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

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
MoOx 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.