Evolution, Speciation, and Distribution of Mo Oxides in MFI-Type Zeolites

Abstract

Mo oxide-impregnated H-ZSM-5, the most extensively studied catalyst for methane dehydroaromatization (MDA), is typically prepared by impregnating Mo precursors into H-ZSM-5 zeolite through physical mixing or incipient wetness impregnation. To understand the transformation of Mo oxide/(H−)ZSM-5 during catalyst preparation and the anchoring stages of MDA, we employ density functional theory (DFT) calculations to investigate changes in the electronic structure, composition, and location of Mo oxide in H-ZSM-5. Using the climbing image nudged elastic band (CI-NEB) method, we explore the anchoring and formation processes of various MoO_x motifs within H-ZSM-5 and provide mechanistic insights into the interconversion between these structural motifs. We also develop a statistical model informed by the kinetics of Mo oxide anchoring and formation, predicting the distribution of Mo oxide catalyst precursors as a function of the synthesis method, zeolite acidity, and Mo loading. Additionally, by employing temperature-programmed calcination under oxidative conditions, we monitor the transformation of Mo oxides by measuring the water released during anchoring. These experimental results are correlated with the modeling predictions, providing insights into the molecular processes during catalyst preparation and how we can rationally control the design of Mo oxides in MFI-type zeolites.