Thermal collapse of sigle-walled aluminosilicate nanotubes: transformation mechanisms and morphology of the resulting lamellar phases

The thermally-induced structural transformations are studied of three imogolite-type nanotube (NT) materials: i) proper imogolite (IMO, (OH)3Al2O3SiOH) with outer surface covered by Al-OH-Al groups and inner one lined by silanols; ii) methyl-imogolite (Me-IMO, (OH)3Al2O3SiCH3), in which at the inner surface silanols have been replaced by methyl groups, while the outer surface is unchanged, and iii) the material Me-IMO-NH2, obtained by grafting the outer surface of Me-IMO with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 IMO only shows loss of water (up to ca. 700 K), while 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 reveals 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 in an overall buckled structure, the strong strain within the silico-alumina layer of the single-walled nanotube providing the driving force against a complete flattening. As a minor feature, decomposition of perchlorate species to chloride anions with release of molecular oxygen is observed with IMO, species which are trapped during the synthesis at the narrow interpores cavitie

CO2 adsorption on Aluminosilicate Single-Walled Nanotubes of Imogolite Type

Adsorption of CO2 at subatmospheric pressure at temperatures about ambient has been studied on three materials: (i) imogolite (IMO, chemical formula (OH)3Al2O3SiOH)) a hydrated alumino-silicate occurring as nanotubes (NTs) with bridged AlOHAl groups at the outer surface and Si−OH groups at the inner surface; (ii) an imogolite-like material (Me-IMO, chemical formula (OH)3Al2O3SiCH3) with Si-CH3 groups replacing Si−OH at NTs inner surface; (iii) a material (Me-IMO-NH2) obtained by grafting 3-aminopropylsilane at the outer surface of Me- IMO. All materials, being in the form of NTs, exhibit rather high specific surface area values (355−665 m2 g−1) and are accessible to CO2 molecules. Infrared spectroscopy shows that carbon dioxide may interact in a variety of ways. At the inner surface of IMO, linear molecular species are reversibly formed by interaction with silanols, whereas at the outer surface carbonate-like species are given rise with partial reversible character. With Me-IMO, no interaction takes place at the inner surface: linear species are formed in the intertube nanopores as well as carbonate species as in the case of IMO. Finally, with Me-IMO-NH2, all species present in Me-IMO are found, as well as reversible carbamate species arising from the reaction with amino groups. Optical isotherms concerning molecular adsorption have Langmuir character, whereas those for the reversible formation of carbonates/carbamates are of Henry-type. Volumetric isotherms are interpreted as due to two independent families of adsorption sites, respectively Langmuir and Henry: comparison between optical isotherms (measured at ca. 33 °C) and volumetric isotherms (measured at 0 °C) allows a semiquantitative estimate of the adsorption enthalpy for molecular species, corresponding to ca. −20 kJ mol−1, for linear species reversibly formed by interaction with inner silanols in IMO, and to a relatively high adsorption enthalpy for molecular species formed in the larger intertube nanopores of Me-IMO (ca. −32 kJ mol−1)