Physico-Chemical Properties of Imogolite Nanotubes Functionalized on Both External and Internal Surfaces

Abstract

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) Postsynthesis 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) infrared spectroscopy, showing that most terminal −NH2 groups are protonated; (iv) X-ray diffraction (XRD) measurements yielding information on the long-range order; and (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.