Hydrotalcites hydrated

Wallace DC (1972) Thermodynamics of Crystals. Dover Publications, Mineloa, New York Wang J, Kalinichev AG, Kirkpatrick RJ, Hou X (2001) Molecular modeling of the stmcture and energetics of hydrotalcite hydration. ChemMater 13 145-150... 

Kumbhar, R S., Sanchez-Yalente, J. and Figueras, F. (1998). Reduction of aromatic nitro compounds with hydrazine hydrate in the presence of iron(III) oxide-MgO catalyst prepared from a Mg-Fe hydrotalcite precursor. Tetrahedron Letts. 39, 2573. 

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-... 

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

Complex precipitates, such as layered double hydroxides, are frequently prepared as precursors of oxide catalysts. Hydrotalcite is a hydrated hydroxy-carbonate mineral with the formula Mg,sAl2(0H)i6C03-4H20. It represents the better known and most popular member of a family of layered double metal hydroxide com-... 

The thermal decomposition of hydrotalcites also is intriguing. They decompose on calcination to nanocrystalline spinel-like phases, and eventually, to crystalline spinels. This pathway initially produces spinels which are hydrated in their large surfaces and poorly ordered in their cation distributions. These materials are very reactive and analogous to precursors sought in ceramic synthesis. The thermochemical study of the evolution of MgAl204 from nitrate precursors (McHale et al. 1998) may in fact have encountered hydrotalcite as an intermediate. Further study of the thermal evolution of hydrotalcite nanomaterials may be applicable to both earth and materials sciences. 

The specific properties of hydrated hydrotalcites appear not only in the aldoli-zation of acetone, but in many other aldolization reactions. For example, in the aldol condensation of benzaldehyde with acetone the hydrated form catalyzes the reaction at 273 K, yielding aldol as the main product instead of benzalacetone, obtained on the calcined sample. Competitive adsorption kinetics are still observed, with a much greater adsorption coefficient for benzaldehyde. As suggested earlier from Hammett relationships, this reaction can be generalized with success to many substituted benzaldehydes [32], although the reaction could be performed selectively at 273 K with benzaldehyde only, and substituted benzaldehydes required a reaction temperature of 333 K. Because of this high temperature the reaction usually gives a, unsaturated ketones isolated yields are > 95 %. 

The aldol condensation of benzaldehyde with acetophenone has been used to compare hydrated hydrotalcites with many solids claimed to be strong solid bases-KF and KNO3 supported on alumina, X zeolites containing excess Cs or Mg, and lanthanum oxide. Only hydrated HDT and KF/AI2O3 could be used to perform this reaction at 273 K-HDT with 100% selectivity for chalcone and KF with lower selectivity, because of secondary Michael addition of the chalcone to acetophenone. The other solids were inactive, but could induce comparable conversions at 423 K. 

Formula Chemical class Chemical type Mg5Fe(C03)(0H)i3 Crystal system Hydrated carbonate with hydroxyl or halogen Mineral group Miscellaneous Space group Trigonal Hydrotalcite (sjogrenite) P3 or P3... 

Formula Chemical class Chemical type MgsAl,(C03)0Hi,-4H,0 Hydrated carbonate with hydroxyl or halogen A B (X03)/,xH30 Crystal system Mineral group Space group Trigonal Sjogrenite (hydrotalcite) R3m or R3m... 

This is the consequence of low mobility of Fe ions which do not migrate but are located in hydrates formed in situ from the ferrite phase [27]. Apart from the iron ions in octahedral sites there are the AF+, Tf + and ions the latter ones are concentrated inbrownmillerite in clinker. The magnesium ions show also a low mobility in pore solution they do not produce bracite but hydrotalcite [Mg Fe, 2(011)2] (C03)o, 25(OH)o5 [27]. The formation of hydrotalcite in situ after brownmillerite is promoted by the highest concentration of magnesium in this phase. 

Co-precipitation is also applied in the synthesis of hydrotalcite-like precursors (HTlcs) for catalysts e.g. based on Cu-Zn-Al, or Co-Al catalysts. Synthetic HTlcs, with a general formula (OHjg] [A ] nHgO, are hydrated hydroxycarbonates of a random lamellar structure. These materials can be visualized as brucite-type octahedral layers, in which cations partially substitute for cations (x = The cations are located in the center of... 

Hydrotalcite is a natural mineral with a white color and pearl-like luster. It is mined in small quantities in Norway and the Ural area of Russia. Other mines of hydrotalcite are located in Austria, Canada, Czech Republic, Germany, Sweden, South Africa, and United States [7]. Natural hydrotalcite is a hydrated magnesium-, aluminum- and... 

Hydrated hydrotalcite is active for cross-aldol condensations of acetone with various substituted benzaldehyde (22). The aldol was the major product at room temperature but the yield was low. At 333 K, yields were improved to give selectively the corresponding dehydration products of aldol condensation (22). 

Nanoclays can be categorized into cationic and anionic types. Cationic nanoclays are based on smectite clays. An example is montmorillonite (MMT), a hydrated Al, Mg silicate that may contain cations such as Na+ and Ca++ between the anionic layers. In contrast, anionic clays contain cationic layers and anions such as Cr and in the interlayer space. Typical examples include layered double hydroxides (LDH) and hydrotalcite (HT), a mostly synthetic hydrated magnesium and aluminum carbonate salt. Whereas MMT is commonly used as a nanofiller to improve thermal, mechanical, and barrier properties, LDHs have many attractive properties that lead to application as surfactant adsorbents, biohybrid materials, antacid food formulations, acid neutralizers, and active pharmaceutical ingredients excipients [37, 13, 28, 14, 35]. 

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