Hydrotalcite, as catalyst

An example of commercial interest is the synthesis of citronitrile (Scheme 17), a compound with a citrus-like odor, which is used in the cosmetics and fragrance industries. The first step in the synthesis of citronitrile is the Knoevenagel condensation of benzyl acetone and ethyl cyanoacetate. This condensation has been carried out with MgO and Al-Mg calcined hydrotalcites as catalysts (148). Similar results were obtained with the two solid catalysts, with yields of 75% of the Knoevenagel adduct. 

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... 

Behler, A., Raths, H. C., Friedrich, K. and Herrmann, K., Use of calcined hydrotalcites as catalysts for ethoxylation or propoxylation of fatty acid esters, German Patent DE 3 914131.4, to Henkel KGaA (1990) Chem. Abstr., 114, 206576. 

The objective of this work is to study the potential of modified ZSM5 zeolite, MCM41 mesoporous silica, hydrotalcites (HD) and HD originated mixed oxides as catalysts for degradation of PE, PP PS and PVC using thermal analytical measurements and laboratory reactor experiments. 

Since the hydrotalcite-like material contains exchangeable interlayer anions, it can be used as a catalyst for reactions for which anion-exchange resins have been used as catalyst. The disproportionation of trimethoxysilane (eq. 2) is... 

A one-pot quinoline synthesis starting with 2-aminobenzyl alcohol and a,/3-unsaturated ketones using ruthenium-grafted hydrotalcites as a heterogeneous catalyst has been reported (Scheme 64) <2004TL6029>. Molecular oxygen was used for the oxidation of ruthenium and the styryl quinoline 42 was produced in good yield. The use of other donors, for example, octanal and phenylacetonitrile, yielded 3-amylquinoline and 2-amino-3-phenylquinoline, respectively. 

We have shown that the isomerization of glucose into fructose (Scheme 7.2) was easily achieved in the presence of cation-exchanged zeolites and hydrotalcites, in water as the solvent, at higher temperatures and higher glucose concentrations than in the case of ion-exchange resins and enzymes as catalysts.[20]... 

While much effort has been put into obtaining materials with basic sites of different strength, the exact nature of the basic sites acting on those catalysts is not always known. For instance, reactions like aldolization, hydrogen transfer or double bond isomerization require either Brpnsted or Lewis type sites or even the association of strong or medium basic sites with weak acid sites/13-161 Many of these requirements (acid-base associations, versatility of the basic strength) can be accomplished by hydrotalcites and this would explain their significant development as catalysts precursors/13-15,171... 

The Tishchenko reaction of furfural has been found to be difficult when carried out by traditional homogeneous catalysis, but excellent results for the Tishchenko reaction of furfural and 3-furaldehyde[90,91] using CaO and SrO as catalysts have been obtained. The use of other solid base catalysts such as La203, Zr02, ZnO, 7-alum in a, hydrotalcite and KOH/alumina, was unsuccessful. An investigation of the influence of the pretreatment temperature of the MgO and CaO catalysts showed that the active basic sites for this transformation are not OH groups, but rather O2 ions on the MgO surface. 

Corma, A., Fomes, V. and Rey, F. Hydrotalcites as base catalysts influence of the chemical composition and synthesis conditions on the dehydrogenation of isopropanol, J. Catal., 1994, 148, 205-12. 

Guida, A., Lhouty, M. H., Tichit, D., Ligueras, L and Geneste, P. Hydrotalcites as base catalysts. Kinetics of Claisen-Schmidt condensation, intramolecular condensation of acetonylacetone and synthesis of chalcone, Appl. Catal., A, 1997, 164, 251-264. 

Lakshmi Kantam, M., Choudary, B. M., Reddy, C. V., Koteswara Rao, K. and Figueras, F. Aldol and Knoevenagel condensations catalyzed by modified Mg-Al hydrotalcite a solid base as catalyst useful in synthetic organic chemistry, Chem. Commun., 1998, 1033-1034. 

Table 5 Oxidation of various alcohols to their corresponding aldehydes or ketones with ruthenium hydrotalcites (HTs) as catalysts using molecular oxygen. 2 mmol substrate, 0.3 g HT (around 14 mol %), in toluene, 60 °C, 1 bar 02. Conversion 100%...

Costantino U, et al. Cu-Zn-Al hydrotalcites as precursors of catalysts for the production of hydrogen from methanol. Solid State Ionics. 2005 176(39-40) 2917-22. 

Excellent results were also obtained using activated hydrotalcite as a solid base catalyst in the Knoevenagel condensation of benzaldehyde with ethylcya-noacetate [110], ethylacetoacetate [111] or malononitrile [112] (see Fig. 2.34). Similarly, citronitrile, a perfumery compound with a citrus-like odor, was synthesized by hydrotalcite-catalyzed condensation of benzylacetone with ethyl-cyanoacetate, followed by hydrolysis and decarboxylation (Fig. 2.34) [113]. 

Mg/Al hydrotalcite as a heterogeneous base catalyst has been used by Samant and co-workers [170] in combination with microwave irradiation for the S3mthesis of 2-aminochromenes 118 via a multicomponent condensation of an aromatic aldehyde, malononitiile and 1-naphthol. The hydrotalcite is a heterogeneous basic catalyst and could easily be separated from the reaction mixture by simple filtration. The recovered catalyst was used for successive runs to investigate its reusability,... 

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. 

Many papers dealing with applications, and variations in synthesis parameters of hydrotalcite-like compounds can be found in the open literature, but it is beyond the scope to present an overview here. On concluding this paragraph, it should be mentioned that hydrotalcite-like compounds are often used as catalyst precursors in the catalyst industry, because of the properties stated above. 

Methylation can be carried out with either acid or basic catalysts, and catalyst properties affect the distribution of the products, especially when different positions for methylation are present. Acid zeolites are very active catalysts in alkylation of phenol derivatives however, the considerable formation of heavy compounds leads to a fast deactivation of the catalyst. Basic catalysts, such as single oxides (MgO) and Mg-Al mixed oxides, have been found to be less active than the acid ones, but did not form heavy products [4]. Mg-Al mixed oxides, prepared starting from hydrotalcite precursors, have shown the best basic features, and indeed in recent years these materials have been reported as catalysts for different basic reactions, such as the Claisen-Schmidt condensation, the Knoevenagel condensation, and many others [5-9]. 

Y Watanabe, T Tatsumi. Hydrotalcite-type materials as catalyst for the synthesis of dimethyl carbonate from ethylene carbonate and methanol. Microporous and mesoporous materials 22 399-407, 1998. 

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