CoO_x/Silicalite-1 Tailored by Calcining Atmosphere for Oxidative Dehydrogenation of Propane With CO₂

The relatively richer reservoir and reasonable performance in activating C─H bonds make cobalt a very competitive catalyst for the oxidative dehydrogenation of propane with carbon dioxide (CO₂-ODP). However, the catalytic nature of different cobalt species is ambiguous, and the development of active and selective Co-based CO₂-ODP catalysts are at the very early stage. In this study, Co species on CoO_x/Silicalite-1 catalysts are regulated by calcining the catalyst precursor at 500°C under Ar, Air, and H₂ atmospheres, respectively. Characterization results reveal that the proportion of Co₃O₄ spinel, metallic Co, and tetrahedrally coordinated Co²⁺ species (Td-Co²⁺) is easily altered by changing the calcining atmosphere. Moreover, Td-Co²⁺ is revealed to be the active site for CO₂-ODP while Co⁰ is active for side reactions such as cracking and reforming. As a result of the richest Td-Co²⁺ and diminished Co⁰ over the CoO_x/Silicalite-1 catalyst calcined in Ar, the highest CO₂-ODP performance is achieved with a propylene space-time yield of 15.88 mmol·g_cat ^-1·h^-1. Besides acting as a mild oxidant, the additional functions of CO₂ are to retard the reduction of Td-Co²⁺ and to reduce the coke accumulation via the reverse Boudouard reaction during CO₂-ODP. These understandings are important guides for optimizing with more efficient Co-based CO₂-ODP catalyst.