Hydrotalcites

Hydrotalcites are layered double hydroxides with the general formula [137]  

The Physical Chemistry of Materials Energy and Environmental Applications 

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. 

Layered double hydroxides with the hydrotalcite stmcture were synthesized with varying Mg A1 atomic ratios and with different contents of exchangeable CT and CO3 anions. The CO2 adsorption isotherms showed an increase of the uptake and consequently of the basicity with initial CO content and calcination temperature up to 800 K. Increasing the Mg A1 ratio of the hydrotalcites from 2.33 to 3 resulted in an increase of the total number of basic sites [94]. 

In another study, Mg-Al hydrotalcite catalysts with different Mg A1 molar ratios (0.6, 1.4, 2.2, 3.0) were characterized by microcalorimetry using CO2 in the gas phase and benzoic acid in toluene [95]. The calcined Al-rich sample (Mg A1 molar ratio of 0.6) possesses Lewis acid sites similar in strength to those found on AI2O3, but stronger than those found on the Mg-rich hydrotalcites. The liquid-phase basicity microcalorimetry measurements with benzoic acid in toluene correlated very well with the catalytic achvity for Michael addihons. 

The strength and accessibihty of the basic sites of hydrotalcites with an Mg Al ratio of 2, prepared via co-precipitahon of the respective nitrates using carbonate or oxalate as the compensating anions, were assessed by calorimetry of CO2 adsorption. Two different methods were used to activate the Mg-Al hydrotalcites and impart Br0nsted basicity. The initial enthalpies of CO2 adsorption at 303 K on the activated hydrotalcites presented very similar values of-108 kj moT [96]. 

The decarbonation of carbonated layered double hydroxides containing Mg with either Al, Fe or Cr bivalent cations or A1 with Mg, Ni, Cu or Zn divalent cations has been studied by thermal analysis. The enthalpy of adsorption of CO2 on the resulting calcined mixed oxides was measured by calorimetry, with initial heats of adsorption close to those reported for MgO (about lOOkJmol ) and a relatively homogeneous strength distribution [100], 

Clay minerals and Hydrotalcite (Mg0-Al203 and related materials, e.g., calcined and 

Non-oxides KF supported on alumina Oxynitride (silicon oxynitride - SiON, aluminophosphate oxynitride -AlPON, zirconophosphate oxynitride - ZrPON) Lanthanide imide and nitride on zeolite Modified natural phosphate (NP) (calcined NaN03/NP) 

The use of these materials in a range of reactions [isomerization of alkenes and alkynes, C—C bond formation, aldol condensation, Knoevenagel condensation, nitroaldol reactions, Michael addition, conjugate addition of alcohols, nucleophilic addition of phenylacetylene, nucleophilic ring opening of epoxides, oxidation reactions, Si—C bond formation, Pudovik reaction (P—C bond formation) and synthesis ofheterocycles] have been discussed in detail by Ono [248], as well as in the other cited reviews. We will thus discuss here only selected examples. 

A survey of industrial applications indicated ten processes catalyzed by solid bases and 14 by solid acid-base bifunctional catalysts with respect to 103 catalyzed by solid acids. Table 2.12 gives a list of industrial examples. 

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 

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 

Natural hydrotalcite deposits are generally found intermeshed with spinel and other materials due to the existence of nonequUibrium conditions during the formation of the deposits. Other minerals, such as penninite and muscovite, as well as heavy metals, are also found in natural hydrotalcite deposits. There are as yet not known the techniques for separating these materials and purifying the natural hydrotalcite. 

The empirical formulae for some synthetic hydrotalcites are as follows [13,14]  

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Hydrotalcite. Synthetic hydrotalcite minerals are gaining commercial acceptance for thein abiHty to costabiHze PVC ia the presence of other primary stabilizers (see Table 2). The performance of the mixed metal stabilizers are particularly boosted when an equal part level, about 2—3 phr, of hydrotalcite is added to the PVC formulation. These minerals function by trapping HCl within the layered lattice arrangement of atoms. The formula. 

Mg4Al2(OH) 2C02 3H20, is commonly written however, these minerals are generally non stoichiometric by nature and can include some amounts of alternative elements in then compositions. They function similarly to the zeoHtes but exist in layered stmctures and have a different trapping mechanism. In addition to then performance enhancement, the hydrotalcite minerals are compatible with PVC and can be used effectively in clear PVC appHcations as well as the pigmented formulations. 

The basic metal salts and soaps tend to be less cosdy than the alkyl tin stabilizers for example, in the United States, the market price in 1993 for calcium stearate was about 1.30— 1.60, zinc stearate was 1.70— 2.00, and barium stearate was 2.40— 2.80/kg. Not all of the coadditives are necessary in every PVC compound. Typically, commercial mixed metal stabilizers contain most of the necessary coadditives and usually an epoxy compound and a phosphite are the only additional products that may be added by the processor. The requited costabilizers, however, significantly add to the stabilization costs. Typical phosphites, used in most flexible PVC formulations, are sold for 4.00— 7.50/kg. Typical antioxidants are bisphenol A, selling at 2.00/kg Nnonylphenol at 1.25/kg and BHT at 3.50/kg, respectively. Pricing for ESO is about 2.00— 2.50/kg. Polyols, such as pentaerythritol, used with the barium—cadmium systems, sells at 2.00, whereas the derivative dipentaerythritol costs over three times as much. The P-diketones and specialized dihydropyridines, which are powerful costabilizers for calcium—zinc and barium—zinc systems, are very cosdy. These additives are 10.00 and 20.00/kg, respectively, contributing significantly to the overall stabilizer costs. Hydrotalcites are sold for about 5.00— 7.00/kg. 

Tetrabutylammonium benzoate has been used as a catalyst for the polymerization of PO over the temperature range 40—108°C and the yield of polymer was typically low (2—78%) a large amount of unsaturation was present due to chain transfer (95). When synthetic hydrotalcite,... 

Although the selectivity is high, minor amounts of by-products can form by dehydration, condensation, and oxidation, eg, propylene [115-07-17, diisopropyl ether, mesityl oxide [141-79-7] acetaldehyde [75-07-0], and propionaldehyde [123-38-6]. Hydrotalcites having different Al/(A1 + Mg) ratios have been used to describe a complete reaction network for dehydrogenation (17). This reaction can also be carried out in the Hquid phase. 

The development of novel materials for applications such as catalysis has been a very important area of work, with several classes of materials being developed and applied (for example, zeolites and their mesoporous analogues, hydrotalcites, hydroxyapatites, clays). 

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. 

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. 

Hydrotalcite clays, for example, are built up of positively charged brucite layers, for reviews see Cavani et al. (1991). Upon calcination they become active as solid bases in e.g. aldol and Knoevenagel condensations (see Fig. 2.26) (Fgueras et al., 1998 Corma and Martin-Aranda, 1993 Climent e/a/., 1995). 

The conversion of acetone to isophorone with dissolved NaOH as a catalyst is industrially practised. In the recent past, however, attempts have been made to use heterogeneous basic catalyst in the form of hydrotalcites. 

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]. 

Lambrecht, N., Korolkiewicz, R and Peskar, B.M. (1992). Effects of endo nous nitric oxide (NO) inhibition on the gas-troprotective activity of the antacid hydrotalcit in rats. Gas-troenterolcgy 102, A105. 

Many inorganic oxides can be manufactured to provide granular, porous materials with high surface areas, which can readily adsorb organic liquids. Preliminary screening of a range of oxides, namely aluminium oxides, titanium dioxides, zinc oxide, hydrotalcites, zeolites and silicas, indicated that the latter two materials were able to retain the largest quantities of biocide. 

Obviously, use of such databases often fails in case of interaction between additives. As an example we mention additive/antistat interaction in PP, as observed by Dieckmann et al. [166], In this case analysis and performance data demonstrate chemical interaction between glycerol esters and acid neutralisers. This phenomenon is pronounced when the additive is a strong base, like synthetic hydrotalcite, or a metal carboxylate. Similar problems may arise after ageing of a polymer. A common request in a technical support analytical laboratory is to analyse the additives in a sample that has prematurely failed in an exposure test, when at best an unexposed control sample is available. Under some circumstances, heat or light exposure may have transformed the additive into other products. Reaction product identification then usually requires a general library of their spectroscopic or mass spectrometric profiles. For example, Bell et al. [167] have focused attention on the degradation of light stabilisers and antioxidants... 

Neutralise acidity (e.g. deriving from acidic catalyst residues) effectively. Commonly salts of weak organic acids (metal stearates) or inorganic bases (hydrotalcite). Also called antiacids. 

Used to kill rest active catalyst residues, e.g. metal stearate and hydrotalcite. 

There are also different hypotheses on the reaction mechanism, as will be discussed in the following chapters. This is still an open area of research and a further understanding will certainly lead to the development of improved catalysts. There are, in particular, three main areas in which further development is necessary (1) improve the low-temperature activity, e.g. below 250°C, (2) improve resistance by deactivation by sulphur and (3) improve the hydrothermal stability. Hydrotalcite-based materials [3la,97] offer interesting opportunities in this direction. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

Ding, Y., Xu, L., Chen, C., Shen, X. and Suib, S.L. (2008) Syntheses of nanostructures of cobalt hydrotalcite like compounds and Co304 via a microwave-assisted reflux method. Journal of Physical Chemistry C, 112, 8177-8183. 

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