Understanding and Breaking Scaling Relations in Single-Site Catalysis: Methane to Methanol Conversion by Fe^IV═O

Abstract

Computational high-throughput screening is an essential tool for catalyst design, limited primarily by the efficiency with which accurate predictions can be made. In bulk heterogeneous catalysis, linear free energy relationships (LFERs) have been extensively developed to relate elementary step activation energies, and thus overall catalytic activity, back to the adsorption energies of key intermediates, dramatically reducing the computational cost of screening. The applicability of these LFERs to single-site catalysts remains unclear, owing to the directional, covalent metal-ligand bonds and the broader chemical space of accessible ligand scaffolds. Through a computational screen of nearly 500 model Fe(II) complexes for CH 4 hydroxylation, we observe that 1) tuning ligand field strength yields LFERs by comparably shifting energetics of the metal 3d levels that govern stability of different intermediates and 2) distortion of the metal coordination geometry breaks these LFERs by increasing the splitting between the d xz /d yz and d z 2 metal states that govern reactivity. Thus, in single site catalysts, low Brønsted-Evans-Polanyi slopes for oxo formation, which would limit peak turnover frequency achievable through ligand field tuning alone, can be overcome through structural distortions achievable in experimentally characterized compounds. Observations from this screen also motivate the placement of strong HB donors in targeted positions as a scaffold-agnostic strategy for further activity improvement. More generally, our findings motivate broader variation of coordination geometries in reactivity studies with single-site catalysts.

Identifying promising metal–organic frameworks for heterogeneous catalysis via high‐throughput periodic density functional theory

“…Motivated by similar schemes for computational catalysis screening of bulk metals and alloys, the general approach for HT‐DFT of MOF catalysts can be outlined as follows: Determine a set of catalytic descriptors that can be used to correlate catalytic activity with readily computed quantities, such as those based on adsorption energies, reaction energies, or electronic structure properties Identify or construct a dataset of MOF crystal structures to study.…”

“…In contrast with the C—H bond activation step, there is currently no reported universal TS scaling relationship for the step in which the metal site is oxidized. Instead, we consider the extrinsic oxidation reaction energy using N 2 O as the proposed oxidant, defined as to determine the thermodynamic favorability of oxidation, as has been done in prior work . Here, E MOF , , and are the electronic energies of the bare MOF (i.e., the reduced state), gas‐phase N 2 , and gas‐phase N 2 O, respectively.…”

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…However, a recent computational study by Gani and Kulik who considered about 500 model Fe 2+ compounds for methane oxidation to methanol with N 2 O oxidant reveal important limitations of such the predictive models based on the linear scaling relationship approximation . It was shown that the linear relationship for single site catalyst screening holds only when the geometric parameters of iron are fixed.…”

Homogenous Meets Heterogenous and Electro‐Catalysis: Iron‐Nitrogen Molecular Complexes within Carbon Materials for Catalytic Applications

“…were able to model Fe(II) single‐site catalyst (though not FeN x ) for methane hydroxylation, demonstrating that i) the tuning of the ligand field strength yields LFERs and ii) scaffold distortion not only breaks strong linear scaling relations arising from ligand field strength tuning but also alters BEP relations for oxo formation (Figure ). We believe that such approach could be very helpful in rationalising reaction mechanisms on FeN x single‐site catalysts in the future …”

“… Understanding and Breaking Scaling Relations in Single‐Site Catalysis: Methane to Methanol Conversion by Fe IV =O. Reproduced with permission from . Copyright 2018 American Chemical Society.…”