Oxidative dehydrogenation of ethane over Cr/ZSM-5 catalysts using CO₂ as an oxidant

Highly active catalysts for oxidative dehydrogenation of ethane with CO₂ were characterized by temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), Fourier transform infrared (FTIR) spectroscopy, and X-ray absorption fine-structure (XAFS) analysis. In the active catalysts, Cr/H-ZSM-5 (SiO₂/ Al₂O₃ > 190), Cr⁶⁺ = O, or possibly Cr⁵⁺ = O was the catalytic species on the zeolite support. In contrast, little Cr⁶⁺ (or Cr⁵⁺) was detected in the less-active catalysts. The Cr⁶⁺ (or Cr⁵⁺) species was reduced to an octahedral Cr³⁺ species by treatment with ethane at 773 K, and the reduced Cr species was reoxidized to the Cr⁶⁺ (or Cr⁵⁺) species by treatment with CO₂ at 673-773 K. The Cr redox cycle played an important role in the catalyst's high activity.