Morphology controlled phosphate grafted SnO2–ZrO2 nanocomposite oxides prepared by a urea hydrolysis method as efficient heterogeneous catalysts towards the synthesis of 3-substituted indoles

Abstract

In this study, a series of phosphate grafted SnO₂–ZrO₂ nanocomposite oxides are prepared via the urea hydrolysis of salt precursors followed by treatment with phosphate ions and calcination. The nanocomposite materials are characterized via XRD, FTIR, Raman, TGA/DSC, BET surface area measurement, XPS, TPD, FESEM and HRTEM techniques. The XRD study indicates the presence of a mixed-phase system consisting of Sn⁴⁺ substituted t-ZrO₂ and Zr⁴⁺ substituted SnO₂ phases. Additionally, the tetragonal ZrO₂ and cassiterite SnO₂ structures are stabilized in their respective solid solutions. Fourier analysis of the broadened XRD profile indicates an average particle size in the range of 15–30 nm. The XPS and IR study confirm the presence of bidentate surface grafted phosphate species coordinated to the metal centers. The TPD study indicates an enhancement in the number as well as strength of the acidic sites as a result of Sn⁴⁺ substitution in the ZrO₂ lattice followed by phosphate grafting. Gradual morphological changes from flake-like to spherical and finally to cuboid shape nanoparticles are observed with an increase in SnO₂ content in the composite. The HRTEM study shows the considerable mismatch along the grain boundary region for the two phases which can serve as potential active sites. Furthermore, the catalytic activity of the phosphate grafted SnO₂–ZrO₂ composites is evaluated for the synthesis of biologically important 3-substituted indoles via the multicomponent one pot condensation of indole, malononitrile and aryl aldehydes. Structurally diverse 3-substituted indoles are synthesized in excellent yield and purity in short reaction times using the phosphate grafted SnO₂–ZrO₂ composites as catalysts.