Decavanadate-pillared hydrotalcite

Kooli, F., Rives, V. and Ulibarri, M. A. (1995). Preparation and study of decavanadate-pillared hydrotalcite-like anionic clays containing transition metal cations in the layers. 1. Samples containing nickel-aluminium prepared by anionic exchange and reconstruction. Inorg. Chem. 34, 5114. 

Figure 4 shows the MAS-NMR spectra of several terephthalate/decavanadate-pillared hydrotalcites. These spectra are consistent with that previously published for (NH4)gVio028 (23). The changes which occur to the decavanadate pillars upon calcination of the pillared hydrotalcite are currently being investigated. It is known, however, that in the case of pillared materials containing as little as 25-30% polyoxometalate pillars the overall layered structure is maintained up to 500°C (Figure 5). 

Figure 3. Model of decavanadate-pillared hydrotalcite viewed normal to the metal hydroxide layers. Atom types are as follows (a) Mg or Al (b) OH group in the metal hydroxide...

Figure 5. TEM micrograph of a terephthalate/decavanadate-pillared hydrotalcite (39% decavanadate exchanged) after calcination at 500 .

Recently, it has been shown that hydrotalcite-type clays interlayered by polyoxometalate anions lead to a new class of pillared materials for selective adsorption and catalysis. The heptamolybdate- and decavanade-pillared hydrotalcite-type clays were synthesized by Drezdzon (1988). An organic-anion-pillared clay precursor was prepared and subsequently exchanged with the appropriate isopolymetalate under mildly acidic conditions (pH = 4.T-4.9). These pillared intercalates were found difficult to obtain in highly crystalline form. This is ascribed to the fact that the Mg and Al hydroxide layers have a basic nature, whereas Mo7024 and anions are stable in acidic conditions. 

Table I. Exchange Reactions of Terephthalate-Pillared Hydrotalcite Clays with Decavanadate...

Figure 1. Schematic representation of terephthalate exchange with heptamolybdate and decavanadate in hydrotalcite. Pillared hydrotalcite d-spacings were calculated from models.

Figure 4. Vanadium-51 solid state MAS-NMR of terephthalate-pillared hydrotalcites partially exchanged with decavanadate (a) 39% exch. (b) 60% exch. (c) 91% exch.

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