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)(3)Al2O3SiOH)) 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)Al2O3SiCH3) 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 m(2) 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 degrees C) and volumetric isotherms (measured at 0 degrees C) allows a semiquantitative estimate of the adsorption enthalpy for molecular species, corresponding to ca. -20 kJ moll, 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)).