(513a) Controlled Synthesis of Supported Low Nuclearity Manganese Oxide and Possible Active Structures
In order to control oxidation catalysts at very mild conditions, nature uses low nuclearity metal oxide active sites in enzymes such as the tetranuclear manganese complex in the oxygen evolving center in photosystem II. Pre-formed coordination complexes are a novel route for creating such structures on supports. We report on the synthesis of defined manganese oxide structures on silica and alumina from triazaocyclononane-derived organic ligands and Mn2+. Mono, di, tetra, and polynuclear structures are selectively formed in solution and used as precursors for surface oxides. Upon deposition onto supports, the structure of the metal oxide core is maintained from less than 0.1 wt% up to 10 wt% loading as demonstrated by diffuse reflectance UV-vis. This synthesis route offers a potential alternative to traditional methods for controlling dispersion of supported metal oxides. Several methods are compared for deposition of the oxide including incipient wetness impregnation, covalent attachment of the ligands, and free exchange with organically functionalized supports. Diffuse reflectance UV-vis was used as the primary tool to determine the nuclearity of the complexes and to track their formation and evolution during temperature treatment and as a function of precursor nuclearity surface density, and deposition method. Selected results in low-T oxidation with added hydroperoxides are given where they lend insight to the structure of the formed surface species. Additional analyses of supported materials and homogenous complexes were performed by electrospray MS, TGA, SEM, vibrational and NMR techniques.
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