Physico-Chemical properties of hybrid organic/inorganic nanotubes of imogolite type

In the beginning of 1950s [1, 2], the usage of microscopic and spectroscopic methods allowed the identification of nanosized tubular clay minerals, such as halloysite, chrysotile and imogolite. However, the concept of nanosized tubular clay minerals was not so popular and a few papers were only published on this topic. After the first observation of carbon nanotubes (CNTs) in 1991 [3], the interest for nanosized tubular clay minerals, which could be synthesized besides being present in nature, was renewed. Nanosized tubular clay minerals, as their name implied, are tubular nanostructures clay minerals with dimensions in the 1 - 100 nm range [4, 5] and a hollow tubular structure. So far, among all clay minerals, both halloysite and imogolite have attracted more attention for their applications in the fabrication of clay polymer [6] nanocomposites [7], catalysis and adsorption [8, 9]. The reason for their popularity is attributed to their unique, one-dimensional tubular structure and properties, which are modifiable by altering the internal and external surfaces [9-11]. Proper imogolite is a hydrous aluminum silicate with chemical formula (OH)3Al2O3SiOH [2, 12], occurring as single walled nanotubes (NTs) with Al(OH)Al and Al-O-Al groups at the outer surface, and silanols (SiOH) at the inner one. This thesis is aimed at assessing the catalytic behavior of the hybrid organic/inorganic analogue of imogolite, i.e. methyl-imogolite (MeIMO, (OH)3Al2O3SiCH3), with an inner surface lined by Si-CH3 groups (hydrophobic) and an outer surface resembling that of imogolite (hydrophilic). The special structure of MeIMO leads to enhanced adsorption properties that render it suitable for gas separation/storage [9]. Furthermore, MeIMO NTs have higher degree of long range order [13], higher production yield, larger pores (> 0.2 nm) and surface area (~ 600 m2.g-1) with respect to proper imogolite [9]. Modification of the outer surface of MeIMO NTs includes either formation of covalent bonding or electrostatic interaction. The former is achieved by the reaction of MeIMO with organosilanes [11], whereas the latter is possible due to the protonation of outer surface of NTs in water, which leads to charge matching between a proper counter-ion and the outer surface of NTs [14]. These specific properties may be exploited in several applications, mainly for removal of organic pollutant from water by the electrostatic interaction of anion/cations with the charged surface of NTs [15-19]. The second topic of this work concerns another modification of the outer surface by isomorphic substitution (IS) of octahedral Al3+‏ in the outer surface by Fe3+. Fe-doped MeIMO was obtained by both ionic exchange (IE) and direct synthesis (DS) method with two Fe contents, i.e. 0.70 wt.% (Fe-0.70-MeIMO) and 1.4 wt% (Fe-1.4-MeIMO). Although IS of Al3+ by Fe3+ is a common process in almost all natural alumino-silicates, little is known about Fe doped imogolite NTs. Several studies have been recently performed dealing with the synthesis, surface characterization and applications of Fe doped imogolite NTs. So far, no specific study has been devoted to the surface properties of Fe-doped MeIMO NTs, nor to their application as the heterogeneous catalyst. The presence of Fe3+ in the structure of NTs induces new chemical and solid state properties. Based on theoretical calculations on Fe-doped imogolite NTs, IS of Fe for Al may create “defective sites” both inside and outside NTs, and reduce the band gap of imogolite (an electrical insulator) from 4.7 eV to 2.0-1.4 eV [20]. On the other hand, the first experimental studies, mainly due to Ookawa and later to Shafia et al., indicate that NTs are preserved up to 1 wt % Fe isomorphically substituted in the NTs. Higher Fe contents lead to the unavoidable formation of iron oxo-hydroxides particles/clusters [21-24]. This study confirms that in Fe-doped MeIMO as obtained by both IE and DS methods, Fe3+ species in the NTs structure decrease the band gap of MeIMO from 4.9 to 2.4 eV. Moreover, with 1.4 wt % Fe, some FexOy oligomeric clusters and Fe2O3 particles are forming. Decreasing the Fe content to 0.7 wt % shows that isolated Fe3+ species are more abundant by DS method, whereas by ionic exchange Fe tends to form more oligomeric iron oxo/hydroxide clusters or Fe2O3 particles. This particular behavior of Fe-doped MeIMO could help one to choose the proper method for specific application. The thermal stability of MeIMO and Fe-doped MeIMO NTs has also been studied. Mechanisms of collapse of NTs (at T > 300 °C dehydroxylation and NTs deformation result in collapse of structure) have been investigated with samples at different Fe loading and also in the different heating environment (either in air or in the vacuum). According to the obtained results, thermal treatment in air results in the faster cleavage of MeIMO NTs by burning the inner methyl groups to CO2. Whereas, thermal treatment in vacuum, triggers the cleavage of NTs from another route which is detectable by more positive chemical shift in 29Si MAS NMR analysis. Although the wall structure of collapsed samples is partially damaged or disordered, and finally collapsed at around 400 °C, the phases stemming by the thermal collapse of NTs show considerably high surface area and high porosity, due to residual microporous regions likely derived from unaffected pristine NTs within the layers [25]. Furthermore, the analysis of IR spectra of pyridine adsorption on collapsed samples show the presence of strong Lewis/Brønsted acidic sites. Another interesting aspect is the deformation of NTs due to dehydroxylation, which may lead to the alteration of light adsorption capacity and to the reduction of band gap in deformed NT with respect to original one. This has been investigated by theoretical calculation (SCC-DFTB) of dehydroxylated imogolite NTs [26]: in the literature, it has been observed by practical band gap evaluation obtained by UV-Vis spectroscopy on MeIMO NTs. Therefore, the dehydroxylated imogolite or MeIMO NTs is considered as a semiconductor and controlling the degree of dehydroxylation in the heat treatment process may open the possibility to adjust and control the electronic and mechanical properties of NTs. All these observations trigger the future investigations on the different structural phases arrived from thermal treatment of NTs. In order to investigate the photocatalytic application of Fe-doped MeIMO NTs, the samples obtained by IE method have been studied for the photo-Fenton oxidation of tartrazine dye, an important pollutant of both wastewater and groundwater. The obtained results imply that IS of Al3+/Fe3+ at low Fe content (i.e. 0.7 wt. % Fe) starts the photo-Fenton process and provides total discoloration and mineralization of tartrazine dye. Whereas, at higher Fe content (i.e. 1.4 wt. % Fe) the higher amount of Fe oxo-hydroxide clusters play a detrimental effect on tartrazine mineralization, due to both a lower photo-Fenton activity of the clusters with respect to isolated Fe3+ species and their ability to catalyze the (undesired) decomposition of H2O2 to oxygen and water. Remarkably, Al-OOH groups at the outer surface of bare MeIMO also enhance the photocatalytic activity towards the dye mineralization.Tartrazine removal was studied also in the presence of the buckled phases stemming from NTs thermal collapse. The results were interesting and show the high dye adsorption and degradation in the presence of collapsed phase, mainly due to new coordination environments for both Al and Fe sites, leading an active site towards the photo-Fenton reaction.