Note: Descriptions are shown in the official language in which they were submitted.
--1--
CATALYST AND PROCESS FOR
MANUFACTURE OF ALKENE OXIDE
Technical Field:
The present invention is directed to improved
processes for the preparation of alkene oxide from
alk~ne and oxygen-containing gas employing a sup-
ported silver catalyst. Particular aspects of the
present invention relate to processes for epoxidizing
alkene in the vapor phase to produce the correspond-
ing alkene oxide at high efficiencies.
Back~round And Background Art:
The production of alkene oxides, or epoxides,
particularly ethylene oxide, by the direct epoxida-
tion of the corresponding alkene in the presence of a
silver-containing catalyst has been known for many
years. One of the earliest disclosures of a process
: : for the direct epoxidation of ethylene was that of
~ Lefort~ U. S. Patent i,998,878, issued in 1935 (re-
:: 30 issued in 1942~as Re. 20,370). Lefort discloses ~hat
: e:~hylene oxide can be formed by reacting ethylene and
~ oxygen according to the followlng equation:
: ~ ,
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~3~2'~
--2--
2cH2=cH2 + 02~ 2CH2-CH2 ~I)
Lefort recognized, however, that some of the
ethylene, when reacted with oxygen, is comDletely
oxidized to carbon dioxide according to the following
equation:
CzH4 + 32 ~2H2O +2C02 (II)
Reaction II, as well as other reactions in which
alkene is converted to products other than alkene
oxide, is undesirable since the alkene reactant is
consumed in the formation of undesired products.
lS Further, the undesired products, e.g., carbon di-
oxide, may adversely affect the reaction systesn. In
general, the overall effectiveness of an alkene oxide
production system is gauged by the performance char-
acteristics of the system. The most important per-
formance characteristics are the efficiency, theactivity, and the useul life of the catalyst, all of
which are defined and more completely described be-
low.
Percent conversion is defined as the percentage
of the alkene introduced to the reaction system that
undergoes reaction. Of the alkene that reacts, the
percenta~e that is converted into the corresponding
alkene oxide is referred to as the selectivity or
efficiency of the process. The commercial success of
a reaction system depends in large measure on the
efficiency of the system. At present, maximum effi-
ciencies in commercial production of ethylene oxide
by epoxidation are in the low 80s, e.g., 80 or 81
percent. Even a very small increase in efficiency
will provide substantial cost benefits in large-scale
lS015
~3~
--3--
operation. For example, taking 100,000 metric tons
as a typical yearly yield for a conventional ethylene
oxide plant, an increase in efficiency of from 80 to
84 percent, all other things being equal, would re-
sult in a savings of 3790 metric tons of ethylene peryear. In addition, the heat of reaction for Reaction
II (formation of carbon dioxide) is much greater than
that of Reaction I (formation of ethylene oxide) so
heat removal problems are more burdensome as the
e'ficiency decreases. Furthermore, as the efficiency
decreases, there is the potential for a greater
amount of impurities to be present in the reactor
effluent which can complicate separation of the de-
sired alkene oxide product. It would be desirable,
therefore, to develop a process for the epoxidation
of alkene in which the efficiency is greater than
that obtained in conventional commercial processes,
e.g~ t with ethylene, efficiences of 84 percent or
greater, while maintaining other performance charac-
teristics, particularly the activity, as de~cribedbelow, in a satisfactory range.
The product of the efficiency and the conversion
is equal to the yield, or the percentage of the al-
kene fed that is converted into the corresponding
oxide. The definitions of conversion, efficiency and
yield may be represented as follows:
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1~3;~ ~
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% Conversion = moles alkene-reacted x 100
moles alkene fed
% Eff iciency a mol~es alkene oxide produced x 100
S moles alkene reacted
% Yield _ moles a1kene oxide produced x 100
moles alkene fed
:
1 0 :
Generally, in a process for the epoxidation of
alkene, a reaction inlet stream containing reactants
and perhaps other additional materials enters the
reactor or reaction zone in which catalytic material
is provided and in which favorable reaction condi-
tions (e.g., temperature and pressure) are maintain-
ed. The reactor effluent is withdrawn or collected
from the reactor. The reactor effluent contains
reaction products, together with unreacted components
~rom the reaction inlet stream.
Since at least some alkene is generally not
converted during its initial pass through the reac-
tor, alkene is generally present in the reactor ef-
fluent. To increase the overall yield of the pro-
cess, at least a portion of the alkene in the reactoreffluent is returned to the reactor via a recycle
stream. Means are provided for removing and recover-
ing at least a portion of the alkene oxide from the
reactor effluent, prior to recycling, to form the
product stream. At least a~portion oE ~he remaining
reactor effIuent (after the product stream has been
withdrawn) becomes the recy~le stream. The recycle
stream preferably contains substantially all of the
alkene that was contained in the reactor effluent.
Reactants, iOe. r oxygen-containing gas and al-
kene, need to be continuously replaced and are pro-
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3~
vided to the reactor by means of a makeup feed-
stream. In general, the makeup feedst:ream and the
recycle stream are combined to form the reaction
inlet stream which is sent into the reactor or reac-
tion zone. Alternatively, the makeup feedstream andthe recycle stream can be introduced into the reactor
separately.
The epoxidation of alkene is generally carried
out in the presence of a supported silver catalyst
located within the reactor or reaction zone. The
silver may be supported by a conventional support
material, for example, alpha-alumina. The
performance of the catalyst may be affected by the
presence of solid, liquid or gaseous compounds which
may, for example, be incorporated in the catalyst or
provided via the makeup feedstream.
The activity of a reaction system is a measure
o the rate o~ production of the desired product,
e~g., ethylene oxide, for a particular reaction sys-
tem at a particular temperature. In order to providea meanin~ful comparison of the effectiveness of two
or more reaction systems or of a single reaction
system at different times, factors, such as feed
rate, feed composition, temperature, and pressure,
that affect the rate of production of the desired
alkene oxide must be normalized or accounted for,
preferably by using a standard or fixed set of oper-
ating conditions. Since the rate of production of
alkene oxide is proportional to the volume of cata-
lyst in the reaction system, the activity is usuallyexpressed in terms oE pounds of alkene oxide produced
per hour per cubic foot of catalyst. It should be
noted that this method of measuring activity does not
take into account variations in the densities of the
catalysts since the controlling factor is the volume
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of the reaction system available, not the weight of
catalyst which will fit into a given volume. Other
factors that have an effect on the rate of production
of the desired compound include the following:
(1) the composition of the reaction stream'
(2) the gas hourly space velocity of the reac-
tion stream;
(3) the temperature and pressure within the
reactor or reaction zone.
In order to compare the efectiveness of two or
more reaction systems or of a single reaction system
at different times, differences in factors 1 through
3 above should be minimized and/or factored into the
evalua~ion of relative effectiveness.
If the activity of a reaction system is low,
then, all other things being equal, the commercial
value of that system will be low. The lower the
activity of a reaction system, the less product pro-
duced in a unit time for a given feed rate, reactor
temperature, catalyst, surface area, etcetera. A low
activity can render even a high efficiency process
commercially impractical. In general, an activity
below 4 pounds of ethylene oxide per hour per cubic
foot of catalyst is unacceptable for commercial prac-
tice. The activity is preferably greater than 8pounds, and in some instances an activity greater
than Il pounds of ethylene oxide per hour per cubic
foot of catalyst is desired.
Reaction systems generally deactivate over time,
i.e., the activity of the catalyst begins to decrease
as~the process is carried out. Activity may be plot-
ted~as a function of time to generate a graph showing
the aging behavior of the catalyst. Experimentation
for the purpose of developing an activity plot is
usually conducted at a set temperature slnce, in
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3;~
general, activity can be increased by raising the
reaction temperature. Alternatively, an activity
plot can be a graph of the temperature required to
maintain a given activity versus time. The rate at
which activity decreases, i.e., the rate of deactiva-
tion at a givçn point in time, can be represented by
the slope of the activity plot, i.e., the derivative
of activity with respect to time:
deactivation = d[activity]/dt.
The average rate of deactivation over a period
; of time can be represented then by the change in
activity divided by the time period:
average deactivation = Q activity/ ~t.
At some point, the activity decreases to an
unacceptable level, for example, the temperature
required to maintain the activity of the system be-
comes unacceptably high or the rate of production
becomes unacceptably low. At this point, the cata-
lyst must either be regenerated or replaced. The
useful life of a reaction system is the length of
time that reactants can be passed through the reac-
tion system during which acceptable activity is ob-
served. The area under a plot of activity versus
time is equal to the number of pounds of alkene oxide
produced during the useul life of the catalyst per
cubic foot of catalyst. The greater the area under
such a plot, the more valuable the process is since
regeneration or replacement of the catalyst involves
a number of expenses~ sometimes referred to as turn-
around costs. More specifically, the replacement of
the catalyst generally requires that the reactor be
~015
-8- ~32 ~
shut down for an extended period of time, e.g., two
weeks or more, to discharge the catalyst, clean the
reactor tubes, etcetera. This operation requires
extra manpower and the use of special equipment. The
costs involved, which may include replacement cata-
lyst t can mount into the millions of dollars.
As used herein, an activity-reducing compound
refers to a compound which, when present in an acti-
vity reducing amount, causes a reduction in activity,
some or all of which activity may subsequently be
regained by returning to a situation in which ~he
concentration of the compound is below the minimum
activity reducing amount. The minimum activity-
reducing amount varies depending on the particular
system, the feedstream and the activity-reducing
compound~
Conversely, deactivation, as used herein, refers
to a permanent loss of activity, i.e., a decrease in
activity which cannot be recovered. As noted above,
activity can be increased by raising the temperature,
but the need to operate at a higher temperature to
maintain a particular activity is representative of
de~activation. Catalysts tend to deactivate more
rapidly when reaction is carried out at higher tem-
peratures.
As previously noted, since the~work of Lefort(U. S. Patent 1,998,878)~ research efforts have been
directed toward improving the performance character-
istics of reaction systems, i.eO, improving the acti-
`
vity, efficiency and useful life. Research has beencon~ducted in~areas such as feedstream additives~
removal of materials in the recycle stream and
methods of catalyst preparation, including the
deposition or impregnation of a particular type or
form of silver. Additionally, research efforts have
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3 ~ ~ ~
g
been directed toward the composition and formation of
the support, as well as toward additives deposited on
or impregnated in the support.
One of the difficulties in carrying out research
is the necessity of considering the interrelationship
of the various variables. The improvement or en-
hancement of one performance characteristic must not
b~ at the expense of, or have ~oo great an adverse
effect on~ one of the other performance characteris-
tics. For example, if a reaction system is designedwhich has a very short u~eful life, the system may be
commercially impractical even though the efficiency
and initial activity of the catalyst are outstand-
ing. Accordingly, a system that provides an increase
in the efficiency of the overall catalytic reaction
system, while only minimally affecting the activity
and useful life of the catalyst, would be particular-
ly beneficial.
Diluents have generally been included in the
gaseous mixture to reduce the likelihood of explo-
sion. Diluents are generally supplied via the makeup
feedstream. Such diluent materials have generally
been b~lieved to be inert, i.e , their function is
primarily to act as a heat sink and to dilute the
gaseous mixture. Nitrogen has been found to be a
suitable diluent material. It is well known to use
air to supply both oxygen and nitrogen to the reac-
tion zone. Another ma~erial that has been used as a
diluent is carbon dioxide. EPO Patent 3642 discloses
that a diluent, for example, heliumt nitrogen, argon,
carbon dioxide, and/or a lower paraffin, for example,
ethane and/or methane, may be present in proportions
of 10-80 percent and preferably 40-70 percent by
volume in total. Similarly, U~ K. Patent Application
GB 2 014 133A mentions carbon dioxide as a possible
1501S
~2932 ~C~
--10--
diluent. Other patents, e.g., U. S. Patents
3,043,854, 4,007,135/ and 4,206,128, Japanese Patent
53-39404, and U. K. Patents 676,358 and 1,571,123,
also mention that carbon dioxide is suitable for use
as an additiveO
Lefort, in U. S. Patent 1,998,878 (Re. 20,370),
states ~hat carbon dioxide may be introduced into the
reactor to limit the rate of complete oxidation of
ethylene to carbon dioxide. Similar disclosure is
found in U. S. Patent 2 f 270,780. U. S. Patent
2,615,900 discloses a process for producing ethylene
oxide in which carbon dioxide gas may be added to the
feed gases to act as a "depressant" or "anti-cata-
lytic material". U. S. Patent 4,007,135 discloses a
process in which, according to the patent, carbon
dioxide may be used to raise the selectivity of the
xeaction. According to Chem. Abstracts, Vol. 80,
Issue 11, ~ection 22, Abstract 059195, the presence
of carbon dioxide tends to retard the deactivation of
~he silver catalyst.
As mentioned above, any alkene contained in the
reactor effluent stream is preferably returned to the
; reaction zone via a recycle stream. It is sometimes
preerred to remove some of the gas contained in the
reactor effluent stream via a purge stream prior to
introducing the recycle stream into the reaction
zone. The purge stream may comprise a straight
purge, i~e., the purge stream can merely draw off a
percentage of the recycle stream. Since a straight
purge stream generally has a composition substantial-
ly similar to that of the stream fro~ which it is
~removed, some alkene will generally be purged when a
straight purge is employed. FGr this reason, means
are sometimes provided to ensure that purge streams
have relatively high concentrations of materials
.
~5015
`~'
Z~
other than alkene, such as nitrogen and carbon diox-
ide~.
U. S. Patent 2,241,019 discloses a process in
which the purge gas is carried through and in contact
with an adsorptive agent which is adapted to adsorb
selectively the ethylene content of the purge gas,
while the nitrogen and much of the carbon dioxide
present in the purge gas pass through the adsorption
agent and are discharged to the atmosphere.
L0 U. S~ Patent 2,376,987 discloses a process for
the two-stage preparation of butadiene in which, in
the first stage, ethylene is oxidized in a converter
to form ~thylene oxide. The converter contains an
oxidizing catalyst which is preferably finely divided
silver on a carrier such as alumina. According to
the patent, if concentrated oxygen is used as the
oxygen source, the ethylerle in the stream containing
the oxidation products from the converter may be con-
centrated and recycled to the process by scrubbing to
remove carbon dioxide, etcetera.
U. S. Patent 2,653,952 discloses a process for
the manu~acture of ethylene oxide in which the pro-
ducts from the reactor, consis~ing essentially of
ethylene oxide, ethylene, oxygen, nitrogen, helium
and carbon dioxide, are dellvered to an ethylene
oxide absorber. The gases are then passed in contact
with a solvent for carbon dioxide. Ordinarily,
ethanolamine is used as a ~solvent in this process.
The gas discharged from ~he;carbon dioxide absorber
contains ethylene and is r~ecycled to ~e again passed
in contact with the catalyst in the reactor. This
patent recognizes that nitrogen tends to build up to
higb concentrations~when the oxygen is supplied by
air. The process of this paten~ therefore employs
relatively nitrogen-free oxygen in the feedstream and
~5015
.
-12- 1 2 9 3 2
dilutes the gaseous mixture with helium.
U. S. Patent 2,799,687 discloses a preferred
embodiment for the oxidation of olefins in which the
reactor effluent may be passed into an ethylene oxide
absorber after which about 70-90 percent of the ef-
fluent from the ethylene oxide absorber is recycled
and the remainder plus additional oxygen is diverted
to a second reactor. According to the patent, by the
di~ersion of a portion of the effluent from the first
reactor to the second reactor, the buildup of carbon
dioxide above certain limits, such as above about 5-7
percent, can be prevented. Similarly, U. S. Patent
4,206,128, Netherlands Patent Ap~lication 6,414,284,
and U. K. Patent 1 1~1 983 all disclose processes in
which some carbon dioxide is removed from the recycle
stream.
According to U. K. Patent 1,055,147, one must
remove carbon dioxide from the ethylene oxide produc-
tion system to keep the carbon dioxide concentration
in an acceptable range since, according to the pat-
ent, carbon dioxide acts as an inhibitor and suppres-
ses the reaction of ethylene to form both ethylene
oxide and carbon dioxide.
U. S. Patent 1,998,878, U. S. Patent 3,904,656,
~he Manufacture Of Ethylene Oxide And Its Deriva-
tives", The Industrial Chemist, February, 1963r Kirk
Othmer, "Ethylene Oxide", Volume 8, pages 5~4,545,
"Ethylene Oxide By~Direct Oxidation Of Ethylene",
Petroleum Processin~, November, 1955r all include, as
a process step, the removal of carbon dioxide from
the alkene oxide for the purification of the alkene
oxide product.
Since the early work on the direct catalytic
oxidation of ethylene to ethylene oxide, it has been
suggested that the addition of certain compounds to
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,
.
13~ 32~
the gaseous feedstream or direct incorporation of
metals or compounds in the catalyst could enhance or
promote the production of ethylene oxide. Such
metals or compounds have been known variously as
'lanti-catalystsn, "promoters" and "inhibitors".
These substances, which are not considered catalysts,
are believed to contribute to the overall utility of
the process by inhibiting the formation of carbon
dioxide or by promoting the production of ethylene
oxide,
Various compounds have been found to provide
some beneficial effects when contained within the
gaseous mixture supplied ~o the reactor. It is well
known that chlorine-containing compounds, when sup-
plied to an ethylene oxide production process, helpto improve the overall effectiveness of the pro-
cess. For example, Law and Chitwood, in U. S. Patent
2,194,602, disclose that higher yields of olefin
oxide are ob~ained by retarding the complete oxida-
tion of the olefin through the addition of very smallamounts of deactivating materials (also reEerred to
by Law and Chitwood as repressants or anti-catalytic
materials) such as ethylene dichloride, chlorine,
sulfur chloride, sulfur trioxide, nitrogen dioxide,
or other halogen-containing or acid-forming
materials. U. S. Patents 2,270,780j 2,279,469,
2,279,470, 2,799,~87, 3,144,416, ~,007,135,
4,206,128, 4,368,144, EPO Patent 11 355, U. K. Pat-
ents 675,358, I,055,147 and 1,571,123 also discuss
the addition of halide compounds, such as ethyl
chloride, ethylene dichloride, potassium chloride,
vinyl chloride and alkyl chloride.
U. S. Patent 2,194,602 discloses a method for
the activation of silver catalysts in which the acti-
vation is accomplished by bringing the catalyst in
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-14-
contact with an aqueous solution of barium, strontium
or lithium hydroxide after the catalyst has first
been treated with a "repressant" such as ethylene di-
chloride, nitrogen dioxide, or other halogen-contain-
ing or acid-forming materialO
U. S. Patents 2,279,469 and 2,279,470 disclose
processes of making olefin oxides in which very small
amounts, i.e., less than 0.1 percent of the total
volume, of "anti-catalysts" are incorpora~ed with the
reactants. Halogens and compounds containing halo-
gens, e.gO, ethylene dichloride, and compounds con-
taining nitrogen, e.g., nitric oxide, can be used as
the anti-catalysts. According to the patents, it is
possible to employ mixtures of the individual anti-
lS catalyst substances.
U. S. Patent 3,144,416 discloses a method ofmanufacturing silver catalysts to be used for the
oxidation of oIefins. According to the patent, in
order to increase the selectivity of the catalyst, a
small quantity of halogen compound or nitrogen com-
pound may be added to the reaction gas or catalyst.
EPO Patent 3642 and U. K. Patent Application GB
2 014 133A disclose processes of producing an olefin
oxide by contacting an olefin with oxygen in the
presence o~ a silver-containing catalyst and a
chlorine-containing reaction modifier, for example,
dichloroethane, methyl chloride, or vinyl chloride.
According to these references, the catalyst per-
formance ~is improved, for example, the selectivity is
increased, by contacting~the catalyst with a nitrate-
or nitrite-forming substance, for example, a gas
containing dinitrogen tetroxide, nitrogen dioxide
and/or a nitrogen-containing compound, together with
an oxidizing agent, such as nitric oxide and oxy-
gen. The catalyst preferably comprises 3 to 50 per-
~5015
~2~
-15-
cent, more preferably 3 to 30 percent, by weight
silver. According to the references, it is preferred
that the catalyst should contain cations, for exam-
ple, alkali and~or alkaline earth metal cations, as
the corresponding nitrate or nitrite, particularly if
the catalyst is treated with the nitrate- or nitrite-
forming substance intermittently. According to the
patents, suitable concentrations of the cations may
be, for example, 5 x 10-5 to 2, preferably 5 x 10 4
to 2, more preferably 5 x ]0~4 to 0.5, gram equiva-
lents per kilogram of catalyst. Suitably, Mo, K, Sr,
Ca and/or Ba are present in amounts of 2 to 20,000,
preferably 2 to 10,000, more preferably 10 to 3,000,
microgram equivalents per gram of silver. In the
processes of these two references, a diluent, for
example, carbon dioxide, may be present and uncon-
verted olefin may be recycled, suitably after removal
of carbon dioxide.
Rumanian Pa~ent No. 53012, published December 2,
1971, discloses a process in which the catalyst is
brought in direct contact w~th a gas ~ixture composed
of 5-15 percent oxygen, 8-20 percent carbon dioxide,
60-80 percent nitrogen, completed by 1-5 percent
nitrogen oxides.
U. ~. Patent 524,007 discloses a method for
activating catalysts which may be accomplished by
contacting the catalyst with an aqueous solution of a
hydroxide of lithium, after the catalyst has first
been treated with an "anti catalyst", such as
ethylene dichloride or nitrogen dioxide. Accordin~
to the patent, the treatment may most advantageously
be conducted simultaneously with the oxidation
reaction of the olefins, inasmuch as the presence of
very small amounts of anti-catalyst (less than 0~1
percent) increases the efficiency by limiting the
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~ 2 9 3
-16-
formation of carbon dioxide.
Scientific literature is replete with examples
of the use of alkali metals and alkaline earth metals
and their cations to promote the efficiency of silver
catalysts used in epoxidation reactions. Numerous
examples may be found in literature regarding prefer-
ence for the inclusion or exclusion oE one or several
metals or cations in silver catalysts. Although many
reports have indicated that no particular effective-
ness is observed with one alkali metal or alkalineearth metal cation vis-a-vis another, several have
suggested clear preferences for particular metal
cations.
Potassium is well known as a catalyst promoter
for the epoxidation o~ alkenes. One of the first
patents to recognize potassium as a suitable promoter
was U. S. Patent 2,177~361. According to this pat-
ent, the catalyst may be promoted by the presence of
very small proportions of alkali or alkaline earth
metals,
U. K. Patent Application 2,122,913A discloses a
catalyst and a process for oxldation of ethylene in
which an amount of alkali metal is deposited on the
catalyst which removes substantially all acti~vity
from the silver catalyst and then activity and selec-
tivity are recovered by heating the catalyst in a
nitrogen atmosphere.
When potassium is employed in the catalyst, it
; is generally introduced in conjunction with an an-
ion. The choice of the anion has not always been
regarded as significant. For example, U. S. Patents
3,962,136~, 4,010,115, 4,012j425, and 4,356,312 state
that no unusual effectiveness is observed ~ith the
~use of any particular anion in the alkali metal salts
used to prepare the catalysts and suggests that ni-
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~Z~3~
-17~
trates, nitrites, chlorides; iodides, bromates, et-
cetera, may be used. Potassium nitrate was employed
in the silver salt solution of Example 1 in each
patent. According to the patents, from about 4.0 x
10 5 to about 8.0 x 10 3 gram equivalent weights of
ionic higher alkali metal, e.g., rubidium, cesium or
potassium or mixtures thereof, per kilogram of cata-
lyst is deposited on the catalyst support simultan-
eously with the deposit of silver. The amount of
higher alkali metal preferably ranges from about 2.0
x 10 4 to about 6.5 x 10-3 gram equivalent weights
per kilogram of finished catalyst. According to the
patents, the amount of the higher alkali me~al (or
metals) present on the catalyst surface is critical
and is a function of the surface area of catalyst.
According to the patents, the alkali metal is present
in final form on the support in the form of its ox-
ide. U. 9. Patents 3,962,136, 4,010,115 and
4,01~,425 note tha~ the highest level of selectivity
obtainable when potassium is employed typically is
lower than that obtainable when rubidium or cesium is
employed while U. S. Patent 4,356,312 notes that
particularly good results are obtained with potas-
sium.
U. S. Patent 4,066,575 notes that alkali metal
nitrate is suitable for supplying an alkali metal
promoter, but it notes that the anion associated with
the promoter metal is not critical. U. S~ Patent
4,207,210 discloses a process for preparing an ethy-
lene oxide catalyst in which higher alkali metals,
such as potassium, rubidium and cesium, are deposited
on a catalyst support prior to the deposition of
silver. According to the patent, the amount of high-
er alkali is a critical function of the surface area
of the support. This patent also notes that no un-
]~015
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usual effectiveness is observed with the use of any
particular anion in preparing the catalysts and lists
nitrates as one type of salt that may be used. Car-
bon dioxide and steam are listed as diluent
; 5 materials.
The use of potassium nitrate, however, to impart
a promotin~ effect on the catalyst has been widely
described. ~or example, U. S. Patent 4,007,135 lists
a number of materials, including potassium, which can
be used as promoters. According to the patent, in
yeneral, 1 to 5,000, preferably 1 to 1,000, more
preferably 40 to 500, and particularly 20 to 200,
atoms of potassium are present per 1,000 atoms of
silver. Suitably an aqueous solution of a compound,
such as a chloride, sulfate, nitrate, nitrite, et-
cetera, of the promoter is used for impregnation.
U. S. Patent 4,094,889 discloses a process for re-
storing the selectivity of silver catalysts in which
alkali metal may be introduced as a nitrate and in
which the preferred content o~ potassium is in the
range of 2 x 10-2 to 3 x 10-5 grams/square meter of
surface area of support. U. S. Patent 4,125,480
discloses a process for reactivating used silver
catalyst comprising (a) washing the used catalyst,
and (b) depositiny from 0.00004 to 0.008~ preferably
from 0.0001 to 0.002 gram equivalent weights per
kilogram of catalyst of ions of one or more of the
alkali metals, su~h as sodium, potassium, rubidium,
or cesium. The ions of, e.g., potassium are
deposited on the ca~alyst by impregnating it with a
solution of one or~more compounds, such as potassium
nitrate. U. S. Patents 4,~26,782, 4,235,757,
4,324,699, 4,342,667, 4,368,144, 4,455,3g2, Japanese
Patent 56/89843, and U. K. Patent 1,571,123 suggest
the use of potassium nitrate in various amounts
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12~3~
--19--
Potassium nitrate may also be formed in situ when a
carrier material is treated with certain amines in
the presence of potassium ions, for example, when
silver is introduced to a carrier material in a sil-
ver-impregnating solution containing an amine and
potassium ions, followed by roasting.
There has been some disclosure directed to cata-
lysts for use in an ethylene oxide production system
in ~hich silver is present in relatively large pro-
portionst e.g., 35 percent or more. For example,U. S. Patents 3,565,828 and 3,654,318 disclose cata-
lysts for the synthesis of ethylene oxide from oxygen
and ethylene. According to the patents, the cata-
lysts contain from 60 percent to 70 percent by weight
of silver.
U. S. Patent 2,593,099 discloses a magnesium
oxide-barium oxide silver catalyst support. Accord-
ing to the patent, the conventional amount of silver
is deposited on the support, namely, 2 to 50 percent,
with the best results being obtained between 4 and 20
percent.
U. S. Patent 2,713,586 discloses a process for
the oxidation of ethylene to ethylene oxide in which,
according to the patent, the conventional amount of
silver is deposited on the support, namely, 5 to 50
percent, with the best results being obtained between
4 and 20 percent.
U. S. Patent 3,793,231 discloses a process for
the preparation of silver cataly~ts for the produc-
tion of ethylene oxide in which the silver content ofthe catalysts generally range between lS to 30 per-
cent by weight, preferably 19 to 27 percent by
weight.
A large body of art directed to various aspects
of alkene oxide production has been developed over
15015
` -20~ 3~
the years since Lefort (U. S. Patent 1,998,~78).
Much of it is contradictory and incapable o~ re~on-
ciliation. None of the art is believed to disclose
or suggest a process for the high-efficiency epoxiaa-
tion of alkene in which the catalyst includes anef~iciency-enhancing salt of a member of a redox-half
reaction pair and which recogni~es the relationship
between ~he amount of efficiency-e~hancing salt pro-
vided in the catalyst and the concentration of carbon
dioxide in th~ reaction inle~ stream and their e~fect
on the activity and ef~iciency of th~ reaction sys-
tem.
Disclosure of the Invention-
Carbon dioxide, when present in a large enough
quantity, can act as an activity-reducing compound in
high eficiency p~ocesses for the epoxidation of
alkene in which alkene and oxygen-containing gas are
contacted in the presence of at least one gaseou~
efficiency-enhancing member of a redox-half reaction
pair, e~g. r nitric oxide and/or nitrogen dioxide,
carbon dioxide, and a supported silver catalyst which
includes a salt of a member o~ a redox-half reaction
2~ pair, e.g., potassium nltrate. Carbon dioxide is
generally continuously produced as a by-product of
the alkene epoxidation reaction. ~5 a result, carbon
: dioxide is generally contained in the reactor ef-
~luent and at least some portion of it is normally
returned to the reaction zone via the recycle
streamc One way to avoid or limit the deleterious
effects o~ carbon dioxide on the catalyst is to re-
move the carbon dioxide from the recycle stream ~y,
`: for example, use o a Benfield scrubber. Such an
additional separa~ion step entails an additional
15015
1~?3Z ~CI
-21-
capital expenditure, particularly with existing
equipment designed for operation with lower efficien-
cy catalysts where carbon dioxide is not a problem.
In commercial operations, removal of carbon dioxide
S to a level at which the adverse effects of carbon
dioxide are minimized can require a ~;ignificant capi-
tal outlay which may not be cost-~ustifiable. The
present invention is directed to a method of minimiz-
ing the adverse effects of carbon dioxide on the
catalyst by providing the efficiency-enhancing salt
in an appropriate amount. The method can be carried
out in conjunction with the removal of some carbon
dioxide to control its concentration in the reaction
zone.
The present invention provides a process then
for the epoxidation of alkene comprising contacting
alkene and oxygen-containing gas under epoxidation
conditions in the presence of at least one gaseous
efficiency-enhancing member of a redox-half reaction
pair, carbon dioxidel and a supported silver cata-
lyst. The catalyst comprises a catalytically-effec-
tive amount of silver on a support and at least one
efficiency-enhancing salt o a member of a redox-half
reaction pair. ~he efficiency-enhancing salt is
Z5 present in an amoun~ sufficient to provide an effi-
ciency of at least about 84 percent under Standard
Test Conditions I (as defined below) but below the
amount which under Standard Test Conditions II (as
defined below) would reduce the activity to less than
30 ~ 4 pounds of ethylene oxide per hour per cubic foot of
catalyst, thereby reducing the activity~reducing
effect of carbon dioxide in carrying out the process
of the invention
.
~;
15015
-22~
Brief D~scription Of The Drawings:
Figure 1 is a flow chart of a process in accord-
ance with the invention; and
Figure 2 is a graph of (1) the activity and (2)
the eficiency of the catalysts of Examples 1-4 ver-
sus the concentration o~ the efficiency-enhancing
salt contained in the catalyst when (a) no carbon
dioxide is present in the reaction inlet stream, and
(b) 3 volume percent of carbon dioxide is present in
the reaction inlet stream.
.
Detailed Description Of The Invention:
The present invention is directed to processes
for the epoxidation of alkene to form alkene oxide by
contacting alkene and oxygen-containing gas under
epoxidation conditions in the presence of a gaseous
eficiency-enhancing member of a redox-half reaction
pair, carbon dioxide, and a supported silver cata-
lyst. The silver catalyst generally comprises a
catalytically-effective amount of silver and an effi-
ciency-enhancing salt o a member of a redox-half
reaction pair on a porous support. The efficiency-
enhancing salt is present in the catalyst in anamount sufficient to provide an eEiciency of at
least about 84 percent under Standard Test Co~ditions
~I but below that amount which, under Standard Test
Conditions II would reduce the activity to less than
4 pounds of ethylene oxide per cubic foot of catalyst
per hour. Preferably, the amount of efficiency-
enhancing salt is~below that amount which, under
Standard Test Conditions II would reduce the activity
to less than 6 pounds of ethylene oxide per cubic
foot of catalyst per hour.
15015
-23~ 3~
The following description of the preferred sys-
tem for epoxidation of alkene in accordance with the
present invention may be better understood by refer-
ence to the flow chart in Figure 1.
In steady-state operation of the preferred sys-
tem, a reaction inlet stream containing reactants,
together with other gaseous materials as discussed
below, is fed to a reactor at a controlled gas hourly
space velocity (GHSV). The reactor may take a vari-
ety of forms, but is preferably a collection of ver-
tical tubes containing a supported silver catalyst.
The reaction inlet stream enters the reactor, passes
through the catalyst, and exi~s the reactor as the
reactor effluent. The desired product, e.g., ethy-
lene oxide, is separated from the other components inthe reactor e~fluent, preferably by a scrubbing oper-
ation. The remainder of the reactor effluent becomes
the recycle stream. It is sometimes preferred to
remove a portion of the material in the recycle
stream in order to, for example, prevent buildup of
certain materials in the system. The removal of
material from the recycle stream may be selective,
i.e., certain compounds may be removed from the re-
cycle stream in greater proportions than other com~
pounds. The remainder of the recycle stream is us-
ually combined with a makeup feedstream to form the
reaction inlet stream. The reaction system will be
; discussed in greater detail below.
Although the present invention can be used with
any size and type of alkene oxide reactor, including
both fixed bed and fluidized bed reactors~ it is
contemplated that the present invention will find
most widespread application in standard fixed bed,
multi-tubular reactors. These generally include
wall-cooled as well as adiabatic reactors. Tube
15015
~s~3;~
-24~
lengths typically range from about 5 to about 60 feet
(1.52 to 18.3 meters), frequently from about 15 to
about 40 feet (1.52 to 12.2 meters). The tubes gen-
erally have internal diameters rom about 0.5 to
about 2 inches (1.27 to 5.08 centimeters), typically
from ab~ut 0.8 to about 1.5 inches (2.03 to 3.81
CentimeteES).
The catalyst generally comprises a support hav-
ing catalyst material or a mixture of catalyst mater-
ial and an efficiency-enhancing material impre~nated
or coated on the support. The support can be gener-
ally described as a porous, inorganic substrate which
is not unduly deleterious to the performance of the
system and .is preferably substantially inert toward
the other materials in the system, i.e., the catalyst
material, any other components present in the cata-
lyst, e.g., eficiency-enhancing salt, and components
in the reaction inlet stream. In addition, the sup-
port should be able to withstand the temperatures
employed within the reactor, as well as, of course,
the temperatures employed in manufacturing the cata-
lyst, e.g., if the catalyst material is reduced to
its free metallic state by roasting. Suitable sup-
ports for use in accordance with the present inven-
tion include silica, magnesia, silicon carbide, zir-
conia, and alumina, preferably alpha-alumina. The
support preferably has a surface area of at least
about 0.7 m2/g~ preferably in the range of from about
0.7 to about 16 m2/g, more preferably about 0.7 to
about 7 m2/gO The surface area is measured by the
B. E. T. nitrogen method described by Brunauer, Emmet
and Teller in J. Am. Chem. Soc. 60, 309-316 (1938).
The support may be composed of a particulate
matrix. In a preferred support, at least about 50
percent of the total~number of support particles
15015
~ 3
-25-
having a particle size greater than about 0.1 micro-
meter have at least one substantially flat major
surface. The support particles are preferably formed
into aggregates or "pills" of such a size and shape
S that they are readily usable in commercially operated
tubular reactors. These aggregates or pills general-
ly range in size from about 2 millimeters to about 15
millimeters, preferably about 3 millimeters to about
12 millimeters. The size is chosen to be consistent
with the type of reactor employed. In general, in
fixed bed reactor applications, particle siæes rang-
ing from about 3 millimeters to about 10 millimeters
have been found to be most suitable in the typical
tubular reactors used in commerce.
The shapes of the carrier aggregates useful for
purposes of the present invention can vary widely.
Common shapes include spheres and cylinders, especi-
ally hollow cylinders.
The preferred support particles in accordance
with the present invention have at least one substan-
tially flat major surface and may be characterized as
having a lamellate or platelet-type morphology. Some
of the particles have two, or sometimes more, flat
surfaces. The major dimension of a substantial por-
tion of the particles having platelet-type morphology
is less than about 50 micr~ns, preferably less than
about 20 microns. When alpha-alumina is employed as
the support material, the platelet-type particles
frequently have a morphology which approximates the
shape of hexagonal plates.
The carrier materials of the present invention
may generally be described as porous or microporous
and they generally have median pore diameters of from
about 0.01 to about 100 microns, preferably about 0.5
to about 50 microns, and most preferably about 1 to
15015
~L293Z ~
-26
about 5 microns. Generally, they have pore volumes
of about 0.6 to about 1.4 cc/g, preferably about 0.8
to about 1~2 cc/g. Pore volumes may be measured by
any conventional technique, such as conventional
mercury porosity or water absorption techniques.
Generally improved results have been demon-
strated when the support material is composition~pure
and also phase-pure. By "composition-pure" is meant
a material which is substantially a single substance,
such as alumina, with only trace impurities being
present. The term "phase-pure" refers to the homo-
geneity of the support with respect to its phase. In
the present invention, alumina, having a high or
exclusive alpha-phase purity (i.e., alpha-alumina) is
preferred. Most preferred is a material composed of
at least 98 percent, by weight, alpha-alumina.
Under some conditions even small amounts of
leachable sodium can adversely affect the service
life of the catalyst. Notably improved results have
been observed when the support contains less than
about 50 parts per million (ppm) by weight, prefer-
ably less than 40 ppm, based on the weight of the
total catalyst. The term leachable sodium, as used
herein, refers to sodium which can be removed from
the support by immersing the support in a 13 percent
by volume nitric acid solution at 90 degrees C for
one hour. Suitable alpha-aluminas~having concentra-
tions of sodium below 50 ppm may be obtained commer-
cially from suppliers such as the ~orton Company.
Alternatively, suitable alpha-alumina support ma`ter-
ials may be prepared so as to obtain leachable sodium
concentrations below 5~ ppm by the method described
by Weber et al in U. S~ Patent 4,379,134.
A particularly preferred support is a high-
purity alpha-alumina support, having platelet
15015
--27--
1~3Z~3
morpht~lo~y, ;.)f the type disclosed in Canadian
Applicat iOII No 515,865~ filed August 13, 1986, in
ttl~ nan-e of TAom~s M. NG~ermann, entitled "ImprGved
C~t~lytic System for Ep(xi~ation of Alkenes".
The present invenl iOIl includes in the
catalyst at least one efficiency-enhancing salt of a
member of a redox-half reaction pair. The term
"redox-half reaction" is defined herein to mean
half~reactions such as those found in equations
presented in tables of standard reduction or
oxidation potentials, also known as standard or
single electrode potentials. These equations are
found in, for instance, "Handbook of Chemistry",
N. A. Lange, Editor, McGraw-Hill Book Company, Inc.,
pages 1213-1218 (1961) or "CRC Handbook of Chemistry
and Physics", 65th Edition, CRC Press, Inc., Boca
Raton, Florida, pages D 155-162 (1984). The term
"redox-half reaction pair" refers to the pairs of
atoms, molecules or ions, or mixtures thereof, which
undergo oxidation of reduction in such half-reaction
equations, A member of a redox-half reaction pair
is, therefore, one of the atoms, molecules or ions
that appear in a particular redox-half reaction
equation. The term redox-half reaction pair is used
herein to include those members of the class of
substances which provide the desired performance
enhancement rather than a mechanism of the chemistry
occurring. Preferably, such compounds, when
associated with the catalyst as salts of members of a
redox-half reaction pair, are salts in which the
anions are oxyanions, preferably an oxyanion of
polyvalent atom, i.e., the atom of the anion to which
oxygen is bonded is capable of existing, when bonded
to a dissimilar atom, in the different valence states.
15015
-28- 12~3~
The preferred efficiency-enhancing salts are potas-
sium nitrate and potassium nitrite.
The catalysts of the present invention are
preferably prepared by depositing catalyst material
and at least one efficiency enhancing salt,
sequentially or simultaneously, on and/or within a
solid porous ~upport. The preferred catalyst
material in accordance with the present invention
comprises silver, preferably of a particle size less
than about 0.5 micron. Any known method of
introducing the catalyst material and efficiency-
enhancing salt into the catalyst support may be
employed, but it is preferred that the support is
either impregnated or coated. The more preferred of
these is impregnation wherein, in general, a solution
of a soluble salt or complex of silver and/or one or
more efficiency-enhancing salt is dissolved in a
suitable solvent or "complexing/solubilizing"
agent. This solution may be used to impregnate a
porous catalyst support or carrier by immersing the
carrier in the silver~ and/or efficiency-enhancing
salt~containing impregnation solution.
Sequential impregnation means that silver is
first deposited within the carrier in one or more
impregnation steps, and then salt is deposited in a
separate impregnation step.
One aspect of the present invention involves the
beneficial effects observed when the catalyst con-
tains high concentrations of silver. In order to
provide such a catalyst by impregnation, it has been
found that it is preferable to deposit the silver via
several impregnation steps. Thus, if a high silver-
content catalyst, e.g., a catalyst containing 30 or
more percent silver, is desired and a sequential
impregnation procedure is to be used, a four-step
15015
-29- ~3~ ~
process may be employed. Such a process would in-
volve three silver-only impregnation steps ~ollowed
by one salt-only impregnation step.
In general, a silver-only impregnation step is
carried out by first immersing the support in a sil-
ver-containing impregnation solution, preferably by
placing the support particles in a vessel, evacuating
the vessel and then adding the impregnation solu-
tion. The excess solution may then be allowed to
drain off or the solvent may be removed by evapora-
tion under reduced pressure at a suitable tempera-
ture. Typically, a silver-containins solution is
prepared by dissolving silver oxide in a sui~able
solvent or complexing/solubilizing agent as, for
example, a mixture of water, ethylenediamine, oxalic
acid, silver oxide, and monoe~hanolamine.
After impregnation, the silver-impregnated car-
rier particles are treated to convert silver salt to
silver metal to effect deposition of silver on the
surace o~ the support. This may be done by treating
the impregnated particles with a reducing agent, such
as oxalic acid, alkanolamine or by roasting at an
elevated temperature on the order of about 100 to
about 900 degrees C, preferably about 200 to about
650 degrees C~ to decompose the silver compound and
reduce the silver to its free metallic state. ~he
duration of roasting is generally for a period of
from about 1 to about 10 minutes, with longer times
for lower temperatures, depending on the temperature
used. As used herein, the term "surface~, as applied
to the support, not only includes the external sur-
faces of the carrier but also the internal surfaces,
that is, the surfaces defining the pores or internal
portion of the support particles.
15015
-30~ 3~
The efficiency-enhancing salt may be introduced
into the catalyst in any suitable manner. In gener-
al, the pr~ferred amount of efficiency-enhancing salt
can be deposited in one impregnation step. After
immersion of the silver loaded support in the effi-
ciency-enhancing salt impregnation solution, the
excess solution is generally drained and the silver-
and efficienoy enhancing salt-containing support is
dried, for example by heating to from 80 to 200 de-
grees C. When more than one salt of a member of aredox-half reaction pair is employed, the salts may
be deposited together or sequentially.
Concurrent or coincidental impregnation means
that generally the final (perhaps the only) impregna-
tion step involves immersion of the support in animpregnation solution which contains silver as well
as one or more efficiency-enhancing salts. Such an
impregnation step may or may not be preceded by one
or more silver-only impregnation steps. Thus, to
make a high silver-content catalyst by coincidental
impregnation, several silver-only impregnation steps
might be carried out, ollowed by a silver- and effi-
ciency-enhancing salt-impregnation step. A low sil-
ver-content catalyst, e.g., from about 2 to about 20
weight percent silver, may be made by a single sil-
ver- and effîciency-enhancing salt-impregnation
step. For the purposes oE this invention, these two
sequences are both referred to as concurrent or coin-
cidental impregnation.
The three types of impregnation solutions, name-
ly, silver-containing, efficiency-enhancing salt-
containing, and silver- and efficiency-enhancing
salt-containing, are discussed in more detail below.
There are a large number of suitable solvents or
complexing/solubilizing agents which may be used to
15015
-31- ~2~3Z~
form the silver-containing impregnating solution. A
suitable solvent or complexing/solubilizing agent,
besides adequately dissolving the silver or convert-
ing it to a soluble form, should be capable of being
readily removed in subsequent stepsl either by a
washing, volatili2ing or oxidizing procedure, or the
like. It is aIso generally preferred that the sol-
vents or complexing/solubilizing agents be readily
;~ miscible with water since aqueous solutions may fre-
quen~ly be employed.
~ mong 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, ammonia~
amines and aqueous mixtures of amines, such as ethy-
lenediamine and monoethanolamine, and carboxylic
acids, such as oxalic acid and lactic acid.
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 known silver salt or
compound generally known to the art which ls soluble
in and does not react with the solvent or complex-
~` ing/solubilizing agent may be employed. Thus, the
silver may be introduced into the solvent or complex-
ing/solubilizing agent as an oxide, a salt, such as a
nitrate or carboxylate, for example, an acetate,
propionate, butyrate, oxalate, lactate, citrate,
phthalake, generally the silver salts of higher fatty
acids, and the like.
Materials which may be employed in the efficien-
cy-enhancing salt-containing impregnation solution to
act as a solvent for the efficiency-enhancing salt
include generally any solvent capable of dissolving
the salt, which solvent will neither react with the
15015
12~2 ~3
-32-
silver nor leach silver from the support. Aqueous
solutions are generally preferred but organic li-
quids, such as alcohols, may also be employed.
In order to perform coincidental impregnation,
S the efficiency-enhancing salt and the silver catalyst
material must both be soluble in the solvent or com-
plexing/solubilizing liquid used.
Suitable results have been obtained with both
the sequential and coincidental procedures. Some
results have indicated that greater amounts of silver
with more uniform distribution of silver throughout
the pill can be obtained by three or more silver-
impregnation cycles. High silver-containing cata-
lysts prepared by a coincidental impregnation techni-
que generally provide better initial performance thanthose prepared by a sequential technique.
If the catalyst material is to be coated on the
catalyst support rather than impregnated in the sup-
port, the catalyst material, e.g., silver, is pre-
formed or precipitated into a slurry, preferably an
aqueous slurry, such that the silver particles are
deposited on the support and adhere to the support
surface when the carrier or support is heated to
removed the liquids present.
Z5 The concentration of silver in the finished
catalyst may vary from about 2 percent to 60 percent
or higher, by weight, based on the total weight of
the catalyst, more preferably from about 8 percent to
about 50 percent, by weight. When a high silver
content catalyst is employed, a silver concentrationrange of from about 30 to about 60 percent, by
weight, is preferred. When a lower silver content
catalyst is used, a preferred range is from about 2
to about 20 weight percent. The silver is preferably
distributed relatively evenly over the support sur
15015
-33- ~3~
faces. The optimum silver concentration for a par-
ticular catalyst must take into consideration per-
formance characteristics, such as catalyst activity,
system efficiency and rate of catalyst aging, as well
as the increased cost associated with greater concen-
trations of silver in the catalyst material. The
approximate concentration of silver in the finished
catalyst can be controlled by appropriate selection
of the number of silver-impregnation steps and of the
concentr~tion of silver in the impregnation solution
or solutions.
When more than one salt of a member of a redox-
half reaction pair is employed, the salts may be
deposited together or sequentiallyO I~ is preferred,
however, to in~roduce the salts to the support in a
single solutlon, rather than to use sequential treat-
ments using more than one solution and a drying step
between impregnation steps, since the latter tech-
nique may result in leaching the first introduced
salt by the solution containing the second salt.
Concurrent or coincidental impregnation may be ac-
complished by forming an impregnating solution which
contains the dissolved efficiency-enhancing salt of a
member of a redox-half reaction pair as well as sil-
ver catalyst material. Silver-first impregnation can
be accomplished by impregnating the support with the
silver-~ontaining solution, drying the silver-con-
taining support, reducing the silver, and impreg-
nating the support with the efficiency-enhancing salt
solution.
Reaction conditions maintained in the reactor
during operation of the process are those typically
used in carrying out epoxidation reactions. Tempera-
tures within the reaction zone of the reactor gener-
ally range from about 180 to about 300 degrees C and15015
-34-
3~
pressures generally range from about 1 to about 30
atmospheres, typically from about 10 to about 25
atmospheres. The gas hourly space velocity (GHSV)
may vary, but it will ~enerally range from about
1,000 to about 16,000 hr 1.
'9Standard Test Conditions I~ (STC-I3 and "Stand-
ard TPst Conditions II~ (STC II), as used herein,
refer to two sets o~ conditions under which catalysts
may be tested to determine if they contain the
10 requisite amount of efficiency-enhancing salt. Note
that STC-I and STC-II are carried out using ethylene
as the alkene in determining the operable level of
the efficiency-enhancing salt. ~owever, the catalyst
may be used in systems for the epoxidation of other
alkenes, e.g., propylene, as described herein. The
reactor used in both STC-I and STC-II is a micro-
reactor, approximately 14 cm in leng~h and having an
inside diameter of approximately 7.8 mm. The micro-
reactor is charged with sufficient fine particulate
catalyst comprisin~ particles of a size Oe from about
0.5 to about 1.5 mm, i.e., from about 35 to about 12
U. S. sieve to provide a catalyst bed height of 5
centimeters~
No recycle stream is used under STC-I and STC-
II, so the makeup feedstream is also the reactioninlet stream. Measurements of efficiency (STC-I) and
activity ~STC-II1 are made 24 hours af~er the respec-
tive test has begun~ i.e., 24 hours a~ter the respec-
tive microreactor has been brought on stream under
the respective conditions described below.
The conditions under STC-I comprise a tempera-
ture of 240 degrees C, a pressure of 150 psigl. a GHSV
15015
_35~ 3~
of 8,000 hr l, a reaction inlet stream containing 30
volume per-cent ethylene, 8 volume percent oxygen, 1
volume percent ethane, 26 parts per milliont by vol-
ume, ethyl chloride, 10 parts per million, by volume,
nitric oxide, and the balance nitrogen.
The conditions under STC-II comprise the same
set of conditions recited above for STC-I except
that, under STC-II, the temperature is 260 degrees C,
the reaction inlet stream contains about 3 volume
percent carbon dioxide and 33 parts per million, by
volume, nitric oxide with the amount of nitrogen
adjusted accordingly.
The product, for example, ethylene oxide, is
recovered from the reactor effluent, e.g., by an
absorption process. One such method comprises sup-
plying the reactor effluent stream to the bottom of
an absorption column while adding a solvent, for
example, water, to the top of the absorption col-
umn. The solvent preferably absorbs the ethylene
~ oxide and carries it out of the bottom of the ab-
sorption column, while the remainder of the reaction
effluent passes out of the top of the absorption
column to form the recycle stream. The desired pro-
duct is thereafter recovered, for example, by passing
the solvent and absorbed product through a stripper.
As noted above, it may be preferable to remove a
portion of the recycle stream prior to returning the
recycle stream to the reaction zone. It is generally
preferable to selectively remove certain compounds.
An absorption column or other types of separation
means can be used to provide a selective purge.
The recycle stream generally contains the dilu-
ents and inhibitors fed to the system, unreacted
alkene and oxygen, together with by-products of the
reaction, such as carbon dioxide and water, and any
15015
~3~
-36-
minor amount of alkene oxide which is not recovered
as product. After removal of the purge stream, the
recycle stream is returned to the reaction zone,
preferably being mixed with the makeup feedstream
- 5 prior to or as it enters the reaction zone.
The makeup feedstream replaces reactants, i.e.,
alkene and oxygen-containing gas, as well as other
materials not contained in the recycle stream in
sufficient amounts. Alkene, as used herein, refers
to cyclic and acyclic alkenes which are in a gaseous
state or have significant vapor pressures under epox-
idation conditions. Typically these compounds are
characterized as having on the order of 12 carbon
atoms or less and which are gaseous under epoxidation
conditions. In addition to ethylene and propylene,
examples of alkenes which may be used in the present
invention include such compounds as butene, dodecene,
cyclohexene, 4-vinylcyclohexene, styrene, and norbor-
nene.
The oxygen-containing gas employed in the reac-
tion may be defined as including pure molecular oxy-
gen, atomic oxygen, any transient radical species
derived from atomic or molecular oxygen capable of
existence under epoxidation conditions, mixtures of
anoth~r gaseous substance with at least one of the
foregoing, and substances capable of forming one of
the foregoing under epoxidation conditions. ~uch
oxygen-containing gas is typically oxygen introduced
to the reactor either as air, commercially pure oxy-
gen or any other gaseous substance which forms oxygenunder epoxidation conditions.
The makeup feedstream may also contain one or
more additives, for example, a performance-enhancing
gaseous halide, preferably an organic halide, includ-
ing saturated and unsaturated halides, such as 1,2-
15015
~37~ ~2~
dichloroethane, ethyl chloride, vinyl chloride,
methyl chloride, and methylene chloride, as well as
aromatic halides. The performance-enhancing gaseous
halide preferably comprises l,2-dichloroethane and~or
ethyl chloride. In addition, a hydrocarbon, such as
ethane, can ~e included in the makeup feedstream.
~he makeup feedstream may also contain a diluent or
ballast, such as nitrogen, as is the case when air is
used as the oxygen-containin~ gas.
The makeup feedstream generally also includes at
least one gaseous efficiency-enhancing member of a
redox-half reaction pair. The phrase "gaseous effi-
ciency-enhancing compoundi', as used herein, is an
alternative expression for the expression "at least
one gaseous efficiency-enhancing member of a redox-
half reaction pair." Both phrases are therefore
meant to include single gaseous efficiency-enhancing
members oE redox-half reaction pairs as well as mix-
tures thereo. The term "redox-half reaction pair"
has essentially the same meaning as defined in con-
nection with efficiency-enhancing salts, above. The
preferred gaseous efficiency-enhancing materials are,
preferably, compounds containing oxygen and an ele-
ment capable of existing in more than two valence
s~.ates. Examples of preferred gaseous efficiency-
enhancing members of redox-half reaction pairs in-
clude NO, NO2, N2O4, N2O3, any substance capable of
forming gaseous NO and/or NO2 under e oxidation con-
di~ions, or mixtures thereof. In addition, mixtures
of one of the compounds listed above, particularly
NO, with one or more of PH3, CO, SO3, and SO2 are
suitable. Nitric oxide is particularly preferred.
In some cases it is preferable to employ two
members of a particular half-reaction pair, one in
the efficiency-enhancing salt and the other in the
15015
-38- ~Z~ J'~
gaseous efficiency-enhancing compound employed in the
feedstream, as, for example, with a preferred combin-
ation of KNO3 and NO. Other combinations, su~h as
KNO3/N2O3, KNO3/NO2, and KNO~/N2O4 may also be em-
ployed in the same system. In some instances, thesalt and the gaseous members may be found in half-
reactions which represent the first and last reac-
tions in a series of half-reaction equations of an
overall reaction.
The gaseous efficiency-enhancing member of a
redox-half reaction pair is preferably present in an
amount that favorably affects the efficiency and/or
the activity. The precise amount is determined, in
part, by the particular efficiency-enhancing salt
employed and the concentration thereo , as well as
the other factors noted above which influence the
amount of efficiency-enhancing salt. Suitable ranges
of concentration for the gaseous efficiency-enhanciny
compound are generally dependent upon the particular
alkene which is being epoxidized, larger amounts of
the gaseous efficiency-enhancing compound generally
being preferable with higher alkenes. For example,
in an ethylene epoxidation system, a suitable range
of concentration for the gaseous eEficiency-enhancing
member of a redox-half reaction pair i5 typically
about 0.1 to about 100 ppm by volume of the reaction
inlet stream. Preferably, the gaseous efficiency-
enhancing compound is present in the reaction inlet
stream in an amount within the range of from 0.1 to
80 ppm, by volume, when about 3 percent, by volume,
carbon dioxide is present in the reaction inlet
stream. When nitric oxide is employed as the gaseous
efficiency-enhancing compound in an ethylene epoxida-
tion system, it is preferably present in an amount of
from about 0.1 to about ~0 ppm by volume. When about
15015
3Z~53
-39-
3 percent, by volume, carbon dioxide is present in
the reaction inlet stream, nitric oxide, if used as
the gaseous efficiency~enhancing compound, is prefer-
ably present in an amount of from about 1 to about 40
ppm. On the other hand, in a propylene or higher
alkene epoxidation system a suitable concentration of
the gaseous efficiency-enhancing compound is typical-
ly higher, e.g., from about 5 to about 2,000 ppm by
volume of the reaction inlet stream when using nitro-
gen ballast.
Similarly, the concentration of the performance-
enhancing gaseous halide, if one is used, is depen-
dent, inter alia, upon the particular alkene which is
being oxidized. A suitable range of concentration
for gaseous halide in an ethylene epoxidation system
is typically from about 0.1 to about 60 pp~ by volume
of the ~eaction inlet stream. A suitable concentra-
tion for gaseous halide in the reaction inlet stream
in a prop~lene epoxidation system is typically from
about 5 to about ~,000 ppm by volume when using ni-
trogen ballast. The preEerred concentration of gas-
eous halids, if one is used, varies depending on the
particular compounds used as the e~ficiency-enhancing
salt an~ the gaseous efficiency-enhancing compound
` 25 and the concentrations thereof, as well as the other
factors noted above which influence the preferred
amount of efficiency-enhancing salt.
The ranges for the concentration of alkene,
oxygen, hydrocarbon, carbon dioxide and nitrogen or
other ballast gas such as methane, in the reaction
inlet stream, are dependent upon the alkene being
epoxidized. The tables below show typical ranges for
the materials~(other than the gaseous halide and the
efficiency-enhancing compound) in the reaction inlet
stream for the epoxidation of ethylene (Table A) and
15015
~ -40- lZ~32'~
.
propylene (Table B).
'.~
; Table A
5 ComPOnent Concentration
~, .
Ethylene at least about 2,
o~ten about 5
~ to about 50,
:~ 10 : volume percent
Oxygen about 2 to about 8
volume percent
15 Hydrocarbon about 0 to about 5
` volume percent
. ,~
Carbon Dioxide up to about 7
volume percent
Nitrogen or other remainder
ballast gas, e.g.,
methane
`
::: : ~ :
:: : 30 ~:
:
:: : :
~: : 35
15015
-
-41~ 3~
Table B
Component Concentration
-
5 Propylene about 2 to about 50
volume percent
Oxygen about 2 to about lO
volume percent
Hydrocarbon about 0 to about S
volume percent
Carbon ~ioxide up to about 15
lS volume percent
Nitrogen or other remainder
ballast gas, e.g~,
methane
The ranges set out in Table B for the concentra-
tion of materials in the reaction inlet stream may be
useful fo.r epoxidation of higher alkenes, e.g., al-
ken~s having from 4 to 12 carbon atoms.
The amount of the efficiency-enhancing salt of a
member of a redox-half reaction pair present in the
catalyst directly affects the activity and/or effi-
ciency of the epoxidation reaction. The most pre~er-
able amount of the salt of a member of a .redox-half
reaction pair varies depending upon the alkene being
~: epoxidized, the compound used as the gaseous effi-
: ciency-enhancing member of a redox-half reaction
pair, the concentration of components in the reaction
inlet stream, particularly the gaseous efficiency-
enhancing compound and carbon dioxide, the amount of
lSOlS
-42~ 3Z
:`:
.,
silver contained in the catalyst, the surface area,
morphology and type of the support, and the process
conditions, e~g., gas hourly space velocity, temper-
ature, and pressure. The approximate concentration
of efficiency-enhancing salt in the finished catalyst
can be controlled by appropriate selection of the
concentration of efficiency-enhancing salt in the
salt impregnation solution. Operable amounts of the
efficiency-enhancing salt can be deter~ined by carry-
ing out tests on similar catalysts containinq varyingamounts of the salt, i.e., by traversing across a
range of salt concentrations from relatively too
small to relatively too high an amount.
It has been noted that when conventional
analyses have been conducted with catalysts prepared
by co-impregnation with silver and efficiency-
enhancing salt, not all the anion associated wi~h the
cation has been accounted for. For example, cata-
lysts prepared by co-impregnation with a potassium
nitrate solution have been analyzed by conventional
techniques and about 3 moles of the nitrate anion
have been observed for every 4 moles of the potassium
cation. This is believed to be due to limitations in
the conventional analytical techniques and does not
necessarily mean that the unaccounted for anions are
not nitrate. ~or this reason, the amount of the
ef~iciency-enhancing salt in the catalyst is given,
in some instances, in terms of the weight percentage
~ of the cation of the efficiency-enhancing salt (based
on the weight of the entire catalyst), with the un-
derstanding that the anion associated with the cation
is also present in the catalyst in an amount roughly
proportional (on a molar basis) to the cation.
The efficiency-enhancing salt is preferably
provided in such an amount, in an ethylene epoxida-
15015
"
-43~
tion system, that the finished catalyst contains from
~bout 0.01 to about 0.7 percent, by weight, of ~he
cation of the salt, based on the total weight of the
catalyst, more preferably from about 0.02 to about
~-5 0.3 weight percent, most preferably from about 0.05
¦to about 0.1 weight precent. The preferred salt is
potassium nitrateO The approximate eoncentration of
efficiency-enhancing salt in the finished catalys~
can be controlled by appropriate selection of the
concen~ration o efficiency-enhancing salt in the
salt-impregnation solution~
E mples 1 through 4:
Exam~les 1 throuyh 4 were carried out in a 14
` centimeter long tubular, s~ainless steel micro-
reactor, having a 7~8 millimeter inside diameter and
a 5 centimeter long catalyst bed. The microreactor
temperature was controlled in each case by tempera-
ture controllers and a rectangular oven with air
circulation. Approximately 1.1 grams of catalyst in
the form of crushed pellets O.S to 1.5 mm in size was
introduced into the microreactors in the amounts
specified in Table 1. Prior to initiating the reac-
tion, the reactor with the catalyst in place washeated to 200 degrees Centigrade in a flowing nitro-
gen atmosphere. The ~ests were then carried out with
a process gas mi~ture con~aining 30 vol~me ~ercent
ethylene, 8 volume percent oxygen, 1 volume percent
eth~ne, 26 parts per million by volume ethyl
chloride, 0 or 2.99 volume percent carbon dioxide as
specified in Table II,10 par~s per million by volume
nitric oxide when no carbon dioxide is present in the
~rocess gas mixture, 33 parts per million by volume
nitric oxide when 2.99 volume percent car~on dioxide
`5015
2~0
-44-
is present, and the balance nitrogen. The tests were
carried out at a pressure of 150 psig and a gas hour-
ly space velocity (GHS~) of 8,000 hr 1.
Examples 1 through 4 each correspond to experi-
mentation with a separate catalyst. The catalysts
were prepared by coincidental impregnation. An im-
; pregnating solution was prepared by dissolving 23.0
grams ethylenediamine with 60.0 grams of distilled
water and stirring for a period of about 10 min-
utes. ro the stirred solution was slowly added 23.0
grams of oxalic acid dihydrate. The resulting solu-
tion was stirred for 10 minutes. To this solution
were added, in portions, 43.3 grams of silver ox-
ide. The resulting solution was therea~ter stirred
for one hour, completely dissolving the silver ox-
ide. To the resulting solution were added 8.1 grams
of monoethanolamine, followed by stirring for an
additional 10 minutes. The resulting solution was
divided into 4 equal parts, each part to be used for
one of the four examples. To each part was added the
following amount of a potassium nitrate in water
solution having a potassium concentration of 0.05
grams of potassium per gram of solution:
Example 1: 0.56 grams KNO3 solution
~xample 20 1.13 grams KNO3 solution
Example 3: 2.26 grams KNO3 solution
Example 4: 6.77 grams RNO3 solution
Each soLution was then diluted with distilled
water to 31.25 cubic centimeters.
15015
" .
-4S~ Z ~0
High-purity alpha-alumina support pellets
(13.8 grams), having a surface area of 1.12 m2/g, a
porosity of 0.78 cc/g and a platelet morphology of
the type disclosed in Canadian Application No.
515,865, were placed in a tube which was then
evacuated, following which the support pellets were
impregnated by immersing them in impregnating
solution formed as described above for one hour. The
excess impregnation solution was then drained. The
resulting pellets were then belt-roasted at 500
degrees C in a 66 SCFH air flow for 2.5 minutes. The
amount of potassium and silver in each of the four
catalysts are set out in Table I.
In testi.ng each catalyst, the activity and
eficiency were determined at specified reaction
temperatures and with specified amounts of carbon
dioxide in the reaction inlet stream, i.e., either 0
volume percent or 2.99 volume percent carbon
dioxide. The results are set out in Table II below
and are depicted in Figure 2.
_ABLE I
Example Catalyst Amount of Amount of
Reactor Silver In Potassium
Charge Catalyst In Catalyst
(grams) (weight (weight
percent) (Percent)
1 1.1 19.8 0.05
2 1.1 19.2 0.11
3 1.1 18.6 0.21
4 1.1 19.7 0.~6
15~15
, ~.
-46- 1Z~3~
TABLE II
Volume Parts Per Temper- Activity Efficiency
; ; Percent Million, ature (Pounds (%)
Of CO2- By Vol- Within Of Ethy
In Re- ume, Of Reac- lene
; action Nitric tion Oxide Pro-
Inlet Oxide In Zone duced Per
Stream Reactlon (C) Hour Per
Inlet Cubic Foot
; Stream Of Cata-
. lyst)
.... , ._. . _ , ._ . ___
Example 1
0.0 10 * 15.7 85.5
~.99 33 258.87.3 87.3
Example 2
0.0 10 239.6L7.0 90.7
2~9g 33 25g.62.9 88.0
Example 3
0.0 10 238.115.3 90.8
2.99 33 259.41.7 86.7
Example 4
0.0 10 237.516.0 91.1
2.99 33 259.2 ;1.7 ~ 85.7
: 30
: *Activity and efficiency corrected to a temperature
of 240 degrees C from data at 225.8 degrees C using
the standard Ar~henius temperature dependence as
determined from experimentation.
15015
4 /
~2~3;~
As can be seen from Table 1 and Figure 2,
foL catalysts of the type described in Examples 1-4,
th~ level of potassium which provides an efficiency
of at least 84 percent is a minimum of about 0.05
weight percent while the maximum amount which can be
present when the reaction inlet stream contains 3
volume percent carbon dioxide without reducing the
activity to less than 4 pounds of ethylene oxide
percubic foot of catalyst per hour was about 0.1
weight percent.
Example 5 - Coinciden-tal or
Coimpregnation Method of Preparation
Of A Potassium Nitrate-Containing
~pE~ d High Silver Concentration Catalyst:
A first silver-containing impregnation
solution was prepared by dissolving 1,292.8 grams
ethylene-diamine with 1.281.6 grams of distilled
water and stirring for a period of 10 minutes. To
the stirred solution were slowly added 1,294.8 grams
of oxalic acid dlhydrate. The resulting soluti.on was
stirred for 10 minutes. To this solution were added,
in portions, 2,268.4 grams of Ag2O. The resulting
silver-containing solution was thereafter stirred for
an additional hour and 453.6 grams of
monoethanolamine were then added to the stirred
silver-containing solution. Stirring was continued
for an additional 10 minutes. This solution was then
diluted to a total volume of 5,000 ml by addition of
distilled water.
High-purity alpha-alumina support pellets
(1,925.4 grams) having platelet morphology of the
type disclosed in Canadian Application No. 515,865,
having a surface area of
15015
~Z~Z ~0
-48-
about 1.2 m2/g and a porosity of about 0.8 cc/g were
` placed in a tube which was then evacuated, following
which a first impregnation was conducted by immersing
the support particles in the first silver-containing
impregnation solution formed as described above, for
one hour. Excess impregnation solution was then
, drained and the resulting pellets were ~hen belt-
- roasted at 500 degrees C in a 66 SCF~ air flow for
2.5 minutes. The resulting material contained 24.9
percent silver by weight.
A co-impregnation solution was prepared by plac-
ing 1,260.5 grams of ethylenediamine into a 5,000 ml
beaker and mixing therewith 1,249.6 grams of dis-
tilled water to form a solution. To the stirred
solution were slowly added 1,262.4 grams of oxalic
acid dihydrate and, with continuous stirring, 2,211.7
grams of silver oxide were slowly added. When dis-
solution was complete, 442.3 yrams of monoethanol-
amine were added direc~ly to the solution. To the
`i 20 silver-containing solution were added 26.4 grams o~
potassium nitr~te dissolved in 50 milliliters of
distilled water. To the resulting solution was added
sufficient water to dilute the solution to 4,875
ml. The silver-impregnated catalyst pellets (2,495.6
grams) were impregnated in a manner similar to the
first impregnation described above. The resulting
catalyst contained 39.8 weight percent silver and
0.098 weight percent potassium.
: This: catalyst may be used in carrying out the
process of this invention.
~`
15015
