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-S catalyst also suggested the formation of a more stable Mo and/or Fe species at 615 degrees C, enhancing carbon nanotube formation. The addition of Fe in the bipromoted Mo/ZSMS catalyst was found to stabilize the catalyst surface and reduce its Zn loss.