Platinum Metals Rev., 1963, 7, (4), 132
Palladium Chloride Catalyst in Olefin Oxidations
New Production Processes for Acetone and Methyl Ethyl Ketone
The use of noble metal salts or compounds in homogeneous catalysis is relatively new, and the oxidation of ethylene to acetaldehyde using palladium chloride as catalyst is among the first industrial scale processes in this field (1, 2).
Since this process was first reported it has become known as the Wacker Process and has found wide acceptance. Studies have been intensified by its originators, the Consortium für Elektrochemische Industrie GmbH, Munich, and by others to extend its scope for the production of other industrially important compounds by analogous techniques.
In a paper presented at the Sixth World Petroleum Congress in Frankfurt in June this year Dr Jürgen Smidt, Director of the Consortium, and Dr Hans Krekeler, of Farbwerke Hoechst AG, Frankfurt, described the production on a pilot plant scale of acetone and methyl-ethyl-ketone from propylene and butylene streams using palladium chloride as catalyst (3). As in the oxidation of ethylene to acetaldehyde, the oxidation of the olefin is believed to proceed via the formation of a PdCl2− olefin π-complex, followed by hydrolysis of the latter:
During the reaction the palladium chloride is reduced to metallic palladium, and in order to carry out the process continuously the palladium must be kept in solution by being re-oxidised. This is accomplished by an instantaneous reaction with cupric chloride which is present in considerable excess. During the re-oxidation of the palladium the cupric chloride is reduced to cuprous chloride; this is in turn oxidised back to cupric chloride by oxygen or air. Under the conditions of operation the rate of oxidation of the cuprous chloride is of the same order as the rate of reduction of cupric chloride. The individual reactions represented by equations 1, 2, 3 and 4 are thus summarised by equation 5:
Comparison of the rates of absorption (equal to the respective rates of reaction) of the lower olefins in aqueous solutions of palladium chloride/cupric chloride at 70°C and atmospheric pressure shows marked differences between ethylene, propylene and the isomers of butylene. The absorption rate of propylene is about one-third, and the rate for butylene-1 about one-quarter, of that of ethylene. The differences in rates of absorption of the butylene isomers are considerable—the absorption rate of a commercial mixture of 60 per cent butylene-1, 34 per cent butylene-2 and 6 per cent butane is one-seventh to one-eighth of that of ethylene; the absorption rate of a 95 per cent mixture of butylene-2 with butane is less than one-fourteenth that of ethylene.
Operation of the oxidation process on a commercial scale is generally carried out using mixtures obtained from cracking processes and containing about 90 per cent olefin with corresponding paraffins. The latter do not take part in the reaction and act only as inert diluents. Olefin reaction and oxidation of the catalyst solution are usually carried out in separate stages, the removal of the acetone or methyl-ethyl-ketone and their subsequent purification being achieved by steam-stripping and distillation operations. Over 95 per cent conversion of the olefins to the ketones is obtained, and final yields of purified acetone and methyl-ethyl-ketone exceed 92 per cent and 85 per cent respectively.
Among the advantages of the direct oxidation of olefins using palladium chloride as a homogeneous liquid phase catalyst is that mixtures of olefins and saturated hydrocarbons may be used as feedstocks without prior separation and that the products are obtained in high purity. Only one main product is obtained, and its output is not dependent on the manufacture or sale of a second product. The life of the palladium chloride/cupric chloride catalyst solution is virtually unlimited, as homogeneous catalysts are not prone to deactivation by poisoning as are supported heterogeneous catalysts.
- 1 J. Smidt et al., Angewandte Chemie, 1959, 71, 176
- 2 J. Smidt,, W. Hafner,, R. Jira,, R. Sieber,, J. Sedlmeier and A. Sabel, ibid., 1962, 74, 93
- 3 J. Smidt and H. Krekeler, Paper 40, Section IV, 6th World Petroleum Congress, Frankfurt, June 1963