CdO Nanostructures as Acid–Base Bifunctional Heterogeneous Catalysts for Making Coumarin-3-carboxylic Acids at Room Temperature

In this work, cadmium oxide (CdO) nanostructures were fabricated in good yields by calcining three different cadmium-based coordination polymer (CP) precursors, with a change in the alkyl group (methyl, ethyl, and tert -butyl) on the tridentate capping ligand, at 600 °C. The precursor-independent spherical morphology was observed in all the cases. On the other hand, a temperature-dependent hierarchical evolution of morphology from spherical (at 600 °C) to octahedron (at 700 °C) to icosahedron (at 800 °C) was observed for each of the precursors, as investigated by field-emission scanning electron microscopy. This hierarchical change in morphology has further been confirmed by the transmission electron microscopy (TEM) analysis. For all three CPs, the increase in the particle size of the nanostructures was correlated to an increase in the calcination temperature. With the help of Raman spectroscopy, the vibrational modes in these nanostructures were depicted, where the transversal and optical phonon modes and second-order optical modes displayed peaks at 270 and 950 cm –1, respectively. The energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy (XPS) were utilized for determining the elemental composition of the nanostructures─the presence of Cd and O only─evidencing their purity. The XPS analysis has also confirmed the Cd 2+ and O 2– oxidation states with the appearance of peaks in the ranges of 403–411 eV and 520–532 eV, respectively. Furthermore, the powder X-ray diffraction revealed the cubic phase of all the nanostructures. This was reaffirmed by the high-resolution transmission electron microscopy. Their characteristic absorption features were examined by the solid-state UV–vis diffuse reflectance spectroscopy, which showed a shoulder peak between 490 and 550 nm, and the resultant Tauc plots provided the band gap values in the range of 1.74–2 eV. A relationship of band gap values with particle sizes was established to justify the presence of the quantum confinement effect. Exploring their acid–base bifunctional nature, the spherical nanostructure (obtained at 600 °C) with the highest band gap value emerges as an extremely efficient, and recyclable heterogeneous catalyst for making coumarin-3-carboxylic acids (100% conversion) from the one-pot Knoevenagel-intramolecular cyclization tandem reaction of substituted salicylaldehydes and Meldrum’s acid in just 1 h at ambient conditions in methanol. The detailed reaction mechanism has been discussed with the help of theoretical calculations and experimental evidence.