Optimizing Open Iron Sites in Metal–Organic Frameworks for Ethane Oxidation: A First-Principles Study

Abstract

Activation of the C-H bonds in ethane to form ethanol is a highly desirable, yet challenging, reaction. Metal-organic frameworks (MOFs) with open Fe sites are promising candidates for catalyzing this reaction. One advantage of MOFs is their modular construction from inorganic nodes and organic linkers, allowing for flexible design and detailed control of properties. In this work, we studied a series of single-metal atom Fe model systems with ligands that are commonly used as MOF linkers and tried to understand how one can design an optimal Fe catalyst. We found linear relationships between the binding enthalpy of oxygen to the Fe sites and common descriptors for catalytic reactions, such as the Fe 3d energy levels in different reaction intermediates. We further analyzed the three highest-barrier steps in the ethane oxidation cycle (including desorption of the product) with the Fe 3d energy levels. Volcano relationships are revealed with peaks toward higher Fe 3d energy and stronger electron-donating group functionalization of linkers. Furthermore, we found that the Fe 3d energy levels positively correlate with the electron-donating strength of functional groups on the linkers. Finally, we validated our hypotheses on larger models of MOF-74 iron sites. Compared with MOF-74, functionalizing the MOF-74 linkers with NH groups lowers the enthalpic barrier for the most endothermic step in the reaction cycle. Our findings provide insight for catalyst optimization and point out directions for future experimental efforts.

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.…”

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

“…Though strictly speaking not a materials characterisation method, increasingly sophisticated systems increasingly rely on supporting understanding of characterisation techniques by computational modelling. This is none the least relevant for catalytic reactions, including single‐site catalysts, an area that has undergone a tremendous amount of progress thanks to method development, increased computational power . Computationally assessing catalytic activity by relating it to descriptors instead of calculating the activation energies of elementary reaction steps in heterogeneous catalytic reactions has been significantly relying on screening linear free energy relationships (LFERs), or linear scaling relations .…”

“…This is none the least relevant for catalytic reactions, including single-site catalysts, an area that has undergone a tremendous amount of progress thanks to method development, increased computational power. [148][149][150][151][152][153][154][155][156][157][158][159][160] Computationally assessing catalytic activity by relating it to descriptors instead of calculating the activation energies of elementary reaction steps in heterogeneous catalytic reactions has been significantly relying on screening linear free energy relationships (LFERs), or linear scaling relations. [161][162][163][164][165][166][167][168][169] However, the direct transposition of LFER on the single-site systems is problematic as the simple models used for bulk metal systems are not satisfactory to describe directional metal-ligand bonds.…”

Metal–Organic Framework (MOF)‐Based Materials as Heterogeneous Catalysts for C−H Bond Activation

“…[4][5][6] In spite of continuous and intensee fforts in this area, the larger obstacles include the relativelys trong saturated hydrocarbons (bond dissociation energy of 431 kJ mol À1 for CH 4 )a nd the ubiquitous natureo f the CÀHb ond, which lead to the inertness of saturated CÀH bonds and selectivity issues. [7][8][9] These limitations greatlyr estrict the application of CÀHa ctivation in industry.T herefore, the development of efficient, stable, and selective catalysts for the activation of the inert apolar CÀH s bonds is supposed to be ad irect andf easible approacht oa ddress this question.I t, however,i ss till af ormidable task.…”