Platinum Metals Rev., 1989, 33, (2), 60
Improved Catalyst System for the Wacker Process
The Wacker process for the palladium catalysed conversion of ethylene to acetaldehyde is one of the oldest industrial homogeneous catalyst systems. The original process was plagued by problems of corrosion within the reactor system, and has been replaced by a heterogeneous catalyst system which suffers, however, from the problem of poor catalyst utilisation.
Acetaldehyde used to be a major source of acetic acid and while this route has largely been superseded by rhodium catalysed carbonylation, the Wacker process remains a major route to acetaldehyde. The problems outlined above that are associated with the Wacker catalyst have been, to a large extent, circumvented by a new catalyst system developed by workers in the Department of Chemical and Materials Engineering at the University of Iowa (V. Rao and R. Datta, J. Catal., 1988, 114, (2), 377–387).
The supported liquid melt catalyst which they have used consists of a spherical silica support which contains a palladium chloride-copper(II) chloride catalyst in a eutectic melt of copper(I) chloride-potassium chloride. The location of the homogeneous catalyst within the pores of the support avoids the problems both of corrosion of the plant and also product/catalyst separation, which are associated with the conventional homogeneous system. However, the system remains in essence a soluble catalyst and retains the benefits of the rates and conversions achieved with the homogeneous catalyst. It shows a better utilisation of metal than the conventional supported liquid phase catalysts. In addition the use of a melt media avoids the rapid deactivation associated with supported liquid phase catalysis containing aqueous catalyst solutions. The use of a copper(I) chloride-potassium chloride with a melting point of 423 K means that the complete oxidation of ethylene to carbon dioxide and water is avoided.
The authors have combined a number of concepts which have been available for some time, and appear to have developed a system which avoids most of the pitfalls associated with other catalyst systems for the Wacker process. If it can be established that the catalyst maintains its activities for substantially greater than one week, does not cause corrosion, and if catalyst attrition is not a problem, then the system may have considerable commercial significance. However, the relative simplicity and high stability of the catalyst components in the Wacker catalyst system may mean that it is difficult to apply this technology in a more general fashion.