Alkaline-earth metals promote propane dehydrogenation with carbon dioxide through geometric effects: Altering the reaction pathway

The synergistic activation of carbon dioxide (CO2) and propane (C3H8) in CO2-assisted C3H8 dehydrogenation (CO2-ODH) poses a significant challenge. While alkali and alkaline-earth metals are known to enhance reactant activation, the precise mechanism by which they influence the reaction pathway remains unclear. This study investigated the role of alkaline-earth magnesium (Mg) in the cobalt-based silicalite-1 catalyst (Co/MgS-1). X-ray absorption spectroscopy (XAS), in-situ Fourier transform infrared spectrometer (FTIR) spectra, Raman spectroscopy, and high-resolution transmission electron microscopy (HR-TEM) revealed that ultrasmall Co-O aggregate ((CoO)x) was anchored to ion exchange sites of skeletal Mg2 +, forming a Mg-(O-Co)x structure. Activity tests combined with isotope tracing demonstrated the Co/MgS-1 catalyst had a higher C3H8 conversion rate and converted CO2 into ethane. In-situ DRIFT and DFT calculations clarified the reaction pathway: the formation of seven-membered cyclic intermediates increased C--O bond stress, facilitating the cleavage of CO2 into O* and CO* . The Co-O* active site promoted the cleavage of the C-H bond, as confirmed by kinetic studies, followed by the continuous hydrogenation and dehydration of CO* to ethane. These findings elucidated the crucial role of Mg in modifying the geometry of the active site and thus influencing the reaction pathway, leading to improved efficiency for propane dehydrogenation coupled with CO2 utilization.