Note: Descriptions are shown in the official language in which they were submitted.
F
HOECHST ARTIENGESELLSCHAFT HOE 89/F' 183 Dr. MA/rh
Description
Process for the preparation of vinyl acetate
It is known that ethylene can be reacted in the gas phase
with acetic acid and oxygen or oxygen--containing gases on
fixed-bed catalysts to form vinyl acetate. Suitable
catalysts contain a noble-metal component and an
activator component. The noble-metal component preferably
comprises palladium and/or compounds thereof; in addi-
20 tion, gold and/or compounds thereof may be presewt
(US Patent 3,939,199, German Offenlegungsschrift
2,100,778, US Patent 4,668,81.9). The activator component
here comprises compounds of elements of main group 1
and/or main group 2 and/or cadmium. Potassium is
Z5 preferred as the element of main group 1. These active
components are applied, in finely divided form, to
supports, the support material used generally being
silica or alumina.
The specific surface area of the supports is generally
20 40-350 m~/g. According to tJS Patent 3,939,199, the total
pore volume should be 0.4-1.2 ml/g, and of this, less
than 10~ should be formed by "micropores°° having a pore
diameter of less than 30 angstrom. Examples of suitable
supports having these properties are aerogenic Si02 or an
25 aerogenic SiOa-A1Z03 mixture. The support particles iwthe
preparation of vinyl acetate generally have a spherical
shape. However, tablets and cylinders have also already
been employed.
The invention relates to a process for the preparation of
30 vinyl acetate in 'the gas phase from ethylene, acetic acid
and oxygen or oxygen-containing gases on a catalyst which
contains palladium and/or compounds thereof, and
optionally in addition, gold and/or gold compounds,
and, as activators, alkali metal compounds and
35 optionally in addition, cadmium compounds on a support
.. v 29381-4 ca o2ois99i 2ooo-oi-3i
2
which comprises Si02 or an Si02-A1203 mixture having a surface
area of 50-250 m2/g and a pore volume of 0.4-1.2 ml/g, and has
a grain size of from 4 to 9 mm, wherein 5 to 20% of the pore
volume of the support is formed by pores having radii of from
200 to 3,000 Angstrom and 50 to 900 of the pore volume is
formed by pores having radii of from 70 to 100 Angstrom.
Preferably, catalysts which comprise palladium and
gold as metals and in which more than 90o by weight of each of
these two metals is in the outer part of the support particles,
extending to a depth of 300 of the support particles are
excluded.
Preferably, 8 to 150 of the pore volume of the
support is formed by pores having radii of from 200 to 3,000
Angstrom, and 55-750 of the pore volume is formed by pores
having radii of from 70 to 100 Angstrom.
Supports of this type are obtained as follows:
first, glassy microspheres are produced, for example by flame
hydrolysis of silicon tetrachloride or a silicon
tetrachloride/aluminum trichloride mixture in an oxyhydrogen
flame (US Patent 3,939,199). The microspheres can also be
produced by melting very fine Si02 dust in a sufficiently hot
flame and subsequently rapidly cooling the melt. The
microspheres produced by one of the two methods have a surface
area of 100-300 m2/g. Particularly suitable microspheres are
those having a surface area of 150-250 m2/g which comprise at
least 95o by weight of Si02 and at most 5o by weight of A1203,
in particular comprise at least 99o by weight of Si02 and at
most to by weight of A1203. Microspheres having the surface
area mentioned are commercially available, for example under
1
_ ~_ 29381-4 CA o2ois99i Zooo-oi-3i
2a
the name (R)Aerosil or (R)Cabosil or as "highly disperse
silica" .
The microspheres are then pressed, for example by
tableting (after precompaction) or extrusion, using organic
fillers (such as sugars, urea, higher fatty acids, longer-chain
paraffins, microcrystalline cellulose) and lubricants (such as
kaolin, graphite, metal soaps) to form moldings. Ignition of
these moldings in 02-containing gases subsequently removes
these auxiliaries again. The surface area of the finished
support, its pore volume and the proportion of the pore volume
formed by pores of a
3 _
certain radius ("pore-radius distribution") are deter-
mined by the type of deformation (tablets, extrudates,
etc.), the temperature and duration of ignition, 'the
relative amounts of fillers, lubricants and microspheres,
and the surface area of the microspheres. The most suit-
able values for these determining parameters can be
determined by sample preliminary experiments.
It is also possible to omit lubricants and fillers and
instead to add a silica sol to the microspheres, and then
to dry and ignite the material. This method is parti-
cularly suitable for extrudates. In this method, the
surface area, pore volume and "pore-radius distribution°'
(see above) are determined by the following parameterse
type of silica sol used (size of tine primary particles,
measurable from the Tyndall effect), the 'type of micro-
spheres used, the drying rate and temperature, and the
ignition duration and temperature. Again, the most
suitable values for these parameters can be determined by
simple preliminary experiments.
The finished supp~rt obtained by one of the two methods
then has a surface area of from 50 to 250 m2/g and a pore
v~lume of from 0.4 to 1.2 ml/g, and a grain size of from
4 to 9 mm (adjustable by tableting or extrusion).
By using supports having the specific pore-radius dis-
tribution mentioned, it is possible to significantly
increase the space-time yield of the catalysts compared
with conventional supports under otherwise identical
conditions (same content of active substances on the
support and same reaction conditions) and simultaneously
to reduce the principal side reaction, the combustion of
ethylene to form CO2, by more than 30~. Likewise, the
formation of ethyl acetate, which proceeds as a further
side reaction, is substantially reduced. The advantages
of the process according to the invention are that this
increase in selectivity from about 92~ to about J5&
allows significant savings to be achieved, and that, in
~o:~.~~~~.
-~_
addition, the increase in performance with significantly
increased selectivity in new plants means that the amount
of catalyst and the reactor volume can be reduced, which
results in considerable reductions in plant costs, or
that, in existing plants, the capacity can be substan-
tially increased witho~.rt rebuilding and the investment
costs for plant expansion can thus be saved.
The surface area of the supports mentioned is in all
cases the so-called BET surface area, measured by the
method of Brunauer, Emmett and Teller. It gives the total
surface area of ~. g of support material, i.e. the sum of
the external surface area of the support and the internal
surface area of all the open pores. The total pore volume
and the proportion thereof provided by pores of a cextain
size (for example those having a diameter of from 70 to
Z00 l~ngstrom) can be measured using mercury porosimetry.
Suitable measurement equipment is produced, for example,
by the Carlo Erba or Micromeritics companies.
The catalytically active substances axe applied to the
support in the customary manner, for example by impreg-
na~ting the support with a solution of the active
substances, subsequently drying and, if necessary,
reducing the material. However, it is also possible to
apply the active substances to the support by, for
example, precipitation, spraying, vapor-deposition ar
dipping.
Suitable solvents for the catalytically active substances
are, in particular, unsubstituted carboxylic acids having
2 to 10 carbon atoms in the molecule, such as acetic
acid, propionic acid, n- and iso-butyric acid and ~.he
various valeric acids. Due to their physical properties
and also for reasons of economy, acetic acid is prefer-
ably used as the solvent. The additional use of an inert
solvent is expedient if the subs~tancos are not suffi-
ciently soluble in the carboxylic acid. Thus, for ex-
ample, palladium chloride is substantially more soluble
~o~.~oo~
-
in aqueous acetic acid than in glacial acetic acid.
Suitable additional solvents are those which are inert
and miscible with the carboxylic acid. Examples which may
be mentioned, besides water, are ketones, such as acetone
5 and acetylacetone, furthermore ethers, such as tetra-
hydrofuran or dioxane, but also hydrocarbons, such as
benzene.
The catalyst contains palladium and/or compounds thereof
as the noble-metal component and alkali metal compounds
~.0 as the activator component. It may contain gold and/or
compounds thereof as an additional noble-metal component
and it may contain cadmium compounds as an additional
activator component.
Suitable palladium compounds are alI salts and complexes
which axe soluble (and also, where appropriate,
reducible) and do not leave any deactivating substances,
such as halogen or sulfur, in the finished catalyst. The
carboxylates, preferably the salts of aliphatic moncar-
boxylic acids having ~. to 5 carbon atoms, for example the
acetate, the propionate or the butyrate, are particularly
suitable. The nitrate, nitrite, oxide hydrate, oxalate,
acetylacetonate or acetoacetate, for example, are also
suitable. However, compounds such as the sulfate and the
halides can also be used if care is taken that the
sulfate radical is removed, for example by precipitation
using barium acetate, or the halogen is removed, for
example by precipitation using silver nitrate, before
impregnation, so that the sulfate or halogen anion does
not enter the support. Due to its solubility and acces-
sibility, palladium acetate i.s the particularly preferred
palladium compound.
In general, the cowtent of palladium in the catalyst is
1.0 to 3~ by weight, preferably 1.5 to 2.5~ by weight, in
particular 2 to 2.5~ by weight, the proportion of metal
being related to the total weight of the supported
catalyst.
6 _
Besides palladium and/or compounds thereof, it is also
possible for gold and/or compounds thereof to be addi-
tionally present. A particularly suitable gold compound
is barium acetoaurate. In general, gold or one of its
compounds, if it is employed, is added in a proportion of
0.2 to 0.7~ by weighty the proportion of metal being
related to the total weight of the supported catalyst.
The catalyst contains, as activators, alkali metal
Compounds arid, o~p$iQSl~lly ir1 addi$iori Cadml.um Com-
pounds. Examples of suitable compounds are alkali metal
carboxylates, such as, for example, potassium acetate,
sodium acetate, lithium acetate and sodium propionate.
Those alkali metal compounds which are converted into the
carboxylates under the reaction conditions, such as, for
example, hydroxides, oxides and carbonates, are also
suitable. Suitable cadmium compounds are those which
contain no halogen or sulfur, for example earboxylate
(preferred), oxide, hydroxide, carbonate, citrate,
tartrate, nitrate, acetacetonate, benzoylacetonate and.
acetylacetate. Cadium acetate is particularly suitable.
It is also possible to employ mixtures of different ac
tivators. Each individual activator is generally added in
a proportion of 0.5-4~ by weight, the proportion of metal
in the activator being related to the total weight of the
supporting catalyst.
The following catalysts are preferred:
Palladium/alkali metal element/cadmium and palladium)
gold/alkali metal element, it being possible for pal-
ladium or gold to be in the form of metals or compounds
in the finished catalyst and the preferred alkali metal
element being potaso:ium (in the form of a carboxyJ.ate).
The K:Pd or K:(Pd+Au) ratio here is preferably 0.?:1 to
201. The Cd:Pd or Cd:(Pd+Au) ratio is preferably 0.6:1 to
2:Z, in par~tioular 0.6x1 to 0.9a~.. Pd, Au, Cd and K are
95 always calculated as elements here, i.e>, for example,
only the metal proportions of Pd acetate, Cd acetate and
- 7 -
K acetate on the support are compared with one another.
The catalysts palladium acetate/potassium acetate/cadmium
acetate and palladium acetatelbarium acetoaurate/potas-
sium acetate are particularly preferred.
The impregnation of the catalyst support with the
solution of the active components is preferably carried
out by coating the support material with the solution and
then pouring off or filtering off 'the excess solution.
With consideration for solution losses, it is
~.0 advantageous only to employ the solution corresponding to
the integral pore volume of the catalyst support and to
carry out mixing carefully so that the particles of the
support material are uniformly wetted. This mixing can
be achieved, for example, by stirring. Tt is expedient to
carry out the impregnation process and the mixing s3.mu1-
taneously, far example in a rotary drum or a drum drier,
it being possible for the drying to be parried out
immediately thereafter. 3t is furthermore expedient to
ad just the amount and composition of the solution used.
for impregnation of the catalyst support in a manner such
that it corresponds to the pare volume of the support
material and that the desired amouwt of active substaxaces
is applied by a single impregnation.
The patalyst support impregnated with the solution of the
active substances is preferably dried under reduced
pressure. The temperature during drying should be below
120°C, preferably below 90°C. In addition, it is
generally advisable 'to carry out the drying in a stream
of inert gas, far examgle in a stream of nitrogen or
carbon dioxide. The residual solvent content after drying
should preferably be less than 8~ by weight, in par-
ticular less than 6~ by weight.
Tf reduction of the palladium cornpounds (and, where
appropriate, the gold compounds ) is parried out, which
may sometimes be useful, this may be carried out in
vacuo, at atmospherip pressure or at elevated pressure of
- 8 --
up to 10 bar. It is advisable here to dilute the reducing
agent with an inert gas the greater the higher 'the
pressure. The reduction temperature is between 40 and
260°C, preferably between 70 and 200°C. It is generally
expedient for the reduction to use an inert gas/reducing
agent mixture which contains 0.01 to 505 by volume,
preferably 0.5 to 20~k by volume, of xeducing agent.
Examples of inert gases which may be used are nitrogen,
carbon dioxide or a noble gas. Examples of suitable
reducing agents are hydrogen, methanol, formaldehyde,
ethylene, propylene, isobutylene, butylene and other
olefins. The amount of reducing agent depends on the
amount of palladium and, where appropriate, on the amount
of gold employed; the reduction equivalent should be a~t
least 1- to 1.5-times the oxidation equivalent, but
larger amounts of reducing agent have no adverse effect.
For example, at least 1 mole of hydrogen should be used
for 1 mole of palladium. The reduction can be carried out
after the drying in the same plant.
The vinyl acetate is generally prepared by passing acetic
acid, ethylene and oxgyen or oxygen-containing gases at
temperatures of from 100 to 220°C, preferably 120 to
200°C, and at pressures of from 1 to 25 bar, preferably
1 to 20 bar, over the finished catalyst, it being pos-
sible to circulate unreacted components. The oxygen
concentration is expediently kept below 10~ by volume
(relative to the acetic acid-free gas mixture). However,
dilution with inert gases, such as nitrogen or carbon
dioxide, may also be advantageous under certain cir-
cumstances. C(32, in particular, is suitable for the
dilution in circulation processes, since it is formed in
small amounts during 'the reaction.
The examples below are intended to illustrate the
invention.
Comparison Example 1
(Spherical support particles comprising conventional
silica gel)
200 g of a silica support comprising conditioned (800°C)
silica gel spheres 5 - 8 mm in diameter were employed.
The (commercially available] support comprising theca
spherical particles had a BET surface area of 169 mz/g and
a pore volume of 0. X48 ml/g, formed by 8~s of pores 70-
100 ~ngs~trom in diameter and 29~ of pores 200-
3,000 angstrom in diameter. The support was impregnated
with a solution (corresponding to this pore volume) of
11.5 g of Pd acetate, 10.0 g of Cd acetate and 10.8 g of
K acetate in: 66 mi of glacial acetic acid and was dried
at 60°C under nitrogen at a pressure of 200 mbar to a
residual solvent content of 2~ by weight. This gave a
doping of 2.3~a by weight of Pd, 1.8~ by weight of Cd and
2.0~ by weight of K (Cd:Pd = 0.78:1, K:Pd = 0.87:1).
50 m1 of the finished catalyst were introduced into a
reaction tube of internal diameter 8 mm and length 1.5 m.
The gas to be reacted was then pried over the catalyst
at a pressure of 8 bar (reactor inlet) and a catalyst
temperature of 150°C. This gas comprised, at the reactor
input, 27~ by volume of ethylene, 55~ by volume of file, 12~
by volume of acetic acid and 6~ by volume of C2. The
results are shown in the table.
Comparison Example 2
(Spherical support particles comprising conventional SiO~)
200 g of a silica support which had been pressed from
bentonite which had been roasted and then washed with HCl
(Si02 content after this washing at 96$ by weight] to form
spheres 5--6 mm in diametex were employed. The support
comprising these spherical particles had a BET surface
area of 121 m~/g and a pare volume of 0.66 ml/g, formed
from 21~ of pores 70-100 .~xgstrom in diameter and ~k2~ of
pores 200-3,000 angstrom in diameter. The support
-
particles were impregnated as in Comparison Example 1
(the only difference being that 114 ml of glacial acetic
acid were used instead of G6 ml) dried, so that the same
doping was present. The catalyst was subsequently tested
5 as in Comparison Example 1. The results are shown in the
table.
Exaromple 1
Ta. support was first prepared from Ei02 mi.crospheres having
a surface area of 200 mz/g and a filler and a lubricant.
10 The finished support had a pore volume of 0,80 ml/g
formed from G2~ of pores 70-100 .$~ngstrom in diameter and
9~ of pores 200-3,000 gstrom in diameter. The support
particles had a cylindrical shape with curved faces (G mm
in diameter and 6 mm in height; the shape is similar to
the shape of knbwn medicament capsules). The surface area
of the support particles was 185 m2/g.
The support particles (200 g) were impregnated as in
Comparison Example 1 (the only difference being that
141 ml of glacial acetic acid were used instead of 6G ml)
and dried, so that the same doping was present. The
catalyst was subsequently tested as in Comparison Example
1. The results are shown in the table.
Example 2
A support was first prepared from Sx.O2/A12O3 microspheres
( 97~ by weight of SiClz, 3~ by weight Of .~12Q3) having a
surface area of 170 m2lg, and a filler and a lubricant.
The finished support had a pore volume of 0.75 m:L/g,
formed from 58~ of pores 70-100 angstrom in diameter and
12~ of pores 200-3,000 angstrom in diameter. The support
particles had the same shape and size as in Example 1., '
but they now had a surface area of 132 m2/g. The support
particles (200 g) were impregnated as in Comparison
Example 1 (the only difference being that 131 m1 of
glacial acetic acid were used instead of 66 ml) and
dried, so that the same doping was present. The catalyst
~o:~.~o~:~
- 11 -
was subsequently tested as in Comparison Example 1. The
results are shown in the table.
Example 3
A silica sol was first mixed with glassy Si02 microspheres
(surface area 200 mz/g), dried and roasted, to produce a
support. The finished support had a pore volume of
0.64 ml/g, formed from 65~ of pores 70-100 angstrom in
diameter and 9~ of pores 200--3,000 angstrom in diameter.
The support particles were extrudate offcuts (diameter
and height each 6 mm) obtained by extrusion and having a
surface area of 1?0 m2/g.
The support particles (200 g) were impregnated as in
Comparison Example 1 (the only difference being that
110 ml of glacial acetic acid were used in place of
66 ml) and dried, so that the same doping was prese;at.
The catalyst was subsequently tested as in Comparison
Example 1. The results are shown in the table.
Example 4
A support was first prepared from Si02 microspheres having
a surface area of 300 m2/g, and a filler and a lubricant.
The finished support had a pore volume of 0.91 ml/g,
formed from 56~k of pores 70-100 ~rn.gstrom in diameter and
10~ of pores 200-3,000 l~ngstrom in diameter. The support
particles had the same shape and size as in Example 1.
However, the surface area of the support particles was
now 184 mz/g.
The support particles (200 g) were impregnated as in
Comparison Example 1 (the only difference being that
163 ml of glacial acetic acid were used instead of 66 ml)
and dried, so that the same doping was present. The
catalyst was subseduently tested as in Comparison
Example 1. The results are shown in the table.
- 12 -
In the table, °'contribution (~)" denotes the percentage
contribution to the pore volume provided by the pores of
diameter 70-100 .d~ngstrom or 200-3,000 angstrom.
"STY" denotes the space-tame yield;
"combustion (~j°' denotes the percentage of reacted
ethylene converted into C02, and
"ethyl acetate content°' refers to the cowtent of ethyl
acetate produced as a by-product in the condensed part of
the reaction product.
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