Evidence from NMR and EXAFS Studies of a Dynamically Uniform Mononuclear Single-Site Zeolite-Supported Rhodium Catalyst

摘要

The best transition-metal complex catalysts are characterized by both high activities and high selectivities, with the latter typically associated with the uniqueness of the catalytic species. Optimal use in technology also demands ease of separation of catalysts from products, and the goal of efficient separation has been a primary motivation for research with supported metal complexes, which offer the added benefit of being resistant to corrosion. General challenges in the synthesis of practical single-site catalysts on supports include the following: a) low catalytic activity associated with non-optimal ligands, which include the support and ligands remaining from the precursor metal complex that is adsorbed on the support; b) non-uniform distribution of metal sites as a consequence of the intrinsic heterogeneity of the surfaces of the inorganic support; and c) loss of structural integrity of the catalysts during operation, sometimes associated with reduction and sintering of the metal to form species of non-uniform (and often poorly characterized) metal nuclearity and often undesired oxidation states of the metal.