Physico-chemical properties of imogolite nanotubes functionalized on both external and internal surfaces

Using a methylated Si precursor instead of Tetraethoxysilane (TEOS), methyl-imogolite (Me-IMO), a nanotube material with formula (OH)3Al2O3SiCH3, is obtained in place of the standard imogolite (OH)3Al2O3SiOH (IMO).21 Post-synthesis grafting of the outer surface of Me-IMO with 3-aminopropyltriethoxysilane (3-APS) yields a new hybrid material (Me-IMO-NH2), with an entirely hydrophobic inner surface and a largely aminated outer surface. In this paper, the structure and stability of Me-IMO-NH2 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-NH2; ii) 1H, 13C, 27Al, 29Si and heteronuclear correlation (HETCOR) 1H-13C Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) experiments, providing evidence for the occurrence of grafting and yielding an estimate of its extent; iii) Infra-Red spectroscopy, showing that most terminal -NH2 groups are protonated; iv) X-Ray Diffraction (XRD) measurements yielding information on the long-range order; v) N2 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.