Interrogating the Lewis Acidity of Metal Sites in Beta Zeolites with 15N Pyridine Adsorption Coupled with MAS NMR Spectroscopy

The Lewis acidity of isolated framework metal sites in Beta zeolites was characterized with 15N isotopically labeled pyridine adsorption coupled with magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The 15N chemical shift of adsorbed pyridine was found to scale with the acid character of both Lewis (Ti, Hf, Zr, Nb, Ta, and Sn) and Bronsted (B, Ga, and Al) acidic heteroatoms. The 15N chemical shift showed a linear correlation with Mulliken electronegativity of the metal center in the order Ti < Hf < Zr < Nb < Ta < Sn < H+. Theoretical calculations using density functional theory (DFT) showed a strong correlation between experimental 15N chemical shift and the calculated metal–nitrogen bond dissociation energy, and revealed the importance of active site reorganization when determining adsorption strength. The relationships found between 15N pyridine chemical shift and intrinsic chemical descriptors of metal framework sites complement adsorption equilibrium data and provide a robust me...