver to the alkaline 
earth metal compound support is by an impregnation process in which a 
solution of a soluble salt or silver compound (which can be a salt or 
complex of silver) in an amount sufficient to deposit the desired weight 
of silver upon the carrier is dissolved in a suitable solvent or 
"complexing/solubilizing" agent. The solution may be used to impregnate 
the support or carrier by immersing the carrier in the silver-containing 
impregnating solution and forming a pasty mixture or slurry. The slurry is 
then dried and calcined by placing the mixture in an oven or furnace at 
about 100.degree. C. to about 120.degree. C. for 0.5 to 6 hours and then 
heating the mixture at a temperature of from about 250.degree. C. to about 
600.degree. C. for another 1 to 6 hours. This procedure accomplishes 
drying of the alkaline earth metal compound/silver mixture, removes 
volatile components and reduces the silver present to its elemental form. 
Selectivity to the desired propylene oxide product may be further optimized 
by the incorporation of one or more promoters, additives, co-catalysts, 
modifying agents or the like into the supported silver catalyst. In one 
desirable embodiment, the catalyst contains not only an alkaline earth 
metal compound support and silver but also a potassium salt. 
The optional potassium salt may be introduced to the catalyst as an 
impregnation solution in a separate impregnation step. Again, this may be 
done by any known manner of impregnating a porous material. Conveniently, 
this may be carried out by placing the catalyst material in a container, 
evacuating the container and thereafter introducing the salt solution. 
Alternatively, the support may be sprayed or sprinkled with the 
impregnating solution. The excess solution may then be allowed to drain 
off or the solvent may be removed by evaporation under reduced pressure at 
a suitable temperature. The catalyst may then be dried at a moderate 
temperature (e.g., at 120.degree. C.) in a oven for one-half to five 
hours. Such a procedure is known as a "sequential" or "consecutive" method 
of preparation. The alkaline earth metal compound-supported catalyst may 
also be prepared by a "simultaneous" or "coincidental" method of 
preparation. With this method, the potassium salt is included in the 
silver compound-containing solution used to impregnate the support. 
The alkaline earth metal compound-coated catalysts are prepared by coating 
a suitable substructure or subsupport material, preferably alumina, and 
most preferably alpha alumina, with an alkaline earth metal 
compound-containing slurry. This may contain only the alkaline earth metal 
compound, in which case the coated support is further treated as indicated 
above to produce a silver or a silver and promoter alkaline earth metal 
compound-coated catalyst. Alternatively, an alkaline earth metal 
compound/silver compound slurry or an alkaline earth metal compound/silver 
compound/promoter slurry may be produced in a sequential or coincidental 
procedure. Thus, in a sequential procedure, particles or pills of a 
suitable subsupport material, such as alpha-alumina, are coated with a 
slurry of an alkaline earth metal compound material and a soluble salt or 
complex of silver dissolved in a complexing/solubilizing agent. The 
particles or pills are thereafter drained and calcined in an oven at a 
temperature of about 250.degree. C. to about 600.degree. C. for about 
three minutes to about four hours, the duration of heating being in 
general inversely proportional to the temperature employed. The catalyst 
is then impregnated in the manner described above with a solution of 
promoter, and then dried. The alkaline earth metal compound-coated 
supports may also be formed by a coincidental procedure in which an 
alkaline earth metal compound/silver compound/promoter slurry is used to 
coat particles or pills of a suitable subsupport. After draining, the 
catalyst is dried at a temperature and for a duration indicated above for 
those carbonate-coated catalysts prepared by the sequential procedure. The 
particular silver salt or compound used to form the silver-containing 
impregnating solution in a solvent or a complexing/solubilizing agent is 
not particularly critical and any silver salt or compound generally known 
to the art which is both soluble in and does not react with the solvent or 
complexing/solubilizing agent to form an unwanted product may be employed. 
Thus, the silver may be introduced to the solvent or 
complexing/solubilizing agent as an oxide or a salt, such as nitrate, 
carbonate, or carboxylate, for example, an acetate, propionate, butyrate, 
oxalate, malonate, malate, maleate, lactate, citrate, phthalate, fatty 
acid ester, and the like or combinations thereof. 
A large number of solvents or complexing/solubilizing agents may be 
suitably used to form the silver-containing impregnating solution. Besides 
adequately dissolving the silver or converting it to a soluble form, a 
suitable solvent or complexing/solubilizing agent should be capable of 
being readily removed in subsequent steps, either by a washing, 
volatilizing or oxidation procedure, or the like. The 
complexing/solubilizing agent, preferably, should also permit solution to 
provide silver in the finished catalyst to the extent of preferably about 
25 to about 60 percent silver, based on the total weight of the catalyst. 
It is also generally preferred that the solvents or 
complexing/solubilizing agents be readily miscible with water since 
aqueous solutions may be conveniently employed. Among the materials found 
suitable as solvents or complexing/solubilizing agents for the preparation 
of the silver-containing solutions are alcohols, including glycols, such 
as ethylene glycol, amines (including alkanolamines and alkyldiamines) and 
carboxylic acids, such as lactic acid and oxalic acid, as well as aqueous 
mixtures of such materials. 
Typically, a silver-containing solution is prepared by dissolving silver in 
a suitable solvent or complexing/solubilizing agent such as, for example, 
a mixture of water, ethylenediamine, oxalic acid, silver oxide, and 
monoethanolamine. The solution is then mixed with support particles and 
drained. Thereafter the particles are suitably dried. 
As indicated above, after impregnation, the silver-impregnated support 
particles are treated to convert the silver salt or complex to silver 
metal and thereby effect deposition of silver on the surface of the 
support. As used herein, the term "surface", as applied to the support, 
includes not only the external surfaces of the support but also the 
internal surfaces, that is, the surfaces defining the pores or internal 
portion of the support particles. This may be done by treating the 
impregnated particles with a reducing agent, such as hydrogen or hydrazine 
and/or by roasting, at an elevated temperature to decompose the silver 
compound and reduce the silver to its free metallic state. Certain 
solubilizing agents such as alkanolamines, alkyldiamines, and the like may 
also function as reducing agents. 
Although at least a catalytically effective amount of silver must be 
present in the finished catalyst (meaning an amount that provides a 
measurable conversion of propylene to propylene oxide), the silver 
concentration preferably is from about 2 percent to 70 percent, by weight, 
based on the total weight of the catalyst. More preferably, the silver 
concentration ranges from about 25 to 60 percent by weight. 
It has been discovered that the presence of certain specific potassium 
salts in the supported silver catalyst significantly enhances the 
efficiency of said catalyst as a propylene epoxidation catalyst. The anion 
preferably is a nitrogen oxyanion (i.e., an anion or negative ion which 
contains both nitrogen and oxygen atoms) such as nitrate and nitrite or a 
precursor thereof (i.e., an anion capable of undergoing displacement or 
other chemical reaction and forming a nitrogen oxyanion under epoxidation 
or catalyst preparation conditions). Potassium nitrate (KNO.sub.3) is the 
preferred potassium salt. Halide salts of potassium such as potassium 
fluoride may also be employed, as halide has been found to function as a 
precursor to nitrate (i.e., is converted to nitrate under the epoxidation 
conditions). 
The efficiency-enhancing potassium salt may be introduced to the catalyst 
in any known manner. Thus, impregnation and deposition of silver and a 
potassium salt may be effected coincidentally or sequentially, as 
described above. 
In order to perform coincidental impregnation, the potassium salt must be 
soluble in the same solvent or complexing/solubilizing liquid used with 
the silver impregnating solution. With the preferred sequential procedure 
in which the silver is added first, any solvent capable of dissolving the 
salt which will neither react with the silver nor leach it from the 
support is suitable. Aqueous solutions are generally preferred, but 
organic liquids, such as alcohols, may also be employed. Suitable 
procedures for effecting introduction of the potassium salt to the solid 
support are well known in the art. 
The optional potassium salt is added in an amount sufficient to provide an 
improvement in one or more of the catalytic properties (e.g., selectivity, 
activity, conversion, stability, yield) of the supported silver catalyst 
as compared to a catalyst not containing the potassium salt (herein 
referred to as "promoting amount"). The precise amount will vary depending 
upon such variables as the nitrogen oxide species and concentration 
thereof employed in the epoxidation procedure, the concentration of other 
components in the feed stream, the amount of silver contained in the 
catalyst, the surface area of the support, the process conditions, e.g., 
space velocity and temperature, and morphology of support. Generally, 
however, a suitable concentration range of the added potassium salt, 
calculated as cation, is about 0.15 to about 5 percent, preferably about 
0.5 to about 3 percent, by weight, based on the total weight of the 
catalyst. Most preferably, the salt is added in an amount of about 1.5 to 
about 2.5 weight percent K. 
Propylene and an oxygen-containing gas (i.e., a gas comprising molecular 
oxygen) are brought together in a reactor in the presence of the 
previously described catalyst under conditions effective to accomplish at 
least partial epoxidation of the propylene. Typical epoxidation conditions 
include temperatures within the reaction zone of the reactor on the order 
of about 180.degree. C. to 350.degree. C. (more preferably, 200.degree. C. 
to 300.degree. C.) and pressures from about 1 to about 30 atmospheres. 
Inlet pressures may be as low as 14 to 75 psig. To favor high selectivity 
to epoxide, it is desirable that the feed stream contain carbon dioxide 
and/or an organic halide (described in more detail hereafter). A gaseous 
nitrogen oxide species (described in more detail hereafter) may also 
optionally be supplied to the reaction zone within the reactor by 
introducing said species to the feedstream containing propylene (fresh 
and/or recycled) and molecular oxygen. 
Examples of nitrogen oxide species suitable for optional introduction in 
the feedstream include at least one of NO, NO.sub.2, N.sub.2 O.sub.4, 
N.sub.2 O.sub.3 or any gaseous substance capable of forming one of the 
aforementioned gases, particularly NO and NO.sub.2, under epoxidation 
conditions, and mixtures of one of the foregoing, particularly NO, with 
one or more of CO, PH.sub.3, SO.sub.3 and SO.sub.2. NO is the most 
preferred nitrogen oxide species. Inclusion of such nitrogen oxide species 
in the feedstream is not necessary, however. 
The amount of gaseous nitrogen oxide species present (if any) is not 
critical. The optimum amount is determined, in part, by the particular 
potassium salt used and the concentration thereof, and by other factors 
noted above which influence the optimum amount of potassium salt. 
Typically, a suitable concentration of the nitrogen oxide species for 
epoxidation of propylene, is about 0.1 to about 2,000 ppm, by volume, when 
N.sub.2 is used as ballast. When NO is used in the epoxidation of 
propylene, the preferred concentration is about 5 to about 2,000 ppm, more 
preferably about 20 to 500 ppm, by volume, with an N.sub.2 ballast. 
However, as explained previously, the nitrogen oxide species concentration 
may be essentially zero. 
The "oxygen" employed in the reaction may be defined as including pure 
molecular oxygen, atomic oxygen, any transient radical species derived 
from atomic or molecular oxygen capable of existence under epoxidation 
conditions, mixtures of another gaseous substance with at least one of the 
foregoing, and substances capable of forming one of the foregoing under 
epoxidation conditions. The oxygen is typically introduced to the reactor 
either as air, commercially pure oxygen or other substance which under 
epoxidation conditions both exists in a gaseous state and forms molecular 
oxygen. 
The gaseous components which are supplied to the reaction zone, or that 
region of the reactor where reactants and catalyst are brought together 
under epoxidation conditions, are generally combined before being 
introduced to the reactor. If desired, however, such components may 
alternatively be introduced separately or in various combinations. The 
feed stream having the particular composition previously described thus 
may be formed prior to or at the time the individual components thereof 
enter the reaction zone. The use of term "feedstream" herein thus is not 
meant to limit the present process to the embodiment where all of the 
gaseous components are combined prior to introduction of said components 
into the reaction zone. The reactors in which the process and catalyst of 
the present invention are employed may be of any type known to the art. A 
brief description of several of the reactor parameters which may be used 
in the present invention is presented below. 
In addition to propylene and oxygen (and, optionally, a nitrogen oxide 
species), the feedstream also desirably contains a performance-enhancing 
organic halide such as an alkyl halide. The organic halide is preferably a 
volatile compound, i.e., a substance which predominantly exists in gaseous 
form under the temperature and pressure conditions present in the reaction 
zone. The normal boiling point of the organic halide is most preferably 
less than about 100.degree. C. at atmospheric pressure. Compounds 
containing from 1 to 10 carbon atoms are preferred. Most preferably, the 
alkyl halide is a chloride species. The term alkyl halide includes both 
saturated and unsaturated halides, such as ethylene dichloride, ethyl 
chloride, vinyl chloride, methyl chloride and methylene chloride. 
Preferably, ethyl chloride is employed as the organic halide. Mixtures of 
different organic halides may be employed. The amount of organic halide 
employed will vary depending upon a variety of factors, including the 
concentration of propylene being oxidized, the particular catalyst 
promoter(s) and nitrogen oxide species and the concentrations thereof, as 
well as other factors noted above as influencing the optimum amount of 
potassium salt and nitrogen oxide species. However, a suitable range of 
concentration for the organic halide in the oxidation of propylene is 
typically about 0.1 to about 2,000 ppm, more preferably about 25 to 500 
ppm by volume, of the feedstream. In addition, a hydrocarbon, particularly 
a saturated hydrocarbon, such as methane, propane, or ethane, may be 
included in the feedstream. The feedstream may also contain a ballast or 
diluent, such as nitrogen, or other inert gas, particularly when air is 
used as the oxygen-containing gas. Varying amounts of water vapor may also 
be present. 
Carbon dioxide is also desirable to include as a component of the 
feedstream in the epoxidation process of this invention. The presence of 
carbon dioxide, within certain limits, has been found to provide 
surprising improvement in propylene oxide selectivity. Desirable 
enhancements in selectivity are generally observed using 1 to 60 volume % 
CO.sub.2 in the feedstream, with 5 to 25 volume % CO.sub.2 being 
preferred. 
The components of the feedstream are most suitably present in the amounts 
shown in the following table. 
______________________________________ 
Volume in % 
Component (or ppm) for Propylene Oxidation 
______________________________________ 
propylene about 2 to about 50% 
oxygen about 2 to about 10% 
organic halide 0 to about 2,000 ppm, 
more preferably, about 20 to 500 
ppm 
nitrogen oxide species 
0 to about 2,000 ppm 
hydrocarbon other 
0 to about 5% 
than propylene 
carbon dioxide 0 to 60%, more preferably 5 to 25% 
nitrogen or remainder. 
other ballast gas 
______________________________________ 
Although the present invention can be used with any size and type of vapor 
phase epoxidation reactor, including both fixed bed and fluidized bed 
reactors known to the art, it is contemplated that the present invention 
will find most widespread application in standard fixed bed, multi-tubular 
reactors such as those now in use as ethylene oxide reactors. These 
generally include wall-cooled as well as adiabatic or non-wall-cooled 
reactors. Tube lengths may typically range from about 5 to about 60 feet 
but will frequently be in the range of from about 15 to about 45 feet. The 
tubes may have internal diameters from about 0.5 to about 2.5 inches and 
are expected to be typically from about 0.8 to about 1.5 inches. A 
plurality of tubes packed with catalyst arranged in parallel within a 
suitable shell may be employed. GHSV generally range from about 500 to 
about 10,000 hr.sup.-1. Typically GHSV values range from about 800 to 
about 3,000 hours.sup.-1 at pressures from about 1 to about 30 
atmospheres, commonly about 1.1 to about 5 atmospheres. Contact times 
should be sufficient to convert 0.5 to 70%, preferably 5 to 30%, of the 
propylene.

EXAMPLES 
Example 1 
A supported silver catalyst in accordance with the invention comprising 39 
weight % Ag and 1.9 weight % K on a tribasic calcium phosphate support 
(Aldrich; CAS 12167-74-7; surface area=65 m.sup.2 /g) was prepared in 
accordance with the following procedure: A 4 oz. jar was charged with 
ceramic stones (5), ethylene diamine (10.30 g), distilled water (10.20 g), 
oxalic acid dihydrate (7.50 g), silver (I) oxide (13.0 g), 
monoethanolamine (3.63 g), potassium nitrate (1.59 g) in distilled water 
(5.17 g), and the tribasic calcium phosphate (17.0 g). The jar was sealed 
and placed on a ball mill for 4 hours. The resulting mixture was dried at 
110.degree. C. for 1 hour and then calcined at 300.degree. C. for hours. 
The material was thereafter pelletized and sieved to 14/30 mesh. The 
supported silver catalyst was tested for activity in propylene oxidation 
using a tubular reactor under the following run conditions: 2 cc catalyst, 
10 volume % propylene, 5 volume % oxygen, 50 ppm ethyl chloride, 200 ppm 
NO, GHSV=1200 hr.sup.-1, 40 cc/min flow rate, 30 psig, 250.degree. C. 
Propylene conversion of 5% with selectivity to propylene oxide of 27% were 
obtained. Increasing the concentration of ethyl chloride to 200 ppm 
improved the propylene selectivity to 34% (5% propylene conversion). 
Example 2 
A supported silver catalyst in accordance with the invention comprising 41 
weight % Ag and 2 weight % K (added as KF) on a tribasic calcium phosphate 
support (Aldrich; CAS 12167-74-7; surface area=65 m.sup.2 /g) was tested 
for activity in propylene oxidation using a tubular reactor using the same 
run conditions as described in Example 1 (50 ppm ethyl chloride). 
Propylene selectivity of 36% at 6% propylene conversion was observed. 
The following Comparative Examples 1-4 demonstrate the superiority of 
tribasic calcium phosphate as a catalyst support over other substances 
which also contain calcium and phosphate components. 
Comparative Example 1 
A supported silver catalyst comprising 40 weight % Ag and 2 weight % K 
(added as KNO.sub.3) on a monobasic calcium phosphate (CAS 7758-23-8) 
support was prepared and tested for activity using the run conditions 
described in Example 1 (50 ppm ethyl chloride; 40 psig). Only 1% propylene 
conversion was achieved; no propylene oxide was detected. 
Comparative Example 2 
A supported silver catalyst comprising 39 weight % Ag and 1.9 weight % K 
(added as KNO.sub.3) on a dibasic calcium phosphate (CAS 7757-93-9) 
support was prepared and tested for activity in propylene oxidation using 
the same conditions as in Example 1. As in Comparative Example 1, no 
propylene oxide was detected and the propylene conversion was low (1%). 
Comparative Example 3 
A supported silver catalyst comprising 43 weight % Ag and 2 weight % K 
(added as KNO.sub.3) on a hydroxyapatite support (CAS 1306-06-5; surface 
area=33 m.sup.2 /g) was prepared and tested for activity in propylene 
oxidation under the run conditions of Comparative Example 1. The results 
obtained (1% propylene conversion; 0% propylene oxide selectivity) provide 
further confirmation of the superiority of tribasic calcium phosphate as a 
catalyst support. 
Comparative Example 4 
A supported silver catalyst comprising 43 weight % Ag and 2.1 weight % K 
(added as KNO.sub.3) on a tricalcium phosphate support (CAS 7758-87-4; 
surface area =47 m.sup.2 /g) was prepared and evaluated for activity as a 
propylene oxidation catalyst using the conditions described in Example 1 
(50 ppm EtCl). Surprisingly, despite the compositional similarities 
between tribasic calcium phosphate and tricalcium phosphate, the latter 
compound when used as a support gave no detectable propylene oxide and 
only 1% conversion of propylene. 
Example 3 
A supported silver catalyst in accordance with the invention comprising 39 
weight % Ag and 2.1 weight % K (added as KNO.sub.3) on a calcium fluoride 
support was prepared and tested for activity in propylene oxidation using 
the same condition described in Example 1 (50 ppm ethyl chloride). 
Propylene conversion was 4%; selectivity to propylene oxide was 35%. When 
the O.sub.2 level was increased to 8 volume %, propylene conversion was 7% 
and propylene oxide selectivity improved to 40%. 
Example 4 
A supported silver catalyst in accordance with the invention comprising 50 
weight % Ag and 2 weight % K (added as KNO.sub.3) on a magnesium aluminate 
support was prepared and tested for activity in propylene oxidation using 
the same conditions described in Example 1 (50 ppm ethyl chloride; 50 
volume % CO.sub.2). Propylene conversion was 6%; propylene oxide 
selectivity was 42%. 
Example 5 
A supported silver catalyst in accordance with the invention comprising 50 
weight % Ag and 1.3 weight % K (added as KNO.sub.3) on a strontium 
titanate support was prepared and tested for activity in propylene 
oxidation using the same conditions described in Example 1 (50 ppm ethyl 
chloride). At 10% propylene conversion, propylene oxide selectivity was 
38%. 
Example 6 
A supported silver catalyst in accordance with the invention comprising 54 
weight % Ag and 1.9 weight % K (added as KNO.sub.3) on a calcium molybdate 
support was prepared and tested for activity in propylene oxidation using 
run conditions identical to those of Example 1 (50 ppm ethyl chloride). 
Propylene conversion was 2%; selectivity to propylene oxide was 26%. 
Example 7 
A supported silver catalyst in accordance with the invention comprising 43 
weight % Ag and 1.6 weight % K (added as KNO.sub.3) on a calcium titanate 
support was prepared and tested for activity in propylene oxidation using 
the same conditions described in Example 1 except for the use of 200 ppm 
ethyl chloride. Propylene oxide selectivity of 36% at 4% propylene 
conversion was observed. 
Example 8 
A supported silver catalyst in accordance with the invention comprising 42 
weight % Ag and 1.1 weight % K (added as KNO.sub.3) on a barium titanate 
support was prepared and tested for activity in propylene oxidation using 
the same conditions described in Example 1 except for the use of 200 ppm 
ethyl chloride. Propylene oxide selectivity of 26% at 3% propylene 
conversion was observed. 
Example 9 
A supported silver catalyst in accordance with the invention comprising 50 
weight % Ag and 1.5 weight % K (added as KNO.sub.3) on a magnesium 
titanate support was prepared and tested for activity in propylene 
oxidation using the same conditions as described in Example 1 except for 
the use of 200 ppm ethyl chloride. Propylene oxide selectivity of 35% at 
4% propylene conversion was observed.