Kinetic study of the condensation of benzaldehyde with ethylcyanoacetate in the presence of Al-enriched fluoroapatites and hydroxyapatites as catalysts

The Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate was performed in different solvents, at room temperature, in the presence of as-synthesized and Al-enriched fluoroapatites and hydroxyapatites as catalysts, and, for comparison, in the presence of a magnesium oxide and a mixed magnesium-aluminium oxide. The catalytic activity is significantly improved for the Al-enriched fluoroapatites and hydroxyapatites compared to their as-synthesized precursors. However, the highest activity is observed for the magnesium oxide and the mixed magnesium-aluminium oxide where the reaction is nearly complete within 15 min in the absence of diffusion limitation at the external surface. The higher activity of magnesium-based oxides can be explained by the differences in the nature of the active sites. MgO possesses principally isolated O2- sites, whereas the mixed oxide would possess Mg2+-O2- and Al3+-O2- acid-base pairs. The presence of Al3+-O2- pairs might also been postulated to account for the higher activity of Al-enriched hydroxyapatite and fluoroapatite, the higher activity being reinforced by the presence of superficial HPO42- basic species in the hydroxyapatite series. When the reaction is carried out in methanol as solvent, it was shown that transesterification of ethyl cyanoacetate into methyl cyanoacetate and trans-alpha-ethyl-2-cyanocinnamate into trans-alpha-methyl-2-cyanocinnamate may occur in the presence of strongly basic catalysts. (C) 2004 Elsevier B.V. All rights reserved.