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Amorphous mesoporous silica–aluminas with different textural properties are prepared by the sol–gel procedure and compared in the “in situ” reaction of methanol by infrared (FT-IR) spectroscopy. Depending on the gelling agent, silica–aluminas with porosity centred in micro and/or mesopores region have been obtained. Acid site characterisation, by IR adsorption/desorption of pyridine and volumetric adsorption of ammonia, shows the low capacity of these materials to interact with probes. Few strong and medium strength acid sites are present. The density of the OH groups and the Al content influence the acidity (density and strength). The nature and distribution of hydroxyls are evaluated by IR spectroscopy in the OH region after treatments at increasing temperature. After treatment at 100°C, these samples show a large amount of silanols (involved or not in H-bonding), decreasing sharply as the temperature of treatment is increased. All materials bond methanol (600–700 μmol/g) and the formed CH 3 O–Si groups are stable during evacuation up to 400°C. The strength of these groups is determined from the shift of the hydroxyls IR signals after methanol interaction. Though no direct bonding between methanol and AlOH groups is monitored, the presence of Al increases the strength of some hydroxyls. |
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