Polyoxometalate-pillared hydrotalcites

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). 

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). 

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. 

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). 

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). 

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. 

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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