MXPA98009846A - Catalyst heterogeneo bimetalico de paladio-oropara the production of vin acetate - Google Patents

Abstract

The invention provides a bimetallic palladium-gold heterogeneous catalyst for the production of vinyl acetate from ethylene, acetic acid and oxygen, a catalyst of the invention is prepared by forming a first layer of colloidal palladium shell dispersion on the surface of the catalyst support, and overlaying a second colloidal metal gold shell dispersion layer on the catalyst support surface, an organometallic gold compound is employed to apply the gold dispersion on the surface of the catalyst support, the organometallic gold compound does not require a fixing process, a preferred palladium-gold catalyst of the invention has a high retention of metallic gold, and exhibits long-term durability and selectivity in the production of vinylacetate.

Description

PALETTE-GOLD BIMETALLIC HETEROGENEOUS CATALYST FOR THE PRODUCTION OF VINYL ACETATE BACKGROUND OF THE INVENTION A well known commercial process for the production of vinyl acetate is by means of the gas phase reaction of ethylene, acetic acid and oxygen in the presence of a supported catalyst which contains palladium. A preferred type of vinyl acetate catalyst is one having a metallic palladium and metallic gold content distributed over the surface of a support substrate such as silica or aluminum. References of the prior art which discloses the palladium-gold supported catalyst for the production of vinyl acetate includes the patent numbers of E.U.A. 3,761,513; 3,775,342; 3,822,308? 3,939,199; 4,048,096; 4,087,622? 4 * 133,962; 4,902,823 .: 5,194,417; 5,314.85BJ and references cited therein; incorporated by reference. The activities and selective properties of the supported palladium-gold catalyst are affected by the physico-chemical form of the metallic palladium and gold content on the support substrate of the catalyst. The patent of E.U.A. No. 4,048,096 discloses a catalyst which consists of a palladium-gold alloy distributed as a shell layer over the outer surface area of a catalyst support such as porous silica. The distribution of the palladium-gold alloy shell provides an improved space-time-yield activity in a vapor phase reaction of ethylene, oxygen and a carboxylic acid for the production of vinyl acetate. The selectivity of a palladium-gold catalyst in the synthesis of vinyl acetate is also influenced by the extension and uniformity of the metallic palladium and the distribution of metallic gold on the exterior and / or interior surfaces of a porous catalyst support substrate, such as the selectivity of carbon dioxide and the conversion of oxygen into a vapor phase reaction of ethylene, oxygen and acetic acid. Accordingly, an object of this invention is to provide one. composition of palladium and gold supported catalyst with enhanced and selective activity in the production of vinyl acetate from ethylene, acetic acid and oxygen. Another object of this invention is to provide a supported catalyst of vinyl acetate which has separately applied shell layers of a dispersed colloid of metallic palladium and metallic gold. Another objective of this invention is to provide a palladium-gold supported vinyl acetate catalyst which has a high gold retention, and which exhibits long-term durability and selectivity in the production of ethylene vinyl acetate, acetic acid and oxygen. A further objective of this invention is to provide a process for preparing a supported vinyl acetate catalyst which has coatings applied separately from metallic palladium and dispersed colloidal gold metal on the support surface. Other objects and advantages of the present invention will be apparent from the accompanying description and examples.

DESCRIPTION OF THE INVENTION One or more objects of the present invention are complemented by the provision of a process for the preparation of a catalyst for the production of vinyl acetate from ethylene, acetic acid and oxygen, said process comprising (1> the formation of a precursor catalyst by impregnating a porous catalyst support medium with a solution of the palladium compound, and reducing the palladium compound to a first colloidal metal palladium dispersion shell layer on the catalyst support surface; > impregnating the parent catalyst with an organic solvent solution of gold organometallic compound, and reducing the gold compound to a second, colloidal metal gold dispersion shell shell on the catalyst support surface to form a catalyst. palladium-bimetallic gold which provides improved carbon dioxide selectivity and conversion of oxygen in ethylene vinyl acetate production, acetic acid and oxygen. The catalyst support medium is selected from porous substrates such as silica, aluminum, silica / aluminum »titanium and zirconium, in the form of spheres» tablets, Raschlg rings, and the like. A typical catalyst support means is illustrated by porous silica spheres, which have a radius of 1-8 mm, a pore volume of 0.1 cc / g, and an internal surface area of 10-350 ma / g. Commercial catalyst support means are widely available, such as the 5 mm silica spheres sold under the trademark KA-160 by Sud-Chemie. In a method of preparing the improved vinyl acetate catalyst of the present invention, the catalyst support is first impregnated with an aqueous solution of a water-soluble palladium compound. Suitable palladium compounds include palladium (II) chloride, palladium nitrate (ID * palladium (II) sulfate, sodium tetrachloropalladium (II), and the like.) The volume of the aqueous impregnating solution is preferably between about 95%. 100% of the absorption capacity of the catalyst support The impregnated catalyst support is treated with an aqueous solution of a basic alkali metal salt, such as sodium silicate, sodium carbonate or sodium hydroxide. Basic alkali metal is used which is sufficient to fix the palladium compound on the catalyst support, i.e. palladium hydroxide is precipitated and is incorporated on the catalyst support surface.In another method of preparing the catalyst, the Catalyst support is first impregnated with the organic solvent solution of at least one palladium organometallic compound. include palladium acetylacetone, palladium acetate, palladium (II) of bis (n3-allyl), palladium (II) of n3-lyl (ns-cyclopentadienyl) »palladium (II) of n3-allyl (1,5-cyclooctadiene) tetrafluoroborate , and similar. Organic solvents that can be employed for the organometallic palladium solution include pentane, hexane, cyclones, anionic acid, octane, isooctane, naphtha, naphthene, benzene, chlorobenzene, nitrobenzene, dichloromethane, and the like. A significant advantage derives from the use of an organic solution of a palladium organometallic compound instead of an aqueous solution of a water-soluble palladium compound. After impregnation of a catalyst support with a solution of an organometallic palladium compound, fixing treatment with a basic alkali salt is not required. The elimination of the amp procedure; Noble metal fixation prevents metal loss which normally occurs during fixing and washing treatment steps. A high charge of noble metal in a catalyst is essential for optimal activity and selectivity in a vinyl acetate process. Subsequent to the step of impregnating the catalyst support with the palladium compound, the catalyst support is treated with a reducing agent to convert the palladium compound into a shell layer of metal palladium colloidal particles on the surface of the catalyst support. . Examples of reducing agents are hydrazine, formaldehyde »ethylene» hydrogen »and the like. The precursor catalyst with a pre-reduced metal palladium content is then impregnated with an organic solvent solution of at least one organometallic gold compound. Suitable organometallic gold compounds include ethyl trimethylsiloxydi gold, trimethylsimethyltriphenylphosphinic gold, gold di-ethylacetate, gold triacetate, and the like. Any suitable organic solvent can be used for the organometallic gold impregnation solution, such as those enumerated above for the palladium organometallic solution. After the impregnation step, the gold organometallic compound is reduced to a second shell layer of dispersed colloidal metallic gold particles on the catalyst support surface. The removal of the fixation process before reduction is particularly significant when the colloidal metal gold is introduced into a vinyl acetate catalyst. Gold is more difficult to fix with a basic alkali metal salt "so that the fixing process causes a low and inconsistent gold retention during catalyst preparation. The process of the present invention for the preparation of the vinyl acetate catalyst allows a high and consistent retention of gold in the catalyst composition. In the process of the invention for the preparation of the vinyl acetate catalyst allows a high and consistent retention of gold in the catalyst composition. In the process of the invention for the preparation of the vinyl acetate catalyst, the gold and palladium part materials are used in amounts which provide about I-10 grams of metallic palladium and about 1-10 grams of metallic gold per gram. liter of finished catalyst. A catalyst of the invention may have a metallic palladium content between about 0.2-2.5 weight percent, and a metallic gold content between about 0.2-2.5 weight percent. The weight ratio of palladium: gold can vary between about 0.5-10: 1.

Optionally, the process of the present invention for the preparation of the catalyst can include a further process for improving the selectivity of the catalyst in the production of vinyl acetate. The palladium-gold catalyst obtained by the process described above is treated with an aqueous solution of the alkali metal acetate such as potassium acetate, and then dried. The content of the alkali metal acetate may be in the range between about 2-10 weight percent, based on the weight of the finished catalyst. Important advantages of the present invention are achieved by providing a palladium-gold bimetallic heterogeneous catalyst composition for the preparation of vinyl acetate from ethylene »acetic acid and oxygen, wherein the catalyst composition comprises a catalyst support medium. which contains a first layer of dispersion shell of a metal palladium colloid on the catalyst support surface. Typically a catalyst of the present invention is employed in a vinyl acetate process by contact with ethylene, acetic acid and oxygen or air with a catalyst at temperatures between about 100 ° -200 ° C and a pressure between about 1-10 atmospheres. The reaction is commonly conducted with an excess of ethylene. A catalyst of the present preferred invention is characterized by a high level of retention of metallic palladium and metallic gold »and exhibits improved long-term durability and selectivity in the production of vinyl acetate from ethylene» acetic acid and oxygen. A catalyst of the present invention can provide an efficient production of vinyl acetate with a low production of carbon dioxide than the conventional Bayer vinyl acetate catalyst of the type described in US Pat.

G. B. 1,246,015; incorporated here by reference. The following examples are even more illustrative of the present invention. The components and specific ingredients are presented as typical, and various modifications may be derived in view of the above description within the scope of the invention. The palladium-gold catalysts in the examples were prepared with different combinations and proportions of palladium and gold starting materials "and compared with the Bayer-type palladium and gold catalyst in the production of vinyl acetate from ethylene, acid acetic and oxygen. Palladium-gold catalysts were prepared from Na2 PdCl ^ / Auf OAc) 3 (OAc = acetate) on silica »as illustrated by catalyst A-E in the examples. Palladium-gold catalysts were prepared from Na2 on silica, as illustrated by catalyst F and catalyst G in the examples. Palladium-gold catalyst was prepared from Nas PdCl.1 / Me; 3AuOSiMe3 on silica, as illustrated by the IO catalyst H and catalyst I in the examples. The unit of the stirred vinyl acetate reactor tank (TRAV) is a Berty reactor, or a continuously stirred reactor tank of the recirculation type that is operating at a constant conversion of oxygen (about 45%). The catalyst (62 ce) is charged into a basket in the reactor, a measured amount of the acetic acid »ethylene» and oxygen is added into a nitrogen diluent »and the reactor is raised to temperature by means of a heating mantle, and the temperature is measured above and below the catalyst. The reaction is terminated after approximately 18 hours at a temperature at which 45% of the oxygen conversion is maintained. Products are measured by gas chromatography. Selectivity of COs tends to be a little high by the same catalyst when tested in a TRAV unit compared to the MUAV and that the vinyl acetate product is recirculated in contact with the catalyst during the sequence reaction. The reactor of the vinyl acetate micro unit (MUAV) in the examples is of the plug flow type operating at constant temperature. The MUAV reactor is a stainless steel tube 0.9 long, and internal diameter of 16 mm with a 3 mm concentric thermocouple box. The reactor is equipped with a heating cover through which hot water and steam circulate. A 30 cm3 sample of the catalyst is diluted with a support up to 150 cm3 and charged to the reactor. The catalyst / support mixture is filled to the top with 30 cm 3 of support. After passing only one time the oxygen, ethylene and acetic acid in a nitrogen diluent, either at a constant temperature or constant conversion of oxygen, the products are analyzed by gas phase chromatography.

EXAMPLES EXAMPLE I This example illustrates the preparation of gold triacetate (III) according to E.U.A. 4,933,204. Gold hydroxide CAu (0H) 33 was made by heating HAuCl in aqueous Ka3COa at a pH of 8 for three hours. The resulting red solution which was filtered, and the Au < OH) .-, precipitate was washed with water and dried with air. The Au (0H) 3 was dissolved in glacial acetic acid with warmth until it formed a gold triacetate solution.

EXAMPLE II This example illustrates the preparation of the pre-reduced palladium-on-silica composition of WajaPdCl, which is employed as an intermediate in the synthesis of the palladium-gold catalyst of the present invention.

An amount of 250 ce 5 mm silica spheres < KA-160 »Sud Chemie) was impregnated with 82.5 ml of aqueous Naa dCl (support of 7 g Pd / L) at an incipient humidity. The impregnated support was treated with 233 sc of aqueous NaOH (50% w / w NaOH / H ^ O, 120% of the amount necessary to convert the metal salt to the hydroxide form). The fixed support was rotated in a Rotovap for 2.5 hours around 5 rpm. After fixation, the treated vehicles were continuously washed with distilled water to remove the chlorine ions until the effluent from the wash was negatively tested with silver nitrate. The speed of the water flow was around 200 cc / min for each wash. The vehicle of each preparation was dried under a continuous stream of nitrogen at a temperature of about 150 ° C. The dried support was reduced with 5% ethylene in nitrogen at 150 ° C for 5 hours. ' EXAMPLE III This example illustrates the preparation of the present invention of Pd-Au catalysts and the properties of the catalysts of the invention in the production of vinyl acetate of ethylene »acetic acid and oxygen in MUAV and TRAV systems compared to the Bayer Pd catalysts -Au. Catalyst A: An amount of 0.88 g of Au (0H3) in ml of acetic acid in a reaction vessel was heated at 60 ° C for 2 hours to produce a clear reddish-brown solution of Au (0Ac) ~ 3. An amount of 35 ml of the solution was added to 100 ml of pre-reduced Pd on silica (example II) at 60 ° C in a reaction vessel, and the impregnation was conducted for about 30 min. The solvent medium was removed at 60 ° C under vacuum. The treated silica support was reduced with 5% ethylene in nitrogen at 120 ° C for 5 hours. The resulting catalyst was impregnated with 4 g of KOAc in 33 ml of water, and then dried in a fluid bed drier at 100 ° C for one hour to provide a Pd-Au A catalyst. Catalyst B: The procedure of catalyst A was followed, except that 0.69 g of Au (OH) 3 in 38 ml of acetic acid was used, to provide a B-Pd-Au catalyst. Catalyst C: The procedure of catalyst A was followed, except that 0.5 g of Au (OH> 3 in 35 ml of acetic acid was used to provide a catalyst C Pd-u Catalyst D: The catalyst A procedure was followed , except that 0.25 g of Au (OH) 3 in 35 ml of acetic acid was used to provide a Pd-Au catalyst. Catalyst E: The catalyst A procedure was followed, except that 0.88 g of Au (0H) 3 in 17 ml of acetic acid »and 45 ml of pre-reduced Pd on silica (example II) was used to provide an E-Pd-Au catalyst. The AD catalysts were tested in a TRAV system compared to the Pd-Ca catalysts. Au of Bayer for the production of vinyl acetate from ethylene, acetic acid and oxygen The comparative data are summarized in Table I. In the catalysts A- »activity increased as the ratio of gold to palladium increased. AD catalysts tend to produce less C O ^ that Bayer catalysts. Catalyst A has improved CO selectivity (8.7 vs. 9.5), higher activity (2.23 vs. 1.37), and lower EtOAc (0.054 vs. 0.06) than Bayer's commercial catalyst. The catalysts A-E were tested in a MUAV system compared to the Bayer Pd-Au catalyst for the production of vinyl acetate. In order to evaluate the activity of the catalyst »an average shell temperature of the unit was recorded at a fixed oxygen conversion (around 45%). A lower shell temperature is an indication of a catalyst activity raised to a constant oxygen consumption. The comparative data are summarized in table II. The catalysts D and E exhibit a high CO selectivity and a catalyst activity lower than the A-C catalysts. A-C catalysts have improved COO selectivity and higher catalyst activity than the Bayer Pd-Au catalyst. The mapping of SEM-EDX with X-rays indicated that the A-E catalysts had metallic palladium dispersed as a shell layer on the outer surface of the silica support. The metallic gold was dispersed mainly on the outer surface of the silica support as a second shell layer, and a smaller proportion of the metallic gold was dispersed on the interior porous surface of the silica support.

TABLE I TRAV data unit for Pd / Au catalysts prepared from NaaPdCl ^ / Au (OAc) .-, Catalyst Selectivity Extreme EtOAc Activity of CO ,. heavy catalyst Bayer 9.51 0.89 0.O6 1.37 Catalyst A 8.70 1.259 0.054 2.23 NasPdCl ^, A < OAc) 3 Catalyst B S.66 1.310 0.048 2.14 NasPdCl ^, Au (OAc) 3 Catalyst C 8.57 1.249 0.O56 2. OI Na3 Cl ^, Au (OAc> a Catalyst D 8.90 O.892 0.078 1.70 Na ^ PdC1_ , Au (OAc) 3 TABLE II Execution unit of the MUAV unit for Pd / Au catalysts prepared from Na ^ PdCl ^ / AuCOAc) Catalyst Loading Selectivity Extremes Temp. Metal conversion of CQ2 heavy shell of O ^ Bayer 6.54 0. 652 153. 9 45. 3 Catalyst Pd: 0.93 5.63 0. 761 140. 9 45. 6 A Na53PdCl ^, Au: 0.69 Catalyst Pd: 1.05 5.89 0.729 144.5 45.5 B Na ^ PdCl ^, Au: 0.31 Au (OAs) - Catalyst Loading Selectivity Extremes Temp. Conversion metal of C02 heavy shell of Oß Catalyst Pd: l.Ol 5.89 0.648 144.4 45. OC Na ^ PdCl ^, Au: 0.25 Au (OAs) 3 Catalyst Pd: 0.97 6.63 0.557 149.2 44.9 D Na ^ dCl ^, Au: 0.14 Au (OAc) s Catalyst Pd: l.OO 6.43 0.76 146.0 45.8 E NaaPdCl ^, Au: 1.11 Au (OAc) _ EXAMPLE IV This example illustrates the preparation of the Pd-Au catalysts of the present invention and the properties of the catalysts of the invention in the production of vinyl acetate from ethylene acetic acid and oxygen in TRAV and MUAV systems compared to the Bayer Pd-Au catalysts. F catalysts and Q catalysts: A 34 ml solution of CHS5C1.-S from h3PAuCHS? Si e3 (1 g) was added to 90 ml of pre-reduced palladium on silica (Example II) in a reaction vessel, and the impregnation was conducted for about 30 minutes. The solvent medium was extracted under vacuum. The treated silica support was reduced with 5% ethylene in nitrogen at 120 ° C for 5 hours. The resulting catalyst was washed with toluene and dried at 120 ° C under vacuum for about 16 hours. The catalyst was impregnated with 3.8 g of KOAc in 30 ml of water »and then dried in a fluid bed dryer at 100 ° C for one hour to provide a Pd-Au F catalyst. Catalyst G was prepared in the same way. Catalyst H: A hexane solution of 16 ml of MeffiAuOSiMe3 (0.38 g) was added to 45 ml of pre-reduced Pd on silica (Example II) in a reaction vessel, and the impregnation was conducted for about 30 minutes. The solvent medium was extracted under vacuum. The treated silica support was reduced with 5% ethylene in nitrogen at 120 ° C for 5 hours. The resulting catalyst was impregnated with 1.8 g in KOAc in 15 ml of water and then dried in a fluid bed dryer at 100 ° C for one hour to provide a Pd-Au catalyst H. Catalyst I: The procedure of catalyst H was followed, except that to a hexane solution of 32 ml of MegjAuOSiMesíO.SS g), and 90 ml of pre-reduced Pd on silica (Example II), were used to provide a catalyst I Pd-Au. The F-I catalysts were tested in a MUAV system compared to the Bayer Pd-Au catalyst for the production of vinyl acetate. The comparative data were summarized in table III. F-I catalysts exhibited improved CO select selectivity over the Bayer catalyst. The catalyst I has a much lower shell temperature (a higher catalyst activity) than the Bayer catalyst. The H-I catalysts had a high metallic gold retention of 86% and 98%, respectively. The F-G and G catalysts have a gold retention of 52% and 57%, respectively. Catalysts F and I were tested in a RAV system compared to the Bayer Pd / Au catalyst for the production of vinyl acetate. The comparative data are summarized in table IV. Catalysts F and I have improved the selectivity of C0S and a higher catalyst activity than the Bayer catalyst. In the SEM-EDX ray mapping it was indicated that catalyst I had a shell dispersion of Pd-Au of metals on the external surface of silica. Catalysts F have a metallic palladium dispersed as a protective layer on the outer surface of silica. The metallic gold was dispersed mainly on the outer surface of the silica support as a second layer of shell. In a smaller proportion the metallic gold was dispersed on the inside of the porous surface of the silica support. The catalyst G was tested in a MUAV system continuously over a period of 7 days for the production of vinyl acetate. The extension of the test period was to monitor the durability of the catalyst, and the long-term selectivity of the catalyst of the invention. The data was recorded every 24 hours. The data was summarized in Table V. The data indicated that the catalyst G of the present invention had a long term pipeline durability and selectivity.

TABLE III MUAV unit data for Na ^ PdCl catalysts ^ Ph ^ P uCH ^ Si e ^ v Na ^ PdCl ^ / Me ^ AuOSiMe ^ Description Analysis Selectivity Extremes Temp. Conversion of heavy C0a from C0a shell Bayer 6.54 0.682 45.3 Catalyst Pd: 1, .01 6.05 0.730 147.1 45.8 Catalyst P Pdd :: 1 1., .0022 6. 53 0.880 144.0 45.3 Catalyst P Pdd :: 0 0 .. .9977 6.26 O. 688 149. 7 45. 6 H a ^ PdCl ^, Au: 0., 77 MeßAuOSiME3 Catalyst P Pdd :: 1 1 .., 0088 6.13 0.915 138.5 45. OI NaMPdCl ^, Au: 0., 94 Me ^ AuOSiME.-, TABLE IV Unit data TRAV for catalysts of a ^ PdCl ^ / Ph ^ PAuCH ^ Si e ^ v Na ,, PdCl-t / Me.-AuOBiMe, Catalyst Selectivity Extremes CO sample activity, Heavy EtOAc catalyst Bayer 9.51 0.89 0.060 1.37 Catalyst F B.31 1.34 0.054 1.97 Na ^ PdCl .. P 3 AuCHjaSiM 3 Catalyst I 9.25 1.75 0.029 2.29 Na ^ PdCl ^, MeS3AuOSiMe3 TABLE V MUAV unit data for a ^ PdCl ^ / h ^ P uCH ^ SiMe ^ in a 7-day A trial period SelectiTempera- Conver- Description of the sample ends of the sample. Hours Co.-, heavy shell O- Bayer 6.54 O.682 153.9 45.3 Catalyst G 24 6.53 0.88 144.0 45, Na3PdCl ^. PhsPAuCHjaS iMe3 Catalyst G 48 6.59 0.91 144. O 45.4 Catalyst G 72 6.24 0.84 145.5 45.4 Catalyst G 96 6.48 0.941 145.2 45.1 Catalyst G 120 6.07 0.817 146.5 45.7 Catalyst G 144 6.15 0.B24 147.3 45.3 Catalyst G 168 6.15 0.828 148.2 45.3

Claims (19)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for the preparation of a catalyst for the production of vinyl acetate from ethylene, acetic acid and oxygen, said process comprising (1) forming a precursor catalyst by impregnating a porous support medium for the catalyst with a solution of a palladium compound, and reducing the palladium compound for a first palladium colloidal metal dispersion shell layer on the surface of the catalyst support; and (2) impregnating a precursor catalyst with a solution of an organometallic gold compound, and reducing the gold compound to a second dispersion shell layer of colloidal metal gold on the catalyst support surface to form a palladium bimetallic catalyst. -gold.

2. A process according to claim 1, further characterized in that the impregnation of the catalyst support medium in the step (1) is conducted by an aqueous solution of a palladium compound »the palladium compound is fixed on the medium of support with an aqueous alkaline solution before reduction.

3. A method according to claim 1 »further characterized in that the impregnation of the catalyst support medium in step (1) is conducted with an organic solvent solution of an organometallic palladium compound.

4. A method according to claim 1, further characterized in that the organometallic gold compound in step (2) is gold triacetate.

5. A process according to claim 1, further characterized in that the organometallic gold compound in step (2) is gold di-ethylacetate.

6. A process according to claim 1, further characterized in that the organometallic gold compound in step (2) is trimethylsiloxydimethyl gold.

7. A process according to claim 1, further characterized in that the organometallic gold compound in step (2) is trimethylsilyl-ethyltriphenylphosphinic gold.

8. A method according to claim 1, further characterized in that the catalyst product has a metallic palladium content between about 0.2-2.5 weight percent, and a metallic gold content between 0.2-2.5. percentage by weight »and a metallic gold content between about 0.2-2.5 weight percent» based on the weight of the catalyst.

9. A process according to claim 1, further characterized in that the catalyst support means is a silica substrate.

10. A method according to claim 1, further characterized in that the catalyst support means is an aluminum substrate.

11. A process according to claim 1 further characterized in that the product of the catalyst has a weight ratio of gold: palladium between about 0.5-10: 1.

12. A process according to claim 1 »further characterized in that in a further process the catalyst product is impregnated with an aqueous solution of an alkali metal alkanoate activator and then dried to provide a catalyst product with improved selectivity for the production of vinyl acetate.

13. A method according to claim 12 »further characterized in that the activating additive is an alkali metal acetate.

14. A composition of the catalyst for the production of vinyl acetate from ethylene »acetic acid and oxygen, prepared according to the process of claim 1. 15.- A bimetallic gold-palladium heterogeneous catalyst composition for the preparation of vinyl acetate from ethylene, acetic acid and oxygen, wherein the composition of the catalyst comprises a porous support medium of the catalyst which contains a first layer of shell of dispersion of the colloidal metal palladium on the catalyst support surface and contains a second layer of dispersion shell of the colloidal metal gold on the catalyst support surface »said catalyst composition prepared according to any of claims 1-13. 16. The composition of the catalyst according to claim 15 which has a metallic palladium content between about 0.2-2.5 weight percent and a metallic gold content between about 0.2-2.5 weight percent. based on the weight of the catalyst. 17. A catalyst composition according to claim 13 which has a palladium: gold weight ratio between about 0.5-10: 1. 18. A catalyst composition according to claim 15, further characterized in that the catalyst support means is a silica substrate. 19. A catalyst composition according to claim 15, further characterized in that the catalyst support means is an aluminum substrate.