Ethanol dehydration on gamma-Al2O3: Effects of partial pressure and temperature

Ethanol dehydration was investigated using platelet gamma-Al2O3 over a wide range of reaction temperature (180-300 degrees C) and ethanol partial pressure (0.5-2 kPa) by X-ray diffraction, ethanol Temperature programmed desorption and reactions. The turnover frequencies for commercial and platelet gamma-Al2O3 were almost identical (1.2-1.3 x 10(-2) ethanol/site s) when normalized to the number of ethoxide quantified by ethanol TPD. The desorption barrier of ethoxide was 183.6 kj/mol, similar to the activation barrier of ethylene formation. These results demonstrate that ethoxide is a key intermediate rather than molecular ethanol, possibly suggesting an El mechanism for ethylene formation, consistent with recent spectroscopic studies. Detailed kinetic measurements demonstrate the nature of the species on alumina surface varied with reaction temperature. At low temperature (180 degrees C), the ethanol dimer, one of which would be the ethoxide, saturated the surface, leading to the inhibition of ethylene formation and constant ether formation rates with ethanol pressure. At high temperature (260 degrees C), the ethanol monomer became dominant, consistent with the constant ethylene formation rates and increased ether formation rates with ethanol pressure. The apparent activation energies also changed with reaction temperature and ethanol partial pressure. Especially, the inhibition by ethanol dimer clearly contributed the increased apparent activation barrier at 180 degrees C