Investigation of catalyst regeneration behavior in ethane dehydrogenation over Co@MFI catalysts

Abstract

Non-noble metal Co-based zeolite catalysts have been extensively investigation for the non-oxidative dehydrogenation of light alkanes to olefins, owing to their excellent activity and low cost. However, few systematic studies have reported the deactivation behavior and mechanisms of Co-based zeolite catalysts during multiple "reaction-regeneration" cycles. This work synthesized 6Co@MFI catalysts via hydrothermal preparation and their acid-washed 6Co@MFI-aw catalysts, systematically investigating the effects of two distinct initial Co species states on catalyst structural evolution and deactivation behavior during repeated "reaction-regeneration" cycles in the ethane dehydrogenation reaction (EDH). The findings reveal that in the initial reaction stage, unstable, ultra-small Co clusters in the pristine 6Co@MFI catalyst readily migrate and agglomerate, sintering into large Co nanoparticles (NPs), concurrently promoting coke deposition. In contrast, acid-washed 6Co@MFI-aw exhibits atomically dispersed Co species in a -Co δ+ -O δ--structure, maintaining stability during reactions while significantly reducing carbon deposition. During the subsequent air-regeneration stage following the initial reaction, the abundant Co NPs on the surface of the 6Co@MFI catalyze the violent combustion of carbon deposition, inducing localized temperature spikes exceeding 40 °C in the catalyst bed. This thermal runaway leads to zeolite framework collapse, severe Co leaching, and further sintering/growth of Co NPs, resulting in irreversible structural damage and unrecoverable activity. In contrast, the 6Co@MFI-aw catalyst, with its highly dispersed and stable Co species that resist NP formation, exhibits a much milder temperature rise during combustion of carbon deposition, thereby the zeolite structure remains complete after regeneration. When subjected to subsequent EDH cycles, the 6Co@MFI catalyst, now containing large Co NPs and a compromised structure, deactivates rapidly. Conversely, the 6Co@MFI-aw catalyst demonstrates remarkable stability over extended testing, maintaining performance throughout 1000 h of repeated reaction-regeneration cycles. Notably, its deactivation rate constant after the third regeneration was as low as 0.0042 h -1 . This work provides fundamental insights into how the initial state of Co species governs deactivation, regeneration ability, and long-term stability in Co-zeolite catalysts for EDH.