Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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WO 96/37455 ''~~4 J ° ° ~ .. a PCT/EP96/01994
Catalyst and process for preparing vinyl acetate
It is known that ethylene can be reacted in the gas phase
with acetic acid and oxygen or oxygen-containing gases
over fixed-bed catalysts containing palladium/cadmium/
alkali metal to give vinyl acetate. In this process, a
space-time yield of more than 200 g/1~h is obtained
(US-A-3 939 199, US-A-4 668 819, US-A-4 902 823,
EP-A-0 403 950, US-A-5 225 388, EP-A-0 565 952,
EP-A-0 634 208, EP-A-0 634 209, EP-A-0 634 214).
It has now surprisingly been found that such catalysts
are considerably improved by addition of at least one
rhenium and/or at least one zirconium compound, i.e. they
give a higher space-time yield at the same or higher
selectivity of the vinyl acetate synthesis and are more
slowly deactivated.
The invention accordingly provides a process for prepar-
ing vinyl acetate in the gas phase from ethylene, acetic
acid and oxygen or oxygen-containing gases over a cata-
lyst comprising palladium and/or its compounds, cadmium
compounds and alkali metal compounds on a support,
wherein the catalyst additionally contains at least one
rhenium and/or at least one zirconium compound.
The invention further provides a catalyst comprising
palladium and/or its compounds, cadmium compounds and
alkali metal compounds on a support, wherein the catalyst
additionally contains at least one rhenium and/or at
least one zirconium compound.
Suitable supports are the'known inert support materials
such as silica, aluminum oxide, aluminosilicates, sili-
Gates, titanium oxide, zirconium oxide, titanates,
silicon carbide and carbon. Particularly suitable are
supports of this type having a specific surface area of
from 40 to 350 m2/g (measured by the BET method) and a
mean pore radius of from 50 to 2000 A (measured using
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mercury porosimetry). especially silicas (Si02) and
Si02/A1203 mixtures. These supports are used in the form
of spheres, pellets, rings, stars or particles of another
shape whose diameter or length and thickness is generally v,,
from 3 to 9 mm.
The total pore volume of the support is preferably
0.4 - 1.2 ml/g. and less than 10% of this volume should
be formed by "micropores" having a pore diameter of below
30 A (Angstrom). Such supports can be prepared from
aerogenic Si02 or an aerogenic Si02/A1203 mixture which is
in the form of vitreous microspheres which can be pre-
pared, for example, by flame hydrolysis of silicon
tetrachloride or a silicon tetrachloride/aluminum
trichloride mixture in a hydrogen/oxygen flame
(US-A-3 939 199). These microspheres are commercially
available under the names ~Aerosil or ~Cabosil.
Particular preference is given to the use of a support
comprising Si02 or a Si02/A1203 mixture having a surface
area of 50 - 250 m2/g and a pore volume of 0.4 - 1.2 ml/g
and pressed from such microspheres using organic fillers
(EP-A-0 403 950). The particles of this support have a
particle size of from 4 to 9 mm, with from 5 to 20 % of
the pore volume of the support being formed by pores
having radii of from 200 to 3000 A and from 50 to 90% of
the pore volume being formed by pores having radii of
from 70 to 100 A. It is particularly advantageous if
these support particles are prepared from the micro-
spheres by tabletting or extrusion with addition of one
or more C2-C2o-carboxylates of Li, Mg, A1, Zn, Fe or Mn as
binders and with addition of organic fillers (such as
sugar, urea, higher fatty acids, relatively long chain
paraffins. microcrystalline cellulose) and lubricants
(such as kaolin, graphite, metal soaps) (US-A-5 225 388).
The particles are subsequently ignited in 02-containing
gases at about 500 - 900°C for about 0.25 - 5 hours.
The catalytically active substances can be applied to the
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~pport in a customary manner, for example by single or
ultiple impregnation of the support with a solution of
he active substances, subsequent drying and, if desired,
reduction. However, the active substances can also be
applied to the support by, for example, single or mul
tiple spraying on, vapor deposition or dipping or by
precipitation onto the support.
Suitable solvents for the catalytically active substances
are, in particular, water or unsubstituted carboxylic
acids having from 2 to 10 carbon atoms, for example
acetic acid, propionic acid, n- and iso-butyric acid and
the various valeric acids. Owing to its physical prop-
erties and also for economic reasons, preference is given
to using acetic acid as carboxylic acid. The additional
use of an inert solvent is advantageous when the
carboxylic acid used is one in which the substances are
not sufficiently soluble. Thus, for example, palladium
chloride can be dissolved significantly better in aqueous
acetic acid than in glacial acetic acid. Suitable
additional solvents are those which are inert and
miscible with the carboxylic acid, for example water or
ethers such as tetrahydrofuran or dioxane, but also
hydrocarbons such as benzene.
It is possible to prepare either fully impregnated
catalysts in which the catalytically active metal com-
pounds have penetrated to the core of the support par-
ticles or else surface-impregnated catalysts in which the
metal salts have not penetrated to the core but only into
a more or less thick outer part of the support particles,
3 0 i . a . the surface zone of the particles . In both cases,
the eiaments to be applied can be applied individually in
the form of solutions of their compounds, or else in any
combinations. Preference is given to using solutions
containing at least one compound of each of the elements
to be applied. Particular preference is given to using a
single solution containing exactly one compound of each
of the elements to be applied. If "the solution" is
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erred to below, this means a solution containing at
.st one compound of one of the elements Pd, alkali
metal, Cd, Re, Zr, or a solution containing at least one
compound of each of two or more of these elements.
To prepare fully impregnated catalysts, the procedure is
preferably as follows (US-A-4 902 823, US-A-3 393 190,
US-A-4 668 819)
The impregnation of the catalyst support with the sol
ution of the active components is undertaken in such a
way that the support material is covered with the sol
ution and any excess solution is then poured off or
filtered off. In consideration of solution losses, it is
advantageous to use only the amount of solution corre-
sponding to the integral pore volume of the catalyst
support and to mix carefully so that the particles of the
support material are uniformly wetted. It is advantageous
to carry out the impregnation process and the mixing at
the same time, for example in a rotating drum or a tumble
dryer, with drying being able to follow immediately.
Furthermore, it is generally advantageous to make the
composition of the solution used for impregnating the
catalyst support such that the desired amount of active
substances is applied by a single impregnation. However,
this amount can also be applied by a plurality of impreg-
nations. with drying preferably being carried out after
each impregnation.
To prepare surface-impregnated catalysts, preference is
given to proceeding according to one of the following
three methods, where a solution of at least one compound
of at least one of the elements Pd, alkali metal, Cd, Re
and/or Zr having a dynamic viscosity of at least
0 . 003 Pa~ s, preferably from 0 . 005 to 0 . 009 Pa~ s is always
used:
1. The support particles are, while being intimately
mixed, sprayed once or a plurality of times with the
solution in the form of droplets having an average
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diameter of at least 0.3 mm or in the form of liquid
jets and are immediately dried after each spraying.
"Immediate" drying here means that drying of the
sprayed particles has to be commenced promptly. It
5 is here generally sufficient if the drying of the
particles is commenced at the latest within 30
minutes of the end~f a spraying. On each spraying,
the solution volume is from 5 to 80% of the pore
volume of the support particles. This method is
comprehensively described in EP-A-O 634 214.
2. The support particles are, while being intimately
mixed, impregnated once or plurality of times with
the solution and immediately dried after each
impregnation. "Immediate" drying here means the same
as in the 1st method, and the solution volume in each
impregnation is from 5 to 80% of the pore volume of
the support particles. This method is
comprehensively described in EP-A-O 634 209.
3. The support particles are impregnated once or a
plurality of times with the solution and dried after
each impregnation, but, in contrast to the 2nd method,
no upper limit is placed on the solution volume: it
is now more than 80% of the pore volume in each
impregnation. Owing to the larger solution volume,
intimate mixing is not absolutely necessary, although
generally useful. Instead, the duration of each
impregnation and the time until the subsequent drying
is commenced, i.e. the time from the commencement of
each impregnation until the commencement of the
subsequent drying, now has to be sufficiently short
for, after completion of the last drying, a surface
zone of 5-80% of the pore volume of the support
particles to contain the catalytically active
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elements. The length of this time which has to be
selected for this purpose can easily be determined by
preliminary experiments. This method is
comprehensively described in EP-A-O 634 208.
A suitable method of determining the thickness of the surface
zone achieved in the surface-impregnated catalysts prepared
comprises cutting open a representative number of impregnated
and dried support particles and measuring the thickness of the
surface zone under the microscope. Here, preferably less than
5% of the particles should have a surface zone thickness which
deviates by more than 15% from the desired value.
The drying of the impregnated or sprayed catalyst support is
preferably carried out under reduced pressure (from 0.1 to 0.8
bar), both in the case of fully impregnated catalysts and in
the case of surface-impregnated catalysts. The temperature
during drying should generally be from 50 to 80°C, preferably
from 50 to 70°C. Furthermore, it is generally advisable to
carry out the drying in a stream of inert gas, for example in a
stream of nitrogen or carbon dioxide. The residual solvent
content after drying should preferably be less than 8% by
weight, in particular less than 6% by weight
The finished catalysts should contain the following amounts of
the catalytically active elements:
The palladium content is generally from 0.6 to 3.5% by weight,
preferably from 0.8 to 3.0% by weight, in particular from 1.0
to 2.5% by weight.
The alkali element content is generally from 0.3 to 10% by
weight.
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6a
Preference is given to using potassium, generally in an amount
of from 0.5 to 4.0% by weight, preferably from 1.0 to 3.0% by
weight, in particular from 1.5 to 2.5% by weight.
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The cadmium content is generally from 0.1 to 2.5% by
weight, preferably from 0.4 to 2.5~ by weight, in par-
ticular from 1.3 to 2% by weight.
The content of rhenium or zirconium is generally from
0.05 to 3% by weight, preferably from 0.05 to 1% by
weight, in particular from 0.05 to 0.5o by weight.
Rhenium and zirconium can also be present together in the
catalyst, in which case the total content of the two
elements is within the specified ranges.
The percentages indicated always refer to the amounts of
the elements Pd, alkali element, Cd, Zr and/or Re present
in the catalyst, based on the total mass of the catalyst
(active elements plus anions plus support material).
Suitable compounds for application to the support are all
compounds of palladium, cadmium, an alkali metal, rhenium
and zirconium which are soluble and contain no constitu-
ents which poison the catalyst, for example sulfur;
preference is given to the acetates and the chlorides .
However, in the case of chlorides it has to be ensured
that the chloride ions are removed before use of the
catalyst for the synthesis of vinyl acetate. This is
achieved by washing the doped support, e.g. with water,
after, for example, the palladium applied as chloride has
been converted into an insoluble form, for instance by
reduction and/or by precipitation with hydroxides.
Particularly suitable compounds of palladium are the
carboxylates, preferably the salts of aliphatic mono-
carboxylic acids having from 2 to 5 carbon atoms, for
instance the acetate, the propionate or the butyrate.
30~ Further examples of suitable co~pou_nds are the nitrate,
nitrite, hydrated oxide, oxalate, acetylacetonate,
acetoacetate. Owing to its good solubility and availabil-
ity, palladium acetate is the particularly preferred
palladium compound.
As alkali metal compound, preference is given to using at
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least one R, Rb or Cs compound, in particular at least
one K compound. Particularly suitable compounds are
carboxylates, in particular acetates and propionates.
Also suitable are compounds which are converted into the
acetate under the reaction conditions, for instance the
hydroxide, the oxide or the carbonate.
As cadmium compound, the acetate is particularly suit-
able.
Particularly suitable zirconium compounds are the acetate
and the acetylacetonate.
Particularly suitable rhenium compounds are Re207 and
(NH4)Re04:
If reduction of the palladium compound is carried out,
which is sometimes useful, a gaseous reducing agent can
be used for this purpose. Suitable reducing agents are,
for example, hydrogen, methanol, formaldehyde, ethylene,
propylene, isobutylene, butylene and other olefins. The
reduction temperature is generally between 40 and 260°C,
preferably between 70 and 200°C. It is generally advan-
tageous to use a reducing agent diluted with inert gas
and containing from 0.01 to 50% by volume, preferably
from 0.5 to 20% by volume, of reducing agent. Inert gases
which can be used are, for example, nitrogen, carbon
dioxide or a noble gas. The amount of the reducing agent
depends on the amount of palladium; the reduction equiv-
alent should be at least from 1 to 1.5 times the oxida-
tion equivalent, but larger amounts of reducing agent do
no harm. The reduction is carried out subsequent to
drying.
The preparation of vinyl acetate is carried out by
passing acetic acid, ethylene and oxygen. ~r oxygen-
containing gases at temperatures of from 100 to 220°C,
preferably from 120 to 200°C, and at pressures of from 1
to 25 bar, preferably from 1 to 20 bar, over the finished
catalyst, with unreacted components being able to be
circulated. The oxygen concentration is advantageously
kept below loo by volume (based on the gas mixture free
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of acetic acid). However, sometimes dilution with inert
gases such as nitrogen or carbon dioxide is advantageous.
Carbon dioxide is particularly suitable for dilution in
the case of a circulation procedure, since it is formed
in small amounts during the reaction.
By means of the catalysts of the invention, a higher
space-time yield and an equal or higher selectivity at a
longer operating life of the catalyst is achieved than
with catalysts which contain no rhenium or zirconium.
The following examples illustrate the invention. The
percentages of the elements Pd, Cd, Zr, Re and K are
percentages by weight based on the total mass of the
catalyst.
As catalyst support, Si02 was used in the form of pellets
having a diameter of 6 mm and a height of 6 mm. The
pellets had been pressed from ~Aerosil powder with the
aid of magnesium stearate as binder as described a.n
US-A-5 225 388. The surface area of the support was
120 m2/g, its pore volume was 0.784 ml/g and its bulk
density was 500 g/l. The pore volume of 1 1 of support
was 392 ml.
I. Fully impregnated catalysts
Comparative Example 1
1 1 of silica supports were impregnated at 60°C with a
solution of 24.3 g of palladium acetate, 21.3 g of
cadmium acetate and 23.8 g of potassium acetate in 392 ml
. of glacial acetic acid (solution volume - 100% of the
pore volume of the support). Subsequently, the supports
were dried in a drying oven at 200 mbar under nitrogen
until the residual acetic acid content was 6% by weight;
the drying temperature was 65°C. The finished catalyst
contained 2.3% by weight of Pd, 1.8% by weight of Cd and
1.9% by weight of R. It was completely impregnated, i.e.
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into the core.
50 ml of this catalyst were placed in a reaction tube
having an internal diameter of 8 mm and a length of
1.5 m. Then, at a pressure of 8 bar (reactor inlet) and
a catalyst temperature of 150°C, the gas to be reacted
was passed over the catalyst for a number of days. This
gas consisted of 27% by volume of ethylene, 55% by volume
of nitrogen, 12% by volume of acetic acid and 6o by
volume of oxygen. The results are shown in Table 1. In
this table, "relative rate of output decrease" is the
quotient, of output decrease (= initial output of the
experiment minus final output of the experiment) and
duration of the experiment, relative to the quotient of
the catalyst used in Comparative Example 1. This catalyst
thus has the quotient (= relative rate of output
decrease) 1.
Example la
The procedure was as in Comparative Example 1, except
that the solution additionally contained 7.5 g of
zirconium acetylacetonate and the amount of glacial
acetic acid was 389 ml. The results are shown in Table 1.
Example lb
1 1 of the catalyst prepared as in Comparative Example 1
was impregnated at room temperature with a solution of
4.2 g of Re207 in 308 ml of water (solution volume = 100%
of the pore volume of the catalyst). The catalyst was
subsequently dried as in Comparative Example 1, until a
residual water content of 6% by weight had been reached.
The catalyst was tested as in Comparative Example 1. The
results are shown in Table 1.
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Table 1 (Fully impregnated catalysts)
Output* Selectivity Relative rate
(g/lh) ( o) of output
decrease
Comparative 813 94.3 1
Example 1
Example la 870 94.5 0.7
Example lb 840 94.8 0.7
* Initial output (gram of vinyl acetate per liter of
catalyst and hour)
II. Surface-impregnated catalysts
Comparative Example 2
At 65°C, 25.3 g of palladium acetate, 25 g of cadmium
acetate and 25.3 g of potassium acetate were dissolved in
130.0 ml of anhydrous acetic acid (glacial acetic acid)
(solution volume = 33% of the pore volume) and the highly
viscous solution (7 mPa ~ s) was placed in a vessel pre-
heated to 65°C. 1 1 of catalyst supports was likewise
heated to 65°C and placed in a flask. The entire impreg-
nation solution was then poured over the support par-
ticles and the particles were intimately mixed until the
solution had been completely absorbed by the particles.
This procedure was complete after 3 minutes.
The catalyst was subsequently dried as in Comparative
Example i. The finis: s3 catalyst contained 2.3 % by weight
of Pd, 1.8% by weight of Cd and 1.9% by weight of K. The
thickness of the surface zone was 0.8 mm.
Testing was carried out as in Comparative Example 1. The
results are shown in Table 2. The "relative rate of
output decrease" is defined as in Comparative Example 1,
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i.e. again relative to the catalyst used there.
Example 2a
The procedure was as in Comparative Example 2, except
that the solution additionally contained 7.0 g of
zirconium acetylacetonate. The thickness of the surface
zone was 0.8 mm. The results are shown in Table 2.
Example 2b
1 1 of the catalyst as prepared in Comparative Example 2
was impregnated at room temperature with a solution of
3.5 g of Re207 in 300 ml of water (solution volume = 100
of the pore volume of the catalyst). The catalyst was
subsequently dried as in Comparative Example l, until a
residual water content of 6o by weight had been reached.
The catalyst was tested as in Comparative Example 1. The
results are shown in Table 2.
Table 2 (Surface-impregnated catalysts)
Output' Selectivity Relative rate
(g/lh) (%) of output
decrease
Comparative 922 95.8 1.4
Example 2
Example 2a 950 96.1 0.9
Example 2b 940 96.0 1.0
* Initial output (gram Qf vinyl acetate per liter of
catalyst and hour)
