Hydrotalcite-like compounds as catalysts in liquid phase organic synthesis I.: Knoevenagel condensation promoted by [Ni0.73Al0.27(OH)2](CO3)0.135

Ni-Al hydrotalcite-like compounds, obtained by precipitation from homogeneous solution accomplished by urea hydrolysis, have been characterised for chemical composition, BET specific surface area, X-ray diffraction and thermogravimetry. The base strength of surface sites was estimated with the method of adsorbed acid-base indicators. A sample of formula [Ni0.73Al0.27(OH)(2)](CO3)(0.135), previously dried at 150degreesC, has been,used as base catalyst at 60degreesC in absence of solvents in the Knoevenagel condensation of different aldehydes with malononitrile and ethylcyanoacetate. Aliphatic and aromatic aldehydes easily react in these reaction conditions and the Knoevenagel adducts were obtained in excellent yield, in short time. Dimethylmalonate, having methylene groups of very low acid strength (pK(a) > 13) did not give the adduct with benzaldehyde because of not sufficiently high base strength of the hydrotalcite-like compound. The catalyst can be recycled several times, without loss of catalytic efficiency, and its performance has been compared with that of other heterogeneous base catalysts such as zeolite A, potassium exchanged zirconium phosphate, amberlist, modified hydrotalcites. (C) 2002 Elsevier Science B.V. All rights reserved.