Thermal Collapse of Single-Walled Alumino-Silicate Nanotubes: Transformation Mechanisms and Morphology of the Resulting Lamellar Phases
Résumé
The thermally induced structural transformations are studied of three imogolite-type nanotube (NT) materials: (i) proper imogolite (IMO, (OH)(3)Al2O3SiOH) with outer surface covered by Al-OH-Al groups and inner surface lined by silanols); (ii) methyl-imogolite (Me-IMO, (OH)(3)Al2O3SiCH3), in which the inner surface silanols have been replaced by methyl groups while the outer surface is unchanged, and (iii) the material Me-IMO-NH2, obtained by grafting the outer surface of Me-IMO with 3-aminopropylsilane (3-APS). TG-MS analysis on the parent IMC shows only loss of water (up to ca. 700 K), whereas XRD indicates the formation of a lamellar phase because of the mutual reaction of inner silanols. With both Me-IMO and Me-IMO-NH2, mass spectrometry and NMR analysis reveal the occurrence of a more complex collapsing mechanism, basically due to the reaction of outer Al-OH groups and inner Si-CH3, following the cleavage of the NTs structure, yielding methane and transient Al-O-CH2-Si species. All three materials show a limited decrease in the interlayer distance caused by collapse as well as a substantial residual porosity. It is concluded that the layered structure can be conceived as consisting of an overall buckled structure, the strong strain within the silico-alumina layer of the single-walled NT providing the driving force against a complete flattening. As a minor feature, decomposition of perchlorate species to chloride anions with the release of molecular oxygen is obse:ved with IMO species that are trapped during the synthesis at the narrow interpores cavities.