(172c) DFT Study on the Catalytic Activity of Oxo-Centered Trimetallic MOF Building Units for Ethane Oxidation to Ethanol

The increasing production of natural gas in the United States has provided new incentives for selectively transforming molecules such as methane and ethane into more valuable chemicals through energy efficient and environmentally friendly processes. Metal-organic frameworks (MOFs) are tunable, attractive supports for well-defined, isolated active sites that can catalyze the conversion of light hydrocarbons found in natural gas. The open metal sites in some classes of MOFs make them promising catalysts for their Lewis-acid/base and redox properties and their constrained environments. The tunability of MOF nodes and linkers also gives us the opportunity to develop structure/activity relationships and gain insight into how to improve the catalytic performance for reactions that are relevant to natural gas conversion.

To examine the catalytic activity of oxo-centered trimetallic MOF nodes, we used density functional theory (DFT) to model the three main steps involved in the conversion of ethane to ethanol using N₂O as the oxidant: (1) N₂O activation, (2) C-H activation, and (3) ethanol desorption. Our results show that the N₂O activation step is the rate determining step for the reaction to proceed.
Analysis of the electron orbital occupation reveals that this energy barrier gets higher as the d-orbitals of the active metal become more populated. This indicates that the electrons in high-energy molecular orbitals are easier to oxidize and the oxygen atom in N₂O binds more strongly. Thus, we find that the oxygen binding strength and energy of the highest occupied molecular orbital (HOMO) describe well the catalytic activity of these clusters and are plausible descriptors that we can use to screen for a larger sample of catalysts.