IN-SITU CHARACTERIZATION OF THE SURFACE INTERMEDIATES FOR THE ETHANOLDEHYDRATION REACTION OVER GAMMA-ALUMINA UNDER DYNAMIC CONDITIONS

The dynamic behavior of the ethanol adsorption on gamma-alumina were i nvestigated at 180 and 200 degrees C by the transient-response method coupled with FT-IR data of the catalyst surface. The existence of thre e adsorbates was demonstrated: a reacting species which is the precurs or for the formation of the gas-phase ethene; an inhibiting species re sponsible for the low steady-state reaction rate; and a spectator spec ies accumulating on the catalyst surface. Their infrared spectra indic ate an ethoxide-like structure for the three adsorbates. Their C-H str etching bands can be depicted by four Lorentzians whose parameters ind icate 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 degree s C. The surface concentration of the spectator species was determined by ex-situ thermogravimetric experiments. A value of 0.13 +/- 0.01 mo l/kg(cat) was found at 180 degrees C. The most likely structure of thi s species corresponded to an ethanol molecule coordinated in a bidenta te manner on Al3+ cations, with stabilisation of the alcoholic-hydroxy l group via lateral hydrogen bonding with an adjacent surface hydroxyl . (C) 1998 Elsevier Science B.V. All rights reserved.