Design of Scandium-Doped USY Zeolite: An Efficient and Green Catalyst for Aza-Diels–Alder Reaction

Scandium-exchanged USY zeolites were synthesized by vapor-phase exchange of parent HUSY with scandium trifluoromethanesulfonate (triflate) Sc(OSO 2 CF 3 ) 3 . Several FAU zeolites having scandium loadings ranging from 2 to 30 mol % were prepared and applied as catalysts to aza-Diels–Alder reaction in order to establish structure and catalytic properties relationships. Calcination at 623 K for 72 h led to preservation of the zeolite crystallinity along with the presence of different extra-framework aluminum species (EFAl) and scandium species, thus providing high Lewis acidity. Observation of the catalyst before, during, and after reaction, by multinuclear MAS NMR techniques and H/D isotope labeling, revealed the modification of aluminum and scandium environments within the zeolite framework. Recycling experiments stated for a truly heterogeneous catalytic path over these Sc–USY zeolites. In contrast to the homogeneous version for the aza-Diels–Alder reaction, the present heterogeneous new version provided a green route for quantitative synthesis of tetrahydropyridines via bifunctional heterogeneous catalysis.