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| United States Patent |
6,069,268
|
|
Hagemeyer
, et al.
|
May 30, 2000
|
Catalyst, process for its production and its use for preparing vinyl
acetate
Abstract
The invention relates to a process for producing supported shell catalysts
comprising noble metals by UV photoreduction of noble metal salt
precursors fixed to a support. For this purpose, the shaped supported body
is impregnated with suitable noble metal salts which are then reduced to
the metals in a zone close to the surface by means of UV radiation,
preferably in the absence of a chemical reducing agent. The metal salts in
the interior of the pellets which have not been irradiated and therefore
have not been reduced are extracted using a solvent.
The noble metal shell catalysts produced in this way can be used for many
heterogeneously catalyzed reactions such as hydrogenations and oxidations.
According to the invention, Pd/Au shell catalysts on porous ceramic
supports, e.g. SiO.sub.2 shaped bodies, produced by this process can be
used in the synthesis of vinyl acetate.
| Inventors:
|
Hagemeyer; Alfred (Rheine, DE);
Dingerdissen; Uwe (Seeheim-Jugenheim, DE);
Kuhlein; Klaus (Kelkheim, DE);
Heitz; Johannes (Linz, AT);
Bauerle; Dieter (Altenberg, AT)
|
| Assignee:
|
Celanese GmbH (Frankfurt, DE)
|
| Appl. No.:
|
089627 |
| Filed:
|
June 3, 1998 |
Foreign Application Priority Data
| Jun 05, 1997[DE] | 197 23 591 |
| Intern'l Class: |
C07C 067/05; B01J 031/00 |
| Field of Search: |
502/170,330
560/245
207/157.41
|
References Cited
U.S. Patent Documents
| 3627821 | Dec., 1971 | Sennewald et al.
| |
| 4264421 | Apr., 1981 | Bard et al.
| |
| 5185308 | Feb., 1993 | Bartley et al.
| |
| 5260108 | Nov., 1993 | Braren et al.
| |
Primary Examiner: Geist; Gary
Assistant Examiner: Deemie; Robert W.
Attorney, Agent or Firm: Frommer Lawrence & Haug LLP
Claims
What is claimed is:
1. A process for producing shell catalysts comprising noble metals on a
porous support, which comprises:
impregnating the support with salt solutions of the noble metals;
exposing the support to UV radiation so that the noble metals in the zone
close to the surface are reduced to metals; and
extracting the pellets in a solvent in order to remove the noble metal
salts in the interior of the pellet which have not been irradiated.
2. The process as claimed in claim 1, wherein the noble metals are selected
from the group consisting of Pd, Au, Pt, Ag, Rh, Ru, Os and Ir.
3. The process as claimed in claim 1 wherein the noble metals are Au and Pd
and the salts thereof which are used are the acetates.
4. The process as claimed in one of claims 1 to 3, wherein the support
material used is SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2 or a
mixture thereof.
5. The process as claimed in one of claims 1 to 3, wherein the UV
photoreduction is carried out by irradiation with light having a
wavelength of from 40 to 400 nm.
6. The process as claimed in claim 5, wherein the UV photoreduction is
carried out by irradiation with light having a wavelength of from 140 to
360 nm.
7. The process as claimed in one of claims 1 to 3, wherein the
photoreduction of the Pd and/or Au is carried out by irradiation with UV
light having a power density of from 0.01 to 100 W/cm.sup.2 from a lamp or
a pulsed laser, where, when a pulsed laser is used, the pulse frequency is
from 1 to 1000 pulses/s and the irradiation time is from 0.01 to 1000 s,
while when a lamp is used the irradiation time is from 0.1 min to 100 min.
8. The process as claimed in claim 7, wherein the UV light has a power
density of from 0.1 to 20 W/cm.sup.2.
9. The process as claimed in one or more of claims 1 to 3, wherein the
impregnated catalyst support is treated with UV sensitizers before
irradiation.
10. The process as claimed in one of claims 1 to 3 carried out without
using a chemical reducing agent.
11. A shell catalyst obtainable by the process as claimed in one of claims
1 to 3, wherein the shell thickness is from 5 to 5000 .mu.m.
12. A shell catalyst as claimed in claim 11, wherein the noble metals
present are Pd and/or Au.
13. A shell catalyst obtainable by the process of claim 9, wherein the
shell thickness is from 5 to 5000 .mu.m.
14. A shell catalyst obtainable by the process of claim 8, wherein the
shell thickness is from 5 to 5000 .mu.m.
15. A shell catalyst obtainable by the process of claim 10, wherein the
shell thickness is from 5 to 5000 .mu.m.
16. A process for hydrogenation or oxidation reactions in the presence of a
catalyst obtainable by a process as claimed in one or more of claims 1 to
3.
17. A process for preparing vinyl acetate in the gas phase from ethylene,
acetic acid and oxygen or oxygen-containing gases in the presence of a
catalyst obtainable by a process as claimed in one of claims 1 to 3.
18. A process for preparing vinyl acetate in the gas phase from ethylene,
acetic acid and oxygen or oxygen-containing gases in the presence of a
catalyst obtainable by a process as claimed in claim 11.
19. A process for preparing vinyl acetate in the gas phase from ethylene,
acetic acid and oxygen or oxygen-containing gases in the presence of a
catalyst obtainable by a process as claimed in claim 8.
20. A process for preparing vinyl acetate in the gas phase from ethylene,
acetic acid and oxygen or oxygen-containing gases in the presence of a
catalyst obtainable by a process as claimed in claim 10.
Description
The present invention relates to a process for producing a catalyst by UV
photoreduction of metal salts on a support, to the catalyst produced in
this way and to its use for preparing vinyl acetate.
It is known that vinyl acetate can be prepared in the gas phase from
ethylene, acetic acid and oxygen. The supported catalysts used for this
synthesis comprise Pd and an alkali metal, preferably K. Further additives
used are Cd, Au or Ba. The metal salts can be applied to the support by
steeping, spraying on, vapor deposition, impregnation, dipping or
precipitation.
In the case of the Pd/Au/K catalysts it has been found to be advantageous
to apply the two noble metals in the form of a shell on the support, i.e.
the noble metals are distributed only in a zone close to the surface while
the regions lying further inside the shaped supported body are virtually
free of noble metal. The thickness of these catalytically active shells is
generally about 0.1-2 mm. Shell catalysts make it possible to carry out
the process more selectively than in the case of catalysts in which the
support particles are impregnated through to the core ("fully
impregnated") or make it possible to increase the capacity. Here, the
reaction conditions can be kept unchanged compared to the fully
impregnated catalysts and more vinyl acetate can be produced for a given
reactor volume and time. This makes the work-up of the crude vinyl acetate
obtained easier, since the vinyl acetate content of the gas leaving the
reactor is higher, which leads to an energy saving in the work-up section.
Suitable work-ups are described, for example, in U.S. Pat. No. 5,066,365,
DE-34 22 575, DE-34 08 239, DE-29 45 913, DE-26 10 624 and U.S. Pat. No.
3,840,590. On the other hand, if the plant capacity is kept constant, the
reaction temperature can be lowered and the reaction can thus be carried
out more selectively at the same total output, resulting in raw material
savings. This also reduces the amount of the carbon dioxide which is
formed as by-product and therefore has to be discharged and consequently
reduces the loss of entrained ethylene associated with this discharge. In
addition, this method of operation leads to a lengthening of the operating
life of the catalyst.
Many documents disclose catalysts and processes for preparing vinyl acetate
and processes for their production. These are fully impregnated catalysts
or shell catalysts which are generally subjected to a chemical reduction
of the noble metal compounds applied to the support to deposit the noble
metals on the catalyst support. It has surprisingly been found that the
catalytically active metals can also be deposited on the support by
photoreduction.
The deposition of a metal on the surface of a support can be carried out
from a gas, a liquid or an adsorbed surface layer. Such processes are
described for many metals (including Ni, Ag, Au, Pd, Pt, Os and Ir) in the
publications "Laser Processing and Chemistry" (Springer-Verlag,
Berlin-Heidelberg-N.Y., 1996) and "Chemical Processing with Lasers"
(Springer-Verlag, Berlin-Heidelberg-N.Y., 1986) by D. Bauerle.
In terms of the process of the invention, the deposition of catalytically
active metals from adsorbed surface layers is of particular interest.
W. Krauter, D. Bauerle, F. Fimberger, Appl. Phys. A 31, 13 (1983) describe
the laser-induced deposition of Ni from the gas phase (Ni(CO).sub.4) using
a krypton ion laser having wavelengths of from 476 to 647 nm. The
substrate used was glass or Si. It is pointed out that the commencement of
the deposition is attributable to the photoreduction of an adsorbed
Ni(CO).sub.4 layer. This photoreduction is significantly more efficient
when ultraviolet (UV) light is used than when visible light is used.
Y. -F. Lu, M. Takai, S. Nagatomo, K. Kato and S. Namba, Appl. Phys. A 54,
51-56 (1992) describe the deposition of Ag from an adsorbed silver acetate
layer on a manganese-zinc ferrite substrate. An argon ion laser having a
wavelength of 514.5 nm was used for the irradiation.
R. C. Sausa, A. Gupta and J. R. White, J. Electrochem. Soc. 134, 2707-2713
(1987) describe the deposition of Pt onto quartz from an organometallic
layer, likewise by irradiation using an argon ion laser. The layer was
produced by evaporation of the solvent from a solution containing the
organometallic compound (Bright Platinum-05X, Engelhard Corporation) plus
varnish-like binders and solvents. The deposited Pt was used as nucleating
layer for electroless deposition of copper.
H. Esrom, J. Demmy and U. Kogelschatz, Chemtronics 4, 202-208 (1989) report
the use of an Xe.sub.2 * excimer lamp (wavelength 172 nm) for depositing
Pd nuclei on aluminum oxide substrates for the electroless deposition of
copper. The adsorbed layer used was palladium acetate.
Y. Zhang and M. Stuke, Chemtronics 4, 212-215 (1989) also describe the
deposition of Pd from a palladium acetate layer on aluminum oxide
ceramics, quartz substrates and silicon wafers. The synchrotron radiation
having a wavelength range of 40-400 nm from an electron synchrotron was
used for irradiation.
H. Esrom and G. Wahl, Chemtronics 4, 216-223 (1989) describe the
photoreduction of palladium acetate by irradiation with light from an ArF
excimer laser (wavelength 193 nm) and a KrF excimer laser (wavelength 248
nm). This process was used to deposit Pd nuclei for the electroless
deposition of copper on quartz and aluminum oxide ceramics.
A. G. Schrott, B. Braren and R. Saraaf, Appl. Phys. Lett. 64, 1582-1584
(1994) report the photoreduction of PdSO.sub.4 to metallic Pd using an
excimer laser. Here too, it could be shown that nucleated substrates
(SiO.sub.2) could be used for electroless deposition of copper.
P. B. Comita, E. Kay, R. Zhang and W. Jacob, Appl. Surf. Sci. 79/80,196-202
(1994) describe the laser-induced coalescence of gold clusters in a thin
fluorocarbon layer which has been produced by plasma polymerization.
During the production of this layer, gold was embedded by ion sputtering.
The polymer matrix was broken up and vaporized by irradiation with an
argon ion laser to leave coherent gold structures.
None of these publications discloses a process for producing catalysts.
The photoinduced deposition of noble metals from adsorbed surface layers
has been carried out using both UV light sources and light sources which
emit visible light. Since the absorption coefficients of the materials
used in the process of the invention are significantly higher in the
ultraviolet spectral region than in the visible spectral region,
correspondingly lower power densities can be employed if UV light sources
are used. Since a significantly higher throughput is achieved in this way,
the use of UV light sources is preferred. The sources having the shortest
wavelengths generally display the highest efficiency and their use is
therefore particularly preferred.
The UV radiation sources used for the photoreduction are prior art. They
are lamps, lasers or other radiation sources such as synchrotrons or
plasma discharger. Lamps which can be used are, in particular, Hg vapor
lamps (with strong emission lines at wavelengths of 185 nm and 254 nm) and
narrow-spectrum excimer lamps in which the UV radiation arises from the
disintegration of excimers or exciplexes such as Kr.sub.2 * (wavelength
146 nm), Xe.sub.2 * (172 nm), KrCl* (222 nm) or XeCl* (308 nm). As
high-power UV lasers, use is made of pulse excimer lasers. Here too, the
light arises from the disintegration of excimers or exciplexes such as
F.sub.2 * (157 nm), ArF* (193 nm), KrF* (248 nm), XeCl* (308 nm) and XeF*
(351 nm). It is also possible to use frequency-multiplied Nd-YAG lasers
(wavelength 1064 nm/n; n=3, 4, 5, . . . ). Further sources of UV radiation
are synchrotrons which produce broad-band radiation extending into the
X-ray region and the light of a plasma discharge at low pressure.
It is an object of the present invention to provide a process for producing
shell catalysts which comprise noble metals, which process does not use
chemical reducing agents and allows the shell thickness to be adjusted in
a simple way. It is a further object of the present invention to produce
an active and selective vinyl acetate shell catalyst based on Pd/Au
quickly and inexpensively using few process steps while making it possible
to control the shell thickness in a simple manner.
According to the invention, these objects are achieved by noble metal salts
on a support being reduced to the metal and fixed in an outer shell of the
shaped support body by means of photoreduction using UV radiation. The
shell thickness can be adjusted via the penetration depth of the UV
radiation. In this way, good uniformity of the catalytically active metal
particles, a narrow particle size distribution and high dispersion of
metal in the shell are achieved.
The present invention provides a process for producing shell catalysts
comprising noble metals on a porous support, which comprises impregnating
the support with salt solutions of the nobel metals and subsequently
exposing it to UV radiation so that the metal salts in the zone close to
the surface are reduced to the metals.
The photoreduction is preferably carried out using monochromatic UV excimer
radiation. The process is preferably carried out in the absence of
chemical reducing agents.
The metal salts in the interior of pellet which have not been irradiated
and therefore have not been reduced are extracted by means of a solvent
after irradiation of the support. The nanosize particles of nobel metal
fixed in the shell are, owing to their insolubility, not washed out and
remain fixed in position.
The invention further provides the shell catalysts which can be produced by
this process.
The shell catalysts produced in this way can be used for many
heterogeneously catalyzed reactions such as hydrogenations and oxidations.
Pd/Au shell catalysts produced by this process are suitable for use in the
synthesis of vinyl acetate. Compared to conventional preparation
techniques for supported noble metal catalysts (impregnation with metal
salts and chemical reduction thereof), the photoreduction according to the
invention makes it possible to omit the chemical reducing agent and thus
avoid the associated disadvantages such as contamination of the support
with extraneous metals, disposal of the salt formed, multistage operation
and energy-intensive and time-consuming handling of often toxic solutions.
Compared to the conventional processes for producing a shell (fixing by
means of base precipitation followed by chemical reduction), the process
of the invention has the advantage that the shell thickness can be readily
controlled and monitored via the the physical parameters significant in
the deposition, e.g. wavelength and power of the UV radiation source, and
also concentration of the impregnation solution and time and temperature
of the photoreduction. In the process of the invention, the reduction and
fixing in the shell occur simultaneously in one step.
Preference is given to catalysts having a shell thickness of from 5 to 5000
.mu.m. Furthermore, preference is given to those catalysts which comprise
Pd and/or Au.
The photoreduction according to the invention takes only a few minutes,
while the conventional base fixing requires about 20 hours.
Owing to the properties mentioned, the shell catalysts produced according
to the invention have high activities and selectivities.
As active metals which can be concentrated in the shell, all metals for
which photoreducible precursors exist are suitable. A prerequisite for
this is sufficient UV absorption by the precursors at the wavelength used
for irradiation. Appropriate selection of the wavelength used for the
irradiation enables UV absorption to be achieved for many simple and
readily available metal salts such as acetates, formates, propionates,
butyrates, nitrates, sulfates or chlorides. The impregnated supports can
also be treated with sensitizers before irradiation with UV light. Owing
to their ready photoreducibility, all noble metals and their mixtures are
particularly suitable. Preference is given to Pd, Au, Pt, Ag, Rh, Ru, Os
and Ir. Particular preference is given to Pd and Au.
Supports used are inert materials such as SiO.sub.2, Al.sub.2 O.sub.3,
TiO.sub.2, ZrO.sub.2 or mixtures of these oxides in the form of spheres,
pellets, rings, stars or other shaped bodies. The diameter or the length
and thickness of the support particles is generally from 3 to 9 mm. The
surface of the support is generally 10-500 m.sup.2 /g, preferably 20-250
m.sup.2 /g, as measured by the BET method. The pore volume is generally
from 0.3 to 1.2 ml/g.
The reduction and fixing to the support of the noble metal precursors is,
according to the invention, carried out by means of UV light. It is
possible to use, for example, the following UV radiation sources: UV
excimer lasers, frequency-multiplied Nd:YAG laser, UV excimer lamps, Hg
vapor lamps, synchrotrons or low-pressure plasma dischargers. Preference
is given to using UV excimer radiation which is monochromatic and has high
power peaks. Suitable wavelengths are in the range from 40 to 400 nm.
Preferred wavelengths are from 140 to 360 nm, in particular 172 nm
(Xe.sub.2 * lamp), 193 nm (ArF* laser), 222 nm (KrCl* lamp), 248 nm (KrF*
laser) and 308 nm (XeCl* lamp). Preferred UV power densities are from 0.01
to 100 W/cm.sup.2, particularly preferably UV power densities of from 0.1
to 20 W/cm.sup.2.
When pulsed lasers are used, suitable pulse frequencies are generally in
the range from 0.1 to 5000 pulses/s. The irradiation times are generally
from 0.01 s to 3600 s. Preferred pulse frequencies are from 1 to 1000
pulses/s and preferred irradiation times are from 0.01 to 1000 s, in
particular from 0.1 to 300 s.
When using UV lamps which are not pulsed and can have significantly greater
spatial and spectral irradiation windows than UV lasers, suitable
irradiation times are from 1 s and 10 h. Preferred irradiation times are
from 0.1 min to 100 min.
As a result of the limited penetration depth of the UV radiation, the
thickness of the shell can be set and controlled easily.
If a plurality of noble metals are to be fixed to the support (e.g. Pd and
Au), these can be photoreduced simultaneously according to the process of
the invention by means of appropriate selection of the physical parameters
significant in the deposition. This generally results in alloy particles.
As an alternative, it is also possible to carry out sequential
photoreduction under irradiation conditions optimized for each of the
individual metals, which can lead to structured noble metal particles.
The photoreduction can also be combined with conventional chemical
reduction. For example, the photoreduction can be used only for
preliminary creation of the nuclei in the shell, which is then reinforced
by renewed impregnation with the same or other metal salts and chemical
reduction of these. Likewise, it is also possible to photoreduce only one
of the two metals and subsequently to apply and reduce the other metal
using conventional methods. For example, the support can first be
impregnated with palladium acetate which is then phototreduced in a shell
to give Pd metal. The support can then be further impregnated with Au
salts which can be reduced wet chemically or else in situ in the reactor
using gaseous reducing agents such as H.sub.2 or ethylene. This ensures
that the actual active metal for the vinyl acetate process, i.e. the Pd,
is fixed in a shell, while the distribution of the activator, i.e. the Au,
is less critical and it can therefore be fixed using conventional chemical
methods.
The noble metal salts in the interior of the pellet which have not been
irradiated and therefore have not been reduced are extracted by means of a
solvent. Suitable solvents are, for example, chloroform, acetic acid or
aqueous solutions of citric acid or oxalic acid.
The process of the invention is particularly suitable for producing vinyl
acetate shell catalysts. There are 3 types of these, which are composed
essentially of Pd/Cd/K, Pd/Ba/K or Pd/Au/K. The finished vinyl acetate
catalysts have the following compositions:
The Pd content of the Pd/K/Cd and the Pd/K/Ba catalysts 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 Pd content of the Pd/Au/K
catalysts is generally from 0.5 to 2.0% by weight, preferably from 0.6 to
1.5% by weight.
The K content of all three types of catalysts is generally from 0.5 to 4.0%
by weight, preferably from 1.5 to 3.0% by weight.
The Cd content of the Pd/K/Cd catalysts is generally from 0.1 to 2.5% by
weight, preferably from 0.4 to 2.0% by weight.
The Ba content of the Pd/K/Ba catalysts is generally from 0.1 to 2.0% by
weight, preferably from 0.2 to 1.0% by weight.
The Au content of the Pd/K/Au catalysts is generally from 0.2 to 1.0% by
weight, preferably from 0.3 to 0.8% by weight.
Suitable salts are all salts of palladium, cadmium, barium, gold and
potassium which are soluble and contain no constituents which act as
catalyst poisons, e.g. sulfur. Preference is given to the acetates and the
chlorides. However, in the case of the chlorides, it has to be ensured
that the chloride ions are removed before the catalyst is used. This is
achieved by washing the doped support, e.g. with water, after Pd and, if
desired, Au have been fixed on the support by reduction to the metal
particles.
Suitable solvents for the impregnation are all compounds in which the salts
selected are soluble and which can be easily removed again by drying after
the impregnation. Suitable solvents for the acetates are first and
foremost unsubstituted carboxylic acids, in particular acetic acid. For
the chlorides, water is especially suitable. The additional use of a
further solvent is advantageous when the salts are not sufficiently
soluble in the acetic acid or in the water. Suitable additional solvents
are those which are inert and miscible with acetic acid or water. Examples
of additives for acetic acid are ketones such as acetone and
acetylacetone, also ethers such as tetrahydrofuran or dioxane,
acetonitrile, dimethylformamide and also hydrocarbons such as benzene.
In general, at least one salt of each of the elements (Pd/K/Au, Pd/K/Cd,
Pd/K/Ba) to be applied to the support particles is applied. It is possible
to apply a plurality of salts of one element, but it is usual to apply
only one salt of each of the three elements. The necessary amount of salt
can be applied in one step or by multiple impregnation. The salts can be
applied to the support by known methods such as steeping, spraying on,
vapor deposition, dipping, impregnation or precipitation.
In the process of the invention, only the noble metal salts, i.e. Pd and Au
salts, are reduced to the corresponding nanosize noble metal particles and
the "base" constituents K, Cd, Ba are not reduced. The latter can be
applied to the support together with the noble metal salts or else
beforehand or afterwards. In the process of the invention, it is usual to
first produce a shell of Pd/Au and then to impregnate the support with
potassium acetate solution, giving a uniform distribution of K over the
pellet cross section.
Vinyl acetate is generally prepared by passing acetic acid, ethylene and
oxygen or oxygen-containing gases at temperatures of from 100 to
220.degree. C., preferably from 120 to 200.degree. 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 10% by volume (based on the gas
mixture without acetic acid). Dilution with inert gases such as nitrogen
or carbon dioxide may also be advantageous under some circumstances.
Carbon dioxide is particularly suitable for dilution since it is formed in
small amounts during the reaction.
The following examples illustrate the invention.
EXAMPLE 1
a) Impregnation of porous SiO.sub.2 pellets with palladium acetate:
50 ml (about 25 g) of Aerosil 200 pellets (5.5.times.6 mm, Degussa) are
placed in a flask. 530 mg of palladium acetate (Aldrich) are dissolved in
30 ml of glacial acetic acid (corresponds to 1% by weight of Pd). The
solution is filtered through a fluted filter paper. The clear Pd solution
is added to the Aerosil pellets and the glacial acetic acid is taken off
again over a period of 2 hours on a rotary evaporator with continuous
rotation. Remaining acetic acid is subsequently taken off in an oil pump
vacuum at 0.2 mbar/60.degree. C.
b) Photoreduction:
The end faces of the pellets were irradiated in air by means of a KrF*
laser (wavelength 248 nm) using 500 laser pulses in each case. The energy
density of the laser on the specimen surface was 350 mJ/cm.sup.2. The
pulse frequency of the laser was 10 pulses/s. After cutting a
representative number of pellets, the shell thickness was measured by
means of optical microscopy and XPS line scans. The shell thickness is
about 0.5 mm.
c) Conversion into the industrial catalyst:
20 ml of irradiated Aerosil pellets are washed with 2 l of acetic acid,
40%+10% of potassium acetate, in a Soxhlet extractor and dried at
110.degree. C. Since the pellets already contain 1% of Pd, only Au is
applied here: 125.4 mg of Au(CH.sub.3 COO).sub.3 (corresponds to 66 mg of
Au), prepared by the method of U.S. Pat. No. 4,933,204, are dissolved in 1
Omi of H.sub.2 O and added to the pellets. The solution is evaporated on a
rotary evaporator with rotation and under a stream of N.sub.2. The pellets
are then dried at 110.degree. C. 0.8 g of potassium acetate is dissolved
in 15 ml of H.sub.2 O and applied to the pellets as above, dried at
110.degree. C. for 4 hours then additionally dried overnight under reduced
pressure.
d) Reactor tests
Reactor tests on the gas phase oxidation of ethylene and acetic acid to
give vinyl acetate:
The catalysts are tested in a fixed-bed tube reactor having a tube diameter
of 2 cm. The reactor is heated externally by means of oil jacket heating.
15 ml of the shaped catalyst bodies are placed in the reactor. The reactor
volume upstream and downstream of the catalyst bed is filled with glass
spheres. The test apparatus is controlled by a process control system and
is operated continuously. The catalyst is first activated and then tested
under constant reaction conditions.
Activation comprises a plurality of steps: heating under N.sub.2, addition
of ethylene, pressure increase, addition of acetic acid, holding of the
conditions, addition of oxygen.
The reaction conditions during the test are 160-170.degree. C. reaction
temperature, 8-9 bar gauge pressure. The feed is composed of 64.5% by
volume of ethylene, 16.1% by volume of N.sub.2, 14.3% by volume of acetic
acid and 5.1% by volume of O.sub.2. A full analysis of the reactor output
is carried out directly at the reactor outlet by means of on-line GC (2
column arrangement).
The test results are shown in the following table. The concentration ratios
of the components are given in GC percentage areas.
TABLE 1
__________________________________________________________________________
GC analysis of the reactor output
T p Vinyl
Acetic
Catalyst (.degree. C.) (bar) CO.sub.2 C.sub.2 H.sub.4 O.sub.2 N.sub.2
H.sub.2 O acetate acid
__________________________________________________________________________
Example 1
160 9 0.85
55.7
3.25
19.5
0.8 1.27
18.5
Example 2 170 9 1.4 53.8 2.59 23.1 1.18 1.15 16.8
__________________________________________________________________________
EXAMPLE 2
a) Impregnation of porous SiO.sub.2 pellets with palladium acetate and gold
acetate:
50 ml (about 25 g) of Aerosil 200 pellets (5.5.times.6 mm, Degussa) are
placed in a flask. 530 mg of palladium acetate (Aldrich) (corresponds to
1% by weight of Pd) are dissolved in 30 ml of glacial acetic acid. 290 mg
of gold acetate (corresponds to 0.6% by weight of Au), prepared by the
method of U.S. Pat. No. 4,933,204, are dissolved in 10 ml of glacial
acetic acid. The two solutions are combined and filtered through a fluted
filter paper. The clear Pd/Au solution is added to the Aerosil pellets and
the glacial acetic acid is taken off again over a period of 2 hours on a
rotary evaporator with continual rotation. Remaining acetic acid is
subsequently taken off in an oil pump vacuum at 0.2 mbar/60.degree. C. The
final weight of the impregnated pellets is 25.8 g.
b) Photoreduction:
The impregnated tablets were irradiated on both end faces in air by means
of a KrF* laser (wavelength 248 nm) using 150 laser pulses in each case.
The energy density (flux) of the laser light on the specimen surface was
350 mJ/cm.sup.2. The pulse frequency of the laser was 10 pulses/s.
After cutting a representative number of pellets, the shell thickness was
measured by means of optical microscopy and XPS line scans. The number of
pulses is selected so that the shell thickness is about 0.9 mm.
The color change from yellow to black/brown induced by the irradiation
could be seen clearly. In contrast to Example 1 (preimpregnation only with
palladium acetate), a color change could be achieved more easily in the
case of the Pd/Au preimpregnation.
c) Conversion into the industrial catalyst
The irradiated tablets are washed in a Soxhlet extractor first with 2000 ml
of 40% acetic acid and then with 1000 ml of water, dried overnight at
110.degree. C. under atmospheric pressure and then dried for another 1
hour under reduced pressure. 2 g of potassium acetate are dissolved in 30
ml of water and added all at once to the pellets. The solution and pellets
are mixed for 15 min with continual rotation and are again dried at
110.degree. C. under atmospheric pressure, finally for another 1 hour
under reduced pressure.
d) Reactor tests
The preparation of vinyl acetate was carried out under the same conditions
as indicated under d) in Example 1. The test results are shown in the
following table. The concentration ratios of the components are given in
GC percentage areas:
TABLE 2
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GC analysis of the reactor output
T p Vinyl
Acetic
Catalyst (.degree. C.) (bar) CO.sub.2 C.sub.2 H.sub.4 O.sub.2 N.sub.2
H.sub.2 O acetate acid
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Example 2
160 9 0.01
56.8
4.3 19.5
0.06
0.34
19.0
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