1. Field of the Invention
The present invention relates to a process of producing palladium-alkali or palladium/promoter metal/alkali metal catalysts useful in the oxacylation of olefins or diolefins. In particular, effecting the production of vinyl acetate from ethylene, acetic acid and an oxygen containing gas. More particular, the present invention relates to the process of producing palladium-gold-potassium fluid bed catalyst useful in the manufacture of vinyl acetate.
The production of vinyl acetate by reacting ethylene, acetic acid and oxygen together in the gas-phase in the presence of a catalyst containing palladium, gold and an alkali metal acetate promoter is known. The catalyst components are typically supported on a porous carrier material such as silica or alumina.
In early examples of these catalysts, both the palladium and gold were distributed more or less uniformly throughout the carrier (see for example U.S. Pat. Nos. 3,275,680, 3,743,607 and 3,950,400 and GB 1333449). This was subsequently recognized to be a disadvantage since it was found that the material within the inner part of the carrier did not contribute to the reaction since the reactants did not diffuse significantly into the carrier before reaction occurred. In other words, a significant amount of the palladium and gold never came into contact with the reactants.
In order to overcome this problem, new methods of catalyst manufacture were devised with the aim of producing catalysts in which the active components were concentrated in the outermost shell of the support (shell impregnated catalysts). For example, GB Patent No.1500167 claims catalysts in which at least 90% of the palladium and gold is distributed in that part of the carrier particle which is not more than 30% of the particle radius from the surface. GB Patent No. 1283737 teaches that the degree of penetration into the porous carrier can be controlled by pretreating the porous carrier with an alkaline solution of, for example, sodium carbonate or sodium hydroxide.
Another approach which has been found to produce particularly active catalysts is described in U.S. Pat. No. 4,048,096. In this patent shell impregnated catalysts are produced by a process comprising the steps of (1) impregnating a carrier with aqueous solutions of water-soluble palladium and gold compounds, the total volume of the solutions being 95 to 100% of the absorptive capacity of the catalyst support, (2) precipitating water-insoluble palladium and gold compounds on the carrier by soaking the impregnated carrier in a solution of an alkali metal silicate, the amount of alkali metal silicate being such that, after the alkali metal silicate has been in contact with the carrier for 12 to 24 hours, the pH of the solution is from 6.5 to 9.5; (3) converting the water-soluble palladium and gold compounds into palladium and gold metal by treatment with a reducing agent; (4) washing with water; (5) contacting the catalyst with alkali metal acetate and (6) drying the catalyst. Using this method, catalysts having a specific activity of at least 83 grams of vinyl acetate per gram of precious metal per hour measured at 150.degree. C. can allegedly be obtained. Shell impregnated catalyst are also disclosed in U.S. Pat. No. 4,087,622. Finally, U.S. Pat. No. 5,185,308 also discloses shell impregnated Pd-Au catalyst and the process of manufacture. Each of the above patents is primarily concerned with the manufacture of fixed bed catalyst useful in the manufacture of vinyl acetate.
It would be economically beneficial if the oxacylation of olefins or diolefins, in particular the manufacture of vinyl acetate from ethylene, acetic acid and oxygen could be performed in a fluid bed process. However, until the discovery of the process of the present invention, the preparation of Pd-Au-alkali metal catalyst in fluid bed form has not led to a catalyst having the necessary properties which can lead to an economically viable fluid bed process for the manufacture of vinyl acetate.