IE44578B1

IE44578B1 - A noble-metal-containing catalyst,its preparation and use

Info

Publication number: IE44578B1
Application number: IE59/77A
Authority: IE
Original assignee: Bayer Ag
Priority date: 1976-01-14
Filing date: 1977-01-13
Publication date: 1982-01-13
Also published as: IE44578L; CS214875B2; GR70649B; NO148664B; AT351003B; DD131133A5; YU3677A; CH630815A5; RO71089A; ATA11677A; IL51242A0; IL51242A; BG29275A3; TR19602A; AU507697B2; FR2338076B1; BE850356A; SU694054A3; ES454991A1; NL7700270A

Abstract

The catalyst comprising mainly noble metal and a support material has, on the one hand, a three-dimensional edge zone which is low in noble metal or free of noble metal and also has a like core, but, on the other hand, has a three-dimensional ring zone which is rich in noble metal. This catalyst has special activity which is obtained reproducibly and persists for a long time. It is used, in particular, in catalytical reactions.

Description

The present.invention relates to a catalyst which contains noble metal and a support material and to its preparation and use.

A process for the preparation of a catalyst which 5 contains a noble metal on an inert support is known from (German Published Specification) 1,944,933 and is characterised in that, by bringing a solution of a salt of the noble metal together with a base-treated inert support which has a moisture content in the range of about 10-90% of saturation, the noble metal is deposited on the resulting support. Catalysts in which the zone of metal penetration amounts to up to about 50% of-, .the radius of the support pellet.are obtained with this process. 15 U.S. Patents Nos. 3,775,342 and 3,822,308 disclose noble metal silica supported catalysts produced by impregnating the support with noble.metal salts followed by treatment with alkali to precipitate noble metal oxides and/or hydroxides which are thereafter reduced to the noble metals. The resulting catalysts are quite satisfactory for the reaction of olefins with.oxygen and acetic acid to form unsaturated acetates, particularly when the noble metal comprises a mixture of gold and palladium.

It was found in practice, however, that the catalyst activity and performance varied somewhat from time to - 3 time and batch to batch and researches were undertaken to minimize the variance and maximize the performance.

It is accordingly an object of the invention to provide a noble metal supported catalyst, particularly gold and palladium supported on silica, characterized by marked uniformity and high activity.

It is a further object of the invention to provide a simple process for producing such a superior catalyst and for using it in catalytic reactions such as making vinyl acetate in the gaseous phase from ethylene, oxygen and acetic acid.

From the researches indicated hereinabove it is believed that best results are realized when the gold and palladium form a mixed crystalline material, the greater the amount of such material the greater the activity. For any given ratio of the metals, more of such mixed crystalline material is believed to be formed if the metals are immediately adjacent and available to one another.

Consequently it was recognized as desirable to provide the gold in the same relatively small space as the palladium but examination of prior art catalyst particles showed that distribution of the noble metals deviated from the ideal.

By means of the present invention, however, it was found that catalyst particles could reproducibly be provided in which the noble metals had substantially identical distribution, the particles being characterized by a long life of high activity and of substantially constant activity.

Specifically, the catalyst support, preferably approximately spherical silica particles, has a relatively noble metal free core and thin outer shell separated by a relatively thin layer rich in noble metal. The noble metal layer, which comprises gold and palla5 dium, has these metals approximately in uniform ratio therethrough, i.e. their distributions on a percentage basis are substantially the same as one another.

According to the present invention there is provided a catalyst in the form of particles, each particle comprising a mixture of palladium and gold as noble metals, and a support material, and having (a) an outer layer of low or zero noble metal content, (b) an inner shell rich in noble metal and (e) a core having a low or zero noble metal content.

Preferably, each particle is spherical and comprises (a) an outer layer ranging in thickness from 5 to 20% of the particle radius (b) an inner shell ranging in thickness from 5 to 45% of the particle radius, and (o) a core ranging in thickness from 50 to 75% of the particle radius.

The amount of the noble metals in the catalyst can vary within wide limits. For example, amounts of noble metals of from 1 to 10 g per litre of catalyst are suitable. Preferably, one litre of catalyst con25 tains 2 to 5 g of palladium and 1 to 2 g of gold.

Palladium and gold form a continuous series of mixed crystals (see Gmelin, Handbuch der anorganischen Chemie (Handbook of Inorganic Chemistry), Volume 68, page 691). Within the idicated quantity ranges, those amounts of palladium and gold which correspond to mixtures which have a maximum adsorption capacity for hydrogen at temperatures of about 200°C are preferred. According to Gmelin, loc. cit., page 702, palladiurr/gold mixed crystals which contain 20—45% by weight of gold have a maximum adsorption capacity for hydrogen at a temperature of 223°C. Preferably, the catalysts according to the invention contain 30—35% by weight of gold, relative to the total amount of palladium and gold.

The support material of the catalyst according to the invention can be of diverse geometrical shapes; for example the support material can be shaped as spheres, tablets or cylinders. The geometrical dimensions of the support material can, for example, be in the range of 1— β mm. They can also be in the ranges of 4'—6 mm or 2— 4 mm or 3—6 mm. A suitable geometrical shape is, in particular, the spherical shape, for example spheres with diameters in the range of 4—6 mm.

The specific surface area of the support material can vary within wide limits. For example, support materials which have an inner surface area of 50—300 2 m /g, and especially 100—200 m /g (measured according to BET), are suitable.

Examples of support materials which can be used are silica, aluminium oxide, aluminium silicates or spinels. Silica is a preferred support material.

A preferred embodiment of the catalyst according to the invention is a spherical catalyst in which the inner shell, rich in noble metal lies within an outer layer which has an internal diameter of 70% of the external diameter of the spherical catalyst and an external diameter of 85% of the external diameter of the spherical catalyst. Preferably, the average thickness of the inner shell is less than 10½ oi the radius of the spherical catalyst.

In a further preferred embodiment of the catalyst according to the invention, 70-100% by weight of the total noble metal present in the catalyst is in the inner shell.

The location of the inner shell can be determined S M by means of the relative parameters P and P , which 20 are determined as follows: catalyst spheres are embedded in plastic and a section is made through the centre of each catalyst sphere. In the section, a white or pale grey edge zone, corresponding to the outer layer, a dark grey or black annular zone, corresponding to the inner shell, and a white or pale grey core, corresponding to the core which has low or zero noble metal content, can be seen. The external diameter of the catalyst sphere (d), the external diameter of the dark grey or block annular zone (d ) and the internal diameter of the dark grey or black annular zone (d^) are determined for each of the 20 spheres, for example with the aid of a measuring g microscope. The relative parameter P is obtained from these measured values by determining the value (d. + d ). 100 1 a . d g for each one of the 20 spheres P 2Q is then defined as arithmetic mean of these individual values.

The procedure used to determine the relative paraIl meter P _ is similar to that used to determine the s relative parameter P 2θ but the annular zone rich in noble metal is measured by means of line analysis along a diameter on the .-100111111 made through tlie eentie oi the catalyst sphere. The line analysis can be carried out by means of an electron-beam microanalyser, the construction and mode of operation of which are described in Farbe und Lack, Volume 76, page 115—123 (1970).

A further preferred embodiment of the catalyst g according to the invention has relative parameters P M and P 20 which lie between 70 and 85.

As noted, the distribution of the individual noble metals is also of significance and it is an advantage of these preferred catalysts that a distribution profile of the gold taken along a radius and as a function of the overall gold content closely resembles that of the palladium therein. Thus, there is no gold in locations substantially free of palladium and vice versa, thereby maximizing the likelihood of mixed crystalline material for any given overall composition.

Since the catalyst is effective only insofar as it contacts olefin or other material being acted upon, noble metal if deep within the catalyst particle will be relatively ineffective ‘This is because of the long diffusion time necessary for olefin molecules to penetrate from outside and then move out before other molecules can take their place. Accordingly, the fact that the core up to a radius of 70% of the total catalyst particle is substantially noble metal-free represents a marked noble metal economy or, conversely, results in high efficiency per unit of noble metal.

While noble metal near the particle surface is highly effective, because of the way the catalyst is 4 8 78 used in fluid processes, the contact between particles results in abrasion and this is not insignificant, particularly when it is realized that particles may be used as long as two years. Accordingly, if noble metal is on or adjacent the particle surfaces initially, it can be lost by abrasion which obviously is economically undesirable. In addition, however, because of a change in the noble metal content, the catalyst activity will change rather than remaining constant.

The novle particles have the noble metal close to the surface for rapid contact with the olefin or other material being acted upon but not so close as to be abraded away in normal use. Thus, there is a thin substantially noble metal-free band on the outside of the particle of about 15% of the overall radius. In absolute dimensions this may range from 0.25 to 0.5 mm, preferably from O.3 to 0.45 mm.

The bulk of the noble metal is between the core and outer shell, both of which are almost noble metal20 free.

It is of significance that within the zone contain ing the bulk of noble metals palladium as well as gold are situated close to each other. A respective measurement is possible by determining the relative parameter M P . separately for palladium and gold. The new relative parameters are named Ρ (P'S) and p 20 ' Preferred catalysts according to the present inven tion show values of and ^20 ran^e of from 70 to 85. A remarkably good activity is found Μ M when the values of P £0 (P<3) and P 20 are not or only slightly different from each other, for example. when pM20 (M> - '">0 <*> is within the range of from +5 to -5.

An important possibility for changing the parameters ΡΜ20ad pM2o ^13 to οθη^Γ°1 the pH value of an aqueous solution which can be obtained, for example, as follows: liter of the carrier material is impregnated with a solution containing palladium salts and gold salts in an amount sufficient to obtain a catalyst with the desired amount of these noble metals. After impregnation the mass is dried and an aqueous alkaline solution is added in an amount corresponding to 30 to 100% of the absorptive capacity of the carrier. The alkaline solution is allowed to act upon the carrier for 24 hours at room temperature. Thereafter distilled or deionized water is added just to cover the carrier material. After 2 hours the aqueous phase is filtered off from the carrier and the pH value of the aqueous phase is determined.

This pH value is further referred to as pH (K).

In a preferred embodiment of the production of catalytically active particles according to the invention the amount of alkaline compounds in the solution to be used for treating the impregnated carrier is regulated in such a manner that the pH (K) is greater than 7, especially between 7 and 9, in particular between 7.5 and 8.5.

The carrier material to be used for preparing the catalytically active particles according to the invention can show different properties, for example different acidities, which can lead to different distributions of 44S78 - ίο the noble metals, even if the same procedure for making the catalytically active particles is applied.

Even carriers which are produced according to the same preparation method can change their properties when the carrier material is subjected to an after - treatment as, for example, drying, calcining or storage. Therefore, it is of advantage to determine the optimum amount of alkaline compounds to be used for preparing the catalyst in a pretrial wherein the pH (K) value is measured according to the method described above.

Customary additives can also be added to the catalyst according to the invention prior to use. Thus, for example, when the catalyst according to the invention is used in the preparation of unsaturated esters from olefines, oxygen and organic acids, additions of alkali metal acetates or of compounds which give alkali metal acetates under the reaction conditions are advantageous. For example, it is possible to add between 1 and 20% by weight of an alkali metal acetate, relative to the sup20 port material.

The preparation of the catalysts according to the invention can be carried out as follows: The support material described above, for example silica in the form of spheres having diameters in the range of 4—6 mm and having an internal surface area 2.. in the range of 50—300 m /g, is impregnated with a solution of one or more noble metal salts. Preferably, an aqueous solution of the noble metal salts is used for this purpose. When palladium is to be applied to the support material, the solution with which the support is to be impregnated can contain, for example. - 11 palladium chloride, sodium palladium chloride, palladium nitrate and/or palladium oulphato. When ijold ls add! · tionally applied to the support material, the solution with which the support material is to be impregnated can, for example, additionally contain gold-III chloride and/or tetrachloroauric acid. An aqueous solution of sodium palladium chloride and tetrachloroauric acid is preferably used. The amount of noble metal salt with which the support is to be impregnated is chosen so that all of the noble metal salt solution is absorbed by the support material. For example, the amount of noble metal salt solution used can be such that it corresponds to 80—100% of the absorptive capacity of the support material and preferably such that it corresponds to 90—95% of the absorptive capacity of the support material. The content of noble metal salts in the solution is so chosen that, after impregnation, the desired amount of noble metal, in the salt form, is present in the support material. After impregnation, the material is dried, preferably at temperatures of below 120°C; for example, the material is dried in a stream of air at 100—120°C. A solution which has an alkaline reaction, for example an aqueous solution which contains alkali metal hydroxides alkali metal bicarbonates and/or alkali metal carbonates, is then added. The alkaline solution is allowed to act for a certain time, for example 1—50 hours at 10—30 hours. The alkaline solution is employed in an amount such that an aqueous extract obtained after the period of action has a pH value of more than 7 and preferably in the range of 7—9. - 12 It is particularly preferred to use aqueous solutions of-sodium hydroxide or potassium hydroxide. Whether or not the proper amount of alkali has been employed to produce the ultimately desired arrangement of noble metal can be determined by the above mentioned measurement of the pK(K) value. If the pH(K) value is below 7, the noble metal distribution, as can be determined by more complex precedures, for example by determination of ?M2o' ^20' ^20 and/or as above, will not be as desired. If the pH (K) value is greater than 9, normally no adverse effects with respect to the distribution of noble metal is observed, however, it might be possible in some cases that the carrier material is attacked or that some parts the unsoluble noble metal compounds dissolve.

It is not possible to change the distribution at that point so that a practical procedure is to take several samples treated with slightly different amounts of alkali and determine which amount gives the desired PH (K) value.

The cross-sectional distribution curves of palladium and gold are the most similar to one another within the indicated range of pH (K). When dissimilar, i.e. outside the range, it means that there will be zones comparatively rich in one of them but poor in the other.

By the treatment with the alkaline solution the noble metal salts are converted to water insoluble compounds. The composition of these insoluble compounds is not known, but it is believed that it might be hydr30 oxides and/or oxides, at least in cases where the alkaline solution is a solution of sodium hydroxide or potassium hydroxide.

A wash, for example with water, and/or drying, can then optionally be carried out. The material is then treated with a reducing agent in order to convert the noble metal salts and compounds which are present into the metallic form. The reduction can be carried out in the liquid phase, for example with aqueous hydrazine hydrate, or in the gas phase, for example with hydrogen or hydrocarbons, for example ethylene. If the reduction is carried out with a solution of hydrazine hydrate, the reaction is preferably carried out at normal temperature; when the reduction is carried out in the gas phase it can be advantageous to carry out the reaction at elevated temperature, for example at 100—200°C in the case of reduction with ethylene. The reducing agent is appropriately employed in excess so that it is certain that all of the noble metal salts and compounds are converted into the metallic form. Subsequently, the catalyst prepared in this way is washed with water in order to remove salts; for example when sodium palladium chloride solution is used, the catalyst is washed until no further chloride ions can be detected. After drying, a catalyst which has a) an outer layer having low or zero noble metal content (b) an inner shell which is rich in noble metal and c) a core which has low or zero noble metal content is obtained. If all the salts have already been removed by a wash carried out after the treatment with alkalis, the wash after the reduction can optionally be dispensed with.

Depending on the use for which the catalyst prepared in this way is intended, the latter can also be provided with customary additives. Thus, for example, additions of alkali metal acetates are advantageous when the catalyst is to be used for the preparation of unsaturated esters from olefines, oxygen and organic acids. In this case, for example, the catalyst can, for this purpose, be impregnated with an aqueous solution of potassium acetate and then dried.

The catalysts according to the invention have the advantage that they contain no noble metal or only very small amounts of noble metal in the proximity of the surface and, therefore, virtually no noble metal can be lost by abrasion. On the other hand, the presence, in the core of the support, of noble metal which takes virtually no part in the chemical reaction is almost or completely avoided so that, overall, optimum distribution of the active noble metal is achieved.

The catalysts according to the invention can advantageously be used in all reactions for which supported catalysts containing noble metal are custo20 marily employed.

The catalysts according to the invention can be used with particular advantage for the preparation of vinyl acetate from ethylene, oxygen and acetic acid in the gas phase. For this purpose, those catalysts accor25 ding to the invention which contain silica as the support material and additives of alkali metal acetates are particularly suitable. In the above mentioned preparation of vinyl acetate, such catalysts are also distinguished by high activity and selectivity and by long life.

When vinyl acetate is prepared using the catalysts according to the invention, a stream of gas, which contains ethylene, oxygen or air and acetic acid as well as, advantageously, small amounts of a gaseous alkali metal acetate is passed over the catalyst. The composition of the stream of gas can be varied within wide limits, taking into account the explosive limits. For example, the molar ratio of ethylene to oxygen can be 80:20 to 98:2 and the molar ratio of acetic acid to ethylene can be 100:1 to 1:100 and the content of gaseous alkali metal acetate can be 2—200 ppmw, relative to the acetic acid employed. The stream of gas can also contain other inert gases, such as nitrogen, carbon dioxide and/ or saturated hydrocarbons. Reaction temperatures which can be used are elevated temperatures, preferably those in the range of 100—25O°C. The pressure employed can be a somewhat reduced pressure, normal pressure or elevated pressure, preferably a pressure of up to 20 atmospheres gauge.

Example 1 (Preparation of a catalyst) litre of a silica support in the form of spheres with diameters in the range of 4—6 mm, an internal sur2 face area of 150 m /g (according to BET) and an absorptive capacity of 350 ml of water per litre was impregnated with 340 ml of an aqueous solution of sodium palladium chloride and tetrachloroauric acid. The amount of sodium palladium chloride and tetrachloroauric acid in the solution corresponded to 3.3 g of palladium and 1.5g of gold. The impregnated support was then dried with hot air at not more than 120°C until the moisture content had fallen below 4%. The dried material was then treated with a solution of 6 g of sodium hydroxide to 340 ml of water. Subsequently it was left to stand for IS hours at room temperature'. Thereafter distilled water was added in such an amount that the catalyst particles were just covered. After 2 hours the water is filtered off and the pH (K) value was determined, the pH (K) value was 7.5. Then the catalyst was washed with distilled water for 24 hours.

No further chloride ions could be detected in the final fractions of the washing water. The material thus obtained was again dried and reduced for 8 hours in a stream of ethylene under normal pressure and at 150°C. Finally, it was impregnated with 30 g of potassium acetate, in the form of an aqueous solution and again dried.

The catalyst prepared in this way had a) an outer layer which was low in noble metal or free from noble metal, b) an inner shell which was rich in noble metal and c) a core which was low in noble metal or free from noble metal.

When the catalyst spheres were embedded in plastic and a section was made through the centre of the spheres a pale grey outer layer, a black inner shell and a white core could be seen. Measurement of the annular zone showed that the annular zone lay within a region which was from 70 to 85% of the diameter of a catalyst sphere.

S M Determination of the parameters Ρ 2Q and Ρ 2Q by the method described above gave the following values: S M P 2O=77 an^ P 20=?θ· avera1e thickness Of the three dimensional annular zone was about 5% of the radius of the catalyst spheres.

About 90% of the noble metal contained in the 1? catalyst was in the three-dimensional annular zone.

This catalyst is designated catalyst A.

Example 2 (Use of a catalyst) 450 ml of the catalyst prepared according to Example 1 were filled into a reaction tube with an internal diameter of 25 mm. Per hour, 14.4 moles of acetic acid, 58 moles of ethylene and 4.4 mols of oxygen were passed in the form of a gas, over the catalyst at a temperature of 160°C and under a pressure of 8 bars absolute. Upstream of the reaction tube, 100 ml per hour of a 0.05% strength by weight solution of potassium acetate in acetic acid were sprayed into this stream of gas. Per hour, 615 g of vinyl acetate were formed per litre of catalyst. Of the converted ethylene, 92% had been converted to vinyl acetate and 8% had been converted to carbon dioxide. No reduction in these catalyst performances could be determined after an operating time of 1,000 hours.

Example 3 A catalyst B was prepared according to Example 1, however, the carrier material was heated for 6 hours to 650°C before impregnation with the aqueous solution of sodium palladium chloride and tetrachloroauric acid. The determination of the pH (K) value gave 6.5.

Example 4 A catalyst C was prepared according to Example 3, however, after heating of the carrier to 650°C the carrier was stored for 3 months at room temperature before the impregnation and the other steps for making the catalyst were carried out. The determination of the pH (K) value gave 7.0.

Example 5 A catalyst D was prepared according to Example 3, however, the amount of sodium hydroxide was increased to such an extent that the determination of the pH (K) value gave 8.5.

Example 6 For catalysts A, B, C and D the differences between Μ M the figures P 20an<3 P 20 were determined to be the following: Catalyst pH (K) Value PM2o (Pd) —PM2o (Au) (in abs olute figures) A 7.5 1 B 6.5 12 C 7.0 5 D 8.5 1 Example 7 Example 2 was repeated using catalysts B, C and D. The following results were obtained: Space time yield (g per Catalyst liter of catalyst per hour Selectivity (%) B 350 91 550 605

Claims

1. CLAIMS:1. A catalyst in the form of particles, each particle comprising a mixture of palladium and gold as noble metals, and a support material, and having (a) an outer layer of low or zero noble metal content, (b) an inner shell rich in noble metal and (c) a core having low or zero noble metal content.

2. A catalyst according to claim 1, wherein 70 to 100 per cent by weight of the noble metals present in the catalyst is in the inner shell.

3. A catalyst according to claim 1 or 2 wherein each particle is spherical.

4. A catalyst according to claim 3, wherein the outer layer (a) ranges in thickness from 5 to 20 per cent of the particle radius, the inner shell (b) ranges in thickness from 5 to 45 per cent of the particle radius and the core (c) ranges in thickness from 50 to 75 per cent of the particle radius. 5. Alkali metal acetate is applied to the support material and the material subsequently dried. 5 obtained afterwards has a pH value greater than 7, (d) treating the support with a reducing agent so as to convert the insoluble noble metal compounds into the corresponding noble metals. 5 10. A catalyst according to any one of claims 1 to 9 wherein the noble metals consist of 30 to 35 per cent.by weight of gold with the balance comprising palladium.

5. A catalyst according to claim 3 or 4 wherein the inner shell lies within a spherical shell which has an internal diameter of 70 per centcf the external diameter of the sphere and an external diameter of 85 per cent of the external diameter of the sphere.

6. A catalyst according to claim 5, wherein the average thickness of the inner shell is less than 10 per cent of the radius of the sphere.

7. A catalyst according to any one of claims 3 to 6 wherein the support material for each particle is in the form of a sphere having a diameter of 4 to 6 mm.

8. A catalyst according to any one of claims 3 14578

9. A catalyst according to claim 8 wherein P M M “Ll (Pd)—P (Au) is between +5 and -5. f 20

10. Ethylene, oxygen and acetic acid substantially as hereinbefore described with reference to Example 2 or 7. 10 palladium nitrate, at least one of gold-III chloride and tetrachloroauric acid and chloride ions, the amount of noble metal salt solution corresponding to 80 to 100 per cent by weight of the absorptive capacity of the support material, the support being dried at a temperature below 10 to 10 wherein the support material comprises silica.

11. A catalyst according to any one of claims 1

12. A catalyst according to any one of claims 1 to 11 which contains 1 to 10 g of noble metals per litre of catalyst.

13. A catalyst according to claim 12 wherein the 15 catalyst contains 2 to 5 g of palladium and 1 to 2 g of gold.

14. A catalyst according to any one of claims 1 to 13 wherein the inner surface area of the support 2 material is 100 to 200 m /g.

15. 120°C after the impregnation step (a), the alkali in step (c) being sodium hydroxide and being employed in an amount to impart to the aqueous extract pH of 7.5 to 8.5, the reduction step (d) being effected at normal temperatures using aqueous hydrazine hydrate or at elevated 15 combination with gold-IH chloride and/or tetrachloroauric acid.

16. A process for the preparation of a catalyst

17. A process according to claim 16 wherein an 10 aqueous solution of noble metal salts is used in step (a).

18. A process according to claims 16 or 17 wherein the noble metal salts are selected from palladium chloride, sodium palladium chloride, palladium nitrate, in

19. A process according to any one of claims 16 to 18 wherein the amount of noble metal salt solution corresponds to 80 per cent to 100 per cent by weight 20. Temperature using a gaseous hydrocarbon or hydrogen, the particles then being washed with water until no further chloride ions are detectable in the wash water and an aqueous solution of alkali metal acetate is applied to the particles which are subsequently dried.

20. A process according to any one of claims 16 to 19 wherein the support material is dried, after the impregnation step (a), at a temperature below 120°C. 20 of the absorptive capacity of the support material. 20 15. A catalyst according to any one of claims 1 to 14 wherein 1 per cent to 20 per cent by weight of an alkali metal acetate, relative to the support material, is present as an additive. - 20 to 7 wherein the relative parameters and (as hereinbefore defined) are between 70 and 85.

21. A process according to any one of claims 16 25 to 20 wherein the alkaline solution comprises an aqueous solution of an alkali metal hydroxide, an alkali metal bicarbonate and/or alkali metal carbonate. - 21 (c) adding alkaline solution to the dried support in an amount such that the alkaline solution is sucked up, thereby converting the noble metal salts to insoluble compounds and such that an aqueous extract

22. A process according to any one of claims 16 to 21 wherein the reduction step (d) occurs at normal temperature using aqueous hydrazine hydrate or at elevated temperature using gaseous hydrocarbons or hydrogen.

23. A process according to any one of claims 16 to 22 wherein, after step (d) an aqueous solution of

24. A process according to claim 16 wherein the noble metal salt solution comprises at least one of palladium chloride, sodium palladium chloride and

25. 25. A process according to claim 16 wherein in step (c) such an amount of alkaline solution is used that the pH value in an aqueous extract is within the range of from 7.5 to 8.5. 25 according to claim 1 comprising the steps of (a) impregnating the support with a solution of a salt of each of gold and palladium so that the support material absorbs the solution, (b) drying the support and its absorbed solution,

26. A process according to claim 16 substantially

27. A catalyst whenever prepared by a process according to one of claims lb to 25.

28. A catalyst comprising a mixture of palladium and gold as noble metals and a support material substantially as hereinbefore described in Example 1, 4 or 5. 5

29. A process for preparing vinyl acetate comprising reacting together ethylene, oxygen and acetic acid in the gas phase in the presence of a catalyst according to any one of claims 1 to 15, 27 or 28.

30. A process for preparing vinyl acetate from 30 as hereinbefore described.

31. Vinyl acetate whenever prepared by a process according to claim 29 or 30.