Pillars polyoxometallates

The main type of inorganic-LDH hybrid system comes from intercalation of oxometalates into the interlayer space of LDHs because of the high catalytic activity of both oxometalates and LDHs. Typical synthetic routes are similar to those of pillared clays, as mentioned before. After pristine nitrate/chloride-LDHs are anion exchanged with polyoxometalates, usually at elevated temperatures, the pillared polyoxometalates are stabilized within the interlayer space by calcination. 

Chibwe, K. and Jones, W. (1989). Synthesis of polyoxometalate-pillared layered double hydroxides via calcined precursors. Chem. Mater. 1, 489. 

Dimotakis, E. D. and Pinnavaia, T. J. (1990). New route to layered double hydroxides intercalated by organic-anions - precursors to polyoxometalate-pillared derivatives. Inorg. Chem. 29, 2393. 

Evans, J., Pillinger, M. and Zhang, J. (1996). Structural studies of polyoxometalate-anion-pillared layered double hydroxides. J. Chem. Soc., Dalton Trans., 2963. 

Narita, E., Kaviratna, P. D. and Pinnavaia, T. J. (1993). Direct synthesis of a polyoxometalate-pillared layered double hydroxide by coprecipitation. J. Chem. Soc., Chem. Commun., 60. 

The anionic polyoxometalates comprise a much larger class of metal oxide-based pillaring agents (18). It was proposed that hydrotaldtes (also referred to as layered double hydroxides, or LDHs) would be suitable host materials for pillaring with polyoxometalates. Subsequently, a route for synthesizing isopol3rmetalate-pillared hydrotalcites via organic-anion-pillared precursors was developed (19-20). 

Previously reported methods for synthesizing polyoxometalate-pillared hydrotalcites involve the exchange of chloride-containing hydrotalcites (21-23). In this work, the novel approach taken in synthesizing pillared hydrotalcites has been to prepare an organic-anion-pillared precursor, which is subsequently exchanged with the appropriate isopolymetalate under mildly acidic conditions (Figure 1). 

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). 

It has previously been reported that hydrotalcite catalyzes the aldol condensation of acetone (25). Polyoxometalates are known to dehydrate alcohols due to their acidic nature (IS ). In order to compare the relative basicity of polyoxometalate-pillared hydrotalcites to that of hydrotalcite itself, a variety of hydrotalcites were screened for 2-propanol conversion (Table II). This reaction is known to give propylene when the catalyst contains acidic sites (such as alumina) and acetone when the catalyst contains basic sites (such as magnesium oxide). 

The results in Table II suggest that molybdate- and vanadate-pillared hydrotalcites contain both acidic and basic sites, the basic sites located on the metal hydroxide sheets, and the acidic sites located on the polyoxometalate pillars. 

Polyoxometalate-pillared hydrotalcites were also screened for activity for the dehydrogenation of t-butylethylbenzene to t-butylstyrene. Table III shows that molybdate-pillared hydrotalcites outperformed other pillared hydrotalcites. 

Pillared montmorillonite (PILC) samples were prepared using 10 g of Na-Mont and 300 cm of polyoxometallate solution. 5% suspension of Na-Mont was produced and the solution was added dropwise at room temperature under vigorous stirring. Stirring was maintained for 8 hours at 353 K. 

Porous metal oxides and related compounds (including pillared oxides, laminar hydroxides, and polyoxometalates)... 

Clays have also been pillared with polyoxometalate ions, such [(PW11VO40)4-].261 Other layered materials can also be pillared. Zirconium phosphate has been pillared with chromium(III) oxide.262 A layered titanate has been pillared with silica.263 The need for a separate pillaring step has been avoided in the preparation of some porous lamellar silicas which are structurally similar to pillared clays.264 Eight to twelve carbon diamines were used with tetra-ethoxysilane in ethanol with added water to make them in 18 h at room temperature. The template was recovered by solvent extraction before the silica was calcined. 

The layered double hydroxides have been of interest as anion exchangers, basic catalysts, and catalyst precursors. Pillared derivatives of the type discussed earlier for the clays have been synthesized by ion exchange with polyoxometallate anions but have lower thermal stability. Several authors have reported synthesis of layered double hydroxides pillared with the Vio028 ion by direct ion exchange or by techniques that involve preswelling the layer structure with an organic anion. For example, the terephthalate anion was used in the synthesis ofVio028 and Mo7 024 intercalated phases. 

On the basis of the above siunmary, there now exist several reliable synthetic pathways to well-ordered microporous LDH derivatives pillared by polyoxometalate anions. The molecular seiving and catalysts properties of these... 

Doeuff, M., Kwon, T., and Pinnavaia, T. J. 1989. Layered double hydroxides pillared by polyoxometalate anions exafs studies and chemical synthesis. Synthetic Metals 34 609-15. 

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. 

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