(728a) Controlled Nuclearity of Hybrid, Supported Manganese Oxides for Controlling Catalytic Oxidation Rates and Selectivities
Nature frequently constructs low nuclearity metal oxide active sites to create unusually active and selective catalysts at mild conditions. The tetranuclear manganese complex in the oxygen evolving center of photosystem II is a prototypical example whose mimicry has been the subject of extensive inorganic chemistry. Such pre-formed coordination complexes in high oxidation states are a route for creating well-defined structures on supports. We report on the use of defined manganese oxide structures complexes that are supported on silica, alumina, and mixed oxides. Mono-, di-, and tetranuclear Mn(IV) oxide clusters are selectively synthesized in solution from triazacyclononane-derived organic ligands, Mn(II), and various oxidants. These large polycations are deposited onto supports with grafted co-catalytic carboxylic acids prior to use or assembled in situ. These assembly routes are compared to covalent grafting of the organic ligand.
Selective oxidation of alkanes and alkenes to their respective epoxides, diols, alcohols and ketones with aqueous hydrogen peroxide demonstrates the importance of nuclearity in overall productivity and selectivity. We have recently developed a supported manganese dimer that is not only active for the epoxidation / dihydroxylation of a number of alkenes with aqueous hydrogen peroxide at 0C, but is at least 10x as productive as the comparable homogeneous catalyst at similar co-catalyst loadings. Diol/epoxide selectivities and productivities are strongly dependant on the pKa and extent of hydroxylation of the surface. In recent work, we have shown that a supported dimer is more than twice as active in cyclohexane oxidation, but half as selective.
X-ray absorption and diffuse reflectance UV-visible spectroscopies and hydrogen-TPR were used to probe the structure of the supported complexes and to track their formation and evolution during temperature treatments and synthesis chemistry. Additional analyses of supported materials and homogenous complexes were performed by electrospray MS and TGA/MS.
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 Schoenfeldt, N. J.; Korinda, A. W.; Notestein, J. M. A heterogeneous, selective oxidation catalyst based on Mn triazacyclononane grafted under reaction conditions. Chem. Commun. 46 (2010) 1640-1642.