Anionic Clays Hydrotalcites

Anionic clays are natural or synthetic lamellar mixed hydroxides with interlayer spaces containing exchangeable anions [10, 104]. The generic terms, layered double hydroxides (LDHs) or hydrotalcites are widely used, the latter because exten- 

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. 

Aldol and related condensation reactions such as Knoevenagel and Claisen-Schmidt condensations are also widely used in the fine chemicals and specialty chemicals, e.g. flavors and fragrances, industries. Activated hydrotalcites have been employed as solid bases in many of these syntheses. Pertinent examples include the aldol condensation of acetone and citral [107, 108], the first step in the synthesis of ionones, and the Claisen-Schmidt condensation of substituted 2-hydroxyacetophenones with substituted benzaldehydes [109], the synthetic 

Excellent results were also obtained using activated hydrotalcite as a solid base catalyst in the Knoevenagel condensation of benzaldehyde with ethylcya-noacetate [110], ethylacetoacetate [111] or malononitrile [112] (see Fig. 2.34). Similarly, citronitrile, a perfumery compound with a citrus-like odor, was synthesized by hydrotalcite-catalyzed condensation of benzylacetone with ethyl-cyanoacetate, followed by hydrolysis and decarboxylation (Fig. 2.34) [113]. 

Other reactions which have been shown to be catalyzed by hydrotalcites include Michael additions, cyanoethylations and alkylations of, e.g. 1,3-dicarbonyl 

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The main classes of catalysts used for heterogeneous WHPCO reaction are clays and anionic clays (hydrotalcites), metal-ion exchanged zeolites and mesoporous silica containing transition metals, and doped metal oxides. Although some other transition metals have been also used (Mn, V), most catalysts contain iron and/or copper as the active elements. Leaching of the active metal is also a significant problem in this case. While different types of catalysts have been reported, only a few of them have been effectively proven to have a stable activity in long-term continuous experiments or at least in several repeated batch tests. Between the stable catalysts, Fe- and Cu-PILC (pillared clays) materials " have the best combination of activity and stability. However, the limited quantity of active elements (around 2% wt. of iron or copper) necessary to achieve stable performances, limits the overall activity. 

Two classes of clays are known [3] (i) cationic clays (or clay minerals) that have negatively charged alumino-silicate layers balanced by small cations in the interlayer space (e.g. K-10 montmorillonite) and (ii) anionic clays which have positively charged brucite-type metal hydroxide layers balanced by anions and water molecules located interstitially (e.g. hydrotalcite, Mg6Al2(0H)igC034H20. 

FIGURE 16.4 Crystal structure of hydrotalcite MggAl2(OH)j8.4.5 H2O. (From Vierheilig, A. A., Process for Making, and Use of, Anionic Clay Materials, U.S. Patent 6,479,421, 2002. With permission from Intercat, Inc.)... 

Cavani, F., Trifiro, F. and Vaccari, A. Hydrotalcite-type anionic clays preparation, properties and applications, Catal. Today, 1991, 11, 173-301. 

New developments in the use of silicates to improve flame retardancy have arisen from the use of synthetic anionic clays that correspond to the family of lamellar mixed metal hydroxides, commonly named layered double hydroxides (LDH) or hydrotalcite-like compounds.17... 

Hydrotalcite (HT) is a clay mineral which on crushing becomes a fine powder, similar to talc. It is a hy-droxycarbonate of Mg and A1, of the general formula Mg6Al2(0H)i6C03 - 4H20, and occurs in nature in foliated and controlled plates and/or fibrous masses. Anionic clays of the HT type have been of importance in catalysis since 1970. Their interesting properties for catalytic applications are ... 

Cavani F, Trifiro F, Vaccari A. Hydrotalcite-type anionic clays Preparation, properties and applications. Catalysis Today. 1991 11 (2) 173—301. 

Trihro, F. and Vaccari, A. (1996). Hydrotalcite-like anionic clays (layered double hydroxides) in Comprehensive Supramolecular Chemistry (G. Alberti and T. Bein, Eds.). Pergamon, Oxford. 

New hydrotalcite-type anionic clays containing noble metals. J. Chem. Soc., Chem. Commun., 2435. 

Velu, S., Sabde, D. P., Shah, N. and Sivasanker, S. (1998). New hydrotalcite-like anionic clays containing Zr4+ in the layers synthesis and physicochemical properties. Chem. Mater. 10, 3451. 

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. 

Another important group of catalytic compounds contain the layered double hydroxides (sometimes called anionic clays or hydrotalcites ). Presently, many researchers study these materials because of their adsorptive and catalytic properties. 

Flash vacuum pyrolysis (FVP) reactions of pyrazole itself and DPP 63 were investigated in the presence of anionic clays having a hydrotalcite structure <2001JOG2943, 2002JOG8147>. Solid catalysts with Mg Al ratio equal to 2 1 containing carbonate, nitrate, and silicate as interlayer anions were employed. Between 400 and 600 °G, compound 1... 

Hydrotalcites (HT) belongs to the class of anionic clay minerals, also known as layered double hydroxides (LDHs). They are probably one of the best known and used solid basic catalyst [212[. Their properties in catalytic organic reactions have been discussed extensively by Jacobs et al. [256[. More recent aspects have been analyzed briefly also... 

To set up a more environmentally friendly process, it is mandatory to replace the caustic soda by solid bases. Many catalysts such as zeolites [2], alumina [3,4], sepiolite [5] and hydrotalcites [6,7] have been regarded as promising solid basic materials. Hydrotalcites, which are anionic clays, are the subject of this investigation. They are layered double hydroxides with the general formula [M(II)i.xM(III)x(0H)2].[A" x/n].mH20. The host layers are charged positively by replacement of divalent metal cations usually Mg ", Ni and... 

Magnesium-aluminium hydrotaldte is a naturally occurring anionic clay that decomposes reversibly upon calcination at high temperatures to form high surface area basic mixed oxide. To generate the hydrotalcite structure, Al cations replace... 

The latter is analogous to the hydrothermal synthesis of zeolites and related molecular sieves (see later). Redox metal ions can be incorporated into acidic clays or zeolites by ion exchange, and oxoanions can be similarly exchanged into hydrotalcite-like anionic clays [30]. 

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