Hydrotalcites condensation catalysts

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. 

Gas-Phase Acetone Condensation over Hydrotalcite-like Catalysts 55... 

Figure 6. Hydrocarbon selectivity (wt%) within condensate fraction from chlorite-and hydrotalcite-derived catalysts. Conditions are shown in Table IV.

Figure 2.42 Examples of reactions catalyzed by hydrotalcite-based catalysts for the synthesis of flavors and fragrances (a) aldol condensation (b) Knoevenagel condensation (c) double-bond isomerization.

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). 

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. 

Roelofs, J. C. A. A., van Dillen, A. J. and de Jong, K. P. Base-catalyzed condensation of citral and acetone at low temperature using modified hydrotalcite catalysts, Catal. Today, 2000, 60, 297-303. 

Abello, S., Medina, F., Tichit, D., Perez-Ramirez, J., Cesteros, Y., Salagre, P. and Sueiras, J. E. Nanoplatelet-based reconstructed hydrotalcites towards more efficient solid base catalysts in aldol condensations, Chem. Commun., 2005, 1453-1455. 

Dumitriu, E., Hulea, V., Chelaru, C., Catrinescu, C., Tichit, D. and Durand, R. (1999). Influence of the acid-base properties of solid catalysts derived from hydrotalcite-like compounds on the condensation of formaldehyde and acetaldehyde. Appl. Catal. A 178, 145. 

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]. 

Interesting recent developments are the use of hydrotalcite supported on carbon nanofibers [119], to facilitate recovery of the catalyst by filtration, and the use of synthetic hydroxyapatite, Ca10(PO4)6(OH)2 as a solid base catalyst in a variety of reactions including Michael additions [120]. The supported hydrotalcite exhibited higher activities and selectivities than the conventional unsupported material in the aldol condensation of citral with acetone [119]. 

Cesium-exchanged zeolite X was used as a solid base catalyst in the Knoevenagel condensation of benzaldehyde or benzyl acetone with ethyl cyanoacetate [121]. The latter reaction is a key step in the synthesis of the fragrance molecule, citronitrile (see Fig. 2.37). However, reactivities were substantially lower than those observed with the more strongly basic hydrotalcite (see earlier). Similarly, Na-Y and Na-Beta catalyzed a variety of Michael additions [122] and K-Y and Cs-X were effective catalysts for the methylation of aniline and phenylaceto-nitrile with dimethyl carbonate or methanol, respectively (Fig. 2.37) [123]. These procedures constitute interesting green alternatives to classical alkylations using methyl halides or dimethyl sulfate in the presence of stoichiometric quantities of conventional bases such as caustic soda. 

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,... 

Both chlorites and hydrotalcites performed as Fischer Tropsch catalysts, with an Anderson-Schulz-Flory value of about 0.8, as expected from the presence of metallic cobalt. Both yielded condensates consisting mainly of n-alkanes, peaking at and... 

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). 

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction has been referred to as a Tollens reaction (see 16-43). However, the classical condensation of an aliphatic nitro compound with an aldehyde or ketone is usually called the Henry reaction or the Kamlet reaction, and is essentially a nitro aldol reaction. A variety of conditions have been reported, including the use of a silica catalyst, Mg—A1 hydrotalcite, a tetraalkylam-monium hydroxide,proazaphosphatranes, " or an ionic liquid.A solvent free Henry reaction was reported in which a nitroalkane and an aldehyde were reacted on KOH powder. Potassium phosphate has been used with nitromethane and aryl aldehydes. The Henry reaction has been done using ZnEt2 and 20%... 

The aldol condensation of formaldehyde and acetone to methylvinylketone has been investigated by Suzuki and Ono [20] on a series of solids MgAl hydrotalcite was found to be the best catalyst, leading to 21 % conversion of acetone at 673 K, with 96 % selectivity for acetone and 64 % for formaldehyde. The carbonate form was the most suitable. Exchange by sulfates or chromates resulted in poor activity. 

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]. 

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. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

The condensation/hydrogenation of acetone to meOiyl isobutyl ketone (MIBK) was studied on a series of hydrotalcite-supported noble metal catalysts in a liquid-phase batch microreactor at 118 C and 400 psig. The O.lwt.% Pd/HTC gave the highest acetone conversion (38%) and selectivity to MIBK (82%). The HTC catalyzes the condensation of acetone to diacetone alcohol (DAA) and its subsequent dehydration to mesityl oxide (MO), whereas the noble metal selectively hydrogenates MO to MIBK. A multifunctional catalyst is required for this acetone-to-MIBK reaction. 

A number of solid base catalysts have been reported in the literature to be active for acetone condensation. These include alkali oxides (NajO, K O, CsjO) (7), alkaline earth oxides (MgO, CaO, BaO) (1,4-6), transition metal oxides (7) and phosphates (8-11), ion-exchange resins (12), zeolites (13) and clay minerals and hydrotalcites (HTC) (14,15). A suitable catalyst for the acetone-to-MIBK reaction must have several properties the condensation of acetone to DAA is catalyzed by either basic or acidic sites, the dehydration of DAA to MO is acid-catalyzed, and the selective hydrogenation of MO to MIBK requires appropriate metal sites (7,8). 

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