Keggin heteropolyacid in auto-tandem catalysis: confinement effects over ordered mesoporous silica in the synthesis of 2-pyridones

Abstract

It is necessary to design and develop simpler, more efficient, and cleaner synthetic methodologies to prepare increasingly complex molecules. One interesting strategy is multicatalysis. The aim of this study was to synthesize a multicatalytic hybrid system by the immobilization of H₃PW₁₂O₄₀ (HPW), a polyoxometalate, on ionic liquid-functionalized mesoporous silica (SBA-15 or MCF). The resulting catalysts were characterized by different techniques, including N₂ adsorption–desorption, Boehm titration of the acid site, SEM-EDS, FT-IR, XPS, CP-MAS, and NMR. To test the active site acid-oxidant duality of HPW, 2-pyridones were synthesized in a one-pot methodology with auto-tandem catalysis involving the ring-opening/ring-closure/oxidation sequence of 4H-pyrans. The morphology of the support structure affects the duality of HPW. Whereas the MCF complex favors oxidation, the SBA-15 complex favors Brønsted acidity (finding greater oxidation when adding water). The hybrid catalyst was recovered and reused for eight consecutive reaction cycles with no significant loss in activity.