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CATALYST FOR VINYL ACETATE hZANIJFACTURE
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a novel catalyst useful in the reaction of
ethylene,
oxygen, and acetic acid in the vapor phase to form vinyl acetate. In
particular, the present
invention is directed to a novel catalyst useful in the catalytic formation of
vinyl acetate in
which said catalyst comprises metallic palladium, gold, and, in addition, at
least one metal of
the group consisting of rhenium and zirconium deposited on a suitable porous
support.
Description of the Prior Art
It is known in the art that vinyl acetate can be produced by reacting
ethylene, oxygen,
and acetic acid in the gaseous phase and in the presence of a catalyst
comprising palladium,
gold, and an alkali metal acetate supported on certain carrier materials such
as silica. Such
catalyst systems exhibit an acceptable activity and, generally, low
selectivity to by-products,
such as carbon dioxide, ethyl acetate, and heavy ends..
Generally, patents describing the manufacture of catalysts for the commercial
2 0 manufacture of vinyl acetate from acetic acid, ethylene, and oxygen
describe and claim
various methods for the deposition of the metallic components , i. e.,
palladium, gold,
cadmium, and the like, to form a narrow band of metal at the surface of the
catalyst
support. Several of the more pertinent vinyl acetate catalysts and their
methods of
manufacture are discussed below.
U.S. Pat. No. 3,190,912 discloses the preparation of catalyst for preparing
unsaturated organic esters wherein the catalyst consists of unsupported or
supported metals
of the platinum or palladium group or oxides or salts thereof, either organic
or inorganic.
Supported catalyst is prepared by dissolving the metal salt or salts in, for
example, water,
adding the support and evaporating the solvent. The noble metal is distributed
uniformly
3 o throughout the support. According to the patent, the activity of the
catalyst can be promoted
by the addition of a metal halide promoter.
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U.S. Pat. No. 3,275,680 discloses a palladium catalyst on aluminum oxide
support
which when promoted with alkali acetate has been found suitable for the
production of
organic acetates, especially vinyl acetate. The palladium catalyst is
deposited throughout the
catalyst support. There is no specific disclosure in the patent of
combinations of two noble
metals or of the use of alloys. The catalyst is prepared by impregnating the
support with an
aqueous solution of the noble metal salt and precipitating the noble metal on
the support by
reduction.
U.S. Pat. No. 3,743,607 discloses a catalyst containing palladium, gold, and
an alkali
metal acetate. The catalyst is prepared by impregnating a carrier such as
silicic acid,
to aluminum oxide, or silicate, aluminum phosphate, etc. with an aqueous
solution of a
palladium salt and of a gold salt and evaporating the resulting mixture to
dryness. The
palladium and gold salts are reduced to their metallic state by a reducing
agent. The catalyst
is then washed with water, impregnated with a solution of sodium acetate and
upon drying is
ready for use.
British Pat. No. 1,333,449 describes a palladium-gold catalyst suitable for
the
preparation of vinyl acetate. It can be established that the process of this
patent results in the
palladium salts being deposited throughout the catalyst support. The catalyst
is prepared by
impregnating the catalyst support with a solution of palladium acetate, barium
aceto-aurate
and potassium acetate in acetic acid and subsequently drying it. The catalyst
is disclosed to
2 0 be a palladium salt and other additives, such as gold, gold salts,
alkaline earth metal salts, and
alkaline metal salts supported on a Garner.
Published Japanese Patent Application No. 48-10135/1973 describes a process
for
preparing surface impregnated supported vinyl acetate catalyst. In a
preliminary step a small
amount of reduced metal (such as gold) is deposited throughout the porous
support. This is
followed by impregnation of the required amount of palladium catalyst which
then is
deposited on the surface around the preforrned metal particles. The palladium
catalyst is
deposited as a surface layer which has a thickness of about 15% or less of the
particle ratios.
The use of alkali metal acetate salts as a co-catalyst is recommended.
British Patent Specification 1,283,737 discloses a method of preparing a
supported
3 0 metal catalyst by contacting the support with a solution of a compound of
a metal such as
platinum, palladium, etc. and converting the deposited compound to the
metallic state, the
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porous catalyst support having been impregnated with an alkaline solution and
saturated with
from 25 to 90% of water or an alcohol. The degree of penetration of metal into
the catalyst
9
support pellet is disclosed to be up to 50% of the pellet radius. The use of a
minor amount
of an activator, such as sodium and potassium acetate, is recommended.
U.S. Pat. No. 3,939,199 discloses the use of a catalyst wherein palladium is
deposited
on a catalyst support which has a pore volume of from 0.4 to 1.0 ml./g. and
wherein less than
10% of the total pore volume is attributable to micropores having a diameter
of less than 30
Angstrom. The usual inert substances such as silicic acid, silicates, aluminum
silicates,
titanium oxide, zirconium oxide, and various glasses are disclosed as the
suitable support
l0 materials. For the preparation of vinyl acetate from ethylene and oxygen,
the catalyst
support is impregnated with a solution of palladium acetate, cadmium acetate,
and potassium
acetate in acetic acid with subsequent drying.
U. S. Patent 3,822,308 describes the manufacture of a vinyl acetate catalyst
containing 0.6-6.0 grams/liter of palladium and 0.1-3.0 grams/liter of gold on
a silica support
wherein the adsorbed metal salts were precipitated on the catalyst support by
use of an
aqueous solution of sodium hydroxide in a quantity equal to 1-110% of the
absorptive
capacity of the support.
U. S. 4,048,096 describes the manufacture of a vinyl acetate catalyst wherein
the
dissolved metal salts were absorbed on the support from a solution equal to
the pore volume
2 0 of the support by mechanically tumbling the supports in a rotating vessel.
The salts were
then fixed with alkali without drying the salt-loaded support.
U. S. 4,087,622 discloses a palladium-gold catalyst manufactured by a two-step
impregnation and precipitation process.
U. S. 3,775,342 describes a method of catalyst preparation generally called
pore
volume impregnation. The patent discloses a vinyl acetate catalyst which
provides an interior
band of palladium-gold alloy deposited on the catalyst support. The catalyst
is disclosed to
be prepared by treating the catalyst support simultaneously or successively,
with or without
intermediate drying, with a solution of palladium and gold salts and a
solution which contains
compounds that are able to react on the catalyst support with the palladium-
gold salts to
3 0 form water insoluble palladium and gold compounds and then transforming
the water
insoluble palladium and gold compounds into the noble metals by treatment with
reducing
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agents, and removing the water soluble compounds by washing. It is recommended
to
employ an alkali metal carboxylate, e.g., alkali metal acetate, on the
catalyst such that after
drying the catalyst contains from 1 to 30 percent by weight alkali metal
carboxylate. In all of
the examples, in which the catalyst support was treated successively with the
solution of
palladium and gold compounds and the precipitating solution the catalyst was
dried between
each of the successive treatments. In pore volume impregnation the amount of
solution
containing a salt of the precious metal is approximately equal to the pore
volume of the
porous support. In that patent there is disclosed a list of other metals which
may be co-
precipitated with the gold and palladium, including copper. There is, however,
no indication
1 o in the disclosure of an example of how to make a catalyst containing
copper as a third metal
species or that the presence of copper is advantageous, either to the make-
rate of vinyl
acetate or to the selectivity towards ethyl acetate, carbon dioxide or other
impurities. It
appears that the inventors were merely speculating that any of the sixteen or
more metals
disclosed in any concentration could exist in the final catalyst.
Other patents and publications of which the inventors are aware describe
catalysts for
the vapor phase reaction of acetic acid, ethylene, and oxygen to form vinyl
acetate and which
contain copper as a reduced metal component. These catalysts, however, do not
contain any
gold component. These include U. S. 3,759,839; U. S. 3,641,121; U. S.
3,671,576; French
Patents 1,566,972 and 1,583,899; Japanese Patent Application (Kokoku) 54-8638;
Japan
46033013 (Chemical Abstracts 76(8):34719a; Japan 46032644 (Chemical Abstracts
76(8):34718z; and Revista de Chemie Vol. 26, No. 9, pages 719-723 (1975).
Several new patents have issued directed to improved methods and to the
resulting
product. Examples ofthese patents are U. S. 5,332,710 and U. S. 5,314,858
directed to new
methods of manufacture of catalysts containing palladium and gold.
In accordance with U. S. 5,314,858, a useful catalyst is formed by the method
of U.
S. Patent 3,775,342 wherein the metals as salts thereof are combined and are
adsorbed into
the pores of the catalyst by the pore volume impregnation method. (1)
simultaneously or
successively impregnating a catalyst support with aqueous solutions of
palladium and gold
salts such as sodium-palladium chloride and auric chloride, (2) fixing the
precious metals on
3 0 the support by precipitating water-insoluble palladium and gold compounds
by treatment of
the impregnated supports with a reactive basic solution such as aqueous sodium
hydroxide
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which reacts to form hydroxides of palladium and gold on the support surface,
(3) washing
with water to remove the chloride ion (or other anion), and (4) reducing the
precious metal
hydroxides to free palladium and gold, wherein the improvement comprises
during fixing
step (2) utilizing two separate precipitation stages wherein the amounts of
the reactive
compound in contact with the salt-impregnated support in each stage is no more
than that
required to react with the water soluble precious metal compounds impregnated
in the
support. Between the separate fixing or precipitation stages, the support
which has been
impregnated with the reactive basic solution is allowed to stand for a set
period of time to
allow precipitation of the water insoluble precious metal compounds before the
second fixing
1o stage in which additional reactive basic compound is added to the support.
U. S. 5,332,710 discloses (1) simultaneously or successively impregnating a
catalyst
support with aqueous solutions of palladium and gold salts such as sodium-
palladium
chloride and auric chloride, (2) fixing the precious metals on the support by
precipitating
water-insoluble palladium and gold compounds by immersion of the impregnated
supports in
a reactive basic solution such as aqueous sodium hydroxide which reacts to
form hydroxides
of palladium and gold on the support surface, (3) washing with water to remove
the chloride
ion (or other anion), and (4) reducing the precious metal hydroxides to free
palladium and
gold, wherein the improvement comprises during fixing step (2) rotating the
impregnated
catalyst supports while such impregnated supports are immersed in the reaction
solution at
2 0 least during the initial precipitation period and prior to letting the
treated catalyst stand for an
extended period of time to continue the precipitation of the water-insoluble
palladium and
gold compounds.
U. S. 5,194,417 describes a process for baking the virgin catalyst in an inert
atmosphere for at lease 15 minutes to improve selectivity of the catalyst
toward the
2 5 manufacture of vinyl acetate.
U. S. 5,185,308 claims a palladium-gold catalyst with a defined ratio of
precious
metals among other defined parameters. According to the disclosure, the
catalyst consists
essentially of (1) a catalyst support having a particle diameter of from about
3 to about 7
mm and a pore volume of from 0.2 to about 1.5 ml/g, (2) gold and palladium in
a ratio in the
3 0 range 0.60 to 1.25 distributed in the outermost 1.0 mm thick layer of the
catalyst support
particles, and (3) from about 3.5 to about 9.5% by weight of potassium
acetate.
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As can be appreciated by the above examples of the prior art, the basic method
of
forming the vinyl acetate catalyst containing palladium and gold deposited on
a catalyst
support has evolved to one which comprises ( 1 ) impregnating the support with
aqueous
solutions of water-soluble palladium and gold compounds, (2) precipitating
water-insoluble
palladium and gold compounds on the catalyse support by contacting the
impregnated
catalyst support with a solution of compounds capable of reacting with the
water-soluble
palladium and gold compounds to form the insoluble metal compounds (3) washing
the
treated catalyst with water to remove anions which are freed from the
initially impregnated
palladium and gold compounds during precipitation and (4) converting the water-
insoluble
1 o palladium and gold compounds to the free metal by treatment with a
reducing agent. A final
treatment usually irrvolves (5) impregnating the reduced catalyst with an
aqueous alkali metal
acetate solution and (6) drying the 5na1 catalyst product.
It is an object of the present invention to provide a virryi acetate catalyst
which
contains a defined quantity and ratio of palladium, gold, and, in addition, at
least one metal
of the group consisting of rhenium and zirconium on a porous support.
SUMMARY OF THE ~OI~I
It has now been found that a particx~lariy active supported catalyst useful
for the
2 o production of vinyl esters from ethylene, lower carboxylic adds with 2-4
carbon atoms, and
oxygen in the gas phase at elevated temperature and at normal or elevated
pressure can be
obtained by airy of the prior art methods of manufacturing the catalyst using
three or four
metals instead of the prior art recogruzed two metals or the newly descn'bed
catalyst
comairung copper as a third metal U. S. 5,347,04b describes a catalyst
containing the third
2 5 metal copper. The metals which comprise the useful catalyst of this
imrention are palladium,
gold, and at least one of rhenium or zirconium. The concentration of palladium
on the
support is from about 2 to about 14 g<ams/liter; the concentration of gold on
the catalyst
support is from about 1 to about 8 gramslliter, and, when used, the
concentration of
zirconium on the catalyst support is from about 0.5 to about 4 gramsNter
and/or the
3 0 concentration of rhenium on the catalyst support is from about 1 to about
8 grams/liter.
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A broad aspect of the invention provides a method
of preparing a catalyst useful for the vapor-phase
manufacture of vinyl acetate from acetic acid, ethylene, and
oxygen which consists of a porous support containing
deposited thereupon metals comprising from about 2 to
about 14 g/1 of palladium, from about 1 to about 8 g/1 of
gold and at least one metal of the group consisting of
about 0.5 to about 4 g/1 of zirconium and from about 1 to
about 8 g/1 of rhenium, said method comprising:
impregnating said support with water soluble compounds of
said metals, converting said water soluble metal compounds
to water insoluble metal compounds by immersing said
impregnated support in a fixing solution containing a
compound reactive with said water soluble compounds to
precipitate on said support said water insoluble metal
compounds and while said impregnated support is immersed in
said fixing solution tumbling said impregnated support
therein for at least 0.5 hour, completing precipitation of
said water insoluble compounds, washing said support and
reducing said water insoluble metal compounds to form free
metals on said support.
Another broad aspect of the invention provides a
method of preparing a catalyst useful for the vapor-phase
manufacture of vinyl acetate from acetic acid, ethylene, and
oxygen which consists of a porous support containing
deposited thereupon metals comprising from about 2 to
about 14 g/1 of palladium, from about 1 to about 8 g/1 of
gold and at least one metal of the group consisting of
about 0.5 to about 4 g/1 of zirconium and from about 1 to
about 8 g/1 of rhenium comprising: contacting said support
with at least one of said water soluble compounds of said
metal to impregnate the water soluble compound on the
support, converting said water soluble metal compounds) to
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water insoluble metal compounds) by contacting said
impregnated support in a first fixing step with a solution
containing a compound reactive with said water soluble
compound to precipitate on said support said water insoluble
metal compound, contacting said first impregnated and fixed
support with at least one other of said water soluble
compounds of said metals to impregnate the water soluble
compounds) on the support, converting said second water
soluble compounds) of said metals to a water insoluble
metal compounds) by contacting said impregnated support in
a second fixing step with a second solution containing a
compound reactive with said second water soluble compound to
precipitate on said support said second water insoluble
metal compound, and thereafter reducing said water insoluble
metal compounds with a reducing gas to form free precious
metals on said support.
A further aspect of the invention provides a
catalyst obtained according to the foregoing method.
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DETAILED DESCRIPTION OF THE INVENTION
The catalyst of the present invention can be made by any of the prior art
methods for
the manufacture of catalysts useful for the manufacture of vinyl acetate by
the reaction of
ethylene, acetic acid, and oxygen. Of particular value are the methods
described in U. S.
Patents 3, 775,342; 5,314,858; or 5,332,710.
The support material for the catalyst according to the present invention can
be of any
diverse geometrical shape. For example, the support can be shaped as spheres,
tablets, or
cylinders having regular or irregular shapes. The geometrical dimensions of
the support
material can, in general, be in the range of 1-8 mm. A most suitable
geometrical shape is, in
particular, the spherical shape, for example, spheres with diameters in the
range of 4-8 mm.
These supports are generally called pills.
The specific surface area of the support material can vary within wide limits.
For
example, support materials which have an inner surface area of 50-300 m2/g and
especially
100-200 m2/g (measured according to BET) are suitable.
Examples of support materials which can be used include silica, aluminum
oxide,
aluminum silicates or spinets. Silica is the preferred support material.
The active metal catalyst can be prepared by any of the above-described
methods for
the preparation of a palladium-gold catalyst wherein zirconium and or rhenium
metal salts
2 0 are added to the metal salt mixture used for impregnation of the catalyst
support. The gold,
palladium, and rhenium and/or zirconium salts in an aqueous solution are
adsorbed into the
pores of the support in what has been described in the prior art as the pore
volume
impregnation method; the treated supports are fixed to precipitate the metals
into water-
insoluble compounds; and the metal salts are reduced by treatment with a
reducing agent, as
2 5 for example ethylene or hydrazine.
In one method for the preparation of the catalyst of this invention, the
appropriate
quantities of water soluble salts of palladium, gold, and rhenium and/or
zirconium are
dissolved in a sufficient amount' of water to provide a solution equivalent to
about 90-110%
' of the pore volume of the porous support. Palladium (II) chloride, sodium
palladium (II)
3 0 chloride and palladium (II) nitrate are examples of suitable water-soluble
palladium
compounds, whereas auric (III) chloride or tetrachloroauric (III) acid and the
alkali metal
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salts thereof can be used as the water-soluble gold compounds. The generally
available
tetrachloroauric (III) acid and the sodium palladium (11) chloride are
preferred because of
their high water solubility. Zirconium sulfate, zirconium nitrate, and
zirconium chloride are
typical of the salts of zirconium useful for the manufacture of the catalyst
of this invention.
Sodium perchenate is an appropriate salt to use as a soluble salt for rhenium
although
rhenium (VII) heptoxide, or rhenium (111) chloride are also useful for the
present invention.
Typically, the quantity of these compounds employed is such as to provide from
about 2 to
about 14 gran~s/liter of palladium, from about 1 to about 8 gtsms/liter of
gold, and from
about 0.5 to about 4 gramslliter of zirconium, and about 1 to about 8
gtarnsniter of finished
1 o catalyst. Accordingly, the amount of gold present in the catalyst will be
from about 10 to
about 70% of the amount of palladium. The solution is adsorbed on the support,
the
support is dried, and the metals are fixed by immersing the support in an
aqueous alkaline
solution of sufficient conarmation for a period of at least about 16 hours to
cause the metal
salts to become insoluble in water.
Alternatively, the 5xing step may be carried out by immersing the treated
supports in
a cuff dent volume of fixing solution to covet the bulk vohime of the supports
and causing
the supports to be rotated as for example according to the method descn'bed in
U. S.
5,332,710. According to that
refereace, the impregnated supports are immersed in the alkaline solution and
tumbled or
2 o rotated therein during the initial stages of precipitation of the water
insoluble precious
metal compounds such as to the oxides or hydroxides. The rotation or tumbling
of the
supports in the allmline fixing solution should prod for at least about 0.5
hour upon
initial treatment and preferably for at least one hour. Rotation-immersion
treatment can
last as long as up to 4 hours. The treated supports may be allowed to stand in
the fining
2 5 solution to ensure that complete precipitation of the water insoluble
precious metal
compounds talaes place.
Any type of rotation, tumbling, or equivalent equipment which will keep the
support in motion can be used as the exact apparatus utilized is not critical.
What may be
critical, however, is the extent of the motion. Thus, the motion should be
sufficient so
3 o that all surfaces of the impregnated supports are evenly contacted with
the alkaline fining
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solution. The motion should not be so harsh that actual abrasion of the
insoluble precious
metal compounds takes place such that the insoluble compounds are abraded off
the
support surface. Generally, the extent of rotation should be about 1 to 10 rpm
and
°
possibly even higher depending upon the exact support utilized and the amount
of
precious metal to be deposited on the support. The rpm to be used is variable
and may
also depend upon the apparatus utilized, the size and shape of the support,
the type of
support, metal loadings, etc., but should fall within the guidelines expressed
above.
While a small amount of abrasion may take place, it is not to be such that the
insoluble
compounds are actually abraded off the support surface to an unacceptable
degree.
1o The fixing solution is one which comprises an alkaline solution, for
example, an
aqueous solution which contains alkali metal hydroxides, alkali metal
bicarbonates and/or
alkali metal carbonates. It is particularly preferred to use aqueous solutions
of sodium
hydroxide or potassium hydroxide. Subsequent to the precipitation or fixing
step, the
S~~lp'F?rt~ ire-~l'a$lleL~-SUFI:-a~~itl:-dl$t:ll~d=~.'ater -s~v a~ tv~ remove
tifG auiorts, Siit~: h its t~fte
~5 chlorides, which are still contained on the support and freed from the
initial impregnating
solution by the precipitation process. Washing is continued until all of the
anions are
removed from the support. No more than about 1,000 ppm of anion should remain
on the
catalyst. To ensure substantially complete removal of the anions such as
chloride ion from
the catalyst, the wash effluent can be tested with silver nitrate. The
catalyst is then dried at
2 0 temperatures not to exceed about 150°C under an inert atmosphere
such as a continuous
nitrogen or air flow.
The fixed and washed material is then treated with a reducing agent in order
to
convert the metal salts and compounds which are present into the metallic
form. The
reduction can be carried out in the liquid phase, for example, with aqueous
hydrazine
2 5 hydrate, or in the gas phase, for example, with hydrogen or hydrocarbons,
for example,
ethylene. If the reduction is carried out with a solution of hydrazine
hydrate, the reaction is
preferably carried out at normal temperature. When the reduction is carried
out in the gas
phase, it can be advantageous to carry out the reaction at elevated
temperature, for example,
at 100-200°C in the case of reduction with ethylene. The reducing agent
is appropriately
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employed in excess so that it is certain that all of the metal salts and
compounds are
converted into the metallic form.
In another method useful for the preparation of the improved catalyst of the
present
invention, a suitable catalyst support is first impregnated with an aqueous
solution containing
water-soluble palladium, gold, and either or both zirconium and rhenium
compounds.
Separate solutions of palladium, gold, and either or both rhenium and
zirconium compounds
could also be used successively, but it is less convenient to proceed in that
fashion. The
volume of solution used for impregnating the support with the metals is
important. For
effective deposition, the volume of the impregnating solution should be from
95 to 100% of
the absorptive capacity of the catalyst support and preferably it should be 98-
99%.
After impregnation of the support with the water soluble palladium, gold, and
either
or both zirconium and rhenium compounds, the impregnated supports are dried
prior to
fixing the palladium, gold compounds , and either or both zirconium and
rhenium
compounds, as water insoluble compounds on the support. The fixing step is
divided into at
least two separate stages of treatment with the alkaline fixing solution. In
each separate
fixing treatment, the amount of the alkaline reactive compound is no more than
that equal to
the molar amount required to react with all of the metal compound which is
present on the
support as a water soluble compound. Preferably, the amount of reactive
compound used in
each fixing stage is less than the molar amount required to react with all of
the water soluble
2 0 metal compound. The first fixing stage is conducted by impregnating the
dried, impregnated
support with the alkaline fixing solution in an amount equal to about the dry
absorptivity of
the support. The amount of the alkaline compound contained in solution should
be such that
the ratio of alkali metal to anion from the water soluble metal salt be from
about 0.7:1 to
about 1:1 molar and the volume should equal the dry support absorptivity. The
alkaline
2 5 fixing solution is simply poured onto the impregnated supports and the
treated supports are
allowed to stand for up to 24 hours or more during the precipitation. The
second fixing
stage is conducted by adding the undried partially fixed pills to a second
portion of aqueous
fixing solution. In this solution the ratio of alkali metal to anion from the
metal salt should
be from about 0.2:1 to about 0.9:1 molar. The volume of solution should cover
the pills.
3 o Preferably, the total amount of alkali metal to anion should range from
about 1.1:1 to about
1.6:1 molar for the combined fixing step. Subsequent to treatment in the first
fixing stage,
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the treated supports are allowed to stand for a sufficient period of time to
allow precipitation
of the insoluble metal compounds. The period of time for this first fixing
step will vary but
typically will range from about 2 to about 8 hours before the support is again
treated with
.: the second portion of alkaline fixing solution.
Subsequent to treatment in the second fixing stage, the treated supports are
allowed
to stand again for at least an additional 2 hours, preferably, at least 4
hours and may stand to
complete precipitation for up to about 16 hours.
The treatment in the second fixing stage can be equivalent to that of the
first stage
wherein the treated dried and partially fixed supports are impregnated with
the fixing
1 o solution at the desired alkaline concentration. The total volume of
solution is sufficient to
cover the volume of the support.
Alternatively, the support can be impregnated in the second fixing stage by a
process
designated rotation immersion which is set forth in U. S. Patent 5,332,710. In
this process,
the once-fixed catalysts are immersed in the alkaline fixing solution and
tumbled or rotated
therein during the initial stages of the precipitation of the water insoluble
metal compounds.
The rotation or tumbling of the supports in the alkaline fixing solution
should proceed for at
least 0.5 hour upon the initial treatment and, preferably, for at least 1
hour. The rotation
immersion treatment can last as long as up to 4 hours before the treated
supports are allowed
to stand in the fixing solution to insure that complete precipitation of the
water soluble metal
2 o compounds take place.
As described above, any type of rotation or tumbling equipment can be used as
the
exact apparatus utilized is not critical. What may be critical, however, is
the extent of the
rotating motion. Thus, the rotation should be sufficient so that all surfaces
of the
impregnated supports are evenly contacted with the alkaline fixing solution.
The rotation
should not be so harsh that actual abrasion of the insoluble metal compounds
takes place
such that the insoluble compounds are abraded offthe support surface.
Generally, the extent
of rotation should be about 1 to 10 rpm and possibly even higher depending
upon the exact .
support utilized and the amount of metal to be deposited on the support. The
rpm to be
used is variable and may also depend upon the apparatus utilized, the size and
shape of the
3 0 support, the type of support, metal loadings, etc., but should fall within
the guidelines
expressed above that while a small amount of abrasion may take place, it is
not to be such
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that the insoluble compounds are actually abraded off the support surface to
an unacceptable
degree.
The fixed pills are then reduced in a stream of ethylene gas or in a solution
of
hydrazine hydrate in a manner well known in the art.
Depending on the use for which the catalyst prepared in this way is intended,
the
latter can also be provided with customary additives. Thus, for example,
additions of alkali
metal acetates are advantageous whem the catalyst is to be used for the
preparation of
unsaturated esters from olefins, oxygen and organic acids. In this case, for
example, the
catalyst can, for this purpose, be impregnated with an aqueous solution of
potassium acetate
l0 and then dried.
The catalysts according to the invention can be used with particular advantage
in the
preparation of vinyl acetate from ethylene, oxygen and acetic acid in the gas
phase. For this
purpose, those catalysts according to the invention which contain silica as
the support
material and additives of alkali metal acetates are particularly suitable. In
the above
mentioned preparation of vinyl acetate, such catalysts are also distinguished
by high activity
and selectivity and by long life.
EXAMPLE 1
2 o Silica catalyst support, 250 g, provided by Sud Chemie having a spherical
shape and
a diameter of 7.3 mm is impregnated with 85 ml. of an aqueous solution
containing 9.248 of
an aqueous solution of sodium palladium chloride containing 18.95% palladium
ions, 1.33g
rhenium heptoxide, and S.Olg of an aqueous solution of sodium
tetrachloroaurate containing
20.02% gold ions. The impregnated support is dried in hot air at a temperature
not
exceeding 100°C. The treated and dried support is impregnated with 85
ml an aqueous
solution containing 1.76 g sodium hydroxide by soaking the pills in the
solution. The volume
of the sodium hydroxide solution is equal to the dry support absorptivity in
the first fixing
stage. After the first stage, the base-treated pills are allowed to stand for
4 hours and then
subsequently poured into a second sodium hydroxide solution (250 ml of an
aqueous
3 o solution containing 1.54 g NaOITj. Subsequent to the second treatment, the
base treated
material is allowed to stand for an additional about 16 hours. After fixing,
the base treated
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material is thoroughly washed with distilled water to remove the chloride ions
to an
acceptable level which is at most 1,000 ppm chloride. The water flow rate is
about 200
cc/min for approximately 5 hours. The catalyst is dried under a continuous
nitrogen flow at
a temperature of no more than 1 SO°C. The dried catalyst is reduced
with a reducing gas
containing 5% ethylene in nitrogen at a temperature of 150°C. The
reducing gas is passed
over the catalyst for 5 hours at atmospheric pressure. The reduced catalyst is
impregnated
with an aqueous solution containing 10 grams of potassium acetate at a
solution volume
equal to the dry absorptivity of the support. The catalyst is dried at a
temperature of 150°C.
to
The method of Example 1 is repeated using 1.59 g of zirconium sulfate instead
of
the rhenium heptoxide. In each of the fixing steps an aqueous solution of 2.24
g NaOIi is
used.
The catalyst is prepared on the same support as used in Example 1. The support
is
impregnated with 85 ml of an aqueous solution containing 9.24 g of an aqueous
solution
2 0 of sodium palladium chloride containing 18.95 % palladium ions, 1.59 g.
zirconium
sulfate, and S.O1 g of an aqueous solution of sodium tetrachloroaurate
containing 20.02
gold ions. Precipitation is accomplished by adding 283 ml aqueous sodium
hydroxide
solution equivalent to 120 % of the stoichiometric equivalent needed to
convert the metal
salts to their hydroxides. The flask is immediately rotated in a roto-
evaporator (without
'vacuum) at approximately 5 rpm and rotation continued for 2.5 hours. After
2.5 hours,
the rotation is stopped and the alkaline treated supports allowed to stand for
an additional
16 hours to insure maximum precipitation of the metal salts as the insoluble
hydroxides.
7Che flask is drained and the alkaline treated material is washed with
distilled water to
remove the chloride ions. A water flow rate of about 200 cc per minute is used
for
3 o approximately 5 hours. The catalyst is reduced with an aqueous solution of
hydrazine
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hydrate (12/1 excess) at ambient room temperature for 4 hours. The pills are
washed and
dried for 1 hour at 100°C. The reduced catalyst is impregnated with an
aqueous solution
containing 10 grams of potassium acetate at a solution volume equal to the dry
support
absorbtivity and the catalyst is dried.
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