Note: Descriptions are shown in the official language in which they were submitted.
WO 96/04989 PCTIUS94108950
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PROCESS FOR PREPARING SILVER CATALYST
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to a supported
silver catalyst useful for the vapor-phase oxidation of
ethylene to ethylene oxide. More particularly, the present
invention relates to a method of preparing an improved
supported silver catalyst post impregnated with cesium.
Related Art
'
i
to The use of supported silver catalysts for the oxidation
of ethylene to ethylene oxide has been long known in the
art. Additionally, over the years various promoting metals
have been added to further enhance performance. In
particular, the use of alkali metals has been disclosed in
various amounts and added by different methods. A very
extensive review of the patent literature is given in G.B.
No. 2,043,481A. Such disclosures have been somewhat
inconsistent in their teachings, as can be seen by
comparing U.S. Pat. No. 2,238,474 in which sodium and
lithium hydroxides were suggested as -promoters and
potassium and cesium were shown to be poisons to U.S Pat.
No. 2,671,764 where rubidium and cesium sulfates were
suggested as promoting compounds.
Although alkali metals were suggested generally in the
earlier disclosures, it is also generally true that more
recent workers in the field have considered potassium,
rubidium, and cesium as the preferred alkali metals. For
example,-see the series of patents to Nielson, et al., in
which these materials were used in small amounts co-
deposited with the silver -- -U. S. Pat. Nos. 3,962,136;
4,010,115, and 4,012,425. Still more recently the art has
emphasized synergistic combinations of the alkali metals.
For example, see G.B. No. 2,043,481A cited above and U.S.
Pat. Nos. 4,212,772 or 4,226,782. The art teaches, in
addition, that the alkali metals may be used to rejuvenate
used catalysts, as for example U.S. Pat. Nos. 4,123,385;
4,033,903; 4,177,169: and 4,186,106. The art teaches that
the alkali metals may be deposited either before the silver
WO 96104989 ~ ~ PCTIUS94108950
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is placed on the support (pre-deposited)--U.S. Pat. No.
4,207,210; at the same time the. silver is deposited (co-
deposited)--U.S. Pat. Nos. .4,066,575 and 4,248,741; or
subsequent to deposition;:of:~'the~silver (post-deposited)-- '
e,. . ,
G.B. No. 2,045,636A.
The amount of alkali metal was suggested to be in quite
a wide range in the older art. It was often indicated that
large quantities,e.g. up to several per cent of an alkali
metal could be used. More recently, the art generally has
taught that small quantities of alkali metals produce the
optimum effect no matter when the silver and the alkali
metals were deposited. ICilty in U.S. Pat. No. 4,207,210
related the optimum amount of alkali metal to the surface
area of the support. Exceptions to the ,above include
patents issued to ICI which teach the use of large amounts
of sodium alone (G.B. No. 1,560,480) and potassium in
combination with smaller amounts of rubidium and cesium
(U. S. Pat. No. 4,226,782). However, the art generally
teaches that the optimum will be found in substant-Tally
lower quantities, perhaps on the order of 50-500 ppm by
weight.
It has long been recognized that the method of preparing
the catalyst affects its performance. The differing heat
"reactivations "' hear witness to this. Additionally, the
impregnating solutions used and the intermediate steps have
been found to effect the final catalyst. For example,
Winnick in commonly assigned U.S. Pat. No. 4,066,575
discloses an impregnating solution containing silver
lactate, lactic acid, barium acetate, hydrogen peroxide and
water. As a class the lactate based catalyst are very
stable but exhibit low selectivity. The support is
impregnated with the solution and then first activated by
heating in an inert atmosphere at 350°C for and then dried
in air at 200°C for 12 hours. The "activated" catalyst is
then impregnated with a cesium solution and dried in air at
130°C for 3 hours. The use of the inert atmosphere during
the activation step produced a catalyst that was more
selective, but much less stable, i.e., the catalyst lost
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,.
its activity fairly quickly resulting in shorter run length
for a given end of run temperature.
Armstrong, in commonly assigned U.S. Pat. No. 4,555,501
' disclosed using an impregnating solution containing the
silver salt of a neo acid. The impregnated support was
then "activated" at temperatures of about 200'C to 600'C in
the presence of air or reduced oxygen atmospheres, the
presence of some oxygen being desirable. The alkali metal,
if desired, Was then deposited in small quantities (in the
range of 260 wppm).
Cesium now appears to be the preferred alkali metal.
Various sources of cesium are catalogued in the prior art,
for example, cesium hydroxide, cesium nitrate, cesium
chloride, cesium chlorate, cesium bicarbonate, cesium
carbonate, and other anion functionalities such as
formates, acetates and the like.
U.S. Pat. No. 4,374,260 discloses the coprecipitation of
silver and cesium salt, such as the carbonate from a silver
carboxylate/amino complex.
U. S. Pat No's 4,350,616 and 4,389,338 both show the
deposition of CsC03 onto activated silver catalyst from
alcohol solution where the silver was derived from aqueous
silver salt solution.
U. S. Pat. No's 4,066,575 and 4,033,903 disclose the
preparation of silver catalyst from both aqueous and non
aqueous salt solutions and subsequent treatment of the
activated silver catalyst with post deposition of an alkali
metal salt such as cesium and anions including carbonate
from lower alcohol and preferably from aqueous solutions.
Similarly U.S. Pat. No. 4,342,667 discloses the post
deposition of cesium on to silver catalyst derived from
aqueous solutions.
What is most clear is from the prior art relating to
post deposition alkali metal is the general
interchangeability of aqueous and non aqueous procedures,
i.e. silver catalyst may be prepared by either aqueous or
non aqueous procedures and the post deposition of alkali
metal may be aqueous or non aqueous. Furthermore, the salt
WO96J04989 219 6 0 ~ 9 PCT/US94108950
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~~.: yL 9
of silver or alkali meta7:'f'~is'not specific. Generally the
a' '..
procedures tended to favor the presence of water.
what has now been found is that water at any stage and
in any amount is detrimental to the performance of the
final catalyst. Thus, the present preparation is
characterized as being substantially anhydrous with post
disposition of cesium.
It is an advantage of the present invention that
catalysts of exceptional stability in use for the
to preparation of ethylene oxide are produced, which have high
selectivity at high conversions for the ethylene oxide
process.
SUMMARY OF THE INVENTION
Briefly stated one aspect of the present invention is a
catalyst prepared by the process of impregnating a porous
support having a low surface area with a hydrocarbon
solution of a silver salt of an organic acid which is
substantially free of water and acid. The impregnated
support is subjected to a low temperature activation in an
atmosphere containing less oxygen than air, preferably an
inert atmosphere, by heating at a temperature not
exceeding 300°C preferably in the range of 25o°C to 300'C
on a moving belt. The activation produces a support
containing the activated silver.
The catalyst is made by impregnating a porous support,
preferably having a surface area in the range of 0.2 to 2.0
m2/g, with a hydrocarbon solution of a silver salt of an
organic acid. The solution should be substantially free of
both water and acid as this aspect has been shown to be
3o especially beneficial to catalyst performance and hence
preferred. The impregnated support is activated as
described above.
In order to modify the activated silver catalyst an '
alkali metal, preferably cesium, is added. Stability as
that term is used herein relates to the temperature in the
catalyst bed in operation as a function of time (time
trend).
Another aspect of the invention the activated silver
CA 02196059 2004-08-11
catalyst is an anhydrous post impregnated with an alkali
metal, preferably cesium, to produce a finished catalyst
by immersing the support in a circulating stream of the
alkali metal in an anhydrous solvent such as ethanol. The
5 optimum amount of alkali metals) added will be selected
to optimize catalyst performance and will be dependent
upon the surface area of the support chosen. That is,
more alkali metal will be used on supports which have
larger surface area than on those having relatively small
surface area.
The catalyst of the present invention may be employed
under oxidizing conditions typical to the art for
preparing ethylene oxide by the vapour phase oxidation of
ethylene with improved results, especially catalyst
stability.
Specifically, the process for producing ethylene
oxide comprises:
A process for preparing a supported silver catalyst
for the vapor-phase oxidation of ethylene to ethylene
oxide, comprising the steps of:
(a) impregnating a porous support having a surface
area of about 0.2 to 2.0 m2/g with a hydrocarbon solution
of a silver salt of an organic acid sufficient to provide
3 to 20 wto silver on the support, wherein said solution
is substantially free of water and acid; and
(b) subjecting the silver impregnated support of step
(a) to activation in an inert atmosphere containing up to
2.5 0 oxygen by heating at a temperature not exceeding
300°C.
The term "inert" as used herein means any gaseous
material under the conditions of activation which does not
react with silver or any other component of the silver
impregnated support. Preferred inert material include
nitrogen, helium and carbon dioxide, but other specific
material include neon, argon, and the like may be used.
The limitation of oxygen during the activation is of
principal concern.
CA 02196059 2004-08-11
5a
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a scanning electron micrograph of a silver
catalyst in which the silver was deposited by heating in
air at 500°C.
FIG. 2 is a scanning electron micrograph of a silver
catalyst in which the silver was deposited by heating in
air at 300°C.
FIG. 3 is a scanning electron micrograph of a silver
catalyst in which the silver was deposited by heating in
an inert atmosphere at 500°C.
FIG. 4 is a scanning electron micrograph of a silver
catalyst in which the silver was deposited by heating in
an inert atmosphere at 300°C.
FIG. 5 is a scanning electron micrograph of a silver
catalyst in which the silver was deposited by heating in
an inert atmosphere at 300°C and thereafter used about
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1000 hours in an ethylene oxide reactor.
FIG. 6 is a scanning electron micrograph of a silver
catalyst in which the silo&~~'iaas deposited by heating in
air at 500'C and therea~t'er used about 1000 hours in an "
ethylene oxide reactor.
DETAILED DESCRIPTION AND PREFERRED EMBODIMENT _ '
The catalyst of the present invention may contain from 3
to 25 wt% silver on the support. Preferred catalysts
prepared in accordance with this invention-contain up to
about 20% by weight of silver, expressed as metal,
deposited upon the surface and throughout the pores of a
porous refractory support. Silver contents higher than 20%
by weight of total catalyst are effective, but result in
catalysts which are unnecessarily expensive. Silver
contents, expressed as metal, of about 5-13% based on
weight of total catalyst are preferred, while silver
contents of 8-12% are especially preferred.
Catalysts may be made with supports comprising alumina,
silica, silica-alumina or combinations thereof. Preferred
supports are those containing principally alpha-alumina,
particularly those containing up to about 15 wt% silica.
Especially preferred supports have a porosity of about 0.1-
1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferred
supports also have a relatively low surface area, i.e.
about 0.2-2.0 m2/g, preferably 0.4-1.6 m2/g and most
preferably 0.5-1.3 m2/g as determined by the BET method.
See J. A. Chem. Soc. 60, 309-16 (1938). Porosities are
determined by the mercury porosimeter method; see Drake
and Ritter, "Ind. Eng. Chem. Anal. Ed.," 17, 787 (1945).
Pore and pore diameter distributions are determined from
the surface area and apparent porosity measurements.
For use in commercial ethylene oxide production
applications, the supports are desirably formed into
regularly shaped pellets, spheres, rings, etc. Desirably,
the support particles used have "equivalent diameters" in
the range from 3-10 mm and preferably in the rang of 4-8
mm, which are usually compatible with the internal diameter
of the tubes in which the catalyst is placed. "Equivalent
CA 02196059 2003-12-09
7
diameter" is the diameter of a sphere having the same
external surface (i.e. neglecting surface within the pores
of the particle) to volume ratio as the support particles
being employed.
The silver is added to the support by immersion of the
support into a solution containing a silver salt of an
organic acid which is substantially free of water and said
acid. The silver containing liquid penetrates by
absorption, capillary action and/or vacuum into the pores
of the support. A single immersion or a series of
immersions, with or without intermediate drying, may be
used, depending in part upon the concentration of the
silver salt in the solution. To obtain catalysts having
silver contents within the preferred range, suitable
impregnating solutions will generally contain from 5-50 wt%
silver, expressed as metal, but supplied as silver salts of
acids. The exact concentrations employed, of course, will
depend upon, among other factors, the desired silver
content, the nature of the support, the viscosity of the
liquid, and solubility of the acid silver salt.
Impregnation of the selected support is achieved in a
conventional manner. The support material is placed in the
silver solution until all of the solution is absorbed by
the support. Preferably the quantity of the silver
solution used to impregnate the porous support is no more
than is necessary to fill the pore volume of the porous
support.
The impregnating solution, as already indicated, is
characterized as a substantially water free and acid free
organic solution of a silver salt of an organic acid, such
as the neo acids (particularly those having at least seven
carbon atoms) disclosed in U.S. Patent No. 4,864,042. A
hydrocarbon solvent is employed, such as toluene, cyclohexane,
xylene, ethyl benzene, cumene or nonene which would normally
be water free. Since water is considered to be detrimental to
the preparation of silver catalysts when the method of the
invention is used, it should be present in no more than
21.9059
R'O 96104989 PCT/US94108950
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about 0.1 vol% in the silver impregnating solution,
preferably less than about 0.01 vol%.
a,
After impregnation with the~ssilver salt the support is
then activated in the low teu~perature process as described, '
preferably on a moving belt4!in the atmosphere as specified.
The silver impregnated support is activated in an inert
atmosphere such as.nitrogen, carbon dioxide or helium, at
a temperature below 300'C as recited and for limited
periods, preferably of Prom about one to ten minutes.
l0 After the low temperature activation the support may be
impregnated with the alkali metal if desired. It is the
purpose of alkali metal to modify the catalyst and raise
selectivity while leaving the improved stability intact.
When used the amount of the alkali metal on the finished
catalyst is generally similar to those employed heretofore.
Thus the amount deposited will be generally up to about 8 X
10-3 gew/kg catalyst, preferably up to about 7 X 10-3
gew/kg, and particularly about 1 to 6 X 10-3 gew/kg (gew =
gram equivalent weight). The alkali metals -of the periodic
table include sodium, lithium, potassium, rubidium and
cesium. For purposes of the present invention, the latter
three alkali metals are particularly preferred, especially
cesium, although sodium and lithium are not necessarily
excluded. The alkali metal salts are dissolved in alcohol
solutions, preferably substantially free of water.
The improvement from the use of cesium salt in a pure
alcohol solvent, substantially free of water is believed to
relate to the relatively poor solubility of cesium salt in
alcohol. In the absence of water in the alcohol solvent,
the cesium compound, although poorly soluble, remains
evenly distributed through the solvent during evaporization
and drying, hence is more evenly distributed over the
silver catalyst. -Active catalyst may also be obtained when
using the present activation followed by deposition of
cesium from alcohol-solution. Preferably the alkali metal
impregnated catalysts are dried rapidly, e.g. one to two
minutes at high temperature, e.g. at least 1D0°C up to
800'C, preferably around 200'C to 600 'C. This may be
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readily achieved by using a moving belt as described
herein. The drying may be conducted in air or an inert
gas.
Catalysts prepared by the procedures above have improved
performance, especially stability, for use in the
' production of ethylene oxide by the vapor phase oxidation
of ethylene with molecular oxygen. These usually involve
reaction temperatures of about 150'C to 400'C, usually
I
about 200°C to 300'C, and reaction pressures in the range
of from 0.5 to 35 bar. Reactant feed mixtures contain 0.5
to 20 % ethylene and 3 to 15 % oxygen, with the balance
comprising comparatively inert materials including such
substances, as nitrogen, carbon dioxide, methane, ethane,
argon and the like. Only a portion of the ethylene usually
is reacted per pass over the catalyst and after separation
of the desired ethylene oxide productand the removal of
appropriate purge streams and carbon dioxide to prevent
uncontrolled build up of inerts and/or by-products,
unreacted materials are returned to the oxidation reactor.
The catalyst prepared by the process of the present
invention has a different silver particle size and
dispersion than catalysts prepared by conventional
processes. In the standard activation step in which the
silver is reduced to its elemental form and deposited and
dispersed upon the surface of the support the support
impregnated with the silver solution is subjected to an
elevated temperature in the presence of a gas containing
oxygen, normally air. This results in very fine silver
particles and is shown in FIG. 1 which is a scanning
electron micrograph (SEM) of a catalyst which was first
activated in air at 500'C. The surface density of the
particles is in the range of 110-125 particles per square
micron (pp~2). Even when the temperature is reduced to
2196059. .
WO 96104989 ~ PCT/US94108950
300'C and air is used the particle density is about the
same as shown in FIG. 2. Also when;the higher temperature
is used with an inert atmosphere the results are about the
same as shown in FIG. 3 where the catalyst was first
5 activated at 500'C in a nitrogen atmosphere. However, when
the lower temperature, i.e., 300°C, is combined with an
inert atmosphere such as nitrogen, the silver particles are
much larger and less dense, e.g. in the range of about 10-
70 pp~e2 as shown in FIG. 4.
10 The catalysts with the finer silver particle dispersion
lose their selectivity during use (over about 1000 hours).
The same catalysts having the 110-125 pp~2 initial
dispersion end up with a silver dispersion of about 1-2
pp~,2 as shown in FIG. 5 which is a SEM of a catalyst having
an initial dispersion of 110-125 pp~2 after about 1000
hours in an ethylene oxide reactor. A catalyst having the
lower initial dispersion of about 10-25 ppu2, however,
after about 1000 hours ends up with a silver particle
dispersion of 25 pp~2 as shown in FIG. 6.
The finished catalysts are then tested for activity and
selectivity by crushing and placing 36 grams in a micro
reactor consisting of a 1/4 inch stainless steel tube which
is heated in a salt bath. A feed mixture of 7% oxygen, 8%
C02, 15% C2H4, 70% N2 is passed over the catalyst with a
gas space velocity of 5500 hr 1. The pressure is
maintained at 300 psig (21.69 bar) and the temperature
between 200'C and 300'C as required to maintain an outlet
concentration of 1.5 vol% (160 Kg per hour per m3 of
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catalyst) ethylene oxide. The activity of the catalyst is
expressed as the temperature necessary to maintain the
outlet concentration at 1.50 vol% ethylene oxide, the lower
the temperature, the more active the catalyst. The
selectivity of the catalyst is expressed as the mole% of
the total ethylene converted to ethylene oxide at the
outlet concentration of 1.50 vol% ethylene. The stability
of the catalyst is measured by the increase in temperature
required to maintain the ethylene oxide productivity.
EXAMPLE 1
The support used for this preparation was obtained from
Norton Company and was made primarily of a-alumina in the
form of 5/I6 inch cylinders. The support has a surface
area of 0.55 m2/g, pore volume of .3 cc/g, and medium pore
diameter of 1.5u. A 95 parts of a cumene solution of
silver neodecanoate, containing 26 wt% silver, was added to
225 parts of the hot support and the mixture was mixed for
minutes. The deposition of the silver was induced by
heating the impregnated support to a temperature that did
20 not exceed 300° on a moving belt in a stream of nitrogen.
The residence time of the catalyst in the heated zone was
two minutes.
The catalyst was then impregnated for two hours at room
temperature in an anhydrous ethanolic solution that
contained 525 ppm cesium bicarbonate. The catalyst was
superficially dried by a stream of nitrogen followed by
heating on a moving belt at 2o0'C.
A sample of the catalyst was tested in a tube that is
WO 96/04989 219 ~ ~ 5 ,~ PCT1US94I08950
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heated by a salt bath. , ~.~:~ gas mixture containing 15%
i\ < <~
ethylene, 7% oxygen,~.~arid 78% inert (mainly nitrogen and
carbon dioxide), was allowed to flow over the catalyst ,
under 300 p.s.i. The temperature of the reaction was
adjusted in order to obtain ethylene oxide productivity of
160 kg/hr/m3 of catalyst. The results of the catalyst test
are summarized in the following table. Additionally
comparative SEM's of the fresh catalyst indicate the
initial silver dispersion was l0-70 ppu2 and the silver
dispersion of the catalyst after removal was 25-75 pp~2.
TABLE I
Life, hr Temp., °C Selectivity, %
150 230 82.3
400 230 82.2
700 231 82.3
977 231 82.2
A similar test was run for a catalyst of the type shown
in Fig. 1 showed lower selectivity, 81.3% and a faster
catalyst deactivation.
EXAMPLE 2
A catalyst was prepared in substantially the same manner
and using the same support as Example 1 except that the
initial calcination was carried out in a gas stream of
nitrogen containing 2.5% oxygen. After 100 hours in the
reactor the selectivity was 82.0% and the reactor
temperature was 228'C.
EXAMPLE 3 (Comparative)
The support used for this preparation was obtained from
WO! 96104989 PCTlUS94108950
13
Norton Company and was made primarily of a-alumina in the
form of 5/16 inch cylinders. The support has a surface
area of 0.55 m2/g, pore volume of .3 cc/g, and medium pore
diameter of 1.5~. A 95 parts of a cumene solution of
salver neodecanoate, containing 26 wt% silver, was added to
225 parts of the hot support and the mixture was mixed for
20 minutes. The catalyst was prepared using activation
with air at 500°C and was impregnated with cesium hydroxide
solution in water/alcohol solvent, which was subsequently
dried with vacuum. The catalyst was tested under the same
condition as in example 1. After 150 hours of reaction
time the selectivity to ethylene oxide was 80.9% and the
reaction temperature was 232'C. The catalyst's
performance did not improve with longer reaction time.
