MXPA97008463A - Process for the production of vin acetate - Google Patents

Abstract

A process for the production of vinyl acetate, which process comprises contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst prepared by a process comprising the steps of: (a) impregnating a catalyst support with a palladium compound, (b) converting the palladium compound to substantially metallic palladium and (c) sintering the supported palladium at a temperature greater than 500

Description

PROCESS FOR THE PRODUCTION OF VINYL ACETATE Field of the Invention The present invention relates to a process for the production of vinyl acetate by contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst. The preparation of supported palladium catalysts for the production of vinyl acetate generally involves the impregnation of a suitable support with a palladium compound followed by the conversion of the palladium compound to substantially metallic palladium. Description of the State of the Art Methods for the preparation of impregnated catalysts in the outer layer or shell are described, for example, in US 3822308, US 4048096, US 5185308, US 5332710, CA 2128162, US 4087622, CA 2128154, CA 2128161 and US 5422329.

Methods for the preparation of catalysts that are not of the type having an outer layer or shell are described, for example, in US 3743607, GB 1333449, US. 3939199, US 4668819, EP 330853, EP 403950, EP 431478 and CA 2071698. US 5336802 discloses a method for the pretreatment of palladium-gold catalysts wherein the catalyst is heated in the presence of an oxidizing agent such as air, at a temperature at least sufficient to partially oxidize the palladium; the oxidizing agent is extracted and an inert gas such as nitrogen is introduced; and then the catalyst is heated again to a temperature of up to 500 ° C in the presence of a reducing agent such as hydrogen or ethylene. The process described therein is illustrated with a "conventional catalyst containing nominally 1% palladium and 0.5% gold". It is known that the activity of supported palladium catalysts for the production of vinyl acetate decreases with use. If the activity of the catalyst and therefore the productivity of the process decrease to a commercially unacceptable level, it is necessary to regenerate and / or replace the catalyst. The deactivation of the catalysts for vinyl acetate is described by Abel et al. in Chem. Eng. Technol. 17 (1994) 112-118. Simply increasing the amount of palladium in the catalyst to increase the service time of the catalyst presents a problem since the initial activity of the catalyst may be too high to be able to achieve a safe and / or controllable operation on an industrial scale, for example , as a consequence of working in a plant that has a limited capacity for heat dissipation. Object of the Invention Therefore, there remains the need to be able to have a process for the preparation of a supported palladium catalyst useful in the production of vinyl acetate that solves said problem. SUMMARY OF THE INVENTION Thus, and according to the present invention, there is provided a process for the production of vinyl acetate, which process comprises contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst prepared by a process comprising the steps of: (a) impregnating a catalyst support with a palladium compound, (b) converting the palladium compound to substantially metallic palladium and (c) sintering the supported palladium at a temperature above 500 ° C. The present invention solves the technical problem defined above as a consequence of sintering the palladium on the support at a temperature above 500 ° C. Description of the Invention Without implying any theoretical limitation, it is believed that this sintering step causes the growth of metallic palladium particles which decreases the initial activity of the catalyst. In this way, catalysts having a high palladium concentration but a commercially acceptable initial activity can be prepared by the process according to the present invention and such catalysts have a longer service life than conventional catalysts. The sintering step also increases the average pore size of the silica supports. Equally, it has been found that the catalysts of the present invention are less susceptible to the adverse effects derived from an excess concentration of promoter, such as potassium acetate. The sintering step (c) is preferably carried out using a reducing gas, but it can be carried out in the presence of an oxidizing gas or in an inert gas. Suitable reducing gases are hydrogen and carbon monoxide. A suitable oxidizing gas is oxygen. These can be diluted with an inert gas. Suitable inert gases to be used on their own or in combination with oxidizing or reducing gases are nitrogen, carbon dioxide and helium. Suitable temperatures for the sintering step range from values above 500 to 1000 ° C, with preferred temperatures being in the order of 650 to 1000 ° C. Preferred times for the sintering step are between 1 and 24 hours. If an oxidizing agent is used, then it will be necessary that the catalyst be subsequently reduced. The catalyst can be purged with an inert gas before sintering and during the heating period (for safety reasons) and during cooling (at temperatures below 100 ° C, more preferably below 60 ° C) to avoid any redispersion of palladium. Any heating and cooling speed that can be achieved in practice can be used. The sintering step (c) can be carried out, on a commercial scale, in a tower or container capable of satisfying the above-mentioned process conditions. The catalyst can be stirred by the gas flow during the process. A rotary screw oven can be used. At the laboratory scale, a horizontally or vertically arranged tube can be used in an electric furnace provided that the gas-solid contact is effective (it will be necessary to consider the length / diameter ratio). It may be necessary to pre-heat the gas stream. The time and temperature in the sintering stage are related factors; the higher the temperature, the shorter the time required. Those skilled in the art will be able to adapt these parameters to the operational scale contemplated. Normally, the sintering step (c) causes the palladium metal particle to grow in diameter from 3-4 nm to 8-15 nm. The conversion of the palladium compound to practically metallic palladium of step (b) can be achieved by a reduction step which can immediately precede the sintering stage (c) and carry out the two stages of the process in the same installation.

The process for preparing the catalyst of the present invention can be used for the preparation of uniformly impregnated catalysts or impregnated in the outer layer, for use in fluid bed or fixed bed processes for the production of vinyl acetate. The catalyst preparation process of the present invention can be used to prepare catalysts having high palladium concentrations, for example, greater than 0.5% by weight, preferably greater than 1% by weight, based on the total weight of the catalyst. catalyst. The concentration of palladium can be as high as 5% by weight for a fluid bed or as high as 10% by weight for fixed bed applications. It is to be expected that the initial activity of a supported palladium catalyst having a high concentration of palladium, if prepared by a conventional process, is very high, and may even be so high that it is unsafe and / or uncontrollable. the case that it is used on a commercial scale. However, when prepared by the process of the present invention, the initial activity of the catalyst is reduced compared to that of a conventionally prepared catalyst, at the same time that the high concentration of palladium results in a commercially acceptable activity during life of extended service of the catalyst. For the preparation of catalysts both impregnated in the outer layer and uniformly impregnated, suitable supports for the catalyst may comprise porous silica, alumina, silica / alumina, titania, zirconia or carbon, preferably silica. Suitably, the support can have a pore volume of 0.2 to 3.5 ml per g of support, a surface area of 5 to 800 m2 per g of support and a bulk density of 0.3 to 1.5 g / ml. For catalysts to be used in fixed bed processes, the support usually has dimensions of 3 to 9 mm. For catalysts to be used in fixed-bed processes, the support may normally have the form of spheres, tablets, extrudates, pills or any other suitable form. For catalysts to be used in fluid bed processes, the support can generally have a particle size distribution such that at least 60% of the catalyst particles have a particle diameter of less than 200 microns, preferably at least 50% they are less than 105 microns and no more than 40% of the catalyst particles have a diameter of less than 40 microns. In step (a) the support is preferably impregnated with a palladium compound in a suitable solvent. Suitable solvents may be water, carboxylic acids such as acetic acid, benzene, toluene, alcohols such as methanol or ethanol, nitriles such as acetonitrile or benzonitrile, tetrahydrofuran or chlorinated solvents such as dichloromethane. Preferably, the solvent is water and / or acetic acid. Suitably, the support is impregnated with acetate, sulfate, nitrate, palladium chloride or palladium salts containing halogen such as H2PdCl4, Na2PdCl4 or K2PdCl4. A preferred water-soluble compound is Na 2 PdCl 4. A preferred palladium soluble in acetic acid is palladium acetate. The impregnation of the support can be carried out by immersing or spraying the support in contact with a solution of the palladium compound. The impregnation can be carried out in one or more stages or according to a continuous process. The support can be brought into contact with the impregnating palladium solution by tumbling, rotating, swirling or by a similar process, in order to obtain a uniform impregnation. The impregnation is usually carried out at room temperature. High temperatures can be used, for example, with palladium acetate in acetic acid, up to 120 ° C, preferably up to 100 ° C and more preferably up to 60 ° C. The impregnation must be done carefully in order to avoid breakage or attrition of the support. The support can be filled with the impregnation solution up to 5-100% of the pore volume. In addition to palladium compounds, the support can also be impregnated in step (a) with gold, copper and / or nickel compounds, preferably gold, which are converted to the metal together with the palladium in step (b) and are present as mixtures and / or alloys with palladium in metal palladium particles. Suitable gold compounds include gold chloride, tetrachloroauric acid (HAuCl4), NaAuCl4, KAuCl4, dimethylol acetate, barium acetoaurate or gold acetate, preferably HAuCl4. These promoters can be used in an amount of 0.1 to 10% by weight of each promoter metal present in the finished catalyst. In addition to palladium and optionally gold, copper and / or nickel, the support can also be impregnated, at any suitable stage during the preparation process, with one or more salts of Group I, Group II, or lanthanide metals. of transition, preferably cadmium, barium, potassium, sodium, iron, manganese, nickel, antimony and / or lanthanum, which are present in the finished catalyst as salts, generally as acetates. In general, potassium will be present. Suitable salts of these compounds are acetates or chlorides, but any soluble salt can be employed. These promoters can be used in an amount of 0.1 to 15%, preferably 3 to 9%, by weight of each promoter salt present in the finished catalyst. The impregnated support can optionally be dried and the impregnation step can be repeated two or more times in the case that palladium or promoter loads higher than those which allow the solubility of the salt in the solvent are required. The drying step can be carried out at temperatures up to 120 ° C, preferably up to 100 ° C and more preferably at 60 ° C. The drying step can be carried out at room temperature and reduced pressure. In the drying step, air, nitrogen, helium, carbon dioxide or any suitable inert gas can be used. The catalyst can be tumbled, rotated or agitated by the gas stream to facilitate drying. To prepare impregnated catalysts in the outer layer, the impregnated support, wet or dry, is brought into contact with a basic solution by means of an action of swirling, tumbling, rotating, mixing and the like. The basic solution can also be applied by spraying on the impregnated support during tumbling, rotating, mixing and the like. The bases can be hydroxides, carbonates or metal silicates of Groups I or II. Typical examples are sodium hydroxide, sodium metasilicate, potassium hydroxide, potassium metasilicate and barium hydroxide. The basic solution can be applied in one or more stages with adequate time delays between the applications. The temperature in the precipitation stage is generally room temperature but can be up to 100 ° C. Any solvent in which the basic material is soluble may be used, with the use of water being preferred. The base should be placed in contact with the impregnated support for an appropriate period of time so that the metal salts precipitate in an outer layer. This usually requires more than 1 hour, preferably between 8 and 24 hours. For precipitation an optimal amount of base will be required and an excess will usually be needed, usually 1.8 times the notional amount required to generate the hydroxides of the metal salts. The impregnated support can be washed to remove anionic contaminants, for example, nitrates, sulfates and usually halides. For the elimination of chlorides, a wash with deionized water should be carried out until the silver nitrate test reveals that there are no chlorides present. The levels of anionic contamination should be reduced to a minimum. Cationic contaminants should be minimized, for example, to values below 0.5% by weight, preferably less than 0.2% by weight of sodium in the dry catalyst. Low levels of these contaminants are likely to remain; it is not essential that such levels are practically nil. On a commercial scale, discontinuous washing may be employed. To accelerate the process, hot water can be used. Likewise, ion exchange solutions (such as potassium acetate) can be used to displace chloride and sodium. Also, the reagents used for the preparation can be selected so as to avoid the use of chloride and sodium, for example, potassium metasilicate instead of a sodium salt. In step (b) the palladium compound can be converted to metal before or after the optional washing step above, depending on the reagents used. Liquid reducing agents may be employed such as aqueous hydrazine, formaldehyde, sodium formate, methanol or alcohols, preferably aqueous hydrazine. The reduction can also be carried out with gases such as carbon monoxide, hydrogen and ethylene. These can be diluted with an inert gas such as nitrogen, carbon dioxide or helium. Normally, the gas reduction takes place at elevated temperatures of 100-500 ° C until the material is reduced. Normally, reduction occurs in liquid reductive agents and ambient temperatures may be employed, but preferably temperatures of up to 100 ° C. Once the palladium has been converted to metal, sintering is carried out as described herein. The sintering step (c) may follow step (b) by additional heating of the catalyst in the reducing gas at temperatures above 500 ° C. the material can then be impregnated with the promoter salts as previously described. Ethylene, acetic acid and oxygen-containing gas can be contacted with the supported palladium catalyst prepared according to the process of the present invention by methods known in the art. In this way, the reactants can be contacted with the catalyst in a fixed bed or in a fluid bed at temperatures of 145 to 195 ° C and pressures of 1 to 20 atmospheres. The vinyl acetate product can be recovered by conventional methods known in the art. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be illustrated with reference to Figures 1 to 3 and through the following examples and experiments. Figure 1 is a schematic representation of some of the possible methods of preparing the catalyst according to the present invention. Figure 2 is a graph comparing the productivity versus time of a catalyst prepared according to the invention with that of a catalyst not prepared according to the invention. Figure 3 is a graph comparing the effect of the amount of potassium acetate promoter on the activity of a catalyst prepared according to the invention in relation to a catalyst not prepared according to the invention. DESCRIPTION OF THE INVENTION IN CONNECTION WITH THE DRAWINGS With reference to Figure 1, catalysts of uniform type (without outer layer) can be prepared by the impregnation steps of a support with palladium salts and optional promoters, followed by drying and reducing the metals. The material can then be optionally washed and dried before sintering according to the present invention and final impregnation with optional promoters such as potassium, sodium, cadmium or barium acetates. To prepare catalysts of the type having an outer layer, the support impregnated with palladium and optional promoters, such as gold, can optionally be dried. The metals are then precipitated. The material can then be passed either to (i) reduction to metals, washing and drying or to (ii) washing and drying followed by reduction to metals. The material is then subjected to the sintering according to the present invention followed by impregnation with promoters such as potassium, sodium, cadmium or barium acetates. EXAMPLES OF PRACTICAL EMBODIMENT OF THE INVENTION EXAMPLE 1 A catalyst A according to the present invention having a notional composition (ie without allowing losses during the preparation) of 1 was prepared., 8% by weight of palladium, 0.8% by weight of gold and 7% by weight of potassium acetate, - Impregnation of the support 15 g of silica support spheres KA160 (4-6 mm, SudChemie) were added to a solution of 1.0264 g of tetrachloropaladate sodium trihydrate (Johnson Matthey) and 0.2665 g of chloroauric acid trihydrate (Aldrich) in 9.1 g of deionized water. The addition was made in a single portion and the mixture was swirled until the solution had been absorbed uniformly. The impregnated support was then allowed to stand covered for 2 hours at room temperature. 2- Precipitation of palladium and gold compounds on the support To the impregnated support of stage 1 was added a solution of 1.7 g of sodium metasilicate pentahydrate (Fisons) in 18 g of water. The mixture swirled briefly several times for 15 minutes to prevent the formation of "spots" and then left to rest overnight. 3. Reduction of palladium and gold to a practically metallic state The aqueous phase of the material from step 2 above was treated with 5 g of 55% hydrazine hydrate (Aldrich). 4. Washing of the supported compounds The aqueous phase was separated by decantation and the material from step 3 was washed four times with approximately 50 ml of water, decanting after each wash. The resulting material was transferred to a glass column provided with a tap and then washed with deionized water at a rate of about 1 liter for 12 hours until the silver nitrate test gave negative results. The material was dried at 60 ° C overnight in an oven with forced air circulation and cooled. 5. Synthesis of palladium (and gold) The supported palladium material from step 4 was transferred to a horizontally arranged and filled homo, in the center of a quartz tube cladding, with quartz wool and a KA 160 support (previously dried completely) filling the empty space. The quartz tube liner was placed inside a steel tube and the gas feed was connected. The furnace temperature was raised to 150 ° C at a rate of 10 ° C / minute and maintained at that temperature for 2 hours under a constant stream of nitrogen. A hydrogen flow was established at a GHSV of 60 / hour and the flow of nitrogen was stopped. The oven temperature was raised to 800 ° C at a rate of 30 ° C / minute and maintained at that temperature for 11 hours. After this period, the resulting material was allowed to cool to room temperature under the flow of hydrogen. The nitrogen flow was started again and the flow of hydrogen was stopped before proceeding with the discharge of the material. 6. Impregnation with metallic acetate The dry material from step 5 was impregnated with 1.16 g of anhydrous potassium acetate (Aldrich) dissolved in 8.8 g of water. The mixture was gently swirled until the liquid was absorbed. The resulting material was dried again overnight at 60 ° C. Example 2 (Comparative) Catalyst B was prepared according to the procedure of Example 1 except that sintering step 5 was omitted. Example 3 (Comparative) Catalyst C was prepared according to the process of Example 1 except that the sintering step 5 was omitted and the metal fillers were reduced to provide the same initial activity as the catalyst prepared in Example 1. Catalyst Assay in a Microreactor The catalysts prepared above were tested in a microreactor using the following general procedure. The tests were carried out at 7.8 bar gauge and at 150 ° C using catalyst pellets (prepared as above, amount specified in Table 1) diluted with 60 ml of 1 mm glass beads and loaded into a steel tube. stainless steel 10-11 mm internal diameter. The catalyst was used at 7.8 bar manometers by heating at 160 ° C for 3 hours in a stream of nitrogen and then at 150 ° C in a stream of ethylene. Acetic acid vapor was then mixed with the ethylene and passed over the catalyst for a period of at least 50 minutes. A mixture of 21% oxygen in helium was gradually added to the feed gas while maintaining the maximum temperature of the catalytic bed at 150 ° C. The hot catalyst spot was maintained at 150 ° C. The final composition of the reactant mixture was ethylene: acetic acid: oxygen: helium = 53.1: 10.4: 7.7: 28.6 by volume and the hourly space velocity of the total gas was 3850 hr "1. The product stream was analyzed in vapor phase at hourly intervals by means of an on-line gas chromatograph.The catalyst activity was calculated as grams of vinyl acetate produced per liter of catalyst per hour (spacetime yield, STY) and The selectivity of the catalyst was calculated as the percentage of ethylene converted present in the product.The data are offered based on the average of the activities and selectivities measured between 17 and 22 hours after achieving full oxygen content. compare the activities of catalysts A, B and C, are shown in Table 1.

TABLE 1 The comparison of the activities of catalysts A and B in Table 1 shows that the sintering step (step 5) caused the decrease in the activity of catalyst A. This is consistent with the palladium particle size growth and with the loss of surface area of metallic palladium. Catalyst C was prepared with a lower metal load than catalysts A and B; the metal fillers were selected to provide the same initial activity as catalyst A. Therefore, it would be expected that catalysts A and C would have a similar initial operational behavior. However, one would expect the catalyst to maintain productivity for a longer period of time than catalyst C in the event that the growth of the palladium particles and the loss of surface area of the metallic palladium are the cause of less activity. initial. This is illustrated in Examples 7 and 8. Catalyst Assay in Larger Reactors Catalysts a and C were tested in larger tubular reactors as follows. 77.5 g of catalyst A (Example 7) and 77.5 g of catalyst C (comparative example 8) were each charged into separate reactor tubes of 1.8 m. These two tubes were placed in the same sand bath in a fluidized bed. The bath temperature could be controlled and each of the tubes had its own gas / liquid feed system and product handling. The nitrogen flow was started at 1106 mi / minute (@ STP) and with an ethylene flow of 2590 mi / minute (@ STP). The sand bath and the tubes were heated to 150 ° C and the reactor pressure was raised to 8 kg / cm 2 gauge. The flow of acetic acid at 155 g / hour (containing 2% by weight of water) was started to a vaporizer and mixed with nitrogen and ethylene. A small stream of acetic acid (2% by weight of water, 0.0285% by weight of potassium acetate) was introduced into the preheater zone at a rate of 13 g / hour to vaporize with the main gas stream. After a few hours the oxygen flow began at a velocity rate of 153 ml / minute (@ STP). The product stream was analyzed by in-line gas chromatography and then condensed to obtain a crude liquid product of vinyl acetate, acetic acid and water, the remaining gases being vented and sampled by the in-line gas chromatograph. The production of vinyl acetate was controlled for both catalysts. As the catalysts were deactivated, a constant production rate was initially maintained by gradually increasing the oxygen feed to a maximum level of 425 ml / minute (@ STP). For a total oxygen flow, the composition of the gaseous feed was ethylene: acetic acid: water: oxygen: nitrogen = 49.7: 19.6: 1.3: 8.2: 21.2 by volume at a total GSHV of 2261 hr "1 (@ STP) Once the total oxygen feed rate was achieved, a constant production was maintained additionally in the case of catalyst A by gradually increasing the temperature of the sand bath from approximately 150 to 160 ° C. Since both tubes were in the same sand bath, the production in the case of catalyst C dropped below that achieved with catalyst A since it was deactivated more rapidly Figure 2 shows the normalized daily production for catalysts A and C as a function of the days in service Figure 2 clearly shows that although the initial production capacity of the two catalysts was similar, after 5 days in service, the productivity of the comparative catalyst, catalyst C, was lower than that of the catalyst of the invention, catalyst A. Examination of the slopes of the productivities of the two catalysts shows that catalyst A maintained production at about 1, while the productivity of catalyst C slowly decreased over time, ending with a productivity of 0.7. Towards the end of the experiment, the production capacities of catalyst A were tested with respect to those of catalyst C by adjusting the oxygen feed levels and / or temperature of the sand bath. It is noted that the production is up and down according to Figure 2 and it can be seen that catalyst A always has a higher productivity than catalyst C. Catalyst A exhibited a slower deactivation rate than catalyst C even though its initial activities they were very similar. Example 9 - Other tests of the catalysts using a microreactor Two other batches of catalyst were prepared according to the procedure of Example 1 except that the amounts of the reagents used were scaled by a factor of 9. After the washing and drying steps, each The batch of catalyst was divided exactly into 9 equal portions and impregnated with the objective fillers in percent by weight of potassium acetate (see Table 2). These catalyst samples were tested according to the procedure of Examples 1 to 3. Figure 3 shows the activity achieved by these catalyst samples and compares it with the activity of the corresponding catalysts described in US 5179056 (this activity is extrapolated according to the model described in US 5179056). Figure 3 shows that the catalyst according to the present invention requires a minimum level of about 1.5% by weight of potassium to be effective, while for the catalyst of US Patent 5179056 a maximum in activity of about 2 was achieved. , 5% by weight of potassium. For the catalyst according to the present invention, the effect of the promoter is approximately constant from about 1.5 to 5% by weight of potassium. For the catalyst according to US 5179056, the activity begins to fall as the promoter loads increase.

TABLE 2 This shows that the catalyst prepared according to the present invention is more tolerant in relation to the excessive concentrations of potassium acetate promoter.

Claims (12)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as novelty and. therefore, the content of the following claims is claimed as property: 1.- A process for the production of vinyl acetate, whose process involves contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst prepared by a process comprising the steps of: (a) impregnating a catalyst support with a palladium compound, (b) converting the palladium compound to substantially metallic palladium, and (c) sintering the supported palladium at a temperature greater than 500 °. C.
2. 2. - A process according to claim 1, characterized in that the support of the catalyst is impregnated with a palladium compound in a solvent chosen from water, carboxylic acid, benzene, toluene, alcohol, or riles, tetrahydrofuran or a chlorinated solvent.
3. 3. - A process according to claim 2, characterized in that the solvent is water and / or acetic acid.
4. 4. A process according to any of the preceding claims, characterized in that the palladium compound is acetate, sulfate, nitrate, palladium chloride or a palladium salt containing halogen.
5. 5. - A process according to claim 4, characterized in that the palladium compound is palladium acetate.
6. 6. - A process according to any of the preceding claims, characterized in that step (b) is carried out by contacting the palladium compound with a liquid or gaseous reducing agent selected from aqueous hydrazine, formaldehyde, sodium formate, alcohol, carbon monoxide , hydrogen or ethylene.
7. 7. - A process according to any of the preceding claims, characterized in that step (c) is carried out at a temperature of 650 to 1000 ° C.
8. 8. - A process according to any of the preceding claims, characterized in that the step (c) is carried out in the presence of a gas.
9. 9. - A process according to any of the preceding claims, characterized in that the palladium catalyst comprises at least 0.5% by weight of palladium based on the total weight of the catalyst.
10. 10. - A process according to any of the preceding claims, characterized in that the support of the catalyst comprises porous silica, alumina, silica / alumina, titania, zirconia or coal.
11. 11. A process according to any of the preceding claims, characterized in that the palladium support is impregnated in stage (a) with gold, copper, nickel, one or more salts of metals of Group I, Group II, of the lanthanides or transition.
12. 12. A process according to any of the preceding claims, characterized in that the ethylene, acetic acid and oxygen-containing gas are brought into contact with the catalyst at a temperature of 145 to 195 ° C and at a pressure of 1 to 20 atmospheres. RESUEN A process for the production of vinyl acetate, whose process involves contacting ethylene, acetic acid and. an oxygen-containing gas with a supported palladium catalyst prepared by a process comprising the steps of: (a) impregnating a catalyst support with a palladium compound, (b) converting the palladium compound to substantially metallic palladium, and (c) sinter the palladium supported at a temperature higher than 500 ° C.