AlCl3@ZnO nanostructured material: an efficient green catalyst for the one-pot solvent-free synthesis of 1,4-dihydropyridines

Abstract

AlCl₃-loaded ZnO nanoparticles have been explored as an efficient catalyst for 1,4-dihydropyridine synthesis under ambient temperature and solvent-free conditions. For this purpose, ZnO nanoparticles were synthesized by a simple solution-based precipitation technique using a stoichiometric amount of zinc sulfate and oxalic acid. The AlCl₃@ZnO nanocrystalline catalyst was prepared by loading 20% AlCl₃ on ZnO nanoparticles by a simple wet-impregnation technique. This catalyst efficiently performed Hantzsch pyridine reactions with various aromatic aldehydes, ethyl acetoacetate and ammonium acetate. The nanostructured AlCl₃-loaded ZnO catalyst was characterized by UV-DRS, XRD, FESEM, EDS, FETEM-STEM-EDS and XPS techniques. The comprehensive characterization reveals the formation of AlCl₃-loaded ZnO catalysts with an average particle size of 70–80 nm. The loading of AlCl₃ on the ZnO surface was confirmed by minor shifts in the XPS and XRD peaks. FETEM-STEM-EDS also indicates reasonable AlCl₃ loading on ZnO nanoparticles. The 20% AlCl₃-loaded ZnO nanocatalyst (AlCl₃@ZnO) confers 92% yield for the synthesis of 1,4-dihydropyridine under solvent-free and ambient temperature conditions. The synthesized 1,4-dihydropyridines were characterized by ¹H-NMR, ¹³C-NMR, HRMS and FT-IR spectroscopic techniques. The reported catalyst is highly efficient, environmentally friendly and could become an alternative to homogenous and heterogenous catalytic reactions.