Rational Tailoring of Metal Precursor Interactions with the Zeolite Support in the Mo/HZSM-5 Catalyst for Methane Dehydroaromatization

Abstract

Two different forms of molybdenum crystals (α-MoO 3 and h-MoO 3 ) are synthesized with an aim to study their role in dictating the catalytic activity of the Mo/HZSM-5 catalyst in the methane dehydroaromatization (MDA) reaction. The catalyst prepared from impregnating the metastable hexagonal crystal form (h-MoO 3 ) on the zeolite support resulted in a higher benzene formation rate (390 nmol/g cat •s) and reduced coke (1.2 wt %) as compared to α-MoO 3 /HZSM-5 (228 nmol/g cat •s and 2.7 wt %, respectively). Insights into the metal precursor interactions with the zeolite surface are obtained using molecular dynamics (MD) and density functional theory (DFT) simulations. MD simulations revealed a greater interaction of the h-MoO 3 crystal structure with the zeolite surface, while DFT calculations indicated a facile anchoring of the molybdenum oxide species derived from h-MoO 3 crystals at the Brønsted acid sites. Following this, the reason for the high activity of h-MoO 3 /HZSM-5 is attributed to the metal precursor interaction with the zeolite during the impregnation process, which resulted in the formation of desired active molybdenum carbide species, as evidenced in temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) experiments.