Hydrotalcites structure

Hydrotalcites are layered double hydroxides with the general formula Mg6Al2(0H)i6[C03].4H20. Loading these compounds with potassium carbonate strongly increases their C02 uptake [25, 35], Notably, the hydrotalcite structure already breaks down below 400 °C [26] into a mixed metal oxide. 

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.,... 

Incorporation of Pt onto calcined Mg(Al)0 was accomplished in several ways. Methods involving aqueous impregnation regenerated the hydrotalcite structure since... 

An alternative approach is precipitation by combining an alkaline NaAlC>2 solution with an acidic nickel nitrate solution [51]. In this case as well, hydrotalcite-like species are probably formed first although, due to the very high nickel contents reported, some nickel hydroxide forms as well. Pure hydrotalcite-like structure are reported to form for Ni Al atomic ratios between 2 and 3 [52] or 4 [53] outside this range the hydroxides of the excess species form as additional phases. Rapid precipitation tends to form pure Al(OH)3 due to the lower solubility. Aging can lead to recrystallization of some of the pure hydroxides to the hydrotalcite structure as well. 

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. 

Figure 21.15 Schematic of the hydrotalcite structure. (Reproduced from [56] with permission).

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... 

Figure 2.39 Schematic representation of a hydrotalcite structure (MggAl2(0H)igC03-4H20).

Fig. 7-3. Representative hydrotalcite structure. The charge imparted by Al3+ incorporation into the edgesharing octahedra of the brucite sheets is compensated by charge balancing anions in the interlayer.

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. 

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... 

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... 

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. 

In short, EXAFS can be used as a fingerprint of the coprecipitate formation onto alumina, providing the hydrotalcite structures are not too poorly crystallized. Furthermore, the M(II)/A1(III) atomic ratio in the coprecipitates can be roughly estimated. Thus, the influence of preparation parameters such as the M(II) concentration in the impregnating solution, can also be evaluated. 

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). 

Figure 6.1 Stacking of brucite-like layers forming the hydrotalcite structure. Reprinted from Coordination Chemistry Reviews, Vol. 181, Vicente Rives and Maria Angeles Ulibarri, Layered double hydroxides (LDH) intercalated with metal coordination compounds and oxometalates, pp. 61 - 120, 1999, with permission from Elsevier.

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 figirre 1 the XRD profiles of the hydrotalcite like precursors substituted by Mn and by binary Mn-Co and Mn-Cu mixes synthesized by the conventional coprecipitation method are shown. The profiles present a group of signals that are characteristic of the hydrotalcite structure (JCPDS No. 89-0460). However, the formation of a contaminant corresponding to rhodocrosite (MnCOs JCPDS No. 44-1472), is identified. This is a crystalline phase which appears with a greater incident in the profiles of mixed binaries, suggesting that the addition of another metal favors the segregation of said phase. 

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