Solvent-Anchored Supported Liquid Phase Catalysis: Polyoxometalate-Catalyzed Oxidations

摘要

An important desired refinement of homogeneous transition metal catalyzed reactions is the heterogenization of the catalysts to achieve facile recovery and recycling of the catalyst. Two basic approaches have been used. The first is immobilization bycovalent binding or adsorption of catalysts on supports. This generally leads to loss of activity and selectivity with respect to the original homogeneous analog. For catalytic oxidations the polar products and oxidants react with the binding units, resulting in leaching of the metal; recent reports describe improved supported catalysts. Loss of catalytic species in oxidations has been prevented by confinement within zeolites or dissolution in nonvolatile siloxanes. The second approach is to carry out the reaction with two liquid phases; the catalyst is dissolved in one phase (water or fluorinated hydrocarbon), and the product is separated into the other. Rates of such reactions are often limited by slow mass transfer.