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 N2O as the oxidant. Ethane ODH activity was tested at contact times varying from 0.46 to 1.0 mg min/cm(-3). N2O 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 N2O 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 N2O 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 N2O 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 N2O and explains the behavior observed during reaction experiments.