Towards highly efficient hydrotalcite/hydroxyapatite composites as novel catalysts involved in eco-synthesis of chromene derivatives

摘要

A series of hydrotalcite/hydroxyapatite composites were investigated as novel catalysts in the green synthesis of chromenes in high conversions (up to 78% after 2 h), under solvent-free and mild conditions, from salycilaldehydes and cyano compounds. Materials were prepared by three different methods: impregnation of hydroxyapatite over hydrotalcite already crystallized; impregnation of hydrotalcite over hydroxyapatite and crystallization of both the components simultaneously. The composites resulted in the materials with high activity and selectivity for the 2-amino-4H-chromene synthesis. The catalytic performance was depending on the catalyst synthesis method and governed by the basicity of the samples and textural properties also varying as a function of the used cyano compound. Observed reactivity was attributed to the high dispersion of the hydrotalcite particles on the hydroxyapatite component, resulting in a considerable increase of the surface areas from 3 to 154 m(2)/g, and thus, given rising to a high concentration of superficial hydroxyls reaching 7.1 x 10(14) sites/g catalyst. Broad pore size distributions in composites enabled a rapid diffusion of the reactants to reach the basic hydroxyl sites.