Bifunctional recyclable ZnO/MgO nanocomposite: solvent-free synthesis of chromenes and efficient water remediation

A ZnO/MgO nanostructured composite was prepared through the thermal degradation of a mixed zinc oxalate/magnesium oxalate precursor. The obtained ZnO/MgO has been characterized using thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The XRD reveals a crystalline structure with a crystal size of 33 nm. The TGA confirmed the purity and stability of ZnO/MgO NPs at high temperatures. The ZnO/MgO NPs served as an efficient heterogeneous catalyst for the solvent-free, room-temperature Knoevenagel-Michael domino assembly of a library of 2-amino-4H-chromenes. The designed catalyst offers short reaction times (8-12 min), excellent yields (91-99%), easy recovery and reuse for at least 4 cycles without a noticeable drop in efficacy, and maintains good performance in scale-up experiments. Furthermore, the composite exhibited promising photocatalytic activity, achieving 96.36% removal of methyl orange dye (5.0 ppm) under solar irradiation within 30 min, outperforming pure MgO (3.64%) and ZnO (18.18%). Comprehensive post-reaction characterization reveals that ZnO/MgO retains its structural and chemical integrity following both heterogeneous organic catalysis and photocatalytic dye degradation. These findings indicate that ZnO/MgO exhibits promising catalytic and photocatalytic performance under the investigated conditions.