Hydrotalcites rehydration

Abello, S., Medina, F., Tichit, D., Perez-Ramirez, J., Groen, J. C., Sueiras, J. E., Salagre, P. and Cesteros, Y. Aldol condensations over reconstructed Mg-Al hydrotalcites structure-activity relationships related to the rehydration method, Chem. Eur. J.,... 

A second batch of the hydrotalcite was activated, rehydrated, filtrated and then impregnated with an aqueous Pt(NH3)4(N03)2 solution. The resulting catalyst precursor was also dried in N2 at 80°C for 18 hours, followed by drying at 200°C (2°C/min) for 1 hour. The sample was directly reduced in H2 at 200°C for 1 hour, passivated and stored in air. The resulting catalyst is denoted as Pt/HT [IM], The resulting Pt particles in both Pt/HT catalysts will be located at the edges of the platelets, near the OH ions, and they will experience the support material as basic. 

Hydrotalcite is a natural mineral of ideal formula Mg6Ab.(()H)1, iCO . 4H20, having a structure similar to brucite, Mg(OH)2. In hydrotalcite the Mg cations are partially replaced with Al3+ and the resulting positive charge is compensated by anions, typically carbonate, in the interlamellar space between the brucite-like sheets. When hydrotalcite is calcined at ca. 500 °C it is decarbonated and dehydrated to afford a strongly basic mixed Mg/Al oxide. Rehydration restores the original hydrotalcite structure and creates Bronsted base sites (OH ) in the interlamellar space. 

Although hydrotalcites are relahvely stable (up to circa 500 °C), they are also of potential applicahon as precursors of mixed metal oxide catalysts, for example Reference [66]. Dehydrahon-rehydration equilibria account for the switching between hydrotalcites and mixed/supported metal oxides, which is somehmes termed the memory effect [67-69]. Recent advances have seen attempts to prepare highly dispersed LDH systems, such as those dispersed within mesoporous carbon [70]. Owing to widespread interest in their application, hydrotalcite catalysts have been the subject of a number of reviews, for example References [71-75]. Other layered-based systems have also attracted attention for application in catalysis, for example Reference [76]. 

Note, however, that rehydration of Mg-Al hydrotalcites in the liquid phase using ultrasound or a high stirring speed [267] leads to nanoplatelets with surface areas of400 m g , displaying catalytic activities in aldol condensations up to eight-times higher than the best catalytic system reported in the literature. There are thus alternative methods to increase the performances of HT materials. 

The original structure can be restored by rehydration. This phenomenon is called the memory effect of hydrotalcite, and suggests that dehydration induces a small displacement of the cations only and not complete rearrangement of structure. Dehydration is accompanied by a spectacular increase of surface area, as illustrated in Table 2, and gas phase rehydration with water-saturated nitrogen by a decrease to very low values of surface area and porosity [12]. 

Table 2. Surface area and porosity of a synthetic hydrotalcite after calcination at 723 K and further rehydration at room temperature for different periods.

Aldolizations on Hydrotalcites First Calcined then Rehydrated... 

Good catalytic properties have also been reported by Teissier and Kervenal [34] for the synthesis of isophorone on an hydrotalcite first calcined then rehydrated. The superiority of the rehydrated solid in the experiment is apparent from the observation that the reaction performed in a batch reactor at 473 K reaches about... 

Reconstructed Hydrotalcite. When calcined hydrotalcite (Mg0-Al203) is rehydrated in water or in flowing nitrogen saturated with water, hydrotalcite structure is reconstructed. This phenomenon is called memory effect. The reconstructed materials contain OH in the interlayers. The reconstructed materials are very useful catalysts for aldol condensation, Knoevenagel condensation, Michel addition, and cyanoethylation of alcohols (22-24). The OH ions in the interlayer are believed to be the active sites for these reactions. 

Various solid-base catalysts are effective for the condensation. Rehydrated hydrotalcite gives a high yield of pseudoionone (96%) with a high selectivity (99%) in 15 min (76). 

The Claisen-Schmit condensation of 2-hydroxyacetophenones and benzaldehyde derivatives proceed at 423 K using calcined and rehydrated hydrotalcites as the catalyst, the latter being more effective (78,79). Nanocrystalline MgO (80) and mesopous silica SBA 15 having propylamino groups (81) are also effective for this reaction. 

The following reaction proceeds to yield Vesidryl (2 ,4 ,4-trimethoxy-chalcone) in a 85% yield at 443 K in the presence of calcined and rehydrated hydrotalcites (78,79). 

Rehydrated hydrotalcite 50 2 mmol toluene room temperature 120 50 23... 

Rehydrate hydrotalcite Fig. 11. Aldol reactions of acetone to diacetone alcohol. 

When hydrotalcite was used as a catalyst in the selfaldol reaction of acetone, the rehydrated hydrotalcite showed higher catalytic activity than MgO, calcined hydrotalcite (mixed oxide), and even NaOH (Fig. 11) (47). The selectivity of hydrotalcite was also much better than other catalysts since mesityl oxide was not formed on the rehydrated hydrotalcite. 

The first step in the base-catalyzed condensation is the proton abstraction of active methylene groups with base catalysts, followed by attack of the anion on a carbonyl moiety. Elimination of water then produces the CMD double bond. Numerous heterogeneous catalysts have been reported to promote these condensations, including the rehydrated hydrotalcites described above for the Aldol reactions (41,51), vanadate apatite (52), and amine-functionalized silica (53). The merits of using these heterogeneous catalysts are the ease of product isolation and catalyst reusability, which is associated with elimination of the necessity to neutralize homogeneous bases such as NaOH and NaOMe. 

There are a number of techniques that have been successfully applied to synthesize modified hydrotalcites (48). The most commonly method used is the co-precipitation of two metal salts in alkaline solution at a constant pH value of about 10. Another method uses the classical ion exchange process in which the guest anions are exchanged with the anions in the interlayer spaces of preformed layered double hydroxides to produce specific anion intercalated modified hydrotalcites. StiU another method is a lattice reconstruction after heating, i.e., calcination, which is based on the structural memory effect of these materials, due to which the original structure is reproduced after rehydration. 

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