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

Abstract

Non-noble metal Co-based zeolite catalysts have been extensively investigated 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 (MFI-type zeolite) catalysts via hydrothermal preparation and their acid-washed 6Co@MFI-aw catalysts, and systematically investigated 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 catalyst catalyze the violent combustion of carbon deposits, 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 and 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 deposits, 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 the deactivation, regeneration ability, and long-term stability of Co–zeolite catalysts for EDH.