Modified Mg-Al hydrotalcite

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). 

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. 

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. 

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>. 

Mg-Al hydrotalcite catalysts were used for the transesterification of rape-seed oil with oil to methanol ratio of 1 6, at a temperature of 65° C with 1.5% catalyst in reaction time of 4 h. It was found that calcined hydrotalcite with Mg/Al molar ratio of 3.0 showed highest transesterification activity with a conversion of 90.5% and the catalyst could be separated easily [61]. Similar types of catalysts were employed for the transesterification of soybean oil with methanol. Oil to methanol ratio of 1 15 at reflux temperature for 9 h with 7.5% catalyst yielded 67% of biodiesel [62]. In a similar study for the transesterification of canola oil with methanol using Mg-Al hydrotalcite catalyst prepared by coprecipitation method, 71.9% conversion was observed. The conditions followed were 1 6 ratio of oil to MeOH 60° C for 9 h with 3% catalyst [63]. Cerium modified Mg-Al hydrotalcites were used for the transesterification of soybean oil with methanol, fith a molar ratio of 1 9 of oil to methanol, at a temperature of 67° C and 5% of catalyst and reaction time of 3 h, a biodiesel yield of 90% was obtained. It was observed that catalysts with Ce/Mg atomic ratio of >0.3 was able to produce biodiesel of 90% yield [64]. Potassium loaded-calcined Mg-Al hydrotalcite was used as a base catalyst for the transesterification of palm oil with methanol. Biodiesel yield of 86.8% was achieved with 7% catalyst at 100° C for 6 h using 1 30 molar ratios of oil to methanol. The blend with prepared biodiesel was tested for its impact on elastomer properties [65],... 

Dias, A. P. S., Bernardo, J., Felizardo, R, and Correia, M. J. N. Biodiesel production over thermal activated cerium modified Mg-Al hydrotalcites. Energy 41, 344-353 (2012). 

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. 

Keywords Aromatic carbraiyl compounds, malononitrile, ethyl cyanoacetate, modified and activated Mg-Al hydrotalcite (MHT), toluene or DMF, room temperature, condensation, Knoevenagel reaction, substituted alkenes... 

The catalyst samples were prepared in our laboratory. The synthesized Na-ZSM-5 zeolite was modified by conventional or solid state ion-exchange [11] to form H-, Fe-, Cu-, Ni- and Ti-ZSM5 samples, while the mesoporous catalysts (Fe- and Ti-MCM-41) were synthesized by isomorphous substitution [12], as well as the hydrotalcites containing Fe-, Cu-, Cr- or Ca-oxide in the Mg,Al-LDH structure [13]. 

Basic supports such as calcined hydrotalcite-like materials are also employed for this purpose. Particularly, Sun et al. proposed Mg(Al)0 type hy-drotalcites to be the ideal catalysts. The Al cations present in the support stabilize the dispersed metal particles against sintering. Most of the reports in this direction suggest Sn as the best element for promotion. The size of Pt ensembles and the bond strength of chemisorbed hydrocarbons are modified by the addition of Sn. Thus, Pt-Sn combination performs better than Pt alone. Sn forms an alloy with platimun. The performance of the catalyst was foimd to be dependent on the composition of the Pt and Sn precursors and the catalyst preparation procedure. PtGa bimetallic nanoparticles were foimd to exhibit not only high activity and selectivity for ethane dehydrogenation, they also... 

Selectivity to ethene achieved about 94-97% (58% conversion, 450°C) on the VMoWNb-0 catalyst. The effect of modifying appeared to be much more poorly under conditions for ODH of propane. The VMoWNb-0 catalysts exhibited high efficiency in oxidative dehydrogenation of ethylbenzene into styrene and dehydrocyclization of -octane (to ethylbenzene and styrene) in the presence of oxygen or CO2. The yield of styrene from ethylbenzene was close to 80% at the 97-98% selectivity (500°C). In a scanty studied reaction of n-octane dehydrocyclization the yields of styrene and ethylbenzene reached 23-28% at the total selectivity of 90%. The efficiency in ODH of several alcohols over V-Mo-W-Nb-Mg-Al-0 catalysts prepared using complex hydrotalcite-like layered hydroxosalts (LDHs) as precursors has been determined. 

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