Microwave-assisted impregnation of highly dispersed Mo over HZSM-5 using various Mo precursors for methane dehydroaromatization

Abstract

The catalytic activity of Mo-doped HZSM-5 in methane dehydroaromatization (MDA) significantly relies on the state/distribution of the initially dispersed Mo-oxide on the zeolite surface, which play an indispensable role in the evolution of the active phase and selectivity toward the desired product. In this scenario, two different forms of Mo oxide precursors (α-MoO₃ and h-MoO₃) were synthesized to study their effect on the catalytic activity toward MDA. To improve the dispersibility, microwave (MW)-assisted impregnation of various Mo precursors over the HZSM-5 support was employed, and the important features of MW-impregnated Mo-containing HZSM-5 catalysts were explored in depth using microscopic/spectroscopic tools. MDA catalytic test showed that the catalyst prepared by the MW-impregnated 4 M HNO₃-treated metastable h-MoO₃ on zeolite support (MoH2-4M) resulted in a higher benzene formation rate (230 nmol g_cat⁻¹ s⁻¹) relative to the conventional Mo/HZSM-5-P (90 nmol g_cat⁻¹ s⁻¹). The superior catalytic activity of the MoH2-4M catalyst is ascribed to the incorporation of highly dispersed initial Mo-oxide species via MW impregnation, which drove part of the Mo-oxide species in the interior of the framework and thereby provided more active Mo species. Further, the spent MoH2-4M catalyst showed enhanced resistance in the formation of polyaromatics/hard coke that is easy to burn off.