Marine carrageenan‐based NiO nanocatalyst in solvent‐free synthesis of polyhydroquinoline derivatives

Abstract

This study presents the preparation of nickel oxide nanoparticles (NiO NPs) in the presence of marine seaweed kappa‐carrageenan (κ‐carrageenan) polysaccharide as a green stabilizer and coating agent under optimal conditions. Thermal gravimetric analyzer (TGA) and Fourier transform infrared (FTIR) results revealed thermal stability and the presence of functional groups of κ‐carrageenan‐coated NiO (NiO@κ‐Car) NPs. The color change of the solution from green to black and advent peak at 320 nm primitively confirmed the formation of NiO NPs. Further, transmission electron microscopy (TEM) images of NiO@κ‐Car demonstrate rather irregular spherical and cubic morphology, showing an average size of 18 ± 1.5 nm. Further, X‐ray diffraction (XRD) analysis confirms that NiO NPs appear as cubic crystal structures with a crystallite size of 23.7 nm. According to the turnover number (TON) and turnover frequency (TOF) results, green NiO@κ‐Car exhibits superior catalytic efficiency in one‐pot multicomponent synthesis of polyhydroquinoline derivatives under free‐solvent conditions. Hydrogen‐1 (1H) and carbon‐13 (13C) nuclear magnetic resonance (NMR) spectra indicated the successful synthesis of various organic products. The key advantages of the proposed efficient synthetic protocol include reusability of the catalyst (four runs), simple workout, high yield of the products, environmental sustainability, and solvent‐free reaction condition. A possible mechanism was also suggested, indicating the role of NiO@κ‐Car as a proficient heterogeneous nanocatalyst in the reaction.