Using a methylated Si precursor instead of tetraethoxysilane (TEOS), methyl-imogolite (Me−IMO), a nanotube material with formula (OH) 3 Al 2 O 3 SiCH 3 is obtained in place of the standard imogolite (OH) 3 Al 2 O 3 SiOH (IMO). 21 Postsynthesis grafting of the outer surface of Me−IMO with 3aminopropyltriethoxysilane (3-APS) yields a new hybrid material (Me−IMO−NH 2 ), with an entirely hydrophobic inner surface and a largely aminated outer surface. In this paper, the structure and stability of Me−IMO−NH 2 are studied in detail and compared with those of Me−IMO by means of (i) X-ray photoelectron spectroscopy (XPS), confirming the surface chemical composition of Me−IMO− NH 2 ; (ii) 1 H, 13 C, 27 Al, 29 Si, and heteronuclear correlation (HETCOR) 1 H− 13 C magic angle spinning nuclear magnetic resonance (MAS NMR) experiments, providing evidence for the occurrence of grafting and yielding an estimate of its extent; (iii) infrared spectroscopy, showing that most terminal −NH 2 groups are protonated; (iv) X-ray diffraction (XRD) measurements yielding information on the long-range order; and (v) N 2 adsorption at −196 °C, yielding specific surface area and pore size distribution. Reaction with 3-APS brings about a limited loss in microporosity, probably caused by functionalization at the mouth of pores, and an increased disorder in the alignment of nanotubes, with neither a big loss of specific surface area nor a sizable change in the distance between nanotubes. As a whole, imogolite-type nanotubes appear to be rather prone to functionalization, which seems to allow the possible tailoring of the properties of both inner and outer surfaces.

Adsorption of CO 2 at subatmospheric pressure at temperatures about ambient has been studied on three materials: (i) imogolite (IMO, chemical formula (OH) 3 Al 2 O 3 SiOH)) 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) 3 Al 2 O 3 SiCH 3 ) with Si-CH 3 groups replacing Si−OH at NTs inner surface; (iii) a material (Me-IMO-NH 2 ) 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 m 2 g −1 ) and are accessible to CO 2 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-NH 2 , 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 ).

The thermally induced structural transformations are studied of three imogolite-type nanotube (NT) materials: (i) proper imogolite (IMO, (OH) 3 Al 2 O 3 SiOH) with outer surface covered by Al−OH−Al groups and inner surface lined by silanols); (ii) methyl-imogolite (Me-IMO, (OH) 3 Al 2 O 3 SiCH 3 ), in which the inner surface silanols have been replaced by methyl groups while the outer surface is unchanged, and (iii) the material Me-IMO-NH 2 , obtained by grafting the outer surface of Me-IMO with 3-aminopropylsilane (3-APS). TG-MS analysis on the parent IMO 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-NH 2 , 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-CH 3 , following the cleavage of the NTs structure, yielding methane and transient Al-O-CH 2 -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 observed with IMO species that are trapped during the synthesis at the narrow interpores cavities.

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