(569df) High-Throughput Screening of the Second Coordination Sphere Effects in the MIL-100 Metal-Organic Framework for Methane Partial Oxidation

For single-site heterogeneous catalysts (SSHCs), the active-site consists of small metal clusters anchored to a solid support, thus generating unique ligand environments more commonly found in homogeneous catalysts. These ligand environments exhibit dynamic changes in composition and structure due to the exposed reaction conditions, with these changes defining and/or altering the catalytic performance of these materials. Presently, the rational design of SSHCs requires further insights into the relationship between the ligand environment and catalytic performance. One such SSHC is the metal node of the MIL-100 porous metal-organic framework (MOF), where divalent sites in tri-metal oxo centered (i.e., Fe(II)M(III)M(III)-O) node partially oxidizes methane into methanol. Prior work demonstrates how residual H₂O ligands anchored to the Fe(III) sites within MIL-100(Fe) indirectly alters the chemical reactivity of the Fe(II) site. With increasing H₂O content, a decrease in reactivity was observed experimentally and explained using quantum chemical calculations as the oxo formation energy decreasing (less stable) and the oxo formation transition state barrier increasing. Herein, these findings are extended to other potential residual ligands at the M(III) sites to understand the influence these ligands have on the Fe(II) site for methane partial oxidation. A structural library of different ligand environments is constructed from six different isostructural MOF variants and eight different ligands (e.g., H₂O, N₂O, CH₃OH, etc.). Using a high-throughput screening approach, the oxo formation, hydrogen atomic transfer (HAT), and methanol release energies are calculated using density functional theory (DFT). Molecular modeling demonstrates the sensitivity of the three key reaction steps as a function of the ligand environment of MIL-100(Fe), with the oxo formation energy exhibiting the largest sensitivity. These findings demonstrate the sensitive relationship between the ligand environment and catalytic performance of MOF-based SSHCs, and highlight the potential of tuning methane partial oxidation in MOFs through these indirect second sphere coordination effects.