Hybrid organic/inorganic nanotubes of imogolite type (2012)

The aim of the present PhD project is to describe the developments obtained in imogolite research field, concerning the synthesis and characterization of two new modified imogolite nanotubes, either by direct or post-synthesis reactions, as compared to unmodified synthetic imogolite. Using a methylated Si precursor instead of TEOS, methyl-imogolite (Me-IMO), a nanotube material with formula (OH)3Al2O3SiCH3, is obtained in place of the standard imogolite (OH)3Al2O3SiOH (IMO) (I. Bottero et al. PCCP 7(13), 2011, 744 -750). Post-synthesis grafting of the outer surface of Me-IMO with 3-aminopropyltriethoxysilane (3-APS) yields a new hybrid material (Me-IMO-NH2) (C.Zanzottera et al. JPCC, DOI: 10.1021/jp301177q) with an entirely hydrophobic inner surface and a largely aminated outer surface. Me-IMO-NH2 structure was studied in detail and compared with those of Me-IMO and IMO by means of: i) 1H, 13C, 27Al, 29Si and HETCOR 1H-13C MAS NMR experiments, providing evidence for the occurrence of grafting and yielding an estimate of its extent; ii) XPS analysis, confirming the surface chemical composition of Me-IMO-NH2; iii) IR spectroscopy, showing that most terminal -NH2 groups are protonated; iv) XRD measurements and FE-SEM microscopy yielding information on the long-range order and morphology; v) N2 adsorption at -196°C, yielding specific surface area and pore size distribution. Adsorptive properties were tested towards CH4 and CO2 by means of volumetric measurments. Curve-fits obtained by assuming a Langmuir model of CH4 adsorption in the 0-35 bar pressure range, show good agreement with experimental points: both the maximum adsorbed volumes (76.81 and 29.65 cm3/g for Me-IMO and IMO, respectively) and equilibrium constants (0.03 and 0.06 bar-1 for Me-IMO and IMO, respectively), were calculated. With CO2, the preliminary results available at low pressure and weighted by the corresponding surface area values, confirm that the interaction, as expected, is stronger in case of IMO, according to the following sequence, IMO > Me-IMO-NH2 > Me-IMO, being Me-IMO the less polar material. Stability of imogolite-like nanotubes was studied by combining MAS NMR, XRD and TGA-Mass techniques. Collapsing of the structure is evidenced by the formation of a lamellar phase at 500 °C. With IMO, the mechanism proposed wants the cleavage of nanotubes to happen across their diameter, causing the formation of the following repeating sequence, Al-O-Si-O-Si-O-Al. With hybrid imogolite-like samples the mechanism is much more complex. Two processes seem to act subsequently: the first one, when methyl groups are removed at high temperature (above 300 °C); the second one, if methyl groups of two close nanotubes are able to create Si-CH2-O-Al links, by the reaction with terminal aluminol groups. Surface properties were studied by FT-IR spectroscopy of adsorbed probe molecules, namely CO2, NH3 (adsorbed at room temperature) and CO (adsorbed at nominal -196°C). IMO and Me-IMO interact reversibly with carbon dioxide showing the presence of physisorbed gas as well as of carbonate-like species. With Me-IMO-NH2, besides the two aforementioned species, carbamic species are also present, thus confirming the successful amination of the outer surface. NH3 adsorption evidenced that pores among aligned tubes of IMO are too narrow to be accessible by the gas. Thus, for geometrical reasons, that is the larger nanotubes diameter external -Al(OH)Al species are detectable only in case of Me-IMO and Me-IMO-NH2. CO adsorption indicated a weak electrostatic interaction. Particularly, with IMO pre-treated at 150 °C, IMO-150, CO interacts with water molecules still adsorbed and removed only by outgassing at a higher temperature; with Me-IMO-150, the slightly shifted 2146 cm-1 band indicates instead a weak perturbation of the gas; Me-IMO-NH2 showed both contributions. In agreement with MAS NMR results, lamellar imogolite phase is obtained at 500 °C. In fact, both NH3 and CO interaction confirmed that with IMO-500, hydroxyls more acidic than silanols are formed. On the contrary, with lamellar Me-IMO and Me-IMO-NH2, physisorbed CO as well as heterogeneous acidic sites are found, analogously to Al-rich microporous silicates. Finally, in spite of their shared fibrous nature, according to the preliminary study performed, imogolite nanotubes were proposed as candidate as a low-toxicity reference material for in vitro toxicological studies on HARNs, High Aspect Ratio Nanomaterials. Moreover, the substantial inertness of macrophages towards the tubes, may ensure high bio-compatibility as a scaffold for cell growth in different medical settings (O. Bussolati et al, manuscript in preparation). As a general conclusion, it is worth noting that, in contrast with the more famous carbon nanotubes, Imo-derived nanotubes may be functionalized under mild conditions by means of direct or post-synthesis procedure. In particular, properties of both the inner and the outer surface may be properly changed, and the hydrophilicity of the material may be modulated as well as porosity and thermal stability. This may open the way to new and promising applications