In-situ characterisation of the surface intermediates for the ethanol dehydration reaction over γ-alumina under dynamic conditions

The dynamic behavior of the ethanol adsorption on gamma-alumina were investigated at 180 and 200 degrees C by the transient-response method coupled with FT-IR data of the catalyst surface. The existence of three adsorbates was demonstrated: a reacting species which is the precursor for the formation of the gas-phase ethene; an inhibiting species responsible for the low steady-state reaction rate; and a spectator species accumulating on the catalyst surface. Their infrared spectra indicate an ethoxide-like structure for the three adsorbates. Their C-H stretching bands can be depicted by four Lorentzians whose parameters indicate different surface environments. The surface concentration of the reacting species was determined on the basis of the transient ethene response. A value of 0.77 +/- 0.07 mol/kg(cat) was found at 180 degrees C. The surface concentration of the spectator species was determined by ex-situ thermogravimetric experiments. A value of 0.13 +/- 0.01 mol/kg(cat) was found at 180 degrees C. The most likely structure of this species corresponded to an ethanol molecule coordinated in a bidentate manner on Al3+ cations, with stabilisation of the alcoholic-hydroxyl group via lateral hydrogen bonding with an adjacent surface hydroxyl. (C) 1998 Elsevier Science B.V. All rights reserved.