USE OF METHANOL AS A PROBE TO STUDY THE ADSORPTION SITES OF DIFFERENT MGO SAMPLES

Methanol adsorption on activated MgO has been studied by gravimetry and FT-IR spectroscopy. It gives rise to three kinds of species: i) one reversibly adsorbed at room temperature (species H) coordinated to Mg2+ sites but also strongly H-bonded to unsaturated O2- sites; ii) two methoxy species characterized by a nu-(O-C) band at 1115 cm-1 (species I) and 1090 cm-1 (species II) resulting both from the dissociation in methanol of the O-H bond as shown by use of CH3(OH)-O-18. Considering that faces of the (100) type are preferentially exposed, species H and I are postulated to be formed on regular faces, while species II formation involves the presence of defects, such as Mg2+ cations in a low coordination state. Extension of the study to five MgO samples, the surface of which varies from 12 m2 g-1 (smoke) to 565 m2 g-1 (ex-carbonate), shows that the proportion of defects increases with the surface area, reaching more than 25 % of the total amount of methanol adsorption sites in the case of high surface area samples.