Hydrotalcite structure/properties

Hydrotalcite-like compounds (HTlcs) have attracted much attention in recent years as catalyst precursors and catalyst support. This is due to (i) their ability to accommodate a large variety of bivalent and trivalent cations (ii) the homogeneous mixture of the cations on an atomic scale and (iii) the formation of thermostable mixed oxides, often denoted as ex-HTlcs, with high surface area upon decomposition. The first two properties are a result of the precursor while the last property appears to be related to the decomposition mechanism. The transitions in the structural properties of Co-based hydrotalcites upon high temperature treatments have been extensively studied in our group.15,16 In the first decomposition step, water is removed from the structure. This transition is followed by dehy-droxylation and decarbonation, as well as carbonate reorganization in the interlayer. Thermal treatment in air finally leads to a solid solution of cobalt spinels (Co(Co, A1)2C>4). Mixtures of CoO and C0AI2O4 are formed upon treatment in inert. 

Table 1 gives the physicochemical properties of the hydrotalcites prepared All the materials showed well crystallized hydrotalcite structures. X-ray results indicate that the HT2 crystallized at 473 R shows very intense and sharp diffractions while HTl crystallized at 333 K shows relatively broad difffaction pattern. MTS containing interlayer S04 ion shows very weak diffraction pattern. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

Hydrotalcite-based materials (Figure 6.4) impregnated with K2CO3 have shown favourable properties for adsorbing CO2 at 4(X) C in the presence of steam [4-6, 21-25]. Potassium-promoted hydrotalcite CO2 sorbents are based on the hydrotalcite structure... 

Only a few studies have been focused on cementitious materials concerning their potential applications in corrosion protection of reinforced concrete structures. The mechanism of corrosion in reinforced concrete and concrete properties that affect corrosion of reinforcement have been briefly detailed. The existing knowledge concerning the synthesis and characterization methods of modified hydrotalcites, ion exchange within the modified hydrotalcite structure as well as the application of modified hydrotalcites in the cementitious materials have been reviewed. It is expected that modified hydrotalcites can improve the durability of reinforced concrete materials (48). 

The synthesized LDHs showed well-ciystallized hydrotalcite structure. The thermal treatment converted the precursors into mixed metal oxides (V, Mo, W, Nb). X-ray diffraction analysis indicates the formation of an orthorhombic Mo7,gVi,2Nbo28,9 and M0O3 phase, the peaks are observed at 20 = 22.1, 28.2,6.2,45.2,50.0° (3). The textnral properties of the systems depend on the eonditions of preparation of LDHs, in particular on the particle size of the sample. For example BET surfaee area increased simultaneously with the partiole size (Table 2). 

In summary, the Zn-Al and Zn-Ga based-hydrotalcites were found as very effective supports for docking a Rh(TPPTS)3Cl complex. The process was carried out via ionic exchange and occurred without any damage of die support structure or of the complex. The resulted catalysts were found to catalyze the cyclization of acetylenic carboxylic acids to the corresponding 5-membered ring heterocycles in good to excellent yields. The basic properties of the support also allow a clean and selective reaction of unsubstituted acetylenic carboxylic acids. 

Structural and textural properties of zinc(II)-chromium(III) spinel oxides prepared using a hydrotalcite-like compound... 

Shen, J. Y., Tu, M. and Hu, C. (1998). Structural and surface acid/base properties of hydrotalcite-derived MgAlO oxides calcined at varying temperatures. J. Solid State Chem. 137, 295. 

Thermal treatments induce dehydration, dehydroxylation and loss of the charge-compensating anions, resulting in mixed oxides with the MgO-type structure. Hydrotalcites are consequently a class of precursors useful for the preparation of catalytically active oxides showing basic properties [94], The acid-base properties of Mg-Al mixed oxides are governed by the Mg Al molar ratio, calcination temperature and preparation conditions. The study of the influence of the acid-base properties and chemical composition on the catalytic performance of calcined hydrotalcites is thus of interest. 

Mckenzie A. L., Fishei C. T. and Davis R. J., Investigation of the surface structure and basic properties of calcined hydrotalcites. J. Catal. 138 (1992) pp. 547-561. Narayann S. and Krishna K., Hydrotalcite-supported palladium catalysts. Appl. Catal. A 174 (1998) pp. 221-229. 

Incorporation of metal phtalocyanine complexes into the inter gallery of hydrotalcite is the subject of many interest owing to their advantageous catalytic properties especially in oxidation reaction [15-18]. These catalysts could be prepared by different methods direct synthesis [18,19], anion exchange [19,20] and structure reconstruction [21,22]. Pinnavaia et al [21] have tested Co (II) phtalocyanine, intercalated in MgAl hydrotalcite as catalyst in... 

Structure and Surface Properties of Hydrotalcites and Calcined Hydrotalcites... 

Because of their basicity, hydrotalcites stored in air become carbonated, then neutral. They can be decarbonated by treatment above 673 K, and a mixed oxide of MgO structure is then obtained which has basic properties. The basic strength can be estimated from the temperature of decarbonation, and it is then observed that this temperature is affected by the presence of chlorine on the surface, even in trace amounts [11]. This temperature is shifted by ca 30 K towards high temperatures when CP has been fully exchanged by carbonate. Because chlorine is normally present at trace levels only, these anions must have a long-range influence and reduce the basicity of the stronger basic sites. 

The structure and properties of hydrotalcites and its application as additives of plastics. Huagong Kgi, 14 (2), 65-69. 

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