Biomimetic Active Sites in Metal-Organic Framework Catalysts for Hydrocarbon Transformations

Metal-organic frameworks (MOFs) are a class of crystalline materials featuring uniform distributions of inorganic nodes interconnected within well-defined organic porous environments. For applications in catalysis, the homogeneity of active centers attainable within MOFs enables achieving a level of clarity into structure-catalytic property relationships and reaction mechanisms beyond which can be realized in synthetic catalysts delimited by a high degree of heterogeneity in active site speciation – a feature of particular importance in the development of biomimetic catalysts. Herein, we demonstrate MIL-100 (MIL = Materials of Institut Lavoisier), a prototypical MOF featuring mixed-valent trinuclear metal nodes [M2+(M3+)2O], features an active site pool which is uniform in structure and catalytic performance. A combination of in-situ infrared spectroscopic characterization and probe molecule adsorption experiments with H2O, NO, and CO evidence the accessibility of the near theoretical density of coordinatively unsaturated divalent and trivalent open-metal sites within the Cr- and Fe-analogues of MIL-100 through facile thermal activation protocols (≤ 523 K, inert or vacuum). Furthermore, we demonstrate the activated catalysts effectuate the gas-phase stoichiometric oxidation of CH4 with N2O to partial oxygenates (methanol and acetaldehyde) through the involvement of every potentially available M2+ open-metal site. Carbon monoxide is applied as a reductant to further elucidate reaction steps that mediate redox turnovers with N2O over MIL-100. Transient, steady-state, and isotopic kinetic analyses provide novel insight into levers for tuning the kinetic relevance of specific reduction and oxidation elementary reaction steps through second-sphere coordination effects and identity of the active metal.