Nanoestructured catalysts for organic reactions: design, synthesis and applications

Abstract

The development of more efficient and selective catalysts actively contributes for the design of safer, cleaner and less energy-demanding processes, and, therefore, catalysis is one of the cornerstones of green chemistry. In this sense, supported-heterogeneous catalysts represent a great advance in this field, especially because they overcome the difficulty in catalyst separation associated to homogeneous catalysts. In this context, this thesis is focused on the development of nanostructured materials for application in Organic Synthesis. In Chapter 1, a brief introduction regarding fundamental concepts in catalysis is presented, focusing on the synthesis and application of magnetically recoverable catalysts. Chapter 2 described the design, preparation and characterization of a new magnetically recoverable niobium nanocatalyst for application in the synthesis of 3,4-dihydropyrimidinones via Biginelli reaction. The catalyst (Fe3O4@Nb2O5) was prepared by coating magnetite nanoparticles with niobium oxide by using a simple wet impregnation method. The developed protocol was applicable to a wide range of aliphatic and aromatic substrates, and structurally diverse products were obtained in excellent yields. Additionally, the nanocatalyst could be easily separated from the reaction mixture with the aid of a magnetic field and reused several times without any losses in its catalytic activity. Moreover, experimental observations provided an insight into the reaction pathway. Chapter 3 describes the synthesis and characterization of a magnetic ZSM-5 zeolite with core-shell type structure for application on the valorization of bio-derived furfuryl alcohol. The magnetic HZSM-5 zeolite catalyst was prepared by the encapsulation of magnetite particles in the zeolite grains using a cationic polymer followed by calcination and next, an ion exchange to obtain the zeolite in its acid form. Remarkably, the catalytic system displayed a tunable selectivity to γ- valerolactone, alkyl levulinates and even levulinic acid by simply changing the reaction conditions. Furthermore, the catalyst could be easily recovered and reused for several reaction cycles without significant losses in its catalytic activity.