Synthesis and Characterization Of Fe-modified Imogolite Nanotubes

During the past decades, and after introducing the most famous carbon nanotubes, the main role in these fields has been playing by the single- and multi-wall carbon nanotubes which have received tremendous research interest due to their superior mechanical, chemical, electrical and thermal properties. However, several problems in carbon nanotube technology, such as high-temperature process with low yield product, imprecise control over nanotube dimensions and chirality, limitations of chemical composition, intrinsic color of nanotubes which limits their application as nanofiller in transparent hybrid materials, low compatibility of carbon nanotubes with human body in bio-applications and the most important, in recent years, effect of carbon nanotubes on human health and environment because of their potential toxic nature, encouraged the research for analogue structure among inorganic materials and the possibility of applying them in other fields, such as catalysis and ion adsorption.Inorganic nanotubes like MoS2 and WS2 were the first inorganic nanotubes that succeeded the discovery of carbon nanotubes and in the following years, other metal oxide/hydroxide nanotube structures have been reported. Among the inorganic nanotubes, which have been developed in recent years, imogolite (IMO, chemical formula (OH)3Al2O3Si(OH)), a natural alumino-silicate clay mineral, recently entered this nanotechnologic scenario. It was discovered for the first time 1962 in Japanese soils of volcanic origins but its structure was determined ten years later. Being an analogue material to carbon nanotubes, IMO type materials can represent an intriguing different source for new technological potential applications as anions/cations retention from water, catalysis, gas adsorption, separation and storage, scaffold for biomedical applications and inorganic nanofiller for polymer matrixes. The main features of imogolite is the crystalline organization of its walls, folded to create a nanotube with two distinct surfaces: an inner surface presenting free silanol groups and an outer surface characterized by hydroxyl groups bridged between two octahedral aluminum atoms. Since, variables such as purity, composition, reproducibility, and specifically designed features can be often better controlled in synthetic procedures rather than using natural clay specimens (which typically contain impurities and are often not easily available), synthetic single walled imogolite nanotunes with high monodispersity in diameter was obtained for the first time in 1972 by a sol-gel synthesis in acid environment. Present PhD project is going to describe the developments obtained in imogolite research field, concerning the synthesis and characterization of a new kind of modified imogolite nanotubes by iron inclution in nanotubes outer surface, either by direct or post-synthesis reactions, as compared to unmodified synthetic IMO. A series of samples of iron-doped IMO, with a range of iron content, were obtained. To investigate the role of Fe ions in nanotube formation and the effect of Fe structural position on nanotubes textural properties, they were characterized by means of low angles X-ray Diffraction (XRD); IR spectroscopy (FT-IR); Transmission Electron Microscopy (HR-TEM); energy dispersive spectroscopy (EDS) and N2 sorption isotherms at -196 °C, indicating the limited level of Fe inclusion into nanotube structure, about up to 1 %wt Fe. The higher amount of Fe will hinder tube formation. The obtained modified samples by direct synthesis show more closely packing in bundles and presents slightly larger inner pores. Then, their physico-chemical properties were compared to those of proper IMO. Several experimental results are reported of nature and structural positions of Fe species in the samples obtained by direct (Fex-IMO) or post synthesis method (Fex-loaded-IMO) by Diffuse Reflectance (DR) UV-Vis pectroscopy, Raman spectroscopy, magnetic test and Electron Paramagnetic Resonance (EPR) spectroscopy which indicate the preferential isomorphic substitution of Fe for Al in the sample prepared by direct synthesis (Fex-IMO) and of the preferential formation of Fe2O3 clusters in that obtained by post-synthesis doping (Fex-loaded-IMO). Same as other nanoporous aluminosilicate materials, IMO nanotubes materials contain considerable amount of water in their pores in ambient condition that influences and governs their properties. Therefore, hydration/dehydration behavior of bare and Fe-modified nanotubes was of paramount importance in dictating the operating conditions for any application requiring a surface interaction like catalytic activity or ion adsorption. Accordingly, the behavior of hydrated Imogolite markedly depends on thermal pre-treatments, which has been investigated by several complementary methods (XRD, IR spectroscopy, TG/DT analysis and N2 sorption isotherms at -196 °C). Obtained results show that, modification of imogolite nanotubes by iron either by direct or post synthesis, accelerate dehydroxylation and decrease their thermal stability, by likely forming some structural defects, able to catalyze silanols condensation. Such defects may be ascribed to those Fe ions substituting for Al ions that likely form also by post-synthesis procedure. After removing of water present at both the external and internal surfaces at 250 oC under residual vacuum equal to 10-3 mbar, inner ≡SiOH groups will be accessible to probes. Surface acidic properties and accessibility of surface groups were investigated in gas and water media. In gas phase, surface acidic properties investigated by combination of IR spectroscopy and interaction with probe molecules (CO and NH3). Moreover a catalytic reaction: epoxidation of propylene by O2 over the obtained samples as catalyst was carried out to provide more information about surface acidity of Fe-modified samples in gas phase. According to the results, with both modified samples, when Fe substitutes for Al, at the outer surface Fe(OH)Al groups occur, the intrinsic acidity of which is only marginally different from that of Al(OH)Al. Fe(OH)Al groups likely act as crystallization centres for the growth of Fe2O3 nanoclusters, bearing less acidic OH groups. The acidity of modified samples were studied in water, as shown by -potential measurements and interaction with (acid orange 7, an organic sodium salt with formula C16H11N2NaO4S) AO7 molecules, for two important reaction: adsorption of AO7 molecules and catalytic reaction azo-dye molecule degradation by H2O2. In water, Fe(OH)Al bridged groups, which are slightly less acidic than Al(OH)Al groups, but provide accessible Fe3+ sites that may be accessible to species able to coordinate iron, as observed in the case of AO7-, leading to a higher efficiency towards the retention of such moiety and, more generally, of anions in aqueous solution. Finally we can conclude that Interaction with AO7- in water solution occurs in different ways, as documented by the observed pH changes: i) with proper IMO, AO7- anions preferentially adsorb via H-bonding; ii) with Fe1.4-IMO, Fe3+ cations of Fe(OH)Al groups act coordination centers for N atoms in the AO7- moiety; iii) with Fe1.4-loaded-IMO, Fe2O3 nanoclusters likely hinder AO7- adsorption. The best condition for degradation if AO7 was observed in the presence of IMO sample as catalyst, by formation of very active OOH groups and then carries out an intermolecular rearrangement with the neighboring adsorbed AO7 molecules to achieve the degradation. In this case higher acidity of Al(OH)Al groups in water provide reacitve sites. Fe(OH)Al groups which are more basic in water environment seems to be weaker in H2O2 decomposition. Therefore IMO with more than 95% of degradation just in a few minutes could be proposed as a new candidate for waste water treatment.