Coke relocation and Mo immobilization in donut-shaped Mo/HZSM-5 catalysts for methane dehydroaromatization

Abstract

Molybdenum-modified HZSM-5 catalysts are widely used for methane dehydroaromatization (MDA) but suffer from rapid deactivation due to coke formation. The limited diffusion in zeolitic catalysts promotes coke accumulation in micropores, creating concentration gradients of reactants and products within the crystals. In this study, we propose a synthesis approach to evaluate the impact of removing the crystal core in the Mo/HZSM-5 catalyst on the MDA performance. To achieve this, a parent zeolite (Z_P) comprising a silicalite-1 core encased by an HZSM-5 crystal is subjected to two distinct treatments: fluorine (Z_F) and alkaline (Z_OH). These treatments dissolve the silicon-rich core, creating a central macropore in the crystals leading to the formation of donut-shaped catalysts. The alkaline treatment further generates additional mesopores and silanols by leaching atoms from the zeolite framework. The MDA reaction on molybdenum-impregnated zeolite catalysts (Z_P, Z_F, and Z_OH) at 973 K was carried out. Interestingly, the “donut”-shaped catalysts modify the nature of the coke deposition, relocating the coke from the micropores to the macropores. Consequently, the removal of the crystal core effectively mitigates catalyst deactivation caused by coke formation. Moreover, the alkaline-treated catalyst exhibits additional beneficial properties, such as increased silanols and a higher surface area, which help to limit the sintering of molybdenum carbides during the MDA reaction. The new strategy improves the performance and stability of Mo/HZSM-5 catalysts in methane dehydroaromatization.