Abstract
An IR spectroscopic study is reported of the surface acidic properties of two nanotube (NT) materials, imogolite (IMO, the aluminosilicate (OH)3Al2O3SiOH) and its hybrid organic/inorganic homologue methyl-imogolite (Me-IMO, (OH)3Al3O3SiCH3). Both are made of closely packed, intertwined bundles of NTs, forming two types of nanotunnels: (i) inner cylindrical ones, pertaining to single NTs, and (ii) intertube pores arising from the contact of adjacent NTs. Mesoporosity is present as a minor feature. The outer diameter of Me-IMO is larger than that of IMO (ca. 3nm against 2nm), and so are the intertube pores of Me-IMO and IMO (ca. 0.3 and 0.4.5nm, respectively). The latter are not accessible even to small molecules: the former become so after proper outgassing, as recently shown by molecular adsorption of CO2 (Zanzottera et al. (2012) [5]).The behaviour is reported with respect to basic molecules (water, CO and ammonia). As a whole, acidic properties are moderate, especially if compared with other aluminosilicates, and depend on the hydration state of the surface. The inner surface of Me-IMO NTs, featuring methyl groups, is inactive. The inner surface of IMO, lined with silanols, behaves closely to amorphous silica: hydronium ions are formed in the presence of water, and the IR frequency of CO molecules H-bonded to silanols after removal of water is very close to that observed with severely outgassed amorphous silica (2156cm−1). With IMO treated at 300°C, some Lewis acidic sites are detected, because of dehydration, as well as new, more acidic, OH groups arising from incipient nanotubes disruption, a prelude to the formation of layered structures (Zanzottera et al. (2012) [4]). The outer surface of a single NT (the same for both materials) features OH species bridged over two octahedral Al atoms, which display an amphoteric nature. In NTs packed into bundles, two types of surfaces occur, that at intertube cavities and the outer at the exterior of bundles. The latter does not exhibit acidic properties. With Me-IMO, AlOH groups in the intertube cavities reveal a moderate acidic character, as shown by ammonia (ammonium species formation) as well as CO adsorption (stretching frequency at 2151–2154cm−1).
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