Magnesium hydrotalcites

While silica-aluminas, as represented in figure 11, still to a larger smaller or degree maintain a basic boehmite structure, there are other examples of alumina mixed oxides where a totally different crystal structure is obtained, such as in the case of aluminum/magnesium hydrotalcites (figure 12). 

MAS NMR study of high-pressure hydrous magnesium silicates has been published. NMR data have also been reported for MgAl204, f Al), CaAUO , Mg-Ca-Si-Al-O-N glasses, f Al), " magnesium hydrotalcite, f Al), " alkali-activated slag cement, f Al, acid-activated Ca-bentonite,... 

MgO samples were prepared by heating basic magnesium carbonate for 3 h. CaO samples were made from calcium carbonate pretreated in N2 at 1173 K for 2 h. MgC-A Os samples were prepared by heating hydrotalcite at 823 K for 4 h. 

Thermally activated mixed metal hydroxides, made from naturally occurring minerals, especially hydrotalcites, may contain small or trace amounts of metal impurities besides the magnesium and aluminum components, which are particularly useful for activation [946]. Mixed hydroxides of bivalent and trivalent metals with a three-dimensional spaced-lattice structure of the garnet type (Ca3Al2[OH]i2) have been described [275,1279]. 

Realizing the importance and effectiveness of the magnesium species in the sorption of SO, in the late 1980s, Akzo Nobel patented the use of hydrotalcite and related compounds for nse in an FCC to reduce SO emissions [8,9,10]. Hydrotalcite contains more active Mg species than spinel. Hydrotalcite-based compounds typically contain 3-4 moles of Mg per mole of Al, while spinels contain only 1 mole of Mg per 2 moles of Al (Figure 16.4). 

Fig. 4. Calculated estimate of the long-term stability of hydrotalcite in seawater at 60 C (a) log ions activity product/equilibrium constant (log IAP/K) (b) quantity of minerals formed (c) magnesium concentration in solution.

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Shen, J., Kobe, J. M., Chen, Y. and Dumesic, J. A. (1994). Synthesis and surface acid/base properties of magnesium-aluminium mixed oxides obtained from hydrotalcites. Langmuir 10, 3902. 

Tu, M., Shen, J. Y. and Chen, Y. (1997). Microcalorimetric studies of surface acid/base properties of magnesium-iron catalysts prepared from hydrotalcite-type precursors. J. Solid State Chem. 128, 73. 

Velu, S. and Swamy, C. S. (1997). Effect of substitution of Fe3+/Cr3+ on the alkylation of phenol with methanol over magnesium-aluminium calcined hydrotalcite. Appl. Catal. A 162, 81. 

The replacement of conventional bases, such as NaOH, KOH and NaOMe, by recyclable solid bases, in a variety of organic reactions, is also a focus of recent attention [27, 40]. For example, synthetic hydrotalcite clays, otherwise known as layered double hydroxides (LDHs) and having the general formula Mg8 xAlx (0H)16(C03)x/2 nH20, are hydrated aluminum-magnesium hydroxides possess-... 

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). 

Catalyst. One catalyst, prepared to contain equi-atomic amounts of lead, magnesium, and aluminum, was prepared for this study. Synthesis was accomplished by the hydrotalcite preparation procedure reported by Reichle 12). It is a standard aqueous precipitation and crystallization procedure that avoids filtering and washing problems associated with gel precipitates. A solution of Pb, Mg, and Al nitrates was added under agitation to a solution of NaOH and Na2C03 and maintained at 70 C for thirty hours. This caused precipitation of crystalline material. 

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