(375d) Well Defined, Supported MnOx Oxidation Catalysts Via Controlled Decomposition of Coordination Complexes

The low nuclearity metal oxide active sites in several metalloenzymes, including catalase or the oxygen evolving center in photosystem II, have inspired the synthesis of numerous homogeneous mimics. Several of these mimics are active catalysts in their own right for low temperature oxidation. Manganese (IV) triazacyclononane, Mn-tacn, coordination complexes are one such family of examples [1]. Here, we describe our results employing Mn-tacn complexes as a route for creating oxide-supported MnO_x sites of controlled nuclearity and reactivity.

Mono-, di-, and tetranuclear Mn-tacn complexes were supported on silica, titania, and alumina, and calcined to produce the final heterogeneous catalysts. The structures of the supported catalysts were tracked during synthesis by diffuse reflectance UV-visible spectroscopy and XANES/EXAFS, and were consistent with retention of the structure of the original complex. For example, EXAFS showed no higher coordination Mn shells for the supported monomer, and 1 or 3 Mn neighbors for the dimer and tetramer, respectively.

Moderate activity in cyclohexane oxidation with H2O2 at 60°C has been observed for the mononuclear species on silica, while no activity was observed for the higher nuclearities under the same conditions. Further correlations between precursor structure, final supported structure, hydrogen-TPR, and reactivity in alkane oxidation will be described.

[1]  Wieghardt, K.; et. al. J. Am. Chem. Soc. 110, 22 (1988).