Oxygen and Nitrous Oxide as Oxidants: Implications for Ethane Oxidative Dehydrogenation over Silica−Titania-Supported Molybdenum

A 10% Mo/Si−Ti catalyst was tested for ethane oxidative dehydrogenation (ODH) activity using either O 2 or N 2 O as the oxidant. Ethane ODH activity was tested at contact times varying from 0.46 to 1.0 mg min/cm −3 . N 2 O gives superior ethylene selectivities at a given ethane conversion outperforming oxygen at all contact times tested. Ethylene selectivities decrease with contact time at a far slower rate when using N 2 O as the oxidant. X-ray photoelectron spectroscopy (XPS) experiments demonstrate that molybdenum in the catalyst is fully oxidized to Mo(VI) during ethane ODH using O 2, whereas the reduction state decreases to an average of +5.8 when using N 2 O as the oxidant. Temperature-programmed oxidation experiments of pre-reduced 10% Mo/Si−Ti were carried out at different temperature ramp rates. The activation energy of re-oxidation when using N 2 O is 98 kJ/mol, whereas that of O 2 is 41 kJ/mol leading to a re-oxidation rate at least 1700 times faster when O 2 is the oxidant. This difference in rates accounts for the less oxidized state of molybdenum during ethane ODH with N 2 O and explains the behavior observed during reaction experiments.