Note: Descriptions are shown in the official language in which they were submitted.
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WO 97/37759 - 1 - PCT/EP97/01327
Description
Catalyst and process for preparing vinyl acetate
The present invention relates to a catalyst comprising palladium, gold and
an alkali metal compound, a process for its preparation and its use for
preparing vinyl acetate from acetic acid, ethylene and oxygen or oxygen-
containing gases.
It is known from the prior art that vinyl acetate can ~e prepared from
ethylene, oxygen and acetic acid by reaction over catalysts comprising
palladium, gold and an alkali metal compound on a support material (for
instance silicon dioxide). Such catalysts display good activity and generally
form little carbon dioxide and ethyl acetate. Although high-boilers are
formed as further by-products in amounts which appear to be small, they
nevertheless present a problem from process and ecological points of
view. The term high-boilers here refers to, in particular, the compounds
ethylidene diacetate, ethylene glycol and diacetoxyethylenes.
Literature references which disclose the preparation of such catalysts for
the commercial production of vinyl acetate generally describe methods of
depositing the noble metals in a shell on the catalyst support.
US-A-4,048,096 describes the preparation of a catalyst for vinyl acetate
production in which the dissolved noble metal salts are adsorbed by the
support from a solution which has the same volume as the pores of the
support material, with the support particles being agitated in a rotating
vessel. The salts are then fixed using alkalis, without drying the support
beforehand.
US-A-5,332,710 discloses the preparation of a catalyst for vinyl acetate
production where the insoluble noble metal salts are precipitated onto the
support particles by agitating these in a rotating drum for at least half an
hour during the precipitation by means of alkalis.
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It has now surprisingly been found that the
addition of boron or boron compounds improves the
selectivity of the catalyst, with, in particular, the
formation of high-boilers being significantly reduced.
High-boilers are, in particular, the compounds mentioned in
the introduction.
The invention provides a catalyst for preparing
vinyl acetate in the gas phase from ethylene, acetic acid
and oxygen or an oxygen-containing gas with, at the same
time, low high-boiler formation, which catalyst comprises
palladium, a palladium compound or a mixture thereof in an
amount of from 2 to 14 g/1, calculated as palladium, gold, a
gold compound or a mixture thereof in an amount of from 1 to
8 g/1, calculated as gold, boron, a boron compound or a
mixture thereof in an amount of from 0.01 to 0.2o by weight,
and potassium acetate in an amount of from 1 to 4a by weight
of potassium on a particulate support.
In a preferred embodiment, the catalyst is
prepared by a) impregnating the support with soluble
palladium and gold compounds; b) converting the soluble
palladium and gold compounds on the support into insoluble
compounds by means of an alkaline solution; c) reducing the
insoluble palladium and gold compounds on the support by
means of a reducing agent in the liquid phase; d) washing
and subsequently drying the support; e) impregnating the
support with a soluble alkali metal compound; and f) finally
drying the support at a maximum of 150°C.
According to the present invention there is
further provided a process for preparing a catalyst for
producing vinyl acetate in the gas phase from ethylene,
acetic acid and oxygen or an oxygen-containing gas with, at
the same time, low high-boiler formation, which catalyst
~
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comprises palladium, a palladium compound or a mixture
thereof in an amount of from 2 to 14 g/1, calculated as
palladium, gold, a gold compound or a mixture thereof in an
amount of from 1 to 8 g/l, calculated as gold, and potassium
acetate in an amount of from 1 to 4% by weight of potassium
on a particulate support, wherein the catalyst is prepared
by a) impregnating the support with soluble palladium and
gold compounds; b) converting the soluble palladium and gold
compounds on the support into insoluble compounds by means
of an alkaline solution; c) reducing the insoluble palladium
and gold compounds on the support by means of a reducing
agent in the liquid phase; d) washing and subsequently
drying the support; e) impregnating the support with
potassium acetate; and f) finally drying the support at a
maximum of 150°C, wherein boron, a boron compound or a
mixture thereof is applied to the catalyst in an amount of
0.01 to 0.2% by weight prior to the final drying.
The invention accordingly provides, on the one
hand, a process for preparing a catalyst for the production
of vinyl acetate in the gas phase from ethylene, acetic acid
and oxygen or oxygen-containing gases with, at the same
time, low high-boiler formation, which catalyst comprises
palladium, palladium compounds or mixtures thereof, gold,
gold compounds or mixtures thereof and alkali metal
compounds on a particulate support, where the catalyst is
prepared by a) impregnating the support with soluble
palladium and gold compounds; b) converting the soluble
palladium and gold compounds on the support into insoluble
compounds by means of an alkaline solution; c) reducing the
insoluble palladium and gold compounds on the support by
means of a reducing agent in the liquid phase; d) washing
and subsequently drying the support; e) impregnating the
support with a soluble alkali metal compound; and f) finally
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drying the support at a maximum of 150°C, wherein 0.01 to
0.2o by weight of boron, boron compounds or mixtures thereof
are applied to the catalyst prior to the final dryingØ2o
by weight of boron, boron compounds or mixtures thereof are
applied to the catalyst prior to the final drying.
On the other hand, the invention provides a
process for preparing vinyl acetate in the gas phase from
ethylene, acetic acid and oxygen or oxygen-containing gases
with, at the same time, low high-boiler formation over the
catalyst of the invention.
The boron content of the catalyst is preferably
from 0.01 to to by weight, in particular from 0.01 to 0.20
by weight. The boron is applied to the support in the form
of its compounds, preferably in the form of borates. The
application can be carried out in the abovementioned step a)
together with the soluble palladium and gold compounds, in
step b) together with the alkaline solution, or using a
borate solution as alkaline solution, in step e) together
with the soluble alkali metal compound or in a separate step
before the final drying of the support. Preference is given
to application in step b).
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The support particles of the catalyst of the invention can have any
geometric shape, for example the shape of spheres, tablets or cylinders of
regular or irregular type. The dimensions of the support particles are
generally between 1 and 8 mm. Preference is given to a spherical shape,
for example spheres having a diameter of from 4 to 8 mm. The support
particles are generally termed pellets. '
Suitable supports are the known inert support materials such as silica,
aluminum oxide, aluminosilicates, silicates, titanium oxide, zirconium oxide,
titanates, silicon carbide and carbon. Particularly suitable supports are
supports of this type having a specific surface area of from 50 to 300 m2/ g
(measured by the BET method) and a mean pore radius of from 50 to
2000 A (measured using mercury porosimetry), especially silica (Si02) and
Si02/A1203 mixtures.
The total pore volume of the support is preferably from 0.4 to 1.2 ml/g.
Less than 10% of this volume should be made up by "micropores" having a
pore diameter of less than 30 A (Angstrom). Such supports can be
prepared from aerogenic Si02 or an aerogenic Si02/AI203 mixture in the
form of vitreous microspheres which can be prepared, for example, by
flame hydrolysis of silicon tetrachloride or a silicon tetrachloride/aluminum
trichloride mixture in an oxyhydrogen flame (US-A-3 939 199). These
microspheres are commercially available under the name ~Aerosil or
~Cabosil.
The dissolved gold and palladium salts are adsorbed in the pores of the
support material, which is referred to in the prior art as the pore volume
impregnation method. The supports impregnated in this way are treated
with an alkaline solution, with the use of a borate solution being preferred,
in order to deposit the noble metals as insoluble compounds. These
compounds are subsequently subjected to a reductive treatment, with the
reducing agent being present in the liquid phase.
Suitable solvents for the catalytically active substances are, in particular,
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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 properties and also for
economic reasons, acetic acid is preferably used 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 dissolves significantly better in an 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.
Two suitable methods for preparing the catalyst of the invention,
designated by I and II, are described below. A particular step in method II
can be carried out in two variants A) and B).
In method I, soluble gold and palladium salts are dissolved in such an
amount of solvent that the solution volume corresponds to about 90-110%
of the pore volume of the support material. Palladium(II) chloride, sodium
palladium(II) chloride and palladium(II) nitrate are examples of suitable
soluble palladium compounds, and gold(III) chloride, tetrachloroauric(III)
acid and its alkali metal salts can be used as soluble gold compounds. In
general, the amounts of these compounds used are such that the finished
catalyst contains between about 2 and about 14 g/I, preferably between
4 and 8 g/I, of palladium and between about 1 and about 8 g/I, preferably
between 2 and 5 g/I, of gold. Accordingly, the gold content of the catalyst is
generally from about 10 to 70% of the mass of palladium present therein.
The solution is adsorbed in the support and the metals are fixed by placing
the support for a sufficient length of time in an alkaline solution which has
a
sufficiently high concentration to precipitate the insoluble metal salts. The
fixing step b) can be carried out by immersing the impregnated supports in
sufficiently alkaline fixing solution for them to be completely covered, with
the support particles preferably being agitated, for instance as described in
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US-A-5,332,710.
According to US-A-5,332,710, the impregnated support materials are
immersed in an alkaline solution and agitated by rotation from the
commencement of the deposition of the insoluble noble inetal compounds.
This rbtation of the support particles in the alkaline solution should be
continued for at least half an hour from the start of the treatment,
preferably one hour. Rotation and immersion can be for up to 4 hours. The
treated support particles can then be left at rest in the fixing soluticn in
order to ensure that complete precipitation of the rioble metal compounds
occurs. The .alkaline,fixing solution can be any solution which is able to
precipitate gold and palladium; preference is given to using borate
solutions.
Any type of rotation or similar treatment which keeps the support particles
in motion can be used, since the precise method is not critical. However,
the intensity of the agitation is important. This should be sufficient
uniformly
to wet the entire surface of the impregnated supports with the alkaline
fixing solution. The agitation of the support particles must not be so
vigorous that the insoluble noble metal compounds are lost as a result, i.e.
~ . that they are rubbed off the support surface. The rotation rate should
preferably be from i to 20 revolutions per minute, but it can also be greater
depending on the type of support material and the amount of noble metal
to be deposited. Different rotation rates can be selected ,and the rotation
rate also depends on the apparatus used, the size and shape of the
supports, the type of support, the metal impregnation, etc., but should
correspond approximately to the abovementioned rotation rates.
The fixing solution is an alkaline solution, preferably an aqueous solution,
containing boron compounds. Particular preference is given to using
aqueous solutions of borax, potassium tetraborate or mixtures of alkali
metal hydroxide solution and boric acid. The alkaline solution can have
buffering properties.
The steps c) to f) are then carried out.
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In method II, a suitable catalyst support is first impregnated with a solution
containing soluble palladium and gold compounds. Separate solutions of
palladium and gold compounds can also be used in succession, although a
drying step then has to be carried out in between. For effective
impregnation, the volume of the impregnation solution should be from 95 to
100% of the pore volume of the catalyst support, preferably 98-99%. After
impregnation of the support with the soluble palladium and gold
compounds, the impregnated support is dried before the palladium and
gold compounds are fixed as insoluble compounds. The fixing step
comprises at least two separate stages of treatment with the alkaline fixing
solution. In each of these stages, the amount of the alkaline reagent used
is at most equal to the amount required to react with the total amount of the
soluble noble metal salts present on the support. This amount can be
greater than the amount stoichiometrically required for the reaction. The
amount of the alkaline reagent employed in each fixing stage is preferably
less than the amount required for complete reaction with the soluble noble
metal salts. The first fixing stage is carried out by bringing the impregnated
and then dried support in contact with the alkaline fixing solution. The
volume of the fixing solution corresponds to the pore volume of the dry
support material. The amount of the alkaline compound present therein
should be such that the molar ratio of alkali metal from the alkaline
compound to anions from the soluble metal salt is from 0.7:1 to 2:1, and
the volume of the solution should correspond to the absorption capacity of
the support in the dry state. The alkaline fixing solution is poured onto the
agitated support particles in order to be absorbed and the support particles
are allowed to stand for up to 24 hours, preferably from 2 to 8 hours.
The second fixing stage can be carried out in 2 variants A) and B).
Variant A) is carried out by treating the undried support particles with a
second fixing solution. In this solution, the molar ratio of alkali metal from
the alkaline compound to anion from the metal salt is from about 0.2:1 to
2:1. The solution volume should at least just cover the supports. The
treatment of the support particles with the second fixing solution should be
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for a time of up to 16 hours, at least 2 hours, preferably at least 4 hours.
In variant B), the supports are treated using the rotation-immersion process
according to US-A 5,332,710. In this process, the supports previously fixed
in the first stage are immersed in the alkaline fixing solution of the second
stage and maintained in rotary motion therein during the initial phase of the
second stage. This rotation should be for a time of at least half an hour,
preferably one hour. The treatment can be for up to 4 hours before the
supports are allowed to stand in the fixing solution in order to ensure
complete deposition. Here too, any type of apparatus can be used for the
rotation of the support particles. The rate of rotation is important. This has
to be sufficient to bring all surfaces of the support particles uniformly into
contact with the alkaline fixing solution. It must not be so great that the
insoluble metal compounds are rubbed off the support surface. The
rotation rate should preferably be from 1 to 20 revolutions per minute, if
desired greater, depending on the type of support material and the amount
of metal which is to be precipitated on the support. The rotation rate also
depends on the type of apparatus used, on the size and shape of the
supports, on the type of supports, on the amount of metal with which they
are treated, etc., but should correspond approximately to the
abovementioned rotation rate.
The treatment in the second stage can be equivalent to that in the first
stage, using a fixing solution of the same concentration. The total molar
ratio of alkali metal to anion from the metal salt for both fixing stages
together should preferably be from 1.1:1 to 3.3:1. Preference is given to
using a borate solution as fixing solution in both stages.
Subsequent to the fixing step of method I or the last fixing step of method
' II, the supports are treated with a reducing agent in order to convert the
precipitated noble metal salts and noble metal compounds present thereon
into the metallic form. This reduction is carried out in the liquid phase, for
example using aqueous hydrazine hydrate or an alkali metal borohydride,
preferably sodium borohydride. The reduction is preferably carried out at
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room temperature. The reducing agent is added in excess so as to ensure
that all the metal salts and metal compounds are converted into the
metallic form. The boron compounds too may, depending on the type of
reducing agent, be converted into elemental boron.
The support particles are washed, preferably with distilled water, in order to
remove any chlorides still present in the support material which originate
from the impregnation step and have been liberated by precipitation of the
noble metals. Washing is continued until the chlorides have been removed
from the support. Not more than 1000 ppm of chlorides should remain on
the catalyst. To ensure the success of the washing procedure, the
washings~can be tested with silver nitrate solution. The washing procedure
also serves to remove residues of reducing agent from step c). The
catalyst is then dried at temperatures of at most 150°C, preferably in
a
stream of nitrogen or air.
Finally, addition of at least one alkali metal compound is necessary. The
catalyst is preferably impregnated with an aqueous solution of potassium
acetate and then dried. The potassium content of the finished catalyst is
between 1 and 4% by weight, preferably between 1.5 and 3% by weight.
The preparation of the vinyl acetate is carried out by passing acetic acid,
ethylene and oxygen or oxygen-containing gases over the finished catalyst
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, with
unreacted components being able to be recirculated. The oxygen
concentration is advantageously kept below 10% by volume (based on the
gas mixture free of acetic acid). However, dilution with inert gases such as
nitrogen or carbon dioxide is sometimes advantageous. Carbon dioxide is
particularly suitable for the dilution in a recirculation procedure since it
is
formed in small amounts during the reaction.
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Example 1
250 ml of silicon dioxide catalyst supports (manufactured by Siadchemie) in
the form of spheres having a diameter of 7.3 mm were impregnated with
85 ml of an aqueous solution containing 4.6 g of Na2PdCl4 and 1.4 g of
NaAuCl4. The precipitation of the insoluble metal compounds was
achieved by addition of 283 ml of an aqueous solution of 17 g of borax.
The vessel was then immediately rotated by means of a rotary evaporator
(without application of vacuum) for 2.5 hours at 5 revolutions per minute
(rpm). The reduction was subsequently achieved by addition of 7 ml of
hydrazine hydrate in 20 ml of water and immediate rotation of the vessel at
5 rpm for 1 hour. The supports thus treated were subsequently allowed to
stand for 16 hours. The liquid was then poured off and the treated supports
were washed with distilled water in order to remove the chloride ions. This
required a water flow rate of 200 ml/minute for about 5 hours. The pellets
thus obtained were dried for 1 hour at 100°C. The reduced catalyst was
impregnated with an aqueous solution containing 10 g of potassium
acetate and having a volume corresponding to the absorption capacity of
the dry support material. The catalyst was then dried again.
200 ml of the finished catalyst were placed in a reaction tube having an
internal diameter of 20 mm and a length of 1.5 m. The gas mixture to be
reacted was then passed over the catalyst at a pressure of 8 bar at the
reactor inlet and a wall temperature of 150°C. This gas mixture
consisted
of 50% by volume of ethylene, 12% by volume of acetic acid, 6% by
volume of oxygen and 32% by volume of nitrogen. The results are shown
in Table I.
Comparative Example 1
The procedure of Example 1 was repeated, except that the precipitation of
the insoluble metal compounds was effected by addition of 283 ml of an
aqueous solution of 2 g of NaOH. The results are shown in Table I.
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Table I
Example 1 Comparative Example
1
Output 837 840
Selectivities:
Carbon dioxide 9.13 9.20
Ethyl acetate 0.09 0.12
High-boilers 0.58 1.53
Units:
Output: Gram of vinyl acetate per liter of catalyst per hour
Selectivities: Mol percent based on ethylene reacted
