Catalyst, process for its production and its use for preparing vinyl acetate

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.

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
 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 2 160 9 0.01 56.8 4.3 19.5 0.06 0.34 19.0