Facile synthesis of imidazoles by an efficient and eco-friendly heterogeneous catalytic system constructed of Fe3O4 and Cu2O nanoparticles, and guarana as a natural basis

•High synergistic catalytic effect between ultrasound waves and a nanoscale system.•A novel eco-friendly heterogeneous catalytic system has been fabricated.•Natural polymer is magnetized with iron oxide nanoparticles.•An appropriate catalyst is prepared by copper(I) oxide nanoparticles.•High reaction yields were obtained for imidazole derivatives.