Photo-activated degradation of tartrazine by H2O2 as catalyzed by both bare and Fe-doped methyl-imogolite nanotubes and the related collapsed phases

Synthetic azo dyes, widely used by textile, cosmetic, photographic, pharmaceutical and food industries, represent an environmental concern and must be removed from both wastewater and groundwater [1] due to their potential toxicity to humans and environment, along with the visibility problems connected to their presence in water. One of the most widely used food coloring agents, i.e Tartrazine E102 (TRZ), is recalcitrant to biodegradation under aerobic conditions, and appears to be responsible of the most allergic and/or intolerance reactions in comparison to the other azo dyes [2]. Among nanomaterials with potential application as heterogeneous catalysts, there is a growing interest for imogolite-related materials, as testified by the recent studies on their synthesis [3], structure [4], electronic states [5], stability [5], and application as catalyst and/or catalytic support [6]. Proper imogolite is a hydrous aluminum silicate with chemical formula (OH)3Al2O3SiOH [7], 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. Subject of this study is instead the catalytic behavior of the hybrid organic/inorganic analogue of imogolite, i.e. methyl-imogolite (MeIMO), along with that of novel Fe-doped MeIMO obtained by ionic exchange and of the phases stemming by the thermal collapse of NTs. MeIMO NTs (chemical formula (OH)3Al2O3SiCH3) have an inner (hydrophobic) surface lined by SiCH3 groups (Figure 1) [8], and an outer surface resembling that of imogolite. In the dry powder, NTs form bundles with hexagonal packing, giving rise to three kinds of pores and related surfaces (Figure 1). In MeIMO, intra-tube pores (A) have  2.0 nm diameter; inter-tube pores (B) among three aligned NTs in a bundle have ca. 0.45 nm diameter and larger C mesopores are disordered slit-pores occurring among bundles.