Improving effect of Fe additive on the catalytic stability of Mo/HZSM-5 in the methane dehydroaromatization

The promotional effects of eight kinds of transition metal additives (0.5 wt%) on the activity and stability of 5 wt%Mo/HZSM-5 were screened at 1073 K and 10,000 mL/g/h in a continuous CH4 feed mode. Only Fe was observed to improve the activity stability. Subsequently, confirmation tests were performed with other three Fe-modified catalysts containing 0.3, 1.0 and 2.0 wt% of Fe in the same continuous operation mode and also with the 0.5 wt%Fe–5 wt%Mo/HZSM-5 under periodic CH4–H2 switch operation for 1650 min. It was confirmed that Fe indeed enhanced the stability of Mo/HZSM-5 catalyst. Then, TPO, TG and BET measurements and SEM observations were performed for almost all spent samples to gain insight into the promotion mechanism of Fe additive. The results revealed that over short time frames in the continuous CH4 feed mode the formation of Fe-induced carbon nanotubes is the origin of its promotional effect, while under the long-term periodic CH4–H2 switch operation the catalytic involvement of Fe in the surface coke removal during the H2 flow periods is the main cause of Fe-modified catalyst being much more stable than the unmodified Mo/HZSM-5 catalyst