Decavanadate pillars

Decavanadate is easily prepared by adjusting the pH of a NaVCh solution to 4.5. The negatively charged pillars are retained essentially by electrostatic attraction to the positively charged brucite-type sheets of the LDH. Each V atom in a decavanadate pillar is linked by at least five V-O-V bonds to the rest of the pillar. Thus, as long as one works under anhydrous conditions, it is unlikely that any of the V is released from the decavanadate-LDH (Vio-LDH) structure. Even complicated allylic alcohols (e.g., those from the group of the terpenes) are epoxidized in high yield ... 

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. 

Thermal stability of decavanadate-pillared LDHs was studied by Twu and Dutta (1990). Their results indicated that the layers remained intact up to 623 K, yet tile interlayer pillars decomposed at ca. 433 K The borate-pillared LDHs prepared... 

In all previous cases, V was incorporated in a monomeric form. There are also methods to introduce oligomeric V into inorganic structures. Choudary et al. (48) advocated the use of a montmorillonite, pillared with V oligomers (V-PILC). V-PILC catalyzes the epoxidation of allylic alcohols with i-BuOOH. Oligomeric V is also used to pillar anionic clays such as layered double hydroxides (LDHs) with decavanadate anions (VioOjg) (49). 

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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