Note: Descriptions are shown in the official language in which they were submitted.
CA 0220~416 1997-0~
Compacts Based On Pyrogenically-Produced Silicon Dioxide
The invention relates to compacts based on pyrogenically-
produced silicon dioxide, a method of producing them, and
use thereof as catalyst carriers or catalysts.
Pyrogenically-produced silicon dioxide is characterised
by extremely fine division and a consequently high
specific surface area, very high purity, spherical shape
of particles and absence of pores. As a result of these
properties, pyrogenically-produced oxides are becoming
increasingly important as carriers for catalysts (D.
Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980)).
Since pyrogenically-produced oxides are very finely-
divided, there is some difficulty in shaping them to form
catalyst carriers or catalysts.
DE-A 31 32 674 discloses a method of producing compacts
from pyrogenically-produced oxides, using silica sol as
the binder.
DE-A 34 06 185 discloses a method of producing compacts
using glaze frit powder as a binder and glycerol as a
mould release agent.
DE-B 21 00 778 discloses use of granulates based on
pyrogenically-produced silicon dioxide for production of
catalyst carriers in the form of vinyl acetate monomers.
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DE-A 39 12 504 discloses a method of producing compacts
wherein aluminum stearate, magnesium stearate and/or
graphite are used as mould release agents and urea and
methyl cellulose are used as pore-forming agents.
These known compacts are commercially available as
Aerosil pellets number 350 (trademark of Degussa AG).
They contain about 0.4 wt.~ Mg.
It is known from EP-B 0 519 435 to compress Si02 into
carriers, using binders, to anneal the resulting carriers
and to wash the annealed carrier particles in acid until
no further binder cations are given off.
The known methods have the disadvantage that in some
catalytic reactions such as vinyl acetate production from
ethylene, acetic acid and oxygen, or hydration of
ethylene to ethanol, the compacts obtained do not have
the required optimum properties such as high purity, high
activity, high selectivity, high yield of product and
high stability.
This invention relates to compacts based on
pyrogenically-produced silicon dioxide and having the
following physical and chemical characteristics:
Outer diameter 0.8 - 20 mm
BET surface area 30 - 400 m2/g
Pore volume 0.5 - 1.3 ml/g
Breaking strength 10 to 250 N
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Composition > 99.8 wt.% Si02
Other constituents < 0.2 wt.
Abrasion < 5 wt.%
Apparent weight 350 - 750 g/l.
The invention also provides a method of producing
compacts based on pyrogenically-produced silicon dioxide
and having the following physical and chemical
characteristics:
Outer diameter 0.8 - 20 mm
BET surface area 30 - 400 m2/g
Pore volume 0.5 - 1.3 ml/g
Breaking strength 10 to 250 N
Composition > 99.8 wt.% Si02
Other constituents < 0.2 wt.%
Abrasion < 5 wt.%
Apparent weight 350 - 750 g/l.
characterised in that pyrogenically-produced silicon
dioxide is homogenised with methyl cellulose, microwax
and/or polyethylene glycol with addition of water, dried
at a temperature of-80 - 150~C., and optionally
comminuted to a powder, and the powder is compressed into
compacts and heat-treated at a temperature of 400 to
1200~C for a time of 0.5 to 8 hours.
The method according to the invention can be worked on
all mixers or mills which ensure good homogenisation,
e.g. paddle mixers, fluidised-bed, gyratory or air-flow
mixers. Mixers of use for additional compaction of the
mixed material are particularly suitable, e.g.
CA 0220~416 1997-0~
ploughshare mixers, pan grinders or ball mills.
Homogenisation can be followed by thorough drying at 80 -
150~C, followed optionally by comminution so as to obtain
a free-flowing powder. The compacts can be produced in
hand presses, eccentric presses, isostatic presses,
extruders or rotary presses or on compactors.
Before compression, in a special embodiment of the
invention, the mixture can have the following
composition:
50 - 90 wt.% silicon dioxide, preferably 65 - 85 wt.%
0.1 - 20 wt.% methyl cellulose, preferably 5 - 15 wt.%
0.1 - 15% microwax, preferably 5 - 10 wt.%
0.1 - 15% polyethylene glycol, preferably 5 - 10 wt.%.
The compacts can have various shapes, such as
cylindrical, spherical or annular, with an outer diameter
of 0.8 to 20 mm. The compacts are heat-treated at 400 -
1200~C for 30 minutes to 8 hours. The amounts used and
the pressures applied can be varied so as to adjust the
breaking strength, the total specific surface area, and
the pore volume within a specific range.
The compacts according to the invention can be used
either directly as a catalyst or as a catalyst carrier.
In the latter case the compacts after manufacture will be
brought into contact with a catalytlcally-active
substance and optionally activated by suitable after-
treatment.
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More particularly, compacts made of pyrogenically-
produced silicon dioxide can be used as carriers for the
catalyst for producing vinyl acetate monomer from
ethylene, acetic acid and oxygen, and as the catalyst
carrier in the olefin hydration process.
The compacts according to the invention have the
following advantages:
Compared with the compacts according to document DE-A 39
12 504, the compacts according to the invention contain
no inorganic constituents apart from silicon dioxide.
The known compacts have the disadvantage of containing
about 0.4 wt.% Mg, which is leached out during the olefin
hydration process.
The compacts according to the invention, by contrast,
have improved hydrothermal stability during hydration
processes of this kind. They also have high purity and a
large pore volume.
The invention also provides a carrier catalyst for
production of vinyl acetate monomer ~VAM) containing
catalytically-active components in the form of palladium,
gold and alkali-metal compounds on the silicon dioxide
carrier according to the invention, and a method of
production and use thereof.
Carrier catalysts containing gold, palladium and alkali-
metal compounds are used for production of vinyl acetate.
To this end, ethene, acetic acid and molecular oxygen or
CA 0220~416 1997-0~
air in the gas phase are reacted, optionally with
addition of inert gases, at temperatures between 100 and
250~C and normal or elevated pressure in the presence of
the carrier catalyst. A method of production of this
kind is known from documents DE 16 68 088, US 4 048 096
and EP 0 519 435 B1.
These patent specifications also disclose a method of
producing carrier catalysts containing gold, palladium
and alkali-metal compounds. Depending on the embodiment,
catalysts are obtained with a homogeneous distribution of
noble metal across the carrier cross-section and with a
relatively marked shell profile.
These catalysts are usually obtained by impregnating the
carriers with a basic solution and a solution containing
gold or palladium salts, the impregnation steps occurring
simultaneously or successively, with or without
intermediate drying. The carrier is then washed in order
to remove any chloride components. Before or after
washing, the insoluble noble-metal compounds precipitated
on the carrier are reduced. The resulting catalyst
precursor is dried and, in order to activate the
catalyst, is impregnated with alkali-metal acetates or
alkali-metal compounds which under the reaction
conditions are converted partly or completely into
alkali-metal acetates in the production of vinyl acetate
monomer.
The catalyst can be reduced in the aqueous phase or in
the gas phase. Formaldehyde or hydrazine are suitable
CA 0220~416 1997-0~
for reduction in the aqueous phase. Reduction in the gas
phase can be brought about with hydrogen or forming gas
(95 vol.% N2 + 5 vol.~O H2) or ethene. According to EP 0
634 209 the reduction is brought about with hydrogen at
temperatures between 40 and 260~C, preferably between 70
and 200~C. Frequently, however, the catalyst is first
activated with alkali-metal acetate before being directly
reduced with ethene in the production reactor.
In the production process the catalyst is first slowly
loaded with the reactants. During this starting phase
the activity of the catalyst increases, usually taking
some days or weeks to reach its final level.
An object of the present invention is to provide a
carrier catalyst for production of vinyl acetate monomer,
having the same or improved selectivity and higher
activity than known catalysts.
The invention provides a carrier catalyst containing
catalytically-active components in the form of palladium,
gold and alkali-metal acetate on the silicon dioxide
carrier.
The carrier material for the catalyst can be the compact
according to the invention based on pyrogenically-
produced silicon dioxide. An important feature is that
the catalyst carriers retain their mechanical strength
under the reaction conditions of the catalytic process,
CA 0220~416 1997-0~
more particularly under the influence of acetic acid.The
compacts according to the invention can be formed into
the form of pellets, cylinders, tablets or other
conventional shapes for fixed-bed catalysts.
The compacts are impregnated with a solution containing
palladium and gold. Together with the solution
containing noble metal or in any sequence, the compacts
according to the invention are impregnated with a basic
solution which can contain one or more basic compounds.
The basic compound or compounds are for converting the
palladium and gold into their hydroxides.
The compounds in the basic solution can comprise alkali-
metal hydroxides, alkali-metal bicarbonates, alkali-metal
carbonates, alkali-metal silicates, or mixtures of these
substances. Preferably potassium hydroxide and sodium
hydroxide are used.
The solution containing noble metals can be produced by
using palladium salts, e.g. palladium chloride, sodium or
potassium palladium chloride or palladium nitrate. The
gold salts can be gold (III) chloride or tetrachloro-
auric (III) acid. Preferably use can be made of
potassium palladium chloride or sodium palladium chloride
and tetrachloro-auric acid.
Impregnation of the compacts with the basic solution
influences deposition of noble metals into the compact.
The basic solution can be brought into contact with the
compact according to the invention either at the same
CA 0220~416 1997-0~
time as the noble-metal solution or in any sequence
therewith. When the compact is impregnated successively
with the two solutions, the first impregnation step can
be followed by intermediate drying.
Preferably the compact is first impregnated with the
basic compound. Subsequent impregnation with the
solution containing palladium and gold results in
precipitation of palladium and gold in a surface shell on
the compact. The reverse sequence of impregnations
usually results in a relatively homogeneous distribution
of the noble metals over the cross-section of the compact
according to the invention. If the process is suitably
guided, however, even with the reverse impregnation
sequence, catalysts with a defined shell can be obtained
(see e.g. US 4 048 096). Catalysts with a homogeneous or
nearly homogenous noble-metal distribution usually have
lower activity and selectivity.
Catalysts with shell thicknesses below 1 mm, preferably
below about O.S mm, are particularly suitable. The shell
thickness is influenced by the quantity of the basic
compound applied to-the carrier material, relative to the
desired quantity of noble metals. The higher this
proportion, the smaller is the thickness of the shell
formed. The proportion of basic compound to noble-metal
compounds required for a desired shell thickness depends
on the nature of the carrier material and on the chosen
basic compound and the noble-metal compounds. The
required proportion is advantageously obtained by a few
g _
CA 0220~416 1997-0~
preliminary tests. The resulting shell thickness can be
determined in simple manner by cutting open the catalyst
particles.
The minimum required amount of the basic solution can be
found from the stoichiometrically calculated quantity of
hydroxide ions needed for converting the palladium and
gold into the hydroxides. As a guiding value, the basic
compound for a shell thickness of 0.5 mm should be used
lo in a 1 to 10-fold stoichiometric excess.
The compacts according to the invention are coated with
the basic compounds and the noble-metal salts by the
method of pore volume impregnation. If the method
includes intermediate drying, the volumes of the two
solutions are chosen so as to make up about 90 to 100% of
the absorption capacity of the compacts. If there is no
intermediate drying, the sum of the individual volumes of
the two impregnating solutions must conform to the above
condition, the proportions of the individual volumes
varying from 1:9 to 9:1. A volume ratio of 3:7 to 7:3,
preferably 1:1, is preferred. In both cases the
preferred solvent is water. However, other suitable
organic or aqueous organic solvents can be used.
The reaction between the noble-metal salt solution and
the basic solution to form insoluble noble-metal
compounds occurs slowly and, depending on the method of
preparation, usually takes from 1 to 24 hours. The
water-insoluble noble-metal compounds are then treated
with reducing agents. The reduction can be wet, e.g.
with aqueous hydrazine hydrate, or in the gas phase with
hydrogen, ethene, forming gas or methanol vapours.
Reduction can
-- 10 --
CA 0220~416 1997-0~
occur at normal temperature or elevated temperature and at
normal pressure or elevated pressure, optionally with
addition of inert gases.
Before or after reduction of the noble-metal compounds, any
chloride on the carrier is removed by thorough washing.
After washing the catalyst should contain less than 500 or
preferably less than 200 ppm of chloride.
The catalyst precursor obtained after reduction is dried
and then impregnated with alkali-metal acetates or alkali-
metal compounds which, under the reaction conditions, are
converted partly or completely into alkali-metal acetates
during production of vinyl acetate monomer. Preferably
potassium acetate is used for impregnation. As before, the
preferred method is pore volume impregnation, i.e. the
required amount of potassium acetate is dissolved in a
solvent, preferably water, having a volume approximately
equal to the capacity of the starting amount of carrier
material to absorb the chosen solvent. This volume is
approximately equal to the total pore volume of the carrier
material.
The finished catalyst-is then dried to a residual moisture
content of less than 2~. Drying can be brought about in
air, or optionally under nitrogen inert gas.
For synthesis of vinyl acetate monomer it is advantageous
to coat the catalyst with 0.2 to 4, preferably 0.3 to 3~
palladium, 0.1 to 2, preferably 0.15 to 1.5 wt.~ gold, and
1 to 10, preferably 3.5 to 10 wt.~ potassium acetate, in
each relative to the weight of~carrier used. In the case
of catalyst carriers with a apparent density of 500 g/l,
these concentrations correspond to concentrations by volume
of 1.0 to 20 g/l palladium, 0.5 to 10 g/l gold and 5 to
50 g/l potassium acetate. The impregnation solutions are
prepared by dissolving the corresponding quantities of
palladium and gold compounds in a volume of water
-- 11 --
CA 0220~416 1997-0~
corresponding to about 90 to 100% of the water absorption
capacity of the starting amount of carrier material. The
basic solution is prepared in similar manner.
The invention also relates to hydration of olefins to the
corresponding alcohols, in the presence of phosphoric
acid or another active component, e.g. a heteropoly acid,
as a catalyst carrier on the compact according to the
invention.
One such method is described e.g. in EP 0 578 441 A2. In
this method, water and ethylene are reacted at
temperatures between 225 and 280~C and pressures between
20 and 240 bar to form ethanol. A water/ethylene molar
ratio in the range from 0.15 to 0.5 is used. The
catalyst load, measured in grams of water/ethylene
mixture per minute and per millilitre catalyst, can be
chosen in the range from 0.01 to 0.1 g/(min x ml). The
by-product of this reaction is diethyl ether.
Isopropanol is prepared by hydration of propylene under
CA 0220~416 1997-0~
similar conditions but at a reduced temperature in the
range between 180 and 225~C. The by-product of this
reaction is n-propanol.
The catalyst carriers for the phosphoric-acid active
component, according to EP 0 578 441 A2, can be pellets
of synthetic silicon dioxide with high breaking strength,
high porosity and few metallic impurities. The pores of
the carrier are for holding the active component. The
average pore radius for the hydration process is in the
range between 1 and 50 nm.
During operation, catalysts are subject to aging, shown
by a reduction in activity and/or selectivity.
Deactivation is frequently due to reduction of the
specific surface area of the carrier caused by high
temperatures.
The specific surface area of the carrier is closely
related to its pore structure. In addition, high-surface
solids usually have a completely or mainly amorphous
CA 0220~416 1997-0~
structure which tends to change to a thermodynamically
stable state, with growth of crystallites and reduction
of the specific surface.
It has been shown that catalyst carriers containing
silicon dioxide are also subject to such aging.
Hydration conditions accelerate aging. It is also known
that impurities, particularly alkali metals, also promote
the aging of carriers containing silicon dioxide under
hydrothermal conditions (see e.g. R. K. Iler in "The
Chemistry of Silica", page 544, John Wiley & Sons
(1979)).
The catalyst carriers described in EP 0 393 356 and based
on pyrogenically-produced silicon dioxide are also
subject, under hydrothermal conditions, to aging, when
small pores grow into larger pores and lose specific
surface area. The pore volume does not initially vary
appreciably.
Another object of the present invention therefore is to
- 14 -
CA 0220~416 1997-0~
disclose silicon dioxide-containing catalyst carriers
which have better resistance to aging when used under
hydrothermal conditions.
This object is achieved by use of catalysts containing an
active component on a compact according to the invention.
The use according to the invention is particularly
advantageous for hydration of olefins. However,
stabilisation of the carrier is also advantageous in
other catalytic reactions under hydrothermal conditions.
In the case of hydration of olefins, the active component
incorporated in the catalyst carrier is phosphoric acid.
To this end, the carrier is immersed in an aqueous
solution of phosphoric acid and impregnated therewith.
Use can be made of phosphoric acid solutions containing
15 to 85 wt.~ phosphoric acid relative to the total
weight of the solution.
One main application of the hydration of olefins is
hydration of ethylene for producing ethanol and diethyl
CA 0220~416 1997-0~
ether, or hydration of propylene for producing
isopropanol. The reaction conditions known from the
prior art are applied.
The following is an investigation of the change in the
pore structure of catalyst carriers containing silicon
dioxide under hydrothermal conditions. A known carrier
is compared with a carrier according to the invention.
This investigation is described with reference to the
accompanying drawings, in which:
Fig. 1 shows the pore construction of a catalyst carrier
containing Mg after a hydrothermal aging test,
and
Fig. 2 shows the pore structure of a catalyst carrier
according to the invention after a hydrothermal aging
test.
The pore distribution curves shown in Figs. 1 and 2 were
- 16 -
CA 0220~416 1997-0~
obtained by known Hg porosimetry. They show the
differential penetration (intrusion) of mercury, in
dependence on pore diameter. Arbitrary units were chosen
for differential intrusion and in each case the curves
were extended over the available region of the graph.
The pyrogenically-produced silicon dioxide can have the
following physical and chemical characteristics:
- 17 -
Aerosil Aerosil 130 150 200 300 380
OX 50 90
BET surface area m2/g 50 + 15 90 ~ 15 130 + 25 150 + 15 200 + 25 300 + 30 380 + 30
Average size of primary
particles, nm 40 20 16 14 12 7 7
Tamping density1~, 9/l about 130 about 80 about 50 about 50 about 50 about 50 about 50
Loss on drying21 < 1.5 < 1 < 1.5 < o 57) < 1 5 < 1 5 < 1 5
(2 hours at 105~C), %
Loss on anno?~ 19~)5) < 1 < 1 < 1 < 1 < 1 < 2 < 2.5
2 hours at 1000~C) %
pH3) 3.8-4.8 3.6-4.5 3.6-4.3 3.6-4.3 3.6-4.3 3.6-4.3 3.6-4.3 D
t (in 4% aqueous di ,pe, ~;on)
SiO26), % > 99.8 > 99.8 > 99.8 > 99.8 > 99.8 > 99.8 > 99.8 '~
Al2036), % < 0.08 < 0.05 < 0.05 < 0-05 < 0-05 < 0-05 < 0-05
Fe2036), % < 0.01 < 0.003 < 0.003 ~ 0.003 < 0.003 < 0.003 < 0.003
TiO26), % < 0.03 < 0.03 < 0.03 c 0.03 < 0.03 < 0.03 < 0.03
HCI6)8) % < 0.025 < 0.025 < 0.025 < 0.025 < 0.025 < 0.025 < 0.025
Retained on sieve4) < 0.2 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05
(after Mocker, 45 IJm) %
1) According to DIN 53 194
2) According to DIN 55 921
3) According to DIN 53 200
4) According to DIN 53 580
5) Relative to substance dried at 105 ~C for 2 hours
6) Relative to substance annealed at 1000~ for 2 hours
8) The HCl content is a component of the loss on ann~ " ~g
CA 0220~416 1997-0~
AEROSIL (trademark) is produced by spraying a volatile
silicon compound into a hydrogen and air detonating gas
flame. In most cases, silicon tetrachloride is used.
This substance, under the influence of the water produced
in the detonating-gas reaction, is hydrolyzed to silicon
dioxide and hydrochloric acid. After leaving the flame,
the silicon dioxide enters a "coagulation zone" in which
the AEROSIL primary particles and primary aggregates form
agglomerates. The aerosol-like product formed at this
stage is separated from the accompanying gaseous
substances in cyclones and then after-treated with moist
hot air. By this method, the residual hydrochloric acid
content can be reduced below 0.025~. Since the AEROSIL
at the end of this process has an apparent density of
only about 15 g/l, the process also includes compaction
in vacuo, which can give tamping densities of about 50
g/l.
The particle sizes of the thus-obtained products can be
varied by means of the reaction conditions, such as the
flame temperature, the proportion of hydrogen or oxygen,
the amount of silicon tetrachloride, the residence time
in the flame or the length of the coagulation section.
-- 19 --
CA 0220~416 1997-0~
The BET surface area is determined to DIN 66 131, using
nitrogen. The pore volume is calculated from the sum of
the micro, meso and macropore volumes. The breaking
strength is determined by using the breaking-strength
tester produced by Messrs Erweka, Type TBH 28.
The micro and meso pores are determined by recording an
N2 isotherm and evaluating it after BET, de Boer and
Barret, Joyner, Halenda.
The macropores are determined by the Hg pressing-in
method.
The abrasion is determined by means of the abrasion and
friability tester produced by Messrs Erweka, Type TAR.
Examples 1 - 5
71.5 wt.% Aerosil 200
13 wt.~ methyl cellulose
7 wt.% microwax and
8.5 wt.% polyethyle glycol
were compacted with adhesion of water, dried at 110~C for
- 20 -
CA 0220~416 1997-0~
16 hours, comminuted to obtain a free-flowing powder and
shaped into compacts in an eccentric press. The crude
pellets were calcined at 750~C for 6 hours.
The resulting compacts had the following physical and
chemical characteristics:
Example 1 2 3 4 5
Shapeof pellet Cylinders Cylinders Cylinders Cylinders Rings
Outer diameter x
height x inner 3x3 4x4 5x5 5x5 9x5x3
diameter (mm)
BET surface area 170 164 168 163 165
(m2l9)
Pore volume (ml/g) 0.75 0.84 0.84 0.97 . Q.79
Breaking strength 49 42 60 31 22
(N)
Abrasion (wt.%) 0.9 1.6 1.3 3.8 3.8
Apparent weight 535 485 470 - 430 400
(9/1)
SiO2 content (wt.%) 99.9 99.9 99.9 99.9 99.9
Example 6
wt.% Aerosil 200
11.5 wt.% methyl cellulose
6 wt.% microwax and
7.5 wt.% polyethylene glycol
- 21 - -
CA 0220~416 1997-0~
were compacted with addition of water, dried at 110~C for
16 hours, comminuted to obtain a free-flowing powder and
shaped into compacts in an eccentric press. The crude
pellets were heat-treated at 750~C for 6 hours.
The resulting compacts had the following physical and
chemical characteristics:
Shape of pellets Cylinders
Outer diameter x height (mm) 5 x 5
BET surface area (m2/g 168
Pore volume (ml/g) 0.71
8reaking strength (N) 61
Abrasion (wt.%) 2.3
Apparent weight t9/1) 510
SiO2 content (wt.%) 99.9
Examples 7 and 8
71.5 wt.% of Aerosil 300 / Aerosil 130
(Example 7) (Example 8)
13 wt.% methyl cellulose
7 wt.% microwax and
8.5 wt.% polyethylene glycol
- 22 -
CA 0220~416 1997-0~
were compacted with addition of water, dried at 110~C for
16 hours, comminuted into a free-flowing powder and
shaped into compacts in an eccentric press. The crude
pellets were calcined at 750~C for 6 hours.
The resulting compacts had the following physical and
chemical characteristics:
Example. 7 8
Outer diameter x height (mm) 5 x 5 5 x 5
BETsurface area (m2/g 210 117
Pore volume (ml/g) 0.85 0.89
Breaking strength (N) 55 39
Abrasion (wt.%) 2.0 2.4
Apparent weight (g/l) 465 450
Si02 content (wt.%) 99.9 99.9
Comparative Example 1
A compact not according to the invention ~catalyst
carrier Aerosil 350, Degussa AG, with 0.4 wt.% Mg
(elementary), BET surface area 180 m2/g, apparent density
490 g/l, total pore volume 0.8 cm3/g, pellets 6 mm in
- 23 -
CA 0220~416 1997-0~
diameter and 5.5 mm high) was loaded with phosphoric acid
(60 wt.%) and left for 41 hours in a high-pressure
installation at a water-vapour pressure of 15 bar and at
350~C. The pore distribution of the aged catalyst was
determined by Hg porosimetry. The measured pore
distribution is shown graphically in Fig. 1.
The hydrothermally aged carriers had a maximum pore
distribution at pore diameters between 20 and 30 ~m. The
proportion of pores smaller than 10 ~m in diameter was
practically zero.
Example 9
A carrier according to the invention as in Example 3 (mg
content < 50 micrograms/g) was loaded with phosphoric
acid (60 wt.%) and left for 40 hours in a high-pressure
installation at a water-vapour pressure of 15 bar and at
350~C. The pore distribution of the aged catalyst was
determined by Hg porosimetry as before. The pore
distribution is shown graphically in Fig. 2.
- 24 -
CA 0220~416 1997-0~
The maximum pore distribution occurs at 30 ~m. Compared
with the catalyst used in comparative Example 1, the
catalyst according to the invention (carrier or
catalyst), even after aging, had a greater proportion of
small pores less than 10 ~m in diameter.
Comparative Example 2
A catalyst support prepared from pyrogenic silica (BET
surface area 180 m2/g, apparent density 490 g/l, total
pore volume 0.8 cm3/g, tablets 6 mm in diameter and 5.5
mm high, with 0.4 wt.% (elemental) Mg) was contacted with
10% hydrochloric acid at room temperature for 14 hours in
accordance with Example 1 of EP 0 519 435, and was then
washed free of chloride under running water, and dried.
A palladium-gold-potassium acetate catalyst was then
prepared on the pre-treated catalyst support.
The concentration of the impregnating solutions was
selected such that the finished catalyst contained a
concentration of 0.55 wt.% palladium, 0.25 wt.% gold and
5.0 wt.% potassium acetate.
CA 0220~416 1997-0~
In a first step, the support was impregnated initially with
a basic solution comprising sodium hydroxide in water. The
volume of the aqueous NaOH solution corresponded to 50 per
cent of the water absorption of the dry support. After
impregnation with sodium hydroxide, the support was
impregnated immediately, without interim drying, with an
aqueous noble metal solution prepared from sodium palladium
chloride and tetrachloroauric acid, the volume of the latter
solution also corresponding to 50 per cent of the water
absorption capacity of the dry support material.
After waiting for 1.5 hours for the noble metal compounds to
hydrolyse, the support particles were washed free of
chloride. The catalyst was dried and was reduced with
forming gas at 450~C in the gas phase. The catalyst was then
impregnated with an aqueous potassium acetate solution and
was onae more dried. Drying was in the gas phase with
nitrogen.
The sodium hydroxide concentration of the basic solution was
calculated such that there formed on the support particles a
noble metal-containing < 1.0 mm shell.
Example 10
A palladium-gold-potassium acetate catalyst as described in
Comparative Example 2 was prepared on the catalyst support
according to the invention in accordance with Example 3
(Mg content c 50 micrograms/g), but 6.0 mm in diameter and
5.5 mm high. By contrast with Comparative Example 2,
however, no pre-treatment with 10~ hydrochloric acid was
carried out.
Example 11
A palladium-gold-potassium acetate catalyst was prepared in
accordance with Example 10 on the catalyst support according
- 26 -
CA 0220~416 1997-0~
to the invention in accordance with Example 5, but having
the dimensions 8 x 5 x 3 mm and having bevelled edges.
Working Example 1
The activity and selectivity of the catalysts from
Comparative Example 2 and Examples 10 and 11 were measured
during a test of up to 24 hours' duration.
The catalysts were tested in an oil-heated tubular-flow
reactor (reactor length 710 mm, internal diameter 23.7 mm)
at standard pressure and at a space velocity (GHSV) of
550/h-1, using the following gas composition: 75 vol.
ethene, 16.6 vol.~ acetic acid, 8.3 vol.~ oxygen. The
catalysts were ~m; ned within the temperature range 120 to
165~C as measured in the catalyst bed.
The reaction products were analysed by on-line gas
chromatography at the reactor discharge. The measure of
catalyst activity adopted was the catalyst space-time yield
in grams of vinyl acetate monomer per hour and kilograms of
catalyst (g VAM/(h x kgcat.).
Carbon dioxide which is formed in particular by ethene
combustion was also determined and was used in evaluating
catalyst selectivity.
Table 1 shows the results of ~m; n; ng the catalysts from
Comparative Example 2 and Examples 10 and 11. The activity
and selectivity of the catalyst in accordance with
Comparative Example 2 were in each case expressed as 100
per cent.
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CA 0220~416 1997-0
Table 1:
Catalyst Activity Selectivity Catalyst
[9 VAM/(h x k9cat )] C02 in exhaust gas in temperature
as [%] of Comp. Ex. 2 [surface area %I, as [~C]
[%] of Comp. Ex. 2
Comp. Ex. 2 100 100 159.6
Example 10 121.1 72.0 155.6
122.7 96.8 161.1
Example 11 106.8 58.5 142.1
124.9 103.9 154.1
The results show that the catalysts according to the
invention have a markedly higher activity than the
Comparative catalyst, with a comparable or even improved
selectivity.
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