Stability of Fe- and Zn-Promoted Mo/ZSM-5 Catalysts for Ethane Dehydroaromatization in Cyclic Operation Mode

The stability of Fe- and Zn-promoted Mo/ZSM-5 catalysts was studied toward the ethane dehydroaromatization reaction. To elucidate the catalyst deactivation and regeneration mechanism, the catalytic performance was evaluated during five consecutive reaction–regeneration cycles. The addition of Zn to Mo/ZSM-5 resulted in an initial increase in aromatic selectivity; however, the loss of about 50% Zn resulted in a total decrease in aromatic selectivity over five reaction and regeneration cycles. The addition of Fe to Mo/ZSM-5 resulted in no decrease in aromatic selectivity or aromatic yield throughout these reaction cycles. The formation of carbon nanotubes was observed on the Fe-promoted Mo/ZSM-5 catalyst, which was believed to improve gas diffusion to micropores. The presence of Fe- and Mo-agglomerated particles on the catalyst surface was observed at low Fe/Mo atomic ratios, resulting in the formation of base-grown carbon nanotubes. When the Fe/Mo atomic ratio increased, the agglomerated particles were able to leave the zeolite surface, resulting in the formation of tip-growth carbon nanotubes. The temperature-programmed reduction profile of the Fe-promoted Mo/ZSM-5 catalyst also suggested the formation of a more stable Mo and/or Fe species at 615 °C, enhancing carbon nanotube formation. The addition of Fe in the bipromoted Mo/ZSM5 catalyst was found to stabilize the catalyst surface and reduce its Zn loss.