FT-IR and catalytic characterization of the acidity of imogolite-based systems

In this work, the surface properties of imogolite are studied, in terms of type, acidic strength, accessibility and stability of hydroxyls groups. Imogolite nanotubes present interesting surface features, in that the outer surface is covered by Al(OH)Al groups and an inner surface lined by isolated ≡SiOH groups. The knowledge of the acidic strength and of the actual accessibility, for example to reactants in a acidic-catalyzed reaction, is of paramount importance for any practical application. Being an hydrated alumino-silicate, the behaviour of imogolite surface resulted to be very sensitive to the pre-treatment temperature: experimental results showed that molecular water is removed only at 300°C, rendering inner silanols accessible to probes like CO, methanol and even phenol. Inner silanols are as acidic as those at the surface of amorphous silica, but the presence of plenty of them at the inner surface enhance the interaction with the adsorbed molecules, thus hindering diffusion. At higher pre-treatment temperatures (500°C), the nanotubes structure is lost, with formation of a lamellar phase, which, interestingly, shows new stronger acidic properties, resembling those of amorphous alumino-silicates with high Al content.