Immobilization of homogeneous oxidation catalysts

Abstract

Homogeneous oxidation catalysts would find more widespread technological application if suitably anchored versions were available. This article discusses the anchoring strategies that have been used for 16 elements. Each element and each type of redox catalysis require a specific approach. Typical supports are oxides such as silica and alumina, zeolites, organic polymers, and activated carbon. The retention of the active metal compound within the catalyst may be based on physisorption, on the formation of covalent bonds between the metal ligand and the support, on ion exchange, or on physical entrapment. Particular attention is devoted to stability tests, which show whether catalytically active metal species are leached from the support. Many metals have the lowest affinity for a support when they are in their most oxidized or most peroxidized state. Therefore, leaching must always be investigated in the presence of the oxidant. It appears that simple adsorption of Mo, V, or Won silica or alumina, for example, does not result in heterogeneous catalysis, whereas ion exchange and covalent methods are often more reliable. In designing a catalyst immobilization method, it is preferable to know all states of the metal during the catalytic cycle; the support should have considerable affinity for all these states. The stability of a catalyst can be promoted by the presence of base, as in the case of metal ion exchanged zeolites, and by appropriate solvent choice. On the other hand, strong or chelating acids tend to cause leaching. Immobilization of a homogeneous oxidation catalyst often greatly enhances its lifetime because of the suppression of bimolecular deactivation. Moreover, unprecedented activities and selectivities may be observed, surpassing the performances of the corresponding homogeneous catalysts.