Oxidation catalysis with semi-inorganic zeolite-based Mn catalysts

The chelation of zeolite-exchanged Mn2+ by N-containing ligands gives rise to a whole class of heterogeneous liquid phase oxidation catalysts. Bi-, tri- or tetradentate ligands can be used. A high degree of metal complexation is required to avoid side reactions due to the presence of zeolite-coordinated manganous ions. Applied physico-chemical techniques include IR, ESR and electronic spectroscopy. Oxidation-resistant chelands, e.g. with aromatic pyridine groups, are employed to ensure long-term catalyst stability. Use of hydrogen peroxide is most successful in combination with 2,2'-bipyridine (bpy) or 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn); with both systems double bond oxidation proceeds with high selectivity. Olefin oxidations with other oxidants, e.g. tert-butylhydroperoxide (tBuOOH) or iodosylbenzene, are less selective or slower. Alkane oxidation with tBuOOH is possible with various tetradentate diimine ligands. A principal effect of the zeolite matrix is that formation of Mn clusters is impeded in comparison with solution chemistry. Other effects of the zeolite matrix include modulation of the acid strength and suppression of side reactions, such as allylic oxidation of olefins or formation of isomerized epoxides.