Bovine bone-derived natural hydroxyapatite-supported ZnCl₂ as a sustainable high efficiency heterogeneous biocatalyst for synthesizing amidoalkyl naphthols

Waste-derived materials have emerged as promising catalysts to meet the requirements for green chemistry. Catalysts and solvents are recognized as the most important components of the 12 principles of green chemistry. The reactivity of members of this class of materials can be increased by loading them with metal salts. Thus, in the present study, we loaded waste bovine bone-derived hydroxyapatite (BBHA) with 10⁻² mol L⁻¹ aqueous zinc chloride (ZnCl₂) solution and used it as a catalyst for synthesizing 1-amidoalkyl-2-naphthols derivatives in a solvent-free, one-pot, three-component condensation method with 2-naphthol, aldehydes, and either urea or acetamides at a reaction temperature of 80 °C. First, we extracted natural hydroxyapatite from waste bovine bone by thermal decomposition at 800 °C for 2 h. The hydroxyapatite was then loaded with ZnCl₂ and its physical and chemical characteristics were assessed. Second, the reaction conditions were optimized in terms of the catalyst loading, solvent, and reaction temperature. Under the optimal reaction conditions, the desired compounds were obtained with high yields (86%–96%) in only 25–40 min using a small amount of catalyst (50 mg). This proof-of-concept study demonstrates the potential use of a waste bone-derived natural hydroxyapatite-supported ZnCl₂ catalyst with high yields and sustainability.