Manipulating the Cobalt Species States to Break the Conversion–Selectivity Trade-Off Relationship for Stable Ethane Dehydrogenation over Ligand-Free-Synthesized Co@MFI Catalysts

Abstract

Nonoxidative dehydrogenation of low-cost alkanes provides a promising route to produce valuable olefins. Herein, we hydrothermally synthesized various non-noble-metal-based, environment-friendly, and Al-free Co@MFI catalysts without the assistance of any additional coordination agents. The Co2+ species were successfully incorporated into well-crystallized MFI to form a stable and atomically dispersed −Coδ+–Oδ−– structure. The Co@MFI catalyst could show a stably high activity for ethane dehydrogenation with equilibrium-approached conversions at 600 °C and at the same time gave an extremely high selectivity to ethylene (∼99%), which is owed to the relatively unreducible −Co–O– species and its appropriate chemical state at the right reaction-temperature window. However, the Co@MFI catalyst showed equilibrium-deviated conversions at lower temperatures (such as 550 °C) and a suppressed activity in the H2O or CO2 co-feeding tests. Then, with characterizations, density functional theory calculations, and abundant experiments over different catalysts, including impregnated Co/MFI and amorphous Co@MFI, this study has impressively demonstrated that the chemical state of Coδ+ species manipulates the conversion–selectivity trade-off relationship in the conversion of alkanes. It is suggested that Co with a lower valence like Co0 promotes both C–C and C–H bond scissions of alkanes into coke and CH4, while Co with a higher one shows a decreased activity or even inactivity for alkane dehydrogenation. In this study, the possible causes for the success in the synthesis of the ligand-unassisted Co@MFI catalyst, the catalyst deactivation modes and the strategies for improving catalyst stability were also demonstrated in detail. This work not only contributes a performance-advanced Co-based catalyst for alkane dehydrogenation but also provides new insights into incorporation of a metal into zeolites.