Unraveling the Origins of the Synergy Effect between ZrO₂ and CrO _x in Supported CrZrO _x for Propene Formation in Nonoxidative Propane Dehydrogenation

In this work, steady-state tests of propane dehydrogenation, density functional theory calculations, operando UV-vis spectroscopy, ex situ and in situ electron paramagnetic resonance spectroscopy, IR spectroscopy, and temperature-programmed techniques were combined to provide fundamentals for tuning activity and onstream stability of low-loaded catalysts with supported CrZrO_x species. Two neighboring Zr_cus (cus = coordinatively unsaturated) sites were concluded to be mainly responsible for propane dehydrogenation to propene. They are formed upon reductive catalyst treatment, and their concentration depends on the strength of interaction among CrO_x, ZrO₂, and support and on the size of ZrO₂ crystallites in CrZrO_x. SiO₂ weakly interacting with CrO_x was found to be a more preferable support than Al₂O₃- or TiO₂-based supports. CrO_x species promotes formation of Zr_cus sites and improves their intrinsic activity for the desired reaction. CrO_x also contributes to coke formation as concluded from operando UV-vis analysis. Cr20Zr80/SiO₂ possessing about 3.9 or 2.5 times lower amounts of chromium or zirconium in comparison with an analogue of industrial K-CrO_x/Al₂O₃ or state-of-the-art Ru/LaZrO_x revealed about 2 times higher space-time yield of propene at 30% propane conversion at 550 °C. Moreover, this catalyst was durable over 50 dehydrogenation/regeneration cycles lasting 150 h.