Palygorskite-indigo and sepiolite-indigo adducts (2 wt.% indigo) were prepared by crushing the two compounds together in a mortar and heating the resulting mixtures at 150 and 120°C, respectively, for 20 h. The samples were tested chemically to ensure that they displayed the characteristic properties of Maya Blue. Textural analysis revealed that no apparent changes in microporosity occurred in sepiolite or palygorskite after thermal treatment at 120°C (sepiolite) and 150°C (palygorskite) for 20 h. Micropore measurements showed a loss of microporosity in both sepiolite and palygorskite after reaction with indigo. The TGA-DTG curves of the sepiolite-indigo and palygorskite-indigo adducts were similar to their pure clay mineral counterparts except for an additional weight loss at ∼360°C due to indigo.The 29Si CP/MAS-NMR spectrum of the heated sepiolite-indigo adduct is very reminiscent of the spectrum of dehydrated sepiolite. Crushing indigo and sepiolite together initiates a complexation, clearly seen in the 13C CP/MAS-NMR spectrum, which can be driven to completion by heat application. In contrast to the broad peaks of the pure indigo 13C CP/MAS-NMR spectrum, the sepiolite-indigo adduct spectrum consists of a well-defined series of six narrow peaks in the 120.0–125.0 ppm range. In addition, the sepiolite-indigo spectrum has two narrow, shifted peaks corresponding to the carbonyl group and the C-7 (C-16) of indigo. A model is proposed in which indigo molecules are rigidly fixed to the clay mineral surface through hydrogen bonds with edge silanol groups, and these molecules act to block the nano-tunnel entrances.
The process of incorporation of pyridine in the nanostructured tunnels of sepiolite was studied in detail, using various complementary characterization techniques, microporosimetry, thermal gravimetric analysis, FTIR, and multinuclear solid-state NMR. It is demonstrated that a remarkable nanohybrid material, SEP-PYR, is formed through the direct coordination of pyridine to the edge Mg(II) sites of the tunnels. This material is formed at temperatures above 140 °C when the sepiolite tunnels are dehydrated and the pyridine molecules are trapped in the tunnels. In a first step toward the formation of SEP-PYR, the pyridine molecules were incorporated at room temperature in the tunnels, by exposing sepiolite to pyridine vapors. The incorporated pyridine molecules are H-bound to the structural water molecules coordinated to the edge Mg(II) cations. In a second step, upon heating to 140 °C, approximately 50% of the pyridine is lost, together with most of the structural water coordinated to Mg(II). This event is accompanied by direct coordination of the remaining pyridine molecules in the tunnels to the edge Mg(II) ions of the octahedral sheets, resulting in a material with a structure similar to the parent sepiolite, but with pyridine molecules coordinated to the Mg(II) edge cations. This material is stable up to 450 °C. At this temperature, the coordinated pyridine molecules escape from the tunnels, resulting in a collapsed sepiolite structure.
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