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

This paper studies the surface properties of three systems based on imogolite, as it concerns the type, acidic strength, accessibility and stability of hydroxyl groups. Proper imogolite presents a peculiar nanotube structure with remarkable surface features being the outer surface covered by Al(OH)Al groups and the inner surface lined by abundant, but presumably non-interacting, triple bond; length of mdashSiOH groups. As a hydrated alumino-silicate, the behaviour of imogolite is very sensitive to thermal pre-treatments. Molecular water is removed only at 300 °C: this makes inner silanols accessible to probes such as CO, ammonia, methanol and, partially, to phenol. Silanols are as acidic as those of amorphous silica, but their abundance enhances the interaction with adsorbed molecules: on the one hand, ammonia is observed to form ammonium species, and on the other hand, diffusion is difficult. At temperatures around 500 °C, the nanotube structure is lost with the formation of a lamellar phase showing new stronger acidic properties, resembling those of amorphous alumino-silicates with high Al content. An amorphous precursor phase with the same chemical composition (proto-imogolite) is also available, and has been studied as were the other forms of imogolite. Catalytic tests of gas-phase phenol reaction with methanol showed that the aromatic compound has a limited access to inner silanols at high temperature, whereas methanol interacts with both inner silanols and outer Al(OH)Al groups. Proto-imogolite shows a remarkable acid-type behaviour in phenol methylation, which, however, rapidly declines because of deactivation phenomena. A peculiar reactivity is shown by the lamellar phase: its amphoteric character, combining both basic and acidic Lewis sites, leads to a high regio-selective behaviour, and to the preferred formation of o-cresol.