Note: Descriptions are shown in the official language in which they were submitted.
' - ~
~29~9~7S
K 702
IMPROVED SILVER C~LYST
The invention relates to an improved silver catalyst and to a
process for preparing such silver catalyst, suitable for use in the
oxidation of ethylene to ethylene oxide, to a process for preparing
ethylene oxide~by the use of such catalyst and to ethylene oxide so
prepared.
It is generally known that silver catalysts are applied in the
oxidation of ethylene to ethylene oxide. It is referred to the
British Patent Specification 1,413,251, in which such silver
catalysts are disclosed. Moreover there is disclosed in the appli-
cation that small am~unts of one or more pramoters are present,
such as oe sium cc~pounds, rubidium conpounds and potassium co~r
po~nds.
Applicant has now found silver catalysts with improved
~electivity and stability.
The invention relates to an improved silver catalyst, suitable
for use in the oxidation of ethylene to ethylene oKide,
characterized by
a) a calcined, alkali metal enriched alumina carrier and
b) from 1 to 25 per cent by weight of metallic silver, based on
the weight of the total catalyst,
c) an alkali metal of the group consisting of potassium, r~bidium
and oe sium, in the form of their oxide or hydroxide as a
promoter and
d) a fluoride-anion,
the latter two under c) and d) each being present in an amcunt
between 10 and 1000 parts by weight per million parts by weight of
the total catalyst.
The invention further relates to a pro oe ss for preparing a
silver catalyst, suitable for use in the oxidation of ethylene to
ethylene oxide characterized in that an alkali metal enriched
-`` lZ9Z97S
alumina carrier, which has been calcined, is impregnated with a
solution of a silver compound, sufficient to cause precipitation on
the carrier of from 1 to 25 per cent by weight, on the total
catalyst, of silver, and before, during or after that impregnation
also with one or more dissolved potassium-, rubidium- or cesium
compounds as promoter and with an additional source of fluoride-
anions, and after precipitation the silver cowpcund on the
impregnated carrier is reduced to metallic silver.
The carrier used in the inventive process for the preparation
of silver catalysts, is an alkali metal enriched alumina carrier,
which has been calcined, preferably to a temperature of between
1200 C and 1700 C. A large part of the calcined material will be
alpha-alumina, but the existence of spinels or other configurations
can not be excluded, since the calcined material is enriched with
alkali metal. Salts or hydroxide of an alkali metal have been mixed
with the original alumina. Suitable salts include fluorides,
nitrates, chlorides and sulphates. Suitable metals are lithium,
sodium, potassium, rubidium and cesium. Preferred oompounds are
cesium fluoride, cesium chloride, lithium fluoride, lithium nitrate
and cesium hydroxide. Preferably the aIkali metal compound is mixed
with the alumina in such quantity that the atamic ratio of alkali/-
aluminium is between 0.0005 and 0.1. If desired silicium dioxide is
additionally mixed with the alumina in such quantity that the
atomic ratio of silicium/aluminium is between 0.1 and 0.5.
m e aluminas may be modifications which by calcination provide
alpha-alumina, such like gamma-alumina. Hydrated aluminas may also
be suitable, such as boehmite, which latter by calcining via
gamma-alumina provides alpha-alumina.
Preferably the carrier is prepared by mixing the alumina with
water and alkali metal salt or hydroxide, extruding the obtained
mixture to shaped particles and calcining the shaped particles,
preferably to a temperature between 1200 C and 1700 C. The
calcination m~y be carried out in one or more steps, dbpendiD~ on
the choice of alumina modification. Generally a sufficient am3unt
of water is added to form a paste suitable for extrusion. m e
`-" 129Z9~75
obtained extrudable paste is then extruded and shaped to particles.
The shaped particles are heated in order to evaporate the water.
me solid particles are then calcined, preferably to a temperature
between 1200 C and 1700 C.
Suitable aluminas are powders of gamma-alumina, alpha-alumina
monohydrate, alpha-alumina trihydrate or beta-alumina manohydrate,
which powder during calcination are sintered. At the calcination
temperature the crystal structure may be mcdified. The cubic
structure of gamma-alumina is converted into the hexagonal
structure of alpha-alumina, depending on the amount and nature of
the additive used. m e catalytically active surface of the enriched
alumina may be between 0.1 and 5 m2/g, preferably between 0.2 and
2 m2/g. me shaped alumina particles comprise i.a. bars, rings,
pellets, tablets and triangles. mey are especially suitable in
fixed bed applications in ethylene oxide preparation.
In order to prepare a suitable catalyst the calcined, aikali
metal enrich~ alumina carrier is impregnated with a solution of a
~ilver compound sufficient to cause precipitation on the carrier of
from 1 to 25 per cent by weight, on the total catalyst, of silver,
the so impregnated carrier is separated from the solution and the
precipitated silver compound is reduced to metallic silver. Herein-
after several detailed methods will be disclosed.
As a promoter is added to the silver solution, one or more of
the alkali metals potassium, rubidium and cesium, preferably in the
form of their salts or hydroxides. Although the metals potassium,
rubidium and cesium in pure metallic form exist, they are in that
form not suitable for use. m erefore they are administered in a
solution of their salts or hydroxide. m e alumuna carrier is
impregnated with the prcmDter before, during or after the
impregnation of the silver salt has taken place. The promoter may
even be brought on the carrier after reduction to metallic silver
has taken place. The amount of promoter generally lies between 10
and 1000 parts by weight of potassium, rubidium or oe sium metal per
million parts by weight of total catalyst. Preferably a~unts
between 250 and 750 parts by weight are present on the total
catalyst.
~Z9Z975
-- 4 --
m e alumina carrier is also impregnated with a source of
fluoride-anions. miS may be done the same time that the pro~oter
is added, before or later. m e function of the F ions is not quite
understood. The amount of fluoride-anions present on the alumina
carrier generally is between 10 and 1000 pæts by weight,
preferably between 100 and 400 parts by weight, per million parts
by weight of the total catalyst. Suitable sources of fluoride-
anions are ammonium fluoride (NH4F), ammoniumhydrogen fluoride
(NH4HF2), lithium fluoride, sodium fluoride and silver fluoride.
Generally the alumina carrier is mlxed with a silver salt or a
silver salt-oomplex containing aqueous solution, so that the
alumina carrier is impregnated with said aqueous solution, there-
after the impregnated carrier is separated from the aqueous
solution, e.g. by filtration and then dried. The thus obtained
impregnated alumina carrier is heated to a temperature in the range
of from 100 C to 400 C, during a period sufficient to cause
reduction of thé silver salt (complex) to metallic silver and to
form a layer of finely divided silver, which is bound to the
surface of the alumina carrier. A reducing gas or an inert gas may
be conducted over the alumina carrier during this heating step.
~here are known several methods to add the silver to the
alumina carrier. m e carrier may be impregnated with an aqueous
silver nitrate containing solution, and then dried after which
- drying step the silver nitrate is reduced with hydrogen or hydra-
zine. m e alumina carrier may also be impregnated with an
ammoniacal solution of silver oxalate or silver carbonate, and then
dried, after which drying step the silver oxalate or silver
carbonate is reduced to metallic silver by heating to e.g. up to
400 C. Specific solutions of silver salts with solubilizing and
reducing agents may be employed as well, e.g. combinations of
vicinal alkanolamines, alkyldiamines and ammonia.
The amount of prom~ter generally lies between 10 and 1000 ppm
of alkali metal calculated on the total carrier material. Pre~er-
ably am~unts between 250 and 750 ppm are especially suitable.
Suitable compounds of potassium, rubidium and cesium are, for
lZ9Z9'7~
example, the nitrates, oxalates, carboxylic acid salts or hydrox-
ides. The most preferred promoter is cesium among the alkali
metals, preferably applied in an aqueous solution of cesium hydrox-
ide or oe sium nitrate.
mere are known excellent methods of applying the promoters
ooincidentally with the silver on the carrier. Suitable alkali
metal salts are generally those which are soluble in the silver-
precipitating liquid phase. Besides the above-mentioned compounds
may be mentioned the nitrites, chlorides, iodides, brcmides,
bicarbonates, acetates, tartrates, lactates and isopropoxides. m e
use of aLkali metal salts which react with the silver salt in
solution must be avoided, e.g. the use of potassium chloride
together with silver nitrate in an aqueous solution, since then
silver chloride is prematurely precipitated. The use of potassium
nitrate is recommended instead of potassium chloride. However
potassium chloride may be used together with a silversalt-amine-
complex in aqueous solution, since then silver chloride is not
precipitated prematurely from the solution.
The amount of promoter on the alumina carrier may also be
regulated within certain limits by washing out the surplus of
aLkali material with methanol or ethanol. Temperatures, contact
times and drying with gases may be regulated. Traces of alcohol in
the pores of the carrier must be avoided.
A preferred process of impregnating the alumina carrier
consists of impregnating the carrier with an aquRous solution
containing a silver salt of a carboxylic acid, an organic amine, a
salt of potassium, rubidium or cesium. A potassium containing
silver oxalate solution may be prepared. Silver oxide (slurry in
water) is reacted with a ~ixture of ethylene diamine and oxalic
acid, so that an aqueous solution of silver oxalate-ethylene
diamine-complex is obtained, to which solution is added a certain
amount of potassium compound. Other amunes, such as ethanolamine~
may be added as well. A potassium contaLning silver oxalate
solution may also be prepared by precipitating silver oxalate from
a solution of potassium oxalate and silver nitrate and rinsing with
129Z~S
-- 6 --
water or alcohol the abtained silver oxalate in order to remove the
adhering potassium salt until the desired potassium content is
abtained. The potassium containing silver oxalate is then
solubilized with ammonia and/or an amine in water. ~ubidium and
cesium containing solution may be prepared also in these ways. The
impregnated alumina carriers are then heated to a temperature
between 100 C and 400 C, preferably between 125 C and 325 C.
It is observed that independent of the form in which the
silver is present in the solution before precipitation on the
carrier, the term "reduction to metallic silver" is used, while in
the meantime often decomposition by heating occurs. We prefer to
use the term ~reduction", since the positively charged Ag+ ion is
converted into metallic Ag atom. Reduction times may generally vary
from 5 min to 8 hcurs, depending on the circumstances.
The promoter on the alumina surface is preferably present in
the form of oxide potassium, rubidium or cesium. Mixtures of oxides
are not excluded.
The silver catalysts according to the present invention have
been shown to be particularly selective and stable catalysts in the
direct axidation of ethylene with molecular oxygen to ethylene
oxide. The conditions for carrying out such an oxidation reaction
in the presence of the silver catalysts according to the pres nt
invention broadly aomprise those already described in the prior
art. This applies, for example, to suitable temperatures,
pressures, residenae times, diluent materials, such as nitrogen,
carbon dioKide, steam, argon, methane or other saturated hydro-
carbons, presence or absence of moderating agen*s to control the
catalytic action, for example, 1-2-dichloroethane, vinyl chloride
or chlorinated polyphenyl ccmF1unds, the desirability of employing
recycle operations or applying successive aonversions in different
reactors to increase the yields of ethylene oxide, and any other
special aonditions which may be selected in processes for preparing
ethylene oxide. Pressures in the range of from atm~spheric to 35
bar are generally emplayed. Higher pressures are, hcwever, by no
means excluded. Mblecular oxygen employed as reactant can be
Z97S
abtained from conventional sources. m e suitable oxygen charge may
consist essentially of relatively pure oxygen, a concentrated
oxygen stream oo~prising oxygen in major amount with lesser amounts
of one or more diluents, such as nitrogen and argon, or another
oxygen-containing stream, such as air~ It is therefore evident that
the use of the present silver catalysts in ethylene oxidation
reactions is in no way limited to the use of specific conditions
among those which are known to be effective.
In a preferred application of the silver catalysts according
to the present invention, ethylene oxide is produced when an
oxygen-containing gas is contacted with ethylene in the presence of
the present catalysts at a temperature in the range of from l90 C
to 285 C and preferably 200 C to 270 C.
Generally in the reaction of ethylene with oxygen to ethylene
oxide, the ethylene present is at least a double amount (on a mol
basis) compared with the oxygen, but the applied amount of ethylene
is often much higher. Therefore the conversion is calculated
accordLng to the mol percentage of oxygen, which has been used.
The oxygen conversion is dependent on the reaction temperature,
which latter is a measure for the activity of the catalyst
employed. The values T30, T40 and T50 indicate the temperatures at
30 mol%, 40 mol% and 50 mol% conversion of the oxygen respectively
in the reactor, and the values T are expressed in C. These
temperatures are higher when the conversion of the oxygen is
higher. Moreover these temperatures are strongly dependent on the
employed catalyst and reaction conditions. The ælectivities (to
ethylene oxide) indicate the molar percentage of ethylene oxide in
- the reaction mixture oompared with the total molar amount of
converted matter. The æ lectivity is indicated e.g. as S30, S40 and
S50, which means the ælectivity at 30, 40 and 50 mol% oxygen
conversion respectively.
The stability of the silver catalyst cannot be expressed
directly. Tb measure the stability experiments during a con-
siderable time, e.g. a year would be ne oe ssary. Applicant has now
found that the æ time consuming tests can be simulated by carrying
1;~9Z975
-- 8 --
out the experiments during about one month under the extreme high
velocity of thirty thousand litres gas.litre catalyst l.h 1 also
indicated as GHSV). This velocity is much higher than that used in
commercial ethylene oxide processes (the latter GHSV = 2800-8000).
During the whole test period the above defined S and T values are
measured regularly. After the reaction has finished, the total
amcunt of produced ethylene oxide per ml of catalyst is determlned.
The selectivity and the activity of the catalyst are extrapolated
on the basis that one ml of catalyst would have produced 1000 g of
ethylene oxide. The new catalyst is considered to be more stable
than a standard catalyst, if the differences in T- and S-values,
measured on the new catalyst (preferably at the beginning and at
the end of the reaction) are smaller than those measured on the
standard catalyst, which in every experiment is present. The
stability tests are carried out at constant oxygen conversion of
35~.
Example 1
A. 8 g of cesium fluoride dissolved in 832 ml water was mixed
with 800 g of Kaiser alumina (26405) ~hl~03.H20) by addition of the
cesium fluoride solution to the alumina, and the mixture was
kneaded during 30 min. The obtained paste was extruded. The
obtained shaped pieces w~re dried at 120 C and then calcined at
periodically increased temperature. Up to 700 C was calcined
firstly at an increase in temperature of 200 C/h, then was
calcined for one hour at 700 C, whereafter the te~perature in
two hours reached 1600 C. Finally was calcined further for one
hour at 1600 C. The pore volume of the alpha-alumina shaped pie oe s
was 0.45 ml/g and the average pore diameter was 1.6 ~m. m e
abtained ring-shaped pieces were impregnated with an aqueous
solution of silver oxalate, to which oe sium hydroxide and a D nium
fluoride was added. m e impregnation was carried out for 10 min
under vacuum, whereafter the shaped pieces were separated from the
aqueous solution, and then placed in a heat air stream at a
temperature of 250-270 C during 10 ~ln, in order to oonvert the
silver oxalate into metallic silver. m e aqueous solution of silver
~ Z92975
oxalate oontained 28 per oe nt by weight of Ag (calculated on the
total weight of the solution), wherein the silver oxalate was
oo~plexed with ethylene diamine and to which solution was added
cesium hydroxide and amm~nium fluoride. The impregnated shaped
pieces before heat treatment contained 17.1 per cent by weight
(calculated one the weight of the total catalyst) of silver and
280 ppm of oe sium and 200 ppm of F ~calculated on one million parts
by weight of total catalyst).
B. A second catalyst was prepared in the same manner as above
described, except that the amount of cesium as promDter was
330 ppm.
Both silver catalysts were employed in the preparation of
ethylene oxide from ethylene and oxygen. A cylindric steel reactor
with a length of 40 cm and a diameter of 5 mm was ,pletely filled
with crushed catalyst particles of about 1 mm. The reactor was
pla oe d in a bath of silica and alumina particles in fluid bed
state. A gas mixture of the following oomposition was introduced
into the reactor: 30 mol% ethylene, 8.5 mol% oxygen, 7 mol% carbon
dioxide and 54.5 1% nitrogen and 5.5 ppm vinyl chloride as
moderator. The GHSV was 3300 h 1 The pressure was maintained at
15 bar and the temperature aepenaent on the oxygen oonversion.
Measuring- mstruments were ccnnectcd to the reactor and to a oom-
puter, such that oonversion and reaction temperature could be
precisely regulated. With the aid of gaschrcmatography and mass-
spectrosoopy the amounts of reaction products were determined. The
oxygen conversion was 40%.
A third catalyst was prepared according to Example lA, withthe exception that ammonium fluoride was not added.
All three catalysts were tested on their selectivity:
The selectivity values (S~0) of the first and the seoond catalyst
were 81.2 and 81.3, while the selectivity (S40) of the third
catalyst was 79.9.
All three catalysts did not differ substantially in activity.
It proved that the addition of fluoride anions oonsiderably
-" ~29Z9~5
-- 10 --
improved the selectivity of the catalyst.
Example 2
5.34 g of cesium fluoride was dissolved in 1070 ml water.
800 g of Kaiser alumina (26405) (A12O3.H2O) and 166.8 g of silicium
dioxide (150 g of dry compound) were muxed and the muxture was
kneaded for 15 min. In one ninute the CsF solution was added to the
mixture and the muxture was again kneaded. The obtained paste was
then extruded. The obtained shaped pieces were dried for one hour
at 120 C and then calcined at periodically increased temperature.
Up to 500 C was firstly calcined at an increase in temperature of
200 C/h, then was calcined for one hour at 500 C, whereafter the
temperature in two hours reached 1600 C. Finally was calcined for
six hours at 1600 C. The pore volume of the shaped pieces was 0.25
ml.g 1 and the average pore diameter 1.3 ~m.
The ring-shaped pieces were impregnated with an aqueous
solution of silver oxalate, to which solution oe sium hydroxide and
ammonium fluoride was added. The impregnation was carried but for
10 min in vacuum, whereafter the shaped pieces were separated from
the solution and then placed in a stream of heat air for 10 mun at
a temperature of 250-270 C, in order to convert the silver oxalate
in metallic silver. The aqueous solution of silver oxalate was a 28
per cent by weight containing silver solution, wherein the silver
oxalate was complexed with ethylene diamine and to which solution
the necessary additives were added. The impregnated shaped pieces
before heat treatment contained 13.4 per cent by weight of silver
(calculated on the total weight of the catalyst), 660 ppm of cesium
and 200 ppm of fluorine (calculated on one million parts by weight
of total catalyst).
The silver catalyst was employed in the preparation of
ethylene oxide from ethylene and oxygen. A cylindric steel reactor
with a length of 40 cm and a diameter of 5 mm was oompletely filled
with crushed catalyst particles of about 1 mm. The reactor was then
placed in a bath of silica- and alumina particles maintained in
fluid bed. A gas muxture of the following oomposition was intro-
duced into the reactor: 30 mol% ethylene, 8.5 mol~ oxygen, 7 mo1%
129;~97S
carbon dioxide and 54,5 mol% nitrogen and 5.5 ppm vinylchloride as
moderator. me GHSV was 3300 h 1 The pressure was maintained at 15
bar and the temperature was dependent on the oxygen conversion, the
latter being 40%. Measuring-instruments were connected to the
reactor and to a computer, such that conversion and reaction
temperature could be precisely regulated. With the aid of gaschroma-
tography and mass-spectroscopy the amounts of reaction products
could be determined.
The selectivity (S40) of the above-mentioned silver catalyst
was 81.5, while the selectivity (S40) of a non-amm~nium fluoride
doped silver catalyst was 80.1.
Example 3
1.79 g of cesium fluoride dissolved in 861 ml water was mixed
with 810 g of Kaiser alumina (26405) (A1203.H2G) by addition of the
cesium fluoride solution to the alumina, and the mLxture was
kneaded during 30 min. m e obtained paste was extruded. The
obtained shaped pieces were dried at 120 C and then calcined at
periodically increased temperature. Up to 500 C wa8 calcLned
firstly at an increase in temperature of 200 C/h, then was
calcined for one hour at 500 C, whereafter the temperature in tw~
hours reached 1600 C. Finally was calcined further for six hours
at 1600 C. The pore volume of the alpha-alumina shaped pieces was
0.50 ml/g and the average pore diameter was 1.2 ~m.
The obtained ring-shaped pieces were impregnated with an
aqueous solution of silver oxalate, to which oe sium hydroxide and
ammonium fluoride was added. The impregnation was carried out for
10 min under vacuum, whereafter the shaped pieoe s were separated
from the aqueous solution, and then placed in a heat air stream at
a temperature of 250-270 C during 10 min, in order to convert the
silver oxalate into metallic silver. The aqueous solution of silver
oxalate contained 28 per cent by weight of Ag (calculated on the
total weight of the solution), wherein the silver oKalate was
complexed with ethylene diamine and to which solution was added
oe sium hydroxide and am~onium fluoride. The impregnated shaped
pieces before heat treabment contained 16.9 per oe nt by weight
lZ5~Z9~5
- 12 -
(calculated on the weight of the total catalyst) of silver and 600
ppm of oe sium and 200 ppm of F (calculated on one million parts by
weight of total catalyst).
The silver catalyst was employed in the preparation of
ethylene o~ide from ethylene and oxygen. A cylindric steel reactor
with a length of 40 cm and a diameter of 5 mm was oompletely filled
with crushed catalyst particles of about 1 mm. me reactor was
placed in a bath of silica and alumina particles in fluid ~ed
state. A gas mLxture of the following composition was introdu oe d
into the reactor: 30 mol% ethylene, 8.5 mol% oxygen, 7 mol% carbon
dioxide and 54.5 mol% nitrogen and 5.5 ppm vinyl chloride as
moderator. The GHSV was 3300 h 1. The pressure was maintained at 15
bar and the temperature dependent on the oxygen conversion.
Measuring-instruments were connected to the reactor and to a
computer, such that conversion and reaction temperature could be
precisely regulated. With the aid of gaschromatography and mass
spectrosoopy the amounts of reaction products were determined. m e
oxygen conversion was 40%.
The selectivity (S40) of the abcve-mentioned silver catalyst was
82,5
Example 4
The silver catalyst prepared according to the process dis-
closed in Example lA and the silver catalyst prepared by the
pro oe ss disclosed in E~ample 3 were both tested on their stability
in the reaction of ethylene to ethylene oxide.
A steel cylindric reactor with a length of 15 cm and a
diameter of 3 mm was filled oompletely with catalyst particles of
about 0.3 mm. The reactor was placed in a bath, which oonsisted of
silicium¦aluminium particles in a fluidized state. A gas mixture
with the followlng oo~position was conducted through the reactor:
30 mol% ethylene, 8.5 mol% oxygen, 7 mol% carbon dioxide and
54.5 mol% nitrogen and 7 parts, per million parts of gas, of vinyl-
chloride as moderator. The GHSV was 30,000 l. il.h 1 The pressure
was 15 bar and the temperature was dependent of the oxygen con-
version. The measuring instruments were oonnec*ed to the reactor
lZ~?29~75
- 13 -
and to a computer, in such a way that conversion and temperature
could be regulated precisely. With the aid of gaschromatography or
mass spectroscopy the conter.t of each reaction component was
determined. The stability test was carried out at a constant oxygen
conversion of 35%. During the test, at regular intervals, the
reaction temperature at 35% oxygen conversion was determined. Also
the selectivity to ethylene oxide was determined at regular inter-
vals. After 40 days the tests were discontinued and the total
amount of produced ethylene oxide per ml catalyst was determined.
lOFrom the measured reaction temperatures, starting at the
beginning of the reaction, the increase in reaction temperature was
calculated in C for the moment at which lO00 g ethylene oxide per
ml catalyst would have been produced (~T1000). From the measured
selectivities the decrease in selectivity in mol% was calculated
for the moment at which 1000 g ethylene oxide per ml catalyst would
have been produced (~S1000).
The same measurements and calculations were carried out with a
third silver catalyst which did not contain fluorine, but which
- still contained oe sium fluoride in it8 carrier and which further in
all aspects was pr~pared in the same way as the inventive
catalysts.
In the Table the ~S1000 and aT1000 are given in the percentage
of the ~S1000 and ~ of the third catalyst.
CATALYST
. . _
CARRIER NH~F QSl ~
enriched appl ed
CsF YES (ex.1~) 81 100
CsF YES (ex.3) 43 91
CsF NO 100 100
