Breaking Linear Scaling Relationships with SecondaryInteractions in Confined Space: A Case Study of Methane Oxidationby Fe/ZSM‑5 Zeolite
2019
Linear
energy scaling laws connect the kinetic and thermodynamic
parameters of key elementary steps for heterogeneously catalyzed reactions
over defined active sites on open surfaces. Such scaling laws provide
a framework for a rapid computational activity screening of families
of catalysts, but they also effectively impose a fundamental limit
on the theoretically attainable activity. Understanding the limits
of applicability of the linear scaling laws is therefore crucial for
the development of predictive models in catalysis. In this work, we
computationally investigate the role of secondary effects of the active
site environment on the reactivity of defined Fe complexes in ZSM-5
zeolite toward methane oxofunctionalization. The computed C–H
activation barriers over Fe-sites at different locations inside the
zeolite pores generally follow the associated reaction enthalpies
and the hydrogen affinities of the active site, reflecting the O–H
bond strength. Nevertheless, despite the close similarity of the geometries
and intrinsic reactivities of the considered active complexes, substantial
deviations from these linear scaling relations are apparent from the
DFT calculations. We identify three major factors behind these deviations,
namely, (1) confinement effects due to the zeolite micropores, (2)
coordinative flexibility, and (3) multifunctionality of the active
site. The latter two phenomena impact the mechanism of the catalytic
reaction by providing a cooperative reaction channel for the substrate
activation or by enabling the stabilization of the intrazeolite complex
along the reaction path. These computational findings point to the
need for the formulation of multidimensional property–activity
relationships accounting for both the intrinsic chemistry of the reactive
ensembles and secondary effects due to their environmental and dynamic
characteristics.
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