TiO₂ and ZnO Nanoparticles Immobilized on Magnetic Sulfonated Melamine‐Formaldehyde: Structure Identification and Performance as Recyclable Catalysts for the Biginelli Reaction

Sulfonated melamine‐formaldehyde (SMF) containing high dispersed TiO 2 and ZnO nanoparticles were fabricated into magnetic nanocomposites by two different method material incorporation protocols, including calcination and wet impregnation. Their catalytic performance was investigated toward Biginelli reaction. For the preparation of the first catalyst series, a two‐step protocol was used. Initially, Fe 3 O 4 nanoparticles were coated with SMF (Fe 3 O 4 /SMF). Subsequently, they were decorated consecutively with TiO 2 (Fe 3 O 4 /SMF/TiO 2 ) and ZnO (Fe 3 O 4 /SMF/ZnO). The second set of catalysts was prepared using a two‐step process. Fe 3 O 4 nanoparticles were first used to modify SMF (SMF/Fe 3 O 4 ). Following the initial step, the surface of SMF/Fe 3 O 4 was decorated with both TiO 2 (SMF/Fe 3 O 4 /TiO 2 ) and ZnO (SMF/Fe 3 O 4 /ZnO). To understand the characteristics of the synthesized nanocomposites, a variety of characterization techniques were employed. New catalysts enabled the rapid synthesis (30–45 min) of a range of biologically active 3,4‐dihydropyrimidinones based on the Biginelli reaction with high yields (70–95 % isolated).