Multiphasic systems transition metal catalysis

Multiphase reaction systems for homogeneous transition metal catalysis consist of one - often polar - phase that contains the dissolved catalyst (the so-called immobilising phase or catalyst phase) and at least one second phase that preferably contains the reaction products. Generally, a suitable catalyst phase must exhibit the following properties ... 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Apart from acidic catalysis, ionic liquids have been intensively tested in the last two decades for the immobilisation of homogeneously dissolved transition metal catalysts. Successful catalyst immobilisation techniques are essential for industrial homogeneous catalysis to solve the problem of catalyst/product separation and to recover and recycle the often very expensive dissolved transition metal complexes. Different immobilisation concepts applying ionic liquids have been developed, including the use of organic-ionic liquid multiphase reaction systems and the use of SILP catalysis. These concepts will be described in the following sections. 

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