Catalytic application of CeO2–CaO nanocomposite oxide synthesized using amorphous citrate process toward the aqueous phase one pot synthesis of 2-amino-2-chromenes

Abstract

A series of CeO2–CaO nanocomposite oxides were synthesized by the amorphous citrate method and characterized by TGA, XRD, UV–Vis-DRS, SEM and TEM experimental techniques. The XRD analysis indicated the presence of a substitutional type solid solution for ceria rich composite oxides whereas for the calcium oxide rich composite oxides, the presence of crystalline CaO was observed along with the solid solution phase. Fourier line profile analysis of the broadened X-ray diffraction profiles revealed the presence of small nanocrystallites of the mixed oxides with size in the range of 5–25nm. UV–Vis-DRS study of the composite oxide showed well dispersion of the ceria in the CaO matrix. The characteristic reflections from the low coordinated surface Ce4+ ions were observed in the UV study along with a blue shift in the absorption edge for samples containing up to 20mol% ceria. The direct band gaps calculated from the UV spectra were found to be in the range of 3.45–3.66eV. The composite oxide materials were found to be porous and spongy in nature observed from the SEM analysis. TEM study of the CeO2(20%)–CaO material indicated the presence of well dispersed mixed oxide nanoparticles in a continuous CaO matrix. The Ce–Ca–O nanocomposite oxides were used as an environmentally benign catalyst for the multicomponent one pot synthesis of 2-amino-2-chromenes by multicomponent condensation of aryl aldehydes, α-naphthol and malononitrile in aqueous media. The nanocomposite oxides were found to be highly efficient for the synthesis of structurally diverse aminochromenes with excellent yield and purity.