Hydrotalcite-based catalysts

Climent, M. J., Corma, A., Guil-Lopez, R., Iborra, S. and Primo, J. Solid catalysts for the production of fine chemicals the use of ALPON and hydrotalcite base catalysts for the synthesis of aryl sulfones, Catal. Lett., 1999, 59, 33-38. 

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.

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. 

The most active Pd based catalysts in the table have been reported by Shishido et al.51 who have employed hydrotalcite precursors, using a solid phase crystallisation (SPC) method. Comparisons made with catalysts prepared via impregnation showed SPC to afford higher activity catalysts, which was ascribed to better dispersion and also easy desorption of CO. The inclusion of a Cr component in these types of catalyst has also been reported to be beneficial.52 In terms of the desorption of CO, isotopic studies performed by McKee53 over ruthenium and platinum catalysts have indicated that the rate determining step of methanol decomposition is the fission of a... 

The authors339 found that the best catalysts were those whereby the ceria was impregnated prior to calcination of the hydrotalcite-based Cu-Zn precursors. The addition of ceria led to an important improvement in catalyst stability. For example, during stability tests (0.7 nl/min N2, 0.3 nl/min CO, 0.3 nl/min H20, T = 300 °C, P = 3 atm), the undoped catalyst calcined at 400 °C dropped to 25% of its initial activity within the first couple of hours on-stream. In contrast, the ceria-doped catalyst slowly decreased to 25% of its initial activity in about 50 hours. The activity was linked to the metallic Cu surface. In either case, the deactivation is considered to be very rapid. 

Air Products and Chemicals pioneered the use of these potassium carbonate promoted hydrotalcite-based materials (K-HTC) for sorption-enhanced reforming of methane [26]. Mixing the K-HTC with a SMR catalyst in a 2 1 ratio gave high (90+%) conversions of methane at temperatures as low as 400 °C. In the first instance Ni-based catalysts were used, but they were not resistant to the environ-... 

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). 

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. 

Corma, A., Iborra, S., Primo, J. and Rey, F. One-step synthesis of citronitrile on hydrotalcite derived base catalysts, Appl. Catal., A, 1994, 114, 215-225. 

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. 

Figure 5.3.7 The variation of CO and methanol production rates with space velocity (A) and methanol synthesis rates on promoted CuZn-based catalysts derived from hydrotalcite and Malachite precursors (B). Reaction conditions 200 mg catalyst, 30 bar, 230°C, 3 1 H2 C02.

Kumbhar, P. S., Sanchez-Valente, J. and Figueras, F. (1998). Modified Mg-Al hydrotalcite a highly active heterogeneous base catalyst for cyanoethylation of alcohols../ Chem. Soc., Chem. Commun., 1091. 

Cabello, F. M., Tichit, D., Coq, B., Vaccari, A. and Dung, N. T. (1997). Hydrogenation of acetonitrile on nickel-based catalysts prepared from hydrotalcite-like precursors. J. Catal. 167, 142. 

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. 

Synthesis of 2-aminothiophenes 564 by using calcined Mg-Al (Mg Al = 4 1) hydrotalcite (HT) as a heterogeneous base catalyst has been reported (Equation 26) <2001SC3113>. 

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,... 

Hydrotalcites are of interest because of their ability to function as base catalysts, for example References [57-61]. In a very topical application, IFP have developed a biofuels process using Al, Zn and Ti mixed oxides [62]. The process is being commercialized by Axens. Figueras and coworkers [63]) have studied Mg-Al... 

Many convention- acid and base catalysts are being replaced by more eco-fnendly solid catalysts. In the present investigation calcined magnesium-aluminium hydrotalcites are used for the tert-butylation of phenol using isobutanol. The aim of the present work is to correlate the acid-base properties of hydrotalcites with the obtained product distribution. 

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