Pillared hydrotalcites

Drezdzon, M. A. (1988). Synthesis of isopolymetalate-pillared hydrotalcite via organic-anion-pillared precursors. Inorg. Chem. 27, 4628. 

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. 

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). 

This paper summarizes work done on the synthesis and characterization of terephthalate-, isopol3rmetalate-, and terephthalate/isopolymetalate-pillared hydrotalcites. Several test... 

Details on the materials and methods used to synthesize and characterize the pillared hydrotalcites discussed in this paper have been previously published (19). For the interested reader s convenience, a brief description of the synthesis of several pillared hydrotalcites follows. 

Synthesis of Terenhthalate/Vanadate-Pillared Hvdrotalcite. To a 1350-g portion of a terephthalate-pillared hydrotalcite was added a solution consisting of 14.0 g NaVOg in 100 mL deionized water. (The amount of vanadate added corresponds to 25% of that needed for stoichiometric exchange of terephthalate with V10O28 .)... 

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). 

In neutral or basic media, terephthalate-pillared hydrotalcite is stable indefinitely. Acidification with dilute HCl or HNO3 results in the formation of the corresponding chloride- or nitrate-... 

Synthesis and Characterization of Isopolymetalate-Pillared Hvdrotalcites. Exchange of terephthalate-pillared hydrotalcite with the appropriate metalate under mildly acidic conditions (pH =4.5) proceeds smoothly to yield the corresponding isopolymetalate-pillared hydrotalcite, as indicated by X-ray diffraction and elemental analysis. The d-spacings of the products confirm that the shortest dimension of the isopolymetalate is perpendicular to the metal hydroxide layers. 

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 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. 

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

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

Table III. Dehydrogenation of t-butylethylbenzene Over Various Isopol3nottetalate-Pillared Hydrotalcites (a)...

It was initially hoped that running the reaction under oxidative dehydrogenation conditions would lead to higher conversions. Studies with molybdate-pillared hydrotalcites under more optimized conditions showed that the presence of oxygen in the feed simply kept the catalyst from coking up as fast as if oxygen were not included. Unfortunately, even with the presence of oxygen in the feed, molybdate-pillared hydrotalcites were found to lose approximately 40% of their activity after 100-150 hours on stream. 

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