Catalyst, process for producing the catalyst and process for preparing vinyl acetate using the catalyst

A process for the production of a catalyst for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases which catalyst comprises palladium and/or its compounds, gold and/or its compounds and also alkali metal compounds on a particulate, porous support obtained by PA1 a) impregnating the support with soluble palladium and gold compounds, PA1 b) converting the soluble palladium and gold compounds into insoluble palladium and gold compounds by addition of an alkaline solution to the support, PA1 c) reducing the insoluble palladium and gold compounds on the support with a reducing agent in the liquid or gaseous phase, PA1 d) impregnating the support with at least one soluble alkali metal compound and PA1 e) finally drying the support at a maximum of 150.degree. C., wherein the catalyst is brought into contact with at least one peroxidic compound in step b).

A catalyst comprising palladium and/or its compounds, gold and/or its 
compounds and also at least one alkali metal compound, a process for 
producing it and its use for preparing vinyl acetate in the gas phase from 
acetic acid, ethylene and oxygen or oxygen-containing gases. 
STATE OF THE ART 
It is known from the prior art that vinyl acetate can be prepared in the 
gas phase from ethylene, oxygen and acetic acid in the presence of 
catalysts which comprise palladium, gold and alkali metal compounds on a 
porous support material such as silicon dioxide. 
The distribution of the noble metals on the support material is of 
particular importance for the activity and selectivity of these catalysts. 
Since the reactants in the reaction to be catalyzed cannot readily diffuse 
into the intermediate or inner regions of the porous support material, the 
reaction takes place essentially only on the outermost or surface regions 
of the catalyst. Thus, the metal components present in the interior or in 
the intermediate regions of the support do not contribute significantly to 
the reaction mechanism, which leads to a reduction in productivity of the 
catalyst based on the weight of the noble metals. 
In the development of effective catalysts for vinyl acetate production, 
efforts have therefore been directed at providing catalysts in which the 
catalytically active noble metals are present in a shell on the support 
particles while the core of the support particles is largely free of noble 
metals. Such catalysts can in principle be produced by impregnation of the 
support material with soluble noble metal compounds, subsequent 
precipitation of insoluble noble metal compounds on the support by 
addition of alkaline compounds and final reduction to the noble metals. 
U.S. Pat. No. 4,048,096 describes a process for producing a palladium-, 
gold- and potassium-containing catalyst for vinyl acetate production. The 
catalyst support is first impregnated with a solution comprising a mixture 
of the dissolved palladium and gold salts. It is essential to that 
invention that the solution has the same volume as the pores of the 
support material in the dry state. During the impregnation step, the 
support particles are kept in motion in a rotating vessel. Without prior 
drying of the impregnated support, the noble metal salts on the support 
particles are subsequently converted into insoluble compounds by addition 
of alkalis and are thus fixed to the support particles. The palladium and 
gold compounds are reduced to the corresponding metals by a final 
treatment with a reducing agent. Application of an alkali metal compound 
in a further impregnation step gives a catalyst which has the desired 
shell structure and comprises palladium and gold in a thickness of 0.5 mm 
on the surface of the support material. 
U.S. Pat. No. 3,775,342 also describes the production of a palladium-, 
gold- and potassium-containing catalyst for vinyl acetate production. In 
this process, the support material is treated in any order with two 
solutions of which one comprises the dissolved palladium and gold salts 
and the other an alkaline substance. After treatment with the first 
solution, the support is dried in an intermediate step before being 
brought into contact with the second solution. The volume of both 
solutions corresponds to the pore volume of the support material. 
Furthermore, U.S. Pat. No. 5,332,710 discloses the production of a catalyst 
for preparing vinyl acetate, in which the insoluble noble metal salts are 
likewise precipitated on the support particles by addition of alkalis. For 
this purpose, the support particles are immersed in the alkaline solution 
and are subjected to rotary motion from the commencement of the 
precipitation for at least half an hour in a drum. This process is known 
as "rotation-immersion." 
In the preparation of vinyl acetate, the catalysts produced as described in 
the above-mentioned process frequently lead to undesirably high formation 
of degradation products and by-products, e.g. carbon dioxide, thus 
adversely affecting activity and selectivity of the overall reaction. 
OBJECTS OF THE INVENTION 
Since vinyl acetate is a volume product produced on a large industrial 
scale, it is an object of the invention to provide a catalyst which has a 
further improved activity and selectivity in the preparation of vinyl 
acetate in the gas phase. 
This and other objects and advantages of the invention will become obvious 
from the following detailed description. 
THE INVENTION 
The invention provides a process for producing a catalyst for the 
preparation of vinyl acetate in the gas phase from ethylene, acetic acid 
and oxygen or oxygen-containing gases, which catalyst comprises palladium 
and/or its compounds, gold and/or its compounds and also alkali metal 
compounds on a particulate, porous support produced by 
a) impregnating the support with soluble palladium and gold compounds, 
b) converting the soluble palladium and gold compounds into insoluble 
palladium and gold compounds by addition of an alkaline solution to the 
support, 
c) reducing the insoluble palladium and gold compounds on the support with 
a reducing agent in the liquid or gaseous phase, 
d) impregnating the support with at least one soluble alkali metal compound 
and 
e) finally drying the support at a maximum of 150.degree. C., the 
improvement comprising the catalyst is brought into contact with at least 
one peroxidic compound in step b). 
The invention also provides a catalyst for preparing vinyl acetate in the 
gas phase from ethylene, acetic acid and oxygen or oxygen-containing 
gases, which comprises palladium and/or its compounds, gold and/or its 
compounds and also alkali metal compounds on a particulate, porous support 
obtained by the above-described process. 
The invention further provides a process for preparing vinyl acetate in the 
gas phase from ethylene, acetic acid and oxygen and/or oxygen-containing 
gases in the presence of a catalyst obtained by the above-described 
process. In the preparation of vinyl acetate, the catalysts of the 
invention surprisingly lead both to an improved activity and to a higher 
selectivity of the reaction. 
The support particles of the catalyst of the invention can have any 
geometric shape, for example spheres, pellets, cylinders, rings or stars 
with a regular or irregular configuration. The dimensions of the support 
particles, i.e. the diameter or the length and thickness are generally 
from 1 to 10 mm, particularly from 3 to 9 mm. Preference is given to using 
spherical support particles having a diameter of from 4 to 8 mm. 
Supports which can be used are the known inert support materials such as 
silica, aluminum oxide, alumino-silicates, silicates, titanium oxide, 
zirconium oxide, titanates, silicon carbide and carbon. Other suitable 
support materials are the pyrogenic silicas obtained by flame hydrolysis 
of silicon tetrachloride or the pyrogenic SiO.sub.2 --M.sub.x O.sub.y 
mixture obtained by flame hydrolysis of silicon tetrachloride and another 
metal chloride such as aluminum chloride (U.S. Pat. No. 3,939,199 and 
EP-A-0 723 810). Preference is given to using silica (SiO.sub.2), 
baddeleyite (ZrO.sub.2) and SiO.sub.2 --Al.sub.2 O.sub.3 mixtures as 
support material. In the case of the pyrogenic support materials, the 
pressed bodies described in DE-A-38 03 895 and DE-A-39 12 504 are 
particularly suitable. 
To be suitable as support material, it is critical that the material 
retains its mechanical strength under the reaction conditions of the 
catalytic process for preparing vinyl acetate, particularly in the 
presence of acetic acid. 
Particularly suitable supports are those of the above-mentioned type having 
a specific surface area of from 50 to 400 m.sup.2 /g (measured by the BET 
method) and a mean pore radius of from 50 to 2000 .ANG. (measured by means 
of mercury porosimetry). 
In step a) of the process of the invention, the impregnation step, the 
support particles are impregnated with the dissolved palladium and gold 
compounds. Suitable palladium and gold compounds are all salts and 
complexes which are soluble in the solvents described below, can be 
precipitated as hydroxide or oxide and in the finished catalyst, possibly 
after a washing step, leave no materials which impair performance of the 
catalyst. 
Examples of suitable palladium compounds are palladium(II) chloride, sodium 
and potassium chloropalladate(II), palladium(II) nitrate, nitrite, 
sulfate, oxalate, acetylacetonate or acetoacetate and hydrated 
palladium(II) oxide. It is also possible to use palladium salts of 
aliphatic monocarboxylic acids of 2 to 5 carbon atoms, preferably 
palladium(II) acetate. Gold(III) chloride, gold(III) acetate, 
tetrachloroauric (III) acid and its alkali metal salts can be used as 
soluble gold compounds. In general, these compounds are used in such 
amounts that the finished catalyst comprises from 2 to 14 g/l, preferably 
from 4 to 8 g/l, of palladium and from 1 to 8 g/l, preferably from 2 to 5 
g/l, of gold. 
Suitable solvents for the palladium and gold compounds and also for the 
alkali metal compounds to be applied in step d) are all compounds in which 
the salts chosen are soluble and which are easy to remove again in an 
optional drying step after the impregnation. Particularly suitable 
solvents are water and unsubstituted carboxylic acids of 2 to 10 carbon 
atoms, e.g. acetic acid, propionic acid, n- and iso-butyric acids and n- 
and iso-valeric acids. Owing to its favorable physical properties and also 
for economic reasons, the preferred carboxylic acid is acetic acid. 
It is advantageous to use an additional solvent when the palladium and gold 
compounds are not sufficiently soluble in the carboxylic acid used. Thus, 
for example, palladium(II) chloride dissolves significantly better in 
aqueous acetic acid than in glacial acetic acid. Suitable additional 
solvents are those which are inert and at the same time are miscible with 
the carboxylic acid, e.g. water, ethers such as tetrahydrofuran or dioxane 
and hydrocarbons such as benzene. 
In the impregnation of the support material, it is possible to use a 
plurality of salts of each of the metals to be applied, but preference is 
given to using only one salt per metal. 
The impregnation of the support material with the soluble palladium and 
gold compounds in step a) can be carried out using a solution which 
simultaneously comprises all soluble palladium and gold compounds. Here, 
the support material can be impregnated once or a plurality of times with 
this solution. Since the amount of palladium and gold compounds used 
should be identical in single and multiple impregnation, the total volume 
of the solution should be divided appropriately in the case of multiple 
impregnation. Preference is given to a single impregnation with the total 
volume of the solution. 
In an alternative embodiment, the impregnation of the support material can 
also be carried out using two separate solutions of which one contains the 
palladium compounds and the other contains the gold compounds. In this 
case, the two solutions can be brought into contact with the support 
material either simultaneously or else in any order. In the latter case, 
the support has to be dried after impregnation with the first solution. 
For effective impregnation, the total volume of the noble metal salt 
solution or of the two noble metal salt solutions should be about 90-100%, 
preferably 95-100% and in particular 98-99%, of the pore volume of the 
support material in the dry state. In practice, it is also possible to 
cover the support particles with an excess of the noble metal salt 
solution and subsequently to pour away or filter off the excess solution. 
However, preference is given to adding only the above-described amount of 
solution corresponding approximately to the pore volume of the catalyst 
support. 
It has been found to be advantageous to keep the support particles in 
motion during the impregnation to achieve intimate mixing. This can be 
done by means of a rotating or shaken flask or a mixing drum. The 
rotational speed or in general terms, the intensity of the motion should 
be sufficient to achieve complete wetting of the support particles with 
the impregnation solution but must not be so great that appreciable 
abrasion of the support material occurs. 
The catalyst can subsequently be dried at temperatures of at most 
150.degree. C., preferably 80-150.degree. C. and more preferably 
100-150.degree. C. This drying procedure can be carried out, for example, 
in a stream of hot air in a fan-forced drier or in a drying oven in a 
stream of inert gas, particularly a stream of nitrogen or carbon dioxide. 
Drying is carried out at atmospheric pressure or under reduced pressure, 
preferably 0.01-0.08 MPa. 
In step b), the fixing step, the soluble palladium and gold compounds 
present on the support particles are converted into insoluble compounds 
with an alkaline solution and are thus fixed to the support. It is assumed 
that the insoluble compounds are the hydroxides and/or oxides of the noble 
metals. 
Suitable alkaline solutions are all solutions which are able to convert the 
soluble palladium and gold compounds into insoluble compounds. Example of 
alkaline reagents which can be used are alkali metal hydroxides, alkali 
metal silicates and alkali metal carbonates. Preference is given to an 
aqueous solution of the alkali metal hydroxides, particularly of potassium 
or sodium hydroxide. Solutions containing boron compounds can also be used 
as alkaline solutions. Here, aqueous solutions of sodium tetraborate 
decahydrate (borax), potassium tetraborate or mixtures of alkali metal 
hydroxide and boric acid are particularly suitable. The alkaline solution 
can have buffer properties. 
The amount of alkaline compound present in the aqueous solution is 
advantageously selected so that it is at least sufficient for the 
stoichiometric reaction with the soluble palladium and gold compounds 
applied. However, it is also possible to use an excess of the alkaline 
compound, usually 1-10 times the stoichiometrically required amount. 
It is essential to the process of the invention that the catalyst is 
brought into contact with at least one peroxidic compound in step b). This 
peroxidic compound can be, for example, a perborate, preferably sodium 
perborate, a percarbonate, preferably sodium percarbonate, a 
peroxodisulfate, preferably sodium peroxodisulfate, or hydrogen peroxide. 
One possible embodiment comprises adding the peroxidic compound to the 
alkaline solution which already comprises one of the above-mentioned 
alkaline substances, preferably an alkali metal hydroxide. In an 
alternative embodiment, a second, separate solution comprising the 
peroxidic compound can be used in step b) in addition to the alkaline 
solution. In this case, the impregnated catalyst support, as described 
below, is first brought into contact with the alkaline solution and 
subsequently treated with the aqueous solution of the peroxidic compound 
before the reduction is carried out in step c). Since some of the 
peroxidic compounds mentioned are themselves alkaline, e.g. the perborates 
and percarbonates, it is also possible, in a third and preferred 
embodiment, for the alkaline solution to be used in step b) to comprise 
only the peroxidic compound which is simultaneously alkaline. 
It has been found to be useful to heat the solution which comprises the 
peroxidic compounds to a maximum of 90.degree. C., preferably to 
60-85.degree. C., before addition to the impregnated catalyst support. 
In all three embodiments, the peroxidic compound is used in a 1-20-fold, 
preferably 5-10-fold, excess based on the concentration of the noble metal 
salt. It is found that contact of the impregnated catalyst support with at 
least one peroxidic compound in the fixing step b) leads to some reduction 
of the noble metals. 
Two methods I and II which are suitable for carrying out the fixing step b) 
and can be employed in the production of the catalyst of the invention are 
described below. 
In method I, the support material impregnated in step a) is placed for a 
sufficient time in an alkaline solution whose concentration is such that 
the desired, insoluble noble metal compounds are precipitated. In 
addition, the volume of the alkaline solution is selected so that it is 
sufficient to completely cover and immerse the impregnated support 
particles. Furthermore, the impregnated support particles immersed in the 
alkaline solution are subjected to rotary motion commencing with the 
precipitation of the insoluble palladium and gold compounds for at least 
half an hour, preferably one hour and at most up to 4 hours. This fixing 
method is known as "rotation-immersion" and is described in detail in U.S. 
Pat. No. 5,332,710, which is hereby incorporated by reference. 
In this variant I, the additional treatment of the catalyst support with 
the peroxidic compound can be carried out as described in the three 
above-mentioned embodiments. 
If the method II described below is employed for fixing the palladium and 
gold compounds to the support particles, the support which has been 
impregnated in step a) should be dried before the fixing step b). 
In method II, the fixing step b) comprises at least two separate stages of 
treatment with the alkaline fixing solution. In the first fixing stage, 
the impregnated and then dried support is brought into contact with the 
alkaline fixing solution. The volume of this first fixing solution 
corresponds to the pore volume and thus the absorptive capacity of the 
support material in the dry state. The amount of alkaline compounds 
present therein should be such that the molar ratio of alkali metal from 
the alkaline compound to anions from the soluble metal salt is in the 
range from 0.7:1 to 2:1. For absorption on the support particles, the 
alkaline fixing solution is poured onto the support particles and they are 
then left to stand for up to 24 hours, preferably 2-8 hours. 
In this method II, the second fixing stage can be carried out in two 
variants A) and B). In both variants, the molar ratio of the alkali metal 
from the alkaline compound to the anion from the metal salt is from about 
0.2:1 to 2:1 in the fixing solution. 
In variant A) of method II, the undried support particles are brought into 
contact with the second fixing solution whose volume should at least just 
cover the supports. For absorption on the support particles, the alkaline 
fixing solution is poured onto the support particles and they are then 
left to stand for up to 16 hours, but at least 2 hours and preferably at 
least 4 hours. 
In variant B), the support material after contact with the first fixing 
solution is, in the second step, treated by the rotation-immersion process 
of U.S. Pat. No. 5,332,710. Here, the support material is immersed in the 
alkaline fixing solution of the second step and at the same time subjected 
to rotary motion. This rotation should continue for at least half an hour, 
preferably one hour and at most up to 4 hours. 
Regardless of whether variant A) or B) is employed, the treatment in the 
second fixing step can be equivalent to the treatment in the first stage 
in that a fixing solution of the same concentration is used and the volume 
of the second fixing solution likewise corresponds to the pore volume and 
thus the absorptive capacity of the support material in the dry state. The 
total molar ratio of alkali metal to anion from the metal salt for both 
fixing stages together is preferably in the range from 1.1:1 to 3.3:1. 
In method II, the additional treatment of the catalyst support with the 
peroxidic compound can in principle be carried out in either of the two 
fixing stages, but it is preferably carried out in the second fixing stage 
as described in the three above-mentioned embodiments. 
After the fixing step of method I or the last fixing step of method II, the 
supports can be washed with water, preferably with distilled water, to 
remove any interfering anions, e.g. chlorides, which originate from the 
impregnation step, have been set free by the precipitation of the noble 
metals and are still present on the support material. This washing 
procedure also removes any excess of alkaline compound which may still be 
present. 
The catalyst can then be dried at temperatures of at most 150.degree. C., 
preferably 80-150.degree. C. and more preferably 100-150.degree. C. This 
drying procedure can be carried out, for example, in a stream of hot air 
in a fan-forced drier or else in a drying oven in a stream of inert gas, 
particularly in a stream of nitrogen or carbon dioxide. Drying is carried 
out at atmospheric pressure or under reduced pressure, preferably 
0.01-0.08 MPa. 
Such a drying procedure is advantageous at this point particularly when the 
reduction step c) described below is carried out in the gas phase. In 
contrast, prior drying is not necessary if the reduction is carried out in 
the liquid phase. 
In step c), the support together with the insoluble palladium and gold 
compounds deposited thereon is treated with a reducing agent to convert 
the precipitated palladium and gold compounds into the metallic form. This 
reduction can be carried out in the liquid phase at a temperature of 
0-90.degree. C., preferably 15-25.degree. C. 
The reducing agent used here is, for example, hydrazine, formic acid or an 
alkali metal borohydride, preferably sodium borohydride. As an 
alternative, it is also possible to carry out the reduction in the gas 
phase using hydrogen, ethylene, propylene, isobutylene, butylene or other 
olefins as reducing agent. In this case, it is advantageous to carry out 
the reduction at an increased temperature of 40-260.degree. C., preferably 
70-200.degree. C. It is also advantageous to dilute the reducing agent 
with an inert gas. The inert gas used can be, for example, nitrogen, 
carbon dioxide or a noble gas. Such a reducing agent/inert gas mixture 
usually contains 0.01-50% by volume, preferably 0.5-20% by volume, of 
reducing agent. 
Regardless of whether the reduction is carried out in the liquid or gas 
phase, the reducing agent should be added in an excess, based on the 
catalyst to be reduced, so as to ensure that all the insoluble noble metal 
compound is converted into the metallic form. 
After the reduction, the support particles can be washed once again or a 
plurality of times, preferably with distilled water, to remove interfering 
anions, e.g. chlorides, and residues of the alkaline solution used. The 
washing procedure can also serve to remove residues of the reducing agent 
from step c). 
Subsequently, the catalyst is dried again under drying conditions which 
should be similar to those of a drying step after the fixing step b). 
Finally, the addition of at least one alkali metal compound is necessary. 
The catalyst is therefore impregnated with an aqueous solution of an 
alkali metal compound in step d). Alkali metal compounds which can be used 
are sodium, potassium, rubidium or cesium compounds; preference is given 
to potassium compounds. 
Suitable anions of these alkali metal compounds are, in particular, 
carboxylates, especially acetates or propionates. Particular preference is 
given to using potassium acetate. However, it is also possible to use 
compounds which liberate alkali metal acetates under the reaction 
conditions, i.e. the alkali metal hydroxides, oxides or carbonates when 
acetic acid is used as solvent. This impregnation is carried out, in 
principle, in the same way as the impregnation of the support material in 
step a). The solvents which can be used are subject to the same conditions 
and definitions as in the case of the solutions in impregnation step a). 
The alkali metal compound is used in such an amount that the catalyst 
after the drying step described below contains 0.1-10% by weight of alkali 
metal, preferably 1-4% by weight of alkali metal, in particular potassium, 
based on the total mass of the catalyst. 
Finally, the catalyst is, in step e), dried at temperatures of at most 
150.degree. C., preferably 80-150.degree. C. and more preferably 
100-150.degree. C. This drying procedure can be carried out, for example, 
in a stream of hot air in a fan-forced drier or in a drying oven in a 
stream of inert gas, particularly in a stream of nitrogen or carbon 
dioxide. Drying is carried out at atmospheric pressure or under reduced 
pressure, preferably 0.01-0.08 MPa. 
The catalyst obtained by steps a) to e) of the process of the invention and 
the treatment with the peroxidic compounds essential to the invention in 
step b) comprises, based on the total mass of the catalyst, 0.2-2.5% by 
weight, preferably 0.6-1.5% by weight, of palladium, 0.2-2.5% by weight, 
preferably 0.3-1.0% by weight, of gold and 0.1-10% by weight of alkali 
metal, preferably 1.0-4.0% by weight of alkali metal, in particular 
potassium. 
Vinyl acetate is prepared by passing acetic acid, ethylene and oxygen or 
oxygen-containing gases at temperatures of from 100 to 220.degree. C., 
preferably from 120 to 200.degree. C., and pressures of from 0.1 to 2.5 
MPa, preferably from 0.1 to 2 MPa, over the catalyst of the invention. 
Unreacted components can be circulated. In some cases, dilution with inert 
gases such as nitrogen or carbon dioxide is also advantageous. Carbon 
dioxide is particularly suitable for dilution in a circulation mode of 
operation since it is in any case formed during the reaction. 
It has been found to be useful to carry out the preparation of the vinyl 
acetate in a stirred reactor, a Berty reactor, in circulation mode in the 
gas phase at a constant oxygen conversion of about 45%. The reactor is 
first charged with the catalyst. Subsequently, a measured amount of acetic 
acid and also ethylene and oxygen diluted with nitrogen is introduced and 
the temperature is increased to the desired value using a heating mantle. 
The reaction is usually stopped after about 18 hours, as long as it has 
been possible to set a temperature at which the oxygen conversion is 
constant at 45%. The composition of the product mixture is determined by 
means of gas chromatography. 
The higher selectivity and activity achievable using the catalysts of the 
invention can in practice be utilized in two ways: 
Firstly, to produce a larger amount of vinyl acetate per unit volume and 
unit time in existing plants while retaining all other reaction 
conditions. Owing to the higher selectivity, the product mixture taken 
from the reactor has a higher proportion of vinyl acetate and contains 
less by-products, particularly carbon dioxide. In this way, the work-up, 
i.e. the isolation of the vinyl acetate, is made easier because, for 
example, the amount of carbon dioxide to be separated off is lower and 
accordingly the loss of entrained ethylene associated with the removal of 
carbon dioxide drops. This leads to a saving in starting material. The 
principles of the work-up of the product mixture after the preparation of 
vinyl acetate are described, for example, in EP-A-0 423 658. 
The second possible way of utilizing the improved properties of the 
catalysts of the invention is to lower the reaction temperature in the 
preparation of vinyl acetate while maintaining the same space-time yield. 
A lower reaction temperature in turn has a positive effect on the total 
operational life of the catalyst.

In the following examples, there are described several preferred 
embodiments to illustrate the invention. However, it should be understood 
that the invention is not intended to be limited to the specific 
embodiments. 
The catalysts in Examples 1-5 are produced using silica based on bentonite 
as support material which is the KA-160 support from Sud-Chemie. Spheres 
having a diameter of 7 mm were employed in Examples 1-4 and 6-8 and 
spheres having a diameter of 5 mm were employed in Example 5. 
EXAMPLE 1 
5.37 g (0.0164 mol) of K.sub.2 PdCl.sub.4 and 3.36 g (0.0089 mol) of 
KAuCl.sub.4 were dissolved together in 80 ml of demineralized water. All 
of this solution was, with gentle motion, applied to 131 g of the support 
material which had been pretreated in this way was placed in a solution of 
18.31 g (0.12 mol) of sodium perborate tetrahydrate (NaBO.sub.3.4H.sub.2 
O) in 300 ml of distilled water. The total reaction mixture was rotated on 
a rotary evaporator at a speed of 5 revolutions per minute for 3.5 hours 
at 85.degree. C. to complete the reaction. The reaction mixture was 
allowed to stand for about 12 hours and then was washed free of chloride 
with demineralized water. The freedom from chloride was checked with the 
silver nitrate test for chloride ions in aqueous solution. The material 
was then dried for 2 hours at 100.degree. C. It was shown by photoelectron 
spectroscopy that after this step, the noble metal shell formed comprised 
metallic gold and palladium in the oxidation state +2. Subsequently, the 
noble metals were reduced completely using diluted ethylene (5% in 
nitrogen). For this purpose, the gas mixture was passed over the catalyst 
for 5 hours at 150.degree. C. 10 g of potassium acetate were then 
dissolved in 75 ml of distilled water and added a little at a time to the 
catalyst and the latter was dried once more for 2 hours at 100.degree. C. 
EXAMPLE 2 
5.37 (0.0164 mol) of K.sub.2 PdCl.sub.4 and 1.92 g (0.0051 mol) of 
KAuCl.sub.4 were dissolved together in 80 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 131 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 14.92 g (0.097 mol) of sodium perborate 
tetrahydrate (NaBO.sub.3.4H.sub.2 O) in 300 ml of distilled water and the 
total reaction mixture was rotated on a rotary evaporator at a speed of 5 
revolutions per minute for 3.5 hours at 85.degree. C. to complete the 
reaction. The reaction mixture was allowed to stand for about 12 hours and 
was then washed free of chloride with demineralized water. The further 
procedure was as described in Example 1. 
EXAMPLE 3 
12.88 g (0.0349 mol) of K.sub.2 PdCl.sub.4 and 4.6 g (0.0122 mol) of 
KAuCl.sub.4 were dissolved together in 192 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 314.4 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 35.8 g (0.23 mol) of sodium perborate tetrahydrate 
(NaBO.sub.3.4H.sub.2 O) in 720 ml of distilled water and the total 
reaction mixture was rotated on a rotary evaporator at a speed of 5 
revolutions per minute for 3.5 hours at 85.degree. C. to complete the 
reaction. The reaction mixture was allowed to stand for about 12 hours and 
was then washed free of chloride with distilled water. The further 
procedure was as described in Example 1. 
EXAMPLE 4 
12.88 (0.0349 mol) of K.sub.2 PdCl.sub.4 and 8.06 g (0.0214 mol) of 
KAuCl.sub.4 were dissolved together in 192 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 314.4 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 35.8 g (0.23 mol) of sodium perborate tetrahydrate 
(NaBO.sub.3.4H.sub.2 O) in 720 ml of distilled water and the total 
reaction mixture was rotated on a rotary evaporator at a speed of 5 
revolutions per minute for 3.5 hours at 85.degree. C. to complete the 
reaction. The reaction mixture was allowed to stand for about 12 hours and 
was then washed free of chloride with distilled water. The further 
procedure was as described in Example 1. 
EXAMPLE 5 
5.37 g (0.0164 mol) of K.sub.2 PdCl.sub.4 and 3.36 g (0.0089 mol) of 
KAuCl.sub.4 were dissolved together in 90 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 147.5 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 18.31 g (0.12 mol) of sodium perborate 
tetrahydrate (NaBO.sub.3.4H.sub.2 O) in 300 ml of distilled water and the 
total reaction mixture was rotated on a rotary evaporator at a speed of 5 
revolutions per minute for 3.5 hours at 85.degree. C. to complete the 
reaction. The reaction mixture was allowed to stand for about 12 hours and 
was then washed free of chloride with distilled water. The further 
procedure was as described in Example 1. 
EXAMPLE 6 
7.67 g (0.0235 mol) of K.sub.2 PdCl.sub.4 and 3.84 g (0.0102 mol) of 
KAuCl.sub.4 were dissolved together in 90 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 133.75 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 23.85 g (0.16 mol) of sodium perborate 
tetrahydrate (NaBO.sub.3.4H.sub.2 O) in 300 ml of distilled water and the 
total reaction mixture was rotated on a rotary evaporator at a speed of 5 
revolutions per minute for 3.5 hours at 85.degree. C. to complete the 
reaction. The reaction mixture was allowed to stand for about 12 hours and 
was then washed free of chloride with distilled water. The further 
procedure was as described in Example 1. 
EXAMPLE 7 
2.69 g (0.0082 mol) of K.sub.2 PdCl.sub.4 and 0.96 g (0.0025 mol) of 
KAuCl.sub.4 were dissolved together in 40 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 65.5 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 1.89 g (0.034 mol) of potassium hydroxide in 150 
ml of distilled water and was rotated in this solution on a rotary 
evaporator at a speed of 5 revolutions per minute for 2.5 hours at room 
temperature. Then, the reaction mixture was allowed to stand for about 12 
hours and then the support was separated from the KOH-solution. The wet 
support material was then brought into contact with 150 ml of an aqueous 
solution which contained 18.84 g (0.12 mol) of sodium percarbonate and 
which had been heated at first to 60.degree. C. Then, the reaction mixture 
was heated immediately on a water bath to 85.degree. C. to complete the 
reaction. Subsequently, the total reaction mixture was rotated the 
reaction. Subsequently, the total reaction mixture was rotated on a rotary 
evaporator at a speed of 5 revolutions per minute for 3.5 hours at 
85.degree. C. to complete the reaction. The reaction mixture was allowed 
to stand for about 12 hours and was then washed free of chloride with 
demineralized water. The further procedure was as described in Example 1. 
EXAMPLE 8 
2.69 g (0.0082 mol) of K.sub.2 PdCl.sub.4 and 0.96 g (0.0025 mol) of 
KAuCl.sub.4 were dissolved together in 40 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 65.5 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 1.15 g (0.029 mol) of sodium hydroxide and 9.94 g 
of a 30% strength hydrogen peroxide solution (corresponds to 0.088 mol 
(2.98 g) of hydrogen peroxide) in 150 ml of distilled water and the total 
reaction mixture was rotated on a rotary evaporator at a speed of 5 
revolutions per minute for 3.5 hours at 85.degree. C. to complete the 
reaction. The reaction mixture was allowed to stand for about 12 hours and 
was then washed free of chloride with demineralized water. The further 
procedure was as described in Example 1. 
Comparative Example 
5.37 g (0.0164 mol) of K.sub.2 PdCl.sub.4 and 1.92 g (0.0051 mol) of 
KAuCl.sub.4 were dissolved together in 87 ml of demineralized water and 
all of this solution was, with gentle motion, applied to 133.75 g of the 
support material. The support which had been pretreated in this way was 
placed in a solution of 19.22 g (0.05 mol) of sodium tetraborate 
decahydrate (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O) in 300 ml of distilled 
water and the total reaction mixture was rotated on a rotary evaporator at 
a speed of 5 revolutions per minute for 3.5 hours at 85.degree. C. to 
complete the reaction. The reaction mixture was allowed to stand for about 
12 hours and was then washed free of chloride with demineralized water. 
The further procedure was as described in Example 1. 
To examine the performance of the catalysts described in the preparation of 
vinyl acetate, tests were carried out in a Berty reactor and the results 
are summarized in the table: 
______________________________________ 
Activity of the 
CO.sub.2 selectivity in % based 
Example Catalyst 
on the amount of ethylene reacted 
______________________________________ 
1 3.2 9.6 
2 10.8 
3 11.4 
4 10.7 
5 10.0 
6 10.7 
7 8.6 
8 10.0 
Comparative 
2.1 
12.4 
______________________________________ 
To determine the activity of the catalyst, the temperature in the middle of 
the wall of the Berty reactor used for testing was recorded at a constant 
oxygen conversion of about 45%. Low wall temperatures at a constant oxygen 
conversion meant a relatively high catalyst activity. 
Various modifications of the catalyst and processes of the invention may be 
made without departing from the spirit or scope thereof and it is to be 
understood that the invention is intended to be limited only as defined in 
the appended claims.