Ammoxidation of Ethane to Acetonitrile and Ethylene: Reaction Transient Analysis for the Co/HZSM-5 Catalyst

Ethane ammoxidation to acetonitrile and ethylene over the Co/HZSM-5 catalysts was revisited based on both transient and steady-state performance evaluation to elucidate the structure/reactivity relationships. We suggested that the exchanged Co²⁺ cation encapsulated in the zeolite favors the formation of acetonitrile and ethylene, whereas nanosized cobalt oxide particles without close proximity with the HZSM-5 only favor CO₂ formation. Excess Brønsted acid sites of the zeolites may act as a reservoir for NH₃, which inhibits the CO₂ formation through the NH₃-mediated oxidative dehydrogenation mechanism. According to the transient kinetic analysis, the time constants τ from the back-transient decay for NH₃ and CO₂ are both 7.7 min, which decreased to 2.7 min for acetonitrile and further decreased to 3-4 s for ethane, ethylene, and O₂. Assuming first-order reaction kinetics, the rate constants for the formation of acetonitrile and CO₂ are 0.37 and 0.13 min^-1, respectively, from their corresponding reactive intermediates.