Oxidative Dehydrogenation of Ethane to Ethylene over Potassium Tungstate Catalysts

This study elucidated the mechanism of catalytic oxidative dehydrogenation (ODH) of ethane over K₂WO₄/SiO₂ by using microkinetic analysis and simulation with gas-phase chemistry. The K₂WO₄/SiO₂ catalyst prepared with amorphous silica as starting material and cristobalite produced during 900 °C calcination exhibited better performance than catalysts prepared with crystallized silica (cristobalite). The kinetic analysis suggested that both C₂H₆ and O₂ show positive dependences and the reaction proceeds by C–H activation via surface O species (O*, likely peroxide species)-mediated H-abstraction at low conversion. Upon addition of H₂O in the reactant stream, the conversion rate drastically improved. Kinetic orders with respective to O₂ and H₂O pressure are consistent with OH radical formation through the reaction of O* species with H₂O, which would then activate the C–H of C₂H₆ molecules in the gas phase. The experimentally observed C₂H₆ conversion rate involved contributions from both O*-mediated (dry) and OH radical-mediated (wet) parallel pathways. By assuming that the main kinetic consequence of cofed H₂O was generating OH radicals, kinetic simulation illustrated the promotional effect of cofed H₂O on C₂H₆ activation while not affecting the product distribution, which matched well with experimentally observed trends.