Ce<sub><i>x</i></sub>Zr<sub>1–<i>x</i></sub>O<sub>2</sub>-Supported CrO<sub><i>x</i></sub> Catalysts for CO<sub>2</sub>-Assisted Oxidative Dehydrogenation of Propane─Probing the Active Sites and Strategies for Enhanced Stability
Jian Dou, Joey Funderburg, Kunran Yang, Junchen Liu, Dennis Chacko, Kui Zhang, Adam Harvey, Vasudev Haribal, S. James. Zhou, Fanxing Li
Abstract
CO2-assisted oxidative dehydrogenation of propane (CO2-ODH) represents an attractive approach for propylene production and CO2 utilization. As a soft oxidant, CO2 can minimize overoxidation of the hydrocarbons to enhance the propylene selectivity while increasing the equilibrium yield. However, a major challenge of CO2-ODH is the rapid deactivation of the catalysts. The current study focuses on designing CexZr1–xO2-mixed oxide-supported CrOx catalysts for CO2-ODH with enhanced product selectivity and catalyst stability. By doping 0–30% Ce in the CexZr1–xO2 mixed oxide support, propane conversion of 53–79% was achieved at 600 °C, with propylene selectivity up to 82%. Compared to the pure ZrO2-supported catalyst (i.e., 5 wt %Cr/ZrO2), 20–30 %Ce doped catalysts (i.e., 5 wt %Cr/Ce0.2Zr0.8O2 and 5 wt %Cr/Ce0.3Zr0.7O2) inhibited the formation of CH4 and ethylene and improved propylene selectivity from 57 to 77–82%. Detailed characterizations of the 5%Cr/Ce0.2Zr0.8O2 catalyst and density functional theory (DFT) calculations indicated that Cr3+ is the active species during the CO2-ODH reaction, and the reaction follows a non-redox dehydrogenation pathway. Coke formation was determined to be the primary reason for catalyst deactivation, and the addition of Ce to the ZrO2 support greatly enhanced the coke resistance, leading to superior stability. Coke removal by oxidizing the catalyst in air is effective in restoring its activity.