Mg—Al hydrotalcites

As part of a search for environmentally friendly solid acid-base catalysts, a modified Mg-Al hydrotalcite has been used as a base catalyst for aldol and Knoevenagel condensations. Yields are often quantitative, reaction times are about Ih, the catalyst can be recovered by filtration, and only moderate temperatures are required (60 °C for the aldol, ambient for the Knoevenagel). 

Alternative approaches for stabilizing the Ni performance include (i) its incorporation in oxide matrices (for instance, controlled Ni metal crystallites were prepared by reduction of Ni-containing Mg/Al hydrotalcite-type precursors by Vaccari and coworkers [128,129]) and (ii) the addition of an active component (Co, Fe and especially noble metals), reducing carbon deposition [130, 131]. 

Sorrentino, A., Gorrasi, G., Tortora, M., Vittoria, V., Costantino, U., Marmottini, F., et al. (2005). Incorporation of Mg-Al hydrotalcite into a biodegradable poly(3-caprolactone) by high energy ball milling. Polymer, 46, 1601-1608. 

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. 

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.,... 

Kumbhar, P. S., Sanchez-Valente, J., Lopez, J. and Figueras, F. Meerwein-Ponndorf-Verley reduction of carbonyl compounds catalysed by Mg-Al hydrotalcite. Chem. Commun., 1998, 535-536. 

Lopez-Salinas, E., Tomita, N., Matsui, T., Suzuki, E. and Ono, Y. (1993). Reactivity of interstitial NiCl4 2 complex anion in a Mg-Al hydrotalcite-like clay towards alkyl bromides in the liquid phase. J. Mol. Catal. 81, 397. 

Kanezaki, E., Kinugawa, K. and Ishikawa, Y. (1994). Conformation of intercalated aromatic molecular anions between layers of Mg/Al-hydrotalcites and Zn/Al-hydrotalcites. Chem. Phys. Lett. 226, 325. 

Bulbule, V. J., Deshpande, V. H., Velu, S., Sudalai, A., Sivasankar, S. and Sathe, V. T. (1999). Heterogeneous Henry reaction of aldehydes diastereoselective synthesis of nitroalcohol derivatives over Mg-Al hydrotalcites. Tetrahedron 55, 9325. 

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,... 

Surpur MP, Kshirsagar S, Samant SD (2009) Exploitation of the catalytic efficacy of Mg/Al hydrotalcite for the rapid synthesis of 2-aminochromene derivatives via a multi-component strategy in the presence of microwaves. Tetrahedron Lett 50 719-722... 

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]. 

Note, however, that rehydration of Mg-Al hydrotalcites in the liquid phase using ultrasound or a high stirring speed [267] leads to nanoplatelets with surface areas of400 m g , displaying catalytic activities in aldol condensations up to eight-times higher than the best catalytic system reported in the literature. There are thus alternative methods to increase the performances of HT materials. 

Figure 2.45 Acid-base bifunctional reaction mechanism in the synthesis of cyclic carbonates over mixed oxides obtained from a Mg-Al hydrotalcite. Source Kaneda et al. [28].

Nucleophilic reactions of unmodified aldehydes are usually diiScult to control, affording complex mixture of products, often due to the high reactivity of the formyl group under either basic or acidic reaction conditions. The activity order of the supported amines was secondary > primary > tertiary, which may suggest the intervention of an enamine pathway the enals were exclusively obtained as ( ) isomers. Notably, FSM-16-(CH2)3-NHMe exhibited higher activity than conventional solid bases such as MgO and Mg-Al-hydrotalcite [hexanal self-aldol condensation FSM-16-(CH2)3-NHMe 97% conversion and 85% yield in 2h, MgO 56% conversion and 26% yield in 20 h, Mg-Al-hydrotalcite 22% conversion and 11% yield in 24 h]. 

The reaction, which could not be catalyzed by ordinary basic metal oxides such as MgO and Mg-Al-hydrotalcite, was efficiently promoted by FSM I6 (CH2)3-NI1Mc, suggesting the intervention of the enamine pathway. 

Hou, W.G. et al.. Studies on zero point of charge and intrinsic ionization constant of Zn-Mg-Al hydrotalcite-like compounds. Colloid Polym. Sci., 281, 738, 2003. 

Tri-component mixed oxides, Mg/Al/La, Mg/Al/Ce and Mg/Al/Zr, were prepared with the co-precipitation technique, with the aim to improve the basic properties with respect to the reference Mg/AI mixed oxides, obtained from Mg/Al hydrotalcite-like precursors. For samples containing Ce and Zr, the basic properties (as determined by CO2 adsorption and TPD) were considerably improved with respect to Mg/Al/0, whereas for the La-containing system a decrease in basicity was observed. These materials were tested in m-cresol methylation, in both liquid- and gas-phase reactions. A relationship was found between basic features and catalytic performance. 

A modified Mg Al hydrotalcite was revealed as an excellent catalyst for the conversion of a-hydroxy carbamates into the corresponding oxazolidin-2-ones. The reaction is performed in refluxing toluene affording the expected products in high yields (83-96%, 6 examples) <05OBC967>. 

This carbanion further adds to the carbonyl compound to form an intermediate, which removes a proton from water to give the final nitro aldol product (Figure 3.9). Solid bases without Bronsted hydroxy groups such as decarbonated hydrotalcites are not active in this reaction. The Mg-Al hydrotalcite catalyst is non-toxic and can be recycled with little loss in activity.42... 

Table 1. Kinetic rate constants K) for the condensation of benzaldehyde with ethyl cyanoacetate (pXa 9.0), ethyl acetoacetate (pATj 10.7), and ethyl malonate (pK 13.2) on alkaline-exchanged zeolites, NaGeX, cesium-exchanged sepiolite, and calcined Mg-Al hydrotalcite.

Kumbhar et al. have used hydrotalcites in the MPV reduction of carbonyl compounds [22]. Mg-Al hydrotalcites with Mg/Al molar ratios varying between 2 and 5, activated at temperatures ranging from 350 up to 550 °C, were both active and selective in the reduction of 4-tert-butylcyclohexanone with / -PrOH. The highest activity was found for a hydrotalcite with an Mg/Al ratio of 3, calcined at 450 °C. This catalyst, which proved recyclable, was also suitable for the reduction of ci-tronellal, cinnamaldehyde, and citral to the corresponding unsaturated alcohols, which are valuable perfumery compounds. The activity was explained by the synergistic effect of both strong Lewis basicity and mild acidity. 

K Koteswara Rao, M Gravelle, J V Valente, F Figueras. Activation of Mg-Al hydrotalcite catalysts for aldol condensation reaction. J Catal 173 115-121, 1998. 

The good selectivity obtained in the iGioevenagel product (95% at 90% conversion) versus the Michael ones could be explained either by a lower basicity of the encaged caesium oxide on zeolite compare to Mg-Al hydrotalcites [26] or by a shape selective properties of this modified zeolites. Complementary work is needed to determine the origin of this last result. 

II) Katsuomi, T., Tetsuya. S., Peng, W., Tokuhisa, K. and Ken, T. (2004). Autothcrmal reforming of CH4 over supported Ni catalysis prepared from Mg Al hydrotalcite-like anionic clay. J. Cata ., 221,43-54. 

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