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

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-α-ethyl-2-cyanocinnamate into trans-α-methyl-2-cyanocinnamate may occur in the presence of strongly basic catalysts.