Note: Descriptions are shown in the official language in which they were submitted.
~7~8
~OECHST AKTIENGESELLSCHAFT HOE 91/F 189 Dr.MA/Dt
Decription
Catalyst and process for the preparation of vinyl acetate
It is known that ethylene can be reacted in the gas phase
S with acetic acid and oxygen or oxygen-cont3ining gases
over fixed bed catalysts to give vinyl acetate. Cataly~ts
for which a space-tLme yield of more than 200 g/l x h ic
stated contain9 wi~hout e~ception, one of the two element
combinations of palladium/cadmium~potassium or
palladium/gold/potassium (US-PS 3 939 199,
US-PS 4 6b8 819, EP-A-0 330 853, EP-A-0 403 950 and
EP-A-0 431 478).
It has now been fou~d, surprisingly, that palladium
catalysts which contain at least one barium compound and
at least one alkali metal compound, but neithex ~admium
nor gold nor compounds thereof, result in a space-time
yield which is at le~st the same as, a combustion of
ethylene to CO2 which is the same as or lower than and an
ethyl acetate formation which is the same as or lower
than that of the catalyst system~; mentioned of Pd/Au/K
and Pd/Cd~K. This is very advantageous, since cadmium is
a disadvantage because of its toxicity and gold because
of its high price, while barium is inexpensive and is
non-toxic by nature, since it i5 converted immediately
into barium sulfate, which is completely inert because it
is sparingly soluble.
The invention accordingly relates to a process for the
preparation of vinyl acetate in the gas phase from
ethylene, acetic acid and oxy~en or oxygen-containing
ases over a catalyst which contains palladium and/or
compounds thereof and alkali metal compounds on a support,
wherein the catalyst comprises at least one barium compound
and in which qold and cadmium compounds thereof are absent.
- ~ ;
: . :
~: :
' ,
_ 2
207~
Furthermore, the inve ion relates to a catalyst which
contains palladium and/or compounds thereof and alkali
metal compounds on a support, wherein the catalyst comprlses
at least one barium compound and in which gold and cadmium
compounds thereof are absent.
The absence of ~hese elements and their compound~ means
that they are pre~ent in not more than trace amounts,
because of impurities in the ~tarting ~ub~tances of the
5 supported catalyst, i.e. in total concentrations of
cadmium and gold of not more ~han 1 ppm, ba~ed on the
total supported catalyst (support plus active
components).
Suitable supports are the known inert support materials,
such as silicic acid, aluminum oxide, alumosilicates,
silicates, titanium oxide, zirconium oxide, titanates,
~ilicon carbide and carbon. 5ilicic acids having a
specific surface area of 40 to 350 m2/g and an average
pore radius of 50 to 2000 A are particularly suitable.
According to US PS 3 939 199, the total pore ~olume of a
highly efficient support should be 0~4-1.2 ml/g, and less
than 10~ of this volume should be formed by "micropores"
having a pore diameter of less than 30 A (Angstr~m). Such
supports can be prepared from aerogenic SiO2 or an aero-
genic SiO2-Al2O3 mixture, which is :in the form of vitreous
microbeads, which can be produced, for example, by flame
hydrolysis of silicon tetrachloride or a silicon tetra-
chloride-aluminum trichloride mixture in an oxyhydrogen
gas flame. These microbeads are commercially available
under the name eAerosil or ~Cabosil.
EP~A-0 403 950 describes a support ~f the type ~ust
mentioned, which compri~es SiO2 or an SiO2-Al2O3 mixture
having a ~urface area of 50-250 m~g and a pore volume of
0.4-1.2 ml/g, the particles of which have a particle size
of 4 to 9 mm, 5 to 20% of the pore volume of the support
being formed by pores having radii of 200 to 3000 A and
50 to 90% of the pore volume being ormed by pores having
radii of 70 to 100 A.
~7~
According to EP~A-0 431 478, shaped articles, i.e. shaped
support particles, are then advantageously produced, for
example by tablet-making or extrusion, from the micro-
beads with the addition of one or more carbo~ylates of
Li, Mg, Al, Zn, Fe or Mn as binders and with th addition
of organic filler~ (such as su~ars, urea, higher fatty
~cids, longer-chain paraffins and microcrystalline
cellulose) and lubricants (such a~ kaolin, graphite and
metal soaps). The ~haped articles are then roasted in
O2-containing gases at about 500-900~C for about 0.25-
5 hours.
The catalytically ~ctive substances are applied to the
~upport ~n khe customary manner, for example by impreg-
nating the support with a solution of the active substan-
ces, subsequently drying it and if appropriate subjectingit to reduction. ~owever, the active substances can also
be applied, for example, by precipitation on the support
or by spraying on, vapour deposition or dipping.
Suitable solvents for the catalytically active substances
are, above all, unsubstituted carboxylic acids having 2
to 10 carbon atoms, such as acetic acid, propionic acid,
n and iso-butyric acid and the various valeric acids.
Acetic acid is preferably employed as the solvent because
of its physical properties and also for economic reasons.
The additional use of an inert solvent is appropriate if
the substances are not sufficiently soluble in the
carboxylic acid. Palladium chloride, for example, can
thus be dissolved considerably more easily in an aqueous
acetic acid than in glacial acetic acid. Possible
additional solvents are those which are inert and are
miscible with the carboxylic acid, for example water and
ether, such as tetrahydrofuran or dioxane, or
hydrocarbons, such as benzene.
The catalyst support is preferably impregnated with the
solution of the active components by covering the support
.~
2 ~
-- 4 --
material with a layer of the solution and then pouring
off or filtering off the excess solution. In consider-
ation of solvent losses, it is advantageous to employ
only the amount of solution which corresponds to the
integral pore volume of the catalyst support and to mix
the components thoroughly, so that the particles of the
suppoxt material are wetted uniformly. This mixing can be
achieved, for example, by stirring. It is advantageous ~o
carry out the Lmpregnating operation and the mixing
simultaneously, for example in a rotary drum or a tumble
dryer, in which case drying can follow immediately. It is
furthermore advantageous to choose the composition of the
solution used to impregnate the catalyst æupport such
that the desired amount of active substances is applied
by a single impregnation. However, it can also be applied
by several impregnation steps.
The catalyst support impregnated with this solution is
preferably dried under reduced pressure. The temperature
during drying should be less than 120C, preferably less
than 90C. It is furthermore in general 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 catalyst contains palladium and/or compounds thereof
as the noble metal component.
Possible compounds of palladium are all the salts and
complexes which are soluble (and if appropriate
reducible) and do not leave behind any deactivating
substances, such as halogen or sulfur, in the finished
catalyst ~washed with water if necessary). The carboxy-
lates, preferably the salts of aliphatic monocarboxylic
acids having 2 to 5 carbon atoms, for ex~mple the
acetate, the propionate or the butyrate, are particularly
,
: ',
- .
2~7~6~8
suitable. The nitrate, nitrite, hydrated oxide, oxalate,
acetylacetonate and acetoacetate, for example, are
furthermore suitable. Palladium acetate is the particu-
larly preferred palladium compound because of its solu-
bility and its accessibility.
The content of palladium in the catalyst is in general
0.3-3~ by weight, preferably 1.5 to 2.5% by weight, in
particular 2-2.5% by weight.
The catalyst contains as activators at least one barium
compound, preferably a carboxylate, such as the formate,
aceta~e, propionate or butyrate. At least one alkali
metal compound, preferably at least one K, Rb or Cs
compound, in particular at least one R compound, is
furthermore pre~ent. Carboxylates above all are suitable
as the alka:Li metal compounds, in particular the acetates
and propionates.
Suitable barium and alkali metal compounds are also those
which are converted into the acetate under the reaction
conditions, such as, for example, hydroxides~ oxides and
carbonates.
The content of barium in the catalyst is in general 0.1-
10~ by weight, preferably 0.2-4~ by weight, in particular
0.4-3% by weight. The content of alkali metal elements is
in general 0.3-10% by weight, preferably 0.5-8% by
weight, in particular 1-4% by weight. The percentage
figures stated always relate to the amounts of the
elements palladium, barium and alkali metal pre~ent in
the catalyst, based on the total weight of ~he catalyst;
any anions are not included in the calculation.
If a reduction is carried out on the palladium compound,
which can sometimes be beneficial, ~his can be carried
out in vacuo, under normal pressure or under an increased
pressure of up to 10 bar. It is advisable here to dilute
- 6 - 2~7~
the reducing agen~ with an inert gas to a higher degree,
the higher the pressure. The reduction temperature is in
general between 40 and 260C, preferably between 70 and
200C. It is in general advantageous to use for the
S reduction an inert gas/reducing agent mixture which
contains O.Ql to 50% by volume, preferably 0.5 to 20~ by
volume, of reducing agent. Nitrogen, carbon dioxide or a
noble gas, for example, can be used as the inert gas.
Examples of possible reducing agents are hydrogen,
methanol, formaldehyde, ethylene, propylene, isobutylene,
butylene and other olefins. The amount of reducing agent
depends on the amount of palladium; the reduction equiva-
lent should in ~eneral be at least 1 to 1.5 times the
oxidation equivalent, but larger amount~ of reducing
agent do no harm. The reduction can be carried out after
the drying, in thP same unit.
The vinyl acetate is in general prepared by passing
acetic acid, ethylene and o~ygen or oxygen-containing
gases over the finished catalyst at temperatures of 100
to 220C, preferably 120 to 200C, under pressures of 1
to 25 bar, preferably 1 to 20 bar, it being possibe for
unreacted components to be passed in circulation. The
oxygen concentration i8 advantageously kept below 10~ by
volume (based on the acetic acid-free gas mixture). Under
certain circum~tances, however, it is also advantageous
to dilute the mixture with inert gases, such as nitrogen
or carbon dioxide. CO2 is particularly suitable for the
dilution in circulation proce~ses, since it is formed in
small amounts during the reaction.
The following examples are intended to illustrate the
invention. The percentage data of the elements Pd, Ba and
are percentages by weight.
Comparison Example 1
100 g of silicic acid tablets which had been produced
from aerogenic SiO2 microbeads having a surface area of
~ ' ~ `: '. . '
:
- i~
: . ,
~7~ 6~
-- 7 --
200 m2/g by pressing with Mg stearate as the binder and
subsequent roasting in accordance with EP-A-0 431 478,
were impregnated with a solution of 5.6 g of Pd acetate,
S.l g of Cd acetate and 5.5 g of K acetate in 78~5 ml of
glacial acetic acid and dried. The finished catalyst
contained 2.3% of Pd, 1.8% of Cd and 1.9% of X, based on
the system consisting of salts and support.
50 ml of the finishQd catalyst were introduced into a
reaction tube having an internal diameter of 8 mm and a
length of 1.5 m. The gas to be reacted was then passed
over the catalyst under a pressur0 of 8 bar (reactor
intake) at a catalyst temperature of 150C. This gas
consisted of 27% by volume of ethylene, 55~ by volume of
N2, 12% by volume of acetic acid and 6% by volume Of 2 at
the reactor intake. The results can be seen from the
table.
Comparison Example 2
The procedure was as in Comparison Example 1, with the
exception that Al stearate instead of Mg stearate was
used as the binder. Testing uncler the conditions of
Comparison Example 1 gave the values shown in the table.
Example 1
100 g of the support mentioned in Comparison Example 1
were impregnated with a solution of 5.6 g of Pd acetate,
5.3 g of Ba acetate and 5.5 g of K acetate in 78.5 ml of
glacial acetic acid and dried. The finished catalyst
contained 2.3% of Pd, 2.2% of Ba and 1.9% of K, based on
the system consisting of support and salts. The values
shown in the table resulted under the conditions of
Comparison Example 1.
Example 2
The support used in Comparison Example 1 was Lmpregnated
with a solution of 5.6 g of Pd acetate, 1.1 g of Ba
acetate and 5.5 g of K acetate in 78.5 ml of glacial
,~ .
.
2 ~
-- 8 --
acetic acid and dried. The finished catalyst contained
2.3% of Pd, 0.5% of Ba and 1.9% of K, based on the system
consis$ing of support and salt~. ~he ~alues achieved
under the conditions of Comparison Example 1 are shown in
the table.
Table
. _
Comparison Example
Example
STY (g/l x h) 838 ~ 840 ~. _
Combustion (%) 5.4 4.6 6 4
Ethyl acetate
content (ppm) 190 218 20 80
"STY" denote the space-tLme yield; ~combusion (~)"
denotes the percentage of ethy.lene reacted which is
converted into COa; "ethyl acetate content~ relates to
the content of ethyl acetate in the condensed p~rt of the
reaction product.
., . ~ . .
