Insights into the rates and kinetic relevance of steps mediating redox turnovers are often limited in clarity and rigor by the heterogeneity of site speciation that is a characteristic feature of most traditional synthetic heterogenous catalytic systems. Metal-organic framework materials (MOFs) offer an opportunity to investigate catalytic properties of well-defined multinuclear sites, the structure of which persist upon subjection to gas phase turnovers. MIL-100 is a MOF material endowed with oxo-bridged trimeric nodes that each carry one M2+ and two M3+ sites that effectuate a variety of acid-catalyzed and redox turnovers. In this study, we elucidate the density and identity of active sites, reaction kinetics, and mechanism of CO oxidation over mixed-valent oxo-bridged trimers. Steady state and transient kinetics experiments evidence steps constituting the oxidation half cycle to be kinetically relevant, with differences in activation energy reflecting standard state free energy differences between oxo-formation transition states and the bare Fe2+ surface. We show that CO oxidation turnover frequencies and apparent activation energies can be systematically tuned through manipulation of second sphere coordination environment, with both rate parameters decreasing monotonically with increasing average trimer electron density (Figure 1). The average electron density of the trimer was varied either by changing the coverage of a specific oxygenate, or by modifying the identity of the oxygenate at constant oxygenate coverage through the use of larger alkyl chain lengths. Overall, the results provide an unprecedented level of clarity as to the effect of second sphere coordination environment on the rates of redox reactions occurring over multinuclear clusters, and point to the utility of MOF materials in addressing questions hitherto rendered intractable by heterogeneity in active site speciation.

On second sphere coordination effects, and the limits of uniformity in heterogeneous redox catalysis

ACS Spring 2022

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