Active species and fundamentals of their creation in Co-containing catalysts for efficient propane dehydrogenation to propylene

Although Co-containing catalysts show attractive performance in the non-oxidative propane dehydrogenation to propylene (PDH), their tailored development is hindered by the ambiguity in the kind of active sites (Co⁰ or Co²⁺O_x) responsible for selective and unselective pathways. Herein, we demonstrate that supported CoO_x species expends its lattice oxygen through oxidation of C₃H₈ to CO₂/C₃H₆ and are accordingly transformed into Co⁰ species. The latter are decisive for the selective dehydrogenation of propane to propylene with high activity. The ability of CoO_x to in situ form Co⁰ is affected by metal-oxide-support interactions (MOSI). In addition to the kind of support, defective OH groups (OH nests) in ZSM-5-type zeolites seem to be relevant for the MOSI effect. In comparison with previous studies highlighting the importance of the size of CoO_x species and their acidity for coke formation, we could not establish any direct correlation. Nevertheless, the presence of CoO_x seems to be relevant for this undesired reaction because the formation of coke is hindered when CoO_x species are transformed to Co⁰ species. Catalyst acidic sites are responsible for cracking reactions. Thus, the derived mechanistic insights reveal the nature of active sites both for selective and unselective pathways and provide fundamentals for catalyst development. From an industrial viewpoint, the developed Co/Silicalite-1 (Co/S-1) catalyst with the weakest MOSI and accordingly highest fraction of Co⁰ showed the space time yield of propylene formation of 1.50 kg·h⁻¹·kg_cat⁻¹ at 67 % equilibrium propane conversion and propylene selectivity of 95.3 % and the Co-related TOF value of 0.145 s⁻¹ at 550 °C. These both activity values exceed those of the state-of-the-art Co-containing catalysts. The productivity of Co/S-1 is comparable or even higher than that of industrially relevant Pt- or Cr-containing catalysts.