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

• Published in: Applied clay science 2020-11, Vol.198, p.105833, Article 105833
• Main Authors: Velázquez-Herrera, Franchescoli D., González-Rodal, Daniel, Fetter, Geolar, Pérez-Mayoral, Elena
• Format: Article
• Language: English
• Subjects: Catalysts; Chromenes; Hydroxyapatite; Layered double hydroxides; Multicomponent synthesis; Solvent-free
• ISSN: 0169-1317; 1872-9053
• DOI: 10.1016/j.clay.2020.105833

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 m2/g, and thus, given rising to a high concentration of superficial hydroxyls reaching 7.1 × 1014 sites/g catalyst. Broad pore size distributions in composites enabled a rapid diffusion of the reactants to reach the basic hydroxyl sites. [Display omitted] •A new family of hydrotalcite/hydroxyapatite (HT/HA) composites were synthesized.•They were successfully applied as catalysts in the chromene synthesis.•The sizes of HT particles in the composites depend on the synthesis method.•High dispersion of HT particles increases the number of basic sites and surface area.•HT/HA composites are cheap, non-toxic and efficient catalysts.