(180e) Single-Atom Catalysts for Selective Reaction of Small Molecules, Including Alkanes
Lewis acids typically coordinate to lone pair electrons of molecules, show little interaction with sigma bonds, and generally do not undergo oxidation state changes when catalyzing transformations. We have prepared a number of singe site, metal ion catalysts on high-surface area silica supports and characterized them by X-ray absorption (XAS) and Raman spectroscopy, infrared spectroscopy of adsorbed pyridine, density functional theory and catalytic measurements. These catalysts, when properly synthesized, give rise to corrdinatively unsaturaged metal centers capable of selectively activating C-H bonds in alkanes at high temperature, e.g., alkane dehydrogenation, and dissociation of H2 at lower temperature, for example, for olefin hydrogenation. Raman spectroscopy suggests specific binding sites on the oxide supports. The observed and predicted stretching modes of the Raman spectrum correspond to metal ions bound in homogeneous-like, well-defined tetrahedral coordination geometry. Infrared spectroscopy of adsorbed pyridine indicates the catalytically active site is a Lewis acid. X-ray absorption spectroscopy under catalytic conditions indicates that the Lewis acid does not change oxidation state. Through the combination of in-situ spectroscopic methods and theoretical studies we propose that the mechanism of Lewis Acid activation of C-H and H-H bonds occurs via non-redox, heterolytic dissociation by direct attack of the sigma bonds, rather than by a redox mechanism typical of transition metals. Lewis acid hydrogenation and dehydrogenation catalysis represents a new opportunity for hydrocarbon transformations.