Mechanism of Fe additive improving the activity stability of microzeolite-based Mo/HZSM-5 catalyst in non-oxidative methane dehydroaromatization at 1073 K under periodic CH4–H2 switching modes

Abstract

The catalytic stabilities of Fe-modified and -unmodified 5% Mo/HZSM-5 catalysts in non-oxidative methane dehydroaromatization were compared at 1073 K and three reaction/H₂-regeneration cycle periods: 5 min CH₄–5 min H₂, 5 min CH₄–10 min H₂, and 5 min CH₄–20 min H₂. Fe addition proved capable of remarkably increasing the catalyst stability over the cycles of 5 min CH₄–20 min H₂ but was hardly effective over the cycles of 5 min CH₄–5 min H₂. On the other hand, SEM observation of all spent samples revealed that Fe addition causes a massive accumulation of carbon nanotubes under the latter cyclic condition but little in the former. Thus several sets of comparative tests were specially designed and performed to gain an insight into the role of Fe-catalyzed cyclic formation of carbon nanotubes in stabilizing the activity under the cyclic condition of 5 min CH₄–20 min H₂. The results further confirmed that at this condition, cyclic formation of carbon nanotubes enhances cyclic evolution of H₂ and increases the H₂ concentration of the system, which is thermodynamically beneficial for suppression of formation of the activity-deactivating surface coke. Finally, it was further confirmed that at least a 20 min H₂ exposure is required to remove most of the carbon nanotubes and surface coke formed during a 5 min CH₄ exposure and reactivate most of the Fe nanoparticles and make them available again for formation of catalytic carbon nanotubes with an enhanced H₂ evolution, i.e., with a controlled formation of the surface coke in the next CH₄ exposure.